Patent Application
20070153084
The invention relates to a method and apparatus for reading identification marks on semiconductor wafers.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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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.
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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
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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.
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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:
