GB1572914A - Process and apparatus for producing multi-doped preformed monocrystals - Google Patents

Description

(54) PROCESS AND APPARATUS FOR PRODUCING MULTI-DOPED PREFORMED MONOCRYSTALS (71) We, PRODUITS CHIMIQUES UGINE KUHLMANN, a French Body Corporate, of 25 Boulevard de l'Amiral Bruix, Paris 16, France, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a process and apparatus for the production of multidoped preformed monocrystals by the hanging drop method.

In this specification, the term "multidoped monocrystal" is used to mean a doped monocrystal in which the quantity or the nature of the doping material, or both, varies or vary along the length of the monocrystal.

Our British Patent No. 1546843 relates to a process for the continuous preparation of a monocrystal of predetermined shape comprising: (a) placing the material serving to produce the monocrystal in a crucible whose bottom part is provided with a die having one or more capillary apertures, each of the capillaries having a height greater than or equal to the height of retention of the molten material in the said capillary at the operative temperature and pressure (b) bringing the said material to a temperature higher than its melting point with the resultant formation of a hanging drop at the lower end of the capillary; (c) bringing the suitably oriented, preformed crystal nucleus into contact with the hanging drop;; (d) moving the nucleus downwards while feeding the said raw material into the crucible at such a rate that the amount of material being introduced per unit of time is at any moment substantially equal to the amount of material drawn out in the form of a monocrystal: and (e) removing the resulting monocrystal at selected intervals of time.

The above-mentioned patent also relates to an apparatus for the continuous production of monocrystals of predetermined shape, comprising: (a) a crucible the bottom part of which is provided with a die having one or more capillary apertures, the axis of which are parallel to that of the crucible, each of the capillaries having a height greater than or equal to the height of retention in the said capillary of the molten material serving to produce the monocrystal, at the temperature and pressure of formation of the monocrystal.

(b) a heating device surrounding the said crucible; (c) a feed system, placed above the crucible, for feeding thereto raw material for the monocrystal; (d) a vessel, provided with a cooling device, surrounding the said crucible and the said feed system and having a bottom aperture permitting the passage of the monocrystal to be formed, apertures permitting the circulation of a gas constituting the working atmosphere, and apertures permitting passage to the heating system; (e) a nucleus carrier system for supporting a preformed monocrystal nucleus; (f) a system permitting the upward and downward displacement of the said nucleus carrier and its rotation about its own axis; and (g) a regulation system for relating the rate at which the monocrystal formed is drawn to the rate of feed of the material serving to produce the monocrystal.

If the composition of the raw material used to produce the monocrystal remains identical during the course of the production, the monocrystal obtained is one which has an identical composition over its entire length.

An object of the present invention is to obtain monocrystals having different compositions at different parts of their length. This is achieved in accordance with the present invention by varying the composition of the raw material which is used to produce the monocrystal during the course of the formation of the monocrystal.

This variation of the composition of the raw material for producing the monocrystal is effected in accordance with the present invention by using a multiple feed system comprising two or more reservoirs from which raw materials of different compositions are simultaneously or successively fed to a crucible where the raw material is melted.

Accordingly, the present invention provides, in one aspect, a process for the continuous preparation of a multi-doped preformed monocrystal, which process comprises: (a) feeding raw material for producing the monocrystal into a crucible having at its bottom part a capillary passage of sufficient length to allow a drop of molten raw material to remain suspended from the lower aperture of the capillary passage, the composition of the raw material which is fed to the crucible being varied in a predetermined and controllable manner throughout the process so as to obtain at the product of step (e) below a doped preformed monocrysral in which the quantity or nature of the doping material, or both, varies or vary along the length of the monocrystal, (b) bringing the raw material to a temperature higher than its melting point with the resultant formation of a drop suspended from the lower end of the capillary passage; (c) bringing a suitably oriented preformed crystal nucleus into contact with the drop formed at the lower end of the capillary passage; (d) drawing the nucleus downwards while feeding raw material into the crucible at such a rate that the amount of raw material fed per unit of time will at any moment be substantially equal to the amount of material withdrawn from the crucible in the form of monocrystal: and (e) removing the monocrystal formed at selected intervals of time.

The raw material for producing the monocrystal is preferably fed in the form of powder or of small pellets.

The process is carried out in a suitable atmosphere depending on the chemical nature of the crystal and of the material of which the crucible is made. For example, it is possible to work in an atmosphere of nitrogen, in an atmosphere of argon free from oxygen, or in air.

The working pressure may be atmospheric pressure or a lower pressure. If necessary it is possible to work in a reduced pressure which may be as low as 10-3 mm Hg.

