Abstract:
An x-ray metrology system includes one or more transmissive x-ray optical elements, such as zone plates or compound refractive x-ray lenses, to shape the x-ray beams used in the measurement operations. Each transmissive x-ray optical element can focus or collimate a source x-ray beam onto a test sample. Another transmissive x-ray optical element can be used to focus reflected or scattered x-rays onto a detector to enhance the resolving capabilities of the system. The compact geometry of transmissive x-ray optical element allows for more flexible placement and positioning than would be feasible with conventional curved crystal reflectors. For example, multiple x-ray beams can be focused onto a test sample using a transmissive x-ray optical element array. Robust zone plates can be efficiently produced using a damascene process.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to metrology tools, and more particularly to a system and method for using transmissive x-ray optical elements to perform x-ray measurements. 
   BACKGROUND OF THE INVENTION 
   X-ray metrology systems are often used to measure and characterize small and/or hidden features in various materials. For example, thin film thickness measurement systems often use a technique known as x-ray reflectometry (XRR), which measures the interference patterns created by reflection of x-rays off a thin film.  FIG. 1   a  shows a conventional x-ray reflectometry system  100 , as described in U.S. Pat. No. 5,619,548, issued Apr. 8, 1997 to Koppel. X-ray reflectometry system  100  comprises a microfocus x-ray tube  110 , an x-ray reflector  120 , a detector  130 , and a stage  140 . A test sample  142  having a thin film layer  141  is held in place by stage  140  for the measurement process. 
   To measure the thickness of thin film layer  141 , microfocus x-ray tube  110  directs a source x-ray beam  150  at x-ray reflector  120 . Source x-ray beam  150  actually comprises a bundle of diverging x-rays, including x-rays  151  and  152 . X-ray reflector  120  reflects and focuses the diverging x-rays of x-ray beam  150  into a converging x-ray beam  160 . Converging x-ray beam  160  includes x-rays  161  and  162 , which correspond to x-rays  151  and  152 , respectively. Converging x-ray beam  160  is then reflected by thin film layer  141  as an output x-ray beam  170  onto detector  130 . Output x-ray beam  170  includes reflected x-rays  171  and  172 , which correspond to x-rays  161  and  162 , respectively. 
   The reflected x-rays in output x-ray beam  170  are actually formed by reflections at both the surface of thin film layer  141  and at the interface between thin film layer  141  and test sample  142 . Detector  130  measures the resulting constructive and destructive interference between the reflected x-rays in output x-ray beam  170  as a reflectivity curve. An example reflectivity curve is shown in  FIG. 2 . By measuring the fringes in the reflectivity curve, the thickness of thin film layer  141  can be determined, as described in U.S. Pat. No. 5,619,548. 
   To ensure accurate measurements in any x-ray metrology system, precise x-ray beam shaping within the system is critical. Due to the small dimensions being measured by x-ray metrology systems, any x-ray beams used within such system must be tightly controlled (e.g., focused, collimated, etc.). Therefore, a critical component in many conventional x-ray metrology systems (such as XRR system  100  shown in  FIG. 1   a ) is an x-ray reflector that focuses the x-ray beam onto the sample being measured. An x-ray reflector (such as x-ray reflector  120  shown in  FIG. 1   a ) is typically a doubly curved crystal formed using high-precision machining and grinding operations. This manufacturing process is very time consuming and expensive. Furthermore, incorporation of a doubly curved crystal into an x-ray metrology system requires large crystal mounts that make the incorporation of multiple crystals into a single tool very difficult. 
   Accordingly, it is desirable to provide a system and method for performing x-ray metrology without using crystal reflectors as a focusing mechanism. 
   SUMMARY OF THE INVENTION 
   The invention provides a method and system for performing x-ray metrology using transmissive x-ray optical elements as beam-shaping elements. For example, a zone plate is a type of transmissive x-ray optical element that comprises a set of concentric metal rings formed on a substrate—essentially a diffraction grating configured to work on x-rays. The beam-shaping properties of a zone plate are defined by the size, shape, and spacing of the metal rings. Because the beam-shaping properties of a zone plate is based upon diffraction, a zone plate can have a much flatter geometry than a curved crystal, which provides beam shaping via reflection. As described by Janoz Kirz in “Phase Zone Plates for X-Rays and the Extreme UV” ( Journal of the Optical Society of America , Vol. 64, No. 3, March 1974, pp. 301–309.), phase reversal zone plates can be used for beam shaping in x-ray astronomy and spectroscopy. 
