Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
2002-02-27
2004-04-20
VerSteeg, Steven H. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C204S192320, C427S531000, C427S295000, C427S248100, C427S377000, C427S383100, C427S350000, C228S193000, C228S194000, C228S195000
Reexamination Certificate
active
06723213
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a titanium target assembly for sputtering, which has a solid phase diffusion joining (bonding) plane, and a method for preparing the same and, in particular, to a titanium target assembly for sputtering obtained by integrating a titanium (Ti) target, which constitutes a cathode used in a thin film-forming sputtering equipment, and a backing plate serving as a support for the target through a coating metal film according to the solid phase diffusion joining (bonding) technique as well as a method for preparing the same.
The sputtering target may serve as a sputtering source for forming a thin film on a substrate in the production of a variety of thin film devices including a semiconductor device. In the case of semiconductor devices, most of the targets are plates each having a disk-like shape. The sputtering target currently used is in the form of a target assembly in which a target is integrated with a support member called a backing plate used for supporting and cooling the target. When a target assembly is fitted to a sputtering cathode and then an electric power is applied to the cathode, the temperature of the target surface is raised due to the application of heat, but the heat is removed from the target by passing cooling water through the back face of the backing plate to thus control any increase of the temperature of the target surface. Materials for the backing plate commonly used are, for instance, metals and alloys thereof such as oxygen free copper, copper alloys, aluminum, aluminum alloys, titanium and titanium alloys.
Conventionally, the target and the backing plate have principally been joined through the use of a low melting brazing filler metal such as an In or Sn alloy. When a high electric power is applied for the purpose of the improvement of the throughput, however, the temperature at the interface to be joined inevitably increases despite of the cooling of the backing plate side and various problems or troubles arise such that the bonding strength is reduced due to the increase in the temperature and that the target material is in turn peeled off due to the melting of the brazing filler metal. For this reason, the joining method has been switched over to the integrated joining method, in which any brazing filler metal is not used, depending on the sputtering target. When a higher electric power is applied to the cathode for the further improvement of the throughput, however, the target undergoes changes in the characteristic properties due to the heat. Moreover, the size of the target assembly has been increased as the size of a wafer increases. This leads to the occurrence of considerable warpage of the target assembly as a joined body of different materials when such a high electric power is applied because of the difference in the coefficient of thermal expansion between these materials for the target assembly and this in turn becomes a cause of inconvenience such as the unevenness of the film thickness and changes in the film quality. Therefore, there has been desired for the appropriate selection of materials for the backing plate, while taking into consideration the control of the cooling efficiency and the warpage.
On the other hand, it has been desired for the target/backing plate assembly to satisfy such a requirement that “the crystalline structure and crystalline orientation of the target material should be maintained and the strength of the backing plate material should be ensured” and that “any deformation of the target material and the backing plate material should be controlled by reducing a joining load” upon the junction of these materials for the assembly, in addition to the foregoing control of the cooling efficiency and the warpage.
In respect of the joining method, in which any brazing filler metal is not used and which is used for producing the target/backing plate assembly, there have been filed a large number of patent applications. For instance, Japanese Un-Examined Patent Publication (hereunder referred to as “J. P. KOKAI”) No. Hei 1-283367 (Japanese Patent No. 2,831,356) discloses that the target and the backing plate are directly welded under pressure without the interposition of any bonding material and lists up an explosive pressure welding technique and a seam welding technique as embodiments thereof. This patent also discloses the results obtained by the hot-rolling and diffusion joining or bonding techniques and states that good bonding strength could be achieved by these techniques.
As another direct joining method, J. P. KOKAI No. Hei 6-65733 discloses a diffusion bonding method comprising the steps of pre-heating a support member and a target member, laying one on top of the other and applying a load to these members by pressing.
As still another direct joining method, J. P. KOKAI No. Hei 9-143704 discloses a diffusion bonding method comprising the steps of bringing a titanium target material into contact with an aluminum backing plate material and then subjecting these materials to hydraulic pressing at temperatures ranging from 300 to 450° C. and pressures ranging from 50 to 200 MPa. This publication also discloses, in Examples, that a bonding strength on the order of 91 MPa (9.3 kg/mm
2
) could be obtained under the conditions of 450° C. and 120 MPa and that a bonding strength on the order of 35 MPa (3.6 kg/mm
2
) could be obtained under the conditions of 450° C. and 45 MPa.
As has been discussed above, there have been filed a large number of patent applications concerning the direct bonding method, but the majority thereof is limited to the combination of Ti/Al and the bonding strength of the assemblies achieved by these methods widely vary.
The inventors of the present invention applied the diffusion bonding method disclosed in J. P. KOKAI No. Hei 1-283367 (Japanese Patent No. 2,831,356), which is not seriously limited in the material for the target assembly among other techniques, to the combination of Ti (target)/Cu (backing plate) and carried out bonding tests. As a result, it was found that the bonding method provided results lacking in bonding stability and reliability and showing high scattering and more specifically, the bonding strength was found to fall within the range of from 6 to 11 kg/mm
2
under the conditions of a bonding temperature of 500 or 450° C. and a pressure of 40 MPa.
The inventors of this invention also attempted to directly bond the combination of Ti/Cu under the conditions of 400° C. and 100 MPa, which were selected on the basis of those disclosed in the other conventional techniques relating to the Ti/Al bonding. In any case, however, the bonding strength attained are low on the order of 1.5 kg/mm
2
and accordingly, this method never ensures the desired or required bonding stability and reliability at all.
On the other hand, as bonding methods, which make use of an insert material for the purpose of the reduction of the load to be applied or the reduction of the bonding temperature, there have been disclosed methods for obtaining “target assemblies each comprising a target material, an insert material and a backing plate material having a solid phase diffusion bonding interface between each neighboring two materials”, in J. P. KOKAI Nos. Hei 6-108246 and Hei 6-172993. More specifically, these publications disclose that a desired assembly can be prepared by solid phase diffusion-bonding the target and backing plate materials through an insert material consisting of Ag, Cu and/or Ni at a temperature falling within the range of from 200 to 600° C. and an applied load falling within the range of from 0.1 to 20 kg/mm
2
. In Examples of these publications, a bonding strength (or shearing strength) of 6 kg/mm
2
can be accomplished by bonding a Ti target material and an oxygen free copper backing plate material (J. P. KOKAI No. Hei 6-108246) or an Al alloy target material and an oxygen free copper backing plate material (J. P. KOKAI No. Hei 6-172993), using an Ag foil having a thickness of 100 &mgr;m as an insert material at a tem
Chai Weiping
Kim Poong
Kodera Masahiro
Nakadai Yasuo
Arent & Fox PLLC
Vacuum Metallurgical Co., Ltd.
VerSteeg Steven H.
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