In x-ray lithography at deep submicron and sub-100-nm linewidths, the effects of diffraction are not negligible. We have investigated the possibility of improving the slope of the irradiance profile at feature edges by using an absorber that produces a pi-phase shift in addition to about 10 dB attenuation. Both numerical simulation and experimental modeling at a longer wavelength (365 nm) were used. These show that the irradiance profile at the edge of features is steeper when using a pi-phase shifting mask. For gold, the condition of pi-phase shift and 10 dB attenuation occurs when the wavelength is 1.15 nm and the thickness is 290 nm; and for tungsten at a wavelength of 1.3 nm and a thickness of 275 nm. X-ray masks made with the proper phase-shift and attenuation yield an increased slope at feature edges which should result in improved process latitude. The phase-shifting scheme introduced here differs from those previously described, and applies to patterns of arbitrary geometry, including isolated lines and spaces.
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Ku et al. (1988) studied this question.