Mechanical Loss Angle Measurement for Stressed thin Film Using Cantilever Ring-Down Method

Mechanical loss of the coating materials, and hence thermal noise from the mirror coatings, is a limiting factor for the sensitivity of the laser interferometer gravitational waves detector at its most sensitive frequency range. Mechanical loss of the thin films are often measured using the cantilever ring-down method. But when the thin film is under stress, the regular ring-down method gives incorrect results. We report a method to obtain the mechanical loss of stressed thin film using the cantilever ring-down technique. A proof-of-concept example is given to demonstrate and verify our method. The method can also be applied to obtain the mechanical loss angle of a rough interface; an example showed that loss angle of the interface between silicon nitride film deposited by plasma enhanced chemical vapor deposition method and silicon substrate is highly frequency dependent.

涂层材料的机械损耗及其引发的反射镜涂层热噪声，是激光干涉引力波探测器（laser interferometer gravitational waves detector）在其最高灵敏频段内灵敏度的核心限制因素。薄膜的机械损耗通常采用悬臂梁振铃法（cantilever ring-down method）进行测量。但当薄膜承受应力时，常规振铃法会得到错误的测量结果。本研究提出一种基于悬臂梁振铃技术的应力薄膜机械损耗测量方法，并通过概念验证实例对所提方法进行了演示与验证。该方法还可用于获取粗糙界面的机械损耗角；示例表明，等离子体增强化学气相沉积（plasma enhanced chemical vapor deposition, PECVD）法制备的氮化硅薄膜与硅基底之间的界面损耗角具有显著的频率依赖性。