Rapid inspection for sub-wavelength line-width

This paper presents a rapid inspection technique used for the evaluation of structures with line-width below sub-wavelength and diffraction limit. Inspections are carried out with an optical microscope via a vertical scanning and through-focus measurement, where the intensities of reflection light from different focal positions of the specimen are transferred into a series of numeric data through the use of an Entropy algorithm. A through-focus focus metric (TFFM) profile is then obtained for the inspection of line-width. The secondary peak in TFFM profile is related to the distance of 180 degrees phase difference of the grating image according to the Talbot effect. This characteristic can be used to determine the pitch of grating specimen. Based on the variance of the secondary peak for different line-width, the line-width of a grating can be obtained from the comparison of simulated and measured data. Experimental results show that the Entropy algorithm can be used to achieve more reliable and fast evaluation in line-width inspection. Furthermore, as through-focus measurement is a non-destructive inspection method, it can be used as another positive element which equals to a traditional nano-scale inspection methods, such as AFM and SEM.