Fluorine etching on the Si(111)-7×7 surfaces using fluorinated fullerene

Fluorine etching on the Si(1 1 1)-7×7 surfaces using fluorinated fullerene molecules as a fluorine source has been investigated. At room temperature, adsorbed fluorinated fullerene molecules reacted with the Si(1 1 1)-7×7 surface to create a localized distribution of fluorine on the surface. Nanoscale etch pits were created by annealing at 300 °C, due to the adsorption of the fluorine localized around the C₆₀F_x molecules. Annealing at 400 °C resulted in the delocalized fluorine distribution on the surface and healing of the etch pits, due to the enhancement of the diffusion of both the fluorine and silicon atoms. Subsequent annealing at 500 °C led to desorption of SiF₂ reactants formed on the surface. The fluorine diffusion process was found to be an elemental process in the etching because the diffusion of adsorbed fluorines is a key for the formation of the SiF₂ species and their subsequent desorption.     

Characteristics of silicon oxide thin films prepared by sol electrophoretic deposition method using tetraethylorthosilicate as the precursor

Silicon dioxide films were prepared on p-type Si (1 0 0) substrates by sol electrophoretic deposition (EPD) using tetraethylorthosilicate (TEOS) at low temperature. According to the variation of sol dipping conditions, we estimated the characteristics of SiO₂ films, such as composition, surface morphology, wet etch rate, breakdown voltage, etc. The growth rate of the film increased linearly with increasing TEOS quantity in solution. It increased exponentially with the increase in deposition time, and the film thickness was saturated at approximately 200 nm on hydrophilic Si surface after more than 6 days. The growth rate of the EPD SiO₂ films on the hydrophobic Si surface was much lower than that of the film on the hydrophilic Si surface.     

Dry etch chemistries for TiO2 thin films

Several different plasma chemistries were investigated for dry etching of TiO₂ thin films. Fluorine-based discharges produced the fastest etch rates (∼2000 Å min⁻¹) and selectivities >1 for Si over TiO₂. Chlorine-based discharges also showed a chemical enhancement over pure Ar sputtering and had selectivities <1 for Si over TiO₂ for a range of plasma conditions. Methane–hydrogen discharges produced very slow etch rates, below those obtained with Ar sputtering. The etched surface morphologies of TiO₂ were excellent in all three types of plasma chemistry. Small concentrations (2 at.%) of chlorine- or fluorine-containing residues were identified on the TiO₂ surface after Cl₂/Ar or SF₆/Ar etching, but these residues were water soluble.     

Scanning probe oxidation of Si3N4 masks for nanoscale lithography,micromachining, and selective epitaxial growth on silicon

For studying the physical, chemical, and electronic properties of ultrasmall man-made structures, the major challenge is to fabricate highly uniform structures and control their positions on the nanometer length scale. Local oxidation of metals and semiconductors using a conductive-probe atomic force microscope (AFM) or other scanning probe microscopes in air at room temperature has emerged as a simple and universal method for this purpose. Here the uses of scanning probe oxidation of Si₃N₄ masks for performing nanolithography, nanomachining, and nanoscale epitaxial growth on silicon are reviewed. The three most unique features of this approach are presented: (1) exceptionally fast oxidation kinetics using silicon nitride masks (∼30 μm/s at 10 V for a ∼5-nm-thick film); (2) selective-area anisotropic etching of Si using a Si₃N₄ etch mask; and (3) selective-area chemical vapor deposition of Si using a SiO₂/Si₃N₄ bilayer growth mask.     

Surface electromigration of In on vicinal Si(0 0 1)

Surface mass transport of In film on vicinal Si(0 0 1) has been systematically investigated by a scanning Auger electron microscopy (SAM), low energy electron diffraction (LEED) and atomic force microscopy (AFM). It was observed that the temperature dependence of the mass transport shows the critical phenomenon. Above a critical temperature T_c, surface electromigration of the In film toward the cathode side dominated the surface mass transport on the vicinal Si(0 0 1) surface. The LEED and AFM observations revealed that the In film surface on the vicinal Si(0 0 1) consists of 3×4 terraces and (3 1 0) facets. The area ratio of the facet to the terrace exhibited abrupt an increase at T_c. It is believed that the change of the mass transport is related to the abrupt change of the area ratio of the facet to the terrace. Both the critical temperature T_c and the spread due to the surface electromigration of the In film depended on the configuration of the DC current direction and the step edge.     

