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Claims  |
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We claim:
1. A method of depositing a transparent, electrically conducting, metal
oxide film onto the surface of a substrate of extended lateral dimensions,
said method comprising the steps of:
a. arranging a cathode assembly whose overall lateral dimensions are not
substantially less than those of the substrate in the vicinity of the
substrate but spaced apart therefrom to define a working space between the
cathode assembly and the substrate surface, the cathode assembly being so
constructed as to present a plurality of elongated, side-by-side strips
comprising a metal capable of being reactively sputtered, said strips
being spaced apart to define passages therebetween:
b. enclosing the cathode assembly and the substrate within a vacuum chamber
containing an atmosphere of oxygen and at least one other gas which is
inert to oxygen and to the other materials in the vacuum chamber, at a
controlled reduced pressure;
e. heating the substrate to a selected, elevated temperature prior to a
reactive sputtering step to be recited;
d. maintaining a substantial degree of uniformity in the oxygen
concentration in said working space by allowing said atmosphere to
penetrate through the spaces between said spaced strips and into said
working space;
e. applying a high negative potential to the cathode assembly to effect
deposition of said metal oxide film by reactive sputtering substantially
perpendicularly from said strips on to the substrate; and
f. maintaining the substrate at the selected, elevated temperature during
the sputtering step;
g. causing relative translatory movement between the cathode assembly and
the substrate in a direction transverse to the length of said strips,
through an amplitude substantially smaller than the overall length of the
cathode assembly, but sufficient to cause all parts of the substrate
surface to be coated by sputtering from at least one of said strips during
the deposition process.
2. A method according to claim 1 wherein the cathode assembly is formed
from a metal having an atomic number between 48 and 51, alloyed with a
metal of higher valency and similar atomic size.
3. A method according to claim 2 wherein the cathode assembly is formed
from an indium/tin alloy,
4. A method according to claim 3 wherein the indium/tin alloy comprises
between 75% and 95% indium and between 5% and 25% tin by weight.
5. A method according to claim 4 wherein the indium/tin alloy comprises 80%
indium and 20% tin.
6. A method according to claim 4 wherein the indium/tin alloy comprises 88%
indium and 12% tin.
7. A method according to claim 1 wherein the relative movement between the
cathode assembly and the substrate is a reciprocating movement.
8. A method according to claim 1 wherein the amplitude of the relative
movement is substantially equal to the spacing between the centre lines of
adjacent strips.
9. A method according to claim 1 wherein the strips move on guide rails
relative to the substrate.
10. A method according to claim 1 wherein the atmosphere is fed into the
vacuum chamber at one end thereof and is exhausted therefrom at the
opposite end thereof so that the atmosphere flow from inlet to exhaust
tends to pass through the working space and thereby assists in maintaining
uniformity of oxygen concentration in the working space.
11. A method according to claim 1, wherein the inert gas is argon.
12. A method according to claim 1 wherein the value of the negative
potential applied to the cathode assembly is selected within the range
-1KV to -5KV, the value of the oxygen concentration in the atmosphere in
the working space is selected in the range between 1.5% and 4.8% by
volume, and the temperature is selected within the range 240.degree. C to
400.degree. C.
13. A method according to claim 1 wherein the oxygen concentration in the
atmosphere in the working space is maintained in the range 1% to 10% by
volume.
14. A method according to claim 1 wherein the negative potential applied to
the cathode assembly is between 1.0 KV and 5.0 KV.
15. A method according to claim 1 wherein the controlled elevated
temperature of the substrate surface is between 240.degree. C and
400.degree. C.
16. A method according to claim 1 wherein the substrate is a sheet of
glass.
17. A method according to claim 1 wherein the pressure in the vacuum
chamber is between 1 .times. 10.sup..sup.-2 mm Hg and 10 .times.
10.sup..sup.-2 mm Hg.
