Field of the Invention The present invention relates to a furnace for the growth of crystalline semiconductor material, together with a method of operating such a furnace.

Background to the Invention

Presently, the bulk volume of the world production of photovoltaic elements comprising solar panels is based on multi-crystalline silicon wafers cut from ingots that are cast by directional solidification (DS) based on the Bridgman method in electrically heated furnaces. The crucible being employed is usually made of silica Si0 ₂ , and the furnaces have heating elements above, below and/or sideways with respect to the crucible to provide the heat for melting and control of heat extraction during the directional solidification. The process may be summarised as follows.

A crucible, open at the top, made of Si0 ₂ is covered in its interior with a silicon nitride containing coating and filled with a silicon feedstock to a predetermined height. The crucible is then placed on the floor of the heating compartment of the furnace. Next, a circumferential support structure of graphite plates is attached along the outer crucible walls to provide mechanical support at elevated temperatures when the Si0 ₂ crucible sags. The furnace compartment is then closed, evacuated and the inert purge gas is supplied during the period when the heating elements are engaged so as to cause the melting/solidification of the silicon feedstock. When the silicon is melted, the heating is adjusted to obtain a directional solidification. An inert purge gas, usually argon, is flushed onto the surface of the silicon to protect against gaseous contamination and to provide effective removal of SiO gas for at least as long as the silicon is in the liquid phase.

A major challenge in these processes is to maintain the purity of the molten silicon material during melting and solidification and to avoid deterioration of the furnace hot zone. The melt is usually protected from gaseous contaminants by a combination of evacuating the atmosphere in the hot zone of the furnace and flushing a cover of inert purge gas over the surface of the liquid silicon phase. However, the amount of purge gas may be insufficient to prevent back flows of CO generated inside the furnace chamber (due to release of SiO from the melt which subsequently contacts the graphite parts of the hot zone) resulting in the formation of SiC impurities in the melt. Build-up of carbon in the silicon melt leads to formation of SiC precipitates responsible for shunting effects (short circuiting of pn-junctions) in solar cells leading to drastic degradation of efficiencies of photovoltaic cells. Especially high levels of CO are generated in cases where the furnace is subject to leakages of ambient air into the interior of the hot zone. Another drawback of low purge gas flow resulting in a reduced evaporation of SiO from the melt, is a reduction of cell efficiencies due to light induced degradation related to higher oxygen content in silicon. However, utilization of higher amounts of gas flows leads to degradation of the silicon nitride coating of the crucible, with subsequent sticking of the silicon to the silica crucible walls, which causes loss of ingots due to their cracking. In addition, improper gas supply increases the speed of the hot zone deterioration due to its contact with aggressive gases originating from the process. Thus proper and adequate control of the gas flow possesses a challenge for the silicon crystallization processes in the photovoltaic industry.

From another perspective, the SiO gas, when in contact with carbon-containing elements of the furnace, reacts releasing CO and converting the graphite parts to SiC. As mentioned above, an increase of the CO concentration inside the furnace causes additional formation of SiC precipitates in the silicon. However, another problem is that the carbon-containing structural elements of the furnace are detrimentally affected by the build-up of SiC deposits. This is especially a problem for the graphite insulation wall of the hot zone. These deteriorations of carbon-containing structural elements of the hot zone of the furnace represent a a substantial cost increase due to reduced service times of the furnace elements and the interruption in production associated with remedial maintenance work. Importantly, the heating elements made of graphite which represent the hottest parts of the furnace, also react with the SiO within the process gas. This causes SiC deposits to form on the heating elements, which changes the electrical properties of the heating elements requiring their frequent replacement. In the worst cases, loss of top heater functionality due to the deterioration of top heaters during the operation of direct crystallization furnaces caused either by aggressive gases or by accidental leakages may lead to the freezing of the silicon melt from the top resulting in subsequent tearing of the crucible and spill out of the melt.

