Wafer inspection using short-pulsed continuous broadband illumination

Abstract

An inspection system may be configured to inspect objects, such as semiconductor wafers, using narrow-pulse broadband illumination. The illumination may be obtained in some embodiments using a laser configured to emit light into a material having a spectral broadening effect. The inspection system can include various filters which may be selectively placed in the illumination and/or imaging path in order to tune the spectrum of light impinging on the wafer and the light that is detected. The filters may include selectable filters, fixed filters, and filters whose characteristics can be adjusted in-place. In some embodiments, filters may be used to match the illumination/detection spectra of different tools. Additionally, the broadband illumination may be tuned between inspections and/or during inspections for best results. The system may support Fourier filtering whereby light, related to repetitive features of the object and in one or more wavelength sub-bands of the illumination, may be filtered.

1 - 32 . (canceled) 
     
     
         33 . A method of inspecting a wafer in an optical inspection system, the method comprising:
 a. emitting a pulse of light, the pulse having a length no longer than 1 μs in time;   b. directing the emitted pulse into a nonlinear optical element to obtain broadband illumination having a continuous spectral range of at least 20 nm;   c. tuning the broadband illumination to at least a first spectrum for inspection of a first region of the wafer and a second spectrum for inspection of a second region of the wafer in order to reduce the color variation in images of the first and second regions;   d. directing the broadband illumination toward the wafer; and   e. capturing at least one image of at least a portion of the wafer using an imager comprising at least one detector.   
     
     
         34 . The method as set forth in  claim 33 , wherein the nonlinear optical element comprises a bundle of photonic crystal fibers, at least some of the fibers in the bundle have differing optical lengths from one another. 
     
     
         35 . The method as set forth in  claim 33 , wherein said step of directing the broadband illumination toward the wafer comprises focusing the broadband illumination such that at least one wavelength band of the broadband illumination is focused at a different location above or below the focus point of another wavelength band of the broadband illumination relative to the surface of the wafer. 
     
     
         36 . The method as set forth in  claim 33 , wherein said tuning step comprises filtering light by placing at least one filter in one or more of the optical path between the nonlinear optical element and the wafer and the optical path between the wafer and the at least one detector included in the imager. 
     
     
         37 . The method as set forth in  claim 36 , wherein said tuning step comprises filtering light by adjusting the optical characteristics of the at least one filter. 
     
     
         38 . The method as set forth in  claim 33 , wherein the pulse of light has a continuous spectral range of at least 50 nm. 
     
     
         39 . The method as set forth in  claim 33 , wherein the pulse of light has a length of at least 1 ns in time. 
     
     
         40 . The method as set forth in  claim 33 , further comprising:
 f. collecting illumination returned from the wafer by using at least one optical element having a Fourier plane and splitting the illumination returned from the wafer into a plurality of wavelength sub-bands, each wavelength sub-band having a separate optical path; and   g. placing at least one filter mask at the Fourier plane in each optical path, wherein the at least one filter mask in each optical path is configured to block light containing information related to the repetitive features at a plurality of the respective optical path's wavelength sub-band.   
     
     
         41 . The method set forth in  claim 40 , further comprising: combining the illumination at each of the plurality of wavelength sub-bands into a single image after the blocking performed by each at least one filter mask. 
     
     
         42 . The method as set forth in  claim 40 , wherein the broadband illumination has a continuous spectral range falling within a range from 180 nm to 450 nm, and wherein each of the plurality of wavelength sub-bands has a continuous spectral range of at least 50 nm. 
     
     
         43 . A wafer inspection system comprising:
 a. an imager operative to image at least one wafer while the wafer is illuminated, the imager including at least one detector;   b. an illumination source comprising a laser and a non-linear optical component having a spectral broadening effect on light from the laser, so that the illumination source is configured to illuminate the at least one wafer with a pulse of illumination of at least 1 ns, but no longer than 1 μs, in time and having a continuous spectral range of at least 20 nm; and   c. a control system configured to adjust the spectrum of light illuminating the wafer in order to reduce the color variation in images of the wafer.   
     
     
         44 . The wafer inspection system as set forth in  claim 43 , wherein the illumination source comprises a pulsed laser configured to emit illumination having a spectral range falling within a range of 180 to 450 nm. 
     
     
         45 . The wafer inspection system as set forth in  claim 43 , wherein the non-linear optical component comprises one or more of photonic crystal fibers and fused silica fibers, at least some of the fibers in the bundle having differing optical lengths. 
     
     
         46 . The wafer inspection system as set forth in  claim 43 , further comprising at least one filter configured to pass one or more specified ranges of wavelengths, wherein the at least one filter is placed in a location selected from the optical path between the nonlinear optical element and the wafer and the optical path between the wafer and at least one detector included in the imager 
     
     
         47 . The wafer inspection system as set forth in  claim 46 , wherein the at least one filter tunes the spectrum of light illuminating the wafer to at least a first spectrum for inspection of a first region of the wafer and a second spectrum for inspection of a second region of the wafer in order to reduce the color variation in images of the first and second regions. 
     
     
         48 . The wafer inspection system as set forth in  claim 46 , wherein the system is configured to select one or more specified ranges of wavelengths by tuning the optical characteristics of the filter or placing or removing the at least one filter in the imaging path based on the optical characteristics of the wafer. 
     
     
         49 . The wafer inspection system as set forth in  claim 43 , further comprising:
 d. at least one optical element positioned to form an image of the wafer, the optical element having a Fourier plane; and   e. at least one filter mask placed in the Fourier plane, wherein the at least one mask is configured to block light corresponding to repetitive features of the wafer under inspection, the light falling in a continuous wavelength band.   
     
     
         50 . The wafer inspection system as set forth in  claim 49 ,
 wherein at least one filter mask comprises a plurality of blocking areas; and   wherein at least some of the blocking areas block light in different wavelength sub-bands within the continuous wavelength band than the other blocking areas.   
     
     
         51 . The wafer inspection system as set forth in  claim 49 ,
 wherein the system comprises a plurality of filter masks; and   wherein at least some of the masks are configured to block light in different wavelength sub-bands within the continuous wavelength band than the other masks.   
     
     
         52 . The wafer inspection system set forth in  claim 43 , further comprising at least one optical element configured to focus at least one wavelength band of the illumination at a different point above or below the focus point of another wavelength band relative to the surface of the wafer.