Abstract:
The present invention is a pre-aligner capable of determining the center of a wafer by casting light onto a wafer that is positioned above a charge-coupled device (CCD). The pre-aligner performs this operation by directing light emitted from a single LED simultaneously onto the wafer and the CCD. The light emitted from the LED is directed through a light guide in order to direct the light onto the wafer and CCD. A lens collimates the light exiting the light guide such that the light, as it passes the wafer&#39;s edge, is substantially perpendicular to the wafer&#39;s edge. The light guide may be removably secured to the pre-aligner housing for easy installation removal. The pre-aligner is capable of self-calibrate the LED.

Description:
CLAIM OF PRIORITY  
       [0001]    This application claims priority to U.S. Provisional Patent Application Serial No. 60/380,993, entitled “Pre-Aligner Light Source,” filed with the U.S. Patent and Trademark Office on May, 16, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention generally relates to a pre-aligner for centering a wafer prior to placing the wafer in a processing tool. More specifically, an embodiment of the present invention aligns a wafer by directing the light emitted from a single LED onto a charge coupled device (CCD) light array with a “light guide.” 
         BACKGROUND OF THE INVENTION  
         [0003]    In the semiconductor industry, it is common to pre-align a semiconductor wafer as part of readying the wafer for processing. The pre-alignment operation conventionally includes locating and precisely positioning the geometric center of the wafer. Once this operation is complete, the wafer is then placed in a selected orientation related to the orientation of its crystalline structure. This pre-alignment procedure is commonly completed as a separate operation before the wafer is transported to a processing apparatus for providing a desired finishing or processing step.  
           [0004]    Conventionally, a robot removes the individual wafers, one at a time, from a transportation carrier and places them at the pre-alignment station. After the wafer is aligned, i.e., after the wafer is properly oriented and its geometric center is located, the wafer may be placed back into the transportation carrier or in a processing carrier in the pre-aligned condition.  
           [0005]    It will be appreciated from the above that precise pre-alignment not only is desirable but can be a major factor in determining ultimate reliability of the integrated circuitry produced on a wafer. Many operations require very accurate alignment, and accurate pre-alignment reduces the mechanical and operational constraints in achieving such alignment.  
           [0006]    [0006]FIG. 1 illustrates a conventional pre-aligner  10 . This pre-aligner  10  includes a “light bar” as a light source. A light bar  12  typically consists of a row of multiple LED&#39;s  13 . Using a light source which consists of multiple, individual LED&#39;s  13 , has several disadvantages. For example, there is often a mismatched brightness between individual LED&#39;s  13 . The mismatched brightness may cause a variation of the image intensity focused at the CCD  14 . A non-uniform image intensity may cause a non-linear detection of the wafer&#39;s edge  16 . Other problems may surface as the pre-aligner  10  ages. Specifically, the brightness of the individual LED&#39;s  13  will fade at different times. This fading will eventually lead to an uneven or non-uniform illumination. Accurate centering of the wafer requires even or uniform illumination. Therefore, a pre-aligner  10  using a conventional “light bar”  12  requires calibration in the factory which must be repeated every time the illumination becomes non-uniform. A diffuser  18  is commonly used to even out the distribution of light and minimize any effect stemming from the non-uniform illumination. The diffuser  18  is also used to control the total amount of light emitted by the array of LED&#39;s  13 . Further, because the light emitted from the light bar  12  is uncollimated (contains light rays that are not parallel to each other), a camera lens  20  is commonly used to focus an image from the wafer  11  onto the CCD  14 . Such a lens  20  is expensive.  
           [0007]    Although a pre-aligner  10  using an array of LED&#39;s  13  as a light source commonly provides high accuracy, there are several disadvantages. By way of example only, it is necessary to perform intricate factory adjustments, tune the diffuser  18  to achieve uniform illumination from the multiple LED&#39;s  13 . Additionally, since the light is uncollimated as it passes the wafer edge  16 , the pre-aligner  10  is very sensitive to vertical runout of the wafer  11 . Because of the non-vertical light, vertical runout of the spindle, which moves the wafer edge  16  up and down, also moves the shadow along the CCD  14 . This up and down movement is indistinguishable from the shadow movement caused by horizontal wafer movement, which is the variable being measured.  
