Edge-on Measurements on Planar Pixel Sensors for the CMS Phase 2 Upgrade

Abstract

In the development process of a new pixel detector for the Phase 2 upgrade of CMS, several variants of new n+p, planar pixel sensors with pixel sizes of 50 x 50 μm2 and 25 x 100 μm2 and an active thickness of 150 μm have been designed and bump bonded to ROC4SENS read out chips. Many weeks of beam tests with sensors irradiated up to fluences of 16E15 neutrons/cm2 have been completed at the DESY test beam facility. Previous studies into the dependence of charge collection on the depth in the silicon sensor have applied the grazing angle technique. This however suffers a lot from resolution effects with the thinner sensors, therefore the grazing angle technique was refined to the edge-on method, where the beam hits the sensor parallel to the sensor surface and travels at one depth through the entire length of the sensor. The edge-on method is used to measure the charge collection of 25 x 100 μm2 pixel sensors as a function of depth for different operating conditions. Additionally, the influence of radiation damage from neutrons with an energy larger than 100 keV and 23GeV protons on the charge collection profile is investigated. The depth performance of the sensors as a function of fluence and bias voltage is then compared to simulations with PIXELAV. Due to the non-zero suppressed chip, it is possible to investigate the charge sharing between the neighboring pixels without any threshold effects. For the first time, with these so called edge-on tomography plots the negative charge induced in the neighboring pixels can be visualized. The position dependent charge collection can be obtained and provides insight into the radiation damage effects on the sensor. Furthermore, the resolution of the pixel cell itself was measured for different angles of beam incidence without an external beam telescope as a reference plane. Instead the pixels before and after the hit pixel itself are used as reference and thus allow for in-silicon tracking as the beam hits the sensor edge-on. The resolution for a 25 x 100 μm2 pixel sensor was determined to be 2.2 μm with errors on the nanometer level.