   Another type of transmissive x-ray optical element, a compound refractive x-ray lens, includes a series of curved structures, each of which acts as a refracting element for an incoming x-ray beam. While the index of refraction of most materials at x-ray energies is very small, the use of many refracting elements in series allows a compound refractive x-ray lens to provide x-ray beam reshaping in a relatively compact form. For example, a compound refractive x-ray lens can be constructed by forming an alternating series of horizontal and vertical holes in a block comprising a low atomic number material (e.g., aluminum, silicon, boron-nitride, diamond, lithium, beryllium, etc.), as described by A. Snigirev et al. in “A Compound Refractive Lens For Focusing High Energy X Rays,” ( Nature , vol. 384, Nov. 7, 1996, pp. 49–51.), herein incorporated by reference. The resulting curved (cylindrical) surfaces within the block form a series of refracting elements that can focus an x-ray beam travelling through the block. Compound refractive x-ray lenses can also be fabricated using semiconductor lithography and etch techniques or by forming thin metal foils into appropriate curved configurations. Various other methods for constructing compound refractive x-ray lenses are discussed by A. Snigirev et al. in “Focusing High Energy X-Rays by Compound Refractive Lenses,” ( Applied Optics , vol. 37, no. 4, Feb. 1, 1998, pp. 653–662.). 
   By incorporating transmissive x-ray optical elements into x-ray metrology systems, the invention advantageously eliminates the need for fragile and expensive crystal reflectors. In addition, transmissive x-ray optical elements are much easier to support and position within an x-ray metrology system (since they do not require the large crystal mounts used by curved crystal reflectors). Therefore, transmissive x-ray optical element provide flexible placement and positioning options, including the use of multiple transmissive x-ray optical elements in series or arrays. Transmissive x-ray optical elements are also capable of focusing x-rays to much smaller spots than curved crystals, thereby enabling the measurement of much smaller spots on test samples. 
   According to an embodiment of the invention, a transmissive x-ray optical element can be used to focus an x-ray beam onto a test sample. An optional order-blocking filter can be used to prevent any unwanted x-rays scattered or diffracted into higher orders by the transmissive x-ray optical element from reaching the test sample. Various x-ray metrology operations can be performed using such a focused beam, including x-ray reflectometry (XRR) and x-ray diffraction (XRD). 
   According to another embodiment of the invention, multiple transmissive x-ray optical elements in series can be used to perform the focusing operation. In this implementation, the total numerical aperture (NA) of the system can be advantageously increased without increasing the overall diameter of the transmissive x-ray optical element. According to another embodiment of the invention, x-rays generated (e.g., reflected or scattered from the test sample) by the focused beam incident on the test sample can be focused onto a detector by a transmissive x-ray optical element (or transmissive x-ray optical elements), thereby increasing the resolving power of the x-ray metrology system without increasing the system footprint. According to another embodiment of the invention, multiple transmissive x-ray optical elements in an array can be used to focus multiple x-ray beams onto the test sample to enable simultaneous measurement of data from multiple incident x-ray beam angles. According to another embodiment of the invention, a transmissive x-ray optical element can be used to collimate and direct an x-ray beam onto a test sample to perform small angle x-ray scattering (SAXS). 
   The invention also provides an improved method for producing zone plates for use in x-ray applications by using standard damascene processing techniques used in integrated circuit (IC) interconnect fabrication. Conventional zone plate production methods involve patterning a substrate using electron beam lithography and deep reactive ion etching and then using multi-level electro-chemical plating to form the final diffraction grating, as described by Chen et al. in “Design and Fabrication of Fresnel Zone Plates With Large Numbers of Zones” (Journal of Vacuum Science Technology, B 15(6), Nov./December 1997, pp. 2522–2527.) and by Fabrizio et al. in “X-Ray Multilevel Zone Plate Fabrication by Means of Electron-Beam Lithography: Toward High-Efficiency Performances” (Journal of Vacuum Science Technology, B 17(6), Nov./December 1999, pp. 3439–3443.). Unfortunately, these conventional zone plate fabrication methods result in very high aspect ratio unsupported metal structures, which are very fragile and difficult to reliably produce. 
