Abstract
For microfocusing x-ray mirrors, an elliptical shape is essen- tial for aberration-free optics. However, it is difficult to polish elliptical mirrors to x-ray quality smoothness. A differential coating method to con- vert a cylindrical mirror to an elliptical one has been previously reported. The coating was obtained by varying the sputter source power over a moving substrate. Here we report a new method of profile coating: the sputter source power is kept constant, while the substrate is passed over a contoured mask at a constant speed to obtain a desired profile along the direction perpendicular to the direction of substrate motion. The shape of the contour depends on the desired profile and the thickness distribution directly above the gun at the substrate level. The thickness distribution was measured on films coated on Si wafers using a spectro- scopic ellipsometer with computer-controlled X-Y translation stages. A model was developed to fit the measured thickness distribution, which determines the relative thickness weightings. When the substrate moves during a deposition, the film thickness is proportional to the length of the opening on the shield can along the direction of motion. By equating the sum of relative weightings to the required relative thickness at the same position, the length of the opening at that position can be determined. By repeating the same process for the whole length of the required profile, a contour can be obtained for a desired thickness profile. The number of passes and the speed of the substrate are determined according to the required thickness and the growth-rate calibration of a test run. The mir- ror coating profile is determined from the difference between the ideal surface figure of a focus ellipse and the surface figure obtained from a long-trace profiler measurement on the substrate. A Kirkpatrick-Baez (KB) mirror pair was made using Au as a coating material and cylindri- cally polished mirrors as substrates. Synchrotron x-ray results using this KB mirror pair showed a focused spot size of 0.430.4 mm 2 . © 2003
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