Download Investigation of plasma effects in silicon sensors for the

Investigation of plasma effects in
silicon sensors for the European XFEL
J. Becker*, D. Eckstein, R. Klanner, G. Steinbrück
University of Hamburg
Institute of Experimental Physics
* email: [email protected]
1. Introduction XFEL
The European XFEL will push the limits of
brilliance farther than any light source today.
Examples of foreseen applications include the
study of structures of complex biomolecules,
resolving tiny structures like viruses and
investigations of the evolution of femtosecond
chemical processes.
The expected dynamics, from single photons
to 105 12 keV photons/pixel/pulse, is a
challenge for the design of silicon sensors and
front end electronics.
Three beamlines differing in photon energy
are foreseen: SASE 1: 12.4 keV, SASE 2:
3.1 - 12.4 keV and SASE 3: 0.8 - 3.1 keV
2. Transient Current
Technique (TCT)
5. Investigated strip sensor
• strip pitch of 50 µm, strip width of 11 µm.
• thickness of 450 µm
• n-type silicon float zone material, p+ readout
• depletion voltage of 155 V
• <111> orientation
• processed by Hamamatsu Photonics
• ≥5 strips neighboring the investigated strips where grounded
8. Impact on sensor performance
From the PSF the modulation transfer function (MTF) has been
calculated. The MTF is the magnitude of the Fourier transform of
the PSF.
The MTF shows contrast as function of spatial frequency,
allowing to quantify the image quality.
6. Increase of charge collection time
Measurements of the charge collection time (time needed to
collect 95 % of the total charge) have been performed as
function of the applied bias voltage for different intensities.
To avoid non-linearities and pile up effects a sufficiently large
bias voltage must be applied (depending on integration time of
the readout)
The Transient Current Technique records the
time resolved current pulse of a sensor.
Photons have an energy dependent attenuation
length in silicon (2.8 µm for 1 keV γ, 250 µm for
12 keV γ) and create electron hole pairs by
ionization along their path. These charge
carriers drift in the electric field and thus cause
a current in the readout electrodes.
Fig. 4: MTF for 660 nm light (1 keV γ) (left) and 1015 nm light (12 keV γ)
(right). Solid lines show data for 500 V, dashed lines for 200 V applied
bias. Black vertical lines mark the Nyquist frequency.
From the value of the MTF at the Nyquist frequency
( f Ny = 12 f sampling = 2 d1pixel ) the imaging performance can be estimated.
The obtained contrast (value of the MTF) decreases for
increased photon densities but stays above 0.5 for all intensities.
3. Electron hole plasmas
When many photons are absorbed the charge
carrier density can exceed the bulk doping
O(1012 cm-3) and e,h plasmas with following
properties are created:
• Local distortions of the electric field inside
the sensor lead to modified transport
properties.
• Field free regions inside the plasma lead to
ambipolar diffusion as dominant transport
process. The plasma dissolves slowly
which affects the pulse shape (plasma
delay).
• Mutual charge carrier repulsion results in
further increased charge carrier spread.
4. Measurement setup
Multi Channel TCT key features:
Use of short laser pulses (~100 ps) of high
energy (up to 105 12 keV γ equivalent) with
660 nm, 1015 nm, 1052 nm wavelength,
corresponding to 1 keV γ, 12 keV γ and mips,
respectively. The laser is focused to a Gaussian
spot with σ ≈ 3 µm. Front and rear side injection
is possible.
32 readout channels with <100 ps risetime are
available (4 simultaneously)
laser
driver
optics
Fig. 2: Time needed to collect 95 % of the total charge. Bias voltage must be
chosen large enough that the collection time is smaller than the integration
time of the readout. Charge collection times without plasma effects range
from 30 ns for 200 V to 12 ns for 500 V.
7. Charge cloud explosion
From position sensitive measurements the point spread
function (PSF) has been determined.
The PSF could be described as the convolution of a circle and
a Gaussian function. For 660 nm light of high intensity the
circular properties are more pronounced, for 1015 nm light the
Gaussian properties.
It was observed that the charge spread is decreasing with
increased applied voltage. The shape of the PSF is a strong
function of the charge carrier density.
The charge spread at high intensities is smaller for 660 nm light
than for 1015 nm light.
residual light
2.5 GHz
control
scope
Fig 5: Contrast at Nyquist frequency. At low intensities contrast for 660 nm
is lower than for 1015 nm due to the larger diffusion of the charge carriers.
At higher intensities the contrast is reduced due to plasma effects.
9. Summary
Plasma Effects in silicon sensors were observed and
studied using a multi channel TCT setup.
• Charge collection times have been measured as function of
intensity and applied bias voltage.
High bias voltages are needed to collect all the
generated charge within the bunch repetition time of
the European XFEL (220 ns).
• The point spread function (PSF) was measured as function
of intensity and applied bias voltage.
The PSFs could be described by the convolution of a
circle and a Gaussian function.
• Modulation transfer functions were calculated allowing to
quantify the impact of plasma effects on imaging
performance
A significant reduction of contrast was found for
high intensities.
More information under DOI:10.1016/j.nima.2010.01.082
10. Acknowledgements
attenuators,
amplifiers
laser
linear tables
cooling
DAQ and
control PC
Fig. 1: Photograph of the setup used for the
measurements. The setup can be closed and flushed
with nitrogen to allow measurements at low
temperatures (down to -30°C).
Fig. 3: PSF for 660 nm light (1 keV γ) (left) and 1015 nm light (12 keV γ) (right).
Solid lines show data for 500 V, dashed lines for 200 V applied bias. The PSF is
symmetric along the x-axis. The black vertical lines mark (half) the AGIPD pixel
size for comparison.
This work was partly funded
by the Helmholtz Alliance
‘Physics at the Terascale’,
the Federal Ministry of
Education and Research
and the European XFEL
Consortium.