GB1559978A

GB1559978A - Chemical vapour deposition processes

Info

Publication number: GB1559978A
Application number: GB5059175A
Authority: GB
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1976-12-01
Filing date: 1976-12-01
Publication date: 1980-01-30

Description

(54) IMPROVEMENTS IN OR RELATING TO CHEMICAL VAPOUR DEPOSITION PROCESSES (71) We, THE GENERAL ELECTRIC COMPANY LIMITED, of 1 Stanhope Gate, London W1A lEM, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment This invention relates to chemical vapour deposition processes that is to say processes of the type in which a chemical reaction is caused to take place in a reaction zone, usually heated, between two or more substances at least one of which is in the gaseous or vapour state and a product of such reaction is deposited or formed, in the liquid or solid state, in a desired location. The invention is more particularly concerned with processes of this type wherein at least one of the reactant substances is transported to the reaction zone by means of a carrier gas stream which has been passed through or over a body of said substance in liquid form so that the substance in vapour andlor liquid form is entrained in the gas stream. The invention also relates to apparatus for use in carrying out the processes described.

In a process of the type referred to, the reaction may be effected between two or more vapours each entrained in a carrier gas stream or between one or more such entrained vapours and one or more other gases and/or vapours, or between one or more components of a gas/vapour mixture and a substance introduced into the reaction zone in the solid or liquid state; the carrier gas may be a reactant, or may dilute the reaction mixture, thus assisting in controlling the rate of reaction, and if desired an additional gas which does not take part in the required reaction may be introduced as a further diluent. The reactant gases and vapours are passed into a reaction zone within a tube or other suitable vessel formed of material which is inert to the reactants under the conditions of thc reaction, while the reaction zone is heated if required, for example by enclosing the vessel in an electrical resistance furnace, or by radio frequency heating of a susceptor placed within the vessel, or by flame heating of the exterior of the vessel.

A process of this type can be designed, for example, to achieve vapour phase nucleation of a reaction mixture in order to effect deposition of a desired solid product on a surface, or to alter the composition of at least the surface layers of a material placed in the reaction zone, or to effect epitaxial growth of a layer of material on a substrate placed in the reaction zone. Such a process can thus be employed, for example, in the manufacture of a semiconductor device by epitaxial deposition of a layer of one conductivity type on a substrate of opposite conductivity type to form a p-n junction, or in the manufacture of a preform for the production of an optical fibre waveguide, for example by depositing material of the desired composition of the fibre core on the interior surface of a tube of the desired composition of the fibre cladding, the preform subsequently being drawn to form the fibre.

Entrainment of a reactant substance in a carrier gas stream for transportation to the reaction zone is usually achieved by bubbling the gas at a suitably controlled rate through the liquid substance maintained at a temperature giving adequate vapour pressure, the liquid being contained in a vessel of the type generally referred to as a "bubbler", which is provided with a gas inlet tube extending below the surface of the liquid and an outlet for the gas/reactant mixture situated above the surface of the liquid, the vessel otherwise being closed. When the carrier gas is bubbled through the liquid, the bubbles bursting at the surface of the liquid initially release a cloud of small droplets, which are entrained in the carrier gas and subsequently evaporate: usually vaporisation of the droplets is com pleted before the mixture reaches the reaction zone, but in some cases, where large mass transfer of the reactant is required, the velocity of the carrier gas stream is so high that there may not be sufficient time for full vaporisation to take place before the mixture enters the reaction zone, so that the reactant is introduced into the reaction zone partly in liquid form and, although vaporisation is rapidly completed in the heated zone, the reaction may commence before this has occurred. At least a proportion of any impurities which may be present in the liquid in the bubbler will escape from the liquid in the droplets entrained in the carrier gas and, especially if vaporisation of the droplets is not completed before the reaction zone is reached, such impurities are carried into the reaction zone and are liable to be incorporated in the reaction product. Such contamination is clearly undesirable in cases where the product is subject to high purity requirements, as for example in the manufacture of optical fibre preforms and semiconductor devices, and it is not always possible to ensure that the liquid initially introduced into the bubbler is of the requisite high degree of purity, or that contamination of the liquid, for example atmospheric contamination during charging of the bubbler, is avoided.

