Injection molding and injection compression molding of three-beam grating of DVD pickup lens

Sensors and Actuators A 125 (2006) 367–375

Injection molding and injection compression molding ofthree-beam grating of DVD pickup lens

Cheng-Hsien Wu∗, Wei-Shiu ChenDepartment of Mechanical and Automation Engineering, Da-Yeh University, Chang-Hwa 51505, Taiwan

Received 21 January 2005; received in revised form 8 July 2005; accepted 14 July 2005Available online 5 October 2005

Abstract

The objective of this paper is to investigate the application of injection molding and injection compression molding processes to producediffraction gratings. A mold was designed to produce a diffraction rating connected with the fixed bushing. The combined part was verified to havea good diffraction performance. Integrated grating eliminates the assembly cost and error. Photolithography was applied to make the mold insert.The Taguchi method and parametric analysis were applied to study the effects of molding parameters on grating quality. The design, fabrication ofstructured mold surfaces and the results of the replication by injection molding (IM) and injection compression molding (ICM) are presented andc page thant©

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ompared. The diffraction angle of ICM grating is more accurate than that of IM grating. Grating made by ICM has a much smaller warhat made by IM. The diffraction pattern shows that ICM is a better process than IM to replicate a diffraction grating.

2005 Elsevier B.V. All rights reserved.

eywords: Injection molding; Injection compression molding; Grating; Replication

. Introduction

All parts in an optical pickup, including the recording sub-trate and a number of components, are required to have superiorptical performance. This requires a very accurate shape repli-ation and low optical anisotropy as induced by the moldingelated stresses.

Whenever a travelling wave encounters an obstruction withimensions similar to its wavelength, some of the energy in theave is scattered[1,2]. If the obstruction is periodic, or indeed

f there is a periodic variation of any parameter which affects theropagation of the wave, energy is scattered into various discreteirections or diffracted orders, and a structure which acts in thisay may be referred to as a diffraction grating.In the optical pickup of the player, diffractive optical elements

DOE) are used to split the main laser beam into three beams forrack following but also to deflect the returning beam onto theetector area. The classical method of manufacturing gratings

s to scribe, burnish or emboss a series of grooves upon a goodptical surface[3].

The master is usually a metal copy of a DOE original facated by one or a combination of high-resolution lithograsteps[4,5]. The material in which the original is fabricated cbe resist, quartz, silicon or virtually any suitable high-resolufilm or substrate. The master is typically fabricated fromoriginal by electroplating in nickel although, in certain casecan be another material such as quartz, plastic or anotheror even the original itself. The master grating is first coateda thin layer of some non-adherent material (e.g. gold). Thfollowed by a substantial layer of aluminum. The master iscemented with the aid of a thin film of a low viscosity resin tocarefully cleaned replica blank, allowing the resin to polymeat a constant temperature, generally a slow process. Wheresin has cured, the replica and the master are separatedever, the process takes too much time and is not suitable forproduction.

To be a major technology for the low-cost, mass producof DOEs, alternative replication methods are applied as follThere are three alternative replication methods to replicatenickel master casting, embossing and injection molding (IM

In casting, a thin film of epoxy is applied on a solid s

∗ Corresponding author. Tel.: +886 4 8511227; fax: +886 4 8511224.E-mail address: [email protected] (C.-H. Wu).

strate blank. Both room temperature and thermally cured epox-ies have been used. The main disadvantage is the long curingtime. A related technique of particular interest for DOEs is the

924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
oi:10.1016/j.sna.2005.07.025

368 C.-H. Wu, W.-S. Chen / Sensors and Actuators A 125 (2006) 367–375

replication into a thin film of UV-curable material coated onto aglass or polymer substrate.

An even faster method of making grating replicas involvesembossing a plastic film by passing it over a heated cylinderunder some pressure from a smooth back-up roll. The cylinderwill typically have a nickel electroplated wrapping around it.Since this is a continuous process the unit cost will be mini-mal, but quality is limited to student experiments, or more likelydecorative devices such as holograms.

Injection molding is a classic low cost process, and in prin-ciple could produce gratings by inserting a nickel electroplatedreplica derived from a precision master into appropriate molds.The accuracy of injection molded gratings is always limited bythe molding conditions of the process. Efforts need to be madeto identify the significant factors that affect micro-filling behav-iors.

In recent years, plastics have begun to show great com-mercial potential especially in manufacturing micro-structuredparts. Injection molding represents the most important processfor manufacturing plastic parts. While many prototype plas-tic micro-devices are fabricated using precision engineeringmethods such as laser machining, micro-injection molding iscurrently being investigated all over the world[6,7]. An impor-tant advantage of injection molding is that with it we canmake complex geometries in one production step in an auto-mated process. Many micro-devices such as watches and camerac nsor sorsp mpon

eltp orma dinfi mess

cese ture[ nyc coldc withh sulw ighet

gess stresm ingu ucind auso oftene freo

ne,W -m e byte . Nofi e of

140◦C. However, SEM micrographs show that there is a shapedeviation, i.e. the structures are thicker at the top than at thebottom. Grating elements were designed to have a pitch below0.5�m and depths in excess of 1.5�m. By splitting the func-tion over the two surfaces of the retardation elements, only ahalf filling depth was necessary. The effect of shape deviationcan be reduced. Grating structures with a record depth of 2�mand a pitch of 0.5�m were replicated by injection molding withpolycarbonate.

Features with high aspect ratio proved difficult to form rou-tinely however it is envisaged that further optimization of injec-tion molding will resolve these difficulties[10]. Parashar etal. [11] employed a two-step pattern transfer to replicate thestructure in glass: first a polydimethylsiloxane (PDMS) replicais obtained from a master structure and secondly, a layer ofsol–gel material is applied on the PDMS soft-replica to getmicro-/nano-structure in glass after drying and annealing. Theyhave established the effectiveness of in-house developed sol–gelmaterials derived from metal alkoxides in nano-replication ofglass micro-/nano-structure.

Obi et al. [12] presented a replication method to fabricatemicro-structures. UV-curable sol–gel is used as base material.The basic fabrication process involves deposition and patterningof a sacrificial spacer layer and a combined molding and pho-tolithography step. This fabrication process can be applied foroptical MEMS devices that incorporate lenses, diffractive opticso

tiono TheT studyt Thed sultso

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ting,i sideb e usedb f theC s ont withp teredo ately2 arep ickupt en thet t andw Thec ckingo ack,t er pitso

thet e

omponents, automotive crash, acceleration, distance seead/write heads of hard discs, CD drives, medical senumps, surgical instruments and telecommunications coents, have been successfully injection molded.

The injection molding process involves the injection of a molymer into a mold where the melt cools and solidifies to fplastic part. It is generally a three-phase process inclu

lling, packing and cooling phases. After the cavity becotable, the product is ejected from the mold.

Despite many advantages, the injection molding proxperiences some inherent problems in molding micro-fea8]. The main difficulty is that the molten polymer in a tiavity instantaneously freezes upon touching the relativelyavity wall. The problem gets worse when micro-featuresigh aspect ratios are to be molded. The best replication reere achieved when melt and mold temperatures were h

han normal values[9].Injection compression molding (ICM) has the advanta

uch as decreasing molding pressure, reducing residualinimizing molecular orientation, packing evenly, reducneven shrinkage, overcoming sink mark and warpage, redensity variation and increasing dimensional accuracy. Becf these advantages, injection compression molding ismployed to produce parts of high accurate dimension andf residual stress especially for the optical parts.

Instead of using a micro-injection molding machiimberger-Friedl[7] fabricated sub-�m grating optical eleents by conventional injection molding. Inserts were mad

wo different processes: reactive-ion etching (RIE) in SiO2 andlectron-beam lithography followed by nickel electroplatinglling problem was encountered with a mold temperatur

rs,,-

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ss

tsr

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r waveguides.The objective of this paper is to investigate the applica

f IM and ICM processes to produce diffraction gratings.aguchi method and parametric analysis were applied tohe effects of molding parameters on the grating quality.esign, fabrication of structured mold surfaces and the ref the replication by IM and ICM are presented.

