US3634045A

US3634045A - Growing of crystals using electron beam heating and annealize

Info

Publication number: US3634045A
Application number: US720113A
Authority: US
Inventors: Ronald Arthur Dugdale; Stanley Denis Ford
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1967-04-14
Filing date: 1968-04-10
Publication date: 1972-01-11
Anticipated expiration: 1989-01-11
Also published as: GB1227331A

Abstract

A method and apparatus for growing a crystal. The method comprising, positioning the crystal and providing material similar to the material of said crystal in a region of intense electron heating whereby said material and a part of said crystal is melted and fuses together, withdrawing the crystal from the region of intense electron heating so as to promote solidification and growth of the crystal, and passing said crystal through a region of less intense electron heating to anneal said crystal.

Description

United States Patent [72] Inventors Ronald Arthur Dugdale [56] References Cited Blewbury, D idcot; UNITED STATES PATENTS 'i' 2,858,199 10/1958 Larson 23 301 21 A l N f' 2,999,737 9/1961 Siebertz 23/301 f A 1968 3,131,051 4/1964 Hanksetal. 13/31 5 Pzifemed g? *1972 3,267,529 8/1966 Gruber 13/31 3,343,828 9/1967 Hunt 13/31 [731 Assgnee fg 'gg gf zr Energy 3,377,419 4/1968 Schiller et al. 13/31 [32] Priority Apr. 14: 1967 Primary Examiner-Wilbur L. Bascomb, Jr. [33] Great Britain Assistant ExaminerR. T. Foster [31] 17,350/67 Anorney- Larson, Taylor and Hinds [54] GROWING 0F CRYSTALS USING ELECTRON ABSTRACT: A method and apparatus for growing a crystal. BEAM HEATING AND ANNEALIZE The method comprising, positioning the crystal and providing 5 Claims, 2 Drawing Figs. material similar to the material of said crystal in a region of intense electron heating whereby said material and a part ofsaid [52] U.S.Cl crystal is melted and fuses together withdrawing the crystal 51 I t Cl from the region of intense electron heating so as to promote 1 n 9 o fic t n and growth of the crystal, an passing Said 5 M is h B0 1 17/24 crystal through a region ofless intense electron heating to an- 0] 1e 0 earc 2732382311, nealsaid crysxal.

M 72 6A5 SUPPLY \73 HEATER POWER SOURCE 77 4 34 35 aw? 33 79 MEL 7' ER 78 CATHODE 77 POWER SOURCE 7 4 22 27 36 x 76 -20 AIV/VEALER 4 14 CATHRODE a 14 POWER c 35 37 SOURCE i 2 PATENTED JAN] 1 m2 4 5 SHEET 1 [IF 2 HEA TER POWER RCE 77 MEL TE R CA THOOE POWER SOURCE A/V/VEAL ER CA THROOE PO WE R SOURCE GROWING OF CRYSTALS USING ELECTRON BEAM HEATING AND ANNEALIZE This invention relates to the growing of crystals.

More particularly the invention relates to the growing of crystals of refractory materials, for example sapphire, using a cold cathode, glow discharge device. The term cold cathode when used in this specification signifies that the cathode is nonthermionic.

At the present time the demand for crystals of refractory materials is increasing, because wafers cut from such crystals are required as substrates for microcircuits, and large crystals are required for lasers.

It is therefore an object of the present invention to provide a new or improved method of growing crystals of materials.

According to the present invention, a method of growing crystals comprises, positioning the crystal and providing material similar to the material of said crystal in a region of intense electron heating whereby said material and a part of said crystal is melted and fuses together, withdrawing the crystal from the region of intense electron heating so as to promote solidification and growth of the crystal, and passing said crystal through a region of less intense electron heating to anneal said crystal.

According to a feature of the invention, a method of growing a crystal comprises, positioning the crystal in a region of intense electron heating whereby a part of said crystal is melted, introducing to said crystal material similar to the material of said crystal whereby said material is melted and fuses with the molten part of said crystal, withdrawing the crystal from the region of intense electron heating so as to promote solidification and growth of the crystal, and passing said crystal through a region of less intense electron heating to anneal said crystal.

According to a further feature of the invention, a method of growing a crystal comprises, positioning material similar to the material of said crystal in a region of intense electron heating thereby forming a pool of molten material, bringing the crystal into contact with the molten material in the region of intense electron heating whereby a part of said crystal is melted and fuses with said molten material, withdrawing the crystal from said molten material to promote solidification and crystal growth, and passing the crystal through a less intense electron heating to anneal the crystal.

