Optimization of sensitivity and noise in piezoresistive cantilevers

In this article, the sensitivity and the noise of piezoresistive cantilevers were systematically investigated with respect to the piezoresistor geometry, the piezoresistive materials, the doping dose, the annealing temperature, and the operating biased voltage. With the noise optimization results, dimension optimized array cantilevers were designed and fabricated by using single-crystal silicon, low-pressure chemical-vapor deposition (LPCVD) amorphous silicon and microcrystalline silicon as piezoresistive layers.

Measurement results have shown that the smallest Hooge factor (alpha) was 3.2x10(-6), the biggest gauge factors was 95, and the minimum detectable deflection (MDD) at 6 V and 200 Hz-measurement bandwidth was 0.3 nm for a single-crystal silicon cantilever. Of the two LPCVD silicon piezoresistive cantilevers, amorphous silicon piezoresistors had relatively lower 1/f noise.

The MDD for a LPCVD silicon cantilever at a 200 Hz-measurement bandwidth was 0.4 nm. For all kinds of piezoresistive cantilevers, the 1/f noises were decreased by 35%-50% and the gauge factors were decreased by 60-70% if the doping dose were increased by ten times. The annealing at 1050 degreesC for 30 min decreased 1/f noise by about 65% compared with the 950 degreesC for 10 min treatments.

The cantilevers with a relatively higher-doping dose gave smaller MDD even though the gauge factors of them were decreased by nearly a factor of 1.8. The higher-biased voltages had no great improvements on the MDD due to the 1/f noise dominance.