US3898051A - Crystal growing - Google Patents

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US3898051A
US3898051A US42914273A US3898051A US 3898051 A US3898051 A US 3898051A US 42914273 A US42914273 A US 42914273A US 3898051 A US3898051 A US 3898051A
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crucible
heat
temperature
exchanger
material
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Frederick Schmid
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GT Crystal Systems LLC
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Crystal Systems Inc
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    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28HEAT EXCHANGE IN GENERAL
    • F28DHEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D7/00Heat-exchange apparatus having stationary tubular conduit assemblies for both heat-exchange media, the media being in contact with different sides of a conduit wall
    • F28D7/10Heat-exchange apparatus having stationary tubular conduit assemblies for both heat-exchange media, the media being in contact with different sides of a conduit wall the conduits being arranged one within the other, e.g. concentrically
    • F28D7/12Heat-exchange apparatus having stationary tubular conduit assemblies for both heat-exchange media, the media being in contact with different sides of a conduit wall the conduits being arranged one within the other, e.g. concentrically the surrounding tube being closed at one end, e.g. return type
    • CCHEMISTRY; METALLURGY
    • C30CRYSTAL GROWTH
    • C30BSINGLE-CRYSTAL-GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B11/00Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method
    • C30B11/003Heating or cooling of the melt or the crystallised material
    • CCHEMISTRY; METALLURGY
    • C30CRYSTAL GROWTH
    • C30BSINGLE-CRYSTAL-GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B17/00Single-crystal growth onto a seed which remains in the melt during growth, e.g. Nacken-Kyropoulos method
    • CCHEMISTRY; METALLURGY
    • C30CRYSTAL GROWTH
    • C30BSINGLE-CRYSTAL-GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B27/00Single-crystal growth under a protective fluid
    • CCHEMISTRY; METALLURGY
    • C30CRYSTAL GROWTH
    • C30BSINGLE-CRYSTAL-GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10Inorganic compounds or compositions
    • C30B29/16Oxides
    • C30B29/20Aluminium oxides
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28HEAT EXCHANGE IN GENERAL
    • F28DHEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D21/00Heat-exchange apparatus not covered by any of the groups F28D1/00 - F28D20/00
    • F28D2021/0019Other heat exchangers for particular applications; Heat exchange systems not otherwise provided for
    • F28D2021/0056Other heat exchangers for particular applications; Heat exchange systems not otherwise provided for for ovens or furnaces
    • F28D2021/0057Other heat exchangers for particular applications; Heat exchange systems not otherwise provided for for ovens or furnaces for melting materials

Abstract

In the process for growing single crystals including the steps of placing material in a crucible, heating the crucible to above the melting point of the material, and thereafter solidifying the melted material by extracting heat from a central portion of the bottom of the crucible, that improvement wherein the temperature of the side walls of the crucible is maintained at temperatures above the melting point of the material until substantially all the material within the crucible has been solidified.

Description

United States Patent 1 1 1 3,898,051

Schmid 1 1 Aug. 5, 1975 I54] CRYSTAL GROWING 3.653.432 4/1972 Schmid et al. 165/61 3.762943 10 1973 W l 'l. 23 273 SP [751 lnventor: Frederick Schmid, Marblehcad, mste eta Mass Primary [:.\'aminerl\10rman Yudkoff [73] Assigncc: Crystal Systems, Inc., Salem, Mass. 14 x1 1!!! Ii\'lH1im'I'D. Sanders 7'7 v l l Filed Dec. 28, 1973 [57] ABSTRACT 1 PP N04 429,142 In the process for growing single crystals including the steps of placing material in a crucible, heating the cru- 521 s Cl u 23 301 p; 23 273 Sp; 1 5/ cible 110 above the melting point Of the material, and {5] Int H B0 17/00; B01 D 9/00 thereafter solidifying the melted material by extracting [58] Field of Search u 23/301 Sp 273 SP; 165/61 heat from a central portion of the bottom of the crucible, that improvement wherein the temperature of the 56] References Cited side walls of the crucible is maintained at temperatures above the melting point of the material until sub- UNITED STATES PATENTS stantially all the material within the crucible has been 1335mm x/wm Hall .1 23/273 SP solidifigd 3,441.385 4/1969 Schmidt. 23/301 SP 3464.812 9/1969 Utech ct a1 .l 23/273 SP 21 Claims, 6 Drawing Figures VACUUM PUMP SHEET PATENTEU AUG 51975 FIG I PUMP VACUUM SHEET PATENTEUAUE 5% TMP 50C "UNMELTED SEED H e M P SOLID CRYSTAL He MP SOLID CRYSTAL LIQUID MINISCUS W MP He MP CRYSTAL GROWING This invention relates to a crystal growing.

