Specific features of synthesis,chemical composition,and thermal stability of anodic borosilicate coatings on silicon

Thickness and resistivity of anodic borosilicate coatings synthesized on n-type silicon were studied in relation to the conditions in which the final forming potentials are reached and to the content of boric acid in an ethylene glycol solution. A method for synthesis of the coatings for use in nanoelectronics was suggested on the basis of data on the thermal stability of their chemical composition.     

Electrochemical formation of conducting polymer coating on porous p‐ and n‐silicon substrates

Electrochemical synthesis of polyaniline was carried out in aqueous sulfuric acid solutions of aniline on porous p‐ and n‐silicon under galvanostatic, potentiostatic and potential pulse regimes. It is shown that the introduction of catalytic quantities of oxidants, potassium hexachloroiridate, potassium dichromate, potassium permanganate and chloranilic acid, in electrolytes accelerates the formation of polymer films and lowers the overvoltage of electrosynthesis. The resulting polymer coatings are characterized by cyclic voltammetry and IR spectroscopy. It is shown that polyaniline coatings on porous p‐ and n‐silicon electrodes are electroactive and conducting in anodic and cathodic ranges. Copyright © 2000 John Wiley & Sons, Ltd.     

An EQCM study on the influence of saccharin on the corrosion properties of nanostructured cobalt and cobalt-iron alloy coatings

Nanostructured cobalt (Co) and cobalt-iron (CoFe) alloy coatings were electrodeposited from sulfate solutions in the presence and absence of saccharin. The effects of saccharin on the corrosion behavior of Co and CoFe alloy coatings were investigated using the electrochemical quartz crystal microbalance (EQCM) technique coupled with cyclic voltammetry (CV) measurements. Saccharin was added to the electrolyte as a grain refiner and brightener. Interestingly, opposite corrosion behaviors were found for all nanostructured coatings in 0.1 M H₂SO₄ and 0.1 M NaOH. The use of saccharin as an additive in the plating solution accelerated the anodic reaction for all deposits in acidic medium. The mass decreases while dissolution rate increased with higher saccharin concentration. Meanwhile, formation of a thick passive film on the Co electrode surface were enhanced while a hindering effect was observed for CoFe alloy coatings deposited in the presence of saccharin in alkaline solution. The anodic and cathodic curves obtained from potentiodynamic polarization experiments were also in agreement with the EQCM results.     

Factors influencing the growth behaviour of nanoporous anodic films on iron under galvanostatic anodizing

The growth behaviour of nanoporous anodic films on iron during galvanostatic anodizing in ethylene glycol electrolytes containing NH₄F and H₂O is examined at various current densities, H₂O concentrations in electrolytes and temperatures. The film morphology is mainly controlled by the formation voltage, regardless of anodizing conditions. Relatively regular cylindrical pores are formed at formation voltages less than 50 V, while rather disordered pores are formed above 100 V. The decrease in the H₂O concentration suppresses chemical dissolution of anodic films in addition to the increased growth efficiency, resulting in the formation of anodic films with a steady thickness of ～7 μm. The cell size of the anodic films depends upon the H₂O concentration as well as the formation voltage, but not upon the current density. Findings in this study will be useful for controlled growth of the anodic films on iron.     

The formation of submonolayer thorium coatings on a silicon oxide surface by electrochemical deposition

Data are reported on the mechanisms of the formation of submonolayer coatings based on thorium oxide on the surface of Si(111) single crystal with natural oxide as a result of electrochemical deposition. It is experimentally shown that the deposition of thorium atoms from an acetone solution of Th(NO₃)₄ onto the surface of natural silicon oxide leads to the formation of defects in the thin silicon oxide layer, which is accompanied by the deposition of thorium nanoclusters onto a pure silicon surface. The observed effects have been qualitatively explained assuming the breakdown of natural silicon oxide due to the presence of an electrical double layer in the near-surface region of the cathode.     

