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

The Belle II experiment at the SuperKEKB e+e− collider is preparing for an upgrade of its vertex detector to cope with an increased luminosity of up to 6 × 1035 cm−2s−1. The new vertex detector (VTX) will consist of six layers of depleted monolithic active pixel sensors (DMAPS), with a total material budget of about 3% of X0. The OBELIX chip, developed for this upgrade, is derived from the TJ-Monopix2 sensor and manufactured using the Tower Semiconductor 180 nm CMOS technology. It features a 33 µm pixel pitch, time-stamping capability with 50 ns resolution, and 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 expected background hit rate at the target luminosity, with high radiation tolerance, up to 5 × 1014 neq/cm2 and 1 MGy, while maintaining a power density in the range of 200-300 mW/cm2, corresponding to hit rates from a few MHz/cm2 up to 120 MHz/cm2. This paper presents results from laboratory measurements and beam tests performed on TJ-Monopix2 chips, including both not-irradiated and irradiated devices. Particular focus is given to the performance of irradiated sensors as a function of temperature, a key aspect for defining the maximum allowable operating temperature for OBELIX. These studies provide essential input for the thermal design of the VTX cooling system, especially for the innermost layers where power density and hit rates are highest.