Copper Deposition onto Silicon by Galvanic Displacement: Effect of Silicon Dissolution Rate

Abstract

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...

The Kinetics and Stoichiometry of Metal Cation Reduction on Multi-Crystalline Silicon in a Dilute Hydrofluoric Acid Matrix

“…It was shown in the study by daRosa et al [ 42 ] that the stoichiometric ratios of Cu 2+ reduction to metallic Cu and the associated oxidation and dissolution of silicon during the reaction of Cu 2+ with silicon in HF matrix varied between 3:4 and 2:1 mol∙kg −1 :mol∙kg −1 . This was interpreted as an indication of a change between the divalent and tetravalent reaction mechanism depending on the concentration of Cu 2+ and HF.…”

Palladium nanostructures from galvanic displacement as hydrogen peroxide sensor

“…XPS studies were also performed several weeks after the deposition, showing the same Pd(0) signals, showing the stable nature of the formed nanofilms and nanostructures. Finally, and in order to confirm the experimental data obtained by XPS and in order to verify the formation of a Pd nanofilm, a 30 and 90 s deposition time sample was softly scratched and the film thickness was measured by AFM as explained elsewhere [26]. The average thickness of the 30-s sample was found to be 33.5 ± 2.4 nm while the 90-s sample presented a film thickness of 52.6 ± 2.3 nm (95% confidence level, n = 10).…”

Copper deposition onto silicon by galvanic displacement: Effect of Cu complex formation in NH4F solutions

“…1b and the associated Eqs. (11)- (14). This model captures the trends of decreasing deposition rates at high rotation speeds in 0.01 M CuSO4 and increasing deposition rates with increasing rotation speeds in 0.001 M CuSO4.…”

“…Experimental methods were similar to those reported previously for deposition of Cu onto Si from HF solutions [13,14]. CuSO 4 ·5H 2 O (99.3%, Fisher) was used as the Cu source (0.0010-0.020 M); NH 4 F (Sigma-Aldrich, 40% w/w) was used as a 6.0-M aqueous solution to dissolve oxidized Si.…”

“…This is consistent with the expressions for Cu reduction rates given in Eqs. (13) and (14). The sum of the reduction rates of Cu(II) to Cu(I) and Cu(I) to Cu(0) (Eq.…”