(i) Field of the Invention
The present invention relates to the field of the preparation of gaseous mixtures containing a silane and an oxidant, in particular carrier gas/oxidizing gas/silane ternary mixtures.
An example which may be mentioned of a technology using atmospheres containing these three types of gases is surface treatment operations performed on polymer substrates by the deposition, under dielectric-barrier electrical discharge, of silicon-based films or layers at the surface of such substrates, with the aim in particular of improving certain properties of such polymer substrates, such as their wettability by inks or adhesives, and also the adhesion of these inks or adhesives to the substrates.
The notion of "film" or "layers" must be understood as meaning the preparation of a continuous or non-continuous deposit, thus including the depositing of only isolated blocks which may cover, for example, only 10 to 15% of the surface of the polymer film.
(ii) Description of Related Art
Reference may be made to the documents EP-A-516,804 and EP-A-622,474, both on behalf of the Applicant Company, which relate to such processes for depositing silicon-based films on polymer substrates.
The very specific properties of silanes mean that it is, in practice, very difficult and expensive to prepare and handle such ternary mixtures.
These properties are well known to a person skilled in the art; here, it will be simply and briefly recalled, taking the example of the monosilane (generally abbreviated to "silane"), that the difficulties of handling silane relate to two aspects:
a) Its self-ignition in the presence of oxygen
This self-ignition property makes it difficult and expensive to handle silane, because it is necessary to use leaktight plants employing isolation valves and other purge lines.
By way of illustration, the self-ignition of monosilane will depend on numerous parameters, such as the supply pressure of the gas, temperature, rate of ejection of the gas or hygrometry. The concentration limit of monosilane in a carrier gas, as generally accepted in the literature, beyond which the mixtures will be regarded as inflammable or spontaneously inflammable, lies between 1 and 4%, depending on the reference source.
b) The formation of particles:
The act of bringing a silane and an oxidant together can result in the formation of silica particles. These particles, by accumulating, can not only obstruct the pipes (and thus dangerously increase the pressure upstream) but, moreover, can damage the various components of the plant, such as valves, flow controllers or other pressure-reducing valves.
As regards the stability of mixtures containing a silane and an oxidant, reference may in particular be made to the article by K. Strater, which appeared in RCA Review in December 1968, in which the author is concerned with the stability of O.sub.2 /N.sub.2 /SiH.sub.4, CO.sub.2 /N.sub.2 /SiH.sub.4 and N.sub.2 O/N.sub.2 /SiH.sub.4 mixtures and in which he demonstrates that the stability of such mixtures at ambient temperature is demonstrated up to 0.8% of monosilane in the mixture.
These results may suggest the following comments:
on the one hand, it may be observed that this upper stability limit is too low for certain applications involving silane and an oxidant and for which a higher silane concentration in the mixture is required; PA1 depending on the equipment used, it would seem difficult to take the risk of forming silica powder, it being known that, in places, for reasons of homogeneity of the mixture, this limit of 0.8% may be exceeded. PA1 preparing the final gaseous mixture (which thus contains a carrier gas, for example a neutral gas or alternatively a mixture of a neutral gas and of a reducing gas such as hydrogen, an oxidizing gas and a silane), in at least two stages, by first preparing a gaseous mixture, which can be described as "primary", which contains a neutral gas and the silane, the silane content in the primary gaseous mixture being below the self-ignition limit of the silane in air; and PA1 the primary gaseous mixture thus formed is mixed with a stream of the carrier gas and, if appropriate, a stream of the oxidizing gas (depending on the residual content of oxidizing gas in the carrier gas), in proportions which make it possible to obtain the required final mixture, the addition of the primary mixture stream or of the oxidizing gas stream, to prepare this final mixture, being carried out under dynamic conditions.
By way of example, the standard use of mass flow controllers employing a capillary seems very risky, even below the concentration limit put forward in this article by K. Strater.
In a way which is consistent with these results, mention may be made of another example from the literature, by M. L. Hammond and M. H. Bowers (which appeared in Transactions of the Metallurgical Society of AIME, March 1968), which relates to the CVD deposition of silica layers on wafers, starting with O.sub.2 /N.sub.2 /SiH.sub.4 mixtures. The equipment used to produce the mixtures employs a conventional gas distribution panel where the three gas lines arrive, the mixtures manufactured thus having a silane content varying from 0.06 to 0.3% by volume and thus very low silane contents.
On reconsidering the example of the deposition of silicon-based films on polymer substrates, by dielectric-barrier electrical discharge, it is then found that, in practice, rather than premixing, it is preferable to convey the various gases constituting the atmosphere separately to different points of the electrical discharge region.
It will be appreciated, therefore, that such separate injections of the various components of the required atmosphere give rise to significant variations in the composition of the treatment atmosphere in the discharge space, which does not allow true control of the composition of the atmosphere to be achieved and thus true reproducibility of the quality of the treatment of the film to be achieved.
Studies completed by the Applicant Company, in particular for the example of plants for the surface treatment of polymer films, show that the gaseous mixtures can be frequently, and for relatively long periods of time, brought to a halt. This is because it is commonplace for the user to bring his production/conversion line to a halt, for example during a maintenance operation or an operation in which the type of film to be treated is changed or alternatively mechanical problems, the surface treatment then being interrupted, which causes the gaseous flow to halt and thus the gaseous mixtures produced prior to the stoppage to be brought to a halt in a large portion of the pipes.