The Optics of Photomasks: From Shadowy Past to Scattered Future

摘要

From an optical imaging perspective, photomask design and topography have evolved over the past 10 years from optically thin, light-blocking apertures to optically thick, volumetric scattering elements. The reduction in mask feature size to the sub-wavelength regime coupled with increases in exposure system numerical aperture will continue to push the optics of a photomask toward a more complex and detailed physical model. Moreover, aggressive implementation of resolution enhancement methods in state of the art lithography create mask layouts that resemble diffractive optical elements rather than the electronic circuit patterns the mask is intended to reproduce. In this paper, we address some of the optical characteristics of photomasks for current lithographic technologies as well as new trends driven by a continued reduction in mask feature size and increased numerical aperture enabled by immersion lithography. As a general conclusion, we find the photomask is best treated and characterized as a true and integrated optical component of the imaging system. By considering the mask from this vantage point, many imaging related issues in advanced lithography may be anticipated and potentially optimized.