奈米雙晶銅熱穩定性與其當作微凸塊金屬墊層在電遷移下的破壞模式研究

Abstract

奈米雙晶銅有許多優秀的性質，有潛力運用於電子產品上。本研究探討奈米雙晶銅的熱穩定性，並利用此當作金屬墊層運用於微凸塊封裝中，觀察此微凸塊在通電下的破壞模式。 研究發現，不同電流密度製程的奈米雙晶銅在300oC以下的溫度退火1小時，大部分的奈米雙晶能夠穩定存在。350oC以上的溫度退火1小時後，大部分試片的雙晶均消失，銅晶粒也發生異常晶粒成長。 接著本研究探討奈米雙晶銅雙晶間距與鍍液溫度的關係，得到一個趨勢：鍍液溫度越大，雙晶間距越大；反之，鍍液溫度越低，雙晶間距越小。同時利用簡單的熱力學，說明此趨勢的原因。 本研究同時比較脈衝電鍍銅與直流電鍍銅在不同溫度下長時間退火的熱穩定性。實驗發現，脈衝電鍍銅與直流電鍍銅在100oC退火2000小時與150oC退火1500小時後，雙晶均能穩定存在。而在200oC長時間退火後，脈衝電鍍銅與直流電鍍銅的熱穩定性有很大的差距，直流電鍍銅在200oC退火500小時後就有雙晶消失且晶粒成長的現象，而脈衝電鍍銅必須在此溫度下退火1500小時才會看到此現象。 最後，本研究使用奈米雙晶銅金屬墊層的微凸塊進行電遷移測試。實驗發現，通電過程中奈米雙晶銅能夠有效的減少Kirkendall孔洞的生成。而在尺寸縮小的情況下，因接點內只剩少許的銲錫晶粒，電子流方向與銲錫c軸方向的角度決定了接點的破壞模式。

Nanotwinned copper has some outstanding properties that could be used in electronic device. In this study, the thermal stability of nanotwinned copper was investigated and used it as under-bump-metallization in microbump to analyze the failure mode with current stressing. First, nanotwinned copper samples were produced with different current density. After annealing at 300oC for 1h, nanotwins were stable in most of samples. After annealing at 350oC for 1h, nanotwin in most of samples disappeared and copper grains exhibited abnormal grain growth. Second, the relationship between the bath temperature and the twin spacing was investigated. When the bath temperature decreases, twin spacing decreases. When the bath temperature increases, twin spacing increases. The thermodynamic model was proposed to explain this phenomenon. Third, the thermal stability of nanotwinned copper produced with different electroplating method was discussed by annealing samples at lower temperature for a long time. The results showed that the microstructure of pulsed electroplating copper and direct current electroplating copper were stable after annealing at 100oC for 2000 h and 150oC for 1500 h. However, the microstructures were different after annealing at 200oC. Nanotwins in DC electroplating samples disappeared and copper grains had abnormal grain growth after annealing for 500 h. The pulsed electroplating sample began to have the same phenomenon after annealing at 200oC for 1500 h. Finally, nanotwinned copper was used as UBM in microbump and performed electromigration test. The nanotwinned copper can reduce the Kirkendall voids caused by Cu3Sn formation. Because of the small size of microbumps, there was a few of tin grains in a microbump. And the relationship between electron direction and tin grain c-axis direction was more important and would determine the failure mode.