GB2173512A

GB2173512A - Apparatus and method for chemical vapour deposition of products

Info

Publication number: GB2173512A
Application number: GB08608070A
Authority: GB
Inventors: Alan John Worrall; Brian Swanson Duchart; Christopher Dawson Bartlett
Original assignee: Thales Optronics Ltd
Current assignee: Thales Optronics Ltd
Priority date: 1985-04-03
Filing date: 1986-04-02
Publication date: 1986-10-15
Also published as: GB8508699D0; GB8608070D0

Abstract

CVD products are formed in a heated pressure chamber (7) from gaseous reactants fed to the chamber interior. A first passageway (10A) feeds a normally gaseous reactant from a source and a second passageway (33) feeds a vapourised reactant from a molten source (4), the first passageway (10A) comprises a nozzle (30A) at its interface with the reaction chamber (7) causing the reactant to enter the chamber (7) in turbulent flow. The molten source (4) includes a heated crucible mounted on a weigh cell (43) located beneath the chamber (7). Gaseous waste materials are removed from chamber (7) through an exhaust duct (8) containing a condenser mechanism (22, 23, 24) which condenses gaseous waste materials into liquid state and retains the condensed liquid.

Description

SPECIFICATION Apparatus and method for chemical vapour deposition of products Apparatus for and methods of chemical vapour deposition of products are already known and involve a chemical reaction between reactants in gaseous form at or near a heated receptor surface on which the product is deposited. The reaction usually takes place in a reaction chamber operating at reduced pressures and elevated temperatures but gives rise to a product deposition rate which is comparatively low so that to obtain thick deposits of product takes many hours and in the known apparatus and methods, uniformity of thickness has been difficult to achieve.

It is an object of the present invention to provide improved apparatus and methods for chemical vapour deposition of products.

According to the present invention there is provided apparatus for chemical vapour deposition of products comprising a heated reaction chamber capable of operating at reduced pressures, means for feeding gaseous reactants to the interior of the chamber, said means comprising a first passageway for feeding a normally gaseous reactant from a source thereof and a second passageway for feeding vaporised reactant from a molten source thereof, said first passageway comprising a nozzle at its interface with the reaction chamber whereby the normally gaseous reactant is caused to enter the reaction chamber in turbulent flow.

By virtue of the present invention mixing of the gaseous reactants is effected rapidly, substantially more rapidly than by diffusion as utilised hitherto, as a result of which transport of the mixed reactants to the heated receptor surface of the reaction chamber is rapid giving rise to rapid deposition of product thereon.

Preferably each nozzle is surrounded by an annular jet supplied with a flow of inert gas for the purpose of preventing vaporised reactant chemically reacting at and clogging or restricting the nozzle.

Preferably said molten source comprises a heated crucible removably located beneath said reaction chamber and connected with said second passageway by means of an inert gas seal.

Preferably also said crucible is mounted on a weigh cell whereby the quantum of vaporised reactant removed from the crucible and entering the reaction chamber is measured.

Preferably also the reaction chamber is provided with an exhaust duct for receiving gaseous waste materials from the chamber, said duct incorporating a condenser mechanism having a portion proximal to the reaction chamber adapted to condense gaseous waste materials into-the-liquid state at elevated temperature, the said condenser portion having liquid retention channels.

Preferably also the condenser mechanism has a portion distal to the reaction chamber adapted to condense gaseous waste materials into solid state at moderate temperature.

The. apparatus of the present invention is particularly suited to production of Zinc sulphide, Zinc selenide, Zinc Telluride, Cadmium Sulphide, and Cadmium Telluride from the appropriate gaseous reactant: hydrogen sulphide, hydrogen selenide or tellurium and the respective molten reactant : Zinc or Cadmium. It will however be appreciated that a wide range of other products could be similarly manufactured.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 schematically illustrates apparatus according to the present invention; Figures 2 and 3 are enlarged views of different details of the Fig. 1 apparatus; and Figure 4 schematically illustrates a detail of Fig. 3.

