Apparatus for making a plasma coating

An improved radio frequency (RF) glow discharge apparatus is disclosed for depositing dielectric films onto substrates in a continuous operation. The apparatus includes one or more capacitively coupled electrodes, with a plate of a dielectric material that overlies the surface of the electrode(s) coupled with the plasma. The thickness of the plate is sufficient such that during deposition with the apparatus of this invention, the amount of material deposited on the electrodes is negligible, and the potential between the electrodes and the substrate remains substantially the same. This, in turn, insures that the chemical structure and properties of the deposited films are reproducible over long periods of operation.

The present invention relates to an improved apparatus for reproducible 
deposition of thin films and more particularly is concerned with plasma 
deposition of dielectric films in a radio frequency (RF) glow discharge 
employing one or more capacitively coupled electrodes. 
BACKGROUND OF THE INVENTION 
As plasma deposition of thin films becomes an increasingly more viable 
process for commercial production, an understanding of those parameters 
which control the chemical structure and properties of the film assumes 
greater importance. A glow discharge system in its simplest form comprises 
a deposition chamber, means for evacuating the chamber, means for 
introducing gases into the chamber, a power source and means for efficient 
transfer of the power to the gases. Kaganowicz et al. show such a glow 
discharge system in U.S. Pat. No. 4,339,471. Glow discharge systems for 
commercial scale production typically include a deposition chamber wherein 
substrates are automatically conveyed under (or over) a series of 
"in-line" electrodes. 
The chemical composition and properties of the deposited film are 
determined by the species generated in the plasma. These species in turn 
depend on the power applied to the electrodes and the potential between 
the electrodes and the substrate to be coated. Since the power supplies 
typically utilized in these systems have a constant power output it 
becomes necessary to optimize and maintain the size and spacing of the 
electrodes with respect to the substrate for a particular coating. In this 
way when a given potential is correlated to a desired film structure, 
reproducibility of that film is insured. 
In practice, however, as much as a 30 percent drop in potential has been 
noted after several hours of operation which results in a change in the 
properties of the deposited film. It is believed the drop is caused by 
substantial deposition of material onto the electrodes. It is necessary 
then to shut down the system and remove the buildup of material on the 
electrodes. 
It would be desirable to be able to deposit reproducible coatings by glow 
discharge in a production environment without having to interrupt the 
process every 2 to 3 hours. 
SUMMARY OF THE INVENTION 
An improved radio frequency (RF) glow discharge apparatus is disclosed for 
depositing dielectric films onto substrates in a continuous operation. The 
apparatus includes one or more capacitively coupled electrodes with a 
plate of a dielectric material that overlies the surface of the 
electrode(s) coupled with the plasma. The thickness of the plate is 
sufficient such that during deposition with the apparatus of this 
invention, the amount of material deposited on the electrodes is 
negligible, and the potential between the electrodes and the substrate 
remains substantially the same. This, in turn, insures that the chemical 
structure and properties of the deposited films are reproducible over long 
periods of operation.

