A built-in SRAM for radiation hard CMOS pixel sensors dedicated to high energy physics experiments

CMOS pixel sensors (CPS) are attractive candidates for charged particle tracking in high energy physics experiments. However, CPS chips fabricated with standard CMOS processes, especially the built-in SRAM IP cores, are not radiation hard enough for this application. This paper presents a radiation hard SRAM for improving the CPS radiation tolerance. The SRAM cell is hardened by increasing the static noise margin (SNM) and adding P+ guard rings in layout. The peripheral circuitry is designed by building a radiation-hardened logic library. The SRAM internal timing control is hardened by a self-adaptive timing design. Finally, the SRAM design was implemented and tested in the Austriamicrosystems (AMS) 0.35μm standard CMOS process. The prototype chips are adapted to work with frequencies up to 80MHz, power supply voltages from 2.9V to 3.3V and temperatures from 0°C to 60°C. The single event latchup (SEL) tolerance is improved from 5.2MeVcm2/mg to above 56MeVcm2/mg. The total ionizing dose (TID) tolerance is enhanced by the P+ guard rings and the self-adaptive timing design. The single event upset (SEU) effects are also alleviated due to the high SNM SRAM cell and the P+ guard rings. In the near future, the presented SRAM will be integrated in the CPS chips for the STAR experiments.