An optical-heterodyne alignment technique for quarter-micron x-ray lithography

Abstract

A new optical-heterodyne interferometry alignment technique with diffraction gratings is developed for quarter-micron x-ray lithography. To obtain detection accuracy as good as a few tens of nanometers, a phase signal is utilized instead of a conventional intensity signal. The relative lateral displacement between mask and wafer is detected by measuring the phase difference between heterodyne beat signals generated by projecting two laser beams from + first-order and − first-order diffraction directions on the mask and wafer grating marks. The displacement signal is only slightly influenced by gap variation using symmetric optics. A lateral displacement detection resolution better than 10 nm is obtained by the experimental alignment setup. A nonsymmetric beam from the − third-order diffraction direction is added to the symmetric beams to detect the gap. The phase difference between two beat signals emitted to the second-order diffraction direction from the same mask and wafer marks is used as the gap detection signal. The cyclic gap signal makes it possible to set an arbitrary gap. A gap detection resolution of <20 nm is realized. Using this optical-heterodyne interferometry alignment method, a four-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.