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Surface MEMS 2014 Part 1
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Applications
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Generic structure Sacrificial material Structural material
Substrate material anchor Structural material Substrate material anchor
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Single mask vs. two mask cantilever
Single mask process Two mask process mask #2 mask #1 mask Etch structural layer with resist mask Etch structural layer with resist mask Etch sacrificial layer without resist Etch sacrificial layer without resist
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Material pairs & etchants
Structural film Sacrificial film Sacrificial etch(es) polysilicon oxide HF, HF vapor silicon nitride oxide HF silicon nitride Al NaOH, H3PO4 nickel Cu HCl nickel resist oxygen plasma aluminum resist oxygen plasma gold Cu HCl gold resist oxygen plasma copper resist oxygen plasma Parylene resist acetone, other solvents SU-8 Cu HCl
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HF etching of SiO2 and other materials
Etchant Material SiO2 TEOS PSG Si3N4 Al Mo HF (49%) BHF 1:10 HF Etch rates in nm/min
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Different silicon dioxide films
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AFM tips: surface release
Condition: Cr release etching must not attack Au or SU-8
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AlN unimorph suspensions
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Thermally excited resonator
Oxide deposition Poly deposition Lithography piezores I/I piezo doping & strip Anneal I/I Au depo Litho for heater Au heater etch & strip Poly etch & strip Oxide etch Rinse & dry poly oxide
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Single mask SOI accelerometer
Device silicon DRIE Buried oxide HF wet etch Rinse & dry
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Optical spectrometer in SOI
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Compressive stresses in film
buckling Depends on span on the structure: short beams do not buckle; and hard materials less prone than soft.
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Tensile stress in film Desired stress state in most cases; too much tensile stress leads to cracking.
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Released structural layers
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Polysilicon Deposited by CVD at 625oC true poly
Can be deposited at 575oC amorphous Anneal after deposition: a-Si poly ! Typical thickness 1-2 µm
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Polysilicon doping Usually deposited undoped (practically insulator)
Doping after deposition diffusion implantation Annealing ~950oC, 1 h to activate dopants Annealing changes film stress (and grain size) Heavy doping ca. 500 µΩ-cm (cf. Al 3 µ Ω-cm ) Grain size ca nm after anneal
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Polysilicon stress anneal
580oC deposited film (a-Si) annealed differently, leading to different final stress
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Marc Madou
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Optical modulator/ interference filter/ display
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Metal micromechanics (1)
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Metal micromechanics (2)
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RF switch
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Electroplated gold switch
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Perforation to release large area structures
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End of surface MEMS 1
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