At present, silicon materials still occupy a major position in the field of semiconductors and solar energy. With the development of science and technology, the production process of integrated circuits and solar cells has put forward new requirements for silicon materials. The growth technology of large-diameter and high-quality silicon single crystals has become a research and development hotspot in the field of semiconductor materials and solar energy. If the diameter of the silicon single crystal increases, the feeding amount will increase, and the crucible diameter and thermal field size will also increase accordingly, which will inevitably lead to intensified thermal convection in the melt. When the crystal is grown by the traditional Czochralski method, the melt is prone to eddy currents, the shape of the solid-liquid interface, the temperature gradient and the uniformity of the oxygen concentration distribution are difficult to control, and it is difficult to achieve the balance of point defects. Applying the magnetic field to the Czochralski-grown single crystal can effectively inhibit the thermal convection, make the impurity content evenly distributed, and significantly improve the crystal quality. PAM-XIAMEN can supply magnetic Czochralski (MCZ) silicon wafers. More about our MCZ silicon wafers please refer to https://www.powerwaywafer.com/pam-xiamen-offers-mcz-silicon-ingot-and-silicon-wafer.html.
1. Magnetic Czochralski Methods
According to whether the magnetic field direction is parallel to the growth axis or perpendicular to the growth axis, there are corresponding longitudinal magnetic field method and transverse magnetic field method. In order to overcome the inherent shortcomings of these two magnetic fields, various non-uniformly distributed magnetic fields, such as cusp magnetic field, have also been developed. The magnetic czochralski method is as follows:
1.1 Transverse Magnetic Field Method
The single crystal furnace is arranged between the two magnetic poles of the transverse magnetic field, and the magnetic field lines are parallel to traverse the silicon single crystal melt in the single crystal furnace, that is, the magnetic field lines are parallel to the radial direction of the single crystal, and the magnetic field lines pass through the furnace body to form a magnetic A transverse magnetic field is formed, as shown in Figure 1. It’s found that the transverse magnetic field can reduce the oxygen content of the crystals and the contamination caused by impurities in the crucible during crystal growth in larger melts.
Fig.1 Schematic Diagram of Transverse Magnetic Field
In the transverse magnetic field (horizontal magnetic field) system, the melt convection in the melt in the axial direction and perpendicular to the magnetic field direction is suppressed, while the melt convection parallel to the magnetic field direction is not affected. The transverse magnetic field applied Czochralski can obtain silicon single crystal with lower oxygen content and better radial uniformity than ordinary Czochralski method, but it cannot inhibit Marangoni convection on the melt surface.
1.2 Longitudinal Magnetic Field Method
By winding a solenoid outside the furnace chamber of a single crystal furnace, a longitudinal magnetic field (vertical magnetic field) can be formed at a lower cost than a transverse magnetic field. The schematic diagram is shown in Figure 2.
Fig.2 Schematic Diagram of Longitudinal Magnetic Field
It is reported that the effect of a 100 mT axial magnetic field on the radial distribution of oxygen and phosphorus in single crystal silicon grown from a 3.5 kg melt, and found that the oxygen content increased in the axial direction, while the radial resistivity uniformity decreased. The resistivity uniformity in the axial direction increases, and the rotational fringes increase at the edge of the crystal.
In the longitudinal magnetic field, the radial melt convection is suppressed, but the axial melt convection is not affected. There is direct oxygen transport from the bottom of the quartz crucible to the crystal/molten silicon interface, which is difficult to control the oxygen content in the crystal. The radial distribution of dopants in crystals, which grown by longitudinal magnetic Czochralski technique is more inhomogeneous, and the oxygen content is higher than that without a magnetic field; in addition, the melt convection at the crystal/fused silicon interface is suppressed.
1.3 Cusp Magnetic Field Method
In order to overcome the limitations of the above two magnetic Czochralski fields applied, various non-uniform magnetic fields have been developed, one of which is the cusp magnetic field (as shown in Figure 3). This magnetic field system consists of two sets of parallel superconducting coils coaxial with the crystal. The two coils pass currents in opposite directions, forming a “sharp-angle” symmetrically distributed magnetic field in the middle of the two sets of coils, so that the solid-liquid interface during the growth of silicon single crystal is located on the symmetry plane between the two sets of coils. It is relatively simple to install sharp-angle magnetic field equipment in large single crystal magnetic Czochralski furnace. Both theory and experiment show that the oxygen content decreases rapidly at low magnetic fields.
Fig.3 Schematic Diagram of Cusp Magnetic Field
In the magnetic Czochralski growth system using the cusp magnetic field, the crystal/fused silicon interface is on the symmetry plane of the symmetrically distributed magnetic field generated by the two coil windings. Therefore, during the magnetic Czochralski crystal growth process, the magnetic field strength at the crystal/molten silicon interface is very small, and the inhibition effect on the forced convection caused by the crystal rotation is small, and the thickness of the boundary layer on the solid-liquid interface is correspondingly small.
The distribution characteristic of the cusp magnetic field is that the magnetic field strength near the inner surface of the quartz crucible is perpendicular to the surface of the quartz crucible, so the thermal convection near the crucible wall is reduced, and the boundary layer and thickness of the molten silicon near the quartz crucible wall are increased. The corrosion rate of the crucible is reduced. The molten silicon in the crucible is generally under a strong magnetic field, the strength of the melt convection in the crucible decreases, and there is no direct oxygen transport from the bottom of the quartz crucible to the crystal interface.
2. Advantages of Magnetic Czochralski Technology
Compared with the CZ method, the MCZ method has the following advantages:
1) The oxygen concentration can be controlled in a wide range (2-20PPm);
2) The oxygen and other impurities are evenly distributed;
3) The probability of crystal defects is small;
4) The warpage caused by thermal stress is small.
3. Applications of CZ and MCZ Silicon Wafers
The large-size heavy/lightly doped Czochralski silicon single crystal wafer prepared by flat shoulder expansion and high pulling speed has low oxygen and carbon content and high minority carrier lifetime, and is suitable for the production of various integrated circuits, diodes, triodes, green energy solar cells, etc. Special elements such as gallium (Ga) and germanium (Ge) can be doped to produce solar cell materials with high efficiency, radiation resistance and anti-decay required for special devices.
However, silicon wafers with low oxygen content and high resistivity uniformity grown by magnetic Czochralski process are suitable for the production of various integrated circuit devices, various discrete devices, and silicon materials for low-oxygen solar cells.
All in all, the applications of MCZ silicon are almost similar to CZ silicon, but the performance of MCZ silicon is better than CZ silicon.
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