使用OTS-SAM和二氧化硅涂料预防小丘形成.docx
【机械类毕业论文中英文对照文献翻译】使用OTS-SAM和二氧化硅涂料预防小丘形成
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机械类毕业论文中英文对照文献翻译
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【机械类毕业论文中英文对照文献翻译】使用OTS-SAM和二氧化硅涂料预防小丘形成,机械类毕业论文中英文对照文献翻译,机械类,毕业论文,中英文,对照,文献,翻译,使用,OTS,SAM,二氧化硅,涂料,预防,小丘,形成
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Prevention of hillock formation during micro-machining of silicon by usingOTS-SAM and SiO2coatingsT-S. Oh, H-J. Kim, D-E. Kim*Yonsei University, Republic of KoreaSponsored by Dong-Yol Yang (1).1. IntroductionVarious techniques related to Micro-Electro-Mechanical Sys-tem (MEMS) have been developed to fabricate micro-patterns andstructures on silicon-based materials 14. These techniques,however, require extensive equipment and high costs are involvedin setting up the fabrication facility. Also, it is often necessary tomake a mask for each optical process which adds to the cost andtime of the total process 5. Although technique such as photo-lithography has a strong meritin mass production, it is not suitablefor flexible machining needed for micro-mold and prototypefabrication for micro-systems.In order to achieve cost effective and flexible machining withoutthe use of a mask, numerous techniques have been developed. IntheMCSPL (Mechano-Chemical Scanning Probe Lithography) processpreviouslydeveloped,athinresistlayerisselectivelyremovedbytheprobetipofanAtomicForceMicroscope(AFM)followedbychemicaletching to fabricate micro-groove patterns on silicon or metalsurfaces 6,7. As the resist layer, materials such as SiO2and Self-Assembled Monolayers (SAMs) have been utilized for their ability toresistchemicaletching.Theresistisremovedbythecuttingactionofthe tip undera constantapplied load. The load isadjusted so that thedesired depth of cut can be achieved. After machining, the pattern isselectivelyfabricatedbychemicaletchingatregionswheretheresisthas been removed. Three-dimensional structures as well as variouslinepatternscanbefabricatedsuccessfullybytheMCSPLprocess8.The dimensions of the fabricated pattern depend on the size of theprobe tip, thickness of the resist, and the applied load during resistmachining 9,10. For example, MCSPL using AFM offers machiningcapability in the tens of nanometer-scale 1113 while patterns oftensofmicrometersorlargercanbefabricatedbyusingasharpstylustype of a tool in conjunction with 3-axis precision actuators 14.Though groove pattern widths of tens of micrometers andhundredsofnanometerscouldbeachievedusingtheMCSPLprocess,patterns in 1mm range could not be fabricated with sufficientreliability. When MCSPL process is applied to a silicon workpiece tofabricate patterns of micrometer in size, hillocks are often formedinstead of grooves 15. This phenomenon is encountered particu-larly when the resist layer is thin and the normal load is relativelyhigh because the tool tip can readily penetrate the resist layercausing direct abrasion between the sharp tip and the siliconsubstrate during the mechanical machining process. One the otherhand, ifthe resist layeristoo thick, it becomes extremely difficult tocontrol the normal load such that only the resist material will beremoved during the mechanical machining process. In such a case,there is a very high probability that direct contact between the toolandtheworkpiecewilloccur.Thus,hillockformationisa significantproblem in micro-patterning of silicon when groove patterns in therange of 1mm width are desired 16,17.In this work, the mechanism of hillock formation during MCSPLprocess applied to silicon workpiece was investigated. The motiva-tion was to better understand the hillock formation process and todevelop a process by which hillock formation can be preventedduring MCSPL process for groove pattern widths in the 1mm range.As a means to eliminate the hillock formation problem variousexperimental conditions were investigated and the results wereanalyzed. Particularly, a layer of octadecyltrichlorosilane (OTS,CH3(CH2)17SiO3) SAM, which is commonly used as an anti-stictioncoating in MEMS applications 18, was exploited as a secondaryresistlayertopreventtheformationofhillocksintheMCSPLprocess.2. Experimental detailsMicro-machining experiments were performed using a custom-built equipment. A diamond tip with a radius of 1mm was used asthecuttingtoolformechanicalmachining.Machiningwasperformed by applying a normal load to the tool against theCIRP Annals - Manufacturing Technology 59 (2010) 259262A R T I C L EI N F OKeywords:Micro-machiningSiliconOTS-SAMA B S T R A C TThe feasibility of using a dual coating system consisting of SiO2and OTS-SAM thin films on the micro-machining characteristics of silicon wafer were investigated with the aim to eliminate the formation ofundesirable hillocks. The outermost OTS-SAM coating was used as a sacrificial layer to pattern the SiO2film, which in turn served to pattern the silicon substrate. After selectively removing the OTS-SAMcoating by micro-machining, HF and KOH chemical etching processes followed to remove the SiO2layerand create patterns on the silicon substrate. By this process, groove patterns of about 1mm width couldbe successfully fabricated on a silicon wafer without the formation of undesirable hillocks.? 