US3336159A - Method for growing single thin film crystals - Google Patents

Description

United States Patent 3,336,159 METHOD FOR GROWING SINGLE THIN FILM CRYSTALS Sidney H. Liebson, Dayton, Ohio, assignor to The National Cash Register Company, Dayton, Ohio, a corporation of Maryland No Drawing. Filed Oct. 7, 1963, Ser. No. 314,541

' 5 Claims. (Cl. 117-201) ABSTRACT OF THE DISCLOSURE A method of making a planar single crystal layer by providing a crystalline material layer in which individual crystals of small dimension may be identified, thereafter building a crystal structure in conformity with a chosen crystallite of the layer, by melting and causing the flow of adjacent crystals to make contact with the chosen crystal, followed by moving the zone of melting outwardly, as by the use of electron bombardment.

The present invention relates to a method for growing thin film crystals and, more specifically, to a method for growing single thin film crystals of selected material and orientation upon amorphous substrates.

Recent developments in molecular electronics have been in the area of electric devices which may be prepared by diffusion techniques in single crystal silicon slices and the epitaxial growth of silicon on single crystal silicon slices.

For devices which are prepared by diffusion techniques in single crystal silicon slices, it has been found that the electrical characteristics of the various diffused layers are interrelated. In view of this, the epitaxial growth of silicon crystals on single crystal silicon slices has been developed and affords the distinct advantage that the electrical properties of the epitaxially deposited layer are substantially independent of those of the substrate, which may be a previously deposited epitaxial layer.

Epitaxial growth of crystals, however, does have some limitations. By their nature, crystals prepared by this technique cannot be grown to an arbitrary size, since the size is limited to the maximum size of the crystal substrate which can be produced economically. With current production techniques, the maximum crystal size is approximately four centimeters in diameter. Furthermore, the epitaxial growth technique cannot be applied for the growth of crystals upon amorphous substrates because of the lack of crystalline structure of substrates of this type.

Truly three-dimensional circuitry could be prepared by growing additional circuitry over the passivated surfaces of planar integrated circuits if single crystal thin film silicon could be deposited upon an amorphous substrate, in that, currently, these passivated surfaces consist of an amorphous silicon oxide or silicon dioxide layer, which precludes the epitaxial crystal growth technique. Therefore, a technique which provides for the growth of single crystals upon such amorphous substrates would be a considerable advancement in the art, in that the crystal size would be limited only by the size of the substrate.

It is, therefore, an object of this invention to provide a novel method for the growth of thin film single crystals.

It is another object of this invention to provide a novel method for the growth of thin film single crystals upon an amorphous substrate.

In accordance with this invention, a film of silicon which has been deposited upon an amorphous substrate member by conventional techniques is scanned, by suitable means, as explained more fully hereinafter, to identify crystallites having certain preferred orientations with respect to the substrate, which crystallites, save a selected one, are then ICC melted by an annular movement of an electron beam, the diameter of which is increased while the power density of the beam is altered to maintain a molten zone within the confines of the beam, and the growth is continued by expanding the sweep of the annular beam until the sub strate is completely covered by a single thin film crystal of silicon.

For a better understanding of the present invention, reference is made to the following description.

In the following description, the unique method of this invention will be described on the basis of depositing a thin film crystal of silicon upon an amorphous substrate such as -silicon oxide or silicon dioxide. It is to be specifically understood, however, that these materials are exemplary only for the purpose of accurately describing this novel technique and that this technique may be employed with other crystalline materials and substrate members. In this regard, it is only necessary that the film material be of a crystalline nature which may be organic, inorganic, metallic, or semi-conductor materials such as germanium, gallium phosphide, or indium antimonide, for example.

With the initial step of this novel method, an amorphous substrate such as silicon oxide or silicon dioxide is coated with a film of crystalline material such as silicon by conventional techniques well known in the art, such as evaporation, disproportionation reaction, or hydrogen reduction. As these techniques are well known in the art, and any of them are acceptable for depositing the silicon layer upon the amorphous substrate, they will not be described in detail herein.

