Study on Device Lifetime Uncertainty asscociated with Substrate Current Fluctuation in Ultrasmall Semiconductor Devices

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We have investigated the device lifetime uncertainty of deep submicron and sub-0.1 micron MOSFET devices from the viewpoint of current fluctuations. More specifically, the substrate current fluctuations associated with anisotropy of impact ionization processes, current noise, and statistical current fluctuations have been investigated along with the device degradation and its reliability. The results we have obtained will be summarized as follows.1. The full-band Monte Carlo simulations have been carried out in Si-MOSFETs to study the anisotropic impact ionization processes and it has been found that the substrate current fluctuates very strongly as the applied drain voltage is reduced. This implies that the reliability of device lifetime based on the substrate current could fluctuate with great uncertainty.2. Current noise under various device structures has been studied via Monte Carlo method. It has been found that the relative strength of current noise could exceed several tens % because of the decrease of the number of conduction electrons.3. Statistical current fluctuation associated with discrete random dopants in Si-MOSFETs has been investigated by the 3-D Drift-Diffusion simulations. It has been pointed out that the 'atomistic' dopant model widely used at present is inconsistent with the concept presumed in the simulation scheme.4. We have developed a new dopant model which overcomes the above-mentioned problems. Its validity has been demonstrated by simulating various device structures.