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Nuclear Instruments and Methods in Physics Research A 607 (2009) 57–60

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/nima

A novel ultra-thin 3D detector—For plasma diagnostics at JET andITER tokamaks

Francisco Garcıa a,�, G. Pelligrini b, J. Balbuena b, M. Lozano b, R. Orava a,c, M. Ullan b

a Helsinki Institute of Physics—University of Helsinki, POBox 64, 00014, University of Helsinki, Finlandb Centro Nacional de Microelectronica, CNM-IMB (CSIC), Barcelona, 08193, Spainc Department of Elementary Particle Physics (AFO), 00014, University of Helsinki, Finland

a r t i c l e i n f o

Available online 25 March 2009

Keywords:

Ultra thin

3D electrodes

Silicon detector

Ions

ITER

Diagnostics

X-ray spectroscopy

02/$ - see front matter & 2009 Elsevier B.V. A

016/j.nima.2009.03.117

esponding author. Tel./fax: +358 9 19151086.

ail address: francisco.garcia@helsinki.fi (F. Ga

a b s t r a c t

A novel ultra-thin silicon detector called U3DTHIN has been designed and built for applications that

range from Neutral Particle Analyzers (NPA) used in Corpuscular Diagnostics of High Temperature

Plasma to very low X-ray spectroscopy. The main purpose of this detector is to provide a state-of-the-art

solution to upgrade the current detector system of the NPAs at JET and also to pave the road for the

future detection systems of the ITER experimental reactor. Currently the NPAs use a very thin

scintillator-photomultiplier tube [F. Garcıa, S.S. Kozlovsky, D.V. Balin, Background Properties of CEM,

MCP and PMT detectors at n-g irradiation. Preprint PNPI-2392, Gatchina, 2000, p. 9 [1]; F. Garcıa, S.S.

Kozlovsky, V.V. Ianovsky, Scintillation Detectors with Low Sensitivity to n-g Background. Preprint PNPI-

2391, Gatchina, 2000, p. 8 [2]], and their main drawbacks are poor energy resolution, intrinsic

scintillator nonlinearity, and relative low count rate capability and finally poor signal-to-background

discrimination for the low-energy channels. The proposed new U3DTHIN detector is based on very thin

sensitive substrate, which will provide nearly 100% detection efficiency for ions and at the same time

very low sensitivity for neutron and gamma backgrounds. To achieve a very fast collection of the charge

carriers generated by the incident ions, a 3D electrode structure [S. Parker, C. Kenney, J. Segal, Nucl. Instr.

and Meth. A 395 (1997) 328 [3]; G. Pellegrini, P. Roy, A. Al-Ajili, R. Bates, L. Haddad, M. Horn, K.

Mathieson, J. Melone, V. O’Shea, K.M. Smith, Nucl. Instr. and Meth. A 487 (2002) 19 [4]] has been

introduced in the sensitive volume of the detector. The geometry of the electrode is known to be rad-

hard. One of the most innovative features of these detectors is the optimal combination of the thin

entrance window and the sensitive substrate thickness, which allows a very large dynamic range for ion

detection. GEANT4 simulations were performed to find the losses of energy in the oxide entrance

window and the energy deposition in the silicon substrate for different types of ions; results from these

simulations and the process used to fabricate the U3DTHIN at the Centro Nacional de Microelectronica

in Barcelona are presented.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

The increase in power of the plasma shots in the JET tokamakhas introduced serious challenges for the operation of the NeutralParticle Analyzers (NPA) detector systems. This type of analyzersis used to perform Corpuscular Diagnostics of plasma. Suchincrease of the plasma burning power has increased the neutronand gamma background to the level where the detectors cannotcope with the particles rate. The detectors get saturated and arenot able to detect the ions that carry the wanted informationabout the plasma parameters. It is expected that this problem willbe even more severe in the new generation of tokamaks, one of

ll rights reserved.

rcıa).

which will be installed in the ITER facility [5]. In order to provide adetector capable of detecting ions under such high intensity ofneutrons and gamma background, a completely new detectorconcept using an Ultra-Thin Silicon detector with 3D electrodes isintroduced.

