GB2279585A

GB2279585A - Crystallising molten materials

Info

Publication number: GB2279585A
Application number: GB9314109A
Authority: GB
Inventors: David Ainsworth Hukin; Barry John Garrard
Original assignee: CRYSTALOX Ltd
Current assignee: CRYSTALOX Ltd
Priority date: 1993-07-08
Filing date: 1993-07-08
Publication date: 1995-01-11
Anticipated expiration: 2013-07-08
Also published as: GB9314109D0; GB2279585B

Abstract

A melt, of eg silicon, can be crystallised uniaxially in an insulated vessel (22) supported on a rotary table (24) that can be raised and lured relative to an induction heater in a furnace. A variable thermal baffle or shutter (50) is located in the table in the heat path between the vessel and a water cooled heat sink (30). The shutter is a fixed apertured insulating disc (52) over a similar rotary apertured insulating disc (51), which is turned to adjust the overlap of the respective apertures (58, 58', 59, 59') and maximise the insulation doing the melting stage, reduce it during the nucleation stage and again during the crystallisation stage, increase it during annealing, and reduce it to a minimum during final cooling to ambient temperature. <IMAGE>

Description

Cl1EniULLISING MOLTEN Nt3ESIA15 This invention relates to crystallising molten materials, and is particularly applicable to use in growing crystals of silicon or the like. A particular use of the invention is in the formation of large photovoltaic multi-crystalline silicon ingots.

During the growth of crystals by the Bridgman process, the material to be crystallised is first melted in a suitable vessel (crucible) in an insulating enclosure. A thermal gradient is then applied to the vessel, by cooling it locally, by translating it so that it is offset fram the heat source used for melting, or by a combination of these methods, in order to induce one or more crystals to nucleate in the bottom of the vessel. Continuation of the cooling in the same region causes further growth of the nucleated crystals.

In its particular application to the growth of multi-crystalline silicon for use in the photovoltaic industry, it is necessary to grow large ingots (more than 50 kg) for economic reasons. In order to be of value, these ingots must be grown with substantially vertical grain structure, which demands that the ingots be cooled uniaxially from the base. Conventional furnaces for this purpose are capable of providing both rotation and translation of the crucible within the insulating enclosure, and achieve local cooling of the base of the crucible by means of a water cooled heat sink below the fixed thermal insulation on which the base of the crucible rests.

This invention addresses the possibility of increasing the production rate of large ingots of silicon or the like by reducing the cycle time of the crystallisation furnace. This cycle time includes melting the initial solid charge, followed by crystallisation under controlled conditions to produce large grain vertically oriented multi-crystalline ingots, ideally with an anneal between crystallisation and removal from the furnace. The insulation between the base of the vessel and the heat sink is a compromise between the high insulation necessary for speeding the melting stage and the lower insulation desirable for speeding the crystallisation of the ingot.Short production cycles are desirable not only in order to make more productive use of the plant and equipment but also because long residence times at high temperatures promotes the migration of impurities into the silicon ingot from the material of the vessel, typically silica in the case of silicon ingots.

The present invention concerns aspects of the crystallisation method and apparatus described below. The scope of the invention extends to all novel aspects thereof whether individually or in ccmbination with other features as described herein. The invention also extends to crystalline materials formed by the method or in the apparatus, and to products made therefrom.

More specifically, in one aspect of the invention a method of crystallising a solid from a melt, especially of crystallising silicon from a silicon melt, may comprise forming a melt in a thermally insulated vessel, cooling the melt locally by extracting heat through a thermal pathway to a heat sink, and regulating the rate of loss of heat by varying the insulating properties of the thermal pathway.

Preferably the local cooling takes place at the base of the vessel.

During the step of forming the melt, the insulation in the thermal pathway may be relatively high, and preferably at a maximum During the step of cooling the melt locally, the insulation in the thermal pathway may be reduced, while nucleation and the start of crystal growth occurs. Following nucleation, the insulation in the thermal pathway may be further reduced, while the melt crystallises to the solid state.

Preferably, the crystallised solid is annealed before being cooled to ambient temperature. The insulating properties of the thermal pathway may be increased during the annealing step, and preferably maximised. me insulation may finally be reduced, preferably to a minimum, during cooling to ambient temperature prior to removal of the crystallised solid fram the vessel.

