Neutron irradiation test of depleted CMOS pixel detector prototypes

TL;DR

This study investigates the charge collection and depletion layer behavior of depleted CMOS pixel detectors on p-type substrates under neutron irradiation, demonstrating their suitability for HL-LHC tracking applications.

Contribution

It provides the first detailed analysis of neutron irradiation effects on depleted CMOS pixel detectors' charge collection properties and depletion thickness.

Findings

Depletion thickness increases after initial irradiation steps due to acceptor removal.

At higher fluences, depletion thickness decreases because of radiation-induced defects.

Depleted layer thickness exceeds 50 μm at 100 V after high fluence irradiation, suitable for HL-LHC detectors.

Abstract

Charge collection properties of depleted CMOS pixel detector prototypes produced on p-type substrate of 2 k$\Omega$cm initial resistivity (by LFoundry 150 nm process) were studied using Edge-TCT method before and after neutron irradiation. The test structures were produced for investigation of CMOS technology in tracking detectors for experiments at HL-LHC upgrade. Measurements were made with passive detector structures in which current pulses induced on charge collecting electrodes could be directly observed. Thickness of depleted layer was estimated and studied as function of neutron irradiation fluence. An increase of depletion thickness was observed after first two irradiation steps to 1$\cdot$10$^{13}$ n/cm$^{2}$ and 5$\cdot$10$^{13}$ n/cm$^{2}$ and attributed to initial acceptor removal. At higher fluences the depletion thickness at given voltage decreases with increasing fluence because of radiation induced defects contributing to the effective space charge concentration. The behaviour is consistent with that of high resistivity silicon used for standard particle detectors. The measured thickness of the depleted layer after irradiation with 1$\cdot$10$^{15}$ n/cm$^{2}$ is more than 50 $\mu$m at 100 V bias. This is sufficient to guarantee satisfactory signal/noise performance on outer layers of pixel trackers in HL-LHC experiments.