【Creative Cultivation Program】Why aren’t chips made into round shapes?

Author: Zhou Lei

Whether you are an electronics enthusiast or not, I believe most people are familiar with the word "chip". From TVs, refrigerators, air conditioners to mobile phones, watches, razors, it can be found in almost every corner around us. But no matter what device it is, the shape of the chip inside is rectangular. Why can't the chip be made into a circle or other shapes?

To explain this problem, we have to start with chip manufacturing.

There are four major processes in chip manufacturing. The upstream IC (Integrated circuit) design company designs the circuit diagram according to the requirements, and the silicon wafer manufacturing company manufactures the silicon wafer. The midstream IC manufacturing company transplants the designed circuit diagram onto the silicon wafer, and finally sends the wafer to the downstream IC packaging factory for packaging and testing.

The main reason why chips are rectangular is due to the wafer manufacturing process. The main component of chips is silicon. To turn humble sand into chips that are more expensive than gold, silicon must first be initially purified. Through high-temperature heating, purification, filtration and other processes, high-purity polysilicon is extracted from silicon dioxide. Then the polysilicon is turned into single crystal silicon with extremely high purity, stable structure, good electrical properties, and suitable for making chips. This series of processes is currently mainly used in the industry by the CZ method invented by the former Soviet scientist Czochralski in 1918.

According to the CZ method, after polycrystalline silicon is extracted from silica sand, it is placed in a quartz crucible and melted at high temperature, and then a small seed crystal or seed is placed in the solution. When the silicon solution touches the single crystal silicon seed, it will grow in an orderly manner at the tail of the silicon seed. By controlling the rotation and pulling rate, single crystal silicon rods of different sizes can be pulled out of the solution. Because it is a rotary pulling, the pulled single crystal silicon rods are all cylindrical, and the silicon wafers cut from them are also round, which is the so-called "wafer". The chip is cut out from the wafer.

The manufacturing process of monocrystalline silicon is so complicated that as a commodity, of course, the more chips can be cut out of it, the better. Therefore, cutting the chips into squares has become the choice to maximize profits.

So the question is, if we use other methods to make single crystal silicon rods into other shapes, can the chips be made into other shapes?

If actual benefits are not taken into consideration, it is theoretically feasible to make single crystal silicon rods into other shapes. This is because in addition to the Czochralski method, there are other methods for producing silicon rods, such as the zone melting method. However, even if silicon rods of other shapes are produced, the silicon wafers cut out can only be round due to the subsequent process.

After the silicon wafer is cut out, it needs to be coated with photoresist before photolithography. The industry currently uses the "spinning method", which is to simply apply photoresist to the center of the silicon wafer and then rotate the silicon wafer. If the silicon wafer is in other shapes, it is likely to cause some places to have more glue and some places to have less glue, resulting in uneven coating. This will not happen with a circular shape that is axially symmetrical in any direction.

Therefore, even if single-crystal silicon rods of other shapes are produced, the most profitable approach is still to cut the silicon wafers into circles, and the chips cut from them will naturally be square in shape, which will bring the most profit.

In addition, the horizontal and vertical cutting method also makes the square product yield the highest. Curved cutting requires a higher degree of machine precision and higher costs, which is not worth the cost.

In summary, the production method of single crystal silicon rods determines the shape of silicon wafers to be round, and the round silicon wafers determine the shape of chips to be square. In addition, subsequent processing techniques including applying photoresist and photolithography are all based on the standard design of "wafers", so square chips are the most beneficial result. Of course, will there be other shapes of silicon wafers and chips in the future? From the current situation, there can be, but it is not necessary.

References

[1] Revealed! The entire process of chip design and manufacturing

https://www.hisilicon.com/cn/about-us/press/news/chip-design-and-manufacturing

[2] Detailed explanation of the chip manufacturing process, down to each step

https://www.guancha.cn/TMT/2017_06_26_415071.shtml

[3] Apple and Huawei are both using glue chips, and this is because the wafers are too round.

https://m.thepaper.cn/newsDetail_forward_17570832

[4]Why aren’t silicon wafers square?

https://www.iszcc.com/news/info12.html

This article is produced by the Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting