EMCCD Sensor Enhancements
Extended QE and Fringe Suppression
Selected iXon models are now available with a new EX2 Dual Anti-Reflection coating
and Fringe Suppression technologies, applied to the back-illuminated sensors. EX2
affords a significant enhancement of the Quantum Efficiency performance in the blue and
red/NIR regions, and Fringe Suppression reduces the effect of etaloning in the NIR.
EX2 Technology - Extended QE with Dual AR coating
Available on the new speed-boosted iXon Ultra 897 and 888 cameras, EX2 technology facilitates
broadening of the QE range of the back-illuminated sensors through
implementation of a new dual layer anti-reflection coating process,
developed by sensor manufacturer e2v (Chelmsford, England). The
net effect is to offer significantly improved sensitivity in both the blue
and NIR wavelength regions, whilst maintaining ~ 90% QE across
the remainder of the visible region.
QE varies somewhat as a function of sensor cooling temperature,
so it can be useful to view reference QE curves at two extreme
temperatures to achieve a sense of this dependence. Figure 1 shows
QE curves for the new EX2 dual AR coated sensors versus that of the
'standard' BV single AR coating (mid-band optimised) at both +20°C
and -100°C sensor cooling temperatures. It is clear, notably under
more typical conditions of deep cooling, that the EX2 curve exhibits
a marked widening of the QE response into both the blue and red/NIR
wavelength ranges, while maintaining ~ 90% QE in the important
region in between.
Figure 2 shows the differential variation in QE for the EX2 sensor
relative to the standard BV curve, presented again for +20°C and
-100°C sensor cooling temperatures. It is evident that a relative QE
boost of up to 130% is possible in the blue region at -100°C.
Furthermore, when compared to a competitive multi-AR coated
back-illuminated EMCCD sensor (available on the same sensor
formats), data for each available for comparison in the approximate
+20°C sensor temperature, the EX2 sensor exhibits higher QE in the
wavelength range between 400-650 nm, i.e. exhibiting less QE drop
off in this important visible region, and near identical NIR sensitivity.
In terms of credibility, it should also be noted that the QE curves
presented by Andor for EX2 and BV coatings are each measured by
the sensor manufacturer e2v on the same apparatus, and thus are truly
comparable. QE curves measured on different set-ups are subject to
Fringe Suppression Technology
Fringing is generated in back illuminated sensors where the extreme
flat surfaces of the substrate effectively create a Fabry-Perot etalon.
The finesse (thickness) of the cavity is such that interference between
reflected radiation within the sensor and/or the incoming radiation
at the same wavelength can occur. The interference pattern level
will be wavelength and cavity finesse dependant. This interference
pattern can affect the acquired image or spectrum. Indeed, for NIR applications, such as imaging of Bose Einstein Condenates, etaloning
effects, often observed more notably beyond ~ 750nm, can sometimes
restrict the ability to perform high integrity quantitative imaging.
Fringe Suppression refers to a mature technology offered by the
sensor manufacturer e2v, and refers to a process whereby structured
'roughening' is introduced on the sensor back surface, which, when
coupled with application of a given anti-reflection coating, leads to
the reduction of the radiation reflections within the sensor, hence
reducing significantly the fringing patterns. This does not affect
other sensor characteristics such as dark noise, readout noise, readout
speeds. The process has been validated over many years of CCD
manufacturer and is now available on EMCCD sensors, specifically
relating to the following iXon models: iXon Ultra 897, and iXon Ultra 888.