           [0008]    In addition to electrical disadvantages, there are also mechanical limitations to the pre-aligner  10 . The lighting assembly is vulnerable to damage. The light bar  12  mounts in a housing  18 , which is suspended above the body  22  of the pre-aligner  10 . Thus, the housing can be easily bent during shipping and handling, or by a collision with a robot arm. Additionally, because the light bar  12  is mounted above the CCD  14 , electrical wiring must be routed out of the main body  22  of the pre-aligner  10  and up into the optical housing  18 .  
           [0009]    Some conventional pre-aligners, such as the pre-aligner  50  shown in FIG. 2, use light emitted from a single LED  52  to cast a shadow of the wafer edge  54  directly onto a CCD  56 . Using a single LED  52  mounted above the wafer  51  (i.e., to a printed circuit board  58 ) and the CCD  56  has several disadvantages. For example, the pre-aligner  50  is very sensitive to vertical mount of the wafer  51 . Similar to the light housing  12  shown in FIG. 1, the light emitted from the single LED  52  as it passes the wafer edge  54  is uncollimated. Thus, movement of the wafer up and down between the LED  52  and the CCD  56  moves the shadow casted onto the CCD  56 .  
           [0010]    Using light from a single source also has a magnification effect. As a result of the non-vertical light emitted from the LED  52 , the movement of the shadow cast onto the CCD  56  is greater than the movement of the actual wafer edge  54 . This exacerbation effect must be compensated for by added complexity in the software that determines the location of the center of the wafer.  
           [0011]    The single LED  52  is also an inefficient use of light energy. Light emitted from the LED  52  fans out to a circular pattern by the time the light reaches the CCD  56 . Thus, a majority of the light actually falls directly on the CCD  56  and not the wafer  51 . Thus, the LED  52  must be closely located above the CCD  56  to provide adequate illumination. However, a LED  52  that is closely positioned to the wafer causes a spread of the shadow of the wafer edge  54  on the CCD  56 . This effect is greatly exaggerated in FIG. 2 for illustration purposes. The “spread shadow” effect increases the uncertainty in determining the actual radius of the wafer edge  54  from the center of the spindle (not shown). As shown in FIG. 2, erroneously, the CCD  56  will measure the wafer edge  54  by a distance d. Further, as the LED  52  ages and its brightness diminishes, the sensitivity of the pre-aligner  50  will change over time.  
           [0012]    Another conventional pre-aligner, such as pre-aligner  80  shown in FIG. 3, uses a laser diode  82  as a light source. A pre-aligner, which uses a laser diode  82  as a light source, has several advantages. With a single light source there is no possibility of a non-uniform light source. In the case of using a laser diode  82  as a light source, the laser circuit produces the same light output throughout the life of the product. Thus, no calibration is necessary to maintain an even or uniform light source. Additionally, because the light passing the wafer edge  84  is vertical, vertical runout of the spindle, which makes the wafer edge  84  move up and down, does not move the shadow along the CCD  86  in response to this runout. Thus, measurements of the movement of the shadow are more accurate. In addition, the vertical light path makes the shadow on the CCD  86  move exactly the same distance as the wafer edge  84 .  
           [0013]    In operation, light from the laser diode  82  naturally fans out in a circular pattern. The cylindrical lens  88  focuses the light into a narrow stripe by allowing the light to continue to diverge along the long axis of the CCD  86 , greatly reducing the divergence of the light rays perpendicular to the long axis of the CCD  86 . The light then encounters the two spherical lenses  90   a  and  90   b , which are intended to refract the light to follow a vertical path past the wafer  81  and onto the CCD  56 .  
           [0014]    However, there are several disadvantages of a laser diode pre-aligner system similar to the pre-aligner  80 . For example, the coherent nature of laser light results in speckles, or dark and light spots, on the CCD  86 . These dark spots cause added uncertainty in the location of the shadow edge. Additionally, the laser diode  82  and associated regulating electronics are relatively expensive. Further, the narrow angle of light spread from the laser diode  82  requires that the light housing  92  be substantially taller than other optical systems (e.g., LED&#39;s). Similar to the previously mentioned pre-aligners, the laser diode  82  is mounted above the CCD  86  and therefore, electrical wiring must be routed out of the main body  94  of the pre-aligner  80  and up into the optical housing  92 .  
           [0015]    Accordingly, there is a need for a pre-aligner utilizing an LED as a light source, yet having the accuracy of a laser diode system. The present invention provides such a system.  
