Patent Number: 053717740
Section: description

DETAILED DESCRIPTION FIG. 5 shows X-ray beamline apparatus in accordance with the present invention, and uses like reference numerals from incorporated U.S. Pat. No. 5,031,199 where appropriate to facilitate understanding. X-ray beamline apparatus 30 receives synchrotron radiation from a synchrotron 31 and delivers an X-ray beam to a target apparatus 32, such as a holder and stepper for mounting semiconductor wafers which are to be exposed by the X-ray beam. The path of the X-ray beam is indicated by the dashed line 34, which path is fully enclosed by enclosing structure, the interior of which is evacuated to a very low pressure. The enclosing mechanical structure for the beamline is preferably constructed to isolate the major components of the system to allow their removal and replacement as modules, and for this purpose gate valves 35 are interposed between the major components. The major components include a fast closure valve 37 which is mounted adjacent the synchrotron and is capable of closing rapidly to isolate the synchrotron from any failure of the vacuum in the beamline system. The beam of synchrotron radiation 34 exiting the synchrotron 31 enters mirror enclosure 38 which mounts mirror 102 and is evacuated by a pump 41. The beam 34 is reflected by mirror 102, to be described, to a focused beam which is then intercepted by a flat scanning mirror 47 mounted within a containment structure 48 and evacuated by a pump 49. The flat mirror 47 is mounted to pivot about a pivot point 50 and is driven upwardly and downwardly at its opposite end by a driveshaft 51 and a linear actuator 52. In another embodiment, FIG. 6, scanning mirror 47 is eliminated, and mirror 102 is mounted to pivot about a pivot point 50a and is driven upwardly and downwardly at its opposite end by a drive shaft 51a and a linear actuator 52a. The embodiment of FIG. 6 is preferred because it eliminates a reflection surface (at mirror 47) and the loss inherent therein. The beam then proceeds down the remaining components of the beam path, which preferably include additional vacuum pumps 54, an acoustic delay line 55, a diagnostic mirror assembly 56, and a shutter assembly 57, finally reaching the mounting assembly 58 for the exit window 59. The window 59 closes the exit of the beamline to seal the interior of the beamline apparatus from the atmosphere so that it may be evacuated down to an ultrahigh vacuum commensurate with that within the synchrotron 31. FIGS. 7 and 8 are top and side views, respectively, of the imaging performed by mirror 102. The high energy electrons orbiting in the synchrotron ring emit broadband X-rays as they are bent by the magnetic field. A fan of X-rays 22, FIG. 2, are emitted tangent to the path of the electrons 21. The synchrotron X-ray source typically has a horizontal divergence of 35 milli-radians and a vertical divergence of 1 milli-radian. The beamline should be capable of accepting all the vertical radiation and as large a horizontal fan as the bending magnets allow. Condensing mirror 102 collects the X-ray radiation from the synchrotron source. The chosen horizontal collection angle is 30 milli-radians, and the vertical collection angle is 2 milli-radians. The distance from source 31 to the center of mirror 102 is 2.5 meters. The distance from image 32 to the center of mirror 102 is 10 meters. The incident angle is 88 degrees to normal of the mirror. The function of a condenser mirror is to focus the synchrotron radiation beam vertically while imaging horizontally to a line, in a non-stigmatic imaging system. The mirror must have different focusing power along two directions. Spherical surfaces have the same curvature along any direction, and thus do not provide enough freedom. Toroidal and elliptical surfaces can have different focusing power along two orthogonal directions. For instance, the equation for a torus as expressed in Cartesian coordinates is EQU x.sup.2 +[(y.sup.2 +z.sup.2).sup.1/2 -(R-r)].sup.2 =r.sup.2(1) where R and r are radius of curvature along major and minor axes, respectively. R and r are the only variables which are determined by the horizontal collimating and vertical focusing condition. However, at grazing incidence conditions the off-axis aberrations cause the line image to be strongly curved, i.e. bent. That is, the imaging property varies with aperture. Unless the curvature of the surface is varied with aperture as well, there is no freedom to correct these unwanted aberrations. Because of the small size of electron trajectory, the synchrotron radiation source acts as a point source. The present invention utilizes a single surface to correct aberrations, which surface is aspherical, and has symmetry only about a plane, without axial symmetry. X-ray source 31 emits X-rays along a y axis, FIG. 9, and diverging along x and z axes. The x, y and