Chemical analysis of acidic silicon etch solutions: II. Determination of HNO3, HF, and H2SiF6 by ion chromatography

The processing of silicon in microelectronics and photovoltaics involves the isotropic chemical etching using HF-HNO₃ mixtures to clean the surface from contaminations, to remove the saw damage, as well as to polish or to texture the wafer surface. Key element of an effective etch process control is the knowledge of the actual etch bath composition in order to maintain a certain etch rate by replenishment of the consumed acids. The present paper describes a methods for the total analysis of the etch bath constituents HF, HNO₃, and H₂SiF₆ by ion chromatography. First step is the measurement of the total fluoride and nitrate content in the analyte. In a second step, H₂SiF₆ is precipitated as K₂SiF₆. After careful filtration of the precipitate, the fluoride concentration in the filtrate is measured and the content of free HF is calculated therefrom. The K₂SiF₆ is dissolved again and the fluoride content measured and recalculated as H₂SiF₆. The results obtained with the presented method are discussed with respect to the results from two other, previously published methods, based on a titration using methanolic cyclohexylamine solution as titrant and based on a method using a fluoride ion selective electrode (F-ISE). An evaluation with respect to the needs for an industrial application is given.     

Determination of total fluoride in HF/HNO3/H2SiF6 etch solutions by new potentiometric titration methods

In the photovoltaic industry the etching of silicon in HF/HNO₃ solutions is a decisive process for cleaning wafer surfaces or to produce certain surface morphologies like polishing or texturization. With regard to cost efficiency, a maximal utilisation of etch baths in combination with highest quality and accuracy is strived. To provide an etch bath control realised by a replenishment with concentrated acids the main constituents of these HF/HNO₃ etch solutions including the reaction product H₂SiF₆ have to be analysed. Two new methods for the determination of the total fluoride content in an acidic etch solution based on the precipitation titration with La(NO₃)₃ are presented within this paper. The first method bases on the proper choice of the reaction conditions, since free fluoride ions have to be liberated from HF and H₂SiF₆ at the same time to be detected by a fluoride ion-selective electrode (F-ISE). Therefore, the sample is adjusted to a pH of 8 for total cleavage of the SiF₆²⁻ anion and titrated in absence of buffers. In a second method, the titration with La(NO₃)₃ is followed by a change of the pH-value using a HF resistant glass-electrode. Both methods provide consistent values, whereas the analysis is fast and accurate, and thus, applicable for industrial process control.     

Titrimetric determination of silicon dissolved in concentrated HF-HNO3-etching solutions

The wet chemical etching of silicon by concentrated HF-HNO₃ mixtures in solar and semiconductor wafer fabrication requires the strict control of the etching conditions. Surface morphology and etch rates are mainly affected by the amount of dissolved silicon, that is continuously enriched in the etching solution with each etching run. A fast and robust method for the titrimetric determination of the total dissolved silicon content out of the concentrated etching solution is presented. This method is based on the difference between the two equivalence points of the total amount of acid and the hydrolysis of the hexafluorosilicic anion. This approach allows a silicon determination directly from the etching process in spite of the presence of dissolved nitric oxides in the etching solution. The influences of different acid mixing ratios and of the etching solution density depending on the silicon content is considered and discussed in detail.     

Contrasting oxide crystal chemistry of Nb and Ta:: The structures of the hexagonal bronzes BaTa2O6 and Ba0.93Nb2.03O6

The high-temperature hexagonal forms of BaTa₂O₆ and Ba_0.93Nb_2.03O₆ have P6/mmm symmetry with unit-cell parameters a=21.116(1) Å, c=3.9157(2) Å and a=21.0174(3) Å, c=3.9732(1) Å, respectively. Single crystal X-ray structure refinements for both phases are generally consistent with a previously proposed model, except for displacements of some Ba atoms from high-symmetry positions. The structures are based on a framework of corner- and edge-connected Nb/Ta-centred octahedra, with barium atoms occupying sites in four different types of [0 0 1] channels with hexagonal, triangular, rectangular and pentagonal cross-sections. The refinements showed that the non-stoichiometry in the niobate phase is due to barium atom vacancies in the pentagonal channels and to extra niobium atoms occupying interstitial sites with tri-capped trigonal prismatic coordination. The origin of the non-stoichiometry is attributed to minimisation of non-bonded Ba-Ba repulsions. The hexagonal structure is related to the structures of the low-temperature forms of BaNb₂O₆ and BaTa₂O₆, through a 30° rotation of the hexagonal rings of octahedra centred at the origin.     

