Laser-generated plasma illuminator with low atomic number low temperature target

Abstract

The invention relates to a laser-generated plasma illuminator with a low atomic number low temperature target. A highly focused, short duration laser pulse is directed to a low atomic number, cryofreeze target, thereby igniting the plasma. In some embodiments, the target material comprises one or more elements having an atomic number of less than 19. In some embodiments, a low atomic number cryogenic target material is coated on a surface of a cryogenic cooled barrel configured to rotate and translate relative to an incident laser. In some embodiments, a low atomic number low temperature LPP light source produces multi-spectral line or broadband X-ray illumination in the soft X-ray (SXR) spectral range for measuring structural and material characteristics of a semiconductor structure. In some embodiments, reflective small angle X-ray scatterometry measurements are performed using a low atomic number low temperature LPP illumination source as described herein.

Description

[0001] Related Application Cross-Reference This application is a divisional application of the invention patent application filed on October 27, 2020, with application number 202080075446.4 and invention title "Laser-generated plasma illuminator with low atomic number low temperature target". Related Application Cross-Reference

[0002] This patent application claims priority to U.S. Provisional Patent Application No. 62 / 929,552, filed November 1, 2019, pursuant to 35 USC §119, the subject matter of which is incorporated herein by reference in its entirety. Technical Field

[0003] The described embodiments relate to x-ray measurement systems and methods, and more specifically to methods and systems for improved measurement accuracy. Background Technology

[0004] Semiconductor devices, such as logic and memory devices, are typically fabricated through a sequence of processing steps applicable to the sample. These processing steps form various features and multiple structural levels of the semiconductor device. For example, photolithography, one of the other processing steps, is a semiconductor fabrication process involving the creation of patterns on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical mechanical polishing, etching, deposition, and ion implantation. Multiple semiconductor devices can be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.

[0005] During semiconductor manufacturing processes, measurement techniques are used at various stages to detect defects on wafers to achieve higher yields. Several measurement-based techniques (including scattering, diffraction, and reflectance measurement implementations) and correlation analysis algorithms are typically used to characterize critical dimensions, film thickness, composition, and other parameters of nanoscale structures.

[0006] Traditionally, critical size measurements using scattering techniques are performed on targets composed of thin films and / or repeating periodic structures. During device fabrication, these thin films and periodic structures typically represent the geometry and material structure of the actual device or an intermediate design. As devices (e.g., logic and memory devices) advance towards smaller nanoscale dimensions, characterization becomes more challenging. Devices incorporating complex three-dimensional geometries and materials with diverse physical properties further complicate characterization.

[0007] In a process development environment at the front end of semiconductor fabrication facilities, accurate information about the material composition and shape of nanostructures is limited. Scatterometry optical metrology systems rely on accurate geometric and dispersion models to avoid measurement bias. With limited knowledge of the material composition and shape of the nanostructures available a priori, measurement recipe development and validation is a slow and lengthy process. For example, cross-sectional transmission electron microscope (TEM) images are used to guide optical scatterometry model development, but TEM imaging is slow and destructive.

[0008] Zeroth order diffraction signals from sub-wavelength structures are measured with infrared to visible scatterometry optical metrology tools. As device critical dimensions continue to shrink, scatterometry optical metrology sensitivity and capability are decreasing. Furthermore, when absorbing materials are present in the structures being measured, penetration and scattering of the illumination light in the optical regime (e.g., 0.5 eV to 10 eV) limits the utility of conventional optical metrology systems.

[0009] Similarly, electron beam based metrology systems have difficulty penetrating semiconductor structures due to absorption and scattering of the illumination, backscattered and secondary emitted electrons.

[0010] Atomic force microscopes (AFM) and scanning tunneling microscopes (STM) are capable of atomic resolution, but they can only probe the surface of a sample. Additionally, AFM and STM microscopes require long scanning times, which makes these techniques impractical in high volume manufacturing (HVM) settings.

[0011] Scanning electron microscopes (SEM) achieve a moderate level of resolution, but cannot penetrate structures to sufficient depth. Therefore, high aspect ratio holes are not well characterized. Additionally, the required charging of the sample has a detrimental effect on imaging performance.

[0012] X-ray based scatterometry systems have shown promise in addressing challenging measurement applications. For example, transmission small angle X-ray scatterometry (T-SAXS) systems employing photons at hard X-ray energy levels (> 15 keV), grazing incidence small angle X-ray scatterometry (GI-SAXS) systems operating near the reflection critical angle with photon energies above 8 keV, and reflection small angle X-ray scatterometry (RSAXS) systems employing photons in the soft x-ray (SXR) regime (80 eV to 5,000 eV) have exhibited the potential to address different metrology applications within the semiconductor industry.

[0013] In some embodiments, the RSAXS system provides a unique combination of sensitivity and speed. Nominal grazing incidence angles in the range between 5 and 20 degrees provide flexibility to select optimal incidence angles to achieve desired penetration into the measured structure and to maximize measurement information content with small beam spot size (e.g., less than 50 pm).

[0014] While X-ray based metrology systems provide an attractive solution for current and future semiconductor measurement applications, the development of reliable and cost effective X-ray illumination sources is challenging. Significant amount of effort has been spent to develop various versions of laser produced plasma (LPP) X-ray illumination sources. In LPP X-ray illumination sources, a target material is irradiated by an excitation source in a vacuum chamber to generate a plasma. In some examples, the excitation source is a pulsed laser beam.

[0015] In general, the peak emission observed in optically thin plasmas of relatively high atomic number (high-Z) elements in the extreme ultraviolet (EUV) and soft X-ray (SXR) spectral regions follows a quasi-Moseley law, as described by H. Ohashi et al., Appl. Phys. Lett. 106, 169903, 2015, the contents of which are incorporated by reference herein in their entirety. The peak wavelength l peak where R ∞ is the Rydberg constant and Z is the atomic number of the element undergoing stimulated emission.

[0016] (1)

[0017] As the atomic number Z increases from Z = 50 (tin) to Z = 83 (bismuth), the emission peak shifts from 13.5 nm to 4.0 nm. Tin-based LPP illumination sources provide optimal conversion efficiency for EUV lithography at 13.5 nm. In addition, light generated by tin-based LPP illumination sources is efficiently reflected by molybdenum / silicon multilayer mirrors (MLMs). Therefore, LPP target elements with relatively high atomic number are typically selected for EUV applications. Tin-based illumination sources are currently employed by leading manufacturers of EUV lithography tools (ASML).

[0018] In some embodiments, for EUV lithography or EUV / SXR metrology applications, EUV or SXR radiation is generated by a discharge of tin. A plasma is ignited in a gas medium between at least two electrodes in a discharge space. The gas medium is generated by partial vaporization of tin by a laser beam from a surface of a rotating disk in the discharge space. Additional explanation is provided in U.S. Patent No. 7,427,766, the contents of which are incorporated by reference herein in their entirety.

[0019] Unfortunately, the difficulty associated with debris mitigation and target replenishment (associated with tin) significantly limits EUV tool availability and results in extremely high tool costs. Tin debris deposition on the chamber walls and optical elements of the EUV tool is significant. In some examples, a hydrogen buffer gas is employed to protect and clean optical devices contaminated by tin debris. However, implementation of a hydrogen buffer gas results in high costs to address safety concerns.

[0020] To avoid the challenges associated with the use of tin targets, xenon (Z = 54) has been considered as a suitable LPP target. Inert cryogenic xenon ice used as an LPP target is chemically inactive and vaporizes immediately at room temperature. Thus, debris generated by a xenon LPP target does not deposit on the optical components. Xenon has an array of unresolved transition arrays (UTAs) in several charge states in the EUV and SXR spectral range. Thus, xenon has the potential to produce useful emissions for lithography and metrology applications.

[0021] In some embodiments, a solid xenon ice target material is formed on the surface of a bucket cooled by liquid nitrogen. A laser pulse irradiates a small area of the solid xenon target material deposited on the bucket. The bucket is rotated, translated, or both, to prevent new solid xenon target material at the irradiation site. Each laser pulse creates a crater in the layer of solid xenon target material. The craters are refilled by a replenishment system that provides new xenon target material to the surface of the bucket. Additional description is provided in U.S. Patent Nos. 6,320,937, 8,963,110, 9,422,978, 9,544,984, 9,918,375, and 10,021,773, the contents of which are incorporated by reference herein in their entirety.

[0022] In some embodiments, a liquid xenon target material stream is used as an LPP target. In one embodiment, a xenon liquefier unit is connected to a xenon mass flow (gas) system within a vacuum chamber, as well as a xenon recovery unit. The xenon recovery unit is connected to the xenon liquefier unit via a capillary tube. The liquid xenon stream flows from the xenon liquefier unit to the xenon recovery unit through the capillary tube. The capillary tube includes an orifice that exposes the liquid xenon stream to a focused laser beam of EUV / SXR radiation that induces an emission plasma. Additional description is provided in U.S. Patent No. 8,258,485, which is incorporated by reference herein in its entirety.

[0023] In some other embodiments, liquid xenon target material droplets are used as LPP targets. In one embodiment, xenon is pressurized and cooled such that it liquefies. The liquid xenon is pumped as a jet through a nozzle. As the jet exits the nozzle, it begins to decay. As the jet decays, a xenon droplet forms. Depending on the conditions, the droplet can be liquid or solid. The droplet travels to a site in a vacuum environment where it is irradiated by a laser beam to generate an EUV / SXR emitting plasma. Additional explanation is provided in U.S. Patent No. 9,295,147 and U.S. Patent Publication No. 2017 / 0131129 Al, the contents of which are incorporated herein by reference in their entirety.

[0024] Unfortunately, implementation of droplet-based LPP targets, such as tin or xenon droplets, introduces additional challenges. For a plasma to be reliably stimulated, droplet position stability is critical. To achieve suitable conversion efficiency, the droplet must accurately reach the irradiation site to ensure sufficient coupling between the target material droplet and the focused laser beam. The environment from the nozzle to the irradiation site significantly affects position stability. Important factors include path length, temperature and pressure conditions along the path, and any gas flow along the path. Many of these factors are difficult to control, which leads to suboptimal LPP illumination source performance.

