以控制銅錫介金屬化合物的晶體方向與形貌改善微電子封裝可靠度之研究

Abstract

本論文主要分為兩部分。第一部分，我們以具有<111>優選方向的奈米雙晶銅控制成長於其上的介金屬化合物η-Cu6Sn5。在三維積體電路每個晶片上，皆有數以萬計的微銲錫凸塊，因此，如何控制每一個微銲錫凸塊的性質是一個很重要的議題。在本實驗中，我們可以藉由<111>奈米雙晶銅墊層控制生長於其上的η-Cu6Sn5。以直流電鍍出大量的奈米雙晶銅墊層以及覆蓋於其上的錫2.3銀，將試片以260 oC迴銲1分鐘後，在銅墊層上成長η-Cu6Sn5。此時，η-Cu6Sn5在<0001>方向上展現出高度的優選性質。當迴銲時間延長時，其優選織構轉變為{21 ̅1 ̅3}。本研究的成果展現出對於η-Cu6Sn5生長方向的控制。以控制銅墊層的微結構，控制生成的η-Cu6Sn5晶體方向，進而控制整顆微銲錫凸塊的性質。 第二部分，則藉由生成均勻厚度的介金屬阻障層，抑制錫晶鬚的生成。自從1940年代以來，錫晶鬚一直都是一個可靠度的議題。經過數十年的研究，銅原子擴散進入錫金屬層且生成Cu6Sn5被視為是成長錫晶鬚的驅動力。在此部分，我們以一個全新的方式抑制錫晶鬚生成。以限制參與反應的錫體積方式，在260 oC迴銲2分鐘後，生成一層包含Cu6Sn5與Cu3Sn兩種介金屬化合物的均勻的雙層結構，此一均勻的介金屬層將作為阻障層阻擋銅原子擴散。在沒有經過任何處理的試片，僅需1至2個禮拜，銅原子就可以從晶界擴散進入錫薄膜，導致錫晶鬚在金屬導線上生成。相較之下，有介金屬阻障層的金屬導線經過7個月後仍然沒有任何錫晶鬚生成。觀察這兩種試片的橫截面，發現在沒有阻障層的試片內，Cu6Sn5的成長較不均勻。因為銅原子在Cu6Sn5與Cu3Sn的擴散係數值比在錫中小了7至8個數量級。因此以這層銅錫原生的介金屬化合物層作為阻障層，將可以有效地限制銅原子的擴散，而抑制錫晶鬚的生成。

This dissertation contains two parts. In the first part, we control the formation of η-Cu6Sn5 by adopting <111> oriented and nanotwinned copper pads. In 3D IC packaging, there are tens of thousands of microbumps in one single chip. Therefore, how to control the properties of the microbumps becomes an important issue. We report here an experimental approach for controlling the microstructure of η-Cu6Sn5 intermetallic compound in microbumps by using <111> oriented and nanotwinned Cu pads as the under-bump-metallization (UBM). By electroplating arrays of large numbers of <111> oriented and nanotwinned Cu pads and by electroplating the Sn2.3Ag solder on the pads, we form η-Cu6Sn5 in the reflow at 260 oC for 1 min. The η-Cu6Sn5 showed a highly preferential growth along the <0001> direction. As reflow time is extended, the preferred texture of η-Cu6Sn5 changed to {21 ̅1 ̅3}. These results indicate that we can control the uniform microstructure of η-Cu6Sn5 IMCs by controlling the microstructure of the Cu under-bump-metallization. In the second part, we attempt to inhibit the formation of tin whiskers by manufacturing a uniform layer of IMCs between solder and copper trances. Whiskers have been one of the most persistent reliability issues in packaging industry since it was discovered in 1940s. After decades of research, the diffusion of Cu atoms into tin layers is considered as one of the driving force for whisker formation. In this study, to inhibit the formation of whiskers, a novel method to hinder the diffusion of copper atoms in Cu-Sn couple was introduced. A uniform Cu6Sn5/Cu3Sn of intermetallics was formed in only two minutes of reflowing with limited volume of tin. The uniform intermetallic layers between copper and tin were served as a barrier layer. In as-electroplated samples without this uniform layer, whiskers were prone to grow on the surface of tin in one to two weeks at room temperature storage. In contrast, the samples with the uniform intermetallic layers between copper and tin were whisker-free even after 7 months of storage. The cross-sectional areas of both conditions were observed. It was found that a fast growth of Cu6Sn5 along grain boundaries of tin occurred in samples without intermetallic layers. The diffusivity of copper in Cu6Sn5, Cu3Sn, and tin has been investigated in previous studies. It is reported that the inter-diffusivities of copper in Cu6Sn5 and Cu3Sn are 7-8 orders smaller in magnitude than that in Sn. Therefore, this IMC barrier layer can effectively block the flux of copper atoms, and eventually inhibit the formation of Sn whiskers.