One approach to address such a problem would be to provide a gas-tight barrier between the heaters and the processing chamber, thereby preventing the graphite from being degraded. It is known, in the separate technical field of solder processing apparatus, to provide a sealed process chamber within which a substrate is heated in a low pressure hydrogen atmosphere. US2009/03251 16A1 describes such an apparatus where a separating wall (crystal board) is used for separating a process chamber from the chamber containing the heater elements (heater lamps). Both chambers are equipped with gas inlet and outlets, and, during heating, gas flows and pressures are adjusted so that the pressure in the heating chamber is higher than in the process chamber. This is due to the reactive nature of hydrogen whereby if there is a leak in the seals between the chambers, then the hydrogen is prevented from reaching the heater lamps which would act as an ignition source. An analogous two chamber arrangement is known from WO2006/082085. However, the crystalline growth of semiconductors such as silicon requires very high temperatures and, as the above discussion demonstrates, involves a number of technical challenges which are not present in lower temperature processing regimes. In the present technical field it is therefore extremely important to control the levels and movement of gases present within the heater and process chambers, and those gases released from the silicon melt. It is these challenges that the invention addresses. Summary of the Invention

In accordance with a first aspect of the present invention we provide a furnace for the growth of crystalline semiconductor material comprising:- a heater chamber comprising one or more heating devices;

a process chamber in which the crystalline semiconductor material is grown when in use;

a separator positioned between the heater chamber and process chamber such that the material receives heat from the said heating devices via the separator and wherein the separator further comprises a number of channels having a predetermined geometry so as to allow the passage of gas through the separator; and,

a gas flow system arranged to cause the flow of a process gas from the heater chamber to the process chamber through the said number of channels of the separator.

The present invention therefore addresses the problems discussed above in a number of ways. The separator provides a partition between the heater and process chambers. It is recognised by the invention that it is difficult to produce a seal between such chambers at the extreme temperatures at which the furnace operates. For this reason the gas flow system ensures that there is a flow of process gas from the heater chamber to the process chamber, thus reducing the possibility of any gases from the process chamber reaching the heating devices and causing their degradation. The gas flow is arranged to be in a controlled and predictable manner through the use of one or more channels.

The invention provides for the reliable and repeatable operation of the heater devices for a service life in excess of prior art furnaces. The furnace also enables the safe completion of the crystallisation process even in the event of significant air ingress from the external environment. Direct benefits attributable to the invention include an increase in operational safety, the decrease in maintenance frequency and costs, and optimisation of the process gas distribution.

The invention is applicable to a number of semiconductor growth furnaces and is particularly applicable to furnaces for melting and directional solidification of silicon under an argon atmosphere. The Bridgman process is one such process. This is because the molten silicon held in silica crucibles of such furnaces typically releases SiO gas which would otherwise subsequently react with and gradually deteriorate the normally graphite based heating devices. The gas supply system is preferably arranged to supply a sufficiently high flow between the chambers so as to dominate any back flow of aggressive gases from the process chamber towards the heating devices. In addition to the Bridgman process, other processes which may be used with the invention include the Czochralski and Vertical Gradient Freeze (VGF) processes. In addition to silicon, the invention is beneficial for the growth of a number of materials, including elements in groups III to V of the Periodic Table, combinations of these elements such as GaAs, GaP, ternary combinations of elements in these groups and compounds of elements in groups II to VI of the Periodic Table. The heater chamber is typically positioned above the process chamber containing the crucible and semiconductor melt. Other arrangements such as the heater chamber being positioned beneath, to one or more sides or surrounding the process chamber (for example circumferentially) are contemplated. In some cases a number of such heater chambers are provided, each with heating devices, positioned according to one or more of the above arrangements. In this case the apparatus may further comprise one or more additional heater chambers comprising additional heating devices and separators, some or all of which may be provided with channels. In this case, for at least one of the additional chambers, the gas flow system is further arranged to cause the flow of the process gas from the additional heater chambers to the process chamber through the said number of channels of the additional separators. These additional chambers may be provided, above, below, to one or more sides or circumferentially with respect to the process chamber. Each heater chamber is typically provided with a gas inlet and the process chamber with a gas outlet and the gas flow system is adapted to supply the process gas, such as argon, to the gas inlet and to extract the process gas from the gas outlet. A number of such inlets and outlets may be provided according to the application. Some such heater chambers may also be supplied by the gas flow system using inlets and outlets with or without passage of the gas into the process chamber. Particularly in the case of the gas outlet, a plurality of gas outlets may be arranged symmetrically upon opposing sides of the process chamber so as to assist with the smooth and symmetrical flow of the process gas within the process chamber.

The channels may comprise holes, slots or passages in the separator passing between respective surfaces of the separator in the heating chamber and the process chamber. The holes may be arranged according to any appropriate geometry desired to control the gas flow. The channels may take the form of through-thickness bores passing between opposing surfaces of the separator in the said heater and process chambers. They may also take more tortuous routes such that the surface openings of the channels upon the respective surfaces of the separator are laterally spaced with respect to one another. The geometries of the openings at each end of the channels may be different. The channels may also bifurcate or join with other channels.

