Method for preparing silicon single crystal through diameter-variable and pulling-speed-variable seeding

Abstract

The invention discloses a method for preparing silicon single crystals through diameter-variable and pulling-speed-variable seeding, and belongs to the technical field of preparation of semiconductor silicon materials through a czochralski method. According to the method, in a seeding stage, the diameter of a seed crystal is controlled to periodically change between the maximum diameter and the minimum diameter, and meanwhile, the pulling speed of the seed crystal is controlled to periodically change between the maximum pulling speed and the minimum pulling speed. Through the collaborative periodic change of the diameter and the pulling speed, the dynamic stress field and temperature field change is introduced into the crystal, the solid-liquid interface fixed morphology is changed, the steady-state path of dislocation extension is broken, and dislocation proliferation, steering and rapid extension to the crystal surface disappear are actively promoted, so that the dislocation discharge efficiency and the seeding success rate are remarkably improved. The method is particularly suitable for challenging conditions such as large thermal field temperature gradient or growth of special crystal orientation, hardware does not need to be changed, and implementation is easy.

Description

Technical Field

[0001] This invention relates to the field of semiconductor material preparation technology, specifically to a method for growing silicon single crystals using the Czochralski method, and particularly to a method for efficiently removing dislocations by controlling the changes in seed crystal diameter and pulling speed. Background Technology

[0002] The Czochralski method is the mainstream method for preparing large-diameter, dislocation-free silicon single crystals. In this method, the seed crystal stage is crucial for obtaining a dislocation-free single crystal. Currently, the industry commonly uses the Dash method for seed crystal production: a large-diameter seed crystal (typically 15-25 mm) is immersed in a silicon melt, and then pulled to rapidly reduce the crystal diameter to a smaller size (e.g., 3-6 mm, i.e., a "neck"). Utilizing the principle that dislocations extend to the crystal surface and disappear during the necking process, the original dislocations in the seed crystal are expelled within a short length (typically 50-600 mm), thus obtaining a dislocation-free single crystal above the neck.

[0003] However, the traditional Dash method for crystal growth has certain limitations. Its dislocation removal effectiveness largely depends on stable thermal conditions. When encountering minor fluctuations in the thermal field, growing single crystals with special crystal orientations (such as the (110) orientation, whose slip system differs from the common (100) orientation), or other process anomalies, dislocation lines may not effectively and quickly extend to the crystal surface, leading to dislocation "necking" failure and a decrease in crystal growth success rate. If dislocations are not completely removed, the entire single crystal growth will fail, resulting in significant material and energy losses.

[0004] To improve the success rate of crystal seeding, those skilled in the art have made various attempts, such as using seed crystals with specific doping or designing seed crystals with special shapes. However, these methods often increase process complexity and cost, and the improvement in effect is limited. Therefore, there is an urgent need for a method that can more effectively and reliably remove dislocations and improve the success rate of dislocation-free single crystal seeding without significantly increasing costs and operational difficulties. Summary of the Invention

[0005] This invention aims to overcome the shortcomings of existing Dash crystal pulling methods and provide an improved crystal pulling method. This method, through dynamic control of crystal pulling process parameters, can effectively cope with thermal field fluctuations and adapt to different crystal orientations during growth, thereby significantly improving the efficiency and success rate of dislocation removal and increasing the yield of dislocation-free silicon single crystals.

[0006] The present invention achieves the above-mentioned objectives through the following technical solutions.

[0007] This invention provides a method for preparing silicon single crystals by variable diameter and variable pulling speed seeding, which is implemented in the seeding stage of silicon single crystal growth using the Czochralski method. The method is characterized in that, during the seeding process, the diameter and pulling speed of the seed crystal are periodically and synergistically changed, wherein the change in diameter and the change in pulling speed are synchronous and opposite in direction.

[0008] Furthermore, the method includes the following steps: a. Seed crystal placement and welding: The seed crystal is placed into the Czochralski single crystal furnace so that its lower end contacts the surface of the molten silicon for welding.