   According to an embodiment of the invention, a zone plate can be manufactured using a damascene process by forming a stack of damascene layers. Each damascene layer can be formed by patterning circular trenches in a dielectric material, depositing a metal seed layer over the patterned surface by physical vapor deposition (PVD), electro-chemically plating onto this seed layer, and then planarizing the top layer of metal to leave an exposed pattern of alternating rings of metal and dielectric material. Intermediate layers of dielectric material can be used to separate the damascene layers. By constructing a zone plate in this staged manner, the problematic high aspect ratio structures required by conventional manufacturing processes can be avoided. Not only does this simplify the manufacture of zone plates, but the zone plates produced using this technique would generally be more robust than conventionally formed zone plates. Furthermore, the actual beam shaping performance of such zone plates can be optimized by tailoring the metal ring widths and thicknesses in individual layers of the zone plate to maximize diffraction efficiency into the desired first order wavelength and cancel out higher diffraction into the unwanted higher order wavelengths. 
   The present invention will be more fully understood in view of the following description and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings. 
       FIG. 1  is a schematic diagram of a conventional x-ray reflectometry system. 
       FIG. 2  is an example of a reflectivity curve. 
       FIG. 3   a  is a schematic diagram of an x-ray metrology system incorporating a transmissive x-ray optical element in accordance with an embodiment of the invention. 
       FIG. 3   b  is a schematic diagram of an x-ray metrology system incorporating a transmissive x-ray optical element and a reflective x-ray optical element in accordance with an embodiment of the invention. 
       FIG. 4  is a schematic diagram of an x-ray metrology system incorporating multiple transmissive x-ray optical elements in series in accordance with another embodiment of the invention. 
       FIG. 5  is a schematic diagram of an x-ray metrology system incorporating multiple transmissive x-ray optical elements in accordance with another embodiment of the invention. 
       FIG. 6  is a schematic diagram of an x-ray metrology system incorporating multiple x-ray beams and multiple transmissive x-ray optical element in accordance with another embodiment of the invention. 
       FIG. 7  is a schematic diagram of an x-ray metrology system incorporating multiple transmissive x-ray optical elements in accordance with another embodiment of the invention. 
       FIGS. 8   a ,  8   b ,  8   c ,  8   d ,  8   e ,  8   f ,  8   g ,  8   h , and  8   i  are cross-sectional views showing a manufacturing process for a zone plate in accordance with an embodiment of the invention. 
       FIG. 9  is a top view of a damascene layer shown in  FIG. 8   h , according to an embodiment of the invention. 
       FIG. 10  is a cross sectional view of a zone plate in accordance with another embodiment of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 3   a  shows an x-ray metrology system  300   a  in accordance with an embodiment of the invention. X-ray metrology system  300   a  includes an x-ray source  310 , a transmissive x-ray optical element  320 , a stage  340  for supporting a test sample  342 , a detector  330 , optional order blocking filters  344   a  and  344   b , and an optional computer  390 . Transmissive x-ray optical element  330  can comprise any x-ray beam reshaping element that operates via transmission of x-rays, such as a zone plate or compound refractive x-ray lens. As described above, a zone plate comprises a set of concentric metal rings that provide x-ray beam shaping via diffraction, with the actual beam shaping properties being determined by the size, shape, and spacing of the concentric metal rings. Note that the relatively flat geometry of a zone plate or compound refractive x-ray lens can provide substantial placement and positioning flexibility within x-ray metrology system  300   a.    
   During a metrology operation, x-ray source  310  generates an x-ray beam  350  that comprises a set of diverging x-rays, as indicated by a diverging beam portion  351 . According to an embodiment of the invention, x-ray source  310  can comprise a microfocus x-ray tube. According to other embodiments of the invention, x-ray source  310  can comprise a laser-plasma or dense plasma source, or a high current capillary discharge source. Transmissive x-ray optical element  320  intercepts beam portion  351  and reshapes it into a converging beam portion  352  focused onto a measurement spot  349  on a thin film layer  341  on test sample  342 . Optional order blocking filter  344  can be positioned above measurement spot  349  to define an opening through which only the focused x-rays of beam portion  352  can pass. Any x-rays scattered or diffracted into non-first order frequencies by transmissive x-ray optical element  320  would then be blocked by order blocking filter  344   a . According to another embodiment of the invention, optional order blocking filter  344   b  can include an aperture placed directly in the path of beam portion  352  to provide a similar filtering effect. Order blocking filters  344   a  and  344   b  can comprise any material that is opaque to the x-rays generated by x-ray source  310 . 
