The design and measurement of low noise 32-channel snapshot X-ray readout integrated circuit (ROIC) are proposed in this paper. This ROIC has 32 analog channels collecting weak current from detectors, then transfers them to voltage and outputs in differential mode. This ROIC adapts low noise preamplifier and S/H circuit. Measure results show a dynamic range larger than 11000:1 with 3.6pF integration capacitor. The dynamic range is higher than the previous design's and very close with a famous commercial IC.

Low noise design of 32-channel snapshot X-ray readout IC

A CMOS compatible P+/Nwell/Psub double junction photodiode pixel was proposed, which can efficiently detect fluorescence from CsI(Tl) scintillation in an X-ray sensor. Photoelectric and spectral responses of P+/Nwell, Nwell/Psub and P+/Nwell/Psub photodiodes were analyzed and modeled. Simulation results show P+/Nwell/Psub photodiode has larger photocurrent than P+/Nwell photodiode and Nwell/Psub photodiode, and its spectral response is more in accordance with CsI(Tl) fluorescence spectrum. Improved P+/Nwell/Psub photodiode detecting CsI(Tl) fluorescence was designed in CSMC 0.5 μm CMOS process, CTIA (capacitive transimpedance amplifier) architecture was used to readout photocurrent signal. CMOS X-ray sensor IC prototype contains 8 × 8 pixel array and pixel pitch is 100 × 100 μm2. Testing results show the dark current of the improved P+/Nwell/Psub photodiode (6.5 pA) is less than that of P+/Nwell and P+/Nwell/Psub photodiodes (13 pA and 11 pA respectively). The sensitivity of P+/Nwell/Psub photodiode is about 20 pA/lux under white LED. The spectrum response of P+/Nwell/Psub photodiode ranges from 400 nm to 800 nm with a peak at 532 nm, which is in accordance with the fluorescence spectrum of CsI(Tl) in an indirect X-ray sensor. Preliminary testing results show the sensitivity of X-ray sensor IC under Cu target X-ray is about 0.21 Vm2/W or 5097e−/pixel @ 8.05 keV considering the pixel size, integration time and average energy of X-ray photons.

Sun Ying

Papers

Double junction photodiode for X-ray CMOS sensor IC