Abstract:
The present invention provides a wafer mapper for imaging large objects such as semiconductor wafers. In operation, the wafer mapper provides spectroscopic ellipsometric data or broadband ellipsometric data for the entire sample being analyzed. This data is provided via either a line scan or a wavelength scan and greatly reduces the time required to map an entire wafer in terms of film thickness, index of refraction, dielectric constant or other measurements.

Description:
PRIORITY CLAIM  
       [0001]    The present application claims priority to U.S. Provisional Patent Application Serial No. 60/430,165, filed Dec. 2, 2002 and U.S. Provisional Patent Application Serial No. 60/452,170, filed Mar. 5, 2003 both of which are incorporated in this document by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This subject invention relates to optical metrology tools that are configured to rapidly analyze large wafer areas at multiple wavelengths.  
         BACKGROUND OF THE INVENTION  
         [0003]    As semiconductor geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. Techniques of this type, known generally as optical metrology, operate by illuminating a sample with an incident field (typically referred to as a probe beam) and then detecting and analyzing the reflected energy off the sample. Ellipsometry and reflectometry are two examples of commonly used optical techniques. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Reflectometry is similar, except that changes in intensity are analyzed. Ellipsometry and reflectometry are effective methods for measuring a wide range of attributes including information about thickness, crystallinity, composition and refractive index. The structural details of ellipsometers are more fully described in U.S. Pat. Nos. 5,910,842 and 5,798,837 both of which are incorporated in this document by reference.  
           [0004]    As shown in FIG. 1, a typical ellipsometer or reflectometer includes an illumination source that creates a monochromatic or polychromatic probe beam. The probe beam is focused by one or more lenses to create an illumination spot on the surface of the sample under test. A second lens (or lenses) images the illumination spot (or a portion of the illumination spot) to a detector. The detector captures (or otherwise processes) the received image. A processor analyzes the data collected by the detector. For systems with polychromatic probe beams, a spectrometer is typically present in front of the detector to disperse light into respective spectrum.  
           [0005]    In production environments, each wafer is typically analyzed at a pre-determined pattern of locations or inspection sites. This is an important step in ensuring the quality of each of the many die that each wafer includes. This process is typically performed in a serial fashion. The wafer is moved (relative to the optical metrology system) to visit each site in turn. As each site is visited, the measurement process is performed and the results are gathered. The entire process is repeated until the entire pattern of inspections sites has been visited and measured. Unfortunately, this sequence of repeated movements and measurements tends to be relatively time-consuming. This is due is large part to the precision with which each inspection site must be located—a process that is typically performed by a human operator using a system of one or more optical microscopes. Although not generally debilitating, the time consumed during the measurement process can be a significant drawback in some environments.  
           [0006]    For these reasons, a need exists for optical metrology systems that can rapidly measure multiple locations within semiconductor wafers. This need is particularly relevant for semiconductor applications where large wafers are used or applications that use a relatively large number of inspection locations.  
           [0007]    One example of an approach that can obtain information across a scan line on a wafer is disclosed in US Patent Application 2002/0030826, incorporated herein by reference. The following disclosure represents different approaches for obtaining information over a large area of a wafer.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a DUV to IR wafer mapper for analyzing large objects such as semiconductor wafers. For one embodiment, the wafer mapper progressively scans the sample under test. Scanning may be accomplished using a number of different patterns. Typically, however a progressive line scan is used in which a line of illumination is scanned over the surface of the sample. Reflected energy is collected for the scanned area and analyzed to determine properties such as film thickness, index of refraction, dielectric constant or other measurements. For typical cases, the illuminating energy is polychromatic light and the reflected energy is analyzed in terms of changes in magnitude (reflectometry) or change in polarization (ellipsometry).  
           [0009]    For a second embodiment of the present invention, the wafer mapper illuminates a sample wafer (or a substantial portion of a wafer) at a single wavelength. The illuminating wavelength is then scanned through a predetermined range (or tuned to a series of different wavelengths). Reflected energy is collected at each illuminating wavelength and analyzed (both ellipsometry and reflectometry are supported) to determine sample properties such as film thickness, index of refraction, or dielectric constant. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a diagram of ellipsometer or reflectometer shown to describe the prior art of the present invention.  
         [0011]    [0011]FIG. 2 is block diagram of a first embodiment of the wafer mapper of the present invention.  