[0009] b. Variable Diameter and Variable Pulling Speed ​​for Crystal Seeding: During the seed crystal pulling process, the diameter of the seed crystal is controlled to alternate between a set maximum diameter (D_max) and a set minimum diameter (D_min). Simultaneously, the pulling speed of the seed crystal is controlled to alternate between a set maximum pulling speed (V_max) and a set minimum pulling speed (V_min). That is, the process of increasing the diameter from D_min to D_max is synchronized with the process of decreasing the pulling speed from V_max to V_min; conversely, the process of decreasing the diameter from D_max to D_min is synchronized with the process of increasing the pulling speed from V_min to V_max.

[0010] c. Shoulder Formation and Constant Diameter Growth: After the crystal pulling stage is completed, the shoulder formation and constant diameter growth stage begins until the growth is complete.

[0011] Furthermore, during the crystal pulling process, the periodic change can be a fixed period length, a combination of multiple different period lengths (e.g., a short period is used in the early stage of crystal pulling and a long period is used in the later stage of crystal pulling), or an asymmetric period (e.g., the proportion of crystal pulling length occupied by the diameter increasing stage is different from that of the diameter decreasing stage).

[0012] Furthermore, the maximum diameter (D_max) is 8 mm to 12 mm, and the minimum diameter (D_min) is 4 mm to 6 mm.

[0013] Furthermore, the maximum pulling speed (V_max) is 3.5 mm / min to 10 mm / min, and the minimum pulling speed (V_min) is 0.5 mm / min to 2.0 mm / min.

[0014] Furthermore, the variation cycle of the diameter and / or pulling speed of the seed crystal is one cycle for every 10 mm to 200 mm of the seed crystal length.

[0015] The key to the above-mentioned technical solution of the present invention does not lie in simple parameter adjustment, but in the discovery that the synergistic periodic change of diameter and pulling speed can produce a synergistic effect, which can drive dislocation discharge more efficiently. The mechanism is as follows.

[0016] (1) Dynamic stress field induces dislocation multiplication and reversal: The traditional Dash method uses a single diameter reduction, resulting in a relatively simple stress field. This invention introduces periodic and varying radial thermal stress and growth front stress into the crystal through periodic changes in the seed crystal diameter (e.g., the seed crystal diameter repeatedly changes between 12 mm and 5 mm). This dynamically changing stress field can continuously apply thermal stress of varying magnitude and direction to the dislocation source point, which not only promotes faster dislocation slip but may also induce dislocation multiplication and cross-slip, increasing the probability of dislocation lines reversing and extending to the crystal surface, rather than simply slipping along the inherent slip plane and slip direction to extend into the crystal.

[0017] (2) Temperature gradient and interface shape controlled by pulling speed variation: Synchronous periodic variation of pulling speed (e.g., repeated between 4 mm / min and 1.2 mm / min) directly changes the shape of the solid-liquid interface and the axial temperature gradient near the interface. Higher pulling speed usually means a more concave interface and greater undercooling, while lower pulling speed has the opposite effect. This periodic perturbation of interface shape and temperature gradient, coupled with the stress field change caused by diameter variation, jointly changes the dynamic environment of dislocation extension, breaks the "steady-state" path of dislocation extension, and makes it easier for dislocations to be "thrown out" of the crystal surface in a complex stress-temperature field.

[0018] (3) Parameter synergy and “resonance” effect: This invention does not view the changes in diameter and pulling speed in isolation, but rather superimposes the effects of the two control methods by setting their change period and phase relationship (such as synchronous change). This synergy creates an unstable, strong driving force environment in the crystal, which can more actively and thoroughly “sweep” dislocations than the traditional single diameter reduction method, especially under challenging conditions such as unstable thermal field or growth of special crystal orientation, showing significant robustness.

[0019] Compared with the prior art, the present invention has the following significant advantages: Higher crystal growth success rate: By dynamically changing stress and temperature fields, the efficiency and thoroughness of dislocation evacuation are significantly improved, especially in the presence of small fluctuations or growth in the thermal field. <110> , <111> The success rate is significantly improved when special crystal orientations are used.

[0020] Enhanced process adaptability: This method is insensitive to fluctuations in process conditions, exhibits good robustness, reduces dependence on the extreme stability of the single crystal furnace thermal field, and improves production stability.

[0021] Economy and simplicity: This method is implemented by optimizing software control logic, without changing hardware (such as seed crystal structure) or adding extra materials, without increasing additional costs, and is easy to implement and promote on existing production equipment. Attached Figure Description

[0022] Figure 1 This is a periodic graph showing the changes in pulling speed and seed crystal diameter with seed crystal length during the Czochralski silicon single crystal growth process in Embodiment 1 of the present invention.

