Abstract:
Aa method for optical inspection and an apparatus for optical inspection of a surface of a substrate, the apparatus includes (i) An optical head comprising a two-dimensional matrix of photodetectors, which is positioned opposite the substrate so as to capture a sequence of area images of respective areas of the surface, (ii) A rotation device, which is coupled to rotate the substrate about a rotation axis, (iii) A translation device, coupled to impart motion to at least one of the optical head and the rotation device so that the optical head is translated radially relative to the substrate while the rotation device rotates the substrate, whereby the area images in the sequence are arrayed in a spiral pattern with respect to the surface, and (iv) An image processor, which is coupled to receive and process the area images so as to determine a characteristic of the surface.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to systems and methods for optical inspection, and specifically to systems for detecting and classifying defects on substrates such as semiconductor wafers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical inspection is commonly used in semiconductor device manufacturing to detect defects on the surface of a wafer, such as contaminant particles, scratches and digs. Undetected defects can cause device failures, thus reducing substantially the process yield. Therefore, careful inspection is required to verify the cleanliness and quality both of unpatterned wafers and of patterned wafers at various stages in the manufacturing process.  
           [0003]    A common method for inspecting semiconductor wafers is to scan a laser beam over the wafer surface, and measure the light scattered from each point on which the beam is incident. One such method, based on dark-field scattering detection, is proposed by Smilansky et al., in U.S. Pat. No. 6,366,690, whose disclosure is incorporated herein by reference. Smilansky et al. describe a wafer inspection system based on an optical detection head that comprises a laser and a number of light sensors, which are fed by fiberoptic light collectors arrayed around the laser. The optical head is positioned over the wafer surface, and the wafer is rotated and translated so that the laser beam scans over the surface. The sensors detect the radiation that is scattered from the surface in different angular directions simultaneously, as determined by the positions of the fiberoptics. The entire wafer surface is thus scanned, one pixel at a time, along a spiral path.  
           [0004]    Another dark-field wafer inspection system is described by Marxer et al., in U.S. Pat. No. 6,271,916, whose disclosure is incorporated herein by reference. In this system, a laser beam is directed toward the wafer surface in a normal direction and scans the surface along a spiral path. An ellipsoidal mirror is used to collect the laser radiation that is scattered from the surface at angles away from the normal. Preferably, light scattered within a first range of angles is collected by one detector, while that scattered within a second range of angles is scattered by another detector. The different detector signals are used to distinguish large defects from small defects.  
         SUMMARY OF THE INVENTION  
         [0005]    It is an object of some aspects of the present invention to provide systems and methods for imaging a substrate at high-speed, and particularly for image-based inspection of semiconductor wafers.  
           [0006]    Spiral scanning patterns have been found to be advantageous for high-speed wafer inspection, because they can be implemented simply and compactly, without the use of moving parts in the optical system. The spiral scan is better-suited for covering the surface of a circular wafer than a conventional X-Y rectilinear scan, and such a scan can easily be adjusted for a larger or smaller inspection area. Spiral scan systems known in the art, however, such as those described in the Background of the Invention, are limited to scanning the wafer surface one pixel at a time, along a single, very long spiral scan path.  
           [0007]    Preferred embodiments of the present invention provide a system for capturing high-resolution images of a substrate, based on spiral scanning of an optical imaging head over the surface of the substrate, such as a semiconductor wafer. The head includes a light source, preferably a pulsed, incoherent light source, which illuminates a succession of small areas along a spiral scan path over the surface. An image sensor, typically comprising a two-dimensional matrix of photodetectors, captures an image of each of the areas illuminated by the light source. The scan path, scanning speed and pulse rate of the light source are preferably chosen so that the area images captured by the image sensor cover substantially the entire surface of the substrate.  
           [0008]    An image processor receives the area images captured by the image sensor and processes them to detect defects on the surface. Optionally, the image processor stitches the area images together to create a combined image of the entire surface. The image processor preferably performs image enhancement operations, as well, such as sharpening and removing noise from the image. These operations most preferably include two-dimensional neighborhood operations, in which each pixel in an area image is processed together with pixels adjoining it in both the vertical and horizontal directions. Operations of this sort cannot be readily performed in spiral scanning systems known in the art, since each pixel along the scan path has neighbors only along the scan path, and not transverse to it.  