References to maximum and minimum insulation refer to the most and least insulation applied during the furnace cycle rather than to any absolute values.

In another aspect of the invention, apparatus for crystallising a solid from a melt may comprise a crystallisation vessel in a thermally insulated enclosure, a thermal pathway between a local portion of the vessel and a heat sink, and means for varying the insulating properties of the thermal pathway.

Regulation of the insulating properties of the thermal pathway may be achieved by adjusting an insulating baffle in the pathway between the melt and the heat sink. In particular, the insulating properties may be regulated by adjusting the position of the baffle within a region of reduced insulation in the thermal pathway. It will be appreciated that at temperatures around the melting points of many of the materials likely to be crystallised, most of the heat transfer takes place by radiation. In such cases, varying the insulating properties of the thermal pathway may involve intercepting the radiant heat energy and absorbing it or reflecting it to the desired extent.

The baffle may take the form of an apertured member, especially a disc, which can be moved, especially rotated, to vary the insulation of the thermal pathway by opening or closing passages through adjacent fixed insulation. The apertures may be circular, or may be partial sectors of circles centred on the rotational axis of the baffle, or may be of any other shape chosen to give desired thermal transmission characteristics upon moving the apertures into and out of alignment with the fixed passages, which may themselves match the shapes and distribution of the apertures.

The melt may be formed by fusing a solid charge in the vessel by the application of a heat source. In accordance with the method of the invention, the thermal gradient in the crystallising melt may be enhanced by a translation of the vessel with respect to the heat source during the crystallisation step. The apparatus may include an insulated table to support the vessel and to raise and lower the same with respect of the heat source. Further, the vessel may be rotated during any or all of the process to distribute the heating and/or the cooling and to reduce the risk of local temperature ananalies. The support table may be rotatable for this purpose.

Preferably, the thermal pathway passes through the table to the heat sink.

By modulating the thermal conductivity of the cooling pathway between the vessel and the heat sink, the thermal losses can be reduced to a minimum during the initial fusion phase of the process, in order to shorten the melting time. The heat leak can then be slowly increased (by reducing the insulation) in order to obtain controlled nucleation during the early growth phase, and then further increased to a high value to achieve maximum growth rate during the main growth rate phase. At the next stage, after completion of crystallisation, the heat leak can be reduced to a minimum, by restoring the insulation, in order to reduce the thermal gradient over the solid ingot during the annealing phase, thereby to relieve stresses in the ingot. Finally, the heat leak may be increased again to allow more rapid cooling down to the unloading temperature.

One embodiment of the invention is illustrated by way of example in the accompanying drawings, in which: Figure 1 is a general sectional view, in elevation, of a silicon crystallisation furnace; Figure 2 is an enlarged sectional view of the vessel support table shown in Figure 1; Figure 3 is a plan view of part of a shutter occupying the thermal pathway in the table of Figure 2; and Figure 4 is a representation, in elevation, of the driving and control mechanism for the table of Figure 2.

Figure 1 shows a crystallisation furnace 10 located centrally in a vacuum chamber 12, defined by casing 14 supported on pedestal 16.

Within the pedestal, colurm 18 extends sealingly into the vacuum chamber from drive, instrumentation and control unit 20 which is adapted to rotate, raise and lower column 18 as may be required as well as providing the interface for temperature measurement and the supply of cooling water to the furnace through the column.

Furnace 10 contains a crucible constituted by a silica vessel 22 supported on a circular insulating support table 24 within an insulating enclosure formed by cylindrical side walls 26 closed by circular top wall 28. Water cooled heat exchanger 30, of substantially the same diameter as vessel 22, lies between table 24 and the top end of column 18. The thermal pathway with which this invention is concerned occupies the cylindrical region between the bottom of vessel 22 and the heat sink formed by the heat exchanger, the local cooling of the vessel being in this case over substantially the whole area of its base.

Since the silica vessel is likely to devitrify during the course of the crystallisation process, it is surrounded by outer graphite supporting walls 32.

A susceptor 34 heated by an induct ion coil (not shown) extends around the upper portion of the insulating enclosure of the furnace. By raising and lowering column 18, the vessel 22 can be translated vertically to influence the temperature gradient between the top and bottom of the vessel.