         [0012]    [0012]FIG. 3A is block diagram of a second embodiment of the wafer mapper of the present invention.  
         [0013]    [0013]FIG. 3B is block diagram of a linear color filter suitable for use in the wafer mapper of FIG. 3A.  
         [0014]    [0014]FIG. 3C is block diagram of a color wheel suitable for use in the wafer mapper of FIG. 3A.  
         [0015]    [0015]FIG. 4 is block diagram of a third embodiment of the wafer mapper of the present invention.  
         [0016]    [0016]FIGS. 5A and 5B show a first detection system suitable for use with the wafer mappers of FIGS. 2, 3, and  4 .  
         [0017]    [0017]FIGS. 6A through 6C show a second detection system suitable for use with the wafer mappers of FIGS. 2, 3, and  4 .  
         [0018]    [0018]FIGS. 7A and 7B show a third detection system suitable for use with the wafer mappers of FIGS. 2, 3, and  4 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    The present invention provides a wafer mapper for imaging large objects such as semiconductor wafers. Unlike traditional optical metrology systems which operate on small (local) portions of semiconductor wafers, the wafer mapper analyzes entire wafers (global or substantial wafer portions) and generates corresponding measurements. As shown in FIG. 2, a first embodiment of the wafer mapper  200  includes a series of illumination sources. For this particular example, the illumination sources are labeled  202   a  through  202   k  but any number of narrowband sources is practical. Each illumination source  202  is typically a light emitting diode (LED) but other sources may be used as well. Typically, each illumination source  202  produces light at a respective spectrum (where illumination sources  202  are polychromatic) or wavelength (where illumination sources  202  are monochromatic). For some implementations, sources  202  may cover the entire wavelength range over deep ultra-violet to near-infrared. For this type of implementation, sources  202  may contain UV-emitting lasers.  
         [0020]    The outputs of the illumination sources  202  are transported using a bundle of optical fibers  204 . Optical fibers  204  are arranged to position the outputs of illumination sources  202  as a linear array  206 . The individual spectra of illumination sources  202  are reproduced by optical fibers  204  so that each point within linear array  206  corresponds to a different illumination source  202  and a spectrum.  
         [0021]    An aperture  208  is positioned to control the light emitted by the linear array  206 . The linear array  206  is movable (typically in translation) to select the output of a single fiber and a single illumination source  202 . The output of the remaining fibers is blocked. The overall result is that a single spectrum (or wavelength) is selected at a time. By moving the linear array  206 , each spectrum (or wavelength) is selected in succession.  
         [0022]    Light from the selected fiber forms a cone of light that illuminates a sample  210 . The illumination is global—a significant portion of sample  210  is illuminated. The shape of the light cone is governed by the numerical aperture of the fiber itself. For a doped, fused silica clad fused silica core fiber this angle is approximately 22 degrees. Thus, to illuminate a 300 mm semiconductor wafer the distance from the linear array to the wafer must be approximately 750-mm.  
         [0023]    Light reflected by sample  210  light is collected by an imaging system, shown here as a lens  212 , to form an image of the sample  210  on a CCD array or other two-dimensional array detector  214 . The imaging process is repeated with the linear array  206  in one or more positions to gather images at one or more different wavelength ranges. That data can then be processed via the techniques of broadband and spectroscopic ellipsometry to determine the index of refraction of the film(s), dielectric constant or thickness. The data obtained by the pixels of the CCD can be mapped to locations over the entire wafer surface.  
         [0024]    For some implementations, the output of the illumination sources  202  may be controlled electronically to select a single illumination source  202  at a time. For this type of implementation, the linear array  206  may be replaced by a multi-input, single-output fixedposition fiber that combines light from optical fibers  204  into a single source and could be laid out in any suitable fashion, such as a circle or square pattern.  
         [0025]    As shown in FIG. 3A, a second embodiment of the wafer mapper  300  includes a broadband illumination source  302  such as a Xenon or Halogen source. The output of illumination source  302  is collected using one or more lenses  304  and projected through an aperture  306 . A color filter  308  follows aperture  306 . Color filter  306  may be a linear filter as shown in FIG. 3B or a color wheel as shown in FIG. 3C. After leaving color filter  308 , the colorized or filtered light illuminates a sample  310 . The illumination is global—a significant portion of sample  310  is illuminated.  
