Scanning microscopes that would be based on microchannel filters and advanced electronic image sensors and that utilize x-ray illumination have been proposed. Because the finest resolution attainable in a microscope is determined by the wavelength of the illumination, the xray illumination in the proposed microscopes would make it possible, in principle, to achieve resolutions of the order of nanometers — about a thousand times as fine as the resolution of a visible-light microscope. Heretofore, it has been necessary to use scanning electron microscopes to obtain such fine resolution. In comparison with scanning electron microscopes, the proposed microscopes would likely be smaller, less massive, and less expensive. Moreover, unlike in scanning electron microscopes, it would not be necessary to place specimens under vacuum.

X-Rays From the Specimen would travel through the microchannels to the photodetectors. The microchannels would define resolution elements. In both the x and y directions, the microchannel plate and photodetectors would be scanned over one pixel pitch, in increments of the microchannel diameter, to acquire high-resolution specimen-image data.
The proposed microscopes are closely related to the ones described in several prior NASA Tech Briefs articles; namely, “Miniature Microscope Without Lenses” (NPO-20218), NASA Tech Briefs, Vol. 22, No. 8 (August 1998), page 43; and “Reflective Variants of Miniature Microscope Without Lenses” (NPO-20610), NASA Tech Briefs, Vol. 26, No. 9 (September 2002) page 6a. In all of these microscopes, the basic principle of design and operation is the same:

The focusing optics of a conventional visible-light microscope are replaced by a combination of a microchannel filter and a charge-coupled-device (CCD) image detector. A microchannel plate containing parallel, microscopic-cross-section holes much longer than they are wide is placed between a specimen and an image sensor, which is typically the CCD. The microchannel plate must be made of a material that absorbs the illuminating radiation reflected or scattered from the specimen. The microchannels must be positioned and dimensioned so that each one is registered with a pixel on the image sensor. Because most of the radiation incident on the microchannel walls becomes absorbed, the radiation that reaches the image sensor consists predominantly of radiation that was launched along the longitudinal direction of the microchannels. Therefore, most of the radiation arriving at each pixel on the sensor must have traveled along a straight line from a corresponding location on the specimen. Thus, there is a one-to-one mapping from a point on a specimen to a pixel in the image sensor, so that the output of the image sensor contains image information equivalent to that from a microscope.

The upper part of the figure depicts a one-pixel portion of a proposed scanning microchannel-type microscope that would utilize x-ray illumination. The lower part of the figure shows a simple square pixel pattern. The CCD could be coated with a phosphor to increase its response to x-ray photons. Provided that the x-ray wavelength was small enough, the diameter of the microchannel would define the resolution element. The microchannels would be much narrower than the CCD pixels. Preferably, the pixel pitch would be an integer multiple of the diameter of a microchannel. Hence, one would acquire a set of high-resolution image data by recording the CCD output while scanning (more precisely, stepping) the specimen under the microchannel plate in increments of the microchannel diameter along both perpendicular axes (x and y) of the pixel pattern.

This work was done by Yu Wang of Caltech for NASA’s Jet Propulsion Laboratory.

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Scanning Microscopes Using X Rays and Microchannels

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This article first appeared in the December, 2003 issue of NASA Tech Briefs Magazine.

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