           [0009]    The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which:  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a schematic, pictorial illustration of an optical inspection system, in accordance with a preferred embodiment of the present invention; and  
         [0011]    [0011]FIG. 2 is a schematic top view of a wafer inspected using the inspection system of FIG. 1, illustrating a pattern of area images of the wafer surface that are captured by the system, in accordance with a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0012]    [0012]FIG. 1 is a schematic, pictorial illustration of a system  20  for optical inspection of a semiconductor wafer  24 , in accordance with a preferred embodiment of the present invention. The wafer may be either unpatterned or patterned, in substantially any stage of its manufacturing process. An optical head  22 , scanning on a spiral path, is used to capture a sequence of area images of the wafer surface, as described in detail hereinbelow.  
         [0013]    Wafer  24  is preferably held by a rotating chuck  26 , as is known in the art, or by another suitable rotation device. A translation device  28  translates optical head  22  over the wafer surface in a direction perpendicular to the rotation axis of the chuck. The rotation of wafer  24  and translation of head  22  are such as to enable the optical head to scan the entire wafer surface in a spiral pattern. Alternatively, the translational motion may be applied to the wafer, rather than to the optical head. Rotational motion may also be applied to the optical head. Suitable mechanical arrangements for scanning a spiral pattern over a semiconductor wafer are described in detail both by Smilansky et al. and by Marxer et al. in the above-cited patents.  
         [0014]    Optical head  22  comprises a light source  30 , which illuminates an area  32  on the surface of wafer  24 , preferably in a dark-field configuration as shown in the figure. For uniform lighting of area  32 , head  22  may comprise multiple light sources or a ring light source positioned around area  32 . An objective lens  34  collects light scattered from area  32  and images the light onto an image sensor  36 . The image sensor comprises a two-dimensional matrix array of photodetector elements  37 . Typically, sensor  36  comprises a CCD or CMOS image sensor, as are known in the art. Sensor  36  forms an area image of area  32  comprising a matrix of pixels, corresponding to elements  37  of sensor  36 . Optionally, optical head  22  also includes a rotating filter (not shown), whose rotation is keyed to the rotation of chuck  26 , in order to enhance or suppress certain image features on wafer  24 . The design and operation of such a filter are described in co-pending U.S. patent application Ser. No. 10/208,113 filed Jul. 29, 2002, titled “Process and assembly for non-destructive surface inspection”, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference.  
         [0015]    Image sensor  36  captures the sequence of area images of the wafer surface while chuck  26  and rotation device  28  rotate an translate the wafer is rotating. Therefore, to ensure that the images are not blurred by motion, light source  30  is preferably pulsed. Most preferably, the light source comprises a non-coherent pulsed source, such as a flashlamp with suitable focusing optics, such as a mirror and/or condenser lens. A laser source may alternatively be used, but is generally not necessary since area  32  is relatively large. The illumination produced by the light source may comprise either narrowband light or broadband, white light, depending on whether sensor  36  is to form monochrome or color images. If light source  30  is sufficiently powerful, it can illuminate the entire wafer surface with each of its pulses, in which case the light source may be mounted separately from the optical head.  
         [0016]    The area images captured by optical head  32  are received by an image processor  38 , which typically processes the area images to detect defects on the surface of wafer  24 . Optionally, the image processor combines the area images into a composite image of the entire surface of the wafer. Alternatively, only a portion of the wafer surface may be imaged if desired. Processor  38  typically comprises a general-purpose computer with suitable image processing software for creating the composite image. In addition, processor  38  may comprise a digital signal processor (DSP) or dedicated hardware logic circuits for carrying out computation-intensive image processing steps, such as digital filtering and image rotation, which may be used in analyzing, enhancing and combining the area images.  