Adsorption of O2 and CO2 on the Si(111)-7 × 7 surfaces

The interaction of O₂ and CO₂ with the Si(111)-7 × 7 surface has been studied with X-ray photoelectron spectroscopy (XPS). It was found that both O₂ and CO₂ molecules can readily oxidize the Si(111)-7 × 7 surface to form thin oxide films. Two oxygen species were identified in the oxide film: oxygen atoms binding to on-top sites of adatom/rest atoms with an O 1s binding energy of ~ 533 eV as well as to bridge sites of adatom/rest atom backbonds at ~ 532 eV. These two oxygen species can be interconverted thermally during the annealing process. Due to the low oxidation capability, the silicon oxide film formed by CO₂ has a lower O/Si ratio than that of O₂.     

Growth and characterization of CoSi2 films on Si (1 0 0) substrates

In the present work, a special solid phase epitaxy method has been adapted for the preparation of CoSi₂ film. This method includes an epitaxial growth of Co films on Si (1 0 0) substrate, and in situ annealing of the Co/Si films in vacuum. It has been found that at the substrate temperature of 360°C, fcc cobalt film grows epitaxially on the Si (1 0 0) surface. The crystallographic orientation relations between fcc Co film and Si substrate determined from the electron diffraction result are: (0 0 1) Co//(0 0 1) Si, [1 0 0] Co//[1 1 0]Si. Upon annealing at temperatures range from 500 to 600°C, Co film reacts with Si substrate and transforms into CoSi₂. The CoSi₂ films prepared by this way are characterized by XTEM, XPS and AFM.     

Evidence for the interfacial reaction between Ni adatoms and H-Si(001) surface

Hydrogen termination of Si surfaces (H-Si) does not stop the interfacial reaction between Ni adatoms and H-Si(001) surface at room temperature. At low Ni coverage of 0.1% (equivalent to 0.02 ML), X-ray photoelectron spectroscopy (XPS) reveals a binding energy shift of Ni 2p_3/2 to 854.0 eV, which corresponds to the formation a NiSi-like environment. As the coverage of Ni increases, the Ni 2p_3/2 eventually shifts to 852.8 eV, indicative of formation of metallic Ni. XPS intensity vs Ni coverage analysis reveals a growth process akin to pseudo-layer-by-layer growth mode, thereby suggesting the formation of bulk-Si(001)/NiSi-like/Ni-rich-silicide-like/metallic Ni structure as growth proceeds. For Ni coverage not more that 33% (equivalent to 12.68 ML), Ni remains protected by the silicide environment and no oxidation of Ni to form Ni-oxides was observed even after exposing the samples to air for 400 days. For samples with Ni coverage above 41%, oxidation of Ni is observed by presence of NiO and NiSiO₃ peaks at 854.5 and 857.0 eV, respectively. The current studies suggest that there is reaction between Ni adatoms and Si at the growth front on H-Si(001) surfaces upon Ni deposition at room temperature and hydrogen termination does not suppress the interface reaction between Ni and Si.     

Effects of ultrasonic agitation and surfactant additive on surface roughness of Si (1 1 1) crystal plane in alkaline KOH solution

In the silicon wet etching process, the “pseudo-mask” formed by the hydrogen bubbles generated during the etching process is the reason causing high surface roughness and poor surface quality. Based upon the ultrasonic mechanical effect and wettability enhanced by isopropyl alcohol (IPA), ultrasonic agitation and IPA were used to improve surface quality of Si (1 1 1) crystal plane during silicon wet etching process. The surface roughness R_q is smaller than 15 nm when using ultrasonic agitation and R_q is smaller than 7 nm when using IPA. When the range of IPA concentration (mass fraction, wt%) is 5–20%, the ultrasonic frequency is 100 kHz and the ultrasound intensity is 30–50 W/L, the surface roughness R_q is smaller than 2 nm when combining ultrasonic agitation and IPA. The surface roughness R_q is equal to 1 nm when the mass fraction of IPA, ultrasound intensity and the ultrasonic frequency is 20%, 50 W and 100 kHz respectively. The experimental results indicated that the combination of ultrasonic agitation and IPA could obtain a lower surface roughness of Si (1 1 1) crystal plane in silicon wet etching process.     

Bi-assisted chemical etching of silicon in HF/Co(NO3)2 solution

The morphology and the photoluminescence (PL) of Bi-assisted electroless etched p-type silicon in HF–Co(NO₃)₂–H₂O solution as a function of etching time were studied. The scanning electron microscopy (SEM) observations have shown that the morphology of etched layers strongly depends on the etching time and it was observed that macropores filled with silicon crystallites are formed for etching time higher than 50 min. Moreover, it was found that the PL spectra show a red emission with a peak centred at 640 nm. The PL peak intensity reaches a maximum for etching time of 50 min, and then it decreases with increasing etching time. The Fourier transform infrared (FTIR) measurements have shown a strong increase in intensities of the relevant Si–H and in the amount of oxide (absorption band at 1070 cm^?1) for long etching time which was ascribed to an increase in the number of Si crystallites formed in the macropores.     

