Magnetic Hard Disks - Fuji Electric€¦ · Recording Density Enhancement and 7 Material-Process Technologies for Magnetic Hard Disks Next Generation HDI Technologies for Magnetic

Whole Number 188

Magnetic Hard Disks

Fuji’s innovation is at the forefrontof technology

Magnetic hard disks are key components of hard disk drivesfor computers. Demand for high-performance disks isrising dramatically, driven by increasingly informationintensive societies and progress in the multimedia age. FujiElectric Co., Ltd. has led the world in sputtering technologysince 1983, which allowed the manufacture of magnetic

hard disks suitable for high-density recording. In the age ofrapid technological development, Fuji Electric has achieveda first-class position in the world market for magnetic harddisks and has earned a strong reputation for quality andreliability from customers worldwide.

Fuji Magnetic Hard Disks

CONTENTS

Present Status and Future Prospects for Magnetic Hard Disks 2

Recording Density Enhancement and 7Material-Process Technologies for Magnetic Hard Disks

Next Generation HDI Technologies for Magnetic Hard Disks 14

Magnetic Hard Disks for Audio-Visual Applications 19

Automation of Magnetic Hard Disk Production Lines 26

Ultra-Clean Technology for High Density Magnetic Hard Disks 31

Cover Photo:The personal computer (PC) is

now an indispensable tool to offices.The magnetic hard disk is one of keycomponents for computers.

The disk requires extremelyhigh technology. Fuji Electric ismaking efforts to enhance thecapacity of disks for home audio-visual devices as well as for comput-ers.

The cover photo shows an imageof the contribution to society withspotlighted disks superposed uponan office worker operating a per-sonal computer which contains adisk storage.

Head Office : No.11-2, Osaki 1-chome, Shinagawa-ku, Tokyo 141-0032, Japan

Magnetic Hard Disks

Vol. 46 No. 1 FUJI ELECTRIC REVIEW2

Tomonobu OgasawaraKenji Ozawa

Present Status and Future Prospectsfor Magnetic Hard Disks

1. Introduction

Since the application of GMR (giant magnetoresis-tive) technology to the heads of magnetic hard disks,increased density has resulted in an increase of over60% a year in the capacity of magnetic hard disk drives(HDDs). Previously, the main applications of HDDswere in personal-use desktop or notebook computers.The range of application has expanded to high-endservers that make use of the large capacity, to microHDDs that utilize the advantages of high density andsmall size, and recently to audio-visual applications.

Characteristic technologies suitable for each appli-cation have been advanced. This paper describes thepresent status and future prospects of magnetic harddisks (hereinafter referred to as the disk) for personalcomputer applications.

2. Trends of the Market and Technology

2.1 Increasing capacityFigure 1 shows the increase in areal recording

density per year. Since the application of the GMRhead, areal density has increased in capacity over 60%a year, and some product segments are competing foran annual capacity increases of over 100%. Theincreased capacity was first utilized with 2.5-inchpersonal computers and then was later applied to thespecifications of desktop personal computers. With theincrease in density, flying head height has becomelower each year as shown in Fig. 2. Recently, a level of0.4 to 0.5 µ inch, the limit of flying height measure-ment by the current valuation method, has generallybeen required. The technology to measure and evalu-ate low flying heights near contact is also an importanttechnical subject.

2.2 Increasing transfer rateIn addition, the transfer rate for high-end servers

has increased, and HDD rotating speed and densityare both increasing at the same time. Currently, 5,400r/min is the standard HDD speed; however, 7,200 and10,000 r/min speeds have become commonplace andproducts with speeds on the order of 15,000 r/min are

introduced to the market. Figure 3 shows the increasein the transfer rate.

Based on technical demands for higher speed andhigher density, it is believed that HDDs mounted withdisks using glass substrates of higher rigidity thanformer aluminum ones will shortly be marketed. Thus

Fig.2 Reduction in flying head height required each year

Fly

ing

hei

ght

(µin

)

4

3

2

1

0

Year20022000199819961994

Are

al d

ensi

ty (

Mb/

in2)

100,000

10,000

1,000

100

10

1200520001995

Year of first shipment

IBM Demo500Mb/in2

IBM Demo1Gb/in2

IBM Demo 5Gb/in2

IBM Demo 10Gb/in2

IBM Demo 11.6Gb/in2 Read Rite Demo13.5Gb/in2

Seagate Demo16.3Gb/in2

High-end areal density (60% rise a year)

Low-end areal density (27% rise a year)

199019851980

IBM Demo (May ’99)20.1 Gb/in2, 490 kBPI × 41 kTPI

+ +Fujitsu Demo (May ’99), 20.1 Gb/in2 (405 kBPI), Hc = 3,380, tMr = 0.42, FH = 0.67 µin

HMT Demo (May ’99), 20.9 Gb/in2, 480 kBPI × 43.5 kTPI, Hc = 2,500, tMr = 0.4 (2 layers), protective film 50Å

Fig.1 Increase in areal recording density per year


far, applications of disks with glass substrates havespread only to the limited market of notebook comput-ers that can take advantage of their impact resistance.However, it is predicted that applications will spreadto the 3.0 and 3.5-inch HDD field in pursuit ofsmoothness, flatness, and low waviness.

2.3 Lower costThere is strong demand for low-priced HDDs in

sub-thousand dollar personal computer marketplace.Because performance improvements and price reduc-tions are continuing simultaneously, the review ofprocesses and component materials has become urgent.There is a growing demand for a low-cost one-head/one-disk HDD that is thoroughly economical.

3. Present Status of Magnetic Hard Disks

Only current topics are described here.

3.1 Substrate size, surface quality, and materialsThere is a great deal of confusion regarding

substrate materials and sizes among recent hard disktechnologies. With the aim of reducing non-repeatablerun-out (NRRO) to suppress so-called track missregistration (TMR) caused by high-speed rotation,various substrates have been used according to variousHDD specifications. Almost all 2.5-inch disks use glasssubstrates. However, both aluminum and glass sub-strates are being considered for 3.0-inch or largerdisks.

Aluminum and glass substrates have become near-ly equal in surface quality, and limitations of charac-teristics, cost, and mass productivity are becoming key

issues. The greatest advantage of glass substrates istheir high resistance against impact (head slapping),considered indispensable to HDDs that use a rampload system. High-end servers require substrate sizesbased on consideration of the balance between capacityand airflow loss of the head. Table 1 shows suitablecombinations of diameters at intervals of 5 mm andsubstrate thicknesses.

With the recent increase in capacity, flying headheights have remarkably decreased as low as 0.4 to 0.5µ inch, near contact.

To attain large capacity, stable flying of the head isabsolutely necessary. When examining hard disksonly, excluding the HDD mechanism, clamping strain,and motor vibration, it is necessary to consider me-chanical characteristics including flatness, smooth-ness, and rigidity, and physical constants such as thedamping constant for resonance. With regard tosubstrate surface quality, attention must be paid to theinfluence of substrate micro-waviness on glide height(GH). The mean surface roughness (Ra) of recent harddisks has been reduced to the order of 5 to 6 Å and theflying head height does not depend on Ra at such alevel. Instead, as shown in Fig. 4, a correlationbetween substrate micro-waviness and takeoff heightis observed and flying head height has a tendency toimprove with reduced micro-waviness.

Further, to suppress the above-mentioned TMR,measures such as increasing the substrate thickness,substrate rigidity, and damping constant are consid-ered. An example of the influence of substratethickness on NRRO is shown in Fig. 5. Increases to thesubstrate thickness are limited by the HDD mecha-nism. A glass substrate is being developed to raise thepresent substrate rigidity from the 100-GPa level tothe 140-GPa level. Because the damping constant isan intrinsic physical constant of the substrate, it isdifficult to control freely. Therefore, it is necessary tochange the substrate material to a quite differentmaterial, or to consider the laminate structures beinginvestigated by some parties.

3.2 Ramp-load systems and padded headsThe ramp load system mentioned above has been

Fig.3 Increase in the transfer rate each year Fig.4 Correlation between disk waviness Wa and flying headcharacteristics

Table 1 Substrate size requirements

10,000

1,000

100

10

Tra

nsf

er r

ate

(Mb/

s)

Year20022000199819961994

Ou

tpu

t vo

ltag

e (V

)

6

5

4

3

2

1

0

Takeoff height (µin)0.50.450.40.350.30.250.2

Wa = 4ÅWa = 6ÅWa = 8ÅWa = 10ÅWa = 12Å

ThicknessDiameter

0.635mm

2.5 in (65 mm) ○

1.0mm

○

1.27mm

○

2.75 in (70 mm)

3.0 in (75 mm)

3.3 in (84 mm)

3.5 in (95 mm)

0.8mm

○

○

○

○

○

○

○

○


installed in many of IBM’s HDD notebook computersand already has been verified technically. However,the HDD speed specification has the comparatively lowvalue of 4,200 r/min, and technology compatible withspeeds higher than 7,200 r/min, believed to be themainstream in the future, has not yet been estab-lished. The impact due to head slapping at the time ofhead loading and unloading is larger than estimated,and there is the danger of serious damage to the headand disk if the mechanism is of poor design orprecision.

On the other hand, contact start stop (CSS)systems using a padded head as shown in Fig. 6 arebeing studied. The style depends upon the HDDmanufacture, and there is an infinite variety, but thesesystems are basically designed to avoid wear with apadded head which has reduced spring force. Asmooth head slider surface can be applied with fairlylarge torque. However, motor torque is minimized inHDDs installed in notebook computers to reduce powerconsumption and heat generation, and therefore, thepossibility of adhesion can not be ruled out and somesurface adjustment is required.

3.3 Magnetic characteristics and noise reductionAlthough the use of a GMR head enables the

reading of feeble signals, how to reduce the media’snoise to improve the signal-to-noise ratio (SNR) re-mains a problem.

As discussed in various institutes and symposiums,reduction in noise basically depends on how a uniformcrystallographic structure of fine magnetic grains canbe formed in a high vacuum and under high-speedconveyance (in a short time). Fuji Electric has madecontinuous efforts to achieve an optimum material,process, and layer configuration.

With regard to the recent increases in density, thecharacteristic of thermal decay has to be considered.Generally at the current technical level, increases infineness and uniformity to reduce noise are notcompatible with thermal stability. For example, use ofmulti-layer magnetic media is very effective for noisereduction, but has the drawback of low thermalstability. Figure 7 shows a comparison of TEM imagesof magnetic grain sizes to different noises, and Fig. 8shows a comparison of thermal characteristics accord-ing to different magnetic materials.

3.4 Hard protective films to reduce magnetic spacingUntil contact recording can be implemented, flying

head recording is the mainstream, and magneticspacing must be minimized to realize high density.

Fig.5 Correlation between substrate rigidity and NRRO

NR

RO

(µ

in)

6

5

4

3

2

1

0

1.25µ in 113 to 130GPa

1.0µ in 120 to 130GPa

0.86µ in Min165GPa

1,100 1,300Servo frequency (Hz)

1,500 1,700 1,900900700500

N : rotating speed D : outside diameter of substrate t : substrate thickness E: Young’s modulus

: damping constant : Poisson ratio

NRRO ∝ (Nm × D2) / t 3/ [E × / (1- 2) ]νξ

ξ ν

Fig.6 Conceptual sketches of padded head

The head slider has 3 to 7 pads on the surface.

Only the pads in contact with the media’s surface reduce friction force by reducing the contact area.

Fig.7 Comparison of TEM images of magnetic grain sizes todifferent noises

(a) An example of highernoise media(coarse grain size)

(b) An example of lowernoise media(fine grain size)

5nm 5nm


Measures for reducing magnetic spacing in the currentsystem are reduction of the disk protective film, thehead protective film, and the head recess as shown inTable 2.

The disk protective film has thus far utilized acarbon or diamond-like carbon film by the sputteringmethod. Because of increasing densities, specificationsof protective film thicknesses have changed from theformer 100-Å level to become as strict as the 60-Ålevel. Further, film made by sputtering, intrinsically aporous film, can not satisfy corrosion resistance anddurability requirements. Manufacturers are tacklingthe development and application of chemical vapordeposition (CVD) films, harder and denser films. Table3 shows typical CVD methods and features. In CVDfilm formation, the development of a film compositionand a film formation method to achieve the desiredfunctionality is of course important, but mass produc-tivity and measures against particles in continuousfilm formation are more important. Fuji Electric isalso striving to develop various CVD films.

3.5 Surface treatment after sputteringRecent media have had problems with head disk

interface (HDI) characteristics that affect reliability.Disk manufacturers have different opinions regardingtreatments after sputtering film formation. FujiElectric is also promoting a review of the work process.The aim is to eliminate irregular projections in filmformation, improve surface coverage of the lubricant,and improve the bonding ratio by surface treatment.In addition to the various improvements that werepreviously implemented, smooth substrates and CVDfilms have been used on a full-scale for protectivefilms, and a post-treatment process appropriate for the

properties of CVD films is considered necessary. Tosuppress lubricant migration when media are used inhigh-speed rotating servers, it is necessary to establishpolymerization and refinement technologies for thelubricant itself. Figure 9 shows an example of GHcharacteristics improved by surface treatment afterlubricant application.

3.6 Error, defect, and automatic viewer/test systemAs densities increase, it is predicted that problems

of errors and defects due to each material, process,production schedule, and working environment willbecome more severe. The recording bit size of 20-Gb/in2 media, soon to be used in practical applications, isat a level of 0.05 µm × 0.5 µm, and a minute cleaningresidue or particle will cause the error characteristicsto deteriorate. A defect in the servo signal portionmakes read/writing impossible.

