Fast timing monolithic silicon pixel sensor for TOF-PET

The Thin-TOF PET (TT-PET) project aims at the construction of a small-animal PET scanner based on silicon monolithic pixel sensors with 30 ps time resolution for 511 keV photons, equivalent to 100 ps time resolution for minimum ionizing particles. The high time resolution of the pixel sensor allows for precise time of flight measurement of the two photons and a significant improvement in the signal-to-noise ratio of images. The TT-PET scanner also has sensitivity for photon depth of interaction, thus improving the spatial resolution across the whole view of the scanner. The performance of the scanner estimated by a GEANT4 Monte Carlo simulation is presented. In order to achieve the high time resolution, the SiGe heterojunction bipolar transistor technology (SG13S from IHP) was chosen for fast integration, low equivalent noise charge and low power consumption. TCAD simulations were used to design the appropriate guard rings for the pixel sensor. Laboratory measurements of the first test chip show the correct functionality of the guard ring up to a bias potential of 200V applied to the pixels and a good signal-to-noise ratio for the detection of electrons from a Sr-90 source. A test beam measurement with an un-thinned chip shows more than 99% efficiency and approximately 200 ps time resolution for minimum ionizing particles despite the absence of wafer thinning and post-processing.