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Novel semiconductor detectors for X-ray astronomy and spectroscopy

Gerhard Lutz*

MPI Semiconductor Laboratory, MPI fur Physik, Otto Hahn Ring 6, Munchen D-81739, Germany

Abstract

Three examples on novel semiconductor detectors developed at the MPI Semiconductor detector laboratory are described: the Semiconductor Drift Diode which is mainly used in spectroscopy , the pn-CCD used as focal detector in the XMM/Newton X-ray observatory and the DEPMOS pixel detector which is under development for the planned XEUS X-ray telescope as well as for applications in future particle physics experiments.

r

2002 Elsevier Science B.V. All rights reserved.

Keywords: Semiconductor detectors; X-ray astronomy; X-ray spectroscopy; XMM; XEUS; DEPFET; Pixel detectors

1. Introduction

Starting from nuclear and particle physics, semiconductor detectors have entered many fields of science and technology. A significant role in this development has been played by the MPI-Munich group, now operating an own semiconductor laboratory containing a complete semiconductor radiation technology. This laboratory is jointly supported by two institutes, the Max-Planck Institute for Physics, mainly active in particle physics, and the Max-Planck Institute for extra- terrestrial physics, active in experimental astro- physics. Detectors for own experiments of these institutes are developed in this laboratory.

Here we will present two projects in X-ray astronomy, one already successful operating, the other being in the development stage. Both detector concepts are based on the semiconductor

drift chamber principle. The working principle of the drift chamber and its use in spectroscopy will precede the presentation of the main activities of the laboratory.

2. MPI semiconductor laboratory

Earlier developments of semiconductor radia- tion detectors in a collaboration between the Max- Planck Institute for Physics and the Technical University of Munich included the first strip detectors used in a particle physics experiment [1], first double-sided detectors [2] as well as the first operating semiconductor drift detector [3].

These developments caught the interest of X-ray astronomers at the Max-Planck Institute for Extraterrestrial Physics who were looking for an imaging device with simultaneous spectroscopic capabilities. An X-ray CCD based on a new principle, derived from the drift chamber, was the driving force for the foundation of the

*Fax: +49-89-83940011.

E-mail address:gerhard.lutz@cern.ch (G. Lutz).

0168-9002/03/$ - see front matterr2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0168-9002(02)02048-X

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laboratory. The laboratory houses a complete double-sided silicon technology, design tools and testing equipment. Thus the whole process is under the control of physicists directly involved in the experiments of the founding institutes, the Max- Planck Institute for Physics and the Max-Planck Institute for Experimental Physics. Several reasons made an own technology necessary: the use of ultra-pure silicon whose properties should not deteriorate throughout the processing and the double-sided defect-free wafer size processing of sophisticated structures based on new concepts.

Dedicated processing equipment and full and detailed knowledge and control of the technology were necessary requirements for a successful development of the devices.

3. Semiconductor detectors for spectroscopy

The spectroscopic detectors developed in the laboratory are based on the semiconductor drift chamber, invented in 1984 by Gatti and Rehak [4].

This device (Fig. 1) is in principle capable of measuring position and energy simultaneously.

Electrons produced by ionizing radiation move towards the potential valley in the center plane of the fully depleted device. The horizontal field component achieved by splitting the p

+

diodes into strips and applying gradually rising potential moves the electrons towards the n

+

anode. Energy and position can be determined from charge and drift time. In the devices presented here, only the excellent spectroscopic capabilities due to the low

capacitive load to the readout amplifier are being used.

Further drastic improvement is obtained by using a point anode in a circular structure (Fig. 2) with a thin homogeneous radiation entrance window [5] and by implementing the first amplifier into this structure (Fig. 3) [6]. The very good spectroscopic performance achieved with such a device is demonstrated on an Iron 55 spectrum taken near room temperature and at high rate (Fig. 4).

Single and multi-cell devices of this kind are used in various spectroscopic applications, as for example, the measurement of induced X-rays for chemical analysis in electron microscopes.