Before describing the regulation of the insulating properties of the thermal pathway, with particular reference to Figures 2, 3 and 4, the overall crystallisation process will be described with reference to Figure 1.

The apparatus is initially set up with the vessel 22 charged with solid silicon pieces, column 18 fully raised, and vacuum chamber 12 evacuated. The temperature of the furnace is raised by means of heater 34, and the temperature at the base of the vessel is monitored by thenmccouple 36. The furnace tenperature is held at about 1540 C, until all the silicon, which has a melting point of about 1410"C, has melted.

Support table 24 is lowered a short distance and the base of the table is cooled, with reduced heating of the upper portion of the melt, until nucleation and the start of crystal growth at the bottom of the vessel occurs. This is illustrated in the right hand side of the view of the interior of furnace 10 in Figure 1, showing a thin layer of solid multi-crystalline vertically oriented crystal grains 38 below the melt 37.

Cooling of the base of the vessel is then increased, and support table 24 is lowered at between 2 and 25mm per hour, eg approximately lOnin per hour, to keep the crystal/melt interface at a steady height corresponding approximately to the lower boundary of the heater 34. The left hand side of the view of the interior of furnace 10 in Figure 1 illustrates the position just prior to complete solidification of a silicon ingot 38', showing the residue of the melt 37'.

After complete solidification, cooling is reduced and the support table 24 is raised to maintain the whole ingot at an annealing temperature within the approximate range 1000 to 1300"C, eg about 12500C, in an environment which is as close to isothermal as possible, for the time necessary to reduce internal stresses in the ingot, which would otherwise tend to relieve as cracks when the ingot was later sliced into wafers for use in photovoltaic cells.

Throughout the whole process, column 18 is rotated at about one revolution per minute to reduce the formation of local hot or cold regions in the silicon.

Insulated circular support table 24 is shown in more detail in Figure 2. It comprises a lower insulating plate 40 separated from an upper insulating plate 42 by an annular insulating spacer 44.

The lower plate is supported on water cooled heat exchanger 30.

The upper plate carries the silica vessel 22 and its surrounding graphite walls 32. The lower plate has a central circular aperture 46 through which passes the shaft of a baffle drive mamber 48, whose upper end above the plate is square in section. The baffle drive member has a central bore through which passes a casing tube for thermocouple 36, which is itself located just below the base of vessel 22 in a recess in the underside of upper plate 42.

The thermal pathway from the vessel to the heat sink passes through the interior of annular insulating spacer 44. This space contains a heat shutter 50 comprising a pair of insulating apertured baffle discs 51, 52. The lower disc 51 lies immediately above lower plate 40 and has a square central aperture 56 engaged with the square upper end of drive member 48, so that. it can be rotated by the drive member. The upper disc 52 lies i m ediately above the lower disc and is supported on an internal shoulder 54 in the annular spacer 44, where it is fixed in position by pins 55 (Figure 3).

Lower baffle disc 51 has four symmetrically disposed radially aligned pairs of circular apertures 58 and 59 (Figure 3).

Aperture 58, lying further from the centre of the disc, is the larger of each pair. Alternative aperture shapes are possible, including apertures formed as partial sectors, bounded by two radial edges and two concentric arcuate edges. Upper baffle disc 52 has matching apertures 58', 59' identically positioned with respect to the centres of the respective baffle discs.Since the aperture pairs are spaced at 900 rotational intervals, rotation of the lower disc by 450 will alter the shutter from a completely closed condition in which the apertures of each disc are occluded by the intermediate solid insulating portions of the other and the insulation is maximised, as shown, to a fully open condition in which all matching apertures are aligned to form eight circular passages through the shutter, minimising its insulating properties by maximising the scope for radiative heat transfer through the shutter.

fntermediate angular displacements of the baffle discs give intermediate insulating properties to the thermal pathway through the table 24.

The baffle discs are constructed fran thermally insulating rigid graphite felt or carbon bonded carbon fibre. Alternative embodiments of the invention may include reflective radiation shields such as polished metal shutters, or other heat screens.

