Fully Depleted Back Illuminated pnCCDs

Silicon is used as a detector material since the early fifties of the last century. The fact that an energy of only 3.7 eV is needed to create an electron-hole pair was soon exploited to fabricate detectors for photons and particles with high sensitivity. In addition, the band gap of silicon (Egap = 1.1 eV) is large enough to operate detectors at moderate temperatures avoiding thermally generated leakage currents. The atomic number of silicon (Z = 14) is still high enough to obtain a high detection probability for e.g., X-rays up to an energy of 30keV for a device thickness of the order of 1 mm.

A major breakthrough in the development of silicon detectors occurred when detector grade silicon was processed in planar technology [6]. It opened the opportunity to directly implement signal processing electronics monolithically into the detector silicon. In the eighties and nineties, more and more complex structures were developed making use of the possibility to shape the potential inside the detectors and of the on-chip electronics for optimum sensor performance (sensor means detector in combination with on-chip electronics).

All experimental results shown here are from devices that have been designed, fabricated, and tested at the MPI Halbleiterlabor.

Conceptually, the pnCCD is a derivative of the silicon drift detector [3]. The development of the pnCCDs started in 1985. In the following years, the basic concept was simulated, modified, and designed in detail [19]. N-channel JFET electronics was integrated in 1992 [14,15] and the first reasonably fine working devices were produced in 1993. Up to then, all presented devices were "small" devices, i.e., 3 cm2 in sensitive area [17]. The flight type large area (36 cm2) detectors were produced from 1995 to 1997, with a sufficiently high yield to equip the X-ray satellite missions ABRIXAS and XMM-Newton [9,18,21] with defect-free focal plane pnCCDs.

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