1Surface Micromachining: Techniques and MaterialsDr. Thara SrinivasanLecture 3Picture credit: Texas Instruments2Reading and Project Info.• Reading• Senturia, sections from Chapter 3. • From reader• Bustillo et al., “Surface Micromachining of MicroelectromechanicalSystems,” pp. 1552-6, 1559-63.• Williams, “Etch Rates for Micromachining Processing,” pp. 256-69.• Print out lecture notes before lecture• http://www-bsac.eecs.berkeley.edu/projects/ee245/index.htm• For the project later on in the semester, you will…• Pick a fabrication method and materials for your MEMS device• Explain fabrication decisions23Bulk Micromachining Review• Definition: Etching pits, trenches or all the way through the silicon wafer to make mechanical structures.• Methods• Wet etching• Dry etching (plasma or vapor)• Etching modes• Isotropic: etches all crystal directions equally• Anisotropic: etches certain directions faster than others• Diffusion (or transport) -limited vs. Reaction-limited4Bulk Micromachining Review• Etch stops for wet etching• Plasma etching• Etching species: ions and neutrals (radicals)• “Plasma” etching: neutrals• Reactive ion etching: ions and neutrals• Deep reactive ion etch (DRIE)• Vertical, deep trenches• Alternate between etch step and protective Teflon deposition step35Bulk Micromachining Review• Bulk etching and metallization• Possible structures6Lecture Outline• Today’s Lecture• Introduction• Thin film deposition • Thin film etching techniques• Material combinations• Lateral resonator process flow47Surface Micromachining8Poly-SiSi substrateDeposit & pattern polyOxideSi substrateDeposit & pattern oxide10 µmCantileverAnchorSi substrateSacrificial etch. This step “releases” the caMicromachining a Cantilever59One Structural Poly, One Oxide ProcessLateral resonator with electrostatic comb drives, Sandia Labs10Surface Micromachining• Basic process sequence• Structural layer• Sacrificial layer • Release etchMeshing gears on a moveable platform, SandiaDigital Micromirror Device, Texas Instruments611History• History of surface micromachining• 1984: Howe and Muller used polysilicon and oxide to make beam resonator as gas sensor• 1988: Pin joints, springs, gears, rotary electrostatic side drive motors (Fan, Tai, Muller)• 1989: Lateral comb drive (Tang, Nguyen, Howe)12History• History of surface micromachining• 1991: Polysilicon hinge (Pister, Judy, Burgett, Fearing)• 1992: MCNC starts MUMPS (a MEMS foundry)• Early 1990’s: First surface micromachined accelerometer sold (Analog Devices, ADXL50)StaplePolysilicon level 2Polysilicon level 1Silicon substratePolysilicon level 1Polysilicon level 2Hinge staplePlateSilicon substrateSupport armProf. Kris PisterAnalog DevicesIntegrated accelerometer713Polysilicon Mechanical Properties• Mechanical properties of polycrystalline silicon (polysilicon or “poly”) • Stronger than stainless steel: fracture strength of poly ~ 2-3 GPa, steel ~ 0.2GPa-1GPa• Young’s Modulus ~ 140-190 GPa• Extremely flexible: maximum strain before fracture ~ 0.5%• Does not fatigue readily• Compatible with IC fabrication processes, process parameters well-known14C. T.-C. Nguyen and R. T. Howe, IEEE IEDM, 1993• Electrostatic force is applied by a drive comb to a suspended shuttle• Motion is detectedcapacitively by a sense comb • Operated at resonant frequency Lateral Resonator 815Today’s Lecture• Introduction • Thin film deposition techniques• This film etching techniques • Material combinations• Lateral resonator process flow16Thin Film Deposition• Chemical Vapor Deposition • Polysilicon• Silicon nitride• Silicon dioxides• Thermal oxidation• Silicon dioxide• Physical Vapor Deposition• Evaporation of metals• Sputtering of metals, dielectrics917Mean Free Path• Definition: distance a molecule travels before hitting another molecule• Mean free path for gases• Atmospheric 760 torr →λ= 40 nm• Low vacuum 0.76 torr →λ= 40 µm• Medium vacuum 7.6 mtorr →λ= 4 mm• High vacuum 7.6 µtorr →λ= 4 m760 Torr = 101 kPa = 1 atm1 mTorr = 0.13 Pa = 1.3×10-6atmPMRTηπλ2=18Chemical Vapor Deposition• Gases react at hot wafer surface to create solid films• Materials: polysilicon, silicon nitride, phosphosilicate glass (PSG), low temperature oxide (LTO)• Parameters: T, P, gas flowratesJensen1019Chemical Vapor DepositionPressure Energy SourceAPCVD 100-760 torr 350-400°C Transport-limitedLPCVD 100-500 mtorr 500-800°C Reaction-limitedPECVD 2-5 torr plasma + Reaction-limited300-400°CAPCVD and PECVD furnaces20Chemical Vapor DepositionLPCVD furnacepumpgate valveN2SiH4Si2H6B2H6PH3mass flowcontrollermass flowcontrollerGeH4injector boats wafers bafflescantilevertubeexhaustdoor1121CVD and Film Conformality• Film coverage• Low pressure = long mean free path • Molecules hit surface with extra energy to migrate • Conformal coverage, case (a)• If there is no surface migration, • Coverage depends on range of arrival angles, case (c)a)b)c)22LPCVD PolysiliconDeposited at 590°C, 5×5 µm2, rms ~ 5 nm• Undoped polycrystalline silicon (poly)• Uses: IC layers, interconnect, MEMS structural material• Pyrolysis of silane, SiH4, and dichlorosilane, SiH2Cl2• T = 550-700°C, P = 100’s mTorr• Deposition rate: 10 nm/min at 630°C, 70 nm/min at 700°C (undoped)• Conformal coverage (aspect ratio < 10)• Large stresses (500 MPa) and stress gradients 1223• Doping “in situ”• n-type: 1 vol % phosphine (PH3), p-type: diborane (B2H6) • [ ] = 1020cm-3, R = 1-10 mΩ/cm• n-type, dep. rate ↓• p-type dep. rate ↑• Large stresses ~ 500 MPa• Diffusion• Use PSG layers• 900-1000°C, hours• Heavy doping possible, R = 0.1 mΩ/cm• Ion implantation• As-deposited doped films also have high stress (500 MPa)Doped LPCVD PolysiliconDeposited at 590°C, 5×5 µm2, rms ~ 12 nm24Residual Stress in Thin Films• Residual film stress• Microstructure• Thermal mismatch• Compressive vs. tensile stressUnder compressive stress, film wants to expand.Constrained to substrate, bends it in convex way.Under tensile stress, film wants to shrinkConstrained to substrate, bends it in concave way.1325Stress Gradients• Stress gradient: (+) or (-)compressivetensile+–26Stress in Polysilicon Films• Stress depends on crystal structure:• ≤ 600°C ~ films are initially amorphous, then crystallize•
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