Effects of Thermal Annealing on the Characteristics of High Frequency FBAR Devices

Yu-Chen Chang,Wei-Che Shih,Chien-Chuan Cheng,Wei-Tsai Chang,Jyun-Min Lin,Ying-Chung Chen,Bing-Rui Li

doi:10.3390/coatings11040397

Yu-Chen Chang, Wei-Che Shih + Show 5 more

Open Access

https://doi.org/10.3390/coatings11040397

Copy DOI

Abstract

In this study, piezoelectric zinc oxide (ZnO) thin film was deposited on the Pt/Ti/SiNx/Si substrate to construct the FBAR device. The Pt/Ti multilayers were deposited on SiNx/Si as the bottom electrode and the Al thin film was deposited on the ZnO piezoelectric layer as the top electrode by a DC sputtering system. The ZnO thin film was deposited onto the Pt thin film by a radio frequency (RF) magnetron sputtering system. The cavity on back side for acoustic reflection of the FBAR device was achieved by KOH solution and reactive ion etching (RIE) processes. The crystalline structures and surface morphologies of the films were analyzed by X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM). The optimized as-deposited ZnO thin films with preferred (002)-orientation were obtained under the sputtering power of 80 W and sputtering pressure of 20 mTorr. The crystalline characteristics of ZnO thin films and the frequency responses of the FBAR devices can be improved by using the rapid thermal annealing (RTA) process. The optimized annealing temperature and annealing time are 400 °C and 10 min, respectively. Finally, the FBAR devices with structure of Al/ZnO/Pt/Ti/SiNx/Si were fabricated. The frequency responses showed that the return loss of the FBAR device with RTA annealing was improved from −24.07 to −34.66 dB, and the electromechanical coupling coefficient (kt2) was improved from 1.73% to 3.02% with the resonance frequency of around 3.4 GHz.

Highlights

In recent years, the development of wireless communication systems is toward high frequency band and high stability, in which, the acoustic devices used in the systems must be small size, low cost and high stability [1,2]
In order to increase the resonance frequency of an film bulk acoustic resonators (FBARs) device, the thickness of the piezoelectric layer should be decreased, which would result in the deterioration of the characteristics of the piezoelectric film and decrease the electromechanical coupling coefficient of the FBAR
The precise thickness with optimized characteristics of the piezoelectric layer is strict for high frequency FBAR devices

Summary

Introduction

The development of wireless communication systems is toward high frequency band and high stability, in which, the acoustic devices used in the systems must be small size, low cost and high stability [1,2]. In order to increase the resonance frequency of an FBAR device, the thickness of the piezoelectric layer should be decreased, which would result in the deterioration of the characteristics of the piezoelectric film and decrease the electromechanical coupling coefficient of the FBAR device. It is expected that the characteristics of piezoelectric ZnO thin films and the frequency responses of FBAR devices can be improved through the treatment of the RTA process. In this study, it was focused on the fabrication of FBAR devices with resonance frequency of about 3.4–3.6 GHz for 5G wireless communication systems. The effects of rapid thermal annealing process on the crystalline characteristics of ZnO thin films and the frequency responses of FBAR devices were investigated and discussed

Materials and Methods

Results

Conclusions