Image sharpness measurement in the scanning electron microscope - Part III

Fully automated or semi-automated scanning electron microscopes (SEM) are n ow commonly used in semiconductor production and other forms of manufacturi ng. Testing and proving that the instrument is performing at a satisfactory level of sharpness is an important aspect of quality control. The applicat ion of Fourier analysis techniques to the analysis of SEM images is a usefu l methodology for sharpness measurement. In this paper, a statistical measu re known as the multivariate kurtosis is proposed as an additional useful m easure of the sharpness of SEM images. Kurtosis is designed to be a measure of the degree of departure of a probability distribution. For selected SEM images, the two-dimensional spatial Fourier transforms were computed. Then the bivariate kurtosis of this Fourier transform was calculated as though it were a probability distribution. Kurtosis has the distinct advantage tha t it is a parametric (i.e., a dimensionless) measure and is sensitive to th e presence of the high spatial frequencies necessary for acceptable levels of image sharpness. The applications of this method to SEM metrology will b e discussed.