Metrology apparatus and method

Abstract

The metrology apparatus includes an optical element in fluid communication with a gain medium of the gas discharge chamber, a probe apparatus configured to generate a probe in proximity to the optical element exposed to one or more dust particles, a detection apparatus configured to detect an interaction of the probe with the one or more dust particles and generate an output signal based on the detected interaction, and a processing apparatus configured to receive the output signal and estimate a characteristic of the one or more dust particles.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. patent application Ser. No. 63 / 283,773, entitled "METROLOGY APPARATUS AND METHOD," filed Nov. 29, 2021, the entire contents of which are incorporated by reference herein.

[0002] The subject matter of this disclosure relates to a metrology apparatus for estimating one or more properties of dust particles in a gas discharge chamber of a deep ultraviolet light source. [Background technology]

[0003]

[0003] One type of gas discharge light source used in photolithography is called an excimer light source, or excimer laser. Typically, an excimer laser uses a combination of one or more noble gases, which may include argon, krypton, or xenon, and a reactive gas, which may include fluorine or chlorine. Under the proper conditions of electrical simulation (energized) and high pressure (of the gas mixture), an excimer laser can form excimers, or pseudo-molecules, which only exist in the energized state. The energized excimers produce amplified light in the ultraviolet range. An excimer light source can use a single gas discharge chamber or multiple gas discharge chambers. When an excimer light source is operating, it generates a deep ultraviolet (DUV) light beam. DUV light can include wavelengths of, for example, about 100 nanometers (nm) to about 400 nm.

[0004]

[0004] The DUV light beam can be directed to a photolithography exposure tool or scanner, which is a machine that applies a desired pattern onto a target portion of a substrate, such as a silicon wafer. The DUV light beam interacts with a projection optical system, which projects the DUV light beam through a mask onto the photoresist of the wafer. In this manner, one or more layers of a chip design are patterned onto the photoresist, and the patterned wafer may then be processed, for example by implantation or etching. Summary of the Invention

[0005]

[0005] In some general aspects, a metrology apparatus includes an optical element in fluid communication with a gain medium of a gas discharge chamber, a probe apparatus configured to generate a probe in the vicinity of the optical element exposed to one or more particles (such as dust particles), a detection apparatus configured to detect an interaction of the probe with the one or more particles and generate an output signal based on the detected interaction, and a processing apparatus configured to receive the output signal and estimate characteristics of the one or more particles.

[0006]

[0006] Implementations may include one or more of the following features. For example, the probe device may be an optical assembly and the probe may be a light sheet. The detection device may be configured to detect the interaction by capturing light generated by the interaction of the light sheet with the one or more particles. The optical assembly may include a laser configured to generate the laser light sheet as a light sheet. The laser may be configured to generate light having a wavelength different from the wavelength of the light generated from the gain medium in the gas discharge chamber. The detection device may include a photodiode or a camera. An imaging surface of the photodiode or camera may face the light sheet such that the extent of the light sheet is observable and imageable. An imaging surface of the photodiode or camera may face a surface of the optical element facing the interior of the gas discharge chamber. The probe axis of the light sheet can be located in the imaging plane of the photodiode or camera, and the long plane of the light sheet can be parallel to the imaging plane of the photodiode or camera, the long plane of the light sheet can be perpendicular to the imaging plane of the photodiode or camera, or the long plane of the light sheet can be positioned to form an angle between an angle parallel to the imaging plane of the photodiode or camera and an angle perpendicular to the imaging plane.

[0007]

[0007] The light sheet can be guided along a path non-parallel to a plane along which the amplified light beam generated by the gain medium during application of energy passes through the gas discharge chamber. The light sheet can be guided along a path adjacent to a surface of an optical element. The optical element can be a hermetically controlled window of the gas discharge chamber configured to pass the amplified light beam between an interior of the gas discharge chamber filled with the gain medium and an exterior of the gas discharge chamber. The light sheet can be guided along a path adjacent to a surface of the window facing the interior of the gas discharge chamber. The light sheet can be guided along a path in the vicinity of a surface of the window facing the interior of the gas discharge chamber. The detection device can be configured to capture light from the light sheet that is scattered or reflected from one or more particles.

[0008]

[0008] The processing device can be configured to estimate the one or more particle properties by estimating one or more of a number of the one or more particles, a position of the one or more particles, a density of the one or more particles, and a velocity of the one or more particles. The probe device can generate a probe in the vicinity of the optical element, and the detection device can generate an output signal based on the detected interaction while the gas discharge chamber generates the amplified light beam. The gas discharge chamber can include a gas that includes a gain medium and electrodes for providing energy to the gain medium such that the gain medium generates a plasma that generates the amplified light beam when a voltage is applied to the electrodes.

[0009] The probe device and detection device may be disposed relative to (eg, disposed within or attached to) a housing that holds the optical element.

[0010] In another general aspect, an apparatus for a deep ultraviolet (DUV) gas discharge light source includes a metrology apparatus and an actuation apparatus. The metrology apparatus includes an optical element in fluid communication with a gain medium of a gas discharge chamber, the optical element being configured to generate a probe proximate the optical element exposed to one or more particles, a detection apparatus configured to detect an interaction of the probe with the one or more particles and generate an output signal based on the detected interaction, and a processing apparatus configured to receive the output signal and estimate a property of the one or more particles. The actuation apparatus is configured to receive the estimated property and adjust one or more characteristics of the gas discharge light source based on the estimated property.

[0011]

[0011] Implementations can include one or more of the following features. For example, the apparatus can include a controller in communication with the processor and the actuator. The controller can be configured to analyze the estimated characteristic and analyze performance of the gas discharge chamber based on the analysis of the estimated characteristic. The controller can be configured to predict a lifetime of the optical element and / or the gas discharge chamber. The actuator can be configured to adjust one or more characteristics of the dust particle capture system.

[0012]

[0012] The particles may include dust particles generated from a gain medium in the gas discharge chamber. The gain medium may include fluoride, and the dust particles may include metal fluoride particles. The gain medium may include argon fluoride, krypton fluoride, or xenon chloride.

