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
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Overview
The document presents a technical support package on "Maskless Gray-Scale X-Ray Lithography," developed by Frank T. Hartley at NASA's Jet Propulsion Laboratory. This innovative technique addresses challenges in traditional lithography by utilizing a maskless approach to achieve sub-micron feature definition over large depths of field.
The core of the method involves the use of a parallel beam of hard x-rays, which are characterized by their large depth of field and minimal reflections from photoresist defects. This allows for precise control over the exposure of photoresist, enabling the creation of complex structures without the need for differential density masks, which are difficult to fabricate. The x-ray exposure is achieved by translating the mask and photoresist-covered substrate across the x-ray beam at a constant rate, generating controlled variations in radiation dose. This results in gradients in the density of the developed photoresist, allowing for the formation of features with varying thicknesses, such as triangular or sawtooth cross-sections for diffraction gratings.
The document highlights the advantages of using x-ray lithography, particularly its ability to maintain sub-micron feature definition over significant variations in topography. The technique is designed to avoid issues associated with traditional optical lithography, such as standing wave interference and reflections from non-planar surfaces, which can degrade feature quality.
Electrophoretic photoresist depositions are also discussed, emphasizing their self-limiting nature, where the insulating properties of the polymer prevent further deposition after a certain thickness is achieved. This method allows for uniform conformal coatings on diverse surfaces, enhancing the versatility of the lithography process.
Overall, the document outlines a significant advancement in lithographic techniques, providing a solution for creating high-precision microstructures in a more efficient and effective manner. The work was conducted under NASA's contract and aims to push the boundaries of current lithography capabilities, making it a valuable contribution to the field of microfabrication and materials science.
