Abstract:
An x-ray laminography imaging system that utilizes a nonplanar anode target to enable objects that are oblique to the direction of projection of electron beams onto the target to be precisely imaged. Because many objects that laminography techniques are used to inspect are oblique or have portions that are oblique, the nonplanar anode target of the present invention enables enables spot patterns to be traced that are parallel to the plane of the object, regardless of whether it is oblique or orthogonal.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to x-ray imaging and, more particularly, to an x-ray laminography imaging system that utilizes a nonplanar target anode to enable oblique objects, or objects having oblique portions or features, to be imaged with preciseness. 
   BACKGROUND OF THE INVENTION 
   Laminography techniques are widely used to produce cross-sectional images of selected planes within objects. Conventional laminography requires a coordinated motion of any two of three main components of a laminography system (i.e., a radiation source, an object being inspected, and a detector). The coordinated motion of the two components can be in any of a variety of patterns, including linear, circular, elliptical and random patterns. Regardless of the pattern of coordinated motion selected, the configuration of the source, object and detector is such that any point in the object plane (i.e., the focal plane within the object) is always projected to the same point in the image plane (i.e., the plane of the detector), and any point outside the object plane is projected to a plurality of points in the image plane during a cycle of the pattern motion. In this manner, a cross-sectional image of the desired plane within the object is formed on the detector. The images of other planes within the object experience movement with respect to the detector, thus creating a blur, i.e. background, on the detector upon which the sharp cross-sectional image of the focal plane within the object is superimposed. This technique results in sharp images of the desired object focal plane. Although any pattern of coordinated motion can be used, circular patterns generally are preferred because they are more easily produced. 
   The laminography techniques described above are currently used in a wide range of applications including medical and industrial x-ray imaging. Laminography is particularly well suited for inspecting objects that comprise several layers, with each layer having distinguishable features. However, laminography systems that produce such cross-sectional images typically experience shortcomings in resolution and/or speed of inspection, thus accounting for the rare implementation of laminography systems for this purpose. These shortcomings are frequently due to the difficulties in achieving high speed coordinated motion of the source and detector to a degree of precision sufficient to produce a high resolution cross-section image. 
   In a laminography system having a field of view that is smaller than the object being inspected, it may be necessary to move the object around within the field of view to obtain multiple laminographs which, when pieced together, cover the entire object. Movement of the object is frequently achieved by supporting the object on a mechanical handling system, such as an X, Y, Z positioning table. The table is then moved to bring the desired portions of the object into the field of view. Movement in the X and Y directions locates the area to be examined, while movement in the Z directions moves the object up and down to select the plane within the object where the image is to be taken. While this method effectively enables various areas and planes of the object to be viewed, there are inherent limitations associated with the speed and accuracy of such mechanical motions. These constraints have the effect of increasing cycle time, thereby reducing the rates at which inspection can occur. Furthermore, these mechanical motions produce vibrations that tend to reduce the system resolution and accuracy. 
   In order to reduce or eliminate the need to move the object, and the problems associated therewith, an off-axis laminography system has been invented, which is disclosed in U.S. Pat. No. 5,259,012 (the &#39;012 patent) and which is incorporated herein by reference in its entirety. The &#39;012 patent discloses a laminography system in which off-axis scanning circles can be used to enable multiple locations on an object to be sequentially imaged without requiring mechanical movement of the object or of the electron beam gun that is used to generate the x-rays. The phrase “off-axis” refers to placing the center of the scan circle in a position that is not concentric with the optical axis of the imaging system. The electron beams are projected from the gun onto a metal target anode. When the electron beams impinge on the target anode, x-rays are produced. The electron beams are deflected by a voltage-controlled yoke that causes the electron beams to impinge on the target anode at selected locations to trace off-axis circles that enable different locations on the object to be scanned. 
     FIG. 1  illustrates a schematic diagram of a laminography system  10  disclosed in the &#39;012 patent. The system  10  comprises a source of x-rays  12  positioned above an object  14  to be imaged, and a rotating x-ray detector  16 , positioned below the object  14  and opposite the x-ray source  12 . The object  14  may be, for example, a printed circuit board, a manufactured item such as, for example, an aircraft part, a portion of a human body, etc. The system  10  is symmetrical about a Z-axis  50 . The system  10  acquires X, Y plane cross-sectional images of the object  14  under inspection using multi-path laminography geometries, which enable multiple locations of the object  14  to be sequentially imaged without requiring mechanical movement of the object  14 . In other words, off-axis (i.e., not about the axis  50 , but about an axis parallel to axis  50 ) scanning patterns are used to image the object over different regions of the object in the X, Y plane. 
