Process quality control and irradiation studies of silicon sensors for the CMS phase-II upgrade

Abstract

High Energy Physics experiments use a wide range of technologies to detect the fundamental building blocks of our universe and test our current understanding of the Standard Model. The Large Hadron Collider (LHC) at the European Organisation for Nuclear Research (CERN) is the world’s largest and most powerful particle accelerator. To extend the discovery potential of the LHC, a major upgrade to the so-called High Luminosity LHC (HL-LHC) will be implemented in the following years. For HL-LHC the received radiation levels of the detectors during their lifetime will be ten times larger than the currently expected limits. These new conditions require a full upgrade of the various detector systems along the LHC’s 27 km accelerator ring. As part of this upgrade, the Compact Muon Solenoid (CMS) experiment will undergo a complete replacement of two of it’s sub-detector systems, namely the silicon Tracker and the calorimeter end caps. Over 50000 new silicon sensors will be manufactured and integrated in the CMS detector during the next years, which requires reliable testing methods to ensure their quality and the stability of production parameters. Process Quality Control (PQC) establishes a fast and highly automated method to access sensor characteristics and additional process parameters from dedicated test structures for the Tracker and new High Granularity Calorimeter (HGCAL). Besides the actual purpose of PQC to monitor mass production of silicon sensors, the present work describes the additional application of PQC tests in the sensor prototyping phase of the HGCAL. Analyses of the measured PQC test structures from various prototypes up to recent Pre-series sensors are discussed in this thesis, which help to evaluate the most suitable sensor parameters for the final production.A further important aspect in the development and testing phase of new silicon sensors is given by simulations of the devices. Therefore, a section of this work is dedicated to simulations of the PQC MOS Field Effect Transistor (MOSFET), which replicates the inter-channel region of the sensor. Altering the properties of the implant that separates the electrodes from each other, correlations can be identified that go beyond the possibilities offered by actual measurements.Studies of irradiated test structures are another part of sensor characterisation to identify the altered properties of silicon induced by hard radiation as experienced in the CMS detector’s environment. Microscopic irradiation damage in the silicon lattice leads to macroscopic changes of sensor properties. Thus, within the scope of this work, irradiation studies of selected PQC test structures with neutrons as well as X-Rays are described. The test structures exposed to neutron irradiation were measured with the PQC method to show its applicability for the altered properties of the irradiated devices. X-Ray studies were carried out at CERN with in-situ measurements that required to build a compact and portable measurement setup. The two kinds of particles allow a separate investigation of the influence of heavy neutrons on the bulk material compared to photons, which mainly cause surface damage.