AVS 52nd International Symposium
    Plasma Science and Technology Thursday Sessions
       Session PS+TF-ThA

Paper PS+TF-ThA9
In Situ Oxidation and Plasma Studies for Magnetic Tunnel Junctions: The Mechanism of Plasma Oxidation of Ultra-Thin Aluminum Films Unraveled

Thursday, November 3, 2005, 4:40 pm, Room 302

Session: Emerging Plasma Applications
Presenter: M.C.M. Van De Sanden, Eindhoven University of Technology, The Netherlands
Authors: M.C.M. Van De Sanden, Eindhoven University of Technology, The Netherlands
K. Knechten, Oce Technology, The Netherlands
B. Koopmans, Eindhoven University of Technology, The Netherlands
H.J.M. Swagten, Eindhoven University of Technology, The Netherlands
W.J.M. de Jonge, Eindhoven University of Technology, The Netherlands
Correspondent: Click to Email
Plasma oxidation of thin aluminum films is a commonly used technique to form thin aluminum oxide barriers for application in magnetic tunnel junctions (typically 1 nm). In this technique a glow discharge in oxygen (P = 5-12 W, p = 5-40 Pa) is used to oxidize ultra thin sputtered aluminum films. In comparison with thermal oxidation the process is faster and provides high values of tunneling magnetoresistance (TMR) but at the cost of higher resistance-area products (RxA). However, whereas thermal oxidation of thin aluminum films is well understood in terms of the original model of Cabrera, where the oxidation rate is limited by field-assisted thermal 'hops' of aluminum ions into the oxide, the detailed mechanism of plasma oxidation of thin aluminum films is still unknown. To unravel the mechanism in situ measurements of the oxidation rate and plasma parameters such as the ion and oxygen density are performed. The oxidation rate is determined from single wavelength ellipsometry. From these measurements we have concluded that not one single particle in the plasma is responsible for the increase in oxidation rate observed. A clear correlation of the oxidation rate with the ion flux towards the sample is observed. In addition the oxidation rate is also correlated with the atomic oxygen density in the gas. These results can be explained within a modified Cabrera model of oxidation in which the oxidation temperature is locally enhanced due to the thermal spike of an impinging ion. Additionally, due to the presence of atomic oxygen in the plasma, the field over the oxide during oxidation is enhanced by the increased adsorption of atomic oxygen on the oxide surface. Including both effects in an adjusted equation for the oxidation rate provides a good agreement between model and experiments. The model provides new insights into plasma based oxidation of ultra thin films and offers oppertunities to further control the quality of the tunnel barrier.