Abstract:
An optical filter for the selective attenuation of specific wavelengths of light includes at least one spectrally dispersive element, such as a diffraction grating or prism, in combination with an optical filter. A dispersive element separates broadband light into a constituent wavelength spread in angle space. An optical filter, or filter array, can block and/or attenuate specific wavelengths or wavelength ranges of interest while the light is angularly dispersed. A second dispersive element can recombine this filtered, separated wavelength fan of light into a coaxial broadband beam having a smoother intensity profile than the unfiltered beam.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority to U.S. Provisional Patent Application No. 60/558,873, entitled “BROADBAND WAVELENGTH SELECTIVE FILTER,” filed Apr. 2, 2004, which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     This invention relates to the non-destructive evaluation of samples, such as semiconductor wafers.  
       BACKGROUND  
       [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 techniques, operate by illuminating a sample with a probe beam, then detecting and analyzing the reflected radiation. Ellipsometry and reflectometry are two examples of commonly used optical metrology techniques. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Changes in intensity are analyzed for reflectometry, while scatterometry is used when the structural geometry of a sample creates diffraction (optical scattering) of the probe beam. Scatterometry systems analyze diffraction to deduce details of the structures that cause the diffraction to occur.  
         [0004]     Various optical techniques have been used to perform optical scatterometry. These include broadband spectroscopy (such as is described in U.S. Pat. Nos. 5,607,800; 5,867,276 and 5,963,329), spectral ellipsometry (U.S. Pat. No. 5,739,909) single-wavelength optical scattering (U.S. Pat. No. 5,889,593), and spectral and single-wavelength beam profile reflectance and beam profile ellipsometry (U.S. Pat. No. 6,429,943). Each of the above-listed patents is hereby incorporated herein by reference. Scatterometry generally refers to optical response information in the form of diffraction orders produced by periodic structures (e.g., gratings on a wafer). In addition, any of these measurement technologies, e.g., single-wavelength laser BPR or BPE, can be employed to obtain critical dimension (CD) measurements on non-periodic structures, such as isolated lines or isolated vias and mesas.  
         [0005]      FIG. 1  shows a diagram for a typical optical metrology tool, in which an illumination source  102  creates a monochromatic or polychromatic probe beam. The probe beam is projected by one or more lenses  104 ,  106  onto the surface of a sample  108 . The sample reflects at least a portion of the probe beam, and the reflected portion is transported to a detector  110 . The detector transforms the received energy into corresponding output signals. A processor  112  analyzes the signals to measure the structure or composition of the sample.  
         [0006]     A polychromatic probe beam typically is used for broadband measurements. The spectral range of these probe beams can be quite large, such as from near-infrared to deep ultra-violet. Creating broadband probe beams is technically challenging. Typically, the outputs of two or more different sources must be combined to create the desired spectral range. The combined outputs then must be filtered to remove undesired components, such as infrared components, as well as to maximize constancy and minimize light contamination over the desired spectral range.  
         [0007]     Optical filters exist in a broad array of configurations. There are filters that absorb or reflect various wavelength bands. These filters can be composed of bulk materials like colored glasses, or composed of dielectric stacks of thin films, for example, that combine to make a filter with the desired properties. It is extremely difficult, if not impossible, to manufacture filters that have specific desired attenuation properties from the deep ultra violet to the infrared because of material properties of bulk materials, such as glasses, and materials used for thin films. For example, there are sources that operate from the deep UV (e.g., 190-nm) to the near-infrared (e.g., 900-nm). Filters for these sources must have high transmissivity in that range and simultaneously block all longer wavelength light (e.g., above 950-nm) in order to prevent overheating of the optical system. While standard approaches for this problem exist, these approaches tend to be highly inefficient. In addition, when an optical spectrum is desired that has a smooth intensity, as a function of wavelength, and the spectral distribution contains a number of intensity spikes, such as in the case of Xenon arc lamps, it is virtually impossible to clean the spectrum with conventional filters without severely impacting the overall light throughput. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a block diagram of a prior art optical metrology tool.  
         [0009]      FIG. 2  shows an optical filter arrangement that can be used in accordance with one embodiment of the present invention.  
         [0010]      FIG. 3  shows a filter array that can be used with the optical filter arrangement of  FIG. 2 .  
         [0011]      FIG. 4  shows an optical filter arrangement that can be used in accordance with another embodiment of the present invention.  
         [0012]      FIG. 5  shows an optical filter arrangement that can be used in accordance with another embodiment of the present invention.  
         [0013]      FIG. 6  shows an optical filter arrangement that can be used in accordance with another embodiment of the present invention.  