z axes are orthogonal to each other. Mirror 102 has a reflecting surface asymmetrical about the y axis and having different focusing power in directions along the x and z axes in an imaging plane orthogonal to the y axis and spaced from the mirror along the y axis such that the mirror reflects and focuses the X-ray beam to a point along the y axis in the imaging plane and to a line, FIG. 7, along the x axis in the imaging plane. The reflecting surface of condenser mirror 102 is generated numerically. The optical path function F from source A to image B is determined by determining the optical path function from source A to the mirror P, and from the mirror P to the image B, according to EQU F=AP+PB=((x.sub.A -x).sup.2 +(y.sub.A -y).sup.2 +(z.sub.A -z).sup.2 ).sup.1/2 +((x.sub.B -x).sup.2 +(y.sub.B -y).sup.2 +(z.sub.B -z).sup.2).sup.1/2 where x.sub.A is the position of the source A along the x axis, y.sub.A is the position of the source along the y axis, z.sub.A is the position of the source along the z axis, x.sub.B is the position of the image along the x axis, y.sub.B is the position of the image along the y axis, z.sub.B is the position of the image along the z axis, x is the position on the mirror along the x axis, y is the position on the mirror along the y axis, and z is the position on the mirror along the z axis, such that A(x.sub.A, y.sub.A, z.sub.A) is the source position, B(x.sub.B, y.sub.B, z.sub.B) is the image position, and P(x, y, z) is the position on the mirror. The partial derivatives of the optical path function F, .differential.F/.differential.x, .differential.F/.differential.y, .differential.F/.differential.z, have the significance of angles. The displacement of reflected rays from the image line along the x axis is zero, .DELTA.x=0, if .differential.F/.differential. x=0. The displacement of reflected rays from the image line along the y axis is zero, .DELTA.y =0, if .differential.F/.differential.y=0. The displacement of reflected rays from the image line along the z axis is zero, .DELTA.z=0, if .differential.F/.differential.z=0. A computational mesh or grid is set up on the reflecting surface having a plurality of grid points, and, upon determining the noted partial derivatives, the slope at the grid points is modified such that the displacement of the reflected rays from the image line vanishes. This tends to make the mirror surface discontinuous. The discontinuities are eliminated by varying surface conditions in z to smooth the surface. The surface is modified recursively until it provides a smooth line image in the image plane along the x axis. The surface is modified recursively according to ##EQU2## where n is the polynomial order, and c is the coefficient of the polynomials. The higher the order, the better the quality, though with the finite size of the mirror, very high orders are usually not necessary. FIG. 10 shows one preferred surface, where the units along the axes are in meters. FIG. 11 shows ray tracing (units in meters) for the mirror of FIG. 10, using the "SHADOW" ray tracing program noted in incorporated U.S. Pat. No. 5,031,199. FIG. 12 shows ray tracing (units in meters) for an unmodified toroidal mirror with major radius R=114.6 meters and minor radius r=0.168 meters. The lower left plot in FIGS. 11 and 12 is x versus z (image), the upper left plot is x versus x', the upper right plot is z' versus x', and the lower right plot is z' versus z, where x', y', z' are the components of the ray direction unit vector, called direction cosines, where EQU (x.sup.').sup.2 +(y.sup.').sup.2 +(z.sup.').sup.2= 1. In the present embodiment, y' is close to 1, and x' and z' are small. The horizontal divergence of the ray is x', and the vertical divergence is z'. Of significance is the change of scale in z in FIGS. 11 and 12. The bending in the z direction is about seventy times smaller in FIG. 11 than in FIG. 12, i.e. only about 0.3 millimeters over a 60 millimeter field. The above modeling is done assuming a point source. FIG. 13 shows the ray tracing result (units in meters) using actual synchrotron radiation source parameters, namely Aladdin at the University of Wisconsin, providing an X-ray lithography beamline image in the 25 millimeter by 50 millimeter exposure field. The image is fairly close to a horizontal line, which is the goal. This proves that the point source model is a good approximation. In proximity lithography, the finite gap between the mask and wafer will cause an overlay error whenever the exposure beam has a variation in incident angle across the whole exposure field. In FIG. 11, the horizontal divergence dx is 1 milli-radian and it has a linear relationship with respect to horizontal position x, which causes a slight but linear displacement, i.e. "runout", which is about 10 nanometers for .