Mark Olde Riekerink

Silicon Pore Optics Development

Future X-ray astrophysics missions, such as the International X-ray Observatory, IXO, require the development of novel optics in order to deliver the missions large aperture, high angular resolution and low mass requirements. A series of activities have been pursued by ESA, leading a consortium of European industries to develop Silicon Pore Optics for use as an x-ray mirror technology. A novel process takes as the base mirror material commercially available silicon wafers, which have been shown to possess excellent x-ray reflecting qualities. These are ribbed, curved and stacked concentrically in layers that have the desired shape at a given radii of the x-ray aperture. Pairs of stacks are aligned and mounted into doubly reflecting mirror modules that can be aligned into the x-ray aperture without the very high angular and position alignment requirements that need to be achieved for mirror plates within the mirror module. The use of this silicon pore optics design substantially reduces mirror assembly time, equipment and costs in comparison to alternative IXO mirror designs. This paper will report the current technology development status of the silicon pore optics and the roadmap expected for developments to meet an IXO schedule. Test results from measurements performed at the PTB lab of the Bessy synchrotron facility and from full illumination at the Panter x-ray facility will be presented.

Making the ATHENA optics using Silicon Pore Optics

Silicon Pore Optics, after 10 years of development, forms now the basis for future large (L) class astrophysics Xray observatories, such as the ATHENA mission to study the hot and energetic universe, matching the L2 science theme recently selected by ESA for launch in 2028. The scientific requirements result in an optical design that demands high angular resolution (5“) and large effective area (2 m2 at a few keV) of an X-ray lens with a focal length of 12 to14 m. Silicon Pore Optics was initially based on long (25 to 50 m) focal length telescope designs, which could achieve several arc second angular resolution by curving the silicon mirror in only one direction (conical approximation). With the advent of shorter focal length missions we started to develop mirrors having a secondary curvature, allowing the production of Wolter-I type optics, which are on axis aberration-free. In this paper we will present the new manufacturing process, discuss the impact of the ATHENA optics design on the technology development and present the results of the latest X-ray test campaigns.

X-ray optics developments at ESA

Future high energy astrophysics missions will require high performance novel X-ray optics to explore the Universe beyond the limits of the currently operating Chandra and Newton observatories. Innovative optics technologies are therefore being developed and matured by the European Space Agency (ESA) in collaboration with research institutions and industry, enabling leading-edge future science missions. Silicon Pore Optics (SPO) [1 to 21] and Slumped Glass Optics (SGO) [22 to 29] are lightweight high performance X-ray optics technologies being developed in Europe, driven by applications in observatory class high energy astrophysics missions, aiming at angular resolutions of 5” and providing effective areas of one or more square meters at a few keV. This paper reports on the development activities led by ESA, and the status of the SPO and SGO technologies, including progress on high performance multilayer reflective coatings [30 to 35]. In addition, the progress with the X-ray test facilities and associated beam-lines is discussed [36].

Aberration-free silicon pore x-ray optics

Silicon Pore Optics is an enabling technology for future L- and M-class astrophysics X-ray missions, which require high angular resolution (~5 arc seconds) and large effective area (1 to 2 m2 at a few keV). The technology exploits the high-quality of super-polished 300 mm silicon wafers and the associated industrial mass production processes, which are readily available in the semiconductor industry. The plan-parallel wafers have a surface roughness better than 0.1 nm rms and are diced, structured, wedged, coated, bent and stacked to form modular Silicon Pore Optics, which can be grouped into a larger optic. The modules are assembled from silicon alone, with all the mechanical advantages, and form an intrinsically stiff pore structure. The optics design was initially based on long (25 to 50 m) focal length X-ray telescopes, which could achieve several arc second angular resolution by curving the silicon mirror in only one direction (conical approximation). Recently shorter focal length missions (10 to 20 m) have been discussed, for which we started to develop Silicon Pore Optics having a secondary curvature in the mirror, allowing the production of Wolter-I type optics, which are on axis aberration-free. In this paper we will present the new manufacturing process, the results achieved and the lessons learned.

