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

Abstract

Microelectromechanical systems (MEMS) often incorporate piezoelectric thin films to actuate and detect motion of mechanical structures. Aluminum nitride is advantageous for MEMS use because it can be deposited at low temperatures, is easily patterned using conventional photolithographic techniques, and is compatible with CMOS contaminant requirements for silicon IC foundries. In this work, AIN thin films were deposited on silicon for use in a MEMS ultrasonic resonator. The resonator is configured as a gravimetric chemical sensor. A rotatable central composite designed experiment was performed to optimize film properties affecting device performance: film crystallinity, stress, and uniformity. Film property response characterization was conducted with x-ray diffractometry, spectroscopic ellipsometry, and surface profilometry. Optimization of film deposition parameters improved AIN film properties in the MEMS sensors. Film property characterization using response surface methodology indicated microstructural changes due to sputtered particle bombardment of the growing film surface. Surface morphology of the sputtered AIN films was assessed using tapping mode atomic force microscopy and scanning electron microscopy. Energetic particle bombardment of the growing film surface helped to yield dense crystalline films with zone T microstructure. Thermalization of the impinging particle flux resulted in voided films with zone 1 microstructure with inferior film properties. Correlation between film crystallinity and oxygen content was explored with x-ray photoelectron spectrometry. Changes in film microstructure and composition are correlated with variations in deposition parameters. Adatom mobility during film growth appears to play an important role in determining final film properties.