Why choose a mask with a high selection ratio for dry etching?

What is the etching ratio of a mask?

s shown in the above figure, resist acts as a mask to etch the Si substrate. Places covered by the photoresist are not etched, and places not covered by the photoresist are etched.
At the end of dry etching, the mask (photoresist) and silicon are eaten d1, d2 thickness respectively, then the etching selection ratio is:

   Selectivity=d2/d1

The larger the selection ratio, that is, the material that needs to be etched is etched quickly, but the mask is etched slowly.

What are the benefits of a high selection ratio?
1. Thinner masks can be made
The mask material with high selection ratio has minimal loss during etching, and even if the mask is very thin, it can protect the underlying pattern during the entire etching process.
2, can achieve accurate etching
The thinner the mask, the more precise the shape of the image can be controlled.
If the selection ratio of the mask is too low, the rate of the mask etching is too fast during the etching process, then it is more difficult to control the rate of the etching material, and the actual etching rate of the target material cannot be obtained.
3, for the underlying material below the target material, it has a good protective effect, thereby reducing the damage and defects caused by etching.