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

Optical characterisation of semiconductor surfaces and interfaces

This paper presents studies of the infrared (IR) absorbance and the intrinsic stress in thermally grown very thin films (60 to 700 A) of SiO2. These data are combined with previously obtained data for thicker thermally grown films (∼1300 A) to study the variation in intrinsic growth stress close to the Si/SiO2 interface. The combined data indicate that the intrinsic stress at Si/SiO2 interfaces extrapolates to the same relatively high values for oxides grown at 700 and 1000 °C, and that the distribution of Si–O–Si bond angles close to the Si/SiO2 interface, as deduced from the IR data, is quantitatively different than in the bulk of the oxide film. These two observations are explained in terms of a model based on a temperature dependent viscoelastic relaxation of the oxide stress. This model emphasizes differences in the thermal history of the SiO2 near the Si/SiO2 interface, as compared to the SiO2 that is well removed from that interface and is in the bulk of the film and/or close to the ‘‘top’’ surface of the film. The observed differences between the 700 and 1000 °C bulk oxides, and the associated Si/SiO2 interfaces are explained in terms of a renormalized time scale that is defined by the ratio of the growth time to the viscoelastic relaxation time at the growth temperature.

Effects of thermal history on stress-related properties of very thin films of thermally grown silicon dioxide, SiO2

Device-grade ultrathin (9–22 A) films of silicon dioxide, prepared from crystalline silicon by remote-plasma oxidation, are studied by soft x-ray photoelectron spectroscopy (SXPS). The 2p core-level spectra for silicon show evidence of five distinct states of Si, attributable to the five oxidation states of silicon between Si0 (the Si substrate) and Si4+ (the thin SiO2 film). The relative binding energy shifts for peaks Si1+ through Si4+ (with respect to Si0) are in agreement with earlier work. The relatively weaker signals found for the three intermediate states (I1, I2, and I3) are attributed to silicon atoms at the abrupt interface between the thin SiO2 film and substrate. Estimates of the interface state density from these interface signals agree with the values reported earlier of ∼2 monolayers (ML). The position and intensity of the five peaks are measured as a function of post-growth annealing temperature, crystal orientation, and exposure to He/N2 plasma. We find that annealing produces more abrup...

/pdf/structure-of-ultrathin-sio2-si-111-interfaces-studied-by-1ghl83njqo.pdf

Structure of ultrathin SiO2/Si(111) interfaces studied by photoelectron spectroscopy

The existence of both strain and disorder at the interface of thermally grown ${\mathrm{SiO}}_{2}$ or plasma-deposited ${\mathrm{Si}}_{3}{N}_{4}$ films on vicinal Si(111) was ascertained unambiguously by frequency, polarization, and crystal orientation dependent studies of optical second-harmonic generation. The strain was seen to cause a redshift of 40 and 70 meV of the interband critical points ${E}_{0}^{\ensuremath{'}}$ and ${E}_{1}$ compared with the bulk silicon values. The disorder is observed by the perturbation of states out of the silicon bulk bands into the gap. Both at terraces and at steps, the density of these charge traps is found to be considerably reduced after rapid thermal annealing.

Electronic transitions at Si(111)/SiO2 and Si(111)/Si3N4 interfaces studied by optical second-harmonic spectroscopy

Si(111) wafers, cut at angles between 0° and 5° in the [112] direction, were prepared by a standard RCA cleaning process followed by a rinse in HF/NH4F solutions with different pH values. Si–SiO2 structures were formed on these surfaces by a two‐step low‐temperature plasma‐assisted oxidation/deposition process, and compared with interfaces formed by thermal oxidation, followed by rapid thermal annealing. The Si(111)–SiO2 interface, subjected to predeposition rinse in a 40 wt. % NH4F solution, displayed a midgap interface trap density Dit of ∼4×1010 cm−2 eV−1. The Dit values increased systematically up to ∼2×1011 cm−2 eV−1 as the pH of the final rinse was decreased. Using a second‐harmonic generation pumped at 1053 nm, the influence of off‐axis orientation and surface structure of Si(111) surfaces was examined. The surface structure was modified by thermal oxidation at 850 °C, annealing at temperatures in the range of 900–1100 °C, and removing the oxide in a HF solution. Changes in the characteristic rota...

Influence of surface roughness on the electrical properties of Si–SiO2 interfaces and on second‐harmonic generation at these interfaces

Second‐harmonic generation (SHG) on vicinal Si(111) surfaces, ∼4° off axis, was studied by varying the chemical bonding state of the surface atoms. O‐terminated surfaces were prepared by thermal oxidation, plasma deposition, and chemical oxidation. H‐terminated surfaces were prepared by removing the oxide layers and subjecting the Si surfaces to RCA cleans terminated by rinses in dilute HF solutions. SHG experiments were performed in the s‐ and p‐polarization modes, using the 1053 nm emission from a Nd:YLF laser. An angular anisotropy, characteristic of vicinal Si(111) surfaces, and including a distinctive SHG feature polarized in the [112] direction of the surface steps, was enhanced by the presence of an oxide layer, independent of how the Si–SiO2 interface was prepared. We show for the first time that H‐terminated Si surfaces exhibited a similar angular anisotropy, and that the SHG signal polarized in the [112] step direction was reduced, rather than enhanced by the change in the surface bonding chem...

Second‐harmonic generation in Si–SiO2 heterostructures formed by chemical, thermal, and plasma‐assisted oxidation and deposition processes

Second-harmonic response of chemically modified vicinal Si(111) surfaces.

Using surface second harmonic generation (SSHG) at 1053nm, we study the influence of off-axis orientation and surface structure of silicon (111) surfaces. We study wafers cut at angles between 0° and 5° in the [112] direction. The surface structure is varied by thermal oxidation at 850°C, annealing, and thinning the oxide in a HF solution. For comparison, nitride films are also investigated. The characteristic rotational symmetry of the SSHG-signal for (111) flat (non-vicinal) surfaces is enhanced by a Si/SiO 2 interface. The oxide layer also influences the signals due to the steps on vicinal surfaces. The results are discussed in comparison with a microscopic model of the oxidized misoriented surface.

Chemically Modified Second Harmonic Generation at Surfaces on Vicinal Si(111) Wafers

This study compares the optical characteristics of SiO2 thin films, grown by high temperature dry thermal oxidation of crystalline Si, or deposited by low-temperature remote plasma enhanced chemical vapor deposition (RPECVD), and subjected to rapid thermal annealing (RTA). Infrared (IR) spectrophotometry is used to determine the frequency of the Si-O-Si bond-stretching vibration for films of varying thickness, both before and after RTA. The thickness dependence of the stretching frequency as a function of distance from the SiO2/Si interface is determined from analysis of these measurements. The resulting profiles combined with a previous study of the index of refraction, n, for thermally grown SiO2 films is used to estimate the variation of n as a function of the distance from the SiO2/Si interface.

Deconvolution of Thickness-Averaged Structural and Optical Properties of Thermally Grown and Rpecvd SiO 2 Films

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