Lithography

Lithography is a method of printing an image by applying patterned layers of color to paper with a series of etched metal or stone plates. This is the process used to print many newspapers and multi-colored lithographs. It is also the general name for the techniques used to fabricate integrated circuits (ICs).

Making integrated circuits

In the manufacture of integrated circuits, the silicon wafer that acts as the base and a light-sensitive polymer material, called photoresist, are used to create the pattern of the circuit's layers. Negative photoresists harden when exposed to light, adhering to the base through the developing process. Positive photoresists degrade when they are exposed to light and developed, leaving a depression. The coated wafer is then dried 10-30 minutes in an oven at 176–194°F (80–90°C).

After photoresist is applied to the silicon wafer, it is selectively exposed to light with the aid of a reticle. A reticle consists of a layer of patterned chrome on transparent glass; gaps in the chrome permit light to reach the resist-covered wafer. Exposure takes place in a device, called a stepper, that shines light on the wafer through the transparent regions of the reticle. Only a small region of the wafer is exposed at a time. Then the wafer is moved, or stepped, forward and a new segment is exposed. After exposure, the wafer is put into developing solution where the positive and unexposed photoresists are removed. It is hard-baked at temperatures between 248–356°F (120–180°C).

After the photoresist is in place, a layer of conducting, semiconducting, or insulating metal solution is applied to the wafer and adheres in the pattern opposite to the photoresist. The application and removal of photoresist and metal solutions is repeated 10-20 times in the manufacture of a single integrated circuit. In addition to the number of steps needed to make the circuit, the complexity of the task is increased by the necessity for precision in the manufacturing process. For instance, some circuits use printed features as small as 0.35 micron. A human hair, on the other hand, is about 100 microns in diameter.