. Transmission diffraction grating

.1. Application example—optical pickup

When the laser beam goes through the diffraction grat is split up into a central bright beam plus a number ofeams. The central beam and one beam on each side ary the CD for the tracking system. Consider a segment oD player containing several tracks. If the optical head i

rack, then the primary beam will be centered on a track (its and bumps) and the two secondary beams will be cenn land. The three spots are deliberately offset approxim0�m with respect to each other. Two additional detectorslaced alongside the main quadrant detector in order to p

hese subsidiary beams. If the three beams are on track, thwo subsidiary photodetectors have equal amounts of lighill be quite bright because they are only tracking on land.entral beam will be reduced in brightness because it is tran both land and pits. However, if the optical head is off tr

hen the center spot gets more light (because there are fewff track) and the side detectors will be misbalanced.

The most common optical train in modern CD players ishree-beam pickup, depicted inFig. 1. The light is emitted by th

C.-H. Wu, W.-S. Chen / Sensors and Actuators A 125 (2006) 367–375 369

Fig. 1. Schematic illustration of an optical pickup.

laser diode and enters a diffraction grating. The grating convertsthe light into a central peak plus side peaks. The main centralpeak and two side peaks are important in the tracking mech-anism. The three beams go through a polarizing beam splitter.This only transmits polarizations parallel to the page. The emerg-ing light (now polarized parallel to the page) is then collimated.The collimated light goes through aλ/4 plate. This converts itinto circularly polarized light. The circularly polarized light isthen focused down onto the disk. If the light strikes land it isreflected back into the objective lens. (If the light strikes the pit,now a bump, it is not reflected.) The light then passes throughtheλ/4 plate again. Since it is going in the reverse direction, itwill be polarized perpendicular to the original beam (in otherwords, the light polarization is now vertical with respect to thepaper). When the vertically polarized light hits the polarizingbeam splitter this time, it will be reflected (not transmitted asbefore). Thus, it will reflect through the focusing lens and thenthe cylindrical lens and be imaged on the photodetector array.The cylindrical lens is important in the auto-focusing mecha-nism.

2.2. The general grating equation

A transmission diffraction grating is a slide with large numberof parallel, closely spaced slits (transparent spaces) drawn on it.E eedlep llenta ave-lt

d

w -t .

2

tin-u ntai ereg uire-m rous

Fig. 2. A transmission grating.

factors: the type and performance of the fabrication technol-ogy used, the grating period to wavelength ratio, the materials,etc. Additionally, the actual value depends on the definition ofdiffraction efficiency, since a unique definition does not exist.Therefore, it is often quite difficult to compare results whichappear in the literature.

In order to characterize the performance of diffraction grat-ings, two numbers are often used:

(1) The overall efficiencyηo,1This overall efficiency is defined asηo,1= intensity of first

order/intensity of incident beam.(2) The diffraction efficiencyηd,1

The diffraction efficiency is defined asηd,1= intensity offirst order/intensity of transmitted beam through unstruc-tured substrate.

With the second definition, losses due to scattering fromsurface roughness at the grating interface are contained in themeasurement of efficiency. In this study, the diffraction effi-ciencyηd,1 is used.

3. Experimental procedures

3.1. Material

radeo eiC ulkd e isb ra-tf

3

Afterm shing

arly ones were carbon covered glass slides etched by a noint—now they tend to be printed onto a slide. It is excet separating the colors in incident light because different w

engths are diffracted at different angles (Fig. 2), according tohe grating relationship:

sinθ = nλ (1)

hered is the distance between the slits,θ the angle of diffracion,λ the wavelength of the light andn is the order of diffraction

.3. Diffraction efficiency

Diffraction efficiencies of standard phase gratings, conously blazed or multilevel approximations, are a fundame

ssue in diffractive micro-optics. In nearly all applications whratings are used, a high diffraction efficiency is a major reqent. Achievable efficiency in practice depends on nume

l

The material used in this study is a high heat injection gf polymethylmethacrylate (PMMA, CM-205, from Chi Morp., Taiwan). The melt flow index is 1.8 g/10 min and the bensity is 1.19 g/cm3. The recommended barrel temperaturetween 210 and 250◦C and the recommended mold tempe

ure between 50 and 70◦C. The material was pre-dried at 90◦Cor 4 h using a dehumidifying drier before molding.