The temperature to which the raw material for producing the monocrystal is brought must be sufficiently higher than the melting point of that material to ensure that the material is thoroughly melted.

The rate at which the nucleus is drawn downwards is usually between 10 and 500 mm per hour.

According to a further aspect of the present invention, there is also provided an apparatus for the continuous preparation of a multi-doped preformed monocrystal, the apparatus comprising: (a) a crucible having at its bottom part a capillary passage of a sufficient length to allow, when the apparatus is in use, a drop of molten material from which the monocrystal is to be produced to remain suspended from the lower aperture of the capillary passage; (b) A heating device surrounding the said crucible; (c) a feed system, placed above the said crucible and comprising at least two independent reservoirs, for the multiple feeding to the crucible of the raw material for producing the monocrystal; (d) a vessel, provided with a cooling device, surrounding the said crucible and the said feed system and having a bottom aperture for withdrawal of the monocrystal formed;; (e) a nucleus carrier system for supporting a preformed monocrystal nucleus; (f) means for moving the nucleus carrier upwards or downwards and for rotating the carrier about a vertical axis; and (g) means for regulating the speed at which the carrier is moved.

The crucible is made of a material which at the working temperature is chemically inert in relation to the raw material for producing the monocrystal.

The vessel is made of any suitable material, for example quartz. It may also be metallic. The vessel must be cooled in certain cases, particularly for the production of monocrystals of alumina and silicon. The cooling may be effected by any suitable means; for example the vessel may be provided with a jacket in which cooling water is cireulated The capillary aperture has a cross-section of a shape adapted to the shape desired for the section of the monocrystal, being for example circular for a monocrystal in the form of a wire and rectangular for a monocrystal in the form of a plate.

The heating system is composed of any suitable device, for example a resistance heater or a high frequency induction heater equipped with windings adapted to the shape and material of the crucible and working at from 20 to 500 kHz in order to ensure coupling with the materials of the crucible, and developing up to 50 kW.

The multiple feed system makes it possible to obtain at the bottom of the capillary a liquid of variable composition which is adjustable at will.

It must be noted that the small amount of molten material used (a few cubic centimetres) makes it possible to obtain a clear change of the doping of the crystal produced, corresponding to the variation of the composition.

It is thus possible to obtain monocrystals having different doping or compositions along their lengths. For example, a band crystal of silicon can be obtained which is of N type over part of its length and then of P type simply by having two reservoirs filled with suitable doped silicon powders to produce P type silicon in one case and N type silicon in the other case.

A crystal the doping of which varies over its entire length in accordance with a predetermined. programme can also be obtained.

For example, it is possible to obtain a laser ruby crystal with chromium doping increasing progressively and linearly throughout the length of the crystal beginning at one end of the crystal and terminating at the opposite end of the crystal.

Fir example, in one process according to the invention, a first feed system is installed for also powder doped with Cr2O3 in such a manner as to obtain 200 ppm by weight of Cr in relation to A1203 in the crystal and a second feed system is installed for A12O3 powder more heavily doped with Cur203.

During the crystallisation the programme of the two feeding systems is arranged for the following operations: a start is made with the first feed the beginning of the crystal contains 200 ppm of Cr/AI203 the second feed is progressively brought into operation so that the chromium content increases linearly. Care is taken that the total amount of Al203 powder supplied into the crucible remains constant, that is to say that the flow of the first feed decreases in proportion as the flow of the second is increased; at the end of the crystallisation a crystal is obtained which contains for example 100 ppm of Cr/Al2O3.

The same result can be obtained with two feeds, one charged with pure AlO3 powder and the other with A12O3-Cr2O3 powder, their mixing being suitably programmed.

The present invention, which makes it possible to obtain variable chemical and/or physical substances on one and the same monocrystalline support, finds numerous applications, particularly in electronics.

The following Examples illustrate the invention.

EXAMPLE 1 Production of a monocrystal of sapphire and ruby A crucible of iridium of a total volume of 20 cc, and having its bottom part pierced by a capillary passage of rectangular section (size lox 15 mm), is enclosed in a vessel flushed by a current of argon without oxygen. With the aid of a first distributor reservoir the crucible is fed with pure alumina monocrystalline pellets of a diameter of 0.05 to 0.1 mm, or with a pure alpha-alumina powder having particles of a diameter of from 5 to 50 ,u. The crucible is brought to a temperature of 2075+10 C (melting point of alpha-alumina=2050 C) with the aid of a high frequency generator working at 20 kHz and developing a permanent power of 20 kW.Once a drop has been formed at the bottom of the capillary passage, there is brought into contact with it a thin, previously oriented monocrystalline plate whose dimensions are lox 15 mm and which serves as nucleus, and once the drop has been bonded to this nucleus the latter is pulled downwards at a speed of 30 cm per hour. At the same time the crucible is fed with alumina at the rate of 18 grammes per hour. After pulling for 10 minutes a thin sapphire plate is obtained of the approximate dimensions It 15 mm, with a rectangular section, and with a length of 50 mm.

