Patent Publication Number: US-11028500-B2

Title: Ingot puller apparatus that include a doping conduit with a porous partition member for subliming solid dopant

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application incorporates herein by reference U.S. Non-provisional patent application Ser. No. 16/220,058, published as U.S. Patent Publication No. 2020/0190689, entitled “Methods for Preparing a Doped Ingot”, filed Dec. 14, 2018. 
     FIELD OF THE DISCLOSURE 
     The field of the disclosure relates to ingot puller apparatus that include a dopant feed system for preparing a doped silicon ingot and, in particular, dopant feed systems having a dopant conduit with a porous partition member disposed across the dopant conduit. 
     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, and a single crystal is grown by slow extraction. 
     In some applications, an 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 can fall into the melt and become incorporated into the growing crystal. These particles can act as heterogeneous nucleation sites, and ultimately result in failure of the crystal pulling process. 
     Some known dopant systems introduce volatile dopants into the growth chamber as a gas. However, such systems must be manually refilled each time a doping procedure is performed. Additionally, such systems cannot be refilled while in use. As a result, such systems have a limited dopant payload capacity for a single growth process. Such systems therefore limit the size of silicon ingots that can be grown. Furthermore, such systems tend to supply dopant non-uniformly during a growth process, thereby increasing the variation in dopant concentration along a grown ingot&#39;s longitudinal axis. 
     Other known systems introduce dopant into an evaporation receptacle near the melt to vaporize dopant. When volatile dopants are used, solid dopant granules may move vigorously and be ejected from the evaporation receptacle causing them to fall into the melt, reducing the consistency of the dopant process. 
     A need exists for improved dopant feed systems for doping a silicon melt to produce a doped silicon ingot by the Czochralski method. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     SUMMARY 
     One aspect of the present disclosure is directed to an ingot puller apparatus for producing a silicon ingot. The ingot puller apparatus includes a crucible for holding a melt of silicon and a growth chamber for pulling a silicon ingot from the melt. The ingot puller apparatus includes a dopant conduit for introducing dopant into the melt. The dopant conduit includes one or more sidewalls. The one or more sidewalls form a conduit chamber through which dopant passes. The conduit chamber has a width. The dopant conduit includes an inlet through which solid dopant is introduced into the dopant conduit. The conduit chamber includes an outlet through which a gaseous dopant is discharged through the dopant conduit. The dopant conduit includes a partition member disposed between the inlet and the outlet of the dopant conduit that supports solid dopant introduced through the inlet. The partition member extends across the width of the conduit chamber. 
     Various refinements exist of the features noted in relation to the above-mentioned aspects of the present disclosure. Further features may also be incorporated in the above-mentioned aspects of the present disclosure as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments of the present disclosure may be incorporated into any of the above-described aspects of the present disclosure, alone or in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section view of an ingot puller apparatus having a dopant feed system; 
         FIG. 2  is a cross-section front view of a dopant conduit of the dopant feed system; 
         FIG. 3  is side view of the dopant conduit rotated from the position of  FIG. 2 ; and 
         FIG. 4  is a box plot of the seed end resistivity for three dopant amounts as described in Example 1. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION 
     An ingot puller apparatus (or more simply “ingot puller”) is indicated generally at “100” in  FIG. 1 . The ingot puller apparatus  100  includes a crucible  102  for holding a melt  104  of semiconductor or solar-grade material, such as silicon, surrounded by a susceptor  106 . The ingot puller apparatus  100  includes a crystal puller housing  108  that defines a growth chamber  126 . The semiconductor or solar-grade material is melted by heat provided from one or more heating elements  110  surrounded by insulation  112 . 
     A pulling mechanism  114  is provided within ingot puller apparatus  100  for growing and pulling ingots  116  out of the melt  104 . Pulling mechanism  114  includes a pulling cable  118 , a seed holder or chuck  120  coupled to one end of pulling cable  118 , and a seed crystal  122  coupled to the seed holder or chuck  120  for initiating crystal growth. 
