Wet etching

Wet etching 
The first etching processes used liquid-phase ("wet") etchants. The wafer can be immersed in a bath of etchant, which must be agitated to achieve good process control. For instance, buffered hydrofluoric acid (BHF) is used commonly to etch silicon dioxide over a silicon substrate. Different specialised etchants can be used to characterise the surface etched.Wet etchants are usually isotropic, which leads to large bias when etching thick films. They also require the disposal of large amounts of toxic waste. For these reasons, they are seldom used in state-of-the-art processes. However, the photographic developer used for photoresist resembles wet etching. As an alternative to immersion, some machines employ a gas (usually, pure nitrogen) to cushion and protect one side of the wafer while etchant is applied to the other side (usually the back). This etch method is particularly effective just before "backend" processing (BEOL), where wafers are normally very much thinner after wafer backgrinding, and very sensitive to thermal or mechanical stress. Etching a thin layer of even a few micrometres dramatically increases the wafer strength at this stage.

Further information:

1. Wolf, S.; R.N. Tauber (1986). Silicon Processing for the VLSI Era: Volume 1 - Process Technology. Lattice Press. pp. 531–534
2. Wolf, S.; R.N. Tauber (1986). Silicon Processing for the VLSI Era: Volume 1 - Process Technology. Lattice Press. p. 546
3. Bahadur, Birendra (1990). Liquid Crystals: Applications and Uses. World Scientific. p. 183
4. Walker, Perrin; William H. Tarn (1991). CRC Handbook of Metal Etchants. pp. 287–291
5. Kohler, Michael (1999). Etching in Microsystem Technology. John Wiley & Son Ltd. p. 329
6. В. В. Усова, Т. П. Плотникова, С. А. Кушакевич. Травление титана и его сплавов
7. М. Беккерт, Х. Клемм. Способы металлографического травления. Справочник
8. Article Wet etching from Wikipedia, the Free Enciclopedia. Available under the license Creative Commons Attribution-Share Alike.

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