Many current outstanding questions in x-ray astronomy were addressed by Lynx, an observatory concept that concluded its study phase in 2019. High-effective area, high angular resolution x-ray missions like Lynx require thin (≤ 0.5 mm thick) mirrors with precise surface figures to maintain high angular resolution (≤ 0.5 arcsec). To study methods of meeting these requirements, adjustable x-ray optics have been fabricated with thin-film piezoelectric actuators to perform figure correction. These adjustable x-ray optics serve to correct low spatial frequency figure errors (⪆ 0.1 mm−1 ). The fabrication and actuator performance for an adjustable x-ray mirror that forms a conical approximation to a Wolter-I telescope are reported. This mirror has a BCB insulating layer with a top level of Ti traces to address its 288 actuator cells. The individual responses of cells are measured and on average they induce a figure change of 0.87 μm Peak-to-Valley (PV) with an associated Root Mean Square (RMS) of 0.10 μm. These measured cell responses are compared to predicted responses generated using a Finite-Element (FEA) analysis algorithm. On average the measured and predicted cell responses agree to within 0.06 μm RMS. The disagreement between predicted and measured cell responses is posited as being due to differences in radial constraints points between the FEA model and the as-built mirror mount.
Thin adjustable X-ray mirrors can correct deformations generated from fabrication, gravity release, mounting stresses, drifting stresses in the reflecting layer(s) and thermal variations while maintaining high angular resolution (< 0.5 arcsecond) and large effective area (< 2 m2) required for future X-ray missions. This work presents fabrication process developments for adjustable mirror segments with actuators for the Lynx X-ray observatory mission concept. Piezoelectric actuator arrays were fabricated on the convex side of precision slumped glass or curved silicon mirror segments using a 1.5 μm thick lead zirconate titanate (PZT) film. A two-layer metal routing scheme with a polymeric insulator was used to independently address 288 actuators on the mirror. The two-layer metal allows narrow kerfs between actuators and increased actuator density. Anisotropic conductive film was used to bond thin flexible copper cables to flat edges of the mirror to interface with external control electronics. This prototype mirror has eight cables with a total of 290 connections to access the array. To reduce the cabling complexity for future mirrors, thin film transistors have been fabricated on the curved mirror to function as access switches. To facilitate this, a mask aligner that allows precision alignment on curved mirror segments was developed and arrays of thin film transistors (TFT) on curved substrates have been tested. TFT and actuator integration on future mirrors will reduce external connections to just two cables with a total of 30 connections. Keywords: Lynx, adjustable optics, X-ray optics, thin film piezoelectric, curved substrate aligner
Piezoelectric adjustable x-ray optics use magnetron sputtered thin film coatings on both sides of a thin curved glass substrate. To produce an optic suitable for a mission requiring high-angular resolution like “Lynx,” the integrated stresses (stress×thickness) of films on both sides of the optic must be approximately equal. Thus, understanding how sputtered film thickness distributions change for convex and concave curved substrates is necessary. To address this, thickness distributions of piezoelectric Pb0.995(Zr0.52Ti0.48)0.99Nb0.01O3 films are studied on flat, convex, and concave cylindrical substrates with a 220-mm radius of curvature. A mathematical model of the film thickness distribution is derived based on the geometric properties of the sputter tool and the substrate, and film thicknesses deposited with a commercially available sputtering tool are measured with spectroscopic ellipsometry. Experiment and modeled results for flat and convex curved substrates demonstrate good agreement, with average relative thickness distribution difference of 0.19% and −0.10% respectively, and a higher average difference of 1.4% for concave substrates. The calculated relative thickness distributions are applied to the convex and concave sides of a finite-element analysis (FEA) model of an adjustable x-ray optic prototype. The FEA model shows that, left uncorrected, the relative film thickness variation will yield an optic with an optical performance of 2.6 arc sec half power diameter (HPD) at 1 keV. However, the mirror figure can be corrected to diffraction-limited performance (0.3 arc sec HPD) using the piezoelectric adjusters, suggesting that the tolerances for applying a balanced integrated stress on both sides of a mirror are alleviated for adjustable x-ray optics as compared to traditional static x-ray mirrors. Furthermore, the piezoelectric adjusters will also allow changes in mirror figure over the telescope lifetime due to drift in the stress states of the x-ray surfaces to be corrected on orbit.
