Comparison of tilting and piston mirror elements for node spatial light modulator optical maskless lithography
Depictions of two types of SLM mirrors, tilt (a) and piston (b). Piston mirrors may be simpler to reduce in size because reducing a tilt torsion arm increases the force required to tilt the mirror more quickly than a reduced piston bending arm. The piston bending member may also be placed under other mirrors and oriented in any direction below the mirror plane.
This aerial image plots the simulated light intensity at a wafer surface in arbitrary energy units vs position in nm. The simulations are of a pair of lines of tilt mirrors illustrated in the inset for a NA of 0.9, of 0.3, and pixels. The plot shows a series of 5 overlapping curves, approximating each tilt mirror by constant phase segments of 2, 4, 8, 16, and 32 divisions. Two segments are enough to adequately model a tilt mirror system. The schematic in the inset shows an example of a single tilt mirror divided into 4 phase segments.
Aerial images of dense and isolated lines, imaged by tilt mirrors with pixel sizes of , with and annular illumination with . Even with extreme off-axis light, contrast of both features is poor and there is no threshold intensity where both can be printed simultaneously.
Aerial images of the same features as in Fig. 3, except (immersion lithography) and . Contrast is improved relative to the images in Fig. 3 and a common threshold level can be used to print all features.
Illustrations of three mirror arrangements. In (a), a line is defined by two rows of tilt mirrors where the tilt axis is perpendicular to the line direction. The schematic below it shows a cross section through one of the rows. In (b), an equivalent piston mirror arrangement is shown. The schematic below it shows that if the tilt mirror approximation is applied to the cross section in (a), the mirror halves can be regrouped to form the piston cross section. In (c), a piston arrangement is shown that forms an identical aerial image to that in (b), but only at best focus.
Aerial image of the tilt mirrors in Fig. 5(a). The solid curve is at best focus, the dashed line is at defocus, and the dotted–dashed line is at defocus. The tilt mirrors are set to a phase shift at their tips.
Aerial image of the piston mirrors in Fig. 5(b). The solid curve is at best focus, the dashed line is at defocus, and the dotted–dashed line is at defocus. The pistons are set at phase shift relative to the surrounding mirrors.
Aerial image of the piston mirrors in Fig. 5(c). The aerial image at best focus is identical to the arrangement of Fig. 7. However, at and of defocus (dashed and dashed–dotted lines), the feature appears to shift in position. This problem is avoided in the configuration in Fig. 5(b), where the sign of the phase of each mirror in a row is alternated.
Schematics of mirror arrangements and aerial images of two configurations of alternating aperture phase shifting with and conventional illumination with . By adding a “dark-center” in the arrangement on the right, contrast and feature size control through focus are improved.
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