An empirical model for early resistance changes due to electromigration

A new heuristic description for electromigration-induced early resistance changes is given. The basis is formed by two coupled partial differential equations, one for vacancies, and one for imperfections. These equations are solved numerically for a grain boundary bamboo structure. It is shown that this model is capable of simulating the typical effects as observed in early resistance change measurements. These early resistance changes are due to the redistributions of the vacancies and the generation of imperfections. Simulations are performed that closely match the measured resistance change curves

Modeling of the Reservoir Effect on Electromigration Lifetime

Electromigration behaviour in W-plug/metal stripe structures is different from conventional metal-strip structures because there is a blocking boundary formed by the immobile W-plug in the contact/via. Electromigration failures occur more readily close to the W-plug than in metal-strip structures because metal ions are forced away from the contacts/vias by electric current, blocking the contacts/vias area. Several works have reported electromigration lifetime of multiple level interconnects to be influenced by the presence of a reservoir around the contacts/vias. Reservoirs are metal parts that are not or are hardly conducting current that act as a source to provide atoms for the area around the blocking boundary where the atoms migrate away due to the electric current. Interconnect lifetime can be prolonged by using the reservoirs, called the ¿reservoir effect¿. 2D simulation of the effects of reservoirs has been performed. The stress build-up during electromigration in the contact area can be simulated for several configurations, separating the effects of overlap, total reservoir area, the reservoir layout directions (vertical and horizontal), number of contacts/vias and contact/via placement. It is very useful for IC design rules to estimate which parameters are important for IC reliability. In this study, we considered the critical stress that the metal line can sustain before void formation as failure criterion. The failure time is determined by the time to reach the critical stres

Effect of Thermal Gradients on the Electromigration Lifetime in Power Electronics

The combined effects of electromigration and thermomigration are studied. Significantly shorter electromigration lifetimes are observed in the presence of a temperature gradient. This cannot be explained by thermomigration only, but is attributed to the effect of temperature gradient on electromigration-induced failures

Avoidance Learning as Predictor of Posttraumatic Stress in Firefighters

BACKGROUND: Avoidance is a well-established maintenance factor in anxiety-related psychopathology. Individuals prone to anxiety show more maladaptive avoidance responses in conditioning paradigms aimed at avoidance learning, which indicates impairments in safety learning. To what extent avoidance learning is associated with posttraumatic stress disorder (PTSD) is still unclear, despite the logical relevance to the symptomatology. In this prospective study, we investigate avoidance learning responses in first responders, a population at high risk for traumatic exposure and thus PTSD development, and studied whether avoidance learning was associated with concurrent and future PTSD symptoms. METHOD: Firefighters (N = 502) performed an avoidance learning task at baseline assessment in which they first learned that two conditioned stimuli (CS+) were followed by an aversive stimulus (US) and one conditioned stimulus (CS-) was not. After that, they could learn to which CS avoidance of the US was effective, ineffective or unnecessary. Self-reported PTSD symptoms were assessed at baseline, and at 6, 12, 18 and 24 months. RESULTS: Participants exhibited comparable avoidance patterns to low anxiety individuals from previous studies. Avoidance learning responses were not associated with PTSD symptoms at baseline nor at follow-up. DISCUSSION: Our study found no evidence that avoidance learning was related to PTSD symptom severity in a high-risk, yet low symptomatic population, nor did it predict the development of PTSD symptoms at a later point in time. Future research should focus on studying avoidance learning in a clinical or high symptomatic sample to further clarify the role of avoidance learning in PTSD development

The role of acute cortisol and DHEAS in predicting acute and chronic PTSD symptoms

Background: Decreased activation of the hypothalamus-pituitary-adrenal (HPA) axis in response to stress is suspected to be a vulnerability factor for posttraumatic stress disorder (PTSD). Previous studies showed inconsistent findings regarding the role of cortisol in predicting PTSD. In addition, no prospective studies have examined the role of dehydroepiandrosterone (DHEA), or its sulfate form DHEAS, and the cortisol-to-DHEA(S) ratio in predicting PTSD. In this study, we tested whether acute plasma cortisol, DHEAS and the cortisol-to-DHEAS ratio predicted PTSD symptoms at 6 weeks and 6 months post-trauma. Methods: Blood samples of 397 adult level-1 trauma center patients, taken at the trauma resuscitation room within hours after the injury, were analyzed for cortisol and DHEAS levels. PTSD symptoms were assessed at 6 weeks and 6 months post-trauma with the Clinician Administered PTSD Scale. Results: Multivariate linear regression analyses showed that lower cortisol predicted PTSD symptoms at both 6 weeks and 6 months, controlling for age, gender, time of blood sampling, injury, trauma history, and admission to intensive care. Higher DHEAS and a smaller cortisol-to-DHEAS ratio predicted PTSD symptoms at 6 weeks, but not after controlling for the same variables, and not at 6 months. Conclusions: Our study provides important new evidence on the crucial role of the HPA-axis in response to trauma by showing that acute cortisol and DHEAS levels predict PTSD symptoms in survivors of recent trauma. © 2014 Elsevier Ltd