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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