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, PA1 (2) radioactive elements such as U and Th, and PA1 (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.