低介電常數材料在積體電路製程整合研究

Abstract

為了建構高效能的積體電路，增加金屬內連線的層數與縮小導線間的距離成為必然的趨勢。此將導致電子訊號在積體電路中的傳送速度大幅降低，衍生所謂電阻-電容時間延遲 (RC delay time)；在內連線結構中，使用低介電常數材料是克服此問題不錯的方法。本研究將探討四種極具潛力的低介電常數材料：Hydrogen silsesquioxane (HSQ)，Methylsilsesquioxane (MSQ)，Hybrid- Organic-Siloxane-Polymer (HOSP) 以及 Porous Organosilicate Glass (POSG)，並探討其製程整合問題及其解決方法，藉以評估其在半導體製程中的相容性。 半導體製程中，光阻去除是必要且不可避免的步驟，製程上常使用氧電漿與光阻去除液去除光阻。吾人發現氧電漿與光阻去除液會造成介電質的嚴重劣化。為了更進一步了解薄膜劣化的成因，本研究針對氧電漿與光阻去除液劣化薄膜的機制進行深入探討。實驗結果顯示，含有甲基的MSQ，HOSP及POSG薄膜對氧電漿極度敏感，氧電漿極易與存在薄膜中的甲基反應，進而形成Si-OH的原子團；另一方面，鹼性的光阻去除液也會水解HSQ與MSQ薄膜，在薄膜表面形成不穩定的懸鍵。水解的過程中，位於薄膜表面的懸鍵極易轉變成Si-OH的原子團。這些親水性Si-OH的原子團，不論由氧電漿或光阻去除液所引發，都將使受損介電質吸收更多外界環境水氣，因而造成介電質的劣化。 為了防止低介電常數材料在光阻去除製程中受到損害，本研究採用氫電漿與氨電漿作事前預防處理。在進行光阻去除步驟前，吾人利用氫電漿與氨電漿處理薄膜，使其在介電質表面形成鈍化層；此鈍化層可阻擋在光阻去除製程中，來自氧電漿與光阻去除液的損害。研究結果顯示，它不僅可鈍化位於薄膜表面不穩定的懸鍵，更將防止薄膜受損引發水氣吸附。故低介電常數材料經電漿處理後，即使再經光阻去除步驟，其優良的介電特性仍將不受影響。另一方面，對於製程中所產生的Si-OH 原子團，使用trimethylchlorosilane (TMCS) 與hexamethyldisilazane (HMDS) 處理受損薄膜，可有效除去位於薄膜表面的Si-OH 鍵結。TMCS與HMDS不但能修補受損薄膜的缺陷，消除親水性的Si-OH，更可在薄膜表面形成Si-O-SiMe3的原子團。因Si-O-SiMe3原子團頗具疏水性，故經TMCS與HMDS的化學處理後，即可將親水性的受損薄膜，修補成為具有疏水性的介電薄膜，使薄膜中水氣含量下降，進而恢復薄膜之介電特性。 在漏電傳導機制方面，對於製程中受損薄膜，本論文有深入的探討。當受損薄膜受到外加電場作用時，所吸附的水氣將解離成帶正電的氫離子與帶負電的氫氧根離子；隨著電場持續作用，這些帶電的正負離子也將隨之移動，進而引發離子傳導電流穿過整個介電薄膜。實驗結果顯示，一但受損的介電薄膜吸收外界的水氣，介電質的漏電流行為將會由原來的Schottky emission 轉變成後來的 ionic conduction，這也是造成介電質劣化最重要的原因之一。另外，由於銅導線與低介電常數材料的組合是未來的趨勢，因此本文也將探討銅金屬與低介電常數材料SILKTM的整合問題。經過BTS測試後，銅會擴散至介電薄膜中，進而導致薄膜介電特性劣化。電性分析顯示，銅擴散將導致介電質的漏電流行為由Schottky emission 轉變成Poole-Frenkel emission。 由於光阻去除製程在介電材料圖形化的過程中，可能劣化介電薄膜；為避免介電層在傳統圖形化的過程中受到損害，於是吾人發展出一種新穎的X-ray微影技術，以有效固化經X-ray曝光的薄膜。經過X-ray曝光照射後薄膜的分子結構，將由原來的籠狀結構(cage)轉變成為後來的網狀結構(network)；再經適當的溶液顯影，薄膜中未經X-ray固化的區域，將迅速溶解於顯影液中，最後留下X-ray固化的薄膜，即為我們所欲完成的介電層圖形。X-ray直接圖形化的技術，可以成功地免除傳統微影技術下對於介電層所造成的傷害，其將是未來多層導體連線結構中，一種極具應用潛力的新技術。

