Characterization of thin irradiated epitaxial silicon sensors for the CMS phase II pixel upgrade

The high-luminosity upgrade fo the large hadron collider foreseen for 2023 resulted in the decision to replace the tracker system of the CMS experiment. The innermost layer of the new pixel detector will experience fluences in the order of $\phi_{eq} \approx 10^{16}$~cm$^{-2}$ and a dose of $\approx 5$~MGy after an integrated luminosity of 3000~fb$^{-1}$. Several materials and designs are under investigation in order to build a detector that can withstand such high fluences. Thin planar silicon sensors are good canditates to achieve this goal since the degradation of the signal produced by traversing particles is less severe than for thicker devices. A study has been carried out in order to characterize highly irradiated planar epitaxial silicon sensors with an active thickness of 100~$\mu$m. The investigation includes pad diodes and strip detectors irradiated up to a fluence of $\phi_{eq} = 1.3 \times 10^{16}$~cm$^{-2}$. The electrical properties of diodes have been characterized using laboratory measurements, while measurements have been carried out at the DESY II test beam facility to characterize the charge collection of the strip detectors. A beam telescope has been used to determine precisely the impact position of beam particles on the sensor. This allows the unbiased extraction of the charge deposit in the strip sensor and a good identification of the noise. In this paper, the results obtained for p-bulk sensors are shown. The charge collection efficiency of the strip sensors is 90\% at 1000~V after a fluence of $\phi_{eq} = 3 \times 10^{15}$~cm$^{-2}$. The irradiated diodes show charge multiplication effects. The impact of the threshold applied to a detector on its efficiency is also discussed.