2010 CIRP.* Corresponding author.Contents lists available at ScienceDirectCIRP Annals - Manufacturing Technologyjournal homepage: /cirp/default.asp0007-8506/$ see front matter ? 2010 CIRP.doi:10.1016/j.cirp.2010.03.084workpiecesurfacefollowedbyascribingmotiontoremovetheresistlayerthatwascoatedontheworkpiece.Thetoolwasmountedattheend of a flexible stainless steel cantilever so that the applied normalloadduringmachiningwouldnotbesensitivetosurfacewavinessaswellashorizontalmisalignmentoftheworkpiece.Themotionofthetool was controlled by a PC-based system using a set of threeprecision linear actuators. The path of the tool could be alteredreadily by programming the tool path in C language. In order tomonitor the state of mechanical machining, the cutting or frictionalforce was measured during the machining process using a forcesensor. Machining was performed in a clean room environment at25 8C and relative humidity in the range of 2530%.After mechanical machining, chemical etching was performedby using appropriate etchants to remove the unprotected regionswhere the resist had been removed. 10 mol KOH solution and 1%buffered HF was used as the etchant for silicon and SiO2,respectively. Silicon (1 0 0) wafer was used as the workpiecespecimen. The specimens were cleaned in ethanol and acetone,followed by a treatment in distilled water. In order to selectivelyetch the silicon surface to fabricate patterns, 10 nm thickness SiO2grown by dry oxidation was deposited as a resist layer. For coatingOTS-SAM, the specimen was first cleaned and then immersed in1 mM OTS-hexadecane solution for 24 h.3. Experimental results3.1. Result of the micro-machining of siliconIn order to verify the formation of hillocks in MCSPL process ofpattern size in 1mm range, a 10 nm thick SiO2film deposited bydry oxidation was used as the only resist layer in the initial set ofexperiments. Also, normal loads of less than 10 mN were used asthe applied load during the mechanical machining process.Fig. 1(a) shows the AFM image of the machined surface beforechemical etching. Each track shown in the image was machinedunder a different applied load. It was clear that the SiO2resist layerhad been penetrated beyond its thickness. Fig. 1(b) shows the AFMimage of the same area that is shown in Fig. 1(a) after etching in10 molKOHsolutionat50 8Cfor30 s.Itwasinterestingtonotethatupon etching the initially grooved machined region was trans-formed to protruding hillock patterns. The width of hillock topswas similar to the width of the machined grooves. The height ofhillocks was in the range of 200250 nm for 30 s of etching and itincreased to 280350 nm after 45 s of etching. However, foretching times longer than 1 min it was found that the height ofhillocks decreased. This was probably due to the under cuttingphenomenon caused by prolonged etching.The formation of hillocks shown in Fig. 1(b) was due to the factthat the area surrounding the mechanically machined region gotetched by the KOH etchant faster than the machined region. Thissuggested that the machined region effectively served as anegative resist that was more etch resistant than the SiO2layer,causing the surrounding areas to be etched away to form aprotruded line pattern. Even though SiO2layer, which is known tobe a fairly good resist material for KOH etchant 19, existed in thesurrounding areas, it did not properly perform the function as aresist since it was too thin. In other words, hillocks were formed asa result of different etching resistant properties of the mechanicalmachined region and the surrounding area that was coated withSiO2. It was postulated that the frictional interaction between thetool tip and the workpiece caused the silicon substrate to betransformed to an amorphous silicon that was resistant to the KOHetchant.It has been previously reported that silicon phase transforma-tion can occur as a result of pressure application which can alsolead to a change in density. Also, phase transformation in silicondue to residual stress and dislocation generation under loadingcondition has been reported 20,21. Thus, both amorphous (a-Si)and grains of crystalline silicon (c-Si) can be formed due topressure applied to crystalline silicon. Hence, the mechanism ofhillock formation during MCSPL process can be explained by thetransformation of the crystalline silicon