Kazuhiro Inoue

Effect of Buffer Electrodes in Crystallization of Zinc Oxide Thin Film for Thin Film Bulk Acoustic Wave Resonator

A thin film bulk acoustic wave resonator (TBAR) has been fabricated using a ZnO thin film on a SiO 2 diaphragm by MEMs techniques. The ZnO/SiO 2 structure TBAR can be designed to cancel a temperature coefficient of frequency (TCF) by the ZnO/SiO 2 thickness ratio, because the TCF of ZnO is negative, and that of SiO 2 is positive. The ZnO thin film on the SiO 2 shows a c-axis orientation almost equivalent to that of the ZnO thin film on a glass substrate by RF sputtering. However, the crystallinity of the ZnO thin film is influenced by the surface conditions of substrates. ZnO thin films have been deposited on Au/Cr, Au/NiCr and Au/Ti. The Au/Ti/ZnO/Au/Ti/SiO 2 structure TBAR shows the best resonant characteristics in this experiment. The resonant characteristics of the TBAR depend on the crystallinity of the ZnO thin film. The resonant resistance of the TBAR at 205MHz using a Au/Ti under electrode is about 10% less than that using an Au/Cr electrode. The x-ray diffraction result shows that the crystallinity of ZnO is greatly influenced by the crystallinity of the lower electrode. The buffer layer between an Au electrode and substrate has an influence on both the crystallinity of the ZnO thin film and the resonant characteristics of the TBAR through the Au electrode.

Fabrication of 5GHz band film bulk acoustic wave resonators using ZnO thin film

We successfully fabricated 5GHz band thin film bulk acoustic wave (BAW) resonators using ZnO thin film. Two types of BAW resonators utilizing fundamental resonance and secondary harmonics were examined. In the fundamental mode resonator, the quality factor (Q factor) and the effective electromechanical coupling coefficient (k/sub teff//sup 2/) were realized at 1000 and 6.7%, respectively. These values for the secondary mode resonator were 700 and 4.7%, respectively. Combining four fundamental mode resonators as a ladder type piezoelectric filter, the bandwidth at 3 dB was achieved to be 191 MHz at 4.9 GHz. Even with the filter using the secondary mode resonator, the bandwidth was 130 MHz at 5.2 GHz.

Internal Stress of ZnO thin Films Caused by Thickness Distribution and Crystallinity

The improvement of thickness distribution and crystallinity in ZnO thin films prepared by radio frequency (rf) planer magnetron sputtering has been studied. Optimum thickness distribution of less than ± 2.2% in a 3-inch wafer is obtained by changing the substrate angle to the ZnO target and is in accordance with cosine law. The c-axis orientation perpendicular to the silicon substrate is confirmed by x-ray diffraction. The stress of ZnO thin films is larger than 0.3GPa and its distribution is independent of the substrate angle that is set at a slant to the optimum angle for thickness distribution. These results indicate that thickness distribution of ZnO thin films heavily depends on the substrate angle, while the stress and its distribution are independent of the setting angle of the substrate. Stress distribution is attributed to the distribution of argon ions and sputtered molecules impinging a wafer.

Fluctuations of Tc in YBCO thin films

Abstract Fluctuations of Tc in YBCO films were investigated using improved rf magnetron sputtering equipment. In fabricating high Tc films, the most important factor was precise control of the substrate temperature. Use of a new designed substrate holder led to films which had superior superconducting characteristics over a wide substrate area.