Under these circumstances, the quality assuranceof magnetic disks requires stricter detection of micro-defects than before. At the same time, to satisfy thecurrent demand for low prices, we must simplify thosetest systems that require the most labor and equip-ment investment. Viewed from the manufacturingprocess, defect and particle inspection just beforesputtering and testing is important.(1) Surface defect inspection before sputtering

Objects for inspection before sputtering are clean-ing residues and scratches due to substrate cleaningand surface machining. When glass substrates areused, improved resolution is required because of thetransparency.(2) Surface defect inspection before testing

The objects for inspection are defects in thesputtering process and final tape polish (FTP) scratch-es. At present, defect sensitivity is 0.8 µm, however

Fig.8 Comparison of thermal characteristics according todifferent magnetic materials

Mag

net

ic r

elax

atio

n (

%)

0

10

20

30

40

50

Time (s)

Applied reverse field = 2,500 Oe

Sample

Type 1/CrY

Type 2/CrY

Type 3/CrX

2,894

3,109

3,012

(Oe)Hc

0.39

0.41

0.39

(memu/cm2)tMr

Type 1/CrY

Type 2/CrY

Type 3/CrX

10,0001,000100

Table 2 Items for reducing magnetic spacing (from StorageResearch Consortium documents)

Surface roughness of the head and disk off 4 nm

Protective film thickness of the head slider off 3 nm off 14 nm in total

Head recess off 2 nm

Protective film of the disk off 5 nm

Table 3 Typical methods of chemical vapor deposition andfeatures

CVD method Film quality

Filament type ion beam ○

Hollow cathode type ion beam ○

ECR (Electron-Cycrotron-　　　　　　　　 Resonance) ◎

Necessity of bias

Yes

Yes

No

Continuousoperation capability

○

△

△

RF discharge type ○ Yes ◎


even higher sensitivity is required to meet the rapidincreases in density.(3) Simplification of testing

A defect inspection system that combines a defectinspection system based on an optical method and astatistical processing technique is being studied and isexpected to simplify the testing process and reduce theproduction cost.

4. Future Prospects

Based on the present status as described above,future prospects for magnetic disk technology arediscussed below.

4.1 Change of substrate material to glassFuji Electric also manufactures aluminum sub-

strates in its factory. Because of rapid technicalinnovation, it is difficult at present to discriminatebetween aluminum and glass substrates with regard tothe characteristics of smoothness, flatness, and micro-waviness. Similarly in the case of server applicationsrequiring rigidity, there is a movement to satisfyimmediate needs by using thick aluminum substrates,

and on the other hand, there is in fact a demand forglass substrates with high rigidity. Cost reduction iscritical for glass substrates to be used for generalpurposes other than notebook computer and high-speed rotation applications. This requires the precon-ditions that a supply of thinner blanks is available anda higher-rate polishing technique is established.

4.2 Limit of longitudinal recording and perpendicularrecording systemsManufacturers have different opinions regarding

the limit of areal recording density by longitudinalrecording. We believe the average opinion is thatlongitudinal recording can definitely reach 40 Gb/in2,based on current technical developments.

At INTER.MAG held by IEEE in Korea in 1999,perpendicular magnetic recording system was the topicof conversation. Because the thermal stability oflongitudinal recording has been highlighted in theStorage Research Consortium (SRC) and various sym-posiums, the superiority of perpendicular magneticrecording was brought into the spotlight. However,perpendicular magnetic recording also has some prob-lems and the road is not yet paved for success. First,even perpendicular recording disk needs to be thinnermagnetic film, and the problem of thermal stability isnot enough eliminated. Secondly, although peripheraltechnologies are somewhat prepared, a technology withhead characteristics comparable to those of longitudi-nal recording is not yet established. Thirdly, anexternal magnetic field causes demagnetization, whichmay be the fate of perpendicular magnetic recording.It will take some time to clear these problems, andtherefore, longitudinal recording has the possibility ofcontinuing until 60 to 80 Gb/in2.

4.3 Lower costThe most effective methods for the cost reduction

of magnetic disks are the simplification of testingsystems by combining optical techniques and statisti-cal treatments and the application of plastic materialto substrates that account for a large proportion ofmaterial costs.

5. Conclusion

For materials and process technology related toincreasing densities, HDI technology, production equip-ment related to reducing errors and defects, and detailsof cleanness control technology, please refer to separatearticles in this special issue. The hard disk will forsome time continue to increase in density with alongitudinal recording system and to maintain itsposition as the major component of external storagedevices while extending its range of applications. FujiElectric will continue to tackle the many technicalproblems.

Fig.9 Example of GH characteristic improved by surfacetreatment after lubricant application

11.9

11.9

22.5 26.25 30 33.75 37.5 41.25 45(mm)

1,000.0

875.0

750.0

625.0

500.0

375.0

250.0

125.0

0

1,000.0

875.0

750.0

625.0

500.0

375.0

250.0

125.0

0

(mV

)

22.5 26.25 30 33.75 37.5 41.25 45(mm)

(mV

)

With post-treatment

Without post-treatment

Recording Density Enhancement and Material-Process Technologies for Magnetic Hard Disks 7

Nobuyuki TakahashiYoshiharu KashiwakuraKazuyoshi Shibata

Recording Density Enhancementand Material-Process Technologiesfor Magnetic Hard Disks

1. Introduction

Recent magnetic recording media for magnetichard disk drives (HDDs) have made remarkable tech-nological advancement. When Fuji Electric startedmanufacturing sputtered thin film magnetic disks in1985, the recording capacity was 10 to 20 Mbytes per3.5-in disk. Currently, disks with a capacity of 7Gbytes are available in the market. Moreover, manu-facturers are striving to develop 20-Gbyte disks andbring them to market in the year 2000.

The technology that has supported this recordingdensity enhancement has largely been due to themagnetic head that has developed from a thin filmhead to a magnetoresistive (MR) head and then togiant magnetoresistive (GMR) head. On the otherhand, the technological advancement of the magneticdisk (hereinafter referred to as the disk) itself thatmaintains the stability of extremely small recordingbits should not be overlooked.

This paper describes Fuji Electric’s magnetic harddisk technology that has supported recording densityenhancement, and in addition, discusses the presentstatus and the problems of future development for thistechnology.

2. Technological Trends of Recording Density

In the 1980s, thin film technology with sputteringinstead of coated disk supported magnetic recordingdensity enhancement at an annual rate exceeding 30%.There is also a history of increasing coercive force anddecreasing magnetic recording layer thickness, indexesof important disk characteristics, to realize very smallrecording bits in the course of increasing magnetichead sensitivity.

In the 1990s, the practical use of MR headsincreased recording density by 60% annually. In themeantime, with regard to media characteristics, reduc-tion of media noise was carefully studied and moreanalytical investigation into magnetic materials andfilm deposition processes was initiated. Also, to solvethe problems of low flying-head height to reducemagnetic spacing and thermal asperity due to contact

with the media, peculiar to MR heads, smooth disksurfaces for the data zone were realized withoutirregular projections and particles, ensuring that theywould be used in practical applications. In addition tothe data zone, a contact start-stop (CSS) zone wasrealized with a laser texturing process to form a newlycontrolled surface of high-density bumps. This allowedfor separate durability characteristics according to thefunction.

The GMR head, introduced in the middle 1990s,has raised recording density by 100% annually in somefields of HDD application. Bit cost lowered byadvances in recording density and new mechanicalcharacteristics such as the ramp load method insteadof the CSS method and high rotational speed technolo-gy have established HDD with distinctive features foreach application field, and the HDD market is showingindications of further expansion. In response to thissituation, there is a clear trend to replace conventionalAl/NiP substrates with glass substrates in fields thatrequire impact resistance and rigidity against high-speed rotation. A successful recording density of20 Gb/in2 at the research level has been announced(1)

and mass production is expected in 2000.

3. Longitudinal Recording System

The details of actual technology for disk used thusfar in the longitudinal recording system, classified intothe requirements of disk composition and the keyformation process, are described below.

3.1 Disk compositionAs shown in Fig. 1, disk is composed of a substrate,

Fig.1 Typical layer structure of magnetic hard disks

Protective layer with lubricant (10 to 15nm)

Magnetic recording layer (20 to 30nm)

Under layer (20 to 40nm)

Substrate (Al/NiP plating)


a magnetic recording layer, and a protective layer withlubricant. Media characteristics required to increaserecording density can be examined by considering howthe magnetic transition length a, inversely proportion-al to linear recording density, can be reduced.

a = [{4Mr·t (d + t /2)}/Hc ]1/2………………………… (1)where Mr : remanence

t : magnetic layer thicknessd : head-disk magnetic spacing

With regard to characteristics required of thesubstrate, to reduce the head-disk magnetic spacing(d), surface smoothness such as surface roughness (Ra)and micro-waviness (Wa) is most important. As shownin Fig. 2, values below 1nm are in practical use, andfurther reduction in the values will be required forlower flying-head height or near-contact spacing in thefuture. To control the circumferential orientation ofmagnetic characteristics to be discussed later, mechan-ical texturing is applied within a range that does noteffect the flying-head height. The opportunity toexpand HDD application fields was made possible bychanging the material from conventional Al/NiP toglass. Glass substrates are being used in the marketfor small, portable personal computers that requireimpact resistivity, and are accumulating good results.Because of its potential for higher rigidity, the practi-cal use of glass substrates in higher rotational speedapplications is expected.

With regard to characteristics required of themagnetic recording layer, maintaining stability of verysmall recording bits is an important issue from the

viewpoints of reading and deterioration with thepassage of time. From the above formula, the ratio ofthe product of remanence and thickness (Br·t) tocoercive force (Hc ) is a basic magnetic parameter. Tosupport the three digit increase in recording densitysince thin film magnetic disks were put to practical useuntil now, Hc has changed from 600Oe to 3,000Oe andBr·t has changed from 700G·µm to 70G·µm [decrease ofone digit in (Mr·t / Hc )1/2 during this period]. Co alloyshave consistently been used for the magnetic material.However, the composition has greatly changed and aneffective composition ratio for optimizing the design ofsaturated magnetization and for decoupling crystalgrains by Cr segregation has been selected from acombination of component elements including Cr, Pt,and Ta. An example of a typical Cr segregationstructure currently in practical use is shown in Fig. 3as examined with a TEM/EDX. Cr segregation can beseen on the grain boundary in the figure. As the layercomposition in Fig. 1 shows, the presence of an underlayer and a seed layer that realize desirable crystalorientation for the magnetic characteristics of a uppermagnetic layer and effect the adjusting of grain sizeare indispensable prior to magnetic layer formation.Formerly, Cr was used for the under layer; however, aCr alloy containing W, Mo, and Ti is being commonlyused to meet changes in the upper magnetic layer andto optimize conditions. An example X-ray diffraction ofa recent typical magnetic layer constructed above anunder layer is shown in Fig. 4. The figure shows thatthe crystal orientation of the Co alloy magnetic layer(110) develops on top of the crystal orientation of theCr alloy under layer (200), and a desirable in-planeorientation of the c-axis (easy magnetization axis) ofthe hexagonal close-packed (hcp) structure is realized.Among magnetic characteristics, c-axis alignment with

Fig.3 Cr segregation structure of a magnetic grainFig.2 Typical surface texture of a magnetic hard disk (Alsubstrate)

R a 9A (AFM)

Wa 3A (Zygo)

(A)

40.0

(nm

)

20.0

0

0

0

00

0.71-100

+100

0.94

2.5

2.55.0

5.0

7.5

7.5

10.0

10.0

(µm)(µ

m)

(mm)

(mm

)

Cr

con

ten

t (a

t %

)

30

25

20

15① ② ③

Point of measurement

Point of measurement

④ ⑤

①

○②

③

④

⑤

○○

○


Fig.5 Edge noise characteristics (oriented and isotropic disk)

the circumferential direction due to anisotropic filmstress along the substrate texture line is characteristicof Al/NiP substrate systems (so-called oriented disk).On the other hand, it has been determined that theformation of a specific seed layer is effective in formingthe desired magnetic layer on a glass substrate, andthe NiAl alloy layer has already been put to practicaluse. As a result, proper magnetic grain size and thein-plane orientation of the c-axis of the magnetic layerare realized even on a texture-less, smooth glasssubstrate. Because of this absence of texture, differentfrom the Al/NiP system, the c-axis is randomly orien-tated within the plane and an isotropic magneticproperty is obtained. At current recording densities,oriented media is generally superior in on-track para-metric performance. However, isotropic disk is superi-or in track-edge noise as shown in Fig. 5, and in orderto improve TPI as recording density increases in thefuture, there is the possibility that isotropic disk willhave the advantage. It is necessary to promote furtheroptimization of high density recording areas, includingmaterials.

The development of magnetic layers has beencontinued with the goal of reducing media noise torealize a high signal-to-noise (SNR) ratio. However,when recording density exceeds 10 Gb/in2 (for example,400 kBPI × 25 kTPI), bit size is less than 0.06 µm × 1µm, and only several grains exist in a bit along thedirection of magnetic transition. In this case, recordedbits cannot be kept stable due to thermal fluctuationwith the passage of time, and the possibility ofspontaneously decaying output signals grows withreduction of magnetization.

It is important to keep a parameter, given by(magnetic anisotropy constant: K u) × (activation vol-ume: V), above a certain value to prevent this thermal

fluctuation phenomenon(2). V is related also with theabove-mentioned grain size and there is a tradeoffbetween V and the preferable method for media noisereduction. A solution advantageous to both is to makethe distribution of grain sizes as narrow as possibleand reduce the proportion of larger and smaller grains.On the other hand, Pt added to the alloy to make smallgrains thermal-fluctuation-resistant and a segregationstructure that diffuses non-magnetic Cr to the grainboundary to raise anisotropy in grains are important.To simply evaluate thermal fluctuation, we applied(3)

measuring methods for Hc dependence on temperatureand magnetization decay in a reverse magnetic field asshown in Fig. 6, and have utilized this method as anevaluation tool. For present disk, the temperaturecoefficient of the former (∆Hc /Hc /∆temp) is approxi-mately - 0.3%/deg, and the latter is approx. 8%/decadeof time (s). These guidelines will be held for futurehigher recording density. Regardless, the coexistenceof noise and thermal fluctuation will surely becomemore difficult, and we shall have to reevaluate record-ing systems.

