Quantitation of latent resist images using photon tunneling microscopy

Photon tunneling microscopy (PTM) is an optical microscopy technique that can, by using frustrated total internal reflection, be sensitive to changes in surface topography as small as 1 nm. We have developed a simple calibration technique that generates plots of sample reflectivity as a function of topography. This empirical data is then used to convert a gray-scale image of a specimen into an accurate topographic image. Errors, such as accurate magnification calibration and detector nonlinearity, lead to only small differences between the empirical data and our theoretical prediction. We have used PTM to study the evolution of latent images in a chemically amplified resist, ARCH2, as a function of dose, both after exposure and after postexposure bake, and have obtained good agreement between the topography measured in the PTM with thickness changes determined by ellipsometry from large exposed areas. Comparison of the results of the PTM with those obtained by near-field scanning optical microscopy demonstrate that changes in topography on the order of 3–5 nm are visible in the PTM. The topography that appears to develop in a latent image is a function of feature size. This effect occurs before the lateral resolution limit of the microscope is reached, and is a result of the mechanical constraint arising from unexposed resist material around the exposed features. Techniques such as adjusting the polarization and wavelength of the illumination can improve the sensitivity of the technique to small changes in topography, while in the case of materials that undergo changes in refractive index, contrast can be seen even in the absence of topography.