Environmental Protection and Patterning of Superconducting Perovskites

This invention relates to a method for the passivation and protection of superconducting materials. More particularly, the present invention relates to the passivation and environmental protection of superconducting rare earth cuprate thin films.

Electronic device applications utilizing high temperature superconductors require stable, patterned thin films. The reliability of these devices is also dependent upon the inertness of these films in diverse operating ambients. Unfortunately, studies have revealed that the superconducting films of interest, namely, the rare earth barium cuprates, decompose in water or humid environments and react rapidly with weak acids or bases. Although the instability of these films occurs in both the fired and unfired states, films in the unfired state are particularly susceptible to degradation. Accordingly, device fabrication utilizing such thin film superconductors is oftimes hindered.

In accordance with the present invention, a technique is described for obviating these prior art limitations. Briefly, the described technique involves depositing a thin layer of carbon upon the superconducting films, such layer serving as a protective measure against deterioration caused by the atmosphere. Carbon deposition is effected either by electron beam deposition or by plasma dissociation of hydrocarbons, resulting in an amorphous or diamond-like carbon film, respectively. The resultant coatings may, subsequently, be removed by annealing in an oxygen atmosphere or by plasma oxidation at low temperatures with essentially no residue.

In the practice of the present invention, the first step involves obtaining a rare earth barium cuprate. Typically, this end is attained by co-evaporation of the components thereof by conventional electron beam deposition techniques at reduced pressures. Alternatively this may also be effected by sputtering or laser deposition at 10.sup.-2 to 10.sup.-6 Torr. Following, carbon deposition may be effected in situ utilizing the same electron beam apparatus using graphite or any other high carbon content source. Thickness is controlled by a quartz crystal or other suitable monitoring source to a value within the range of 100 .ANG.-10 microns, such range being determined by considerations relating to pinhole content at the lower range and stress in the film at the upper range beyond which peeling occurs.

If the underlying film is already superconducting, no further treatment is required and the structure is then suitable for device fabrication utilizing photolithographic techniques or storage. In the event the underlying film is not superconducting, annealing must be effected at 600.degree.-850.degree. C. to remove the carbon film and convert the underlying film to a superconducting phase.

An example of the application of the present invention is set forth below. This example is solely for purposes of exposition and is not to be construed as limiting.

Barium metal, yttria (Y.sub.2 O.sub.3) and copper were evaporated in a vacuum evaporator at 10.sup.-5 Torr oxygen by electron beam and thermal resistive heating for a time period sufficient to yield a thin film deposit of 1 micron upon a strontium titanate degreased substrate. Before breaking the vacuum, a crucible containing graphite was rotated into the electron beam trajectory path and deposition of carbon (amorphous) effected at room temperature, the thickness of the carbon film being controlled to yield a film of approximately 200 .ANG..

The substrate bearing the thin film was then removed from the apparatus and thermally annealed at 850.degree. C. for 30 minutes in an oxygen atmosphere (1 atmosphere) to remove the carbon film to convert the film to superconducting YBa.sub.2 Cu.sub.3 O.sub.7-x.

It will be understood that diamond-like carbon coatings can be deposited upon the thin film materials by plasma dissociation of hydrocarbons, such as butane, methane and the like using any plasma (RF). Studies have revealed that the YBa.sub.2 Cu.sub.3 O.sub.7-x films bearing a carbon coating are less degradable in the ambient than those without any coating.

Encapsulation of the YBa.sub.2 Cu.sub.3 O.sub.7-x films with a 20 nm thick film of diamond-like carbon was effected by dissociation of butane in a radio frequency (13.56 MHz) plasma with a 20 millitorr gas pressure at room temperature. These coatings were transparent to light and Raman spectra demonstrated that the YBa.sub.2 Cu.sub.3 O.sub.7-x films retained their good quality after the coating procedure. Furthermore, no deterioration was detected in the Raman spectra over the course of a day in contrast with uncoated samples.

Water permeability of both the amorphous and diamond-like carbon films was measured as follows. An amorphous carbon film of 45 nm was electron deposited on a thin sheet of cellulose acetate at 4.times.10.sup.-6 Torr. A diamond-like film of the same thickness was deposited on another sheet of cellulose acetate for comparison. A blank cellulose sheet was used as a control. The coated and blank sheet were stretched over and clamped on the rims of 2 inch diameter stainless steel cups with viton seals. The cups were half filled with water. The weight of each of the three assemblies was measured before and after a week of storage over P.sub.2 O.sub.5 inside a desiccator. From the loss in weight, relative water vapor permeability through the three sheets at room temperature was estimated. The diamond-like coated cellulose acetate sheet was found to be two times less permeable than the amorphous coated cellulose acetate sheet which, in turn, was four times less permeable than the uncoated sheet.

The protective carbon films have also been found suitable for preparing patterned superconducting films. Specifically, the carbon film may be deposited upon an as-deposited film of superconducting material. Following, a patterned organic film is placed upon the carbon film using conventional photolithographic techniques. Then a low temperature oxygen plasma may be used to etch away the carbon film from these areas not covered by the organic film, which, while it is also etched by the plasma, is not removed because it is thicker than the carbon film. Nitric acid may then be used to chemically etch away the exposed superconducting films, so leaving a patterned as-deposited superconducting film covered by the carbon and organic films. This structure may then be annealed in an oxygen rich environment to produce the superconducting phase and to remove both the carbon and organic films.