Abstract:
Infrared detection is used to monitor the temperature within a vapor transport deposition processing chamber. Changes in temperature that occur when a substrate passes an infrared detector are detected and used to precisely locate a position of the substrate within the chamber. Position correction of the substrate can also be implemented.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/598,086, filed Feb. 13, 2012, which is hereby fully incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to using infrared detection to detect the position of a substrate that is being transported within a substrate processing system such as e.g., a vapor transport deposition system. 
       BACKGROUND 
       [0003]    Photovoltaic devices such as e.g., photovoltaic modules or cells can include semiconductor and other materials deposited over a substrate using various deposition systems and techniques. One example is the deposition of a semiconductor material such as cadmium sulfide (CdS) or cadmium telluride (CdTe) thin fihns over a glass substrate using a processing chamber such as e.g., a vapor transport deposition (VTD) chamber. 
         [0004]    During the processing, it is important for a system controller to know the positions of the substrates within the processing chamber to ensure, among other things, that there is proper spacing between the substrates and to know what process the substrates are currently undergoing. Typically, the edge position of each substrate is checked before the substrate is placed into the chamber. This gives the controller an initial point to track the substrate as it progresses through the chamber. Unfortunately, the actual substrate position can be shifted/offset from the controller&#39;s calculated substrate position at different times during the process. For example, a position shift can occur as the substrate is entering the processing chamber due to the high speed transfer used to place the substrate into the chamber. Other shifts between the actual and controller calculated positions can occur due to speed changes arising from material build up on the rollers transporting the substrates. 
         [0005]    Differences between the controller calculated substrate position and the actual substrate position can adversely impact the processing or lead to mishandling of the substrates. Thus, processing systems will incorporate edge detection mechanisms, portions of which reside within the chamber. Each detection mechanism includes a laser that emits a light beam through a chamber window to a reflector located within the chamber at a point somewhere along the substrate travelling path. The reflector reflects the beam back to a detector. When the substrate passes by, the beam is interrupted, signaling the presence of the substrate. The controller can use this information to try to compensate for the new substrate position. 
         [0006]    The above detection mechanism has some shortcomings and relies on several factors to be successful, some of which cannot be controlled. For example, the chamber windows must be clean to allow the light beam to enter the chamber, be reflected back and detected. Keeping the windows clean will require additional maintenance and down time, which is undesirable. Moreover, the laser, reflector and detector must remain properly aligned, which is difficult to achieve due to the processing and vibrations within the chamber. Furthermore, there is the general need to prevent the light beam path from being blocked or disrupted by anything other than the substrate to prevent false detections, which is an onerous task. 
         [0007]    Accordingly, there is a need and desire for a better way to detect a position of substrate that is being transported within a substrate processing chamber. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  shows a system for detecting substrate position within a first portion of a processing chamber in accordance with an embodiment of the invention. 
           [0009]      FIG. 2  shows an example detector output from the detector illustrated in  FIG. 1 . 
           [0010]      FIG. 3  shows the  FIG. 1  system detecting substrate position within a second portion of the processing chamber in accordance with an embodiment of the invention. 
           [0011]      FIG. 4  shows an example detector output from the detector illustrated in  FIG. 3 . 
           [0012]      FIG. 5  shows another example system in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  shows a system  10  for detecting substrate position within a first portion  20   a  of a processing chamber (hereinafter “chamber portion  20   a ”) in accordance with an embodiment of the invention. In the example embodiment, the chamber portion  20   a  is VTD chamber included within a VTD processing system; it should be appreciated, however, that the system  10  and chamber portion  20   a  can be any processing system or chamber that utilizes physical vapor deposition, chemical vapor deposition, sputtering or the like. In the illustrated embodiment, the substrate  28  is a glass sheet and the chamber portion  20   a  is used for one or more processes needed to prepare thin film photovoltaic devices such as e.g., thin film photovoltaic modules or cells. The illustrated chamber portion  20   a  includes a first set of heaters  22 , second set of heaters  24  and rollers  26 . It should be appreciated that other pieces of equipment often found in a processing chamber (e.g., the equipment required to deposit materials) are not shown for clarity purposes. In the illustrated embodiment, the substrate  28  is transported through the chamber via the rollers  26  in the direction of the substrate flow arrow. 
