Plasma x-ray source for semiconductor metrology

Abstract

An X-ray source for semiconductor metrology that includes (a) a target delivery system configured to deliver a liquid jet to a plasma interaction region within a vacuum chamber, wherein the liquid jet comprises a cryogenic liquid or an aqueous solution of a compound; (b) a pulsed beam system configured to focus a beam onto the liquid jet at the plasma interaction region to generate plasma that emits X-rays at a plurality of X-ray emission lines; and (c) X-ray focusing optics configured to receive the X-rays emitted from the plasma and to focus the X-rays onto a semiconductor wafer, wherein the X-ray focusing optics comprise at least one of a multilayer mirror or a crystal monochromator.

Description

, PLASMA X-RAY SOURCE FOR SEMICONDUCTOR METROLOGY CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of U.S. Provisional Patent No. 63 / 771,604 filing date March 13, 2025, which is hereby incorporated by reference in its entirety.FIELD OF INVENTION

[0002] The present disclosure relates to X-ray sources for semiconductor metrology, and more particularly to plasma X-ray sources utilizing liquid or frozen target materials to generate multiple X-ray emission lines for advanced in-line semiconductor wafer metrology applications.BACKGROUND

[0003] Advanced semiconductor manufacturing processes demand precise metrology techniques capable of characterizing increasingly complex device structures. As semiconductor devices continue to shrink in size and increase in complexity, the ability to accurately measure film composition, thickness, dopant levels, impurities, critical dimensions, nanostructures, surface roughness, and potential defect levels becomes increasingly challenging. These measurements are performed to optimize semiconductor manufacturing processes and ensure device performance and yield.

[0004] X-ray based metrology techniques offer capabilities for characterizing semiconductor structures at the nanoscale level. Such techniques include X-ray reflectivity, X-ray scatterometry, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF), Auger spectroscopy, and ptychography. Each of these techniques may benefit from X-ray sources that can provide radiation at specific wavelengths or energy levels suited to the particular measurement being performed.

[0005] Semiconductor metrology applications typically require X-ray sources capable of producing small spot sizes, often less than 50 micrometers, with minimal scatter outside the measurement spot. X-ray radiation that falls outside the intended measurement area can lead to signal contamination from structures adjacent to the target measurement region, thereby reducing measurement accuracy. The degree of acceptable scatter may vary depending on the X-ray wavelength being utilized.

[0006] Conventional electron-gun-based X-ray sources have been employed in various metrology applications. However, such sources may present limitations in termsof brightness and footprint when applied to advanced semiconductor metrology requirements. Alternative approaches to X-ray generation, including plasma (LPP) sources, have been explored in research settings. LPP sources generate X-rays by focusing high-intensity laser pulses onto target materials, creating a plasma that emits X-ray radiation.

[0007] The characteristics of plasma formation and X-ray emission can vary based on numerous parameters, including laser pulse duration, wavelength, intensity, and the properties of the target material. Nanosecond, picosecond, and femtosecond laser pulses interact with target materials through different mechanisms, affecting the resulting plasma conditions and emission spectra. Target materials for LPP sources have included various solids, liquids, and gases, each presenting different advantages and challenges related to debris generation, chamber contamination, and X-ray emission characteristics.

[0008] For semiconductor metrology applications, X-ray sources that can operate at high repetition rates while maintaining peak powers below damage thresholds for integrated circuit components are desirable. Additionally, the ability to generate X-rays at multiple wavelengths or energy levels from a single source system could provide flexibility for different metrology techniques and measurement requirements.SUMMARY

[0009] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0010] According to an embodiment, there is provided an X-ray source for semiconductor metrology that includes (a) a target delivery system configured to deliver a liquid jet to a plasma interaction region within a vacuum chamber, wherein the liquid jet comprises a cryogenic liquid or an aqueous solution of a compound; (b) a pulsed beam system configured to focus a beam onto the liquid jet at the plasma interaction region to generate plasma that emits X-rays at a plurality of X-ray emission lines; and (c) X-ray focusing optics configured to receive the X-rays emitted from the plasma and to focus the X-rays onto a semiconductor wafer, wherein the X-ray focusing optics comprise at least one of a multilayer mirror or a crystal monochromator.

[0011] According to an embodiment, there is provided an X-ray source system for semiconductor in-line metrology that includes (a) a vacuum chamber; (b) a reservoir configured to store a cryogenic liquid; (c) a cooling system coupled to the reservoir and configured to maintain the cryogenic liquid in a liquid state; (d) a nozzle configured to eject the cryogenic liquid into the vacuum chamber as a liquid jet; (e) a pulsed beam system configured to direct a beam through a window of the vacuum chamber and onto the liquid jet to generate a plasma that emits X-rays, wherein the plasma emits X-rays at Kr L lines; (f) X-ray collecting optics configured to receive the X-rays through an X-ray window of the vacuum chamber and to focus the X-rays onto a sample; and (g) a liquid circulation system configured to collect the cryogenic liquid after plasma generation and return the collected cryogenic liquid to the reservoir.

[0012] According to an embodiment, there is provided a method for generating X-rays for semiconductor metrology, the method includes: (a) delivering a liquid jet comprising a cryogenic liquid or an aqueous solution of a compound to a plasma interaction region within a vacuum chamber; (b) focusing a pulsed beam onto the liquid jet at the plasma interaction region to generate a plasma that emits X-rays at a plurality of X-ray emission lines; and (c) directing the X-rays emitted from the plasma through X-ray focusing optics onto a semiconductor wafer, wherein the X-ray focusing optics comprise at least one of a multilayer mirror or a crystal monochromator.

[0013] The foregoing general description of the illustrative embodiments and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.BRIEF DESCRIPTION OF FIGURES

[0014] Non-limiting and non-exhaustive examples are described with reference to the following figures.

[0015] FIG. 1 illustrates a transmission mode configuration for an X-ray source system, according to aspects of the present disclosure.

[0016] FIG. 2 illustrates an angle transmission mode configuration for an X-ray source system, according to aspects of the present disclosure.

[0017] FIG. 3 illustrates a reflection mode configuration for an X-ray source system, according to aspects of the present disclosure.