It is an object of the present invention to provide an improvement in a chemical vapour deposition process of the type referred to which involves transportation of at least one of the reactants to the reaction zone by entrainment in a carrier gas stream, whereby impurities initially present in the liquid reactant can be to a large extent eliminated from the said reactant before it is introduced into the reaction zone, and hence a product of high purity can be obtained from the reaction. It is a further object of the invention to provide apparatus suitable for use in effecting such improvement in the process.

According to the invention, in a chemical vapour deposition process, as hereinbefore defined, which includes the step of transporting at least one reactant substance to the reaction zone by entrainment in a stream of carrier gas, a body of liquid, consisting of said reactant substance in liquid form containing one or more impurities of vapour pressure different from that of the liquid reactant substance, is contained in a vessel which is provided with inlet and outlet means for liquid and for gas and is otherwise closed, a porous mass of inert (as hereinafter defined) solid material, preferably in fibrous or particulate form, is partially immersed in said body of liquid so as to be wholly impregnated with the liquid, and the carrier gas stream is passed through the said porous mass of liquid-impregnated material, whereby differential evaporation of the liquid reactant and said impurity of ompurities is promoted by the presence of said porous mass of material and the impurity or impurities is or are substantially eliminated from the reactant substance before it is entrained in the gas stream and introduction into the reaction zone.

The term "inert", as used herein with reference to the porous mass of material immersed in and impregnated with the said liquid, is to be understood to mean that the said material does not react chemically with the liquid under the conditions of the process; the material should also be free from impurities which are capable of being extracted into the liquid.The said material may consist of a mass of fibres, or may be in the form of powder, granules, small pellets, or shavings, or in any other suitably porous form; glass or silica is suitable in many cases, for example in the form of glass wool or silica wool or small glass balls or powder. The said porous mass of material preferably substantially fills that part of the vessel which is occupied by the body of liquid in use, as well as extending above the surface of the body of liquid so as to perform the function of a wick in conveying, by capillary action, some of the liquid above the said surface, thus facilitating rapid evaporation of the liquid.

Th ecarrier gas stream may be passed through the body of liquid in which the said porous mass of inert solid material is immersed: in this case the effect of the immersed material is to modify the formation of bubbles, which results from the passage of the gas stream through the liquid, in such a way that the release of droplets of the liquid into the gas stream is prevented, so that liquid entrained in the gas stream is vaporised substantially immediately on leaving the body of liquid, the bubble modification and rapid evaporation being assisted by the extension of the said immersed material above the liquid surface. Alternatively, the gas stream may be passed only through a portion of the said porous mass of material which lies above the surface of the body of liquid and which is impregnated with liquid by virtue of capillary action: in this case no bubbles are formed, but the gas stream will entrain vapour already present above the "wick" by virtue of rapid evaporation of the liquid which is promoted by the presence of the "wick" material above the said surface.

The rapid vaporisation of the liquid pro moted by either of the arrangements described above enables any impurity in the reactant substance, which impuity has a different vapour pressure from that of the reactant substance in liquid form, to be largely removed from the reactant vapour before the latter is introduced into the reaction zone.

Thus an impurity whose vapour pressure is higher than that of the liquid reactant substance will evaporate preferentially and will therefore be mainly removed from the liquid in the early stages of the passage of the carrier gas stream through the liquidcontaining vessel, so in this case the introduction of the gas stream with entrained vapour into the reaction zone is delayed until after substantially all of such impurity has been removed from the body of liquid in this way and the liquid remaining in the vessel is thus substantially free from this impurity. On the other hand, if the vapour pressure of an impurity is ]ower than that of the liquid reactant substance, the latter will evaporate preferentially and the carrier gas stream emerging from the liquid-containing vessel will be substantially free from the said impurity of lower vapour pressure: in this case, therefore, the gas stream, with entrained vapour, initially emerging from the vessel is immediately introduced into the reaction zone. The concentration of the impurity of lower vapour pressure in the liquid remaining in the vessel will increase until it becomes unacceptably high, when the liquid is drained off and the vessel is recharged with fresh liquid.

