WO1993019222A1

WO1993019222A1 - Treatment chamber

Info

Publication number: WO1993019222A1
Application number: PCT/GB1993/000461
Authority: WO
Inventors: Brian Turner
Original assignee: Metals Research Semiconductors Ltd.
Priority date: 1992-03-25
Filing date: 1993-03-05
Publication date: 1993-09-30
Also published as: EP0586651A1; AU3641993A; GB9206442D0; JPH06508661A

Abstract

A chemical vapour deposition chamber for forming layered wafers used in electronic devices such as laser diodes, transistors, opto-electronic integrated circuits and the like. The chamber has a substrate supporting surface which is so adapted that gaseous reactants pass thereover in a converging manner so that the reactant concentration is not depleted as deposition takes place and a uniform thickness of chemical is deposited across the wafer. Furthermore, the supporting surface is located symmetrically around an exit through which the excess reactants pass, to be conveniently absorbed thereafter on a charcoal filter. The chamber is particularly suitable for metal organic chemical vapour deposition in that carrier gas flows over the chamber surfaces during and after reaction thereby reducing the amounts of unwanted chemicals deposited in the chamber.

Description

Title: Treatment Chamber

Field of invention

This invention concerns the deposition of material on substrates or wafers to form layers particularly for use in electronic devices such as laser diodes, transistors opto-electronic integrated circuits and the like.

Background to the invention

There are several known techniques associated with the growth of epitaxial films. One such process has involved chemical vapour deposition (CVD) in which reactive gases are thermally decomposed over a heated substrate.

CVD is carried out in reaction chambers in which a substrate is heated typically by conduction from a graphite susceptor which itself is heated typically using RF induction heating techniques. However, it is to be understood that other forms of radiant heating may be employed such as using infrared.

The reactive gases are normally combined with a carrier gas such as hydrogen before being introduced into the chamber. As the mixture of gases flows over the heated substrate thermochemical reactions occur causing constituent parts of the reactive gases to combine with the heated substrate and form a surface layer. Since such systems are used to lay down molecular thickness layers of material, changes of temperature, concentration of reactive agents, speed of gas flow and the like can all have significant effects on the uniformity of the deposited epitaxial layer. Indeed it has been noted that as the reactive gases flow across a surface, the removal of some of the -reactive material during their passage means that the layer becomes progressively depleted in the direction of gaseous travel across the surface. Accordingly chamber design and gas flow have been adjusted so as to attempt to compensate for this effect.

Likewise where thin epitaxial layers of different materials are required, the reactants producing, the first layer must be replaced abruptly and swiftly by the reactants which will produce the next layer. Manifold systems have been designed to produce appropriate abrupt changeover and further design refinements have been incorporated to remove unwanted pressure transients in the gaseous stream.

It has been known for some time that increased gas velocity can be obtained by operating such systems at below atmospheric pressures. Consequently the reaction chamber and exhaust arrangements of known systems are normally operated at pressures of the order of 1/lOth atmospheric.

Reaction chambers vary from horizontal reactors in which a wafer is located on a susceptor with its face substantially parallel to the gas stream to vertical arrangements in which the substrate surface is substantially perpendicular to the gas flow. A combination approach is the so-called barrel reactor in which a large number of individual substrates can be located on a susceptor which itself conforms to part of the internal surface of the barrel and the carrier and reactant gases are introduced at the top and exit from the lower end of the reactor. By rotating the substrate material usually by rotating the susceptor, a more uniform deposition is achieved since the rotation seems to iron out local hot spots and also seems to accommodate non- uniformity in the gas supply.

A particular problem associated with many CVD systems is the contamination of the reactor walls with some of the reactive components of the gases which have been decomposed in the chamber during the reaction. Although the heated substrate attracts these materials, during prolonged treatment processes, considerable build-up of the reactive materials can occur on parts of the reactor which are not so well cooled as others and in view of the nature of such materials cleaning the chamber can be difficult and sometimes hazardous.

It is_^an object of the present invention to provide an improved CVD reactor which is particularly suitable for metal organic chemical vapour deposition (MOCVD) in that it reduces the amount of unwanted reactive material deposition within the chamber during a reaction process.

Summary of the invention

According to one aspect of the present invention a reaction chamber for chemical vapour deposition particularly metal organic chemical vapour deposition comprises an enclosure having a substrate supporting surface located generally symmetrically around a central exit, means within the enclosure to cause gaseous reactants introduced therein to pass in a generally radial and converging manner over the substrate material in order to reach the exit, heating means for heating the substrate material and a body located centrally within the enclosure to reduce the volume available to the gaseous reactants and define a generally annular cavity through which the gaseous reactants have to pass.