18. A method of depositing a transparent, electrically conducting, metal
oxide film onto the surface of a substrate of extended lateral dimensions,
said method comprising the steps of:
a. arranging a cathode assembly whose overall lateral dimensions are not
substantially less than those of the substrate in the vicinity of the
substrate but spaced apart therefrom to define a working space between the
cathode assembly and the substrate surface, the cathode assembly being so
constructed as to present a plurality of elongated, side-by-side strips
comprising a metal capable of being reactively sputtered, the strips being
spaced apart to define passages therebetween;
b. enclosing the cathode assembly and the substrate within a vacuum
chamber;
c. supplying a sputtering atmosphere of oxygen and another gas or gases to
the vacuum chamber;
d. heating the substrate to an elevated temperature selected in accordance
with criteria to be recited prior to a reactive sputtering step to be
recited;
e. applying a high negative potential to the cathode assembly to effect
deposition of the metal oxide film by reactive sputtering;
f. controlling the oxygen concentration in the vacuum chamber, the
substrate temperature, the vacuum chamber pressure, and the cathode
potential during the sputtering step at values selected such that the
deposition coating is haze-free and its specific electrical resistivity
lies at or close to the minimum of the curve which is obtained by plotting
specific electrical resistivity against oxygen concentration while
maintaining the substrate temperature, vacuum chamber pressure, and
cathode potential all constant at selected values;
g. maintaining a substantial degree of uniformity in the oxygen
concentration in the working space by:
i. allowing the sputtering atmosphere to penetrate through the spaces
between the spaced strips and into the working space and
ii. causing relative translatory movement between the cathode assembly and
the substrate in a direction transverse to the length of the strips,
through an amplitude substantially smaller than the overall length of the
cathode assembly, but sufficient to cause all parts of the substrate
surface to be coated by sputtering from at least one of the strips during
the deposition process.
19. A method according to claim 18 wherein the cathode assembly is formed
from an indium/tin alloy.
20. A method according to claim 19 wherein the indium/tin alloy comprises
between 75% and 95% indium and between 5% and 25% tin by weight.
21. A method according to claim 20 wherein the indium/tin alloy comprises
80% indium and 20% tin.
22. A method according to claim 21 wherein the indium/tin alloy comprises
88% indium and 12% tin.
23. A method according to claim 18 wherein the atmosphere comprises a
mixture of oxygen and a gas which is at least substantially inert to the
remainder of the atmosphere and the materials in the vacuum chamber.
24. A method according to claim 23, wherein the inert gas is argon.
25. A method according to claim 18 wherein the amplitude of the relative
movement is substantially equal to the spacing between the centre lines of
adjacent strips.
26. A method according to claim 18 wherein the strips move on guide rails
relative to the substrate.
27. A method according to claim 18 wherein the atmosphere is fed into the
vacuum chamber at one end thereof and is exhausted therefrom at the
opposite end thereof so that the atmosphere flow from inlet to exhaust
tends to pass through the working space and thereby assists in maintaining
uniformity of oxygen concentration in the working space.
28. A method according to claim 18 wherein the value of the negative
potential applied to the cathode assembly is selected within the range
-1KV to -5KV, the value of the oxygen concentration in the atmosphere in
the working space is selected in the range between 1.5% and 4.8% by
volume, and the temperature is selected within the range 240.degree. C to
400.degree. C.
29. A method according to claim 18 wherein the oxygen concentration in the
atmosphere in the working space is maintained in the range 1% to 10% by
volume.
30. A method according to claim 18, wherein the cathode assembly is formed
from a metal having an atomic number between 48 and 51, alloyed with a
metal of higher valency and similar atomic volume.
31. A method according to claim 18 wherein the substrate is a sheet of
glass.
32. A method according to claim 18 wherein the pressure in the vacuum
chamber is between 1 .times. 10.sup..sup.-2 mm Hg and 10 .times.
10.sup..sup.-2 mm Hg.
33. A method of depositing a transparent, electrically conducting film of
an oxide of metal of atomic number 48 to 51 on to the surface of a
substrate of extended lateral dimensions, said method comprising the steps
of:
a. arranging a cathode assembly whose overall lateral dimensions are not
substantially less than those of the substrate in the vicinity of the
substrate but spaced apart therefrom to define a working space between the
cathode assembly and the substrate surface, the cathode assembly being so
constructed as to present a plurality of elongated, side-by-side strips
comprising said metal, said strips being spaced apart to define passages
therebetween; b. enclosing the cathode assembly and the substrate within a
vacuum chamber containing an atmosphere of oxgyen and an inert gas with a
controlled oxygen concentration of between 1% and 10% by volume, at a
controlled reduced pressure of between 1 .times. 10.sup..sup.-2 mm Hg and
10 .times. 10.sup..sup.-2 mm Hg;
c. heating the substrate to a selected, elevated temperature between
240.degree. C and 400.degree. C prior to a reactive sputtering step to be
recited;
d. maintaining a substantial degree of uniformity in the oxygen
concentration in the working space by allowing said atmosphere to
penetrate through the spaces between said spaced strips into the working
space;
c. applying a high negative potential of between -1 KV and -5 KV to the
cathode assembly to effect deposition of said metal oxide film by reactive
sputtering substantially perpendicularly from said strips on to the
substrate surface;
f. maintaining the substrate at the selected, elevated temperature during
the sputtering step; and
g. causing relative reciprocating movement between the cathode assembly and
the substrate in a direction transverse to the length of said strips,
through an amplitude substantially smaller than the overall length of the
cathode assembly, but sufficient to cause all parts of the substrate
surface to be coated by sputtering from at least one of said strips during
the deposition process.
