R.F.-magnetron-sputtered AlN films for microwave acoustic resonators

Abstract

Aluminum nitride (AlN) is becoming one of the more promising thin film piezoelectric materials because of its high ultrasonic velocity, low acoustic loss and fairly large piezoelectric coupling factor. Previous studies have shown that, using d.c. magnetron sputtering from an aluminum target in N 2, bulk acoustic wave resonators exhibiting a high electromechanical quality factor Q and good temperature stability may be prepared on silicon substrates. In the present work r.f. planar magnetron sputtering was employed for the AlN film deposition. The films are currently being made by reactive sputtering from an aluminum target in an Ar-N 2 atmosphere. X-ray diffraction data show that films deposited at above 200°C are highly oriented with the c axis normal to the substrate. Scanning electron microscopy examination of the film structure, together with reflection electron diffraction and optical transmission data, are used to determine the film quality. Variations in the film characteristics such as X-ray diffraction data and microstructure as a function of the deposition parameters such as the composition and pressure of the sputtering gas, the power and the substrate temperatures are investigated in order to optimize resonator properties. AlN films on silicon membranes are used for bulk acoustic wave measurements. Acoustic resonant responses of such composite resonators together with their applications will be discussed.