Abstract:
A device for monitoring a melt for the production of crystals. A camera is provided which images at least portions of the surface of the contents of a crucible. An evaluating device is used to evaluate the camera&#39;s images with respect to solid and liquid portions of the surface of the crucible contents.

Description:
INTRODUCTION AND BACKGROUND 
     The invention relates to a device for monitoring a melt for the production of crystals and a process therefor. 
     When a given material, for example, Si polymaterial is thoroughly melted in a quartz crucible, the transition from a solid to a liquid state does not, as a rule, occur abruptly, but rather, gradually. In the actual practice of creating crystals it is very important that the given melt state be precisely known or identifiable, since this state dictates the procedural steps that are to follow. 
     There are experienced melt specialists who are able to precisely infer the state of the melt from the surface appearance of a crucible. Also known is the automatic identification of a melt state using pyrometers as sensors, though this results in an unreliable kind of regulation involving long time constants. 
     Various processes have already been suggested for an improved automatic regulation of the drawing process of a monocrystal. For example, a process for drawing a monocrystal from the melt is known in which the individual crystals are drawn up while the data influencing the drawing process, which are based on numerous conditions, are recorded and compared with other data (EP 0 536 405 A1). In the process, a laser beam, for example, strikes the surface of molten material located in a crucible. Identification of a reflected laser beam allows the position of the molten surface to be determined, and the crucible is elevated according to the difference between the measured position and a predetermined position. However, this known process does not permit reliable process monitoring during the melting phase. 
     Also known is an optical system or process for regulating the growth of a silicon crystal in which the diameter of a silicon crystal drawn from a melt is measured with the assistance of a television camera; here the surface of the melt exhibits a meniscus which is visible as a bright area in close proximity to the silicon crystal (EP 0 745 830 A2). In this system, a camera is used to produce an image pattern of a portion of the bright area near the silicon crystal. The characteristics of the image pattern are then detected. A characteristic of the visual pattern would be, for example, the light intensity gradient. After this, one edge of the bright area is defined as a function of the detected characteristics. Then an outline is defined which closes the defined edge of the bright area; finally the diameter of the defined outline is determined, and the diameter of the silicon crystal is determined as a function of that defined outline diameter. A disadvantage of this known system is that its accuracy is insufficient in several applications and external influences, in particular, are not adequately taken into account. 
     To eliminate these disadvantages it has already been suggested to add to the camera imaging the first area of the crystal a second camera which images a second portion of the crystal; the diameter of the crystal is determined in an evaluation circuit using the images of both cameras (unpublished patent application 197 38 438.2). In this manner it is possible to precisely record the actual crystal diameter in all phases of the cultivation process. The melting process itself cannot be monitored with the proposed device. 
     An object of the invention is to overcome the problem of monitoring the melting process for the basic materials from which monocrystals are drawn. 
     SUMMARY OF THE INVENTION 
     The above and other objects of the invention can be achieved with a device for monitoring a melt for the production of crystals, wherein a camera is provided which images at least portions of the surface of the contents of a crucible and with an evaluating device provided which evaluates the camera&#39;s images with respect to solid and liquid portions of the surface of the crucible contents. 
     A feature of the invention resides in a process for monitoring a melt serving the production of crystals, wherein: 
     a) at least portions of the surface of the contents of the crucible are imaged with a camera; and, 
     b) the image made by the camera is evaluated with respect to solid and liquid components on the surface of the contents of the crucible. 
     The advantage achieved with the invention particularly consists in shortening the process time, avoiding overheating of the melt and the crucible, and minimizing the O 2  content of the melt. The use of a special sensor thus makes it possible to implement a melt control before complete fusion of the molten material. In addition, individual differences can be identified per batch and taken into account. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     An example of embodiment of the invention is shown in the drawings and is described below in greater detail. Shown are: 
     FIG. 1 is a general depiction of the device according to the invention; 
     FIG. 2 a  is a top view of a crucible containing molten material; 
     FIG. 2 b  is a graphic depiction of the brightness distribution along a horizontal line on the crucible; 
     FIG. 3 is an enlarged depiction of the measuring window of a camera directed at the surface of a crucible; and 
     FIG. 4 is a flow chart for the process of identifying solid or liquid components of the melt. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     FIG. 1 shows a device which makes it possible to identify the fusion of molten material at an early stage. This device is based on an optical principle according to which those areas of a melt that are liquid emit less visible light than areas that are still solid or that have solidified out of the melt. Instead of, or in addition to, this brightness principle, the principle of hue or chrome and/or color saturation can be used—inasmuch as molten materials differ from non-molten materials not only in brightness but also in chrome and/or color saturation. 
     To identify the brightness of the melt, a camera  2  is provided; the camera can be a CCD video camera. This camera  2  is positioned diagonally over the crucible  7  in which the melt  3  is located. The camera  2  is used to observe the surface  4  of the melt  3 , or at least a portion of this surface  4 . 
     The crucible  7  can be moved by means of a shaft  5  and a gear  6  using a motor  9 , for example, from above downwards. It is also possible to make it rotate. The crucible  7  is located in a housing consisting of an upper part  12 , a middle part  13 , and a lower part  14 . The lower part  14  is furnished with two gas outlets  25 ,  26 . An electrical heating system  16  is positioned around the crucible  7  and is provided with electrical energy from a heater current source  17 . Parts that are not depicted can brought into the proximity of the melt  3  with a threaded rod  18  driven by a motor  19 . The threaded rod  18  is enclosed in a cylindrical unit or pipe  23  belonging to the upper part  12 , which is provided with a gas inlet opening  24 . 
