三維積體電路微接點中介金屬作為結構材之應用

Abstract

由於現今正面臨到摩爾定律的終結，致使半導體業嘗試以其他方式克服電子元件製造上的物理極限以及逐漸提升的成本。其中3D IC是近年來最受到各大知名半導體廠青睞的方法。3D IC係以垂直方式堆疊晶片，其優勢在於增加接點密度的同時，可以大幅縮短訊號傳輸距離，進而提升系統效能，更重要的是這種架構能將異質晶片整合在一起。3D IC微接點與傳統覆晶技術主要的不同之處在於接點中銲料體積大幅縮小，僅有覆晶銲點的千分之一。因銲料體積大幅減少，組裝完畢後微接點內會充滿大量介金屬化合物。因此決定3D IC 微接點性質與可靠度的關鍵不再是銲料本身而轉變為占滿接點的介金屬，介金屬將會取代銲料成為微接點的結構材料。值此，工業界缺乏機械性質之數據据以模擬微接點可靠度之際，研究量化介金屬之性質乃當務之急。 在本論文的第一部分將藉由微米柱壓縮測試，針對單晶介金屬Cu6Sn5與Ni3Sn4進行機械性質之測量。測試結構將利用聚焦離子束切割製備，接著以Picoindenter在SEM下進行即時影像測試。實驗結果顯示雖然Cu6Sn5與Ni3Sn4之破壞模式皆為劈裂，但他們仍然能夠利用差排滑移產生應變突進。所以他們不像人們想像中的脆。在Cu6Sn5中，晶粒c軸垂直於施力方向時具有較佳的機械性質。而Ni3Sn4，在滑移系統(100)[010]中，能夠耐受4 %以上之應變。也就是說異相性在介金屬的機械強度提升上扮演重要角色，控制其晶粒方向能提升其塑形形變及機械性能。相較於Cu6Sn5，Ni3Sn4具有較佳之機械性質與韌性，也因此更適合用於3D IC微接點之結構材。 在製作一個全Ni3Sn4其高度小於10 μm的微接點時，由鎳與錫之間的反應所生成的微孔洞引起嚴重的可靠度問題。雖然銀的添加已被證明可以抑制這種微孔洞的形成，但是銀的最佳濃度尚未確立。本論文的第二部分將針對銀濃度在0 – 8 wt. %的範圍內的影響進行探討，以確定最佳的銀添加的量。發現銀的可消彌孔洞之最佳重量百分比為3.5 %左右，當它小於3.5 %時，微孔洞將無法完全消彌，而當其高於3.5 %時，銀將會形成的Ag3Sn連續層。此外，銀的添加對Ni3Sn4的生長動力學則沒有影響。另外在固液反應中，即使不添加的任何銀於錫之中，也可以獲得完全無孔洞之結構。 本論文的第三部分，旨在探討添加微量合金元素鈦對空間侷限下的Ni-Sn和Cu-Sn介面反應的影響。固態擴散偶Ni/Sn2.4AgX/Ni和Cu/Sn2.4AgX/Cu將利用熱壓接合製備，接著在150 - 200 °C下進行熱儲藏。鈦無論是在Ni-Sn或Cu-Sn系統皆以Ti2Sn3的形式析出。而添加鈦對Ni-Sn或Cu-Sn介金屬的生長動力學的其介面形貌沒有明顯的影響。

Considering the substantial reduction of solder volume in 3D IC micro joint, it is expected that a large portion of the joints will be occupied by intermetallic compounds (IMC) after assembly. Unlike flip-chip joints, the mechanical properties and reliability of joints are no longer dominated solely by solders but rather by IMCs. Thus, it is anticipated that IMCs will be used as structural materials instead of solder in commercial scale in a few years. However, mechanical property data for industrial reliability modeling are lacking. To characterize mechanical properties of IMCs becomes an urgent issue. In the first part of this thesis, micromechanical behaviors of single-crystalline Cu6Sn5 and Ni3Sn4 which are anticipated to be used as potential structural materials in micro joints in a few years are studied by micropillar compression. Micropillars were fabricated by focused ion beam machining and subsequently tested by Picoindenter. The failure mode of single-crystalline Cu6Sn5 and Ni3Sn4 are cleavage, but they both performed strain bursts as a result of dislocation gliding with certain slip system before failure. They are not as brittle as people thought and have superior mechanical properties. In Cu6Sn5, grains whose angle between c-axis and load direction is larger has better mechanical compatibility. Ni3Sn4 can withstand more than 4 % strain in preferred slip system, (100)[010]. The anisotropy becomes so important that the specific grain orientation shows the better plastic behavior and mechanical properties. Compared to Cu6Sn5, Ni3Sn has better mechanical performance as well as toughness and should be more auspicious to be adopted as structure materials of 3D IC microbumps. To form a full Ni3Sn4 micro joints whose height of solder is smaller than 10 μm, microvoids formation induced by Ni/Sn reaction raises a serious reliability issue. Although it was recently shown that the addition of Ag can inhibit the formation of such microvoids, the optimal concentration of Ag has not yet been established. The second part of this thesis is systematically conducted to investigate the effects of Ag concentration in the range of 0-8 wt.%, with the objective of identifying the optimal Ag addition. It is found that the optimal weight percentage of Ag is between 2.4 and 3.5 wt.%.; when it is lower than 2.4 wt.%, not all of the microvoids are eliminated, while when it is substantially higher than 3.5 wt.%, Ag3Sn becomes the primary solidification phase and large Ag3Sn plates form. Furthermore, the addition of Ag is found to have no effect on the growth kinetics of Ni3Sn4. In solid-liquid reactions, void-free structures could be obtained after Sn was fully consumed, even in the absence of added Ag. Minor alloying element addition in Sn-rich solder joint not only improve the mechanical properties of joints but also have several effect on interfacial reaction. Therefore, the information of minor element addition established on modification of properties of Sn is not suitable for reference anymore. It is a new challenge on the effects of minor addition on such small joints with high portion of IMC. The third part of this thesis aims to uncover the effect of minor alloying element, Ti, in the miniaturization of solder volume on the Ni-Sn and Cu-Sn solid-state reactions. The solid-state diffusion couples, Ni/Sn-2.4AgX/Ni and Cu/Sn-2.4AgX/Cu, were prepared by thermal compression and then isothermally aged at 150 – 200 °C. Ti participated in the solder as Ti2Sn3 in both Ni-Sn and Cu-Sn system. In addition, Ti addition have no apparent effect on the growth kinetics and morphology of Ni-Sn or Cu-Sn IMCs.