Patent Publication Number: US-2022230833-A1

Title: Target Features to Increase X-Ray Flux

Description:
CLAIM OF PRIORITY 
     This application claims priority to US Provisional Patent Application Numbers U.S. 63/139,403, filed on Jan. 20, 2021, and U.S. 63/231,917, filed on Aug. 11, 2021, which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present application is related generally to x-ray sources. 
     BACKGROUND 
     An x-ray tube can make x-rays by sending electrons, in an electron-beam, across a voltage differential, to a target. X-rays can form as the electrons hit the target. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS (DRAWINGS MIGHT NOT BE DRAWN TO SCALE) 
         FIG. 1 a    is a cross-sectional side-view of a transmission-target x-ray tube  10   a  including a cathode  11  configured to emit electrons in an electron beam to a target  14 . X-rays  17  can emit out of the x-ray tube  10  through the target  14  and an adjacent x-ray window  13 . 
         FIG. 1 b    is a cross-sectional side-view of a transmission-target x-ray tube  10   b , similar to transmission-target x-ray tube  10   a . Transmission-target x-ray tube  10   b  has a differently shaped anode  12  and electrically-insulative structure  15 . 
         FIG. 2  is a cross-sectional side-view of a reflective-target, side-window x-ray tube  20 . A cathode  11  can emit electrons in an electron beam to a target  14 . X-rays  17  can transmit through an interior of the x-ray tube  20 , and out of the x-ray tube  20  through an x-ray window  13 . 
         FIG. 3  is an expanded cross-sectional side-view of a target  14  with an array of holes  33 , preferably for transmission-target x-ray tubes  10   a  and  10   b.    
         FIG. 4  is an expanded cross-sectional side-view of a target  14  with an array of posts  43 , preferably for transmission-target x-ray tubes  10   a  and  10   b.    
         FIG. 5  is an expanded cross-sectional side-view of a target  14  with an array of holes  33 , preferably for a reflective-target, side-window x-ray tube  20 . 
         FIG. 6  is an expanded cross-sectional side-view of a target  14  with an array of posts  43 , preferably for a reflective-target, side-window x-ray tube  20 . 
         FIG. 7  is an expanded cross-sectional side-view of a hole  33  in a target  14  with bumps  73  on sidewalls  33   s  of the hole  33 . 
         FIG. 8  is an expanded cross-sectional side-view of a hole  33  in a target  14 . A diameter D h , of the hole  33  decreases moving deeper into the hole  33 . 
         FIG. 9  is an expanded cross-sectional side-view of a hole  33  in a target  14 . A diameter D h , of the hole  33  increases moving deeper into the hole  33 . 
         FIG. 10  is an expanded cross-sectional side-view of a target  14  with a top-layer  14   t  closest to the cathode  11 , a bottom-layer  14   b  farther from the cathode  11 , and a hole  33  extending through the top-layer  14   t.    
         FIG. 11 a    is an expanded cross-sectional side-view of a target  14 , similar to the target of  FIG. 10 , except that a diameter D h  of the hole  33  in  FIG. 11 a    increases linearly moving deeper into the hole  33 , closer to the bottom-layer  14   b.    
         FIG. 11 b    is an expanded cross-sectional side-view of a target  14 , similar to the target of  FIG. 10 , except that a diameter D h , of the hole  33  in  FIG. 11 b    increases in a step, moving deeper into the hole  33 , closer to the bottom-layer  14   b.    
         FIG. 12  is an expanded cross-sectional side-view of a target  14 , similar to the targets of  FIGS. 10 and 11   a - b , except that the target  14  of  FIG. 12  has gap G between the top-layer  14   t  and the bottom-layer  14   b.    
         FIG. 13  is a top-view of a target  14  with a grid array of holes  33  with aligned columns  131  and rows  132 . 
         FIG. 14  is a top-view of a target  14  with an array of holes  33 . Each hole  33  has a hexagonal shape. The array of holes  33  combine to form repeating hexagonal shapes  141  and  142 . 
         FIG. 15  is a top-view of a target  14  with an array of holes  33 . Each hole  33  has a circular shape. The array of holes  33  combine to form repeating hexagonal shapes  141 . 
         FIG. 16  is a top-view of a target  14  with an array of posts  43 . 
         FIG. 17  is a perspective-view of a target  14  with alternating wires  44  and channels in an elongated, parallel array. The wires  44  are posts  43  and the channels are holes  33 . 
         FIG. 18  is a top-view of a target  14  with alternating wires  44  and channels in a zig-zag pattern. The wires  44  are posts  43  and the channels are holes  33 . 
         FIG. 19  is a cross-sectional side-view of a target  14  with a bottom-layer  14   b  that is a continuous film, and an array of wires  44  on the bottom-layer  14   b.    
         FIG. 20  is a cross-sectional side-view of a target  14  with a bottom-layer  14   b  that is a continuous film, and posts  43   A ,  43   B , and  43   C  on the bottom-layer  14   b . The target  14  has multiple thicknesses T B , T PA , T PB , and T PC . 
         FIG. 21  is a perspective-view of a step  210  in a method of making a target  14  for an x-ray tube, including patterning and etching a first array of channels  211  in a target material, or patterning and sputtering an array of wires  44  of target material, in a first direction D 1 . 
         FIG. 22  is a top-view of a step  220  in a method of making a target  14  for an x-ray tube, including patterning and etching a second array of channels  221  in the target material, or patterning and sputtering an array of wires  224  of target material, in a second direction D 2 . The second direction D 2  is different from the first direction D 1 . This step  220  forms an array of posts  43  extending from the bottom-layer  14   b.    
         FIG. 23  is a side-view of a method  230  of making a target  14  for an x-ray tube, including using a laser  231  or  232  to form holes  33  in the target  14 , posts  43  on the target, or both. 
     
    
    
     Definitions. The following definitions, including plurals of the same, apply throughout this patent application. 
     As used herein, the face  14   e  of the target  14  is a face or side of the target  14  that faces the electron beam, and into which the holes  33  penetrate or from which the posts  43  protrude. 
     As used herein, the terms “on”, “located on”, “located at”, and “located over” mean located directly on or located over with some other solid material between. The terms “located directly on”, “adjoin”, “adjoins”, and “adjoining” mean direct and immediate contact. 
     As used herein, the term “parallel” means exactly parallel, or within 10° of exactly parallel. The term “parallel” can mean within 0.1°, within 1°, or within 5° of exactly parallel if explicitly so stated in the claims. 
     As used herein, the term “unparallel” means the lines or surfaces intersect at an angle greater than 10°. 
     As used herein, the term “perpendicular” means exactly perpendicular, or within 10° of exactly perpendicular. The term “perpendicular” can mean within 0.1°, within 1°, or within 5° of exactly perpendicular if explicitly so stated in the claims. 
     As used herein, terms like “same”, “equal”, and “identical” mean (a) exactly the same, equal, or identical; (b) the same, equal, or identical within normal manufacturing tolerances; or (c) nearly the same, equal, or identical such that any deviation from exactly the same, equal, or identical would have negligible effect for ordinary use of the device. 
     Shapes described herein can have (a) the exactly described shape (e.g. circular, hexagonal, etc.); (b) the described shape within normal manufacturing tolerances; or (c) nearly the exactly described shape, such that any deviation from the exactly described shape would have negligible effect for ordinary use of the device. 
     As used herein, the term “x-ray tube” is not limited to tubular/cylindrical shaped devices. The term “tube” is used because this is the standard term used for x-ray emitting devices. 
     As used herein, the term “nm” means nanometer(s), the term “μm” means micrometer(s), and the term “mm” means millimeter(s). 
     DETAILED DESCRIPTION 
     An x-ray tube can make x-rays by sending electrons, in an electron-beam, across a voltage differential, to a target. X-rays can form as the electrons hit the target. Some electrons rebound without interacting atomically to form x-rays. Thus, x-ray flux is reduced. 
     The rebounded electrons can charge electrically-insulative components of the x-ray tube, which may result in deflection of the electron beam, and increased chance of electrical breakdown of the x-ray tube. 
     The invention reduces electron rebound to the electrically-insulative components of the x-ray tube. The invention can increase x-ray flux, decrease electron beam deflection, and decrease x-ray tube electrical breakdown failure. 
     As illustrated in  FIGS. 1 a   - 2 , x-ray tubes  10   a ,  10   b , and  20  include a cathode  11  and an anode  12  electrically insulated from one another. For example, an electrically-insulative structure  15  can separate and insulate the cathode  11  from the anode  12 . Example materials for the electrically-insulative structure  15  include glass and ceramic. The electrically-insulative structure  15  can be a cylinder, as illustrated in  FIGS. 1 a    and  2 . 
     The cathode  11  can be configured to emit electrons (e.g. from an electron emitter  11   EE , such as a filament) in an electron beam to a target  14  at the anode  12 . The target  14  can be configured to emit x-rays  17  out of the x-ray tube  10   a ,  10   b , and  20  in response to impinging electrons from the cathode  11 . The target  14  can include high melting point material(s) for generation of the x-rays, such as rhodium, tungsten, or gold. 
     Transmission-target x-ray tubes  10   a  and  10   b  are illustrated in  FIGS. 1 a -1 b   . The target  14  can be attached to the x-ray window  13 . The target  14  can adjoin the x-ray window  13 . X-rays  17  generated in the target  14  can transmit through the target  14  and the x-ray window  13 , and out of the x-ray tube  10   a  or  10   b.    
     A reflective-target, side-window x-ray tube  20  is illustrated in  FIG. 2 . The x-ray window  13  can be spaced apart from the target  14 . A region of an evacuated interior of the x-ray tube can be between the x-ray window  13  and the target  14 . X-rays  17  generated in the target  14  can transmit through an internal vacuum of the x-ray tube  20  to the x-ray window  13 , and out of the x-ray tube  20 . 
     The invention is applicable to both transmission-target x-ray tubes  10   a  and  10   b  and to reflective-target, side-window x-ray tubes  20 . The invention can increase electron interactions with the target  14 . 
     Holes  33  in the target  14 , posts  43  on the target  14 , or both can increase electron interaction with material of the target  14 . Rebounding electrons can hit a sidewall or a bottom of the hole  33 , or hit a post  43 , instead of hitting and charging the electrically-insulative structure  15 . There is a chance of forming an x-ray  17  each time a rebounded electron hits the target  14 . Thus, by adding holes  33 /posts  43  to the target  14 , x-ray flux can increase for a given electron beam. Alternatively, the power of the electron beam can be reduced while achieving the same x-ray flux. Reducing the electron beam power can increase x-ray tube life and reduce power requirements. 
     Holes  33 , posts  43 , or both can also allow the target  14  to effectively generate x-rays  17  of different energies by providing a target  14  with multiple thicknesses. When the x-ray tube  10   a ,  10   b , or  20  is operated at a larger voltage, x-rays  17  can be generated in thicker regions Th 1  of the target  14 . When the x-ray tube  10   a ,  10   b , or  20  is operated at a smaller voltage, x-rays  17  can be generated in thinner regions Th 2  of the target  14 . 
     As illustrated in  FIGS. 3 and 5 , the target  14  can include an array of holes  33 . The target  14  can encircle each hole  33 , at a face  14   r  of the target  14  and along an entire depth of the hole  33 . A bottom  33   b  and a sidewall  33   s  of the holes  33  can have an identical material composition. The sidewall  33   s  of the holes  33  can have an identical material composition along an entire depth of the hole  33 . All holes  33  can be identical with respect to each other. Sidewalls  33   s  of all the holes  33  can have an identical material with respect to each other. The bottom  33   b  of all the holes  33  can have an identical material with respect to each other. 
     A longitudinal-axis  31  for each of the holes  33  can be parallel to a longitudinal axis  16  of the x-ray tube, parallel to the electron beam, or both. The longitudinal axis  16  of the x-ray tube can extend between the cathode  11  and the target  14 . This parallel arrangement can increase electron capture, which can increase x-ray flux. 
     The target  14  in  FIG. 3  is preferred for a transmission-target x-ray tubes  10   a  and  10   b . The longitudinal-axis  31  for holes  33  of target  14  in  FIG. 3  can be perpendicular to a plane  32  of a face  14   f  of the target  14 . 
     The target  14  in  FIG. 5  is preferred for a reflective-target, side-window x-ray tube  20 . The longitudinal-axis  31  for holes  33  of target  14  in  FIG. 5  can be non-perpendicular to a plane  32  of a face  14   f  of the target  14 . For example, 100°≤A h , 110°≤A h , or 120°≤A h ; and A h ≤120°, A h ≤130°, or A h ≤140°; where A h  is an angle between the longitudinal-axis  31  of the holes  33  and the plane  32 . 
     In the target  14  of  FIG. 3  or of  FIG. 5 , a depth d h  of the holes  33  can be the same with respect to each other. This can simplify design and manufacturing. Alternatively, hole depth d h  and longitudinal axis  31  of the holes  33  can be adjusted according to the angle of incidence for electrons at the specific location of the target  14 . Hole depth d h  is measured at a center of the hole  33 . 
     Minimum hole diameter D h1 , as measured at a face  14   f  of the target  14 , can be selected for increased capture of electrons, and increased x-ray flux. For example, 10 nm≤D h1 , 100 nm≤D h1 , or 1 μm≤D h1 ; and D h1 ≤1 μm, D h1 ≤10 μm, D h1 ≤20 μm, D h1 ≤50 μm, or D h1 ≤100 μm. 
     Proper selection of aspect ratio AR h  of the holes  33  can increase capture of electrons. The equation for aspect ratio is AR h =d h /D h1  (d h  and D h1  are defined above). 
     A relatively higher aspect ratio AR h  is preferred for transmission-target x-ray tubes  10   a  and  10   b , because generated x-rays  17  must pass through the target  14  anyway. Thus, there is no concern of generating these x-rays  17  deep in the target  14 . Example aspect ratios AR h  for transmission-target x-ray tubes  10   a  and  10   b  include 0.5≤AR h , 1≤AR h , or 5≤AR h ; and AR h ≤5, AR h ≤10, or AR h ≤20. 
     In contrast, a relatively lower aspect ratio AR h  is preferred for a reflective-target, side-window x-ray tube  20  because x-rays  17  generated deep in the target  14  must pass through the target  14  back into the evacuated enclosure of the x-ray tube  20 . X-rays  17  thus generated deep in the target  14  can be unduly attenuated. Example aspect ratios AR h  for a reflective-target, side-window x-ray tube  20  include 0.1≤AR h , 0.5≤AR h , or 1≤AR h ; and AR h ≤1, AR h ≤3, or AR h , ≤6. 
     Optimal selection of minimum distance S h  between adjacent holes  33  can increase capture of electrons. If the minimum distance S h  is too small, then electrons can pass through the sidewall of one hole  33  and into another hole  33  without generation of an x-ray  17 . Alternatively, if the minimum distance S h  is too large, then there are fewer holes  33  for capture of electrons. Example ranges for the minimum distance S h  between adjacent holes  33  include 50 nm≤S h , 300 nm≤S h , or 1 μm≤S h ; and S h ≤1 μm, S h ≤10 μm, S h ≤20 μm, or S h , ≤50 μm. S h  is measured at a face  14   f  of the target  14 . 
     As illustrated in  FIGS. 4 and 6 , the target  14  can include an array of posts  43  on a bottom-layer  14   b . The bottom-layer  14   b  can be a continuous film. The posts  43  and the bottom-layer  14   b  can have an identical material composition. Alternatively, the posts  43  and the bottom-layer  14   b  can be made of different materials. Adjacent posts  43  can be separated from each other (not touching) from a proximal-end  43   p  at the bottom-layer  14   b  to a distal-end  43   d  farthest from the bottom-layer  14   b.    
     Each post  43  can have an identical material composition along an entire height h p  of the post  43 . All posts  43  can have an identical material composition with respect to each other. All posts  43  can be identical with respect to each other. 
     In the targets  14  of  FIG. 4  or  FIG. 6 , a longitudinal-axis  41  for each of the posts  43  can be parallel to the electron beam, parallel to a longitudinal axis  16  of the x-ray tube, or both. This parallel arrangement can increase electron capture and electron rebound, which can increase x-ray flux. 
     The target  14  in  FIG. 4  is preferred for transmission-target x-ray tubes  10   a  and  10   b . The longitudinal-axis  41  for the posts  43  can be perpendicular to a plane  42  of a face  14   f  of the target  14 . 
     The target  14  in  FIG. 6  is preferred for a reflective-target, side-window x-ray tube  20 . The longitudinal-axis  41  can be non-perpendicular to a plane  42  of a face  14   1  of the target  14 . For example, 100°≤A p , 110°≤A p , or 120°≤A p ; and A p ≤120°, A p ≤130°, or A p ≤135°; where A p  is an angle between the longitudinal-axis  31  for the posts  43  and the plane  32 . 
     In the targets  14  of  FIG. 4  or  FIG. 6 , a height h p  of the posts  43  can be the same with respect to each other. This can simplify design and manufacturing. Alternatively, post height h p  can be adjusted according to the angle of incidence for electrons at the specific location of the target  14 . Post height h p  is measured at a center of the post  43 . 
     Minimum post diameter D p1 , measured perpendicular to the longitudinal-axis  41 , can be selected for increased capture of electrons, and increased x-ray flux. If the minimum post diameter D p1  varies along the height h p  of the post  43 , then the minimum post diameter D p1  is defined as the smallest diameter at a midpoint on the post  43  between the proximal-end  43   p  and the distal-end  43   d . If the minimum post diameter D p1  is too small, then electrons can pass through the post  43  without generation of an x-ray  17 . Alternatively, if the minimum post diameter D p1  is too large, then there are fewer posts  43  for capture of electrons. Example minimum post diameters D p1  include 10 nm≤D p1 , 100 nm≤D p1 , or 1 μm≤D p1 ; and D p1 ≤1 μm, D p1 ≤10 μm, or D p1 ≤100 μm. 
     Proper selection of aspect ratio AR p  of the posts  43  can increase capture of electrons. The equation for aspect ratio is AR p =h p /D p1  (h p  and D p1  are defined above). 
     A higher aspect ratio AR p  is preferred for transmission-target x-ray tubes  10   a  and  10   b , because generated x-rays  17  must pass through the target  14  anyway. Thus, there is no concern of generating these x-rays closer to the proximal-end  43   p  of the post  43 . Example aspect ratios AR p  for a transmission-target x-ray tube  10  include 0.5≤AR p , 1≤AR p , or 5≤AR p ; and AR p ≤5, AR p ≤10, or AR p ≤20. 
     In contrast, a relatively lower aspect ratio AR p  is preferred for a reflective-target, side-window x-ray tube  20  because x-rays  17  generated deep in the target  14  must pass through the target  14  back into the evacuated enclosure of the x-ray tube  20 . X-rays  17  thus generated deep in the target  14  can be unduly attenuated. Example aspect ratios AR p  for a reflective-target, side-window x-ray tube  20  include 0.1≤AR p , 0.5≤AR p , or 1≤AR p ; and AR p ≤1, AR p ≤3, or AR p ≤6. 
     Proper selection of minimum distance S p  between adjacent posts  43  can increase capture of electrons. The minimum distance S p  between any two adjacent posts  43  is the closest straight-line path between these posts  43 , measured at the distal-end  43   d . 
     If the minimum distance S p  is too small, then too many electrons won&#39;t enter gaps between posts. Alternatively, if the minimum distance S p  is too large, then too many electrons will hit the bottom-layer  14   b  and reflect away from the target  14 . Example ranges for the minimum distance S p  between adjacent posts  43  include 50 nm≤S p , 300 nm≤S p , or 1 μm≤S p ; and S p ≤1 μm, S p ≤10 μm, or S p ≤50 μm. S p  is measured at a face  14   f  of the target  14 . 
     As illustrated in  FIGS. 7-9 and 11   a , an average direction of sidewalls  33   s  of the holes  33  can be unparallel with respect to the electron beam, unparallel with respect to the longitudinal axis  16  of the x-ray tube, or both. The direction of the electron beam is based on an average direction of electrons travelling from the electron emitter  11   EE  to the target  14 . The hole  33  shapes of  FIGS. 7-9 and 11   a - b  are applicable to both transmission-target x-ray tubes  10   a  and  10   b  and to reflective-target, side-window x-ray tubes  20 . The hole  33  shapes of  FIGS. 7-9 and 11   a - b  can be combined with the other details of the target  14  in  FIGS. 3-6 and 12-16 . 
     As illustrated in  FIG. 7 , bumps  73  on the sidewall  33 , can cause a direction of the sidewalls  33   s  of the holes  33  to be unparallel with respect to the longitudinal axis  16  of the x-ray tube. This direction can change, and a majority of this direction can be unparallel with respect to the electron beam, unparallel with respect to the longitudinal axis  16  of the x-ray tube, or both. The bumps  73  can increase x-ray production by reflecting electrons that hit a base of the hole  33 , back to the target  14 . It is preferable for the bumps  73  to be angled to reflect electrons to the bottom  33   b  or other sidewalls  33   s , in order to increase electron interaction with the target  14 . See for example the path  76  followed by an example electron. 
     The bumps  73  can cover a large percent of a surface of the sidewalls  33   s , in order to increase electron interaction with the target  14 . For example, ≥25%, ≥50%, ≥80%, ≥90%, or ≥99% of a surface of the sidewalls  33   s  can be covered by the bumps  73 . 
     The bumps  73  can be ribs  75  with channels  74  between the ribs  75 . The ribs  75  can encircle the longitudinal-axis  31  along sidewalls  33   s  of each hole  33  and can extend into each hole  33 . The ribs  75  can be pointed ridges. Each concave channel  74  can encircle the longitudinal-axis  31  along sidewalls  33   s  of each hole  33 . The ribs  75  can be relatively easy to make and can increase electron interaction with the target  14  by encircling each hole  33 . 
     Example numbers of ribs  75  in each hole  33  include≥3 ribs, ≥5 ribs, ≥10 ribs, or ≥25 ribs. Example widths W r  of the ribs (parallel to the longitudinal-axis  31 ) include 10 nm≤W r , 50 nm≤W r , or 200 nm≤W r ; and W r ≤300 nm, W r ≤1500 nm, or W r ≤6000 nm. Example thicknesses Th r  of the ribs (perpendicular to the longitudinal-axis  31 , into the hole) include 5 nm≤Th r , 15 nm≤Th r , or 45 nm≤Th r ; and Th r ≤150 nm, Th r ≤500 nm, or Th r ≤1500 nm. 
     The bumps  73  and ribs  75  are applicable to both transmission-target x-ray tubes  10   a  and  10   b  and to reflective-target, side-window x-ray tubes  20 , and can be combined with other target  14  features described herein. 
     The bumps  73  can be formed by alternating isotropic and anisotropic etching (e.g. ≥2, ≥4, or ≥8 of each type of etch). The isotropic etching can form wider regions of the holes  33  (e.g. between ribs  75 ) and the anisotropic etching can form narrower regions of the holes  33  (e.g. where the ribs  75  protruded into the hole  33 ). Deep reactive-ion etching milling can also form the holes  33  with the bumps  73 . 
     As illustrated in  FIGS. 8-9 , a narrowing or widening of the holes  33  can cause an average direction of the sidewalls  33   s  of the holes  33 , or a majority direction of the sidewalls  33   s  of the holes  33 , to be unparallel with respect to the electron beam, unparallel with respect to the longitudinal axis  16  of the x-ray tube, or both. The narrowing or widening of the holes  33  in  FIGS. 8-9  are applicable to both transmission-target x-ray tubes  10   a  and  10   b  and to reflective-target, side-window x-ray tubes  20 , and can be combined with other target  14  features described herein. 
     In  FIG. 8 , the holes  33  decrease in diameter D h  moving deeper into the holes  33 . Thus, a minimum diameter D h1  of the hole  33  measured at a face  14   f  of the target  14  can be greater than a minimum diameter D h3  of the hole  33  measured at a bottom  33   b  of the hole  33 . Example relationships between these diameters include D h1 /D h3 ≥1.25, D h1 /D h3 ≥1.5, D h1 /D h3 ≥2, and D h1 /D h3 ≥10. 
     A linear decrease in diameter D h  is shown in  FIG. 8 , but this change in diameter D h  can be a step (opposite of  FIG. 11 b   ). This decrease in diameter D h , moving deeper into the holes  33 , can be formed by a laser or by etching. This shape has the disadvantage that electrons entering the hole  33  can more easily reflect back towards the cathode or the electrically-insulative structure  15 . This shape has the advantage that the holes  33  can be placed closer together (decreased S h ). 
     In  FIG. 9 , the holes  33  increase in diameter D h  moving deeper into the holes  33 . A linear increase in diameter D h  is shown in  FIG. 9 , but this change in diameter can be a step, as illustrated in  FIG. 11 b   . Thus, a minimum diameter D h1  of the hole  33  measured at a face  14   f  of the target  14  can be smaller than a minimum diameter D h3  of the hole  33  measured at a bottom  33   h  of the hole  33 . Example relationships between these diameters include D h3 /D h1 ≥1.1, D h3 /D h1 ≥1.25, D h3 /D h1 ≥1.5, and D h3 /D h1 ≥2. 
     This shape can be formed by isotropic etching. This shape has the disadvantage that the holes  33  may need to be placed farther apart (increased S h ). This shape has the advantage that electrons entering the hole  33  can more easily reflect back towards a bottom  33   b  of the hole  33  or sidewalls of the hole  33 . 
     Each hole  33  can have a conical shape ( FIG. 8 ) or a conical frustum shape ( FIGS. 9 and 11 ). 
     As illustrated in  FIGS. 10-12 , the target  14  can include a top-layer  14   t  closest to the cathode  11  and a bottom-layer  14   b  farther from the cathode  11 . The top-layer  14   t  and the bottom-layer  14   b  are applicable to both transmission-target x-ray tubes  10   a  and  10   b  and to reflective-target, side-window x-ray tubes  20 , and to other target features described herein. 
     The array of holes  33  can be in the top-layer  14   t . Each hole  33  can extend through the top-layer  14   t  to expose the bottom-layer  14   b . A side of the bottom-layer  14   b  facing the top-layer  14   t  can be free of holes  33 . Boring holes  33  completely through the top-layer  14   t , then attaching the top-layer  14   t  to the bottom-layer  14   b , can improve consistency in manufacturing hole depth d h . 
     The top-layer  14   t  can have a different material composition from the bottom-layer  14   b . The top-layer  14   t  can have≥75, ≥85, or ≥95 weight percent of one chemical element and the bottom-layer  14   b  can have ≥75, ≥85, or ≥95 weight percent of another chemical element. Example chemical elements for the top-layer  14   t  and the bottom-layer  14   b  include transition metals, lanthanoids, some specific refractory metals (such as Zr, Mo, W, Hf, Ta, Re, Os, Ir), precious metals (such as Au, Pt, Pd, Rh, and Ag), and other metals (such as Ti, Cr, Fe, Co, Ni, and Cu). An atomic number of a majority element (by atomic weight) in the top-layer  14   t  can be greater than an atomic number of a majority element (by atomic weight) in the bottom-layer  14   b.    
     As illustrated in  FIG. 11 a   , the holes  33  through the top-layer  14   t  can have conical frustum shape. These can be formed by laser cutting from the wider diameter side, then placing this wider diameter side adjacent to the bottom-layer  14   b.    
     As illustrated in  FIG. 11 b   , the holes  33  through the top-layer  14   t  can have widening diameter D h , moving deeper into the hole. The widening diameter D h  can be abrupt, like a step. These can be formed by laser cutting (a) across the wider diameter with limited time to avoid cutting all the way through, and (b) cutting the center all the way through. The wider diameter side can be placed next to the bottom-layer  14   b.    
     As illustrated in  FIG. 12 , the top-layer  14   t  and the bottom-layer  14   b  can be spaced apart, with a gap G between them. The gap G can be filled with vacuum, gas, or both. Benefits of the gap G include (a) avoiding damage to the target  14  caused by differences in the coefficient of thermal expansion between the top-layer  14   t  and the bottom-layer  14   b , (b) avoiding trapped gas between the top-layer  14   t  and the bottom-layer  14   b , (c) increased rate for forming a vacuum in the x-ray tube, and (d) increased capture of electrons that pass all the way through the holes  33 . 
       FIGS. 13-15  are top-views of the array of holes  33  in the target  14 . The hole  33  arrangements and shapes of  FIGS. 13-15  are applicable to both transmission-target x-ray tubes  10   a  and  10   b  and to reflective-target, side-window x-ray tubes  20 . Any of the hole  33  cross-sectional shapes of  FIGS. 7-9  may be combined with the hole  33  arrangements and shapes of  FIGS. 13-15 . Any of the layered targets of  FIGS. 10-12  may be combined with the hole  33  arrangements and shapes of  FIGS. 13-15 . 
     Example numbers of holes  33  in the target  14  include ≥5, ≥25, ≥75, or ≥150. By proper selection of the number of holes  33  and minimum hole diameter D h1 , a large percent of the electron beam can enter the holes  33 . For example, ≥25%, ≥50%, ≥75%, or ≥90% of the electron beam can enter the holes  33 . 
     As illustrated in  FIG. 13 , the rows  132  and columns  131  can be aligned in a grid array. A disadvantage of the example in  FIG. 13  is variable distance between adjacent holes  33  and reduced packing of holes  33 . 
     As illustrated in  FIGS. 14-15 , the holes  33  and the adjacent rows of the array of holes can be offset with respect to each other for more consistent and/or reduced spacing between adjacent holes  33 . This can allow more holes  33  to be packed into the target  14 , and thus capture more electrons. This offset can be described by (a) a line  152  across each row, through a center of holes  33  in that row, can cross holes  33  of every other column; (b) an X shape  151  can be formed by each group of five holes  33 , with one of the five holes  33  at a center of the X shape  151 ; (c) the array of holes  33  can form repeating hexagonal shapes  141  and  142 ; or (d) combinations thereof. Hexagonal shape  141  includes nineteen holes. Hexagonal shape  142  includes seven holes. 
     As illustrated in  FIG. 14 , each hole  33  can have a hexagonal shape at a face  14   f  of the target  14 . The hexagonal shape can further provide more consistent wall thickness between adjacent holes  33 ; but hexagonal-shaped holes  33  can be more difficult to manufacture. The hexagonal shaped hole  33  can apply to other target  14  examples herein. 
     The holes  33  can have other shapes, including triangle, square, rectangle, circular, or elliptical at a face  14   f  of the target  14 . The target  14  of  FIG. 13  has an elliptical hole  33   e  with a minimum diameter D h1  and a maximum diameter D h2 , both measured at a face  14   f  of the target  14 . Example relationships between these diameters include 1.05≤D h2 /D h1 , 2≤D h2 /D h1 , 10≤D h2 /D h1 , D h2 /D h1 ≤1.1, D h2 /D h1 ≤2, D h2 /D h1 ≤5, D h2 /D h1 ≤10, D h2 /D h1 ≤100. 
       FIG. 16  is a top-view of the array of posts  43  on the target  14 . Example numbers of posts  43  on the target  14  include ≥5, ≥10, ≥25, ≥75, or ≥150. All posts  43  can be identical with respect to each other. Rows and columns of posts  43  can be aligned in a grid array, similar to the holes  33  of  FIG. 13 . Alternatively, as illustrated in  FIG. 16 , the posts  43  can be offset with respect to each other for more consistent and/or minimized average distance between adjacent posts  43 . This can allow more posts  43  to be packed into the target  14 , and thus capture of more electrons. This offset can be described by (a) a line  152  across each row can cross posts  43  of every other column; (b) an X shape  151  can be formed by each group of five posts  43 , with one of the five posts  43  at a center of the X shape  151 ; (c) the array of posts  43  can form repeating hexagonal shapes  142 ; or (d) combinations thereof. 
     The posts  43  can have a hexagonal shape at its proximal end  43   p , at its distal end  43   d , or both, similar to the shape of the holes  33  in  FIG. 14 . One post  43   h  with a hexagonal shape is illustrated in  FIG. 16 . The hexagonal shape can provide a consistent distance between adjacent posts  43  and closer packing of posts  43 ; but hexagonal-shaped posts  43  can be more difficult to manufacture. 
     The posts  43  can have other shapes, including triangle, square, rectangle, or elliptical. The target  14  of  FIG. 16  has an elliptical post  43   e  with a minimum diameter D p1  and a maximum diameter D p2 , both measured perpendicular to the longitudinal-axis  41  at a midpoint between the proximal-end  43   p  and the distal-end  43   d . Example relationships between these diameters include 1.05≤D p2 /D p1 , 2≤D p2 /D p1 , 10≤D p2 /D p1 , D p2 /D p1  1.1, D p2 /D p1 ≤2, D p2 /D p1 ≤5, D p2 /D p1 ≤10, D p2 /D p1 ≤100. 
     Illustrated in  FIG. 17  is a perspective-view of a target  14  with an array of holes  33  and an array of posts  43  as alternating ribs and channels.  FIG. 18  is a top-view of a target  14  with an array of holes  33  and an array of posts  43  as alternating ribs  44  and channels  33  in a zig-zag pattern. The zig-zag can improve capture of electrons, but can be more complicated to manufacture than the straight channels and ribs of  FIG. 17 . 
     Illustrated in  FIGS. 19-20  are targets  14  for x-ray tubes with posts  43  arising out of a bottom-layer  14   b . The bottom-layer  14   b  can be a continuous film in a single plane  191 . There can be holes  33  between adjacent posts  43 . 
     In the target  14  of  FIG. 19 , the holes  33  can be channels and the posts  43  can be an array of wires  44 . The wires  44  can be separated from each other from a proximal-end  44   p  at the bottom-layer  14   b  to a distal-end  44   D  farthest from the bottom-layer  14   b . The array of wires  44  can be parallel and elongated. 
     In the target  14  of  FIG. 20 , the posts  43   A ,  43   B , and  43   C  have three different thicknesses T PA , T PB , and T PC . The bottom-layer  14   b  has a thickness T B  at a bottom of the holes  33 . Thus, the target  14  of  FIG. 20  has four different thicknesses T PA , T PB , T PC , and T B . Each thickness can be measured perpendicular to the single plane  191 . 
     The targets  14  of  FIGS. 19 and 20 , and associated description below, are designed to produce x-rays of different energies. The x-ray tube with these targets  14  can operate at a high voltage (e.g. 55 kV) and produce x-rays primarily in thicker posts  43  ( FIG. 19 ) or  43   c  ( FIG. 20 ). The x-ray tube with these targets  14  can operate at a low voltage (e.g. 10 kV) and produce x-rays primarily in the bottom-layer  14   b  between posts  43 . The x-ray tube with the target  14  of  FIG. 20  can operate at intermediate voltages, such as 25 kV or 40 kV, and produce x-rays primarily in intermediate-sized posts  43   B  and  43   C  respectively. 
     A relationship of a pitch P between adjacent wires ( FIG. 19 ) can be selected relative to a width W beam  of the electron beam, for increased production of x-rays. For example, 1.5≤W beam /P, 2≤W beam /P, or 4≤W beam /P; and W beam /P≤6, W beam /P≤12, W beam /P≤20, W beam /P ≤100, or W beam /P≤250. The width W beam  includes 90% of the electron beam at the target  14 . A higher value for W beam /P has the benefit of less variation in x-ray flux as the electron beam shifts. But, it is more difficult to make a target  14  with higher W beam /P. In  FIG. 19 , W beam /P=3.8. 
     An area A P  of the bottom-layer  14   b  covered by the posts  43  can be selected for better x-ray production. Fewer low-energy x-rays are typically produced, because flux is proportional to voltage, and low-energy x-rays are produced at a lower voltage. Therefore, in order to increase production of low-energy x-rays, it is useful for the area A B  of the bottom-layer  14   b  not covered by posts  43  to be greater than the area A P  of the bottom-layer  14   b  with posts  43 . For example, 1≤A B /A P , 3≤A B /A P , 6≤A B /A p , or 9≤A B /A p ; and A B /A p ≤9, A B /A P ≤15, or A B /A P ≤30. In  FIG. 19 , A B /A P =1.5. Areas A p  and A B  are measured parallel to the single plane  191 . 
     The target  14  can include multiple layers of different material, such as for example two or three layers of different material. Each layer can be perpendicular to the single plane  191 . The most expensive of these layers can be the bottom-layer  14   b , which isn&#39;t etched. For example, the bottom-layer  14   b  can be ≥75 weight percent or ≥95 weight percent rhodium. The posts  43  can be ≥75 weight percent or ≥95 weight percent silver or tungsten. Each layer can be optimized for a different voltage range. Each subsequent layer can be sputter deposited on top of lower layer(s). 
     A thickness T P  of the posts  43  and a thickness T B  of the bottom-layer  14   b  can be selected to improve x-ray generation at both low and high x-ray tube voltages, and to increase x-ray production from sidewalls of the posts  43 . For example, 2≤T P /T B , 3≤T P /T B , 6≤T P /T B , or 9≤T P /T B ; and T P /T B ≤11, T P /T B ≤15, T P /T B ≤25, or T P /T B ≤50. Each thickness T P  and T B  can be measured perpendicular to the single plane  191 . 
     This thickness ratio T P /T B  can be related to the voltage that each thickness T P  and T B  is designed for. For example, T P /T B  can be greater than kV B /kV P , where kV P  is a voltage that the thickness T P  of the posts  43  are optimized for, and kV B  is a voltage that the thickness T B  of the bottom-layer  14   b  is optimized for. 
     A method of making a target  14  for an x-ray tube can include step  210  ( FIG. 21 ), patterning and etching a first array of channels  211  in a target material in a first direction D 1 , forming an array of wires  44  extending from a bottom-layer  14   b . Adjacent wires  44  can be separated from each other by a channel  211 . 
     The method can further comprise step  220  ( FIG. 22 ), patterning and etching a second array of channels  221 , or patterning and sputtering an array of wires  224  of target material, in a second direction D 2 . The second direction D 2  can be different from the first direction D 1 . The second direction D 2  can be perpendicular to the first direction D 1 . This step  220  can form an array of posts  43  extending from the bottom-layer  14   b . There can be additional patterning and etching step(s) in different directions, to form additional posts  43  of additional thicknesses. 
     The etching of steps  210  and  220  can be different depths with respect to each other, resulting in posts  43   A ,  43   B , and  43   C  that have three different thicknesses T PA , T PB , and T PC , as illustrated in  FIG. 20 . Alternatively, the etching of steps  210  and  220  can be the same depth with respect to each other, resulting in posts  43   A ,  43   B , and  43   C  that have two different thicknesses T PA =T PB , and T PC . 
     Another method of making a target  14  for an x-ray tube with step  210  ( FIG. 21 ) can include patterning and sputtering an array of wires  44  on a bottom-layer  14   b . Adjacent wires  44  can be separated from each other by a channel  211 . 
     The wires  44  and the bottom-layer  14   b  can be a target material. Target material of the bottom-layer  14   b  can be different from, or the same as, target material of the wires  44 . 
     A first array of wires  44  of target material can be patterned and sputtered on the bottom-layer  14   b  in a first direction D 1 , then a second array of wires  244  can be patterned and sputtered in a second direction D 2 . The second direction D 2  can be different from the first direction D 1 . 
     The patterning and sputtering of steps  210  and  220  can be different thicknesses with respect to each other, resulting in posts  43   A ,  43   B , and  43   C  that have three different thicknesses T PA , T PB , and T PC , as illustrated in  FIG. 20 . Alternatively, the patterning and sputtering of steps  210  and  220  can be the same thickness with respect to each other, resulting in posts  43   A ,  43   B , and  43   C  that have two different thicknesses T PA =T PB  and T PC . 
     A method of making a target  14  for an x-ray tube  10   a ,  10   b , or  20  can comprise using a laser  231  or  232  to form holes  33  in the target  14 , posts  43  on the target, or both. The laser  231  or  232  can be a high power laser, so that material of the holes  33  is removed by ablation. Ablation is preferred over melting because melting can change or damage the grain structure of remaining target material. This change or damage can be avoided by a high power laser  231  or  232  that uses picosecond pulses, femtosecond pulses, or both to form the holes  33  or posts  43  by ablation. A large portion of material of the holes  33  can be removed by ablation, such as for example ≥25%, ≥50%, ≥75%, or ≥90%. The laser  232  can be tilted at an oblique angle, with respect to the target  14 , to form the holes  33  of  FIG. 5  or the posts  43  of  FIG. 6 . 
     Another method of making the target  14  for the x-ray tube  10   a ,  10   b , or  20  can comprise isotropic etching, anisotropic etching, or alternating isotropic and anisotropic etching. Other methods include deep reactive-ion etching and focused ion beam milling.