[0013]

[0013] The metrology device can be associated with a powering ring amplifier of a DUV gas discharge light source, and the optical element can be a window of a gas discharge chamber of the powering ring amplifier. The probe can be positioned proximate a window of the gas discharge chamber of the powering ring amplifier in fluid communication with the gain medium, the window being exposed to the one or more particles. The window of the gas discharge chamber of the powering ring amplifier can be a window at an output side of the gas discharge chamber of the powering ring amplifier. The window can be made of a crystalline structure configured to transmit light having a wavelength in the DUV range. The window can be made of calcium fluoride, magnesium fluoride, or fused silica.

[0014]

[0014] The probe device may be an optical assembly including a laser configured to generate a laser light sheet as a probe. The detection device may be configured to detect the interaction by capturing light generated by the interaction of the light sheet with one or more particles. The laser may be configured to generate light having a wavelength different from the wavelength of the light generated from the gain medium in the gas discharge chamber. The detection device may include a photodiode or a camera. The light sheet may be guided along a path non-parallel to a plane along which the amplified light beam generated by the gain medium passes through the gas discharge chamber during the application of energy. The light sheet may be guided along a path adjacent to a surface of the optical element.

[0015]

[0015] In another general aspect, a metrology method includes generating a probe in the vicinity of an optical element in fluid communication with a gain medium of a gas discharge chamber, the optical element being exposed to one or more dust particles, detecting an interaction between the generated probe and the one or more dust particles, generating an output signal based on the detected interaction, and estimating characteristics of the one or more dust particles based on the output signal.

[0016]

[0016] Implementations may include one or more of the following features. For example, the probe may be generated by generating a laser light sheet, and the interaction may be detected by capturing light generated by the interaction of the light sheet with one or more dust particles. The laser light sheet may have a wavelength different from that of the light generated from the gain medium in the gas discharge chamber. The light generated by the interaction of the light sheet with one or more dust particles may be captured by capturing light from the laser light sheet that is scattered or reflected from the one or more dust particles. The light from the laser light sheet that is scattered or reflected from the one or more dust particles may be captured by generating a potential difference on an exposed surface or generating a two-dimensional image on the exposed surface, the exposed surface receiving the scattered or reflected light from the laser light sheet. The laser light sheet may be generated by directing the laser light sheet along a path that is non-parallel to a plane along which the amplified light beam generated by the gain medium passes through the gas discharge chamber during application of energy. The laser light sheet may be directed along a path by directing the laser light sheet along a path adjacent to a surface of an optical element.

[0017]

[0017] The one or more dust particle characteristics can be estimated by estimating one or more of a number of the one or more dust particles, a position of the one or more dust particles, a density of the one or more dust particles, and a velocity of the one or more dust particles. A probe can be generated in the vicinity of the optical element, and an output signal can be generated based on the detected interaction while the gas discharge chamber generates the amplified light beam. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a block diagram of a metrology apparatus positioned relative to a gas discharge chamber of a light source to estimate properties of one or more particles in the vicinity of an optical element within the cavity of the gas discharge chamber. [Figure 2A]

[0019] 2 is a schematic diagram of an embodiment of a probe and detection apparatus including a light sheet of the metrology apparatus of FIG. 1; [Figure 2B]

[0020] 2 is a schematic diagram showing an embodiment of the relative arrangement of the light sheet and the imaging plane of the detector; FIG. [Figure 2C]

[0021] 4 is a schematic diagram showing another embodiment of the relative arrangement of the light sheet and the imaging plane of the detector; FIG. [Figure 3A]

[0022] 1 is a schematic diagram of an embodiment of a probe device generating a light sheet as a probe and a detection device arranged relative to an optical element. [Figure 3B]

[0023] FIG. 3B is a schematic diagram of a camera sensor of an embodiment of the detection device of FIG. 3A. [Figure 4]

[0024] 2 is a block diagram of an embodiment of the gas discharge chamber and metrology apparatus of FIG. 1 as part of a deep ultraviolet (DUV) light source. [Diagram 5]

[0025] 2 is a block diagram of an embodiment of a processor of the metrology apparatus of FIG. 1; [Figure 6]

[0026] FIG. 5 is a block diagram of a two-stage light source that is an embodiment of the DUV light source of FIG. [Figure 7]

[0027] FIG. 7 is a cross-sectional view of an embodiment of the power amplifier gas discharge chamber of the two-stage light source of FIG. 6, in which a metrology device is positioned relative to a window of the power amplifier gas discharge chamber and the probe is a light sheet traveling along a probe axis that is non-parallel to the operating light beam. [Figure 8A]

[0028] FIG. 8 is an enlarged detailed cross-sectional view of a window of the power amplifier gas discharge chamber of FIG. 7 along the probe axis of the probe light sheet. [Figure 8B]

[0029] FIG. 8B is an enlarged detail of a cross-sectional view of the window of FIG. 8A taken along plane 8B-8B. [Figure 9A]

[0030] An enlarged detailed cross-sectional view of the window of the power amplifier gas discharge chamber of FIG. 7, taken along the probe axis of the probe light sheet, showing another possible position of the probe light sheet. [Figure 9B]

[0031] An enlarged detailed cross-sectional view of the window of the power amplifier gas discharge chamber of FIG. 7, taken along the probe axis of the probe light sheet, showing another possible position of the probe light sheet. [Figure 10A]

[0032] A schematic diagram of an embodiment of a detection device arranged with respect to an optical element such that an imaging surface of a probe device that generates a light sheet as a probe is different from the imaging surface of the detection device of FIG. 3A. [Figure 10B]

[0033] A schematic diagram of the sensor of the camera of the embodiment of the detection device of FIG. 10A. [Figure 11]

[0034] A flowchart of a procedure performed by a metrology device. [Figure 12]

[0035] An example of an image captured by the camera of the detection device of FIGS. 3A and 3B. [Figure 13]

[0036] A composite image captured by the camera of the detection device of FIGS. 3A and 3B, which is an example of a composite image showing the flow path or trajectory of dust particles over time. [Figure 14A]

[0037] An example of an image captured by the camera of the detection device of FIGS. 3A and 3B, taken at time T1. [Figure 14B]

[0038] An example of an image captured by the camera of the detection device of FIGS. 3A and 3B, taken at time T2 after time T1, showing the region of interest captured at time T1. [Figure 14C]

[0039] An example of an image captured by the camera of the detection device of FIGS. 3A and 3B, taken at time T3 after time T2, showing the regions of interest captured at times T2 and T1.

