Definition
A wafer is a thin, round disc of high-purity silicon extracted from sand, sliced thin and polished smooth as a mirror. Many advanced chips, like the CPU that acts as a computer's brain or the storage inside a smartphone, are made on this round silicon disc through precise processing, many at a time.
Slice a log thin and you get pizza dough
Picture a log of slice-and-bake cookie dough being cut into neat round discs. The starting point of making a semiconductor, a marvel of cutting-edge technology, is surprisingly similar.
Silicon extracted from sand is melted with intense heat, then slowly hardened into a giant, flawless cylinder. This is called a silicon ingot. It has almost no impurities and boasts an extreme purity of 99.9999999%.
This big silicon column is sliced precisely with a diamond wire into discs thinner than 1 millimeter. Then the surface is ground until it shines like a mirror, with not even a tiny scratch left. The result is the wafer, the most perfect canvas for etching complex, microscopic circuits.
Hundreds of chips get etched onto one round disc
Just as you spread toppings evenly over a big pizza crust, light and chemicals build up dozens of layers of electronic pathways, too small to see, on the mirror-smooth surface of the wafer (in 3D NAND memory, the storage layers alone can number in the hundreds).
When all the precision steps are done, hundreds of identical square chips are packed tightly across the shiny wafer like a checkerboard grid. Since one wafer stamps out so many parts at once, it's the key to mass production.
The finished wafer is carefully cut along scored lines, with a diamond blade or a laser, into individual square pieces. Each tiny piece is called a "die," or a chip. It only becomes a finished product for electronics once it gets a protective casing against shocks and wires attached.
A bit more precisely: why a circle and not a square?
Finished chips are all square, so why is the wafer, their canvas, round instead of square? A square plate could be filled with chips right out to the corners, and there would be far less wasted scrap.
There's a reason rooted in physics and the manufacturing process. An ingot is made by slowly pulling a crystal upward out of hot, molten silicon while rotating it. So the column that hardens and comes out has a cross-section that is naturally circular.
Also, when chemicals are applied to a wafer, the whole disc is spun at thousands of rotations per minute. A round shape lets the liquid spread evenly instead of pooling in the corners. Today, the most advanced factories around the world use the 300-millimeter wafer, about the size of a large pizza, as their main standard to maximize productivity.
Common misconceptions
A wafer is already a finished semiconductor chip.
A wafer is just the blank base plate (the canvas) before any circuits are etched. It only becomes a finished chip after hundreds of processing steps, then cutting and packaging.
Where you meet it
Slicing thin discs from a cylinder made by melting and hardening the silicon in sand
Making hundreds of smartphone brain chips at the same time from a single 300-millimeter wafer
In one sentence
A wafer is a round disc sliced thin from a silicon column, the basic canvas for stamping out many semiconductor chips at once.