         [0014]      FIG. 7  shows a filter array that can be used with the optical filter system of  FIG. 6 .  
         [0015]      FIG. 8  is a graph showing optical intensity as a function of wavelength for an un-filtered probe beam.  
         [0016]      FIG. 9  is a graph showing optical intensity as a function of wavelength for a probe beam after processing by the optical filter arrangement of  FIG. 6 .  
         [0017]      FIG. 10  shows a filter array that can be used with the optical filter arrangement of  FIG. 6 .  
         [0018]      FIG. 11  is a graph showing optical intensity as a function of wavelength for a probe beam after processing by the optical filter of  FIG. 6  and filter array of  FIG. 10 .  
         [0019]      FIG. 12  shows a filter array that can be used with the optical filter arrangement of  FIG. 6 .  
         [0020]      FIG. 13  is a graph showing optical intensity as a function of wavelength for a probe beam after processing by the optical filter of  FIG. 6  and filter array of  FIG. 12 .  
     
    
     DETAILED DESCRIPTION  
       [0021]     Systems and methods in accordance with various embodiments of the present invention overcome deficiencies in existing broadband optical devices by changing the way in which light is filtered. A number of optical filter systems and devices in accordance with embodiments of the present invention can be used for the selective attenuation and/or exclusion of specific wavelengths of light across a broadband spectrum.  
         [0022]      FIG. 2  shows an arrangement for a filter system  200  that can be used in accordance with one embodiment of the present invention. The filter system includes a first grating  202  and a second grating  204 , each of which can be any appropriate grating known in the art for broadband dispersion. A polychromatic beam of light  206  is focused by a lens element  218  onto the first grating  202 , which disperses the beam into a ray fan  208  of constituent wavelengths. An optical filter  210  is positioned to receive the ray fan  208  as the fan comes to focus in a spectrum. The optical filter system  200  can include a long wavelength blocker  212  positioned to intercept the ray fan  208  and redirect (or alternately absorb) the long wavelength light rays  214 . After filtering and (any) long wavelength blocking, the ray fan  208  is recombined by grating  204 . The recombined light is collimated by lens  220 , producing a filtered optical beam  216  in which selected spectral regions have been attenuated or eliminated.  
         [0023]     As shown in  FIG. 3 , the optical filter  210  can include a substrate  302  having a series of blocking and/or attenuation regions  304 . Each of these regions  304  can be tailored to transmit a desired amount of light, as well as to absorb, reflect, scatter, and/or diffuse the remainder of the light.  
         [0024]      FIG. 4  shows an optical filter system  400  in accordance with another embodiment of the present invention. Instead of using reflective gratings as in the system of  FIG. 2 , this optical filter includes transmission gratings  402  and  404 . The transmission gratings can be any appropriate gratings known in the art for broadband dispersion. A polychromatic beam of light  406  can be passed through at least one focusing optical element, such as a focusing lens  414 , and transmitted to the first transmissive grating  402 . The first grating  402  can disperse the focused light into a ray fan  408  of constituent wavelengths. An optical filter  410  can be positioned near where the ray fan  408  comes to focus. The filter  410  can be similar or identical to the filter  210  described with respect to  FIGS. 2 and 3 . The optical filter arrangement  400  also can include long and/or short wavelength blockers (not shown) for absorbing and/or redirecting specific rays across the spectrum. After filtering, and any wavelength blocking, the ray fan  408  can be recombined by the second transmissive grating  404 . The recombined beam can be collimated by at least one optical element, such as a collimating lens  416 , to produce a filtered optical beam  412  in which selected spectral regions have been attenuated and/or eliminated.  
         [0025]      FIG. 5  shows an optical filter system  500  in accordance with another embodiment of the present invention. In this arrangement, gratings such as those described with respect to  FIGS. 2 and 4  can be replaced with dispersing prisms  502  and  504 . These dispersing prisms can be any appropriate prisms known for use with broadband light. A polychromatic beam of light  506  is focused by an optical element, such as an appropriate focusing lens  514 , and is dispersed by the first prism  502  into a ray fan  508  of constituent wavelengths. An optical filter  510  is positioned near a focus point of the ray fan  508 . The optical filter  510  can be similar or identical to filter  210  described with respect to  FIGS. 2 and 3 . The optical filter  500  also can include long and/or short wavelength blockers (not shown), capable of absorbing and/or redirecting selected wavelengths of the ray fan. After filtering and any wavelength blocking, the ray fan  508  is recombined by a second prism  504  and collimated by at least one optical element, such as a collimating lens  516 . A filtered optical beam  512  is produced, in which selected spectral regions have been attenuated and/or eliminated.  
         [0026]     It should be noted that the arrangements described above are meant to be exemplary, and that any of a number of different arrangements and/or systems can be used to achieve similar results. For example, any of the above systems can include elements for blocking long or short wavelengths. Alternatively, any wave blocking can be done through the optical filter element. A system also can include more than one optical filter element. The filter elements in these systems can be reflective, transmissive, absorptive, or scattering, and the optical path of each system can be adjusted accordingly. Any lens can be replaced by an equivalent mirror or other appropriate optical element as known in the art. Lenses can be positioned on either side of the dispersive elements.  
         [0027]     Each of the filter arrangements described above is based on the spectral separation of an incoming beam into a ray fan, followed by optical filtering and recombination. These arrangements typically are used to shape spectrums near the light sources. In optical metrology tools, however, there are situations in which optical filtering needs to be done at or near the detector. In these instances, recombination may not be required. For such a situation, an optical filter arrangement  600  can be used such as is shown in  FIG. 6 . In this arrangement, a dispersive element  602 , such as a reflective diffraction grating, receives a converging polychromatic light beam  604  from a focusing optical element. The dispersive element  602  diffracts the incident light into a wavelength dispersed ray fan  606 , as well as a zero order, non-dispersed beam  608  that is typical of diffraction gratings. The ray fan and non-dispersed beam are incident on a filter array  610 . The filter array  610  can be similar to the filter arrays described above, except that this filter array can be configured to selectively attenuate the zero order non-dispersed beam  608 . This filter array also can be configured to smooth other spectral components. For this particular example, the filtered light then is incident on a detector or detector array  612 .  
         [0028]      FIG. 7  shows a filter array  700  that can be used with the arrangement of  FIG. 6 . This array includes a substrate  702  having a series of blocking and/or attenuation regions  704 ( a )-( e ). Each blocking/attenuating region can be tailored to transmit a desired amount of light, while absorbing, reflecting, or scattering diffuse non-desired light. This filter array  700  can be constructed using any appropriate technology, such as patterned dielectric or metallic coating technology. An exemplary filter array is formed of a broadband transmitting material, such as fused silica, with a blocking region  704 ( a ) that eliminates the diffraction grating zero-order beam from interacting with the detector. This exemplary filter array also includes a tailored transmission dielectric filter stack coated region  704 ( b ) for reducing the intensity of the visible portion of the spectrum by 50% and smoothing out the roughness of the curve. The filter array also includes a variety of attenuating dielectric filter stack coatings  704 ( c ),  704 ( d ), and  704 ( e ), wherein each coating has a unique set of characteristics for altering the intensity of rays passing through that filter by a specified amount.  
         [0029]      FIG. 8  is a graph showing a representation of the energy received at the detector  612  in  FIG. 6  as a function of wavelength. For this example, it is assumed that the filter array  610  is transparent and does not attenuate light at any wavelength. Two waveforms are shown, with the first waveform  802  corresponding to the wavelength dispersed ray fan  606  and the second waveform  804  corresponding to the zero order non-dispersed beam  608 . As can be seen, the spectrum contains a number of spikes and is not very uniform.  
         [0030]      FIG. 9  is a graph showing another representation of the energy received at the detector as a function of wavelength, except that this plot assumes that the filter array  610  is configured as shown in  FIG. 7 . As discussed above, such an array is not completely transparent but instead attenuates the beam as a function of coating characteristics and position on the filter array. The effect is shown by waveform  902 , where the zero order beam  608  has been removed and the waveforms across the spectral range of dispersed ray fan  606  have been attenuated. A significant difference can be seen for the infrared (IR) region, or the region above about 7500 Angstroms, where the presence of spikes has been greatly reduced relative to the plot of  FIG. 8 .  
         [0031]     The filter arrays described above, including arrays  210 ,  410 ,  510  and  610 , can be implemented using a range of different technologies. In one approach shown with respect to  FIG. 10 , a filter array  1000  can be formed as a substrate  1002  coated with a variable reflective neutral density filter coating. In this specific example, the coating includes a blocking region  1004 ( a ), a clear region  1004 ( b ), and specific spike attenuating segments  1004 ( c )- 1004 ( e ) that are selected to again remove large intensity fluctuations by wavelength. These regions can be formed in the variable coating by use of moving masks in the coating process or lithography techniques. The filter array alternatively can have a programmed attenuation coating, where the optical properties of the coating are actually programmed as a function of the linear position to correspond to, and compensate for, the actual fluctuations of the light source with wavelength. The stark discontinuities represented by segments  1004 ( c ),  1004 ( d ), and  1004 ( e ) in the figure can be minimized or eliminated using such programming, whereby the coating reflectivity can smoothly track the inverse of the curve generated by the light source as a function of wavelength. Similar smoothing properties can be obtained using a laser writing technique on a photosensitive material, such as HEBS glass available from Canyon Materials of San Diego, Calif.  