Lastly, details of the protective layer with lubri-cant are omitted since it is described separately. Withmagnetic spacing d (magnetic flying height + protec-tive layer thickness), since linear recording density canincrease in proportion to 1 / d , reduction in protectivelayer thickness is effective when the head flying heightis low. Lubricant on the surface that reduces thecoefficient of friction between the head and the disk isimportant not only from the viewpoint of tribology, butalso because flying height instability due to thetransference of contamination and lubricant to thehead will cause defects in parametric performance.

3.2 Process technologyActual process technology is described below.

2.01.51.00.50

108

12

14

16

18

20

22

24

2628

-0.5-1.0-1.5-2.0

Med

ia n

oise

(µV

)

Offset from the head track center (µm)

Oriented diskIsotropic disk

Disk noise increases at track edges on the oriented disk

Fig.4 X-ray diffraction pattern of a magnetic layer

XRD pattern (2 : 30° to 90° )θ

θ(a) 2 = 60° to 67° θ(b) 2 = 70° to 77°

300

250

500

750

1,000

1,250

40

Inte

nsi

ty (

cou

nt)

50 60 70 80 90

Cr (200)

Cr (200)

Co (110)

Co (110)


3.2.1 Substrate processThe correlation between requested characteristics

and process parameters in Al/NiP substrates is shownin Fig. 7.

To ensure head stability at a low flying height,local projections or scratches must also be consideredin addition to the average smoothness and flatness ofthe substrate surface, and each step of every machin-ing process requires precise control. In particular, thetexturing process that creates the final finish on thesubstrate surface has to realize extremely low rough-ness while removing enough stock to eliminate abnor-malities (scratches, etc.) on the surface. It is importantto select shape-optimized abrasive grids for sufficientstock removal and tape material to fix the grids inplace. However, there is a tradeoff between stockremoval and surface roughness. To ensure a uniformtextured surface with a low rate of stock removal,greater smoothness and flatness at the stage of thepolishing process is important. Improvements in Algrinding and NiP plating as well as the utilization ofnew processing conditions for the polishing processrealize very smooth surfaces as shown in Fig. 8. Thechallenge for future processes will be to realize asurface that ensures stable head flying, assuming thatflying-head height is made as low as possible (glideheight < 0.3 µin), requiring much more precision

Fig.7 Correlation between characteristics and processes in Al/NiP substrates

Fig.8 Surface image of a polished substrate (Al/NiP)

(A)

00

0.71-100

+100

0.94(mm)

(mm

)

Characteristics

Smoothness

Flatness

Cleanness

(Roughness)

(Micro-waviness)

(Roll-off)(Flatness)(Freedom from contamina-tion and foreign matter) Cleaning

Process

Al grinding

NiP plating

NiP polishingNiP texturing

Table 1 Correlation between disk characteristics and deposi-tion processes

Process

Notes*: Degree of vacuum indicates evacuation to lower pressure. Magnetic layer composition indicates increase in the content of a component.

Remarks

Noise

Degree of vacuum Higher vacuum necessary

Optimization necessary

Change in media character-istics* due to increase in a process parameter

Increases Decreases

Magnetic layer composition

Cr

Substrate temperature

There is a maximum value

There is a minimum value

Ta

Ar gas pressure Decreases Decreases

Pt

Under layer thickness (Cr alloy) Increases Increases

Substrate biasDependent upon layer structure

Dependent upon layer structure

Decreases Decreases

Decreases Decreases

Increases Increases

Hc

control in each process. Smoothness and flatnessrequirements for processing glass substrates are simi-lar, however changes in the processes are necessary tocomply with special physical property requirements(high hardness and high insulating property).3.2.2 Magnetic layer deposition process

As mentioned previously, the characteristics re-quired of a magnetic layer to attain high-densityrecording are high coercive force Hc and low noise. The

Fig.6 Dependence of Hc on temperature and magneticviscosity in a reverse field

10

8

6

4

2

0

- 2

A

EDCBA

-10.6-11.8-9.3-8.9-9

-0.32-0.35-0.32-0.28-0.32

3,3563,0632,6202,9302,552

0.580.450.660.550.68

2323232321

88383

AI上

BCDE

6 7 8 9

(a) Dependence of Hc on temperature

10

∆ Hc / HcMr . t

(emu/cm2)∆ T ∆ Hc / ∆ T / Hc

(Oe/°C) (Oe) Cr+Ta(at %)

Pt(at %)(273K) /°C

11 12

2.52.01.5

Applied reverse field H/ Hr

1.00.50Mag

net

ic v

isco

sity

S (

% /

deca

de)

(b) Magnetic viscosity in a reverse field

Isotropic disk 1 (3,650/3,400/0.4)Isotropic disk 2 (3,650/3,400/0.4)Oriented disk 1 (3,250/3,000/0.55)Oriented disk 2 (3,550/3,300/0.55)

∆ Hc / ∆ temp (-Oe/°C)

(Hr / Hc / Mr . t )


Fig.9 Dependence of Hc and noise on several processparameters

10

9.8

9.6

9.4

9.2

9180 200 220 240

Temperature (°C)260 280

300260220180140Temperature (°C)

Noi

se (

µV)

8.6

8.4

8.2

8.0

7.8

7.60.5 1 1.5 2

Ar gas flow (sccm)

Noi

se (

µV)

3,600

3,200

2,800

2,400

2,000

1,6001,200

Hc (

Oe)

Fig.10 Dependence of Hc on seed layer thickness

Fig.11 Parametric performances (isotropic and oriented disk)

1.0

0.9

0.8

0.7

0.6

0.5

0.40 25 50 75

0 25 50 75

Seed layer thickness (NiAl) (nm)

3,6003,5003,4003,3003,2003,1003,0002,9002,800

Hc (

Oe)

S

Sample A Sample BSample C Sample D

Sample A Sample BSample C Sample D

Seed layer thickness (NiAl) (nm)

relation between these and actual film depositionprocesses is shown in Table 1. It has already beenshown that highly purified operation gas, reducedoutgas in the vacuum chamber, and increased vacuumlevel in the chamber increase effectiveness in principle.Other deposition parameters, such as substrate tem-perature, gas pressure, under layer thickness, andsubstrate bias, are related to each other and mutuallyinfluence the layer structure and film composition, andtherefore, it is necessary to optimize each individually.Figure 9 shows the dependence upon several processparameters of Hc and noise on an Al/NiP substrate.Substrate temperature is an important parameter toincrease Hc, but from the viewpoint of reducing noise,the optimum value is a lower temperature than thevalue for maximum Hc. Increased Ar gas flow andhigher gas pressure settings are effective in reducingnoise. Actually, the magnetic layer components (Pt,Cr, etc.) are optimized to meet the required Hc and Mr·tvalues as shown in Table 1. In the course of processoptimization, the goal is to realize higher Hc whiledecreasing the Pt quantity and to prevent Hc fromdeclining while adding as much nonmagnetic elements,such as Cr and Ta, as possible which are effective inreducing noise. Overall, higher Pt and higher Cr andTa are desirable to increase recording density in the

-35

-40-45

-50A

Iso2,900/0.56

B AIso

3,100/0.50

B

OW (dB)

AOri

2,900/0.56

B

ODMDID

15131197

AIso

2,900/0.56

B AIso

3,100/0.50

B

Pw50 (ns)

AOri

2,900/0.56

B

13.012.512.0

11.011.5

AIso

2,900/0.56

B AIso

3,100/0.50

B

SNR (dB)

AOri

2,900/0.56

B

Hc (Oe) / Mr.t (emu/cm2)

ODMDID

8

7

6

5A

Iso2,900/0.56

B AIso

3,100/0.50

B

NLTS (%)

AOri

2,900/0.56

B

future. Further, the use of effective nonmagneticelements in addition to or instead of Cr and Ta is alsoa subject for further development.

In addition to the above subjects which apply todifferent substrates, the necessity of a seed layer torealize an isotropic layer, typically utilized with glasssubstrates, has been described above. By optimizingprocessing for this seed layer (temperature and filmthickness), the desired crystal orientation of a magnet-ic layer and grain size can be controlled on a glasssubstrate, and characteristics similar to an orientedmagnetic layer on a NiP/Al substrate can be obtained.Examples of experiments are shown in Fig. 10 (process)and Fig. 11 (parametrics). These figures show thatseed layer thickness can control Hc, and isotropic diskcan have almost the same characteristics (excludingthe OW characteristic) as oriented disk by adjustingmagnetic characteristics. Irrespective of isotropic ororiented disk, the realization of a magnetic layer for


Table 2 Comparison between longitudinal and perpendicular recording

Conventinal recording

Classification Longitudinal recording

Thermal stability KuV/kT≧60

Item

Magnetic layer thickness

Reduction in noise

Increase in Ku

™Unstable at high density recording™Stable at low recording density

™Cr segregation to the grain boundary

™Reduction in grain size

™Dispersion of magnetic particles such as SiO2 into the nonmagnetic matrix

Problems ™Reconciliation between thermal stability and noise reduction ™Thermal stability

Decreasing

Solutions ™Uniform distribution of grain sizes™Increase Ku and Hc

Decreasing

Required Required

Granular type longitudinal recording

™Unstable at high density recording™Stable at low recording density

™Uniform distribution of grain sizes™Increase Ku and Hc

™Cr segregation to the grain boundary

™Reduction of reverse magnetic domains

™Reduction in media noise

Thinness not strongly required

Not strongly required

Perpendicular recording

™Unstable at low recording density™Stable at high density recording

™Reduce interactions among magnetic grains

™Increase the squareness ratio™Increase the magnetic field to form

reverse magnetic domain neuclei

Notes: Ku: magnetic anisotropy energy V : activation volume k : Boltzmann constant T : absolute temperature

recording densities greater than 10 Gb/in2 is a large,challenging subject. To obtain a high SNR whilepreventing characteristic deterioration due to thermalfluctuation as mentioned above, more precise processcontrol as well as fine adjustment of the optimalmaterial composition is required.

4. Future Recording Systems

In the previous chapter, examination focused onthe longitudinal system. However, there is a commonperception in the industry that the current diskconfiguration will limit enhancement of recordingdensity in the near future. In the longitudinalrecording system, the so-called granular structure hasbeen proposed as a method of reducing noise to makethe grain boundary more definite using oxides such asSiO2 from Cr segregation in order to decouple themagnetic grains. Further, to increase areal recordingdensity effectively, the importance of lowering theconventional aspect ratio (BPI/TPI), i.e. raising trackdensity (TPI) rather than linear recording density(BPI), is recognized and is being implemented. Even-tually, the problem of thermal fluctuation has to besolved. Factors that have an effect on the SNR in ahigh density recording area are given by Bertram etal(4) as follows:

SNR ∝ 1/[Density3/2(W/B)1/2D3]where

B : bit spacingW : read track widthD : grain diameter

As discussed before, this formula supports reduc-ing the micro-grain size and aspect ratio to raise theSNR. Because thermal fluctuation is directly relatedto grain volume D2t (t : magnetic layer thickness) and acomparatively thicker magnetic layer is desirable for

the perpendicular recording system to stabilize magne-tization, the perpendicular recording system is superi-or with respect to the limit of SNR restricted bythermal fluctuation because it can take advantage offilm thickness.

Simplified features of the above-mentioned tech-nologies are listed in Table 2. Although Fuji Electrichas just started development, examples of its experi-ments are presented below. The perpendicular coerciv-ity obtained with a configuration of NiFe/TiCr/CoCr-TaPt on an Al/NiP substrate was 2,000 Oe. This wasevaluated with a GMR head in the setup shown inFig. 12. Noise reduction was observed to depend on thethickness of the intermediate layer TiCr, and typically,the frequency characteristics were as shown in Fig. 13.This proved that the noise characteristics lag those ofthe longitudinal recording system.

Perpendicular recording media has thus far faced a

Fig.12 Measuring system for perpendicular recording disk

Head amplifier

GMR head

Differential filter

Oscilloscope

Tester setting

(Read back waveform : Freq. 1MHz)

(a) Without differential filter:TAA = 1.144 (mVpp)

(b) With differential filter:TAA = 0.786 (mVpp)

C

RIA amplifier


great problem in media noise reduction. However, thedirection of the solution has been clarified in a recentinstitute paper(5). It is desired that through furtherprocess development in the future, performance superi-or than that of longitudinal recording in applicationsabove 20 Gb/in2 will be ensured.

As shown in Table 3, various new recordingprinciples and concepts other than current longitudinalrecording have been proposed, and their application toHDDs is expected in the future.

5. Conclusion

Details of the recent technological development ofmagnetic hard disks for HDDs have been examined,focusing on materials and processes. Recently, the

Table 3 Promising new technologies for high density recording

development of conventional longitudinal recordinghas reached a more difficult technological phase. Thispaper has proposed important subjects for expandingthose boundaries. On the other hand, new recordingsystems are under development in various locations asreplacements to longitudinal recording. It is expectedthat these new systems will expand the role of HDDsin the future memory market and further widen themain fields of application. Fuji Electric will alsosupport development of the HDD industry throughcontinuous contribution to the technical developmentof magnetic hard disks.

References(1) Okamoto, I. et al.: Co Alloy Thin Film Media and

Granular Media for 40-Gb/in2 Longitudinal Recording(Projection), Intermag ’99, Korea, AA-03

(2) Charap, S. H. et al.: Thermal Stability of RecordedInformation at High Densities. IEEE Trans.Magn.Vol.33, 978 (1997)

(3) Uwazumi, H. et al. : Time Decay of Remanent Magneti-zation, Remanent Coercivity, and Readback Signal ofUltrathin Longitudinal CoCr12Ta2/Cr Thin Film Media.IEEE Trans.Magn. Vol.33, 3031 (1997)

(4) Bertram, H. N. et al.: SNR and Density Limit Esti-mates: A Comparison of Longitudinal and Perpendicu-lar Recording. TMRC ’99, San Diego, Al

(5) Wu, L. et al.: Low Noise Co/Pd Multilayer Media forPerpendicular Magnetic Recording. Intermag ’99,Korea, GB-05

Fig.13 Example frequency characteristics of perpendiculardisk

60

50

40

30

20

10

0

TA

A (

mV

pp)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

00 100 200 300 400

Linear recording density (kFCI)

SN

R (

dB),

Med

ia n

oise

(µV

)

SNR

TAA

Noise

New technology Content Features Problems

Magnetic particles such as Co and Pt scattered in the nonmagnetic matrix

Magnetization in the direction of the C-axis of Co alloy crystals perpendicular to the film

Granular disk

Perpendicular recording

Low noise and high thermal stability

High thermal stability

The ratio of width to length of bit domain is nearly 1 (currently 10 or more) with a small demagnetized field.

