FWF - SIRENS - Studies of Ionizing Radiation Effects in NanoScale CMOS

Ionizing radiation, like X- and gamma rays, cause a gradual damage in semiconductor devices. Radiationinduced defects build up throughout the exposure time leading to change of transistor characteristics. This leads to reliability issues in integrated circuits (IC). Over the years scaling of the integrated circuit process improved the robustness to radiation thanks to thinner silicon oxides under MOS transistor gate and higher bulk doping levels. However, starting with 40 nm and 28 nm process nodes to reduce high gate leakage currents the gate material had to change to one with high-dielectric constant. This new gate stack in combination with aggressive scaling is expected to uncover new radiation effects. The project SIRENS will focus on these modern process nodes and examine device-level effects and mechanisms of defects forming due to X-rays. Custom integrated test circuits in 40 nm and 28 nm processes will be designed. These will include dedicated test structures, above all arrays of different size and type transistors. Test structures will be exposed to X-ray radiation to characterize parameters drift. Also energy and spatial distributions of traps in the new gate stack will be studied. The effects will be examined from three perspectives. First, the transistor geometry ependence will be examined. The reason of the apparent lack of radiation induced narrow channel effect in p-channel MOS transistor will be investigated. The radiation-induced short channel effect will be examined to determine the dominating effect amongst the three hypotheses: of charge trapping in sidewall spacer, of halo implant increasing the effective doping, and of gate extension area influencing the electric field. The second perspective covers examination of the extent of possible radiation damage. For this, the evolution of traps density will be evaluated up to very high total ionizing dose levels, reaching 1 Grad, including annealing. The final third aspect is studies of dose rate sensitivity of MOS transistors. The hypothesis, suggested in several recent papers, states that high dose rate in accelerated X-ray testing could lead to illusionary lower damage effects than the stress with low dose rate. Today’s understanding of radiation effects in scaled devices down to 28 nm process is only superficial. The proposed studies encompassing two process nodes (40 nm and 28 nm) and three foundries (Fab40, Fab28- A and Fab28-B) will greatly enrich the state of the current knowledge.