Edgeless Medipix sensors for medical X-ray imaging applications

1. Edgeless Medipix sensorsfor medical X-ray imaging applications Marten Bosma BND School in Rathen, 2009-Sep-14

2. 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. Pixelated 300 μm silicon sensor (256 x 256 pixels) 14 mm • 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) Detector bias voltage Medipix2 readout ASIC Marten Bosma, BND School in Rathen, 2009-Sep-14

3. MedipiX-ray Imaging 0.5 cm Large surface areas (ReLAXD) {  Use of Medipix in medical applications High-Z (HiDRaLoN) • Medipix is a SPC →: • Ultra-low noise • Large Dynamic range • Therefore, low contrast detectability. → potential dose reduction! Colour X-ray imaging (Medipix 3) Marten Bosma, BND School in Rathen, 2009-Sep-14

4. “Edgeless” Quad Sensor with Guard Ring Edgeless Assemblies TSV’s + BGA Medipix ASIC Medipix ASIC Large Surface Area PCB Edgeless Sensors  Deep Reactive Ion Etching (DRIE) instead of conventional dicing Narrow n+ or p+ Stop Ring (SR) instead of wide Guard Ring Ball-Grid-Array Through-Silicon-VIa Conventional structure Edgeless structure ‘Edgeless’ Quad Sensor Medipix ASIC Medipix ASIC PCB Marten Bosma, BND School in Rathen, 2009-Sep-14

5. Large Area X-ray Detector (RELAXD) SenterNovem Eureka project • 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 Bosma

6. Different types of structures Circular diode Rectangular test diode Pixel detector Circular detector 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. SR: 75 µm Gap AA-Edge Marten Bosma, BND School in Rathen, 2009-Sep-14

7. Leakage Current Three types: • 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 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. Reduction of edge leakage current : Marten Bosma, BND School in Rathen, 2009-Sep-14

8. Concerns In order to allow the structure to settle, the actual measurement of Ileak was done 120 seconds after applying Vbias • 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). • Temperatureand humiditychanges → 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%) Marten Bosma, BND School in Rathen, 2009-Sep-14

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