Optimization of laser machining processfor the preparation of photomasks, andits application to microsystemsfabrication

Avinash KumarAnkur GuptaRishi KantSyed Nadeem AkhtarNachiketa TiwariJanakrajan RamkumarShantanu Bhattacharya

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Optimization of laser machining process for thepreparation of photomasks, and its application tomicrosystems fabrication

Avinash KumarAnkur GuptaRishi KantSyed Nadeem AkhtarNachiketa TiwariJanakrajan RamkumarShantanu BhattacharyaIndian Institute of Technology KanpurDepartment of Mechanical EngineeringKanpur, Uttar Pradesh 208016, IndiaE-mail: bhattacs@iitk.ac.in

Abstract. Conventional photolithography normally utilizes a photomaskfor patterning light onto a chemical resist film. Therefore, the accuracyof microfabrication is highly dependent on the accuracy of the photo-masks. Fabrication of hard masks involves the use of expensive laserpattern generators and other sophisticated machines using very high-precision stages and the necessary control instrumentation; therefore,an inexpensive strategy is highly necessary for laboratory-level fabrica-tion. As this technology is primarily based on raster scanning of a laserbeam, the mask making as such becomes a low-throughput process. Astrategy of high-throughput manufacturing of hard masks with laser micro-machining using a one-step exposure process of a chromated glass slidethrough a micromachined aluminum shadow mask is proposed. The fea-tures that are finally embedded in the mask are highly demagnified andwell focused. Optimization of the laser machining process is carried outby considering all processing parameters. The features are characterizedusing an optical microscope, a scanning electron microscope, and aself-developed image analysis code. Geometrical methods are usedto estimate the average edge roughness and feature size. We havealso validated the usage of these masks by performing microfabricationon films made of photoresist. © 2013 Society of Photo-Optical InstrumentationEngineers (SPIE) [DOI: 10.1117/1.JMM.12.4.041203]

Subject terms: laser; photolithography; photomask; microfabrication; processoptimization.

Paper 13072SS received May 4, 2013; revised manuscript received Jul. 2, 2013;accepted for publication Jul. 17, 2013; published online Sep. 25, 2013; correctedJan. 8, 2014.

1 IntroductionWith the burgeoning demand of microtechnology, a growingnumber of industrial applications requiring submicrometerfeatures in various materials have become the need of theday.1 To fabricate these features, a multistep hybrid machin-ing technique combining laser machining and an existingMEMS process become highly relevant. An excimer laseris one of the key nontraditional micromachining technolo-gies regularly used in microfabrication.2 One of the majormotivations in MEMS technology is to enable processeswith high capabilities to obtain features and structures alongwith microspacing such as micropillars, microelectrodes,microseparator structures, etc. The fine resolution neededfor the fabrication of such features can easily be obtainedby using a variety of MEMS processing techniques likephotolithography, deposition, and lift-off. As the processof photolithography uses a mask for the selective exposureof photoresist films, the fabrication quality is highly relianton the quality of the photomask in terms of the sharpness andaccuracy of the features and structures. For a relativelyhigher feature size (25 μm and above), soft masks are com-monly used. They are realized in Mylar transparencies byusing relatively inexpensive photoprinting methodologies atvery high resolution (5000 dpi and above). For feature sizes<25 μm, an accurate process necessitates a hard mask, which

is typically a laser-scribed chrome-coated glass frame madeon fused silica, soda lime glass, or polyester film.

Photomasks, requiring sophisticated manufacturing tech-niques and complex mathematical algorithms to design,are at the forefront of the microminiaturization of chips.However, their production is a low-throughput processowing to the finite scanning speeds of laser pattern genera-tors. A highly iterative process, MEMS design, needs multi-ple changes based on device performance so that a finaldesign may be arrived at. This necessitates easy access tofabrication of such masks. Therefore, a high-throughputprocess for fabricating hard masks more amenable to anyMEMS research laboratory setup is needed for the easiercontrol on the mask-making processes.3,4 Nonconventionalmachining processes have been repeatedly used in microma-nufacturing, although their application to mask making hasnot previously been explored. Laser ablation, for example,has been repeatedly used for micromachining applica-tions.5–14 Laser ablation being localized and noncontactmachining is very useful for the fabrication of small featuresand structures which are useful in MEMS devices. Thesetechniques are well suited with a high level of integrabilitythrough complementary and postprocessing means whichallows a completely unified fabrication strategy. Machiningby laser occurs in three steps: (a) interaction between thematter and the laser beam, (b) absorptive heat conduction andtemperature rise, and (c) melting and vaporization of the mat-ter causing its rapid ablation. The various advantages of laser0091-3286/2013/$25.00 © 2013 SPIE

