Impact of probe penetration on the electrical characterization of sub-50 nm profiles

The spreading resistance probe (SRP) and four point probe (FPP) use, respectively, two or four metal probes with loads in the range of 5–100 g to obtain indispensable information on the electrical characteristics (sheet resistance, junction depth, activation degree, profile shape) of impurity depth profiles in silicon. Recently, however, it has been reported that FPP sheet values are becoming irreproducible and that SRP depth profiles can be significantly shallower than the corresponding secondary ion mass spectrometry dopant profiles for ultrashallow structures. In this work we analyze the impact of probe penetration on the accuracy of SRP and FPP measurements for a series of sub-50 nm profiles with different steepness and substrate level. The probe penetration has been measured by atomic force microscopy and ranged from 5 to 130 nm. The FPP sheet resistance errors of up to 300%, as found for the higher loads, can be correlated with the penetration of the probes through the electrical junction taking into account material removal effects. Experimental data and simulations indicate that SRP raw data are relatively insensitive to probe penetration for source/drain implantation and diffusion profiles. Dependent on the structure involved, however, the SRP depth scale must be corrected for an offset due to probe penetration.