Systems and methods for controlling a gas dopant vaporization rate during a crystal growth process

A method of growing a single crystal ingot includes growing a single crystal silicon ingot from a silicon melt in a crucible within an inner chamber, adding a volatile dopant into a feed tube, positioning the feed tube within an inner chamber at a first height relative to a surface of the melt, adjusting the feed tube within the inner chamber to a second height at a speed rate, and heating the volatile dopant to form a gaseous dopant as the feed tube is moved from the first height to the second height at the speed rate. Each of the second height and the speed rate are selected to control a vaporization rate of the volatile dopant. The method also includes introducing dopant species into the melt while growing the ingot by contacting the surface of the melt with the gaseous dopant.

FIELD

The field relates generally to preparation of single crystals of semiconductor material and, more specifically, to systems and methods for controlling a gas dopant vaporization rate during a crystal growth process.

BACKGROUND

Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared by the so-called Czochralski (“Cz”) method. In this method, polycrystalline silicon (“polysilicon”) is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon or silicon melt, and a single crystal ingot is grown by slow extraction.

A certain amount of dopant is added to the melt to achieve a desired resistivity in the silicon crystal. Conventionally, dopant is fed into the melt from a feed hopper located a few feet above the silicon melt level. However, this approach is not favorable for volatile dopants because such dopants tend to vaporize uncontrolled into the surrounding environment, resulting in the generation of oxide particles (i.e., sub-oxides) that may fall into the melt and become incorporated into the growing crystal. These particles act as heterogeneous nucleation sites, and ultimately result in failure of the crystal pulling process.

Further, in conventional systems, the sublimation of dopant granules at the melt surface often causes a local temperature reduction of the surrounding silicon melt, which in turn results in the formation of “silicon boats” adjacent the dopant granules. These silicon boats, along with the surface tension of the melt, prevent many of the dopant granules that do reach the melt surface from sinking into the melt, thus increasing the time during which sublimation to the atmosphere may occur. This phenomenon results in a significant loss of dopant to the gaseous environment and further increases the concentration of contaminant particles in the growth chamber.

Some known dopant systems introduce volatile dopants into the growth chamber as a gas. Gas dopants may be formed by vaporizing the volatile dopants in the feed hopper. The gas dopants so formed exit the feed hopper and subsequently contact a surface of the melt and flux into the melt. The dopant species in the melt are then transported, by diffusion and convection, from the surface of the melt toward the solid-liquid interface formed by the growing single crystal ingot. However, such systems tend to supply dopant non-uniformly during a growth process, thereby increasing the variation in dopant concentration in the radial and/or axial direction of the grown ingot.

In some doping systems, inert gas is used to feed volatile dopants into a growth chamber and/or carry gas dopants from the feed hopper to the surface of the silicon melt. However, the use of inert gas tends to dilute the gaseous dopant, thereby decreasing the dopant concentration, and purge the evaporated dopant from the growth chamber too quickly. For example, dopants with low segregation coefficients such as arsenic (0.3) and phosphorus (0.35) require dopant concentrations in the melt of about 3 times higher than the desired dopant concentration in the grown crystal. As a result, the evaporated dopant does not have sufficient time to flux into the silicon melt, and more dopant is needed to achieve a desired dopant concentration in the silicon melt.

Accordingly, a need exists for a simple, cost-effective approach to produce doped single crystal silicon by a crystal pulling process. A need also exists for doping methods that facilitate control over the introduction of gas dopant species into a silicon melt during a crystal pulling process. Further, a need exists for gas doping methods that allow for use of dopant source materials that are readily available and/or relatively inexpensive and that allow the melt to be doped with relative ease.

BRIEF SUMMARY

In one aspect, a method of growing a doped single crystal silicon ingot using an ingot pulling apparatus is disclosed. The ingot pulling apparatus includes an inner chamber, a crucible disposed within the inner chamber, a heat source, and a feed tube having an open end. The feed tube includes a capsule proximate the open end. The method includes adding polycrystalline silicon to the crucible, heating, by the heat source, the crucible to form a silicon melt from the polycrystalline silicon in the crucible, and growing a single crystal silicon ingot from the melt by contacting the melt with a seed crystal and pulling the seed crystal away from the melt to grow the single crystal silicon ingot. The single crystal silicon ingot has a neck region, a shoulder region, and a body region. The method also includes adding a charge of a volatile dopant into the feed tube, the charge of the volatile dopant being received by the capsule. The method further includes positioning the feed tube within the inner chamber such that the open end of the feed tube has a first height relative to a surface of the melt. The method also includes adjusting the feed tube within the inner chamber to move the open end of the feed tube from the first height to a second height relative to the surface of the melt. The second height is smaller than the first height, and the open end of the feed tube is moved from the first height to the second height at a speed rate. The method also includes heating, by the heat source and radiant heat from the surface of the melt, the capsule containing the volatile dopant to form a gaseous dopant as the open end is moved from the first height to the second height at the speed rate. Each of the second height and the speed rate are selected to control a vaporization rate of the volatile dopant. The method also includes introducing dopant species into the melt while growing the body region of the single crystal silicon ingot by contacting the surface of the melt with the gaseous dopant. The vaporization rate is controlled such that the dopant species are introduced at a rate sufficient to maintain a resistivity of the body region over an axial length of the body region.

In another aspect, an ingot pulling apparatus for growing a doped single crystal silicon ingot is disclosed. The apparatus includes an outer housing defining an inner chamber and a crucible disposed within the inner chamber for holding a silicon melt. The apparatus also includes a gas doping system for introducing dopant species into the melt. The gas doping system includes a feed tube extending between a first end and a second end, the second end located in the inner chamber. The feed tube also includes a capsule disposed proximate the second end. The gas doping system also includes a dopant feed source coupled in flow communication with the first end of the feed tube, the dopant feed source being configured to add a volatile dopant to the feed tube. The gas doping system also includes a positioning system configured to adjust the position of the feed tube between a first position, in which the second end of the feed tube is at a first height above a surface of the melt, and a second position, in which the second end of the feed tube is at a second height above the surface of the melt that is smaller than the first height. The gas doping system also includes a controller communicatively coupled to the dopant feed source and the positioning system. The controller is configured to cause the dopant feed source to add a targeted amount of the volatile dopant to the feed tube and to cause the positioning system to move the feed tube to the second position at a speed rate. Each of the second height and the speed rate are selected to control a vaporization rate of the volatile dopant in the feed tube during an ingot pulling process.

In another aspect, an ingot pulling apparatus for growing a doped single crystal silicon ingot is disclosed. The apparatus includes an outer housing defining an inner chamber and a crucible disposed within the inner chamber for holding a silicon melt. The apparatus also includes a first gas doping system and a second gas doping system for introducing dopant species into the melt. Each of the first and second gas doping systems includes a feed tube extending between a first end and a second end, the second end located in the inner chamber. The feed tube also includes a capsule disposed proximate the second end. Each of the first and second gas doping systems also includes a dopant feed source coupled in flow communication with the first end of the feed tube, the dopant feed source being configured to add a volatile dopant to the feed tube. Each of the first and second gas doping systems also includes a positioning system configured to adjust the position of the feed tube between a first position, in which the second end of the feed tube is at a first height above a surface of the melt, and a second position, in which the second end of the feed tube is at a second height above the surface of the melt that is smaller than the first height. The gas doping system also includes a controller communicatively coupled to the dopant feed source and the positioning system. The controller is configured to cause the dopant feed source to add a targeted amount of the volatile dopant to the feed tube and to cause the positioning system to move the feed tube to the second position. The second height is selected to control a vaporization rate of the volatile dopant in the feed tube during an ingot pulling process.

