銅導線化學機械研磨之研究—機制,膠體研磨粒子漿料及在嵌入式製程之應用

Abstract

隨著積體電路(IC)製作技術之成長，元件尺寸必須縮減以大幅提升效能。為了減少多層金屬導線所造成的RC延遲，其必須以低電阻金屬導線及低介電常數介電材料來建構多層金屬連線來加以改善。銅因其有較低之電阻值以及較高抵抗電致遷移的能力，以成為取代鋁導線的另一選擇。在多層金屬化導體連線的製作上，由於微影曝光解析度的要求更加的嚴苛，全域性平坦化是必須的，而化學機械研磨是目前唯一可達全域性平坦化的技術。本論文中，主要是針對研磨液中研磨粉體其不同的化學特性對於嵌鑲式銅導線化學機械研磨製程的影響。 實驗中所採用的研磨粉體是 ” 氧化鋁表面改質矽膠(Alumina Modified Colloidal silica)”和氧化鋁粉末。兩者的表面都是由氧化鋁組成，但膠體的氧化鋁比氧化鋁粉末，具較好的懸浮性、較小的粒徑、及呈圓球狀。 針對銅薄膜之化學機械研磨技術探討，實驗中以雙氧水(H2O2)為氧化劑，因H+及OH-的同時存在，故對銅金屬導線提供了腐蝕環境。藉由電化學分析可知以雙氧水為氧化劑的酸性氧化鋁表面改質矽膠(pH=3.8~4.0)中，隨著雙氧水的加入，在5vol.% H2O2以下為銅的純腐蝕，但7vol.% H2O2以上銅的表面會生成金屬氧化物導致腐蝕鈍化層的形成。以不同大小之氧化鋁表面改質矽膠粒子進行銅的化學機械研磨來探討銅薄膜在不同H2O2 vol.%之下，其與研磨粉體之間的研磨作用時，可以得知不管膠體氧化鋁粒子大小為何，在H2O2濃度低於5vol.% 時銅的研磨速率較高於7vol.%低，但高於7vol.% H2O2時銅的研磨速率會有一極大值產生，發生極值的H2O2濃度會隨氧化鋁表面改質矽膠粒子大小改變。因此可知H2O2濃度低於5vol.%銅的研磨速率由腐蝕速率主導，與氧化鋁表面改質矽膠粒子大小無直接的關連。但隨著H2O2濃度增加7vol.%以上，銅的表面同時生成金屬氧化物，此時銅的研磨速率是由研磨粉体的機械磨耗能力即氧化鋁表面改質矽膠粒子大小決定。將氧化鋁粉末和氧化鋁表面改質矽膠粒子在10vol.% H2O2溶液之下進行銅導線之化學機械研磨，由實驗結果發現，不同結晶相之氧化鋁粉末對銅的研磨速率較氧化鋁表面改質矽膠粒子低，而兩者移除率的不均勻度也都在百分之十五以上。然而以粒徑較小(10nm~70nm)且似圓球狀的氧化鋁表面改質矽膠粒子為研磨粉體，經研磨後銅表面仍然有刮痕的殘留。 在銅鑲嵌式製程研究中，針對濺鍍銅和電鍍銅我們做了一詳細的闡述。在第一步驟的研磨，階差（step hight）必須先消除，故除選擇對銅有一高移除率的研磨條件外，尚須一具有較大硬度、較小可壓縮性的研磨墊，如Rodel IC1400™ 研磨墊，來避免較低的區域(low features)也被磨到，導致無法達到平坦化的目的。而當溝渠外的銅膜快被移除完時，我們選擇一低的銅移除速率，銅對鉭有高移除選擇比的條件，來完成溝渠外銅完全移除的的步驟。在此步驟我們應用了兩種研磨液來進行比較，一為氧化鋁粉末與硝酸及檸檬酸混合的研磨液，另一種則為氧化鋁表面改質矽膠(Alumina Modified Colloidal silica)與雙氧水混合的研磨液。進入到第二步驟，即面臨到鉭、銅、氧化矽介電層移除的選擇比率問題。鑲嵌式製程結果的好與壞決定於第二步驟中，如何選擇銅對鉭與氧化矽的移除選擇比。所以我們以兩種不同製程方式的氧化矽膠體（colloidal silica）與雙氧水混合的研磨液應用在此步驟，以調整H2O2濃度或pH值來控制銅、鉭、氧化矽三者之間的研磨選擇率，借此方式，則兩種氧化矽膠體研磨粒子皆能降低原本在第一步驟研磨就已造成的碟陷（dishing）現象，且在過研磨（over-polish）的階段，都能使碟陷變得更小。

As IC fabricating technology developed, the device size has to be decrease for improving its effectiveness. For reducing RC delay, copper interconnection is used to instead aluminum owing to its low electrical resistivity and high electromigration resistance. In order to satisfy the requirement of the exposure of photolithography process, the globe planar surface is necessary. However, chemical mechanical polishing is the only way can approach this requirement. In this study, we research the effect of chemical characteristics of abrasives in copper damascene CMP. In our experiments, we applied alumina modified colloidal silica and pure aluminum powder to be our abrasives. Both they are composed by aluminum, but alumina modified colloidal silica has higher suspension, smaller size and circular morphology. In this study, we applied hydrogen peroxide to be an oxidizer. By the existence of H+ and OH –, copper interconnection could be corrosive in this surrounding. From the electrochemical technology analysis, while the alumina colloidal silica mixed with H2O2 in acidic media, copper would be corrosive below 5 vol.% H2O2 content, and form stable metal oxide on its surface over 7 vol.% H2O2 concentration. Applied different size alumina modified colloidal silica to investigate the corrosive behavior of copper in deferent H2O2 concentration. It was found that copper removal rate is higher below 5 vol.% H2O2 concentration and there would be a maximum value of copper removal rate existence over 7 vol.% H2O2. However, the maximum value is affected by the size of alumina colloidal silica abrasives. Therefore, the corrosive behavior of copper determined the removal rate below 5 vol.% H2O2 concentraion and the size of abrasives did not work. Till the H2O2 concentration over 7 vol.%, there was stable metal oxide existing copper surface, the removal rate determined by the wear ability of abrasives affected by their size. Mixed pure aluminum powder and alumina colloidal silica with 10 vol.% H2O2 respectively, different phase of pure aluminum has lower removal rate than alumina modified colloidal silica based slurries. However, both the aluminum system had high non – uniformity over 15 % and there were still scratches left on polished copper surface in smaller size (10nm ~ 70nm) alumina modified colloidal silica based slurries. In copper damascene CMP process, we still research the deferent polishing phenomenon between sputtering and electroplating copper. In first step polish, step hight has to be removed first. It is necessary to select a polishing condition with high removal rate and a harder and less deformation pad, like Rodel IC1400™ for avoiding serious thickness reduction of low features. While the copper film on space is almost removed, we select a polishing condition with lower removal rate and high selectivity of copper to tantalum. In this polishing phase, we compare two different polishing slurries. One is pure aluminum powder mixed with nitric acid and citric acid, the other is alumina modified colloidal silica mixed with hydrogen peroxide aqueous. In secondary polishing step, the most important thing is the selectivity of copper, tantalum and oxide. We used two kinds of colloidal silica from different fabricating process mixed with hydrogen peroxide. By adjusting the H2O2 concentration and pH values of slurries to control the polishing selectivity. By the way, both the colloidal silica abrasives would reduce the dishing amount during secondary step and over – polish.