Square bit High thermal stability

Magnetic domains of several tens of nm in size arranged in a lattice pattern, having little interaction. Can be used for longitudinal and perpendicular media.

Patterned disk Low noise and high thermal stability

Writes and reads with a collimated sub-micron size light beam based on near field optics theory. The solid immersion lens system under development by Tera Store, the same as a magneto-optical disk

Near field recording High thermal stability

™Increase in coercivity

™Reduction in noise™Thermal stability in low recording

density™Stabilization of stray fields

™Reduction in the track pitch™Increase in precision of track servo

Writes bits with the help of light-induced heat, the same as a magneto-optical disk. Light transmitted to the HDD head through optical fiber is irradiated on the media by micro-mirror and lens to write and read with the magnetic domain. Being developed by Quita/Seagate.

Optically assisted Winchester High thermal stability

™Increase in writing and reading speed

™Reduction in cost

™Dust control™Strict tolerance to optical

configuration™Low efficiency in light utilization

™Noise caused by optical fibers™Dust control


Mitsuo KobayashiMichinari KamiyamaAkiyasu Kumagai

Next Generation HDI Technologiesfor Magnetic Hard Disks

1. Introduction

With enhanced recording density of magnetic harddisk drives (HDDs), requirements for reducing the“magnetic spacing”, the distance between the read-write head and the magnetic layer, are becoming moreand more critical. In order to reduce magnetic spacing,the most effective means is to reduce flying height ofthe head. Following current trends, the flying heightwill decrease below 20nm and various demands arebeing placed on the hard disk (hereinafter referred toas the disk) (Fig. 1).

From this viewpoint, development in various fieldsis being actively promoted in order to define thephenomena involved in the head disk interface (HDI),that is, interactions between the head and disk.Problems with HDI technology have become increas-ingly important together with reliability improvementof a hard disk drive.

2. Problems of HDI Technology Related to theDisk

Firstly, to reduce substantially the magnetic spac-ing, it is strongly required that the disk has both athinner carbon overcoat layer, covering the surface ofthe magnetic layer, and a thinner lubricant layer ontop. Furthermore, the surface roughness (Ra) and thesurface undulation (waviness) (Wa) of the disk must bemade small so that the surface will not come in contactwith the head, even if the head flying height isreduced. Precise substrate processing techniques toobtain a flat and smooth surface have become morecritical.

Secondly, another problem is the contact of thehead and disk that necessarily occurs during startsand stops. In the current CSS (contact start stop)system broadly used, the laser bump system is appliedas means to make the contact area as small aspossible. The development of design and processingtechniques for a laser bumps of smaller diameter andhigh density has been accelerated, keeping pace withthe reduction in magnetic spacing.

On the other hand, there is a demand for a disk

design able to withstand temporary contact of the headin a load-unload system, which will broadly be used infuture.

In this paper, we suggest a fabrication method forthe CVD (chemical vapor deposition) carbon overcoatlayer, and discuss the analysis technology to micro-scopically observe this layer and the distribution of thelubricant used.

3. CVD Overcoat Layer

3.1 CVD and carbon layer characteristicsAt present, the carbon overcoat layer is deposited

by sputtering a graphite target. If the layer is notmore than 10nm thick, it is likely to be defective withregard to durability or corrosion-resistance for protec-tion. A layer formed by CVD has superior coveragecharacteristics and protective performance for underlayers because the layer formation is accompanied by asurface chemical reaction. Because the layer materialof carbon can be controlled with ion energy, comparedwith a sputtered layer, a finer and more rigid layer canbe formed. Accordingly, fabrication of the carbonovercoat layer will employ the CVD method in thefuture.

Among CVD applications, filament type ion beamdeposition (IBD), hollow cathode type ion beam deposi-tion, ECR (e1ectron cyclotron resonance)-CVD and RF(radio frequency)-CVD are currently suggested forapplications to disk. The carbon overcoat layer and

Fig.1 Cross section of flying head and hard disk

Lubricant layer (1 to 2nm)

F1ying height of head(25 to 30nm)Magnetic spacing

Read-write head

Carbon overcoat layer (8nm)

Flying head

Carbon overcoat layer (10nm)Magnetic layer (20 to 30nm)

Substrate


related equipment of ECR-CVD, IBD and RF-CVDhave already been evaluated. Hollow cathode type ionbeam deposition will be evaluated in the future. Thispaper describes layer deposited by ECR-CVD and IBDin particular.

3.2 Carbon overcoat layer by ECR-CVDWhen microwaves of 2.45GHz are introduced into a

magnetic field of 875 G, electron cyclotron resonanceoccurs, converting the microwave energy into kineticenergy with high efficiency, and high-density plasma isformed. ECR-CVD utilizes this principle as shown inFig. 2. The same structure is arranged also on theopposite side of substrate and microwaves are intro-duced differing in phase angle by 90° so as to preventinterference.

Raman scattering spectroscopy is generally used toevaluate the carbon layer material. Figure 3 shows a

Fig.2 Principle of ECR-CVD Fig.4 Process dependence of B/A characteristic (ECR-CVD)

Fig.3 Result of Raman spectroscopy measurement

Raman spectrum for the case where a carbon layer hasbeen deposited with C2H4 of 0.03 SLM, microwavepower of 70W, and a bias voltage of - 200V. Weobserved the carbon layer, exciting it with an Ar laserlight of 514nm. A peak known as the D-peak appearsnear 1,350cm-1, possibly inherent in the carbon on theborder of a graphite crystallite. Another peak knownas the G-peak appears near 1,570cm-1, inherent in theplanar vibration of the graphite. Both peaks superim-pose on the broadband fluorescent component having apeak at approximately 3,000cm-1. As shown in Fig. 3,we assume the total intensity of G-peak to be B.Subtracting the intensity of the fluorescent componentfrom B, we assume this intensity to be A. Theintensity ratio B/A correlates with the hydrogen con-centration in the carbon layer. We know that if theratio B/A increases, the hydrogen concentration in thecarbon layer becomes higher.

Among the deposition parameters such as micro-wave power, flow rate of raw material gas (ethylene:C2H4), bias voltage applied on the substrate, etc., theparameter most influential on the layer material is thebias voltage applied on the substrate. Figure 4 showsthe bias voltage dependence of the ratio B/A character-istic obtained by Raman scattering spectroscopy mea-surement. As the negative bias voltage increases, theratio B/A becomes smaller, the hydrogen concentrationin the layer decreases, and the layer goes finer. It isbelieved that the impact energy of ions becomes largerwith increasing bias voltage, and as a result, weaklycombined hydrogen atoms are separated and carbonatoms are rearranged in the layer.

3.3 Carbon layer by IBDThe principle of IBD equipment is shown in Fig. 5.

Thermoelectrons are emitted from a heated filamentand attracted to an anode electrode. They collide withraw material gas flowing from the anode side andionize. The ionized gas is repelled by the anode voltageand attracted by the negative bias voltage applied onthe substrate. The ionized gas arrives at the substrateto form the layer. Similar to ECR, IBD controls thecarbon layer material with a voltage applied on the

Magnet

Substrate

Plasma

Microwave power

Gas

Bias power supplier

Inte

nsi

ty (

a.u

.)

40.0

32.0

24.0

16.0

8.0

0900.01,080.01,260.0

Raman shift ( cm-1)∆

Raman shift ( cm-1)∆1,440.01,620.01,800.0

Inte

nsi

ty (

a.u

.)

40.0

32.0

24.0

16.0

8.0

0900.01,080.01,260.01,440.01,620.0

G peak

BA

D peak

1,800.0

1.60

1.55

1.50

1.45

1.40

1.35

1.30

1.25500400300200

Bias voltage (-V)1000

B/A

A sideB side


Fig.6 Process dependence of B/A characteristic

substrate.IBD in particular controls the layer material with

the sum of the absolute values of the anode voltage andbias voltage. With the anode voltage (Va) as aparameter, the dependency of the ratio B/A upon thebias voltage applied on the substrate is shown in Fig. 6.As in the case of ECR, the ratio B/A decreases as thebias voltage increases. However, as the anode voltagebecomes higher, its effect on the bias voltage becomessmaller and the ratio B/A converges to approximately1.4. If the anode voltage is higher, the initial energy ofthe ions becomes larger and therefore the effect of thebias voltage is barely noticeable.

At first, we thought it possible to develop a methodto deposit a layer on an insulating substrate withoutapplying a bias voltage and increasing the anodevoltage. However, unless a bias voltage is applied, theions repelled by the anode voltage spring toward thespace around the substrate. In other words, the samematerial as deposited on the substrate is deposited onmany places in the chamber. This deposited materialhas a relatively high internal stress, and if made thick,will exfoliate to create particles. We determined thatthis process could not be applied to mass production.By lowering the anode voltage and applying a biasvoltage on the substrate, we utilized a fabricationprocess capable of suppressing particle scatter byattracting the ions to the substrate.

3.4 Characteristics of the CVD layerWe evaluated the durability of the carbon overcoat

layer by a CSS test that repeats starts and stops. Inaddition to the CSS test, we investigated the perfor-mance of the carbon overcoat layer and the resistanceof the layer surface against gas adsorption in order tosecure high reliability.

For the coating performance, we applied a methodto investigate the amount of cobalt (Co), the mainmaterial of the magnetic layer, that corrodes the diskin a high temperature and humidity environment.According to the result shown in Fig. 7, ECR and IBDdo not differ much in coating performance for layerthicknesses of 5nm or more, but ECR seems superiorfor layer thicknesses of 5nm or less. Supposedly, IBDdeposits the layer mainly by ions, but ECR forms thelayer with a surface reaction of neutral radicals.

In addition, we measured and evaluated the resis-tance of the layer surface against gas contaminationwith time-of-flight secondary ion mass spectroscopy(TOF-SIMS) after leaving it for 24 hours in atmo-spheres of SO2 gas of 0.1ppm concentration and NH3

gas of 1ppm concentration. When acid gas SO2 isadsorbed, the ions of sulfuric acid cause Co corrosion ofthe magnetic layer. When alkaline gas NH3 isadsorbed, the ammonium ions cause the head tobecome dirty.

CVD carbon layers will become, without a doubt,the mainstream of protective overcoats in the futuredue to their superior performance in fineness andsurface coating. However, many unresolved issuessuch as long term stability of the fabrication equip-ment, particle scattering and related suppressingmethods need to find solutions as quickly as possible.We will aggressively promote further developmentaiming at the highest technical levels.

4. Observation Techniques of High Space Reso-lution for Liquid Lubricant

Analysis techniques that provide much chemicalinformation, such as Fourier transform infrared spec-troscopy (FTIR), X-ray photoelectron spectroscopy(XPS) and time-of-flight secondary ion mass spectrom-

Fig.7 Co separation versus carbon layer thickness

2.0

1.8

1.6

1.4

1.2120100806040200

B/A

Va = 200VVa = 180VVa = 160VVa = 120V

Bias voltage (-V)

10,000

1,000

100

10

1

0.1

Co corrosionH/W:85°C/85RH%Measured by ICP n = 2

120100806040200

ECR (Ra = 4Å)ECR (Ra = 12Å)IBD (Ra = 4Å)IBD (Ra = 12Å)

Co

(µg/

m2)

Carbon (Å)δ

Fig.5 Principle of IBD-CVD

e-e-e-

C2H4

CxHy+

CxHy+CxHy+

Filament

Power controller

Power supply Bias supply

SubstrateAnode

electrodeMagnet


etry (TOF-SIMS), are utilized to investigate extremelythin layers (several nm or less) of a liquid lubricantplaced on the disk media. It is not easy, however, forthe aforementioned techniques to evaluate the distri-bution of lubricant at high spatial resolutions that donot exceed the sub-micron level.

The tribology between the head and disk shouldnaturally be discussed from a microscopic viewpoint.Therefore, we have developed a new method to evalu-ate liquid lubricant distribution utilizing a scanningprobe microscope (SPM) that sensitively evaluates thesample surface at a high spatial resolution.

4.1 PrinciplesSPM is the general term for analysis techniques to

investigate the surface state of a sample by detectinginteractions between the surface and a sharpened tip.

The probe used is fabricated by silicon micro-machining and is integrated into the edge of acantilever. While vibrating the cantilever at a frequen-cy near the resonance frequency, the sample of thedisk media surface is brought close to the probe. Atthe position where it touches the probe, the vibrationamplitude of the probe decreases due to the meniscusforce of lubricant. Then, SPM works by feedback tomove the sample away from the probe until theoriginal vibration amplitude is restored. When theprobe is peeled away from the lubricant, a phase delayoccurs in the vibration of the cantilever due to viscosityof the lubricant. We can observe the thicknessdistribution of the lubricant layer by monitoring thisphase delay (see Fig. 8).

The phase imaging mode itself is a well-knownmethod to detect phase. We applied this technique tothe lubricant of the disk and developed a new evalua-tion method that could quantitatively measure thick-ness distribution of the liquid lubricant layer at highspace resolution.

4.2 ResultsWhen we observed the lubricant layer of the disk

with the above-mentioned method, we found that thephase delay did not continuously change with the localdistribution of lubricant layer thickness. In the local

area of observation, where the lubricant layer thick-ness exceeds a certain threshold value, the probebecomes suddenly trapped in the lubricant layer andproduces a large phase delay.

In other words, the phase image is displayed as abinary figure, clearly separating the thick and thinparts of the lubricant layer. We also found that wecould control the threshold value with the vibrationamplitude of the cantilever. By superimposing phaseimages of different vibration amplitudes, we can obtaina topological image of the liquid lubricant layer’sthickness.

