Charge collection of irradiated silicon pad diodes and characterization of pixel sensor for the CMS experiment

Abstract

Silicon sensors are considered for the Phase-2 upgrade of the Inner Tracker of the Compact Muon Solenoid (CMS) detector. The high radiation environment changes the properties of the sensors and degrades their performance. In this regard, two types of sensors are studied: pad diodes and hybrid pixel sensors. Both types of sensors are p-type with a thickness of 150 µm produced by Hamamatsu Photonics K.K on the same wafer. Sensors are irradiated with 23 MeV protons at the same irradiation facility.

Using Transient Current Technique with α-particles and red-light laser, Charge Collection Efficiency (CCE) of two irradiated pad diodes are measured close to their n^+ and p^+ implants. Irradiation is done at 1 MeV neutron-equivalent fluences of 2 and 8E15 cm^-2. For the n^+ implant, CCE measured by α-particles shows a higher value than measurements with a red-light laser for all bias voltage. By reducing the energy of α-particles, CCE results became similar to the CCE measured by a red-light laser. The results can be understood by assuming a "non-active region" with a reduced CCE. A model is developed to extract the width and CCE of this region. Next, charge collection profiles of non-irradiated and irradiated pad diodes are measured using a 5.2 GeV electron beam traversing the diode parallel to the readout electrode. The CCE profiles as a function of depth are extracted by unfolding the measured charge collection profiles. The results of the measurements are compared to the simulation using three radiation damage models from the literature.

The second half of the work is related to the characterisation of hybrid pixel modules in the test-beam and lab. Planar sensors with various designs and pixel sizes of 25 x 100 µm^2 and 50 x 50 µm^2 are bump bonded to the RD53A readout chip. To extract parameters such as hit efficiency, spatial resolution, and cluster size, the irradiated and non-irradiated modules are tested at Deutsches Elektronen-Synchrotron (DESY) with a 5.2 GeV electron beam. Irradiation is done up to the 1 MeV neutron-equivalent fluence of 2.0E16 cm^-2. All irradiated modules, except one with a bias-dot, reach a hit efficiency of 0.98 at a bias voltage below 800 V. The estimated spatial resolution degrades after irradiation due to the reduction of cluster size. The RD53A readout chip has a non-staggered bump-bond pattern. Therefore, the opening for passivation of the sensor pixel is not directly above its implant and routing is needed. This routing increases the cross-talk effect between neighbouring pixels in adjacent rows. Cross-talk biases the hit reconstruction by the sensor and deteriorates spatial resolution. In this work, cross-talk of three non-irradiated sensors with different designs is characterised using the charge injection method.