Calorimetric study and thermal analysis of [Gd4/3Y2/3(Gly)6(H2O)4](ClO4)6·5H2O and [ErY(Gly)6(H2O)4](ClO4)6·5H2O (Gly: glycin)

Two solid-state coordination compounds of rare earth metals with glycin, [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O and [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O were synthesized. The low-temperature heat capacities of the two coordination compounds were measured with an adiabatic calorimeter over the temperature range from 78 to 376 K. [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O melted at 342.90 K, while [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O melted at 328.79 K. The molar enthalpy and entropy of fusion for the two coordination compounds were determined to be 18.48 kJ mol⁻¹ and 53.9 J K⁻¹ mol⁻¹ for [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O, 1.82 kJ mol⁻¹ and 5.5 J K⁻¹ mol⁻¹ for [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O, respectively. Thermal decompositions of the two coordination compounds were studied through the thermogravimetry (TG). Possible mechanisms of the decompositions are discussed.     

Fluoride modification of Mo/Al2O3 catalysts: Characterization of the changes induced in support and Mo phases

The characterization of fluoride-modified Mo/Al₂O₃ catalysts was performed in order to investigate on the effect that low levels of fluoridation of the alumina support (0-2.0 wt.%) cause on the support itself and on the supported Mo oxide and sulfide phases. Fluoride-modified Al₂O₃ supports and Mo/Al₂O₃ catalysts where characterized by nitrogen physisorption, scanning electronic microscopy (SEM-EDX), isoelectric point (IEP), Fourier transform infrared spectroscopy (FT-IR), infrared spectroscopy of adsorbed CO₂ (IR-CO₂), and temperature programmed reduction (TPR). The dispersion of the sulfided catalysts was estimated by dynamic NO chemisorption. The results indicate that the different hydroxyl types present on the alumina surface react to a different extent with fluoride and that it is the most basic hydroxyl groups that are titrated first.The consumption of the alumina OH by F⁻, inhibits, during the deposition of Mo, the formation of tetrahedral molybdenum oxide species in strong interaction with the support, leading to an increased number of polymeric octahedral Mo surface species. The NO adsorption results put in evidence a drop in the dispersion of the MoS₂ phase present in the sulfided samples.     

Effect of heat treatment on the structure of L-Ta2O5:: a study by XRPD and HRTEM methods

The effect of heat treatment on the structure of L-Ta₂O₅ has been studied by X-ray powder diffraction and high-resolution transmission electron microscopy, complemented by density measurements. Two stable low-temperature forms of L-Ta₂O₅ were found: one below about 1000°C with a b^* multiplicity of m≈13.5 and the other at 1350°C with m=11. The former modification was disordered, containing defects and twins, while the latter seemed to be more ordered. At intermediate temperatures, ordered and disordered mixtures of L-Ta₂O₅ slabs with m values in the range m=11-14 were seen. A new model of a structure of L-Ta₂O₅ (m=11) is proposed. The model can be described as an ordered intergrowth of slabs of α-U₃O₈ and β-U₃O₈ types. The α-U₃O₈ slabs are wider and contain somewhat larger three-sided tunnels that appear to be more suitable for interstitial Ta atoms than the β-U₃O₈ slabs. The density measurements confirm that additional Ta atoms are present in the structure.     