It is important, however, that care be exercised during this original deposition to achieve a polycrystalline layer of the largest size of crystals which can be conveniently attained.

After the film of silicon has been deposited upon the amorphous substrate, a single crystallite thereof having the desired orientation with respect to the substrate is selected as the seed from which the single crystal will be grown. As is well known in the crystallography art, the individual crystallites of silicon upon the amorphous substrate are randomly oriented relative to the surface thereof. For any specific application, there usually is a preferred crystal orientation. For example, a crystal orientation may be eX- pressed by the Miller indices 1:1:1 for a specific application. Other applications may require different crystal orientation. After the preferred crystal orientation for a specific application has been determined, the surface of the substrate is scanned by X-ray or electron diffraction to identify crystallites deposited on the substrate which have certain preferred orientations, as dictated by the desired final result, and one chosen from among them.

When a satisfactory crystallite has been identified and located, this selected seed crystallite is centered within a source of heat having an annular pattern. This heat source may be but is not limited to an electron beam heating apparatus. The electron beam heating apparatus may consist of a simple electron beam gun operating in a vacuum and deflected in a manner to produce a circular sweep, thereby providing a source of heat having an annular pattern. This equipment is commercially available, and the circular sweep deflection techniques are well known in the cathode ray deflection art.

Dependent upon substrate temperatures, material used, thickness of substrate, and thickness of the initial film, the electron beam voltage and current are adjusted to increase the intensity of the source of heat to provide an annular melting of the film about the selected crystallite. Because the beam diameter will in all probability be greater than the film thickness, it is important that these parameters be varied to minimize separating effects of surface tension in the molten film within the annular area and the probabilities of dendritic growth formation within the molten zone.

After the intensity of the annular electron beam heating pattern has been adjusted to the melting point of the deposited film of silicon, the annular electron beam heating pattern radius is slowly increased, and the power density is simultaneously varied to increase the intensity of the source of heat to maintain the molten zone within the area of the annular electron beam heating pattern. The annular-like beam heating pattern is then constantly expanded, the beam intensity always being made sufficient to bring about melting of the crystalline material Where the beam is incident. The moving beam leaves in its wake a cooling melt path which crystallizes according to the pattern of the seed crystal in abutment thereto. This results in the growth of a single crystal of silicon over the entire surface area of the amorphous substrate member.

In the event that an unwanted crystal structure is detected within the molten area, the procedure is reversed, and the entire surface is melted back to the point that the undersized structure is eliminated, and then the crystal growth is continued therefrom by expanding the annular beam pattern in a manner thus previously described.

While a preferred embodiment of the present invention has been shown and described, it will be obvious to those skilled in the art that various modifications and substitutions may be made without departing from the spirit of the invention, which is to be limited only within the scope of the appended claims.

What is claimed is:

1. A method for growing single thin film crystals upon an amorphous substrate comprising the steps of depositing a film of crystalline material upon an amorphous substrate, selecting a crystallite of the film of the desired orientation as the seed from which a single crystal will be grown, centering the selected crystallite within a source of heat having an annular pattern, adjusting the intensity of the source of heat to provide an annular melting of the film about the selected crystallite, and simultaneously increasing the radius of the annular heating pattern and the heating intensity thereof to maintain a molten zone within the area of the annular heating pattern until the entire deposited film upon the substrate has been melted to produce a single crystal of the deposited crystalline material layer over the entire surface of the substrate.

2. A method for growing single thin film crystals upon an amorphous substrate comprising the steps of depositing a film of silicon upon an amorphous substrate, selecting a crystallite of the film of the desired orientation as the seed from which a single silicon crystal will be grown, centering the selected crystallite within a source of heat having an annular pattern, adjusting the intensity of the source of heat to provide an annular melting of the silicon film about the selected crystallite, and simultaneously increasing the radius of the annular heating pattern and the heating intensity thereof to maintain a molten zone within the area of the annular heating pattern until the entire deposited silicon film upon the substrate has been melted to produce a single silicon crystal over the entire surface of the substrate.

