Flyable media for slider based ultra-high densityoptical recording

C. Morsbach, C. Dubarry, M. Gabriel, M. Hoyer, S. Knappmann, F. Piazza, J. Robertson, R. Vullers and H.H.Gatzen

Abstract: An overview of the research project ‘Flyable media for slider based ultra-high densityoptical recording’ (FAMOUS) in the European Union’s Framework Programme 5 is presented.The project aims to investigate the potential of slider technology for use in optical data storage onremovable plastic media. The investigations performed in this joint research effort focus on themanufacturing of flat plastic media, sliders for optical data storage in the optical far-field and near-field regime, protective coatings, and flyability testing. The flying heads developed in this projecttake advantage of glass patterning for the air bearing surface generation. Glass slider prototypeswere produced and evaluated, yielding a head-to-disk spacing within the desired range of500–700nm. To improve the flatness and roughness of injection moulded plastic media the Nielectroplating technology for stamper production was improved to yield a roughness below 3nm.The coatings used to protect disks and heads from damage consist of amorphous diamond-likecarbon (DLC). First investigations on DLC deposition showed the potential to achieve a bandgapin the order of 3 eV, as is required for blue laser optical recording technology.

1 Project objectives

Optical storage is the preferred technology when high-density storage on removable and exchangeable media isrequired. The advantages are its low cost, media exchange-ability (standardisation) and robustness. One approach tomeeting the demand for high data capacity of optical disksis the use of flying head technology. A flying headcontributes to a substantial increase in data density byreducing the spacing between media and coil for magneto-optical recording with high numerical aperture (NAB0.8–0.9). It also allows the use of a solid immersion lens (SIL)for optical near-field recording using phase change media. Asolid immersion lens requires a lens-to-media spacing of lessthan approximately 120nm, and thus calls for highlyaccurate gap control [1]. A flying head comprises a stiffdynamic air bearing, thereby offering a proven and reliablemeans of maintaining a stable distance between recordingmedium and recording coil or lens, respectively [2]. Since therequirements for disk-to-slider spacing are different for

magneto-optical and optical near-field recording, tworanges of slider flying height will be investigated in thisproject: a slider flying in the optical far-field (head-to-diskspacing approximately 1mm) for domain expansion mag-neto-optical recording utilising magnetic amplifying mag-neto-optical system technology or domain walldisplacement detection [3, 4]. The far-field slider is intendedto carry a magnetic field modulation (MFM) coil. A similarsystem, featuring a flying head with a flying height of 3mm,carrying a coil and a lens has been proposed by Shimokawaet al. [5]. The second approach pursued in this project is touse a slider-to-media distance in the optical near-field range(approximately 50nm) with the slider being capable ofcarrying a solid immersion lens. One major task within theproject is the development of the manufacturing technologyfor optical near-field as well as far-field sliders made out ofglass. Critical issues for applying flying head technology onremovable media are: the flying behaviour on rather wavyinjection moulded plastic disks, and the influence ofcontamination on the head disk interface. Both aspects willbe investigated within the scope of the project.The second focus of the project has been on the media

substrates for slider based optical data storage. The aimhere is to develop CD-type removable plastic media,manufactured by injection moulding. The injection mould-ing process as well as the machine and tooling need to beadapted to meet the roughness and flatness requirements offlying head technology. Therefore, within the project aprecision moulding machine will be developed, and themoulding and stamper technology will be optimized.The head–media interface is the third technology

challenge in the project. Protective coatings based onDLC films are state-of-the-art in magnetic data storage[6, 7]. However, that DLC does not need to transmit light.Since the plastic substrates are softer and more sensitive tothermal influences, an adaptation of the DLC coatingspecification is needed to achieve the desired low residualstress level as well as a reduced thermal load. To meet the

C. Morsbach and H.H. Gatzen are with the Institute for Microtechnology,Hanover University, Callinstrasse 30A, Hanover D-30167, Germany

