US2988433A - Method of forming crystals - Google Patents

Description

June 13, 1961 J. 5. HANSON METHOD OF FORMING CRYSTALS 2 Sheets-Sheet 1 FIG.2

Flled Dec 31, 1957 Hi] AXIS FIGJ INVENTOR.

JAMES S. HANSON ATTORNEY June 13, 1961 J. 5. HANSON 2,988,433

METHOD OF FORMING CRYSTALS Filed Dec. 51, 1957 2 Sheets-Sheet 2 2,988,433 METHOD OF FOG CRYSTALS James S. Hanson, Poughlreepsie, N.Y., assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Dec. 31, 1957, Ser. No. 706,449 6 Claims. (Cl. 23-401) This invention relates to a method of forming a monocrystalline ingot having facets displaying the preferred planes of crystallization of the material.

In the manufacture of crystal semiconductive devices, it is highly desirable that certain surfaces of each device be parallel to certain crystallographic planes of the material from which it is formed. To so form the devices, it is essential that the orientation of these crystallographic planes be known.

These planes are planes of preferred growth of the particular material involved. In other words, during growth of a crystal, there is a natural tendency to form surfaces parallel to those planes.

In a copending application, now Patent No. 2,858,730, issued November 4, 1958, I have disclosed a method of orienting a monocrystalline ingot for the purpose of cutting it into semiconductive bodies having certain surfaces parallel to certain crystallographic planes. That method involves the use of optical reflections off the surfaces of certain small areas, or facets, of the ingot.

Difficulties had been encountered in the use of that orienting method, in that not all ingots exhibited the facets which form the basis of the method. As described in said patent, it was then necessary in such cases, to utilize selective etching methods to produce facets.

A known method of making monocrystalline ingots for the manufacture and semi-conductive devices such as diodes and transistors involves the steps of inserting a seed crystal in a melt of the material, for example germanium, then slowly withdrawing or pulling the seed crystal from the melt while maintaining the melt substantially at the melting temperature of the material. A crystal is thereby grown on the seed and follows the crystallographic orientation of the seed. In general, it is advantageous to have the pulling direction as nearly co incident as possible with a desired crystallographic axis.

Several factors affect the formation of orientation facets during the manufacture of an ingot. One important factor is the characteristics of the particular furnace in which the ingot is formed. All furnaces for forming pulled ingots tend to form generally cylindrical ingots. It has been found that some furnaces produce ingots which are almost completely cylindrical, and in which facets are absent. The characteristics are apparently determined by the furnace design, and it is presently impossible to predict, before a furnace is built and operated, whether its design is such as to promote the formation of facets.

Another factor which affects facet formation is excessive angular error between the pulling direction of the ingot from the melt and the desired crystallographic axis of the ingot. Ingots so formed tend to have very few, or even no facets.

An object of the present invention is to provide an improved method of growing a crystalline ingot having facets of suitable quality for optical orientation.

A further object is to provide a completely reliable method of producing facets which is independent of both furnace design and large angular errors between the pulling axis and the preferred axis of orientation.

A further object is to provide a method which produces facets for any desired pulling axis however far it may be from any crystallographic plane or axis.

The foregoing and other objects of the invention are attained in the method described herein. The first essential step in that method is to vary the conditions of crystal formation, just before the withdrawal of the crystal from the melt, so as to produce an outward flare of the surface of the generally cylindrical crystal at the bottom. This flare should be rather small, i.e., about 0.005 to 0.010" in radial width. It is preferred to form this flare by rapidly reducing the temperature of the melt a sufficient amount and only for a long enough time to form a flare of the desired dimension. Alternatively, with some materials, the flare may be formed by reducing the withdrawal rate of the crystal from the melt. Another essential step is that of suddenly withdrawing the crystal from the melt after the flare has reached a sufi'icient size.

Other objects and advantages of the invention will become apparent from a consideration of the following specification and claims, taken together with the accompanyin g drawings.