Advantageously the body includes cooling means to reduce the temperature of its surface, and that of adjoining devices, below that at which vapour deposition will normally occur thereon during operation of the enclosure.

The centrally located body is itself preferably a hollow container the outside surface of which is shaped so as to define with at least the interior wall of the main enclosure, the said annular passage "for the gaseous reactants.

The interior of the centrally located body may be filled at least in part with a cooling fluid such as water.

Where the heat generated during reaction is such that the fluid itself needs to be cooled, a fluid circulation means may be provided to circulate fluid to and from the body, the flow rate being selected so as to maintain the temperature of the body at the desired level.

The space between the enclosure and the centrally located body may be divided into a plurality of annular paths by the interposition of hollow shells spaced from each other and from the central body and from the inside surface of the enclosure, the said spacing in each event defining an annular passage through which gaseous products can pass. Thus a first passage exits between the inside wall surface of the enclosure and one of the shells, another passage exists between two adjacent intermediate shells and a further passage exists between the inside surface of the innermost shell and the external surface of the central body.

Thus the interposition of two such shells defines three such paths one each of those just described.

The interposition of more than two such shells merely defines additional parallel annular paths should these be required.

In a preferred embodiment of the invention three parallel paths are defined by means of two such shells and a region of the outer of the said two shells- which lies symmetrically around the gas exit which communicates with each of the three passages, is formed as a susceptor on which substrate material can be located.

In the preferred embodiment, means is provided for introducing carrier gas such as hydrogen into the outer and inner said parallel passages and further means is provided for introducing the carrier gas together with gaseous reagents in combination therewith into the central passage, which combination of gases are thereby forced to flow over the heated substrate material before reaching the exit. The carrier gas passing internally and externally of the annular passage containing the carrier gas and gaseous reactants, serves not only to purge but also cool the regions of the enclosure through which it is flowing .

By ensuring that the substrate material is at a sufficiently high temperature before the reactant gases are introduced, some of the reactive material will be deposited on the heated substrate but inevitably some of the reactive material passes to the exit and according to a preferred feature of the invention, filter means is provided between the exit and exhaust to absorb any reactive materials leaving the enclosure, before they pass to exhaust.

A charcoal filter may for example be used to collect reactive materials such as phosphorus, aresnic, gallium and indium all of which are typical components of reactive gaseous products which will be supplied to such an enclosure to form epitaxial layers on substrate materials.

According to another preferred feature of the invention, the exit from the main enclosure is in the form of a tubular extension which is adapted to be connected to a housing containing the filter, and a removable inner tube is provided for conveying the mixture of unused reactant gases and at least some of the carrier gas straight to the filter. The annular space between the central tube and the said tubular extension of the main enclosure can be used to convey only uncontaminated carrier gas into the filter housing so that after the reaction process has been completed and treated substrate is ready to be removed, the enclosure can be purged with an appropriate gas and before opening the enclosure, the filter chamber can be removed from the outlet extension of the main enclosure and the inner tube removed therefrom into a safe place to reduce the possibility of fire or explosion which might arise if the reactive deposition on its inside surface is allowed to come into contact with air for any extended period of time.

This is of particular importance where reactant gases incorporating phosphorus are employed.

Preferably the removable tubular passage for conveying carrier gases and surplus reactant gases to the filter chamber is formed from quartz.

Preferably the main outer enclosure is formed in two parts, a lower cup shaped housing from quartz and an upper domed lid formed from stainless steel.

Conveniently and preferably the internal central body is formed from stainless steel and is hollow to allow it to contain water or other cooling liquid.

The intermediate shells interposed between the outer housing and the inner central body are formed from quartz.

Preferably that part of the assembly which carries the substrate which is to be coated by vapour deposition is rotated relative to the gas flow so as to produce a more uniform deposition of material during the vapour deposition process.

The susceptor may be in two parts, a stationary outer member and an inner member of complementary shape, to permit rotation of it within the outer member. The inner member may be driven in rotation by the appropriate supply of carrier gas under pressure to its underside, which may be formed with grooves to assist this rotation.

The susceptor on which the substrate is mounted may be formed from graphite and where a so-called barrel reactor design is adopted for the overall enclosure, such that the susceptor surface on which the substrate is mounted forms the inside of the inverted hollow frustoconical shell, the susceptor is conveniently formed from two such inverted frustoconical shells one lying within the other, each having an aligned central aperture to define part of the exit from the chamber, and the inner surface of the inner shell is machined to form a plurality of trapezoidally shaped facets onto each of which can be secured a wafer of substrate material. Typically the latter are in the form of thin wafers and the inner shell may be formed with appropriate wells or recesses into which the wafers fit.

