Abstract:
An apparatus and method for optically characterizing the reflection and transmission properties of a sample with a beam of light having a small diameter on a surface of the sample over a broadband of wavelengths, from 190 nm to 1100 nm. Reflective optical components, including off-axis parabolic mirrors with a collimated incident or reflected broadband beam of light, minimize non-chromatic aberration. The apparatus and method further disclose an optical light path that can be focused by adjusting the position of an off-axis parabolic mirror and a planar mirror.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is co-filed with application “Apparatus and Method for Optical Characterization of a Sample Over a Broadband of Wavelengths While Minimizing Polarization Changes” by Ray Hebert, Marc Aho and Abdul Rahim Forouhi, which is herein incorporated by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to an apparatus and method for optically characterizing the properties of a sample on reflection and transmission of light over a broadband of wavelengths with a small spot size on a surface of the sample.  
       BACKGROUND OF THE INVENTION  
       [0003]     Advances in microelectronics necessitate components with ever smaller critical dimensions. Manufacturing such components requires the use of shorter wavelengths of light in the lithography processes employed in component fabrication. This, in turn, has lead to a need to measure the optical characteristics of samples such as, among other, photolithographic masks and fabricated components over a broad range or band of wavelengths including the UV. Typically, in these measurements the cross-sectional diameter of a beam of light focused on the sample is large enough to spatially average the optical characteristic being measured yet small enough to resolve spatial variations across the sample. As the critical dimensions have decreased so too has the required diameter of the beam of light on the sample. It is now desirable to have a diameter of less than 100 micron.  
         [0004]     As with many engineering problems, the design of an optical system to measure the optical characteristics of such a sample represents a tradeoff. For example, when illuminating the beam of light with the broadband of wavelengths from a light source onto a surface of the sample, it is desirable to have a small spot size but not a diffraction limited spot. In addition, this should be accomplished in an optically efficient manner. There is, therefore, a tradeoff in this regard between a need for optical components with a low f-number (for higher optical efficiency) and a need for optical components with a high f-number (for a small spot size over a practical depth of field with minimal aberration and angles of incidence) and thus a small cone of rays corresponding to the beam of light that is used in the optical system, i.e., the useful light. Similar design tradeoffs occur in the collection and illumination on a detector of the beam of light reflected from the sample and the beam of light transmitted through the sample.  
         [0005]     The need to operate over the broadband of wavelengths is a further design constraint for many optical components because they are subject to a variety of effects such as chromatic aberration and absorption. For refractive optical components these effects become pronounced as the wavelengths approach the UV. There exist optical systems based on refractive optical components in the prior art that operate over a broadband of wavelengths with a small diameter of the beam of light on the sample. In these systems, attempts are made to compensate for chromatic aberration and absorption effects. However, this adds expense and complexity to these optical systems.  
         [0006]     Reflective optical components are a suitable solution to this technical challenge. A wide variety of components are available including mirrors with non-spherical shape, such as an off-axis paraboloid shape, henceforth called an off-axis parabolic mirror. However, non-spherical shaped mirrors can add expense to the optical system, especially when such mirrors are manufactured by diamond turning. Optical systems including torroidal, spherical and elliptical mirrors are disclosed in the prior art. For examples, see U.S. Pat. No. 5,910,842, U.S. Pat. No. 6,583,877 and U.S. Pat. No. 6,128,085.  
         [0007]     In addition, many prior art broadband optical systems combine refractive and reflective optical components. However, such catadioptric systems do not avoid the complexity and expense needed to overcome the chromatic aberration and absorption issues associated with refractive optical components.  
         [0008]     Furthermore, when different samples are characterized, the beam of light in the optical system will need to be focused on the sample to correct for effects such as varying surface topography. Such an adjustment is problematic if the adjustment of the position of certain optical components in the optical system necessitates the adjustment of the position of many other optical components, since this can easily lead to misalignment. A preferred solution would allow the beam of light to be focused on the sample by adjusting a minimum number of components in the optical system or a simple assembly of components. Furthermore, such a preferred solution would be a sufficiently compact and simple optical system that a single light source could be used to optically characterize the reflection and transmission properties of the sample.  