Nonselective vertical etching of GaAs and AlGaAs/GaAs in high pressure capacitively coupled BCl3/N2 plasmas

We studied nonselective, vertical dry etching of GaAs and AlGaAs/GaAs structure in high pressure capacitively coupled BCl₃/N₂ plasmas. The operating pressure was fixed at 150 m Torr. We found that there was an optimized process condition for nonselective and vertical etching of GaAs and AlGaAs/GaAs at the relatively high pressure. It was noted that there was a range of % N₂ (i.e. 20–40%) where nonselective etching of GaAs over AlGaAs could be achieved in the BCl₃/N₂ mixed plasma. We also found that dry etching of GaAs and AlGaAs/GaAs structure provided quite vertical and smooth surface when % N₂ was in the range of 0–20% in the BCl₃/N₂ plasma. The maximum etch rates for GaAs (0.41 μm/min) and AlGaAs/GaAs structure (0.42 μm/min) were obtained with 20–30% N₂ composition in the plasma.     

Ferrofluidic masking of solid state nuclear track detectors during etching

This work presents a novel method for determining bulk etch rate of CR-39 during prolonged etching by masking the surface with a ferrofluidic film held in position by magnetostatic forces. The CR-39 etching conditions were 6.25 M NaOH solution for 24 h at temperatures ranging from 50 to 80 °C. After etching, the heights of the resulting un-etched plateaus were measured using a Talyscan 150 profilometer. The removed layer thicknesses ranged from 12 to 85 μm, giving corresponding bulk etch rates in the range 0.5–3.54 μm/h.     

Epitaxial growth of well-ordered ultra-thin Al2O3 film on NiAl (1 1 0) by a single-step oxidation

Well-ordered ultra-thin Al₂O₃ films were grown on NiAl (1 1 0) surface by exposing the sample at various oxygen absorption temperatures ranging from 570 to 1100 K at dose rates 6.6 × 10⁻⁵ and 6.6 × 10⁻⁶ Pa. From the results of low-energy electron diffraction (LEED), Auger electron spectrometer (AES) and X-ray photon spectroscopy (XPS) observations, it was revealed that oxidation mechanism above 770 K is different from well-known two-step process. At high temperature, oxidation and crystallization occurred simultaneously while in two-step process oxidation and crystallization occurred one after another. At high-temperature oxidation well-ordered crystalline oxide can be formed by a single-step without annealing. Well-ordered Al₂O₃ layer with thickness over 1 nm was obtained in oxygen absorption temperature 1070 K and a dose rate 6.6 × 10⁻⁶ Pa at 1200 L oxygen.     

Electric field effect on low temperature nanoscale oxidation

The influence of electric fields on the low temperature oxidation of individual nanoscale tungsten wires was investigated. In the experiments at room temperature, the nanowires were biased as anode opposite to a macroscopic cathode and H₂O-vapor with a pressure of 10^?7–10¹ mbar was provided as oxygen source. Under the influence of an electric field, a dramatic change of the oxidation behavior is observed with the formation of several 10 nm thick oxide layers for electric fields exceeding a threshold. The chemical composition of the layers formed is determined with laser-assisted atom probe tomography to be slightly understoichiometric WO₃. After an initial period of fast growth, the oxidation rate later rapidly decreases to immeasurable low values. Evaluation of the electric field distribution in the vicinity of the sample by the finite element method reveals that oxide formation only proceeds if a critical field in the range of 0.7–5.0 V/nm, depending on the H₂O-pressure, is present. This critical field is attributed to a field-activated reaction of H₂O at the oxide–vapor interface. Besides for tungsten, field-induced oxidation is also observed for aluminum and p-doped silicon and thus apparently is a widely material independent phenomenon.     

Wet and dry etching of Sc2O3

Wet chemical and plasma etch processes were developed for pattering of Sc₂O₃ films on GaN. Chlorine-based plasma chemistries produced a significant chemical enhancement of removal rate over pure Ar sputtering. The etching was anisotropic and did not significantly alter the surface composition of the Sc₂O₃ films. Reaction-limited wet etching in the HNO₃/HCl/HF system was investigated as a function of solution formulation and temperature. The activation energy for the wet etching ranged from 8 to 14 kcal/mol and the etch rates were independent of solution agitation.     

