Patent Number: 
Section: claims

1. An electron beam source comprising:a cathode having a hemispherical electron discharge surface;an anode disposed opposite said cathode and having a first aperture on an optical axis;a first bias electrode disposed between said cathode and said anode, to which is applied an electric potential that is lower than an electric potential to said cathode, and having a second aperture on said optical axis that is larger than said electron discharge surface of said cathode;a second bias electrode disposed adjacent to said cathode as seen from said first bias electrode, to which is applied an electric potential that is lower than the electric potential to said cathode, and having a third aperture on said optical axis that is larger than said electron discharge surface of said cathode; anda controller that controls the electric potential applied to said first bias electrode and the electric potential applied to said second bias electrode. 2. The electron beam source according to claim 1, wherein a tip of said electron discharge surface of said cathode is disposed atop or outside a shell having a diameter equal to an inner diameter of said second aperture of said first bias electrode. 3. The electron beam source according to claim 1, wherein, in a case where a radius of said second aperture of said first bias electrode is R1, a radius of said third aperture of said second bias electrode is R2, a distance from the tip of said electron discharge surface of said cathode to a center of said second aperture of said first bias electrode is D1, and a distance from the center of said second aperture of said first bias electrode to a center of said third aperture of said second bias electrode on said optical axis is D2, a relation between R1 and R2 is 0.8R1≦R2≦1.2R1 and a relation between D1 and D2 is 0.8D1≦D2≦1.2D1. 4. The electron beam source according to claim 3, wherein said controller, in a case where the electric potential E1 applied to said first bias electrode is a relative electric potential relative to the electric potential of said cathode and the electric potential E2 applied to said second bias electrode is a relative electric potential relative to the electric potential of said cathode, controls the electric potential applied to said first bias electrode and the electric potential applied to said second bias electrode so as to maintain a relation between E1 and E2 such that 0.9k≦|E1+E2|≦1.1k, where k is a positive constant which has a unit of volts. 5. The electron beam source according to claim 3, wherein one or more bias electrodes, to which is applied an electric potential lower than the electric potential applied to said cathode, and which have an aperture on said optical axis larger than said electron discharge surface of said cathode, are aligned between said first bias electrode and said second bias electrode. 6. An electron beam exposure apparatus, comprising:the electron beam source according to claim 1;a formation system for applying an exposure beam having desired characteristics to an electron beam from said electron beam source;a deflection system for controlling a position of said exposure beam; anda projection system for concentrating said exposure beam on a substrate that is a specimen and drawing a desired pattern thereon.