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EM-CCD Technology

On-chip Mulitplication Gain

Figure 1. This example of an electron-multiplying CCD has a frame transfer architecture

High Performance in Low Light

Recently, CCD manufacturers have introduced novel, high-sensitivity CCDs engineered to address the challenges of ultra-low-light imaging applications – without the use of external image intensifiers. The new detectors utilize revolutionary on-chip multiplication gain technology to multiply photon-generated charge above the read noise, even at supravideo frame rates.

This special, signal-boosting process occurs before the charge reaches the on-chip readout amplifier, effectively reducing the CCD read noise by the on-chip multiplication gain factor, which can be greater than 1000x. The main benefit of the technology, therefore, is a far better signal-to-noise ratio for signal levels below the CCD read-noise floor.

The principal difference between a charge-multiplying CCD and a traditional CCD is the presence of a special extended serial register, known as a multiplication register, in the new device (see Figure 2). Note that since the on-chip multiplication gain takes please after photons have been detected in the device’s active area, it is possible to adapt the new technology to all current CCD formats and architectures. Recently, for example, cameras utilizing back-illuminated versions of these new charge-multiplying CCDs have been introduced (e.g., the Photometrics Cascade:512B).

Electrons are accelerated from pixel to pixel in the multiplication register by applying higher-tha-typical CCD clock voltages (up to 50 V). Secondary electrons are generated via an impact-ionization process that is initiated and sustained when these voltages are applied. The on-chip multiplication gain can be controlled by increasing or decreasing the clock voltages; the result gain is exponentially proportional to the voltage.

Fact
On-chip multiplication gain is achieved by generating secondary electrons via impact ionization.

Technology Description

As mentioned earlier, the gain factor achieved via the impact-ionization process can be greater than 1000x. In fact, on-chip multiplication gain is actually a complex function of the probability of secondary-electron generation and the number of pixels in the multiplication register.

Mathematically, it is given by

G = (1+g)N,

where N is the number of pixels in the multiplication register and g is the probability of generating a secondary electron. The probability of secondary-electron generation, which is dependent on the voltage levels of the serial clock and the temperature of the CCD, typically ranges from 0.01 to 0.016. Although this probability is low, the total gain on actually be quite high, owing to a large number of pixels in the multiplication register. For example, a CCD with pixels N equal to 400 and probability g equal to 0.012 produces on-chip multiplication gain G of 118.
 

Fact
On-chip multiplication gain has an exponential relationship to the CCD’s high-voltage serial clock.