高度(110)方向性電鍍金薄膜的金對金直接接合研究

Abstract

傳統銲錫具有介金屬化合物(IMC)和孔洞電遷移的問題，常是造成元件失效的原因。直接接合是電子封裝的新技術。若能除去銲錫，我們能夠避免前面提到的問題。 先前的研究已經在較低的溫度和一般真空條件下展示出了銅對銅具有高度<111>方向性Cu薄膜直接接合成功的例子。這是因為銅原子在{111}平面中的擴散率是銅的所有平面中最大的，銅對銅直接接合可以在這樣的條件實現，這個突破可以節省大量的熱源成本，並降低了接合中對真空環境的要求，為了防止銅膜的氧化，銅直接接合的條件應控制在10-3 torr下進行，並配合0.78 MPa的壓力，可以在30 min接合成功。 雖然已經有許多銅對銅直接接合的成功例子，但銅在鍍膜製程至接合中間的保存仍然會因氧化使得製程的成本上升。在這項研究中，我們提出以金用來作為Cu或Ni膜彼此直接接合的抗氧化層，因為金具有高的化學穩定性和低的活性，可以解決此問題，Au本身的直接接合變成為我們主要探討的目標，高度<220>方向性金薄膜可以透過直流電鍍鍍出，在金電鍍液中施加通以電流密度5 mA/cm2的直流電流，並施加1200 rpm的攪拌速度，可以製造出具有細晶粒和奈米雙晶結構的金薄膜。 本實驗在熱壓接合之前，需要對Au表面清潔，首先，將樣品浸泡在丙酮中超聲波振動清洗5分鐘，再將樣品浸入稀釋的HCl溶液30秒，以消除在Au表面上的氧化物。熱壓接合在10-3torr真空中施加0.78MPa壓力在150,200和250oC持溫不同時間。 金薄膜表面的方向性及晶粒尺寸通過XRD和EBSD測量而得，薄膜平整度則以AFM測量，並通過使用FIB觀察薄膜及接合面的橫截面微結構，幫助我們了解接合機制及孔洞遷移的機制。 兩金薄膜在10-3 torr真空環境中，施加0.78 MPa之壓力，並在200oC持溫1 h可以接合成功。並發現當薄膜粗糙度較低時，有助於增加接合面的接觸面積。

Traditional solder joints have brittle IMC(intermetallic compound) issues. Therefore, they cannot be scaled down below 10μm pitch. Direct bonding is a new technology of electronic packaging. For fine pitch by removing the solder, we can continue to scale down the joints. Previous studies have shown the successful results for Cu-Cu direct bonding with highly <111>-orientated Cu film at lower temperature and with regular vacuum condition. It is because the diffusivity of Cu atoms on {111} plane is the largest among all planes of Cu. This breakthrough lowers thermal budget and lowers the requirement for vacuum environment on bond process. In order to prevent the oxidation of Cu films, the conditions of bonding process should be controlled in 10-3 torr level. In this study, we examine another direct bonding with gold, which has been used to passivate Cu or Ni films. Gold films with <220>-preferred orientation could be made by DC electroplating. The gold films possess the nanocrystalline structure and with nano twins. The direct current of 5 mA/cm2 was applied with the stirring rate at 1200 rpm in a plating bath. Gold is very resistant to oxidation. Thus the cleaning process is simple. Before the bonding process, it takes several cleaning steps for Au surface. First, the sample is cleaned by ultrasonic vibration in acetone for 5 minutes. Afterward, the sample was immersed into diluted solution of HCl for 30s to eliminate any possible oxides on Au surface. Two Au films with <220> preferred orientation were then bonded together directly under 0.78 MPa compression stress in 10-3 torr vacuum condition at 150, 200 and 250 oC for various time durations. The orientation of Au surface was measured by x-ray diffraction(XRD).The microstructure with different thermal compression conditions was observed by using focused ion beam (FIB).The microstructures at different bonding temperatures will help us to figure out the bonding mechanisms and void migration mechanism for [220] Au films.