Silicon Piezoresistive Transducer Carrier Density Fundamentals and Gauge Factors
P-type silicon piezoresistors doped with boron to 1e18 cm-3 achieve an optimal balance between gauge factor magnitude and thermal coefficient stability.
Silicon Anisotropic Etch
Chemical Technique
Selective material removal occurs at different rates along distinct crystallographic planes in the substrate. A silicon anisotropic etch creates precise geometric features like V grooves or diaphragm cavities. The speed of the reaction depends heavily on the concentration of the alkaline bath used during the process.
Geometric Precision
Precise angles are naturally defined by the atomic structure of the crystal itself. Using a silicon anisotropic etch ensures that the resulting pits have uniform wall slants exactly consistent with the 111 crystal plane. This reliability allows for mass production of consistent pressure sensors across thousands of wafers.
Process Monitor
Etch stops identify the points where the chemical reaction must cease to maintain the desired thickness. Verification of a silicon anisotropic etch involves measuring the depth of the resulting trench with an optical profilometer. If the temperature of the chemical bath fluctuates, the etch rate will shift and potentially over cut the features.
Integration Limit
Selectivity between different masks prevents the unintended removal of protective oxides or nitrides. While a silicon anisotropic etch is effective for broad structures, its reliance on crystal orientation limits the design to certain rectangular or hexagonal patterns. Designers follow these geometric constraints to ensure the structural integrity of the final suspended membrane.