Improvements to the X-ray source and improvements related to the X-ray source

Abstract

The X-ray source is configured to generate X-rays (11) by means of electron collisions with the anode (2), and comprises an electron beam generator (5) for generating an electron beam (3) of electrons guided along the beam axis. The rotating anode assembly (1) comprises a pivot (17) and an anode surface portion (2) mounted on the pivot for receiving electrons in the electron beam for generating X-rays. The suspension assembly (19) is configured to apply a restoring force to the rotating anode assembly to bias the rotating anode assembly towards a stationary position in which the anode surface portion is stationary relative to the pivot. The displacer assembly (21) is configured to apply a periodic displacement force to the rotating anode assembly to displace the rotating anode assembly from the stationary position. The position of the anode assembly oscillates about the stationary position with a lateral movement relative to the beam axis in response to the periodic displacement force and the restoring force, thereby causing the anode surface portion to oscillate laterally across the beam axis.

Description

Technical Field

[0001] This application claims the benefit of priority of earlier filed application number UK 2209809.9, filed on July 4, 2022, the content and elements of which are incorporated herein by reference for all purposes.

[0002] The present invention relates to an X-ray source configured to generate X-rays by electron bombardment of an anode. Although not exclusive, in particular, the present invention relates to an X-ray source used in the field of X-ray photoelectron spectroscopy (XPS).

Background Art

[0003] X-ray sources typically take the form of a cathode facing an anode within a vacuum chamber (often referred to as a "tube"). The cathode is heated to emit electrons into the vacuum. A high-voltage power source (e.g., from tens of kV to 100 kV or more) is applied between the cathode and the anode to accelerate the electrons towards the anode. This creates a flow of current through the tube, known as an electron beam. The electrons of the electron beam collide with the atoms forming the anode material (e.g., copper, molybdenum, or tungsten). When doing so, the colliding beam electrons create inner-shell holes, decelerate and scatter, and in doing so, emit X-rays in several processes. One such process known as bremsstrahlung: electromagnetic radiation created by the acceleration or particularly the deceleration of charged particles after passing through the electric and magnetic fields of an atom. Further, the incident electrons create inner-shell vacancies by ejecting inner-shell electrons, which then create "line" or "characteristic" radiation as outer-shell electrons fill the vacancies while emitting photons of energy equal to the energy difference between the energy levels of the outer-shell and inner-shell electrons. The spectrum of the emitted X-rays depends on the anode material and the kinetic energy of the electrons in the electron beam, which is determined by the acceleration voltage between the anode and the cathode.

[0004] Only approximately 1% of the energy generated by the electron beam collision process is emitted as bremsstrahlung and "line" X-rays. The remaining energy is emitted as heat created at the focal point on the anode. The focal point is the location where the electron beam is induced (e.g., focused) to strike the anode using an electron optical element disposed between the cathode and the anode for this purpose. The amount of heat (E, joules) created at the focal point is: E = V × I × t where V is the accelerating voltage (volts) between the anode and the cathode, I is the current (amperes) of the electron beam passing through the tube, and t is the exposure time (seconds) of the beam to the focal point at a given fixed point on the anode. High temperatures can be generated at the focal point of the stationary anode. The focal point temperature can reach a temperature higher than the melting point of the anode material, and thus there is a risk of damaging the anode surface. However, prior art devices enable the electron beam to scan along a circular path on the surface of the anode by rotating the anode around an axis offset from the electron beam, thereby reducing the exposure time of a given point on the anode to the focal point of beam damage on the anode. However, rotating the anode configuration is complex and difficult to implement in an ultra-high vacuum environment for continuous operation in an economical manner.

[0005] An example of the use of an X-ray tube is in X-ray photoelectron spectroscopy (XPS). This is a spectroscopic technique that utilizes the photoelectric effect, whereby X-rays (typically characteristic line radiation) from an X-ray source are induced onto the sample material under investigation, and photoelectrons are emitted from the surface of the sample. This technique identifies the elements present within or on the sample material. Additionally, the electronic structure of the material and the density of the electronic states of the material can be estimated. The characteristics of the material can be estimated from the measurement results of the number of photoelectrons with respective kinetic energies within a certain range by a photoelectron detector: these two quantities can generate a photoelectron spectrum including characteristic spectral peaks of the sample material.

[0006] The precision depends on several parameters, among others, the signal-to-noise ratio of the photoelectron signal from the photodetector, and the photoelectron peak intensity (i.e., the spectral peak). It is known that by increasing the intensity of the X-ray illumination on the target sample, a higher photoelectron flux can be generated. This higher flux corresponds to a higher signal in the photodetector, and thus, with a higher signal-to-noise ratio, improved measurement precision is achieved. The increased X-ray illumination requires an increased electron beam current I of the electron beam incident on the anode. This leads to the problems of anode heating and damage identified above.

[0007] The present invention has been devised in light of the above considerations.

SUMMARY OF THE INVENTION

PROBLEM TO BE SOLVED BY THE INVENTION

[0008] In its most general form, the present invention relates to an anode for an X-ray source, where there is an anode on a pivot-mounted mechanical structure that is periodically displaced from a stationary or reference position and returned to the stationary or reference position in a vibration mode (e.g., forced vibration) by a suspension configuration (e.g., spring-type) that suspends the mechanical structure. This results in a simple configuration that allows the electron beam of the X-ray source to scan along a path on the surface of the anode by vibrating the anode relative to the electron beam, thereby reducing the exposure time of a given point on the anode to the focus of the electron beam. This reduces the temperature rise on the surface of the anode. The speed and trajectory (e.g., length) of the path of the focus of the electron beam on the anode surface can be precisely controlled by controlling the periodic displacement. By appropriately configuring the mechanical structure and the suspension configuration to achieve a preferred mass distribution (e.g., moment of inertia), an improved dynamic response to the periodic displacement can be achieved.

MEANS FOR SOLVING THE PROBLEM

[0009] In a first aspect, the present invention can provide an X-ray source comprising the following, configured to generate X-rays by electron bombardment of an anode: an electron beam generator for generating a beam of electrons induced along a beam axis; a rotating anode assembly comprising a pivot and an anode surface portion mounted on the pivot for receiving electrons in the electron beam to generate X-rays; a suspension assembly configured to apply a restoring force to the rotating anode assembly to bias the rotating anode assembly towards a stationary position in which the anode surface portion is stationary relative to the pivot; a displacer assembly configured to apply a periodic displacement force to the rotating anode assembly to displace the rotating anode assembly from the stationary position; wherein the position of the anode assembly oscillates about the stationary position in a lateral motion relative to the beam axis in response to the periodic displacement force and the restoring force, thereby oscillating the anode surface portion laterally across the beam axis.

[0010] In this way, the dynamic interaction between the restoring force from the suspension system and the periodic displacement force of the displacer assembly, both acting about the same pivot of the rotating anode assembly, determines the speed and trajectory of the path of the point of impact of the electron beam on the anode surface. By controlling the frequency and / or magnitude of the periodic displacement force, and by controlling the restoring force from the suspension assembly, the position of the anode assembly can be oscillated with a desired dynamic response to the periodic displacement. The appropriate mass distribution (e.g., moment of inertia) of the anode assembly can significantly improve the energy efficiency and / or the oscillation frequency of this dynamic response. The suspension assembly can comprise a spring-type suspension configuration. For example, the suspension assembly can comprise one or more springs configured to provide a restoring force.

[0011] Preferably, the pivot is fixed or stationary relative to the suspension assembly and / or relative to the displacer assembly. Preferably, the pivot comprises a single pivot axis configured such that the swivel anode assembly pivots about the single pivot axis. The single pivot axis constrains the movement of the anode assembly and thus the movement of the anode surface to be substantially linear (e.g., movement along a slightly curved arc) or the linear path of the trajectory of the point of impact of the electron beam on the anode surface. For example, the pivot may comprise a fixed or stationary rotating axle or axis, such as a hinge or bearing (e.g., a flexure bearing), about which the anode assembly is configured to rotate. Alternatively, the pivot may comprise a universal joint. The universal joint can allow for a longer path for the movement of the anode surface (e.g., a looped or circular path rather than an approximately linear path). As a result, the path of the electron beam collision point on the anode surface can be made longer with a given angular deflection from a reference position. This can result in a lower average power density along that path.

[0012] Desirably, the pivot comprises one or more flexure bearings configured to bend about the pivot axis of the swivel anode assembly. The flexure bearings have been found to be advantageous in that they provide a configuration with negligible "backlash". The flexure bearings further have a very long service life when the angular deflection and load are kept within appropriate limits. The appropriate limits can be, for example, about ±6°, preferably about ±5°, more preferably about ±4°, even more preferably about ±3°, or still more preferably about ±2°. The swivel anode assembly can be configured to limit the angular deflection within such limits.

