Abstract:
An x-ray generating device includes at least one field-emission cold cathode having a substrate and incorporating nanostructure-containing material including carbon nanotubes. The device further includes at least one anode target. Associated methods are also described.

Description:
[0001]     The present application is a Continuation of U.S. patent application Ser. No. 10/614,787 filed on Jul. 9, 2003, which is a Continuation-in-Part of co-pending U.S. patent application Ser. No. 10/309,126 filed on Dec. 4, 2002, now U.S. Pat. No. 6,850,595, which is a Continuation of U.S. patent application Ser. No. 09/679,303 filed on Oct. 6, 2000, now U.S. Pat. No. 6,553,096. U.S. patent application Ser. No. 10/614,787 is also a continuation of U.S. patent application Ser. No. 10/448,144, filed on May 30, 2003. The disclosure of each of these applications is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is directed to devices and techniques for producing a plurality of X-ray beams from multiple locations. For example, methods and devices using a field emission cathode with a plurality of individually addressable electron-emitting pixels are contemplated. Electrons emitted from the pixels can be directed towards different focal points on the anode, thus producing multiple x-ray beams from multiple locations of the same device.  
       BACKGROUND OF THE INVENTION  
       [0003]     Various constructions and techniques will be described below. However, nothing described herein should be construed as an admission of prior art. To the contrary, Applicants expressly preserve the right to demonstrate, where appropriate, that anything described herein does not qualify as prior art under the applicable statutory provisions.  
         [0004]     Conventional x-ray tubes comprise a cathode, an anode and a vacuum housing. The cathode is a negative electrode that delivers electrons towards the positive anode. The anode attracts and accelerates the electrons through the electric field applied between the anode and cathode. The anode is typically made of metals such as tungsten, molybdenum, palladium, silver and copper. When the electrons bombard the target most of their energy is converted to thermal energy. A small portion of the energy is transformed into x-ray photons radiated from the target, forming the x-ray beam. The cathode and the anode are sealed in an evacuated chamber which includes an x-ray transparent window typically composed of low atomic number elements such as Be.  
         [0005]     X-ray tubes are widely used for industrial and medical imaging and treatment applications. All x-ray imaging is based on the fact that different materials have different x-ray absorption coefficients. Conventional x-ray imaging techniques produce a 2-dimensional projection of a  3  dimensional object. In such process the special resolution along the x-ray beam direction is lost.  
         [0006]     Although also based on the variable absorption of x-rays by different materials, computed tomography (CT) imaging, also known as “CAT scanning” (Computerized Axial Tomography), provides a different form of imaging known as cross-sectional imaging. A CT imaging system produces cross-sectional images or “slices” of an object. By collecting a series of projection images of the same object from different viewing angles, a  3 -D image of the object can be reconstructed to reveal the internal structure to a certain resolution. Today CT technology is widely used for medical diagnostic testing, industrial non-destructive testing for example for inspection of semiconductor printed circuit boards (PCBs), explosive detection, and airport security scans.  
         [0007]     In the semiconductor industry, the features on printed circuit boards are becoming smaller, and circuits with multi-layer architectures are becoming more common. There is an increasing demand for machines that can perform 3-D inspection at rapid speed. The most common medical CT scanners today use one x-ray tube that rotates around the patient and in the process takes hundreds of projection images necessary for re-constructing one slice image. The x-ray tube used in the medical CT scanners has a single electron emitting cathode and a single focal spot. For industrial inspection and in particular for PCB inspection, only a small number of projection images are taken from a narrow range of viewing angles. For this special purpose, several devices have been developed to generate multiple x-ray beams from multiple focal points on the anode surface. The purpose is to produce multiple projection images with different viewing angles without mechanically moving the x-ray tube. Such devices are all based on a thermionic cathode that produces the electrons. The electrons produced from the same cathode are steered to different points of the anode by complicated electrical and magnetic devices built inside the x-ray tube. This type of device is generally illustrated in  FIG. 1 . This device  1000  includes a thermionic cathode  1002  that emits a beam of electrons e which pass through an arrangement of focus and steering coils  1004 ,  1006 , thereby directing the electron beam e onto different locations of an anode surface  1008  having multiple x-ray emitting focal points that produce x-rays  1010 .  
