Study of Monolithic Active Pixel Sensors for the Upgrade of the ALICE Inner Tracking System

The upgrade of the ALICE vertex detector, the Inner Tracking System (ITS), is scheduled to be installed during the next long shutdown period (LS2 in 2019-2020) of the CERN Large Hadron Collider (LHC). The current ITS will be replaced by seven concentric layers of Monolithic Active Pixel Sensors (MAPS) with total active surface of $\sim$10 m$^2$, thus making ALICE the first LHC experiment implementing MAPS detector technology on a large scale. The scope of this thesis is twofold; to report on the activity on the development and the characterisation of a MAPS for the ITS upgrade and to study the charge collection process using a first-principles Monte Carlo simulation. The performance of a MAPS depends on a large number of design and operational parameters, such as collection diode geometry, reverse bias voltage, and epitaxial layer thickness. I have studied this dependence by measuring the INVESTIGATOR chip response to X-rays emitted by an $^{55}$Fe source and to minimum ionising particles. In particular, I have examined the influence of the parameters considered in the design of the MAPS for the ITS upgrade, on the $Q/C$ ratio, i.e.\ the ratio of the collected charge in a single pixel and the pixel input capacitance. The ALPIDE chip, based on TowerJazz 180~nm CMOS Imaging Process, has been developed for the ITS upgrade. I have performed extensive laboratory studies on the full-scale prototypes as well as on the final sensor. I have studied in detail the analogue front-end response in terms of timing characteristics using an infrared laser beam. I have significantly contributed to the measurements of the prototype and final sensors under charged particle beams. The laboratory and test-beam measurements allowed me to verify the compliance of the ALPIDE sensor to the ITS upgrade requirements. The layers of the new ITS will be azimuthally segmented into the independent units called staves, which integrate the ALPIDE sensors and the mechanical and electrical support elements. The NA61/SHINE collaborations offered the opportunity to test an ITS stave in the Pb--Pb collision environment of their experiment. I have integrated a stave consisting of nine ALPIDE sensors in the NA61 experiment and tested its performance in terms of detection efficiency and spatial resolution at the multiplicities comparable to those expected in the ITS after the LS2. While the successful operation of the sensors for the ITS upgrade has been confirmed, a great deal still remains to be understood about the charge collection process of MAPS. The exact response of MAPS is challenging to model due to contributions from both epitaxial layer and substrate, a typically only partially depleted active volume, and complex well structures. Transient TCAD simulations provide a good description but are not sufficiently fast to be used in the experiments simulations. Therefore, I have developed a \textit{fast tool} to model the response of ALICE ITS MAPS. The basic concept is a first principles MC simulation, using electric fields extracted from a TCAD simulation to model the charge carrier drift. That is, the more complex part is handled by a TCAD simulation while preserving the speed of a MC simulation with only one free parameter. The tool is versatile, any MAPS architecture can be simulated once the proper external electric field is provided. So far, I have simulated the response of the INVESTIGATOR (analogue output) and ALPIDE (digital output) chips. An excellent agreement between data and simulation has been achieved, both for $^{55}$Fe X-rays and minimum ionising particles.