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micromachining are the controlled and planned formation ofdesired properties after fabricating and patterning, preci-sion of achieving exact features dimension, and rapidity ofprocessing, although the process optimization of the machin-ing parameters like laser power, pulse duration, and numberof pulses are critical to establish a well-controlled process.

In this article, we have used a one-step demagnificationand laser ablation technique using an excimer laser (wave-length ∼248 nm) to print hard masks on chromate glassslides with high resolution.

We have realized a hard mask by working on regular lab-oratory glass slides sputter coated with a thin film of chro-mium (thickness ¼ 1000 nm) followed by a guided-laserablation with an excimer laser (wavelength 248 nm) on thesedeposited films. A much larger replica (10×) of the object tobe engraved is realized by a shadow mask fabricated throughmicromilling on a thin aluminum sheet. The aluminum maskis mounted on the excimer laser, and a demagnification ofthe shadow is carried out over the chrome film after properalignment and focusing. The laser machining parameters arefully optimized using the design of experiment technique inwhich a central composite design is used to fit a model byleast square techniques.11 An optimum solution is extractedfrom the different machining parameters, including energy,pulse frequency, pulse duration, and number of pulses, etc.,which minimize the edge roughness of the features ablatedon the chromium film. An energy optimization is performedby calculating the energy value used for ablating the metalfilm specifically without affecting the substrate surface.Furthermore, an algorithm is prepared to calculate the boun-dary roughness value to optimize the unevenness of maskboundary obtained from excimer laser machining. Finally,the chrome mask thus realized is used to perform two-dimen-sional (2-D) lithography on positive and negative tone resists,whose features are compared with those made using other softand hard masks. We have fabricated various features such asarray of micropillars of 10 μm diameter with 5 to 10 μm gap,microchannels of 10 μm width with 10 μm gap, etc.

2 Materials and MethodsA laboratory glass slide (borosilicate glass) of 75 × 25×1.35 mm3 was used as the substrate for mask making. Theglass wafer was thoroughly cleaned using piranha and AMD(acetone, methanol, de-ionized water) wash and well dried ina gravity fed-convection oven (M/s Khera Instruments Pvt.Ltd., Kanpur, Uttar Pradesh, India). A 1-μm thick layer ofchromium was sputter coated on this cleaned glass slideusing a NSC400 sputtering system (M/s Nano Master,Austin, Texas). Our experiments necessitated the use of avery high speed precise laser machining system. The meth-odology of laser writing employed a masking and a demag-nification strategy with a completely opaque aluminum plate.

Slots were cut in this plate with a CNC-controlledmicrotooling center (Integrated Multi-process MachineTool DT-110) using micromilling. The slots machined onthe aluminum plate would allow the laser beam to pass, andsuch regions would be ablated on the chrome surface aftera demagnification of 1∕10th by alignment and focusing.

2.1 Micromachining of Aluminum Mask

Micromachining (milling and drilling) (electro dischargemachining) was performed on an aluminum sheet of 200-μm

thickness according to a user-defined tool path through adrawing exchange format interface, which corresponded tothe negative of the feature that we needed to finally obtainon the photoresist film. This tooling center could machinewith a resolution of around 0.1 μm, which correspondedto the minimum feature size on the aluminum plate. Thespindle head was servo controlled and could operate overspeed ranges from 1 to 5000 rpm. After the aluminum platewas fully machined, it was taken out and sonicated for30 min using ultrasonicator (Digital ultrasonicator, CD4820)for deburring.