         [0008]     Another apparatus is described, for example, in U.S. Pat. No. 5,594,770 and includes an x-ray source having a cathode for producing a steerable electron beam. A controller directs the electron beam to predetermined locations on a target anode. The user may flexibly select appropriate predetermined positions. A detector receives x-rays that are transmitted through the test object from each of the predetermined locations, and produces images corresponding to each of the predetermined locations. The images are digitized and maybe combined to produce an image of a region of interest. Alternatively, as described in U.S. Pat. Nos. 4,926,452 and 4,809,308, an electron beam is deflected in a circular scan pattern onto the tube anode in synchronization with a rotating detector that converts the x-ray shadow-graph into an optical image which is converted and viewed on a stationary video screen. A computer system controls an automated positioning system that supports the item under inspection and moves successive areas of interest into view. In order to maintain high image quality, a computer system also controls the synchronization of the electron beam deflection and rotating optical system, making adjustments for inaccuracies of the mechanics of the system. Such a device is generally illustrated in  FIG. 2 . The illustrative device  2000  includes a thermionic electron beam source  2002  which generates an electron beam e that passes through an arrangement of focus coils  2004 ,  2006  that direct the beam onto a tube angle  2008 , thereby generating a pattern of x-rays  2010 .  
         [0009]     A third way to get x-ray beams emanating from different angles is to mechanically rotate a single beam x-ray tube/source, as schematically illustrated in  FIG. 3 .  
         [0010]     Although the above listed techniques can serve the purpose, these single electron beam based x-ray inspection have several drawbacks related to limitations in resolution, limited viewing angles, cost and efficiency. These prior devices and techniques suffer from a common drawback in that they all rely on one single source of electrons to generate x-rays and obtain multiple images of the PCBs from different angles. Thus, inherently they are slow and cannot simultaneously generate multiple images of the object under inspection from different angles. In addition, they all require mechanical motion of either the x-ray source or the x-ray detector, which will lead to inconsistency in x-ray focus spot size and imaging quality. Furthermore, these x-ray systems all rely on thermionic electron emitters which are sensitive to temperature, require long warm up time, and can not turn on/off easily, thus they can not be easily programmed and waste large amount energy and x-ray system lifetime.  
         [0011]     The concept of field-emission x-ray tubes has been investigated. In such devices a field emission cathode replaces the thermionic filament. Electron field emission can be accomplished via a simple diode mode where a bias voltage is applied between the target and the cathode. Electrons are emitted from the cathode when the electrical field exceeds the threshold field for emission. A triode construction can also be employed wherein a gate electrode is placed very close to the cathode. In such configurations, electrons are extracted by applying a bias field between gate electrode and the cathode. The field-emitted electrons are then accelerated by a high voltage between the gate and the anode. Here the electron current and energy are controlled separately.  
         [0012]     Recently discovered carbon nanotubes have larger field enhancement factors (β), thus lower threshold fields for emission are required relative to conventional emitters such as Spindt-type tips. Carbon nanotubes are stable at high currents. A stable emission current of 1 μA or greater has been observed from an individual single-walled carbon nanotube and an emission current density greater than 1 A/cm 2  from a macroscopic cathode containing such material, has been reported. Carbon nanotubes are also thermally stable and chemically inert. These properties make carbon nanotubes attractive electron field emitters for field emission x-ray devices.  
         [0013]      FIG. 4  and its inset show the typical emission current-voltage characteristics of a CNT cathode. It shows the classic Fowler-Nordheim behavior with a threshold field of 2 V/μm for 1 mA/cm 2  current density. Emission current density over 1 μA/cm 2  was readily achieved. Field emitted electrons from carbon nanotubes have a very narrow energy and spatial distribution. The energy spread is about 0.5 eV and the spatial spread angle in the direction parallel to the electrical field is 2-5° degree half angle. The potential of using carbon nanotubes as a cold-cathode has been demonstrated in devices such as the field emission flat panel displays (FEDs), lighting elements, and discharge tubes for over-voltage protection.  
         [0014]     U.S. Pat. No. ______ (Ser. No. 09/296,572 entitled “Device Comprising Carbon Nanotube Field Emitter Structure and Process for Forming Device”), the disclosure of which is incorporated herein by reference, in its entirety, discloses a carbon nanotube-based electron emitter structure.  
         [0015]     U.S. Pat. No. ______ (Ser. No. 09/351,537 entitled “Device Comprising Thin Film Carbon Nanotube Electron Field Emitter Structure”), the disclosure of which is incorporated herein by reference, in its entirety, discloses a carbon nanotube field emitter structure having a high emitted current density.  
         [0016]     U.S. Pat. No. 6,553,096 entitled “X-Ray Generating Mechanism Using Electron Field Emission Cathode”, the disclosure of which is incorporated herein by reference, in its entirety, discloses an x-ray generating device incorporating a cathode formed at least in part with a nanostructure-containing material.  
         [0017]     U.S. patent application Publication No. US-2002/0094064, entitled “Large-Area Individually Addressable Multi-Beam X-Ray System and Method of Forming Same”, the disclosure of which is incorporated herein by reference, in its entirety, discloses structures and techniques for generating x-rays which includes a plurality of stationary and individually electrically addressable field emissive electron sources.  
         [0018]     U.S. Pat. No. ______ (Ser. No. 10/358,160 entitled “Method and Apparatus for Controlling Electron Beam Current”), the disclosure of which is incorporated herein by reference, in its entirety, discloses an x-ray generating device which allows independent control of the electron emission current by piezoelectric, thermal, or optical means.  
         [0019]     U.S. patent application Publication No. US-2002/0140336, entitled “Coated Electrode with Enhanced Electron Emission and Ignition Characteristics”, the disclosure of which is incorporated herein by reference, in its entirety, discloses a coated electrode construction which incorporates nanostructure-containing materials.  
         [0020]     U.S. Pat. No. ______ (Ser. No. ______ Attorney Docket No. 033627-003, entitled “Nano-Material Based Electron Field Emission Cathodes for Vacuum and Gaseous Electronics”), the disclosure of which is incorporated herein by reference, in its entirety, discloses electronics incorporating field emission cathodes based at least in part on nanostructure-containing materials.  
         [0021]     U.S. Pat. No. 6,385,292 entitled “Solid State CT System and Method”, the disclosure of which is incorporated herein by reference, in its entirety, disclose an x-ray source including a cathode formed from a plurality of addressable elements.  
         [0022]     U.S. patent application Publication No. US-2002/0085674 entitled “Radiography Device With Flat Panel X-Ray Source”, the disclosure of which is incorporated herein by reference, in its entirety, discloses a radiography system having a solid state x-ray source that includes a substrate with a cathode disposed thereon within a vacuum chamber.  
         [0023]     U.S. Pat. No. 6,385,292 entitled “X-Ray Generator”, the disclosure of which is incorporated herein by reference, in its entirety, discloses an x-ray generator which includes a cold field-emission cathode. The emissive current of the cathode can be controlled by various means.  
         [0024]     Thus, it is highly desirable to have an x-ray imaging system which can generate multiple beams of x-ray simultaneously from different positions and radiation angles. Utilizing nanostructure-containing field emissive cathodes, the present invention provides methods and apparatus for making such multi-beam x-ray imaging systems, and techniques for their use.  
       SUMMARY OF THE INVENTION  
       [0025]     According to the present invention, devices and techniques are provided that are more efficient in producing multi-beam x-rays, provide more flexible controllability and are equipped with highly integrated multiple functions. According to the present invention, an x-ray source that can provide x-ray beams shooting to the scanned objects from different angles is provided.  
         [0026]     Apparatus for making non-destructive x-ray measurements are also provided. The apparatus includes single or multiple field emission cold cathodes. The electrons generated from the nanostructure-containing cold cathodes will be accelerated to certain desired sites in the target anode therefore to generate x-rays beam from different angles respective to the scanned object. Detectors will be used to collect the x-rays transmitted through the scanned objects to form images from different angles. The images can be used to reconstruct a 2-D or 3-D images revealing the internal structure of the object.  
         [0027]     According to the present invention, a field emission cathode which comprises nanostructure materials is used in the x-ray tubes as electron source for generating x-rays in this invention. This new x-ray generation mechanism provides many advantages over the conventional thermionic based x-ray source in the sense of eliminating the heating element, operating at room temperature, generating pulsed x-ray radiation in a high repetition rate and making multi-beam x-ray source and portable x-ray devices possible.  
         [0028]     According to a first aspect, the present invention provides an x-ray generating device for scanning an object under inspection comprising: at least one addressable field emission cathode, the cathode comprising a substrate and a nanostructure-containing material comprising carbon nanotubes; and at least one anode target; wherein the device lacks a heater for the cathode. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]      FIG. 1  is a schematic illustration of a known configuration and technique for manipulating an electron beam to form plurality of x-rays.  
         [0030]      FIG. 2  is a schematic illustration of another known technique and construction for manipulation of an electron beam to produce a plurality of x-rays.  
         [0031]      FIG. 3  is yet another schematic illustration of a known arrangement and technique for scanning an object with x-rays provided at multiple angles relative thereto.  
         [0032]      FIG. 4  is a plot of current versus voltage behavior for a carbon-nanotube-based cathode.  
         [0033]      FIG. 5  is a schematic illustration of an x-ray source with multiple stationary electron sources formed according to the principles of the present invention.  
         [0034]      FIG. 6  is a bottom view of the configuration illustrated in  FIG. 5 .  
         [0035]      FIG. 7  is a bottom view of an alternative embodiment for producing x-rays with multiple electron sources, formed according to another aspect of the present invention.  
         [0036]      FIG. 8  is a bottom view schematically illustrating yet another alternative arrangement of multiple electron emission sources according to yet another aspect of the present invention.  
         [0037]      FIG. 9  is also a bottom or planar view of a further alternative embodiment formed according to the principles of the present invention.  
         [0038]      FIG. 10  is a schematic illustration of electron emission source, or pixel, provided with a multilayer gated construction formed according to the principles of the present invention.  
         [0039]      FIG. 11  is a schematic illustration of an alternative arrangement and technique including a rotating gate structure formed according to the principles of the present invention.  
         [0040]      FIG. 12  is a schematic illustration of a gate electrode construction formed according to the present invention.  
         [0041]      FIG. 13  is a schematic illustration of an inspection arrangement or system incorporating an x-ray source according to the present invention.  
         [0042]      FIG. 14  is a schematic illustration of a further arrangement for providing multi-beam x-rays based on laminography, formed according to the principles of the present invention.  
         [0043]      FIG. 15  is a schematic illustration of an x-ray collimator device which may be utilized with various constructions and techniques performed according to the principles of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0044]     Exemplary arrangements and techniques according to the present invention will now be described by reference to the drawing figures.  
         [0045]     According to one embodiment of the invention, as illustrated in  FIG. 5 , an x-ray source comprises a field emission cathode  12  with multiple individually-addressable electron-emitting elements or “pixels”  11 . The cathode  12  has a planar geometry as shown in  FIG. 6 . The anode  13  is opposing and is separated from the cathode  12  by a finite gap distance within a vacuum chamber  14 . Electron emission from the pixels  11  on the cathode can be controlled by a gate electrode. Details of possible gate electrode constructions and arrangements that can be utilized in this embodiment, and others, are described in later portions of the disclosure. The x-ray source may comprise a single gate electrode or more preferably a gate electrode with a plurality of individually addressable units, each unit controls a corresponding pixel  11  on the cathode  12 . Electrons are extracted from an emission pixel  11  when the applied an electrical field between the said pixel  11  and its corresponding controlling unit on the gate electrode exceeds a threshold value. A high voltage is applied between the cathode  12  and anode  13 . When an individual pixel  11  is turned on, the emitted electron beam is accelerated by the high tension electrical field to gain enough kinetic energy and bombard a corresponding point on the anode  13 . The anode  13  could be made of any suitable material such as copper, tungsten, molybdenum, or an alloy of different metals. X-ray is produced from the anode at the point the electrons impinge, or a so-called “focal spot.” 
         [0046]     The anode  13  comprises a plurality of discrete focal spots  10  wherein each focal spot comprises a different material with a different atomic number or a different alloy; wherein each focal spot  10  produces x ray with a different energy distribution when bombarded with the emitted electrons.  
         [0047]     In the illustrated embodiment, the x-ray focal points  10  on the anode  13  have a one-to-one relationship with the electron emitting pixels  11  on the cathode  12 . So when a pixel  11  is turned on, an x-ray beam is generated from the corresponding spot on the anode  13 . Therefore by turning on the pixels  11  at different positions will generate x-ray beams from different focal points  10  on the anode  13 . As a result, for imaging purpose, x-ray beams from different viewing angles are realized without physical motion of the x-ray generating device. The pixels at different positions can be programmed and controlled by computer to be turned on in a sequence, in certain frequency, duty cycle, and dwell time.  
         [0048]     The cathode  12  can have a plurality of emission pixels  11  arranged in any pre-determined pattern. In one particular embodiment, the emission pixels  11  are arranged along the circumference of a circle with a finite diameter as illustrated in  FIG. 6 . The electrons emitted from each pixel  11  can be directed towards a corresponding focal spot  10  on the anode  13 , wherein the focal spots  10  on the anode  13  are positioned along the circumference of a circle, wherein each focal spot  10  corresponds to a field emission pixel  11  on the cathode.  
         [0049]     A cathode constructed according to the principles of the present invention preferably incorporates a field-emissive material. More preferably, a cathode formed according to the principles of the present invention incorporates a nanostructure-containing material. The term “nanostructure” material is used by those familiar with the art to designate materials including nanoparticles such as C 60  fullerenes, fullerene-type concentric graphitic particles, metal, compound semiconductors such as CdSe, InP, nanowires/nanorods such as Si, Ge, SiO x , Ge, O x , or nanotubes composed of either single or multiple elements such as carbon, B x N y , C x , B y , N z , MoS 2 , and WS 2 . One of the common features of nanostructure materials is their basic building blocks. A single nanoparticle or a carbon nanotube has a dimension that is less than 500 nm in at least one direction. The term “nanostructure-containing” is intended to encompass materials which are composed entirely, or almost entirely of nanostructure materials, as well as materials composed of both nanostructures as well as other types of materials, thereby forming a composite construction. A cathode formed according to the principles of the present invention can be formed entirely of the above-described nanostructure-containing materials. Alternatively, the cathode may comprise a substrate or base material, which is then provided with the one or more coating layers which include the above-described nanostructure-containing materials. The nanostructure-containing material coating may be applied directly to the cathode substrate material surface. Alternatively, an intervening adhesion-promoting layer may also be provided. According to an illustrative, embodiment, the cathode formed according to the principles of the present invention is formed, at least in part, from a high-purity material comprising single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes or mixtures thereof.  
         [0050]     In some applications, high x-ray flux is needed and the focal spot size is not important, in such cases, a pixel with a bigger emission area which can produce higher current is desired. One can prepare the pixels with different sized emission areas  110 ,  111  as shown in  FIG. 7 . In this way, a multifunctional x-ray source can be achieved. The emission area of each field emission pixel  110 ,  111  varies according to a predetermined pattern, wherein under the same applied electrical field the total emission current from each pixel is commensurate with the emission area of the pixel, wherein a scanning x-ray beam with programmable intensity from each focal spot is achieved by applying the electrical field with the same amplitude to each pixel. As shown in  FIG. 7 , the emission areas of field emission pixel set  111  and field emission pixel set  110  are different. In the event that a high x-ray intensity is desired, with the applied electrical field remaining unchanged, field emission pixel set  110  is used.  
         [0051]     According to alternative constructions, as illustrated in  FIGS. 8 and 9 , a plurality of field emission pixels  11  on the cathode  12  are arranged into a predetermined pattern, and are programmed into groups of emission units wherein each emission unit comprises a sub-set  31 ,  32  and  33  of emission pixels with different diameters b, c and d ( FIG. 8 ), or form clusters  41 ,  42  ( FIG. 9 ), wherein electrons emitted from each emission unit are directed towards corresponding focal spots on the anode. The focal spots on the anode can be positioned according to the same pattern as the emission units on the cathode.  
         [0052]     To focus the electron beam extracted from each pixel  11 , multi-layer electrical gates or coils  11   g  separated by insulator layers  11   s  can be built on top of each pixel  11  in the path of the electron beam “e” as shown in  FIG. 10 . When appropriate voltage is applied on these gates or current pass through the coils, the electron beam can be focused or steered to certain degree.  
         [0053]     An alternative technique and arrangement formed according to the principles of the present invention is illustrated in  FIG. 11 .  
         [0054]     In this embodiment the cathode  55  has a planar geometry and comprises an electron emissive material disposed on either the entire planar surface, or on parts thereof. A gate electrode  52  is placed parallel to and separate from the cathode  55  with a finite gap. An anode  53  is opposing and is separated from the cathode  55  by a finite gap distance and are both enveloped by vacuum chamber  54 . The gate electrode  52  contains one or a plurality of openings which can have mesh grids  51  disposed therein, wherein the positions of the mesh grids  51  with respect to the cathode  55  can be arranged such that the a specific area or areas on the cathode can be selected as the emission pixel or pixels to produce field emitted electrons that are directed towards a specific location or locations on the anode  53 . Electrons are extracted from an emission pixel when the applied an electrical field between the pixel and its corresponding controlling unit on the gate electrode  52  exceeds a threshold value. A high voltage is applied between the gate electrode and the anode. When an individual pixel is turned on, the electron beam is accelerated by the high tension to gain enough kinetic energy and bombard a corresponding point on the anode  53 . The anode  53  could be made of any suitable material such as copper, tungsten, molybdenum, or an alloy of different metals. X-ray is produced from the anode at the point the electrons impinge (referring to as “focal point” thereafter).  
         [0055]     The mesh grids  51  can be made of a material with high melting temperature such as tungsten, molybdenum or nickel etc. The size of the openings in the mesh influences the amount of emitted electron current passing therethrough. Thus, the layer the size of the mesh openings the more emitted electron passing through and impinging the anode, and visa versa Preferably, a plurality of mesh grids  51  are utilized. Each of the grids can be provided with the same mesh opening size. Alternatively, the mesh grids can be provided with different sized openings.  
         [0056]     The mesh grids  51  can be in the form of independently addressable units. For example, each grid can be electrically addressed, or opened and closed, independently from the others.  
         [0057]     The gate electrode  52  can rotate around the axis  56  at various speeds controlled by a motor unit. When the applied an electrical field between the said emission area(s) and its corresponding controlling unit on the gate electrode  52  exceeds a threshold value, electrons are extracted from emission area(s). During the rotation of the gate  52  at certain speed, the emission current can be generated from anywhere in the emission ring of the cathode. A scanning x-ray beam is generated from the corresponding spots  50  on the anode  53  in a continuous or pulsed mode depending on whether a continuous or pulsed electrical potential is applied between the selected mesh grid  51  and the cathode  55 . As a result, for imaging purposes, x-ray beams from different viewing angles are realized. The rotation speed and the voltage pulsation applied on the electrode can be programmed and controlled by computer to be turned on in a sequence, in certain frequency, duty cycle, and/or dwell time.  
         [0058]     The emitted-electron current of the device can be controlled by choosing mesh grids with different mesh opening sizes, the rotation speed of the gate electrode, and/or the frequency and dwell time of the pulsation applied on the mesh grids.  
         [0059]     To control the electron beam extracted from each pixel, a gate construction can be used, such as the one illustrated in  FIG. 12 . One or more gates  55   g  may be provided which is separated by at least one insulating spacer  55   s.  A grid  51  may be incorporated into the gate  55   g  to selectively regulate the flow of emitted electrons therethrough.  
         [0060]     An exemplary embodiment of an x-ray inspection arrangement or system is illustrated in  FIG. 13 . The arrangement includes an x-ray source  151  constructed according to any of the previously-described embodiments. X-rays generated by the x-ray source  151  are directed onto the object under inspection  152 , which can be located on a movable stage  153 . When utilized, the stage  153  is preferably translatable in the x, y and z directions, and/or rotatable about a given axis.  
         [0061]     An x-ray detector  74  is provided which may include an array of individual detectors  731 ,  732  at different locations. X-rays passing through the object  152  are received by the detector  74 . Preferably, a controller is provided that can be utilized to control the movement of stage  153 , and thereby position the object  152 , as well as control operation and/or location of the detector(s)  74 ,  731 ,  732 . An image analysis device may also be incorporated to receive, manipulate and/or output data from the detector  74 .  
         [0062]     In another embodiment of the invention, an ultra-fast all stationary x-ray imaging and inspection technique and system is constructed utilizing the field emission multi-beam x-ray source. One version of this system is illustrated in  FIG. 14 . An object  72  to be inspected, e.g.—a circuit board  70 , is placed between an x-ray source  14  and an x-ray detector  74 . The x-ray source  14  is preferably the field emission multi-beam x-ray source disclosed herein. The x-ray detector  74  can be either an array of detectors  731 ,  732  placed at different locations on the same plane, or an area detector with a matrix of pixels. To collect the data, the x-ray source is turned on. All the electron emitting pixels on the cathode are turned on at the same time. Each pixel produces an electron beam that bombards on a corresponding focal spot  101 ,  102  on the anode  13  of the x-ray source. The x-ray generated from each focal spot on the anode  13  produces one image of the object from different angles which is recorded by a corresponding detector. For example, the x-ray beam generated from focal spot  101  produces one image of the object that is recorded by detector  732 . The x-ray beam generated from focal point  102  produces one image of the object that is recorded by detector  731 . In the case where a large area detector is used,  731  and  732  are specific regions of the area detector.  
         [0063]     Since the different focal spots are located at different points of the anode, images of the object produced by the x-ray beams originated from the different focal spots have different projection angles relative to the object being imaged. Structures obscured from one projection angle can be revealed by the x-ray beam coming from a different focal spot and thus different viewing angle. By turning on all the electron-emitting pixels on the cathode, x-ray beams are generated from all the different focal spots at the same time, and therefore the different projection images of the same object can be collected at the same time. Optionally, all the projection images are displayed on a monitor. Further, the imaging and inspection system may comprise a computer and software to reconstruct an image which reveals the internal structure of the object under examination using the different projection images collected. Since all the projection images are collected at the same time, the system enables instantaneous reconstruction and display of an image which reveals the internal structure of the object. This is advantageous compared to other inspection systems where the different projection images have to be collected one at a time. The capability of the present invention can significantly increase the rate by which objects can be imaged.  
         [0064]     According to an alternative embodiment, the x-ray beam from each pixel  101 ,  102  will produce an x-ray image of the plane  70  in the object  72  on the corresponding x-ray detector. The image plane  70  is the intersection area of the x-ray beams from each pixel  101 ,  102  of the x-ray source  14 . During the operation, each of the pixels  101 ,  102  will be turned on to provide an x-ray beam from different directions respective to the scanned object. Thus, the x-ray images of the object from different angles will be recorded by the corresponding x-ray detectors. This information will be further used to reconstruct a 2-D or 3-D image. During the reconstruction of the collected image data, structure in the object  72  which is outside of the scanned plan  70  will produce a blurred image on the detectors  731 ,  732  while the structure on the scanned plane  70  will form a sharp image. A different plane can be selected for examination by changing the location at which the x-ray beams intersect within the object  72 . This can be accomplished by moving the object  72  relative to the x-ray source  14 , or changing the angle at which the x-rays are incident upon the object  72  by moving the pixels  101 ,  102 .  
         [0065]     In one particular mode of operation of this system, all the pixels can be turned on at the same time. The detector array will be arranged and programmed in such a way that different regions of the detector array  731 ,  732  will only collect x-ray signals from one corresponding pixel  101 ,  102  of the x-ray source  14 . For example, region  732  of the detector array will only collect the x-rays from the particular pixel  101  and region  731  will only collect the x-rays from the pixel  102 . When all the pixels are programmed to be turned on at once, the detectors will collect all of the x-ray images of the scan plane simultaneously, so an x-ray image can be obtained instantly. This imaging geometry is shown in  FIG. 14 .  
         [0066]     According to another embodiment of this invention, the x-ray source  14  is turned on to collect data. All the electron emitting pixels on the cathode are turned on in a programmable sequence, therefore one or multiple pixels, but not all pixels, are turned on at one time. Each pixel produces an electron beam that bombards on a corresponding focal spot  101 ,  102  on the anode  13  of the x-ray source  14 . The x-ray generated from each focal spot on the anode produces one image of the object from different angles which is recorded by a corresponding detector. The x-ray detector  74  can be constructed and operate as described above. For example, when the x-ray beam is generated from focal spot  101 , the image of the object is recorded by detector  732 , when the x-ray beam is generated from focal point  102 , the image of the object therefore is recorded by detector  731 . Detector  731  and detector  732  could be different detectors, different regions of a detector array, or they could be the same detector which is positioned at different places. Since the different focal spots are located at different points of the anode  13 , images of the object produced by the x-ray beams originated from the different focal spots have different projection angles. Structures obscured from one projection angle can be revealed by the x-ray beam coming from a different focal spot and thus different viewing angle. By turning on different electron-emitting pixels on the cathode, x-ray beams are generated from all the different focal spots and therefore different projection images of the same object can be collected.  
         [0067]     According to an alternative, the system may further comprise a collimator  82  or a group of collimators, as shown in  FIG. 15 , to define the spread angle of the x-ray fan beam  81  with certain spread angle from each focal spot  80 . The collimator(s)  82  are designed such that the x-ray beam from each focal spot on the anode illuminates only the area to be imaged, and such that the x-ray photons originated from a focal spot reaches only the corresponding detector.  
         [0068]     While the present invention has been described by reference to the above-mentioned embodiments, certain modifications and variations will be evident to those of ordinary skill in the art. Therefore, the present invention is limited only by the scope and spirit of the appended claims.