Abstract:
A phase contrast x-ray microscope has a phase plate that is placed in proximity of and attached rigidly to the objective to form a composite optic. This enables easier initial and long-term maintenance of alignment of the microscope. In one example, they are fabricated on the same high-transmissive substrate. The use of this composite optic allows for lithographic-based alignment that will not change over the lifetime of the instrument. Also, in one configuration, the phase plate is located between the test object and the objective.

Description:
RELATED APPLICATIONS 
     This application is a Division of U.S. application Ser. No. 10/331,108, filed Dec. 27, 2002, now U.S. Publication Number US 2004/0125442 A1, published on Jul. 1, 2004, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Short wavelength microscopes are devices that produce a magnified image of an object utilizing electromagnetic radiation in the extreme ultraviolet (EUV) and x-ray regime. The wavelengths range from 20 nanometers (nm) to 0.02 nm. These microscopes typically develop image contrast by relying on photoelectric absorption in the test object. Different elements, or structures formed of elements, attenuate the x-rays to varying degrees. 
     In a typical short wavelength microscope configuration, a focusing element, such as a condenser, is used to concentrate the radiation on the test object. An objective, such as a zone plate, collects the radiation after transmission through the object and forms an image on a detector, such as a charge-coupled device or film. Intervening scintillators are sometimes required, depending on the specific wavelength and detector used. 
     Absorption-based x-ray microscopy, however, tends to impose certain limitations on the types of structures that can be imaged. Generally, absorption contrast decreases in proportion to the third power of the photon energy of the radiation. This tends to motivate for the use of lower energies, but lower energies may not provide sufficient penetration through the object. 
     Moreover, absorption-based x-ray microscopy may also fail to provide sufficient contrast between structures within the object of interest when those structures are composed of elements that have similar atomic numbers. Absorption contrast is generally proportional to the fourth power of the atomic number, away from an absorption edge. Absorption contrast x-ray microscopy therefore works well when imaging structures consisting of mostly high atomic number elements, such as gold or tungsten, in a host material consisting of mostly low atomic number elements, such as silicon. It is generally difficult, however, to use absorption contrast to image small structures consisting of mostly low atomic number elements in a test object containing non-negligible amounts of high atomic number elements, such as imaging cracks in dielectrics of a multilevel integrated device. 
     An alternative to absorption contrast x-ray microscopy is sometimes termed phase contrast x-ray microscopy. Here, the phase shifting properties of the structures within the object of interest are used to create the image contrast between the structures. To utilize the phase contrast, a phase shifting element is typically placed at the back focal plane of the objective to impart a suitable phase shift to the direct beam, i.e., radiation that passed directly through the test object. The focal plane is the plane parallel to the lens that passes through the point at which parallel rays of light meet after being focused by the lens. The phase shifted direct beam interferes at the image plane with radiation that was scattered and diffracted in the test object. Thus, contrast is produced in response to the phase shifting properties or refractive indices of structures within the test object. 
     Phase contrast x-ray microscopy has some intrinsic characteristics that render it more effective in many types of imaging applications. First, phase contrast is generally significantly larger than absorption contrast in the 0.02-20 nanometer (nm), short wavelength spectral region. As a consequence, exposure time can be substantially reduced. Secondly, phase contrast is inversely proportional to the energy except for a narrow spectrum near an absorption edge. As a result, doubling the energy decreases the phase contrast only by a factor of two while the penetration power increases by a factor of eight. This allows for thicker samples, easing the requirements for sample preparation, or allows for the imaging of samples nondestructively. Moreover, phase contrast between elements is roughly related to the mass density, rather than the atomic number, except for a narrow spectrum near an absorption edge. This enables the imaging of structures comprising low atomic numbered elements alone or in a host materials matrix containing high atomic number elements. It is especially applicable to imaging structures comprising organic compounds, silicon, and/or oxygen, for example. 
     In the past, the optical trains of phase contrast x-ray microscopes were similar to the trains used for optical frequencies. In a conventional configuration, the scattered light was phase shifted relative to the direct beam by, typically, 90 degrees with a quarter wave plate or 270 degrees with a three quarter wave plate that was located at the back focal plane of the objective to retard or advance the phase of the direct beam. There was typically a requirement to attenuate the direct beam so that it had comparable intensity as the collected, scattered signal radiation. This provided higher contrast because complete extinction occurred during destructive interference when the two interfering beams are of the same amplitude. 
