Radiation tolerance of the readout chip for the phase I upgrade of the CMS pixel detector

For the Phase I upgrade of the CMS pixel detector a new digital readout chip (ROC) has been developed. An important part of the design verification are irradiation studies to ensure sufficient radiation tolerance. The paper summarizes results of the irradiation studies on the final ROC design for the detector layers~$2-4$. Samples have been irradiated with 23\,MeV protons to accumulate the expected lifetime dose of 0.5\,MGy and up to 1.1\,MGy to project the performance of the ROC for layer~1 of the detector. It could be shown that the design is sufficiently radiation tolerant and that all performance parameters stay within their specifications. Additionally, very high doses of up to 4.2\,MGy have been tested to explore the limits of the current chip design on 250\,nm CMOS technology. The study confirmed that samples irradiated up to the highest dose could be successfully operated with test pulses.