金屬墊層及銲錫高度對微凸塊之電遷移破壞模式研究

Abstract

隨著電子元件微小化與多功能化，以往的二維封裝技術已漸漸不敷使用，逐漸由三維封裝技術取代，因為三維封裝可以有高密度的I/O接點。而電遷移一直是嚴重的可靠性議題，因此本研究要探討不同結構的微凸塊在電遷移下的破壞模式。 實驗中使用了四種菊鍊微凸塊，微凸塊結構分別為Ni/16μm SnAg/Ni、Ni/8μm SnAg/Ni、Cu/16μm SnAg/Cu以及Cu/8μm SnAg/Cu。每條菊鍊由四十顆微凸塊串聯組成，均在150℃的加熱板上通以8x104 A/cm2的電流密度，當阻值上升起始值的20%、50%以及100%時，停止通電，將試片研磨拋光並透過掃描式電子顯微鏡以及能量散佈光譜儀觀察分析實驗結果。 由於銅與銲錫反應速率非常快，在以往的覆晶銲錫中會添加一層 鎳當做擴散阻擋層或是將銅金屬墊層加厚，用以延長接點壽命。但在本實驗結果中得知當微凸塊中的銲錫迅速的完全形成介金屬化合物時，其整體微凸塊的抵抗電遷移破壞能力非常好，因此迅速形成介金屬化合物會讓微凸塊有良好的抗電遷移能力，跟在覆晶銲錫中要阻止銅金屬墊層溶解的觀點不同。

As electronic devices become more functional and miniature, two dimensional packaging becomes insufficient. Three dimensional packaging is gradually replacing two dimensional packaging, because of its high-density input/output (I/O) interconnects. Electromigration is always a serious and persistent reliability issue. This study focuses on electromigration failure mode in different types of microbumps.

In this study, we used four types of microbumps. Their structures are Ni/16μm SnAg/Ni, Ni/8μm SnAg/Ni, Cu/16μm SnAg/Cu and Cu/8μm SnAg/Cu, respectively. Daisy chains with forty microbumps were used. Each daisy chain was heated on 150℃ hotplate and the applied current density was 8x104 A/cm2. When resistance increased 20%, 50% and 100% of their initial values, we stopped the experiments and polished the samples to observe their failure mechanism by scanning electronic microscope and energy dispersive spectrometers. Due to the fast reaction rate of copper and tin, thick copper underbump metallization (UBM) or nickel UBM was added in flip chip to elongate lifetime of the microbumps. In this study, we find out that the ability of interconnects to resist electromigration improved greatly when microbumps transformed into intermetallic compound (IMC) microbump. The fast dissolution of UBM and fast formation of IMC are good for microbumps to resist electromigration. The approach to improve electromigration lifetime in microbump is quite different from flip chip joints.