Optical diffraction-based methodology to measure on-product EUV exposure focus variations

Stochastic printing variations are a challenge for EUV lithography and it is well known that these variations worsen if exposed out-of-focus because the EUV image contrast degrades. The introduction of 0.55NA will improve image contrast at a reduced depth-of-focus. This paper will describe how best focus planes differences between features can be used to design focus-sensitive metrology targets that can report EUV focus if used in combination with an optical metrology tool. Moreover, the developed target methodology ensures design rules compliance. The focus metrology target concept is experimentally demonstrated using a 24nm pitch line/spacer in combination with a low-n EUV mask absorber material, metal-oxide-resist (MOR), and a 0.33NA EUV scanner. The observed focus variation is modeled to quantify how much content is correctable using scanner feedback. This illustrates that on-product focus metrology can improve focus performance if combined with advanced process control.