Quantum efficiency 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 in which 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 sensitivity of CCDs in the blue visible and ultraviolet wavelengths (200 nm - 400 nm), it is also possible to coat a CCD with Metachrome® II, a proprietary phosphor developed by Photometrics and Unichrome, a coating optimized for back-illuminated CCDs developed by Princeton Instruments.
ITO, A new Technology improves Blue/Green
Sensitivity of Frontside-Illuminated CCDs
In an effort to boost the sensitivity of frontside-illuminated CCDs in the blue/green region of the spectrum, the Eastman Kodak Company has pioneered a new gate structure based on indium tin oxide (ITO). Indium tin oxide was a logical candidate for a new gate material, as it has been used for many years to provide a clear conductive coating in a wide variety of applications. Kodak's Microelectronics Technology Division has developed a new fabrication process that produces gates that are more transparent to light. Based on indium tin oxide, these gates provide higher light throughput into the photoconversion layer of the CCD. The resultant imaging devices have higher QE levels than those attainable with conventional frontside-illuminated CCDs. This improvement extends across the visible spectrum, including the blue/green region.
The quantum efficiency of ITO imagers in the blue/green region exceeds the QE performance generally seen in lens-on-chip and open-electrode designs. Furthermore, ITO devices have no inherent reduction of dynamic range and carry a fairly comparable price tag. All three of these designs are outperformed by backside-illuminated CCDs in terms of quantum efficiency, but as was pointed out earlier, price and availability may be the largest issues when considering thinned devices. Overall, ITO technology represents an excellent " price and performance" option for many low-light-level applications that require imaging in the blue/green.