Vapor-phase Surface Cleaning of Electroplated Cu Films Using Anhydrous N2H4
Authors: Hwang Su Min , Peña Luis Fabián , Tan Kui , Kim Harrison Sejoon , Kondusamy Aswin L. N. , Qin Zhiyang , Jung Yong Chan , Veyan Jean-Francois , Alvarez Daniel , Spiegelman Jeff , Kim Jiyoung 
Published: February 2020

Copper is widely used in the semiconductor industry as interconnects due to its low resistivity, high resistance to electromigration, low temperature coefficient of resistance, and good thermal stability (1). However, the exposed Cu interconnects during via-opening and post chemical mechanical polishing/planarization (CMP) process, are vulnerable to oxidation with water rinse and exposure to air, resulting in reliability degradation (2). Therefore, an additional process to reduce the copper oxide would be required. Copper cleaning can be achieved by either physical Ar sputtering or chemical reduction process (3). Recent demonstration of chemical-based cleaning of Cu interconnects is expected to overcome disadvantages of physical Ar sputtering process, such as chamfering and re-deposition on vias and trenches. A number of studies on vapor-based reduction of copper oxide under ambient pressure conditions and at temperatures below 400 °C using hydrogen, ammonia, carbon monoxide, forming gas, acetic acid, formic acid, and ethanol as reducing agents have been reported (4,5). On the other hand, Hydrazine (N2H4) can be used in the reduction of copper oxide due to its higher reduction capability (6). Inspired by hydrazine’s unique characteristics, we explore the feasibility of vapor-phase reduction of copper oxide using anhydrous N2H4 to achieve an ideal metallic Cu film in an ALD environment. Additionally, a detailed surface analysis and reaction pathway of reduction with N2H4 has not been reported yet due to lack of in-situ experiment.

In this work, reduction of Cu samples with a native oxide were evaluated using N2H4 in a rapid thermal ALD system, as shown in Figure 1 (a). Before introducing the samples, the Cu surface was swept with compressed N2 gas, without any prior solvent cleaning, followed by loading into the ALD chamber. The representative time sequence of one cycle of the N2H4 treatment is illustrated in Figure 1 (b). The chamber was pumped down to 0.2 Torr without Ar carrier gas flow. N2H4 was exposed for 5 s with trapping for 120 s, followed by a purging time of 120 s.

From ex-situ XPS analysis, the initial sample surface showed contamination with adventitious carbon species, resulting in relatively low intensity in Cu 2p narrow scan (Fig. 2(a)). In addition, the surface contained Cu(OH)x and a CuxO film approximately 2 nm thick, indicating the metallic copper surface had formed CuxO and Cu(OH)x from exposure to air. With N2H4 treatment at 200 oC, a significant amount of copper oxide and hydroxide were reduced to metallic Cu, as observed in a decrease in the O 1s peak. It implies that N2H4 can clean the surface by reducing the oxide to metallic Cu as well as removing the surface contaminants. In addition, in-situ reflection absorption infrared spectroscopy (RAIRS) was employed to elucidate the individual surface chemistry of copper films during the N2H4 exposure. By monitoring the interaction of N2H4 with the surface species, we found both removal of surface contaminants and reduction of CuxO to metallic Cu (Figure 3). The detailed experimental results will be presented.

This work is supported by Rasirc Inc. by providing the anhydrous N2H4.

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Figure 1