Silicon doping

Silicon mono crystal for the semiconductor industry. (Photo: Siltronic)

The silicon doping system (SDA) is the only purely commercial production facility at the FRM II. It is used for the doping of high purity silicon that is instrumental in the semiconductor industry, for example for high- power electronics such as long-range DC power transmission, or in the automotive industry.

Pure silicon is a very poor conductor of electricity. However, it is industrially viable for semiconductors when it contains a small amount of impurities (such as phosphorus). The introduction of these impurities is called doping. At the research neutron source, doping is achieved using neutrons. The silicon crystal is placed in the irradiation position and subjected to a well-defined thermal neutron flux. The effect of the doping originates from the fact  that the silicon isotope 30Si (this isotope constitutes about 3 % of the naturally occurring silicon) is activated during irradiation by the capture of neutrons in a way calculated beforehand, and is converted, with a half-life of 2.6 hours, into stable 31P (phosphorus). In contrast to other doping techniques, the so-called neutron transmutation doping (NTD) guarantees a particularly homogeneous P-distribution in the silicon crystal.

The SDA at the FRM II can irradiate cylindrical Si mono-crystals up to 50 cm in height and up to 20 cm diameter. To achieve a homogeneous doping, the neutron flux is smoothed in the irradiation position of the SDA by a suitable nickel coating and, in addition, the basket is rotated about its own cylinder axis during irradiation. The demand for doped silicon by neutron irradiation is so high that, at the FRM II, up to 15 tons are commissioned annually by companies from Europe and Asia. After irradiation, the silicon crystals have a resistivity of 25 to 1000 Ωcm.