The need for nitridation of Hf silicate is controversial. On one hand, it has not been proven that the nitridation is mandatory to have working devices and on the other hand, it is known to increase the charge density. In this paper, we present a detailed comprehensive study of the role and the need for nitridation of Hf-based silicates deposited by Atomic Layer Deposition (ALD). The results are based on a correlation of Fourier-Transformed Infrared Spectroscopy (FT-IR), X-ray Photoelectron Spectroscopy (XPS), High-resolution Transmission Electron Microscopy (HR-TEM) and electrical measurements (gate leakage and mobility).
It was observed that the phase segregation in gate dielectrics is not detrimental for the gate leakage density at room temperature. However, the leakage current is significantly increased at higher temperature. The incorporation of nitrogen was either done by NH3 anneal (at 800C) or by Decoupled Plasma Nitridation (DPN – 25.9kJ). While the DPN or NH3 anneal prevent phase segregation for 50% Hf silicate, only the NH3 anneal helps prevent the phase segregation of Hf-rich silicate. Furthermore, the NH3 anneal increases the interfacial thickness, which produces a very low gate leakage with only 10% loss in mobility at high field. Interestingly, DPN followed by O2 anneal leads to an advantageous phase segregation of the Hf-rich silicate by transforming the silicate in a HfO2/SiO2-like stack.
As a conclusion, not only the phase segregation of the silicate does not always lead to shorted devices, but it can be beneficial in terms of mobility. However, the phase segregation seems to be responsible for an enlarged trap-assisted conduction mechanism at high temperature. But even if the 50% Hf silicates non-nitrided leads to working devices, the incorporation of nitrogen in the stack improves the Jg/CET trends and is therefore beneficial.