This detector concept (see Fig. 1) fulfills the main requirementsfor the operation under high-radiation environment in terms ofthe count rate capability and radiation hardness. Complementaryto this, the detector will have nearly 100% efficiency for detectionof ions and new clusterization schemes can be explored to furtherimprove the background rejection capability, thus increasing thesignal-to-background ratio.

Simulations using GEANT4 have been carried out in order tobetter understand the detector performance by irradiating withbackground radiation (neutrons and gammas) and with ions.Complementary to this, a SENTAURUS Technology Computer

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F. Garcıa et al. / Nuclear Instruments and Methods in Physics Research A 607 (2009) 57–6058

Aided Design (TCAD) model has been created to study theelectrical performance for different geometry cells in order toget some figures of merit for the fabrication process.

Finally the first fabrication run has been performed at CNM inBarcelona.

2. Simulations using GEANT4

In order to carry out a preliminary test of this detector concept,various simulation models were created. In particular, a GEANT4model for MonteCarlo simulations of the interaction of radiationis shown in Fig. 2.

The GEANT4 model geometry description includes all thecomponents of the detector. The model was used to obtainaccurate energy deposition values in its sensitive volume.

The geometry description of the detector has the followingcomponents: a very thin entrance window of Silicon Oxide of20 nm, a supporter silicon frame of 300mm, the silicon-sensitivedetector of 10mm, holes of 5mm and metallic strips on the backside made of Aluminum with a thickness of 1mm.

With the GEANT4 simulations, an evaluation of detectorresponse to background radiation and incident ions was per-formed. The simulated results obtained from the irradiation withphotons indicate that the detector sensitivity was of 10�6, whichis four orders of magnitude less than the previously usedscintillator detectors. In addition, the cluster size for the

5mm

1cm� = 3um

10um

300um

γ n α

Oxide 20nm

p-type

p-type

Fig. 1. Detector concept of the Ultra Thin S

Fig. 2. Geometry description in the G

interaction of the Compton electrons in the sensitive volumewas found to be of the order of 10mm.

Similar simulations were performed for incident ions: alphaparticles, deuterons, tritons and protons. In the figure below, theenergy deposition of ions for a wide range of the incident energy isshown. It was found that the deposited energy has similarbehavior for all the ions; it first shows linear increase with theincident particle energy, then a saturation point and, finally, withfurther increase of the incident particle energy smaller energy isdeposited. As we can see in Fig. 3, the saturation point is reachedat different energy levels for different types of particles, e.g. alphaparticles reach the saturation at 2.5 MeV, then tritons at evenlower with 950 keV, then the deuterons with 850 keV and thelowest ones were the protons with 700 keV.

The results presented in Fig. 4 clearly show that in the lineardetector response interval, less fluctuations of the depositedenergy are seen for the 10-mm-thick detector.

The cluster size between the ions and the secondary electronsgenerated by the interactions of the photons with the detectorwas found to be of the order of 10mm for the photons and 1mm forthe ions.

3. Fabrication run

A first a fabrication test run was done in order to prove thefeasibility of this new detector concept. The main step in the

t d p

Low Resistivity

High Resistivity

Oxide 2um

ilicon with 3D electrodes—U3DTHIN.

EANT4 model of the U3DTHIN.

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Fig. 3. Energy deposition for alpha particles, tritons, deuterons and protons in a

wide energy range.

100k

10k

1k

1000 500 1000 1500 2000 2500 3000 3500 4000

Particle Energy, keV

ΔE, e

V

Energy Deposition FluctuationsDetector Thickness 10 μm

Alphas

Tritons

Deuterons

Protons

Fig. 4. Energy deposition fluctuations for alpha particles, tritons, deuterons and

protons in a wide energy range.

