Reactive plating method and product

A method is disclosed for the reactive plating of substrates to produce transparent conducting films and photoactive coatings. Reactive gases at low pressures are introduced into a vacuum chamber having a partial vacuum therein. A substrate located in the vacuum chamber is subjected to a glow discharge in the partial vacuum. A coating material, such as zinc or silicon is vaporized in the vacuum chamber to react with the gases, with the resulting compound being deposited on the substrate by the effect of the glow discharge. The power in the glow discharge and the partial pressures of the vaporized coating material and gases introduced into the vacuum chamber can all be controlled separately to vary the stoichiometric ratios and the properties of the coatings. The electrode geometry is arranged and the operation maintained such that the power density distribution in the discharge is fixed and controlled.

BACKGROUND OF THE INVENTION 
This invention relates to the coating or plating of substrates using glow 
discharge and thermal vaporization in a vacuum chamber. 
In the past, various techniques have been used to deposit a film or coating 
on a substrate located in a vacuum chamber. One technique simply is to 
thermally vaporize or evaporate a metal, permitting the vapor to condense 
and be deposited on the substrate. Another method is referred to as 
chemical vapor deposition wherein different gases are introduced into the 
vacuum chamber to react and form a compound on the substrate. Yet another 
prior art method is referred to as sputtering. In this method, a vacuum or 
gas filled discharge tube has a cathode that is disintegrated by 
bombardment, so that the cathode material is vaporized and deposited on 
the substrate. 
A variation of these methods is shown in U.S. Pat. No. 2,501,563 issued to 
W. H. Colbert. In this patent, vacuum evaporation is used to vaporize and 
deposit a metal substrate. Gas is then introduced into the vacuum chamber 
to oxidize the metal and form a metallic compound coating. Another 
variation of the prior art methods is shown in U.S. Pat. No. 3,318,790 
issued to B. G. Carbajal III, et al. In this patent, a gaseous organic 
source material is introduced into a vacuum chamber and a glow discharge 
is established to polymerize the source material and deposit same on the 
substrate. 
A difficulty with the prior art methods is that they generally do not 
permit individual and independent control of each parameter of the process 
and hence do not permit the fabrication of "tailor-made" compounds having 
predetermined atom ratios and hence predetermined properties. 
SUMMARY OF THE INVENTION 
In the present invention, a glow discharge in a reacting gas is provided in 
a vacuum chamber and a coating material is vaporized therein to be 
deposited on a substrate. Independent adjustment of each of the parameters 
of the process is possible allowing for control of the reaction and 
deposition of the coating material, and thus the properties of the film 
deposited on the substrate. 
According to the invention, there is provided a method of coating a 
substrate comprising the steps of providing a vacuum chamber communicating 
with a surface of the substrate to be coated. A glow discharge is produced 
in the vacuum chamber in contact with the substrate surface. Coating 
material is vaporized in the chamber, and a gas is introduced into the 
vacuum chamber to react with the vaporized coating material forming a 
compound to be deposited on the substrate by the glow discharge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following description of a preferred embodiment is given in reference 
to a laboratory test and demonstration unit for carrying out the preferred 
method to produce the preferred coated substrate. Specific values are 
given for the various parameters, but it should be emphasized that these 
values are only exemplary. It will be appreciated that all of the 
parameters have a range of values as indicated. Further, the ranges of 
values may vary as the invention is scaled up for commercial production. 
It is believed, however, that appropriate values for the parameters for 
apparatus on a larger scale will be readily apparent or perceivable by 
those skilled in the art. 
Referring to the drawings, a vacuum chamber for carrying out a preferred 
method of the present invention is generally indicated in FIG. 1 by 
reference number 10. Vacuum chamber 10 is in the form of a bell jar 12 
supported on a base 14. Bell jar 12 has a removable lid 16 which seals the 
upper opening of the bell jar when the latter is evacuated. Bell jar 12 
also has an evacuation port 18 which is connected to a vacuum pump (not 
shown) for evacuating vacuum chamber 10. 
A substrate holder 20 is connected to lid 16 by a support rod 22. Substrate 
holder 20 is in the form of a loop having a central opening 24 (see FIG. 