[0013] The swivel anode assembly may be configured to limit the load applied to the pivot (e.g., a flexure bearing, or in multiple cases, each bearing) to less than about 650 lb (2891 N), less than about 500 lb (2224 N), preferably less than about 450 lb (2002 N), or less than about 425 lb (1890 N), e.g., less than about 403 lb (1793 N).

[0014] The pivot can comprise one or more flexure bearings configured with a diameter between about 0.375 inches (0.952 cm) and about 0.625 inches (1.588 cm), such as about 0.5 inches (1.270 cm). The pivot can comprise two pairs of flexure bearings configured and aligned with mutually orthogonal axes of rotation to form a universal joint that can be used to achieve the desired accuracy of movement of the swivel anode assembly.

[0015] The flexure bearing(s) can comprise a single unit (e.g., one semi-cylindrical part concentrically inside the outer cylindrical part) including two rigid structures rotatably connected by a thin bridging part of a flat elastic material that can be repeatedly bent without falling apart. The bridging part can be integrally formed with both of the two rigid structures and can be their only connection to each other. The bridging part acts as a "hinge" between the two rigid structures. The flexure bearing has the advantage of having essentially zero (or negligibly low) friction from a dynamics perspective and thus not significantly contributing to the damping of the harmonic oscillatory motion of the anode assembly. Since the flexure bearing does not require lubrication, it can be used in a vacuum state and thus may be desirable for an X-ray source. Alternatively, the pivot can comprise an elastically resilient member (e.g., a spring) on which the anode assembly is mounted (on the end of the spring). This elastically resilient member can further act as a suspension assembly (or at least a part thereof).

[0016] For example, the swivel anode assembly can include a pivot fixture that is not a rotational axis, but instead is a spring member (e.g., a coil spring, a helical spring, or a torsion spring) that can be deformed in a direction transverse to the longitudinal axis. The pivot fixture can be a fixture on the spring member, for example, a fixture at an end of its length or at a position along its length. The deformation of the spring member can be bent or flexed to allow the anode assembly to swivel. In some examples, the spring member of the pivot can act not only as a pivot but also as a suspension assembly. The movement of the surface of the anode surface portion can be sufficiently controlled to avoid significant longitudinal movement. For example, if the spring member comprises a coil spring or a helical spring, the displacer assembly can be configured to apply a periodic displacement force to the swivel anode assembly to displace the swivel anode assembly and limit the displacement of the spring member, thereby limiting the excitation of the extension and contraction of the spring member. In this way, appropriate limits or constraints can be applied to the maximum displacement position of the anode surface portion. For example, a torsion spring can function both as a pivot and to provide a restoring force to bias the swivel anode assembly to its rest position when the anode surface portion reaches its maximum displacement position (e.g., angular displacement).

[0017] Desirably, the swivel anode assembly is configured to oscillate about a stationary position such that the intersection of the electron beam axis on the anode surface portion scans a substantially straight path across the anode surface portion. In this way, the trajectory of the path of the focus of the electron beam on the anode surface can be precisely controlled to be straight by controlling or constraining such that the periodic displacement of the anode assembly is in (or parallel to) the plane of vibration; the plane of vibration can include the path of the focus of the electron beam on the anode surface. A particular benefit of this configuration is that the position of the anode assembly is configured to be displaceable adjustably in a direction transverse to the plane of vibration, whereby the oscillatory motion of the displaced anode assembly can be constrained to be in (or parallel to) a new plane of vibration that is substantially parallel to the previous plane of vibration. Thereby, when an old surface area of the anode surface previously traversed by the focus of the electron beam becomes unusable, for example due to cumulative electron collision damage, the path of the focus of the electron beam can cross a "unused" new surface area of the anode surface.

[0018] For example, the X-ray source can comprise a lateral displacer configured to adjustably displace the rotating anode assembly in a direction transverse to the plane of vibration of the anode surface portion. In some embodiments, the lateral displacer can comprise one or more stepper motors or actuators configured to laterally displace (preferably slowly) the rotating anode assembly and the suspension assembly to different positions on a plane parallel to the anode surface portion. Thereby, when a particular area has been used up, the unused area of the anode surface can be used, thereby reducing the frequency of anode replacement. Thus, the X-ray source comprises a lateral displacer configured to adjustably displace the position of the anode assembly in a direction transverse to the plane of vibration to an adjusted position and to enable the anode assembly to vibrate in (or parallel to) a new plane of vibration that is substantially parallel to the plane of vibration in which the displacement takes place. The lateral displacer can be configured to adjustably displace the position of the anode assembly in a direction substantially parallel to the anode surface (for example, the anode surface can be planar). Desirably, the lateral displacer can be configured to adjustably displace the position of the anode assembly in a direction such that the focus of the electron beam continues to coincide with the anode surface or such that the distance between the electron source and the point of impact of the electron beam on the anode surface remains substantially unchanged.

[0019] Even if the surface of the anode portion presented toward the electron beam generator is substantially flat or planar, the lateral portions of the surface can move so as to periodically approach the electron beam generator as the surface tilts or wobbles relative to the electron beam. Those lateral portions include the changing points where the electron beam impinges. Thus, the changing points on the anode surface can move periodically forward and backward along the electron beam axis. Desirably, the X-ray source comprises a longitudinal displacer configured to adjustably displace the rotating anode assembly in a direction substantially parallel to the plane of vibration of the anode surface portion so as to adjustably displace the anode surface portion selectively in a direction toward the electron beam generator in the longitudinal direction or in a direction away from the electron beam generator in the longitudinal direction. The longitudinal displacer can be configured to adjustably displace the rotating anode assembly so as to substantially keep the separation distance between the electron beam generator and the point of collision of the electron beam on the anode surface portion substantially constant during the rotation of the anode assembly. In this way, the changing separation distance can be made substantially constant in another situation.

[0020] Preferably, the X-ray source comprises a position sensor or transducer configured to determine the position of the anode surface portion during vibration of the anode surface portion. The position sensor or transducer may be configured to determine the angular position of the rotating anode assembly (e.g., by sampling the position at discrete points in time that are continuous, or sufficiently high sampling rate to be quasi-continuous). The position of the anode surface varies according to the angular position of the rotating anode assembly to which it is attached. The location of any given fixed point on the anode surface can be constrained to be on an arc orbit or path defined by the angular position of the rotating anode assembly and the radial distance of the given point from the pivot. However, the longitudinal displacer is configured to adjustably displace the rotating anode assembly so as to substantially keep the separation distance between the electron beam generator and the point of impact of the electron beam on the anode surface portion substantially constant in response to the determined position of the anode surface portion, in response to the determined angular position of the rotating anode assembly, or in response to another determined position measurement proportional to the angular position of the rotating anode assembly during rotation of the anode assembly. In this way, the separation distance, which varies in other situations, can be made substantially constant.

[0021] As a result, during normal use, the separation distance between the electron beam generator (e.g., electron gun) and the pivot is made to vary longitudinally properly during vibration of the rotating anode assembly, so that the separation distance between the point of impact of the electron beam and the electron beam generator is substantially constant. The electron beam generator can usually be provided with electron optics configured to focus the electron beam onto a focal point that coincides with the surface of the anode portion, so any change in the separation distance between the point of impact of the electron beam and the electron beam generator should mean that the anode surface moves away from the focal point of the electron beam, and in some cases, results in a change in the size of the electron beam on the anode surface. The longitudinal displacer can prevent this from occurring.

[0022] The longitudinal displacer can comprise a stepper motor or an actuator (e.g., a piezoelectric element) configured to adjustably displace the swivel anode assembly in the longitudinal direction (e.g., selectively, in a direction towards or away from the anode surface portion, or in a direction towards or away from the electron beam generator). For example, the longitudinal displacer can comprise a position sensor or transducer configured as described above to generate a signal / information representing the angular position of the swivel anode assembly relative to a stationary position. The longitudinal displacer may be configured to use this angular information to control (extend or retract) the position of the actuator or stepper motor in proportion to the angular position.

[0023] It should be noted that the angular position of the swivel anode assembly is directly converted to the corresponding position of the point of impact of the electron beam on the anode surface portion relative to the position of the point of impact of the collision when the swivel anode assembly is in the stationary position. For example, when the swivel anode assembly is vibrating, it will periodically pass through the position it should have when at rest (i.e., the reference position). The longitudinal displacer can be configured to displace the swivel anode assembly in a first direction away from the electron beam generator when the swivel anode assembly is moving away from the reference position, and to displace the swivel anode assembly in a second direction towards the electron beam generator (i.e., opposite to the first direction) when the swivel anode assembly is moving towards the reference position.

[0024] For example, let l0 be the radial distance from the pivot to the point of impact of the electron beam on the surface of the anode portion when the oscillating position of the pivoting anode assembly instantaneously coincides with the stationary position. Let l be the radial distance from the pivot to the point of impact of the electron beam on the surface of the anode portion when the angular position of the pivoting anode assembly relative to the stationary position is θ. When the anode tilts to θ, the distance from the pivot point to the electron beam impact point increases from l0 to l0sec(θ), and thus, the rate of change differential defines the rate of change of the radial distance as a function of the changing angular position, which is:

Equation

[0025] The longitudinal displacer can be configured to apply a changing longitudinal displacement (Δ) to the pivoting anode assembly, and thus, the rate of change of the longitudinal displacement (Δ) as a function of the changing angular position is:

Equation

[0026] The effect is to balance the change in the radial distance from the pivot to the point of impact of the electron beam on the surface of the anode portion caused by changing the angular position of the pivoting anode. By integrating the rate of change of the longitudinal displacement (Δ) as a function of the changing angular position, the value of the longitudinal displacement (Δ) at a given angular position (θ) is given as follows: Δ = -l0(sec(θ) - 1)

[0027] The negative sign indicates that when the pivoting anode assembly is moving away from the reference position, i.e., when the magnitude of θ is increasing, the displacement is in the aforementioned first direction away from the electron beam generator. The longitudinal displacer may be configured to apply a longitudinal displacement (Δ) that changes according to this equation to the pivoting anode assembly.

[0028] The spatial distribution of the mass of the swivel anode assembly can be asymmetric with respect to the pivot axis. For example, separate portions of the swivel anode assembly can be located on opposite sides of the pivot. In this configuration, the pivot can be located between the two ends of the anode assembly (see, for example, FIG. 5, where a part of the swivel anode structure, in particular, certain water-cooled pipes and connections are to the right of pivot 17).

[0029] Alternatively, substantially all of the mass of the swivel anode assembly may be located on the same side of the pivot. In this alternative configuration, the pivot may be located at one end of the anode assembly (see, for example, FIG. 1).

[0030] Desirably, the location of the center of gravity of the swivel anode assembly is closer to the pivot than to the anode surface portion. It has been found that this results in a lower moment of inertia, allowing the oscillation frequency of the oscillatory motion of the anode assembly to be made faster. The center of gravity of the swivel anode assembly is located between the pivot and the anode surface portion. Alternatively, the center of gravity of the swivel anode assembly and the anode surface may be located on the opposite side of the pivot. Careful balancing of the moment of inertia and the suspension assembly (e.g., the spring constant of the spring when used within the suspension assembly) has been found to improve sensitivity and mechanical efficiency. Preferably, the anode assembly is configured such that most of its mass is near the pivot / axis of rotation, so that the moment of inertia is relatively low. This means that the relatively high oscillation frequency required to minimize the peak instantaneous temperature rise of the anode surface can be achieved by an appropriate restoring force (e.g., an appropriate spring constant) from the suspension assembly. This means that the resonance frequency of the oscillatory motion of the anode assembly can be adjusted to increase or decrease by varying the restoring force (e.g., spring constant) of the element (e.g., spring(s)) either up or down.

[0031] It is desirable to achieve the highest possible resonance frequency within the mechanical limitations of the overall structure. The purpose of achieving a high resonance frequency is to limit the peak temperature at the electron beam collision spot when the electron beam scans across the anode surface. It is desirable to keep the center of gravity of the rotating anode assembly as close as possible to the pivot point within the other constraints of the structure.

[0032] Preferably, the rotating anode assembly comprises cooling channels configured to pass a cooling fluid (e.g., water) in a direction away from the pivot towards the anode surface portion. The cooling channels are preferably further configured to pass the cooling fluid back towards the pivot and away from the anode surface. This has been found to achieve a desirable effect such that the mass and the cooling flow have only a minor effect on the oscillatory motion of the anode assembly (e.g., its effect on the moment of inertia). This avoids or reduces the effect of interfering with the oscillatory motion due to the air flow space within the cooling flow.

[0033] Preferably, the suspension assembly and the displacer assembly are configured such that the displacer assembly is operable to reinforce the driving oscillatory motion of the anode assembly at a frequency of the oscillatory motion corresponding to the resonance frequency. Preferably, the range of the oscillation frequency can be from about 10 Hz to about 1 kHz. Preferably, the path length of the electron beam focus on the anode surface can be any value up to about 10 mm. Preferably, the length of the anode assembly measured from the pivot point to the anode surface can be in the range from about 50 mm to about 600 mm.

[0034] Preferably, the displacer assembly is configured to apply to the rotating anode assembly the periodic displacement force having a periodicity corresponding to the resonance frequency of the oscillation of the rotating anode assembly.

[0035] Desirably, the suspension assembly comprises one or more springs configured to apply a restoring force, and the resonance frequency of the vibration is substantially proportional to the value(s) of the spring constant(s) of the one or more springs. The desirable range of the spring constant(s) of such spring(s) can be calculated according to the desirable vibration frequency and considering the resulting load of the pivot assembly. Optionally, instead of such spring(s), piezoelectric actuators can be used and selected to obtain their spring rate and actuation force. Similarly, instead of such springs, a pneumatic actuator in which pressurized gas can act as a spring element may be used.

[0036] The resonance frequency of the system depends on the value of the moment of inertia of the mass supported by the pivot and the value of the spring rate / constant of the support spring of the suspension system. By proper selection of these values, the desirable resonance frequency of the oscillatory motion of the anode assembly can be achieved.

[0037] Preferably, the anode surface portion is electrically floating and maintained at a positive potential of about 5 kV or more, more preferably about 10 kV or more, or even more preferably in the range of about 5 kV to about 20 kV (e.g., about 15 kV). This simplifies the requirements for the control electronics for the electron beam generator (e.g., electron gun) of the system, because if an electron collision energy of about 15 keV is required, it is essentially close to the local ground potential. A further advantage is that secondary electrons backscattered from the anode surface cannot reach the surrounding ground potential region (e.g., in XPS, this should be the analysis chamber and the sample region) because their energy values are not high enough. As a result, in some embodiments, there is no need for a window surrounding the anode surface within the volume of the X-ray source, but in other situations, a window may be required for the purpose of blocking the entry of backscattered secondary electrons into the surrounding environment beyond the X-ray source.

[0038] The displacer assembly preferably comprises one or more actuators, more preferably a single actuator. The actuator(s) may comprise a piezoelectric actuator. The displacer assembly may be configured to apply a periodic displacement force in a direction substantially parallel to the direction of the restoring force applied by the suspension assembly. For example, if the displacer assembly comprises a linear actuator and the suspension assembly comprises one or more linear (e.g., helical) springs, the extension / retraction axis of the actuator may be substantially parallel to the longitudinal axis of the one or more linear springs. A particular advantage of this configuration is that it is convenient in terms of the constraints of other parts of the instrument. Alternatively, the displacer assembly may be configured to apply a periodic displacement force in a direction transverse to the direction of the restoring force applied by the suspension assembly.

[0039] The displacer assembly may be configured to apply a periodic displacement force to the anode assembly at a position laterally offset from the pivot such that the periodic displacement force generates a periodic displacement torque about the pivot with respect to the anode assembly. The position of the actuator may be selected such that the required amplitude of movement of the actuator corresponds to the required amplitude of movement of the anode surface and is within a desired range.

[0040] The displacer assembly may be configured to apply a periodic displacement force to the pivoting anode assembly to apply a torque to the pivoting anode assembly at a location on the pivoting anode assembly between the anode surface portion and the pivot.

[0041] The displacer assembly may be configured to apply a periodic displacement force to the pivoting anode assembly to apply a torque to the pivoting anode assembly at a location on the pivoting anode assembly, and the pivot is located between the said location of the applied torque and the anode surface.

[0042] It should be understood that the above-described aspects and features of the present invention implement the present invention from the perspective of an X-ray source. However, the present invention is further or alternatively configured for use within an X-ray source that does not include an electron beam generator but has an electron beam generator for generating a beam of electrons induced along the beam axis with respect to the anode surface portion, and is intended to be manufactured and sold as an anode device. In other words, each of the above-described aspects and features of the X-ray source according to the present invention in its first aspect is equally applicable to the present invention in its second aspect where the electron beam generator is omitted.

[0043] For example, in the second aspect, the present invention can provide an anode device for an X-ray source having an electron beam generator for generating a beam of electrons induced along the beam axis for use in X-ray generation by electron bombardment of the anode device, the anode device comprising: a rotating anode assembly comprising a pivot and an anode surface portion mounted on the pivot for receiving electrons in the electron beam to generate X-rays; a suspension assembly configured to apply a restoring force to the rotating anode assembly to bias the rotating anode assembly towards a stationary position where the anode surface portion is relatively stationary with respect to the pivot; a displacer assembly configured to apply a periodic displacement force to the rotating anode assembly to displace the rotating anode assembly from the stationary position. comprising, wherein the position of the anode assembly is configured to oscillate about the stationary position in a lateral motion with respect to the beam axis in response to the periodic displacement force and the restoring force, thereby oscillating the anode surface portion laterally across the beam axis.

[0044] In this aspect, the present invention can be manufactured and sold without including an electron gun, as an "anode device", that is, as a package including a rotating anode assembly, a suspension assembly, and a displacer assembly. The "anode device" may be inserted into an existing X-ray apparatus that already has an electron beam generator, as a replacement / upgrade of the anode that has been in the existing X-ray apparatus. Thereby, the X-ray gun product can be upgraded together with this anode assembly.

[0045] It should be understood that the above-described aspects and features of the present invention implement corresponding methods for generating X-rays encompassed by the present invention. For example, in a third aspect, the present invention may provide a method for generating X-rays by electron collisions with an anode, the method comprising: generating a beam of electrons induced along a beam axis; receiving electrons from the beam of electrons at an anode surface portion, thereby generating X-rays, wherein the anode surface portion is mounted on a rotating anode assembly having a pivot, which is mounted on a suspension assembly; applying a restoring force to the rotating anode assembly via the suspension assembly to bias the rotating anode assembly towards a stationary position where the anode surface portion is relatively stationary with respect to the pivot; applying a periodic displacement force to the rotating anode assembly to displace the rotating anode assembly from the stationary position including whereby the position of the anode assembly is vibrated about the stationary position in a lateral motion with respect to the beam axis in response to the periodic displacement force and the restoring force, thereby vibrating the anode surface portion laterally across the beam axis.

[0046] Thus, the present invention can provide an apparatus and method for increasing the maximum electron beam power density that the anode part can withstand without damage, which is achieved by rapid lateral vibration of the anode surface to spread heat dissipation on the anode surface, whereby a higher total power load for a given surface temperature is allowed. This can be used in any application when a more intense X-ray beam, generated by electron collisions with the target material, is required than would be allowed by a completely stationary anode. The present invention can provide an electron collision X-ray source for use in the field of XPS, where the requirement for a high signal level for rapid XPS analysis leads to the use of a high electron beam power density. Thus, in prior art devices, this leads to a significant temperature rise at the anode surface and, in extreme cases, to melting of the metal surface layer and subsequent failure. Current commercially available X-ray sources in XPS instruments operate with the anode in a fixed position. In contrast, according to the present invention, the anode structure can be rapidly vibrated laterally under the electron beam to expose a larger area of the anode surface and reduce the average power density. Thus, the anode surface can withstand a higher total power before damage occurs, which leads to an increase in the XPS signal intensity compared to what might be achievable by other means with an electron beam generator. The position of the electron beam generator is preferably fixed in space (i.e., relative to the pivot), and as a result, the position of the electron beam is similarly fixed in space. As a result, the generated X-rays originate from an area determined by the lateral size of the electron beam rather than by the lateral size of the area on the anode surface illuminated by the electron beam.

[0047] In XPS, the signal intensity in a small analysis area is an important metric for the performance and competitiveness of commercial XPS instruments. To achieve high intensity, various strategies are utilized. The strategies include maximizing the heat dissipation capacity of the anode through the use of a high-speed water flow inside the anode structure and a high thermal conductivity diamond embedded in the area collided by the electron beam, and a high-efficiency electron optical system for collecting as many of the emitted photoelectrons as possible for the strategy. Even with these strategies, the surface temperature of the anode can quite easily reach the melting point of the anode coating (typically aluminum), thereby potentially causing rapid deterioration of the metal surface. As is well known, since very high temperatures can be achieved by electron beam welding of high melting point materials, it is not surprising that aluminum can be easily melted by a focused electron beam.

[0048] It is well known in the art to reduce the power density, and thus the anode surface temperature, by spreading the electron beam over the anode surface. The common way to achieve this is to use a rotating anode structure. This allows the electron beam to be spread around the periphery of the anode disk while maintaining the apparent size of the X-ray source at a size closely related to the apparent size of the colliding electron beam. These systems often have high melting point materials and a low duty cycle to avoid the need for continuous cooling. In the case of XPS instruments, continuous cooling is required because of the occasional long acquisition times that necessitate continuous operation. This presents a difficult technical problem of cooling an anode structure that rotates at high speed in an ultra-high vacuum (UHV) state.

[0049] The present invention enables an increase of up to about six times or more compared to prior art systems at the electron beam power level at which the X-ray source can be operated, which results in a significant commercial benefit. The present invention enables the anode to be rapidly vibrated in a direction parallel to the anode surface. Typically, in a commercially available XPS measuring instrument, the electron beam can have a diameter of 150 μm. Thus, with anode vibration with an amplitude “A” mm, this will enable the power delivered to this 150 μm diameter to spread over an area of 150×A×1000 μm 2 plus the area of the spot. For example, when A = 1, this enables illumination of approximately 9.5 times the area of the anode surface with a constant electron beam intensity. Thus, this should enable an increase of up to about 9.5 times the electron beam power and thus the generated X-ray intensity.

[0050] The rotating anode assembly can be an elongated structure (e.g., a cylindrical structure) where the anode surface is located at one longitudinal end of the elongated structure and the pivot is located at or adjacent to the other opposite longitudinal end of the elongated structure. For example, the longitudinal length of the rotating anode assembly can be at least about 50 mm long, or at least about 100 mm long, or at least about 150 mm long, or at least about 200 mm long, or at least about 250 mm long, or at least about 300 mm long, or at least about 350 mm long, or at least about 400 mm long, or at least about 450 mm long. Most preferably, the longitudinal length of the rotating anode assembly is in the range of about 50 mm to about 500 mm. For example, the longitudinal length of the rotating anode assembly can be in the range of about 100 mm to about 450 mm, or about 200 mm to about 500 mm, or more preferably in the range of about 250 mm to about 400 mm, for example, about 300 mm long.

[0051] An X-ray source according to any aspect may be configured such that the anode surface portion during use is electrically floating and is maintained at a positive potential (e.g., 5 kV or higher). Alternatively, an X-ray source according to any aspect may be configured such that the anode is at ground potential (earth) and the electron emitter (cathode) within the electron gun is at a negative potential. The value of the negative potential may be selected to be high enough to provide the collision energy required for X-ray generation. This version of the X-ray source may be used to directly irradiate a test object with X-rays. For example, in that case, the analytical application will accept both characteristic radiation and bremsstrahlung.

[0052] In a further aspect, the invention may provide an X-ray monochromator comprising an X-ray source disclosed according to any aspect herein. The ability of the invention to compensate for the movement of the X-ray source spot (i.e., the electron beam collision spot on the anode surface) caused by the movement of the anode means that the invention can significantly reduce the variation in the intensity, energy, or position change of the reflected X-ray spot on the surface of the sample being irradiated with X-rays from the X-ray source during use.

[0053] To create an X-ray radiation beam with a narrow wavelength distribution, a monochromator optical element (e.g., a single crystal) can be used. The monochromator optical element functions by receiving X-rays from an X-ray source and reflecting the incident X-ray wavelength that follows Bragg's law. Thereby, on a dedicated instrument or beamline for further purposes, a defined wavelength of X-ray radiation can be selected. In this way, the X-ray monochromator operates through a diffraction process according to Bragg's law. The X-ray monochromator optical element is similar to a diffraction grating monochromator and spectrometer in the visible part of the spectrum. The X-ray monochromator optical element can include a crystal lattice structure selected for this purpose. When the lattice spacing of the crystal is precisely known, the observed diffraction angle can be used to select the X-ray wavelength. Due to the high-sensitivity wavelength dependence of Bragg reflection exhibited by a certain material (e.g., quartz crystal), a small portion of the continuous spectrum of radiation can be separated. This separated spectral portion is considered to be practically monochromatic, i.e., the separated spectral portion has a narrower energy spread than the characteristic X-ray lines separated therefrom. Furthermore, when the crystal lattice structure is bent into an appropriate shape, the separated X-rays can be induced into an intense focusing spot that will be used for subsequent XPS analysis.

[0054] The present invention includes the described aspects and combinations of preferred features, except when such combinations are clearly unacceptable or particularly avoided.

[0055] Here, embodiments and experiments for explaining the principles of the present invention will be discussed with reference to the accompanying drawings.

