METHOD AND DEVICE FOR PRODUCING A SINGLE CRYSTAL OF SILICON DOPED WITH N-TYPE DOP.

Description

[0001] The invention relates to a method for producing a silicon single crystal doped with n-type dopant and exhibiting a specific electrical resistivity of no more than 2 mΩ·cm in a cylindrical section, by pulling the single crystal from a melt contained in a crucible according to the CZ method. The invention further relates to an apparatus for carrying out the method. State of the art / Problems

[0002] The production of a silicon single crystal containing a relatively high concentration of n-type dopant is particularly challenging. The dopant is highly volatile and can form deposits that may induce dislocations. It has proven advantageous to introduce the dopant, either initially or additionally, during the drawing of a cylindrical section of the growing single crystal into the melt.

[0003] US 2010 / 0 294 999 A1 describes such a process, which involves blowing a gas stream containing elemental dopant through a tube to the surface of the melt. For this purpose, the solid dopant is sublimated in the drawing chamber and, mixed with a carrier gas, blown to the surface of the melt. A similar process is described in JP 2013-129 551 A.

[0004] In CN 1 600 905 A and in DE 11 2017 004 790 T5 it is recommended to provide several openings through which gaseous dopant is blown to the surface of the melt.

[0005] A disadvantage of these proposals is that they require modifications to the immediate surroundings (hot zone) of the growing single crystal to create space for the sublimation of the dopant, and that the distribution of the dopant is uneven despite the provision of multiple openings for blowing in the dopant.

[0006] Furthermore, WO 2009 / 119342 A1 describes a method and a device for pulling a doped silicon single crystal from a melt according to the Czochralski method with a plurality of storage means for storing a sublimable dopant, a feeding device for supplying the sublimable dopant to the melt and a drive device for separately supplying the sublimable dopant from the storage means to the feeding device.

[0007] The object of the invention is to minimize modifications of the hot zone and to ensure a more uniform distribution of the dopant in the melt. In particular, local increases in the concentration of dopant are to be avoided, the presence of which increases the risk of dislocations being triggered in the growing single crystal.

[0008] The problem is solved by a method for producing a silicon single crystal doped with n-type dopant and exhibiting a specific electrical resistance of no more than 2 mΩ·cm in a cylindrical section, by pulling the single crystal from a melt contained in a crucible according to the CZ method, comprising, during the pulling of the cylindrical section of the single crystal, supplying a gas stream containing gaseous dopant to a surface of the melt, wherein the gas stream is directed from a sublimation device outside a pulling chamber in a piping system into the pulling chamber and through a heat shield surrounding the growing single crystal, or along an outer surface of the heat shield, to an annular channel at a lower end of the heat shield and from there through nozzles to the surface of the melt, characterized in that a load cell is used to measure the gas stream.The consumption of dopant is determined using a camera with image processing or by means of the load cell and the camera with image processing.

[0009] The cylindrical section of the single crystal exhibits, partially or preferably completely, a specific electrical resistance of no more than 2 mΩcm. In the case of phosphorus as a dopant, the specific electrical resistance in the cylindrical section of the single crystal is preferably no more than 1.2 mΩcm, and particularly preferably no more than 1 mΩcm.

[0010] Specific modifications to the hot zone are avoided, in particular, by locating the point where solid dopant sublimates into a gaseous state outside the drawing chamber. Furthermore, the gaseous dopant travels through a piping system, either through a heat shield or along an outer surface of the heat shield, to an annular channel at the lower end of the heat shield, and from there through nozzles to the melt surface.

[0011] The outer surface of the heat shield refers to the side surface of the heat shield that first receives the heat radiation from a heating device (crucible heater) arranged around the crucible.

[0012] The dopant is sublimated outside the drawing chamber and, together with a carrier gas, is flushed through the piping system into the annular channel and from there guided through the nozzles to the surface of the melt.

[0013] At least two, preferably four to 100 nozzles are provided. The nozzle tips, where the gas stream consisting of carrier gas and dopant gas exits, are preferably no more than 20 mm away from the surface of the melt.

[0014] The arrangement of the nozzles directs the gas flow to the surface of the melt in a predetermined manner.

