Adjustment of Membrane Stress Using Aluminum Oxide and Silicon Dioxide Multi Layer Structure
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Adjustment of Membrane Stress Using Aluminum Oxide and Silicon Dioxide Multi Layer Structure Ryuichi Kubo, Yukio Yoshino, Kazuhiro Inoue, Takahiro Makino, and Seiichi Arai Murata Mfg. Co., Ltd., Kyoto, JAPAN ABSTRACT Relaxation of internal stress for micro-electromechanical systems (MEMS) using SiO2 / Al2O3 / SiO2 membrane has been studied. The aluminum oxide thin films were formed by electron beam evaporation at room temperature. No peaks were observed in the X-ray diffraction pattern of the films. The ratio of Al and O in aluminum oxide was stoichiometric compared with Al2O3 target material. The internal stress was tensile at about 300-400 MPa. Bottom SiO2 thin film was formed by thermal oxidation and the top one by RF magnetron sputtering method. The internal stress of thermally oxidized SiO2 film was compressive at about 440 MPa, while that of the films deposited by sputtering, was compressive at about 100 MPa. The ratio of Si and O in each SiO2 thin films remained stoichiometric. The total stress of the membrane was controlled by optimizing the thickness of each film for relaxing the total stress of the membrane. The total stress of the membrane became almost zero under optimum conditions of SiO2 and Al2O3 films. INTRODUCTION Relaxation of internal stress is necessary for micro-electromechanical systems (MEMS), using membrane, such as thin film resonators, infrared sensors, etc. This is because the internal stress causes destruction or bending of MEMS devices[1, 2]. Both silicon nitride film and silicon dioxide thin films are usually used for the relaxation of internal stress in the MEMS structures. In these cases, silicon nitride films have tensile stress and they were commonly deposited by chemical vapor deposition (CVD) method[3, 4, 5]. This is a high temperature process, therefore, it makes heat migration and residual stress in the structure of MEMS[6]. Whereas Al2O3 thin film has tensile stress and can be deposited by electron beam evaporation at room temperature. In this paper, we propose Al2O3 thin film deposited by the electron beam evaporation for the relaxation of internal stress of MEMS structures. And the characteristics of SiO2 thin films prepared by sputtering and thermal oxidization are described. EXPERIMENTAL The Al2O3 thin films were deposited by electron beam evaporation, using the Al2O3 granule target. The filament current of electron beam gun was adjusted to keep the deposition rate about 50nm/min during the evaporation. Substrate temperature was kept at room temperature. Crystallinity of the films was identified by x-ray diffraction (XRD). Compositions of the films were estimated by means of energy dispersive x-ray spectroscopy (EDX). Two different methods of SiO2 thin film preparation were adopted in our experiment, thermal oxidation (1100 90min) and radio frequency (RF) sputtering. Deposition conditions of sputtering are shown in Table . EE5.3.1
Table . Deposition conditions of SiO2 sputtering Target Substrate temperature Ambient gas Ambient pressure Target-substrate distance Depositio
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