     The ingot puller apparatus  100  also includes a dopant feed system  130  for introducing gaseous dopant into the melt  104 . The dopant feed system  130  includes a dopant conduit  134  and a partition member  138 . In the illustrated embodiment, the dopant feed system  130  also includes a dopant feeding device  140 , a positioning system  142 , and an inert gas source  144 . Generally, the dopant feed system  130  is configured to sublime solid dopant (e.g., dopant granules) and cause gaseous dopant to flow across a melt surface  146 . Gaseous dopant may be introduced into the ingot puller  100  before crystal growth commences and/or during crystal growth, as shown in  FIG. 1 . 
     In operation, a solid dopant  148 , such as arsenic, phosphorous, or any other element or compound with a suitably low sublimation or evaporation temperature that enables the dopant feed system to function as described herein, is introduced into the dopant conduit  134  through an inlet  124  positioned toward a first end  150  of the conduit  134 . Solid dopant  148  falls downwardly through the dopant conduit  134 , passes through a conduit chamber  164 , and comes to rest on a porous partition member  138 . Heat supplied to the conduit  134  causes the solid dopant  148  resting on the partition member  138  to vaporize into a gaseous dopant. The gaseous dopant mixes with an inert gas  152 , supplied by inert gas source  144 , which passes through the dopant conduit  134  and passes through the porous partition member  138 . The inert gas having dopant mixed therein or “doped inert gas”  198  flows out of the dopant conduit  134  through outlet  190  and across the melt surface  146 . 
     As solid dopant  148  is consumed by vaporization, more solid dopant  148  may be fed into the dopant conduit  134  by the feeding device  140 . By continuously or intermittently supplying solid dopant  148  to the dopant conduit  134 , a relatively constant concentration of gaseous dopant may be maintained above the melt surface  146  during the doping process and/or the crystal growth process. 
     Referring now to  FIG. 2 , dopant conduit  134  includes a dopant conduit sidewall  154  and includes an inlet  124  near a first end  150  of the sidewall  154  through which solid dopant  148  is introduced. The conduit  134  includes an outlet  190  near a second end  136  of the sidewall  154  through which gaseous dopant is discharged from the conduit  134 . In the embodiment shown in  FIGS. 1-3 , the dopant conduit  134  has a generally cylindrical shape defined by a single dopant conduit sidewall  154 . Generally, the dopant conduit  134  may have any suitable shape and/or any number of dopant conduit sidewalls that enable dopant feed system  130  to function as described herein (e.g., may be circular, oval, triangular, square, or rectangle in cross-section). In some embodiments, the sidewall  154  is made of quartz. 
     The dopant conduit  134  has a width W 134  and the partition member  138  extends across the width W 134  of the conduit  134  to catch all solid dopant that falls through the conduit chamber  164  (i.e., the partition member  138  fully extends across the conduit  134  to occlude the chamber  164 ). In embodiments in which the conduit  134  is cylindrical, the partition member  138  is a disk having a diameter at least the diameter of the conduit chamber  164 ). The partition member  138  is disposed between the conduit inlet  124  and the outlet  190  and may be closer to the outlet  190  (e.g., within about the last 25% of the length of the conduit or about the last 10% or about the last 5% of the length of the conduit). The partition member  138  may have any suitable thickness that allows the partition member  138  to support dopant while allowing doped inert gas to pass through the partition member  138 . 
     The partition member  138  may be configured (e.g., be porous) to enable sublimed dopant to pass through the partition member  138  and toward the melt  104 . In some embodiments, the partition member  138  has a nominal maximum pore size of less than about 1 mm, less than about 750 μm, less than about 675 μm, less than about 600 μm, less than about 500 μm or less than about 400 μm, from about 10 μm to about 1 mm, from about 10 μm to about 750 μm or from about 10 μm to about 750 μm. In some embodiments, the partition member  138  is made of quartz (e.g., type 00, type 0, type 1, type 2, or the like). The partition member  138  and dopant conduit  134  may be made from separate components with the partition member  138  being welded or otherwise connected to the sidewall(s) of the dopant conduit  134 . 
