Laser bandwidth and other sources of focus blur in lithography

It is well known that the refractive optics used in today's exposure tools are highly chromatic, meaning that small wavelength shifts will cause large focus shifts. Even a line-narrowed excimer laser has a large enough range of wavelengths that we can no longer think of an infinitely thin image plane. The concept of "focus blur" can be generalized to encompass the effect of laser bandwidth chromatic aberrations, vertical stage vibrations, and stage tilts, which cause the focus to change during the scan. We introduce a new parameter called mean absolute defocus that can characterize the focus blur and is shown to correlate with the lithographic effects. Focus blur can be incorporated into simulation models, in a manner similar to the way that stage vibration is modeled. New simulation results illustrate the impact of focus blur on modern lithographic processes. Process stability and machine-to-machine matchina issues are discussed. © 2006 Society of Photo-Optical Instrumentation Engineers.