Anodic etching

Electrolytic (anodic) etching of siUcon occurs in HE solutions (5). Using concentrated HE, the siUcon is dissolved in the divalent state ... 

For doping-dependent anodic etch stops in HF, a general hierarchy of dissolution is observed [La5] illuminated n-doped and n+-doped areas are most easily dissolved, followed by p+-doped areas. Next likely to be dissolved are p-type areas. Moderately n-type doped areas kept in the dark are least likely to be etched. This hierarchy corresponds to the potential shift of the I-V curve in the regime of PS formation [Gal, Zh5]. 

The preferred method for preparing STM tips is the dc dropoff method. The basic setup is shown in Fig. 13.1. It consists of a beaker containing an electrolyte, typically 2M aqueous solution of NaOH. A piece of W wire, mounted on a micrometer, is placed near the center of the beaker. The height of the W wire relative to the surface of the electrolyte can then be adjusted. The cathode, or counterelectrode, is a piece of stainless steel or platinum placed in the beaker. The shape and location of the cathode has very little effect on the etching process, which can be chosen for convenience. A positive voltage, 4 V to 12 V, is applied to the wire, which is the anode. Etching occurs at the air-electrolyte interface. The overall electrochemical reaction is (Ibe et al., 1990) ... 

Electrochemical properties of silicon single crystals, usually cuts of semiconductor wafers, have to be considered under two distinct respects (1) As an electrode, silicon is a source of charge carriers, electrons or positive holes, involved in electrochemical reactions, and whose surface concentration is a determining parameter for the rate of charge transfer. (2) As a chemical element, silicon material is also involved in redox transformations such as electroless deposition, oxide generation, and anodic etching, or corrosion processes. 

A similar procedure can be used to determine the space charge distribution in n-type Si in the dark with a positive bias polarization so as to generate a depletion layer within the semiconductor substrate. In this case, the situation is somewhat different because the positive polarization in HF results in an anodic etching of the sample with a nonnegligible current density near 7 pA cm . Nevertheless, similar results were obtained, the components of the equivalent circuit were a capacitance of a few 10 F cm , and a resistance term ranging from 1 to 10Mf2cm for a bias potential varying from —0.1 to -1-0.9 V vs. SCE. 

Several results concerning the electrochemical reactions at the Si/HF interface have been published. Some were focused on the cathodic processes, others on the anodic etching reaction, and the influence of various parameters, such as doping level, influence of light, surface structure or presence of redox reactants, was investigated. A synthetic picture of anodic and cathodic behavior of both p- and n-type Si substrate, in the simplest condition of a pure 5% HF aqueous solution, in the dark is presented (see Fig. 5). 

The electrochemical reaction rate for the anodic etching of Si in HF was very rapid. This is confirmed by the electrochemical impedance diagram of Fig. 7 that shows a real component equal to 150 cm, and is the result of the high reactivity of the transient bare —Si sites that appear under anodic current. The detailed mechanism of the transformation was investigated by FIS, which revealed quite an unusual inductive loop, which is shown in Fig. 7. Such a diagram was obtained by modeling the reaction kinetics based... 

A spatial distribution of Pb centres (g = 2.0029, g — 2.0086) has been revealed in porous silicon formed by anodic etching of crystalline silicon in hydrofluoric acid.144 Oxygen ions were also implanted using accelerator at 2 MeV and compared with photoluminescence. 

Several groups have then used an anodic etch in the ionic liquid prior to deposition. Anodic etch potentials and times are dependent on the substrate and the... 

GaN 0.1 M NaOH 25 Y Pankove [18] First anodic etching of GaN. Used jet spray of electrolyte... 

Fig. 4. 65/35 brass annealed for 15 hrs at 200°C, polished to dissolve 100ft, annealed again for 3 hrs at 400°C, repolished to dissolve 15/i, annealed for l hr at 500°C and finally polished to dissolve 10ft. The specimen is anodically etched in a thiosulfate solution and the colored film is dissolved (30) X630. 

The electrochemistry of the anodic etching of semiconductors is similar in most respects to the anodic dissolution of metals. The main difference in the electrolytic behavior of metals and semiconductors is in the electrode material itself. 

Ellis (13) has shown that the anodic etch will not be uniform, however, if small-angle grain boundaries are present at the surface. The dislocations that form the small-angle grain boundaries are the sites of traps which permit hole-electron recombination. The number of holes available for the anode reaction at the dislocation is therefore reduced. 

Another source of additional holes for anodic etching of n-type semiconductors is the avalanche breakdown. The break-... 

A number of different fluoride salts have been used as electrolytes for electropolishing silicon. Not only must the fluoride salt be sufficiently water soluble but the anode reaction product, the corresponding fiuosilicate, must also be readily soluble in water. Potassium fluoride is highly water soluble but the fiuosilicate is not. At a critical anodic etch rate silicon will passivate in KF solutions due to the precipitation of SiFg on the surface. Ammonium fluoride and ammonium fiuosilicate are both sufficiently water soluble. The acids HF and SiFg are even more water soluble. 

There are essentially three different ways how to prepare nanometer sized silicon particles. The porous silicon is, as already mentioned, prepared by anodic etching of silicon wafers in an HF/ethanol/water solution [6, 7]. The microporous silicon has typically a high porosity of 60-70 vol.%, and it consists of few nm thin wires which preserve the original orientation of the wafer. The thickness of the wires varies within the PS layer and the material is very brittle. Free standing PS films can be prepared by application of a high current density after the usual etching of the desired thickness of the PS. 

Reaction Mechanism for Anodic Etching of Silicon in Fluoride Electrolytes. 104... 

As stated in the introduction, wet etching processes may proceed either with or without external current flow. In the former case, the semiconductor crystal is incorporated as an electrode in an electrochemical cell and polarized anodically under illumination or in darkness for n- and p-type samples, respectively, leading to dissolution of the sample (see Sec. 2). This is referred to as the (photo)anodic etching process. 

---