[0018] FIG. 4 illustrates a system diagram of an X-ray source system using liquid krypton, according to aspects of the present disclosure.

[0019] FIG. 5 illustrates a flowchart for a method, according to aspects of the present disclosure.DETAILED DESCRIPTION

[0020] The following description sets forth exemplary aspects of the present disclosure. It should be recognized, however, that such description is not intended as a limitation on the scope of the present disclosure. Rather, the description also encompasses combinations and modifications to those exemplary aspects described herein.

[0021] The following refers to pulsed beams such as a pulsed laser beam or a pulsed electron beam. Any reference to a laser beam should be applied mutatis to an electron beam.

[0022] Any reference to an electron beam should be applied mutatis mutandis to a laser beam.

[0023] Referring to FIGS. 1-3, an X-ray source system for semiconductor metrology may be configured in one or more of several operational modes. Thus - the X-ray source system may have a single configuration of the configurations illustrated in FIGS. 1-3. Alternatively - the X-ray source system may be configured to one of the configurations at one point in time and may be configurated to another configuration and another point in time.

[0024] The X-ray source system may be configured to a transmission mode configuration 31, an angle transmission mode configuration 32, or a reflection mode configuration 33. Each configuration mode may provide different geometric arrangements for X-ray generation and collection while sharing common components.

[0025] The X-ray source system may include a laser beam or an electron beam (denoted laser or electron beam 12) that serves as the excitation source for plasma generation. In some cases, the laser or electron beam 12 may be a pulsed laser beam. In other cases, the laser or electron beam 12 may be a high brightness electron beam.

[0026] With continued reference to FIGS. 1-3, the X-ray source system may include a focusing lens 22 positioned to receive and focus the laser or electron beam 12. The focusing lens 22 may direct the laser or electron beam 12 onto a liquid jet 16. The liquid jet 16 may serve as the target material for plasma generation. When the focused laser or electron beam 12 interacts with the liquid jet 16, a plasma may be generated that emits an X-ray beam 14.

[0027] The X-ray beam 14 may have a spot size ranging from 1 micrometer to 50 micrometers full width at half maximum (FWHM). The X-ray source system may further include a collection lens 24 positioned to collect and direct the X-ray beam 14 for downstream metrology applications. A collection nozzle 20 may be arranged to collect the liquid jet 16 material after the liquid jet 16 passes through the plasma interaction region.

[0028] As further shown in FIGS. 1-3, the transmission mode configuration 31 may arrange the collection lens 24 on an opposite side of the liquid jet 16 from the focusing lens 22, such that the X-ray beam 14 passes through the liquid jet 16 in a direction aligned with the laser or electron beam 12. The angle transmission mode configuration 32 may position the collection lens 24 at an angle relative to the liquid jet 16, such that the X-ray beam 14 exits at an angle from the direction of the incoming laser or electron beam 12. The reflection mode configuration 33 may position the collection lens 24 on the same side of the liquid jet 16 as the focusing lens 22, such that the X-ray beam 14 exits in a reflection geometry relative to the incoming laser or electron beam 12.

[0029] In the transmission mode configuration 31, the X-ray beam 14 may be collected in line with the laser or electron beam 12 through the liquid jet 16. The focusing lens 22 may receive the laser or electron beam 12 and focus the laser or electron beam 12 onto a region of the liquid jet 16. The focused laser or electron beam 12 may interact with the liquid jet 16 to generate a plasma at the interaction region. The plasma may emit X-rays that propagate through the liquid jet 16 in a direction substantially aligned with the propagation direction of the incoming laser or electron beam 12.

[0030] The collection lens 24 may be positioned downstream of the liquid jet 16 along the propagation axis of the laser or electron beam 12. The collection lens 24 may receive the X-ray beam 14 that exits the liquid jet 16 on a side opposite to the side wherethe laser or electron beam 12 enters the liquid jet 16. The collection lens 24 may collect and direct the X-ray beam 14 toward downstream optical components or toward a sample for metrology applications.

[0031] The transmission mode configuration 31 may provide a geometric arrangement where the X-ray generation and collection occur along a common optical axis defined by the laser or electron beam 12. The liquid jet 16 may be positioned such that the plasma interaction region is located at a focal point of the focusing lens 22. The X-ray beam 14 generated at the plasma interaction region may propagate through the remaining portion of the liquid jet 16 before being collected by the collection lens 24.

[0032] In the angle transmission mode configuration 32, the X-ray beam 14 may be collected at an angle relative to the laser or electron beam 12 through the liquid jet 16. The focusing lens 22 may receive the laser or electron beam 12 and focus the laser or electron beam 12 onto a region of the liquid jet 16. The focused laser or electron beam 12 may interact with the liquid jet 16 to generate a plasma at the interaction region. The plasma may emit X-rays that propagate at various angles from the plasma interaction region.

[0033] The collection lens 24 may be positioned at an angle relative to the propagation direction of the laser or electron beam 12. The angular positioning of the collection lens 24 may allow the collection lens 24 to capture X-rays generated from the plasma interaction that exit the liquid jet 16 at an angle different from the propagation direction of the incoming laser or electron beam 12. The angle between the collection lens 24 and the laser or electron beam 12 may be selected based on the desired X-ray collection geometry and the characteristics of the X-ray emission from the plasma.

[0034] The angle transmission mode configuration 32 may provide a geometric arrangement where the X-ray generation occurs at the plasma interaction region within the liquid jet 16, and the X-ray collection occurs along an axis that is angularly offset from the axis of the laser or electron beam 12. The collection lens 24 may receive the X-ray beam 14 that exits the liquid jet 16 at the selected angle and may collect and direct the X-ray beam 14 toward downstream optical components or toward a sample for metrology applications.

[0035] In the reflection mode configuration 33, the X-ray beam 14 may be collected on the same side as the laser or electron beam 12 relative to the liquid jet 16. The focusing lens 22 may receive the laser or electron beam 12 and focus the laser or electron beam 12 onto a region of the liquid jet 16. The focused laser or electron beam 12 may interact with the liquid jet 16 to generate a plasma at the interaction region. The plasma may emit X-rays that propagate in various directions from the plasma interaction region, including directions that return toward the same side of the liquid jet 16 from which the laser or electron beam 12 originated.

