Patent ID: 12247314

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring initially toFIGS.1and2, a crucible of one embodiment is indicated generally at10. A cylindrical coordinate system for crucible10includes a radial direction R12, an angular direction θ14, and an axial direction Z16. Coordinates R12, θ14, and Z16are used herein to describe methods and systems for producing low oxygen silicon ingots. The crucible10contains a melt25having a melt surface36. A crystal27is grown from the melt25. The melt25may contain one or more convective flow cells17,18induced by heating of the crucible10and rotation of the crucible10and/or crystal27in the angular direction θ14. The structure and interaction of these one or more convective flow cells17,18are modulated via regulation of one of more process parameters to reduce the inclusion of oxygen within the forming crystal27as described in detail herein below.

FIG.3is a block diagram illustrating a cusped magnetic field being applied to crucible10containing melt25in a crystal growing apparatus. As shown, crucible10contains silicon melt25from which a crystal27is grown. The cusped magnetic field configuration is designed to overcome deficiencies of axial and horizontal magnetic field configurations. A pair of coils31and33(e.g., Helmholtz coils) are placed coaxially above and below melt surface36. Coils31and33are operated in an opposed current mode to generate a magnetic field that has a purely radial field component (i.e., along R12) near melt surface36and a purely axial field component (i.e., along Z16) near an axis of symmetry38of crystal27. The combination of an upper magnetic field40and a lower magnetic field42produced by coils31and33, respectively, results in axial and radial cusped magnetic field components.

FIG.4is a block diagram of a crystal growing system100. The crystal growing system100, elements of the crystal growing system100, and various operating parameters of the crystal growing system100are described in additional detail in PCT Published Application 2014/190165, which is incorporated by reference herein in its entirety. Referring again toFIG.4, system100employs a Czochralski crystal growth method to produce a semiconductor ingot. In this embodiment, system100is configured to produce a cylindrical semiconductor ingot having an ingot diameter of greater than one-hundred fifty millimeters (150 mm), more specifically in a range from approximately 150 mm to 460 mm, and even more specifically, a diameter of approximately three-hundred millimeters (300 mm). In other embodiments, system100is configured to produce a semiconductor ingot having a two-hundred millimeter (200 mm) ingot diameter or a four-hundred and fifty millimeter (450 mm) ingot diameter. In addition, in one embodiment, system100is configured to produce a semiconductor ingot with a total ingot length of at least nine hundred millimeters (900 mm). In other embodiments, system100is configured to produce a semiconductor ingot with a total ingot length ranging from approximately nine hundred millimeters (900 mm) to twelve hundred millimeters (1200 mm).

Referring again toFIG.4, the crystal growing system100includes a vacuum chamber101enclosing crucible10. A side heater105, for example, a resistance heater, surrounds crucible10. A bottom heater106, for example, a resistance heater, is positioned below crucible10. During heating and crystal pulling, a crucible drive unit107(e.g., a motor) rotates crucible10, for example, in the clockwise direction as indicated by the arrow108. Crucible drive unit107may also raise and/or lower crucible10as desired during the growth process. Within crucible10is silicon melt25having a melt level or melt surface36. In operation, system100pulls a single crystal27, starting with a seed crystal115attached to a pull shaft or cable117, from melt25. One end of pull shaft or cable117is connected by way of a pulley (not shown) to a drum (not shown), or any other suitable type of lifting mechanism, for example, a shaft, and the other end is connected to a chuck (not shown) that holds seed crystal115and crystal27grown from seed crystal115.

Crucible10and single crystal27have a common axis of symmetry38. Crucible drive unit107can raise crucible10along axis38as the melt25is depleted to maintain melt level36at a desired height. A crystal drive unit121similarly rotates pull shaft or cable117in a direction110opposite the direction in which crucible drive unit107rotates crucible10(e.g., counter-rotation). In embodiments using iso-rotation, crystal drive unit121may rotate pull shaft or cable117in the same direction in which crucible drive unit107rotates crucible10(e.g., in the clockwise direction). Iso-rotation may also be referred to as a co-rotation. In addition, crystal drive unit121raises and lowers crystal27relative to melt level36as desired during the growth process.

According to the Czochralski single crystal growth process, a quantity of polycrystalline silicon, or polysilicon, is charged to crucible10. A heater power supply123energizes resistance heaters105and106, and insulation125lines the inner wall of vacuum chamber101. A gas supply127(e.g., a bottle) feeds argon gas to vacuum chamber101via a gas flow controller129as a vacuum pump131removes gas from vacuum chamber101. An outer chamber133, which is fed with cooling water from a reservoir135, surrounds vacuum chamber101.

The cooling water is then drained to a cooling water return manifold137. Typically, a temperature sensor such as a photocell139(or pyrometer) measures the temperature of melt25at its surface, and a diameter transducer141measures a diameter of single crystal27. In this embodiment, system100does not include an upper heater. The presence of an upper heater, or lack of an upper heater, alters cooling characteristics of crystal27.

