Heater, monocrystalline silicon drawing method and monocrystalline silicon

Abstract

The invention relates to the technical field of monocrystalline silicon preparation, and discloses a heater, a monocrystalline silicon drawing method and monocrystalline silicon. The heater comprises an annular heating part, two opposite ends of the annular heating part are a high-position end and a low-position end, the height of the heater is H, and the thickness of the heater is D; the heater is composed of a high-resistance area and a low-resistance area. The section of the high-resistance area passing through the axis of the heater is in a wedge shape, the small head end of the wedge shape is located at the low-position end, the large head end of the wedge shape is located at the high-position end, the thickness of the small head end is 0, the thickness of the large head end is d, and d is (0.5-1) D; and the resistivity of the high-resistance region is 20-50% higher than that of the low-resistance region. The heater provided by the invention is used for heating a silicon material, so that the content of free oxygen can be reduced, and the quality of the prepared monocrystalline silicon is improved.

Description

Technical Field

[0001] This invention relates to the field of monocrystalline silicon preparation technology, and more specifically, to a heater, a method for pulling monocrystalline silicon, and monocrystalline silicon. Background Technology

[0002] Monocrystalline silicon, as a key basic material in core fields such as semiconductor devices and photovoltaic cells, directly determines the performance and reliability of downstream products through its crystal quality. Currently, the large-scale preparation of monocrystalline silicon mainly adopts the Czochralski (CZ) method and the improved multiple-feed Czochralski (RCZ) method, which guides the directional crystallization of silicon melt to form monocrystalline silicon rods through seed crystals. However, the above-mentioned pulling process generally suffers from the technical pain points of free oxygen generation and residue, which seriously restricts the improvement of monocrystalline silicon quality.

[0003] During the pulling of monocrystalline silicon, the quartz crucible (mainly composed of SiO2) containing the molten silicon reacts with the molten silicon under high temperature conditions. Simultaneously, the high-temperature radiation from the thermal field system causes oxygen to be released from the crucible surface. Some of this oxygen dissolves in the molten silicon and is ultimately trapped inside the crystallized monocrystalline silicon rod as free oxygen. Free oxygen in monocrystalline silicon easily forms oxygen donors, oxygen precipitates, and lattice defects. On the one hand, this significantly reduces carrier lifetime and resistivity uniformity, leading to deterioration of the electrical properties of monocrystalline silicon and affecting the conductivity and stability of downstream devices. On the other hand, it can induce internal stress within the crystal, increasing the risk of wafer breakage and edge chipping during subsequent slicing processes, thus reducing the production yield.

[0004] Although various oxygen reduction schemes have been proposed in existing technologies, including oxygen-reducing heaters with shortened widths, adjusting the angle of the flow guide tube, adjusting the crucible rotation speed, and adjusting argon and furnace pressure parameters, these methods have a significant adverse impact on crystal formation quality in practical applications, making it difficult to achieve a balance between oxygen reduction and crystal formation stability. Furthermore, as monocrystalline silicon develops towards larger sizes and higher purity, and as the oxygen content requirements for semiconductor-grade and high-efficiency photovoltaic-grade monocrystalline silicon become increasingly stringent, existing control methods are insufficient to meet the needs of high-quality monocrystalline silicon production.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a heater, a method for pulling monocrystalline silicon, and monocrystalline silicon.

[0007] This invention is implemented as follows: In a first aspect, the present invention provides a heater, the heater including an annular heating part, the annular heating part having a high end and a low end at opposite ends, the heater having a height of H and a thickness of D; The heater consists of a high-resistance region and a low-resistance region; The high-resistance region has a wedge-shaped cross-section passing through the heater axis. The small end of the wedge is located at the low end, and the large end is located at the high end. The thickness of the small end is 0, and the thickness of the large end is d, where d is (0.5~1)D. Alternatively, the high-position end is located at the top, and the high-resistance region is located at the top of the heater. The shape of the cross-section passing through the heater axis is rectangular, with a length of h and a thickness of L. h is (0.5~0.8)H and L is (0.3~0.8)D. The resistivity of the high-resistivity region is 20-50% higher than that of the low-resistivity region.

