Etaloning in Back Illuminated CCDs
also eXcelon new-generation reduced etaloning CCD/EMCCD
Backilluminated CCD's are
thin devices (typically 10-20 microns thick) which become
semitransparent in the near infrared. Reflections between
the nearly parallel front and back surfaces of these
devices cause them to act as etalons.. This etalon-like
behavior leads to unwanted fringes of constructive and
destructive interference which artificially modulate
a spectrum. The extent of modulation can be very significant
(over 20%) and the spectral spacing of fringes, typically
5 nm, is close enough to make them troublesome for almost
all NIR spectroscopy.
A Review of Etalons
An etalon is a thin
flat transparent optical element with both surfaces
highly reflective. This forms a resonant optical cavity
and only wavelengths which fit an exact integer number
of times between the surfaces can be sustained in it.
Because of this property, etalons can be used as comb
filters, passing just a series of uniformly spaced wavelengths.
In an imperfect etalon, the reflectance of the surfaces
becomes less than 100% and the spectral characteristics
soften from a spiky comb to a smooth set of fringes.
How Etaloning Works in a Backilluminated CCD
NIR wavelengths, the silicon that CCD's are made of
becomes increasingly transparent The back surface, where
light enters a CCD in the backilluminated configuration
is typically antireflection coated. These coatings are
not perfect and their effectiveness varies by wavelength.
four). At wavelengths where silicon is transparent enough
that light can traverse the thickness of the CCD several
times, one can expect light to bounce back and forth
between the two surfaces. This increases the effective
path length in the silicon and thus the QE, but it also
sets up a standing wave pattern. At long wavelengths,
several passes cause significant constructive or destructive
interference. A backilluminated CCD is typically about
17 microns thick. Since the index of refraction of silicon
is almost four, the effective optical thickness is about
60 microns. The round trip optical path length between
the surfaces is thus about 120 microns. At 750 nm, this
is 160 wavelengths. Thus there will be constructive
interference at 750 nm. The next wavelength where there
will be constructive interference will be where 161
wavelengths fit in 120 microns, about at 745 nm. Thus
a pattern of constructive and destructive interference
will repeat at intervals of about 5 nm. In a thin CCD,
there are two types of etalon pattern, one spatial and
the other spectral a is not evident. In a backilluminated
the fringes are due to the variation of the wavelength,
not the thickness.
How to Avoid Etaloning
To tackle this problem,
Princeton Instruments has develop a backilluminated
CCD which has maximum etalon reduction. It combines
1. The CCD is made on thick silicon, 40-50 microns.
Since this is more than double the thickness of a normal
backilluminated CCD, it significantly contributes to
the absorption of NIR light, reducing the amount of
light which survives a round trip path (and which can
then interfere). This increased thickness also increases
the QE at NIR wavelengths.
2. The antireflection coating has been optimized
for NIR wavelengths. This reduces the amount of light
which is reflected back into the CCD when it comes to
the back surface from the polysilicon side. It also
increases the amount of light which passes into the
CCD in the first place, increasing the QE and reducing
stray light in the spectrometer.
3. The back surface is processed in a proprietary
way that helps to break up the etalon effect. The combination
of these approaches has resulted in a CCD which is uniquely
suited to low light NIR spectroscopy. This CCD is available
exclusively from Princeton Instruments.