It is a primary object of the present invention to provide a method for growing very high quality single crystals of far larger size than heretofore possible. Other objects include providing such a method for growing single crystals of ceramic, metal, or composite materials (including sapphire, ruby, spinel. eutectics, and the like) in which the problems normally caused by convection currents or other turbulence, gas bubbles, constitutional supercooling, high impurity levels and high temperature gradients are substantially eliminated.

The invention features, in the process for growing single crystals including the steps of placing material in a crucible, heating the crucible to above the melting point of the material, and thereafter solidifying the melted material by extracting heat from a central portion of the bottom of the crucible, that improvement wherein the temperature of the side walls of the crucible is maintained at temperatures above the melting point of the material until substantially all the material within the crucible has been solidified. In preferred processes in which heat is extracted by a cooling gas heat exchanger engaging the bottom of the crucible and the diameter of the crucible is at least twice that of the heat exchanger and the crucible height is not less than its radius, there is featured placing in the crucible a seed crystal not smaller than the area of top of the heat exchanger and positioning the seed crystal over the heat exchanger, initiating flow of coolant gas through the heat exchanger and superheating the crucible side walls to not less than about 50C. above the material melting point to seed the melt, maintaining the superheated crucible side wall temperature while increasing flow of coolant gas through the heat exchanger to solidify a portion of the melted material, thereafter slowly decreasing the temperature of the crucible side walls at a rate of not more than about C. per hour to a temperature not less than the material melting point while continuing to increase flow of cooling gas at such a rate that the heat exchanger temperature decreases at not over 100C. per hour to solidify substantially all of the remaining portion of the melted material, and then controlling the crucible and heat exchanger temperatures as required to anneal the solidified material.

Other objects, features, and advantages will appear from the following detailed description of a preferred embodiment of the present invention, taken together with the attached drawings in which:

FIG. I is a plan, somewhat schematic, view of a system used in the practice of the present invention;

FIG. 2 is a perspective view, partially in section, of portions of the system of FIG. I; and

FIGS. 3a3d are diagrammatic views illustrating various stages in the growth of a large single crystal using the system of FIGS. 1 and 2 according to the present invention.

Referring more particularly to the drawings, there is shown in FIG. I a vacuum graphite resistance furnace I0 (manufactured by Advanced Vacuum Systems of Woburn, Mass.) connected to a vacuum pump 12. Within furnace I0 is a double-walled heating chamber, generally designated 14. As shown. the outer walls (peripheral side. top and bottom) of heating chamber 14 are of stainless steel and are spaced from the adjacent walls of vacuum furnace 10. Heating chamber 14 is supported within the vacuum furnace by an annular flange l6 projecting inwardly from the cylindrical wall 11 of furnace l0 and engaging the outer rim of the bottom 15 of chamber '14.

The inner walls of heating chamber 14 are defined by a cylindrical graphite sleeve 18, a top cover plate 20, and a bottom plate 22. The volume between the inner and outer walls is filled with graphite felt insulation 24. To permit access into the interior of the heating chamber, the top 13 of vacuum furnace l0 and the top 17 of heating chamber 14 (including graphite top plate 20, stainless steel top 19, and the insulation 24 between the two top plates) are removable.

A cylindrical resistance heater 26 is mounted in the cylindrical cavity 28 within heating chamber 14. The electrical power and control leads 30 of the heater pass through the peripheral walls of the heating chamber 14 and furnace 10.

A helium-cooled, tungsten/molybdenum heat exchanger 32 is mounted on the bottom of furnace l0 and projects into the furnace and then through a graphite sleeve 33 extending through the bottom of heating chamber 14 up into cavity 28. As shown more clearly in US. Pat. No. 3,653,432, heat exchanger 32 includes a base segment 34 secured to the outside of the bottom of furnace 10, and a hollow cylindrical rod segment 36 extending from base segment 34 into cavity 28. The top 38 of rod segment 36 is flat. A tungsten inlet tube 40 and a thermocouple 44 extend within heat exchanger 32 from below base 34, through rod segment 36 to closely adjacent top 38. An outlet tube 42 extends from an outlet aperture (communicating with the interior of rod 36) in base 34. Inlet tube 40 and outlet tube 42 are both connected to a helium source 45. Helium from source 45 can be either recirculated or, if desired, released into the atmosphere.

The size of rod segment 36 depends to some extent on the particular material to be crystallized. For ceramic materials (such as sapphire) having a relatively low thermal conductivity and diffusivity, the overall diameter of rod segment 36, and thus of top 38, will typically be about inch. For metals, which have higher thermal conductivity and diffusivity, a smaller heat exchanger typically be used sothat the rate of heat extraction can be decreased. Alternatively, insulation may be placed between the heat exchanger top and crucible bottom, or the position of the heat exchanger in the heat zone may be raised. All these latter measures will decrease the rate at which heat can be extracted with any particular rate of helium flow.

