Improving frequency control of temperature compensated surface acoustic wave devices

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.