An upper magnet, such as solenoid coil31, and a lower magnet, such as solenoid coil33, are located above and below, respectively, melt level36in this embodiment. The coils31and33, shown in cross-section, surround vacuum chamber (not shown) and share axes with axis of symmetry38. In one embodiment, the upper and lower coils31and33have separate power supplies, including, but not limited to, an upper coil power supply149and a lower coil power supply151, each of which is connected to and controlled by control unit143.

In this embodiment, current flows in opposite directions in the two solenoid coils31and33to produce a magnetic field (as shown inFIG.3). A reservoir153provides cooling water to the upper and lower coils31and33before draining via cooling water return manifold137. A ferrous shield155surrounds coils31and33to reduce stray magnetic fields and to enhance the strength of the field produced.

A control unit143is used to regulate the plurality of process parameters including, but not limited to, at least one of crystal rotation rate, crucible rotation rate, and magnetic field strength. In various embodiments, the control unit143may include a processor144that processes the signals received from various sensors of the system100including, but not limited to, photocell139and diameter transducer141, as well as to control one or more devices of system100including, but not limited to: crucible drive unit107, crystal drive unit121, heater power supply123, vacuum pump131, gas flow controller129(e.g., an argon flow controller), upper coil power supply149, lower coil power supply151, and any combination thereof.

Control unit143may be a computer system. Computer systems, as described herein, refer to any known computing device and computer system. As described herein, all such computer systems include a processor and a memory. However, any processor in a computer system referred to herein may also refer to one or more processors wherein the processor may be in one computing device or a plurality of computing devices acting in parallel. Additionally, any memory in a computer device referred to herein may also refer to one or more memories wherein the memories may be in one computing device or a plurality of computing devices acting in parallel.

The term processor, as used herein, refers to central processing units, microprocessors, microcontrollers, reduced instruction set circuits (RISC), application specific integrated circuits (ASIC), logic circuits, and any other circuit or processor capable of executing the functions described herein. The above are examples only, and are thus not intended to limit in any way the definition and/or meaning of the term “processor.”

As used herein, the term “database” may refer to either a body of data, a relational database management system (RDBMS), or to both. As used herein, a database may include any collection of data including hierarchical databases, relational databases, flat file databases, object-relational databases, object oriented databases, and any other structured collection of records or data that is stored in a computer system. The above examples are example only, and thus are not intended to limit in any way the definition and/or meaning of the term database. Examples of RDBMS's include, but are not limited to including, Oracle® Database, MySQL, IBM® DB2, Microsoft® SQL Server, Sybase®, and PostgreSQL. However, any database may be used that enables the systems and methods described herein. (Oracle is a registered trademark of Oracle Corporation, Redwood Shores, California; IBM is a registered trademark of International Business Machines Corporation, Armonk, New York; Microsoft is a registered trademark of Microsoft Corporation, Redmond, Washington; and Sybase is a registered trademark of Sybase, Dublin, California.)

In one embodiment, a computer program is provided to enable control unit143, and this program is embodied on a computer readable medium. In an example embodiment, the computer system is executed on a single computer system, without requiring a connection to a server computer. In a further embodiment, the computer system is run in a Windows® environment (Windows is a registered trademark of Microsoft Corporation, Redmond, Washington). In yet another embodiment, the computer system is run on a mainframe environment and a UNIX® server environment (UNIX is a registered trademark of X/Open Company Limited located in Reading, Berkshire, United Kingdom). Alternatively, the computer system is run in any suitable operating system environment. The computer program is flexible and designed to run in various different environments without compromising any major functionality. In some embodiments, the computer system includes multiple components distributed among a plurality of computing devices. One or more components may be in the form of computer-executable instructions embodied in a computer-readable medium.

The computer systems and processes are not limited to the specific embodiments described herein. In addition, components of each computer system and each process can be practiced independent and separate from other components and processes described herein. Each component and process also can be used in combination with other assembly packages and processes.

In one embodiment, the computer system may be configured as a server system.FIG.10illustrates an example configuration of a server system301used to receive measurements from one or more sensors including, but not limited to: temperature sensor139, diameter transducer141, and any combination thereof, as well as to control one or more devices of system100including, but not limited to: crucible drive unit107, crystal drive unit121, heater power supply123, vacuum pump131, gas flow controller129(e.g., an argon flow controller), upper coil power supply149, lower coil power supply151, and any combination thereof as described herein and illustrated inFIG.4in one embodiment. Referring again toFIG.10, server system301may also include, but is not limited to, a database server. In this example embodiment, server system301performs all of the steps used to control one or more devices of system100as described herein.