As shown most clearly in FIG. 2, the refractory crucible 48 in which the crystals are grown is supported within cavity 14 by the top 38 of the heat exchanger 32 and eight tungsten plates 50 mounted vertically in radially extending grooves 52 in the upper surface of a graphite support plate 54, one-inch thick and about 7 /2 inches in diameter. Support plate 54 rests on bottom plate 22. Grooves 52 in plate 54 are regularly spaced at 45 intervals. Each tungsten plate 50 is about 1 inch long, inch high and 0.040 inch thick, and engages the outer annular portion of the bottom of crucible 48. Heat exchanger rod segment 36 extends through a hole 55 in the center-of support plate 54, and the flat top 38 of the rod segment engages the center of the bottom 49 of crucible 48.

- the crucible must be greater (generally at least twice) than that of heat exchanger top 36, and it should have a height not less than its radius. Typically, the crucible diameter will be much greater than (for example, about eight times) the heat exchanger top diameter, and its height will be about the same as its diameter. Crucible 48 has an overall diameter of 6 /2 in. and an overall height of 6 inches.

The crucible is typically formed by spinning a disc. Thus, the thickness of its bottom is greater than that of its sides. The thickness of crucible bottom 49 is 0.040 in. and that of cylindrical wall 56 is about 0.030 in. To reduce flow of heat from the cylindrical wall to the crucible bottom, a thin wall annular portion 58 (thickness, 0.020 in.) is provided about V8 in. above the crucible bottom.

The top of the crucible is covered by a cover plate 60, made of the same material as crucible 48, having a sight hole 62, one inch in diameter, in the center thereof.

Sight holes 64, 66 extend through, respectively, the top 13 of furnace 10 and the top 17 of heating chamber 14, and are axially aligned with sight hole 62 in crucible cover 60. Sight hole 64 through furnace top 13 is, of course, vacuum tight and is defined by lens assembly 68. Sight hole 66 through heating chamber top 15 is defined by a cylindrical graphite sleeve extending between the -double walls 19, of heating chamber top 17.

Two other sight hole assemblies, generally designated 70 and 72 respectively, permit the temperatures of heating element 26 and vertical side wall 56 of crucible 48 to be monitored during crystal growth. Each assembly includes three axially aligned sight holes-one through the peripheral wall 11 of furnace 10, defined by a vacuum tight lens assembly at the cylindrical periphery of the furnace l0, and designated 74, 76 respectively; a second, defined by a graphite sleeve extending through the cylindrical double side wall of heating chamber 14 and designated 78, 80 respectively; and a third, extending through heating element 26 and designated 82, 84 respectively. Pyrometers 71, 73 are mounted adjacent the exterior end of, respectively, sight hole assemblies 70, 72. As shown, sight hole assembly 70 is located so as to permit pyrometer 71 to view the interior surface of the far vertical wall of heating element 26,just above the top of crucible 48. Sight hole assembly 72 is below assembly 70 and permits pyrometer 73 to view the side wall 56 of crucible 48, about /2 inch above bottom 49 and just above thin wall portion 58.

Pyrometers 71 or 73 and thermocouple 42 are connected to a controller 85. One output of controller 85 is connected to the source of power 86 for heating element 26. A second controller output is connected to helium source 45. Controller 85 is responsive to the temperatures sensed by pyrometers 71, 73 to vary the power supplied by heating source 86 as required to maintain the temperatures of heating element 26 and crucible 48 at the proper level; and to the temperature sensed by thermocouple 42 to vary the flow from helium source 45 as required properly to vary the temperature of heat exchanger top 38.

In practice, crucible 48 is first washed with, for example, nitric acid and chlorox, to remove impurities. In those growth processes where one is used, a seed crystal 100, shown in dashed lines in FIG. 3a and having an overall diameter slightly greater than the diameter of the top 38 of heat exchanger 32, is placed in the center of the bottom 49 of the crucible. The crucible is then filled with small pieces of the material to be melted. If a seed crystal is used. the first pieces are placed tightly around the crystal to hold it in place. To obtain maximum loading, the pieces are all placed in the crucible one-by-one and are fitted closely together.

The loaded crucible is then placed in heating chamber 14 with crucible bottom 49 seated on heat exchanger top 38. The height of the heat exchanger, i.e., the distance it protrudes into the heating chamber, is determined by experimentation. The heat exchanger is positioned so that, when the crucible side walls are superheated over the melting point of the material therein (typically about 50C.), a relatively small flow of helium through the heat exchanger (typically at a rate of about 40 c.f.h.) will prevent the seed crystal from melting. As shown, the seed crystal slightly overhangs all sides of heat exchanger top 38. Slots 52 are cut into support plate 54 to such a depth that, when the crucible is cold, the tops of tungsten plates 50 are slightly below the crucible bottom. When the temperature of the crucible is increased, the crucible slightly sags and its bottom 49 rests on plates 50.

