Effect of gas atmospheres on degradation of MgO thin film magnetic tunneling junctions by deionized water

Abstract

The degradation of the MgO thin film barrier of magnetic tunneling junctions caused by the dissolution of the film in deionized water during manufacturing processes is critical to the performance of spintronic devices. In this work, the influence of gas atmospheres including oxygen (O2), nitrogen (N2) and carbon dioxide (CO2) on dissolution behavior of the film was investigated by using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic absorption spectroscopy (AAS). XPS depth profile results reveal that deionized water reacts with the film and infiltrates into the deeper layer of the film resulting in the chemical transformation of MgO to Mg(OH)2. AAS results confirm that the films dissolve because Mg2+ ions are released into deionized water. Consequently, Mg concentration in deionized water increases. It is found that the concentration of Mg2+ ions in deionized water is highest in the dissolution of CO2-saturated deionized water and lowest in those of N2-saturated deionized water. Simultaneously, the release of Mg2+ causes the development of the coral-like clusters over their surface and a decrease of the film thickness. XPS depth profile and SEM cross-sectional results demonstrate that the film thickness slightly decreases in sample N2 and decreases considerably in sample CO2. This is due to the fact that CO2 gas reacts with water, reducing its pH and thus increasing of the dissolution rate, while N2 gas has no reaction with water. Finally, this work proposes a qualitative model based on chemical reaction to explain the dissolution behavior of the film.