What is a CPU wafer

This is how the production of CPUs works

Friedrich Stiemer

Processors are extremely complex chips that are essential for the operation of PCs. Today's models perform millions of arithmetic operations per second. They are the result of decades of research and engineering. Because the transistors - basically the building blocks of every processor - have been shrinking for years. Since they are now microscopic, manufacturing is becoming more and more complicated.

EnlargeThis is how the production of CPUs works

Because of this, manufacturers cannot produce a processor using conventional methods. In the meantime, specialized 3D printers and precision machines are suitable for extremely complex things, but they usually only achieve an accuracy in the micrometer range (one millionth of a meter). Today's chips are manufactured on a nanometer scale, which corresponds to a billionth of a meter.

For this reason, companies make do with so-called photolithography. With this technology, an image is transferred to the chip surface with the help of light - similar to outdated overhead projectors, but vice versa. Because the template itself is reduced to the required precision. This makes it unnecessary to have complex machines perform extremely precise movements.

The image is projected onto a so-called silicon wafer, which is processed with the highest precision under strict controls in the laboratory. Even a single speck of dust could destroy the entire wafer and thus thousands of euros. The wafer itself is coated with a photoresist that reacts to light and is washed away after transfer. What remains is the transferred engraving, which is filled with copper. This process repeats several times to form the transistors.

But downsizing the transistors doesn't matter as long as they don't work properly. Since we are in the nanometer range here, manufacturers run into various physical problems. Transistors are supposed to prevent the flow of current when they are switched off. But since these transistors are so small, the electrons can now simply flow through them. This tunnel effect poses massive problems for the engineers. In addition, even photolithography is reaching its limits of precision. Manufacturers try to get around this problem by using extremely ultraviolet light. This light has such a long wavelength that the human eye can no longer perceive it. However, the problem remains as the transistors continue to shrink.

EnlargeThis is what a silicon wafer looks like - in this case from Intel. It belongs to the most sensitive part of a CPU.

However, defective chips can be sorted out relatively well during what is known as binning. This procedure tests the quality of the chips. If a core of the processor is found to be defective, it is deactivated and sold for less. In fact, many CPU generations share the same blueprint, but with the cores disabled, they are sold at more affordable prices. However, if a defect affects more critical components such as the cache, the chip is rejected. The result: the yield drops. The sales prices increase accordingly. Especially with chips that are manufactured using the 7 or 10 nanometer process, the defect rate is quite high, which makes them more expensive.

In the next step in the CPU manufacturing process, the silicon wafer must be prepared so that it can be connected to the rest of the system. For this purpose, the silicon is combined with the so-called PCB, i.e. the printed circuit board - it is precisely these components that most users would call the actual CPU. Here, too, the highest precision is called for, because this sensitive combination is mounted on a silicon circuit board, which in turn has a large number of electrical connections to connect to the main circuit board. Since a CPU generates a lot of heat, the components must be protected accordingly. For this purpose, a heat spreader is integrated on the top. It has direct contact with the chip surface and conducts the waste heat to the CPU cooler, without which the processor cannot be operated. The CPU is now ready for sale and ends up in stores and online shops with the appropriate packaging.

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