65-nm CMOS front-end channel for pixel readout in the HL-LHC radiation environment

A charge preamplifier has been developed in a 65-nm CMOS technology for processing the signals from the inner pixel layers of the CMS detector, in view of the experiment upgrade for the High-Luminosity (HL) Large Hadron Collider (LHC). The circuit is part of a readout channel implementing a time-over-threshold method for the analog-to-digital conversion of the input charge signal. Samples of the circuit have been exposed to high doses of ionizing radiation, up to 500 Mrad(SiO$_{2}$), from a low-energy proton source and from an X-ray tube. The test results show that the performance degradation, in terms of charge sensitivity, signal shape, and noise, is compatible with operation in the harsh environment of the HL-LHC. Measurement data from the characterization of preamplifiers using different kinds of capacitors in the feedback network help to gain some insight into the damage mechanisms in pMOS transistors irradiated with large ionizing doses. The dependence of performance degradation on the kind of radiation source used in the tests is also discussed.