Simulation of dense contact hole (κ1=0.35) arrays with 193-nm immersion lithography

The resolution limits of optical lithography are usually described by the well-known Raleigh criterion, CD = kappa(1) (lambda/NA). One of the biggest challenges in optical lithography is to reliably print contact holes patterns with kappa(1) - 0.35 using a hyper NA system (NA > 1) especially for relatively small (m x n) arrays. Polarization effects cause deviations from a simple (lambda/NA) scaling large NA values. For an isolated hole, n = I and for large arrays, n > 15, the spectral content is mainly contained in the lowest diffracted orders that are captured within the NA of the imaging lens. The most difficult situation is for small arrays (m, n approximate to 2, 3, 4) where the spectral features are broader more of the important image information is contained in the higher diffraction orders. The patterning of contact holes also suffers from tight dose tolerances and high mask error enhancement factors (MEEF) as both the feature and array sizes decrease. A detailed PROLITH (TM) vector simulation study is reported for three different approaches to printing, isolated contact holes and small to large contact hole arrays with a kappa(1) of 0.35 and NAs of 1.05 and 1.3: 1) imaging interferometric lithography (IIL, with a single mask and multiple exposures incorporating pupil plane filters), 2) two-exposure dipole illumination, and 3) alternating phase shift masks (alt-PSM). Only the IIL scheme is capable of printing smaller (m, n <= 10) at this low kappa(1) factor. Single exposure alt-PSM does not allow for the necessary polarization control. Periodic assist features provide improved resolution, depth of focus and MEEF, at the expense of a more complex mask and additional nonprinting area surrounding the contact holes.