太曼-格林干涉儀量測藍寶石晶圓之薄膜應力

Abstract

在藍寶石基板上沉積薄膜的製程中，由於薄膜材料與藍寶石基板的材料性質不同，造成薄膜與基板內皆有殘留應力使得基板變形，殘留應力產生主因可分為基板與薄膜兩者晶格的不匹配與兩者的膨脹係數不同，分別產生了本質應力與熱應力。本研究使用太曼-格林干涉儀的系統量測晶圓表面變形並推估殘留應力。利用形態學影像處理消除干涉儀系統拍攝所得干涉條紋影像內的雜訊，將消除雜訊後的干涉影像細線化以定位干涉條紋中心位置。利用干涉條紋中心位置的資訊以Zernike多項式做曲面擬合計算各項的待定係數得到晶圓表面形貌，根據擬合的晶圓表面形貌資訊可成功推估藍寶石晶圓與薄膜內的應力。

In the epitaxial process of gallium nitride (GaN) deposition on a sapphire wafer, the large residual stress usually results in wafer bowing because of lattice and thermal misfit between both thin film and substrate. The residual stress comprises of intrinsic stress and thermal stress. The Stoney stress formula for wafer bowing was amended, and the residual stresses in the thin film are modeled as biaxial forces acting at the sapphire substrate which is an anisotropic material. In this study, Twyman-Green interferometer was used to measure the surface topography of GaN deposited sapphire substrate. After reduction of interferogram noise by the morphology image process, the image thinning algorithm was used to locate the center of interference fringes. The Zernike polynomials were employed to describe a wave front of sapphire substrate by surface fitting. The residual stress was determined according to the curvatures calculated by finite differences of those surface fitting data of the measured topography.