Abstract:
A feed assembly and method of use thereof of the present invention is used for the addition of a high pressure dopant such as arsenic into a silicon melt for CZ growth of semiconductor silicon crystals. The feed assembly includes a vessel-and-valve assembly for holding dopant, and a feed tube assembly, attached to the vessel-and-valve assembly for delivering dopant to a silicon melt. An actuator is connected to the feed tube assembly and a receiving tube for advancing and retracting the feed tube assembly to and from the surface of the silicon melt. A brake assembly is attached to the actuator and the receiving tube for restricting movement of the feed tube assembly and locking the feed tube assembly at a selected position.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to the preparation of semiconductor grade silicon crystals, used in the manufacture of electronics. More particularly, the invention relates to a device for feeding arsenic dopant into an apparatus for producing low resistivity silicon crystals. 
     Silicon crystal growth using the Czochralski (CZ) method involves changing the characteristics and properties of the silicon ingot being grown by adding a dopant material to the molten silicon before silicon ingot growth. A common dopant material used in this process is arsenic. Arsenic, however, is a volatile substance and problems often arise through conventional methods of introducing the dopant to the silicon melt. 
     One such method is to dump the dopant from a port positioned above the melt. However, because of the high temperatures of the process, there is a violent loss of arsenic to the argon gas environment above the melt. This results in the generation of oxide-particles which can prolong and compromise the crystal growing process. Thus, this method is very inefficient. 
     Another method uses a quartz vessel containing the dopant above the melt for introducing the volatile gas to the melt. This method can reduce loss of vaporized dopant if the vessel has a port extending into the melt. Regardless, these methods result in complicated operation and loss of volatile dopant. The present invention overcomes these difficulties and disadvantages associated with prior art processes by introducing the dopant to the melt at an upper surface of the melt. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention includes a feed assembly for feeding a dopant to a silicon melt in a crystal growing apparatus. The assembly comprises a vessel for holding and releasing a dopant solid material and an elongate feed tube operatively connected to the vessel. The feed tube comprises a fixed tube and a movable tube concentrically arranged with the fixed tube. The assembly also includes an actuator connected to the moveable tube for moving the moveable tube relative to the fixed tube for advancing the moveable tube toward an upper surface of the silicon melt in the apparatus and retracting the moveable tube away from the upper surface of the silicon melt to selectively position the moveable tube for introducing the dopant material released from the vessel to the silicon melt when the feed assembly is mounted on the crystal growing apparatus. 
     In another aspect, the present invention includes a method for feeding arsenic dopant to a silicon melt in a silicon crystal growing apparatus having a crystal growing chamber. The method includes placing granular solid arsenic dopant in a vessel attached to a feed tube comprising a fixed tube and a movable tube in concentric arrangement. The moveable tube is lowered toward the silicon melt with an actuator connected to the moveable tube to selectively position the moveable tube at the surface of the silicon melt. In addition, the dopant is released from the vessel to allow dopant to travel down the feed tube and into the melt at an upper surface of the melt. 
     In still another aspect, the present invention includes a method for feeding arsenic dopant to a silicon melt in a silicon crystal growing apparatus having a crystal growing chamber. The method comprises placing granular solid arsenic dopant in a vessel attached to a feed tube comprising a fixed tube and a moveable quartz tube having an angled tip. The fixed tube and moveable quartz tube are in concentric arrangement. Further, the method includes lowering the moveable tube toward the silicon melt with an actuator connected to the moveable tube to selectively position the moveable tube at the surface of the silicon melt. Still further, the method comprises releasing the dopant from the vessel to allow the dopant to travel down the feed tube to a catch located in the moveable tube for catching the dopant material when it is released from the vessel. In addition, the method comprises introducing argon gas into the feed tube below the vessel causing sublimation of the dopant resulting in dopant laden argon exiting the angled tip of the moveable quartz tube at an upper surface of the silicon melt. 
