GB2194966A

GB2194966A - Deposition of films

Info

Publication number: GB2194966A
Application number: GB08709629A
Authority: GB
Inventors: Noorallah Nourshargh
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1986-08-20
Filing date: 1987-04-23
Publication date: 1988-03-23
Also published as: GB8709629D0; GB8620273D0; DE3726775A1; FR2603049A1

Description

1 GB2194966A 1

SPECIFICATION

Deposition of thin films 1 This invention relates to a method of deposit70 ing thin films using a plasma assisted chemical vapour deposition technique. The method is intended primarily for depositing thin optical waveguiding films for use in integrated optics, but it has a much wider range of applications 75 and can be used, for example, for depositing passivating buffer layers, metal films and sem iconductor films; these could be used in inte grated optics, integrated circuits and a variety of guided wave optical sensors.

One method of depositing thin films in the manufacture of integrated optical waveguides by such a technique is described in copending Patent Application No. 8514618 (Serial No.

2160226). In the method described in the aforesaid patent application a substrate is mounted so as to be exposed to the interior of a reaction tube having an inner perforated tube extending into it, and mounted within an outer tube, reaction vapours in a carrier gas and oxygen are introduced into the reaction tube through the perforations in the inner per forated tube whilst the pressure within the re action tube is maintained at around 10 Torr, and the microwave power and flow rates are 95 controlled to produce a plasma column at least over the region occupied by the sub strate, whilst the latter is heated, to produce said chemical reaction and the deposition of a layer of glass on the substrate.

An object of the present invention is to pro vide an alternative method of depositing thin films which enables deposition to take place over larger substrates than has been found practicable with the method described above. 105 Accordingly in a method of depositing a thin film on a substrate using a plasma assisted chemical vapour deposition technique, the sub strate is supported within an evacuated cham ber, and a gaseous reaction mixture, compris- 110 ing two or more gases and/or vapours capable of reacting together to form a solid deposit, is directed towards a surface of the substrate through a multiplicity of perforations in a plate spaced from and substantially paral- 115 lei to the surface whilst a microwave plasma is produced between the perforated plate and the substrate such as to cause said chemical reaction to take place and form a film on the substrate surface.

Both the substrate surface and the adjacent surface of the perforated plate are preferably planar, and are conveniently spaced apart a distance of between 0.5 and 1.5cm, although the most satisfactory spacing for any particu lar application of the invention may readily be found by trial.

Not only does the present invention enable films to be formed over a. greater area than the earlier microwave plasma deposition pro- 130 cess referred to above, but, because it can be carried out with a smaller reaction zone volume, it does not require as high an electrical or microwave power consumption. Preferably the chamber containing the substrate is continuously evacuated during the deposition process to a pressure of the order of 1 millibar. The substrate may be heated or cooled during the process which may be desirable in some cases, for example, to improve adhesion of the deposited film, or to prevent overheating of the substrate by the plasma, which could be harmful for some forms of substrate. 80 The perforations in the plate are conveniently disposed in a regular array, and have a diameter of the order of 100 um. Such perforations can readily be produced by C02 laser drilling. 85 For the manufacture of integrated optical waveguides by this technique the waveguide is conveniently composed of a glass deposited as a first film on the substrate in a desired pattern, and a second film of glass, having a lower refractive index, is subsequently deposited on the first film.

The desired pattern of glass may be formed by selective removal of part of the first film, after deposition, to leave the pattern remaining, the removal conveniently being effected by masking the first film with the pattern, and then etching away the remainder of the film, prior to the deposition of the second film. Both the first and the second films may be formed by the method of the invention, although it will be apparent that in the process as described above the two films will need to be deposited in separate stages.

However the desired pattern may alternatively be achieved by producing grooves in the desired pattern in the surface of the substrate, depositing the first film on the surface so that the desired pattern is formed at the bottom of the pattern of grooves, and then depositing the second film over the first film. In this case the substrate need not be removed from the deposition apparatus between the application of the first and second films. The second film preferably has the same refractive index as the material of the substrate.

