The present invention relates to a method of forming a multilayer thin film which includes a conductive film of a predetermined pattern formed on a substrate and another conductive film of a different predetermined pattern formed on the above-mentioned conductive film, and to a method of forming a multilayer thin film for use in a magnetoresistive element, and more particularly to a method of forming a multilayer thin film suitable for use in a barber pole type magnetoresistive element.
A conventional method of forming the above-mentioned multilayer thin films comprises the steps of: (i) forming a first thin conductive film 1 on a substrate 10, as shown in FIG. 1a of the accompanying drawings; (ii) processing the first thin conductive film 1 so as to form a thin conductive film 1' having a first predetermined pattern, as shown in FIG. 1b; (iii) depositing a second thin conductive film 2 which is made of a material different from the material of the thin film 1', on a surface of the substrate 10 which is provided with the thin film 1', as shown in FIG. 1c; and (iv) processing the second thin conductive film 2 so as to form a thin conductive film 2' having a second predetermined pattern, as shown in FIG. 1d. In the above-mentioned prior art, when the thin conductive film 1' is made of permalloy (namely, an Ni--Fe alloy) or the like to be used as a magnetoresistive film and the thin conductive film 2' is made of aluminum or the like to be used as a finely patterned electrode and a conductor, the element shown in FIG. 1d constitutes a barber pole type magnetoresistive element.
According to the above-mentioned conventional method, after the thin film 1' has been formed (as shown in FIG. 1b), the surface of the thin film 1' is usually cleaned by chemical etching, electrolytic etching, sputter etching, ion milling, or the like, and then the thin film 2 is formed on the thin film 1' and the exposed surface of the substrate. When the above-mentioned surface treatment is performed in the air, or when the thin film 1' is exposed to the air after the above-mentioned surface cleaning treatment, an oxide film may be formed on the surface of the thin film 1' and therefore metallic bonding (or interconnection) at an interface between the thin film 1' and the thin film 2 or 2' may become defective. Also, the bonding strength between the film 1' and the thin film 2 or 2' may be reduced, and the contact resistance at the interface may become high. That is, undesirable results may be produced.
In order to solve the above-mentioned problem, for example, the thin film 1' having the first predetermined pattern is first subjected to sputter cleaning in a vacuum vessel, and then the formation of the thin film 2 is carried out in the same vessel without exposing the cleaned thin film 1' to the air, so as to deposit the thin film 2 on the clean surface of the thin film 1' through the vacuum evaporation method. However, this method has the following drawbacks: (1) it is difficult to form a reproducible, clean surface by the present-day sputtering technique for a thin film having a thickness of hundreds of angstroms or less, and therefore sputtering is not suited for cleaning the surface of a very thin film; (2) residual nitrogen and oxygen molecules give an adverse effect to the surface of the thin film 1' when sputtering is performed; and (3) the sputter cleaning cannot be used in the case where the thin film 1' is readily damaged by sputtering, especially when this damage is a fatal one.
The following reference is cited to show the state of the art; a Japanese Patent Application Laid-open Specification (Laid-open No. 134624/1975).
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