IMIC — needle-shaped low-power monolithic active pixel sensor for molecular neuroimaging on awake and freely moving rats

IMIC is a Monolithic Active Pixel Sensor prototype for the MAPSSIC project dedicated to direct detection of low energy β+ rays in the brain of awake and freely-moving rats using CMOS technology. Former experiments using a β+ Si probe developed within the PIXSIC project validated a methodological proof of concept. However, conducting routinely such measurements would require improvements with respect to the passive pixel sensors employed in PIXSIC. The new IMIC circuit is fabricated in a 180 nm CMOS Image Sensor Technology and features a matrix of 16 × 128 pixels, which are 30 × 50 μm
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large. The sensor has a needle-like aspect ratio (610 μm × 12 000 μm). The chip is produced on a 18 μm high-resistivity epitaxial layer substrate. The foreseen application requires high sensitivity to β-rays while being immune to background γ-rays. Another severe constraint is the limited power dissipation in order to minimize the thermal impact on the brain. IMIC is a fully-programmable digital sensor. The pixel design is based on the front-end architecture of the ALPIDE chip. However modifications have been made to store the information inside fired pixels between two readouts allowing low data throughput. The circuit is controlled through the SPI protocol, which allows for setting all the necessary polarization signals. The results of post-layout simulations show a high signal to noise ratio (>40) and low power dissipation of 115 μW/matrix. Laboratory characterization using β-rays validate these predictions and demonstrated that the slow readout can cope with the expected low activity (≈ 120 hits/matrix/s).