Electronic – How is lithography actually used to “print” transistors


In one of my classes, we skimmed over lithography, but mostly the optics side of things (the diffraction limit, liquid immersion to increase angle of incidence, etc).

One point that was never covered is how the light actually dopes the silicon, and creating a transistor. I've tried to stumble around on the net but every article is either way over my head, or way too vague.

In short, how does a focused beam of light directed at a compound like silicon lead to a "printed" transistor, for lack of a better term?

Best Answer

There are multiple steps but the basic process is that you use a photoresist.

At the beginning of a process step, a photoresist is "spun" on to the wafer. It is a very literal thing, they spin the wafer while dripping the polymer onto the surface which spreads out into a thin layer of precise thickness. This is cured and then placed into a photolitographic machine, which projects an image onto the wafer that leaves latent images in the Photoresist (AKA PR).

The PR is developed (some resists are negative and some are positive, which means the exposed areas stay or the exposed areas are eliminated). the development process removes the parts of the PR that are to be removed leaving behind the desired pattern.

The PR can define areas that are etched (removed) or windows through which ions are implanted. Implanting is the process through which the Si is doped.

Once the area is implanted, the remaining PR is removed and the wafer is thermally treated to anneal the implant damage.

In between litho steps are depositions, growths, etches, wet baths, plasma treatments etc.