Polysilicon

Hole filled with polysilicon

10umMembrane

10 um

Fig. 5. Cross-section of the hole filled with polysilicon.

Fig. 6. Finished wafer with the first U3DTHIN mechanical samples.

F. Garcıa et al. / Nuclear Instruments and Methods in Physics Research A 607 (2009) 57–60 59

realization of the U3DTHIN is to combine the fabricationtechnology of standard 3D detectors [6] with thinning of planardevices. The process of drilling the holes to make the cylindricalcontacts into the silicon substrate is done using an InductivelyCoupled Plasma process by an Alcatel 601E dry etching machine.The process was optimized to stop the etching at the oxideinterface of the Silicon on Insulator (SOI) wafer. In this first testrun only one type of holes was drilled, filled with polysilicon andthen doped. Since these devices were only mechanical samples totest whether the thin membrane would break when filled with5mm holes, no metal was deposited on the surface to form thecontact.

The final step in this fabrication process is thinning of the backsurface of what would be the active detector area. The thinning isdone using a TMAH solution, which stops etching at the oxideinterface of the SOI wafer. This oxide is etched and then depositedwith an atomic layer deposition (ALD).

Fig. 5 shows a cross-section of a hole filled with polysilicon,where the diameter of the hole is 5mm and the depth is 10mm;the thickness of the active volume of the detector also can beenseen.

Four wafers (see Fig. 6) were processed with this method andthen the single chips were diced with a diamond saw. None of thewafers or the single chips was damaged during this process. Thefeasibility of the process was successfully proved and therefore adedicated mask set will be designed and produced.

4. Conclusions

The detector concept has been tested with this fabrication run,the design shown to be very robust, because none of the sensorswere broken during the process.

Results of the GEANT4 simulations show that the integralsensitivity at very low threshold is four orders of magnitudeless than with the scintillator detectors. Increasing theenergy threshold will drastically improve the signal-to-back-ground ratio for ions. The cluster sizes for both radiationtypes, background and ions, are much less than the strip width.Further simulations with SENTAURUS TCAD will be necessary todetermine more realistic cluster size; this will be mainly given bythe simulation of the diffusion of charge carriers in the detectorbulk.

The detection efficiency for ions is nearly 100%, the loss inefficiency is mainly due to the hits on the area of the electrodes;

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F. Garcıa et al. / Nuclear Instruments and Methods in Physics Research A 607 (2009) 57–6060

further reduction of the electrodes size will further increase theion detection efficiency.

The signal-to-noise ratio and energy resolution for ions willdepend mainly on the readout electronics characteristics.

Acknowledgments

This work is supported and financed by the Academy of FinlandGrant and by the Spanish Ministry of Education and Sciencethrough the Particle Physics National Program (ref. FPA2006-13238-C02-02) and co-financed with FEDER funds.

References

[1] F. Garcıa, S.S. Kozlovsky, D.V. Balin, Background properties of CEM,MCP and PMT detectors at n-g irradiation, Preprint PNPI-2392, Gatchina,2000, p. 9.

[2] F. Garcıa, S.S. Kozlovsky, V.V. Ianovsky, Scintillation detectors with lowsensitivity to n-g background, Preprint PNPI-2391, Gatchina, 2000, p. 8.

[3] S. Parker, C. Kenney, J. Segal, Nucl. Instr. and Meth. A 395 (1997) 328.[4] G. Pellegrini, P. Roy, A. Al-Ajili, R. Bates, L. Haddad, M. Horn, K. Mathieson,

J. Melone, V. O’Shea, K.M. Smith, Nucl. Instr. and Meth. A 487 (2002) 19.[5] ITER Facility /http://www.iter.orgS.[6] B. Miller, in: Proceedings of the sixth Australian Electrochemical Conference,

Geelong, Vic., 19–24 February, 1984.B. Miller, J. Electroanal. Chem. 168 (1984) 91.