Institut de Physique Nucléaire, Atomique et de Spectroscopie

QUANTUM EFFICIENCY OF
BACK-ILLUMINATED CCD DETECTORS
IN THE VUV REGION (30-200 nm)
H.P. Garnir , P.H. Lefèbvre
Institut de Physique Nucléaire, Atomique et de Spectroscopie, Université de Liège,
Sart Tilman B 15, B-4000 Liège, Belgium
[email protected]
RESULTS AND DISCUSSION
Our results are in good agreement with the QE
obtained by other authors (fig 3). This suggests that
the overall efficiency of a CCD is not too dependant
of the manufacturing process and that the proposed
curves could reliably by trusted for all thinned
backilluminated CCD.
INTRODUCTION
Charge-coupled devices (CCD's) detectors are
commonly used for spectroscopy of highly ionized
ions. Their efficiency could be improved at short
wavelengths by illuminating the CCD from the back.
To get the light directly into the sensitive area, the
substrate is thickened to less than 20µm by a
chemical process (a.e. immersion in an acid bath). A
simplified diagram of a back illuminated CCD
structure is presented on Fig.1.
Quantum efficiency (QE) is the measure of the
effectiveness of an imager to produce electronic
charge from incident photons. QE determines the
fraction of photons incident on a device that is
actually detected.
Let us note that for photons having an energy higher
than the electron-hole pair creation energy threshold
(3.65 eV or 340 nm for Si) more that one pair could
be produced. To take this effect into account, quoted
QEs for Si based detectors at wavelengths shorter
than 340 nm are corrected by a factor of λ(nm) / 340.
Due to that definition QE can never exceed 100 %.
60
QE (%)
EXPERIMENT
The CCD detector system was supplied by the
Universities of Leicester and Lund. It is based on a
EEV CCD15-1 chip of 27.6x6.9 mm (1040x280 of
27x27 µm square pixels) specially conditioned for
UV light detection [1]. The CCD works under
vacuum and is cooled by liquid nitrogen to -90 C for
noise reduction.In order to evaluate the efficiency of
our detector in the extreme UV region we have
+
recorded spectra produced by an N ion beam
excited in a thin foil (beam-foil light source) with our
newly installed CCD (Fig. 2) and with a channeltron
detector or a PM whose efficiencies are well known.
By comparing the intensities of many observed lines,
we have been able to establish an quantum efficiency
curve for our CCD detector between 30 and 200 nm.
Fig. 1 - CCD Structure simplified diagram
right : front illuminated ; left: back illuminated
40
20
0
40
60
80
100
120
140
160
180
200
Wavelength (nm)
Vacuum seal
Fig. 3 - QE of a back illuminatd CCD in the 30 -200 nm
region.
Data are from : • this work ; [2] ; [3] [4]
The dotted line is a “best fit” through all the data.
CCD chip
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copper screen
rotating support
10 mm
Fig. 2 - Picture of our CCD mounting. The CCD is
cooled and work under vacuum.
REFERENCES
[1] R. Hutton et al., Physica Scripta, T80, (1999), 552
[2] R. Stern et al., Applied Optics, 28 (1994), 2521
[3] J.P. Delaboudinière et al., Sol. Phys. 162 (1995), 291
[4] L. Poletto et al., Ap. Optics, 38, (1999), 29