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High-resolution maskless lithography

Kin Foong Chan *Zhiqiang FengRen Yang †

Akihito IshikawaWenhui MeiBall Semiconductor, Incorporated415 Century ParkwayAllen, Texas 75013-8043

Abstract. An innovative high-resolution maskless lithography system isdesigned employing a combination of low- and high-numerical-aperture(NA) projection lens systems along with integrated micro-optics, and us-ing Texas Instruments’ super video graphic array (SVGA) digital micro-mirror device (DMD) as the spatial and temporal light modulator. A mer-cury arc lamp filtered for the G-line (l5435.8 nm) is used as the lightsource. Exposure experiments are performed using data extraction andtransfer software, and synchronous stage control algorithms derivedfrom a point array scrolling technique. Each exposure scan produces afield width (W) of approximately 8.47 mm with a field length (longitudinalfield) limited only by onboard memory capacity. DMD frame rates of up to5 kHz (kframes/s), synchronized to the stage motion, are achievable. Inthis experiment, TSMR-8970XB10 photoresist (PR), diluted to 3.8 cPwith PR thinner is prepared. The PR is spin-coated onto a chrome-coated glass substrate to 1.0-mm thickness with 0.1-mm uniformity. A0.4-mm scan step is used and 27,000 DMD data frames are extractedand transferred to the DMD driver. Results indicate consistent 1.8-mmline space (L/S) resolved across the entire field width of 8.47 mm. Givenoptimized exposure and development conditions, 1.5-mm L/S is also ob-served at certain locations. The potential of this maskless lithographysystem is substantial; its performance is sufficient for applications in mi-croelectromechanical systems (MEMS), photomasking, high-resolutionLCD, high-density printed circuit boards (PCBs), etc. Higher productivityis predicted by a custom H-line (l5405 nm) lens system designed andused in conjunction with a violet diode laser systems and the develop-ment of a real-time driver. © 2003 Society of Photo-Optical Instrumentation Engi-neers. [DOI: 10.1117/1.1611182]

Subject terms: exposure; microlens; microlithography; micro-optics; microelectro-mechanical systems; liquid crystal display; optics; pattern; printed circuit board;photomask; reticle.

Paper 02052 received Dec. 11, 2002; revised manuscript received Feb. 25, 2003,May 8, 2003, and May 27, 2003; accepted for publication Jun. 13, 2003.

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1 Introduction

Demands in the design and manufacturing of high-denprinted circuit boards~PCBs!, high-definition liquid crystaldisplays~LCDs!, microelectromechanical systems~MEMS!prototyping, biosensor applications, etc., have falteredcause of the high costs associated with the making of ptomasks or reticles in the lithography process. Especifor research or small-volume production requiring featusizes in the 1.0- to 50.0-mm range, the costs of photomasing have become logistically unreasonable. As a resvarious maskless lithography techniques have rececaught widespread attention. These maskless methodbased on different technologies, utilizing resnanodroplets,1 electron beams,2,3 the atomic force micros-copy ~AFM!-based technique,4 direct extreme ultraviolet~EUV! or laser writing,5,6 or incorporating various types ospatial light modulators and optical element.7–10 However,

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most of these maskless techniques are highly ineffic~i.e., slow exposure time, light efficiency!, or are not able toachieve sufficiently small feature [email protected]., line space~L/S!. 20 mm#. Currently, none of these technologies possescombination of sufficient throughput or productivity, optcal efficiency, feature size, and cost efficiency to be comercialized.

Wang and Bokor1 attempted to directly deposit photoresist droplets on wafers or substrates by microfabricatinthermal bimembrane actuator. A 6-mm-scale drop was recently reported, but this technology seems far from marity. Droplet generation was highly irregular and requirmore in-depth investigation. Chen et al.6 produced arraynanomirrors intended for the design of an EUV masklelithography system. However, this modulating devicestill in the early stage of characterization and refinemeand not yet available for integration with a complete opticsystem for lithographic experimentation.

