錫晶粒方向對65μm厚覆晶銲錫接點的電遷移破壞研究

Abstract

隨著積體電路的微小化趨勢，覆晶封裝技術已成為重要的晶片接合技術。現在電子產品越來越小且輕，功能也越來越強大的趨勢，一般傳統的覆晶封裝方式已經無法滿足需求，3D積體電路堆疊整合技術(3D Integrated Circuit Stacking Technology)就開始蓬勃發展。剛開始發展的3D封裝技術是用覆晶技術為基礎，使用微凸塊(Micro Bump)來連接各晶片。不過當銲錫凸塊的高度發展到20微米或更微小時，銲錫凸塊接點就容易因電遷移效應而反應成為介金屬化合物。這生成介金屬化合物將影響覆晶銲錫接點的破壞模式，是本研究分析的主要課題。 本研究中，所使用是上下端都是鎳/銅金屬墊層(Under Bump Metallization)的試片。此試片上方金屬墊層是鎳/銅，分別是3微米/5微米，銲錫的高度約65微米。測試電流密度為1.51 x〖 10〗^4 A/cm^2，實際通電溫度分別為151.9°C、175.3°C以及199.1°C。利用凱文結構來量測銲錫接點在電遷移下電阻變化的狀況，並且在電阻值增加不同階段，作微結構分析，溫度越高，銲錫凸塊電阻增加越快，利用背向散射電子繞射儀來分析錫的晶粒方向，不同錫晶粒方向對電遷移效應有明顯的差異，假如錫晶粒的c軸與電子流方向平行，在陰極端的金屬墊層會迅速溶解，在陽極端會堆積〖(Ni,Cu)〗_6 Sn_5；假如錫晶粒c軸與電子流方向垂直，銲錫電阻增加會較緩慢。

As trends in integrated circuits miniaturization continues, flip-chip packaging became main method for packing of microelectronic devices. Recently the size of electronic devices become smaller and lighter. The required functions become higher than before. Traditional flip-chip packaging technology cannot meet the requirement of high density packaging. Three dimensional ICs packaging technology has been developed. Micro bumps with 20μm diameter has been adopted for the interconnects between chips. The height of the microbumps is less than 20μm. Thus, microbumps are easily to transform into intermetallic compounds (IMCs) during the electro-migration tests. The IMCs would affect the failure mode of flip-chip solder joints. As a result, it is of interests how the IMCs would affect the EM behavior of microbumps. In this study, the SnAg solder joints with Ni-Cu UBM (under bump metallization) was used. The solder joints were with 3μm thick Ni and 5 μm thick Cu column UBMs. The bump height of the SnAg solder joints is 65 μm. Test current density was 1.51x〖10〗^4 A / cm^2 , and the actual temperature was measured in 151.9°C 、175.3°C and 199.1°C. For electrical observation, Kevin structure was used to measure the resistance change during electromigration tests. As a result, the highter temperature made the resistance of the SnAg solder joints increased faster and faster. And we used flip-chip samples to study the Sn grain orientation by electron backscatter diffraction (EBSD). the effect of Sn grain orientation on Ni-Cu IMCs formation was obvious. If the c-axis of Sn grain was aligned with along flow, the IMCs and the Ni-Cu UBM dissolved quickly at the cathode side and the IMCs accumulated at the anode side. If the c-axis of Sn grain and electron flow was in the vertical direction, the resistance of the SnAg solder joints will increase slowly, became IMCs grew slowly during electromigration.