[0025] Additionally, as a xenon liquid jet or series of column droplets travel, a portion of the xenon evaporates and creates a xenon gas cloud surrounding the emission site. Xenon gas strongly absorbs EUV / SXR light, resulting in very inefficient extraction of available EUV / SXR light from the LPP light source.

[0026] Moreover, xenon supply is limited and expensive. Xenon is a trace component in the atmosphere (eighty-seven parts per billion). A complex and expensive air separation process is required to extract xenon from the atmosphere. In response, expensive recirculation equipment is required to recapture as much xenon as possible from the LPP illumination source environment to minimize xenon loss.

[0027] As an LPP target material, xenon atoms are highly ionized and excited into various high-energy ion states under electron impact or laser field. One or more buffer gases, such as argon, neon, oxygen, nitrogen, and hydrogen, are employed to decelerate and eventually stop the high-energy xenon ions to prevent etching of the chamber and optical elements. To recover xenon swept by the buffer gas, the gas within the LPP chamber is continuously pumped out by a vacuum pump and sent to a noble gas recovery unit. The gas recovery unit separates xenon from the buffer gas using one or more gas separation techniques and purifies the recovered xenon.

[0028] Unfortunately, xenon gas recovery units are extremely expensive and do not achieve 100% recirculation efficiency. The long-term cost of ownership (COO) of a tool utilizing a xenon gas recovery system can be extremely significant. Figure 1The diagram illustrates how the annual cost of ownership of xenon attributable to losses varies with the nominal flow rate of xenon for tools in continuous operation. Figure 10 .like Figure 1 The diagram illustrates the annual costs plotted for different recycling efficiencies. Lines 11, 12, 13, and 14 depict the annual costs associated with recycling efficiencies of 98%, 98.5%, 99%, and 99.5%, respectively. Each of these recycling efficiencies is extremely difficult to achieve in practice, yet the annual costs remain quite high.

[0029] Finally, xenon's SXR emission spectrum is broadband, similar to other high atomic number elements. Delivery optics used to extract SXR illumination from LPP illumination sources and deliver it to semiconductor wafers are limited in their ability to maintain spectral purity and minimize photon flux loss, because SXR optics typically trade off photon flux for spectral purity.

[0030] In summary, the semiconductor industry continues to shrink device size while increasing its complexity. To achieve process optimization and yield improvements, new online measurement tools are needed to provide process developers with accurate structural information in a fast and non-destructive manner. X-ray-based measurement systems show promise, but improvements are expected in the LPP illumination source used to deliver X-rays to the measured structure. Summary of the Invention

[0031] This paper presents a method and system for generating X-ray illumination from laser-generated plasma employing a low atomic number cryogenic target. Additionally, it presents a method and system for measuring the structural and material properties (e.g., material composition, dimensional properties, etc.) of semiconductor structures associated with different semiconductor fabrication processes based on the generated X-ray illumination.

[0032] In some embodiments, a low atomic number cryogenic LPP source guides a highly focused, short-duration laser source to a low atomic number cryogenic target. The interaction between the focused laser pulse and the low atomic number cryogenic target ignites the plasma. In some embodiments, the low atomic number cryogenic LPP source produces multi-line or broadband X-ray illumination within the soft X-ray (SXR) spectral range (e.g., 10 electron volts to 5,000 electron volts). As described herein, the low atomic number cryogenic target comprises one or more elements, each having an atomic number less than 19.

[0033] In some embodiments, a low atomic number cryogenic target material is coated onto the surface of a cryogenically cooled barrel configured to rotate and translate relative to the incident laser. When the low atomic number cryogenic target material is removed from the barrel surface by plasma, replacement target material is deposited onto the barrel surface in a liquid or gas phase. The deposited material freezes onto the barrel surface. The thickness of the frozen low atomic number target material on the barrel surface is maintained by a scraper mechanism.

[0034] Low atomic number (LNP) cryogenic LPP (low atomic number plasma) sources have a relatively large lateral range (e.g., several hundred millimeters in both lateral directions). This large lateral range minimizes the lateral stability requirements for target positioning, as the target area is so large compared to droplet-based targets. Similarly, the repositioning of the plasma source can be easily achieved by simply controlling the target of the pump laser beam to reposition the incident point to another location on the target. Finally, using low atomic number materials as the emission material minimizes cost because many low atomic number materials (e.g., carbon, oxygen, nitrogen, etc.) are readily available in the environment. Therefore, expensive rare gas recirculation systems are unnecessary. These materials can be frozen and used as LNP cryogenic targets in their pure form, or dissolved in a solvent, then frozen and used as LNP cryogenic targets.

[0035] In one aspect, RSAXS measurements are performed using X-ray radiation generated by a low atomic number cryogenic LPP illumination source. The X-ray illumination radiation emitted from the low atomic number cryogenic LPP source passes through a beamline and is focused onto the semiconductor wafer being measured.

[0036] In another further aspect, the low atomic number cryogenic LPP light source includes a debris management system comprising a guided buffer gas flow in the plasma chamber and a vacuum pump for extracting the buffer gas and any contaminants.

[0037] In another further aspect, the low atomic number cryogenic LPP light source includes a magnetic field source that spans a portion of the plasma chamber to drive dynamic ions toward a buffer gas flow within the plasma chamber. In this way, the magnetic field facilitates the removal of dynamic ions by driving them into the buffer gas flow as the buffer gas flows through the plasma chamber toward a vacuum pump used to exhaust the buffer gas from the plasma chamber.

[0038] In another aspect, X-ray-based measurement systems comprise multiple detectors for individually detecting the zero diffraction order and higher diffraction orders. Generally, any combination of multiple detectors can be considered for detecting the zero diffraction order and higher diffraction orders.

[0039] In another approach, the X-ray-based measurement system incorporates a multi-layered diffractive optical structure within the illumination path to filter the X-ray illumination light. This eliminates the need for a vacuum window within the illumination path.

[0040] In another approach, an X-ray-based measurement system incorporates a zone plate structure located in the illumination path to refocus the excitation light back onto the laser-generated plasma source. In this way, radiation that would otherwise be discarded is used to excite the plasma.

[0041] The foregoing description is an inventive summary and therefore necessarily contains simplifications, summaries, and omissions of details; thus, those skilled in the art will understand that the inventive summary is merely illustrative and not in any way limiting. Other aspects, inventive features, and advantages of the apparatus and / or process described herein will become apparent from the non-limiting detailed description set forth herein. Attached Figure Description

[0042] Figure 1 This is a simplified diagram illustrating the cost associated with xenon loss during continuous operation of a laser-generated plasma (LPP) illumination source.

[0043] Figure 2 This is a simplified diagram illustrating an embodiment of a measurement system in at least one novel aspect, the measurement system comprising a laser-generated plasma (LPP) X-ray illumination source having a low atomic number cryogenic target for measuring the properties of a sample.

[0044] Figure 3 Figure 140 is a simulation illustrating the change in molecular density over time in a dielectric barrier discharge plasma during a discharge with a specific energy input of 129 joules / cubic centimeter.

[0045] Figure 4 Figure 150 is a simulation illustrating how the stopping range of carbon, oxygen, and xenon ions in nitrogen (N2) gas varies with the energy of the ions.

[0046] Figure 5 Figure 170 depicts the simulated emission spectrum associated with an LPP X-ray illumination source that uses carbon as a low atomic number cryogenic target.

[0047] Figure 6 Figure 173 depicts the simulated emission spectrum associated with an LPP X-ray illumination source employing nitrogen as a low atomic number cryogenic target.

[0048] Figure 7 Figure 176 depicts the simulated emission spectrum associated with an LPP X-ray illumination source that uses oxygen as a low atomic number cryogenic target.

[0049] Figure 8 It is a simplified diagram illustrating the exemplary model construction and analysis engine.

[0050] Figure 9 This is a simplified diagram illustrating another embodiment of a measurement system in at least one novel aspect, the measurement system comprising a laser-generated plasma (LPP) X-ray illumination source having a low atomic number cryogenic target for measuring the properties of a sample.

[0051] Figure 10 This is a simplified diagram illustrating yet another embodiment of a measurement system in at least one novel aspect, the measurement system comprising a laser-generated plasma (LPP) X-ray illumination source having a low atomic number cryogenic target for measuring the properties of a sample.

[0052] Figure 11 This is a simplified diagram illustrating yet another embodiment of a measurement system in at least one novel aspect, the measurement system comprising a laser-generated plasma (LPP) X-ray illumination source having a low atomic number cryogenic target for measuring the properties of a sample.

[0053] Figure 12 This is a flowchart of a method for performing measurements on a semiconductor wafer using a measurement system based on the method described herein, wherein the measurement system employs an LPP X-ray illumination source with a low atomic number cryogenic target. Detailed Implementation

[0054] Examples of the present invention will now be illustrated in detail with reference to background art examples and some embodiments of the present invention, and in the accompanying drawings.

[0055] This paper presents a method and system for generating X-ray illumination from laser-generated plasma employing a low atomic number cryogenic target. Additionally, it presents a method and system for measuring the structural and material properties (e.g., material composition, dimensional properties, etc.) of semiconductor structures associated with different semiconductor fabrication processes based on the generated X-ray illumination.

[0056] In some embodiments, a laser-generated plasma (LPP) light source produces high brightness (i.e., greater than 10). 13 Photon / (sec . mm 2. mrad 2. 1% bandwidth) X-ray illumination. To achieve this high brightness, the LPP source guides a highly focused, short-duration laser pulse to a low atomic number cryogenic target. The interaction between the focused laser pulse and the low atomic number cryogenic target ignites the plasma. Radiation from the plasma is collected by collecting optics and guided to the sample being measured.

[0057] In some embodiments, low atomic number cryogenic LPP sources produce multi-line or broadband X-ray illumination within a soft X-ray (SXR) spectral range (e.g., 10 eV to 5,000 eV). The SXR spectral range as defined herein may include all or part of the vacuum ultraviolet (VUV), extreme ultraviolet (EUV), soft X-ray, and hard X-ray ranges as defined in other literature. As described herein, low atomic number cryogenic targets comprise one or more elements, each having an atomic number less than 19.

