次0.15 微米溝渠與引洞之電鍍銅技術

Abstract

在超大型積體電路製程中，銅配線技術已被世界各大半導體公司持續開發且已正式投入量產。而因應銅製程的整合考慮，崁入式結構是目前公認為最實用的後段銅導線設計。但當因應漸趨複雜且高速元件產品的需求時，高深寬比的崁入式製程也產生一系列的技術挑戰。如為增強銅電鍍製程的填洞能力，須在電鍍液的組成上加入多重功能的添加劑以利鍍液能克服狹小孔洞入口的限制而進入底部隨之由平整劑之作用使電鍍具非等向性鍍膜能力。 在過去發表的文獻中，大家研究填洞能力的電鍍液，不是使用商用的配方，就是未有明確的定義或研究。而在本論文實驗研究中，所提出的最大貢獻，就是自己能夠配置出高填洞效能的電鍍液，並且說明添加劑填洞的機制。依據我們的研究成果，一些特殊有效的鍍液組成能填入深寬比約7:1，小於0.15 um尺吋的介層洞或溝渠已被開發出來。我們發現具有足夠活化過電壓、選擇抑制梯度和低電阻的特性才是有效的平整劑(leveling agents)。 電鍍銅的填洞能力，不但與進行電鍍的條件有關，其所使用之電鍍液成分更是一項不可或缺的因素。一般傳統的銅電鍍液只注重電鍍銅後之銅層外觀，並未考慮銅層形成後之電性問題，所以，加入過量之平整劑來維持優良的填洞能力，以致殘留微量之平整劑於銅層中，進而造成銅層之電性不良問題。因此，尋找一種適用於超大型積體電路中的高效添加劑，已成為研究銅電鍍液配方之重要課題。所謂高效添加劑亦即加入少量添加劑就能於深次微米金屬鑲嵌製程中達到超填塞（super-filling）能力。 本論文提供一種高效銅電鍍液之配方，適用於超大型積體電路製程。於銅電鍍液中加入少量之添加劑，即可得到小於0.15 um，高深寬比的銅導線，且不會產生孔洞或是縫隙。此電鍍液配方中具有高的酸／銅離子濃度比值、二種不同聚合度之聚乙烯二醇（polyethylene glycol；PEG）當作潤濕劑(wetting agents)，以及具有有效抑制能力(adequate inhibition ability)及選擇性抑制梯度（selective inhibition gradient）之平整劑。其中平整劑主要是由具有位於氮原子鄰位之硫醇基（mercapto-group）的pyridine衍生物或同時具有苯基（benzyl group）與氨基（amino-group）的thiazole衍生物組成。 在鍍液中，PEG可降低鍍液表面張力。其中PEG（2000）可改善晶粒品質，PEG（200）可促進填洞能力。另外，平整劑可與銅層表面之銅原子形成螯狀化合物。因此藉由平整劑與銅晶種層與新成長之銅核團的強吸附作用，以降低銅晶體成長的速率。同時因平整劑在開口頂端與開口側壁之濃度高於開口底部的濃度。所以，相較於開口底部，較強的抑制效應會發生在開口頂端與開口側壁。如此，於開口底部之銅晶體成長速率相對的較高，因而可達到超填塞的效果。 因此有效之電鍍液除了需要潤濕劑提供足夠低的表面張力外，還要加入平整劑來提供有效的抑制能力以及選擇性抑制梯度。只有同時具備此三種條件之電鍍液，才能在小於0.15 mm，高深寬比的溝渠或介層洞中，達到超填塞的效果。

In ultra large integrated circuit (ULSI) technology, copper interconnect technology has been developed continuously and incorporated in commercial products. As considering the constrain of integration of copper metallization, damascene structure promises a practical wiring design in IC backend process. While function complexity in device and the need of high-speed product is increasing, the damascene process with high aspect ratio has met a series of obstacles on copper process. Those technical thresholds included such as the electrolyte perfectly diffusing into the quite small gaps achieving the anisotropic deposition in trenches or vias. Copper electroplating for damascene interconnection by adding leveling agents in the electrolyte to enhance the filling capability has been investigated. But most of previous studies utilized either unidentified additives or commercial electrolytes. In this study, we would research additives from the literatures that they were typically for electroplating unpatterned substrates and found the recipe of the composition and concentration of electrolyte for sub-150 nm, 7:1 AR copper metallization. We also investigated that the effective leveling agent must offer sufficient inhibition ability, selective inhibition gradient and low electrical resistance. In order to achieve the defect-free filling in sub-0.15 um vias and trenches, the electrolyte must be composed of appreciate amount of cupric ions, H2SO4, chloride ions, wetting agent and leveling agent. The supplied current must be controlled at lower current density and the agitation must be acceded to the electroplating process. Typically, a wide variety of additive compositions for acid copper electroplating baths are compatible to the large scale such as material surface treatment or PCB application. But in ULSI technology, the process window we met is sub 0.2 um and high aspect ratio. However, not all known leveling agents are suitable for electroplating process in ULSI, since the conventional copper electroplating baths only focus on the appearance without considering electric properties of copper deposits and, generally, a large amount of leveling agent should be added to attain a good filling performance. Too large amount of the added leveling agent may cause little leveling agent to remain in the copper deposit, which would deteriorate the electric properties of the copper deposits. Therefore, the best way to seek an additive for accomplishing super-filling is to try the various leveling agents and find out the highly efficient leveling agents; in other words, a small-added amount of the highly efficient leveling agent may attain super-filing ability for application in ULSI process. In this study, an acid copper electroplating bath, which used to achieve gap-filling capability for deep sub-0.15 um damascene metallization in ULSI technology, is comprised of (1) high acid/cupric ion concentration ratio, (2) two different average molecular weight of polyethylene glycol, which one has lower molecular weight with DP ranging from 200 to 600 and the other has higher molecular weight with DP ranging from 2,000 to 2,0000, (3) The levelers consist essentially of pyridine derivatives with the mercapto-group (-SH) at ortho position neighboring on N atom or thiazole derivatives with benzyl groups and amino-group (-NH2). The wetting agent system in the electrolyte was consisted of two different average molecular weight of polyethylene glycol, which one has smaller molecular weight with DP of 200 and the other has larger molecular weight with DP of 2,000. The smaller PEG (200) with better diffusion ability enhances filling capability and deposit quality within deep feature. The larger PEG (2000) enhances grain growth control and improves uniformity of deposited films. In addition, the leveling agent improved filling capability in sub-micro vias and trenches because of effective chelate formation between leveler and copper surface. There are three indispensable requirements for achieving super-filling property; they are low surface tension of electrolyte, sufficient inhibition on copper depositing and selective inhibition gradient. If those conditions were present simultaneously during electroplating, then super-filling could be achieved.