1ksjp, 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 fromthe 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 wafersurface, 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 substratebut looks like surface micromachining!2ksjp, 7/01MEMS Design & FabBulk Micromachining•Many liquid etchants demonstrate dramatic etch ratedifferences 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?3ksjp, 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 scalarproduct: the angle between [abc] and [xyz] is given byax+by+cz = |(a,b,c)|*|(x,y,z)|*cos(theta)e.g.:))3)(1/()001((1)111(),100(++=−Cosθksjp, 7/01MEMS Design & FabMiller indices[100][010][001]abc[abc]4ksjp, 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)5ksjp, 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] direction6ksjp, 7/01MEMS Design & Fab(111)(111)(111)(110)<111><100>ksjp, 7/01MEMS Design & FabRosetteUn-etched siliconLateral undercut“Amplified” etch rateMasking layer7ksjp, 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 crystallineplanes with the highest density• Commonly used anisotropic etches in silicon include PotasiumHydroxide (KOH), Tetramethyl Ammonium Hydroxide (TmAH), andEthylene Diamine Pyrochatecol (EDP)<111><100>Silicon Substrate54.78ksjp, 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 Berlin9ksjp, 7/01MEMS Design & FabBulk Micromachining• Anisotropic etching allowsvery precise machining ofsilicon• Silicon also exhibit a strongpiezoresistive effect• These properties, combinedwith silicon’s exceptionalmechanical characteristics,and well-developedmanufacturing base, makesilicon the ideal material forprecision sensors• Pressure sensors andaccelerometers were the firstto be developedSilicon pressure sensor chipPackaged pressure sensorksjp, 7/01MEMS Design & FabKOH etching: atomic viewSTM image of a (111) face with a ~10 atom step. FromWeisendanger, et al., Scanning tunnelling microscopy study ofSi(111)7*7 in the presence of multiple-step edges, EurophysicsLetters, 12, 57 (1990).10ksjp, 7/01MEMS Design & FabBulk micromachined cavities• Anisotropic KOH etch (Upperleft)• Isotropic plasma etch (upper right)• Isotropic BrF3 etch withcompressive oxide still showing(lower right)ksjp, 7/01MEMS Design & Fab(110)(111)(100)Clever KOHetching of (100)(110)(111)(110)(110)(111)Clockwise from above:Ternez; Rosengren; Keller11ksjp, 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 water12ksjp, 7/01MEMS Design & FabResidual stress gradientsMore tensile on topMore compressive on topJust right! The bottom line: annealpoly between oxides with similarphosphorous content. ~1000C for~60 seconds is enough.ksjp, 7/01MEMS Design & FabResidual stress gradientsA bad day at MCNC (1996).13ksjp, 7/01MEMS Design & FabHingesDeposit first sacrificialDeposit and pattern first polyEtch sacrificialPattern contactsDeposit and pattern 2ndpolyDeposit and pattern secondsacrificialksjp, 7/01MEMS Design & FabDeep Reactive Ion EtchBOSCH PatentSTS, Alcatel, Trion, Oxford Instruments …Uses high density plasma to alternativelyetch silicon and deposit a etch-resistantpolymer on side wallsPolymer deposition Silicon etch usingSF6chemistryPolymerUnconstrained geometry90° side wallsHigh aspect ratio 1:30Easily masked (PR, SiO2)Process recipe depends ongeometry☺14ksjp, 7/01MEMS Design & Fab1 mScalloping and Footing issues of DRIEScalloped sidewallTop wafer s urfacecathodeTop wafer surfaceanodeTip precursor sScalloped sidewallTop wafer s urfacecathodeTop wafer surfaceanodeTip precursor s<100 nm silicon nanowireover >10 micron gapmicrogridFooting at the bottom ofdevice layerMilanovic et al, IEEE TED, Jan. 2001.ksjp, 7/01MEMS Design & Fab1) Begin with a bonded SOI wafer. Growand etch a thin thermal oxide layer to actas a mask for the silicon etch.2) Etch the silicon device layer to exposethe buried oxide layer.3) Etch the buried oxide layer in bufferedHF to release free-standing structures.Si device layer, 20 µmthickburied oxide layerSi handle waferoxide mask layersiliconThermal oxideTypical simple SOI-MEMS Process15ksjp, 7/01MEMS Design & FabDRIE structures• Increased capacitancefor actuation andsensing• Low-stress structures• single-crystal Si onlystructural material• Highly stiff in verticaldirection• isolation of motion towafer plane• flat, robust structures2DoF
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