Dose rate dependence of TID damage to 65 nm CMOS transistors in X-ray irradiations of the ATLAS ITk Pixel ASIC (ITkPix)

The ATLAS Inner Tracker (ITk) upgrade for the High-Luminosity LHC (HL-LHC) requires a radiation-tolerant pixel readout chip, which must withstand a total ionising dose (TID) of up to 1 Grad. The readout ASIC for the ITk upgrade has been designed by the RD53 collaboration using 65 nm CMOS technology. In order to characterise the radiation tolerance of the chip digital logic, the RD53 ASICs include ring oscillators, which can be used to measure gate delay degradation. Extensive X-ray irradiation studies of the ring oscillators have been performed on the ITk Pixel pre-production readout ASIC, ITkPixV1. A dependence of radiation damage on dose rate has been observed in 65 nm CMOS technology. This paper aims to quantify the dose rate dependence of TID damage to the ITkPix ring oscillators and, therefore, the ITkPix ASIC digital logic. X-ray irradiations at different dose rates between 20 krad/h and 30 Mrad/h are compared. A dose rate dependence is observed, with 2-3 times more damage at the lowest dose rate of 20 krad/h, compared to 4 Mrad/h. The dose rate dependence was also observed to be dependent on transistor size and type.