Like reference symbols used in the various drawings indicate like elements.

DETAILED DESCRIPTION

An example ingot pulling apparatus or ingot puller is indicated generally at100inFIG.1. The ingot puller100is used to produce single crystal (i.e., monocrystalline) ingots102of semiconductor or solar-grade material such as, for example, single crystal silicon ingots102. In some embodiments, the ingot102is grown by the so-called Czochralski (CZ) process in which the ingot102is withdrawn from a silicon melt104held within a crucible106of the ingot puller100. In some embodiments, the ingot102is grown by a batch CZ process in which polycrystalline silicon is charged to the crucible106in an amount sufficient to grow one ingot102, such that the crucible106is essentially depleted of silicon melt104after the growth of the one ingot102. In other embodiments, the ingot102is grown by a continuous CZ (CCZ) process in which polycrystalline silicon is continually or periodically added to the crucible106to replenish the silicon melt104during the growth process. The CCZ process facilitates growth of multiple ingots102pulled from a single melt104. Unless stated otherwise, embodiments of the subject matter described herein are not limited to a particular crystal growth process. For example, in other embodiments, a polycrystalline silicon ingot may be grown using a directional solidification process for solar applications.

Referring toFIG.1, the ingot puller100includes an outer housing108that defines an inner chamber110within the housing108. The crucible106is disposed within the inner chamber110. The crucible106contains the silicon melt104from which the silicon ingot102is pulled. The crucible106may be supported by a susceptor (not shown). The ingot puller100may be configured to rotate the crucible106and/or move the crucible106vertically within the inner chamber110.

To prepare the silicon melt104, polycrystalline silicon is added to the crucible106. The polycrystalline silicon is heated to above the melting temperature of silicon (about 1414° C.) to cause the polycrystalline silicon to liquefy into the silicon melt104. A heat source112is operated to melt-down the polycrystalline silicon. For example, the heat source112includes one or more heaters114mounted within the inner chamber110below or to the side of (i.e., radially outward from) the crucible106are operated to melt-down the polycrystalline silicon to prepare the silicon melt104.

Before or after the melt104is produced, the melt104may be doped with a dopant, typically an n-type dopant, to compensate for p-type impurities (e.g., boron) in the melt. The n-type dopant may be added before growth of the ingot102. By compensating the melt104, the resistivity of the resulting ingot102may be controlled to a targeted resistivity (e.g., increased). For example, the seed end of the ingot102(i.e., the portion of the ingot nearest the ingot crown) may have a resistivity of at least about 30 Ω-cm or, as in other embodiments, at least about 35 Ω-cm, at least about 40 Ω-cm, at least about 45 Ω-cm, at least about 50 Ω-cm, at least about 55 Ω-cm, at least about 60 Ω-cm, or from about 30 Ω-cm to about 100 ohm-cm, or from about 60 ohm-cm to about 80 ohm-cm. Suitable n-type dopants include phosphorous and arsenic.

Once the melt104is prepared, the single crystal silicon ingot102is pulled from the melt104using a pulling system116. The pulling system116includes a pulling mechanism (not shown) attached to a pull wire122that extends down from the mechanism. The mechanism is capable of raising and lowering the pull wire122along a pull axis X1and rotating the pull wire122about the pull axis X1. The ingot puller100may have a pull shaft rather than a wire, depending upon the type of puller. The pull wire122terminates at a seed chuck120that holds and/or is secured to a seed crystal118. The pulling mechanism lowers the seed chuck120and crystal118along the pull axis X1until the seed crystal118contacts the surface of the silicon melt104. Once the seed crystal118begins to melt, the pulling mechanism slowly raises the seed crystal118up along the pull axis X1to grow the single crystal ingot102. The seed crystal118may also be rotated about the pull axis X1by the pulling mechanism as the pulling mechanism raises the seed crystal118. As the seed crystal118is slowly raised from the melt104along the pull axis X1, the silicon ingot102begins to solidify and to be extracted from the melt104.

A process gas (e.g., argon) is introduced through a gas inlet port128into inner chamber110and is withdrawn through an outlet port (not shown) in fluid communication with an exhaust system (not shown) of the ingot puller100. The process gas creates an inert atmosphere within the housing. The surface of the melt104and the inert atmosphere form a melt-gas interface126. The melt-gas interface126is located radially outward from a solid-melt interface124along which the ingot102is grown. As the description proceeds, the melt-gas interface126and the surface of the melt104may be used interchangeably.

The ingot102is shrouded by an annular heat shield130and a cooling jacket132. The annular heat shield130and the cooling jacket132are each mounted within the inner chamber110above the melt104. The heat shield130is mounted radially outward from the cooling jacket132, and defines an elongate passage134sized and shaped to receive the ingot102as the ingot102is pulled up from the melt104along the pull axis X1. The heat shield130is mounted above the melt-gas interface126such that a gap136is defined therebetween. The cooling jacket132is positioned radially inward from the heat shield130, and within the elongate passage134. The cooling jacket132is concentrically arranged with the heat shield130along the pull axis X1, and defines a central passage140for receiving the ingot102as the ingot102is pulled along the pull axis X1by the pulling system116. A portion138of the passage134defined by the heat shield130is located below the cooling jacket132. The heat shield130insulates and/or reflects radiant heat away from the ingot102as the ingot is pulled through the passage134. The cooling jacket132may be in the form of a cylindrical, fluid-cooled heat exchanger that facilitates cooling of the ingot102as the ingot102is pulled through the passage140. The heat shield130and the cooling jacket132may facilitate controlling axial and radial temperature gradients, which drive solidification and crystallization of molten silicon in the melt104into the growing ingot102. The configuration of the heat shield130and the cooling jacket132may vary to enhance temperature effects within the passages134,140as the ingot102is pulled therethrough.

The ingot puller100also includes a gas doping system200for introducing gaseous dopant (indicated by arrows202) into the melt104. The doping system200includes a dopant feed source204, a feed tube206, and an evaporation capsule208. Generally, the gas doping system200causes the gaseous dopant202to flow across the melt-gas interface126during growth of the ingot102. The gaseous dopant202fluxes into the melt104through the melt-gas interface126. Dopant species is then transported, by diffusion and convection, toward the solid-melt interface124whereby the ingot102is doped to control resistivity variations that occur during growth of the ingot102.

The feed tube206extends along a feed tube axis X2between a first end210and a second end212(FIG.2). The first end210is located adjacent to the dopant feed source204, which may be positioned outside of the outer housing108. The second end212is located within the inner chamber110and oriented toward the surface of the melt104. The feed tube206may extend through a valve assembly (not shown) that provides an ingress point for the feed tube206through the outer housing108, and seals the ingress point when the feed tube206is removed from the ingot puller100. The feed tube206is open at the first end210to receive volatile dopant (indicated by arrows216shown inFIG.2) from the dopant feed source204and open at the second end212to allow gaseous dopant202to flow out from the second end212of the feed tube206toward the surface of the melt104and, particularly, toward the melt-gas interface126.

As shown inFIG.1, the feed tube206may be angled with respect to the pulling axis X1and the surface of the melt104to facilitate the distribution of gaseous dopant202across the melt-gas interface126. For example, the feed tube206may be angled such that the feed tube axis X2forms an angle of between about 10 degrees and about 80 degrees, such as between about 15 degrees and about 45 degrees, relative to the pulling axis X1. The orientation of the feed tube206may translate to an orientation of the open end212. In some embodiments, the open end212may be angled with respect to the feed tube axis X2. For example, the open end212may be angled relative to the feed tube axis X2such that an opening formed by the open end212is substantially parallel to the melt-gas interface126. In other embodiments, the feed tube206may be positioned substantially perpendicular to the melt-gas interface126such that the feed tube axis X2is parallel to the pulling axis X1. In yet other embodiments, the feed tube206and/or open end212of the feed tube206may have any other suitable configuration or orientation that enables gas doping system200to function as described herein.

