Frequency responses of AIN/IDT/Si surface acoustic wave structures with single and double interdigital transducer (IDT) patterns were characterized to investigate the grain-size effects of AIN piezoelectric thin films deposited by a sputtering method. In order to control grain size of films, r.f. sputtering powers were varied at 200 and 300 W. The as-deposited AIN film prepared at 300 W had a larger grain size than the film deposited at 200 W, but the grain shape of the as-deposited films was almost identical irrespective of r.f. power, which indicates larger grain-boundary area for the as-deposited film at 200 W. The as-deposited films prepared at 300 W and 200 W showed full width half at maximum values of 3.54 degrees and 4.41 degrees at the X-ray peak of 2 theta = 36 '', respectively. In the frequency response measurements, better frequency properties were obtained in the as deposited AIN film with an r.f. power of 300 W. For example, the electromechanical coupling factor of the as-deposited AIN film at 300 W was 0.13%, while that of the as-deposited AIN film at 200 W was 0.10%. From a frequency response analysis, the grain size was found to affect the frequency response strongly. These results indicate that AIN thin film promises high potential for a high frequency device if a highly-textured AIN film with large grain size can be deposited.

Keywords: piezoelectric thin film; sputtering; high frequency; grain size