At the beginning of a polysilicon production applying the Siemens process, the slim rods are far too cold. The required temperature for deposition is approximately 1.000 °C. To increase the temperature and to achieve the necessary conductivity, the slim rods are ignited. As soon as the required temperature has been reached, the standard power supply (usually Thyristor based) can be switched on to continue to heat up the slim rods to deposition temperatures.
The main standard technologies for ignition are either based on heating elements (infrared) or on medium voltage systems. Technologies like plasma, additional graphite rods or hot gas injection are used in small quantities only.
centrotherm SiTec has developed a medium voltage ignition system (MVI) to ignite the silicon slim rods in the CVD reactors. The standard medium voltage ignition systems are available for 12, 18 and 24 pair CVD reactors and for slim rods up to 12 kV and can be easily adapted to any other CVD reactor configuration.
Depending on slim rod parameters (conductivity or doping, length, diameter) and supply parameters (cooling water temperature, ignition voltage), medium voltage ignition systems can be applied. The main limitation is shown in the diagram for the 24 pair centrotherm SiTec MVI system. The central objective of the MVI system is to generate a small current flow within an acceptable time. This small current will further heat up the slim rods and increase their conductivity. At the end of the ignition process, the current has to be limited to avoid rod melting.
The centrotherm SiTec MVI system uses a two stage transformer solution. At the first stage, the ignition will be started with high voltage and low current. The latter is limited by an inductance. At the second stage, the current will be increased to a value, suitable for switching to the standard Thyristor power supply. The transformers of the MVI system are connected to the main CVD transformer, hereby increasing safety and reducing installation costs. Hence, no additional power supply system is necessary.
The Medium Voltage Ignition System is characterized by high availability and safety, it leads to a shorter process cycle time and secures a contamination-free process. Compared to other MVI systems, it requires less space, thus leading to cost and time savings in installation and commissioning. The product is available as of June this year.