DETAILED DESCRIPTION OF THE INVENTION 
The apparatus of the present invention stabilizes the dielectric properties 
of the space between an electrode and a substrate in an RF glow discharge 
system. This is accomplished by placing a plate of dielectric material 
over the surface of the capacitive electrode which is coupled with the 
plasma. 
It has been determined that during glow discharge deposition, the 
deposition of material on the electrodes tends to change the dielectric 
properties of, and thereby the potential across, the space between the 
electrode and the substrate. The thickness of the plate should be 
sufficient to render negligible any deposition of material on the 
electrode. It has been found that if the plate is at least about 50 times 
thicker than the anticipated deposit of material on the electrode, there 
is no drop in potential. Without the dielectric plate, it would be 
necessary to periodically cease operation every few hours and clean the 
coated electrodes in order to maintain uniform chemical structure and 
properties of the deposited films. 
While maintaining the plate at least 50 times thicker than the deposit on 
the electrodes stabilizes the dielectric properties of the space between 
electrode and substrate for a single deposition, from a production 
perspective, it is advantageous to be able to operate a glow discharge 
system with a stabilized potential until the system would normally be shut 
down for cleaning. A preferred embodiment of the present invention 
provides this result by incorporating into the glow discharge apparatus a 
plate of dielectric material whose thickness is at least about 50 times 
greater than the maximum tolerable buildup level for that system. In this 
way the plate is at all times a minimum of 50 times thicker than the 
competing deposition on the electrode thereby maintaining stability of 
composition and properties of the deposited films until the apparatus 
would otherwise have to be cleaned. As a practical matter the dielectric 
plate thickness is limited on the upper extreme by the physical 
specifications of the particular deposition system. 
Another consideration is the material of the dielectric plate. In practice 
any dielectric material, otherwise compatible with the apparatus, is 
suitable. However, because many of the dielectric materials suitable would 
tend to sputter, especially at high power levels, contamination of the 
deposited film may result. Therefore, the material used for the plate 
should preferably be of the same chemical structure and properties of the 
film of material to be deposited. In this way, any secondary deposition 
onto the substrate by sputtering will not compromise the integrity of the 
deposited film. 
The present apparatus will now be described with reference to the Drawing. 
Further, the invention will be described with reference to the deposition 
of silicon dioxide (SiO.sub.2) onto a substrate. 
In FIG. 1 is shown an embodiment of the improved apparatus suitable for use 
in the present invention generally as 10. The glow discharge apparatus 10 
includes a vacuum chamber 12 which can be a glass bell jar. In the vacuum 
chamber 12 are two electrodes 14, 18 which can be a screen, coil or plate 
of a material that is a good electrical conductor and does not readily 
sputter, such as aluminum. Affixed to the surfaces of the electrodes 14 
and 18, which will face the plasma, are two 0.125 inch (31.75 cm) thick 
silicon dioxide (SiO.sub.2) plates 15, 17. The thickness of the plates 15, 
17 has been chosen to be about 50 times greater than the maximum tolerable 
thickness of SiO.sub.2 buildup in this apparatus 10. By maintaining a 
minimum 50:1 ratio of the plates 15, 17 to any coating deposited thereon, 
the potential drop as a result of such coating will be negligible. A radio 
frequency, constant ouput power supply 16, is employed to obtain a voltage 
potential between the electrodes 14, 18. An outlet 20 into the vacuum 
chamber 12 allows for evacuation of the system and is connected to a 
mechanical pump, not shown. A first inlet 22 and a second inlet 24 are 
connected to gas bleed systems, not shown, for adding the reactants 
employed to prepare the SiO.sub.2 coating. 
In carrying out the process a substrate 26 to be coated is placed between, 
and spaced apart from, the electrodes 14, 18, typically maintained about 5 
to 10 centimeters apart. The vacuum chamber 12 is then evacuated through 
the outlet 20 to a pressure of about 0.5-1.times.10.sup.-5 torr. Nitrous 
oxide (N.sub.2 O) is added through the first inlet 22 to its desired 
partial pressure and silane (SiH.sub.4) is added through the second inlet 
24 until the desired partial pressure ratio of silane (SiH.sub.4) to 
nitrous oxide (N.sub.2 O) is obtained, for example, about 1 to 6. 
In order to begin deposition of the silicon dioxide (SiO.sub.2) coating on 
the substrate 26, a glow discharge is initiated between the electrodes 14 
and 18 by energizing the power source 16. For deposition the power should 
be in the range of 0.05 to 5 watts/cm.sup.2 of electrode, preferably 0.1 
to 1 watts/cm.sup.2 of electrode, and should be at a radio frequency. The 
potential between electrodes I4 and 18 is generally about 1,000 volts. 
In FIG. 2 is shown another embodiment of the present apparatus suitable for 
mass production, an "in-line" coater, generally as 30. Analogous in many 
respects to the apparatus 10 of FIG. 1, this "in-line" coater 30 comprises 
a deposition chamber 32 with load/unload chambers 34 on either end. A 
series of in-line electrodes 36 with dielectric plates 38 affixed thereto 
are connected to a power supply 40. An outlet means 42 is connected to an 
evacuation means (not shown). Inlets 44 and 46 allow introduction of 
plasma precursors into the chamber 32. 
The deposition process is carried out substantially as described 
hereinabove, but with the substrates 48 to be coated being conveyed across 
the plasma area, i.e. between the electrodes 36 and substrates 48, by the 
conveyor means 50 which is spaced apart from the electrodes 36. 
While the apparatus of the invention has been described with respect to the 
deposition of silicon dioxide (SiO.sub.2), it should be appreciated that 
deposition of other dielectrics, such as silicon oxide, silicon 
oxynitride, silicon nitride, etc., as well as semiconductors with other 
RF, capacitively coupled glow discharge systems would fall within the 
scope of the invention.