Electron-beam lithography2,3 has recently garnered atention because of the limitations of conventional opticlithography on feature sizes less than 65 nm. Ware3 re-ported an electron projection lithography~EPL! systemcomprising of a 2-D 64364 e-beam columns, covering

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*Present address: Reliant Technologies, Inc., Palo Alto, California. [email protected].

†Present address: Louisiana State University, Baton Rouge, LouisE-mail: [email protected].

total exposure area of 0.0625 mm. Despite having a longer

331© 2003 Society of Photo-Optical Instrumentation Engineers

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Chan et al.: High-resolution maskless lithography

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depth of focus than that of optical lithography, the penettion depth of an e-beam is usually shorter and henceconducive for use on thick resists. Additionally, the expsure area is too small for large exposure areas in the hdreds of square millimeter range.

A scanning probe lithography~SPL! technique based4 onAFM produced a larger exposure area of 100 mm2. Thistechnique employs a 1-D array of 50 cantilevered tispaced at 200-mm intervals, to obtain patterns at 26-nm linwidth on polymethyl methacrylate~PMMA! at a scan speedof 10 mm/s. This technology is still in the developmestage and is likely to be limited by the scan rate. Howevbecause of the extremely small feature size it is capablproducing, it offers great potential.

At the expense of small feature size, maskless lithogphy systems designed with higher throughput in mind hbeen demonstrated. Takahashi and Setoyama8 showed50-mm feature size on a 1.2-mm negative photoresist layethrough direct projection of a Texas Instruments~TI! digitalmicromirror device ~DMD!-generated pattern. Seltmanet al.,7 using a custom-designed spatial light modula

~SLM!similar to TI’s DMD, produced 0.6-mm L/S featurescovering an image field of 102393mm. This was done byprojection through a 100:1 reduction lens having an Exmer laser (l5308 nm) as the light source. The authoemployed a step and stitch method on a 4-in. waferestimated a throughput of 1 wafer/h.

Carter et al.9 and Gil et al.10 demonstrated anothemaskless technique termed zone-plate-array lithogra~ZPAL!. This lithographic technique used an arrayFresnel zone plates with very high numerical apert(NA50.95), and the researchers succeeded in produsubmicrometer feature sizes reported to be 360 nm. Hever, it has extremely low throughput, covering onlyarea of 0.25 cm2 over a period of 20 min.

We have developed maskless lithography systems oing minimum feature sizes at optimized exposure contions approaching 20, 10, and 1.5-mm L/S. Productivity canbe as fast as 20 mm/s for the equivalent of an area of3200 mm over 150 s of exposure time, depending onsystems and the illumination intensity of the laser ligsource.

1.1 High-Resolution Maskless Lithography System

The high-resolution maskless lithography system11,12 ~Hi-Res MLS! consists of two major subassemblies. First,DMD field image is projected onto an integrated opticdiffractive element using a low-NA 1:3~in actuality 1:2.94!magnifying projection lens (NA50.12). The focal plane othe diffractive element becomes the second object plaThe second lens assembly consists of a high-NA 5:1 redtion lens (NA50.5), which serves to project the focpoints ~point array! from the second object plane onto thsubstrate.

Figure 1 shows a block diagram of the lithography stem. The light source is provided through an optical fibbundle or liquid waveguide to a diffuser and a line filter~ifnecessary!, after which it is collimated before it is reflecteto the DMD surface with a UV-enhanced mirror. The supvideo graphic array~SVGA! 8483600 array DMD, havinga pitch size of 17mm, was a product of TI. The micromir

