Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
2000-03-08
2001-09-04
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C075S247000, C075S369000, C075S410000, C075S631000, C075S361000, C075S367000, C419S048000
Reexamination Certificate
active
06284013
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a high-purity ruthenium sputtering target which can be used as a material for the lower or upper electrodes of ferroelectric semiconductor capacitors, and to a method for manufacturing such a high-purity ruthenium sputtering target.
BACKGROUND OF THE INVENTION
In the manufacture of semiconductors, the use of a thin film of a ferroelectric substance such as a BaSrTi composite oxide, a PbZrTi composite oxide, and a SrBiTa composite oxide on a wafer such as a silicon wafer constituting semiconductor elements as a capacitor has recently been studied. For using as the lower or upper electrode of such a thin ferroelectric capacitor film, the formation of a ruthenium oxide film by sputtering a target member consisting of ruthenium in an oxygen atmosphere has been studied.
In order to guarantee reliable semiconductor performance for semiconductor members formed by sputtering, it is critical that the content of metal impurities harmful to semiconductor devices is minimal. That is, impurities such as
(1) alkali metal elements such as Na and K,
(2) radioactive elements such as U and Th, and
(3) heavy metal elements such as Fe and Ni, must be removed as much as possible.
Alkali metal elements such as Na and K move easily in insulating films, and radioactive elements such as U and Th emit alpha rays, causing soft errors. Heavy metals such as Fe and Ni may arise problems of interfacial junctions.
In general, the following method is used for the industrial manufacture of ruthenium. That is, crude ruthenium is mixed with potassium hydroxide and potassium nitrate, and undergone oxidation melting to convert ruthenium to soluble potassium ruthenate. This salt is extracted with water, and heated while blowing chlorine gas in to form ruthenium tetroxide, which is collected in diluted hydrochloric acid containing methanol. This solution is evaporated to dryness, the residue is sintered in an oxygen atmosphere to form ruthenium dioxide, which is further ignited in hydrogen to form metal ruthenium.
However, commercially available ruthenium powder manufactured by the above-described method contains large quantities of alkali metals such as Na and K, heavy metals such as Fe and Ni, and radioactive elements such as U and Th, and is not suitable as a material for the electrodes of ferroelectric capacitors.
For solving this problem, efforts have been done for highly purifying ruthenium.
For example, Japanese Patent Laid-Open No. 8-199350 discloses a method for manufacturing a high-purity ruthenium sputtering target having a purity of 5 N or higher, comprising the steps of alkali-melting commercially available ruthenium powder, leaching it with water, adding excess NaOH to it, being saturated with chlorine gas, and heating to convert ruthenium into ruthenium tetroxide; separating the ruthenium tetroxide by evaporation, absorbing the separated ruthenium tetroxide in a solution of hydrochloric acid and methanol, purifying it three times by distillation, refluxing and evaporating the solution to dryness to form gel-like precipitate of ruthenium hydroxide; drying this precipitate, heating the precipitate in the air to form ruthenium dioxide; heating this ruthenium dioxide in a hydrogen stream to form ruthenium powder of 5-N purity, hot-pressing this powder into a disc, and subjecting this disc to electron beam melting to remove Na, K, Mg, and Ca.
However, these conventional methods have problems in that a large number of process steps, complicated operations, and high manufacturing costs are required; the products are easily contaminated during the process steps; and the yield is poor. With increase in the density of wiring in semiconductor thin films, the formation of particles during sputtering arises a large problem. The formation of particles cannot be reduced by high-purity ruthenium target manufactured by conventional methods.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a method for manufacturing a high-purity ruthenium sputtering target having a low impurity content, and in particular, producing few particles, which is suitable for applications such as the formation of semiconductor thin films.
SUMMARY OF THE INVENTION
In order to solve the above-described problems, the present inventors repeated experiments, and found that most of crude ruthenium could be converted into ruthenium tetroxide by blowing chlorine gas into the NaOH solution of crude ruthenium, then blowing ozone into the solution, and the crystals of a ruthenium salt could be obtained by evaporating to dryness a solution of the ruthenium tetroxide in hydrochloric acid or in a mixed solvent of hydrochloric acid and an organic solvent, and that a high-purity ruthenium powder could be obtained by reducing these in a hydrogen atmosphere, and further treating them in an inert atmosphere. The inventors also found that the high-purity ruthenium sputtering target obtained by such a method has a low content of not only metal impurities which have arisen problems, but also gaseous components, thereby significantly decreasing the formation of particles.
On the basis of these findings, according to a first aspect of the present invention there is provided a method for manufacturing a high-purity ruthenium sputtering target comprising the steps of feeding crude ruthenium powder into a sodium hydroxide solution; blowing an ozone-containing gas during or after blowing chlorine gas into the solution to form ruthenium tetroxide; absorbing the ruthenium tetroxide in a hydrochloric acid solution or a mixed solution of hydrochloric acid and ammonium chloride, and evaporating the solution to dryness; sintering the resultant ruthenium salt in a hydrogen atmosphere to form high-purity ruthenium powder; and hot-pressing the ruthenium powder into a sputtering target.
According to a second aspect of the present invention there is provided a method for manufacturing a high-purity ruthenium sputtering target comprising the steps of feeding crude ruthenium powder into a sodium hydroxide solution; blowing an ozone-containing gas during or after blowing chlorine gas into the solution to form ruthenium tetroxide; absorbing the ruthenium tetroxide in a hydrochloric acid solution or a mixed solution of hydrochloric acid and ammonium chloride, and evaporating the solution to dryness; sintering the resultant ruthenium salt in a hydrogen atmosphere to form high-purity ruthenium powder; and subjecting the ruthenium powder to electron-beam melting to produce a sputtering target.
According to a third aspect of the present invention, there is provided a high-purity ruthenium sputtering target, wherein the content of each element of carbon, oxygen, and chlorine is 100 ppm or below.
According to a fourth aspect of the present invention, there is provided the high-purity ruthenium sputtering target according to the third aspect, wherein the purity of ruthenium after removing gaseous component is 99.995% or higher.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiment of the present invention will be described in detail below.
Crude ruthenium powder used in the present invention is not specifically limited, but it is generally crude ruthenium powder with a purity of about 98 to 99.9% commercially available. Such crude ruthenium normally contains 10 to 1000 ppm of Na, 10 to 1000 ppm of Fe, and 0.5 to 100 ppb of each of U and Th.
On the other hand, the purity of NaOH used for dissolving the crude ruthenium and the purity of chlorine gas blown in the solution are not limited, but those of generally used industrial grades can be used. This is because impurities contained in these can be separated efficiently from ruthenium tetroxide.
The concentration of NaOH is 10 to 400 g/l, preferably 100 to 350 g/l. If the concentration is less than 10 g/l, little ruthenium can be dissolved, and if the concentration exceeds 400 g/l, NaOH which has not been dissolved may remain, or a large quantity of the reaction product, NaCl, may deposit from the solution and interfere with reaction.
The rea
Shindo Yuichiro
Suzuki Tsuneo
Coy Nicole
Howson and Howson
Japan Energy Corporation
King Roy
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