Process for producing monocrystalline silicon doped with n-type dopant by cz method

Abstract

The invention relates to a method for producing single crystal silicon doped with n-type dopants by pulling a single crystal surrounded by a heat shield having a lower end in a reactor chamber according to the CZ method from a melt contained in a crucible, the method comprises: heating, by means of a crucible heater, a solid dopant in a dopant crucible of a sublimation unit outside the reactor chamber to a temperature at which a gaseous dopant is formed; a gaseous dopant in the form of a volumetric flow of dopant gas is supplied to the surface of the melt through a conduit having a lower end, characterized in that a control valve between the sublimation unit and the reactor chamber is brought into a defined open state once the pressure difference between the pressure in the sublimation unit and the pressure in the reactor chamber has increased to a predetermined value; and controlling an open state of the control valve with a set point pressure in the sublimation unit as a command variable and the pressure difference as a controlled variable of control.

Description

Technical Field

[0001] The subject of the invention is a process (or method) for producing single crystal silicon doped with an n-type dopant by pulling the single crystal from a melt contained in a crucible in a reactor chamber by the CZ method. Background Art

[0002] Single crystal silicon containing a relatively large amount of n-type dopant is required primarily as a raw material for producing power semiconductor components.

[0003] Sublimated dopant elements such as phosphorus, arsenic or antimony are usually brought into contact with the melt while they are in the gaseous state. The challenge here is to ensure a sufficiently high concentration of the dopant in the melt despite the high vapor pressure of the dopant. Furthermore, the introduction of the dopant must be regulated with the utmost precision in order to prevent the formation of dislocations that would terminate the single crystal growth of the crystal.

[0004] WO 2021 / 115 904 A1 describes a process in which a dopant gas is generated in a sublimation facility and, together with a carrier gas, is passed through a conduit into a reactor chamber until it reaches an annular channel at the lower end of a heat shield.

[0005] US 2010 / 0 294 999 A1 describes the sublimation of a solid dopant in a heated post-unit in a reactor chamber and the control of the sublimation rate and the passing of the resulting dopant gas together with a carrier gas through a conduit to the melt. Summary of the invention

[0006] The object of the invention is to achieve, by simple means, an extremely uniform distribution of the dopant over the axial length of a cylindrical portion of a single crystal, independently of the length of the cylindrical portion.

[0007] This object is achieved by means of a process for producing single crystal silicon doped with an n-type dopant by pulling a single crystal surrounded by a heat shield (or heat shield) having a lower end from a melt contained in a crucible in a reactor chamber by a CZ process, comprising:

[0008] heating a solid dopant in a dopant crucible of a sublimation unit outside the reactor chamber to a temperature for forming a gaseous dopant by means of a crucible heater;

[0009] A gaseous dopant is supplied to the surface of the melt in the form of a volume flow of dopant gas through a conduit (or pipe) having a lower end, comprising:

[0010] once the pressure difference between the pressure in the sublimation unit and the pressure in the reactor chamber has increased to a predetermined value, bringing a control valve between the sublimation unit and the reactor chamber into a prescribed open state; and

[0011] The opening state of the control valve is controlled with the set point pressure in the sublimation unit as a command variable and the pressure difference as a controlled variable of control.

[0012] The present invention enables a substantially (or almost) constant dopant gas flow to be delivered to the melt over a relatively long period of time during the pulling of the single crystal, and reloading of solid dopant when necessary to ensure a continuous supply of dopant gas to the melt. The solution is simple and reliable.

[0013] The gaseous dopant is conveyed to the melt without a carrier gas (e.g. argon) or together with a carrier gas as a volume flow of the dopant gas. If the concentration of the gaseous dopant is relatively low, the carrier gas is used in particular to stabilize the gaseous dopant flow. The supply of the carrier gas is preferably controlled by means of a mass flow controller (MFC).

[0014] Solid dopants can be loaded into the dopant crucible before and / or during pulling the single crystal. In order to reload the dopant crucible with solid dopants, the conduit outside the reactor chamber is interrupted by completely closing the control valve. For safety purposes, an isolation valve is preferably also activated, which valve may be arranged between the reactor chamber and the control valve. The isolation valve ensures reliable access to the sublimation unit regardless of the tightness of the control valve. The temperature of the dopant crucible and therefore the temperature in the sublimation unit is preferably increased shortly before the solid dopant contained therein is exhausted. This ensures that the dopant has completely escaped before the sublimation unit is opened. The escape of the dopant can be monitored via the associated pressure drop in the sublimation unit.

[0015] The sublimation unit comprises a housing (or shell) which accommodates the dopant crucible and provides access to the dopant crucible.The housing is preferably of double-walled construction and can be actively cooled by means of a cooling circuit.

[0016] In addition to the crucible heater, other heaters are preferably present, in particular to counteract resublimation of the dopant; examples include internal heaters for the housing, external heaters for the control valves and optional isolation valves and those areas of the conduits that are located outside the reactor chamber. Additional heaters may be provided to heat the valves for introducing carrier gas and ambient air into the sublimation unit. For red phosphorus as dopant, the sublimation temperature is 362° C. at a pressure of 10 000 Pa; for arsenic, the sublimation temperature is 508° C. at this pressure. In particular, the temperature of the conduits that carry (or carry) the dopant should be higher than the corresponding sublimation temperature.

[0017] 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 (or differential) between the pressure in the sublimation unit and the pressure in the reactor chamber and is regulated via a control valve actuator. The actuator regulates the stroke of an element that determines the flow through the valve, such as the position of a barrier that can interrupt the flow through the valve. The actuator is preferably electrically (or electrically) operated and forms a control element of the control. During the supply of gaseous dopants to the surface of the melt, the pressure in the sublimation unit is greater than the pressure in the reactor chamber.

[0018] In order to ensure that the volume flow of dopant gas reaches the melt in a reproducible and regulated manner during the process (or process) of pulling single crystal silicon, the volume flow preferably passes through the control valve only after the stabilization phase. The stabilization phase includes heating the dopant crucible to a temperature below the temperature at which the solid dopant starts to sublimate with the control valve closed, until the pressure in the sublimation unit increases to a predetermined value. After the stabilization phase, the dopant crucible is heated to a temperature above the temperature at which the solid dopant starts to sublimate. The rate of sublimation of the solid dopant (evaporation rate) is determined in particular by the temperature to which the dopant crucible is heated. The further pressure increase that occurs in the sublimation unit when the temperature of the dopant crucible heater is constant or slightly increased indicates the start of sublimation. Once the pressure difference between the pressure in the sublimation unit and the pressure in the reactor chamber grows to a predetermined value, the control valve is opened. The pressure difference is preferably at least 1000 Pa, more preferably 5000 to 10 000 Pa. The opening condition of the control valve is then controlled by means of a control valve actuator as part of the control, with the set point pressure in the sublimation unit as the command variable and the pressure difference as the controlled variable of the control. The control preferably takes the form of a PID control. Apart from periodic pressure fluctuations due to the control, the pressure in the sublimation unit remains substantially (or almost) constant until a pressure drop occurs due to a reduced supply of solid dopant in the dopant crucible. When the pressure drops to a desired value, the control valve is also closed.

[0019] If it is necessary to replace dopant that has escaped from the melt, the dopant crucible can be refilled with solid dopant. This may be the case if the crystal is remelted after dislocation development and a new single crystal pulling campaign is started, or if the dopant stock in the sublimation cell is exhausted due to the length of the cylindrical portion of the growing single crystal. In this case, with the control valve closed and any isolation valves closed, the sublimation cell is purged and cooled, and the required amount of additional solid dopant is placed in the dopant crucible.

[0020] The supply of a volume flow of dopant gas to the surface of the melt via the conduit preferably starts before pulling the cylindrical portion of the single crystal, for example immediately after the seed crystal comes into contact with the melt, or before or during pulling the conical portion of the single crystal. An alternative possibility is to start the process only at the beginning of pulling the cylindrical portion or during it. Independently of this, even before the supply of the volume flow of dopant gas, the melt may already be enriched with the dopant, for example due to the immersion of a doping bell with solid dopant into the silicon melt.

[0021] The lower end of the conduit (where the volume flow of the dopant gas occurs) is preferably arranged above the melt at the same height as the lower end of the heat shield surrounding the growing single crystal. In addition, the lower end of the conduit is preferably located between the lower end of the heat shield and the wall of the crucible with the melt. It is also preferred that the conduit in the reactor chamber is arranged so that it passes through the wall of the heat shield. In the area of ​​the wall of the reactor chamber where the conduit can be cooled by water, the conduit is preferably composed of metal (for example, composed of stainless steel) and is exposed to a temperature of no more than 600° C. There. Around the growing single crystal and relatively hot graphite parts, i.e. the hot zone, temperature-stable materials such as graphite, carbon fiber reinforced composite (CFC) materials and / or quartz must be used. It is particularly preferred to subdivide the conduit in the reactor chamber into an upper part made of stainless steel, a middle part made of CFC, and a lower part made of quartz and coated with CFC at its lower end. The coating protects the quartz tube from overheating.

[0022] The present invention is further described below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A vertical cross-section of a reactor chamber and a sublimation unit suitable for practicing the present invention is shown.

[0024] Figure 2 Shows Figure 1 Zoomed in details.

[0025] Figure 3 A graph showing the resistivity R of single crystal silicon as a function of the axial position P of a cylindrical portion of the single crystal is shown.

[0026] List of reference symbols used

[0027] 1 Reactor room

[0028] 2 sublimation units

[0029] 3 Crucible

[0030] 4 Melt

[0031] 5Single crystal

[0032] 6 Catheter

[0033] 7. Heating device

[0034] 8 lifting device

[0035] 9 Magnetic field coil

[0036] 10 Heat shield

[0037] 11 Dopant Crucible

[0038] 12 Crucible heater

[0039] 13 Load Cell

[0040] 14 Mass Flow Controller

[0041] 15 Controller

[0042] 16 Control valve

[0043] 17 Isolation valve

[0044] 18 Upper part

[0045] 19 Middle part

[0046] 20 Lower part

[0047] 21CFC sheath (or jacket) DETAILED DESCRIPTION

[0048] DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS ACCORDING TO THE PRESENT INVENTION

[0049] Figure 1 A vertical section of a reactor chamber 1 and a sublimation unit 2 suitable for implementing the present invention is shown. The following description does not mention the various features shown, which do not contribute to the explanation of the present invention. In the reactor chamber 1, a crucible 3 with (i.e. filled with) a silicon melt 4 is arranged, which is doped (from the melt) with a gaseous n-type dopant during the pulling of a single crystal 5, which is conveyed to the surface of the melt 4 via a conduit 6. The melt 4 is kept in a liquid state by means of a heating device 7 surrounding the crucible 3. The single crystal 5 is surrounded by a heat shield 10 shielding the thermal radiation of the heating device 7. The crucible 3 is raised and rotated while the single crystal 5 is rotated and pulled from the melt by means of a pulling device 8. It is possible, but not absolutely necessary, to cause a magnetic field (e.g. a horizontal magnetic field) to act on the melt 4 by means of one or more magnetic field coils 9 arranged around the reactor chamber 1. During the pulling of the single crystal 5, the reactor chamber 1 is purged, for example, by means of an argon flow passing through the reactor chamber 1.

[0050] The gaseous dopant is generated in a sublimation unit 2 located outside the reactor chamber and is guided from the sublimation unit 2 to the surface of the melt 4 via a conduit 6 in the form of a volume flow of dopant gas. The volume flow may contain a portion of a carrier gas, for example argon, which is conveyed into the sublimation unit via a mass flow controller 14. In order to generate the gaseous dopant, a solid dopant, for example red phosphorus, is introduced into a dopant crucible 11 and the dopant crucible 11 is heated by means of a crucible heater 12 in the sublimation unit 2. The sublimation unit 2 may have a weighing sensor 13, which provides information about the current weight of the solid dopant in the dopant crucible 11. On its way through the conduit 6, the volume flow of the dopant gas passes a control valve 16 and an optional isolation valve 17 after leaving the sublimation unit 2 and before entering the reactor chamber 1.