Silicon pore optics developments and status

Silicon Pore Optics (SPO) is a lightweight high performance X-ray optics technology being developed in Europe, driven by applications in observatory class high energy astrophysics missions. An example of such application is the former ESA science mission candidate ATHENA (Advanced Telescope for High Energy Astrophysics), which uses the SPO technology for its two telescopes, in order to provide an effective area exceeding 1 m2 at 1 keV, and 0.5 m2 at 6 keV, featuring an angular resolution of 10” or better [1 to 24]. This paper reports on the development activities led by ESA, and the status of the SPO technology. The technology development programme has succeeded in maturing the SPO further and achieving important milestones, in each of the main activity streams: environmental compatibility, industrial production and optical performance. In order to accurately characterise the increasing performance of this innovative optical technology, the associated X-ray test facilities and beam-lines have been refined and upgraded.

Qualification of silicon pore optics

Silicon Pore Optics (SPO) are the enabling technology for ESA’s second large class mission in the Cosmic Vision programme. As for every space hardware, a critical qualification process is required to verify the suitability of the SPO mirror modules surviving the launch loads and maintaining their performance in the space environment. We present recent design modifications to further strengthen the mounting system (brackets and dowel pins) against mechanical loads. The progress of a formal qualification test campaign with the new mirror module design is shown. We discuss mechanical and thermal limitations of the SPO technology and provide recommendations for the mission design of the next X-ray Space Observatory.

Silicon pore optics for astrophysical missions

The establishment of Silicon Pore Optics (SPO) as the technology of choice for the implementation of future large X-ray space optics has opened up the road to its use in all classes of X-ray missions with varying scientific goals. This interest has given us the possibility to broaden the design parameter space which is normally considered for SPO optics. In doing so a number of classical space X-ray optics design issues (e.g., field of view, stray light, baffling, aberrations) have been tackled. In this paper we report on recent results achieved in this effort. Particular attention will be given to the issues of stray light and baffling, a topic upon which a combination of analytical, simulation, and data analysis means can be effectively brought to bear. Missions considering the use of SPO optics have requirements spanning more than two orders of magnitude in energy, and a factor 20 in focal length. The possibilities that can be considered and the trade offs that must be made when applying SPO to such a wide range of optical designs will be illustrated, and some of the possible solutions discussed.

Preparing the optics technology to observe the hot universe

With the selection of “The hot and energetic Universe” as science theme for ESAs second large class mission (L2) in the Cosmic Vision programme, work is focusing on the technology preparation for an advanced X-ray observatory. The core enabling technology for the high performance mirror is the Silicon Pore Optics (SPO) [1 to 23], a modular X-ray optics technology, which utilises processes and equipment developed for the semiconductor industry. The paper provides an overview of the programmatic background, the status of SPO technology and gives an outline of the development roadmap and activities undertaken and planned by ESA on optics, coatings [24 to 30] and test facilities [31, 33].

Novel applications of silicon pore optics technology

In this paper we present several novel applications using X-ray mirrors based on Silicon Pore Optics technology, the present baseline technology for large effective area space based X-ray telescopes. By cutting, bending and direct bonding of mirrors cut from silicon wafers we can create a variety of structures in a number of well-defined shapes. One novel application is an X-ray half-mirror for X-ray interferometry applications based on flat, structured Si mirrors bonded to a glass support structure with a large open area ratio. A second application is to use bent silicon single crystals as a focusing Laue lens for soft gamma rays.

Laser assisted and hermetic room temperature bonding based on direct bonding technology

A novel method for laser assisted room temperature bonding of two substrates is presented. The method enables the packaging of delicate (bio)structures and/or finished (MEMS) devices, as there is no need for a high temperature annealing process. This also allows the bonding of two substrates with non-matching thermal expansion coefficients. The basis of the presented technology is the ability to create a direct pre-bond between two substrates. These can be two glass substrates, of which one has a thin film metal coating (e.g. Cr. Ti, Ta, Au…), or a silicon-glass combination. After (aligned) pre-bonding of the two wafers, a laser (e.g. a Nd:YAG laser) is used to form a permanent bond line on the bond interface, using the metal layer as a light absorber (or the silicon, in the case of a glass-silicon combination). The permanent bond line width is in the order of 10-50μm. The use of a laser to form the permanent bond ensures a hermetic sealing of the total package; a distinctive advantage over other, more conventional methods of room temperature bonding (e.g. adhesive bonding). He-leak testing showed leak rates in the order of 10-9 mbar l/s. This meets the failure criteria of the MIL-STD-883H standard of 5x10-8 mbar l/s. An added functionality of the proposed method is the possibility to create electrical circuitry on the bond interface, using the laser to modify the metal interlayer, rendering it electrically non-conductive. Biocompatible packages are also possible, by choosing the appropriate interlayer material. This would allow for the fabrication of implantable packages.