34. A method according to claim 33 wherein said metal is alloyed with a
metal of a higher valency and similar atomic volume.
35. A method according to claim 34 wherein the cathode assembly is formed
from an indium/tin alloy.
36. A method according to claim 34 wherein the indium/tin alloy comprises
between 75% and 95% indium and between 5% and 25% tin by weight.
37. A method according to claim 35 wherein the indium/tin alloy comprises
80% indium and 20% tin.
38. A method according to claim 35 wherein the indium/tin alloy comprises
88% indium and 12% tin.
39. A method according to claim 33 wherein the amplitude of the relative
movement is substantially equal to the spacing between the centre lines of
adjacent strips.
40. A method according to claim 33 wherein the strips move on guide rails
relative to the substrate.
41. A method according to claim 33 wherein the atmosphere is fed into the
vacuum chamber at one end thereof and is exhausted therefrom at the
opposite end thereof so that the atmosphere flow from inlet to exhaust
tends to pass through the working space and thereby assists in maintaining
uniformity of oxygen concentration in the working space.
42. A method according to claim 33 wherein the inert gas is argon.
43. A method according to claim 33 wherein the value of the oxygen
concentration in the atmosphere in the working space is selected in the
range between 1.5% and 4.8% by volume.
44. A method according to claim 33 wherein the substrate is a sheet of
glass.
45. An article having its smallest lateral dimension greater than 30 cm.
and having a stable transparent electrically conductive film deposited on
a surface thereof, said film:
a. having a specific electrical resistivity of between 2 .times.
10.sup..sup.-4 ohm cm. and 20 .times. 10.sup..sup.-4 ohm cm., which
resistivity is substantially uniform over the whole of said film;
b. having a thickness which is everywhere less than 10,000A;
c. having a light transmission figure of over 70%; and
d. having been deposited by a reactive sputtering method comprising the
steps of:
i. arranging a cathode assembly whose overal lateral dimensions are not
substantially less than those of the substrate in the vicinity of the
substrate but spaced apart therefrom to define a working space between the
cathode assembly and the substrate surface, the cathode assembly being so
constructed as to present a plurality of elongated, side-by-side strips
comprising a metal capable of being reactively sputtered, said strips
being spaced apart to define passages therebetween;
ii. enclosing the cathode assembly and the substrate within a vacuum
chamber containing an atmosphere of oxygen and at least one other gas
which is inert to oxygen and to the other materials in the vacuum chamber,
at a controlled reduced pressure;
iii. heating the substrate to a selected, elevated temperature prior to
sputtering;
iv. maintaining a substantial degree of uniformity in the oxygen
concentration in said working space by allowing said atmosphere to
penetrate through the spaces between said spaced strips into said working
space;
v. maintaining the substrate at the selected, elevated temperature during
sputtering;
vi. applying a high negative potential to said cathode assembly to effect
deposition of said metal oxide film by reactive sputtering substantially
perpendicularly from said strips onto the substrate;
vii. selecting the value of the oxygen concentration, substrate
temperature, vacuum chamber pressure, and cathode potential such that the
deposited coating is haze-free and its specific electrical resistivity
lies at or close to the minimum of the curve which is obtained by plotting
specific electrical resistivity against oxygen concentration while
maintaining the substrate temperature, vacuum chamber pressure, and
cathode potential all constant at selected values and
e. causing relative translatory movement between said cathode assembly and
the substrate in a direction transverse to the length of said strips,
through an amplitude substantially smaller than the overall length of the
cathode assembly, but sufficient to cause all parts of the substrate
surface to be coated by sputtering from at least one of said strips during
the deposition process.
46. An article as recited in claim 45 wherein the thickness of said film is
substantially uniform over the whole of said film.
47. An article as recited in claim 45 wherein the substrate is a sheet of
glass.
48. An article as recited in claim 45 wherein the thickness of said film is
everywhere greater than 500.degree. A. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Cross-Reference to Related Application
The invention is related to that described in co-pending Application Ser.
No. 144,541 filed May 18, 1971.
2. Field of the Invention
This invention relates to methods and apparatus for depositing transparent,
electrically conducting, metal oxide films on to substrates, such as
glass, and to articles having such metal oxide films applied thereto. By
way of example, the article may be a windscreen, e.g. an aircraft
windscreen, on which the film can provide electrical resistance heating
means for de-icing or de-misting.