As the device dimensions continue to shrink, interconnection delay becomes a limiting factor for increasing device speed. Since interconnection delay is the product of the resistance in metal interconnection and the capacitance between the metal lines, we have to minimize the parasitic capacitance and the resistance in interconnection. Incorporation of low-dielectric-constant materials in multilevel interconnect can effectively reduce parasitic capacitance, thus decreasing the transmission delay. In this study, four kinds of low-dielectric-constant materials are investigated: Hydrogen Silsesquioxane (HSQ), Methylsilsesquioxane (MSQ), Hybrid-Organic- Siloxane-Polymer (HOSP) and Porous Organosilicate Glass (POSG). Moreover, the effects of integration issue on the low-dielectric-constant materials are also studied to evaluate the compatibility of low-k materials on semiconductor process. Photoresist removal is an indispensable process in integrated circuit fabrication. O2 plasma and wet stripper are commonly used to remove photoresist during photoresist stripping process. It is found that serious dielectric degradation often occurs in low-dielectric-constant materials during photoresist removal process. We have found that methyl-containing films (MSQ, HOSP and POSG) are very sensitive to O2 plasma. The O2 plasma can easily react with Si-CH3 groups in methyl-containing films, converting Si-CH3 groups into Si-OH groups. We have also found that alkalinity-containing wet stripper solution can lead to a hydrolysis reaction of HSQ and MSQ films, forming dangling bonds on the film surface. The dangling bonds on film surface can react with hydroxide ion (OH—) in wet stripper solution and form hydrophilic Si-OH groups. Consequently, these hydrophilic Si-OH groups caused by either O2 plasma or wet stripper can effectively absorb moisture in low-k film, resulting in dielectric degradation. In order to protect low-k films from damage during photoresist stripping, H2 plasma and NH3 plasma pre-treatments are proposed. Prior to photoresist stripping process, plasma pre-treatments should be performed on low-k films so as to form a passivation layer on the surface of low-k film. The passivation layer, generated from either H2 plasma or NH3 plasma, is thought to be a barrier against O2 plasma and wet stripper damages. This layer can passivate dangling bonds on the surface of low-k film, and prevent low-k film from process-induced damage and moisture uptake during photoresist removal. As a result, low-k film can still keep its excellent properties even after photoresist removal process. In case of Si-OH group formation during photoresist removal, the troublesome Si-OH group is the major issue to induce moisture absorption, causing dielectric degradation. Hence, we can utilize trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDS) to eliminate hydrophilic Si-OH groups on damaged surface of low-k film. Both TMCS and HMDS treatments can react with Si-OH groups. Consequently, Si-OH groups are not only eliminated to repair the damaged low-k films, but also undergoing reaction with TMCS and HMDS, which form the Si-OSi(CH3)3 groups. Owing to hydrophobic Si(CH3)3 groups, the TMCS and HMDS treatments can change the film surface from hydrophilic to hydrophobic. For this reason, moisture content in low-k film will be reduced by TMCS or HMDS treatment and thus both the leakage current density and dielectric constant can be reduced for the damaged film. The leakage current behavior of damaged low-k film is investigated. Moisture uptake is easily induced in damaged low-k film. Thus, when the exterior electric field is applied to the moisture-absorbing film, the mobile ions (H+, OH—) supported by absorbing moisture will move, leading to the ionic conduction leakage current across the whole dielectric film. Therefore, once moisture uptake occurs in damaged film, the leakage-current mechanism will transform from Schottky emission into ionic conduction, which is the most important reason to cause dielectric degradation in dielectric film. In addition, the integration issue between copper and low-k SiLK is also studied. After bias temperature stress (BTS) test, copper will diffuse into SiLK, causing dielectric degradation. Electrical analyses show that the behavior of leakage conduction transforms from Schottky to Poole-Frenkel mechanism. Finally, a novel X-ray direct patterning is proposed to avoid process-induced damage during photoresist removal. The X-ray can cure exposure region of HSQ and HOSP films, which transforms dielectric films from cage-like structure into network structure. Then, X ray-uncured region can be dissolved by suitable solvent to achieve the desired pattern after development. After furnace annealing, the superior dielectric characteristics of X ray-cured films (desired pattern) can be achieved. Consequently, SiO2-based low dielectric constant materials can successfully avoid dielectric degradation caused by traditional lithography. Therefore, X-ray direct patterning is a key candidate for the next generation of multilevel interconnect technology.