In a proposed technique of maskless gray-scale x-ray lithography, a photoresist to be patterned would be exposed to a parallel beam of hard x-rays. As explained below, the photoresist would be translated across the beam at a varying rate to effect one-dimensional spatial variations in the radiation dose received by the photoresist. The technique would be particularly suitable for making diffraction gratings and similar items.
In gray-scale lithography in general, the radiation dose to a photoresist on a substrate is made to vary spatially, within a range in which the solubility of the exposed photoresist in a developer liquid varies with the dose. In customary gray-scale x-ray lithography, the required spatial variation in the dose is achieved by use of a mask. The mask and the photoresist-covered substrate are translated as a unit across an x-ray beam at a constant rate to obtain the required integrated dose to the mask.
The lithographically desirable characteristics of a parallel beam of hard x-rays include a large depth of field (typically characterized by image dispersion less than 1 µm over a depth of 15 mm) and negligible reflections from photoresist defects and surfaces. A parallel beam of hard x-rays (wavelengths < 10 Å) for use in the proposed technique could be generated by a synchrotron source in conjunction with a slit filter (typically 50 nm wide).
In the proposed technique, the photoresist would not be masked. The gradients in the radiation dose needed to obtain gradients in the density of the developed photoresist would be generated by controlled variations in the rate of translation of the x-ray beam across the photoresist. These controlled variations would suffice to define the desired features (variations of the height of the subsequently developed photoresist) to within submicron dimensions, within the 15-mm depth of field.
After exposure to x-rays, the photoresist would be developed in the customary manner. After development, the photoresist would be dried, giving rise to spatial consolidation of the photoresist into thickness gradients corresponding to the density gradients. The dosage gradients could be chosen to achieve desired final thickness gradients; for example, to produce triangular- or sawtooth-cross-section blazes for diffraction gratings. The large depth of field could be exploited to form such blazes on curved surfaces.
This work was done by Frank Hartley of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Manufacturing/Fabrication category.
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Maskless Gray-Scale X-Ray Lithography
(reference NPO20445) is currently available for download from the TSP library.
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