US3188182A - Use of the working material as part of the crystal making apparatus - Google Patents

Description

J1me 1965 c. R. MORELOCK 3,

USE OF THE WORKING MATERIAL AS PART OF THE CRYSTAL MAKING APPARATUS Filed May 51, 1962 7'0 Vacuum I I 2a W 29M 56 f I I/4Z g 4, 1 3 n 43 LV 2/ o I I Q O 54 O I o 53 o ,+-20

lnvenfor: Char/es R More/oak,

His Afforney' United States Patent 7 F 3,188,182 USE OF THE WORKKNG MATERIAL AS PART 025 THE CRYSTAL MAKING APPARATUS Charles R. Moreiock, Ballston Spa, N.Y., assignorv to General Electric (Iompany, a corporation of New York Filed May 31, 1962, Ser. No. 2%,023 3 Claims. (Cl. 223-273) This application is a continuation-in-part of applicants copending application, Serial No. 120,560, filed June 29, 1961, now abandoned, and assigned to the same assignee as the present invention.

This invention relates to metal single crystals and more particularly to improved processes and apparatus for producing such crystals.

Prior to this invention, the production of vapor-deposited single crystals has been limited to those materials having low melting points and relatively high oxide dissociation pressures. The greatly improved physical properties offered by single crystals, as compared to polycrystalline materials, have not been attainable due to the absence of processes for etfectively producing such crystals.

ildhdd Patented June 8, 1955 ture of the growth substrate body below that of the source It is a principal object of this invention to provide an improved process for producing metal singlecrystals or whiskers.

Another object of this invention is to provide an improved process for producing single crystals of metals having low oxide dissociation pressures.

An additional object of this invention is to provide improved apparatus for producing metal single crystals.

Other objects and advantages of the present invention will be in part obvious and in part explained by reference to the accompanying specification and drawings.

In the drawings: 7

FIG. 1 is a side elevation, partly in section, showing an apparatus for producing metal crystals according to the present invention;

FIG. 2 is a perspective showing a combined source metal and substrate growth body for use in the the apparatus of FIG. 1;

FIG. 3 is a perspective of another form of source metalgrowth substrate combination; and

FIG. 4 is yet another form of source metal and growth substrate combination for growing single crystals.

The process of this invention generally comprises 10- eating a source metal body, which may be made of a metal suchas chromium, copper, gold, nickel, iron, cobalt, or binary or ternary alloys of iron, nickel and cobalt in a chamber isolated from the outside atmosphere, which chamber can be heated to a temperature sufiicient to vaporize the source metal. A growth substrate body which may be compositionally identical to the source metal, or of a different composition if desired, is also located Within the vaporizing chamber that is maintained at a temperature lower than that of the source metal body. As the chamber is heated to a temperature sufiicient to vaporize the source metal, the chamber is continuously evacuated so that some portion of the source metal vapor in the chamber escapes toward the outlet and getters oxygen from' the chamber to maintain the oxygen pressure therein below the dissociation pressure of the source metal oxide.

The apparatus of this invention generally comprises means defining a vaporizing chamber, such asa fused silica tube, evacuating means operably connected to the vaporizing chamber so that the atmosphere within the chamber can be continuously removed, a body of source metal and a growth substrate body located within the metal body.

The apparatus of the invention can be described more completely by referring to FIG. 1 of the drawings. In this figure, the numeral 19 indicates the vaporizing chamber, this chamber being defined by a fused silica tube 11 and surrounded by heating means, here shown as a plati mum-wound furnace 12. The furnace 12 surrounds only a portion of the entire length of tube 11 so that the heating zone where vaporization and deposition of metal single crystals is carried out is not coextensive with the entire length of tube 11. A Pyrex tube 15 has been integrally joined to the upper end of tube 11 and the upper end of this tube is connected to a source of vacuum, not shown. Extending outwardly from but integrally joined to the tube 15 is a lateral extension 16 which is completely sealed so that no opening is present into the volume defined by tubes 11 and 15. The Pyrex glass was substituted for quartz in the manner described because it is easier to work and shape and is cheaper in the overall construction of the apparatus.

A generally cylindrical source metal body 2 9 is shown positioned within the hot zone of vaporizing chamber 16, the body 20 having, of course, an axial opening extending I through it. A growth substrate body is located approximately equidistant from the inner surface of body 2t) and extends upwardly beyond the upper limit of body 20 a distance sufficient to permit a portion of its length to be located outside of the hot zone defined by furnace 12.

The growth substrate body 21 can be lowered into the operating position shown and, conversely, raised completely out of the hot zone defined by furnace 12 bymeans of the winch mechanism indicated generally by numeral .25. This mechanism comprises a tungsten wire 26 which can be wound and unwound onto reel 27. The reel 27 is fixedly attached to rotatable shaft 28 mounted in journal 29. That end of shaft 28 furthest removed from reel 27 has a magnet 30 connected to it, which magnet can be operated by the rotatable operating member 31. Member 31 comprises a body portion 32 constructed of any suitable plastic or non-conducting material and contains a plurality of magnets 33 which cooperate with the magnet 30 to rotate shaft 28 as member 31 is turned. As shown in the figure, clockwise rotation of member 31 will wind fiber 26 on reel 27 and raise substrate body 7 21 out of operating position within the vaporizing chamber 10.

