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The Development of Coplanar CZT Strip Detectorsfor Gamma-Ray Astronomy
The Development ofThe Development of Coplanar CZT Strip Detectors Coplanar CZT Strip Detectorsfor Gamma-Ray Astronomyfor Gamma-Ray Astronomy
M. McConnell, J. R. Macri, M. McClish, J. M. Ryan, Space Science Center, University of New Hampshire, Durham, NH
L.-A. HamelPhysics Department, University of Montreal, Montreal, Quebec, Canada
GAMMA 2001, Baltimore, Md, 4-6 April 2001
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Traditional CZT Strip DetectorsTraditional CZT Strip Detectors
Concept
� Uses orthogonal strips on opposite sides of the detector.
� One set of parallel strips collects holes; the other collects electrons.
Advantages
� Effectively provides N2 pixels with only 2N electrical channels.
� Considerably reduces complexity and power requirements.
Disadvantages
� Effective detector thickness is limited by hole trapping to a few mm.
� Requires signal connections to both top and bottom surfaces.
z
x
y
x-strips(top)
y-strips(bottom)
CZT
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Orthogonal Coplanar Anode DesignOrthogonal Coplanar Anode Design
Concept
� Both sets of orthogonal �strips� on same side of detector.
� Rows of interconnected �pixels� collect electrons.
� Orthogonal strips, at slightly different bias, act as steering electrodes andregister induced-charge signals.
� Pixel row signals can be interpolated to get sub-pitch Y-coordinate.
� Strip signals can be interpolated to get sub-pitch X-coordinate.
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Orthogonal Coplanar Anode DesignOrthogonal Coplanar Anode Design
Advantages
� Provides N2 pixels with only 2N electrical channels.
� Considerably reduces complexity and power requirements.
� Electron-only device.
� Permits thicker detectors (> 1 cm). Limited by electron mobility.
� All signal connections on one side ⇒ close-packing.
CZT substrate with goldanode contact pattern.
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Prototype CZT Detector ModulesPrototype CZT Detector Modules
Prototype detectors have been fabricated and tested
� 5 mm thick CZT substrate (single-crystal, discriminator grade, eV Products)
� Gold anode contact pattern provides an 8 × 8 array of 1 mm �pixels�.
Assembly of prototype detectors involves two key technologies
� Low-Temperature Co-fired Ceramics (LTCC)
� Polymer Flip-Chip (PFC) Bonding (no wire bonds)
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Low-Temperature Co-fired CeramicsLow-Temperature Co-fired Ceramics
� Substrate fabrication featuring 170 µm filled vias for electrical connections.
� Provides low leakage under HV bias.
� Has thermal expansion coefficient similar to that of CZT.
Underside of LTCC carriershowing electrical connections.
Topside of LTCC carrier that isbonded to CZT.
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Polymer Flip-Chip BondingPolymer Flip-Chip Bonding
SEM photos showing polymer bumps on the patterned CZT substrate
� A low-temperature bonding process (T < 80° C).
� Conducting polymer bumps are stencil printed on CZT and the LTCC carrier.
� Bumps are 120 µm diameter and 20 µm high.
� A non-conducting epoxy is used as an underfill between the mating surfaces.
� Underfill provides both a strong mechanical assembly and thermal isolation.
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Single Single ��PixelPixel�� Spectra Spectra
8000
6000
4000
2000
0
Cou
nts
400350300250200150100Channel Number
241Am
60 keV,5.7% FWHM
1500
1000
500
0
Cou
nts
10008006004002000Channel Number
57Co
122 keV,2.6% FWHM
136 keV,2.2%FWHM
pulser
7000
6000
5000
4000
3000
2000
1000
0
Cou
nts
400300200100Channel Number
137Cs
pulser
662 keV,0.9% FWHM
� Required coincidence between one strip and one pixel row.
� Bias levels: cathode = �800 V, anode pixels = 0 V, anode strips = �30 V.
� Measured FWHM resolutions are 3.4 keV (at 60 keV), 3.2 keV (at 122keV), and 6.0 keV (at 662 keV).
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��BulkBulk�� Spectrum Spectrum
Spectrum generated from the full detector volume.
Bias levels: cathode = �550 V, anode pixels = 15 V, anode strips = 0 V.
2000
1500
1000
500
0
Cou
nts
1000900800700600500400
Energy (keV)
137Cs
662 keV2.9% FWHM
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Response UniformityResponse Uniformity
6
5
4
3
2
1
0
Ene
rgy
Res
olut
ion,
FW
HM
(%
)
87654321Pixel Row ID
122 keV photopeak
662 keV photopeak
Pixel Row Uniformity800
600
400
200
0
Pea
k C
hann
el
87654321Strip ID
Strip Signal Uniformity(122 keV)
Uniformity measurements :
1) Energy resolution at 122 keV and 662 keV for each pixel row.