Cover plate 60 is placed on the crucible with its sight hole 62 axially aligned with the crucible and heat exchanger, and the tops 17, 13 of heating chamber 14 and furnace 10 are replaced. Vacuum pump 12 is then started and the furnace is evacuated to a pressure of about 01 torr. It should be noted that in some growth processes, discussed hereinafter, the furnace pressure is increased.

When the furnace pressure has reached the desired level, heat source 86 is actuated. The power supplied to heating element 26 is gradually increased, typically so that the temperature within heating chamber will rise at a rate not over about 250C. per hour. The power supplied to the heating element is increased until, as observed through sight holes 62, 64, 66, the material within the crucible begins to melt.

The first material to melt is the pieces adjacent the outer cylindrical wall of the crucible. As soon as such melting is observed, the temperatures of heating element 26 (T crucible side wall 56 (T and heat exchanger 36 (T as indicated by, respectively, pyrometers 71 and 73 and thermocouple 42, are measured and recorded. Although the actual temperature at which any particular material melts does not change, the melting point, T,,,,,, indicated by the different instruments may vary somewhat depending on such things as the contact between the thermocouple and crucible,

size and length of the sight hole assemblies, cleanliness of the windows, and the like. For accurate control of the process it is important that the instruments be calibrated.

When a seed crystal is used, it is important that it be kept from melting. Therefore, helium source 46 is actuated to cause the aforementioned initial flow of room temperature helium. typically at a flow rate of about 40 c.f.h., as soon as melting of pieces within crucible 36 begins.

The amount of power from source 86 applied to heating element 26 is further increased to superheat the crucible side walls to above, typically about 50C., the initial melting point. The power input is then held constant until all temperatures within the furnace have stabilized.

At this stage, conditions are substantially as shown in H6. 3a. The temperatures of the heating element, T and crucible side walls, T are substantially equal, and above (typically about 50C.) the melting point of the material in the crucible. All material within the crucible, with the exception of seed crystal 100 assuming one is provided, has melted to form a liquid 102. The liquid has melted the edges of seed crystal 100 (to the extent shown by solid lines) to promote nucleation, but melting of the major portion of the seed crystal above the heat exchanger has been prevented by the flow of helium through heat exchanger 32. Because of the helium flow, the temperatures of the top 38 of heat exchanger 32, T and hence of the adjacent engaged portion of crucible bottom 49, are below the melting point of the seed crystal, even though the temperatures of the heating element and crucible cylindrical wall are above the melting point.

The extent to which the heating element and crucible side wall are superheated above the melting point of the material to be crystallized depends on several factors, particularly the conductivity of the material, the desired growth rate, and the ratio of crucible diameter to heat exchanger diameter. Typically, the superheat is about 50C. For processes involving relatively slow growth rates, materials having higher thermal conduc tivities, and/or crucibles and heat exchangers having lower diameter ratios, it may be desirable to superheat to lOC. or more above the melting point.

In all processes according to the present invention, the material within the crucible is grown into a single crystal by independently controlling the temperatures of the crucible side walls and heat exchanger top to provide the desired and necessary temperature gradi ents in the solid and liquid portions of the material.

lnitial crystal growth is commenced by gradually increasing the rate of helium flow through heat exchanger 32, typically at a rate of about l0 to l5 c.f.h. per hour, to slowly decrease the temperature of the heat exchanger and increase the rate at which heat is drawn from the center bottom of the crucible. Simultaneously, the amount of power from source 86 applied to heating element 26 is increased as required to maintain the temperatures of heating element 26 and of crucible vertical wall 56 (as observed by pyrometers 71, 73) constant.

The duration of this initial period of crystal growth depends on the size of the crucible and on the particular material being crystallized. Typically it extends through about six to eight hours. At the end of the initial period. conditions are substantially as shown in FIG. 3b. The temperature of the top 38 of the heat exchanger, T,,,;, has decreased to well below the melting point. T,,,,,. The temperatures of the heating element 26 and cylindrical side wall 56 ofcrucible 48 are still at the initial superheated level. typically 50C. above the ob served melting point. Crystal growth (solidification of the liquid in the melt) has progressed to a stage where the solidified crystal or boule 104 is more or less ovoid in shape. The entire boule, with the exception of that portion overlying the top 38 of heat exchanger 32, is surrounded by molten material and its exact size and shape cannot directly be observed. The general shape of the boule is known from the facts that the material at the top of the crucible is liquid, the entire side wall of the crucible is well above the melting point, and the portions of the crucible wall adjacent the top and bottom of the crucible are even hotter (due to heat reflected from the top and bottom of heating chamber 14).

