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The manufacturing of lightweight silicon x-ray mirrors requires the application of a low stress thin film coating to the optical reflecting surface to achieve high performance. Coating of high-density materials such as iridium, is necessary to increase reflectivity at high energies above 4 keV, but presents many challenges, primarily the large distortions to the thin silicon substrates that occur from these highly stressed layers. The ideal solution to this problem is to perfectly balance the thin film stress on the front reflective surface with an equal layer on the backside of the mirror.
Two approaches will be discussed in this paper. First is magnetron sputtering of thin film iridium using Ion beam figuring (IBF) of a thick silicon oxide layer on the back surface derived algorithmically from optical metrology data to compensate the residual stresses from the imperfections of the sputter coating process. Second is the application of thin coating using atomic layer deposition (ALD), a process that is uniform at the atomic layer. Results of experiments from both processes will be presented, showing that either process is suitable for future x-ray telescopes, with the ALD process being preferable for its consistent higher performance and simplicity.
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