The temperature of the susceptor is preferably monitored by means of thermocouples or the like which may be embedded within the susceptor and electrical conductors connected thereto for feeding signals therefrom to an appropriate monitoring circuit and control of display are conveniently fed via the outlet extension of the main casing to extend laterally through the junction between it and the filter housing where the latter is fitted.

Heating may be by means of electrical resistance heating, RF induction heating, infrared radiation or any other form of heating which is convenient having regard to the circumstances. RF induction heating is preferred. Where it is desired that the substrate is supported in a substantially horizontal plane, the upper surface of the susceptor must likewise be substantially horizontal and planar and in this event the walls of the intermediate shell and central cooling body within the overall enclosure must likewise possess substantially planar underside surfaces to extend substantially parallel to and spaced from the susceptor and the shell respectively. In this event the susceptor may be in the form of an inverted truncated cone or may itself be a substantially planar member the depth of which is sufficient to produce a substantially uniform temperature when heated for raising the temperature of the substrate material on its upper surface.

Advantage gained by providing a horizontal susceptor surface is that planetary motion of the wafers can be accommodated.

According to another aspect of the present invention there is provided a method of forming epitaxial layers of material on substrate comprising the steps of locating one or more pieces of the substrate on the surface of a susceptor which is either generally circular or frustoconical and which contain a central aperture through which gases can escape, causing a gaseous mixture containing at least one reactive component which will react with the surface of the heated substrate, to pass over the substrate in a radially inwardly converging manner to exit from the central aperture in the susceptor, and constraining the gaseous mixture into a relatively narrow space between the susceptor and wall of a member having a similar complimentary shape to that of at least part of the susceptor thereby to constrain the gaseous mixture into close proximity with the heated substrate material as the gaseous mixture passes thereover.

Preferably the method additionally includes the step of rotating part of the assembly relative to the gaseous mixture so as to improve the uniformity of deposition of reactive material onto the substrate.

The method also preferably includes the step of circulating cooling fluid through at least part of the overall assembly so as to cool some of the surfaces within the overall assembly and thereby reduce the prereaction and courage deposition of exhaust reactive materials thereon.

The method also includes the step of conveying the exiting gaseous mixture to exhaust through a filter such as a charcoal filter to assist in removing undesirable reactive components and gases from the mixture before it exhausts.

The method also includes the step of conveying the exiting gases to the filter through a removable tube, typically a quartz tube.

The invention also lies in a method of preparing a vapour deposition chamber, and exhaust system before a subsequent deposition process to enable volatile products of reaction such as reactive components deposited during a previous vapour deposition process can be removed, comprising the steps of flushing the chamber and exhaust system with carrier gas such as hydrogen after the deposition process has been carried out and thereafter removing the filter and its housing from the exhaust line and removing from the exhaust line the removable tube and if appropriate immersing the tube and the filter in a liquid such as water so as to keep air from the components and thereby prevent oxidation and possible spontaneous combustion or explosion occurring, as may occur if for example phospherus has been coated on the transfer tube or in the filter, replacing the tube and filter with uncontaminated items and reassembling the apparatus.

The replacement items may be fresh tube and filter elements or may be the original items after suitable cleaning as by abrasion or chemical cleaning.

The invention also lies in wafers of substrate having epitaxial layers formed thereon by vapour deposition when the method is carried out in a vapour deposition chamber constructed and operated in accordance with the invention.

The invention will now be described-by way of example with reference to the accompanying drawings in which:

Figure 1 is a cross section through a vapour deposition chamber, transfer tube and filter constructed in accordance with the invention.

Figure 2 is a cross section through the lower part of a chamber such as is shown in Figure 1 in which the substrate supporting susceptor presents a generally flat horizontal upper surface for mounting the substrate, and

Figure 3 illustrates a still further embodiment of the invention in which the susceptor is itself a generally thin cylindrical plate thereby enabling the lower part of the overall casing to be in the form-of a cylindrical member having a generally flat base.

Detailed description of the drawings

Figure 1 shows a reaction chamber for performing MOCVD in combination with a filter for removing unwanted bi- products and excess reagent from the exhaust gases and illustrates one embodiment of the invention disclosed herein.

The reaction chamber is constructed from an assembly of shells some made of steel and others of quartz so as to form a series of annular paths through which gases can flow from an inlet region to an outlet region. In passing through the chamber the gases pass over the surface of heated substrate so as to produce molecular deposition thereon to form epitaxial layers of material.

The outermost casing is formed from,two shells 10 and 12 which co-operate to form a barrel shaped structure. The upper shell 10 is of stainless steel whilst the lower shell 12 is of quartz. The shell 10 includes a peripheral flange 14 containing an annular groove 16 within which an O-ring 18 is fitted for sealing against the upper surface of an enlarged peripheral edge region 20 of the shell 12. The two shells are held together by means of a ring 22 which is secured to the underside of the flange 14 and itself extends below the enlarged periphery 20 of the shell 12 and urges the latter into contact with the seal 16.

The central upper region of the shell 10 contains an opening 24 which is closed by a plate 26 through which tubes 28 and 30 extend and which additionally includes drillings such as 32 for the supply of gas to the enclosure.

The opening 24 is surrounded by an annular platform 34 on which the plate 26 sits and an O-ring 36 in a groove 38 in the platform 34 ensures a gas tight fit.

Within the first chamber formed by the shells 10 and 12, there is located another pair of shells 40 and 42 which are joined by simple abutment between the lower peripheral edge 44 of the shell 40 and the peripheral platform 46 of the lower shell 42. The upper shell 40 is formed from quartz whilst the lower shell is formed from graphite.

Whilst the upper shell 40 is in the form of a bell housing, the lower shell 42 is more in the form of an upturned frustoconical shell and a complimentary frustoconical shell 48 is fitted within the lower shell 42, the inner shell 48 having formed thereon a series of flat trapezoidal surfaces on which substrate wafers can be affixed. The latter may for example be located within wells or recesses formed in the facets formed on the internal surface of the shell 48 or may simply be stuck to the surface using an appropriate adhesive.

The inner graphite shell 48 is designed to rotate within the shell 42 about the central axis of the assembly under the action of gas flow to be described. The relative rotation achieved between (the wafers of substrate material located on the shell 48) and the radially converging gas flow passing over the substrate wafers has been found to produce more uniform deposition on the surface of the wafers. Within the shells 40 and 42, are located two further shells 50 and 52 both formed from quartz and as with shells 40, 42, 50 and 52 are also sealed by means of an abutting join at 54. An inturned lip 56 in the lower shell 54 further assists in maintaining the seal.

The two shells 50 and 52 form a body which is somewhat similar in shape but overall smaller than that of the two shells 40 and 42 and generates between the two pairs of shells an annular space 58 around which gases can flow from the top of the assembly towards the bottom. The input and exit of the gas flows will be described later.

The innermost body making the assembly comprises a single shell 60 having a generally similar shape to that of the body formed by the two shells 50 and 52 but again being smaller so as to be capable of fitting therewithin. The shell 60 includes an opening 62 surrounded by a peripheral platform 64 which is secured to the underside of a cylindrical manifold 66 which extends from the underside of the plate 26. An O-ring 68 provides a gas tight seal between the interior of the shell 60 and the space 70 between the shell 60 and the shells 50 and 52.

Tubes 28 and 30 are of different length and extend through the opening 62 into the interior of the shell 60. Liquid such as water passes down pipe 28 and up pipe 30 at a rate so as to maintain a given level such as that shown at 72 within the shell 60. The flow of liquid may be employed to transfer heat from the shell 60 to a cooling plant (not shown) .

The lower end of the shell 52 is formed centrally with a cylindrical housing 74 the wall of which is castellated to define apertures such as 76 through which gas can flow. The housing 74 supports the shell 52 within the graphite shell 42 by fitting within a cylindrical well 78 formed centrally in the shell 42. An intermediate inverted top hat member 80 formed from graphite and thereby providing a bearing surface is fitted into a central aperture 82 in the shell 42, the out turned lip of the top hat member providing a bearing surface on which a radially outwardly extending flange 84 of a quartz tube 86, rests.

The cylindrical section of the top hat member 80 also closes off an annular groove 88 formed around the opening in the shell 42 through which the cylindrical section 80 extends, the purpose of which will be described later.

The graphite shell 42 constitutes a susceptor which is heated by RF induction from an induction coil shown in cross section at 90. In known manner the coil is formed from conductive tubing through which a cooling fluid such as water is passed whilst in use.

The heating of the susceptor 42 in turn heats the internal graphite shell 48 and the wafers of substrate located thereon.

Thermocouples or like temperature sensing devices such as 92 are embedded in the graphite shell 42 and conductors 94 communicate between the thermocouples and temperature indicating means (not shown) .

