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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