[0051] 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 control the supply of dopant gas to the surface of the melt 4. In the embodiment shown, the controller 15 is also used to guide the pulling operation - for example, the rotation and extraction of the single crystal 5 and the rotation and raising of the crucible 3. In order to start the supply of gaseous dopants to the surface of the melt 4, the dopant crucible 11 is heated by means of the crucible heater 12. During this time, the controller 15 keeps the control valve 16 and the isolation valve 17 (if present) closed. The solid dopant in the dopant crucible 11 is initially preheated to a temperature below the sublimation temperature until the pressure in the sublimation unit 2 reaches a prescribed threshold. The temperature of the crucible heater 12 is then increased to a target temperature above the sublimation temperature, resulting in a further pressure increase in the sublimation unit 2. Once the pressure difference between the pressure in the sublimation unit 2 and the pressure in the reactor chamber 1 has grown to a predetermined value, the controller 15 brings the control valve 16 into a prescribed open state and, where appropriate, opens the isolation valve 17. The opening state of the control valve 16 is then controlled, wherein the command variable controlled is the set point pressure in the sublimation unit 2. If the solid dopant inventory in the dopant crucible is reduced, the pressure in the sublimation unit 2 drops. When the pressure drops to a prescribed value, the control valve 16 and the isolation valve 17 (when present) are closed. Afterwards, if necessary, still during the pulling process of the single crystal 5, the dopant crucible 11 can be loaded with additional solid dopant and a new cycle of supplying gaseous dopant to the surface of the melt 4 can be started.

[0052] according to Figure 1 In the preferred embodiment shown in , the distance between the lower end of the conduit 6 and the surface of the melt 4 is the same as the distance between the lower end of the heat shield 10 and the surface of the melt 4. In addition, in the case of this embodiment, the profile of the conduit 6 in the reactor chamber 1 is designed so that the conduit 6 passes through the heat shield 10, and the lower end of the conduit 6 is arranged between the wall of the crucible 3 and the lower end of the heat shield 10.

[0053] According to this preferred configuration and Figure 2 As shown in FIG. 1 , the conduit 6 in the reactor chamber 1 is subdivided into three parts, which are an upper part 18 made of stainless steel, a middle part 19 made of CFC (carbon fiber composite material), and a lower part 20 made of quartz and surrounded by a CFC sheath at its lower end.

[0054] Basically including Figure 1 The effectiveness of the invention was tested as an example by producing single crystal silicon with a diameter of 200 mm in a facility of the characteristics shown. The sublimation unit was from the French manufacturer Riber.

[0055] First, the single crystal is pulled to a point (or extent) in the upper cylindrical portion having a basic phosphorus doping (which has been added to the melt by means of a doping bell before pulling). The process of the present invention is used to show that the resistivity of the single crystal can be further reduced and can be kept substantially (or almost) constant between the lower limit LSL and the upper limit USL until the end of the pulling process. Figure 3 The results are presented in the form of resistance measurements. The resistance distribution is shown from the start of the cylindrical portion at position P=0 to the end of the cylindrical portion at position P=100. Supplying the gaseous dopant in the form of a volume flow of the dopant gas according to the invention takes place during the pulling of the cylindrical portion of the single crystal between the positions indicated as start and end. By applying the process of the invention, the resistivity in the single crystal can be reduced to less than 1 milliohm centimeter (mΩcm).

Claims

1. A process for producing single crystal silicon doped with an n-type dopant by pulling a single crystal surrounded by a heat shield having a lower end from a melt contained in a crucible in a reactor chamber by a CZ method, comprising: heating a solid dopant in a dopant crucible of a sublimation unit outside the reactor chamber to a temperature for forming a gaseous dopant by means of a crucible heater; Supplying a gaseous dopant in the form of a volume flow of dopant gas to a surface of the melt through a conduit having a lower end, comprising: once the pressure difference between the pressure in the sublimation unit and the pressure in the reactor chamber has increased to a predetermined value, bringing the control valve between the sublimation unit and the reactor chamber into a prescribed open state; as well as The opening state of the control valve is controlled with the set point pressure in the sublimation unit as a command variable and the pressure difference as a controlled variable of control.

2. The process of claim 1, wherein: The control adopts the form of PID control.

3. The process of claim 1 or claim 2, wherein: The distance between the lower end of the conduit and the surface of the melt is the same as the distance between the lower end of the heat shield and the surface of the melt.

4. The process according to any one of claims 1 to 3, wherein The conduit passes through the heat shield, and the lower end of the conduit is disposed between the wall of the crucible and the lower end of the heat shield.

5. The process according to any one of claims 1 to 4, wherein The conduit is divided into an upper portion made of stainless steel, a middle portion made of CFC, and a lower portion made of quartz and coated with CFC.

6. The process according to any one of claims 1 to 5, wherein Before the dopant crucible is reloaded with further solid dopant, the conduit is shut off between the sublimation unit and the reactor chamber by means of an isolation valve.

7. The process according to any one of claims 1 to 6, wherein The temperature of the dopant crucible is increased shortly before the solid dopant contained in the dopant crucible is exhausted.

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