     The output signals of camera  2  are fed to an image evaluating unit  37 , which exchanges data with a control unit  38 . This control unit  38  can be influenced by means of an operating unit  39 , for example, a keyboard. The control unit  38  makes it possible to control the drive  6 ,  9  for the crucible  7  and to control the heater current source  17 . 
     FIG. 2 a  depicts the crucible  7  in a view from above. Located in the crucible  7  is liquid material  50 , e.g., liquid silicon, in which there are several islands  51  to  54  of hardened material. The liquid melt  50  emits less brightness than the hardened islands  51 - 54 , because the hardened material better reflects visible light than the liquid material. As a result, the islands  51 - 54  have a brighter appearance than the surrounding liquid material  50 . 
     Reference numeral  55  designates a measuring window which corresponds to the viewing angle of the camera  2 , i.e., the camera  2  records the area of the crucible  7  defined by the measuring window  55 . Reference numeral  56  designates a measuring line whose meaning will be explained below. 
     In FIG. 2 b  the brightness of the crucible contents is depicted in an x coordinate system. It will be seen that those points where solid islands  51 - 54  are located exhibit a greater brightness B 1  . . . B 4  than at those points where the liquid melt is located. 
     If only the islands  52 ,  53  through which measuring line  56  passes are taken into account, then brightnesses B 1  and B 3  in FIG. 2 b  would fall out. 
     FIG. 3 again shows the measuring window  55  with the melt  50  and the islands  51  to  54  located therein, but on an enlarged scale. Superimposed on the measuring window are a number of horizontal and vertical measuring lines  60  to  69  and  70  to  86 , so as to form a lattice. These measuring lines  60  to  69  and  70  to  86  are scanning lines and columns for the CCD camera  2 . The measuring window recorded by the CCD camera  2  is thus scanned line by line or column by column, i.e., the scanning lines and columns shown in FIG. 3 are approached in time-multiplex fashion. 
     With this kind of scanning, the brightness transitions between solid and liquid Si are recorded; this is marked by points P 1  . . . P 9  with reference to island  52 . By identifying these brightness transitions, it is possible to precisely establish the aggregate state of the surface of the Si melt. 
     If the individual points P 1  . . . P 9  are known, the surface area of the island  52  can be calculated. The surface areas of the other islands  51 ,  53 ,  54  can be calculated in corresponding fashion. This in turn creates the possibility of determining the ratio of liquid surface to solid surface. Different values can be established for these ratios, which, when reached, will result in the execution of certain procedural steps. Of particular interest here is the disappearance of the solid surface portion of the melt, since this state indicates a finished melt. Since the disappearance of solid bodies can be simulated by natural events, a predetermined waiting period is observed during which the crucible continues to rotate. Only after this waiting period is over is it assumed that the solid parts of the surface have actually disappeared; the appropriate procedural step is then initiated. 
     FIG. 4 shows the process flow of the invention in the form of a flow chart. After the startup indicated by block  100 , the surface of the melt reproduced by the CCD camera  2  is read as an image in digital form into a storage unit (not shown), as suggested by block  101 . The image thus read in is now scanned line by line and/or column by column, cf. block  102 , and monitored for brightness. Whenever the difference in brightness between adjacent points of a line and/or column exceeds a predetermined threshold value, the local coordinate of the transition point—the so-called edge—is identified and stored, cf. block  103 . 
     In this manner it is possible to locate, e.g., points P 1  . . . P 9  of island  52 . With a suitable interpolative process a path can be drawn joining these points P 1  . . . P 9 , giving the outline of island  52 . From this the surface area of the island  52  can be calculated. 
     If the islands  51  to  54  become larger or increase in number, the number of identified edges also increases. The number of identified edges is thus a measure of the molten state on the surface of the melt. If a specific melt value is established for the number of edges—cf. block  104 —a predetermined melt-solid body behavior can be defined, arrival at which will trigger a given process step. This is indicated by block  105 . A process step of this kind might be, for example, a reduction in the heat in heating element  16  or an increase in the crucible&#39;s rate of revolution. When all process steps have been executed, the process can be terminated. 
     The threshold value for the number of edges could be input via, e.g., the operating unit  39 . 
     The invention thus makes it possible to monitor and influence the individual steps in the production of a crystal, even at an early point in the process. If solid semiconductor lumps, for example, are left hanging on the wall of the crucible or if lumps drifting in the melt threaten to touch equipment above the melt, and to damage it, the heating output, for example, and/or the rate of rotation of the crucible can be modified appropriately in order to accelerate the melting process. 
     If the process according to the invention establishes that the solid/liquid ratio=0, a waiting period of 1 to 5 minutes is introduced, until the next process step is introduced, since, as mentioned above, disruptive influences can simulate a complete liquefaction of the melt when some unmolten parts still remain in it. This waiting period assures that all material has actually melted and that the process of drawing a crystal can begin. The length of the waiting period depends on the crucible&#39;s rate of rotation. If the crucible is rotated quickly, the waiting period can be shortened, since the increased rate of rotation will more rapidly bring the unmelted lumps into the area where they are melted. 
     Further variations and modifications of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto. 
     German priority applications 197 38 438.2 and 198 17 709.7 are relied on and incorporated herein by reference.