Cu Electrode Surfaces

Fig. 2. (a) 3 nm x 3 nm AFM image of electropolished Cu(100) in 0.1 M HC104 at -0.45 V vs. Hg2S04. The structure is square with an interatomic spacing of 0.36 nm. (b) 9 nm x 9 nm AFM image of two-domain region with Cu(l 10) (upper right) and (2x1) rows (lower left) observed at after a potential of -550 mV was applied.

most negative potentials. This (V2 x structure was anion independent and appeared even when rigorous measures were taken to exclude CI" from the electrolyte. The structure was thus associated with O or OH" adsorbed on the surface. However, the same

0/2 x V2)R45° structure appeared when CI" was deliberately added to the solution, which complicated definitive assignment of these features. On Cu(lll) surfaces, only a diñuse structure was observed, which is considerably different from the (V3 x V3)R30°

adlattice formed on the surface following deliberate addition of CI" [12]. In order to show definitively that the adlattice structures were not associated with CI", we imaged Cu(l 10) single crystals in aqueous solutions during the initial stages of oxidation. Images obtained in pH 2.5-2.7 HCIO4 and H2SO4 solutions revealed the growth of oxide monolayers consisting primarily of [001] oriented chains (Fig. 2(b)). A majority of these chains (ca. 70%) were arranged in (2 x 1) and (3 x 1) structures. Images obtained following addition of CI" yielded a completely different structure. These observations strongly indicate that the adlattices observed on the Cu(100) and Cu(l 10) surfaces are not related to ad-ventitious CI", but rather are associated with O or OH" as an adsórbate. The (n x 1) structures found on the Cu(l 10) surface are in

850 nm

Fig. 3. 850 nm x 850 nm AFM images showing the Cu surface before and after enhanced electrochemical deposition of Cu. (A) The Cu(110) surface under open-circuit conditions in an HCIO4 solution (pH = 2.45) prior to deposition. The black square outlines the region in which the AFM tip was scanned during deposition. (B) After the deposition caused by a potential step of -70 mV, a single 60 mn-high feature was evident on the surface.

850 nm

Fig. 3. 850 nm x 850 nm AFM images showing the Cu surface before and after enhanced electrochemical deposition of Cu. (A) The Cu(110) surface under open-circuit conditions in an HCIO4 solution (pH = 2.45) prior to deposition. The black square outlines the region in which the AFM tip was scanned during deposition. (B) After the deposition caused by a potential step of -70 mV, a single 60 mn-high feature was evident on the surface.

addition strongly reminiscent of the structures formed in the UHV environment following exposure of the surface to small amounts of oxygen. However, definitive identification of these adlattices must await interrogation of these surfaces with a chemically sensitive probe. Both the (^2 x V2)R45° and the chain structures were observed in the thermodynamically forbidden region for copper oxide in the pH-potential phase diagram, which indicates that stable oxide monolayers develop prior to bulk oxide formation. In turn, this has important consequences for interaction of Cu surfaces with bulk plating additives and other adsorbates.

During the course of our work on Cu adsorbate systems, we discovered that the AFM tip-sample interaction could enhance Cu electrodeposition on Cu single-crystal electrodes [13, 14]. Figure 3 shows a sequence of AFM images taken before and after electrodeposition of Cu with concurrent scanning of the AFM tip at high force in a restricted spatial region. The image in Fig. 3(b) clearly shows that Cu deposition occurs preferentially in the area in which the AFM tip was scanning; the magnitude of this enhancement is approximately 15-folg. The magnitude of the enhanced deposition effect depends primarily on tip-sample force, crystallographic orientation, and solution pH. Enhanced Cu deposition is stronger on Cu(llO) than CU(lll) which correlates with the reactivity of these orientations towards oxide monolayer formation, described above. For a specific orientation, enhanced Cu deposition becomes less pronounced with decreasing solution pH. These results are consistent with a modification mechanism in which partially passivating oxide layers mediate both normal and enhanced Cu deposition. The AFM tip-sample force physically creates defects in these adlayers, thus forming active sites for Cu adsorption.

Finally, we have examined bulk Cu electrodeposition from acidic solutions both with and without organic additives [15]. The electrodeposited surfaces were analyzed quantitatively by scaling analysis of electrodeposited surface roughness during the course of deposition and by modeling the spectral power density (SPD) of the surface shape evolution. These analyses provide insight into the specific mechanisms giving rise to the observed textures. Deposition from additive-free solutions (Fig. 4(a)) leads to rough surface textures due to roughening originating from surface diffusion. Addition of benzotriazole (BTA), a commonly used organic additive, acts to smooth the deposit (Fig. 4(b)) by diminishing surface diffusion. Deposits grown from thiourea-containing solutions (Fig. 4(c)) exhibit formation of three-dimensional islands atop initially flat plates, reflecting a two-stage growth mechanism. These results emphasize the close interplay between molecular functionality and gross topology of the growing electrodeposit.

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