IC/MEMS Fabrication - OutlineFabricationMaterialsSilicon propertiesDiamond LatticePeriodic TableBand Structure of Electron EnergiesBand structure of SemiconductorsConductors, insulators, semiconductorsDoping semiconductorsDiodesSilicon wafer fabricationSilicon wafer fabrication – slicing and polishingProcess FlowDeposition Issues - CompatibilityDeposition Issues - ConformalityEtching Issues - AnisotropyEtching Issues - SelectivitySpin CastingPhysical Vapor Deposition - EvaporationPhysical Vapor Deposition - SputteringThermal OxidationSlide 23Chemical Vapor DepositionLPCVD SystemsWet etchingPlasma EtchingSlide 28Plasma EtchersPowerPoint PresentationLAVA/SAMPLE2D – www.eecs.berkeley.edu/~neureuthLine-of-sight depositionStep coverage problemsHemispherical source improves step coverageBut isn’t perfect - Keyhole effectEtching simulationSummaryksjp, 7/01MEMS Design & FabIC/MEMS Fabrication - Outline•Fabrication overview•Materials•Wafer fabrication•The Cycle:•Deposition•Lithography•Etchingksjp, 7/01MEMS Design & FabFabrication•IC Fabrication•Deposition•Spin Casting•PVD – physical vapor deposition•CVD – chemical vapor deposition•Lithography•Removal•Wet etching•Plasma etching•Bulk Micromachining•Surface Micromachining•MUMPS•DRIE – deep reactive ion etchksjp, 7/01MEMS Design & FabMaterials•Single crystal silicon – SCS•Anisotropic crystal•Semiconductor, great heat conductor•Polycrystalline silicon – polysilicon•Mostly isotropic material•Semiconductor•Silicon dioxide – SiO2 •Excellent thermal and electrical insulator•Thermal oxide, LTO, PSG: different names for different deposition conditions and methods•Silicon nitride – Si3N4•Excellent electrical insulator•Aluminum – Al•Metal – excellent thermal and electrical conductorksjp, 7/01MEMS Design & FabSilicon properties•Semiconductor•Electrical conductivity varies over ~8 orders of magnitude depending on impurity concentration (from ppb to ~1%)•N-type and P-type dopants both give linear conduction, but from fundamentally different mechanisms•N-type touching P-type forms a diode•Cubic crystal•Diamond lattice•Anisotropic mechanical propertiesksjp, 7/01MEMS Design & FabDiamond Latticeksjp, 7/01MEMS Design & FabPeriodic Tableksjp, 7/01MEMS Design & FabBand Structure of Electron Energies•http://www.physics.udel.edu/wwwusers/watson/scen103/99s/clas0416.html•Bohr atom, Pauli exclusion principle •(not exactly right, but gives some intuition)ksjp, 7/01MEMS Design & FabBand structure of Semiconductorsksjp, 7/01MEMS Design & FabConductors, insulators, semiconductorsksjp, 7/01MEMS Design & FabDoping semiconductors•Two different types of conduction•Electrons (negative, N-type)•Holes (positive, P-type)-o- - - - - -- - - - - -ksjp, 7/01MEMS Design & FabDiodesN PN PV+V-V-V+Tiny current~pACurrent exponentiallydependent on voltagepA to kA in ~1Vksjp, 7/01MEMS Design & FabSilicon wafer fabrication•Taken from www.egg.or.jp/MSIL/english/index-e.htmlksjp, 7/01MEMS Design & FabSilicon wafer fabrication – slicing and polishing•Taken from www.egg.or.jp/MSIL/english/index-e.htmlksjp, 7/01MEMS Design & FabProcess Flow•Integrated Circuits and MEMS identical•Process comlexity/yield related to # trips through central loopDeposition Lithography EtchWafersChipsksjp, 7/01MEMS Design & FabDeposition Issues - Compatibility•Thermal compatibility•Thermal oxidation and LPCVD films are mutually compatible•Thermal oxidation and LPCVD are not compatible with polymers (melting/burning) and most metals (eutectic formation, diffusion, furnace contamination)•Topographic compatibilitiy•Can not spin-cast over large step heights•Distributed-source deposition over deep trenches leaves keyholesksjp, 7/01MEMS Design & FabDeposition Issues - Conformality•A conformal coating covers all surfaces to a uniform depth•A planarizing coating tends to reduce the vertical step height of the cross-section•A non-conformal coating deposits more on top surfaces than bottom and/or side surfacesConformalPlanarizingNon-conformalksjp, 7/01MEMS Design & FabEtching Issues - Anisotropy•Isotropic etchants etch at the same rate in every directionIsotropicAn-isotropicmaskksjp, 7/01MEMS Design & FabEtching Issues - Selectivity•Selectivity is the ratio of the etch rate of the target material being 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 stopksjp, 7/01MEMS Design & FabSpin Casting•Viscous liquid is poured on center of wafer•Wafer spins at 1000-5000 RPM for ~30s•Baked on hotplates 80-500C for 10-1000s•Application of etchants and solvents, rinsing•Deposition of polymers, sol-gel precursorsksjp, 7/01MEMS Design & FabPhysical Vapor Deposition - Evaporation•Evaporated metals in a tungsten crucible•Aluminum, gold•Evaporated metals and dielectrics by electron-beam•Refractory metals (e.g. tungsten)•Dielectrices (e.g. SiO2)•Typically line-of-sight deposition•Very high-vacuum required to prevent oxidation of e.g. aluminumksjp, 7/01MEMS Design & FabPhysical Vapor Deposition - Sputtering•Sputtered metals and dielectrics•Argon plasma sputters material (small #s of atoms) off target•Ejected material takes ballistic path to wafers•Typically line-of-sight from a distributed source•Requires high vacuum depending on materialksjp, 7/01MEMS Design & FabThermal OxidationSilicon is consumed as the silicon dioxide is grown.Growth occurs in oxygen and/or steam at 800-1200 C. ~2um films are maximum practical SiliconO2SiliconSiO2ksjp, 7/01MEMS Design & FabThermal Oxidation•Oxidation can be masked with silicon nitride, which prevents O2 diffusionSiliconSiO2Silicon nitrideksjp, 7/01MEMS Design & FabChemical Vapor Deposition•Gases dissociate on surfaces at high temperature•Typically done at low pressure (LPCVD) rather than atmospheric (APCVD)•LPCVD pressures around 300mT (0.05% atmosphere)•Moderate Temperatures•450 SiO2•580-650 polysilicon•800 SixNy •Very dangerous gases•Silane: SiH4•Arsine, phosphine, diborane: AsH3, PH3, B2H6ksjp, 7/01MEMS Design & FabLPCVD Systemsksjp, 7/01MEMS Design & FabWet etching•Extremely selective•Typically isotropic•Not widely usedksjp, 7/01MEMS Design & FabPlasma Etchingksjp, 7/01MEMS Design & FabPlasma Etchingksjp, 7/01MEMS Design & FabPlasma Etchersksjp, 7/01MEMS
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