[0036] The collection lens 24 may be positioned on the same side of the liquid jet 16 as the focusing lens 22. The positioning of the collection lens 24 on the same side as the incoming laser or electron beam 12 may allow the collection lens 24 to capture X-rays that are emitted from the plasma in a reflection geometry. The X-ray beam 14 may exit the liquid jet 16 on the same side where the laser or electron beam 12 enters the liquid jet 16, and the collection lens 24 may receive and collect the X-ray beam 14 for downstream use.

[0037] The reflection mode configuration 33 may provide a geometric arrangement where the X-ray generation occurs at the plasma interaction region within the liquid jet 16, and the X-ray collection occurs along an axis that is oriented to receive X-rays propagating back toward the source side of the liquid jet 16. The collection nozzle 20 may be positioned to collect the liquid jet 16 material after the liquid jet 16 passes through the plasma interaction region. The collection nozzle 20 may receive the liquid jet 16 material following plasma generation and may direct the collected material for recirculation or disposal.

[0038] FIG. 4 refers to liquid krypton - but any other cryogenic liquid may be provided.

[0039] FIG. 4 refers to laser beam 52 - but an electron beam may replace the laser beam.

[0040] Referring to FIG. 4 an X-ray source system may be configured in a reflection mode implementation using liquid krypton as the source material. The X-ray source system may include a vacuum chamber 42 that contains a plasma generation region where the laser or electron beam interacts with the target material to generate X-rays. Thevacuum chamber 42 may provide a controlled environment for plasma generation and X-ray emission.

[0041] The X-ray source system may include a reservoir 48 that stores liquid krypton 50. The reservoir 48 may be positioned to supply the liquid krypton 50 to a nozzle that delivers the liquid krypton 50 into the vacuum chamber 42 for plasma generation. The liquid krypton 50 may serve as the target material for plasma X-ray generation.

[0042] With continued reference to FIG. 4, the X-ray source system may include a cooling system to maintain the krypton in a liquid state. The cooling system may include a cryo-chiller 40 that provides cooling capacity to the system. The cryo-chiller 40 may be connected to cooling lines 46 that distribute cooling to various components of the X-ray source system. The cooling lines 46 may cool the reservoir 48 to maintain the krypton in a liquid state within the reservoir 48. The cooling lines 46 may also cool krypton release lines that deliver the liquid krypton 50 from the reservoir 48 to the nozzle within the vacuum chamber 42.

[0043] The liquid jet may be composed of liquid krypton or frozen krypton. In some cases, the liquid jet composed of liquid or frozen krypton may produce emission spectra in a wavelength range from 5.0 nanometers to 8.5 nanometers. In some cases, the liquid jet composed of liquid or frozen krypton may produce Kr L lines at 1,586 electron volts (eV) and 1,637 eV. The emission spectra and Kr L lines may be suitable for various semiconductor metrology applications including X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF) analysis.

[0044] As further shown in FIG. 4, the use of liquid or frozen gas targets such as liquid krypton 50 may mitigate chamber and laser / X-ray window contamination issues. Liquid or frozen gas targets may reduce debris generation compared to liquid metal jets or plasma from solid targets that emit in similar energy or wavelength ranges. The reduced debris generation may extend the operational lifetime of optical components within the vacuum chamber 42 and may reduce maintenance requirements for the X-ray source system.

[0045] The X-ray source system may include krypton release lines 44 that deliver the liquid krypton 50 from the reservoir 48 to a nozzle positioned within the vacuum chamber 42. The krypton release lines 44 may be cooled by the cooling lines 46P0346connected to the cryo-chiller 40 to maintain the krypton in a liquid state during delivery. A high pressure pump 56 may supply recycled krypton to the krypton release lines 44 for delivery to the vacuum chamber 42.

[0046] The liquid krypton 50 may be ejected into the vacuum chamber 42 through a high-pressure nozzle connected to the krypton release lines 44. The high-pressure nozzle may eject the cryogenic liquid krypton 50 into the vacuum chamber 42 as either a continuous stream of fluid or as a stream that breaks up into droplets. In some cases, the liquid jet 16 may be a continuous stream of liquid krypton 50. In other cases, the liquid jet 16 may break into droplets that can be individually targeted by the laser or electron beam 12.

[0047] The X-ray source system may include a liquid circulation system for returning liquid from a krypton liquid collector 62 to the reservoir 48 for recirculation. The krypton liquid collector 62 may be positioned to receive the liquid krypton 50 after the liquid krypton 50 passes through the plasma interaction region. The krypton liquid collector 62 may include a cooled element in a collection zone to maintain the krypton in a liquid state for recirculation rather than allowing the krypton to evaporate. The cooled element may receive cooling from the cooling lines 46 connected to the cryo-chiller 40. The high pressure pump 56 may draw the collected liquid krypton from the krypton liquid collector 62 and return the collected liquid krypton to the reservoir 48 through the krypton release lines 44.

[0048] The X-ray source system may include a gas collection nozzle 60 positioned within the vacuum chamber 42 to collect gas that evaporates from the liquid krypton 50 during operation. The gas collection nozzle 60 may be positioned opposite a generation nozzle to collect the liquid jet 16 while the liquid jet 16 is still in a liquid state. A blower 58 may direct gas from the gas collection nozzle 60 to an exhaust 64 for removal from the vacuum chamber 42. The blower 58 and exhaust 64 may reduce the gas load within the vacuum chamber 42 by removing evaporated krypton gas from the system.

[0049] The plasma generated by the interaction between the laser or electron beam 12 and the liquid jet 16 may create a shock wave that disturbs neighboring portions of the liquid jet 16. The X-ray source system may require a relaxation time between laser shots to allow the liquid jet 16 to stabilize after plasma shock wave disturbance. The relaxationP0346time may allow the liquid jet 16 stream to stabilize before a subsequent laser shot can produce a plasma. In some cases, the liquid jet 16 may be configured as a pulsed jet of droplets to avoid plasma shock wave disturbance of neighboring droplets.