[0008] In an optional implementation, the high-resistivity region is made of at least one of ordinary petroleum coke graphite and pitch coke graphite.

[0009] In an optional embodiment, the material of the low-resistivity region is at least one of needle-shaped coke graphite and metal-doped graphite.

[0010] In an optional implementation, H is 270~290mm and D is 20~30mm.

[0011] Secondly, the present invention provides a method for pulling single-crystal silicon, comprising heating silicon material by means of an annular heating section arranged around the crucible as described in any of the foregoing embodiments.

[0012] In optional implementations, the process includes melting, temperature control, crystal pulling, shoulder formation, equal diameter setting, and finishing steps. During the melting process, AC power is applied to the heater to heat the silicon material. When the silicon material melts to a solid-liquid ratio of 0.15~0.35, DC power is switched on. Optionally, when the silicon material is melted to a solid-liquid ratio of 0.15 to 0.25 during the initial melting, DC power is switched; after RCZ is added, DC power is switched when the solid-liquid ratio is melted to a solid-liquid ratio of 0.25 to 0.35.

[0013] In an optional implementation, the AC frequency is 9000~11000Hz and the power is 2000~2140Kw.

[0014] In an optional implementation, the current when AC and / or DC is applied is 1900~2100A.

[0015] In an optional implementation, the temperature control process includes heating and natural cooling processes; The heating process uses AC electric heating with a frequency of 600~800Hz, a power of 9.6~10.6Kw, and a current of 1900~2100A; Heat to 1600~1650℃, then let it naturally cool down to the temperature required for crystal development.

[0016] Thirdly, the present invention provides a single-crystal silicon prepared by any of the preparation methods described in the foregoing embodiments.

[0017] The present invention has the following beneficial effects: The heater provided by this invention has its high-resistance components mainly distributed in the upper part of the heater. Higher resistance in the upper part of the heater results in higher heating power, shifts the high-temperature line upwards, and reduces heat flux at the tangent point between the sidewall and bottom of the quartz crucible, thereby reducing free oxygen in the molten silicon.

[0018] In a preferred embodiment, using AC heating during the melting process of single crystal pulling can further reduce the amount of free oxygen. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of an existing heater, where the ring-shaped structure represents the heating element. Figure 2 A schematic diagram showing the position of the high-temperature wire of the heater after adjusting the resistance of the heater; Figure 3 This is a schematic diagram of the convection diffusion of free oxygen through molten silicon. Figure 4 Cross-sectional views of three specific types of structures shown in the first structure of the heating section of the heater provided in the embodiment of the present invention; Figure 5 Cross-sectional views of two specific types of structures shown in the second type of structure of the heating part of the heater provided in the embodiment of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0022] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0023] Based on the problems existing in the current technology, the inventors considered the following: like Figure 1 As shown, Figure 1This is an existing heater where the annular heating element has a uniform thickness throughout and is made of the same material. The high-temperature wire of this heater is located in the center. For example... Figure 2 As shown, if a portion of the upper part of the heater is cut off to reduce the thickness of the annular heating element, it will move upwards at high temperatures; for example... Figure 3 As shown, 1. Oxygen is mainly generated at the solid-liquid interface; 2. Most of the oxygen diffuses into the oxygen-deficient zone; 3. A small portion of the oxygen migrates under the influence of vortex 1 and gravity and settles at the point where the sidewall is tangent to the bottom of the pot (part R). Figure 3 The oxygen-rich zone is shown; 4. After the overall temperature rises, the free oxygen in the oxygen-rich zone diffuses into vortex 2, is carried by vortex 2 to vortex 3, and finally enters the crystal. Therefore, the upward movement of the high-temperature line reduces the heat flux at the tangent point between the sidewall and the bottom of the quartz crucible, thereby reducing the free oxygen in the molten silicon. However, if the free oxygen is reduced by thinning a portion of the heater, the thinned area is prone to corrosion.

