Patent Number: 
Section: description

In accordance with the present invention, a parallel x-ray nanotomography system and method are provided for forming an image of a target sample. As illustrated in FIG. 1, an x-ray source 10 is provided. A laser-based x-ray source 10 is illustrated in FIG. 1, which preferably is a point-like x-ray source. As seen in FIG. 1, the laser-based x-ray source includes a laser source 20, generating an output laser beam 30 and a laser plasma x-ray target 40. Any laser source 20 may be used that can provide light at a desired wavelength, power level and beam quality. It is preferred that the laser source 20 provides an output beam 30 that has good beam quality (i.e. focused close to its diffraction limit). A preferred x-ray source 10 is described in U.S. Pat. Nos. 5,003,779, 5,089,711, 5,539,764 and preferably includes as the laser source 20 a BriteLight(trademark) laser available from JMAR Technologies, Inc. of San Diego, Calif., and as described in U.S. Pat. Nos. 5,434,875, 5,491,707 and 5,790,574, all of said patents being referred to and incorporated in this description by this reference. However, it should be understood that these particular laser sources and x-ray sources are mentioned as examples and any x-ray source generating a sufficient x-ray flux (i.e. photons per unit area, per unit time, per unit of solid angle) such as a point-like x-ray source at the sample 80 can be used. Alternative embodiments of x-ray sources are illustrated in FIGS. 2 and 3. In FIG. 2, a synchrotron 41 is provided, although this is not preferred because of the large size and high cost of currently available synchrotron. In FIG. 3, an x-ray tube 24 is provided. In an exemplary embodiment, an x-ray flux of between 0.01 and 1 watt per square centimeter (cm2) at the sample 80 is preferred. However, it should be understood that any x-ray flux suitable for generating an image at the x-ray image formation and acquisition apparatus 160 may be used and the acceptable x-ray flux may be above or below this range. In alternative embodiments an electron beam excited x-ray source is used instead of the laser beam source 20. This may be particularly suitable for thicker samples 80, which tend to require illumination with harder x-rays to ensure a good transmission through the specimen. Nevertheless, the use of harder x-rays also can have an adverse effect of decreasing the imaging resolution of the apparatus. The x-rays 50 generated in the x-ray source 10 are collected and focused using collector optic 60. The x-rays 70 exiting from the collector optic pass to sample 80, which optionally is on a rotating or translating apparatus 100. The image generating x-rays 110 are received in the composite objective 120 (which preferably includes a Fresnel zone plate array, as described below) which creates a detectable image of the x-rays. The detectable image optionally is further refined using an aperture assembly 140. X-ray image formation and acquisition apparatus 160 detects the x-ray image. The aperture assembly 140 is illustrated in more detail in FIG. 7. The aperture assembly 140 includes an aperture structure (illustrated with the thick lines labeled 140 in FIG. 7) having plural through-holes therein (illustrated with reference number 142 in FIG. 7). The aperture assembly preferably is placed midway between the plane of the composite objective 120 (indicated in FIG. 7 as zone plate array including ZP1 and ZP2) and the image plane, where preferably the X-ray image formation and acquisition apparatus 160 is located. In FIG. 7, the image plane is indicated with reference number 161. Preferably the aperture assembly 140 is an order sorting aperture. The aperture assembly 140 blocks the positive and negative odd-order diffracted images except the first order from overlapping with the image forming first order x-rays, as illustrated in FIG. 7. In order to increase the proportion of the x-rays 50 that ultimately impinge on sample 80 (the x-rays that impinge, i.e. illuminate the sample 80 are illustrated with reference number 70), it is preferred to use a grazing incidence collector in an ellipsoid shape for the collecting optic 60. Preferably, the source of the x-rays 40 is viewed as a point source and is at one focus of the collecting optic 60 and the sample 80 is located at a second focus of collecting optic 60. Improved control over the fidelity of the image of the source 40 may be obtained through the use of a Wolter optics as the collector optics 60. Wolter optics combine a reflection off an ellipsoid with one off a hyperboloid. In the preferred embodiment, the collecting optic 60 includes a cylindrical collector having a multi-layer coating 63 on a mirror 65. A Wolter optic can be used. Such a multi-layer coating 63 serves to enhance the reflectivity of the mirror 65. Such a collecting optic can collect incident x-rays 50 incident at an angle that is less than the critical angle of the mirror 65. Using a multi-layer coating 63 can serve to increase the critical angle, thereby increasing the collection efficiency of the collecting optic 60. A additional feature of the multi-layer coating 53 is that it serves to monochromatize the light. Preferably its bandpass is matched to an emission line of the x-ray source. The monochromized light is required to keep the resolution length scale of the Fresnel zone plate array 200 (discussed in greater detail below) small, due to the chromatic dispersion of the Fresnel zone plate array 200. This is because the multilayer coating selects a single line from the emission spectrum and the intrinsic line width of the x-ray emission is small enough to suppress significant chromatic dispersion of the Fresnel zone plate. Tomography may be performed in a raster scan mode or an imaging mode. If scanning is used, the length scale of the reconstructed volume elements are limited to the length scale of the spot size of the source, such as described in A. C. Kak and M. Slaney, Principles of Computerized Tomographic Imaging, IEEE Press, NY, 1986, which is referred to and incorporated herein by reference. If imaging is used, the length scale of the reconstructed volume elements are limited to the length scale of the resolution of the composite objective lens 120, or the resolution of the x-ray image formation and acquisition apparatus 160 demagnified by the composite objective lens 120, whichever is larger. Since the imaging mode is orders of magnitude faster than the scanning mode, scanning is not discussed here (although it does minimize the radiation dose to he sample). The x-rays 70 from the collecting optic 60 are received by the sample 80, as illustrated in FIG. 1. The sample 80 is the item to be imaged in the x-ray imaging system of the present invention. In one example, the sample 80 is a silicon based wafer incorporating microcircuit elements and connectors disposed in a three-dimensional configuration within the processed wafer. Exemplary microcircuit elements are gates, transistors and interconnect wiring (that may be with or without defects) in x- y- and/or z-directions, although any elements may be included in the sample. Exemplary dimensions of such elements are 20-250 nanometers, although any other dimensioned elements may also be imaged using the present system. The sample 80 is mounted on receiving apparatus 100. Any rotating and/or translating receiving apparatus that can receive and retain the sample may be used. Preferably, the receiving apparatus can rotate or translate the sample 80 to allow different views to be generated. The rotating or translating apparatus 100, positions the target (i.e. sample) by rotating or moving in linear directions (or combinations thereof) such as horizontally, vertically or diagonally. Exemplary rotation of the sample 80 is illustrated in FIG. 1 with arrow 90 and translation is illustrated with arrows 92 and 94. In one embodiment, a sample 80 is mounted on a receiving apparatus 100. The sample includes a microchip having various gates, transistors, connectors etc. thereon. A microcircuit failure analysis is performed by imaging the sample 80 using the apparatus of the present invention. This analysis includes, for example determining if any of the interconnects have been or might become damaged. Downstream of the sample 80 is a composite objective 120. The composite objective 120 receives the x-rays 110 downstream of the sample 80 and creates a readable image in any desired fashion for receipt by the x-ray image formation and acquisition apparatus 160. The composite objective 120 includes an array of micro-objectives 200, which preferably includes a Fresnel zone plate array, such as illustrated in the exemplary embodiment of FIG. 4. The composite objective 200 such as a Fresnel zone plate array, includes plural micro-objectives or Fresnel zone plates 210 arranged into any desired pattern. Any pattern incorporating plural micro-objective plates 210 may be used to achieve a desired imaging of the x-rays and the desired properties of the exit x-rays 130. Any suitable type of micro-objective or Fresnel zone plate can be used in the array 200, which suitably form an image of the x-rays 110. For example, the Fresnel zone plates can be amplitude zone plates, phase zone plates, blazed zone plates, or any other suitable form of zone plate. Likewise any shape of micro-objective 210 or Fresnel zone plate may be used, such as annular, elliptical, square or rectangular. Alternatively, x-ray reflective or refractive lenses or zone plate lenses may be used in place of, or intermingled with the Fresnel zone plates. Each individual micro-objective or Fresnel zone plate creates an individual image received in the x-ray image formation and acquisition apparatus. In one embodiment, the micro-objective array 200 is a portion of a two-dimensional hexagonally closed packed lattice, such as illustrated in FIG. 4A. Such a pattern achieves six-fold rotational symmetry. Each micro-objective 210 or Fresnel zone plate 210 can have any desired shape, although it is preferred that each be generally circular. The individual micro-objectives 210 may be arranged to touch, or to have varying sized gaps in between. Preferably the individual micro-objectives 210 are relatively close to one another making approximately 91% of a plane of the array 200 contained within the touching or almost touching micro-objectives 210. The 91% figure is given more precisely as xcfx80/2{square root over (3)}, the ratio of the area of a circle to its circumscribed hexagon. The x-rays falling within this fraction will be imaged by each individual micro-objective or Fresnel Zone Plate 210. It should be appreciated that any suitable array pattern may be selected which will form an image from the incoming x-rays 110. Preferably, multiple micro-objectives 210 are used, each forming one image per micro-objective. In a preferred example, the micro-objectives 210 are arranged in a circular pattern, as illustrated in FIG. 4B. In this embodiment, the individual micro-objectives are situated in a pattern adapted