Upgrade of the Belle II Vertex Detector with Depleted Monolithic CMOS Active Pixel Sensors

TL;DR

The paper discusses the upgrade of the Belle II vertex detector using depleted monolithic CMOS sensors, focusing on the development and testing of the OBELIX chip for high-luminosity conditions.

Contribution

It introduces the OBELIX chip, a new DMAPS sensor designed for Belle II upgrade, with enhanced radiation tolerance and digital features compatible with the trigger system.

Findings

OBELIX chip achieves radiation tolerance up to 5×10^14 n_eq/cm^2 and 100 Mrad.

Beam tests show sensor operation below 40°C after irradiation.

Sensor supports high hit rates up to 120 MHz/cm^2.

Abstract

The Belle II experiment, operating at the asymmetric SuperKEKB $e^+e^-$ collider, is preparing an upgrade of its vertex detector to cope with an increased luminosity of $6 \times 10^{35}$ cm$^{-2}$s$^{-1}$. The upgraded vertex detector (VTX) will consist of five or six layers of depleted monolithic active pixel sensors (DMAPS), with a total material budget of approximately $3\%$ $X/X_0$. The OBELIX chip, derived from the TJ-Monopix2 sensor and fabricated using Tower Semiconductor 180 nm CMOS technology, is being developed for this upgrade. It features a 33 $\mu$m pixel pitch with a time-stamping binning of $50-100$ ns, along with a dedicated digital periphery compatible with the Belle II trigger system, supporting rates up to 30 kHz. The sensor is designed to operate under the high background conditions expected at the target luminosity, with radiation tolerance up to $5 \times 10^{14}$ $n_{eq}$/cm$^2$ and 100 Mrad, while targeting a power density of about 200 mW/cm$^2$. This corresponds to hit rates up to 120 MHz/cm$^2$. Beam test and irradiation studies of TJ-Monopix2 demonstrate that the operating sensor temperature should stay below $40^\circ$C after irradiation up to $5 \times 10^{14}$ $n_{eq}$/cm$^2$. This report reviews the proposed VTX concept, sensor performance, and ongoing R$\&$D activities.