Sputtered thin magnetic films

Abstract

Sputtered magnetic thin films have been studied for the purpose of evaluating both the deposition technique and the films fabricated by this technique. Results of these studies show the sputtering process to be relatively simple to control and the films to be highly uniform and reproducible. Some indications of correlation between crystalline structure and the magnetic properties of the films have been obtained. Control of deposition is relatively simple, since sputtering is a mechanical surface erosion phenomena [1] rather than a thermal process. This control is particularly important in the deposition of magnetic films, since fractional distillation is avoided [2], [3] and film composition is, consequently, reproducible. Control and reproducibility of film thickness is positive because deposition is initiated and terminated by applying or removing the target accelerating potential. Angle of incidence of the deposited particles can be controlled by the proper choice of target geometry, and there is a possibility of controlling film structure by varying the target voltage and, therefore, the energy of the deposited particles [4]. Fixturing can be kept mechanically simple, even in multilayer deposition systems, both because the target (or source) is used at low enough temperatures to be mechanically self-supporting, and only high-voltage low-current electrical contacts need be made to the target. The gas discharge used in these experiments was similar to that used by Gawehn [5]. Argon at a pressure of 10-3torr was used as the ionizing gas. Target current densities ranged from 5-10 mA/cm2, and the partial pressures of impurity gases were controlled to 10-6torr or less. Gross magnetic parameters show correlations similar to those found in evaporated and plated films, with a number of factors related to the deposition process. That is, by varying substrate temperature [6], partial pressures of impurity gases, and substrate coatings [7], it is possible to vary gross magnetic parameters in a predictable manner. These process variations tend to change the crystalline growth of the films, and it is this variation which appears to influence the magnetic parameters. X-ray diffraction measurements of crystalline texture show that the average size (thickness and average density) of the single crystallites and the orientation of those crystallites are important factors in determining the magnetic characteristics of the films. It has been found that most films have a predominately {111} orientation, but that this is somewhat dependent upon the substrate coating. Measurements have also been made to evaluate sputtered films as memory elements. This was done in a qualitative way by plotting switching asteroids. These asteroids consist of plots of the word current vs. the digit current necessary to 1) write both ones and zeros, 2) self-disturb the film, and 3) adjacent-word disturb the film. Proper interpretation of these plots indicates how well the film will perform in an operating memory. The use of this method in conjunction with X-ray diffraction measurements indicates that the amount of crystalline texture may well have an important effect on the reversible rotation limits of thin magnetic Ni-Fe films.