Abstract:
An imaging system including a back-plane reflector having a concave aspherical reflecting surface and an outer diameter that is no greater than a first distance, with an aperture formed in the back-plane reflector, the aperture for admitting light from a field of view to the imaging system, a fore-plane reflector having a concave aspherical reflecting surface and an outer diameter that is no greater than the first distance, with an aperture formed in the fore-plane reflector, the aperture for discharging the light from the imaging system to an image plane, and a central reflector having a convex aspherical reflecting surface for receiving light from the fore-plane reflector and discharging the light from the imaging system through the aperture in the fore-plane reflector.

Description:
FIELD 
       [0001]    This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to ultraviolet optical inspection of integrated circuits. 
       INTRODUCTION 
       [0002]    Optical inspection systems are used to detect very small defects in the substrates and structures of which integrated circuits are formed. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices. 
         [0003]    There is a continual need to increase both the throughput and the sensitivity of such inspection systems. In optics-based inspection tools, the use of very high numerical aperture (NA) optics, and high-power, short-wavelength (λ) radiation (typically within the deep ultraviolet range) provided by emission-pulsed lasers are ways to achieve these goals. The optical resolution (R) of such a system is defined by the Rayleigh criterion, where R=(0.61)λ/NA. The smaller the value of the optical resolution (R), the higher the resolution of the optical system. 
         [0004]    Pulsed ultraviolet lasers are currently the only source for generating powers that are high enough to enable the inspection tool to achieve the required throughput and sensitivity. These laser sources tend to have a fairly wide wavelength emission bandwidth in comparison to low-power continuous-wave laser sources, which tend to be pure single-wavelength emission sources. 
         [0005]    Optical systems based solely on refractive (glass) elements suffer from a lack of color correction and chromatic aberrations in the ultraviolet range, due to the limited selection of glass materials with good ultraviolet transmission characteristics. Thus, refractive optics tend to produce poor image qualities at higher numerical apertures. One solution to that problem is the use of catadioptric systems, which are combinations of reflective elements and refractive elements. These systems provide a limited correction of the chromatic aberrations within a small bandwidth of the source light. 
         [0006]    What is needed, therefore, is a system that reduces problems such as those described above, at least in part. 
       SUMMARY OF THE CLAIMS 
       [0007]    The above and other needs are met by an imaging system including a back-plane reflector having a concave aspherical reflecting surface and an outer diameter that is no greater than a first distance, with an aperture formed in the back-plane reflector, the aperture for admitting light from a field of view to the imaging system, a fore-plane reflector having a concave aspherical reflecting surface and an outer diameter that is no greater than the first distance, with an aperture formed in the fore-plane reflector, the aperture for discharging the light from the imaging system to an image plane, and a central reflector having a convex aspherical reflecting surface for receiving light from the fore-plane reflector and discharging the light from the imaging system through the aperture in the fore-plane reflector. 
         [0008]    By forming a main imaging channel using only the catoptric elements described, the system exhibits a broad numerical aperture, is wavelength insensitive, has extremely high resolution, and is very compact. 
         [0009]    In various embodiments according to this aspect of the invention, the imaging system is a pure catoptric system. In some embodiments the first distance is no more than about 172 millimeters. In some embodiments the central reflector is disposed between the back-plane reflector and the fore-plane reflector. In some embodiments the reflecting surface of the back-plane reflector faces the reflecting surface of the fore-plane reflector. In some embodiments the light admitted to the image system from the field of view reflects twice off the fore-plane reflector, and once each off of both the back-plane reflector and the central reflector. In some embodiments the system admits from the field of view and discharges to the image plane a fan of light having a numerical aperture of from about 0.375 to about 0.93. In some embodiments a dioptric element is disposed so as to receive the light discharged from the central reflector through the aperture in the back-plane reflector. 
         [0010]    In some embodiments a right angle fold prism is disposed between the back-plane reflector and the central reflector for capturing a portion of the light from the field of view and discharging the portion of light from the system between the fore-plane reflector and the back-plane reflector. In some embodiments the right angle fold prism has curved input and output faces. In some embodiments the right angle fold prism admits from the field of view and discharges from the system a fan of light having a numerical aperture of about 0.3. Some embodiments include an illumination source for directing illumination into the system between the back-plane reflector and the fore-plane reflector, into the right angle fold prism and onto the field of view. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0011]    Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the FIGURE, which is not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout, and which depicts an imaging system according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    The various embodiments of the present invention are directed toward a three-reflector, all reflective, catoptric imaging system having no refractive elements, and thus are free of all forms of chromatic aberrations and, in principle, will operate within any bandwidth and all wavelengths of the laser source. 