Indirect atomic absorption spectroscopic determination of SiO2 in copper composite coatings

A fast, sensitive and reliable method for the indirect atomic absorption determination of SiO₂ in copper composite coatings after extraction of silicomolybdic acid in a mixture of isobutyl methyl ketone and butanol (volume ratio 5?:?1) and measurement of the molybdenum absorbance in an air/acetylene flame is described. The experimental conditions are optimized for (i) prevention of the silicon polymerization during sample preparation and storage; (ii) for overcoming of the Cu(SiF₆) – complex formation and (iii) for quantitative extraction of the yellow complex into the organic phase for a wide silicon concentration range. The method permits the determination of 0.5–10 mg/g Si in copper. The analytical performance of the proposed method is compared with direct Si determination using ICP-AES.     

发光多孔硅的光谱及电化学研究

单晶硅是E_g为1.1eV的间接带隙半导体材料,在可见光区不发光,不能应用于光电子领域.但是,Canham 1990年首次发现^[1],适当条件下形成的多孔硅在室温下就可发出强度能与Ⅲ-Ⅴ族半导体发光二极管相媲美的可见光。     

Porous silicon: repeatability of generation?

A systematic investigation of the preparation of porous silicon layers (PSLs) on silicon wafers by anodic treatment in hydrofluoric acid/water/ethanol mixtures under various conditions was carried out with the aim to develop a repeatable process for homogeneous layers. The preparations were controlled by FTIR spectroscopy supported by TOF-SIMS measurements of the uppermost surface area. The repeatability of PLS generation was proved on the grounds of the IR-absorption of SiH_x and O_xSiH_y surface groups with respect to their frequencies and intensities. Uniform properties of PSLs are an essential assumption for defined chemical modifications.     

Indirect atomic absorption spectroscopic determination of SiO2 in copper composite coatings

A fast, sensitive and reliable method for the indirect atomic absorption determination of SiO₂ in copper composite coatings after extraction of silicomolybdic acid in a mixture of isobutyl methyl ketone and butanol (volume ratio 5 : 1) and measurement of the molybdenum absorbance in an air/acetylene flame is described. The experimental conditions are optimized for (i) prevention of the silicon polymerization during sample preparation and storage; (ii) for overcoming of the Cu(SiF₆) – complex formation and (iii) for quantitative extraction of the yellow complex into the organic phase for a wide silicon concentration range. The method permits the determination of 0.5–10 mg/g Si in copper. The analytical performance of the proposed method is compared with direct Si determination using ICP-AES. Received: 15 May 1997 / Revised: 19 September 1997 / Accepted: 25 September 1997     

Effect of TMAB concentration on structural,mechanical and corrosion properties of electrodeposited Ni–B alloys

The influence of trimethylamine borane (TMAB) concentration as a boron (B) source on the structural and corrosive properties of Ni–B alloy coatings produced by electrodeposition was investigated. The crystal structure of the Ni–B coatings is influenced by the B content in the coating, having a slight (220) preferred orientation. The B content in the coating increased from 16 to 34 at.% with the corresponding increase in the concentration of TMAB from 1 to 20 g/L, while interestingly retaining a crystal structure. The hardness of the coatings increased with increasing B content owing to the formation of smaller crystallites. An increase in B content in the alloy coatings, led to a shift in the E_corr values to more anodic potentials, indicating increase corrosion protection for the Ni–B coatings. This study achieved to reach 34 at.% B content in Ni–B alloy coatings produced by electrodeposition while preventing amorphization of the coating layer.     

Electrochemical preparation and characterization of a cobalt oxide–platinum composite with promising capacitive and electrocatalytic features

In the present work, boron-doped diamond polycrystalline films were used as support for direct anodic deposition of the cobalt oxide, and continuous Co₃O₄ coatings with reasonably good conductivity were obtained by appropriately adjusting the deposition charge. Further electrochemical deposition of platinum particles on the oxide substrate enabled the formation of a stable composite with a specific capacitance of ca. 431 F?cm^?3 that compares well with that available with similar materials obtained by non-electrochemical methods. Additional advantages of electrochemically obtained composites are the lower content of noble metal, the uniform distribution of the charge over an extended potential range, and, importantly, the simplicity of the preparation method. It was also found that when deposited on a Co₃O₄ substrate, Pt particles show, besides an enhanced active surface area, an improved catalytic activity for methanol anodic oxidation. This behavior was tentatively ascribed to the presence of a high amount of platinum-oxidized species.     

