覆晶15μm厚銲錫接點在高溫下的電遷移研究

Abstract

因為積體電路的微小化，覆晶封裝成為主要的晶片端接合技術。近年來，電子產品微小輕便及功能多樣化的趨勢並未減緩，傳統覆晶封裝方式已不敷使用，三維積體電路堆疊整合技術 (3D Integrated Circuit Stacking Technology) 因而對應發展。初期的3D IC技術仍以覆晶技術為基礎，以微凸塊 (Micro Bump) 來連結各晶片。但是，當銲錫凸塊的高度縮減到20微米以下，銲錫接點容易因為電遷移現象而轉變成介金屬化合物。此介金屬化合物的性質將決定覆晶銲錫接點的失效模式，因此成為研究分析的新課題。 本研究中，將使用上下端皆為銅墊層的試片。此結構的上端為銅柱，其高度約為50微米，銲錫接點的高度約15微米。測試電流密度為1.32x104 A/cm2，實際通電溫度為198 oC。在電性觀察上，利用凱文結構來量測銲錫接點在電遷移下電阻變化的情形，並且在不同的阻值增加階段做剖面微結構觀測。在剖面微結構觀測上所發現的多孔狀Cu3Sn結構，將主要影響銲錫接點的可靠度。在研究中發現低高度銲錫試片，在高溫下會生成多孔狀Cu3Sn；如果再加以高電流密度，將加速生成多孔狀Cu3Sn。

Because the miniaturization of integrated circuits (ICs), flip-chip become main packaging technology for wafer-end jointing. In recent years, the miniaturization trend still continues for portable and functional product. 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 easy 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 Cu UBM (under-bump-metallization) was used. The solder joints were with 50 μm thick Cu column UBMs. The bump height of the SnAg solder joints is 15 μm. Test current density was 1.32x104 A / cm2, and the actual temperature was measured to be 198°C. For electrical observation, Kevin structure was used to measure the resistance change during electromigration tests. Cross-section microstructure observations were performed at different stages of the bump resistance increases. Porous Cu3Sn structure was found after the bump resistance increased to 29%, and it would affect the reliability of solder joints. The porous Cu3Sn would be formed on thinner Sn2.3Ag solder joints at high temperature and current stressing. High current density stressing would accelerate the formation of the porous Cu3Sn.