Light turns a circuit design into a physical pattern
The light-sensitive coating is called photoresist. A patterned mask—or, in some systems, a reflective template—controls where ultraviolet light reaches the wafer. In the process described here, exposure changes the resist so the lit regions stay behind when a developer washes away the rest. Other photoresists work the opposite way, removing the exposed regions instead.
The key is that light does not carve the silicon directly. It creates a sharply defined stencil in the coating, which can then guide steps such as etching away exposed material or adding a thin film only in selected areas. The resist is later stripped off, leaving the pattern in the layer beneath it.
Why the same wafer passes through the process repeatedly
A chip is built from many patterned layers: some form transistor components, while others connect them with tiny wires. Each layer requires its own pattern, and the wafer must be lined up precisely with the ones already made. A slight shift can leave a connection misplaced or a transistor misshapen, so alignment is as important as projecting the pattern itself.
The optical system also limits how small a feature can be made. Shorter-wavelength light and carefully designed optics can resolve finer details, but shrinking patterns is not simply a matter of choosing a brighter lamp: the resist, mask, optics, and later processing must all preserve the intended shape.
That is why a microscopic circuit is printed through a sequence of exposures and material-processing steps, not in one flash. A speck of dust or a small alignment error can affect features too small to see, while repeating the patterning process lets a flat silicon wafer become a dense, connected electronic device.
