Changes in mass and stress during anodic oxidation and cathodic reduction of the Cu/Cu₂O multilayer film

Abstract

The anodic oxidation and cathodic reduction processes of the Cu/Cu₂O multilayer film and pure Cu film in pH 8.4 borate buffer solution were analyzed by electrochemical quartz crystal microbalance (EQCM) for gravimetry and bending beam method (BBM) for stress measurement. The mass loss of the multilayer film during anodic oxidation at 0.8 V (SHE) in the passive region was less than that of the pure Cu film. The comparison between current transients and mass changes during anodic oxidation has succeeded in separating the anodic current density into two partial current densities of oxide film growth, $i_{{\text{O}}^{2-}}$, and of Cu²⁺ dissolution through the passive film, $i_{{\text{Cu}}^{2+}}$. As a result, in the case of the pure Cu film, the anodic current density was mainly due to $i_{{\text{Cu}}^{2+}}$, while in the case of the multilayer film, $i_{{\text{Cu}}^{2+}}$ was almost equal to $i_{{\text{O}}^{2-}}$. The compressive stress for the multilayer film was generated during anodic oxidation, while the tensile stress for the pure Cu film was generated.

The mass loss of the multilayer film during cathodic reduction at a constant current density (i_c = −20 μA cm⁻²) was significantly less than that estimated from coulometry, suggesting that H₂O produced by cathodic reduction remained in the multilayer film. The compressive stress was generated during cathodic reduction of the multilayer film, which was ascribed to H₂O remained in the multilayer film.

Introduction

Copper has been widely used as key materials in microelectronics such as interconnections in integrated circuits because of its superior electronic conductivity and high electromigration resistance as compared to aluminum. The fundamental knowledge of the copper passivation and of the structure of anodic oxide film on copper is essentially necessary for further development of microelectronics and of its related industry. It has been recently found that multilayer films of Cu and Cu₂O are electrodeposited from alkaline solutions of Cu(II) lactate at room temperature, accompanying spontaneous oscillation of electrode potential [1], [2], [3]. Moreover, it has been revealed that the Cu/Cu₂O multilayer films have the specular reflectance spectra with blue-shift due to quantum-confined effect of layered nanostructure and the anisotropic resistivities ranged from 10⁻⁴ ohm cm to 10⁶ ohm cm, depending on Cu content in the multilayer films [4]. The Cu/Cu₂O multilayer films, therefore, will be situated under a variety of potential applications for microelectronics and optoelectronics. The chemical and mechanical stabilities of the Cu/Cu₂O multilayer films as well as surface oxide films on copper in environments are of great importance for their applications.

There have been many studies on chemical and semiconductive properties [5], [6], [7] of surface oxide films on copper in relation to their chemical stabilities. An electrochemical quartz crystal microbalance (EQCM) is powerful and useful for evaluation of chemical stabilities of thin metal films in aqueous environments [8], [9] because it is capable of measuring simultaneously small changes in mass and current density. The measurement of stresses generated in films is indispensable for evaluation of their mechanical stabilities. However, the studies on stresses of surface oxide films on copper were few because there were no reliable tools for measurement of small changes in stress. In our previous studies [10], [11], it has been reported that a bending beam method (BBD) is effective and reliable for measurement of small changes in stress.

In this study, the changes in mass and stress during anodic oxidation and cathodic reduction of Cu/Cu₂O multilayer and pure copper films in pH 8.4 borate solution were measured by using EQCM and BBM to discuss from the viewpoint of chemical and mechanical stabilities of these films in aqueous environments.