Crystallinity effect of AlN thin films on the frequency response of an AlN/IDT/Si surface acoustic wave device

AIN thin films are deposited by using RF reactive magnetron sputtering at room temperature to investigate the possibility of AIN/IDT/Si surface acoustic wave structures with single and double interdigital transducer (IDT) patterns. Of the AIN films with different deposition conditions, the as-deposited AIN film at an RF power of 300 W and a working pressure of 5 mTorr has the most highly textured structure. Its textured structure is indicated by the following characteristics: the narrowest full width at half maximum (FWHM) of the X-ray diffraction (XRD) theta-rocking curve, namely, 1.52 degrees, and a high intensity value of the (002) plane of the wide-angle XRD pattern. The as-deposited AIN film at working pressure of 20 mTorr shows a relatively wide FWHM, 2.41 degrees, and a low intensity value for the (002) plane of the. wide-angle XRD pattern, thereby indicating a structure with a low degree of texture. The grains of the as-deposited films have almost the same sizes and shapes at various working pressure. In the frequency response measurements, better frequency properties are observed in the as-deposited AIN film at a working pressure of 5 mTorr. For example, the band width of the as-deposited AIN film at a working pressure of 5 mTorr is 12.5 MHz, which is larger than the theoretical value; in contrast, the band width of the as-deposited AIN film at a working pressure of 20 mTorr is 12.8 MHz. From the frequency response analysis, the grain boundary is found to affect the electromechanical coupling factor and the insertion loss. These results indicate that a high-performance AIN/IDT/Si device might be possible if a much better textured or epitaxial AIN film with a high grain boundary quality can be deposited on an IDT/Si substrate.