Radiation hardness studies on Monolithic Active Pixel Sensors

CMOS Monolithic Active Pixel Sensors (MAPS) form a promising sensor technology for the micro vertex detector (MVD) of the Compressed Baryonic Matter (CBM) experiment. The technology allows for building very thin (∼ 50 μm) detectors with a spatial resolution of few μm and a satisfactory time resolution of 10 μs. It is expected that within one run of CBM, the MVD will be exposed to a radiation dose of ∼ 3 Mrad and 10 neq/cm. A joined research program of GSI, the Goethe University Frankfurt and the IPHC Strasbourg aims to expand the radiation tolerance of MAPS beyond this level. Our activities in 2008 followed three research lines which were the study of the Random Telegraph Signal observed in irradiated sensors[1], the search for improved sensor designs based on conventional CMOS processes and the identification and evaluation of dedicated CMOS processes allowing for building depleted sensors. RTS is a non-Gaussian noise of CMOS components. As illustrated in figure 1, it manifests itself as a rectangular modulation of the dark signal of neutron irradiated MAPS, which is sufficient to exceed the discrimination threshold of the sensor and to generate false hit indications. We studied RTS as function of temperature and radiation dose with MIMOSA-181 and MIMOSA-192 prototypes, which were irradiated with fission neutrons [2] at the MEDAPP facility of the FRM II reactor. We observed that up to few 10% of all pixels show occasionally RTS signatures. However, at a temperature of −20C, the SB-pixels of MIMOSA18 showed a reasonably low fake hit rate of ≪ 10 after a dose of 10 neq/cm. This low rate (one order of magnitude lower than observed with the conventional 3Tpixels of MIMOSA-19) is due to the intrinsic leakage current compensation of the SB-pixels. Our studies on MIMOSA-18 and -19 aimed also to test strategies to improve the signal over noise ratio (S/N) of MAPS being irradiated with non-ionizing doses. This S/N is mostly degraded by a drop of the charge collection efficiency (CCE) of the sensor, which is due to the shrinking lifetime of the signal electrons in silicon suffering from radiation induced bulk damage. The CCE is partially recovered if an accelerated charge collection process collects the electrons before they recombine. As already demonstrated [3], this acceleration may be reached by reducing the pixel pitch, which however substantially increases the number of channels in the MVD. An alternative strategy, the use of