+-.25 millimeter wide field and a 20 micrometer gap. Vertical divergence dz causes a distortion of 15 nanometers at the edge of the field. There is also image blur of 0.5 milli-radians. Exposure is accomplished by scanning the line shape X-ray beam over the range of the wafer, preferably by oscillating mirror 102, FIG. 6. The horizontal intensity distribution in the image produced by mirror 102 should not change with scanning. The change in overall reflectivity can be easily compensated by tuning the scanning speed. For a mirror exposure stage distance of 10 meters, to pattern a one inch wafer, the scanning angle is .+-.1.3 milliradians. Thus, the vibrating range of the mirror is .+-.0.65 milliradians. Mirror 102 provides a uniform exposure X-ray beam within this scan range, as shown in FIG. 14 (units in meters) which shows the image shape at the marginal scanning position corresponding to the 25 millimeter vertical field of FIG. 13. At the marginal scanning position, the image keeps the same shape and the vertical divergence is only shifted by 1.3 milliradians, which corresponds to a 26 nanometer "runout" at the edge of a 1 inch vertical field for the same 20 micrometer gap. FIG. 15 shows X-ray power density in arbitrary units versus horizontal image width in meters, and illustrates power density uniformity of the condenser mirror. There is less than a 4 percent variation in power density across the 1 inch scanning range. This is an improvement over multiple mirror systems. The change of X-ray power between different scan positions can be easily corrected by adjusting the speed of scanning, if desired. FIG. 16 shows ray tracing plots (units in meters) for the two mirror system of incorporated U.S. Pat. No. 5,031,199, and FIG. 17 shows the plot (units in meters) for the single aspherical mirror of the present invention, using the synchrotron source. The ray tracings show that the image characteristics are comparable. The horizontal and vertical divergences are smaller in FIG. 17 than FIG. 16, though the focus is widened by a small amount due to the finite size of the synchrotron source. Condenser mirror 102 forms a focus line 50 millimeters long with only 0.3 millimeter bending, which is 70 times less than the bending produced by a single toroidal mirror. The condenser mirror produces uniform exposure over a 50 millimeter field .+-.4 percent, which is comparable to the two mirror system of incorporated U.S. Pat. No. 5,031,199. The loss of X-ray flux for the present single aspherical mirror is only half that of the two mirror system. As in incorporated U.S. Pat. No. 5,031,199, small corrections in power non-uniformity can be compensated by using thin filters with tapered thickness profiles for the exit window. Corrections that rely on the absorptive characteristics of various thicknesses of filters will have an impact on the shape of the transmitted spectrum. Thus, starting with an optical system that delivers an extremely uniform power distribution and spectral response, as does both the two mirror system of incorporated U.S. Pat. No. 5,031,199 and the single aspherical mirror system of the present invention, carries with it less need to clean up the image afterwards. As noted, correcting the residual non-uniformities in the power at the image can be accomplished using filters or windows with tailored thickness profiles. As long as the required tailoring is slight, the attenuation produced will not affect the transmitted spectra. The procedure requires first mapping the power-density at the exposure-field. This can be accomplished by rastering any suitable detector across the exposure field. Knowing that the attenuation produced by an absorbing material is related to the thickness of the material by I=I.sub.o (e.sup.-.gamma.t), on can easily calculate the additional absorber thickness required to attenuate the beam to the desired intensity, I. The analysis then provides an `additional thickness` map. Various techniques are commonly employed during vacuum deposition to provide coatings with varying thickness profiles; such techniques can be as simple as positioning one part of the substrate closer to the source than the other, or as elaborate as utilizing a planetary motion system and a series of baffles. Another approach to improve the uniformity in the horizontal direction is to use a scanning thin wire mesh positioned vertically across the beam. A feedback system can be used to monitor the X-ray intensity at the wire (e.g. by measuring the photo-current) and used to control the wire scanning speed (V(x)) according to the equation: ##EQU3## where .DELTA. is the width of the wire, T.sub.o is the scanning time period. I.sub.o is the current intensity, and .delta.(x) is the intensity variation added to I.sub.o, FIG. 18. In this way a beam of high uniformity can be delivered. It is recognized that various equivalents, alternatives and modifications are possible within the scope of the appended claims.