ATHENA Wide Field Imager

The ATHENA Mission

ATHENA, the Advanced Telescope for High ENergy Astrophysics is one of three cornerstone L-class missions within the current Cosmic Vision programme of the European Space Agency ESA (fig. 1). Among ATHENA’s prime science objectives are phenomena of the early universe like hot gas structures and supermassive black holes. The launch date is scheduled for the early 2030s. With its large-area Silicon Pore Optics mirror and high-resolution focal plane instruments, ATHENA will be the successor of the long-lasting and successful XMM-Newton (ESA) and Chandra (NASA) missions and the next-generation X-ray observatory with unprecedented observing power. The science payload includes two imaging spectrometers: the cryogenic transition edge sensor X‑ray Integral Field Unit (X-IFU) with ultimate spectral resolution in the eV-range and the Wide Field Imager (WFI).

The Wide Field Imager

The Wide Field Imager is a development under the responsibility of the Max-Planck-Institute for Extraterrestrial Physics (MPE) with MPG HLL as detector supplying partner. It is composed of two specialised sub-instruments: a large-area detector covering a field of view of 40 arcmin and a small-area detector capable of processing high count rates as produced by the brightest point sources in the X‑ray sky. Despite their different scientific tasks, both components of the WFI are based on the same sensor principle: Silicon Active Pixel Sensors using the integrated detector/amplifier structure DePFET (Depleted P-channel Field Effect Transistor) as a unit cell. The large-area detector will have a format of 1024 x 1024 pixels of 130 µm pixel size divided into four monolithic quadrant tiles. The fast-timing detector will have a format of 64 x 64 pixels of the same pixel size and be placed out of focus to distribute the incoming high photon flux over the sensitive area. Both will use equal readout and control systems, making use of the synergy of a common effort.

In the first project phase of sensor development, small format prototype devices of DePFET Active Pixel Sensors had been fabricated. This prototype production included a large variety of design and technology flavours. Devices of this production have been pre-tested, assembled, and characterised spectroscopically to filter out the best performing DePFET type out of the many variants. The selected DePFET type (fig. 2) is the basic cell of the layout of the next, so-called pre-flight production.

The layout of the pre-flight production’s wafer (fig. 3) contains one full-size quadrant tile with a format of 512 x 512 pixels and a chip size of 78 x 76 mm². Because of the focal plane arrangement of four of these tiles, the connection wires for readout and control electronics are grouped along two sides of the chip. Their bond pads are displaced with respect to the pixel matrix because of the constraints given by the mechanical and thermal construction of the focal plane. Besides the quadrant chip, the wafer layout includes several samples of the fast-timing detector. The fast-timing detector has a format of 64 x 64 pixels and a chip size of 16 x 16 mm². It is read out in a split frame mode, i.e. two sub-matrices with 32 rows each will be processed in parallel to keep the frame time short. Spectroscopic tests of small format devices show an excellent energy resolution of 128 eV (FWHM at 5.9 keV) and homogeneity over the pixel matrix.

The final ‘flight’ production includes the large area and fast detectors consolidated in the pre-flight production, ideally without changes in the layout or in the production technology. The challenge of the flight production is to organize and process a large volume of wafers to get a sufficient stock of devices for the spacecraft hardware and all kinds of qualification models. Devices are expected to be available in 2022/23.

Figure 1

Artistic view of Athena space observatory and Sagittarius (Sgr) A*, the central supermassive black hole of the Milky Way. The mirror of the telescope is located upfront. The science payload is located at the opposite. The overall dimension of the spacecraft is about 15 m, derived from the 12 m focal length of the telescope. The Sgr A* image comes from a mosaic of XMM-Newton observations (credit: XMM-Newton. Athena mission: IRAP, CNES, ESA & ACO).

Figure 2

DepFET cell for the ATHENA Wide Field Imager. The linear arrangement of drain, gate and source allows for compact device dimensions, excellent spectroscopic performance and fast operation.

Figure 3

Six-inch silicon wafer of the ATHENA WFI pre-flight production. The pre-flight wafer contains sensors in the final format and layout: large area detector with 512 x 512 pixels and a sensitive area of 6.7 x 6.7 cm² (blue frame), fast detector with 64 x 64 pixels and split frame readout (red frame).

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