2.2 Excimer Laser Machining on Cr-Coated Glassfor Photomask

An excimer laser machine (Coherent, Variolas Pro 248 nm)was used to machine the features on the chrome-coated glassslide. The maximum pulse energy of this machine is 700 mJ.The wavelength and maximum repetition rate of the laser are248 nm and 50 Hz, respectively. The original beam dimen-sions at the mask plane are 20 × 20 mm2, and the pulse dura-tion is 20. The beam delivery system comprises three sets oflenses (Fig. 1). The first set contains three telescopic lensesto shape and collimate the beam from an initial dimension of10 × 24 to 20 × 20 mm3. The second set contains a pair of8 × 8 array of insect eye lenses to homogenize the intensityof the beam over the cross-section of the beam. The thirdset contains a Fourier, projecting, and condensing lensesto guide the beam into the mask and to project a 0.1× demag-nified image of the mask on to the work plane with a spot sizeof 2 × 2 mm2. The total demagnification that is provided bythis system is 0.1×. To make a feature on the excimer lasermachining, i.e., to obtain a feature of 10 μm, we requirea shadow mask with a feature of 100 μm. Images of the fea-tures realized on the work surface are digitally acquired andanalyzed for parameters like overall dimensions, surfaceroughness, etc.

Figure 2 shows a process flow for the mask making andcharacterization operation. The image analysis code has beendeveloped specially for comparing the mask features toplanned dimensions, and the laser machining is optimizedso that accurate hard masks with minimum edge roughnessare fabricated.

Fig. 1 Schematic representation of the placement of demagnificationlens after shadow mask.

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2.3 MATLAB Code for the Image Analysis ofLaser-Scribed Features

The code has an input in the form of a grayscale imageof the scribed-feature imaged over a high-resolution opticalmicroscope. The input raw image data is converted into abinary image. The acquired binary image is digitized intoblack and white pixels marked as “1” and “0.” From thisdata, the image boundary is identified by exploiting the sud-den contrast obtained at the boundary due to a change inpixel luminance. The mean roughness can be obtained bydrawing a line of best fit and calculating the root mean squareof the perpendicular distance of the points on the imageboundary from the line of best fit.

Another part of the code is used for pulse energyoptimization of the laser head for lasing on the thin metalfilm of the photomask with the optimization criteria assurface roughness. The code takes inputs from discreteexperimental points, and then interpolates the data over

the whole range of pulse energy using three-dimensional(3-D) plots.

3 Results and Discussion

3.1 Image Analysis on Acquired Images for SurfaceRoughness

Figure 3(a) shows the optical micrograph of the laser-machined photomask surface. The black region of theimage is the part scribed on the chrome-coated glass plate(blue background). Figure 3(b) shows the correspondinggrayscale image processed using Image-J software. Thered region on this image corresponds to the ablated blackregion in the optical micrograph. Figure 3(c) shows thebinary image, which also leads to the creation of a pixelmatrix with values “1” and “0.” The image boundary shownby the green line is traced in the matrix as a locus of all suchpoints showing an abrupt change in pixel type in the imme-diate neighborhood. The line of best fit is generated by look-ing at the (x, y) coordinates of the boundary pixels and fittinga straight line across these pixels. The code output, obtainedby calculating the root mean square of the perpendicular dis-tance of the points on the image boundary from the line ofbest fit, is mentioned below. The output is also expressed interms of a theoretical energy value needed to perform thelaser micromachining of the thin chromium film on theglass surface as detailed in the next section.

3.2 Theoretical Assumption for Power Estimation

For calculating the optimum lasing power, we have used aone-dimensional (1-D) transient heat flow equation in cylin-drical coordinates assuming a circular laser spot. The upperdiameter of the cylinder was assumed as the diameter of thelaser spot, and its height was equated to the thickness ofthe chromium layer. The underlying assumptions of thetheoretical model assumed minimal damage to the glasssurface underneath the chromium film. This assumptionwas also appropriate, as in general, the glass surface is highly

Fig. 2 Process parameter optimization on excimer laser.

