錫的微結構對覆晶封裝銲錫接點的電遷移破壞之影響

Abstract

由於高密度以及多功能設計下，有越來越多凸塊微小化以及高腳

數(I/O)的整併應用在三微積體電路(3DIC)的科技中。一般來說，

3DIC是目前利用微凸塊結構讓電子元件微小化與多功能化的重點

結構之一，解決尺寸的問題，其中接點的微小化是主要的核心技

術。本研究探討了不同高度的銅柱金屬墊層電遷移現象。藉由觀

察不同高度的銅柱凸塊電阻上升的現象發現較高的銅柱金屬墊層

有助於提高銲錫接點的電遷移壽命。我們也同時模擬並探討凸塊

結構的電流集中效應(Current crowding effect) 和電流分布的現

象，讓我們可以更了解狀況。另外，本研究也同時探討了含與不

含電鍍鎳層對於較高銅柱金屬墊層在電遷移上的效應，尤其是邊

牆潤溼效應(side wall wetting effect)的影響，當不含電鍍鎳層的微

凸塊因為較少side wall wetting效應，而有效降低了孔洞的生成和

電阻上升的比率。含電鍍鎳層的微凸塊在通電情況下，有大量的

I

錫原子越過鎳層在側邊銅層形成Cu6Sn5，進而在中間銲錫層形成

孔洞。

另一方面，銲錫所佔體積的百分比大幅降低，相關的可靠度

以及失效模式，也會不同，例如以往銲錫內所含的錫晶粒數量較

多，微凸塊結構製造後，銲錫中的錫晶粒甚至只會剩下一兩顆，

此時的銲錫晶粒方向特性將會直接影響其破壞的模式。本研究利

用銲錫高度為15微米的覆晶銲錫Cu/SnAg/Cu結構在不同的溫度與

電流做通電測試來模擬微凸塊電遷移的情形，並利用背向散射電

子繞射儀 (EBSD) 來分析錫晶粒的方向來觀察錫晶粒。發現銅原

子在錫晶粒內的擴散非等向性差異以及不同方向的錫晶會造成不

一樣的失效模式差異。

With high density and multifunctional design, there is increasing integration of tiny bumps and high pin-count (I/O) for design. Generally, 3D IC is one of important structures which utilize micro bumps to facilitate miniaturization and multi-functionalization of electronic components to solve the size problem. Among that, the miniaturization of joints is the primary core technology. This study discusses the electromigration phenomena in solder joints with copper pillar UBM (under bump metallization) of different heights. By observing the rise phenomena of bump resistance of solder joints with different heights, it is found that higher copper pillar UBM will contribute to improve the service life of electromigration of solder joints. Meanwhile, we also III

simulate and discuss the current crowding effect and current distribution within the bump structures to further understand the conditions. Additionally, the study also probes into the effects of Ni layers on the electromigration of higher copper pillar UMB, especially on the side wall wetting effect. Without the electroplated nickel coatings, less side wall wetting effects of micro-bump will effectively reduce the generation of voids.

On the other hand, the volume fraction of soldering tins will dramatically decline for microbumps. Consequently, failure modes will also be different. For instance, the quantity of tin grains in flip-chip solder joints is more. Yet, for micro-bumps, only one or two tin grains exit in micro bumps. In this case, directions and features of Sn grains will directly influence its failure modes. In flip-chip Cu/SnAg/Cu solder structures with 15µm height stressed under different temperatures and electric currents, we also conduct simulation on current density simulation on the solder joints during current stressing. Furthermore, the study employs EBSD to analyze orientations tin grains. It is found that the difference in anisotropic diffusion of copper atoms in tin grains will result in differences of failure modes.