C. Dubarry is with the CEA-DRT-LETI/DOPT, CEA/GRE, 17 rue deMartyrs, 38054 Grenobles CEDEX 9, France

M. Gabriel is with the AWM Mold-Tech AG, Gotthardstrasse 3, Muri CH-5630, Switzerland

M. Hoyer is with the Arburg GmbH & Co, Lossburg, Postbox 1109, LossburgD-72286, Germany

S. Knappmann is with the Thomson Deutsche Brandt GmbH, Postbox 1307,Villingen Schwenningen D-78003, Germany

F. Piazza and J. Robertson are with the Department of Engineering, CambridgeUniversity, Cambridge CB2 1PZ, UK

R. Vullers is with the Philips Research Laboratories, Prof. Holstlaan 4 (WY31),Eindhoven 5656 AA, The Netherlands

r IEE, 2003

IEE Proceedings online no. 20030869

doi:10.1049/ip-smt:20030869

Paper first received 13th February and in revised form 31st July 2003

IEE Proc.-Sci. Meas. Technol., Vol. 150, No. 5, September 2003 203

optical constraints for optical data storage, the film needsto be transparent at the recording wavelength l with abandgap 43eV (corresponding to an app. l of 400nm).The slider flying performance on plastic substrates will beinvestigated using an adapted DVD drive. The static flightattitude is measured by a laser interferometer flying heighttester, while the dynamic flight attitude will be measured byusing a thermoresistive sensor [8].

2 Slider technology

To meet the optical requirements sliders were manufacturedout of glass. The sliders glide over the rotating disk on anaerodynamic bearing formed by the smooth disk surface onone side and the patterned lower slider surface on the otherside, called the air bearing surface (ABS). The slider ABShave been designed for a flying height of around 1mm (far-field) or 50nm (near-field). The 1mm flying height is lowenough to have sufficient magnetic field intensity, whilebeing mechanically sufficiently stable and having sufficienttribological robustness. For both slider types the light isdelivered by a lens located perpendicular above the slider. Inthe case of the far-field slider, no second lens on the slider isrequired, whereas the near-field slider is designed to carry aSIL. The flying characteristics of these zero-load sliders [9]were modelled with a finite element method solution of amodified version of the Reynolds equation. The sliderdimensions are fairly large compared to the present-dayhard-disk sliders, but this is necessary because part of thesurface is taken up by the hemispherical lens (radius0.5mm) and by the suspension’s contact area. Thehemispherical lens is a composition of a plane glass waferbody and a complementary part of a glass sphere with thesame refractive index, where the thickness of the assembledlens is equal to the radius of curvature.The ABS was generated on glass wafers using photo-

lithography and subsequent wet etching. Especially for thenear-field sliders, it is important to reach the required depthwith considerable accuracy. By closely monitoring theetching speed the error in depth could be kept within a fewpercent. In a final machining step, the single slider wereseparated by a dicing process. Dicing wheels consisting of adiamond abrasive grit embedded in a resinoid binder wereused to separate the sliders. The optimisation of the dicingprocess included screening of different dicing wheels and thedevelopment of a dicing process yielding minimal edgechipping. Table 1 summarises the investigation parametersof the wheel screening.As a result, the optimum grain size of the diamond

abrasives was determined to be 15mm. As presented in

Fig. 1 the 5mm wheel failed during dicing, and themaximum chipping increases with larger diamond grains.The maximum chipping is the width of the largest chips in adicing lane measured perpendicular to the edge.

After dicing, the sliders were assembled to heads byattaching a suspension to the sliders’ upper surface, whichacts as a preloaded spring opposing the forces at the ABS.The suspension was a standard suspension commonly usedin rigid disk drives. Figure 2 shows an optical recordinghead assembly featuring the glass slider with patterned ABSand the suspension attached to its back side.

3 Manufacturing of flat media

To manufacture flat plastic disks as required for a datastorage system comprising removable media an injectionmoulding process optimisation as well as the improvementof the manufacturing technology for the stamper isrequired. The stamper is a patterned disk containingformatting structures which is inserted in the mould. Sinceit determines the surface topography of the disk side facingthe slider’s ABS, strict requirements with regard to thestamper’s roughness and flatness need to be met. Themanufacturing process for the stamper is nickel electroplat-ing, which was subject to investigations. Both the influenceof the electroplating bath composition and the depositionparameters were investigated to improve the stamper’sroughness. An average roughness (Ra) of 3nm could beobtained using suitable additives at a current density of28mA/cm2 (Fig. 3).

