Abstract
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.
Highlights
IntroductionAdjustable x-ray optics are curved thin shell mirrors, in which the figure can be adjusted postmanufacture by applying a voltage to piezoelectric actuators on the back (convex side) of the mirror.[13,14,15] When a voltage is applied, a strain is produced by means of the converse piezoelectric effect and this is used to make precise changes to the mirror figure, correcting distortions introduced from processing, mounting, or on-orbit
The thickness distribution of a PZT thin film deposited by Radio frequency (RF) magnetron sputtering using an off axis angled circular planar magnetron with a rotating substrate was mapped on 101.6 mm[2] flat, convex, and concave curved substrates (ROC of 220 mm) using spectroscopic ellipsometry
The key geometric parameters influencing the thickness distribution as a function of substrate curvature were the distance of a given point on the substrate surface from the target and the angle between the mass trajectory and the normal to the substrate surface at a given point
Summary
Adjustable x-ray optics are curved thin shell mirrors, in which the figure can be adjusted postmanufacture by applying a voltage to piezoelectric actuators on the back (convex side) of the mirror.[13,14,15] When a voltage is applied, a strain is produced by means of the converse piezoelectric effect and this is used to make precise changes to the mirror figure, correcting distortions introduced from processing, mounting, or on-orbit. Additional sources of distortion must be minimized to keep the mirror figure within the dynamic range of the actuators, and correctable to the required performance. Identifying, understanding, and minimizing sources of figure distortion associated with the mirror processing is necessary
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