c軸傾斜反転配向ZnO多層膜の作製と横波共振子への応用

摘要

In a previous study, we have reported c-axis 23° tilted ZnO film for shear wave excitations. We here propose a new multilayered resonator consisting of the c-axis tilted ZnO films. Every layer has same thickness, and the c-axis tilt direction in odd and even layers are symmetric with respect to the film surface normal. In this resonator structure, we can increase its thickness without change of shear mode resonant frequency. This provides the expansion of sensing area (in 50 Ω matching) and high power operation. Moreover, longitudinal wave excitation is expected to be suppressed because thickness extensional mode resonant frequency should shifts to lower frequency.rnIn this study, we have fabricated a multilayered resonator consisting of six layered ZnO films. To prevent degradation of crystalline orientation, SiO_2 buffer layers were inserted between each ZnO layer. The influences of the method for fabricating SiO_2 on the crystalline orientation also have been investigated. Crystalline orientation of the c-axis tilted ZnO films were confirmed by XRD pole figure analysis and FE-SEM. In the sixth order mode shear mode resonant frequency, high efficient shear mode excitation with relatively suppressed extensional mode excitation was observed. More extensional mode suppression is expected by increasing the number of the layer.%本研究では，これまでに電界方向に対してc軸が23°傾いたZnO膜の作製に成功している．この薄膜を用いてc軸の傾斜方向を交互反転させて積層すれば，高次の厚みすべりモード共振子を作製することができる．この共振子では共振周波数を維持したまま膜厚を大きくすることができる．また測定回路とインピーダンス整合を取るときの電極面積（センサ面積）も大きくでき，更に耐電力性の向上や，不要な縦波の励振も抑制できると考えられる．本研究では，RFマグネトロンスパッタ法を用いて，c軸を交互反転させた6層ZnO多層膜を作製した．結晶配向性を維持したまま多層構造を実現するのに有効なSiO_2緩衝層の作製方法について検証した．ZnO膜の三次元的な配向性は極点X線回折法を用いて評価した．さらに断面SEM画像からZnO膜の構造を評価した．共振子の変換損失を測定した結果，横波最小変換損失の6次モード共振周波数において，厚みすべりモード共振は維持されたまま，厚み縦モード共振は抑制された．膜をより多層化することで，さらに高次のすべりモード共振も可能と考えられる．