Effect of Heat Treatment on the Performance of Silicon Single Crystals

Effect of Heat Treatment on the Performance of Silicon Single Crystals

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Heat treatment refers to the heat treatment of silicon single crystals (or silicon wafers) for a certain period of time at a certain temperature and protective atmosphere, with the aim of improving their performance. Heat treatment is usually carried out in a heat treatment furnace protected by nitrogen or argon gas. The typical heat treatment methods for silicon single crystal (or silicon wafer) are as follows:

1. Eliminating Thermal Donors in Silicon Wafer

During the cooling process of monocrystalline silicon grown by Czochralski method to room temperature, oxygen impurities in the body will generate thermal donors in the temperature range of 350-500 °C. 450 °C is the most effective temperature for the formation of thermal donors. After annealing at 450 degrees for 100 hours, the concentration of thermal donors can reach about 10 (16) cm-3. The head of Czochralski silicon crystals in the temperature range of about 450 °C for the longest time during the cooling process, and has a higher oxygen content, so there are more thermal donors compared to the tail. Thermal donors can distort the resistivity of monocrystalline silicon, leading to a decrease in the resistivity of n-type materials and an increase in the resistivity of p-type materials.

Generally, a heat treatment at 650°C is used for 30-60 minutes, followed by rapid cooling to below 300°C, which can effectively eliminate thermal donors. If the cooling rate is not fast enough, a small portion of the thermal donor will be preserved. For large-diameter single crystal rods, the thermal stress generated by rapid cooling can cause cracks in the rods, so it is necessary to use silicon wafer annealing to restore the resistivity of the silicon wafer to its original value.

2. Eliminating Neutron Irradiation Damage of Silicon Wafers

Neutron irradiation doping can be used for doping high resistivity FZ monocrystalline silicon. This doping process is a nuclear reaction process: silicon is composed of three isotopes: 28Si, 39Si, and 30Si, with 30Si accounting for 3.09%; In an atomic reactor, after neutron irradiation of Si, 30Si reacts with neutrons to form an unstable isotope 31Si with a half-life of 2.6 hours, which then decays into a stable phosphorus isotope 31P. The irradiation process will cause lattice damage within the silicon crystal, which can be eliminated by high-temperature annealing. The typical annealing temperature is 750~850 °C, and the annealing time is 1-2 hours.

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