For crystal growth to proceed further, it is necessary not only to continue to increase the rate of helium flow through the heat exchanger, but also to decrease the temperature of the crucible vertical wall. During the next crystal growth period, therefore, helium flow is further increased, typically at the same rate of l0 to 15 c.f.h. per hour, and the observed temperature of the top 38 of the heat exchanger continues to drop. Additionally, the power applied to heating element 26 is reduced at such a rate that the temperatures of the heating element 26 and cylindrical wall of crucible 48 will slowly decrease, typically at a rate of less than l5C. per hour and preferably less than 5C. per hour, until the observed temperatures have reached a level about 5C. above the observed melting point.

At about this time, solidification has advanced to the point shown in FIG. 3c. The top of the solidified crystal boule 104 has just broken through the top of the melt, as observed through sight holes 62, 64, 66. Except for a thin annulus 106 of liquid between the boule 104 and the vertical cylindrical wall of the crucible, which is still above the melting point, the boule fills substantially the entire crucible. Annulus 106 is thickest near its top 108 and bottom 110, which as indicated previously, are adjacent the hottest points of the crucible.

To complete crystallization, the slow increase in helium flow and slow decrease of furnace temperature are continued until it is observed (through sight holes 62, 64, 66) that the only liquid left in the crucible 48 is a very thin film or miniscus, which runs back and forth over the top of the solid crystal boule 104 and creeps over the side of the crucible. At this point, the temperature of the crucible; side wall, except for the slightly warmer extreme top and bottom, is approximately equal to the melting temperature and solidification is substantially complete. The final miniscus is solidified by further the decreasing the power supplied to heat element 26, to drop temperature of the heating chamber and crucible to slightly below the melt temperature.

The rates at which the heating chamber and heat exchanger temperatures are decreased during crystal growth are critical. If either drops too rapidly, gas bubbles and high dislocation densities will result in the crystal boule. The exact limits depend on the particular crystal being grown. For growth of ceramics such as sapphire, for example, the furnace and crucible wall temperatures generally should not decrease at a rate over 10C. per hour, and the heat exchanger temperature should not drop faster than 5O"Cv per hour. For metal crystals, the rates of decrease should be lower, typically not over. respectively, 5C. per hour and 25C. per hour.

After solidification is complete, the boule is cooled to room temperature in such a way as to relieve solidification stresses therein. This involves three steps. First, the flow of helium gas through the heat exchanger is slowly terminated. while at the same time the furnace power is decreased so that the furnace temperature drops to about 50C. below the initial melting point. Second, the boule is held at about 50C. below the melting point temperature for an annealing period of the order of a few hours. Third, furnace power is further decreased to slowly cool the furnace and boule to room temperature. Typically, the first period takes about three or four hours, and the temperature is decreased during the third phase at the rate of about 50C. per hour.

The following Examples will serve further to illustrate the invention.

EXAMPLE I A sapphire seed crystal about one inch in diameter was placed in a molybdenum crucible and the crucible filled with cracked pieces of Verneuil sapphire. The filled crucible was placed in the furnace, the furnace evacuated, and the power source turned on.

The power was increased at such a rate that the furnace temperature increased at a rate of 250C. per hour, and after about eight hours the sapphire at the crucible side walls began to melt. When melting was observed, the instruments were calibrated, and helium source was turned on to force helium through the heat exchanger at an initial rate of 40 c.f.h. The temperature of the furnace was then further increased until it was 50C. above the observed initial melting point, and was held at this temperature for four hours to permit conditions within the crucible to stabilize.

To commence crystal growth, the rate of helium flow through the heat exchanger was then increased from the initial flow of 40 c.f.h. per hour, at a rate of about 10 c.f.h. per hour, until the flow reached 100 c.f.h. This period of flow increase extended through about 6 hours, during which time the power applied to the furnace was adjusted as required to hold the observed temperature of the crucible side walls constant, at 50C. above the observed initial melting temperature.

For the next stage of crystal growth, which extended through approximately 18 hours, the power applied to furnace was decreased as required to drop the observed temperature of the crucible side walls at a rate of 3 C. per hour, and the rate of helium flow through the heat exchanger was further increased, still at the rate of IO c.f.h. per hour. When the temperature of the crucible side walls fell to a level only slightly above the observed initial melting temperature, substantially all liquid in the crucible had solidified. The only remaining liquid was a very thin and discontinuous miniscus that ran back and forth from side to side over the top of the boule. The miniscus was solidified by continuing to drop the crucible side wall temperature until it was slightly below the initial melting temperature.

