Illuminator for dark field inspection

Abstract

Light from a single source is divided among several illumination arms, each of which directs light via a multimode fiber bundle from the source to the wafer location. The arms are arranged circumferentially around a common illumination region, so that the region is illuminated from several directions. For each arm, light exiting the fiber bundle enters a turning prism, reflects off the hypotenuse of the prism, and is diverged in one dimension by a negative cylindrical surface on the exiting face of the prism. The beam then reflects off an anamorphic mirror and propagates to the illumination region on the wafer. The beam has an asymmetric footprint, so that it illuminates a nearly circular region of the wafer when viewed at normal incidence. The fiber bundle is at the front focal plane in the meridional dimension. The illumination region is at the rear focal plane in both dimensions.

1 . An illuminator for dark field inspection of a surface of a target, comprising: 
 one or more illumination arms disposed about an illumination region, each arm comprising: 
 an extended, substantially uniform source of diverging light;  
 a turning element having a first face optically coupled to the light source, and a second face having optical power in a meridional direction; and  
 an anamorphic element having a first optical power in an azimuthal direction and a second optical power in the meridional direction, the first and second optical powers being unequal, and the anamorphic element being optically coupled to the second face of the turning element.  
   
     
     
         2 . The illuminator of  claim 1 , wherein the second face of the turning element has no power in the azimuthal direction.  
     
     
         3 . The illuminator of  claim 1 , wherein the second face of the turning element has optical power in the azimuthal direction.  
     
     
         4 . The illuminator of  claim 1 , wherein the anamorphic element comprises a mirror.  
     
     
         5 . The illuminator of  claim 1 , wherein the anamorphic element comprises a lens.  
     
     
         6 . The illuminator of  claim 1 , wherein the turning element comprises: 
 a turning prism; and    a negative cylindrical lens attached to a face of the turning prism;    wherein the first face of the turning element is an incident face of the turning prism, and the second face of the turning element is an exiting face of the cylindrical lens.    
     
     
         7 . The illuminator of  claim 1 , wherein: 
 the turning element comprises a turning prism having an incident face and an exiting face, the exiting face containing a concave cylindrical surface; and    the first face of the turning element is the incident face of the turning prism, and the second face of the turning element is the exiting face of the turning prism.    
     
     
         8 . The illuminator of  claim 1 , wherein each arm has an azimuthal front focal plane, an azimuthal rear focal plane, a meridional front focal plane, and a meridional rear focal plane, and wherein for each arm: 
 the azimuthal front focal plane and the meridional front focal plane are not coincident;    the light source is located substantially at the meridional front focal plane;    the meridional rear focal plane and the azimuthal rear focal plane are substantially coincident;    the illumination region is substantially circular with a center located substantially at the meridional rear focal plane and at the azimuthal rear focal plane; and    the light source, turning element, and anamorphic element are arranged to illuminate the illumination region at a non-zero angle of incidence relative to a normal of the illumination region to achieve specular reflection from the target surface under defect-free conditions.    
     
     
         9 . The method of  claim 8 , wherein the illumination region is within a field of view of a collection lens having a numerical aperture that defines a maximum collection angle of light for the collection lens, the angle of incidence being greater than the maximum collection angle.  
     
     
         10 . The illuminator of  claim 8 , wherein the angle of incidence is greater than approximately 53 degrees.  
     
     
         11 . The illuminator of  claim 10 , wherein the angle of incidence is greater than approximately 81 degrees.  
     
     
         12 . The illuminator of  claim 1 , wherein the light source operationally produces light having a finite spectrum.  
     
     
         13 . The illuminator of  claim 1 , wherein the light source comprises a high pass ultraviolet filter.  
     
     
         14 . The illuminator of  claim 1 , wherein the cylindrical optical power is negative.  
     
     
         15 . The illuminator of  claim 1  wherein the illuminator comprises five arms.  
     
