A vertical stepper for synchrotron x-ray lithography

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Abstract
A newly developed synchrotron x-ray stepper installed at a port of an electron storage ring of the NTT Synchrotron Facility1–3 is described. The stepper exposes wafers in a normal atmosphere to vertically scanned synchrotron x rays with continuous alignment control throughout the exposure. The key devices of the stepper are a vertical x–y stage and an optical-heterodyne alignment system. The x–y stage fully utilizes air lubricated ceramic components including air bearing lead screws and achieves 5 nm motion precision over a 200 mm stroke. The optical-heterodyne alignment system detects both lateral and gap displacements between the mask and wafer gratings with accuracies of 10 and 100 nm, respectively. After wafer step positioning using a laser interferometer, alignment between the mask and wafer is executed using the optical-heterodyne signal. In several exposure experiments, alignment capability is estimated to be better than ±0.14 μm as a 3σ value of machine repeatability. Also, several applications to device fabrication confirmed that the vertical stepper is feasible in quarter- to half-micron x-ray lithography.

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