In the drawings:

FIG. 1 is a geometrical representation of an octahedron, the faces of which represent the [111] crystallographic planes;

FIG. 2 is a somewhat diagrammatic illustration of apparatus used in optically orienting monocrystalline in gots;

FIG. 3 is a diagrammatic illustration of a. conventional crystal pulling process;

FIG. 4 is a diagrammatic illustration of one step in the preferred method of carrying out the present invention;

FIG. 5 is a diagrammatic illustration of a further and essential step in carrying out the process of the present invention.

FIG. 6 is a view, partly in elevation and partly in section, showing a monocrystalline ingot formed in accordance with the invention;

FIG. 7 is a fragmentary view on an enlarged scale, showing a portion of the ingot of FIG. 6; and

FIG. 8 is a fragmentary bottom plan view showing the portion of the ingot illustrated in FIG. 7.

Referring to FIG. 1, there is shown an octahedron having its apices identified respectfully as A, B, C, D, E and F. All the surfaces of such an octahedron correspond to [111] crystallographic planes. The planes defined by the quadrilaterals, ABED, ACEF and BCDF are known as crystallographic planes.

The octahedron shown in FIG. 1 may be considered as representing an idealized perfect crystal of certain substances, including germanium.

During the formation of crystals from any crystallized material, it tends to grow most readily in certain directions. In the case of germanium, it tends to grow in directions which preserve the [111] crystallographic planes as external surfaces. In the growing of germanium crystals for use in transistors, it is essential that the ingots be monocrystalline, i.e., that they follow one single crystal lattice pattern throughout the whole crystal. The methods of making such germanium crystals which are in current use have been particularly selected for their adaptability for production of monocrystaline structures. While the crystals produced by those methods may occasionally have polycrystalline portions, which must be rejected for purposes of transistor manufacture, nevertheless, the major portions of the crystals so produced are monoorystalline.

It is essential that some transistors be formed with certain of their surfaces parallel to a [111] plane of the crystal. In order to ensure such parallelism, the location of [111] planes must be accurately established.

My copending application, now Patent No. 2,858,730,

describes one method of locating a [111] axis. Once three such planes have been located, the location of all the [111] planes is definitely determined. This method will now be briefly described.

An axis which is perpendicular to a [111] plane is referred to herein as a [111] axis. In FIG. 1, the Y-Y axis is such an axis, since it is perpendicular to the ABC face. Note that there is one set of three plane surfaces, identified by the triangles ACD, ABP and BCE, which, if extended, intersect'the [111] axis at the same point P and which ,form equal angles with the [111] axis at that point. This angle is approximately 19 /2 and is identified in the drawing as the angle alpha (on).

Another set of plane surfaces, identified by the triangles CDE, BEF and ADF, if extended, intersect the [111] axis at the point P and at equal angles, identified as alpha (a'). If a beam of collirnated light is directed toward the crystal and the crystal is slowly rotated about a [111] axis, then the rays of light reflected from the three planes which are equiangular with respect to that axis, will follow the same paths as the crystal rotates. By locating three facets on the surface of the crystal which produce light reflections which may be made to strike the same target by rotating the ingot about a given axis, it may be ascertained that the axis of rotation coincides with the [111] axis of the crystal. By cutting the crystal perpendicular to that axis of rotation, it may be ensured that all the dice cut from the crystal for manufacturing transistors have surfaces parallel to a [111] plane.

FIG. 2 shows a crystal ingot 1 mounted in an optical apparatus for locating the [111] axis of the crystal. A shaft 2 is fixed to one end of the crystal by sealing wax 3, or cement, or other suitable means. The shaft 2 is supported in a cylinder 4 by two sets of three screws, each respectively identified as the sets 5 and 6. The cylinder 4 rests in a pair of V-shaped supports 7 and 8 so that the cylinder is free to rotate about its geometrical axis. This fixture including the cylinder 4 and the sets of screws 5 and 6 permit the geometric axis of the ingot 1 to be shifted or tilted with respect to the rotational axis of the cylinder 4. In order to determine the location of the [111] axis of the crystal 1, light from a suitably collimated source or sunlight from a mirror, is directed against the facets on the crystal 1, and from there is reflected to a target 10. If three facets can be found at different points on the surface of one end of the crystal, all of which produce reflections which hit the target 10, as the crystal is rotated, then it is determined that the crystal is being rotated about a [111] axis.

