celeste fleta instituto de microelectrónica de barcelona

RAD2012 Ultra-Thin 3D detectors

Celeste Fleta Instituto de Microelectrónica de Barcelona

Celeste Fleta

Instituto de Microelectrónica de BarcelonaCentro Nacional de Microelectrónica - CSIC

Spain

[email protected]

Ultra-Thin 3D Silicon Detectors for Active Neutron Detection



Motivation

Radiotherapy linacs

Working at > 8MeV generate neutrons by fotonuclear reaction

Pulsated radiation

Usually dosemeters passive Real time counting and high gamma

rejection factor required

Other applications: nuclear security, avionics, space, …



Neutron detection with silicon sensors

Silicon radiation sensors

Low bias (few V) Low battery consumption for

transportability Compact: small sizes (mm) and

weight (g) Resistant to shock and inmune to

magnetic fields Fast response (ns)

Adaptation for neutron detection

Converter with high thermal neutron cross section Reaction products with enough energy to reach the detector

Will use 10B-based compounds

)01.1()78.1(

)48.0()84.0()47.1(710

710

MeVLiMeVBn

MeVMeVLiMeVBn

93.7 %6.3 %)01.1()78.1(

)48.0()84.0()47.1(710

710

MeVLiMeVBn

MeVMeVLiMeVBn

93.7 %6.3 %

t

I

particle

Maximum detection efficiency 4.7%



Traditional “planar PIN” silicon sensors

Thickness: 300 – 1000 µm Depletion voltage: 80-200 V Low charge collection time: ns High γ-ray absorption probability

Standard sensors

Thickness: 10-20 µm Depletion voltage < 10 V Low charge collection time: ns Low γ-ray absorption High capacitance and electronic

noise

Thin sensors

-VCC

P+

N+++---

N+

SCR

gamma rejection/capacitance trade-off

0.001%

0.010%

0.100%

1.000%

10.000%

100.000%

0 2000 4000 6000 8000 10000 12000 14000 16000

Photon energy (keV)

% a

bsor

bed

300 microns10 microns20 microns



Simulated capacitance

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

0 10 20 30 40 50 60

silicon thickness (microns)

Cap

acita

nce

(nF/

cm2) planar

3D (80µm pitch)

New “Ultra-thin 3D” silicon sensors

3D is advantageous if thickness < 50 µm

Thickness: 10-20 µm Columnar electrodes passing through

substrate Depletion voltage: few V Low charge collection time: ns Low γ-ray absorption Resistant to radiation damage Capacitance lower than the

planar equivalent



Design and fabrication

Design and fabrication done in-house 0.5 cm2 active area, window <400nm SiO2

Electrode fabrication:1. ICP etching of the holes: ALCATEL 601-E2. Holes partially filled with LPCVD polysilicon3. Holes doped with P or B4. Holes passivated with TEOS SiO2

Electrodes: 5µm diameter, 10µm deep SOI wafer 10µm active thickness

Detail of a sensor designn-contact

n-holes connected together with thin metal lines

p-contact on other side

p-holes



In pictures



Electrical test

Sensors depleted at ~5 volts

50-70 nA/cm2 at 10V 60-80 pF/cm2

Capacitance vs. voltage Current vs. voltage

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

0 10 20 30 40 50 60silicon thickness (microns)

Cap

acita

nce

(nF/

cm2)

planar3D (80µm pitch)3D Measured

0

100

200

300

400

500

600

0 10 20 30 40 50

Vbias (V)

Cur

rent

(nA

/cm

2)

0

100

200

300

400

500

0 5 10 15 20

Vbias (V)

Cap

acita

nce

(pF/

cm2)



Electronics

The detector is mounted on a separated board to allow testing different detectors with the same system

Compact (50g, 13x2.5cm) Fast (80000 counts/s) Cheap (<100€) towards a portable system



137Cs 10µm sensor Threshold = 100keV 1 count/10 min 1 gamma counted

every 108

300µm sensor 29.0±0.2 c/s90 mCi (2.8x109 γ/s in 4π)

1m distance

Lab source tests

241Am-Be + 137Cs

40 mCi AmBe (88000 n/s)8 mCi 137Cs

10cm polyethilene

800µm sensor 204.0±0.6 c/s



Hospital tests

sensor

Elekta Synergy (Hospital General de Catalunya) and Siemens Primus (Hospital de Santiago de Compostela)

6MV (γ only) and 15MV (γ + n) 10x10 cm2 field 50 to 500 MU/min*

*MU: a Monitor Unit is a measure of the machine output of a linac which is calibrated to deliver an absorbed dose under particular conditions, e.g. 100 MU gives 1 Gray in water at 100 cm SSD for a 10x10 cm2 field



Hospital tests

No pile-up counts observed At 6 MV (γ only): 3.8 counts/min

1E6 γ/cm2s Gamma rejection factor: 2x10-9

At 15 MV: γ/n counts = 0.002 (sensor without/with H310BO3)

Fixed rate: Elekta Synergy, 400MU/min

H310BO3



Hospital tests

Variable rates: Siemens Primus, 15 MV

Linear response: no pile-up up to 500 MU/min• 50 MU/min: 4010±100 counts in 10 minutes• 500 MU/min: 4062±48 counts in 1 minute

γ/n rate = 0.02System works well in pulsated gamma/neutron environment

H310BO3



Summary and outlook

Summary Innovative ultra-thin 3D silicon sensors with 10 µm thickness have been

successfully fabricated and adapted to detect neutrons with 10B-enriched compounds.

The detectors show a gamma rejection factor higher than 10-8 for 137Cs for a threshold of 100 keV, and 2x10-9 in a radiotherapy field.

Preliminary tests of the detectors in clinical linacs show their usefulness in these complex gamma-neutron pulsated radiation fields: low gamma count rate, linear up to at least 500 MU/min.

Ongoing work Working to integrate the system in a fully portable dosemeter. Developing 10B- based converter deposition tecniques. Working to obtain absolute efficiencies/gamma rejection factors with

calibrated sources. Developing microstructured detectors for higher neutron detection efficiency.

C. Guardiola et al., “Ultra-thin 3D silicon sensors for neutron detection”, 2012 JINST 7 P03006



Thanks for your attention!Full author list:

C. Fleta, C. Guardiola, D. Quirion, J. Rodríguez, G. Pellegrini, J.P Balbuena, M. LozanoInstituto de Microelectrónica de Barcelona, Barcelona, Spain

F. Gómez, X. González, D. González, J. Pardo Universidad de Santiago de Compostela, Santiago de Compostela, Spain

F. GarcíaHelsinki Institute of Physics, University of Helsinki, Helsinki, Finland

celeste fleta instituto de microelectrónica de barcelona

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