Abstract
A high performance piezoelectric film, such as Aluminum nitride (AlN), combined with high acoustic wave velocity substrate, such as diamond, is a promising for applications as high frequency SAW devices. However, diamond films with smoother surface are urgently needed. We first grow diamond films using CH4/Ar plasma to result in a diamond film with very small grain size (< 10 nm), the ultra-nano-crystalline films (UNCD). Then we grow high quality AlN thin films on UNCD, using TiN 150 nm/Ti 100 nm buffer layer for enhancing the adhesion of the AlN on UNCD. The scratch test shows the critical load of the sample was 33 mN. By tuning the buffer layer and AlN deposition parameters, c-axis oriented AlN with a thickness of 1 µm were obtained by reactive RF-sputtering technique. The columnar structured AlN grains, with c-axis oriented almost perpendicular to the diamond surface were obtained. AFM image showed the average surface roughness (Ra) was less than 15 nm and the d33 value measured by PFM wax 4.9 pm/V. The AlN/TiN/Ti/UNCD thin films show good potential for Diamond SAW device applications. The measurement by using 3 □m linewidth IDT reveal the frequency was 850 MHz and the surface wave velocity was 10200 m/s. For the film bulk acoustic wave device(FBAR), effect of buffer layer on the characteristics of the AlN thin films deposited on SiNx/Si substrate was systematically examined. Among the buffer layers examined, both Mo and Ti buffer layers can not only greatly enhance the (002) preferred orientation of the films, but also improve the smoothness of the AlN films, whereas the Al thin films contain large grains microstructure and resulting in rough surface and wide distribution of (002) preferred orientation of the films. AlN thin films with smooth surface with (r.m.s.< 5 nm) and narrow distribution of grains’ orientation, which is suitable for fabricating the devices. A thin film bulk acoustic wave resonator with resonance frequency around 2.7 GHz was fabricated from thus obtained AlN thin films and the insertion loss was about -5 dB. The electro-mechanical coupling coefficient was 4.0~5.8% of Mo/AlN/Mo was close to the theoretical value. Based on the resonator measurement, we can transform the characteristics into BVD equivalent circuit. The model not only provides the information of the loss term of the resonator but also can anticipate the performance of the filter. Based on the simulation result, we can develop thin film bulk acoustic wave filter and improves its characteristics.