After solidification was complete, the flow of helium gas through the heat exchanger was decreased at the rate of 100 c.f.h. per hour. At the same time, the furnace power was decreased at such a rate that, when helium flow through the heat exchanger terminated, the observed temperature of the crucible side walls was about 50C. below the initial melting point. The furnace was then held at this temperature for two hours, after which the power supplied to the furnace was again decreased, at the rate of about 50C. per hour, until it reached room temperature. The furnace was then opened and the crucible and boule removed therefrom.

EXAMPLE It Single crystal sapphire was grown without a seed crystal. The process of Example I was followed, with the following changes:

a. Sintered alumina pellets were used in lieu of cracked pieces of Verneuil sapphire;

b. ln place of a seed crystal. a molybdenum washer having a restricted upper opening was placed in the center of the crucible bottom;

e. The heat exchanger was not activated until the crucible had been superheated to 50C. above the initial melting point; and

d. The heat exchanger was then activated at a flow rate of 50 c.f.h.

EXAMPLE Ill To grow a metal (germanium) single crystal, the inside of a high purity graphite crucible was machined very smooth. Since germanium is lighter in the solid than in the liquid, the inside center bottom of the eruci ble was formed in such a way as to hold the seed crystal in place over the heat exchanger and prevent it from floating away when the germanium was melted.

A thin wafer of metal was placed on the top of the heat exchanger, the seed and pieces of germanium placed in the crucible, and the crucible placed in the furnace.

The furnace was then closed and heated, as in Exam ple l, to 50C. over the observed melting point. Crystal growth was accomplished as in Example I. except that the rate of helium flow through the heat exchanger was increased at a slower rate, about 5 c.f.h., since the thermal conductivity and diffusivity of germanium are much greater than those of sapphire. The helium flow through the heat exchanger and power input to heating element 26 were both varied so that the rate of temperature decrease of the heat exchanger and furnace, respectively, did not exceed 50C. per hour and 5C. per hour.

EXAMPLE lV It is often desirable to grow single crystals of socalled 3/5 compounds. One of the most difficult to grow is gallium phosphide, which is extremely unstable at its melting point. To prevent it from dissociating, an inert gas pressure of 35 atmospheres and a liquid encapsulant of B 0 are required.

Because of the high pressure requirements, the apparatus of FIGS. 1-2 was somewhat modified. The heating element and heating changer were placed in a high pressure graphite resistance furnace. In lieu of sight hole assemblies 70, 71, Platinum/Platinum Rodium thermocouples were provided at the crucible wall near its bottom, and centralized in the furnace heat zone. Heat exchanger 32 was made thick walled to withstand the high pressure.

The gallium phosphide seed crystal was attached to the bottom of a quartz crucible, the crucible filled with gallium phosphide pieces in a manner similar to the loading of sapphire in Example 1. and a B 0 encapsulant placed in the crucible.

The entire system was placed in a block house, and the thermocouple wires and control leads were taken to a remote control station. Since visual inspection is not necessary, remote TV monitoring is not needed.

The material in the crucible was melted and subsequently solidified as in Example 1. The rate of temperature decrease of the furnace and heat exchanger were controlled so as not to exceed, respectively. 10 and 75C. per hour.

EXAMPLE V Stoichiometric MgAl O, spinel single crystals were grown from the melt. A single crystal disc 2.5 cm. in diameter was used as a seed crystal, and a molybdenum crucible was filled with mixture of high purity alumina crackle and high purity magnesia chips in the correct proportions.

The crucible and material were heated to 1500C. in a vacuum of0.02 Torr. and the furnace was then backfllled with inert gas at an overpressure of 0.3 atmospheres. The overpressure was maintained throughout the crystal growth process, which proceeded as in Example 1.

EXAMPLE VI A seed crystal was placed in a crucible as in Example 1. A molybdenum screen and a molybdenum plate were placed vertically in the crucible, on opposite sides of the seed crystal, with the plate extending generally radially and the screen generally perpendicular to the plate. The crucible was then filled with cracked pieces of sapphire, and crystal growth accomplished as in Example I.

The sapphire grew through the screen without change in crystal orientation. A good bond existed between the sheet and the sapphire crystal, and there was no cracking or change in orientation.

Other embodiments and examples will be within the scope of the following claims.

What is claimed is:

1. 1n the process for growing single crystals including the steps of placing material in a crucible, heating the crucible to above the melting point of the material to melt the material therein. and thereafter solidifying the melted material by extracting heat from a bottom portion of the crucible. that improvement comprising:

controlling the heating of said crucible and the extracting of heat from said bottom portion of said crucible so that the temperature of at least those portions of the crucible side walls that are in contact with the material within said crucible is maintained above the melting point of the material within said crucible until substantially all the material within said crucible has been solidified while simultaneously reducing the temperature of said bottom portion below said melting point of the material.

