Charged particle detection performances of CMOS pixel sensors produced in a 0.18 um process with a high resistivity epitaxial layer

摘要

The apparatus of the ALICE experiment at CERN will be upgraded in 2017/18during the second long shutdown of the LHC (LS2). A major motivation for thisupgrade is to extend the physics reach for charmed and beauty particles down tolow transverse momenta. This requires a substantial improvement of the spatialresolution and the data rate capability of the ALICE Inner Tracking System(ITS). To achieve this goal, the new ITS will be equipped with 50 um thin CMOSPixel Sensors (CPS) covering either the 3 innermost layers or all the 7 layersof the detector. The CPS being developed for the ITS upgrade at IPHC(Strasbourg) is derived from the MIMOSA 28 sensor realised for the STAR-PXL atRHIC in a 0.35 um CMOS process. In order to satisfy the ITS upgraderequirements in terms of readout speed and radiation tolerance, a CMOS processwith a reduced feature size and a high resistivity epitaxial layer should beexploited. In this respect, the charged particle detection performance andradiation hardness of the TowerJazz 0.18 um CMOS process were studied with thehelp of the first prototype chip MIMOSA 32. The beam tests performed withnegative pions of 120 GeV/c at the CERN-SPS allowed to measure asignal-to-noise ratio (SNR) for the non-irradiated chip in the range between 22and 32 depending on the pixel design. The chip irradiated with the combineddose of 1 MRad and 10^13 n_eq/cm^2 was observed to yield a SNR ranging between11 and 23 for coolant temperatures varying from 15 C to 30 C. These SNR valueswere measured to result in particle detection efficiencies above 99.5% and 98%before and after irradiation respectively. These satisfactory results allow tovalidate the TowerJazz 0.18 um CMOS process for the ALICE ITS upgrade.