TPA-TCT Analysis of the RD50-MPW4 Monolithic Pixel Particle Detector

TL;DR

This paper uses TPA-TCT to analyze the RD50-MPW4 monolithic pixel detector, providing detailed 3D maps of its electric field, depletion depth, and charge sharing with micrometer precision.

Contribution

It introduces a TPA-TCT based method for 3D characterization of monolithic pixel sensors, revealing detailed internal electric and charge collection properties.

Findings

Depletion depth of 226 μm was measured.

Charge collection efficiency is 100%.

Charge sharing occurs at pixel peripheries.

Abstract

The RD50-MPW4, a Depleted Monolithic Active Pixel Sensor (DMAPS) was analyzed using a Two Photon Absortion Transient Current Technique (TPA-TCT). This technique provides sensitivity maps with micrometer-scale spatial resolution, enabling the resolution of the boundaries of the detector's sensitive volume, even for small-area pixels (62x62 squared micrometers in this study). With a full 3D resolution, the depletion depth, the boundaries of the detector electric field, the 3D hit detection efficiency and the charge sharing between neighboring pixels were measured. The RD50-MPW4, a multi-project wafer chip developed by the HV-CMOS working group within the CERN RD50 collaboration, features a 64x64 DMAPS pixel matrix. Illuminating the chip from the backside, the TPA-TCT technique can characterize any pixel element in the matrix because silicon is transparent for near infrared laser light (1550 nm). Electron-hole pairs are generated only around the light focal point, deep in the silicon, so that any charge collected is precisely only from the focal point. With the TPA-TCT technique, the RD50-MPW4 was found to be have a 100\% charge collection efficiency and a depletion depth of 226 $\upmu$m. It was also found that part of the charge in the periphery of the pixel was collected in the neighboring pixel. A 3D map of the sensor clearly shows the in-pixel electronics and the limits of the depletion region.