12.008-spring-20032.008 Design & Manufacturing IISpring 2004MEMS, Tiny Products2.008-spring-2003 C. Wong & S. KimTunable microcavity waveguide fabrication process flowCrSi SiO2Pt/TiPZT1. RIE Si trench 2. Pattern Pt bottom electrode3. Pattern PZT and anneal4. Pattern Pt top electrodefront-viewside-view (thru’ waveguide)airsubstrateSiO2 deformable membraneintegrated tri-la yer piezoelectric microactuator1D photonic crystal microcavitySi waveguide2.008-spring-2003Tunable microcavity waveguide1,2fabrication process flow5. RIE SiO26. X-ray lithography of PMMA for Cr mask7. CF4Si waveguide RIE8. XeF2isotropic release etchCrSi SiO2Pt/TiPZTfront-viewside-view (thru’ waveguide)2.008-spring-2003Process FlowDepositionLithography EtchWafers DevicesPhoto resist coatingPattern transferPhoto resist removalOxidationSputteringEvaporationCVDSol-gelEpitaxyWet isotropicWet anisotropicPlasmaRIEDRIE2.008-spring-2003Wet or dry? Low resolution feature size  Low cost Undercut for isotropic Wider area needed for anisotropic wafer etch Sticking High resolution feature size Expensive Vertical side wall Avoid stickingWetDry2.008-spring-2003Etching Issues - AnisotropyIsotropicAn-isotropicmask-Structural layer-Sacrificial layer22.008-spring-2003Etching Issues - Selectivity Selectivity is the ratio of the etch rate of the target materialbeing etched to the etch rate of other materials  Chemical etches are generally more selective than plasma etches Selectivity to masking material and to etch-stop is importantMasktargetEtch stop2.008-spring-2003Bulk Micromachining KOH etches silicon substrate V-grooves, trenches Concave stop, convex undercut (100) to (111) Æ 100 to 1 etch rate Masks: SiO2: for short period SixNy: Excellent heavily doped P++silicon: etch stop<111><100>Silicon Substrate54.7a0.707a2.008-spring-2003Dry etching RIE (reactive ion etching) Chemical & physical etching by RF excited reactive ions Bombardment of accelerated ions, anisotropic SF6Æ Si, CHF3Æ oxide and polymers Anisotropy, selectivity, etch rate, surface roughness by gas concentration, pressure, RF power, temperature control Plasma etching Purely chemical etching by reactive ions, isotropic Vapor phase etching Use of reactive gases, XeF2 No drying neededsticktion2.008-spring-2003DRIE (Deep RIE) Alternating RIE and polymer deposition process for side wall protection and removal Etching phase: SF6 /Ar Polymerization process: CHF3/Ar forms Teflon-like layer Only 9 years after the Bosch process patent, 1994-1.5 to 4 µm/min-selectivity to PR 100 to 12.008-spring-2003Deposition processes Chemical CVD(Chemical Vapor Deposition) Thermal Oxidation Epitaxy Electrodeposition Physical PVD Evaporation Sputtering Casting2.008-spring-2003Thermal Oxidation Silicon is consumed as the silicon dioxide is grown. Growth occurs in oxygen and/or steam at 800-1200 C. Compressive stress  ~2um films are maximum practically.  Simple, easy process for electrical insulation, intentional warpage, etc.SiliconO2SiliconSiO232.008-spring-2003Thermal Oxidation Oxidation can be masked with silicon nitride, which prevents O2diffusionSilicon nitrideSiliconSiO22.008-spring-2003Chemical Vapor Deposition Thermal energy to dissociate gases and deposit thin films on surfaces, high productivity, better step coverage low pressure (LPCVD), atmospheric pressure (APCVD), plasma enhanced (PECVD), horizontal, vertical LPCVD pressures around 300mT (0.05% atmosphere) Moderate Temperatures 450oC SiO2 : PSG, LTO 580-650oC polysilicon 800oC SixNy–SiH4 + NH3 Very dangerous gases Silane: SiH4 Arsine, phosphine, diborane: AsH3, PH3, B2H6>252.008-spring-2003Physical Vapor Deposition Evaporated metals in a tungsten crucible Aluminum, gold, Pt, W Evaporated metals and dielectrics by electron-beam or resistance heating Typically line-of-sight deposition Very high-vacuum required to prevent oxidation, load lockEvaporationE-beam evaporatorShadowing2.008-spring-2003Physical Vapor Deposition –Sputtering Sputtered metals and dielectrics Argon ions bombards target Ejected material takes ballistic path to wafers Typically line-of-sight from a distributed source Requires high vacuum, but low temperatureEvaporation vs. SputteringRF sputter2.008-spring-2003Spin Casting Viscous liquid is poured on center of wafer Wafer spins at 1,000-5,000 RPM for ~30s Baked on hotplates 80-500oC for 10-1000s Application of etchants and solvents, rinsing Deposition of polymers, sol-gel PZT dispenservacuum chuckPRwaferωtslowcoatspindownlevel out2.008-spring-2003Deposition Issues - Compatibility Thermal compatibility Thermal oxidation and LPCVD films  Thermal oxidation and LPCVD vs. polymers (melting/burning) and most metals (eutectic formation, diffusion) Topographic compatibilitiy Spin-casting over large step heights Deposition over deep trenches-key hole42.008-spring-2003Deposition Issues - Conformality A conformal coating covers all surfaces to a uniform depth A non-conformal coating deposits more on top surfaces than bottom and/or side surfacesConformalNon-conformal Non-conformal2.008-spring-2003 M. Koo & S. KimPhoto 1 . Cracking of sol-geldeposited PZT after 650C firingPhoto 2. Poor step coverage and high stress evolved at corner of a step2.008-spring-2003Lithography (Greek, “stone-writing”) Pattern Transfer Appication of photosensitive PR Optical exposure to transfer image from mask to PR Remove PR -Æ binary pattern transfer2.008-spring-2003Photo resist Spin coat phto-resist 3000 – 6000 rpm, 15-30 sec Viscosity and rpm determine thickness Soft bake ->alignment ->exposure Develop PR after exposure Hardbake Positive Negativedispenservacuum chuckPRwaferωtslowcoatspindownlevel out2.008-spring-2003Photomasks Master patterns to be transferred Types: Photographic emulsion on soda lime glass (cheap) Fe2O3or Cr on soda lime glass Cr on quartz (expensive, for deep UV light source) Polarity Light field: mostly clear, opaque feature Dark field: mostly opaque, clear feature2.0082.0082.008-spring-2003Types of AlignerContact Proximity ProjectionReduction ratio 1:1Array of the same pattern Æ Stepper5IC manufacturingThis picture comes from an excellent introductory discussion about IC fabrication at: