Doping Technology of FZ Silicon

Silicon is a semiconductor material. In the absence of impurities, its own electrical conductivity is very weak. Impurities and crystal defects within the crystal are the main factors affecting its electrical properties. Since the purity of FZ silicon single crystals is very high, in order to obtain certain electrical properties, some impurities must be added to improve its electrical activity. The impurity content and type in the polysilicon raw material and the electrical properties of doped single crystal silicon are important factors affecting its doping substances and doping amounts. Then, through calculation and actual measurement, the pulling parameters are corrected, and finally high-quality single crystals are obtained. The main doping methods for FZ silicon single crystals include core doping, solution coating doping, filling doping, neutron transmutation doping (NTD) and gas phase doping.

1. Core doping method

This doping technology is to mix dopants into the entire raw material rod. We know that the raw material rod is made by CVD method, so the seed used to make the raw material rod can use silicon crystals that already contain dopants. When pulling silicon single crystals, the seed crystals that already contain a large amount of dopants are melted and mixed with the polycrystalline with higher purity wrapped outside the seed crystals. The impurities can be evenly mixed into the single crystal silicon through the rotation and stirring of the melt zone. However, the single crystal silicon pulled in this way has a low resistivity. Therefore, it is necessary to use the zone melting purification technology to control the concentration of dopants in the polycrystalline raw material rod to control the resistivity. For example: to reduce the concentration of dopants in the polycrystalline raw material rod, the number of zone melting purification must be increased. Using this doping technology, it is relatively difficult to control the axial resistivity uniformity of the product rod, so it is generally only suitable for boron with a large segregation coefficient. Because the segregation coefficient of boron in silicon is 0.8, the segregation effect is low during the doping process and the resistivity is easy to control, so the silicon core doping method is particularly suitable for the boron doping process.

2. Solution coating doping method

As the name implies, the solution coating method is to coat a solution containing doping substances on a polycrystalline raw material rod. When the polycrystalline melts, the solution evaporates, mixing the dopant into the molten zone, and finally pulling it into a silicon single crystal. At present, the main doping solution is an anhydrous ethanol solution of boron trioxide (B2O3) or phosphorus pentoxide (P2O5). The doping concentration and doping amount are controlled according to the doping type and target resistivity. This method has many disadvantages, such as difficulty in quantitatively controlling dopants, dopant segregation, and uneven distribution of dopants on the surface, resulting in poor resistivity uniformity.

3. Filling doping method

This method is more suitable for dopants with low segregation coefficient and low volatility, such as Ga (k=0.008) and In (k=0.0004). This method is to drill a small hole near the cone on the raw material rod, and then plug Ga or In into the hole. Since the segregation coefficient of the dopant is very low, the concentration in the melting zone will hardly decrease too much during the growth process, so the axial resistivity uniformity of the grown single crystal silicon rod is good. Single crystal silicon containing this dopant is mainly used in the preparation of infrared detectors. Therefore, during the drawing process, the process control requirements are very high. Including polycrystalline raw materials, protective gas, deionized water, cleaning corrosive liquid, purity of dopants, etc. Process pollution should also be controlled as much as possible during the drawing process. Prevent the occurrence of coil sparking, silicon collapse, etc.

4. Neutron transmutation doping (NTD) method

Neutron transmutation doping (NTD for short). The use of neutron irradiation doping (NTD) technology can solve the problem of uneven resistivity in N-type single crystals. Natural silicon contains about 3.1% of the isotope 30Si. These isotopes 30Si can be converted into 31P after absorbing thermal neutrons and releasing an electron.

With the nuclear reaction carried out by the kinetic energy of neutrons, the 31Si/31P atoms deviate a small distance from the original lattice position, causing lattice defects. Most of the 31P atoms are confined to the interstitial sites, where the 31P atoms do not have electronic activation energy. However, annealing the crystal rod at about 800℃ can make the phosphorus atoms return to their original lattice positions. Since most neutrons can pass through the silicon lattice completely, each Si atom has the same probability of capturing a neutron and converting into a phosphorus atom. Therefore, 31Si atoms can be evenly distributed in the crystal rod.

5. Gas phase doping method

This doping technology is to blow volatile PH3 (N-type) or B2H6 (P-type) gas directly into the melting zone. This is the most commonly used doping method. The doping gas used must be diluted with Ar gas before being introduced into the melting zone. By stably controlling the amount of gas filling and ignoring the evaporation of phosphorus in the melting zone, the doping amount in the melting zone can be stabilized, and the resistivity of the zone melting single crystal silicon can be stably controlled. However, due to the large volume of the zone melting furnace and the high content of the protective gas Ar, pre-doping is required. Make the concentration of the doping gas in the furnace reach the set value as soon as possible, and then stably control the resistivity of the single crystal silicon.

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