         [0032]      FIG. 11  is a graph showing energy received at the detector as a function of wavelength. Plot  1100  shows the energy received using a system such as the system of  FIG. 6  without a variable filter. Plot  1102  shows the energy received when using a filter array as described with respect to  FIG. 10 . It can be seen that the filtered waveform  1102  is relatively flat and smooth when compared to the unfiltered waveform  1100 . It should be noted that any zero order beam that would have been present in the unfiltered system would have been removed by the corresponding blocking region  1004 ( a ) of the filter array.  
         [0033]      FIG. 12  shows a filter array  1200  that can be used in accordance with another embodiment of the present invention. This exemplary filter array  1200  can be assembled to include a number of regions. The array can be assembled by any appropriate means, such as by bonding together materials of appropriate thickness and optical transmissivity/reflectivity to form the desired filter array. For example, a dark glass (or metal) region  1204 ( a ) can be used that is capable of blocking the zero order beam. A clear region  1204 ( b ) can be included that is formed from a material such as fused silica or calcium fluoride, in order to transmit the UV portion of the spectrum. A visible attenuating filter glass region  1204 ( c ), formed from a material such as Schott KG5, can be used to smooth or “bring down” a central peak. A section of fused silica  1204 ( d ) or other appropriate material can allow the near-IR light to be un-attenuated. Specific segments of attenuating glasses  1204 ( e )- 1204 ( g ) can be used to minimize spikes over the IR region of the spectrum.  
         [0034]      FIG. 13  is a graph showing energy received at the detector as a function of wavelength. Plot  1300  shows the energy received using a system such as the system of  FIG. 6  without a variable filter. Plot  1302  shows the energy received when using a filter array as described with respect to  FIG. 12 . The filtered waveform  1302  is relatively flat and smooth when compared to the unfiltered waveform  1300 . Any zero order beam is removed by the blocking region  1204 ( a ) of the filter array. When forming the array, it can be desirable to first obtain plot  1300  in order to get a measure of the shape of the energy spectrum. An analysis then can be done on the curve, using parameters such as tolerance thresholds, local slope, or roughness values, in order to determine how the light would need to be filtered to produce a desired energy curve. A filter array then can be produced that contains a number of filter regions, having determined optical characteristics, necessary to bring the curve to within a desired shape range or smoothness. A balance can be met between the complexity of the filter and the shape of the curve, where necessary, in order to obtain acceptable results while keeping costs and complexity at a reasonable level.  
         [0035]     Any of a number of other approaches can be used to create filter arrays similar to those described with respect to  FIGS. 10 and 12 . For example, filter arrays such as filter arrays  210 ,  410 ,  510  and  610  can be formed from photosensitive materials, such as laser written HEBS glass, as well as from liquid crystal material. Methods for forming and operating liquid crystal materials are known in the art and will not be discussed in detail herein.  
         [0036]     As described above, the present invention provides a range of optical filters for the selective attenuation and/or exclusion of specific wavelengths of light from at least one broadband source. Each filter can use an initial optical element, such as a grating or prism, to spread an incoming beam of light into a ray fan. The incoming beam of light can include undesired spectral ranges (e.g., infrared radiation), and can lack the required degree of uniformity (e.g., includes too much illumination at one or more spectral ranges). The spread optical beam, or ray fan, can be passed through a filter array that can include transparent, translucent, and/or opaque regions. Specific spectral ranges can be eliminated by corresponding opaque regions, attenuated by corresponding translucent regions, or unaffected by corresponding transparent regions. Once filtered, the spread optical beam can be recombined by a second optical element, such as a grating or prism. The second optical element can be of a different type than the first optical element (e.g., a grating may be followed by a prism). Alternately, the second (recombining) optical element optical element can be eliminated, such as where the filter is used as part of a spectrometer or other wavelength resolving detector. The filter array can be designed to act in reflection or in transmission, or in a combination of reflection and transmission.  
         [0037]     It should be recognized that a number of variations of the above-identified embodiments will be obvious to one of ordinary skill in the art in view of the foregoing description. Accordingly, the invention is not to be limited by those specific embodiments and methods of the present invention shown and described herein. Rather, the scope of the invention is to be defined by the following claims and their equivalents.