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The exceptional efficiency and wavelength response of CCDs are illustrated in Fig 3. In the case of a front-illuminated CCD (Fig 1) the loss of efficiency to the blue is due to absorption of blue photons by the electrodes before they can penetrate to the substrate. Redder photons have no such difficulty, but penetration into the substrate increases strongly with wavelength. Generally electrons generated further into the substrate diffuse back up to be successfully collected in the potential wells under the electrodes. However, with very deep penetration by the reddest photons, photoelectrons are progressively lost, and indeed infra- red photons pass straight through the substrate without generating any photoelectrons. To improve the blue response, the thinned, back- illuminated CCD was developed, in which the device is turned around so that the substrate faces the light and the blue photons are not faced with the immediate hurdle of impenetrable electrodes. The substrate is thinned to maximize the collection of photoelectrons by the now backward- facing electrodes. The remarkable feature of the design is that the thinning of the substrate does not seriously impair the red response.

Fri Jun 10 17:31:56 BST 1994