This paper describes an acid-based electroless Cu deposition system employing CuCl 2-HNO 3 chemistry in a HF-NH 4 F buffer solution. With the help of nitric acid, Cu can be deposited on SiO 2 /Ta/TaN substrate without the insertion of a Cu seed layer. The deposition rate is found to decrease with increasing ͓HNO 3 ͔ in the solution. The roles of ͓NO 3 Ϫ ͔, ͓F Ϫ ͔, ͓Cl Ϫ ͔, ͓NH 4 Ϫ ͔, Si, and TaN in the system are identified based on split experiments. Si acts as the reducing agent while ͓F Ϫ ͔ and ͓Cl Ϫ ͔ ions form complexions with and transmit electrons to Cu 2ϩ. Overall, a deposition rate of 2700 Å/min with a Cu resistivity of 2.35 ⍀ cm can be achieved.
This paper describes the kinetics involved in the acid-based electroless Cu deposition system employing CuCl 2 -HNO 3 chemistry in a HF-NH 4 F buffer solution. The rate equation is set up as a function of concentrations of active chemical components involved, and rate orders are determined to evaluate the contribution from each component. The deposition rate of Cu is found most sensitive to variation in ͓Cl Ϫ ͔ concentrations, followed by that of ͓Cu 2ϩ ͔ and ͓F Ϫ ͔. The activation energy derived from deposition at different temperatures is 0.445 eV. Grain size of deposited Cu films is influenced strongly by deposition rate. Electrical resistivity of Cu films is dominated by the amount of point defects and microvoids present right after deposition, and by grain size after a 300°C anneal.Electroless plating provides a viable alternative to electroplating for Cu thin-film deposition in the era of Cu-based multilevel interconnect for microelectronic devices. 1 Besides its capability of filling high-aspect-ratio pattern structure, electroless plating offers additional advantages such as low cost and simpler chemistry.Most existing electroless Cu plating processes adopt base solutions with copper sulfate as the Cu ion source, ethylenedinitroloacetate ͑EDTA͒ as the complexing agent, formaldehyde as reducing agents, tetramethylammonium hydroxide ͑TMAH͒ or alkali solution as pH buffer, and some surfactant and stabilizer added to improve step coverage and bath stability, respectively. This chemical formulation has been studied extensively over the past twenty years and has been applied to the manufacture of circuit boards.In the first part of this study, 2 we proposed an acid-based CuCl 2 -HNO 3 electroless Cu plating solution with HF-NH 4 F as pH buffer. An optimum deposition rate of ϳ2700 Å/min directly on TaN substrate and a resistivity of 2.35 ⍀ cm can be achieved at pH 4.70. CuCl 2 , when dissolved in the solution, provides the ͓Cu 2ϩ ͔ ions, which are catalyzed by HNO 3 on TaN surface and reduced by Si to form Cu deposits. ͓Cl Ϫ ͔ and ͓F Ϫ ͔ ions act as complexing agents and transmit the electrons from Si to ͓Cu 2ϩ ͔. Cu deposition rate exhibits a decreasing trend with increasing ͓HNO 3 ͔ concentration in the bath. Adding ͓HF͔ or ͓NH 4 F͔ into the bath, on the other hand, enhances the deposition rate to a peak level, after which it saturates and declines.As an extension of our previous study, we explore the kinetics and material issues of the system in this contribution. Concentration of the active components and temperature used in the system are varied in order to deduce the reaction rate orders and activation energy. Evolution of grain size and resistivity with ͓Cu 2ϩ ͔ concentration in the chemical bath are analyzed. The effects of postdeposition anneal on resistivity, microstructure, and preferred crystal orientation of Cu thin films are also investigated. ExperimentalDetermination of reaction rates.-Chemical components and substrate materials used for the Cu deposition experiment basically follow those from ou...
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