Spectroscopic silicon imaging detectors: Past achievements and new
Transcript of Spectroscopic silicon imaging detectors: Past achievements and new
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Abstract
The need of high quality spectroscopic semiconductor imaging detectors in X-ray astronomy was the
principal driving force in founding the MPI Semiconductor Laboratory. Detectors developed in this
laboratory are based on new function principles and are processed in the silicon detector processing line
established within the laboratory. We describe the development of pnCCDs as already used in the
XMM-Newton European X-ray observatory and foreseen for eROSITA, the DEPFET based pixel
detector for XEUS and a new development which makes it possible to measure charge with a precision
below one elementary charge. A noise value of 0.25 electrons r.m.s. has already been reached.
Keywords: Silicon detector, pixel detector, imaging, spectroscopy, DEPFET, RNDR, XMM, XEUS
I. INTRODUCTION
The detectors to be described in this article have been developed for application in astronomy, mostly for direct
detection in silicon of individual photons in the energy range up to 20keV. Although the standard applications in
medical imaging do not require the detection and energy measurement of individual photons these detectors may
still offer interesting properties. The very high quantum efficiency down to low photon energies for example
would reduce patient radiation exposure. Combining silicon detectors with scintillating crystals extends the X-ray
energy to higher values. The function principles of most of the detectors to be described have been presented in a
Spectroscopic silicon imaging detectors: Past achievements and new developments
G. Lutz b), R. Andritschke a) , L. Andricek b) , R. Eckhardt c), J. Englhauser a) , G. Fuchs b), O. Hälker a), R. Hartmann c), K. Heinzinger c), S. Hermann a),
P. Holl c), N. Kimmel a), P. Lechner c), N. Meidinger a), M. Porro a), R.H. Richter b), G. Schaller a), M. Schnecke b), F. Schopper a), H. Soltau c), L. Strüder a),
J. Treis a), U. Weichert c), S. Wölfl a)
a) Max Planck Institut für extraterrestrische Physik, München b) Max Planck Institut für Physik, München
c) PNSensor GmbH, München
all at MPI Semiconductor Laboratory, Otto Hahn Ring 6, D 81739 München
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very basic fashion to the Synchrotron Community [1] that has in some respect similar requirements as medicine.
Here they are presented in the framework of concrete projects in astronomy.
II. BASIC SEMICONDUCTOR STRUCTURES
The simplest and most common semiconductor structure is a reverse biased diode (Fig. 1a). Applying a reverse
bias mobile charge carriers (electrons and holes) are removed from the bulk. New carriers generated by radiation
are swept towards the electrodes. Inventing the semiconductor drift chamber [2] E. Gatti and P. Rehak
introduced the sideward depletion structure (Fig. 1b). Holes will move towards the reverse biased large area p+
contacts and electrons will assemble in the centre plane, slowly diffusing towards the n+ electrode. Dividing the
two p+ electrodes into strips (Fig. 1c) and applying a strip by strip rising potential one adds a horizontal drift
field that moves the electrons towards the n+ node. The pnCCD (Fig. 1d) has p+ strips on the top side only and a
negative potential is applied to the uniformly doped backside. A periodically varying potential on these strips,
creates potential wells for storing the electrons.
Varying these potential the charges are moved
towards the n+ doped anodes.
III. SILICON DRIFT DIODES FOR SPECTROSCOPY
In many cases excellent spectroscopic capabilities
are needed while imaging is of no or secondary
importance. For such applications the
semiconductor drift diodes [3] or arrays of them
Figure 2. Silicon Drift Diode (SDD). The signal electrons move along an inclined path toward the anode that is connected to the gate of an integrated JFET transistor
Figure 1.Basic semiconductor detector structures: a) Reverse biased diode; b) Sideward depletion structure, basis of the semiconductor drift detector; c) Drift detector; d) pn-CCD
a) b) c) d)
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are a good choice. This device (Fig. 2) has a
uniform thin radiation entrance window on the
backside, cylindrical geometry and a transistor
integrated in the centre. Two of those devices
are part of the APXS (Alpha Particle X-ray
Spectrometers) on the Mars Rovers [4]
analyzing the chemical composition of the
Mars soil by measuring the alpha induced X-
ray spectrum (Fig. 3).
Figure 3. Alpha induced X-ray spectrum from the APX Spectrometer of the Mars rover.
