CALIBRATION OF DIFFRACTION GRATINGS USED AS LINE STANDARDS IN SEM (SCANNING ELECTRON MICROSCOPE) MAGNIFICATION CALIBRATION

摘要

An investigation into the standards used for the calibration of the magnification of Scanning Electron Microscopes is performed and a system developed to calibrate these standards. The ASTM E 766-98 standard practice (Calibrating the Magnification of a Scanning Electron Microscope) recommends a calibration grid as preferred calibration standard. The reason for recommending a grid is that it does not have to be moved to calibrate both the x and y axes. It does however state that other standards such as line gratings, can also be used to perform the calibration. It was decided to use the line grating as the standard for discussion in this paper, as methods for calibrating line gratings have been developed internationally and are well established. The system developed was built around projecting a laser, with a known wavelength, onto the grating and measuring the angle at which the light is diffracted. From the wavelength and the angle, the pitch between the line gratings can be calculated. The two main sources of uncertainty are the laser wavelength and the angle measuring capability which must be accurate to sub arc-seconds. A comparison with another national metrology institute (NMI) proves that the current system can measure down to an uncertainty of +- 0,2 nm (nanometer) but with improvements planned to the detector, an uncertainty of +- 0,05 nm or 50 pm (picometer) would theoretically be achievable. This paper discusses the design of the system, the measurements performed and comparison results obtained.