Process and device for the deposition of thin layers and product made thereby

This invention relates to a process and a device for the deposition of thin layers on a substrate using a plasma-CVD technique. The substrate itself, which previously has been made conductive by the deposition of conductive layers, is used as an electrode to create the discharge. In particular, the technique can be applied to the deposition of organosilicon layers on glass plates of large dimensions. The invention also relates to a glass substrate covered by thin layers including at least one metal layer, in particular silver, on which the organosilicon layer is deposited according to the process.

BACKGROUND OF THE INVENTION 
This invention relates to a process and a device for depositing a thin 
layer using a plasma-CVD technique on a conductive substrate where the 
substrate itself is used as an electrode for an electrical current. Of 
particular interest, an insulating substrate can be used that is first 
made conductive by the deposition of a thin conductive layer. 
Advantageously, the thin conductive layer will remain intact when the 
final product is obtained. 
The invention also relates to a substrate covered by thin layers including 
at least one metal layer, preferably silver, on which a layer such as an 
organosilicon layer is deposited according to the process of the 
invention. 
It is known to protect the surfaces of various materials by depositing on 
the surface a thin layer, such as one consisting of an organic compound 
obtained by the chemical reaction of a monomer under a low pressure 
plasma. Thus, French patent application FR 85 18673 and corresponding U.S. 
patent applications Nos. 515,539 and 306,960 describe an organosilicon 
film obtained by the introduction, under primary vacuum, of an 
organosilicon monomer gas into an electric discharge created by the 
capacitive coupling between two electrodes. One electrode is grounded and 
supports the substrate to be coated, the other is located a small distance 
above the substrate and is connected to a power generator operating at 
frequencies below one megahertz. This process makes it possible to obtain 
thin layers which have many applications including, the protection of thin 
layers with silver bases deposited on a glass substrate. 
In the above mentioned process, the substrate remains immobile during the 
deposition. In other known processes the plasma is contained with a 
magnetic field and the substrate is displaced relative to the electrodes 
and therefore relative to the plasma. 
Such a process is cumbersome to use because it requires a mechanical drive 
system under vacuum which is difficult to achieve and maintain. Further, 
since the thin organosilicon layers are most often intended to protect the 
more fragile operational layers, such as silver, an additional chamber may 
be required to facilitate industrial operations. For example, on an 
industrial production line where the operational layers are deposited, it 
would be necessary to have a special chamber equipped with a mechanical 
drive system where the deposition of the final protective layer could be 
performed. This chamber would have to be similar to those where the first 
layers are deposited, aside from its mechanical requirements, and be 
placed before the output lock chamber (a chamber where the pressure is 
normally brought to atmospheric pressure) and after the other deposition 
chambers. 
The requirement of a special chamber can be obviated by using the above 
mentioned process of EP 230 188 where the substrate remains immobile. 
Since this process makes it possible, under certain limits, to perform the 
deposition without relative displacement between the glass and the plasma, 
it can be performed, for example, during the substrate's stop in the input 
lock chamber or the output lock chamber of an industrial 
cathode-sputtering line. 
However, this process also has its limits. It has been observed that the 
zone where the major portion of the polymer deposition is performed is 
that closest to the electrode located above the substrate. Additionally, 
the deposition is performed as quickly on the upper electrode as on the 
substrate to be treated. This has multiple negative consequences. First, 
the quantitative efficiency of the process is low since only a small 
proportion of the monomers introduced in the chamber are found in the 
polymer on the glass. Furthermore, the operation of the process itself is 
impeded by the preferred deposition on the upper electrode because the 
deposition, which is insulating, modifies the characteristics of the 
electrical field created under the electrode. This in turn creates a drift 
that must be corrected by adjusting the electrical parameters of the 
process. Additionally, a harmful mechanical effect can also be manifested 
when several glass sheets are treated successively. In this situation the 
deposition on the electrode increases in thickness as the successive 
sheets are treated; and the thickening layer becomes friable and 
eventually disintegrates. Chips then fall on the substrate where they 
disturb the deposition process. Further, the rate of deposition on the 
glass is not very high and the thickness of the final layer is greater at 
the center than it is opposite the edges of the electrode. 
SUMMARY OF THE INVENTION 
An object of this invention is to eliminate these various drawbacks. 
In accordance with the invention a thin layer is deposited by a plasma-CVD 
technique on a conductive substrate where the substrate itself is used as 
an electrode to create the discharge. Preferably, the substrate consists 
of an insulating material covered by a thin conductive layer. The 
deposition is performed up to 180 cm from the power lead-in and preferably 
up to about 160 cm. Moreover, the frequency is advantageously between 10 
and 400 kHz. When the deposition is of an organosilicon compound, it is 
performed in the presence of oxygen supplied by nitrous oxide. 
The invention also relates to the product formed by this process, namely a 
substrate covered by thin layers including at least one metal layer, in 
particular silver, on which a thin layer such as an organosilicon layer is 
deposited.