   Note that the beam shaping characteristics and position of transmissive x-ray optical element  320  can be selected based on the design parameters of x-ray metrology system  300   a , such as the specific metrology operation being performed, desired system footprint, measurement spot size, and measurement throughput. For example, to perform x-ray reflectometry (XRR), transmissive x-ray optical element  320  could be selected to be a zone plate producing a first order diffraction of the x-rays in beam portion  351  that focuses beam portion  352  into a spot no larger than 1 μm (diameter) at a focal point 300 mm from transmissive x-ray optical element  320 . Similarly, transmissive x-ray optical element  320  could comprise a compound refractive x-ray lens that refracts the x-rays in beam portion  351  into a similar beam portion  352 . Transmissive x-ray optical element  320  could then be positioned two focal lengths (i.e., 2×150 mm) from both x-ray source  310  and measurement spot  349 , to form a 1:1 imaging system, such that beam portion  352  takes the shape of a cone having a half angle Ab roughly equal to 0.03° and incident to test sample  342  at an incident angle Ai roughly equal to 0.2°. Note that while beam portion  352  as a whole has an incident angle Ai with thin film layer  341 , the individual x-rays (not shown for clarity) beam portion  352  have a variety of different incident angles with thin film layer  341 . Those individual x-rays are then reflected across a corresponding range of reflected angles, thereby forming an output beam portion  353 , which is measured by detector  330 . 
   Depending on the type of x-ray metrology process being performed, detector  330  can comprise various detector elements. For example, to measure reflectivity curves for x-ray reflectometry (XRR) or diffraction patterns for x-ray diffraction (XRD), detector  330  can comprise a position-sensitive charge-coupled device (CCD) sensor (linear array or 2-dimensional), photodiode array, or image plate, among others. By simulatneously detecting reflected x-rays from incident x-rays having a variety of incident angles, the position sensitive detector provides measurements that can then be stored or processed by computer  390  to determine thin film properties associated with test sample  342 . Note that thin film layer  341  can comprise various materials, including metal, dielectric, and semiconducting, and the measured film properties can include film thickness, density, roughness, and composition, among others. Furthermore, thin film layer  341  can even comprise multiple layers which can be simultaneously measured (e.g., simultaneous measurement of the thickness for each layer). 
   As is described below with respect to  FIG. 9 , a zone plate includes concentric rings of a first material formed in a second material. The zone plate material diffracts the incident x-rays to reshape the incident x-ray beam into a desired form. By properly sizing the concentric rings (according to the characteristics of the incident x-ray beam and the properties of the first material and the second material) the x-rays in the x-ray beam exiting from the zone plate can be made to constructively interfere, thereby ensuring a strong output signal. Note that a compound refractive x-ray element can likewise be optimized to ensure a strong output signal. 
   The specific configuration and positioning of transmissive optical element  320  can be adjusted depending on the particular requirements of the measurement operation being performed. For example, an XRR operation could incorporate a zone plate or compound refractive x-ray lens configured as described above (i.e., producing a cone of x-rays having a half angle Ab equal to roughly 0.03° and an incident angle Ai roughly equal to 0.2°). For XRD measurements, larger values for the incident angle Ai could be used. Note that while a focusing operation is depicted in  FIG. 3   a  for explanatory purposes, a transmissive x-ray optical element can provide any other desired beam shaping, such as collimating (as described below with respect to  FIG. 7 ). 
   Note further that according to other embodiments of the invention, transmissive x-ray optical elements can be used in conjunction with reflective x-ray optical elements within an x-ray metrology system.  FIG. 3   b  shows an x-ray metrology system  300   b  that is substantially similar to x-ray metrology system  300   a  shown in  FIG. 3   a  except that x-ray metrology system  300   b  includes a reflective x-ray optical element  301  (similar to x-ray reflector  120  shown in  FIG. 1 ) in accordance with an embodiment of the invention. Reflective x-ray optical element  301  reflects x-ray beam portion  351   a  onto transmmissive x-ray optical element  320 , which then focuses the beam onto thin film layer  341 . Various other combinations of reflective and transmissive x-ray optical elements to reshape different portions of an x-ray beam (or beams) in an x-ray metrology system can be incorporated into other embodiments of the invention. 
