RAD2012
Ultra-Thin 3D detectors
Ultra-Thin 3D Silicon Detectors
for Active Neutron Detection
Celeste Fleta
Instituto de Microelectrónica de Barcelona
Centro Nacional de Microelectrónica - CSIC
Spain
Celeste.Fleta@csic.es
Celeste Fleta
Instituto de Microelectrónica de Barcelona
RAD2012
Ultra-Thin 3D detectors
Radiotherapy linacs
Motivation
 Usually dosemeters  passive
 Real time counting and high gamma
rejection factor required
 Working at > 8MeV generate
neutrons by fotonuclear reaction
 Pulsated radiation
Other applications: nuclear security,
avionics, space, …
Celeste Fleta
Instituto de Microelectrónica de Barcelona
RAD2012
Neutron detection with silicon sensors
Ultra-Thin 3D detectors
Silicon radiation sensors
Adaptation for neutron detection

particle

I
Converter with high thermal
neutron cross section
Reaction products with enough
energy to reach the detector
t





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)
Celeste Fleta

Will use 10B-based compounds
n10B   (1.47MeV ) 7Li(0.84MeV )   (0.48MeV )
n10B   (1.78MeV ) 7Li(1.01MeV )
93.7 %
6.3 %
Maximum detection efficiency 4.7%
Instituto de Microelectrónica de Barcelona
RAD2012
Ultra-Thin 3D detectors
Traditional “planar PIN” silicon sensors
-VCC
Standard sensors
P+
 Thickness: 300 – 1000 µm
 Depletion voltage: 80-200 V
 Low charge collection time: ns
 High γ-ray absorption probability
N
-
-
+
+
- +
SCR
N+
Absorption efficiency for silicon as a function of
photon energy
100.000%
Thin sensors
300 microns
10 microns
20 microns
 Thickness: 10-20 µm
 Depletion voltage < 10 V
 Low charge collection time: ns
 Low γ-ray absorption
 High capacitance and electronic
noise
% absorbed
10.000%
1.000%
0.100%
0.010%
0.001%
0
2000
4000
6000
8000
10000 12000 14000 16000
Photon energy (keV)
gamma rejection/capacitance trade-off
Celeste Fleta
Instituto de Microelectrónica de Barcelona
RAD2012
Ultra-Thin 3D detectors
New “Ultra-thin 3D” silicon sensors
Simulated capacitance
1.E+02

Thickness: 10-20 µm

Columnar electrodes passing through
substrate





Capacitance (nF/cm2)
1.E+01
planar
3D (80µm pitch)
1.E+00
1.E-01
1.E-02
Depletion voltage: few V
Low charge collection time: ns
Low γ-ray absorption
1.E-03
0
10
20
30
40
silicon thickness (microns)
Resistant to radiation damage
Capacitance lower than the
planar equivalent
Celeste Fleta
3D is advantageous if
thickness < 50 µm
Instituto de Microelectrónica de Barcelona
50
60
RAD2012
Ultra-Thin 3D detectors
Design and fabrication

Design and fabrication done in-house

0.5 cm2 active area, window <400nm SiO2

Electrode fabrication:
Detail of a sensor design
n-contact
1.
ICP etching of the holes: ALCATEL 601-E
2.
Holes partially filled with LPCVD polysilicon
3.
Holes doped with P or B
4.
Holes passivated with TEOS SiO2
Electrodes: 5µm diameter, 10µm deep
SOI wafer 10µm active thickness
n-holes
connected
together
with thin
metal lines
p-holes
p-contact on other side
Celeste Fleta
Instituto de Microelectrónica de Barcelona
RAD2012
Ultra-Thin 3D detectors
In pictures
Celeste Fleta
Instituto de Microelectrónica de Barcelona
RAD2012
Ultra-Thin 3D detectors
Electrical test
Capacitance vs. voltage
Current vs. voltage
600
500
Current (nA/cm2)
400
300
200
100
400
300
200
100
0
0
0
5
10
15
20
0
10
20
Vbias (V)
30
40
Vbias (V)
1.E+02

Sensors depleted at
~5 volts

50-70 nA/cm2 at 10V

60-80 pF/cm2
planar
3D (80µm pitch)
3D Measured
1.E+01
Capacitance (nF/cm2)
Capacitance (pF/cm2)
500
1.E+00
1.E-01
1.E-02
1.E-03
0
10
20
30
40
50
60
silicon thickness (microns)
Celeste Fleta
Instituto de Microelectrónica de Barcelona
50
RAD2012
Ultra-Thin 3D detectors
Electronics
 Compact (50g, 13x2.5cm)
 Fast (80000 counts/s)
 Cheap (<100€)
 towards a portable system
The detector is mounted on a separated board to allow
testing different detectors with the same system
Celeste Fleta
Instituto de Microelectrónica de Barcelona
RAD2012
Ultra-Thin 3D detectors
137Cs
90 mCi (2.8x109 γ/s in 4π)
1m distance
241Am-Be
+
Lab source tests
10µm sensor
 Threshold = 100keV
 1 count/10 min
 1 gamma counted
every 108
300µm sensor
 29.0±0.2 c/s
800µm sensor
 204.0±0.6 c/s
137Cs
40 mCi AmBe (88000 n/s)
8 mCi 137Cs
10cm polyethilene
Celeste Fleta
Instituto de Microelectrónica de Barcelona
RAD2012
Ultra-Thin 3D detectors
Hospital tests
 Elekta Synergy (Hospital
General de Catalunya) and
Siemens Primus (Hospital de
Santiago de Compostela)
 6MV (γ only) and 15MV (γ + n)
 10x10 cm2 field
sensor
 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
Celeste Fleta
Instituto de Microelectrónica de Barcelona
RAD2012
Ultra-Thin 3D detectors
Hospital tests
Fixed rate: Elekta Synergy, 400MU/min
H310BO
3
 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)
Celeste Fleta
Instituto de Microelectrónica de Barcelona
RAD2012
Ultra-Thin 3D detectors
Hospital tests
Variable rates:
Siemens Primus, 15 MV
H310BO
3
 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 = 0.02
System works well in pulsated gamma/neutron environment
Celeste Fleta
Instituto de Microelectrónica de Barcelona
RAD2012
Ultra-Thin 3D detectors
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
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.
137Cs
for
Ongoing work

Working to integrate the system in a fully portable dosemeter.

Developing

Working to obtain absolute efficiencies/gamma rejection factors with
calibrated sources.

Developing microstructured detectors for higher neutron detection
efficiency.
10B-
based converter deposition tecniques.
C. Guardiola et al., “Ultra-thin 3D silicon sensors for neutron detection”, 2012 JINST 7 P03006
Celeste Fleta
Instituto de Microelectrónica de Barcelona
RAD2012
Ultra-Thin 3D detectors
Thanks for your attention!
Full author list:
C. Fleta, C. Guardiola, D. Quirion, J. Rodríguez, G. Pellegrini, J.P Balbuena, M. Lozano
Instituto de Microelectrónica de Barcelona, Barcelona, Spain
F. Gómez, X. González, D. González, J. Pardo
Universidad de Santiago de Compostela, Santiago de Compostela, Spain
F. García
Helsinki Institute of Physics, University of Helsinki, Helsinki, Finland
Celeste Fleta
Instituto de Microelectrónica de Barcelona