Embodiments for Carrying Out the Invention

[0019]

[0040] Referring to FIG. 1, metrology apparatus 100 is positioned relative to cavity 105 of gas discharge chamber 110. Metrology apparatus 100 is configured to estimate properties of one or more particles (which may be, for example, dust particles) flowing in the vicinity of optical element 120 within cavity 105 of gas discharge chamber 110. For example, metrology apparatus 100 may detect and / or track one or more of these dust particles 115. Optical element 120 is an element that includes a surface that is in fluid communication with cavity 105 of gas discharge chamber 110 and thus may become exposed to dust particles 115. Optical element 120 is an element that optically interacts with light beam 127p, which is either amplified light beam 127 generated by gas discharge chamber 110 or a precursor light beam that becomes amplified light beam 127 (e.g., after interacting with other components or elements of gas discharge chamber 110). For example, optical element 120 can be a window in gas discharge chamber 110 or an optical element used in various metrology operations. Metrology apparatus 100 can estimate properties of one or more particles 115 while gas discharge chamber 110 produces (or is producing) an amplified light beam 127 for use by an output device, e.g., shown in FIG. 4 below. Thus, metrology apparatus 100 operates in real time and operation of metrology apparatus 100 does not disrupt operation of gas discharge chamber 110 (or the light source in which gas discharge chamber 110 is used, e.g., shown in FIG. 4).

[0020]

[0041] During operation of the gas discharge chamber 110, the gain medium 130 (located within the optical cavity) is excited with a short (e.g., nanosecond) current pulse in a high-voltage discharge from an energy source 125 (e.g., a pair of electrodes) that creates a plasma that results in optical amplification, and an amplified light beam 127 having a wavelength in the ultraviolet range (e.g., deep ultraviolet or DUV range) is generated and output from the gas discharge chamber 110. The gain medium 130 is typically a gas mixture that includes a noble gas (e.g., argon, krypton, xenon) and a halogen (e.g., fluorine, chlorine), in addition to a buffer gas. Thus, for example, the gain medium 130 can include argon fluoride (ArF), krypton fluoride (KrF), or xenon chloride (XeCl). When the gain medium 130 includes argon fluoride (ArF), the wavelength of the amplified light beam 127 is about 193 nm. The electrodes 125 corrode during normal operation and such corrosion can generate metal fluoride (or metal chloride, if the chloride is a halogen) particles. Such particles generated due to corrosion are referred to herein as dust particles 115, but may alternatively be described simply as particles.

[0021]

[0042] Typically, these dust particles 115 do not approach the optical element 120 because the gas discharge chamber 110 is fitted with a dust particle capture system 135. The dust particle capture system 135 provides a cleaning purge gas configured to push the purge gas along a path against the optical element 120 to prevent or reduce the possibility of the dust particles 115 contacting the optical element 120. For example, the dust particle capture system 135 can be a metal fluoride trap (MFT) that can capture metal fluoride particles or other particles using a mechanical mesh and electrostatic forces. In some implementations, as a portion of the gas discharge gain medium passes through the MFT, the metal fluoride dust in the contaminated gas discharge gain medium is adsorbed onto a capture filter and any remaining particles are collected by an electrostatic precipitator. For example, certain MFTs have been previously described in U.S. Pat. No. 6,240,117, issued May 29, 2001, and U.S. Pat. No. 7,819,945, issued October 26, 2010, which are incorporated herein by reference in their entireties.

[0022]

[0043] Nevertheless, even with the dust particle capture system 135, there are certain circumstances in which dust particles 115 may still approach (and contaminate) the optical element 120. For example, contamination may occur during a gas refill procedure (where the gain medium 130 is replaced or refilled). As another example, contamination may occur during normal operation of the gas discharge chamber 110 when the dust particle capture system 135 is leaking or filling up (with dust particles 115). If a significant number of dust particles 115 are deposited on the optical element 120, damage to the optical element 120 may occur. As the optical element 120 interacts with the light beam 127, the dust particles 115 (such as dust particles 115s) on the surface 121 of the optical element 120 also absorb energy from the light beam 127, which may cause the dust particles 115s at the surface of the optical element 120 to heat up and melt to the surface of the optical element. Damage to surface 121 of optical element 120 can be a significant problem, especially when the life of gas discharge chamber 110 is to be extended and the energy of amplified light beam 127 is to be increased.

[0023]

[0044] Metrology apparatus 100 can track and / or detect these dust particles 115 / 115s as they flow near optical element 120. Information obtained by metrology apparatus 100 about dust particles 115 / 115s can be used to determine whether performance issues related to gas discharge chamber 110 are due to contamination of optical element 120 with dust particles 115 / 115s. Furthermore, metrology apparatus 100 enables tracking and / or detection of these dust particles 115 / 115s and enables determinations regarding the performance of gas discharge chamber 110 without requiring shutdown of gas discharge chamber 110, without requiring disassembly of gas discharge chamber 110, and without requiring removal of optical element 120 from cavity 105 to directly inspect optical element 120.

[0024]

[0045] The metrology apparatus 100 includes a probe apparatus 102, a detection apparatus 106, and a processing apparatus 108. The probe apparatus 102 is configured to generate a probe 104 in the vicinity of an optical element 120. The probe 104 is in the vicinity of the optical element 120 if the probe 104 is located adjacent or near the optical element 120 or is close enough to the optical element 120 to be able to estimate a characteristic of a dust particle 115 that affects the operation of the optical element 120. Furthermore, the probe 104 is in the vicinity of the optical element 120 if there is a path for the dust particle 115 to travel between the probe 104 and the optical element 120 and there is no obstruction between the dust particle 115 and the probe 104. The probe 104 interacts with the dust particle 115 that is blocked by the probe 104. The detection apparatus 106 is configured to detect this interaction between the probe 104 and one or more dust particles 115. The detection device 106 generates an output signal 107 based on the detected interaction. The processing device 108 is configured to receive the output signal 107 and estimate characteristics of one or more dust particles 115.

