Abstract:
A method of continuously measuring an elevation and shape of an unmelted polycrystalline silicon island during a silicon meltdown process. The method comprises projecting a focused bright light on the silicon island to produce a bright dot on the silicon island. The method also includes electronically determining an elevation and a shape of the silicon island by tracking the bright dot during the meltdown process.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to a system and method for measuring and monitoring the elevation and shape of a silicon island during charge meltdown, granular silicon feeding in a crystal grower used to grow mono-crystalline silicon ingots. 
     The elevation and shape of the silicon island is critical to the quality of the silicon meltdown process which is in turn essential to the success of crystal growth from the silicon melt. The elevation and shape are affected by many variables in the meltdown process such as heater powers, granular poly feed rates, feeding locations, crucible positions, etc. To better ensure that the crystal growth is performed under optimal conditions, the silicon island should be measured and continuously monitored throughout the meltdown process. This can be difficult because the elevation of the silicon island is constantly changing and the shape of the island is also constantly changing and has very complex variations. 
     One existing method of measuring and monitoring a silicon island includes using a photo multiplier tube or charge-coupled device (CCD) cameras with a conventional light source (i.e., standard LEDs or the background radiation in the grower). This particular method, however, is not accurate enough to satisfy the control needs and is not capable of monitoring the silicon island in all meltdown conditions. Another method includes the use of a laser range finder or similar device. However, this method is not suitable for use in a crystal growth furnace because the laser beam generates reflection or scattering signals from the windows and heat shield on the furnace, causing significant errors in range finder measurements. 
     Thus, there exists a need for an effective means of continuously measuring and monitoring the silicon island during the meltdown process regardless of the conditions inside or outside of the crystal growth furnace. Furthermore, such means should not affect the meltdown or crystal growth process or pose harm to the operators. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention includes a method of continuously measuring an elevation and a shape of an unmelted polycrystalline silicon island during a silicon meltdown process. The method comprises projecting a focused bright light on the silicon island to produce a bright dot on the silicon island. Further, an elevation and a shape of the silicon island are electronically determining by tracking the bright dot during the meltdown process. 
     In another aspect, the present invention includes a system for use in combination with apparatus for growing a silicon crystal from a silicon melt to measure an elevation and a shape of an unmelted polycrystalline silicon island of the silicon melt during a silicon meltdown process. The apparatus includes a housing having an interior in which the silicon melts. The system measures an elevation and a shape of an unmelted polycrystalline silicon island. The system comprises a focused bright light source directed into the interior of the housing for projecting a bright dot onto the silicon island. Further, the system includes a camera directed into the interior of the housing for generating a continuous image pattern of a portion of the silicon island including the bright dot. In addition, the system includes a programmable controller remote from the housing for determining a location and an elevation of the bright dot and continuously calculating a shape and an elevation of the silicon island therefrom based on the image pattern. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a crystal grower including apparatus of the present invention for preparing a molten silicon melt from polycrystalline silicon; 
         FIG. 2  is a side view of a camera and a laser source of the apparatus 
         FIG. 3  is a block diagram of a control unit and camera of the apparatus of  FIG. 1 ; 
         FIG. 4  is a fragmentary section view of the crystal grower including a schematic of the apparatus of the present invention; 
         FIG. 5  is a partial perspective of the apparatus and the crystal grower including a schematic of optical shields of the present invention; 
         FIG. 6  is an illustration of a second embodiment of the present invention illustrating a side view of the camera and laser source of the apparatus and a schematic of an elevation control scanner; and 
         FIG. 7  is an expanded view of the scanner of  FIG. 6 . 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now referring to the drawings, and in particular to  FIG. 1 , an apparatus of the present invention, generally indicated at  11 , is illustrated for use with a crystal grower, generally indicated at  13 , of the type used to grow mono-crystalline silicon ingots by a Czochralski method. The crystal grower  13  includes a housing, generally indicated at  15 , for isolating an interior including a crystal growth chamber  17 . A quartz crucible  19  is supported in the growth chamber  17  and contains molten semiconductor source material M from which the mono-crystalline silicon ingot is grown. A heater power supply  20  energizes a resistance heater  21  surrounding the crucible  19  to form the molten silicon M in the crucible. Insulation  23  lines the inner wall of the housing  15 . A crucible drive unit  25  rotates the crucible  19  about a vertical axis X, as indicated by the arrow, and raises and lowers the crucible during the growth process. 
     As illustrated in  FIG. 2 , the apparatus  11  comprises a laser or focused bright light source  27 , and a camera system, generally indicated at  29 . The camera system includes a camera  31  and detection and tracking software. The focused light source  27  is mounted on the housing  15  of the crystal grower  13 . The light source  27  comprises a short wavelength laser (such as a green/blue laser) or a focused ultra-bright light source (high powered green/blue LEDs). A green laser (e.g. about 532 nm), blue diode laser or high power blue LED (e.g. about 405 nm) is used to avoid a cut-off wavelength of a typical heat shield window/filter on a crystal grower. Generally, the cut-off wavelength is from about 600 nm to about 650 nm for a shield window/filter. Because these light sources have wavelengths below the cut-off wavelength, they will achieve a high transmission and high signal to noise ratio through a protective window  33  and into the crystal grower  13 , which has a strong ambient radiation. It is understood that alternative light sources could be used without departing from the scope of the invention. For instance, an LED having a wavelength less than the cut-off wavelength of the window  33  (e.g., less than about 600 nm) can also be used provided the LED is sufficiently bright. In the preferred embodiment, the requirement for the optical output of the focused light source  27  is typically greater than about 10 mW. However, light sources having optical outputs of 10 mW or less are also within the scope of the invention. 