Table 1: Glass slider dicing parameters

Dicing blade parameter

Parameter Value

Binder Resin

Abrasive Diamond

Grit, mm 5, 15, 30, 45

Thickness, mm 125

Diameter, mm 55.6

Process parameters

Feed rate, mm/s 0.1

Cut speed, m/s 55

Depth of cut, mm 0.4

25

20

15

10

0

5max

. chi

ppin

g, µ

m

blad

e fa

ilure

5 15 30 45

diamond grain size, µm

Fig. 1 Chipping as a function of the diamond grain size duringdicing of glass wafers

Fig. 2 Head assembly of an optical recording head

204 IEE Proc.-Sci. Meas. Technol., Vol. 150, No. 5, September 2003

The roughness values achieved by the electroplatingoptimisation are sufficient to meet the requirements forpatterning plastic substrates in optical data storage. Furtherinvestigations need to be performed to evaluate thereplication accuracy during the injection moulding process.

4 Protective coatings

The protective DLC coatings were deposited by an electroncyclotron wave resonance (ECWR) plasma source [10]. Theaim of the work was to deposit low stress DLC films with ahigh optical gap. Investigations were performed to increasethe bandgap by varying gas flow and pressure. CH4 wasused as a precursor gas, as DLC films deposited using thisgas exhibit lower stress compared with C2H2. Table 2 showsthe results of the optical gap measurements for two givenvalues of gas flow and pressure. The magnetic field strengthB and the RF power PRF were kept constant: B¼ 13G andPRF¼ 300W.

Films deposited under these conditions are polymeric,independent of pressure and gas flow. It is possible toincrease the hardness of the film by increasing the ionenergy. This leads to a higher sp3 fraction, i.e. the amount ofdiamond-like bonds in the film. At the same time thetransparency of the film decreases, as shown in Fig. 4. Thepressure during deposition was 1.7� 10�3mbar.

The results show that the optical gap decreases from amaximum of 4.2 eV to 3eV by increasing the ion energy.Any higher ion energy causes a high residual stress, resultingin film delamination.

5 Flying height evaluation

Flying height measurements were performed to verify theflying height which was modelled and calculated usingFEM. A commercial phase metrics flying height tester withthe sliders flying on a rotating glass plate was used for thistask. Fig. 5 shows a comparison of calculated and measuredflying height (Fig 5a) and the flying attitude as a function ofthe disk velocity (Fig 5b). The flying attitude describes theslider’s roll and pitch angle, i.e. the rotation around the twohorizontal axes. Because this rotation affects the alignmentof the two lenses in the optical system of a near field set-up,they are crucial for the recording and readout performance.The slider investigated at this stage of the project was a ‘far-field slider’, designed for a flying height in the optical far-field regime. The comparison indicates a good matchbetween calculated and measured flying behaviour. Theflying height for the designated velocity in the range 3–6m/sis 500–700nm. Further investigations need to be performed

70

60

50

40

30

20

10

0

roug

hnes

s, n

m

0 10 20 30 40 50

without additive

with additives

current density, mA/cm2

Fig. 3 Roughness of electroplated nickel as a function of thecurrent density

Table 2: Optical gap E04 as a function of the pressure andgas flow

Gas flow F, sccm Pressure, mbar Optical gap E04, eV

30 1.7�10�3 3.8

100 8.4�10�3 4.6

4.5

4.0

3.5

2.5

3.0

E04

, eV

40 50 60 70 80mean ion energy, eV

Fig. 4 Optical gap E04 as a function of the mean ion energy forfilms deposited at 1.7� 10�3 mbar

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

flyin

g he

ight

, µm

phase metricscalculationsinterferometer

0 2 4 6 8 10 12 14 16 18 20 22

disc velocity, m/s0 1 2 3 4 5 6 7 8 9

0.02

0.01

0

− 0.01

− 0.02

− 0.03

− 0.04

− 0.05

− 0.06

roll

pitch

0.8

0.6

0.4

0.2

0

pitc

h, m

rad

roll,

mra

da

b

disc velocity, m/s

Fig. 5 Flying height and pitch and roll of glass sliders as a functionof disc velocitya Flying heightb Flying attitude

IEE Proc.-Sci. Meas. Technol., Vol. 150, No. 5, September 2003 205

to evaluate the slider designs with regard to flying attitudeon plastic disks in the far-field and near-field regime.