         SUMMARY OF THE INVENTION  
         [0016]    One aspect of the present invention is to provide a pre-aligner that utilized light emitted from a single LED as its primary light source. As a light source that comprises a single LED reduces or eliminates the possibility of producing an uneven or non-uniform array of light.  
           [0017]    It is another aspect of the present invention to provide a pre-aligner that may self-adjust the light source once the pre-aligner is installed. In one embodiment, the pre-aligner includes an LED brightness calibration function to maximize the energy efficiency of the LED. This calibration process may be performed upon startup of the pre-aligner or during routine intervals.  
           [0018]    Yet another aspect of the present invention is to provide a pre-aligner that incorporates a “light guide” assembly that directs the light emitted from the LED simultaneously partially onto a wafer edge and a sensor. In a preferred embodiment, the light guide directs the light onto a wafer, that extends over the sensor the wafer casts a shadow onto the sensor, which can determine the radial turnout of the wafer based upon the amount of light that strikes the sensor.  
           [0019]    A further aspect of the present invention is to provide a light guide assembly that may be produced inexpensively. In one embodiment, the light guide housing is manufactured from plastic. In another embodiment, the light guide may be easily removed and replaced.  
           [0020]    Still another aspect of the present invention is to provide a pre-aligner whereby little or no recalibration of the LED is required after the light guide assembly has been removed and replaced. In one embodiment, the LED is held in place by a magnet that centers itself within the light guide housing. Thus, the light guide assembly may be removed from the pre-aligner during shipment, allowing a simpler, more compact packaging for shipment, which may be better protected in shipping.  
           [0021]    Yet another aspect of the present invention is to provide a pre-aligner with a simplified electrical system. In one embodiment, the LED and the CCD are electrically connected to the same printed circuit board (PCB). By doing so, the separate cables and PCB conventionally associated and dedicated to the LED may be eliminated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a representative drawing of an embodiment of a pre-aligner, according to the prior art;  
         [0023]    [0023]FIG. 2 is a representative drawing of another embodiment of a pre-aligner, according to the prior art;  
         [0024]    [0024]FIG. 3 is a representative drawing of still another embodiment of a pre-aligner, according to the prior art;  
         [0025]    FIGS.  4 A- 4 B; FIG. 4A is a representative drawing of an embodiment of the pre-aligner, according to the present invention; FIG. 4B is a cross-sectional view of the pre-aligner shown in FIG. 4A, along view line A-A;  
         [0026]    FIGS.  5 A- 5 B; FIG. 5 is a representative drawing of another embodiment of the pre-aligner, according to the present invention; and FIG. 5B is a cross-sectional view of the pre-aligner shown in FIG. 5A, oblong view line A-A;  
         [0027]    [0027]FIG. 6 is a representative drawing of an embodiment of the present invention;  
         [0028]    [0028]FIG. 7 is a representative drawing of another embodiment of the present invention;  
         [0029]    [0029]FIG. 8 is an assembly view of an embodiment of the light guide assembly, according to the present invention.  
         [0030]    [0030]FIG. 9 is a graphical representation of the voltage input of the CCD during one revolution of the workpiece; and  
         [0031]    [0031]FIG. 10 is a graphical representation of the data shown in FIG. 9. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    The present invention, and all the embodiments, will now be described with reference to FIGS.  8 - 10 . In a preferred embodiment, the pre-aligner  100  uses the method of casting a shadow of a wafer edge directly onto a charge coupled device (CCD) linear array.  
         [0033]    As shown in FIGS.  4 A- 4 B, the pre-aligner  100  comprises an LED  102 , a PCB  104 , a CCD  106 , and a light guide  108 . An LED  102  is commonly known within to one skilled in the art and does not require further disclosure. The LED  102  is preferably the primary light source for the pre-aligner  100 . In a preferred embodiment, a single LED  102  is the only light source for the pre-aligner  100 . It is within the scope and spirit of the invention for the pre-aligner  100  to use more than one LED  102 . As shown in FIG. 4A, the LED  102  has a light emitting portion  103 . The LED  102  is mounted to the PCB  104  such that the light-emitting portion  103  is facing upward—toward the light guide  108 .  
         [0034]    As previously discussed above, the LED and CCD in a conventional aligner must be mounted opposite, or facing, each other so the light emitted from the LED will shine onto the CCD. To significantly reduce or eliminate the wiring conventionally associated with a pre-aligner device, the LED  102  and the CCD  106  are mounted on the same PCB  104 . It is possible to mount the LED  102  and CCD  106  on the same circuit board  104  since the light emitted from the LED  102  is reflected back towards the wafer by a light guide  108 . Thus, the LED  102  does not have to directly face the CCD  106 . The CCD  106  is known to one skilled in the art and does not require further disclosure. The light guide  108  is preferably made from optical grade clear acrylic sheet. It is within the scope and spirit of the invention to manufacture the light guide  108  from other material.  
