rad 2012 some advances in dose measurement with mosfet for portable instrumentation alberto j. palma...
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RAD 2012
SOME ADVANCES IN DOSE MEASUREMENT WITH MOSFET FOR
PORTABLE INSTRUMENTATION
Alberto J. Palma
Professor of the Department of Electronics
University of Granada (Spain)
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AGENDA
Research group Introduction Low cost pMOS as dosemeter Procedures of measurement
• Pulsed biasing (PB)• Two currents method (2CM)• Three currents method (3CM)
Portable dosimetry system Conclusions Acknowledgements
RAD 2012, Niš (Serbia)
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RESEARCH GROUP
Interdisciplinary Spanish group:• Department of
Electronics• Department of Atomic,
Nuclear and Molecular Physics
• Radiotherapy Units of several Hospitals in Spain (Granada and Málaga)
RAD 2012, Niš (Serbia)
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RESEARCH GROUP
Our interest in dosimetry:• MOSFET: electrical and thermal
characterization• Monte Carlo simulation of radiation-matter
interaction• Measurement science• Design and testing of electronic
instrumentation
RAD 2012, Niš (Serbia)
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INTRODUCTION
Scope: Dose verification systems (DVS) based on MOSFETs mainly for medical use.
High reliable commercial available DVS:• Best Medical Canada Systems (BM)• One Dose by Sicel Tech.? (OD)• REM Oxford Ltd. (REM)
Some small disadvantages• Wired sensors for bias purposing (BM, REM)• Short dose range (OD)• Expensive specific RADFETs (BM, REM, OD)
RAD 2012, Niš (Serbia)
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INTRODUCTION
Our approach• Use of low-cost general-purpose MOSFET as dosemeters• Wireless and reusable sensors without bias during irradiation
Drawbacks of our approach:• Low sensitivity to radiation.• Low signal-to-noise ratio (SNR).• Low linear range.
Objective: overcoming the above problems
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INTRODUCTIONIrradiation set-up
Irradiation conditions • Theratron-780 with a 60Co source and
a field of 25 x 25 cm2
• MOSFETs, in electronic equilibrium condition, located at 80 cm of the isocentre
RAD 2012, Niš (Serbia)
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LOW COST pMOS
3N163 (Vishay-Siliconix, USA)• Response to radiation from ID-VGS curves
Sensitivity 30 mV/Gy
RAD 2012, Niš (Serbia)
Longitudinalsection
Bottom view
Sealing
resin
Plastic
support
pMOS
transistor
Gold-plated circle
5.3
4.3
D
C
GS
Nickelcasing
Longitudinalaxis
y = 0.0297x + 0.0037
R2 = 0.9999
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
0 10 20 30 40 50 60
Dose (Gy)
D |V
T| (
V)
5.65E-04
5.70E-04
5.75E-04
5.80E-04
5.85E-04
5.90E-04
5.95E-04
6.00E-04
6.05E-04
Db (
A/V
2 )D|Vt|Db
L
WCVvI oxptGSD bb
;2
2
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PROCEDURES OF MEASUREMENTState of the art
Ionizing radiation creates charge in the oxide: Threshold voltage, Vt ,is the dosimetric parameter
Measurement of Vt at constant drain current
(VGD = 0). (BM, REM, OD)
RAD 2012, Niš (Serbia)
0
0
DD
b|V| t
ID
VS
VDD
S
G B
Dcteb St VV DD
22 tGSD VVI b
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PROCEDURES OF MEASUREMENTState of the art
Thermal compensation techniques
Biasing the pMOS at IZTC Two identical pMOS with different (REM, OD) sensibilities (BM)
RAD 2012, Niš (Serbia)
Di
SGv
ZTC
321 TTT 3T2T
1T
M1M2
During irradiation
Read-out configuration
DVD
+ -
M1 M2
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PROCEDURES OF MEASUREMENTPulsed biasing (PB)
SNR improvement• Read-time instabilities caused by low frequency noise (LFN)
(due to near-interface and interface traps)• How can LFN be reduced? Chopping the drain current as in
other experimental techniques (i.e. spectroscopy) and averaging
RAD 2012, Niš (Serbia)
pMOS Switch
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PROCEDURES OF MEASUREMENTTwo currents method
Linear range improvement• Accounting for Db effect in DVt
• pre- and post-irradiation parameters at constant current I:
RAD 2012, Niš (Serbia)
bI
VV St
2
prepreS
pret
IVV
b2
postpostS
postt
IVV
b2
DD
prepostSt IVVbb
112 2
2 tS VVI b
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PROCEDURES OF MEASURMENTTwo currents method
Linear range improvement• Using two drain currents during read-out phase:
• Threshold voltage shift without neglecting Db
RAD 2012, Niš (Serbia)
DD
prepostSt IVVbb
112 11
DD
prepostSt IVVbb
112 22
1
2
121
1I
I
VVVV SSSt
DDDD
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PROCEDURES OF MEASUREMENTResults (PB+2CM)
Linearity improvement
Sensitivity decay coefficient, mean sensitivity and the linear limit (up to 5%).