According to a further feature of the invention an apparatus for growing a crystal comprises an enclosure; means to maintain the enclosure at a low gas pressure; an electrode arrangement including an anode mounted within or forming at least part of aninterior wall of the enclosure, and first and second cathodes mounted within the enclosure; means to apply electrical potentials to the electrode arrangement such that during operation a glow discharge occurs, a region of intense electron heating is set up in association with the first cathode and a region of less intense electron heating is set up in association with the second cathode; a hearth; means to position the hearth in the region of intense heating; and means to supply material similar to the material of said crystal to the region of intense heating, the arrangement being such that the said material and a part of said crystal is melted in the region of intense heating, and when said crystal is moved out of the region of intense heating solidification occurs and the crystal grows on the hearth and as the growth progresses the crystal is withdrawn through the region of less intense heating whereby the grown crystal is annealed.

According to a further feature of the invention an apparatus for growing a crystal comprises an enclosure; means to maintain the enclosure at a low gas pressure; and electrode arrangement including an anode mounted within or forming at least part of an interior wall of the enclosure; and first and second cathodes mounted within the enclosure; means to apply electrical potentials to the electrode arrangement such that during operation a glow discharge occurs, a region of intense electron heating is set up in association with the first cathode and a region of less intense electron heating is set up in association with said second cathode; a hearth adapted to hold material similar to the material of said crystal in the region of intense heating; and moving means to move the said crystal into contact with the material in said hearth, to move said crystal out of the region of intense heating to promote solidification and crystal growth and to move the crystal through the region of less intense heating to anneal the crystal,

Preferably the first cathode is of hallow hemispherical or part spherical shape, the region of intense heating being at the center of the sphere of which the cathode forms a part, and the second cathode is of hollow, right-circular, frustoconical shape, the region of less intense heating being around the axis of the cone.

It is to be understood that the methods and apparatus described above may be applied to growing crystals of refractory materials.

The crystal grown may be a sapphire.

The invention will now be described with reference to the accompanying drawings in which:

FIG. 1 shows diagrammatically an apparatus for growing crystals according to the present invention, and

FIG. 2 shows diagrammatically a second form of apparatus for growing crystals according to the present invention.

The apparatus and methods of carrying the invention into effect will be described as used for growing sapphire crystals, but it will be appreciated that the invention is applicable to the growing of crystals of other refractory materials.

Referring to FIG. 1 of the drawings, the apparatus comprises an electrode structure 1 which in operation produces a cold cathode, glow discharge, a hearth 2 on which the crystal 3 is grown, and a mechanism 4 to feed powdered alumina to the hearth 2.

The electrode structure 1 and the hearth 2 are contained within a tubular metal vessel 5 which has a bore of 6 inches and which is encircled by a coil 6 through which cooling water is pumped during operation. The vessel 5 is oriented as shown in the drawing and the top and bottom ends are closed by

earthed metal plates

7 and 8 respectively. An outlet tube 9 passes through the side of the vessel 5 to a vacuum pump 10. During operation the pump 10 operates continuously and the required pressure is maintained in the vessel 5 by leaking gas by way of the top portion 4 into a metal tube 11 which passes through the plate 7. The gas is derived from a supply 12 and the flow is controlled by a valve 13.

The electrode structure 1 comprises a hollow hemispherical cathode 14 having a central aperture to the periphery of which the lower end of the tube I] is secured, and a hollow, right-circular, frustoconcial-shaped cathode 15 positioned below, and symmetrically with respect to, the cathode l4.

Wound on the outside of the cathode 14 is a heater element 16 across which can be connected a heater power supply [7. Connection between the power supply 17 and the element 16 is by way of a lead 18 which has bead insulation and which passes through the top portion 4 and the tube I1. Power for the cathode 14 is derived from a power source 19 which is capable of supplying up to 0.5 amp. at a negative potential of l to 5 kilovolts. Connection is made from the source 19 to the cathode 14 by way of the tube 11. To prevent unwanted discharges the outside of the cathode 14 is surrounded by an earthed wire mesh screen 20 held in place by a metal tube 21 which is secured to a metal plate 22 secured to the underside of the plate 7. The plate 22 has the additional functions of lengthening the path for the conduction of heat and of acting as a radiation shield.

The cathode 15 is carried by three supports 23 which extend from an apertured plate 24 which in turn is secured to supports 25 on the wall of the vessel 5. Power for the cathode 15 is derived from a power source 26, similar to the source 19, connection being made by way of a screened lead 27 which passes through the wall of the vessel 5.

The

cathodes

14 and 15, and the screen 20 are made of stainless steel, although other materials, such as mild steel, may be used,

The hearth 2 is a depression formed in the top of a ceramic cylinder 28 carried on a metal tube 29 which slides within a metal tube 30 that passes through the plate 8, and is sealed at its lower end The hearth 2 can be raised and lowered by rotation ofa threaded rod 31 by a motor 32.