         [0013]    Various embodiments include a finite-conjugate system with a single magnification stage. One of the reflector surfaces is used two times, and thus all of the optical rays emanating from the source are reflected twice by this reflector, resulting in a four-reflection system that meets the performance requirements of a diffraction-limited lens. 
         [0014]    With reference now to the FIGURE, there is depicted an embodiment  100  of the present invention. The system  100  uses three reflecting surfaces, including a fore-plane reflector  102 , a back-plane reflector  104 , and a central reflector  106 , one surface  104  of which is used two times. Because of this, an optical ray  118  emanating from the object plane  124  undergoes four reflections  108 ,  110 ,  112 , and  114  before focusing into an image plane (not depicted). One reason for using four reflections  108 ,  110 ,  112 , and  114  (instead of a lesser number of reflections) is that these four reflections allow a higher numerical aperture beam to exit the confinements of the folding reflectors and form an image. 
         [0015]    The radii of curvature and reflector separations of all of the reflecting surfaces are chosen to achieve specific design requirements. The radius of curvature of the reflector  104  is chosen to reduce the height (convergence) of the marginal ray  116  emanating from the object plane  124  as it hits reflector  102  after its first reflection from reflector  104 . The radius of curvature of reflector  102  is chosen to reduce the convergence of the marginal ray  116  as it hits reflector  106  after its second reflection from reflector  104 , thereby reducing the obscuration ratio. Finally, the radius of curvature of reflector  106  is chosen to reduce the size of the hole in reflector  104  that is required for the marginal ray  116  to exit the system  100 , which also reduces the obscuration ratio. 
         [0016]    Due to this convergence of the marginal rays, a very compact system can be realized with a reduced obscuration ratio. In other reflecting systems, the primary and secondary mirrors are spaced farther apart to achieve the desired beam convergence. The base curvature of the reflectors  102 ,  104 , and  106  are chosen to achieve this rapid convergence and then the aspheric coefficients are added to correct the primary and higher order aberrations of the optical system  100 . The sag of a surface with aspheric coefficients is defined by the following polynomial: 
         [0000]    
       
         
           
             
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                             c 
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                             h 
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         [0017]    Where: s is the sag of the surface, 
         [0018]    c is the base radius of curvature, 
         [0019]    h is the height from the axis, 
         [0020]    k is the conic constant, and 
         [0021]    the A&#39;s are the aspheric coefficients. 
         [0022]    The A&#39;s are added during the optimization procedure in a systematic manner to reduce the aberrations of the rays emanating from the object plane  124  until the desired degree of correction is achieved, while maintaining the first principle of the design, which is the rapid convergence of the marginal rays. 
         [0023]    In-line reflective optical systems have a central obscuration that prevents collection of a portion of the numerical aperture, thereby reducing its sensitivity. In the system  100 , the location and size of the reflector  106  is selected so as to reduce the central obscuration. As the reflector  106  is brought closer to the reflector  104 , the central obscuration is reduced. However, care must be taken that the marginal rays  116  do not miss the reflector  106  as they exit the system  100 , which would reduce the numerical aperture of the system  100 . 
         [0024]    This balance is achieved in the present system  100  by forming the reflector  106  with a non-spherical surface profile, in this embodiment, an aspheric profile with higher polynomial coefficients. In the system  100 , the location and size of the reflector  106  reduces the obscuration to no more than about sixteen percent, whereas a typical Schwarzschild objective has an obscuration of about twenty percent. 
         [0025]    The overall length  120  of the system  100  and the inter-reflector separation are also adjusted to achieve the results described above, and to reduce the size of the system  100 . As a result, the admitted range of numerical apertures is from about 0.375 (beam boundaries  118 ) to about 0.93 (beam boundaries  116 ), as depicted. This design produces a compact system  100  that in one embodiment is enclosed within a cylindrical housing measuring only about 172 millimeters in diameter  122 , with a length  120  of about 65 millimeters. 