     SUMMARY OF THE INVENTION 
     One challenge that arises in the operation of these conventional phase contrast microscopes is achieving and maintaining alignment between the objective and the phase plate. The objective and the phase plate must be aligned in the two axes that are perpendicular to the optical axis of the microscope. In some implementations, the elements must be aligned to better than about 0.5 to 1 micrometers for short wavelength microscopes using x-rays, for example. Also, there are typically minimum tolerances for angular alignments between these two optical elements and the surrounding optical train. 
     In the prior art devices, this alignment has been difficult to achieve during the initial assembly of the microscope and to maintain the alignment over the microscope&#39;s lifetime, because the distance between the phase plate and the objective is substantially larger than the alignment tolerance. Misalignment induced by thermal drift or mechanical vibration, for example, can render the alignment dynamic over an exposure and lead to performance degradation over time. 
     According to the present invention, the phase plate is placed away from the objective&#39;s back focal plane and preferably in close proximity to the objective, instead of at the back focal plane of the objective as in the prior art. The close proximity facilitates the initial alignment between the zone plate and the phase plate, and the maintenance of that alignment over time. In one example, the zone and phase plates are fabricated on the same substrate or mounted rigidly together, producing a composite optical device, resulting in accurate alignment over a long period of time. The present invention limits the field of view to be a fraction of the entrance pupil of the objective but this limitation is acceptable for many practical applications, especially in the short wavelengths. 
     In general, the invention features a phase contrast x-ray microscope, which comprises a source for irradiating a test object with short wavelength irradiation and a detector for detecting the radiation from the test object. An objective images and magnifies the radiation from the object onto the detector, with the direct beam (undiffracted beam by the object) suitably phase shifted by a phase plate. A field aperture defines an appropriate field of view for imaging. 
     According to a preferred embodiment of the invention, a phase plate and the objective are rigidly attached at a close proximity by, for example, fabricating on or mounting them to the same substrate. 
     In the typical implementation, a condenser is used to relay the radiation from the source to the test object. A reflective optic, such as a capillary or Wolter optics, or a diffractive condenser zone plate, is used in the current implementation. 
     In the current embodiment, the objective comprises a zone plate lens. 
     In an alternative embodiment, the objective comprises a Fresnel optic. In yet another alternative embodiment, the objective comprises a Wolter optic. 
     Depending on the implementation, the radiation can be detected, for example, using standard silver-based film or electronically using a detector array, such as a charged coupled device (CCD). 
     In general, according to another aspect, the invention also features a composite optic for a phase contrast x-ray microscope. This optic comprises a substrate and a lens attached to the substrate. A phase plate is also attached to this same substrate. 
     According to still another aspect, the invention features a method for imaging structures of a test object. The process comprises configuring an optical train including an objective, having a back focal plane, and a phase plate, which is located in the optical train away from the back focal plane of the objective. The test object is irradiated with short wavelength radiation. A differential phase shift is induced between direct radiation and diffracted radiation from the test object with the phase plate. The radiation is imaged onto a detector with the objective. 
     In general, according to still another aspect, the invention features a method for fabricating a composite optic. It comprises forming fiducials on a substrate. An objective and a phase plate are also formed on the substrate. The phase plate is aligned relative to the objective using the fiducials. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
         FIGS. 1A and 1B  are schematic views of optical trains of a phase contrast microscope illustrating why the phase plate  116  (not shown in figure) is placed at the back focal plane  130  of the objective  118  in prior art implementations and the validity conditions for the implementation of phase contrast imaging according to the present invention; 
         FIG. 2A  is a schematic view of an optical train of a phase contrast x-ray microscope according to the present invention; 
         FIG. 2B  is a schematic view of an optical train of a phase contrast x-ray microscope according to a current implementation of the present invention; 
         FIGS. 3A and 3B  are cross-sectional views of composite optics comprising zone plates that are formed on one side of a substrate and phase rings that are formed on the other side of the substrate, according to the present invention; 
         FIGS. 4A and 4B  are cross-sectional views of inventive composite optics comprising a Fresnel lens that is formed on one side of a substrate and the phase ring that is formed on the other side of the substrate; 
         FIGS. 5A and 5B  are cross-sectional views of composite optics comprising zone plates and phase plates according to still other embodiments; 
         FIG. 6  is a plan view of a composite optic showing alignment fiducials according to the present invention; and 
         FIG. 7  is a cross-sectional view of composite optic comprising an achromatic Fresnel optic (AFO) and a phase plate according to still another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1A and 1B  schematically illustrate why phase plates are placed at the back focal planes of objectives in the typical phase contrast microscope configuration and the validity conditions for the phase contrast imaging configuration according to the present invention. 
