錫銀銅覆晶微接點暨鋁導線外加機械應力之電遷移研究

Investigation of Electromigration Characteristic in SnAg3.0Cu0.5 Flip Chip Interconnection and the External Mechanical Stress Impact of Al Thin Film

李建成(Chien Chen Lee)

指導教授 ： 江國寧

國立清華大學/工學院/動力機械工程學系/博士(2008年)

摘要

積體電路的鋁導線中所施加電流密度若高於105 A/cm2時，將在陰極產生孔洞而在陽極產生凸起，此電致遷移現象導致導線損壞為常見的破壞模式。當現今覆晶封裝微接點的尺寸到達35 μm時，其電流密度將達到104 A/cm2，在此電流密度下，電致遷移將會造成覆晶封裝微接點的損壞。因此在覆晶封裝微接點中，由於電致遷移所產生的各種行為及效應值得深入探討。
本研究首先討論在高電流密度運作下，錫銀銅無鉛凸塊的電致遷移行為，來進行平均失效時間預估的探討，研究中也應用了有限元素法來分析因為電流擁擠所產生的最大電流密度所發生的位置，以及微接點的電流密度分布。在電致遷移所產生的孔洞擴散現象方面，除了文獻中所提出的薄餅狀孔洞現象，本研究也觀察到另一種因電致遷移行為所產生的棉花狀孔洞，此棉花狀孔洞是由於在施加電流之前、或是在施加電流的同時，於陰極/晶片端已經有微小裂縫存在所導致。且由棉花狀孔洞所造成的損壞時，其於陽極/基板端的金屬界層厚度，大於由薄餅狀孔洞所產生的厚度。在電致遷移所產生的介面反應現象方面，除了文獻中所提出的Polarity效應，本研究也觀察到在較高電流密度下才較易觀察得到的Tilting效應，此效應顯示，只有在陽極/晶片端會觀察到金屬界層會延著電流流動的路徑形成。此一現象與本研究中有限元素分析所得到結果一致。
在外加機械應力對鋁導線的影響之研究中，本研究設計並製造了沒有保護層的Blech strip以及相關治具來討論外加機械應力對鋁導線電致遷移的影響。在試件實驗條件達到平衡後，陰極端的鋁原子濃度會由原始的約74%降低至約60%，而陽極端的鋁原子濃度會由原始的約74%增加至約85%。本研究亦提出當外加-50 MPa至+50 Mpa拉伸應力下，此外加機械應力對鋁導線電致遷移的影響，屬於微小但仍可以被觀察到程度。

關鍵字

電致遷移 ； 電流擁擠 ； 焦耳熱 ； 平均失效時間 ； 金屬界層 ； 錫銀銅無鉛凸塊 ； Polarity效應 ； Tilting效應 ； 外加機械應力 ； 四點彎折

並列摘要

Electromigration damage was examined under a current density magnitude higher than 105 A/cm2, which led to the void in the cathode and the hillock in the anode and consequently, the failure of the aluminum conductor. Electromigration has also become one of the failure mechanisms when the flip chip solder joint interconnects shrink to 35 μm, in which the current density magnitude will increase up to 104 A/cm2. With these, the reactions and effects of flip chip solder joint electromigration have become to be of great interest.
Thus, this study investigates the electromigration of SnAg3.0Cu0.5 flip chip bumps by adopting Black’s equation to investigate the mean-time-to-failure prediction. Maximum current density was simulated using the finite element method, thereby providing a better understanding of local heat as well as current crowding. The current crowding phenomenon enhances the void formation at the entry points of the cathode side of the solder bumps. In the aspect of electromigration-induced void phenomenon, in addition to the pancake-type void failure mode proposed in the literature, a cotton-type void failure mode was also found in this study. Cross-sectional scanning electro microscopy images showed that the cotton-type void formation may due to the bump with a crack in the cathode/chip side before or during the current stressing. The cotton-type void failure mode intermetallic compounds layer at the anode/substrate side was thicker than that of the pancake-type void failure mode. In the aspect of electromigration effects on interfacial reaction, in addition to the polarity effect proposed in the literature, a more obvious tilting effect was found at the anode/chip side along the electron flow path under a higher current stressing condition.
The unpassivated aluminum thin film test specimens and the four-point-bend equipment have been successfully fabricated to investigate external mechanical stress impact during the electromigration test. This study also found that once the thin films reach equilibrium after the electromigration test, the aluminum atomic concentration, originally at 74%, decreases to about 60% at the cathode and increases up to about 80% at the anode. In addition, this study revealed that applying external mechanical stress has a small but finite effect on critical length. Under the external mechanical stress range of –50 to +50 MPa tensile stress, the higher the tensile stress applied, the shorter the critical length becomes.

並列關鍵字

Electromigration ； Current crowding ； Joule heating ； Mean-time-to-failure ； Intermetallic compound ； Polarity effect ； Tilting effect ； Blech strip ； External mechanical stress

參考文獻

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