Interfacial Reactions Between Cu Substrates and Lead Free Solder for 3D IC Applications

近半世紀以來，半導體產業一直遵循摩爾定律的預測而發展。然而，對於電子產品的小尺寸與高功能性的不斷追求，摩爾定律持續發展的瓶頸已然迫在眉睫。因此，封裝技術轉而朝另一維度展開，3D IC技術與其相關領域的研究遂逐漸為業界及學術界所共同關注。藉由穿晶矽導孔技術，訊號的傳遞距離縮短，並可降低電子元件的功率損耗，隨著I/O數的提升，封裝模組也能有效達到尺寸減小的效果。然而，在小尺度銲點中，必須仔細考慮接點空間受限效應（space confinement）對銲點所造成的影響。由於銲點尺寸較小，介金屬化合物（IMC）在銲料體積中勢必佔有較高的比例，同時也影響了銲點的機械性質，介金屬化合物因此在3D IC結構中扮演十分重要的角色。此外，在RoHS與WEEE環保法令實行之後，無鉛銲料的相關研究至今持續蓬勃發展，基板與無鉛銲料之間的介面反應也成為各界相當關心的議題。 本研究主要觀察3D IC尺度之微銲點下銅基板與無鉛銲料之介面反應，並針對其與接點空間受限效應相關之三項關鍵議題進行討論。第一項議題與介面反應之微結構有關。將以熱壓接合法製備的Cu/Sn（10 μm）/Cu三明治結構試片分別在高溫下進行固液反應以及固態時效反應，觀察介面上所生長出的介金屬化合物形貌。在固液反應中，分別來自介面兩側的扇貝狀Cu6Sn5晶粒在經過短時間後即發生上下碰撞的現象，介金屬碰撞時間也在此被定義。而在固態時效反應中，層狀的Cu3Sn中有大量Kirkendall voids生成，對銲點的機械性質有造成嚴重威脅的可能。此外，本實驗也針對銲料中微量元素添加後的介面進行形貌變化的觀察。 第二項議題主要研究介金屬化合物之生長動力學。為了解3D IC中微小的銲料體積是否會對介金屬之生長速率造成影響，本研究將在小尺度下不同反應時間長度之介金屬厚度對反應時間作圖分析，並與文獻之數據互相比較，發現介金屬的生長速率與銲料體積並無明顯關聯，證實銲料體積對介金屬之生長機制的影響相當有限。若在銲料中微量添加其他元素，則可對Cu3Sn的生長造成抑制的效果。 第三項議題與介金屬的合併行為和晶粒方向性（grain orientation）有關。在固液反應中，由介面兩側生長出的Cu6Sn5在垂直接觸後合併成為較大的晶粒，並由EBSD的鑑定結果證實，合併後的Cu6Sn5已密合為一個具有相同晶粒方向性的單晶，並以極圖（PF）顯示晶粒之從優取向，說明在合併的Cu6Sn5之間是否有特殊之晶粒方向性存在。另一方面，在固態時效反應中，Cu6Sn5在互相碰撞之初並不立即密合，而是在反應時間拉長之後才會發生合併，其尺寸不但較固液反應中為小，晶界也相對較不平整，這些行為可歸因於晶粒之粗化（coarsening）現象，以及在較低反應溫度中原子的擴散速率較慢所致。在本研究中，除了實驗數據之外，亦針對理論假設與結果分析進行詳細而深入的探討，盼能對3D IC產業的發展有所助益。Semiconductor industry has been guided by Moore’s law in the past 50 years. However, smaller size and better functionality of electronic devices is now forcing Moore’s law to face its imminent limitation. As a result, 3-Dimensional Integrated Circuit (3D IC) is regarded as a promising innovation in packaging industry. By means of through silicon via (TSV) technique, shorter interconnect distance is provided, which decreases power consumption of the electronic devices. With higher input/output (I/O) density, the package volume is able to be effectively scaled down. Due to the miniaturized size of the solder joints in 3D IC packages, intermetallic compound (IMC) forming at the interface occupies higher percentage in the solder volume and plays a very important role in the joints, raising the concern about mechanical properties. Therefore, the space confinement effect on the interface should be under careful consideration. Furthermore, research of lead free solder has been under vigorous development since RoHS and WEEE were put into practice. The interfacial reactions between lead free solder and substrates in the micro jointing for 3D IC applications are worthy of investigation. In this study, interfacial reactions between Cu substrates and lead free solder for 3D IC micro joints are revealed. Three critical issues arising from the space confinement effect are discussed. The first issue is about