Nonlinear processes to extend interferometric lithography

The linear-systems spatial frequency limit of diffraction-limited optical lithography is similar to NA/lambda, where NA is the optical system numerical aperture, and lambda is the exposure wavelength. Optical resolution enhancement techniques (RETs) such as optical proximity correction, phase-shifts masks and off-axis illumination extend this resolution towards 2NA/lambda. Interferometric lithography (IL) for periodic patterns and imaging interferometric lithography (IL) for arbitrary patterns extends the frequency space coverage out towards the fi-ee-space linear systems transmission limit of 2/lambda. By taking advantage of inherent processing nonlinearities, higher spatial frequencies beyond these linear systems limits can be realized. Two nonlinear processes aimed at extending the spatial frequencies available by IL are reported. The first process is a variant of spatial frequency doubling in which two maskless IL processes combined with a moire alignment scheme are used to form a spatial frequency doubled grating at a period of d/2, where d is the original grating period. A first grating is written at period d, and linewidth similar to d/4, and transferred to a thin nitride film. A second IL grating, at the same period but shifted in phase by pi, is then interpolated to divide the period by two. A moire interference scheme is used to ensure proper alignment over large areas. This process has been used to frequency double a 360-nm period at I-line exposure to 180-nm period that was transferred into (110) Si using KOH etching with the nitride film as the etch mask. A second example, using photoresist nonlinearities, is the formation of nanoscale square vias (similar to 80-nm at I-line wavelengths) in a single photoresist level. This structure is formed by sequence including exposure of a grating, partial development of the resist, exposure of a second grating at right angles, and final development.