[0058] Low atomic number (LNP) cryogenic LPP (low atomic number plasma) sources have a relatively large lateral range (e.g., several hundred millimeters in both lateral directions). This large lateral range minimizes the lateral stability requirements for target positioning, as the target area is so large compared to droplet-based targets. Similarly, the repositioning of the plasma source can be easily achieved by simply controlling the target of the pump laser beam to reposition the incident point to another location on the target. Finally, using low atomic number materials as the emission material minimizes cost because many low atomic number materials (e.g., carbon, oxygen, nitrogen, etc.) are readily available in the environment. Therefore, expensive rare gas recirculation systems are unnecessary. These materials can be frozen and used as LNP cryogenic targets in their pure form, or dissolved in a solvent, then frozen and used as LNP cryogenic targets.

[0059] Figure 2 An x-ray-based metrology system 100 is depicted in one embodiment. By way of non-limiting example, the x-ray-based metrology system 100 is configured as a small-angle reflected X-ray scattering (RSAXS) system. In some embodiments, RSAXS measurements are performed at one or more wavelengths in the soft x-ray (SXR) region (e.g., 10 eV to 5000 eV) with a nominal grazing incidence angle ranging from 1 degree to 45 degrees. The grazing incidence angle for a particular measurement application is selected to achieve the desired penetration into the measured structure and to maximize the measurement information content with a small beam size (e.g., less than 50 micrometers). The RSAXS system (e.g., metrology system 100) performs measurements of parameters of interest, including critical dimensions, overlap, and edge placement errors. SXR illumination enables overlap measurements on a design-regular target because the illumination wavelength is shorter than the period of the measured structure. This provides a significant advantage over the prior art, in which overlap is measured on targets larger than the design regulations. Using SXR wavelengths allows for target design according to process design rules, i.e., without "non-zero offset". In some embodiments, the overlay measurement target for RSAXS measurements can be used to measure both overlay and critical dimensions. This also enables the measurement of edge placement errors (EPE) (e.g., end-line shortening, line-to-contact distance, etc.).

[0060] In one aspect, RSAXS measurements are performed using x-ray radiation generated by a low-atomic-number low-temperature LPP illumination source. For example... Figure 2 As depicted, the X-ray-based metrology system 100 includes a low atomic number cryogenic LPP light source 101, a beamline 200, and a wafer metrology subsystem 300. X-ray illumination radiation emitted from the low atomic number cryogenic LPP light source 101 passes through the beamline 200 and is focused onto a semiconductor wafer 306. X-ray radiation is collected from the semiconductor wafer 306 in response to the incident X-ray illumination radiation and the X-ray radiation is detected. Based on the detected X-ray radiation, values ​​of one or more parameters of interest characterizing one or more structures 307 disposed on the semiconductor wafer 306 are estimated.

[0061] like Figure 2 As depicted, the low atomic number cryogenic LPP light source 101 includes a barrel 106 coated with a layer of low atomic number cryogenic target material 107. A rotational actuation system 108 rotates the barrel 106 about axis A. Additionally, a linear actuation system 109 translates the barrel 106 along axis A. Figure 2 In the embodiment depicted, the computing system 130 transmits control commands to the rotary actuator system 108 and the linear actuator system 109, the control commands causing the rotary actuator system 108 to rotate the barrel 106 at a desired angular velocity and causing the linear actuator system 109 to drive the barrel 106 at a desired linear velocity. In this way, the computing system 130 controls the trajectory of the surface of the barrel 106 exposed to illumination light from the laser illumination source 114.

[0062] A controlled flow of liquid nitrogen 102 circulates through a container 106 to maintain the surface of the container 106 at a temperature that keeps the low atomic number target material 107 in a solid state. When the low atomic number cryogenic target material 107 is removed from the surface of the container 106 by plasma 103, a replacement target material is deposited onto the surface of the container 106 in a liquid or gas phase, and this replacement target material is then frozen onto the surface of the container 106. Figure 2 As depicted, target material source 110 provides low atomic number target material in gaseous or liquid phase to pump 112. Pulse damper 113 is located near the output of pump 112 to remove any high-frequency pressure pulsations that may be introduced by pump 112. Pump 112 pressurizes low atomic number target material stream 124, which is delivered through nozzle 104 to the surface of barrel 106. The thickness of the frozen low atomic number target material on the surface of barrel 106 is maintained by scraper mechanism 105 (e.g., blades located at a fixed distance from the surface of cryogenically cooled barrel 106). In some embodiments, the thickness of the low atomic number target material deposited on the cryogenically cooled barrel is between 200 micrometers and 1 millimeter.

[0063] A pulsed laser illumination source 114 emits a series of excitation (pumping) light pulses directed toward the surface of barrel 106. For example... Figure 2 As depicted, the excitation light passes through a beam expander 115, one or more focusing optics 116, and an optical window 117 to reach a low atomic number cryogenic target material deposited on the surface of barrel 106. The interaction between the excitation light pulse and the target material causes the target material to ionize to form a plasma 103, which emits X-ray illumination light with extremely high brightness. In a preferred embodiment, the brightness of plasma 103 is greater than 10. 13 Photon / (sec) . (mm 2 ) . (mrad 2 ) . (1% bandwidth).

[0064] Focusing optics 116 focuses the excitation light onto the target material within a very small spot size. In some embodiments, the excitation light is focused onto the target material with a spot size of less than 100 micrometers. In some embodiments, the excitation light is focused onto the target material with a spot size of less than 20 micrometers. In a preferred embodiment, the excitation light is focused onto the target material with a spot size of less than 10 micrometers. As the spot size of the excitation light decreases, the spot size of the induced plasma decreases. In some embodiments, the spot size of the plasma 103 is less than 400 micrometers. In some embodiments, the spot size of the plasma 103 is less than 100 micrometers. In some embodiments, the spot size of the plasma 103 is less than 20 micrometers.

[0065] In some embodiments, the pulsed laser illumination source 114 is a ytterbium (Yb)-based solid-state laser. In some other embodiments, the pulsed laser illumination source 114 is a neodymium (Nb)-based solid-state laser. In some embodiments, the pulsed laser illumination source 114 is, for example, a picosecond laser operating at wavelengths within the IR range (e.g., 1 micrometer). In some embodiments, the excitation light has a beam quality factor M² < 2.0, a pulse duration in the range of 5 picoseconds to 500 picoseconds, a pulse energy in the range of 10 millijoules to 500 millijoules, a peak power in the range of 50 megawatts to 1,000 megawatts, and is maintained at 10 13 W / cm 2 Or higher focused laser intensity and a contrast ratio greater than 200.

[0066] As the barrel 106 rotates and translates, pits are formed along a spiral path on the surface of the barrel 106 due to exposure to excitation light from the pulsed laser illumination source 114. However, the nozzle 104 deposits new target material, and the scraper mechanism 105 smooths the deposited material on the surface of the barrel 106. Therefore, the pits are filled before the next exposure to excitation light from the pulsed laser illumination source 114. Figure 2 As depicted, nozzle 104 has an outlet orifice located at a fixed distance from the surface of barrel 106. In some embodiments, nozzle 104 is mechanically coupled directly or indirectly to plasma chamber 125 to maintain a fixed distance from the surface of barrel 106 with high stability. Low atomic number target material stream 124 exits the outlet orifice of nozzle and is deposited onto the surface of cryogenically cooled barrel as the barrel rotates and translates. In some embodiments, the low atomic number target material stream exits the outlet orifice of nozzle 104 in a gaseous phase. In some embodiments, the low atomic number target material stream exits the outlet orifice of nozzle 104 in a liquid phase. Similarly, scraper mechanism 105 is located at a fixed distance from the surface of barrel 106. In some embodiments, scraper mechanism 105 is directly or indirectly coupled to plasma chamber 125 to maintain a fixed distance from the surface of cryogenically cooled barrel. In this way, as the cryogenically cooled barrel rotates and translates, scraper mechanism 105 scrapes the low atomic number target material cryogenically frozen to the surface of cryogenically cooled barrel to a predetermined thickness.

[0067] Generally, low atomic number cryogenic LPP X-ray illumination sources can employ any suitable material or combination of materials as low atomic number cryogenic targets. However, materials comprising elements having relatively low atomic numbers are preferred. In some embodiments, the low atomic number cryogenic target comprises one or more materials, each comprising one or more elements having an atomic number less than 19 (Z<19). The low atomic number cryogenic target is maintained in a solid or gas phase during delivery to container 106 by providing suitable pressure and temperature conditions. In some embodiments, the low atomic number cryogenic target comprises a liquid solvent that maintains another material in solution. In some of these embodiments, the solvent comprises one or more materials, each comprising one or more elements having an atomic number less than 19 (Z<19). By way of non-limiting examples, suitable low atomic number cryogenic target materials include ethanol, water, hydrocarbons, CO2, N2O, CO, N2, O2, F2, H2O2, urea, ammonium hydroxide, sodium hydroxide, magnesium hydroxide, aluminum hydroxide, silicon hydroxide (e.g., hydroxides in the form of soda ash, such as NaOH (caustic soda), Na2CO3 (washing soda), NaHCO3 (baking soda)), salts (e.g., fluoride salts, chloride salts soluble in liquid solvents), and any low atomic number material (Z<19) soluble in liquid solvents.

[0068] Figure 5 Figure 170 depicts the simulated emission spectrum associated with the spectral contribution of carbon to radiation emitted from an LPP X-ray illumination source employing a target material containing carbon as a component. Line 171 depicts the emission spectrum associated with a plasma temperature of 100 electron volts. Line 172 depicts the emission spectrum associated with a plasma temperature of 500 electron volts.

[0069] Figure 6 Figure 173 depicts the simulated emission spectrum associated with the spectral contribution of nitrogen to radiation emitted from an LPP X-ray illumination source employing a target material containing nitrogen as a component. Line 174 depicts the emission spectrum associated with a plasma temperature of 100 electron volts. Line 175 depicts the emission spectrum associated with a plasma temperature of 500 electron volts.

[0070] Figure 7 Figure 176 depicts the simulated emission spectrum associated with the spectral contribution of oxygen to radiation emitted from an LPP X-ray illumination source employing a target material containing oxygen as a component. Line 177 depicts the emission spectrum associated with a plasma temperature of 100 electron volts. Line 178 depicts the emission spectrum associated with a plasma temperature of 500 electron volts.

[0071] like Figures 5 to 7 The diagram illustrates that strong spectral line emission exists across a wide plasma temperature range for all these low atomic number materials. Furthermore, the spectral line emission lies entirely within the reflectivity bandwidth of the MLM optics. Therefore, it is expected that the spectral purity of low atomic number low-temperature LPP sources should be significantly better compared to LPP sources using tin-based or xenon-based target materials.

[0072] Figure 3 Figure 140 depicts the simulated molecular density over time in a dielectric barrier discharge plasma targeting a CO2 cryogenic target material during a discharge with a specific energy input (SEI) of 129 joules / cm³. (As shown in Figure 140...) Figure 3 The illustration shows that the molecular density in dielectric barrier discharge plasma is comparable to that in LPP plasma in terms of plasma dynamics and chemical properties. Additional notes are provided by A. Robby et al., Chemsuschem - ISSN 1864-5631 - 8:4 (2015), pp. 702-716 (the contents of which are incorporated herein by reference in their entirety). Figure 3 The diagram illustrates that the main dissociation pathway is the splitting of CO2 into CO and O. Both CO2 and CO are stable molecules. After 100 nanoseconds, for example, CO2... + Other carbon-containing molecules are at least three orders of magnitude smaller than CO. Therefore, CO2 is an effective debris-free target for LPP plasma. Furthermore, CO2 acts as a cleaner for oxygen from the plasma chamber.

[0073] In another further aspect, the low atomic number cryogenic LPP light source includes a debris management system comprising a guided buffer gas flow within the plasma chamber and a vacuum pump for extracting the buffer gas and any contaminants. Figure 2 As depicted, the plasma chamber 125 includes one or more walls that contain a buffer gas flow 121 within the plasma chamber. The buffer gas prevents high-energy ions and neutral particles from depositing on sensitive optical elements near the plasma 103. Figure 2As depicted, a buffer gas flow 119 is distributed within the plasma chamber 125 via one or more gas cones 120. In some embodiments, each gas cone 120 directs a high-velocity longitudinal gas flow toward the source of debris (i.e., plasma 103) to prevent debris from reaching one or more optical elements. In some embodiments, one or more gas cones are positioned before the window 117, beamline 200, and flux monitor 118. In some embodiments, a buffer gas flow is provided around the location of plasma 103 to facilitate contaminant flow away from the immediate vicinity of plasma 103. Further description of a debris mitigation technique incorporating gas cones is provided in U.S. Patent No. 10,101,664, the contents of which are incorporated herein by reference in their entirety. Figure 2 As depicted, a vacuum pump 122 is used to extract the contaminated buffer gas stream 121 from the plasma chamber 125. The extracted material 123 is discharged from the system without recirculating the buffer gas material or target material extracted by the vacuum pump 122, because these materials are low cost.

[0074] Figure 4 Figure 150 illustrates the stopping range of oxygen, carbon, and xenon ions in nitrogen (N2) gas as a function of ion energy. Line 151 depicts the average stopping range associated with each xenon ion in the set of xenon ions that stops at the plotted ion energy. Line 152 depicts the average stopping range associated with each oxygen ion in the set of oxygen ions that stops at the plotted ion energy. Line 153 depicts the average stopping range associated with each carbon ion in the set of carbon ions that stops at the plotted ion energy. Figure 4 The diagram illustrates that when using N2 buffer gas, both carbon and oxygen ions require a larger stopping range compared to xenon ions.

[0075] like Figure 4 The illustration shows that oxygen ions with an initial kinetic energy (i.e., ion energy) of 30 keV require a stopping range of 30 mbar-cm in a nitrogen buffer gas. For example, maintaining a nitrogen buffer gas at 3 mbar will have a high probability of stopping oxygen ions with an initial kinetic energy of up to 30 keV within a path length of 10 cm. In another example, maintaining a nitrogen buffer gas at 1 mbar will have a high probability of stopping oxygen ions with an initial kinetic energy of up to 30 keV within a path length of 30 cm. In some embodiments, the distance between the window of plasma chamber 125 and plasma 103 is at least 10 cm.

[0076] In another further aspect, the low atomic number cryogenic LPP light source includes a magnetic field source that spans a portion of the plasma chamber to drive dynamic ions toward a buffer gas flow within the plasma chamber. In this way, the magnetic field facilitates the removal of dynamic ions by driving them into the buffer gas flow as the buffer gas flows through the plasma chamber toward a vacuum pump used to remove the buffer gas from the plasma chamber. In some instances, a set of permanent magnets, electromagnets, etc., is positioned across the field of the buffer gas flow to generate a magnetic field that drives ions into the buffer gas flow before it is extracted.

[0077] like Figure 2 As depicted, X-ray illumination light emitted from plasma 103 exits plasma chamber 125, passes through beamline 200, and enters wafer measurement subsystem 300. Generally, the X-ray illumination path from plasma 103 to wafer 306 includes a number of illumination control elements for shaping, guiding, and filtering the X-ray illumination light. In some embodiments, an energy filter is included in the illumination path to select the desired beam energy. In some embodiments, one or more optical elements are located in the illumination path to control beam divergence, incident angle, azimuth angle, or any combination thereof. In some embodiments, a vacuum window is located in the illumination path to separate the environment of plasma chamber 125 from the environment of wafer measurement subsystem 300. In some of these embodiments, a vacuum window material, one or more thin films deposited on the vacuum window, or both are selected to filter the energy of the X-ray illumination light passing through the vacuum window. In some embodiments, one or more optical elements are located in the illumination path to amplify or reduce the X-ray illumination beam. In some embodiments, diffraction grating structures are formed on the surfaces of one or more illumination optics to enhance the spectral purity of the X-ray illumination light.

[0078] exist Figure 2 In the embodiment depicted, X-ray illumination emitted by plasma 103 enters beamline 200 and passes through pneumatic gate valve 201A, vacuum window 202, orifice system 203, vacuum window monitoring and safety device 204, and pneumatic gate valve 201B. Pneumatic gate valves 201A and 201B are located at both ends of beamline 200. During measurement system operation, pneumatic gate valves 201A and 201B remain open. However, in cases where isolation between plasma chamber 125 and measurement chamber 311 is desired, one or more of pneumatic gate valves 201A and 201B are closed. When both pneumatic gate valves 201A and 201B are closed, beamline chamber 205 is formed, environmentally isolated from both plasma chamber 125 and measurement chamber 311.

[0079] exist Figure 2In the embodiment depicted, vacuum window 202 is located in the illumination path between pneumatic gate valves 201A and 201B to separate the vacuum environment of plasma chamber 125 from metrology chamber 311. In one embodiment, vacuum window 202 includes a thin coating to prevent infrared wavelengths generated by pulsed laser illumination source 114 from reaching metrology subsystem 300.

[0080] The aperture system 203 controls the numerical aperture, nominal grazing incidence angle (AOI), and azimuth of the X-ray illumination beam at the wafer 306. In some embodiments, the aperture system 203 is a four-bladed programmable aperture device. In some embodiments, the computing system 130 transmits control commands (not shown) to the aperture system 203 to control the position of each of the four blades relative to the X-ray illumination beam 302 to achieve the desired beam numerical aperture, nominal grazing incidence angle (AOI), and azimuth at the wafer 306.

[0081] Generally, an RSAX measurement system (e.g., measurement system 100) includes one or more beam slits or apertures that shape the X-ray illumination beam incident on wafer 306 and selectively block portions of the illumination light that would otherwise illuminate the target being measured. The beam slits define the beam size and shape such that the X-ray illumination spot is suited to the region of the target being measured. Additionally, the beam slits define the beam divergence to minimize the overlap of diffraction orders on the detector.

[0082] like Figure 2 As illustrated in the diagram, the vacuum window monitoring and safety device 204 is positioned across the beamline 200 between the vacuum window 202 and the measurement chamber 300. The vacuum window monitoring and safety device 204 monitors the integrity of the vacuum window 202. If the vacuum window 202 experiences a mechanical failure (i.e., breaks or otherwise fractures into one or more pieces), the vacuum window monitoring and safety device 204 rapidly closes the space across the beamline 200 to capture any fragments of the vacuum window 202 and prevent contamination of the measurement chamber 300. In some embodiments, the vacuum window monitoring and safety device 204 includes a rapid mechanical shutter or pneumatic actuator to rapidly close any space across the beamline 200. In some embodiments, activation of the vacuum window monitoring and safety device 204 also triggers the closure of pneumatic gate valves 201A and 201B to provide additional protection. However, due to its relatively large mass, a considerable amount of time may be required for the pneumatic gate valves 201A and 201B to fully close and isolate the beamline chamber.

[0083] exist Figure 2In the embodiments depicted herein, X-ray illumination light entering the metrology subsystem 300 from beamline 200 is incident on an ellipsoidal mirror 303. In some embodiments, the ellipsoidal mirror 303 images the X-ray illumination source spot onto a metrology target 307 mounted on a wafer 306 with a reduction factor ranging from 0.5 to 0.1 (i.e., projects an image of the source onto a wafer that is 1 / 2 to 1 / 10 the size of the source). In one embodiment, the RSAXS system as described herein employs an X-ray illumination source having a source region characterized by a lateral dimension of 20 micrometers or less (i.e., a source size of 20 micrometers or less) and a focusing mirror with a reduction factor of 0.1. In this embodiment, the focusing mirror projects illumination onto the wafer 306 with an incident illumination spot size of two micrometers or less.