DETAILED DESCRIPTION OF THE INVENTION 
The deposition method according to the invention can be used on any 
installation under vacuum. In particular, it does not require any 
displacement either of the sample or of any part in the installation when 
the vacuum is achieved. FIG. 1 diagrams the installation. A vacuum chamber 
1 is evacuated utilizing a pipe 2 which is connected to a primary and/or 
secondary conventional pumping system, not shown. A valve 3 makes it 
possible to separate the installation from the pumps. This is particularly 
useful when it is desired to link the inside of the chamber with the 
ambient atmosphere. Gas is introduced into chamber 1 through pipe 6 under 
control of valve 7 which is regulated by a gas managment system (not 
shown). A substrate 4 covered by a conductive layer 5 is located within 
chamber 1. 
At least one power lead-in 8 is used to connect the substrate to an 
alternating voltage source 9 (FIG. 1) by a suitable cable 10. The chamber 
itself is grounded by a cable 11 as is a second pole 12 of alternating 
current source 9. 
The power lead-in itself is placed over the entire length of the edge of 
substrate 4. It comprises a conductive element 13, copper in the example, 
which is surrounded by an insulating material 14 on all sides except the 
side in contact with layer 5. 
FIG. 2 shows a detailed view of a particular embodiment of power lead-in 8. 
Power lead-in 8 comprises two blocks 15, 16 of an insulating material 14 
such as PTFE. These two blocks are connected together and clamped onto 
substrate 4, by bolts 17. A copper conductor 13 is mounted in block 15 and 
extends all along the edge of sheet 4 in contact with conductive layer 5. 
Conductor 13 is itself pressed to conductive layer 5 by a series of 
insulating bolts 18 and it is connected by cable 10 to alternating current 
source 9. 
Illustratively, substrate 4 is a glass sheet covered by a conductive layer 
5 which consists, for example, of a layer of silver that is 10 nm thick 
deposited between two layers of tin oxide (SnO.sub.2), each 40 nm thick. 
These layers are relatively sensitive to outside influences; and silver, 
in particular, is a mechanically and chemically fragile metal. When it is 
desired to produce an insulating glazing over a layer with a silver base 
and when it is desired to make such glazing independently from the 
production of the sheet glass itself, i.e., at a separate location and at 
a later time, it is necessary to take very strict precautions in storing 
and processing. These precautions include storage in a dry environment, 
special washing machines and other appropriate measures. 
Due to the above mentioned frailties associated with a conductive layer 
such as silver, it is desired to have a colorless protective layer to 
reduce the severity of the required precautions in handling such a glass 
sheet. This layer requires a unique set of properties which are difficult 
to combine, since it is necessary that in addition to being invisible, the 
layer must provide mechanical and chemical protection while at the same 
time being thin enough not to modify the infrared properties of the 
underlying layer, particularly its low emissivity. 
A layer incorporating these characteristics has been described in French 
patent application FR 85 18673. As described in this prior application, 
the layer is formed by introducing a mixture of a plasma-generating gas 
and an organosilicon monomer such as, for example, hexamethyldisiloxane 
(HMDSO) or hexamethyldisilazane (HMDS) and, in general, alkylsilazanes, 
vinyltrimethoxysilane (VTMOS), vinyldimethylethoxysilane (VDMEOS), 
dimethyldiethoxysilane (DMDEOS), trimethylethoxysilane (TMEOS), 
tetramethoxysilane (TMOS), tetraethoxysilane (TEOS) and, in general, 
alkoxysilanes, into an electrical discharge within a chamber as discussed 
below. 
However, unlike French patent application FR 85 18673 which utilizes two 
slightly spaced electrodes to create the discharge, the present invention 
uses the substrate itself as an electrode thus avoiding the problems 
associated with the two electrode system discussed above. 
The process of this invention also works for other gases making it possible 
to obtain layers of different materials. For example, while the 
above-mentioned alkoxy silanes are used in producing layers with a silica 
base, methane gas can be used to produce a carbon layer or an 
organometallic can be used to produce a layer whose main component is the 
oxide of the metal concerned. 
Likewise, the process of this invention also works for other conductive 
layers such as Al, Ti, Ta, Cr, Mn, Zr and Cu, as well as for multiple 
conductive layers. Additionally, the conductive layer can be entirely 
omitted where a conductive substrate is employed. 
The monomer proportion in the plasma-generating gas depends on the 
respective natures of the gases used and the conditions of the deposition. 
The conditions of the following example provide good results. 
In a laboratory installation, a cylindrical chamber was used having a 
vertical axis of a height of 35 cm and a diameter of 60 cm. The sample was 
of a plane window glass 4 mm thick made of a soda-lime-silica glass with 
dimensions of 30.times.30 cm. This window glass was covered by a triple 
layer comprising a dielectric underlayer of SnO.sub.2, 40 nm thick 
deposited by reactive cathode sputtering from a tin target; an "active" 
layer of silver, 10 nm thick deposited by the same method in an argon 
atmosphere; and a third dielectric layer similar to the first. This 
covered glass was placed at the midheight of the chamber with its layered 
side facing upward and resting on a suitable support. 
