Edgeless Medipix sensors for medical X-ray imaging applications Marten Bosma BND School in Rathen,...

Edgeless Medipix sensorsfor medical X-ray imaging applications

Marten Bosma

BND School in Rathen, 2009-Sep-14

Medipix • Originated from the hybrid pixel technology developments for vertex detection at the LHC experiments.

• Collaboration of more than 20 institutes.• Grown to a product in X-ray diffraction. Aim

for an X-ray imaging device with Medipix.

Marten Bosma, BND School in Rathen, 2009-Sep-14

• Single photon counter• Pixel size: 55 x 55 μm2 • Active area per chip ≈ 14 x 14 mm2 (256 x 256 pixel matrix)• I/O periphery (wire bonded)

Pixelated 300 μm silicon sensor (256 x 256 pixels)

Medipix2 readout ASIC

Detector bias voltage

14 mm

Use of Medipix in medical applications

High-Z(HiDRaLoN)

Colour X-ray imaging(Medipix 3)

Large surface areas(ReLAXD)

{0.5 cm

Medipix is a SPC →:• Ultra-low noise• Large Dynamic range

Therefore, low contrast detectability. → potential dose reduction!

MedipiX-ray Imaging


Large Surface Area

TSV’s + BGAEdgeless Assemblies

Edgeless Sensors

Deep Reactive Ion Etching (DRIE) instead of conventional dicing

Narrow n+ or p+ Stop Ring (SR) instead of wide Guard Ring

Conventional structure Edgeless structure

“Edgeless”


Quad Sensor with Guard Ring

Medipix ASIC Medipix ASIC

PCB

‘Edgeless’ Quad Sensor

Medipix ASIC Medipix ASIC

PCB

Ball-Grid-Array Through-Silicon-VIa

Technologies:

Edgeless sensor technology (Canberra) Through-Silicon-Via and Ball-Grid-Array (IMEC) 1 Gbit/s ethernet data transmission (Nikhef) CO2-cooling (Nikhef) Product development (PANalytical)

Marten Bosma5

Large Area X-ray Detector (RELAXD)

Gap AA-Edge

SR: 75 µm

Different types of structures

Each structure type is characterised by its Stop Ring (SR) width and the gap in between the Active Area (AA) edge and the trench (i.e. edge of the structure). We are interested

in the influence of both on the leakage current density in the AA.

Circular diode Rectangular test diode Pixel detector Circular detector


Leakage Current

• We want to have a high SNR in the Active Area.• By biasing the Stop Ring at the same potential as the Active Area, it is possible to eliminate

leakage currents from the Stop Ring to the Active Area and at the same time prevent the edge leakage current from reaching the Active Area.

• Generation current → Electron-hole pair generation. Number is proportional to the area and depletion thickness → proportional to number of intrinsic carriers.

• Diffusion current → Pair generation outside depletion region, which drift into depleted volume before recombination.

• Surface Current → Current flowing around edges, caused by possible damage and contamination

Reduction of edge leakage current :

The depletion layer width increases with the square root of the applied potential → leakage current characteristic can be determined by studying the current as a function of the voltage.

2d V

Three types:


Concerns• Light tightness → structure fully

enclosed by probe station.• Guarding → use of triax cables.• Avoidance of ground loops → can

cause a shift in the measured voltage or fluctuating currents

• Settling time (we took 60 s)• Probing needles → the thicker the

needle, the higher the measured current (we used two 407-A needles).

• Temperature and humidity changes → stable clean room (~ 50 % hum.) and possible chuck temperature control.

• Averaging → in each configuration, the leakage current characteristic has been measured five times.

• Accuracy → in terms of ±(% of reading + fraction of range) Voltage: ±(0.015% + 3 mV)Current: ±(0.05%)

In order to allow the structure to settle, the actual measurement of Ileak was done 120 seconds after applying Vbias


Edgeless Medipix sensors for medical X-ray imaging applications Marten Bosma BND School in Rathen,...

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