Multi-spectral techniques for defocus detection

Abstract

A method and apparatus for improved defocus detection on wafers. The use of hyperspectral imaging provides increased sensitivity for local defocus defects, and the use of Fourier Space analysis provides increased sensitivity for extended defocus defects. A combination of the two provides improved overall sensitivity to local and extended defocus defects.

1 . A method for detecting defocus defects on an illuminated sample surface comprising: 
 using a spectroreflectometric technique to form at least one intensity image of outgoing light from a first portion of said sample surface, I(x,y, λ) including a plurality of wavelengths x;    comparing said at least one intensity image with at least one reference intensity image; and    from said comparison, locating defocus defects on said sample surface.    
   
   
       2 . The method of  claim 1 , wherein said spectroreflectometric technique is selected from the group consisting of: 
 a) sequentially illuminating said sample surface with narrow-band light having a plurality of wavelengths, and analyzing outgoing light from said sample surface with an analyzer;    b) illuminating said sample surface with broadband light, then selecting a plurality of narrow wavelength bands of outgoing light for detection and analysis by an analyzer using at least one bandpass filter;    c) illuminating said sample surface with broadband light, then angularly separating light of different wavelengths outgoing from said sample surface using a dispersive element, and analyzing said light outgoing from said sample surface with an analyzer; and    d) illuminating said sample surface with broadband light, then using an interferometer for one of the group consisting of: selecting the wavelengths of outgoing light from said sample surface for observation, and spreading said wavelengths of outgoing light from said sample surface in one dimension only.    
   
   
       3 . The method of  claim 2 , wherein said analyzer includes a computer for data analysis.  
   
   
       4 . The method of  claim 2 , wherein said spectroreflectometric technique comprises sequentially illuminating said sample surface with narrow-band light having a plurality of wavelengths, and analyzing outgoing light from said sample surface with an analyzer.  
   
   
       5 . The method of  claim 4 , wherein said analyzer comprises an imaging camera.  
   
   
       6 . The method of  claim 4 , wherein said step of sequentially illuminating said sample surface with narrow-band light having a plurality of wavelengths comprises illuminating with a collection of LED's of different wavelengths.  
   
   
       7 . The method of  claim 2 , wherein said spectroreflectometric technique comprises illuminating said sample surface with broadband light, then selecting a plurality of narrow wavelength bands of outgoing light for detection and analysis using at least one bandpass filter positioned between said sample surface and said analyzer.  
   
   
       8 . The method of  claim 7 , wherein said at least one bandpass filter is selected from the group consisting of: 
 a) a plurality of fixed absorption filters having differing bandpass regions;    b) a plurality of fixed interference filters having differing bandpass regions; and    c) a continuously adjustable bandpass filter positioned between said sample surface and said analyzer.    
   
   
       9 . The method of  claim 8 , wherein said at least one bandpass filter is a continuously adjustable bandpass filter selected from the group consisting of: Fabry-Perot Etalon interferometer and Lyot filter.  
   
   
       10 . The method of  claim 2 , wherein said spectroreflectometric technique comprises illuminating said sample surface with broadband light, then angularly separating light of different wavelengths outgoing from said sample surface using a dispersive element, and analyzing said light outgoing from said sample surface with an analyzer.  
   
   
       11 . The method of  claim 10 , wherein said dispersive element is selected from the group consisting of prism and diffraction grating.  
   
   
       12 . The method of  claim 10 , wherein said analyzer is a point-measuring system, and said intensity image is obtained by scanning image building.  
   
   
       13 . The method of  claim 10 , wherein said analyzer is an imaging spectrometer.  
   
   
       14 . The method of  claim 2 , wherein said spectroreflectometric technique comprises illuminating said sample surface with broadband light, then using an interferometer for one of the group consisting of: selecting the wavelengths of outgoing light from said sample surface for observation, and spreading said wavelengths of outgoing light from said sample surface in one dimension only.  
   
   
       15 . The method  claim 14 , wherein said steps of forming an intensity image of outgoing light from a portion of said sample surface, and comparing said intensity image with a reference intensity image, are performed utilizing a computer.  
   
   
       16 . The method of  claim 14 , wherein said interferometer is used to select the wavelengths of outgoing light from said sample surface for observation.  
   