332 J. Microlith., Microfab., Microsyst., Vol. 2 No. 4, October 2003


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rors on the DMD can be instructed to produce a patternway of tilting them 6 10 deg off their normal positionLight reflected off the micromirrors is projected into thlow-NA 1:3 magnifying lens, forming a DMD image coplanar to the back plane of the integrated microlens and spafilter array13,14 ~MLSFA!. This is the first image planewhere the individual DMD pixel image is focused by thcorresponding microlens onto a second surface coplawith the spatial filter array. Perfectly designed microlearray foci are diffraction-limited to about 3mm. In practice,the foci or focal points are approximately 5mm. The mi-crolens array~MLA ! foci serve as the second object plafor the high-NA 5:1 reduction lens, imaged on to the sustrate as a point array to perform lithography on photossitive materials. Background illumination due to opticscattering or noise, crosstalk, high-spatial-frequency coponents, as well as imperfect focusing by the microlearray are eliminated at the foci with a corresponding arof spatial filters. This array filtering technique12,14 increasesthe system contrast and produces better lithographic resWith the 5:1 reduction lens, the focal points or point arrcarrying the DMD pattern is projected onto the surfacethe substrate.

Since the DMD pixel images are focused into tiny do~point array! by the MLSFA, they are disjointed duringeach exposure data frame. Hence, a point artechnique11,12,15 having unique formulations of overlayinarray of dots is used to link lines and other patternsflipping the DMD micromirrors at very high frame rate~up to 5000 frames/s! while moving the substrate on thstage.

As indicated in Fig. 2, this technique requires the DMfield to be rotated at a small angle relative to the scann

Fig. 1 Illustration of the Hi-Res MLS. Four diagrams on the right(from top to bottom) describe the appearance of the DMD, the DMDinverted image coplanar with the MLA back plane, the MLA focicoplanar with the spatial filter array, and the point array projected tothe substrate. These diagrams are not adjusted for image magnifi-cation or reduction.


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Fig. 2 Illustration of the point array technique. Each data frame ofpoint array or DMD micromirrors is turned on according to the dataextraction algorithm, given a set of input parameters such as the Kvalue and the scan step, where K is the number or repetition of pointarray exactly overlapping one another at a coordinate position alonga line drawn parallel to the scanning direction, d is the pitch size ofthe point array, u is the discrete rotational angle, w is the point arraysize, x is the minimum horizontal separation, M is the number ofpoint array pixels in the long axis, and N is the number of point arraypixels in the short axis. A virtual row is added beyond the DMD fielddimensions along the short axis to facilitate computation, resulting inN85N11, and thus K85K11. Note that the expression K5K821is true only if N/K is a positive integer with an M3N array largerthan 131. In this case, K54, highlighted with black dots to indicatefour-point array repetition along the scanning direction. In this figure,for illustration purpose only, N520 and K54 (K855), or N/K55,hence, u'11.31.

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the scanning direction coincides with four-point arrays,dicating aK value of 4. GivenN520 in Fig. 2,N/K55,and henceu'11.31.

To expose one uninterrupted~nondisconnected! line or-thogonal to the scanning direction, the minimum horizonseparationx among the point array must be small enougiven a finite point array size or diameterw ~Fig. 2!, wherex,w. Thus, as long asx,w, the minimum horizontalseparationx is usually chosen as at least 20% of the smaest feature size a lithography system is capable of proding. For example, if the lithography system is expectedproduce 2-mm L/S features, thenx<0.4mm. The mini-mum horizontal separation determines the horizontal ovlaying density of the point array, shown in Fig. 2 as

x5d sinu, ~2!

whered is the point array pitch size, andu is the discreterotational angle. Equation~2! indicates thatx influences theselection ofK value throughu.

In addition, a stage ‘‘step-size’’ or ‘‘scan step’’ durinlithographic exposure along the scanning direction malso be smaller than the expected performance of the smest feature size the lithography system is capable of.example, if the lithography system is expected to produ2-mm L/S features, then a stage scan step of at least 20%the feature, or 0.4mm, is recommended. Scan step detmines the vertical overlaying density of the point arralong the stage scanning direction, and each scan stesynchronized to its corresponding DMD frame data. Thorizontal separationx and scan step of at least 20%~or,20%! of feature size is a criterion that was determinexperimentally. Through our observation, this criterionsulted in feature sizes that are610% of the intended pattern. For example, a 1.5-mm L/S design pattern will resulin a 1.65-mm line/1.35-mm space or 1.35-mm line/1.65-mmspace on the substrate, which we deem acceptable.