[0084] The X-ray illumination source spot is located at one focal point of ellipsoidal mirror 303, and the measuring target 307 is located at the other focal point of ellipsoidal mirror 303. Ellipsoidal mirror 303 includes a film mirror modulator (MLM) with a gradient thickness to compensate for variations in the grazing incidence angle across the surface of ellipsoidal mirror 303. The aperture of ellipsoidal mirror 303 defines the maximum numerical aperture (NA) 301 from the X-ray illumination source spot and the maximum NA 305 to wafer 306. By controlling the aperture system 203, the grazing AOI, NA, and azimuth angle of wafer 306 can be scanned within the maximum NA cone 305. For example, Figure 2 The diagram illustrates NA 304 within the largest NA cone 305.

[0085] Generally, focusing optics (e.g., elliptical mirror 303) collect the source emission and select one or more discrete wavelengths or spectral bands, and focus the selected light onto the wafer 306 at a nominal grazing incidence angle in the range of 1 degree to 45 degrees.

[0086] In some embodiments, the focusing optics includes a graded multilayer that selects a desired wavelength or wavelength range for projection onto wafer 306. In some instances, the focusing optics includes a graded multilayer structure (e.g., a layer or coating) that selects a wavelength and projects the selected wavelength onto wafer 306 at a range of incident angles. In some instances, the focusing optics includes a graded multilayer structure that selects a wavelength range and projects the selected wavelength onto wafer 306 at a single incident angle. In some instances, the focusing optics includes a graded multilayer structure that selects a wavelength range and projects the selected wavelength onto wafer 306 at a range of incident angles.

[0087] Graded-multilayer optics preferably minimize optical loss when the single-layer grating structure is too deep. Generally, multilayer optics select the reflected wavelength. The spectral bandwidth of the selected wavelength is optimized to provide flux to wafer 306, information content in the measured diffraction order, and to prevent signal degradation through angular dispersion and diffraction peak overlap at the detector. In addition, graded-multilayer optics are used to control divergence. Angular divergence at each wavelength is optimized to achieve flux at the detector and minimal spatial overlap.

[0088] In some instances, graded-multilayer optics select wavelengths to enhance the contrast and information content of diffraction signals from specific material interfaces or structural dimensions. For example, the selected wavelengths can be chosen to span element-specific resonance regions (e.g., silicon K-edge, nitrogen K-edge, oxygen K-edge, etc.). Additionally, in these instances, the illumination source can be tuned to maximize flux in the selected spectral region (e.g., HHG spectral tuning, LPP laser tuning, etc.).

[0089] exist Figure 2 In the embodiments depicted, the X-ray-based measurement system 100 includes a wafer positioning system 320 for positioning and orienting a wafer 306 relative to incident X-ray illumination. In some embodiments, the wafer positioning system 320 is configured to rotate the wafer 306 to perform angle-resolved measurements of the wafer 306 at any number of locations on the surface of the wafer 306. In one example, a computing system 130 transmits command signals (not shown) to a motion controller of the wafer positioning system 320, the command signals indicating the desired position and orientation of the wafer 306. In response, the motion controller generates command signals to various actuators of the wafer positioning system 320 to achieve the desired position and orientation of the wafer 306.

[0090] In some embodiments, the measurement system 100 includes one or more collecting optics that collect light from wafer 306 and guide at least a portion of the collected light 308 to detector 310. In some embodiments, one or more aperture elements (e.g., slits) are located in the X-ray collection path to block some of the reflected light, one or more diffraction orders. In some embodiments, one or more spatial attenuators are located in the collection path to selectively attenuate some of the reflected light (i.e., reduce its intensity), for example, selectively reducing the intensity of one or more diffraction orders. Figure 2In the embodiment depicted, spatial attenuator 309 is located in a portion of the collection path associated with the zero order. In this way, before detection by detector 310, spatial attenuator 309 makes the intensity of the zero diffraction order equal to the intensity of the higher diffraction orders. Attenuating the intensity of the zero order relative to the higher diffraction orders can be advantageous to avoid saturating detector 310 when the intensity of the zero order is significantly greater than any of the higher diffraction orders. In other embodiments, a beam blocker is used to block the zero order to prevent undesirable flares across the photosensitive surface of the detector caused by strong zero-order reflections.

[0091] The measurement system 100 also includes one or more detectors for measuring intensity, energy, wavelength, etc., associated with the diffraction order. In some embodiments, detector 310 detects diffracted light at multiple wavelengths and incident angles. In some embodiments, the position, orientation, or both of detector 310 are controlled to capture diffracted light from measurement target 307.

[0092] like Figure 2 As depicted, the X-ray detector 310 detects X-ray radiation scattered from the wafer 306 according to the RSAXS measurement mode and generates an output signal 135 indicating the properties of the wafer 306 that are sensitive to incident X-ray radiation. In some embodiments, the X-ray detector 310 collects the scattered X-rays, while the sample positioning system 320 positions and orients the wafer 306 to produce angle-resolved scattered X-rays.

[0093] In some embodiments, the RSAXS system includes a high dynamic range (e.g., greater than 10). 5 One or more photon counting detectors. In some embodiments, a single photon counting detector detects the position and number of detected photons.

[0094] In some embodiments, the x-ray detector resolves one or more x-ray photon energies and generates a signal indicating the properties of the sample for each x-ray energy component. In some embodiments, the x-ray detector 310 includes any of the following: a CCD array, a microchannel plate, a photodiode array, a microstrip proportional counter, a gas-filled proportional counter, a scintillator, or a fluorescent material.

[0095] In this way, in addition to pixel location and count, energy is used to distinguish X-ray photon interactions within the detector. In some embodiments, X-ray photon interactions are distinguished by comparing the energy of the X-ray photon interactions with predetermined upper and lower thresholds. In one embodiment, this information is transmitted to a computing system 130 via an output signal 135 for further processing and storage.

[0096] Due to the angular dispersion in diffraction, the diffraction pattern produced by simultaneously illuminating a periodic target with multiple illumination wavelengths is separated at the detector plane. In these embodiments, an integrating detector is employed. A surface detector (e.g., a vacuum-compatible back-side CCD or a hybrid pixel array detector) is used to measure the diffraction pattern. Angle sampling is optimized for integration over the Bragg peak. If a pixel-level model fitting is used, then angle sampling is optimized for the signal information content. The sampling rate is selected to prevent saturation of the zero-order signal.

[0097] In a further aspect, the RSAXS system is employed to determine the properties of a sample (e.g., structural parameter values) based on one or more diffraction orders of the scattered light. Figure 2 As depicted, the measurement system 100 includes a computing system 130 for acquiring a signal 135 generated by the detector 310 and determining the properties of the wafer 306 based at least in part on the acquired signal 135.

[0098] In some instances, RSAXS-based measurements involve determining the sample size through the inverse solution of a predetermined measurement model with the measured data. The measurement model contains several (approximately ten) adjustable parameters and represents the geometry and optical properties of the sample, as well as the optical properties of the measurement system. Inverse solution methods include, but are not limited to, model-based regression, computed tomography, machine learning, or any combination thereof. In this way, target profile parameters are estimated by solving for values ​​in a parameterized measurement model that minimize the error between the measured scattered X-ray intensity and the modeled result.

[0099] In some instances, it is desirable to perform measurements over a wide range of wavelengths, incident angles, and azimuths to increase the precision and accuracy of the measured parameter values. This method reduces the correlation among parameters by expanding the number and diversity of datasets available for analysis.

[0100] The intensity of diffracted radiation is measured as a function of the illumination wavelength and the angle of incidence of the X-rays relative to the normal to the wafer surface. Information encompassed across multiple diffraction orders is typically unique among the model parameters considered. Therefore, X-ray scattering yields estimates of the parameters of interest with small errors and reduced parameter correlation.

[0101] In another aspect, the computational system 130 is configured to generate a structural model (e.g., a geometric model, a material model, or a combination of geometric and material models) of the measured structure of the sample, generate an X-ray scattering measurement response model including at least one geometric parameter from the structural model, and resolve at least one sample parameter value by performing a fitting analysis of the X-ray scattering measurement data using the X-ray scattering measurement response model. An analysis engine is used to compare the simulated X-ray scattering measurement signal with the measured data, thereby allowing the determination of geometric and material properties of the sample, such as electron density. Figure 2 In the embodiments depicted herein, computing system 130 is configured as a model building and analysis engine configured to implement model building and analysis functionality as described herein.

[0102] Figure 8 This is a diagram illustrating the exemplary model building and analysis engine 180 implemented through the computing system 130. For example... Figure 8 As depicted, the model building and analysis engine 180 includes a structure model building module 181 that generates a structure model 182 of the measured structure of the sample. In some embodiments, the structure model 182 further includes the material properties of the sample. The structure model 182 is received as input to the RSAXS response function building module 183. The RSAXS response function building module 183 generates an RSAXS response function model 184 based at least in part on the structure model 182. In some instances, the RSAXS response function model 184 is based on an x-ray form factor, also referred to as a structure factor.

[0103] (2)

[0104] Where F is the formal factor, q is the scattering vector, and ρ(r) is the electron density of the sample in spherical coordinates. The X-ray scattering intensity is then given by the following equation.

[0105] (3)

[0106] The RSAXS response function model 184 is received as input to the fitting analysis module 185. The fitting analysis module 185 compares the modeled RSAXS response with the corresponding measured data to determine the geometry and material properties of the sample.

[0107] In some instances, the modeled data is fitted to the experimental data by minimizing the chi-square value. For example, for RSAXS measurements, the chi-square value can be defined as...

[0108] (4)

[0109] in It is the measured RSAXS signal 135 in "channel" j, where the exponent j describes a set of system parameters (e.g., diffraction order, energy, angular coordinates, etc.). It is the modeled RSAXS signal S of the "channel" j for evaluating group structure (target) parameters. j These parameters describe geometry (CD, sidewall angles, stacking, etc.) and materials (electron density, etc.). This is an uncertainty associated with the j-th channel. N SAXS L is the total number of channels in the X-ray measurement. L is the number of parameters characterizing the measurement target.