2) Signal pulseheight (at 122 keV) for each strip.
These data indicate that the detector fabrication yielded reliableinterconnections for all 64 �pixels�.
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X-Y Spatial ResolutionX-Y Spatial Resolution
Charge sharing between adjacent strips permits sub-strip spatial resolution in X.
Limited charge sharing between pixels reduces the ability to interpolate in Y.
Lab measurements with a collimated alpha source.
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Depth MeasurementDepth Measurement
Using both the cathode and anode signals, the interaction depth is given by,
where L is the detector thickness (= 5 mm in our prototypes).
z cathode signal
pixel signalL= −
1
Measurements with a Tungstensheet at two different Z-
positions differing by 500 µm.
The difference between the twodepth distributions yielded aneffective slit measurement.
σz ≈ 350 µm.
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Measured and Simulated SignalsMeasured and Simulated Signals
Simulated signalscompare well withmeasured data.
Here are seencomparisons for signalsat three different depths
within the detector.
cathode surface
anode surface
pixel signal
strip signal
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Signal CharacteristicsSignal Characteristics
0.5
0.4
0.3
0.2
0.1
0.0
Tim
e (µ
s)
543210
Interaction Depth (mm)
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
Residual (relative units)
time-over-threshold
rise time
residual
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
Pre
amp
Sig
nal (
rela
tive
units
)
0.300.250.200.150.100.050.00
Time (µs)
pixel row signal
strip signal
rise time
time-over-threshold
residual
threshold
SignalCharacteristics
The characteristics of the anode strip signals can be used to define ameasurement of the interaction depth without the cathode signal.
Simulated StripSignal Parameters
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Measurements of Strip Signal ParametersMeasurements of Strip Signal Parameters
0.20
0.15
0.10
0.05
0.00
Ris
etim
e of
Str
ip S
igna
l (µs
)
-0.15 -0.10 -0.05 0.00
Residual of Strip Signal (relative to pixel residual)
Z =
3 m
m
Z =
2 m
m
Z =
1 m
m
60Co (1.33 MeV)
The plot below shows the measured relationship between the strip signalrisetime and the strip signal residual.
The solid line represents the relationship expected based on simulations.
These data support the claim that depth measurements will be possibleusing just the anode signals.
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Time-Over-Threshold Time-Over-Threshold vsvs. Depth. Depth
We have chosen to explore the use of the time-over-threshold (TOT) ofthe strip signal for determining the interaction depth.
An analog circuit design has been developed to measure TOT.
600
500
400
300
200
100
0
Tim
e-O
ver-
Thr
esho
ld (
nsec
)
543210
Interaction Depth (mm)
Measured Data
Simulation
Event trigger came from asingle pixel row with no
coincident strip requirement.
Lack of a strip coincidencerequirement introduces events
from adjacent �pixels�.
These data were collectedusing a prototype TOT circuit,
measuring a single strip.
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Three-dimensional ImagingThree-dimensional Imaging
A VME-based DAQ system (developed at the Univ of Montreal) providesreadout of all signal channels.
Plot of interaction locationsfor a collimated beam of 122keV photons (spot size ~200
µm) obliquely incident oncathode surface.
incident beam
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Conceptual Module Packaging DesignConceptual Module Packaging Design
PFCbondinglayer
PatternedCZTSubstrate
LTCCSubstrate(MCM)
cathode
anode
ContactPlate
ImagePlaneBoard(section)
FEE ASICs underneath
The current concept for thepackaging of a single CZT
module is based on experiencewith the prototype.
The concept involves a singlemodule with 16 × 16 (256)logical pixels (32 channels)on a 1 mm pitch (2.56 cm2
active area).
All front-end electronics will fitwithin the foot-print of the CZT
substrate.
PassiveCircuitComponents
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Closely-Packed Array of CZT ModulesClosely-Packed Array of CZT Modules
connectors(to controller)
Image Plane Board
Module Array
mounting frame (thermal path)
The module design will provide a packing fraction of ~90-95%.
An array of 20 × 20 modules with a total active area of 1024 cm2.
Total power of 26 W for 12,800 channels, assuming 2 mW/channel(vs. 205 W for a 102,400 channel pixellated array).
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Current Status and Future EffortsCurrent Status and Future Efforts
Future efforts will be focused on:
� optimizing the anode design using simulation tools.
� fabrication and testing of thicker (10 mm) prototypes.
� continued development of circuitry to process the bipolar strip signalfor the depth measurement.
� ASIC development.
� continued packaging development.
� studying the effects of multiple interaction sites at higher energies.
� evaluating the ability to measure incident photon polarization.
We have successfully demonstrated the viability of a coplanaranode design for CZT strip detectors.
We have developed a compact, reliable packaging concept thatwill permit the fabrication of large-area closely-packed arrays.