Experimental study of threshold voltage fluctuation due to statistical variation of channel dopant number in MOSFET´s

Threshold voltage fluctuation has been experimentally studied, using a newly developed test structure utilizing an 8 k-NMOSFET array. It has been experimentally shown that both V_th and the channel dopant number n_a distributions are given as the Gaussian function, and verified that the standard deviation of n_a , can be expressed as the square root of the average of n_a , which is consistent with statistics. In this study, it has been shown that V_th fluctuation (δV_th) is mainly caused by the statistical fluctuation of the channel dopant number which explains about 60% of the experimental results. Moreover, we discuss briefly a new scaling scenario, based on the experimental results of the channel length, the gate oxide thickness, and the channel dopant dependence of δV_th. Finally, we discuss V_th fluctuation caused by the independent statistical-variations of two different dopant atoms in the counter ion implantation process