Patent ID: 8368016

Claim:
An electron beam apparatus for evaluating a sample surface comprising: an electron gun having a cathode and an anode for emitting an electron beam which is focused and irradiated as a primary electron beam onto the sample surface, wherein the cathode comprises a thermionic emission cathode for emitting a thermionic beam as the primary electron beam toward the sample surface to thereby irradiate the thermionic beam on the sample surface; a detector for detecting secondary electron beams emanated from the sample surface to provide an electric signal corresponding to an intensity of the detected secondary electron beam; a noise meter connected to the detector, for detecting a noise level from the electric signal; and a system for evaluating signal/noise (S/N) ratios or noise levels, and determining a value of electric power to be supplied for heating the thermionic emission cathode, on the basis of the evaluation results of the signal/noise ratios or the noise levels, in which the noise levels are measured by the noise meter while changing electric power for heating the thermionic emission cathode, and the signal/noise ratios are obtained from the measured noise levels.