1. Preparation of single crystal silicon ingots
There are many preparation methods for single crystal silicon ingots. At present, the melt Czochralski method and the floating zone melting method are mainly used in the production in China.
(1) Straight pull method
The Czochralski method, also known as the Czochralski method, referred to as the CZ method, is a method of preparing a single crystal by pulling a rotating seed crystal from a melt in a crucible. At present, most Chinese solar cell monocrystalline silicon wafer manufacturers use this technology. Its basic principle is shown in Figure 1.
The processed high-purity polysilicon or the defective silicon produced by the semiconductor industry (monocrystalline silicon and polycrystalline silicon heads and tails) are loaded into the quartz crucible of the single crystal furnace. In a reasonable thermal field, heat silicon in a vacuum or atmosphere to melt it, and use a processed seed crystal to fully weld it with the molten silicon, and rotate and lift at a certain speed. Under the induction of the crystal nucleus, the specific process conditions and doping technology are controlled, so that the single crystal with the expected electrical properties solidifies, nucleates and grows along the seed crystal direction, and is slowly pulled out from the melt.
Doping can be done before melting the silicon. Using the difference between the melting temperature of many impurities when silicon is condensing and melting, some harmful impurities are concentrated at the bottom of the crucible, so the pulling process also has a purification effect. At present, this method has been able to draw large single crystal silicon ingots with a diameter of more than 6 inches and a weight of 100 kilograms.
(2) Zone melting method
The zone melting method means that a water-cooled high-frequency coil surrounds a silicon single crystal rod, so that eddy currents are generated in the silicon rod to heat itself, the silicon rod is partially melted, a floating area appears, and the high-frequency coil is moved slowly in time, and one end of the silicon rod is rotated, and the molten silicon is recrystallized. Using the segregation phenomenon of impurities in silicon, the purity of silicon is increased, and the high-frequency coil is reversely moved, so that the middle section of the silicon rod can be repeatedly purified until extremely high purity. The zone melting method, also known as the floating zone melting method, is currently the only method for producing high-efficiency and ultra-high-efficiency monocrystalline silicon solar cell raw materials. Due to the limitation of the coil power, the diameter of the molten silicon rod in the area should not be too large. The schematic diagram of the zone melting method is shown in Figure 2.
2. Preparation of polycrystalline silicon ingots
(1) DuPont method
The DuPont method is the SiCI4 zinc reduction method for silicon raw materials.
(2) Trichlorosilane method
The trichlorosilane method is also called the Siemens method. The trichlorosilane method mainly has three key processes:
①Silicon powder reacts with HCI to generate trichlorosilane (TCS)
② fractional distillation of TCS to achieve PPb-level ultra-pure state.
③ The ultra-pure TCS is reduced to polysilicon by chemical vapor deposition (CVD) method with H2.
(3) Silane method
Komatsu’s process for silane production is based on the following chemical reactions:
The cost of the silane method is higher than that of the trichlorosilane method, but the quality of polysilicon is also higher.
(4) Preparation of ingot polycrystalline silicon
The polycrystalline silicon rods directly obtained by the Siemens method are not easily used to directly manufacture polycrystalline silicon solar cells due to undoped and other reasons. After the molten silicon is directionally coagulated, a polycrystalline silicon ingot with uniform doping, large grain size and fibrous shape can be obtained. Compared with pulling monocrystalline silicon ingots, the processing cost of ingot polycrystalline silicon can be reduced by 10 times. The principle of directional solidification of polysilicon is shown in Figure 3.
