Modeling Switched Bias Irradiations on Floating Gate Devices: Application to Dosimetry

Abstract

The response of floating gate (FG) devices to 60Co gamma -rays under switched bias conditions is studied by real-time monitoring of the threshold voltage evolution with accumulated dose. Samples were fabricated in a 1.5- mu ext{m} CMOS process, suitable for dosimetry applications. A physics-based numerical model of total ionizing dose (TID) effects in FG MOS devices is developed, taking into consideration the dominant microscopic processes leading to charge accumulation/neutralization across the structure. The main parameters of the model are the capture and neutralization rates in both oxides (tunnel and interpoly), and the initial FG charge. A dataset of experimental measurements is used to extract those parameters. Then, the model is applied to predict the response of later experiments. In one case, the device is irradiated under different switching bias voltages, yielding a mean absolute error of 1.8%. With the same set of parameters, but using another device, the model is able to predict with an error of 2.1%, an exposition performed with a V{t} -shift range twice the one in the first experiment. The predictive power of the model in a wide range of experimental conditions relies on the physics principles behind it. Combined experiments plus simulations allow to distinguish how charges in each layer of the structure evolve and contribute to the overall response. As an application of the model, cycled measurement technique for dosimetry is investigated.