Materials and Fill Studies of Electroplated Copper by Pulse Current
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.
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