Dilute HF Solution Research Articles

SiO2/Ta2O5 Core–Shell Nanowires and Nanotubes

Tantalizing silica: SiO2/Ta2O5 core–shell nanostructures were formed by reducing SiO2 nanowires under a Ta atmosphere at 950 °C. The diameter of the SiO2 nanowires inside the core–shell nanostructures could be manipulated by modifying the reduction time. Treatment of the core–shell structures in dilute HF solution yielded Ta2O5 nanotubes (see TEM image), whereas Ta2O5 nanowires were formed directly by increasing the reduction time. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2006/z602228_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Cathodic electrochemiluminescence of metal ions in acid fluoride solutions

The conditions for exciting the cathodic electrochemiluminescence (ECL) of mercury-like ions and manganese in dilute HF solutions at aluminum and magnesium electrodes are described. ECL spectra are measured. It is shown that Mn(II) can be determined with a detection limit of 10−7 M.

Depth Profiling of Chemical and Electronic Structures and Defects of Ultrathin HfSiON on Si(100)

We present in-depth profiling of chemical bonding features and defect state density in ultrathin HfSiOxNy (Hf/(Hf+Si)=~43%) films with average nitrogen contents up to ~18at.% by using x- ray photoelectron spectroscopy (XPS) and total photoelectron yield spectroscopy (PYS) in combination oxide thinning in a dilute HF solution. It is found that the oxygen content becomes its minimum around ~1.2nm from the surface while the nitrogen content becomes its maximum within ~1.5nm from the surface. The result suggests that the surface re-oxidation is promoted coincidentally with the diffusion of N atoms generated by thermal decomposition of the Hf-Nx units during the N2-annealing. The depth profile of the defect states, which was derived from the change in the yield, shows that the defect state density becomes its maximum in the near-surface region where oxygen deficiency becomes significant. It is likely that the imbalance in chemical coordination between anions and cations is responsible for the defect generation.

Self-aligned multi-level air gap integration

Dual damascene self-aligned air gap structures have been fabricated through selective removal of interline plasma-damaged SiOC material using dilute HF solutions after metal CMP. The extent of the gaps was shown to be tuneable. The creation of interline air gaps through removal of damaged dielectric yielded significant capacitance reduction plus in addition dielectric reliability improvement. The via-reliability of 2 metal-build air gap structures was tested by thermal cycling and constant thermal stress. No significant difference in via reliability was observed between SiOC interconnects with and without air gaps. However, failure analysis showed weak spots near the bottom of the barrier, which could be detrimental for dual damascene reliability. These weak spots at the barrier bottom could lead to catastrophic failures in both via and lines during electromigration stressing. Moreover, process-related issues such as bottom liner undercut and copper corrosion need to be controlled more stringent before this air gap approach can be successfully implemented.

Photoluminescence properties of porous silicon layers prepared by electrochemical etching in extremely dilute HF solutions

Dilute HF solutions with concentrations down to 0.03% have been used to obtain luminescent porous silicon (PSi) layers on p-type Si wafers. The experimental results show that with a constant etching time of 30 min, PSi layers with sufficient luminescence efficiencies can be formed for HF concentrations as low as 0.1%. Because of a significantly lowered critical current density, only very low etching current densities of ≤0.1 mA cm−2 can result in the formation of luminescent PSi samples in 0.1% HF solutions. A notable result is that these low etching current densities cannot be used to form luminescent PSi layers in concentrated ( ≥1%) HF solutions. The behavior of PL intensity as a function of etching current density has been analyzed over a wide range of HF concentration. The PL intensity is determined by the ratio of the etching current density to the critical current density, suggesting that the presence of silicon oxides plays an important role in the formation of luminescent Si nanostructures in PSi layers.

