In-Situ Scanning Probe Microscopy of Solid-Liquid Interfaces: Role of Epitaxial Oxide Adlayers on Cu Electrodeposition
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Figure 1. Schematics of low index faces of Cu with proposed oxide (hydroxide) adlayer structures. (A) c(2x2) adlattice was observed on Cu(100) down to pH values near 1 (see Ref. [4]). (B) (2xl) adlattice was observed on Cu(1 10) down to pH values near 2.5 (see Ref. [5]). (C) No ordered oxide (hydroxide) adlattice was observed on Cu(1 11) between pH values of 1 to 3 (see Ref. [61). The white and black circles represent Cu and oxide (hydroxide), respectively.
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Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
passivating adlayer conditioned and/or modified through additives introduced into the plating solution. These additives modify the initial nucleation and subsequent morphology of the Cu deposits in ways which are not fully understood on a fundamental level. In this paper we demonstrate that the AFM can also modify the nucleation and growth of Cu on Cu single crystal surfaces. In particular, we summarize in-situ AFM results [7,8] which demonstrate that the tipsample interaction locally enhances the deposition rate of Cu under controlled electrochemical conditions. This process is consistent with a mechanism in which partially passivating oxide monolayers are physically removed by the AFM tip. EXPERIMENTAL In-situ AFM images of Cu( 110) crystals were collected in constant force repulsive mode on a Nanoscope II Atomic Force Microscope (Digital Instruments) equipped with a 1 Rm scanning head, microfabricated square-pyramidal Si3 N4 tips, and a 0.2 ml fluid cell. The working electrode was a Cu(1 10) single crystal (Monocrystals Company, 99.999%) prepared by standard techniques [9]. Cu was introduced into the solution through either the application of a small anodic potential (a few mV above open-circuit conditions) to the Cu single crystal surface or by the direct addition of 10-4 M Cu(C104)2. Deposition was induced by a cathodic overpotential step rl of 70 mV negative of the open-circuit potential E0 , which was +280 mV versus the normal hydrogen electrode (NHE). RESULTS Figure 2 shows long range AFM images (850 x 850 nm) of the enhanced deposition effect. Figure 2A is a typical Cu(1 10) surface under open circuit conditions prior to Cu deposition. The surface was smooth, exhibiting features no more than 10 nm in height. Atomic resolution of either the bare Cu(1 10) lattice or the Cu(1 10) (nxl)-O adlattice (n = 1,2,3,4) has been obtained under these conditions [5]. Cu deposition, initiated and continued with the AFM tip retracted 15 gm above the surface, yielded images [8] which showed uniform three-dimensional deposits on the surface with lateral dimensions of 50-200 nm which were much greater than their heights (1-10 nm). Similar morphologies have been reported elsewhere [10]. However, when Cu is deposited while the tip is engaged and scanning, a completely different morphology develops. Figure 2B reveals a single large Cu deposit in the center of the image which possessed a "truncated-cone" morphology with a height of 60 nm. (The observed feature shape is most likely a convol
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