超薄層微銲點界面反應之最終端行為時介金屬剝離現象

Abstract

電子封裝產業之線寬微縮製程因為物理限制與複雜昂貴的微影製程而逼近極 限，三維積體電路藉由微銲點與矽穿孔技術，進行多種不同功能晶片之垂直性整 合，被視為最有潛力延續摩爾定律之封裝技術。然而，隨著微銲點的體積與銲點底 層金屬厚度的縮減，界面反應將在晶片組裝或時效處理過後耗盡銲錫，形成全介金 屬化合物接點，甚至更進一步耗盡潤濕層，使得介金屬化合物直接與黏著層直接接 觸。此行為稱為界面反應之最終端行為，這是在文獻中至今缺乏相關研究一塊。本 研究探討超薄層銅錫微銲點於固態時效處理後，界面反應之最終階段時的顯微結 構演變，並觀察剝離現象的發生與其發生機制。另外，有鑑於在金相製備時微銲點 與銲點底層金屬的界面易於產生裂紋影響觀測，故我們利用氬離子切割拋磨機來 進行金相製備，並藉由掃描式電子顯微鏡進行微結構觀察。 由顯微結構觀測的結果發現，當潤濕層(銅)在固態時效處理時耗盡後，界面 反應產生的 Cu6Sn5 與 Cu3Sn 就會從黏著層剝離，而此剝離現象的驅動力為介金屬 化合物與潤濕層之間的高界面能。此外，由微銲點破斷面分析結果顯示，已經產生 剝離的微銲點強度會大幅下降，對電子元件銲點的可靠度造成嚴重的威脅。故為了 避免剝離現象的產生，潤濕層的厚度一定要足夠到不被銲料所消耗殆盡。 本研究亦探討介金屬化合物的生長動力學行為，實驗結果發現在固態時效處 理與不考慮表面擴散的條件下，超薄層微銲點中的 Cu3Sn 符合拋物線生長速率定 律，而且銲料與銲點底層金屬體積的減少對於 Cu3Sn 成長速度沒有顯著的影響。 最後，穿透式菊池繞射分析結果指出 Cu3Sn 與 Cu 並無存有明顯的晶粒方向對應的 關係。本研究的結果建立了超薄層微銲點界面反應之最終端行為。

As the electronics packaging industries encounter physical limitation and more sophisticated and expensive of lithography in further scaling down transistor size, three- dimensional integrated circuits (3D ICs) provide the most promising approach to extend Moore’s law by vertically stacking of multiple functional chips with micro joints and Through-Silicon-Vias. Consequently, with the miniaturization of solder joints and reduction in thickness of the Under Bump Metallurgy, solders would form full intermetallic compound (IMC) joints and even deplete the wetting layer after assembling or aging. Therefore, IMCs will directly attach to the adhesion layer at the terminal stage of interfacial reactions, which has never been discussed in previous studies. In this study, we explored the microstructure evolution of ultra-thin Cu/Sn micro joints and occurrence and mechanisms of the spalling phenomenon during solid-state aging. Ar ion milling system (Hitachi Ion Milling System IM4000Plus) was applied in our study to create artifact-free cross-sections. The microstructure evolution was analyzed by scanning electron microscopy (SEM). The microstructure evolution of ultra-thin Cu/Sn micro joints shows that spalling of Cu6Sn5 and Cu3Sn from the adhesion layer occurs when Cu is consumed during solid-state aging. The driving force for the spalling of Cu-Sn IMCs is the high interfacial energy between IMCs and the adhesion layer. Also, the fracture surface analysis shows that the strength of spalled specimens is significantly reduced, which deteriorates solder joint reliability. To prevent the spalling of IMCs, the wetting layer must be sufficient to not be consumed by solders. Besides, the growth kinetics show that the growth of Cu3Sn follows a parabolic law and the volume of Sn and Cu has little influence on the Cu3Sn growth rate during solid- state reactions without considering the effect of surface diffusion. Transmission Kikuchi diffraction analysis shows that no preferred orientation between Cu and Cu3Sn in ultra- thin micro joints was found. These results of this study establish terminal reaction behaviors in ultra-thin micro joints