     In yet another aspect, the present invention includes a feed assembly for feeding a dopant to a silicon melt in a crystal growing apparatus. The feed assembly comprises a vessel for holding and releasing a dopant solid material and an elongate feed tube attached to the vessel. The feed tube includes a fixed tube and a movable tube concentrically arranged with the fixed tube. Further, the feed assembly includes a catch located within the moveable tube for catching the dopant material when it is released from the vessel. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross section of a first embodiment of a feed assembly in a retracted position; 
         FIG. 2  is a front view of a vessel-and-valve assembly of the feed assembly with a portion broken away showing the flow of dopant material; 
         FIG. 3  is a cross section of a second embodiment of the feed assembly in an extended position; 
         FIG. 4  is a perspective of the feed assembly attached to a crystal grower furnace chamber; 
         FIG. 5  is a perspective of the vessel-and-valve assembly and actuator of the feed assembly; 
         FIG. 6  is a perspective of an isolation valve of the feed assembly attached to the crystal grower. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Multiple embodiments for an arsenic dopant feed assembly are illustrated.  FIG. 1  illustrates a first embodiment of an arsenic dopant feed assembly, generally designated by the reference number  10 . In the first embodiment, the dopant feed assembly  10  is fabricated from a refractory material that is non-contaminating and non-reactive with arsenic, silicon and graphite. 
     The first embodiment of the feed assembly  10  comprises a vessel-and-valve assembly  11  for holding dopant solid (not shown), and a feed tube assembly, generally indicated at  15 , attached to the vessel-and-valve assembly  11  for delivering the dopant to a silicon melt (not shown). An actuator  19  is operatively connected between the feed tube assembly  15  and a receiving tube  21  for advancing and retracting the feed tube assembly to and from an upper surface of the silicon melt. A brake assembly  25  is operatively connected between the actuator  19  and the receiving tube  21  for restricting movement of the feed tube assembly  15  and locking the feed tube assembly at a selected position. An isolation valve  27  is provided at a bottom of the feed tube assembly  15 . The valve  27  is configured for placing the feed assembly  10  in communication with a crystal growing apparatus  31  (see  FIG. 6 ). 
     Referring to  FIG. 2 , the vessel-and-valve assembly  11  includes a dopant cartridge  41  configured for holding the dopant solid and a valve  43  attached to the feed tube assembly  15  that can be opened to release the dopant down the feed tube assembly. The valve  43  has a handle  45  for opening and closing the valve. 
     The feed tube assembly  15  comprises a series of elongate concentric tubes including a fixed tube  51  and one or more moveable tubes  53  situated around the fixed tube and arranged in a telescoping fashion (see  FIG. 1 ). The fixed tube  51  is closed at a first end  54  by a vacuum flange  55  and is received by the moveable tubes  53  at a second end  57  (see  FIG. 3 ). An end cap  59  at the first end  54  attaches the fixed tube  51  to the receiving tube  21 . The end cap  59  includes a seat  61  having an opening  63  which receives the first end  54  of the fixed tube  51 . An annular seal  65  seals the opening  63  between the end cap  59  and the fixed tube  51 . 
     A vacuum fitting  67  connects each moveable tube  53  to an adjacent moveable tube. Each vacuum fitting  67  includes two opposing ring fittings  69  connected to each other by a threaded coupling  71  engaging threads  73  on the ring fittings. The embodiment illustrated in  FIG. 1  shows two moveable tubes, however a single moveable tube or three or more moveable tubes are contemplated as being within the scope of the present invention. 
     The feed tube assembly  15  provides a passage  81  through which dopant material travels when it is released from the vessel-and-valve assembly  11 . An outlet  83  of the moveable tubes  53  is in fluid communication with the vessel-and-valve assembly  11  for introducing the dopant to the silicon melt (see  FIG. 3 ). In this first embodiment, the feed tube assembly  15  can be made of any refractory material that is non-contaminating and non-reactive with arsenic, silicon and graphite. As will be explained in greater detail later, a moveable tube  53 ′ of the second embodiment that is positioned at the surface of the melt is fabricated from quartz. 
     Referring to  FIG. 1  the actuator  19  comprises a linear translator  85  including an annular magnetic sleeve  87  attached to the moveable tubes  53  and an annular magnetic slide  89  adjacent and magnetically coupled to the sleeve. The magnetic sleeve  87  is sized and shaped for receiving the moveable tubes  53  in the sleeve. The sleeve  87  is secured to the moveable tubes  53  by friction fitting. The magnetic slide  89  is sized and shaped for receiving the receiving tube  21  and directly engages the outer surface of the receiving tube  21 . A small clearance  90  between the receiving tube  21  and the magnetic slide  89  allows the magnetic slide to slide along the length of the receiving tube. The slide  89  is aligned with the magnetic sleeve  87 , creating a magnetic coupling due to the opposite polarization of the two structures. This coupling secures the slide  89  to the receiving tube  21  at the same height that the magnetic sleeve  87  is positioned on the moveable tubes  53 . As a result, movement of the slide  89  along the receiving tube  21  causes the magnetic sleeve  87  to move under the force of magnetic attraction. As the slide  89  moves up and down the receiving tube  21 , the moveable tubes  53  slide away from and toward the fixed tube  51  for positioning a tip  91  of the moveable tubes  53  at the surface of the silicon melt (see  FIG. 3 ). As will be explained in greater detail below, the magnetic slide  89  also includes an extension  93  having an annular teardrop shape with an aperture  95  at its tapered end. The aperture  95  is configured for attaching to the brake assembly  25 . Although the preferred embodiment of the invention incorporates the magnetically coupled linear translator, it is envisioned that other suitable actuators (e.g., mechanical, electrical, or electromechanical) could be used without departing from the scope of this invention. 
