Chemical etching is basically a process used to remove selectively controlled amounts of material from the substrate by a solution. The etch depth can be designed to be just tens of angstroms or up to a few hundred of micrometers. Concerning the fabrication of microelectronic devices, the etching process is very important at different fabrication stages. Errors during the etching process will severely impair the performance of devices. More than that, due to the disadvantages of dry etching, among which is the need for specialized and expensive equipment, chemical etching is still unavoidable .
Non-oxidative chemical etching is a potential-independent dissolution and does not involve exchange of free charge carriers between the solid and solution. This concept was introduced for III-V compounds by Gerischer et al [37, 38]. They suggested that during a purely chemical etching process a synchronous bond-exchange occurs. As a result, the bonds between the atoms in the solid break and new bonds are formed with reactive molecules from the electrolyte. This process is shown schematically in Figure 3.2.
Figure 3.2: A schematic representation of the model proposed by Gerischer et al for chemical etching of III-V compounds.
In order to etch chemically a III-V compound according to Gerischers model, it is necessary to have a non-oxidizing chemical enchant, such as undissociated molecules of HCl, HBr, Br2 etc. . These molecules should be capable to break the III-V bonds and consequently to saturate the resulting dangling bonds. Therefore, the chemical etching rate depends strongly on the concentration of undissociated molecules in the electrolyte. It is well known that the number of undissociated acid molecules (HCl, HBr) increases in aqueous solutions by increasing the concentration of the acid. On the other hand, the number of undissociated molecules can be also increased by using a solvent like acetic acid instead of water. Consequently, in such solutions the chemical etch rate will be strongly increased.
A chemical dissolution reaction is usually divided in some slow and fast reaction steps. The slowest step in the whole process is the rate determining one. For example, during the chemical reaction of GaAs in H2O2 solution, breaking of a Ga-As bond, followed by the formation of GaOH and AsOH bonds, is the rate-determining step. The subsequent steps are much faster than the first one, therefore they do not contribute much to the overall dissolution rate.
For pore formation purposes, chemical etching can be used as a pre-anodization treatment, e.g., for generation of randomly or uniformly distributed defects at the surface of the samples, in order to improve the nucleation stage of the pore formation process. Pyramid-like pits on Si surfaces, formed by means of a mask and applied chemical etching in KOH solutions, are good examples for controlled generation of surface defects by means of pure chemical etching.
It is important to note that chemical etching can significantly influence the total etching rate of III-V semiconductors during the pore formation process . This should be taken into account when choosing the concentration of the electrolyte for electrochemical etching. By increasing the concentration of the acid in solution, the number of undissociated acid molecules increases as well, and thus the chemical etch reaction can proceed in parallel with the electrochemical etch reaction. This is normally not desired during a pore formation process due to the fact that a chemical reaction is less anisotropic as compared with electrochemical ones. This could increase uncontrollably the diameters of the pores up to the destruction of the porous structure.
However, chemical reactions could be of interest for increasing pore diameters after the porous structure was already formed. As mentioned in Chapter 1 - Porous silicon, the pore wall thicknesses are mainly determined by the double width of the space charge region. Thus, for a given doping of the substrate we do not have the freedom to decrease the pore walls arbitrarily. This barrier can be overcome by chemical etching after the electrochemical process finishes.