Brandon Chalifoux

Toward Large-Area Sub-Arcsecond X-Ray Telescopes

The future of x-ray astronomy depends upon development of x-ray telescopes with larger aperture areas (≈ 3 m2) and fine angular resolution (≈ 1″). Combined with the special requirements of nested grazing-incidence optics, the mass and envelope constraints of space-borne telescopes render such advances technologically and programmatically challenging. Achieving this goal will require precision fabrication, alignment, mounting, and assembly of large areas (≈ 600 m2) of lightweight (≈ 1 kg/m2 areal density) high-quality mirrors at an acceptable cost (≈ 1 M

High-precision figure correction of x-ray telescope optics using ion implantation

/m2 of mirror surface area). This paper reviews relevant technological and programmatic issues, as well as possible approaches for addressing these issues—including active (in-space adjustable) alignment and figure correction.

Shaping of thin glass X-ray telescope mirrors using air bearing slumping and ion implantation

Achieving both high resolution and large collection area in the next generation of x-ray telescopes requires highly accurate shaping of thin mirrors, which is not achievable with current technology. Ion implantation offers a promising method of modifying the shape of mirrors by imparting internal stresses in a substrate, which are a function of the ion species and dose. This technique has the potential for highly deterministic substrate shape correction using a rapid, low cost process. Wafers of silicon and glass (D-263 and BK-7) have been implanted with Si+ ions at 150 keV, and the changes in shape have been measured using a Shack-Hartmann metrology system. We show that a uniform dose over the surface repeatably changes the spherical curvature of the substrates, and we show correction of spherical curvature in wafers. Modeling based on experiments with spherical curvature correction shows that ion implantation could be used to eliminate higher-order shape errors, such as astigmatism and coma, by using a spatially-varying implant dose. We will report on progress in modelling and experimental tests to eliminate higher-order shape errors. In addition, the results of experiments to determine the thermal and temporal stability of implanted substrates will be reported.

Non-touch thermal air-bearing shaping of x-ray telescope optics

Air bearing glass slumping followed by ion implantation for fine figure correction constitutes a promising process for fabricating thin glass segmented mirrors for future high-resolution x-ray telescopes. We have previously demonstrated the feasibility of both air bearing slumping and ion implantation figure correction to produce mirrors with good figure and without introducing mid spatial-frequency errors or roughness. In this work, we describe a mechanically-robust slumping tool design that can be adapted to Wolter I mirror shapes; and we describe progress on understanding ion implantation for use as a figure correction process, by using in-situ curvature measurements in a tandem ion accelerator.

Thermal oxide patterning method for compensating coating stress in silicon x-ray telescope mirrors

Molding glass by using air bearings is a promising procedure for inexpensive and high precision glass shaping. Thin glass sheets are sandwiched between air bearings and pushed flat while being thermally cycled. In this study, a novel device for shaping glass is created and tested using 0.5 mm thick, 100 mm round, Schott D263 wafers. Numerous samples were shaped with varying values for bearing-to-glass gap and maximum temperature, and were measured with a Shack Hartmann metrology tool. Glass was shaped with bearing-to-glass gaps of >50 μm, 36±2.5 μm, and 30.5±2.5 μm. The best peak-to-valley (P-V) flatness achieved is 6.7/3.6±0.5 μm for front/back of the glass sheet, using a gap of 36±2.5 μm. The average steady-state P-V achieved is 12 μm. Using the same device parameters, the best repeatability achieved over the whole 100 mm wafer is 2.7±0.5 μm P-V and 9.5 arcseconds RMS slope error. When looking at 60 mm sections, the repeatability improves to <1 μm P-V and 5±0.5 arcsec.

Progress report on air bearing slumping of thin glass mirrors for x-ray telescopes

