MiniCACTUS: A 65 ps Time Resolution Depleted Monolithic CMOS Sensor

Abstract

MiniCACTUS is a monolithic sensor prototype optimised for timing measurement of charged particles. It has been designed in a standard 150 nm CMOS process without dedicated amplification layer. It is intended as a demonstrator chip for future large scale timing detectors, like upgrades of timing detectors at LHC, or future high energy physics detector projects. The sensor features an active array of 2 x 4 diodes, analog and digital Front-Ends (FEs), a slow control interface, and bias circuitry programmable through internal DACs. The sensing element is a deep n-well/p-substrate diode. Thanks to the optimized guard-rings surrounding the whole chip, it is possible to apply safely more than 450 V on the high-resistivity substrate allowing fast charge collection. The baseline pixel dimensions are 1.0 mm x 1.0 mm and 0.5 mm x 1.0 mm. The analog FEs and the discriminators for each pixel are implemented outside the pixel, at the column level. The power consumption is approximately 300 mW/cm$\mathbf {^2}$, which is compatible with cooling infrastructure available at LHC experiments, and making integration of this concept viable in future high energy physics experiments. After fabrication, the sensors have been thinned to 100 $\mu$m, 200 $\mu$m and 300 $\mu$m total thickness and then post-processed for backside biasing. The time resolution of several sensors with different thicknesses has been measured in 3 test-beam campaigns using high energy muons (Minimum Ionizing Particles) at CERN SPS in 2021 and 2022. A resolution of 65.3 ps has been measured with on-chip FE and discriminator. This paper will focus on the results of these test-beam campaigns.