Method of manufacturing a conductive layer on a substrate

A conductive layer comprising metal-oxide or metal-oxides is applied to a substrate by PA1 providing a layer comprising a metal-salt or a mixture of metal-salts on the substrate, PA1 bringing the layer into contact with an alkaline solution, and, subsequently, PA1 subjecting the layer to a thermal treatment (heating).

BACKGROUND OF THE INVENTION 
The invention relates to a method of applying a conductive, transparent 
metal-oxide layer to a surface of a substrate. 
Such layers are applied, inter alia, to windows of cathode ray tubes. The 
conductive layer has an anti-static effect and a shielding effect, that 
is, the intensity of the electromagnetic stray field emitted by the 
cathode ray tube is reduced by applying said conductive transparent layer. 
A method of the type mentioned in the opening paragraph is disclosed in WO 
95/29501. In said document, a description is given of a method in which a 
sol/gel coating of ITO (indium-tin oxide, i.e. a layer containing 
SnO.sub.2 /In.sub.2 O.sub.3) is cured by means of a laser in a 
hydrogen-containing atmosphere. Within the scope of the invention, a 
conductive metal-oxide layer must be understood to mean a layer which 
comprises an oxide or oxides of a metal or of a mixture of metals and 
which is capable of conducting electric current. 
As regards the known method and methods of applying conductive layers in 
general, the relatively high resistance and the instability of the 
conductive layer constitute a problem. 
Although conductive layers manufactured in accordance with the known method 
have a lower resistance than conductive layers manufactured by earlier 
methods, their resistance is still relatively high for specific 
applications and they often exhibit, as a function of time, an instability 
in the conductivity. In general, the conductivity decreases as a function 
of time. A decrease in conductivity causes a reduction of the shielding 
and anti-static function of the conductive layer. A reduction of the 
resistance leads to an improvement of the shielding effect. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a method of the type mentioned 
in the opening paragraph, which method leads to a reduction of one or more 
of the above problems and, in particular, to an improved shielding effect. 
To achieve this, the method in accordance with the invention is 
characterized in that the surface is provided with 
a layer comprising a metal-salt or a mixture of metal salts, whereafter 
the layer is brought into contact with a solution to form metal-hydroxides, 
after which 
the layer is subjected to a thermal treatment. 
By bringing the layer into contact with the solution, a reaction of the 
metal-salt(s) with the (e.g. alkaline solution); solution causes 
metal-hydroxides to be formed in situ (i.e. in the layer), which 
metal-hydroxides are converted in situ to conductive metal-(di)oxides by 
the subsequent thermal treatment. In comparison with conductive layers 
manufactured according to the known method, the resultant conductive 
layers exhibit a relatively low resistance and an increased stability. The 
increase in stability can probably be attributed to the reaction with the 
(e.g. alkaline) solution and the relatively low porosity of the conductive 
layers formed. 
Preferably, the layer comprises a salt or a mixture of salts of one or more 
of the metals indium, tin and antimony. 
The oxides of these metals exhibit a good conductivity. 
A preferred embodiment of the method in accordance with the invention is 
characterized in that the layer is applied to the surface in the form of a 
salt-solution. 
Sol/gel solutions as used in the known method are generally instable and 
exhibit poor keeping properties. This means that the preparation, storage 
and processing of the solutions requires great care and that, preferably, 
a relatively small quantity of the solution is available from stock, the 
sol/gel solution has to be prepared at a small distance from the device 
used to carry out the method, and the time period between the preparation 
and the use of said solution should be as small as possible. Such 
conditions lead to a substantial increase in cost, and there is a 
substantial risk that, despite due care, the solution and hence the 
conductive layer do not meet the quality requirements. 
Salt solutions are more stable than sol/gel solutions and hence exhibit 
better keeping properties. In addition, by applying the salt layer as a 
solution, a proper distribution of the salt or salts over the surface of 
the substrate can be achieved. 
For the salts, use is preferably made of carbonates, nitrates, chlorides, 
acetates, acetyl acetonates and/or formiates. 
For the solution use is preferably made of e.g. an ammonia solution or a 
hydrogen-peroxide solution. 
These salts and alkaline solutions have the advantage that no, or hardly 
any, residual products remain in the conductive layer.

DETAILED DESCRIPTION OF THE INVENTION 
The invention will now be described in greater detail with reference to the 
figures of the drawing. 
FIG. 1 is a schematic, cut-away view of a cathode ray tube 1 with a glass 
envelope 2 comprising a display screen 3, a cone 4 and a neck 5. Said neck 
accommodates an electron gun 6 for generating an electron beam. This 
electron beam is focused on a phosphor layer on the inside 7 of the 
display screen 3. In operation, the electron beam is deflected across the 
display screen 3 in two mutually perpendicular directions by means of a 
deflection coil system (not shown). An anti-static coating 8 in accordance 
with the invention is applied to the outer surface of the display screen 
3. 
FIG. 2 is a schematic, cross-sectional view of a display screen in 
accordance with an inventive embodiment. An anti-static coating 8 is 
applied to the display screen 3. This anti-static coating 8 comprises a 
first layer 9 (AS), a second layer 10 and, in this example, a third layer 
11. The first layer 9 is prepared in accordance with the inventive method 
and comprises, in this example, tin-oxide. The second layer is made of 
silicon dioxide. The first layer and the second layer together form an 
anti-reflection filter (AR). For this purpose, the thickness of both 
layers 9 and 10 is, for example, approximately .lambda./4, where .lambda. 
ranges in the visible region, for example, between 500 and 600 nm. The 
second layer may be provided with polypyrrole-latex particles, enabling 
the transmission properties of the second layer to be determined. The 
third layer 11 (AG) is responsible for an anti-glare effect and is made, 
for example, of sprayed-on silicon dioxide. 
FIGS. 3A through 3C illustrate a method in accordance with the invention. 
The method in accordance with the invention is characterized in that the 
surface 3 is provided with a layer containing a salt, whereafter the salt 
layer is brought into contact with an alkaline solution and, subsequently, 
subjected to a thermal treatment. In this example, a film of a salt 
solution 31 comprising a salt or a mixture of salts of indium and/or tin 
is provided, on the surface (FIG. 3) whereafter the film is fixed on the 
surface by a thermal treatment (for example drying). The salt layer (that 
is, a layer comprising a metal-salt) is subsequently brought into contact 
with an alkaline solution, for example, an ammonia solution 32 (FIG. 3B) 
whereafter the layer is subjected to a further thermal treatment. In the 
course of this further thermal treatment, the volatile constituents 
disappear and a conductive layer 33 is formed (FIG. 3C). 
Salt solutions are more stable than sol/gel solutions. The salt solutions 
may be solutions, inter alia, of carbonates, nitrates, acetates, acetyl 
acetonates, formiates, for example, of indium, tin, indium-tin, antimony 
or mixtures thereof. A film of the solution may be applied, for example, 
by means of flow-coating, curtain-coating, spinning or spraying. Also 
after fixing (drying), a salt solution is more stable than a sol/gel 
solution. A salt solution can be fixed by means of hot air, in a furnace, 
by means of a laser or otherwise. The salt layer may also be provided in a 
different manner, for example, by spreading very fine salt powder over the 
surface. However, the use of a salt solution has the advantage that a 
uniform or substantially uniform distribution of the salt layer over the 
surface can be readily achieved. A non-uniform distribution of the salt 
layer results in inhomogeneities of the resistance of the conductive 
layer, which adversely affects the shielding effect. 
The relatively low resistance and the high stability of the layers formed 
can possibly be attributed to a reaction between the alkaline solution and 
the salt or the salt mixture, which probably leads to a conversion of the 
salts into metal hydroxides (for example, indium, tin or indium-tin 
hydroxides). The subsequent, further thermal treatment, for example by 
means of hot air, in a furnace or, preferably, by means of a laser leads 
to a conversion of the (non-conductive or low conductive) hydroxides into 
electroconductive oxides, for example ITO (indium-tin oxide) or ATO 
(antimony-doped tin oxide). The resultant conductive layers exhibit a 
relatively low surface resistance and an increased stability. Said 
increase in stability can probably be attributed to the reaction between 
the salts and the alkaline solution, and the relatively low porosity of 
the conductive layers thus formed. The alkaline solution may be, for 
example, an ammonia solution or ammonia vapors or a hydrogen-peroxide 
solution, and may be a liquid or a gas. 
Subsequently, a description will be given of a few examples in accordance 
with the invention. 
EXAMPLE 1 
A solution of In(NO.sub.3).sub.3 is prepared in ethanol. The solution 
comprises 0.15 M In(NO.sub.3).sub.3, 0.3 M acetate acid (HAc) and 0.015 M 
SnCl.sub.4. The solution is subsequently applied to a substrate by 
spin-coating. For the substrate use can be made of Corning 7059 or Schott 
AF45 borosilicate glass. The spinning rate ranges, for example, between 
400 and 800 rpm (revolutions per minute). Subsequently, the substrates 
provided with a film of said solution are heated, in succession, for 5 
minutes at 150.degree. C., 2 minutes at 300.degree. C. or 1 minute at 
500.degree. C., whereafter they are immersed in a 6% ammonia solution and, 
subsequently, cured by heating (30 minutes) in air at 550.degree. C. and 
15 minutes in a forming gas at 325.degree. C. The following chemical 
reactions probably occur: 
metal-salt+base gives metal hydroxide+salt, the subsequent thermal 
treatment brings about a thermal decomposition leading to the formation of 
a conductive oxide layer and further, preferably volatile, constituents. 
