Charge collection properties of irradiated depleted CMOS pixel test structures

TL;DR

This study investigates the charge collection efficiency of irradiated depleted CMOS pixel structures, demonstrating their potential for use in high-radiation environments like the HL-LHC, even after significant neutron irradiation.

Contribution

It provides new data on the radiation tolerance of depleted CMOS sensors, especially comparing thinned and unthinned devices with different backplane processing.

Findings

Depleted depth decreases with neutron fluence but remains >70 μm at 250 V after high irradiation.

Thinned detectors with backplane processing collect significantly more charge post-irradiation.

Collected charge over 5000 electrons is maintained after irradiation to 2×10^15 n_eq/cm^2.

Abstract

Edge-TCT and charge collection measurements with passive test structures made in LFoundry 150 nm CMOS process on p-type substrate with initial resistivity of over 3 k$\Omega$cm are presented. 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 substrate biased through the implant on top and thinned (200 $\mu$m) with processed and metallised back plane. Depleted depth was estimated with Edge-TCT and collected charge was measured with $^{90}$Sr source using an external amplifier with 25 ns shaping time. Depleted depth at given bias voltage decreased with increasing neutron fluence but it was still larger than 70 $\mu$m at 250 V after the highest fluence. After irradiation much higher collected charge was measured with thinned detectors with processed back plane although the same depleted depth was observed with Edge-TCT. Most probable value of collected charge of over 5000 electrons was measured also after irradiation to 2$\cdot$10$^{15}$ n$_{\mathrm{eq}}$/cm$^2$. This is sufficient to ensure successful operation of these detectors at the outer layer of the pixel detector in the ATLAS experiment at the upgraded HL-LHC.