The quartz tube 86 serves to convey gases from the exit region of the shells defined by the castellated cylindrical housing 74 and a filter housing generally designated 96. The housing 96 is comprised of an upper inverted cup shaped housing 98 closed at its lower end by a plate 100 having a central aperture leading to an exhaust pipe 102 through which exhaust gases flow in the direction of the arrow 104. Within the housing is located a cartridge typically of charcoal designated 106 and in known manner the cannister is sealed so as to prevent gases from the tube 86 passing to the exhaust pipe 102 other than through the charcoal filter cartridge 106.

The upper end of the housing 98 is secured to the lower flanged end of a tubular extension 108 of the outer lower shell 12 and is held captive thereagainst by means of a split ring 110 having a radially inwardly directed flange 112 which overlies the radially outwardly extending flange 114 at the lower end of the cylindrical extension 108. A sealing ring 116 prevents gases from escaping between the abutting surfaces.

A drilling 118 in the upper solid end of the housing 98 communicates with a vertical drilling 120 within which is located as a sliding fit the lower end of a small diameter pipe 122, an O-ring seal 124 engaging the outside of the small diameter tube 122 and providing a gas tight seal within the drilling 120. The tube 122 communicates through a passage 126 with the annular groove 88 previously mentioned in the lower end of the shell 42 and drillings in the graphite shell 42 such as 128 enable gas supplied through the drilling 118, tube 122 and annular groove 88 to pass to grooves such as 130 in the surface of the shell 42 to lift and rotate the inner shell 48 as previously described.

The passage 126 is countersunk to facilitate the entry of the upper end of the tube 122 and assembly tolerances are accommodated by spring loading the lower end of the tube

122 by means of a compression spring 132 urging the tube

122 in an upward direction into engagement with the countersink of the passage 126.

Assembly and manufacturing tolerances are also accommodated by means of the sliding sealing engagement of the annular flanges 134 and 136 (provided at the upper central regions of the shells 40 and 50 respectively) , with the generally cylindrical housing 66 which extends between the platform 64 at the upper end of the shell 60 and the underside of the plate 26. The housing 66 includes two cylindrical sections 138 and 140 and the annular flanges 134 and 136 include O-ring seals 142 and 144 respectively and are capable of sliding axially up and down the cylindrical surfaces 138 and 140 respectively whilst still maintaining a gas tight seal therewith. In this way as the shells are assembled within the housing 12, so manufacturing or assembly tolerances which may arise as shell 50 is fitted onto shell 52 (and 40 is fitted onto 42) , can be accommodated.

Within the housing 66 are a number of drillings through which gases can pass from manifold inlets in the plate 26. Thus a drilling 146 supplies hydrogen gas to an annular groove 148 to allow hydrogen to pass into the annular space 70 to pass around the outside of the shell 60 and exit through an aperture 150 in the centre of the lower shell 52 to pass into the tube 86.

Other drillings such as 32 allow a carrier gas such as hydrogen together with reagent gases containing reactive ingredients such as arsine, phosphine, and organo metallic components of gallium, and indium and the like to be introduced into the annular space 152 between the housing formed by the two shells 50 and 52 and the housing formed by the shells 40 and 42. The mixture of carrier and reagent gases is thereby forced to travel through the annular space between these shells and finally after passing over the substrate (not shown) located on the slαsceptor 48, to pass through the castellations in the castellated cylindrical housing 74 into the tube 86 along with the hydrogen passing through aperture 150. As before the drilling 32 communicates with an annular groove 154 between the two cylindrical sections 138 and 140 of the member 66.

Another set of drillings 156 in the member 66 serves to allow hydrogen to pass via an annular groove 158 into the annular space 160 between the outer housing formed by the shells 10 and 12 and the next inmost housing formed by the shells 40 and 42. Hydrogen introduced into the space 160 passes below the underside of the shell 42 into the cylindrical space 162 within the cylindrical extension 108 of the outermost shell 12, and can pass into the filter chamber containing the filter 106 through the annular gap 164 between the outside of the cylindrical tube 86 and the inside surface of the cylindrical aperture 166, through which the tube passes, where it extends through the upper plate 98 of the housing 96.

Although lamina flow of the mixture of gases is desirable as they pass over the heated wafers of substrate, mixing, particularly in the region 152 is desirably achieved by a turbulance. To assist this each of the grooves 148, 154 and 158 is surrounded by a castellated ring 168, 170 and 172 respectively. The inside surface of each of the rings is tapered and the external surface of the cylindrical member 66 is likewise tapered in the region of each of the grooves concerned so that each of the rings is a force fit on the taper. Typically a 10° taper is employed. The passage of the gases from the grooves through the castellations into the chambers 70, 152 and 160 produces sufficient turbulance to cause satisfactory mixing of mixtures of gases introduced particularly into the space 152, and the presence of the castellations produces a more uniform distribution of the bas mixture within the chamber.

