Characterization of MEMS piezoresistive pressure sensors using AFM

Atomic force microscope (AFM) is adapted to characterize an ultrasensitive piezoresistive pressure sensor based on microelectromechanical system (MEMS) technology. AFM is utilized in contact mode to exert force on several different micromachined diaphragm structures using a modified silicon cantilever with a particle attached to its end. The applied force is adjusted by changing the trigger voltage during each engage step of the probe-tip on the diaphragm surface. The contact force is determined from the force plots obtained for each trigger voltage in advanced force mode. Low force values in the range of 0.3-5 μN have been obtained with this method. This force induces strain on the bridge-arm of the diaphragm where the polysilicon resistor is located. The resultant change in the resistance produced due to varying force/pressure is measured using a delta mode current-voltage (I-V) measurement set-up. The contact mode AFM in conjunction with a nanovoltmeter enables the calibration of very sensitive force sensors down to 0.3 μN.