Method of producing transparent zinc oxide films

There is disclosed a method of producing a transparent zinc oxide film having an even thickness especially useful as a ultraviolet rays cut off. The method comprises applying a solution of an organic zinc compound selected from the group consisting of a zinc salt of a fatty acid of 3-7 carbons and a chelate compound of a diketone of 5-8 carbons in an organic solution on a substrate, and then baking the resultant coating at temperatures of 300.degree.-600.degree. C. It is preferred that the solution of an organic zinc compound further contains at least one organometallic compound of a metal selected from the group consisting of aluminum, indium, tin and titanium.

This invention relates to a method of producing a zinc oxide film which has 
a high transparence and an even thickness, especially useful as a 
ultraviolet rays cut off or interceptor. The invention further relates to 
a method of producing a composite film containing a transparent zinc oxide 
film which has electroconductivity as well as a high transparence and an 
even thickness, useful as a ultraviolet rays cut off or interceptor and 
many other uses. 
The transparent film composed of zinc oxide exhibits a very sharp 
ultraviolet rays cut off, or intercepts or absorbs ultraviolet rays very 
effectively, and thus it is in wide use in such appliances as fluorescent 
lights, high luminance lamps and display panels of various office 
automation appliances as ultraviolet rays cut off. The film effectively 
prevents deterioration of fluorescent materials or liquid crystals 
incorporated therein or, eliminates or reduces harmful influence upon the 
eyes or skin. 
The film also attracts much attention as a material for surface-treatment 
of glass used in hothouses or biochemical apparatus from the biochemical 
standpoint as technology in the field of ecology and biology progresses. 
Various methods are already known for the production of such transparent 
zinc oxide films, among which is a baking method (J. Material Science 
Letters, 9 (1990), 127). According to the method, a solution of zinc 
2-ethyl hexanoate in butanol is applied to a substrate and the resultant 
coating is baked to form a zinc oxide film. This method is inexpensive and 
has a high productivity. However, the method has been found to fail to 
provide transparent zinc oxide films having an even thickness since the 
resultant coatings shrink when being heated. 
It is, therefore, an object of the invention to provide a method of 
producing a zinc oxide film which has a high transparence and an even 
thickness, and which is optionally electroconductive, especially useful as 
ultraviolet rays cut off.

According to the invention, there is provided a method of producing a 
transparent zinc oxide film, which comprises applying a solution of an 
organic zinc compound selected from the group consisting of a zinc salt of 
a fatty acid of 3-7 carbons and a chelate compound of a diketone of 5-8 
carbons in an organic solvent on a substrate, and then baking the 
resultant coating at temperatures of 300.degree.-600.degree. C. 
The organic zinc compound used in the invention is selected from the group 
consisting of a zinc salt of a fatty acid of 3-7 carbons and a chelate 
compound of a diketone of 5-8 carbons. By way of example, zinc valerate or 
zinc caproate is preferably used as the fatty acid salt, while zinc 
acetylacetonate as the chelate compound. 
To prepare an organic solution of these organic zinc compounds, it is 
generally preferred that an organic solvent having relatively high boiling 
points is used. Thus, there may be preferably used n-butanol, dimethyl 
formamide, acetylacetone or ethyl cellosolve, although not limited to 
these exemplified. 
The solution contains the organic zinc compound preferably in an amount of 
not less than 1% by weight, more preferably in the range of 2-20% by 
weight, and most preferably in the range of 3-15% by weight, based on the 
solution. 
In accordance with the invention, it is further preferred that the solution 
of organic zinc compound contains at least one organometallic compound of 
a metal selected from the group consisting of aluminum, indium, tin and 
titanium, thereby to provide zinc oxide films having a higher 
transparence. 
Preferred organometallic compounds include, for example, a salt of a fatty 
acid of up to 20 carbons, preferably of 6-18 carbons, an acetylacetone 
type complex of an .alpha., .beta.-unsaturated diketone, preferably of 5-7 
carbons, or an alkoxide of an aliphatic alcohol of 3-5 carbons. 
Accordingly, the fatty acid salt includes, for example, aluminum laurate, 
indium 2-ethyl hexanoate and tin 2-ethyl hexanoate; the acetylacetone type 
complex includes, for example, aluminum acetylacetonate; and the alkoxide 
includes, for example, titanium tetrabutoxide. The organometallic compound 
is used in an amount of 1-50% by weight of the orgaic zinc compound. 
According to the invention, there is further provided a method of producing 
a transparent and electroconductive film of zinc oxide, which comprises 
forming an electroconductive film of stannic oxide on a substrate, and 
then forming a transparent film of zinc oxide thereon. 