[0050] The X-ray source system may include a laser beam 52 that enters the vacuum chamber 42 through a window 71. The window 71 may be positioned on the vacuum chamber 42 to allow the laser beam 52 to pass into the vacuum chamber 42 and reach the liquid krypton 50 at the plasma interaction region. The laser beam 52 may be focused onto the liquid krypton 50 to generate a plasma that emits X-rays.

[0051] The laser beam 52 may have wavelengths selected to optimize laser-target interactions and laser-plasma coupling efficiencies. In some cases, the laser beam 52 may have ultraviolet wavelengths including 193 nanometers, 266 nanometers, or 355 nanometers. In some cases, the laser beam 52 may have wavelengths of 800 nanometers or 1064 nanometers. In some cases, the laser beam 52 may have wavelengths in the range of 2 micrometers to 4 micrometers, 5 micrometers, or 10.6 micrometers. The selection of laser wavelength may influence penetration depth, spot size, absorption efficiency, and plasma shielding characteristics.

[0052] The laser beam 52 may have pulse durations ranging from 100 femtoseconds to 300 nanoseconds. In some cases, the laser beam 52 may have nanosecond pulse durations where initial absorption is predominantly linear following Beer-Lambert law absorption, with transitions to nonlinear absorption at higher intensities. In some cases, the laser beam 52 may have femtosecond or picosecond pulse durations characterized by high peak intensities that drive nonlinear absorption processes. Femtosecond laser plasma formation may be governed by electron impact ionization and strong electric field ionization. The pulse duration may be selected to optimize plasma conditions and emission spectrum characteristics. The laser beam may have a wavelength selected from 193 nm, 266 nm, 355 nm, 800 nm, 1064 nm, 2-4 pm, 5 pm, and 10.6 pm.

[0053] The pulsed beam may have a repetition rate ranging from 1 kHz to 50 kHz.

[0054] The laser beam 52 may have peak intensities ranging from approximately 10 2 18 21x10 W / cm to 1x10 W / cm . The peak intensity may be selected based on the target material and the ionization level required for proper X-ray line emission. The peak intensity may be configured to achieve appropriate ionization level, plasma temperature,P0346and electron critical temperature to optimize desired emission lines from the liquid krypton 50.

[0055] The laser system generating the laser beam 52 may operate at high repetition rates for semiconductor metrology applications. In some cases, the laser system may operate at repetition rates ranging from 1 kilohertz to 50 kilohertz or greater. High repetition rate operation may provide quasi-continuous X-ray output suitable for metrology applications while maintaining peak powers below damage thresholds for integrated circuit components.

[0056] The interaction between the laser beam 52 and the liquid krypton 50 may generate a plasma that emits an X-ray beam 70. The X-ray beam 70 may exit the vacuum chamber 42 through an X-ray window 72. The X-ray window 72 may be positioned on the vacuum chamber 42 to allow the X-ray beam 70 to pass out of the vacuum chamber 42 toward downstream optical components.

[0057] The X-ray source system may include X-ray collecting / focusing optics 54 positioned to receive the X-ray beam 70 exiting through the X-ray window 72. The X-ray collecting / focusing optics 54 may collect the X-ray beam 70 and direct a focused X-ray beam toward a sample 90. The sample 90 may be a semiconductor wafer or other substrate for metrology analysis. The X-ray collecting / focusing optics 54 may focus the X-ray beam 70 to a spot size suitable for semiconductor metrology applications, enabling measurements on specific regions of the sample 90 with reduced signal contamination from structures outside the intended measurement area.

[0058] The X-ray source system may utilize a pre-pulse laser to create pre-ionization of the target material prior to delivery of a main pulse. The pre -pulse laser may generate initial ionization of the liquid jet material to prepare the target for interaction with the main pulse. The main pulse may have different characteristics from the pre-pulse laser, including different peak intensity, pulse width, repetition rate, or spot size. The preionization created by the pre -pulse laser may modify the plasma conditions to optimize X-ray emission when the main pulse interacts with the pre-ionized target material.

[0059] The pre-pulse excitation may be generated by the same laser system that generates the main pulse. In some cases, the pre -pulse and main pulse may be generated with different laser parameters to optimize spectral line emission. The pre-pulse mayP0346have lower peak intensity than the main pulse to create controlled pre-ionization without generating substantial X-ray emission. The main pulse may then interact with the preionized plasma to achieve higher ionization levels and optimized X-ray line emission. The temporal separation between the pre-pulse and main pulse may be selected to allow the pre-ionized plasma to expand to a desired density profile before the main pulse arrives.

[0060] The X-ray source system may utilize two or more laser systems simultaneously (or one or more electron beam systems simultaneously) to optimize plasma shape, emission spot, and spectral emission characteristics. The multiple laser systems may be configured to deliver laser pulses to the liquid jet from different angles or with different timing relationships. The combination of multiple laser systems may allow independent control of plasma heating, ionization, and expansion dynamics. The multiple laser systems may have different wavelengths, pulse durations, or peak intensities to provide complementary effects on the plasma generation process.

[0061] In some cases, a first laser system may provide absorption at a longer wavelength for initial plasma generation, and a second laser system may provide subsequent plasma heating and ionization at a shorter wavelength. The longer wavelength laser may exhibit more efficient absorption in the target material, while the shorter wavelength laser may penetrate deeper into the plasma and heat the plasma core to higher temperatures. The combination of wavelengths may optimize both the initial plasma formation and the subsequent ionization required for X-ray emission at desired spectral lines.

[0062] The X-ray source system may be configured to optimize a specific spectrum range by adjusting laser parameters and liquid jet parameters for specific metrology needs. The laser parameters that may be adjusted include peak intensity, pulse width, repetition rate, spot size, and wavelength. The liquid jet parameters that may be adjusted include jet velocity, jet diameter, jet shape, and material composition or concentration. The combination of laser and liquid jet parameter optimization may allow the X-ray source system to be tuned for specific X-ray emission characteristics required by different metrology applications.P0346

[0063] The X-ray source system may emit multiple emission lines simultaneously under appropriate laser and plasma conditions. The simultaneous multi-line emission may be achieved through selection of optimum laser wavelength and utilization of laser prepulse excitation prior to a main high-power pulse. The plasma conditions including plasma temperature and electron density may be controlled through laser parameter selection to achieve simultaneous emission at multiple X-ray wavelengths or energies. The simultaneous emission of multiple X-ray lines may enable metrology applications that utilize different X-ray energies for complementary measurements on the same sample.