Bonding of soda-lime glass microchips at low temperature

A simple, low-temperature bonding process is described for the fabrication of soda-lime glass microfluidic chips. Due to its chemical inertness and temperature stability, glass remains a popular material for microfluidic chips despite the advances that have been made with polymer materials. Conventional thermal bonding is performed over the course of 24 h at 600°C and requires a precise temperature-controlled furnace. Here we introduce a simple low-temperature alternative for the high-strength bonding of soda-lime glass wafers based on the use of diluted HF solution in combination with pressure at a temperature of 65°C.

Synthesis of Au nanotubes with SiOx nanowires as sacrificial templates

Gold nanotubes with SiOx nanowires as sacrificial templates have been synthesized. SiOx nanowires were functionalized by 3-aminopropyl trimethoxysilane that generates a charged surface. The attachment of negatively charged Au nanoparticles was followed. The coverage of Au nanoparticles was initially less than 30%. Further coverage was achieved by the reduction of gold hydroxide to grow the continuous nanoshell on Au nanoparticles, which serve as nucleation sites. The final coverage of Au nanoshells on SiOx nanowires depends strongly on the relative amount of SiOx nanowires in gold hydroxide solution. Both transmission electron microscope and scanning electron microscope images revealed the formation of Au nanotubes with the removal of SiOx nanowires by etching in a dilute HF solution.

Chemical Structure of Nitrogen Atoms in Thin-Film Nitrided Silicon Dioxide Formed on Silicon Substrate after Hydrogenation Reaction in Hydrofluoric Acid

Hydrogenated structures on a nitrided silicon dioxide SiO(N) film, which had been thermally grown on Si(100) wafers in a NO atmosphere to contain 4 atomic percent of N atoms, were detected after the film was etched slightly in a dilute HF solution. The N atoms on the surface of the etched SiO(N) film were observed as N–H monohydride structures by infrared spectroscopy. The depth profile and chemical structure of the N atoms were deduced on the basis of the infrared absorption intensity and the frequency of N–H stretching vibrations. In the depth profile, the concentration of N atoms had a maximum, which was distant from the interface with the Si crystal and was within the film. The frequencies of the N–H stretching vibrations were from 3380–3390 cm-1 in the film and they shifted down to 3300 cm-1 at the interface. In the film, the N atoms were not directly bound to the O atoms, but instead were found to be connected to the O atoms through the Si atoms. At the interface, the secondary-bonded atoms of the N atoms changed from O atoms to Si atoms.

Atomic Force Microscopic Nanolithography on Hafnium Oxide Thin Film Grown on Si(100)

As a high-dielectric-constant material, hafnium oxide is one of the promising gate oxides to replace SiO2 for future applications in microelectronics. By atomic force microscopy (AFM), we investigate the AFM-induced anodic oxidation on a hafnium oxide surface and compare the wet etching property of this local oxide with the as-deposited 3 nm HfO2 film in dilute HF solution. Oxide patterns can be reproducibly fabricated and their protruded feature mainly results from the local oxidation of the Si(100) substrate. The AFM-induced oxide pattern has a higher etch rate than the as-deposited HfO2 film due to a greater amount of oxygen.

Surface Preparation Techniques for the Atomic Layer Deposition of Hafnium Oxide

AbstractAtomic layer deposition (ALD) has been successfully used over the years for the deposition of conformal dielectric layers with precise thickness down to the nanometer scale. Therefore, optimization of the growth behaviour of the dielectric is mandatory. Since ALD is a surface sensitive growth technique, determined by the amount of available reaction sites at the starting surface, the impact of various wet chemical and thermal Si treatments on the HfO2 growth was evaluated. Thin SiO2 starting layers, based on wet chemical processing, were prepared by using a diluted peroxide mixture and an ozone/DI-water treatment. The thermally grown oxides were gradually etched by slowly immersing the oxidized Si substrate into a diluted HF solution, creating a thickness range on a single wafer. Our results demonstrate that the HfO2 deposition is more dependent on the thickness of the SiO2 layer than on the chemistry used to grow the oxide layer. For all studied oxides, two regions can clearly be distinguished. First, a linear relationship between the oxide thickness and the amount of deposited HfO2 is seen. Because chemical oxides tend to grow in islands, this trend can be explained by an increase in density of surface OH groups when the oxide grows thicker. When an ellipsometric thickness of ~0.8 nm is reached, saturation of the HfO2 growth is obtained. We believe that, from this thickness on, the starting surface is completely covered with hydroxyl groups, leaving the HfO2 growth only dependent on the ALD process itself. Since both the wet chemical as the thermal oxides are showing the same trend in HfO2 deposition, it can be stated that surface preparations can be selected solely based on their ease of processing. However, the electrical results show that there may be a difference between the ozone based wet chemical oxides and the etched thermal oxides, since the latter seems to possess slightly more leakage current. The electrical results will be discussed in more detail during the presentation.

