Polycrystalline Silicon

Polycrystalline silicon of electronic quality is mainly used to produce cylindrical crystals for electronics using the Czochralski method and crucible free zone melting . Polycrystalline silicon of solar quality is used to produce rectangular multicrystalline blocks, cylindrical crystals, plates for the solar energy using directed crystallization methods, Stepanov , Czochralski . It is mainly used in the manufacture of crystalline and thin-film photoconverters based on silicon, LCD screens, substrates and technological layers of integrated circuits. Most of the ultrapure polysilicon is obtained from monosilane , due to the cost-effectiveness of the method.

In the USSR

In the 1950s, the production of electronic polysilicon in the world was mastered. The production of cheaper and dirty polysilicon “solar” quality was mastered much later. The USSR had its own production of polysilicon of electronic quality for the needs of the military-industrial complex:

• “ Podolsk Chemical and Metallurgical Plant” since the 1960s (production stopped and capacities completely destroyed by 2001);
• “ Zaporizhzhya Semiconductor Plant” - Production was stopped due to lack of budget for the purchase of raw materials in 2017;
• “ Donetsk Chemical and Metallurgical Plant” since 1980 (up to 45% of the total volume of the former USSR), production was stopped in 1993, equipment was mothballed;
• In the 80s of the XX century. The construction of a plant in Tash-Kumyr was started but not completed.

The expansion of photovoltaic production in the late 90s of the 20th century led to the depletion of silicon scrap, which was removed from circulation due to insufficient purity in the manufacture of electronic devices. As a result, polysilicon consumption increased in the industry, which in the 2000s led to a shortage of primary polysilicon raw materials for both photovoltaics and the electronics industry.

In the CIS

Against the backdrop of scarcity, many large-scale projects for the construction of polysilicon plants both electronic and solar quality were launched around the world.

As part of overcoming the deficit in the CIS , several industries have developed:

• The existing production expanded in the city of Tash-Kumyr (Kyrgyzstan, OJSC Crystal). In 2009, the quality of silicon allows it to be used for the production of relatively low-resistance crystals of electronic quality.
• Nitol Solar Group launched a polysilicon plant in Usolye-Sibirsky ). In 2009, the quality of silicon allows you to use it for the production of crystals of solar quality.
• In 2008, production began at the Federal State Unitary Enterprise Mining and Chemical Combine (MCC) of Rosatom in Zheleznogorsk .
• In 2008, the Podolsk Chemical and Metallurgical Plant began developing a polysilicon project for the revival of production halted in 2000.
• The revival of polysilicon production at the Zaporizhzhya titanium-magnesium plant has begun.
• In November 2008, the construction of a plant for the production of polycrystalline silicon in the Astana Industrial Park began with a division of Lancaster Group - KUN Renybills JSC.

By 2012, the erupted polysilicon overproduction crisis led to price collapse to the payback threshold, which led to the closure of all polysilicon production in the CIS. Including:

• As of July 2013, production at the Kristall Group of Companies in Tashkumir was stopped, the columns were sawn into scrap metal , the company has large debts and lawsuits against illegal nationalization from investors.
• By the beginning of 2013, the Nitol Solar group halted the production of polysilicon in Usolye-Sibirsky due to loss-making.
• In 2013, a lawsuit was going on around the Federal State Unitary Enterprise Mining and Chemical Combine (MCC) ^[1] , production was not operational.

For 2014, according to the analyst in the field of polysilicon Bibisheva D.O. , 100% of production capacities are controlled by 9 largest companies from the USA, Japan, Germany, Italy, Singapore and China. The main production facilities are located in China, Singapore and the USA.

Most of the polycrystalline silicon in the world is made in the form of cylindrical rods (for 2009: Russia - up to 140 mm in diameter, outside the CIS - up to 300 mm in diameter) of gray color with a rough dendritic surface. The rods themselves do not always go on sale. Typically, the rods break into fragments (“chunk”), which are packed in measured (5-10 kg) clean bags of thick polyethylene. Chipped rods have a conchoidal fracture, similar to fractures of amorphous materials. The section (thin section) of the polysilicon rod is usually studied in the quality control of the obtained silicon and in the analysis of the process.

In the center of the rod is a "seed" made of mono- or polysilicon. Previously, the seeds were obtained by pulling in an atmosphere of polysilicon of electronic quality (the so-called oxygen rods). With the development of wire and tape cutting technologies, seed crystals began to be obtained by longitudinally cutting ingots of mono- and polysilicon rods into square bars (5 × 5, 7 × 7, 10 × 10 mm, etc.). The purity and, accordingly, the electrical resistivity of the seed have a decisive influence on the purity of the resulting polycrystalline rod. This is due to the fact that the process of hydrogen reduction of silanes is carried out at temperatures of 900–1100 ° C for a long time, which leads to active diffusion of impurities from the seed crystal into the material deposited onto the seed. On the other hand, a decrease in the content of impurities and, correspondingly, an increase in the electrical resistivity of the seed prevents both resistive and high-frequency heating of the seed crystals at the starting phase of the process, which requires the use of more expensive equipment providing significantly higher voltages at the ends of the rods at the start of the process (or higher electromagnetic field strength in the chamber when using high-frequency heating).

