脈波電流電鍍對電鍍銅材料特性及填孔能力之研究

Abstract

當元件大小縮小至次微米的尺寸時，訊號傳遞的速度不是由電晶體轉換的速度來決定，而是由配線的性能來決定，配線的性能是依據它的電阻及電容來判定。為了增加訊號傳遞的速度，低電阻係數之製程材料在超大型積體電路製程中扮演非常重要的角色。銅因為比鋁有更低的電阻係數，更高的抗電致遷移係數，近來在超大型積體電路製程材料中是相當受到重視的。銅配線可以用電鍍方式來沉積；用電鍍來沉積銅配線雖然是一種最傳統的方式，但卻是最簡單的一種方式。跟物理氣相沉積技術（PVD）與化學氣相沉積技術（CVD）比較起來，電鍍有低成本、低製程溫度、高填洞能力等優勢。因此，電鍍已成為銅配線之主要技術。 依據電鍍過程中所施加的電流形式，一般可分為兩種形式：直流電及脈波電流電鍍。其主要的分別在於直流電只能控制電壓（或電流）大小，而脈波電流可分別控制其頻率、能率時間、及電流密度大小。當電鍍高深寬比的引洞與溝渠時，因為晶種層比較突出而導致引洞開口部分較緊束，或者是在高電流密度下銅沉積過於迅速，會導致孔洞之產生，這時就必須使用脈波電流電鍍。在含有添加劑之酸性硫酸銅溶液中，脈波電流電鍍被證明會增加均擲力。因此，我們可以相信脈波電流電鍍可以促進電鍍液之控制，以及減少鍍層中之雜質，來減少在電鍍過程中添加劑之使用量。 在本論文中，我們首先討論電流密度大小、能率時間、及頻率對鍍層性質的影響。我們可以發現電阻係數、表面粗糙度、及晶粒尺寸會隨著電流密度、能率時間、及頻率的增加而減小。電阻係數可減少至18.3 μΩ-cm, 而晶粒尺寸可減少至94.3nm. 當電流密度、能率時間、及頻率達到一定值時，電阻係數、表面粗糙度、及晶粒尺寸會隨著電流密度、能率時間、及頻率的增加而增加。另外，用脈波電流電鍍之鍍層其電阻係數、表面粗糙度、及晶粒尺寸會比用直流電電鍍之鍍層來的低。在填洞能力方面，電鍍液中只含有兩種不同分子量之聚乙烯二醇（polyethylene glycol；PEG）當作濕潤劑，用脈波電流電鍍即可得到小於0.15μm，高深寬比的銅導線，且不會產生孔洞或細縫。因為電鍍液中所含的添加劑量不多，所以銅導線之電阻係數比較低。

As device scales are narrowed down to submicron dimensions, the singal propagation is dominated by interconnection performance. But not by transistor switching speed. The performance of interconnection is dependent on its resistance and capacitance. In order to increase signal propagation speed, low resistivity metallization materials play very important roles in the ultralarge scale integration (ULSI) metallization. Recently, copper has attracted considerable attention as a potential ULSI metallization materials, because of its low electrical resistivity and high electromigration resistance compared with aluminum. Copper interconnection could be deposited by most conventional methods. However, a simple method of electrochemical plating could be used to deposit Copper interconnection as well. Electrochemical plating has several advantages compared to PVD and CVD due to its low cost, low processing temperature, and good ability to fill vias. Therefore, electroplating is becoming the leading technique in copper metallization for feature ULSI interconnection. According to the electrical current applied for electrochemical deposition, one can, in general, categorize the process into two types: direct current (DC) and pulse current electroplating. The main practical difference between DC and pulse current electroplating is that with DC electroplating only voltage (or current) can be controlled, but with pulse current electroplating three parameters-frequency, duty cycle and current density can be varied independently. In practice while filling a high aspect ratio vias or trenches the top of the via pinches off due to overhang of seed layer or rapid deposition of copper at high current density during electroplating thereby leading to void. This necessitates the need for pulse current electroplating. Pulse current electroplating has been proven to produce an increase in throwing power of acid copper sulfate solution containing organic additives. Furthermore, it is believed desirable to reduce the use of additives in these processes for at least two reasons: to facilitate the control and monitoring of electroplating baths and to minimize impurity levels in electro-deposition thin films. In this study, we discussed the qualities of deposit films from different parameters in pulse current electroplating such as supplied current density, duty cycle, and frequency first. We could see that the resistivity, roughness and grain size diameter of copper films decreased with increasing the supplied current, duty cycle, and frequency, and then which were increased with increasing the supplied current, duty cycle, and frequency until a threshold quantity reached. We could see that the resistivity of deposit films decreased to 1.83 μΩ-cm. The grain size diameter of deposit films decreased to 94.3 nm. Another, the resistivity, roughness and grain size diameter of deposit films by pulse current electroplating was lower than direct current electroplating. In filling power, the additives of electrolyte only included two different average molecular weight of polyethylene glycol (PEG) as wetting agents, which used to achieve gap-filling capability for deep submicron damascene metallization in pulse current electroplating. The resistivity of copper interconnect was lower, it is because that the quantities of additives was decreased in pulse current electroplating.