In our experimental setup, the Hi-Res MLS has a finpoint array sizew of approximately 1.0mm ~the MLSFAfoci are 5.0mm, which are reduced by the 5:1 lens to poiarray sizes of 1.0mm!, and a point array pitch sized of 10.0mm ~the DMD pitch size is 17mm, expanded to 50.0mmby the 1:2.94 lens, and then reduced to 10.0mm by the 5:1lens!. Assuming that the Hi-Res MLS can resolve at leas2.0-mm L/S feature, we setx to 0.4mm using the 20% rule,which also satisfies the criterionx,w. From Eq.~2!, wesee that sinu50.04. The TI SVGA DMD has 8483600micromirrors;N is 600. WhenN andu are substituted intoEq. ~1!, we obtainedK;24.02. SinceN/K must be a posi-tive integer, we selectedK520 for convenience of dataextraction, and thusN/K530. Using Eq.~1! again,u be-comes 1.909 deg. The final minimum horizontal separatx is now 0.333mm. Because the Hi-Res MLS stage hasprecision step size of 100 nm, it is prudent to set our sstep at 0.4mm while noting the slight difference in overlaying density between the horizontal/orthogonal avertical/scanning directions.

To facilitate data extraction, and synchronization btween frame data and stage motion,M, N, K, d, w, scanstep, and their derivatives~i.e.,u andx! are important input

direction of the moving stage or substrate. AK value deter-mines the discrete rotational angleu. This relation can bewritten as

u5arcsinS K2

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whereN is the number of point arrays along the axis havithe smallest incidence angle to the stage scanning direcas shown in Fig. 2;u is termed the discrete rotational angbecause for reasons of convenience in handling frameextraction, we have required thatN/K be a positive integerhence restrictingu to certain discrete values; andK is theKvalue defined as the number or repetition of point arrexactly overlapping one another at a coordinate posialong a line drawn parallel to the scanning direction. Finstance in Fig. 2, each virtual line drawn in parallel wi

333J. Microlith., Microfab., Microsyst., Vol. 2 No. 4, October 2003



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Fig. 3 Series of diagrams showing the progress of an exposure based on the point array technique. The exposure progresses from the leftcolumn (top to bottom) gradually to the right columns. The diagrams are labeled according to temporal increment from T50, T5t, T52t,..., to T5nt, where n is an integer, and with the stage moving forward with a fixed scan step. An intended outline of the line patterns(white) is shown on the stage. As the stage scans underneath the DMD (or the Hi-Res MLS), some point array is activated as they pass withinthe intended pattern, exposing points that eventually connected and filled in to complete the line patterns (black) at T5nt.

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parameters in the software algorithm for data extractiondata transfer. The discussion of software manipulationbeyond the scope of this paper, but can be referred fMei12 and Zhou et al.16 Figure 3 illustrates a sequencepoint array data frames in the temporal domain to perfo



an exposure using the Hi-Res MLS. Based on the inparameters in the previous paragraph, we performed asimulation of cross-sectional overlaying intensity of 2.01.5-, and 1.0-mm L/S features. Figure 4 shows the ideinput profiles and the corresponding simulation results



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Fig. 4 Comparison between (a), (b), and (c) ideal input profiles and (d), (e), and (f) their corresponding simulation results of 1-D cross-sectionaloverlaying intensity profiles for 2.0, 1.5-, and 1.0-mm L/S. The simulations were performed using (M,N)5(848,600), d510 mm, w51.0 mm,K520, and scan step50.4 mm as input parameters. The results indicate 2.0- and 1.5-mm L/S features (d) and (e) are resolvable on photore-sists with high-contrast characteristic curves treated with the appropriate preparation and development processes. The overlaying intensitycontrast ratio for 1.0-mm L/S, as seen in (f), is reduced, suggesting that 1.0-mm features may not be easily produced.

