Raegan L. Johnson-Wilke

Sputter deposition of PZT piezoelectric films on thin glass substrates for adjustable x-ray optics

Piezoelectric PbZr(0.52)Ti(0.48)O(3) (PZT) thin films deposited on thin glass substrates have been proposed for adjustable optics in future x-ray telescopes. The light weight of these x-ray optics enables large collecting areas, while the capability to correct mirror figure errors with the PZT thin film will allow much higher imaging resolution than possible with conventional lightweight optics. However, the low strain temperature and flexible nature of the thin glass complicate the use of chemical-solution deposition due to warping of the substrate at typical crystallization temperatures for the PZT. RF magnetron sputtering enabled preparation of PZT films with thicknesses up to 3 μm on Schott D263 glass substrates with much less deformation. X-ray diffraction analysis indicated that the films crystallized with the perovskite phase and showed no indication of secondary phases. Films with 1 cm(2) electrodes exhibited relative permittivity values near 1100 and loss tangents below 0.05. In addition, the remanent polarization was 26 μC/cm(2) with coercive fields of 33 kV/cm. The transverse piezoelectric coefficient was as high as -6.1±0.6 C/m(2). To assess influence functions for the x-ray optics application, the piezoelectrically induced deflection of individual cells was measured and compared with finite-element-analysis calculations. The good agreement between the results suggests that actuation of PZT thin films can control mirror figure errors to a precision of about 5 nm, allowing sub-arcsecond imaging.

In situ measurement of increased ferroelectric/ferroelastic domain wall motion in declamped tetragonal lead zirconate titanate thin films

Ferroelectric/ferroelastic domain reorientation was measured in a 1.9 μm thick tetragonal {001} oriented PbZr0.3Ti0.7O3 thin film doped with 1% Mn under different mechanical boundary constraints. Domain reorientation was quantified through the intensity changes in the 002/200 Bragg reflections as a function of applied electric field. To alter the degree of clamping, films were undercut from the underlying substrate by 0%, ∼25%, ∼50%, or ∼75% of the electrode area. As the amount of declamping from the substrate increased from 0% to ∼75%, the degree of ferroelectric/ferroelastic domain reorientation in the films increased more than six fold at three times the coercive field. In a film that was ∼75% released from the substrate, approximately 26% of 90° domains were reoriented under the maximum applied field; this value for domain reorientation compares favorably to bulk ceramics of similar compositions. An estimate for the upper limit of 90° domain reorientation in a fully released film under these condition...

Development status of adjustable grazing incidence optics for 0.5 arcsecond X-ray imaging

We describe progress in the development of adjustable grazing incidence X-ray optics for 0.5 arcsec resolution cosmic X-ray imaging. To date, no optics technology is available to blend high resolution imaging like the Chandra X-ray Observatory, with square meter collecting area. Our approach to achieve these goals simultaneously is to directly deposit thin film piezoelectric actuators on the back surface of thin, lightweight Wolter-I or Wolter- Schwarschild mirror segments. The actuators are used to correct mirror figure errors due to fabrication, mounting and alignment, using calibration and a one-time figure adjustment on the ground. If necessary, it will also be possible to correct for residual gravity release and thermal effects on-orbit. In this paper we discuss our most recent results measuring influence functions of the piezoelectric actuators using a Shack-Hartmann wavefront sensor. We describe accelerated and real-time lifetime testing of the piezoelectric material, and we also discuss changes to, and recent results of, our simulations of mirror correction.

Ferroelectric/Ferroelastic domain wall motion in dense and porous tetragonal lead zirconate titanate films

Direct evidence of ferroelectric/ferroelastic domain reorientation is shown in Pb(Zr0.30Ti0.70)O3 (PZT30/70) thin films clamped to a rigid silicon substrate using in situ synchrotron X-ray diffraction during application of electric fields. Both dense films and films with 3 to 4 vol% porosity were measured. On application of electric fields exceeding the coercive field, it is shown that the porous films exhibit a greater volume fraction of ferroelastic domain reorientation (approximately 12 vol% of domains reorient at 3 times the coercive field, Ec) relative to the dense films (~3.5 vol% at 3Ec). Furthermore, the volume fraction of domain reorientation significantly exceeded that predicted by linear mixing rules. The high response of domain reorientation in porous films is discussed in the context of two mechanisms: local enhancement of the electric field near the pores and a reduction of substrate clamping resulting from the lowering of the film stiffness as a result of the porosity. Similar measurements during weak-field (subcoercive) amplitudes showed 0.6% volume fraction of domains reoriented for the porous films, which demonstrates that extrinsic effects contribute to the dielectric and piezoelectric properties.

