DC magnetron reactive sputtering of low stress AlN piezoelectric thin films for MEMS application

Many MEMS devices require piezoelectric excitation and readout to actuate and sense motion of mechanical structures. Aluminum nitride is advantageous for MEMS fabrication because it is compatible with silicon integrated circuit foundry impurity contamination requirements, can be deposited at low temperatures, provides a high piezoelectric coefficient, and is easily patterned using conventional photolithographic techniques. In this work, AIN thin films were deposited on silicon substrates for use in a MEMS silicon membrane ultrasonic resonator. The ultrasonic resonator is configured as a gravimetric sensing device for chemical detection. Issues of concern with regard to device performance and yield include the maximization of the electromechanical coupling constant (k(2)), film stress control, and film uniformity; these issues were addressed through a central composite design set of experiments to resolve the film property responses as a function of the deposition parameters. Film characterization was conducted with x-ray diffraction, spectroscopic ellipsometry, and surface profilometry. Optimization of film deposition parameters improved sensor performance and enabled further device miniaturization with the use of thinner films.