[0024] Therefore, this application provides the following technical solution: This invention provides a heater, which includes an annular heating part, with two opposite ends being a high end and a low end. The height of the heater is H and the thickness is D. The heater consists of a high-resistance region and a low-resistance region; (1) such as Figure 4 As shown, the high resistance region has a wedge-shaped shape on the cross section passing through the heater axis. The small end of the wedge is located at the low end, and the large end of the wedge is located at the high end. The thickness of the small end is 0, and the thickness of the large end is d, where d is (0.5~1)D. Or, (2) such as Figure 5 As shown, the high-position end is located at the top, and the high-resistance region is located at the top of the heater. The shape of the cross-section passing through the heater axis is rectangular, with a length of h and a thickness of L. h is (0.5~0.8)H and L is (0.3~0.8)D. The resistivity of the high-resistivity region is 20-50% higher than that of the low-resistivity region.

[0025] The heater provided by this invention has high resistance mainly distributed in the upper part of the heater. The higher the resistance in the upper part of the heater, the higher the heating power, the higher the temperature line moves upward, and the heat flux of the R part of the quartz crucible decreases, thereby reducing the free oxygen in the molten silicon. When the specific structure of the heater is (1), the volume of the high resistance area (wedge-shaped) gradually increases from the bottom to the top of the heater, and the heating power of the heater gradually increases from the bottom to the top. Compared with structure (2), it can better reduce free oxygen, make the working temperature distribution more uniform, and thus better maintain the stable growth of crystal; and there is no structural weak area, better corrosion resistance, and longer service life. For the heater structure of type (1), when the ratio of the thickness of the large end to the thickness of the entire heater is in the range of 0.5 to 1, the amount of free oxygen can be reduced to a greater extent. If the thickness of the large end is too small, the proportion of high resistance in the upper part of the heater is small, and the effect on reducing free oxygen is weak. For the heater in (2), when the ratio of L and h to the overall thickness and height of the heater is within the range required by the present invention, the amount of free oxygen can be reduced to a greater extent. If the size is too small, the effect on reducing free oxygen is weak. If the size is too large, the heating effect is not good.

[0026] Furthermore, for the first type of heater structure, there are three specific forms: such as Figure 4 As shown, the high-resistance part is located close to the inner side (left figure), the high-resistance part is located in the middle (middle figure), and the high-resistance part is located close to the outer side (right figure). For the second type of heater structure, there are two specific forms: such as... Figure 5 As shown, the high-resistance part is placed closer to the inner side (left figure), and the high-resistance part is placed closer to the outer side (right figure).

[0027] Optionally, the material of the high-resistivity region is at least one of ordinary petroleum coke graphite and pitch coke graphite.

[0028] Optionally, the material of the low-resistivity region is at least one of needle-shaped coke graphite and metal-doped graphite.

[0029] Optionally, H is 270~290mm and D is 20~30mm. The height of the heater is matched to the height of a conventional crucible, and its thickness ensures good heating of the silicon material while avoiding corrosion due to excessive thickness.

[0030] This invention also provides a method for pulling single-crystal silicon, which includes heating silicon material by using an annular heating section provided in this invention, which is arranged around the outside of a crucible.

[0031] Because this drawing method uses the heater provided in the embodiments of the present invention for heating, less free oxygen is generated.

[0032] Specifically, the drawing method includes melting, temperature adjustment, crystal pulling, shoulder formation, equal diameter forming, and finishing processes; During the melting process, AC power is applied to the heater to heat the silicon material. When the silicon material melts to a solid-liquid ratio of 0.15~0.35, DC power is switched on.

[0033] The solid-liquid ratio here is, for example, the area ratio of solid to liquid captured by a CCD camera.

[0034] Compared to direct current heating of the crucible, using alternating current (AC) to directly heat the silicon material after adding it avoids the phenomenon of a large amount of free oxygen being generated by the reaction between the quartz crucible and the molten silicon. The reason for stopping AC heating only after the silicon has nearly completely melted, rather than after it has completely melted, is that the residual heat in the crucible allows the remaining silicon material to continue melting, thus eliminating the need for complete melting. Switching to direct current heating after stopping AC heating, rather than stopping heating altogether, helps maintain the silicon material's stability and uniformity within a suitable range. Therefore, using AC heating primarily in the melting process yields higher quality monocrystalline silicon.