     Four important parameters characterize the illumination beam  112 : 1) the brightness B, 2) the field of view (FOV), 3) the mean numerical aperture NA c =sin θ, and 4) the angular spread Δθ. 
     The photon flux incident on the test object  10  within the field of view (FOV) is proportional to B*2π*(1−cos θ), which approximately equals to (B*2π*sin θ*Δθ) for small θ. The exposure time is inversely proportional to this photon flux. 
     The brightness B is typically constrained by the device used. The source brightness and the throughput of the optical system relaying the photons from the source to the test object  10  to thus produce the illumination beam  112  dictate the level of the brightness. 
     Thus, for a given brightness, it is important to maximize the sin θ*Δθ product to reduce exposure time, for example. In general, the mean numerical aperture NA c  is equal to or smaller than the numerical aperture NA of the objective  118 . In order to reduce exposure time, one consequently needs to increase Δθ. There are consequences, however, to increasing Δθ on the imaging property of the phase contrast microscope, which must be considered, however. 
       FIG. 1A  schematically shows that the width of the phase ring required at the back focal plane  130  is very narrow for a very small Δθ. 
       FIG. 1B  shows that the width of the phase ring required at the back focal plane  130  is finite when Δθ is a finite yet still small. Generally, the width of the phase ring is approximately equal to f*Δθ, where f is the focal length of the objective  118 . 
     Because of the finite phase ring width required for small Δθ in combination with the intended phase shift for the direct beam, the low spatial frequency component of the beam diffracted by the test object  10  will also be unintentionally phase shifted. This unintentional phase shift results in the so-called “halo-effect” at edges of images corresponding to large features. Thus, for a given Δθ, the minimum feature size that may have “halo-effect” is approximately equal to 2λ/Δθ, where λ is the average wavelength of the illumination beam. 
     The “halo effect” is acceptable in most practical phase contrast microscopes and what is the acceptable minimum feature size is often an important consideration in designing a phase contrast microscope. 
       FIG. 1B  shows that when the FOV is substantially small, the phase ring width at the back focal plane  130  is approximately equal to its projected width on the objective  118 , especially when the NA of the objective is large. The present invention recognizes that in this case the phase ring can be placed away from the back focal plane  130  of the objective  118  and in proximity to the objective  118 , for example, without affecting the imaging property of the phase contrast microscope. The acceptable FOV may be defined by FOV&lt;O*Δθ, where O is the distance between the test object  10  and the objective  118 . 
       FIG. 2A  shows an optical train for a phase contrast short wavelength microscope, which has been constructed according to the principles of the present invention. It preferably utilizes electromagnetic radiation in the extreme ultraviolet (EUV) and x-ray regime, between about 20 nanometers (nm) to 0.02 nm in the present embodiment. 
     Specifically, the optical train  100  comprises a source  110 . In one example, the primary X-ray radiation is generated by bombarding a solid target with energetic electrons, or by focusing a sufficiently intense laser beam on a solid or liquid target to generate plasma of a sufficiently high temperature. A synchrotron can also be used. 
     A condenser  120  is used to collect and relay the radiation from the source  110  to the test object  10 . The condenser  120  preferably performs three functions: increasing the flux density of the illumination beam  112  at the test object  10  to speed the image collection time, increasing resolution of the microscope by delivering the illumination beam  112  with an appropriate numerical aperture, and shaping the illumination beam  112  with a suitable angular spread Δθ in conjunction with a suitably shaped aperture  115 . In one example, a capillary optic condenser with a suitably configured reflecting surface is used. For example, the reflecting surface is usually an ellipsoid reflecting surface to form an image of the source  110  on the test object  10 . Single layer or multilayer coatings are preferably used to increase reflection efficiency. In another example, a Wolter optic with suitable optical parameters is used. A reflecting surface of the Wolter optic comprises a single layer or a multilayer coating to increase reflection efficiency. In yet another example, the condenser  120  is a zone plate. 
     The beam  112  from the condenser  120  is generally conically shaped. An aperture  115  is typically used to render this beam hollow to form a hollow cone illuminating beam. That is, substantially no radiation is present that is directed along or at small angles to the optical axis  20 . Depending on the implementation, the aperture  115  is placed before, after, or in the condenser  120 . 
     According to the present invention, the area of the illumination beam projected on the entrance pupil plane is typically a fraction of the area of the objective pupil, i.e., the illumination beam  112  incident on the objective  118  occupies a small fraction of the surface of the objective  118 . This fraction is typically limited to less than 0.2. It is typically between 0.01 and 0.2. 
