On the reliability of SIMS depth profiles through HfO2-stacks

Measuring HfO2/Si stacks-by secondary ion mass spectroscopy (SIMS) has become a common task in the semiconductor industry. Although the reliability of the SIMS-profiles from multilayer structures can be influenced by sputter yield and ionization yield variations in the interface region, the part of the dopant profile located beyond the interface can normally be obtained with excellent SIMS-characteristics. By comparing the SIMS-profiles from B-profiles in Si, measured with and without a HfO2-cap layer, it becomes clear that the high k film has a strong impact on the resulting dopant profiles. With the high k film present, a significant distortion of the depth and intensity scale can be observed and the B-profile (which is, in principle, fully contained in the Si) appears almost twice as deep when profiling through the HfO2-layer as compared to the uncapped sample. Normalizing the depth and concentration scale with the variations of the SiO-signals, seems to lead to an acceptable B-profile. The distortion results from a strong reduction in sputter yield due to the presence of (even small) amounts of HE Apparently, Hf segregates towards the surface forming a (fractional) layer, which shields the Si-atoms from sputtering. The sputter yield of this Hf-layer on the other hand is very low leading to a drastic reduction in average erosion rate. For conditions where normally no full oxidation is obtained (high energy, non-normal incidence beams on Si and Ge-substrates), this reduction leads to a temporary enhancement of the oxygen incorporation and the ionization probability. (C) 2004 Elsevier B.V. All rights reserved.