Photoresist_Spray_Using_Ultrasonics (1063669)
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Journal of the Microscopy Society of Thailand 24 (1), 42-46 (2010)Improvement of Photoresist Film Coverage onHigh Topology Surface withSpray Coating TechniqueNithi Atthi, Karoon Saejok, Jakrapong Supadech, Wutthinan Jeamsaksiri, Oraphan Thongsuk,Paweena Dulyaseree, Charndet Hruanun, and Amporn PoyaiThai Microelectronics Center, National Electronics and Computer Technology Center, Chacheongsao, Thailand.*Corresponding author, e-mail: nithi.atthi@nectec.or.thAbstractThis paper compares a current single-spin coating method to deposit a thick photoresist film on 14 μm ofhigh topology micro-structure to a multi-spin coating and a direct spray coating method.
By using a single-spincoating method, the Clariantz AZ-P4620 photoresist film uniformity was not so good and the remainingphotoresist was not enough to block the plasma during a dry plasma etching process. When using a multi-spincoating, the multi-layers of medium viscous, Sumitomo PFI-34A, photoresist film were applied over ClariantzAZ-P4620 photoresist initial layer.
The photoresist film thickness has enough to block the plasma but thephotoresist film peeled off during the development step due to an undercutting trouble. Then, low viscousClariantz AZ-P4999 photoresist film was coated with spray coating method. The photoresist film was thickerthan a multi-spin coating method and the photoresist step coverage over the topology surface was better than theother methods. Moreover, the developed photoresist pattern has a final photoresist film thickness around 4.0 μmon the topology surface, which is suitable for a dry etching process.
It can be concluded that, the spray coating isa more suitable photoresist film coating technique on high topology surface than spin coating techniques.IntroductionFor several micro-electro mechanical systems(MEMS) applications, pattern transfer on the waferwith extensive topography requires a thickphotoresist (PR), of which the film thickness is in arange of 10 μm to 100 μm, with a uniform PR layerover a non-planar surface. Thus, the conformal PRcoating of wafers with 3D micro-structuringbecomes a crucial step in the photolithographyprocess. There is some experiment, which efforts toget a conformal coating layer by using spin coatinghave been reported [1].
However, this technique,even after certain modifications, presents somesevere limitations for truly 3D structures. Anothermethod is electro-deposition (ED) of PR [2]. Itrequires a conductive plating base layer. Therefore,electroplating of PR is restricted to the backendprocessing. A new alternative method, direct spraycoating of PR [3-4], appears to be a promisingtechnique as it does not require a conductive seedlayer and can therefore be used at all stages of thewafer processing. In this paper, various PR coatingtechniques, including current single-spin coating(SS), multi-spin coating (MS) technique, and aspray coating (SC) technique, have been applied tocoat a thick PR film on 14 μm of high topologystructure.
The PR film thickness and film stepcoverage for all coating techniques have beenstudied.Spin coating techniqueThe spin coating (SC) is the prevalent methodof applying PR, which is widely used insemiconductor devices manufacturing. It isintensively used to deposit uniform thin PR films tothe flat substrates in photolithography process.There are four process steps in SC process, whichare (1) Dispensing, (2) Spin-up, (3) Spin-off, and(4) Evaporation [5]. During a dispensing step, itcan either be accomplished by flooding the entirewafer with PR solution before beginning thespinning (Static dispense), or by dispensing asmaller volume of PR solution at the center of thewafer and spinning at low speed to produce auniform liquid layer across the wafer (Dynamicdispense).
During spin-up step, the wafer isnormally accelerated to the first spin speed to makethe PR liquid spread off and cover the whole waferarea by using a centrifugal force. After that, thewafer is normally accelerated as fast as possible tothe final spin speed. This stage is a spin-off step,which is used to control the PR film thickness.Finally, in the evaporation step, the wafer is spin atthe constant final speed to make a solventevaporate off the PR film. This stage is use tocontrol the film uniformity. By using a spin coatingtechnique, less than 10% of the applied PR remainson the substrate [6].
The schematic diagram of thespin coating technique has shown in Figure 1.42Journal of the Microscopy Society of Thailand 24 (1), 42-46 (2010)Materials and Methods(a)(b)(c)(d)Micro-needles process flowIn this work, 6 inch p-type (100) silicon waferswere used as a substrate.