   To further enhance the measurement capabilities of an x-ray metrology system, multiple transmissive x-ray optical elements can be used. For example,  FIG. 4  shows an x-ray metrology system  400  according to another embodiment of the invention. X-ray metrology system  400  includes an x-ray source  410 , transmissive x-ray optical elements  421  and  422 , a stage  440  for supporting a test sample  442 , a detector  430 , optional order blocking filters  444   a  and  444   b , and an optional computer  490 . X-ray metrology system  400  is substantially similar to x-ray metrology system  300   a  shown in  FIG. 3   a , except that two transmissive x-ray optical elements are used for focusing the x-ray beam onto the test sample. 
   During a metrology operation, x-ray source  410  generates an x-ray beam  450  that comprises a set of diverging x-rays, as indicated by an initial beam portion  451 . Transmissive x-ray optical element  421  intercepts beam portion  451  and reshapes it into a converging beam portion  452 . Transmissive x-ray optical elements  422  further reshapes beam portion  452  into a focused beam portion  453  that is directed onto a measurement spot  449  on a thin film region  441  on test sample  442 . Optional order blocking filter  444   a  can be positioned above measurement spot  449  to define an opening through which only the focused x-rays of beam portion  453  can pass. Any x-rays scattered or diffracted into non-first order frequencies by transmissive x-ray optical element  421  and/or  422  would then be blocked by order blocking filter  444   a . According to another embodiment of the invention, optional order blocking filter  444   b  can include an aperture placed directly in the path of beam portion  453  to provide a similar filtering effect. Order blocking filters  444   a  and  444   b  can comprise any material that is opaque to the x-rays generated by x-ray source  410 . 
   Because the focusing of initial beam portion  451  is performed partially by transmissive x-ray optical element  421  and partially by transmissive x-ray optical element  422 , the beam shaping characteristics for each of transmissive x-ray optical elements  421  and  422  can be much more moderate than those of a single transmissive x-ray optical element that independently provides the same focusing behavior. Relatedly, multiple transmissive x-ray optical elements can provide a much larger numerical aperture than a single zone plate of similar diameter, and therefore can be significantly more space-efficient. Note that while two transmissive x-ray optical elements are shown in  FIG. 4  for explanatory purposes, according to other embodiments of the invention, any number of transmissive x-ray optical elements could be used to focus initial beam portion  451  onto test sample  442 . 
     FIG. 5  shows an x-ray metrology system  500  that includes multiple transmissive x-ray optical elements in accordance with another embodiment of the invention. X-ray metrology system  500  includes an x-ray source  510 , transmissive x-ray optical elements  521  and  522 , a stage  540  for supporting a test sample  542 , a detector  530 , optional order blocking filters  544   a  and  544   b , and an optional computer  590 . X-ray metrology system  500  is substantially similar to x-ray metrology system  300   a  shown in  FIG. 3   a , except that a second transmissive x-ray optical element is used for focusing the output (reflected) x-ray beam onto the detector. 
   During a metrology operation, x-ray source  510  generates an x-ray beam  550  that comprises a set of diverging x-rays, as indicated by an initial beam portion  551 . Transmissive x-ray optical element  521  intercepts beam portion  551  and reshapes it into a converging beam portion  552  that is directed onto a measurement spot  549  on a thin film region  541  on test sample  542 . Optional order blocking filter  544   a  can be positioned above measurement spot  549  to define an opening through which only the focused x-rays of beam portion  552  can pass. Any x-rays scattered or diffracted into non-first order frequencies by transmissive x-ray optical element  521  would then be blocked by order blocking filter  544   a . According to another embodiment of the invention, optional order blocking filter  544   b  can include an aperture placed directly in the path of beam portion  552  to provide a similar filtering effect. Order blocking filters  544   a  and  544   b  can comprise any material that is opaque to the x-rays generated by x-ray source  510 . Beam portion  552  is reflected by test sample  542  as an output beam portion  553 . Transmissive x-ray optical element  522  intercepts the diverging x-rays of beam portion  553  and reshapes them into a converging beam portion  554  that is then measured by detector  530 . Note that transmissive x-ray optical element  522  does not focus beam portion  553  down to a small spot (in contrast to transmissive x-ray optical element  521 ), but instead merely reduces the size (diameter) of the beam portion to be measured by detector  530 . The measurement data can then be stored or processed by optional computer  590  according to the type of metrology operation being performed. 
