ULSI 嵌入式銅導線之電解拋光平坦化研究

Abstract

本實驗的目的是建立銅電解拋光(Cu Electropolishing)的方法應用於半導體後段製程中多層導體連線的製作。研究重點包括以電解拋光的方法製作銅導線、電鍍銅層(Cu Electroplating)階梯差之平坦化 以及釐清圖型效應(pattern effect)對銅電解拋光之影響。技術發展的重點為電鍍銅膜電解拋光液的研發及製程的最佳化參數，以達平坦化及導線可靠度要求。 電解拋光在傳統表面處理工業上為一重要的技術。此方法能夠均勻的溶解金屬表面，而得到一平坦且光亮的外層。在電解拋光的過程中，電解拋光液組成及操作參數，皆為影響平坦化能力的關鍵因素。如何開發一有效電解拋光液及添加劑，為電解拋光方法應用在深次微米銅導線製程中，成為全域性平坦化技術的一大挑戰。 在深次微米半導體製程中，化學機械研磨技術 (Chemical Mechanical Polishing, CMP) 是目前積極發展全域性平坦化的技術。但化學機械研磨應用於玻璃絕緣膜平坦化及金屬導線製作時，機械應力(mechanic stress)、溶液浪費(waste stream)、刮痕(scratch)、以及化學機械研磨後清潔(post-clean)，都是目前所需克服的關鍵問題。 因此，我們可利用電解拋光的技術來增進電鍍銅化學機械研磨的效能。例如：在兩階段(two steps)的銅化學機械研磨過程中，我們可採用電解拋光的技術來替代第一階段(first step)的化學機械研磨，這樣就可以解決目前第一階段化學機械研磨中，研磨漿料(slurry)所造成銅膜表面刮傷(scratch)的問題，進而增進第二階段化學機械研磨的平坦化效果。實驗中為了更進一步改善銅電解拋光之平坦效能，我們於電解液中加入檸檬酸作為添加劑; 藉由微量的添加劑於具有階梯差的銅膜表面形成一擴散梯度、促使溝渠內外之凹陷和突起部分產生局部之酸鹼與導電度之變化，進而增進銅電解拋光時銅膜階低差之降低能力。此外，我們也使用聚乙烯二醇(polyethylene glycol; PEG)於銅電解拋光製程上以抑制較高操作電壓時之氧氣泡產生而造成孔洞(pitting)生成，並藉由其較佳之潤濕能力(wetting ability)而得到一較平整且光滑之銅膜表面。 在未來我們更可以電解拋光的技術配合無阻障層的低介電質材料(non-barrier low-dielectric materials)取代化學機械研磨製程來完成後段銅導體連線。由於此製程只對金屬銅膜做均勻的溶解，所以，可避免銅層與阻障層磨除選擇率的問題及絕緣膜磨耗現象，並可解決化學機械研磨後清潔的問題，增加線上量產的效能(throughput)。

The purpose of this thesis is to establish Cu electropolishing technology for the back-end multi-layer interconnects, including the production of copper wires by electropolishing process, the planarization of step-height for Cu electroplating layers and the pattern effects on Cu electropolishing. The main issue for the requirement of planarization and reliability is the development of the electropolishing electrolyte and the optimum of operation parameters. In tradition, electropolishing is the important technology of surface treatment. It can dissolve metallic film uniformly and produce a smooth and bright surface. In electropolishing processes, the composition of an electropolishing and operation parameters are the key points affecting the capability of the planarization. The challenge for electropolishing being a technology of globe planarization in sub-micron copper interconnects is how to develop an effective electropolishing electrolyte and proper additives. In sub-micron semiconductor interconnects, chemical-mechanical polishing (CMP) is one candidate technology for global planarization. However, mechanic stress, waste stream, scratches, and post-CMP clean are critical problems in CMP applications. Therefore, we could utilize the electropolishing technology to improve the performance for CMP of electroplated copper. For example, in the typical two-steps Cu-CMP processes, we can employ electropolishing technology as the first-step CMP to eliminate scratches produced by CMP slurry and then improve the planarization capability of CMP. In this study, citric acid was added into the electrolyte as additives in order to further improve the planarization efficiency of Cu electropolishing. With the few additives producing a diffusion gradient on the step-height surface of Cu films, those local acidities and conductivities at the recess and protrusion between inside and outside of the trenches will change. Furthermore, the capability of step-height reduction on Cu electropolishing will improve. Besides, The polyethylene glycol (PEG) was also employed to suppress the pitting resulting from the production of Oxygen bubbles at highly operation voltages on Cu electropolishing; then a smooth and bright surface of Cu films would be accomplished due to the better wetting ability of PEG. In the future, we could furthermore integrate copper electropolishing with non-barrier low-dielectric materials (non-barrier low-k materials) to replace CMP processes. Since uniformly dissolved copper film is intrinsic characteristic of electropolishing, the problems of removal selectivity between copper film and barrier film, oxide erosion, and post-CMP clean could be disappeared naturally. As we mentioned above, the throughput will become higher.