The doping system200may also include an inert gas supply214coupled in fluid communication with the feed tube206to guide gaseous dopant202out from the feed tube206through the second end212and to reduce back flow of gaseous dopant202. An inert gas (indicated by arrows218shown inFIG.2) introduced into the feed tube206by the inert gas supply214and/or the process gas introduced through the gas inlet port128may guide the gaseous dopant202to flow across the melt-gas interface126and flux into the melt104. The inert gas218and/or the process gas introduced through the gas inlet port128is also used to produce an inert atmosphere above the melt-gas interface126. The inert gas218may be introduced into the feed tube206from the inert gas supply214at a suitable flow rate, such that the inert gas218flows downwardly towards the second end212. The inert gas218may be argon, although any other suitable inert gas may be used that enables the gas doping system200to function as described herein. The flow rate of the inert gas218is suitably sufficient to guide the gaseous dopant202out from the second end212toward the melt-gas interface126without substantially causing undesirable dilution of gaseous dopant202and/or substantially causing gaseous dopant202to flow out from the inner chamber110without fluxing into the melt104. For example, the inert gas flow rate may be less than about 10 normal-liters per minute, less than about 5 normal-liters per minute, or even less than about 2 normal-liters per minute.

The first end210of the feed tube206is coupled in flow communication with the dopant feed source204. The dopant feed source204feeds a volatile dopant216, which may be in the form of solid-phase dopant or liquid phase dopant, into the first end210of the feed tube206. As used herein, the term “volatile dopant” generally refers to dopants that have a sublimation or evaporation temperature at or below the melting temperature of silicon (about 1414° C.), such that the volatile dopant216may be vaporized into the gaseous dopant202under thermal conditions within the inner chamber110during a crystal growth process.

The dopant feed source204may be automated, partially automated, or manually operated. Automated control of the dopant feed source204may be facilitated by a controller220communicatively coupled to the dopant feed source204. The dopant feed source204may automatically feed volatile dopant216into the feed tube206based upon one or more user-defined parameters, and/or environment-specific parameters. For example, the dopant feed source204may feed volatile dopant216into the feed tube206based upon any one or more of the following parameters: preset time(s) during a growth process, user defined interval(s), a targeted resistivity of the ingot102during a growth process, a mass of the volatile dopant216within the feed tube206and/or evaporation capsule208, a concentration of the gaseous dopant202within the feed tube206, the evaporation capsule208, and/or the inner chamber110, a vaporization rate of the volatile dopant216, and a volumetric or mass flow rate of the gaseous dopant202and/or the inert gas218. The continuous and/or intermittent feeding of the volatile dopant216may facilitate a relatively constant gaseous dopant concentration to be maintained within the inner chamber110, and in the melt104, during the crystal growth process, resulting in a more uniform dopant concentration profile in grown ingots.

The controller220may be programmed to control the frequency and/or amount of volatile dopant216being fed into the feed tube206by the dopant feed source204. The controller220includes a processor222that sends and receives signals to and from the controller220and/or the dopant feed source204based on one or more user-defined parameters and/or environment-specific parameters. The controller220also includes a user interface224communicatively coupled to the processor222, and a sensor226communicatively coupled to the processor222. The user interface224receives user-defined parameters, and communicates user-defined parameters to the processor222and/or the controller220. The sensor226receives and/or measures environment-specific parameters, and communicates such environment-specific parameters to the processor222and/or the controller220. For example, the sensor226may be a pyrometer that measures a temperature of the melt104and/or a temperature of the growing ingot102. The environment-specific parameters communicated by the sensor226may be used by the processor222and/or the controller to cause dopant feed source204to feed volatile dopant216to the feed tube206.

The evaporation capsule208is disposed within the feed tube206proximate the second end212, and within the inner chamber110. With additional reference toFIG.2, the volatile dopant216fed to the feed tube206flows downward through a channel228defined by an annular sidewall230of the feed tube206and is received by the evaporation capsule208which forms a receptacle within the feed tube proximate the second end212. The evaporation capsule208includes a base232extending inward from the sidewall230, and a capsule sidewall234joined to the base232and extending upwardly from the base232along the feed tube axis X2. In other embodiments, the evaporation capsule208may have any other suitable configuration that enables gas doping system200to function as described herein. For example, in some embodiments, the capsule208may be removably connected to the tube to allow the capsule to be separated from the tube for loading of dopant.

A portion236of the channel228through which the gaseous dopant202flows is partially defined by the capsule sidewall234and the feed tube sidewall230. The portion236of the channel228facilitates fluid communication between the evaporation capsule208and the open end212of the feed tube206. Thereby, gaseous dopant202that is formed by vaporizing volatile dopant216received by the evaporation capsule208is enabled to flow from the evaporation capsule208to the second end212and out from the feed tube206. The cross-sectional area of portion236of the channel228, viewed perpendicular to the feed tube axis X2, may be adjusted in order to increase or decrease the flow rate of gaseous dopant202passing therethrough. For example, the cross-sectional area of the portion236of the channel228may be decreased by extending the length of the base232of the evaporation capsule208. Similarly, the length of the portion236of the channel228may be increased or decreased by varying the height of capsule sidewall234.

The feed tube206may also include a guide238disposed in the channel228above the evaporation capsule208for facilitating flow of the volatile dopant216into the evaporation capsule208. The guide238may also act as a fluid flow restrictor, permitting the passage of the volatile dopant216therethrough, and also restricting backflow of the gaseous dopant202therethrough. Alternatively stated, the guide238may act as a one-way valve for dopants supplied to the inner chamber110via the gas doping system200.

The guide238may be conically shaped and extends inwardly from the feed tube sidewall230and downwardly towards the second end212. In alternative embodiments, guide238may have any suitable configuration that enables the gas doping system200to function as described herein. The guide238funnels the volatile dopant216through the opening240and into the evaporation receptacle. As the volatile dopant216is vaporized in the evaporation capsule208, the guide238diverts gaseous dopant202flowing upwards away from the opening240, thereby limiting back flow of the gaseous dopant202. Additionally, the guide238directs the inert gas218through the opening240(which has a smaller diameter than the channel228), creating a localized high pressure area of the inert gas218near the opening240, thereby restricting back flow of the gaseous dopant202therethrough.

The feed tube206may also include a fluid-distribution plate (not shown) that facilitates distributing the gaseous dopant202across the melt-gas interface126. The fluid-distribution plate may be coupled to the feed tube206at the second end212. The fluid-distribution plate may have a hemi-spherical, conical, rectangular, or square shape, or any other suitable shape that enables gas doping system200to function as described herein.

The evaporation capsule208and the feed tube206, as well as the guide238and the fluid-distribution plate (when included), may be made of any suitable material that enables gas doping system200to function as described herein. In some embodiments, these components are each suitably made from fused quartz (e.g., a single piece of fused quartz). In yet other embodiments, the evaporation capsule208and the feed tube206, as well as the guide238and the fluid-distribution plate may be fabricated as separate components. Integrating the evaporation capsule208within the feed tube206may provide a relatively simple construction of the gas doping system200, and may reduce the overall size of the gas doping system200. As a result, positioning the feed tube206and the evaporation capsule208within the furnace using a positioning system, such as positioning system242, is simplified.