Server system301includes a processor305for executing instructions. Instructions may be stored in a memory area310, for example. Processor305may include one or more processing units (e.g., in a multi-core configuration) for executing instructions. The instructions may be executed within a variety of different operating systems on the server system301, such as UNIX, LINUX, Microsoft Windows®, etc. It should also be appreciated that upon initiation of a computer-based method, various instructions may be executed during initialization. Some operations may be required in order to perform one or more processes described herein, while other operations may be more general and/or specific to a particular programming language (e.g., C, C #, C++, Java, or any other suitable programming languages).

Processor305is operatively coupled to a communication interface315such that server system301is capable of communicating with a remote device such as a user system or another server system301. For example, communication interface315may receive requests (e.g., requests to provide an interactive user interface to receive sensor inputs and to control one or more devices of system100from a client system via the Internet.

Processor305may also be operatively coupled to a storage device134. Storage device134is any computer-operated hardware suitable for storing and/or retrieving data. In some embodiments, storage device134is integrated in server system301. For example, server system301may include one or more hard disk drives as storage device134. In other embodiments, storage device134is external to server system301and may be accessed by a plurality of server systems301. For example, storage device134may include multiple storage units such as hard disks or solid state disks in a redundant array of inexpensive disks (RAID) configuration. Storage device134may include a storage area network (SAN) and/or a network attached storage (NAS) system.

In some embodiments, processor305is operatively coupled to storage device134via a storage interface320. Storage interface320is any component capable of providing processor305with access to storage device134. Storage interface320may include, for example, an Advanced Technology Attachment (ATA) adapter, a Serial ATA (SATA) adapter, a Small Computer System Interface (SCSI) adapter, a RAID controller, a SAN adapter, a network adapter, and/or any component providing processor305with access to storage device134.

Memory area310may include, but is not limited to, random access memory (RAN) such as dynamic RAM (DRAM) or static RAM (SRAM), read-only memory (ROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), and non-volatile RAM (NVRAM). The above memory types are exemplary only, and are thus not limiting as to the types of memory usable for storage of a computer program.

In another embodiment, the computer system may be provided in the form of a computing device, such as a computing device402(shown inFIG.11). Computing device402includes a processor404for executing instructions. In some embodiments, executable instructions are stored in a memory area406. Processor404may include one or more processing units (e.g., in a multi-core configuration). Memory area406is any device allowing information such as executable instructions and/or other data to be stored and retrieved. Memory area406may include one or more computer-readable media.

In another embodiment, the memory included in the computing device of the control unit143may include a plurality of modules. Each module may include instructions configured to execute using at least one processor. The instructions contained in the plurality of modules may implement at least part of the method for simultaneously regulating a plurality of process parameters as described herein when executed by the one or more processors of the computing device. Non-limiting examples of modules stored in the memory of the computing device include: a first module to receive measurements from one or more sensors and a second module to control one or more devices of the system100.

Computing device402also includes one media output component408for presenting information to a user400. Media output component408is any component capable of conveying information to user400. In some embodiments, media output component408includes an output adapter such as a video adapter and/or an audio adapter. An output adapter is operatively coupled to processor404and is further configured to be operatively coupled to an output device such as a display device (e.g., a liquid crystal display (LCD), organic light emitting diode (OLED) display, cathode ray tube (CRT), or “electronic ink” display) or an audio output device (e.g., a speaker or headphones).

In some embodiments, client computing device402includes an input device410for receiving input from user400. Input device410may include, for example, a keyboard, a pointing device, a mouse, a stylus, a touch sensitive panel (e.g., a touch pad or a touch screen), a camera, a gyroscope, an accelerometer, a position detector, and/or an audio input device. A single component such as a touch screen may function as both an output device of media output component408and input device410.

Computing device402may also include a communication interface412, which is configured to communicatively couple to a remote device such as server system302or a web server. Communication interface412may include, for example, a wired or wireless network adapter or a wireless data transceiver for use with a mobile phone network (e.g., Global System for Mobile communications (GSM), 3G, 4G or Bluetooth) or other mobile data network (e.g., Worldwide Interoperability for Microwave Access (WIMAX)).

Stored in memory406are, for example, computer-readable instructions for providing a user interface to user400via media output component408and, optionally, receiving and processing input from input device410. A user interface may include, among other possibilities, a web browser and an application. Web browsers enable users400to display and interact with media and other information typically embedded on a web page or a website from a web server. An application allows users400to interact with a server application. The user interface, via one or both of a web browser and an application, facilitates display of information related to the process of producing a single crystal silicon ingot with low oxygen content.

In the example embodiment, system100produces silicon crystal ingots suitable for use in device manufacturing. Advantageously, system100may be used to produce silicon crystal27, a substantial portion or all of which is substantially free of agglomerated intrinsic point defects. Furthermore, system100may be used to produce crystal27having substantially no agglomerated defects that are larger than approximately one hundred twenty nanometers (nm) in diameter, or more particularly, approximately ninety nm in diameter. The shape of the melt-solid or melt-crystal interface and the pull speed is controlled during crystal growth to limit and/or suppress the formation of agglomerated intrinsic point defects.