One form of apparatus for carrying out a deposition process in accordance with the invention, and its manner of use, will now be described by way of example with reference to Figs. 1 and 2 of the accompanying schematic drawing, in which Figure 1 represents perspective views of various parts of the apparatus, and Figure 2 represents a sectional view of the assembled apparatus.

Referring first to Fig. 1, the main part of the reaction chamber consists of three sections 1, 2, 3, namely a first section 1 provided by a silica outer tube having a main portion 4 of relatively larger diameter and a neck portion 5 2 GB2194966A 2 of relatively smaller diameter at one end, a second section 2, comprising a cylindrical silica inner tube having an external diameter sightly less than the internal diameter of the main portion 4 of the outer tube 1, and closed at one end by a silica plate 6, and a third section 3 forming a dispenser tube, in the shape of an inverted funnel or shower head, closed at its wider end by a perforated -silica disc 7, and terminating at the opposite end in a narrow inlet portion 8 having an external diameter which is smaller than the internal diameter of the neck portion 5 of the first section 1. The three sections fit together as shown diagrammatically in Fig. 2 to provide a reaction zone between the perforated disc 7 and the plate 6, the latter providing a support for a substrate plate 9 on to which a film is required to be deposited; an O- ring vacuum sea[ 12 is formed between the inner tube 2 and the main portion 4 of the outer tube 1.

In use of the apparatus, the substrate plate 9, for example of silica, is placed on the support plate 6 and the inner tube 2 is introduced into the outer tube 1, so that the upper surface of the substrate plate 9 is closely adjacent the lower surface of the perforated disc 7, for example at a distance of approximately 1 cm therefrom.

Gases and/or vapours which are capable of reacting together to form a required deposit are then fed into the upper end of the inlet portion 8 of the dispenser tube 3 and enter the reaction zone through the perforations 13 in the disc 6, whilst exhaust gases are removed through the annular space between the inlet portion 8 of the dispenser tube 3 and the neck 5 of the outer tube 1 using a vacuum system, the pressure in the reaction zone be- ing maintained at, typically, 1 millibar.

A chemical reaction is initiated by means of a plasma produced in the reaction zone as indicated at 14, so as to cause a layer of material to be deposited on the upper surface of the substrate plate 9. The plasma is excited by introducing microwaves into the reac- -tion zone using a microwave cavity as shown diagrammatically at 15. It will, however, be appreciated that both the design of the cavity and its position relative to the reaction zone can be changed to suit the particular application.

Provision can be made for either heating or cooling the substrate from below the platform, as may be desirable in some cases, as above 120 described.

The perforations 13 in the dispenser tube disc 6 consist of a regular array of several hundred holes each with a diameter of about 100,um. The holes are most conveniently produced by C02 laser drilling. In this way, both the pattern and the size of the holes can be readily altered. By adjusting the pattern one can alter the thickness profile of the deposited layer. The diameter of the holes, on the other hand, has a direct bearing on the chemical flow rates that can be used and also on the pressure in the reaction zone.

The invention may be used to produce an integrated optical waveguide on the substrate plate in a generally similar manner to the methods described with reference to Figs. 2 and 3 of Patent Application No. 8514618.

However, the present technique allows de- position over much larger substrates, for example up to 3 inches in diameter, than has been found practicable with the apparatus illustrated in Fig. 1 of that application. In addition, it has much lower running costs, since it requires less electrical power and substantially less microwave power than the former apparatus, because of the smaller volume of the reaction zone.

For the manufacture of an integrated optical waveguide on a silica substrate the first film is conveniently formed of silica incorporating a suitable dopant, for example GeO2, such that it has a suitably higher refractive index than the silica substrate, the second film then conveniently being formed of undoped silica. The gaseous mixture used in such a deposition process will usually consist of oxygen plus the vapours of one or more halides, depending upon the particular film required, as well as an easily ionizable gas such as argon.