Since the method of introducing the reactant substance into the carrier gas stream, in accordance with the invention, enables a reactant vapour to be separated from impurities intially present in said reactant in the liquid form, this method reduces the necessity of employing a liquid reactant which is initally in a high state of purity, and also reduces the need to exercise extreme care in avoiding contamination of the liquid reactant before and during its introduction into the vessel through which the carrier gas stream is passed.

The liquid-containing vessel employed for carrying out the method of the invention is provided with an inlet tube for liquid, an inlet tube for the carrier gas terminating at the desired level in the vessel, that is to say either in the body of liquid or in that portion of the porous mass of inert solid material extending above the body of liquid, an outlet for the gas/vapour mixture located in the upper part of the vessel, above the levels to which the vessel is filled, in use, with liquid and the porous mass of inert solid material partially immersed therein, and an outlet for liquid located at the bottom of the vessel.

Apart from these inlets and outlets, the vessel is normally closed but is capable of being opened for the introduction of the said porous mass of solid material, and is provided with external heating means if required. If the said solid material to be partially immersed in the liquid is in such a form that it would be capable of floating on the liquid, such as glass wool or silica wool, means are provided for retaining the mass of material in the desired positions, such that it will be partially immersed in the liquid: such retaining means may consist, for example, of a plate of disc supported by the side walls of the vessel at a suitable level below which it is desired that the said mass is held, and having an aperture permitting the insertion of the gas inlet tube therethrough and egress of the gas/vapour mixture to the appropriate outlet. All parts of the vessel and the fluid inlet and outlet tubes must, of course, be constructed of material which is chemically inert to the liquid and the immersed porous solid material to be contained in the vessel, and which is capable of withstanding the temperatures to which the vessel is required to be heated in use.

Any number of the vapour reactants in a chemical vapour deposition process may be introduced into a carrier gas stream, for transportation to the reaction zone, by the method in accordance with the invention, a vessel containing a porous mass of inert solid material as aforesaid being provided for containing each such reactant in liquid form.

The method of the invention is advantageously applied to chemical vapour deposition processes employed for the manufacture of preforms to be subsequently drawn to form optical fibre waveguides, which preforms are composed mainly of vitreous silica and are at least partly formed by the chemical vapour deposition of silica, or of mixtures of silica with one or more dopants, especially other oxides for example titanium dioxide, phosphorus pentoxide boron oxide or alumina for modifying the refractive index of the sliica.

Such optical fibre peforms are usually composite rods consisting or a core and cladding of different compositions with respect to the presence or absence or to the relative proportions, of additional oxide or oxides in the silica, such that the refractive index of the core is greater than that of the cladding, and either or both of these components is or are conveniently produced by a chemical vapour deposition process. The materials deposited are required to be of the highest possible degree of purity, since high purity is one of the factors contributing to the desirable property of low attenuation of radiation in operation of optical fibre waveguides.

In a preferred process for the manufacture of an optical fibre peform, material of the composition of the fibre core, consisting of vitreous silica and one or more dopant oxides for modifying the refractive index of the silica, is produced from a vapour phase reaction mixture and deposited on the interior surface of a tube formed of vitreous silica, possibly with a dopant or dopants, which tube is to constitute the fibre cladding, if desired the core material being deposited in a pluality of layers of varying dopant concentration to give a graded refractive index profile across the cross-section of the preform and hence of the fibre drawn therefrom; in such a process, in accordance with the in vention, at least one of the reactant vapours from which such core material is produced is introduced into the tube by entrainment in a gas stream passed through a porous mass of material as aforesaid partially immersed in the reactant liquid, for substantially eliminating one or more impurities from the entained reactant.