     
         16 . An illuminator for dark field inspection of a surface of a target, comprising: 
 one or more illumination arms disposed about a substantially circular illumination region substantially at the target surface, and each the arm comprising:    a light source;    a coupling element having an incident face optically coupled to the light source, and an exiting face with negative cylindrical optical power in a meridional direction; and    an anamorphic element having a first optical power in an azimuthal direction and a second optical power in the meridional direction, the first and second optical powers being unequal, and the anamorphic element being optically coupled to the exiting face of the coupling element;    wherein each of the arms has an azimuthal front focal plane, an azimuthal rear focal plane, a meridional front focal plane, and a meridional rear focal plane; and    wherein for each of the arms: 
 the azimuthal front focal plane and the meridional front focal plane are not coincident;  
 the light source is located substantially at the meridional front focal plane;  
 the meridional rear focal plane and the azimuthal rear focal plane are substantially coincident;  
 the illumination region has a center located substantially at the meridional rear focal plane and at the azimuthal rear focal plane; and  
 the light source, turning element, and anamorphic element are arranged to illuminate the illumination region at a non-zero angle of incidence relative to a normal of the illumination region to achieve specular reflection from the target surface under defect-free conditions.  
   
     
     
         17 . The method of  claim 16 , wherein the illumination region is within a field of view of a collection lens having a numerical aperture that defines a maximum collection angle of light for the collection lens, the angle of incidence being greater than the maximum collection angle.  
     
     
         18 . The illuminator of  claim 16 , wherein the angle of incidence is greater than approximately 53 degrees.  
     
     
         19 . The illuminator of  claim 16 , wherein the angle of incidence is greater than approximately 81 degrees.  
     
     
         20 . The illuminator of  claim 16 , wherein the light source operationally produces light having a finite spectrum.  
     
     
         21 . The illuminator of  claim 16 , wherein the light source comprises a high pass ultraviolet filter.  
     
     
         22 . The illuminator of  claim 16 , wherein the anamorphic element comprises a mirror.  
     
     
         23 . The illuminator of  claim 16 , wherein the anamorphic element comprises a lens.  
     
     
         24 . The illuminator of  claim 16 , wherein the coupling element comprises a turning element.  
     
     
         25 . The illuminator of  claim 16 , wherein the coupling element comprises a non-turning element.  
     
     
         26 . The illuminator of  claim 16  wherein the light source is an extended, substantially uniform source of diverging light.  
     
     
         27 . The illuminator of  claim 16  comprising five illumination arms.  
     
     
         28 . An illuminator for dark field inspection of a surface of a target, comprising: 
 a plurality of illumination arms disposed about an illumination region substantially at the target surface, and each of the arms comprising: 
 an extended, substantially uniform source of diverging light; and  
 an assembly of optical elements for producing from the diverging light an illuminating beam directed to the illumination region, the illuminating beam having a nominal incident angle, an incident angle range, and an azimuthal angle range;  
   wherein both the nominal incident angle and incident angle range are essentially invariant throughout the illumination region;    wherein the azimuthal angle range is essentially invariant throughout the illumination region; and    wherein light intensity throughout the illumination region is essentially invariant.    
     
     
         29 . An illuminator for dark field inspection of a surface of a target, comprising: 
 a plurality of illumination arms disposed about a substantially circular illumination region substantially at the target surface, and each of the arms comprising: 
 means for establishing an azimuthal rear focal plane;  
 means for establishing a meridional rear focal plane, the meridional rear focal plane and the azimuthal rear focal plane being substantially coincident;  
 means for establishing an azimuthal front focal plane;  
 means for establishing a meridional front focal plane separated from the azimuthal front focal plane;  
 means for locating a center of the illumination region substantially at the meridional rear focal plane and at the azimuthal rear focal plane;  
 means for introducing light having extended, substantially uniform, and diverging characteristics substantially at the meridional front focal plane; and  
 means for forming a beam from the light introduced at the meridional front focal plane, the beam being incident on the illumination region at a non-zero angle of incidence relative to a normal of the illumination region to achieve specular reflection from the target surface under defect-free conditions.  
   
