Design%20of%20semiconductor%20detectors%20for%20digital%20mammography

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IWORID 2002 Amsterdam, 8-12 September

Design of semiconductor detectors for digital
mammography
S.R.Amendoliaa, M.Boscardind, M.G.Bisognib,
G.F. Dalla Bettad, P.Delogub, M.E.Fantaccib,
M.Novellib, M. Quattrocchic, V.Rossob,
A.Stefaninib, S.Zuccab a Istituto di
Matematica e Fisica dellUniversita di Sassari e
INFN, Sezione di Pisa, Italy b Dipartimento di
Fisica, Universita di Pisa and Sezione INFN
Pisa, Italy c Dipartimento di Fisica,
Universita di Napoli and Sezione INFN Pisa,
Italy d ITC-irst, Divisione Microsistemi, 38050
Povo (TN), Italy

• marzia.novelli_at_pi.infn.it

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Outline

• Study on Gallium Arsenide and Silicon detectors
• to realize a digital imaging mammographic system
• Characterization of GaAs detectors to find a
  material with good c.c.e. and detection
  efficiency properties
• Simulation of Silicon detectors to choose a
  structure to avoid the risk of electrical
  discharge between detector and electronic chip

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Introduction
Digital imaging mammographic system based on a
semiconductor pixel detector
Detector

• Detector
• semiconductor
• Si thickness 300 mm
• GaAs thickness 200 mm
• pixel 170 x 170 mm2
• Schottky 150x150 mm2
• chanel 64 x 64
• area 1.2 cm2

25 mm
Electronic chip
Images of the RMI 156 mammographic phantom
film 12 bit 100mm
Si 300 mm (scansion 6x6)
Sourcemammographic tube
(Mo target) Dose 4 mGy
6 cm
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GaAs pixel detectors
It s important to define the optimal reverse
voltage for the detector because Operating
bias (Vbias ) influences image
quality Increasing Vbias
leakage current and noise increase
breakdown limit in
order to have response uniformity, a good c.c.e
and detection efficiency the detector must be
overdepleted Vbias
300 Volt

• Conditions to have a good detector
• Breakdown voltage gt 500 Volt
• Current Density lt 50 nA/mm2 (_at_ 300
  Volt)
• Charge Collection Efficiency gt 75 (_at_
  300 Volt)

We have observed that its important to have a
well defined concentration of Carbon in the bulk
of GaAs so we have studied the behaviour of
single diodes, some with a dopant concentration
known and some unknown
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GaAs detectors
Single diode diameter 2 mm and thickness 200 mm
Schottky Contact multilayer Ti/Pt/Au Ohmic
Contact non alloyed (AMS in Rome,
Italy)
Contacts
Dopants concentrations
MCP (LEC) C 1015 cm-3 AMXC 2065
(VGF) C 1015 cm-3 FRE (LEC)
C lt 1015 cm-3 ACR 68 (LEC) C 1.3
1015 cm-3 ACR 13 (LEC) C 5 1015
cm-3 ACR 79 (LEC) C 1 1016cm-3 SMT1CR
(LEC) C lt 1015 cm-3 , Cr 1017 cm-3
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Electrical Characterization
Current densities as a function of reverse
voltage have shown the behavior typical of a
diode under reverse voltage.
Current density decreases when Carbon
concentration become higher.
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Spectroscopic Characterization
ACR 013 C 5 1015 cm-3
ACR 068
C 1.3 1015 cm-3
300 Volt
500 Volt
ACR 079 C 1 1016 cm-3
Irradiation with 231Am Source (59,54 KeV)
Comparison of spectra acquired using diodes with
the different Carbon concentrations
400 Volt
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Spectroscopic Characterization
MCP C 1 1015 cm-3
AMXC 2065 C 1 1015 cm-3
300 Volt
200 Volt
SMT 1 CR C 1 1015 cm-3 Cr 1017 cm-3
FRE C lt 1 1015 cm-3
200 Volt
300 Volt
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Spectroscopic Characterization
ACR 068 C 1.3 1015 cm-3
500 Volt
ACR 068 is the material which shows the best
compromise between electrical and spectroscopic
characteristic
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Spectroscopic Characterization
Systematic study on the material ACR 068 with the
characterization of 10 single diodes
C 1.3 1015 cm-3
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Simulation of Silicon detectors ISE-TCAD
Creation of a geometry
Creation of dopant distribution

