Roper Photometric manual Thick and Thinned CCDs

Spectral Response Light normally enters the CCD through the gates of the parallel register. These gates are made of very thin polysilicon that is reasonably transparent at long wavelengths but becomes opaque at wavelengths shorter than 400 nm. Thus, at short wavelengths the gate structure attenuates incoming light.

Thinning With acid etching techniques, CCDs can be uniformly thinned to approximately 10 µm, and an image can be focused on the backside of the parallel register where there is no gate structure. These thinned, or backside-illuminated, CCDs exhibit high sensitivity to photons from the soft x-ray to the near-infrared regions of the spectrum. Thick and Thinned CCDs illustrates the structures of both kinds of device.

Thick and Thinned CCDs

Incoming light

Polysilicon gate

Silicon dioxide

Silicon

Thinned silicon

Incoming light

Down Converters CCD spectral response can also be extended with the use of a light-emitting phosphor called a down converter. A down converter absorbs light in the ultraviolet range of the spectrum and re-emits it in the visible range.

A CCD with a Down Converter demonstrates the use of the phosphor down converter Metachrome® II in conjunction with a front-illuminated CCD.

A CCD with a Down Converter

120-450 nm light

400-1000 nm light

Phosphor

Polysilicon gate

Silicon dioxide

Epitaxial silicon

Bulk silicon

560 nm emission

The phosphor absorbs photons with short wavelengths and emits photons with a 560 nm wavelength. These photons pass through the polysilicon gates into the photon-sensitive region of the CCD. The down converter is transparent between 400 and 1000 nm, so CCD performance is not affected at these longer wavelengths.

Down converters exhibit very high quantum efficiency and, when properly processed and applied to CCDs, can produce a significant improvement in UV sensitivity.

38Advanced Camera Operation Manual