Micromachining Chemistry
Aqueous alkaline solutions etch single-crystal silicon along distinct crystallographic planes at highly differentiated reaction velocities. The anisotropic TMAH etching process employs tetramethylammonium hydroxide to sculpt microelectromechanical sensor structures without introducing toxic heavy metals into fabrication facilities. Etch rates along the (100) plane exceed those along the (111) plane by more than twenty to one, producing clean pyramidal cavities and angled sidewalls.
Etching action ceases when exposed surfaces consist entirely of self-limiting (111) atomic lattice planes.
Selectivity Ratio
Microfabrication workflows depend on high selectivity between silicon substrates and masking dielectric layers. Silicon dioxide exhibits etch rates below one nanometer per minute in five to twenty-five weight percent solution concentrations. Silicon nitride masks demonstrate exceptional stability, withstanding prolonged exposure without measurable degradation.
Doped silicon regions containing high boron concentrations function as effective chemical etch stops.
Process Control
Solution temperature and chemical concentration govern both the vertical etch velocity and final surface micromorphology. Bath temperatures maintained near eighty degrees Celsius optimize reaction rates while limiting solvent evaporation. Continuous bubbling of inert nitrogen prevents carbon dioxide absorption, which degrades chemical purity and alters reaction kinetics.
Dissolved silicon additives reduce aluminum metallization attack in integrated sensor wafers.
Fabrication Qualification
Microelectromechanical pressure sensor membranes require precise dimensional validation following wet chemical release. Optical reflectometry and white light interferometry verify cavity depth, membrane thickness, and surface roughness across the processed wafer. Undercutting at convex corners demands compensation geometry in photolithographic mask layouts to prevent structural collapse.
Incomplete mask adhesion causes pinhole defects and irregular boundary termination along structural edges. Silicon piezoresistor placement requires sub-micrometer alignment relative to etched diaphragm boundaries to ensure repeatable pressure sensitivity. Solution contamination with alkali metal ions compromises complementary metal-oxide-semiconductor compatibility in co-integrated sensor designs.
Anisotropic TMAH etching provides the geometric precision required to fabricate repeatable mechanical suspensions for inertial and pressure sensors.