3. Description of Prior Art
Various proposals have been made for reactively sputtering a transparent,
electrically conducting, metal oxide film on to the surface of a substrate
supported in a vacuum vessel having an atmosphere of oxygen and another
gas or gases, from a metal cathode near the substrate surface to be
coated. One example of such a process is described in U.S. Patent
application Ser. No. 144,541.
Such processes have been successful in producing transparent, electrically
conducting, films, of reasonably uniform characteristics on substrates of
relatively small lateral dimensions, e.g. 4 cm. in width, but difficulties
have been experienced with larger articles. Even though the cathode is
enlarged commensurately with the article, so as to cover the whole
substrate area and to maintain the direct sputtering path from cathode to
substrate at the optimum value (between 2 cm. and 10 cm. depending on the
applied potential difference), it is found that the film produced tends to
be non-uniform. Variations are found in the thickness and specific
electrical resistivity of the film, which result in wide variations in its
resistance and diminish or destroy its utility as a resistance heating
means. In extreme cases, the film is also found to be less transparent
near the middle of the article. Any such reduction in transparency is
unacceptable in a windscreen, for example.
The present inventors have deduced that the non-uniformity of the film is
due to a reduction in the oxygen concentration in the atmosphere in the
working space between the cathode and the substrate, which is believed to
be caused by the consumption of the oxygen originally present during the
formation of the film, and by the slow rate at which fresh oxygen can
diffuse into this space. As the process proceeds, a gradient of oxygen
concentration is thus established in the atmosphere in the working space
in a plane parallel to the cathode surface, the concentration falling
towards the centre of the cathode.
For economy in production, it is desirable to have a high deposition rate
and to achieve a minimum specific resistivity (.rho.). However, as the
rate of deposition is increased, the rate of consumption of oxygen is also
increased and the effect of the gradient in the oxygen concentration will
become more severe. Further, as the area of the substrate and cathode is
increased so the gradient of oxygen concentration is established over
greater distances, the oxygen starvation at the centre of the substrate
becomes more pronounced. Consequently it is no longer possible to maintain
the desired oxygen concentration necessary to provide a minimum specific
electrical resistivity and adequate transparency over the whole area of
the substrate to be coated.
The existence of the above-mentioned oxygen gradient has been found to be
most detrimental to the deposition of a uniform film. The effect can
generally be detected where each lateral dimension, i.e. length and width,
of the substrate is substantially greater than the distance between the
cathode and substrate, e.g. is greater than 10 cm., and particularly if
they are greater than 30 cm.
OBJECT OF THE INVENTION
An object of this invention is to provide an improved method and apparatus
for depositing films on larger substrates than hitherto practicable, e.g.
substrates having both lateral dimensions greater than 30 cm., and in
particular on substrates such as windscreens for aircraft and land
vehicles, whose dimension can reach 100 cm. and more.
SUMMARY OF THE INVENTION
According to the invention, we provide a method of depositing a
transparent, electrically conducting metal oxide film by reactive
sputtering on to the surface of a substrate of extended lateral dimensions
which is maintained at a controlled elevated temperature in a vacuum
chamber containing an atmosphere of oxygen and another gas or gases at a
controlled reduced pressure, a high negative potential being applied to a
cathode assembly of the metal which is arranged in the vicinity of the
substrate and presents a surface or surfaces extending substantially
parallel to the substrate surface so that sputtering takes place
substantially perpendicularly on to all parts of the substrate, wherein
access is provided for the atmosphere to penetrate into the whole of the
working space between the cathode assembly and the substrate so as to
maintain a substantial degree of uniformity in the oxygen concentration in
the working space.
The term "extended lateral dimensions" is to be understood to mean having
lateral dimensions substantially greater than the distance between the
cathode and substrate, and generally having its smallest lateral dimension
greater than 30 cm.
Variation in the oxygen concentration has been found to affect the specific
electrical resistivity (.rho.) and the thickness (t) deposited in a given
time and hence the resistance in ohm/square of the sputtered film, as
disclosed in the Specification of co-pending U.S. Pat. application Ser.
No. 144,541. It will be understood that the resistance in ohm/square is
independent of the size of the square under consideration and is related
to the specific resistivity and thickness by the equation
##EQU1##
By providing access for the atmosphere into the whole of the working space
so as to maintain the oxygen concentration substantially uniform, we have
found it possible to produce articles of considerable size coated with
films having substantially uniform low resistance in ohm/square and
substantially uniform high light transmission.
In one form of the invention, passages extend through the cathode assembly
to provide the access for the atmosphere into the working space.
