Removal Of SiO2 Research Articles

A novel etching and reconstruction route to ultrathin porous TiO2 hollow spheres for enhanced photocatalytic hydrogen evolution

Decreasing the shell thickness of TiO2 hollow spheres is a promising route to achieve an overall enhanced photocatalytic performance. However, in traditional synthetic approaches, the thin shell can not guarantee the structural stability of the TiO2 hollow spheres in the following calcination process to achieve a high degree of crystallinity. In this paper, ultrathin porous TiO2 hollow spheres with good structural stability, high degree of crystallinity as well as in small grain size were prepared through a novel etching and reconstruction route using amorphous SiO2/TiO2 hollow spheres as precursor. With hot water as etchant, the removal of SiO2 leads to the reconstruction of TiO2 hollow spheres and receiving a thinner and thinner shell. Such thin shell enables the exposure of more catalytic active sites of TiO2 nanocrystals and the improved photogenerated charge transfer and separation. Meanwhile, the residual SiO2 on the TiO2 shell protects the TiO2 nanocrystals from growing larger during high-temperature calcinations. The as-obtained ultrathin porous TiO2 hollow spheres exhibited enhanced photocatalytic activity in hydrogen production from water in comparison with those with thicker shell.

Selective Removal of Native SiO2 Using XeF2

The selective removal of native SiO2 with respect to Si and thermal SiO2 using XeF2 was studied with ellipsometry, temperature-programmed desorption (TPD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). The native SiO2 layer was removed with a static process at a pressure of 1 Torr and room temperature, but the Si substrate was roughened on a micron scale. To avoid roughening, a continuous flow of XeF2 was used at pressures lower than 10 mTorr. XPS analysis confirmed that the XeF2 exposure fluorinated the native oxide and both the oxide and fluorine were removed by heating to 925 K (650 °C). AFM showed no change in surface roughness after the low pressure exposure and heating, and together with XPS peak area ratios before and after XeF2 show that the native oxide was etched without etching the underlying Si substrate.

Selective Removal of Native SiO2 Using XeF2

Selective removal of native silicon dioxide (NOx) relative to the underlying Si substrate and to other films present in a device structure is important in forming low resistance electrical contacts. Xenon difluoride (XeF2) was chosen as the F-source because of the weak molecular bonds and the absence of precursors that could promote the uncontrolled formation of water on the surface. Etching of SiO2 with XeF2 has been shown to require the etching byproducts of elemental Si in close proximity.1 Both NOx and thermal SiO2 (TOx) films were etched with XeF2, titanium tetrachloride (TiCl4), and water vapor at different temperatures, pressures and process sequences. Samples were analyzed in situ with x-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). The O 1s XPS spectrum in Fig. 1 after static XeF2 exposure at room temperature and 1 Torr showed a decrease in the O coverage to near the detection limit. The surface was visibly roughened by this process because the Si substrate was etched spontaneously by F atoms produced by the reaction of XeF2. Flowing XeF2 at a pressure below 10 mTorr, NOx was fluorinated without removing it and without roughening the underlying substrate. The fluorinated NOx was desorbed by heating above 677 °C. The TPD spectra in Fig. 2 show that F disrupted the oxide causing it to desorb beginning at about 190 °C. Si desorbed at this temperature likely as SiF4 based on the SiF3 daughter species observed. Control experiments showed that the onset of native oxide desorption was about 770 °C without XeF2. Atomic force microscopy (AFM) images showed that the surface roughness was unchanged by low pressure XeF2 and heating to high temperature. In order to understand the reaction kinetics and reduce the high temperature anneal required for NOx removal, XeF2 was dosed at 120 °C and TiCl4 was used as a co-reactant because of the facile reaction that it undergoes with silanol groups. Three treatments of XeF2 at 120 °C removed all oxide as shown by the O 1s XPS spectra in Fig. 3. More controlled NOx removal was achieved via the introduction of TiCl4 prior to XeF2. The Ti 2p XPS spectra in Fig. 4 shows that TiO2 was formed after TiCl4 pulses at 250 °C, and the TiO2 was removed by XeF2 under continuous flow conditions. In addition, peak area ratios between O 1s and Si 2p states decreased by 20% after XeF2 exposure. Based on an initial thickness measured via ellipsometry of 15.8±0.2 Å, the amount of NOx etched was about 3 Å or approximately one monolayer. The surface was fluorinated after XeF2 exposure and unreactive to further TiCl4 pulses. Two process sequences were demonstrated for the controlled removal of NOx, one involving XeF2 and high temperature annealing and the other involving sequential doses of TiCl4 and XeF2. 1. Veyan, J.F.; Halls, M.D.; Rangan, S.; Aureau, D.; Yan, X.M.; Chabal, Y.J. J. Appl. Phys. 108, 114914 (2010). Figure 1