Low-temperature heat capacity and standard enthalpy of formation of neodymium glycine perchlorate complex [Nd2(Gly)6(H2O)4](ClO4)6·5H2O

Rare-earth perchlorate complex coordinated with glycine [Nd₂(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O was synthesized and its structure was characterized by using thermogravimetric analysis (TG), differential thermal analysis (DTA), chemical analysis and elementary analysis. Its purity was 99.90%. Heat capacity measurement was carried out with a high-precision fully-automatic adiabatic calorimeter over the temperature range from 78 to 369 K. A solid-solid phase transformation peak was observed at 256.97 K, with the enthalpy and entropy of the phase transformation process are 4.438 kJ mol⁻¹ and 17.270 J K⁻¹ mol⁻¹, respectively. There is a big dehydrated peak appears at 330 K, its decomposition temperature, decomposition enthalpy and entropy are 320.606 K, 41.364 kJ mol⁻¹ and 129.018 J K⁻¹ mol⁻¹, respectively. The polynomial equations of heat capacity of this compound in different temperature ranges have been fitted. The standard enthalpy of formation was determined to be −8023.002 kJ mol⁻¹ with isoperibol reaction calorimeter at 298.15 K.     

Alkalimetry in alcohol-water mixtures with potentiometric end-point detection: Critical remarks on a newer method of European Pharmacopoeia

A new method of the European Pharmacopoeia (Ph. Eur.)—alkalimetry in alcohol-water mixture with potentiometric end-point detection—for the assay of halide salts of alkaloids and other organic N-bases has been investigated using 13 substances. The results were compared to those obtained by nonaqueous direct acidimetry. In general, our measurements show that the Ph. Eur. method may be regarded as an environment-friendly, precise, reproducible approach, with average S.D. ±0.33. However, in six cases out of 13, the method did not work. In these cases, the first inflexion point, which should appear due to the neutralization of a defined, small volume of HCl added to the solution of the sample before the titration, was missing on the potentiometric titration curves. This was observed for papaverine chloride, pilocarpine chloride, pyridoxine chloride, thiamine chloride, histamine dichloride and noscapine chloride; this missing inflexion point hampered the exact measurement of the alkaloid content. This study shows that the method, in the present official form, can be applied only for compounds with pK_a values of seven or higher. For the salts of weaker bases, a modified approach (titration in 70% ethanol, without addition of HCl) is proposed.     

The reactions of dialkyl and diarylethoxysilanes with T6 silsesquioxane cages: X-ray crystallographic studies of the mono-T6D1 and bis-T6D2 insertion ring expansion products

Successful ring-expanding insertion reactions of T₆ silsesquioxane cages using dialkyl and diarylethoxysilanes have been performed to give the first reported mixed T₆D₁ and T₆D₂ silsesquioxane cages. The reactions of hexacyclohexylsilsesquioxane (T₆) with dialkyl and diaryldiethoxysilanes give predominantly T₆D₂ bis-insertion compounds while the reaction of T₆ with dimethylethoxysilane gives one T₆D₁ mono-insertion product and various T₆D₂ bis-insertion products as isolable components. Three of the ring-expanded products are chiral and it has been shown from their X-ray crystal structures that the pairs of enantiomers, formed as racemic mixtures, co-crystallise together. As well as comparing these structures with related ones in the literature, the possible mechanism of their formation is discussed.     

Synthesis and crystal structure of Bi6.4Pb0.6P2O15.2: A new polymorph in the series Bi6+xM1−xP2O15+y