the morphology of IMC forming at the interface. After thermal compression, Cu/Sn (10 μm)/Cu sandwiches experienced solid-liquid and solid state aging reactions. During solid-liquid reactions, scallop-type Cu6Sn5 grains which originally formed at opposite interfaces were able to impinge on each other after a short time frame. The time-to-impinge of IMC is defined in the case. In contrast, Kirkendall voids formed in the thick Cu3Sn layer after solid state aging process, which threaten mechanical properties of the joints. Moreover, the effect of minor element addition on morphology is also proposed. The second issue relates to the IMC growth kinetics. In order to investigate whether solder volume affects the growth rate of the IMC on 3D IC scale, diagrams of thickness of IMC versus reaction time was illustrated. Compared to literature, the growth rate was independent of solder volume in the diagrams. That is, the limited solder volume had little impact on the IMC growing mechanism before impinged. When the minor elements were added, the growth rate of Cu3Sn was retarded. The last issue arises from the merging behavior and grain orientation. After solid-liquid reactions, the vertically contacted Cu6Sn5 grains merged into a larger one and were identified as a single crystalline grain by electron back-scattered diffraction (EBSD) analysis. The pole figure (PF) showed the preferred orientation among the Cu6Sn5 grains after merging. On the other hand, Cu6Sn5 grains did not merge immediately after impingement during solid state aging reactions but after aging for longer time. After merging, the grains had smaller size and the grain boundaries were not as smooth as those in solid-liquid reactions. These behaviors could be attributed to coarsening process of the grains and slower diffusion rate at the lower temperature. The experimental results are reported in this study. Theoretical hypothesis and detailed analysis of above-mentioned issues will also be discussed.致謝 i 摘要 iii ABSTRACT v 目錄 ix 圖目錄 xi 表目錄 xvii 第1章 緒論 1 1.1 3D IC封裝簡介 1 1.2 無鉛銲料的使用 6 1.3 銅與無鉛銲料的介面反應 7 1.4 接點空間受限效應 11 1.5 介金屬的晶粒成長與合併 16 第2章 研究動機與目標 19 第3章 實驗步驟與方法 21 3.1 實驗設備與儀器 21 3.2 試片的準備 21 3.2.1 銲料 21 3.2.2 熱壓接合 21 3.2.3 單側迴銲試片 24 3.2.4 銲料微添加試片 24 3.3 實驗步驟 25 3.3.1 固液反應 25 3.3.2 固態時效反應 25 3.3.3 單側迴銲 25 3.3.4 金相分析試片準備 26 3.4 實驗結果分析 27 3.4.1 掃描式電子顯微鏡（SEM）觀察 27 3.4.2 電子微探儀（EPMA）分析 27 3.4.3 電子背向散射繞射儀（EBSD）分析 27 3.4.4 生長動力學分析假設與理論 33 第4章 結果與討論 35 4.1 介面形貌觀察 35 4.1.1 Cu/Sn/Cu固液反應 35 4.1.2 Cu/Sn3.5Ag/Cu固液反應 38 4.1.3 Cu/Sn/Cu固態時效反應 40 4.1.4 微添加的影響 42 4.2 生長動力學分析 49 4.2.1 Cu/Sn/Cu固液反應 49 4.2.2 Cu/Sn/Cu固態時效反應 50 4.2.3 微添加的影響 52 4.3 晶粒方向性分析 56 4.3.1 Cu/Sn/Cu固液反應 56 4.3.2 Cu/Sn單側迴銲 62 4.3.3 Cu/Sn/Cu固態時效反應 68 第5章 結論 71 參考文獻 7