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.
Four Point Probe
Measurement Instrument
Arrangements of collinear contacts enable the accurate determination of sheet resistance without interference from lead impedance. A standard four point probe applies current through the outer pair of tips while measuring potential across the inner pair. This technique separates the source current from the voltage sensing path to eliminate contact error.
Probe Calibration
Regular checks against reference wafers ensure that tip spacing stays within strict tolerance levels. If a four point probe suffers from uneven wear or contamination, the resulting calculations for resistivity will drift. Verification targets the mechanical alignment and the electrical insulation between adjacent pins.
Substrate Assessment
Testing occurs primarily during the early stages of wafer fabrication to confirm doping uniformity. The four point probe identifies areas where ion injection has failed to meet the specified concentration. Reliable data allows for the early rejection of material before expensive lithography begins.
Installation Effect
Pressure between the probe tips and the sample must be consistent to avoid cracking the silicon. Excessive force with a four point probe can introduce local strain that momentarily shifts the resistivity readings. Soft landing mechanisms allow the pins to touch the surface without compromising the integrity of the material under test.