     
     
         30 . A method for illuminating an illumination region on a surface of a target from a plurality of different directions with respective optical systems to perform a dark field inspection of the target surface, comprising for each of the optical systems: 
 establishing an azimuthal rear focal plane;    establishing a meridional rear focal plane, the meridional rear focal plane and the azimuthal rear focal plane being substantially coincident;    establishing an azimuthal front focal plane;    establishing a meridional front focal plane separated from the azimuthal front focal plane;    establishing the illumination region as a substantially circular region with a center substantially at the meridional rear focal plane and at the azimuthal rear focal plane;    introducing light having extended, substantially uniform, and diverging characteristics substantially at the meridional front focal plane; and    forming a beam from the light introduced at the meridional front focal plane, the beam being incident on the illumination region at a non-zero angle of incidence relative to a normal of the illumination region to achieve specular reflection from the target surface under defect-free conditions.    
     
     
         31 . The method of  claim 30 , wherein the beam forming step comprises forming the beam within a field of view of a collection lens having a numerical aperture that defines a maximum collection angle of light for the collection lens, the angle of incidence being greater than the maximum collection angle.  
     
     
         32 . The method of  claim 30 , wherein the angle of incidence in the beam forming step is greater than about 53 degrees.  
     
     
         33 . The method of  claim 32 , wherein the angle of incidence in the beam forming step is greater than about 81 degrees.  
     
     
         34 . The method of  claim 30 , wherein the beam forming step comprises spectrally filtering the beam.  
     
     
         35 . A wafer inspection system comprising: 
 a fiber optic dark-field illuminator comprising: 
 a source of spectrally filtered, spatially uniform, extended and diverging light; and  
 a plurality of illumination arms optically coupled to the light source and circumferentially disposed about a substantially circular illumination region; and  
   a camera disposed at normal or near-normal incidence with respect to the illumination region, the illumination region being within a field of view of the camera, and the camera comprising a collection lens having a numerical aperture that defines a maximum collection angle of light for the collection lens;    wherein each of the illumination arms comprises:    a turning element having an incident face optically coupled to the light source, and an exiting face with negative cylindrical optical power in a meridional direction; and    an anamorphic mirror having a first optical power in an azimuthal direction and a second optical power in the meridional direction, the first and second optical powers being unequal, and the anamorphic mirror being optically coupled to the exiting face of the turning prism;    wherein each of the illumination arms has an azimuthal front focal plane, an azimuthal rear focal plane, a meridional front focal plane, and a meridional rear focal plane; and    wherein for each of the arms:    the azimuthal front focal plane and the meridional front focal plane are not coincident;    the light source coupling is located substantially at the meridional front focal plane;    the meridional rear focal plane and the azimuthal rear focal plane are substantially coincident;    the illumination region has a center located substantially at the meridional rear focal plane and at the azimuthal rear focal plane; and    the light source coupling, turning element, and anamorphic mirror are arranged to illuminate the illumination region at an angle of incidence relative to a normal of the illumination region that is greater than the maximum collection angle.    
     
     
         36 . The method of  claim 35 , wherein the turning element comprises a turning prism.  
     
     
         37 . An illuminator for dark field inspection of a surface of a target, comprising: 
 a plurality of illumination arms disposed about an illumination region, each arm comprising: 
 a light source;  
 a coupling element having a first face optically coupled to the light source, and a second face having optical power in a meridional direction; and  
 an anamorphic element having a first optical power in an azimuthal direction and a second optical power in the meridional direction, the first and second optical powers being unequal, and the anamorphic element being optically coupled to the second face of the turning element.  
   
     
     
         38 . The illuminator of  claim 37 , wherein the second face of the turning element has no power in the azimuthal direction.  
     
     
         39 . The illuminator of  claim 37 , wherein the second face of the turning element has optical power in the azimuthal direction.