Thermal conductivity of underlayers for EUV lithography and its effect on sensitivity of metal oxide resist

One running hypothesis is that the dose shift of metal oxide resists in extreme ultraviolet lithography is caused by the thermal insulation effect of the underlayers that hinders the thermolysis. To prove or disprove this hypothesis we prepared a series wafers with varying dielectric materials and thickness as stack, and compared the experimental dose to gel of the metal oxide resist with the total thermal resistance of those stacks. As the thickness of these “back end of line” dielectrics was below sub-100nm, their thermal resistance becomes vanishingly small, which poses a metrology challenge. We used steady state thermoreflectance, a highly sensitive thermo-optical technique, to characterize the thermal properties of these films and stacks and to make a model for the temperature drop across the photoresist film. It was found that the temperature drop is smaller than 1 °C and the thermal hypothesis can be therefore rejected.