Résumé
Some radionuclides are known to spontaneously emit alpha particles, which have been identified as a reliability issue in microelectronics. Despite their low stopping powers (<1.45 MeV / (mg/cm2) in silicon), these alpha particles are able to trigger Single Event Transient (SET) and Single Event Upset (SEU) in modern technologies and it is useful to evaluate the effect of alpha emitters on the Soft Error Rate (SER). The radionuclides that are likely to be introduced during semiconductor processing have been previously identified. Here, we focus on the decay chain of Uranium 238 which is very abundant in nature and thus in the raw materials that can be used for microelectronics processing. Uranium 238 chain contains 8 radiaoactive nuclei which produce alpha particles. Along their paths the alpha particles are able to generate electron-hole pairs, which diffuse and may lead to transient currents that affect directly the device reliability. In order to accurately evaluate the reliability of a given device, it is mandatory to understand in detail the main properties of the transient currents (e.g. the shape) that are generated during particle emission. It has already been asserted that a double exponential law can model the shape of the transient current, but it is a priori difficult to determine the values of the parameters of this law. Moreover, these parameters strongly depend on the technology as well as the nature and energy of the ionizing particle. Focusing on the Uranium decay chain we investigated the pollution of the bulk and the properties of the transient currents which are generated at an electrode of a 65 nm and a 40 nm technologies. To do so, we used a Monte Carlo approach. The simulated structure is a silicon layer with a surface of 30 µm x 30 µm and a thickness of 20 µm. Above this bulk, we added a layer of 10 µm of silicon dioxide to account for the Back End Of Line (BEOL). At the Si/SiO2 interface, we simulate 12 x 12 drain electrodes whose characteristic dimensions are extrapolated from actual sequential logic circuits implemented with 65 nm and 40 nm technologies. Then we trigger alpha emission at random in the structure assuming that the contamination is uniform. We assume the secular equilibrium for the decay chain which means that even if the concentrations of the emitters are different, their activities are equal. The concentration of the Uranium 238 nucleus is typically around 100 ppt after recent works. Then its activity is easily obtained. Following the alpha particle along its path, we use the diffusion law model to determine how the carrier concentrations vary in space and time. We then numerically divide the ion track into small fragments that spherically diffuse their charges which are calculated using SRIM results. A portion of these electrical charges is collected by the drain electrode that is itself divided into elemental sections. The integration of the collected charge along the whole track and over the whole drain surface gives the final current pulse. A high number of disintegrations is chosen to have a good enough statistic. Yet, because the critical charge is typically above 0.5-1 fC for the investigated technologies, we trigger enough disintegration to have hundreds of transient pulses for which the total collected charge is higher than 0.5 fC. Finally, we present the characteristics of these pulses such as their amplitude and full width at medium height. It allows having guidelines on the kind of transient pulses that can be produced by alpha particles.