Image quality and spectroscopic Image quality and spectroscopic characteristics of different silicon characteristics of different silicon

pixel imaging systemspixel imaging systems M. G. Bisogni, D.Bulajic, M. Boscardin, G. F. Dalla Betta, P.

Delogu, M. E. Fantacci, M. Novelli, C. Piemonte, M. Quattrocchi, V. Rosso*, A. Stefanini and N. Zorzi

valeria.rosso@pi.infn.it

* Universita’ degli Studi eSezione I.N.F.N., Pisa, Italy

14TH INTERNATIONAL WORKSHOP ON ROOM-TEMPERATURE SEMICONDUCTOR X-RAY AND GAMMA-RAY

DETECTORS ROME 18-21 OCTOBER 2004

OutlineOutline

3 imaging systems based on different thickness Si pixel detectors were compared:

Spectroscopic characteristics109Cd source

Imaging qualitycontrastsignal to noise ratio (SNR)

Spatial resolutionmodulation transfer function (MTF)

Si detector

p+ side

Imaging systemImaging system

ITC-Irst DetectorSi <111>300-800 m thickpixel 170 x 170 m2

p+ side 150x150 m2

64 x 64 chs1.2 cm2 area

0 200 400 600 800 1000 1200 14000

20

40

60

80

100

with multiguardrings without multiguardrings

elec

tric

pot

entia

l (V

olt)

width (m)

Photon Counting ChipMedipix collaboration

SACMOS 1 m technology

pixel: 170 x 170 m2

64 x 64 channelsarea 1.7 cm2

threshold adjust 3-bit15-bit counter

VTT Bump-bonding:

http://medipix.w

eb.cern.ch/ME

DIP

IX/

http://medipix.w

eb.cern.ch/ME

DIP

IX/

http://medipix.w

eb.cern.ch/ME

DIP

IX/

http://medipix.w

eb.cern.ch/ME

DIP

IX/

Spectroscopic capabilitySpectroscopic capability

109Cd Integral spectra 109Cd Differential spectra

Thickness ratio

Calculated ratio

Experimental ratio

525/300 1.62 1.61

800/525 1.39 1.38

800/300 2.25 2.23

109Cd Integral spectra

Wafers characteristics 

Thickness (m) resistivity J (nA/cm2) VDEP (V) V OVER-DEP (V)

300+15 >= 6 K cm 0.5 30 60

525+20 >= 5 K cm 1.2 90 120

800+25 12 ÷ 30 K cm 26 80 200

Detection efficiencies ratioDetection efficiencies ratioVVthth=11keV=11keV

Contrast measurementsContrast measurements

Al thickness 75 m

Air

X-ray (W-anode) settings : 40 kV, 25 mA, 500 ms

air

Alair

N

NNC

Si detector

140

cm

Collimator

X-ray focus

1.5

cm

Al

8 10 12 14 16 18 20 22 242,4

2,6

2,8

3,0

3,2

3,4

3,6

3,8 800m detector

Con

tras

t (%

)

Energy threshold (keV)

ContrastContrast

8 1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4

2 , 8

3 , 0

3 , 2

3 , 4

3 , 6

3 , 8

4 , 0

4 , 2

C (

%)

E n e r g y t h r e s h o l d ( k e V )

0 5 10 15 20 25 30 35 40 450

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

15000 40kV W seen by 800 m seen by 525 m seen by 300 m

Ph

oto

ns

per

(m

A s

mm

2 ) at

750

mm

Energy (keV)

xdEESE

dEESEEE airAl

eC

)()(

)()())()((

1

Al(E) and air(E) are the absorption

coefficients at the energy E (E) is the detector efficiency at the energy E S(E) is the incident spectrum

Signal to Noise RatioSignal to Noise Ratio

22Alair

Alair NNSNR

Thickness ratio Calculated ratio Experimental SNR ratio

525/300 1.24 1.25

800/300 1.41 1.42

800/525 1.14 1.14

75 m Al

air

5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 50 , 0

0 , 2

0 , 4

0 , 6

0 , 8

1 , 0

No

rm

ali

ze

d c

ou

nts

P i x e l n u m b e r

Spatial resolutionSpatial resolution800 m Vth=11keVSettings : 40 kV, 20 mA, 4000 ms

W Slit :width: 10±1mlength: 5.5±0.1mmthikness: 1.5mm

4 , 5 5 , 0 5 , 5 6 , 0 6 , 50

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

6 0 0 0

7 0 0 0

LS

F

p o s i t i o n

5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 50 , 0

0 , 2

0 , 4

0 , 6

0 , 8

1 , 0

No

rm

ali

ze

d c

ou

nts

P i x e l n u m b e r

Counts/row pixel number

experimental finely sampled LSF

2 Boltzman functions

 LSFfitted fft MTF

xdxxdxxx ee

AAAy /)(/)(212 00 1

1

1

1)(

MTFMTFNyquist Freq. (2.94 lp/mm)

MTF: 64 %Evaluated aperture 168 m

Detector pitch 170 m

Vth (keV)

800 m aperture

(m)

11 168

15 161

19 155

23 146

Exposure condition: W anode, 40 kV, 40 mA, 630 ms

300 m 525 m 800 m

C=4,8%

C=1,2%

C=2,4%

C=3,7%

25 micron

50 micron

75 micron

100 micron

Image of different Al thickness

ConclusionsConclusions

Increasing the detector thickness:

increases the detection efficiency

SNR increases

contrast decrease as expected

spatial resolution unchanges

More information on a PCC based digital mammographic system: poster session R11-51 Thursday 11:00-12:30

width

Electric Field and PotentialElectric Field and Potential

0 200 400 600 800 1000 1200 14000

20

40

60

80

100

with multiguardrings without multiguardrings

elec

tric

po

tent

ial (

Vol

t)

width (m)0 200 400 600 800 1000 1200 1400

0

5000

10000

15000

20000

25000

30000

with multiguardrings without multiguardrings

ele

ctri

c fie

ld (

Vo

lt/cm

)width (m)

Photos of some detailsPhotos of some details

Medipix 1

Medipix 2

Pixel 150 m x 150 m

Pixel 45 m x 45 m

guardring

guardring

multiguardrings

multiguardrings

multiguardrings

Snake pads

Diffusione di caricaCon la diffusione, una carica che impiega untempo t per raggiungere l’elettrodo avrà una fluttuazione sulla posizione cor rms diff

2diff = D*t D alla mobilita’ dei portatori di carica.

Ediff

2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 02

3

4

5

6

7

8

9

1 0

1 1

1 2

1 3

1 4

d

iff

us

ion

(

m)

d e t e c t o r t h i c k n e s s ( m )

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5

0 , 0

0 , 2

0 , 4

0 , 6

0 , 8

1 , 0

4 0 k e V s i m u l a t e d 5 2 5 m

No

rm

ali

ze

d c

ou

nts

E n e r g y [ k e V ]