Patent Number: 051777746
Section: description

DETAILED DESCRIPTION OF THE INVENTION As previously indicated, the present invention is intended for use in the microscopy of biological elements and observation of very fine details of semiconductor elements, for example. However, its use is not meant to be so limited, and many other uses will occur to those of skill in the art. Various embodiments of the present invention are described below, all of which may be used in constructing commercial reflection soft X-ray microscopes. It is expected that such a refection soft X-ray microscope will permit the observation of surfaces of materials and thin (1-10 nm) layers near the surfaces with ten times higher resolution than that presently available from optical microscopes. The basis of the present invention is that reflection coefficients for soft X-ray radiation (around 10.0 to 20.0 nm) can be substantial, and significantly different for different materials. As described below, in the present reflection soft X-ray microscope, the source of soft X-ray radiation, and the detector or imaging system for the X-rays are on the same side of the specimen or object under observation. The detector or imaging system records only soft X-rays reflected from the surface or layers near the surface of the specimen or object under observation. In the preferred embodiment of the invention, as shown in FIG. 1, a soft X-ray generator 2, in this example a soft X-ray laser source 2 is shown, provides a soft X-ray beam 4 at an angle .alpha. of 30.degree. to 60.degree. to the horizontal plane of an object or specimen 6, for example. A Schwarzschild mirror condenser 8 is located between the source of soft X-rays 2 and the object or specimen 6 for focusing the soft X-ray beam 4 onto a predetermined region on the surface of the object 6, as shown. The condensed soft X-ray beams 10 illuminate a region under observation on object 6 and reflect off of this region as shown by the reflected soft X-ray beams 12. A Schwarzschild mirror objective 14 is placed at an appropriate angle to the normal and located for receiving the reflected soft X-ray beam 12, and focusing the same onto a detector 16 such as a CCD (charge coupled device) array or photographic film sensitive to soft X-ray beams, for forming an image. As would be known to one of skill in the art, the use of a CCD array for the detector 16 could provide electrical signals to a television system, for example, for permitting a television image to be produced of the region of the surface of the object 6 under observation. An optical microscope 18 is shown for permitting a user represented by eye 20 to visually align the object 6 for permitting observation of a desired region of the object. In certain applications, a bandpass filter 27 can be added between condenser 8 and object 6 for passing the soft X-ray beam 10 while substantially blocking other radiation (see below description for FIG. 5 for details of the filter). The Schwarzschild condenser 8 and objective 14, as tested in a laboratory configuration, are easy to match in that they have a common f or numerical aperture. Also, Schwarzschild mirrors are commercially available. Spatial resolutions of 0.2 to 0.5 micrometers have been demonstrated in a laboratory for such mirrors. In one experimental configuration for the present reflection soft X-ray microscope, Schwarzschild Optics manufactured by T. R. Optics, Ltd., of England, were used. The surfaces of the optics were highly polished for providing a surface finish of approximately 0.5 nm rms surface roughness. As shown in FIG. 1, the Schwarzschild mirrors 8 and 14 each include a convex mirror 24, and a concave mirror 22, which for the sake of simplicity is shown as two concave reflective surfaces 22. In the experimental system, the diameter of the convex mirror 22 was 66.3 millimeters, and its radius of curvature was 68.5 millimeters for example. The relatively smaller convex mirror had a diameter of 14.5 millimeters with a radius of curvature of 23 millimeters, for example. The Schwarzschild mirror systems 8 and 14 each had a numerical aperture of 0.4, and a focal length of 14.0 millimeters, in this example. The reflective surfaces of the concave mirror 22 and convex mirror 24 of the Schwarzschild configurations each included a multi-layer coating consisting of fifteen layer-pairs of molybdenum and silicon layers, having thicknesses of 3.0 nm and 9.0 nm, respectively. In the embodiment of the invention of FIG. 2, the Schwarzschild objective 14 is replace by an elliptical zone plate 28. With present technology, this embodiment of the invention is less preferred in that the elliptical zone plate is not commercially available and is presently a laboratory type device. Nonetheless, it is expected that the elliptical zone plate 28 will provide an effective objective for focusing the reflected soft X-ray laser beams 12 onto the detector 16, for providing good image resolution. In FIG. 3, a third embodiment of the invention includes an elliptical zone plate 30, and a Schwarzschild objective 14 as shown. As previously indicated, the elliptical zone plate 30 is a specialty item not commercially available at the present time. However, it is expected that the elliptical zone plate 30 will provide adequate condensing of the soft X-ray beam 4 for illuminating a predetermined region on the surface of the object 6. Another embodiment of the invention is shown in FIG. 4. In this embodiment elliptical zone plates are included for providing both the objective and condenser elements 28, 30, respectively. In FIG. 5, another embodiment of the invention includes an elliptical zone plate objective 28, and an ellipsoidal mirror 26, as shown. A filter 27, in this example an 80 nm thick aluminum filter with a coating of 10.0 nm of carbon film, is used in the soft X-ray laser beam path between mirror 26 and object 6 for blocking out VUV (vacuum ultraviolet), UV (ultraviolet) and visible light. In this example, an 18.2 nm soft X-ray laser beam was utilized, whereby the filter 27 had a transmission of about 56.0% at 18.2 nm. Accordingly, filter 27 acts as a bandpass filter for passing the soft X-ray laser beam, while blocking other radiation. If another type of laser source 2 was used having a wavelength other 18.2 nm, filter 27 would be adjusted for maximizing the transmission of the soft X-ray beam. In the configuration or embodiment of the invention shown in FIG. 6, the soft X-ray reflection microscope configuration includes an ellipsoidal mirror condenser 26, and a Schwarzschild objective 14, as shown. In this example, an 18.2 nm laser beam was aligned to a reflection object 6 via an ellipsoidal mirror 26. The reflection object 6 was constructed by evaporating gold onto a polished surface through a TEM number 200 grid. The angle of incidence of the 18.2 nm laser beam to the reflection object was limited by the vacuum chamber utilized to 70.degree., or an angle of 20.degree. between the reflection surface of object 6 and the incident beam 10. The image of the reflection object or grid on the surface of object 6 was recorded in this example on Kodak 101-07 X-ray film providing the detector 16. This configuration verified the operation of the Schwarzschild optics 14, in this example, using an 18.2 nm laser beam 4. A filter 27 is used as indicated above for this embodiment of FIG. 5. In the embodiments of the invention shown in FIGS. 5 and 6, each embodiment includes ellipsoidal mirror 26 for a condenser. Such a mirror has a small numerical aperture or a high f number. Note also that one would normally not use an ellipsoidal mirror on the imaging side, that is as an objective focusing means, due to the poor resolution of such mirrors resulting from their high f, and the present poor quality of optics available for such mirrors. Although various embodiments of the invention have been shown and described herein, they are not meant to be limiting. Different modifications may occur to those of skill in the art relative to the embodiments described herein, which modifications are meant to be covered by the spirit and scope of the claims appended hereto. For example, while the elliptical zone plates 28 and 30 in the embodiments of FIGS. 2 through 5 are described in the preferred, embodiment as being elliptical zone plates 28 and 30 can also each be circular zone plates. Also, future cavity based X-ray lasers may be sufficiently collimated so as to permit the elimination of condenser elements 8, 30 and 26 of FIGS. 1-6.