Abstract
摘要:International Technology Roadmap for Semiconductors 是世界半導體業具領導地位之科技發展趨勢圖。2006 年12 月最新改版之 Roadmap 指出覆晶科技之電遷移問題將成為半導體業之主要限制性因素之一 [1],並鼓勵產業及學術界積極投入研究此一問題。本計劃之主要目的是深入探討覆晶封裝內,原子因電遷移所產生之通量與因化學位能梯度所產生之通量之交互作用及其機制, 及此一交互作用對覆晶封裝可靠度之影響。
電遷移通量指的是,當銲點通電時,電子會撞擊導線金屬原子,足夠的動量轉移而輔助了原子擴散。由於電遷移現象中之原子遷移是電子動量轉移所致, 因此原子遷移的方向也與電子流的方向相同。化學位能梯度通量指的是原子因化學位能梯度,而由該元素之高化學位能區域朝低化學位能區域之擴散。Cu 及Ni 都是UBM 及surface finish 所廣泛使用之材料,而Cu/solder/Ni 此一三明治結構又是封裝中橫跨銲點最常見之材料順序。由本實驗室過去的研究成果可知,當電流通過此類銲點時,電遷移通量與化學位能通量將同時存在,並產生交互作用。以下圖中Cu/solder/Ni 銲點中之Cu 原子為例[2], 銅原子因電遷移所產生之通量將與電子流方向一致;而銅原子因化學位能所產生之通量只會由Cu/solder 端流向solder/Ni 端。因這兩種通量之同向或逆向,造成如下圖所示之相當特別之微結構。左圖中Cu 之電遷移通量與化學位能通量是一致往下,故Cu6Sn5 皆於solder/Ni 端且成一連續層;而右圖中Cu 電遷移通量與化學位能通量則方向相反,造成Cu6Sn5散佈於銲點內。此一結果是文獻中僅有之兩種通量交互作用之具體證據。
此一結果顯示,化學位能通量與電遷移通量其數量級已相當接近。因此電遷移研究中對此兩通量之交互作用的了解就顯的相當重要,然而目前為止,學術界對此兩現象的相互影響甚少著墨。此一現象可能成為影響覆晶銲點中金屬層的可靠度的議題之一, 急需進一步探討。
本計劃之主要目的是更進一步深入探討化學位能通量與電遷移通量相互影響之機制。詳細之目標有以下幾點:( 1) 釐清恆溫通電時化學位能通量與電遷移通量對介金屬型態之影響及其機制、( 2) 探討恆溫下不同電流密度, 亦即不同電遷移通量之影響。( 3) 探討恆電流下不同溫度對機制之影響。上述目標若能順利達成,不但對工業界解決electromigration 問題有實質幫助,亦對吾人更進一步瞭解electromigration 此一基礎現象有所幫助, 在學術研究上做出貢獻。
Abstract: International Technology Roadmap for Semiconductors is the leading roadmap guiding the development of the semiconductor industry. According to the most recent update [1], issued in December 2006, electromigration is becoming a main limiting factor for the flip-chip technology, and more efforts should be devoted to this area by all parties involved. The objective of this proposed study is to investigate the cross-interactions between two atomic fluxes, the one due to electromigration and the one due to chemical potential gradient, which might co-exist in the solder joints. The impact of such cross-interaction on the device reliability will also be investigated.
The electromigration arises from the momentum transfer between the flowing electrons and the atoms, and therefore the resulting atomic flux has the same direction as that of the electrons. The chemical potential flux arises from the chemical potential gradient of a certain species exists in a solder joint. The direction of the chemical potential flux is toward the direction of decreasing chemical potential. The elements Cu and Ni are both popular materials for UBM and surface finish applications, and the Cu/solder/Ni material sequence is the most common one across a flip-chip solder joint. Our past studies had shown that when high current density was applied to such a Cu/solder/Ni structure, both the electromigration flux and the chemical potential flux existed simultaneously, and these two fluxes interacted. [2] Taking the Cu atoms in Cu/solder/Ni as an example, the Cu electromigration flux is always in the same direction as that of the electrons, but the Cu chemical potential flux only goes from the Cu/solder side to the solder/Ni side. The resulting microstructures are shown in the flowing two micrographs.
In the micrograph on the left, the electromigration flux and the chemical potential flux were in the same direction, and Cu6Sn5 had a layered structure attached to the solder/Ni interface. In the micrograph on the right, the electromigration flux and the chemical potential flux were in the opposite direction, and Cu6Sn5 particles scattered inside the solder joint. This was the first direct evidence for the cross-interaction of these two fluxes that ever published in the literature. This result shows that two types of flux are in the same order of magnitude. Consequently, the understanding of the nature of this cross-interaction becomes very important. Unfortunately, there are very few related studies in the literature. There is clearly an urgent need to investigate this effect further.
The objective of this proposed study is to investigate in detail the mechanism for the cross-interaction of these two types of flux. Specific studies to be carried out include (1) the mechanism for the cross-interaction of these two types of flux at constant temperature, (2) the relative importance of these two fluxes at different current density levels, and (3) the effect of temperature on such cross-interactions. We expect that the proposed study will have a deep and profound impact on our understanding of electromigration in flip-chip solder joint, and the obtained results will be world-leading research.
Keyword(s)
電遷移
覆晶
化學位能通量
Electromigration
Flip-chip
Chemical potential flux