Although turbulance achieves good mixing as the gases enter the top end of the reaction chamber, laminar flow is preferred as the gases flow over the wafers of substrate (not shown) in the lower part of the enclosure. The relatively long path with changes of direction between the upper end of the enclosure and the region where the wafers of substrate are located, enables the mixed gases to assume laminar flow and achieve more uniform deposition on the wafers.

The flow of gas under pressure such as hydrogen through the pipe 118 to exit via the grooves such as 130 provides a gas bearing on which the graphite susceptor shell 48 is supported. The virtually frictionless bearing so produced enables the shell 48 to be rotated by the gas flow in known manner without the need for excessive gas energy to produce the rotation.

The apparatus shown in Figure 1 may be used in the following manner:-

1. The susceptor shell 48 is fitted into position and wafers are loaded onto it. Shell 52 is positioned into the central recess in susceptor 48. The lower housing shell 12 containing these components is then offered up to the upper part of the enclosure and secured in position by means of ring 22.

2. The enclosure is then evacuated to a full vacuum or pumped full of hydrogen to expel all gas other than hydrogen.

3. Hydrogen is then pumped through the enclosure to purge the system entirely.

4. Energy is supplied to the RF induction coil 90 to heat the susceptor.

5. If a pre-bake step is required temperature to which the susceptor is raised is above that at which vapour deposition would normally be expected to occur, so as inter alia to drive off impurities. -

Before the reagent gas or gases is introduced into the gas stream, the temperature of the susceptor and wafers is reduced to the temperature at which deposition will occur, if a prebake step has been involved. Otherwise the chamber is merely heated up to the desired reaction temperature.

6. When the temperature is correct, the first reagent gas is introduced into the space 152 by opening an appropriate valve (not shown) feeding the appropriate reagent gas to the drilling 32. If the substrate material disassociates on heating, the disassociating component may be replenished in known manner by introducing the apropriate component into the gas stream before the temperature at which disassociation occurs, is reached.

7. When sufficient of a first gas has been caused to flow through the system, it is rapidly substituted by another gas and similar swift transitions are made between it and any following gas until all the necessary gaseous reagents have been introduced into the system and caused to flow therethrough, so as to be deposited as layer upon layer on the wafers.

8. After all the layers which are required have been produced, a stabilising gas is introduced into the hydrogen flow which will prevent the last layer from decomposing whilst it is still above the congruent evaporation temperature whilst the temperature of the susceptor and wafers is reduced below the congruent evaporation temperature of the last layer of material.

9. Once the congruent evaporation temperature has been reached and the system is cooling further below that temperature, the flushing gas can be reduced to hydrogen alone and the heating can be turned off completely allowing the reaction chamber to cool down towards room temperature.

Towards the end of the process the hydrogen flush can be replaced by a dry nitrogen flush.

10. The charcoal filter housing 96 is then broken open and the charcoal filter is removed and put into an appropriate spent filter box such as a nitrogen box. A new uncontaminated charcoal filter is put in place and the housing resealed. Nitrogen purge is continued whilst the housing is split to remove the lower part 12 from the upper part 10. Shell 52 is removed and placed in a nitrogen vessel and thereafter the tube 86 can be lifted out and likewise placed in a safe environment such as nitrogen vessel. Thereafter the treated wafers can be removed from the susceptor and the susceptor itself can be removed and placed in a nitrogen box if required. Typically a fresh susceptor will be used each time. The shell 52 can be cleaned chemically such as using aqua regia or may be abraded to as to remove any reagent material clinging thereto.

The tube 86 can be cleaned chemically or by abrasives and reused or a fresh clean tube put in place during reassembly.

Figure 2 illustrates a variation of the arrangement shown in Figure 1 wherein the inner shell 60 denoted by 60a in Figure 2 is a flat bottomed container and thereby allows the shell 52 (52a in Figure 2) to be. a similarly shallow tray-like member in place of the f ustoconical shell of item 52 in Figure 1.

The susceptor must now present the wafers in a substantially horizontal manner so as to preserve a generally constant spacing between them and the underside of the shell 52a. One of the wafers is denoted by reference number 53 in Figure 2 and the modified support susceptor 48a is shown fitted to the rest of the susceptor 42a within which the thermocouples such as 92 are embedded.

