光ヘテロダイン干渉式ギャップ検出法

Abstract

A new gap detection technique is developed for quarter micron X-ray lithography, whitch technique depends on an optical-heterodyne interferometry with diffraction gratings. To obtain detection accuracy as good as a few tens of nanometers, a phase signal is utilized instead of a conventional intensity signal. The gap between mask and wafer is detected by measuring the phase difference between two heterodyne beat signals generated by projecting two laser beams from-1st-order and-3rd-order diffraction directions on the mask and wafer grating marks. The gap signal with a few tens of microns cycle makes it possible to set an arbitrary wide range gap. The gap can be detected in resolution of less than 20 nm by an experimental setup. Combining this gap detection method with the relative position detection technique using symmetric optics, a 4-channel alignment system is developed for synchrotron X-ray lithography. Six-axis alignment servo control is established by combining this system with highly accurate stages. The total overlay accuracies of ± 0.05 μm (3σ) are achieved.