SAW characteristics of AlN films sputtered on silicon substrates

Abstract

This article is focused on the analysis of the electroacoustic response of surface acoustic wave (SAW) filters made of aluminium nitride (AlN) thin films on various types of Si wafers. AlN films with (0 0 · 2) orientation were deposited by RF reactive sputtering of an Al target in Ar and N 2 admixtures on Si(1 0 0) and (1 1 1) wafers with resistivities ranging between 10 and 2000 Ω cm. The electroacoustic response of SAW filters with an acoustic wavelength of 40 μm was analysed by measuring the S ij parameters with a network analyser. We have determined that the out-of-band loss is directly related to the Si substrate resistivity, varying from 26 dB for 10 Ω cm to 55 dB for 2000 Ω cm. The SAW velocity depends on the orientation of the Si wafer, being approximately 4700 m/s for Si(1 1 1) and 5100 m/s for Si(1 0 0). The electroacoustic responses of the SAW filters were fitted by computations based on a simple circuital model that takes into account parasitic effects such as airborne electromagnetic coupling and conduction through the substrate. This procedure provides accurate values of the electromechanical coupling factor k 2 even for devices with poor characteristics. Good quality SAW filters of AlN on high resistivity Si(1 0 0) wafers with k 2 larger than 0.12% are demonstrated.