phase to an inherentlysuperior etch resistant structure during the mechanical machiningprocess 22,23. Thus, upon chemical etching the machined areagets etched at a much slower rate resulting in silicon protrusionalong the machined lines in the form of protruded hillocks.3.2. Effects of OTS-SAM coated silicon for patterning groovesIn another set of experiments, the method to prevent hillockformation during MCSPL process applied to silicon workpiece wasinvestigated. As the modified MCSPL process to eliminate thehillock formation problem, a layer of OTS-SAM was exploited as asecondary resist layer in addition to the SiO2primary resist layer toprevent the formation of hillocks during the MCSPL process. Themost significant advantage of OTS-SAM, which is commonlyutilized as an anti-stiction coating for MEMS application, is its highchemical stability with ultra-thin thickness. Even though it hasonly23 nmthickness,OTS-SAMcanmaintainitsstructureinbothacid andalkalinesolutions suchasHCl and NaOHfor afew minutesFig. 1. AFM images and cross-section profiles of the silicon specimen with 10 nm thick SiO2resist layer (a) before chemical etching and (b) after chemical etching. The appliedload (mN) for each track is indicated below the AFM image.T.-S. Oh et al./CIRP Annals - Manufacturing Technology 59 (2010) 25926226024. Furthermore, due to its spring-like molecular structure OTS-SAM can sustain normal contact pressures up to 1 GPa 25.The specimens were ultrasonically cleaned in ethanol andacetone, followed by rinsing in distilled water. After treatment byhydrogen peroxide (H2O2) for surface hydration, OTS-SAM wascoated on the surface by immersing the workpieces in 10 mol OTS-toluene solution for 10 h. Mechanical machining was performedwith applied normal loads of 0.50 and 1.00 mN to remove the OTS-SAM and part of the SiO2primary resist layer without scratchingthe silicon substrate. Following the mechanical machining processthe workpiece was first etched in 1% buffered HF solution for 10 sto remove the remaining SiO2layer. Then, it was etched in 10 molKOH solution for 3 min at 50 8C to pattern the silicon substratewhere the SiO2had been completely removed by the previousetching process. It is know that SiO2can be etched in HF but not soreadily in KOH solution, and vice versa for silicon 26. Also, OTS-SAMisresistanttobothHFandKOHetchantsuptoseveralminutesunder controlled etching conditions.Fig. 2 shows the silicon pattern fabricated by the modifiedMCSPL process that exploited OTS-SAM as the secondary resistlayer. Fig. 2(a) shows the surface of silicon workpiece coated withOTS-SAM and SiO2after mechanical machining and beforechemical etching. It can be seen that the machined lines on theOTS surface have depth of less than 10 nm. Thus, it is most likelythatthetipdidnotpenetratetheSiO2resistlayer.However,eveninthe case where the tip did penetrate the resist layer, themechanical interaction between the tip and the silicon substratewas weak enough not to cause hillock formation. In Fig. 2(b) and(c),itisclearthatafterKOHetching,groovepatternsofwidthinthe1mm range could be successfully fabricated without any indica-tion of hillock formation. Table 1 shows the resulting width anddepth of the groove patterns obtained from the two differentapplied normal loads.Hillock formation could be prevented because direct contactbetween the tool and the silicon substrate could be avoided duringthemechanicalmachiningprocess.Thatis, theabrasive interactionwas limited to within the SiO2primary resist layer as intended.When HF etchant was used to remove the SiO2resist layer toexpose the silicon substrate, the OTS-SAM layer served as a resistlayer against the HF etchant and successfully protected thesurrounding area from being etched. Furthermore, when KOHetchant was used to pattern the silicon substrate, the OTS-SAMtogether with the SiO2resist protected the remaining area leadingto successful fabrication of the groove pattern on silicon.3.3. Machining application using the modified MCSPL processIn order to demonstrate the merits of the modified MCSPLprocess, the number 123 with different line widths of 1.0 and2.5mm were fabricated on a silicon workpiece as shown in Fig. 3.The numbers could be patterned by programming the path of thetool.Also,bycontrollingtheappliednormalloadduringmechanical machining, the width of the pattern could be variedat desired locations. The relationship between the applied normalload and the resulting pattern width was established in a separateset of experiments. The relationship was quite nonlinear and wasdifficult to predict theoretically. Nevertheless, a fairly consistentpattern width could be obtained under a given applied load. For1.0mm width a normal load of 1.00 mN was applied as before. For2.5mm width a normal load of 3.00 mN was applied. As shown inthe image, the junction of the number3 where the direction of thetool was changed by 908 and the applied load was increased from1.00to 3.00 mN to fabricatea thicker linewidth is quiteintact. ThisFig. 