Method of manufacturing ITO sputtering target

In manufacturing a high-density ITO sputtering target by press-molding a powder mixture consisting essentially of indium oxide and tin oxide is compacted and the resulting compact is sintered. The sintering is performed in an atmosphere of oxygen under a pressure of at least one atmosphere (gauge pressure). The present invention offers very advantageous effects from the industrial viewpoint in that it permits mass production of high-performance, high-density ITO targets at low cost using mass productivity.

FIELD OF THE INVENTION 
This invention relates to a method of manufacturing a "high-density 
sintered target" for use in forming an ITO Indium-Tin Oxide film by 
sputtering. 
BACKGROUND OF THE INVENTION 
The oxide film of In.sub.2 O.sub.3 -SnO.sub.2, called "ITO film, " exhibits 
the semiconductor characteristics of n-type conductivity. Its high 
conductivity plus visible light transmissivity make the ITO film useful in 
a wide range of applications. Recent applications include liquid-crystal 
display devices, thin-film electroluminescent display devices, radiation 
detector elements, transparent tablets of terminal equipments, defogging 
heater films for windowpanes, antistatic films, and selective permeable 
membranes for solar heat collectors. The ITO film is formed by a variety 
of known techniques, such as chemical processes, e.g., spraying which 
utilizes the pyrolysis of a compound, and chemical vapor deposition (CVD); 
and physical processes, e.g., vacuum evaporation and sputtering. Of these, 
sputtering is finding extensive adoption because of growing interest in 
its advantage that the production of films having large-area is permitted. 
Further films of low resistivity may be formed with good reproducibility. 
In forming an ITO film by sputtering, a sputtering target consisting of 
indium oxide and tin oxide (hereinafter called "ITO target" for brevity) 
is used. ITO targets usually have been made by press-molding a powder 
mixture of indium oxide and tin oxide, with or without the addition of a 
dopant, at ordinary temperature and by then sintering the resulting green 
compact in an ambient air atmosphere at 1250.degree. to 1650.degree. C. 
Unfortunately, indium oxide powder and tin oxide powder are difficult to 
sinter. The ITO targets made by the above process (cold-pressing and 
atmospheric sintering), therefore, have densities of only 4.2 to 5.0 
g/cm.sup.3, or at most about 60 to 70% of the theoretical density value. 
Another factor placing a limit upon the density and hampering its rise to 
higher levels is the consideration of density reproducibility. It is 
generally accepted knowledge in the art is that if large targets are to be 
manufactured steadily on an industrial scale, it is necessary to keep 
their density from reaching above approximately 5.0 g/cm.sup.3. 
Disadvantages of these low density ITO targets so far pointed out have 
included: 
a) Low sputtering rate (film forming rate) 
b) Short target life 
c) Pronounced surface blackening tendency of ITO, and 
d) Increased dust (particle) deposition on the substrate. 
In view of these disadvantage, and for the enhancement of sputtering 
operation efficiency, and for the qualitative improvement of the resulting 
ITO film, there has been strong demand for ITO targets of higher 
densities. 
An attempt has been made to apply hot pressing in the step of compacting 
the starting powder material in a series of, for the purpose of improving 
the density of ITO target. The compaction step is one in a series of the 
target-making steps. It was found possible in that way to increase the ITO 
target density with relative ease to about 6.7 g/cm.sup.3 (about 94% of 
the theoretical). Thus, hot pressing has come into use for the manufacture 
of high density (6.0 to 6.7 g/cm.sup.3) products. 
However, the use of hot pressing in the manufacture of ITO targets has 
caused new problems, such as 
(a) very high initial investment and difficulty of building large-size 
equipment, 
(b) high running cost due to the necessity of expensive mold and associated 
parts, and 
(c) lowered mass-production ability resulting from operation rates of at 
most one batch or two per day. 
Because of the adverse eventual effects of these difficulties upon the 
target price, the hot press technique can scarcely be regarded as a fully 
satisfactory means for commercial production. 
OBJECT OF THE INVENTION 
In view of the above, the present invention is aimed at establishing means 
capable of producing high-density ITO targets stably with high 
productivity, without the need for any extraordinarily expensive new 
equipment. 
SUMMARY OF THE INVENTION 
After extensive studies with diverse experiments performed from various 
angles, it has now been found that in the manufacture of a sintered ITO 
target, sintering a compression-molded oxide powder mixture in a specific 
atmosphere where partial pressure of oxygen is high, will give a sintered 
ITO product with a very high density and fully satisfactory performance as 
a sputtering target. This is so even when cold pressing has been used for 
the compression molding of the mixture, rather than hot pressing. 
