A 900 MHz SAW resonator filter on Li2B4O7 crystal

A 900-MHz SAW (surface acoustic wave) resonator filter for the front end has been fabricated on a 45° rotated X cut Z propagation Li₂B₄O₇ crystal. The SAW resonator filter consists of three IDTs (interdigital transducers) and two reflectors with Al electrodes. When the Al thickness is below 500 Å, the insertion loss increases steeply. On the other hand, when the Al thickness is over 800 Å, the insertion loss rises in the filter. Therefore, the Al thickness is determined at 740 Å to improve the insertion loss on the basis of experimental results. The filter has a center frequency of 914 MHz, an insertion loss of 3 dB, and a rejection level of more than 60 dB. The aperture is determined to reduce transversal modes. The center frequency varies 350 ppm and the passband ripple is less than 1.5 dB in the temperature range from -10°C to +55°C. The filter has sufficient electrical characteristics for a front-end filter