Abstract:
A method and apparatus is provided for illuminating a wafer during wafer alignment using machine vision. An illumination device is fabricated using electroluminescent material, that provides diffuse illumination uniformly over the surface of the lamp to provide backlighting of the wafer. Contrast between the image of the wafer and the diffuse illumination produce edge features in the image that can be analyzed to determine the position and orientation of the wafer.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to illumination of a silicon wafer when using a machine vision system to obtain alignment characteristics of the wafer. 
   Silicon wafer fabrication is generally described as a series of sequential photo-chemical processing steps that create an array of semiconductor devices. The silicon wafer, the substrate upon which the semiconductor devices are fabricated, is a flat single monocrystal of silicon. It is typically in the shape of a circle of diameter 150 mm, 200 mm or 300 mm. The various wafer fabrication processing steps require precise alignment of the silicon wafer. Precise alignment may be required for a particular processing step because that step depends on precise crystal alignment either for manufacturing efficacy or manufacturing repeatability such as an ion beam deposition step. Alternatively, precise alignment may be required because a particular processing step is pattern-dependent and needs to be photographically registered with one of the previous steps such as a photolithography step. 
   In order to facilitate this precise alignment, wafers are manufactured to agreed upon standards with specific features. For example, SEMI M1-0305 Specifications for Polished Monocrystalline Silicon Wafers defines some of these standards and describes notches or flats cut into the outside perimeter of the wafer permitting wafer orientation to be determined by examination of the wafer perimeter. 
   Wafer Prealignment (sometimes called Wafer Coarse Alignment) is an automated process of examining the shape of a silicon wafer and its notches, flats or other geometric shape characteristics to determine the alignment of a wafer. Alignment of the wafer means determining the position and orientation of the wafer relative to a particular coordinate system. The physical positional accuracy of such an alignment could range from a fraction of a micron to a few millimeters in position and from a few thousands of a degree to a degree or two in orientation. It is possible for some of the processing steps described above to involve creating fiducial marks on the surface of the wafer. In later processing steps, those newly created fiducial marks can also be used for wafer alignment. Such a wafer alignment step that uses fiducial marks on the surface of the wafer is called a fine alignment step and is not the subject of this application. However, it is important to note that even when a fine alignment step is performed to align a wafer, a coarse alignment step is typically performed first in order to reduce the search area of the fiducial mark. 
   Conventional wafer alignment systems and methods employ LED illumination that provides backlight illumination of the wafer. To provide the requisite uniform diffuse illumination, the LED illuminators require a diffuser that distributes the point source illumination of the LED into a wide area diffused mode of illumination proximate to the peripheral region of the wafer. While effective, the conventional illumination systems and methods become increasingly complex and expensive as the wafer fabrication industry continues in its trend toward larger wafer sizes. 
   Accordingly, there is a need for a low cost method and apparatus for providing uniform and diffuse illumination over a wide area for backlighting wafers during coarse alignment. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention provides a method and apparatus for backlighting a wafer during wafer alignment processing. In a particular embodiment, the invention provides illumination using a sheet of electroluminescent material to backlight a wafer. An image of the backlit wafer is acquired using a camera coupled to a machine vision system. In this embodiment, the wafer is positioned on a stage, and the position and orientation of the wafer can be determined from the image. 
   In accordance with the principles of the present invention, the sheet of electroluminescent material can be operated while adhered to a rigid substrate and applying electrical power. Alternate embodiments of the invention include the use of a robotic end effector to position the wafer between the electroluminescent lamp and the camera. In this embodiment, the wafer can be held stationary during image acquisition, or dynamically moved through the field of view. Alternatively, the electroluminescent material can be operated in a strobed mode of operation by momentarily cycling the power application when the wafer is in the field of view of the camera. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention will be more fully understood from the following detailed description, in conjunction with the following figures, wherein: 
       FIG. 1  is an illustrative diagram of an exemplary wafer alignment system according to the present invention; 
       FIG. 2  is an illustrative diagram of an alternative exemplary wafer alignment system according to the present invention; 
       FIG. 3  is a representation of an image of the wafer acquired during wafer alignment according to the present invention; 
       FIG. 4  is an exploded view of the electroluminescent material according to the present invention; and 
       FIG. 5  is an isometric view of the electroluminescent lamp according to an illustrative embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , in accordance with the present invention, there is provided a wafer alignment system that can be deployed in a wafer fabrication process for providing coarse alignment of a silicon wafer. A silicon wafer  100  is presented to a machine vision camera  120  using an alignment stage  130 . An electroluminescent lamp  110  projects illumination toward the camera  120  to provide backlight illumination of the wafer  100  through the application of power from a power supply  140 . 
