A Monolithic Approach to Downscaling Silicon Piezoresistive Sensors

Multi-scale integration remains the primary challenge in the fabrication of miniature piezoresistive sensors, as the co-fabrication of a silicon nanowire along with a microscale shuttle is the main architecture facilitating high-sensitivity transduction. The efforts in this field are marred by the lack of batch techniques compatible with semiconductor manufacturing. A technology is introduced here that leads to the fabrication of a piezoresistive silicon nanowire sharing the same single-crystalline device layer of a thick silicon-on-insulator wafer as the microscale component. The approach is based on a combination of high-resolution lithography with a two-stage etching process. The demonstration is carried out by spanning an electrostatic comb-drive actuator and a micromechanical amplifier by a single nanowire. A gage factor range of 135-145 is obtained, corresponding to an almost 20% resistance change for a nanowire strain of 1.26 × 10
^-3
. The technique is shown to generate a two-order-of-magnitude scale difference within the same silicon crystal. It also provides ease of electrical access to the nanowire, as the nanowire does not remain buried underneath the thick micromechanical system. With the associated lack of high-temperature processes and its CMOS-compatibility, the technique is a promising enabler for future miniaturized piezoresistive sensors.