Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
1997-05-27
2001-01-23
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C419S049000, C419S019000
Reexamination Certificate
active
06176986
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dielectric sputtering target having high strength for forming capacitor films of highly integrated semiconductor memories of the next generation and to thin films of functional dielectric, and to a method for producing the same.
2. Description of the Background
It is known, as disclosed in Japanese Unexamined Patent Publication 6-330297, that sputtered films can be formed at considerably high speeds by stable direct current sputtering. This technique is made possible because the electric resistivity of sintered body targets can be reduced to 10 &OHgr;-m or less when powders of (Ba, Sr) Ti oxides are sintered in a vacuum or in an inert atmosphere to form a target of oxygen-deficient oxides.
Sputtering methods which operate at higher electric power have been required in recent years in order to form sputtered films at higher speeds. However, there are some problems which are known concerning these higher power methods, which include the fact that the dielectric targets are brittle and easily crack during the sputtering process under high electric power, thereby interfering with stable film formation at high speeds. A need, therefore, continues to exist for improvements in the dielectric targets employed in rapid sputtering processes at high speed.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a sputtering target of a dielectric for high speed sputtering which has a high thermal shock resistance and excellent properties in comparison to others, which dielectrics do not form cracks during sputtering under high electric power.
Briefly, this object and other objects of the invention as hereinafter will become more readily apparent can be attained by a dielectric sputtering target of a sintered barium, strontium titanate body, which is oxygen deficient, of the formula: Ba
1-x
Sr
x
TiO
3-y
, (wherein 0<x<1 and 0<y≦0.03), said sintered body having a mean grain size of 0.3 to 5 &mgr;m, a maximum grain size of 20 &mgr;m or less, a relative density of 95% to 99%, a purity of 4N or more, a K content of 1 ppm or less, a Na content of 2 ppm or less, an Al content of 5 ppm or less, a Si content of 20 ppm or less and a Fe content of 2 ppm or less, and a flexural strength of 150 MPa or more.
In another embodiment of the invention, the present dielectric having high strength is prepared by hot pressing a powder material having a mean primary particle size of 1 &mgr;m or less at a sintering temperature of 1100° C. to 1300° C. for a sintering time of 1 hr to 10 hrs at maximum temperature under a pressure of 10 MPa to 50 MPa in a vacuum or under an inert gas atmosphere.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The sputtering target of the invention prepared from the present sintered body has a high density, as well as a fine micro-structure. It has an improved three point flexural strength of 150 MPa or more which is 2 to 5 times as large as that of the (Ba,Sr)TiO
3
sintered body. Improvements in the strength of dielectric sputtering targets achieved in the present invention comply with the requirements for sputtering under high DC power. A part of the invention is the discovery that the trace impurities content of such as K, Na and the like, have great influence on the flexural strength of the sintered body.
In the method of the invention highly purified powders of BaO, SrO and TiO
2
are first prepared by recrystallization or distillation. These powders are mixed together in a prescribed ratio and the mixture is heat-treated at 1100° C. in the air followed by crushing and grinding, thereby preparing powdered (Ba,Sr) TiO
3
having a perovskite crystal structure with a purity of 4N or more. The mean primary particle size of this powder is 0.05 to 1 &mgr;m.
A hot-press graphite mold is filled with the above powder and the powder is subjected to hot-pressing in a vacuum or in an atmosphere of an inert gas at a sintering temperature of 1100° C. to 1300° C. for a sintering time of 1 hr to 10 hrs at maximum temperature under a pressure of 10 MPa to 50 MPa. A sintered body, having the formula: Ba
1-x
Sr
x
TiO
3-y
(0<x<1.0, 0<y≦0.03), which is oxygen deficient, having a mean grain size of 0.3 to 5 &mgr;m, a maximum grain size of 20 &mgr;m or less and a relative density of 95% to 99% is thus prepared.
Flexural strength is measured using a test piece that is cut from the sintered body prepared above. The mean flexural strength of the test piece is 150 MPa or more, and is 2 to 5 times as high as that of conventional sintered bodies.
The sintered body obtained is then formed into a plate having a dimension of 125 mm (diameter)×5 mm (thickness), which is bonded to a backing plate made of copper using In—Sn solder. The sputtering target thus prepared is subjected to a break resistance test under direct current sputtering power and to a film forming speed test. The results show that sputtering is possible without breaking the test target up to a sputtering power of DC 600 (W) and at a film-forming speed of 250 Å/min or more. This value is 2 to 3 times as high as the values of conventional targets. These results indicate that the sintered body of the present invention is useful as a high strength, dielectric sputtering target, which has an excellent performance even under high sputtering power conditions.
The powdered material for producing the present sintered body is highly purified by repeated recrystallization and distillation, thereby adjusting the amounts of the trace impurities in the powder material such that the content of K is 1 ppm or less, Na is 2 ppm or less, Al is 5 ppm or less, Si is 20 ppm or less and Fe is 2 ppm or less. The sputtering target thus obtained is able to be subjected to sputtering under a higher sputtering power without causing any breakage besides having a higher maximum film-forming speed. These observations indicate that purification of the powder material improves the characteristic of the high strength dielectric sputtering target.
The reason why the values are limited to the aforementioned ranges are:
(a) Mean grain size of the sintered body:
If the value exceeds 5 &mgr;gm, the mean break-resistance strength decreases which results in breakage of the sputtering target material during sputtering even at low sputtering power. If the value is less than 0.3 &mgr;m, on the other hand, it is difficult to produce a sintered body having a high density by the production technique of the present invention. Therefore, the value is limited to the range of 0.3 to 5 &mgr;m.
(b) Maximum grain size of the sintered body:
The value is limited to 20 &mgr;m or less because, if the value is over 20 &mgr;m, the mean break-resistance strength of the sintered body decreases substantially, thereby resulting in breakage of the target material during sputtering even at low sputtering power.
(c) Relative density of the sintered body:
The reason why the value is limited to the range of 95% to 99% is, that if is less than 95%, the high strength of the sintered body is sacrificed, while the thermal shock resistance reduces if the value exceeds 99%.
(d) Purity of the sintered body
The purity of the sintered body is established to at least 4N because the growth of grains in the sintered body tends to accelerate at a purity below 4N.
(e) Contents of trace impurities
The contents of K, Na, Al, Si and Fe, as impurities, are adjusted as required to 1 ppm or less, 2 ppm or less, 5 ppm or less, 20 ppm or less and 2 ppm or less for the purpose of preventing the target material from breaking.
(f) The mean primary particle size of the powder material.
The value should be desirably 1 &mgr;m or less, because, if it exceeds 1 &mgr;m, the grain size of the sintered body would be more than 5 &mgr;m, which results in a decrease in the mean break-resistant strength and breakage of the target material during sputtering under low DC power.
(g) Sintering temperature and sintering time
The sintering temperature and sintering time have the effects of cont
Mishima Akifumi
Watanabe Kazuo
Cantelmo Gregg
Mitsubishi Materials Corporation
Nguyen Nam
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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