Generally speaking, when the various elements of the apparatus are in the positions indicated in FIG. 1, metal single crystals, or whiskers, can be grown by operating furnace 12 to raise the temperature within vaporizing chamber 10 to a point sufiicient to effect vaporization of p the source metal body 20. Since a portion of the growth radiated outwardly from that portion of body 21 which is located beyond the limits of the hot zone defined by furnace 12. Thus, metal vapor present in chamber 10 will deposit as single crystals. 2

It is felt that the single crystals or whiskers nucleate V on emergent screw dislocations located on or in the surface of the substrate body. Once started by this initial nucleation, the screw dislocation becomes built into the crystal and axial growth proceeds by metal atoms being c1) adsorbed on the sides of the crystal, migrating to the tip and becoming incorporated in the lattice at the step of the screw dislocation.

According to this mechanism, crystals should grow at supersaturation ratios which are less than some critical supersaturation ratio, oc required for measurable twodimensional homogenous nucleation. The supersaturation ratio, a, is equal to p/p where p-is the actual pressure of-the depositing-vapor and p the saturation pressure of the deposited vapor. These pressures are established by the temperatures of the evaporation and growth sub stratesurfaces.

According to the proposed growth mechanism, there should be no lower vapor pressure limit for the occurrence of-single crystalgrowth. For the metals with which this invention is particularly concerned, viz., chromium, copper, gold, nickel, iron, cobalt, or binary or ternary alloys of iron, nickel and cobalt, no single crystals were grown at vapor pressures less than mm. of mercury. It is probable that the gettering action of the metal vapor, made possible by continuous evacuation of the vaporiz ing chamber, is sufficient to keep the oxygen pressure in the growth zone below the dissociation pressure of the metal oxide only at metal vapor pressures of 10 or greater. The dissociation pressures for the oxides of chromium, iron, cobalt, nickel and copper are approxi mately 10- 1O 10* 10' and 10* mm. of mercury, respectively. Since in all the experiments conducted the metal vapor pressures were all nearly the same, it would be expected that oxides in the chromium experiments would be more severe than the copper experiments. This was observed to be true.

It is therefore important in carrying out the present process that'before the condition of saturation ratio is established, the partial pressure of oxygen must be below the dissociation pressure of the particular metal oxide if whisker growth is to occur. The upper vapor pressure limit should only be p at the melting point, T of the metal being investigated. The closest approach to this limitation has been in experiments with gold and copper, whiskers "having been grown when the substrate temperature was held within C. of their respective melting points.

Whisker growths, then, are not restricted .to any particular magnitude of the metal vapor pressures but rather to the upper and lower limits determined as above. This is substantiated by the fact that the vapor pressures used in whisker growth investigations have ranged from 10* mm. of mercury for gold, to 0.1 mm. of mercury for cadmium.

' Several experiments were run to grow metal single crystals or whiskers of various types of metals. In these experiments, growth substrate bodies and source metal bodies were positioned in the vaporizing chamber as shown in FIG. 1 of the drawings while the chamber was beingheated continuously by furnace 12 and was also beingcontinuously evacuated. Inthese experiments, the apparatus was heated to operating temperature prior to the lowering of the growth substrate body into the vaporizing chamber so that any oxygen or other atmospheric contaminants inthe apparatus were removed as low as possible. The useful ranges of the evaporation temperatures'for the various materials are listed in the following Table I.

Table I Evaporation temperature Metal: Range, C. Chromium 830l890 Nickel 10804455 'Gold 700-1063 Copper 820-1083 Iron 1050-1539 Cobalt 1000-1495 40 in FIG. 3 of the drawings. -metal is a flat sheet 41 which is connected to a larger The source metal, evaporation temperature, growth time-whisker length and the composition of the growth substrate for these various samples are all listed in Table 11 following:

Table 11 l l Evap. Growth Longest Growth Evap. Metal Temp. Time Whiskers Substrate C.) '(Hrs.)

1, 050 16 70 Mo Screen. l

900 None Do. 1, 200 16 100 D0. 1, 050 48 None Do. 1, 050 16 300 Au Sheet,

875 120 150 Do. 1 830 70 None On Sheet. 930 16 200 Do. i 1, 020 1 None D0. 1, 000 3 100 Do. 1 1,000 16 100 Do. l 950 50 300 D0. j 1, 030 16 None Fe Sheet 1 1,060 16 2 Do. 1 1,150 None Mo Screen 1 1,150 1 15 Do. 1, 060 so 200 Fe Sheet. i 1,150 16 100 D0. Co 1,020 $6 5 Mo Sheet. 1 Co (23%)Fe 77%) 1,120 50 Do. i

As mentioned previously, it isnecessary that the growth 3 substrate bodybe maintained at a temperature below that i of the source metal body if crystal growth is to be obtained. This temperature diiterential can be obtained in any one of several different ways. For example, in FIG. 2 of the drawings, the growth substrate body and source 1 metal body exist as'a single unit, here shown as a spirallyi wound coil 35. 'In this case, metal is evaporated from i the inner surface of the outermost'spiral' of coil 35, since i it is subjected directly-to radiant heating from the furnace and deposited on the outermost surface of the second spiral 37 since this surface is protected from direct heating of the furnace and is therefore somewhat cooler. This same -vaporizing and'depositing process takes place on each successive spiral, progressing inwardly and ending at the innermost spiral 38. It has been found that the normal temperature differential existing between suc- 1 cessive spirals is about 17 C., temperature differentials on I the order of 15 and more normally being sufficient to deposit vaporized metal for whisker growth.

