電遷移效應對銅薄膜表面粗糙度影響之研究

A Study of Effect of Electromigration on Surface Roughness of Copper Thin Film

詹宗晟(HASH(0x1c13aaa0))

指導教授 ： 廖建能

國立清華大學/工學院/材料科學工程學系/碩士(2008年)

https://doi.org/10.6843/NTHU.2008.00486

摘要

在銅製程中，銅晶種層的表面粗糙度對於之後的電鍍銅導線性質會有很密切的影響。隨著電子元件的不斷縮小，到了45奈米製程節點，銅晶種層的厚度會下降到30-50奈米且其表面粗糙度需低於1奈米。然而在本實驗中，銅薄膜在200 °C退火1分鐘之後，其表面粗糙度會高達7.5奈米。在電鍍過程中，ㄧ個表面粗糙的銅晶種層會在銅晶種層跟電鍍銅導線之間產生孔洞，進而造成元件可靠度下降的問題。在本研究中，我們利用電致遷移的方法，試圖改善銅薄膜的表面粗糙度。藉由原子力顯微鏡的分析結果，我們發現在通電流30分鐘之後，銅薄膜的表面粗糙度由原本的7.5奈米下降至4奈米，在通電流10小時之後，銅薄膜的表面粗糙度更進一步下降至1.4奈米，這個現象可藉由表面晶粒薄化(grain thinning)來解釋。另外，一般來講，施加交流電通過金屬薄膜並不會造成任何電致遷移的現象發生，然而在本實驗中，我們發現施加交流電通過銅薄膜也可讓其粗糙度下降，這部份推測與棘輪作用(ratchet effect)有關。

關鍵字

銅導線 ； 平坦化 ； 電遷移 ； 銅製程 ； 表面粗糙度

並列摘要

For conventional copper (Cu) damascene processes employed in integrated-circuits fabrication technology, a seed layer is required prior to electroplating of copper metallization. As a result, the surface roughness of the seed layer is expected to affect the structure and quality of the electroplated Cu layer. For 45 nm technology node, the thickness of Cu seed layer will be 30-50 nm with less than 1 nm surface roughness. However, the surface roughness of Cu films after annealing at 200 °C for 1 minute would be 7.5 nm. A rough Cu surface would cause voiding problem between Cu seed layer and electroplated Cu, leading to electromigration reliability issue. In the present study, we have proposed a method to improve the surface roughness of copper thin films by electric current induced surface smoothing. Due to fast surface diffusion of copper atoms, the surface morphology of copper thin film can be modified by electrical current induced atomic migration. Our results show that the surface morphology and film texture of the electrically stressed Cu thin films change notably with time according to the atomic force microscopy (AFM) and X-ray diffraction (XRD) analysis. The roughness of copper thin film was found to improve from the initial 7.5 nm to 4 nm after 30-minute electrical current stressing, and eventually down to 1.4 nm after 10 hours of electrical current stressing. In general, no long range mass transport is expected for metal films stressed by alternating electrical current. However, the surface roughness of Cu films is reduced after alternating electrically stressing. The mechanism of electric current induce surface smoothing is speculated to be related to the ratchet effect.

並列關鍵字

copper interconnect ； planarization ； electromigration ； copper metallization ； surface roughness

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