Precision Thin Film Metallization for Space-borne SAW Narrow Band Pass Filters using E-Beam Evaporation

Surface acoustic wave (SAW) filters provide an efficient and compact means of narrow band channelization in Satellite Communication payloads. The efficiency of this narrow band filtering is critically dependent on temperature and fabrication induced frequency shifts in the filter centre frequency. The temperature induced drifts can be minimized by the choice of a suitable piezoelectric substrate. However, tolerances associated with the fabrication processes, especially the thin film metallization thickness, can have a deleterious effect on the repeatability and accuracy of the centre frequency. Here, we present results demonstrating a reliable and repeatable fabrication process for SAW narrow band pass filter (BPF) development. We have achieved precise thickness tolerance of 1% for NiCr/Al metallization, using e-beam evaporation. The thickness characterization is conducted using stylus profilometer and X-ray reflectivity (XRR) measurement, using high resolution x-ray diffraction (HRXRD) system. We have showcased that a change of just ±120 Å in the metal thickness, about the nominal value, leads to a deviation of approximately ∓ 146 kHz in the centre frequency of a UHF SAW filter. Process details to minimize the metallization thickness tolerance are discussed and the ensuing benefits in achieving the desired centre frequency is presented through fabrication and measurement.