1

Three-Dimensional Imaging Three-Dimensional Imaging Performance Performance

of Orthogonal Coplanar CZT Strip of Orthogonal Coplanar CZT Strip DetectorsDetectors

Three-Dimensional Imaging Three-Dimensional Imaging Performance Performance

of Orthogonal Coplanar CZT Strip of Orthogonal Coplanar CZT Strip DetectorsDetectors

M. McConnell1, J. R. Macri, J. M. Ryan, K. Larson, L.-A. Hamel, G. Bernard, C. Pomerleau,

O. Tousignant, J.-C. Leroux, and V. Jordanov

1 Space Science Center, University of New Hampshire, Durham, NH2Physics Department, University of Montreal, Montreal, Quebec, Canada

3Yantra, Durham, NH

45th SPIE Meeting San Diego, CA 30 July - 4 August 2000

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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.

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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 and register 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 gold anode 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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Assembly of Prototype ModulesAssembly of Prototype Modules

CZT substrate LTCC carrier

assembled CZT module

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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 carrier showing electrical connections.

Topside of LTCC carrier that is bonded 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 “Pixel” SpectraSingle “Pixel” Spectra

8000

6000

4000

2000

0

Counts

400350300250200150100Channel Number

241Am

60 keV,5.7% FWHM

1500

1000

500

0

Counts

10008006004002000Channel Number

57Co

122 keV,2.6% FWHM

136 keV,2.2%FWHM

pulser

• 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 122 keV), and 6.0 keV (at 662 keV).

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Response UniformityResponse Uniformity

800

600

400

200

0

Peak Channel

87654321

Strip 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 reliable interconnections 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= 1−cathodesignalpixelsignal

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟ L

Measurements with a Tungsten sheet at two different Z-

positions differing by 500 µm.

The difference between the two depth distributions yielded an effective slit measurement.

z ≈ 350 µm.

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Attenuation Length MeasurementsAttenuation Length Measurements

5000

4000

3000

2000

1000

0

Simulated Counts

4321Interaction Depth (mm)

150

100

50

0

Measured Counts

Simulated Counts

Measured Counts

57Co (122 keV)15000

10000

5000

0

Simulated Counts

4321Interaction Depth (mm)

250

200

150

100

50

0

Measured Counts

Simulated Counts

Measured Counts

241Am (60 keV)

Ratio of cathode to anode pulse heights used to determine the interaction depth for incident photon energies of 60 keV and 122 keV.

Distribution of interaction depth measures the attenuation length (µ).

These data demonstrate the ability to measure Z at all depths.

µmeasured = 0.34 ± 0.04 mm

µpublished = 0.27 mm

µmeasured = 1.72 ± 0.15 mm

µpublished = 1.65 mm

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Measured and Simulated SignalsMeasured and Simulated Signals

Simulated signals compare well with measured data.

Here are seen comparisons for signals at three different depths

within the detector.

cathode surface

anode surface

pixel signal

strip signal

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Signal CharacteristicsSignal Characteristics

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

Preamp Signal (relative 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 a measurement of the interaction depth without the cathode signal.

Simulated Strip Signal Parameters

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Measurements of Strip Signal ParametersMeasurements of Strip Signal Parameters

0.20

0.15

0.10

0.05

0.00

Risetime of Strip Signal (µs)

-0.15 -0.10 -0.05 0.00

Residual of Strip Signal (relative to pixel residual)

Z = 3 mm Z = 2 mm Z = 1 mm

60Co (1.33 MeV)

The plot below shows the measured relationship between the strip signal risetime 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 possible using just the anode signals.

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Time-Over-Threshold vs. DepthTime-Over-Threshold vs. Depth

We have chosen to explore the use of the time-over-threshold (TOT) of the strip signal for determining the interaction depth.

An analog circuit design has been developed to measure TOT.(poster paper by UNH student Kipp Larson, paper 4141-40, presented Monday)

600

500

400

300

200

100

0

Time-Over-Threshold (nsec)

543210

Interaction Depth (mm)

Measured Data

Simulation

Event trigger came from a single pixel row with no

coincident strip requirement.

Lack of a strip coincidence requirement introduces events

from adjacent “pixels”.

These data were collected using 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) provides readout of all signal channels.

Plot of interaction locations for a collimated beam of 122 keV photons (spot size ~200

µm) obliquely incident on cathode surface.

incident beam

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Conceptual Module Packaging DesignConceptual Module Packaging Design

The current concept for the packaging of a single CZT

module is based on experience with the prototype.

The concept involves a single module with 16 16 (256) logical pixels (32 channels)

on a 1 mm pitch (2.56 cm2 active area).

All front-end electronics will fit within the foot-print of the CZT

substrate.

Passive Circuit Components

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Closely-Packed Array of CZT ModulesClosely-Packed Array of CZT Modules

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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Importance of Thicker CZT DetectorsImportance of Thicker CZT Detectors

1.0

0.8

0.6

0.4

0.2

0.0

Total Detection Efficiency

2 3 4 5 6 7

102

2 3 4 5 6 7

103

2 3 4 5 6 7

104

2

Energy (keV)

5mm thick

10mm thick

15mm thick

1.0

0.8

0.6

0.4

0.2

0.0

Full Energy Peak Efficiency

2 3 4 5 6 7

102

2 3 4 5 6 7

103

2 3 4 5 6 7

104

2

Energy (keV)

5mm thick

10mm thick

15mm thick

In many applications (e.g., astrophysics) good sensitivity is

required at higher energies (above several hundred keV).

These simulated results show the detection characteristics for a

12 mm 12 mm block of CZT with thicknesses between 5 and 15 mm.

Even thicker detectors (> 15 mm) would possibly be required in some

applications.

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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 signal for the depth measurement (Larson et al., paper 4141-40).

• 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 coplanar anode design for CZT strip detectors.

We have developed a compact, reliable packaging concept that will permit the fabrication of large-area closely-packed arrays.