Brief Description of the Drawings

[0056]

Figure 1

Figure 2A

Figure 2B

Figure 3A

Figure 3B

Figure 4

Figure 5

Figure 6A

Figure 6B

DETAILED DESCRIPTION OF THE INVENTION

[0057] Here, aspects and embodiments of the present invention will be discussed with reference to the accompanying drawings. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0058] FIG. 1 shows a schematic cross-sectional view of an X-ray source according to an embodiment of the present invention. The X-ray source is configured to generate X-rays 11 by the collision of an electron beam 3 on the surface portion of the anode 2. The X-ray source includes an electron beam generator 5 for generating an electron beam 3 of electrons induced to the anode 2 along the beam axis. The anode 2 is mounted on the end portion of a rotating anode assembly 1 for receiving electrons in the electron beam 3 to generate X-rays. The anode surface portion is electrically floating and maintained at a positive potential of 5 kV or more. The proximal end portion of the rotating anode assembly is mounted on a pivot 17 having one or more flexible bearings configured to bend about the pivot axis of the rotating anode assembly. In an alternative configuration, the anode is at ground potential and the electron emitter in the electron gun is at a high negative potential to provide the collision energy required for X-ray generation. If desired, a high voltage insulator (not shown in FIG. 1) may be provided for insulating the rotating anode assembly 1 in order to maintain the potential of the anode at ground potential in a suitable manner that will be readily apparent to those skilled in the art.

[0059] The X-ray source includes a suspension assembly 15b configured to apply a restoring force to the rotating anode assembly 1 with a plurality of springs laterally offset from the pivot 17 to bias the rotating anode assembly toward a stationary position (as shown in FIG. 1) where the anode surface portion 2 is relatively stationary with respect to the pivot 17. A displacer assembly 21 is configured to apply a periodic displacement force 27 to the rotating anode assembly 1 such that the rotating anode assembly 1 is displaced from the stationary position. As a result, the angular position of the rotating anode assembly 1 is configured to oscillate about the stationary position in a motion transverse to the beam axis 3 in response to the periodic displacement force 27 and simultaneously in response to the restoring force from the suspension spring 19. Thereby, the rotating anode assembly vibrates the anode surface portion 2 laterally (in direction 23) at its end portion intersecting the beam axis of the electron beam.

[0060] Thereby, the pivot anode assembly oscillates about a stationary position such that the intersection of the electron beam axis on the anode surface portion 2 scans a substantially straight path across the anode surface portion. The pivot 17 comprises a single pivot axis about which the pivot anode assembly pivots when the angular position of the pivot anode assembly is oscillated. The angular position can be considered as the angle defined at the pivot by the longitudinal axis of the pivotable anode assembly 1 with respect to the longitudinal axis of the anode assembly 1 when the anode assembly 1 is in its stationary position.

[0061] In particular, the spatial distribution of the mass of the pivot anode assembly 1 is asymmetric with respect to the pivot axis of the pivot 17. In this example, the position of the center of gravity of the pivot anode assembly 1 is located between the pivot 17 and the anode surface portion 2. The location of the center of gravity of the pivot anode assembly is closer to the pivot than to the anode surface portion. The pivot anode 1 has an elongated shape such that its longitudinal dimension in the direction of its longitudinal axis extending from the pivot 17 to the anode surface portion 2 exceeds its dimension in any direction perpendicular to the longitudinal axis. In one example, the pivot anode assembly can have a maximum lateral dimension (width) of about 70 mm and a length of about 100 mm. In contrast, in other examples, the pivot anode assembly can have a maximum lateral dimension (width) of about 70 mm and a length of about 400 mm.

[0062] In other embodiments, such as those shown in FIGS. 3, 4, and 5, the position of the center of gravity of the pivot anode assembly 1 and the position of the anode surface portion 2 are located on the opposite side of the pivot, and the location of the center of gravity of the pivot anode assembly is still closer to the pivot than to the anode surface portion.

[0063] The displacer assembly is configured to apply a periodic displacement force having a periodicity corresponding to the resonance frequency of the oscillation of the swivel anode assembly. The periodic displacement force is applied at a location on the swivel anode assembly that is laterally offset from the longitudinal axis of the swivel anode assembly and further longitudinally offset from the pivot in a direction toward the anode surface portion. As a result, a torque on the swivel anode assembly is generated at a location on the swivel anode assembly between the anode surface portion and the pivot. That is, the pivot is farther from the anode surface portion than from the location where the force is applied. In other embodiments, such as those shown in FIGS. 3, 4, and 5, the periodic displacement force applies a torque to the swivel anode assembly at a location on the swivel anode assembly such that the pivot is located between the location where the torque is applied and the location of the anode surface. That is, the pivot is closer to the anode surface portion than to the location where the force is applied in these other embodiments.

[0064] The suspension assembly includes one or more springs configured to apply a restoring force. The resonance frequency of the oscillation is substantially proportional to the value of the combined spring constant of the one or more springs. In the example of FIG. 1, two springs are utilized, where the two springs are each mechanically coupled to the proximal end portion of the swivel anode assembly at a location that is laterally offset from the longitudinal axis of the swivel anode assembly and the pivot 17 (the longitudinal axis passes through the pivot). The two springs 19 share the same dimensions and spring constant. The displacer assembly 21 includes a single actuator, such as a piezoelectric actuator, and is configured to communicate with a control unit 25 configured to issue a control signal to the actuator to control the extension and retraction of the actuator to control both the amplitude and the frequency of the periodic displacement applied by the actuator.

[0065] In all embodiments of the present invention, the proper alignment of the center of gravity of the swivel anode assembly relative to the pivot in response to a relatively small periodic displacement at the resonant frequency of a mechanical system comprising a swivel anode assembly and a suspension assembly mechanically coupled thereto has a significant benefit in reducing the moment of inertia of the swivel anode assembly to allow for rapid oscillatory movement 23 of the anode surface portion 2 with large amplitude angular displacement.

[0066] In another context, the moment of inertia of a rigid body, known as the mass moment of inertia, angular mass, second mass moment, or most precisely, the moment of rotation, is an amount that determines the torque required for a desired angular acceleration about an axis of rotation, similar to how mass determines the force required for a desired acceleration. The moment of inertia of a rigid body depends on the mass distribution of the rigid body and the chosen axis, and a larger moment requires a larger torque to change the rotational speed of the rigid body. In the case of a point mass, the moment of inertia is simply the mass multiplied by the square of the perpendicular distance to the axis of rotation. The moment of inertia of a rigid body composite system is the sum of the moments of inertia of its component subsystems (all taking the same axis).

[0067] FIG. 6A shows a schematic depiction of the basic elements of the configuration of forces, motion, and mass distribution according to an example of the present invention. Here, the suspension assembly comprises one spring with a spring constant k. The following analysis is applicable to other embodiments having a suspension system including additional springs as shown in FIGS. 1 and 3, 4, and 5, with modifications as necessary.

[0068] Consider a swivel anode assembly having a mass distribution equivalent to a center of gravity "m" located at a distance "h" from a pivot "p" of the swivel anode assembly. The anode surface portion (not shown) defines the distal end of the swivel anode assembly, distal from the pivot. The suspension assembly comprises a lateral arm "a" of length "L", the lateral arm "a" extending laterally from the longitudinal axis of the swivel anode assembly at a point adjacent to the pivot "p" located at a longitudinal distance "h" from the center of gravity "m". The suspension assembly comprises a spring of spring constant "k" configured to apply a restoring force to the distal end of the lateral arm "a", the restoring force biasing the swivel anode assembly towards a rest position (as shown) where the swivel anode assembly is relatively stationary with respect to the pivot. A displacer assembly (not shown) is configured to apply a periodic displacement force "F" to the lateral arm "a" to displace the swivel anode assembly from the rest position. As a result, in response to the periodic displacement force and the restoring force, the position "x" of the center of gravity "m" of the swivel anode assembly oscillates about the rest position in a lateral motion with respect to its longitudinal axis. This causes the anode surface portion (not shown) at the distal end of the swivel anode assembly to oscillate laterally in a direction intersecting the longitudinal axis and reciprocating. The nature of this vibration with respect to the center of gravity of the swivel anode assembly, as well as the characteristics of the suspension assembly and the displacer assembly, are as follows.

[0069] Let the periodic displacement force "F" take the following equation: F1 = F0sin(ωt)

[0070] The torque applied to the swivel anode assembly by this displacement force is: T1 = F1L = F0Lsin(ωt) That is.

[0071] By displacing the spring of the suspension assembly by a displacement "y" by the displacement force, i.e., compressing or stretching it, the lateral arm "a" pivots about the pivot point "p" by a small angle θ, and as a result, the spring force exerted on the lateral arm by the spring is: F2 = ky = kLθ It is as follows.

[0072] The torque applied to the swivel anode assembly by this spring force: T2 = F2L = kL 2 θ It is as follows.

[0073] In response to the net effective applied torque T1 - T2, the center of gravity "m" of the swivel anode assembly is displaced by a lateral displacement "x". As a result of this acceleration of the mass corresponding to the force, and its longitudinal separation by a distance "h" from the pivot point, this combines to give the net torque:

Equation

[0074] Here, for small θ, we use x = hθ. Therefore, the equation of motion of the center of gravity "m" of the swivel anode assembly is:

Equation

[0075] That is:

Equation

[0076] As a first approximation, if it is reasonably assumed that there is a damping force (F d ) proportional to the angular velocity of the motion of the swivel anode assembly for this system, the final equation of motion of the center of gravity "m" of the swivel anode assembly is:

Equation

[0077] Here, α is the damping constant of the damping force F d = αdθ / dt. The solution of this differential equation takes the following form:

Number

[0078] Here, θ0 is the amplitude factor, M is the magnification of the amplitude, where:

Number

Number

[0079] The response of the swivel anode assembly to the periodic force (F) provided by the displacer assembly depends on the ratio ω / Ω of the forcing frequency to the natural frequency and the damping component ζ. Figure 6B shows the change in the amplitude magnification M in response to the change in these components. The resonant drive frequency ω at which the maximum amplitude “M” occurs (resonance) Res is slightly less than the undamped natural frequency Ω:

Number

[0080] The natural frequency Ω and the resonant drive frequency ω Res are, respectively, inversely proportional to the moment of inertia obtained by I m = mh 2 of the center of gravity of the swivel anode assembly. It can be understood that by the “Parallel Axis Theorem”, when a system of mass “m” is rotated about an axis offset from the center of gravity of the system by a displacement “h”, the moment of inertia of the system is defined as the sum of the following: (1) the moment of inertia of the system (I CM ) when considering the first axis of rotation passing through the center of gravity of the system, and (2) the moment of inertia of the center of gravity of the system (I m = mh2 )。That is: I = I CM + I m = I CM + mh 2

[0081] The inventors have found that by reducing the value of the offset distance "h", the moment of inertia (I) of the swivel anode assembly is reduced, thereby reducing the energy required to vibrate the anode assembly. Moreover, this further causes the resonance drive frequency ω when the maximum amplitude "M" occurs due to the vibrational motion of the anode surface portion intersecting the colliding electron beam Res to be higher. As a result, it has been found that the time period during which the electron beam spot-collides with a given surface point on the anode surface is very effectively reduced, thereby reducing the heating effect at the point of collision of the electron beam on the anode material and increasing the electron beam intensity without overheating the anode surface. The resonance frequency and amplitude can be easily changed by an appropriate selection of the spring constant "k" of the suspension assembly and / or the lateral arm length "L", and / or by the amplitude (F0) of the driving force applied by the displacer assembly.

[0082] In FIG. 1, the X-ray source includes a longitudinal displacer 29 coupled to the proximal end of the pivot and configured to displace the pivot adjustably, and a rotating anode assembly 1 connected to the pivot in a direction substantially parallel to the plane of vibration of the anode surface portion 2. The longitudinal displacer includes an actuator (e.g., a piezoelectric element) configured to displace the rotating anode assembly adjustably in the longitudinal direction. This displacement is controlled so as to displace the anode surface portion 2 adjustably in the longitudinal direction toward the electron beam generator 5 or away from the electron beam generator 5 in the longitudinal direction. The longitudinal displacer 29 includes a position transducer configured to determine the angular position of the anode surface portion (i.e., relative to the pivot) during vibration of the anode surface portion. This angular position corresponds to the angular position of the rotating anode assembly. The longitudinal displacer adjusts the pivot 17 displaceably in response to the determined angular position of the anode surface portion, thereby displacing the rotating anode assembly 1 so as to substantially keep constant the separation distance between the electron beam generator 5 and the point of collision of the electron beam 3 on the anode surface portion during vibration of the anode surface portion. By maintaining this separation distance constant, the point of collision of the electron beam 3 on the anode surface portion does not move along the electron beam axis 3. This maintains the focus of the electron beam on the anode surface and thus eliminates the change in the size of the focus associated with the angular position. The position transducer is configured to directly sense the angular position, and this sensing can be provided by a sensor integrated with the pivot 17, such as a strain gauge attached to the bent portion (angular displacement) of the pivot or to an additional element that bends according to the angular displacement of the pivot.

[0083] The longitudinal displacer converts the angular position of the rotating anode assembly into a correction value for this longitudinal distance so as to keep constant the longitudinal distance of the position of the point of collision of the electron beam on the anode surface portion relative to the position of the point of collision when the rotating anode assembly is in the stationary position.

[0084] Referring to FIG. 2A, when the swivel anode assembly is vibrating, it periodically passes through a reference position 1a that it would have at rest. The longitudinal displacer 29 is configured to displace the pivot 17 in a first direction away from the electron beam generator 5 when the swivel anode assembly moves away from the reference position 1a to a position 1b having an angular displacement θ. This serves to displace the swivel anode assembly 1 away from the electron beam generator 5. The point "a" of the electron beam collision on the anode surface is separated from the pivot 17 by a separation distance l0 at rest, but the point "b" of the collision is separated from the pivot 17 by a separation distance l0 + Δ when the anode assembly 1 is at the position 1b having an angular displacement θ. This means that the collision point will move closer to the electron gun and thus out of the focal region of the electron beam. At this angular position, the previous collision point "a" has moved along an arc trajectory 24 away from the electron beam due to the angular displacement of the anode assembly. Due to this arcing motion, the adjacent part of the flat surface of the anode tilts towards the electron gun 5, thereby moving the collision point "b" closer to the electron gun.

[0085] Therefore, the longitudinal displacer applies a changing longitudinal displacement (Δ) to the swivel anode assembly, and thus the rate of change of the longitudinal displacement (Δ) as a function of the changing angular position is:

Equation

[0086] The effect is to balance the change in the longitudinal distance from the pivot 17 to the point of collision of the electron beam 3 on the surface 2 of the anode part caused by changing the angular position of the swivel anode assembly 1. The longitudinal displacer applies a changing longitudinal displacement (Δ) to the swivel anode assembly at a given angular position (θ): Δ = -l0(sec(θ) - 1) is.

[0087] This displacement is shown graphically in Figure 2B, where, for example, the parameters are l0 = 300 mm and the maximum value of θ = 0.01 radians.

[0088] In accordance with this relationship, the longitudinal displacer moves the anode surface 2 as a whole away from (or towards) the electron gun 5 as the angular displacement of the anode assembly increases (or decreases).

[0089] The swivel anode assembly 1 includes a cooling channel 31 configured to pass a cooling fluid (e.g., water) in a direction away from the pivot towards the anode surface portion 2. The cooling channel is configured to pass the cooling fluid in a direction 32 towards the pivot such that the cooling fluid is in thermal communication with the back side of the anode surface (i.e., the side opposite the surface portion that receives electrons from the electron beam 3), thereby enabling the cooling fluid to remove heat from the anode surface portion. The longitudinal separation 33 within the cooling channel defines a first conduit part in communication with the back side of the anode surface portion for passing the flow of the cooling fluid towards the anode surface portion, and a second conduit part for passing the flow of the cooling fluid away from the anode surface. The first and second conduit parts may be separate pipes arranged in fluid communication at their respective end portions adjacent to the back side of the anode surface, where the fluid flows in the reverse direction while remaining in thermal communication with the back side of the anode surface portion as it proceeds from the first conduit part to the second conduit part.

[0090] The X-ray sources of FIGS. 1 and 3, 4, and 5 include a lateral displacer 37 (not shown in FIG. 3) comprising a stepper motor and a coupling 36 with the swivel anode assembly configured to displace the swivel anode assembly and the suspension assembly as a unit in a direction 35 transverse to the plane of vibration of the anode surface portion. These components are shown in FIGS. 1 and 3

Number

[0091] For this reason, the X-ray source includes a lateral displacer 37 configured to adjustably displace the swivel anode assembly 1 in a direction 35 transverse to the plane of vibration of the anode surface portion 2. The lateral displacer 37 comprises a stepper motor 36 configured to laterally displace the swivel anode assembly 2 and the suspension assemblies 19, 30 to different positions on a plane parallel to the anode surface portion 2. Thereby, when a particular area has been exhausted, the unused area of the anode surface can be used, thereby reducing the frequency of anode replacement. This enables the position of the anode assembly in a direction transverse to the plane of vibration to be adjustably displaced to an adjusted position and enables the anode assembly to vibrate in (or parallel to) a new plane of vibration that is substantially parallel to the plane of vibration in which the displacement takes place. The anode surface 2 is planar and is tilted towards the window 9 and tilted with respect to the electron beam axis 3. The lateral displacer is configured to adjustably displace the position of the anode assembly in the tilted direction 35 as well by pushing the portion of the X-ray source where the pivot is mounted along a tilted sliding / bearing surface 32 that is substantially parallel to the anode surface. As a result, the adjusted position of the anode assembly is such that the focus of the electron beam continues to coincide with the anode surface and the distance between the electron supply source and the point of collision of the electron beam on the anode surface remains substantially unchanged.

[0092] Example Example 1 Referring to FIGS. 1, 3, 4, and 5, the metal anode 2 is bombarded by an electron beam 3 from an electron gun 5. The electron gun 5 has focusing means therein which enable the lateral size and energy of the electron beam to be varied to conform to the required operating conditions. Typically, the electron beam may have an energy of 15 keV and a diameter of 0.15 mm when it strikes the metal anode 2. These components are installed within a vacuum chamber 7 which is connected to the vacuum of the XPS chamber. A thin window or barrier material 9 may be included to prevent backscattered electrons from the metal anode 2 from reaching the XPS chamber, and the X-rays 11 generated by the impact of the electrons 3 on the metal anode 2 can pass through the window 9 and be used for the subsequent excitation of photoelectrons for XPS analysis.

[0093] The metal anode 2 is attached to the vacuum chamber 7 via a flexible bellows 13 which allows the anode to move relative to the chamber but excludes ambient air to maintain a high or ultra-high vacuum environment suitable for the operation of the electron gun 5. The metal anode 2 has internal water channels 31 which carry a flow of water 32 to the metal anode 2, and these water channels are arranged in a range up to a point near the tip of the rotary anode assembly 1 so as to maximize the cooling efficiency between areas heated by the electron beam 3. Thereby, the anode is liquid-cooled via these coaxial water channels which feed water to the tip of the rotary anode assembly and return the water to the outside.

[0094] The metal anode 2 is connected to rigid structures 15a, 15b attached to the vacuum chamber via several elements. These are a pivot 17, a spring element 19 of a suspension assembly, and an actuator 21 of a displacer assembly. The pivot 17 (e.g., a hinge or a flexure bearing) has its pivot axis orthogonal to the plane of the figure such that the movement at the tip of the anode assembly 1 and at that tip of the metal anode 2 is as indicated by the two-directional arrow 23.

[0095] Although the spring element 19 of the suspension assembly is a compression coil spring in this example, any suitable means for providing spring action may be used. The actuator 21 is a piezoelectric actuator that exerts a force between the rigid structures 15a, 15b and the pivot 17 mounted behind the metal anode 1. This actuator 21 exerts a displacement force related to the electrical signal from the electrical signal source 25 in the direction of the arrow 27 directed substantially in two directions. This force is created periodically in time such that the anode assembly 1 is caused to move along a path indicated by the arrow 23 directed substantially in two directions with the metal anode 2 at its tip.

[0096] To obtain a slow rate of change of the displacement force 27, the movement along the path 23 will depend on the combined spring rate of the displacement force applied by the actuator 21 and the spring element 19. If the rate of change of the displacement force 27 increases steadily, the amplitude of the movement 23 will increase substantially as it approaches the resonance frequency of the anode assembly 1 supported by the pivot 17 and the spring 19. This is a favorable operating condition where a small excitation force from the actuator 21 leads to a large amplitude of the movement 23.

[0097] The resonance frequency of the system as described depends on the moment of inertia of the mass supported by the pivot 17 and the spring rate of the support spring 19, and to a lesser extent, the inherent spring rate of the actuator 21. The flexible bellows 13 will also contribute to the overall spring rate of the system. This means that the resonance frequency can be adjusted upward or downward by changing the spring constant of the spring element 19 either upward or downward. The change in the spring constant of the spring can be achieved simply by replacing the spring.

[0098] The actuator 21 more preferably includes a transducer that enables real-time measurement of its length / extension. The signal from the transducer may be used as a control signal for the longitudinal actuator 29.

[0099] The effect of the movement of the surface of the metal anode 2 along the path 23 is to expand the area struck by the electron beam 3 along a length determined by the length of the path 23. Since the position of the electron beam 3 is fixed within the space and within the reference frame of the vacuum chamber 7, this means that the generated X-ray 11 appears to originate from the intersection area between the electron beam 3 and the surface of the metal anode 2. The heat to be dissipated in the metal anode due to the impact of the electron beam spreads over an area (A) equal to A = W × L + the area of the electron beam spot, where W is the width of the electron beam 3 in the direction out of the plane of the figure and L is twice the amplitude of the movement 23 (i.e., the "peak-to-peak" displacement). Adopting a typical example, the electron beam diameter 3 can be 150 μm and twice the amplitude of the movement 23 (i.e., the "peak-to-peak" displacement) can be 1 mm.

[0100] This means that the average power density in this elongated area is 9.5 times lower. If the frequency of the movement is high enough, the power of the electron beam (which is the product of the electron acceleration voltage and the current in the beam) can be increased by approximately this factor for the same surface temperature of the metal anode 2. By this means, the intensity of the X-ray beam 11 can be increased proportionally to the electron beam current. Therefore, this necessarily leads to an increase in the XPS signal intensity, which brings commercial benefits.

[0101] Due to the pivot 17, the path 23 of the anode tip 2 is a very shallow arc. For typical dimensions of the anode and the length of the path 23, the depth of the arc is a few μm. To keep the separation distance constant between the electron gun 5 and the point of impact of the electron beam on the anode surface 2, the piezoelectric element 29 expands and contracts in accordance with the angular displacement of the anode assembly 1 from its stationary (reference) position, thereby keeping the separation distance in question constant. To achieve this purpose, the piezoelectric element 29 is controlled using information regarding the extension / retraction state of the actuator 21 of the displacement assembly provided by the position transducer within the actuator 21.

[0102] The X-rays emitted along path 11 are passed to an X-ray mirror (not shown), which refocuses the X-rays onto the sample surface, reducing the spread of the X-ray energy and making them suitable for XPS analysis. The X-ray mirror assembly is susceptible to the position of the X-ray source in a direction transverse to the shown path 11. Such small changes can further result in the position of the refocused X-ray spot on the sample and the reflected X-ray energy, which vary with the amount of displacement in the transverse direction. Both of these effects can be detrimental to the quality of the subsequent XPS spectrum. The ability of the present invention to compensate for the movement of the X-ray source spot (i.e., the electron beam collision spot on the anode surface) caused by the movement of the anode means that the present invention can significantly reduce the variation in the intensity, energy, or position change of the reflected X-ray spot on the surface of the sample being irradiated with X-rays from the X-ray source during use.

[0103] The extended / retracted state of the actuator 21 is directly interrelated to the angular displacement of the pivoting anode assembly. For example, referring to FIG. 6a, to obtain a force “F” at a lateral distance “a” from the pivot “p” in a part of the pivoting anode assembly, the extension “δ” of the actuator results in an angular displacement “θ” with respect to the pivoting anode assembly, which is given by:

Equation

[0104] Optimal operating conditions can be achieved at the highest vibration frequency and amplitude. The pivoting anode assembly can comprise bellows (e.g., edge-welded bellows) connected to the remainder of the X-ray gun structure via edge-welded bellows utilized to enable precise alignment using the lateral displacer described above and alignment with the electron gun of the anode.

[0105] The example of FIG. 5 shows a mechanical configuration different from that of FIG. 1. The main difference is that the pivot is in front of, rather than behind, the displacement actuator 21 (in FIG. 5, 21 is an extension rod insulated from the actuator 25), as in FIG. 1. FIGS. 3A and 3B are simplified schematic views of the main elements of the design of FIG. 5. Here, the difference is shown as a change in the structure of the “suspension assemblies” 19, 30, and 30a. In this example, the base 42 of the swivel anode assembly 1 is fixed to a “swinging basket” type configuration 30, and the basket arm 30a is attached to the remainder of the frame 15b via the pivot 17. It is suspended behind the pivot 17 and has a spring 19 that couples it to the frame 15a on either side of the pivot 17. These springs provide a restoring force to bias the “basket” to a rest position corresponding to the rest position of the anode assembly 1. By applying a periodic displacement force 27 to one side of the base of the basket, the basket is forced to vibrate, thereby vibrating the anode assembly fixed thereto. In the schematic view of FIG. 3A, the displacement actuator 21 acts on the back surface of the “swinging basket” configuration 30. In the schematic view of FIG. 3B, the displacement actuator 21 acts on the lateral side of the “swinging basket” configuration 30. The advantage of this configuration is that since the longitudinal actuator 29 is on the moving side of the pivot point 17, it will not be subject to the “preload” spring force of the spring 19, and thus, an actuator that is not as robust (e.g., inexpensive) may be sufficient.

[0106] The main difference between the design shown in FIG. 3 and the design shown in FIG. 5 is that in FIG. 5, the plane containing the vibration direction 23 is not in the same plane as the plane containing the electron beam and the outgoing X-ray beam, as in FIG. 3, but is orthogonal. This feature of the design shown in FIG. 5 may be advantageous for the angled end of the anode, which will increase the longitudinal movement of the X-ray spot deviated from the rest position in another situation. This angled anode construction is convenient in the use of the X-ray gun of FIG. 5 when used in an X-ray monochromator. The anode surface in FIG. 3 is not angled with respect to the electron beam incident direction.

[0107] Regarding the means for performing the functions expressed or disclosed in those specific forms, or regarding the methods or processes for obtaining the disclosed results, the features disclosed in the foregoing description, in the following claims, or in the accompanying drawings may, as appropriate, be used separately or in any combination of such features in their various forms to implement the present invention.

[0108] The present invention has been described in conjunction with the above exemplary embodiments, but many equivalent modifications and variations will become apparent to those skilled in the art when the present disclosure is provided. Therefore, the exemplary embodiments of the present invention described above are considered to be illustrative but not limiting. Various changes can be made to the described embodiments without departing from the spirit and scope of the present invention.

[0109] To avoid any doubt, any theoretical explanations provided in this specification are provided to improve the reader's understanding. The inventors do not wish to be limited by any of these theoretical explanations.

[0110] The title of any paragraph used in this specification is for organization purposes only and should not be construed as limiting the described subject matter.

[0111] Throughout this specification, including the following claims, unless the context requires otherwise, the words "comprise" and "include" and variations such as "comprises," "comprising," and "including" are to be understood to mean the inclusion of the stated integers, steps, or groups of integers or steps, but not the exclusion of other integers, steps, or groups of integers or steps.

[0112] It should be noted that, as used in this specification and the appended claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from "about" one particular value and / or to "about" another particular value. When such a range is expressed, another embodiment includes from one particular value and / or to another particular value. Similarly, when values are expressed as approximations by use of the preceding "about", the particular value is understood to form another embodiment. The term "about" with respect to a numerical value is optional and means, for example, + / - 10%.

Claims

1. An X-ray source configured to generate X-rays by electron collisions with an anode, an electron beam generator for generating a beam of electrons induced along a beam axis, a pivot, and a rotating anode assembly comprising an anode surface portion mounted on the pivot for receiving electrons in the electron beam for generating X-rays, a suspension assembly configured to apply a restoring force to the rotating anode assembly to bias the rotating anode assembly toward a stationary position where the anode surface portion is stationary relative to the pivot, a displacer assembly configured to apply a periodic displacement force to the rotating anode assembly to displace the rotating anode assembly from the stationary position comprising, wherein the position of the anode assembly oscillates about the stationary position with a lateral motion relative to the beam axis in response to the periodic displacement force and the restoring force, thereby oscillating the anode surface portion laterally across the beam axis, an X-ray source.

2. The X-ray source according to claim 1, wherein the rotating anode assembly is configured to oscillate about the stationary position such that an intersection of the electron beam axis on the anode surface portion scans a substantially straight path across the anode surface portion.

3. The X-ray source according to claim 1 or 2, wherein the pivot comprises a single pivot axis about which the rotating anode assembly is configured to rotate about a single pivot axis.

4. The X-ray source according to claim 3, wherein a spatial distribution of the mass of the rotating anode assembly is asymmetric with respect to the pivot axis.

5. The X-ray source according to claim 4, wherein the center of gravity of the rotating anode assembly and the anode surface are located on opposite sides of the pivot.

6. The X-ray source according to claim 4 or 5, wherein the location of the center of gravity of the rotating anode assembly is closer to the pivot than to the anode surface portion.

7. The X-ray source according to any one of claims 4 to 6, wherein the center of gravity of the rotating anode assembly is located between the pivot and the anode surface portion.

8. The X-ray source according to any one of claims 1 to 7, wherein the rotating anode assembly comprises a cooling channel configured to pass a cooling fluid in a direction away from the pivot toward the anode surface portion.

9. The X-ray source according to claim 8, wherein the cooling channel is configured to pass the cooling fluid in a direction toward the pivot and away from the anode surface.

10. The X-ray source according to any one of claims 1 to 9, wherein the displacer assembly is configured to apply the periodic displacement force having periodicity corresponding to the resonance frequency of the vibration of the rotating anode assembly to the rotating anode assembly.

11. The X-ray source according to claim 10, wherein the suspension assembly includes one or more springs configured to apply a restoring force, and the resonance frequency of the vibration is substantially proportional to the value of the spring constant of the one or more springs.

12. The X-ray source according to any one of claims 1 to 11, wherein the anode surface portion is electrically floating and maintained at a positive potential of 5 kV or more.

13. The X-ray source according to any one of claims 1 to 12, wherein the displacer assembly includes a single actuator.

14. The X-ray source according to claim 13, wherein the actuator includes a piezoelectric actuator.

15. The X-ray source according to any one of claims 1 to 14, wherein the pivot includes one or more flexure bearings configured to bend about the pivot axis of the rotating anode assembly.

16. The X-ray source according to any one of claims 1 to 15, wherein the displacer assembly includes a lateral displacer configured to displace the rotating anode assembly and the suspension assembly as a unit in a direction transverse to the plane of vibration of the anode surface portion in an adjustable manner.

17. The X-ray source according to any one of claims 1 to 16, comprising a longitudinal displacer configured to displace the rotating anode assembly in a direction substantially parallel to the plane of vibration of the anode surface portion in an adjustable manner, so that the anode surface portion is selectively displaced in a direction toward the electron beam generator in the longitudinal direction or away from the electron beam generator in the longitudinal direction.

18. The longitudinal displacer comprises a position transducer configured to determine the position of the anode surface portion during vibration of the anode surface portion, and the longitudinal displacer is configured to displace the swivel anode assembly adjustably in response to the determined position of the anode surface portion so as to substantially keep constant the separation distance between the electron beam generator and the point of impact of the electron beam on the anode surface portion during vibration of the anode surface portion. The X-ray source according to any one of claims 1 to 17.

19. The displacer assembly is configured to apply a periodic displacement force to the swivel anode assembly to apply torque to the swivel anode assembly at a location on the swivel anode assembly between the anode surface portion and the pivot. The X-ray source according to any one of claims 1 to 18.

20. The displacer assembly is configured to apply a periodic displacement force to the swivel anode assembly to apply torque to the swivel anode assembly at a location on the swivel anode assembly, and the pivot is located between the said location of the applied torque and the anode surface. The X-ray source according to any one of claims 1 to 19.

21. An X-ray monochromator comprising the X-ray source according to any one of claims 1 to 20.

22. An anode device for an X-ray source having an electron beam generator for generating a beam of electrons induced along a beam axis for use in X-ray generation by electron collision with the anode device, comprising a swivel anode assembly having a pivot and an anode surface portion mounted on the pivot for receiving electrons in the electron beam for generating X-rays, a suspension assembly configured to apply a restoring force to the swivel anode assembly to bias the swivel anode assembly towards a stationary position where the anode surface portion is relatively stationary with respect to the pivot, a displacer assembly configured to apply a periodic displacement force to the swivel anode assembly to displace the swivel anode assembly from the stationary position comprising The position of the anode assembly is configured to vibrate about the stationary position in a lateral motion with respect to the beam axis in response to the periodic displacement force and the restoring force, thereby vibrating the anode surface portion laterally across the beam axis. An anode device.

23. A method for generating X-rays by electron collision on the surface portion of an anode, comprising: generating a beam of electrons induced along a beam axis; receiving electrons from the electron beam at the anode surface portion, thereby generating X-rays, wherein the anode surface portion is mounted on a pivoted rotating anode assembly mounted on a suspension assembly; applying a restoring force to the rotating anode assembly via the suspension assembly to bias the rotating anode assembly towards a stationary position where the anode surface portion is stationary relative to the pivot; applying a periodic displacement force to the rotating anode assembly to displace the rotating anode assembly from the stationary position; comprising; thereby, the position of the anode assembly is vibrated about the stationary position in a lateral motion in response to the periodic displacement force and the restoring force with respect to the beam axis, thereby vibrating the anode surface portion laterally across the beam axis.

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