High-purity lanthanum, sputtering target comprising high-purity lanthanum, and metal gate film mainly comprising high-purity lanthanum

1. A sputtering target comprising high purity lanthanum, wherein the purity excluding rare-earth elements and gas components is 4N or higher, contents of aluminum, iron and copper in the lanthanum are respectively 100 wtppm or less, oxygen content is 1500 wtppm or less, contents of alkali metal elements and alkali earth metal elements are respectively 1 wtppm or less, contents of transition metal elements and high-melting-point metal elements other than those above are respectively 10 wtppm or less, and contents of radioactive elements are respectively 10 wtppb or less.

2. A sputtering target according to claim 1 , wherein the alkali metal elements include lithium, sodium, potassium, rubidium, cesium and francium, and the alkali earth metal elements include calcium, strontium, barium and radium, and wherein the content of each is 1 wtppm or less.

3. A sputtering target according to claim 2 , wherein the content of each of lithium, sodium, potassium, rubidium, cesium, calcium and strontium is less than 0.1 wtppm.

4. A metal gate film mainly comprising high purity lanthanum, wherein the purity excluding rare-earth elements and gas components is 4N or higher, contents of aluminum, iron and copper in the lanthanum are respectively 100 wtppm or less, oxygen content is 1500 wtppm or less, contents of alkali metal elements and alkali earth metal elements are respectively 1 wtppm or less, contents of transition metal elements and high-melting-point metal elements other than those above are respectively 10 wtppm or less, and contents of radioactive elements are respectively 10 wtppb or less.

5. A metal gate film according to claim 4 , wherein the alkali metal elements include lithium, sodium, potassium, rubidium, cesium and francium, and the alkali earth metal elements include calcium, strontium, barium and radium, and wherein the content of each is 1 wtppm or less.

6. A metal gate film according to claim 5 , wherein the content of each of lithium, sodium, potassium, rubidium, cesium, calcium and strontium is less than 0.1 wtppm.

7. A metal gate film according to claim 6 , wherein the metal gate film is a gate insulator film of a MOSFET.

8. A method of producing high purity lanthanum, comprising the steps of:

subjecting a lanthanum raw material to nitric acid treatment to form an oxide film on the lanthanum raw material,

washing and ultrasonic cleansing the lanthanum raw material to remove nitric acid and moisture from the lanthanum raw material after said step of subjecting the lanthanum raw material to nitric acid treatment,

after said step of washing and ultrasonic cleansing, EB (electron beam) melting the lanthanum raw material to eliminate the oxide film on a surface of the lanthanum raw material and oxide contained in the lanthanum raw material,

repeating said electron beam melting,

after said step of repeating said electron beam melting, solidifying and forming an ingot of high purity lanthanum such that oxygen content is 1500 wtppm or less, the purity of the lanthanum excluding rare-earth elements and gas components is 4N or higher, contents of aluminum, iron and copper in the lanthanum are respectively 100 wtppm or less, contents of alkali metal elements and alkali earth metal elements are respectively 1 wtppm or less, contents of transition metal elements and high-melting-point metal elements other than those above are respectively 10 wtppm or less, and radioactive elements are respectively 10 wtppb or less,

cutting the ingot of high purity lanthanum to produce a sputtering target, and

machining and polishing the sputtering target.