From Silicon to Circuitry

From Silicon to Circuitry

The microchips used in computers, mobile phones and electronic appliances throughout the world all have a curious beginning: wafers of crystalline silicon. The journey from silicon wafer suppliers to integrated circuits in your laptop is easily taken for granted, but the process of manufacturing those wafers, refined since their invention in the 60s, is the basis for an immense part of our lives.

Creating the Wafers

Silicon wafers have extremely strict specifications for their creation—very high purity, nearly defect-free and made from a single crystal. To begin, a seed crystal is introduced to a melt of poly-crystalline silicon doped with trace amounts of impurities—boron or phosphorus—required for particular semiconducting properties. This seed crystal is slowly pulled up from the melt to form a cylindrical crystal ingot called a boule. The boule is sliced with a fine wire saw and polished to produce individual wafers. A weak acid bath washes away residual particles, preparing the wafer for fabrication.

Microfabrication

A wafer of silicon crystal is not capable of much yet. To make proper circuits, a lengthy process of microfabrication takes place in a clean room over the course of 10 to 30 days. Select portions of the surface are masked via thin-film deposition and photolithography, inducing a reaction in the deposited coating. Then, the wafer is treated with liquid etchants to wear away layers. The masked patterns are resistant to etching, so patterns are created where the underlying silicon is left untouched. This is repeated multiple times.

Dicing and Testing

A single wafer is used to create many microchips; this is done by dicing the wafer, dividing it into blocks of self-contained integrated circuitry. Before separating the microchips, though, the wafer must be tested via tiny electrical probes that contact each chip to measure conductive properties. Numerous “testability features” built in, as well as careful statistical analysis, speed up the process and allow defective wafers and dice to be filtered out. Finally, the wafer is scored and the dice are separated and packaged as individual chips.

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