Surface MEMS 2014Part 1

sami.franssila@aalto.fi

Applications

Generic structure

Sacrificial material

Structural material

Substrate material

Structural material

Substrate material

anchor

anchor

Single mask vs. two mask cantileverSingle mask process Two mask process

mask #2

mask #1

mask

Etch structural layer with resist mask

Etch sacrificial layer without resist

Etch structural layer with resist mask

Etch sacrificial layer without resist

Material pairs & etchants Structural film Sacrificial film Sacrificial etch(es)

polysilicon oxide HF, HF vaporsilicon nitride oxide HFsilicon nitride Al NaOH, H3PO4

nickel Cu HCl nickel resist oxygen plasma aluminum resist oxygen plasmagold Cu HClgold resist oxygen plasmacopper resist oxygen plasmaParylene resist acetone, other

solventsSU-8 Cu HCl

HF etching of SiO2 and other materials

Etchant MaterialSiO2 TEOSPSG Si3N4 Al Mo

HF (49%) 176339694778 15 38 0.15BHF 133 107 1024 1 3 0.51:10 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

1.Oxide deposition2.Poly deposition3.Lithography piezores4. I/I piezo doping & strip5.Anneal I/I6.Au depo7.Litho for heater8.Au heater etch & strip9.Poly etch & strip10.Oxide etch11.Rinse & dryoxide

poly

Single mask SOI accelerometer

1. Device silicon DRIE

2. Buried oxide HF wet etch

3. 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