On the influence of BTI and HCI on parameter variability

BTI parameter degradation of MOSFETs shows a statistical variation. The distribution of the threshold voltage Vth after NBTI stress originates from a convolution of the distribution of the virgin devices together with the additional distribution of the BTI degradation itself. The variability of the Vth (and other electrical parameters) of the virgin devices bases on process induced fluctuations of dopant atoms, oxide thickness, channel length, etc [1]. The dependence on the transistor size is proven by several publications [e.g. 2,3]. The variability of the BTI parameter degradation itself and the convolution are not fully understood yet. The impact of the recovery behavior on the distribution of the Vth - values is, to our knowledge, not yet studied at all. In this paper we investigate the increase (degradation) of variability due to NBTI and the statistical behavior of Vth after end of stress (recovery). Furthermore we analyze the dependency of the additional variability of Vth on the transistor size and geometry. For the necessary statistical relevance we perform long term NBTI experiments with a special smart array test-structure at a large amount of pMOSFETs with various geometries. We prove our results for a second technology node with a different oxide thickness. We demonstrate for the first time that also the induced additional variability recovers. Furthermore, we show that the variability of pMOSFETs after NBTI depends not only on the size of the active area (w×t) but also on its geometry (w/l). Finally we check the influence of HCS on parameter variability.