.2. Part geometry and mold design

Diffraction gratings were machined on a glass substrate.achining, the grating has to be assembled into a fixing bu


Fig. 3. The grating and the fixing bushing.

(as shown inFig. 3). Assembly requires much production timeand labor cost. It also creates positioning error and angle error,and then reduces the accuracy of an optical pickup. In this study,the product was designed to combine grating and bushing. Thewhole part has a diameter of 7 mm. The grating portion, with adiameter of 4 mm, is located at the center. The nickel mold insertwas designed with a periodicity of 20�m. The notch depth was1.5�m.

The electroplated nickel mold insert was installed in a center-gated mold base as shown inFig. 4. The mold has two cavitiessymmetrically located on the opposite sides of the sprue. Themold plates are made of S45C tool steel. The cavity is fed bya sprue, two runners and two fan gates as shown inFig. 5. Thesprue is 48mm long and 5 mm in diameter. The dimensions ofrunners are 1.55 mm× 3.50 mm× 5.68 mm.

3.3. Mold insert fabrication

Our photolithography process involves photomask fabrica-tion, wafer cleaning, spin coating, soft baking, exposure, post-exposure baking, developing and hard baking.

A high-resolution transparency containing the features’design, created in a CAD program, was used as the mask inphotolithography. In this study, the mask pattern was trans-ferred onto a transparency using a high-resolution laser printer(16,000 dpi).

iredt aferw step to

F t.

Fig. 5. Schematic illustration of cavity arrangement.

remove contaminants from the wafer surface in order to obtainhigh performance and high reliability of products, and to pre-vent contamination of process equipment. The wafer surface wascleaned with a 4:1 H2SO4/H2O2 for 10 min at 120◦C to removeorganic contaminants. The wafer surface was rinsed using deion-ized (DI) water until the water resistance was higher than 8�. A50:1 H2O/HF step for 10 min at room temperature was appliedto remove chemical oxides. The wafer surface was rinsed usingdeionized water again. After wafer cleaning, the substrate wasspun, blown dry using heated nitrogen and then placed on ahot plate (120◦C for 3 min) to drive off any water vapor on thesurface. This step is called dehydration baking.

To improve the adhesion of resists to the silicon wafer, hex-amethyldisilane (HMDS) is often applied. HMDS was applied tothe wafer by spinning at room temperature. HMDS was dried byplacing the wafer on a hot plate for 2 min at 90◦C. The next stepis spinning the resist on to the wafer and it should be done imme-diately after the HMDS application. A positive resist, AZ9260,was used in this study. The resist is dispensed onto the waferwhile the wafer is spinning to produce a uniform layer on thewafer. The spin speed of the spin coater was increased to 500 rpm

F s.

Fig. 6 shows the schematic of the fabrication steps requo produce a nickel electroplated mold insert. A silicon was used as the substrate. Wafer cleaning is a necessary

ig. 4. A center-gated mold base with an electroplated nickel mold inser
ig. 6. Fabrication schematic to produce nickel electroplated mold insert


with an acceleration of 500 rpm/s for 10 s. For another 30 s, thespin speed and acceleration of the spin coater were 300 rpm and300 rpm/s, respectively. The spin speed at this step determinesthe final thickness of the resist (about 50�m in this study).

