Microprocessor Design/Photolithography

Microprocessor Design

The current state-of-the-art process for manufacturing processors and small ICs in general is to use photolithography. Photolithography is a complicated multi-step process.

A Photolithography lab.

Wafers

A wafer is a large circular disk, typically made of doped silicon. Each wafer can hold multiple chips arranged like tiles. The number of chips per wafer is known as the yield.

Basic Photolithography

Diagram of Photolithography process.

In photolithography, there are typically two important chemicals: an acid and a resist. A photo-negative of the design is exposed to light, and the pattern is projected onto the wafer. Resist is applied to the wafer, and it sticks to the portions of the wafer that are exposed to light. Once the resist is applied to the wafer, it is dipped in the acid. The acid eats away a layer of everything that is not covered in resist. After the top layer has been disolved, the wafer is washed (to remove any remaining acid and resist), and a new layer of doped silicon is applied to the top of the wafer. Once the new layer of silicon has been applied, the process is repeated again.

The first two applications of resist are used to convert thin, carefully shaped regions of the base silicon wafer into n-type and p-type w:doping (semiconductor) (no net material is added or taken away after these steps). Wait up -- I thought doping occurred after the polysilicon was added? Is that an additional doping stage, or is doping not really the first 2 stages?

After that, layers of polysilicon, silicon oxide, and metal are added, coating the entire wafer. After each layer of desired material is added, resist and acid are used to "pattern" the layer, keeping the desired regions and removing the undesired regions of that layer.

Packaging

After all the layers specified by the design have been applied, the wafer is "diced" into individual rectangular "die". Then each die packaged.

... does testing happen before the wafer is diced? Before and after? ...

further reading

• MOSIS (Metal Oxide Semiconductor Implementation Service) is probably the oldest (1981) integrated circuit (IC) foundry service. Many VLSI students have sent their chips to MOSIS for fabrication.