What is masking corresponding with IC fabrication?

Masking is an important process in IC fabrication that is used at every step whether its doping, oxidization, metallization, passivization, etc.

Masking at its simplest is done as follows:

1. coat the wafer surface with a photosensitive plastic resin
2. let the plastic resin set (sometimes mild heating assists this step)
3. expose the plastic resin to light through a photomask
4. develop the plastic resin with a chemical that dissolves away the unwanted parts (the undissolved parts of the plastic resin form the mask)
5. perform the desired fabrication step
6. remove the plastic resin with another chemical to give a clean wafer surface

More advanced forms of masking to produce ICs with features too small for ordinary light to resolve use x-rays, electron beams, etc. to expose the plastic resin.

The presence of the plastic resin prevents the given fabrication step from affecting that area of the wafer surface while the absence of the plastic resin allows the given fabrication step to affect that area of the wafer surface.

In certain fabrication steps the plastic resin would not be a satisfactory mask itself (e.g. steps performed in a furnace). For these steps a layer of silicon dioxide or silicon nitride is grown on the wafer surface and the masking with the plastic resin is used to etch away the silicon dioxide or silicon nitride which is then used as the mask to complete the difficult fabrication step. The silicon dioxide or silicon nitride layer is then removed.

A similar masking process is used in etching printed circuit boards.

The first known historical examples of masking being used was by metal engravers, who would take the piece of metal to be engraved and paint it with tar. When the tar set they would draw the pattern to be engraved on the tar with sharp tools, exposing bare metal in the scratches. The tar covered metal object was then immersed in a strong acid for a period of time. When removed from the acid the tar would be cleaned off, leaving an acid engraving.