It is preferred that the zinc oxide film is formed by the method as 
hereinbefore set forth, however, the zinc oxide film may be formed by any 
known conventional method, among which is a thermal decomposition method 
wherein an aqueous solution of zinc acetate is sprayed on a preheated 
substrate having a stannic oxide film thereon and is then thermally 
decomposed thereon to zinc oxide. 
According to the method, when a pattern of electroconductive stannic oxide 
film is first formed as an undercoat on a limited area on a substrate, and 
then a transparent zinc oxide film is formed on the entire surface of the 
substrate, there is obtained such a transparent zinc oxide film which is 
electroconductive only on the stannic oxide film, with the other surface 
being insulative. Namely, the zinc oxide film has the same pattern on the 
substrate as the stannic oxide film with respect to electroconductivity. 
Moreover, the resultant layered or composite film composed of stannic 
oxide and zinc oxide intercepts ultraviolet rays effectively. It is also 
more transparent than the stannic oxide film. 
According to the invention, an electroconductive indium (III) oxide-stannic 
oxide film (hereinafter referred to the ITO film, as accepted in the art) 
may be formed in place of an electroconductive film of stannic oxide. 
The ITO film may be formed on a substrate by any known conventional method 
such as a sputtering method or an ion plating method. The transparent zinc 
oxide film formed on the ITO film is electroconductive. In addition, the 
substrate having the composite film has a higher transmittance to visible 
lights since the zinc oxide film has a smaller refractive index than the 
ITO film. The substrate is also useful as a ultraviolet rays cut off. 
Further according to the invention, an electroconductive zinc oxide film 
may be prepared by first forming an insulative film of stannic oxide on a 
substrate, and then a transparent zinc oxide film thereon in the same 
manner as hereinbefore set forth, although the electroconductivity of the 
film being of a much smaller level. 
As above set forth, the method of the invention provides zinc oxide films 
which have a high transparence and an even thickness, and which is 
optionally electroconductive, and the films are especially useful as 
ultraviolet rays cut off. 
The invention will now be described in more detail with reference to 
examples, however, the invention is not limited to the examples. 
EXAMPLE 1 
A solution of 10 parts by weight of zinc valerate and 90 parts by weight of 
acetylacetone was applied on a glass substrate. The resultant coating was 
dried at room temperatures for 30 minutes, at a temperature of 110.degree. 
C. for 30 minutes, and then baked at 550.degree. C. for 30 minutes to 
provide a transparent zinc oxide film. 
As shown in FIG. 1, the glass substrate having the transparent zinc oxide 
film thereon is a sharper ultraviolet rays cut off than the glass 
substrate itself. 
EXAMPLE 2 
A solution of 5 parts by weight of zinc caproate and 95 parts by weight of 
n-butanol was applied on a glass substrate. The resultant coating was 
dried and baked in the same manner as in Example 1 to provide a 
transparent zinc oxide film which was also found to be useful as a 
ultraviolet rays cut off. 
EXAMPLE 3 
A solution of 5 parts by weight of zinc acetylacetonate, 47.5 parts by 
weight of acetylacetone and 47.5 parts by weight of ethyl acetate was 
applied on a glass substrate. The resultant coating was dried and baked in 
the same manner as in Example 1 to provide a transparent zinc oxide film 
which was also found to be useful as a ultraviolet rays cut off. 
EXAMPLE 4 
An electroconductive film of stannic oxide having an area of 10 cm.times.10 
cm and an average surface resistivity of about 600 .OMEGA./.quadrature. 
was formed on a glass substrate. 
The substrate was immersed in a solution of 10 parts by weight of zinc 
valerate in 90 parts by weight of acetylacetone and then baked at 
520.degree. C. over one hour to provide a transparent zinc oxide film 
which was found to have an average surface resistivity of about 900 
.OMEGA./.quadrature.. 
EXAMPLE 5 
A patterned electroconductive stannic oxide film having an average surface 
resistivity of about 600 .OMEGA./.quadrature. was formed as an undercoat 
at a limited area on a glass substrate with the use of a mask pattern, and 
then a transparent zinc oxide film was formed on the entire surface of the 
substrate in the same manner as in Example 4. 
The portion of the zinc oxide film having no undercoat of electroconductive 
stannic oxide film was found to be insulative, but the portion of the zinc 
oxide film having an undercoat of electroconductive stannic oxide film was 
found to have the same electroconductivity as the stannic oxide film. 
EXAMPLE 6 
An insulative stannic oxide film having an area of 10 cm.times.10 cm and an 
indifinite surface resistivity was formed on a glass substrate, and then a 
transparent zinc oxide film was formed in the same manner as in Example 4. 
The zinc oxide film was found to have an average surface resistivity of 
about 400 K.OMEGA./.quadrature.. 