[0035] Optionally, during the initial melting, DC power is switched when the silicon material melts to a solid-liquid ratio of 0.15~0.25, and DC power is switched again when the RCZ material is added and the melting reaches a solid-liquid ratio of 0.25~0.35. Since the furnace temperature is generally low during the initial melting, AC power is used to heat the material until a greater degree of melting is achieved, at which point DC power is switched to ensure complete melting.

[0036] Optionally, the AC frequency is 9000~11000Hz and the power is 2000~2140 kW. Optionally, the current when AC and / or DC is applied is 1900~2100A.

[0037] At the aforementioned frequencies, power, and currents, the penetration depth is still greater than the crucible radius, enabling heating of the silicon material near the center. Since the power of induction heating is positively correlated with both frequency and current, and higher frequencies result in shallower penetration depths (skin depth), while lower frequencies result in deeper penetration depths. Therefore, the frequency cannot be too low, nor is higher always better; only a suitable frequency range can ensure sufficiently uniform heating of the silicon material.

[0038] Optionally, the temperature control process includes both heating and natural cooling. The heating process uses alternating current (AC) heating at a frequency of 600-800 Hz, a power of 9.6-10.6 kW, and a current of 1900-2100 A to raise the temperature to 1450-1480°C, followed by natural cooling to the temperature required for crystal development. AC heating directly heats the silicon material, further preventing the generation of free oxygen. The aforementioned frequency, power, and current are sufficient to heat the central part while avoiding excessive energy output that could damage the quartz crucible.

[0039] Optionally, the operating parameters for the crystal-drawing process are: crucible rotation 5~6 rpm, crystal rotation 5~6 rpm, crystal pulling rate 180 mm / min~210 mm / min, argon flow rate 90~120 SLM; the operating parameters for the shoulder-forming process are: crucible rotation 5~6 rpm, crystal rotation 5~6 rpm, crystal pulling rate 30 mm / min~60 mm / min, argon flow rate 90~120 SLM; the operating parameters for the equal diameter process are: crucible rotation 3~5 rpm, crystal rotation 5~6 rpm, crystal pulling rate 90 mm / min~100 mm / min, argon flow rate 90~120 SLM; and the operating parameters for the finishing process are: crucible rotation 3~5 rpm, crystal rotation 5~6 rpm, crystal pulling rate 150 mm / min~180 mm / min, argon flow rate 90~120 SLM.

[0040] Explanation based on the principle of alternating current melting: In the process of manufacturing single-crystal silicon using the RCZ method, most of the oxygen comes from the reaction between the crucible and the molten silicon at 1420℃. The reaction process and mathematical expression are as follows: Equations 1 and 2 describe the process by which a quartz crucible reacts with molten silicon to produce free oxygen: Equation 1: SiO2 + Si → 2SiO; Equation 2: 2SiO → 2Si + O2; Combining Equations 1 and 2, we can obtain the simplified process equation 3: SiO2 + Si → 2Si + O2; According to the Arrhenius equation, the relationship between reaction rate and temperature is given by equation 4: ; k: Reaction rate constant (unit depends on reaction order); A: Pre-exponential factor (or frequency factor) represents the frequency and direction of molecular collisions. It is not significantly related to temperature and is measured in kJ. Ea: Activation Energy, the energy barrier that must be overcome for a reaction to occur, measured in J / mol or kJ / mol. This is a key parameter determining the temperature sensitivity of the reaction rate. R: constant (8.314 J / mol·K); T: Absolute temperature (unit: Kelvin K).

[0041] As can be seen from Equation 4, during the RCZ farad process for producing single-crystal silicon, the rate of oxygen generation at the interface between the quartz crucible and the molten silicon is positively correlated with temperature, while the increase in oxygen generation is positively correlated with reaction time.

