Essays/linkedin/26-01-23 semicon
🎰 Making a wafer requires anywhere between 500 and 1500 process steps. Sometimes it's easy to forget how many things need to work together to deliver a chip that fits in the palm of your hand, goes to space, or sits in a server.
While lithography is the step that takes all the views, mostly due to the challenges around, wafers have layers, and each layer has to be constructed step by step. These are the most common steps in the process:
Material is deposited on a pristine silicon wafer: conductive, dielectric, etc. depending on what is being built. These thin films have to be precisely created, with very tight layer tolerances.
On top, a layer of photoresist is applied. This layer is crucial, since it's the one that will be exposed to the lithographic process.
Photolithography transfers a shape from mask onto the resist. The wavelength defines the resolution, but also the type of polymer that can be used as a resist. The areas exposed to light become soluble, exposing the underlying layer.
Etching removes the soluble part of the resist and the underlying material. Of course etching has to be done precisely, not to disrupt underlying layers already in place.
Ion implantation allows to tune the electrical properties of the material. This is what makes transistors possible, fine-tuning the band gap of silicon, doping just the junction, etc.
Finally, the photoresist is lifted off, and the wafer is polished to achieve a very flat surface, ready for the next layer.
All these steps are repeated cyclically, each one creating a single layer in a wafer. Today, some chips have 175 layers. Those are more than 1000 process steps.
Finally, wafers are cut (diced) into individual chips, and packaged. This last step is also fundamental for stability and inter-connectivity. The black boxes surrounding almost every chip are there to ensure thermal and mechanical stability, including the small exposed legs that allow the chip to be connected to other components.
Each step in the wafer fabrication process requires top notch machines. From those doing the depositions, to those inspecting the results. Tolerances are incredible tight, cleanliness is super strict.
Going from vacuum tubes to this level of control took few decades. What will the next few decades bring?
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