[0023] Figure 2 This is a periodic graph showing the changes in pulling speed and seed crystal diameter with seed crystal length during the Czochralski silicon single crystal growth process in Embodiment 6 of the present invention.

[0024] Figure 3 This is a periodic graph showing the changes in pulling speed and seed crystal diameter with seed crystal length during the Czochralski silicon single crystal growth process in Embodiment 7 of the present invention.

[0025] Figure 4 This is a periodic graph showing the changes in pulling speed and seed crystal diameter with seed crystal length during the Czochralski silicon single crystal growth process in Embodiment 8 of the present invention. Detailed Implementation

[0026] The present invention will be further described in detail below with reference to embodiments, but the scope of protection of the present invention is not limited thereto.

[0027] In the following embodiments, a Czochralski single crystal furnace is used to prepare polycrystalline silicon material and a specific crystal orientation (e.g., <100> , <111> or <110> The seed crystal is used to melt and stabilize polycrystalline silicon material in a furnace to form a uniform silicon melt. The seed crystal is then rotated downwards so that its lower end contacts the surface of the melt, completing the fusion. The subsequent steps of shoulder formation, shoulder rotation, equal-diameter growth, and finishing after the seeding stage are all conventional operations in the field and will not be described in detail in this invention. Example 1

[0028] Step 1), Seed crystal placement and welding: Use a seed crystal with an initial diameter of about 18mm to complete the welding.

[0029] Step 2), Variable Diameter and Variable Pulling Speed ​​Seeding: During the seed crystal pulling process, the diameter and pulling speed of the seed crystal are controlled to change periodically. In this embodiment, the variation of the seed crystal diameter and pulling speed with the seed crystal length during the seed crystal pulling process is shown in the attached figure. Figure 1 As shown.

[0030] Starting with the initial diameter of the seed crystal, the diameter is first reduced to a minimum diameter D_min≈5mm over a seed crystal length of approximately 50m, while the pulling speed is increased to a maximum pulling speed V_max≈4.0 mm / min. This stage utilizes the high axial temperature gradient and large radial stress generated by high-speed crystal pulling and the small diameter to initially promote rapid dislocation propagation. Subsequently, when the seed crystal length reaches approximately 100 mm, a periodic change begins, gradually increasing the diameter from D_min≈5 mm to a maximum diameter D_max≈12 mm. Simultaneously, the pulling speed decreases from a maximum pulling speed V_max≈4.0 mm / min to a minimum pulling speed V_min≈1.2 mm / min. At this point, the seed crystal length is approximately 150 mm. During this stage, the increased diameter introduces compressive stress, while the decreased pulling speed flattens the interface and reduces the temperature gradient. This reversal of stress and temperature fields effectively causes dislocation lines to change their slip direction, resulting in cross-slip and increasing their chance of being exposed on the crystal surface. Then, as the seed crystal length increases from 150 mm to 200 mm, the seed crystal diameter gradually decreases from D_max to D_min, while the pulling speed gradually increases from V_min to V_max, completing one cycle. This cycle (from D_min / V_max to D_max / V_min, and then from D_max / V_min to D_min / V_max) is approximately 100 mm long. This process is repeated 2-3 times, resulting in a total lead length of approximately 400 mm. Through this periodic perturbation, dislocations are efficiently "sweeped up".

[0031] Step 3), Shoulder Formation and Constant Diameter Growth: After the crystal pulling stage is completed, the shoulder formation and constant diameter growth stage begins until the growth is complete.

[0032] In the four embodiments of Examples 2-5 below, the length of a single change cycle is the same as that of Example 1, which is approximately 100 mm of crystal pulling length. However, the embodiments show that the diameter and pulling speed change range are adjusted relative to Example 1. Table 1 below shows the parameters of Examples 1-5.

[0033] Table 1

[0034] Example Maximum diameter (mm) Minimum diameter (mm) Maximum pulling speed (mm / min) Minimum pulling speed (mm / min) Periodic lead length (mm) Example 1 12 5 4 1.2 100 Example 2 11 5 4 1.3 100 Example 3 10 5 4 1.4 100 Example 4 9 5 4 1.5 100 Example 5 8 5 4 1.6 100 The crystal pulling process of specific embodiments 2-5 is described in detail below. Example 2

[0035] Step 1) Same as Example 1.

[0036] Step 2) Variable diameter and variable pulling speed crystal pulling: In this embodiment, the diameter variation range is narrowed, D_max≈11mm, D_min≈5mm; at the same time, the pulling speed variation range is also reduced, V_max≈4 mm / min, V_min≈1.3 mm / min. The variation period length is approximately 100mm. Example 3

[0037] Step 1) Same as Example 1.

[0038] Step 2) Variable diameter and variable pulling speed crystal pulling: In this embodiment, the diameter variation range is further narrowed, D_max≈10mm, D_min≈5mm; at the same time, the pulling speed variation range is also reduced, V_max≈4 mm / min, V_min≈1.4 mm / min. The variation period length is approximately 100mm. Example 4

[0039] Step 1) Same as Example 1.

[0040] Step 2) Variable diameter and variable pulling speed crystal pulling: In this embodiment, the diameter variation range is further narrowed, D_max≈9mm, D_min≈5mm; at the same time, the pulling speed variation range is also reduced, V_max≈4 mm / min, V_min≈1.5mm / min. The variation period length is approximately 100mm. Example 5

[0041] Step 1) Same as Example 1.

[0042] Step 2) Variable diameter and variable pulling speed crystal pulling: In this embodiment, the diameter variation range is further narrowed, D_max≈8mm, D_min≈5mm; at the same time, the pulling speed variation range is also reduced, V_max≈4 mm / min, V_min≈1.6mm / min. The variation period length is approximately 100mm.

[0043] In Examples 2-5, compared to Example 1, the diameter variation range gradually narrows, and the pulling speed variation range also gradually decreases, while the length of the variation period remains at approximately 100 mm for the crystal pulling length. This combination of parameters with narrower ranges produces relatively mild stress field and interface shape changes, suitable for applications with relatively stable thermal fields but requiring consideration of crystal orientation (e.g.,...). <110> In cases where dislocation reorientation is inherently difficult, gentle but continuous perturbations can effectively promote the reorientation and reorientation of dislocations without introducing excessive thermal stress that could lead to the generation of new dislocations. Example 6

[0044] Step 1) Same as Example 1.

[0045] Step 2) Variable diameter and variable pulling speed crystal pulling: The parameter variation law of the crystal pulling process in this embodiment is shown in the appendix. Figure 2 As shown.

[0046] Compared to Example 2, this example shortens the length of a single variation cycle to approximately 80 mm. That is, the seed crystal diameter and pulling speed alternate between D_min≈5 mm / V_max≈4.0 mm / min and D_max≈11 mm / V_min≈1.3 mm / min, each changing in opposite directions. One alternation occurs every approximately 40 mm of growth, and two alternations complete one cycle. The shorter variation cycle implies a higher frequency of dynamic modulation of the internal stress field and temperature gradient of the crystal. This high-frequency perturbation can more frequently interrupt the stable path of dislocation extension, making it particularly suitable for situations with high dislocation density or continuous small fluctuations in the thermal field. It can more proactively prevent dislocations from restabilizing and extending into the crystal. Example 7

[0047] This embodiment demonstrates an implementation of an asymmetric periodic change.

[0048] Step 1) Same as Example 1.

[0049] Step 2) Variable diameter and variable pulling speed crystal pulling: The parameter variation law of the crystal pulling process in this embodiment is shown in the appendix. Figure 3 As shown.

[0050] In this embodiment, the process of increasing the diameter from D_min to D_max (corresponding to the pulling speed decreasing from V_max to V_min) takes a relatively long time, accounting for approximately 70% of a complete cycle (e.g., Figure 3 Approximately 70mm growth length); while the process of returning from D_max to D_min (corresponding to the pulling speed increasing from V_min to V_max) is faster, accounting for about 30% of the cycle (e.g. Figure 3 (Approximately 30mm growth length). This asymmetric design focuses on giving dislocations more time to slip and deflect in a relatively gentle stress field during the larger diameter and lower pulling speed stages, which may be more conducive to the discharge of certain types of dislocations. Example 8

[0051] This embodiment demonstrates a more complex multi-period combination implementation.

[0052] Step 1) Same as Example 1.