   By reshaping output beam portion  553  in this manner, transmissive x-ray optical element  522  increases the apparent distance between measurement spot  549  and detector  530 . This in turn enhances the angular resolution of the measurements taken by detector  530 , thereby improving the metrology results. Selecting transmissive x-ray optical element  522  to have a shorter focal length than transmissive x-ray optical element  521  allows x-ray metrology system  500  to be constructed in a space-efficient manner, while positioning detector  530  at the focal point of transmissive x-ray optical element  522  optimizes the resolving power of x-ray metrology system  500 . Note that according to various other embodiments of the invention, transmissive x-ray optical element  521  could be replaced by multiple transmissive x-ray optical elements, as described previously with respect to  FIG. 4 . 
     FIG. 6  shows an x-ray metrology system  600  that includes multiple transmissive x-ray optical elements in accordance with another embodiment of the invention. X-ray metrology system  600  includes an x-ray source  610 , transmissive x-ray optical elements  620   a  and  620   b , a stage  640  for supporting a test sample  642 , a detector  630 , an optional order blocking filter  644 , and an optional computer  690 . X-ray metrology system  600  is substantially similar to x-ray metrology system  300   a  shown in  FIG. 3   a , except that microfocus x-ray source  610  is configured to provide multiple x-ray beams, and a second transmissive x-ray optical element is used to focus a second x-ray beam onto the test sample. 
   During a metrology operation, microfocus x-ray source  610  generates x-ray beams  650   a  and  650   b , each of which comprises a set of diverging x-rays, as indicated by an initial beam portions  651   a  and  651   b , respectively. According to an embodiment of the invention, microfocus x-ray source  610  comprises a single multi-spot microfocus x-ray tube, wherein a large spot x-ray source is filtered by a multi-hole mask to produce the multiple x-ray beams. According to another embodiment of the invention, microfocus x-ray source  610  comprises multiple single-spot microfocus x-ray tubes. Transmissive x-ray optical element  620   a  intercepts beam portion  651   a  and reshapes it into a converging beam portion  652   a  that is directed onto a measurement spot  649  on a thin film region  641  on test sample  642 . Similarly, transmissive x-ray optical element  620   b  intercepts beam portion  651   b  and reshapes it into a converging beam portion  652   b  that is directed at measurement spot  649  on test sample  642 . Optional order blocking filter  644   a  can be positioned above measurement spot  649  to define an opening through which only the focused x-rays of beam portions  652   a  and  652   b  can pass. Any x-rays scattered or diffracted into non-first order frequencies by transmissive x-ray optical element  62   a  and  620   b  would then be blocked by order blocking filter  644   a.    
   According to another embodiment of the invention, optional order blocking filter  644   b  can include an aperture or apertures placed directly in the paths of beam portion  652   a  and  652   b  to provide a similar filtering effect. Order blocking filters  644   a  and  644   b  can comprise any material that is opaque to the x-rays generated by x-ray source  610 . Beam portions  652   a  and  652   b  are reflected by test sample  542  as output beam portions  653   a  and  653   b , respectively, which are then measured by detector  630 . 
   According to an embodiment of the invention, detector  630  can comprise a single large detector for measuring all output beam portions. According to another embodiment of the invention, detector  630  can comprise a discrete detector for each output beam portion (as indicated by the dotted line). The measurement data can then be stored or processed by optional computer  590  according to the type of metrology operation being performed. 
   By focusing multiple x-ray beams onto the test sample, measurements for multiple incident beam angles (e.g., incident angles Aia and Aib in  FIG. 6 ) can be taken simultaneously. According to an embodiment of the invention, transmissive x-ray optical elements  620   a  and  620   b  can be formed in a single substrate (as indicated by the dashed lines), thereby improving relative positioning accuracy and simplifying system setup. According to other embodiments of the invention, either or both of transmissive x-ray optical elements  620   a  and  620   b  can be replaced with multiple transmissive x-ray optical elements, as described with respect to  FIG. 4 . According to other embodiments of the invention, x-ray metrology system  600  can include additional transmissive x-ray optical elements to focus output beam portions  653   a  and  653   b  onto detector  630 . Note that while two transmissive x-ray optical elements and two x-ray beams are shown in  FIG. 6  for explanatory purposes, according to other embodiments of the invention, any number of transmissive x-ray optical elements and beams can be included in x-ray metrology system  600 . 