Based on the data thus provided, we can calculatethe volume of the liquid lubricant in the area ofobservation. The calculated volume is only a relativequantity of lubricant, because thickness of the lubri-cant layer is given by the vibration amplitude of thecantilever as a quantity proportional to the layerthickness. However, the volume has a close correlationwith the average quantity of lubricant evaluated byother techniques (FTIR, etc.) and we consider that thismethod can quantitatively evaluate the distribution ofliquid lubricant.

By comparing the relative quantity of lubricantobtained by this method with the average quantity oflubricant measured by the other techniques, we canconvert the vibration amplitude of the cantilever intothe thickness of the lubricant layer.

Figure 9 shows a distribution of liquid lubricant onAl substrate disk evaluated by the above-mentionedmethod (overcoat layer; a-C:H:N). A brighter colorindicates a thicker lubricant layer.

In this technique, the vibration amplitude of thecantilever is gradually increased until finally the probebecomes untrapped in the area of observation, and theshape of the disk itself can be seen by using atomicforce microscope mode. In other words, the disksurface image can be compared with the lubricantdistribution.

Fig.8 Principle of liquid lubricant thickness evaluation by SPM Fig.9 Distribution of liquid lubricant on disk

Tip /Cantilever

Lubricant

Feedback

Phase delay; small

Phase delay; large

1.1 1.3 1.5 1.7 1.9

Lube thickness (nm)0.1 µm


In Fig. 9, the area between two arrows is a texturedgroove. We understand that, in this disk , lubricant isthick at the bottom of the groove and thin at the slopeof the groove. Definition of correct spatial resolutionmay be difficult, but lubricant structures on the orderof approximately 10nm can be observed. The spatialresolution that can be observed is enhanced by approx-imately two digits compared with conventional tech-niques.

We will be able to use this technique as an analysistool for microscopic phenomena involved in the headand disk interface in future.

5. Conclusions

As examples of HDI technology, we described thelatest fabrication methods of the CVD carbon overcoatlayer and observation techniques of the CVD carbonovercoat layer and related lubricant distribution. HDItechnology involves a wide range of technical fields. Toenhance the level of these important techniques insecuring reliable HDDs, we will continue to tacklefurther development in these fields.


Youichi TeiShoji SakaguchiHiroyuki Uwazumi

Magnetic Hard Disks forAudio-Visual Applications

Fig.1 Flow of “convergence” advancing in the 21st century1. Introduction

Since about three years ago, it has been expected(mainly in the United States) that a new home-oriented service, in which communications, computersand consumers converge, will grow to become agigantic market in the 21st century along with thedevelopment of digital broadcasting (see Fig. 1). The“convergence” of this service will create a trend ofglobal cooperation, absorption and mergers to over-come business or national barriers and existing indus-trial structures will evolve to reintegrate their overallassociated infrastructures (see Fig. 2).

More specifically, this trend will, for example,develop at homes in the following way.

A set top box (STB) capable of temporarily storingaudio-visual (AV) information such as a digital broad-cast will be set up at input and output points thatinterface between users and society. The STB will beconnected with home AV equipment such as digitalTVs through an IEEE 1394 high-speed communicationsystem. Needless to say, handheld AV equipment canalso be connected to the STB. Home AV equipmentwill be controllable by a home server equipped with aman-machine interface that is superior than that oftoday’s personal computers (PCs). In the 21st century,consumers will be able to freely edit and distributemusical or pictorial data or personal AV information.

The “convergence” will tend to popularize theabove home information service in the short-term anddata broadcasting service in the mid-term. In the long-term, it will lead to a future system that guaranteesconsumer comfort with regard to electronic commerce(EC) and security control.

More accurately, the “convergence” is expected tospread throughout homes and society in the followingthree stages. In the first stage, the diffusion of databroadcasting after the year 2000 will become a turningpoint, and individual AV equipment and associatednetworks will come into wide use responding toincreasing information volume. In the second stage,utilizing the opportunity provided by the world cupsoccer games, home servers with higher functions willspread into homes as broadcasting stations. In the

third stage, from the year 2004 or 2005 and thereafter,new on-demand service will burgeon for EC andsecurity control.

The key technology for achieving “convergence” isto develop an “AV information cash memory” capableof handling quality AV information with a fast,versatile and cost-effective means. As shown in Table1, among the candidates for external memory devices,those having all of the above features are unexpectedlyfew (see Table 1). The hard disk drive (HDD), having

Fig.2 Evolution of industrial structures with “convergence”

Consumer

Computer Communication

Convergence

The above 3Cs are converging!

Free from the existing framework, industrial structures are evolving in a new trend!

Home appliance business

Broad-casting business

Digital television (TV)

2000 2002 2004

Change of paradigm and evolution of industrial structures

Conver-gence of TV and PC

Personalcomputer

Adver-tising business

Publishing businessNewspaper businessService business

Data broadcasting

Computer business

One-stop shopSystematizingElectronic commerce trade

Access to homesInternetBroadbandBusiness-user oriented

Multiple functionsDual-directiveCommunicative functionsAdditional new service

Multiple channelsHigh quality picture imageHigher functions

Financial businessRetailer business


been an indispensable device for PCs, is once again inthe spotlight.

These trends will finally lead to use of various AV-HDDs (custom HDDs for the AV equipment) havingdifferent specifications and shapes in the STB, digitalTV, home server and handheld AV devices. The erawhen consumers can readily walk around with hand-held AV information will arrive very soon.

In consideration of the future aspects of 3-Dtelevisions and handheld devices for personal use, AV-HDDs as well as their core elements, that is, magnetichard disk (hereinafter referred to as the disk) for AVuse, will be most promising.

This paper describes the development of the latestAV magnetic hard disk that Fuji Electric is focusingon.

2. Plastic Disks

An HDD uses magnetic heads flying above its disksurfaces for fast read-out and write-in. Therefore,aluminum or glass substrate has mainly been utilizedto create a super smooth surface. On the other hand,optronical devices such as CDs, MDs, PDs and DVDsuse injection-molded plastics, because their opticalheads do not require high surface precision for readingand writing, but only need groove formation. There-fore, plastics are most advantageous in cost since

plastic disks can be fabricated by injection molding.Recently, injection-molding techniques have advancedto fabricate plastic substrates having surface precisionthat is suitable for the flying head of HDDs.

Moreover, digitalization in the AV field has beenadvancing. It is expected that the start of digitalbroadcasting will accelerate expansion of the digitalAV market, and HDDs that can quickly handle largevolumes of data will play an essential role. Thismarket necessarily demands large-capacity and price-effective HDDs.

Plastic disks completely match these needs. Plas-tic disks not only have inexpensive material costs butalso require few fabricating processes, and thereforeoffer lower cost, because substrates can be formedsimply by injection molding. Their light weight lowersthe energy consumption and reduces acoustic noisecaused by rotation. These features are optimal for AVrecording devices. Elasticity of the plastic disk absorbsmechanical shocks from the head, and therefore makesit possible to design damage-resistant disk for theramp load mechanism.

Different from the case of ordinary processing ofcomputers, defective bits of AV data are not as seriousa problem as long as people cannot sense the defect inpictorial images or musical sounds. However, it causesserious problems when the pictorial images areblacked out or the music does not sound smooth.

HDDs are sometimes handled by people not famil-iar with mechanics, or are exposed to direct sunlight ormechanical impacts. The data processing of computersand AVs is different as summarized in Table 2. Inview of this table, HDDs for AV applications demandanother kind of reliability, not as computers but ashome appliances.

Let us imagine, for instance, the situation whendigital broadcasting gets started in the future.

You have a handheld set with an HDD and animage display device. Before leaving home, you canrecord the news of the day from an STB in an instant,and watch it on the way to work. During the recessafter lunch, you can also watch the drama that you

Table 2 Comparison of the hard disk drives for PC and AV applications

Use Data processing

Error rate 10-10

Environment 33°C, 80%RH

Data preservation term 2 to 3 years

Frequency of operations A few times per day

Access speed High speed rotationLow speed seek

Media Al, glass

PC applications

PC Home server

Video editorHome TVInternet data

10-10

80°C, 80%RH

3 to 5 years

A few times per day

Medium speed rotationHigh speed seek

Al, glass

AV applications

Set top box

Image dataDigital TV

10-3 to 10-4

80°C, 80%RH

1 to 7 days

Continuous operation

Low speed rotationHigh speed seek

Glass, plastics

Handheld drive

Video cameraCellular phoneNavigator

10-3 to 10-4

80°C, 80%RH

1 to 2 weeks

A few times per day

Low speed rotationLow speed seek

Glass

Handheld media

Video cameraPortable video setMusic distribution

10-3 to 10-4

80°C, 80%RH

1 to 2 weeks

A few times per day

Low speed rotationLow speed seek

Plastics

Category

Item

Catego-ry

Item Flash memory HDD

Product

Example from products on sale

SanDisk Corp.Compack Flash

Toshiba Smart Media

Memory size

Data transfer velocityAccess time

48 MB

24 Mbit/s

10 µs 7 µs

16 MB

8 Mbit/s

Quantum Corp.Eagle

25 GB

12ms

Magnetoptical disk drive

Sony MD data

650 MB

288 Mbit/s 4.6 Mbit/s

500ms

Table 1 Performance comparison of main external memories


Table 3 Comparison of material performance

missed on the day before. Or, you can recorddistributed music and listen to it at any time you want.

The HDD you use should be compact, lightweight,shock-resistant, low in acoustic noise, and energyconsumption. Above all, it should be low in price.Only the plastic disk can achieve these features.

To form the plastic disk, we have to solve threemajor problems:

™ Technology of plastic materials™ Precision molding technology for plastic substrates™ Low temperature deposition of the magnetic layer

on the disk surface for high recording density useSince plastics are weak at high temperature, lowtemperature deposition is required.

2.1 Technology of plastic materialsApplication of conventional plastic substrates for

MDs, DVDs, etc. to magnetic disk substrates thatusually demand precise surface characteristics resultin difficulties in reading and writing due to theinsufficient head flying-height caused by undulation(waviness) of the substrate surface. Even if the surfacecharacteristics are sufficient, ordinary plastic sub-strates such as those made of polycarbonate resin aremechanically weak, thermally transformable and hu-midity-absorbent, and therefore cause deformation ofthe substrates in hot or humid environmental condi-tions.

The performance required of plastic substratematerials for magnetic disks are as follows:(1) Extremely small residual stress on the molded

substrate and preservation of stable shape underhot and humid environmental conditions

(2) Preservation of low roughness, no defects andprecision of the substrate surface

(3) Cleanliness with controlled outgas and contami-nants

Conventional resin materials such as polycarbon-ates or polyachrilates cannot attain the above level ofperformance. We are currently promoting develop-ment of new resins and their applications.

Required performance characteristics are higherrigidity, three-dimensionally combined molecularstructure, high mechanical strength, creep-resistanceand small thermal expansion.

Table 3 shows a comparison of the main character-istics of the resin materials.

The new resin is superior to polycarbonate in

mechanical strength, heat-resistance, creep character-istic and thermal expansion. From the viewpoint ofmolecular structures having high rigidity, resin iscopolymerized with more three-dimensionally com-bined and heat-resistant monomer units.

The above-mentioned shape stability is the mostimportant requirement. Shape deformation is consid-ered to take place by the following mechanism.

Upon filling resin into the molding die, the hotresin in the cavity center flows in a radial directionwith a small gradient of temperature and velocity, andorientates the substrate bulk in this direction. On theother hand, the hot resin cools and solidifies at theinside surface of the die. The gradient temperatureand velocity between the boundary of the above tworesin fluids generates shearing stress, and thereforethe entangled molecules solidify without orientation.The larger the difference in the molecule orientationsat the surface and bulk of the substrate, the strongerthe residual stress will be near the substrate surface.A molded substrate with large residual stress willundergo a large transformation of shape while leftunder hot and humid environmental conditions. More-over, accelerated by the humidity-absorbent character-istic inherent in the resin, a substrate with largerresidual stress deforms so much that it cannot pre-serve its original shape.

The degree of deformation and residual stress ofthe substrate depends upon the creep and thermalexpansion characteristics inherent in the resin struc-tures.

Figure 3 shows the relation of the longitudinalexpansion coefficient and the shape deformation ratioof substrates under various molding conditions of resinA, B and polycarbonates. In these experiments, thesubstrates were left for 500 hours under 60°C and 80%relative humidity.

The longitudinal expansion coefficients decrease inorder of polycarbonate, resin B and then resin A. Afterthese materials are left under hot and humid environ-mental conditions, their shape deformation ratios arealso smaller in this order. However, the longitudinalexpansion coefficient and the shape deformation ratio

Fig.3 Comparison of shape deformation rate versus longitudi-nal expansion coefficient

CategoryItem Resin A

＜5 × 10-5Longitudinal expansion coefficient (1/°C)

Temperature of thermal decomposition (°C)

Water contents (ppm)

Resin B

6 × 10-5

Poly-carbonate

8 × 10-5

420 400 360

5 5 150

Sh

ape

defo

rmat

ion

rat

e (%

) 250

200

150

100

50

01086

Longitudinal expansion coefficient (cm/cm·°C)42

New resinPolycarbonate


of resin A varies in a wide range depending upon themolding conditions.

The following investigations are necessary to ob-tain plastic substrates with smaller shape deformationratios.

™ Investigation of excellent precision molding tech-nology attaining smaller residual stresses and lon-gitudinal expansion coefficient (＜ 4 × 10-5)

™ Application of new heat-resistant materials hav-ing a more rigid three-dimensional structure withbetter creep characteristics and mechanicalstrength

On the other hand, in order to attain “precisesurface characteristics”, it is necessary to lower theviscosity of the molten resin and use clean resin. Wemust inject and mold resin in a better fluid state. Asdiscussed below in section 2.2, we must remove asmany contaminants as possible such as outgas compo-nents and particles causing surface defects.(1) Upgrade of resin fluidity

(a) Control of molecular weight distribution tolower the viscosity of melting resin

(b) Application of heat-resistant resin to mold athigh temperature and speed

(2) Prevention of surface defects(a) Preservation of a clean environment during

fabrication and conveyance process to reduceparticle contamination

The HDDs for PC applications have become morecomplex due to the explosive increase in recordingdensity. On the other hand, HDDs for AV applicationsare required to be less expensive than before, whilepreserving their high performance. Plastic disks aremost promising to solve the above contradictory prob-lem.

