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MEMS Fabrication I : Process Flows and Bulk …

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. 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.

1 U. Srinivasan © EE C245 Dr. Thara Srinivasan Lecture 2 MEMS Fabrication I : Process Flows and Bulk Micromachining Picture credit: Alien Technology

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Transcription of 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. 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.

2 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. 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?

3 Package Sealing in gas environments EE C245. Interconnect - electrical, mechanical, fluidic Dice Testing electrical, mechanical, fluidic 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 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.

4 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). <100> y y y [010]. [100] x (100) (110) (111). x x Miller Indices (h k l).

5 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}. (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}.

6 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. 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.

7 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. 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.

8 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). 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.

9 {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.), stirred bath @ 80 C. Si (100) m/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 additives EE C245. U. Srinivasan . Maluf Undercutting Convex corners bounded by {111} planes are attacked Maluf EE C245. Ristic U. Srinivasan . 11. Undercutting Convex corners bounded by {111} planes are attacked EE C245. U. Srinivasan . Corner Compensation Protect corners with compensation.

10 Areas in layout Mesa array for self-assembly test structures, Smith and coworkers (1995). EE C245. Hadley Alien Technology U. Srinivasan . Chang 12. Corner Compensation Self-assembly microparts, Alien Technology EE C245. U. Srinivasan . Other Anisotropic Etchants TMAH, Tetramethyl ammonium hydroxide, 10-40 (90 C). Etch rate (100) = m/min Al safe, IC compatible Etch ratio (100)/(111) = 10-35. Etch masks: SiO2 , Si3N4 ~ nm/min Boron doped etch stop, up to 40 slower EDP (115 C). Carcinogenic, corrosive Etch rate (100) = m/min Al may be etched R(100) > R(110) > R(111). Etch ratio (100)/(111) = 35. Etch masks: SiO2 ~ nm/min, Si3N4 ~ nm/min EE C245. Boron doped etch stop, 50 slower U. Srinivasan.


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