Copper was deposited onto silicon rotating-disk electrodes in 3.0 M HF to determine the effects of boundary-layer mass transfer and surface reaction kinetics on Cu deposition rates. For all solution compositions tested ͓͑CuSO 4 ͔ = 0.0010 − 0.020 M, ͓HF͔ = 3.0 M͒, the Cu deposition rate was proportional to the concentration of Cu 2+ and was under mixed diffusion and surface kinetic control. HF was used to dissolve oxidized Si, and a stoichiometry of one Cu atom deposited for each Si atom that is oxidized and dissolved was deduced from a combination of UV-visible spectroscopy and mass change experiments. These results indicate that Si dissolves as Si 2+ , not Si 4+ , as is commonly assumed for galvanic displacement. Mixed-potential theory was used to further analyze the reaction mechanism, and deposition rates and open-circuit potential values predicted from this method agreed well with experimental results during the initial stages of Cu deposition. Deposition rates decreased abruptly when film thicknesses exceeded 150 nm, and optical microscopy revealed buckling in these Cu films. This buckling occurred in response to compressive stresses in the films and decreased deposition rates by partially detaching Cu films from the Si substrate.

The rates of Cu deposition onto rotating Si electrodes were measured to probe the effects of mass transfer, Cu 2+ reduction, and Si oxidation and dissolution on deposition dynamics. Cu deposition rates were proportional to CuSO 4 concentration and limited by Cu 2+ diffusion and subsequent reduction at high HF concentrations ͓͑HF͔/͓CuSO 4 ͔ Ͼ 20͒. In contrast, Si dissolution limited film growth at low HF concentrations ͓͑HF͔/͓CuSO 4 ͔ Ͻ 10͒, and HF 2 − was identified as the most active Si etchant. The observed effects of rotation rate indicate that mass transfer of Cu 2+ limits deposition rates, but mass transfer of HF does not. Open-circuit potential measurements and mixed-potential theory were used to develop a reaction-transport model that accurately predicts deposition rates over a broad range of Cu and HF concentrations. The structure of the films formed was probed by atomic force microscopy. The roughness of the Cu films decreased with increasing ͓HF͔/͓CuSO 4 ͔ ratios, as Si surfaces became less oxidized, and lateral connectivity between Cu nuclei increased.Galvanic displacement has been used for metal deposition on Si for applications including microelectromechanical system, 1,2 surface-enhanced Raman spectroscopy, 3-5 and catalysis. 6,7 Galvanic displacement offers the advantage of selective deposition because reduction of metal ions is coupled with oxidation and dissolution of the substrate. A schematic diagram of galvanic displacement of Cu onto Si is shown in Fig. 1, indicating the mass transfer and kinetic processes required for deposition. Fluoride species, HF in this case, must be added to dissolve oxidized Si and expose additional Si to continue galvanic displacement. Galvanic displacement occurs spontaneously when the depositing metal is more noble ͑easier to reduce͒ than both the substrate and hydrogen. Thus, Cu can be deposited by galvanic displacement because of its favorable reduction potential 8where NHE is the potential of the normal hydrogen electrode. Si is easily oxidized ͑Si 2+ /Si E 0 = −0.81 V 9 ͒, making it a suitable substrate for galvanic displacement. In this work, the concentrations of Cu 2+ and HF are varied to determine the effects of the oxidation and reduction half-reactions on Cu deposition rates and the structure Cu films. Several studies have examined the effects of Cu 2+ concentration on galvanic displacement rates. Deposition rates were found to be first-order in Cu 2+ concentration for deposition on substrates including Si, Fe, and Zn. [10][11][12][13][14] Less is known about the effect of HF concentration on deposition rates. Deposition rates increased with increasing HF concentration for deposition of Cu and Ag on Si, but few HF concentrations were studied, and the relationship between HF concentration and Cu deposition rates was not quantified. 10,15 In this work, we investigate the effect of Si dissolution on Cu deposition rates by systematically varying the HF concentration.In addition to the effects of formal HF concentration, the fluoride species responsible for Si...

No abstract

The open-circuit deposition of copper onto silicon rotating disks has been studied to determine the deposition mechanism and the effects of deposition conditions on the properties of the resulting copper films. Cu2+ reduction limits the rate when HF is present in large excess, and Si oxidation and dissolution limit the rate at lower ratios of [HF] to [CuSO4]. The rate-limiting process strongly affects the resulting film properties, as the Cu films are smoother and more reflective when Cu2+ reduction limits the rate. Deposition rates increased with increasing rotating speed, and Cu2+ was found to be the diffusion- limited species over a wide range of CuSO4 (0.001- 0.04 M) and HF (0.02-0.4 M) concentrations. Open circuit potential measurements confirmed that Cu2+ was the species responsible for diffusion limitations through the external boundary layer.