Nano-Structured Silicon Derived from Silica Precursors as Anode Material for Lithium Ion Batteries

Silicon is valuable as a high energy density anode material for Li-ion battery applications. However, the high lithium storage capacity leads to extreme volume expansion during lithiation and delithiation that can lead to mechanical deterioration and poor cycle life. To improve cyclability, nano-structured materials can be used to facilitate the volume change without deterioration. By using structured SiO2 precursors, the goal is to reduce SiO2 to Si while maintaining the structure and porosity of SiO2 and thus, mitigating degradation effects during cycling.Nanostructured Si has been synthesized by Mg vapor reduction from SiO2 precursors [1] (e.g. SiO2 microspheres from 3M, SiO2 aerogel, and mesoporous SiO2 [2]). Following the reduction to Si, the materials were chemically treated in HCl and HF to remove remaining MgO and SiO2 respectively. Figures 1 and 2 confirm the presence of Si in reduced samples and successful removal of MgO and SiO2. TGA/DSC has been used to optimize the reduction time and temperature. CV data (Figure 3) and cyclability tests (Figure 4) demonstrate a specific capacity of up to 2065 mAh/g with a low charge transfer resistance (Figure 3, inset).SEM, TEM, BET, EDS, XRD, and Raman spectroscopy data will be presented for SiO2 and reduced Si to describe chemical composition and morphology. Structure dependent electrochemical properties such as impedance spectroscopy, cyclic voltammetry, galvanostatic discharge, and cycle stability will be presented and discussed.

Formation of a hydrothermal kaolinite deposit from rhyolitic tuff in Jiangxi, China

The Longmen kaolinite deposit is one of the largest hydrothermal clay deposits of Ganxi volcanic basin (northern Wuyi Mountain area, China). The pristine host rocks are rhyolitic crystal-vitric tuff and minor lapilli tuff from the Late Jurassic Ehuling Formation. The ore consists of kaolin-group minerals (kaolinite, dickite), pyrophyllite with minor quartz, sericite, pyrite, etc.. From the host rocks to the transition zones (altered rocks) then to the vein ores, contents of SiO2 and TFe2O3 decrease, whereas Al2O3 and LOI increase, consistent with the contents increase of kaolin minerals and pyrophyllite in the samples. The total REE abundances of the ores are much lower than that of the host and altered rocks, Rb, Nb, Nd, Zr, Ti and Y are significantly depleted. Apparent zoning features of bulk geochemistry and mineral component reflect that the kaolinite deposit occurred at the expense of the host rock by ascending hydrothermal fluids with distinct removal of SiO2, TFe2O3, Na2O, K2O. According to the mineral assemblage, the formation temperature of this deposit falls within the range of 270–350 °C. With regard to the industrial applications, the kaolinized ores are suitable for use in ceramics and gemologic materials crafted for seal stones. Moreover, in mineralogical terms, this deposit is also proved to be an excellent example for studying channeled hydrothermal alterations of rhyolitic tuff.

Role of the Surface Chemistry of Ceria Surfaces on Silicate Adsorption

Ceria nanoparticles (NPs) have been widely explored as a promising material in various fields. As synthesized under various physicochemical conditions, it exhibits the different surface chemistry. Here, the role of hydroxyl and nitrate group on ceria surface, formed under various physicochemical conditions, for the silicate adsorption was experimentally and theoretically investigated based on the adsorption isotherms and theoretical analyses using density functional theory (DFT) calculation. Experimental results acquired from adsorption isotherms with Freundlich model indicated that the nitrate group shows a much higher affinity with silicate than the hydroxyl groups. These phenomena were demonstrated through the theoretical approaches that exhibit the binding energy of the NO3-ceria (-4.383 eV) on the SiO2 surface being much higher than that of the OH-ceria (-3.813 eV). In good agreement with the experimental and the theoretical results based on adsorption properties, the results of chemical mechanical planarization (CMP) also show that the nitrate groups significantly enhance the removal of SiO2 than the hydroxyl groups. The results investigated in this study will provide researchers, studying the ceria NPs, with guidelines on the importance of exploring the surface chemistry of ceria.