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IV. PNCCD AT THE XMM-NEWTON SPACE BASED X-RAY
OBSERVATORY
The goal of the XMM-Newton X-ray Observatory (Fig.4) is
the detailed investigation of the physics of X-ray sources as
for example black holes. The large aperture and the
spectroscopic capability make it possible to investigate faint
sources and to analyze element composition. It was launched
into orbit December 1999. The largest (6x6 cm2) existing
monolithic CCD [5] is based on the principle explained in
section II (Fig.1). The signal charge is stored and shifted at a
depth of ~10μm. Parallel architecture – each column having
its own amplifier – allows fast readout (4ms for the complete
device) at low noise (4 electrons r.m.s) and high quantum
efficiency (>90% in the range of 300eV to 10keV). The
device is working perfectly since launch, still delivering
excellent images [6].
Figure 4: Artist’s view of the XMM-Newton X-ray Observatory. Each of the three Wolter I mirror telescopes consist of 57 nested parabolic-hyperbolic mirror shells with 7.5 m focal length. A 6x6 cm2 pn-CCD forms the focal plane image detector.
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V. FRAME STORE PNCCDS FOR EROSITA
ROSITA will make an all sky survey of X-ray
sources. Seven Wolter I type mirror telescopes,
each one with
its own focal
detector will
look in parallel
to the same
object (Fig. 5).
Again pnCCDs
will be used. The CCDs possess a 2x2 cm2 image area with 256x256 75x75
μm2 size pixels and a smaller frame store area with the same number of pixels
(Fig. 6). Image transfer to the frame store area is rather fast (100 μs) and the
full image collection time (5ms) is available for low noise readout. Compared to XMM a very strong
improvement of all key performance parameters has been reached as is demonstrated in Fig. 7 which compares
the Carbon Kα spectrum with that obtained at XMM.
Figure 5: (a) eROSITA concept (b) photo of frame store pnCCD
Figure 6: Layout of the eROSITA frame store pnCCD
Figure 7: Carbon-K spectrum measured with (a) eROSITA frame store and (b) XMM pnCCD
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VI. DEPFET PIXEL DETECTORS FOR XEUS
XEUS (X-ray Evolving Universe Spectroscopy) is the
planned follow up mission of XMM-Newton. Its scientific
aim is the investigation of the universe at an early
evolution stage by studying early black holes, the
evolution and clustering of galaxies and the evolution of
element synthesis. The collection area will be increased by
more than one order of magnitude. Consequently the focal
length raises from 7.5 to 50m making it necessary to fly
mirror and focal plane instrumentation on separate
satellites (Fig. 8) following each other with a precision
of 1mm3.
The focal plane detector is based on the DEPFET
(Depleted Field Effect Transistor) concept [7]. A p-
Figure 9. The Concept of a DEPFET
Figure 10. Schematics of a DEPFET pixel detector.
Figure 11. 55Fe spectrum measured at room temperature with a cylindrical DEPFET at 6 μs Gaussian shaping.
Figure 8. An artist’s view of XEUS. A DEPFET pixel detector located on the focal plane satellite provides a spectroscopicX-ray image.
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channel field effect transistor is placed on the fully depleted bulk (Fig.9). By suitable doping a potential
maximum (internal gate IG) is created below the transistor channel. Electrons created anywhere in the depleted
bulk are collected in the IG, inducing a mirror charge within the channel, thus increasing its conductivity. The
unique properties of this device as for example combined detector and amplification properties, signal charge
storage and non destructive readout make it useful for many applications. Of particular interest is its use as
building block of a pixel detector with very low noise and power consumption. An extended area is covered with
properly connected DEPFETs (Fig. 10). An individual row of transistors can be selected for readout while all
other DEPFETs are turned off able to
collect signals without consuming power.
The collected charge is measured by
subtracting the drain current before and
after clearing the internal gate. Single
DEPFETs and 64x64 prototype pixel
matrices have been produced. Fig. 11 shows
the noise performance of a single XEUS-
type DEPFET at room temperature
resulting in a noise figure of 2.2 electrons
r.m.s. An 55Fe measured with the fully
operational 64x64 pixel matrix at -50°C is
seen in Fig. 12.
0 1 2 3 4 5 6 71
10
100
1000
10000
Si-KαAl-Kα
Mn-Kβ
Mn-KαC
ount
s
Energy (keV)
Escape Peak
0 1 2 3 4 5 6 71
10
100
1000
10000
Si-KαAl-Kα
Mn-Kβ
Mn-KαC
ount
s
Energy (keV)
Escape Peak
Figure 12. 55Fe spectrum measured at -50°C with a 64x64 prototype DEPFET pixel detector. The pixel current was 30μA, the line processing time 25μs. The energy resolution of 126 eV FWHM of the Mn-Ka line corresponds to 4.9 electrons ENC.
