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Dark current and cooling next up previous
Next: Modes of Operation Up: CCD Scientific Performance Previous: Field of View

Dark current and cooling

Thermal excitation of charge carriers from the valence to the conduction band creates a dark current signal which may vary with position in the array, and also has temporal shot noise variation. Therefore CCDs must be cooled to ensure this dark current and associated noise is small compared with the noise of the readout amplifier. The temperature dependence of dark current is of the form

equation92

where tex2html_wrap_inline309 is in general the bandgap energy, but for individual generation sites E may be a trapping state energy. In a device devoid of localized defects which give rise to individual dark current spikes, most dark current arises from electrons generated at interface states at the surface. This surface component of dark current is described by an equation of the form

equation98

The first terms are silicon material constants whose magnitudes are usually related to the quality of silicon processing. tex2html_wrap_inline243 is the trap capture cross-section, tex2html_wrap_inline315 is the thermal velocity of the charge carriers and tex2html_wrap_inline317 the surface state trap density. In the case of a depleted condition at the surface, the free charge carrier densities, tex2html_wrap_inline319 and tex2html_wrap_inline321 are tex2html_wrap_inline323 (the intrinsic carrier density), but if the surface is forced into inversion then tex2html_wrap_inline325 and tex2html_wrap_inline327 . The inverted surface case offers a reduction in dark current by a factor tex2html_wrap_inline329 , or tex2html_wrap_inline331 in principle.

By adjusting the surface potential to a sufficiently low level compared with the substrate, free holes from surface implants may populate the surface, changing the dominant carriers from n- to p-type, and as can be seen above, the dark current decreases by orders of magnitude. This feature has been exploited in technologies such as Open Pinned Phase, Multi Pinned Phase and Virtual Phase. In practice, a conventional CCD operates at room temperature with a dark current density of tex2html_wrap_inline333 nA cm tex2html_wrap_inline335 , but a a device operating with a pinned inverted surface has tex2html_wrap_inline207 pA cm tex2html_wrap_inline335 [Janesick et al 1989a]. This allows a tex2html_wrap_inline341 temperature advantage for a given dark current performance. Integration times for X-ray astronomy are short compared with optical applications, and less emphasis is placed on dark current performance.

Besides using special CCD architectures, dark current may be minimized by appropriate choice of clocking schemes. For example, during integration of images some of the clocks may be biased to place a large fraction of the surface into the inverted state. However, this implies a reduced surface potential and hence a lower depletion depth, such that there is a trade-off between ease of cooling the CCD and hard X-ray sensitivity. A recent discovery [Thorne et al] shows that rapidly switching the clocks between on and off states, so that stored charge moves from one electrode to the next and back, should produce a factor tex2html_wrap_inline207 reduction in dark current. The physical principle behind this technique is that dark charge generation is a time dependent phenomenon, and takes tex2html_wrap_inline333 msec to reach steady state. The so-called ``Dithering'' or ``wobbling'' of the clocks at a tex2html_wrap_inline347 sec rate therefore prevents the equilibrium generation rate from being reached.

The choice of operating temperature is complicated by the trade-off of CTE, readout noise and dark current performance. Optimum performance can be obtained with a temperature typically in the range -100 tex2html_wrap_inline233 C to -70 tex2html_wrap_inline233 C, depending on the details of the application.

Such temperatures allow cooling without consumable cryogens. In low earth orbit a Peltier effect cooler can be employed, but with a penalty of substantial power requirements. In the cases of XMM and JET-X a highly eccentric orbit, with minimal thermal load from earthshine, permits the use of passive radiators.


next up previous
Next: Modes of Operation Up: CCD Scientific Performance Previous: Field of View

Dave Lumb
Fri Aug 30 07:43:04 MET DST 1996