Regardless of the common conception of conven-tional plastic substrates, it is most important toenhance the technology of plastic materials as well asthat of molding and dies. It is necessary not only topositively improve performance and quality of thecurrent plastic materials but also to actively promotedevelopment of new materials of higher performance.

2.2 Precision molding technologyAdditionally, magnetic disk substrates for AV

applications have the following quality requirements:(1) Precise surface characteristics

Surface roughness and surface undulation (wavi-ness)

(2) Good mechanical characteristicsFlatness, total index runout and surface accelera-tion

(3) Few surface defectsAbnormal bumps, pits, scratches and particles

(4) Heat-resistance stabilityMaterial physical property and after treatment

Plastic substrates for conventional optical disksare controlled for the performance of pattern copying

and light refraction in addition to the above items (2),(3) and (4). Item (1), “precise surface characteristics”,of the optical disks is controlled above a certain levelbut not more than necessary. Therefore, actual surfacecharacteristics of optical disks are quite different fromthose of current magnetic disks (see Fig. 4).

The important factors that affect molding of mag-net disk substrates are as follows:

™ Pellet materials™ Accuracy of dies™ Molding conditions™ Atmospheric conditions™ Fabrication facilitiesIn this paper, we discuss the molding conditions

and the results of their study.Important factors related to precision injection

molding and the mold shape are assumed as follows:™ Die temperature™ Resin temperature (nozzle temperature)™ Injection speed™ Molding pressure™ Cooling time (molding cycle)We assessed the effects of these five factors on the

shape figures by varying them parametrically. Inaddition, we have found that annealing after moldingreleases internal stress caused by the molding processand annealing is also another influential factor in thefabrication of precision plastic substrates. At the sametime, we have also confirmed the actual effects ofannealing (see Fig. 5).

The resin property is closely related to the appro-priate die temperature, the die temperature should behigher for resins with higher glass transition tempera-tures (Tg). Regarding die temperature settings, thetemperature of the movable die should be differentfrom that of the fixed die, as this is also an importantfactor affecting surface characteristics.

The larger the temperature difference, the betterthe surface flatness before annealing. After annealing,

Fig.4 Comparison of surface characteristics

Item Magnetoptical disk (PC) Hard disk (Al)

Shape

Surface flatness (AFM)

Undula-tion (ZYGO)

Wa 3.2 nm Wa 0.4 nm

Ra 1.2 nm Ra 0.4 nm

Flatness 3 µmFlatness 50 µm


however, the surface flatness worsens. The storedstresses after molding differ in the outermost surfacelayer and the internal bulk. These stresses arereleased by annealing. However, residual stress maystrain the substrate at a side where final residualstresses remain. Analysis demonstrates that thelongitudinal expansion coefficients are not equal atboth sides of the substrate. This phenomenon hasbeen verified as the relation between shape deforma-tion by annealing and stress.

As shown in Fig. 6, each other important factoraffecting the shape has its own optimum range. It isnecessary to ultimately set up the molding conditionsin view of the mutual influence of these factors.

Figure 7 shows the shape and surface characteris-tics of plastic substrates fabricated under optimummolding conditions currently available.

A microscopic assessment demonstrates that it ispossible to fabricate substrates that exceed the Aldisks in precision surface characteristics.

When we observe the overall surface of the sub-strate, we will find some partial molding defects suchas surface roughness, silver streaks, pits, etc.

However, the problems of these defects are cur-rently unresolved and solutions must be found in the

future.About heat-resistance stability, it is necessary to

control fluidity inherent in the resin after molding bymeasures that suppress changes of the physical prop-erty and shape of the substrates due to heat. Theessential points are how to make rigid molecularstructures and uniformly distributed internal stresses.It is necessary, for the former, to develop new resinmaterials as discussed in section 2.1, and for the latter,to investigate post-molding treatments.

After investigating the post-treatments, we havefound that “heat processing; annealing” can suppressshape deformation to some extent.

More accurately, the two factors of “temperatureand time” are capable of controlling both shape qualityand heat-resistance stability of the substrate. Figure 5refers to the surface characteristics of the substrateversus thermal conditions of the die. The result of theenvironmental test (60°C-80%RH-500h) for heat-resis-tance stability is shown in Fig. 8. If optimal annealingconditions to release internal stresses are found, it ispossible to suppress deformation of the substrateshape after the environmental test and to offer morestabilized substrates fabricated by plastic molding.

In order to produce precision moldings of stable

Fig.6 Flatness versus important factors for molding

Fig.8 Change in flatness versus annealing time

Fig.7 Substrate shape and related surface characteristicsunder optimal molding conditions

Fla

tnes

s (µ

m)

Fla

tnes

s (µ

m)

Fla

tnes

s (µ

m)

Fla

tnes

s (µ

m)

70

60

50

40

30

20

10

0390380370360350

Resin temperature (°C)

After molding

After annealing

After molding

After annealing

After molding

After annealing

After molding

After annealing

50

40

30

20

10

01701207050

Injection velocity (mm/s)

70

60

50

40

30

20

10

0403020100

Cooling time (s)

70

60

50

40

30

20

10

070/60/50 60/50/40 30/30/30

Molding pressure (kg/cm2)

Flatness 7 µm

TIR 6 µm

Ra 0.3 nm

Shape charac-teris-tics

Surface charac-teris-tics

Wa 0.2 nm

Ch

ange

in f

latn

ess

afte

r th

e en

viro

nm

enta

l tes

t (µ

m)

25

20

15

10

5

03025201510

Annealing at 110°C

Annealing at 90°C

Annealing at 70°C

50Annealing time (h)

Fig.5 Flatness of the substrate versus die temperaturedifference

Fla

tnes

s (µ

m)

80

60

40

20

0130/120130/122.5130/125

Die temperature difference (°C)

Before annealing

After annealing

130/127.5130/130


Fig.9 TEM image of granular magnetic layer

quality in a quick and inexpensive way, it is essentialto systematize overall processes involving productdesign, die work, injection molding, release of stressand verification as well as to shorten these processingtimes. Most molding techniques have been establishedby trial and error. Accumulation of the data thusobtained and its effective use as a database will lead toenhancement of the precision molding techniques.

2.3 Low temperature deposition technology for magneticlayersConventional magnetic hard disks are generally

produced at high temperature in the following manner.After heating the substrate up to approximately

300°C, a Cr-alloy is sputtered onto a substrate as anunder layer, and then a CoCrPtTa-alloy is sputteredonto the top surface as a magnetic layer. The highsubstrate temperature accelerates the Cr in the mag-netic layer to segregate towards the grain boundary ofthe Co-alloy grains, and therefore to reduce themagnetic interactions between the Co-alloy grains.This process helps create a disk of high magneticcoercive force H c and low noise. These features aredesirable for high recording density.

On the other hand, plastic substrates are heat-resistant only up to approximately 100°C. Therefore,it was necessary to develop a significantly new deposi-tion technology for the magnetic layer, which themagnetic layer was deposited at low temperature sothat Co-alloy grains could be isolated from each otherand their magnetic interactions could be reduced.Because oxide additives such as SiO2 are almostinsoluble in a Co-alloy solid at equilibrium, they wereadded to help isolate Co-alloy grains in the magneticlayer. This layer is called the granular magnetic layer.Instead of the conventional DC magnetron sputteringmethod, we introduced the RF sputtering methodcapable of depositing insulation additives such asoxides.

Figure 9 demonstrates a TEM planar image of thegranular magnetic layer. As the under layer, the Cr-alloy was deposited onto the plastic substrate, andthen RF deposition was performed using the compositetarget of a CoCrPt-alloy and an additive of 5 mol%SiO2. We can observe that the grain boundary phase(whitish area) surrounding the magnetic alloy grainforms the so-called granular structure. The magneticproperties and noise of the disk are superior to those ofthe low temperature-deposited CoCrPt disk withoutoxide additives. We consider that boundary segrega-tion by oxide additives has the favorable effect ofreducing the magnetic interactions between the mag-netic alloy grains and the granular disk, resulting inexcellent characteristics even for a low temperature-deposited magnetic layer.

However, the characteristics of the granular diskare inferior to those of the conventional high tempera-ture-deposited disk. Therefore, it is important toenhance grain segregation with oxide additives and tolower transition noises. It is believed that themicrostructure of the grains and their related bound-ary segregation and grain size distributions dependupon the method with which the deposited layer isgrown initially. Therefore, we changed the conditionsfor depositing the outermost surface of the Cr-alloyunder layer, to control the physical conditions border-ing the magnetic layer and finally control the initialgrowth of this layer. As the result, we have enhancedthe granular structure and have found that its magnet-ic property and electromagnetic conversion characteris-tics can greatly be improved.

Figure 10 shows a TEM image of the magneticlayer of the disk, which is sequentially composed of aplastic substrate, a Cr-alloy under layer, a modifiedoutermost surface of the under layer and a CoCrPt-SiO2 granular magnetic layer. Only the depositingconditions of the outermost surface of the under layerdiffer from those of Fig. 9. Comparing Fig. 9 and

10nm

Fig.10 TEM image of granular magnetic layer with modifiedoutermost surface

10nm


Fig. 10, we can clearly identify the grain boundaryareas in Fig. 10, and understand that grain boundarysegregation is greatly enhanced with a SiO2 additive.

Table 4 compares the above two disks with regardto magnetic coercive force Hc, Mr t (product of rema-nence magnetization and layer thickness), O/W valueand SNR. An AMR head with recording density of200kFCI is used to measure the O/W value and SNR.

We have changed the depositing conditions of theoutermost surface of the under layer to control thephysical conditions that border the magnetic layer.Under the low temperature-deposited conditions, wefinally have achieved better performance of the newdisk comparable with that of the conventional one.

The conventional CoCrTaPt high temperature-deposited magnetic layer has encountered the otherdifficulty of “thermal decay of magnetism” whichcauses loss of the recording bits under normal ambienttemperature. In order to suppress noise, the hightemperature-deposited magnetic layer increases its Cr

contents for the grain boundary segregation. At thesame time, however, the level of saturating magneticmoment is reduced and thermal energy becomesgreater than magnetic energy. On the other hand, theoxide additive promotes the grain boundary segrega-tion for the granular magnetic layer, without reducingthe level of saturating magnetic moment. Therefore,an excellent magnetic hard disk of high recordingdensity, high thermal stability and low noise can befabricated. Low temperature-deposited magnetic layertechnology can also be applied to Al or glass substrateswithout difficulties.

We are currently promoting development of higherperformance disk media in this way.

3. Conclusion

In the 21st century, the information revolutionthat personal computers and the Internet havebrought about will also extend into homes, and every-one will have the ability to handle images, music andinformation as desired. In this situation, one of theindispensable devices will be a capable but inexpensivehard disk drive. We have been promoting the develop-ment of magnetic hard disk for this drive.

In the future, society will demand a variety ofHDDs in applications not only for computers but alsofor audio-visual devices, and will encourage furtherexpansion of these devices. To respond to the impend-ing demand and supply of versatile magnetic harddisk, we will successively improve hard disk so that itis optimized for each application.

Table 4 Comparison of main figures with or without modifiedoutermost surface

Modified layer

Without modified outermost layer

Hcr(Oe)

Mrt(menu/cm2)

O/W(dB)

2,200 36.7

SNR(dB)

26.60.77

With modified outermost layer

2,500 38.3 32.50.73


Hiroshi HashidaEiichi FujisawaMotonori Nakazawa

Automation of Magnetic Hard DiskProduction Lines

Fig.1 Schematic diagram of an automated transfer system1. Introduction

In industry, the main aim of production lineautomation is to reduce the workforce. However in themagnetic hard disk (hereinafter referred to as the disk)industry, the aim is to improve quality and to enhancestability of the disk through a cleaner working environ-ment as well as to reduce the workforce.

The surface area of the disk per bit (0.8 × 0.07 µmat present) and the size of particles and defects underconsideration are decreasing year by year as thestorage capacity of disks increases. To cope with this,automation of the disk production line can preventdamage to the disk caused by manual transfer and canprevent dust from adhering to the disk in the case of aclean transfer.

Ever-increasing demand for higher-quality disk,however, cannot be satisfied only by automation of thedisk production line, resulting in the necessity of anautomated transfer system integrated with clean roomdesign.

Since an automated transfer system equipped witha cleaning function for a disk production line is highlyspecialized, there are very few dedicated systemscompared with other industries. Thus, transfer sys-tems for this purpose have mainly been developedjointly with transfer system manufacturers based ontheir existing systems.

This paper describes examples of the introductionof an automated transfer system into an existingproduction line, replacing a previous manual transfersystem, and introduction into a newly built productionline. Also described is an example of an automatedtransfer system as a total system for integrating cleanroom design to meet the requirements of a cleanerwork environment.

2. Introduction of an Automated Transfer Sys-tem into an Existing Production Line

Figure 1 shows a schematic diagram of an automat-ed transfer system introduced between processeswhere disk cassettes have traditionally been manuallytransferred. This automated transfer system aims to

improve cleanliness of the disk.

2.1 Space savings in the clean roomAutomated transfer systems within a single pro-

cess or between several processes require a ratherlarge amount of space in a clean room for installation.Especially when installed on the floor, such a systemcan obstruct passage and reduce workspace, resultingin increased cost of the clean room.

As for the introduction of an automated transfersystem into an existing production line, it is difficult tosecure space for its installation. Thus, we designed anoverhead-transfer system that saves space by utilizingthe space near the clean room ceiling.

Consequently, the space under the transfer systemcould be used for the existing passage and workspace.This has enabled the introduction of a clean transfersystem without requiring expansion of the clean room.

2.2 Purification due to the overhead-transfer systemSince the transfer system was installed close to the

ceiling, the clean area directly underneath the fanfilter unit has become a transfer route for diskcassettes. This separates the disk from the workers, asource of particles.

