- Etch-Stop is defined as a technique that allows termination of the etching process at a controllable depth.
There are two popular techniques used in etch stop :-
- Dopant-controlled Etch stop :- A peculiar phenomenon that can be used to control the etching of silicon is that doped Silicon Substrates, whether they are doped with boron for p-type silicon or phosphorus or arsenic for n-type silicon show a different etching rate than pure silicon. In this case when the isotropic HNA etchants are used, the P or n-doped areas are dissolved significantly faster than the undoped regions. However, excessive doping of boron in silicon for faster etching can introduce lattice distortion in the silicon crystal and
thus produce undesirable internal (residual) stresses.
- Electrochemical Etch Stop :- This technique is popular for controlling anistropic etching. As illustrated in fig below, a lightly doped p-n junction is first produced in the silicon wafer by a diffusion process. The n-type is phosphorous doped at $10^{15} / cm^{3} $ and the p-type is boron doped at 30Ω cm.
The doped silicon substrate is then mounted on an inert substrate container made of a material such as sapphire.
The n-type silicon layer is used as one of the electrodes in an electrolyte system with a constant voltage source as shown in figure below.
As we may observe from the arrangement in the figure, the unmasked part of the p-type substrate face is in contact with the etchant. Etching thus takes place as usual until it reaches the interface of the p-n junction, at which point etching stops because of the rate difference in p and n-doped silicon.
Consequently, one can effectively control the depth of etching simply by establishing the p-n silicon boundaries at the desired locations in a doped silicon substrate.
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