         [0035]    In general, the light guide  108  defines an enclosed reflective enclosure that includes an inlet  114  and an outlet  112 . When the pre-aligner  100  is assembled, the inlet  114  of the light guide  108  is preferably located substantially above the LED  102  while the outlet  112  of the light guide  108  is preferably positioned substantially over the CCD  106 . The reflective enclosure preferably includes a light pipe portion  110 , a first mirrored surface  20 , a second mirrored surface  122 , and a lens  112 . In a preferred embodiment, the inlet  114  is comprised of the light pipe portion  110  and includes an entry window  124  positioned directly above the light-emitting portion  103  of the LED  102 .  
         [0036]    The light emitted from the LED  102  passes through the entry window  124  and enters the light pipe  110 . As shown in FIG. 4B, the light emitted from the LED  102  is repeatedly reflected within the light pipe  110  as it travels upward towards the first mirrored surface  20 . In optics, this is know an total internal reflection. The first mirrored surface  20  is located above the light pipe  110  and the LED  102 . The location of the first mirrored surface  20  allows substantially all of the light reflected within the light pipe  110  to strike the first mirrored surface  20 . The first mirrored surface  20  is preferably placed at a 45° angle with respect to the longitudinal axis of the light pipe  110 . As shown in FIGS.  4 A- 4 B, the light guide  100  is substantially “cone”-shaped and thus the second mirrored surface  122  is located substantially opposite the first mirrored surface  20 . The relative angle degree between the second mirrored surface  122  and the first mirrored surface  120  is preferably a 90° angle. However, one skilled in the art will appreciate the mirrored surfaces  120  and  122  may be positioned at various angles within the light guide  108  and achieve a similar result. For example, FIGS.  5 A- 5 B illustrate that the relative angle degree may be less than 90°.  
         [0037]    Light paths L 1  and L 2  are shown in FIG. 4A to illustrate examples of the path the light ma travel from the LED  102 , through the light guide  108 , and eventually onto the CCD  106 . For example, the light path L 1  travels from the LED  102  into the light pipe  110 . The light pipe  110  guides the light along light path L 1  until the light strikes the first mirrored surface  20 . The light path L 1  reflects off the first mirrored surface  20  towards the second mirrored surface  122 . The light path L 2  reflects off of the second mirrored surface  122  towards the outlet  112 . The light path L 1  exits the light guide  108  and strikes the top surface of the wafer  111 . The light path L 2  follows a similar route as the light path L 1 . However, the light path L 2  exits the light guide  108  and strikes the top surface of the CCD  106  instead of the wafer  111 .  
         [0038]    As shown in FIG. 4A, the light paths L 1  and L 2  exit the light guide along a path that is substantially perpendicular to the top surface of the CCD  106  and the wafer  111 . The wafer  111  is preferably situated close to the CCD  106 —shown as distance D in FIG. 4A. Minimizing the distance D between the bottom of the wafer  111  and the top surface of the CCD  106  improves the accuracy of the pre-aligner  100 . Casting a shadow onto the CCD  106  is less precise the further the light has to travel. In a preferred embodiment, the wafer  111  is located in a rotating chuck (not shown) such that the wafer&#39;s edge  161  hangs over at least a portion of the top surface of the CCD  106  and is located below the output  112  of the light guide  108 . When the wafer  111  interrupts the light exiting the light guide  108 , it casts a shadow on the CCD  106 . For example, the wafer  111  prevents the light path L 1  from reaching the CCD  106  and thus casts a shadow on the CCD  106  at that location.  
         [0039]    The outlet  112  of the light guide  108  includes a lens  109  (see FIG. 8) seated within the outlet  112 . In one embodiment, the lens  109  has a convex surface that faces the wafer  111  and the CCD  106 . The lens  109  converges the outgoing light (e.g., light paths L 1 , L 2 , and L 3 ) exiting the outlet  112  of the light guide  108 . Thus, the light exiting the light guide  108  (e.g., light paths L 1 , L 2 , and L 3 ) travels along a path that is substantially perpendicular to the top surface of the wafer  111  and the top surface of the CCD  106 .  