The high topology surfaceused in the experimental was a micro-needle, whichcan be fabricated by using a process flow shown inFigure 3 (a) to Figure 3 (f). First, the Si wafer wascoated with 2 μm of PR film and patterned byphotolithography process. This PR film acts like asoft mask to protected an underneath Si surfaceduring a plasma deep reactive ion etching (DRIE)process.
The etch depth through the Si surface was13 μm. Then, 2 μm of SiO2 film was deposited onSi etched pattern surface, as a schematic shown inFigure 3 (a). Then, coated a thin PR film, which thefilm thickness of 2 μm, and did the patterns on PRfilm as the schematic shown in Figure 3 (b).
Afterthat, 2 μm of SiO2 film was etched by using RIE asthe schematic shown in Figure 3 (c). Then, strippedthe remained PR film and coated the thick PR film,which the thickness was greater than 10 μm, onSiO2 etched surface and did the patterns on thickPR as the schematic shown in Figure 3 (d). Laterthat, using thick PR film as a soft mask and etchedthough the center of SiO2 opaque pattern for 150μm depth. The fabrication structure now is shownin Figure 3 (e). Noted that, the etch selectivitybetween silicon (Si) and PR was 15:1. The etchedSiO2 structure will acts as a hard mask for Si deeptrench etching process.
The final micro-structureafter deep trench etching process is shown inFigure 3 (f).Figure 1 The schematic view of spin coatingprocess, (a) Dispensing, (b) Spin-up, (c) Spin-off,and (d) Evaporate.Spray coating techniqueA direct spray coating (SC) system has beendeveloped and introduced by the Electronic VisionGroup (EVG) in 1999 The direct spray systemincludes an ultrasonic spray nozzle, whichgenerates a distribution of droplets of micrometersize. During spray coating, the wafer is rotatedslowly while the swivel arm of the spray coatingunit is moved across the wafer.
The low spinningspeed (30-60 rpm) is to minimize the centrifugalforce. The rotation also allows resist coverage in allangles of the cavities [3]. Because the parts arestatic, no residual stresses in the coating areencountered as in spinning and the meniscusproblem is significantly reduced [5].Key parameters that influence the quality of thespray coated layer are (1) solid content of the spraysolution, (2) resist dispensed volume, (3) angle ofthe atomizer, (4) scanning speed of atomizer, and(5) spray pressure [4]. The advantage of spraycoating method is a much smaller resist amount isconsumed when thick resist layers are needed.
Toobtain a layer of the same thickness generally a PRvolume 10-15 times smaller than SS method. Thedisadvantage of the SC method is that the PRsolution should have a viscosity less than 20 cSt.Moreover, an excessively rapid evaporation ofsolvents from the resist can cause "orange peel" onthe substrate surface. The schematic diagram ofspray coating technique is shown in Figure 2.Spray head(a)(b)(c)(d)(e)(f)Wafer chuckFigure 3 Micro-needles fabrication process flow, (a)SiO2 deposition, (b) Patterning#1, (c) SiO2 etch, (d)Patterning#2, (e) Shallow trench etch, (f) Deep trenchetch.Figure 2 The schematic view of spray coating process.43Journal of the Microscopy Society of Thailand 24 (1), 42-46 (2010)Table 1 The process conditions for SS and MS coatingmethod.Thick photoresist coatingThis experiment is focusing on the thick PRfilm coating on 15 μm of high topology surface.The high topology surface consists of 13 μm ofdeep etched Si surface and covered with 2 μm ofetched SiO2 film.
This PR coating process is at theprocess step shown in Figure 3 (d). Three kinds ofPR coating methods, which are single-spin coating(SS) method, multi-spin coating (MS) and a spraycoating (SC) method, have been studied.For SS coating method, Clariantz AZ-P4620commercial PR was coated by using TEL Mark Vtrack coating system with a dynamic dispensingprocess. The PR dispensing spin speed was 500rpm for 15 sec. After that, the wafer wasaccelerated at 10,000 rpm/sec to the final spinspeed at 750 rpm, and span at the constant finalspin speed for 10 sec. Then, PR film was baked at100°C for 180 sec on a hot plate. The target PRfilm thickness was 20 μm.For MS coating method, it was a technique toincreases a total PR film thickness beyond thelimitation of the PR materials on the first layer. 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.
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