The invention is also advantageously applied to any other chemical vapour deposition processes in which the purity of the product is important, for example in the manufacture of a semiconductor device by a vapour phase epitaxial deposition process as mentioned above, again at least one of the reactant vapours being entrained in a gas stream passed through a said porous mass of material partially immersed in the reactant liquid, for substantially eliminating impurities from the entrained reactant.

Some specific processes for the manufacture of optical fibre preforms, in which at least one reactant vapour is introduced into a carrier gas stream by the method of the invention, which we have carried out, will be described in the following examples, with reference to the diagrammatic drawing accompanying the Provisional Specification which shows, in sectional elevation, the apparatus which we have used for carrying out the said method.

The apparatus shown in the drawing is formed entirely of fused silica, and comprises a closed cylindrical vessel 1, surrounded by an outer jacket 2 filled with silicone oil for maintaining the vessel 1 at a constant temperature, and fitted with a gas inlet tube 3, a gasjvapour outlet tube 4, a liquid inlet tube 5, and a liquid drainage tube 6. A baffle 7, in the form of a silica disc with an aperture- 8 therein, is sealed to the cylindrical wall of the vessel 1: the aperture 8 permits insertion of the gas inlet tube 3 into the space below the baffle, and permits a gas/vapour mixture to flow from said space to the outlet tube 4. The space between the baffle 7 and the bottom of the vessel 1 is substantially filled with a mass of pure, clean silica wool 9. All parts of the apparatus are sealed together; thus the top of the outer jacket 2 is sealed to the wall of the vesel 1, the tubes 3, 4 and 5 are sealed to the top of the vessel 1, and the liquid drainage tube 6 is sealed to the walls of the vessel 1 and the outer jacket 2. When it is required to introduce silica wool into the vessel 1, the top of the vessel is cut off, at a level above that of the top of the outer jacket, and is removed together with the attached tubes 3, 4 and 5; after insertion of the requisite quantity of silica wool into the space below the baffle 7, the top of the vessel 1- is re-sealed in position.

In use of the apparatus, the required reactant liquid is introduced through the tube 5, to fill the vessel up to the approximate level indicated at 10, such that the silica wool extends above the surface of the liquid, the silica wool being prevented from floating on the liquid by the baffle 7. During the passage of a carrier gas stream through the vessel 1, silicone oil is circulated through the outer jacket from a thermostatically controlled reservoir maintained at the desired constant temperature, circulation being effected by leans of a pump: the temperature control and oil circllation system is of conventional form and is not shown in the drawing; the coil inlet to and outlet from the jacket 2 are shown respectively at 11 and 12, the direction of flow of the oil being indicated by arrows.

Example 1.

For the manufacture of an optical fibre preform consisting of a core composed of silica doped with phosphorus pentoxide and a cladding layer of silica free from dopant, the interior surface of a fused silica tube was coated with a combination of silica and phosphorus pentoxide in the required relative proportions by a chemical vapour deposition process involving a reaction between silicon tetrachloride, phosphorus oxychloride (PO Cl3), and oxygen. The silicon tetrachloride and phosphorus oxychloride vapours were both transported to the reaction zone by entrainment in a stream of oxygen which had been passed through the appropriate liquid.

The liquid phosphorus oxychloride was contained in a vessel of the form described above with reference to the drawing, containing silica wool, and the oxygen stream was passed through the vessel at a rate of 310 ml per minute while the silicone oil bath was maintained at a temperature of 15"C. The liquid silicone tetrachloride was contained in a conventional bubbler, also provided with a silicone oil bath for heat transfer, and the bath was maintained at a temperature of 15"C while the oxygen stream was bubbled through the liquid at a rate of 125 ml. per minute. The two oxygen/vapour streams thus produced were mixed, together with an additional stream of oxygen flowing at the rate of 200 ml per minute: since the main impurities present in the liquid phosphorus oxychloride had vapour pressures lower than that of phosphorus oxychloride, the oxygen-phosphorus oxychloride stream emerging from the liquid-containing vessel was introduced into the reaction mixture without any initial delay. The resulting mixture of oxygen and the reactant vapours was led into a silica tube which was rotated in a glass working lathe and at the same time was heated by the application of an oxy-hydrogen flame to the exterior of the tube, the flame being moved along the tube in the direction of the gas/ vapour flow. The flame heating promoted the reaction between the oxygen and silicon tetrachloride and phosphorus oxychloride vapours to produce a mixture of silica and phosphorus pentoxide which was deposited progressively on the interior surface of the silica tube downstream of the flame, and was converted into a vitreous layer by the subsequent passage of the flame along the tube. When the requisite thickness of phospho silicate glass had been formed in this way, the tube was cuased to collapse and was then drawn to fibre in an electrical resistance furnace of the form described in the specification of co-pending Patent Application No. 41336/73 /Serial No. 1,475,997).