to receive and image a ring field emission pattern of the incoming x-rays 110, such as can be generated with a cylindrical collector x-ray mirror 60, that generally produces a ring field of illumination. Other array shapes may also be selected to optimally receive and image the x-rays 110. Likewise, a single micro-objective may be used, but an array with multiple micro-objectives is preferred as a view is captured by each micro objective or zone plate in the array 200 thereby increasing the total x-ray radiation collected by the imaging system, and also reducing the number of times the sample needs to be moved and the system realigned to produce an image, reducing the total time to acquire an image. In an illustrative example, zone plates are used in the composite objective 120 and an order order-sorting aperture 140 is used in order to refine the image. To separate the first-order diffracted x-rays (which are the imaging x-rays) from the zero-order diffracted x-rays (which are non-imaging), a central stop 220 is introduced into the zone plate, as illustrated in FIG. 6. In one example, a sample radius is r, the zone plate radius is R, and the radius of the central stop be Ro. In this example, the sample radius r is half the length of the largest two-dimensional distance in the object plane between any two points in the sample 80 that are illuminated by x-rays. For a single zone plate to avoid overlap between the zero and first orders, the following relationship holds:             2      ⁢      r        ≤                  (                  1          +                      1            M                          )            ⁢              R        0              , where M is the unsigned magnification of the system, which is equal to the ratio of the distance between the image plane 161 and the zone plate (such as depicted as ZP1 and ZP2 in FIG. 7, which are within composite objective 120) and the distance between object plane (i.e. the location of sample 80) and the zone plate (such as depicted as ZP1 and ZP2 in FIG. 7, which are within composite objective 120). To avoid an overlap between the zero-order x-rays of one zone plate and the first-order x-rays of another zone plate, the following condition must hold, where xe2x80x9caxe2x80x9d is the distance between the centers of the two zone plates:       2    ⁢    r    ≤            (              1        +                  1          M                    )        ⁢                  (                  a          -          R                )            .       If the zone plates are no closer than touching, then 2Rxe2x89xa6a. Also, R0 less than R, as the central stop may not have a width greater than that of its zone plate. From the above equation, we can obtain:             2      ⁢      r        ≤                  (                  1          +                      1            M                          )            ⁢              R        0               less than                   (                  1          +                      1            M                          )            ⁢      R        =                    (                  1          +                      1            M                          )            ⁢              (                              2            ⁢            R                    -          R                )              ≤                  (                  1          +                      1            M                          )            ⁢              (                  a          -          R                )             In the above example, any zone plate array having non-overlapping imaging zone plates with a central stop will not suffer from an overlap of the image formed by the first-order diffracted x-rays and the zero-order diffracted x-rays of a neighboring zone plate. In the exemplary Fresnel zone plate 210 illustrated in FIG. 6, there is illustrated a central stop 220 and zones 230, 240 and 250 of the zone plate 210. Although it is preferred that the micro-objectives be Fresnel zone plates, as already discussed, other types of micro-objectives may also be used. For example Wolter microscopes or Kirkpatrick-Baez microscopes suitable for use with x-rays also may be used. Likewise, other types of microlenses or micromirrors also may be used as the micro-objectives 210 in the array 200. Combinations of different types of such micro-objectives may also be used in the array 200. Alternatively, if the imaging is done with photons, electrons, neutrons, positrons or photons or other forms of matter, other forms of suitable micro-objectives 210 may be selected which are suitable for receiving and imaging. The preferred embodiment of the collector optic 60 includes a multilayer reflective coating with a band pass which is matched to the number of zones in each Fresnel Zone Plate 210. The zones are illustrated in FIGS. 5 and 6 by the rings illustrated therein. The central pass frequency is matched to bright transition lines emanating from the x-ray source 10. The role of the multilayer coating 63 is both to filter the x-ray light and to allow the collection of a larger solid angle (by a factor of about 3) than an uncoated surface. Although a laser-based x-ray source 10, as illustrated in FIG. 1 is preferred, a sychrotron-based x-ray source also may be used, as illustrated in FIG. 2. In such an alternative embodiment, the array 200 may be used to improvve synchrotron-based tomography for bending magnet x-ray sources that emit less intense x-rays, but are less