MEMS Fabrication I Process Flows and Bulk Micromachining EE C245 Dr Thara Srinivasan Lecture 2 Picture credit Alien Technology U Srinivasan Lecture 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 EE C245 Today s Lecture Tools Needed for MEMS Fabrication Photolithography Review Crystal Structure of Silicon Bulk Silicon Etching Techniques U Srinivasan 1 IC Processing Cross section Masks Cross section Masks EE C245 N type Metal Oxide Semiconductor NMOS process flow Jaeger U Srinivasan CMOS Processing Processing steps EE C245 Oxidation Photolithography Etching Chemical Vapor Deposition Diffusion Ion Implantation Evaporation and Sputtering Epitaxy Jaeger Complementary Metal Oxide Semiconductor deposit etch pattern U Srinivasan 2 MEMS Devices Polysilicon level 1 Plate Polysilicon level 2 Polysilicon level 2 Staple Polysilicon level 1 EE C245 Silicon substrate Hinge staple Silicon substrate Prof Kris Pister Support arm U Srinivasan MEMS Devices Microoptomechanical switches Lucent Caliper EE C245 Thermally isolated RMS converter Reay et al U Srinivasan Analog Devices Integrated accelerometer Microturbine Schmidt group MIT 3 MEMS 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 sacrificial layer structural layer Unique to MEMS packaging and testing EE C245 Delicate mechanical structures Packaging before or after dicing Sealing in gas environments Interconnect electrical mechanical fluidic Testing electrical mechanical fluidic Package Dice Release U Srinivasan Photolithography Masks and Photoresist Photolithography steps Photoresist spinnning 1 10 m spin coating Optical exposure through a photomask Developing to dissolve exposed resist Bake to drive off solvents Remove using solvents acetone or O2 plasma Photomasks EE C245 Layout generated from CAD file Mask reticle chrome or emulsion on fused silica 1 3 k U Srinivasan light field dark field 4 Photoresist Application Spin casting photoresist Polymer resin sensitizer carrier solvent Positive and negative photoresist Thickness depends on Concentration Viscosity Spin speed Spin time EE C245 www brewerscience com U Srinivasan Photolithography Tools Contact or proximity Projection EE C245 Resolution Contact 1 2 m Proximity 5 m Depth of focus poor U Srinivasan Reduce 5 10 stepper mode Resolution 0 5 NA 1 m Depth of focus Few ms 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 back 5 Materials for MEMS Substrates Silicon Glass Quartz Thin Films EE C245 Polysilicon Silicon Dioxide Silicon Nitride Metals Polymers Silicon crystal structure 5 43 Wolf and Tauber U Srinivasan Silicon Crystallography 001 z z y y 100 z 110 y 010 100 100 x x 110 x 111 Miller Indices h k l Planes EE C245 Reciprocal of plane intercepts with axes Intercepts of normal to plane with plane unique family Directions Move one endpoint to origin unique family 111 U Srinivasan 6 Silicon Crystallography 0 1 2 0 1 4 3 4 1 2 1 2 0 1 4 3 4 0 0 1 2 Angles between planes between abc and xyz given by ax by cz a b c x y z cos EE C245 100 111 Cos 1 1 0 0 1 3 100 and 110 45 100 and 111 54 74 110 and 111 35 26 90 and 144 74 U Srinivasan Silicon Crystal Origami 111 111 110 111 111 101 100 111 111 Judy 110 101 101 101 100 111 111 100 110 111 111 011 110 101 111 111 110 011 110 110 010 110 100 110 100 110 100 110 001 100 110 010 001 100 110 110 Adapted from Profs Kris Pister and Jack Judy Print onto transparency Assemble inside out Visualize crystal plane orientations intersections and directions 110 EE C245 Silicon fold up cube 011 011 001 100 110 011 111 111 110 101 010 111 111 110 011 010 100 001 Judy UCLA U Srinivasan 7 Silicon Wafers Location of primary EE C245 and secondary flats shows Crystal orientation Doping n or p type Maluf U Srinivasan Mechanical 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 mm 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 piezoresistivity EE C245 U Srinivasan 8 EE C245 What is Bulk Micromachining U Srinivasan Bulk 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 EE C245 Choosing a method U Srinivasan Desired shapes Etch depth and uniformity Surface roughness Process compatibility Safety cost availability environmental impact Maluf adsorption surface reaction desorption slowest step controls rate of reaction 9 Wet 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 EE C245 Etch rate variation due to layout Distribution of exposed area loading Structure geometry U Srinivasan Anisotropic Etching of Silicon Etching of Si with KOH Si 2OH Si OH 2 2 4e4H2O 4e 4 OH 2H2 Crystal orientation relative etch rates 110 100 111 600 400 1 111 plane has three of its bonds below the surface 111 may form protective oxide EE C245 quickly 111 smoother than other crystal planes U Srinivasan 100 Maluf 10 KOH Etch Conditions 1 KOH 2 H2O wt stirred bath 80 C Si 100 1 4 m min Etch masks Si3N4 0 SiO2 1 10 nm min Photoresist Al fast Micromasking by H2 bubbles leads to roughness EE C245 Stirring displaces bubbles Oxidizer surfactant additives Maluf U Srinivasan Undercutting Convex corners bounded by 111 planes are attacked EE C245 Maluf Ristic U Srinivasan 11 Undercutting Convex EE C245 corners bounded by 111 planes are attacked U Srinivasan Corner Compensation Protect corners with compensation EE C245 areas in layout Mesa array for self assembly test structures Smith and coworkers 1995 Hadley U Srinivasan
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