Surface MEMS 2014 Part 1 sami.franssila@aalto.fi

Applications

Generic structure Sacrificial material Structural material Substrate material anchor Structural material Substrate material anchor

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

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

HF etching of SiO2 and other materials Etchant Material SiO2 TEOS PSG Si3N4 Al Mo HF (49%) 1763 3969 4778 15 38 0.15 BHF 133 107 1024 1 3 0.5 1:10 HF 48 157 922 1.5 320 0.15 Etch rates in nm/min

Different silicon dioxide films

AFM tips: surface release Condition: Cr release etching must not attack Au or SU-8

AlN unimorph suspensions

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

Single mask SOI accelerometer Device silicon DRIE Buried oxide HF wet etch Rinse & dry

Optical spectrometer in SOI

Compressive stresses in film buckling Depends on span on the structure: short beams do not buckle; and hard materials less prone than soft.

Tensile stress in film Desired stress state in most cases; too much tensile stress leads to cracking.

Released structural layers

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

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. 200-300 nm after anneal

Polysilicon stress anneal 580oC deposited film (a-Si) annealed differently, leading to different final stress

Marc Madou

Optical modulator/ interference filter/ display

Metal micromechanics (1)

Metal micromechanics (2)

RF switch

Electroplated gold switch

Perforation to release large area structures

End of surface MEMS 1