GB2196019A

GB2196019A - Metalorganic chemical vapour deposition

Info

Publication number: GB2196019A
Application number: GB08624006A
Authority: GB
Inventors: Dr Michael Roger Czerniak; Michael Franks Robinson
Original assignee: Cambridge Instruments Ltd
Current assignee: Cambridge Instruments Ltd
Priority date: 1986-10-07
Filing date: 1986-10-07
Publication date: 1988-04-20
Also published as: DE3733499A1; JPS63128622A; GB8624006D0

Description

1 GB2196019A 1

SPECIFICATION

Metalorganic chemical vapour deposition cell The invention relates to metalorganic chemical vapour deposition and to apparatus (cells) for 5 carrying out the method.

Metalorganic chemical vapour deposition is a method of depositing a layer of material, such as a semiconductor (gallium arsenide or cadmium mercury telluride, for example) on a substrate which may be of similar or different material. It is known to carry out such a process by the chemical dissociation (usually thermal dissociation) of reagent gases in a container or cell in a 10 space immediately above a heated substrate. In a known arrangement, the reagent gas mix contains a carrier gas together with metalorganic compounds of the elements which are required for forming the desired layer; hydride forms of elements for which suitable metalorganic com pounds are not available may also be incorporated in the gas mix. In this known process, the gas mix is passed through the cell which is in the form of a vessel also containing the substrate 15 on which deposition is to take place. The substrate is held at an elevated temperature which is high enough to cause the thermal dissociation of the reagent molecules. This process releases free atoms of the desired elements which subsequently combine on the substrate surface to form the required layer.

According to the invention, there is provided a method of vapour deposition on a substrate in 20 which material containing a substance to be deposited on the substrate is thermally dissociated so as to release the said substance for deposition, wherein the position of thermal dissiciation is spatially separated from the position of deposition.

According to the invention, there is also provided a method of vapour deposition on a substrate in which material containing a substance to be deposited on the substrate is thermally 25 dissociated so as to release the said substance for deposition, wherein the substrate is held at a temperature substantially below the temperature at which dissociation takes place.

According to the invention, there is further provided a method of metalorganic chemical vapour deposition, comprising the steps of producing a relatively high temperature zone adjacent to a relatively lower temperature zone, placing a substrate on which deposition is to take place in the 30 relatively lower temperature zone, and supplying material to the relatively higher temperature zone in predetermined gaseous form containing metalorganic compounds, at least, of the ele ments to be deposited, the relatively higher temperature being sufficient to cause thermal dissociation of the gaseous material whereby to release the elements for deposition in the substrate on the relatively lower temperature zone. 35 According to the invention, there is still further provided chemical vapour deposition apparatus in which material containing a substance to be deposited on the substrate is thermally dissoci ated so as to release the said substance for deposition, comprising means defining a first zone in which the thermal dissocciation takes place and means defining a second zone in which the deposition takes place, and temperature control means for controlling the temperatures of the 40 zones so that the temperature of the first zone is significantly greater than that of the second zone.

According to the invention, there is yet further provided a reactor cell for metalorganic chemical vapour deposition, comprising an enclosure defining a volume of predetermined shape and size, a carrier within the volume for receiving a substrate on which deposition is to take 45 place, a susceptor within the volume but physically spaced from the carrier, temperature control means for controlling the temperatures of the carrier and susceptor so that the susceptor is held at a significantly higher temperature than that of the carrier, and means for passing into the region of the susceptor a reagent gas mix containing metalorganic compounds of the elements required to form the deposition, whereby the temperature of the susceptor causes thermal 50 dissociation of the gas mix to release free atoms of the desired elements which combine on the surface of the substrate.

A metalorganic chemical vapour deposition (MOCVD) method according to the invention, and apparatus for carrying out metalorganic chemical vapour deposition and embodying the invention, will now be described with reference to the accompanying diagrammatic drawing which is a 55 cross-section through a cell used in the method and incorporated in the apparatus.

The apparatus shown in the Figure comprises a vessel or cell 10 having a gas inlet 12 and a gas outlet 14. The gas inlet and outlet would be of appropriate design to prevent the gas inadvertently leaking into the atmosphere and to prevent the atmosphere from entering the vessel. The inlet and outlet could incorporate suitable control valves. 60 Inside the cell 10 are mounted a carrier 16 and susceptor 18. Each has associated with it a respective temperature control device 20,22. The purpose of each temperature control device is to control the temperature of the carrier or susceptor as the case may be and the devices may take any suitable form incorporating heating and cooling means. Suitable heating means could operate by RF induction, infra-red illumination or resistive heating, but other means of heating 65 2 GB2196019A 2 could be used instead. Examples of suitable cooling means are by means of water or gas cooling but again other cooling means could be used.

