Silicon is a non-casting material due to its physical and chemical properties. Because its high capacity to enter chemical reactions with other substances in the melted state, crystal growing process needs to take place either in a vacuum, or in the atmosphere of an inert gas. The process of crystallization involves a significant increase in the volume of silicon, which creates the need to apply specialized technologies that mitigate the risk of solidification of some parts of the material before the silicon bar is crystallized.
The following picture shows how the starting raw silicon looks like.
There are two main methods of crystal growing: the Chokhralsky method (named after the Polish scientist Jan Chokhralsky), and the crucibleless zone smelting method, a specific kind of zone melting without using a crucible.
In the 1990s, more than 80% of monocrystalline silicon crystals were formed on the basis of Chokhralsky method. These crystals were successfully applied in the field of solar energy and power electronics. The Chokhralsky method is based on the growth of a monocrystal by the transition of atoms from the liquid (or gaseous) state into the solid state in their separation area.
The Chokhralsky method allows using polycrystalline silicon formed at the first stage to later grow new monocrystals from a quartz crucible. In this process, polysilicon is getting melted in the crucibles by putting into them a high quality seed monocrystal. As silicon grows, the rod with a seed is lifted up, spinning around the vertical axis. The crucible spins in the opposite direction. This double spinning allows for a proper mixing of the melt and reduces the risk of uneven distribution of the temperature.
The Chokhralsky method can also be called the crystal pulling method. The following picture illustrates different stages of the process.
In the case of multicrystalline silicon, the crystal needs to be homogenized and then cooled. To form monocrystals, part of the seed is melted in order to eliminate zones with the excess concentration of mechanical tensions and defects. Then the crystal is slowly pulled out of the melt. Formed crystals are then cut into the separate wafers 200–220 micrometer thick.
Nowadays, to form monocrystals according to Chokhralsky method a novel equipment is used: a special installation consisting of a spinning device, a device for moving the rods, an electric power system, vacuum apparatus, water cooling block, inert gas supply, regulation and purification block, as well as an automatic control system.
In spite of all the advantages of Chokhralsky method, it is not without certain shortcomings: the melt may become contaminated with admixtures and oxygen that is present in the quartz crucible. In order to prevent such contamination another method of crystal growing was invented — the method of the crucibleless zone smelting. It is based on the idea that one does need a crucible to grow crystals if the smelting area is placed within the rod itself.
Multicrystalline silicon ingot (brick)
The idea is that the seed is placed under the rod, while its end point is getting smelted. In this process, the melt area emerges within the vertical rod. The mass of the melt increases the pressure in the smelting area. The surface tension force stabilizes the melt and creates a stable smelting area with the height of 1.5 centimeter. After the seed is put into the melt, during starting phase of the crystal growth, the seed is slowly lowered down together with the growing crystal. The main rod moves in the same direction. Similarly to the Chokhralsky method, the seed and the melting rod are spinning in opposite directions around the vertical axis. In the industry the most frequently used method of heating is induction heating, with the ring inductor, which allows a high-frequency electric current to pass through it, being encircled around the silicon rod.
Monocrystalline silicon ingots
Despite the fact that the crucibleless zone smelting methods allows to form purer crystals, both methods are used in practices. Usually the crucibleless method has an auxiliary significance and is applied in the hydrogen reduction of trichlorosilane in the process of polycrystalline silicon production. For the formation of monocrystalline silicon the Chokhralsky method is more effective when it is based on the application of a combined magnetic field, which allows to form crystals with more refined structure, and also speeds up the process of crystallization.
After the crystals of multi- and monocrystalline silicon are formed, their edges are getting cut, because they contain the highest concentration of admixtures. The resulting crystals are then getting formed (“squaring”, that is, making them square-shaped), polished and cut into wafers. As a result, one gets almost square-shaped wafers of multicrystalline silicon and pseudo square-shaped wafers of monocrystalline silicon. The picture below shows the finished square bars of crystalline silicon.
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Cleaned ingots that are ready to further slicing (multicrystalline)