EXAMPLE 2 
A solution of In(AcAc) (indium acetate) is prepared in ethanol. The 
solution comprises 0.15 M In(AcAc).sub.3, 0.3 M HNO.sub.3 and 0.015 M 
SnCl.sub.4. The solution is subsequently spun onto a substrate. For the 
substrate use can be made of Corning 7059 or Schott AF45 borosilicate 
glass. The spinning rate ranges, for example, between 400 and 800 rpm 
(revolutions per minute). The substrates provided with a film of said 
solution are subsequently heated, in succession, for 5 minutes at 
150.degree. C., 2 minutes at 300.degree. C. or 1 minute at 500.degree. C., 
whereafter they are immersed in a 6% ammonia solution and, subsequently, 
cured by heating (30 minutes) in air at 550.degree. C. and in a forming 
gas at 325.degree. C. for 15 minutes. 
EXAMPLE 3 
A solution as described in example 1 is spin-coated (spinning rate 200 rpm) 
onto a display window of a cathode ray tube. The solution is dried by 
scanning it with a heating laser. In this process, a dried layer 
containing salts is formed. Subsequently, a 5 M ammonia mist is sprayed 
over the dried layer. Next, the layer is cured by means of a laser. For 
this purpose, the layer is scanned by means of a laser beam. 
Immediately after the conductive layers have been formed in accordance with 
the above method, they exhibit a surface resistance which is generally 
below 1000 Ohm, customarily in the range from 200 to 500 Ohm (said 
resistance values always are square resistance values). Conductive layers 
made in accordance with the known method have surface resistances of the 
order of 1000-10,000 Ohm or higher. Apart from the surface resistance of 
the layer immediately after the conductive layer has been formed, also the 
variation of the surface resistance is an important aspect. In general, 
the surface resistance of a conductive layer is not constant but varies as 
a function of time, with, in general, the resistance increasing from a 
lower initial value to a more or less stable final value. As regards 
conductive layers made in accordance with the inventive method, the 
surface resistance generally increases by a factor of 2 to 4, the increase 
being smaller as the initial value is lower. At an initial resistance also 
value below 500 Ohm, the resistance increases by a factor of 2-3, at a 
higher initial resistance value, the resistance increases by a factor of 
3-4. In the known method, the surface resistance increases by a factor of 
5-20, so that the final resistance is much higher than 1000 Ohm. 
The generally lower surface resistance of the conductive layers made in 
accordance with the inventive method can possibly be attributed to the 
higher density of the layer and the larger grain size. Conductive layers 
made by means of wet-chemical processes generally exhibit a granular 
structure. The interfaces between the grains are an important factor as 
regards the value of the surface resistance. Investigations by means of a 
transmission-electron-microscope have revealed that the reaction with an 
alkaline solution leads to an increase of the average grain size in a 
layer and to a higher density. This may be responsible for the substantial 
reduction in surface resistance relative to the known method. The use of 
an alkaline solution has a positive effect on the initial resistance and 
on the final surface resistance. 
Preferably, the conductive layer is covered with a sealing layer, such as a 
second conductive layer and/or a silicon-dioxide layer. Such sealing 
layers lead to a reduction of the initial resistance and/or the 
degradation (increase of the surface resistance as a function of time) of 
the surface resistance. This positive effect is illustrated by the 
following example: 
A conductive layer is manufactured in accordance with one of the above 
examples. The initial resistance is approximately 500 Ohm. After 50 hours, 
the surface resistance has increased to a stable value of 2000 Ohm. For 
comparison, subsequently a conductive layer is formed and covered with a 
second layer in accordance with one of the above examples. The initial 
resistance of the double, conductive layer thus formed is approximately 
200 Ohm. After 50 hours, the surface resistance has increased to a stable 
value of approximately 500 Ohm. Consequently, the final surface resistance 
has decreased by a factor of 4. 
For the salts use is preferably made of carbonates, nitrates, chlorides, 
acetates, acetyl acetonates and/or formiates. 
For the solution use is preferably made of an alkaline solution, e.g. an 
ammonia solution or of hydrogen-peroxide solutions. 
These salts and alkaline solutions have the advantage that no, or hardly 
any, residual products remain in the conductive layer. 
It will be obvious that within the scope of the invention many variations 
are possible to those skilled in the art. 
The invention can be summarized as follows: 
A conductive layer comprising metal-oxide or metal-oxides is applied to a 
substrate by 
providing a layer comprising a metal-salt or a mixture of metal-salts on 
the substrate, 
bringing the layer into contact with an alkaline solution, and, 
subsequently, 
subjecting the layer to a thermal treatment (heating). 
The layer comprising metal-salts is preferably provided as a salt solution 
which is subsequently dried.