Silicon Sensors for the Upgrades of the CMS Pixel Detector

Abstract

The Compact Muon Solenoid (CMS) is a general purpose detector at the Large Hadron Collider (LHC). The LHC luminosity is constantly increased through upgrades of the accel- erator and its injection chain. Two major upgrades will take place in the next years. The rst upgrade involves the LHC injector chain and allows the collider to achieve a luminosity of about 2 10 34 cm-2 s-1 A further upgrade of the LHC foreseen for 2025 will boost its luminosity to 5 10 34 cm-2 s1. As a consequence of the increased luminosity, the detectors need to be upgraded. In particular, the CMS pixel detector will undergo two upgrades in the next years. The rst upgrade (phase I) consists in the substitution of the current pixel detector in winter 2016/2017. The upgraded pixel detector will implement new readout elec- tronics that allow ecient data taking up to a luminosity of 2 10 34 cm-2s-1,twice as much as the LHC design luminosity. The modules that will constitute the upgraded detector are being produced at dierent institutes. Hamburg (University and DESY) is responsible for the production of 350 pixel modules. The second upgrade (phase II) of the pixel detector is foreseen for 2025. The innermost pixel layer of the upgraded detector will accumulate a radiation damage corresponding to an equivalent uence of eq = 2 10 16 cm-2 and a dose of 10 MGy after an integrated luminosity of 3000 fb-1. Several groups are investigating sensor designs and congurations able to withstand such high doses and uences. This work is divided into two parts related to important aspects of the upgrades of the CMS pixel detector. For the phase I upgrade, a setup has been developed to provide an absolute energy calibration of the pixel modules that will constitute the detector. The cali- bration is obtained using monochromatic X-rays. The same setup is used to test the buering capabilities of the modules' readout chip. The maximum rate experienced by the modules produced in Hamburg will be 120 MHz/cm 2 . For this rate the modules' eciency has been measured to be 99%. In view of the module production, the energy calibration procedure has been automated. The modules assigned to the Hamburg production center should be completed by the end of February 2016. For the phase II upgrade, thin silicon sensors with an active thickness of 100 m irradi- ated with protons up to eq = 1 : 3 10 16 cm-2 have been characterized. The charge collection eciency has been measured using pad diodes. Charge multiplication eects have been ob- served for both n- and p-bulk sensors. P-bulk strip sensors with an active thickness of 100 and 200 m have been characterized with a beam test. The signal of these sensors lies be- tween 4000 and 5000 e- after a uence of 1 : 3 10 16 cm-2. The 200 m thick sensors require a higher bias voltage than the 100 m thick sensors to reach this signal height. The threshold necessary to obtain 95% detection eciency is found to be around 2000 e- for the 100 m thick sensors