Chemical dry etchings fall into two categories depending on the etching principle.
1. A more chemical etching refers to a process in which the substrate is attacked by radicals and converted into volatile compounds. This procedure is highly material-selective since the etching gasses specifically attack certain substrate materials. Silicon or silicon dioxide for example is quickly converted in the presence of CF4 into the volatile SiF4 and consequently removed in this process. The procedure is carried out at relatively high pressure (approx. 1 Torr). One consequence of the “high” pressure is that particles increasingly show isotropic behavior. The etching process not only proceeds in downward direction, but also a certain amount of lateral underetching takes place. The following gives an example of the chemical reaction:
1. e- + CF4 → CF3 + F + e-
2. Si + 4F → SiF4
The CF4 is decomposed by electrons and radicals are formed which react in a second step with silicon. The resulting SiF4 is a volatile gas and escapes. The etching rates can be adjusted specifically. An addition of oxygen to CF4 increases the etching rate for silicon, but the oxygen content also leads to an increased attack of the resist mask (ashing). Addition of hydrogen will reduce the etching rate in the above described system.
2. Secondly, physical effects are exploited for this purpose. Ions are accelerated in an electrical field and collide with high energy with the substrate. Similar to the effect of sandblasting, the material to be processed is successively removed. This process takes place at low pressures (up to 0.01 Torr). Due to the linear acceleration, an anisotropic etching results and structures e.g. in silicon show vertical side walls.
Both reaction principles are used for dry chemical etching procedures, either alone or in combination. Plasma etching is a radical etching procedure, while the physical principle is used in the so-called sputtering (sputter etching). Combined mechanisms are used for the reactive ion etching (RIE) or the reactive ion beam etching (RIBE).
During all dry etching procedures, the resist mask is subjected to a high thermal load, and particularly intense in this respect is sputtering. A cooling in the plasma etcher can reduce this effect. An additional bake of the resist mask (tempering) or UV curing (see UV curing) stabilizes the resist. It is therefore generally recommended to utilize temperature-stable resists for dry etching processes (see temperature-stable negative resist).