2). A substrate 26 is shown located on holder 20 over opening 24. In the 
embodiment shown in the drawings, substrate 26 is a square glass pane 
approximately 2 inches square. For the purposes of this disclosure, the 
term substrate includes any object or article which is desired to be 
coated. The substrate does not have to be flat and the substrate can be 
made of materials other than glass. In the vacuum chamber shown in the 
drawings, the underside or lower side of substrate 26 is the surface that 
is to be coated in vacuum chamber 10. 
A screen or shutter 28 is located adjacent to, but spaced closely to the 
underside of substrate holder 20. Shutter 28 is connected to lid 16 by an 
upright support rod 30. Support rod 30 is rotatably mounted in lid 16 
using a suitable seal 32. Rod 30 has an upper end portion 34 projecting 
above lid 16. End portion 34 may be rotated to rotate shutter 28 
transversely away from substrate 26 when it is desired to commence the 
coating of the substrate, as described further below. 
A high voltage electrode forms a cathode 36 in vacuum chamber 10. Another 
high voltage electrode forms an anode 38. As seen in FIG. 2, cathode 26 is 
in the form a circular loop spaced below shutter 28, the loop having a 
diameter slightly larger than substrate holder 20. Cathode 36 includes an 
upright portion 40 which is electrically connected to a connector 42. 
Connector 42 provides a seal for lid 16 and permits cathode 36 to be 
connected to a source of power. Anode 38 also has an upright portion 44 
electrically connected to a similar sealing connector 46 for connecting 
the anode to the source of power. Shields 48, 50 are provided around the 
upright portions 44, 40 of the respective anode and cathode. Shields 48, 
50 are in the form of inverted insulating cups, and they are provided in 
the event that a conducting film is being deposited on substrate 26. 
Without the shields, stray conducting coating material could be deposited 
on upright portions 44, 46 and the underside of lid 16 causing a short or 
arcing between the cathode and anode, which is undesirable. Shields 48, 50 
also serve to maintain the electrode geometry and enhance the plasma power 
density when conducting films are deposited. 
Referring in particular to FIGS. 1, 3 and 4, a coating material source is 
generally indicated by reference number 54. Source 54 is formed of carbon 
and has a conical upper portion 56 and a cylindrical lower portion 58. 
This results in a high resistance joint 77 (see FIG. 4) between upper and 
lower portions 56, 58, so that electrical current passing therethrough 
causes heating in the area of joint 77. This heating in the area of joint 
77 causes material in source 54 to be heated and vaporized mainly at the 
top of the material. Conical upper portion 56 has a top orifice 78 
typically 0.024 inches in diameter in the laboratory apparatus shown in 
the drawings. Vapor produced in source 54 emerges through orifice 78 into 
vacuum chamber 10 as the material in source 54 is heated. 
Upper portion 56 is conductibly or electrically connected to an upper 
element 60, which, in turn, is conductibly connected to a terminal 62 
projecting from the underside of base 14. The lower portion 58 of material 
source 54 is conductibly connected to a lower element 64, which in turn is 
conductibly connnected to a terminal 66 projecting from the underside of 
base 14. 
Terminals 62, 66 are connected to a suitable source of electrical power to 
produce the desired heating and vaporization of the material inside source 
54. 
A thermocouple 68 communicates with the inside of material source 54 for 
measuring the temperature of the coating material therein. Theremocouple 
68 has leads 71, 73 passing through base 14 for connection to a 
temperature indicating device (not shown). 
A gas inlet tube 70 passes through base 14 and has an upper circular loop 
72. Loop 72 is formed with a plurality of equally spaced openings 74 
through which gas emerges into vacuum chamber 10. Gas inlet tube 70 has a 
needle valve 76 attached thereto outside vacuum chamber 10 to control the 
flow of gas passing through tube 70 and entering the vacuum chamber. 
The operation of vacuum chamber 10 and the preferred method of the present 
invention will now be described with reference to the coating of a glass 
substrate 26 using zinc as the source material and oxygen as the gas 
inside vacuum chamber 10, to produce a thin, zinc oxide type transparent 
film on the substrate. Coating material source 54 is filled with zinc 
metal, and a glass substrate 26 is positioned on substrate holder 20. 