         [0026]    Sample  310  reflects the colorized light and the reflected light is collected by an imaging system, shown here as a lens  312 , to form an image of sample  310  on a CCD array or other detector  314 . The imaging process is repeated with the color filter  308  in one or more positions to gather images at one or more different wavelengths. That data can then be processed via the techniques of broadband and spectroscopic ellipsometry to determine the index of refraction of the film(s), dielectric constant and thickness.  
         [0027]    As an alternative to the color filter, the light from the source could be passed through a monochrometer for selecting particular wavelengths of light. The monochrometer can include a dispersive element such as a grating or a prism and an aperture.  
         [0028]    As shown in FIG. 4, a third embodiment of the wafer mapper  400  includes a broadband illumination source  402  such as a Xenon or Halogen source. The output of illumination source  402  is through a lens  404  and into a fiber bundle  406 . The output of illumination source  402  passes through fiber bundle  406  to a fiber bar array  408  where it is projected to form a line on a sample  410 . The reflected light is collected by an imaging system, shown here as a lens  412 , and passed through (or off of) a grating  414  before reaching a CCD array or other detector  416 . Grating  414  creates a two-dimensional image on at the detector  416 . One axis of the two-dimensional image includes spatial information while the second axis includes spectral information. Sample  410  is stepped to scan the line across the entire wafer surface. Alternately, sample  410  may remain motionless and fiber bar array  408  moved, either in translation or by pivoting to perform the scan operation.  
         [0029]    In an alternative to the FIG. 4 embodiment, it is possible to time multiplex multiple narrowband illumination sources. This is similar to the case where time multiplexing is used with the multiple sources of FIG. 2. In this case, there would be no need for grating  414  and the detector could be a linear array.  
         [0030]    In another alternative to the FIG. 4 embodiment, a variable color filter of the type shown in FIG. 3 could be used. In this case, there would be no need for grating  414  and the detector could be a linear array.  
         [0031]    [0031]FIG. 5A shows a detection system  500  suitable for use with any of the embodiments described above. As shown, detection system  500  uses an array of detection optics ( 504   a  through  504   c  for this example) to image a sample wafer  502 . As shown in FIG. 5B, each detection optic  504  views a two-dimensional segment of wafer  502 . Each detection optic  504  includes a spherical mirror  506 , a cubical beam splitter  508  and a detector array  510 . Spherical mirrors  506  collect light reflected by sample  502 . The collected light is directed by beam splitters  508  to detector arrays  510 . In addition to supplying detector arrays  510 , the combination of beam splitters  508  and spherical mirrors  506  collapses the path length used in detection system  500 .  
         [0032]    In another alternate to FIG. 5A, spherical mirror  506  and cube beam splitter  508  in detector optic  504  are replaced by a lens array. The lens array collects light reflected off the sample  502 , at multiple discreet points over sample  502 . The collected light is directed to detector arrays  510 . In this type of implementation, lens diameter controls the sampling frequency at sample  502  and lens NA controls spatial resolution of the image. Lens array implementations may be implemented using lithographic techniques which, increases, in many cases the density with which the individual lenses are grouped.  
         [0033]    [0033]FIG. 6A shows a second detection system  600  suitable for use with any of the embodiments described above. As shown, detection system  600  includes one or more refractive optical elements  602 . The individual optical elements  602   a  through  602   c  are shown more clearly in FIG. 6B. FIG. 6C shows detection system  600  used in combination with reflective elements  602   a  and  602   b . Reflective elements  604  fold the beam path of detection system  600 , reducing its physical size.  
         [0034]    [0034]FIGS. 7A and 7B shows a third detection system  700  suitable for use with any of the embodiments described above. As shown, detection system  700  uses reflective optical elements. Detector system  700  includes a flat mirror  702  for gathering energy reflected by a sample (sample not shown). The energy gathered by mirror  702  is projected to a sequence that includes a convex mirror  704  followed by a concave mirror  706 , a flat mirror  708 , an aperture  710  and a concave mirror  712 . Concave mirror  712  is followed by a detector  714 . Mirrors  704 ,  706  and  712  set system magnification. Mirrors  704  and  708  fold the system for packaging purposes. FIG. 7B is a perspective view of FIG. 7A.