The feed tube206is slidingly coupled to a positioning system242that raises and/or lowers feed tube206along the feed tube axis X2. The positioning system242includes a rail244, a coupling member246, and a motor (e.g., a step motor) (not shown). The coupling member246slidingly couples the feed tube206to the rail244. The motor moves the coupling member246and the feed tube206along the rail244. The rail244extends in a direction substantially parallel to the feed tube axis X2. Using the positioning system242, the second end212of the feed tube206and the evaporation capsule208is raised and lowered into and out of the inner chamber110. Additionally, the positioning system242facilitates adjusting a height H of the second end212above the melt-gas interface126.

The positioning system242is communicatively coupled to the controller220. The controller220may be programmed to control the positioning system242to dynamically adjust the height H of the second end212of the feed tube206above the melt-gas interface126during a crystal growth process as well as the speed at which the feed tube206moves along the rail244to adjust the height H of the second end212. For example, the processor222may send and receive signals to and from the controller220and/or the positioning system242based on one or more user-defined parameters and/or environment-specific parameters. The user-defined parameters for controlling the positioning system242may be received by the user interface224which communicates the user-defined parameters to the processor222and/or the controller220. As described above, the sensor226receives and/or measures environment-specific parameters, and communicates such environment-specific parameters to the processor222and/or the controller220. The user-defined parameters and the environment-specific parameters may be used by the processor222and/or the controller220to cause the positioning system242to adjust the height H of the second end212of the feed tube206and/or control the speed at which the height H of the second end212changes by movement of the feed tube206along the rail244.

In operation, the Czochralski method (batch or continuous) begins by loading polycrystalline silicon (or “polysilicon”) into the crucible106. The solid polysilicon added to the crucible106is typically granular polysilicon, although chunk polysilicon may be used, and it is fed into the crucible106using a polysilicon feeder (not shown) that is optimized for use with granular polysilicon. Chunk polysilicon typically has a size of between 3 and 45 millimeters (e.g., the largest dimension), and granular polysilicon typically has a size between 400 and 1400 microns. Granular polysilicon has several advantages including providing for easy and precise control of the feed rate due to the smaller size. However, the cost of granular polysilicon is typically higher than that of chunk polysilicon due to the chemical vapor deposition process or other manufacturing methods used in its production. Chunk polysilicon has the advantage of being cheaper and being capable of a higher feed rate given its larger size.

Generally, the melt104from which the ingot102is drawn is formed by loading the polycrystalline silicon into the crucible106to form an initial silicon charge. In general, an initial charge is between about 100 kilograms and about 1000 kilograms of polycrystalline silicon, which may be granular, chunk, or a combination of granular and chunk. The mass of the initial charges depends on the desired crystal diameter and configuration of the ingot puller100. In some embodiments, the initial polycrystalline silicon charge is sufficient to grow one single crystal silicon ingot, i.e., in a batch Czochralski method. In general, the total axial length (measured in a direction of the pulling axis X1) of the solid main body of the single crystal silicon ingot102is at least about 1100 millimeters (mm). In a continuous Czochralski method, the initial charge does not reflect the length of crystal, because polycrystalline silicon is continuously fed during crystal growth. Accordingly, the initial charge may be smaller, such as between about 100 kg and about 200 kg. If polycrystalline silicon is fed continuously and the height of the inner chamber110is sufficiently tall, axial length of the ingot102may be extended in length, such as to 4000 mm. A variety of sources of polycrystalline silicon may be used including, for example, granular polycrystalline silicon produced by thermal decomposition of silane or a halosilane in a fluidized bed reactor or polycrystalline silicon produced in a Siemens reactor. Once polycrystalline silicon is added to the crucible to form a charge, the charge is heated to a temperature above about the melting temperature of silicon (e.g., about 1414° C.) to melt the charge, and thereby form the silicon melt104comprising molten silicon. The silicon melt104has an initial volume of molten silicon and has an initial melt elevation level, and these parameters are determined by the size of the initial charge. In some embodiments, the crucible106containing the silicon melt is heated to a temperature of at least about 1425° C., at least about 1450° C. or even at least about 1500° C. The initial polycrystalline silicon charge is heated by supplying power to the heaters114.

Once the solid polycrystalline silicon charge is liquefied to form the silicon melt104, the seed crystal118is lowered to contact the melt104. The seed crystal118is then withdrawn from the melt104with silicon being attached thereto to thereby forming the solid-melt interface124near or at the surface of the melt104. Generally, the initial pull speed of the seed crystal118is high to form a neck portion of the ingot having a relatively smaller diameter relative to a diameter of a main body of the ingot102subsequently grown. In some embodiments, the seed crystal118is withdrawn at a neck portion pull rate of at least about 1.0 mm/minute, such as between about 1.5 mm/minute and about 6 mm/minute. In some embodiments, the seed crystal118and the crucible106are rotated in opposite directions, i.e., counter-rotation. Counter-rotation achieves convection in the silicon melt104. Rotation of the seed crystal118is mainly used to provide a symmetric temperature profile, suppress angular variation of impurities and also to control a shape of the solid-melt interface124. In some embodiments, the seed crystal118is rotated at a rate of between about 5 rpm and about 30 rpm. In some embodiments, the seed crystal rotation rate may change during growth of the main body of the single crystal silicon ingot102. In some embodiments, the crucible106is rotated at a rate between about 0.5 rpm and about 10 rpm. In some embodiments, the seed crystal118is rotated at a faster rate than the crucible106. In general, the neck portion has a length between about 300 millimeters and about 700 millimeters, and the length of the neck portion may vary outside these ranges.

After formation of the neck, an outwardly flaring seed-cone portion of the ingot102adjacent the neck is grown. In general, the pull rate of the seed crystal118is decreased from the neck portion pull rate to a rate suitable for growing the outwardly flaring seed-cone portion. For example, the seed-cone pull rate during growth of the outwardly flaring seed-cone is between about 0.5 mm/min and about 2.0 mm/min. In some embodiments, the outwardly flaring seed-cone has a length between about 100 millimeters and about 400 millimeters. The length of the outwardly flaring seed-cone may vary outside these ranges. In some embodiments, the outwardly flaring seed-cone is grown to a terminal diameter of between about 150 millimeters to about 450 mm. The terminal diameter of the outwardly flaring seed-cone is generally equivalent to the diameter of the constant diameter of the main ingot body of the single crystal silicon ingot102.

After formation of the neck and the outwardly flaring seed-cone adjacent the neck portion, the main ingot body of the ingot102having a constant diameter adjacent the outwardly flaring seed-cone is grown. The constant diameter portion of the main ingot body has a circumferential edge, a central axis that is parallel to the circumferential edge and the pulling axis X1, and a radius that extends from the central axis to the circumferential edge. The central axis also passes through the cone portion and neck. The diameter of the main ingot body may vary and, in some embodiments, the diameter may be about 150 mm, at least about 150 mm, about 200 mm, at least about 200 millimeters, about 300 mm, at least about 300 mm, about 450 mm, or even at least about 450 mm. Alternatively stated, the radial length of the solid main ingot body of the single crystal silicon ingot102is about 75 mm, at least about 75 millimeters, about 100 mm, at least about 100 millimeters, about 150 mm, at least about 150 mm, about 225 mm, or even at least about 225 mm. The main ingot body of the single crystal silicon ingot102is eventually grown to an axial length (measured along the pulling axis X1) at least about 1000 mm, such as at least 1200 mm, such as at least 1250 mm, such as at least 1400 mm, such as at least 1500 mm, or at least 2000 mm, or at least 2200 mm, such as 2200 mm, or at least about 3000 mm, or at least about 4000 mm. In some preferred embodiments, the total axial length of the solid main ingot body of the single crystal silicon ingot102is at least about 1100 mm, such as between about 1200 mm and about 1300 mm, such as between about 1200 mm and about 1250 mm.

