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Displacement damage for CCDs

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Introduction

Silicon optoelectronic sensing arrays (visible, ultraviolet, and x-ray) have been developed for a wide variety of scientific, commercial, and military uses in space. Charge Coupled Devices (CCDs) are available with picoampere dark currents and Charge Transfer Efficiencies (CTE) in excess of 0.999999 per pixel. During the development of these sensors, their susceptibility to ionising radiation effects has been characterised and hardening solutions have been successfully implemented in many cases. For example, oxides have been hardened to reduce flatband shifts and surface related leakage currents, and some CCDs can be run with the Si surface inverted to keep the interface traps filled to minimise dark currents. Bulk displacement damage effects are seen to dominate the radiation response in state of the art scientific imagers when operated in natural particle environments [Janesick et al., 1989, 1991; Holland, 1993; Holland et al., 1991]. The flatband shifts and dark current increases that occur for ionising dose levels below 10-20 krad (Si) 60Co are not serious and can be overcome with minor changes in voltages and operating temperature. In contrast, significant CTE losses are observed for proton exposures of less than 1 krad (Si).

Particle induced displacement damage degrades performance parameters in solid state imagers due to decreased charge transfer efficiency, increased dark current, and dark current spikes. A good estimate of the CTE degradation in a specified radiation environment can be accomplished with minimal device testing, as the displacement damage effects in a device for a given particle and energy can be correlated to the effects of other energies and even other particles [Summers et al., 1987, 1989; Dale et al., 1989]. This correlation has been demonstrated for a number of device types. It is based on the concept of non-ionising energy loss (NIEL), which is the energy a particle imparts to a solid through mechanisms other than ionisation. Non-ionising energy deposition plays the same role in displacement damage effects that ionising energy deposition (i.e. dose) plays in ionising effects. Once the response to a set of test environments has been determined for the detector in question, the correlation can be used to predict the imager performance degradation following exposure to the spectrum of particles and energies expected for a particular space mission.

Heavy shielding is often proposed to ameliorate the CTE degradation of CCDs in particle environments. For thick, high atomic number shields, the displacement damage energy per gram silicon in the CCD from secondary particles produced in the shield is significant, and in some cases dominant.

Experimental CTE damage factors

The first step towards the prediction of CTE behaviour in orbit is a characterisation of the corresponding damage factor as a function of proton energy. Figure 1 presents proton CTE measurements on different CCDs at various energies. The measured parallel CTE per pixel scales linearly with proton fluence so the data are presented as damage factors equal to the change in CTE per unit fluence.

Experimental CTE damage factors
Figure 1. Nonionising energy loss rate compared to experimental CTE damage factors (from [Dale et al., 1993])

The solid line in Figure 1 is the calculated nonionising energy loss (NIEL) rate as a function of proton energy. The data from each CCD type exhibits nearly the same energy dependence as the NIEL, but each data set has been scaled independently by a constant C, which has units of CTE change per unit of nonionising energy deposited. A scale factor is necessary because it is not presently possible to make a first principles calculation of the final stable proton induced defect inventory (defect types and quantities), and its effectiveness at causing CTE changes. The constant also depends on the particular imager design and the readout conditions.

NIEL, which includes the damaging effects of nuclear elastic and inelastic events, predicts the energy dependence of the CTE damage factors reasonably well. Still, the available test data do fall below NIEL at the higher energies, and fall above the 1/E dependence expected for the Coulombic component of the nonionising energy loss rate. The discrepancy at higher energies is possibly due to recoil equilibrium not being established in the beam experiment where normally incident protons generate recoils with ranges longer than the depth to the active volume of the CCD. Hence, NIEL should be employed for the space prediction, since equilibrium conditions are satisfied for an omnidirectional flux of incident particles.

Calculation of displacement damage behind shielding

In SPENVIS, the attenuation of incident protons by an aluminium shielding is evaluated using a routine from the CREME programme suite.

Once the particle environment at the CCD is established, the change in CTE expected on orbit is calculated as follows. The damage constant K(E) is defined as:

DeltaCTE(E) = K(E) Phi(E) ,

where Phi(E) is the fluence of particles with energy E, and

K(E) = C NIEL(E) .

The differential proton spectrum averaged over the orbit and attenuated by a given shielding is used to calculate the amount of damage at each proton energy. The total damage follows from integrating the damage over all energies reaching the CCD:

equation for integrated damage

In practice, the lower limit of the integration is set by the lowest energy of the trapped and solar proton models. Except for lightly shielded imagers, most of the damage results from particles with energies above 10 MeV. Also, the relative gains from adding shield mass diminish as the shield gets thicker.

References

Dale, C., P. Marshall, B. Cummings, L. Shamey, and A. Holland, Displacement Damage Effects in Mixed Particle Environments for Shielded Spacecraft CCDs, IEEE Trans. Nucl. Sci., 40, 1628-1637, 1993.

Holland, A., The Effect of Bulk Traps in Proton Irradiated EEV CCDs, NIM A326, 335-343, 1993.

Holland, A., A. Holmes-Siedle, B. Johlander, and L. Adams, Techniques for Minimizing Space Proton Damage in Scientific Charge Coupled Devices, IEEE Trans. Nucl. Sci., 38, 1663-1670, 1991.

Janesick, J., G. Soli, T. Elliott, and S. Collins, The Effects of Proton Damage on Charged-Coupled Devices, Proc. SPIE, 1447, 87-108, 1991.

Janesick, J., T. Elliott, and F. Pool, Radiation Damage in Scientific Charged-Coupled Devices, IEEE Trans. Nucl. Sci., 36, 572-578, 1989.

Summers, , 1987.

Summers, , 1989.

This text is based on the paper by Dale et al. [1993].

Last update: Mon, 12 Mar 2018