Cassette transfer point

Empty cassette transfer point

Loaded cassette transfer point

Overhead transfer track

Sputter

Loading machineLift for overhead transfer


Automation can also reduce the quantity of goods-in-process between operations and the time duringwhich disk is left exposed.

The above measures were implemented to realizeclean media transfer that prevents particles fromadhering to the disk.

2.3 Considerations regarding the automation of anexisting lineIn the above example of the automation of an

existing line, the transfer system could not be suspend-ed from the ceiling of the clean room due to insufficientstrength, but was instead supported by standaloneposts and by using existing facilities. However, thelocations of supporting posts and the transfer routewere restricted to secure space for the existing produc-tion line.

In this manner, installation of the automatedtransfer system in the clean room was subjected tovarious constraints due to the construction of the cleanroom and the facilities layout.

Thus, it has become important to design anintegrated production line and clean room that incor-porates the concept of an automated transfer system inthe early stages of the clean room design.

Based on this overhead transfer system, we havedeveloped subsequent automated transfer systems.

3. Introduction of an Automated Transfer Sys-tem into a New Production Line

3.1 Selection of a transfer system in accordance with itsusesFigure 2 shows a schematic diagram of an overhead

automated transfer system introduced between pro-cesses of a new production line. The system, jointlydeveloped in collaboration with a transfer systemmanufacturer, is based on the overhead disk cassettetransfer system that has been introduced into existingproduction lines.(1) Straight type transfer system (linear transfer

system)Applicable to a high-class clean area where trans-fer distance is short, buffers between processes arenot required, and high transfer speed is required

(2) Loop type transfer system (track feed system)Applicable to a middle- or low-class clean areawhere multiple input and output devices areinstalled to transfer multiple transfer cassettes

(3) Clean conveyer type transfer systemApplicable to a high- or middle-class clean areawhere a buffer function for disk cassettes isrequired between processes

An automated transfer system that complies withhigh quality requirements was realized by choosingfrom among the above-mentioned systems according tothe use and operating environment of the transfersystem.

3.2 Automated production line integrated with clean roomdesignIn the introduction of a transfer system, the design

and installation of the transfer system along with thedesign of the clean room take into consideration, fromthe outset, the required transfer area, attachment ofhanging bolts to the ceiling, ceiling strength, arrange-ment within the clean room and air flow in the cleanroom due to cover mounting.

Consequently, an automated transfer system canbe introduced smoothly without impairing the functionof a clean room and resulting in reduced installationcost.

After the installation, minor changes such asalterations of floor openings and the addition of coverswere performed to improve the cleanliness in the roombased on the results of investigation and analysis ofparticles and airflow in the clean room.

If even higher quality is required, it is necessary todesign a clean room and automated transfer systemsbetween processes as a total system, including theplacement of peripheral equipment of the transfersystem and clean room construction.

Fig.2 Schematic diagram of an overhead automated transfersystem introduced in a new production line

Fan filter unit

Straight type transfer unit

Cassette

Cassette

Loop type transfer unit

Clean conveyer type transfer unit

Cassette

Fan filter unit

Rail

(a) Straight type transfer unit

(b) Loop type transfer unit

(c) Clean conveyer type transfer unit


Fig.3 Schematic diagram of a total system

4. Design as a Total System

To cope with higher quality requirements for thedisk, an example of an automated transfer systemintroduced between processes will be described.

Figure 3 shows a schematic diagram of the totalsystem.

4.1 Area separationIn designing an automated transfer system, areas

in and between processes were separated from eachother in accordance with the required cleanliness.

Due to economical and technical limitations, it isimpossible to make dust source free of the system.Therefore, disk cassette transfer routes, facilities in-stallation areas, and maintenance and working areaswere completely separated to isolate the disk and dust

sources. In addition, no shielding materials were usedover the disk cassette transfer routes in order to securea laminar flow of air. Figure 4 shows a schematicdiagram of the area separation.

Thus, dust sources and the disk were completelyisolated in the clean room to create a clean environ-ment that maintained the required cleanliness in eacharea even if there was dust from human bodies andfacilities.

4.2 Development of a transfer system for improvingcleanlinessTraditionally, transfer systems in areas requiring

a high level of cleanliness were not popular because oftheir high cost.

By improving the track feed system (the loop typetransfer system mentioned above) and integrating itwith a clean room, Fuji Electric has developed a low-cost clean transfer system which can be used in anultra high-class clean area. Previously, track feedsystems could only be applied to medium- and low-class clean areas. Figure 5 shows a schematic diagramof the newly developed low-cost clean transfer system.

In the previous type of transfer systems, diskcassette chucks were provided under the transfersystem body. In the new system, the chucks wereremoved from the system and placed outside the bodycover to separate them from dust sources. In addition,to provide a clean power supply system to the transfersystem body, an induction type feed system (non-contact type) was developed to replace the brush typefeed system (contact type) because the latter emits arelatively large amount of dust.

The transfer system was developed as part of thetotal system, integrating a clean room design aroundthe transfer system.

Fig.5 Schematic diagram of low cost clean transfer system

Fig.4 Schematic diagram of area separation

Chemical filter

Clean unit(ULPA)

Clean unit(ULPA)

Clean unit(ULPA)

Cassette

Exhaust

Overhead transfer system

Transfer point

Production equipment

Maintenance area Class 100

Cassette transfer area Class 10

Returnarea

Return area

OA OA

(ULPA) (ULPA) (ULPA) (ULPA) (ULPA) (ULPA) (ULPA)

(Passage)

Equipment area Class 10

Equipment

Exhaust

Working areaClass 10

Auto-mated transfer

Opening for exhaust

Rail

Transfer unit

Cassette chuck

Cassette

Class 10area

Return areaOpening for exhaust

Opening for cassette chuck

(ULPA)


4.3 Improvement of exhaust routesThe exhaust of air from facilities and dust sources

is important in maintaining cleanliness of the cleanroom. The exhaust of a large amount air required forautomated transfer systems within a single process orbetween several processes also requires a large amountof intake air, resulting in increased cost.

Realizing that exhaust air does not contain hazard-ous gases but only particles, Fuji Electric has devel-oped a system that, utilizing pressure differences inthe clean room to reduce the amount of intake air,directly exhausts dust sources to a return area andthen removes particles from the exhaust. Figure 5shows a schematic diagram of a low cost clean transfersystem.

More specifically, exhaust pipes are attached to thetop of the main body of the transfer system that isinstalled close to the ceiling. Usually these pipes areconnected to exhaust ducts, but instead they passexhaust air to the return area. In addition, since diskcassette chucks are provided outside the cover, slitsare needed. An exhaust system was constructed toallow the exhaust air to be drawn into and to flowinside the cover and to directly return to the returnarea utilizing the pressure in the clean room.

Since media cassettes are transferred in an areaoutside the cover, which is installed immediately belowclean units (ULPA) where a laminar flow of air flows,the clean transfer of disk was realized.

This type of transfer system was employed in otherceiling transfer systems [items (1) and (3) of section 3.1above].

The driving unit of the transfer system was placedbelow disk cassettes and shielded with covers toseparate it from the cassettes. The covers wereprovided with openings so that the exhaust air woulddirectly return to the return area.

These measures can reduce costs in the exhaustsystem of an automated transfer system.

4.4 Configuration of peripheral equipmentFigure 6 shows the configuration of an automated

transfer system and peripheral equipment that, as atotal system, aim at improving cleanliness.

As a total system, measures for improving cleanli-ness were implemented for production facilities as wellas for the automated transfer system. Particularattention was paid to the cassette transfer points of theautomated transfer system, areas where cassettes andthe disk passed, and their surrounding areas.

As higher quality products are required, specialattention must be paid to the residence period betweenprocesses, and thus the buffer function was improved.4.4.1 Areas where cassettes and the disk pass

To secure a laminar flow of air from the ceiling,measures where implemented in the facility at areaswhere cassettes and the disk passed.

Partition covers are attached to the ceiling toseparate areas for cassettes and the disk from theareas for facilities. Disk cassettes were placed on aframework construction to allow the laminar flow of airto and under the floor bottom.

Since cassettes and the disk are placed on transferunits at the transfer points, and robots for chuckingare operated from the side, there are no shieldingmaterials over the cassettes and the disk while theytravel, preventing clean-air flow turbulence and secur-ing a laminar flow of air.4.4.2 Buffer function of cassettes

To cope with higher quality requirements for thedisk, the residence period between processes should beshortened to reduce the quantity of adhered particlesand ion contamination. In some cases, there arelimitations on the residence period for products be-tween processes. An automated transfer system wasdesigned taking into consideration not only tact timebut also the residence period of disk cassettes.

In addition, a buffer function for disk cassettes isrequired for short time stoppage of the facilities. Thenumber of buffer cassettes is designated in the produc-tion specifications for each unit, taking into consider-ation the number of units in each process, unitreliability and tact time at the time of total systemdesign.

Buffer areas for disk cassettes were constructed inthe same manner as cassette transfer points to securea laminar flow of air from the ceiling. Utilizing theadvantage of an overhead transfer system, the bufferfunction was incorporated into the design of overheadtransfer routes.

Specifically, an area for receiving disk cassetteswas secured on the overhead transfer routes. Al-though that area is not normally used, during opera-tion of the production line it provides functions toremove and buffer all of the disk cassettes from the

Fig.6 Configuration of an automated transfer system andperipheral equipment

Partition cover

Buffer cassette

Cassette transfer point

Cassette

Opening for exhaust

(ULPA) (ULPA) (ULPA) (ULPA) (ULPA)

Production units


previous process in the event of a short stop of theproduction line when disk cassettes being transferredto the next process.

Since disk cassettes are buffered directly below theoverhead clean units, they are shielded from other dustsources, and particles are prevented from adhering tothe disk cassettes even if the buffer time is short.

5. Conclusion

In order to construct automated transfer systemswithin a single process or between several processes, itis necessary to design as a total system all facilitiesincluding the transfer systems, clean room and eacharea.

Cost and technical limitations in improving cleanli-ness can be overcome by total system design, leading toa substantial cost reduction.

As the drive toward lower cost has intensifiedmuch faster than expected, cost reduction of the

production line and transfer systems has becomeimperative. On the other hand, total automation of theproduction line is required to meet the demands ofhigher quality disk.

To satisfy these mutually contradictory require-ments, it is necessary to shorten the residence periodto prevent contamination, to reduce the quantity ofgoods in process and simplify production management,to reduce the cost of the transfer system and to savespace, and as a result, to provide a production line witha minimized transfer system.

Minimization of the transfer system can be real-ized by connecting processes, eliminating distancesbetween several processes, and improving the reliabili-ty of facilities.

Fuji Electric will continue to develop total systemsin which the transfer function is incorporated intoproduction facilities, beginning from the early stage ofproduction specification determination.


Kurao HabayaHideyuki IwataYoshitomo Ogimura

Ultra-Clean Technology forHigh Density Magnetic Hard Disks

1. Introduction

With the dramatic proliferation and higher perfor-mance of personal computers, higher quality is increas-ingly demanded of magnetic hard disks (hereinafterreferred to as the disk) mounted on hard disk drives.

One of the factors affecting the trend mentionedabove is the widespread use of hermetically sealeddrives without any openings for air intake. As the useof drives become more widespread, applications inadverse environments coupled with mobile usage willdisrupt the humidity and temperature balance in thedrives, leading to dew condensation on the surface ofthe disks. Adsorbed water due to dew condensationreacts with ionic contaminants, and can corrode anddissolve thin film magnetic materials and lubricant.

Another factor is the challenge of higher recordingdensity. In recent years, the size of the recording bit ofdisk platters where digital data is recorded has becomeextremely small, 0.8 µm × 0.07 µm in currently mass-produced types. If there are defects of approximatelythe same size as mentioned above, some reproduceddata will be missing. On the other hand, since the gapbetween a disk and a recording head (flying height ofthe head) is small (0.03 µm), dust on the surface of thedisk will scratch the surfaces of the recording head anddisk, and in the worst case, recorded data will be lost.The size of the recording bit and flying height havebeen exponentially decreasing as shown in Fig. 1, andthis trend will continue in the future.

With higher recording density, contamination ofdisk surfaces during processing must be kept to aminimum, necessitating a total control of environment,materials and equipment in a clean room, (refer toTable 1).

This paper presents an overview of contaminationcontrol in a clean room and contamination preventionof disk surfaces and countermeasures against electro-static hazards.

2. Contamination Control in a Clean Room

Contaminants in a clean room are divided into twomajor categories: particle contaminants, and gaseous

chemical contaminants that cannot be removed with adust removal filter. The recent development of highefficiency filters such as HEPA (high efficiency particu-late air-filter) and ULPA (ultra low penetration air-filter) has made it possible to control dust contami-nants. Thus, the control of gaseous chemical contami-nants has become commonplace within a clean room.

Chemical contaminants are acids, alkalis andorganic gases, that come from equipment, workers,construction materials, cassette materials and outsideair. The contaminants are selectively adsorbed by thesurfaces of a disk and recording head, exerting anadverse effect on product quality.

Chemical contaminant control can be broadly clas-sified into three methods: “do not bring in,” “do notgenerate,” and “removal.” The “do not bring in” and “donot generate” methods use construction materials andcassette materials in the clean room having a lowamount of outgassing. The “removal” method isimplemented by installing a chemical filter and an airwasher. Figure 2 shows a schematic diagram of an airconditioning system in a clean room.