The substrate on which deposition is to take place is placed on carrier 16 as shown at A. In practice, more than one substrate could be placed there so as to carry out simultaneous deposition on several substrates. 5 In operation, the carrier 16 and susceptor 18 are controlled by their respective temperature control devices 20 and 22 to be held at predetermined temperatures, susceptor 18 being significantly hotter than the carrier 16. The reagent gas mix is then introduced through the gas inlet 12. The gas mix contains the metalorganic compounds of the elements required to form the desired layer on the substrate (together with hydride forms of elements for which suitable 10 metalorganic compounds are not available), and may also contain a carrier gas. The process proceeds by the reagent gases thermally decomposing on the susceptor 18 which is held at a sufficiently high temperature to cause thermal dissociation so as to form free atoms or molecules of the desired elements. Carrier 16 is held at a temperature which is in general (but not necessarily) insufficient to cause such thermal dissociation. The free atoms or molecules formed 15 as described diffuse towards the cooler carrier 16 and the substrate A so they combine to form the desired layer of material on the substrate A. The temperatures of the carrier 16 and susceptor 18, and the thermal dissociation process, combine to set up thermal and concentra tion gradients which are perpendicular to the direction of gas flow between inlet and outlet, and the free atoms and molecules diffuse down these gradients onto the carrier 16 and the substrate 20 A.

Spent gases are extracted via the outlet 14.

Effective control of the whole process is achieved by varying the temperatures of the carrier 16 and susceptor 18 and the velocity and composition of the reagent gas mix passing through the cell. Further control is achieved by varying the dimensions and shape of the cell itself, 25 particularly as this affects the free space between the susceptor and carrier.

The process described, and the cell construction, are such that the actual deposition of the required layer of material on the substrate A proceeds at temperatures substantially below those at which the dissociation or pyrolysis reaction proceeds. This contrasts with the known pro cesses and cells in which the substrate is held at an elevated temperature high enough to cause 30 the required thermal dissociation of the reagent gases. In the latter case, the actual deposition process inevitably proceeds at this high temperature, producing a number of undesirable effects.

Thus, firstly, excess interdiffusion between layers of differing composition can occur, making impossible the formation of abrupt junctions in certain materials-between cadmium telluride and mercury telluride for example. Secondly, if one of the elements which make up the layer has a 35 high partial vapour pressure, the reagent gas mix has to contain an excess of that element to prevent its loss from the layer; for example, such action may have to be taken to prevent escape of arsenic from gallium arsenide or mercury from cadmium mercury telluride. Thirdly, deposition by such a method on certain substrates may not be possible at all because the substrate actually thermally decompose at the high temperatures involved. Fourthly, the high 40 temperatures of the substrates can under certain circumstances form undesirable by-products immediately adjacent to the substrate, these by-products becoming incorporated into the deposi ted layer and adversely affecting its characteristics.

These disadvantages are overcome by the cell illustrated and the method used with it because the deposition process takes place at temperatures substantially below those of the pyrolysis 45 reaction.

Claims

1. A method of vapour deposition on a substrate in which material containing a substance to be deposited on the substrate is thermally dissociated so as to release the said substance for 50 deposition, wherein the position of thermal dissociation is spatially separated from the position of deposition.

2. A method of vapour deposition on a substrate in which material containing a substance to be deposited on the substrate is thermally dissociated so as to release the said substance for deposition, wherein the substrate is held at a temperature substantially below the temperature at 55 which dissociation takes place.

3. A method according to claim 1 or 2, in-which the said material is a gaseous material.

4. A method according to any preceding claim, in which the said material contains a metalorganic compound of one or more elements which form the said substance.

5. A method of metalorganic chemical vapour deposition, comprising the steps of producing 60 a relatively high temperature zone adjacent to a relatively lower temperature zone, placing a substrate on which deposition is to take place in the relatively lower temperature zone, and supplying material to the relatively higher temperature zone in predetermined gaseous form containing at least metalorganic compounds of the elements to be deposited, the relatively higher temperature being sufficient to cause thermal dissociation of the gaseous material 65 3 GB2196019A 3 whereby to release the elements for deposition in the substrate on the relatively lower tempera ture zone.

6. A method according to claim 5, including the step of setting up at least a thermal gradient between the relatively higher temperature zone and the relatively lower temperature zone for assisting in driving the elements to the substrate. 5

7. A method according to claim 6, including also the step of setting up a concentration gradient for at least assisting in driving the elements to the said substrate.

8. Chemical vapour deposition apparatus in which material containing a substance to be deposited on the substrate is thermally dissociated so as to release the said substance for deposition, comprising means defining a first zone in which the thermal dissociation takes place 10 and means defining a second zone in which the deposition takes place, and temperature control means for controlling the temperatures of the zones so that the temperature of the first zone is significantly greater than that of the second zone.

9. Apparatus according to claim 8, in which the zones are different parts of a closed volume.

10. A reactor cell for metalorganic chemical vapour deposition, comprising an enclosure 15 defining a volume of predetermined shape and size, a first susceptor within the volume for receiving a substrate on which deposition is to take place, a second susceptor within the volume but physically spaced from the carrier, temperature control means for controlling the tempera tures of the carrier and susceptor so that the susceptor is held at a significantly higher tempera ture than that of the carrier, and means for passing into the region of the susceptor a reagent 20 gas mix containing metalorganic compounds of the elements required to form the deposition, whereby the temperature of the susceptor causes thermal dissociation of the gas mix to release free atoms of the desired elements which combine on the surface of the substrate.

11. A cell according to claim 10, including a gas inlet for entry of the reagent gas mix and a gas outlet for exhaustion of spent gas. 25,

12. A metalorganic chemical vapour deposition method, substantially as described with refer- ence to the accompanying drawing.

13. A metalorganic chemical vapour deposition reactor cell, substantially as described with reference to the accompanying drawing.

Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC 1 R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.