The present invention, based on the above discovery, provides a method of 
the manufacture of an ITO target by press-molding a powder mixture 
consisting chiefly of indium oxide and tin oxide followed by sintering the 
resulting compact, characterized in that the sintering is conducted in a 
pressurized oxygen atmosphere of at least one atm., whereby an ITO target 
having as high a density as close to the theoretical can be produced with 
good productivity and at low cost. 
As used herein the expression "a pressurized oxygen atmosphere of at least 
one atm." means not merely an atmosphere of pure oxygen under a pressure 
of no less than one atm. (gauge pressure), but also a mixed gas atmosphere 
in which the partial pressure of O.sub.2 is no less than one atm..

DETAILED DESCRIPTION 
The present invention, together with its functions and effects, will be 
better understood from the following detailed description. 
The reasons why high-density products with good performance can be obtained 
by the use of a pressurized oxygen atmosphere in the sintering step during 
the manufacture of an ITO target are yet to be fully clarified. In this 
connection, it has been confirmed that sintering in an atmosphere of an 
inert gas such as N.sub.2 or Ar can cause decomposition of ITO, resulting 
in a sintered body of reduced density. This fact suggests that ITO by 
nature will easily release oxygen upon heating to elevated temperature. 
Hence it is presumed that sintering in oxygen under pressure effectively 
prevents the dissociation of ITO upon high-temperature heating, while 
oxygen acts like a sintering assistant, and thereby contributes to the 
improvement in density. 
If the partial pressure of O.sub.2 in the sintering atmosphere is less than 
one atm. (gauge), the density-increasing effect on the sintered body is so 
small that no ITO target with quite satisfactory performance will result. 
It is for this reason that the sintering atmosphere in the present 
invention is restricted to "a pressurized oxygen atmosphere of at least 
one atm." To secure a more favorable effect, it is preferable to set the 
partial pressure of O.sub.2 in the sintering atmosphere to 3 atm. or 
upwards. While no upper limit is specified for the pressure, it would be 
advisable to fix the upper limit on the order of 10 atm., in consideration 
of the pressure resistance of ordinary equipment. 
It is desired that the sintering atmosphere is, where available, an 
atmosphere of pure oxygen under pressure, the pressure being between 1 and 
10 atm., preferably between 3 and 10 atm.. As noted already, a mixed gas 
atmosphere of oxygen gas and one or more inert gasses which does not react 
with oxygen may be used as well, provided the partial pressure of O.sub.2 
in the mixed gas is one atm. or more. 
As with ordinary sintering, the sintering temperature in the pressurized 
oxygen atmosphere is appropriately between 1600.degree. and 1700.degree. 
C. Also, the sintering time is desired to be 3 hours or more (the longer 
the sintering time the better). 
As described above, the method of the invention for manufacturing ITO 
targets achieves advantageous effects over the conventional 
cold-pressing-atmospheric-sintering method in that it yields products of 
higher densities (97 to 99% of the theoretical density) with good density 
reproducibility Compared with the ordinary hot-pressing method, the 
present method permits enjoyment of the marked advantages of low 
investment in equipment and low running cost, yet has a far greater 
capacity for mass production. These advantages are particularly pronounced 
when ITO targets with a SnO.sub.2 content of 5 to 10% by weight are made. 
The advantageous effects of the invention will now be more concretely 
described with the reference to the accompanying drawings illustrating 
examples thereof and comparative examples. 
EXAMPLE 1 
In this example, an ITO target was made in the sequence of steps 
represented by the flow sheet in FIG. 1. 
First, indium oxide powder with a means particle diameter of 2 .mu.m and 
tin oxide powder of the same particle size were mixed uniformly in a 
weight ratio of 95:5. After the addition of a molding binder, the mixture 
was evenly charged into a mold of 165 mm width by 520 mm length, and 
compacted by a hydraulic press at a pressure of 750 kg/cm.sup.2. The 
density of the green compact thus obtained was 4.06 g/cm.sup.3. 
The compact then was dried by heating to 80.degree. C. and evaporating 
water from the binder. 
Next, using a pressure sintering furnace, the dry compact was sintered in a 
pure oxygen atmosphere at 3.5 atm. (gauge, the absolute pressure being 4.5 
atm.) at 1650.degree. C. for 9 hours. The surface of the sintered body was 
ground by a surface grinder, and cut on sides by a diamond cutter into a 
target product. 