     FIG. 7  shows an x-ray metrology system  700  in accordance with another embodiment of the invention. X-ray metrology system  700  is configured to perform small angle x-ray scattering (SAXS) on a test sample  742 . Small angle scattering using visible light sources are presently used in areas such as polymer analysis and biological analysis to determine the size (and to some degree the shape) of small particles. A collimated beam of light is directed onto the test sample and the resulting distribution of scattered light rays are analyzed to characterize the structures within the test sample. However, the technique cannot be used for structures that are smaller than the wavelength of the measurement light. For example, dielectric materials for use in semiconductor devices have been proposed that are filled with tiny pores (i.e., porous dielectric material) to reduce the dielectric constant of the material. The pores can be on the order of two nanometers, which is far less than the wavelength of visible light (roughly 400–700 nm), and therefore cannot be resolved by visible light-based techniques. However, such pores can be measured using SAXS, since x-ray wavelengths can be well below the nanometer level. 
   X-ray metrology system  700  includes an x-ray source  710 , a transmissive x-ray optical element  721 , a stage  740  for supporting test sample  742 , an optional transmissive x-ray optical element  721 , a detector  730 , and an optional computer  790 . As described above with respect to  FIG. 3   a , x-ray source  710  can comprise any x-ray beam-producing component, including a microfocus x-ray tube, a plasma source (laser-plasma or dense plasma), or a capillary discharge source. During an SAXS operation, x-ray source  710  generates an x-ray beam  750  that comprises a set of diverging x-rays, as indicated by an initial beam portion  751 . Transmissive x-ray optical element  720  intercepts beam portion  751  and reshapes it into a collimated beam portion  752  that is directed onto a thin film region  741  on test sample  742 . The scattering distribution of x-ray set  770  (with individual x-rays  771 ,  772 , and  773  shown for explanatory purposes) is then measured by detector  730 . An optional transmissive x-ray optical element  721  can be placed in the path of the set of scattered x-rays  730  to enhance the resolving power of detector  730 , as described above with respect to  FIG. 5 . The measurement data from detector  730  can then be stored or processed by optional computer  790  to determine the desired characteristics of thin film region  741 . 
     FIGS. 8   a – 8   i  show a method for fabricating a zone plate using a damascene process according to an embodiment of the invention. Referring to  FIG. 8   a , the fabrication process begins by forming a dielectric layer  820  on a substrate  810 . Dielectric layer  820  can comprise elements having low atomic numbers (e.g., silicon (14) and lower) to minimize interaction with the x-rays of interest. According to various embodiments of the invention, dielectric layer  820  can comprise silicon dioxide (SiO2), silicon nitride (SiN), silicon carbide (SiC), or even a porous dielectric. In  FIG. 8   b , a resist layer  830  is formed over dielectric layer  820 , and is then patterned with the desired concentric ring pattern to form a patterned resist layer  831  in  FIG. 8   c . According to an embodiment of the invention, the patterning operation can be performed using standard lithography techniques such as optical lithography (using optical proximity correction or phase shift masking) or electron beam lithography. Therefore, the dimensions of the final zone plate are only limited by the resolution limit of the lithography processes being used. Then in  FIG. 8   d , the exposed portions of dielectric layer are etched away to form a patterned dielectric layer  821  made up of concentric trenches of circular, elliptical, or other oval shapes. 
