錫的微結構對於錫銀微凸塊電遷移及熱遷移的影響

Abstract

微凸塊銲錫在三維積體電路的發展下，已經廣泛的應用在先進半導體封裝製程中。然而，隨著銲錫體積的縮小下，相較於過往的覆晶銲錫接點，其在電遷移及熱遷移的可靠度上將會變得更加危險。由於在許多文獻上觀察到，不同的銲錫金屬層(under bump metallization)對於銲錫晶粒的影響；晶粒方向使得金屬層的原子在電遷移及熱遷移的測試過程中，沿著c-axis擴散較快導致金屬層快速溶解；以及，晶界的錯位角度將影響金屬層原子沿著晶界擴散的速度。因此，在本實驗中，我們將廣泛應用背向散射電子繞射儀來研究微凸塊中的銲錫微結構，其中包含晶粒大小的分布、晶粒方向以及晶界錯位角度。我們發現到當銲錫在銅-銅金屬層以及銅-鎳金屬層之間時，銲錫較容易呈現大晶粒，但在鎳-鎳金屬層之間時，其晶粒較小；另外，金屬墊層對於銲錫晶粒的優選方向並無明顯影響。此外，在電遷移及熱遷移的測試過程，當錫晶粒的c軸越接近電子流方向或者熱梯度方向時，銅金屬層的銅原子將會快速溶進銲錫中，並大量形成銅錫的介金屬化合物，隨著c軸與電子流或熱梯度方向的角度變大，銅金屬層溶解的速度有明顯的變慢。從結果中明顯看到錫晶粒方向的影響，然而卻沒有看到界金屬化合物沿著晶界成長，最重要的原因是來自於錫晶界大量的雙晶結構 (cyclic-twin boundary)有效阻擋銅原子沿著晶界擴散。

Microbump plays an important role to connect chips in three-dimensional integrated circuit (3D IC) technology. With the reduction of solder volume, the increase in current density and thermal gradient has raised serious reliability concern during electromigration and thermomigration. Tin microstructures are very important on testing the reliability of solder joints； Tin grain orientation affects electromigration and thermomigration in solder joints due to the high anisotropic diffusivity of Cu and Ni in Tin crystals; In addition, Sn grain boundaries could provide a fast path for the diffusion of Cu and Ni atoms. In this study, we investigate the microstructures of microbumps with Cu-Cu, Cu-Ni, and Ni-Ni UBM. The Sn grains between Cu-Cu- and Cu-Ni UBMs are larger than those between Ni-Ni UBM. However, the UBM materials have no significant effect on the orientation of Sn grains. Furthermore, we observe that Sn grain orientation plays a critical role in the growth of Cu-Sn intermetallic compounds (IMCs) during electromigration and thermomigration. For Sn grains with low α-angles, which is the angle between the c-axis of a Sn grain and electron direction or the direction of thermal gradient, Cu-Sn IMCs grew very fast during electromigration and thermomigration. On the other hand, the thickness of interfacial IMCs did not grow much in Sn grains with high α angles. This is because Cu diffusion was very slow in high-α-angle Sn grains. In addition, for Sn-Ag solder joints, most of the grain boundaries consist of cyclic twins, unideal paths for fast diffusion for Cu atoms. Therefore, the grain-boundary diffusion of Cu is less important than Sn orientation in the formation of Cu-Sn IMCs during electromigration.