Chips and Wafers

- Wafers

In electronics, a chip is an integrated circuit with millions of transistors, resistors, and capacitors, while a wafer is the base of any integrated circuit.

Wafers (also known as slices or substrates) are thin slices of semiconductors, such as crystalline silicon (c-Si), used to make integrated circuits and, in photovoltaics, to make solar cells. 

Wafers serve as substrates for microelectronic devices built into and on the wafers. It undergoes many microfabrication processes such as doping, ion implantation, etching, thin film deposition of various materials, and photolithographic patterning. Finally, individual microcircuits are separated and packaged into integrated circuits by wafer dicing.

Several types of solar cells are also made from such wafers. On a solar wafer, solar cells (usually square) are made from an entire wafer.

- The Foundation of the Semiconductor Industry: Wafers

Wafer refers to a silicon wafer used to manufacture silicon semiconductor integrated circuits. Because of its circular shape, it is called a wafer. Wafers are the carriers used to produce integrated circuits. 

Generally speaking, a wafer refers to a single crystal silicon wafer. Wafer is the most commonly used semiconductor substrate, which is divided into 4 inches, 5 inches, 6 inches, and 8 inches according to the diameter. The larger the wafer, the more ICs that can be produced on the same wafer, reducing costs. However, larger wafers place higher demands on materials technology and production techniques. It is generally believed that the larger the diameter of the silicon wafer, the better the technology of the fab. In the process of producing wafers, yield rate is a very important condition. 

Wafers are the building blocks for making computer chips. We can compare chip manufacturing to building a house with Lego blocks. By stacking layers one by one, we can complete the shape we want (that is, the desired chip). But if there is no good foundation, the house built will be deformed and not ideal. In order to build a perfect house, a stable floor is needed.

Wafer fabrication holds the subsequently added atoms together with the substrate in a similar fashion. Therefore, we need to find a substrate with a neat surface to meet the conditions required for subsequent fabrication.

- Monocrystalline

In solid materials, there is a special crystal structure - Monocrystalline. It has the property that atoms are closely arranged to form a flat atomic surface. Therefore, using a single crystal to make a wafer can meet the above requirements. However, to produce such materials, there are two main steps, purification and crystal pulling.

- How to Make Monocrystalline Silicon Wafer?

Purification is divided into two stages. The first step is metallurgical purification. The process is mainly to add carbon to silicon oxide and convert it into silicon with a purity of more than 98% through redox. Refining of most metals, such as iron or copper, is done in this way to obtain metals of sufficient purity. However, 98% is still not enough for wafer manufacturing and needs to be further improved. Therefore, the Siemens process will be used for further purification to obtain the high-purity polysilicon required for semiconductor processes. 

Next, is the step of pulling the crystal. The previously obtained high-purity polysilicon is melted to form liquid silicon. Afterwards, the monocrystalline silicon seed contacts the surface of the liquid and is slowly pulled up while spinning. Single crystal silicon seeds are needed to initiate the order in which the silicon atoms line up on the crystal as it grows. Finally, after the silicon atoms leaving the liquid solidify on the surface of the crystal, the well-arranged columns of single-crystal silicon are completed.  

However, what do 8 inches and 12 inches represent? This is the diameter of the crystal column when it is finished. How hard is it to make large wafers? As mentioned earlier, the buildup on the crystals is like the process of making marshmallows. As the center rotates, multiple layers are added until the desired diameter is achieved. If you've ever made marshmallows, you've noticed that it becomes more difficult to create a uniform end product as the core gets larger. The drawing of crystal is the same. The speed of rotation and temperature control will affect the quality of the crystal column. Therefore, the larger the size, the higher the speed and temperature requirements for crystal pulling, so it is more difficult to make high-quality 12-inch wafers than 8-inch wafers.

To obtain a wafer, this pillar of silicon now needs to be sliced. The silicon pillars are cut laterally into wafers using a diamond knife, and the wafer surface is then polished to form the final finished wafer. Once the wafer substrate is complete, the next step is to stack the circuits onto the wafer to complete chip fabrication.