         [0009]     There is a continued need, therefore, for a compact optical system for optical characterization of a sample, which operates over a broadband of wavelengths with a small diameter of the beam of light on the sample and which employs reflective optics with a minimum number of optical components such that advantageous components such as off-axis parabolic mirrors can be used. There is also a need for such an optical system that can be focused by adjusting the position of the minimum number of optical components or a simple assembly of components.  
       OBJECTS AND ADVANTAGES  
       [0010]     In view of the above, it is a primary object of the present invention to provide an apparatus and method that enables optical characterization of the properties of a sample on reflection and transmission of a beam of light over a broadband of wavelengths with a small spot size on the surface of the sample. More specifically, it is an object of the present invention to provide a broadband apparatus with a small spot size on the surface of the sample, and a method of using this apparatus, for optical characterization of the properties of the sample on reflection and transmission of the beam of light through the use of optical light paths comprising reflective optical components, including off-axis parabolic mirrors. It is a further object of the present invention to provide an apparatus, and a method of using this apparatus, where the spot size on the surface of the sample can be brought into focus without extensive adjustment of the position of these optical light paths.  
         [0011]     These and numerous other objects and advantages of the present invention will become apparent upon reading the following description.  
       SUMMARY  
       [0012]     The objects and advantages of the present invention are secured by an apparatus and method for the optical characterization of the properties of a sample on reflection and transmission of a beam of light, with a small spot size on the sample, over a broadband of wavelengths. A broadband beam of light from a light source is fractionally magnified and illuminated onto a top surface of the sample. A portion of the broadband beam of light is reflected from the top surface of the sample, a portion of the broadband beam of light is transmitted through the sample and a portion of the broadband beam of light is absorbed. The portion of the broadband beam of light reflected from the top surface of the sample is redirected and illuminated onto a first detector. The portion of the broadband beam of light transmitted through the sample is redirected from a bottom surface of the sample and illuminated onto a second detector. These functions are accomplished using an illumination optical light path, a reflection optical light path and a transmission optical light path, each of which comprises reflective optical components, thereby eliminating chromatic aberrations from these components. Pairs of planar and off-axis parabolic mirrors are used to redirect and magnify the broadband beam of light. In a preferred embodiment, the planar and off-axis parabolic mirrors are coated with a UV-enhancing aluminum coating. The broadband beam of light in the illumination optical light path, the reflection optical light path and the transmission optical light path is collimated between the pair of parabolic mirrors in each optical light path. This configuration allows focusing of the broadband beam of light on the top surface of the sample by adjusting a position of one of the pairs of planar and off-axis parabolic mirrors without requiring adjustment of the position of other components in each of the optical light paths.  
         [0013]     In another embodiment of this invention, an optical fiber is used to redirect the portion of the broadband beam of light transmitted through the sample to illuminate the second detector.  
         [0014]     In another embodiment of this invention, a polarizing means is incorporated into at least one of the optical light paths to adjust the polarization of the broadband band beam of light.  
         [0015]     In another embodiment of this invention, the portion of the broadband beam of light reflected from the sample and the portion of the broadband beam of light transmitted through the sample are each redirected and illuminated onto a common detector.  
         [0016]     A detailed description of the invention and the preferred and alternative embodiments is presented below in reference to the attached drawing figures. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0017]      FIG. 1  is a diagram illustrating an apparatus according to the invention.  
         [0018]      FIG. 2  is a diagram illustrating another embodiment of an apparatus according to the invention.  
         [0019]      FIG. 3  is a diagram illustrating a side view of one of the planar mirrors of the apparatus in  FIG. 1  or  FIG. 2 .  
         [0020]      FIG. 4  is a diagram illustrating a side view of one of the off-axis parabolic mirrors of the apparatus in  FIG. 1  or  FIG. 2 .  
         [0021]      FIG. 5  is a diagram illustrating the focusing of the broadband beam onto the sample surface.  
