Abstract:
A reader for semiconductor wafers includes a camera for reading a mark on a semiconductor wafer. The wafer is positioned adjacent a surface of the housing including a reading window and an illumination device. The illumination device provides both bright and dark field illumination to the wafer, and light reflected from the wafer is directed to a mirror inside the housing, which directs the light along a camera axis to a lens of the camera. The alignment of the reflected illumination can be adjusted by changing the angle of the single mirror within the reader, thereby limiting the complexity of the device. The illumination device can be an array of light emitting diodes arranged in rows. The rows are separated by baffles which restrict dispersion of light from the light emitting diodes to provide directed bright field illumination.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a method and apparatus for reading identification marks on semiconductor wafers.  
         [0002]     Semiconductor wafers used in the manufacture of integrated circuits are often marked with identification marks or other identifying information to facilitate tracking during production of semiconductor chips. These identifying marks, known as scribe marks, typically comprise a series of characters, bar codes, or other two-dimensional codes, each of which is formed from depressions in the substrate.  
         [0003]     To provide an efficient production process, these marks must be reliably read by automated process equipment. In typical systems, a camera forms an image of the scribe mark, and converts the image into a digital format. The digitized image is then interpreted using, for example, optical character recognition or decoding software that determines letters, numbers, bar codes, or other symbols in the digitized image. For the mark to be properly interpreted by the software, however, the digitized image must be relatively clear. The image, therefore, must include adequate contrast between the background and the remainder of the mark.  
         [0004]     Forming a clear image of the scribe mark, however, can be difficult for a number of reasons. First, because typical scribe marks comprise a group of relatively shallow depressions in the substrate, and the marks are of the same color as the substrate background, the marks can be hard to differentiate. Furthermore, the substrates are typically highly polished and, therefore, reflect a large amount of light into the camera, which tends to obscure the mark. Additionally, during chip production, material coatings, etching sequences, and other process steps adversely affect the marking, decreasing the quality of marking as the production process proceeds.  
         [0005]     The optical properties of the wafer surfaces, moreover, can vary not only from wafer to wafer but also across the surface of an individual wafer. Imperfect formation or etching of the layers can also lead to variations in the thickness of the layers, which can produce artifacts in the image. Furthermore, materials used to treat or coat wafers, particularly photoresist, can accumulate in the depressions of the scribe marks, further obscuring the mark by affecting the optical properties of the substrate surface and scribe marks.  
         [0006]     Because the problems described above make it difficult to form a clear image of the scribe marks, optical systems providing various lighting conditions, and particularly both dark and bright field illumination, have been developed. These systems typically include both a bright field light source and a dark field light source. The bright field light source provides a light to the wafer surface via a beam splitter and associated mirror system, which reflects the light in a direction normal to the surface of the wafer. A mirror re-directs light from the surface of the wafer back to the beam splitter, which reflects light from the mirror to a camera. A separate dark field light source is directed to the surface of wafer at an angle that is not normal to the surface of the wafer. The scribe imperfections in the surface of the wafer scatter the dark field light and reflect some light in a direction normal to the surface of the wafer. The mirror assembly re-directs this light to the beam splitter, which directs it to the camera. Since the mirror assembly and beam splitter redirect light reflected from the scribe back to the camera, the dark field light source causes the background of the scribe to look dark and the scribe itself to look bright.  
         [0007]     Illumination systems of this type have been largely successful in providing efficient wafer reading systems. However, these prior art systems require elaborate optical components to direct bright field illumination directly coincident with the reflected illumination path. Therefore, these prior art systems are complicated, difficult to construct, and expensive. The present invention addresses these problems.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a reader for optically reading a scribe or mark on a semiconductor wafer that is effective, simple to construct and inexpensive. An illumination device providing both bright and dark field illumination is provided on a surface reader and is arranged to illuminate the scribe or mark. Angled light reflected from the wafer is directed through a reader aperture to a reflector inside the reader. The reflector is angled to direct the light toward a camera including an image sensor, which acquires an image of the mark or scribe. The reader images the mark, and decodes the mark or scribe.  
