Question

What process would you use to make regions of silicon resistant
to etching by KOH?

Answer 1

To make silicon resistant to KOH etching, a protective layer of silicon dioxide is applied and patterned using photolithography. This layer acts as a mask during the etching process, ensuring only the unprotected regions are etched away, leaving the designated regions intact.

Step-by-step explanation:

To make regions of silicon resistant to etching by KOH, you would need to follow a process of creating a protective layer over the silicon that KOH cannot penetrate. This process might involve depositing a thin film of silicon dioxide (SiO₂) or metal that can act as a mask. Silicon dioxide is a common choice because it forms a natural barrier to etchants like KOH. Here's a general step-by-step process to achieve this:

1. Clean the silicon wafer to remove any organic residues and to ensure a uniform oxide layer can be grown.
2. Grow or deposit an oxide layer on the silicon wafer. This can be done thermally in a furnace (thermal oxidation) to create a layer of silicon dioxide.
3. Apply a photoresist layer on top of the oxide, and then use photolithography to pattern the photoresist with the desired design. This includes exposing the photoresist to UV light through a mask, and developing the pattern by removing either the exposed or unexposed photoresist, depending on whether a positive or negative photoresist is used.
4. Use the developed photoresist to protect areas of silicon dioxide during an etch process. This etch process, often using buffered hydrofluoric acid (BHF), removes the unprotected oxide, leaving the desired pattern of oxide on the silicon.
5. Remove the remaining photoresist, typically with a solvent or oxygen plasma, leaving the patterned silicon dioxide on top of the silicon.
6. Finally, the silicon wafer is exposed to the KOH etchant. The silicon regions protected by the patterned silicon dioxide will be resistant to the KOH, while the exposed silicon will be etched away.

The silicon dioxide serves as an etchant barrier, and the patterned wafer can now undergo KOH etching without affecting the protected areas.