Charged particle beam apparatus and method for calculating roughness index

A method implemented by a scanning electron microscope (SEM) type length measurement apparatus comprising an objective lens, a deflector, a detector, an analog-to-digital (A/D) converter, an image processing unit, and a power spectral density (PSD) input unit for decreasing variation in roughness measurements made by SEMs in semiconductor fabrication, the method comprising:

• receiving, by the PSD input unit, a first power spectral density (PSD) that indicates a power spectral density of a line pattern measured for a line pattern formed on a first wafer in advance by a second SEM;
• acquiring, by a first SEM under a specified optical condition including at least one of irradiation energy, probe current, detector type, or scanning speed, a scanning image of the line pattern formed on the first wafer by two-dimensionally scanning the line pattern with a charged particle beam, detecting electrons emitted from the line pattern with the detector, and converting a detection signal into a digital image via the A/D converter;
• determining a second PSD that indicates a power spectral density of the line pattern formed on the first wafer from the acquired scanning image by the first SEM;
• obtaining a correction method for correcting the power spectral density of the second PSD to the power spectral density of the first PSD;
• acquiring, by the first SEM under the same specified optical condition, a scanning image of a line pattern formed on a second wafer by two-dimensionally scanning the line pattern with the charged particle beam, detecting electrons emitted from the line pattern with the detector, and converting the detection signal into a digital image via the A/D converter;
• determining a third PSD that indicates a power spectral density of a line pattern formed on the second wafer from the acquired scanning image by the first SEM;
• calculating a corrected power spectral density obtained by correcting the power spectral density of the third PSD by the correction method;
• and
• calculating a roughness index of the line pattern formed on the second wafer by the first SEM using the corrected power spectral density, wherein the roughness index comprises at least one of a line edge roughness (LER) or a line width roughness (LWR).