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Deep Depletion

Work at Westinghouse [Peckerar et al] first demonstrated the possibility for enhancing X-ray detection efficiency by utilising deep depletion layers. The depth of a depletion layer varies as

where is the depleting potential, and is the acceptor concentration. The applied potential is limited by the breakdown voltage of the MOS structure, and hence the use of high resistivity silicon (low ) is required. Work was also carried out at Bell Northern [Griffiths 1985] but neither manufacturer was able to attain good noise and charge transfer performance. This was attributed to the difficulty in fabricating CCD structures and output MOSFETs on the new form of silicon.

The first successful demonstration of a low noise deep depletion CCD with good X-ray energy resolution was obtained with GEC/EEV devices [Chowanietz et al]. Those investigations identified epitaxial silicon as the most promising source of silicon. In this method, a high resistivity layer is deposited on a low resistivity bulk substrate, and the cosmetic quality is enhanced through the gettering action of the high dopant concentration of the low resistivity bulk. In addition the epi/bulk boundary forms a natural edge for thinning by resistive selective etches, when fabricating thinned back-illuminated devices.

Work at MIT Lincoln Laboratories in support of the ASTRO-D and AXAF missions [Burke et al] has shown that excellent performance can be obtained on bulk silicon with resistivity has high as 6500 -cm. Devices have been fabricated with noise below 4 electrons rms, charge transfer inefficiency better than 10 , depletion depths of approximately 80 microns, and detection efficiencies exceeding 80% at 5.9 keV.

In practice the production of suitable CCDs on high resistivity material has been sorely limited by the difficulties encountered in obtaining supplies of high quality silicon with the wafer sizes matched to the chosen production line.