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intensity dosage as a consequence of overlaying forthree cases of feature sizes. Results indicated that thetrast ratio of overlaying intensity in the case of a 1.0-mmL/S feature is reduced compared to those of the 1.5-2.0-mm L/S features, suggesting that the 1.0-mm L/S fea-tures may not be achievable in actual experiments. Theand 1.5-mm L/S features are, however, possible when usphotoresists with threshold~high contrast! characteristiccurves.17

2 Methods

2.1 Exposure Experiment

The performance of the Hi-Res MLS was evaluated byamining the best possible L/S pattern it was capableresolving. The lithography system subassembly was dusing propriety technologies, ensuring alignment accurbetween the DMD image and the MLSFA with toleranless than 1/20 of a pixel for the whole field. The first po


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tion of 1:3 projection lens and MLSFA were then alignedthe high-NA 5:1 reduction lens. The assembled lithograpsystem was then aligned to the stage scanning direcbased on the desiredu ~or K value!. The focal plane of thesystem was aligned to the substrate surface. The scannscrolling and stitching~not used in this experiment! stagesused in this experiment had a precision step size of 0.1mmon the horizontalx and y axes~scrolling and stitching di-rections!, 0.01 mm on the z axis ~work distance adjust-ment!, and 0.01 deg on the rotational stage. Any stitchimisalignment~if used! can be compensated through soware control since the stage absolute position is highlypeatable.

Chrome-coated glass wafers were used as substratthis experiment. An in-house spin-coater was used to uformly spread a layer of photoresist~PR! on the substratesTSMR-8970XB10 PR was prediluted to 3.8 cp with thiner. The substrates were initially spin-coated with OAP~anadhesion-promoting agent between substrate and pho




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Fig. 5 Exposure results of vertical lines (left column), horizontal lines (center column), and 45-deg angled lines (right column) relative to thestage scanning or point array pattern scrolling direction. Vertical lines were generated in the direction of the stage motion or point array scrolling.The top row shows scanning electron microscope (SEM) images of 1.5-mm L/S, while the bottom row shows those of 1.8-mm L/S. Themagnified SEM on the top left corner shows the 1.5-mm L/S in more detail. It is important to evaluate results of line patterns in differentorientation to verify the system’s alignment and stage synchronization during an exposure run.

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sist, by Tokyo Ohka Kogyo Co., Ltd.! surface wetting at3000 rpm for 25 s. Later, the diluted PR was spin-coatedthe substrate at 3000 rpm for 30 s. On evaluation, thethickness was determined to be approximately60.1mm. The substrates were then prebaked at 120 °C18 min.

In this experiment, a total of 27,000 DMD data framwere generated using data extraction software algoritgiven (M ,N)5(848,600), d510mm, w51.0mm, K520, and scan step50.4mm as input parameters. The dawere then transferred to local memories within the DMdriver. A diffuser was used to uniformly illuminate the e

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tire field of the DMD surface, while aG-line filter was usedto extract the optical power at a 436-nm wavelength frothe low-power mercury arc lamp. Because of the relativlow illumination intensity, only a 0.04 mm/s scroll or scarate was used. The Hi-Res MLS was then instructedexpose a total area of 8.47 (W)310.80 mm~L!.

3 Results

3.1 Maskless Lithography Exposure Results

Figure 5 shows typical exposure results of pattern genated on a 1.0-mm PR-coated chrome-coated glass substr

Fig. 6 Exposure results of two star patterns on a glass substrate taken with SEM. The star pattern on the left (a) shows 1.0-mm lines, whereasthat on the right (b) shows 2.0-mm lines. V-edges in (a) form a less perfect circle than that in (b), indicating synchronization error (this case)and/or discrete rotational misalignment limiting feature sizes of the Hi-Res MLS to 1.5 to 1.8 mm.


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Fig. 7 Exposure results of (a) a circular-ring pattern on a glass substrate. The magnified SEM image on the right (b) shows a partial arc of2.0-mm L/S from the inner ring of the SEM image in (a).

