Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
2002-03-29
2003-12-30
Ver Steeg, Steven H. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C432S013000, C438S618000, C438S622000, C438S624000
Reexamination Certificate
active
06669830
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a sputtering target (which may be referred to merely as a target hereinafter), a transparent conductive oxide made of the sputtering target, and a process for producing the sputtering target.
The present invention relates particularly to a target making it possible to suppress nodules generated when sputtering is used to form a transparent conductive oxide into a film, and to make the sputtering stable, a transparent conductive oxide made of such a target, and a process for producing such a target.
BACKGROUND ART
In recent years, display devices have been remarkably developed. Liquid crystal display devices (LCD), electroluminescence display devices (EL), field emission displays (FED) or the like are used as display devices for office machines such as personal computers and word processors, or for control systems in factories. These display devices have a sandwich structure wherein a display element is sandwiched between transparent conductive oxides.
As such transparent conductive oxides, indium tin oxide (which may be referred to ITO hereinafter) formed into a film by sputtering, ion plating or vapor deposition accounts for the main current, as disclosed in document 1: “Technology on Transparent Conductive Film” (published by Ohmsha Ltd., 166th committee on transparent oxides and photoelectric materials in Japan Society for the Promotion of Science, 1999).
Such ITO is made of indium oxide and tin oxide of given amounts and is characterized by having superior transparency and conductivity, being capable of being etched with a strong acid, and having good adhesion to a substrate.
In the meantime, as disclosed in JP-A-3-50148, JP-A-5-155651, JP-A-5-70943, JP-A-6-234565 and so on, there is known a target made of indium oxide, tin oxide and zinc oxide of given amounts, or a transparent electrode made from such a target into a film (which may be referred to as IZO hereinafter). This can be etched with a weak acid, and is widely used because it has good sinterability and transparency.
As described above, ITO or IZO has superior performances as the material of a transparent conductive oxide, but has a problem that when a target is used to form ITO or IZO into a film, nodules (swellings) as shown in
FIG. 2
(photograph) are easily generated in the surface of the target.
Particularly when an amorphous ITO film is formed to improve etching property, there arises a problem that the surface of a target is reduced because of a very small amount of water or hydrogen gas introduced into a sputtering chamber for the deposition, so that nodules are more easily generated.
When such nodules are generated in the surface of the target, the nodules are easily scattered by power of plasma during the sputtering. As a result, there arises a problem that the scattered substances adhere, as alien substances, to the transparent conductive oxide during or immediately after the formation of the film.
The nodules generated in the surface of this target account for one of causes of abnormal electric discharge.
Thus, as disclosed in JP-A-8-283934, as measures for suppressing the generation of nodules in a target, efforts are made to reduce pores by making the material of the target denser through sintering at high temperature. That is, a target having a 99%-density relative to the theoretical density thereof is produced, but even in this case nodules cannot be completely removed.
In such a situation, it is desired to develop a target making it possible to suppress the generation of nodules when a film is formed by sputtering and make the sputtering stable.
Thus, the present inventors made eager investigations repeatedly to solve the above-mentioned problems. As a result, the inventors have found out that nodules generated in the surface of a target are basically the remainders of excavation and the cause for generating this remainder of excavation depends on the value (for example, 10 &mgr;m or more) of the crystal grain size of metal oxides constituting the target.
Namely, in the case that a target is struck out from the surface thereof by sputtering, the speeds of the striking-out are different dependently on the directions of crystal planes so that unevenness is generated in the surface of the target. It has been proved that the size of the unevenness depends on the crystal grain size of metal oxides present in the sintered body.
Therefore, it can be considered that when a target made of a sintered body having a large crystal grain size is used, unevenness generated in the surface of the target gradually gets larger so that nodules are generated from the convex portions of the unevenness.
That is, an object of the present invention is to provide a target making it possible to suppress nodules generated when a transparent conductive oxide is formed into a film by sputtering, and to make the sputtering stable, a transparent conductive oxide made of such a target, and a process for producing such a target.
DISCLOSURE OF THE INVENTION
[1] According to the present invention, provided is a sputtering target comprising at least indium oxide and zinc oxide, wherein the atomic ratio represented by In/(In+Zn) is set to a value within the range of 0.75 to 0.97, a hexagonal layered compound represented by In
2
O
3
(ZnO)
m
wherein m is an integer of 2 to 20 is contained, and the crystal grain size of the hexagonal layered compound is set to a value of 5 &mgr;m or less.
That is, by limiting the value of the crystal grain size into the given range, the size of unevenness generated in the surface of the target is controlled so that the generation of nodules can be effectively suppressed.
[2] When the sputtering target of the present invention is constructed, it is preferred that 67 to 93% by weight of indium oxide, 5 to 25% by weight of tin oxide and 2 to 8% by weight of zinc oxide are contained, and the atomic ratio of tin to zinc is set to a value of 1 or more.
Such a construction makes the target dense and makes it possible to suppress the generation of nodules more effectively.
By setting the atomic ratio of tin to zinc to a value of 1 or more in this way, the electric resistance after crystallization is effectively lowered so that a transparent conductive oxide having superior conductivity can be obtained.
[3] When the sputtering target of the present invention is constructed, it is preferred that a spinel structural compound represented by Zn
2
SnO
4
is contained instead of the hexagonal layered compound or together with the hexagonal layered compound, and the crystal grain size of the spinel structural compound is set to a value of 5 &mgr;m or less.
Such a construction makes the target dense so that the generation of nodules can be more effectively suppressed.
Such a construction also makes it possible to yield a transparent conductive oxide having superior transparency and conductivity by sputtering.
[4] When the sputtering target of the present invention is constructed, it is preferred that the bulk resistance thereof is set to a value less than 1×10
−3
&OHgr;·cm.
Such a construction makes it possible to reduce abnormal electric discharge (spark) during the sputtering and yield a sputtering film stably. Conversely, if the bulk resistance is a value of 1×10
−3
&OHgr;·cm or more, charges are accumulated in the surface of the target so that abnormal electric discharge is easily generated.
[5] When the sputtering target of the present invention is constructed, it is preferred that the density thereof is set to a value of 6.7 g/cm
3
or more.
Such a construction makes it possible to give superior mechanical property, and makes the target dense, whereby the generation of nodules can be more effectively suppressed.
[6] Another embodiment of the present invention is a transparent conductive oxide (amorphous transparent conductive oxide) comprising a sputtering target wherein the atomic ratio represented by In/ (In+Zn) is set to a value within the range of 0.75 to 0.97, a he
Inoue Kazuyoshi
Kaijo Akira
Shibuya Tadao
Idemitsu Kosan Co. Ltd.
Ver Steeg Steven H.
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