Metal dissolution silicon

A piece of the metal of about 0.5 g is weighed and dissolved slowly with 30 ml nitric acid (A) and 10 ml hydrochloric acid (B) in a 100 ml beaker After dissolution, silicon is filtered off by using a medium size filter. To the filtrate 2 ml sulphuric acid (C) are added and the solution is boiled down until sulphuric acid starts fuming. The filter with the non dissolved silicon is dried at 110°C, the silicon is transferred from the filter onto a beaker cover and the filter is ashed in a platinum dish. After ashing, the silicon is introduced into the platinum dish. 5 ml hydrofluoric acid (D), 5 ml nitric acid (E) and 1 ml sulphuric acid (C) are added and one heats until the whole is dissolved. The boiled down filtrate is added to the solution in the dish and heated until the sulphuric acid starts fuming. After cooling off, 40 ml water are added and the solution is transferred into a 100 ml beaker. The final determination is carried out as described above. The accuracy is 15 for concentrations of the order of 10 Mg/g. The duration of the analysis is approximately 5 hours. 

Titanium Silicides. The titanium—silicon system includes Ti Si, Ti Si, TiSi, and TiSi (154). Physical properties are summarized in Table 18. Direct synthesis by heating the elements in vacuo or in a protective atmosphere is possible. In the latter case, it is convenient to use titanium hydride instead of titanium metal. Other preparative methods include high temperature electrolysis of molten salt baths containing titanium dioxide and alkalifluorosiUcate (155) reaction of TiCl, SiCl, and H2 at ca 1150°C, using appropriate reactant quantities for both TiSi and TiSi2 (156) and, for Ti Si, reaction between titanium dioxide and calcium siUcide at ca 1200°C, followed by dissolution of excess lime and calcium siUcate in acetic acid. 

The semi-consumable electrodes, as the name implies, suffer rather less dissolution than Faraday s law would predict and substantially more than the non-consumable electrodes. This is because the anodic reaction is shared between oxidising the anode material (causing consumption) and oxidising the environment (with no concomitant loss of metal). Electrodes made from silicon-iron, chromium-silicon-iron and graphite fall into this category. 

Figure 7 (a) Sartorius absorption model (b) Sartorius dissolution model, a, Plastic syringe b, timer c, safety lock d, cable connector e, silicon tubes f, silicon-O-rings g, metal filter h, polyacryl reaction vessel. 

Preparation. Oxidation of the chromite ore by air in molten alkali gives sodium chromate, Na2Cr04 that is then converted to Cr203. The oxide is further reduced with aluminium or silicon to form chromium metal. Solutions suitable for electrolytic production of chromium (for plating) can be obtained from ore by oxidative roasting in alkali or by dissolution of chromite in H2S04 and especially by dissolving ferro-chromium in sulphuric acid. 

For homogeneously doped silicon samples free of metals the identification of cathodic and anodic sites is difficult. In the frame of the quantum size formation model for micro PS, as discussed in Section 7.1, it can be speculated that hole injection by an oxidizing species, according to Eq. (2.2), predominantly occurs into the bulk silicon, because a quantum-confined feature shows an increased VB energy. As a result, hole injection is expected to occur predominantly at the bulk-porous interface and into the bulk Si. The divalent dissolution reaction according to Eq. (4.4) then consumes these holes under formation of micro PS. In this model the limited thickness of stain films can be explained by a reduced rate of hole injection caused by a diffusional limitation for the oxidizing species with increasing film thickness. 

Silicon reacts with strong bases forming silicates and hberating hydrogen. Sihcon is attacked by hydrofluoric acid if there is no oxide layer over it. Flowever, since the metal has a very thin oxide film over its surface, a mixture of nitric and hydrofluoric acid is effective in dissolution of the metal. While nitric acid dissolves the oxide layer, the metal is then attacked by hydrofluoric acid. 

The direct electrochemical synthesis of metal alkoxides by the anodic dissolution of metals into alcohols containing conducting electrolytes was initially demonstrated by Szilard in 1906 for the methoxides of copper and lead.19 More recently the method has received some attention particularly in the patent literature.29-25 The preparation of the ethoxides of silicon, titanium, germanium, zirconium and tantalum by electrolysis of ethanolic solutions of NH Cl has been patented, although the production of the ethoxides was found to cease after several hours.24,25... 