         [0022]      FIG. 6  is a diagram illustrating a cross-sectional view of the broadband beam of light. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0023]     A preferred embodiment of the invention is illustrated in  FIG. 1 . An apparatus  100  according to the invention comprises a first optical light path  110 , a second optical light path  160  and a third optical light path  180 . A light source  112  produces a broadband beam of light  114  between 190 nm and 1100 nm (broadband beam of light  114  is identified by its extremal rays in  FIG. 1 ). An arc source, such as a Hamamatsu L2-2000 series deuterium lamp, is suitable for the UV portion of the spectrum, and a tungsten lamp for the visible and IR portions of the spectrum. A combination of the deuterium lamp and the tungsten lamp is suitable as the light source  112 . The broadband beam of light  114  is redirected on reflection off of a first planar mirror  116 . A model 01-MGF-005/028 planar mirror from Melles-Griot is suitable as the planar mirror  116 . The first planar mirror  116  is positioned relative to a first off-axis parabolic mirror  126  such that the broadband beam of light  114  is collimated on reflection from the first off-axis parabolic mirror  126 . A suitable off-axis parabolic mirror can be custom manufactured by Edmond Industrial Optics using diamond turning. A commercially available example of such an off-axis parabolic mirror is model H47-085 from Edmond Industrial Optics.  
         [0024]     The broadband beam of light  114  is redirected on reflection off of a second planar mirror  138 . The broadband beam of light  114  incident and reflected off of the second planar mirror  138  is collimated. The second planar mirror  138  is positioned relative to a second off-axis parabolic mirror  140  such that the broadband beam of light  114  illuminates and is brought into focus on a sample  144 .  
         [0025]     Referring to  FIG. 5 , the broadband beam of light  114  is collimated when incident on the second off-axis parabolic mirror  140 . This ensures that the broadband beam of light  114  will come to focus at a distance  142  from the second off-axis parabolic mirror  140 . There is a known relationship between the distance  142  and focal length along axis of the second off-axis parabolic mirror  140 . A person of skill in the art will be able to determine the focal length from the curvature of the second off-axis parabolic mirror  140 . By adjusting the position of the second off-axis parabolic mirror  140  relative to the sample  144 , the broadband beam of light  114  is brought into focus on a top surface  146  of the sample  144 . It is important, however, that the position of the second planar mirror  138  also be adjusted such that the second planar mirror  138  maintains the same position relative to the second off-axis parabolic mirror  140 . In this way, the collimated light reflected off of the second planar mirror  138  remains parallel to the axis (not shown) of the second off-axis parabolic mirror  140 . Since the broadband beam of light  114  incident on the second planar mirror  138  is collimated, this adjustment of the position of the second planar mirror  138  and the second off-axis parabolic mirror  140  does not necessitate adjustment of the position of the other components in the first optical light path  110 .  
         [0026]     Referring to  FIG. 6 , the broadband beam of light  114  has a cross-section  210  with a diameter  220  defined as twice the distance from the center of the cross-section  210  where the light intensity is reduced by a factor of 1/e. The broadband beam of light  114  has the diameter  220  greater than 500 microns at the light source  112  and the diameter  220  between 50 microns and 80 microns on the top surface  146  of the sample  144 .  
         [0027]     Referring back to  FIG. 5 , the small diameter  220  of the broadband beam of light  114  illuminated on the top surface  146  of the sample  144  corresponds to a small spread of angles in the cone of rays in the broadband beam of light  114  incident on the sample  144 . The broadband beam of light  114  incident on the top surface  146  of the sample  144  has a minimum angle of incidence  152  and a maximum angle of incidence  154  relative to a normal  150  to the top surface  146  of the sample  144 . This reduction is proportional to the ratio of the focal lengths of off-axis parabolic mirror  140  and off-axis parabolic mirror  126 .  
