Acoustic Modes in Lithium Tetraborate

Acoustic modes propagating in X-cut. Z-propagating lithium tetraborate (Li2B407) substrates have been analyzed theoretically and experimentally. The theoretical predictions were made using a computer program written to search for all possible acoustic modes propagating in arbitrary cuts of piezoelectric materials. A new photolithographic process has been developed to fabricate interdigital transducers (IDTs) on Li2B407 to avoid using acids which attack and damage the substrates. Using this process, split-electrode SAWIDTs were fabricated with a period of 16 pm. The frequencies of the observed modes agreed to better than 0.5% with the theoretical predictions. The insertion loss of the SAW delay line was less than 10 dB at 219 MHz and the temperature coefkient of delay was measured to be about 6 ppm/"C. These numbers suggest that Li2B407 is a candidate to replace quartz as a substrate for narrowband SAW filter applications. A detailed description of the properties of the observed modes will be given. Methods used to suppress the non-SAW modes will also be described.