Photoresist_Spray_Using_Ultrasonics (1063649), страница 2
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Thetopology substrate was coated with Clariantz AZP4620. The PR solution was dispensed for 15 secduring a wafer was spinning at 1,000 rpm. Then,the wafer was accelerated at 30,000 rpm/sec to thefinal spin speed at 1,500 rpm for 20 sec. Then, PRfilm was baked at 100°C for 180 sec on a hot plate.Then, multi-layers film stack of Sumitomo PFI34A PR were applied over AZ-P4620 film. ThePFI-34A PR was dispensed during a wafer wasspinning at 1,000 rpm for 4 sec.
Then, the waferwas accelerated at 30,000 rpm/sec to the final spinspeed at 3,800 rpm. Hold the wafer at final spinspeed for 20 sec and baked the PFI-34A PR film at90°C for 80 sec, layer-by-layer, on a hot plate. Thetarget film thickness of the first AZ-P4620 layerwas 15μm, and the target film thickness of totalPFI-34A film stacks was 5 μm. This means, thetotal PR film thickness of AZ-P4620 PR and PFI34A PR, which coated by MS coating method,were 20 μm. The process conditions for SS and MScoating methods are shown in Table 1.For SC coating method, Clariantz AZ-P4999PR was coated on a high topology substrate.
Thedispensed volume of PR was 70 ml/s, spin rate ofwafer was 60 rpm and the scan speed was 1,000steps/sec across the wafer. The angle of atomizerwas 45° and the spray pressure was 500 mbar.After spraying the PR layer, the wafer was bakedon a hot plate at 90° C for 10 min. The target PRfilm thickness by SC coating method was 25 μm.The process condition for SC coating method isshown in Table 2.Finally, the PR film thickness and PR filmcoverage were investigated by Field-EmissionScanning Electron Microscope (FE-SEM), Hitachimodel S4700.Process stepSSAZP4620First spin speed(rpm)Dispensingtime (sec)Acceleration(rpm/sec)Second spinspeed (rpm)Second spintime (sec)Soft baketemperature(°C)Soft bake time(sec)No.of loopsTarget filmthickness (μm)PR coating methodMSAZPFI-34AP46205001,0001,0001515410,00030,00030,0007501,5003,80010202010010090180*180*80420155Table 2 The process conditions for SC coating method.Photoresist typeAZ-P4999Dispense volume (ml/s)70Wafer spin rate (rpm)60Scan speed (step/s)1,000Angle of atomizer (°)45PR spray pressure (mbar)500Soft bake temperature (°C)90Soft bake time (sec)10Target film thickness (μm)25Dispense volume (ml/s)70Results and DiscussionFollowing by the process steps shown in Figure3, the real micro-structure after SiO2 deposition isshown in Figure 4.
First, high viscous ClariantzAZ-P4620 PR film was applied on the surface bySS method. The result in Figure 5 shows, PR filmthickness (FT) on the flat surface (F-pos), pillarsurface (P-pos), and the corner of the pillar (C-pos)are 12.50 μm, 5.95 μm, and 3.05 μm, respectively.After patterning, the PR film thickness coatedwith SS method at C-pos decreases down to1.39 μm, as the SEM image shown in Figure 6.
Theremaining PR at the C-pos was not enough to blockthe plasma during a dry plasma etching processbecause the etch selectivity between Si and PR was15:1. So the Si substrate under a thin PR has beenetched as the result is shown in Figure 7.44Journal of the Microscopy Society of Thailand 24 (1), 42-46 (2010)SiO2Deep trenchSiOver-etchedregionSiFigure 4 SEM image of SiO2 film coated on Si substrate.Figure 7 SEM image of a PR film coated by using SSmethod after etched.PRSiO2PRSiO2SiSiFigure 8 SEM image of a PR film coated by using MSbefore exposed.Figure 5 SEM image of a PR film coated by using SSmethod before exposed.PRPRLifted-off PRSiO2Exposed regionSiSiSiO2Figure 6 SEM image of a PR film coated by using SSmethod after developed.Figure 9 SEM image of a PR film coated by using MSafter developed.For MS coating method, multi-layers of mediumviscous, Sumitomo PFI-34A, PR film were appliedover Clariantz AZ-P4620 PR initial layer.
