Proceedings Article | 9 November 2015
KEYWORDS: Polarization, Refractive index, Phase measurement, Near field, Absorption, Opacity, Silica, Photomasks, 193nm lithography, Semiconducting wafers, Diffraction, Phase shift keying, Critical dimension metrology, Airborne remote sensing, Finite-difference time-domain method, Panoramic photography, Binary data, Deep ultraviolet
Mask topography contributes to phase at the wafer plane, even for OMOG binary masks currently in use at
the 22nm node in deep UV (193nm) lithography. Here, numerical experiments with rigorous FDTD simulation
are used to study the impact of mask 3D effects on aerial imaging, by varying the height of the absorber stack
and its sidewall angle. Using a thin mask boundary layer model to fit to rigorous simulations it is seen that
increasing the absorber thickness, and hence the phase through the middle of a feature (bulk phase) monotonically
changes the wafer-plane phase. Absorber height also influences best focus, revealed by an up/down shift in the
Bossung plot (linewidth vs. defocus). Bossung plot tilt, however, responsible for process window variability
at the wafer, is insensitive to changes in the absorber height (and hence also the bulk phase). It is seen to
depend instead on EM edge diffraction from the thick mask edge (edge phase), but stays constant for variations
in mask thickness within a 10% range. Both bulk phase and edge phase are also independent of sidewall angle
fluctuation, which is seen to linearly affect the CD at the wafer, but does not alter wafer phase or the defocus
process window. Notably, as mask topography varies, the effect of edge phase can be replicated by a thin mask
model with 8nm wide boundary layers, irrespective of absorber height or sidewall angle. The conclusions are
validated with measurements on phase shifting masks having different topographic parameters, confirming the
strong dependence of phase variations at the wafer on bulk phase of the mask absorber.
