Lec_24 - EE143 F2010 Lecture 24 Micro-Electro-Mechanical...

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Professor N Cheung , U.C. Berkeley Lecture 24 EE143 F2010 1 Fabrication Considerations – Stress-Strain, Thin-film Stress, Stiction Special Process Modules for MEMS – Bonding, Cavity Sealing, Deep RIE, Spatial forming (Molding), Layer Transfer Principle of Sensing and Actuation – Beam and Thin-Plate Deflections Micromachining Process Flows – MEMS-IC Integration – BioMEMS, PhotoMEMS Micro-Electro-Mechanical Systems (MEMS) Fabrication
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Professor N Cheung , U.C. Berkeley Lecture 24 EE143 F2010 2 Axial Stress and Strain Stress s : force per unit area acting on a material [unit: Newtons/m 2 (pascal)] s = F/A , A = area s > 0 tensile s < 0 compressive Strain e : displacement per unit length (dimensionless) e = L/ L o * Figure assumes there is no change in lateral dimensions
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Professor N Cheung , U.C. Berkeley Lecture 24 EE143 F2010 3 E = s / e [ in N/m 2 (Pascal) ] Poisson’s Ratio = 0.5 volume conserved E in GPa ( 1E9 N/m2) Si 190 SiO2 73 Diamond 1035 Young’s Modulus of a material
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Professor N Cheung , U.C. Berkeley Lecture 24 EE143 F2010 4 Stress-Strain Characteristic For low stress: material responds in elastic fashion (Hooke’s Law) stress/strain = constant s y = yield stress Ultimate stress - material will break; For Si (brittle) ultimate stress ~ yield stress
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Professor N Cheung , U.C. Berkeley Lecture 24 EE143 F2010 5 Mechanical Properties of Microelectronic Materials
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Professor N Cheung , U.C. Berkeley Lecture 24 EE143 F2010 6 Material Choices (a) Stiffness (b) Strength
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Professor N Cheung , U.C. Berkeley Lecture 24 EE143 F2010 7 Poly-Si For MEMS Structure • Effect of substrate: single-crystal substrate (clean surface) epitaxial layer amorphous substrate polycrystalline film • Average grain size depends on deposition & annealing conditions
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Professor N Cheung , U.C. Berkeley Lecture 24 EE143 F2010 8 Stress in LPCVD Poly-Si Films Stress varies significantly with process conditions – strong correlation between microstructure and stress Strain vs. t anneal : T dep ~620 o C
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Professor N Cheung , U.C. Berkeley Lecture 24 EE143 F2010 9 1) Begin with a bonded SOI wafer. Grow and etch a thin thermal oxide layer to act as a mask for the silicon etch. 2) Etch the silicon device layer to expose the buried oxide layer. 3) Etch the buried oxide layer in buffered HF to release free-standing structures. Si device layer, 20 μ m thick buried oxide layer Si handle wafer oxide mask layer silicon Thermal oxide Use of SOI for MEMS Process
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Professor N Cheung , U.C. Berkeley Lecture 24 EE143 F2010 10 Origins of Thin-film Stress Extrinsic – Applied stress – Thermal expansion – Plastic deformation Intrinsic – Growth morphology – Lattice misfit – Phase transformation s tot = s th + s int + s ext
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Professor N Cheung , U.C. Berkeley Lecture 24 EE143 F2010 11 substrate
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This note was uploaded on 03/03/2012 for the course EECS 142 taught by Professor Ee142 during the Spring '04 term at Berkeley.

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Lec_24 - EE143 F2010 Lecture 24 Micro-Electro-Mechanical...

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