RAD 2012 SOME ADVANCES IN DOSE MEASUREMENT WITH MOSFET FOR PORTABLE INSTRUMENTATION Alberto J. Palma Professor of the Department of Electronics University.

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)

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)

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)


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


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)


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


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


LOW COST pMOS

3N163 (Vishay-Siliconix, USA)• Response to radiation from ID-VGS curves

Sensitivity 30 mV/Gy


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

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)


0

0

DD

b|V| t

ID

VS

VDD

S

G B

Dcteb St VV DD

22 tGSD VVI b

PROCEDURES OF MEASUREMENTState of the art

Thermal compensation techniques

Biasing the pMOS at IZTC Two identical pMOS with different (REM, OD) sensibilities (BM)


Di

SGv

ZTC

321 TTT 3T2T

1T

M1M2

During irradiation

Read-out configuration

DVD

+ -

M1 M2

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


pMOS Switch

PROCEDURES OF MEASUREMENTTwo currents method

Linear range improvement• Accounting for Db effect in DVt

• pre- and post-irradiation parameters at constant current I:


bI

VV St

2

prepreS

pret

IVV

b2

postpostS

postt

IVV

b2

DD

prepostSt IVVbb

112 2

2 tS VVI b

PROCEDURES OF MEASURMENTTwo currents method

Linear range improvement• Using two drain currents during read-out phase:

• Threshold voltage shift without neglecting Db


DD

prepostSt IVVbb

112 11

DD

prepostSt IVVbb

112 22

1

2

121

1I

I

VVVV SSSt

DDDD

PROCEDURES OF MEASUREMENTResults (PB+2CM)

Linearity improvement

Sensitivity decay coefficient, mean sensitivity and the linear limit (up to 5%).


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

PROCEDURES OF MEASUREMENTThree currents method (3CM)

Thermal compensation• Starting hypothesis:

• Experimentally verified


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

)

PROCEDURES OF MEASUREMENTThree currents method (3CM)

Thermal compensation• D|Vt| thermal compensated

• Additional current: I3

• Simplification I1 = IZTC


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

PROCEDURES OF MEASUREMENTResults (3CM)

Thermal compensation: • Reduction in a factor of 50 in the thermal drift

Compensated


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

PORTABLE DOSIMETRY SYSTEM

Sensor Module• JFET as a switch (shorted during irradiation and storage and open

during read-out)



Reader Unit


VDD

Programablecurrentsource

VS

ID

Timer555

pMOSFET

BJT

GS

D

B

Microcontroller

VS aconditionatingcircuit

A.I. A/D

DAC

A/D


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)


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


D

VS TD

PORTABLE DOSIMETRY SYSTEMCalibration curves

Radiation response Dpre=30 Gy


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)

PORTABLE DOSIMETRY SYSTEMTechnical specifications


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

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)


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


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)

RAD 2012 SOME ADVANCES IN DOSE MEASUREMENT WITH MOSFET FOR PORTABLE INSTRUMENTATION Alberto J. Palma Professor of the Department of Electronics University.

Documents

ni serbiaintroductionscope

ni serbia20

low linear range

drain currents

low sensitivity

linear range gy6

dose measurement

cmthree currents method