Indirect X-ray photon counting image sensor with 27T pixels and 15 electrons RMS accurate threshold

Transcription

1 Indirect X-ray photon counting image sensor with 27T pixels and 15 electrons RMS accurate threshold B. Dierickx 1,2, B. Dupont 1,3, A. Defernez 1, N. Ahmed 1 1 Caeleste, Antwerp, Belgium 2 Vrije Universiteit Brussel, Belgium 3 Université Paris Nord XIII, France 22 February 2011 Indirect X-ray photon counting image sensor 1

2 Challenge addressed An X-ray photon counting pixel array Indirect (using a scintillator ) X-ray detection Charge packet = 100 to 500 electrons per X-photon Counts up to Counting speed 100 to Hz Si demonstrator realization as a 16x16 pixel array, 100µm pixel pitch transistors per pixel (8 variants) Detection threshold noise & variability ~15e - RMS Covered with a GdOS scintillator sheet 22 February 2011 Indirect X-ray photon counting image sensor 2

3 Outline Direct and indirect X-ray detection Indirect Photon counting pixel concept and layout Pixel key subcircuits Measurement results & performance Future developments and conclusions 22 February 2011 Indirect X-ray photon counting image sensor 3

4 Direct and Indirect X-ray detection pixel X photon VBIAS X photon Direct detector =semiconductor -Si -α Se -InP -CdTe -CdZnTe - Excited electron Electron-hole cloud Charge collection By electric field Visible photons (fluorescence) Excited electron Electron-hole cloud indirect = Scintillator -CsI -Bi4Ge3O12 -ZnSe -Gd2O2S -NaBiW2O8 -CdWO4 - - ROIC Si charge sensitive amplifier Si photodiode ROIC Electron-hole pairs Si charge sensitive amplifier 22 February 2011 Indirect X-ray photon counting image sensor 4

5 Outline Direct and indirect X-ray detection Indirect Photon counting pixel concept and layout Pixel key subcircuits Measurement results & performance Future developments and conclusions 22 February 2011 Indirect X-ray photon counting image sensor 7

6 First generation of 2D photon counting pixel array 22 February 2011 Indirect X-ray photon counting image sensor 8

7 750µm wafer thickness Pixel sees both direct and indirect X From X-ray source Si photodiodes in 5µm epitaxial layer 100µm From X-ray source GdOS scintillator sheet 100 µm thick Optical glue Visible light flash ROIC e- charge packet ROIC e- charge packet Direct detection: Rare in Si, spurious Localized and large charge packet Indirect detection: efficient in high-z scintillator Indirect charge packet is small and smeared out 22 February 2011 Indirect X-ray photon counting image sensor 9

8 X-photon counting pixel topology Light flash ΔV C=0.1fF Q=ΔV*C Charge packet Some pixels have an electrical input to inject a precise charge packet, for calibration Pulse shaper = band filter + preamp reference Analog Voltage pulse train Comparator Binary pulse train These pixels can output also a realtime view on the internal comparator signal counter Discrete count multiplexor To imager output Photo diode counter staircase 22 February 2011 Indirect X-ray photon counting image sensor 10

9 Pixel layout photodiode Pulse shaper 100µm pitch 0.18µm CMOS technology % FF Variants with MOSFETs Comparator Analog counter Multiplexor 22 February 2011 Indirect X-ray photon counting image sensor 11

10 Outline Direct and indirect X-ray detection Indirect Photon counting pixel concept and layout Pixel key subcircuits Measurement results & performance Future developments and conclusions 22 February 2011 Indirect X-ray photon counting image sensor 12

11 Charge integrator (CTIA) radiation current -A V General concept charge sensitive amplifier Integrate charges without seeing individual photons High Z out 1 Photo diode or direct detector C feedback 0.1fF I V Actual concept charge sensitive amplifier CTIA with resistive feedback = pulse shaper Q to comparator 22 February 2011 Indirect X-ray photon counting image sensor 13

12 analog count signal [AU] Very compact analog counter DC Comparator toggle GND Non-overlapping clock C1 C2 Reset counter analog count Analog counting is valid as long as ΔV is visible by an external ADC s LSB, or photon shot noise > ADC LSB 1 V ΔV 0,5 linear non-linear (exponential) PSN linear PSN exponential time photons counted 22 February 2011 Indirect X-ray photon counting image sensor 14

13 Outline Direct and indirect X-ray detection Indirect Photon counting pixel concept and layout Pixel key subcircuits Measurement results & performance Future developments and conclusions 22 February 2011 Indirect X-ray photon counting image sensor 15

14 Under LED illumination: pixel s analog count Blue: programmed sequence of LED pulses Red: Analog count ~40mV/count 22 February 2011 Indirect X-ray photon counting image sensor 16

15 X-ray illumination: comparators s real time signal BEAM ON X-ray source: 70kVp, 1mA Beam hardened with 1/2mm Cu Distance 1.5m Scintillator: GdOS 100µm thick Peaks correspond to electrons Photon rate is about 0.5 X-photon/ms BEAM OFF noise corresponds to about 70 electrons p/p, electrons RMS 22 February 2011 Indirect X-ray photon counting image sensor 17

16 X-ray illumination: pixel s analog count Reset counter Reset counter Reset counter BEAM ON count 2 count 1 count 2 22 February 2011 Indirect X-ray photon counting image sensor 18

17 Outline Direct and indirect X-ray detection Indirect Photon counting pixel concept and layout Pixel key subcircuits Measurement results & performance Future developments and conclusions 22 February 2011 Indirect X-ray photon counting image sensor 19

18 Conclusions Indirect X-ray photon counting demonstrated Detects and counts charge packets <100e- Pixel has only 27 MOSFETs Pixel has ~80% fill factor Future challenges Increasing array size Improving performance (noise, speed) Enhancing functionality (energy discrimination) 22 February 2011 Indirect X-ray photon counting image sensor 21

19 Thank you Acknowledging for the X-ray experiments 22 February 2011 Indirect X-ray photon counting image sensor 22