直徑30 µm微凸塊的冶金、電遷移、熱循環測試的可靠度議題研究

Abstract

現今電子元件趨向尺寸微縮和密度增大，三維電子堆疊為重點發展導向，其中銲錫微凸塊為主要用來連接晶片的封裝技術，然而銲錫接點縮小所造成的可靠度問題目前仍鮮少有研究討論。本實驗利用銲錫直徑30微米的微凸塊做三種可靠度測試：銅、鎳金屬墊層組合搭配（銅/銅、銅/鎳、鎳/鎳）的高溫儲存測試，以及使用相同銲錫量但不同銲錫高度的銅/錫銀/銅的微凸塊結構，做電遷移與熱循環測試。在150°C、1000小時高溫儲存測試的結果，顯示銅/銅金屬墊層最快達成電阻上升20%的破壞，因為其介金屬化合物成長速率常數較銅/鎳、鎳/鎳兩者高。而針對較生成介金屬化合物較快的銅/銲錫/銅作電遷移測試，通以8×104 A/cm2在150°C的加熱板上，發現當銲錫高度從34 µm降低至5 µm時，同樣的電阻上升比例時壽命可以增加兩百倍；高銲錫高度的微凸塊會發生銅墊層快速溶解的破壞模式，還有錫回填的現象；低銲錫高度的微凸塊則容易變成介金屬化合物接點，並且因錫含量大於銅，在高電阻上升比例時會發生Cu6Sn5轉回Cu3Sn釋出銅與殘餘的銲錫反應的情形。而熱循環測試則同樣觀察到低銲錫高度的微凸塊表現最佳，循環數大於最高銲錫高度的微凸塊三倍，因為介金屬化合物快速生成使強度增加，也觀察到當微凸塊的錫晶粒為類單晶或是為a、b軸垂直載板方向時裂縫破壞明顯較少，有助於增加熱循環測試的壽命；晶粒延a、b軸較抗熱應力，也呼應微凸塊的應力方向來自底膠與微凸塊之間的差異。

Nowadays, smaller form factor, higher density, and more functional are the prime demands for the ICs (Integrated Circuits), and 3D-IC (3-Dimensional IC) is one of the most promising approaches to achieve the demands. Microbumps have been adopted for the vertical interconnects between stacked chips. However, the downsizing of the solder joints raises many reliability issues, and they are not well studied. In this study, 30-µm microbumps are used for three reliability tests: high-temperature storage (HTS) tests, electromigration (EM) tests, and thermal cycling tests (TCT). In the HTS, microbumps with three kinds of under-bump metallizations (UBMs), which are Cu-Cu, Cu-Ni, Ni-Ni, were conducted at 150°C isothermally for 1000 h. The results showed that only Cu-Cu reached to the failure criterion of the resistance increment of 20%. It was owning that its faster growth of the intermetallic compounds (IMCs) than the other two kinds of microbumps. For the EM and TCT tests, microbumps with Cu/Cu UBMs with four different bump heights but the same amount of the solder were conducted in the experiments. The condition of the EM test was under 8×104 A/cm2 at 150°C. The results showed that when the bump height decreased from 34 µm to 5 µm, the lifetime increased by 200 times. The open failure of the microbumps with high bump heights was due to UBM dissolution, and Sn backfill effect was also observed in the microbumps. In the lowest bump height, IMC joint was easily formed. Since the quantity of Sn was much larger than Cu, it was found that the Cu3Sn might transform back into Cu6Sn5 at a longer stressing time, and the released Cu would react with the remaining Sn. For the TCT, the micro bump with the lowest bump height also performed the best reliability, and the lifetime is three times higher than the highest ones. The higher ratio of the IMC made the micobumps harder and more resistant to stress. EBSD results also showed that when the Sn grains were single crystalline or the shear stress was along the a- or b-axis, the lifetime would increase as well. It also shows that in the microbumps, the strain was along the direction vertical to the carrier after TCT.