As is shown in Fig. 1 of the drawings a chemical vapour deposition apparatus comprises a cylindrical vacuum furnace 1 the casing of which is double-skinned to permit circulation of a liquid coolant 2, such as water, whereby the exterior surfaces of the furnace 1 are maintained at an acceptably low temperature.

Internally the furnace 1 comprises a reaction chamber 7 above which is located a condenser 8 and beneath which is located a crucible 4 containing a molten reactant (such as Zinc).

The chamber 7 is maintained at a constant elevated temperature by means of electrical heating elements 12 and the crucible 4 is heated by further electrical heating elements 9, the elements 9,12 being externally surrounded by a thermal insulation jacket 3.

In order to introduce gaseous reactants into the interior of the chamber 7 a manifold block 6 is provided between the chamber 7 and the crucible 4, block 6 having internal passageways communicating with both chamber 7 and crucible 4 (via spigot 11 as will be explained) and these passageways extend to the exterior of the furnace 1 to connect with inlet pipes 10 to which various gases are fed from pressurised sources thereof.One inlet pipe 10A is fed with reactant gas (such as Hydrogen sulphide) which is directed straight into chamber 7; whereas another inlet pipe 10B is fed with an inert gas (such as Argon) which is directed straight into crucible 4 there to entrain molten metal vapour prior to this gas flow being redirected into chamber 7 where chemical reaction occurs and the product (Zinc sulphide) is deposited on the walls of the chamber 7 which act as receptor surfaces. The thickness of deposited material increases as the process continues and waste gases such as the inert gas, excess gaseous reactant, and non-reacted metal vapour pass through the chamber 7 into the condenser 8, where solid and liquid waste is collected, and gaseous waste is exhausted along duct 13 by means of a vacuum pumping system (not shown).Gaseous waste is released to atmosphere after traversing a gas scrubbing plant.

The gaseous reactant entering inlet pipe 1 0A is determined by valve 1 4A and metered by a conventional gas flow rate device 1 4B whereas the molten reactant is metered by a weigh system 5 supporting the crucible 4, the output of measure determined by system 5 being used to servo control the flow of inert entrainment gas entering inlet pipe 10B at valve 14C. A control system 15 correlates the settings of valves 14A, 14C, whereby the quantum of vaporised reactant and the quantum of gaseous reactant entering the chamber 7 is controlled precisely so that the quality of the deposited product within chamber 7 is controlled.

In the case where the deposited product is Zinc sulphide arising from the reaction between Zinc vapour and Hydrogen sulphide gas, the reaction chamber is held at a reduced pressure of between 20 and 100 torr and at temperatures in the range 600-900"C.

The condenser 8 is illustrated in greater detail in Fig. 2 and is in the form of a housing 20 secured to the top end of the reaction chamber 7, the housing 20 in its lower face being provided with a series of entry ports 21 through which the gaseous waste products pass from chamber 7. Mounted within and extending across the full width of housing 20 is a series of internal baffles 22 located at different positions within the housing 20 and arranged to cause the gaseous waste products to traverse a serpentine path before existing from housing 20 via duct 13. The lower portion of condenser 8 which is proximal to chamber 7 is provided with thermal insulation 23 which may take the form of carbon felt, graphite felt, rigidised carbon foam or a similar graphite-based material secured to the baffles 22 and, if required, to the housing -20.With this construction of condenser 8 the speed of the gas flow through the condenser is reduced so that metal vapour content therein is caused to condense. For example, the gas velocity is reduced by a factor of the order of 50 and a temperature reduction of the qrder of 300-400"C is provided over the vertical height of the condenser 8 which may be limited to as -little as 300 mm. This arrangement enables the condensing metal vapour to be retained in liquid form in the lower portion of the condenser 8 and in solid form in the upper portion of the condenser 8. The solid matter adheres to the uppermost baffle 22 and to the housing 20 whilst condensed liquid is captured by liquid retention channels 24.