[0053] Step 2) Variable diameter and variable pulling speed crystal pulling: The parameter variation law of the crystal pulling process in this embodiment is shown in the appendix. Figure 4 As shown.

[0054] This embodiment employs a combination of two different cycles within the total pilot length. In the early stages of piloting (e.g., the first 200mm of pilot length), a shorter cycle is used (e.g., ...). Figure 4 Within the 50mm section, high-frequency perturbations are applied to rapidly excite and initiate dislocation movement. This is done in the later stages of crystal development (e.g., during the seeding process). Figure 4 The last 200mm of the lead-in length), using a longer variation period (e.g. Figure 4 The crystal can be further processed (up to 100mm) for deeper dislocation removal and crystal quality optimization. This combined strategy can be adaptively adjusted according to the objectives of different stages of crystal development, demonstrating the flexibility of the method of this invention.

[0055] The inventors adopted the variable diameter and variable pulling speed crystal pulling methods of the above embodiments and conducted repeated experiments to verify them. Each implementation method was verified by adopting... <100> , <111> , <110> Different seed crystal orientations and more than 10 repeated verifications all yielded excellent dislocation results, with a crystal germination success rate of over 80% for complete dislocation removal. For example, in Example 5, the crystal germination success rate for complete dislocation removal reached 90% after 10 repeated experiments; in Example 4, the crystal germination success rate for complete dislocation removal reached 100% after 10 repeated experiments.

[0056] In addition, based on the variable diameter and variable pulling speed seeding method in the above embodiments, the inventors have achieved a dislocation removal effect comparable to that in the above embodiments by adjusting the minimum diameter (D_min) of the seed crystal to 6 mm, the maximum pulling speed (V_max) from 3.5 mm / min to 10 mm / min, the minimum pulling speed (V_min) from 0.5 mm / min to 2.0 mm / min, and the periodic seeding length from 10 mm to 200 mm.

[0057] The above embodiments are intended to illustrate the essential content of the present invention, but are not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of protection of the present invention.

Claims

1. A method for preparing a silicon single crystal by a diameter- varying and a speed-varying pulling method, characterized by, In the seeding stage of the Czochralski method for growing silicon single crystals, the diameter of the seed crystal and the pulling speed are periodically and coordinately changed.

2. The method of claim 1, wherein the diameter of the seed crystal is changed during the pulling of the single crystal silicon ingot. The diameter of the seed crystal is alternately changed between a set maximum diameter (D_max) and a set minimum diameter (D_min); the pulling speed of the seed crystal is alternately changed between a set maximum pulling speed (V_max) and a set minimum pulling speed (V_min), and the direction of the diameter change is opposite to that of the pulling speed change.

3. The method of claim 2, wherein the diameter of the seed crystal is changed by changing the diameter of the nozzle. The diameter change and the pulling speed change are synchronous.

4. The method according to claim 2 or 3, wherein the diameter of the seed crystal is changed during the pulling of the single crystal silicon. The maximum diameter (D_max) is 8mm to 12mm, and the minimum diameter (D_min) is 4mm to 6mm. ​ 5. The method according to claim 2 or 3, wherein the diameter of the seed crystal is changed during the pulling of the single crystal silicon. The maximum pulling speed (V_max) is 3.5mm / min to 10mm / min, and the minimum pulling speed (V_min) is 0.5mm / min to 2.0mm / min. ​ 6. The method according to claim 2 or 3, wherein the diameter of the seed crystal is changed during the pulling of the single crystal silicon. The change period of the diameter and / or pulling speed is 10mm to 200mm for each change period of the seeding length.

7. The method according to claim 2 or 3, wherein the diameter of the seed crystal is changed during the pulling of the single crystal silicon. The periodic change includes at least two different change period lengths.

8. The method according to claim 2 or 3, wherein the diameter of the seed crystal is changed during the pulling of the single crystal silicon. In one change period, the seeding length corresponding to the process of increasing the diameter is not equal to the seeding length corresponding to the process of decreasing the diameter.

9. A method for preparing silicon single crystals by variable diameter and variable pulling speed according to any one of claims 1-8, characterized in that, The method is suitable for improving the seeding success rate when there is fluctuation in the thermal field and / or <110> or <111> crystal orientation silicon single crystals are grown.

10. A silicon single crystal, characterized by, The silicon single crystal is prepared by the method of any one of claims 1-9 in the seeding stage.

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