[0110] Equation (4) assumes that the uncertainties associated with different channels are uncorrelated. In instances where uncertainties associated with different channels are correlated, the covariance between the uncertainties can be calculated. In these instances, the chi-square value of the RSAXS measurement can be expressed as...

[0111] (5)

[0112] Where V SAXS It is the covariance matrix of the uncertainties in the SAXS channel, and T represents the transpose.

[0113] In some instances, the fitting analysis module 185 resolves at least one sample parameter value by performing a fitting analysis on RSAXS measurement data 135 and RSAXS response model 184. In some instances, Optimization.

[0114] As described above, RSAXS data fitting is achieved by minimizing the chi-square value. However, in general, RSAXS data fitting can be achieved using other functions.

[0115] Fitting RSAXS metric data is advantageous for any type of RSAXS technique that provides sensitivity to the geometric and / or material parameters of interest. Sample parameters can be deterministic (e.g., CD, SWA, etc.) or statistical (e.g., RMS height of sidewall roughness, roughness-related length, etc.), provided that an appropriate model describing the interaction of the RSAXS beam with the sample is used.

[0116] Generally, the computing system 130 is configured to access model parameters in real time using Real-Time Critical Size Indication (RTCD), or a library of pre-computed models, to determine the value of at least one sample parameter associated with sample 101. Generally, some form of CD engine can be used to evaluate the difference between the assigned CD parameters of the sample and the CD parameters associated with the measured sample. An exemplary method and system for calculating sample parameter values ​​is described in U.S. Patent No. 7,826,071, issued November 2, 2010, to KLA-Tencor Corp., the entire contents of which are incorporated herein by reference.

[0117] In some instances, the model building and analysis engine 180 improves the accuracy of measured parameters through any combination of feedforward, feed-forward, and parallel analysis. Feedforward analysis involves taking multiple datasets from different regions of the same sample and transferring common parameters determined from the first dataset to a second dataset for analysis. Feedforward analysis involves taking datasets from different samples and using a progressively replicating accurate parameter feedforward method to pass common parameters forward to subsequent analyses. Parallel analysis involves applying nonlinear fitting methods in parallel or simultaneously to multiple datasets, where at least one common parameter is coupled during fitting.

[0118] Multi-tool and structured analysis refers to feedforward, side-feedback, or parallel analysis based on another fitting procedure for regression, lookup tables (i.e., "library" matching), or multiple datasets. An exemplary method and system for multi-tool and structured analysis is described in U.S. Patent No. 7,478,019, issued to KLA-Tec Inc. on January 13, 2009, the entire contents of which are incorporated herein by reference.

[0119] In another further aspect, initial estimates of one or more parameters of interest are determined based on RSAXS measurements performed at a single orientation of the incident X-ray beam relative to the measurement target. These initial estimates are implemented as starting values ​​for the parameters of interest to regress a measurement model using measurement data collected from RSAXS measurements at multiple orientations. In this way, a near estimate of the parameters of interest is determined with relatively little computational effort, and a precise estimate of the parameters of interest is obtained with less overall computational effort by implementing this near estimate as the starting point for regression on a much larger dataset.

[0120] In a further aspect, RSAXS measurement data is used to generate an image of the measured structure based on the measured intensity of the detected diffraction order. In some embodiments, the RSAXS response function model is generalized to describe scattering from a universal electron density grid. Matching this model to the measured signal while constraining the modeled electron density in this grid to enhance continuity and sparse edges provides a three-dimensional image of the sample.

[0121] While geometric model-based parametric inversion is preferred for critical size (CD) measurements based on RSAXS measurements, spectra of samples derived from the same RSAXS measurement data can be used to identify and correct model errors when the measured sample deviates from the assumptions of the geometric model.

[0122] In some instances, the image is compared with the structural characteristics estimated by parametric inversion of a geometrically based model using data measured through the same scattering technique. The differences are used to update the geometric model of the measured structure and improve measurement performance. The ability to converge to an accurate parametric measurement model is particularly important when measuring integrated circuits to control, monitor, and troubleshoot their manufacturing processes.

[0123] In some instances, the image is a two-dimensional (2-D) spectrum of electron density, absorptivity, complex refractive index, or a combination of these material properties. In some instances, the image is a three-dimensional (3-D) spectrum of electron density, absorptivity, complex refractive index, or a combination of these material properties. The spectrum is generated using relatively few physical constraints. In some instances, one or more parameters of interest (e.g., critical dimension (CD), sidewall angle (SWA), overlap, edge placement error, pitch walk, etc.) are directly estimated from the resulting spectrum. In some other instances, the spectrum can be used for wafer fabrication debugging when the sample geometry or material deviates from the expected range considered by the parametric structure model used for model-based CD measurements. In one instance, the parametric structure model is updated and its measurement performance is improved using the difference between the spectrum and the reproduction of the structure predicted by the parametric structure model based on its measured parameters. Further details are described in U.S. Patent Publication No. 2015 / 0300965, the contents of which are incorporated herein by reference in their entirety. Additional details are described in U.S. Patent Publication No. 2015 / 0117610, the contents of which are incorporated herein by reference in their entirety.

[0124] In a further aspect, a model building and analysis engine 180 is employed to generate models for combined X-ray and optical measurement analysis. In some instances, the optical simulation is based on, for example, rigorous coupled-wave analysis (RCWA), where Maxwell's equations are solved to calculate optical signals, such as reflectivity at different polarizations, ellipsometric parameters, phase transitions, etc.

[0125] Values ​​of one or more parameters of interest are determined using a combined geometrically parameterized response model based on a combined fitting analysis of the detected intensity and detected optical intensity of X-ray diffraction orders at multiple different incident angles. The optical intensity is measured using an optical measurement tool, which may or may not be connected to an X-ray measurement system (e.g., Figure 2 The system 100 depicted herein is mechanically integrated. Further details are described in U.S. Patent Publication No. 2014 / 0019097 and U.S. Patent Publication No. 2013 / 0304424, the contents of each of which are incorporated herein by reference in their entirety.

[0126] On the other hand, X-ray-based measurement systems include multiple detectors for individually detecting zero diffraction order and higher diffraction orders. Figure 9 An X-ray-based measurement system 400 is depicted in another embodiment. Figure 9 The same numbered elements and references depicted in the text Figure 2 The components described are similar.

[0127] like Figure 9 As depicted, the wafer measurement subsystem 300 includes detectors 310A and 310B. Detector 310A is located in the collection path for diffraction orders greater than zero. Detector 310B is located in the collection path for the zero diffraction order. In this manner, the risk of signal overflow from the zero order contaminating higher-order measurements is minimized. In some other embodiments, three detectors may be used: one detector for detecting the zero order, another detector for collecting positive non-zero orders, and a third detector for collecting negative non-zero orders. Generally, any combination of multiple detectors can be considered to detect the zero diffraction order and higher diffraction orders.

[0128] refer to Figure 2 The described embodiment includes a vacuum window 202 for filtering X-ray illumination and separating the vacuum environment of the plasma chamber 125 from that of the wafer measurement chamber 311. The vacuum window 202 must be made of an extremely thin layer of material to minimize absorption of desired X-ray illumination light and maximize absorption of unwanted infrared light from the pulsed laser illumination source (pump excitation source) 114. The thermal load on the vacuum window 202 due to radiation absorption is significant. Furthermore, the vacuum window 202 must also be mechanically strong and stable to withstand the pressure difference between the plasma chamber 125 and the wafer measurement chamber 311. Mechanical failure of the vacuum window 202 threatens the integrity of both the plasma chamber 125 and the wafer measurement chamber 311. In practice, it is difficult to achieve a vacuum window that meets the system requirements for filtering, X-ray transmission, and mechanical stability.

[0129] In another approach, the X-ray-based measurement system incorporates a multi-layered diffractive optical structure within the illumination path to filter the X-ray illumination light. This eliminates the need for a vacuum window within the illumination path. Figure 10 An X-ray-based measurement system 500 is depicted in another embodiment. Figure 10 The same numbered elements and references depicted in the text Figure 2 The components described are similar.

[0130] like Figure 10 As depicted, the ellipsoidal mirror 501 is coated with a three-dimensional multilayer diffractive optical structure 502. In some embodiments, the 3D multilayer structure 502 is a scintillation grating. In other embodiments, the 3D multilayer structure 502 is a layered grating. The angular dispersion of different wavelengths from the three-dimensional multilayer diffractive optical structure 502 filters out unwanted radiation from the X-ray illumination light, thereby enhancing spectral purity. Light from the pulsed laser illumination source 114 (e.g., IR light) and unwanted wavelengths (e.g., UV, EUV, or both) generated by the plasma 103 are diffracted at different angles than the light generated by the plasma 103 (e.g., SXR light). Undesired IR light 503 is directed to the beam trap 504, and desirable SXR light propagates to the wafer 306.

[0131] To maintain the difference between the vacuum plasma chamber 125 and the wafer measurement chamber 311, the two chambers are differentially pumped at the orifice system 203. Figure 10 In the embodiment depicted, the aperture system 203 is sealed relative to the beamline 200 around the outside of the aperture. Therefore, the only light path between the plasma chamber 125 and the wafer measurement chamber 311 is through the tiny aperture of the aperture system 203. Differential pumping is sufficient to maintain separate vacuum levels in the plasma chamber 125 and the wafer measurement chamber 311.

[0132] In another aspect, the X-ray-based measurement system includes a zone plate structure located in the illumination path to refocus the excitation light back to the laser-generated plasma source. Figure 11 An X-ray-based measurement system 600 is depicted in another embodiment. Figure 11 The same numbered elements and references depicted in the text Figure 2 The components described are similar.

[0133] like Figure 11As depicted, a zone plate structure 603 is fabricated on an ellipsoidal mirror 601. A three-dimensional multilayer diffractive optical structure 602 is deposited on the zone plate structure 603 and the ellipsoidal mirror 601. In some embodiments, the 3D multilayer structure 602 is a glazing grating. In other embodiments, the 3D multilayer structure 602 is a layered grating. Incident infrared light from the pulsed laser illumination source 114 is scattered by the zone plate structure 603 onto the reflective surface of the ellipsoidal mirror 601, which refocuses the scattered infrared light back to the plasma 103. Additional unwanted wavelengths 605 (e.g., UV, EUV, or both) generated by the plasma 103 are diffracted by the 3D multilayer structure 602 to the beam trap 604, and desired SRX light propagates to the wafer 306.

