Patent Application
20150092924
The embodiments of the invention disclosed herein relate to specially designed targets for electron beams that can be used in high-brightness sources of x-rays. Such high brightness sources may be useful for a variety of applications in which x-rays are employed, including manufacturing inspection, metrology, crystallography, structure analysis and medical imaging and diagnostic systems.
The initial discovery of x-rays by Röntgen in 1895 [W. C. Röntgen, “Eine Neue Art von Strahlen (Würzburg Verlag, 1895); “On a New Kind of Rays,” Nature, Vol. 53, pp. 274-276 (Jan. 23 1896)] occurred by accident when Röntgen was experimenting with electron bombardment of targets in vacuum tubes. These high energy, short wavelength photons are now routinely used for medical applications and diagnostic evaluations, as well as for security screening, industrial inspection, quality control and failure analysis, and for scientific applications such as crystallography, tomography, x-ray fluorescence analysis and the like.
The laboratory x-ray source was later improved by Coolidge in the early 20th century [see, for example, William D. Coolidge, U.S. Pat. No. 1,211,092, issued Jan. 2, 1917, U.S. Pat. No. 1,917,099, issued Jul. 4, 1933, and U.S. Pat. No. 1,946,312, issued Feb. 6, 1934], and, later in the 20th century, systems generating very intense beams of x-rays using synchrotrons or free electron lasers (FELs) have been developed. These synchrotron or FEL systems, however, are physically very large systems, requiring large buildings and acres of land for their implementation. For compact, practical lab-based systems and instruments, most x-ray sources today still use the fundamental mechanism of the Coolidge tube.
An example of the simplest x-ray source, a transmission x-ray source 08, is illustrated in
Inside the vacuum tube 02, an emitter 11 connected through the lead 21 to the high voltage source 10 serves as a cathode and generates a beam of electrons 111, often by running a current through a filament. The target 01 is electrically connected to the opposite high voltage lead 22 to be at low voltage, thus serving as an anode. The emitted electrons 111 accelerate towards the target 01 and collide with it at high energy, with the energy of the electrons determined by the magnitude of the accelerating voltage. The collision of the electrons 111 into the solid target 01 induces several effects, including the emission of x-rays 888, some of which exit the vacuum tube 02 through a window 04 designed to transmit x-rays. In the configuration shown in
Another example of a common x-ray source design is the reflection x-ray source 80, is illustrated in
Inside the tube 20, an emitter 11 connected through the lead 21 to the high voltage source 10 serves as a cathode and generates a beam of electrons 111, often by running a current through a filament. A target 100 supported by a target substrate 110 is electrically connected to the opposite high voltage lead 22 and target support 32 to be at low voltage, thus serving as an anode. The electrons 111 accelerate towards the target 100 and collide with it at high energy, with the energy of the electrons determined by the magnitude of the accelerating voltage. The collision of the electrons 111 into the target 100 induces several effects, including the emission of x-rays, some of which exit the vacuum tube 20 and are transmitted through a window 40 that is transparent to x-rays.
In an alternative prior art embodiment for a reflective x-ray source (not shown in
When the electrons collide with a target 100, they can interact in several ways. These are illustrated in
An equation commonly used to estimate the penetration depth for electrons into a material is Pott's Law [P. J. Potts, Electron Probe Microanalysis, Ch. 10 of A Handbook of Silicate Rock Analysis, Springer Netherlands, 1987, p. 336)], which states that the penetration depth x in microns is related to the 10% of the value of the electron energy E0 in keV raised to the 3/2 power, divided by the density of the material:
For less dense material, such as a diamond substrate, the penetration depth is much larger than for a material with greater density, such as most elements used for x-ray generation.
There are several energy transfer mechanisms that can occur. Throughout the interaction volume 200, electron energy may simply be converted into heat. Some absorbed energy may excite the generation of secondary electrons, typically detected from a region 221 located near the surface, while some electrons may also be backscattered, which, due to their higher energy, can be detected from a somewhat larger region 231.
Throughout the interaction volume 200, including in the regions 221 and 231 near the surface and extending approximately 3 times deeper into the target 100, x-rays 888 are generated and radiated outward in all directions. The x-ray emission can have a complex energy spectrum. As the electrons penetrate the material, they decelerate and lose energy, and therefore different parts of the interaction volume 200 produce x-rays with different properties. A typical x-ray radiation spectrum for emission from the collision of 100 keV electrons with a tungsten target is illustrated in
As shown in
These continuum x-rays 388 are generated throughout the interaction volume, shown in
As was shown in
When the electron energy is larger than the binding energy of an inner-shell (core-shell) electron of an element within the target, ejection of the electron (ionization) from the shell may occur, creating a vacancy. Electrons from less strongly bound outer shell(s) are then free to transition to the vacant inner shell, filling the vacancy. As the filling electron moves down to the lower energy level, the excess energy is emitted in the form of an x-ray photon. This is known as “characteristic” radiation because the energy of the photon is characteristic of the chemical element that generates the photon.