Bi_6.4Pb_0.6P₂O_15.2 is a polymorph of structures with the general stoichiometry Bi_6+xM_1−xP₂O_15+y. However, unlike previously published structures that consist of layers formed by edge sharing OBi₄ tetrahedra bridged by PO₄ and TO₆ (T=transition metal) tetrahedra and octahedra the title compound's structure is more complex. It is monoclinic, C2, a=19.4698(4) Å, b=11.3692(3) Å, c=16.3809(5) Å, β=101.167(1)°, Z=10. Single-crystal X-ray diffraction data were refined by least squares on F² converging to R₁=0.0387, wR₂=0.0836 for 7023 intensities. The crystal twins by mirror reflection across (001) as the twin plane and twin component 1 equals 0.74(1). Oxygen ions are in tetrahedral coordination to four metal ions and the O(BiPb)₄ units share corners to form layers that are part of the three-dimensional framework. Eight oxygen ions form a cube around the two crystallographically independent Pb ions. Pb-O bond lengths vary from 2.265(14) to 2.869(14) Å. Pairs of such cubes share an edge to form a Pb₃O₂₀ unit. The two oxygen ions from the unshared edges are part of irregular Bi polyhedra. Other oxygen ions of Bi polyhedra are part only of O(BiPb)₄ units, and some oxygen ions of the polyhedra are also part of PO₄ tetrahedra. One, two, three and or four PO₄ moieties are connected to the Bi polyhedra. Bi-O bond lengths ?3.1 Å vary from 2.090(12) to 3.07(3) Å. The articulations of Pb cubes, Bi polyhedra and PO₄ tetrahedra link into the three-dimensional structure.     

Synthesis, heat capacity and enthalpy of formation of [Ho2(l-Glu)2(H2O)8](ClO4)4·H2O

A complex of holmium perchlorate coordinated with l-glutamic acid, [Ho₂(l-Glu)₂(H₂O)₈](ClO₄)₄·H₂O, was prepared with a purity of 98.96%. The compound was characterized by chemical, elemental and thermal analysis. Heat capacities of the compound were determined by automated adiabatic calorimetry from 78 to 370 K. The dehydration temperature is 350 K. The dehydration enthalpy and entropy are 16.34 kJ mol⁻¹ and 16.67 J K⁻¹ mol⁻¹, respectively. The standard enthalpy of formation is −6474.6 kJ mol⁻¹ from reaction calorimetry at 298.15 K.     

La3Ru8B6 and Y3Os8B6, new members of a homologous series R(A)nM3n−1B2n

Two new compounds, La₃Ru₈B₆ and Y₃Os₈B₆, were synthesized by arc melting the elements. Their structural characterization was carried out at room temperature on as-cast samples by using X-ray diffractometry. According to X-ray single-crystal diffraction results these borides crystallize in Fmmm space group (no. 69), Z=4, a=5.5607(1) Å, b=9.8035(3) Å, c=17.5524(4) Å, ρ=8.956 Mg/m³, μ=25.23 mm⁻¹ for La₃Ru₈B₆ and a=5.4792(2) Å, b=9.5139(4) Å, c=17.6972(8) Å, ρ=13.343 Mg/m³, μ=128.23 mm⁻¹ for Y₃Os₈B₆. The crystal structure of La₃Ru₈B₆ was confirmed from Rietveld refinement of X-ray powder diffraction data. Both La₃Ru₈B₆ and Y₃Os₈B₆ compounds are isotypic with the Ca₃Rh₈B₆ compound and their structures are built up from CeCo₃B₂-type and CeAl₂Ga₂-type structural fragments taken in ratio 2:1. They are the members of structural series R(A)_nM_3n−1B_2n with n=3 (R is the rare earth metal, A the alkaline earth metal, and M the transition metal). Structural and atomic parameters were also obtained for La_0.94Ru₃B₂ compound from Rietveld refinement (CeCo₃B₂-type structure, P6/mmm space group (no. 191), a=5.5835(9) Å, c=3.0278(6) Å).     