[0134] To maintain the difference between the vacuum plasma chamber 125 and the wafer measurement chamber 311, the two chambers are differentially pumped at the orifice system 203. Figure 11 In the embodiment depicted, the aperture system 203 is sealed relative to the beamline 200 around the outside of the aperture. Therefore, the only light path between the plasma chamber 125 and the wafer measurement chamber 311 is through the tiny aperture of the aperture system 203. Differential pumping is sufficient to maintain separate vacuum levels in the plasma chamber 125 and the wafer measurement chamber 311.

[0135] In a further aspect, the flux of X-ray illumination light generated by a low atomic number low-temperature LPP illumination source is monitored and controlled. Figure 2 A flux sensor 118 is located near the entrance of beamline 200. The measured value of the X-ray flux is transmitted to a computing system 130. In response, the computing system 130 compares the measured flux with the desired flux and transmits a control command 136 to a pulsed laser illumination source 114 to adjust the output of the pulsed laser illumination source 114 to reduce the difference between the measured flux and the desired flux.

[0136] In some embodiments, the wavelength emitted by plasma 103 is selectable. In some embodiments, the pulsed laser illumination source 114 is controlled by a computing system 130 to maximize the flux generated by plasma 103 in one or more selected spectral regions. The peak intensity of the pump laser at the target material controls the plasma temperature and thus the spectral region of emitted radiation. The peak intensity of the pump laser is varied by adjusting the pulse energy, pulse width, or both. In one example, a 100 picosecond pulse width is suitable for generating SXR radiation. Figure 2 As depicted, the computing system 130 transmits a command signal 136 to the pulsed laser illumination source 114, which causes the pulsed laser illumination source 114 to adjust the spectral range of wavelengths emitted from the plasma 103.

[0137] It should be understood that the various steps described throughout this invention can be performed by a single computer system 130 or (alternatively) multiple computer systems 130. Furthermore, different subsystems of system 100 (e.g., sample positioning system 320) may include computer systems suitable for performing at least a portion of the steps described herein. Therefore, the foregoing description should not be construed as a limitation of the invention but merely as illustrative. Additionally, one or more computing systems 130 may be configured to perform any (any) other steps of any of the method embodiments described herein.

[0138] Additionally, computer system 130 can be communicatively coupled to pulsed laser illumination source 114, orifice system 203, sample positioning system 320, and detector 310 in any manner known in the art. For example, one or more computing systems 130 can be coupled to computing systems respectively associated with pulsed laser illumination source 114, orifice system 203, sample positioning system 320, and detector 310. In another example, any of pulsed laser illumination source 114, orifice system 203, sample positioning system 320, and detector 310 can be directly controlled by a single computer system coupled to computer system 130.

[0139] Computer system 130 can be configured to receive and / or acquire data or information from subsystems of the system (e.g., pulsed laser illumination source 114, orifice system 203, sample positioning system 320, and detector 310, etc.) via a transmission medium that may include wired and / or wireless components. In this way, the transmission medium can be used as a data link between computer system 130 and other subsystems of system 100.

[0140] The computer system 130 of the measurement system 100 can be configured to receive and / or acquire data or information (e.g., measurement results, modeling inputs, modeling results, etc.) from other systems via a transmission medium that may include wired and / or wireless components. In this manner, the transmission medium can serve as a data link between the computer system 130 and other systems (e.g., on-board measurement system 100, external memory, or external systems). For example, the computer system 130 can be configured to receive measurement data (e.g., signal 135) from a storage medium (i.e., memory 132 or 190) via a data link. For example, intensity results obtained using detector 310 can be stored in a permanent or semi-permanent memory device (e.g., memory 132 or 190). In this context, measurement results can be imported from on-board memory or from an external memory system. Additionally, the computer system 130 can transmit data to other systems via the transmission medium. For example, sample parameter values ​​186 determined by the computer system 130 can be stored in a permanent or semi-permanent memory device (e.g., memory 190). In this respect, the measurement results can be transmitted to another system.

[0141] The computing system 130 may include, but is not limited to, personal computer systems, mainframe computer systems, workstations, graphics computers, parallel processors, cloud-based computing systems, or any other devices known in the art. Generally, the term "computing system" can be broadly defined to encompass any device having one or more processors that execute instructions from memory media.

[0142] The program instructions 134 for implementing methods such as those described herein can be transmitted via a transmission medium (e.g., wires, cables, or wireless transmission links). For example, such as Figure 2 As illustrated in the diagram, program instructions stored in memory 132 are transferred to processor 131 via bus 133. Program instructions 134 are stored in a computer-readable medium (e.g., memory 132). Exemplary computer-readable media include read-only memory, random access memory, magnetic disk or optical disk, or magnetic tape.

[0143] Figure 12 The illustrations depict method 700 suitable for implementation by the measurement systems 100, 400, 500, and 600 of the present invention. In one aspect, it should be appreciated that the data processing portion of method 700 may be executed via a pre-programmed algorithm executed by one or more processors of computing system 130. Although the following description is presented in the context of measurement systems 100, 400, 500, and 600, it is understood herein that specific structural aspects of measurement systems 100, 400, 500, and 600 are not intended to be limiting and should be interpreted only as illustrative.

[0144] In frame 701, a cryogenically cooled barrel is rotated and translated within a plasma chamber. The cryogenically cooled barrel has a surface coated with a predetermined thickness of a low atomic number target material. The low atomic number target material comprises one or more elements, each having an atomic number less than 19. The plasma chamber has at least one wall that is partially operable to confine a buffer gas flow within the plasma chamber.

[0145] In box 702, an excitation pulse is generated and directed to a low atomic number target material at a location on the surface of a cryogenically cooled chamber. The interaction between the excitation pulse and the low atomic number target material causes the target material to ionize, forming a plasma that emits illumination light. The illumination light comprises emission of one or more spectral lines in the spectral region from 10 electron volts to 5,000 electron volts.

[0146] In box 703, the amount of light from the sample is detected in response to illumination light.

[0147] In box 704, the value of at least one parameter of interest for the measured sample is determined based on the detected light intensity.

[0148] In some embodiments, RSAXS measurements as described herein are implemented as part of a fabrication process tool. Examples of fabrication process tools include (but are not limited to) photolithography tools, thin film deposition tools, implantation tools, and etching tools. In this manner, the results of RSAXS analysis are used to control the fabrication process. In one example, RSAXS measurement data collected from one or more targets is sent to the fabrication process tool. The RSAXS measurement data is analyzed as described herein, and the results are used to adjust the operation of the fabrication process tool to reduce errors in semiconductor structure fabrication.

[0149] The properties of a variety of semiconductor structures can be determined using scattering measurements as described herein. Exemplary structures include, but are not limited to, FinFETs, low-size structures (e.g., nanowires or graphene), structures smaller than 10 nm, photolithographic structures, through-substrate vias (TSVs), memory structures (e.g., DRAM, DRAM 4F2, flash memory, MRAM), and high aspect ratio memory structures. Exemplary structure properties include, but are not limited to, geometric parameters (e.g., line edge roughness, linewidth roughness, aperture size, aperture density, sidewall angles, profile, critical size, pitch, thickness, and stacking) and material parameters (e.g., electron density, composition, particle structure, morphology, stress, strain, and element identification). In some embodiments, the metric target is a periodic structure. In some other embodiments, the metric target is aperiodic.

[0150] In some instances, the RSAXS measurement system, as described herein, is used to perform measurements of critical dimensions, thickness, stack-up, and material properties of high aspect ratio semiconductor structures, including but not limited to spin torque random access memory (STT-RAM), three-dimensional NAND memory (3D-NAND) or vertical NAND memory (V-NAND), dynamic random access memory (DRAM), three-dimensional flash memory (3D flash), resistive random access memory (Re-RAM), and phase change random access memory (PC-RAM).

[0151] As described herein, the term "critical dimension" includes any critical dimension of a structure (e.g., bottom critical dimension, middle critical dimension, top critical dimension, sidewall angle, grating height, etc.), any critical dimension between two or more structures (e.g., distance between two structures), and displacement between two or more structures (e.g., stacking displacement between stacked grating structures, etc.). Structures may include three-dimensional structures, patterned structures, stacked structures, etc.

[0152] As described herein, the terms “critical size application” or “critical size measurement application” include any critical size measurement.

[0153] As described herein, the term "metric system" includes any system used at least in part to characterize samples in any aspect, including critical size applications and stack-up metric applications. However, these technical terms do not limit the scope of the term "metric system" as described herein. Furthermore, the metric systems described herein can be configured for measuring patterned wafers and / or unpatterned wafers. Metric systems can be configured as LED inspection tools, edge inspection tools, backside inspection tools, macroscopic inspection tools, or multi-mode inspection tools (involving data simultaneously from one or more platforms), and any other metric or inspection tool that benefits from the measurement techniques described herein.

[0154] This document describes various embodiments of semiconductor processing systems (e.g., inspection systems or lithography systems) that can be used to process samples. The term "sample" is used herein to refer to a wafer, photomask, or any other sample that can be processed by means known in this art (e.g., printing or inspecting for defects).

[0155] As used herein, the term "wafer" generally refers to a substrate formed of semiconductor or non-semiconductor materials. Examples include, but are not limited to, single-crystal silicon, gallium arsenide, and indium phosphide. Such substrates are typically found and / or processed in semiconductor fabrication facilities. In some cases, a wafer may consist only of a substrate (i.e., a bare wafer). Alternatively, a wafer may contain one or more layers of different materials formed on the substrate. The one or more layers formed on the wafer may be "patterned" or "unpatterned." For example, a wafer may contain multiple bare wafers with repeatable pattern features.

