Crystalline Selectivity
Chemical micromachining relying on liquid potassium hydroxide removes silicon along specific crystallographic planes to form thin sensing membranes. Process engineers employ anisotropic KOH diaphragm etching to exploit the high etch rate ratio between (100) and (111) crystal planes, forming silicon diaphragms with smooth sidewalls inclined at 54.74 degrees. Dissolution stops on (111) planes because atomic packing density slows the reaction rate down by up to two orders of magnitude compared to (100) planes.
Thickness Control
Wafers placed in heated potassium hydroxide baths undergo material removal at rates dictated by bath temperature and concentration. High-precision pressure sensor fabrication requires diaphragm thickness tolerances within 0.5 micrometres across an eight-inch wafer. Etch stop techniques using heavy boron doping or electrochemical bias halt the etching action when the target thickness is reached, preventing over-etching caused by minor bath temperature gradients.
Geometry Distortion
Thermal convection and hydrogen bubble accumulation alter localized etch rates during chemical processing. Gas bubbles clinging to the etching surface cause localized masking, creating micro-pits on the diaphragm back. Agitation systems and surfactant additions flush bubbles away, maintaining uniform surface morphology across micromachined features.
Yield Boundary
Etch mask alignment relative to the primary crystal wafer flat determines final diaphragm dimensional accuracy. Misalignment by a fraction of a degree leads to mask undercut and lateral dimension enlargement.