     The dopant conduit  134  is positioned within the crystal puller housing  108  ( FIG. 1 ), and extends through a valve assembly  158  and outside of the housing  108 . In the embodiment shown in  FIG. 1 , the dopant conduit  134  is slidingly coupled to a positioning system  142 . The positioning system  142  is configured to raise and/or lower the dopant conduit  134 . The positioning system  142  includes a rail  160 , a coupling member  162 , and a motor (not shown) configured to move coupling member  162  along the rail  160 . The rail  160  extends in a direction substantially parallel to the longitudinal axis A ( FIG. 2 ) of the dopant conduit  134 . The coupling member  162  is slidingly coupled to the rail  160  and is affixed to dopant conduit  134 . The dopant conduit  134  may move relative to a feed conduit  196  that extends into the inlet  124  of the dopant conduit  134 . Using the positioning system  142 , the dopant conduit  134  may be raised and lowered into and out of the ingot puller apparatus  100 . In other embodiments, the dopant conduit  134  may be positioned wholly within the housing. In some embodiments, the dopant conduit  134  is permanently attached to the ingot puller apparatus  100  (e.g., by connecting a tube housing to the puller housing  108 ). In yet other embodiments, the dopant conduit  134  may be positioned within and/or secured within or outside the housing  108  in any manner that enables the dopant feed system  130  to function as described herein. 
     In the embodiment shown in  FIGS. 1 and 2 , the dopant conduit  134  is angled with respect to the melt surface  146  to facilitate the distribution of gaseous dopant across the melt surface  146 . The dopant conduit  134  is angled such that the longitudinal axis A ( FIG. 2 ) of the dopant conduit  134  forms an angle (i.e., acute angle) of between about 45 degrees and about 90 degrees with respect to the melt surface  146  and/or to a horizontal plane P that extends across a top of the crucible  102 . In other embodiments, the angle formed between the longitudinal axis A of the dopant conduit  134  and the melt surface  146  and/or the horizontal plane P is between about 45 degrees and about 75 degrees. 
     The partition member  138  may be angled with respect to the longitudinal axis A of the dopant conduit  134 . As shown in  FIG. 3 , the partition member  138  has a supporting surface  192 . The supporting surface  192  has an uppermost point P H  and a lowermost point P L  relative to the longitudinal axis A. The uppermost and lowermost points P H , P L  define a plane P 138  that extends through the uppermost and lowermost points P H , P L . The longitudinal axis A of the conduit  134  and the plane P 138  form an angle between about 45 degrees and about 90 degrees (e.g., between 45 degrees and 90 degrees). In some embodiments, this angle is the same as the angle formed between the dopant conduit  134  and the melt surface  146  to allow the partition member  138  to be substantially parallel to the melt surface  146  (and to the horizontal plane P that extends across a top of the crucible  102  and to the outlet  190  of the conduit  134 ). 
     The outlet  190  of the dopant conduit  134  may also be angled with respect to the longitudinal axis A of the conduit  134  to facilitate the distribution of gaseous dopant across melt surface  146 . For example, the outlet  190  may be angled such that outlet  190  is substantially parallel to the melt surface  146 , and angled at an angle of between about 45 degrees and about 75 degrees with respect to longitudinal axis A of the dopant conduit  134 . In other embodiments, the dopant conduit  134  is positioned substantially perpendicular to the melt surface  146  such that the longitudinal axis A of the dopant conduit  134  forms an angle of about 90 degrees with respect to the melt surface  146  and/or horizontal plane P that extends across a top of the crucible  102 . The arrangement of the dopant conduit  134 , outlet  190 , and partition member  138  shown in the illustrated embodiment is exemplary and the dopant conduit  134 , outlet  190 , and partition member  138  may have any suitable configuration or orientation that enables dopant feed system  130  to function as described herein. 
