From Flat Substrate to Elliptical KB Mirror by Profile Coating

Abstract

For microfocusing x‐ray mirrors, an elliptical shape is essential for aberration‐free optics. However, it is difficult to polish elliptical mirrors to x‐ray‐quality smoothness. Profile coatings have been applied on both cylindrical and flat Si substrates to make the desired elliptical shape. In a profile‐coating process, 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 substrate‐moving direction. The shape of the contour was derived from a desired profile and the thickness distribution of the coating material at the substrate level. The thickness distribution was measured on films coated on Si wafers using a spectroscopic ellipsometer with computer‐controlled X‐Y translation stages. The mirror coating profile is determined from the difference between the ideal surface figure of a focusing ellipse and the surface figure obtained from a long trace profiler measurement on the substrate. The number of passes and the moving speed of the substrate are determined according to the required thickness and the growth‐rate calibration of a test run. A KB mirror pair was made using Au as a coating material and cylindrically polished mirrors as substrates. Synchrotron x‐ray results using this KB mirror pair showed a focused spot size of 0.4 × 0.4 μm2. This technique has also been applied for making elliptical KB mirrors from flat Si substrates. The challenges and solutions associated with elliptical profile coating on flat substrates will be discussed.