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For designing sensors, it is very important to know the mechanical strength of constituent thin film materials. Although material strength is generally measured by tensile experiment, it is difficult to apply it to thin films. In this work, we developed a new method which can easily evaluate the fracture strength of thin film. In the method, first, silicon nitride and silicon oxide are alternatively deposited on a silicon substrate. Then, a target thin film is deposited on the top surface of the specimen and the silicon substrate of the back side is removed to form a diaphragm at the central region. Since the substrate around the diaphragm is thick, the specimen can be easily handled without fracture. The multilayered diaphragm realizes to break the target film by tensile deformation. After an air pressure is applied to the top surface of specimen until the diaphragm is broken, the fracture strength of the target film is obtained by stress analysis based on the experiment results. Adapting this method to a titanium thin film of 230 nm thickness, the fracture strength was evaluated to be 2.3 ± 0.2 GPa. 1. 緒 言 MEMS(Micro-Electromechanical System)技術を用いたセンサは,加速度センサ,圧力センサ及び角速度センサ などがあり,様々な製品の性能向上や新製品を開発する上で必須なデバイスである.これらセンサを高信頼に設 計するには,使用する薄膜材料の機械強度を知る必要がある.しかしながら,その強度を計測するには試験片を 単層膜にしなければならず,この場合,試験片のハンドリングが困難なために特殊な加工を施した試験片及び試 験装置が必要(安藤他,2000,小川,2006, Tsuchiya et al.,1998,上杉他,2013)であった.そのため,薄膜材料の強度 データは多くは得られていない.本研究では簡便な薄膜強度試験方法として多層バルジ法を新たに開発した.バ ルジ法(Vlassak and Nix,1992)は,シリコン基板に単層の薄膜を堆積し,裏面のシリコン基板の一部を除去して ダイヤフラムを形成し,これに圧力を加える方法であり,試験片周囲は 1 mm厚程度のシリコン基板となるので, ハンドリングが容易,且つ試験装置もシンプルにできる.しかし,上記バルジ法を薄膜の破壊強度評価に適用し た場合,ダイヤフラム端部の圧縮場に応力特異場が形成される.その場合,得られる破壊応力は形状等で決まる

Development of evaluation method for fracture strength of thin films using multilayer bulge specimen