The gas port 128 of Figure 1 is now denoted by 128a but this still feeds the groove 130 formed in the upper surface of the lower susceptor part 42a for the purposes hereinbefore described.

In all other regards the central region of the susceptor and the method by which gases exit and pass through the removable tube 86 are the same as herein described with reference to Figure 1.

Figure 3 illustrates a still further arrangement in which the mass of the susceptor can be even further reduced if the lower housing part 12a is formed more as a flat bottomed cylindrical member in place of the wine glass • shaped member of Figure 1 and 2. Reference numerals denoting parts which are similar to those in Figure 2 are denoted by the same reference numeral and the element which is of significant difference e.g. the lower susceptor part, is denoted by reference numeral 42b. Likewise the gas port 128a is denoted by reference numeral 128b.

The mode of operation of an arrangement incorporating the modified lower chamber section such as shown in Figure 2 or Figure 3 is susbstantially the same as that described in relation to Figure 1. The advantage of Figure 2 is that the wafers are supported in a generally horizontal manner and the arrangement of Figure 3 possesses the same advantage of Figure 2 but the added advantage of the reduced susceptor mass so that the susceptor has a lower thermal hysteresis, although this chamber design does not normally allow significantly reduced or elevated pressures to be employed.

Claims

1. Apparatus for chemical vapour deposition from gaseous reactants onto a substrate comprising an enclosure forming a reaction chamber, a substrate supporting surface therewithin and guide means also within the enclosure to cause gaseous reactants introduced therein to pass in a converging manner over the substrate supporting surface to effect deposition on any substrate located thereon, before leaving through the exit in the chamber.

2. Apparatus according to Claim 1, wherein the supporting surface is located symetrically relative to the exit.

3. Apparatus according to Claim 1 wherein the exit is located centrally of the substrate supporting surface.

4. Apparatus according to any one of Claims 1 to 3, in which the enclosure is internally circular in cross section.

5. Apparatus according to any one of Claims 1 to 4, wherein the guide means comprises a body located within the enclosure to reduce the volume available to the gaseous reactants in the enclosure and to define a cavity through which the gaseous reactants have to pass.

6. Apparatus according to Claim 5, wherein the said cavity is generally annular.

7. Apparatus according to any one of Claims 1 to 6, which also incorporates heating means for heating the substrate material.

8. Apparatus according to any one of Claims 1 to 7, which also includes cooling means for cooling at least the guide means.

9. Apparatus according to any one of Claims 1 to 8 wherein the said guide means is hollow.

10. Apparatus according to Claim 9, wherein the guide means is filled with a cooling fluid.

11. Apparatus according to Claim 10, further comprising means for transferring heat from the cooling fluid to maintain the temperature thereof substantially constant to effect the said cooling.

12. Apparatus according to any of Claims 1 to 11 wherein the guide means is a body shaped so as to define with at least the interior wall of the chamber enclosure an annular passage for the gaseous reactants.

13. Apparatus according to any one of Claims 1 to 12 wherein the guide means is centrally located within the enclosure.

14. Apparatus according to any one of Claims 1 to 13 wherein a carrier gas is introduced to flow through the chamber to remove reactive gaseous deposits from the chamber surfaces to prevent the build-up of unwanted material in the chamber.

15. Apparatus according to any one of Claims 1 to 14 wherein the space between the enclosure and the centrally located body is divided into a plurality of annular paths by the interposition of hollow shells spaced from each other and from the central body and the inside surface of the enclosure.

16. Apparatus according to Claim 15, wherein three parallel paths are defined by means of two such shells and a region of the outer said two shells comprises the substrate supporting surface.

17. Apparatus according to Claim 16, wherein first means is provided for introducing carrier gas into the outer and inner annular passages and second means is provided for introducing the gaseous reagents in combination with the carrier gas, into the central passage, so that the reactants are thereby forced to flow over the heated substrate material before reaching the exit.

18. Apparatus according to any one of Claims 1 to 17, further comprising means whereby that part of the assembly which carries the substrate, is rotatable relative to the gas flow, so as to produce a more uniform deposition of material during the deposition process.

19. Apparatus according to Claim 18 wherein the susceptor is in two parts, a stationery outer member and an inner member of complementary shape and which is rotatable within the outer member, and wherein the inner member is driven in rotation by the application of a stream of carrier gas under pressure to its underside which underside is grooved to assist in producing this rotation.

20. Apparatus according to any one of the preceding Claims wherein the substrate support is formed from graphite.

21. Apparatus according to any one of the preceding Claims wherein the enclosure is constructed in the form of a barrel reactor.

