Charge-collection properties of irradiated depleted CMOS pixel test structures

The edge transient-current technique (Edge-TCT) and charge-collection measurements with passive test structures made with the LFoundry 150-nm CMOS process on a p-type substrate with an initial resistivity of over 3 k$\Omega$cm are presented. The measurements were made before and after irradiation with reactor neutrons up to $2\cdot 10$¹⁵ n${}_{\mathrm{eq}}$/cm${}^2$. Two sets of devices were investigated: unthinned (700 $\mu$m) with the substrate biased through the implant on top and thinned (200 $\mu$m) with a processed and metallised backplane. The depletion depth was estimated with the Edge-TCT and the collected charge was measured with a ⁹⁰Sr source using an external amplifier having a 25-ns shaping time. The depletion depth for a given bias voltage decreased with the increasing neutron fluence, but it was still larger than 70 $\mu$m at 250 V after the highest fluence. After irradiation a much higher collected charge was measured for the thinned detectors with a processed backplane compared to the unthinned devices, although the same or an even larger depletion depth was measured in the unthinned devices. The most probable value of the collected charge of over 5000 electrons was measured with a thinned device also after irradiation to $2\cdot 10$¹⁵ n${}_{\mathrm{eq}}$/cm${}^2$. This is sufficient to ensure the successful operation of these detectors at the outer layer of the pixel detector in the ATLAS experiment at the upgraded HL-LHC.