During production, oxygen is introduced into silicon crystal ingots through the melt-solid or melt crystal interface. However, oxygen may cause various defects in wafers produced from the ingots, reducing the yield of semiconductor devices. Accordingly, it is desirable to produce silicon crystal ingots with a low oxygen concentration. Using the methods described herein, silicon crystal ingots are produced having an oxygen concentration less than approximately 5 parts per million atoms (ppma).

Without being limited to any particular theory, oxygen is introduced into the growing silicon crystal ingot emerging from the melt by an interacting series of events, each of which is influenced by at least one process parameter as described herein below. SiO is introduced into the melt via dissolution at the crucible wall. The SiO introduced at the crucible wall may be moved elsewhere in the melt via flow induced by buoyancy forces created by localized heating of the melt neat the crucible wall. The SiO may be further moved by additional flow induced by the rotation rate of the crystal at the melt-crystal interface as well as rotation rate of the crucible itself. The concentration of SiO in the melt may be reduced via evaporation from the melt at the exposed surface of the melt. The interaction of any combination of dissolution, convection, and evaporation of SiO within the melt influences the concentration of SiO in the melt situated near the crystal-melt interface that is formed into the silicon crystal ingot. In various aspects, any one or more process parameters are simultaneously regulated to reduce the concentration of SiO situated near the melt-crystal interface, and consequently reduce the oxygen concentration within the silicon crystal ingot formed according to the method.

In various embodiments, various process parameters are regulated simultaneously to facilitate producing silicon crystal ingots with a low oxygen concentration. In one embodiment, the various process parameters are regulated in at least two stages that include an intermediate body growth stage corresponding to growth of the silicon crystal ingot up to an intermediate ingot lengths of approximately 800 mm, and a late body growth stage corresponding to growth of the silicon crystal ingot from an intermediate ingot length of approximately 800 mm up to the total ingot length. In this embodiment, the regulation of the various process parameters in at least two different stages accounts for changes in the nature of the interaction of dissolution, convection, evaporation of SiO within the melt, depth of the melt in the crucible, and the flow cells within the melt in the crucible as the silicon crystal ingot grows in length.

In particular, the role of convection is modified over the formation of the entire silicon crystal ingot due to a decrease in the depth of the melt within the crucible associated with growth of the silicon crystal ingot, as described in detail below. As a result, at the late body growth stage, the regulation of at least one process parameter is modified differently relative to the regulation of these same parameters at the intermediate body growth stage. In some embodiments, at the late body growth stage, the regulation of at least three process parameters is modified differently relative to the regulation of these same parameters at the intermediate body growth stage. As described herein below, the regulation of the process parameters modulate various factors related to the convection of SiO within the melt at the late body growth stage. In one embodiment, the process parameters with modified regulation during the late body growth stage include, but are not limited to: seed rotation rate, crucible rotation rate, and magnetic field strength.

Referring again toFIG.4, seed rotation rate refers to the rate at which pull shaft or cable117rotates seed crystal115about axis38. Seed rotation rate impacts the flow of SiO from crucible10to crystal27and a rate of SiO evaporation from melt25. Referring again toFIG.2, the flow of SiO from crucible10to crystal27is influenced generally by interactions between crystal flow cell18driven by the rotation of crystal27at the seed rotation rate within melt25and buoyancy flow cell17driven by heating of melt25within crucible10. The impact of seed rotation rate on the flow of SiO from crucible10to crystal27differs depending on the stage of growth of crystal27.

FIG.5Ais a cross-sectional view of simulated flowlines and oxygen concentrations within melt25at an intermediate body growth stage, corresponding to growth of crystal27up to an intermediate ingot length of approximately 800 mm. At the intermediate body growth stage, depth200of melt25within crucible10is sufficiently deep to effectively decouple interactions between fluid motion induced by crystal flow cell18and buoyancy flow cell17. A high seed rotation rate (i.e. 12 rpm) reduces the boundary layer thickness between melt line36and the gas above melt25to increase SiO evaporation. Further, a high seed rotation rate decreases melt flow from crucible10to crystal27by suppressing buoyancy flow cell17with induced crystal flow cell18, as illustrated inFIG.5A. Moreover, a high seed rotation rate creates an outward radial flow that retards the inward flow (i.e., transport) of SiO from crucible10, reducing the oxygen concentration in crystal27.