In a continuous Czochralski method, during growth of the main ingot body of the single crystal silicon ingot102, polycrystalline silicon, i.e., granular, chunk, or a combination of granular and chunk, is added to the melt104to thereby achieve a constant volume of molten silicon in the melt104and constant melt elevation level. Maintenance of a substantially constant melt volume during growth of a substantial portion of the axial length of the main body of the single crystal silicon ingot102enables the achievement of high ingot quality over a substantial portion of the axial length of the main body of the single crystal silicon ingot102at a constant pull rate. The constant melt volume regardless of the crystal length enables maintaining a constant solid-melt interface124and thus uniform crystal quality over a substantial portion of the main body of the ingot102. Accordingly, in some embodiments, the volume of melt104varies by no more than about 1.0 volume % during growth of at least about 90% the main body of the single crystal silicon ingot102, or by no more than about 0.5 volume % during growth of at least about 90% the main body of the single crystal silicon ingot102, or even by no more than about 0.1 volume % during growth of at least about 90% the main body of the single crystal silicon ingot102. Alternatively stated, in some embodiments, the melt elevation level varies by less than about +/−0.5 millimeter during growth of at least about 90% the main body of the single crystal silicon ingot102.

In a batch Czochralski method, the initial charge of polycrystalline silicon is sufficient to grow the entire length of the ingot102. Rather than maintain a constant melt elevation level, the volume of the melt104is depleted as the ingot102grows. In some embodiments, growth of the ingot102by a batch Czochralski method may necessitate the vertical movement of the crucible106in the same direction in which the ingot is pulled, that is, along the pulling axis X1.

During growth of the ingot102, the gaseous dopant202is introduced into the melt104using the gas doping system200to modify and/or control a base resistivity of the ingot102. Suitably, the gas doping process may be initiated once a predetermined axial length of the main body of the ingot102has been grown. In batch Czochralski growth processes, for example, significant resistivity variations in the main body of the ingot102and/or a type-change of the ingot102may occur as the melt104is depleted during growth of a single ingot102. As the melt104is depleted, dopants (e.g., n-type dopants such as arsenic or phosphorus) that were previously introduced into the melt104and have low segregation coefficients accumulate in the melt104altering the resistivity of the growing ingot102. Accordingly, the gas doping process may be initiated to dope or counter-dope the ingot102with the gaseous dopants202suitable to modify and/or control resistivity variations in later stages of the growth process. The time at which the gas doping process is initiated may be controlled manually or automatically controlled by controller220based on user-defined parameters or environment-specific parameters. In some instances, the gas doping process is initiated at a suitable time based on empirical data and a predicted resistivity drop once a certain axial length of the main body of the ingot102has been grown.

To initiate the doping process, an initial charge of solid-phase or liquid-phase volatile dopant216, such as volatile p-type dopants (e.g., boron), or volatile n-type dopants (e.g., arsenic, phosphorous), or any other element or compound with a suitably low sublimation or evaporation temperature that enables the gas doping system200to function as described herein, is introduced into the feed tube206from the dopant feed source204. The amount and type of the initial charge of volatile dopant216is selected to achieved targeted modification of the base resistivity of the ingot102. Solid dopant falls downwardly through the channel228of the feed tube206, and is funneled by the guide238to pass through the opening240and into the evaporation capsule208.

At this stage, the second end212of the feed tube206may be at an initial height (not shown inFIG.1) above the melt-gas interface126. The initial height may also be referred to herein as a first height. In order to vaporize the volatile dopant216, heat is supplied to the evaporation capsule208. In the example shown inFIG.1, heat is supplied to the evaporation capsule208in the form of radiant heat from the melt104and the heat source112which may include the heaters114. When the second end212of the feed tube206is at the initial height above the melt-gas interface126, the evaporation capsule208that is positioned proximate the second end212may not receive sufficient radiant heat from the melt104and the heat source112to increase a temperature of the evaporation capsule208to a temperature sufficient to vaporize the volatile dopant202. The positioning system242is used to lower the feed tube206along the feed tube axis X2so that the second end212is located at the height H (shown inFIG.1) above the melt-gas interface126. When the second end212is at the height H, the evaporation capsule208is positioned sufficiently near the melt104such that radiant heat from melt104and the heaters114is sufficient to vaporize the volatile dopant216within the evaporation capsule208into the gaseous dopant202. For example, the height H may be about 1 centimeter and about 15 centimeters above melt-gas interface126. In other embodiments, a separate heating element (not shown) may be used to supply heat to the evaporation capsule208to vaporize the volatile dopant216therein into the gaseous dopant202.

As shown inFIG.1, the second end212at the height H may be located in the portion138of the passage134defined by the heat shield130that is located below the cooling jacket132. Alternatively, the second end212at the height H may be located in the gap136defined between the melt-gas interface126and the heat shield130. Movement of the feed tube206to position the second end212at the height H may require the feed tube206to pass through the heat shield130. The heat shield130may include a conduit (not shown) formed therein sized and shaped to receive the feed tube206and allow movement of the feed tube206therethrough.

The gaseous dopant202formed by heating the initial charge of volatile dopant216in the evaporation capsule208subsequently flows out from the second end212and toward the melt-gas interface126. The gaseous dopant202contacts the surface of the melt104at the melt-gas interface126and fluxes into the melt104. Dopant species introduced in the melt104are transported, by diffusion and convection, toward the solid-melt interface124and are taken up by the growing ingot102.

Doping the melt104with the gaseous dopant202may present several challenges to producing the growing ingot102with targeted resistivity. Initially, a target dopant concentration in the melt104to achieve a targeted resistivity of the ingot102must be determined. The target dopant concentration is determined based on numerous transport mechanisms that affect the amount of dopant in the melt104that is taken up by the ingot102, including, for example, convective mass transport, diffusion resulting from dopant concentration gradients, and dopant segregation from the ingot102. Also, in the case of continuous Czochralski growth processes, additional dopant and melt material added to the melt104throughout the growth process affect the dopant concentration in the ingot102.

These transport mechanisms may be simulated to study thermal heat flow and dopant concentration distribution to estimate the path of dopant species through the melt104from the melt-gas interface126to the solid-melt interface124. A study of the transport mechanisms may be used to control variations in the dopant concentration at the solid-melt interface124, which may otherwise cause non-uniform axial and radial resistivity profiles of the growing ingot102. For example,FIG.3illustrates melt convection patterns (left) and dopant diffusion patterns (right) in the melt104, depicted by arrows inFIG.3indicating the direction of dopant transport. In the example shown, the melt convection pattern includes two vortexes and the average melt velocity is around 2 mm/second. From a point where the gaseous dopant202fluxes into the melt104, dopant species may need more than 16 minutes to reach the solid-melt interface124. These transport patterns are shown for example only. The transport mechanisms depend upon the environmental conditions and operating parameters within the ingot puller100, including, for example, a pull rate of the seed crystal118, a pressure within the inner chamber110, a rotation rate of the crucible106, a rotation rate of the seed crystal118, a gas flow rate across the surface of the melt104, and a size of the gap136between the heat shield130and the melt-gas interface126. Transport characteristics, such as the diffusion coefficients for various volatile dopants216, may be empirically determined for a specific configuration of the ingot puller100and growth process based on one more Czochralski growth procedures performed using the ingot puller100and used to generate simulations such as the one shown inFIG.3to study transport mechanisms. For example, in the case of boron dopant species, a boron dopant diffusion coefficient in the silicon melt104may be 6×10−5kg/m/s.