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Results indicated full-field exposure of 1.8-mm L/S on allline and space orientations. Patterns of 1.5-mm L/S, how-ever, were observed only in several locations where liuniformity and exposure conditions were optimized.

The star pattern in Fig. 6 provides convenient evaluatof the performance of the Hi-Res MLS because of itsverging lines at many angles relative to the stage motionpoint array scrolling direction. Any stage misalignmediscrete rotational misalignment, or synchronization erwill result in nonuniform linewidths of the star patterndifferent angles.

Manifestation of the limitation of the point array technique in the Hi-Res MLS can be seen in Fig. 7. The mnified SEM image@Fig. 7~b!# shows an arc-shaped 2.0-mmL/S pattern after zooming in from the original circular-rinpattern @Fig. 7~a!#. The arc-shaped pattern has pointedges that were attributed to the nature of data generand the finite size of the point array inherent in the poarray technique. Such pointed edges may be minimizedreducing the synchronization scan step and/or by refinthe focal size of the point array.

4 Discussion

4.1 Discussions on Exposure Results

It was difficult to detect discrete rotational misalignmeand synchronization error based on the 1.5- and 1.8-mmL/S results in Fig. 5. That is partly because 1.5 and 1.8mmwere well within the performance capability of the Hi-RMLS. In Fig. 6~a!, however, the discrepancies between dcrete rotational angle of the system relative to the stmotion and synchronization became more apparent o1.0-mm star pattern. The horizontal lines in Fig. 6~a!seemed thinner and weaker than the vertical lines, andtips of V-edges toward which the lines converged did nform a near circle as in the case of Fig. 6~b! ~where thelines were 2.0mm wide!. Thus, Fig. 6~a! shows a higherdegree of error in synchronization. Because of synchrozation error between the stage scan step and DMD fradata, the activated point array spreads over a wider cr


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sectional area along the scanning direction, resulting iweaker dose of average intensity, and thus a thinner/weline pattern with a shallower profile. This is in contrastthe vertical line, which has a strong and prominent liprofile. If discrete rotational misalignment were more seous than synchronization error, the reversed scenario wooccur; the vertical line would be weaker and less prominthan the horizontal line. Analyses of discrete rotational malignment and synchronization error prior to exposure ton a substrate can be performed. An imaging deviceplaced on the scanning stage and the point array imaduring an exposure trial~images formed by changing DMDframe data as the stage is being moved! are observed on themonitor. We assume, for instance, that the vertical lineparallel to and the horizontal line is perpendicular to tstage scanning direction. If there were significant synchnization errors, the horizontal line would gradually drupward~or downward! along the scanning direction as thframe data changes. Synchronization errors may be limby tolerance in lens magnification that resulted in sligerror in the desired point array pitch sized or the scan stepaccuracy of the stage. The errors can be compensaterefining the input parameterd and reextracting the framedata using the software algorithm. However, if the verticline gradually drifts left~or right!, there is discrete rotational misalignment of which is a result of imperfect adjusment of the discrete rotational angle for the correspondpresetK value. Discrete rotational misalignment and sychronization errors are deemed acceptable only if the dis within 610% of the smallest intended linewidth of thlithography system. For example, if the maskless lithogphy system is designed for exposure feature sizes2.0-mm L/S, the error~or drift! must be within60.2 mmwhen analyzing the frame images during a trial exposu

The limitation of the point array technique can be sein Fig. 7~b!, where pointed edges appeared on 2.0-mm L/Sover an arc when magnified from a circular-ring-shappattern. These edges were a result of the finite focal sizthe point array limited by Rayleigh’s criterion, as well athe software algorithm used for data extraction inheren




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Fig. 8 Portion of a photomask fabricated using the Hi-Res MLS,shown here after chrome etching. The smallest resolution on thisphotomask sample is 8-mm L/S, easily achieved by this lithographysystem. The photomask is being used in testing some of our pro-cess technologies.

