METHOD FOR PRODUCING A SILICON SINGLE CRYSTAL DOPED WITH N-TYPE DOP, ACCORDING TO THE CZ METHOD

Description

[0001] The invention relates to a method for producing a single crystal of silicon doped with n-type dopant by pulling the single crystal in a reactor chamber according to the CZ method from a melt contained in a crucible. State of the art / problems

[0002] Single crystals of silicon, which contain comparatively large amounts of n-type dopant, are mainly required as raw materials for the production of semiconductor power devices.

[0003] Sublimating dopant elements such as phosphorus, arsenic, or antimony are typically brought into contact with the melt in the gaseous state. The challenge is to ensure a sufficiently high dopant concentration in the melt, despite the dopant's high vapor pressure. Furthermore, the dopant introduction must be controlled as precisely as possible to avoid the formation of dislocations that would terminate single-crystal growth.

[0004] WO 2021 / 115 904 A1 describes a method according to which dopant gas is generated in a sublimation device and, together with a carrier gas, is passed through a pipeline into the reactor chamber to an annular channel at a lower end of a heat shield.

[0005] US 2010 / 0 294 999 A1 describes sublimating solid dopant in a heated unit within the reactor chamber and regulating the sublimation rate, as well as passing the resulting dopant gas together with a carrier gas through a pipeline to the melt.

[0006] The object of this invention is to achieve in a simple manner that the dopant distribution over the axial length of a cylindrical section of the single crystal is as uniform as possible, regardless of the length of the cylindrical section.

[0007] The object is achieved by a method for producing a single crystal of silicon doped with n-type dopant by pulling the single crystal, which is surrounded by a heat shield with a lower end, in a reactor chamber according to the CZ method from a melt contained in a crucible, comprising Heating solid dopant in a dopant crucible of a sublimation unit outside the reactor chamber by means of a crucible heater to a temperature at which gaseous dopant is formed; supplying the gaseous dopant in the form of a volume flow of dopant gas through a pipeline with a lower end to a surface of the melt, characterized by bringing a control valve between the sublimation unit and the reactor chamber into a predetermined opening state as soon as a pressure difference between the pressure in the sublimation unit and the pressure in the reactor chamber has increased to a predetermined value; and regulating the opening state of the control valve with a target pressure in the sublimation unit as a reference variable and the pressure difference as the controlled variable of the control.

[0008] Using the invention, a nearly constant flow of dopant gas can be supplied to the melt over a comparatively long period of time during single crystal pulling, and solid dopant can be reloaded as needed to ensure a continued supply of dopant gas to the melt. The solution is simple and reliable.

[0009] The gaseous dopant is fed to the melt as a volume flow of dopant gas, either without or in combination with a carrier gas such as argon. The carrier gas is used in particular to stabilize the flow of gaseous dopant when the concentration of gaseous dopant is comparatively low. The supply of carrier gas is preferably controlled by a mass flow controller (MFC).

[0010] Solid dopant can be filled into the dopant crucible before and / or during the pulling of a single crystal. To refill the dopant crucible with solid dopant, the piping outside the reactor chamber is interrupted by completely closing the control valve. For safety reasons, an isolation valve, which can be arranged between the reactor chamber and the control valve, is preferably also activated. The isolation valve ensures that safe access to the sublimation unit is guaranteed, regardless of the tightness of the control valve. Preferably, the temperature of the dopant crucible, and thus the temperature in the sublimation unit, is raised shortly before the solid dopant contained therein is used up. This ensures that the dopant has completely escaped before the sublimation unit is opened.The escape of the dopant can be monitored by the associated pressure drop in the sublimation unit.

[0011] The sublimation unit comprises a housing that houses the dopant crucible and provides access to the dopant crucible. The housing is preferably double-walled and can be actively cooled by means of a cooling circuit.