Effects of N2, O2, and Ar plasma treatments on the removal of crystallized HfO2 film

The effects of plasma treatment using Ar, N2, and O2 on the removal of crystallized HfO2 films in a dilute HF solution were studied. The resulting damage in source and drain regions, and recess in isolation regions were also investigated. It was found that plasma nitridation with an ion energy of several hundred electron volts can lower the wet etch resistance of crystallized HfO2 films up to 70Å thick through the generation of Hf–N bonds. However, thermal nitridation did not introduce sufficient nitrogen into bulk crystallized HfO2 films to lower wet etch resistance. Plasma nitridation without bias power introduced nitrogen to the crystallized HfO2 in the region only within 10Å of the surface. The enhancement of the etch rate of crystallized HfO2 in dilute HF and the amount of recess in the active and isolation regions using N2, O2, and Ar plasma treatment have been evaluated. Results show that N2 plasma treatment is the most effective in enhancing the removal rate of crystallized HfO2 in dilute HF and minimizing recess on substrate among the plasmas studied.

Origin of the Anisotropy in the Anodic Dissolution of Silicon

We report on an in situ, real-time, electrochemical, polarized, attenuated total reflection Fourier transform infrared spectroscopic investigation of Si(100), (111), and (110) surfaces in diluted HF solution (, pH 3). Prior to the IR measurements, cyclic voltammetry was carried out on the samples in a standard three-electrode arrangement, then IR spectra were recorded as a function of potential using 50-mV potential increments. The results give evidence for potential-induced absorption changes of the chemical surface species (Si–O, Si–H, Si–OH), which depend somewhat on substrate orientation. The potential-induced changes regard the shape and magnitude of all the above vibration bands and include a shift for some vibrations. In addition, the SiOH contribution appears about twice as large on (100) as on (110) and (111). The SiOH species, present just above the onset of anodic current flowing, at potentials around 0 V, appear as a key factor in the anisotropic dissolution of silicon. These observations are accounted for semiquantitatively in the framework of a kinetic model. The main factor responsible for the anisotropic effects appears to be the ability of surface silanols to couple and form siloxane bridges, a reaction governed by the surface concentration of available sites and the geometry of their neighborhood.

Chemical procedure applied for the identification of Rf/Db produced in the 48Ca +243Am reaction

Summary A chemical separation procedure for Rf/Db is described which was applied to a long-lived decay product from the nuclear fusion reaction 48Ca+ 243Am. A 1.2 mg thick 243Am target was bombarded by 247 MeV 48Ca particles. The recoiling products were collected in a thick Cu catcher for about one day and then subjected to a chemical separation procedure that included an ion exchange from dilute HF solutions. Final samples were prepared on 30 μg/cm2 thick polyethylene (PE) foils and counted in 4π-geometry for α-particles and spontaneous fission (SF) coincidences. The detector arrays were surrounded by 3He detectors to also assay prompt neutrons. Decontamination factors from actinides of about 105 were achieved. Group 6 (W) to 14 (Pb) elements as models for their heavier homologues were shown to be separated from the Rf/Db fraction with more than 90%. In eight final samples, representing a total beam dose of 3.4 × 1018 particles, 15 SF events were detected. The decay pattern points to a single component with a half-life of ≈32h, which shows a chemical behavior similar to the lighter homologues of group 4 and 5 elements.