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Fig. 9 Small portion of a large 2-D array of concave microlens takenwith a SEM. As shown, the pitch size of this microlens array is 17mm. The role of the Hi-Res MLS was critical in making this uniquemicrolens array fabrication process possible.

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the technique. To minimize this effect in the future, smalpoint array sizes, reduced scan step, as well as moditions in the process of data extraction may be necessa

Our results indicated consistent exposure of 1.8-mm L/S,with marginally acceptable results at 1.5-mm L/S and yetunacceptable results at 1.0-mm L/S. Actions are being takento enhance the Hi-Res MLS performance to at least 1.0-mmL/S as our target specification under the best conditionsthe appropriate substrate handling and processing pre-postexposure. These future improvements are discussethe next section.

4.2 Examples of Various Applications

The Hi-Res MLS has been a workhorse within our resealaboratory in the development of various technologies.expect to further enhance the capability of the Hi-Res Mat several fronts, which include faster data extraction atransfer rate, higher optical power and more efficient walength for illumination, and maximization of the DMD’capability. We currently manufacture standard 300-mW a1-W violet diode (l5405 nm) laser systems for use in thdesign of the next generation18 Hi-Res MLS. These lasesystems are already being used for some of the compaproducts. Under development is also a high-speed drfor real-time data extraction and data transfer subsystwhich will tremendously improve the Hi-Res MLS’s peformance. In addition, future maskless lithography systdesign may employ DMD structures requiring much ledata management19 or even eliminating the DMD entirely.20

Figure 8 shows a portion of a photomask made usingHi-Res MLS on a chrome-coated glass substrate. Wecesfully applied this system to make photomasks to furtenhance the capability of our technologies and improveunderstanding of our fabrication processes. Such goalsbe achieved swiftly because of the short turn-around tfacilitated by the Hi-Res MLS to produce new or modifiepatterns.

In general, the exposure field length in the scanningrection is limited only by the amount of memory allocatin our electronics hardware. However, the horizontal fidimension~perpendicular to the scanning direction! is lim-ited by the DMD cross-sectional field length. To expandexposure field in the horizontal direction, multiple scansperformed adjacent to or overlapping one another, depeing on the software algorithm used; thus termed a stitchfunction. Figure 9 illustrates a small section of a large 2array of a microlens measuring 14.416310.200 mm. Inthis case, the stitching function was applied. The Hi-RMLS was used to generate patterns on fused silica or ooptical glasses that have undergone preexposure pre

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tion. After lithographic exposure with the Hi-Res MLS, thsubstrate was further treated through a series of semiductor processes before the concave microlens arrayproduced as shown.13,14

5 Conclusion

The Hi-Res MLS has potential for numerous applicatioThese include but are not limited to high-definitioLCD screen manufacturing, MEMS/microopticalelectromechanical systems~MOEMS! prototyping,photomask fabrication, high-resolution PCB productiobiomedical device design, optical component fabricatiacademic or institutional research, etc. The Hi-Res MLSparticularly suited for fast turn-around designs, device teing, modifications, and product improvement. We have scessfully applied this technology to fabricate varioushouse components to enhance our company’s competedge in other areas, be it semiconductor electronics ortical device fabrications. The market opportunities for tHi-Res MLS are enormous. We are currently promotinguse of the Hi-Res MLS for academic and institutional rsearch to expand the bases of applications in differentence and engineering disciplines.

Acknowledgments

Funding for this research was provided by Ball Semicoductor Inc. and its shareholders. The authors would likethank Mr. Akira Ishikawa for his dedication and suppoThe DMD used in this paper was a product of Texas Instments, Inc. Also, many thanks to the time and efforts puby the Exposure Systems Division team members to mthis a successful project.

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http://buffy.eecs.berkeley.edu/IRO/Summary/02abstracts/yanw.1.h2. C. S. Whelan, D. M. Tanenbaum, D. C. La Tulipe, M. Isaacson, a

H. G. Craighead, ‘‘Low energy electron beam top surface image pcessing using chemically amplified AXT resist,’’J. Vac. Sci. Technol.B 15~6!, 2555–2560~1997!.