At the moment the feeding of pure alumina is stopped and at the same time the crucible is fed by a second distributor reservoir which contains alpha-alumina powder doped with 500 parts per million of chromium in relation to the weight of alumina. The pulling of the plate is continued at the same speed and at the end of 10 minutes the plate has a total length of 100 mm. This plate is transparent and monocrystalline, with the first part colourless and the second part pink and with a zone of separation between the two parts of a few millimetres which passes progressively from colourless to pink.

EXAMPLE 2 Production of a monocrystal of Si of type N & P A crucible of sintered silicon carbide of a total volume of 20 cc and having its bottom part pierced by a capillary passage of rectangular section (lx15 mm), is enclosed, together with a feed system, in a quartz vessel flushed by a current of argon without oxygen. The crucible is fed with silicon powder of high purity doped with boron, of a granulometry of 0.1 to I mm, and is brought to a temperature of 1500"C+10"C (melting point of Si: 470"C) with the aid of a high frequency generator working at 300 kHz and permanently developing a power of 10 kW.There is brought into contact with the drop formed at the bottom end of the capillary passage an oriented silicon plate which serves as nucleus, its dimensions being 1 x 15 mm, and the drop is bonded to this nucleus. The nucleus is then pulled down at the rate of 50 cm per hour and at the same time the crucible is fed at a mean rate of 19 grammes per hour of silicon powder. After pulling for 10 minutes a silicon plate is obtained whose section is about 1x15 mm and whose length is about 75 mm.

At the moment the feeding with boron doped silicon powder is stopped and at the same time the crucible is fed by a second distributor reservoir containing silicon powder doped with phosphorus. The pulling of the plate is continued at the same speed and at the end of 10 minutes the plate attains a total length of 150 mm. This plate is monocrystalline with a first boron doped part (type P) and a second phosphorus doped part (type N). The zone of separation between the two parts is a few millimetres in length, in which the transition from boron doping to phosphorus doping occurs.

Our application No. 4243/77 (Serial No.

1572915) describes and claims a process and apparatus for producing monocrystalline plates.

WHAT WE CLAIM IS: 1. A process for the continuous preparation of a multi-doped preformed monocrystal, which process comprises: (a) feeding raw material for producing the monocrystal into a crucible having at its bottom part a capillary passage of sufficient length to allow a drop of molten raw material to remain suspended from the lower aperture of the capillary passage, the composition of the raw material which is fed to the crucible being varied in a predetermined and controllable manner throughout the process so as to obtain as the product of step (e) below a doped preformed monocrystal in which the quantity or nature of the doping material, or both, varies or vary along the length of the monocrystal, (b) bringing the raw material to a temperature higher than its melting point with the resultant formation of a drop suspended from the lower end of the capillary passage;; (c) bringing a suitably oriented preformed crystal nucleus into contact with the drop formed at the lower end of the capillary passage; (d) drawing the nucleus downwards while feeding raw material into the crucible at such a rate that the amount of raw material fed per unit of time will at any moment be substantially equal to the amount of material withdrawn from the crucible in the form of monocrystal; and (e) removing the monocrystal formed at selected intervals of time.

2. A process according to Claim 1, in which the composition of the raw material is varied by using a multiple feed system comprising two or more reservoirs from which raw materials of different composition can be fed to the crucible.

3. A process according to Claim 2, in which raw material is fed simultaneously from each reservoir at progressively different rates.

4. A process according to Claim 2, in which raw material is fed successively from each reservoir.

5. A process according to Claim 1, substantially as described in the foregoing Example 1 or 2.

6. A multi-doped monocrystal prepared by a process as claimed in any one of Claims 1 to 5.

7. An apparatus for the continuous preparation of a multi-doped preformed monocrystal, the apparatus comprising: (a) a crucible having at its bottom part a capillary passage of a sufficient length to allow, when the apparatus is in use, a drop of molten material from which the monocrystal is to be produced to remain suspended from the lower aperture of the capillary passage; (b) a heating device surrounding the said crucible; (c) a feed system, placed above the said crucible and comprising at least two independent reservoirs, for the multiple feeding to the crucible of the raw material for producing the monocrystal; (d) a vessel, provided with a cooling device, surrounding the said crucible and the said feed system and having a bottom aperture for withdrawal of the monocrystal formed; (e) a nucleus carrier system for supporting a preformed monocrystal nucleus; ; (f) means for moving the nucleus carrier upwards or downwards and for rotating the carrier about a vertical axis; and (g) means for regulating the speed at which the carrier is moved.

