The surface figure of x-ray mirrors can be improved by differential deposition of thin films. To achieve the required corrections, WSi2 layers of variable thickness were deposited through beam-defining apertures of different openings. The substrates were moved in front of the particle source with specific velocity profiles that were calculated with a deconvolution algorithm. Two different DC magnetron sputter systems were used to investigate the correction process. Height errors were evaluated before and after each iteration using off-line visible light surface metrology. Four 300mm long flat Si mirrors were used to study the impact of the initial shape errors on the performance of the correction approach. The shape errors were routinely reduced by a factor of 20 to 30 down to levels below 0.5nm RMS.
The surface figure of an x-ray mirror was improved by differential deposition of WSi2 layers. DC magnetron sputtering through beam-defining apertures was applied on moving substrates to generate thin films with arbitrary longitudinal thickness variations. The required velocity profiles were calculated using a deconvolution algorithm. Height errors were evaluated after each correction iteration using off-line visible light surface metrology. WSi2 was selected as a promising material since it conserves the initial substrate surface roughness and limits the film stress to acceptable levels. On a 300 mm long flat Si mirror the shape error was reduced to less than 0.2 nm RMS.
The process of differential deposition is currently applied at the ESRF in order to correct figure errors of x-ray optics substrates, prior to multilayer deposition. The substrate is moved at a controlled speed in front of a sputtering source to precisely control the deposition profile. This work will describe the concept of differential deposition at the ESRF as well as recent results of its implementation to correct a real mirror substrate surface. Finally, initial studies using a synchrotron beamline characterization technique based on x-ray total reflection are presented.
The figure errors of an x-ray mirror were reduced by differential deposition of C/Pt layered structures. Different apertures were inserted into the particle beam to correct height errors on variable length scales down to less than 10 mm. The required velocity profile was calculated using a deconvolution algorithm. The film thickness profiles were measured directly by xray reflectivity. Height errors were evaluated using visible light surface metrology. The results of these different techniques are compared and discussed. After two iterations the shape error of a 300 mm long flat Si mirror was reduced by a factor of 5 to less than 1 nm RMS. This work describes the experimental techniques and discusses the achieved accuracy. It also addresses open questions such as roughness evolution, layer stress, and the interpretation of metrology data.
Differential deposition techniques were applied to reduce the figure error of x-ray mirrors. Cr layers were sputtered on flat substrates that were moved with variable speed in front of a beam defining aperture. The required velocity profile was calculated using a deconvolution algorithm. The Cr thickness profiles were derived in two ways: directly, using x-ray reflectivity and indirectly, by measuring the surface figure before and after the deposition. After two iterations the mirror surface figure could be improved by almost one order of magnitude. This work will describe the experimental techniques and discuss the achieved accuracy. It will also address open questions such as layer stress, roughness evolution, and limitations of the available instrumentation.
KEYWORDS: Mirrors, Polishing, Surface roughness, Additive manufacturing, Finite element methods, Space mirrors, Aluminum, Single point diamond turning, Lightweight mirrors, Error analysis, 3D printing
Additive manufacturing (AM), more commonly known as 3D printing, is a commercially established technology for rapid prototyping and fabrication of bespoke intricate parts. To date, research quality mirror prototypes are being trialled using additive manufacturing, where a high quality reflective surface is created in a post-processing step. One advantage of additive manufacturing for mirror fabrication is the ease to lightweight the structure: the design is no longer confined by traditional machining (mill, drill and lathe) and optimised/innovative structures can be used. The end applications of lightweight AM mirrors are broad; the motivation behind this research is low mass mirrors for space-based astronomical or Earth Observation imaging. An example of a potential application could be within nano-satellites, where volume and mass limits are critical. The research presented in this paper highlights the early stage experimental development in AM mirrors and the future innovative designs which could be applied using AM.
The surface roughness on a diamond-turned AM aluminium (AlSi10Mg) mirror is presented which demonstrates the ability to achieve an average roughness of ~3.6nm root mean square (RMS) measured over a 3 x 3 grid. A Fourier transform of the roughness data is shown which deconvolves the roughness into contributions from the diamond-turning tooling and the AM build layers. In addition, two nickel phosphorus (NiP) coated AlSi10Mg AM mirrors are compared in terms of surface form error; one mirror has a generic sandwich lightweight design at 44% the mass of a solid equivalent, prior to coating and the second mirror was lightweighted further using the finite element analysis tool topology optimisation. The surface form error indicates an improvement in peak-to-valley (PV) from 323nm to 204nm and in RMS from 83nm to 31nm for the generic and optimised lightweighting respectively while demonstrating a weight reduction between the samples of 18%. The paper concludes with a discussion of the breadth of AM design that could be applied to mirror lightweighting in the future, in particular, topology optimisation, tessellating polyhedrons and Voronoi cells are presented.
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