稀土脯氨酸配合物[RE2(L-Pro)6(H2O)4](ClO4)6的标准生成焓测定

合成了两种稀土高氯酸盐与L 脯氨酸配合物的晶体.经热重、差热、化学分析及对比有关文献,知其组成是[Pr2(L Pro)6(H2O)4](ClO4)6和[Er2(L Pro)6(H2O)4](ClO4)6,质量分数为99.24％和98.20％.选用RE(NO3)3•6H2O(RE=Pr,Er)、L Pro、NaClO4•H2O和NaNO3作辅助物,使用具有恒温环境的反应热量计,以2 mol•L－1 HCl作溶剂,分别测定了[2RE(NO3)3•6H2O＋6L Pro＋6NaClO4•H2O]和{[RE2(L PrO)6(H2O)4](ClO4)6＋6NaNO3}在298.15 K时的溶解热.设计一热化学循环求得化学反应的反应焓ΔrHm分别是：63.904 kJ•mol－1和91.017 kJ•mol－1,经计算得配合物[RE2(L Pro)6(H2O)4](ClO4)6(s)在298.15 K时的标准生成焓ΔfHm(298.15 K)分别是－6 594.78 kJ•mol－1和－6 532.87 kJ•mol－1.     

Pentafluoro-λ-sulfanyl (SF5) perfluoroalkyl iodides-synthesis and reaction with ethylene and tetrafluoroethylene : Crystal structure of SF5(CF2)4CH2CH2I

A series of reactions between SF₅CF₂CF₂I and SF₅(CF₂)₄I with F₂CCF₂ was carried out in an effort to find the most effective methods for chain-extension. Also, for the first time, SF₅(CF₂)₈I and SF₅(CF₂)₁₀I have been prepared and isolated. The reaction conditions for the addition of H₂CCH₂ were also investigated. A determination of the crystal structure of the SF₅(CF₂)₄CH₂CH₂I has been carried out: the crystal system is monoclinic, with space group P2(1)/n and a=23.465(5) Å; b=6.0971(12) Å; c=44.892(9) Å; α=90°; β=99.38(3)°; γ=90°; Z=20.     

Structural disorder along the lithium diffusion pathway in cubically stabilized lithium manganese spinel: I. Synchrotron X-ray studies

Electron density distribution, EXAFS, and transmission electron microscope studies on stoichiometric Li(Mg_1/6Mn_11/6)O₄ have revealed that: (1) the structure is essentially of spinel-type with slight diffuse scattering, (2) the Li atoms are not all located at the ideal 8a site of the spinel structure, but are partially tetrahedrally distributed along the 8a-16c tie line, (3) the O atoms also exhibit a statistical distribution about their ideal positions and (4) the Mn 3d electrons are squeezed out toward the open space of the coordinating octahedra with D_3d distortion. The present results indicate the possible existence of many metastable Li positions in the structure, suggestive of complicated Li atom hopping routes in conjunction with a local distortion of neighboring atoms at least up to the second shell.     

Low-temperature heat-capacity and standard molar enthalpy of formation of copper l-threonate hydrate Cu(C4H6O5)·0.5H2O(s)

The solid copper l-threonate hydrate, Cu(C₄H₆O₅)·0.5H₂O, was synthesized by the reaction of l-threonic acid with copper dihydrocarbonate and characterized by means of chemical and elemental analyses, IR and TG-DTG. Low-temperature heat-capacity of the title compound has been precisely measured with a small sample precise automated adiabatic calorimeter over the temperature range from 77 to 390 K. An obvious process of the dehydration occurred in the temperature range between 353 and 370 K. The peak temperature of the dehydration of the compound has been observed to be 369.304 ± 0.208 K by means of the heat-capacity measurements. The molar enthalpy, Δ_dH_m, of the dehydration of the resulting compound was of 16.490 ± 0.063 kJ mol⁻¹. The experimental molar heat capacities of the solid from 77 to 353 K and the solid from 370 to 390 K have been, respectively, fitted to tow polynomial equations with the reduced temperatures by least square method. The constant-volume energy of combustion of the compound, Δ_cU_m, has been determined as being −1616.15 ± 0.72 kJ mol⁻¹ by an RBC-II precision rotating-bomb combustion calorimeter at 298.15 K. The standard molar enthalpy of formation of the compound, , has been calculated to be −1114.76 ± 0.81 kJ mol⁻¹ from the combination of the data of standard molar enthalpy of combustion of the compound with other auxiliary thermodynamic quantities.     