A slightly modified combined form of source metal body and growth substrate body is indicated by numeral In this case, the source growth substrate body 42 by means of connecting pieces 4-3. The body 42 is somewhat longer than body 41 so that it can extend-upwardly out of the hot zone defined by the heating .furnace in much the ame manner as the growth substrate body 21 shown in FIG. 1 of the drawportion located outside of the hot zone.

tion which is particularly useful in the present invention.

In this instance, the source metal body is shown as a wire 50 which extends down through the center of a completely closed growth substrate body 51. The wire 50 is insulated from body 51 by means of an insulating grommet 52 but is electrically connected to the bot-tom plate of body 51- so that electricity can be made to flow through both body 50 and body 51. The electricity is supplied from a source 53 via the wires 54. The operation of this device is eifected by placing the structure in the vaporizing zone, such as shown in FIG. 1, and heating. the assembly to a temperature below that Where any metal condensation will occur. By then conducting a flow of electricity through the two bodies, the temperature of Wire 50 can be raised beyond the temperature of body 51. This temperature differential results from the carrying capacity of the two bodies. That is, the relatively small size of wire 50 results in a resistance greater to the passage of electricity than is present in body 51, so that electrical heating of wire 50 is effected beyond that which takes place in body 51. This apparatus makes it possible to accurately control the temperature differential between the growth substrate body and the source metal body.

The function of continuously evacuating the vaporizing chamber is important to this invention. Since metals having extremely low oxide dissociation pressures are contemplated, it is necessary that any oxygen in the system be removed to a level as low as possible. It is further essential that oxygen diffusing into the system through the walls of the various vessels be prevented from coming into contact with metal vapor in the growth zone. These objectives are attended by continuously evacuating the atmosphere from vaporizing zone in a manner such that a continuous supply of metal vapor is directed upwardly through tubes 11 and 15 to getter out any oxygen which may be present. This operation makes it possible for the first time to maintain the oxygen pressure in the vaporizing chamber below the dissociation pressure of the source metal oxides, where the metals being vaporized are extremely strong oxide formers. Of course, the apparatus and process are equally etfective in growing crystals of those materials where oxygen contamination is a relatively minor problem.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An apparatus for growing metal single crystals comprising, means defining a vaporizing chamber for growing single crystals of a source metal on a substrate, means operably connected to said vaporizing chamber to elfect continuous evacuation of the atmosphere therefrom during operation, means for heating said source metal body and said substrate body, and means separate from said heating means for rendering the temperature of said substrate body below that of said source metal body, said separate means being in part said source metal body.

2. An apparatus for growing metal single crystals comprising, means defining a vaporizing chamber for growing single crystals of'a source metal on a substrate, means operatively connected to said vaporizing chamber to effect continuous evacuation of the atmosphere therefrom during operation, means for heating said source metal body and said substrate body, means separate from said heating means for rendering the temperature of said substrate body below that of said source metal body, and means electrically connecting said source metal body and said substrate body to a source of electricity to provide for a flow of electricity through said bodies, to render the temperature of said substrate body below that of said source metal body.

3. An apparatus as defined in claim 1 wherein said substrate body includes a portion extending beyond the hot zone created by said heating means to withdraw heat from a portion located within the hot zone and thereby maintain the portion within the hot zone at a temperature lower than that of said source metal body.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Griest et al., article in the Journal of Applied Physics, vol. 27, September 1956, pages 1022-1024.

Holland, Vacuum Deposition of Thin Films, John Wiley and Sons, Inc., 1956, page 20.

DAVID L. RECK, Primary Examiner.

Claims (1)

1. AN APPARATUS FOR GROWING METAL SINGLE CRYSTALS COMPRISING MEANS DEFINING A VAPORIZING CHAMBER FOR GROWING SINGLE CRYSTALS OF A SOURCE METAL ON A SUBSTRATE, MEANS OPERABLY CONNECTED TO SAID VAPORIZING CHAMBER TO EFFECT CONTINUOUS EVACUATION OF THE ATMOSPHERE THEREFROM DURING OPERATION, MEANS FOR HEATING SAID SOURCE METAL BODY AND SAID SUBSTRATE BODY, AND MEANS SEPARATE FROM SAID HEATING MEANS FOR RENDERING THE TEMPERATURE OF SAID SUBSTRATE BODY BELOW THAT OF SAID SOURCE METAL BODY, SAID SEPARATE MEANS BEING IN PART SAID SOURCE METAL BODY.

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