[0042] All other things being equal, lowering the temperature of the crucible-silicon molten interface can reduce the oxygen content in the silicon molten material.

[0043] Therefore, the key to effectively reducing the free oxygen content in molten silicon is to reduce the high-temperature baking time of the quartz crucible. 1. During the melting process, the power supply mode of the heater is switched to AC power. According to the eddy current heating penetration depth formula, Equation 5: Where δ (penetration depth), ρ (resistivity), f (excitation frequency), and μ (permeability).

[0044] When silicon material is heated by eddy current, the following conversion process occurs: electrical energy → magnetic energy → electrical energy → thermal energy; In the traditional RCZ method, the temperature of the central part is the lowest because it is far from the heater during the heating process of silicon material.

[0045] When using eddy current heating, the penetration depth can be changed by adjusting the frequency, thereby heating a specified depth.

[0046] For example, taking a mainstream 36-inch hot zone as an example, with a quartz crucible inner diameter of approximately 920 mm, a polycrystalline silicon ohm value of 1.6 Ω·cm, a permeability of 1, and a frequency of 10000 Hz: ; Depth 1: At a frequency of 10000Hz, the skin depth is 0.6366m, which is still greater than the radius of 0.456m; Depth 2: At a frequency of 700Hz, the skin depth is 2.406m.

[0047] After completing the final stage, the next round of RCZ begins. When silicon material is added, the center temperature decreases. Traditional heaters require a significant increase in power, with the main heater increasing from 45kW to 85kW and the bottom heater from 0kW to 95kW. The total power received by the quartz crucible is approximately 180kW. The quartz crucible is heated first, and then the heat is conducted to the molten silicon. This process increases the temperature at the interface between the quartz crucible and the molten silicon. According to the conclusion drawn from Equation 4, this process generates a large amount of free oxygen through the reaction between the quartz crucible and the molten silicon.

[0048] The heater in this application is switched to AC power supply to directly heat the silicon material.

[0049] According to a commonly used formula for eddy current heating, Equation 6: ; Integrating Equation 6, we get Equation 7: ; P: Total power absorbed by the heated material (W); B: Peak magnetic flux density (T) of the alternating magnetic field acting on the material surface; f: Frequency (Hz) of the alternating magnetic field; ω: ω = 2πf; d: Thickness (m) or characteristic dimension of the material being heated; V: Volume of the material being heated (cubic meters, m³); ρ: Resistivity of the material being heated at the heating temperature; σ: σ = 1 / ρ; According to the commonly used formula for magnetic flux density, Equation 8: ; μ0: Vacuum permeability; N: Total number of turns in the coil; Ip: Peak current in the coil; L: Length of the object being heated (height of silicon material inside the quartz crucible); In this case, the corresponding thermal field R = 0.456m, length L = 0.64m, resistivity ρ = 1.6Ω·cm, and frequency f = 10000Hz, substituting these values ​​into Equations 8 and 7 respectively, yields the power 1: P ≈ 2.07 × 10⁻⁶. 6 =2.07MW=2070kW; Heating 974 kg of silicon material from 25℃ to 1450℃, with a specific heat capacity of 700 J / kg·K, yields the energy requirement: Q = 974 * 700 * 1425 = 9.72 * 10 8 J≈270kWh; If the coupling efficiency is 60%, approximately 450 kWh of electrical energy is required, and heating under power condition 1 would take 13 minutes. However, DC resistance heating typically outputs 85 kW of power, requiring 450 / 85 = 5.3 hours = 318 minutes. The comparison clearly shows that the heating time required by DC is significantly longer than that by AC.

[0050] Example 1 like Figure 4 As shown, this embodiment provides a heater, the structure of which is as follows: Figure 4 As shown in the left figure, H is 280 mm, D is 25 mm, and d is 0.8D. The material in the high-resistivity region is ordinary petroleum coke graphite (resistivity 24.3 μΩ·m), and the material in the low-resistivity region is needle coke graphite (resistivity 18 μΩ·m), with a resistivity difference of 35%.

