Random Dopant Fluctuation (RDF)

Abstract

Following Moore's Law [1], semiconductor industries have doubled the density of transistors in integrated circuits (ICs) every two years. This has rapidly increased the performance of ICs because the degree of integration has grown exponentially. However, below the 1 μm technology node, a serious technical issue was encountered that frustrated further shrinking of the gate pitch, namely, the short channel effect (SCE) [2,3]. The short channel effect brings about other undesirable effects, such as threshold voltage (V TH ) roll off [4-6] and drain-induced barrier lowering (DIBL) [7][8][9]. As the portion of the channel region depleted by the source/drain junction increases as much as the channel length is shortened, the threshold voltage is reduced. In other words, lower gate voltage is required to invert the channel region because a larger part of the channel region is already depleted by the source/ drain-to-channel junctions. The second phenomenon, namely, drain-induced barrier lowering (DIBL), occurs when the source-to-channel potential barrier is affected by the drain bias. If the gate length of metal oxide semiconductor field effect transistors (MOSFETs) is scaled down to such an extent that the energy barrier height at the source-to-channel interface is decreased because of the electric field emanating from the drain region, more electrons can diffuse from the source to the drain over the energy barrier at the source/channel interface. This causes higher off-state leakage current, degraded subthreshold slope, and a lowered threshold voltage. In order to alleviate short channel effects, the channel region of MOSFETs should be sufficiently doped to minimize (i) the depletion charge induced by the source/drain junction, and (ii) the impact of drain bias on the modulation of the height of the energy barrier at the source/channel interface [10]. In very scaled MOSFETs in sub-32 nm technology nodes, the doping concentration in the channel (or the halo doping concentration) is close to 10 18 cm −3 or even higher [11,12].