SAW oscillator frequency stability at high temperatures

A broad overview of various factors affecting the frequency stability of surface-acoustic-wave (SAW) resonators is given. Two major causes of rapid degradation in the long-term frequency stability are the presence of a chromium interface between aluminum and quartz, and moderate to high drive levels in SAW devices with pure aluminum fingers, resulting in metal migration in the region of high thin-film stresses. On the other hand, devices with copper-doped aluminum electrodes maintained excellent long-term stability, even when operating at 175 degrees C and at moderately high drive levels. Experimental data on both the long-term and short-term frequency stabilities of SAW devices at 25 degrees C and 175 degrees C for moderate to high drive levels are presented. Results obtained for the frequency stabilities of SAW devices with pure aluminum and copper-doped aluminum electrodes are compared. It is shown that the short-term frequency stability of SAW devices with copper-doped aluminum electrodes is a few parts in 10/sup 10/, even at 175 degrees C and for moderately large drive levels. Overall, the best short-term frequency stability is found to be for a gate time of 0.1 s. >