Creation of elementary domain
Iterative Calculus
Physical Equation

y electrostatic potential e electric
permittivity q elementary charge n, p electron
and hole densities ND, NA donors and
acceptors Jn, Jp current densities R
recombination rate m mobility F quasi-Fermi
potential
Poisson Equation
Continuity Equation
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Electric Potential at the cutting edge
Electric Potential
p junction NB 2 1019 cm-3 n
contact NP 1 1020 cm-3 Bulk
NP 5 1011 cm-3 Scribe Line NP 1 1017
cm-3 Oxide fixed charge 4 1011 e- cm-2
p
guardring p
oxide
scribe line
150
20
300
300
300
p Junction at ground Guardring at ground n
Contact at 100 Volt
1070
n
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New structure Electric Potential
p junction NB 2 1019 cm-3 n
contact NP 1 1020 cm-3 Bulk
NP 5 1011 cm-3 scribe Line NP 1 1017
cm-3 Oxide fixed charge 4 1011 e- cm-2
Multiguardring (450 mm) 3 x (oxide 15 mm p
15 mm) 3 x (oxide 20 mm p 15 mm)3 x (oxide
25 mm p 15 mm)3 x (oxide 30 mm p 15 mm)
p
guardring p
multiguardring
scribe line
oxide
150
200
450
150
20
300
n
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Electric Potential and Electric Field
width
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Wafer layout at ITC-IRST
12 wafers 4 inches in diameter Thickness 300 mm
and 500 mm Type of production p/n
18 detectors for Medipix 1 6 detectors for
Medipix 2 (a matrix of 256x256 square
pixels of 55 mm in side) Several test
structures These detectors are ready and they
are in phase of bump-bonding to the electronic
chip at VTT
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Photos of some details
Pixel 150 mm x 150 mm
multiguardrings
Medipix 1
guardring
multiguardrings
Pixel 45 mm x 45 mm
guardring
Medipix 2
Snake pads
multiguardrings
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Test structure in the wafer
To check the properties of the wafers we have
tested the electrical characteristic of some
single diodes (3 diodes for each wafer always in
the same position)
Central diode Diameter 2 mm
Gap 8 mm First guardring thickness 300 mm
Gap 30 mm Second guardring thickness 150 mm
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Electrical Characterization
Current in the central diode with guardring at
ground
Thickness 300 mm
T 21.5 C U65
Wafer 15
Wafer 9
Wafer 7
Wafer 5
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Electrical Characterization
Thickness 500 mm
T 21.5 C U65
Wafer 4
Current in the central diode with guardring at
ground
Wafer 5
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Detector of area larger than the electronic chip
Another approach for the solution of the HV at
the cutting edge
Wafer 4 inches in diameter, type of production
p/n, thickness 300 500 mm
flip-chip bonding I/O pads
flip-chip bonding
PCC - electronic chip
semiconductor detector
multiguardring around the matrix and the lines
for the fanout
guardring around the active matrix of pixels
Series of p implantation to isolate the active
region from the lines
These detectors will be ready at the end of the
year
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14200
Chip dimension
Active matrix (64 x 64)
Main guard-ring
18600
Termination structure (12 rings)
Resistors to isolate the active region from the
I/O pads
Bus line for the fanout
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Conclusions

• Research of the optimal concentration of dopants
  in GaAs detectors
• Realization of the optimal geometry of Silicon
  detector with simulation and characterization
  of some test structures to check the properties
  of the new wafers
• Status in the production of the detectors
• The GaAs detectors with the optimal Carbon
  concentration for imaging
• applications are ready and they are in phase of
  bump bonding by AMS.
• The Si dectors are in phase of bump bonding by
  VTT.
• Future works
• When the assemblies will be ready we want to
  check their performance and to compare the
  results with those obtained in the past years.