A shape of the solid-melt interface124may also be controlled by adjusting the above-described parameters, and the shape of the solid-melt interface124may affect axial and radial resistivity gradients of the growing ingot102. As shown inFIG.3, the solid-melt interface124has an M-curve shape, and the center of the concave portion of the solid-melt interface124extends about 8.5 mm into the melt104. The ingot102will solidify from the crystal edge at the intersection of the solid-melt interface124and the melt-gas interface126. When the dopant species enter the melt104and are transported to the solid-melt interface124, the resistivity of the ingot102will be modified starting at the crystal edge.

By simulating the transport mechanisms of the dopant species within the melt104, the dopant concentration at the solid-melt interface124may be estimated based on a concentration of dopant species that flux into the melt from the gaseous dopant202. The concentration of dopant taken up by the crystal ingot102may be determined from the dopant concentration in the melt by considering the segregation coefficient and the solidification ratio of the dopant. In batch Czochralski growth processes, the following equation may be used:
CC(η)=CC,0(1−η)ki−1Eq. 1
where Ccrepresents the dopant concentration in the crystal ingot, kirepresents the effective segregation coefficient of the dopant (which may be known or empirically determined), η represents the fraction of the ingot102grown, and Cc,0represents the dopant concentration of the melt104from which the crystal ingot is grown. The resistivity of the crystal ingot102may be determined based on the dopant concentration in the ingot102using standard conversion tables and/or formulas known in the art, such as standards SEMI MF723-0307 and SEMI F723-99, published by SEMI International Standards, which are incorporated herein by reference for all relevant and consistent purposes.

Accordingly, the above simulations and calculations may be used to predict resistivity, and axial and radial resistivity gradients, in an ingot102pulled from the melt104over the course of a Czochralski growth process. These may be used as a basis to set a targeted amount of dopant concentration in the melt104at various stages of the growth process. However, the use of gaseous dopant202presents an additional challenge of controlling the amount of dopant added to the melt104to meet the targeted dopant concentration and, consequently, to control the axial and radial resistivity profile of an ingot grown from the melt104. In addition to the above-described parameters related to transport of dopant species within the melt104, numerous factors affect the doping efficiency of the volatile dopant216contained in the evaporation capsule208. The “doping efficiency” refers to the amount of dopant species that are taken up by the growing ingot102versus the amount of volatile dopant216added to the feed tube206in a single charge. For example, the gas transport characteristics of the gaseous dopant202that travels to the melt-gas interface126from the second end212of the feed tube206affect the amount of gaseous dopant202that fluxes into the melt104. The path of the gaseous dopant202is guided by diffusion of the gaseous dopant202and force convection by the inert gas218and/or the process gas supplied by the gas inlet port128. Similar to the dopant species transport in the melt104(shown inFIG.3), gas transport of the gaseous dopant202toward the melt-gas interface126may be simulated as shown inFIG.4. Specifically,FIG.4shows simulated profiles of velocity distribution (left) and gaseous dopant202concentration (right) within the inner chamber110of the ingot puller100.

In addition, the amount of gaseous dopant202produced depends on both the amount of volatile dopant216fed to the evaporation capsule208, as well as the thermal conditions used to vaporize the volatile dopant216in the evaporation capsule208. As discussed above, the evaporation capsule208must reach a suitable temperature in order to vaporize the volatile dopant216and form a suitable amount of the gaseous dopant202that may flow to the melt-gas interface126. Otherwise, volatile dopant216remains in the evaporation capsule208and is not introduced into the melt104as the gaseous dopant202. The temperature conditions necessary to vaporize the volatile dopant216depend on phase changes that the volatile dopant216undergoes and the temperatures at which these changes occur. For example, in examples where solid boric acid powder is fed to the evaporation capsule208to introduce boron as dopant species in the melt104, the following phase changes and reactions occur:1. H3BO3(s)→HBO3(s)+H2O (g), T>170° C. (443K);2. 4HBO2(s)→H2B4O7(l)+H2O (g), T>300° C. (573K);3. H2B4O7(l)→2B2O3(l)+H2O (g), T>330° C. (600K);4. B2O3(l)→B2O3(g), T>510° C. (783K).
See High-Temperature Vaporization of B2O3(l) under Reducing Conditions, J. Phys. Chem. B 2011, 115, 45, 13253-13260. Temperatures within the inner chamber near the melt104may reach 1200K to 1400K. Thus, at the working temperatures experienced by the capsule208when the second end212is at the height H causes congruent vaporization of B2O3(l) according to the equilibrium state.

The phase change and reaction characteristics of the volatile dopant216may be used to set targeted temperatures of the evaporation capsule208so that sufficient vaporization of the volatile dopant216occurs.

As discussed above, heat supplied to the evaporation capsule208depends on the height of the second end212of the feed tube206above the melt-gas interface126. Vaporization of the volatile dopant216to produce the gaseous dopant202may thereby be controlled by controlling the height H of the second end212, as well as the speed at which the positioning system242lowers the feed tube206to move the second end212from the initial height to the height H (which may also be referred to as the second height H). Specifically, the speed at which the second end212is moved from the initial height to the second height H controls the rate of temperature increase of the evaporation capsule208, which in turn controls the vaporization rate of the volatile dopant216. Thus, by controlling the height H and the speed at which the second end212moves from the initial height to the height H, the amount of gaseous dopant202that fluxes into the melt104may be controlled, taking into account the above-described considerations related to gas transport characteristics.

An additional challenge, however, is that the vaporization rate of the volatile dopant216and the temperature of the evaporation capsule208are difficult to measure directly during a crystal growth process. For example, temperature sensors that measure the temperature of the evaporation capsule208during a growth process may undesirably impact or influence the gas dopant process and, accordingly, may not be used. It is difficult, therefore, to determine a suitable height H and movement speed of the feed tube206to achieve targeted vaporization of the volatile dopant216and, in turn, targeted dopant concentration in the melt104. A temperature profile of the second end212of the feed tube206vs. height above the melt-gas interface126may be generated by two or more thermal simulations with the height H being varied during the thermal simulations (seeFIG.5). Based on the simulated temperature profile, a targeted height H of the second end212may be set. The thermal simulations were performed using commercially available tools and other software codes that model the heat transmission in a puller (e.g., ingot puller100) during the growth of a crystal ingot (e.g., ingot102). The temperature of the second end212may also be measured experimentally by use of temperature sensors (e.g., sensor226) during sample runs of the puller.

In addition to the simulated temperature profile, a doping efficiency of the volatile dopant216may be estimated based on the height H and used to control the height H of the second end212of feed tube as well as the speed at which the second end212moves from the initial height to the height H. The doping efficiency is the ratio of the amount of volatile dopant216fed to the evaporation capsule208to the amount of dopant species taken up in the ingot102. A doping efficiency profile (shown inFIG.6) may be generated based on simulation or experimentally. To determine the doping efficiency experimentally, test runs may be performed, and a resistivity of the resulting ingot102may be measured against an amount of volatile dopant216added. As shown inFIG.6, the doping efficiency increases as the height H decreases. The doping efficiency influences the amount of volatile dopant216charged to obtain a targeted resistivity in the ingot102. The doping efficiency remains below 1% even at the higher range of doping efficiencies (0.3%-0.35%). This may indicate that longer hold times of the second end212of feed tube206at the height H should be used. The low doping efficiencies may be due to a significant portion of the volatile material216remaining in the capsule208, or being carried out with the process gas and/or inert gas218through the exhaust of the puller100.

