三維積體電路封裝接點之研究：微凸塊可靠度與銅對銅直接接合

Abstract

近年來隨著科技的進步，大量新穎的可攜式電子產品不斷問世。這些產品都是在追求更多的功能，但同時又要以輕薄短小為目標設計。傳統的二維電子封裝似乎已經無法再快速的進展已實踐單位體積或面積內有更多的訊號輸出入。於是將不同元件以垂直堆疊整合的三維積體電路開始受到人們的注目。 在垂直堆疊的結構中，接點扮演著極為重要的角色以穩定地連接不同晶片。目前來說，直徑不到30微米的微凸塊在三維積體電路中是最常做為接點。隨著微凸塊也必須跟著微縮化，當中的銲錫比例逐漸下降，甚至將無銲錫列入微縮化的途徑。而本論文將會逐步研究微凸塊內銲錫減少過程將遇到的難題。 第一部分是討論當微凸塊的銲錫體積已經縮小到直徑約30微米時，一個微凸塊內的銲錫將變成僅由個位數錫晶粒所構成。由於錫有非等向的機械性質，因此在熱循環測試後，容易產生應變產生裂痕。本論文發現有三種裂痕傳播途徑，包含錫晶粒間的高角度晶界、介金屬化合物與銲錫介面以及銲錫與錫銀析出化合物的介面。 當微凸塊微縮化，內部的銲錫量與金屬墊層的體積比率變少時，很容易所有銲錫與金屬墊層反應而消耗殆盡，而留下全介金屬化合物接點。此外，接點本身的重要功能其中之一就是傳遞電流，但是由於微凸塊的電流通過面積小將造成較大的電流密度。因此，在第二部分我們對全介金屬化合物微凸塊進行高電流密度之電遷移測試，發現多孔狀Cu3Sn生成為主要的破壞機制。 第三部分則是研究不使用銲錫作為接點材料，利用銅與銅直接完成連接兩晶片的目的。我們使用電鍍奈米雙晶銅膜與一般銅膜來做接合的材料並且進行比較。從實驗結果發現，奈米雙晶銅膜可在較短的加熱時間或較低溫度完成接合，並且經過適當熱處理能夠發生非等向大晶粒成長，幫助銅晶利成長以消除接合介面。

With evolution of technology in the recent years, more portable electronic products come out. These are pursuing multifunction but also become light and thin in the same time. Traditional 2D electronic packaginghas limited inputs and outputs in unit volume. Therefore, vertical stacking of different devices is needed. In a vertical stacking structure, joints play important roles to stably connect chips. So far, microbumps with less than 30 μm diameter are commonly used as joints in 3D IC.Continuous miniaturization is needed to provide more inputs and outputs. For this reason, the solder volume will shrink down and may be decreased to zero in the future.. In this study, we studied the problems as the solder volume continue to reduce. The first issue is the miniaturization of microbumps because the solder in amircobump consist of only few Sn grains. Tin has anisotropic thermal expansion and mechanical behavior. Therefore, cracks formed during thermal cycling tests. We found that cracks may propagate along three path: Sn grainboundaries with high misorientation angles, SnAgsolder/Ni3Sn4 IMC interfaces and Sn/Ag3Sn interfaces. Transformation of all solders into intermetallic compounds (IMCs) in microbumps is possible when the solder amount is small. In particular, current stressing can accelerate the formation of IMCs. Microbumps have to bear high current densities due to its small diameter and high performance requirements. Therefore, in the second part, we do electromigration tests found that the failure mechanism is due to the formation of porous Cu3Sn. In the third part, we removed solders as the joint material but chose Cu-to-Cu direct bonding to connect two chips. Electroplated nanotwinned copper and regular copper films were used as bonding materials. The results indicated that it took less time or lower temperatures to accomplish bonding by using nanotwinned copper films. Anisotropic grain growth also happened after proper heat treatment to eliminate the bonding interfaces.