Surface Preparation for Transistor Performance Improvement in Triple Gate Oxide Integration

Fabrication of current high-performance metal-oxide-semiconductor field effect transistors (MOSFETs) requires a multiple gate oxide integration to obtain different oxide thicknesses and applied voltages. However, channel mobility and drive current are degraded as the number of oxide growth and etching steps increases during the multiple gate oxide integration. In multiple gate oxide integration, a shorter overetch time with a more dilute HF solution for the removal of preexisting oxides exhibits improved surface mobility and transistor drive current resulting from the suppression of surface roughness deterioration. The elimination of SC1 cleaning or the addition of a lower-temperature dilute SC1 cleaning in the pregate cleaning sequence also produces improved transistor mobility. Considering that the addition of SC1 cleaning in the pregate cleaning sequence is needed to remove particles on the surface, the use of a dilute SC1 cleaning may be a realistic method to achieve both improvement of transistor performance and removal of particles. Mobility and drive current of are found to be more sensitive to surface preparation changes than , which may be explained by the closer proximity of electrons to the interface compared to holes.

Bias voltage dependent electrochemical impedance spectroscopy of p- and n-type silicon substrates

For a comprehensive interpretation of the electrochemical processes occurring at a semiconductor/electrolyte interface, an accurate value of the flatband potential versus the reference electrode is needed together with the current/voltage response. The present work is mainly devoted to the analysis of impedance diagrams recorded with n- and p-type silicon electrodes in the dark, in a pure diluted HF solution under different bias values, the potential range being chosen so as the dc current is maintained at a low value, e.g. less than 5 μA cm −2, and even less than 10 nA cm −2 in the case of p-type Si under cathodic polarization. Two series of impedance diagrams were recorded. Firstly, the bias value was settled in the potential range where the depletion layer was generated within the semiconductor substrate. In this condition, the high resistance of the space charge region and the resulting low value of the dc current permitted to obtain well-defined semicircles leading to the derivation of both R and C values equivalent to the depletion layer charge distribution. The method based on the analysis of the voltage dependent R-value was compared to the usual Mott–Schottky plot treatment, and proved to be efficient for the determination of a reliable value of the flatband potential versus the reference electrode. In the second series, the analysis of the impedance diagrams was focused on the range of potentials corresponding to the onset of an accumulation layer within the semiconductor. In the case of p-Si, this condition corresponds to the rapid anodic etching of the silicon substrate. At a particular value of the bias, an important induction loop was observed in addition to the usual capacitive behaviour. A Fourier transform treatment demonstrated that the impedance components were interrelated to the time dependent current response under constant bias potential. Then, simultaneous experiments based on chronoamperometry measurements suggested that the electrochemical processes involved in the reaction of Si substrate with HF solution was correlated to a two-step charge transfer mechanism. This interpretation is valid for both the inductive loop obtained in electrochemical impedance spectroscopy and the rise of current versus time observed in chronoamperometry.

Electrical profiles of ultrashallow p+ layers formed in Si by low-energy BF2+ ion implantation

Carrier-concentration and mobility profiles formed in Si by BF2 implantation and by subsequent rapid thermal annealing (RTA) have been examined by differential Hall-effect measurements. B-concentration profiles are measured before and after RTA by a secondary-ion-mass spectroscopic technique. It is shown that a significant amount of B atoms, over 40% of an effective implant dose, is lost during RTA if the substrate is dipped in a dilute HF solution prior to annealing. The loss of B atoms gives rise to the increase in sheet resistivity of the formed p-type layers. It is revealed that an approximately 1.3–1.5-nm-thick, native oxide layer acts as an encapsulant and depresses the loss of implanted B atoms from the substrate during RTA at/above 950 °C, resulting in the formation of an electrical profile with a long tail. The redistribution of B atoms during RTA is suppressed in the sample from which the native oxide has been stripped. Also, an approximately 25-nm-thick, p+ layer with a maximum carrier concentration of 1×1020∕cm3 is formed by 3-keV BF2 implantation and subsequent RTA at 1000 °C. Experimental data indicate that the HF dipping effect is negligibly small if BF2 implantation is carried out above 40 keV.