4. Semiconductor detectors for X-ray astronomy

Only recently, semiconductor detectors came into use for imaging in X-ray astronomy, replacing gas detectors as used in earlier satellite missions.

Two space-based X-ray observatories are operat- ing successfully at present, both launched in 1999,

Fig. 1. Drift detector principle.

P+ D

P+

D1

VB1 R A

VB2 VD

Fig. 2. Drift diode.

Integrated FET SG D

Collecting Anode

Field Rings -V

Back Contact n Si

Fig. 3. Drift diode with integrated detector technology compa- tible JFET transistor.

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the USA CHANDRA [7] and the European XMM/Newton mission [8].

The X-ray optics principle is the same in the two missions and follows the one used in the ROSAT observatory [9]. It makes use of the fact that X- rays incident under glancing angle are reflected by metallic surfaces of extremely low surface rough- ness. The Wolter-type telescope (Fig. 5) consists of a combination of hyperbolic and parabolic mir- rors. Many nested mirror shells focus the image onto the focal plane with the image sensor.

CHANDRA has one telescope aimed at high angular resolution; XMM is equipped with three X-ray telescopes optimized for high collecting power, so as to be sensitive to very distant objects.

All telescopes but one are equipped by mosaics of

MOS CCDs, which are modifications of optical CCDs.

The only principally new device is the pn-CCD on one of the three XMM telescopes. It has been optimized for the specific application and is significantly superior in its performance. In the following section, the principle and the perfor- mance of this device will be described which has been the major project in our laboratory over a period of 12 years.

4.1. The pn-CCD on the XMM/Newton X-ray observatory

An artist’s view of the XMM/Newton Observa- tory with its three X-ray and one optical telescope, all pointing to the same object is shown in Fig. 6.

The three mirror telescopes each consist of 58 gold-coated shells, with a wall thickness of 0.5–1 mm (Fig. 7).

A cross-section through the pn-CCD [10] is shown in Fig. 8. The working principle of the device is derived from the Silicon Drift Chamber (Fig. 1) by applying non-equal voltages on the front and backside p-regions, such that the electron potential valley moves close to the top surface. In addition, instead of applying a graded potential onto the strips on the top, a periodically varying potential is applied and the bottom side is a large area diode. Increased n-doping near the top surface (epitaxial layer) prevents hole injection from the top when the potential valley is brought

Fig. 5. Principle of a Wolter-type X-ray telescope. Fig. 6. The XMM/Newton Observatory.

counts/ADC channel

energy [eV]

4000 0 500 1000 1500

2000 Fe source

-10oC

FWHM = 147 eV

* * * exp. data gouse fit 2500

5000 6000 7000

Fig. 4. Iron 55 spectrum measured with Silicon Drift Diode.

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to roughly 10 mm from the surface. Suitable changing of the three transfer register potentials moves the signal charge towards the readout node.

Table 1 compares the properties of the pn-CCD to those of the more conventional ones used in CHANDRA and XMM.

For CHANDRA we have taken the properties before the severe radiation damage which occurred in the radiation belt already in the first few revolutions. One notices the superiority of the pn-CCDs in all respects which is explained by the working principle and the layout optimized for the specific application. The high quantum effi- ciency over the whole energy range and in

Fig. 7. Mirror telescope seen from the backside.

Fig. 8. Working principle of the pn-CCD, cut along the channel.

Table1 CCDpropertiesatCHANDRAandXMM ComparisonofCCDS TypeEnergyres.at272and 5900eV(eV)Quantumefficiencyat 272eVand10keV(%)Readout timeper cm2(ms)

Pixelcellsize (mm)Detectorsize (cm2) CHANDRA (original)MIT/LL-FI6013032050024242.52.5 MIT/LL-BI125200501050024242.52.5 XMMLeicester80130253050040402.42.4 SRON100145651550027272.82.8 pn-CDD7013090901.315015066

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particular at low and high energies is due to the thin homogeneous backside radiation entrance window and the sensitivity over the whole wafer thickness (Fig. 9). Particularly striking is the factor 400 in readout speed, which is due to two reasons.