   
       17 . The method of  claim 16 , wherein said interferometer is selected from the group consisting of Fabry-Perot, Sagnac, and Michaelson interferometers.  
   
   
       18 . The method of  claim 14 , wherein said interferometer is used for single dimension wavelength spreading.  
   
   
       19 . The method of  claim 18 , wherein said interferometer is selected from the group consisting of Sagnac, generic whiskbroom, and generic pushbroom interferometers.  
   
   
       20 . The method of  claim 1 , further including utilizing imaging polarimetry to provide polarimetric information to said intensity image.  
   
   
       21 . The method of  claim 20 , wherein said polarimetric information is obtained using a point measurement spectroreflectometer.  
   
   
       22 . The method of  claim 21 , wherein an image is built by scanning said sample.  
   
   
       23 . The method of  claim 22 , wherein said steps of scanning said sample to build said image, and comparing said intensity image with a reference intensity image, are performed utilizing a computer.  
   
   
       24 . The method of  claim 1 , wherein said at least one intensity image comprises a defocus signature of said portion of said sample surface, and wherein said at least one reference intensity image comprises a reference defocus signature.  
   
   
       25 . The method of  claim 24 , wherein said reference defocus signature is selected from the group consisting of: 
 a) a defocus signature from a second portion of said sample surface, said first and second portions of said sample surface being equivalent regions on different die,    b) a defocus signature obtained from a portion of a focus-exposure matrix wafer, said portion of said focus-exposure matrix wafer being an equivalent region to said first portion of said sample surface, or    c) a defocus signature obtained from a library spectrum set for a library structure equivalent to the structure on said portion of said sample surface.    
   
   
       26 . The method of  claim 25 , wherein said reference defocus signature is obtained from a library spectrum set for a library structure equivalent to the structure on said portion of said sample surface, and wherein said library spectrum set is comprised of one from the group consisting of: real data and simulated data.  
   
   
       27 . The method of  claim 24 , wherein said step of comparing said at least one intensity image with said at least one reference intensity image comprises making a functional comparison.  
   
   
       28 . The method of  claim 27 , wherein said functional comparison comprises comparison of spectrum statistics.  
   
   
       29 . The method of  claim 24 , wherein said step of comparing said at least one intensity image with at least one reference intensity image is performed utilizing a computer.  
   
   
       30 . An apparatus for detecting defocus defects on an illuminated sample surface, said apparatus comprising: 
 a sample holder for holding a sample to be illuminated;    an illuminator arranged to provide incident light onto said sample;    a detector for detecting outgoing light from said illuminated sample surface;    a hyperspectral imager arranged to form an intensity image I(x, y, λ) of outgoing light from said illuminated sample surface; and    an analyzer configured to compare said intensity image to a reference intensity image.    
   
   
       31 . The apparatus of  claim 30 , wherein said analyzer includes a computer for data analysis.  
   
   
       32 . The apparatus of  claim 30 , wherein said analyzer includes an imaging camera.  
   
   
       33 . The apparatus of  claim 30 , wherein said intensity image is a defocus signature, and wherein said reference intensity image is a reference defocus signature; 
 said reference defocus signature being selected from the group consisting of:    a) a defocus signature from a second portion of said sample surface, said first and second portions of said sample surface being equivalent regions on different die;    b) a defocus signature obtained from a portion of a focus-exposure matrix wafer, said portion of said focus-exposure matrix wafer being an equivalent region to said first portion of said sample surface;    c) a defocus signature obtained from a library spectrum set for a library structure equivalent to the structure on said portion of said sample surface, said library spectrum set comprised of real data; or    d) a defocus signature obtained from a library spectrum set for a library structure equivalent to the structure on said portion of said sample surface, said library spectrum set comprised of simulated data.    
   
   
       34 . The apparatus of  claim 30 , wherein said hyperspectral imager comprises a plurality of illuminators of differing wavelengths, arranged to illuminate said sample surface sequentially.  
   
   
       35 . The apparatus of  claim 30 , wherein said illuminator is a broadband illuminator, and said hyperspectral imager comprises a selector for selecting a plurality of narrow wavelength bands of outgoing light.  
   
