The principle of evanescent interferometric lithography (EIL) is based on the nature of interference between evanescent diffracted orders, particularly close to a resonance of the grating that is illuminated with UV light (Blaikie 2001 ).
Fig. 17.17 compares the intensity distribution for two identical TM-illuminated chrome gratings. Both have a period of 270 nm and are imbedded in a medium with a refractive index n = 1.6. For this grating period and refractive index the cutoff for first order diffraction is at a wavelength of 432 nm, and this is the illumination wavelength chosen for Fig.17.16(a). In this case the ±1 diffracted orders, which propagate at close to a grazing angle, interfere to form a period-halved intensity distribution beneath the mask. Zeroth order diffraction is suppressed (the so-called Wood's anomaly) which results in a high contrast in the interference pattern. This near field interference mimics that obtained in conventional interferometic lithography, and the period halving effect is useful for giving a 2x reduction of the grating.
In Fig. 17.17(b) the illumination wavelength has been increased to 438 nm, for which the ±1 diffracted orders are now evanescent. No far field diffraction would be obtained for this grating, however a strong, high-contrast interference pattern is retained in the near field. The intensity and contrast in this interference pattern decay exponentially away from the mask; in this case the depth of field is more than 300 nm, which would be adequate for exposure of conventional resist layers. The most interesting point to note about the EIL exposure is the intensity enhancement that can be obtained. The peak near field intensity in Fig. 17.17(b) is more than five times the incident intensity, which would result in a significant lowering of exposure time compared with conventional interferometric lithography.
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