Saikat Adhikari

Chemical Mechanism of Suppression of Copper Electrodeposition by Poly(ethylene glycol)

Poly(ethylene glycol) (PEG) 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 (1984) 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 (2003)], 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.

Formation of Aluminum Hydride during Alkaline Dissolution of Aluminum

The role of hydrogen-containing surface species in the alkaline dissolution of aluminum was studied by secondary ion mass spectrometry SIMS and atomic force microscopy AFM. 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 NaOHD at room temperature. SIMS results using deuterated solutions, and prior Auger microprobe measurements, indicated that the particles were composed of aluminum hydride deuteride, with an aluminum hydroxide deuteroxide surface layer. The measured open-circuit potential during dissolution was close to the Nernst potential of hydride oxidation. It was concluded that AlH3 forms continuously during dissolution by reaction of cathodically generated hydrogen with the Al metal and is oxidized to aluminate ions AlOH4

Participation of Aluminum Hydride in the Anodic Dissolution of Aluminum in Alkaline Solutions

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 AlOH4 � 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.

Electron Microscopic Observations of Interfacial Voids in Aluminum Created by Alkaline Dissolution

Electron Microscopic Observations of Interfacial Voids in Aluminum Created by Alkaline Dissolution Saikat Adhikari, L. S. Chumbley b A. C. Geiculescu c and K. R. Hebert a Department of Chemical and Biological Engineering a Iowa State University, Ames, IA 50011 b Ames laboratory – USDOE and Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011 c St. Jude Medical CRMD, Liberty, SC 29657

Injection of Hydrogen and Vacancy-Type Defects during Dissolution of Aluminum

Positron annihilation spectroscopy (PAS) investigations have found evidence for nm-scale voids in aluminum, located in the metal within tens of nm from the metal/oxide interface (1-2). These interfacial voids result from either dissolution or oxide growth, suggesting that they form from vacancy-type defects injected by metal atom oxidation. PAS indicated that the internal surface of the voids was oxide-free and thus would be highly reactive if exposed at the surface; in fact, correspondence between voids and pit sites was demonstrated (3). However, questions remain about voids because the vacancy formation energy in Al apparently prohibits roomtemperature vacancy injection.

Surface processes accompanying corrosion-induced hydrogen absorption into aluminum

The role of hydrogen-containing species in the alkaline dissolution of aluminum was studied by secondary ion mass spectrometry (SLMS). Large number densities of submicron particles nucleated and then disappeared during dissolution, at intervals of approximately 3 min. The particles were composed of aluminum hydride, with an aluminum hydroxide surface layer. When particles first appeared, the aluminate ion concentration near the surface was at the solubility of Al(OH)3, and the potential was close to the Nernst potential for oxidation of AlH3 to Al(OH)3. The observed formation of AlH3 indicates that the dissolving Al surface was poised near this equilibrium potential, i. e. that a hydride species serves as an intermediate in the dissolution process.