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A Study on ZnO Based Lateral Field Excited Thin Film Bulk Acoustic Resonator (FBAR)

Author: ZhuZuo
Tutor: WangDeMiao; JinHao
School: Zhejiang University
Course: Electronics and Communication Engineering
Keywords: FBAR lateral field excitation ZnO embedded electrodes
CLC: TN65
Type: Master's thesis
Year: 2014
Downloads: 25
Quote: 0
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Abstract


Film bulk acoustic resonator (FBAR) is an radio frequency micro-electromechanical Systems (RF-MEMS) device based on piezoelectric effect. Owing to its high resonant frequency, high mass sensitivity, small size and CMOS-compatibility, FBAR has made great commercial success in the field of wireless communication. Meanwhile, biochemical sensors, another important application of FBAR technology, have got increasingly close attention of academia and market.Biochemical sensors generally work in liquid environment, however, as for conventional longitudinal wave FBAR, its quality factor value (Q) in liquid environment is significantly attenuated, and therefore, shear wave which can maintain a high Q value in liquid environment becomes a hot research direction. There are two main methods to excite the shear acoustic wave, thickness excitation and lateral field excitation. Thickness excitation need piezoelectric film grows at a tilt angle with c-axis, which is complex and hard to repeat. Lateral field excitation take advantage of a mature c-axis oriented piezoelectric film growth process, as well as the simplification of electrodes structural, which make it the best choice for bio-chemical FBAR sensor.We deposited highly c-axis oriented ZnO piezoelectric film, studied the processes of manufacture of back-etching type FBAR based on ZnO film, and successfully obtained two series of devices:the thickness excitation longitudinal wave FBAR and the lateral field excitation shear waves FBAR, and finally proposed a novel lateral shear-wave excitation FBAR device with embedded electrodes. The main content and results of this research are listed as follows:We had got theoretical derivation of the principle of the lateral field excited shear FBAR resonator, and establish a corresponding mathematical model.We had studied the effect of deposition parameters on the characteristics of ZnO films by DC reactive magnetron sputtering, and successfully obtained ZnO thin films with high c-axis orientation. The optimal deposition parameters are:oxygen and argon gas flow rate are50sccm and100sccm respectively, substrate temperature is200℃, sputtering power is200W, sputtering pressure is2Pa, bias voltage is-75V. The surface of prepared ZnO thin films are flat and rule, grains are ovate particles with columnar growth, which showed good (002) orientation and the surface roughness (RMS) is only7.5nm.We had thoroughly studied the preparation process of back etching type FBAR, and successfully manufactured two representative longitudinal wave FBAR devices with the resonant frequencies (Q values) of2.69GHz (1116) and3.4GHz (170) respectively.We had prepared lateral field excited shear wave FBAR devices based on back etching process. Results show that there are parasitic noises around the dominant resonant frequency, which reduce the resolution as well as the Q value. The resonant frequency of the prepared device is522MHz, with Q value of30. Using Comsol Multiphysics finite element software simulating the device structure, we found that there also exist the parasitic resonances near the main resonant peak, which was similar to the measured results. We theoretically analyzed this phenomenon, and then proposed a novel structure with embedded electrodes, which can successfully eliminate the influence of noises in theory. And then we prepared a lateral field excited FBAR with embedded electrodes based on back etching process, the resonant frequency is809.4MHz with no distinct noise around the main resonance peak. The Q value improved slightly to80, these verified that the embedded electrode structure could improve the performance of devices.

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