The characterization of SAW coupling factor from paired IDT insertion loss measurements

The extent to which the measured transmission insertion loss between paired identical unapodized IDTs can be correlated to the coupling factor of a piezoelectric substrate or piezo-film layer is considered in this paper. The paired transducer insertion loss of standard and rotated cuts of commonly used piezoelectric substrates with coupling factors in the range from 0.000 1 to 0.1 were measured using a harmonic rich split finger IDT. The network analyzer measurements were made in a test fixture having a 50-ohm connection in close proximity to the transducer electrodes. The reported coupling factors were compared with the measured insertion loss values for fundamental and harmonic modes over a frequency range from 30 MHz to above 1.0 GHz. On the average a 20-dB increase in insertion loss represented a factor of 10 decrease in coupling factor. Graphical data was developed and applied to estimate the coupling factor values for AlN film layers on silicon substrates under various deposition conditions.