     In this embodiment, the dopant conduit  134  is in fluid communication with an inert gas source  144  to carry gaseous dopant out of the conduit  134  through the outlet  190  and toward the melt  104 . Inert gas may reduce or eliminate back flow of sublimed dopant in the upper portion of the dopant conduit  134  and other upstream components of the dopant feed system  130  (e.g., to reduce build-up of flammable phosphorous). An inert gas  152  may be introduced into the dopant conduit  134  from the inert gas source  144  at a given flow rate, such that the inert gas  152  flows downwardly towards the outlet  190  of the conduit  134 . For example, inert gas flow rates of 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 can be used while maintaining a sufficient supply of gaseous dopant to the melt surface  146 . The inert gas  152  may be argon, although any other suitable inert gas may be used that enables the dopant feed system  130  to function as described herein. The flow rate of the inert gas within the conduit chamber  164  may be controlled based, as least in part, on the sublimation rate of the dopant to prevent backflow of dopant through the dopant conduit  134 . 
     In some embodiments, the dopant conduit  134  does not include flow restrictors that restrict the flow of inert gas  152  in the conduit  134  and/or does not include vertical partitions such as partitions connected to the partition member  138  to divide the conduit  134  into additional chambers or receptacles. 
     The dopant conduit  134  may be communicatively coupled to a dopant feeding device  140  configured to feed solid dopant  148  into the dopant conduit  134 . The dopant feeding device  140  may be automated or, as in other embodiments, may be manually operated, or only partially automated. The feeding device  140  may be configured to automatically feed solid dopant  148  into the dopant conduit  134  based upon one or more user-defined parameters, and/or environment-specific parameters. For example, the automated feeding device  140  may feed solid dopant  148  into the dopant conduit  134  based upon any one or more of the following parameters: preset time(s) during a growth process, user defined interval(s), the mass of solid dopant  148  within dopant conduit  134 , a concentration of gaseous dopant within the conduit chamber  164 , and a volumetric or mass flow rate of gaseous dopant and/or inert gas  152 . The continuous and/or intermittent feeding of solid dopant  148  to conduit  134  enables a relatively constant gaseous dopant concentration to be maintained within the growth chamber  126  during the crystal growth process, resulting in a more uniform dopant concentration profile in grown ingots. 
     The feeding device  140  may be coupled to a controller  182  configured to control the frequency and/or amount of solid dopant  148  being fed into the dopant conduit  134  by the feeding device  140 . The controller  182  includes a processor  184  configured to send and receive signals to and from controller  182  and/or feeding device  140  based on one or more user-defined parameters and/or environment-specific parameters. In this embodiment, the controller  182  includes a user interface  186  coupled to a processor  184 , and a sensor  188  coupled to the processor  184 . User interface  186  is configured to receive user-defined parameters, and communicate user-defined parameters to processor  184  and/or controller  182 . The sensor  188  is configured to receive and/or measure environment-specific parameters, and communicate such environment-specific parameters to the processor  184  and/or controller  182 . 
     The partition member  138  is positioned within dopant conduit  134  near the outlet  190 . The partition member  138  is configured to hold solid dopant  148  and allow heat to be transferred from the melt  104  to the solid dopant  148  to vaporize dopant while allowing vaporized gas to pass through the partition member  138 . The partition member  138  may be positioned sufficiently near the melt  104  such that radiant heat from the melt  104  vaporizes solid dopant  148  that falls onto the partition member  138 . For example, the partition member  138  may be positioned between about 1 mm to about 15 mm above the melt surface  146 . In some embodiments, the outlet  190  of the dopant conduit  134  is disposed within the melt  104  to position the partition member  138  near the melt surface  146 . In other embodiments, a separate heating element (not shown) may be used to supply heat to vaporize dopant  148  therein. 
