Phase-Shift Mask Issues for 193 nm Lithography

As feature sizes below 0.25 micron are pursued, it becomes apparent that there will be few lithographic technologies capable of such resolution. Of these, deep-UV lithography at 193 urn is being investigated, which may prevail over X-ray lithography in terms of manufacturability. Furthermore, through the use of image enhancement techniques such as phase-shift masking, 193 rim lithography may dominate for feature resolution below 0.20 micron. This paper presents results from investigations into phase-shift mask issues for 193 nm excimer laser lithography. A small field refractive projection system for operation at the 193 .3 nm wavelength of a spectrally narrowed ArF excimer laser has been constructed for lithographic research. The small field, 20X system operates with a variable objective lens numerical aperture from 0.30 to 0.60, variable partial coherence, and control over illumination fill. Through the use of attenuated and alternating phase-shifting techniques resolution can be pushed to the 0.2 micron range with depth of focus as large as 2 microns. Problems do arise, though, as these techniques are applied to such short wavelengths of an excimer laser. Sensitivities to shifter deviations and resist interaction increase. Shifter etch influences on fused silica surface characteristics need to be addressed. Transmission effects of attenuating materials becomes increasingly important. Resist imaging and simulation results presented will shed some light on the potential of phase-shift masking for 193 nm lithography, along with inherent difficulties