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Kin Foong Chan received his BS, MS,and PhD degrees, all in electrical engineer-ing, from the University of Texas (UT) atAustin in 1996, 1997, and 2000, respec-tively. Between 1994 and 1998, he workedon electronics design for data acquisitionand instrumentation control with NationalInstruments Corporation in Austin, Texas.He has been an optical research engineerwith Ball Semiconductor, Inc., since 2000,and has been an appointed adjunct profes-

sor in biomedical engineering at UT Southwestern Medical Centersince 2002, both in Dallas, Texas. He joined Reliant Technologies,Inc., Palo Alto, California, in August 2003, to develop advanced laserand optical technologies for clinical dermatology. His research inter-ests include biomedical optics and instrumentation, electro-opticaldevices, lithography, and optical nanotechnology. He has publishednumerous journal and proceedings papers related to laser-tissueinteraction and microlithography, and has several issued or pendingU.S. and foreign patents. Dr. Chan is a member of Tau Beta Pi, EtaKappa Nu, SPIE, and the IEEE.

Zhiqiang Feng received his BS degree inphysics from Fudan University in 1984.During the following 6 years he was a re-search associate in physics with theShanghai Institute of Mechanical Engineer-ing. He entered the Yokohama NationalUniversity of Japan as a research studentin 1991, where he received his MS degreein 1994 and his PhD degree in physicalchemistry from in 1996. After receiving hisdoctorate, he spent a year as a postdoc-

toral fellow with the Department of Chemistry, Iowa State University,


working under Dr. Therese M. Cotton on the electron transfer prop-erty of redox reactive proteins. Since 1997 Dr. Feng has been withBall Semiconductor, Inc., in the Ball Lithography Group as seniorR&D and process engineer. From 1997 to 2000 he was a corporateresearcher with Tohoku University of Japan, working in Dr. Masay-oshi Esashi’s Lab on the application of microelectromechanical sys-tems (MEMS) technology for Ball new devices. He is now with Ball’sExposure System Division working on the development of a high-resolution exposure system and microlens array fabrication. Dr.Feng was a member of the Electrochemistry Society of Japan from1994 to 1996 and is a member of the American Physical Society andthe IEEE.

Ren Yang attended Tsinghua University,majoring in optical precision instruments,and received his BS engineering degree inoptical precision instruments in May 1996and his MS degree in optical engineering inMarch 1999. Beginning June 1999, he fur-thered his graduate studies in microelectro-mechanical systems (MEMS) with Louisi-ana State University, where he receivedMS degree in engineering science in May2002. Mr. Yang became an optical engi-

neer with Ball Semiconductor, Inc., in September 2001.

Akihito Ishikawa joined Ball Semiconduc-tor, Inc. (Ball), when the company wasfounded in October 1996, as a photolithog-raphy process engineer, and led much ofthe development activities for variousresist-coating techniques and equipmentfor the spherical devices. Prior to joiningBall, he founded AtoZ Systems Perfor-mance Motoring (AZSys) in 1992, whilepursuing his BS degree in mechanical en-gineering at University of Texas, Arlington.

AZSys served the auto racing industry, supporting international raceteams in vehicle testing, vehicle preparation, and logistical services.Since his appointment as the Marketing Director in January 2001,he has been promoting the ‘‘maskless exposure’’ and ‘‘spherical de-vice’’ technology products at Ball Semiconductor, Inc.

Wenhui Mei holds a PhD degree in optico-electronic engineering from the Beijing In-stitute of Technology (BIT). He is the presi-dent of the Exposure Systems Division andwas elected a vice president of Ball Semi-conductor, Inc., in March 2002. He workedin design engineering for GL AutomationInc. and USA Display L.L.C. before comingto Ball Semiconductor, Inc., in May 1998 asan optics and mechatronics engineer. Dr.Mei is a member of the Optical Society of

America and the SPIE and was formerly a professor at BIT.



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