         [0040]    FIGS.  5 A- 5 B illustrate the light paths L 1 , L 2 , and L 3  to further demonstrate how the pre-aligner  100  aligns a wafer  111 . As shown in FIG. 5A, the light path L 1  travels through the light pipe  110 , strikes the first mirrored surface, reflects towards the second mirrored surface  122 , and exits through the outlet  112  of the light guide  108 . The light path L 1  exits the outlet  112  along a substantially vertical path that strikes the CCD  106 . The light path L 2  travels through the light guide  108  in a similar fashion—exiting the light guide  108  and striking the CCD  106 . The light path L 2  travels close to the wafer edge  161  along the way to the CCS  106 . L 3  also travels through the light guide  108 . However, the light path L 2  exits the light guide  108  and strikes the top surface of the wafer  111 .  
         [0041]    If the wafer  111  is misaligned on the rotating chuck  150  (see FIG. 6) the wafer edge  161  will oscillate left and right (from the perspective of FIG. 5A) and occasionally interrupt the light path L 2 . When the wafer  111  is aligned on the rotating chuck  150  the wafer edge  161  will prevent a constant number of light paths from striking the CCD  106 . In other words, a similar number of light paths will strike the CCD  106  while the wafer  111  is rotating. FIG. 5B illustrates that the light paths will repeatedly reflect as it travels up the light pipe  110 .  
         [0042]    [0042]FIG. 6 illustrates the space saving characteristics achieved by the present invention by placing the LED  102  and the CCD  106  adjacent to each other instead of opposite or facing each other. The wafer  111  is supported by the rotating chuck  150  in such a manner that the edge of the wafer  111  hangs over a portion of the CCD  106 . The LED  102  is shown adjacent the CCD  106 . The light-emitting portion  103  of the LED  102  does not directly or partially face the CCD  106 . The light guide  108  directs the light emitted from the LED  102  onto the top surface of the wafer  111  and the top surface of the CCD  106 . The lens  109  seated within the outlet  112  of the light guide  108  collimates the light exiting the light guide  108 .  
         [0043]    As previously discussed, conventional aligners (e.g., aligner  10  in FIG. 1) require the LED  102  to directly face the CCD  106  because the aligner requires a straight light path from the LED to the CCD. The apparent light source  152  shown in FIG. 6 illustrates where the LED  102  would be located in relation to the CCD  106  in a conventional aligner. The light guide  108  provides a more compact design than the housing  18  that encloses the LED&#39;s  13  in the conventional aligner  10 . Further, the light guide  108  does not contain any electrical wiring, which is required in the housing  18  to power the LED&#39;s  13 . In the present invention, substantially all of the electrical circuits for powering the LED  102  are contained within the PCB  104 .  
         [0044]    The light striking the CCD  106  is effectively from a small aperture source (e.g., LED  102 ) at a large distance. Thus, the shadow cast by the wafer  111  onto the CCD  106  is minimally dispersed along the line of the CCD&#39;s pixel array. The light diverging out of the LED  102  is initially constrained by the light pipe  110  and remains within the light guide  108  until the light passes through the exit window  112 . At this point, the light resumes its original divergence. Thus, a single LED  102  is able to cast enough light on the CCD  106  to saturate it if need be.  
         [0045]    The curvature of the lens  109  serves to refract the exiting light to a substantially vertical path prior to passing the wafer edge  161  or striking the CCD  106 . This feature prevents vertical motion of the wafer edge  161  (vertical runout) causing radial motion of the wafer edge&#39;s shadow.  
         [0046]    Because the light passing the wafer edge is substantially vertical, vertical runout of the spindle, which makes the wafer edge move up and down, does not move the shadow along the CCD  106  in response to this runout. Thus, measurements of the movement of the shadow are more accurate than many conventional pre-aligners. In addition, the vertical light path makes the shadow on the CCD  106  move exactly the same distance as the wafer edge. The light from the LED  102  is concentrated on the area of the CCD  106  that is not covered by the wafer. This allows an increased optical path length from the LED  102  to the CCD  106  resulting in a reduced spread of the shadow of the wafer edge on the CCD  106 .  