[0012] In addition to the crucible heater, additional heaters are preferably present, particularly to counteract resublimation of the dopant, for example, an internal heater for the housing, an external heater for the control valve and any isolation valve present, and the piping sections located outside the reactor chamber. A further heater can be provided to heat valves for introducing the carrier gas and ambient air into the sublimation unit. For red phosphorus as the dopant, the sublimation temperature is 362 °C at a pressure of 10,000 Pa; for arsenic, the sublimation temperature at this pressure is 508 °C. In particular, the temperature of the dopant-carrying piping should be above the corresponding sublimation temperature.

[0013] The pressure is measured in the reactor chamber and in the sublimation unit. The opening state of the control valve depends on the pressure difference in the sublimation unit and the reactor chamber and is adjusted using a control valve actuator. The actuator adjusts the stroke of an element that determines the flow through the valve, for example, the position of a plug that can interrupt the flow through the valve. The actuator preferably operates electrically and forms the final control element. During the supply of the gaseous dopant to the surface of the melt, the pressure in the sublimation unit is greater than the pressure in the reactor chamber.

[0014] In order to ensure that the volume flow of dopant gas reaches the melt in a reproducible and controlled manner during a process of pulling a single crystal of silicon, the volume flow is preferably only passed through the control valve after a stabilization phase. The stabilization phase comprises heating the dopant crucible with the control valve closed to a temperature below the temperature at which the solid dopant begins to sublimate until the pressure in the sublimation unit has risen to a predetermined value. After the stabilization phase, the dopant crucible is heated to a temperature above the temperature at which the solid dopant begins to sublimate. The rate (evaporation rate) at which the solid dopant sublimates is determined in particular by the temperature to which the dopant crucible is heated.A further pressure increase in the sublimation unit at a constant or slightly increasing temperature of the dopant crucible heater indicates the start of sublimation. The control valve is opened as soon as the difference between the pressure in the sublimation unit and the pressure in the reactor chamber has increased to a predetermined value. The pressure difference is preferably at least 1000 Pa, particularly preferably 5000 to 10000 Pa. The opening state of the control valve is then controlled by a control valve actuator within the framework of a closed-loop control system, with a target pressure in the sublimation unit as the reference variable and the pressure difference as the controlled variable of the control system. The control system is preferably designed as a PID control system.The pressure in the sublimation unit remains virtually constant, except for periodic pressure fluctuations due to regulation, until a pressure drop occurs, which occurs due to the depletion of the solid dopant in the dopant crucible. When the pressure drops to a predetermined value, the control valve is also closed.

[0015] The dopant crucible can be reloaded with solid dopant if it becomes necessary to replace dopant that has escaped from the melt. This can occur when the crystal is remelted after dislocation formation and a new attempt to grow a single crystal is initiated, or when the dopant supply in the sublimation unit is exhausted due to the length of the cylindrical section of the growing single crystal. The sublimation unit is then rinsed and cooled with the control valve and, if applicable, the isolation valve closed, and the required amount of additional solid dopant is deposited in the dopant crucible.

[0016] The supply of the volume flow of dopant gas through the pipeline to the surface of the melt preferably begins before pulling a cylindrical section of the single crystal, for example, immediately after bringing a seed crystal into contact with the melt, or before or during pulling a conical section of the single crystal. However, it is also possible to start at the beginning or during pulling of the cylindrical section. Irrespective of this, the melt can already be enriched with dopant before the supply of the volume flow of dopant gas, for example, by immersing a dopant bell containing solid dopant into the silicon melt.

[0017] The lower end of the pipeline, at which the volume flow of dopant gas emerges, is preferably arranged at the same height above the melt as the lower end of a heat shield surrounding the growing single crystal. Furthermore, the lower end of the pipeline is preferably located between the lower end of the heat shield and the wall of the crucible containing the melt. It is also preferable to arrange the pipeline in the reactor chamber so that it passes through the wall of the heat shield. In the region of the wall of the reactor chamber, where it can be cooled with water, the pipeline is preferably made of metal, for example stainless steel, and is exposed there to a temperature of no more than 600°C. In the area surrounding the growing single crystal and warmer graphite parts, the hot zone, temperature-resistant material must be used, for example graphite, a carbon fiber reinforced composite (CFC) material and / or quartz.It is particularly preferred to divide the piping in the reactor chamber into an upper section made of stainless steel, a middle section made of CFC, and a lower section made of quartz, which is coated with CFC at its lower end. The coating protects the quartz tube from overheating.