Adjustable X-ray optics is the technology under study by SAO and PSU for the realization of the proposed X-ray telescope Lynx. The technology is based on thin films of lead-zirconate-titanate (PZT) deposited on the back of thermally formed thin substrates, and represents a potential solution to the challenging trade-off between high-surface quality and low mass, that limits the performance of current generation of X-ray telescopes. The technology enables the correction of mirror fabrication figure, mounting induced distortions, and on-orbit correction for variations in the mirror thermal environment. We describe the current state of development, presenting updated test data, anticipation of performances and expectations.
Thin x-ray optics with high angular resolution (≤ 0.5 arcsec) over a wide field of view enable the study of a number of astrophysically important topics and feature prominently in Lynx, a next-generation x-ray observatory concept currently under NASA study. In an effort to address this technology need, piezoelectrically adjustable, thin mirror segments capable of figure correction after mounting and on-orbit are under development. We report on the fabrication and characterization of an adjustable cylindrical slumped glass optic. This optic has realized 100% piezoelectric cell yield and employs lithographically patterned traces and anisotropic conductive film connections to address the piezoelectric cells. In addition, the measured responses of the piezoelectric cells are found to be in good agreement with finite-element analysis models. While the optic as manufactured is outside the range of absolute figure correction, simulated corrections using the measured responses of the piezoelectric cells are found to improve 5 to 10 arcsec mirrors to 1 to 3 arcsec [half-power diameter (HPD), single reflection at 1 keV]. Moreover, a measured relative figure change which would correct the figure of a representative slumped glass piece from 6.7 to 1.2 arcsec HPD is empirically demonstrated. We employ finite-element analysis-modeled influence functions to understand the current frequency limitations of the correction algorithm employed and identify a path toward achieving subarcsecond corrections.
Lynx is a concept under study for prioritization in the 2020 Astrophysics Decadal Survey. Providing orders of magnitude increase in sensitivity over Chandra, Lynx will examine the first black holes and their galaxies, map the large-scale structure and galactic halos, and shed new light on the environments of young stars and their planetary systems. In order to meet the Lynx science goals, the telescope consists of a high-angular resolution optical assembly complemented by an instrument suite that may include a High Definition X-ray Imager, X-ray Microcalorimeter and an X-ray Grating Spectrometer. The telescope is integrated onto the spacecraft to form a comprehensive observatory concept. Progress on the formulation of the Lynx telescope and observatory configuration is reported in this paper.
Piezoelectric adjustable optics are being developed for high throughput, high resolution, low mass Xray mirror assemblies. These optics require robust piezoelectric thin films and reproducible lithographic patterning on curved glass substrates. This work details the cleaning of Corning Eagle XG glass substrates for thin shell X-ray mirrors by a three stage acid and solvent cleaning procedure before a 0.02 μm Ti adhesion layer and a 0.1 μm Pt bottom electrode layer was deposited using DC magnetron sputtering. Piezoelectric Pb(Zr0.52Ti0.48)0.99Nb0.01O3 thin films with a thickness of 1.5 μm were then deposited by radio frequency magnetron sputtering in three 0.5 µm layers with intermittent annealing steps in a rapid thermal annealing furnace at 650°C for 60 seconds. Defects observed in the piezoelectric thin films were linked to residue remaining on the glass after cleaning. 112 piezoelectric cells and 100 μm wide conductive Pt traces were patterned using bilayer photolithography. The photoresist layers were deposited using spin coating at 2000 and 4000 RPM to achieve uniform 1 μm thick layers, resulting in reproducibly resolved features with limiting resolutions of approximately >25 μm. The resulting mirror pieces achieved a 100% yield, with average relative permittivity of 1270, dielectric loss 0.047, coercive field 30 kV/cm and remanent polarization of 20 μC/cm2 . While the defects observed in the films appeared to have not influence on the electrical properties, additional cleaning steps using DI water were proposed to further reduce their presence.
Thin X-ray optics with high angular resolution (≤ 0.5 arcseconds) over a wide field of view enable the study of a number of astrophysically important topics, and feature prominently in Lynx, a next-generation X-ray observatory concept currently under NASA study. To produce such optics, we propose to use piezoelectrically adjustable, thin mirror segments capable of figure correction after mounting and on-orbit. In the present work, we report on the fabrication and characterization of an adjustable cylindrical slumped glass optic. This optic has realized 100% piezoelectric cell yield and employs lithographically patterned traces and anisotropic conductive film (ACF) connections to address the piezoelectric cells. The response of the piezoelectric cells are found to agree with finite-element analysis models, and simulated corrections to distortions are found to improve 7 – 10 arcsecond mirrors to 1 – 2 arcseconds (HPD, single reflection at 1 keV). Moreover, such a figure change is empirically demonstrated using an adjustable slumped glass optic, and we identify a path for achieving subarcsecond corrections.
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