[0064] In some cases, a shutter may be positioned in one of multiple X-ray beam paths to selectively block one of the emission lines while allowing another emission line to reach the sample. The shutter may allow the X-ray source system to switch between different X-ray emission lines without changing laser parameters or target materials. The selective blocking of emission lines may enable sequential measurements using different X-ray energies from the same X-ray source system.

[0065] The liquid jet may have various shapes depending on nozzle design, flow dynamics, pressure, and external forces applied to the liquid during ejection. The shape of the liquid jet may be selected based on the requirements of the plasma generation process and the desired X-ray emission characteristics.

[0066] The liquid jet may have a cylindrical shape configured as a continuous column-like stream ejected from a circular nozzle. The cylindrical jet may provide a uniform cross-sectional area along the length of the jet, which may facilitate consistent plasma generation when the laser or electron beam interacts with the cylindrical jet at different positions along the jet length.

[0067] The liquid jet may have a flat sheet shape configured as a thin wide sheet produced by a slit or fan-shaped nozzle. The flat sheet jet may provide an extended interaction region in one dimension while maintaining a thin profile in the perpendicular dimension. The flat sheet configuration may allow the laser or electron beam to interact with a larger surface area of target material compared to cylindrical jet configurations.

[0068] The liquid jet may have an annular shape, also referred to as a hollow shape, created by a nozzle with a central obstruction leaving a hollow core. The annular jet mayP0346form a ring-shaped stream of liquid with an open center region. The hollow core of the annular jet may provide different plasma generation characteristics compared to solid jet configurations.

[0069] The liquid jet may be configured as a pulsed jet that is periodically interrupted, forming discrete droplets or bursts of liquid. The pulsed jet configuration may avoid plasma shock wave disturbance that can affect neighboring portions of a continuous liquid stream. The discrete droplets of the pulsed jet may be individually targeted by the laser or electron beam, with sufficient spacing between droplets to allow plasma shock waves from one droplet interaction to dissipate before the next droplet arrives at the interaction region.

[0070] The liquid jet may have a ligament shape that breaks into thin liquid threads or ligaments before fragmenting into droplets. The ligament jet may represent an intermediate state between a continuous stream and fully formed droplets. The thin liquid threads of the ligament jet may provide target material with reduced cross-sectional dimensions compared to the original jet diameter.

[0071] The liquid jet may have a Rayleigh jet shape that undergoes surface tension-driven instabilities leading to droplet formation. The Rayleigh jet may transition from a cylindrical stream into a series of droplets through natural instability mechanisms driven by surface tension forces. The droplet formation in the Rayleigh jet may occur at predictable intervals based on the jet diameter and fluid properties.

[0072] The liquid jet may have a helical shape, also referred to as a swirling shape, with spiral or vortex-like motion induced by tangential or swirling flow at the nozzle. The helical jet may rotate about the jet axis as the liquid travels away from the nozzle. The swirling motion of the helical jet may be generated by nozzle geometries that impart angular momentum to the liquid during ejection.

[0073] The liquid jet may have a sheet configuration with rim instabilities that form waves breaking into ligaments and droplets at the edges. The sheet with rim instabilities may provide a thin sheet of target material in the central region while generating ligaments and droplets at the peripheral edges of the sheet. The rim instabilities may result from aerodynamic forces or surface tension effects acting on the edges of the liquid sheet.P0346

[0074] The liquid jet may be configured as an electrospray jet emitted under the influence of an electric field leading to fine mist or droplet formation. The electrospray jet may utilize electric field forces to atomize the liquid into fine droplets or mist. The electric field applied to the electrospray jet may control the droplet size and spatial distribution of the atomized liquid.

[0075] The liquid jet may have diameters of 3 micrometers and larger. The jet diameter may be selected based on the desired plasma generation characteristics and the spot size requirements for X-ray emission. Smaller jet diameters may provide smaller X-ray source sizes, while larger jet diameters may provide increased target material volume for plasma generation.

[0076] The liquid jet may have speeds ranging from 1 meter per second to 550 meters per second. The jet speed may be selected based on the repetition rate of the laser system and the required spacing between plasma interaction events. Higher jet speeds may allow higher repetition rate operation by providing fresh target material at the interaction region more rapidly.

[0077] The liquid jet may be composed of carbon-oxygen compounds as an alternative to liquid or frozen gas targets. In some cases, the liquid jet may be composed of carbon monoxide (CO). In some cases, the liquid jet may be composed of carbon dioxide (CO2). In some cases, the liquid jet may be composed of carbonate compounds such as CO3 or carbonic acid (H2CO3). The carbon-oxygen compound targets may produce a laser-produced carbon and oxygen plasma when interacting with the laser beam. The carbon-oxy gen plasma may produce an X-ray spectrum ranging from 200 electron volts (eV) to 900 eV with spectral lines within this range. The X-ray emission from carbon-oxygen compound targets may be suitable for metrology applications requiring X-ray energies in the 200 eV to 900 eV range.

[0078] The liquid jet may be composed of aqueous solutions of compounds as a viable alternative to liquid or frozen gas sources. The aqueous solution targets may provide the ability to optimize solution concentration for best quantum yield under plasma conditions. The solution-based targets may include X-halides or organometallic solutions. X-halides may refer to metal halides dissolved in water or other solvents.P0346

[0079] In some cases, the liquid jet may be composed of aqueous solutions of Gadolinium (III) Chloride (GdCh). The GdCh aqueous solution may be delivered to the laser plasma interaction region as a liquid jet. The GdCh solution may produce emission lines in the extreme ultraviolet (XUV) range from 6.5 nanometers to 7.5 nanometers. The GdCh solution may also produce characteristic X-ray emissions suitable for X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF) analysis.

