Crystal growing is a lengthy and energy-intensive process. The first step is to produce pure silicon from silicon dioxide by chemical methods. Then the material must first be melted and crystallized by cooling. The monocrystal should not contain foreign atoms, so the process must take place under special conditions. The best results are achieved in a vacuum furnace, where crystallization takes place without access of gases and especially oxygen. The method was developed by the Polish scientist Jan Czochralski [1]. A schematic of the silicon monocrystal culture process is shown in Fig. 1.

The first stage of production is the chemical process of obtaining pure silicon \( SiO_{2} \). The resulting pieces of pure silicon are placed in a vacuum chamber in a quartz crucible.

After the silicon is loaded into the vacuum chamber, the silicon is melted in a quartz crucible, and then a small silicon crystal is lowered from the top vacuum chamber of the device, which becomes a crystallization nucleus. The embryo with a specific crystallographic orientation touches the surface of the molten silicon. The molten silicon (liquid) adheres to the surface of the embryo, which rises minimally. Moving away from the surface, it enters a region of reduced temperature which starts the crystallization process. In order to homogenize the crystallization process, the embryo rotates around its axis. By adjusting the speed of rotation of the embryo and the speed of ejection upwards, we obtain cylindrical crystals, initially of varying diameter. Once stable process parameters are achieved, the diameter of the grown monocrystal maintains its value until the molten material is exhausted. The orientation of the growing monocrystal is identical to that of the embryo initiating crystallization. The result of the process is the large cylindrical monocrystal shown in Fig. 2 [2].

Figure 2: Silicon monocrystal (ingot) grown by Czochralski method. Photo by Massimiliano Lincetto, licensed under CC BY-SA 4.0, source: Wikimedia Commons.

The steps of this process, the principle of the Czochralski method and the wafer processing steps are presented in the film "Silicon Wafer Production by MicroChemicals".

Mimotec SA, Silicon Wafer Production by MicroChemicals (Technological stages of silicon wafer production), 09.10.2015 (accessed 12.12.2020). Available on YouTube: https://youtu.be/2qLI-NYdLy8.

Due to the purity requirements of the monocrystals produced, the manufacturing processes take place in high purity class halls. The crystal shown in Fig. 2 is about 1 meter long. With the current demand for monocrystalline silicon for both electronic and photovoltaic production, dozens of Czochralski furnaces are installed in Chinese factories in the halls, and the lengths of monocrystals reach 4.5 m.
The next step in the production of a silicon wafer, because silicon wafers are the material for electronic and photovoltaic production, is the mechanical processing of the ingot (this is what the monocrystal is called after being removed from the furnace). The first step of mechanical processing is to cut off ends of variable diameter and roll the surface of the ingot into a cylinder with fixed geometric parameters. The cylinder is cut into pieces corresponding to the dimensions of a wire saw, which will cut it into wafers 180 micrometres thick. It used to be 300 micrometres, then 200 micrometres, and now it is also cut to a dimension of 160 micrometres. The problem after reducing the wafer thickness is the mechanical resistance of the monocrystalline to cracking, during further processing up to and including panel assembly. Cut wafers have edges as sharp as broken glass, also they need to be blunted before further processing. Each wafer is numbered by laser, which allows further identification of the production process and its parameters. The next process is polishing to the required level. In many cases, this is the atomic level. The next steps are chemical etching, polishing and cleaning the surface from residues of previous processes. Depending on the customer, wafers are covered with a layer of silicon oxide of a specified thickness and then an electronic inspection of the correctness of the state of the wafer and its surface is carried out. The final stage is packing of the wafers into transport trays.

Silicon wafer production has always been catered to the needs of electronic factories and wafers were initially produced in small sizes of 1 inch, then 2 inches, etc. The increase in wafer diameter was driven by the rapidly increasing industrial demand for monocrystalline silicon and the wafer diameter varied from 2 inches in 1970 to 18 inches in 2014. An example of wafers with different diameters is shown in Fig. 3 [3].

Figure 3: Successive generations of silicon wafers with increasing size. Photo license CC BY-SA 3.0, source: Wikimedia Commons.

Each change in dimension meant new generations of tooling in processor and memory factories. In photovoltaics, where wafer parameters and properties are not as stringent, the standard size for a photovoltaic cell has become 15.6 x 15.6 cm and a wafer from which such a square can be cut with minimal chipping. As a result, monocrystalline silicon photovoltaic panels show an empty space at the junction of the four cells, resulting from the cylindrical symmetry of the starting material. Extremely sterile silicon production conditions also allow the production of silicon with strictly defined dopants. This is particularly important in photovoltaic production. By obtaining a wafer with a specific doping we have a ready component for the production of a diode, which is every photovoltaic cell.