The large pixel size of 150 150 mm matched the optical properties of the telescope and the parallel column readout through a readout chip perform- ing four-fold correlated double sampling and multiplexing readout.

As an example of the excellent performance of the X-ray camera, we show the elemental decom- position of the remnants of Tycho Brahe’s super- nova observed in 1572 (Fig. 10).

4.2. The XEUS X-ray observatory

The XEUS project [11] (Fig. 11) is a corner- stone of the European Space Agency (ESA) aimed at looking into very deep space, close to the origin of the birth of the universe. This project is assumed to be launched around the year 2015.

Prominent aims are, among others, the observa- tion of early black holes, the evolution and clustering of galaxies and the evolution of element synthesis. The collecting power will be two orders larger than at XMM. Correspondingly, the re- quirements on the focal detectors will increase dramatically.

Fig. 10. Remnants of Tycho Brahe’s supernovae; image with energy spectrum and element decomposition.

Quantum Efficiency Quantum Efficiency

1.0

0.8

0.6

0.4

0.2

0.0

1.00 0.98 0.96 0.94

0.15

Energy [keV]

Energy [keV]

1 10

1.836 1.846 1.856 1.866 1.876

15

Fig. 9. Quantum efficiency of pn-CCD.

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These requirements are listed together with some other design parameters in Table 2.

Note the increase of focal length from 7.5 to 50 m and of the mirror collection area from 0.05 to 6 m

2

in the fist stage and to 30 m

2

in the second stage. As a consequence, the mirror and focal plane are housed on separate satellites that will keep their distance precise to 1 mm and the mirrors will be assembled on the International Space Station for the second stage.

The dramatic increase of photon flux in the focal plane due to the large collection power and large increase of readout rate, which can hardly be accomplished with the CCD concept, is realized in the XMM mission. Therefore another type of detector based on the DEPleted Field Effect Transistor (DEPFET) concept [12] is under devel-

opment. This development runs in parallel with that of a vertex detector for the TESLA linear collider. This development is described in the following section.

5. The DEPFET pixel detector 5.1. Device structure

The DEPFET detector–amplifier concept (Fig. 12) [13] is based on the combination of the sideward depletion principle as used in the semiconductor drift detector with the field effect transistor. Signal charge is generated in the fully depleted bulk assembles in an electron potential minimum below the transistor channel. Charge of opposite sign and same magnitude is induced in the transistor channel, thus increasing the channel conductance and therefore the transistor current.

This structure has some extraordinary properties that make it very suitable as a basic element of a pixel detector: very low capacity and therefore low noise; possibility of repeated readout without destruction of the signal charge; absence of reset

Fig. 11. The XEUS observatory consisting of two separate satellites, one housing the mirrors, the other carrying the focal instruments.

Cell SourceGate

A1 Drain Clear

Clear S

Blas

D

G G

P+

N+

N

S10 P+

P+

Fig. 12. The DEPFET detector–amplifier principle, cross-sec- tion and device symbol.

Table 2

Comparison of XMM and XEUS requirements

Feature XMM XEUS Focal detector

Energy range 0.1–15 keV 0.1–20 keV Increase thickness from 300 to 500mm

Focal length 7.5 mm 50 m

Angular resolution 15 arcsec 1–2 arcsec

Focal plane position resol. 36mm/arcsec 250mm/arcsec Decrease pixel size to 75mm

Field of view 30 arcmin 5–10 arcmin Increase detector area to 77cm2

Collection area at 1 keV 0.5 m2 6 and 30 m2

Collection area at 8 keV 0.05 m2 3 m2

Operating temperature 130–180 K >180 K Increase operating temperature

Full frame time resolution 70 ms 1–5 ms Drastic increase of readout speed

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noise with complete clearing of the signal charge.

The good properties of the pn-CCDs as for example the sensitivity over the full depth and the thin radiation entrance window are retained.

