Hi, I'm Steve Shannon,
Associate Professor in the Department Nuclear Engineering at NC State University.
Today, we'll be talking about chemical wet etching.
Wet etching uses liquid etchings to remove undesired material.
The substrates are placed directly into a bath containing an etching solution.
These etchings are often strong acids or bases.
Because the wafers are submerged in the bath,
both sides of the substrate will be exposed to the etching and can be etched.
The exposed material is etched by chemical processes.
The reaction products are typically soluble in the bath solution or gaseous.
As can be seen in this image,
the user works directly with the etching.
As these are harsh chemicals,
the user works in a fume hood to protect themselves from harmful fumes.
The user also wears the appropriate personal protective equipment including;
safety glasses, a face shield,
chemical resistant gloves, and a chemical apron to
avoid direct contact with etch chemicals.
A chemical reaction between the reactant in the film etches the surface of the film.
Depending on the reactant in the film,
the reaction can be very selective,
only etching the film.
However, wet etching is isotropic leading to pattern under-cutting.
One common process uses
the reactant hydrofluoric acid to etch the thin film silicon dioxide.
Buffering agents are also often added to etch solutions to
maintain PH and keep a consistent chemical reaction over time.
In the case of HF etching,
ammonium fluoride is used as the buffering agent to
prevent depletion of the floride ions and maintain PH.
The etch rate can also be controlled to some degree by the user.
For example, we can add more HF
to increase the number of reactants that reach the surface.
We can also increase the reaction rate by increasing the temperature.
And we can remove the reaction products by stirring,
which will also increase the reaction rate.
After etching, imaging by scanning electron microscopy or
profilometry measurements should be conducted to validate the etching process.
The sample should also be characterized prior to etching.
Here, we can see a list of common material and etchant pairs for wet etching.
It is important to consider all of
the material components when you are selecting an etchant.
For example, if you want to etch
titanium and you are using photo resist as a protective layer,
you may not want to select an etchant like
hydrogen peroxide because it will also etch the photo resist.
One of the main advantages of wet etching when properly done,
is that it uses simple equipment.
The process is straightforward and it also has high throughput.
Wafers can be loaded into the container called the boat.
Boats hold multiple wafers which allow
simultaneous etching of several wafers at one time.
Due to the chemical reaction that occurs during the etching,
the process is highly selective.
One of the main disadvantages of wet etching,
is that the reactions in most cases are isotropic.
This makes etching small geometries very difficult.
It also requires large amounts of chemicals which
can be costly to purchase and dispose of.
The etchants used are also typically very dangerous chemicals.
So safety can be a concern as users work directly with these chemicals.
Further, find process control in these systems is difficult.
This can make reproducibility hard.
When selecting an etching process,
It is important to take these pros and cons under consideration.
This concludes our overview of chemical wet etching. Thank you for joining us today.