Abstract
Future space telescopes, especially X-ray telescopes, will require thin mirrors to achieve high optical throughput. Thin mirrors are more difficult to fabricate than thick mirrors, but recent advances have made accurate fabrication of thin mirrors possible. However, mirrors must have a reflective coating, which typically has non-repeatable and non-uniform intrinsic stress that deforms a thin mirror. Reducing coating stress by controlling deposition parameters typically reduces reflectivity. Non-uniform integrated stress compensation (NISC) methods, in which spatially controlled stress is applied to the mirror substrate backside to balance the frontside coating stress, decouple the film stress from the reflectivity. Ion implantation is one NISC method, where high-energy ions are implanted into a glass or silicon substrate to generate stress near the substrate surface. In this paper, we demonstrate the use of ion implantation for stress compensation of 30 nm thick chromium films applied to the front of five silicon wafers. The reflective films have mean integrated stress between -8 and -35 N/m, which cause deformations between 400 and 1600 nm RMS. We demonstrate that these wafers can be restored to the pre-coating shape to within 60 nm RMS, in most cases within 1/20th of the coating deformation.
Highlights
For optical applications where mass is constrained, such as in space telescopes, the use of thin mirrors may allow larger collecting area and/or lower cost than thick mirrors
We demonstrate that ion implantation is capable of reducing film stressinduced deformation by a factor of about 20 on flat wafers
We demonstrate the use of ion implantation for film stress compensation by restoring five flat wafers, coated with stressed chromium films, close to their pre-coating shape
Summary
For optical applications where mass is constrained, such as in space telescopes, the use of thin mirrors may allow larger collecting area and/or lower cost than thick mirrors. Relying on in situ curvature measurement, Broadway et al have deposited an iridium film with integrated stress of −0.05 N/m, and 5 Å roughness [6] This approach may meet Lynx requirements if it can be applied to curved mirrors with similar results. In addition to ion implantation, there are several non-uniform integrated stress compensation (NISC) methods under development, which are intended to balance a nonuniform film stress. These methods include silicon oxide patterning [17,18], active mirrors [19], differential deposition [20,21], substrate bias during deposition [22], magneto-strictive films [23], and laser micro-stressing [24]. Further improvement to the accuracy of ion implantation may be possible
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