   In an embodiment of the invention, a robot end effector or a person places the wafer  100  upon the alignment stage  130 . The wafer can be inaccurately placed on the stage and can have any orientation—the objective of the wafer alignment process is to determine the relative position and angular orientation of the wafer  100  to a reference location or position, so that an automated material transfer system, like a robotic end effector, can accurately pick up and transfer the wafer using the determined relative positional information. 
   In the wafer alignment process using the present invention, a machine vision system  150  acquires at least one image of the wafer on the alignment stage using the camera  120 . In an illustrative embodiment, the machine vision system is a personal computer with a frame grabber, like the MVS-8100 PCI frame grabber available from Cognex Corporation. The camera  120  can be an RS-170/CCIR industry standard 640×480 monochrome camera, coupled to the machine vision system  150  using standard video interconnection cables. Alternatively, the camera  120  and the machine vision system  150  can be an integrated sensor, for example, an In-Sight 1700 Series Wafer Reader, also available from Cognex Corporation, where the functionality of the camera  120  is internally coupled to the machine vision system  150 . In a clean-room environment, the integrated camera/system solution will be preferred. 
   The acquired image of the wafer  100 , backlit by the electroluminescent lamp  110 , is represented by  FIG. 3 . An apparent edge is visible in the image between the electroluminescent lamp  110  and the wafer  100 ; the lamp  110  will appear bright, while the wafer  100  will appear dark. Using boundary tracking methods of fitting a circle template to the image of the wafer  100 , a center position of the wafer can be determined. The angular orientation of the wafer can be determined by finding the notch  115 , using conventional pattern matching or correlation matching tools commonly known in the art. In 200 mm wafer fabrication implementations, the notch  115  may appear as a short chord feature, or “flat” in the circular profile of the wafer, though the same, or similar, feature locating methods can be similarly applied. One method of performing the machine vision methods for determining the position and orientation of wafer using an acquired backlit image of the wafer is described in commonly assigned U.S. Pat. No. 5,825,913, the entirety of which is herein incorporated by reference. 
   In an alternate embodiment of the invention shown in  FIG. 2 , a robot end effector  160  passes the wafer between the lamp  110  and the camera  120  so that the wafer alignment process can be performed, with the determined wafer position and orientation passed to the robot controller. The wafer alignment process in this embodiment is the same as that described above with reference to the camera  120  coupled to the machine vision system  150 , or through the use of an integrated sensor. In this alternate embodiment, features of the robot end effector  160 , typically a vacuum grip or edge grip configuration, will appear in the acquired image. The wafer alignment process must be tolerant of these extraneous features, and there is the possibility that the notch or flat features may be obscured by the robot end effector  160 . In this embodiment, the robot end effector  160  may pause in the predetermined position so that the wafer alignment process can be performed, or the image can be acquired as the end effector  160  dynamically passes the wafer through the predetermined position. In the latter configuration, a strobed actuation of the lamp  110  is preferred, by momentarily actuating the power supply  140 , as described below. 
   An exploded view of a section of the electroluminescent material  112  used in the electroluminescent lamp  110  is shown in  FIG. 4 . A thin layer of light emitting phosphor  185  is placed between a translucent electrode  195  and an opaque electrode  175 . When alternating current (400-1600 Hz) is applied to the translucent electrode  195  and the opaque electrode  175 , the phosphor layer  185  rapidly charges and discharges, resulting in the emission of light. An insulating layer  165  electrically isolates the active layers of the composite structure from the base structural material  155 . The typical thickness of the electroluminescent material is approximately 0.30+/−0.03 mm. 