FIG.5Bis a cross-sectional view of simulated flowlines and oxygen concentrations within melt25at a late body growth stage, corresponding to growth of crystal27from an intermediate ingot length of approximately 800 mm up to the total ingot length. Due to removal of melt25from crucible10associated with formation of crystal27, depth200at the late body growth stage is shallower with respect to depth200at intermediate body growth stage as illustrated inFIG.5A. At a similarly high seed rotation rate to that used to perform the simulation illustrated inFIG.5A(i.e. 12 rpm), crystal flow cell18contacts the inner wall of crucible10, causing convection of SiO formed at the inner wall of crucible10into crystal27formed at the late body growth stage.

FIG.5Cis a cross-sectional view of simulated flowlines and oxygen concentrations within melt25at a late body growth stage calculated at a lower (e.g., 8 rpm) seed rotation rate. Crystal flow cell18induced by the lower seed rotation rate does not extend to the inner wall of crucible10, but instead is excluded by buoyancy cell17. As a result, the flow of SiO produced at the inner wall of crucible10to crystal27is disrupted, thereby reducing the oxygen concentration within crystal27formed at the late body growth stage at reduced seed rotation rate.

As described herein, the transition from an intermediate to a late body growth stage is a soft transition. The transition may vary depending on various parameters of the process, such as crucible size, shape, depth of melt, modeling parameters, and the like. Generally, at the intermediate body growth stage, parameters are such that there are limited or no interactions between fluid motion induced by crystal flow cell18and buoyancy flow cell17; the crystal flow cell18and buoyancy flow cell17are effectively decoupled. At the late body growth stage, parameters are such that there are interactions between fluid motion induced by crystal flow cell18and buoyancy flow cell17; the crystal flow cell18and buoyancy flow cell17are effectively coupled. By way of non-limiting example, late body growth stage occurs when less than about 37% of the initial mass of melt25is left in crucible10in an embodiment that includes an initial melt mass of 250 kg in a crucible10with an inner diameter of about 28 inches. In various embodiments, depth200of melt25within crucible10is monitored to identify the transition from the intermediate to a late body growth stage. In other examples, the late body growth stage occurs when less than about 35%, less than about 40%, less than about 45%, or less than about 50% of the initial mass of melt25is left in crucible10. In some embodiments, the transition from intermediate to late body growth stage is determined based on the depth of melt25, or any other suitable parameter.

In various embodiments, the method includes regulating the seed rotation rate in at least two stages including, but not limited to, the intermediate body growth stage and the late body growth stage. In one embodiment, the method includes rotating crystal27during the intermediate body growth stage at a seed rotation rate ranging from approximately 8 to 14 rpm, and more specifically 12 rpm. In this embodiment, the method further includes reducing the seed rotation rate at the late body growth stage to a seed rotation rate ranging from approximately 6 rpm to 8 rpm, and more specifically 8 rpm.

In another embodiment, the seed rotation rate may be reduced according to the intermediate ingot length. By way of non-limiting example, the seed rotation rate may be regulated to approximately 12 rpm for intermediate ingot lengths up to approximately 850 mm, and may be further regulated to linearly decrease to approximately 8 rpm at an intermediate ingot length of approximately 950 mm, and then regulate seed rotation rate at approximately 8 rpm up to the total ingot length, as illustrated inFIG.9. As also illustrated inFIG.9, the oxygen content of the crystal within the body length ranging from approximately 800 mm to the total ingot length is reduced compared to a crystal formed at a constant seed rotation rate of approximately 12 rpm.FIG.6is a graph comparing the simulated oxygen concentration of crystals formed at seed rotation rates according to three rotation schedules: a) rotation at 12 rpm for the formation of the entire crystal; b) rotation at 12 rpm up to an intermediate crystal length of 900 mm followed by rotation at 8 rpm for formation of the remaining crystal length; and c) rotation at 12 rpm up to an intermediate crystal length of 900 mm followed by rotation at 6 rpm for formation of the remaining crystal length. As illustrated inFIG.6, lower seed rotation rates reduced oxygen concentration within the portion of the crystal formed at the late body growth stage.

Crucible rotation rate may further influence the oxygen concentrations within crystals27formed according to embodiments of the method. Crucible rotation rate refers to the rate at which crucible10is rotated about axis38using crucible drive unit107. Crucible rotation rate impacts the flow of SiO from crucible10to crystal27and an amount of SiO evaporating from melt25. A high crucible rotation rate reduces both a boundary layer thickness between crucible10and melt25, and a boundary layer thickness between melt line36and the gas above melt25. However, to minimize the oxygen concentration in crystal27, a thicker boundary layer between crucible10and melt25is desired to reduce the SiO transport rate, while a thinner boundary layer between melt line36and the gas above melt25is desired to increase the SiO evaporation rate. Accordingly, the crucible rotation rate is selected to balance the competing interests of a high boundary layer thickness between crucible10and melt25resulting from slower crucible rotation rates and a low boundary layer thickness between melt line36and the gas above melt25resulting from higher crucible rotation rates.

Changes in depth200of melt10between intermediate body growth stage and late body growth stage described herein above influence the impact of modulation of crucible rotation rate on oxygen concentration in a manner similar to the influence of seed rotation rate described herein previously. In various embodiments, the method includes regulating the crucible rotation rate in at least two stages including, but not limited to, the intermediate body growth stage and the late body growth stage. In one embodiment, the method includes rotating crucible10at the intermediate body growth stage at a crucible rotation rate ranging from approximately 1.3 rpm to approximately 2.2, and more specifically 1.7 rpm. In this embodiment, the method further includes reducing the crucible rotation rate at the late body growth stage to a crucible rotation rate ranging from approximately 0.5 rpm to approximately 1.0 rpm, and more specifically 1 rpm.

FIGS.7A and7Bare graphs showing a simulated oxygen concentration within silicon ingots as a function of the crucible rotation rate at late body growth stage. The silicon ingots ofFIG.7Awere formed using an embodiment of the method in which the seed rotation rate was reduced from 12 rpm to 6 rpm at late body growth stage, and the crucible rotation rate was reduced from about 1.7 rpm to 1 rpm or 1.5 rpm at late body growth stage. The silicon ingots ofFIG.7Bwere formed using an embodiment of the method in which the seed rotation rate was reduced from 12 rpm to 8 rpm at late body growth stage, and the crucible rotation rate was reduced from about 1.7 rpm to 0.5 rpm, 1 rpm, or 1.5 rpm at late body growth stage. In both simulations, lower crucible rotation rates were associated with lower oxygen concentrations within the resulting silicon ingot.

The method may further include regulating magnet strength in at least two stages including, but not limited to, the intermediate body growth stage and the late body growth stage. Magnet strength refers to the strength of the cusped magnetic field within the vacuum chamber. More specifically, magnet strength is characterized by a current through coils31and33that is controlled to regulate magnetic strength. Magnetic strength impacts the flow of SiO from crucible10to crystal27. That is, a high magnetic strength minimizes the flow of SiO from crucible10to crystal27by suppressing a buoyancy force within melt25. As the magnetic field suppresses the buoyancy flow, it decreases the dissolution rate of the quartz crucible, thus lowering the interstitial oxygen incorporated into the crystal. However, if the magnetic field strength increases beyond a certain level, further retardation in the buoyancy flow may result in decreasing the evaporation rate at the melt free surface, thus raising the interstitial oxygen levels. Due to differences in the relative contribution of buoyancy flow to the oxygen content of the crystal at the late body formation stage relative to the intermediate body formation stage as described previously herein, an adjustment to the magnet strength at the late body formation stage enables appropriate modulation of buoyancy flow to reduce oxygen within the crystal formed at the late body formation stage.

In various embodiments, the method includes regulating the magnetic field strength in at least two stages including, but not limited to, the intermediate body growth stage and the late body growth stage. In one embodiment, the method includes regulating the magnetic field strength at the intermediate body growth stage such that the magnetic field strength is approximately 0.02 to 0.05 Tesla (T) at an edge of crystal27at the melt-solid interface and approximately 0.05 to 0.12 T at the wall of crucible10. In another aspect, the method includes regulating the magnetic field strength at the late body growth stage such that the magnetic field strength is approximately 150% of the magnetic field strength used during the intermediate body growth stage, corresponding to approximately 0.03 to 0.075 T at an edge of crystal27at the melt-solid interface and approximately 0.075 to 0.18 T at the wall of crucible10.

FIGS.8A,8B, and8Care cross-sectional views of simulated flowlines and total speeds within melt25at a late body growth stage.FIG.8Awas simulated using magnetic field strengths corresponding to 50% of the magnetic field used at intermediate body growth stage (i.e., approximately 0.01 to 0.025 T at an edge of crystal27at the melt-solid interface and approximately 0.025 to 0.06 T at the wall of crucible10).FIG.8Bwas simulated using magnetic field strengths corresponding to 95% of the magnetic field used at intermediate body growth stage (i.e., approximately 0.019 to 0.0475 T at an edge of crystal27at the melt-solid interface and approximately 0.0475 to 0.114 T at the wall of crucible10).FIG.8Cwas simulated using magnetic field strengths corresponding to 150% of the magnetic field used at intermediate body growth stage (i.e., approximately 0.03 to 0.075 T at an edge of crystal27at the melt-solid interface and approximately 0.075 to 0.18 T at the wall of crucible10). ComparingFIGS.8A,8B, and8C, as the strength of the magnetic field increases, flow300from the bottom of crucible10to melt-crystal interface302transitions from relatively high convection to melt-crystal interface302at low magnetic field strength (FIG.8A) to a relatively little convection at higher magnetic field strengths. This suppression of buoyancy flow within melt25by the increased magnetic field results in lower oxygen concentration in the resulting silicon ingot, as summarized in Table 1 below. At 150% magnetic field strength, the simulated oxygen concentration was within the desired range below 5% parts per million atoms (ppma).