     The converging, hollow beam  112  irradiates the test object  10 . Different structures within the test object  10  are comprised of different constituent elements with different refractive indices and therefore phase shifting properties. The patterns of these structures in the plane that is orthogonal to the optical axis  20  further have different spatial frequencies. As a result, the radiation tends to be scattered and diffracted out of the path of the direct radiation beam occupying angle Δθ and the diffracted radiation covering a larger cone is partially or completely collected by the objective  118  and delivered to the image plane to produce an image. 
     The exiting radiation from the test object  10  including both the direct beam and the diffracted radiation passes through a phase plate  116 . In the illustrated example, the phase plate  116 - 1  is implemented as a phase ring to induce a phase shift between the radiation of the direct beam in angle Δθ and the radiation diffracted from the object  10  and collected by the objective  118  but not passing through the phase plate  116 . Specifically, in the typical implementation, material and thickness of the phase plate  116 - 1  are selected to induce a relative 90 or 270 degree phase retardation between the diffracted radiation and the direct beam radiation to produce a positive phase contrast or negative phase contrast. Sometimes, this relative phase change is more than 270 degrees and is equal to substantially a product of 90 degree with an odd integer, such as 5, 7, 9, etc. The ring  116  has a uniform thickness and extends parallel to the objective  118 . 
     In the preferred embodiment, the material of the phase ring  116 - 1  in the path of the direct radiation is selected to achieve a desired attenuation to improve image contrast by balancing the relative strength of the interfering diffracted and direct beams because, typically, the direct beam radiation is much more intense that the scattered radiation. 
     After the selective phase shifting, the radiation passes through an objective  118 . This objective  118  forms an image on a detector  125  by creating the focused radiation  122 . In the preferred embodiment, the objective  118  is a zone plate lens, a Wolter optic, or a Fresnel optic. 
     In an alternative embodiment, the phase plate  116 - 1  is placed after the objective  118  in the optical train. The critical notion is that the phase plate  116 - 1  is moved away from the back focal plane of the objective  118  and preferable nearer to the objective  118  in order to improve the alignment between the objective  118  and the phase plate  116 - 1 . 
     According to one aspect of the invention, the phase plate  116  and the objective  118  are attached to and preferably fabricated on the same high-transmissive substrate  140  to form a composite optic  138 . In one example, the high-transmissive substrate  140  is a low stress Si 3 N 4  membrane of a thickness between 100-1000 nm. The high-transmissive substrate  140  is more than 80%, transmissive to the radiation of the illumination beam  112  in most implementations, and is preferably greater than 95% transmissive in the preferred embodiment. The objective  118  and the phase plate are fabricated by patterning a resist material that has been deposed on the high-transmissive substrate  140 , and then plating into the patterned resist and onto the silicon wafer. 
     An important advantage of having the phase plate  116  and the objective  118  attached to a common high-transmissive substrate  140  is achieving and maintaining alignment. The two optical elements are aligned to lithographic accuracies in the situation where they are fabricated on the same high-transmissive substrate. In the situation in which they are mechanically attached to a common high-transmissive substrate, the phase plate  116  and the objective  118  are fabricated on separate high-transmissive substrates, which are then attached to each other. In this implementation, standard wafer-to-wafer alignment and bonding techniques are used. 
     In the preferred embodiment, the detector  125  is a detector array, such as a charged couple device (CCD) array. Alternatively, a film-based detection is implemented. Further, in other implementations, a scintillator may be necessary, depending on the wavelength of radiation and the bandwidth of the detector  125 . 
       FIG. 2B  shows a current implementation of the optical train for the phase contrast short wavelength microscope. It shows the preferred capillary optic condenser  120 . It further shows the general arrangement in which the distance between the test object  10  and the objective  118  is much shorter than the objective  118  and the detector  125  to obtain a magnified image. Typically, distances are about 1-200 millimeters (mm) for the object distance, i.e., the distance between the test object  10  and the objective  118 , and 100-3000 mm for the image distance, i.e., the distance between the objective  118  and the detector  125 . 
       FIGS. 3A and 3B  show two related implementations of a composite optic  138 , which have been constructed according to the principles of the present invention. 
     With reference to  FIG. 3A , the phase ring  116  and a zone plate objective  118  are fabricated on a common high-transmissive substrate  140 . Specifically, the zone plate  118  is formed on one side of high-transmissive substrate  140  and the phase ring  116  is formed on the other side of the high-transmissive substrate  140 , in the illustrated example. 
