MEMS-specific fabricationBulk, Surface, DRIEBulk MicromachiningKOH EtchingCrystal Planes & Miller IndicesMiller indicesPowerPoint PresentationSlide 8Slide 9Typical 100 waferSlide 11RosetteSlide 13Anisotropic Etching of SiliconEtch stops in anisotropic silicon etchingMicromachining Ink Jet NozzlesSlide 17KOH etching: atomic viewBulk micromachined cavitiesClever KOH etching of (100)Surface MicromachiningSurface micromachining material systemsSlide 23Slide 24HingesDeep Reactive Ion EtchScalloping and Footing issues of DRIETypical simple SOI-MEMS ProcessDRIE structuresSlide 30Slide 31Slide 32Slide 33Slide 34Slide 35Sub-Micron Stereo LithographySlide 37Combining Microstereolithography and Thick Resist UV LithographySlide 39Slide 40ksjp, 7/01MEMS Design & FabMEMS-specific fabrication•Bulk micromachining•Surface micromachining•Deep reactive ion etching (DRIE)•Other materials/processesksjp, 7/01MEMS Design & FabBulk, Surface, DRIE•Bulk micromachining involves removing material from the silicon wafer itself•Typically wet etched•Traditional MEMS industry•Artistic design, inexpensive equipment•Issues with IC compatibility•Surface micromachining leaves the wafer untouched, but adds/removes additional layers above the wafer surface, First widely used in 1990s•Typically plasma etched•IC-like design philosophy, relatively expensive equipment•Different issues with IC compatibility•Deep Reactive Ion Etch (DRIE) removes substrate but looks like surface micromachining!ksjp, 7/01MEMS Design & FabBulk Micromachining•Many liquid etchants demonstrate dramatic etch rate differences in different crystal directions•<111> etch rate is slowest, <100> and <110> fastest•Fastest:slowest can be more than 400:1•KOH, EDP, TMAH most common anisotropic silicon etchants•Isotropic silicon etchants•HNA•HF, nitric, and acetic acids•Lots of neat features, tough to work with•XeF2, BrF3 •gas phase, gentle•Xactix, STS selling research & production equipmentksjp, 7/01MEMS Design & FabKOH Etching•Etches PR and Aluminum instantly•Masks:•SiO2•compressive•SixNy•tensile•Parylene!•Au?ksjp, 7/01MEMS Design & FabCrystal Planes & Miller Indices•[abc] in a cubic crystal is just a direction vector•(abc) is any plane perpendicular to the [abc] vector•(…)/[…] indicate a specific plane/direction•{…}/<…> indicate equivalent planes/directionAngles between directions can be determined by scalar product: the angle between [abc] and [xyz] is given by ax+by+cz = |(a,b,c)|*|(x,y,z)|*cos(theta)e.g.:))3)(1/()001((1)111(),100(Cosksjp, 7/01MEMS Design & FabMiller indices[100][010][001]abc[abc]ksjp, 7/01MEMS Design & Fab[100][010][001]abc[abc]1/a1/b1/c(abc)ksjp, 7/01MEMS Design & Fab[100][010][001](100){100}(001)(010)ksjp, 7/01MEMS Design & Fab[100][010][001](110)(111)ksjp, 7/01MEMS Design & FabTypical 100 waferCross-section in (110) plane<111><100>The wafer flat is oriented in the [110] directionksjp, 7/01MEMS Design & Fab(111)(111)(111)(110)<111><100>ksjp, 7/01MEMS Design & FabRosetteUn-etched siliconLateral undercut“Amplified” etch rateMasking layerksjp, 7/01MEMS Design & Fabksjp, 7/01MEMS Design & FabAnisotropic Etching of Silicon•Anisotropic etches have direction dependent etch rates in crystals •Typically the etch rates are slower perpendicularly to the crystalline planes with the highest density•Commonly used anisotropic etches in silicon include Potasium Hydroxide (KOH), Tetramethyl Ammonium Hydroxide (TmAH), and Ethylene Diamine Pyrochatecol (EDP) <111><100>Silicon Substrate54.7ksjp, 7/01MEMS Design & FabEtch stops in anisotropic silicon etching•Electrochemical etch stop•High boron doping (~1e20/cm)ksjp, 7/01MEMS Design & FabMicromachining Ink Jet NozzlesMicrotechnology group, TU Berlinksjp, 7/01MEMS Design & FabBulk Micromachining•Anisotropic etching allows very precise machining of silicon•Silicon also exhibit a strong piezoresistive effect•These properties, combined with silicon’s exceptional mechanical characteristics, and well-developed manufacturing base, make silicon the ideal material for precision sensors •Pressure sensors and accelerometers were the first to be developed Silicon pressure sensor chipPackaged pressure sensorksjp, 7/01MEMS Design & FabKOH etching: atomic viewSTM image of a (111) face with a ~10 atom step. From Weisendanger, et al., Scanning tunnelling microscopy study of Si(111)7*7 in the presence of multiple-step edges, Europhysics Letters, 12, 57 (1990).ksjp, 7/01MEMS Design & FabBulk micromachined cavities•Anisotropic KOH etch (Upperleft)•Isotropic plasma etch (upper right)•Isotropic BrF3 etch with compressive oxide still showing (lower right)ksjp, 7/01MEMS Design & Fab(110)(111)(100)Clever KOH etching of (100)(110)(111)(110)(110)(111)Clockwise from above: Ternez; Rosengren; Kellerksjp, 7/01MEMS Design & FabSurface MicromachiningDeposit sacrificial layerPattern contactsDeposit/pattern structural layer Etch sacrificial layerksjp, 7/01MEMS Design & FabSurface micromachining material systems•Structure/ sacrificial/ etchant•Polysilicon/ Silicon dioxide/ HF•Silicon dioxide/ polysilicon/ XeF2•Aluminum/ photoresist/ oxygen plasma•Photoresist/ aluminum/ Al etch•Aluminum/ SCS EDP, TMAH, XeF2•Poly-SiGe poly-SiGe DI waterksjp, 7/01MEMS Design & FabResidual stress gradientsMore tensile on topMore compressive on topJust right! The bottom line: anneal poly between oxides with similar phosphorous content. ~1000C for ~60 seconds is enough.ksjp, 7/01MEMS Design & FabResidual stress gradientsA bad day at MCNC (1996).ksjp, 7/01MEMS Design & FabHingesDeposit first sacrificialDeposit and pattern first polyEtch sacrificialPattern contactsDeposit and pattern 2nd polyDeposit and pattern second sacrificialksjp, 7/01MEMS Design & FabDeep Reactive Ion EtchBOSCH PatentSTS, Alcatel, Trion, Oxford Instruments …Uses high density plasma to alternatively etch silicon and deposit a etch-resistant polymer on side wallsPolymer depositionSilicon etch using SF6 chemistryPolymerUnconstrained geometry90° side wallsHigh aspect ratio 1:30Easily masked (PR, SiO2)Process recipe depends ongeometryksjp, 7/01MEMS Design & Fab1 µmScalloping and Footing issues of DRIEScalloped sidewallTop wafer surfacecathodeTop wafer surfaceanodeTip precursorsScalloped sidewallTop wafer surfacecathodeTop wafer surfaceanodeTip
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