ArF solutions for low-k₁ back-end imaging

The requirements stated in the ITRS roadmap for back-end-of-line imaging of current and future technology nodes are very aggressive. Therefore, it is likely that high NA in combination with enhancement techniques will be necessary for the imaging of contacts and trenches, pushing optical lithography into the low-k₁ regime. In this paper, we focus more specifically on imaging solutions for contact holes beyond the 90 nm node using high NA ArF lithography, as this is currently seen as one of the major challenges in optical lithography. We investigate the performance of various existing enhancement techniques in order to provide contact holes imaging solutions in a k₁ range from 0.35 to 0.45, using the ASML PAS5500/1100 0.75NA ArF scanner installed at IMEC. For various resolution enhancement techniques (RET), the proof of concept has been demonstrated in literature. In this paper, we propose an experimental one-to-one comparison of these RET’s with fixed CD target, exposure tool, lithographic process, and metrology. A single exposure through pitch (dense through isolated) printing solution is preferred and is the largest challenge. The common approach using a 6% attenuated phase-shifted mask (attPSM) with a conventional illumination fails. The advantages and drawbacks of other techniques are discussed. High transmission (17%) attenuated phase shift, potentially beneficial for part of the pitch range, requires conflicting trade-offs when looking for a single exposure through pitch solution. More promising results are obtained combining a BIM or a 6% attPSM with assist slots and off-axis illumination, yielding a depth of focus (DOF) at 8% exposure latitude (EL) greater than 0.31 µm from 200 nm pitch through isolated. Chromeless phase lithography (CPL) is also discussed with promising results obtained at the densest pitch. At a 0.4 k₁, an experimental extrapolation to 0.85NA demonstrates that a pitch of 180 nm can be resolved with 0.4 µm DOF at 8% EL. For all of these imaging solutions, various metrics are studied to compare printing performance. In addition to process latitude, we consider forbidden pitches, sidelobes printability, and mask error enhancement factor (MEEF).