         [0009]     In one aspect, the present invention provides an optical wafer reader including a camera having a camera axis substantially parallel to the wafer surface, an illumination device disposed on the surface of the reader facing the wafer surface and aimed to illuminate a mark on the wafer with bright and dark field illumination, a window in the surface of the reader facing the wafer surface for passage of light reflected off of the mark, and a reflector positioned to receive illumination reflected from the wafer and to direct the light along the camera axis.  
         [0010]     In another aspect of the invention, the illumination device comprises an array of light emitting diodes arranged in a plurality of rows. A baffle can be positioned between each of the adjacent rows of light emitting diodes that are adapted to direct the light emitted from the light emitting diodes to provide bright field illumination to direct the light and limit light dispersion. A diffusing cover can also be provided over the light emitting diodes producing bright field illumination to disperse the light in a more uniform fashion.  
         [0011]     In another aspect of the invention, the optical wafer reader can include a gear assembly for adjusting a focus of the camera. The reflector, moreover, can be pivotally mounted to the housing opposite the camera, and both the gear assembly and pivotal mounting of the reflector can be adjusted through devices accessed externally to the housing.  
         [0012]     In yet another aspect, the present invention provides an optical wafer reader including a housing including a reader aperture positionable adjacent a semiconductor wafer for reading a mark on the wafer, a camera including a lens coupled to the housing, and a mirror pivotally coupled to an opposing end of the housing and configured to reflect light from the reader aperture toward the lens of the camera. An array of light emitting diodes are coupled to the housing adjacent the reader aperture and are adapted to provide bright and dark field illumination to the wafer positioned below the reader aperture such that light reflected from the wafer is directed by the mirror along a camera axis substantially parallel to the reading aperture.  
         [0013]     In yet another aspect of the invention, an optical wafer reader is provided including a housing having a reflector pivotally coupled to a first end and a camera coupled to an opposing end. A light emitting device is coupled to a bottom of the housing, and a reading aperture provided in the bottom of the housing adjacent the first end of the housing and adjacent the light emitting device. The reflector is pivotally adjustable to direct light reflected through the reading aperture along a camera axis parallel to the bottom surface of the housing and toward a lens of the camera.  
         [0014]     These and other aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0015]      FIG. 1  is a perspective view of a semiconductor wafer reader constructed in accordance with the present invention;  
         [0016]      FIG. 2  is a bottom plan view of the semiconductor reader of  FIG. 1 ;  
         [0017]      FIG. 3  is a side perspective view of the semiconductor reader of  FIG. 1 , with the sides removed;  
         [0018]      FIG. 4  is a perspective view of the semiconductor reader of  FIG. 1  at a different angle from  FIG. 3 , and with additional parts removed to illustrate the internal reflective components;  
         [0019]      FIG. 5  is an exploded view of the illumination device of the semiconductor wafer reader of  FIG. 1 ;  
         [0020]      FIG. 6  is a cutaway view of the reader of  FIG. 1  taken along the line  6 - 6  of  FIG. 2 ; and  
         [0021]      FIG. 7  is an end view of the reader of  FIG. 6 ; 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Referring now to the figures and more particularly to  FIGS. 1 and 2 , a wafer reader  10  constructed in accordance with the present invention is shown. The wafer reader  10  comprises a housing  11 , with a reading window  12  and illumination device  14  provided in a bottom surface  13 . A semiconductor wafer  42  ( FIG. 7 ) is positionable adjacent the bottom surface  13 , wherein light from the illumination device  14  can be radiated onto the wafer  42 . The light is reflected from the wafer and directed through the reading window  12  onto optical components provided inside the housing  11 , which image and read or decode the scribe marks on the wafer  42 , as described more fully below.  