Preferably relative movement is provided between the cathode assembly and
the substrate in a direction parallel to the substrate surface. In a
preferred embodiment of the invention, the cathode assembly is divided
into spaced parallel strips so as to provide the passages for the
atmosphere between the strips, and the relative movement is provided
between the strips and the substrate in a direction transverse to the
length of the strips so that the strips cover all parts of the substrate
surface for equal deposition periods during one part or another of the
deposition process. Preferably the relative movement between the strips
and the substrate is a reciprocating movement. Advantageously the relative
movement is substantially equal to the spacing between the centre lines of
adjacent strips. The strips may move on guide rails relative to the
substrate.
The invention also provides an article of extended lateral dimensions
having a transparent electrically conducting film deposited on a surface
thereof by a method as described above, said film having a specific
electrical resistivity between 2 .times. 10.sup..sup.-4 ohm. cm. and 20
.times. 10.sup..sup.-4 ohm. cm. and preferably between 2 .times.
10.sup..sup.-4 ohm. cm. and 4 .times. 10.sup..sup.-4 ohm. cm., a thickness
of between 500 A and 10,000 A, and a light transmission figure of over
70%. Where the film thickness is below 5000 A, the light transmission
figure may be over 80%.
The invention further provides a glass article of extended lateral
dimensions having a transparent electrically conducting film of indium/tin
oxide deposited on a surface thereof, said film having a substantially
uniform resistance of between 2 and 40 ohm/square and a light transmission
figure of over 80%.
The invention further provides apparatus for depositing a transparent,
electrically conducting metal oxide film by reactive sputtering on to the
surface of a substrate of extended lateral dimensions, comprising a vacuum
chamber, means for supporting the substrate in the vacuum chamber, means
for maintaining the substrate at a controlled elevated temperature in the
vacuum chamber, means for supplying an atmosphere of oxygen and another
gas or gases at reduced pressure into the vacuum chamber, a cathode
assembly arranged in the vacuum chamber in the vicinity of the substrate
and presenting a surface or surfaces capable of extending over the whole
of the substrate surface and substantially parallel thereto, and means for
applying a high negative potential to the cathode assembly, wherein means
is provided for allowing access for the atmosphere to penetrate into the
whole of the working space between the cathode assembly and the substrate
so as to maintain a substantial degree of uniformity in the oxygen
concentration in the working space.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a diagrammatic layout of a known type of apparatus for reactively
sputtering a film on a relatively small substrate surface;
FIG. 2 is a graph of the resistance in ohm/square, thickness and specific
electrical resistivity of a sputtered film as a function of the distance
along the cathode in an experiment in which an oxygen concentration
gradient is deliberately established along the cathode, starting with a
known initial oxygen concentration;
FIG. 3 is a similar graph to FIG. 2 in respect of a similar experiment but
starting with a higher initial oxygen concentration;
FIG. 4 is a schematic perspective view of a first form of modified cathode
assembly for sputtering a large area substrate surface in accordance with
the method of the present invention;
FIG. 5 is a schematic section through the cathode assembly of FIG. 4;
FIG. 6 is a perspective view of an apparatus according to the invention
incorporating a cathode assembly as illustrated in FIGS. 4 and 5;
FIG. 7 is a longitudinal axial section through the apparatus of FIG. 6,
modified to deposit a film on a substrate which is curved from end to end;
FIG. 7A is a detail sectional view to a larger scale, of one of the cathode
sections used in the apparatus of FIG. 6 and FIG. 7;
FIG. 8 is a schematic section through a fourth form of modified cathode
assembly;
FIG. 9 is a similar view of a fifth form of modified cathode assembly; and
FIG. 10 is a graph of specific electrical resistivity against percentage
oxygen concentration in the atmosphere of sputtering apparatus having one
of the modified cathode assemblies of FIGS. 4 to 9, for a series of
examples in which the oxygen concentration is maintained substantially
uniform between the cathode or cathodes and a large area of substrate
surface.
DETAILED DESCRIPTION
Referring to FIG. 1, there is shown a vacuum vessel 10 for connection by
conduit 11 to a vacuum pump (not shown). A further conduit 12 passing
through the wall of the vacuum vessel 10 is connected via gas flow meters
13, 14 to separate sources of oxygen and argon respectively. These gas
flow meters 13, 14 are provided to ensure accurate control of the rate of
flow of the oxygen into the argon and thence into the atmosphere of the
vacuum vessel 10.