The density of the ITO target product so obtained was calculated from its 
volume and weight, and an exceptionally high value of 6.95 g/cm.sup.3 was 
confirmed. 
For comparison purposes, another ITO target was made on trial, this time 
under the following conditions. 
A dry compact was obtained under exactly the same conditions a in the above 
example of the invention down to the drying step. In the following step of 
sintering, an "air atmosphere" as in the prior art was used instead. The 
compact was sintered in an air atmosphere at 1650.degree. C. for 9 hours, 
and the sintered body was machined into an ITO target. Density measurement 
of the product obtained in the comparative example gave a value of as 
small as 5.56 g/cm.sup.3. 
As is obvious from the results of the example, sintering in a pressurized 
oxygen atmosphere as specified by the invention is highly effective in 
improving the density of a sintered product for use as an ITO target. 
EXAMPLE 2 
In this example an ITO target was made following the sequence of steps in 
the flow sheet of FIG. 2. 
First, indium oxide powder having a mean particle diameter of 0.4 .mu.m and 
tin oxide powder of the same particle size were thoroughly mixed in a 
weight ratio of 90:10. With the addition of a molding binder, the mixture 
was filled evenly into a mold 165 mm wide by 520 mm long and molded at a 
pressure of 1000 kg/cm.sup.2 by a hydraulic press. The density of the 
green compact was 3.98 g/cm.sup.3. 
The compact was heated to 80.degree. C. and dried through evaporation of 
water from the binder. 
Next, using a pressure sintering furnace, the dried compact was sintered in 
a pure oxygen gas atmosphere under a pressure of 8.9 atm. (gauge, the 
absolute pressure being 9.9 atm.) at 1650.degree. C. for 9 hours. The 
surface of the sintered body was ground by a surface grinder and cut on 
sides by a diamond cutter into a target product. 
The density of the ITO target product thus obtained was calculated from its 
volume and weight and found to be as high as 7.06 g/cm.sup.3. 
Comparatively, another ITO target was made using the following conditions. 
A dry compact was obtained in exactly the same manner as in the preceding 
example of the invention down to the drying step. In the ensuing step of 
sintering, ordinary "air atmosphere" was employed instead of pure oxygen. 
The compact was sintered in that atmosphere at 1650.degree. C. for 9 
hours, and the sintered body machined into an ITO target. Density 
measurement of the comparative product gave a value of as low as 6.20 
g/cm.sup.3. 
The results of this example demonstrate that sintering in a pressurized 
oxygen atmosphere as specified by the invention is highly effective in 
improving the sintered density of ITO target. 
EXAMPLE 3 
In this example, an ITO target was made following the sequence of steps in 
the flow sheet of FIG. 3. 
First, indium oxide powder having a mean particle diameter of 2 .mu.m and 
tin oxide powder of the same particle size were thoroughly mixed in a 
weight ratio of 95:5. With the addition of a molding binder, the mixture 
was filled evenly into a mold 165 mm wide by 520 mm long and molded at a 
pressure of 750 kg/cm.sup.2 by a hydraulic press. The density of the green 
compact was 4.09 g/cm.sup.3. 
The compact was heated at 80.degree. C. to evaporate water from the binder 
until it was dry. 
Next, by means of a pressure sintering furnace, the dried compact was 
sintered in a pure oxygen gas atmosphere under a pressure of 1.0 atm. 
(gauge, the absolute pressure being 2.0 atm.) at 1650.degree. C. for 9 
hours. The surface of the sintered body was ground by a surface grinder 
and cut on sides by a diamond cutter into a target product. 
The density of the ITO target product thus obtained was calculated from its 
volume and weight and found to be as high as 6.75 g/cm.sup.3. 
For comparison, another ITO target was made under the following conditions. 
A dry compact was obtained by exactly the same procedure as in the 
preceding example of the invention down to the drying step. In the ensuing 
step of sintering, an "air atmosphere" was employed instead. The compact 
was sintered in that atmosphere at 1650.degree. C. for 9 hours, and the 
sintered body was machined to obtain an ITO target. Density measurement of 
the comparative product gave a value of 5.60 g/cm.sup.3. 
The results of this example prove that sintering in a pressurized oxygen 
atmosphere as taught by the invention is effective to improve the sintered 
density of ITO target. 
As described above, the present invention offers very advantageous effects 
from the industrial viewpoint in that it permits mass production of 
high-performance, high-density ITO targets with good productivity and at 
low cost.