Development of high precision pixel sensors for the CBM vertex detector

The first full scale sensor featuring fast read-out with integrated zero-suppression, called MIMOSA-26 [1], was manufactured in 2008/2009 in a 0.35 μm CMOS technology based on a standard, low resistivity, epitaxial layer. More recently, high resistivity epitaxial layers became available, reducing the thermal diffusion of the signal charges, for the benefit of the SNR and thus the radiation tolerance. MIMOSA-26 was re-fabricated with a 400 Ω·cm resistivity, 10, 15 or 20 μm thick, epitaxial layer. Laboratory measurements were performed by illuminating these sensors with Fe and Ru sources[2]. The SNR obtained with the high-resistivity epitaxial layer (∼ 35-40) was about twice higher than with the standard layer (∼ 20). Sensors were tested with a ∼ 100 GeV particle beam at the CERN-SPS. The results are summarised in figure 1. The latter shows that the detection efficiency remains close to 100 % for high discriminator threshold values, indicating that even after a potential increase of the noise, consecutive e.g. to irradiation, a threshold value should still be found with a detection efficiency well above 99% and an affordable fake hit rate. The sensitivity to non-ionising radiation was investigated for a fluence of 10neq/cm. The sensor performances were measured at the CERN-SPS for a temperature of 0◦C (see figure 1). A threshold of 5-6 mV allows keeping the fake hit rate below 10−4 with a detection efficiency of nearly 100 %.