Preferably the channels are positioned according to a predetermined regular pattern, such as an array, within a distributor region of the separator and wherein the channels produce a generally even distribution of process gas on a downstream side of the separator adjacent to the region. An even distribution of gas emitted from the channels prevents turbulence and allows substantially laminar flow which is beneficial in removing contaminant gases such as SiO from a silicon melt and to protect the separator. Thus the apparatus is preferably arranged so as to produce a generally smooth lateral flow of process gas above the semiconductor material so as to remove contaminant gas from the vicinity of the material surface. At the edge of or surrounding the distributor region, or spaced therefrom, the number or cross section of the channels may be reduced (in a protector region) so as to provide a reduced gas flow rate thereby reducing degradation in the walls of an adjacent component such as a crucible or a coating of its walls.

A number of additional channels may be positioned within a shield region of the separator so as to generate a process gas shield or curtain through which other gases (desorbing from furnace insulation or originating from accidental leakages of the air atmosphere into the furnace) are substantially prevented from passing laterally towards the melt. Such a gas shield may take the form of an increased gas velocity on the downstream side of the separator adjacent the shield region in comparison with the gas provided by other channels such as those of the distributor region. The shield region may be arranged to be positioned adjacent to the external walls of the crucible when in use so as to prevent gases flowing over the melt from a location originating to the side of the crucible.

The separator may be thought of as a member for closure of the heating chamber(s) and process chamber containing the material to be heated. In the case of a "top" or "upper" separator this typically consists of a horizontally oriented partition wall or ceiling dividing the upper compartment of the hot zone into two horizontally separated chambers: the lower becoming the process chamber housing the crucible and the upper heater chamber containing upper heating elements. This has the advantage of protecting the top heating elements from deteriorating gases originating from the material to be heated and/or the material constituting the interior of the hot zone. A top separator may be suspended in the top part of the hot zone which may or may not be an integral part of the furnace cover. A bottom separator may be provided, similar to a top separator, but may or may not be an integrated part of a bottom support structure for the crucible. A side separator may have a cylindrical shape or any geometrical structures forming a circumferential enclosure. The latter may include n (n>2) flat plates joined together to form a regular or irregular polygon. Typically, the side separator is attached to the bottom support structure and aligned with the top separator underneath the top heater chamber or the ceiling of the process camber in cases which there are no overlaying heater chamber. Other separator geometries are also contemplated. Preferably such separators have conformal opposing surfaces (for example parallel surfaces in the case of flat surfaces). An example of such a separator is a frusto-conical top separator in which the apex of the frusto-conical shape is directed towards the centre of the crucible and positioned above the semiconductor material.

A bottom support structure may be provided as a load carrying structure of graphite or another heat conductive and mechanically rigid material, for supporting the crucible and other structures/devices placed in the hot zone. The bottom support structure may form a horizontally oriented partition (wall) or floor covering the entire cross-section area of the hot zone and thus dividing the lower part of the hot zone into two chambers; one upper chamber where the crucible is to be placed and one lower chamber containing the heating means for heating from below. Thus the bottom support may be a separator also. This provision has the advantage of protecting the lower heating means from deteriorative gases stemming from the melt. However, the present invention may also be applied to furnaces with any type of conceivable load carrying support structure for carrying the crucible, including furnaces where the crucible is placed directly onto the bottom floor of the hot zone.

The separator may typically have a substantially planar form, for example with two opposed planar surfaces facing the heater and process chambers respectively. The thickness of the separator normal to the plane may be 1 to 10 mm, preferably 2 to 8 mm, more preferably 2 to 5 mm, and most preferably 2 to 3 mm. The separator advantageously is formed from a material with a number of desirable properties. One such property is that the separator is formed from a material having a thermal conductivity of 1 W/mK or higher. Another and preferably an additional such property is that the separator is formed from a material which is able to withstand the high temperatures associated with the semiconductor growth processes without decomposing or in other ways releasing (by gassing, perspiration, flaking, etc.) contaminating compounds. Another alternative or additional desired property is that the material should also withstand the temperatures without loss of its desired mechanical rigidity.

In the case of processing silicon, the separator material preferably should be chemically inert in the chemical environment encountered in crystallization furnaces for manufacturing mono- and multi-crystalline silicon ingots. Suitable separator materials include ceramic materials preferably with a SiC coating, such as carbon fibre-reinforced carbon (known as CFRC, C/C, CFC etc) with a SiC-coating, graphite coated with SiC, silicon carbide ceramics (C-C/SiC or C/SiC composites), silicon carbide fibre composite (SiC/SiC) carbon fibre- reinforced carbon, graphite coated with silicon carbide, silicon carbide ceramic composite, or silicon carbide fibre composite. These materials provide the desired high temperature stability both physically and chemically and have desired levels of thermal conductivity.