Processing Challenges of CMOS Integration of Finfets with All-Last Gate Stacks

FinFETs with 20nm BEOL with one generation improvement in performance and power efficiency has announced for mass production by leading IC companies. The processing details, however, have never been reported. In this talk, processing challenges of CMOS integration of FinFETs with all-last gate stacks is presented based on our recent results. Special processing issues in Fin and the replacement gate (RMG) formation, RMG filling with high-k/metal gate and self-aligned contact (SAC) module will be discussed. It is found that the etching of dummy gate of FinFETs behaves differently from that in planar MOSFETs. Thick enough dummy SiO2 is needed for protecting the Fin from plasma damage during dummy gate etch. Wet removal of dummy SiO2 in gate area faces also the risk of Fin damage, due to galvanic corrosion, and thus prefers to a thin SiO2 for control of the wet processing window. It is shown that TaN coverage of the channel area is important in single dielectric dual metal gate stack, where a p-FET stack is first deposited in both n- and p-area, and then the top work function metals in n-area will be removed before n-FET metal deposition. ALD TaN is studied systematically for the the etch stopper application. It is found that the largest issue in the CMOS integration of the FinFETS is n-work function metal deposition. Poor TiAl coverage induces large Vt variation. ALD TiAl could be a good solution for n-work function metal deposition. Filling of the RMG, which has extremely large AR in FinFETs, and SAC is a big challenge. A systematic study of ALD W shows that its advantage in properties such as adhesion, conformality and good constraint by TiN barrier enables good fill of the gaps. Electrical characterizations of the FinFETs are presented in order to show the impact of the above issues to device behavior variation.

Electrospun mesoporous carbon nanofibers produced from phenolic resin and their use in the adsorption of large dye molecules

Mesoporous carbon nanofibers (CNFs) were prepared by a sol–gel/electrospinning process using phenolic resin precursor as carbon source and triblock copolymer Pluronic F127 as template. The final CNFs were obtained after carbonization of as-spun nanofibers and removal of SiO2. Three samples (C-1, C-2, and C-3) with different pore textures were synthesized. The CNF structures were characterized by scanning and transmission electron microscopy, and N2 adsorption–desorption measurements, demonstrating that the samples consisted of nanofibers with mesopores and the mesopore volumes depended on the amount of tetraethyl orthosilicate in the spinnable sols. According to thermogravimetric analysis, the CNF yields of 2.57%, 2.78%, and 2.13% from the spinnable sols for sample C-1, C-2, and C-3 were obtained, respectively. The mesoporous CNFs were used as highly efficient adsorbents for large dye molecules. The relationship between the pore textures and adsorption properties was studied. It is suggested that the adsorption of different dyes depend on an appropriate pore size distribution in addition to surface area. However, the adsorption capacity of the regenerated adsorbents gradually decreased with the number of regeneration cycles. The adsorption of acid red 1 could reach 186mgg−1 for C-3 after seven regeneration cycles. Furthermore, the mechanical strength of CNFs needs improvement.

The sol–gel template synthesis of porousTiO2 for a high performance humidity sensor

This research develops a simple template assisted sol–gel process for preparing porousTiO2 for a highperformance humidity sensor. Tetraethyl orthosilicate (TEOS) as a template was directly introduced intoTiO2 sol formedby the hydrolysis and condensation of titanium alkoxide; the following calcination led to the formationof TiO2–SiO2 composite, and theselective removal of SiO2 by dilute HF solution led to the formation of porous structure inTiO2. The resultingporous TiO2-based sensor exhibits high sensitivity and linear response in the widerelative humidity (RH) range of 11%–95%, with an impedance variation offour orders of magnitude to humidity change. Moreover, it exhibits a rapidand highly reversible response characterized by a very small hysteresis of < 1% RH and a short response-recovery time (5 s for adsorption and 8 s for desorption), and a30-day stability test also confirms its long-term stability. Compared with pureTiO2 prepared by the conventional sol–gel method, our product shows remarkably improvedperformance and good prospect for a high performance humidity sensor. The compleximpedance spectra were used to elucidate its humidity sensing mechanism in detail.

