Physical method solar cell polysilicon

What is the Czochralski method for preparing single crystal silicon?

admin on March 4, 2022 0 Comments • Tags: #basicproblemsofczochralskimethod #czochralskimonocrystallinesiliconprocess #technicalimprovementofczochralskimethod

What is the Czochralski method for preparing single crystal silicon?

1. Czochralski monocrystalline silicon process
The technique of growing crystals by the Czochralski method was invented by J.Czchralksi of Poland in 1917, so it is also called the Czchralksi method. In 1950, Teal and others used this technology to grow semiconductor germanium single crystal, and then he used this method to grow Czochralski monocrystalline silicon. On this basis, Dash proposed the “necking” technology of Czochralski monocrystalline silicon growth. , G.Ziegler proposed the technology of fast necking, which constitutes the basic method of Czochralski monocrystalline silicon. First, the silicon material is heated and melted in a quartz crucible, and then the seed crystal is placed in the molten silicon. After the solution around the seed crystal is cooled, the silicon crystal will adhere to the seed crystal. After the temperature and pulling speed meet the requirements Pull the crystal upwards. After the crystal is pulled to the predetermined requirements, the tail will be pulled into a cone shape, so that a complete single crystal is formed. Because it has to go through a solid-liquid interface process, which is equivalent to a directional solidification process, it is also a purification process. Process.

The specific method is: the raw material is heated and melted in a crucible, and the end of a fine single crystal (called a seed crystal) cut into a specific orientation is immersed in the solution and slightly melted. Then, the temperature is controlled, the seed crystal is slowly lifted vertically, and the pulled-out liquid solidifies into a single crystal. The required diameter of the single crystal rod can be obtained by adjusting the heating power (Figure 1). The furnace body of the Czochralski body growth equipment is generally made of metal (such as stainless steel), and is clamped by a seed rod and a crucible rod respectively. Holds the seed crystal and supports the crucible, and can rotate and move up and down. The crucible is generally heated by resistance or high frequency induction. The furnace atmosphere can be inert gas or vacuum.

What is the Czochralski method for preparing single crystal silicon?
Figure 1 Schematic diagram of the Czochralski process

There are the following specific stages.

①Seeding, through resistance heating, melt the polysilicon in the quartz crucible, and keep the temperature slightly higher than the melting point of silicon, immerse the seed crystal in the melt, then pull the seed crystal upward at a certain speed and rotate the crystal at the same time.

② neck down. A crystal with a narrowed elongated neck is grown to a certain length to prevent dislocations in the seed crystal from extending into the crystal.

③ Put your shoulders down. Control the crystal to the desired diameter.

④ isometric growth. According to the melt and single crystal furnace conditions, control the crystal to grow to the desired length with equal diameter.

⑤ Finishing. The diameter gradually decreases, leaving the melt.

⑥ Cool down, lower the temperature, take out the crystal, and wait for subsequent processing.
Figure 2 shows the Czochralski production process.

What is the Czochralski method for preparing single crystal silicon?
Figure 2 Czochralski production process

2. Several basic problems of the straight pull method
(1) Maximum growth rate
The maximum speed of crystal growth is related to the longitudinal temperature gradient in the crystal, the thermal conductivity of the crystal, the crystal density, etc. Increasing the temperature gradient in the crystal can increase the crystal growth rate; however, if the temperature gradient is too large, it will produce larger crystals. Thermal stress can lead to the formation of crystal defects such as dislocations, and even cracks in the crystal. In order to reduce the dislocation density, the actual growth rate of the crystal is often lower than the maximum growth rate.

(2) Convection in the melt
The forced convection created by the oppositely rotating crystal (clockwise) and crucible is driven by centrifugal and centripetal forces, and ultimately by the melt surface tension gradient. The larger the diameter of the crystal grown (the larger the crucible), the stronger the convection, which will cause the melt
Medium temperature fluctuations and local remelting of crystals lead to uneven distribution of impurities in crystals, etc. In actual production, the rotation speed of crystals is generally 1 to 3 times faster than that of crucibles, and the mutually opposite movement of crystals and crucibles causes melt The relative movement between the central area and the peripheral area is beneficial to form a relatively stable area under the solid-liquid interface, which is beneficial to the stable growth of crystals.

