Device for producing a single crystal by a float zone method and method of use

Abstract

Induction coil for producing a single crystal by a float zone method, comprising: an annular coil body having an opening in its centre and having a first and a second end separated from each other by a slot extending substantially in a radial direction from the opening, characterised in that the induction coil is made of a material selected from a group consisting of molybdenum, tungsten and tantalum.

Description

[0001] 202500001 / Kl

[0002] Apparatus for producing a single crystal by a zone-pulling process and method for using

[0003] The invention relates to a device for producing a monocrystalline rod made of silicon in a zone drawing system and a method for this purpose.

[0004] Zone growth of single crystals is a known technique (J. Bohm et al. / 'Handbook of Crystal Growth', Ed.: DTJ Hurle, Vol. 2, Part A, 213-257, 1994) and is used on an industrial scale for the production of single-crystal materials. In this process, an induction coil carrying a high-frequency current melts the starting material in the zone. As the material is pulled vertically, it solidifies as a single crystal, which is usually rotated. Depending on the specific design of the process, the single crystal can be pulled upwards or downwards. The electromagnetic field of the induction coil generates a flow with a double-vortex structure in the molten zone. This flow is always directed inwards in the center of the zone, while near both ends of the molten zone, the flow is always directed radially outwards.The resulting flow in the melting zone is generated not only by electromagnetic forces but also by buoyancy and Marangoni forces, as well as by the rotation of the rod or crystal. The geometry of the solidifying phase boundary adjusts itself according to the temperature distribution prevailing in the rod, which in turn is influenced by the flow conditions.

[0005] Investigations have been conducted on flow control during zone growing and the associated improvements in crystal quality and process stability (A. Mühlbauer et al.: Journal of Crystal Growth, Vol. 151, 66-79, 1995; S. Otani et al.: Journal of Crystal Growth, Vol. 66, 419-425, 1984; S.Y. Zhang et al.: Journal of Crystal Growth, Vol. 243, 410-418, 2002), proposing optimization of the process parameters: geometry of the induction coil, current in the induction coil, rotation of the rod or crystal, and growing speed. Attempts have been made to achieve homogenization of the dopant distribution by varying the crystal rotation, by shifting the induction coil relative to the crystal axis, or by using an optimized induction coil shape.

[0006] 2

[0007] On an industrial scale, zone pulling is used particularly for the production of silicon single crystals. A single crystal is obtained from a polycrystalline feed rod, although a feed rod made of monocrystalline silicon can optionally be used.

[0008] For this process, one end of the stock rod is melted using a high-frequency coil ("inductor"), and a single-crystal seed crystal is attached to the resulting droplet of melt. Material gradually melted from the stock rod serves as a feed for the single crystal that subsequently grows on the seed crystal. First, a section of the crystal known as the "neck" is crystallized to direct dislocations out of the crystal lattice. Then, the diameter of the growing single crystal is expanded to a target diameter in a section known as the "seed cone." Next, a section is created in which the single crystal has the target diameter. Finally, a section known as the "end cone" is formed.Alternatively, the process can also be completed without an end cone, but then a part at the end of the length section with the target diameter is unusable for the intended further processing because it has dislocations.

[0009] The stock rod is mounted at one end to a rotating shaft (drawing shaft) in such a way that it experiences no slippage, even if the direction of rotation changes abruptly. Furthermore, the center of the other end of the stock rod must remain on the axis of rotation of the drawing shaft at all times during the crystal drawing process. If the center of the other end were to move away from the axis of rotation of the drawing shaft, this would significantly affect the melting process by the drawing coil, which in turn could negatively impact the entire drawing process.

[0010] In the prior art, a suitably shaped body made of silver is used as the induction coil, which is permeated with cooling channels to keep the temperature of the silver below the melting point.

[0011] Document EP 4450683 A1 describes an induction coil comprising a ring-shaped coil body which has an opening in its center and a first end202500001 / Kl

[0012] 3

[0013] and has a second end, which are separated from each other by a slit extending in an essentially radial direction from the opening.

[0014] Document CN 112813490 A describes an induction coil comprising a coil body and a doping tube used to supply doping gas to a melting zone, wherein the doping tube is arranged in the coil body, a gas outlet of the doping tube is formed in a cutting edge on the inside of the coil body, and the doping tube is arranged in the radial direction of the coil body.

[0015] Document JP2022101774 A2 describes an induction coil used for the production of a single crystal by the FZ process, comprising a cyclic coil body with an opening.

[0016] JP 2022101 775 A2 describes an induction coil comprising an annular coil body; a slot extending from an opening provided in the center of the coil body; a flow passage for annular cooling water formed inside the coil body; wherein the inlet and outlet of the flow passage are provided at one and the other end, adjacent to each other, by the slot extending circumferentially around the coil body; and at least one guiding element provided in the center of the flow channel, which directs the flow of cooling water on the outer circumferential side of the flow channel 25 to the inner circumferential side.

[0017] The inventors recognized that the induction coils used had disadvantages that could lead to failures (e.g., flashovers) and significantly restrict the necessary degrees of freedom for pulling larger crystals (such as crystals with a diameter greater than 200 mm).

[0018] As a solution, the inventors propose a device as described in the claims.

[0019] The solution consists of an induction coil for the production of a single crystal by an FZ process, comprising: an annular coil body having an opening in its center and a first and a second end, which are connected by a 202500001 / Kl

[0020] 4

[0021] The slots are separated from each other, extending essentially in a radial direction from the opening, characterized in that the induction heating coil is made of a material consisting of a group comprising molybdenum, tungsten and tantalum.

[0022] Brief description of the characters

[0023] Fig. 1 shows a preferred embodiment of the inventive device for growing a single crystal of silicon. A silicon stock rod (101) is melted by means of an induction coil (104) made of a material from the group consisting of molybdenum, tungsten, and tantalum. The induction coil (104) has two effects: firstly, it causes the stock rod (101) to melt, and secondly, it exerts force on the melt (102) forming below it, from which the monocrystalline rod (103) can grow downwards in a monocrystalline form.

[0024] Fig. 2 shows another preferred embodiment of the inventive device for drawing a single crystal from silicon (103). The induction coil comprises two induction coils. Both induction coils are made of a material from the group consisting of molybdenum, tungsten, and tantalum. The effect of the upper induction coil (105) is essentially to heat the material of the stock rod, while the lower induction coil (106) exerts a force on the melt via the applied high frequency.

[0025] Fig. 3 also shows another preferred embodiment of the inventive device, wherein the induction coils known from Fig. 2 (upper induction coil (107) and lower induction coil (108)) are additionally encapsulated. The encapsulation (109) preferably consists of quartz glass. The resulting space between the induction coils and the encapsulation is designed so that it can be purged with an inert gas. This space can be purged with gas via an inlet (110) and an outlet (111).

[0026] 5

[0027] Detailed description of embodiments according to the invention

[0028] The present invention relates to a device for growing a single crystal using the zone-pulling (float zone) method.

[0029] Many variations of induction coils are described in the prior art. For example, German patent application DE 102010006724 A1 describes a device comprising an induction coil that allows the use of polysilicon granules instead of a storage rod. Solutions to similar problems are offered in German patent applications DE 102009051 010 A1, DE 102008038810 A1, and DE 102008013326 A1.

[0030] The inventors recognized that a significant disadvantage of the induction coils described in the literature lies in their inherent size. This is due to the necessary water cooling system, which ensures that the induction coil does not heat up above the melting point of the material used during operation.

[0031] However, using water cooling for the coil doesn't only offer advantages. The inventors realized that the water cooling not only dissipates heat generated by the coil's ohmic resistance, but also cools the molten material located in close proximity to the coil. Therefore, the water cooling requires additional electrical power through the coil to compensate for this effect.

[0032] To compensate for this or at least limit it, the coil is made of polished silver according to the state of the art.

[0033] The inventors now propose replacing the material described in the prior art (silver) with a material that has a significantly higher melting point. This would, firstly, eliminate the need for water cooling.

[0034] Secondly, a higher coil temperature results in a better balance of thermal radiation between the coil and the melt. Significantly less power is transferred to the coil via thermal radiation. 202500001 / Kl

[0035] 6

[0036] The higher the temperature of the coil, the more pronounced this effect becomes. This effect is also known as the Stefan-Boltzmann law for non-blackbody radiators. Both the temperature and the emissivity (s) of the material play a role here.

[0037] The induction coil according to the invention for producing a single crystal by a zone-pulling process comprises an annular coil body which has an opening in its center and has a first and a second end which are separated from each other by a slot which extends substantially in a radial direction from the opening, characterized in that the induction coil is made of a material or contains a material which is from a group consisting of molybdenum, tungsten and tantalum.

[0038] Preferably, the induction coil is made of a material or contains a material that is additionally made of graphite, carbon fiber composites, silicon carbide, titanium, vanadium, chromium, zirconium, hafnium, yttrium, niobium, platinum.

