Evaluating Thermal Hysteresis Limits in MEMS Accelerometer Static Leveling
MEMS thermal hysteresis creates non-repeatable zero-g offset shifts up to 3 mg, driving uncompensable arcsecond tilt errors in static leveling systems.
Bulk Micromachining
Fabrication Foundation
Chemical etching of single crystal silicon wafers defines the primary physical geometry of the substrate. Bulk micromachining removes substantial portions of the silicon material to create structural membranes, cantilevers, or deep cavities. Photolithographic masks protect selected regions from an anisotropic etchant like potassium hydroxide.
This selective material removal shapes mechanical components directly from the host wafer mass.
Etching Dynamics
Orientation dependent etching rates determine the final dimensions of the structural features. Potassium hydroxide travels along the silicon (100) plane much faster than the (111) plane, creating specific v-shaped troughs or rectangular trenches. Temperature stability remains the primary constraint during this bath process.
Minor fluctuations in solution concentration or heater output cause significant deviations in the wall angle or the depth of the finished cavity.
Dimension Control
Precise etch stop techniques manage the thickness of diaphragms and beams after the bulk material removal. Boron doping at high concentrations slows the etch rate to near zero at the target boundary. Electrochemical potential control provides a secondary method to halt the process when the etchant reaches a specific pn junction.
These methods ensure that structural dimensions remain consistent across the entire wafer surface.
Metrological Variance
Interferometric measurement verifies the etch depth and the wall profile uniformity against the design specifications. Thermal stresses induced by high-temperature dopant diffusion introduce latent mechanical drift that changes the resonant frequency of the finished silicon structure. Compensation for this offset requires calibration during the final assembly of the transducer.
The physical geometry of the resulting device dictates its mechanical response under operating pressures.