[0025]

[0046] Referring to FIG. 2A, an embodiment 202 of the probe apparatus 102 generates a light sheet 204 as the probe 104. The probe apparatus 202 is an optical assembly including a light source 212 configured to generate a light beam, which is optically modified by an optical component 216 that directs and shapes the light beam into the light sheet 204. In a particular example, the light source 212 is a laser, such as a He-Ne laser, a Nd / YAG laser, or any laser or laser source having a wavelength different from that of the light beam 127. The output of the light source 212 is a light beam or laser beam, which can be directed through an optical fiber 213 toward the optical component 216, which is positioned or disposed along a path toward the optical element 220. The optical component 216 can include one or more mirrors that redirect the light beam and one or more windows through which the light beam passes. One or more of the optical components 216 (such as windows) can be used to hermetically seal the optical component 216 to the gas discharge chamber 110 of FIG. 1. Alternatively, the optical component 216 can be mounted in a housing 240 that is hermetically sealed to the gas discharge chamber 110. In this way, the light beam from the light source 212 can be generated outside the cavity 105 and then transported through the optical component 216 into the fluidly connected area or inside the cavity 105. Additionally, the optical element 220 can be mounted in a housing 240 that is hermetically sealed to the gas discharge chamber 110 so that the optical element 220 is exposed to the gain medium 130. In this case, the window of the optical component 216 can hermetically seal the housing 240. The optical component 216 also includes a cylindrical lens that transforms the light beam into a light sheet 204 with a spread along a first transverse direction that is much larger than the spread along a second transverse direction, the transverse direction being perpendicular to the direction along which the light sheet 204 travels. The window of the optical component 216 can be made of CaF2. In this way, the probe device 202 is retrofitted into the housing 240 of the optical element 220.

[0026]

[0047] The light sheet 204 is aligned along the probe axis A. P The probe is guided along a path defined by the probe axis A. P must be non-parallel to the plane or path that light beam 127 follows through gas discharge chamber 110. In this way, light sheet 204 does not interfere with light beam 127 because it cannot follow the same path that light beam 127 takes in passing through gas discharge chamber 110. For example, light beam 127 may travel along the XY plane of chamber 110 and probe axis A may travel along the XY plane of chamber 110 and probe axis B may travel along the Y plane of chamber 110 and probe axis C may travel along the Y plane of chamber 110 and probe axis D may travel along the Y plane of chamber 110 and probe axis E may travel along the Y plane of chamber 110 and probe axis F ... P is generally aligned with the Z axis. In the illustrated embodiment, the light sheet 204 is directed along a path adjacent to a surface 221 of an optical element 220 that is in fluid communication with the cavity 105 of the gas discharge chamber 110.

[0027]

[0048] 2A shows an embodiment 206 of the detection device 106. The detection device 206 is configured to detect light 242 generated by the interaction of the light sheet 204 with one or more dust particles 115. The light 242 can be light from the light sheet 204 that is scattered, reflected or deflected by the dust particles 115 and directed along the field of view of the detection device 206. The detection device 206 can include a photodiode or a camera. The photodiode measures the intensity of the light 242 and converts this light energy into an electrical current.

[0028]

[0049] 3A and 3B, on the other hand, the camera's sensor 244 captures a two-dimensional visual image 246 of the field of view facing the light sheet 204, and thus allows the dust particle 115 to be visualized in two dimensions in the XY plane of the sensor 244. In particular, the dust particle 115 appears in the image as a shape (or region of interest) 248 corresponding to the intensity change of the pixels underlying the shape 248 relative to other pixels in the image 246.

[0029]

[0050] 2B and 2C, in either scenario (where detection device 206 includes a photodiode or camera), an image plane IP244 of such photodiode or camera should face light sheet 204. In particular, image plane IP244 of the photodiode or camera can be perpendicular to transverse plane TP204 of light sheet 204, which transverse plane TP204 of light sheet 204 is perpendicular to probe axis A. P 2. Thus, in some embodiments, the image plane IP 244 of the photodiode or camera is perpendicular to the probe axis A. P is parallel to the probe axis A P is located in the image plane IP244 of the photodiode or camera. If the light sheet 204 has a long extent defined by a long plane LP204 perpendicular to the transverse plane TP204 of the light sheet 204, the image plane IP244 of the photodiode or camera can be parallel to the long plane LP204 of the light sheet 204, as shown in Figure 2B, or perpendicular to the long plane LP204 of the light sheet 204, as shown in Figure 2C and also shown in Figure 10A, or along any direction between these two extremes.

[0030]

[0051] Referring to FIG. 4, an embodiment 410 of the gas discharge chamber 110 is shown as part of a deep ultraviolet (DUV) gas discharge light source 450, which generates an amplified light beam 427 (corresponding to the amplified light beam 127). The light source 450 may include other devices and optical elements not shown in FIG. 4. For example, FIG. 6 shows an embodiment 650 of a two-stage light source 450. The light source 450 further outputs an operating light beam 451 for use by an output device 455, which may be a photolithography exposure device. Depending on the position of the gas discharge chamber 410 within the light source 450, the operating light beam 451 may correspond to the amplified light beam 427. Alternatively, the amplified light beam 427 may be directed through other optical devices and elements within the light source 450 before forming the operating light beam 451 for use by the output device 455. The metrology apparatus 100 is in communication with an actuator 452, which receives estimated properties 453 of one or more particles 115 flowing near the optical element 420 in the cavity 405 of the gas discharge chamber 410. The actuator 452 is configured to adjust one or more characteristics of the DUV gas discharge light source 450 based on the estimated properties 453. For example, the actuator 452 may be configured to adjust one or more characteristics of the dust particle capture system 435.

[0031]

[0052] Additionally, controller 454 can be in communication with metrology apparatus 100 (specifically processor 108) and actuator 452. Controller 454 can know more information about the operation of light source 450 than processor 108. In this manner, controller 454 can analyze estimated characteristic 453 (output from processor 108 of metrology apparatus 100) and further analyze the performance of gas discharge chamber 410 and / or light source 450 based on estimated characteristic 453. For example, controller 454 can be configured to predict the lifetime of optical element 420 and / or gas discharge chamber 410.

[0032]

[0053] 5, an embodiment 508 of the processing device 108 is shown. The processing device 508 includes a signal processing module 522 configured to receive the output signal 107 from the detection device 106.

[0033]

[0054] If the output signal 107 is provided by the sensor 244 of the detector 206, the signal processing module 522 receives a two-dimensional representation (image) from the detector 206 and performs processing on the image. To this end, the signal processing module 522 can include various sub-modules configured to perform various types of analysis on the image. For example, the signal processing module 522 can include an input sub-module that receives the image from the detector 206 and converts the data into a suitable format for processing. The signal processing module 522 can include a pre-processing sub-module that makes the image from the detector 206 usable (e.g., removes background noise, filters the image, and compensates for gain). The signal processing module 522 can include an image sub-module that performs processing of the image data, such as identifying one or more regions of interest (ROI) in the image, each ROI being one of the shapes 248 corresponding to the location of a dust particle 115. The image sub-module can also calculate characteristics of each ROI, such as, for example, the area of ​​each ROI in the image and the center of gravity of each ROI. The analysis signal processing module 522 may include an output sub-module that prepares the calculated data (such as the area and centroid of the ROI) for output.

[0034]

[0055] If the detection device 106 includes a photodiode, the output signal 107 is provided by the photodiode, and the output signal 107 is a voltage signal related to a current generated from the light detected at the photodiode of the detection device 106. Generally, the signal processing module 522 analyzes the output signal 107 from the photodiode. For example, the signal processing module 522 can analyze a set of timestamps corresponding to how each dust particle 115 interacts with the probe light sheet 204, determine whether the amplitude of the output signal 107 is greater than a threshold, identify the magnitude (e.g., area) of the output signal 107 that is greater than the threshold, and / or determine the start and end times when the output signal 107 crosses a threshold.

[0035]

[0056] The processing unit 508 may also include or have one or more programmable processors 523 and one or more computer programs 524 tangibly embodied in machine-readable storage devices for execution by the programmable processors. The one or more programmable processors 523 may each execute a program of instructions that performs a desired function by operating on input data to generate appropriate output. Generally, the processor 523 receives instructions and data from a memory 526. The memory 526 may be a read-only memory and / or a random-access memory. Suitable storage devices for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices such as EPROM, EEPROM, and flash memory devices, magnetic disks such as internal hard disks and removable disks, magneto-optical disks, and CD-ROM disks. Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits). The processing unit 508 may also include one or more input devices 528 (e.g., a keyboard, a touch screen, a microphone, a mouse, a handheld input device, etc.) and one or more output devices 529 (e.g., a speaker or a monitor).

[0036]

[0057] Additionally, if the metrology apparatus 100 is in communication with an actuator 452 (FIG. 4), the processing apparatus 508 also includes an actuator module 514 in communication with the actuator 452, which in turn is in communication with the DUV light source 450. The actuator module 514 can be located within the processing apparatus 508 (as shown in FIG. 5) or can be incorporated into the actuator 452.

[0037]

[0058] The modules within the processing unit 508 (such as the signal processing module 522 and the actuation module 514) each include their own digital electronic circuitry, computer hardware, firmware, and software, as well as dedicated memories, input / output devices, programmable processors, and computer programs. Similarly, any one or more of the modules 522, 514 can access and use a memory 526, one or more input devices 528, one or more output devices 529, one or more programmable processors 523, and one or more computer programs 524.

[0038]

[0059] Although the processing unit 508 is shown as a separate complete unit, each of the components and modules of the processing unit 508 may be separate units. The processing unit 508 may also include other components, such as dedicated memory, input / output devices, processors, computer programs, etc., that are not shown in FIG. 5. Alternatively, the processing unit 508 may be integrated with the controller 454.

[0039]

[0060] Referring to FIG. 6, as described above, the light source 450 can be a two-stage light source 650. The light source 650 includes a master oscillator 660A as its first stage and a power amplifier 660B as its second stage. The master oscillator 660A includes a master oscillator gas discharge chamber 610A, and the power amplifier 660B includes a power amplifier gas discharge chamber 610B. The master oscillator gas discharge chamber 610A includes two elongated electrodes 625A as an energy source that provide a pulsed energy source to the gain medium 630A in the chamber 610A. The power amplifier gas discharge chamber 610B includes two elongated electrodes 625B as an energy source that provide a pulsed energy source to the gain medium 630B in the chamber 610B.

[0040]

[0061] A master oscillator 660A provides a pulsed amplified light beam (called a seed light beam) 661 to a power amplifier 660B. A master oscillator gas discharge chamber 610A houses a gain medium 630A in which amplification occurs, and the master oscillator 660A includes an optical feedback mechanism, such as an optical resonator. The optical resonator is formed between a spectroscopic optical system 662A on one side of the master oscillator gas discharge chamber 610A and an output coupler 663A on a second side of the master oscillator gas discharge chamber 610A. A power amplifier gas discharge chamber 610B houses a gain medium 630B in which amplification occurs when seeded with a seed light beam 661 from the master oscillator 660A. If the power amplifier 660B is designed as a regenerative ring resonator, it is described as a power ring amplifier, in which case sufficient optical feedback can be provided from the ring design. The power amplifier 660B also includes a beam return (such as a reflector) 662B that returns the light beam (e.g., by reflection) back into the power amplifier gas discharge chamber 610B to form a circulation and loop path (where the input to the ring amplifier intersects with the output from the ring amplifier), and an output coupler 663B for inputting the seed light beam 661 and outputting an amplified light beam 667. The operating light beam 651 provided to the output device can correspond to the amplified light beam 667 that is output from the power amplifier 660B and that is additionally modified by other optical components 664, such as beam steering and redirection and pulse stretching optics.

[0041]

[0062] The gain medium 630A, 630B used in each discharge chamber 610A, 610B can be any combination of gases suitable to generate an amplified light beam around the desired wavelength, bandwidth and energy. For example, as described above, the gain medium 630A, 630B can include argon fluoride (ArF), which emits light at a wavelength of about 193 nm, or krypton fluoride (KrF), which emits light at a wavelength of about 248 nm.

