Defects in CMOS gate dielectrics

The electrical and optical behavior of semiconducting devices is often dominated by the quantity, energy, and physical location of defects. Five decades of research on Si-based devices have led to a reasonable (although not definitive) consensus concerning the nature of defects in complementary metal-oxide semiconductor (CMOS) gate stack dielectrics [1–3]. This understanding has resulted from a continuous interplay between theoretical computation of model structures and experimental measurements of films and devices using a variety of methods. Some of the defects involve changes in local structure or stoichiometry. For example, a slight excess of Si atoms in an otherwise perfect SiO₂film will result in the appearance of Si–Si bonds. These bonds result in electronic states in the SiO₂band gap that can become charged under certain conditions. Other defects involve dangling bonds, either at the Si=SiO₂interface or in the bulk of the SiO₂film. A third class of defect involves changes in local coordination (Si becoming three- or fivefold coordinated, or O becoming threefold coordinated). Yet another class involves impurity atoms in the film, hydrogen being predominant. Although most impurities degrade device performance, hydrogen can also improve device properties when present at appropriate concentrations and in the appropriate location (usually by bonding with uncoordinated=dangling bonds). The role of nitrogen incorporation into these films as industry has moved from SiO₂to SiON dielectrics has been extensively studied [2]. Finally, the role of radiation damage in dielectrics has also received much attention over the past few decades, especially for space and some military applications.