H. C. Lu

Intermixing at the tantalum oxide/silicon interface in gate dielectric structures

Metal oxides with high dielectric constants have the potential to extend scaling of transistor gate capacitance beyond that of ultrathin silicon dioxide. However, during deposition of most metal oxides on silicon, an interfacial region of SiOx can form that limits the specific capacitance of the gate structure. We have examined the composition of this layer using high-resolution depth profiling of medium ion energy scattering combined with infrared spectroscopy and transmission electron microscopy. We find that the interfacial region is not pure SiO2, but is a complex depth-dependent ternary oxide of Si–Tax–Oy with a dielectric constant at least twice that of pure SiO2 as inferred from electrical measurements. High-temperature annealing crystallizes the Ta2O5 film and converts the composite oxide to a more pure SiO2 layer with a lower capacitance density. Using low postanneal temperatures, a stable composite oxide structure can be obtained with good electrical properties and an effective SiO2 thickness of...

High resolution ion scattering study of silicon oxynitridation

High resolution medium energy ion scattering was used to characterize the nitrogen distribution in ultrathin silicon oxynitrides with sub‐nm‐accuracy. We show that nitrogen does not incorporate into the subsurface region of the substrate after oxidation of Si(100) in NO. Core‐level photoemission experiments show two bonding configurations of nitrogen near the interface. Oxynitridation in N2O results in a lower concentration and a broader distribution of nitrogen than in the NO case.

The composition of ultrathin silicon oxynitrides thermally grown in nitric oxide

The thermal oxynitridation of Si(100) in nitric oxide (NO) has been studied by high resolution medium energy ion scattering for ultrathin films. The nitrogen depth distribution and the composition of the films have been accurately determined. It is observed that for NO-grown films the nitrogen is distributed relatively evenly in the film, unlike the sharply peaked distribution observed in the case of SiO2 films that were subsequently annealed in NO. The width of the nitrogen distribution, as well as the oxynitride thickness, increase with temperature. It is further found that the total amount of nitrogen in the film and the ratio of nitrogen to oxygen increases with increasing oxynitridation temperature. These results have significant impact on our understanding of how nitrogen can be positioned in next-generation gate dielectrics.

NITROUS OXIDE GAS PHASE CHEMISTRY DURING SILICON OXYNITRIDE FILM GROWTH

N2O gas phase chemistry has been examined as it relates to the problem of ultrathin film silicon oxynitridation for semiconductor devices. Computational and analytical kinetics studies are presented that demonstrate: (i) there are 5 main reactions in the decomposition of N2O, (ii) the gas composition over a 1000K – 1400K temperature range is as follows: N2 (65.3 − 59.3%); O2 (32.0 − 25.7%); NO (2.7 − 15.0%), (iii) the N2O decomposition obeys first-order kinetics, and the initial rate law for N2O decomposition is Rinit = 2k1[N2O] which rapidly changes to Rlate = k1[N2O] as the reaction proceeds, (iv) the branching ratio for the two reactions: N2O + O → 2NO and N2O + O → N2 + O2 lies between 0.1 and 0.5 (0.1 < α < 0.5) and varies with conditions, (v) the apparent activation energy for the decomposition of N2O is 2.5 eV/molecule (2.4×102 kJ/mole), (vi) the rate law for NO formation is R = k1N2O], and (vii) the apparent activation energy for the formation of NO is 2.4 eV/molecule (2.3×102 kJ/mole).

Nitrogen engineering of ultrathin oxynitrides by a thermal NO/O2/NO process

The paper discusses nitrogen engineering of ultrathin (<5 nm) oxynitride gate dielectrics. The dielectric film that we have aimed for has two nitrogen enhanced layers: one at the SiO2/polysilicon interface to retard boron diffusion from the gate, and a smaller one peaked at the Si/SiO2 interface to increase the hot electron degradation resistance. We were able to produce this dielectric by a thermal (NO/O2/NO) process, aided by an understanding of the kinetics and thermodynamics of nitrogen incorporation in SiO2.