[0042]

[0063] As discussed above, metrology apparatus 100 may be associated with gas discharge chamber 110. An embodiment 600 of metrology apparatus 100 may be associated with either or both of gas discharge chambers 610A, 610B in light source 650. In one particular embodiment, as shown in FIG. 6 and shown in detail in FIGS. 7-8B, metrology apparatus 600 is associated with power amplifier gas discharge chamber 610B, and in particular with an optical element in the form of window 620oB located on an input / output side of chamber 610B. In other embodiments, metrology apparatus 600 is associated with windows at other locations, or additional instances of metrology apparatus are associated with additional windows at other locations within light source 650. For example, the metrology device 600 can be associated with a window 620rB disposed on the other side of the power amplifier discharge chamber 610B, a window 620oA disposed as the output side of the master oscillator discharge chamber 610A, or a window 620rA disposed on the side of the master oscillator discharge chamber 610A facing the spectroscopic optical system 662A. The windows 620oA, 620rA, 620oB, 620rB are made of materials that are compatible with the gain media 630A, 630B, respectively. Additionally, the windows 620oA, 620rA, 620oB, 620rB are made of materials that can transmit the light generated by the gain media 630A, 630B. Thus, in this example, the windows 620oA, 620rA, 620oB, 620rB must transmit light having a wavelength in the DUV range, since the light generated is in the DUV range. In some embodiments, the windows 620oA, 620rA, 620oB, 620rB are made of a crystalline structure. For example, the windows 620oA, 620rA, 620oB, 620rB can be made of calcium fluoride, magnesium fluoride, or fused silica.

[0043]

[0064] 7-8B, metrology apparatus 600 is associated with a power amplifier gas discharge chamber 610B, specifically with a window 620oB located on the input / output side of chamber 610B. Window 620oB is secured to a wall 666B of chamber 610B and within a window housing 640. Window housing 640 serves two functions: it secures window 620oB in place, and it hermetically seals window 620oB to wall 666B such that gas discharge chamber 610B remains hermetically sealed and holds gain medium 630B. Window 620oB is configured to pass seed light beam 661 into cavity 605B of chamber 610B, and is also configured to pass output amplified light beam 667.

[0044]

[0065] 8A and 8B show the arrangement of the window housing 640 and the window 620oB in more detail. In this embodiment, the probe is a light sheet 604 guided along a path adjacent to the surface 621oB of the window 620oB facing the cavity 605B of the chamber 610B. The housing 640 includes a receiving cavity 640c in fluid communication with the cavity 605B of the chamber 610B. Moreover, an optical path is defined in a passage 668 of the housing 640 opposite the window 620oB. The passage 668 allows the light beams 661, 667 to pass through the window 620oB into and out of the cavity 605B of the chamber 610B. The light beams 661, 667 generally travel in the XY plane of the power amplifier gas discharge chamber 610B. This means that the light beams 661, 667 are generally not configured to move along the Z axis of the chamber 610B. As described above, metrology apparatus 600 is operated while gas discharge chamber 610B is generating amplified light beam 667, and thus light sheet 604 is directed so that light sheet 604 is positioned proximate window 620oB at the same time that light beams 661, 667 pass through window 620oB. To prevent any optical interference between light sheet 604 and light beams 661, 667, light sheet 604 is aligned along probe axis A, which is non-parallel to the XY plane along which light beams 661, 667 pass through power amplifier gas discharge chamber 610B. P (See, e.g., FIG. 2A ). In one example, as shown in FIGS. 8A and 8B , the probe axis A P is generally aligned with the Z-axis, and the light sheet 604 is guided along a path adjacent to a surface 621oB of a window 620oB facing the cavity 605B of the gas discharge chamber 610B. Moreover, the wavelength of the light sheet 604 is different from the wavelengths of the light beams 661, 667. In this way, the detection device 606 can detect the interaction of the light sheet 604 with one or more dust particles 115 even during the generation of the amplified light beams 661, 667.

[0045]

[0066] The light sheet 604 can be configured to pass through another region of the cavity 605B or the receiving cavity 640c (in fluid communication with the cavity 605B). For example, as shown in Figures 9A and 9B, the respective light sheets 904A and 904B are positioned at other locations within the receiving cavity 640c. Other locations of the light sheets 604, 904A, 904B are also possible, so long as they are close enough (near) the window 620oB to allow an accurate estimation of the number of dust particles 115 that are in contact with or near the window 620oB.

[0046]

[0067] 10A and 10B, an embodiment 1006 of the detection device 206 has an imaging plane (X 1044) relative to the imaging plane of the sensor 244 of the detection device 206 shown in FIG. S Y S In both cases, the detectors 206 and 1006 are positioned so that their imaging plane (X 1004) faces the light sheet 204 and can image the full extent of the light sheet 204. This enables the detector 1006 to capture a two-dimensional visual image 1046 of the field of view facing the light sheet 204, with the dust particles 115 becoming visible as shapes or regions of interest 1048 in the image 1046. The detector 206 is positioned so that the imaging plane (X 1004) of the sensor 244 is rotated. S Y S The imaging plane (X plane) of the sensor 244 can be positioned along any direction as long as it can visualize a wider extent of the light sheet 204. S Y S plane) is the probe axis A P should not be perpendicular to the

[0047]

[0068] 11, a metrology procedure 1170 is performed. The metrology procedure 1170 may be performed by a metrology apparatus 100 associated with a gas discharge chamber 110. A probe 104 is generated (1172) in proximity to an optical element 120. An interaction between the probe 104 and one or more dust particles 115 is detected (1174). An output signal 107 is generated (1176) based on the detected interaction. Then, a property of the one or more dust particles 115 is estimated (1178) based on the output signal 107.

[0048]

[0069] For example, probe 104 may be created 1172 by creating a light sheet (such as light sheet 204) in proximity to optical element 220. With reference to the embodiment of Figures 7, 8A and 8B, light sheet 604 may be aligned along a probe axis A that is non-parallel to the XY plane along which actuation light beams 661, 667 pass through gas discharge chamber 610B. P Additionally, the light sheet 604 can be directed along a path adjacent to the surface 621oB of the optical element 620oB. The probe 104 is generated (1172) while the gas discharge chamber 110 is operating to generate the operating light beam 127.

[0049]

[0070] 2A, the interaction is detected (1174) by capturing light 242 generated by the interaction of the light sheet 204 with one or more dust particles 115. The light 242 captured (e.g., by the detection device 206 in step 1174) may be light from the light sheet 204 that is scattered or reflected from the one or more dust particles 115. Moreover, the light 242 may be captured by a photodiode in the detection device 206 or by a camera with a sensor 244 or the like of the detection device 206.