In the example shown in
Because these discrete emission lines depend on the atomic structure of the target material, the emission is generally called “characteristic lines”, since they are a characteristic of the particular material. The sharp lines 988 in the example of an x-ray emission spectrum shown in
Returning to
For some applications, broad-spectrum x-rays may be appropriate. For other applications, a monochromatic source may be desired or even necessary for the sensitivity or resolution required. In general, the composition of the target material is selected to provide x-ray spectra with ideal characteristics for a specific application, such as strong characteristic lines at particular wavelengths of interest, or bremsstrahlung radiation over a desired bandwidth.
Control of the x-ray emission properties of a source may be governed by the selection of an electron energy (typically changed by varying the accelerating voltage), x-ray target material selection, and by the geometry of x-ray collection from the target.
Although the x-rays may be emitted isotropically, as was illustrated in
The brightness can be increased by adjusting the geometric factors to maximize the collected x-rays. As illustrated in
In principle, it may appear that a source mounted at θ=0° would have all sources apparently overlapping, accumulating the emitted x-rays, and therefore would have the largest possible brightness. In practice, emission at 0° occurs parallel to the surface of a solid metal target for conventional sources, and since the x-rays must propagate along a long length of the target material before emission, most of the produced x-rays will be attenuated (reabsorbed) by the target material, reducing brightness. In practice, a source with take-off angle of around 6° to 15° (depending on the source configuration, target material, and electron energy) will often provide the greatest practical brightness, concentrating the apparent size of the source while reducing re-absorption within the target material and is therefore commonly used in commercial x-ray sources.
The effective source area is the projected area viewed along the direction along which x-ray are collected for use, i.e. along the axis of the x-ray beam. Because of the limited electron penetration depth, the effective source area for an incident electron beam with a size comparable or larger than the electron penetration depth is dependent on the angle between the axis of the x-ray beam and the surface of the target, referred to as the “take-off angle”. When the electron beam size is much larger than the electron penetration depth, the effective source area decreases with decreasing take-off angle. This effect has been used to increase x-ray source brightness. However, with an extensive flat target, there is a limit to this benefit, due to the increasing absorption of x-rays from their production points inside the target as they propagate to the surface, which increases with a smaller take-off angle. Typically, a compromise between improved brightness from a lower angle and reduced brightness from reabsorption is reached around a take-off angle of ˜6 degrees.
Another way to increase the brightness of the x-ray source for bremsstrahlung radiation is to use a target material with a higher atomic number Z, as efficiency of x-ray production for bremsstrahlung radiation scales with increasingly higher atomic number materials. Furthermore, the x-ray emitting material should ideally have good thermal properties, such as a high melting point and high thermal conductivity, in order to allow higher electron power loading on the source to increase x-ray production. For these reasons, targets are often fabricated using tungsten, with an atomic number Z=74. Table I lists several materials that are commonly used for x-ray targets, several additional potential target materials (notably useful for specific characteristic lines of interest), and some materials that may be used as substrates for target materials. Melting points, and thermal and electrical conductivities are presented for values near 300° K (27° C.). Most values are cited
from the CRC Handbook of Chemistry and Physics, 90th ed. [CRC Press, Boca Raton, Fla., 2009]. Other values are cited from various sources found on the Internet. Note that, for some materials, such as sapphire for example, thermal conductivities an order of magnitude larger may be possible when cooled to temperatures below that of liquid nitrogen (77° K) [see, for example, Section 2.1.5, Thermal Properties, of E. R. Dobrovinskaya et al., Sapphire: Material, Manufacturing, Applications, Springer Science+Business Media, LLC (2009)]
Other ways to increase the brightness of the x-ray source are: increasing the electron current density, either by increasing the overall current or by focusing the electron beam to a smaller spot using, for example, electron optics; or by increasing the electron energy by increasing the accelerating voltage (which increases x-ray production per unit electron energy deposited in the target, and may excite more emission in the characteristic lines as well).
However, these improvements have a limit, in that all can increase the amount of heat generated in the interaction volume. The problem is exacerbated by having the target in a vacuum, so no air cooling from the surface by convection may occur. If too much heat is generated within the target, the target material may undergo phase changes, even as far as melting or evaporating. Because the vast majority of the energy deposited into the target by an electron beam becomes heat, thermal management techniques are an important tool for building better x-ray sources.
One prior art technology that has been developed to address this problem is the rotating anode system, illustrated in
The surface of the target anode 500 is shown in more detail in
Another approach to mitigating heat is to use a target with a thin layer of target x-ray material deposited onto a substrate with high heat conduction. Because the interaction volume is thin, for electrons with energies up to 100 keV the target material itself need not be thicker than a few microns, and can be deposited onto a substrate, such as diamond, sapphire or graphite that conducts the heat away quickly. However, as noted in Table I, diamond is a very poor electrical conductor, so the design of any anode fabricated on a diamond substrate must still provide an electrical connection between the target material of the anode and the positive terminal of the high voltage. [Diamond mounted anodes for x-ray sources have been described by, for example, K. Upadhya et al. U.S. Pat. No. 4,972,449; B. Spitsyn et. al. U.S. Pat. No. 5,148,462; and M. Fryda et al., U.S. Pat. No. 6,850,598].
The substrate may also comprise channels for a coolant, for example liquids such as water or ethylene glycol, or a gas such as hydrogen or helium, that remove heat from the substrate [see, for example, Paul E. Larson, U.S. Pat. No. 5,602,899] Water-cooled anodes are used for a variety of x-ray sources, including rotating anode x-ray sources.
The substrate may in turn be mounted to a heat sink comprising copper or some other material chosen for its thermally conducting properties. The heat sink may also comprise channels for a coolant, to transport the heat away [See, for example, Edward J. Morton, U.S. Pat. No. 8,094,784]. In some cases, thermoelectric coolers or cryogenic systems have been used to provide further cooling to an x-ray target mounted onto a heat sink, again, all with the goal of achieving higher x-ray brightness without melting or damaging the target material through excessive heating.
Another approach to mitigating heat for microfocus sources is to use a target created by a jet of liquid metal. Electrons bombard a conducting jet of liquid gallium (Z=31), and because the heated gallium flows away from the electron irradiation volume with the jet, higher current densities are possible. [See, for example, M. Otendal, et al., “A 9 keV electron-impact liquid-gallium-jet x-ray source”, Rev. Sci. Instrum., vol. 79, 016102, (2008)].
Although effective in certain circumstances, there is still room for improvement in these sources. Jets of liquid metal require an elaborate plumbing system and consumables, are limited in the materials (and thus values of Z and their associated spectra) that may be used, and are difficult to scale to larger output powers. In the case of thin film targets of uniform solid material coated onto diamond substrates, there is still a limitation in the amount of heat that can be tolerated before damage to the film may occur, even if used in a rotating anode configuration. Conduction of heat only occurs through the bottom of the film. In a lateral dimension, the same conduction problem exists as exists in the bulk material.
There is therefore a need for a practical method for fabricating a target for use in an x-ray source that may be used to achieve higher x-ray brightness through the use of a higher electron current density. Once fabricated, such a target can be used as a component of brighter x-ray source, enabling x-ray based tools that offer better signal to noise ratios in imaging and other applications.
This disclosure presents novel configurations for x-ray targets for use in generating x-rays from electron beam bombardment. The x-ray target configurations comprise a number of microstructures of a selected x-ray generating material fabricated in close thermal contact with (such as embedded in) a substrate with high thermal conductivity, such that the heat is more efficiently drawn out of the x-ray generating material. This in turn allows irradiation of the x-ray generating material with higher electron density or higher energy electrons, which leads to greater x-ray brightness.
The microstructures may comprise any number of conventional x-ray target materials (such as copper (Cu), and molybdenum (Mo) and tungsten (W)) that are patterned as features of micron scale dimensions on (or embedded in) a thermally conducting substrate, such as diamond or sapphire. In some embodiments, the microstructures may alternatively comprise unconventional x-ray target materials, such as tin (Sn), sulfur (S), titanium (Ti), antimony (Sb), etc. that have thus far been limited in their use due to poor thermal properties. The microstructures may take any number of geometric shapes, such as cubes, rectangular blocks, regular prisms, right rectangular prisms, trapezoidal prisms, spheres, ovoids, barrel shaped objects, cylinders, triangular prisms, pyramids, tetrahedra, or other particularly designed shapes, including those with surface textures or structures that enhance surface area, to best generate x-rays of high brightness and that also efficiently disperse heat.
In some embodiments, the target may additionally comprise an electrically conducting overcoat or internal layer to give the electrons a path to complete an electrical circuit, for example, serving as an anode relative to a high voltage source of electrons.
In some embodiments, the target comprising microstructures may be incorporated into a rotating anode geometry, to enhance x-ray generation in such systems.
In some embodiments, the target comprising microstructures may be structured such that, with certain incident electron beam orientations, increased x-ray transmission at near-zero take-off angles will be observed.
In some embodiments, the target comprising microstructures may be designed to improve thermal dissipation.
In this application, embodiments of novel designs for targets for generating x-rays using electron beams are disclosed, along with their method of manufacture. A target according to the invention comprises a number of regions fabricated from an x-ray generating material arranged in close thermal contact with a substrate such that heat is more efficiently drawn out of the x-ray generating substance. This in turn allows irradiation of the x-ray generating substance with higher electron density or higher energy electrons, which leads to greater x-ray brightness. To achieve this increased heat transfer, the target comprises a plurality of regions of x-ray generating material in close thermal contact with a substrate made from a material selected for its high thermal conductivity. The x-ray generating material may be any material known to generate x-rays, including conventional materials such as copper (Cu), molybdenum (Mo) or tungsten (W). The substrate may be any material that has high thermal conductivity, and may also be chosen to be a low Z material, so that the electron energy deposition rate in the substrate is less than in the x-ray generating material, and the substrate is less likely to heat up with exposure to the incident electrons.
A target 1100 according to the invention may be inserted as a replacement for the target 01 for the transmission x-ray source 08 illustrated in
It should be noted here that, when the word “microstructure” is used herein, it is specifically referring to microstructures comprising x-ray generating material. Other structures, such as the cavities used to form the x-ray microstructures, have dimensions of the same order of magnitude, and might also be considered “microstructures”. As used herein, however, other words, such as “structures”, “cavities”, “holes”, “apertures”, etc. may be used for these structures when they are formed in materials, such as the substrate, that are not selected for their x-ray generating properties. The word “microstructure” will be reserved for structures comprising materials selected for their x-ray generating properties.
Likewise, it should be noted that, although the word “microstructure” is used, x-ray generating structures with dimensions smaller than 1 micron, or even as small as nano-scale dimensions (i.e. greater than 10 nm) may also be described by the word “microstructures” as used herein.
A disadvantage of the embodiment of
To address this, some embodiments of the invention may use a configuration like that shown in
As discussed in Eqn. 1 above, the depth of penetration can be estimated by Pott's Law. Using this formula, Table II illustrates some of the estimated penetration depths for some common x-ray target materials.
For the illustration in
The majority of characteristic Cu K x-rays are generated within depth D. The electron interactions below that depth typically generate few characteristic K-line x-rays but will contribute to the heat generation, thus resulting in a low thermal gradient along the depth direction. It is therefore preferable in some embodiments to set a maximum thickness for the microstructures in order to limit electron interaction in the material and optimize local thermal gradients. One embodiment of the invention limits the depth of the microstructured x-ray generating material to between one third and two thirds of the electron penetration depth at the incident electron energy. In this case, the lower mass density of the substrate leads to a lower energy deposition rate in the substrate material immediately below the x-ray generating material, which in turn leads to a lower temperature in the substrate material below. This results in a higher thermal gradient between the x-ray generating material and the substrate, enhancing heat transfer. The thermal gradient is further enhanced by the high thermal conductivity of the substrate material.
For similar reasons, selecting the depth D to be less than the electron penetration depth is also generally preferred for efficient generation of bremsstrahlung radiation, because the electrons below that depth have lower energy and thus lower x-ray production efficiency.
Note: Other choices for the dimensions of the x-ray generating material may also be used. In some embodiments, the depth of the x-ray material may be selected to be 50% of the electron penetration depth. In other embodiments, the depth of the x-ray material may be selected to be 33% of the electron penetration depth. Other depths may be specified depending on the x-ray spectrum desired and the properties of the selected x-ray material.
Note: In other embodiments, a particular ratio between the depth and the lateral dimensions (such as width W and length L) of the x-ray generating material may also be specified. For example, if the depth is selected to be a particular dimension D, then the lateral dimensions W and/or L may be selected to be no more than 5×D, giving a maximum ratio of 5. In other embodiments, the lateral dimensions W and/or L may be selected to be no more than 2×D. It should also be noted that the depth D and lateral dimensions W and L (for width and length of the x-ray generating microstructure) may be defined relative to the axis of electron propagation, or defined with respect to the orientation of the surface of the x-ray generating material. For normal incidence electrons, these will be the same dimensions. For electrons incident at an angle, care must be taken to make sure the appropriate projections are used.
It should be noted that, although the illustration of
It should also be noted that materials are relatively transparent to their own characteristic x-rays, so that
In this embodiment, the microstructures have been fabricated such that they are in close thermal contact on five of six sides with the substrate. As illustrated, the top of the microstructures 700 are flush with the surface of the substrate, but other embodiments in which the microstructure is recessed may be fabricated, and still other embodiments in which the microstructures present a topographical “bump” relative to the surface of the substrate may also be fabricated.
An alternative embodiment may have several microstructures of right rectangular prisms simply deposited upon the surface of the substrate. In this case, only the bottom base of the prism would be in thermal contact with the substrate. For a structure comprising the microstructures embedded in the substrate with a side/cross-section view as shown in
With a small value for D relative to W and L, the ratio is essentially 1. For larger thicknesses, the ratio becomes larger, and for a cube (D=W=L) in which 5 equal sides are in thermal contact, the ratio is 5. If a cap layer of a material with similar properties as the substrate in terms of mass density and thermal conductivity is used, the ratio may be increased to 6.
The heat transfer is illustrated with representative arrows in
where κ is the thermal conductivity in W/(m ° C.) and ΔT is the temperature difference across thickness d in ° C. Therefore, an increase in surface area A, a decrease in thickness d and an increase in ΔT all lead to a proportional increase in heat transfer.
An alternative embodiment is illustrated in
Other embodiments may be understood or devised by those skilled in the art, in which the substrate may, for example, be bonded to a heat sink, such as a copper block, for improved thermal transfer. The copper block may in turn have cooling channels within it to assist in carrying heat away from the block. Alternatively, the substrate may be attached to a thermoelectric cooler, in which a voltage is applied to a specially constructed semiconductor device. In these devices, the flow of current causes one side to cool while the other heats up. Commercially available devices, such as Peltier coolers, can produce a temperature difference of up to 70° C. across the device, but may be limited in their overall capacity to remove large amounts of heat from a heat source.
Alternatively, the substrate can be attached to a cryogenic cooler, such as a block containing channels for the flow of liquid nitrogen, or be in thermal contact with a reservoir of liquid nitrogen or some other cryogenic substance, to provide more extreme cooling. When the substrate comprises a material such as diamond, sapphire, silicon, or silicon carbide, thermal conductivity generally increases with decreasing temperature from room temperature. In such a case, designing the target so that it can withstand cooling to these lower temperatures may be preferred.
This can be addressed by the deposition of a thin layer of conducting material that is preferably of relatively low atomic number, such as aluminum (Al), beryllium (Be), carbon (C), chromium (Cr) or titanium (Ti), that allows electrical conduction from the discrete microstructures 700 to an electrical path 722 that connects to a positive terminal relative to the high voltage supply. This terminal as a practical matter is typically the electrical ground of the system, while the cathode electron source is supplied with a negative high voltage.
It should be clear to those skilled in the art that although several embodiments have been presented separately in
Although the microstructures illustrated in
Likewise, although some embodiments have been described with microstructures in, for example, the shape of right rectangular prisms, fabrication processes may create structures that have walls at angles other than 90°, or do not have corners that are exactly right angles, but may be rounded or beveled or undercut, depending on the artifacts of the specific process used. Embodiments in which the microstructures are essentially similar with the shapes described herein will be understood by those skilled in the art to be disclosed, even if process artifacts lead to some deviation from the shapes as illustrated or described.
Likewise, although the various examples disclosed herein may be illustrated with ordered periodic arrays of microstructures, the relative position, size and shape of the discrete microstructures need not be regular, periodic, or uniform. Arrangements of microstructures that have a distribution of sizes, with spacing between microstructures that can have a range of distances, may also be functional.
Certain embodiments of the invention have been described in the previous section. However, aside from variations in layers and structures, various embodiments will comprise microstructures of various sizes and shapes as well.
As illustrated in
On the other hand, the configuration illustrated in
To address this, as illustrated in
For some fabrication processes, the etching process can be tuned to provide an undercut. [See, for example, D. S. Hwang, T. Saito and N. Fujimori, “New etching process for device fabrication using diamond”, Diamond & Related Materials vol. 13, pp. 2207-2210 (2004) for examples of both isotropic and anisotropic etching of diamond.] If a process with an undercut is selected to etch the cavities in the substrate that are used to form the microstructures, and the microstructures are formed using an isotropic process such as electroplating, which can fill all portions of the cavity, microstructures that are “secured” in place may be formed, as is illustrated in
Fabrication of a region of embedded spheres as illustrated in
As discussed earlier, although the various examples disclosed herein may be illustrated with ordered periodic or regular arrays of microstructures, the relative position, size and shape of the discrete microstructures need not be regular, periodic, or uniform. Arrangements of microstructures that have a distribution of sizes, with spacing between microstructures that can have a range of distances, may also be functional.
For some embodiments, various metrics to determine the size and distribution of the microstructures comprising x-ray generating material may be used. Microstructures may be designated to be a predetermined thickness D within the target, where, as discussed before, D may be selected to be a certain fraction of the electron penetration depth for a electrons of a given energy in a given x-ray generating material (such as, for example, 30% or 50%) or may be a range of allowed depths. Microstructures may be specified such that their lateral dimensions L and W do not exceed D by more than a specific factor (e.g. a factor of 2 or 3), and that any individual microstructure be no closer than a predetermined distance d to a neighboring microstructure. Alternatively, microstructures may be specified such that their lateral dimensions L and W do not exceed the x-ray attenuation length (the length at which the intensity of an x-ray beam of a specific energy falls off by a factor of 1/e), which will be different for different applications.
In the most general case, the region comprising microstructures may be specified by defining a volume fraction of the entire area to be exposed to electrons up to the thickness D that will comprise x-ray generating material. For example, if microstructures of tungsten (W) are to be used with 60 keV electrons, the penetration depth from Table II is 2.41 microns, and a value of D=1 micron would represent a thickness of 41% of the penetration depth. Defining a volume fraction of 50% for the initial layer of thickness D=1 μm could either be achieved by using, for example, the checkerboard array of
The volume fraction for the x-ray generating region may also be set to varying values, depending on the electron energy, the x-ray generating material properties, and the substrate reabsorption properties. For some applications requiring specific characteristic lines, configurations with a lower volume fraction may be preferred. For other variations on the emission spectrum, such as those with a range of wavelengths, a higher volume fraction that increases the bremsstrahlung may be preferred. In general, volume fractions in the first layer of thickness D may be set to be between 15% and 85% for various applications.
Alternatively, in some embodiments, spherical microstructures of x-ray generating material may be prepared in advance, and then dispersed onto the surface of the substrate. A process that fixes the microstructures in place may be used, particularly if it is desired that the microstructures be positioned in a regular array (as was illustrated in
In another embodiment, the dispersal of microstructures comprising x-ray generating material need not be confined to uniform spheres, but may be a number or particles of various sizes and shapes. This is illustrated in
Up to this point, embodiments that are arranged in planar configurations have been presented. These are generally easier to implement, since equipment and process recipes for deposition, etching and other planar processing steps are well known from processing devices for microelectromechanical systems (MEMS) applications using planar diamond, and from processing silicon wafers for the semiconductor industry.
However, in some embodiments, a surface with additional properties in three dimensions (3-D) may be desired. As discussed previously, when the electron beam is larger than the electron penetration depth, the apparent x-ray source size and area is at minimum (and brightness maximized) when viewed parallel to surface, i.e. at a zero degree (0°) take-off angle. As a consequence, the apparent brightest of x-ray emission occurs when viewed at 0° take-off angle. The emission from within the x-ray generating material will accumulate as it propagates at 0° through the material.
However, with an extended target of substantially uniform material, the attenuation of x-rays between their points of origin inside the target as they propagate through the material to the surface increases with decreasing take-off angle, due to the longer distance traveled within the material, and often becomes largest at or near 0° take-off angle. Reabsorption may therefore counterbalance any increased brightness that viewing at near 0° achieves. The distance through which an x-ray beam will be reduced in intensity by 1/e is called the x-ray attenuation length, and therefore, a configuration in which the emitted x-rays pass through as little additional material as possible, with the distance selected to be related to the x-ray attenuation length, may be desired.
An illustration of one embodiment of a target is presented in
The thickness of the bar D (along the surface normal of the target) is selected to be between one third and two thirds of the electron penetration depth of the x-ray generating material at the incident electron energy for optimal thermal performance. It may also be selected to obtain a desired x-ray source size in the vertical direction. The width of the bar W is selected to obtain a desired source size in the corresponding direction. As illustrated, W≈1.5D, but could be substantially smaller or larger, depending on the size of the source spot desired.
The length of the bar L as illustrated is L≈4D, but may be any dimension, and may typically be determined to be between ¼ to 3 times the x-ray attenuation length for the selected x-ray generating material. The distance between the edge of the shelf and the edge of the x-ray generating material p as illustrated is p≈W, but may be selected to be any value, from flush with the edge 2003 (p=0) to as much as 1 mm, depending on the x-ray reabsorption properties of the substrate material, the relative thermal properties, and the amount of heat expected to be generated when bombarded with electrons.
An illustration of an alternative embodiment of the invention is presented in
In this embodiment, the total volume of x-ray generating material is the same as in the previous illustration of
However, the single bar 2700 of length L as illustrated in
Likewise, the distance between the edge of the shelf and the edge of the x-ray generating material p as illustrated is p≈W, but may be selected to be any value, from flush with the edge 2003 (p=0) to as much as 1 mm, depending on the x-ray reabsorption properties of the substrate material, the relative thermal properties, and the amount of heat expected to be generated when bombarded with electrons.
For a configuration such as shown in
The bars as shown may be embedded in the substrate (as shown), but if the thermal load generated in the x-ray generating material is not too large, they may also be placed on top of the substrate.
An illustration of another embodiment of the invention is presented in
As in other embodiments, these microstructures 2790 and 2791 are embedded in the surface of the substrate. However, the surface of the substrate comprises a predetermined non-planar topography, and in this particular case, a plurality of steps along the surface normal of the substrate 2000. As illustrated, the height of each step is h≈D, but the step height may be selected to be between 1× and 3× the thickness of the microstructures. The total height of all the steps may be selected to be equal or less than the desired x-ray source size along the vertical (thickness) direction.
The total width of the microstructured region may be equal to the desired x-ray source size in the corresponding direction. The overall appearance resembles a staircase of x-ray sources.
The brightness of x-rays from each prism will therefore be increased, especially when compared to the x-ray emission from the embodiment of, for example,
Such an embodiment comprising a target with topography may be manufactured by first preparing a substrate with topography, and then embedding the prisms of x-ray material following the fabrication processes for the previously described planar substrates. Alternatively, the initial steps that create cavities to be filled with x-ray material may be enhanced to create the staircase topography structure in an initially flat substrate. In either case, additional alignment steps, such as those known to those skilled in the art of planar processing, may be employed if overlay of the embedded prisms with a particular feature of topography is desired.
Microstructures may be embedded with some distance to the edges of the staircase, as illustrated in
The embodiments described so far describe a variety of x-ray target configurations that comprise a plurality of microstructures comprising x-ray material that can be used as targets in x-ray sources to generate x-rays with increased brightness. These target configurations have been described as being bombarded with electrons and emitting x-rays, but may be used as the static x-ray target in an otherwise conventional source, replacing either the target 01 from the transmission x-ray source 08 of
It is also possible that the embodiments as described could be equally well applied to a moving x-ray target, replacing, for example, the target 500 from the rotating anode x-ray source 80 of
As in the conventional rotating anode, electrons bombard the target anode 2500 at the edge, which may be beveled, just as a conventional rotating anode is beveled, and the source of the electron beam directs the electron beam onto the beveled edge 2510 of the target anode 2500, generating x-rays from a target spot 2501. As the target spot 2501 generates x-rays, it heats up, but as the target anode 2500 rotates, the heated spot moves away from the target spot 2501, and the electron beam now irradiates a cooler portion of the target anode 2500. The hot spot has the time of one rotation to cool before becoming heated again when it passes through the hot spot 2501. By continuously rotating the target anode 2500, a single spot never becomes too hot, yet a continuous source of x-rays can be provided.
As in the previously described rotating anode system, additional cooling channels may be provided in the rotating anode to further cool the anode, allowing bombardment with electrons at higher voltages or with higher current densities to make a brighter x-ray source. However, if the target material in the rotating anode uses a plurality of microstructures according to the invention disclosed herein, the improved thermal properties may allow higher electron power loading. This enables an x-ray source of higher brightness, because the electron energy and current may be increased once the additional heat load can be accommodated. Alternatively, the thermal benefits may be used to enable a rotating anode source of the same brightness, but with components that are easier to engineer, such as lower voltage, lower current, or slower anode rotation speed.
The methods for fabricating the targets according to the invention involve a number of steps that are outlined in the flow chart of
4.1. Selecting a Substrate.
In the initial step, a substrate 3000 of a suitable material is selected. In
Wafers of CVD grown diamond up to 120 mm in diameter and with diamond coatings up to 2 mm thick may be may be purchased from Diamond Materials GmbH of Freiburg, Germany. Substrates of silicon coated with diamond or diamond on insulator (DOI) may also be purchased from, for example, Advanced Diamond Technologies, Inc. of Romeoville, Ill. or sp3 Diamond Technologies of Santa Clara, Calif. Diamond-like carbon (DLC) films such as those manufactured by Richter Precision, Inc. of East Petersburg, Pa. may also be useful as substrate materials.
Beryllium may also be a candidate for a substrate material. With a low atomic number (Z=4), beryllium is very lightweight and is especially transparent to x-rays, and therefore less likely to be a source of continuum x-rays that might interfere with the x-ray emission from the plurality of microstructures embedded within the substrate. Beryllium wafers may be commercially purchased from, for example, American Elements, Inc. of Los Angeles, Calif., and Atomergic Chemetals Corporation of Farmingdale, N.Y.
Other materials that may be suitable as substrates are graphite, silicon, boron nitride, gallium nitride, silicon carbide and sapphire. Other suitable materials may also be known to those skilled in the art.
4.2. Patterning the Substrate.
Once a substrate is selected, the next step 3100, as shown in
In the process cited above by Masuda et al., a polished polycrystalline diamond film ˜3 mm thick is patterned using a porous alumina mask. The mask is prepared in advance using a silicon carbide mold to texture an aluminum surface, which is subsequently oxidized through an anodization process. The alumina film so formed has pores with positions determined by the texture on the SiC mold. The film is then removed from its aluminum substrate and transferred to the diamond surface. The diamond is then subjected to an oxygen reactive ion etch process, in which the porous alumina film acts as a mask.
Representative steps for this process step 3100 are illustrated with the corresponding steps in
In
Alternatively, the substrate may be patterned using conventional lithographic processes. These may include coating the substrate with a photoresist, such as HSQ, and exposing the resist using electron beams or ultraviolet photons in a pattern that represents the desired structure to be formed on the wafer. The resist is then developed to remove the exposed regions, laying the substrate bare. The substrate and patterned resist combination are then processed with a suitable etching process (such as a reactive ion etch (RIE) with oxygen gas) that transfers the pattern in the resist into the substrate. Once this is completed, the excess resist is removed, leaving a patterned substrate essentially the same as the patterned substrate 3001 designated by step “4)” in
In a variation on the lithographic patterning process described above, the substrate may be coated with a specially selected material that serves as a hard mask for patterning the substrate. The steps in this case are: coating of a hard mask onto the substrate, coating resist onto the hard mask, patterning the resist with either electron or optical exposure, developing the resist, transferring the pattern from the resist into the hard mask, and transferring the pattern from the hard mask into the substrate, leaving a patterned substrate essentially the same as the patterned substrate 3001 designated by step “4)” in
In other alternatives to the lithographic patterning processes discussed above, the substrate may also be directly ion milled using a machine such as a focused ion beam. Other techniques, such as laser etching, may also be used to pattern the substrate.
4.3. Embedding the X-ray Material.
Once a substrate has been patterned, the next step is the deposition of a material that can produce x-rays of desired characteristics into the patterned cavities 3300. This may be through any number of well-known deposition techniques, depending on the material, including chemical vapor deposition (CVD), sputtering, electroplating, mechanical stamping, or others that will be known to those skilled in the art. Various materials may be selected for use as x-ray generating materials, including aluminum, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, gallium, zinc, yttrium, zirconium, molybdenum, niobium, ruthenium, rhenium, rhodium, palladium, silver, tin, iridium, tantalum, tungsten, indium, cesium, barium, gold, platinum, lead and combinations and alloys thereof.
Representative steps for this process step 3300 are illustrated with the corresponding steps in
Once the x-ray material has been deposited, there is typically an excess of material on the substrate. The following step of polishing 3500 with either a mechanical/abrasive polishing process, or a chemical-mechanical polishing (CMP) process, removes the excess material, leaving behind the cavities in the patterned substrate 3001 now filled with discrete microstructures 3401 of x-ray material, as illustrated with the corresponding step in
4.4. An Adhesive Alternative
Some materials, when used in combination with certain substrates, may form an interface layer that provides a good bond between the two. For example, for a selected x-ray material such as tungsten, CVD deposition of tungsten material 3400 onto a patterned diamond substrate 3001 can be adequate to fill the cavities in the diamond, with the tungsten forming a strong carbide bond in the boundary between the tungsten and the carbon. However, for other materials, such as copper, the use of an adhesive layer, such as the deposition of a 10 nm thick layer of titanium (Ti) or chromium (Cr) between the copper and a diamond substrate may be preferred to improve the mechanical integrity of the anode, both by increasing the adhesion between the two materials, and also in some cases by preventing diffusion of material from one region into the other.
Representative steps for this process step are illustrated with the corresponding steps in
In
4.5. Overcoats.
Once a substrate has been patterned and the cavities filled to create microstructures of x-ray material, the next step is the deposition of a conducting layer, so that electrons impinging on the x-ray material will have a path to ground.
A representative illustration for this process step 3700 is shown with the corresponding result in
The deposited material 3750 may be any one of a number of electrically conducting materials, such as beryllium (Be), aluminum (Al), chromium (Cr), titanium (Ti), silver (Ag), gold (Au), copper (Cu), or carbon materials such as graphite or carbon nanotubes. The material may be as thin as 5 nm or as thick as 100 nm, and in some circumstances, such as if there are larger topography variations in the substrate with filled cavities, the material may be as thick as 500 nm.
The deposition techniques may be any number of a variety of deposition techniques, including but not limited to chemical vapor deposition (CVD), sputtering, electroplating, mechanical stamping, or others that will be known to those skilled in the art.
Once the electrically conducting layer has been deposited, a final protective overcoat, or cap layer, may also be deposited.
A representative illustration for this process step 3900 is shown with the corresponding result in
The deposited material 3950 may be any one of a number of materials, but may be typically selected to be the same material used for the substrate, such as diamond (C) diamond-like carbon (DLC), or beryllium (Be), or another materials, such as silicon carbide (SiC), chromium (Cr), molybdenum (Mo), rhodium (Rh) and palladium (Pd). The material may be as thin as 100 nm, or may be as thick as 50 μm.
The deposition techniques may be any number of a variety of deposition techniques, including but not limited to chemical vapor deposition (CVD), sputtering, electroplating, mechanical stamping, or others that will be known to those skilled in the art.
To have good adhesion to the layers below, the deposition of the cap layer may be preceded by the deposition of an adhesion layer, such as a titanium carbide (TiC) to form a seed layer for the growth of diamond. The deposition may be very thin, perhaps between 1 and 5 nm, to provide this seed layer.
4.6. Process Combinations.
Once these steps have been completed, the final object, denoted with “8)” in
Although certain processes steps have been described in certain sequences, it should be known that certain steps may be executed in a different order or combined with each other to achieve a similar result.
For example, the electrically conducting layer has been described as occurring before the deposition of a cap layer, but a layer that combines these functions (i.e. an electrically conducting cap layer) such as that illustrated in
Furthermore, multiple layers of microstructures, as were illustrated in
With this application, several embodiments of the invention, including the best mode contemplated by the inventors, have been disclosed. It will be recognized that, while specific embodiments may be presented, elements discussed in detail only for some embodiments may also be applied to others.
While specific materials, designs, configurations and fabrication steps have been set forth to describe this invention and the preferred embodiments, such descriptions are not intended to be limiting. Modifications and changes may be apparent to those skilled in the art, and it is intended that this invention be limited only by the scope of the appended claims.
This Patent Application claims the benefit of U.S. Provisional Patent Application Nos. 61/873,735, filed on Sep. 4, 2013, 61/880,151, filed on Sep. 19, 2013, 61/894,073, filed on Oct. 22, 2013, and 61/931,519, filed on Jan. 24, 2014, all of which are incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61873735 | Sep 2013 | US | |
| 61880151 | Sep 2013 | US | |
| 61894073 | Oct 2013 | US | |
| 61931519 | Jan 2014 | US |
