Ultrahigh vacuum plasma oxidation in the fabrication of ultrathin silicon dioxide films

摘要

This thesis discusses the fabrication and characterization of ultrathin insulator films. These are essential for nanoscale semiconductor device fabrication. When the insulator layer thickness is only a few times the molecule diameter, it is crucial that both film homogeneity and the insulator/semiconductor interface quality are as high as possible. The small dimensions must also be taken into account in characterization, because the established measurement techniques and analyses used in more traditional MOS characterization are not necessarily valid any more.In this work, the metal/silicon dioxide/silicon structure (especially oxide and oxide/semiconductor interface quality) and various silicon dioxide fabrication methods are discussed. The focus in the experimental work is on the studying the plasma assisted oxidation of silicon in an ultra high vacuum chamber and on characterizing the fabricated films. Some of the film properties are found to be excellent: interface smoothness is of a very high quality and interface state densities are low (1011 eV-1cm-2 or lower in the mid-gap) even without any annealing. Process control also seems to be good, as is the breakdown field. The oxide charge, however, is quite high. This may cause considerable harm. One of the consequences is an increased leakage current. This also significantly decreases the device life time by increasing current generated defects. In the annealing experiments carried out, the oxide charge was seen to decrease, indicating that the quality of the silicon dioxide films can be significantly improved by optimization of the thermal treatments.The molecular beam epitaxy system used in processing is designed mainly for research purposes, offering possibility to gain much information about the oxidation process itself. Other, and cheaper, thermal oxidation procedures have in recent years already been developed to a very high level, which means that the process developed is not necessarily the best choice for conventional IC manufacturing purposes. It offers, however, useful applications for research into silicon-based nanostructures, such as silicon/silicon dioxide heterostructures.