Effect of near-interfacial nitrogen on the oxidation behavior of ultrathin silicon oxynitrides

Medium energy ion scattering has been used to study the role of nitrogen in the thermal oxidation kinetics of ultrathin silicon oxynitrides. Oxynitride films with different amounts of nitrogen near the SiOxNy/Si interface and pure (control) SiO2/Si films were reoxidized in dry 18O2 under equivalent conditions. The spatial distribution of 18O incorporated into the films was analyzed by high-resolution depth profiling methods. Analogous to the pure SiO2 case, we observed two distinct regions where oxygen incorporation into the oxynitride films occurs: at/near the interface and near the outer oxide surface. The (near) interface oxide growth reaction is found to be significantly retarded by the presence of near-interfacial nitrogen (with a higher degree of the retardation for higher concentrations of nitrogen). The presence of nitrogen near the interface does not affect the surface exchange reaction.

AN ISOTOPIC LABELING STUDY OF THE GROWTH OF THIN OXIDE FILMS ON SI(100)

The mechanism of thin (<8 nm) oxide growth on Si(100) has been studied by high‐resolution medium energy ion scattering in combination with oxygen isotope substitution in the T=800–900 °C and 0.1–1 Torr oxygen pressure regime. Isotopic labeling experiments demonstrate that the Deal–Grove model breaks down for these films. In addition to the traditional oxidation reaction at the Si/SiO2 interface, two other spatially specific reactions take place during thermal oxidation: an exchange reaction at the oxide surface and an oxidation reaction in the near‐interfacial region.

Surface structure of MgO(001): a medium energy ion scattering study

Abstract We have used medium-energy ion-scattering with channeling and blocking to study the surface structure of MgO(001). the ion scattering data show that UHV-cleaved crystals result in well ordered surfaces, while sputtered/ annealed samples show structural disorder. Surface blocking dips in both the O and Mg spectra are found to occur at angles very close to the corresponding bulk blocking directions, indicating very small surface relaxation and rumpling. A detailed R -factor analysis comparing the experimental spectra with results of Monte Carlo simulations for different structures gives a surface relaxation of −1.0% ± 1.0% and a rumpling of 0.5% ± 1.0%. These results are in good agreement with preliminary results from all-electron total-energy calculations and imply that the surface O 2− ions have small polarizations.

High-resolution depth profiling of ultrathin gate oxides using medium-energy ion scattering

Abstract Medium-energy ion scattering (MEIS) has been used to characterize the composition of ultrathin gate dielectrics. Examples covering investigations on silicon-oxides and oxynitrides as well as high-dielectric constant (high- K ) films on silicon substrates are discussed, with special emphasis on understanding film growth. In the MEIS spectra obtained from ultrathin films, the signals from different oxygen and nitrogen isotopes are well separated. By analyzing samples that have undergone different thermal processing steps in gases of different isotopes, both growth mechanisms and also atomic exchange effects can therefore be monitored directly. For various high- K dielectric films, we found that oxygen-isotope exchange is significant even at temperatures below 500°C. This may point to a serious limitation for the application of such materials as gate dielectrics in semiconductor devices.

An ion scattering study of the interaction of oxygen with Si(111): surface roughening and oxide growth

High-resolution medium energy ion scattering (MEIS) has been used to examine the initial oxidation of Si(111) at elevated temperatures (870–1170 K) and low oxygen pressures (5 × 10−8–10−5 Torr). These oxidation parameters cover three different parts of the (p,T) phase space: the oxide growth regime (passive oxidation), the surface etching regime (active oxidation), and a transition (“roughening”) regime. We show that in the passive oxidation regime, a non-stoichiometric oxide grows initially only in the surface layer. This stage is followed by “bulk” (3D) oxidation. Our results demonstrate that under certain conditions, the initial oxidation may be accompanied by roughening of the surface. During active oxidation, Si leaves the surface as SiO. Vacancies are formed stochastically, and the vacancies can form vacancy islands. At relatively low temperatures, the vacancy island formation occurs much faster than step flow can smooth the surface, leading to a roughened surface. Finally, our experiments also support the existence of a transition regime between “passive” and “active” oxidation. We obtain direct evidence for the presence of oxygen on the surface in this regime. In addition, both silicon and oxygen spectra show clear evidence of roughening. Our interpretation, consistent with that of others, is that the surface is partially covered by passive surface oxide islands with the remaining area free of oxygen. Continuous etching of the bare silicon areas results in the surface developing a vertical roughness as high as 20–30 A.