Background: To enable printing of smaller feature sizes, lithography has progressed into the extreme ultraviolet lithography (EUVL) regime. Alongside the wavelength scaling, a reduction in resist film thickness (FT) is observed to avoid large aspect ratios that would lead to pattern collapse. The further progression to high numerical aperture (NA) EUVL will require a further reduction in resist FT moving toward an ultra-thin film regime (<30-nm resist FT). This reduction in resist FT will make the interfacial interactions between resist and underlayer more dominant, potentially influencing material behavior and making resist design challenging.Aim: We assess a reflow process as a means to investigate interfacial effects and in this way deconvolute the correlation between resist line volume, interfacial effects, and the reflow temperature (T R ), defined as the temperature at which the resist line starts broadening, which is indicative of the glass transition temperature (T g ).Approach: We pattern a model EUV chemically amplified resist at different nominal resist FTs and different critical dimensions (CDs) and half-pitch (HP) combinations to quantify changes in the T R . Results:The T R increases with the inverse of the CD, as well as the inverse of the resist FT. Moreover, the T R also scales with the area ratio (the ratio of the area in contact with the ambient to the area in contact with the underlayer).Conclusions: A linear relationship between T R and its volume factor (CD × FT) normalized for the area ratio (area in contact with the ambient to the area in contact with the underlayer) is found, revealing a combined dependency on line volume and interfacial interactions. This opens the potential for the use of the reflow methodology in investigating interfacial interactions.