The deposition technique requires a particularly clean substrate. This can 
be achieved by carefully cleaning the substrate or by performing the 
deposition immediately following the deposition of the underlying layers 
without returning to the atmosphere. In the first case, i.e., where the 
two production phases are separated, the cleaning advantageously is ended 
by a discharge. In the example cited, after creating a vacuum of 0.013 
hPa, argon is introduced and the pressure is then allowed to increase to 
0.40 hPa. A discharge is then created with an alternating current that is 
established between the conductive surface of the substrate and the 
chamber with a frequency of 100 kHz and a power of 100 watts. After 30 
seconds, the current is turned off. After such cleaning, or immediately 
following the deposition of the underlying layers, the protective layer is 
deposited by introducing a monomer gas which is carried by the 
plasma-generating gas. 
In the example, the plasma-generating gas was the nitrous oxide N.sub.2 O, 
which supplied the oxygen necessary for the reaction. Tests with an 
argon-oxygen mixture have also been fully satisfactory. The organosilicon 
gas in the experiment was hexamethyldisiloxane (HMDSO) in a proportion of 
10%. After the vacuum pressure was stabilized at 0.15 hPa, the mixture was 
introduced with a delivery of 200 cm.sup.3 /mn (standard temperature and 
pressure (STP)). 
As soon as the delivery-pressure conditions are stabilized, the plasma is 
ignited by applying an alternating voltage to conductor 13. In the example 
chosen, the frequency was 100 kHz and the best results were obtained with 
a power of 300 watts for a power lead-in length of 30 cm. Tests showed 
that, in a general, the deposition conditions were comparable if the power 
varied proportionally to the length of the power lead-in. In the case of 
the example, a homogeneous and regular layer 30 nm thick was obtained in 8 
seconds. 
Observing the plasma at the time of deposition shows that the location 
where it is most luminous is on the surface of the substrate used as an 
electrode. This indicates the location where the density of the positive 
and negative dissociated ions created in the plasma is the greatest 
Additionally, it is here where the kinetic energy of the ions is the 
greatest. As a result, the probability is very high that the ions will 
recombine with other ions in the gas just above the layer and also with 
ions on the layer. The result is a highly uniform layer that adheres 
surprisingly well to the substrate with a surprisingly high growth rate. 
These properties distinguish the layer obtained in the present invention 
from those obtained in a standard device using two electrodes like that in 
French patent application FR 85 18673. 
Some of the limits of this invention were also explored in the laboratory. 
In particular, a study was made of the maximum distance from the power 
lead-in 8 at which the plasma is intense enough to be effective. More 
specifically, the maximum distance was determined at which the plasma is 
intense enough that the dissociation of the monomer of the organosilicon 
compound is sufficient so that enough active ions are created to recombine 
and form on the substrate. 
In this study a sample was prepared on a glass plate of 30.times.30 cm on 
which the above-mentioned combination of silver and dielectric layers had 
been deposited. However, in certain zones, marked 19 in FIG. 3, the layer 
was totally abraded with a grinding wheel. The remaining zone 20 is a 
circuitious path or labyrinth of about 180 cm. in length through which the 
alternating current is to pass from a power lead-in 21 to a farthest 
corner 22. As in the embodiment shown in FIGS. 1 and 2, power lead-in 21 
contacts the sample over the entire length of one edge. When the 
luminescent plasma is observed during the deposition, it is found that it 
is located exclusively above zone 20 where the conductive layer is present 
and not above zone 19. Additionally, an extreme luminosity close to power 
lead-in 21 decreases quickly over about 10 centimeters to remain 
apparently constant up to farthest point 22. 
At the end of the deposition which is performed under the same conditions 
as in the preceding example, it is found that the deposition occurs all 
along track 20 up to and including farthest zone 22. Its thickness is 
regular, with a thickness of 20.+-.2 nm being measured. These thickness 
variations are virtually undetectable by the eye. Thus, it is found that 
the process makes it possible to obtain quality depositions at large 
distance, i.e., in the order of at least 180 cm, from the power lead-in. 
If two power lead-ins are used, each on one of the two opposite sides of a 
substrate of float glass coated by a conductive layer, the technique makes 
it possible to deposit a layer by plasma-CVD on a typical industrial plate 
such as those having a width of 3.20 m or 4.00 m. 
The preceding exemplifies that the process of the invention provides a very 
advantageous technique to produce layers. In particular, thin 
organosilicon layers of good quality can be formed on substrates which are 
conductive or made conductive due to the prior deposition of conductive 
layers. This technique is compatible with the industrial production of 
large layered glass plates particularly those in which an underlayer is 
deposited on a glass already equipped with a conductive layer, for 
example, by pyrolization, or an over layer, before or after the deposition 
of dielectric and/or metallic layers by cathode sputtering. This process 
is then performed preferably in the input or output lock chamber of an 
industrial production line.