In fibres drawn from preforms manu factured by the process of this example we have found that the attenuation of radiation of wavelength 850 nm was 8 dBkm-1, and that of radiation 1060 nm was 7 dBkm-1; an absorption peak occurred at wavelenth 940 nm, with an increase in attenuation of 2 dBkm-l, due to the presence of hycroxyl groups in the silica lattice of the fibre. By comparison, a fibre produced by the same process, but without employing the method of the invention for the introduction of the phosphorous oxychloride vapour into the carrier gas stream, gave an attenation of 15 dBkm-l at 850 nm and of 12 dBkm-1 at 1060 nm, with an excess attenuation of 3 dKkm-1 at 940 nm.

Example 2.

An optical fibre preform consisting of a phosphorus pentoxide-doped silica core and a silica cladding layer was made by the process described in Example 1, with the modification that the silicon tetrachlroide vapour, as well as the phosphorus oxychloride vapour, was introduced into the carrier gas stream by the method of the invention, the oxygen stream being passed at the rate of 125 ml per minute through a vessel of the form shown in the drawing, containing liquid silicon tetrachloride and maintained at a temperature of 15"C. Fibres drawn from preforms produced by the process so modified have been found to give an attenuation of 1-8 dBkm-1 for radiation of wavelength 850 nm and an attenuation of 1 1 dBkm-l for radiation of wavelength 1060 nm, with a peak increase in attenuation of 0 3 dBkm,, at 940 nm due to hydroxyl groups present.

The above examples demonstrates that purification of the reactant vapours is effected by the method of introducing the vapours into the carrier gas stream, in accordance with the invention. It is known that the principal impurities in both liquid silicon tetrachcloride and liquid phosphorous oxychloride, as commerically available, are chlorides of iron.

The reduction in attenuation of the fibre when the method of the invention is used for the introduction of one or both of these reactants into a carrier gas stream, shown by the figures quoted above, is mainly due to a reduction in the iron content of the phosphosilicate glass core material produced, as a result of differential evaporation of the reactant liquid and the iron chlorides present therein effected by the silica wool, and the consequent retention of substantially all of the iron chlorides content in the liquid containing vessel. This effect is the more marked when the method of the invention is employed for the entrainment of the silicon tetrachloride vapour, since the proportion of silicon tetrachloride employed in the pro duction of the glass is considerably greater than the proportion of phosphorus oxychlo ride. It may further be noted that the excess attenuation figures for the 940 nm peak, due to hydroxyl groups present, indicate that the amount of water carried over in the trans ported silicon tetrachloride vapour is also reduced by the method of the invention. As the conentration of iron chlorides, and to a lesser extent, of water, in the reactant liquid is progressively increased as the passage of the carrier gas through the vessel proceeds, after a time it is necessary to drain off the liquid and refill the vessel with fresh liquid.

Example 3.

