| 摘要: | 本研究目的在透過有限元素分析(FEM)，計算一有機金屬氣相沉積(MOCVD)反應腔體之旋轉載台在製程時受到高溫熱負載及不同轉速作用時的應力分佈與變形。考慮的負荷條件分別為無轉速只受溫度負載之狀態，以及主軸轉速10 rpm、100 rpm、500 rpm、1000 rpm、1500 rpm之情況。另外，本研究亦以系統化觀點考慮整個旋轉載台的溫度分布對氮化鎵薄膜磊晶翹曲及薄膜殘留應力的影響。此外，本研究亦以簡易模型分析不同晶圓直徑與材料、不同薄膜與晶圓厚度、加入緩衝層與否及溫度梯度對於晶圓翹曲及氮化鎵薄膜殘留應力的影響。為驗證本研究所建立有限元素分析模型之有效性，將模擬結果與前人以不同厚度氮化鎵磊晶在藍寶石晶圓量測實驗結果作比對，模擬結果之晶圓翹曲及晶圓曲率半徑改變趨勢和實驗結果一致，證實本研究所建立模型之有效性，可適用於評估各種磊晶參數對於晶圓翹曲及薄膜殘留應力的影響。
旋轉載台結構應力分析結果顯示此旋轉載台在同時受到溫度負載及各轉速的作用下，各個組件將不會有永久變形之情形發生。此外，轉速的提升對於結構應力的影響很小。溫度分布分析結果顯示，在同一個穩定的熱源條件下，以藍寶石晶圓磊晶氮化鎵薄膜的製程，旋轉載台上半部零件包含主承載盤、晶圓承載盤及晶圓，其溫度高於以矽晶圓磊晶的製程，且前者上半部零件溫度梯度皆小於後者。研究結果顯示有溫度梯度將增加晶圓翹曲及薄膜殘留應力，所以溫度均勻性對於磊晶製程是一重要參數。此外，氮化鎵薄膜磊晶於藍寶石晶圓，其薄膜殘留應力為壓應力，而磊晶於矽晶圓，其殘留應力為張應力；以裂縫生成的觀點來看，氮化鎵薄膜磊晶於藍寶石晶圓優於矽晶圓。
薄膜磊晶分析結果顯示不論磊晶於藍寶石晶圓或矽晶圓，增加磊晶薄膜厚度，晶圓翹曲量亦會增加，但薄膜殘留應力下降。增加晶圓厚度能有效的減少晶圓翹曲量，亦是業界常用的方法之一，但薄膜殘留應力會上升。考慮同厚度的晶圓，晶圓直徑的增大會造成翹曲量增加，甚至超越其本身厚度，此為大尺寸薄膜磊晶的一大挑戰，因此實務上直徑越大的晶圓將搭配越厚的晶圓以減緩翹曲。模擬結果顯示於薄膜及晶圓間添加緩衝層能夠降低薄膜殘留應力，且降低的程度隨著緩衝層厚度的增加而提升，因此，薄膜的可靠性可以透過加入緩衝層來改善。
;The aim of this work is using finite element analysis (FEM) to study the effects of thermal load and rotation speed on the structural integrity of a substrate holder module in an MOCVD reaction chamber. Several loading conditions are considered, including thermal load alone and thermal load plus rotation speeds of 10 rpm, 100 rpm, 500 rpm, 1000 rpm, and 1500 rpm. In addition, the wafer bow and residual stress of GaN growth on silicon or sapphire are systematically studied for various scenarios. The effects of size and material of wafer, thickness of film and substrate, buffer layer, and temperature gradient are characterized. Moreover, in order to validate the FEM model constructed in the current study, experimental results of a previous study are applied to assessing the credibility of the numerical methods by comparison of the simulation results with the experimental measurements of wafer bow. The variation trends of wafer bow and curvature radius in simulation agree well with those in experiment such that the constructed model is validated. Therefore, the constructed model is effective in assessing the effect of various parameters acting on a film-substrate system.
As the calculated critical stress is less than the strength of material, no structural failure is predicted for all the components in the given substrate holder module under all of the given loading conditions. The variation of critical stress with rotation speed in all of the components is small. Given a similar heat source in the MOCVD reaction chamber, temperature of the upper components such as susceptor, substrate holders, and wafers is higher in the case of sapphire wafer than that in the case of silicon wafer. The temperature gradient of upper components is greater for the silicon wafer case. A greater temperature gradient in the film-substrate system generates a greater wafer bow and residual stress. Therefore, the temperature uniformity is an important parameter for the epitaxial process. The sign of residual stress is different between a GaN film grown on a sapphire wafer and a silicon wafer (compressive for sapphire wafer and tensile for silicon wafer). For growing a GaN thin film, GaN thin film, sapphire wafer is better than silicon wafer in terms of lessening cracking in film.
No matter GaN is grown on sapphire wafer or silicon wafer, wafer bow increases and residual stress in the film decreases with an increase in thickness of film. Increasing the thickness of wafer can effectively reduce wafer bow, which is also a method commonly used in industry, but the residual stress in the film is increased. Given a wafer thickness, the size of bow is increased with wafer diameter, which is one of the major challenges in growth of a large-size epitaxial wafer. The magnitude of residual stress in a thin film can be reduced when a thick buffer layer is added between film and wafer. For a lower residual stress, the reliability of a thin film can be improved by the addition of buffer layer. |