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VII. SUBELECTRON PRECISION CHARGE MEASUREMENR WITH DEPFETS
The precise measurement of charge down to one electron is required
in High Time Resolution Astronomy. There one wants to observe
optical photons rapidly and periodically changing objects as are for
example rotating neutron stars or binary objects. Rotation periods
down to milliseconds have been observed. Looking at faint distant
objects, one or even fewer photons will reach the telescope during
one rotation period. The non destructive readout possible with
DEPFETs allows the measurement of charge with a precision well
below one elementary charge. Avalanche multiplication that does
not allow the distinction between one or few electrons therefore is not needed.
The possibility of multiple reading with the help of a pair
of DEPFETs has already been proposed in the original
publication on DEPFETs (Fig. 13) and other structures
[7]. RNDR (Repeated Non-Destructive Readout) can be
done for example by measuring the DEPFET current with
the signal charge in the internal gate, moving the charge
Figure 14. Cell topology of the RNDR DEPFET pixel matrix. (TG…transfer gate, CLG… clear gate)
Figure 15. Layout of the 4x4 RNDR DEPFET pixel matrix.
Figure 13. Duble gate DEPMOS transistor with transfer gates TG1 and TG from Ref. 7.
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to an intermediate storage place, re-measuring the current and taking the difference, moving the charge back into
the internal gate, repeating this procedure N times and taking the average. The noise will then decrease by the
square root of N. This holds for serial noise including the 1/f noise that cannot be reduced by standard methods.
A more elegant and efficient arrangement is a pair of DEPFETs with a transfer structure allowing shifting of
charge from one internal gate to the other. These DEPFETs may be part of a differential amplifier.
A matrix with 4x4 DEPFET pairs and 75x75 μm pixel size has been built and tested. Fig.14 shows the cell
topology, Fig. 15 the matrix layout.
The double DEPFET structure has only one transfer gate. As a consequence one of the two DEPFETs has to be
turned off during transfer. This and the fact that no specially adapted readout electronics was available is the
reason that the two DEPFETs were read out separately and the differences taken arithmetically. The
measurements presented in Figs. 16 and 17 were taken at -45°C. Each histogram entry represents the average of
180 loops. One loop contains four current measurements (each transistor with and without charge in the internal
gate). The loop time was 51 μs, the leakage current 1 electron in 14 ms. A weak laser pulse of adjustable
intensity could be injected before the start of the readout.
Without laser pulse (Fig. 16) the single charge peak is located at zero with a width of 0.25 electrons r.m.s and a
tail to higher values. This tail is due to electrons generated thermally during readout that are only partially
measured. At very low laser pulse intensity individual peaks for 0,1,2,3,... electrons are seen. Increasing the
pulse intensity further (Fig. 17) one sees a multi peak distribution that can be fitted by a Poisson distribution with
mean value of twelve, folded with the measurement resolution.
Figure 17. RNDR charge measurement with more intense laser pulse illumination. Figure 16. RNDR charge measurement without and with
very weak laser pulse illumination.
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A further production with 64x64 RNDR matrices and triple transfer gates is in production. These devices promise
in combination with a custom built microelectronics readout chip faster and even lower noise readout.
VIII. SUMMARY
A variety of detectors developed in the MPI Semiconductor Laboratory for application in MPI experiments have
already and will find further applications in other fields of science. A rather significant achievement is the
measurement of charge with a resolution much below one electron. This result has been presented for the first
time at this conference.
References
[1] G. Lutz: “Silicon drift and pixel devices for X-ray imaging and spectroscopy”, J. Synchrotron Rad. (2006). 13, 99-109
[2] E. Gatti and P. Rehak: "Semiconductor drift chamber - an application of a novel charge transport scheme", NIM A, 225, p.608 − 621, 1984.
[3] J. Kemmer and G. Lutz: “Low capacitive drift diode”, Nucl.Instr. Meth. A 253 (1987) 378-381 [4] R.Rieder et al: The new Athena alpha particle X-ray spectrometer for the Mars Exploration Rovers, Journal of
Geophysical Research 108(E12), 8066, doi:10.1029/2003JE002150, 2003. [5] L. Strüder et al.: The European Photon Imaging Camera on XMM – Newton : The pnCCD camera. Astronomy and
Astrophysics, 365:18-26, 2001 [6] L. Strüder et al.: pnCCDs on XMM-Newton – 42 months in orbit, NIM A512 (2003) 386-400 [7] J. Kemmer and G. Lutz: ''New semiconductor detector concepts'', Nucl. Instr. & Meth. A 253 (1987) 365-377 [8] XEUS Astrophysics working group: "X-ray Evolving - Universe Spectroscopy - The XEUS scientific case", ESA SP-
1238 (1999), 30 pages