The baffle drive member 48 is turned, to rotate the lower baffle disc, by a shaft 62 which extends through the length of column 18 (Figure 4). An electric motor 64 is fixed to the column, to turn with it as it rotates the whole of table 24 in the furnace, and the motor turns shaft 62 relative to the column by means of drive pinion 66. Alternatively, a manually operated handle may be used to turn the shaft. The column drive, the heater and instrumentation connections, and the provision for the cooling water supply, are conventional and are not shown. Motor 64 is provided with power by means of a suitable electrical slip ring assembly 68.

In accordance with the invention, the rate of loss of heat of the melt is regulated by varying the insulation of the thermal pathway between the vessel 22 and the heat sink 30, by using motor 64 to move the baffle disc 51 in either direction, according to the need to open or close shutter 50 to any desired extent.

Claims

AA:95S

1 A method of crystallising a solid fran a melt, comprising forming a melt in a thermally insulated vessel, cooling the melt locally by extracting heat through a thermal pathway to a heat sink, and regulating the rate of loss of heat by varying the insulating properties of the thermal pathway.

2 A method as claimed in claim 1 in which the local cooling takes place at the base of the vessel.

3 A method as claimed in claim 1 or 2 in which the thermal pathway is relatively highly insulating when forming the melt, and its insulation is lower during the step of cooling the melt locally, while nucleation and the start of crystal growth occurs.

4 A method as claimed in claim 3 in which the insulation in the thermal pathway is further reduced after nucleation.

5 A method as claimed in any one of the preceding claims in which the crystallised solid is annealed before being cooled to ambient temperature and the insulation of the thermal pathway is greater during the annealing step than during solidification.

6 A method as claimed in claim 5 in which the insulation is reduced after annealing, for cooling to ambient temperature prior to removing the crystallised solid from the vessel.

7 A method as claimed in any one of the preceding claims in which the insulating properties of the thermal pathway are varied by adjusting an insulating baffle in the thermal pathway.

8 A method as claimed in claim 7 in which the position of the baffle is adjusted within a region of reduced insulation in the thermal pathway to vary the absorption or reflection of radiant heat energy by the baffle.

9 A method as claimed in any one of the preceding claims in which the thermal gradient in the crystallising melt is enhanced by a translation of the vessel with respect to the heat source during the crystallisation step.

10 A method as claimed in any one of the preceding claims during which the vessel is rotated.

11 A method as claimed in any one of the preceding claims in which the melt is formed by fusing a solid charge in the vessel.

12 A method as claimed in any one of the preceding claims in which a solid silicon ingot is crystallised from a silicon melt.

13 A method as claimed in claim 12 in which the solid ingot has a substantially vertical grain structure.

14 A method according to claim 1 substantially as herein described with reference to and as illustrated in the accompanying drawings.

15 Apparatus for crystallising a solid fran a melt comprising a crystallisation vessel in a thermally insulated enclosure, a thermal pathway between a local portion of the vessel and a heat sink, and means for varying the insulating properties of the thermal pathway.

16 Apparatus as claimed in claim 15 which comprises a baffle with adjustable insulating properties in the pathway between the melt and the heat sink.

17 Apparatus as claimed in claim 16 in which the baffle comprises an apertured member, which is movable to vary the insulation of the thermal pathway by opening or closing passages through adjacent fixed insulation.

18 Apparatus as claimed in claim 17 in which the movable apertured member is a disc, which is movable by rotation relative to adjacent apertured fixed insulation, within an annular insulating surround.

19 Apparatus as claimed in claim 18 in which the apertures in the fixed insulation match the apertures in the rotatable disc, whereby rotation of the disc can vary the baffle between a completely closed condition in which the apertures are occluded and the insulation is maximised, and a fully open condition in which all matching apertures are aligned and the insulation is minimised.

20 Apparatus as claimed in any one of claims 15 to 19 including an insulated table to support the vessel and to raise and lower the same with respect to the heat source.

21 Apparatus as claimed in claim 20 in which the support table is rotatable.

22 Apparatus as claimed in claim 21 in which the thermal pathway passes through the table to the heat sink.

23 Apparatus as claimed in any one of claims 15 to 22 in which the thermally insulated enclosure is located in a vacuum chamber.

24 Apparatus for crystallising a solid from a melt, substantially as herein described with reference to and as illustrated in the accompanying drawings.

25 A crystalline material formed by a method as claimed in any one of claims 1 to 14, or in apparatus as claimed in any one of claims 15 to 24.