2. The process of claim 1 including an initial period of solidification wherein said temperature of said side wall portions is maintained substantially constant, and said temperature of said bottom portion is reduced at substantially a constant rate.

3. The process of claim 2 wherein during said initial period said temperature of said side wall portions is maintained at a level not less than about 50C. above said melting point.

4. The process of claim 2 including a period of solidification subsequent to said initial period wherein said temperature of said side wall portions is reduced at a substantially constant rate. and said temperature of said bottom portion is further reduced at substantially a constant rate.

LII

5. The process of claim 2.wherein said rate is not more than C. per hour- 6-. The process of claim 4 wherein said temperature of said side wall portions is reduced at a rate of not greater than 15C. per hour and said temperature. of said bottom portion is reduced at a rate of not greater than 100C. per hour.

7. The process of claim 1 wherein said heat is extracted from a central bottom portion of said crucible by placing the top of a gaseous flow heat exchanger in engagement with said bottom portion and passing gase ous fluid through said heat exchanger.

8. The process of claim 1 wherein said crucible is cy'- lindrical and has an overall diameter not less than twice or more than 10 times the major dimension of said bottom portion.

9. The process of claim 8 wherein the height of said crucible is not less than its radius.

10. The process of claim 1 including the step of decreasing the temperature of said portions to below said melting point after substantially all the material in the crucible has been solidified.

11. The process of claim 1 including the step of placing a seed crystal having a major dimension not less than the major dimension of said bottom portion over and in substantial alignment with said bottom portion, and extracting heat from said bottom portion throughout the period that the temperature of said walls is above said melting point to prevent melting of said seed crystal.

12. The process of claim 1 including the steps of:

placing a circular heat exchange surface in engagement with a central bottom portion of said crucible;

placing pieces of said material within said crucible;

heating said portions of said side walls of said crucible to a temperature not less than about 50C. above said melting point; decreasing the temperature of said heat exchange surface while maintaining the temperature of said portions of said side walls at said temperature not less than about 50C. above said melting point; and

thereafter simultaneously decreasing the temperatures of said heat exchange surface and of said portions of said side walls.

13. The process of claim 12 including the step of decreasing the temperature of said portions of said side walls to below said melting point after substantially all the material in said crucible has been solidified.

14. The process of claim 12 including the steps of placing a seed crystal having a major dimension not less than the diameter of said heat exchange surface over and in alignment with said central bottom portion prior to placing pieces of said material within said crucible, heating said portions of said side walls to about said melting point and commencing extraction of heat from said bottom portion to prevent melting of said seed crystal, and thereafter heating said portions of said side walls to said temperature not less than about 50C. above said melting point while continuing to extract heat from said bottom portion at such a rate as to prevent said melting of said seed crystal.

15. The process of claim 12 wherein said temperature of said heat exchange surface is reduced at a rate of not more than 100C. per hour, and said temperature of said portions of said side walls is reduced at a rate of not more than 15C. per hour.

16. The process of claim wherein said material is a ceramic and said temperatures are reduced at rates of. respectively. not more than 50C. and 10C. per

19. The process of claim 15 wherein said heat exchanger surface is the top of a gaseous flow heat exchanger and said temperature of said heat exchanger surface is reduced by passing cooling gas through said heat exchanger. the rate of flow of said gas being substantially continuously increased to reduce said temperature of said heat exchanger surface at said rate.

20. The process of claim 19 wherein said temperatures of said heat exchanger surface and of said portions are independently controlled.

21. The process of claim 15 wherein said material is sapphire and said temperatures are reduced at rates of.

respectively, about 25C. and 3.()C. per hour.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,898,051

|N\/ ENTOR(S) Frederick id It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 10, line 28, Claim 11, after "said" insert --side--.

Signed and Scaled this twenty-first D ay Of October 1975 [SEAL] A ttes t:

RUTH C. MASON Arresting Officer C. MARSHALL DANN Commissioner oflarents and Trademarks

Claims (21)