TABLE 1Effect of Magnetic Field Strength at Late Body GrowthStage on Oxygen Concentration in Silicon IngotsMagnetic Field StrengthSimulated Oxygen(% intermediate bodyConcentrationgrowth stage field strength)(ppma)50%9.395%6.4150%4.5

One or more additional process parameters may be regulated to facilitate producing silicon crystal ingots with a low oxygen concentration. However, the effects of these additional process parameters are not sensitive to the changes in the depth200of melt25within crucible10during growth of crystal27. Consequently, the regulation of the additional process parameters described herein remains essentially the same between different stages of crystal growth, as described in additional detail below.

One additional process parameter that is controlled, at least in some embodiments, is wall temperature of crucible10. The wall temperature of crucible10corresponds to a dissolution rate of crucible10. Specifically, the higher the wall temperature of crucible10, the faster that portions of crucible10will react with and dissolve into melt25, generating SiO into the melt and potentially increasing an oxygen concentration of crystal27via the melt-crystal interface. Accordingly, reducing the wall temperature of crucible10, as used herein, equates to reducing the dissolution rate of crucible10. By reducing the wall temperature of crucible10(i.e., reducing the dissolution rate of crucible10), the oxygen concentration of crystal27can be reduced. Wall temperature may be regulated by controlling one or more additional process parameters including, but not limited to heater power and melt to reflector gap.

Heater power is another process parameter that may be controlled in some embodiments to regulate the wall temperature of crucible10. Heater power refers to the power of side and bottom heaters105and106. Specifically, relative to typical heating configurations, by increasing a power of side heater105and reducing a power of bottom heater106, a hot spot on the wall of crucible10is raised close to the melt line36. As the wall temperature of crucible10at or below melt line36is lower, the amount of SiO generated by melt25reacting with crucible10is also lower. The heater power configuration also impacts melt flow by reducing the flow (i.e., transport) of SiO from crucible10to single crystal27. In this embodiment, a power of bottom heater106is approximately 0 to 5 kilowatts, and more specifically approximately 0 kilowatts, and a power of side heater105is in a range from approximately 100 to 125 kilowatts. Variations in the power of side heater105may be due to, for example, variation in a hot zone age from puller to puller.

In some embodiments, melt to reflector gap is an additional process parameter that is controlled to regulate the wall temperature of crucible10. Melt to reflector gap refers to a gap between melt line36and a heat reflector (not shown). Melt to reflector gap impacts the wall temperature of crucible10. Specifically, a larger melt to reflector gap reduces the wall temperature of crucible10. In this embodiment, the melt to reflector gap is between approximately 60 mm and 80 mm, and more specifically 70 mm.

Seed lift is an additional process parameter that is controlled to regulate the flow of SiO from crucible10to crystal27. Seed lift refers to the rate at which pull shaft or cable117lifts seed crystal115out of melt25. In one embodiment, seed crystal115is lifted at a rate in a range of approximately 0.42 to 0.55 millimeters per minute (mm/min), and more specifically 0.46 mm/min for 300 mm product. This pull rate is slower than pull rates typically used for smaller diameter (e.g., 200 mm) crystals. For example, the seed lift for 200 mm product may be in a range of approximately 0.55 to 0.85 mm/min, and more specifically 0.7 mm/min.

Pull speed is an additional process parameter that may be regulated to control the defect quality of the crystal. For example, using SP2 laser light scattering, the detected agglomerated point defects generated by the process described herein may be less than 400 counts for defects less than 60 nm, less than 100 counts for defects between 60 and 90 nm, and less than 100 counts for less defects between 90 and 120 nm.

In some embodiments, inert gas flow is an additional process parameter that is controlled to regulate the SiO evaporation from melt25. Inert gas flow, as described herein, refers to the rate at which argon gas flows through vacuum chamber101. Increasing the argon gas flow rate sweeps more SiO gas above melt line36away from crystal27, minimizing a SiO gas partial pressure, and in turn increasing SiO evaporation. In this embodiment, the argon gas flow rate is in a range from approximately 100 slpm to 150 slpm.

Inert gas pressure is an additional process parameter also controlled to regulate the SiO evaporation from melt27in some embodiments. Inert gas pressure, as described herein, refers to the pressure of the argon gas flowing through vacuum chamber101. Decreasing the argon gas pressure increases SiO evaporation and hence decreases SiO concentration in melt25. In this embodiment, the argon gas pressure ranges from approximately 10 torr to 30 torr.

