1. Principle
The principle of wet etching is the conversion of the solid material of the layer into liquid compounds with the help of a chemical solution. The selectivity is very high, since the chemicals used can be precisely tailored to the layers present; for most solutions, it exceeds 100:1.
The process proceeds in three sub-steps, which must occur one after another:
- The reactive species travel from the solution to the surface of the layer to be etched.
- The chemical reaction takes place at the surface, forming a soluble compound.
- The reaction product detaches from the surface and is transported away into the solution.
Whichever of these steps is the slowest determines the behavior of the entire process. If the actual reaction is the bottleneck, the process is called reaction-limited: the etch rate then depends strongly on temperature and increases roughly exponentially with it, but the process etches uniformly across the entire wafer. If, on the other hand, the transport to and from the surface is the bottleneck, the process is diffusion-limited. The temperature dependence is then weaker, but any non-uniformity in the flow directly affects the removal rate – the edges of the wafer are etched faster than the center.
For practical purposes, this means: reaction-limited processes are controlled through very precise temperature control, while diffusion-limited processes are controlled through movement of the solution, i.e. through circulation, rotation of the wafer, or ultrasound. In narrow trenches and contact holes, every process becomes diffusion-limited, because the solution there is hardly exchanged at all. This is one of the reasons why wet etching is not suitable for fine structures.