   
       36 . The apparatus of  claim 35 , wherein said selector is at least one bandpass filter positioned between said sample surface and said analyzer; 
 said at least one bandpass filter being selected from the group consisting of:    a) a plurality of fixed absorption filters having differing bandpass regions;    b) a plurality of fixed interference filters having differing bandpass regions; and    c) a continuously adjustable bandpass filter.    
   
   
       37 . The apparatus of  claim 36 , wherein said selector is a continuously adjustable bandpass filter selected from the group consisting of: Fabry-Perot Etalon interferometer and Lyot filter.  
   
   
       38 . The apparatus of  claim 30 , wherein said illuminator is a broadband illuminator, and wherein said hyperspectral imager includes one selected from the group consisting of: 
 a) a dispersive element for angularly separating light of different wavelengths outgoing from said sample surface;    b) an interferometer configured to select the wavelengths of outgoing light from said sample surface for observation; and    c) an interferometer configured to perform single dimension wavelength spreading.    
   
   
       39 . The apparatus of  claim 1 , further including a polarimeter arranged to provide polarimetric information about outgoing light from said illuminated sample surface.  
   
   
       40 . The apparatus of  claim 39 , wherein said polarimeter is a point measurement spectroreflectometer configured to build an image of said sample surface when said sample is scanned.  
   
   
       41 . The apparatus of  claim 30 , wherein said analyzer is configured to compare said intensity image to a reference intensity image by means of a functional comparison.  
   
   
       42 . A computer-readable storage medium containing computer executable code to: 
 compare at least one intensity image of outgoing light from a first portion of an illuminated sample surface, said intensity image I(x,y, λ) including a plurality of wavelengths λ and formed using a spectroreflectometric technique, with at least one reference intensity image.    
   
   
       43 . A method for detecting extended defocus defects on a sample surface comprising: 
 illuminating said sample surface with coherent monochromatic light;    observing a far-field intensity image of outgoing light from said sample surface which is the spatial Fourier transform of said sample surface;    comparing said far-field intensity image with a reference far-field intensity image; and    from said comparison, locating extended defocus defects on said sample surface.    
   
   
       44 . The method of  claim 43 , wherein said step of observing a far-field intensity image of outgoing light from said sample surface which is the spatial Fourier transform of said sample surface comprises providing an imaging surface at a distance sufficiently far from said sample surface to see said far-field pattern thereon.  
   
   
       45 . The method of  claim 44 , further including the step of imaging said imaging surface with a camera.  
   
   
       46 . The method of  claim 45 , further including the step of digitizing the data obtained from said imaging and analyzing said digitized data using a computer.  
   
   
       47 . The method of  claim 44 , further including the step of relaying said outgoing light to said imaging surface comprising an image plane.  
   
   
       48 . The method of  claim 47 , wherein said step of relaying said outgoing light to said imaging surface comprising an image plane is performed using one selected from the group consisting of: directly capturing outgoing pencils of light with a large diameter lens, and using a large-size replicated mirror.  
   
   
       49 . The method of  claim 43 , wherein said step of comparing said far-field intensity image with a reference far-field intensity image comprises: 
 generating one image from a sample field, which contains a signature of the field; and    comparing said image to other similar images from other similar fields.    
   
   
       50 . The method of  claim 46 , wherein said step of analyzing said digitized data using a computer comprises a simple subtraction of patterns followed by a thresholding step.  
   
   
       51 . The method of  claim 43 , wherein said reference far-field intensity image is a Fourier signature chosen from a library of Fourier signatures collected using focus-exposure matrix wafers.  
   
   
       52 . An apparatus for detecting extended defocus defects on a sample surface comprising: 
 a holder for holding a sample thereon;    a coherent monochromatic light source arranged to provide incident light onto said sample surface;    an imaging surface positioned at a distance sufficiently far from said sample surface to see said far-field pattern of outgoing light from said sample surface thereon; and    an imager for imaging said imaging surface.    
   
   
       53 . The apparatus of  claim 52 , wherein said sample holder includes an x-y stage.  
   
   
       54 . The apparatus of  claim 52 , wherein said imager comprises a lens positioned to direct said outgoing light onto said imaging surface, and a camera.  
   
   
       55 . The apparatus of  claim 52 , further including a relay for relaying said outgoing light to said imaging surface comprising an image plane.  
   