[0039] The inventors realized that this made it possible to operate the induction coil without a complex water cooling system. The result is significantly lower energy loss and, at the same time, improved energy input into the silicon through thermal radiation from the induction coil. This reduces the probability of RF arcs.

[0040] Furthermore, the induction coil has a significantly lower overall height.

[0041] It is preferred to encapsulate the induction coil, i.e., to equip the induction coil with a surrounding solid casing.

[0042] The encapsulation is particularly preferably carried out using quartz glass.

[0043] The inventors have recognized that the encapsulation is capable of preventing contact between any metal vapors that may be generated from the induction coil and the molten metal located in the immediate vicinity of the induction coil. 202500001 / Kl

[0044] 7

[0045] Particularly preferably, a space is located between the encapsulation and the induction coil, which can very preferably be purged with an inert gas. For this purpose, the encapsulation can have at least one inlet and at least one outlet that allows the gas in the space to be exchanged or purged.

[0046] The inventors realized that rinsing the space between the components could prevent the formation of a high-frequency mirror.

[0047] A noble gas such as argon or nitrogen is particularly preferred. Mixtures of these gases are also preferred.

[0048] The inventors realized that this would allow any metal vapors, plasma, ions and metallic vapors that might be produced to be removed.

[0049] Preferably, a second induction coil is located in the resulting space between the induction coil and the encapsulation.

[0050] The second induction coil is preferably made of the same material as the first, particularly preferably of a material from a group consisting of molybdenum, tungsten and tantalum.

[0051] This also makes it possible, for example, to accommodate two or more coils in a very limited technically usable space.

[0052] The upper induction coil influences the melting of the silicon in the stock rod, while the lower coil influences the shape of the liquid silicon.

[0053] It is possible to add further coils, whether to treat the shape or temperature of the molten silicon separately.

[0054] The use of multiple coils operated at different frequencies makes it possible to independently model the shape of the melt, especially the neck and shoulder, due to different field directions and penetration depths. This also makes it possible to control the temperature independently. 202500001 / Kl

[0055] 8

[0056] The present invention also relates to a method for producing a single crystal of silicon using the inventive apparatus in the zone-pulling process.

[0057] According to the invention, a method for producing a single crystal from silicon using an inventive induction coil is provided. The method comprises melting a silicon stock rod with the induction coil and drawing a cylindrical portion of the single crystal.

[0058] According to the invention, the induction coil has a temperature of at least 600°C.

[0059] The inventors recognized that the higher temperature of more than 600 °C results in significantly less energy loss and at the same time an improved energy input into the silicon through thermal radiation from the induction coil.

[0060] Preferably, the temperature of the induction coil is greater than 1000°C.

[0061] Preferably, the storage rod has a polycrystalline line structure.

[0062] Preferably, the diameter of the cylindrical part of the single crystal is more than 200 mm, particularly preferably more than 300 mm.

Claims

202500001 / Kl 9 Patent claims 1. Induction coil for the production of a single crystal by a zone-pulling process, comprising: a ring-shaped coil body having an opening in its center and a first and a second end separated from each other by a slot extending substantially in a radial direction from the opening, characterized in that The induction coil is made of a material that comes from a group consisting of molybdenum, tungsten and tantalum.

2. Induction coil according to claim 1, characterized in that The induction coil is encapsulated.

3. Induction coil according to claim 1 or 2, characterized in that the encapsulation consists of quartz glass.

4. Induction coil according to claim 2, wherein any resulting gap between the induction coil and the encapsulation is designed in such a way that it can be purged with an inert gas.

5. Induction coil according to claim 2, wherein a second induction coil is located in an intermediate space created between the induction coil and the encapsulation.

6. Induction coil according to claim 5, wherein the second induction coil is made of a material consisting of the group consisting of molybdenum, tungsten and tantalum.

7. Method for producing a single crystal from silicon using an induction coil according to one of claims 1 with 6, comprising: melting a silicon rod with the induction coil, and drawing a cylindrical part of the single crystal with a diameter, characterized in that The induction coil has a temperature of at least 600°C. 202500001 / Kl 10 8. The method of claim 7, wherein the storage rod has a polycrystalline structure.

9. Method according to claim 7 or 8, wherein the induction coil has a temperature of more than 1000°C.

10. Method according to one of claims 7 with 9, wherein the induction coil has a temperature of more than 1100°C.

11. Method according to one of claims 7 with 10, wherein the diameter of the cylindrical part of the single crystal is more than 200 mm.

12. Method according to one of claims 7 with 11, wherein the diameter of the cylindrical part of the single crystal is more than 300 mm.

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