Fig. 3 Input and output of image acquisitions and image analysis for excimer laser. (a) optical micrograph of a laser etched microchannel, (b) imageanalyzed gray scale version of themicrograph acquired in (a), (c) binary image generated from the gray scale image after intensity adjustment, (d) proc-essed output of the code with an input of binary image of (c) reporting optimized laser machining parameter for optimum average surface roughness.

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Fig. 4 Surface plot for basic feature size for (a) 10-μm feature size and (b) 20-μm feature size, and edge roughness for (c) 10-μm feature size and(d) 20-μm feature size.

Fig. 5 Minimization of edge roughness with energy and number of pulses features of size (a) 10 μm and (b) 20 μm, and minimization of deviationfrom basic size with energy and number of pulses for features of size (c) 10 μm and (d) 20 μm.

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reflective. It is correct to interpret that not enough laserpower would be attenuated on the glass, and a major partof this would be reflected. Vaporization time is given by a1-D transient heat equation:15

∂θðz; tÞ∂z2

−1

α•

∂θðz; tÞ∂t

¼ 0: (1)

By solving Eq. (1), we get temperature as a function ofdepth from the top surface and time:

θðz; tÞ ¼ 2Hffiffiffiffiffiαt

pK

•

264ierfc

�z

2ffiffiffiffiffiαt

p�− ierfc

0B@

ffiffiffiffiffiffiffiffiffiffiffiffiffiz2 þ d2

4

q2

ffiffiffiffiffiαt

p

1CA375:(2)

Putting z ¼ 0 (as the heat boundary is considered to be thetop surface of the chromium film) in Eq. (2), we get

θðz; tÞ ¼ 2Hffiffiffiffiffiαt

pK

•

�1ffiffiffiπ

p − ierfc

�d

4ffiffiffiffiffiαt

p��

: (3)

Equation (3) gives the relation between temperature “θ”of the melting surface and time required for “θ” to reach themelting point of the metal.15 We have assumed for the sake ofsimplicity that the melting temperature of chromium metaland that of sputtered-thin film of chromium are similar.16 In

Eq. (3), as θ approached θm (melting temperature), the time tapproached tm (the vaporization time). In summary, Eq. (2)was used with a “z” value of the film thickness, a “θðz; tÞ”value equal to the melting temperature of the film obtaininga set of input lasing power (with a surface-dependentattenuation level). Corresponding pulse times indicative ofthe pulse frequency were found, and the lasing energywas predicted. The thickness of the film was taken at1 μm, as the whole film was physically ablated by thelaser during the laser exposure. The melting point of chro-mium was assumed to be 1900°C, and spot size was variedbetween 1 and 1000 μm, and the temperature versus time ofablation was plotted using MATLAB version 7.1. The outputis represented as Fig. 9 and a tabulation is made (Table 1)about the spot diameter versus the pulse time for the lasingprocess. We found the pulse time for the temperature to riseto 1900°C to be approximately 20 ns which was made com-mensurate with all the experiments related to the mask-making process. Ablation was physically performed usingcombinations of the energy and pulse time obtained bythe above methodology. Basic feature size and edge rough-ness of the features were used as the target optimizationcriteria.

3.3 Surface Contour Plots Through MATLAB AfterImage Analysis

The features scribed on the surface are analyzed for edgeroughness. Surface and contour plots are made with pulse

Fig. 6 Optical micrographs of enlarged aluminum mask manufactured through micromilling and drilling for (a) micropillar array (imaged at 4×)(magnified view of microdrilled area in callout), (b) microchannel array (imaged at 4×), and laser micromachined image on the chromatedglass slide for (c) micropillar array (imaged at 4×) and (d) microchannel array (imaged at 4×).

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energy, number of pulses, and percentage deviation frombasic size of the features, as shown in Figs 4(a) and 4(b).The basic feature size used with operating points in the nearvicinity of the theoretically obtained values is a minimumprocess resolution of 10 to 20 μm. Similar plots are generatedfor average edge roughness of the features [Figs. 4(c) and4(d)].

