MEMS Fabrication I : Process Flows and Bulk …

1 MEMS Fabrication I : Process Flows and bulk micromachining Dr. Thara Srinivasan Lecture 2. EE C245. U. Srinivasan Picture credit: Alien Technology 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.. Today's Lecture Tools Needed for MEMS Fabrication Photolithography Review Crystal Structure of Silicon bulk Silicon Etching Techniques EE C245. U. Srinivasan . 1. IC Processing Cross-section Masks Cross-section Masks N-type Metal Oxide Semiconductor (NMOS) Process flow EE C245.

2 Jaeger U. Srinivasan . CMOS Processing Processing steps Oxidation Photolithography Etching Chemical Vapor Jaeger Deposition Complementary Metal-Oxide-Semiconductor Diffusion Ion Implantation Evaporation and deposit Sputtering Epitaxy EE C245. etch pattern U. Srinivasan . 2. MEMS Devices Polysilicon level 1. Plate Polysilicon level 2. Polysilicon level 2 Staple Polysilicon level 1. Silicon substrate EE C245. Hinge staple Silicon substrate Prof. Kris Pister Support arm U. Srinivasan . MEMS Devices Microoptomechanical switches, Lucent Caliper Thermally isolated RMS. converter Reay et al. EE C245. Analog Devices Integrated U.

3 Srinivasan . 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 Delicate mechanical structures Packaging: before or after dicing? Package Sealing in gas environments EE C245. Interconnect - electrical, mechanical, fluidic Dice Testing electrical, mechanical, fluidic Release U. Srinivasan.

4 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 Layout generated from CAD file Mask reticle: chrome or emulsion on fused silica 1-3 $k EE C245. 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.

5 U. Srinivasan . Photolithography Tools Contact or proximity Projection Resolution: Contact - 1-2 m, Reduce 5-10 , stepper mode Proximity - 5 m Resolution - ( /NA) ~ 1 m Depth of focus poor Depth of focus ~ Few ms Double-sided lithography EE C245. 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 U. Srinivasan . 5. Materials for MEMS. Substrates Silicon Glass Quartz Thin Films Polysilicon Silicon Dioxide, Silicon Nitride Metals Silicon crystal structure Polymers = . EE C245. Wolf and Tauber U. Srinivasan . Silicon Crystallography [001] z z z (110).

6 <100> y y y [010]. [100] x (100) (110) (111). x x Miller Indices (h k l). Planes Reciprocal of plane intercepts with axes Intercepts of normal to plane with plane (unique), {family}. Directions EE C245. Move one endpoint to origin {111}. [unique], <family>. U. Srinivasan . 6. Silicon Crystallography 0 1/2 0. 3/4 1/4. 1/2 0 1/2. 1/4 3/4. 0 1/2 0. Angles between planes, . between [abc] and [xyz] given by: ax+by+cz = |(a,b,c)|*|(x,y,z)|*cos( ). (100),(111) = Cos 1 ((1 + 0 + 0) /(1)( 3 )). {100} and {110} 45 . EE C245. {100} and {111} . {110} and {111} , 90 and . U. Srinivasan . Silicon Crystal Origami {111} {110} {111}.

7 (111) (101) (111). {100}. (100). [101][101]. {111} {110} {111}. (111) (101) (111). Judy {100}. (001). Silicon fold-up cube [011][011]. {110} {111} {110} {111} {110}. Adapted from Profs. Kris (011) (111) (101) (111) (011). Pister and Jack Judy [110] [110]. {110}. {110}. {110}. [001]. {110}. {100} {100} {100}. (110). (110). (110). (110). Print onto transparency [001]. (010) (100) (010). Assemble inside out [100]. {110}. [100]. {111} {110} {111} {110}. Visualize crystal plane (011) (111) (101) (111) (011). [010] [010]. orientations, intersections, EE C245. {100}. and directions (001). Judy, UCLA. U.

8 Srinivasan . 7. Silicon Wafers Location of primary and secondary flats shows Crystal orientation Doping, n- or p-type EE C245. U. Srinivasan . Maluf 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.

9 U. Srinivasan . 8. What is bulk EE C245. 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 Maluf Choosing a method adsorption desorption Desired shapes surface reaction Etch depth and uniformity Surface roughness . EE C245. Process compatibility Safety, cost, availability, slowest step controls environmental impact U. Srinivasan . 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).

10 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 geometry EE C245. U. Srinivasan . Anisotropic Etching of Silicon Etching of Si with KOH. Si + 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 of its bonds below the surface {111} may form protective oxide quickly {111} smoother than other crystal EE C245. planes <100>. U. Srinivasan . Maluf 10. KOH Etch Conditions 1 KOH : 2 H2O (wt.)