Negative Chemically Amplified Resist (nCAR) for DRAM Mask Fabrications

Abstract

In this contribution we will demonstrate how the use of negative tone CAR can significantly improve the CD control of mask layers in which CD is measured on opaque features. A thorough investigation of the individual contributions of sequential process steps in mask making revealed that the final CD uniformity can by improved by 20% when a negative tone resist is used. In case of 50 keV electron beam (EB) mask writing systems, that employ variable shaped beam (VSB) writing, the writing time can be reduced by 40-50 % when a chemically amplified resist (CAR) is applied. Therefore we have evaluated and characterized a commercially available negative-tone CAR. The resist showed good pattern performance down to 150 nm for isolated and semi-isolated opaque lines thus having the ability to form assist bar features. Vertical profiles have been obtained. Line edge roughness (LER) is more pronounced for this material when compared to standard EB resist ZEP 7000. But analysis of CD uniformity (3σ) of 500 nm opaque lines in local area with negative CAR and with positive tone ZEP 7000 showed 4,8 nm and 6,2 nm, respectively. Thus substantiating that the negative CAR is advantageous in terms of opaque line CD control. Regarding soft bake (SB) and post exposure bake (PEB) latitude, the CAR is stable with respect to soft bake temperature variation (3,7 nm/°C). Much more severe is the steep PEB latitude with respect to dose of 0,7-1,3 (μC/cm2)/°C. This requires the use of high precision baking tools for the PEB step. Since all mask blanks have been coated in-house, we have investigated a variety of pre-treatment steps. The influence of each step was characterized by contact angle measurement. We found out that the best results have been achieved when the sequence H2SO4/H2O2-cleaning-UV/ozone-clean-dehydration bake is applied to virgin blanks as delivered by the blank supplier.