Influence of laser microfabrication on silicon electrochemical behavior in HF solution

Influence of direct laser writing with femtosecond pulses on electrochemical etching of n-type low conductivity (>1,000 Ωcm) silicon is demonstrated. It has been shown that thermal 1-μm-thick SiO₂ layer on silicon surface can be used as a protective layer in the electrochemical etching process. It has been found that laser ablation changes not only the surface morphology and structure of silicon samples but also the character of their anodic etching in aqueous solution of hydrofluoric acid. Formation of microvoids and caverns of irregular shape has been observed at the laser-ablated sites. It is proposed that the change of silicon conductivity from n- to p-type takes place at the laser fabricated regions. Processes of Si anodic oxidation and electrochemical etching are discussed.     

Formation and transformation of Mg(OH)2 in anodic coating using FTIR mapping

FTIR microscopic mapping was used to investigate the surface distribution of Mg(OH)₂ on anodic coatings. The results indicated Mg(OH)₂ was formed accompanying with micro-arc sparking and the amount of Mg(OH)₂ was related to the intensity of sparking, which depends on the applied voltage. The distribution and amount of Mg(OH)₂ varied with immersion time in 3.5 wt.% NaCl solution. The transformation process of anodic coatings was researched by FTIR microscopic mapping and smart map. The Cl^? could be absorbed on the surface of anodic coatings where Mg(OH)₂ is present and promote Mg(OH)₂ to transform into the soluble magnesium salt MgCl₂.     

Anodic film formation on osmium electrodes: Part II. Galvanostatic and potentiostatic measurements

The formation and reduction of anodic films on Os electrodes in 2 M HCl and HClO₄ solutions were studied by anodic and cathodic charging curves. The galvanostatic oxidation of Os in HClO₄ shows the formation of OsO₂ as an intermediate step to OsO₄ that goes in the solution. The cathodic charging curves at Os electrodes previously oxidized at constant potential reveal the anodic film to be made up of a reversibly desorbed oxygen layer and an oxide phase reduced irreversibly. Both layers increase with time under potentiostatic conditions following a logarithmic equation until a constant value is reached. At all times, the content of OsO₂ in the anodic film at high potentials is larger than that of chemisorbed oxygen.In HCl solutions only the reversible reduction of an oxygen layer is observed. The growth of this film also complies with a direct logarithmic law before attaining a limiting coverage. The charge involved in the reduction increases linearly with the potential at a given time of formation. The results are discussed in terms of a Temkin-type isotherm and a place-exchange mechanism.     

Hybrid plasma bonding for void-free strong bonded interface of silicon/glass at 200 °C

A novel hybrid plasma bonding (HPB) that combines sequential plasma activation (reactive ion etching followed by microwave radicals) with anodic bonding has been developed to achieve void-free and strong silicon/glass bonding at low temperature. The interfacial voids were observed at the silicon/glass interface both in the anodic bonding and in the plasma activated anodic bonding, but the voids were completely disappeared in the HPB method at 200 °C. The bonding strength of the silicon/glass in the HPB was as high as 30 MPa at 200 °C, which was higher than that in the individual treatment of anodic and plasma activated bonding methods. The improved characteristic behavior of the interface in the HPB is attributed to the higher hydrophilicity and smooth surfaces of silicon and glass after sequential plasma activation. These highly reactive and clean surfaces enhance the mobility of alkaline cations from the glass surface across the interface toward the bulk of glass in the HPB. This transportation resulted in a ∼353 nm thick alkaline depletion layer in the glass and enlarged the amorphous SiO₂ across the interface. The void-free strong bonding is attributed to the clean hydrophilic surfaces and the amorphous SiO₂ layer across the interface.     

Anodic treatment of tin in alkaline electrolytes

Processes of the formation of anodic films on the surface of tin in sodium hydroxide solutions were studied. The optimal conditions of the high-speed anodization, at which conversion coatings with high corrosion resistance can be formed, were determined.     

