Non-invasive imaging of brain activity enables novel studies in neuroscience and provides an alternative modality for clinical monitoring applications. Near-infrared spectroscopic imaging (NIRSI) is non-invasive, cheap, portable, and immune to electro-magnetic interference. NIRSI is also superior in terms of spatial or temporal resolution when compared to electro-encephalography (EEG) or magnetic resonance imaging (MRI), respectively. Utilizing the time-domain (TD) technique offers the richest information at the cost of being the most complex. TD NIRS imagers utilize time-correlated single-photon-counting (TCSPC) measurements which require detectors with single-photon sensitivity like single-photon avalanche diodes (SPADs) or Silicon photomultipliers (SiPMs) and very fast time-discriminators like time-discriminator circuits (TDCs). Implementing these circuits in well-established and mature CMOS technologies is advantageous.
The main challenges in designing a TD NIRSI sensor deal with four issues: spectral responsivity, noise, fill-factor, and throughput. The spectral responsivity and noise are shaped by the technology process and structure (see figure 2) of the SPAD. The fill-factor is determined by the ancillary circuitry needed to maintain the TD operation of the SPAD (i.e., frontend, gating, and TDC). The throughput is dependent on the readout architecture of the array of pixels.
We are proposing a fully integrated scalable array of time-gated actively-quenched SPADs with shared time-gated ring-oscillator-based TDCs following H-tree-based architecture and a 3-transistor active-pixel sensor (3T APS) readout scheme with in-pixel storage capability to be implemented using standard deep sub-micron CMOS technology. For example, using 130 nm CMOS technology SPADs can be built with an area of 50 um2 which exhibit a dark count rate (DCR) of 18 KHz. At a wavelength around 600 nm and an excess bias of 2 V, their photon detection probability (PDP) could reach 22%. These SPADs breakdown at 20 V and could resolve down to 90 ps for wavelengths around 654 nm. With the same technology a 0.04 mm2 TDC can be designed with a resolution or least significant bit (LSB) of 6 ps for a range of 11-bits. Moreover, SPAD structure variations, novel gating schemes, smart resource sharing, and efficient array architecture a CMOS TD NIRSI sensor can be realized to meet requirements of functional human brain imaging applications.
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