| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 93 === Hafnium oxide (HfOx) and hafnium oxynitride (HfOxNy) gate dielectrics were prepared by reactive magnetron sputtering from Hf target, following by postdeposition annealing at 700 ℃ in N2 ambient. We investigated the material and electrical characteristics to establish the feasibility of applying sputtered HfOx and HfOxNy as the gate oxide. In addition, low-temperature (450 ℃) N2O or NH3 plasma nitridation on Si surface was also carried out. The influence of surface nitridation on the interface quality and electrical properties is also discussed.
Rutherford backscattering spectrometry (RBS) was utilized to examine the composition and density of the thin films. The crystal structure of the thin films was identified by glacing incident angle x-ray diffraction (GIAXRD). X-ray photoelectron spectroscopy (XPS) was applied for surface chemical bonding analysis of nitrided Si and the dielectric films. The thicknesses of films were determined by high-resolution transmission electron microscopy (HRTEM), and the microstructure was also characterized. For electrical properties, picoampere meter/DC voltage source (HP4140B) was used to measure the I-V curves, and the C-V curves were obtained by LCR meter (HP4284). For optical properties, ellipsometry was used not only to estimate the refractive index of the thin films, but also to simulate the percentage of void in the films.
Based on RBS analysis, as-deposited HfOx film possesses excess oxygen atoms, and both HfOx and HfOxNy films transform to HfO2 after annealing at 700 ℃. According to the results of GIAXRD and HRTEM, ultra-thin HfOx and HfOxNy films are amorphous even after annealing at 700 ℃ in N2 ambient for 15 minutes. From HRTEM images and XP spectra, there exist apparent interfacial layers with the structure close to SiO2 at the HfOx/Si and HfOxNy/Si interfaces. However, HfOxNy shows a thinner interlayer during deposition because of the nearly oxygen-free ambient. Besides, HfOxNy film also presents a denser structure than the HfOx film does.
Nitrogen atoms are introduced into the interfacial layers by plsma nitridation on Si before dielectrics deposition. For HfOxNy dielectrics system, the nitrogen-doped process in interfacial layers also occurs during sputtering, and the effects are more significant than surface nitridation treatment. For both HfOx and HfOxNy dielectrics systems, interfacial layers will still further grow during post-annealing process even if Si substrates have been passivated through surface nitridation. Nevertheless, ΔVFB is reduced by plasma nitridation, indicating that the number of fixed oxide charges are effectly decreased, especially via N2O plasma.
In comparison to HfOx, MOS stacks with HfOxNy dielectics present smaller hysteresis loops, higher refractive index, and less percentage of voids, which expresses that the number of oxide trapped charges are evidently decreased due to nitrogen incorporation in the dielectrics. Furthermore, HfOxNy dielectrics also show higher ability to suppress leakage current than HfOx.
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