is an integral part of the manufacture
of integrated circuits
. It allows one to etch certain parts of a substrate (such as a doped silicon wafer
) while leaving others intact. Here is a brief procedure
for a photolithography step:
Photoresist (positive or negative) is spun onto the wafer to be processed at a set rotational velocity to insure an even coating across the surface of said wafer.
The wafer is placed in a pre-bake oven to harden the photoresist.
Next, the wafer is secured into a mask aligner and a test pattern is used to aid in aligning the wafer so that successive masks overlap perfectly.
A UV light is used to expose the image from the mask onto the photoresist-covered wafer for a pre-determined amount of time. Depending on the type of photoresist used, this light causes the photoresist to either harden or become more soluble when immersed in a developing solution.
The wafer is placed in developer, causing the exposed photoresist to either harden or strip away.
A buffered oxide etch is then used to etch exposed areas of the wafer. This can be used to open up areas which will then be doped by diffusion, creating n-wells or p-wells. Likewise, these areas could undergo a metallization process to create contacts. HF, or hydrofluoric acid, which is present in a buffered oxide solution, will also do a number to your bones about 6-8 hours after contact. The time used for this etching need to be controlled precisely to prevent errors such as overcut/undercut.
Finally, a warm stripper solution is used to remove the remnants of the photoresist.
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