During reactive ion etching (RIE), gaseous species from the plasma react with the surface atoms forming compounds or molecules. These species then leave the surface thermally, if the vapor pressure is high enough, or as a result of ion bombardment. Depending on the specific nature of the individual solid and gas species, the reaction may be rate-limited by the production rate of reactive species in the plasma, by the surface reaction rate, or by the emission rate of products. In many cases the ion bombardment is very useful to maintain the directionality, or anisotropy, of the etching process. The ion bombardment may clean the surface, allowing the reaction to occur; it may stimulate the reaction itself; or it may help desorb or detrap the product molecule. In many of these reactive ion etching chemistries, a product is formed that is not volatile. In some cases this effect could be exploited to protect vertical walls from isotropic chemical etching, while the same protecting film is removed from the bottom surface by energetic ions. Obviously, depending on the available starting gases, several different process can be realized. This aspect make the development of fluorine based process highly preferred both for safety and economical reason (safe environments are quite expensive to realize and all the necessary safety procedures are normally time consuming.