藉由同步輻射X光源深入探討同分異構相轉變及高電流驅動下的原子擴散行為

隨著半導體製程持續進步，單一晶片之電晶體數量不斷增加，其所搭配的電子封裝技術往往需朝高功率密度及微型化來進行設計。這些發展趨勢將使得金屬導線與微接點所乘載的電流密度因此急遽上升，容易觸發電遷移(electromigration)現象。電遷移可能使導體之陰極端產生孔洞，而造成線路斷路;相對的，陽極端則易產生凸塊或鬚晶。而一旦具有導電性的鬚晶生長至足夠長度時，可能與其鄰近線路相接觸，進而造成電子電路的短路。Sn材料多用於電子封裝之微接點材料，因Sn具有優異的潤濕性(wetting)、延展性、低熔點及機械強度等。為此，本論文第一部分將利用同步輻射(synchrotron radiation)設施設計即時觀測系統，以深入探討Sn金屬導線之電遷移行為。本研究透過Sn Blech結構，並採用同步輻射X光螢光光譜即時觀測錫電遷移行為。同時也利用同步輻射白光勞厄繞射(Laue diffraction)，以建立Sn晶粒大小/取向及應力累積/釋放與電遷移的關聯。本論文第二部分係以同步輻射白光勞厄繞射探討微接點結構。Sn基銲點及Cu金屬銲墊為微電子工業最主要的冶金系統。在一次迴焊反應後，界面會生成典型的Cu-Sn介金屬化合物(intermetallic compound, IMC) (Cu6Sn5 and Cu3Sn)。Cu6Sn5作為Sn/Cu銲點界面主要之IMC，其機械性質之評估尤其重要。然而，Cu6Sn5存在η及η’兩同素異形體(allotropes)。η及η’之同素異形體相變態點約在186 °C。此相變態點是電子封裝製程中會經過的溫度，此時便可能引發Cu6Sn5由高溫相(η)至低溫相(η’)的相變態行為。同時，因η及η’之晶體於特定取向具有假對稱結構，不易進行相變態分析。為此，本研究藉由同步輻射白光勞厄繞射，以進行η及η’之相鑑定，並建立Cu6Sn5相變態隨時間的演變情形。未來即將邁入第五代行動通訊，其操作頻率將大幅提升。隨著傳輸頻率升高，導線及微接點內的材料缺陷及表面的形變將影響高頻傳輸品質。本論文結果非但能用來作為改善金屬導線及微接點可靠度的立論依據，亦能對材料物理界長期以來所關心的電遷移基礎原理，Sn鬚生長與應力釋放機構，及Cu-Sn銲點特性有更加深層的認識。

關鍵字

同步輻射；電遷移；錫鬚； Cu6Sn5介金屬化合物；同分異構相轉變； X光螢光光譜；白光勞厄繞射

並列摘要

With the advance of semiconductor processing technology, the number of transistors on a silicon chip increase rapidly. Accompanying this advance, the back-end process and packaging technology also progress rapidly, resulting in rapid miniaturization and increased power density. Such trend makes the current densities carried by various metal lines increase quickly. A direct consequence of a high current density is the electromigration effects. Electromigration drives atoms diffusion from the cathode side toward the anode side. Excessive electromigration cause the formation of voids near the cathode side, which in turn may induce open circuit failures. Excessive electromigration also induces the formation of hillocks or even whiskers near the anode side. Long whiskers present a serious concern for short circuits in fine spacing conducting traces. Sn based alloys are widely used as soldering material in microelectronic industry due to its good wettability, low melting point and mechanical property. In this thesis (chapter 3), synchrotron X-ray analysis of the Sn electromigration behavior and Sn whisker growth in a Blech structure using nano-X-ray fluorescence microscopy and white beam Laue nanodiffraction was conducted. Sn depletion at the cathode and whisker/extrusion formation at the anode were characterized in-situ, and the results obeyed the electromigration kinetics. This electromigration scenario gradually decayed because of the counterbalance between electron wind force and back stress. White beam Laue nanodiffraction analysis showed that a noticeable compressive deviatoric stress in the direction of electron flow built up at the anode of Sn strips, particularly in the roots of Sn whiskers, confirming that electromigration-induced atomic accumulation occurred downstream in a strip and that Sn whiskering was closely related to internal stress resulting from atomic accumulation in confined segments. Finally, a theoretical model based on fundamental electromigration theory revealed that Sn diffused predominately through lattice and grain boundary paths at Sn homologous temperature of 0.6. These new data will not only advance our knowledge for the fundamental understanding in the solid-state diffusion and stress evolution of the interconnects, but will also be helpful in developing advanced 3D IC technologies for the semiconductor industry. Additionally, in pursuit of the high reliability of microelectronic devices, researchers and engineers have devoted their efforts to the improvements of micro joint integrity. Cu6Sn5 and Cu3Sn are two typical intermetallic compounds nucleated in the Cu/Sn system, governing the mechanical/electrical reliability of micro joints. The allotropic phase transformation of Cu6Sn5 from a hexagonal (η) to monoclinic (η’) structure at 186 °C is scientifically interesting and technologically important. As the η’ lattice is crystallographically pseudosymmetrical with η, the identifications of η and η’ and their transformation are extremely difficult via traditional techniques, such as electron backscatter diffraction, transmission electron microscopy, or, X-ray powder diffraction. In this thesis (chapter 4), a study of the η-to-η’ transformation and phase distribution by Laue diffraction via synchrotron white X-ray radiation with a 70 × 70 (nm) focused beam is conducted. The allotropic species (η and η’) can be well distinguished, and two-stage phase transformation kinetics is proposed. This non-destructive method can further provide the crystallographic information associated with the underlying Cu3Sn nanolayer and Cu crystal and their crystallographic correlations with respect to the phase transformation, which advances fundamental understanding of Cu/Sn metallurgical reaction.

並列關鍵字

Synchrotron radiation ； Electromigration ； Sn whiskers ； Cu6Sn5 ； Allotropic phase transformation ； Nano X-ray fluorescence ； Laue diffraction

參考文獻

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