MEMS Fabrication: Process Flows and Bulk Silicon EtchingLecture OutlineIC ProcessingCMOS ProcessingMEMS DevicesMEMS DevicesMEMS ProcessingPhotolithography: Masks and PhotoresistPhotoresist ApplicationPhotolithography ToolsMaterials for MEMSSilicon CrystallographySlide 13Silicon Crystal OrigamiSilicon WafersProperties of SiliconBulk Etching of SiliconWet Etch VariationsAnisotropic Etching of SiliconKOH Etch ConditionsUndercuttingSlide 22Corner CompensationSlide 24Other Anisotropic EtchantsBoron-Doped Etch StopMicroneedlesSlide 28Slide 29Electrochemical Etch StopSlide 31Pressure SensorsIsotropic Etching of SiliconIsotropic Etching ExamplesDry Etching of SiliconPlasma Etching of SiliconHigh-Aspect-Ratio Plasma EtchingDRIE IssuesDRIE ExamplesVapor Phase Etching of SiliconEtching with Xenon DifluorideLaser-Driven EtchingThara SrinivasanLecture 2MEMS Fabrication: Process Flows and Bulk Silicon EtchingPicture credit: Alien TechnologyLecture Outline•Reading•Reader: Kovacs, pp. 1536-43, Williams, pp. 256-60.•Senturia, Chapter 2. •Today’s Lecture•Tools Needed for MEMS Fabrication•Photolithography Review•Crystal Structure of Silicon•Silicon Etching TechniquesIC ProcessingCross-sectionJaegerMasksCross-section MasksN-type metal oxide semiconductor (NMOS) process flowCMOS Processing•Processing steps•Oxidation•Photolithography•Etching•Diffusion•Evaporation and Sputtering•Chemical Vapor Deposition•Ion Implantation•EpitaxyComplementary Metal-Oxide-SemiconductorJaegerdepositpatternetchMEMS Devices Integrated accelerometer chipFord MicroelectronicsMicromachined turbine Schmidt group, MIT Angular rate sensorDelphi-Delco Electronic SystemsMicrooptomechanical switches, LucentMEMS DevicesStaplePolysilicon level 2Polysilicon level 1Silicon substratePolysilicon level 1Polysilicon level 2Hinge staplePlateSilicon substrateSupport armMEMS Processing•Unique to MEMS fabrication•Sacrificial etching•Thicker films and deep etching•Mechanical properties critical•Etching into substrate•3-D assembly•Wafer-bonding•Molding•Unique to MEMS packaging and testing•Delicate mechanical structures•Packaging: before or after dicing?•Sealing in gas environments•Interconnect - electrical, mechanical, fluidic•Testing – electrical, mechanical, fluidicPackageDiceReleasesacrificial layerstructural layerPhotolithography: Masks and Photoresistdark-fieldlight-field•Photolithography steps•Photoresist spinnning, 1-10 µm spin coating•Optical exposure through a photomask•Developing to dissolve exposed resist•Photomasks•Layout generated from CAD file•Chrome or emulsion on glass•1-3 $kPhotoresist Application•Spin-casting photoresist•Polymer resin, sensitizer, carrier solvent•Positive and negative photoresist•Thickness depends on•Concentration•Viscosity•Spin speed•Spin timewww.brewerscience.comPhotolithography Tools•Contact or proximity•Resolution: Contact - 1-2 µm, Proximity - 5 µm•Depth of focus•Projection •Resolution - 0.5 (/NA) ~ 1 µm•Depth of focus ~ Few µmsMaterials for MEMS•Substrates•Silicon•Glass•Quartz •Thin Films•Polysilicon•Silicon Dioxide, Silicon Nitride•Metals•PolymersWolf and TauberSilicon crystal structure = 5.43 ÅSilicon Crystallography•Miller Indices (hkl)•Normal to plane •Reciprocal of plane intercepts with axes•(unique), {family}•Direction •Move one endpoint to origin•[unique], <family>x x xyyyz z z(100)(110) (111){111}[001][100][010](110)Silicon Crystallography•Angles between planes, • between [abc] and [xyz] is given by: ax+by+cz = |(a,b,c)|*|(x,y,z)|*cos()•{100} and {110} – 45°•{100} and {111} – 54.74°•{110} and {111} – 35.26, 90 and 144.74°0 1/2 001/203/41/41/43/401/2 1/2))3)(1/()001((1)111(),100(CosSilicon Crystal Origami•Silicon fold-up cube•Adapted from Profs. Kris Pister and Jack Judy•Print onto transparency•Assemble inside out•Visualize crystal plane orientations, intersections, and directions{111}(111){111}(111){111}(111){111}(111){111}(111){111}(111){111}(111){111}(111){100}(100){110}(110){100}(010){110}(011){110}(011){110}(110){110}(110){100}(010){110}(011){110}(011){110}(110){110}(101){100}(001){100}(100){110}(101){110}(101){100}(001){110}(101)[010] [010][001][001][100][100][101][101][011][011][110][110]Silicon Wafers•Location of primary and secondary flats shows•Crystal orientation•Doping, n- or p-typeMalufProperties of Silicon•Crystalline silicon is a hard and brittle material that deforms elastically until it reaches its yield strength, at which point it breaks.•Tensile yield strength = 7 GPa (~1500 lb suspended from 1 mm²)•Young’s Modulus near that of stainless steel•{100} = 130 GPa; {110} = 169 GPa; {111} = 188 GPa •Mechanical properties uniform, no intrinsic stress•Good thermal conductor•Mechanical integrity up to 500°CBulk Etching of Silicon•Etching modes•Isotropic vs. anisotropic•Reaction-limited•Etch rate dependent on temperature•Diffusion-limited•Etch rate dependent on mixing•Also dependent on layout and geometry, “loading”•Choosing a method •Desired shapes•Layout and uniformity•Surface roughness•Process compatibility•Safety, cost, availabilityadsorption desorptionsurfacereactionslowest step controls rate of reactionMalufWet Etch Variations•Etch rate variation due to wet etch set-up•Loss of reactive species•Evaporation of liquids•Poor mixing (etch product blocks diffusion of reactants)•Contamination•Applied potential•IlluminationAnisotropic Etching of Silicon•Etching of Si with KOHSi + 2OH- Si(OH)2 2+ + 4e- 4H2O + 4e- 4(OH) - + 2H2•Crystal orientation relative etch rates•{110}:{100}:{111} = 600:400:1•{111} plane has three backbonds below the surface•Energy explanation•{111} may form protective oxide quickly<100>MalufKOH Etch Conditions •1 KOH : 2 H2O (wt.), stirred bath @ 80°C •Si (100) 1.4 µm/min•Etch masks•Si3N4 0•SiO2 1-10 nm/min •Photoresist, Al ~ fast•“Micromasking” by H2 bubbles leads to roughness•Stirring displaces bubbles•Oxidizer, surfactant additivesMalufUndercutting•Convex corners bounded by {111} planes are attackedMalufRisticUndercutting•Convex corners bounded by {111} planes are attackedCorner Compensation•Protect corners with “compensation” areas in layout, Buser et al. (1986)•Mesa array for self-assembly test structures, Smith and
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