Technology

We offer our customers technology on a one-stop shop basis for the production of polysilicon through to ingots and wafers. Our experts’ experience in these areas guarantees extensive process know-how. Tried and tested technology and efficient processes ensure quality and a high degree of silicon purity, as well as production capacities guaranteed beforehand. Our technologies also help to reduce the use of resources – thanks to low equipment energy consumption and vent gas recovery, for example – and contribute significantly to cost reduction and environmental protection.

We use our own CVD reactors (Chemical Vapor Deposition) for silicon production, which we have developed based on the Siemens process. Our CVD-Reactors impress with low energy consumption, high production capacity and an outstanding degree of end product purity. The STC-TCS converter, which is also a proprietary development, transforms silicon tetrachloride produced in the CVD process into trichlorosilane that is subsequently recycled into the process. This significantly reduces the cost of silicon production by a significant measure. Reactors and converters must be able to withstand significant stress – high pressure and extreme temperatures. Glatt GmbH, which is integrated in the group produces special technology for pressure system construction.

Our vent gas recovery system breaks down the gas mixture created in silicon production into its individual components in a chemical process in order to recycle them back into the production process. This considerably reduces production and energy costs and also contributes to environmental protection.

Silicon is the key basic substance in the production of solar cells and semiconductors. It is commonly found in nature including in the form of quartz (silicon dioxide, SiO₂). To make this supply of SiO₂ useable for the solar and semiconductor industry, the silicon (Si) has to be separated from oxygen (O₂) and polluting substances. The purity requirements of the silicon (solar silicon) used in the photovoltaic industry are lower than those of the semiconductor industry.

Industry principally uses the “Siemens process” to produce solar silicon. The metallurgic purification process, which is still under development, has also been tested in industry recently. centrotherm photovoltaics’ services and production equipment for the manufacture of silicon are currently only based on the proven Siemens process. However, there are plans to extend the service portfolio for systems using the metallurgic purification process.

Both the semiconductor and solar cell industries predominantly utilize the Siemens process for the production of silicon, whereby silicon dioxide and carbon are united in a reduction furnace at temperatures of 2,000° C.The chemical reduction of the silicon dioxide to metallurgic silicon takes place which can reach a degree of purity of up to 98% with certain quartz sand. The metallurgic silicon undergoes further treatment in a second phase where it is converted with gaseous hydrogen chloride (HCl) to trichlorosilane (HSiCl₃) at temperatures of between 300 and 350° C. After several distillation steps, the trichlorosilane is thermally broken down into silicon and hydrogen chloride in the “CVD reactor" by adding hydrogen (H₂) at 1,100 to 1,200° C. The silicon is refined to highly pure silicon rods.

Silicon tetrachloride (SiCl₄) is another reaction by-product, which is converted to trichlorosilane and recycled into the process. Alternatively, the silicon tetrachloride can be burnt down to silicic acid by introducing oxygen and finds use in the pharmaceutical, packaging and fiber-glass industries, as well as in the production of colors and paints. centrotherm photovoltaics has developed a silicon tetrachloride-trichlorosilane converter for this purpose.

Currently, this technology does not enable complete breakdown of the gaseous trichlorosilane in the CVD reactor. However, the remaining trichlorosilane can be retreated in a converter and recycled into the CVD reactor to produce silicon, thereby increasing output.

An alternative to this process is the use of monosilane (SiH₄), which is produced from the previously mentioned elements. Monosilane breaks down after a purification process involving heated surfaces or passing through fluidized bed reactors and deposits silicon.

The silicon produced in both processes has a level of purity of 8 N to 9 N (up to 99.9999999%), making it suitable for use in solar cells.

Waste gas is emitted from the CVD reactor. Our trichlorosilane recovery system converts this waste gas into useful process gas. The trichlorosilane is recycled into the silicon production process in a closed loop. The reuse of the gas enables high production and energy costs in the production of trichlorosilane to be reduced.

Thin wafers have to be produced from the solar silicon obtained for the manufacture of solar cells. The solar silicon is initially crystallized to ingots – silicon blocks or columns of mono or multi-crystalline silicon – which are later cut into wafers. Mono-crystalline ingots are primarily produced using the “Czochralski process”, where melted solar silicon crystallizes to seed crystal. This is attached to a slowly rotating metal rod and extracted from melted silicon. To produce multi-crystalline ingots, melted solar silicon is slowly cooled to a state where the silicon block’s multi-crystalline structure forms. Boron is generally added to the melted solar silicon in both processes and increases the crystal’s electrical conductivity. centrotherm photovoltaics, by contrast, introduces an additional procedure based on the directional solidification principle, and has developed a multi-crystalline ingot furnace specifically for this purpose.

The ingots are cut into thin sheets of silicon – wafers – using a wire saw with suspension and then cleaned. A slurry management system is used to optimize the cutting process.