         [0014]    The chamber portion  20   a  operates at a sufficient temperature (e.g., 600° C.) that can emit detectable infrared radiation. In accordance with the disclosed principles, the infrared radiation from the chamber portion  20   a  will be detected from outside the chamber, providing advantages discussed below in more detail. As such, the system  10  also comprises at least one infrared detector  30  externally mounted to the chamber portion  20   a.  In a desired embodiment, the detector  30  is mounted on a window of the chamber portion  20   a  with a focusing lens and wave filter pointed into the chamber. The detector  30  will have a “line of sight” into the chamber portion  20   a  at a point along the substrate flow path. By mounting the infrared detector  30  in this manner, the quantity of infrared radiation from within the chamber portion  20   a  at the “line of sight” can be detected and reported to a controller  50  as e.g., a varying output voltage. As is discussed below in more detail, the controller  50  will input the output voltage from the detector  30  and use the voltage to determine the location of the substrate  28 , one of its edges or gaps between substrates  28 . The controller  50  can use the determined location(s) to control, among other things, the rollers  26  to adjust the substrate&#39;s  28  position. 
         [0015]    It is desirable to place the detector  30  at points where the substrate&#39;s  28  temperature will be different from the background temperature (i.e., temperature of the heaters  22 ,  24 ) within the chamber portion  20   a.  In the illustrated embodiment, the background temperature within the chamber portion  20   a  is much hotter than the temperature of the substrate  28 . This condition may arise e.g., when a newly inserted substrate  28  has not undergone any processing within the chamber portion  20   a,  or has undergone processing at a lower chamber temperature. As is explained below with reference to  FIGS. 3 and 4 , it is also possible for the background temperature to be cooler than the temperature of the substrate  28 . According to the  FIG. 1  example, the heaters  22 ,  24  are e.g., 600° C., causing the background temperature to be at least 600° C., and the substrate temperature is&#39;e.g., less than 400° C. 
         [0016]    Referring also to  FIG. 2 , when gaps between substrates  28  are within the “line of sight” of the infrared detector  30 , the detector  30  will output a voltage within a certain voltage range corresponding to the background temperature. When the substrate  28  is within the “line of sight” of the infrared detector  30 , the detector  30  will output a voltage within a certain lower voltage range (compared to the background detection) corresponding to the cooler temperature of the substrate  28 . The differences between the two voltage ranges can be used to detect trailing and leading edges of substrates  28 . 
         [0017]    For example, after the entire substrate  28  passes by the detector  30 , there will be an abrupt rise in the detector&#39;s  30  output voltage due to the detection of the much higher background temperature. This spike in the output voltage, corresponding to the spike in detected temperature, can be used as a signal that a substrate edge was just detected. In  FIG. 2 , the rightmost portion of a spike indicates that a trailing edge of a substrate  28  has just completely passed the detector  30  while the leftmost portion of the same spike corresponds to a leading edge of a substrate  28  that has just come into the “line of sight” of the detector  30 . The exact position of the edges is determined from the location of the detector  30 . As can be seen, a gap length between substrates can also be computed from the same information illustrated in  FIG. 2 . 
         [0018]    The disclosed principles can also be used to detect whether the substrate  28  has been improperly rotated or skewed from its intended position within the chamber portion  20   a.  It is possible for the substrate  28  to rotate or shift from its intended position. Thus, the detection of a gap or edge of the shifted substrate  28  may not represent the “true” gap or orientation of the substrate  28 . Accordingly, the chamber portion  20   a  could include multiple detectors  30  at the same point (separated by a known distance) along the substrate path (see e.g., detectors  130   a,    130   b  in  FIG. 5 ). Having detections from two different detectors, separated by a known distance, the controller  50  will be able to use the output voltages from the detectors to determine if a plate was rotated or skewed. It should be appreciated that the controller  50  may also be able to detect a skewed or rotated substrate using the output voltage from one detector  30 . For example, if the controller  50  detects a gradual change in output voltage, instead of the abrupt changes illustrated in  FIG. 2 , the controller  50  can determine that something is wrong with positioning of the substrate  28 . 
         [0019]      FIG. 3  shows the system  10  detecting substrate position within a second portion  20   b  of the processing chamber (hereinafter “second chamber portion  20   b ”) in accordance with an embodiment of the invention. Similar to  FIG. 1 , an infrared detector  30  is externally mounted to the second chamber portion  20   b.  In a desired embodiment, the detector  30  is mounted on a window of the second chamber portion  20   b  with a focusing lens and wave filter pointed into the chamber. The detector  30  will have a “line of sight” into the second chamber portion  20   b  at a point along the substrate flow path. As such, the quantity of infrared radiation at the “line of sight” can be detected by the detector  30  and reported to the controller  50  as e.g., a varying output voltage. The controller  50  will input the output voltage from the detector  30  and use the voltage to determine the location of the substrate  28 , one of its edges or gaps between substrates  28 . The controller  50  can use the determined location(s) to control, among other things, the rollers  26  to adjust the substrate&#39;s  28  position. 