Segmented X-ray telescope mirrors fabricated from thin silicon substrates are being developed by a group at the NASA Goddard Space Flight Center for future generation telescopes such as the Lynx mission concept. The Goddard team has demonstrated high precision silicon mirrors with high angular resolution (~1’’) manufactured by a simple, low cost process. However, the required high-Z optical coatings on mirror front surfaces are difficult to deposit without significant compressive thin film stress, which threatens to distort mirrors and negate the benefits of the high quality substrates. Coating stress reduction methods have been investigated by several groups, but none to date have reported success on real mirrors to the required tolerances. In this paper, we report a new method for correcting mirrors with stress-induced distortion which utilizes a micro-patterned silicon oxide layer on the mirror’s back side. Due to the excellent lithographic precision of the patterning process, we demonstrate stress compensation control to a precision of ~0.3%. The proposed process is simple and inexpensive due to the relatively large pattern features on the photomask. The correction process has been tested on flat silicon wafers with 30 nm-thick chrome coatings under compressive stress and achieved surface slope improvements of a factor of ~80. We have also successfully compensated two iridium-coated silicon mirrors provided by the Goddard group. The RMS slope errors on coated mirrors after compensation were only degraded by ~0.06 arc-seconds RMS axial slope compared to the initial uncoated state.

Ion implantation for figure correction of thin X-ray telescope mirror substrates

The successful NuSTAR telescope was fabricated with thin glass mirrors formed into conic shapes by thermal slumping of thin glass sheets onto high precision mandrels. While mirrors generated by this process have very good figure, the best mirrors to date have a resolution limited to ~7 arc sec, due primarily to mid-range scale spatial frequency errors. These mid-range errors are believed to be due to clumping and particulates in the anti-stick coatings used to prevent sticking between mandrel and mirrors. We have developed a new slumping process which avoids sticking and surface-induced mid-range error by floating hot glass substrates between a pair of porous air bearing mandrels through which compressed nitrogen is forced. We report on the design and testing of an improved air bearing slumping tool and show results of short and long slumping cycles.

X-ray telescope mirror mounting and deformation reduction using ThermoYield actuators and mirror geometry changes

Figure correction of X-ray telescope mirrors will be critical for future missions that require high angular resolution and large collecting areas. In this paper, we show that ion implantation offers a method of correcting figure errors by imparting sub-surface in-plane stress in a controllable magnitude and location in Schott D-263 glass, Corning Eagle XG glass, and crystalline silicon substrates. In addition, we can in theory achieve nearly exact corrections in Schott D-263 glass, by controlling the direction of the stress. We show that sufficient stress may be applied to Schott D-263 glass to achieve figure correction in mirrors with simulated initial figure errors. We also report on progress of a system that will be capable of correcting conical shell mirror substrates.

Compensating film stress in silicon substrates for the Lynx X-ray telescope mission concept using ion implantation

Recently, the X-ray optics community has been developing technology for high angular resolution, large collecting area X-ray telescopes such as the Lynx mission concept. To meet the high collecting area requirements of such telescope concepts, research is being conducted on thin, segmented optics. The precision mounting posts that fixture and align segmented optics must be the correct length to sub-micron accuracy to satisfy the angular resolution goals of such a concept. Mirror distortion caused by adhesive shrinkage at mount points on the mirror surface also needs to be controlled to micron-radian tolerances. We report on two solutions to these problems. Set-and-forget adjustable length optical mounting posts have been developed to control mirror spacer length. The actuator consists of a metal cylinder with a cylindrical neck cut halfway along the length. To change the length of this actuator, an axial force is applied and the neck is momentarily heated to the plastic deformation temperature via resistive heating. All of the plastic deformation that occurs becomes permanent after cooling. Both compression and expansion of these actuators has been demonstrated in steps ranging from 6 nm to several microns. This paper will describe an experimental setup, show, and discuss data. Additionally, a stress relief technique to reduce mirror distortion caused by shrinkage of the adhesive bond to the actuator is proposed and demonstrated by modelling.

Ultrafast laser micro-stressing for correction of thin fused silica optics for the Lynx X-Ray Telescope Mission

Ion implantation is used to correct figure errors resulting from film stress in thin silicon mirror substrates. The Lynx mission concept requires mirrors with extremely small figure errors and excellent X-ray reflectivity, and only a small portion of the mirror error budget may be allocated to distortion from film stress. While reducing film stress in itself is ideal, compensation of film stress may be required. In addition, compensation, in combination with other film stress reduction techniques, may allow freedom in making coatings with optimal x-ray performance while minimizing distortion. Ion implantation offers a rapid method of applying a precise stress distribution to the backside of a mirror, which may be used to compensate for a uniform or non-uniform film stress. In this paper, we demonstrate the use of ion implantation to achieve a roughly 10x reduction in deformation from film stress, and that the stress from ion implantation is stable over at least five months.