XeF 2 -induced removal of SiO2 near Si surfaces at 300 K: An unexpected proximity effect.

XeF 2 interaction with SiO2/Si stacks has been investigated to understand the role of Si in proximity of SiO2 during XeF2 exposures of Si/SiO2 stacks. In situ Fourier transform infrared absorption spectroscopy, using a custom-made reaction cell compatible with high XeF2 pressures, reveals that, while pure SiO2 is not etched by XeF2, the oxide in SiO2/Si stacks is effectively removed when XeF2 has access to the silicon, i.e., when the Si in close proximity to the oxide is etched. Thick oxides (∼1–2 μm) are removed if sample edges are accessible, while thinner oxides (50–100 nm) are removed without requiring edge access. This unexpected SiO2 removal is found to be due to the formation of reactive fluorine species (XeF and F) evolved by the reaction of XeF2 with Si, which can, subsequently, etch SiO2. Calculations based on density functional theory provide critical insight into the underlying energetics and reaction pathways controlling XeF2 etching of both Si and SiO2.

Selectivity Enhancement in the Removal of SiO2 and Si3N4 Films with Addition of Triethanolamine in a Ceria Slurry during Shallow Trench Isolation Chemical Mechanical Polishing

Selectivity Enhancement in the Removal of SiO2 and Si3N4 Films with Addition of Triethanolamine in a Ceria Slurry during Shallow Trench Isolation Chemical Mechanical Polishing

Improvement of Ion-Exchange Capacity of Indonesian Natural Zeolite by Sequential Washing Using Hydrofluoric Acid and EDTA Solutions

To improve ion-exchange capacity of zeolite towards Pb (II) ion, the sequent removal of free SiO2 using HF and of free Al2O3, CaO, Fe2O3, and MgO using EDTA solution has been performed. It was observed that the large amount of the metal ions can be dissolved from the zeolite without any significant damage to the structure of zeolite. The optimum condition was reached when 1.0 g zeolite was washed with 25 cm3 of 0.05 % (v/v) HF solution for 3 hours, and with 200 cm3 of 0.075 M EDTA solution at pH 5 for 50 hours. The HF washing improved the zeolite capacity in adsorbing Pb (II) ions from 87.7 mg g-1 to 95.2 mg g-1, and sequent washing using EDTA solution could further increase the capacity up to 135 mg g-1, which is comparable to that absorbed by synthetic mordenite, ca. 152 mg g-1.

Two-dimensional arrays of nanometre scale holes and nano-V-grooves in oxidized Si wafersfor the selective growth of Ge dots or Ge/Si hetero-nanocrystals

Two-dimensional (2D) arrays of nanometre scale holes were opened in thinSiO2 layerson silicon by electron beam lithography and chemical etching. Oxidized silicon wafers with a 5 nm thickSiO2 layer on top were used in this respect. Pattern transfer involved either only removal ofSiO2 or a two-step process of oxide removal and anisotropic silicon chemical etching to formnanometre scale silicon V-grooves. The size of the holes in the photoresist layer varied inthe range 40–80 nm, depending on the exposure dose used. The smallest holes in the oxidewere about 50 nm in diameter, while in V-grooves the smallest width was nm. 2D arrays of Ge dots or Ge/Si hetero-nanocrystals were selectively grown on thesepatterned silicon wafers. In small windows only one Ge island per hole was nucleated.

Guided self-assembly of metallic nanowires and channels

A method is presented to form metallic nanowires and nanochannels by guided self-assembly. The method relies on an initial plasma-enhanced chemical vapor deposition of a silicon oxide film with altered chemistry on a silicon wafer, and the cracking of the film due to tensile stresses upon annealing. The fabricated stress concentration features on the Si substrate control the number of cracks and their orientation. These cracks are then filled with electroless nickel, and the subsequent removal of SiO2 produces a controlled network of nanowires of about 100 nm in dimension. In addition to nanowires, nanobridges, and nanocantilevers have also been fabricated by releasing the wires, confirming that the resulting structures are rather robust.