1. IN THE PROCESS FOR GROWING SINGLE CRYSTALS INCLUDING THE STEPS OF PLACING MATERIAL IN A CRUCIBLE, HEATING THE CRUCIBLE TO ABOVE THE MELTING POINT OF THE MATERIAL TO MELT THE MATERIAL THEREIN, AND THEREAFTER SOLIDIFYING THE MELTED MATERIAL BY EXTRACTING HEAT FROM A BOTTOM PORTION OF THE CRUCIBLE, THAT IMPROVEMENT COMPRISING: CONTROLLING THE HEATING OF SAID CRUCIBLE AND THE EXTRACTING OF HEAT FROM SAID BOTTOM PORTION OF SAID CRUCIBLE SO THAT THE TEMPERATURE OF AT LEAST THOSE PORTIONS OF THE CRUCIBLE SIDE WALLS THAT ARE IN CONTACT WITH THE MATERIAL WITHIN SAID CRUCIBLE IS MAINTAINED ABOVE THE MELTING POINT OF THE MATERIAL WITHIN SAID CRUCIBLE UNTIL SUBSTANTIALLY ALL THE MATERIAL WITHIN SAID CRUCIBLE HAS BEEN SOLIDIFIED WHILE SIMULTANEOUSLY REDUCING THE TEMPER-
2. The process of claim 1 including an initial period of solidification wherein said temperature of said side wall portions is maintained substantially constant, and said temperature of said bottom portion is reduced at substantially a constant rate.
3. The process of claim 2 wherein during said initial period said temperature of said side wall portions is maintained at a level not less than about 50*C. above said melting point.
4. The process of claim 2 including a period of solidification subsequent to said initial period wherein said temperature of said side wall portions is reduced at a substantially constant rate, and said temperature of said bottom portion is further reduced at substantially a constant rate.
5. The process of claim 2 wherein said rate is not more than 100*C. per hour.
6. The process of claim 4 wherein said temperature of said side wall portions is reduced at a rate of not greater than 15*C. per hour and said temperature of said bottom portion is reduced at a rate of not greater than 100*C. per hour.
7. The process of claim 1 wherein said heat is extracted from a central bottom portion of said crucible by placing the top of a gaseous flow heat exchanger in engagement with said bottom portion and passing gaseous fluid through said heat exchanger.
8. The process of claim 1 wherein said crucible is cylindrical and has an overall diameter not less than twice or more than 10 times the major dimension of said bottom portion.
9. The process of claim 8 wherein the height of said crucible is not less than its radius.
10. The process of claim 1 including the step of decreasing the temperature of said portions to below said melting point after substantially all the material in the crucible has been solidified.
11. The process of claim 1 including the step of placing a seed crystal having a major dimension not less than the major dimension of said bottom portion over and in substantial alignment with said bottom portion, and extracting heat from said bottom portion throughout the period that the temperature of said walls is above said melting point to prevent melting of said seed crystal.
12. The process of claim 1 including the steps of: placing a circular heat exchange surface in engagement with a central bottom portion of said crucible; placing pieces of said material within said crucible; heating said portions of said side walls of said crucible to a temperature not less than about 50*C. above said melting point; decreasing the temperature of said heat exchange surface while maintaining the temperature of said portions of said side walls at said temperature not less than about 50*C. above said melting point; and thereafter simultaneously decreasing the temperatures of said heat exchange surface and of said portions of said side walls.
13. The process of claim 12 including the step of decreasing the temperature of said portions of said side walls to below said melting point after substantially all the material in said crucible has been solidified.
14. The process of claim 12 including the steps of placing a seed crystal having a major dimension not less than the diameter of said heat exchange surface over and in alignment with said central bottom portion prior to placing pieces of said material within said crucible, heating said portions of said side walls to about said melting point and commencing extraction of heat from said bottom portion to prevent melting of said seed crystal, and thereafter heating said portions of said side walls to said temperature not less than about 50*C. above said melting point while continuing to extract heat from said bottom portion at such a rate as to prevent said melting of said seed crystal.
15. The process of claim 12 wherein said temperature of said heat exchange surface is reduced at a rate of not more than 100*C. per hour, and said temperature of said portions of said side walls is reduced at a rate of not more than 15*C. per hour.
16. The process of claim 15 wherein said material is a ceramic and said temperatures are reduced at rates of, respectively, not more than 50*C. and 10*C. per hour.
17. The process of claim 15 wherein said material is a metal and said temperatures are reduced at rates of, respectively, not less than 25*C. and 5*C. per hour.
18. The process of claim 15 wherein after substantially all the material in said crucible has been solidified, the temperature of said crucible is reduced to a level about 50*C. below said melting point, is maintained at said level below said melting point for a predetermineD period, and is thereafter reduced to ambient temperature.
19. The process of claim 15 wherein said heat exchanger surface is the top of a gaseous flow heat exchanger and said temperature of said heat exchanger surface is reduced by passing cooling gas through said heat exchanger, the rate of flow of said gas being substantially continuously increased to reduce said temperature of said heat exchanger surface at said rate.
20. The process of claim 19 wherein said temperatures of said heat exchanger surface and of said portions are independently controlled.
21. The process of claim 15 wherein said material is sapphire and said temperatures are reduced at rates of, respectively, about 25*C. and 3.0*C. per hour.
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