8. Apparatus according to Claim 7 substantially as hereinbefore described.

9. Apparatus according to Claim 7

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   powder of high purity doped with boron, of a granulometry of 0.1 to I mm, and is brought to a temperature of 1500"C+10"C (melting point of Si: 470"C) with the aid of a high frequency generator working at 300 kHz and permanently developing a power of 10 kW. There is brought into contact with the drop formed at the bottom end of the capillary passage an oriented silicon plate which serves as nucleus, its dimensions being 1 x 15 mm, and the drop is bonded to this nucleus. The nucleus is then pulled down at the rate of 50 cm per hour and at the same time the crucible is fed at a mean rate of 19 grammes per hour of silicon powder.After pulling for 10 minutes a silicon plate is obtained whose section is about 1x15 mm and whose length is about 75 mm.

   At the moment the feeding with boron doped silicon powder is stopped and at the same time the crucible is fed by a second distributor reservoir containing silicon powder doped with phosphorus. The pulling of the plate is continued at the same speed and at the end of 10 minutes the plate attains a total length of 150 mm. This plate is monocrystalline with a first boron doped part (type P) and a second phosphorus doped part (type N). The zone of separation between the two parts is a few millimetres in length, in which the transition from boron doping to phosphorus doping occurs.

   Our application No. 4243/77 (Serial No.

   1572915) describes and claims a process and apparatus for producing monocrystalline plates.

   WHAT WE CLAIM IS: 1. A process for the continuous preparation of a multi-doped preformed monocrystal, which process comprises: (a) feeding raw material for producing the monocrystal into a crucible having at its bottom part a capillary passage of sufficient length to allow a drop of molten raw material to remain suspended from the lower aperture of the capillary passage, the composition of the raw material which is fed to the crucible being varied in a predetermined and controllable manner throughout the process so as to obtain as the product of step (e) below a doped preformed monocrystal in which the quantity or nature of the doping material, or both, varies or vary along the length of the monocrystal, (b) bringing the raw material to a temperature higher than its melting point with the resultant formation of a drop suspended from the lower end of the capillary passage;; (c) bringing a suitably oriented preformed crystal nucleus into contact with the drop formed at the lower end of the capillary passage; (d) drawing the nucleus downwards while feeding raw material into the crucible at such a rate that the amount of raw material fed per unit of time will at any moment be substantially equal to the amount of material withdrawn from the crucible in the form of monocrystal; and (e) removing the monocrystal formed at selected intervals of time.
2. 2. A process according to Claim 1, in which the composition of the raw material is varied by using a multiple feed system comprising two or more reservoirs from which raw materials of different composition can be fed to the crucible.
3. 3. A process according to Claim 2, in which raw material is fed simultaneously from each reservoir at progressively different rates.
4. 4. A process according to Claim 2, in which raw material is fed successively from each reservoir.
5. 5. A process according to Claim 1, substantially as described in the foregoing Example 1 or 2.
6. 6. A multi-doped monocrystal prepared by a process as claimed in any one of Claims 1 to 5.
7. 7. An apparatus for the continuous preparation of a multi-doped preformed monocrystal, the apparatus comprising: (a) a crucible having at its bottom part a capillary passage of a sufficient length to allow, when the apparatus is in use, a drop of molten material from which the monocrystal is to be produced to remain suspended from the lower aperture of the capillary passage; (b) a heating device surrounding the said crucible; (c) a feed system, placed above the said crucible and comprising at least two independent reservoirs, for the multiple feeding to the crucible of the raw material for producing the monocrystal; (d) a vessel, provided with a cooling device, surrounding the said crucible and the said feed system and having a bottom aperture for withdrawal of the monocrystal formed; (e) a nucleus carrier system for supporting a preformed monocrystal nucleus; ; (f) means for moving the nucleus carrier upwards or downwards and for rotating the carrier about a vertical axis; and (g) means for regulating the speed at which the carrier is moved.
8. 8. Apparatus according to Claim 7 substantially as hereinbefore described.
9. 9. Apparatus according to Claim 7

   substantially as described in the foregoing Example 1 or 2.
10. 10. A multi-doped monocrystal produced using a device as claimed in any one of Claims 7 to 10.