The powder feed mechanism 4 comprises an Archemedian screw device 33 driven by a motor 34. The operation is such that an open end of the screw 33 rotates within a reservoir 35 of powdered or granulated alumina, and as a result alumina is fed at a controllable rate to the tube II and thence to a tube 36 from which it falls onto a hearth 2.

Spaced at intervals around the walls of the vessel are silica windows 37 through which the operation can be observed.

The operation of the apparatus of FIG. 1 is as follows.

A small seed crystal 3 having been placed in the hearth 2, the pump and valve 13 are operated to bring the air pressure within the vessel 5 to a value within the range 30 to I00 microns Hg. (Other gases, such as hydrogen or helium can be used, and in these cases the pressure is somewhat higher). The cathode I4 is then heated to about 300 C. using the heater element 16, which is then disconnected.

A glow discharge is then initiated within the cathode 14 by switching on and controlling the source 19. While this is done the cathode is isolated so that electrons from the glow discharge strike the cathode l5 and heat it. This preliminary heating of the cathodes l4 and 15 is necessary to prevent arcing while the crystal 3 is being grown. The minimum necessary temperature appears to be about 200 C.

When the cathode 15 has been heated to the predetermined temperature it is connected to the source 26 which is switched on and controlled such that the glow discharge extends to the region within the cathode 15. Within the cathode 14 are streams of electrons (indicated by the broken lines 38) which tend to be focused by the electric field associated with the cathode 14 in a region at the center of the sphere of which the hemispherical cathode 14 forms a part. Within the cathode 15 are streams of electrons (indicated by the broken lines 39) which converge on the axis of the cathode 15 The hearth 2, which until now has been kept below the cathode 15, is then raised to bring the seed crystal 3 to the region of intense electron heating at the focus of the stream of electrons associated with the cathode 14, and the powder feed mechanism 4 is started. The intense heating causes localized melting of the seed crystal 3 and of the powdered alumina. The motor 32 is operated so that the hearth 2 is drawn slowly downwards away from the region of intense heating. As this is done, therefore, the seed crystal 3 grows by the addition of melted material. Furthermore, as the crystal 3 is withdrawn it passes through a region of less intense electron heating where it is heated by electrons associated with the cathode 15. This heating is controlled, by adjustment of the source 26, such that the grown crystal 3 is annealed. The operation is continued until a crystal of the required size has been grown.

Referring to FIG. 2, there is shown a second form of apparatus which in many respects is similar to that shown in FIG. 1.

Where a component of the apparatus of FIG. 2 is similar to that of FIG. 1, the same reference number has been used.

The apparatus of FIG. 2 differs from that of FIG. 1 in that the hearth 2 is replaced by a ceramic tray 40 which in operation of the apparatus contains molten alumina. Also the internal bore of the tube 11 has been increased and a axially movable rod 41 having an end portion 42 shaped to receive and hold a seed crystal 43 is suspended in the space beneath the concave surface of the cathode 14. The radius of curvature of the cathode 14 of FIG. 2 has been increased and the cathode takes the shape of a segment of a hollow sphere. The cathode 15 of FIG. 2 has been positioned nearer to the cathode 14 so that the resulting beam of electrons 39 from the cathode l5 cross the path of the beam of electrons 38 from the cathode 14. Some of the electrons 39 collide with some of the electrons 38 and cause ionization locally within the beam of electrons 38; this tends to make more ions bombard the cathode 14 with the result that more electrons are released from the cathode l4 and thus the power of the beam of electrons 38 is increased.

A further modification to the apparatus of FIG. 2 is that the lead 18 supplying the heater 16 from the power source 17 is inserted through an insulating bush 44 in the wall of the vessel 5 instead of being passed down through the inner bore of the tube 11.

The operation of the apparatus of FIG. 2 is as follows.

Powdered alumina is placed in the tray 40 and a small seed crystal 43 is attached to the end 42 of the rod 41. The pump 10 and valve 13 are then operated to bring the air pressure within the vessel 5 to a value within the range of 30 to I00 microns HG. (Other gases, such as hydrogen or helium can be used, and in these cases the pressure is somewhat higher). The cathode I4 is then heated to about 300 C. using the heater element 16, which is then disconnected.

A glow discharge is then initiated within the cathode 14 by switching on and controlling the source 19. Whilst this is done the cathode 15 is isolated so that electrons from the glow discharge strike the cathode l5 and heat it. The preliminary heating of the cathodes l4 and I5 is necessary to prevent arcing while the crystal 43 is being grown.

When the cathode 15 has been heated to the predetennined temperature it is connected to the source 26 which is switched on and controlled such that the glow discharge extends to the region within the cathode 15. The electrons (denoted by the broken lines 38) leaving the cathode 14 converge at one point, and the tray 40 containing the powdered alumina, which up until now has been kept below the cathode I5, is then raised by operation of the motor 32 to bring the powdered alumina in the path of the beam of electrons 38.