     Generally, the partition member  138  causes a pressure difference in the gas upstream and downstream of the partition member such that the pressure of the inert gas is reduced upon passing through the partition member  138 . In some embodiments, the pressure of the inert gas is reduced by at least about 5 mbar as it passes across the partition member  138 . In some instances, solid dopant  148  resting on the partition member  138  contributes to the pressure difference across the partition member  138 . This allows the pressure difference to be correlated to the amount of solid dopant  148  in the dopant conduit  134 . The pressure difference when no dopant is present may be used for calibration. 
     In accordance with embodiments of the present disclosure, a doped ingot may be prepared by an embodiment of an ingot puller apparatus  100  described above. Polycrystalline silicon is melted by heating elements  110  to form a melt of silicon in the crucible  102 . Solid dopant  148  is introduced into the dopant conduit  134  through the inlet  124 . The solid dopant  148  falls through the conduit  134  to partition member  138  where the solid dopant  148  comes to rest. Solid dopant  148  loaded onto the partition member  138  is heated (e.g., by radiant heat from the melt  104 ) to cause the solid dopant  148  to sublime. 
     Inert gas  152  is introduced into the conduit chamber  164  and mixes with sublimed dopant to form a doped inert gas  198 . The doped inert gas  198  passes through the partition member  138  to cause dopant to contact the melt surface  146  and be absorbed into the melt  104 . A doped ingot is pulled from the silicon melt having dopant therein. The ingot may be grown from a pre-loaded charge of polycrystalline silicon (e.g., a batch process) or in a continuous Czochralski process or even a semi-continuous Czochralski process. 
     The ingot puller apparatus of embodiments of the present disclosure has several advantages over previous ingot pullers. Use of a partition member that extends across the width of the dopant conduit allows solid dopant to sublime relatively vigorously without dopant particles falling into the melt (e.g., by jumping from an evacuation receptacle during sublimation). This is particularly advantageous for dopants that tend to sublime vigorously such as phosphorous. The partition member also increases the surface of the dopant granules exposed to radiant heat from the melt which allows for more efficient heat exchange with the ingot puller environment, reducing the time at which the ingot melt is initially doped. 
     EXAMPLES 
     The processes of the present disclosure are further illustrated by the following Examples. These Examples should not be viewed in a limiting sense. 
     Example 1: Stability and Repeatability by Use of the Dopant Feed System 
     Single crystal silicon ingots were prepared with different amounts of phosphorous doping (light doping, medium doping, heavy doping). The silicon melt was doped before ingot growth by the dopant feed system of  FIG. 1  having a dopant conduit with a partition member therein. The outlet of the dopant conduit was about 5 mm from the surface of the melt. A box plot of the distribution of the resistivity of the seed end of a number of ingots is shown in  FIG. 4 . As shown in  FIG. 4 , for all three dopant amounts, the distribution of the seed end resistivity was relatively tight which demonstrates good stability and repeatability of doping by use of the dopant feed system described herein. 
     The number of attempts to grow a single crystal ingot decreased by 4% when using the dopant feed system of  FIG. 1  having a dopant conduit with a partition member therein. This demonstrates an improvement in repeatability and better control of resistivity at the seed end. 
     As used herein, the terms “about,” “substantially,” “essentially” and “approximately” when used in conjunction with ranges of dimensions, concentrations, temperatures or other physical or chemical properties or characteristics is meant to cover variations that may exist in the upper and/or lower limits of the ranges of the properties or characteristics, including, for example, variations resulting from rounding, measurement methodology or other statistical variation. 
     When introducing elements of the present disclosure 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,” “containing” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of terms indicating a particular orientation (e.g., “top”, “bottom”, “side”, etc.) is for convenience of description and does not require any particular orientation of the item described. 
     As various changes could be made in the above constructions and methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawing[s] shall be interpreted as illustrative and not in a limiting sense.