An optical fibre preform composed of a germania-silicate glass core and a silica cladding layer was manufactured by a chemical vapour deposition process in which silicon tetrachloride and germanium tetrachloride were caused to react with oxygen inside a tube of fused natural quartz. Silicon tetra-, chloride and germanium tetrachloride vapours were both entrained in oxygen carrier gas streams by passing oxygen through the respective liquids contained in conventional bubblers, the liquid silicon tetrachloride and germanium tetrachloride both being maintained at a temperature of 20"C and the respective rates of flow of the oxygen streams through the silicon tetrachloride and germanium tetrachloride bubblers being 250 ml per minute and 400 ml per minute. The oxygen/vapour streams were mixed, with an additional stream of oxygen flowing at a rate of 500 ml per minute, and the resulting mixture was passed through the silica tube while the latter was rotated and heated in the manner described in Example 1, repeated deposition of the mixed oxides of silicon and germanium and subsequent glass formation being achieved by multiple passes of the oxy-hydrogen flame over the exterior of the tube. The tube was collapsed when a sufficient thickness of the core glass had been formed, and was then drawn to fibre. The optical fibre waveguide so produced showed an attenuation in excess of 10 dBkm,, for radiation throughout the visible region and in the infra red region up to 1200 nm wavelength; the minimum attenuation was 11 dBkm-1 at 850 nm, and an excess attenuation of 6 dBkm-1 was observed at 940 nm.

A further pre form of the same composition was manufactured by the same process, with the exception that the liquid silicon tetrachloride and germanium tetrachloride were each contained in a vessel of the form described above with reference to the drawing, containing silica wool. The fibre obtained by drawing this preform showed a minimum attenuation of 5 dBkm-1 at a wavelenth of 850 nm, the attenuation throughout the visible and infra red regions up to a wavelength cf 1200 nm being less than 10 dBkm-1, with an excess attentuation at 940 nm of only 3 dBkm-1. Hence purification of the reactant vapours was again achieved as a result of the immersion of a mass of silica wool in the respective reactant liquids.

In a modification of the apparatus described above with reference to the drawing, the gas inlet tube 3, instead of extending nearly to the bottom of the vessel 1 as shown, may terminate in the mass of silica wool 9 above the surface of the liquid. The processes described in the above examples can be carried out in the same way, using the apparatus so modified.

WHAT WE CLAIM IS:- 1. A chemical vapour deposition process, as hereinbefore defined, which includes the step of transporting at least one reactant substance to the reaction zone by entrainment in a stream of carrier gas, wherein a body of liquid, consisting of said reactant substance in liquid form containing one or more impurities of vapour pressure different from that of the liquid reactant substance, is contained in a vessel which is provided with inlet and outlet means for liquid and for gas and is otherwise closed, a porous mass of inert (as hereinbefore defined) solid material is partially immersed in said body of liquid so as to be wholly impregnated with the liquid, and the carrier gas stream is passed through the said porous mass of liquid-impregnated material, whereby differential evaporation of the liquid reactant and said impurity or impurities is promoted by the presence of said porous mass of material and the impurity or impurities is or are substantially eliminated from the reactant substance before it is entrained in the gas stream and introduced into the reaction zone.

2. A process according to Claim 1, wherein the said porous mass of inert solid material is in fibrous or particulate form.

3. A process according to Claim 2, wherein the said porous mass of inert solid material consists of glass wool or silica wool.

4. A process according to Claim 1, 2 or 3, wherin the said porous mass of inert solid material substantially fills that part of the said vessel which is occupied by the body of liquid, as well as extending above the surface of the body of liquid.

5. A process according to Claim 1, 2, 3 or 4, wherein the carrier gas stream is passed through the said body of liquid and the inert porous solid material

Claims

**WARNING** start of CLMS field may overlap end of DESC **. was manufactured by the same process, with the exception that the liquid silicon tetrachloride and germanium tetrachloride were each contained in a vessel of the form described above with reference to the drawing, containing silica wool. The fibre obtained by drawing this preform showed a minimum attenuation of 5 dBkm-1 at a wavelenth of 850 nm, the attenuation throughout the visible and infra red regions up to a wavelength cf 1200 nm being less than 10 dBkm-1, with an excess attentuation at 940 nm of only 3 dBkm-1. Hence purification of the reactant vapours was again achieved as a result of the immersion of a mass of silica wool in the respective reactant liquids. In a modification of the apparatus described above with reference to the drawing, the gas inlet tube 3, instead of extending nearly to the bottom of the vessel 1 as shown, may terminate in the mass of silica wool 9 above the surface of the liquid. The processes described in the above examples can be carried out in the same way, using the apparatus so modified. WHAT WE CLAIM IS:-