Fusion with anhydrous potassium fluoride in a platinum dish is undoubtedly the simplest, most effective and reliable method available for the complete dissolution of a wide variety of siliceous materials. The potassium fluoride cake can then be transposed in the same container to a pyrosulfate fusion with rapid and complete volatilisation of both hydrogen fluoride and silicon tetrafluoride [54]. Except for a small quantity of barium sulfate, the pyrosulfate cake will dissolve completely in dilute hydrochloric acid. The resulting pyrosulfate fusion is one of the simplest and most effective methods available for rapid, complete and dependable dissolution of nonsiliceous materials, particularly high-fired oxides. This fusion has the distinct advantage that the flux can be obtained by simply adding easily purified alkali metal sulfates to sulfuric acid, and the fusion can be carried out in either borosilicate flasks or platinum vessels with very little contamination from either reagents or containers. 

Vapor phase dissolution (VPD) is commonly used for surface and contamination analysis of semiconductor wafers [374-379]. HF vapor is used to remove a silicon oxide or native silicon layer. A drop of hydrofluoric acid or deionized water (with a volume of 50 to 200 jxL) is placed on the surface and rolled around the surface to dissolve the metals. The small drop is then analyzed by ICP-MS by using either a direct injection nebulizer, a micronebulizer, or ETV. The ability of ICP-MS to measure several elements rapidly in a small volume of solution is essential. 

Deposition of metals on a silicon surface can be either a conduction band process or a valence band process depending on the redox potential of the metal and solution composition. Deposition of Au on p-Si in alkaline solution occurs only under illumination indicating that it is a conduction band process due to the unfavorable position of the redox couple for hole injection. " On the other hand, deposition of platinum on p-Si can occur in the dark by hole injection into the valence band. For Cu, although the deposition proceeds via the conduction band as shown in Fig. 6.9, it can also proceed via the valence band because a large anodic current of n-Si occurs in the dark in copper-containing HF solution as shown in Fig. 6.10. The reduction of copper under this condition is via hole injection. The holes are consumed by silicon dissolution and the silicon reaction intermediates then inject electrons into the conduction band, resulting in the anodic current on n-Si in the dark. 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

Thus, whether a metal can be deposited by electroless deposition onto a silicon surface depends on the redox potential and its relative position to the band edges and on whether the silicon can be dissolved under those conditions. On the other hand, whether the deposition can be sustained to cover the entire surface area depends, on nucleation and growth kinetics of the deposits, the catalytic effect of the deposits on silicon dissolution and hydrogen evolution and the evolution of the morphology of the surface. The formation of a continuous and uniform metal film by electroless deposition is intrinsically difficult because a certain amount of bare silicon surface area is required for silicon dissolution in order to sustain the deposition. 

Electroless deposition of Au in KAu(CN)2 -I- HF can be controlled by both the kinetic process and the diffusion process. The deposition is a two-step process, with initial diffusion-limited deposition of the intermediate species, followed by surface-limited reduction of this species. For electroless deposition of Pt, it has been reported that the rate-determining step is the deposition on n-Si, whereas it is the dissolution of silicon on p-Si. Electroless copper deposition does not occur on Si02-covered silicon surface due to the lack of anodic dissolution of silicon In a non-HF solution, the deposition of copper on a bare silicon surface results in the formation of oxide aroimd the metal particles. In HF solutions, the deposition of copper proceeds very slowly in the dark on both p-Si and n-Si samples due to the lack of carriers. The... 

Electroless metal deposition at trace levels in the solution is an important factor affecting silicon wafer cleaning. The deposition rate of most metals at trace levels depends mainly on the metal concentration and some may also depend on the interaction with other species as well. For copper the deposition rate at trace levels in HF solutions is different for n and p types. It depends on illumination for p-Si but not for n-Si. It is also different in HF and BHF solutions. In a HF solution the deposition process is controlled by both the supply of minority carriers and the kinetics of cathodic reactions. Thus, a high deposition rate occurs on p-Si only when both and illumination are present. In the BHF solution, the corrosion process is limited by the supply of electrons for p-Si whereas for n-Si it is limited by the dissolution of silicon because the reaction rate is indepaidmt of concentration and illumination. The amount of copper deposition does not correlate with the corrosion current density, which may be attributed to the chemical reactions associated with hydrogen reduction. More information on trace metal deposition can be found in Chapters 2 and 7. 

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