practical implementation. The most fundamental limitation appears to be photoresist roughness, which becomes predominant for shorter sub-wavelength periods where only the zeroth diffraction order is expected to propagate. All the pattern details are in this case conveyed by the evanescent waves, which decay more rapidly for finer resolution. As a result, the photoresist's inherent roughness following development can become more significant than the pattern. Defect and contamination issues As with any technology that relies on surface contact, defects are a strong concern. Defects are particularly detrimental to contact lithography in two respects. First, a hard defect can widen the gap between the mask and the substrate. This can easily cause images based on evanescent waves or surface plasmon interference to disappear. Second, smaller, softer defects attached to the metal surface of the mask may not disturb the gap but can still alter the evanescent wave distribution or destroy the surface plasmon interference condition. Oxidation of the metal surface also destroys plasmon resonance conditions (as the oxide surface is not a metal). References Photographic processes