Mixed conductivity and oxygen permeability of doped Pr2NiO4-based oxide

Pr₂NiO₄-based oxide was studied as a new mixed electronic and oxide ionic conductor for the oxygen permeation membrane. It was found that Pr₂NiO₄ doped with Cu and Fe for Ni site exhibits the relatively high oxygen permeation rate. Doping second cation to Ni site is effective for improving the oxygen permeation rate and the trivalent cation seems to be effective for increasing the oxygen permeation rate. Among the examined cation, the highest oxygen permeation rate was obtained by doping 5 mol% Fe. The oxygen permeation rate was also significantly affected by the surface catalyst and the highest oxygen permeation rate of 80 μmol·min^− 1·cm^− 2 at 1273 K was achieved by using La_0.1Sr_0.9Co_0.9Fe_0.1O₃ for the surface catalyst. Since the electrical conductivity slightly decreased with decreasing P_O2 and it dropped significantly at P_O2 = 10^− 19 atm, chemical stability of Pr₂NiO₄-based oxide seems to be reasonably high. Application of this new mixed conductor for the oxygen permeation membrane under the CH₄ partial oxidation was also studied and it was confirmed that the oxygen permeation rate much improved under the CH₄ oxidation condition and this Pr₂NiO₄ can be used for the oxygen permeation membrane for the CH₄ partial oxidation.     

Interface strain profiling in ultrathin SiO2 gate oxides with medium energy ion scattering spectroscopy

Medium energy ion scattering spectroscopy (MEIS) could identify ∼1 nm interface layer with compressive strain, which depends sensitively on the interface treatment conditions such as oxynitridation, ozone oxidation, tilt of Si(0 0 1) substrates. The interface strain relaxation always shows improvements in gate oxide reliability. Atomic scale investigations of strain profiles with MEIS are reviewed for SiO₂/Si(0 0 1) interfaces.     

The interaction of O2 with the surface of polycrystalline gadolinium at the temperature range 300–670 K

Auger-Electron-Spectroscopy (AES) and Direct-Recoils-Spectrometry (DRS) were applied to study the interaction of O₂ with a polycrystalline gadolinium surface, in the temperature range 300–670 K and oxygen pressure up to 2 × 10^? 6 Torr. It has been found that initial uptake of oxygen, at coverage measurable by the techniques used here, results in rapid oxide island formation. The subsurface is believed to be a mixture of oxide particles and oxygen dissolved in the Gd metal, the latter being the mobile species, even at relatively low temperatures.Enhanced inward diffusion of oxygen starts as early as 420 K and dictates the surface oxygen concentration and effective thickness of the forming oxide. The oxygen accumulation rate at the near-surface region, as measured by the O(KLL) AES signal intensity, goes through a maximum as a function of temperature at 420 K. This is a result of the combination of still efficient oxygen chemisorption that increases surface occupation and slow inward diffusion. The thickest oxide, ~ 1.7 nm, is formed at 300 K and its effective thickness was found to decrease with increasing temperature (due to oxygen dissolution into the metal bulk).Diffusion coefficients of the oxygen dissolution into the bulk were evaluated for various temperatures utilizing models for infinitely thin oxide layer and thick oxide layer, respectively. The best fit under our experimental procedure was obtained by the thick layer model, and the coefficients that were calculated are D₀ = 2.2 × 10^? 16m²s^? 1 and E_a = 46kJ/mol.     

Soot low-temperature combustion on Cu–Zr/ZSM-5 catalysts in O2/He and NO/O2/He atmospheres

Zirconium doped Cu/ZSM-5 catalysts were prepared and characterized in this investigation. Catalytic activity during soot combustion was determined in both O₂/He and NO/O₂/He atmospheres by temperature-programmed oxidation. The use of zirconium reduces the temperature of maximum soot oxidation rate by 229 °C in O₂/He atmosphere and 270 °C in NO/O₂/He atmosphere. The promoting effect of zirconium is discussed in terms of surface dispersion, enrichment of active components, and creation of oxygen vacancies where molecular oxygen or NO_x is adsorbed forming basic surface oxygen species active for soot oxidation. The NO₂ formed at the copper–zirconium interface sites leads to the ignition temperature being significantly decreased to 93 °C, which is inside the exhaust temperature range of diesel engines. To understand the combustion reaction kinetics, the activation energy and reaction order of soot combustion were evaluated. According to the Redhead method, the activation energy for non-catalyzed reaction is 164 kJ/mol under the O₂/He atmosphere. For the Cu/ZSM-5 and Cu–Zr/ZSM-5, the activation energies under the O₂/He atmosphere (134–151 kJ/mol) are slightly higher than those under the NO/O₂/He atmosphere (128–135 kJ/mol). The Freeman–Carroll method is suitable to describe the soot combustion in the NO/O₂/He atmosphere, with the activation energies for the catalysts in the range of 97–112 kJ/mol and the average value of reaction order equal to 1.36.