Production of zirconia from zircon using a plasma‐rotating furnace

Desilicated zirconia (88.60–94.21 mass% ZrO2) and stabilized zirconia (ZrO2≥ 91.54 mass%, MgO ≤ 5.39 mass%) have been produced successfully from zircon in a 150 kW plasma facility. Effects of temperature, time and carbon content on the removal of SiO2 from zircon have been investigated. According to preliminary applications in continuous‐casting nozzles at a steel works, the obtained desilicated ZrO2 can substitute for traditional ZrO2 made by the chemical‐purifying method. The main characteristics of the trial product meet the requirements for industrial production.

Decomposition of interfacial SiO2 during HfO2 deposition

Growth of HfO2 by Hf deposition in an oxidizing ambient is found to cause removal of interfacial SiO2. Medium-energy ion scattering results show that the reaction takes place during growth, and involves transport of oxygen through the HfO2 layer. An examination of the temperature dependence suggests that oxygen vacancy reactions are responsible.

Selective desorption of interfacial SiO2

In this letter, we report the selective thermal decomposition of SiO2 from a silicate/Si(001) interface, without silicidation of the dielectric. We observe kinetics that are similar to SiO2 desorption from Si(001), which takes place by SiO volatilization, suggesting that a similar mechanism is responsible at a buried interface. Interface desorption is a route to direct dielectric/silicon structures without prior removal of SiO2.

Patterning SiO2 thin films using synchrotron radiation stimulated etching with a Co contact mask

Patterning SiO2 thin film on the Si(100) surface was successfully demonstrated using a synchrotron radiation (SR) stimulated etching technique with SF6 + O2 as the reaction gas and a Co contact mask. The etching stopped completely at the SiO2/Si(100) interface. After the complete removal of SiO2 by SR etching, the Si surface was flat (Ra = 0.1 nm) and a well-ordered self-assembled monolayer (SAM) of dodecene was deposited on the SR etched region area selectively. Co was found to show sufficient resistivity against the SR etching and to be easily removed by dilute (∼ 0.01 N) HNO3, without damaging the SAM. The SR etching of the SiO2 thin films on the Si surface with the Co contact mask was found to be a suitable patterning technique for the area-selective deposition of alkyl SAMs.

Hysteresis of the photonic band gap in VO2 photonic crystal in the semiconductor-metal phase transition

VO2 photonic crystals exhibiting a semiconductor-metal phase transition at 55–75°C have been synthesized by the infiltration of vanadium dioxide (VO2) into opal crystals and the subsequent removal of SiO2 by etching. A study of the optical reflection spectra of such crystals demonstrated that they are characterized by a wide photonic band gap (in the [111] direction of light propagation) in the visible spectral range. The energy position of this band gap changes abruptly upon a phase transition. The temperature shift and hysteresis of the position of the photonic band gap were measured. Quantitative calculations of the reflection spectra of photonic crystals of opal and VO2 were performed in terms of the model of a layered periodic medium, and numerical values of the geometric parameters and optical constants of the studied three-dimensional periodic structures were obtained.

The modeling of excimer laser particle removal from hydrophilic silicon surfaces

We summarize experimental results on the successful removal of submicron-sized polystyrene latex, carboxylate-modified latex, SiO2, and Al2O3 particles from hydrophilic silicon surfaces by excimer laser, using both dry and steam cleaning; the cleaning and damage thresholds have also been determined for these particles. Adhesion and removal models for an ideal sphere particle, that include van der Waals forces, hydrogen bonding, and thermoelastic effects, theoretically explain the laser cleaning results. Two models of the removal force due to the explosive evaporation of liquid film have been calculated and compared. The effects of both asperities on the particle surface and particle aggregation have also been considered. The results of the calculations show that even those surface asperities which are small compared with the particle dimension can cause a large reduction in both adhesion and thermoelastic removal forces. The theoretical predictions are consistent with the experimental observations.