1/8 MU-M OPTICAL LITHOGRAPHY

The critical bottleneck to extending optical lithography down to the 1/8 mum level is the performance of the projection optics. The Markle-Dyson configuration is virtually free of all geometric and chromatic aberrations. A prototype system has been constructed and characterized. The system uses 248 nm light from a mercury arc lamp at a numerical aperture of 0.7. 0.25 mum resolution has been demonstrated with non phase shifting masks: using phase shifting Levenson-type masks, a grating consisting of 0.125 mum lines and spaces has been printed. Two possible extensions of the existing design are proposed which would allow general 1/8 mum geometries to be patterned. The first is a 0.7 numerical aperture (NA) system working at a wavelength of 157 nm, and the second is a 1.05 NA immersion system working at 193 nm. At these high NAs the depth-of-focus (DOF) of the image becomes very small if a clear aperture is used. However, if the aperture is apodized, the DOF can be increased considerably, and a procedure for optimizing apodization functions has been devised. This makes it possible to consider high NA optical techniques as candidates worthy of further investigation for 1/8 mum lithography.