The interline CCD is a hybrid sensor with photosensitive diodes on one part of the pixel which are electrically coupled to a CCD type storage region which resides under a mask structure. The masks are long structures running along the vertical axis of the CCD alternating with the open regions, hence the name interline CCD. The diode portion of the pixel has very good QE properties, but the diode only takes up approximately 25% of the pixel area (25 % fill factor), reducing the number of photons converted per unit area.
Lens on Chip
As a way to drive up the fill factor, higher quality interline CCDs have small lenses annealed to the CCD that bring the light from a larger area down to the photodiode. This brings up the fill factor to around 70% and results in a better net QE across the visible spectrum than a CCD without the lenses. In this type of CCD, the signal accumulates on the photodiode and is then rapidly shifted to the adjacent CCD structure, taking approximately 1 microsecond to perform the transfer. This extremely fast transfer means that the smearing will be non-detectable for any exposure of a millisecond or longer. In addition, this type of rapid shifting allows for some interesting uses of the interline CCD such as very short exposures for fast moving objects and is an ideal CCD to use with slow gating types of measurements.
a new Technology Improves Overal Quantum Efficiency of Interline
Sony improved the microlens technology with the introduction of Super HAD CCD (HAD, stands for Hole Accumulation Diode). Super HAD CCDs have much closer spacing between microlenses, thus further increasing the light collection efficiency.The latest of Sony's innovations is introduction of additional layer of on-chip microlens very close to the pixel area. When used with wider f-stops, single array of microlens can not focus the parallel beams on to the sensing area of the pixel, reducing the sensitivity. Second layer of lenses helps alleviate this problem by further condensing the beam path, consequently increasing QE. Another improvement in this technology is thinning of insulating layer between Si substrate and poly Si gate structures, reducing the smear factor (light leaking under the mask). With the introduction of EXview HAD CCDs, Sony took the interline technology another step by improving the QE in near infrared (NIR) region. As the NIR photons are absorbed in deeper part of the Si, using thicker Si, increases the probability of photon-Si interaction and thus QE. Our new CoolSNAP HQ uses the EXview HAD technology
Roper Scientific has engineered the read out of the Exview HAD CCDs to bring the full well to the absolute highest level while maintaining linearity throughout the imaging range of the device. Getting the readout noise even at 20 MHz to the lowest level possible allows the maximum in dynamic range performance.
Quantum efficiency (QE) is the probability of a photon being absorbed in the Si. By allowing the more light reach the Si, QE is increased. Quantum efficiency, often expressed as a percentage, indicates the effectiveness of an imager to produce electronic charge from incident photons. The greater the QE at a given wavelength, the more efficient the imager at that wavelength.