An R&D on monolithic silicon pixel sensors aiming at time resolutions below 100 ps without an internal gain mechanism was started in the framework of the TT-PET medical-physics project. A series of small ASICs was produced in the 130 nm SiGe processes of IHP. These prototypes include a low-noise and very-fast SiGe HBT amplifier able to provide an equivalent-noise charge of 90 electrons on 70 fF capacitance and a total time walk of less than 1 ns. In a testbeam experiment with minimum-ionizing particles, a first monolithic prototype, containing a TDC with a 50 ps binning and a matrix of pixels of 500x500 µm2 area with 750 fF capacitance, provided full efficiency and 110 ps time resolution at a power consumption of 375 µW/channel. Recently, in a lab test with a 90Sr source, a second prototype with pixels of smaller area provided a time resolution of 50 ps on 70 fF, a result that is competitive with silicon technologies that integrate an avalanche mechanism.
The choice of SiGe HBT technology and a sensor design optimized for timing measurement were key elements to this R&D.