Contact Lithography - Resolution Enhancements

Resolution Enhancements

As noted above, thinner photoresist can help improve image contrast. Reflections from the layer underlying the photoresist also have to be taken into account when absorption and evanescent wave decay are reduced.

The resolution of contact lithography has been predicted to surpass λ/20 periodicity.

The pitch resolution of contact lithography can be readily enhanced by multiple exposures generating feature images between previously exposed features. This is suitable for nested array features, as in memory layouts.

Surface plasmons are collective oscillations of free electrons confined to metal surfaces. They couple strongly to light, forming surface plasmon polaritons. Such excitations effectively behave as waves with very short wavelength (approaching the x-ray regime). By exciting such oscillations under the right conditions, multiple features can appear in between a pair of grooves in the contact mask. The resolution achievable by surface plasmon polariton standing waves on a thin metallic film is <10 nm with a wavelength in the 380-390 nm range using a <20 nm silver film. In addition, deep narrow slits in metallic transmission gratings have been shown to allow resonances that amplify light passing through the slits.

A layer of metal film, has been proposed to act as a 'perfect lens' for amplifying the evanescent waves, resulting in enhanced image contrast. This requires tuning the permittivity to have a negative real part, e.g., silver at 436 nm wavelength. The use of such a lens allows imaging to be achieved with a wide tolerance of distance between mask and photoresist, while achieving extreme resolution enhancement by use of surface plasmon interference, e.g., a half-pitch of 25 nm with 436 nm wavelength. The perfect lens effect is only effective for certain conditions, but allows a resolution roughly equal to the layer thickness. Hence a sub-10 nm resolution appears feasible with this approach as well.

The use of surface plasmon interference gives an edge over other lithography techniques, as the number of mask features can be much less than the number of features in the desired image, making the mask easier to fabricate and inspect. While silver is the most commonly used metal for demonstrating surface plasmons for lithography, aluminum has also been used at 365 nm wavelength.

While these resolution enhancement techniques allow 10 nm features to be contemplated, other factors must be considered for 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.