     Referring to  FIG. 4 , the receiving tube  21  is an elongate tube made of stainless steel. The receiving tube  21  separates a portion of the actuator  19  and feed tube assembly  15  from the surrounding environment (see  FIG. 1 ). The feed assembly  10  is illustrated as having two receiving tube members  99  connected in series. However, any number of receiving tube members  99  is foreseen. A first seal assembly  101  connects the receiving tubes  21 . The seal assembly  101  comprises an o-ring  103  and a clamp  105  having semi-circular clamp halves  107 . A second seal assembly  109  connects the receiving tube  21  to the isolation valve  27 . One clamp half  111  of the second seal assembly  109  has a threaded extension  113  for connecting the receiving tube  21  to the isolation valve  27  as will be explained in greater detail below. 
     Referring to  FIGS. 1 and 5 , the brake assembly  25  comprises stops  121 ,  122  positioned on the receiving tube  21 , a rod  123  (broadly, a “braking member”) disposed between the stops and a screw  125  (broadly, a locking member) engaging the braking member and the magnetic slide  89  for locking the slide at a selected position along the receiving tube  21 . Similar to the extension  93  on the magnetic slide  89 , the stops  121 ,  122  have an annular teardrop shape with a central opening  127  at its bulbous end for receiving the receiving tube  21  and a hole  129  at the tapered end extending from a top face  131  to a bottom face  133  for receiving the braking member  123 . A side face  135  on the tapered end has an adjustment opening  137 . The stops  121 ,  122  are positioned on the receiving tube  21  above and below the magnetic slide  89 . The braking member  123  passes through the hole  129  in the first stop  121 , the aperture  95  in the slide  89  and the hole  129  in the second stop  122 . Thus, the braking member  123  links the stops  121 ,  122  to the slide  89  creating a track  139  to guide the slide along the receiving tube  21 . 
     The stops  121 ,  122  are also adjustable. The central opening  127  is sized and shaped for receiving the receiving tube  21 . Similar to the magnetic slide  89 , a small clearance  140  between the stops  121 ,  122  and the receiving tube  21  allow the stops to slide along the length of the receiving tube. On the receiving tube  21  the stops  121 ,  122  can be slid to a selected position. Once the selected position for the stops  121 ,  122  is achieved, a stop screw  141  can be inserted into the adjustment bore  137  to lock the stops in place. The tip of the stop screw  141  presses against the braking member  123  holding the stops  121 ,  122  in position. The stop screw  141  can then be unscrewed to allow the stops  121 ,  122  to move to another position on the receiving tube  21  and re-tightened to lock the stops in place again. 
     Referring to  FIGS. 1 and 6 , in one embodiment the isolation valve  27  comprises a ball valve  151  having a body  153  and a passageway  155  with a ball  157  disposed in the passageway mounted for selective rotation between open and closed positions (illustrated embodiment shown in open position. A pair of valve seats  159 ,  161  are provided in the passageway  155  on opposing sides of the ball  157 . In the preferred embodiment, the valve seats  159 ,  161  are located substantially equidistant from an axis of rotation of the ball  157  and include radial openings  163 . The ball  157  and valve seats  159 ,  161  are enclosed within the body  153  by a pair of end fittings  165 . The end fittings  165  can be mounted to the body  153  by any sufficient means. In the present invention, mounting bolts  167  are utilized. At least one end fitting  165  is also provided with internal threads  169  to facilitate connecting the isolation valve  27  to the feed tube assembly  15  by the threaded extension  113  on the second seal assembly  109 . It is understood that any other convenient means of connecting the isolation valve to the feed tube assembly is within the scope of the present invention. 
