Piezoelectric thin film stress sensors for metal-forming operations

Sensor arrays are being developed to determine the friction distribution of the die (metal-working tool)-work material interface during metal forming operations. The basic element of the sensors are thin films of piezoelectric ZnO and AlN. Normal (compressive) stress sensors use films having their C-axis normal to their interface and shear-stress sensors use films having their C-axis ideally (for zero compressional stress response) inclined at 40 degrees to the interface normal. The details of the RF-diode sputter deposition of the films and the characterization of the piezoelectric properties of the films are described. A prototype multi-element normal stress sensor having 125- mu m feature size has been fabricated on a silicon wafer, and preliminary test results are described.<>