The crystal and its fixture are mounted in such a manner that light from some suitably collimated source is reflected from one of the required three areas on the crystal and to the target 10. The crystal is then turned by rotating the cylinder 4 through approximately 120 to bring the next flat area into position for reflection. The position of the reflected light on the target will be an indication of the amount and the direction in which the [111] axis of the crystal must be tilted to bring it into parallel with the rotational axis of the cylinder 4. Adjustment of the two sets of screws 5 and 6 will accomplish this movement. The crystal is again rotated to the third flat area and the position of the reflected light on the target is again observed and proper adjustment of the crystallographic axis of the ingot is again made. This is continued until a reflected light spot from each of the three flat areas passes in turn through the same point on the target as the cylinder 4 is rotated.

The orientation process as described above is no part of the present invention, but is described and claimed in my copending application, now Patent No. 2,858,730.

FIGS. 3, 4 and 5 As shown in FIG. 3, a conventional crystal pulling process is carried out by inserting a seed crystal such as that shown at 11 in FIG. 3 into a melt 12 of the same 4 material, contained in a crucible 13 of material which will not contaminate the melt. The crucible is placed in a furnace, which is maintained at the melting point of the material. The seed crystal 11 is slowly rotated, and is slowly withdrawn from the melt. As the seed crystal is withdrawn, the material of the melt recrystallizes or grows on the seed crystal, producing a large, generally cylindrical crystal ingot, such as shown at 14.

In carrying out the present invention when the crystal 14 has reached the desired length, the temperature of the melt is rapidly reduced by a suflicient amount, say 10 to 20 C. This results in a fairly sudden increase in the diameter of the ingot at the solid-liquid interface, and, hence produces a flare at the bottom end of the ingot. This reduced temperature is maintained until the flare reaches a desired radial width, preferably in the range of 0.005" to 0.010". When the required flare width is reached, the crystal is lifted suddenly from the melt, as shown in FIG. 5.

In conventional crystal growing processes, the growing ingot is substantially at or slightly below the melting point of the material, while the melt is commonly held somewhat above that temperature. It is best produced by reducing the rate of heat supply by an amount dependent upon the furnace design and other factors. It is considered that a reduction in the temperature of the melt by about 10 to about 20 C. produces the best results. This temperature drop readily causes flaring but does not risk recrystallizing the melt in the crucible. The actual temperature used must, for that purpose, remain above the melting point of the material.

While it is of course impossible to ascertain exactly what transpires at the liquid-solid interface between the ingot 14 and the melt 12, it is believed that there is a strong tendency for the crystal to grow at that interface in directions which tend to preserve the preferred [111] planes. Consequently facets coinciding with those planes commonly exist at the liquid-solid interface throughout the drawing of the crystal ingot. When the crystal is slowly withdrawn from the melt, as is the case in prior art crystal forming techniques, these facets are obscured by rounding ofi of the crystal bottom. By suddenly withdrawing the ingot after forming the flare, the facets are preserved.

The production of facets is greatly enhanced, even for pulling directions far from any crystallographic axis, by the flaring technique described above.

The increase in growth rate necessitated by this flare technique, while undesirable, is limited to the bottom end of the ingot, which is discarded in conventional semiconductor techniques because of crystallographic imper-. fections and undesirable resistivity variations.

Although, in the embodiment of my invention described above, the flare is produced by reducing the melt temperature, it is possible to produce a flare by varying other melt conditions, e.g., lowering the withdrawal rate, or by simultaneously reducing the temperature and the withdrawal rate.

FIG. 6 shows a crystal 14 grown in accordance with the present invention, and having a flare 15 formed thereon at its lower periphery. FIG. 7 shows the crystal 14 and flare 15 of FIG. 6 on a greatly enlarged scale, with a facet appearing at 16. FIG. 8 is a bottom view of the crystal of FIG. 7 showing the facet 16 as it appears from below.

It is essential. that the facets be small for the purposes of use according to the orientation method described above. Preferably, the facets should be between and long by not more than 0.010" wide. In general, the smaller the dimensions, the more accurate is the orientation of the ingot.

