Axial asymmetry for improved sensitivity in MEMS piezoresistors

Abstract

The strain induced resistance change is compared for asymmetric, symmetric and diffused piezoresistive elements. Finite element analysis is used to simulate the performance of a T-shaped piezoresistive MEMS cantilever, including a lumped parameter model to show the effect of geometric asymmetry on the piezoresistor sensitivity. Asymmetric piezoresistors are found to be much more sensitive to applied load than the typical symmetric design producing about two orders of magnitude higher resistance change. This is shown to be due to the difference in the stress distribution in the symmetric and asymmetric geometries resulting in less resistance change cancellation in the asymmetric design. Although still less sensitive than diffused piezoresistors, asymmetric piezoresistors are sensitive enough for many applications, and are much easier to fabricate and integrate into MEMS devices.

Buckled beam mechanical memory using an asymmetric piezoresistor for readout

“…In this manner, one piezoresistor increases in resistance, and one piezoresistor decreases in resistance, permitting a differential resistance measurement to be performed. Unlike the design reported in [22], there is a strong transverse force on the piezoresistor due to the buckled beam. This transverse force must be supported, necessitating the use of a short piezoresistor.…”

“…To differentiate the direction that the beam is buckled, asymmetry must be introduced. We utilize the asymmetric piezoresistor design that we introduced in [22] where one length of the piezoresistor is wide and the other length of the piezoresistor is narrow. Two piezoresistors with different orientations are used at either side of the buckled beam (figure 1).…”

“…A larger beam width would result in a substantially larger switching voltage and require a longer beam to buckle. The piezoresistor base widths of 5 µm and 11 µm are chosen because our group has obtained a good response from this design in the past [22]. Figure 3 shows the simulated resistance change between the two stable states as a function of beam length for a series of different base lengths.…”

“…The piezoresistor on the bottom anchor is wider on the left than on the right. The stress profile within the piezoresistor creates a net change in resistance that depends upon the direction of the beam buckling [22]. The piezoresistance increases when the beam bends towards the narrow section of the piezoresistor.…”

Near-Surface Electronic Contribution to Semiconductor Elasticity

“…The resistance of the base changes with the strain of the moving cantilever due to the piezoresistivity of the silicon, so that cantilever motion can be detected by monitoring the ac voltage generated across a sense resistor in series with the base. The base has an asymmetric design to maximize the resistance change [22]. Figure 1(b) shows the frequency response of a representative device measured in a vacuum chamber at a pressure of 2 x 10 −6 mbar.…”