In suitable embodiments, cusp position is an additional process parameter that is controlled to regulate the wall temperature of crucible10and the flow of SiO from crucible10to crystal27. Cusp position, as described herein, refers to the position of the cusp of the magnetic field generated by coils31and33. Maintaining the cusp position below melt line36facilitates reducing the oxygen concentration. In this embodiment, the cusp position is set in a range from approximately 10 mm to 40 mm below melt line36, more specifically, in a range of approximately 25 mm to 35 mm below melt line36, and even more specifically, at approximately 30 mm.

By controlling process parameters (i.e., heater power, crucible rotation rate, magnet strength, seed lift, melt to reflector gap, inert gas flow, inert gas pressure, seed rotation rate, and cusp position) as described above, a plurality of process parameters (i.e., a wall temperature of a crucible, a flow of SiO from the crucible to a single crystal, and an evaporation of SiO from a melt) are regulated to produce silicon ingots having a low oxygen concentration. In one embodiment, the methods described herein facilitate producing a silicon ingot with an ingot diameter greater than approximately 150 millimeters (mm), a total ingot length of at least approximately 900 mm, and an oxygen concentration less than 5 ppma. In another embodiment, the methods described herein facilitate producing a silicon ingot with an ingot diameter ranging from approximately 150 mm to 460 mm, specifically approximately 300 mm, and an oxygen concentration less than 5 ppma. In another additional embodiment, the methods described herein facilitate producing a silicon ingot with a total ingot length ranging from approximately 900 mm to 1200 mm, and an oxygen concentration less than 5 ppma.

Wafers having low oxygen concentration using the systems and methods described herein may be advantageous in a variety of applications. For example, insulated-gate bipolar transistors (IGBTs), high quality radio-frequency (RF), high resistivity silicon on insulator (HR-SOI), and charge trap layer SOI (CTL-SOI) applications may benefit from the low oxygen concentration because they achieve high resistivity and do not have p-n junctions. Wafers produced for IGBT applications using the methods described herein may, for example, have 30 to 300 ohm-centimeter (ohm-cm) N-type resistivity or greater than 750 ohm-cm N/P-type resistivity. Further, wafers produced for RF, HR-SOI, and/or CTL-SOI applicants using the methods described herein may have, for example, greater than 750 ohm-cm P-type wafers. Wafers produced by the described systems and methods may also be used as handle wafers.

For P-type wafers produced using the methods described herein, boron, aluminum, germanium, and/or indium may be suitably used has a majority carrier, and red phosphorus, phosphorus, arsenic, and/or antimony may be used as a minority carrier. For N-type wafers produced using the methods described herein, red phosphorus, phosphorus, arsenic, and/or antimony may be used as the majority carrier, and boron, aluminum, germanium, and/or indium may be used as the minority carrier.

To improve mechanical strength and slip performance, wafers produced using the methods described herein may be co-doped (e.g., by doping the single crystal that forms the ingot) with nitrogen or carbon, due to the relatively low Oi of the wafers. For example, the nitrogen concentration may be varied between 0 to 8e15 atoms per cubic centimeter, and the carbon concentration may be varied between 0.0 to 2.0 ppma.

Example systems and methods of producing single crystal silicon ingots with relatively low oxygen concentration from a melt formed from polycrystalline silicon are described herein. These methods take advantage of changes in the structure of flow cells in the melt between a first and second stage of production of the ingot to produce relatively low oxygen silicon. During the first stage, the silicon ingot is relatively small and the depth of the melt is relatively deep. The second stage is characterized by a depleted melt depth within the crucible due to formation of the silicon ingot. In this second stage, a flow cell induced by rotation of the silicon ingot within the melt may contact the bottom of the crucible, causing unwanted inclusion of silicon oxide formed at the crucible bottom into the growing crystal ingot. The methods and systems described herein control production of the ingot to limit the including of the unwanted silicon oxide. Generally, at least one process parameter is changed during the second stage relative to its value during the first stage. Non-limiting examples of changes in process parameters from the first stage to the second stage include: reduced crystal rotation rate, reduced crucible rotation rate, increased magnetic field strength, and any combination thereof. For example, in some embodiments, the silicon ingot is rotated more slowly during the second stage to reduce contact of the rotation induced flow cell with the bottom of the crucible, and thereby reduce the amount oxygen included in the silicon ingot.

The systems and methods described herein enable the formation of single crystal silicon ingots with low oxygen concentration maintained over a longer ingot length than was achieved using previous methods. A detailed description of the effects of these changes in process parameters on the structure of flow cells within the crucible and the oxygen content of the silicon ingots formed using the method on various embodiments, are described in further detail herein.

Embodiments of the methods described herein achieve superior results compared to prior methods and systems. For example, the methods described herein facilitate producing silicon ingots with a lower oxygen concentration than at least some known methods. Further, unlike at least some known methods, the methods described herein may be used for the production of ingots having a diameter greater than 150 mm.

When introducing elements of the present invention or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” “approximately,” and “substantially,” is not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged; such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.

As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.