Tadao Tsuruta

Issues and method of designing lenses for optical lithography

A variety of technologies to improve optical performance in photolithography are surveyed. The technologies are classified according to which component of the optics they work on: the source, the mask, the pupil, or the image. Emphasis is put on, first, the advantages and disadvantages of annular illumination, which are studied by lithography simulation. Second, a polarized mask is proposed. The effect of polarization on imaging, which has not been explored yet, is studied and the result is that TE polarization produces images of the best quality. Third, as a part of pupil function, aberration optimization is discussed. The conclusion is that positive spherical aberration is suitable for projection optics.

Resolution versus depth of focus in the resolution-enhanced optical system for lithography

Practical resolution of optical lithography is often defined as the minimum feature size which can be fabricated with acceptable depth of focus. It can be predicted by calculating diffraction image contrast in various defocused plane. However since large volume of calculation is required to know image contrast under partially coherent illumination, evaluation of optical lithography systems with regard to use of the practical resolution is time consuming and does not give us quick and clear forecast on optimum optical parameters for a given lithography specifications. In this paper, we propose an analytical and intuitive method for getting image contrast in defocused planes, by use of the theory of interference fringe formation. By using this method, relations among defocus, numerical aperture, wavelength, coherence factor and image contrast are derived analytically and these parameters can be optimized for given lithography methods, which employ not only the conventional but also various resolution-enhanced methods, such as annular illumination, phase-shift, illumination control and others.