   
       56 . The apparatus of  claim 55 , wherein said relay is one of the group consisting of: a large diameter lens for directly capturing outgoing pencils of light, and a large-size replicated mirror.  
   
   
       57 . The apparatus of  claim 52 , further including an analyzer coupled to said imager for analyzing data.  
   
   
       58 . The apparatus of  claim 57 , wherein said analyzer includes a computer.  
   
   
       59 . The apparatus of  claim 57 , wherein said analyzer is configured to: 
 generate an image from a sample field, which contains a signature of the far-field pattern of the field; and    compare said image to other similar images from other similar fields.    
   
   
       60 . The apparatus of  claim 59 , wherein said analyzer is further configured to compare said image to other similar images from other similar fields by a simple subraction of patterns followed by a thresholding step.  
   
   
       61 . A computer-readable storage medium containing computer executable code to: 
 generate an image from a sample field from a sample surface illuminated with monochromatic coherent light, said image containing a signature of the far-field pattern of the field; and    compare said image to other similar images from other similar fields.    
   
   
       62 . A method for detecting local and extended defocus defects on a sample surface comprising: 
 illuminating said sample surface with coherent monochromatic light;    observing a far-field intensity image of outgoing light from said sample surface which is the spatial Fourier transform of said sample surface;    comparing said far-field intensity image with a reference far-field intensity image;    further using a spectroreflectometric technique to form an intensity image of outgoing light from a portion of said sample surface, I(x,y, λ) including a plurality of wavelengths λ;    comparing said intensity image with a reference intensity image; and    from said comparisons, locating both local and extended defocus defects on said sample surface.    
   
   
       63 . The method of  claim 62 , wherein said steps of: 
 illuminating said sample surface with coherent monochromatic light;    observing a far-field intensity image of outgoing light from said sample surface which is the spatial Fourier transform of said sample surface; and    comparing said far-field intensity image with a reference far-field intensity image;    are performed for gross defect detection; and    wherein said steps of: 
 further using a spectroreflectometric technique to form an intensity image of outgoing light from a portion of said sample surface, I(x,y, λ) including a plurality of wavelengths λ;  
   comparing said intensity image with a reference intensity image    are performed on selected spots on said sample surface for detailed defect detection.    
   
   
       64 . The method of  claim 62 , wherein said spectroreflectometric technique is selected from the group consisting of: 
 sequentially illuminating said sample surface with narrow-band light having a plurality of wavelengths, and analyzing outgoing light from said sample surface;    f) illuminating said sample surface with broadband light, then selecting a plurality of narrow wavelength bands of outgoing light for detection and analysis using at least one bandpass filter;    g) illuminating said sample surface with broadband light, then angularly separating light of different wavelengths outgoing from said sample surface using a dispersive element, and analyzing said light outgoing from said sample surface; and    h) illuminating said sample surface with broadband light, then using an interferometer for one of the group consisting of: selecting the wavelengths of outgoing light from said sample surface for observation, and spreading said wavelengths of outgoing light from said sample surface in one dimension only.    
   
   
       65 . An apparatus for detecting local and extended defocus defects on a sample surface comprising: 
 a sample holder for holding a sample to be illuminated;    an illuminator arranged to provide a first incident light onto said sample;    a detector for detecting outgoing light from said illuminated sample surface;    a hyperspectral imager arranged to form an intensity image I(x, y, λ) of first outgoing light from said illuminated sample surface;    an analyzer configured to compare said intensity image to a reference intensity image;    a coherent monochromatic light source arranged to provide a second incident light onto said sample surface;    an imaging surface positioned at a distance sufficiently far from said sample surface to see said far-field pattern of second outgoing light from said sample surface thereon; and    a far field imager for imaging said imaging surface.    
   
   
       66 . The apparatus of  claim 65 , wherein: 
 said illuminator is a broadband illuminator;    said imaging surface is a screen having an aperture therethrough to admit said first incident light onto said sample surface.    
   
   
       67 . The apparatus of  claim 66 , wherein said hyperspectral imager is an imaging spectrometer, and further including a mirror positioned to deflect said first outgoing light such that said imaging spectrometer does not impede line of sight of said far field imager onto said imaging surface.