New methods for CD measurements on photomasks using dark field optical microscopy

We present novel methods of dark field optical microscopy for CD measurements on photomasks in the sub lambda regime. Optical CD inspection systems usually suffer from limited resolution and from linear and nonlinear superposition of the light diffracted at both edges of a line structure (optical proximity effects). These disadvantages partly can be overcome using alternating grazing incidence illumination of the specimen, where the angle of incidence of the illumination is perpendicular to the edges or grooves of the specimen. Especially for s-polarised light the diffraction efficiencies of the edges directed towards the illumination are at least an order of magnitude stronger than that of the averted edges. Thus an efficient suppression of optical proximity effects is achieved. Additionally a reduction of the widths of the diffraction limited images of structure edges due to a high pass characteristic of the optical image takes place. Both effects result in a significantly increased resolution power. We developed both a reflection mode (alternating grazing incidence dark field microscopy, AGED) and a transmission mode method (Frustrated internal total reflection microscopy, FIRM). Line width measurements on high quality photomasks will be presented. The experimental results will be compared with theoretical simulations and the applicability for sub lambda structures will be discussed.