Correct Answer - Option 3 : CF4
Etching refers to the removal of material from the wafer surface.
The process is usually combined with lithography in order to select specific areas on the wafer from which material is to be removed.
There are two main types of etching.
Dry etching and Wet etching
- Dry etching, as the name suggests, is the removal of material in the absence of solvent.
- Plasma etch is a type of dry etching. In the plasma etching, the chemical etchant is introduced in the gas phase.
- During plasma etching, the highly energetic and reactive species produced from the selected process gas.
- Such as O2 or a fluorine bearing gas, bombard and react with the sample surface.
- As a result, the materials at the surface are broken down into volatile and/or smaller molecules which are then removed by the vacuum system.
- For etching silicon oxide, CF4 (tetrafluoromethane) is used.
So, the correct answer is an option (3)
In wet etching, the wafers are immersed in a tank of the etchant (mix of chemicals.
There is a chemical reaction between the wafer surface and the etchants that helps in material removal.
Either a photoresist layer or a hard mask-like oxide or nitride layer is used to protect the rest of the wafer
The process was introduced because wet etching has some limitations in its applicability.
- Wet etching is used for large pattern sizes, usually larger than 2 µm.
- It is an isotropic process - sloped sidewalls rather than straight walls.
- Wet etch has to be combined with subsequent rinse and dry steps. This increases the chances of defects or contamination.
- Hazardous chemicals and conditions are used, so safety is an issue. Safe disposal of chemicals is essential.
- Undercutting and resist peeling off can happen if time is not controlled or etch conditions change during the process.