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‘Double’ Displacement Talbot Lithography (D2TL) is proposed as a new, high-throughput and rapid tool that allows the fabrication of complex periodic nano- to micron patterns over a large area (up to 100 200 mm). This method is a combination of conventional Displacement Talbot Lithography (DTL) [1] and a planar displacement to make more complex shapes. The more straight-forward DTL technique utilizes the periodic diffraction pattern on all space axes that results when a periodic mask is illuminated by a planar wave (Talbot effect); by displacing the wafer along the axis of illumination over integer spatial periods, called the Talbot length, the low depth of field is overcome. In D2TL, by additionally moving the wafer perpendicular to the axis of illumination, the range and complexity of features possible across whole wafers is dramatically extended. Furthermore, a single periodic mask can be used to create a large number of different patterns by using different lateral displacements.
In order to demonstrate the capability of this technique, a number of proof-of-principle experiments have been performed using a hexagonal mask with a 1.5 micron pitch with a positive i-line resist on silicon wafers. Firstly, dual exposures before and after sub-micron, nm-precision, translations show the transition between coincident, merged and isolated features. Secondly, circular displacements of differing amplitudes were applied during single exposures, which led to a large variety of shapes being obtained: positive or negative honeycomb, large dot or large holes, rings and periodic cancelation of dot site with a change of pitch. These results compare well with simulations created using MATLAB that calculate the integrated intensity distribution for each lateral position in the photoresist. Importantly, the simulations take into account weaker, secondary features in the intensity map that result from unwanted constructive interference. The modelling allows greater insights into these secondary patterns, helping to diminish their impact through tuning the filling factor and pitch of the hexagonal mask.
The complexity of patterns that can be produced using this technique depends on the minimum feature size for a given periodicity and the relative intensity of any unwanted secondary features. Simulations of the intensity maps coupled with the development model of high-contrast, i-line resists show a strong influence on whether an amplitude or phase mask is used, with a strong non-linear dependence on pitch and filling factor of the mask. Specific optimum masks can be identified that offer the greatest opportunity for exploiting this new technique.
In summary, D2TL offers the possibility to create a multitude of patterns from a single mask by simply controlling the lateral displacement during or between successive exposures. Successful implementation of this technique requires an in-depth understanding of the DTL process and simulation tools to correctly account for proximity effects.
[1] H H Solak, C Dais, and F Clube, Displacement Talbot lithography: a new method for high-resolution patterning of large areas, Opt. Express 19, 10686-10691 (2011).
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