         [0047]    [0047]FIG. 7 illustrates another embodiment of the pre-aligner  100 . In this embodiment, the vent guide  108  is removably attached to the housing  154  (See FIG. 6). Specifically, the light pipe portion  110  is removably attached to a pair of steel pole pieces  132 . A magnet  130 , located between the two pole pieces  132 , holds the poles  132  in place by magnetic attraction. In a preferred embodiment, the poles  132  are also mounted on an armature plate  134  and secured to the main body  154  of the pre-aligner  100 .  
         [0048]    Thus, the light guide  108  may be removed from the housing  154  without breaking any electrical connections. In other words, there are no electrical components within the light guide  108 . Thus, the light guide  108  may be removably attached to the pre-aligner  100  housing  154  and can be detached from the housing  154  in response to any mishandling or robot collision. The light guide  108  may be reattached to the pre-aligner  100  housing  154  in the correct location with minimal effort.  
         [0049]    The LED  102  must be calibrated to ensure that the pre-aligner  100  accurately centers the wafer  111 . The calibration process must be performed with the wafer  111  absent so that the entire CCD  106  is illuminated by the light emitted by the LED  102 . The LED current is preferably adjusted so that it is bright enough to put the CCD  106  output on the “light” side of the detection threshold over the whole CCD  106  length, within some reasonable margin. Calibrating the LED  102  also maximizes the LED life by minimizing its power consumption. Calibrating the LED  102  also ensures that the pre-aligner functions properly while the LED  102  ages and its intensity changes. Another reason to calibrate the LED  102  is to ensure the pre-aligner  100  works properly with the wide variation of initial LED intensities without human intervention in the factory.  
         [0050]    The single LED  102  is capable of providing enough light to saturate the CCD  106 . Electronics exist today, and are well known within the art, to set the current delivered to the LED  102 . It is necessary to make the LED brightness sufficient to make the CCD output exceed the program light detection threshold, plus some margin. It is also necessary to avoid total saturation of the CCD  106 . It is desirable to use a minimum amount of light so that stray light paths have minimal effect on the detection of the wafer&#39;s shadow edge. Thus, a calibration step is desirable in order to select an appropriate value for the LED current.  
         [0051]    By way of example only, one embodiment of the calibration algorithm is as follows:  
         [0052]    1. Set the CCD detection threshold to a lower level than is normally used. The specific value must be determined by consideration of the margin desired, i.e., the desired minimum voltage difference between the threshold and the illuminated level.  
         [0053]    2. Reduce the LED current until the light to dark transition is detected. The radial location is unimportant.  
         [0054]    3. Increase the LED current until no light to dark transition is detected. This is now the desired LED current for normal operation.  
         [0055]    4. Restore the normal detection threshold.  
         [0056]    Conventionally, the LED calibration function is performed during power up of the pre-aligner  100 . However, if the pre-aligner  100  is left on continuously, the LED calibration function can be performed at some reasonable time intervals after power up, for instance, once each week. Alternately, the LED calibration function may be performed when instructed by a diagnostic command from either a local processor or a central, remotely processor.  
         [0057]    In a preferred embodiment, there is no need for recalibration upon replacing guide  108  onto the pre-aligner  100 . This feature may allow simpler, more compact packing for shipment, and better protects the pre-aligner  100  in shipping. It also reduces the chance of damage in case a robot crashes into the light guide  108 .  
         [0058]    The pre-aligner  100  may accommodate various sizes of wafers. To accommodate 200 mm and 300 mm wafer sizes, the light guide  108 , LED  102 , and CCD  106  may be moved as a single unit by repositioning the housing  154  to which they all mount.  
         [0059]    It is common within the industry to isolate the LED  102  from the rest of the pre-aligner  100 , and typically the LED  102  extends out from a surface, exposing it to damage. For example, operators can grab the light source or LED when they pick up the pre-aligner or a robot may strike the LED in error. If the LED is touched, it is likely that the LED will become damaged or require recalibration. In one embodiment of the present invention, the LED  102  detaches from the light pipe  110 . This is accomplished by securing the LED  102  within the light pipe  110  by a magnetic force. As previously mentioned, the magnet  150  in combination with the two magnetic plates holds the LED  102  in position. In a preferred embodiment, dowel pin locating features are used in addition to the magnet  150  and the magnetic plates to ensure that the LED  102  is always in proper alignment when it is replaced back into the light pipe  110 .  
         [0060]    Although the invention has been described in detail herein, it should be understood that the invention is not limited to the embodiments herein disclosed. Various changes, substitutions and modifications may be made thereto by those skilled in the art without departing from the spirit or scope of the invention as described and defined herein.