[0080] The characteristic X-ray emissions from GdCh aqueous solution may include Gadolinium L-lines at 6,057.2 eV, 6,025.0 eV, 6,713.2 eV, 7,102.8 eV, and 7,785.8 eV. The GdCh solution may also produce a Gadolinium M-line at 1,185 eV. The Chlorine component of the GdCh solution may produce Chlorine K-lines at 2,622.39 eV, 2,620.78 eV, and 2,815.6 eV. The water solvent of the aqueous GdCh solution may produce an Oxygen K-line at approximately 524.9 eV. The multiple emission lines from the GdCh aqueous solution may provide a range of X-ray energies for different metrology applications from a single target material.

[0081] In some cases, the liquid jet may be composed of aqueous solutions of Gadolinium (III) Bromide (GdBrs). The GdBr3 aqueous solution may provide shifted halide emission lines compared to the GdCh solution. The Bromine component of the GdBr3 solution may produce Bromine L-lines at 1,480.43 eV and 1,525.90 eV. The GdBrs solution may also produce Bromine K-lines at approximately 11,925 eV. The selection between GdCh and GdBrs solutions may allow selection of different halide X-ray emission lines based on the requirements of specific metrology applications.

[0082] The spectrum range produced by solution-based targets may depend on the solution composition. The GdBrs solution may differ from the GdCh solution in the halide X-ray generation, with Bromine X-ray lines replacing Chlorine X-ray lines. The relative intensities of the various emission lines may depend on the solution concentration of the dissolved compound.

[0083] The solution concentration of compound targets may be adjusted to optimize the X-ray yield for specific applications. The solution concentration may be biased toP0346maximize the X-ray yield of interest for a particular metrology application. In some cases, a lower concentration of the metal salt may be used when the application focus is on X-ray lines from the solvent or halide component rather than the metal component. For example, a lower concentration of Gadolinium halide may be used when the application requires high yield from Oxygen X-rays rather than Gadolinium X-rays. The ability to tune solution concentration may provide flexibility in optimizing the X-ray source for different metrology requirements without changing the fundamental target delivery system.

[0084] The X-ray collecting / focusing optics may include a bent crystal monochromator configured to select and focus X-ray lines of interest. The bent crystal monochromator may have an intrinsic blur of less than 2 micrometers for substrate surface figure and crystal alignment errors. The crystal alignment errors of the bent crystal monochromator may not exceed 2 arcseconds. The crystal alignment errors may depend on the monochromator figure tolerances, including the c / b and a / b tolerance parameters of the monochromator substrate shape.

[0085] The bent crystal monochromator may have an ellipsoidal substrate figure. The ellipsoidal monochromator may have a substrate shape matching the equation x2 / a2+ y2lb + z2 / b2= 1. The ellipsoidal substrate figure may be configured to focus X-rays onto a wafer or sample location. The focusing properties of the bent crystal monochromator may be determined by the ellipsoidal substrate shape, which may be configured to match the source-to-wafer focusing distance and the Bragg angle for a given X-ray energy.

[0086] The bent crystal monochromator may include a Quartz 100 crystal to diffract X-ray lines of interest. The Quartz 100 crystal may meet Bragg diffraction conditions for selecting specific X-ray wavelengths or energies. The crystal lattice spacing of the Quartz 100 crystal may be suitable for diffracting X-ray lines in the energy range produced by the X-ray source. The Quartz 100 crystal may be used to diffract Kr L lines or other X-ray emission lines produced by the laser-produced plasma.

[0087] The X-ray collecting / focusing optics may include crystals with lattice spacing suitable for respective X-ray energies when targeting emission of X-ray lines in different energy ranges. The substrate figure of the bent crystal monochromator may be adjustedP0346based on the X-ray energy to be selected and focused. The combination of crystal lattice spacing and substrate figure may allow the monochromator to be configured for different X-ray energies while maintaining appropriate Bragg diffraction and focusing properties.

[0088] The X-ray collecting / focusing optics may include collimating optics configured to provide a monochromatic parallel beam. The collimating optics may be reflective optics or polycapillary optics. The collimating optics may receive divergent X-rays from the X-ray source and produce a collimated beam of X-rays propagating in a parallel direction.

[0089] The X-ray collecting / focusing optics may include angle and position adjustable flat double crystals configured to work with the collimating optics. The flat double crystals may receive the collimated X-ray beam from the collimating optics and select a monochromatic X-ray wavelength through Bragg diffraction. The angle and position adjustability of the flat double crystals may allow selection of different X-ray wavelengths by adjusting the Bragg angle. The monochromatic parallel beam produced by the combination of collimating optics and flat double crystals may then be focused by reflective or polycapillary optics to a small spot on the wafer or sample.

[0090] The X-ray collecting / focusing optics may include a multilayer mirror configured to transport and focus X-rays at specific wavelengths. The multilayer mirror may have a material combination and d-spacing dependent on the X-ray energy to be transported. The multilayer mirror may be engineered through a combination of layer materials of different density and relative layer thickness. The number of layers deposited onto the substrate of the multilayer mirror may also be selected based on the X-ray energy and desired reflectivity characteristics.

[0091] The multilayer mirror may be configured to match the Bragg condition for the X-ray wavelength to be transported, analogous to crystal monochromators. The d-spacing of the multilayer mirror may be selected to satisfy the Bragg condition for the target X-ray wavelength. The substrate figure of the multilayer mirror may be configured as a focusing ellipsoidal mirror. The substrate figure of the focusing ellipsoidal mirror may be determined by the Bragg angle of the multilayer for a given X-ray energy and the source -to- wafer focusing distance.P0346

[0092] The substrate for multilayer deposition may have a surface finish with roughness of the order of 0.2 nanometers or less. The low surface roughness of the substrate may reduce scattering losses and maintain high reflectivity of the multilayer mirror. The surface finish requirement may apply to substrates used for both flat and curved multilayer mirror configurations.

[0093] The multilayer mirror may include Cr / C repeating multilayers for efficient diffraction of Carbon K-alpha radiation. The Cr / C multilayer composition may provide appropriate contrast between high-density chromium layers and low-density carbon layers for X-ray reflection. The Cr / C repeating multilayer structure may be configured with d-spacing and number of layer pairs selected to optimize reflectivity at the Carbon K-alpha wavelength.

