pn-CCDs are backside illuminated, 3-phase CCDs on a fully depleted silicon substrate. Their functioning is based on the principal of sideward depletion. A schematic drawing is shown in the picture below. In difference to commonly used MOS-CCDs, the transfer registers of a pn-CCD are formed by pn-junctions. This allows for high radiation hardness, fast transfer modes and high full-well capacitances. The ultrathin and unstructured radiation entrance window enables a high quantum efficiency from the near IR and optical to the X-ray regime.

Schematic cut through a pn-CCD.

To enable low noise and high frame rates, each channel of the CCD is has its own on-chip amplifier. The amplified signals of one CCD line are fed in parallel into a amplification chip (CAMEX), where all signals are further filtered and shaped in parallel. The following picture shows a zoom of the readout region of the CCD, comprising two readout anodes, connected to the on-chip amplifiers.

Readout region of a pn-CCD, showing two readout anodes, being connected to on-chip amplifiers.

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pn-CCDs for X-ray Astronomy

The Max-Planck-Institute for Extraterrestical Physics has developed together with the Semiconductor Laboratory of the Max-Planck-Institutes and the staff of PNSensor the largest defect free X-ray CCD ever built with a sensitive area of 6 x 6 cm² for the XMM-Newton satellite mission which was launched successfully in the year 1999. The CCD in the central focal area follows the pn-CCD concept.

The largest X-ray CCD ever build, with a sensitive area of 60 x 60 mm². The CCD was developed for the XMM-Newton satellite mission.

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Detector Modules

For operation, pn-CCDs are packed onto a ceramic carrier board, together with the CAMEX readout chip and RC-filters to ensure clean and stable supply voltages.

Picture of a 64 x 200 pixel pn-CCD, mounted on a ceramic carrier together with the CAMEX readout chip. The CCD has a sensitive area of 3 cm².

256 x 256 pixel CCD with 50 µm pixel size. The 256 channels of the CCD are read out by two CAMEX chips, having 128 channels each.

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System Set-Up

In set-up of the pn-CCD system the ceramic boards of the CCD module are fixed between cooling masks in order to enable a cooling of the CCD to its operating temperature. The masks are connected by flexible copper wires to a cooling system and the vacuum chamber. For the cooling system several types have between tested and are operated in our laboratory:

• a closed cycle He-system
• stirling cooler
• cryo Tiger
• liquid nitrogen dewar.

A ceramic board with CCD mounted in a laboratory testing facility.

The system is operated and read out by the compact pn-CCD controller containing all power supplies, the sequencer and ADC allowing for high-speed high-precision data acquisition.

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