In order to improve the chemical inertness of the reactive materials one or more of the separator surfaces are preferably coated with a layer of silicon carbide having a thickness of about 10 to 200 micrometres, preferably 20 to 150 micrometres, more preferably 40 to 120 micrometres and most preferably 60 to 100 micrometres.

In principle the separator may be arranged to be substantially transparent to infra red radiation from the heating devices. However, since such devices have an associated geometry this inherently causes modulation of the infra red radiation received within the process chamber as a function of position. Preferably therefore the separator acts as a diffuse source of infra-red radiation. The separator may therefore be substantially translucent to the infra-red radiation or may even be substantially opaque. In this way the separator may act as a primary or a secondary heat source of the material in comparison with the heating devices. A lateral spacing of the openings of the channels in the separator may prevent direct line-of-sight to any particular parts of the semiconductor material, thereby improving the homogeneity of heating. The heating devices themselves may take any number of suitable forms, typically comprising electrical resistance heaters, either AC or DC, or inductive coil heaters. A crucible is typically provided for retaining the semiconductor material when in solid and liquid form. The crucible may be formed from a material which is inert with respect to the semiconductor material or it may be provided with a coating which is inert to the semiconductor material. A further support (for example formed from graphite) may be used to provide physical support to the crucible at high temperatures during the melting or growth processes.

The process and heater chambers form part of the "hot zone" of the furnace. The term "hot zone" as used herein refers to a compartment of the furnace where the heating takes place and which has thermally insulating walls, floor and ceiling. The hot zone is typically a compartment shaped as a parallelepiped, but may have other geometries.

The walls of the heater and process chambers containing the hot zone are typically formed from thick carbon or graphite felt (about 30 cm thick) and provide good thermal insulation. However, the hot zone is not gas tight. In order to provide a gas-tight chamber and some further thermal insulation the hot zone is typically provided within a housing. Preferably such a housing is typically a water-cooled housing and surrounds the walls of heater and process chambers. The walls of the housing are separated from those of the hot zone by a gas-filled volume, the volume providing further thermal insulation and containing exhaust gas (process gas plus any contaminants) from the hot zone. The housing may comprise a water-cooled steel shell or other type of externally cooled, mechanically rigid and gas-tight structure enclosing the hot zone. The housing is gas tight to allow for elevated temperatures at vacuum pressures. For loading and unloading the material to/from the furnace hot zone the housing normally contains a detachable arrangement (cover or door) in which parts of the hot zone may be integrated. When in use the furnace benefits from accurate control of the gas flow and the temperature within the heater and process chambers. Whilst such control may be effected manually though various controls, preferably the furnace further comprises a control system adapted to operate the heating devices and gas flow system so as to effect the controlled crystalline growth of the semiconductor material. This may take the form of a processor and appropriate further hardware and software.

Although it is expected normally that the outlet for the gas from the heater chamber will be positioned in the process chamber, it is also contemplated that a further outlet may also be positioned in the heater chamber which may for example enter a further heater chamber. In this gas the pressures and flow rates in the furnace will need further control so as to ensure that a suitable flow of process gas continues to flow through the separator into the process chamber. A similar consideration is also required if a further inlet for process gas is provided in the process chamber.

In accordance with a second aspect of the invention we provide a method of operating a furnace for the growth of crystalline semiconductor material in which a separator provides heat from heating devices in a heater chamber, to the material in a process chamber, the method comprising growing the crystalline semiconductor material using the heat from the heating devices whilst causing the flow of process gas from the heater chamber to the process chamber through a number of channels in the separator which have a predetermined geometry. The method is preferably performed using apparatus in accordance with that of the first aspect of the invention.

Specifically it is preferred that the method may include causing the gas flow to be generally evenly distributed on a downstream side of the separator. In addition or alternatively the method may include causing the gas flow to be increased in a localised region on the downstream side of the separator with respect to the evenly distributed gas flow (or the flow from other channels), thereby generating a gas shield. When the semiconductor material is retained within the walls of a crucible the gas shield may be positioned substantially at the crucible walls so as to separate a region above the crucible from a region to the side of the crucible. In addition or alternatively it may be positioned adjacent to the outside of the crucible walls.