After a monocrystalline ingot is formed according to the process described above, and its [111] axis has been accurately located, it may thereafter be positioned in any desired angular orientation, by means of a gom'ometer or the like, and semi-conductor devices may be cut from it having surfaces with any desired angular relation with respect to the crystallographic axis.

If the process described above is followed, facets usable for the optical orientation process are reliably pro duced, regardless of the furnace design and regardless of the angle between the pulling direction and the various crystallographic axes.

While I have shown and described certain preferred embodiments of my invention, other modifications thereof will readily occur to those skilled in the art, and I therefore intend by invention to be limited only by the appended claims.

I claim:

1. The method of growing a monocrystalline ingot having facets coinciding with crystallographic planes of the material of said ingot, so as to enable the grown ingot to be oriented with respect to its crystallographic planes by optical methods, comprising the steps of inserting a seed crystal of the same material as said ingot in a molten mass of said material, slowly withdrawing the seed crystal from the molten mass while maintaining the molten mass at a temperature slightly above the melting point of the material, thereby growing a crystal on the seed crystal, continuing the slow withdrawing movement of the seed crystal until the grown crystal reaches a desired length, increasing the radial rate of crystal formation with respect to the rate of such slow movement, so as to produce an outward flare on the crystal, and then immediately and suddenly withdrawing the crystal from the molten mass.

2. The method in accordance with claim 1, in which the increased rate of crystal formation with respect to the rate of the slow movement of the crystal is maintained 6 until the resulting flare thus formed has a radial width of about 0.005 inch to 0.010 inch.

3. The method in accordance with claim 1, in which the increased rate of crystal formation with respect to the rate of the slow movement of the crystal is accomplished by reducing the temperature of the molten mass by about 10 to 20 C., while still maintaining the molten mass at a temperature about its melting point.

4. The method in accordance with claim 1, in which the increased rate of crystal formation with respect to the rate of the slow movement of the crystal is accomplished by reducing the rate at which the crystal is Withdrawn from the molten mass.

5. The method in accordance with claim 1, in which the increased rate of crystal formation with respect to the rate of the slow movement of the crystal is accomplished in part by reducing the temperature of the molten mass by about 10 to 20 C., while still maintaining the molten mass at a temperature above its melting point, and in part by reducing the rate at which the crystal is withdrawn from the molten mass.

6. The method in accordance with claim 1, in which the material of which the ingot is formed is germanium.

References Cited in the file of this patent UNITED STATES PATENTS 2,631,356 Sparks Mar. 17, 1953 2,683,676 Little July 13, 1954 2,809,135 Koury Oct. 8, 1957 2,858,730 Hanson Nov. 4, 1958 OTHER REFERENCES Bradley: Transistor Technology 3), chap. 7, pages 143-155.

Claims (1)

1. THE METHOD OF GROWING A MONOCRYSTALLINE INGOT HAVING FACETS COINCIDING WITH CRYSTALLOGRAPHIC PLANES OF THE MATERIAL OF SAID INGOT, SO AS TO ENABLE THE GROWN INGOT TO BE ORIENTED WITH RESPECT TO ITS CRYSTALLOGRAPHIC PLANES BY OPTICAL METHODS, COMPRISING THE STEPS OF INSERTING A SEED CRYSTAL OF THE SAME MATERIAL AS SAID INGOT IN A MOLTEN MASS OF SAID MATERIAL, SLOWLY WITHDRAWING THE SEED CRYSTAL FROM THE MOLTEN MASS WHILE MAINTAINING THE MOLTEN MASS AT A TEMPERATURE SLIGHTLY ABOVE THE MELTING POINT OF THE MATERIAL, THEREBY GROWING A CRYSTAL ON THE SEED CRYSTAL, CONTINUING THE SLOW WITHDRAWING MOVEMENT OF THE SEED CRYSTAL UNTIL THE GROWN CRYSTAL REACHES A DESIRED LENGTH, INCREASING THE RADIAL RATE OF CRYSTAL FORMATION WITH RESPECT TO THE RATE OF SUCH SLOW MOVEMENT, SO AS TO PRODUCE AN OUTWARD FLARE ON THE CRYSTAL, AND THEN IMMEDIATELY AND SUDDENLY WITHDRAWING THE CRYSTAL FROM THE MOLTEN MASS.

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