[0094] The X-ray source system may accommodate two or more monochromators or multilayer mirrors to enable a range of metrology capabilities. In some cases, the system may include multiple monochromators configured for different X-ray energies. In some cases, the system may include multiple multilayer mirrors configured for different X-ray wavelengths. In some cases, the system may include a combination of at least one monochromator and at least one multilayer mirror.

[0095] The multiple optics configurations may enable X-ray reflectivity (XRR), X-ray scatterometry (XRS), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF), Auger spectroscopy, and Ptychography capabilities. The different optics configurations may provide X-rays at different energies or wavelengths suitable for each metrology technique. The ability to accommodate multiple optics configurations may allow the X-ray source system to support multiple metrology applications using a single X-ray source with selectable optical paths.

[0096] The X-ray source system may target emission of X-ray lines in an energy range between 1,460 eV and 8,000 eV. The X-ray source system may utilize focusing optics configured with crystals having lattice spacing suitable for the respective X-ray energies within this range. The crystal lattice spacing may be selected to satisfy Bragg diffraction conditions for the target X-ray energy. The focusing optics may include bent crystal monochromators or flat crystal configurations with substrate figures adjusted based on the X-ray energy to be selected and focused. The 1,460 eV to 8,000 eV energyP0346range may encompass characteristic X-ray emission lines from various target materials including Kr L lines, Gadolinium L-lines and M-lines, Chlorine K-lines, and Bromine L-lines.

[0097] The X-ray source system may target peak wavelengths in soft X-ray wavelength bands for use with multilayer mirrors. In some cases, the X-ray source system may target peak wavelengths between 5.7 nanometers and 5.8 nanometers. In some cases, the X-ray source system may target peak wavelengths between 5.9 nanometers and 6.5 nanometers. In some cases, the X-ray source system may target peak wavelengths between 6.8 nanometers and 7.4 nanometers. In some cases, the X-ray source system may target peak wavelengths between 7.6 nanometers and 8.25 nanometers. The multilayer mirror material combination and d-spacing may be configured based on the target wavelength band to satisfy Bragg conditions for efficient X-ray transport and focusing.

[0098] The X-ray source system may include an optical path shutter mechanism for selectively using either soft X-ray or hard X-ray emission lines. The shutter may be positioned in one of multiple optical paths extending from the X-ray source. The shutter may be configured to block one of the optical paths while allowing X-rays to propagate through an alternative optical path. In some cases, the shutter may block a soft X-ray optical path to allow hard X-ray emission lines to reach a sample. In some cases, the shutter may block a hard X-ray optical path to allow soft X-ray emission lines to reach a sample. The shutter mechanism may enable switching between different X-ray emission lines without requiring changes to laser parameters or target materials. The selective blocking capability may allow the X-ray source system to provide different X-ray energies or wavelengths for sequential metrology measurements on the same sample using a single X-ray source.

[0099] Referring to FIG. 5, a method 100 for generating X-rays using a plasma system for semiconductor metrology applications may include a sequential progression of operations. The method 100 may begin with a step 110, proceed to a step 120, and conclude with a step 130. The method 100 may be performed using the X-ray source system components described herein, including the vacuum chamber 42, the reservoir 48,P0346the cryogenic liquid (such a liquid krypton 50), and may receive laser beam 52 or an electron beam.

[0100] Step 110 may include delivering a liquid jet target material to a plasma interaction region within the vacuum chamber 42. In some cases, step 110 may include ejecting the liquid krypton 50 from the reservoir 48 through the krypton release lines 44 and a high-pressure nozzle into the vacuum chamber 42. Step 110 may include maintaining the target material in a liquid or frozen state using the cooling lines 46 connected to the cryo-chiller 40. In some cases, step 110 may include delivering aqueous solutions of compounds such as GdCh or GdBr3 to the plasma interaction region as the liquid jet 16. Step 110 may include configuring the liquid jet 16 with a selected jet shape, jet diameter, and jet speed based on the requirements of the plasma generation process and the desired X-ray emission characteristics.

[0101] With continued reference to FIG. 5, step 120 may include directing the laser beam 52 through the window 71 and focusing the laser beam 52 onto the liquid jet 16 at the plasma interaction region. Step 120 may include generating a plasma through the interaction between the focused laser beam 52 and the liquid jet 16. In some cases, step 120 may include delivering a pre-pulse laser to create pre-ionization of the target material prior to delivery of a main pulse. Step 120 may include selecting laser parameters including peak intensity, pulse width, repetition rate, spot size, and wavelength to optimize plasma conditions and X-ray emission characteristics. Step 120 may include generating the plasma at peak intensities ranging from approximately 1x10 W / cm to 18 21x10 W / cm depending on the target material and the ionization level required for proper X-ray line emission.

[0102] Step 130 may include collecting the X-ray beam 70 emitted from the plasma and directing the X-ray beam 70 toward the sample 90 for semiconductor metrology applications. Step 130 may include passing the X-ray beam 70 through the X-ray window 72 and receiving the X-ray beam 70 at the X-ray collecting / focusing optics 54. In some cases, step 130 may include selecting specific X-ray emission lines using a bent crystal monochromator or a multilayer mirror. Step 130 may include focusing the X-ray beam 70 to a spot size ranging from 1 micrometer to 50 micrometers FWHM on the sample 90.P0346Step 130 may include collecting the liquid jet 16 material after plasma generation using the gas collection nozzle 60 and the krypton liquid collector 62, and recirculating the collected material to the reservoir 48 using the high pressure pump 56.

[0103] A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims.

[0104] Any reference to the term "comprising" or "having" should be applied, mutatis mutandis to "consisting of" or "essentially consisting of".

[0105] Any reference to a system should be applied, mutatis mutandis to a method executable by the system and / or should be applied, mutatis mutandis to a non-transitory computer readable medium that stores instructions executable by the system.

[0106] Any reference to a method system should be applied, mutatis mutandis to a system configured to execute the method and / or should be applied, mutatis mutandis to a non-transitory computer readable medium that stores instructions executable by the system.

[0107] Any reference to a computer readable medium should be applied, mutatis mutandis to a method executed based on instructions stored in the computer readable medium and / or should be applied, mutatis mutandis to a system configured toe xecute the instructions.