On comparison of Fig. 4(a)–4(d), we have obtained 20 to50 mJ energy with 1 to 2 laser pulses considering the opti-mization criteria as the minimum deviation from the basicsize and minimum edge roughness. The maximum edgeroughness obtained is around 1.5 to 2 μm. The allowabledeviation from basic size has a maximum value of 10%to 15%.

3.4 Validation of Our Optimization Strategy UsingDesign of Experiments

The Design Expert Software was used for parametricoptimization of the lasing process. The entire range ofpulse energy and number of pulses were used for lasermicromachining of the chromate slides, and optimizationwas performed with target criteria of edge roughness anddeviation from basic size in a similar manner.

Figures 5(a)–5(d) show various plots for roughness anddeviation from basic size minimization with varying numberof pulses and pulse energy. We have obtained 105-mJ energywith a single pulse to satisfy the minimum roughness criteria,and the roughness value reported for this combination ofpulse energy and number of pulses is 1.92 μm. For the20-μm feature, the best combination corresponds to 30-mJenergy with the single pulse, and the observed roughness

Fig. 7 (a) Aluminum mask for the confined micropillar arrays withinchannel-like structures. (b) Micrograph of the ablated, chromatedglass after the one-step laser exposure.

Fig. 8 Micrograph of the patterned photoresist for (a) micropillar array of 10-μm resolution and (b) microchannel array of 10-μm resolution. (c, d)Scanning electron micrograph of the pillars confined within channel arrays.

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comes to be 1.10 μm. Similarly, the deviation from basic sizecomes to be in the range of 12% to 20%, which is also veryclose to the findings in our code.

3.5 Optical Micrographs of Various FeaturesEmbedded on the Chromated Glass Surfaces

Optical micrographs of patterns and features printed on thechromated glass slides were imaged with the help of an opti-cal microscope (M/s Nikon80i, Chiyoda-ku, Tokyo, Japan)mounted with a Peltier-cooled CCD camera. Figures 6(a) and6(b) show the optical micrographs of the aluminum shadowmask that is micromachined using micromilling and drilling.Figures 6(c) and 6(d) show the corresponding features on thelaser micromachined, chromated glass surface after the one-step exposure process.

We have also fabricated confined micropillar arrayswithin microchannels, as indicated in Figs. 7. Figure 7(a)shows the aluminum mask, and Fig. 7(b) shows the laser-ablated chromate slide.

3.6 Photolithography Using the Hard Mask(Chromated Glass Slide) Developed inthe Earlier Step

The photolithography step is carried out using SU-82025 negative photoresist (M/s Microchem, Newton,Massachusetts). The resist is spin coated at 1500 rpm ona well-cleaned silicon wafer up to a thickness of 20 μmfollowed by the standardized soft bake, exposure, hard bake,and development steps.

Figures 8(a) and 8(b) are the micrographs of the patternedphotoresist where the chromated glass slide has served asthe mask. The resolution of 10 μm is easily achievable usingthis process, and the throughput of the manufacture of sucha mask is very high.

4 ConclusionThe methodology described in this article explores a one-step high-throughput shadow masking process to make hardmasks at high resolution. The minimum resolvable featuresize that can be arrived at through this process is roughly10 μm. The mask-making strategy with a combination ofadvanced machining technologies, easily available withinan advanced machining laboratory, can be very helpful foriterative microsystems designing. Through this work, therepeatability of the different processes have been validatedwith different designs, like micropillar arrays, microchannelarrays, micropillar arrays confined within microchannels,etc. The strategy so developed can cater to a large high-resolution feature printing requirements without waiting fora large turnaround time for obtaining the laser pattern gen-erator-based masks which are commercially obtained froma mask manufacturer.

AcknowledgmentsThe authors would like to acknowledge the NanoscienceCentre, 4i Laboratory, IIT Kanpur, for providing fabricationand characterization facilities. The authors would also like togratefully acknowledge the valuable advice of ProfessorShubhra and Keshab Gangopadhyay of the University ofMissouri, Columbia.

Appendix

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Fig. 9 Output plot of MATLAB from Eq. 2 where the time of laser pulseneeded for the whole chrome thickness (1 μ) to achieve a melting tem-perature is plotted. The callouts are indicative of the operating points.

