Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Sintering which includes a chemical reaction
Reexamination Certificate
1998-12-24
2001-12-11
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Sintering which includes a chemical reaction
C419S049000
Reexamination Certificate
active
06328927
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the fabrication of high-purity, high-density tungsten sputter targets for use in physical vapor deposition of thin films.
BACKGROUND OF THE INVENTION
In the manufacture of sputter targets used in the semi-conductor industry, and more particularly to sputter targets used in physical vapor deposition (PVD) of thin films onto complex integrated circuits, it is desirable to produce a sputter target that will provide film uniformity, minimal particle generation during sputtering, and desired electrical properties. Furthermore, to meet the reliability requirements for diffusion barriers or plugs of complex integrated circuits, the sputter target must have high-purity and high-density.
Current methods to achieve suitable sputter targets for use in complex integrated circuits involve either hot-pressing or cold-isostatic-pressing followed by high temperature sintering. Using either of these techniques, the density of the pressed target material is about 90% of theoretical density. To obtain that 90% density, the sintering process needs to proceed at a minimum of 1800° C. This high temperature results in a significant growth of the grains. Large grain size in sputter targets is deleterious to the uniformity of the deposited films. Furthermore, the sputter targets fabricated by these methods have a high oxygen content, which results in a high film electric resistivity. In addition, these processes typically involve pressing in a graphite die mold. Some volatile contaminations are contributed to the targets by this graphite mold, which results in an increase in the impurities in the films and deteriorates the reliability of the sputter devices. For example, graphite has a high alkaline element which evaporates out of the mold and is absorbed by the sputter target during pressing and sintering. Thus, the sputter targets fabricated by the hot-press or cold-isostatic-press followed by high temperature sintering have proved unreliable for use in complex integrated circuits.
There is thus a need to develop a method for fabricating high-purity, high-density tungsten sputtering targets that will meet the reliability requirements for complex integrated circuits.
SUMMARY OF THE INVENTION
The present invention provides a tungsten sputter target having a density of at least about 97% of theoretical density and an oxygen content of at least about 100 ppm less than the starting powder. Furthermore, this high-density, low oxygen sputter target may be produced with a metallic purity of at least about 99.9995. This high-density, high-purity tungsten sputter target is fabricated by (a) providing a tungsten powder having a purity higher than about 99.999%, a powder size smaller than about 50 &mgr;m and preferably smaller than about 20 &mgr;m, and an oxygen content less than about 500 ppm; and (b) hot-isostatic-pressing the powder at a temperature between about 1200° C. to about 1600° C. at a pressure of at least about 15 ksi for at least about 3 hours. In a preferred embodiment of the invention, the tungsten powder has a purity of at least 99.9995%, a powder size of less than about 10 &mgr;m, and an oxygen content less than about 300 ppm.
In a further preferred embodiment of the present invention, hot-isostatic-pressing is performed at a temperature of about 1400° C. and a pressure of about 40 ksi for about 7 hours. Where a desired target diameter to height ratio is greater than about 3, the method of fabricating the sputter target preferably includes the additional step of cold-isostatic pressing the powder prior to hot-isostatic-pressing.
In a preferred embodiment of the invention, the powder is pressed in a powder capsule made of either titanium, iron, or an alloy thereof, to reduce the oxygen level of the tungsten.
These and other objects and advantages of the present invention shall become more apparent from the accompanying description thereof.
DETAILED DESCRIPTION
According to the principles of the present invention, a tungsten sputter target is fabricated having an oxygen content of at least about 100 ppm less than the starting powder and a density higher than 97% of theoretical density. By starting with a high purity powder, such as 99.999% purity or higher tungsten powder, a high-purity sputter target may also be achieved. This high-purity, high-density tungsten sputter target can be used in the physical vapor deposition of thin films as diffusion barriers or plugs in complex integrated circuits.
To achieve the high-density tungsten sputter target, a tungsten powder is provided having a powder size smaller than about 50 &mgr;m. To obtain a high-purity tungsten sputter target, the tungsten powder is further provided with an oxygen content less than about 500 ppm (such as that commercially available from Sumitomo, Tokyo, Japan). In a preferred embodiment of the present invention, the tungsten powder is provided with a powder size smaller than about 20 &mgr;m, and more preferably smaller than about 10 &mgr;m, and an oxygen content less than about 300 ppm. This tungsten powder is then hot-isostatic-pressed at a temperature in the range of 1200° C. to 1600° C. at a pressure higher than about 15 ksi for a period of at least 3 hours in an inert environment such as argon. The temperature of the hot-isostatic-press (HIP) is preferably about 1400° C. with a pressure of about 40 ksi. At these preferred temperature and pressures, the hot-isostatic-pressing (HIPing) step is preferably performed for about 7 hours.
The HIPing method requires the use of a capsule for containing the powder material during pressing. The capsule material must be capable of substantial deformation because the HIPing method uses high pressure to achieve about a 50-70% volume reduction. Furthermore, the capsule material must have a melting point higher than the HIPing temperature. Thus, any material of sufficiently high melting point that can withstand the degree of deformation caused by the HIPing process is suitable for the present invention. Suitable materials may include, for example, beryllium, cobalt, copper, iron, molybdenum, nickel, titanium or steel.
A cold-isostatic-pressing step prior to the HIPing step is recommended for target diameter/height ratios of greater than about 3. This consolidated powder can then be machined using known techniques, such as electro-discharge machining, water-jet cutting or a regular mechanical lathe. Once machined, the consolidated target blank can be bonded to a backing plate using known methods, such as soldering with a lead-tin or indium/tin solder.
The sputter targets fabricated by this process have a density higher than 97% of theoretical density, and normally, a 99% density can be achieved through the use of the smaller particle size starting powder. Thus, the combination of the HIPing process with a small particle size starting powder produces a highly dense tungsten sputter target. This high-density reduces particle generation from the targets during sputtering.
High-purity sputter targets may also be produced by using a starting powder of high purity. For example, a starting powder having a metallic purity higher than about 99.999% consolidated by HIPing can produce a sputter target having a metallic purity of at least about 99.9995.
The oxygen level of the targets produced by the present invention is at least about 100 parts per million less than the starting powder. This can further be achieved by using a powder capsule made of such materials as titanium, iron or alloys thereof. It is believed that because oxygen in the powder material, such as in the form of WO
2
, is not stable at the high temperatures used in the HIPing process, the capsule material will react with the oxygen. Thus, it is believed that these powder capsules can act as an oxygen absorber to reduce the oxygen level of the tungsten by more than 100 ppm. The lower oxygen content in the sputter targets of the present invention results in a decrease in the resistivity in conducting films. Thus, any material that reacts with unstable oxygen and meets the melting point and
Draper Darryl
Gilman Paul S.
Lo Chi-Fung
Beiderman Blake T.
Jenkins Daniel J.
Praxair Technology Inc.
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