How are Pentium chips manufactured?

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1. How are Pentium chips manufactured?

Semiconductor manufacturing process can be summarized in the following steps:

Chip design: Where engineers design the chip, i.e. how it will work. From this step come several masks (which are sort of blueprints on how the chip must be manufactured) that will be used on the wafer fabrication.
Wafer fabrication: This is the main chip manufacturing process, which we will be exploring on this tutorial.
Die preparation: This step consists basically in cutting the chips from the wafer.
Packaging: In this step terminals and a body are added to the chip.
Testing: The chip is tested and then sold.
Each one of these steps can be broke down into several other steps.

When we say "chip manufacturing" usually we think about the wafer fabrication step, which is the most complicated one. That is the step we will be explaining in this tutorial.

Raw Wafer Fabrication Process
The wafer is the substrate where the chips will be built on. Raw wafers are made of silicon, which comes from beach sand. They are created thru a method called Czochralski process, where a seed crystal (a piece of silicon crystal) is mounted on a rod and then dipped into molten silicon. The rod is pulled upwards and rotated at the same time, making a big cylindrical piece of silicon crystal, also known as ingot.

The ingot resulted from this process measures from three to six feet (one to two meters) long and can have up to 12 inches (300 mm) in diameter (this is where terms like 12-inch or 300-mm wafers come from). The ingot is then sliced into wafers. These wafers are polished and sent to the chip manufacturers. As mentioned, these raw ("virgin") wafers are where the chips will be manufactured on.

Figure 1: The ingot is sliced to create raw wafers.

One common question is why wafers are rounded and not squared. The answer is simple. Since thru the Czochralski process the ingot is created by pulling and rotating the molten silicon, the natural shape for the silicon crystal resulted from this process is rounded, not squared.

Photolithography
Chips are mounted on the wafer thru a process called photolithography. Under this process, chemicals sensitive to ultraviolet light are used. When exposed to ultraviolet light, they can become "soft" or "hard". So basically this process consists in blocking the ultraviolet light from the chemicals applied to the wafer using stencils (the masks created by the engineers), removing the "soft" parts, and then repeating the process again with another mask, until the chip is finished.

Figure 2: How photolithography works.

Of course each mask has a different pattern and they tell how the transistors and wires inside the chip will be manufactured. The numbers of masks used vary depending on the project. A Pentium 4 processor, for example, uses 26 masks.

Let's see exactly how this process is done.
The first thing that is done to the raw wafer is to grow silicon dioxide (SiO2) on it, by exposing the wafer to extreme heat and gas. This growth is similar to the way rust grows on metal when ...