     Referring to  FIG. 2 , the laser  27  also includes a protection shield  35 . The protection shield is attached to the camera  31  and mounted on a camera window  37 . The shield  35  consists of an elongate tube open at both ends for receiving the laser beam  39  at a first end  41  and permitting the beam to exit at a second end  43 , opposite the first end. The side walls of the tube are made of a suitable material for preventing any misdirected or reflected laser beams from being projected to the surrounding environment. To protect operators from laser radiation, optical shields  45  with corresponding wavelengths can also be installed on all the observation and auxiliary windows as shown in  FIG. 5 . Those skilled in the art will be familiar with suitable protection and optical shields for use in the present invention. 
     The two-dimensional camera  31  is also mounted on the housing  15  of the crystal grower  13  and is in electrical communication with a control unit  47  (see  FIG. 1 ). The camera  31  is represented as a box to indicate that one or more than one camera may be used without departing from the scope of the present invention. As is well known in the art, the control unit  47  is electrically connected to various operating components of the crystal grower  13  to control operation of the grower. The camera  31  is mounted in a viewport  49  of the crystal grower housing  15  and is aimed generally at an intersection of the central axis X of the grower and an upper surface U of the molten silicon M in the crucible  19 . 
     For example, the camera  31  may be mounted at an angle from about 15° to about 34° measured with respect to the central axis X of the crystal grower  13 . The camera  31  is preferably a monochrome charge coupled device (CCD) camera, such as Sony XC-75 CCD video camera having a resolution of 768×494 pixels. 
     Additionally, depending on the type of light source  27  used, a corresponding laser line interference filter (laser) or band pass filter (LED) can be used on the detection CCD camera  31  so the camera selects dot signals that are not affected by most of the ambient radiation in the crystal grower  13 . The type of filters suitable for use in the present invention will be known to those skilled in the art. 
       FIG. 3  illustrates a preferred embodiment of the control unit  47  in block diagram form. The camera  31  communicates video images via line  51  (e.g., an RS-170 video cable) to a vision system  53 . The vision system includes a video image frame buffer  55  and an image processor  57  for capturing and processing the video images. As an example, the vision system  53  is a Cx-100 Imagenation Frame grabber or a Cognex CVS-4400 vision system. In turn, the vision system  53  communicates with a programmable logic controller (PLC)  59  via line  61 . In one preferred embodiment, the PLC  59  is a Model 575 PLC or a Model 545 PLC manufactured by Texas Instruments and line  61  is a communication interface (e.g., VME backplane interface). 
     The vision system  53  also communicates with a video display  63  via line  65  (e.g., an RS-170 RGB video cable) for displaying the video image generated by the camera  31  and with a computer  67  via line  69  (e.g., an RS-232 cable) used to program the vision system. As illustrated in  FIG. 2 , the PLC  59  communicates with one or more process input/output modules  71  via line  73  (e.g., an RS-485 cable). An operator interface computer  75  also communicates with the PLC via line  77  (e.g., an RS-232 cable) to permit the crystal puller operator to input desired operating parameters to the PLC and/or to retrieve operating information from the PLC during operation of the crystal grower  13 . 
     The detection and tracking software detects and tracks the location of a bright dot produced by the beam  39  of the light source  27  on the silicon melt M. The software continuously calculates the actual location and elevation of the dot. From theses calculations, the software can continuously measure and monitor the elevation and shape of the silicon island I. The computer  67  is programmable such that it can access and execute the detection and tracking software. Software of the type described above is commonly known to those skilled in the art. Therefore, no further explanation is needed. 
     In a second embodiment, the apparatus  11  includes an elevation control scanner  81  attached to the end of the laser shield  35  and arranged to scan in one or more directions to provide real time 3D measurements of the silicon island I (see  FIGS. 6 and 7 ). A first mirror  83  reflects the beam  39  from the light source  27  to a second mirror  85  having a piezoelectric motion drive (not shown) on its back configured to reflect the beam  39  along a radius of the island I. The beam  39  can be arranged to scan along the island radius at a predetermined speed or controlled by the camera PC  67 . 
     In operation, the light source  27  of the apparatus  11  projects a bright green or blue laser dot, clearly visible in the ambient radiation of the crystal grower chamber  13 , at a desired location on the silicon island I. Typically, this location is where the elevation is most representative of the changing silicon island I and it is determined based on a slope of a surface of the silicon island. Preselecting the laser dot location allows the operator to optimize tracking and controlling quality of the silicon island I. 
     The detection and tracking software detects and tracks the dot and continuously calculates a location and an elevation of the dot. Because the silicon island I moves rotationally and vertically during the meltdown process, the dot accurately correlates with the elevation and shape of the moving silicon island at a certain time and location. Through this continuous detecting, tracking and calculating process, the elevation and shape of the silicon island I at desirable locations are measured, monitored and controlled. 
     In the alternative embodiment, the dot can be scanned by the elevation control scanner  81  in one or more dimensions to provide 3D measurements of the silicon island I. Since the island I is rotating at a predetermined rotation rate during meltdown, the continuous tracking of the scanning laser dot on the island radius provides real time 3D shape measurements. This can be very helpful to provide more dynamic meltdown control. 
     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.