Linewidth measurement comparison between a photometric optical microscope and a scanning electron microscope backed with Monte Carlo trajectory computations

摘要

Abstract: Linewidth measurements of clear lines on anti-reflection coated photomasks are obtained with a photometric optical microscope and a scanning electron microscope (SEM). Both microscopes have stages fitted with helium-neon laser interferometers and thus provide directly traceable length measurements. In both instruments, measurements of pitches are independent of the choice of image edge intensity threshold, while linewidth measurements are greatly affected by the threshold levels. Simple scalar optical theory predicts that in the absence of aberrations, the position of the edge of a totally opaque film should be at the 25% intensity threshold for coherent illumination. The 50% backscattered electron intensity threshold has proved to be a repeatable point for measurements as it is very insensitive to such parameters as beam diameter, accelerating voltage and exact focus. The relationship between the 50% threshold and the dimensions of the sample itself is obtained with the aid of Monte Carlo electron trajectory calculations. Linewidth measurements made with the two different techniques on the same photomask are presented with particular interest in lines with widths below the resolving power of the optical microscope operated at visible wavelengths and using coherent illumination. The measurement agreement is better than 0.03 $mu@m for lines which are fully resolved and increasingly worse for lines with widths below 0.7 $mu@m. The degree to which theory predicts agreement between the two techniques and the resulting confidence in the SEM measurements are discussed.!