Oxidizing agents etching

Alkaline etchants are anisotropic. The etch rate for the (111) crystal planes of the Si crystal is smaller by about two orders of magnitude than the etch rate of any other crystal plane. The etch rate ratio between other crystal planes like (100) and (110) depends on etchant concentration and temperature, but doesn t usually exceed a factor of two [Sa6]. Addition of oxidizing agents reduces the anisotropy. The etch rate of (100) Si and Si02 in KOH at different temperatures is shown in Fig. 2.2. 

Etching of silicon in alkaline solutions occurs under evolution of hydrogen with a ratio of two molecules H2 per dissolved Si atom. This ratio is found to be reduced under positive bias [Pa6] or by addition of oxidizing agents like H202 [Sc6], If the anodic bias is increased beyond the passivation potential (PP), the dissolution rate is reduced by orders of magnitude. 

Etchants for defect and junction delineation are usually composed of HF and an oxidizing agent such as HN03 [Dal, Gr4, Ka4, Nel], K2Cr207 [Se5] or Cr03 [Sil, Jel, Sc7, Ya4, Me5]. Alkaline solutions are rarely used for defect delineation [Mal2], An etch pit will form on a silicon surface if the dissolution rate is enhanced locally. Enhancement of the etch rate may occur for various reasons ... 

Under photochemical etching, special demands should be made on the oxidizing agent in the solution. Above all, it must be reduced with the participation of conduction-band electrons alone (see above) and at potentials less negative than the potential of reducing water or H+ ions to molecular hydrogen, in order to avoid gas evolution. 

The removal of electrons from a metal resulting in the formation of positive ions is achieved by the application of an emf (in the case of electroetching) or by an oxidizing agent (in the case of chemical etching). Electrolytic or chemical etching may lead to the formation of soluble or insoluble oxides depending upon the metal and the electrolyte employed. 

It is still difficult to propose simple rules for defining the desirable composition of the electrolyte. Nevertheless many of the electrolytes employed are oxidizing agents, hi order to show the possibilities of these reagents, a comparison will be made of the different techniques proposed recently for the etching of easily oxidized metals, such as Ti, Zr and U. 

When etching is carried out at open circuit potential, these holes must be supplied by an oxidizing agent of a rather positive standard potential, such as Ce [230],... 

The conditions for the formation of stain etch films are similar to the case for chemical etching of silicon in HF or NH4F, except for the the addition of an oxidizing agent. As for any open-circuit process, the etching involes coupled oxidation and reduction reactions. For the FIF/HNO3 case, these are thought to be ... 

In the case of photoetching, the mechanism is again electrochemical, with two partial currents cancelling each other electrically but not chemically [5, 71]. Under illumination, electron-hole pairs are created the conduction band electrons reduce a dissolved oxidizing agent, whereas the holes oxidize the semiconductor. This model implies that at rest potential K, the photoetch rate (i.e., the etch rate under illumination minus the etch rate in the dark, both at a potential corresponding to under illumination) should correlate with the net photocurrent density at Vr under... 

Fig. 9. Schematic representation of the electron transfer process from a surface bond towards an oxidizing agent in solution, (a) Electroless etching, (b) chemical etching, (c) photoetching.

A high etch rate is only obtained when both rates of oxidation and dissolution are high. The etch rate can be increased either by addition of effective oxidizing agents into the solution or by anodic polarization which increases the surface concentration of carriers. As shown by the i-V curve (Fig. 5.1) the anodic dissolution of silicon in HF solutions consists of three regions. At potentials below the current peak, nonuniform anodic... 

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