Table 1 The time (in ‘ns’) to reach melting temperature of a 1 μ thickfilm with respect to an attenuated pulse energy (‘mJ’) for different laserspot diameters. If the energy is lowered then the time would increase.

Pulse Energy (mJ)Spot Dia

Assumption (μm)Time to reach Tm ata depth of 1 μm (ns)

300 1 35

300 2 11

300 3 9.4

300 4 9.2

300 10 9.2

300 100 9.2

300 1000 9.2

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Avinash Kumar received his BE in mechani-cal engineering in 2010 fromOriental Instituteof Science and Technology, Bhopal, India,and M. Tech. in 2012 from Indian Instituteof Technology Kanpur, India. He worked asa research associate with the Bio-MEMSand Micro-fluidics Laboratory, IIT Kanpur,India, for 4 months in 2012 and currentlyworking as a research associate in RobertBosch Centre for Cyber Physical Systems,Mechanical Engineering Department, Indian

Institute of Science, Bangalore, India. His research interests comprisemicro/nanofabrication, MEMS, microfluidics, compliant mechanisms,biorobotics, and biomechanics.

Ankur Gupta received his B. Tech. inmechanical engineering from Uttar PradeshTechnical University, India, and M. Tech.from Indian Institute of Technology, BanarasHindu University, India. He is currently adoctoral student at IIT Kanpur, India. Hisresearch interests comprise fabrication andcharacterization of nanostructured materials,and its application toward gas/biosensing.

Rishi Kant received his B. Tech. in mechani-cal engineering from the University Instituteof Engineering and Technology, ChhatrapatiShahu Ji Maharaj University, Kanpur, India,in 2004, and the ME degree in mechanicalengineering from Delhi College of Engineer-ing, University of Delhi, New Delhi, India, in2007. He is currently working toward thePhD degree in mechanical engineering atthe Indian Institute of Technology Kanpur.He was a research assistant with the Design

Manufacturing Integration (DFM) Laboratory, Indian Institute of Tech-nology, New Delhi, from 2007 to 2008. His research interests includebio-MEMS and micro/nanofabrication for fluidic and otherapplications.

Syed Nadeem Akhtar received his B. Tech.and M. Tech. in mechanical engineering in2007 from Indian Institute of TechnologyKanpur, India. He worked for Hindustan Uni-lever Limited, India, for a period of 3½ years,and then joined IIT Kanpur for his PhD. Hisresearch interests comprise micromachining,specially laser micromachining, and productdesign.

Nachiketa Tiwari received his MBA fromBabson College and PhD from VirginiaTech, USA. He is an associate professor inIndian Institute of Technology Kanpur,India. His research interests comprise acous-tics and noise control, solid mechanics,composite structures, vibrations, productdesign, automotive systems, and MEMS.

Janakrajan Ramkumar received his BE in1996 from Regional Engineering College,Trichy, India, and M. Tech. and PhD fromIndian Institute of Technology Madras, Chen-nai, India. Thereafter, he joined IndianInstitute of Technology Kanpur, India, asan assistant professor, where he is an asso-ciate professor now. He has taught severalcourses at both undergraduate and post-graduate levels. His research interestscomprise micro- and nanomachining, nano-

finishing, composites, and product design.

Shantanu Bhattacharya received the BSdegree in industrial and production engineer-ing from the University of Delhi, New Delhi,India, in 1996, the MS degree in mechanicalengineering from Texas Tech University,Lubbock, Texas, in 2003, and the PhDdegree in biological engineering from theUniversity of Missouri, Columbia, Missouri,in 2006. He was a senior engineer withSuzuki Motors Corporation from 1996 to2002. He also completed postdoctoral

training at the Birck Nanotechnology Center, Purdue University,West Lafayette, Indiana, for 1 year. He was an assistant professorwith the Department of Mechanical Engineering, Indian Institute ofTechnology Kanpur, India, from 2007 to 2012, where he is currentlyan associate professor. His research interests include design anddevelopment of microfluidics and MEMS platforms for varied engi-neering applications.

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