Dark current

The CCD output is proportional to the exposure, in which ER is the illumination on the surface of the  CCD sensor and the integration time. The output signal can be enhanced by increasing the integration time.... and long integration times are generally used for low light level applications. However, this approach is quickly limited by the generation of dark current, which is integrated such as photocurrent. The dark current is expressed in current density [ [A/m2] or in electron/pixel/second [e -/pix/s] (Fig.15). For a large pixel (24 x 24µm2), we can reach a dark current density of 1000pA/cm2, producing 36,000 electrons/pixel/second. If the system has a full well capacity (FWC) of 360,000 electrons, the well is filled in 10 seconds. The dark current is sensible only when is large. It can be the case in scientific applications at low light level (studies of plasmons, photoemissions, low-reflection materials, astronomy....

A critical parameter in the design of the chip will be to reduce significantly the dark current. There are three potential sources of dark current:

• Thermal generation and diffusion in the bulk,

• Thermal generation in the depletion zone,

• Thermal generation due to surface states.

The dark current density varies significantly according to the manufacturers and in a range between 0,1nA/cm2 and 10nA/cm2 for silicon-based CCDs. The dark current due to the thermal generation of electrons can be solved by cooling the system. In principle, the dark current density can be made negligible by adequate cooling. The dark current density decreases approximately by a factor two for each decrease of 7 to 8°C f the temperature of the matrix and vice versa. Cooling is particularly important in scientific applications at low light level where high precision on charge level of the wells (greyscale) is required. TEC (ThermoElectric Cooling) systems, are Peltier systems driven by electric current pumping the heat of the CCD to a radiator. The radiator is cooled by air (forced or not), or by a circulating liquid (water, liquid nitrogen...).

As the liquid nitrogen is at a temperature of -200°C, the optimal working temperature is between -60°C and -120°C because the charge transfer efficiency (CTE: reliability to transfer the charge for site to site) and the quantum efficiency decrease at inferior temperatures. Condensation is a problem and matrices should be placed in a low pressure chamber or a chamber filled with a dry atmosphere. The dark current can approximate 3.5 electrons/pixel/second at -60°C and 0.02 electrons/pixel/hour at -120°C.

The output amplifier continuously dissipates heat. It results in the local heating of the silicon chip. To minimize this effect, output amplifiers are more often separated by several insulation pixels to insulate locally the amplifier from other active pixels.