Combination multiple focal planes and PSM for sub 120 nm node with KrF lithography: study of the proximity effects
In optical lithography, the use of multiple focal planes with different energy and focus improves the photolithography performances like the Depth of Focus (DOF) and the Energy Latitude (EL). We have chosen to use a symmetrical double exposure (symmetrical in focus) with equal energy, an attenuated (6%) phase shift mask and the standard KrF photolithography process in the study of 180 nm holes. The ASM 5500/700 and /900 steppers make this double exposure possible. The study of the process window versus the distance between the two focal planes (?F) shows that the multiple focal planes technique generates proximity effects namely increases the difference between dense holes Critical Dimension (CD) and isolated one for the same parameters (energy, focus and ?F). We study the evolution of these proximity effects for KrF lithography and propose solutions to minimise them.
psm krf proximity effects symmetrical double exposure contacts vias side lobes