Clock-induced charges (CIC) are an important source of noise in low imaging with EMCCD cameras. These false counts are created when reading a CCD, as the photoelectrons are transferred to the storage area or as they are travelling through the EM register. While these charges are also generated in conventional CCDs, they are negligible since readout noise and thermal noise are orders of magnitude higher. In contrast, the EMCCD technology renders the readout noise negligible, thanks to EM Gain, and is deep cooled to significantly reduce thermal noise. This means clock-induced charges are the dominant source of noise in most EMCCD acquisitions.
In CCDs, the charges must be transferred down to a single component, the output amplifier, where they are read and can be accessed by the user. In order to move the charges, an electronic oscillator, or clock, generates a patterned electric signal that goes from a high voltage to a low voltage. This clock signal oscillation transfers the signal charges, photoelectrons, on the sensor down to the readout area.
When these clock signals reach a certain phase, the inversion phase, positive charges (“holes”) are created in the silicon sensor chip. Some of these holes stay trapped on the chip and, once the clock signal ends the inverted phase, they are accelerated. These fast-moving charges can collide with the silicon atoms of the chip with enough force to create an electron. The electron created is then trapped in a pixel with the photoelectrons where it becomes indistinguishable from the true signal.
As clock signals are intrinsic to the CCD readout process, they cannot be eliminated. The approach to reducing clock-induced charges is thus to optimize the clock signals.
Other EMCCD manufacturers use 2 or 3-level clocks that send rectangular signals on the chip, which gives limited flexibility for improvements; other than achieving a high precision pattern. While it is also possible to control the amplitude of the clock signals, lowering the amplitude will diminish the charge transfer efficiency (CTE) which decreases the quality of the EMCCD image.
However, Nüvü Camēras has created the innovative and patented CCD Controller for Counting Photons (CCCP), which generates both finer and more adjustable clock signals. Nüvü™’s CCCP generates multi-level clocks that can be adjusted on a 14 bits range at a rate of 250 MHz, this means a signal on any 214 levels can be sent each 4 ns. This allows optimizing a specific clock signal for each individual sensor unit, using the precise optimal pattern (sinusoidal, triangular, etc.) to push each EMCCD unit to its maximal performance.
This unique technology has allowed Nüvü Camēras to boast industry-leading performances ever since its creation, with the lowest CIC levels on the market and the highest single-photon detection probabilities.