Within the vacuum vessel 10 there is mounted a substrate 17 which is to be
coated with an electrically conductive film by sputtering from a water
cooled cathode 18. The substrate 17 is supported on a heated block 19
which is heated by an internal electric heating element 20 connected by
leads 21, 22 to an external source of low voltage electrical power. The
cathode 18 is connected by a lead 23 to the negative terminal of an
external source of high voltage. An earthed electrostatic screen 24 is
provided round the cathode 18, and the heated block 19 and vacuum vessel
10 are also earthed as indicated at 25. Instead of the heated block 19
being heated internally, the heating element 20 may be disposed on
insulated support pillars on the upper surface of the block 19 so that the
substrate is heated mainly by radiation.
In order to determine and to control the temperature of the substrate 17 at
the required value, a thermocouple 26 is attached to the edge of the
substrate 17 so as to be in thermal contact with it. The thermocouple
provides a measure of the surface temperature of the substrate 17, as it
is heated by the heated block 19. As the substrate 17 is exposed to the
plasma induced by the electric field existing between the cathode and the
substrate 17, the additional power injected by the plasma heats the
substrate, and it is consequently necessary to gradually reduce the
electric power supplied to the heating element 20 inside the block 19, in
order to maintain the temperature of the substrate at the required value.
The substrate 17, which may be of toughened soda-lime-silica glass, is
placed on the heated block 19 in the vacuum vessel 10. The vacuum vessel
10 is then evacuated to a pressure of say 5 .times. 10.sup..sup.-4 mm Hg
as measured on a Penning vacuum pressure gauge 101, and low voltage (say
10 volts) electric power applied to heating element 20 through leads 21,
22. The substrate surface is thereby heated to a selected temperature
between 240.degree. C and 400.degree. C. Oxygen gas is then admitted to
the vacuum vessel 10 through the gas flow meter 13 and argon gas through
the gas flow meter 14 at desired flow rates. This flow of gas results in
the presence of a selected percentage of oxygen between 1% and 10% by
volume of the total gas in the vacuum vessel and produces an increase in
the pressure in the vessel to a selected value of the order of 5 .times.
10.sup..sup.-2 mm Hg as measured on a Macleod vacuum pressure gauge 102.
The cathode 18 is supported at a selected distance of say 3 to 4 cm. from
the glass substrate surface to be coated, and a selected negative voltage
of between -1.0 KV and -5 KV is applied to the cathode. The power applied
to the heating element 20 is gradually reduced in order to maintain the
glass substrate surface at the desired temperature, this step being
necessary owing to the heating effect produced by ionic and electronic
bombardment from the glow discharge between the cathode and substrate.
The ionised argon ions bombard the surface of the cathode 18 thus removing
metal from the cathode and thereby reactively sputtering a film of oxide
on to the upper surface of the glass substrate 17. At the completion of
this process, the power supplies to the cathode 18 and the heating element
20 are disconnected, the gas flows turned off and the glass substrate
allowed to cool.
The coated glass substrate is then removed from the vessel and the physical
characteristics of the film may be determined by measurement and
calculation.
The above method is applicable to the coating of a transparent and
haze-free film on a glass substrate from a single, stationary cathode
having a lateral dimension, e.g. a width or length, of 10 cm. or less.
However, for substrates and cathodes of greater lateral dimensions, it has
been found that the method results in the production of a non-uniform film
which does not have the desired resistance in ohm/square and may not be
haze-free, and the present inventors deduced that this effect is due to
variation of the oxygen concentration in the atmosphere in the working
space between the cathode and the substrate surface. To investigate and
measure this effect, there was employed a cathode measuring 45 cm. long by
15 cm. wide. A 60 cm. long by 30 cm. wide by 4 mm thick soda-lime-silica
glass substrate was placed at a distance 38 mm from the cathode surface.
The gaps formed between the edges of the cathode and the surface of the
substrate were blanked off along the two long sides and one end by three
pieces of glass. There was thus access for the sputtering atmosphere at
one end only.
The vacuum vessel was evacuated to a pressure or 5 .times. 10.sup..sup.-4
mm Hg and a voltage of 10 volts was applied to the heater 20 to raise the
temperature of the substrate surface to 300.degree. C. A gas mixture
consisting of 96% argon and 4% oxygen by volume, was admitted to the
vacuum vessel thereby increasing the pressure in the vessel to 6.5 .times.
10.sup.-.sup.2 mm Hg. The cathode was then energised to a voltage of -3.0
KV and sputtering was allowed to take place for a period of 10 minutes.
Upon removal of the glass substrate from the vessel it was immediately
obvious that the characteristics of the film varied considerably from one
end to the other. In particular, at the end where access of the sputtering
atmosphere had been allowed the film was substantially transparent
although slightly hazy. At the other end where access of the sputtering
atmosphere had been restricted the film was completely opaque and metallic
in appearance. Indeed the film showed characteristics over the 45 cm.
length of the cathode which would be consistent with a considerable drop
in the percentage of oxygen concentration.
The above experiment was repeated employing an atmosphere of 94% argon and
6% oxygen, with a cathode voltage of -2.75 KV. As expected, it was found
that the reduction in cathode voltage and increase in oxygen concentration
reduced the degree of haze and shifted the specific resistivity curve to
the right. At the end of the film where access of the sputtering
atmosphere had been allowed, a highly transparent, haze-free film having a
specific resistivity less than 10 .times. 10.sup.-.sup.4 ohm. cm. was
obtained. The film at the other end was very hazy and had a much higher
resistivity. The results of this experiment are shown in FIG. 2 which
comprises a graph of the resistance in ohm/square, the film thickness in A
and the specific resistivity (.rho.) in ohm. cm. as a function of the
distance along the cathode. This graph clearly shows that there is a
percentage of oxygen concentration which results in a minimum specific
resistivity.
The experiment as again repeated using the same reduced cathode voltage of
-2.75 KV but with an increased oxygen concentration of 7% in the
sputtering atmosphere. The results of this experiment are shown in FIG. 3
which is again a graph of the resistance in ohms/square, the film
thickness in A and the specific resistivity (.rho.) in ohm. cm. as a
function of the distance along the cathode. As before the graph shows that
there is a percentage of oxygen concentration which results in a minimum
specific resistivity.
These results indicated to the inventors that if the oxygen concentration
in the sputtering atmosphere in the working space between the cathode and
substrate could be controlled, it should be possible to maintain adequate
uniformity of transparency, specific resistivity, thickness, and thus
resistance in ohm/square of the sputtered film.
According to the invention, access is provided for the atmosphere to
penetrate into the whole of the working space so as to maintain a
substantial degree of uniformity of the oxygen concentration between the
cathode and the substrate. Such access may be provided by a particular
construction of the cathode and/or by the provision of relative motion
between the cathode and the substrate surface. Some examples will now be
described of suitable methods of controlling the uniformity of oxygen
concentration over the area of the substrate surface.
FIGS. 4 and 5 illustrate diagrammatically a first type of modified cathode
assembly 27 for carrying out the method of the present invention. The
cathode assembly 27 is divided into four parallel sections or strips 271,
each strip measuring 60 cm. in length and 8 cm. in width and being
surrounded by a separate earthed electrostatic shield 28. The cathode
sections or strips 271 are spaced apart by gaps 29 of equal width. The
individual gaps may be varied in width, for example between 1.0 cm. and 10
cm., and are provided to allow access for diffusion of the atmosphere from
the under side of the cathode assembly 27 to the side adjacent to the
substrate 31, as indicated by the arrows 30 in FIG. 5.
Means (not shown in FIGS. 4 and 5) are provided which effect relative
motion between the substrate 31 and the cathode assembly 27 in a direction
parallel to their facing surfaces. The preferred direction of relative
motion is perpendicular to the length of the cathode strips 271. The
motion is preferably an oscillatory motion of the cathode assembly 27 with
a constant speed of traverse between reversing points, as shown by the
double headed arrow 32a in FIG. 4, the amplitude of the oscillations being
equal to the spacing between the centre lines of adjacent cathode strips
271. By this means, during sputtering the gradient of oxygen concentration
in the atmosphere in the working space 32 between the cathode and
substrate, which would result from the use of a single large area cathode,
is reduced to an acceptable limit. As a consequence of the reduction in
the oxygen gradient in the sputtering atmosphere, a substantially uniform
conductive film of lower specific resistivity can be produced. The spacing
between adjacent strips 271 is chosen to be the minimum which will provide
adequate diffusion of the sputtering atmosphere into the working space 32,
while maintaining a sufficient coating rate.
FIG. 6 illustrates an apparatus incorporating a cathode assembly of the
kind described above with reference to FIGS. 4 and 5. The apparatus
comprises a cylindrical vacuum vessel 40 with removable vacuum-tight end
closures (not shown). The cathode assembly 27 comprises a plurality of
spaced, parallel sections of strips 271 having upper surfaces of
indium/tin alloy. Each strip 271 has an earthed electrostatic shield 28.
Only three sections or strips 271 are shown in FIG. 6, for clarity. In
practice, the number of strips used will depend on the length of the
substrate to be coated, being generally chosen so that an oscillation
having an amplitude equal to the spacing between the centre lines of the
strips will cause all parts of the substrate to be covered. The strips 271
are mounted on pairs of rollers 41 at each of their ends, and these
rollers run on horizontal guide rails 42 secured to opposite sides of the
vessel 40. The strips 271 are connected to one another by adjustable link
rods 43 which maintain their spacing and parallel alignment with one
another and ensure that all the strips can move together along the guide
rails in the direction perpendicular to their length. A flexible
high-tension lead 44 connects the strips 271 to the negative terminal of a
high-voltage source 45.