[0018] The invention is further described below with reference to drawings. Short description of the characters

[0019] Fig. 1 shows a vertical section through a reactor chamber and a sublimation unit suitable for implementing the invention. Fig. 2 shows an enlarged section of Fig. 1 Fig. 3 shows the curve of the specific electrical resistance R of a single crystal of silicon as a function of the axial position P of the cylindrical section of the single crystal. List of reference symbols used

[0020] 1 reactor chamber 2 Sublimation unit 3crucible 4 melt 5 single crystal 6 pipeline 7 Heating device 8 pulling device 9 Magnetic field coil 10 heat shield 11 Dopant crucible 12 Crucible heating 13 load cell 14 Mass flow controller 15 Controller 16 control valve 17 Isolation valve 18 upper section 19 middle section 20 lower section 21 CFC sheath Detailed description of embodiments according to the invention

[0021] Fig. 1shows a vertical section through a reactor chamber 1 and a sublimation unit 2, which are suitable for implementing the invention. The following description omits some shown features that do not contribute to the explanation of the invention. In the reactor chamber 1, a crucible 3 is arranged with a melt 4 of silicon, which is doped with gaseous n-type dopant during the pulling of a single crystal 5 from the melt, which is led through a pipe 6 to the surface of the melt 4. The melt 4 is kept liquid by means of a heating device 7 surrounding the crucible 3. The single crystal 5 is surrounded by a heat shield 10, which shields the heat radiation from the heating device 7. While the single crystal 5 is rotated by means of a pulling device 8 and pulled from the melt, the crucible 3 is lifted and rotated.Applying a magnetic field, for example a horizontal magnetic field, to the melt 4 by means of one or more magnetic field coils 9 arranged around the reactor chamber 1 is possible, but not mandatory. During the pulling of the single crystal 5, the reactor chamber 1 is purged, for example, by means of a flow of argon.

[0022] The gaseous dopant is generated in the sublimation unit 2, which is located outside the reactor chamber, and from there, in the form of a volume flow of dopant gas, is fed through the pipeline 6 to the surface of the melt 4. The volume flow may contain a portion of carrier gas, for example, argon, which is fed into the sublimation unit via a mass flow controller 14. To generate the gaseous dopant, solid dopant, for example, red phosphorus, is filled into a dopant crucible 11, and the dopant crucible 11 is heated by a crucible heater 12 in the sublimation unit 2. The sublimation unit 2 may have a load cell 13 that provides information about the current weight of solid dopant in the dopant crucible 11. On its way through the pipeline 6, the volume flow of doping gas passes a control valve 16 and, if necessary, a separation valve 17 after leaving the sublimation unit 2 and before entering the reactor chamber 1.

[0023] The pressure and temperature in the reactor chamber 1 and in the sublimation unit 2 are transmitted by sensors to a controller 15, which is used to regulate the supply of dopant gas to the surface of the melt 4. In the illustrated embodiment, the controller 15 is also used to control the pulling process, for example, the rotation and withdrawal of the single crystal 5 and the rotation and lifting of the crucible 3. To initiate the supply of gaseous dopant to the surface of the melt 4, the dopant crucible 11 is heated by the crucible heater 12. During this time, the controller 15 keeps the control valve 16 and the isolation valve 17 (if present) closed. First, the solid dopant in the dopant crucible 11 is preheated to a temperature below the sublimation temperature until the pressure in the sublimation unit 2 has reached a predetermined threshold.Subsequently, the temperature of the crucible heater 12 is increased to a target temperature above the sublimation temperature, which results in a further increase in pressure in the sublimation unit 2. The controller 15 brings the control valve 16 into a predetermined opening state and, if necessary, opens the isolation valve 17 as soon as the difference between the pressure in the sublimation unit 2 and the pressure in the reactor chamber 1 has increased to a predetermined value. Subsequently, the opening state of the control valve 16 is regulated, with a target pressure in the sublimation unit 2 serving as the control variable. When the supply of solid dopant in the dopant crucible runs low, the pressure in the sublimation unit 2 drops. Once the pressure has dropped to a predetermined value, the control valve 16 and, if necessary, the isolation valve 17 are closed.Thereafter, if necessary, the dopant crucible 11 can be loaded with further solid dopant while the single crystal 5 is being pulled and a new cycle of supplying the gaseous dopant to the surface of the melt 4 can be started.

[0024] According to the Fig. 1 In the preferred embodiment shown, the lower end of the pipe 6 has the same distance from the surface of the melt 4 as the lower end of the heat shield 10. Furthermore, in this embodiment, the course of the pipe 6 in the reactor chamber 1 is designed such that the pipe 6 passes through the heat shield 10, and the lower end of the pipe 6 is arranged between a wall of the crucible 3 and the lower end of the heat shield 10.

[0025] According to this preferred embodiment and the representation in Fig. 2the pipe 6 in the reactor chamber 1 is divided into three sections, namely an upper section 18 made of stainless steel, a middle section 19 made of CFC (carbon fiber carbon composite) and a lower section 20 made of quartz, which is surrounded at its lower end by a CFC jacket.

[0026] The effectiveness of the invention was tested using the example of the production of a single crystal of silicon with a diameter of 200 mm in a plant which essentially uses the Fig.1 The sublimation unit was manufactured by Riber in France.

[0027] First, the single crystal was pulled to an upper, cylindrical section with a basic doping of phosphorus, which had been added to the melt prior to pulling using a doping bell. The inventive method was used to demonstrate that the specific electrical resistance of the single crystal could be further reduced and maintained nearly constant between a lower limit (LSL) and an upper limit (USL) until the completion of the pulling process. Fig. 3The result is shown in the form of a resistance measurement. The resistance curve is shown from the beginning of the cylindrical section at position P = 0 to the end of the cylindrical section at position P = 100. The inventive supply of gaseous dopant in the form of a volume flow of dopant gas took place during the pulling of the cylindrical section of the single crystal between the positions labeled Start and End. By applying the inventive method, the specific resistance in the single crystal could be reduced to less than 1 mΩcm.

Claims

1. Process for producing a single silicon crystal doped with n-type dopant by pulling the single crystal, surrounded by a heat shield having a lower end, in a reactor chamber by the CZ method from a melt contained in a crucible, comprising heating solid dopant in a dopant crucible of a sublimating unit outside the reactor chamber by means of a crucible heater to a temperature at which gaseous dopant is formed; supplying the gaseous dopant in the form of a volume stream of dopant gas through a conduit having a lower end to a surface of the melt, characterized by bringing a control valve between the sublimating unit and the reactor chamber into a mandated open state as soon as a pressure difference between the pressure in the sublimating unit and the pressure in the reactor chamber has grown to a predetermined value; and controlling the open state of the control valve with a setpoint pressure in the sublimating unit as command variable and the pressure difference as controlled variable of the control.

2. Process according to Claim 1, characterized in that the control takes the form of PID control.

3. Process according to Claim 1 or Claim 2, characterized in that the lower end of the conduit has the same distance from the surface of the melt as does the lower end of the heat shield.

4. Process according to any of Claims 1 to 3, characterized in that the conduit passes through the heat shield, the lower end of the conduit being disposed between a wall of the crucible and the lower end of the heat shield.

5. Process according to any of Claims 1 to 4, characterized in that the conduit is divided into an upper portion of stainless steel, a middle portion of CFC and a lower portion of quartz clad with CFC.

6. Process according to any of Claims 1 to 5, characterized in that the conduit is closed between the sublimating unit and the reactor chamber by an isolation valve before the dopant crucible is reloaded with further solid dopant.

7. Process according to any of Claims 1 to 6, characterized in that the temperature of the dopant crucible is raised shortly before the solid dopant contained therein is used up.