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

A combination of two complementary depth profiling techniques with sub-nm depth resolution, nuclear resonance profiling and medium energy ion scattering, and cross-sectional high-resolution transmission electron microscopy were used to study compositional and microstructural aspects of ultrathin (sub-10 nm) Al2O3 films on silicon. All three techniques demonstrate uniform continuous films of stoichiometric Al2O3 with abrupt interfaces. These film properties lead to the ability of making metal-oxide semiconductor devices with Al2O3 gate dielectric with equivalent electrical thickness in the sub-2 nm range.

High-resolution depth profiling in ultrathin Al2O3 films on Si

Using narrow nuclear reaction resonance profiling, aluminum profiles are obtained in ∼3.5 nm Al2O3 films deposited by low temperature (<400 °C) chemical vapor deposition on Si(100). Narrow nuclear resonance and Auger depth profiles show similar Al profiles for thicker (∼18 nm) films. The Al profile obtained on the thin film is consistent with a thin aluminum silicate layer, consisting of Al–O–Si bond units, between the silicon and Al2O3 layer. Transmission electron microscopy shows evidence for a two-layer structure in Si/Al2O3/Al stacks, and x-ray photoelectron spectroscopy shows a peak in the Si 2p region near 102 eV, consistent with Al–O–Si units. The silicate layer is speculated to result from reactions between silicon and hydroxyl groups formed on the surface during oxidation of the adsorbed precursor.

Evidence of aluminum silicate formation during chemical vapor deposition of amorphous Al2O3 thin films on Si(100)

The growth of ultrathin oxide films by the thermal oxidation of Si(100) at 1020--1170 K and in ${10}^{\mathrm{\ensuremath{-}}1}$--${10}^{\mathrm{\ensuremath{-}}3}$ Torr ${\mathrm{O}}_{2}$ pressure has been studied by high-resolution medium-energy ion-scattering spectroscopy (MEIS). To develop a fundamental understanding of very thin oxide film growth, we utilize sequential isotopic exposures ${(}^{18}$${\mathrm{O}}_{2}$ followed by $^{16}\mathrm{O}_{2}$). MEIS readily distinguishes $^{18}\mathrm{O}$ from $^{16}\mathrm{O}$ and the depth distribution for both species can be determined quantitatively with high accuracy. Our results show that the traditional phenomenological models for silicon oxidation cannot be applied to the initial oxidation. For very thin oxide films (15--25 \AA{}), we find overlapping isotope depth profiles in the film. For thicker films (g40 \AA{}), we find that several key aspects of the Deal-Grove model (oxygen diffusion to the Si-${\mathrm{SiO}}_{2}$ interface and oxide formation at and/or near that interface) are consistent with our results. We also observe $^{18}\mathrm{O}$ loss from the surface after reoxidation in $^{16}\mathrm{O}_{2}$. The complex oxidation behavior during the initial oxidation is likely to be a combination of interfacial, near-interfacial, and surface reactions.

Growth mechanism of thin silicon oxide films on Si(100) studied by medium-energy ion scattering.

Atomic transport during growth of ultrathin dielectrics on silicon

The exceptional depth profiling potential of the ≈ 100 eV narrow resonance in the 18 O(p, α) 15 N nuclear reaction at 151 keV is hindered by the low counting rates due to its low cross section. This drawback is in fact associated with Van de Graaff type accelerators, that provide small proton beam currents at these low energies. In this paper we demonstrate experimentally that this disadvantage may be overcome with ion implantation type accelerators, which may deliver high proton currents. The very high depth resolution of this method in the first hundreds Angstroms of solids was put to benefit efficiently and without hindrance in 18 O isotopic tracing experiments applied to the systematic study of oxygen transport processes in various oxides. The method is illustrated by the study of various oxidation processes of silicon and of recoil implantation of oxygen into the silicon substrate from very thin thermal oxide layers under As and Sb ion bombardment. Deep 18 O profiles could be measured in perpendicular geometry with energy straggling limited resolution. Shallow 18 O profiles in SiO 2 films with thicknesses below 100 A could be deduced from excitation curves recorded in grazing incidence geometry.

High resolution low energy resonance depth profiling of18O in near surface isotopic tracing studies

The growth mechanisms of very thin silicon oxide films formed during rapid thermal oxidation were studied using ion beam analysis and 18 O isotopic tracing methods. In this paper we report on the effects of different cleaning procedures of silicon wafers prior to oxidation in dry oxygen ( 16 O 2 followed by 18 O 2 ) on the growth mechanisms and kinetics. Typical oxide thicknesses ranging from 0.2 to 10 nm were studied. The 18 O and 16 O isotopic profiles were determined by ion beam analysis methods, namely: the 18 O(p, α ) 15 N narrow resonance at 151 keV, and the 18 O(p , α) 15 N and the 16 O(d, p) 17 O reactions associated with step-by-step chemical dissolution. The profiles could be related to current theories on the initial stages of thermal growth of silicon oxide layers allowing us to draw some conclusions regarding the role of surface cleaning of the silicon wafers on the formation of silicon fragments in the volume of the very thin oxide layer. The influence of rapid thermal processing parameters like temperature, time and oxygen partial pressure on the growth mechanisms were also studied and discussed here.

IBA study of the growth mechanisms of very thin silicon oxide films: the effect of wafer cleaning

High resolution depth profiling in near-surface regions of solids by narrow nuclear reaction resonances below 0.5 MeV with low energy spread proton beams

Gas turbine blades are covered with an outer ceramic top coat and an inner metallic bond coat, namely a thermal barrier coating system (TBC). The stability of the TBC is strongly influenced by the thermally growing oxide (TGO) which forms between the top and bond coat during turbine operation. This work is focused on the role of hydrogen in the adhesion of the top coat after oxidation at 1100 °C in dry and wet air at various time steps between 75 and 1150 h. To obtain the essential hydrogen information from the TGO the nuclear reaction 1H(15N, αγ)12C is used with a unique scattering chamber (SDIBA). This equipment combines the defined exfoliation of the top coat by using a 4-points bending mechanism followed by IBA. This allows the determination of hydrogen concentration depth profiles at the TGO and first results are presented.

H.W. Becker

Papers

High resolution low energy resonance depth profiling of18O in near surface isotopic tracing studies

IBA study of the growth mechanisms of very thin silicon oxide films: the effect of wafer cleaning

High resolution depth profiling in near-surface regions of solids by narrow nuclear reaction resonances below 0.5 MeV with low energy spread proton beams

Detection of hydrogen in hidden and spalled layers of turbine blade coatings