Silicon (Si) is a solid with a high melting point, hard and brittle, and a silvery-white metallic luster. It is the second most abundant element on Earth, and silicon accounts for 26.4% of the total mass of the Earth's crust. Silicon is located in the main group IV of the periodic table, with an atomic number of 14, a relative molecular mass of 28.09, a density of 2.33 g/cm3, a melting point of 1420°C, and a boiling point of 2355°C.
Silicon exists mainly in the form of silicon dioxide and silicates in nature. It requires a relatively complex smelting process and ultra-high purification processing technology to meet the requirements of semiconductor industry production and manufacturing. The purity requirement of single crystal silicon used for semiconductors is 99.9999%, or even more than 99.9999999%. The single crystal silicon growth method can be divided into the Czochralski method, the zone melting method, and the epitaxial method according to the different growth methods of the crystal. Among them, the Czochralski method introduced below is the more mainstream single crystal silicon growth method. Single crystal silicon is obtained through slicing, rounding, grinding, and other processes to obtain silicon wafers.
The following is a detailed description of the process:
(1) Melting. Put the block polycrystalline silicon that meets the high purity requirements into the crucible of the single crystal furnace, add a specific amount of metal or other impurities according to the electrical characteristics of the product, and heat it to a melting temperature above 1420°C to melt the polycrystalline silicon.
(2) Crystal growth. When the temperature of the silicon slurry stabilizes, slowly lower the seed crystal into the silicon melt (the seed crystal will also be melted in the silicon melt), and then lift the seed crystal with a certain rotation speed upward at a certain speed, and finally produce a qualified silicon crystal column.
The key to the quality of silicon crystal columns lies in the purity and single crystal of silicon. The larger the diameter of the silicon crystal column, the more difficult it is to control the single crystal of silicon and the more difficult it is to ensure the quality. Therefore, the larger the diameter of the silicon crystal column, the higher the technical threshold of the process. The manufacturing process of growing silicon crystal columns mainly includes complex process flows such as crystal wetting, necking, shouldering, equal diameter growth, and finishing.
(3) Slicing. After the silicon crystal column is completed, it needs to be cut and tested. Samples are cut from silicon crystal columns to test their technical parameters such as resistivity, oxygen/carbon content, and crystal defects. The slices are first processed using an industrial-grade diamond mold to grind the crystal column into a smooth cylinder, and the head and tail of the conical crystal ingot are cut off to form a standard cylinder, which is then sliced using an inner diameter saw blade. The thickness, bow, and deflection of the sliced silicon wafers are the key to the quality control of the slicing process.
(4) Rounded edges. The edges of the freshly cut silicon wafers are all sharp right angles. Due to their hard and brittle material properties, right angles are easy to break, and other quality defects such as thermal stress, cracking, and edge collapse are easy to occur in subsequent processes. In addition to affecting the strength of the silicon wafer, it will also become a source of contamination particles in the entire process. Rounding the edges is the process of chamfering the edges of the silicon wafers, also known as chamfering. The rounded silicon wafers have smooth edges and lower center stress, which can effectively improve and enhance the overall mechanical strength and processability of the silicon wafers.
(5) Grinding, etching, defect removal, and polishing. Grinding is to remove saw marks, adhered debris, and stains caused by cutting and wheel grinding, so that the surface of the silicon wafer can reach a flatness that can be further polished. After the above-mentioned processing, a damaged layer (Damaged Layer) is formed on the surface of the silicon wafer due to processing. Before polishing, it is etched with a chemical solution to remove it, and then rinsed and dried with pure water. The defects on the silicon wafer are processed and perfected by sandblasting and other processes, to produce a complete and defect-free wafer material.
Silicon wafer cutting is a very important link in the photovoltaic industry chain. The quality of silicon wafer cutting directly affects the downstream battery production, and the level of slicing technology determines the production cost of the enterprise. The slicing machines currently used in China are mainly HCT in Switzerland, MB in Germany, and Toko in Japan. The following introduces the process of MB slicing machines in Germany.
1. Configure crystal rods
Configure the required crystal rods according to the "Crystal Rod Receiving Record Table" provided by the cutting process. When a group of long rods needs to be pieced together by several small segments of crystal rods, the length difference between the crystal rods corresponding to the same position in each group shall not exceed 10mm. There shall be no more than 4 segments of crystal rods at each position. The total length of each group of crystal rods shall not be less than 450mm and not more than 515mm. After configuration, fill in the crystal rod number, length, and location on the "Crystal Rod Gluing Record Sheet".
2. Receive the crystal rod
According to the crystal rod number configured in the "Crystal Rod Gluing Record Sheet", receive the crystal rod in the square cutting process and sign on the "Crystal Rod Gluing Record Sheet". The received crystal rods are placed on the crystal rod gluing workbench according to the position marked on the "Crystal Rod Gluing Record Sheet" and kept clean. When receiving the crystal rod, if there is oil, silicon powder, water stains, and other dirt on the crystal rod, this crystal rod shall not be used. Report to the supervisor and wait until the crystal rod is cleaned in the square cutting process before receiving it.
3. Bonding the crystal rod
Place the metal pad on the crystal rod gluing limiter, with one side close to the positioning screw. Clean the surfaces of the metal pad and the glass pad with anhydrous alcohol, mix 15 grams of glue and 15 grams of curing agent, stir evenly, and evenly apply them on the metal pad, then put the glass pad on it, squeeze and move the glass pad hard to exhaust all the air between the glass pad and the metal pad and make full contact. Then press the weight and wait for 20 minutes. Remove the weight and clean the glass pad and the surface of the crystal rod with anhydrous alcohol. Adjust 20-25 grams of glue and 20-25 grams of curing agent according to the length of the crystal rod, stir evenly and evenly apply them on the glass pad, put the crystal rod on it, squeeze and move the crystal rod hard to exhaust all the air in it, and make full contact, then press the weight, wait for 20 minutes and turn it over, place the clamping rail on the metal pad, tighten 5 bolts coated with butter with 40 Newtons, and leave it for 6 hours for cutting.
Silicon wafers are the most expensive part of crystalline silicon photovoltaic cell technology, so reducing the production cost of this part is crucial to improving the competitiveness of solar energy over traditional energy. This article will provide an overview of the silicon wafer slicing process, the challenges of the manufacturing industry, and how the new generation of wire saw technology can reduce slicing costs.