Achieving zero stress in iridium, chromium, and nickel thin films

Broadway, David M.; Weimer, Jeffrey J.; Gurgew, Danielle; Lis, Tomasz; Ramsey, Brian D.; O’Dell, Stephen L.; Gubarev, Mikhail V.; Ames, A.; Bruni, R.

doi:10.1117/12.2180641

Cited by 16 publications

(16 citation statements)

References 8 publications

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“…However, the integrated stress changes as the deposited thickness changes, at a rate of −2.8 to −4 N∕m∕nm, depending on the deposition pressure. 6 This implies that a small variation in thickness across a mirror would result in varying integrated stress. For example, a AE1% thickness variation (as measured by Mori et al 9 ) on a nominal 15 nm film would result in a variation in integrated stress of AE0.4 to AE0.6 N∕m.…”

Section: Nonuniform Integrated Stressmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Chalifoux

Yao

Heilmann

et al. 2019

J. Astron. Telesc. Instrum. Syst.

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The Lynx X-ray Observatory concept, under study for the 2020 NASA Decadal Survey, will require a telescope with ∼2 m 2 of effective area and a point-spread function (PSF) with ∼0.5-arc sec half-power diameter (HPD) to meet its science goals. This requires extremely accurate thin grazing-incidence mirrors with a reflective x-ray coating. A mirror coating, such as 15-nm-thick iridium, can exhibit stress exceeding 1 GPa, significantly deforming segmented mirrors and blurring the PSF. The film stress and thickness are neither perfectly repeatable nor uniform. We use finite element analysis and ray tracing to quantify the effects of integrated stress inaccuracy, nonrepeatability, nonuniformity, and postmounting stress changes on segmented mirrors. We find that if Lynx uses segmented mirrors, it will likely require extremely small film stress (∼10 MPa) and nonuniformity (<1%). We show that realigning mirrors and matching complementary mirror pairs can reduce the HPD from uniform film stress by a factor of 2.3× and 5×, respectively. Doubling mirror thickness produces much less than the 4× HPD reduction that would be expected from a flat mirror. The x-ray astronomy community has developed numerous methods of reducing the PSF blurring from film stress, and Lynx may require several of these in combination to achieve 0.5 arc sec HPD using segmented mirrors.

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Section: Nonuniform Integrated Stressmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Chalifoux

Yao

Heilmann

et al. 2019

J. Astron. Telesc. Instrum. Syst.

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“…5 (2) the argon pressure to achieve a stress reduction of nearly three orders of magnitude in iridium films, while also maintaining the surface roughness within acceptable limits for soft x-ray reflectivity. 15 The stress evolution in polycrystalline metal films, such as iridium, can be associated with various features of film growth including nucleation, island growth, and island coalescence. The film stress reaches a tensile maximum during coalescences and then passes through zero stress before reaching a compressive steady state.…”

Section: Coatingsmentioning

confidence: 99%

Full-shell x-ray optics development at NASA Marshall Space Flight Center

Kilaru

Ramsey

Baumgartner

et al. 2019

J. Astron. Telesc. Instrum. Syst.

Self Cite

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NASA's Marshall Space Flight Center (MSFC) maintains an active research program toward the development of high-resolution, lightweight, grazing-incidence x-ray optics to serve the needs of future x-ray astronomy missions such as Lynx. MSFC development efforts include both direct fabrication (diamond turning and deterministic computer-controlled polishing) of mirror shells and replication of mirror shells (from figured, polished mandrels). Both techniques produce full-circumference monolithic (primary + secondary) shells that share the advantages of inherent stability, ease of assembly, and low production cost. However, to achieve high-angular resolution, MSFC is exploring significant technology advances needed to control sources of figure error including fabrication-and coating-induced stresses and mounting-induced distortions. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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“…According to Broadway et al [29] the sputtering pressure is a relevant parameter influencing the intrinsic stress of coating. Coating experiments were therefore performed using different sputtering pressures to investigate the influence of this parameter on the film properties.…”

Section: A Coating Methodsmentioning

confidence: 99%