Charge collection properties of TowerJazz 180 nm CMOS Pixel Sensors in dependence of pixel geometries and bias parameters, studied using a dedicated test-vehicle: the Investigator chip

TL;DR

This study investigates how pixel geometry and bias parameters affect charge collection in TowerJazz 180 nm CMOS sensors, providing insights for optimizing sensor design and guiding future simulations.

Contribution

It offers a comprehensive analysis of parameters influencing charge collection in partially depleted CMOS pixel sensors, aiding optimization and future device modeling.

Findings

Charge collection can be fast (<10 ns) even in partially depleted sensors.

Electrode size and spacing significantly influence signal sharing and amplitude.

Reverse substrate bias impacts charge collection efficiency.

Abstract

This paper contains a compilation of parameters influencing the charge collection process extracted from a comprehensive study of partially depleted Monolithic Active Pixel Sensors with small (<25 um$^2$) collection electrodes fabricated in the TowerJazz 180 nm CMOS process. These results gave guidance for the optimisation of the diode implemented in ALPIDE, the chip used in the second generation Inner Tracking System of ALICE, and serve as reference for future simulation studies of similar devices. The studied parameters include: reverse substrate bias, epitaxial layer thickness, charge collection electrode size and the spacing of the electrode to surrounding in-pixel electronics. The results from pixels of 28 um pitch confirm that even in partially depleted circuits, charge collection can be fast (<10 ns), and quantify the influence of the parameters onto the signal sharing and amplitudes, highlighting the importance of a correct spacing between wells and of the impact of the reverse substrate bias.