以直流電製備奈米雙晶銅及其在3D IC 封裝之應用

Abstract

本論文主要研究具[111]優選方向的奈米雙晶銅材料的電鍍沉積技術與其在3D IC電子接點上的應用。奈米雙晶的微結構鑑定必須使用聚焦離子束技術(Focused ion beam,FIB)以及電子背向散射技術(electron backscatter diffraction,EBSD)，為因應3D IC微凸塊的尺度特徵，我們特別發展出一種低掠角聚焦離子束研磨技術，並收錄於第二章。同時也節錄數種因孔洞形成導致接點失效的線路修復方法。 第三章敘述以控制電流密度與轉速的製程條件，沉積具備高度優選方向以及高密度雙晶體堆疊銅膜的詳細方法，並且對奈米雙晶銅進行詳盡的微結構鑑定。相較於過去文獻的結果，無論在沉積速率、晶格方向優選性均有突破性的進度。XRD繞射法分析指出:鍍層中(111)繞射峰強度是(222)強度的506倍，也遠高於過去文獻中電鍍雙晶銅層的紀錄。所生成的雙晶晶格間距約為 10 nm到 100 nm之間，高密度的銅雙晶提供優異的機械性質，機械硬度高達2.23GPa。藉由直流電鍍可高速沉積20 μm的銅層厚度，使此材料非常適合用來製造3D IC 中的微凸塊結構。第四章研究此奈米雙晶銅在微凸塊冶金反應下的行為特性。由於雙晶晶界(twins boundary)上聚集著大量的原子差排與扭結可以用來作為空位阱(vacancy sink)。因此奈米雙晶銅可以消除克氏孔洞(Kirkendall voids)。在實際應用於晶片封裝的表現上，證實奈米雙晶銅可提升接點可靠度。 克式孔洞的形成會使銅錫界面的機械強度變弱。我們藉由奈米雙晶銅不會產生孔洞的特性，可以單獨觀察到Cu/solder/Ni 微凸塊內部的交互作用對介金屬化合物成長的影響。10 μm的銲錫接點內形成的鎳濃度梯度，驅動鎳原子快速穿透到對向端影響介金屬化合物生成反應，形成比Cu6Sn5熱力學性質更穩定的三元合金相的(Cu,Ni)6Sn5，因此可以減緩Cu3Sn的成長。完整的結果已收錄於第五章。

This research proposed a novel approach for the fabrication of an [111]-oriented nanotwinned Cu (nt-Cu) material by utilizing electrodeposition technique and its application in microbumps of 3D IC. The material characterization of nt-Cu was performed with focused ion beam (FIB) and electron backscatter diffraction (EBSD). A developed low-angle FIB polishing technique for cross-sectioned microbumps is described in Chapter 2. In addition, several methodologies of circuits repair for the void-damaged solder joints have also herein. Chapter 3 proposes particularly the fabrication of preferred-oriented Cu films with densely packed nanotwins with various current and stirring speeds. Furthermore, the material characterization has been completed and reported. Compare with the previous literatures, this technique provides the high deposition rate and the intensity of preferred orientated microstructure. X-ray diffraction indicates the intensity ratio of (111) to (220) is as high as 506, which is the highest among the reported electroplated Cu films. The twins spacing ranges from 10 nm to 100 nm, which reveals a high hardness value of 2.23GPa. The film thickness of nt-Cu can be grown to exceed 20 μm thick with DC electrodeposition; therefore, which possesses the capability of the manufacturing of 3D IC microbumps. Chapter 4 demonstrates the metallurgy reaction of the nt-Cu-containing microbumps. As the high densities of steps and kinks at the nano twin boundaries which serve as vacancies sinks, the nt-Cu can eliminate the Kirkendall void. We found no formation of Kirkendall voids in solder reactions on the nano-twinned Cu. In practice, the joint reliability of chip packaging can be enhanced by nt-Cu . The formation of Kirkendall void can weaken the mechanical properties of the microbumps. Therefore, the void-free nt-Cu can be performed to observe the cross-interaction independently affecting to the interfacial reaction in Cu/solder/Ni microbump. The results indicates that the metallurgical reaction caused the Ni atoms diffusing to the Cu side to form the (Cu,Ni)6Sn5, and the growth of the Cu3Sn IMCs was inhibited due to the formation of the ternary intermetallic compounds, which possesses a lower free energy than Cu6Sn5 does. A considerable concentration gradient of Ni was detected in 10 µm solder sample, which triggers the diffusion of Ni atoms to the Cu side.