When the powdered alumina has been melted by the electron beam from the cathode 14, the rod 41 is lowered and the seed crystal 43 is immersed in the molten alumina. The intense heating causes localized melting of the seed crystal and maintains the alumina in the tray 40 in molten form. The rod 41 and seed crystal 43 is slowly withdrawn from the molten alumina in the tray 40 resulting in solidification of the molten alumina and thus causing growth of the crystal. As the seed crystal 43 is withdrawn from the molten alumina it passes through the beam of electrons (denoted by the dotted lines 39) leaving the cathode l5 and is annealed.

The powder feed mechanism is selectively operated to maintain a constant supply of molten alumina in the tray 40.

The annealing heat applied to the crystal by the electrons leaving the cathode l5 and which converge on the axis of the frustoconical cathode 15 is controlled by adjustment of the power source 26.

The operation of the apparatus is continued until a crystal of the required size has been grown.

We claim:

1. Apparatus for growing a crystal comprising:

an enclosure;

means for maintaining a low gas pressure in said enclosure;

means supplying feedstock material to a melt zone;

an electrode arrangement in said enclosure comprising an anode and first and second cathodes, said first cathode being hemispherical and directing an electron beam to point concentration in the melt zone as an intense zone of electron beam heating;

a hearth located in said enclosure and moveable from a first position along an axis with the hearth located in said first zone of electron beam heating and a second position with the hearth located remote from said first zone of electron beam heating; and

means for moving said hearth from said first position to said second position, said second cathode being a conical section and directing an electron beam of less intensity than in said first zone and concentrating along the axis, the second cathode being stationary with respect to hearth movement, whereby said hearth moves through said second zone of electron beam heating on movement of said hearth from said first position to said second position, means for applying electrical potentials to the electrode arrangement to provide said first and second zones of electron beam heating; said first zone of electron beam heating being sufficiently intense to melt a crystal-forming material located in said first zone, said second zone of electron beam heating being sufficiently intense to anneal a grown crystal located in said second zone, said second zone being separate from said first zone.

2. An apparatus according to claim 1 wherein said first cathode is in the shape of a segment of a hollow sphere, the first zone of electron beam hearing being at the center of the sphere of which the first cathode forms a part.

3. An apparatus according to claim 1 wherein the said second cathode is a hollow, right-circular, frustoconical shape, the second zone of electron beam heating being around the axis of the cone.

4. A method of growing a crystal comprising, the steps of positioning a seed crystal in a first region of electron heating which is sufficiently intense to melt a part of the seed crystal, contacting the seed crystal with a feedstock material similar to the material of the seed crystal while holding the seed crystal in the first region of electron beam heating to cause the feedstock material to melt and fuse with the molten part of the seed crystal, withdrawing the seed crystal from the first region of electron beam heating so as to promote solidification and growth of the crystal, and passing the solidified crystal through a second region of electron beam heating which is sufficiently intense to anneal the crystal but not intense enough to melt the crystal.

5. A method of growing a crystal comprising positioning a feedstock material similar to the material of said crystal in a first region of electron beam heating to form a pool of molten material, bringing a seed crystal into contact with the molten material in the first region of electron heating to melt a part of the seed crystal and fuse the seed crystal and the molten feedstock material, withdrawing the crystal from said molten feedstock material to promote solidification and crystal growth, and passing the crystal through a second region of electron heating which is sufficiently intense to anneal the crystal but not intense enough to melt the crystal.

Claims

2. An apparatus according to claim 1 wherein said first cathode is in the shape of a segment of a hollow sphere, the first zone of electron beam hearing being at the center of the sphere of which the first cathode forms a part.
3. An apparatus according to claim 1 wherein the said second cathode is a hollow, right-circular, frustoconical shape, the second zone of electron beam heating being around the axis of the cone.
4. A method of growing a crystal comprising, the steps of positioning a seed crystal in a first region of electron heating which is sufficiently intense to melt a part of the seed crystal, contacting the seed crystal with a feedstock material similar to the material of the seed crystal while holding the seed crystal in the first region of electron beam heating to cause the feedstock material to melt and fuse with the molten part of the seed crystal, withdrawing the seed crystal from the first region of electron beam heating so as to promote solidification and growth of the crystal, and passing the solidified crystal through a second region of electron beam heating which is sufficiently intense to anneal the crystal but not intense enough to melt the crystal.
5. A method of growing a crystal comprising positioning a feedstock material similar to the material of said crystal in a first region of electron beam heating to form a pool of molten material, bringing a seed crystal into contact with the molten material in the first region of electron heating to melt a part of the seed crystal and fuse the seed crystal and the molten feedstock material, withdrawing the crystal from said molten feedstock Material to promote solidification and crystal growth, and passing the crystal through a second region of electron heating which is sufficiently intense to anneal the crystal but not intense enough to melt the crystal.