1U. Srinivasan ©EE C245Dr. Thara SrinivasanLecture 2MEMS Fabrication I :Process Flows and Bulk MicromachiningPicture credit: Alien TechnologyU. Srinivasan ©EE C245Lecture Outline• Reading• Reader is in! (at South side Copy Central)• Kovacs, “Bulk Micromachining of Silicon,” pp. 1536-43.• Williams, “Etch Rates for Micromachining Processing,” pp. 256-60.• Senturia, Chapter 3, “Microfabrication.” • Today’s Lecture• Tools Needed for MEMS Fabrication• Photolithography Review• Crystal Structure of Silicon• Bulk Silicon Etching Techniques2U. Srinivasan ©EE C245IC ProcessingCross-sectionJaegerMasksCross-section MasksN-type Metal Oxide Semiconductor (NMOS) process flowU. Srinivasan ©EE C245CMOS Processing• Processing steps• Oxidation• Photolithography• Etching• Chemical Vapor Deposition• Diffusion• Ion Implantation• Evaporation and Sputtering• EpitaxyComplementary Metal-Oxide-SemiconductorJaegerdepositpatternetch3U. Srinivasan ©EE C245MEMS DevicesStaplePolysilicon level 2Polysilicon level 1Silicon substratePolysilicon level 1Polysilicon level 2Hinge staplePlateSilicon substrateSupport armProf. Kris PisterU. Srinivasan ©EE C245MEMS Devices Microoptomechanicalswitches, Lucent Analog DevicesIntegrated accelerometerMicroturbine, Schmidt group MITThermally isolated RMS converter Reay et al.Caliper4U. Srinivasan ©EE C245MEMS Processing• Unique to MEMS fabrication• Sacrificial etching• Mechanical properties critical• Thicker films and deep etching• Etching into substrate• Double-sided lithography• 3-D assembly• Wafer-bonding• Molding• Integration with electronics, fluidics• 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 layerU. Srinivasan ©EE C245Photolithography: Masks and Photoresistdark-fieldlight-field• Photolithography steps• Photoresist spinnning, 1-10 µm spin coating• Optical exposure through a photomask• Developing to dissolve exposed resist• Bake to drive off solvents• Remove using solvents (acetone) or O2plasma• Photomasks• Layout generated from CAD file• Mask reticle: chrome or emulsion on fused silica• 1-3 $k5U. Srinivasan ©EE C245Photoresist Application• Spin-casting photoresist• Polymer resin, sensitizer, carrier solvent• Positive and negative photoresist• Thickness depends on• Concentration• Viscosity• Spin speed• Spin timewww.brewerscience.comU. Srinivasan ©EE C245Photolithography Tools• Contact or proximity• Resolution: Contact - 1-2 µm, Proximity - 5 µm• Depth of focus poor• Projection• Reduce 5-10×, stepper mode• Resolution - 0.5 (λ/NA) ~ ≤ 1 µm• Depth of focus ~ Few µms• Double-sided lithography• Make alignment marks on both sides of wafer • Use IR imaging to see through to back side• Store image of front side marks; align to back6U. Srinivasan ©EE C245Materials for MEMS• Substrates• Silicon• Glass• Quartz• Thin Films• Polysilicon• Silicon Dioxide, Silicon Nitride• Metals• PolymersWolf and TauberSilicon crystal structureλ = 5.43 ÅU. Srinivasan ©EE C245Silicon Crystallography• Miller Indices (h k l)• Planes• Reciprocal of plane intercepts with axes• Intercepts of normal to plane with plane• (unique), {family}• Directions • Move one endpoint to origin• [unique], <family>xxxyyyzzz(100)(110) (111){111}[001][100][010](110) <100>7U. Srinivasan ©EE C245Silicon Crystallography• Angles between planes, ∠• ∠ between [abc] and [xyz] given by: ax+by+cz = |(a,b,c)|*|(x,y,z)|*cos(Θ)• {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(++=−CosθU. Srinivasan ©EE C245Silicon 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]Judy, UCLAJudy8U. Srinivasan ©EE C245Silicon Wafers• Location of primary and secondary flats shows• Crystal orientation• Doping, n- or p-typeMalufU. Srinivasan ©EE C245Mechanical Properties 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• Mechanical integrity up to 500°C• Good thermal conductor, low thermal expansion coefficient• High piezoresistivity9U. Srinivasan ©EE C245What is Bulk Micromachining?U. Srinivasan ©EE C245Bulk 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• Etch depth and uniformity• Surface roughness• Process compatibility• Safety, cost, availability, environmental impactadsorption desorptionsurfacereactionslowest step controls rate of reactionMaluf10U. Srinivasan ©EE C245Wet Etch Variations, Crystalline Si• Etch rate variation due to wet etch set-up• Loss of reactive species through consumption• Evaporation of liquids• Poor mixing (etch product blocks diffusion of reactants)• Contamination• Applied potential• Illumination• Etch rate variation due to material being etched• Impurities/dopants• Etch rate variation due to layout • Distribution of exposed area ~ loading• Structure geometryU. Srinivasan ©EE C245Anisotropic Etching of Silicon• Etching of Si with KOHSi + 2OH-→ Si(OH)22++ 4e-4H2O + 4e-→ 4(OH)-+ 2H2<100>Maluf• Crystal orientation relative etch rates• {110}:{100}:{111} = 600:400:1• {111} plane has three of its bonds below the
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