Optical alignment system for submicron x-ray lithography

Abstract

The alignment system described in this paper is applicable to all proximity mode replication systems. The inherent high accuracy of the method makes it particularly attractive for submicron lithography applications requiring positioning accuracy of better than 0.1 μm. The basic alignment mark on the mask is a one dimensional Fresnel zone whose primary focal length is equal to the proximity gap spacing. Helium–Neon laser illumination is used. Elimination of unwanted diffraction orders is achieved by incorporating a transverse grating structure on the alignment mark on the wafer. The alignment signal sensitivity to gap variation is such that it allows very precise setting of the gap spacing in an unambiguous fashion. Three pairs of such alignment marks are necessary to provide x–y–ϑ registration as well as gap spacing and parallelism settings. A key feature of the alignment system is the time varying incidence angle of the laser beam upon each Fresnel zone mark, which results in a small amplitude scanning of the wafer surface and allows continuous registration throughout the entire exposure duration. A description of the alignment system designed for a submicron x-ray lithography system is given, together with some preliminary results.

High-resolution proximity lithography for nano-optical components

“…This allows calibration-free measurement of the mask-to-substrate separation in a range of 10-400 lm with 100 nm accuracy and can be used for calibration of the length gauges at any time. The system described is less complex but offers similar measurement accuracies like the early approaches [107][108][109]. No pre-structured substrates are required.…”

“…Based on mutual interference of diffraction orders generated by gratings on both mask and substrate [107,108] or masks with Fresnel lenses having a retro-reflected focus on the substrate [109], gap accuracies in the order of 100 nm were demonstrated. These and also more recent refinements with e.g.…”

An optical alignment microscope for x-ray lithography

“…8 We begin with a heuristic discussion of the instrument function by considering the optics of Fig. In the first, the mask and wafer alignment marks are active components of the imaging system.…”

Moiré-Based Phase Imaging for Sensing and Adjustment of In-Plane Twist Angle

“…The early optical imaging method directly seeks the aligned points by geometrically referring two cross or bar like marks on wafer and mask through a microscope or graphic detector [8], the intensity based method detects the aligned position by means of locating the critical intensity value of reflected beam or symmetrically diffracted interfering beams from zone plates [9] or gratings [10], the optical heterodyne method associates the aligned state with the phase of a beat signal generated by interference of two-frequency laser beams. [11], [12], and the recent moiré imaging method encodes the aligned point into the spatial phase of moiré fringes [13]- [15].…”