A 0.18μm CMOS fluorescent detector system for bio-sensing application

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.

Double junction photodiode for X-ray CMOS sensor IC

A linear array of 128 Active Pixel Sensors has been developed in standard CMOS technology and a Linear Variable
Optical Filter (LVOF) is added using CMOS-compatible post-process, resulting in a single chip highly-integrated highresolution
microspectrometer. The optical requirements imposed by the LVOF result in photodetectors with small pitch
and large length in the direction normal to the dispersed spectrum (7.2μ;m×300μm). The specific characteristics of the
readout are the small pitch, low optical signals (typically a photocurrent of 100fA~1pA) and a much longer integration
time as compared to regular video (typically 100μs~63s). These characteristics enable a very different trade-off between
SNR and integration time and IC-compatibility. The system discussed in this paper operates in the visible part of the
spectrum. The prototype is fabricated in the AMIS 0.35μm A/D CMOS technology.

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Chi Liu

Papers

A CMOS 128-APS linear array integrated with a LVOF for highsensitivity and high-resolution micro-spectrophotometry