Etching is a process that removes a thin film from a defined area after patterning the wafer using photolithography. This process can be used to create circuits on the wafer.
Of course, you can create the desired thin film shape using the Lift-off or Damascene method, but the Lift-off method is difficult to create a fine pattern, so it is only used at the laboratory level, and the Damascene method, which creates a trench to create the desired thin film, requires an additional CMP (Chemical Mechanical Flashing) process, so the etching process is mainly used to optimize the process.
Our lab used the Lift-off method for patterning during our master's degree period. For more information on this issue, please refer to this article.
Types of Etching
The etching process is broadly divided into wet etching, which uses a chemical solution, and dry etching, which does not use a solution.
Wet etching
Wet etching is a method of removing a thin film on a wafer by inducing a chemical reaction with a chemical solution. For the solution to react with the thin film, contact between the surface and the solution is required. This chemical reaction between the chemical and the thin film is facilitated by mechanically moving the process equipment or circulating the solution.
Wet etching offers the advantages of fast etching rates and excellent selectivity depending on the type of thin film and chemical solution. However, because the chemical reaction proceeds isotropically and in all directions, it can cause undercutting, etching even beneath the masked area, as shown in the figure below. Furthermore, due to surface tension, it has the disadvantage of not etching fine patterns.
Dry etching
Dry etching uses reactive gases instead of chemical solutions to etch. Similarly, because it requires a chemical reaction with the thin film, the gas is typically used in a plasma state.
While it offers the advantage of being usable for micropatterning, it also has the disadvantages of slow etching speed and high cost. A number of other issues also arise.
Etch rate
Etch rate refers to how fast etching can proceed.
Naturally, the faster the etch rate, the better the productivity. The etch rate can be controlled by the type of gas required for the reaction, the gas flow, and the energy of the ions when plasma is generated.
Also, although less than wet etching, undercut occurs (etch bias), which is an important factor in fine patterns and should be considered.
Selectivity
Dry etching involves both chemical etching and physical etching via plasma. Therefore, not only the thin film being removed is etched, but also other materials. Because the etching rate differs from the target material, the thicknesses of the two materials etched after the process differ. This ratio of the target material to be etched and the remaining material is called the etching selectivity. A higher selectivity indicates a more favorable process environment.
If you look at the picture below, you can see that when we tried to remove the BPSG, a small amount of Si was also etched away after the BPSG on top of the Poly-Si was removed.
Etch uniformity
Etch uniformity refers to how evenly etching occurs across the entire wafer. Poor uniformity results in a poor post-etch profile, degrading characteristics and yield, negatively impacting chip performance.
Dry etching profile
In summary, dry etching
must have a sufficiently fast etch rate,
high selectivity,
a well-defined process recipe to ensure an ideal profile,
and uniform etching with good reproducibility.
Wet etching vs Dry etching
Finally, we will summarize and conclude by discussing the differences between wet etching and dry etching.
| wet etch | dry etch | |
|---|---|---|
| Method | chemical reaction (solution) | Physical, chemical reactions |
| Environment / Equipment | Atmosphere / Bath | Vacuum / Chamber |
| Advantage | – Low cost, easy process – Fast etching speed - Good selectivity | – Good accuracy – Fine patterning possible |
| Disadvantage | – Poor accuracy – Wafer contamination – PR undercut | – High cost, difficult process – Slow etching speed - Bad selectivity |
| Etching direction | isotropic | Anisotropy |
References: Samsung Electronics