Close-packed crystallites in the form of short needles with a cross section of less than 1 mm sprout from the seed perpendicularly to the generatrix. At a high deposition rate, polysilicon grains often begin to grow dendritic (like “popcorn”), in case of an abnormal flow of the process, dendrites can even form exfoliating crusts. The quality and purity of such polysilicon is usually lower.

A small part of polycrystalline silicon is made from monosilane in a fluidized (boiling) layer in the form of dark gray granules with a diameter of 0.1 to 8 mm ( MEMS ). Fluidized bed production is more advantageous due to orders of magnitude larger deposition surface and, accordingly, more complete consumption of the reaction mixture; due to the possibility of continuous withdrawal from the reaction zone of particles having reached a certain limiting size. On the other hand, such silicon contains a certain amount of amorphous material and small particles of the lining of the reactor (including coated with deposited silicon). Due to the developed surface, granular silicon is easily contaminated, adsorbs a lot of water and air gases. In general, granular silicon has a markedly lower degree of purity than silicon obtained by deposition on fixed rods, and is more often used for less demanding production of solar-grade crystals.

Polycrystalline silicon is traditionally obtained from technical silicon by converting it to volatile silanes (monosilane, chlorosilanes, fluorosilanes), followed by separation of silanes, distillation purification of the gas and its reduction to crystalline silicon.

Initially, in the industrial production of polysilicon, chlorosilanes were used. For 2011, trichlorosilane-based technologies remain dominant. Fluorosilane technologies replacing chlorosilane are considered to be cheaper, but less environmentally friendly.

To restore silicon in technologies using trichlorosilane, the Siemens process is mainly used: in the flow of a reaction gas-vapor mixture of silanes and hydrogen on the surface of silicon rods heated to 650–1300 ° C (or crumbs in a fluidized bed), silane is reduced and free silicon is deposited. The temperature regime of the reaction substantially depends on the design features of the reactor and technology ^[2] . Due to the high temperature of the rods, the released silicon atoms are immediately embedded in the crystal lattice, forming crystals of the dendritic structure. Gaseous products formed during the reaction are carried away by the duct of the unreacted vapor-gas mixture and, after purification and separation, can be reused.

Siemens Stages

The preparation of polysilicon in the Siemens process ^{[3] is} based on the conversion of silicon tetrachloride to trichlorosilane with reuse of the resulting silicon-containing substances, which reduces production costs and eliminates environmental problems.

1. Synthesis of trichlorosilane by the low-temperature catalytic hydrogenation of silicon tetrachloride

3SiCl ₄ + 2 H ₂ + Si _met. ↔ 4 SiHCl ₃

2. Successive reduction of silicon on a substrate

2SiHCl ₃ ↔ SiH ₂ Cl ₂ + SiCl ₄
2SiH ₂ Cl ₂ ↔ SiH ₃ Cl + SiHCl ₃
2SiH ₃ Cl ↔ SiH ₄ + SiH ₂ Cl ₂
SiH ₄ ↔ Si + 2H ₂

3. Reuse

The evolved hydrogen and derivatives can be reused.

Technology Improvement

EPC Company Group proposed EPC-SCHMID technology based on the disproportionation of chlorosilanes, purification and subsequent pyrolysis of monosilane. According to the assurances of the developers ^{[4] [5]} , in terms of energy and material consumption, the technology gives a gain of up to 30% compared to the traditional Siemens process and ensures a product yield of 80% with additional purification of polysilicon from boron.

Known, but not yet widely used, are methods for producing polycrystalline silicon through an amorphous phase by hydrolysis of silanes, as well as reduction of silanes in the plasma of high-frequency and microwave discharges due to the light contamination and the difficulty of transferring amorphous silicon to the crystalline phase. Siemens technologies are developing, for example, using proteins , polymers , etc.

• Nitol Solar - Russia's largest silicon producer
• Giredmet - a research and design institute, designed the largest plants for the production of polycrystalline silicon
• Khimprom , Novocheboksarsk
• Khimprom , Volgograd
• Zheleznogorsk plant of semiconductor silicon - current production
• POLYSIL (Baltic Silicon Valley)
• Siberian Silicon (RUSAL)
• Solar power
• Podolsk Chemical and Metallurgical Plant
• Tash-Kumyr Silicon Productions , Tashkumyr Semiconductor Technology Plant Tash-Kumyr Silicon Producitons, Tash-Kumyr, Kyrgyzstan

• Bask in the sun - an article about polysilicon in the journal "Expert"