Sensitivity analysis of a thin film bulk acoustic resonator ladder filter

In this paper, an analysis of sensitivity of the filter performance of a solidly mounted thin film bulk acoustic wave resonator ladder filter is presented. The ladder filter investigated consists of three series and two shunt resonators. Frequency sensitivity analysis of a single resonator and changes in out-of-band rejection, filter bandwidth, and insertion loss as a result of variation in thicknesses of piezoelectric layers and mounting interlayers is presented. Also, resonator and reflector layer thickness tolerances that must be held during filter fabrication are presented. Analysis of a single resonator shows that the thickness of the piezoelectric layer has the most dominant effect on the resonant frequency. For the acoustic stack, frequency sensitivity increases as the layers are traversed from the bottom layer to the topmost layer. The use of the multilayer acoustic stack resulted in increase in the Q of the resonator. Small variations in resonator material thicknesses show negligible effects on the out-of-band rejection level and an approximately linear relationship with filter bandwidth. Insertion loss sensitivity on the other hand indicated a non-linear relationship between thickness variations and corresponding effects on insertion loss.