         [0017]    [0017]FIG. 2 is a schematic top view of wafer  24 , showing a spiral pattern  40  of area images  42  of the wafer captured by optical head  22 , in accordance with a preferred embodiment of the present invention. The spiral pattern of the area images corresponds to the spiral scan of optical head  22  over wafer  24 , due to the simultaneous operation of chuck  26  and translation device  28 . Each of area images  42  preferably overlaps its neighbors, so that the area images cover substantially the entire wafer surface. It is desirable, however, to reduce the amount of overlap to the minimum necessary to ensure coverage of the entire wafer, in order to reduce the amount of data that processor  38  must handle. For this purpose, either the speeds of chuck  26  and translation device  28  may be varied, or the rate at which sensor  36  captures images may change as a function of position, or both.  
         [0018]    The requirements of optical head  22  may be estimated based on the pattern of area images shown in FIG. 2. Assuming that wafer  24  is 30 cm in diameter, and that the wafer surface is to be imaged with spatial resolution of 2 μm, the number of pixels in the composite image of the entire surface will be about 1.8×10 10 . If sensor  36  is made up of 4 million detector elements  37  (for example, a 2000×2000 matrix), it will be necessary for head  22  to capture about 5000 area images  42 , allowing for about 10% overlap between area images. If wafer  24  is to be scanned completely in one minute, light source  30  and sensor  36  should operate at 83 frames/sec. These requirements are cited only by way of example and may be adjusted depending on the actual size of wafer  24  and the actual resolution and throughput requirements of system  20 .  
         [0019]    Photodetector elements  37  in modern image sensors typically have a pitch of 4 μm. To achieve the desired resolution, objective  34  should thus have a magnification of approximately 2×. Image sensor  36  is typically a monochrome sensor, but it may alternatively comprise a color mosaic filter, as is known in the art, so as to form color area images of wafer  24  (albeit at reduced resolution). Alternatively, although only a single image sensor  36  is shown in FIG. 1, multiple sensors may be used, with suitable dichroic beamsplitters, so as to form color area images without loss of resolution.  
         [0020]    In order to process all of area images  42 , processor  38  must handle approximately 333 million pixels/sec. The operations performed by processor  38  may include rotation of the axes of area images  42  so that all the images are aligned within a common coordinate system. The pixels in the original area images are replaced by new, “virtual” pixels on the rotated axes. The intensity value of each new pixel is typically a weighted linear combination of a number of the values of the original pixels in its neighborhood, wherein the weighting coefficients are determined according to the known angular orientation of the original area image. As noted above, in order to perform this operation at the requisite speed, processor  38  preferably comprises special image-processing hardware, such as a programmable DSP or a suitable custom or semi-custom integrated circuit.  
         [0021]    These elements of processor  38  may also be used to perform other image processing operations, particularly other neighborhood operations, such as edge enhancement and noise masking. Such operations typically involve convolution of the original pixel values in a given neighborhood with a suitable filter kernel, so as to determine enhanced output pixel values. The neighborhood used may extend over substantially any practical one- or two-dimensional domain. Image rotation, if required, may be carried out together with other image enhancement functions in a single two-dimensional computation.  
         [0022]    After performing any desired image rotation and/or image enhancement operations on the area images, processor  38  analyzes the area images to find defects on wafer  24 . Typical defects detected in this manner include contaminant particles, scratches and digs. Processor  38  then outputs data indicating the locations, types and other characteristics (such as size) of the defects it has discovered. Preferably, the defects are presented to a operator of system  20  in the form of a map, which can then be used by the operator in taking appropriate corrective action. Optionally, processor  38  stitches the area images together to form a composite image of the entire wafer. Where image features in adjacent area images are discernable, processor  38  preferably uses these features to register the area images precisely one with another. Values of pixels in areas of overlap between adjacent area images are typically averaged to provide a smooth transition from one area to the next.  
         [0023]    Although preferred embodiments are described hereinabove with reference to inspection of semiconductor wafer  24 , system  20  and the principles embodied therein may similarly be applied to inspection of substrates of other types, both in the microelectronics field and in other spheres. It will thus be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.