         [0023]     Referring now to  FIGS. 3 and 5 , the illumination device  14  includes an illumination board  18 , a baffle assembly  20 , and a plastic cover  22 . The illumination board  18  is a printed circuit board with a plurality of rows of light emitting diodes (LEDs) which, when properly positioned in the housing  11 , extend across the housing  11  from side  19  to side  21 . Rows of bright field LEDs  34  are provided in a center portion of the board  18 , and rows of dark field LEDs  32  are provided on opposing ends of the illumination board  18  adjacent the outer rows of bright field LEDs  34 .  
         [0024]     Referring still to  FIGS. 3 and 5 , the baffle assembly  20  comprises a plurality of baffles  23 , extending between and perpendicular to connectors  19  and  21  provided at opposing ends of the assembly  20 . There are a sufficient number of baffles  23  in the assembly  20  to provide one baffle between each adjacent row of bright field LEDs  34  and to separate the outer row of bright field LEDs  34  from the adjacent dark field LEDs  32 . The baffles  23  are therefore sized and dimensioned to be received between adjacent rows of LEDs  34 , and are further dimensioned to provide an opaque “wall” of sufficient height to limit the dispersion of light from the sides of the LEDs  34  such that illumination from the LEDs  34  is directed straight ahead, perpendicular to the board  18  and to the bottom surface  13  of the housing  11 , thereby providing bright field illumination. The dark field LEDs  32 , however, emit diffuse, angled light, and are not directed specifically in a direction perpendicular to the illumination board  18 . These lights, therefore, provide dark field illumination.  
         [0025]     The cover  22  is sized in dimensioned to extend over the rows of bright field LEDs  34  and associated baffles  23 , and is constructed of a transparent, preferably plastic material, selected to diffuse the light emitted by the bright field LEDs  34  to make the emitted light more uniform and to limit the appearance of “dots” within the light. Apertures  25  are provided on opposing sides of the cover  22 , and are sized and dimensioned to allow the dark field LEDs  32  to extend through the cover  22 , thereby maintaining the normal diffusion properties of the dark field LEDs  32 .  
         [0026]     Referring still to  FIGS. 3 and 5 , and now also to  FIG. 4 , the reading window  12  comprises a glass, Plexiglas, plastic or other transparent material provided over an aperture cut into the bottom surface  13  of the housing  11 . Light emitted through the reading window  12  is directed to a reflector  16 , typically a mirror, coupled to a first end wall  17  of the housing  11 . The reflected light is then directed along a camera axis  30  to a camera  24  that is mounted to the opposing end wall  15  of the housing  11 .  
         [0027]     Referring still to  FIGS. 3, 4 , and  5  and now also to  FIG. 6 , the camera  24  includes a lens  26 , an image sensor (not shown), and gears  28  and  29  for focusing the lens  26 . As shown here, the gears  28  and  29  are adjustable by rotating a threaded fastener  44  which, in turn, rotates the gear  28 , causing the gear  29  to focus the lens  26  in the camera  24 . Although a threaded fastener  44  is shown for making this adjustment, the gears  28  can also be motor-driven, manually operated or otherwise controlled. The end of the threaded fastener  44  is accessible from outside the housing, such that the focus can be adjusted without removing the housing  11 .  
         [0028]     As described above, the camera  24  includes an image sensor such as a CMOS imaging sensor or CCD device, along with a processor which can, for example, include a microprocessor and/or digital signal processor provided on a control board  41  in the housing  11 , as shown in  FIG. 6 . In a preferred embodiment, the image sensor is a high speed 1280×1024 CMOS sensor and the processor is a high speed digital signal processing device, such as a chip from the TI 64X family, commercially available from Texas Instruments of Dallas, Tex. The control board  41  includes software for decoding symbols such as one-dimensional barcode, data matrix, or other symbols. The control board  41  can also include software providing optical character recognition for identifying characters provided on the wafer  42 .  
