非真空環境下之奈米雙晶銅-銅直接接合技術研究

Copper-to-copper direct bonding on highly (111) oriented and nano-twinned copper in no-vacuum ambient

莊敬業(Jing-Ye Juang)

指導教授 ： 陳智

國立交通大學/工學院/材料科學與工程系所/博士(2019年)

摘要

為因應下世代高階封裝技術的需求，本研究成功開發出能取代傳統銲錫材料的高強度、低電阻之奈米雙晶銅-銅接點及相應之氮氣環境下的銅-銅直接接合技術。此研究之目的在於解決傳統銲錫在微縮化的過程中，接點內存在大量比例之介金屬化合物(Intermetallic compounds, IMCs)，而介金屬化合物的存在會造成的接點結構的脆性表現並最終導致電子產品在運作時的失效，這是極待解決的問題。再者，為了滿足未來5G高頻通訊以及5奈米元件所需之高頻、低電阻接點，使得使用銲錫材料的接點結構碰到前所未有的挑戰。因此本研究投入於應用奈米雙晶銅材料的高階封裝技術，並深入於金屬對金屬接合時的接合機制，及其奈米雙晶銅之晶粒成長行為，為的提供低點阻高強度的新接點結構於整合型封裝結構中。
本論文規劃一系列之系統化研究方法來針對奈米雙晶銅材料及其銅-銅直接接合方法進行研究，開發出不需要真空環境下之銅-銅接合技術，探討溫度、壓力與時間三個製程因子下對銅-銅接點的微結構影響關係為何，並對銅-銅接合的機制以及接合界面微結構分析，分析異方性晶粒成長行為，應用表面擴散之數學模型來計算不同製程溫度下之孔洞生成率並與實驗結果進行驗證比較。然後量測銅-銅接點的機械與電阻特性，與銅接點的溫度循環測試與電遷移可靠度測試下的表現。

研究成果顯示我們可以在氮氣環境下，施予40.6 MPa，在200 °C~450°C持溫 20 min 完成接合。接合介面只有少許奈米級孔洞。並能達成30微米(111)雙晶銅微凸塊的對接，剛接好的單點銅-銅接點電阻只有 4.12 mΩ ，contact resistivity 是3.98 × 10-8 Ω·cm2。當再施予400°C第二階段退火，接點的電阻及contact resistivity 能夠分別再下降到3.27 mΩ 及 3.14 × 10-8 Ω·cm2. 比起相同直徑的錫銀銲錫接點，大約有50% 電阻值的下降。最後，電遷移測試結果顯示銅-銅微凸塊接點的電遷移壽命比銲錫微凸塊接點至少長750倍。在取得低電阻(Low resistance)、高剪力強度(High shear strength)、與高抗電遷能力(Long EM life)的比較結果後，確認以奈米雙晶銅材料所形成的銅-銅接點，能擁有優異的結構強度與低阻值的電氣特性，它能解決傳統銲錫因IMC生成造成接點結構的弱化問題，也能提供低阻值、低功耗與低延遲的銅接點結構(Cu Interconnects)，相信外來能有機會應用於高階封裝結構與技術的應用之中。

關鍵字

銅-銅直接接合 ； 奈米雙晶銅 ； 表面擴散接合 ； 晶粒成長 ； 機械力強度 ； 電遷移 ； 應力遷移

並列摘要

To meet the packaging requirements for next-generation high-end devices, this dissertation investigates Cu direct joints with high strength and low resistance to replace traditional solder joints. As solder joints continue to shrink in size, brittle intermetallic compounds (IMCs) will cause fabrication and reliability problems for devices. These are urgent issues that needs to be solved.
In this dissertation, we perform a systematic study of Cu-to-Cu direct bonding in no-vacuum ambient using nanotwinned nt-Cu materials. Several factors influencing the bonding quality as well as the grain growth are investigated, including bonding temperatures, pressure, and time. We examine the microstructures of the Cu joints, and propose a model to explain the mechanism for the bonding. We also analyze abnormal grain growth behaviors, and apply mathematic models using surface diffusion to calculate the void formation rate at different bonding conditions. This is then compared to the experimental results. Finally, we measure mechanical strength and resistance of the Cu joints, as well as the electromigration reliability.
The results indicate that excellent bonding of Cu joints in N2 ambient can be achieved at 40.6 MPa and 200°C~450°C for 20 min. There are only a few nanoscale voids at the bonding interface. We can also bond arrays of Cu microbumps, 30 μm in diameter, using (111) nt-Cu bumps. The average resistance and contact resistivity for the as-fabricated Cu microbumps are 4.12 mΩ, and 3.98 × 10-8 Ω·cm2, respectively. After applying a second-step annealing at 400°C, the resistance and contact resistivity can be further reduced to 3.27 mΩ, and 3.14 × 10-8 Ω·cm2, respectively. Compared with Cu/SnAg/Cu solder joints of the same diameter, there is a 50% reduction in resistance. Finally, electromigration tests show that the lifetime for Cu joints is at least 750 times longer than that of Cu/SnAg/Cu joints. Therefore, Cu joints show great potential in application for interconnects in 3D IC packaging.

並列關鍵字

Cu-to-Cu direct bonding ； nano-twinned Cu ； fast surface diffusion character ； surface creep ； bonding interface ； grain growth ； grain orientation ； shear strength ； Electro-migration ； and stress migration

參考文獻

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