The Improvement of Temperature Coefficient of Frequency in Thin Film Bulk Acoustic Wave Resonator using Secondary Harmon
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J10.4.1
The Improvement of Temperature Coefficient of Frequency in Thin Film Bulk Acoustic Wave Resonator using Secondary Harmonics Yukio Yoshino, Masaki Takeuchi, Hajime Yamada, Yoshihiko Goto, Tadashi Nomura, Takahiro Makino and Seiichi Arai Murata Mfg. Co., Ltd., Nagaoka-kyo, Kyoto, JAPAN ABSTRACT We have succeeded in making an 870MHz-range thin film bulk acoustic wave (BAW) resonator that has a small temperature coefficient of frequency (TCF) using secondary harmonics. The 870MHz-range BAW resonator has been requested to have nearly zero TCF, because it will be used in an oscillator for remote keyless entry systems. The BAW resonator has composite structure that consists of Al electrodes and ZnO/SiO 2 . We directed our attention to the fact that ZnO and Al have negative TCF, and SiO 2 has a positive one. It is theoretically possible to make zero TCF BAW resonators by optimizing the thickness ratio of ZnO and SiO 2 . However, using fundamental resonance, TCF is so sensitive to the thickness ratio that it cannot be easily controlled by MEMS techniques. We found in finite element method simulation and confirmed by experiment that the TCF of secondary harmonics has a local minimum when changing the ZnO/SiO 2 thickness ratio. As the result, a nearly zero TCF resonator without strict control of ZnO/SiO 2 thickness ratio has been realized by adopting Al/ZnO/SiO 2 /ZnO/Al/SiO 2 structure and combining thermal oxidized Si and sputtered SiO 2 . The resonator has the TCF of -1.86ppm/degree in the range of -40 to 85 degrees centigrade. INTRODUCTION A bulk acoustic wave (BAW) resonator using piezoelectric thin film is suitable for high frequency telecommunication devices in quasi-microwave and microwave range [1]. Many studies of BAW resonators for high frequency devices have been carried out in recent years [2,3,4,5,6]. BAW resonators generally have a high quality factor (Q factor), stable resonant frequency and small temperature coefficient of frequency (TCF) by design. In case of ZnO/SiO 2 structure BAW resonator, the resonant frequency and TCF depend on both the thickness ratio of each thin film consisting of membrane and the temperature dependence of sound velocity of the materials. It is possible to make BAW resonators from about 100MHz to several GHz by changing the thickness and thickness ratio [7]. We have recently studied the interface microstructure among ZnO, buffer electrodes and substrates, and succeeded in making a 200MHz BAW resonator with good resonant characteristics [8]. We showed TCF of BAW resonator could be controlled by changing membrane composition in the last study [9]. In this article we designed and made the BAW resonator that had a very small TCF for 870MHz remote keyless entry
J10.4.2
(RKE) system using secondary harmonics. EXPERIMENTAL Non-doped Si wafers with high resistance (>10 3 ohm-cm) are used for substrates to make 870MHz-range BAW resonators. Figure 1 shows a schematic structure of a BAW resonator. The membrane is composed of a top electrode, a ZnO piezoelectric thin film, and a bott
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