合金元素添加對3D IC封裝中微接點之界面反應研究

隨著半導體製程技術開始遭遇其物理極限，三維晶片堆疊(3D IC)技術的興起，被視為跳脫製程技術極限框架暨延續摩爾定律的重要關鍵。與直徑約為100微米之覆晶銲點相比，目前應用於三維晶片堆疊構裝技術（3D IC）之微銲點直徑約為10微米，其銲點體積被大幅縮小約500倍。可以想見，這些微銲點於迴銲組裝接合(Assembly)或長時間固態熱時效後，整個微銲點將有機會被大量介金屬化合物所佔滿，此時微銲點的機械性質已不再由原始的Sn基銲料所主導，而將由介金屬化合物來決定。由於銲點的尺寸相當微小，在此空間受限（Space confinement）下的界面反應及其微結構變化特徵勢必與傳統覆晶銲點截然不同，特別是因為在空間受限下所誘發的一些新效應更值得本研究深入探討。本研究分為兩大部分，第一部分為探討合金元素添加對3D IC封裝中微接點之界面反應影響。本部分研究著重在探討合金元素添加（如Ag、Bi、Zn）對於微銲點的界面反應影響及銲點因空間受限所誘發出的相關可靠度議題。透過挑選三種最具代表性之合金元素，包括(1)易與Sn反應生成介金屬化合物，但幾乎不溶於Cu-Sn介金屬化合物之元素，如Ag，(2)完全不參與界面反應的鈍態元素，如 Bi，(3)積極參與界面反應的非鈍態元素，如 Zn，來進行相關研究。本部分研究主要以Cu/solder (10μm)/Cu三明治結構來進行120 oC、150 oC、180 oC、200 oC之固態熱處理，並觀察微銲點中因界面反應所誘發之相關效應。擬於本部份研究探討之議題包括 (1)固態熱時效下，來自上、下端之介金屬化合物的碰觸行為。(2)固態熱時效下，微銲點中之合金元素濃度因界面反應而不斷提高。(3) 固態熱時效下，有效抑制Cu-Sn介金屬化合物成長的方法。第二部分主要探討新穎之改良式固液擴散接合技術於3D IC晶片堆疊之應用。如前所述，目前應用於三維晶片堆疊之微接點尺寸約為10 μm，當欲垂直堆疊多層同質或異質晶片時，需能有效避免各層晶片於堆疊時發生微接點內銲錫重熔，進而發生晶片塌陷之現象，降低製程良率。由於固液擴散接合技術的最終產物「介金屬化合物」其熔點遠高於原本銲錫之熔點，將可有效避免上述現象的發生，因此在三維晶片堆疊應用上極具吸引力。然而，「接合過程相當耗時」是傳統固液擴散接合技術的一大限制，相較於傳統的固液擴散接合原理，本研究透過引入一可調控之參數而首度提出「新穎之改良式固液擴散接合技術」。實驗結果證實該創新之改良技術具有(1)大幅縮短固液擴散接合所需之時間，(2)可任意控制介金屬化合物的生長方向，(3)可有效控制介金屬化合物的晶粒取向(即具有從優取向)，(4)有效抑制基材端金屬墊層消耗，大幅降低金屬墊層之製備成本，等絕對優勢。本研究將先以Cu/Sn/Cu、Ni/Sn/Ni、Ni/Sn/Cu等常見之微銲點結構，首度揭露並實際展示傳統固液擴散接合與新穎之改良式固液擴散接合的差別，亦將深入剖析此方法於現今封裝製程上之適用性、接合材料之選擇性(含銲料與金屬墊層)、介金屬化合物形貌暨其微結構特性(EBSD)等。

關鍵字

3D IC ；微銲點；界面反應；接點空間限制；固液擴散接合；晶粒取向；合金元素添加

並列摘要

Recently, three-dimensional integrated circuit (3D IC) integration technology consisting of TSV and micro solder joints has been viewed as one of the very promising solutions to go beyond Moore’s law. Typical solder joints for 3D IC integration under development today has a solder volume about 1000 μm3, which is roughly 1/500 that of a conventional flip-chip solder joint with usually an 100 μm in diameter. One well-perceived effect of such a small solder volume is the solder joint would have very high possibility to be occupied by a relatively large fraction of intermetallic compounds (IMCs) after common reflowing or thermal compression bonding process. Under this circumstance, instead of solder, the mechanical properties of IMCs and other reaction-induced microstructural changes and characteristics will undoubtedly play dominant roles on the reliability of the micro joints. Some new concerns resulted from interfacial reactions are worthy to keep an eye on from the aspect of a reliable and robust solder joint. This research work is divided into two major parts. The first part systematically investigates solid-state interfacial reactions under a very confined space between Cu and Sn, Sn-Ag, Sn-Bi, Sn-Zn solders alloy. Sandwich structures of Cu/Solder/Cu are fabricated through chip-to-chip (C2C) thermal compression bonding process. The thickness of the solder layer in this study is well-controlled at 10 μm. High temperature storage tests are conducted by isothermal aging at 120 oC, 150 oC, 180 oC, and 200 oC for different time periods, respectively. Issues to be discussed in this paper include (a) Impingement behavior of IMC grains during solid-state isothermal aging, (b) Rise of concentrations of minor inert alloying constituents in low solder volume joint, (c) Promising approach to reduce the growth rate of Cu-Sn reactants within limited solder volume joint. These critical issues will be proposed and discussed through experimental evidence, and the implications based on these findings will be discussed as well. The second part mainly focuses on proposing a novel modified Solid-Liquid Interdiffusion (M-SLID) bonding technique for 3D chip-stacking applications. Due to extremely low solder volume in micro solder joint, such a key feature is particularly suitable to implement the concept of Solid-Liquid Interdiffusion (SLID) Bonding technique, where a layer of low melting solder materials, such as Sn, is taken as an intermediate layer to interconnect two higher melting point substrate materials, typically Cu. Under appropriate temperature and reflowing time frame, a configuration of Cu/IMC/Cu sandwich can be obtained as the mediate layer is completely reacted with Cu to form IMCs. Compared with conventional SLID boning technique, a novel modified SLID bonding technique (M-SLID) through applying one more controllable factor is firstly proposed, uncovered, and developed in this part, which possess some significant advantages for multi-layer chip to chip (C2C) stacking, including (1) effective shrinking the processing time for forming demanded IMCs during bonding process, (2) controlled growth direction of the IMCs in assigned directions (i.e. either forming at the chip side or the substrate side), (3) controlled crystallographic orientation of the Cu6Sn5 grains (i.e. having preferred orientation), (4) markedly inhibited consumption of the metallization layer at the substrate side during bonding process. Three kinds of typical sandwich structures, Cu/Sn/Cu, Ni/Sn/Ni, and Ni/Sn/Cu are purposely used to show and demo the major difference between conventional SLID (C-SLID) and modified SLID (M-SLID) bonding technique in this study.

並列關鍵字

3D IC integration ； micro joints ； interfacial reactions ； space confinement ； Solid-Liquid Interdiffusion bonding ； grain orientation ； alloy constituent additions.

參考文獻

1.5 Reference

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