expensive than later generation synchrotrons, such as third generation synchrotrons. An electron beam 31 is injected from the injector 22 into the synchrotron 41. In the case of a bending magnet, the x-rays 50 leave the synchrotron 41 in a direction tangent to the circular electron trajectory; typically these are collimated in the vertical direction but not in the horizontal direction. Using the Fresnel Zone Plate array 200, the requirement for horizontal collimation may be relaxed by the number of zones placed in a row. This increases the amount of x-rays passing through the sample and entering the detector. In an alternative embodiment, an x-ray tube is used, as illustrated in FIG. 3. In typical x-ray tubes harder x-rays are typically emitted than with laser-plasma x-ray sources. Where relatively thick samples 80 are used, harder x-rays are preferred so as to increase the x-ray transmission through the specimen. As illustrated in FIG. 3, an example of an x-ray tube includes an electron source 24 generating an electron beam 32. The electron beam 32 impinges on a target 42 generating x-rays 50. In embodiments where it is desired to further increase the brightness of the x-ray source, a microfocus x-ray source may be used, for example, in which the emitted x-rays 50 have a very small cross-sectional width, such as between 4 xcexcm and 30 xcexcm, although any dimension may be selected that provides sufficient x-ray flux for imaging the sample 80. In one embodiment, such a relatively small x-ray source can be achieved by focusing the electron beam 32 on the target 42 by means of an electromagnetic lens. The x-ray image formation and acquisition apparatus 160 can include any apparatus that can detect the image from the composite objective 120. In one embodiment, as illustrated in FIG. 10, the x-ray image formation and acquisition apparatus 160 includes a 2D imaging detector 162, such as a phosphor screen and visible light CCD camera combination or an x-ray CCD array or x-ray CCD camera 162. The 2D imaging detector 162 detects the plural 2D images produced by the composite objective 120. Optionally the 2D image is stored in a storage device 164 that can be read by a processing device 166. Any storage device can be used that can store the 2D image, preferably a digital storage device, such as a computer readable media. Examples of suitable media are magnetic or optical media such as hard disks, floppy disks, CD-ROMs, flash memory, RAM etc. Likewise any processing device 166 can be used, but preferably is a computing device that includes a processor and which also can display images on a display 168. Plural 2D images are combined in the processing device 166 to create a 3D image. The display 168 can include any form of display that can depict a desired image. Examples of such displays include a printer to make a hard copy, or a display screen, such as a CRT monitor, television monitor or LCD display. A system of apertures 140, as illustrated in FIG. 7, allows the image forming positive 1st diffraction orders to pass through the system, but blocks the 0th and all odd orders of diffraction, including negative 1st orders, from reaching the image areas within the common image plane. When the width ratio of alternating zones in a zone plate is close to 1, the even orders are generally significantly reduced or substantially absent. In operation, the present invention is practiced using steps such as illustrated in FIG. 8 in which an image of a sample is formed by providing x-rays 310, exposing the sample to the x-rays 320 such as by positioning it in the path of the x-rays, and focusing the x-ray light downstream of the sample using a composite objective lens comprising a plurality of micro-objectives, such as Fresnel zone plates 330 and forming an image based on the x-rays 340. The step of forming the image 340 includes in the preferred embodiment recording a plurality of 2D images using the 2D image detector 162 and optionally storing them in storage device 164. Then the 2D images are combined to create a 3D image or images using an image reconstruction processing conducted by processor 166. Then the image is optionally displayed or printed out using display device 168. As discussed above, the Fresnel zone plates 210 preferably are arranged in an array 200 such as a generally hexagonal and/or substantially planar array as illustrated with step 330 in the figures. Another illustration of the practice of the present invention is illustrated in FIG. 9. As seen in FIG. 9, an image of a sample is formed by providing x-rays 310, collecting the x-rays 350 and transmitting or reflecting them in a desired fashion so that they can go to the sample, positioning the sample in the path of said transmitted or reflected x-rays 320, imaging the x-rays downstream of the sample using a composite objective lens comprising micro-objectives, such as a plurality of Fresnel zone plates 360 and forming an image using the imaged x-rays 380. Optionally the x-rays are refined using one or more apertures 370 between the composite objective lensing step 360 and the image formation 380. Thus it is seen that a tomography imaging method and apparatus is provided. One skilled in the art will appreciate that the present invention can be practiced by other than the preferred embodiments, which are presented in this description for the purposes of illustration and not limitation. It is noted that equivalents for the particular embodiments discussed in this description may practice the invention as well.