Shutter 28 is rotated into position so that it is directly beneath 
substrate 26 to block the substrate and prevent coating material from 
being deposited on substrate 26 until conditions have reached steady state 
inside vacuum chamber 10. Evacuation port 18 is connected to a vacuum pump 
and vacuum chamber 10 is evacuated. Needle valve 76 is opened to allow 
oxygen to flow through inlet tube 70 and out through loop 72 to fill the 
vacuum chamber with oxygen. Vacuum chamber 10 is partially evacuated until 
the oxygen pressure therein is approximately 0.035 torr., and a small 
amount of oxygen is continuously fed into vacuum chamber 10 to maintain 
this pressure level. 
Cathode 36 and anode 38 are connected to respective negative and positive 
terminals of a DC power source through connectors 42, 46. Power is applied 
to the cathode and anode to produce a glow discharge. The power dissipated 
in the glow discharge is approximately 0.5 watts in the laboratory 
apparatus shown in the drawings, and this is just above the threshold 
power required to maintain the glow discharge in the vacuum chamber. 
The zince in coating material source 54 is heated by passing a current 
therethrough; terminals 62 and 66 being connected to a suitable power 
source for this purpose. The zinc is typically heated to a temperature of 
about 585 degrees Centigrade in the laboratory apparatus shown, where it 
is vaporized at the top of coating material source 54. The zinc vapor tus 
produced in source 54 then passes through orifice 78 in the top of conical 
upper portion 56 into vacuum chamber 10. 
The glow discharge produced between cathode 36 and anode 38 envelops or 
contacts the underside of substrate 26. Once this glow discharge has been 
established, and the oxygen pressure is vacuum chamber 10 is at the 
desired level, and the zinc is being vaporized in coating material source 
54, shutter 28 is rotated out of the way. The zinc vapor and oxygen react 
forming a compound which is deposited on the underside of substrate 26 by 
the glow discharge. After the compound has been deposited for 50 to 100 
seconds, the glow discharge can be turned off and the oxygen and zinc 
reaction continued. The coating will continue to be deposited on substrate 
26 without the glow discharge, and the coating will grow to the thickness 
desired. 
The coating produced as described above is a transparent conducting film 
having low resistivity, in which the film is ZnO.sub.x, wherein x is less 
than one. Typical properties of this non-stoichiometric transparent 
conducting film are as follows: 
1. 93% light transmission over the visible spectrum with much less than 1% 
absorption losses 
2. 1.3.times.10.sup.-3 ohm cm. resistivity 
3. 10.sup.20 per cm..sup.3 carrier density (electrons) 
4. 25 to 35 cm..sup.2 /volt-second mobility 
5. 4.24 e.v. work function 
Having described the preferred method of operation to produce transparent 
high conductivity films, many variations are possible to obtain other 
properties. For example, in the laboratory test and demonstration unit 
shown in the drawings, the power in the glow discharge for the entire film 
deposition process can range from 0.1 watts to 20 watts to produce a very 
large change in the film resistivity. Different power levels, source 
temperatures and gas pressures produce films with different opacity and/or 
different electrical resistivities, which can range from opaque to clear 
and resistivities differing by as much as eleven orders of magnitude. The 
glow discharge appears to "clean" the substrate and promote uniform 
nucleation of the coating thickness. Once a continuous molecular layer of 
coating compounds is deposited on the substrate, the glow discharge can be 
turned off and a film will continue to grow uniformly. In fact, with a 
higher power glow discharge (near 20 watts) operating for about 200 
seconds, in the laboratory unit the zinc source can remain cold until 
after the glow discharge is terminated, and if the zinc source is then 
heated to vaporize the zinc, a film will be deposited, but it may not have 
the desired properties. 
The amount of zinc being vaporized in coating material source 54 can also 
be varied by changing the diameter of orifice 78 and by changing the 
temperature to which the zinc is heated in source 54. The zinc source 
orifice size and the temperature can also be varied to produce the same 
rate of vaporization of zinc. For example, orifice 78 could have a 
diameter of 0.030 inches, and the zinc heated to a temperature of 430 
degrees Centigrade or lower, or an orifice diameter of 0.024 inches and a 
temperature of 585 degrees Centigrade might be used as described above. 