   In  FIG. 8   e , an optional barrier layer  844  and a seed layer  845  are formed over the entire patterned region (i.e., patterned dielectric layer  821  and the exposed portions of substrate  810 ) using physical vapor deposition (PVD) or chemical vapor deposition (CVD). Then in  FIG. 8   f , a metal layer  840  is electro-chemically plated over seed layer  845 . Note that if migration of the atoms of metal layer  840  is not a concern, then barrier layer  844  can be eliminated. According to various embodiments of the invention, metal layer  840  can comprise copper, tungsten, cobalt, or any other metal or metal compound compatible with the damascene process. Then, in  FIG. 8   g , the top portion of metal layer  840  is planarized via chemical-mechanical polishing (CMP) until patterned dielectric layer  821  is exposed, thereby forming a damascene layer  850  made up of patterned dielectric layer  821  and concentric metal rings  841 . The metal rings will generally introduce significantly more phase shift to the transmitted x-rays than will the dielectric rings, and the thickness Th of damascene layer  850  is selected to ensure proper constructive interference of the x-rays that exit the metal and dielectric rings.  FIG. 9  shows a plan (top) view of damascene layer  850 , which clearly reveals the concentric rings formed by the damascene process. The performance of a zone plate including damascene layer  850  can be optimized by sizing concentric metal rings  841  and dielectric spacer rings  821  such that they all have the same plan view areas. Equal plan areas ensures complete constructive and destructive interference from the metal and dielectric rings, respectively. 
   To complete the zone plate, additional damascene layers are then formed over damascene layer  850  using substantially the same processes (described with respect to  FIGS. 8   a – 8   h ) used to form damascene layer  850 .  FIG. 8   i  shows a completed zone plate  800  that includes damascene layers  850 ,  851 , and  852 , formed one over the other, and separated by dielectric layers  860  (e.g., silicon nitride). By “stacking” damascene layers in this manner, high aspect ratio metal structures can be created in a very structurally sound manner. Note that while the outer diameters of corresponding metal rings in each damascene layer are aligned, the inner diameter of corresponding metal rings in each damascene layer get progressively larger in each successive damascene layer, so that the width of corresponding metal rings decreases in each successive damascene layer. This width variance creates the angled profile metal structures required to provide the desired x-ray beam shaping. For example, for a beam traveling in the Y direction, the metal rings of damascene layers  852 ,  851 , and  850  will tend to cause the x-rays exiting the zone plate to converge (i.e., the x-ray beam will be focused (or collimated if the original x-rays entering the zone plate were diverging)). Note that the x-rays in an x-ray beam traveling in the opposite direction through zone plate  800  (i.e., in the negative Y direction) would be affected in the same manner—i.e., the exiting x-rays would also converge. Note that while increasing metal ring inner diameters in damascene layers  850 – 852  are shown in  FIG. 8   i  for explanatory purposes, according to other embodiments of the invention, the metal ring inner diameters can decrease in successive damascene layers, or the outer diameters of the metal rings can be increased or decreased (while holding the inner diameters constant between damascene layers) to provide the desired beam shaping. The details of how the rings in different levels change in thickness and position affect the intensity of various orders of diffraction and can be tailored to ensure that the great majority of x-rays diffract into the desired order. When tailored in this way, the zone plate will have maximum efficiency and contrast. According to various other embodiments of the invention, different dielectric materials and different metals can be used in (and/or between) the different damascene layers to adjust the overall beam shaping properties of zone plate  800 . Note that while three damascene layers are shown in  FIG. 8   i  for explanatory purposes, a zone plate in accordance with the invention can include any number of damascene layers. 
     FIG. 10  shows a zone plate  1000  in accordance with another embodiment of the invention. Zone plate  1000  includes three damascene layers  1050 ,  1051 , and  1052 , each of which is substantially similar to damascene layers  850 ,  851 , and  852 , respectively, shown in  FIG. 8   i , except that each of damascene layers  1050 ,  1051 , and  1052  includes two sets of concentric metal rings. Therefore, zone plate  1000  includes two diffraction grating regions  1001  and  1002 , each of which is substantially similar to zone plate  800  shown in  FIG. 8   i . Because diffraction grating regions  1001  and  1002  can be formed simultaneously on the same substrate  1010  (using substantially the same process described with respect to  FIGS. 8   a – 8   i ), zone plate  1000  effectively provides a zone plate array that can be efficiently and accurately manufactured. Note that while two diffraction grating regions having three damascene layers each are shown in  FIG. 10  for explanatory purposes, a zone plate in accordance with the invention can include any number of diffraction grating regions, with each of the diffraction grating regions having any number, type, and configuration of damascene layers. 
   The various embodiments of the structures and methods of this invention that are described above are illustrative only of the principles of this invention and are not intended to limit the scope of the invention to the particular embodiments described. Thus, the invention is limited only by the following claims.