fig1 a and 1b therefore provide partial schematic representations of a coating installation according to the abovementioned prior art ( european documents ). shown schematically in fig1 by the reference 4 is an apparatus for forming excited or unstable gaseous species , supplied at its gas inlet 5 with an initial gas treatment mixture 7 . a primary gas treatment mixture 8 is then obtained at the gas outlet 6 of the apparatus . moreover , a substrate 1 , placed opposite this gas outlet 6 , is exposed to an adjacent gas treatment mixture which is fed , in the embodiments shown , via one or two gas inlets ( 10 in the case of fig1 a and 9 and 10 in the case of fig1 b ), this adjacent treatment mixture not passing through the apparatus 4 for forming excited or unstable gaseous species . shown symbolically in these figures by the dashed rectangle 30 is the zone in which the primary and adjacent gas treatment mixtures interact so as to produce the required coating on the substrate 1 . fig2 shows one particular embodiment of a coating installation according to the prior art already mentioned , such as comprising a tunnel 3 defining an internal space 31 through which the substrate 1 is conveyed with the aid of a conveying means 2 ( for example a conveyor belt ). the substrate is then brought opposite the gas outlet 6 of the apparatus 4 mentioned above within the context of fig1 a and 1b , where it comes into contact with the primary gas treatment mixture 8 and with the adjacent treatment mixture being fed via the two gas inlets 9 and 10 . it will be noted that the embodiment shown in fig2 allows the substrate 1 to be treated by several apparatuses for forming excited or unstable gaseous species placed in series , the apparatuses placed at 11 and 12 not having been shown in order not to unnecessarily clutter up the figure , while the reference numbers 13 and 29 for their part illustrate additional examples of inlets of adjacent treatment mixtures that have not passed through the apparatuses . fig3 shows a schematic and partial representation of a device for forming a coating on a substrate suitable for implementing the process according to the present invention . this fig3 shows an apparatus 4 for forming excited or unstable species provided with its gas inlet 5 , capable of admitting the initial gas mixture 7 that has to be transformed in the apparatus , and with a gas outlet 6 capable of producing the primary gas treatment mixture which includes the excited or unstable gaseous species . as will be more clearly seen below in the context of fig4 the installation is also equipped with means capable of separating the stream of primary gas mixture from the gas outlet of the apparatus into two separate streams , the means comprising here a cylindrical component 40 provided with a slot 42 over all or part of the length of the component 40 . as may be very clearly seen in fig3 and 4 , the stream of primary gas mixture from the gas outlet 6 of the apparatus is therefore in fact divided into two separate streams 8 a and 8 b , whereas an adjacent gas treatment mixture , which includes at least one of the gaseous precursors needed to produce the coating on the component 1 , for example a gaseous silicon precursor , again for example , an organometallic compound , escapes from the slot 42 of the component 40 . such a configuration therefore makes it possible for this adjacent gas mixture to be injected into the stream of primary gas mixture as obtained at the gas outlet of the apparatus 4 . as will be clearly apparent to those skilled in the art , although the embodiment shown in fig4 illustrates an injection of the adjacent mixture using a slot , many other methods of injection may be envisaged without departing from the scope of the present invention , such as a porous medium , a pipe with orifices ( holes ), multiple slot systems , etc . it has therefore been possible to compare the effectiveness of the devices according to fig1 a , fig1 b and fig3 in order to produce silicon - based coatings on specimens consisting of polished glass plates , approximately 50 mm in size , the specimen holder moving at a rate of about 10 cm / minute beneath the discharge . for safety reasons and for ease of implementation , the adjacent gas mixture used was obtained from a bottle containing a n 2 / 1 . 9 % sih 4 mixture . the comparative results obtained may be summarized in the following way : a ) first test series : using the device of fig1 a ( unsymmetrical lateral injection of the adjacent mixture ). the adjacent mixture was injected through a lateral slot about 10 mm in width , located about 30 mm from the primary gas mixture jet 8 coming from the discharge . by means of this injection , it was therefore possible to demonstrate that silicon oxide layers could be deposited at a mean rate of about 1 å / s , the amount of silane used with respect to the rate of deposition then being about 3 . 3 × 10 − 3 liters of sih 4 per å . moreover , it was found that such an arrangement generates a not insignificant amount of silica powder , which signifies that a not insignificant part of the silane flow does not reach the region of the primary gas mixture jet because of the remoteness of the adjacent mixture injection . b ) for a second test series using the device in fig1 b ( symmetrical lateral injections of adjacent mixture ), we have therefore injected the adjacent gas mixture on each side of the discharge , at approximately 5 mm from the central jet of primary gas mixture . the adjacent injections were made through slots approximately 0 . 5 mm in thickness , ensuring an ejection rate close to 3 m / s ; the adjacent gas flow rate was close to 1 liter / h . such operating conditions have made it possible to deposit silicon oxide layers at a mean rate of about 1 . 2 å / s , therefore corresponding to an amount of silane used with respect to the rate of deposition of 2 . 31 × 10 − 4 liters of sih 4 per å . it may therefore be seen without any difficulty that the efficiency of silane transformation , obtained by means of such a symmetrical injection , closer to the jet of primary gas mixture obtained in remote plasma mode , is approximately 15 times greater than that obtained previously within the context of the configuration of fig1 a ; c ) third test series using the device of fig3 ( injection into the stream of primary mixture ). as clearly shown schematically in this fig3 this third test series used a separation of the jet of primary gas mixture output by the discharge head into two , and the injection of the adjacent gas mixture comprising the gaseous silicon precursor between the two separate jets 8 a and 8 b . such a configuration therefore clearly makes it possible to trap the stream of adjacent mixture in the volume bounded by the two jets of primary gas mixture in order to facilitate the mixing and the reaction of the adjacent gas on the surface of the substrate . in this configuration , the slot 42 for injecting the adjacent gas was decreased to about 0 . 25 mm in order to allow the rate of output of the reactive gas to be increased to approximately 5 to 6 m / s . the two spaces for outputting the primary gas treatment mixture around the component 40 had a width of about 0 . 5 mm , making it possible to retain velocities close to 10 m / s for the two corresponding gas jets . the flow rate of adjacent gas was then close to 0 . 2 liters / h . such a configuration therefore made it possible to deposit silicon oxide layers at a rate close to 2 . 5 å / s , corresponding to an amount of silane consumed with respect to the rate of deposition close this time to 2 . 2 × 10 − 5 liters of sih 4 per å . it may therefore be very clearly concluded that the silane transformation efficiency for such a central configuration of the adjacent gas mixture within the remote plasma is approximately 150 times greater than that obtained within the context of the first configuration according to fig1 a and approximately 10 times greater than the results obtained within the context of the configuration according to fig1 b . it is therefore apparent how much such a configuration correspondingly improves the economic but also the qualitative aspects of the process , the amount of silica powder produced by such a configuration being markedly less than that produced by the previous configurations .