The excellent spectral resolution obtained with such a structure is demonstrated in Fig. 13. Slight cooling improves the noise from 4.6 to 3.6 electrons rms.

5.2. The pixel matrix

A pixel detector is formed by arranging a multitude of DEPFETs in the form of a two-

dimensional matrix and connecting them in such a way that individual transistors can be turned on individually as indicated in Fig. 14. In this example, the device is read out through a single node. Alternatively one may equip each row of pixels with a separate output, similar to the solution chosen for the pn-CCD and obtain a drastic increase of readout speed. One can read the device row by row, clearing each row after readout. Compared to CCDs, one of the impor- tant properties of this device is the signal readout at the place of origin, which results in the avoidance of ‘‘out of time events’’; signals

Fig. 13. Iron 55 spectrum taken with a DEPFET detector at room temperature and with moderate cooling.

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generated during readout with wrong position assignment.

Clearing of the DEPFET is achieved by remov- ing the potential barrier between the internal gate and the clear electrode while applying a positive voltage pulse on the clear electrode. This can be achieved in several ways and will not be described here. However, some conditions to be met will be pointed out: complete charge removal is desired while re-injection of charge at the end of the clear pulse and the loss of signal to the clear electrode during the charge collection period has to be avoided.

To meet these partially conflicting requirements, geometry and doping profiles have to be chosen with the help of extensive, partially three-dimen- sional, device simulations. This will be demon- strated on a particular device, which makes use of the possibility of reading the signal several times without destruction of the signal charge. In this case the signal charge is switched several times between the internal gates of two neighboring DEPFETs which form the input transistors of a differential amplifier as shown in Fig. 15. Shifting the signal charge n times will reduce the noise by a factor square root of n; even in the case of 1=f noise.

Fig. 16 shows the design of a pixel device with charge-switching option.

5.3. Device simulations

Fig. 17and 18 demonstrate signal charge collection and clearing properties as obtained from two-dimensional device simulations in which the signal was generated by a 5.9 keV X-ray photon.

Observing the drain current, one concludes that all signal charge is collected within 30 ns. A 12 V clear pulse with 10 ns rise time completely clears the device within 15 ns. No signal charge is lost to the clear contact and no charge is injected into the internal gate at the end of the clear cycle.

Decoder Clear

D e c o d e r D r a i n s

νc νc

νc

Decoder Gates

Fig. 14. Schematics of a DEPFET pixel detector with readout through a single output node.

back contact

doploted n-Si bulk shift of signal charg

'internal gata' es gata 1 drain

gata 2

source 2 clear different

amplifierial Source 1

Fig. 15. DEPFET pair with charge switching.

Fig. 16. Layout of a DEPFET pixel matrix with charge- switching option.

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Fig. 19 demonstrates that the signal charge can be transferred fast (25 ns) and completely from one internal gate to that of its neighbor.

Finally, a three-dimensional simulation of the potential in the charge collection state (the normal situation which is interrupted only during readout and clearing) shows that all signal charge gener- ated in the bulk will find its way towards the internal gate. This is seen in Fig. 20 where the potential along a cut through gate and clear contacts is shown up to a depth of 5 mm in a view from the inside of the bulk towards the surface.

The potential maximum at the left (the internal gate) is due to a buried n-type doping, the potential minimum at the right side due to a p- type implantation below the clear electrode. It makes sure that all charge created at the bulk is steered towards the internal gate.

5.4. Applications

The unique properties of DEPFET pixel ma- trices offer a variety of interesting applications

cut at y = 20.0 µm

6

4

2

0

-2

-4

-6

potential energy [eV]

[µm]

Fig. 20. Three-dimensional simulation of matrix cell; potential along cut through gate and clear electrode.

DEPMOS hit response - 1600 ekectrib/hole pairs generated

W = 10um

0 20 40 60 80 100

104

102

100

98

source current [uA]

⊥ = 5um 0.49nA /electron

⊥ = 3um 1.19nA /electron

Fig. 17. Simulation of charge collection; DEPFET current response for a 5.9 keV photon.