RAD 2012, Niš (Serbia)
y = -3.35E-03x + 1.01E+00
R2 = 9.17E-01
y = -1.06E-03x + 1.01E+00
R2 = 5.20E-01
0.92
0.94
0.96
0.98
1.00
1.02
0 5 10 15 20 25Dose (Gy)
No
rmliz
ed S
ensi
tivi
ty
Sen VS (IZTC)
Sen Vt , 2 Ids
MOSFET P2
DC modes PB
IZTC 2CM IZTC 2CM
m (mV/Gy2) -0.153 -0.095 -0.113 -0.057
Mean Sensitivity (mV/Gy) 20.6 19.7 20.0 19.2
Linear Range (Gy) 6.8 10.3 8.8 16.8V
V
PBVs
DCVs
4.36
8.45
20 % reduction of SD
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PROCEDURES OF MEASUREMENTThree currents method (3CM)
Thermal compensation• Starting hypothesis:
• Experimentally verified
RAD 2012, Niš (Serbia)
1
2
121
1I
I
VVVV SSSt
DDDD
TV)T(V
TV)T(V
SS
SS
DDD
DDD
2022
1011
00)( TTVTV Vttt DD
ZTC
iVti I
I1
y = 201.797x - 3.059
R2 = 0.999
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0 0.005 0.01 0.015 0.02 0.025
Id1/2 (A1/2)
(
mV
/ºC
)
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PROCEDURES OF MEASUREMENTThree currents method (3CM)
Thermal compensation• D|Vt| thermal compensated
• Additional current: I3
• Simplification I1 = IZTC
RAD 2012, Niš (Serbia)
1
2
01
020
1
1I
I
VVVV SSSt
DDDD
31
1131
01
II
IIVVVV ZTCSSSS
DDDD
32
2232
02
II
IIVVVV ZTCSSSS
DDDD
ZTC
ZTCSSZTCSt
I
I
VVVV
2
,02
,
1
DDDD
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PROCEDURES OF MEASUREMENTResults (3CM)
Thermal compensation: • Reduction in a factor of 50 in the thermal drift
Compensated
RAD 2012, Niš (Serbia)
AI
AI
AI
ZTC
225
120
30
3
2
-20
-15
-10
-5
0
5
10
15
20
17 19 21 23 25 27 29 31T (ºC)
D| V
t| (m
V)
D|Vt| not compensated
D|Vt| comp. Method (i)
D|Vt| comp. Method (ii)CVT
Vt /º70DD
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PORTABLE DOSIMETRY SYSTEM
Sensor Module• JFET as a switch (shorted during irradiation and storage and open
during read-out)
RAD 2012, Niš (Serbia)
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PORTABLE DOSIMETRY SYSTEM
Reader Unit
RAD 2012, Niš (Serbia)
VDD
Programablecurrentsource
VS
ID
Timer555
pMOSFET
BJT
GS
D
B
Microcontroller
VS aconditionatingcircuit
A.I. A/D
DAC
A/D
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PORTABLE DOSIMETRY SYSTEM
Dose measurement process • Zeroing:
• Measurement and storage of pre-irradiation VS at the two (three) drain currents
• Irradiation• Timeout for short-term fading (120 -180 s)• Dose measurement
• Recovery of the pre-irradiation values from the memory.
• Measurement and storage of the post-irradiation VS at the two (three) currents
• Dose calculation (calibration is required)
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22
23
24
25
26
27
28
29
0 20 40 60 80
D (Gy)
Se
n (
mV
/Gy
)
1
2
3
4
5
Previous irradiations
Dose measurements
PORTABLE DOSIMETRY SYSTEMResults
Sensitivity per session
A pre-irradiation of 30 Gy is required for reproducibility of S
RAD 2012, Niš (Serbia)
D
VS TD
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PORTABLE DOSIMETRY SYSTEMCalibration curves
Radiation response Dpre=30 Gy
RAD 2012, Niš (Serbia)
y = 0.0233x
R2 = 1.0000
y = 0.0240x
R2 = 1.0000
y = 0.0240x
R2 = 1.0000
y = 0.0243x
R2 = 1.0000
y = 0.0232x
R2 = 1.0000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 10 20 30 40 50 60
D-Dpre (Gy)
DV
t (V
)
12345Lineal (5)Lineal (3)Lineal (4)Lineal (2)Lineal (1)
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PORTABLE DOSIMETRY SYSTEMTechnical specifications
RAD 2012, Niš (Serbia)
Temperature range 10 – 40 ºC Resolution 1 cGy Accuracy ± 3 %
Linear range 15 Gy > 80 Gy*
Thermal drift < 3mGy/ºC Delay after irradiation 2 – 3 minutes * with recalibrations every 15 Gy
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CONCLUSIONS
We have presented methods of dose measurement for
linearity and SNR improvement and thermal drift reduction
Portable dosimetry system based on commercial and
standard MOSFET sensor• Wireless sensor
• Reusable sensor (up to 80 Gy with recalibration each 15 Gy)
• Thermal compensation without additional devices (3 mGy/ºC)
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AKNOWLEDGEMENTS
University Hospitals “San Cecilio” and “Virgen de las
Nieves” of Granada (Spain) for permitting us to use their
installations
Spanish and Andalusian (regional) Governments for
funding, partially supported by European Regional
Development Funds
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RAD 2012
SOME ADVANCES IN DOSE MEASUREMENT WITH MOSFET FOR
PORTABLE INSTRUMENTATION
Alberto J. Palma
Professor of the Department of Electronics
University of Granada (Spain)