Integrated Control Electronics for Adjustable X-Ray Optics

The next generation space-based X-ray observatories will seek to broaden significantly our understanding of the Universe. In order to improve upon the existing state of the art demonstrated by the grazing incidence X-ray telescope in the Chandra Observatory, future X-ray telescopes need to achieve similar sub-arcsecond angular resolution, but with much larger collection area. The present target for optics systems is to achieve 0.5 arcsecond half-power diameter (HPD) resolution at 1 keV with > 2 m2collecting area. At this level of resolution and area, the telescope could potentially observe the formation of the first galaxies and black holes in the Universe. Previous work has demonstrated that the angular resolution target can be achieved using active mirrors that allow for the adjustment of figure errors with spatial periods longer than 20 mm [1, 2]. The Square Meter Arcsecond Resolution X-ray Telescope (SMART-X) has been conceived with an effective area of approximately 2.3 m2, about 30 times that of Chandra. The design will consist of mirror segments that form nested shells, similar to the design proposed for the International X-ray Observatory (IXO) observatory [3]. The largest mirror segments for SMART-X will be approximately 200 mm x 400 mm. Achieving high resolution with large area requires high precision actuation elements that are light weight, thin, with low power consumption. The proposed optics system for future X-ray telescopes uses bimorph mirrors that are actuated by piezoelectric thin films deposited on the back side of thin, flexible glass substrates. One surface of the mirror will be uniformly coated with a Cr/Ir stack and serve as the reflective mirror side of the telescope. On the back side, a piezoelectric thin film will be uniformly coated over a bottom electrode. Individual pixels will be defined using an array of top electrodes that can be addressed separately. On application of a voltage across the thickness of the piezoelectric film, an in-plane stress develops, whose magnitude is governed by the transverse piezoelectric coefficient (e31,f) of the film. This locally deforms the mirror’s surface. Current designs employ 200 mm long mirrors with piezoelectric cells 1 cm long and 2 cm wide. This results in 200 to 400 cells per mirror segment, depending upon the segment size. The post-correction figure error provides sub-half arc sec HPD imaging at 1 keV. Deconvolving the influence functions from the mirror figure error yields a weighting factor for each influence function. By independently adjusting the voltage to each piezoelectric cell, we can compensate for figure errors in thin mirrors to an unprecedented level. The goal of this program is to develop technology for integrating electronics onto adjustable optics for X-ray space telescope applications so that the large number of piezoelectric cells can be sensibly controlled without running lead wires to each individual element. The approach adopted is to integrate ZnO thin film transistors onto the PbZr0.52Ti0.48O3(PZT) piezoelectric layers.
•Developed a processing procedure for ZnO thin film electronics on adjustable mirror segments using piezoelectric cells.  
•Demonstrated row-column addressing of the adjustable mirror was demonstrated using ZnO control electronics
•Developed and deployed ESD protection circuitry for the adjustable optics
•Demonstrated in simulations of mirror performance that reduction of the piezoelectric cell size from 10mm x 20 mm to 5 mm x 10mm reduces the residual slope error to 0.4 arc second
•Fabricated a 5 x 5 array of piezoelectric cells with ZnO thin film transistors for row-column addressing on a glass substrate and sent to SAO for lifetime testing 
•Determined that anisotropic conductive film bonding can be used to reduce the complexity of electrical interconnections between the mirror optic and the body of the telescope
•Developed a process flow for integrating active electronics onto the piezoelectric cells for adjustable optics using a process that enables construction of the electronics on separate substrates, and transferring the electronics to a curved array.