Experimental evaluation of bull's-eye illumination for assist-free random contact printing at sub-65nm node

摘要

Patterning of random CH for the 65nm node and below has proven to be a very difficult task. As a rule of thumb, difficulties in contact patterning are driven by the low depth of focus towards isolated contacts and/or the lower contrast combined with higher mask error factor (MEEF) for denser contact arrays. In this work, we experimentally investigate the use of illumination modes consisting of the combination of annular plus conventional illumination, so-called "Bulls-Eye" illumination. This study is a search for the optimal sigma settings for the annular and conventional parts, with respect to process window and MEEF through pitch. Also, the extend to which the Bulls-Eye is advantageous is demonstrated by means of experimental comparison to wafer prints by conventional illumination. Besides regular grid CH arrays, the Bulls-Eye performance is evaluated for different 2D contact patterns. Experimental results are obtained on ASML ArF scanners at various NAs up to 0.93. Additionally, immersion lithography and Focus Drilling are considered at given exposure setting as techniques to increase the focal depth. The experiments show promising results, printing contacts from k1 = 0.40 onwards with acceptable process, MEEF, and proximity through pitch, and this without side-lobe printing.