[0050]

[0071] As mentioned above, in an embodiment where the detection arrangement 106 includes a two-dimensional imaging device such as a camera with a sensor 244, the output signal 107 generated (1176) is a two-dimensional representation or image 246 of the field of view of the sensor 244. With reference to FIG. 12, a region or shape of interest 248 is displayed in the image 246. From this data, for example, the processing unit 108 can estimate or count (1178) the number of dust particles 115 present in the vicinity of the optical element 120 (over a period of time). Additionally, since the position of the light sheet 204 within the cavity 105 (or cavity 605B, 640c) is known, the processing unit 108 can also estimate the position of one or more dust particles 115. The processing unit 108 can also estimate or count (1178) the number of dust particles 115 present in a particular area in the vicinity of the optical element 120 at a time (density) or over a period of time (varying density).

[0051]

[0072] The processing unit 108 may continuously store the positions of one or more dust particles 115 in the memory 523 and use the stored positions to track (1178) the path of the dust particle and also the velocity (speed and direction) of the dust particle 115 in the vicinity of the optical element 220 over time. For example, referring to Figure 13, the processing unit 108 tracks the trajectories (or flow patterns) of several dust particles 115 over time. For simplicity, only three trajectories 1185i, 1185ii, 1185iii are labeled in Figure 13, although many more trajectories have been observed.

[0052]

[0073] The estimated properties 1178 of the one or more dust particles can be used to adjust 1180 one or more characteristics of the DUV light source 450 in which the gas discharge chamber 410 is implemented. In one example, the adjustment to the DUV light source 450 can be to empty or replace the capture system 135 (if the capture system 135 is deemed full). For example, visualization of the dust particle 115 flow patterns (such as the flow patterns in FIG. 13 ) allows for further analysis of the dust particle flow behavior, which can be used to improve the design of the gas discharge chamber 410 to reduce the likelihood of dust particles flowing near the optical element 420. For example, the gas discharge chamber 410 can be modified by changing the rate at which gas is circulated in the cavity of the chamber 410. As another example, information obtained by tracking and counting the dust particles 115 in the vicinity of the optical element 120 can be used to resolve performance issues with the gas discharge chamber 110. As a further example, it is also possible to predict the lifetime of the optical element 120 based on information obtained by tracking and counting the dust particles 115 in the vicinity of the optical element 120, or to determine how the number of dust particles in the vicinity of the optical element 120 affects the lifetime of the optical element 120. Estimating the velocity of the dust particles 115 near the optical element 120 enhances the overall understanding of the flow behavior of the dust particles 115 near the optical element 120.

[0053]

[0074] In one embodiment, the processing device 108 tracks the dust particle 115 as follows, with reference to Figures 14A-14C: At time T1, the detection device 106 captures an image 1446-1 (Figure 14A), at time T2, the detection device 106 captures an image 1446-2 (Figure 14B), and at time T3, the detection device 106 captures an image 1446-3 (Figure 14C). At time T1 (Figure 14A), the processing device 108 (specifically the signal processing module 522) identifies the dust particle 115 (shown in Figure 14A as a region or shape of interest 1448A) in the image 1446-1. At time T2 (FIG. 14B), the processing device 108 (specifically the signal processing module 522) identifies the dust particles 115 in the image 1446-2 (shown in FIG. 14B as regions of interest or shapes 1448B) and also locates the dust particles 115 (shown in FIG. 14B as shapes 1448A) in the vicinity of the dust particles 115 detected in the previous image 1446-1. The dust particles 115 in the vicinity of the dust particles 115 detected in the previous image 1446-1 are represented by a faded fill pattern (shape 1448A) relative to the dust particles 115 detected in the current image 1446-2 (shape 1448B). At time T3 (FIG. 14C), the processing device 108 (specifically the signal processing module 522) identifies the dust particles 115 in the image 1446-3 (shown in FIG. 14C as a region of interest or shape 1448C) and also locates the dust particles 115 (shown in FIG. 14C as shape 1448B) in the vicinity of the dust particles 115 detected in the previous image 1446-2. The dust particles 115 in the vicinity of the dust particles 115 detected in the previous image 1446-2 are represented by a faded fill pattern (shape 1448B) relative to the dust particles 115 detected in the current image 1446-3 (shape 1448C). The dust particles 115 in the vicinity of the dust particles 115 detected in the image 1446-1 taken at time T1 are also shown in FIG. 14C as shape 1448A for reference. In this manner, the processing device 108 can track each dust particle over time.

[0054]

