Approach to on-wafer controllable trimming of SAW filters

We have developed the concept and the processing details of SAW filter trimming by thin film deposition having in mind the applicability of such a process to controllable on-wafer trimming. Fast and repeatable deposition is the key element for individual processing of multiple wafer regions. We have discussed and experimentally compared the following options: deposition of platinum (or other metals) instead of gold, and deposition of selected dielectrics, notably alumina and ytterbium oxide. We show that the latter is a promising candidate for trimming due to high density and low stiffness while the former does not prove to be useful for leaky waves in LiTaO₃. We have obtained trimming ranges of about 0.1% for ladder structures, 1% for DMS filters and 4% for resonators on lithium tantalate with negligible loss increase and little distortion in response.