Moreover although the invention is particularly suitable for use in the manufacture of integrated optical waveguides it can also be used for a variety of applications in which a film can be formed by a plasma assisted chemical vapour deposition technique, for example passivating buffer layers, metallic films and semiconductor films.

Moreover the composition of a film can be varied across its thickness by varying the relative proportion of the reaction gases or vapours during a deposition process.

It will be understood that the invention includes within its scope apparatus for use in carrying out the method of plasma deposition above described.

Claims

1. A method of depositing thin films on a substrate using a plasma assisted chemical vapour deposition technique, in which the substrate is supported within an evacuated chamber and a gaseous reaction mixture, comprising at least two gases capable of reacting chemically together to form a solid deposit, is directed towards a surface of the substrate through a multiplicity of perforations in a plate spaced from and substantially parallel to the surface whilst a microwave plasma is produced between the perforated plate and the substrate such as to cause said chemical reaction to take place and form a film on the substrate surface.

2. A method of depositing thin films as claimed in Claim 1 in which the substrate sur- i 3 GB2194966A 3 i face is planar.

3. A method of depositing thin films as claimed in Claims 1 or 2 in which the surface of the perforated plate adjacent to the sub5 strate surface is planar.

4. A method of depositing thin films as claimed in any preceding claim in which the substrate surface and the adjacent surface of the perforated plate are spaced apart a dis- tance of between 0.5 and 1.5 cm.

5. A method of depositing thin films as claimed in any preceding claim in which the evacuation chamber is continuously evacuated during deposition to a pressure of the order of 1 millibar.

6. A method of depositing thin films as claimed in any preceding claim in which the substrate is heated during deposition.

7. A method of depositing thin films as claimed in any preceding claim in which the substate is cooled during deposition.

8. A method of depositing thin films as claimed in any preceding claim in which the perforations are disposed in a regular array.

9. A method of depositing thin films as claimed in any preceding claim in which the perforations have a diameter of the order of 100'am.

10. A method of depositing thin films as claimed in any preceding claim in which a glass is deposited on the substrate as a first film in a desired pattern and a second film of glass, having a lower refractive index, is subsequently deposited on the first film.

11. A method of depositing thin films as claimed in any preceding claim in which a desired pattern is formed by selective removal of part of a film after deposition to leave the pattern remaining.

12. A method of depositing thin films as claimed in Claim 11 in which the selective removal is effected by masking the film with a pattern and then etching away the remainder of the film.

13. A method of depositing thin films as claimed in Claims 1 to 10 in which a desired pattern is formed in a film by producing grooves in the surface of the substrate in the desired pattern and depositing a film on the surface so that the desired pattern is formed in the film.

14. A method of depositing thin films as claimed in Claims 11, 12 or 13 in which a second film is deposited over the film in which the desired pattern is formed.

15. A method of depositing thin films, as claimed in Claim 14, in which the second film has the same refractive index as the substrate.

16. Apparatus for depositing thin films on a substrate using a microwave plasma assisted chemical vapour deposition technique consisting of a silica outer tube, a silica inner tube having an external diameter slightly less than the internal diameter of a main portion of the outer tube, the inner tube being closed at one end by a silica plate, on which the substrate is placed, a dispenser tube in the shape of a funnel closed at its wider end by a perfo- rated disc and terminating at the opposite end in a narrow inlet portion having an external diameter which is smaller than the internal diameter of a neck portion of the outer tube and an 0-ring vacuum seal formed between the inner tube and the main portion of the outer tube.

17. A method of depositing thin films as hereinbefore described and illustrated in Figs. 1 and 2 of the accompanying drawings.

18. Apparatus for depositing thin films on a substrate as hereinbefore described and illustrated in Figs. 1 and 2 of the accompanying drawings.

Published 1988 at The Patent Office, State House, 66/7 1 HighHolborn, London WC1R 4TP. Furthercopies 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.