         [0020]    It is desirable to place the detector  30  at points where the substrate&#39;s  28  temperature will be different from the background temperature (i.e., temperature of the heaters  22 ,  24 ) within the second chamber portion  20   b.  In the illustrated embodiment, the background temperature within the second chamber portion  20   b  is much cooler than the temperature of the substrate  28 . This condition may arise after the substrate  28  has undergone some processing and is about to undergo different processing at a lower chamber temperature. According to the  FIG. 3  example, the heaters  22 ,  24  are e.g., less than 600° C., causing the background temperature to be less then 600° C., and the substrate&#39;s  28  temperature is e.g., greater than 600° C. 
         [0021]    Referring also to  FIG. 4 , when gaps between substrates  28  are within the “line of sight” of the infrared detector  30 , the detector  30  will output a voltage within a certain voltage range corresponding to the cool background temperature. When the substrate  28  is within the “line of sight” of the infrared detector  30 , the detector  30  will output a voltage within a certain higher voltage range (compared to the background detection) corresponding to the higher temperature of the substrate  28 . The differences between the two voltage ranges can be used to detect trailing and leading edges of substrates  28 . 
         [0022]    For example, after the entire substrate passes by the detector  30 , there will be a drop in the detector&#39;s  30  output voltage due to the detection of the much cooler background temperature. This drop in the output voltage, corresponding to the drop in detected temperature, can be used as a signal that a substrate edge was just detected. In  FIG. 4 , the rightmost portion of the voltage drop indicates that a trailing edge of a substrate  28  has just completely passed the detector  30  while the leftmost portion of the same drop corresponds to a leading edge of a substrate  28  that has just come into the “line of sight” of the detector  30 . The exact position of the edges is determined from the location of the detector  30 . A gap length between substrates can also be computed from the same information illustrated in FIG  4 . Moreover, as mentioned above, the controller  50  will be able to determine if a substrate has been rotated or skewed from its intended orientation (as discussed above). 
         [0023]      FIG. 5  illustrates another system  110  constructed in accordance with the disclosed principles. The system  110  comprises a plurality of infrared detectors  130   a,    130   b,    130   c,    130   d  externally mounted to windows of a processing chamber  120 . As with the other detectors  30  ( FIGS. 1 and 3 ), the illustrated detectors  130   a,    130   b,    130   c,    130   d  will have a “line of sight” into the chamber  120  at points where it is desirable to detect the presence or absence of a substrate or a substrate edge or whether the substrate has been improperly rotated. It should be appreciated that the system  110  could use more or less than three infrared detectors  130   a,    130   b,    130   c,    130   d  depending upon the application, and that the disclosed principles should not be limited to a particular number of detectors used. 
         [0024]    Voltage outputs from the infrared detectors  130   a,    130   b,    130   c,    130   d  are input into a controller  150 . The controller  150  will monitor the input voltages to detect the positions of substrates within the chamber  120  and, if necessary, adjust controls to slow down or speed up portions of the process. One example adjustment would be to control the speed of rollers within the chamber  120  to alter the position of certain substrates  10  keep consistent gaps between the substrates. It should be appreciated that the monitoring and detection of temperature changes will follow the principles discussed above with respect to  FIGS. 2 and 4 . In the illustrated example, the controller  150  could use the inputs of detectors  130   a,    130   b,  positioned within a know distance from each other, to determine if any substrate has been rotated or skewed. 
         [0025]    The disclosed systems  10 ,  110  will experience improved cycle times in the process e.g., a VTD process, because of accurate substrate detection. It should also be appreciated that using infrared detectors reduces the likelihood of false detections and other failures experienced by conventional substrate detection mechanisms using lasers, light reflectors and light detectors. The disclosed systems  10 ,  110  are better suited for a deposition or sputtering environment and have many advantages over other detection mechanisms. For example, the systems  10 ,  110  disclosed herein will not require clean chamber windows because infrared wavelengths pass through typical deposits used within the chamber. Thus, additional maintenance and down time will not be required. Moreover, the alignment issues experienced by detection mechanisms relying on lasers, light reflectors and light detectors will not exist in the systems  10 ,  110  disclosed herein because a reflector is not required. As can be seen, one major advantage of the disclosed systems  10 ,  110  is that there is no need to ensure a clean beam path into the chamber because detection is being based on temperature and not reflected light. 
         [0026]    Details of one or more embodiments are set forth in the accompanying drawings and description. Other features, objects, and advantages will be apparent from the description, drawings, and claims. Although a number of embodiments of the invention have been described, it will be understood that various modifications can be made without departing from the scope of the invention. Also, it should also be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features and basic principles of the invention. The invention is not intended to be limited by any portion of the disclosure and is defined only by the appended claims.