   An illustrative embodiment of the present invention is shown in  FIG. 5 . The electroluminescent lamp  110  is fabricated by attaching the electroluminescent material  112 , which is typically flexible, to a rigid substrate  145 . The electroluminescent material can be obtained from MKS, Bridgeton, N.J., as Quantaflex 1600. The rigid substrate material  145  can be, for example, polycarbonate, or any similar material that is suitable for use in a wafer fabrication clean-room environment, such as G10 epoxy-glass composites, or anodized aluminum. To attach the electroluminescent material  112  to the rigid substrate  145 , an adhesive suitable for use in a wafer fabrication clean-room environment, such as Dymax “Multi-cure 427”, UV cured epoxy. Electrical power supplied from the power supply  140  is connected to the electroluminescent material  112  via a cable  135  that is attached using a suitable connector  125  in a manner specified by the material provider. During operation, the power applied to the lamp  110  is 60-120 Volts AC at 400-800 Hz. As shown in  FIG. 5 , the illustrative embodiment of the present invention is a circular shape in an annular ring, with a void area in the center to accommodate the wafer stage  130 . In the illustrative embodiment, which is sufficient for illuminating a 300 mm wafer, the lamp is approximately fourteen inches in diameter overall with an six inch center diameter. One skilled in the art will appreciate that nearly any geometric shape is suitable for the design of the lamp  110 , so long as the expected edge of the wafer  100  is illuminated in the field of view of the camera  120  by the lamp  110 . 
   During operation, the power supply  140  supplies alternating current to the translucent electrode layer  195  and the opaque electrode layer  175  so that light emits from the surface of the lamp  110 . In a static wafer alignment process, the power supply continuously applies current to the lamp  110  during image acquisition. Alternatively, in either a static analysis, or in an implementation according to the alternate embodiment wherein an end effector dynamically passes the wafer through the field of view of the camera  120 , the power supply can strobe the lamp  110  with an intermittent actuation in response to a system trigger. In a strobed implementation, the power can be optionally overdriven according to manufacturer specification to increase illumination intensity over a short duration, at the expense of potential reduction in expected life cycle of the lamp. When strobing power supplied to the lamp, the latency is reasonably predictable over the area of illumination, which can be calibrated with the system timing requirements. 
   One skilled in the art will appreciate that variations to the illustrative embodiment can be contemplated within the purview of the appended claims. For example, the electroluminescent material  112  can be captured between a sheet of transparent glass, quartz, or plastic and a substrate. The flexible material need only be held relatively flat on a plane substantially parallel to the surface of the wafer under alignment so that its perceived illumination is evenly distributed over the area at the expected edge of the wafer in the field of view of the camera  120 . Since the illumination output from the material is highly efficient, the material does not generate thermal management issues with respect to the construction or particular design of the electroluminescent lamp  110 . 
   The electroluminescent material can be obtained in any of a variety of illumination colors. In the illustrative embodiment of the present invention, a lime-green color has been selected, since it has been found to be non-reactive to semiconductor fabrication processes that are associated with, or in near proximity to, the wafer alignment processes. 
   The illustrative embodiment has been shown to be effective for wafer alignment even in installations where ambient light is not controlled. Reflection of ambient light from the lamp  110  can be distinguished from specular reflections of ambient light from the wafer under alignment by the machine vision system  150  such that sufficient contrast at the wafer edge can permit an effective analysis of an acquired image. Alternatively, a band-pass filter (not shown) that is tuned to the wavelength (color) of the light projected from electroluminescent material  112  can be installed in the optical path of the camera  120  to reduce the potential for susceptibility of ambient light reflections. 
   While the invention has been described with reference to certain illustrated embodiments, the words which have been used herein are words of description rather than words of limitation. Changes may be made, within the purview of the appended claims, without departing from the scope and spirit of the invention and its aspects. Although the invention has been described herein with reference to particular structures, acts and material, the invention is not to be limited to the particulars disclosed, but rather extending to all equivalent structures, acts, and materials, such as are within the scope of the appended claims.