Alloys or metallic compositions – Titanium base
Reexamination Certificate
1999-03-29
2001-04-03
Jenkins, Daniel J. (Department: 1742)
Alloys or metallic compositions
Titanium base
Reexamination Certificate
active
06210634
ABSTRACT:
FIELD OF THE INVENTION OF THE RELATED ART STATEMENT
The present invention relates to highly purified titanium material suitable for target material used for forming Ti wiring network on a semiconductor device surface, a method for preparation of it, and a sputtering target using it.
Wiring network having a complicated pattern is formed on a surface of various kinds of semiconductor devices with conductive metallic materials according to a purpose of use. The wiring network is formed by forming a conductive metallic film made of Al, Au or the like by sputtering process and applying a certain etching treatment of the film for patterning.
Currently, the wiring is required to have a narrow width and a thin thickness as the semiconductor device is highly integrated. But high minute wiring network causes delay of signals due to the wiring resistance of used wiring material, or burn-out of wiring due to resistance heating during operation of the device because of using a material having a low melting point. Therefore wiring material not to need a drastic alteration of any forming processes of LSI, VLSI, and ULSI, and having a high melting point is demanded. Ti attracts attention as one of the materials for that purpose, as well as Mo, W and Ta.
When Ti is used for wiring network of a semiconductor device, usually a film made of Ti is formed by sputtering process as described above. Therefore, a sputtering target made of Ti material is required to be prepared and the Ti target must have high purity. When the Ti target contains oxygen as an impurity, increased electric resistance of the film prepared causes a problem of delay or a trouble of burn-out of wiring network. A heavy metal such as Fe, Ni or Cr makes a factor causing leakage at the joint of the film interface. An alkali metal such as Na or K readily sets Si free and degrades the characteristics of the device.
Besides the high purity mentioned above, the Ti target forming wiring network of, for example, VLSI is highly required to be uniform without any cracks or creases externally or internally, and to uniformly discharge the thermal energy produced during sputtering.
This is because the film thickness and the uniformity of inside composition of a film formed by sputtering, besides effects of an impurity, are becoming more important as high integration requires the wiring to be more minute. When scratches exist on the target surface or inside, flight of sputtering particles falls into disorder at that place, and the uniformity of bonding condition on a substrate is lowered, causing varying electric resistances or burn-out. When thermal energy applied to the target during sputtering is not uniformly discharged to the side of a backing plate, uniform bonding condition is deteriorated because of heterogeneity of temperature distribution of the target.
The above-mentioned Ti target is generally prepared by the method described below.
Crude Ti material is prepared by one of the following three methods. First method is called Kroll method or Hunter method which is to thermally reduce a Ti compound such as TiCl
4
with an active metal such as Na or Mg. Second is called Iodide method which is to thermally decompose a titanium compound such as TiI
4
. And third is a method for molten salt electrolysis in salt such as NaCl or KCl. Because the crude Ti material prepared as above is in the shape of sponge, crystal or a needle, it is generally arc-melted in a vacuum of about 10
−2
Torr to 10
−3
Torr to be made into an ingot, which is then shaped into a target form for use.
Because the purity of the Ti target prepared by the prior methods mentioned above is about 2N to 3N, it can be used as a sputtering target for 64 Kbits but not for 256 Kbits, 1 Mbit, 4 Mbits or higher because of low purity as a target material for wiring or barrier material.
Besides, the Ti target prepared by the prior methods is insufficient in view of the surface and inside conditions mentioned above. To prevent any surface or inside scratches of the Ti target from causing a change of electric resistance or burn-out, processability shall be raised. On the other hand, to prevent contamination during preparation, highly purified Ti is shaped into a certain form by cold working. The processability of the Ti material by the prior methods is not enough to achieve sufficient homogeneity. Besides, thermal energy is insufficiently discharged. In short, the prior methods do not provide a Ti material having high thermal conductivity sufficient to uniformly discharge the thermal energy which is applied to a target during sputtering, to the side of a backing plate.
OBJECT AND SUMMARY OF THE INVENTION
An object of the present invention is to provide a highly purified Ti material having sufficient purity, processability and thermal conductivity required for a Ti target when forming a wiring layer or a barrier layer of LSI by sputtering process.
Another object of the present invention is to provide a method for preparing a highly purified Ti material capable of preparing a highly purified Ti material having high purity, excellent processability and thermal conductivity required for a Ti target when forming a wiring layer or a barrier layer of LSI by sputtering process.
And a further object of the present invention is to provide a sputtering target capable of forming a uniform Ti film including only a few impurities which have an adverse effect on the function of a device.
The highly purified Ti material of the present invention has an oxygen content of not more than 350 ppm, Fe, Ni and Cr contents of not more than 15 ppm each, Na and K contents of not more than 0.5 ppm each, a reduction of area as a material characteristic of not less than 70%, and a thermal conductivity of not less than 16 W/m K. Besides, a sputtering target of the present invention is made by shaping the above highly purified Ti material into a desired form.
First method of preparation of the highly purified Ti material of the present invention consists of a step of sieving crude Ti particles to classify them into each type of particle diameters according to contents of impurities, a step of sorting them having the desired particle diameters from the above sieved crude Ti particles as one type or a mixture of two or more types, and melting by using an electron beam.
Second method consists of a step of acid-treating the crude Ti particles to remove a contaminated layer on the crude Ti particle surfaces and a step of melting the acid-treated crude Ti particles by using an electron beam.
The highly purified Ti material having good processability and thermal conductivity of the present invention has been achieved by finding out the facts that the crude Ti particles prepared by molten salt electrolysis or Iodide method have different purities, and particularly oxygen contents depending on the particle diameter, that the impurities contained in the crude Ti particles exist as concentrated in the surface layer, and that a thermal conductivity of not less than 16 W/m K and a reduction of area of not less than 70%, as well as high purity, which heretofore could hardly be obtained by the prior arts can be obtained by using sieving and acid-treating methods.
The highly purified Ti material of the present invention can be prepared for example as follows.
First, crude Ti particles which make a starting material of the present invention are prepared by molten salt electrolysis for example. As the Ti material used for molten salt electrolysis, for example, sponge Ti, preferably one including less U and Th contents is used. KCl—NaCl is preferably used for electrolytic bathing (electrolyte), and an electrolytic temperature of 730 to 755° C. and a voltage of 6.0 to 8.0 V are preferable. The Ti particles prepared by molten salt electrolysis usually has high contents of Na and K and relatively less heavy metals such as Fe and Ni or oxygen. The crude Ti particles as a starting material of the present invention are not limited to those prepared by molten salt electrolysis, but include those which can have a desired purity
Ishigami Takashi
Kawai mituo
Yagi Noriaki
Foley & Lardner
Jenkins Daniel J.
Kabushiki Kaisha Toshiba
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