         [0028]     Referring back to  FIG. 1 , the broadband beam of light  161  is reflected from the top surface  146  of the sample  144  (broadband beam of light  161  is identified by its extremal rays in  FIG. 1 ). The broadband beam of light  161  is redirected and magnified in the second optical light path  160 . The broadband beam of light  161  is redirected on reflection off of a first off-axis parabolic mirror  162  and then redirected on reflection off of a first planar mirror  164 . In a manner similar to that used in adjusting the position of second planar mirror  138  and second off-axis parabolic mirror  140  in the first optical light path  110 , the position of the first off-axis parabolic mirror  162  and the first planar mirror  164  relative to the top surface  146  of the sample  144  are adjusted such that the broadband beam of light  161  incident and reflected from the first planar mirror  164  is collimated. This ensures that the adjustment of the position of the first off-axis parabolic mirror  162  and the adjustment of the position of the first planar mirror  164  does not necessitate adjustment of the position of other components in the second optical light path  160 . Referring back to  FIG. 5 , the small diameter  220  of the broadband beam of light  114  on the top surface  146  of the sample  144  corresponds to a small spread of angles in the cone of rays in the broadband beam  161  of light with a minimum angle of reflection  156  and a maximum angle of reflection  158 .  
         [0029]     Referring back to  FIG. 1 , the broadband beam of light  161  is redirected on reflection off of a second of axis parabolic mirror  168 . The broadband beam of light  161  is redirected on reflection off of the second planar mirror  170  and illuminates a first detector  172 . The entrance aperture  171  of the first detector  172  is positioned at the focal length of the second off-axis parabolic mirror  168 . A person of skill in the art will be able to determine the focal length from the curvature of the second off-axis parabolic mirror  168 .  
         [0030]     Referring back to  FIG. 5 , after transmission through the sample  144  the broadband beam of light  181  exits the sample through a bottom surface  148  of the sample  144  (broadband beam of light  181  is identified by its extremal rays in  FIG. 5 ). The cone of rays in the broadband beam of light  181  transmitted through the sample  144  has minimum angle of transmission  184  and maximum angle of transmission  186  relative to a normal  182  to the bottom surface  148  of the sample  144 . Referring back to  FIG. 1 , the broadband beam of light  181  is redirected and magnified by the third optical light path  180 . The broadband beam of light  181  is redirected on reflection off of a first off-axis parabolic mirror  188  and then redirected on reflection off of a first planar mirror  190 . In a manner similar to that used in adjusting the position of second planar mirror  138  and second off-axis parabolic mirror  140  in the first optical light path  110 , the position of the first off-axis parabolic mirror  188  and the first planar mirror  190  relative to the top surface  146  of the sample  144  are adjusted such that the broadband beam of light  181  incident and reflected from the first planar mirror  190  is collimated. This ensures that the adjustment of the position of the first off-axis parabolic mirror  188  and the adjustment of the position of the first planar mirror  190  does not necessitate adjustment of the position of other components in the third optical light path  180 . Since the broadband beam of light  114 ,  161  and  181  is collimated substantially perpendicular to the sample  144  over a portion of the first optical light path  110 , the second optical light path  160  and the third optical light path  180 , in an embodiment of this invention the adjustment of the second planar mirror  138  and second off-axis parabolic mirror  140 , the first off-axis parabolic mirror  162  and the first planar mirror  164 , and the first off-axis parabolic mirror  188  and the first planar mirror  190  relative to the top surface  146  of the sample  144  is accomplished with a group of mechanically coupled elements. The broadband beam of light  181  is redirected on reflection off of a second of axis parabolic mirror  194 . The broadband beam of light  181  is redirected on reflection off of the second planar mirror  196  and illuminates a second detector  198 . The entrance aperture  197  of the second detector  198  is positioned at the focal length of the second off-axis parabolic mirror  194 . A person of skill in the art will be able to determine the focal length from the curvature of the second off-axis parabolic mirror  194 .  
         [0031]      FIG. 3  illustrates a side view of the first planar mirror  116  in the first optical light path  110 . In a preferred embodiment of the invention, the planar mirror  116  includes a UV-enhancing aluminum coating  124 . As an example, the model 01-MGF-005/028 planar mirror from Melles-Griot has a UV-enhancing aluminum coating  124 . In a preferred embodiment, such UV-enhancing aluminum coatings are used on the other planar mirrors  138 ,  164 ,  170 ,  190  and  196  in the first optical light path  110 , the second optical light path  160  and the third optical light path  180 .  
