銅電鍍與電解拋光於銅鑲嵌金屬連導線應用之研究

Abstract

本論文含有三個部分，第一部份介紹銅電鍍液中抑制劑PEG與加速劑SPS 經由電流驅使的衰敗對填孔能力的影響。第二部分則研究具有氫氧基的醇類與有機酸添加劑加入電解液後對銅電解拋光的影響。第三部分則是因為考量到成本與許多製程風險的影響，所以提出一種電化學方法將電鍍與電解拋光整合於同一電解液並利用電腦程式的控制能將填孔與平坦化以一個程序完成。 首先，我們研究了經過一段長時間或多次電鍍後，鍍液中添加劑PEG與SPS的衰敗對填孔能力與表面形貌的影響。當同一溶液中只具有PEG時，多次電鍍與高電流密度將使得高分子類的PEG產生裂解。PEG的裂解不但會降低PEG的電流抑制效果，而且會使得許多短鏈PEG與銅的錯合物產生在溶液擴散層中。因此，經過越多次的電鍍，越不佳的填孔能力與越粗糙的銅膜表面將會呈現。我們利用即時觀測電鍍時鍍液電壓突然升高的現象來推測PEG在銅表面的吸附脫附行為能力與銅離子的還原速度。電壓突然升高的程度與電化學陰極交流阻抗分析將可推測PEG的劣化程度與電鍍液的可靠度。 除此之外，我們證明了兩種加速劑SPS在經過多次電鍍後所可能劣化的原因。第一、一個SPS會經由電流驅使分裂成為兩個MPS，MPS比SPS更具有去極化的效果。當電鍍進行時，越來越多的MPS將會使得電鍍液的填孔能力喪失。第二、SPS會經由空氣氧化或電流驅使在一段時間後生成不再具有加速效果的硫化物（S-product）。 本論文第二部份，我們發展了一種雙添加劑系統的電解拋光溶液來達到銅鑲嵌構造拋光後大小線寬(1-50μm)皆達到高度平坦化的效果。這種電解液含有具有氫氧基的醇類與有機酸，再加上原本的主體溶液磷酸。在銅表面上有高潤濕能力的醇類能加強保護溝渠（Trench底部）的效果。而因為醇類吸附銅表面也會增加表面黏度更能抑制銅的拋光速度。這個對銅Trench底部卓越的保護能力使得此雙添加劑成為銅電解拋光後階段性高低差（Step-height）能快速平坦的原因。 另外，我們還研究了含醇電解拋光液中有機酸加速拋光劑對銅溶解的影響。我們建立的有機酸雙添加劑系統為醋酸、檸檬酸、Citrazinic酸以及苯甲酸。在溝渠的底部，被證明酯化反應有效的生成一層抵抗電解拋光的黏膜且降低了局部區域的酸度。因此在Trench外部，銅的溶解速率由電解液中的酸度主導。而在Trench底部，則由此生成的高黏度的電阻值主導。對於弱酸系統例如醋酸來說，比較高濃度的添加劑量較能維持較低的酸度與緻密的酯化層，所以能在Trench底部抑制銅的溶解。所以能得知為何此含有醋酸加上醇類的電解液能在電解拋光後得到最高的銅平坦化效率。 論文第三部份，我們提出了一種方法：銅雙重模式電鍍，寄望能使得銅電鍍與電解拋光能在一個電解槽完成。最重要的參數除了為電解液的選用外，還包含電化學程式的最佳化。目前為止，我們發展了幾種有效的電解液配方與電脈衝頻率來促進銅雙重電鍍模式的效能。目前發現有效的電解液中，包含了銅標準電鍍液、抑制劑、平整劑以及電解拋光的必備磷酸。對於小線寬銅線，已經有特定參數能達到電鍍後的表面平整與高度填孔能力。對於大線寬銅線，則在良率60%的情形下達到階段性高低差的減少。

There are three parts in this study. The first introduces aging influence of polyethylene glycol (PEG) and PEG- bis-3-sodiumsulfopropyl disulfide (SPS) containing bath on gaps filling during Cu electrodeposition. The second part introduces the role of alcohols and organic acids additives in electrolyte of damascene Cu electropolishing. In order to save many risks and cost at the back end of the interconnect fabrication, third part demonstrates a method that can integrate Cu electrodepostion and electropolishing in one electrolyte and electrochemical tank. The developing method can be called dual-mode plating. First part, we investigate how the degradation of poly (ethylene glycol) (PEG) additives-containing and PEG-bis-(3-sodiumsulfopropyl disulfide) (SPS)-containing Cu electroplating electrolytes influences the gaps filling of damascene features and the roughness of plated surfaces. For only PEG-containing bath, the cleavage of PEG whose reaction is enhanced by a high bias current and more plating cycles, not only diminishes the inhibition effect of the electrolytes, but also enables the formation of enormous complexes of short-chain PEG–Cu far from the reacting surfaces. Hence, the more plating cycles performed, the worse the gaps filling characteristic and the rougher the plated surfaces. Furthermore, an overshoot phenomenon on the transient cells voltage for PEG-containing electrolytes is observed and explained well by a dynamic equilibrium between PEG absorption ability and Cu reduction speed. Accordingly, the change in overshoot shape is closely related to PEG aging and this fact is employed to examine the reliability of electrolytes. Moreover, we verify the two possible mechanisms of degradation of SPS. First explanation: SPS will crack into two MPS driven by the overpotential of plating, and MPS is more active depolarization than SPS. Moreover, the more and more existed MPS will make the electrolyte more ineffective for filling capability during aging process. Second explanation is SPS will lose all the accelerating ability after becoming S product which is some kinds of derivatives of SPS. Second part, we demonstrate a two-additive electropolishing (EP) electrolytes that exhibit an extremely high planarization-efficiency in Cu damascene schemes, independent of pattern sizes (1-50 μm). This electrolyte is displayed by adding alcohols and organic acids to the H3PO4 electrolyte. The high wetting ability of alcohols allows such additives to easily access the damascene bottom. This mechanism, assisted by the reduced polishing rate associated with the high surface viscosity caused by alcohol additives, greatly passivates the damascene bottom from elecropolishing. Accordingly, the superpolishing functionality of the two-additive electrolyte outperforms additive-free and one-additive electrolyte. Furthermore, this study also explores how the dissolution of damascene Cu depends on accelerators of organic acids in alcohol-containing H3PO4 electropolishing electrolytes. Four two-additive electrolytes that contain different accelerators, acetic, citric, citrazinic, and benzoic acids, are evaluated. At the bottom of damascene features, an esterification reaction between alcohols and organic acids efficiently forms a highly resistive layer and reduces the acidity of the solution. Accordingly, outside the damascene features, the rate of removal of Cu is dominated by the acidity of electrolytes but, inside the features, it is also determined by the resistance of the viscous layer. For a weak acidic additive, acetic acid, an extremely high additive concentration is introduced to sustain the moderate acidity of the solution to initiate intense esterification. Therefore, how acetic-acid-based two-additive electrolytes exhibit excellent Cu planarization capability is realized. In third part, an effective technology (Cu dual-mode plating) containing two functions of Cu film depositing and polishing in one-step electrochemical process in one tank has been developed. We have identified chemical additives and processing parameters in dual-mode plating. By mixing inhibitors, leveler, and phosphoric acid with standard copper electroplating solution, we are able to obtain similar planarization and gap-filling performance in narrow trenches (350nm) and step-height reduction in wide trenches (50 μm) to those of standard electroplating solution with identical processing time and yield of 60%.