The SEMimage in Figure 8 shown that, the PR film thicknessat the F-pos, P-pos, and C-pos are 23.00 μm, 10.00μm, and 12.20 μm, respectively. The PR filmthickness by the MS method was enough to blockthe plasma. However, the PR film stacks peeled offduring the development step due to a film adhesionand undercutting trouble as shown in Figure 9.
Lowviscous Clariantz AZ-P4999 PR film has beenapplied on the surface by the SC method. Figure 10shows the PR film thickness at F-pos, P-pos, and Cpos, which were 28.60 μm, 13.40 μm, and 12.60μm, respectively. The PR film thickness coatingwith SC method was higher than MC method at Cpos and the PR step coverage and film uniformity45Journal of the Microscopy Society of Thailand 24 (1), 42-46 (2010)over the topology surface was better than the SSand MS method as the results are shown in Figure6, Figure 8, and Figure 10, respectively.
Thedeveloped pattern remains intact the PR filmthickness at C-pos around 4.0μm on the topologysurface, as shown in Figure 11. Moreover, thesummarized of PR film thickness and film qualityfor various coating methods are shown in Table 3.ConclusionA comparison of three photoresist coatingmethods for MEMS structures fabrication has beenpresented.
By using a single-spin coating, the PRfilm coverage was not suitable for high topologysurface and the PR film thickness was not enoughto block the plasma during a deep reactive ionetching process. When using a multi-spin coating,the PR film thickness and film step coverage wasimproved but the PR film peeled off during thedevelopment step. The results show that the spraycoating is a promising technique for pattern transferon wafers with extensive topography due to itsgood film thickness, good film uniformity, andgood film step coverage.PRAcknowledgementsThe authors would like to thank Western Digital(Thailand) Co., Ltd. for the use of spraydevelopmentmachineandthankstoNanoelectronics and MEMS laboratory, NationalElectronics and Computer Technology Center(NECTEC) to advice for the MEMS fabricationprocess step.SiO2SiFigure 10 SEM image of a PR film coated by using SCbefore exposed.References1. V.G.
Kutchoukow, J.R. Mollinger, A.Bossche, Novel method for spinning ofphotoresist on wafers with through-hole,Eurosensors XIII, 256-272 (1999).2. P. Kersten, S. Bouwstra, J.W. Petersen,Photolithography on micromachined 3Dsurfaces using electrodeposited photoresists,Sensor & Actuator, A51, 51-54 (1995).3. T. Luxbacher, A. Mirza, Spray Coating forMEMS, Interconnects, and AdvancedPackaging Applications, Sensors, 16, 7, 61-64(1999).4.
N. Pham, T. Scholtes, R. Klerks, E. Boellaard,P.M. Sarro and J.N. Burghartz, Direct spraycoating of photoresist–a new method forpatterning 3-D structures, Eurosensors XVI,182-185 (2002).5. S. Wolf, R.N. Tauber: Silicon processing forVLSI era: Process technology, Vol.1, seconded., Lattice Press, California, 2000.6. J.R. Sheat, B.W. Smith: Microlithographyscience and technology, Marcel Dekkar, Inc.New York, 1998.PRExposed regionSiO2SiFigure 11 SEM image of a PR film coated by using SCafter exposed.Table 3 The summarized of PR film thickness and filmquality for various coating methods.PR typeC-Pos FT (μm)P-Pos FT (μm)F-Pos FT (μm)PR FTPR coveragePR lifted-offFilmuniformityover topologyPR coating methodMSSCAZAZP4620+P4999PFI-34A3.0512.2012.205.9510.0013.4012.5023.0028.60NotEnoughEnoughenoughBadGoodVery goodNoYesNoSSAZP4620BadGoodVery good46.
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