Adjustable grazing incidence x-ray optics based on thin PZT films

The direct deposition of piezoelectric thin films on thin substrates offers an appealing technology for the realization of lightweight adjustable mirrors capable of sub-arcsecond resolution. This solution will make it possible to realize X-ray telescopes with both large effective area and exceptional angular resolution and, in particular, it will enable the realization of the adjustable optics for the proposed mission Square Meter Arcsecond Resolution X-ray Telescope (SMART-X). In the past years we demonstrated for the first time the possibility of depositing a working piezoelectric thin film (1-5 um) made of lead-zirconate-titanate (PZT) on glass. Here we review the recent progress in film deposition and influence function characterization and comparison with finite element models. The suitability of the deposited films is analyzed and some constrains on the piezoelectric film performances are derived. The future steps in the development of the technology are described.

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

Technology development of adjustable grazing incidence x-ray optics for sub-arc second imaging

/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.

Improving yield of PZT piezoelectric devices on glass substrates

We report on technical progress made over the past year developing thin film piezoelectric adjustable grazing incidence optics. We believe such mirror technology represents a solution to the problem of developing lightweight, sub-arc second imaging resolution X-ray optics. Such optics will be critical to the development next decade of astronomical X-ray observatories such as SMART-X, the Square Meter Arc Second Resolution X-ray Telescope. SMART-X is the logical heir to Chandra, with 30 times the collecting area and Chandra-like imaging resolution, and will greatly expand the discovery space opened by Chandra’s exquisite imaging resolution. In this paper we discuss deposition of thin film piezoelectric material on flat glass mirrors. For the first time, we measured the local figure change produced by energizing a piezo cell – the influence function, and showed it is in good agreement with finite element modeled predictions. We determined that at least one mirror substrate material is suitably resistant to piezoelectric deposition processing temperatures, meaning the amplitude of the deformations introduced is significantly smaller than the adjuster correction dynamic range. Also, using modeled influence functions and IXO-based mirror figure errors, the residual figure error was predicted post-correction. The impact of the residual figure error on imaging performance, including any mid-frequency ripple introduced by the corrections, was modeled. These, and other, results are discussed, as well as future technology development plans.

Toward Active X-ray Telescopes II

The proposed SMART-X telescope includes adaptive optics systems that use piezoelectric lead zirconate titanate (PZT) films deposited on flexible glass substrates. Several processing constraints are imposed by current designs: the crystallization temperature must be kept below 550 °C, the total stress in the film must be minimized, and the yield on 1 cm2 actuator elements should be < 90%. For this work, RF magnetron sputtering was used to deposit films since chemical solution deposition (CSD) led to warping of large area flexible glass substrates. A PZT 52/48 film that wasdeposited at 4 mTorr and annealed at 550 °C for 24 hours showed no detectable levels of either PbO or pyrochlore second phases. Large area electrodes (1cm x 1 cm) were deposited on 4” glass substrates. Initially, the yield of the devices was low, however, two methods were employed to increase the yield to near 100 %. The first method included a more rigorous cleaning to improve the continuity of the Pt bottom electrode. The second method was to apply 3 V DC across the capacitor structure to burn out regions of defective PZT. The result of this latter method essentially removed conducting filaments in the PZT but left the bulk of the material undamaged. By combining these two methods, the yield on the large area electrodes improved from < 10% to nearly 100%.

Technology Requirements For a Square-Meter, Arcsecond-Resolution Telescope for X-Rays: The SMART-X Mission

In the half century since the initial discovery of an astronomical (non-solar) x-ray source, the observation time required to achieve a given sensitivity has decreased by eight orders of magnitude. Largely responsible for this dramatic progress has been the refinement of the (grazing-incidence) focusing x-ray telescope, culminating with the exquisite subarcsecond imaging performance of the Chandra X-ray Observatory. The future of x-ray astronomy relies upon the development of x-ray telescopes with larger aperture areas (< 1 m2) and comparable or finer angular resolution (< 1″). Combined with the special requirements of grazing-incidence optics, the mass and envelope constraints of space-borne telescopes render such advances technologically challenging—requiring precision fabrication, alignment, and assembly of large areas (< 200 m2) of lightweight (≈ 1 kg m-2 areal density) mirrors. Achieving precise and stable alignment and figure control may entail active (in-space adjustable) x-ray optics. This paper discusses relevant programmatic and technological issues and summarizes current progress toward active x-ray telescopes.