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 Cu_xO film approximately 2 nm thick, indicating the metallic copper surface had formed Cu_xO 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 Cu_xO 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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