[0108] In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims.

[0109] Moreover, the terms "front, " "back, " "top, " "bottom, " "over, " "under " and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.P0346

[0110] Any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as "associated with " each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "operably connected, " or "operably coupled, " to each other to achieve the desired functionality.

[0111] Furthermore, those skilled in the art will recognize that boundaries between the above described operations are merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time.Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.

[0112] While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

CLAIMS1. An X-ray source for semiconductor metrology, comprising:a target delivery system configured to deliver a liquid jet to a plasma interaction region within a vacuum chamber, wherein the liquid jet comprises a cryogenic liquid or an aqueous solution of a compound;a pulsed beam system configured to focus a beam onto the liquid jet at the plasma interaction region to generate plasma that emits X-rays at a plurality of X-ray emission lines; andX-ray focusing optics configured to receive the X-rays emitted from the plasma and to focus the X-rays onto a semiconductor wafer, wherein the X-ray focusing optics comprise at least one of a multilayer mirror or a crystal monochromator.

2. The X-ray source of claim 1 , wherein the cryogenic liquid comprises liquid krypton.

3. The X-ray source of claim 2, wherein the liquid jet composed of liquid krypton produces emission spectra in a wavelength range from 5.0 nanometers to 8.5 nanometers and Kr L lines at 1,586 eV and 1,637 eV.

4. The X-ray source of claim 1 , wherein the aqueous solution of a compound comprises gadolinium (III) chloride, and wherein the liquid jet produces characteristic X-ray emissions comprising:gadolinium L-lines at 6,057.2 eV, 6,025.0 eV, 6,713.2 eV, 7,102.8 eV, and 7,785.8 eV; gadolinium M-line at 1 , 185 eV;chlorine K-lines at 2,622.39 eV, 2,620.78 eV, and 2,815.6 eV; andoxygen K-line at approximately 524.9 eV from a water solvent.

5. The X-ray source of claim 1, wherein the aqueous solution of a compound comprises gadolinium (III) bromide, and wherein the liquid jet produces bromine L-lines at 1,480.43 eV and 1,525.90 eV.

6. The X-ray source of claim 1 , wherein the pulsed beam is a laser beam having a pulse duration ranging from 100 femtoseconds to 300 nanoseconds.

7. The X-ray source of claim 6, wherein the laser beam has a wavelength selected from 193 nm, 266 nm, 355 nm, 800 nm, 1064 nm, 2-4 pm, 5 pm, and 10.6 pm.P03468. The X-ray source of claim 1, wherein the pulsed beam is a laser beam having repetition rate ranging from 1 kHz to 50 kHz.

9. The X-ray source of claim 1, wherein the pulsed beam is a laser beam having a peak intensity ranging from lxl0A10 W / cmA2 to lxl0A18 W / cmA2.

10. The X-ray source of claim 1, wherein the crystal monochromator comprises a bent crystal monochromator having an ellipsoidal substrate figure configured to focus X-rays onto the semiconductor wafer.

11. The X-ray source of claim 10, wherein the bent crystal monochromator has an intrinsic blur of less than 2 micrometers for substrate surface figure and crystal alignment errors, and wherein crystal alignment errors do not exceed 2 arcseconds.

12. A method for generating X-rays for semiconductor metrology, comprising:delivering a liquid jet comprising a cryogenic liquid or an aqueous solution of a compound to a plasma interaction region within a vacuum chamber;focusing a pulsed beam onto the liquid jet at the plasma interaction region to generate a plasma that emits X-rays at a plurality of X-ray emission lines; and directing the X-rays emitted from the plasma through X-ray focusing optics onto a semiconductor wafer, wherein the X-ray focusing optics comprise at least one of a multilayer mirror or a crystal monochromator.

13. The method of claim 12, wherein the cryogenic liquid comprises liquid krypton, and wherein the plasma produces emission spectra in a wavelength range from 5.0 nanometers to 8.5 nanometers and Kr L lines at 1,586 eV and 1,637 eV.

14. The method of claim 12, wherein the aqueous solution of a compound comprises gadolinium (III) halide, and wherein delivering the liquid jet comprises adjusting a solution concentration of the gadolinium (III) halide to optimize X-ray yield for a selected emission line.

15. The method of claim 12, further comprising delivering a pre-pulse laser beam to the liquid jet to create pre-ionization of the liquid jet prior to focusing a pulsed laser beam onto the liquid jet.

16. The method of claim 15, wherein the pre -pulse laser beam and the pulsed laser beam are generated by a same pulsed beam system with different laser parameters comprising at least one of peak intensity, pulse width, repetition rate, or spot size.

17. An X-ray source system for semiconductor in-line metrology, comprising: a vacuum chamber;a reservoir configured to store a cryogenic liquid;a cooling system coupled to the reservoir and configured to maintain the cryogenic liquid in a liquid state;a nozzle configured to eject the cryogenic liquid into the vacuum chamber as a liquid jet;a pulsed beam system configured to direct a beam through a window of the vacuum chamber and onto the liquid jet to generate a plasma that emits X-rays, wherein the plasma emits X-rays at Kr L lines;X-ray collecting optics configured to receive the X-rays through an X-ray window of the vacuum chamber and to focus the X-rays onto a sample; anda liquid circulation system configured to collect the cryogenic liquid after plasma generation and return the collected cryogenic liquid to the reservoir.

18. The X-ray source system of claim 17, wherein the liquid circulation system comprises:a cryogenic liquid collector positioned to receive the cryogenic liquid after the cryogenic liquid passes through a plasma interaction region; anda high pressure pump configured to draw collected cryogenic liquid from the cryogenic liquid collector and return the collected cryogenic liquid to the reservoir.

19. The X-ray source system of claim 18, wherein the cryogenic liquid collector comprises a cooled element configured to maintain the cryogenic liquid in a liquid state for recirculation.

20. The X-ray source system of claim 17, wherein the X-ray collecting optics comprise at least one of a multilayer mirror or a bent crystal monochromator configured to select X-ray emission lines in an energy range between 1,460 eV and 8,000 eV.

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