The narrowest feature on present-day integrated circuits is the gate oxide—the thin dielectric layer that forms the basis of field-effect device structures. Silicon dioxide is the dielectric of choice and, if present miniaturization trends continue, the projected oxide thickness by 2012 will be less than one nanometre, or about five silicon atoms across1. At least two of those five atoms will be at the silicon–oxide interfaces, and so will have very different electrical and optical properties from the desired bulk oxide, while constituting a significant fraction of the dielectric layer. Here we use electron-energy-loss spectroscopy in a scanning transmission electron microscope to measure the chemical composition and electronic structure, at the atomic scale, across gate oxides as thin as one nanometre. We are able to resolve the interfacial states that result from the spillover of the silicon conduction-band wavefunctions into the oxide. The spatial extent of these states places a fundamental limit of 0.7 nm (four silicon atoms across) on the thinnest usable silicon dioxide gate dielectric. And for present-day oxide growth techniques, interface roughness will raise this limit to 1.2 nm.

The electronic structure at the atomic scale of ultrathin gate oxides

Characterization of amorphous and nanocrystalline carbon films

Scanning tunneling microscopy (STM) and ab initio calculations based on density functional theory (DFT) were used to study the self-aligned silicon nanoribbons on Ag(110) with honeycomb, graphene-like structure. The silicon honeycombs structure on top of the silver substrate is clearly observed by STM, while the DFT calculations confirm that the Si atoms adopt spontaneously this new silicon structure.

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Graphene-like silicon nanoribbons on Ag(110): A possible formation of silicene

The outstanding properties of SiO2, which include high resistivity, excellent dielectric strength, a large band gap, a high melting point, and a native, low defect density interface with Si, are in large part responsible for enabling the microelectronics revolution. The Si/SiO2 interface, which forms the heart of the modern metal–oxide–semiconductor field effect transistor, the building block of the integrated circuit, is arguably the worlds most economically and technologically important materials interface. This article summarizes recent progress and current scientific understanding of ultrathin (<4 nm) SiO2 and Si–O–N (silicon oxynitride) gate dielectrics on Si based devices. We will emphasize an understanding of the limits of these gate dielectrics, i.e., how their continuously shrinking thickness, dictated by integrated circuit device scaling, results in physical and electrical property changes that impose limits on their usefulness. We observe, in conclusion, that although Si microelectronic devices...

Ultrathin (<4 nm) SiO2 and Si-O-N gate dielectric layers for silicon microelectronics: Understanding the processing, structure, and physical and electrical limits

Degradation of silicon dioxide films is shown to occur primarily near interfaces with contacting metals or semiconductors. This deterioration is shown to be accountable through two mechanisms triggered by electron heating in the oxide conduction band. These mechanisms are trap creation and band‐gap ionization by carriers with energies exceeding 2 and 9 eV with respect to the bottom of the oxide conduction band, respectively. The relationship of band‐gap ionization to defect production and subsequent degradation is emphasized. The dependence of the generated sites on electric field, oxide thickness, temperature, voltage polarity, and processing for each mechanism is discussed. A procedure for separating and studying these two generation modes is also discussed. A unified model from simple kinetic relationships is developed and compared to the experimental results. Destructive breakdown of the oxide is shown to be correlated with ‘‘effective’’ interface softening due to the total defect generation caused by both mechanisms.

Impact ionization, trap creation, degradation, and breakdown in silicon dioxide films on silicon

The bonding of Si atoms at the SiO2/Si interface is determined via high-resolution core level spectroscopy with synchrotron radiation. For oxides grown in pure O2, the SiO2/Si interface is found to contain Si atoms in intermediate oxidation states with a density of 1.5 ± 0.5 × 1015 cm−2. From the density and distribution of intermediate oxidation states, models of the interface structure are obtained. The interface is not abrupt, as evidenced by the non-ideal distribution of intermediate oxidation states and their high density (about 2 monolayers of Si). The finite width of the interface is explained by the bond density mismatch between SiO2 and Si. Annealing in H2 is found to influence the electrical parameters by removing the Pb centers that pin the Fermi level. The distribution of intermediate oxidation states is not affected.

Microscopic structure of the SiO 2 /Si interface

The nature of fluorosilyl species existing on silicon surfaces during the initial stages of fluorine attack has been investigated with use of high-resolution soft-x-ray photoemission spectroscopy. In this study, several different silicon surfaces [Si(111)-(2\ifmmode\times\else\texttimes\fi{}1), Si(111)-(7\ifmmode\times\else\texttimes\fi{}7), Si(100)-(2\ifmmode\times\else\texttimes\fi{}1), and silicon rendered amorphous by argon-ion bombardment] were exposed to fluorine via the dissociative chemisorption of Xe${\mathrm{F}}_{2}$. For fluorine coverages in the monolayer regime we observed Si $2p$ levels chemically shifted by approximately 1, 2, and 3 eV corresponding, respectively, to Si${\mathrm{F}}_{1}$, Si${\mathrm{F}}_{2}$ and Si${\mathrm{F}}_{3}$. The relative abundance of the specific fluorosilyl species varied significantly among the different silicon surfaces. Recent experiments indicate atomic fluorine is the primary agent in reactive ion etching (RIE) of silicon by ${\mathrm{CF}}_{4}$/${\mathrm{O}}_{2}$ plasmas. Based on such experiments several reaction models hypothesizing the existence of specific surface fluorosilyl species have been proposed, although no direct evidence for any of these surface species has been presented. The results presented here provide the first direct measurement of the composition and relative abundance of the fluorosilyl species remaining on silicon surfaces after exposure to fluorine.

Synchrotron photoemission investigation of the initial stages of fluorine attack on Si surfaces: Relative abundance of fluorosilyl species

The chemisorption of chlorosilanes and chlorine on Si(111)7 × 7

Soft X-ray photoemission study of the silicon-fluorine etching reaction

Unoccupied electronic states at the diamond (111) surface are studied by measuring both bulk- and surface-sensitive C 1s partial-yield soft-x-ray absorption spectra. Several absorption features are observed in the bulk band gap below the 289.19-eV bulk-C 1s absorption edge. They are associated with transitions from the C 1s surface core level to unoccupied surface states by their sensitivity to chemisorbed species and changes in their intensity as the electron escape depth is varied. These states have been detected previously with electron-energy-loss spectroscopy but no structure was resolved. The close proximity of the observed surface absorption (onset at h\ensuremath{\nu}=284 eV) with the h\ensuremath{\nu}=285.35 eV, C 1s\ensuremath{\rightarrow}${\ensuremath{\pi}}^{\mathrm{*}}$ transition in graphite indicates the existence of unoccupied ${\ensuremath{\pi}}^{\mathrm{*}}$ bands at the diamond (111) surface. An interpretation of these results in terms of the \ensuremath{\pi}-bonded chain model for the diamond (111) 2\ifmmode\times\else\texttimes\fi{}1 reconstructed surface is given.

F. R. McFeely

Papers

Microscopic structure of the SiO 2 /Si interface

Synchrotron photoemission investigation of the initial stages of fluorine attack on Si surfaces: Relative abundance of fluorosilyl species

The chemisorption of chlorosilanes and chlorine on Si(111)7 × 7

Soft X-ray photoemission study of the silicon-fluorine etching reaction

C 1s excitation studies of diamond (111). II. Unoccupied surface states