The "process gas" as used herein refers to a gas which at the inlet normally, but not necessarily, consists of a gas with a certain purity level which furthermore is inert to the object to be heated and the material constituting the interior of the hot zone. During its residence time inside the hot zone the process gas mixes with gases being released from the material inside the hot zone and/or the object to be heated. In the example of a Bridgman type of furnace for making silicon ingots for subsequent processing to photovoltaic application, typically the inlet gas which is pure argon, mixes with CO and SiO hence the exit process gas also contains these species. Whilst argon is the preferred process gas, other inert gasses may be used, including helium, hydrogen and mixtures of gases containing such elements. The process gas supplied to the heater chamber and process chamber is typically at least a gas inert to the material to be heated and the material constituting the interior of the hot zone. More than one different process gas or mixture of gases may be provided to the heater chambers and/or the process chamber independently.

In accordance with a third aspect of the present invention, a method of growing a crystalline semiconductor material comprises:- loading a furnace process chamber with material to be heated;

closing the furnace;

evacuating the air within the process chamber;

causing a flow of process gas at desired pressures from one or more heating chambers of the furnace to the process chamber;

heating the material while performing the method according to the second aspect of the invention;

cooling the material;

aligning the pressure in the process chamber to atmospheric pressure; opening the furnace; and removing the material.

In particular, the growth process may consist of loading the furnace hot zone with a crucible filled with the material to be heated, closing the cover, evacuating the air, introducing a flow of process gas at the desired pressures in the heater chamber and process chamber, heating, melting and solidifying the material directionally by extracting heat from the bottom of the crucible while constantly purging with process gas, cooling down the solidified material, aligning the pressure to atmospheric pressure, opening the cover and unloading. A suitable directional growth process is the Bridgman process and a material especially suitable for directional growth is silicon.

In accordance with the novel flow paths of the process gas through the separator, the furnace design should ensure that the temperature profile and heat fluxes inside the hot-zone of the furnace should not be detrimentally affected by the introduction of the separator in order to preserve the intended heat extraction rates and control of the directional solidification process. In the case of processing silicon, the heat resistance across the separators should not influence heat extraction through the bottom of the crucible. Thus, in summary, the separators are preferably made of a material with sufficient mechanical rigidity to form a rigid partition with a plate thickness of the separator in the range of 1 - 20 mm, which is preferably chemically inert with respect to the chemical environment in the furnace during process conditions, and which preferably also has a thermal conductivity of at least 1 W/mK or higher.

Brief Description of the Drawings

Some examples of furnaces and associated methods according to the present invention are now described with reference to the accompanying drawings, in which :-

Figure 1 is a schematic drawing showing the general principle underlying the embodiments; Figure 2 shows a schematic section through the hot zone of a first embodiment, showing three heater chambers;

Figure 3 shows a schematic section through the hot zone of a second embodiment, having two heater chambers;

Figure 4 shows a schematic section through an upper part of the hot zone of a third embodiment having a separator with distributor and shield regions;

Figures 5a and 5b show modelled flow in two different implementation examples of separators;

Figures 6a and 6b show representative gas flow paths relating to Figures 5a and 5b;

Figure 7a and 7b show further details of the distributor regions of Figures 5a and 5b respectively;

Figure 8a and 8b show further details of the shield regions of Figures 5a and 5b respectively;

Figure 9 shows a general example of a method applicable to the embodiments; Figure 10a shows the reduction in heater device resistance as a function of the number of furnace runs in the absence of the process gas protection resulting in a frequent exchange of heaters; and,

Figure 10b shows the increased heater device resistance performance due to the presence of the process gas and continuous operation without exchange of heaters.

Description of Examples

The invention will now be described in a greater detail. Firstly a description of the generalised concept is given (this including some specific details for illustration only), followed by a discussion of three specific embodiments, each implementing the generalised concept.

Generalised Concept

A furnace 500 is provided as is shown in Figure 1. This has a "hot zone" 50 embodied as the internal volume of a chamber. The chamber 51 is bounded by thick thermally insulating graphite walls 3 (in the form of a graphite felt with a carbon fibre composite lining. Heating devices in the form of a number of electrical resistance heaters 12 are distributed in a horizontal plane in an upper part of the hot zone 50. An ingot of semiconductor material 1 (which is silicon in this case) is positioned centrally on a support in a lower part of the hot zone 50. In practice this is held within a crucible (not shown).

An inlet 13 provides process gas to the top of the hot zone through the roof of the chamber 51. In the bottom part of the hot zone 50 two outlets 14 are provided to remove contaminated process gas from the chamber 51 , these being positioned symmetrically upon either side of the crucible (not shown).

