No abstract

Poly͑ethylene glycol͒ ͑PEG͒ is an important additive to electroplating baths used for the deposition of copper interconnects on semiconductor wafers. In an earlier paper, Yokoi et al., Denki Kagaku oyobi Kogyo Butsuri Kagaku, 52, 218 ͑1984͒ found a direct relationship between the deposition rate in the presence of PEG and chloride ions with the open-circuit potential measured after plating, suggesting that the rest potential reflects the chemical state of reactive copper ions within a surface polymer film. Here, these measurements were corroborated and then interpreted in terms of a proposed mechanism of copper deposition in the presence of PEG. In this mechanism, aqueous Cu 2+ ions are reduced to an intermediate complex at the PEG-Cu interface detected earlier by Raman spectroscopy ͓Z. V. Feng et al., J. Phys. Chem. B, 107, 9415 ͑2003͔͒, in which Cu + ions associate with adsorbed Cl − ions and ether oxygen ligands of PEG. The rest potential measurements are quantitatively explained on the basis of competition for these ligands at open circuit with Cu 2+ ions absorbing from solution. The results indicate that deposition is mediated though ions partially solvated with the polymer, the concentration of which is controlled by the PEG concentration and molecular weight. PEG then behaves as a polymer electrolyte film as opposed to a passive barrier.

The role of hydrogen-containing surface species in the alkaline dissolution of aluminum was studied by secondary ion mass spectrometry ͑SIMS͒ and atomic force microscopy ͑AFM͒. The measurements revealed quasi-periodic nucleation and dissolution of large number densities of 10-100 nm size particles, during open-circuit dissolution in 1 M NaOH͑D͒ at room temperature. SIMS results using deuterated solutions, and prior Auger microprobe measurements, indicated that the particles were composed of aluminum hydride ͑deuteride͒, with an aluminum hydroxide ͑deuteroxide͒ surface layer. The measured open-circuit potential during dissolution was close to the Nernst potential of hydride oxidation. It was concluded that AlH 3 forms continuously during dissolution by reaction of cathodically generated hydrogen with the Al metal and is oxidized to aluminate ions ͓Al͑OH͒ 4 − ͔ in the accompanying anodic process. The present results are a direct confirmation of hydride formation on Al accompanying corrosion.

Metallization layers nanometers to tens of nanometers thick are desirable for semiconductor interconnects, among other technologically relevant nanostructures. Whereas aqueous deposition of such films is economically attractive, fabrication of continuous layers is particularly challenging on oxidized substrates used in many applications. Here it is demonstrated that galvanic displacement can deposit thin adherent copper layers on aluminum foils and thin films from alkaline copper sulfate baths. According to scanning electron microscopy and quartz crystal microbalance measurements, the use of relatively low CuSO4 concentrations produced films composed of copper nanoparticles overlying a uniform continuous copper layer on the order of nanometers in thickness. It seems that there are no precedents for such thin layers formed by aqueous deposition on oxidized metals. The thin copper layers are explained by a mechanism in which copper ions are reduced by surface aluminum hydride on Al during alkaline dissolution. Keywords Ames Laboratory Disciplines Chemical Engineering CommentsReprinted with permission from Journal of Physical Chemistry C 115 (2011) Because of their cost effectiveness, solution-based thin-film deposition techniques are widely integrated in semiconductor interconnect and MEMS technology. "Ultrathin" metallization layers of a few nanometers thickness are particularly beneficial in applications involving nanostructured substrates. In semiconductor interconnects, the ability to produce such deposits would eliminate the need for vapor-deposited seed layers for copper electrodeposition on TaN or TiN barrier materials. Efforts to produce seed layers by direct electrodeposition onto the barrier are hindered by the insulating, spontaneously formed oxide films on these materials. 1À4 Electroless deposition (ELD) methods, in which the deposited metal ion is chemically rather than electrochemically reduced, are attractive because they do not require conductive substrates. ELD of Cu on barrier materials has been demonstrated. 5À7 Galvanic displacement, in which either the substrate itself or adsorbed atoms reduce the deposited metal ions, has been explored extensively for fabrication of metal films on semiconductors and noble metals, 8,9 and copper layers on barrier materials have been reported. 10,11 However, it seems that ultrathin films on oxidized substrates have not yet been fabricated by either ELD or galvanic displacement.The present Article concerns the deposition of thin copper layers on aluminum by galvanic displacement. Aluminum exemplifies substrates having a surface oxide that interferes with solution-phase thin film deposition. Micron-thick particulate Cu films displaying good adhesion can be electrodeposited from alkaline sulfate baths; 12 also, several techniques have been developed to disrupt Al oxide layers to promote electrodeposition. 13À16Evidence of these papers suggests that deposition is promoted by alkaline solutions or by cathodic applied potentials at which hydrogen evolution...

No abstract

Vacancy and hydrogen concentrations in Al were determined by first-principles calculations and statisticalmechanics modeling, as functions of temperature and hydrogen chemical potential μH.

No abstract

The mechanism of anodic alkaline dissolution of aluminum was investigated through the analysis of cyclic voltammetry (CV) and potential step experiments. Attention was focused on the role of aluminum hydride (AlH3) as a reaction intermediate, as suggested by the recent detection of AlH3 formation during open-circuit dissolution. Potential step experiments at pH 11.75 revealed that the potential at the metal–surface film interface was close to the Nernst potential of AlH3 oxidation. This finding suggested a reaction mechanism in which an interfacial AlH3 layer is formed continuously by reaction of cathodically formed H with Al, and is then oxidized to the dissolution product, aluminate [Al(OH)4−] ions. However, potential step experiments at pH 11 did not indicate the presence of interfacial AlH3 ; instead, the metal–film interface was close to the equilibrium potential of Al oxidation. Analysis of the CV indicated an abrupt transition in dissolution behavior between the two pH values, from a relatively rapid dissolution controlled by diffusion and film conduction in highly alkaline solutions, to a slow dissolution at a lower pH controlled by a highly resistive surface film. The formation of interfacial AlH3 occurs readily at the higher pH, but is suppressed as the pH approaches neutrality.