selectiveetch

Selectiveetch is a term used to describe a family of material removal processes in which an etchant or etching condition preferentially removes one material over another within a layered or multi-material structure. This differential etching enables pattern transfer, etch-stop behavior, and selective exposure of underlying layers, which are fundamental to microfabrication and materials processing.

Selectivity is typically quantified by etch-rate ratios, defined as S_A/B equals the etch rate of material A divided by the etch rate of material B. A high selectivity means material B is preserved while material A is removed; low or negative selectivity leads to undesired erosion of protected layers. Processes can be wet chemical or dry (plasma or reactive ion etching). Wet etching uses chemical reactions in solution, offering simplicity and good selectivity for certain material pairs but often suffering from isotropy and undercutting. Dry etching combines chemical reactions with physical sputtering in a plasma, providing higher directionality (anisotropy) and the ability to tune selectivity with gas chemistries, ion energies, and deposition inhibitors.

Common examples include achieving silicon dioxide selectivity over silicon with hydrofluoric-acid–based solutions, or silicon selectivity over silicon dioxide using alkaline etchants. In plasma-based routes, selectivity is influenced by the chemistry of the etch gases, sidewall passivation, and the materials’ surface properties.

Applications span semiconductor device fabrication, MEMS, microfluidics, and optical coatings. Limitations include narrow processing windows, potential damage to sensitive layers, surface roughening, and safety concerns with hazardous chemistries.