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Dry etching

Dry etching, also known as Plasma etching, is the universal tool for structure etching since about 1980s.

What is Plasma etching? In the most simple way of looking at it, you just replace a liquid etchant by a plasma. The basic set-up is not unlike sputtering, where you not only deposit a layer, but etch the target at the same time.

So what you have to do is to somehow produce a plasma of the right kind between some electrode and the wafer to be etched. If all parameters are right, your wafer might get etched the way you want it to happen.

Here we compare chemical etching and plasma etching for the same materials to be etched - lets take SiO₂

Chemical etching of SiO₂	Plasma etching of SiO₂
Etchant: HF + H₂O (for etching SiO₂.	Gases: CF₄ + H₂ (or almost any other gas containing F).
Species in solution:: F^–, HF^–, H⁺SiO₄^2–, SiF₄, O₂ - whatever chemical reactions and dissociation produces.	Species in plasma and on wafer: CF_x⁺ (x £ 3), and all kinds of unstable species not existent in wet chemistry. Carbon based polymers, produced in the plasma which may be deposited on parts of the wafer.
Basic processes: SiO₂ dissolves	Etching of SiO₂, formation of polymers, deposition of polymers (and other stuff) and etching of the deposited stuff, occurs simultaneously
Driving force for reactions: Only "chemistry", i.e. reaction enthalpies or chemical potentials of the possible reactions; essentially equilibrium thermodynamics	Driving force for reactions: "Chemistry", i.e. reaction enthalpies or chemical potentials of the possible reactions, including the ones never observed for wet chemistry, near equilibrium, and non-equilibrium physical processes", i.e. mechanical ablation of atoms by ions with high energies.
Energy for kinetics: Thermal energy only, i.e. in the 1 eV range	Energy for kinetics: Thermal energy, but also kinetic energy of ions obtained in an electrical field. High energies (several eV to hundreds of eV) are possible.
Anisotropy: None; except some possible {hkl} dependence of the etch rate in crystals.	Anisotropy: Two major mechanisms 1. Ions may have a preferred direction of incidence on the wafer. 2. Sidewalls may become protected through preferred deposition of e.g. polymers Completely isotropic etching is also possible
Selectivity: Often extremely good	Selectivity: Good for the chemical component, rather bad for the physical component of the etching mechanism. Total effect is open to optimization.


	The two perhaps most essential parameters are: 1. the relative strength of chemical to physical etching, and 2. the deposition of polymers or other layers on the wafer, preferably on the sidewalls for protection against lateral etching.
		The physical part provides the absolutely necessary anisotropy, but lacks selectivity
		The chemical part provides selectivity.
	Polymer deposition, while tricky, is often the key to optimized processes. In our example of SiO₂ etching, a general finding is:
		Si and SiO₂ is etched in this process, but with different etch rates that can be optimized
		The (chemical) etching reaction is always triggered by an energetic ion hitting the substrate (this provides for good anisotropy).
		The tendency to polymer formation scales with the ratio of F/H in the plasma. The etching rate increases with increasing F concentration; the polymerization rate with increasing H concentration.
		Best selectivity is obtained in the border region between etching and polymer formation. This will lead to polymer formation (and then protecting the surface) with Si, while SiO₂ is still etched. The weaker tendency to polymer formation while etching SiO₂ is due to the oxygen being liberated during SiO₂ etching which oxidizes carbon to CO₂ and thus partially removes the necessary atoms for polymerization

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