The next step in the lithography is the pre-bake. Pre-bakeconditions depend on the thickness of the photoresist. In thisstudy, the substrate was placed on a leveled hot plate at 50◦Cfor 10 min and at 90◦C for 10 min. The pre-bake step accom-plishes three things. Firstly, the remaining solvent in the resistlargely evaporates. Secondly, adhesion of the resist improves.Finally, stresses in the resist are relieved through thermal relax-ation. The substrate was then gradually cooled down to roomtemperature in order to minimize residual stresses.The next stepis wafer exposure which can be divided into three basic meth-ods: contact printing, proximity printing and projection printing.Contact printing is the oldest and the simplest printing process.The mask is placed print side down in direct contact with theresist layer on the wafer. Exposure of the resist then takes placeby shining light through the mask. Aligning the mask to patternswas carried out with a contact mask aligner (Karl Suss MA-6).The hard contact between the mask and the wafer results indamage to both the mask and the resist layer. However, contactprinting was applied in this study for its high-resolution printingcapability. Exposure time was 3 min.

The next step is a post-exposure bake (PEB). PEB was car-ried out at 90◦C for 1 min. After PEB, the substrate was againg ini-m stratew (1:3A ered andb oldi

ost-b etchr t-b ndingt Thep ickelm areaf

ram lat-i . Then th DIw udin1

3

old-i inec eteim cess

Fig. 7. Experimental set-up used to measure the diffraction efficiency of gratingsand diffraction pattern of a laminar grating structure under laser illumination.

Under each set of process conditions, 10 shots were made toensure that the process was stable before samples were col-lected. If no significant variation was observed during these first10 runs, the molded parts from the next 5 runs were collected asthe samples for product characterization.

3.5. Quality measurement

Two major performances of a diffraction grating are the accu-racy of diffraction angle and the diffraction efficiency. An exper-imental set-up, shown inFig. 7, was designed to measure thediffraction efficiency and the diffraction angle. A He–Ne laserwas placed on an accurate rotary indexing table and directedtoward the center of the table. The tested grating was placedat the table center. The grating was oriented so that the laserbeam was perpendicular to the grating surface. A power metertogether with a photodetector was used to measure light inten-sity. Firstly, the rotary table was rotated to such a direction thatthe photodetector received the largest intensity. This means thatthe light of order 0 irradiated exactly at the center of the pho-todetector. Then the rotary table was zeroed. Secondly, the rotarytable was rotated to such a direction that the light of order +1irradiated exactly at the center of the photodetector. The rotationangle is the diffraction angle of order +1. Experimental resultsshowed that the diffraction angle of order−1 was very close tothe diffraction angle of order +1. Therefore, only the diffractiona

4

ngleso sionm uchim tageo ame-t sses

radually cooled down to room temperature in order to mize stresses and prevent the resist from cracking. The subas immersed into the beaker containing the developerZ400K/DI water) for about 5 min. After all the features weveloped, the photoresist was rinsed in fresh DI waterlown dry with nitrogen. This gave rise to an AZ9260 m

nsert with positive features.The final step in the photolithography process is the p

ake which is designed to harden the resist and improve itsesistance. The temperature is set at 110◦C for 10 min. The posaked photoresist mold is not as strong as metals in withsta

he high pressure and temperature in injection molding.hotoresist mold acted as a pattern for electroplating of the nold. A thin gold layer was sputtered to create a conductive

or nickel growth.The surface was pre-treated in light sulfuric acid at 60◦C and

insed in DI water. Electroplating was conducted at 50◦C withpH of 4, and at a low current density of 4 A/dm2 in order toinimize internal stresses in the nickel mold. After electrop

ng, the photoresist was stripped with an ultrasonic shakerickel structure was placed in acetone and then rinsed wiater. The whole process requires less than 2 days (incl-h photolithohgraphy and 40-h electroplating).

.4. Molding

Molding operations were conducted with an injection mng machine (FANUC ROBOSHOT S-2000i50A). The machan offer a clamping force up to 50 tonnes. The screw diams 22 mm and the maximum injection volume is 29 cm3. The

achine can provide an ICM process as well as an IM pro

g

r

.

ngle of order +1 was measured in this study.