Study of the Electroless Silver Seed Formation on Silicon Surface

The silver seed on silicon was prepared through aqueous I-IF and AgNO3 solution at room temperature. In order to explore the formation process of silver seed on silicon, the methods of open circuit potential with time （OCP-t）, anodic stripping sweep voltammetry （ASV） and scanning tunneling microscope （STM） were used in this work. The procedure of silver nucleus growing into large particles was explained by electro-negativity. The growth mechanism of silver seed on silicon has been presented： at first, the silver monolayer and multilayer firstly grows onto silicon without fully covering the surface at the expense of silicon etching due to the silver seed attracting the electron from silicon, after that, the monolayer coalesces together, forming continuous grain film with some silver atoms diffusing into the silicon and the multilayer still grows thick simultaneously.     

Kinetics of Silicon Nitride Formation on SiO2‐Derived Rice Husk Ash Using the Chemical Vapor Infiltration Method

Since silicon nitride coatings on silicon dioxide are attractive for the semiconductor and electronics industries, cognizance of their formation kinetics is crucial for optimization of production parameters. In this contribution, the deposition kinetics (rate constant and activation energy) of Si₃N₄ by the hybrid system chemical vapor infiltration route (HYSY‐CVI), starting from N₂:NH₃ and SiF₄ (produced by the decomposition of Na₂SiF₆) has been studied. The deposition rate equation for Si₃N₄ was established from several possible gas‐phase or surface reaction steps involved in the growth of Si₃N₄ coatings onto silica‐derived rice husk ash (RHA). Based on a judicious analysis of four different models, it was found that Freundlich's adsorption model satisfactorily represents the rate of Si₃N₄ deposition process onto RHA.     

Mass spectrometric investigation of the thermal decomposition of various organosilicon compounds in SixC1-x(H) chemical vapour deposition

Thermal decomposition studies on various organometallic molecules containing silicon have been performed to obtain amorphous silicon and silicon carbide coatings by chemical vapour deposition. In particular, some organometallic compounds containing weak silicon-carbon bonds were employed. The thermal decomposition of four molecules was investigated: two cyclic molecules, sila-5-spiro[4.4]nona-2,7-diene (I_A) and dimethyl-2,7-sila-5-spiro[4,4]nona-2,7-diene (I_B), and two non-cyclic molecules, tetrapropargylsilane (II_A) and tetraethynylsilane (II_B). We observed that the thermal decomposition of II_A proceeds via the formation of I_A.The vapour-phase analysis was carried out by mass spectrometry and decomposition schemes are proposed. The influence of various experimental parameters on the amounts of by-products of decomposition was investigated. It is important to note in particular that the temperature of decomposition is a function of the structure of the compounds, especially with respect to the position of the methyl substituent (for cyclic molecules) or the acetylenic bond in the α- or β-position relative to the silicon atom (for non-cyclic molecules).     

Fast migration of fluoride ions in growing anodic titanium oxide

The rapid inward migration of fluoride ions in growing anodic titanium oxide under a high electric field has been elucidated by anodizing a Ti–12 at% silicon alloy, where film growth proceeds at nearly 100% efficiency in selected electrolytes. Further, incorporated silicon species in the anodic film are immobile, acting as marker species. The migration rate of fluoride ions is determined precisely by three-stage anodizing, consisting of initial anodic film formation at a constant current density to 50 V in ammonium pentaborate electrolyte, subsequent incorporation of fluoride ions by reanodizing to 55 V in ammonium fluoride electrolyte and, finally, anodizing again in ammonium pentaborate electrolyte at high current efficiency. The resultant films were analyzed by glow discharge optical emission spectroscopy to reveal the depth distribution of fluoride ions and the location of the silicon marker species. The fluoride ions migrate inward at twice the rate of O²⁻ ions. Consequently, anodizing of titanium in fluoride-containing electrolytes develops a fluoride-rich layer that separates the alloy substrate from the anodic oxide, with eventual detachment of the film from the substrate.