Wet-Chemical Treatment of Si3 N 4 Surfaces Studied Using Infrared Attenuated Total Reflection Spectroscopy

Infrared attenuated total reflection spectroscopy has been used to observe the surface chemistry of films (grown on Si by low-pressure chemical vapor deposition) under steady-state conditions during exposure to dilute aqueous HF solutions. Surfaces etched in HF do not exhibit rapid growth of an layer when subsequently exposed to either humid room air at room temperature or to liquid However, some evidence is found for the formation of an ultrathin oxide-like layer when the etched surface is rinsed in deionized Removal of this layer in HF results in no detectable signal in the Si-H stretching region, unlike the case for Si subjected to a similar oxide-removal treatment. The presence of SiOH groups has been detected by observing their removal by reaction with aqueous acetic acid or HCl solutions, which suggests that groups are rapidly hydrolyzed. An attempt was made to detect surface groups by protonation in aqueous acid to form which should be more readily observable in the infrared spectrum. However, the surface coverage of if any, is too small to be detected by this means. © 2005 The Electrochemical Society. All rights reserved.

Synthesis of Cu Nanotubes with Silicon Oxide Nanowire Templates by MOCVD

Cu nanotubes have been synthesized using silicon oxide nanowires as templates. The silicon oxide nanowires were then coated with a thin, uniform copper layer by metallorganic chemical vapor deposition (MOCVD). Hollow Cu nanotubes were then produced by etching away the inner silicon oxide templates with dilute HF solution. The synthesis process was simple and of low cost. A large quantity of the Cu nanotubes can be readily grown.

Impact of Rapid Thermal O2 Anneal on Dielectric Stack Structures of Hafnium Aluminate and Silicon Dioxide Formed on Si(100)

3–7-nm-thick HfAlOx (Hf/Hf+Al=29%) films were deposited on thermally grown ∼0.9-nm-thick SiO2/Si(100) by atomic layer chemical vapor deposition (ALCVD) at 350°C. After postdeposition anneal (PDA) in 0.2% O2 ambience at 1050°C for 1 s, we evaluated the compositional profile of HfAlOx/SiO2 dielectric stack structures by X-ray photoelectron spectroscopy (XPS) in combination with oxide thinning in a dilute HF solution and observed Si diffusion and incorporation into the HfAlOx network by a few at.% through the pregrown SiO2 layer. The energy band alignment in the dielectric stack structure on Si(100) after the PDA was determined by the energy band gap of the ALCVD HfAlOx films and the valence band offset at the interfaces between HfAlOx and Si(100), which were measured by analyzing the energy loss spectra of O1s photoelectrons and the valence band spectra of the dielectric stack structures, respectively. Also, by the total photoelectron yield spectroscopy (PYS) of the dielectric stack structures, filled defect states as many as 1–3×1017 cm-3 eV-1 were estimated in the HfAlOx layer in the energy region of 0.4 to 0.8 eV from the energy position corresponding to the Si valence band top.

Analysis of the aperture formation mechanism in the fabrication process of nano-aperture arrays

The fabrication of nano-aperture arrays has attracted much interest for its potential application in near-field optical recording, one of the most probable candidates for high-density storage devices. A parallel-processing technique based on nano-aperture arrays has been investigated in order to achieve fast scan rate. In this work, the fabrication process of nano-aperture arrays and the analysis on the aperture opening mechanism will be presented. A series of oxide pyramid arrays have been fabricated by generating inverted pyramid array followed by stress-dependent oxidation on inner surfaces and backside Si etching. Nano-apertures have been opened at the apexes of pyramids by diluted HF solution. The relation between aperture diameters and etch time showed good linearity with proper aperture opening rate. The variation of opening rate upon HF concentration was examined and measured to be ∼23 nm/min for 50:1 HF solution. Investigations on the parameters determining aperture-opening rate will be discussed.