     A stem assembly  171  and handle  173  are provided for actuating the isolation valve  27 . The handle  173  is releasably secured to the stem assembly  171  by a nut  175  that clamps to the tip of a packing nut  177  and also helps to support the ball  157  in the body  153 . The ball  157  is supported in the passageway  155  such that the ball can shift axially along the passageway. The ball valve  151  can be manually actuated with the handle  173 , or an actuator (not shown) may be provided to actuate the valve. The positions of the handle  173  and the ball  157  are limited by a depending catch member  179  carried by the handle. The catch member  179  engages a surface of the body  153  to provide fixed stops for the isolation valve  27 . 
     The structure of the isolation valve as described above reflects a preferred embodiment. It will be readily apparent to those skilled in the art that changes and additions to the structure may be made to accommodate specific operational requirements. Such modifications are not deemed to affect the scope of the present invention. 
     Operation of this first embodiment of the feed assembly  10  is as follows. Once the silicon melting process is complete, the actuator  19  advances the moveable tubes  53  of the feed tube assembly  15  so the outlet  83  of the moveable tubes  53  is located at the upper surface of the silicon melt. The locking member  125  of the brake assembly  25  is tightened to lock the magnetic slide  89  in place, thus locking the outlet  83  of the moveable tubes  53  in position at the surface of the silicon melt. The dopant held in the vessel-and-valve assembly  11  is released when the valve  43  in the dopant cartridge  41  is opened. The dopant will travel through the feed tube assembly  15 , past an opened isolation valve  27  and into the silicon melt at the surface of the melt. The moveable tubes  53  are retracted by the actuator  19  and argon gas is released below the vessel-and-valve assembly  11  into the feed tube assembly  15  for cooling the assembly  10 . Finally, the assembly  10  is isolated from the crystal growing apparatus  31  by closing the isolation valve  27 . 
     As illustrated in  FIG. 3 , a second embodiment of the feed assembly  10 ′ is designated in its entirety by the reference number  10 ′. The components of the second embodiment are exactly the same as the first embodiment except for a modified feeding tube assembly  15 ′. The feeding tube assembly  15 ′ of the second embodiment comprises a moveable tube  53 ′ made of a special quartz material. This material is used primarily to accommodate gas phase doping. The quartz tube  53 ′ is a thick walled clear fused quartz tube with an outside diameter of about 25 mm, a wall thickness of about 3 mm and a length of about 711 mm. This tube  53 ′ has an angled tip  91 ′ allowing a maximum melt surface area to be exposed to dopant gasses flowing from the tip. Additionally, the moveable tube  53 ′ includes a guide  201  aligning the quartz tube  53 ′ and a perforated disk  203  preventing the dopant from exiting the tube  53 ′ into the silicon melt  17 . When the dopant material trapped in the tube  53 ′, argon gas can be introduced into the feed tube assembly  15 ′ causing the dopant laden argon to travel down the feed tube assembly  15 ′ under sublimation and exit the angled tip  91 ′ at the upper surface of the silicon melt. 
     This second embodiment of the feed assembly  10 ′ operates as follows. The process is similar to the process described for the first embodiment except the actuator  19  advances the quartz tube  53 ′ of the feed tube assembly  15 ′ so the angled tip  91 ′ is positioned at the upper surface of the silicon melt. The locking member  125  of the brake assembly  25  is tightened to lock the magnetic slide  89  in position, thus locking the angled tip  91 ′ of moveable tubes  53 ′ in position at the upper surface of the silicon melt. The dopant held in the vessel-and-valve assembly  11  is released when the valve  43  in the dopant cartridge  41  is opened by the handle  45 . The dopant travels through the feed tube assembly  15 ′ and is collected on the perforated internal disk  203 . Argon gas is introduced into the feed tube assembly  15 ′ below the vessel-and-valve assembly  11 . Sublimation of the dopant will occur as the dopant is captured at the perforated disk  203 . This process results in dopant laden argon traveling past the opened isolation valve  27  and out the angled tip  91 ′ of the quartz tube  53 ′ at the upper surface of the silicon melt. After the dopant has undergone sublimation, the quartz tube  53 ′ is retracted by the actuator  19  and the feed tube assembly  15 ′ is cooled with the flow of argon gas. Finally, the assembly  10 ′ is isolated from the crystal growing apparatus  31  by closing the isolation valve  27 . 
     Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. 
     When introducing elements of the present invention or the preferred embodiments(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.