A differentiation of the etching procedures with respect to their spatial effect is of particular importance for the generation of small structures. The spatial distribution of the etching results determines (in addition to the mask geometry) to a great extent the three-dimensional shape resulting from etching-technical treatment. The spatial efficiency of the etchant depends on the local rate-limiting factor. If a chemical reaction on the surface is involved, the local distribution of the limiting material property is projected. This is the basis for etching processes applied for the generation of material textures, e. g., as used in microscopy. The local geometries created for amorphous, partially amorphous or polycrystalline materials often only exhibit short characteristic lengths, the orientation of the grains and crystallites and sometimes also their shape are distributed irregularly and are therefore not applicable for a controlled shaping process. On the other hand, anisotropy can be used for shaping in the case of mono-crystalline materials. More than three orders of magnitude difference in etching rate between different crystal orientations can be controlled by selection of the etch bath
composition. One example is the highly reproducible etching of Si(111) facets of monocrystalline Si.
When the transport of particles to and from the surface is rate-limiting (rather than the surface reaction), the distribution ofthe direction ofthis transportation determines whether isotropic or anisotropic etching dominates. In the case of frequent strikes between particles involved in the etching, which means it is controlled by diffusive transportation, their trajectories are isotropic. Therefore, an isotropic etching occurs, yielding hemispheric profiles under the mask edges. For an isotropic concentration distribution, the attack of the etchant occurs perpendicular and parallel to the substrate plane at the same rate (Fig. 44). Isotropic etching is observed for many wet etching processes, but also for etching procedures with reactive gases or plasmas, as long as the pressure is not too low and the kinetic energies of the particles are predominantly in the thermal range.
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