A separator 9 is positioned between the crucible containing the silicon 1 and the heating devices 12. The separator is planar and is arranged horizontally. This divides the chamber 51 into a heater chamber 1 1 containing the heating devices 12, and a process chamber 7 containing the silicon 1. A number of channels 17 are provided in the separator 9, distributed in the plane of the separator, each providing a gas flow path between the heating chamber 1 1 and the process chamber 7.

The chamber 51 containing the hot zone 50 is contained within a gas-tight housing 60. Aside from supporting members (not shown) a space 61 is generally provided between the chamber 51 and housing 60 so as to provide further thermal insulation between the two members. The housing 60 may be formed from a material such as steel and provided with water cooling. In this way the outer surface of the housing may be cool enough to touch by hand. The outlets 14 of the hot zone 50 vent into the space 61. The space 61 is therefore filled with process gas from the chamber 7, thus containing any contaminants from the hot zone. Beneath the chamber 51 , an exhaust outlet 62 is provided to remove the contaminated process gas from the furnace. External to the furnace a gas flow control system 70 is illustrated which provides the process gas to the inlet 1 1 and removes the contaminated process gas (that is, the process gas plus unwanted reactive gases) from the chamber through the outlet 14. A computer controller 80 is also illustrated schematically, providing control signals to the heating devices 12 and gas flow system 80 (in response to feedback supplied by appropriate sensors).

During operation of the furnace 500, under the supervision of the computer controller 80, the gas flow system 70 provides a steady flow of inert process gas to the inlet 13. The gas flows through the heater chamber 1 1 and passes through each of the channels 17 into the process chamber 7. The design of the separator and channels, in association with the crucible geometry, process chamber geometry and outlets 14 is such that a smooth gas flow is provided over the surface of the silicon 1 , this providing the effective removal of unwanted gases from the melt whilst minimizing the possibility of generating carbon monoxide. Any carbon monoxide generated, together with gases from the melt, is swiftly removed by the gas flow through the outlets 14, into the space 61 and out of the housing via the exhaust outlet 62. Simultaneously the computer controller 80 controls the heating devices so as to initially cause the melting of the silicon 1 , followed by its controlled solidification (typically directionally).

A general method of operating furnaces according to the invention is now described with reference to Figure 9.

At step 400, a crucible is filled with a feedstock of semiconductor material. The furnace, having an interior temperature which is ambient, is opened at step 401. The crucible with feedstock is then placed within a crucible and positioned upon a support within the process chamber 7 of the hot zone at step 402. Thereafter a graphite support material is positioned around the crucible at step 403. The furnace is then closed and sealed (step 404). The gas flow system is then operated at step 405 to evacuate the chamber, thereby reducing the pressure to less than 0.1 mbar. The gas flow system is then operated at step 406 to generate a stable gas flow (for example of argon) through the heater chamber, separator and process chamber at a pressure of about 600 to 900 mbar. After a period of time, when it is determined that the system has been flushed (purged) fully, the computer controller operates the heating devices (step 407). This causes the heating of the silicon above its melting temperature. A typical temperature for this is about 1420 to 1480°C. The heating to melt persists until all of the silicon material is melted. Thereafter, at step 408 the computer controller modulates the heating devices so as to cool the silicon material from the bottom of a crucible below its melting point (meanwhile, maintaining the temperature of the top of the crucible above the silicon melting point) and provide the controlled directional solidification of the material (a typical growth rate being about 1 to 3 cm/hour). Following completion of the solidification and subsequent annealing the heating devices are then deactivated at step 409. The silicon is then allowed to cool at step 410 as the flow of process gas continues, this flow providing an additional cooling effect. Once cooled, the furnace hot zone is then returned to atmospheric pressure at step 41 1. The chamber is opened when the temperature has reduced to below about 350°C and the solidified material is removed at step 412. First example embodiment

A first example embodiment, shown in partial section in Figure 2, comprises a hot zone 100. The hot zone 100 has an inner furnace space for performing melting and solidification of silicon 1 in a crucible 2 by the Bridgeman method. The hot zone is the inner compartment confined by the heat insulating walls 3. The crucible is a conventional silica crucible 2 coated with a slip coating of silicon nitride.

The crucible 2 is placed onto a graphite floor 4 which acts as a separator (with bottom channels 23) and divides the inner compartment of the hot zone into a bottom heater chamber 5 housing electric resistance heaters 6 made of graphite for heating the crucible 2 from below, and a process chamber 7 housing the crucible 2. The sides of the crucible 2 are mechanically supported by plates of graphite or, preferably, SiC-coated graphite 8 (alternatively silicon nitride or any other rigid chemically inert material with respect to SiO may be used) extending above or below the level of the crucible. The separator 9 in the form of a horizontal partition wall divides the upper part of the hot zone further to form an upper heater chamber 11 housing electric resistance heaters 12, again made of graphite for heating the crucible 2 from above. A flow control system supplies a first process gas to the process chamber via a central inlet or distributor 13.