         [0032]      FIG. 4  illustrates a side view of the first off-axis parabolic mirror  126  in the first optical light path  110 . In a preferred embodiment of the invention, the off-axis parabolic mirror  126  includes a UV-enhancing aluminum coating  134 . Edmond Industrial Optics is a supplier of such UV-enhanced aluminum coatings. In a preferred embodiment, such UV-enhancing aluminum coatings are used on the other off-axis parabolic mirrors  140 ,  162 ,  168 ,  188  and  194  in the first optical light path  110 , the second optical light path  160  and the third optical light path  180 .  
         [0033]      FIG. 2  illustrates alternate embodiments of the invention. The first optical light path  110  includes a polarizing means  136  for polarizing the broadband beam of light  114  in one of two orthogonal directions. A suitable device is a model PTH-SMP Glan Thompson-type calcite polarizer made by Harrick. The second optical light path  160  includes a polarizing means  166 , such as a polarizing analyzer. Once again, the model PTH-SMP Glan Thompson-type calcite polarizer made by Harrick is suitable. The third optical light path  180  includes a polarizing means  192 , such as a polarizing analyzer.  
         [0034]     In another embodiment of this invention, the third optical light path  180  also includes an optical fiber  199  for redirecting the broadband beam  181  from the third optical light path  180  to the second detector  198 .  
         [0035]     In another embodiment of this invention, the broadband beam  181  from the third optical light path  180  is redirected and illuminated onto the first detector  172  eliminating the need for the second detector  198 . Additional optical components, such as a beam splitter, may be added as is known in the art to ensure that broadband beam  161  and broadband beam  181  are coaxial when they illuminate the first detector  172 . A chopper may also be added.  
         [0036]     Referring back to  FIG. 1 , the first detector  172  and the second detector  198  depend on the type of optical characterization to be performed on the sample  144 . For measurements of reflected or transmitted intensity as a function of wavelength, the first detector  172  and the second detector  198  with a monochromator, a diode array or a photomutiplier tube is suitable. A monochromator with a 512-element diode array (Model PDA-512) is available from Control Development. A mechanically scanned monochromator is known in the art. A suitable photomultiplier is model R928 from Hamamatsu. For a spectroscopic ellipsometer, a polarization analyzer, such as the model PTH-SMP Glan Thompson-type calcite polarizer made by Harrick, in addition to the monochromator, the diode array or the photomultiplier tube is suitable. In one embodiment, the polarization analyzer can be incorporated in the first detector  172  and the second detector  198 . The analysis techniques in United States patent U.S. Pat. No. 4,905,170 to Forouhi et al. and United States patent application U.S. Ser. No. 10/607,410 to Li et al., hereby incorporated by reference, can be used to determine optical characteristics of the sample  144  from the measurements.  
         [0037]     By employing substantially reflective optical components and off-axis parabolic mirrors with collimated incident broadband beam of light  114 , reflected broadband beam of light  161 , and transmitted broadband beam of light  181 , the invention minimizes chromatic aberration in the first light path  110 , the second light path  160  and the third light path  180 . This enables the small diameter  220  of the broadband beam of light  114  and  161  on the top surface  146  of the sample  144  as well as optical characterization of reflection and transmission properties using the single light source  112 . The diameter  220  of the broadband beam of light  114  and  161  is small enough to resolve spatial variations in optical characteristics on the top surface  146  of the sample  144  yet large enough to spatially average the optical characteristics of the sample  144 . Artifacts associated with diamond-turned parabolic mirrors are not a concern in this invention since the diameter  220  of the broadband beam of light  114  and  161  on the top surface  146  and the diameter  220  of the broadband beam of light  181  on the bottom surface  148  of the sample  144  are not diffraction limited. The principle impact of such artifacts is scattering of the broadband beam of light  114 ,  161  and  181 , which is not a concern in this invention since these scattered rays will not be illuminated onto the first detector  172  or the second detector  198 .  
         [0038]     The first, second and third optical light paths  110 ,  160  and  180  in this invention have been described with parabolic mirrors  126 ,  140 ,  162 ,  168 ,  188  and  194 . One skilled in the art will recognize that other mirror shapes such as a toroidal mirror as well as those based on conic sections, such as elliptical, hyperbolic and spherical, are also suitable. In addition, another reflective surface may be substituted for the planar mirrors  116 ,  138 ,  164 ,  170 ,  190  and  196 .  
         [0039]     In view of the above, it will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.