[0015] The nozzles can be arranged such that the gas flow is directed in a direction perpendicular to the surface of the melt, or towards the surface of the melt with a directional component towards an axis through the center of the growing single crystal, or towards the surface of the melt with a directional component away from the axis through the center of the growing single crystal.

[0016] Preferably, the nozzles are arranged such that the gas flow of adjacent nozzles is directed in different directions, for example alternately in two or three of the three directions mentioned above, i.e., alternately in the direction perpendicular to the surface of the melt and in the direction towards the axis through the center of the growing single crystal, or alternately in the direction towards the axis through the center of the growing single crystal and in the direction away from the axis through the center of the growing single crystal.

[0017] It is particularly preferred to design the nozzles to be pivotable and to align their outlet openings before the installation of the heat shield so that the nozzles assume a desired arrangement during the drawing of the cylindrical section.

[0018] Directing the gas flow to the surface of the melt during the drawing of the cylindrical section of the single crystal can be continuous or intermittent. If the melt contains relatively little or no dopant at the start of the cylindrical section drawing process, the gas flow is directed to the surface of the melt at that point. The gas flow rate can be constant or variable. For example, the gas flow rate can be reduced as the single crystal crystallizes to compensate for the increase in dopant in the melt through segregation.

[0019] The dopant preferably consists of one or more of the elements phosphorus, arsenic, and antimony, which are sublimated into a gaseous state in a dopant sublimation unit. However, it is also possible to use dopant consisting of one or more chemical compounds of these elements that are already gaseous, for example, compounds of these elements with hydrogen. In this case, the sublimation unit is used only to mix the dopant gas and the carrier gas.

[0020] The invention also relates to a device for producing a single crystal of silicon doped with n-type dopant and having a specific electrical resistance of no more than 2 mOhmcm in a cylindrical section, comprising a drawing chamber for growing the single crystal according to the CZ method; a crucible for receiving a silicon melt; a heat shield for shielding a growing single crystal; a sublimation device for converting solid dopant into gaseous dopant; and a piping system for supplying a gas stream containing the gaseous dopant to a surface of the melt, wherein the sublimation device is arranged outside the drawing chamber and the piping system leads through the heat shield or along an outer surface of the heat shield and terminates in an annular channel at the lower end of the heat shield, and the annular channel has nozzles for directing the gas stream to the surface of the melt, characterized by a load cell, by a camera with image processing, or by the load cell and the camera with image processing, for determining the consumption of dopant.

[0021] The annular channel and the nozzles are preferably made of a particularly corrosion-resistant material such as molybdenum, as is at least the part of the piping system whose distance to the surface of the melt is no more than 200 mm.

[0022] Outside the drawing chamber, a sublimation unit is arranged, comprising a dopant container and a surrounding heater. The heater can have several separately controllable heating zones, which improves control over the amount of sublimating dopant. A load cell is also present, which detects the change in weight of the container's contents during single-crystal drawing. This information allows for the determination of dopant consumption and precise control of the further supply of dopant to the melt. Alternatively, or in addition to the load cell, a camera with image processing can be used to determine dopant consumption.

[0023] On the inlet side, the sublimation unit is connected to a flow regulator, which mixes the carrier gas and dopant gas within the unit and directs them into the drawing chamber. On the outlet side, part of the piping system connects the sublimation unit to a flange on the outer wall of the drawing chamber. From there, the piping system continues to an annular channel at the lower end of a heat shield that surrounds the growing single crystal. The heat shield is mounted in a fixed position relative to the surface of the melt.

[0024] It should be prevented that sublimated dopant precipitates as a solid in the piping system. It is therefore preferred to thermally insulate the part of the piping system located between the sublimation unit and the drawing chamber, including the area of ​​the flange on the outer wall of the drawing chamber, or to provide it partially or completely with a pipe heater and heat it from the outside.

[0025] The flow controller, the load cell, and the vessel heater are preferably connected to a control device of the single-crystal pulling apparatus. This device processes the data from the single-crystal pulling apparatus and regulates the heating power of the vessel heater and the delivery of carrier gas by the flow controller to ensure a predetermined dopant supply to the melt, or a supply calculated within a control loop.

[0026] According to a first embodiment of the invention, the piping system extends through the heat shield to the annular channel. According to a second embodiment of the invention, the piping system extends along the outer surface of the heat shield to the annular channel.

[0027] Nozzles are evenly distributed along the underside of the annular channel, pointing towards the surface of the melt. The number of nozzles is preferably 4 to 100. The nozzles are arranged in a predetermined manner. One possible arrangement is such that all nozzles are oriented perpendicular to the surface of the melt, so that the gas flow strikes the surface of the melt at nearly a right angle. Another arrangement is such that the nozzles are tilted radially towards the single crystal, i.e., with a directional component pointing towards an axis through the center of the growing single crystal. A further arrangement is such that the nozzles are tilted radially away from the single crystal, i.e., with a directional component pointing away from the axis through the center of the growing single crystal.

[0028] Particularly preferred is an arrangement in which the orientation of adjacent nozzles is different, for example one in which the orientation of the nozzles alternates between the arrangement perpendicular to the surface, the tilted arrangement towards the single crystal and the tilted arrangement away from the single crystal.

[0029] The invention is further explained below with reference to the drawings. Brief description of the characters

[0030] Fig. 1 shows a device according to a first embodiment of the invention. Fig. 2 shows a device according to a second embodiment of the invention. Fig. 3 shows possible arrangements of the nozzles, which are directed from the annular channel to the surface of the melt. List of reference symbols used

[0031] 1 growing single crystal 2 Draw chamber 3 Thermal insulation 4 Gas intake 5 Gas outlet 6 Crucible heater 7Ring canal 8 outer crucible 9 inner crucible 10 Heat shield 11 Piping system 12 Crucible shaft drive 13 Crucible shaft 14 Draw shaft 15 Drawshaft drive 16 Germ crystal holder 17 germ crystal 18 Device for generating a magnetic field 19 Control device 20 surface of the melt 21 Device for sublimating dopant 22 nozzle 23 Container for dopant 24 Container heating 25 load cell 26 Flow regulator 27 Carrier gas 28 Axis through the center of the growing single crystal 29 Pipeline heating

[0032] The device according to Fig. 1The apparatus comprises a drawing chamber 2, which houses an inner crucible 9 that can be rotated, raised, and lowered by means of a crucible shaft 13. The inner crucible 9 is supported by an outer crucible 8. The crucible shaft 13 is set in motion by a crucible shaft drive 12, which is connected to a control device 19 of the single-crystal growing apparatus. A thermal insulation 3 shields the inner wall of the drawing chamber from thermal radiation emitted by a crucible heater 6, which is arranged around the crucibles 8 and 9. The drawing chamber 2 further comprises a gas inlet 4 and a gas outlet 5 for passing through a process gas such as argon or a process gas mixture such as argon and hydrogen during the growing of a single crystal. A heat shield 10, which is fixedly mounted, surrounds a growing single crystal 1 during operation of the apparatus.The single crystal 1 grows on a seed crystal 17, which is clamped in a seed crystal holder 16, and is drawn upwards from the surface 20 of a silicon melt by means of a drawing mechanism, for example a drawing shaft 14 and a drawing shaft drive 15, and is rotated about the axis 28 through its center during this process. The drawing chamber 2 is surrounded by a device 18 for generating a magnetic field, for example a horizontal magnetic field or a CUSP magnetic field.

[0033] According to the invention, a device 21 for sublimating dopant (sublimation device) is arranged outside the drawing chamber 2 and connected to a piping system 11. The sublimation device carries the dopant, which has been converted into a gaseous state in the sublimation device 21, in the form of a gas stream containing the gaseous dopant and a carrier gas 27, into an annular channel 7 and from there through nozzles 22 to the surface 20 of the melt. The annular channel is located at the lower end of the heat shield 10. The sublimation device 21 comprises a container 23 for dopant, a container heater 24, and a load cell 25. The container heater 24, the load cell, and a flow regulator 26, through which the carrier gas is supplied to the gaseous dopant, are connected to the control device 19.Preferably, a pipe heating system 29 is arranged partially or completely around the pipe system 11 in the area outside the drawing chamber 2 for heating the pipe system 11.

[0034] According to the in Fig. 1 In the embodiment shown, the piping system 11 passes through the heat shield 19 and opens into the ring channel 7.

[0035] According to the in Fig. 2 In the embodiment shown, the piping system 11 leads along the outer surface of the heat shield 19 into the annular channel 7.

[0036] The nozzles 22 are arranged distributed over the side of the annular channel 7 opposite the surface of the melt, preferably at uniform intervals, and directed towards the surface 20 of the melt, either with only one directional component parallel to the axis 28 through the center of the growing single crystal or with an additional directional component towards this axis or with an additional directional component away from this axis. Fig. 3 Examples of such nozzle arrangements are shown. A nozzle arrangement that alternately implements two or three of these three possibilities is particularly preferred.

[0037] The foregoing description of exemplary embodiments is to be understood as illustrative. The disclosure thereby enables the person skilled in the art, on the one hand, to understand the present invention and its associated advantages, and, on the other hand, also includes, in the understanding of the person skilled in the art, obvious modifications and alterations of the described structures and methods.

Claims

1. Method for producing a single silicon crystal doped with n-type dopant and having in a cylindrical portion an electrical resistivity of not more than 2 mohmcm, by pulling of the single crystal by the CZ method from a melt contained in a crucible, comprising in the course of the pulling of the cylindrical portion of the single crystal, the supplying of a gas stream comprising gaseous dopant to a surface of the melt, the gas stream being guided from a sublimation facility outside a pulling chamber in a tubing system into the pulling chamber and through a heat shield which surrounds the growing single crystal, or along an outer surface of the heat shield, to an annular channel on a lower end of the heat shield and from there through nozzles to the surface of the melt, characterized in that, by means of a weighing cell, a camera with image processing or by means of the weighing cell and the camera with image processing, the dopant consumption is ascertained.

2. Method according to Claim 1, characterized in that the dopant in the form of a chemical compound or in a molecular or atomic form of an element of the dopant is guided to the surface of the melt, and the dopant comprises one or more of the elements P, As and Sb.

3. Method according to Claim 1 or Claim 2, characterized in that the dopant is guided through at least two nozzles to the surface of the melt.

4. Method according to any of Claims 1 to 3, characterized in that the gas stream is guided by the nozzles in a direction perpendicular to the surface of the melt or toward the surface of the melt in a direction away from an axis through the middle of the single crystal or toward the surface of the melt in a direction toward the axis through the middle of the single crystal.

5. Method according to Claim 1 or Claim 2, characterized in that the gas stream is guided to the surface of the melt by adjacent nozzles in respectively different directions on account of their arrangement, with alternation of an arrangement perpendicular to the surface of the melt, a tilted arrangement toward the single crystal and a tilted arrangement away from the single crystal.

6. Method according to any of Claims 1 to 5, characterized in that the tubing system is heated regionally or completely from the outside in the region outside the pulling chamber.

7. Apparatus for producing a single silicon crystal doped with an n-type dopant and having in a cylindrical portion an electrical resistivity of not more than 2 mohmcm, comprising a pulling chamber for pulling the single crystal by the CZ method; a crucible for accommodating a melt of silicon; a heat shield for shielding a growing single crystal; a sublimation facility for converting solid dopant into gaseous dopant, and a tubing system for supplying a gas stream comprising the gaseous dopant to a surface of the melt, wherein the sublimation facility is arranged outside the pulling chamber, and the tubing system, leading through the heat shield or along an outer surface of the heat shield, opens out in an annular channel at the lower end of the heat shield, and the annular channel has nozzles for guiding the gas stream to the surface of the melt, characterized by a weighing cell, by a camera with image processing or by the weighing cell and the camera with image processing, for ascertaining the dopant consumption.

8. Apparatus according to Claim 7, characterized in that there are at least two nozzles.

9. Apparatus according to Claim 7 or Claim 8, characterized in that the nozzles are distributed with uniform spacing over the annular channel.

10. Apparatus according to any of Claims 7 to 9, characterized in that adjacent nozzles are arranged such that the gas stream is guided to the surface of the melt in respectively different directions, with alternation of an arrangement perpendicular to the surface of the melt, a tilted arrangement toward the single crystal and a tilted arrangement away from the single crystal.

11. Apparatus according to any of Claims 7 to 10, characterized by a tubing heater which heats the tubing system regionally or completely from the outside in the region outside the pulling chamber.