[0051] The method for pulling single-crystal silicon provided in this embodiment is as follows: Molten material: The crucible containing silicon material is heated by passing an alternating current (AC) through the heater. The AC frequency is 10000Hz, the power is 2070 kW, and the current is 2000A. When the solid-liquid ratio is detected to be 0.2, the current is switched to direct current (DC) with a current of 2000A and a power of 180kW. Temperature adjustment: Once the melting point is detected as complete, switch to AC power at 700Hz, 2000A, and 10.1 kW. When the temperature reaches approximately 1450~1480℃ (the color of the molten silicon is observed via a CCD camera and vision software; the peak brightness wavelength of 1680nm is in the infrared band, and this color corresponds to this temperature range), stop all heating and allow the temperature to drop naturally to approximately 1414~1416℃ (the peak brightness wavelength of the molten silicon is observed via a CCD camera and vision software; this color corresponds to this temperature range), and then begin crystal development. Seeding: The crucible rotates at 6 rpm, the crystal rotates at 6 rpm, the crystal pulling rate is 180 mm / min to 210 mm / min, and the argon flow rate is 90 SLM. Relax your shoulders: The crucible rotates at 5 rpm, the crystal rotates at 6 rpm, the crystal pulling rate is 30 mm / min to 60 mm / min, and the argon flow rate is 90 SLM. Equal diameter: The crucible rotates at 5 rpm, the crystal rotates at 6 rpm, the crystal pulling rate is 90 mm / min to 100 mm / min, and the argon flow rate is 90 SLM. ending: The crucible rotates at 5 rpm, the crystal rotates at 5 rpm, the crystal pulling rate is 150 mm / min to 180 mm / min, and the argon flow rate is 90 SLM.

[0052] RCZ charging, followed by melting: The crucible for adding silicon material is heated by passing AC power to the heater. The AC power frequency is 10000Hz, the power is 2070 kW, and the current is 2000A. When the solid-liquid ratio is detected to be 0.3, the power is switched to DC power with a current of 2000A and a power of 180 kW.

[0053] The subsequent temperature adjustment and final procedures are the same as those described above.

[0054] The process of pulling monocrystalline silicon continues, and the process is the same as the melting and finishing steps after adding RCZ material.

[0055] Example 2 This embodiment is basically the same as Embodiment 1, except that the structure of the heater is as follows. Figure 4 As shown in the middle figure, d is 0.5D. The material in the high-resistivity region is ordinary petroleum coke graphite (21.6 μΩ·m), and the material in the low-resistivity region is needle coke graphite (resistivity 18 μΩ·m), with a resistivity difference of 20%.

[0056] Example 3 This embodiment is basically the same as Embodiment 1, except that the structure of the heater is as follows. Figure 4As shown in the right figure, d represents D. The material in the high-resistivity region is ordinary petroleum coke graphite (resistivity 27 μΩ·m), and the material in the low-resistivity region is needle coke graphite (resistivity 18 μΩ·m), with a resistivity difference of 50%.

[0057] Comparative Example 1 This comparative example is basically the same as Example 1, except that the heater is an existing heater, and the difference from the heater in Example 1 is that it does not have a high-resistance region and is made entirely of a material with a low-resistance region.

[0058] Comparative Example 2 This comparative example is basically the same as Comparative Example 1, except that: the melting process does not use AC heating, but uses DC heating according to the current conventional method, with a specific heating method of voltage 50V, current 2100A, and output power of 105kw.

[0059] Comparative Example 3 This comparative example is basically the same as Comparative Example 1, except that: the melting process does not use AC heating, but uses DC heating according to the current conventional method, with heating parameters of voltage 50V, current 2100A, and output power of 105kW; the temperature adjustment process also does not use AC heating, but uses DC heating, with heating parameters of 50V, current 200A, and output power of 10kW.

[0060] Comparative Example 4 This comparative example is basically the same as Comparative Example 1, except that: during the melting process, AC power is used to heat the material until the solid-liquid ratio reaches the target ratio, and then the heating is stopped directly without switching to DC power until all the material is melted.

