"In manufacturing ITO film, in addition to the sputtering method that is generally used, ion plating methods such as electron beam vapor deposition method and high-density plasma assist vapor deposition method, and other vacuum depositions method are often used, and as the vapor source in the vacuum deposition method, an ITO tablet (also called an ITO pellet) that is composed of an ITO sintered compact is used.
"However, ITO film, even though it is easy to obtain the desired product quality during manufacturing, the main raw material indium is an expensive rare earth metal, so there are limits to how much the cost can be reduced.
"On the other hand, in the case of a transparent zinc oxide conductive film that is composed of zinc oxide, or zinc oxide that includes aluminum or gallium as a dopant, the main raw material zinc is very inexpensive, light transmittance is high, and the plasma-resistant characteristics are good, so is widely used as electrodes in thin-film silicon solar batteries. Moreover, the band gap of zinc oxide is wide at approximately 3.4 eV, and the exciton energy is high, so in recent years, there have been many reports of applications in light-emitting diodes. Furthermore, application in transparent thin-film transistors is also expected. In a transparent zinc oxide conductive film, the reason for including boron group elements such aluminum and gallium, which have a larger valence number than zinc, is that it is possible to reduce the resistivity (specific electrical resistance).
"In the manufacturing of transparent zinc oxide conductive film as well, mainly the sputtering method is used. In the sputtering method, a zinc oxide sintered compact target is used as the raw material, however, and up until now zinc oxide sintered compact targets having various crystalline orientation having been proposed in order to obtain a film with excellent homogeneity. For example, in JP 06-88218 (A), a zinc oxide sintered compact target in which the crystallinity of surface (002) is greater than the crystallinity of surface (101) is proposed; in JP 06-340468 (A), a zinc oxide sintered compact target in which the crystallinity of surface (101) is large is proposed; and in JP 2002-121067, a zinc oxide sintered compact target in which the crystallinity of surface (110) is large is proposed.
"For this transparent zinc oxide conductive film as well, as in the case of ITO film, manufacturing the film using the vacuum deposition method is being studied, and various types of zinc oxide sintered compact tablets have been proposed such as disclosed in JP 06-248427 (A), JP 2006-117462 (A), JP 2007-56351 (A) and JP 2007-56352 (A).
"The zinc oxide sintered compact tablet used in this vacuum sputtering method, from the aspect of preventing fracturing or cracking during film formation, is a tablet that has a relative density (ratio of the bulk density with respect to the theoretical density) of about 50% to 70%. However, the zinc oxide sintered compact is a material having higher resistance than an ITO sintered compact, so when compared with a sintered compact target having a high relative density of 90% or more such as used in the sputtering method, the resistivity value increases by the amount that the relative density is low. When the resistivity value of the sintered compact target becomes high, uniform sublimation by the plasma beam or electron beam becomes difficult, and there is a possibility that a splashing phenomenon will occur in which vapor deposition material having a size of several .mu.m to 1000 .mu.m that is mixed with evaporative gas is scattered and collides with the vapor deposition film. This splashing phenomenon is the cause of film defects such as pinhole defects and the like. Therefore, achieving a zinc oxide sintered compact tablet for which the occurrence of this kind of splashing phenomenon is suppressed is desired.
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