Microelectronics is based on the electrical properties of semiconductors, which can be altered by implanting them with impurities, some of which produce positive charges and some, negative charges. Transistors, diodes, capacitors, and other components of electrical circuits can be created on a tiny chip of silicon by altering its electrical properties into a correct pattern of positive and negative charges. The process begins on powerful computer work stations, where engineers design the circuits. Modern integrated circuits have become extremely dense, with thousands of tiny components squeezed on to each chip. To make room for more components engineers design circuitry to be very thin. Once the design is complete, a master image (mask) of the circuit is created.
The next step is microlithography. A silicon wafer is placed in an ``aligner.'' The wafer, which has a layer of oxide to protect its electrical properties, has been coated with photosensitive chemicals. The wafer is below the mask, through which light is focused, much like a negative is used to make prints from camera film. Once the circuit is ``printed'' on the chip, excess chemicals are removed. The printed image of the circuit provides a guide for selective implanting of impurities, which creates the desired electrical properties in the silicon.