MEMS 2 based on chapter 29 sami franssilaaalto

MEMS 2 (based on chapter 29) sami. franssila@aalto. fi

Isotropic etching • Proceeds as a spherical wave • Undercuts structures (proceeds under mask) • Most wet etching processes are isotropic e. g. HF etching of oxide, H 3 PO 4 etching of Al

Isotropy: good and bad If you want closely spaced lines (as in comb-drive capacitor), undercutting is bad, but if. . .

Undercutting in action: dome resonator RIE etching a small hole in polysilicon Isotropic HF wet etching of oxide under polysilicon Membrane can move H. G. Craighead

Applications

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

Single mask SOI accelerometer 1. Device silicon DRIE 2. Buried oxide HF wet etch 3. Rinse & dry

Single mask vs. two mask cantilever Two mask process Single mask process mask #1 Etch structural layer with resist mask #2 Etch sacrificial layer without resist Etch structural layer with resist mask Etch sacrificial layer without resist

Material pairs & etchants Structural film Sacrificial etch(es) polysilicon nitride nickel aluminum gold copper Parylene SU-8 oxide Al Cu resist resist Cu HF, HF vapor HF Na. OH, H 3 PO 4 HCl oxygen plasma acetone, other solvents HCl

HF etching of Si. O 2 and other materials Etchant Material Si. O 2 TEOS HF (49%) 1763 3969 BHF 133 107 1: 10 HF 48 157 PSG 4778 1024 922 Si 3 N 4 Al 15 38 1 3 1. 5 320 Etch rates in nm/min Mo 0. 15

Different silicon dioxide films

Thermally excited resonator poly oxide 1. Oxide deposition 2. Poly deposition 3. Lithography piezores 4. I/I piezo doping & strip 5. Anneal I/I 6. Au depo 7. Litho for heater 8. Au heater etch & strip 9. Litho for poly hole 10. Poly etch & strip 11. Oxide wet etch in HF 12. Rinse & dry

Lateral-field-excited (LFE) piezoelectric Al. N contourmode Zuo et al. : CMOS oscillator based on contour-mode MEMS resonators, IEEE 2010

LFE Al. N fabrication (a) Al. N sputter deposition on top of Si wafers (b) top Pt electrode deposition by lift-off (c) Al. N lithography and plasma etching using Cl 2 and BCl 3, into Si (d) structure release by Si dry etching in Xe. F 2. Zuo et al. : CMOS oscillator based on contour-mode MEMS resonators, IEEE 2010

Lateral-field-excited (LFE) piezoelectric Al. N contourmode Zuo et al. : CMOS oscillator based on contour-mode MEMS resonators, IEEE 2010

Perforation to release large area structures

Microphone aluminum membrane oxide N+ diffusion 0. Wafer silicon 1. Thermal oxide 2. 1 st litho for diffusion 3. Etch oxide & strip resist 4. Clean 5. N+ diffusion 6. Etch all oxide away 7. CVD oxide deposition 8. 2 nd litho: contact to n+ 9. Etch oxide & strip resist 10. Aluminum sputtering 11. 3 rd litho: aluminum pattern 12. Etch aluminum & strip resist 13. Etch oxide, rinse & dry

Stiction ( sticking + friction) Capillary force of liquid exceeds mechanical strength of the released beam

Stiction prevention • Replace water by something that has lower surface tension, like isopropyl alcohol • Use stronger structures (H, T, I, U beams) • Change structural material • Redesign so that shorter beams needed • Use more elaborate drying • Use dry release no drying needed

Alternative drying

Stiffening beam by 3 D shaping

Stiction prevention: dry release Other dry releases: • Xe. F 2 dry etching of silicon • SF 6 isotropic plasma etching of silicon • HF-vapor etching of oxide

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.

Marc Madou

RF switch Off-state when up. On-state when pulled down.

Electroplated gold switch

Electroplated gold switch

Critical release gap Gap height is critical for device operation. These gaps are often ~ 1 µm in size. Actuation voltage depends on gap height. Optical path length depends on gap.

Non-critical release gap Gap provides space so that elements can move; or gap provides thermal isolation. Large: >> 1 µm

Sideways movement thermal switch

Sideways movement thermal relay/switch

Hinged structures (1)

Hinged structures (2)

Hinged structures (3)

µ-mirror array on CMOS

Zero-level package by thin films

Packaging by deposition

Problems with thin film roofs a) cracks a b) outgassing b c) collapse c

Packaging by wafer bonding -needs an additional wafer -electrical feedthroughs ? -degree of hermeticity needed ?

Bulk vs. surface • in bulk micromechanics <Si> is etched by either KOH or DRIE • structure heights 380 µm/500 µm • in surface micromechanics thin films (oxide, nitride, poly, aluminum) are used • structure heights 1 -2 µm typically (=sputter & CVD film thicknesses) or 5 -50 µm if SOI or thick poly used