Measurement of the relative response of small-electrode CMOS sensors at Diamond Light Source

This paper outlines the results of investigations into the effects of radiation damage in the mini-MALTA depleted monolithic pixel sensor prototype. Measurements were carried out at Diamond Light Source using a micro-focus X-ray beam, which scanned across the surface of the device in 2 μm steps. This allowed the in-pixel photon response to be measured directly with high statistics. Three pixel design variations were considered: one with the standard continuous n$^{−}$ layer layout and front-end, and extra deep p-well and n$^{−}$ gap designs with a modified front-end. Five chips were measured: one unirradiated, one neutron irradiated, and three proton irradiated. The standard design showed a decrease of 12% in pixel response after irradiation to 1e15 n$_{eq}$∕cm$^{2}$ . For the two new designs the pixel response did not decrease significantly after irradiation. A decrease of pixel response at high biasing voltages was observed. The charge sharing in the chip was quantified and found to be in agreement with expectations.