錫非等向性及銀添加對電遷移影響之研究

Effects of Sn Anisotropy and Ag Addition on Electromigration

黃挺嘉(Ting-Chia Huang)

指導教授 ： 高振宏

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.02675

摘要

本論文含有兩部份之研究主題，分別為銀濃度添加及錫晶粒方向性對於電遷移效應之影響。在所有實驗中，利用半導體製程所得之直線型Cu/solder/Cu及 Ni/solder/Ni結構將可有效避免許多在真實焊點中因電路設計所引發之現象，包括熱遷移 (thermomigration)或電流擁擠 (current crowding)，進而使對於電遷移現象之討論更加的單純。
由於有害物質管制指令 (Restricted of Hazardous Substances, RoHS)的實施，電子產品封裝已走向無鉛焊料的方向，而其中錫銀及錫銀銅焊料更被視為最有潛力之共晶錫鉛焊料替代品之一，且已被廣泛使用。然而目前許多針對銀添加對於焊料影響之相關研究，大多著重於冷卻速率、銀濃度對於Ag3Sn析出及相關之機械性質之觀察，鮮少加入電遷移之考量。在此研究中，將使用Cu/Sn1.2Ag/Cu、Cu/Sn4.0Ag/ 及 Ni/3.0Ag/Ni 進行銀添加對於電遷移效應之探討。根據實驗結果，銀添加可以造成陰極端金屬墊層溶解效應形貌明顯之改變，我們更進一步配合掃描式離子影像(Scanning ion microscopes, SIM)及電遷移中金屬墊層溶解之動力學分析，推論傾向析出於介金屬化合物晶界之Ag3Sn具有抑制介金屬中快速晶界擴散之作用。另外由實驗結果也可發現，析出於焊料及介金屬化合物介面之Ag3Sn具有保護下方介金屬免於因電遷移而溶解之效果，由此角度推論，控制Ag3Sn的析出可能為有效抑制通電下陰極端金屬墊層大量溶解之方法之一。
另外，由於錫晶粒的高度非等向性及焊點體積上持續地縮小，錫晶粒方向對於電遷移效應之影響在近年來漸漸受到關注。根據文獻，在現今之焊點尺度下，一單一接點在迴焊之後將會由少數之粗大晶粒所組成，在此情形下這些粗大晶粒之方向對於通電下接點之可靠度將有決定性的影響。在本論文中，利用試片及製程的設計所得具有高度優選方位 (prefer orientation)所組成之錫特殊織構結構 (texture)將做為此部分電遷移試驗之試片，並配合背向式電子繞射 (electron backscattered diffraction, EBSD)進行錫晶粒方向性之觀察。根據實驗結果，特殊的結晶方向將會加速陰極端金屬墊層之消耗，而當錫晶粒之優選方位發生劇烈改變時，錫晶粒在擴散上之非等向性具有阻止金屬原子進一步向前擴散之能力，進而控制介金屬化合物在焊料內部之析出位置。除此之外，由較大且無優選方位之錫晶粒組成之直線型結構也進行相同之電遷移試驗，令人震驚的是，在一個寬度僅200micron 、且通以均勻電流密度之接點中，錫晶粒之方向即足以造成陰極端在金屬墊層消耗速率上明顯之差異。另外，根據陽極端介金屬化合物累積位置及錫晶粒之個別方向，足以推論當金屬離子在焊料內部進行擴散時，會傾向延著錫晶粒之c軸方向移動而非外部之電子流方向。由以上實驗結果，本論文可提供在錫晶粒方向性對於電遷移影響更進一步之了解，進而指出未來微小焊點中抵抗電遷移上可能之改善方向。

關鍵字

錫非等向性 ； 銀添加 ； 電遷移

並列摘要

Two main topics were discussed in this thesis, including the effect of Ag addition and the Sn grain orientation on the electromigration. The line-type solder joints were designed for the electromigration test in both topics in order to exclude some additional concerns that sometimes disturbed the direct explanation of the electromigration phenomenon, including the thermomigration and the current crowding.
Referring to some previous studies, the studies of the Ag addition largely focused on the mechanical properties and the solification of Ag3Sn. Though the SnAg and SnAgCu solder were acknowledged as one of the most promising candidates in lead-free solders, nearly no studies had discussed about the effect of Ag addition on the electromigration. In this thesis, the Cu/Sn1.2Ag/Cu, Cu/Sn4.0Ag/Cu and Ni/Sn3.0Ag/Ni solder joints were used for discussing the effect of Ag addition on electromigration, especially focusing on the dissolution phenomenon at the cathode interface. With the observation with the scanning ion microscopes (SIM) and the kinetic calculation, we concluded that the Ag3Sn would be competent to repress the grain boundary diffusion through the IMC that once prevailed in the systems without Ag addition. Base on this assumption, the difference of the cathode dissolution morphology between the solder joints that with and without Ag addition was well explained. Besides, the shielding effect in which the Ag3Sn could prevent the underlying IMC from stress-induced dissolution also provided a possible method to suppress the serious metallization consumption in electromigration.
Another topic was related to the Sn grain orientation, and the electron backscattered diffraction (EBSD) was utilized to observe this phenomenon. With the trend of miniaturization of the microelectronic devices, many studies had found that a single solder joint would tend to be simply composed with several large Sn grains, and thus the grain orientation of these Sn grains would highly decide the behavior of electromigration. In this thesis, a unique Sn texture in the Ni/Sn/Ni solder joints was available and shown that some specific Sn grain orientations could not only intensify the dissolution at the cathode but also obstruct the diffusion flux, deciding the location of the IMC precipitation. Additionally, larger Sn grains without unique texture were also achieved by adjusting the manufacturing parameters. In this case, the Sn grains seemed to decide the degrading behavior of the cathode interface that was bordered on them. Striking results had been observed that, a dramatically differential dissolution rate appeared in a 200 micron -wideness solder joint with a uniform current density could simply caused by the the Sn grain orientation. Besides, a strong correlation between the Sn grains and the accumulation of the IMC at the anode interface also suggested that the diffusion flux would get along the c-axis of the Sn grain rather than the direction of the electron flow. With this thesis, the behavior of the electromigration related to the Sn grain orientation was further understood and also provided a strong evidence for the importance of the effect of the Sn grain orientation on electromigration.

並列關鍵字

Sn anisotropy ； Ag addition ； electromigration

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