Referring toFIG.7, an example method300of growing a doped single crystal silicon ingot (e.g., ingot102) using an ingot pulling apparatus (e.g., ingot puller100) and a gas doping system (e.g., gas doping system200) is shown. In a first step302, the single crystal silicon ingot102is grown by contacting the silicon melt104with the seed crystal118and pulling the seed crystal118away from the melt104at pull rates sufficient to grow the ingot102having a neck region, an outwardly flaring seed-cone or shoulder region adjacent the neck region, and a main body region adjacent the shoulder region. The melt104is prepared by adding polycrystalline silicon to the crucible106and heating, by the heaters114, the crucible106to form the melt104.

The single crystal silicon ingot102grown in the first step302may suitably be a low resistivity ingot. That is, the ingot102, a segment of the ingot (e.g., the neck region, the outwardly flaring seed-cone, and/or the main body region), and/or any single crystal silicon wafer sliced from the ingot102has a relatively low minimum bulk resistivity, such as below about 200 ohm-cm, below about 150 ohm-cm, below about 100 ohm-cm, below about 50 ohm-cm, below about 20 ohm-cm, below about 1 ohm-cm, below about 0.1 ohm-cm, or even below about 0.01 ohm-cm. In some embodiments, the ingot, segment, or any single crystal silicon wafer sliced therefrom has a relatively low minimum bulk resistivity, such as below about 200 ohm-cm, or between about 0.01 ohm-cm and about 200 ohm-cm, such as between about 1 ohm-cm and about 200 ohm-cm, between about 20 ohm-cm and about 200 ohm-cm, between about 40 ohm-cm and about 100 ohm-cm, or between about 60 ohm-cm and about 80 ohm-cm. Low resistivity ingots and wafers sliced therefrom may include electrically active dopants, such as p-type dopants such as boron, aluminum, gallium and indium and/or n-type dopants such as phosphorous, arsenic and antimony.

At a second step304, during growth of the ingot102in the first step302, a charge of volatile dopant216is added into the feed tube206and is received by the evaporation capsule208disposed within the feed tube206proximate the second end212. The charge of volatile dopant216is added into the feed tube206by the dopant feed source204. The amount of volatile dopant216added in the first step302is selected to achieve suitable modification of a base resistivity of the ingot102being grown. The charge amount added by the feed source204may be automatically controlled by the controller220, or may be manually controlled. Before, during, and/or after adding a charge of volatile dopant216into the capsule208at the first step302, the feed tube206is positioned, at step306, within the inner chamber110of the ingot puller100such that the open second end212of the feed tube206has an initial height relative to a surface of the melt104. The initial height is a suitable distance from the melt104so that radiant heat does not substantially cause the capsule208to increase to a vaporizing temperature of the volatile dopant216. The positioning system242is used to position the feed tube206at the appropriate initial height, which may be automatically controlled by the controller220, or may be manually controlled.

At step308, the feed tube206is adjusted within the inner chamber110to move the open second end212of the feed tube206from the initial height to a height H relative to the surface of the melt104. The height H is smaller than the initial height and is a distance suitable to allow radiant heat, from the melt104and the heaters114, to cause the capsule208to increase to a vaporizing temperature of the volatile dopant216. The open second end212of the feed tube206is moved from the initial height to the height H at a speed rate. The positioning system242is used to adjust the feed tube206to move the second open end212to the appropriate height H, which may be automatically controlled by the controller220, or may be manually controlled. At step310, the capsule208containing the volatile dopant216is heated by radiant heat from the melt104and the heaters114as the open second end212is moved from the initial height to the height H at the speed rate. As the capsule208moves toward the surface of the melt104, the temperature of the capsule208increases to a suitable temperature to vaporize the volatile dopant216, thereby producing the gaseous dopant202. Accordingly, both the height H and the speed rate affect the rate at which the volatile dopant216vaporizes to form the gaseous dopant202, also referred to herein as the vaporization rate.

Each of the height H and the speed rate are selected to control a vaporization rate of the volatile dopant216in the capsule208. For example, the height H and the speed rate may be selected based on a temperature profile generated for the second end212at various heights above the surface of the melt104(shown inFIG.5) and the phase change and reaction characteristics of the particular volatile dopant216that is being used. Additionally or alternatively, the height H and the speed rate are selected based on a doping efficiency profile generated for the volatile dopant216at various heights of the second end212above the surface of the melt104(shown inFIG.6). The doping efficiency may also be used to select an amount of the volatile dopant216that is included in the charge during step304.

At step312, dopant species are introduced into the melt104while growing the body region of the single crystal silicon ingot102by contacting the surface of the melt104(i.e., at the melt-gas interface126) with the gaseous dopant202. As described herein, by simulating the transport mechanisms of the dopant species within the melt104, the dopant concentration at the solid-melt interface124may be estimated based on a concentration of dopant species that flux into the melt from the gaseous dopant202. The concentration of dopant taken up by the crystal ingot102may be determined from the dopant concentration in the melt by considering the segregation coefficient and the solidification ratio of the dopant, and the resistivity of the growing ingot102may thereby be determined. In this regard, the vaporization rate is controlled to introduce a suitable amount of dopant species into the melt104that are taken up by the ingot102, and to control the rate at which the dopant species are taken up by the ingot102to maintain a resistivity of the body region over an axial length of the body region. On this basis, the height H and the speed at which the second end212moves to the height H may be selected as described above.

For example, the vaporization rate may be controlled such that the dopant species is introduced at a rate sufficient to maintain a resistivity of the body region within a predetermined range (e.g., between about 0.01 ohm-cm and about 200 ohm-cm, between about 20 ohm-cm and about 200 ohm-cm, between about 30 ohm-cm and about 100 ohm-cm, or between about 60 ohm-cm and about 80 ohm-cm) and to control variations in the resistivity of the body region to within +/−15% over an axial length of at least (no less than) 300 mm, at least 500 mm, or even at least 800 mm. In some examples, the vaporization rate may be controlled such that the dopant species is introduced at a rate sufficient to maintain a resistivity of the body region within a predetermined range (e.g., between about 30 ohm-cm and about 100 ohm-cm, or between about 60 ohm-cm and about 80 ohm-cm) and to control variations in the resistivity of the body region to within +/−10% over an axial length of at least 300 mm, of at least 500 mm, or even at least 800 mm.

At step312, the feed tube206remains at the position where the open second end212has the height H above the surface of the melt104for a suitable duration to allow gaseous dopant202to exit the feed tube206at the evaporation rate. The stay time of the feed tube206at the position during step312may be an additional control variable determined by the above-described considerations. After step312, the positioning system242may raise the feed tube206so that the second end212is at the initial height. The positioning system242is used to adjust the feed tube206to move the second open end212to the initial height, which may be automatically controlled by the controller220once the controller220determines that the stay time of the second end212at height H has elapsed, or may be manually controlled.

In some embodiments, the method300may repeat at step304and one or multiple subsequent charges of the volatile dopant216are successively added into the feed tube206, vaporized into gaseous dopant202, and dopant species from the subsequent batch of gaseous dopant202are introduced into the melt104at step312. Each of the multiple charges of the volatile dopant216are added after dopant species from a previous charge of the volatile dopant216have been introduced into the melt104. Each of the multiple charges may include the same type of volatile dopant216or a different volatile dopant216as a previous charge. The feed tube206may be repositioned so that the second end212is at the initial height for each subsequent charge of volatile dopant216.

