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[Industry knowledge] What is a wafer?

2020-09-19

In semiconductor news, there are always fabs marked by size, such as 8-inch or 12-inch fabs. However, what is the so-called wafer? What part does 8 inches refer to? How difficult is it to produce large-size wafers? The following will introduce step by step the most important foundation of semiconductor-what is "wafer".

Wafer is the basis for manufacturing all kinds of computer chips. We can compare chip manufacturing to building a house with Lego bricks, stacking one after another to complete the shape (that is, various chips) that we want. However, if you don't have a good foundation, the built house will be crooked and not what you want. In order to make a perfect house, you need a stable substrate. For chip manufacturing, this substrate is the wafer that will be described next.

First of all, recall that when we were playing with Lego bricks when we were young, there will be small round protrusions on the surface of the bricks. With this structure, we can stack two bricks firmly without the use of glue. . Chip manufacturing also uses a similar way to fix the subsequently added atoms and the substrate together. Therefore, we need to find a substrate with a neat surface to meet the conditions required for subsequent manufacturing.

Among solid materials, there is a special crystal structure ─ ─ Monocrystalline. It has the characteristic that atoms are closely arranged one after another and can form a flat atomic surface. Therefore, the use of single crystals to make wafers can meet the above requirements. However, how to produce such a material? There are mainly two steps, namely purification and crystal pulling, after which the material can be completed.

How to manufacture single crystal wafers

Purification is divided into two stages. The first step is metallurgical purification. This process is mainly to add carbon to convert silicon oxide into silicon with a purity of more than 98% by means of redox. Most metal extractions, such as iron or copper, use this method to obtain metals of sufficient purity. However, 98% is still not enough for chip manufacturing and still needs further improvement. Therefore, the Siemens process will be further used for purification, so that the high-purity polysilicon required for the semiconductor process will be obtained.

Next, is the step of pulling crystals. First, melt the high-purity polysilicon obtained above to form liquid silicon. After that, the single crystal silicon seed (seed) is in contact with the surface of the liquid, and it is slowly pulled up while rotating. As for why single-crystal silicon seeds are needed, it is because the arrangement of silicon atoms is the same as that of people queuing up. It will need to be the first to allow later people to arrange them correctly. Silicon seeds are the important leader, so that later atoms know how to line up. Finally, after the silicon atoms leaving the liquid surface solidify, the neatly arranged single crystal silicon columns are completed.

However, what do 8 inches and 12 inches represent? He refers to the diameter of the crystal column that we produce, which looks like a pencil barrel, whose surface is processed and cut into thin discs. As for the difficulty of manufacturing large-size wafers? As mentioned earlier, the process of making crystal pillars is like making marshmallows, forming while rotating. If you have made marshmallows, you should know that it is quite difficult to make large and solid marshmallows, and the process of pulling crystals is the same. The speed of rotation and the control of temperature will affect the quality of the crystal column. Therefore, the larger the size, the higher the speed and temperature requirements of crystal pulling, so it is more difficult to make high-quality 12-inch wafers than 8-inch wafers.

However, a whole piece of silicon pillar cannot be made into a chip manufacturing substrate. In order to produce a piece of silicon wafer, then the silicon crystal pillar needs to be cut into wafers laterally with a diamond knife, and the wafer can be polished to form a chip. Manufacture the required silicon wafers.

Source: The content comes from the WeChat official account of "Flash Market", thank you.


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