Heiji Watanabe

Interface engineering of a ZrO2/SiO2/Si layered structure by in situ reoxidation and its oxygen-pressure-dependent thermal stability

Reactions at ZrO2/SiO2/Si interfaces during fabrication and postannealing have been studied in detail. The layered structures were fabricated by deposition of a thin Zr layer on a chemical oxide, followed by oxidation in an UHV chamber without air exposure (i.e., in situ reoxidation). On-line x-ray photoelectron spectroscopy was used to show that in situ reoxidation can be used for precisely designing interfacial structures. It was found that the thermal stability of ZrO2/SiO2/Si interfaces crucially depends on oxygen ambient; that is, while the interfaces are stable up to 900 °C under UHV conditions, annealing in 1×10−4 Torr oxygen results in formation of interfacial Zr silicate. Moreover, the ZrO2 overlayer was found to accelerate the interfacial oxidation reaction.

Thermal decomposition of ZrO2/SiO2 bilayer on Si(001) caused by void nucleation and its lateral growth

The stability of a ZrO2/SiO2 bilayer on a Si(001) substrate was investigated in terms of thermal decomposition during ultrahigh-vacuum annealing. In spite of the intrinsic thermal stability of the ZrO2/SiO2 system, void nucleation initiated by local defects and the following lateral growth of the voids proceed at temperatures over 900 °C. The remaining Zr atoms accumulate and react with the Si substrate to form silicide (ZrSi2) islands within the voids. It was found that the decomposition temperature of the ZrO2/SiO2 bilayer is lower than that of a SiO2 single layer, which suggests defect generation in the SiO2 underlayer by ZrO2 deposition.

La–silicate gate dielectrics fabricated by solid phase reaction between La metal and SiO2 underlayers

La-based high-k gate dielectrics were fabricated by reoxidation of thin La layers deposited on SiO2 underlayers. Interface reaction that causes metal diffusion through the oxide underlayer increases permittivity of the oxide and forms high-quality La–silicate films. We successfully fabricated ultrathin La–silicates of equivalent oxide thickness ranging from 0.75 to 1.6 nm with low-leakage current by controlling the interface solid phase reaction. We characterized degradation in the silicate film caused by electrical stressing and demonstrated the effectiveness of high-temperature annealing to improve the reliability of silicate dielectrics. Moreover, it was found that water absorption during exposure to air causes positive fixed charge in the silicate (flatband voltage shift), but degradation can be annealed out at relatively low temperatures.

High-quality HfSixOy gate dielectrics fabricated by solid phase interface reaction between physical-vapor-deposited metal–Hf and SiO2 underlayer

We fabricated high-quality Hf–silicate (HfSixOy) gate dielectrics by utilizing the solid phase interface reaction between physical-vapor-deposited metal–Hf (typically 0.5nm thick) and SiO2 underlayers. Metal diffusion to the SiO2 layer increases the permittivity of the underlayer, while preservation of the initial SiO2∕Si bottom interface ensures good electrical properties of the gate dielectrics. The Hf–silicate layer remains amorphous and the poly-Si∕HfSixOy gate stack endures activation annealing at 1000°C. The interface trap density was comparable to that of conventional SiO2 dielectrics and the hysteresis of capacitance–voltage curves was as low as 4mV for a bias swing between −2 and +2.5V. Moreover, high electron mobility, equal to 89% of the universal mobility, was obtained for the high-k gate transistor.

Characterization of local dielectric breakdown in ultrathin SiO2 films using scanning tunneling microscopy and spectroscopy

Local dielectric breakdown of ultrathin SiO2 films grown on silicon substrates has been investigated by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). We found that STM observation can reveal individual quasibreakdown spots created by hot-electron injection into the oxide, as well as features of the topography such as atomic steps on the oxide surface. STS was used to study the local electrical properties of the oxide films before and after electrical stressing. We observed a leakage current at the quasibreakdown spots that passed through defect levels in the ultrathin oxide films. We also found that several tunneling spectra obtained from near leakage sites showed clear negative differential resistance. This phenomenon was attributed to the conductance change in the leakage path due to electron charging effects. Moreover, we confirmed the stressing polarity dependence of the leakage-site creation, and that atomic steps on the oxide and at the SiO2/Si interface did no...

Roughness at ZrO2/Si interfaces induced by accelerated oxidation due to the metal oxide overlayer

We investigated reactions at ZrO2/Si interfaces by means of interface oxidation and atomic-scale roughness caused by postoxidation annealing. When fabricating ZrO2 by in situ reoxidation of a thin Zr metal layer, the interface oxidation that results in Zr–silicate formation accelerates, when the initial Zr layer becomes thicker. This implies that interface oxidation is not dominated by oxidant diffusion through the high-k film, but is accelerated by catalytic effects that occurs within the metal oxide. We also found that, in contrast with a SiO2/Si interface, roughness at the high-k/Si interface monotonously increases with the growth of the interface silicate layers. These results demonstrate the importance of postannealing conditions on high-k gate dielectrics.

Reflection high-energy electron diffraction intensity oscillation during layer-by-layer oxidation of Si(001) surfaces

We have studied initial layer-by-layer oxidation of Si(001)-2×1 surfaces by using reflection high-energy electron diffraction (RHEED). We observed an intensity oscillation and a change in the streaky profile of a specular reflection spot in RHEED patterns during initial oxidation. These results indicate that layer-by-layer oxidation of Si surfaces is promoted by nucleation and lateral growth of two-dimensional oxide islands. We have also confirmed that a 1-monolayer-thick oxide has an ordered structure originating from the initial 2×1 reconstruction.

Ultrathin zirconium silicate gate dielectrics with compositional gradation formed by self-organized reactions

Zirconium silicate gate dielectrics with compositional gradation in depth were fabricated by in situ reoxidation of thin metal layers on oxidized Si surfaces. The silicate is composed of a trilayer structure, in which Si-rich layers are formed both at the top and the bottom of the film. The zirconium element is localized at the center of the silicate and, thus, the advantages of the silicate material can be obtained, while keeping sufficient permittivity. The compositionally graded silicates show promising electrical properties, such as a leakage current of less than 0.11 A/cm2 for an equivalent oxide thickness of 1.1 nm and an improved flatband voltage shift that is a result of postdeposition annealing and oxidation.

Electron‐beam‐assisted dry etching for GaAs using electron cyclotron resonance plasma electron source

Electron‐beam (EB)‐assisted dry etching of GaAs using Ar electron cyclotron resonance (ECR) plasma as an electron shower source is developed to achieve a low energy and high current density electron beam (EB). The rate of EB‐assisted dry etching is more than ten times larger than for Cl2 gas etching.It is confirmed, through photoluminescence measurement, that this etching method causes less damage than ion beam techniques and is very effective for damaged layer removal. Using this technique, a 0.4 μm linewidth low‐damage fine structure of GaAs was fabricated.

Scanning tunneling microscopy and spectroscopy characterization of ion-beam-induced dielectric degradation in ultrathin SiO2 films and its thermal recovery process

We have investigated dielectric degradation in ultrathin SiO2 films induced by ion-beam irradiation and its thermal recovery by using scanning tunneling microscopy (STM) and spectroscopy. Our STM results showed that although the ion-beam-induced damage uniformly spreads on the oxide, the dielectric properties are locally recovered by thermal annealing. We found that the tunneling spectra obtained from the ultrathin SiO2 films are sensitive to the process-induced damage, and observed both a leakage current through the damaged oxide film and the thermal recovery of the insulating features. In addition, we confirmed a local charging phenomena of the damaged oxide caused by electron traps around the STM tip.