以化學機械研磨之奈米雙晶銅薄膜應用於低溫銅對銅直接接合研究

Abstract

現今的微電子產品在效能上大幅的提升同時也希望能夠達到體積輕薄化的訴求，為了提高晶片之間接點數目而使用了微凸塊銲錫作為封裝的接點。但微凸塊銲錫在通電後容易形成硬脆的介金屬化合物(IMC)和發生電遷移的問題，導致元件產生失效。金屬直接接合被視為是取代銲錫避免上述可靠度問題的新一代微電子封裝技術。 一般的銅對銅的直接接合的溫度約在300°C-400°C才能夠接合成功，然而這個溫度對於微電子元件製程來說太高。先前的研究以直流電電鍍的方式製備出具有高度<111>方向性的奈米雙晶銅薄膜在低溫與一般真空的環境下進行銅對銅直接接合成功。因為銅原子在{111}平面中擁有最快的擴散係數，銅對銅直接接合在低溫以及一般的真空環境下達成，可以取代銲錫成為下一代微電子封裝的技術。奈米雙晶銅薄膜直接接合的條件控制在10-3 torr下，並施加0.78 MPa的壓力，可以低溫的環境接合成功。 除了接合溫度與接合時間外，表面的粗糙度在金屬的直接接合中扮演著相當重要的角色，先前研究中的奈米雙晶銅薄膜在進行接合之前會先使用電解拋光的方式來使表面平整化再進行直接接合。本實驗中使用化學機械研磨( Chemical Mechanical Planarization, CMP) 的方式取代先前的電解拋光進行表面的平整化處理。 經過化學機械研磨的奈米雙晶銅薄膜在10-3torr的真空環境下施加0.78MPa的壓力能夠成功的接合。由於表面較平整，粗糙度約為2nm，在與先前研究相同的實驗條件(150°C-60min,200°C-30min)下比較：經過化學機械研磨的奈米雙晶銅薄膜擁有更好的接合介面；而在同樣200°C的條件下，經過化學機械研磨的奈米雙晶銅薄膜能夠在10分鐘內就達成接合，相較於電解拋光的奈米雙晶銅薄膜必須在200°C 持溫30分鐘才能完成接合。由此得知：導入化學機械研磨製程能夠有效縮短奈米雙晶銅薄膜的接合時間以及提升接合介面的品質。

Nowadays, the requirement of performance on microelectronic devices becomes higher while the size shrinks smaller. People use solder micro-bumps as the joints in microelectronic packaging. Solder joints have brittle IMCs (intermetallic compounds) and electromigration issues. Metal direct bonding is a promising approach to replace solder in microelectronic packaging, which can avoid the reliability issues caused by solders. Cu-to-Cu direct bonding was achieved at the temperature of 300°C-400°C, which is too high for microelectronic devices. Previous studies have shown the successful results of bonding two highly <111>-orientated nano-twinned Cu films fabricated by DC electroplating at low temperatures and with regular vacuum condition. It is because the Cu atoms have largest diffusivity on {111} planes. This breakthrough can lower the thermal budget and high vacuum environment on the bonding process. In this study we performed the DC electroplating to fabricate the <111>-oriented nt-Cu films. The current density is 80mA/cm2. In previous study, the nt-Cu films were electropolished at 1.75 V for 10 min and the roughness was 7nm. In this experiment, we perform the chemical mechanical planarization (CMP) to make the surface smooth for direct bonding. The nt-Cu films prepared by the CMP process can be bonded successfully in 10-3torr vacuum environment under the compressive stress of 0.78MPa. Owing to the lower surface roughness of 2nm, the CMP nt-Cu films bonding shows better bonding quality than the electropolished Cu films. The CMP nt-Cu films can be bonded successfully at 200°C in 10 min. In comparison, the bonding time for the electropolished nt-Cu films was 30min at 200°C.