[0156] A "photomask" can be a photomask at any stage of the photomask fabrication process or a completed photomask that may or may not be released for use in a semiconductor fabrication facility. A photomask or "mask" is generally defined as a generally transparent substrate on which opaque areas are generally formed and configured as patterns. The substrate may contain, for example, a glass material such as amorphous SiO2. The photomask can be deposited on a resist-covered wafer during the exposure step of a photolithography process, such that the pattern on the photomask can be transferred to the resist.

[0157] One or more layers formed on a wafer can be patterned or unpatterned. For example, a wafer can comprise multiple dies, each having repeatable pattern features. The formation and processing of such material layers can ultimately produce a finished device. Many different types of devices can be formed on a wafer, and the term wafer, as used herein, is intended to encompass wafers on which any type of device known in the art is fabricated.

[0158] In one or more exemplary embodiments, the described functionality may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functionality may be stored as one or more instructions or code on or transmitted via a computer-readable medium. Computer-readable medium includes both computer storage media and communication media, including any media that facilitates the transfer of a computer program from one place to another. Storage media may be any available media accessible by a general-purpose or special-purpose computer. By way of example, and not limitation, such computer-readable media may include: RAM, ROM, EEPROM, CD-ROM or other optical disc storage devices, magnetic disk storage devices or other magnetic storage devices, or any other media that can be used to carry or store desired program code elements in the form of instructions or data structures and is accessible by a general-purpose or special-purpose computer or a general-purpose or special-purpose processor. Furthermore, any connection may be properly referred to as computer-readable media. For example, if software is transmitted from a website, server, or other remote source using coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technology (such as infrared, radio, and microwave), then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technology (such as infrared, radio, and microwave) are all included in the definition of media. As used herein, disks and platters include optical discs (CDs), laser discs, XRF discs, digital versatile discs (DVDs), floppy disks, and Blu-ray discs, wherein disks typically copy data magnetically while platters use lasers to copy data optically. The combination of the above should also be included within the scope of computer-readable media.

[0159] While specific embodiments have been described above for illustrative purposes, the teachings of this patent document are generally applicable and not limited to the specific embodiments described above. Therefore, various modifications, alterations, and combinations of features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.

Claims

1. A laser-generated plasma light source, comprising: a plasma chamber having at least one wall operable in part to enclose a buffer gas flow within the plasma chamber; a cryogenically-cooled barrel located in the plasma chamber, the cryogenically-cooled barrel configured to rotate about an axis and translate along the axis; a low atomic number target material deposited on a surface of the cryogenically-cooled barrel, wherein the low atomic number target material includes one or more elements having an atomic number less than 19; and a pulsed laser generating excitation light pulses directed to the low atomic number target material at a location on the surface of the rotating cryogenically-cooled barrel, wherein interaction of the excitation light pulses with the low atomic number target material causes the low atomic number target material to ionize to form a plasma that emits illumination light, wherein the illumination light includes one or more spectral line emissions from a spectral region of 10 electron volts to 5,000 electron volts, wherein the illumination can be used to illuminate a sample under measurement.

2. The laser-generated plasma light source of claim 1, further comprising: one or more rotational actuators configured to rotate the cryogenically-cooled barrel about the axis; and one or more linear actuators configured to translate the cryogenically-cooled barrel along the axis.

3. The laser-generated plasma light source of claim 1, further comprising: a nozzle mechanically coupled to the plasma chamber, the nozzle having an exit orifice located a distance away from the surface of the cryogenically-cooled barrel, wherein a flow of low atomic number target material exits the exit orifice of the nozzle and is deposited onto the surface of the cryogenically-cooled barrel as the cryogenically-cooled barrel rotates and translates; and a wiper mechanism coupled to the plasma chamber at a fixed distance from the surface of the cryogenically-cooled barrel, wherein the wiper mechanism scrapes the low atomic number target material that is cryogenically frozen to the surface of the cryogenically-cooled barrel to a predetermined thickness as the cryogenically-cooled barrel rotates and translates.

4. The laser-generated plasma light source of claim 3, wherein the flow of low atomic number target material exits the exit orifice of the nozzle in a gas phase or a liquid phase.

5. The laser-generated plasma light source of claim 3, wherein the predetermined thickness is in a range between 200 micrometers and 1 millimeter.

6. The laser-generated plasma light source of claim 1, wherein the low atomic number target material includes a first low atomic number target material including one or more elements each having an atomic number less than 19 dissolved in a solvent including elements each having an atomic number less than 19.

7. The laser-generated plasma light source of claim 1, further comprising: one or more gas manifolds disposed within the plasma chamber, wherein the one or more gas manifolds disperse a buffer gas flow into the plasma chamber; and a gas source coupled to the one or more gas manifolds. a vacuum pump coupled to the plasma chamber, wherein the vacuum pump evacuates the buffer gas flow from the plasma chamber along with debris generated by the plasma entrained in the buffer gas flow.

8. The laser-produced plasma light source of claim 7, wherein the buffer gas is nitrogen, hydrogen, oxygen, argon, neon, or any combination thereof.

9. The laser-produced plasma light source of claim 1, wherein a distance between a window of the plasma chamber and the plasma is at least 10 centimeters.

10. The laser-produced plasma light source of claim 1, wherein the brightness of the plasma is greater than 10 13 photons / (sec) . (mm 2 ) . (mrad 2 ) . (1% bandwidth).

11. The laser-produced plasma light source of claim 1, wherein a spot size of the plasma is less than 100 microns.

12. A metrology system comprising: a laser-produced plasma light source comprising: a plasma chamber having at least one wall operable in part to enclose a buffer gas flow within the plasma chamber; a cryogenically cooled bucket located in the plasma chamber, the cryogenically cooled bucket configured to rotate about an axis and translate along the axis; a low atomic number target material deposited on a surface of the cryogenically cooled bucket, wherein the low atomic number target material includes one or more elements having an atomic number less than 19; a pulsed laser generating excitation light pulses directed to the low atomic number target material at a location on the surface of the rotating cryogenically cooled bucket, wherein interaction of the excitation light pulses with the low atomic number target material causes the low atomic number target material to ionize to form a plasma that emits illumination light, wherein the illumination light includes one or more spectral line emissions from a spectral region of 10 electron volts to 5,000 electron volts, wherein the illumination light is usable to illuminate a sample under measurement; one or more optical elements located in an illumination path between the plasma and the sample under measurement; one or more x-ray detectors that detect an amount of light from the sample under measurement in response to the illumination light incident on the sample; and a computing system configured to determine a value of a parameter of interest characterizing the sample under measurement based on the detected amount of light.

13. The metrology system of claim 12, wherein the metrology system is configured as a small angle x-ray scattering measurement system.

14. The metrology system of claim 12, the one or more optical elements located in the illumination path include an ellipsoidal mirror that focuses the illumination light incident on the sample.

15. The metrology system of claim 14, the ellipsoidal mirror includes a multilayer diffractive optical structure fabricated on the ellipsoidal mirror, wherein the multilayer diffractive optical structure diffracts a first portion of the illumination light incident on the ellipsoidal mirror toward a beam dump and a second portion of the illumination light incident on the ellipsoidal mirror toward the sample under measurement.

16. The metrology system of claim 14, the ellipsoidal mirror including a zone plate structure fabricated on the ellipsoidal mirror, and a multilayer diffractive optical structure fabricated on the ellipsoidal mirror over the zone plate structure, wherein the zone plate structure scatters a first portion of the illumination light incident on the ellipsoidal mirror back to the plasmon, wherein the multilayer diffractive optical structure diffracts a second portion of the illumination light incident on the ellipsoidal mirror toward a beam dump and a third portion of the illumination light incident on the ellipsoidal mirror toward the measured sample.

17. The metrology system of claim 12, the laser-generated plasmonic light source further comprising: a nozzle mechanically coupled to the plasmonic chamber, the nozzle having an exit orifice located a distance away from the surface of the cryogenically-cooled barrel, wherein a stream of low atomic number target material exits the exit orifice of the nozzle and is deposited onto the surface of the cryogenically-cooled barrel as the cryogenically-cooled barrel is rotated and translated; and a squeegee mechanism coupled to the plasmonic chamber at a fixed distance from the surface of the cryogenically-cooled barrel, wherein the squeegee mechanism scrapes the low atomic number target material cryogenically frozen to the surface of the cryogenically-cooled barrel to a predetermined thickness as the cryogenically-cooled barrel is rotated and translated.

18. The metrology system of claim 17, wherein the stream of low atomic number target material exits the exit orifice of the nozzle in a gas phase or a liquid phase.

19. The metrology system of claim 17, wherein the predetermined thickness is in a range between 200 micrometers and 1 millimeter.

20. A method comprising: rotating and translating a cryogenically-cooled barrel within a plasmonic chamber, the cryogenically-cooled barrel having a surface coated with an amount of a low atomic number target material to a predetermined thickness, the low atomic number target material comprising one or more elements each having an atomic number less than 19, the plasmonic chamber having at least one wall operable in part to enclose a flow of buffer gas within the plasmonic chamber; generating an excitation light pulse directed to the low atomic number target material at a location on the surface of the cryogenically-cooled barrel, wherein interaction of the excitation light pulse with the low atomic number target material causes the low atomic number target material to ionize to form a plasmon that emits an illumination light, wherein the illumination light comprises one or more spectral line emissions from a spectral region of 10 electron volts to 5,000 electron volts; detecting an amount of light from a sample in response to the illumination light; and determining a value of at least one parameter of interest of a measured sample based on the detected amount of light.

21. The method of claim 20, further comprising: depositing a stream of the low atomic number target material onto the surface of the cryogenically-cooled barrel as the cryogenically-cooled barrel is rotated and translated; and The low atomic number target material is scraped to the predetermined thickness as the cryogenically cooled barrel is rotated and translated to cryogenically freeze the surface of the cryogenically cooled barrel.

22. The method of claim 21, wherein the low atomic number target material stream is in a gas phase or a liquid phase.

23. The method of claim 20, wherein the predetermined thickness is in a range between 200 micrometers and 1 millimeter.

Images