DEMPOS clear behavior - charge of internal gate clear pulse

time [ns]

O_ln [electrons] V_clear [V]

2000

1500

1000

500

0

20

15

10

5

0

0 5 10 15 20

Fig. 18. Simulation of DEPFET clearing; charge in internal gate with application of a 12 V clear pulse.

DEPMOS switch behavior - charge transfer between two internal gates

lleft internal gate right internal gate

0 100 200 300 400

time [ns]

O_im [electrons]

2000 1500 1000 500 0

lleft ext. gate right ext. gate drain

0 100 200 300 400

time [ns]

clock voltages [volts]

5 0 -5 -10 -15

Fig. 19. Charge transfer between neighboring DEPFETS;

signal charge in the two internal gates (top) and voltages applied to external gates and drain.

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besides the main driving force of our development in the field of X-ray astronomy.

In charged particle tracking, the very low electronic noise, in combination with low power consumption allows the building of very thin low mass detectors, thus reducing strongly particle scattering. This is very important in elementary particle spectroscopy. However, DEPFET pixel detectors as presented here can only be operated with moderate readout speed. Nevertheless, for applications in a future linear collider as TESLA, this speed is still adequate.

6. Summary

Out of the various developments performed at the MPI Semiconductor Laboratory, two exam- ples from X-ray astronomy have been selected for prominent presentation, the already success- fully operating XMM pn-CCD focal instrumenta- tion and the preparation for the even much more challenging XEUS project. For the latter we have started the development of a detector which is based on a further new principle. The pn-CCD as well as the drift diode which has also been described here have found many applications outside the traditional fields of X-ray astronomy and particle physics. The same is to be expected of the DEPFET devices now under development.

Acknowledgements

A very large number of colleagues from the MPI Semiconductor Laboratory and of other collabor- ating institutes was and still is involved in the projects described in this paper. I would like to express my thanks for their enthusiasm and perseverance during the many years of common work.

References

[1] B. Hyams, U. Koetz, E. Belau, R. Klanner, G. Lutz, E.

Neugebauer, A. Wylie, J. Kemmer, Nucl. Instr. and Meth.

205 (1983) 99;

E. Belau, et al., Nucl. Instr. and Meth. 214 (1983) 253;

E. Belau, et al., Nucl. Instr. and Meth. 217(1983) 224.

[2] G. Lutz, Vertex Detectors, Plenum Press, New York, 1988, pp. 195–224.

[3] P. Rehak, et al., Nucl. Instr. and Meth. A 235 (1985) 224.

[4] E. Gatti, P. Rehak, Nucl. Instr. and Meth. 225 (1984) 608.

[5] J. Kemmer, G. Lutz, Nucl. Instr. and Meth. A 253 (1987) 356.

[6] P. Lechner, et al., Nucl. Instr. and Meth. A 377 (1996) 346.

[7] M.C. Weisskopf, et al., Proc. SPIE 4012 (2000) 2.

[8] F. Jansen, et al., Astron. Astrophys. 365 (2001) L1.

[9] J. Trumper, Science 260 (1993) 1769..

[10] L. Struder, et al., Astron. Astrophys. 365 (2001) L18.. [11] X-ray Evolving-Universe Spectroscopy—The XEUS

Science Case, ISBN No 92–9092–548–5, European Space Agency, SP-1238, March 2000.

[12] J. Kemmer, G. Lutz, Nucl. Instr. and Meth. A 253 (1987) 356.

[13] J. Kemmer, et al., Nucl. Instr. and Meth. A 288 (1990) 92.

Abbildung

Fig. 1. Drift detector principle.
Fig. 5. Principle of a Wolter-type X-ray telescope. Fig. 6. The XMM/Newton Observatory.
Fig. 8. Working principle of the pn-CCD, cut along the channel.
Fig. 10. Remnants of Tycho Brahe’s supernovae; image with energy spectrum and element decomposition.
+5

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