Improving frequency control of temperature compensated surface acoustic wave devices

Abstract

In this paper, we demonstrated improvement of frequency uniformity and stability for Temperature Compensated Surface Acoustic Wave (SAW) devices. SiO2 has been used to obtain low Temperature Coefficient (TempCo) in SAW devices for more than three decades [1]. One of the big issues is that SAW devices have to be processed at temperatures below 300C. When low temperature SiO2 is exposed to the ambient environment, it interacts with ambient humidity [2], [3], [5], [6]. Such interactions can change frequency of the SAW devices and can make frequency trimming with a focused Ion Beam [7], [8] extremely challenging. UV and steam treatment of SiO2 improved trimming rate stability on the first trimming [4] and [9], but was not sufficient to provide tight frequency control required for SAW devices after the second trimming. Using silicon nitride (Si3N4) capping layer on top of SiO2 showed some improvement in frequency control after trimming process. Most improvement was obtained using aluminum nitride (AlN) capping layer on top of SiO2 followed by two trimming steps.