In this first embodiment, a second process gas is also supplied separately to the top heater chamber 11 overlying the process chamber 7 through inlet 15. The second process gas passes through channels in the form of holes or perforations 17 in the separator 9. The channels 17 act as additional gas distributors to the process chamber. The first and second process gases, together with any contaminants of SiO evaporated from the silicon melt, or carbon monoxide, are extracted from the process chamber via one or several gas outlets 14.

The separation of the hot zone into chambers, where at least one of the chambers is a process chamber, and the other(s) is(are) heat chamber(s), allows the use of more than one process gas, organised as independent streams of different gases. One or more of the process gases may not necessarily be inert, but rather a reactive or doping gas like hydrogen or phosphane which may have a functional role with respect to the material quality. Such a reactive or doping gas is provided as a first process gas through the distributor 13 in Figure 2. Meanwhile, a second process gas in the form of an inert gas is used in a role of heater protection and serves as a gas carrier for the volatile impurities (gaseous contaminants) released in the process chamber. The second process gas therefore "washes" the surface of the melt.

In this example embodiment, a further process gas (similar to the second process gas in this case) is supplied separately through an inlet 18 to a side heater chamber 19 housing electrical resistance heaters 20 made of graphite for heating the crucible 2 from the side. Furthermore, the side heater chamber 19 completely surrounds the process chamber 7 circumferentially and is equipped with a gas outlet 21. In practice the circumferential separator between the side heater chamber 19 and the process chamber 7 will not be perfectly sealed so, when in use, a slight overpressure exists in the side heater chamber 19 compared to the process chamber 7 which causes a fraction of the gas introduced to the side heater chamber 19 to pass into the process chamber 7.

Finally, in this example embodiment, a further process gas (again similar to the second process gas) is supplied separately through an inlet 22 to the bottom heater chamber 5 housing electric resistance heaters 6 made of graphite for supplying heat to the crucible 2 from the bottom. The graphite floor plate 4 separating the bottom heater chamber 5 and the process chamber 7 acts as a bottom separator with channels positioned upon either side of the crucible 2 and support 8. The channels of the plate 4 allow the venting through the channels 23 of the bottom heater chamber 5 into the process chamber.

Each of the inlets and outlets of the various chambers are connected to and controlled by the gas flow system which controls the pressure and therefore the flow of the process gas, which in this case is argon, through the various chambers. The circumferential heater chamber 19 and the bottom heater chamber 5 are approximately sealed from the process chamber, although any leakage results in the process gas flowing into the process chamber. The graphite heaters are thereby maintained in a pure argon atmosphere during the use of the apparatus. The outward flow path is promoted by the position of the outlets 14 on opposing sides of the process chamber 7. The gas supply system is operational so as to produce the flow of the process gas within the chambers in the manner described at least during a heating phase of operation of the furnace in which the silicon is melted and also a growth phase during which, in this case, directional growth, is provided.

Second example embodiment

A second example embodiment, shown in Figure 3, includes a hot zone 200 with a somewhat simpler arrangement in comparison with Figure 2. In this second embodiment, analogous components to the first embodiment are referred to using similar reference numerals. The crucible 2 is placed onto a graphite floor 4 which divides the inner compartment of the hot zone into one bottom chamber 5 housing electric resistance heaters 6 made of graphite for heating the crucible 2 from below, and the process chamber 7 housing the crucible 2. The sides of the crucible 2 are again mechanically supported by plates of SiC-coated graphite. The separator 9, again in the form of a horizontal partition wall, divides the upper part of the hot zone further to form an upper heater chamber 1 1 housing electric resistance heaters 12 made of graphite for heating the crucible 2 from above. In this case, in comparison with the first embodiment, there is no side heating or side/circumferential heater chamber, as well as no gas supply to the bottom heater chamber. Again a purge gas (process gas) is supplied to the upper heater chamber 1 1 via a central inlet 13 and extracted together with SiO evaporated from the melt via one or several gas outlets 14.