Referring toFIGS.8-11, gas dopant control and resulting resistivity in an ingot102grown according to the method300are shown.FIG.8is a plot showing gaseous dopant control over time during a gas doping process. As shown inFIG.8, the evaporation rate of the volatile dopant216may be controlled, based on the selected height H and the speed rate of the second end212moving to the second height H, such that the amount of gaseous dopant202produced increases and subsequently decreases at controlled, linear rates.FIG.9is a plot showing average dopant concentration in the silicon melt104(at the solid-melt interface124) and radial resistivity change in a growing ingot102during the gas doping process shown inFIG.8. As shown inFIG.9, the amount of the dopant at the solid-melt interface124increases to a maximum dopant concentration at a constant rate and subsequently decreases at a constant rate under the control region (gas dopant process time), consistent with the rate of production of the gaseous dopant202shown inFIG.8.FIG.9also shows that the radial resistivity slope across a diameter of the ingot102, calculated as the difference of the resistivity at the axial center of the ingot102and the edge of the ingot102divided by the average resistivity, (res cent-res edge)/average, decreases and subsequently decreases at a controlled rate under the control region. Thus, controlling the vaporization rate translates to control of dopant species at the solid-melt interface124and radial resistivity control of the growing ingot102.FIG.10is a plot showing axial resistivity change along a growing ingot during a crystal growth process, with and without a gas doping process. As shown inFIG.10, without the gas doping process, axial resistivity of the main body portion of the ingot102steadily decreases and falls below the lower resistivity limit (LRL) once the main body portion reaches a certain axial length (e.g., about 500 mm). When the controlled gas doping process according to the present disclosure is used, which may be initiated at the point where the resistivity would otherwise fall below the LRL, axial resistivity is maintained within a range between the LRL and an upper resistivity limit (URL) over a substantial portion of the axial length of the main ingot body (e.g., about 1000 mm). Axial resistivity may further be improved and maintained within the range between the LRL and the URL using multiple loads of volatile dopants216at suitable intervals (shown inFIG.11). The range defined by the URL and the LRL may be, for example, +/−15% of a targeted resistivity, +/−13% of a targeted resistivity, or +/−10% of a targeted resistivity. The axial resistivity of the main body ingot may be maintained within the range defined by the URL and the LRL over an axial length of at least 300 mm, of at least 500 mm, or even at least 800 mm.

Referring toFIG.12, another example ingot puller is indicated generally at150. The ingot puller150includes the same features and elements of ingot puller100shown inFIG.1and described herein. In addition, the ingot puller150includes a second gas doping system400for introducing the gaseous dopant202into the melt104. The doping system400includes features and elements similar to those included in the gas doping system200shown inFIG.1and described herein. The ingot puller150that includes both the gas doping system200and the gas doping system400may facilitate improving control of base resistivity of ingots102grown by the ingot puller150.

As above for the gas doping system200(which may be referred to as a “first gas doping system200”), the gas doping system400(which may be referred to as a “second gas doping system400”) includes a dopant feed source404, a feed tube406, and an evaporation capsule408. The feed tube406extends along a feed tube axis X3between a first end410and a second end412. The first end410is located adjacent to the dopant feed source404, and the second end412is located within the inner chamber110and oriented toward the surface of the melt104. The feed tube406may extend between the first end410and the second end412through a valve assembly (not shown) that provides an ingress point for the feed tube406through the outer housing108, and seals the ingress point when the feed tube406is removed from the ingot puller150. The feed tube406is open at the first end410to receive volatile dopant (e.g., volatile dopant216shown inFIG.2) from the dopant feed source404and open at the second end412to allow gaseous dopant202to flow out from the second end412of the feed tube406toward the surface of the melt104and, particularly, toward the melt-gas interface126. The feed tube406may be angled with respect to the pulling axis X2and may have similar configurations or orientations as those described above for feed tube206.

The doping system400may also include an inert gas supply414coupled in fluid communication with the feed tube406to guide gaseous dopant202out from the feed tube406through the second end412and to reduce back flow of gaseous dopant202, as described above for inert gas supply214in the doping system200.

The first end410of the feed tube406is coupled in flow communication with the dopant feed source404. The dopant feed source404feeds a volatile dopant216, which may be in the form of solid-phase dopant or liquid phase dopant, into the first end410of the feed tube406, as described above for the dopant feed source204of doping system200. The dopant feed source404may be automated, partially automated, or manually operated. Automated control of the dopant feed source404may be facilitated by the controller220communicatively coupled to the dopant feed source404. The controller220may be programmed to control the frequency and/or amount of volatile dopant416being fed into the feed tube406by the dopant feed source404.

The evaporation capsule408is disposed within the feed tube406proximate the second end412, and within the inner chamber110, as described above for the evaporation capsule208shown inFIG.2. The feed tube406and the evaporation capsule408may have the same configuration as the feed tube206and evaporation capsule208shown inFIGS.1and2and described above. The feed tube406may include a guide (e.g., guide238) and a fluid-distribution plate as described above for the feed tube206.

The feed tube406is slidingly coupled to a positioning system442that raises and/or lowers feed tube406along the feed tube axis X3. The positioning system442includes a rail444, a coupling member446, and a motor (not shown). The coupling member446slidingly couples the feed tube406to the rail444. The motor moves the coupling member446and the feed tube406along the rail444. The rail444extends in a direction substantially parallel to the feed tube axis X3. Using the positioning system442, the second end412of the feed tube406and the evaporation capsule408raised and lowered into and out of the inner chamber110. Additionally, the positioning system442facilitates adjusting a height of the second end412above the melt-gas interface126(which may be similar to the height H of the second end212of the feed tube206described above). The positioning system442is communicatively coupled to the controller220, and the controller220may be programmed to control the positioning system442to dynamically adjust the height of the second end412of the feed tube406above the melt-gas interface126during a crystal growth process as well as the speed at which the feed tube406moves along the rail444to adjust the height of the second end412, as described above with respect to the positioning system242. The height to which the second end412of the feed tube406is moved may be the same as the height H of the second end212of the feed tube206, described above, and the speed at which the feed tube406moves to adjust the height of the second end412may be the same as the speed at which the feed tube206moves to adjust the height of the second end212. Alternatively, the second end212of the feed tube206may be moved to the height H at a first speed rate, and the second end412of the feed tube406may be moved to a second height that is different from the height H and/or may be moved at a second speed rate that is different than the first speed rate.

Ingot pullers and associated gas doping systems and methods of the present disclosure provide an improvement over known ingot pullers that facilitate doping of ingots using gaseous dopants. By controlling a height of a feed tube above a silicon melt surface, a movement speed of the feed tube, and a stay time of the feed tube at a controlled height during a doping process, the vaporization rate of volatile dopant in the feed tube may be finely tuned and controlled. Consequently, the amount of dopant species introduced into the melt and the rate at which dopant species are introduced are controlled to achieve and maintain a base resistivity of a growing ingot.

Unless otherwise indicated, approximating language, such as “generally,” “substantially,” and “about,” as used herein indicates that the term so modified may apply to only an approximate degree, as would be recognized by one of ordinary skill in the art, rather than to an absolute or perfect degree. 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. Additionally, unless otherwise indicated, the terms “first,” “second,” etc. are used herein merely as labels, and are not intended to impose ordinal, positional, or hierarchical requirements on the items to which these terms refer. Moreover, reference to, for example, a “second” item does not require or preclude the existence of, for example, a “first” or lower-numbered item or a “third” or higher-numbered item.