Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2000-08-31
2003-07-22
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S298130, C428S702000
Reexamination Certificate
active
06596135
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a sputtering target, a transparent conductive film and a method for producing the same.
BACKGROUND ART
It is considered preferable to interpose zinc oxide type transparent conductive film between an amorphous Si layer and an Ag film as a back surface electrode of an amorphous solar cell, from the viewpoint of durability and photoelectric conversion efficiency.
However, the light absorptivity tends to be high (i.e. the light transmittance tends to decrease), when it is attempted to obtain an oxide type transparent conductive film having a low resistivity. Of a conventional zinc oxide type transparent conductive film, although the resistivity is low, the transmittance of light (particularly the transmittance of light having a wavelength of from 400 to 1000) is not high.
On the other hand, of an oxide type transparent conductive film having a low light absorptivity (i.e. a high light transmittance), the resistivity tends to be high. Accordingly, it has been difficult to obtain an oxide type transparent conductive film having a resistivity of from 10
−2
to 10
10
&OHgr;·cm and having a low light absorptivity.
In a case of forming an oxide type transparent conductive film having a resistivity of from 10
−2
to 10
10
&OHgr;·cm by a sputtering method, it has been conventionally known to carry out sputtering by using a target having such an electrical conductivity that the resistivity of the target itself is from 10
−1
to 10
10
&OHgr;·cm, at a low oxygen concentration such that the oxygen in the atmosphere is at most 3 vol %.
However, this method is limited to a high-frequency sputtering material, as the electrical conductivity of the target is poor. Accordingly, a direct current (DC) sputtering method can not be used which is excellent in industrial productivity such that the structure of the equipment is simple, whereby the operation efficiency is high, and the rate of film formation is high.
It is preferred to use a target of low resistance having a resistivity of at most 10
−1
&OHgr;·cm when the DC sputtering method is employed, since stable discharge can be obtained.
However, in the case of forming a film by the DC sputtering method by using a ZnO type target of low resistance having a resistivity of from 10
−1
to 10
−1
&OHgr;·cm, in an atmosphere of a low oxygen concentration (e.g. an argon gas alone), only a transparent conductive film of low resistance having a resistivity of a level of 10
−4
&OHgr;·cm will be obtained (JP-A-2-149459).
In order to obtain a film having a resistivity of from 10
−2
to 10
10
&OHgr;·cm by using a target having a low resistivity of at most 10
−1
&OHgr;·cm, it is required to introduce an oxidative gas (such as oxygen gas) into the atmosphere so as to carry out the sputtering at a high oxygen concentration.
However, the oxidative gas in the atmosphere may affect also the transparent conductive film to be formed, and the transparent conductive film may be damaged, such being problematic.
Accordingly, a method has been desired to produce a transparent conductive film having a resistivity of from 10
−2
to 10
10
&OHgr;·cm, by using the DC sputtering method which is excellent in productivity, in an atmosphere of a low oxygen concentration in which the transparent conductive film is less likely to be damaged.
It is an object of the present invention to provide a sputtering target which can be used for the DC sputtering method, and with which a transparent conductive film having a resistivity of from 10
−2
to 10
10
&OHgr;·cm can be stably produced.
Further, it is an object of the present invention to provide a zinc oxide type transparent conductive film having a low light absorptivity, and its production method.
Still further, it is an object of the present invention to provide a zinc oxide type transparent conductive film having a resistivity of from 10
−2
to 10
10
&OHgr;·cm, and its production method.
DISCLOSURE OF THE INVENTION
The present invention provides a sputtering target comprising an oxide containing Zn, Al and Y (hereinafter referred to as ZAY target).
By using the ZAY target, a transparent conductive film having a low light absorptivity (having substantially no coloring) will be obtained without introducing a reactive gas such as oxygen during sputtering (e.g. by an argon gas alone).
The resistivity of the ZAY target is preferably at most 10
−1
&OHgr;·cm. If it is higher than 10
−1
&OHgr;·cm, the discharge during the DC sputtering tends to be unstable.
By using the ZAY target, an oxide type transparent conductive film having a resistivity of from 10
−2
to 10
10
&OHgr;·cm will be obtained stably.
The ZAY target contains preferably from 0.2 to 15.0 atomic % of Al and from 0.2 to 75.0 atomic % of Y, based on the total amount of Zn, Al and Y, since a transparent conductive film having a resistivity of from 10
−2
to 10
10
&OHgr;·cm will be obtained more stably. Particularly, in order to obtain a transparent conductive film having a resistivity of from 10
−2
to 10
4
&OHgr;·cm, the ZAY target contains preferably from 0.2 to 8.0 atomic % of Al and from 0.2 to 20.0 atomic % of Y, more preferably from 0.2 to 8.0 atomic % of Al and from 0.2 to 10.0 atomic % of Y, based on the total amount of Zn, Al and Y.
In a case of an oxide target containing from 0.2 to 15.0 atomic % of Al based on the total amount of Zn, Al and Y, and containing no Y, when the DC sputtering is carried out in an atmosphere at a low oxygen concentration (e.g. an argon gas alone), of the obtained transparent conductive film, the resistivity will be less than 10
−2
&OHgr;·cm, however, the light absorptivity tends to be high.
Further, in a case of an oxide target containing from 0.2 to 75.0 atomic % of Y based on the total amount of Zn, Al and Y, and containing no Al, the target will have a high resistance (exceeding 10
−1
&OHgr;·cm), whereby the DC sputtering tends to be difficult. Further, in a case of using, instead of Al, B of the same Group 3 of Periodic Table, the target tends to be poor in moisture resistance.
By properly adjusting the amount of Al and Y, a transparent conductive film having a desired resistivity of from 10
−2
to 10
10
&OHgr;·cm will be obtained.
Further, the atomic ratio of Y/Al in the ZAY target is preferably from 0.1 to 25. If the atomic ratio of Y/Al is less than 0.1, the light absorptivity of the obtained transparent conductive film tends to be high. On the other hand, if the atomic ratio of Y/Al exceeds 25, the resistivity of the obtained transparent conductive film tends to exceed 10
10
&OHgr;·cm.
The method for producing the ZAY target is not particularly limited, and a common method for sintering ceramics such as atmospheric pressure sintering method or a hot pressing method may be employed.
The atmospheric pressure sintering method is preferred since a sintered body (target) can be produced at a low cost without using a huge equipment such as a vacuum equipment.
The ZAY target can be obtained, for example, by mixing each powder of alumina, yttria and zinc oxide as materials to obtain a powder mixture, filling the powder mixture in a mold for pressing, followed by press molding by a press machine or a rubber press machine (or molding by plaster casting), and then sintering the obtained molded product at from 1400 to 1550° C. in the air.
If it is less than 1400° C., the sintering degree will be poor, whereby no dense target will be obtained. Further, if it is higher than 1550° C., the evaporation from the sintered body will be active, and pores in the sintered body will be enlarged by the vaporized gas, whereby the density tends to decrease. The holding time during sintering is preferably from 1 to 10 hours. If it is shorter than 1 hour, the sintering tends to be inadequate, whereby no dense product tends to be obtained. Further, if it is longer than 10 hours, the amount of evaporation tends to be large, whereby the density tends to decrease.
The density of the Z
Asahi Glass Company Limited
McDonald Rodney G.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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