Introduction to HF Vapor Etch
Nearly all silicon MEMS devices are created using a sacrificial silicon oxide layer, which when removed, "releases" the silicon MEMS structure and allows free movement.
Silicon oxide is typically etched by hydrogen fluoride :
SiO2 + 4 HF → SiF4 (g) + 2 H2O
The most widespread method of HF based etch release is wet chemical etching using a mixture of HF and water. However, as the HF, or subsequent rinsing solutions, dry it can cause "stiction", by pulling the free-moving microstuctures together which remain adhered to each other after release, reducing device yields. Another potential issue with wet HF etching is that it will corrode any exposed metals, most notably aluminium, on the wafer.
To avoid these issues, dry HF vapor can be used. A gaseous etchant also penetrates smaller features more easily and allows longer undercuts.
Alcohol (A) ionises the HF vapor and acts as a catalyst:
SiO2 (s) + 2HF2- (ads) + 2AH+ (ads) → SiF4 (ads) + 2H2O (ads) + 2A (ads)
Water, a by-product of the reaction, also acts as a catalyst and must therefore be carefully controlled and removed from the system.
SPTS' patented Primaxx HF/Alcohol process employs a reduced pressure, gas phase environment for the isotropic etch removal of sacrificial oxide layers.
The process is generally carried out at pressures between 75 and 150 torr providing controlled, residue-free etching. Typical vertical and lateral oxide etch rates are in the 0.1 - 10 microns/minute range.
Why Dry Etch Release?
- Eliminates stiction with device yields typically ~ 100%
- Provides repeatable, stable performance with a wide process window
- Compatible with a wide range of metals, especially unprotected Al mirrors and bond pads
- No complex waste management issues, small footprint, no process consumables
- Low cost of ownership
Why Reduced Pressure?
- Keeps etch byproducts in the gas phase ensuring high selectivities to metals
- Maximum feature penetration without localized loading effects
- Broad process window for optimizing productivity and etch results
- For more information of SPTS' HF release etch products
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