Coating thin mirror segments for lightweight x-ray optics

Abstract

Next generation's lightweight, high resolution, high throughput optics for x-ray astronomy requires integration of very thin mirror segments into a lightweight telescope housing without distortion. Thin glass substrates with linear dimension of 200 mm and thickness as small as 0.4 mm can now be fabricated to a precision of a few arc-seconds for grazing incidence optics. Subsequent implementation requires a distortion-free deposition of metals such as iridium or platinum. These depositions, however, generally have high coating stresses that cause mirror distortion. In this paper, we discuss the coating stress on these thin glass mirrors and the effort to eliminate their induced distortion. It is shown that balancing the coating distortion either by coating films with tensile and compressive stresses, or on both sides of the mirrors is not sufficient. Heating the mirror in a moderately high temperature turns out to relax the coated films reasonably well to a precision of about a second of arc and therefore provide a practical solution to the coating problem.

Iridium coatings for space based x-ray optics

“…Iridium thin films sputtered at lower total pressures (5.0·10 -3 mbar) depict in contrast high density and low surface roughness, but high intrinsic stress. Annealing those Ir films should involve a rearrangement of the Ir atoms, which should reduce the coating stress [35]. A further way is to compensate the deformation by applying the same coating on both sides of the mirror or by deposition of an additional layer with the opposite stress between the mirror substrate and the Ir coating.…”

Thermal oxide patterning method for compensating coating stress in silicon substrates

“…A group at GSFC tried to relax the coating stress after deposition by annealing at 350 °C, but they found that the residual RMS slope error of coated wafers after annealing could not be improved to better than 1-2 arc-seconds [8], which is not compatible with Lynx requirements. They also attempted to balance the compressive stress in the iridium film by depositing chromium film under tensile stress beneath the iridium, or by using atomic layer deposition to coat iridium films on both sides of the mirror, but stress nonuniformity resulted in a poor balance and a 1-2 μm mirror sag error which is far beyond the Lynx requirement [9]. Another group, at the NASA Marshall Space Flight Center (MSFC), monitored stress in situ during deposition onto 50 mm-diameter flat silicon wafers, and was able to reduce the stress in 15 nm-thick iridium films to ~3 MPa (~0.05 N/m of integrated stress, i.e., the stress in the film integrated over its thickness, which is equivalent to the mean film stress multiplied by the film thickness) [10], which may meet Lynx requirements [11].…”

Simulations of film stress effects on mirror segments for the Lynx X-ray Observatory concept

“…This paper is restricted to the investigation of film stress effects on the shape of segmented silicon mirrors. To produce an x-ray mirror segment, a silicon substrate must be fabricated 5 and then coated, typically with a thin metal film, such as iridium [6][7][8][9] or a multilayered coating, [10][11][12] to efficiently reflect x-rays. Coatings are often deposited using magnetron sputtering, usually to maximize x-ray reflectivity, which results in a stressed film that deforms the thin mirror substrates, especially segmented mirrors.…”

“…One approach is to use deposition conditions that result in minimum integrated film stress while not significantly degrading x-ray reflectivity. 6,11,17,18 Depositing the same film (or a different film with the same integrated stress) on both sides of the mirror 8,9,19 enables a wider range of deposition conditions to optimize reflectivity and can also result in a thermally balanced mirror. The ability of either of these methods to solve the film stress problem may be limited by uniformity of the integrated stress, since thickness uniformity alone is typically around ±1%, 9 and stress may also vary over the mirror surface.…”

“…Annealing can reduce, but may be incapable of eliminating, the iridium film stress, and also results in some nonuniformity. 8 Several approaches are under development to provide nonuniform integrated stress compensation (NISC), such as silicon oxide patterning, 20,21 ion implantation, 22 active mirrors, 3 differential deposition, 23,24 substrate biasing during deposition, 25 magneto-strictive films, 26 and laser microstressing. 27 As of this writing, silicon oxide patterning has been the only method applied to x-ray mirror segments to compensate for nonuniform film stress.…”