Radiation effects on signal and noise parameters of submicron CMOS technologies

G. Gaioni, M. Manghisoni, L. Ratti, V. Re, V. Speziali, G. Traversi The aim of this research activity is to study the properties of the most recent commercial CMOS technology nodes (down to 0.09 micron), in view of their application to rad-hard detector front-end systems. Standard devices in this technologies have a very thin gate oxide which is expected to lead to an increased ionizing radiation tolerance with respect to previous CMOS generations. This property can be exploited where rad-hard CMOS integrated circuits are required, e.g. in space, imaging and high energy physics detector applications. In some cases, such as high energy physics experiments, these circuits have to retain adequate performances even after the exposure to high doses, in the order of several MRad or even tens of MRad. The focus of this activity is to assess analog performance, providing information to rad-hard IC analog designers, who are gradually shifting their effort to nanoscale technologies. The examined devices are fabricated with a standard interdigitated layout, without using the special techniques that were adopted to avoid radiation-induced leakage current increase in technologies with a larger feature size. An interesting point is to verify if open structure devices can be safely used in rad-hard circuits. Besides key parameters such as the threshold voltage shift, special attention is given to the behavior of the white and 1/f components in the noise voltage spectrum, which are often of fundamental importance in amplifying and filtering stages. The 1/f noise term is a figure of merit in radiation effects for MOSFETs. In previous CMOS generations, it was found to be one of the most sensitive parameters to ionizing radiation, and its behavior can be correlated to the threshold voltage shift. In the framework of this activity, the radiation hardness of deep submicron CMOS processes (from 350 nm to 90 nm) has been evaluated. CMOS transistors belonging to the mentioned technologies have been exposed to gamma-rays from a 60-Co source and changes in static, signal and noise parameters have been monitored. This study is focused on applications to high density mixed-signal integrated circuits, where power dissipation is an important issue and critical devices in the analog processing section have to operate in the low current density region, where MOSFETs are often biased in weak or moderate inversion. Therefore, a large part of the experimental analysis presented in this work is focused on device behavior under irradiation in these operating regions. The goal is to define design criteria for the next generation front-end chips to be operated in future experiments at facilities such as SLHC, ILC, Super B-Factory.

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