Sira works on doping to solve harder imaging problems

Citation

(1998), "Sira works on doping to solve harder imaging problems", Aircraft Engineering and Aerospace Technology, Vol. 70 No. 1. https://doi.org/10.1108/aeat.1998.12770aab.002

Sira works on doping to solve harder imaging problems

Sira works on doping to solve harder imaging problems

Sira Electro-optics Division has been asked by the Defence and Evaluation Research Agency (DERA) to see if the next generation of satellite-based charge-coupled imaging devices (CCDs) should store charge in p-type rather than n-type silicon to minimise the problems of radiation damage.

Trapped and solar-flare protons represent a large component of the radiation found in space and are especially damaging to CCDs, introducing defects into the bulk silicon.

Additional shielding against proton radiation is largely ineffective, but the damage it causes can be the limiting factor in the performance of instruments such as star trackers, which are used to control the pointing direction of satellites.

The fundamental problem, according to Sira's CCD expert Dr Gordon Hopkinson, is the generation of dark-current spikes. CCDs intended for space applications usually need a low dark-current (background noise) level and a high charge transfer efficiency.

Proton damage to the crystal lattice of the CCDs' bulk silicon introduces noisy signal interference in the form of sharp bursts of high-current, which seriously disrupt the performance of the devices.

"These spikes can only be controlled by arranging for special cooling of the detectors," says Dr Hopkinson. "But this complicates the design, increases the weight and consumes more power; all things you want to avoid on a satellite." "By simply changing from n-type to p-type channels to handle the signal charge, we expect to eliminate the dark-current spikes."

Sira's feasibility study will be looking at how altering the doping changes the performance of the CCDs and hardens them to cosmic rays and space radiation. EEV of Chelmsford is supplying the specially-doped devices.

Dr Hopkinson says he still expects radiation-induced defects with p-channel CCDs, but for the problems to be far less significant to the devices' performance.

Figure 1: Effect of trapped and solar flare protons on dark current of an MPP CCD