The effect of the fabrication process in propagation and reflectivity in an IDT

Narrow band SAW bandpass filter design requires an accurate computation of phase velocity and strip reflectivity within the IDT. It is well known that the influence of the technological process on both key parameters can not be neglected. With the help of the recently developed numerical FEM BEM model it is now possible to characterize with an excellent accuracy the effects of the shape of the electrode (up to a few thousand nodes can be used to mesh the electrode). In this paper, using an isotropic chemical etching model, a comparison between chemical etching and liftoff processes as well as the influence of the overetching parameter for the 38° Y rotated quartz will be presented. Three kinds of frequency ranges are studied corresponding to four, three and two fingers per wavelength. We will also show that a simple phenomenological model can be used to explain most results. Comparisons between simulations and measurements for low loss filters built using both liftoff and chemical etching processes will be shown