METHOD FOR PRODUCING A SINGLE CRYSTAL OF SILICON

Description

[0001] The invention relates to a method for producing a monocrystalline silicon rod in a zone pulling plant.

[0002] Zone pulling of single crystals is well known in the art (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 to produce single-crystal materials. An induction coil carrying a high-frequency current melts the starting material in the zone. This material then solidifies as a single crystal as the material is pulled vertically, with the resulting single crystal usually being rotated. Depending on 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 inward in the center of the zone, while near both ends of the melt zone, the flow is always directed radially outward.The resulting flow in the melt 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 is determined by the temperature distribution prevailing in the rod, which in turn is influenced by the flow conditions.

[0003] Investigations have been conducted into flow control during zone pulling and the associated improvements in crystal quality and process stability (A. Mühlbauer, et al.: Journal of Crystal Growth, Vol. 151, pp. 66-79, 1995; S. Otani, et al.: Journal of Crystal Growth, Vol. 66, 419-425, 1984; SY Zhang, et al.: Journal of Crystal Growth, Vol. 243, pp. 410-418, 2002). These studies propose optimizing the process parameters of induction coil geometry, current in the induction coil, rotation of the rod or crystal, and pulling speed. Attempts have been made to homogenize the dopant distribution by varying the crystal rotation, by shifting the induction coil relative to the crystal axis, or by optimizing the shape of the induction coil.

[0004] On an industrial scale, zone pulling is used primarily to produce single crystals of silicon. A single crystal is obtained from a polycrystalline feed rod, although a feed rod made of monocrystalline silicon can also be used.

[0005] For this purpose, the stock rod is melted at one end using a high-frequency coil ("inductor"), and a single-crystal seed crystal is applied to the resulting melt droplet. Material gradually melted from the stock rod serves as a supply for a single crystal subsequently growing on the seed crystal. First, a length known as the thin neck ("neck") is crystallized to direct dislocations from the crystal lattice. The diameter of the growing single crystal is then expanded to a target diameter in a length known as the seed cone. A length is then created in which the single crystal has the target diameter. At the end of the process, a length known as the end cone is created.Alternatively, the process can also be completed without an end cone, but in this case a part at the end of the length section with the target diameter is unusable for the intended further processing because it has dislocations.

[0006] The supply rod is mounted at one end to a rotating shaft (drawing shaft) in such a way that it does not slip, even if the direction of rotation changes abruptly. Furthermore, the center of the other end of the supply rod must remain on the rotational axis of the drawing shaft at all times during crystal pulling. If the center of the other end were to move away from the rotational axis of the drawing shaft, this would significantly impair melting by the drawing coil, which in turn could negatively impact the entire pulling process.

[0007] Currently, only monocrystalline silicon rods with a nominal diameter of up to 200 mm, produced using the zone pulling process, are available on the market.

[0008] Achieving a larger target diameter than 200 mm while maximizing pulling speed has always been an unfulfilled wish of the industry, as it promises greater returns (especially in the production of components on the resulting semiconductor wafers). State of the art

[0009] From the document DE 101 37 856 A1 a method is known for producing a single crystal of silicon by crucibleless zone pulling, which single crystal has a diameter of at least 200 mm over a length of at least 200 mm and is dislocation-free in the region of this length, wherein during zone pulling a melt neck is formed between a supply rod and the single crystal.

[0010] However, it turns out that the pulling speed is too low to pull crystals economically.

[0011] Patent specification EP2142686 A1 discloses a method for producing a single crystal, in which a polycrystalline rod is passed through a heating region to create a molten zone, a magnetic field is applied to the molten zone, and the growth of a single crystal is induced upon solidification of the molten material on a single crystal seed. The growing single crystal is rotated in an alternating clockwise and counterclockwise pattern. The method is useful for producing silicon single crystals with uniform electrical characteristics. An apparatus for carrying out the method is also disclosed. Although the claims claim a crystal larger than 200 mm, no method is specifically provided for a diameter of 300 mm.

[0012] US 2016 053 401 AA discloses a supplementary heating device for a zone melting furnace and a heat preservation method for a single-crystal rod. The supplementary heating device comprises a supplementary heater arranged below a high-frequency heating coil inside the zone melting furnace and formed by winding a hollow metallic round tube. The winding start end of the supplementary heater is positioned at the upper part, the winding stop end of the supplementary heater is positioned at the lower part, and an upper end part and a lower end part are respectively led out from the two ends. A hollow cylindrical heating load is arranged on the inside of the supplementary heater, and an insulating part is arranged between the heating load and the supplementary heater.The present invention can solve the problem of cracking of single crystal rods caused by inappropriate thermal field distribution and excessive thermal stress in the growth process of zone-melted silicon single crystals over 6.5 inches.

[0013] DE 3 805 118 A1 discloses induction heating coils suitable for the crucibleless drawing process. It also shows methods for adapting coils if necessary.

[0014] The document US 2010 / 307406 A1 describes a method for producing a crystal for zone pulling. The type of material to be used is not discussed. The core of this document deals with the method by which the semiconductor material is to be heated. The use of an induction coil is completely omitted here, as the heating of the semiconductor material is achieved using infrared lamps and appropriately dimensioned mirrors.

[0015] Dold et al. ("In situ Observation of Growth Interfaces by Ultrasound", Journal of Crystal Growth, Elsevier, Amsterdam, NL, Vol. 256, No. 3-4, 1 September 2003 (2003-09-01), pages 352-360) describes a method for measuring growth interfaces using ultrasound during crystal pulling.

[0016] US 2019 / 032242 A1 describes a process for producing a single crystal, whereby a so-called "powder raw material" is applied to an already produced monocrystalline rod and is apparently melted there using lamps.

[0017] The object of the invention is to provide a method which makes it possible to produce a monocrystalline rod by means of the FZ drawing process, wherein the diameter is greater than 200 mm, without having the disadvantages of the prior art - such as too low drawing speed.

[0018] The problem is solved by the methods described in the claims. Short description of the characters

[0019] Figure 1 shows an axial section of a zone pulling system during crystal pulling as required for the process according to the invention. Shown is a stock rod (102) with a diameter D p, a first hollow cylinder (105) with an inner diameter dc1 , a second hollow cylinder (109) with an inner diameter dc2 , a monocrystalline rod (101), a melt (106) and a coil (104). The size h 1 defines the vertical distance of the upper edge of the second hollow cylinder (109) from the crystallization edge (103). The size h 2 defines the vertical distance of the lower edge of the first hollow cylinder (105) from the outer melting edge (110) of the stock rod. The vertical distance between the outer melting edge (110) and the crystallization edge (103) will be with h ak During crystal pulling, the supply rod (102) at the melting front (107) molten. The point in the figure where the stock rod, the melt and the gas space meet is called the internal triple point (108) called. Figure 2shows the contour of a growth stripe (200), which was determined as a function of the radius of the crystal and the length D of the crystal in the growth direction. The value d 0 denotes the maximum deflection of the growth strip. A characteristic parameter for the method according to the invention is the angle of attack β, between a horizontal line and a tangent drawn to growth strips, determined at the radial position 85 mm. Abbreviations

[0020] 101 monocrystalline rod with a diameter D c 102 Silicon stock rod with a diameter D p 103 Crystallization edge of the single crystal 104 Sink 105 first hollow cylinder with an inner diameter dc1 106 melt 107 Melting front of the stock rod 108 internal triple point 109second hollow cylinder with an inner diameter dc2 110 outer melting edge of the stock rod h 2 vertical distance of the lower edge of the first hollow cylinder from the outer melting edge h 1 vertical distance of the second cylinder from the crystallization edge h ak vertical distance between the melting edge and the crystallization edge 200 Contour of a growth stripe determined as a function of the radius of the crystal and the length D of the crystal in the growth direction. 201 Surface area of ​​the crystal βAngle of incidence between a horizontal line and a tangent applied to growth stripes, which is determined at the radial position 85 mm. d 0 maximum deflection of the growth strips Detailed description of embodiments according to the invention

[0021] The present invention relates to a method for producing a single crystal of silicon by means of the zone pulling method.

[0022] In order to achieve the largest possible diameters and at the same time high growth rates, the inventors recognized that it is necessary to install a stock rod in the drawing system that has a diameter of not less than 230 mm and not more than 270 mm.

[0023] Furthermore, it is apparently necessary for a first hollow cylinder to surround the previously installed stock rod during crystal pulling. The inner diameter of the stock rod must be no less than 30 mm and no more than 50 mm larger than the diameter of the stock rod. Essentially, the longitudinal axis of the first hollow cylinder and the longitudinal axis of the stock rod should be aligned. Small radial deviations of less than 3 mm are sometimes unavoidable, but it is advantageous to minimize these deviations during installation.

[0024] It is also necessary to install a second hollow cylinder prior to crystal pulling so that it surrounds the subsequently drawn single-crystal rod. Its inner diameter must be no less than 20 mm and no more than 60 mm larger than the target diameter of the single-crystal rod. For example, if a crystal with a target diameter of 300 mm is to be pulled, it is preferable for the inner diameter to be between 320 mm and 360 mm.

[0025] It is preferred if the cylindrical part of the single crystal has a diameter of no less than 290 mm and no more than 310 mm and a length of no less than 15 cm. The maximum length of the cylindrical part of the rod depends essentially on the dimensions of the crystal pulling system.

[0026] The diameter of a single crystal is subject to minor process-related fluctuations that can be minimized but not completely eliminated. Therefore, the term "target diameter" is understood as the average diameter of the single crystal.

[0027] As in Figure 1 As shown, the stock rod forms an outer melting edge at the melting front and the monocrystalline rod forms a crystallization edge at the growth side.

[0028] The inventors have recognized that during crystal pulling, the vertical distance between the lower edge of the first hollow cylinder and the outer melting edge of the stock rod is preferably less than 2 mm. The lower edge of the first hollow cylinder is located above the outer melting edge of the stock rod. The first hollow cylinder is therefore shifted upward relative to the melting edge.

[0029] Particularly preferably, the length of the first hollow cylinder is at least 10 cm and less than 50 cm. The material from which the first hollow cylinder is made is preferably silver, with a material having high emissivity being most preferably selected for the coating of the inner surface of the first hollow cylinder.

[0030] The emissivity of a body indicates how much radiation it emits compared to an ideal heat radiator, a black body.

[0031] Gold, silver, silver alloys, carbon or copper and the like are good candidates for coating, although the use of gold, silver or silver alloys is preferable as there is no risk of contamination of the melt or the single crystal.

[0032] Particularly preferably, the first hollow cylinder can be composed of two hollow cylinders, wherein a lower hollow cylinder can preferably be provided with an active heater. The active heater could preferably be implemented using a device similar to that described in US 2016 053 401 AA.

[0033] The inventors paid particular attention to ensuring that the upper edge of the second cylinder extends beyond the crystallization edge. The vertical distance between the crystallization edge and the upper edge of the second hollow cylinder is particularly preferred to be no less than 1 mm and no more than 10 mm.

[0034] As with the first hollow cylinder, the material from which the second hollow cylinder is made should be selected so that the emissivity on the inside of the hollow cylinder is as high as possible. Additionally, it is preferred that the second hollow cylinder be composed of two hollow cylinders made of different materials.

[0035] Preferably, the lower part of the second hollow cylinder, i.e., the part further away from the coil, is made of silver. Most preferably, it contains a surface treatment, such as a coating of silver or gold, or alloys thereof, on the inside to maximize emissivity.

[0036] The upper part of the second hollow cylinder, i.e., the hollow cylinder facing the coil, is preferably made of a material that, on the one hand, has high emissivity on the inside and, at the same time, is robust against high temperatures (i.e., greater than 1000°C). Recommended candidates for this include ceramic materials, platinum, or platinum-coated ceramics.

[0037] The second hollow cylinder also preferably has apertures and holes that allow an image processing system to have a clear view of the crystallization edge of the single crystal. Particular care must be taken to ensure that these apertures and holes are as small as possible and as large as necessary, as they can negatively impact the pulling process.

[0038] The pulling speed is preferably no less than 1.3 mm / min and no more than 1.5 mm / min, preferably no less than 1.35 mm / min and no more than 1.45 mm / min. The pulling speed is defined as the rate at which the single-crystal rod grows in the axial direction. The rate at which the stock rod must be fed can be easily calculated from the corresponding mass balance for a given pulling speed.

[0039] The length of the second hollow cylinder is preferably more than 10 cm and not more than 40 cm.

[0040] The wall thickness of the two hollow cylinders is preferably not more than 10 mm and not less than 3 mm.

[0041] As is usual in the state of the art for the zone pulling process, the gas space contains nitrogen, which is incorporated into the pulled crystal.

[0042] Crystals grown using the method just described can be further processed like conventional crystals from the Czochralski growing process.

[0043] Preferably, the further processing comprises the steps of circular grinding of the single crystal, cutting the rod into rod pieces, sawing a rod piece into slices, grinding and polishing the slices of the single crystal.

[0044] Semiconductor wafers manufactured from ingots produced using the process described above are ideally suited for manufacturing power components because they have very few defects. This is due to the fact that this process essentially lacks the interstitial oxygen that would otherwise form oxygen precipitates in the crystal lattice. A nominal diameter of 300 mm and a high pulling speed make this process highly economical and therefore previously unattainable.

[0045] By applying the process according to the invention, a monocrystalline rod with a nominal diameter of not less than 290 mm and not more than 330 mm is obtained. This rod is preferably cut into rod pieces with a length of not less than 15 cm and not more than 50 cm.

[0046] For example, if a piece of rod obtained in this way with a diameter of 300 mm and a length of 20 cm is cut lengthwise (i.e. axially), a so-called board with a width of 300 mm and a length of 20 cm can be obtained.

[0047] Measurements can be made on a board that characterize both the crystal and the pulling process used to produce the crystal.

[0048] The dopant, which is usually added in gaseous form to the melt during the zone pulling process, is incorporated irregularly into the crystal. The dopant preferably contains boron or phosphorus. This leads to a locally inhomogeneous resistance distribution of the silicon, which is called "striation."

[0049] Although great efforts are made to avoid striations in order to avoid negative effects during the device process, striations are always measurable once the drawn rod has been doped in the zone drawing process.

[0050] Since the dopant is incorporated from the melt into the crystal along the melt / crystal interface, the original shape of the crystal-melt interface can be determined in the form of growth stripes by analyzing the measured resistance distribution. Two examples of this measurement and evaluation method are given below: Investigation of defects and striations in as-grown Si crystals by SEM using Schottky diodes, Appl. Phys. Lett. 27, 313 (1975); https: / / doi.org / 10.1063 / 1.88482, AJR de Kock, SD Ferris, LC Kimerling, and HJ Leamy and Lüdge, A., Riemann, H.: Doping in homogeneities in silicon crystals detected by the lateral photovoltage scanning (LPS) method. Inst. Phys. Conf. Ser. 160, 145-148 (1997).

[0051] The latter source (Lüdge et al.) describes the method of "lateral photovoltaic scanning" (LPS), which is suitable for reconstructing the interface between crystal and melt, i.e. the growth stripes, even when the resistance adjusted by doping is high, i.e. the dopant concentration is low.

[0052] If the lateral photovoltaic scanning (LPS) method is applied to the board described above, contours of the growth stripes can be determined, which accurately reflect the deflection of the interface between melt and crystal.

[0053] A contour of a growth strip of a crystal (200) grown according to the inventive method is shown in Figure 2 shown.

[0054] Two characteristic quantities can be derived that describe the properties of the crystal piece: (1) the maximum deflection d 0 the growth strips and (2) the angle of incidence β between a horizontal line and a tangent to the growth strips, which is determined at the radial position 85 mm.

Claims

1. Process for producing a single crystal of silicon, comprising: the installing of a feed rod of silicon in a float-zone apparatus, the feed rod having a diameter of not less than 230 mm and not more than 270 mm, the installing of a first hollow cylinder having a bottom edge and an internal diameter which is larger by not less than 30 mm and not more than 50 mm than the diameter of the feed rod, so that during pulling, the hollow cylinder surrounds the feed rod and the longitudinal axis of the first hollow cylinder and the longitudinal axis of the feed rod lie one above the other, the installing of a second hollow cylinder having a top edge and an internal diameter which is larger by not less than 20 mm and not more than 60 mm than the target diameter of the single crystal, so that the second hollow cylinder is installed in such a way that it surrounds the single crystal to be pulled; pulling a cylindrical part of the single crystal which has a target diameter of not less than 290 mm and not more than 310 mm, where the feed rod at the melting front forms an outer melting edge and the monocrystalline ingot on the growth side forms a crystallizing edge, where the pulling speed is not less than 1.3 mm / min and not more than 1.5 mm / min, preferably not less than 1.35 mm / min and not more than 1.45 mm / min, where the vertical distance of the bottom edge of the first hollow cylinder from the outer melting edge is smaller than 2 mm, and the top edge of the second cylinder protrudes not less than 1 mm and not more than 10 mm over the crystallizing edge, removing a length of the single crystal to form an ingot piece having a length of not less than 15 cm and not more than 50 cm.

2. Process according to Claim 1, characterized in that the vertical distance between the melting edge and the crystallizing edge is not smaller than 35 mm and not larger than 40 mm.

3. Process according to any of the preceding claims, characterized in that the feed rod has been produced by means of a CZ process.

4. Process according to any of the preceding claims, characterized in that the process further comprises the steps of circularly grinding the single crystal, sawing the ingot piece into wafers, the grinding and the polishing of the wafers.