22. Apparatus according to any one of Claims 15 to 21 wherein the substrate support comprises a pair of spaced inverted hollow frustoconical shells, one lying within the other and, each having an aligned central apperture to define part of the exit from the chamber.

23. Apparatus according to Claim 22 wherein the inner surface of the inner shell is machined to form a plurality of trapezoidally shaped facets onto each of which can be secured a wafer of substrate material.

24. Apparatus according to Claim 23 wherein the substrate material is in the form of thin wafers and the inner shell is formed with appropriate wells or recesses into which the wafers fit.

25. Apparatus according to any one of Claims 1 to 24 wherein the temperature of the substrate support is monitored by means of thermocouples located therewithin.

26. Apparatus according to Claim 25 wherein electrical conductors connected to the thermocouples feed signals therefrom to an appropriate monitoring circuit and control or display.

27. Apparatus according to Claim 26 wherein the electrical conductors are fed via an outlet extension of the main casing to extend laterally through the junction between it and a filter housing.

28. Apparatus according to any one of Claims 7 to 27 wherein the heating means comprises electrical resistance heating, or RF induction heating means, or infra-red radiant heating means.

29. Apparatus according to any one of the preceding Claims 15 to 28 wherein the upper suface of the substrate support is substantially horizontal and planar so that the substrate is supported in a substantially horizontal plane and the walls of the intermediate shell and central cooling body within the overall enclosure also possess substantially planar underside surfaces to extend substantially parallel to and spaced from the susceptor and shell respectively.

30. Apparatus according to Claim 29 wherein the substrate support is in the form of an inverted truncated cone.

31. Apparatus according to Claim 29 wherein the substrate support is a substantially planar member the depth of which is sufficient to produce a substantially uniform temperature, when heated, so as to uniformly heat a substrate located on its upper surface.

32. A method of forming epitaxial layers of material on a substrate comprising the steps of locating substrate material on the surface of a susceptor so as to define a central exit and guiding a gaseous mixture containing at least one reactive component which will react with the surface of the substrate thereover in a converging manner, leaving through the exit.

33. A method according to Claim 32, wherein the susceptor surface contains a central aperture which comprises the said exit.

34. A method according to either of Claims 32 and 33, wherein the susceptor is generally circular or is frustoconical.

35. A method according to any one of Claims 32 to 34 wherein the gaseous mixture is forced to pass through a relatively narrow space between the susceptor and a member having a similar complementary shape to that of at least part of the susceptor, thereby to constrain the gaseous mixture into close proximity with the substrate material as the gaseous mixture passes thereover.

36. A method according to any one of Claims 32 to 35 wherein the substrate is heated.

37. A method according to Claim 36 which includes the step of rotating part of the assembly relative to the gaseous mixture so as to improve the uniformity of deposition of reactive material onto the substrate.

38. A method according to any one of Claims 32 to 37 which includes the step of circulating cooling fluid through at least part of the overall assembly so as to cool some of the surfaces within the overall assembly and thereby reduce pre-reaction and discourage the deposition of exhaust reactive materials thereon.

39. A method according to any one of Claims 32 to 38 which includes the step of conveying the exiting gaseous mixture to exhaust through a filter to assist in removing undesirable reactive components and gases from the mixture before it exhausts.

40. A method as claimed in Claim 39 wherein the filter is a charcoal filter

41. A method according to any one of Claims 32 to 40 which includes the step of conveying the exiting gases to the filter through a removable tube, typically a quartz tube.

42. A method according to Claim 41 which includes the steps of preparing a vapour deposition chamber and exhaust system before a deposition process to enable volatile products of reaction such as reactive components deposited during a previous vapour deposition process to be removed, which steps comprise flushing the chamber and exhaust system with carrier gas such as hydrogen, removing the filter, its housing and the removable tube from the exhaust line and if appropriate immersing the tube and the filter in a liquid such as water so as to keep air from the components, and replacing the tube and filter with fresh items u contaminated with the product of reaction and reassembling the apparatus.

43. A method according to Claim 42 wherein the fresh items are new tube and filter elements or are the original items after suitable cleaning as by abrasion and/or chemical cleaning.

44. Wafers of substrate having epitaxial layers formed thereon by vapour deposition when formed by a method as claimed in any of claims 32 to 43.

45. Apparatus for vapour deposition constructed substantially as herein described and with reference to the accompanying drawings.

46. Methods for producing epitaxial layers by vapour deposition substantially as herein described and with reference to the accompanying drawings.