2. AFM and optical images of groove patterns fabricated by using the modified MCSPL process for (a) after mechanical machining and before etching, (b) 0.50 mN and (c)1.00 mN applied normal load after etching.Table 1Width and depth profiles of the groove pattern shown in Fig. 2.Applied load (mN)Width (nm)Depth (nm)(b) 0.50800450(c) 1.001050550T.-S. Oh et al./CIRP Annals - Manufacturing Technology 59 (2010) 259262261demonstrates the ability of the modified MCSPL process tofabricate various groove patterns of about 1mm width on asilicon surface through tool path programming and normal loadcontrol.4. ConclusionWhen normal loads of 0.5 mN or higher were used in theconventional MCSPL process to fabricate groove patterns withwidth in the range of 1mm on a silicon workpiece, undesirablehillocks were readily formed. The mechanism hillock formationwas attributed to direct contact between the tool and the siliconsubstrate which caused the contacted to region to be transformedinto to a silicon phase that was highly resistant to KOH etchant. Inorder to fabricate groove patterns in the range of 1mm widthwithout hillock formation, the MCSPL process was modified toinclude an OTS-SAM coating layer, which served as the secondaryresist. By doing so direct contact between the tool and the siliconsubstrate could be prevented, thus avoiding the phase transforma-tion of silicon and formation of hillocks. Through the modifiedMCSPL process, groove patterns of 0.8 and 1.0mm width and about0.5 aspect ratio could be fabricated successfully on a siliconworkpiece with an applied load of 0.50 and 1.00 mN, respectively.Furthermore, the flexibility of the process was demonstrated withthe fabrication of 123 groove pattern on silicon with varyingwidths. The desired pattern and width could be accomplished byprogramming the tool path and controlling the applied normalload, respectively, during the mechanical machining process.References1 Chun DM, Kim MH, Lee JC, Ahn SH (2008) A Nanoparticle Deposition Systemfor Ceramic and Metal Coating at Room Temperature and Low VacuumConditions. International Journal of Precision Engineering & Manufacturing9(1):5153.2 Shin W, Park S, Kim H, Joo S, Jeong H (2009) Local/Global Planarization ofPolysilicon Micropatterns by Selectivity Controlled CMP. International Journalof Precision Engineering & Manufacturing 10(3):3136.3 Lee ES, Hwang SC, Lee JT, Won JK (2009) A Study on the Characteristic ofParameters by the Response Surface Method in Final Wafer Polishing. Inter-national Journal of Precision Engineering & Manufacturing 10(3):2530.4 Cheng X, Nakamoto K, Sugai M, Matsumoto S, Wang ZG, Yamazaki K (2008)Development of Ultra-precision Machining System with Unique Wire EDMTool Fabrication System for Micro/nano-machining. CIRP Annals-Manufactur-ing Technology 57(1):415420.5 Sung IH, Kim DE (2005) Nano-scale Patterning by Mechano-chemical ScanningProbe Lithography. Applied Surface Science 239(2):209221.6 Sung IH, Lee HS, Kim DE (2003) Effect of Surface Topography on the FrictionalBehavior at the Micro/nano-scale. Wear 254(10):10191031.7 Sung IH, Kim DE (2007) Study on Nanoscale Abrasive Interaction BetweenNanoprobe and Self-assembled Molecular Surface for Probe-based Nanolitho-graphy Process. Ultramicroscopy 107(1):17.8 Lee JM, Sung IH, Kim DE (2002) Process Development of Precision SurfaceMicro-machining using Mechanical Abrasion and Chemical Etching. Micro-system Technologies 8(6):419426.9 Kim HJ, Kim DE (2009) Nano-scale Friction: A Review. International Journal ofPrecision Engineering & Manufacturing 10(2):141151.10 Rathinam M, Thillaigovindan R, Paramasivan P (2009) Nanoindentation ofAluminum (1 0 0) at Various Temperatures. Journal of Mechanical Science &Technology 23(10):26522657.11 Sung IH, Yang JC, Kim DE, Shin BS (2003) Micro/nano-tribological Character-istics of Self-assembled Monolayer and its Application in Nano-structureFabrication. Wear 255(712):808818.12 Sung IH, Kim DE (2003) Fabrication of Micro/Nano-patterns using MC-SPL(Mechano-Chemical Scanning Probe Lithography) Process. International Jour-nal of KSPE 4(5):2226.13 Kwon EY, Kim YT, Kim DE (2009) Investigation of Penetration Force of LivingCell Using an Atomic Force Microscope. Journal of Mechanical Science &Technology 23(7):19321938.14 Lee JM, Jin WH, Kim DE (2001) Application of Single Asperity Abrasion Processfor Surface Micromachining. Wear 251(112):11331143.15 Chung KH, Lee YH, Kim DE (2005) Characteristics of Fracture During theApproach Process and Wear Mechanism of a Sili
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