COMATIVE EXAMPLE 1 
A glass substrate having an area of 10 cm.times.10 cm was half immersed in 
the same solution of zinc valerate as in Example 4, and then baked at 
520.degree. C. for 30 minutes to provide a transparent zinc oxide film on 
the half of the substrate. Thereafter an electroconductive stannic oxide 
film having a surface resistivity of the order of 10.sup.3 
.OMEGA./.quadrature. was formed on the entire surface of the substrate by 
a CVD method. 
The stannic oxide film on the zinc oxide film was found to have a surface 
resistivity of the order of 10.sup.6 .OMEGA./.quadrature., about 1000 
times as much as the surface resistivity of the electroconductive film on 
the other half of the substrate, on account of autodoping of zinc into the 
stannic oxide film. 
EXAMPLE 7 
An electroconductive stannic oxide film having an area of 10 cm.times.10 cm 
and an average surface resistivity of about 600 .OMEGA./.quadrature. was 
formed on a glass substrate. 
The substrate was immersed in a solution of 5 parts by weight of zinc 
valerate and 1 part by weigh of titanium tetrabutoxide in 94 parts by 
weight of acetylacetone and then baked at 520.degree. C. over one hour to 
provide an electroconductive and transparent zinc oxide film which was 
found to have an average surface resistivity of about 900 
.OMEGA./.quadrature.. 
FIG. 2 shows the transparence of glass substrate having an 
electroconductive stannic oxide film thereon designated by 1, the 
transparence of glass substrate having an electroconductive stannic oxide 
film and zinc oxide film thereon according to the invention designated by 
2, and the glass substrate designated by 3, each over a wavelength of 
350-700 m.mu.. The substrate having the composite film of the invention is 
larger than the substrate 1 in transparence over a wavelength more than 
about 370 m.mu., but also it is much more interceptive against ultraviolet 
rays over a wave length of not more than about 380 m.mu.. 
EXAMPLE 8 
A solution of 5 parts by weight of zinc valerate and 1 part by weight of 
titanium tetrabutoxide in 94 parts by weight of acetylacetone was applied 
on a glass substrate and then baked at 520.degree. C. over one hour to 
provide a first transparent zinc oxide film. 
A further solution of 5 parts by weight of zinc valerate in 95 parts by 
weight of acetylacetone was applied on a glass substrate and then baked at 
520.degree. over one hour to provide a second transparent zinc oxide film. 
FIG. 3 shows the transparence of glass substrate having the first zinc 
oxide film thereon designated by 1, in comparison with the transparence of 
glass substrate having the second zinc oxide film thereon designated by 2, 
and of the glass substrate itself designated by 3, each over a wavelength 
of 350-700 m.mu.. 
The substrate having the first film thereon is larger in transparence than 
the substrate over a wavelength more than 430 m.mu., but also it is more 
interceptive against ultraviolet rays than the substrate itself 3. 
In contrast, the substrate having the second film thereon has an uneven 
thickness so that the substrate is smaller in transparence over a 
wavelength of 350-700 m.mu. than the substrate itself. 
EXAMPLE 9 
Aluminum acetylacetonate, indium 2-ethylhexanoate and tin 2-ethylhexanoate 
were used respectively as the organometallic compound in place of titanium 
tetrabutoxide, and otherwise in the same manner as in Example 7, 
transparent and elecroconductive films were prepared. These films were 
also found to have the same high level of electroconductivity and 
transparence as the film of Example 7. 
EXAMPLE 10 
A glass substrate having an ITO film formed thereon was immersed in a 
solution of 10 parts by weight of zinc valerate in 90 parts by weight of 
acetylacetone and then baked at 320.degree. C. over 30 minutes to provide 
a transparent zinc oxide film on the ITO film. The zinc oxide film was 
found to have an average surface resistivity of 30 .OMEGA./.quadrature.. 
The ITO film on the glass substrate was found to have a surface resistivity 
of 25 .OMEGA./.quadrature., but was found to have a surface resisitivity 
of 65 .OMEGA./.quadrature. after the baking. It is well known that when an 
ITO film is heated, its surface resistivity increases. 
FIG. 4 shows the transparence of glass substrate having the composite film 
formed as above in comparison with the glass substrate having the ITO film 
only, each over a wavelength of 350-700 m.mu.. The glass substrate having 
the ITO film only has a reduced transmittance at a wavelength more than 
about 500 m.mu.. In contrast, the glass substrate having the composite 
film thereon has an increasing transmittance towards a longer wavelength. 
Moreover, the glass substrate of the invention intercepts ultraviolet rays 
more effectively at a wavelength less than about 400 m.mu..