The higher the zinc temperature, the higher is the zinc partial pressure 
or the amount of zinc vapor in vacuum chamber 10. Higher temperatures 
result in a coating compound having a higher percentage of zinc. This 
would result in a coating or film being deposited on the substrate having 
an even lower resistivity (at the same O.sub.2 pressure), but increased 
optical absorption. Lowering the zinc partial pressure by decreasing the 
zinc temperature (again at the same 0.sub.2 pressure) could produce a zinc 
oxide compound coating at the stoichiometric ratio, and this would have a 
very high resistivity. Normal operating temperatures for a zinc source 
orifice size of 0.024 inches are between 550 and 600 degrees Centigrade. 
The oxygen gas pressure inside vacuum chamber 10 can also be varied 
independently. For a fixed glow discharge power and source temperature, 
the range can be between 0.010 and 0.070 torr.. Higher oxygen pressures 
result in coatings having higher resistivity and lower opacity, and vice 
versa. Increasing the amount or pressure of the oxygen causes the coated 
film to approach the stoichiometric ratio of zinc oxide and high 
resistivity films, but for low resistivity, a non-stoichiometric compound 
should be deposited on the substrate. It will be appreciated that the 
stoichiometric ratio of the coating can be varied by changing either the 
rate of vaporization of the zinc or the oxygen pressure, so stoichiometric 
coatings can be produced by increasing the oxygen pressure. 
Other materials and gases could also be used with the method of this 
invention. For example, silicon can be vaporized in coating material 
source 54, and hydrogen can be introduced into the vacuum chamber through 
gas inlet tube 70. In this case, the glow discharge must be operating for 
the entire deposition process. This produces hydrogenated silicon. Other 
gases, such as nitrogen, fluorine and oxygen, and mixtures of same such as 
nitrogen and oxygen, could also be used with silicon to produce 
correspondingly similar types of coatings. 
The method as described above could be adapted to a continuous process 
wherein the substrate passes through a vacuum chamber, or a vacuum chamber 
passes over the substrate. It will be appreciated that forms of vacuum 
chambers can be used other than the bell jar type container described 
above. The polarity of the electrodes forming the glow discharge can be 
reversed, and in fact, alternating current can be used to produce the glow 
discharge. It is preferred to have the dimensions of the cathode and anode 
comparable to the substrate size and the cathode spaced about 1 cm. from 
the substrate, but this can be varied. However, the film uniformity and 
the rate of deposition would change. The term "transparent" as used in 
association with the zinc and oxygen coating described above refers to the 
visible spectrum only, and the degree of transparency is dependent upon 
the opacity or stoichiometric ratio of the compound deposited on the 
substrate. 
From the above, it will be appreciated that the present invention provides 
a very flexible method of coating substrates wherein the properties of the 
coatings can be changed as desired by varying the three parameters 
involved, namely, the glow discharge power, the vaporization rate of the 
coating material, and the reactive gas pressure in the vacuum chamber. 
These parameters are interrelated but can be controlled and adjusted 
separately. For example, in the case of ZnO.sub.x, the three parameters, 
discharge power density, Zn partial pressure, and oxygen gas pressure can 
be varied to produce a range of properties from good conductivity and good 
transparency to high resistivity and good transparency by adjusting one, 
two or all three of these parameters. Any given set of parameters is not 
necessarily unique, since for example, an increase in the gas pressure can 
be offset by an appropriate change in the Zn partial pressure, and an 
appropriate change in the discharge power density to achieve films with 
similar properties. 
The parameters can be changed even during the coating process to produce a 
coating with varying properties across the coating thickness. For example, 
the index of refraction can be varied from 2.3 to 1.8 for ZnO.sub.x, or a 
non-stoichiometric zinc and oxygen coating can be produced by this method 
having very low resistivity and high visible light transmission 
characteristics. In fact, the resistivity of this coating can be varied 
between 6.times.10.sup.-4 and 10.sup.8 ohm cm.