6 Conclusions

The goal of the work performed within the FrameworkProgramme 5 project FAMOUS is to investigate thepotential of flying head technology for advanced opticalrecording. Sliders for flying heights in the optical far-fieldand near-field have been developed and manufactured. Aviable manufacturing technology for the glass sliders wasfound to be photolithography and etching technology forABS patterning combined with dicing technology toseparate the sliders. The flight attitude of far-field sliderswas investigated while flying on a glass substrate, andyielded a good match between calculated and measuredvalues. Flying heights are between 500nm and 700nm.Further investigations need to be performed to evaluate thenear-field sliders’ performance and the flyability on plasticdisks. Nickel deposition for stamper manufacturing couldbe improved with respect to roughness by using additives.The achieved roughness of 3nm is sufficient to manufacturea stamper for the injection moulding of optical disks. Thefeasibility of these disks for the use as a removable mediumin a slider-based system needs to be verified in tests.Investigations of DLC deposition technology for protectivecoatings of head and disk showed, that a compromisebetween optical gap, hardness, and residual stress of thefilms needs to be achieved. Carbon films with an optical gapbetween 3eV and 4.2 eV could be deposited. While theoptical properties match the requirements of an optical datastorage system, the mechanical properties still need to beinvestigated.

7 Acknowledgments

The work presented in this paper was sponsored by theEuropean Commission within the scope of a FrameworkProgramme 5 research and technical development project.

8 References

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2 Ohkubo, T., Tanaka, K., Hirota, T., Itao, K., Niwa, T., Maeda, H.,Shinohara, Y., Mitsuoka, Y., and Nakajima, K.: ‘Mechanicalcharacteristics of a contact optical head slider operating on a metal-layered medium’, Microsyst. Technol., 2002, 8, pp. 235–243

3 Awano, H., and Ohta, N.: ‘Magnetooptical recording technologytoward 100Gb/in2’, IEEE J. Sel. Top. Quantum. Electron., 1998, 4, (5),pp. 815–820

4 Sakamoto, T., Fujita, G., and Nakaoki, A.: ‘Large areal density MOdisk using DWDD and a blue laser diode’, J. Mag. Soc. Jpn., 2001, 25,(5), pp. 1261–1266

5 Shimokawa, S., Matsumoto, T., Kawasaki, G., Nakao, Y., Kanto,N., and Tezuka, K.: ‘An optical flying head system that features adual-stage tracking mechanism for use in high-density MO recording’.Proc. Int. Symp. on Optical memory and optical data storage,Waikoloa, Hawaii, 7–11 July 2002, pp. 40–42

6 Prabhakaran, V., and Talke, F.E.: ‘Wear and hardness of carbonovercoats on magnetic recording sliders’, Wear, 2000, 243, (1–2),pp. 18–24

7 Robertson, J.: ‘Ultrathin carbon coatings for magnetic storagetechnology’, Thin Solid Films, 2000, 383, (81), pp. 81–88

8 Smith, G.J.: ‘Dynamic in-situ measurements of head-to-disk spacing’,IEEE Trans. Magn., 1999, 35, (5), pp. 2346–2351

9 White, J.W.: ‘Flying characteristics of the ‘zero-load’ slider bearing’,J. Lub. Technol., 1983, 105, pp. 484–490

10 Morrison, N.A., Muhl, S., Rodil, S.E., Ferrari, A.C., Nesladek, M.,Milne, W.I., and Robertson, J.: ‘The preparation, characterization andtribological properties of ta-C:H deposited using an electron cyclotronwave resonance plasma beam source’, Phys. Stat. Sol. A, 1999, 172,(1), pp. 79–90

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