3. A method for growing single thin film crystals upon an amorphous substrate comprising the steps of depositing a film of crystalline material upon an amorphous substrate, selecting a crystallite of the film of the desired orientation as the seed from which a single crystal will be grown, centering the selected crystallite within a source of heat comprising an electron beam deflected to produce an annular heating pattern, adjusting the intensity of the electron beam annular heating pattern to provide an annular melting of the film about the selected crystallite, and simultaneously increasing the radius of the electron beam annular heating pattern and the heating intensity thereof to maintain a molten zone within the area of the electron beam annular heating pattern until the entire de posited film upon the substrate has been melted to produce a single crystal of the deposited crystalline material layer over the entire surface of the substrate.

4. A method for growing single thin film crystals upon an amorphous substrate comprising the steps of depositing a film of silicon upon an amorphous substrate, selecting a crystallite of the film of the desired orientation as the seed from which a single silicon crystal will be grown, centering the selected crystallite within a source of heat comprising an electron beam deflected to produce an annular heating pattern, adjusting the intensity of the electron beam annular heating pattern to provide an annular melting of the silicon film about the selected crystallite, and simultaneously increasing the radius of the electron beam annular heating pattern and the heating intensity thereof to maintain a molten zone within the area of the electron beam annular heating pattern until the entire deposited silicon film upon the substrate has been melted to produce a single silicon crystal over the entire surface of the substrate.

5. A method of making a planar single crystal layer from crystalline material susceptible to melting by electron beam bombardment, comprising the steps of (a) disposing on a heat-resistant substrate a layer of selected crystalline material of the same composition in which individual crystals of small dimension may be identified;

(b) choosing a small crystal of proper size around which an electron beam may be directed, and

(c) directing an electron beam close to the chosen crystal to melt and cause flow of adjacent crystals to make contact with the chosen crystal; and

(d) moving the beam in substantially circular motion that constantly increases in radius, to build a crystal structure in conformity to the chosen crystal, the beam intensity always being made sulficient to bring about melting of the crystalline material where incident.

References Cited UNITED STATES PATENTS 2,813,048 11/1957 Pfann 148-16 2,816,050 12/1957 Hunter 1481.6 2,926,075 2/1960 Pfann 148-1.6 2,968,723 1/1961 Steigerwald 65 2,992,903 7/1961 Imber 148-1.6 3,160,522 12/1964 Heywang et al 1481.6

ALFRED L. LEAVITT, Primary Examiner.

A. ROSENSTEIN, Examiner.

Claims (1)

1. A METHOD FOR GROWING SINGLE THIN FILM CRYSTALS UPON AN AMORPHOUS SUBSTRATE COMPRISING THE STEPS OF DEPOSITING A FILM OF CRYSTALLINE MATERIAL UPON AN AMORPHOUS SUBSTRATE, SELECTING A CRYSTALLITE OF THE FILM OF THE DESIRED ORIENTATION AS THE SEED FROM WHICH A SINGLE CRYSTAL WILL BE GROWN, CENTERING THE SELECTED CRYSTALLITE WITHIN A SOURCE OF HEAT HAVING AN ANNULAR PATTERN, ADJUSTING THE INTENSITY OF THE SOURCE OF HEAT TO PROVIDE AN ANNULAR MELTING OF THE FILM ABOUT THE SELECTED CRYSTALLITE, AND SIMULTANEOUSLY INCREASING THE RADIUS OF THE ANNULAR HEATING PATTERN AND THE HEATING INTENSITY THEREOF TO MAINTAIN A MOLTEN ZONE WITHIN THE AREA OF THE ANNULAR HEATING PATTERN UNTIL THE ENTIRE DEPOSITED FILM UPON THE SUBSTRATE HAS BEEN MELTED TO PRODUCE A SINGLE CRYSTAL OF THE DEPOSITED CRYSTALLINE MATERIAL LAYER OVER THE ENTIRE SURFACE OF THE SUBSTRATE.

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