[0061] Experimental Example The prepared single-crystal silicon rods were tested, and the oxygen content was measured by sampling the crystal head using Fourier transform infrared spectroscopy (FTIR). The melting time and temperature control time were also recorded.

[0062] Record the test results in Table 1.

[0063] Table 1 Test results for each embodiment and comparative example

[0064] As can be seen from the test results in Table 1, the single crystal silicon oxygen content obtained by the preparation methods provided in each embodiment of the present invention is lower than that of conventional methods, which indicates that less free oxygen is generated during the preparation process. Comparing Comparative Example 1 with Example 1, the oxygen content of Comparative Example 1 is worse, indicating that more free oxygen is generated during the preparation process using a conventional heater. It can be predicted that the photoelectric conversion efficiency of the single crystal silicon prepared in Example 1 will be reduced. Comparing Comparative Example 2 with Comparative Example 1, Comparative Example 1 has a lower oxygen content, indicating that using AC heating during the melting process will reduce the generation of free oxygen. Comparing Comparative Example 3 with Comparative Example 2, Comparative Example 2 has less oxygen content, indicating that using AC heating during the temperature adjustment process will also reduce the generation of free oxygen. Comparing Comparative Example 4 with Comparative Example 1, Comparative Example 4 stopped heating directly after AC heating without switching to DC heating, and the temperature adjustment time was significantly longer.

[0065] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A heater, characterized in that, The heater includes an annular heating section, with a high end and a low end at opposite ends. The height of the heater is H and the thickness is D. The heater consists of a high-resistance region and a low-resistance region; The high-resistance region has a wedge-shaped shape in the cross section passing through the axis of the heater. The small end of the wedge is located at the low end, and the large end of the wedge is located at the high end. The thickness of the small end is 0, and the thickness of the large end is d, where d is (0.5~1)D. Alternatively, the high-position end is located at the upper part, the high-resistance region is located at the upper part of the heater, and the shape of the cross section passing through the axis of the heater is rectangular, the length of the rectangle is h, the thickness is L, h is (0.5~0.8)H, and L is (0.3~0.8)D; The resistivity of the high-resistivity region is 20-50% higher than that of the low-resistivity region.

2. The heater according to claim 1, characterized in that, The high-resistivity region is made of at least one of petroleum coke graphite and pitch coke graphite.

3. The heater according to claim 1, characterized in that, The material of the low-resistivity region is at least one of needle-shaped coke graphite and metal-doped graphite.

4. The heater according to claim 1, characterized in that, H is 270~290mm, and D is 20~30mm.

5. A method for pulling single-crystal silicon, characterized in that, This includes heating the silicon material by means of an annular heating element as described in any one of claims 1 to 4, which is arranged around the outside of the crucible.

6. The drawing method according to claim 5, characterized in that, This includes processes such as melting, temperature control, crystal pulling, shoulder formation, equal diameter setting, and finishing. During the melting process, the heater is powered by alternating current to heat the silicon material. When the silicon material melts to a solid-liquid ratio of 0.15 to 0.35, the power is switched to direct current. Optionally, when the silicon material is melted to a solid-liquid ratio of 0.15 to 0.25 during the initial melting, DC power is switched; after RCZ is added, DC power is switched when the solid-liquid ratio is melted to a solid-liquid ratio of 0.25 to 0.

35.

7. The drawing method according to claim 5, characterized in that, The AC frequency is 9000~11000Hz, and the power is 2000~2140kW.

8. The drawing method according to claim 7, characterized in that, The current when AC and / or DC is applied is 1900~2100A.

9. The drawing method according to claim 5, characterized in that, The temperature regulation process includes heating and natural cooling. The heating process uses AC electric heating with a frequency of 600~800Hz, a power of 9.6~10.6 kW, and a current of 1900~2100A; The temperature is raised to 1600~1650℃ and then naturally drops back to the temperature required for crystal development, 1410~1420℃.

10. A type of monocrystalline silicon, characterized in that, It is obtained by the drawing method as described in any one of claims 5 to 9.

Images