         [0026]    The three reflecting surfaces  102 ,  104 , and  106  in this embodiment are used to generate a diffraction-limited spot within the field of view  124 , which is from about one hundred and fifty microns in diameter to about three hundred microns in diameter. This focused spot is magnified by the system  100  by a factor of about 52.8× at the image plane (not depicted). The compactness of this system  100  prevents the formation of intermediate images, which commonly occurs in catoptric systems. The compactness of the system  100  also reduces the needed diameter of the reflectors  102  and  104 . The diameter of reflector  104  is determined by its physical distance from the object plane  124 , and the diameter of reflector  102  is determined by the path of the marginal rays  116 . 
         [0027]    The surface profile of the reflector  104  dictates the path and direction of the marginal rays  116 . If the rays  116  diverge after reflection  108  from reflector  104 , they will hit the reflector  102  at a position  110  that is radially greater than position  108 , and thus the diameter of reflector  102  would need to be larger than it is in the depicted embodiment. 
         [0028]    In some embodiments, aberrations in the system  100  are reduced by forming some or all of the reflectors  102 ,  104 , and  106  with aspherical surface profiles. A surface with a spherical profile has only a limited capability to correct aberrations of a high numerical aperture optical system. Surfaces with aspherical profiles, on the other hand, can provide greater design freedom, which can be used to reduce the aberrations of the lens that are associated with higher numerical aperture inputs. In the current embodiment  100 , the reflectors  102 ,  104 , and  106  have profiles that are aspherical, which reduces the size of the system  100 , and improves the collection efficiency. 
         [0029]    In order to collect the lower numerical aperture scattering from the substrate plane  124  (low numerical aperture dark field), a second optical channel  126  is added in one embodiment to the system  100 , which makes use of a curved right angle fold prism  128 , with an effective focal length of about 19.28 millimeters. The fold prism  128 , with curved input and output faces, enables the second channel  126  to be very compact so that it fits in the dead space of the large numerical aperture collection optics without blocking any of the useable areas of the reflector system  100 . The curvatures of the prism  128  input and output surfaces are configured to capture a 0.3 numerical aperture fan of rays  130  and contain it within the free space available between the reflectors  102  and  104  before exiting between them. 
         [0030]    All of the elements  132  of the second channel  126  are refractive, and thus the second channel  126  exhibits relatively higher dispersion. Therefore, this second channel  126  is corrected for a design wavelength of 266 nanometers+150 picometers Full Width Half Maximum (266 nm+150 pm FWHM). The dispersion of this channel  126  is tolerable because its numerical aperture coverage is much lower than the main channel. 
         [0031]    Because the second channel  126  collects a numerical aperture up to about 0.3, and the main optical channel collects a numerical aperture of from about 0.375 to about 0.93, only numerical apertures within a range of from about 0.30 to about 0.375 are not collected by the system  100 . 
         [0032]    The fold prism  128  in the second channel  126  can also be used as part of the illumination channel  134 , using a light source  136 . The embodiment depicted generates focused spots of about 1.5 microns in diameter on the focal plane  124 , extending to a field diameter of about 150 microns. 
         [0033]    As described above, the aspherical components of the surfaces  102 ,  104 , and  106  play an important role in achieving a high degree of correction and resolution. The base spherical curves are set so as to form a rapid convergence of the bundle of rays from the object plane  124 . This limits their capability to correct the system aberrations, and thus the need for the higher order polynomial coefficients. 
         [0034]    Another characteristic of this design is the use of two reflections off of the reflector  104 . This has the advantage of reducing cost, but imposes limitations on the degree of correction that can be achieved by this surface. Initially, the marginal rays  116  are diverging as they hit the surface  104  at  108 , and converge after the second reflection off surface  102 . The direction cosine of the rays changes sign in the process, yielding the aberrational characteristics of the rays. This imposes additional constraints on the design, making it virtually impossible to correct without the aid of the aspheric coefficients. 
         [0035]    In other reflective designs, each reflector is used only once, and thus can have purely spherical forms. However, such systems will also be larger than the present system  100 . 
         [0036]    These two characteristics of the system  100 —base curves that impart a rapid convergence of the marginal rays and the systematic addition of the aspheric coefficients to correct the system aberrations—make the system  100  unique in the art of optical design. 
         [0037]    In another embodiment a thin refractive element  138  is added after the reflector  106 , just outside the mirror box of the system  100 , to provide additional correction for off-axis fields. This element  138  is thin enough so as to not have any impact on the chromatic correction of the system  100 , but doubles the field size  124 . 
         [0038]    The foregoing description of embodiments for this invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.