Czochralski method

Wafers are sliced from crystal rods, which are produced from polycrystalline and pure undoped intrinsic materials. The process of transforming polycrystalline material into single crystals through melting and recrystallization is known as crystal growth. Currently, two main methods are employed for this process: the Czochralski method and the zone melting method. Among these, the Czochralski method (often referred to as the CZ method) is the most significant for growing single crystals from melts. In fact, over 85% of single crystal silicon is produced using the Czochralski method.

The Czochralski method involves heating and melting high-purity polycrystalline silicon materials into a liquid state under high vacuum or an inert gas atmosphere, followed by recrystallization to form single crystal silicon. The equipment necessary for this process includes a Czochralski single crystal furnace, which consists of a furnace body, a mechanical transmission system, a temperature control system, and a gas transmission system. The design of the furnace ensures uniform temperature distribution and effective heat dissipation. The mechanical transmission system manages the movement of the crucible and seed crystal, while the heating system melts the polysilicon using either a high-frequency coil or a resistance heater. The gas transmission system is responsible for creating a vacuum and filling the chamber with inert gas to prevent oxidation of the silicon solution, with a required vacuum level below 5 Torr and an inert gas purity of at least 99.9999%.

The purity of the crystal rod is critical, as it significantly impacts the quality of the resulting wafer. Therefore, maintaining high purity during the growth of single crystals is essential.

Crystal growth involves using single crystal silicon with a specific crystal orientation as the starting seed crystal to cultivate silicon ingots. The resulting silicon ingot will "inherit" the structural characteristics (crystal orientation) of the seed crystal. To ensure that the molten silicon accurately follows the crystal structure of the seed crystal and gradually expands into a large single crystal silicon ingot, the conditions at the contact interface between the molten silicon and the single crystal silicon seed crystals must be strictly controlled. This process is facilitated by a Czochralski (CZ) single crystal growth furnace.

The main steps in growing single crystal silicon through the CZ method are as follows:

Preparation Stage:

1. Begin with high-purity polycrystalline silicon, then crush and clean it using a mixed solution of hydrofluoric acid and nitric acid.

2. Polish the seed crystal, ensuring that its orientation matches the desired growth direction of the single crystal silicon and that it is free of defects. Any imperfections will be "inherited" by the growing crystal.

3. Select the impurities to be added to the crucible to control the conductivity type of the growing crystal (either N-type or P-type).

4. Rinse all cleaned materials with high-purity deionized water until neutral, then dry them.

Loading the Furnace:

1. Place the crushed polysilicon into a quartz crucible, secure the seed crystal, cover it, evacuate the furnace, and fill it with inert gas.

Heating and Melting Polysilicon:

1. After filling with inert gas, heat and melt the polysilicon in the crucible, typically at a temperature of around 1420°C.

Growing Stage:

1. This stage is referred to as "seeding." Lower the temperature to slightly below 1420°C so that the seed crystal is positioned a few millimeters above the liquid surface.

2. Preheat the seed crystal for about 2-3 minutes to achieve thermal equilibrium between the molten silicon and the seed crystal.

3. After preheating, bring the seed crystal into contact with the molten silicon surface to complete the seeding process.

Necking Stage:

1. Following the seeding step, gradually increase the temperature while the seed crystal begins to rotate and is slowly pulled upward, forming a small single crystal with a diameter of about 0.5 to 0.7 cm, smaller than the initial seed crystal.

2. The primary goal during this necking stage is to eliminate any defects present in the seed crystal as well as any new defects that may arise from temperature fluctuations during the seeding process. Although the pulling speed is comparatively fast during this stage, it must be maintained within appropriate limits to avoid excessively rapid operation.

Shouldering Stage:

1. After necking is complete, decrease the pulling speed and reduce the temperature to allow the crystal to gradually achieve the required diameter.

2. Careful control of temperature and pulling speed during this shouldering process is essential to ensure even and stable crystal growth.

Equal Diameter Growth Stage:

1. As the shouldering process nears completion, slowly increase and stabilize the temperature to ensure uniform growth in diameter.

2. This stage requires stringent control of pulling speed and temperature to guarantee the uniformity and consistency of the single crystal.

Finishing Stage:

1. As single crystal growth approaches completion, moderately raise the temperature and accelerate the pulling rate to gradually taper the crystal rod's diameter into a point.

2. This tapering helps to prevent defects that could arise from a sudden temperature drop when the crystal rod exits the molten state, thereby ensuring the overall high quality of the crystal.

After the direct pulling of single crystal is completed, the raw material crystal rod of the wafer is obtained. By cutting the crystal rod, the most original wafer is obtained. However, the wafer cannot be used directly at this time. In order to obtain usable wafers, some complex subsequent operations such as polishing, cleaning, thin film deposition, annealing, etc. are required.

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