         [0029]     Referring still to  FIGS. 3-6 , the reflector  16  is mounted to the end wall  17  of the housing  11  though a spring  40  and mounting bracket  38 . The reflector  16  is typically a rectangular mirror, and includes pins  36  extending from opposing sides of the reflector body that are sized and dimensioned to be rotatably received in an aperture  35  provided in the mounting bracket  38 , and therefore allow the mirror to be pivoted about the mounting brackets  38 . A threaded connector  39  extends through the end wall  17  toward the reflector  16 , and can be adjusted externally to the housing  11  to pivot the reflector  16  about the mounting bracket  38 , and therefore to adjust the angle of the reflector  16 , as described more fully below. To read a wafer positioned beneath the illumination device  14 , the reflector is angled at an angle of greater than forty-five degrees. This angle is adjusted depending on the distance that a wafer is positioned beneath the bottom surface  13  of the reader  10  as described below.  
         [0030]     Referring still to  FIG. 6  and now also to  FIG. 7 , the reader  10  can be used for reading wafers  42  at varying distances from the bottom surface  13  of the housing  11 , and is typically used in an automated manufacturing environment or similar application in which wafers are continually fed to a known position adjacent the reading window  12  for evaluation and decoding. Prior to use, the expected location for the wafer is determined, including the distance beneath the bottom surface  13  of the reader  10 , and the reader  10  is adjusted to provide proper reflection and focus for the selected location. To properly adjust the reader, the threaded fastener  39  is rotated to adjust the angle of the reflector  16  to reflect light from the wafer  42  along the camera axis  30  to the lens  26  of the camera  24 . The threaded fastener  44  can then be rotated, causing the gears  28  and  29  to rotate, adjusting the focus of the lens  26  of the camera  24  for the selected reading distance and location.  
         [0031]     For the establishment of a proper optical path for illumination and light reflection, the wafer  42  is positioned directly below the bottom surface  13  of the housing  11 , and parallel to the plane of the bottom surface  13 . The wafer  42  is positioned beneath the bottom surface  13  of the reader  10  with the scribe or other mark to be read positioned directly below the illumination device  14 , such that the bright field illumination is directed onto the wafer  42  by the bright field LEDs  34 , which is directed by the baffles  23  toward the wafer  42  in a direction substantially perpendicular to the bottom surface  13  of the housing  11 . Dark field illumination is provided by the rows of LEDs  32  at the opposing ends of the illumination board  18 , which are not directed on the surface below and provide diffuse angled light onto the scribe or mark. Light reflected from the wafer  42  is received in the housing  11  through the reading window  12  by the reflector  16 . The reflected light is then reflected by the reflector  16  along a camera axis  30  and toward the lens  26  of the camera  24 . An image sensor in the camera  24  receives the reflected light, and the acquired image is decoded by a processing device and associated hardware provided on the control board  41 .  
         [0032]     Referring still to  FIGS. 6 and 7 , by way of example, in the embodiment shown here, the reader  10  can read wafers at distances between about twenty and forty millimeters below the bottom surface  13  of the housing  11 . As shown here, the wafer is positioned thirty millimeters below the bottom surface  13  of the housing  11 , and is about twenty-seven millimeters long (where this dimension is along the length of the reader  10 ) and thirty-six millimeters wide (along the width of the reader  10 ). The reflector  16  is angled at an angle of about fifty-eight degrees relative to the camera axis  30 . Light from the illumination device  14  is reflected from the surface of the wafer  42  toward the reflector  16  at angles from about 58.79 degrees at the edge of the wafer furthest from the end wall  17  of the housing  11  to an angle of about 69.21 degrees at the edge closest to the end wall  17 . At the approximate center of the wafer  42 , the light is reflected toward the reflector  16  at an angle of about sixty-four degrees.  
         [0033]     The invention therefore provides a method and apparatus for reading semiconductor wafers which is inexpensive, simple to construct and easy to adjust. It should be understood, however, that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. For example, although the invention is described for use in a semiconductor reader, it will be apparent that the principles described herein could be applied to various other reader applications. To apprise the public of the scope of this invention, the following claims are made: