Fabrication of Micro Gyroscope on the SOI Substratewith Enhanced Sensitivity for Detecting Vertical MotionSeung-Jae MoonDepartment of Mechanical Engineering, University of California, Berkeley, CA 94720ABSTRACTReactive ion etching lag is used to fabricatecapacitance sensor to detect vertical movement in agyroscope. Aspect ratio of each trenches arecontrolled by inlet opening width. In comparison topresent gap closing-type sensors, the sensorstructure fabricated with RIE lag on silicon oninsulator wafer shows bigger sensitivity.INTRODUCTIONMicromachined gyroscopes for measuringthe rate and/or angle of rotation have found manyapplication areas such as automotive safety andstability control systems, video camera stabilization,and 3-D input devices for computers and personaldata assistance (PDA) systems [1]. Those wideapplications require a smaller and cheaper gyroscopeinstead of bulky and expensive gyroscopes such asrotating wheel gyroscopes, fiber optic gyroscopesand laser gyroscopes.Microsensors that consist of mechanical andelectrical elements are commonly referred to asmicromechanical systems (MEMS). MEMS sensorspresent the advantages of being light in weight,being small in size, having low power consumptionand low cost, due to standard IC fabricationtechniques. With these benefits, research in MEMSgyroscopes has been accelerated and theperformance of MEMS gyroscopes has beenimproved. Most MEMS gyroscopes are silicon-based vibratory sensors, which utilize the energytransfer between two vibrating modes of amechanical structure[2]. The principle most-oftenexploited in building micromachined gyroscopes isthe Coriolis force, expressed by theυ×Ω,terminthe equation of motion. One effective way to usethis force is to create resonant motion of fixedamplitude in a direction perpendicular to the axis ofrotation. The Coriolis force then induces motion inthe third direction perpendicular both to thedirectionofrotationandtothedrivenmotion[3].To sense this rotation in a MEMS device, thevibration of suspended mechanical structures, i.e.comb structure, beam, disk or ring structures isused. To fabricate comb structure on singlecrystalline silicon (SCS) wafer, bulk (TMAH, orKOH etching) and surface micromachining(Reactive ion etching) is used. Instead of usingdeposited poly silicon (p-Si) as structure material,SCS wafers are used to obtain uniform propertiesand parameters of micromachined structures since ittakes several hours or more to deposit a 2- m thickfilm and the material properties of amorphous silicon(a-Si) and p-Si vary significantly. In this work, afabrication process that can increase sensitivity ofsensorsispresented.REACTIVE ION ETCHING (RIE)RIE process is a critical process in thefabrication to define structural shapes since micro-structures with larger height and smaller gap arepreferred because they enhance the deviceperformance significantly by increasing theelectrostatic force between elements. Typicaltrenches and structures in the range of 10-500 µminthe MEMS fabrication have been etched over theentire substrate and processes are becoming wellcontrolled. In the trench formation by RIE, etchrate is dependent on feature opening width. This iscalled RIE lag. Figure 1 shows etch rate depends onthe opening width. Therefore, trench depth can becontrolled by opening width defined by a lithographystep.Figure 1 Etch rate dependence on opening width byRIEAPPLICATION OF RIE TO MICRO-GYROSCOPE FABRICATION PROCESSSCS wafers are widely used in fabrication ofmicrogyroscope since stable and reproduciblemechanical properties provide enhanced productivityand reliability and it is easy to fabricate high-aspect-ratio microstructure for enhanced performance.However, gap closing type sensors shown in Figure2 have several disadvantages: Metal deposition forthe bottom electrodes, stiction problem and etc.Figure 3 shows process flow for SOI wafertechnology. As shown in Figure 3(d) those combstructures vibrate in a way of gap closing mode.Therefore reverse biased p-n junction can be used toisolate two electrodes. To increase the sensitivityfor vertical motion detection in a microgyroscope,following technology for fabricating vertical combstructure with RIE is provided. The depth oftrenches are different by utilizing RIE lag in Sietching process as shown in Figure 4(a). Aspresented in Figure 4(e), the gap between movingcantilever beams and an oxide layer increasescompared to the microgyroscope sensor structuresshowninFigure2&3. Therefore,thestictionproblem can be overcome. Since the biggerdeflection can be allowed in the moving cantileverbeam, detection area can be enlarged. Moreover, incomparison to the micro comb structure shown inFigure 3 shown, by enlarging the surface area andincreasing numbers of capacitors, the sensitivity ofsensor can increase.Figure 2 Microgyroscope fabricated on SOI or SCSwafersSCSMetal electrodeSCSGlassSiSiPR(a) PhotolithographySiSiOxidePR(b) Deep Si etchingSiOxide(c) Sacrificial oxide etching(HF wet etching)SiSiOxide(d)Capwaferbonding(Anodic bonding)GlassSi1.52.02.50 5 10 15 20Etch rate (µm/min)Opening width (µm)Figure 3 Process flow for SOI technologyFigure 4 Process flow chart of fabrication of verticalcomb structures on SOI wafer.EXPECED RESULTSFigure 5 presents simple model to comparethe capacitance increase calculation by applying RIElag technology. Vertically-placed parallel plates areshown in Figure 5(a) and vertical combs in Figure5(b). The thickness of a cantilever is 5 µmandagap between two electroplates is a minimum featuresize of lithography, 2 µm. Therefore, Ngis 60, i.e. 60capacitors. The deflection dz =1µminthe(a) Oxide patterning and deep Si etchingSiSi(b) Thermal oxidation for trenches(c) Deep Oxide and Si etching(d) Sacrificial etching (TMAH) of Si;oxide mask and passivation layer etching(e) Cap wafer bonding (Anodic bonding)GlassStationaryMovingestimation. Capacitance between 2 electroplates isas follows:dACoε= ,(1)where,εois permittivity in the air gap, A is area ofcapacitor and d is a gap between electroplates. TheCois calculated as follows:(a)pF464.0mµ4mµ210mµ1000F/m1085.812oo=××==−dACε,(b)pF66.260mµ2mµ10mµ1000F/m1085.812goo=××==−NdACεThe sensitivity of sensorsC∆with displacement ofdz =1µm:(a)fF92)(ooo=+=+−=∆dzddAdzdzdAdACεεε,(b)fF265)(ogogog=×=−×−×=∆ddzLNddzwLNdwLNCεεε.By simple calculation, the sensitivity of the sensorsmade with RIE lag technique is around 3 timeshigher. Figure 5 Comb structure
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