As shown in Fig. 3 the lower end of the reaction chamber 7 is supported by the manifold block 6 and inlet pipe 10A which receives a supply of gaseous reactant is connected to passageway 30 which terminates at its interface with the reaction chamber in a nozzle 30A. Inert gas is supplied via inlet pipe 10C to an annular jet 31 which surrounds nozzle 30A for the purpose of preventing vapourised reactant clogging or restricting nozzle 30A by way of deposit of reactant in the vicinity.thereof. The vapourised reactant is itself fed into chamber 7 by way of a plurality of jets 32 arranged in a ring around nozzle 30A and connected in common to a passageway 33 leading into the crucible 4 via the spigot 11, the spigot 11 being of such length as to terminate above the level of molten metal within crucible 4.In order to extract the metal vapour from the crucible 4 inlet pipe 10B is connected to passageway 34 which extends via spigot 11 into the crucible-4 such- that the entrainment gas emerges from the spigot 11 in a radial manner and is caused to circulate within crucible 4 to enable vapour entrainment prior to the entrained vapour traversing passageway 33 towards the chamber 7.

For the purpose of sealing the- junction between crucible 4 and manifold block 6 to prevent escape of vapourised reactant, a further inlet pipe 10D is provided and which is con necked to passageway 35 along which an inert gas flow is directed in order to form a gas seal annularly surrounding spigot 11. It is preferred that the inert gas supplied to inlet pipe 10D is the same as that supplied to inlet pipe 10B.

As is shown in Fig. 3 for a typical reaction chamber 7, several nozzles 30A each associated with jets 31,32, are provided with respective nozzles and jets connected in parallel to the pertaining feed passageways for the purpose of enhancing uniformity of gas flow within the chamber 7.

In order for the weigh system 5 on which crucible 4 is mounted to be fully effective as regards accuracy of measurement (to about one part in ten thousand), it is of considerable importance that the gas seal at spigot 11 be fully effective which means that the annular gap between the spigot 11 and the crucible 4 must be of constant width and that at least in the vicinity of the gas seal the gap between the crucible 4 and the manifold block 6 must be of constant height. To achieve this crucible 4 is supported rigidly on a three-point mount comprising one fixed leg and two inverted vee-blocks 40 which, via respective cylinders 41, mate with vee-blocks 42 secured to load cells 43. With this arrangement a limited extent of lateral self-alignment is achieved between the load cell 43 and the load which ensures that the force supplied to the load cell acts vertically downwardly. Furthermore, by virtue of the vee-block and cylinder.arrange- ment crucible 4 is easily removed for replenishment with fresh metal and is quickly relocated in precisely the same position as previously so that the separation of the crucible from the manifold block 6 and from the spigot 11 remains the same.

In order to remove the crucible 4 from the furnace 1 the tines of a forklift truck are entered beneath the crucible 4 to raise it by about 1 mm which is typically the size of the gap between the crucible 4 and the manifold block 6. This enables cylinders 41 to be removed so that crucible 4 can thereafter be lowered until spigot 11 is withdrawn from the crucible and thereafter crucible 4 can be removed laterally from the furnace 1. In this connection it will be understood that furnace 1 is itself formed with a clam shell construc tion so that the components illustrated in Fig.

1 are located in one half of the clam shell construction.

Fig. 4 illustrates nozzle 30A in greater detail where the nozzle diameter is d and the gas flowrate is V for a gas having a density p and.yiscosity The Reynolds number Re is givers by the formulation: dVp Re=- and turbulent flow occurs at the nozzle outlet when the Reynolds number Re exceeds a critical value. This critical value varies according to individual gases and conditions being used but is easily established on an experimental basis so that to enable implementation of turbulent flow within chamber 7 in accordance with the present invention all that is required for a given construction of manifold block 6 is that the gas flow rate V be maintained at or above a predetermined level.

In a modification of the above described apparatus the output of measure determined by the weigh system 5 is used to servo control the temperature of the crucible 4 so that without varying the flow of inert entrainment gas the quantum of vaporised reactant carried therewith is adjusted in the desired manner.

