Implant dose profile dependence of electrical characteristics of ion-implanted MOS transistors

Abstract

Ion-implanted MOS transistors were fabricated and their electrical characteristics, such as threshold voltage, effective mobility, etc., were measured. In the 11B+-implanted p-channel case, threshold voltage VT can be shifted linearly with implant dose. These shifts ΔVT were entirely determined by the net dose entering silicon. On the other hand, in the 11B+-implanted n-channel case, threshold voltage shift ΔVT varied sublinearly with dose and showed strong dose profile dependence. The profiles were varied with changing implantation energies and annealing times. These results can be interpreted in accordance with the rapid decrease of the maximum surface depletion layer Xd max with the implant dose increase. Numerical calculations of threshold voltage shifts accounting for nonuniformly implanted profiles were compared with observed results. Good agreement was obtained. Effective mobilities μeff of 11B+-implanted p - and n -channel MOS transistors also showed different dose dependences. In the low-dose region, effective mobilities of 11B+-implanted p -channel MOSFET remained almost unchanged, but those of the n -channel case decreased monotonically with dose increase. Qualitative arguments, taking into account surface scattering and impurity scattering effects, and rough calculations are presented.