   The laminography system  10  may be interfaced with an analysis system  15  that automatically evaluates the cross-sectional image generated by the system  10  and provides a report to a user indicating the results of the evaluation. The source  12  is positioned adjacent the object  14 , and comprises an electron gun  18 , a set of electrodes  20  for electron beam acceleration and focus, a focus coil  60 , a steering yoke or deflection coil  62 , and a substantially flat target anode  24 . An electron beam  30  emitted from the electron gun  18  along the Z-axis  50  is incident upon the target anode  24  and causes an x-ray spot  32  to be produced, which serves as an approximate point source of x-rays  34 . The x-rays  34  emanate from a point on the target anode  24  where the electron beam  30  impinges upon the target anode  24 . At least a portion of these x-rays pass through various regions of the object  14  and impinge on the detector  16 . 
   The object  14  is mounted on a platform  48  which may be affixed to, for example, a granite table  49 , so as to provide a rigid, vibration-free platform for structurally integrating the functional elements of the system  10 , including the x-ray source  12  and the turntable  46 . It is also possible that the platform  48  comprises a positioning table that is capable of moving the object  14  along three mutually perpendicular axes; labeled X, Y, and Z in FIG.  1 . As stated above, with off-axis scanning, it is not necessary to physically move the object  14 . However, it may be desirable to move the object  14  to some degree to improve image quality. At any rate, with off-axis scanning, it is not necessary to move the object anywhere near as much as with on-axis scanning. 
   The rotating x-ray detector  16  comprises a fluorescent screen  40 , a first mirror  42 , a second mirror  44 , and a turntable  46 . The turntable  46  is positioned adjacent the object  14  on the side of the object  14  opposite the x-ray source  12 . A camera  56  is positioned opposite the mirror  44  for capturing images reflected into the mirrors  42 ,  44  from the fluorescent screen  40 . The camera  56  may comprise a low light level, closed circuit television camera that produces a video image of the x-ray image formed on the fluorescent screen  40 . The camera  56  may be, for example, connected to a video terminal  57  so that a user may observe the image appearing on the detector  40 . The camera  56  may also be connected to the image analysis system  15 . 
   In operation, x-rays  34  produced by the x-ray source  12  illuminate and penetrate regions of the object  14  and are intercepted by the screen  40  of detector  16 . Synchronous rotation of the x-ray source  12  and detector  16  about the axis  50  causes an x-ray image of a plane within the object  14  to be formed on the detector  16 . Although the axis of rotation  50  illustrated in  FIG. 1  is the common axis of rotation for both the source  12  and detector  16 , as stated above, these axes of rotation are not collinear in an off-axis system, but rather, are parallel to one another. The electron beam  30  is emitted from the electron gun  18  and travels in a region between the electrodes  20  and steering coils  60 ,  62 . The steering coils  60 ,  62  are separate X and Y electromagnetic deflection coils that deflect the electron beam  30  discharged from the electron gun  18  in the X and Y directions, respectively.! Electrical current flowing in the coils creates a magnetic field that interacts with the electron beam  30 , thereby causing the beam  30  to be deflected. The configuration of the x-ray spot pattern on the target  24  depends on where the beam  30  strikes the target  24 , which depends on the manner in which the beam  30  is steered. Electrostatic deflection techniques could also be used to deflect the electron beam  30  in the desired directions. 
   A lookup table (LUT)  63  outputs voltage signals that are applied to the X and Y deflection coils  60 ,  62  to cause the electron beam spot  32  ( FIG. 2 ) to rotate, thus producing a circular spot pattern on the surface of the target anode  24 . The values stored in the LUT  63  are predetermined using a calibration technique that correlates the position of the turntable  46  (i.e., the rotational position of the detector  16  and the position of the x-ray beam spot  32 ). The values stored in the LUT  63  correspond to the rotational positions of the turntable  46 . The turntable outputs electrical signals as it rotates that correspond to its rotational position. Once calibration has been performed using these electrical signals, the calibrated electrical signals are converted into digital values and stored the LUT  63  at appropriate addresses and off-axis laminography is then performed. 
   It should be noted that the target anode  24  in the &#39;012 patent is flat. Because the target anode  24  is flat, it is difficult for the system  10  to focus on oblique objects, or oblique portions of otherwise planar objects. The term “oblique”, as that term is used herein, is intended to indicate a position that is not in the X, Y plane represented by the X, Y and Z axes shown in FIG.  1 . The term “planar”, as that term is used herein, is intended to denote a position that is in the X, Y plane. Thus, the flat target anode  24  shown in  FIG. 1  is in the X, Y plane. 
   Some objects, such as printed circuit boards, for example, are warped or bowed in some fashion, and therefore are oblique or have portions or features that are oblique. It would be desirable to provide an off-axis scanning system that traces circular scan patterns on a target anode in a manner similar to the manner in which the system  10  of the &#39;012 patent operates, but that has the ability not only to precisely image planar objects, but that is also well suited for imaging oblique objects. A need exists for such a system because many objects that laminography techniques are used to inspect are oblique or have portions that are oblique. In addition, such a system could increase the types of objects that can be precisely imaged using laminography. 
   SUMMARY OF THE INVENTION 
   The present invention provides an x-ray laminography imaging system that utilizes a stationary x-ray source and generates a moving pattern of x-ray spots on a nonplanar target anode synchronously with rotation of an x-ray detector. Because the target anode is nonplanar, objects that are oblique, or oblique portions of generally planar objects, can be precisely imaged. 
   The x-ray laminography imaging system comprises an electron beam source that projects a beam of electrons along a Z-axis of the system, a nonplanar anode target upon which the electron beam impinges, an electron beam deflection controller that controls the deflection of the beam of electrons produced by the electron beam source onto the anode target, and an x-ray detector that receives x-rays that emanate from the target and pass through the object and converts the received x-rays into electrical signals from which an image of at least a portion of the object can be constructed. The electron beam deflection controller causes the electron beams to be deflected in an X-direction and a Y-direction in a selected manner so that the beam of electrons impinges on the target at particular locations on the target to produce x-ray spot paths having selected configurations. The X-direction and the Y-direction are perpendicular to each other and perpendicular to the Z-direction. The configurations of the x-ray spot paths are selected based on the plane in which the object being imaged lies with respect to the X, Y and Z-directions. 
   The present invention also provides a method of performing x-ray laminography. The method comprises the steps of projecting a beam of electrons along a Z-axis of the system from an electron beam source onto a nonplanar metallic anode target at particular locations on the target to produce paths of x-ray spots having selected configurations. X-rays emanate from the target at the locations of the spots on the target. 
   Deflecting the beam of electrons produced by the electron beam source with an electron beam deflection controller to cause the electron beams to be deflected in an X-direction and a Y-direction in a selected manner so that the electron beams impinge on the target at particular locations on the target to produce x-ray spot paths having selected configurations, the X-direction and the Y-direction being perpendicular to each other and perpendicular to the Z-direction; and
         selecting the configurations of the x-ray spot paths based on a plane in which an object being imaged lies with respect to the X, Y and Z-directions.       

   These and other features and advantages of the present invention will become apparent from the following description, drawings and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a known x-ray laminography system that is capable of performing off-axis scanning by steering an electron beam over a planar target anode. 
       FIG. 2  is a schematic view of the laminography system of the present invention comprising a nonplanar target anode that enables laminography to be used to precisely image oblique objects. 
       FIG. 3  is a schematic view of the nonplanar target anode of the present invention in accordance with an example embodiment. 
       FIG. 4  is a flow chart illustrating the method of the present invention in accordance with an embodiment. 
       FIG. 5  illustrates a target anode that is concave in shape. 
       FIG. 6  illustrates a target anode that is parabolic in shape. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  is a schematic view of the laminography system  100  of the present invention, which comprises a nonplanar target anode  110  that enables oblique objects to be precisely imaged. The laminography system  100  of the present invention may be, but need not be, identical in all respects to the laminography system  10  shown in  FIG. 1 , with the exception that the target anode  110  of the laminography system  100  of the present invention is nonplanar. The shape of the nonplanar target anode  110  of the present invention is not limited to any particular shape, but preferably is symmetrical. The target anode  110  may have, for example, a convex spherical shape, as shown in  FIGS. 2 and 3 , a concave shape, as shown in  FIG. 5 , a parabolic shape, as shown in  FIG. 6 , etc. Preferably, the nonplanar target anode  110  has the shape of an axially symmetric shell of revolution about the Z-axis. 
   The electrodes  101  and coils  103  and  104  produce electromagnetic fields that interact with the electron beam  102  to focus and direct the beam  102  onto the nonplanar target anode  110 , thereby forming an electron beam spot on the nonplanar target anode  110  from which x-rays are emitted, at least some of which pass through the object  111  and impinge on x-ray detector  112 . The steering coils  103  and  104  enable the x-ray source  115  to provide x-rays from the x-ray spots on the nonplanar target anode  110  such that the locations of the spots move in a desired pattern around the nonplanar target anode  110 . It is the creation of the desired x-ray spot patterns on the nonplanar target anode  110  that eliminate or reduce the need to physically move the object  111  to obtain images of different regions of the object  111  in the X, Y plane in different Z-planes, and in planes that are at oblique angles to the X, Y plane. 
   As indicated above with reference to  FIG. 1 , electrical current flowing in the steering coils  103  and  104  creates a magnetic field that interacts with the electron beam  102 , thereby causing the beam  102  to be deflected. The configuration of the x-ray spot pattern on the nonplanar target anode  110  depends on the where the beam  102  strikes the target  110 , which depends on the manner in which the beam  102  is steered. A lookup table (LUT)  120  can be used to store voltage values that are applied to the X and Y deflection coils  103  and  104  to cause the electron beam spot to rotate, thus producing a circular spot pattern on the surface of the nonplanar target anode  110 . Although a LUT is preferred, any memory device may be used for this purpose. The laminography system  100  also comprises a processor  140  of some type that is programmable to cause the stored values to be read out and applied to the deflection coils  103  and  104  in a particular order to produce circular spot patterns having preselected configurations. The values that are read out of memory will be converted into analog values by a digital-to-analog converter (not shown), and preferably amplified by an amplifier (not shown) before being applied to the deflection coils  103  and  104 . 
   As shown in  FIG. 2 , the object  111  being imaged is somewhat warped or bowed. In other words, any given cross-section of the object  111  is not entirely in the same X, Y plane. For this reason, alterations in spot patterns generated on a flat anode target, such as target  24  of the system  10  shown in  FIG. 1 , will not precisely image all regions of the object  111 . In accordance with the present invention, various circles of given radius are traced on the nonplanar anode target  110  to ensure that the spot pattern is coplanar with the object  111 . Furthermore, in addition to providing spot patterns having orientations that can be changed to ensure that the path of the spot is coplanar with the object  111 , the nonplanar anode target also provides the ability to vary the axial position of the spot path in the Z-direction, which facilitates changes in focus and magnification. 
     FIG. 3  is a diagram illustrating first and second scan paths  131  and  132  formed by tracing spots on the nonplanar target  100  that are coplanar with an orthogonal object  133  (i.e., an object in the X, Y plane) and with an oblique object  134 , respectively. The nonplanar anode target  100  in this example embodiment is a concave, semi-spherical shell that is axially symmetric about the Z-axis of the system, which is co-linear with the electron beam  102 . It can be seen that the scan path  131  is in an X, Y plane as is the orthogonal object  132 . Therefore, scan path  131  is in a plane that is parallel to the plane in which the orthogonal object  133  lies. The x-ray scan path  132  is not in an X, Y plane, but is oblique. Likewise the oblique object  134  is not in an X, Y plane. However, the plane of the scan path  132  is parallel to the plane of the oblique object  133 . Therefore, the scan path  131  will cause the orthogonal object  133  to be precisely imaged, but would not result in the oblique object  134  being precisely imaged. Likewise, the scan path  132  will cause the oblique object  134  to be precisely imaged, but would not result in the orthogonal object  133  being precisely imaged. 
   By steering the electron beam in the appropriate manner, spot paths that are parallel to the plane of the object can always be traced, which means that the object of interest can always be precisely imaged, regardless of whether it is orthogonal or oblique to some degree. 
   The method of the present invention will now be described with reference to FIG.  4 . The configurations of the spot paths to be formed on the nonplanar anode target  100  are preselected, as indicated by block  161 , based on the shape of the target (e.g., whether it is orthogonal, oblique, degree of obliqueness, etc.). The preselected spot paths will correspond to the preselected values to stored in the LUT  120 . After the spot paths to be formed on the target have been selected and the corresponding values have been stored in the LUT  120 , the object is imaged by projecting the beam of electrons  102  from the electron beam source  107  onto the target  100 , as indicated by block  162 . The beam of electrons  102  is deflected by the deflection coils  103  and  104  in the manner dictated by the values read out of the LUT  120  to cause the spot patterns to be appropriately formed on the target  100  so that the object is precisely imaged, as indicated by block  163 . 
   It should be noted that the present invention has been described only with reference to preferred embodiments for example purposes and in the interest of brevity, and that the present invention is not limited to these embodiments. Those skilled in the art will understand, in view of the present disclosure, the manner in which embodiments not disclosed herein can be developed by utilizing the principles and concepts of the present invention. These undisclosed embodiments are also within the scope of the present invention. Those skilled in the art will also understand that modifications can be made to the embodiments discussed herein and that all such modifications are within the scope of the present invention.