(3) Growth interface shape (solid-liquid interface)
The shape of the solid-liquid interface has an important influence on the uniformity and integrity of the single crystal. Normally, the macroscopic shape of the solid-liquid interface should coincide with the melt isothermal surface determined by the thermal field. During the seeding and shouldering stage, the solid-liquid interface is convex to the melt, and after the single crystal grows with equal diameter, the interface becomes flat and then concave to the melt. The shape of the solid-liquid interface can be adjusted by adjusting the crystal pulling speed, crystal rotation and crucible rotation speed.

(4) Differences in growth conditions at various stages during the growth process
In the seeding stage of the Czochralski method, the height of the melt is the highest, and the height of the bare crucible wall is the smallest. During the crystal growth process until the end stage, the height of the bare crucible wall continues to increase, which causes the growth conditions to change constantly (convection of the melt, heat transfer , solid-liquid interface shape, etc.), that is, the entire ingot experiences different thermal histories from the beginning to the end, the head is heated for the longest time and the tail is the shortest, which will cause uneven distribution of impurities in the axial and radial directions of the crystal.

The advantage of the Czochralski method is that the crystal is pulled out of the liquid surface without contact with the vessel wall and is not restricted by the container, so the stress in the crystal is small, and at the same time, it can prevent the wall of the vessel from being stained or contact may cause disordered crystallographic directions. The Czochralski method also uses the oriented seed crystal as the growth nucleus, so a crystal with a certain crystal orientation can be obtained. The crystal orientation of the Czochralski method is better than that of the casting method, but the growth rate is low and the relative cost is also high. .

3. Technical improvements of the Czochralski method
(1) Magnetically controlled straight-pull technology
In the Czochralski method, the oxygen content and its distribution are very important and difficult to control parameters, mainly because the thermal convection in the melt intensifies the effect of the molten silicon and the quartz crucible, that is, impurities such as O2, B, A1 in the crucible are prone to into the melt and crystals. Thermal convection also causes temperature fluctuations in the melt, leading to the formation of impurity streaks and vortex defects in the crystal. Silicon melts are good conductors. When a magnetic field is applied to the melt, the melt will be affected by the Lorentz force in the opposite direction of its motion, which can hinder the convection in the melt, which is equivalent to increasing the viscosity in the melt. In production, technologies such as horizontal magnetic field and vertical magnetic field are usually used. The advantage of the magnetron Czochralski technique compared to the Czochralski method is that it reduces temperature fluctuations in the melt. Generally, the temperature fluctuation in the melt near the solid-liquid interface in the Czochralski method is more than 10°C, but when a 0.2T magnetic field is applied, the temperature fluctuation is less than 1°C. In this way, the uniformity of the impurity distribution in the crystal can be significantly improved, and the radial distribution uniformity of the crystal can also be improved: the defect density in the single crystal is reduced; the entry of impurities is reduced, and the purity of the crystal is improved. This is because under the action of the magnetic field, the interaction between the molten silicon and the crucible is weakened, so that less impurities in the crucible enter the melt and crystal. Combining the magnetic field strength with the process parameters such as crystal rotation and crucible rotation can effectively control the crystal Changes in oxygen concentration; due to the magnetic viscosity, the thickness of the diffusion layer increases, which can improve the uniformity of the longitudinal distribution of impurities; it is beneficial to improve the productivity, using the magnetron Czochralski technology, such as using a horizontal magnetic field, when the growth rate is normal Czochralski When the method is doubled, higher quality crystals can still be obtained.

(2) Continuous growth technology
In order to improve productivity and save quartz crucibles, continuous Czochralski growth technology has been developed, mainly including recharging and continuous feeding: ① Recharging Czochralski growth technology can save a lot of time (cooling after growth, opening the furnace, loading furnace, etc.) One crucible can be used multiple times; ② In addition to the advantages of recharging, the continuous feeding Czochralski growth technology can keep the volume of the chamber constant throughout the growth process, improve the basically stable growth conditions, and thus obtain the resistivity There are two feeding methods for the continuous feeding Czochralski growth technology of single crystal with uniform longitudinal distribution: continuous solid feeding method and continuous liquid feeding method.

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