Layer placed between a photoresist and a substrate to prevent reflection from the latter resulting in a reduction in photoresist pattern quality, as characterized by resolution,
line edge roughness, etc.
Source:
PAC, 2020, 92, 1861. 'Terminology of polymers in advanced lithography (IUPAC Recommendations 2020)' on page 1868 (https://doi.org/10.1515/pac-2018-1215)