. Taguchi parameters design

The effects of processing parameters on the diffraction af an injection molded grating and an injection compresolded grating were analyzed and compared. The Tagethod was applied in this study. The contribution percenf each factor can be found and the optimum set of par

ers driving the effective factors in these two molding proce


Table 1The three levels of effective factors for injection molding of PMMA

Factors Levels

1 2 3

Melt temperature, A (◦C) 230 240 250Injection velocity, B (mm/s) 110 130 150Mold temperature, C (◦C) 50 60 70Packing pressure, D (MPa) 100 150 200

Table 2The three levels of effective factors for injection compression molding of PMMA

Factors Levels

1 2 3

Mold temperature, A (◦C) 50 60 70Injection velocity, B (mm/s) 110 130 150Compression speed, C (mm/s) 5 10 15Compression period, D (s) 30 40 50

can be determined to obtain a product with the most accuratediffraction angle.

In the analysis, a signal-to-noise (S/N) ratio is the statisti-cal quantity representing the power of a response signal dividedby the power of the variation in the signal due to noise. Themaximization of the S/N ratio leads to the minimization of anyproperty that is sensitive to noise. In order to optimize the diffrac-tion angle of IM and ICM gratings, the equations describing “thenominal is the best” characteristic can be used for the analysis:

S/N = 10 log10µ2

σ2 , (2)

Fig. 8. Effects of processing parameters on diffraction angle for IM.

.

Table 3Measured diffraction angle and corresponding S/N ratio for injection molding

Experimentalrun

Factor Measureddiffraction angle

S/N ratio

A B C D

1 1 1 1 1 1.105 48.312 1 2 2 2 1.071 54.063 1 3 3 3 1.068 57.564 2 1 2 3 1.049 63.425 2 2 3 1 1.033 63.296 2 3 1 2 1.026 51.197 3 1 3 2 1.019 51.138 3 2 1 3 1.015 49.169 3 3 2 1 1.004 63.04

µ = 1

n

n∑

i=1

yi (3)

and

σ2 = 1

n − 1

n∑

i=1

(yi − µ)2 (4)

whereµ is the average property,σ the standard deviation,yi themeasured property andn corresponds to the number of samplesin each test trial. The optimum factor levels with the largest S/Nratios can then be summarized, which will minimize sensitivityover the range of noises.

The objective of the experiments was to determine the effectsof molding factors on the diffraction angle and the optimumset of factors. With this optimum set of process conditions, thediffraction angle would be closest to the theoretical value. Fromour initial tests and a previous literature review[13–15], fourmolding factors were chosen for both processes. Melt temper-ature, injection speed, mold temperature and packing pressurewere selected for injection molding. Each factor is designedwith three levels, as shown inTable 1. This is because the influ-ence of these factors on the results may vary nonlinearly. Asfor injection compression molding, mold temperature, injec-tion speed, compression speed and compression period wereselected as shown inTable 2. According to the levels of eachp thet

TM pres-s

Er

123456789

Fig. 9. Effects of processing parameters on diffraction angle for ICM

arameter, a L9(34) orthogonal table was used to performests.

able 4easured diffraction angle and corresponding S/N ratio for injection com

ion molding

xperimentalun

Factor Measureddiffraction angle

S/N ratio

A B C D

1 1 1 1 1.01 57.081 2 2 2 1.032 45.221 3 3 3 1.068 57.562 1 2 3 1.062 51.492 2 3 1 1.091 43.772 3 1 2 1.049 57.413 1 3 2 0.998 47.433 2 1 3 1.129 58.043 3 2 1 0.997 47.42


Fig. 10. The diffraction patterns of (a) IM grating and (b) ICM grating.

5. Results

5.1. Optimum set of molding parameters

Figs. 8 and 9illustrate the effects of processing factors onthe diffraction angle accuracy for IM and ICM, respectively.

FI

These response plots show how the accuracy changes with eachfactor. On the basis ofFig. 8, the levels of the factors that willgenerate the smallest error are a melt temperature of 240◦C,an injection speed of 150 mm/s, a mold temperature of 60◦Cand a packing pressure of 100 MPa. Under optimum processconditions, confirmation tests were conducted four times. Themeasured diffraction angles were 0.988◦, 0.985◦, 0.987◦ and0.984◦. The averaged value is 0.986 and the corresponding S/Nratio is 54.65. The calculated S/N ratio from the confirmationtests locates inside the confirmation interval [52.38, 83.42]. Theresults of confirmation tests tell that the Taguchi method is wellimplemented in this study (Tables 3 and 4).