Claims

1. Apparatus for chemical vapour deposition of products comprising a heated reaction chamber capable of operating at reduced pressures, means for feeding gaseous reactants to the interior of the chamber, said means comprising a first passageway for feeding a normally gaseous reactant from a source thereof and a second passageway for feeding vaporised reactant from a molten source thereof, said first passageway comprising a nozzle at its interface with the reaction chamber whereby the normally gaseous reactant is caused to enter the reaction chamber in turbulent flow.

2. Apparatus as claimed in claim 1, wherein each nozzle is surrounded by an annular jet supplied with a flow of inert gas for the purpose of preventing vaporised reactant chemically reacting at and clogging or restricting the nozzle.

3. Apparatus as claimed in claim 1 or claim 2, wherein said molten source comprises a heated crucible removably located beneath said reaction chamber and connected with said second passageway by means of an inert gas seal.

4. Apparatus as claimed in claim 3, wherein said crucible is mounted on a weigh cell whereby the quantum of vaporised reactant removed from the crucible and entering the reaction chamber is measured.

5. Apparatus as claimed in any preceding claim, wherein the reaction chamber is provided with an exhaust duct for receiving gaseous waste materials from the chamber, said duct incorporating a condenser mechanism having a portion proximal to the reaction chamber adapted to condense gaseous waste materials into the liquid state at elevated temperature, the said condenser portion having liquid retention channels.

6. Apparatus as claimed in claim 5, wherein the condenser mechanism has a portion distal to the reaction chamber adapted to condense gaseous waste materials into solid state at moderate temperature.

7. Apparatus for chemical vapour deposition of products comprising a heated reaction chamber capable of operating at reduced pressures, means for feeding gaseous reactants to the interior of the chamber, said means comprising a first passageway for feeding a normally gaseous reactant from a source thereof and a second passageway for feeding vaporised reactant from a molten source thereof, said molten source being supported by weight-measuring means, a gas seal interfacing the second passageway with the molten source, and means for directing a flow of entrainment gas at the molten source to thereby entrain vaporised reactant and to deliver said vaporised reactant via said second passageway to the reaction chamber, said weight measuring means providing a measure of the depletion rate of the molten source for controlling the apparatus to produce a product a predetermined quality.

8. Apparatus as claimed in claim 7, wherein the entrainment-gas-flow directing means includes flow-rate control means the setting of which is determined by said measure provided by the weight measuring means.

9. Apparatus as claimed in claim 7, including temperature control means for the reaction chamber the setting of which control means is determined by said measure provided by the weight measuring means.

10. Apparatus as claimed in any one of claims 7-9, wherein said molten source is supported on V-block mounts including removable cylinders to permit said source to be removed for replenishment and to be re-located precisely.

11. Apparatus for chemical vapour deposition of products, comprising a heated reaction chamber capable of operating at reduced pressures, means for feeding gaseous reactants to the interior of the chamber, said means comprising a first passageway for feeding a normally gaseous reactant from a source thereof and a second passageway for feeding vaporised reactant from a molten source thereof, and an exhaust duct for receiving gaseous waste materials from the chamber, said duct including a condenser mechanism having a portion proximal to the reaction chamber adapted to condense gaseous waste materials into liquid state at elevated temperature, said condenser portion having liquid retention channels.

12. Apparatus as claimed in claim 11, wherein the condenser mechanism has a portion distal to the reaction chamber adapted to condense gaseous waste materials into solid state at moderate temperature.

13. Apparatus as claimed in claim 11 or claim 12, wherein said proximal portion comprises at least one transverse baffle providing a serpentine path for travel of gaseous waste materials, said at least one baffle being provided with a layer of thermal insulation.

14. Apparatus as claimed in any one of claims 11-13, wherein said distal portion comprises at least one transverse baffle providing a serpentine path for travel of gaseous waste materials.

15.- A chemical vapour deposition product when produced by the apparatus as claimed in any preceding claim.

16. A chemical vapour deposition product as claimed in claim 15, wherein the product is any one of the materials: Zinc sulphide Zinc selenide Zinc- telluride Cadmium sulphide Cadmium telluride