     It should be noted that this composite optic  138 - 1  could be oriented in either direction in optical path between the test object  10  and the detector  125 . In one embodiment, the phase ring  116  is located between the zone plate objective  118  and the test object  10 . Alternatively, as shown, the composite optic  138  is oriented such that the zone plate objective  118  is between the test object  10  and the phase plate  116 . 
     Because the objective  118  and the phase plate  116  are formed on the same high-transmissive substrate  140 , a single positional and angular alignment of the high-transmissive substrate  140  is required to the optical axis  20  of the optical train  100 . 
     In a second implementation of  FIG. 3B , the high-transmissive substrate  140  of composite optic  138 - 2  is fabricated to have a curve. Preferably, this curve is spherical, with the curve being in the direction of the test object  10 . This minimizes changes in the angle of incidence across the phase plate  116 . This curvature can be induced by using a flexible high-transmissive substrate  140  and controlling the stress properties of the material deposited on the high-transmissive substrate  140 . 
       FIGS. 4A and 4B  illustrate other embodiments of the composite optic  138  that have Fresnel-lens type objectives, as illustrated, or achromatic Fresnel optic objectives. 
     With reference to  FIG. 4A , the objective  118  is configured as a Fresnel lens. It is formed on a common high-transmissive substrate  140  with the phase ring  116  to form composite optic  138 - 3 . In the illustrated optical train  100 , the composite optic  138 - 3  is oriented such that the Fresnel objective  118  is between the phase plate  116  and the test object  10 . However, in other implementations, this composite optic  138 - 3  can be flipped so that the phase plate  116  is adjacent to the test object  10 . 
     As illustrated in  FIG. 4B , in some embodiments, the high-transmissive substrate  140  of composite optic  138 - 4  is fabricated with an arcuate, or specifically spherical shape, curving in the direction of the test object  10 . In this embodiment, the Fresnel lens objective  116  is fabricated on the side of the high-transmissive substrate  140  adjacent the test object  10 , or the concave side. In other implementations, the phase plate  118  is fabricated on side of the high-transmissive substrate  140  adjacent the test object  10 , or the concave side. 
     It should be noted that, although in the previous embodiments, the phase plate  116  and the objective  118  have been shown to be fabricated on opposite sides of the high-transmissive substrate, in other embodiments, they are fabricated on the same side, using a two-step patterning and plating process. 
     This arrangement is shown in  FIG. 5A . Specifically, the phase plate  116  is shown as being fabricated over the zone plate objective  118 , which in turn is fabricated on the high-transmissive substrate  140  to form the composite optic  138 - 5 . This configuration is deemed preferable since the yields associated with the fabrication of the zone plate  118  are typically lower than the phase plate  116 , thereby improving overall yields when the zone plate  118  is fabricated first. Further, a planarizing filler is usually coated over the zone plate  118  before the phase plate  116  is fabricated on the zone plate  118 . 
     As shown in  FIG. 5B , in still another embodiment, the composite optic  138 - 6  is fabricated by fabricating each of the zone plate  116  and the phase plate  118  on separate high-transmissive substrates  140 -A and  140 -B and then bonding or otherwise attaching the high-transmissive substrates  140 -A,  140 -B. 
     As shown in  FIG. 6 , the alignment between the objective  118  and the phase plate  116  is accomplished, in the preferred embodiment, by including alignment fiducials  210  on the high-transmissive substrate or high-transmissive substrates  140  of the phase plate  116  and the zone plate objective  118 . In the situation where the phase plate and zone plate are fabricated on the same high-transmissive substrate, the alignment fiducials are used during the lithographic patterning steps. If the phase plate  116  and zone plate  118  are fabricated on opposed sides of the high-transmissive substrate  140 , the high-transmissive substrate  140  is either thinned such that the fiducials are discernable from the backside or transferred to the backside in a frontside/backside alignment process. 
       FIG. 7  shows still another implementation of the composite optic. Here, the composite optic  138 - 7  is a combination of a phase plate  116  and an objective, which is an achromatic Fresnel optic (AFO)  310 . The configuration of the AFO  310  is preferably as described in U.S. patent application Ser. No. 10/134,026, which application is incorporated herein in its entirety by this reference. Specifically, the AFO  310  is a combination of a Fresnel refractive lens  312  and a zone plate  314 . In the illustrated embodiment, the phase plate  116  is fabricated on the zone plate  314 , which is fabricated on a substrate  140 . The Fresnel lens  312  is fabricated on the other side of the substrate  140 . 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.