Specifications

Version 1.1 rev Jan 2013
Page 46
iXon Ultra
, Features and Functionality
3.1.7 - EM Gain Ageing: What causes it and how is it countered?
As already noted in Section 3.1.5, EMCCD sensors can suffer from EM Gain ageing. This is the phenomenon whereby
the EM Gain falls off over a period of time, when operating at the same clock voltage and cooling temperature. This
ageing effect appears to be dependent on the amount of charge that is passed through the gain register, combined
with the actual EM electric eld strength that it is transferred through. It seems to be very strongly dependent on the
EM electric eld strength. Therefore when operating at high EM Gains the ageing rate can be disproportionately greater.
Fortunately, it has been observed that this ageing effect itself decreases with time, meaning that, with proper use, the
device should remain useful for many years. As part of Andor’s EMCCD production process, all sensors are conditioned
so much of the “shorter-term ageing” has already occurred prior to calibration and setting of the EM Gain.
The rationale for this ageing effect is not fully understood, but it is assumed that accelerating charge through the high
electric elds is causing a tiny fraction of that charge to become permanently embedded in the insulator (typically
silicon dioxide) between the EM electrode and the active silicon. This slow build-up of charge effectively reduces the
eld strength produced by the electrode. The signal electrons, therefore, experience a lower accelerating potential,
which subsequently produces fewer secondary electrons from the impact ionisation process resulting in less electron
multiplication and, in effect, a lower EM Gain.
In order to minimise the effect of EM Gain ageing it is recommended that the following guidelines are always adhered to:
• Do not use EM Gains greater than necessary to overcome the read noise (please refer to Figure 13 on page
44 and Figure 14 on page 45). A gain of x4 or x5 of the root-mean-square read noise (accessible from the
performance sheet) is normally more than sufcient to render this noise source negligible. In practice, this can
always be achieved with EM Gain of less than x500 (much less for the slower readout speeds). Pushing gain
beyond this value would give little or no extra S/N benet and would only reduce dynamic range.
• OnlyselecttheextendedEMGainscaleofx1000whensingle photon counting and always ensure that the
signal falling onto the sensor is within the regime of low numbers of photons per pixel.
• TurntheEMGainOFF when not in use.
• Trynot to over-saturate the EMCCD detector.
Andor’s linear and quantitative gain calibration scale (RealGain
TM
) is one of a number of systems that prevent conditions
from being selected that may result in accelerated sensor ageing in addition to allowing the optimum imaging
performance to easily be achieved.