2.1 Chemical contaminant control in a clean room2.1.1 Contamination control due to air flow control

If dust contaminants are adsorbed by products in aclean room, they exert an adverse effect on product

Fig.1 Transition of bit size and head flying height versus year

Bit

len

gth

, bit

wid

th a

nd

flyi

ng

hei

ght

(µm

)

10

1

0.1

0.011996 1998

Bit width

Bit length

Flying height

2000(Year)

2002


Table 1 Summary of cleaning items and their evaluation

Item

Clean room environment

Filtration

Contamination check of finished products

Materials, utility

Object Evaluation

Air flow control

Monitoring

Selection of construction materials

Filter

Air washer

Lubricant, solvent

Ultra pure water

Cassette, bag

Air flow visualization systemAir flow

Ion components in the air

Organic components in the air

Organic components

Particles

Ion components

Automatic particle counterDust in the air

Organic components

Automatic monitorTemperature, humidity, room atmospheric pressure

Ion components

Impinger collection → IC

Organic components

Witness disk → IC

Water

Adsorption by the disk when left standing → FTIR

Ion component in the water

Adsorption by activated carbon → GC-MS

Ion components

Ultra pure water extraction → IC

Corrosion

Concentrated collection → GS-MS

Organic components


Ion components

Adsorption by activated carbon → GC-MS

Organic components

Electrical conductivity sensor

Particles, ion components



Ultra pure water extraction → ICP

Ultra pure water extraction → FTIR

IC

FTIR

Particle counter, IC


Ultra pure water extraction → FTIR

Concentrated collection → GS-MS

Particle counter

Ion components

Measurementfrequency

Whenever necessary

Continuously

Every week

Whenever necessary

When constructing or reconstructing the clean room

Every month

Whenever necessary

Continuously

Every month

Every week

Every week

Continuously

Every week

When selected

Every week

Fig.2 Air conditioning system in a clean room

Gas removal air filterHEPA filterFan

Gas removal air filter for FFU use

FFU

Free access floor grating

Air conditioner

Outer air conditioner

Air washer

Roll filter

Differential pressure damper

quality. Although a dust removal filter is generallyused for this purpose, that by itself is insufficient. Airflow control is necessary to exhaust dust contaminantsgenerated in the clean room out of a production processwithout contaminating products.

Air flow control requires that parameters such as

the positions of air inlet and outlet openings, air flowrate and the layout of equipment in the room beadjusted to prevent turbulence, including the consider-ation of a push-pull construction for air inlet and outletopenings. At present, design of air flow control ismainly based on a simulation conducted at the time ofconstruction. Special attention must be paid to the factthat even if there was no trouble in air flow control atthe beginning, turbulence caused by changes to thelayout of line equipment or other factors could contami-nate products as time goes by.

Therefore, after layout, air flow in the clean roommust regularly be monitored by an air flow visualiza-tion system. Points that are detected as causingturbulence are altered. Figure 3 shows the verificationof air flow with an air flow visualization system.2.1.2 Monitoring of dust contaminants

Maintaining cleanliness in a clean room requiresregular monitoring based on control criteria. Acentralized supervisory system is constructed to simul-taneously monitor in real-time the dust suspended inthe air, dust suspended in liquids, temperature andhumidity in the clean room so that countermeasurescan be implemented against sudden abnormalities.


2.2 Countermeasures against chemical contaminants in aclean room

2.2.1 Contamination control due to selection of construc-tion materials in a clean room

Organic gases from chemical contaminants aremainly emitted by construction materials used for theclean room’s interior and cassette materials. To copewith this, construction and cassette materials that onlyemit a small amount of outgas were selected toconstruct a clean room having few organic gases.(1) Selection of construction materials

Construction materials used in a clean roominclude paint, adhesive, wall and floor materials, resinmold, tape and other materials.

First, materials emitting low aggregate amounts oforganic gases and specific components with lowamounts of outgases were selected from among poten-tial construction materials using a gas chromatograph-mass spectrometer (GC-MS). Specific componentsinclude siloxane and dioctyl phthalate (DOP), possiblyharmful to the quality of products.

Secondly, construction materials with ionic compo-nents below reference values were selected by analyz-ing them with an ion chromatograph (IC). Figure 4shows sealant analysis results as an example of theanalysis of construction materials. It was determinedfrom the results that sealant A emitted a smallamount of gases and their ion components were belowreference values. On the other hand, sealant Bemitted a rather large amount of gases, and gases suchas DOP were identified, and the material was judgedas improper.(2) Confirmation of effect of selection of construction

materialsTo confirm the effect of the selection, organic gas

concentration was verified for clean rooms A (CR-A)and B (CR-B), which were constructed in differenttimes. CR-B contains the latest high-tech equipmentand was constructed 18 months after the installation ofCR-A. CR-B was provided with construction materials

Fig.4 Sealant analysis results with a GC-MS

Fig.5 Transition of organic gas concentration

Pea

k in

ten

sity

100000000TIC

5

1

2

10 15

1 2-Butanone2 2-Butanone, oxime

20 25 30Retention time (min)

(a) Sealant-A

Pea

k in

ten

sity

100000000TIC

5

0

0

1

2

3

4

56

10 15

1 Butanol2 Benzene, 1,2-dimethyl-3 Butylated Hydroxytoluene4 Diethyl Phthalate5 Benzenesulfonamide,4-methyl-6 Di-n-octyl phthalate


(b) Sealant-B

Con

cen

trat

ion

(µg

/m3)

500

400

300

200

100

00 5 10 15

CR-ACR-B

Time (month)

selected for their low outgassing.Figure 5 shows the transition of the organic gas

concentration of CR-A and CR-B. It was confirmedthat CR-B had much lower organic gas concentrationimmediately after and since its construction, comparedwith CR-A. This demonstrated the effect and impor-tance of material selection.2.2.2 Contamination control due to installation of a

chemical filter and other devicesChemical contaminants enter a clean room from

equipment and workers in the clean room and fromoutside air, in addition to construction and cassette

Fig.3 Verification of air flow with a visualization system


materials. Countermeasures only against constructionand cassette materials are insufficient. Thus, achemical filter and an air washer are used to reduce orremove harmful gases.(1) Contamination control with a chemical filter

Various types of chemical filters have been devel-oped and an appropriate filter for the gas componentsshould be used. At present, an ion exchange filter isused for alkaline gases, a chemical adsorption filter foracidic gases, and an activated carbon filter for organicgases. When required, filters are installed in the aircirculation system and in an outer air conditioner. Thenumber of filters installed varies with gas concentra-tion settings.(2) Contamination control with an air washer

Chemical contaminants also enter a clean roomfrom outside air entering a circulating system. Theouter air contains SOx, NOx, etc., and their concentra-tion varies with meteorological conditions and thedirection of a wind, affecting gas concentration in theair circulation of the clean room. An air washer isused together with a filter to remove the gases,because providing the outer air conditioner with only afilter is neither sufficient not cost-effective.

An air washer is a system intended to removechemical contaminants in the outer air by absorbingthem with water. This system passes the air through aspray of pure water before taking the air into the cleanroom circulation system.

Special care must be taken because the contami-nant removal rate decreases when the contaminantconcentration in the water increases. Pure water usedfor the air washer is automatically controlled with anelectrical conductivity sensor to maintain a specifiedremoval rate.2.2.3 Monitoring of chemical contaminants

Chemical contaminants are monitored based onestablished methods of sample collection and analysis.At present SOx, NOx and ammonium are automatical-ly monitored with a sulfur oxide meter and others.

Detection limits can be circumvented and a largeamount of information can be processed by combiningsample collection with analysis. Sample collectionincludes collection with an impinger and activated-carbon tube. Analysis is conducted with IC and GC-MS. Automatic monitoring with IC is being consideredby some manufacturers.

3. Contamination Control of Disk Surfaces

Contamination control of disk surfaces requires acomprehensive control of environment, processes,packing materials and workpiece materials to besurface-processed. As mentioned before, with theincrease of packing density, stricter contaminationcontrol is required. The present challenge is how toachieve an effective contamination control. Contami-nation control currently in use for disk surfaces ispresented below.

3.1 Control of residence time in the processIn the processes following sputtering, control of the

environment during the lubricant coating process andresidence time during disk processing is important forcontamination control of disk surfaces. Carbon layersclosest to the disk surface are, in general, sensitive tocontamination until they are coated with lubricant. Asshown in Fig. 6, the amount of contaminants adsorbedby the disk increases with residence time.

Based on the findings, strict control of residencetime during processing is conducted to minimize theadsorption of contaminants. Time control is conductedusing a production control system and the disk qualityis verified through analysis using IC of pure waterextracted from the disk.

3.2 Control of disk contamination due to cassettes andpackingSince disk loaded on cassettes for a long time is

subject to outgas from the cassettes, examination of

Fig.6 Witness of the disk Fig.7 Analysis of a PP cassette cover before implementingmeasures against outgassing

Incr

ease

of

con

tam

inan

ts (

µg/m

2)

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

0170144120

Disk A

SO42-

Disk B

967248240Number of days witness (h)

Pea

k in

ten

sity

50000000TIC

50

1

2

3

4

5

10 15

1 Octane, 4-methyl-2 1-Butanol, 2-ethyl-3 Decane4 1-Nonene, 4, 6, 8-trimethyl-5 1-Undecene, 7-methyl-



Fig.8 Analysis of a PP cassette cover after having implementedmeasures against outgassing

Fig.9 Water vapor permeability characteristics versus time

Pea

k in

ten

sity

50000000TIC

50

1

2

34

5

10 15

1 Octane2 Heptane,2,5-dimethyl-3 Undecane4 Tridecane5 Butylated Hydroxytoluene


Hu

mid

ity

(%)

80

70

60

50

40

30

20

10

0120967248240

Standing time (h)

Al bag (newly developed)Conventional bagSample ASample B

resins used for cassettes and confirmation of cleaningquality are very important.

Today, the dominant cassettes to be antistaticfinished consist of a body made of polycarbonate (PC)and upper and lower covers of polypropylene (PP).Both PC and PP must fully be checked for dustgeneration and outgassing. Quantitative control mustregularly be made to check if the cassettes contain alarge quantity of plasticizer and sizing materials,contaminants easily adsorbed by the disk surface. Inparticular, special attention must be paid to specificpolymeric compounds such as BHP, DBP, DEP andsiloxane, contaminants requiring extra caution, be-cause they were claimed to be responsible for someaccidents in the past. Analysis is regularly performedto check the quality of the disk by an FTIR(Fouriertransform infrared spectrophotometer), extracting sol-vent from the disk, and by a high sensitivity concentra-tion method based on GC-MS purges and traps.3.2.1 Improvement in the quality of cassette materials

Outgassing from cassette materials can be sub-stantially reduced by joint efforts with the cassettemanufacturer, resulting in simplification of the cas-sette cleaning process.

Figures 7 and 8 show an example of improvementin the quality of cassette materials. Due to thisimprovement, the gas cure process during cleaning canbe omitted.3.2.2 Improvement in the quality of packing materials

Contamination control against packing materialsand enhancement of environmental barrier character-istics are also important topics in the prevention ofdisk surface contamination.(1) Outgassing

Materials having a low amount of outgassing wereexamined and selected with a GC-MS as is the casewith cassettes.(2) Environmental barrier characteristics (water va-

por permeability)Figure 9 shows an example of the improvement

from implementing measures against humidity duringtransportation. Fuji Electric, in collaboration with apacking material manufacturer, has developed and putto practical use a new four-layer bag using aluminumand new-generation packing materials. This bag isprovided with measures against static electricity andfeatures enhanced strength.

3.3 Monitoring of contaminants (workpiece materials,clean room, process, finished goods)Surface contamination of the disk and its environ-

ment are continuously monitored with regard to theitems listed in Table 1. These items are regularlymeasured by week, by month and by lot. A database iscreated from these results and necessary data is fedback to production processes for the purpose of contam-ination control and improving product quality.

In the future, Fuji Electric will clarify the relation-ship between disk reliability evaluation items andcontamination control items and will reconstruct anefficient quality assurance system to meet the needs ofnext-generation disk.

4. Static Electricity

Static electricity causes contamination due to dustdeposition and electrostatic discharge failure in work-piece materials.(1) Countermeasures against static electricity are the

last to be implemented against contamination dueto dust deposition. Charged electric potentialmust be 50V or less to prevent the deposition ofdust particles of 0.5 µm or more in diameter. In adisk production process, grounding of the producttransfer system and use of conductive materialsfor cassettes and humidity control are implement-ed as countermeasures against contamination dueto dust deposition.

(2) As for the breakdown of workpiece materials,countermeasures against electrostatic dischargefailure are implemented on the GMR (gaint mag-netoresitive) head during testing. The counter-


measures include ① grounding, ② use of conduc-tive materials, ③ humidity control and ④ use ofan ionizer.① A testing machine and workbench for exchang-

ing GMR(giant magneto-resistive) heads weregrounded. The charged electric potential of ahuman body can be reduced from 1,500V to 3Vor less by the use of a wristband.

② Use of conductive materials permitted thecharged electric potential of substrates in thecassettes to be reduced from 2,310V to 5V orless, covers to be reduced from 2,200V to 3V orless and chairs from 7,910V to 5V or less.

③ Humidity is controlled and maintained atgreater than 45%.

Fig.10 Charged electricity potential characteristic of cleangarments versus time

Ch

arge

d el

ectr

icit

y po

ten

tial

(- V

)

1,600

1,400

1,200

1,000

800

600

400

200

060302520151050

Time (s)

Company A productCompany B product 1Company B product 2Company B-developed product

④ An ionizer with an AC special-type fan whichcan control the ion balance within a range of±5V was selected and used for GMR headexchange on the workbench.

Fuji Electric has asked a manufacturer to developclean garments equipped with countermeasuresagainst static electricity. Changing the fibers in cleangarments from hydrophobic to hydrophilic reduced thecharged electric potential from 350-470V to 10V or lesswhen wearing the garments on a conductive floor, andfrom 600-1,500V maximum to 300V when wearingthem on an insulated material. These results areshown in Fig. 10, illustrating the advantage of chang-ing fibers. Fuji Electric is also considering thedevelopment of gloves.

5. Conclusion

This paper has presented improvements to theclean room environment and management, and meth-ods to verify the products. These procedures mustcontinuously be improved as higher performance harddisks are required.

On the other hand, within several years revolu-tionary changes in the concepts for magnetic harddisks will possibly be needed, particularly when pro-duction processes are substantially changed due to theintroduction of a perpendicular magnetic recordingsystem and an optically assisted recording system, andalternative substrates are utilized to meet cost reduc-tion goals. To satisfy these needs, the developmentand enhancement of clean technology will be required.

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