1. A chemical vapour deposition process, as hereinbefore defined, which includes the step of transporting at least one reactant substance to the reaction zone by entrainment in a stream of carrier gas, wherein a body of liquid, consisting of said reactant substance in liquid form containing one or more impurities of vapour pressure different from that of the liquid reactant substance, is contained in a vessel which is provided with inlet and outlet means for liquid and for gas and is otherwise closed, a porous mass of inert (as hereinbefore defined) solid material is partially immersed in said body of liquid so as to be wholly impregnated with the liquid, and the carrier gas stream is passed through the said porous mass of liquid-impregnated material, whereby differential evaporation of the liquid reactant and said impurity or impurities is promoted by the presence of said porous mass of material and the impurity or impurities is or are substantially eliminated from the reactant substance before it is entrained in the gas stream and introduced into the reaction zone.

5. A process according to Claim 1, 2, 3 or 4, wherein the carrier gas stream is passed through the said body of liquid and the inert porous solid material immersed therein, for entrainment of reactant vapour in the gas stream.

6. A process according to Claim 1, 2, 3 or 4, wherein the carrier gas stream is passed only through a portion of said porous mass of inert solid material which extends above the surface of the body of liquid and is impregnated with liquid by virtue of capillary action, for entrainment of reactant vapour in the gas stream.

7. A process according to any preceding Claim, wherein the reactant substance contains an impurity having a vapour pressure higher than that of the said substance in liquid form, so that the said impurity is preferentially evaporated, and wherein the introduction of the carrier gas stream with entrained vapour is delayed until after substantially all of such impurity has been removed from the body of liquid in the said vessel by preferential evaporation and entrainment of said impurity in the gas stream.

8. A process according to any of the preceding Claims 1 to 6, wherein the reactant substance contains an impurity having a vapour pressure lower than that of the said substance in liquid form, so that the reactant liquid is preferentially evaporated and the vapour entrained in the carrier gas stream is substantially free from said impurity, and wherein the gas stream, with entrained vapour, initially emerging from the said vessel is immediately introduced into the reaction zone.

9. A process according to any preceding Claim, for the manufacture of a preform to be subsequently drawn to form an optical fibre waveguide consisting of a core and cladding of different compositions, wherein material of the composition of the fibre core, consisting of vitreous silica and one or more dopant oxides for modifying the refractive index of the silica, is produced from a vapour phase reaction mixture and deposited on the interior surface of a tube formed substantially of vitreous silica, which tube is to constitute the fibre cladding, and wherein at least one of the reactant vapours from which the said core material is produced is introduced into the tube by entrainment in a gas stream passed through a said porous mass of material partially immersed in the reactant liquid, for substantially eliminating one or more impurities from the entrained reactant.

10. A process according to any of the preceding Caims 1 to 8, for the manufacture of a semiconductor device by epitaxial deposition, from a vapour phase reaction mixture, of a layer of material of one conductivity type on a substrate composed of material of opposite conductivity type, to form a pn junction, wherein at least one of

the reactant vapours from which said deposited material is formed is entrained in a gas stream passed through a said porous mass of material partially immersed in the reactant liquid, for substantially eliminating one or more impurities from the entrained reactant.

11. Apparatus for use in carrying out a process according to any preceding Claim, substantially as shown in, and as hereinbefore described with reference to, the drawing accompanying the Provisional Specification.

12. A chemical vapour deposition process according to Claim 1, for the manufacture of a preform for an optical fibre waveguide, carried out substantially as hereinbefore described in Example 1 or Example 2 or Example 3.

13. An optical fibre waveguide preform manufactured by a process according to Claim 9 or Claim 12.