A pair of pulleys 46 is mounted on a transverse shaft 47 at each end of the
vessel 40 and a pair of traction wires or cables 48 connected at each end
to the electrostatic shields 28 of the end strips 271 are led over the
pulleys 46 to form drive means. One of the shafts 47 passes through the
wall of the vessel 40 and is connected via a variable-amplitude
oscillatory motion device 49 to an electric motor 50.
Each of the strips 271 is hollow, as shown in FIG. 7A, its interior being
filled with cooling water which is supplied through a flexible pipe 52
which enters near one end of the strip. The water leaves through a second
flexible pipe 51 near the other end of the strip 271. The pipes 51, 52
connect the strips 271 in series, but the pipes extending between the
adjacent strips have been omitted from FIGS. 6 and 7 for clarity. The
high-tension lead 44 from source 45 is of the co-axial cable type, the
outer conductor being earthed. Similar cables 44 connect the strips 271 to
one another.
Above the horizontal guide rails 42, a pair of horizontal support rails 53
(only one of which is shown) are secured to opposite sides of the vessel
40 to support a substrate 31 which is to be provided with a transparent
conducting film.
Above the position of the substrate 31, radiant heater 54 is secured in the
vessel 40, fed through low-tension leads 55 and busbars 56 from a low
voltage power unit 57. The heater 54 extends over the whole area of the
substrate 31.
A thermocouple 58 is placed on the upper surface of the substrate 31 and
connected through leads 59 to a calibrated dial instrument 60 to indicate
the temperature of the substrate.
A vacuum pump (not shown) is connected to the interior of the vessel 40
through an exhaust connection 61 and a gas supply 62 of the selected
atmosphere is connected through a flow meter 63 and needle valve 64 to an
inlet 65 opening into the vessel. The inlet 65 is at the opposite end of
the vessel 40 from the exhaust connection 61 so that gas flow from inlet
to exhaust tends to pass through the working space between the cathode
assembly and substrate and thereby assists in maintaining uniformity of
the oxygen concentration in the working space.
In use, when the substrate 31 has been placed on the support rails 53 and
the end closures have been sealed, the vessel 40 is evacuated through the
exhaust connection 61 and the selected sputtering atmosphere is supplied
through the inlet 65, while the substrate is heated to the desired
temperature by the heater 54. The cathode assembly 27 comprising the
strips 271 is oscillated back and forth along the guide rails 42 by the
motor 50 and the high negative voltage is applied to the strips 271 by the
source 45. The vessel 40 and rails 42, 53, as well as the electrostatic
shields 28, are earthed. A film of indium/tin oxides is thus sputtered on
to the lower surface of the substrate 31. The heating effect on the
substrate of the plasma in the working space is such that the heating
current supply from the low voltage power unit has to be reduced to
maintain the substrate temperature constant within .+-. 10.degree. C of
the desired value. An automatic control circuit of known type (not shown)
can be used for this purpose.
The amplitude of the oscillatory motion of the strips 271 is adjusted to
equal the spacing between the centre lines of the strips. This spacing can
be adjusted by means of the link rods 43. All parts of the substrate 31
are effectively covered for equal deposition times by the strips during
one part or another of each oscillatory cycle.
The spaces between the strips 271 allow free circulation of the sputtering
atmosphere so that no substantial oxygen gradient can become established.
With an oxygen content of 3.0% by volume in the atmosphere supplied, it is
believed that the reduction in the oxygen content in the working space is
not more than 0.2%, i.e. a reduction from 3.0% to 2.8%. A substantially
uniform highly transparent film of low specific resistivity can thus be
deposited on the substrate. Variations in the specific resistivity can
readily be kept within .+-. 10% of a means value.
FIG. 7 illustrates a modification of the apparatus of FIG. 6 for use in
depositing a film on to a substrate 311 which is longer than the substrate
31 shown in FIG. 6 and is curved from end to end, e.g. a windscreen for a
motor vehicle. The support rails 531 and the guide rails 421 are similarly
curved, as seen in elevation, and can be supported from the sides of the
vacuum vessel 40 through separate brackets (not shown). The curvature of
the guide rails 421 is such that the cathode sections or strips 271, shown
here as being five in number, always remain parallel to the tangent to the
adjacent portions of the substrate surface and at the required
substantially constant distance from that surface, during their
oscillatory movement. The heater 54 is formed in sections disposed on
tangents to an arc which corresponds approximatel | | |