In this example embodiment, the inlet 13 supplies process gas directly to the upper heater chamber 1 1. In this case all of the gas supplied to the upper heater chamber 11 passes through the separator 9. Here, numerous channels 17 are positioned as gas pathways through the separator 9. The channels 17 are arranged in an array, each generally having a similar geometry. The flow of the gas from the channels 17 is illustrated by the adjacent arrows in Figure 3. The channels 17 provide a distributor region of the separator 9 such that a smooth flow of gas is provided on the downstream side of the separator 9. This is directed within the crucible over the surface of the silicon melt where it mixes with SiO gas and the mixture is removed by the symmetrically positioned outlets 14 from the process chamber.

In this example embodiment there is no separate gas supply to the bottom heater chamber 5. However, the action of the channels 17 as a distributor and the positions of the outlets 14 distal from the bottom heater chamber 5 ensures that reactive species are kept away from the heating elements 6.

Third Embodiment

A third embodiment is illustrated in Figure 4, this relating to a hot zone 300. In comparison with the second embodiment, the lower heater chamber 5 is removed. The main difference is in the form of the separator 9. In this case, unlike in the other embodiments where similar channels were provided, for example in a regular array, here there are three different types of channel arrangements. In the central part of the separator a number of regularly spaced channels are provided, these forming a distributor zone 30 of channels. Thus a middle area of the separator is provided with small holes (2 mm diameter) covering or overlying the majority of the melt area except the periphery close to the crucible walls. It should be noted that some further channels 31 , at an increased spacing are provided on the periphery of the distributor zone 30. The periphery of the melt in a vicinity of the crucible walls therefore has a reduced gas supply in order to decrease the extent to which the silicon nitride coating (covering the silica walls and preventing contact with molten silicon) is deteriorated. Furthermore, at a location above the edge of the crucible 2 and adjacent support 8, a number of channels are provided having a significantly greater spatial density and/or great cross section with respect to the gas flow direction. These channels form a shield region 32. Thus the outer periphery of the crucible is encompassed by a "gas curtain" in order to prevent furnace ambient atmosphere containing CO from penetration into the area above the melt. The peripheral pattern is formed by 3 perimeters of holes with 2, 3, 4 mm in diameter respectively in order to achieve a "gas curtain" effect. In use the inert process gas is supplied through the upper heater chamber, thereby protecting the top heaters. It is then distributed through the pattern of holes in the separator. The purpose of the arrangement of channels is firstly, in the case of silicon growth, to remove oxygen dissolved in the silicon melt by effectively perturbing the diffusion layer, which is an evaporation-limiting stage. Secondly, the purpose is to protect the melt from contact with CO gas which is the main transport mechanism of carbon into silicon melt. Thirdly, the purpose is to protect separator material from destructive contact with SiO gas originating from the melt. The distribution of the holes, their size and gas pressure in the upper heater chamber produces a desirable gas pattern supply, which may vary depending on application area and crucible shape and size. Verification of the invention

Figures 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b show computer simulations of two example implementations of the third embodiment. In Figure 5a distributor and shield zones are provided (as can be seen in Figure 7a and 8a respectively). Here the spacing of the channels in the distributor zone is relatively small. In comparison Figure 5b has a distributor zone with a much larger channel spacing (again shown more clearly in Figures 7b and 8b). In each of these figures, the gas flow velocity is inversely proportional to the shading density (light shading denoting high velocity, dense shading denoting low velocity). For clarity, Figures 6a and 6b show gas streamlines to illustrate the flow. It can be seen how the distributor zone channels cause a flow direction which is primarily parallel to the melt surface. The shield zone channels can be seen to provide an effective gas curtain of higher velocity gas. The use of the gas flow to effectively protect the heating devices is illustrated in Figures 10a and 10b. Each figure shows the electrical resistance (ordinate) of the top heaters (for example in Ohms) as a function of the number of furnace runs (abscissa). Figure 10a shows the performance of the heaters in the absence of a process gas atmosphere and demonstrates the frequency of heater exchange. It can be seen that the electrical resistance increases as the heater material degrades. In comparison, Figure 10b shows the equivalent results for a heater chamber containing process gas. Here the electrical resistance remains generally constant after a significantly larger number of furnace runs and does not require further heater exchange.

During normal operation of a prior art furnace the top heater elements are routinely replaced after 6-7 furnace runs. This is done because formation of SiC- deposits changes the electrical properties of the heating elements, resulting in less power and increased risk for complete heater failure. The changing electrical properties also extend the process time and reduce the productivity. In addition, the replacement of heater elements is a time consuming operation which is done at the expense of valuable process time. The use of the process gas to protect the heater elements as part of the gas flow allows a significant increase in the number of furnace runs which may be completed between heater device maintenance or replacement procedures.