Crystal structure and magnetic properties of the solid-solution phase Ca3Co2-vMnvO6

The homogeneity range of the Ca₃Co_2-vMn_vO₆ solid-solution phase covers the entire composition interval from v=0 to 1. A systematic powder X-ray and neutron diffraction, magnetic susceptibility, and magnetization study has been carried out to investigate effects of the Mn-for-Co substitution on structural and magnetic properties. The Mn substitution concerns primarily only the octahedral Co1 site of the Ca₃Co1Co2O₆ crystal structure, whereas the trigonal-prismatic Co2 site structurally is left essentially unaffected. The Ca₃Co_2-vMn_vO₆ crystal structure belongs to space group with unit-cell dimensions (in hexagonal setting) 9.084?a?9.134 Å and 10.448?c?10.583 Å. A cut through the magnetic phase diagram at 10 K shows a ferrimagnetic domain for 0?v<∼0.3 and an antiferromagnetic domain for ∼0.50<v<∼1. The magnetic ordering temperatures are quite low (<∼25/18 K), and even so further magnetic transitions appear to take place at still lower temperature. The legitimity and reliability of the different indicators used to establish the magnetic transitions, their individual accuracy, and mutual consistency are briefly discussed. Variable parameters of the crystal and magnetic structures of Ca₃Co1_1-vMn_vCo2O₆ are determined and their variation with v is briefly discussed in relation to chemical bonding. The magnetic structure in the ferrimagnetic region is essentially the same as that of the pristine v=0 phase, but since the moments at the Co2 site decrease and those at the (Co1,Mn) site increase with increasing v; characteristic traits of ferrimagnetism in magnetic susceptibility and magnetization gradually disappear. The magnetic arrangement in the antiferromagnetic region is characterized by differently sized moments at the (Co1,Mn) and Co2 sites, moments at adjacent sites in each of these sublattices being oppositely oriented along [001].     

Thermodynamic properties of hydrated sodium l-threonate Na(C4H7O5)·H2O(s)

Low-temperature heat capacities of the compound Na(C₄H₇O₅)·H₂O(s) have been measured with an automated adiabatic calorimeter. A solid-solid phase transition and dehydration occur at 290-318 K and 367-373 K, respectively. The enthalpy and entropy of the solid-solid transition are Δ_transH_m = (5.75 ± 0.01) kJ mol⁻¹ and Δ_transS_m = (18.47 ± 0.02) J K⁻¹ mol⁻¹. The enthalpy and entropy of the dehydration are Δ_dH_m = (15.35 ± 0.03) kJ mol⁻¹ and Δ_dS_m = (41.35 ± 0.08) J K⁻¹ mol⁻¹. Experimental values of heat capacities for the solids (I and II) and the solid-liquid mixture (III) have been fitted to polynomial equations.     

Neutron diffraction study of the crystal structure and structural phase transition of La0.7Ca0.3−xSrxCrO3 (0≤x≤0.3): The relationship between thermodynamic behavior and crystal structure changes at the phase transition

The crystal structure of the La_0.7Ca_0.3−xSr_xCrO₃ series, including the compositional and temperature dependence of the structural parameters, has been studied by variable temperature neutron diffraction measurements. The extent of the distortions from the ideal cubic perovskite structure has been evaluated quantitatively using the average bond lengths and the mean volumes of the [CrO₆] octahedron and [(La/Ca/Sr)O₁₂] polyhedron, and has been shown to decrease with increase of Sr content or temperature. At the structural phase transition from the orthorhombic (Pnma) structure to the rhombohedral one, the volume of the [CrO₆] octahedron decreases whereas that of the [(La/Ca/Sr)O₁₂] polyhedron shows little difference, resulting in an overall decrease in the level of distortion. The change in the degree of distortion at the phase transition decreases with increase of Sr content, in agreement with the smaller variation of the enthalpy and volume for the specimens with higher Sr content.