以微米柱壓縮試驗探討三維積體電路微銲點中Cu6Sn5之機械行為

Mechanical Behaviors of Cu6Sn5 in 3D IC Micro Joints Using Micropillar Compression

吳瑞洋(Jui-Yang Wu)

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/博士(2019年)

https://doi.org/10.6342/NTU201901885

摘要

半導體產業之線寬微縮製程日益逼近物理極限，三維積體電路被視為最有潛力能延續、甚至超越摩爾定律之封裝技術；然而，有鑑於微銲點體積急劇縮減，界面反應將在短時間內耗盡銲錫並生成大量介金屬化合物，使得介金屬有機會佔據整個微銲點。換言之，微銲點機械可靠度之關鍵已迥異於傳統上受銲料主導之球矩陣封裝接點及覆晶銲點，而係轉為由介金屬化合物決定。基於對微銲點可靠度評估之迫切需求，本研究採微米柱壓縮測試與奈米壓痕試驗來仔細探究不同晶粒取向Cu6Sn5介金屬於微米尺度下之機械行為。
首先，本研究對於微米柱壓縮測試提出校正方法，並成功取得符合奈米壓痕結果之楊氏模數以消弭探針、底材、錯位及重力所造成之誤差。其次，實驗之應力應變曲線顯示，Cu6Sn5微米柱在脆性破斷前會發生不連續應變之跳升，穿透式電子顯微鏡證實該塑性現象涉及微米柱內部之差排滑移。同時，本研究亦建立出Cu6Sn5晶粒取向與機械性質之關係，發現沿六方晶體之c軸測試時，Cu6Sn5展現出較高之楊氏模數。此外，微添加Ni元素至Cu6Sn5可大幅提高機械強度並穩定其六方晶體之結構。最後，實驗亦測量Cu6Sn5/Sn/Cu6Sn5及Cu6Sn5/Cu3Sn/Cu多層微米柱之機械行為，以期瞭解真實微銲點中之介面強度及破壞模式。結果顯示，Cu6Sn5/Sn/Cu6Sn5三層結構之微米柱會以介面滑動來發生塑性形變，在承受10%以上之應變量後，仍保有其結構完整性且於其內部介面無孔洞生成。另一方面，Cu6Sn5/Cu3Sn/Cu三層微米柱在受到壓應力時，此結構之塑性變形係以底材Cu之滑移達成，代表介金屬之間的介面具有足夠之強度。本研究結果將有助於三維積體電路微銲點之可行性及可靠度之評估。

關鍵字

三維積體電路 ； 微銲點 ； 微米柱壓縮測試 ； 奈米壓痕試驗 ； 介金屬

並列摘要

As semiconductor industry is confronted with the challenge of physical limit in scaling down transistor size, three dimensional integrated circuits (3D ICs) serve as one of the most promising technologies to extend Moore’s law (More-Moore) or even go More-than-Moore. Yet, the miniaturization of micro solder joints, an essential joining structure in 3D IC technology, leads to quick transformation from solders into intermetallic compounds (IMCs) in the joint. When a large portion of micro joints is occupied by IMCs, the mechanical properties and deformation behaviors of micro joints are no longer dominated by solder as in conventional flip-chip joints and ball grid array (BGA) joints, and instead intermetallics are anticipated to shoulder primary responsibility of mechanical reliability of micro joints. In light of this, this study puts emphasis on mechanical reliability assessment of single crystalline IMCs and IMC-based multiphase structures by means of nanoindentation and micropillar compression.
In order to eliminate the influence of indenter, substrate, misalignment and gravity effect under tilt condition, a calibration approach for micropillar compression is firstly established such that Young’s moduli measured from micropillar compression tests are in good agreement with those from nanoindentation. Also, the stress-strain curves show that Cu6Sn5 exhibited one or more strain bursts prior to brittle fracture, and the post-mortem transmission electron microscopy (TEM) analysis suggests that dislocation gliding inside micropillars is involved in strain jumps. Besides, a statistical analysis of the anisotropy in hexagonal Cu6Sn5 is also carried out based on micro-compression as well as nanoindentation results, where Cu6Sn5 exhibits a higher value of Young’s modulus when the loading direction is closer to c-axis. Furthermore, Ni addition into Cu6Sn5 can notably enhance mechanical strength and stabilize the hexagonal structure. Last but not least, multilayered micropillars (Cu6Sn5/Sn/Cu6Sn5 and Cu6Sn5/Cu3Sn/Cu) containing heterogeneous phase interfaces provide insight into overall deformation behaviors in real Cu-Sn interconnects. On one hand, Cu6Sn5/Sn/Cu6Sn5 cylinders exhibit remarkable plasticity in the manner of interface sliding at around 100 MPa, accommodating at least 10% strain and retaining its integrity without any void formation at the interface. On the other hand, multilayered Cu6Sn5/Cu3Sn/Cu micropillars undergo plastic deformation through slip deformation at Cu substrate, suggesting sufficient strength of intermetallic compounds and the interfaces. These results are beneficial in evaluation of validity and reliability of 3D IC micro-joints for chip-stacking applications.

並列關鍵字

3D IC ； Micro Joints ； Micropillar Compression ； Nanoindentation ； Intermetallic Compound

參考文獻

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