Deposition of Piezoelectric ZnO Films by RF Magnetron Sputtering and its Application to SAW Devices
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Deposition of Piezoelectric ZnO Films by RF Magnetron Sputtering and its Application to SAW Devices Seol Hee Choi, and Chan Hyoung Kang Department of Advanced Materials Engineering, Korea Polytechnic University, 2121 JungwangDong, Shihung-Si, Kyonggi-Do, 429-793, Korea, Republic of ABSTRACT Highly c-axis oriented, dense, and fine-grained polycrystalline ZnO films with smooth surface and high resistivity were deposited on 4 inch silicon wafers by employing ZnO targets in a radio-frequency (RF) magnetron sputtering system. By changing applied RF power, substrate temperature and O2/Ar gas ratio, the optimum process parameters were found to be 150 W, 200 °C and 30/70, respectively. Applying the ZnO films deposited under these optimum conditions, surface acoustic wave (SAW) devices of ZnO/IDT/SiO2/Si structure were fabricated by conventional photolithography and etching processes. The interdigital transducers (IDT), made of the aluminum deposited by DC magnetron sputter, were patterned as 2.5/2.5 µm of finger width/spacing. Another type of SAW filter of IDT/ZnO/diamond/Si structure was fabricated. In this structure, high-quality nanocrystalline diamond (NCD) films were deposited on 4 inch silicon wafers by direct current (DC) plasma assisted chemical vapor deposition method using H2-CH4 mixture as precursor gas. On the top of the diamond films, ZnO films were deposited under the optimum conditions. The aluminum IDT pattern was fabricated on the ZnO/diamond layered films. The characteristics of the fabricated SAW devices were evaluated in terms of center frequency, insertion loss, and wave propagation velocity. INTRODUCTION The ZnO crystal with hexagonal wurtzite structure shows a good piezoelectric behavior. Piezoelectric ZnO thin film has a cheaper manufacturing cost than piezoelectric single crystal materials and can be deposited on other substrates. As a SAW device material, ZnO film should show an excellent c-axis orientation, flat surface roughness, and high resistivity of more than 106 Ωcm. Many deposition techniques have been developed to prepare ZnO thin films, i.e., radio frequency (RF) sputtering, direct current (DC) sputtering, chemical vapor deposition (CVD) and pulsed laser deposition. With the development of mobile telephones and wireless local area network systems, high frequency surface acoustic wave (SAW) devices in the GHz range are increasingly on demand. The existing SAW filter has been made of piezoelectric single crystal materials like quartz and LiNbO3, but the miniaturization tendency of electronics technology has required the studies on the SAW filter made of ZnO piezoelectric thin films [1]. The multi layers including piezoelectric thin films coupled with high phase velocity transmitting substrates such as diamond and sapphire are reported to achieve high-frequency SAW devices [2-8]. In this research, ZnO films were deposited by RF magnetron sputtering with a highly c axis orientation and a high resistivity on the nanocrystalline diamond substrate and SAW filter devices were fabrica
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