雙光束量測藍寶石晶圓曲率及薄膜應力

Abstract

摘 要 氮化鎵發光二極體晶圓在磊晶製程，因為薄膜與基材間的晶格不匹配，造成本質應力。製程中溫度變化產生的熱應力也會形成薄膜殘留應力，使基板彎曲變形。過大的殘留應力使得薄膜品質不穩定，甚至破裂或皺摺而脫落，高品質的磊晶製程需要精確地監測晶圓曲率，以便控制製程參數，降低薄膜殘留應力，提高良率。 本研究採用非接觸之雙光束量測法，測量氮化鎵薄膜覆蓋之藍寶石基板的曲率。量測系統以兩平行雷射光入射翹曲晶圓表面，反射光的平行度改變，採用電荷耦合相機偵測投影至旋轉屏幕上的兩光點中心位置，依據幾何關係計算晶圓表面的曲率。另以古典板理論推導非等向性基材的薄膜應力，修正一般習用的Stoney薄膜應力公式。並對雙光束量測法之元件及系統進行測試，降低量測偏差量。曲率量測結果與太曼-格林干涉儀量測之曲率比較，兩者趨勢相符。本研究採用的量測系統不屬於光學干涉儀，反應快，不易受環境擾動影響，所需空間小，適合應用於磊晶製程監測。

關鍵詞：雙光束量測法、晶圓曲率量測、薄膜殘留應力

ABSTRACT

Lattice misfit between the gallium nitride thin film and sapphire substrate often induces intrinsic stress in light-emitting diode (LED) wafers in the epitaxial process. Besides, the thermal mismatch also yields wafer bowing and residual stress in the thin film. The quality of thin-film deposition becomes unstable and followed by peeling or wrinkling if large residual stress remains in the thin film. It results in a need for precise evaluation methods to measure wafer curvature during and after the epitaxial deposition process. The process parameters can then be controlled to reduce the thin film residual stress and to increase the yield rate of epitaxial wafers. This study determines the thin-film residual stress by using the curvature of a GaN-based sapphire wafer measured by a non-contact, dual beam optical method. Two parallel beams of laser light are incident to the warped surface of a test wafer. The laser beams reflected from the curved surface do not remain parallel. A charge-coupled device camera was used to detect the relative distance of two reflected beam spots projected on a rotating ground glass screen. The curvature is determined in accordance with the geometric relation between the wafer bow and deflection of light beams. In addition, the conventional Stoney formula for thin film residual stress was amended for the anisotropic substrates under bi-axial stresses using the classical plate theory. The measuring components and system were evaluated to reduce the standard deviation of data set. The measured results were compared to those obtained by Twyman-Green interferometer. Both have a good agreement in tendency. Dual beam optical method follows geometric optics and differs from interference method. The present method is not easily interrupted by environment, and it is suitable to be applied in a compact space for epitaxial process monitoring.

Keywords：dual beam optical method, measurement of wafer curvature, thin-film residual stress