Test-beam results from MiniCACTUS-v2: A depleted monolithic CMOS timing sensor prototype

TL;DR

MiniCACTUS-v2 is a CMOS-based timing sensor prototype capable of detecting individual particles with sub-100 ps accuracy, demonstrated through test-beam results showing a best time resolution of 48.88 ps.

Contribution

This paper presents the first test-beam results of MiniCACTUS-v2, a novel monolithic CMOS sensor with optimized design for high-precision timing of ionizing particles.

Findings

Achieved a best time resolution of 48.88 ps.

Sensor breakdown voltages exceed 500 V, enabling full depletion.

Effective time tagging demonstrated at various sensor thicknesses and bias voltages.

Abstract

MiniCACTUS-v2 is a monolithic sensor prototype designed in LF 150 nm CMOS process for time tagging of individual Minimum Ionizing Particles with an accuracy better than 100 ps. The sensing element is a deep n-well/p-substrate diode without internal amplification. To minimize detector capacitances, the analog front-ends and the discriminators for each pixel have been implemented outside the pixel, at the column level. After fabrication, the sensors have been thinned to 150 microns, 175 microns and 200 microns and then post-processed for backside biasing. The breakdown voltages measured on these sensors are higher than 500 V, ensuring the complete depletion of the charge collection volume. In this paper, we will focus on the time resolution measurements from a test-beam campaign conducted in July 2025 at SPS-CERN. During this period, several pixels from the 3 different sensor thicknesses have been tested at different bias voltages. The best time resolution measured is 48.88 ps on a 0.5 mm x 0.5 mm pixel from a 175 microns-thick sensor at 500 V, with nominal settings for the on-chip analog front-end and discriminator.