5.2. Effects of molding factors on diffraction angleaccuracy

In the previous analysis, each factor’s contribution to thetotal effect of diffraction angle accuracy was presented as apercentage. For an IM grating, mold temperature was the mostinfluential factor with a contribution of 57.41%, followed bymelt temperature at 19.23%, packing pressure at 19.1% andinjection speed at 4.25%. For an ICM grating, the compres-sion speed was the most influential factor with a contributionof 55.7%, followed by compression period at 26.0%, injectionspeed at 14.3% and mold temperature at 4.02%.

ig. 11. Scanning electron microscope of a diffraction grating by (a) IM and (b)CM.

FI

ig. 12. Microscopic stereopictures of a diffraction grating by (a) IM and (b)CM.


5.3. Comparison between IM and ICM parts

With the corresponding optimum set of process conditions,the parts were made by IM and ICM. These two processes werestudied by comparing the qualities of IM part and ICM part.The diffraction performances of IM grating and ICM grating areshown inFig. 10. ICM grating gives a more focused diffractionpattern than IM grating. It shows that ICM grating has a betteroptical quality than IM grating. Grating was also studied bymeasuring the diffraction angle and observing surface quality.The surface’s characteristic was also studied by measuring thesurface profile of the grating surface.

5.3.1. Diffraction angle measurementThe wavelength of the laser is 685 nm. The pitch of the

diffraction grating is 40�m in this study. Based on Eq.(1),the predicted diffraction angle can be found to be 0.9812◦.The results from the confirmation tests show that the averageoptimum diffraction angle is 0.986◦ for IM. As for ICM, thediffraction angle is 0.983◦, which is a little closer to the pre-dicted diffraction angle.

5.3.2. MicroscopyThe molded gratings were examined in a scanning elec-

tron microscope (JOEL, JSM-6400). Specimens were cut intosmaller pieces and gold was sputtered onto the surfaces, fol-lowed by inspection in the scanning electron microscope (SEM).Fig. 11shows SEM micrographs of both IM grating and ICMgrating. Their shapes and sizes look similar. To have a clearstereopicture, another microscope (SONY Exwave HAD cam-era with OPTEM Zoom 125 optical system) was also used.After a deep observation, an ICM grating reveals a better repli-cation than an IM grating (Fig. 12). An ICM grating has anotch with a sharper edge than an IM grating does. Further-more, an IM grating contains more void than an ICM gratingdoes.

5.3.3. Surface profile measurementTo measure the micro-structure profile, a high performance

surface profiler (XP-2, Ambios Technology Inc.) was used. BothIM grating and ICM grating were measured. The surface pro-files are shown inFig. 13. Measurement was conducted onthe grating surface near the center. The central point of the

Fig. 13. Surface profiles of diffractio
n gratings by (a) IM and (b) ICM.


grating is indicated as point A on the surface profile. Thepoint, which is at a 17 periods of point A, is indicated aspoint B. The horizontal distance between points A and B isabout 670�m for both profiles. The periods are about the same.However, the vertical distance of IM profile (23.6�m) is muchlarger than the vertical distance of ICM profile (9.2�m). Ver-tical deviation reveals grating warpage. The results show thatICM grating has a much smaller warpage than IM gratingdoes.

6. Conclusion

The objective of this paper is to investigate the applicationof IM and ICM processes to produce diffraction gratings. TheTaguchi method and parametric analysis were applied to studythe effects of molding parameters on the grating quality. Thedesign, fabrication of structured mold surfaces and the resultsof the replication by IM and ICM are presented.

