Bulk Micromachining

EE485A Lab Preparation

29 August 2009

• Portion of substrate removed to create structure.

Microchannels for on-chip cooling system

AFM tip

MIT microturbine

Silicon etchant classifications• Isotropic vs. Anisotropic

– Isotropic– etch rate is same in all directions– Anisotropic– etch rate is orientation dependent

• Dry vs. Wet– Dry– uses gas-phase process– Wet– uses liquid-phase process

Issues with etchant processes• Etch rate

• Etch rate selectivity

• Processing temperature

• Etch uniformity

• Sensitivity to over-time etch

• Safety and cost of etchants

• Surface finish and defects

Overtime etch sensitivityA particular etchant has an etch rate which is temperature dependent, according to the formula:

Small temperature variations can lead to large changes in etch rate!

Result is that “timed etches” are not very accurate.

Hence the search for “etch stops”--- ways to self-limit the etch besides time.

aE kTor r e k = Boltzmann constant =8.6 x 10-5 eV/K

Ea = activation energy

Anisotropic wet etching• Selective to <111> crystal plane.• KOH (potassium hydroxide)

– Safe. Cheap. Not very compatible with CMOS

• EDP (ethylene diamine pyrocatechol)– Safe. More expensive. More compatible

with CMOS

• TMAH (tetramethylammonium hydroxide)– Very toxic. Not used much now.

• Typical masking materials: SiO2, Si3N4

– Not photoresist!

• Inexpensive compared to dry methods

Etch Profile Evolution

Predicting etch profiles

Self-Limiting Stable Profile (SLSP) Transitional Profile (stable type– changes predictably)

54.75º

Calculations

w = 100 m, d = ?

Footprint of Self-Limiting Cavity

• SLSP are inverted pyramids with sides aligned to <110>

<110>

Etch Stops for Wet Anisotropic Etch of Silicon

• Heavy p-doping• Electrochemical etch

stop

Wet Isotropic Etching• HNA:

– Hydrofluoric acid

– Nitric acid

– Acetic acid

– Also very inexpensive

Dry Etching Methods• DRIE: Deep Reactive Ion

Etching– Anisotropy not dependent

on crystal orientation– Selective to SiO2, Si3N4,

photoresist– Anisotropic or isotropic– Expensive equipment

• XeF2:– Equipment is cheap, but

XeF2 is not.– Isotropic– Selective to SiO2, Si3N4,

photoresist

Wafer Bonding• Characterized by temperature

– Low-temperature (< 100 ºC)• Adhesive bonding

– Using photoresist, other polymers, spin-on-glass, solder

– High-temperature (>100 ºC)• Anodic bonding (glass to silicon)

– 400 ºC with large electric field

• Fusion bonding (silicon to silicon)– High temperature (~1000 ºC)– Very sensitive to surface defects and particles

• Low Temperature Fusion Bonding– Requires long-term storage or heat treatment

• Eutectic bonding (gold to silicon)– 450-550 ºC

Creation of membranes in bulk micromachining

• Single wafer process:– Etch stop layer as

membrane– Etch stop layer

underlying membrane

• Wafer bonding approaches

• See Fig. 10.30

Suspended beams and plates