Towards Radiation Hard Sensor Materials for the CMS Tracker Upgrade

During the high luminosity phase of the LHC starting around 2020 (HL-LHC), the machine is expected to deliver an instantaneous luminosity of $5 \cdot 10^{34}\ \tt{cm}^{-2} s^{-1}$. A total of $3000\ {\tt fb}^{-1}$ of data is foreseen to be delivered, hereby increasing the discovery potential of the LHC experiments significantly. However, the radiation dose of the inner tracking systems will be severe, requiring new radiation hard sensors for the CMS tracker. Up to now, typically p-in-n float zone devices with a thickness of at least 300 micrometers have been used for silicon strip detectors at CMS and other experiments. However, the signal-to-noise ratio for sensors implemented in this technology would be severely reduced for the inner layers of the tracker at the HL-LHC. Many measurements are described in literature, performed on a variety of silicon materials and technologies, but they are often hard to compare, because they were done under different conditions. To systematically compare the properties of different silicon materials and design choices and identify a solution suited for the upgrade, CMS has initiated a large irradiation and measurement campaign. Several test structures and sensors have been designed and implemented on 18 different combinations of wafer materials, thicknesses and production technologies. The structures are electrically characterized before and after irradiation with different fluences of neutrons and protons, corresponding to the expected fluences at different radii of the outer tracker after $3000\ \tt{fb}^{-1}$. The tests performed include studies with $\beta$-sources, lasers and beam tests. In this talk, results from the ongoing campaign are presented.