Quantum efficiency (QE) is the measure of the effectiveness of an imager to produce electronic charge from incident photons. This is an especially important property when doing low-light-level imaging. Because most CCD imagers are made from silicon, it is useful to examine the properties of this element and the way that it interacts with light.
In the high-purity crystalline form, each atom of silicon is covalently bonded to its neighbor. Energy greater than the band gap energy, about 1.1 eV, is required to break a bond and create an electron/hole pair. The wavelength of incoming light and photon absorption depth are directly related; the shorter the wavelength, the shorter the penetration depth into the silicon.
Light normally enters the CCD through gates of the parallel register (front-illuminated CCD). These gates are made of very thin polysilicon, which is reasonably transparent at long wavelengths, but becomes opaque at wavelengths shorter than 400 nm. Thus, at short wavelengths, gate structure attenuates incoming light.
It is possible, using acid-etching techniques, to uniformly thin a CCD to a thickness of approximately 10 μm and focus an image on the backside of the CCD register where there is no gate structure (back-illuminated CCD). Thinned CCDs exhibit high sensitivity to light from the soft x-ray to the near-infrared regions of the spectrum.
To improve the sensitivity of CCDs in the blue-visible and ultraviolet wavelengths (200 nm to 400 nm), it is also possible to coat a CCD with Metachrome II, a proprietary phosphor developed by Teledyne Photometrics.