Some conclusions can be drawn from the experimentalresults:

(1) Diffraction grating can be produced by IM and ICM. Themold is designed to produce a diffraction grating connectedwith the fixed bushing. The combined part was verifiedto have a good diffraction performance. Integrated gratingeliminates assembly cost and error. The mold insert is made

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References

[1] E. Hecht, Optics, Addison-Wesley, Boston, 1990.[2] J.W. Goodman, International Trends in Optics, Academic Press, San

Diego, 1991.[3] H.H. Karow, Fabrication Methods for Precision Optics, John Wiley &

Sons, New York, 1993.[4] M.T. Gale, Replication techniques for diffractive optical elements,

Microelectron. Eng. 34 (1997) 321–339.[5] E.G. Loewen, E. Popov, Diffraction Gratings and Applications, Marcel

Dekker, New York, 1997.[6] D. Yao, B. Kim, Simulation of the filling process in micro channels for

polymeric materials, J. Micromech. Microeng. 12 (2002) 604–610.[7] R. Wimberger-Friedl, Injection molding of sub-�m grating optical ele-

ments, J. Injection Molding Technol. 4 (2000) 78–83.[8] D. Yao, B. Kim, Injection molding high aspect ratio microfeatures, J.

Injection Molding Technol. 6 (2002) 11–17.[9] K. M onkkonen, et al., Replication of sub-micron features using amor-

phous thermoplastics, Polym. Eng. Sci. 42 (2004) 1600–1608.[10] D. Macintyre, S. Thomas, The fabrication of high resolution features by

mould injection, Microelectron. Eng. 41–42 (1998) 211–214.[11] V.K. Parashar, A. Sayah, M. Pfeffer, F. Schoch, J. Gobrecht, A.M.

Gijs, Nano-replication of diffractive optical elements in sol–gel derivedglasses, Microelectron. Eng. 67–68 (2003) 710–719.

[12] S. Obi, M.T. Gale, A. Kuoni, N.D. Rooij, Replication of opticalMEMS structures in sol–gel materials, Microelectron. Eng. 73–74 (2004)157–160.

[13] S.C. Chen, Y.C. Chen, N.T. Cheng, Simulation of injection-compressionmold-filling process, Int. Comm. Heat Mass Transfer 25 (1998) 907–917.

[14] T.C. Chang, E. Faison, Shrinkage behavior and optimization of injection2001)

[ lemcess.

B

C y in1 rch isf fessori -YehU oingr ing,h

W 04.D Uni-v rchera entC delingo

by a photolithography process.2) For an injection molded grating, mold temperature

the most influential factor, followed by melt temperatupacking pressure and injection speed. For an injectionpression molded grating, compression speed was theinfluential factor, followed by compression period, injectspeed and mold temperature.

3) The diffraction pattern shows that ICM is a better procthan IM to replicate a diffraction grating. The diffractiangle of ICM grating is more accurate than that of IM ging. ICM provides a more accurate shape replicationinjection molding does. Grating made by ICM has a msmaller warpage than that made by IM.

cknowledgement

This work was supported by the National Science Counder NSC grant NSC-92-2216-E-212-001.

-st

molded parts studied by the Taguchi method, Polym. Eng. Sci. 41 (703–710.

15] M.C. Huang, C.C. Tai, The effective factors in the warpage probof an injection-molded part with a thin shell feature, J. Mater. ProTechnol. 110 (2001) 1–9.

iographies

heng-Hsien Wu received his MS degree from National Taiwan Universit990 and PhD degree from the Ohio State University in 1995. His resea

ocused on polymer processing. Currently, Dr. Wu is an associate pron the Department of Mechanical and Automation Engineering of Daniversity, Taiwan. Dr. Wu’s group at Da-Yeh University has an ong

esearch program on polymer microfabrication, including injection moldot embossing and thermoforming.

ei-Shiu Chen received his MS degree from Da-Yeh University in 20uring his graduate studies in Polymer Processing Laboratory at Da-Yehersity, he worked on polymer microfabrication. He is currently a reseat the Injection Molding Technology Group of Plastics Industry Developmenter, Taiwan. His research is focused on design, fabrication and mof polymer processing as well as product mechanism design.

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Injection molding and injection compression molding of three-beam grating of DVD pickup lens