[0075] The embodiments may be further described using the following clauses. 1. A metrology apparatus comprising: an optical element in fluid communication with a gain medium of the gas discharge chamber, the probe apparatus configured to generate a probe proximate the optical element that is exposed to the one or more particles; a detection device configured to detect an interaction between the probe and one or more particles and generate an output signal based on the detected interaction; a processor configured to receive the output signal and estimate a characteristic of the one or more particles; 1. A metrology apparatus comprising: 2. A metrology apparatus as described in clause 1, wherein the probe apparatus is an optical assembly, the probe is a light sheet, and the detection apparatus configured to detect the interaction includes a detection apparatus configured to capture light generated by the interaction of the light sheet with one or more particles. 3. The metrology apparatus of clause 2, wherein the optical assembly includes a laser configured to generate the laser light sheet as a light sheet. 4. The metrology apparatus of clause 3, wherein the laser is configured to generate light having a wavelength different from a wavelength of light generated from the gain medium in the gas discharge chamber. 5. The metrology apparatus of clause 2, wherein the detection device comprises a photodiode or a camera. 6. A metrology apparatus as described in clause 2, wherein an image plane of the detection device faces the light sheet such that the extent of the light sheet is observable and imageable. 7. The metrology apparatus of clause 6, wherein an image surface of the detection device faces a surface of an optical element that is in fluid communication with the interior of the gas discharge chamber. 8. A metrology apparatus as described in clause 2, wherein the light sheet is guided along a path non-parallel to a plane along which the amplified light beam passes through the gas discharge chamber, the amplified light beam being generated by the gain medium during application of energy. 9. A metrology apparatus as described in clause 2, wherein the light sheet is guided along a path adjacent to a surface of the optical element. 10. The metrology apparatus of clause 2, wherein the optical element is a window in the gas discharge chamber disposed between an interior of the gas discharge chamber filled with the gain medium and an exterior of the gas discharge chamber, the window hermetically sealing the discharge chamber and configured to allow the amplified light beam to pass through the window. 11. The metrology apparatus of clause 10, wherein the light sheet is guided along a path adjacent a surface of a window facing the interior of the gas discharge chamber. 12. The metrology apparatus of clause 10, wherein the light sheet is guided along a path proximate a surface of a window facing the interior of the gas discharge chamber. 13. A metrology apparatus as described in clause 2, wherein the detection device configured to capture light generated by interaction of the light sheet with one or more particles includes capturing light from the light sheet that is scattered or reflected from the one or more particles. 14. A metrology apparatus as described in clause 2, wherein the probe axis of the light sheet is located in the imaging plane of the detection device and the long plane of the light sheet is perpendicular to the imaging plane or the long plane of the light sheet is positioned at an angle between an angle parallel to the imaging plane and an angle perpendicular to the imaging plane. 15. A metrology apparatus as described in clause 2, wherein the probe axis of the light sheet is located in an image plane of the detection device and the long plane of the light sheet is parallel to the image plane. 16. The metrology apparatus of clause 1, wherein the processing device configured to estimate the properties of the one or more particles includes a processing device configured to estimate one or more of: a number of one or more particles, a position of one or more particles, a density of one or more particles, and a velocity of one or more particles. 17. A metrology apparatus as described in clause 1, wherein the probe apparatus generates a probe in the vicinity of the optical element and the detection apparatus generates an output signal based on the detected interaction while the gas discharge chamber generates the amplified light beam. 18. The metrology apparatus of clause 17, wherein the gas discharge chamber includes a gas that includes the gain medium and electrodes for providing energy to the gain medium such that the gain medium generates a plasma that produces the amplified light beam when a voltage is applied to the electrodes. 19. A metrology apparatus as described in clause 1, wherein the probe and detection devices are disposed within or attached to a housing that holds the optical element. 20. An apparatus for a deep ultraviolet (DUV) gas discharge light source, comprising: 1. A metrology apparatus comprising: an optical element in fluid communication with a gain medium of the gas discharge chamber, the probe apparatus configured to generate a probe proximate the optical element that is exposed to the one or more particles; a detection device configured to detect an interaction between the probe and one or more particles and generate an output signal based on the detected interaction; a processor configured to receive the output signal and estimate a characteristic of the one or more particles; a metrology apparatus comprising: an actuator configured to receive the estimated characteristic and adjust one or more characteristics of the gas discharge light source based on the estimated characteristic; 2. An apparatus for a deep ultraviolet (DUV) gas discharge light source, comprising: 21. The apparatus of clause 20, further comprising a controller in communication with the processing device and the actuator, the controller configured to analyze the estimated characteristic and analyze performance of the gas discharge chamber based on the analysis of the estimated characteristic. 22. The apparatus of clause 21, wherein the control device is configured to predict the lifetime of the optical element and / or the gas discharge chamber. 23. The apparatus of clause 21, wherein the actuator is configured to adjust one or more characteristics of the dust particle capture system. 24. The apparatus of clause 20, wherein the particles include dust particles generated from a gain medium within the gas discharge chamber. 25. The apparatus of claim 24, wherein the gain medium comprises a fluoride and the dust particles comprise metal fluoride particles. 26. The apparatus of claim 20, wherein the gain medium comprises argon fluoride, krypton fluoride, or xenon chloride. 27. The apparatus of clause 20, wherein the metrology apparatus is associated with a powering ring amplifier of a DUV gas discharge light source and the optical element is a window in a gas discharge chamber of the powering ring amplifier. 28. The apparatus of clause 27, wherein the probe is positioned proximate a window of a gas discharge chamber of a powering amplifier that is in fluid communication with the gain medium and that is exposed to the one or more particles. 29. The apparatus of claim 27, wherein the window of the gas discharge chamber of the powering ring amplifier is a window at an output side of the gas discharge chamber of the powering ring amplifier. 30. The apparatus of clause 27, wherein the window comprises a crystalline structure configured to transmit light having a wavelength in the DUV range. 31. The apparatus of claim 30, wherein the window comprises calcium fluoride, magnesium fluoride, or fused silica. 32. An apparatus as described in clause 20, wherein the probe apparatus is an optical assembly including a laser configured to generate a laser light sheet as a probe, and the detection apparatus configured to detect the interaction includes a detection apparatus configured to capture light generated by the interaction of the light sheet with one or more particles. 33. The apparatus of clause 32, wherein the laser is configured to generate light having a wavelength different from a wavelength of light generated from the gain medium in the gas discharge chamber. 34. The apparatus of clause 32, wherein the detection device comprises a photodiode or a camera. 35. An apparatus as described in clause 32, wherein the light sheet is directed along a path non-parallel to a plane along which the amplified light beam produced by the gain medium during application of energy passes through the gas discharge chamber. 36. An apparatus as described in clause 32, wherein the laser light sheet is directed along a path adjacent to a surface of the optical element. 37. Producing a probe proximate an optical element in fluid communication with a gain medium of a gas discharge chamber, the optical element being exposed to one or more dust particles; detecting an interaction of the generated probe with one or more dust particles; generating an output signal based on the detected interaction; Estimating one or more dust particle characteristics based on the output signal. A metrology method comprising: 38. A metrology method as described in clause 37, wherein generating the probe includes generating a laser light sheet and detecting the interaction includes capturing light generated by interaction of the light sheet with one or more dust particles. 39. The metrology method of clause 38, wherein the laser light sheet has a wavelength different from a wavelength of light generated from the gain medium in the gas discharge chamber. 40. A metrology method as described in clause 38, wherein capturing light generated by interaction of the light sheet with one or more dust particles includes capturing light from the laser light sheet that is scattered or reflected from the one or more dust particles. 41. The metrology method described in clause 40, wherein capturing light from the laser light sheet that is scattered or reflected from one or more dust particles includes generating a potential difference on the exposed surface or generating a two-dimensional image on the exposed surface, wherein the exposed surface receives the scattered or reflected light from the laser light sheet. 42. The metrology method of clause 38, wherein generating the laser light sheet includes directing the laser light sheet along a path non-parallel to a plane along which the amplified light beam generated by the gain medium during application of energy passes through the gas discharge chamber. 43. The metrology method of claim 42, wherein directing the laser light sheet along a path includes directing the laser light sheet along a path adjacent to a surface of the optical element. 44. The metrology method of clause 37, wherein estimating the one or more dust particle characteristics includes estimating one or more of a number of one or more dust particles, a position of one or more dust particles, a density of one or more dust particles, and a velocity of one or more dust particles. 45. The metrology method of claim 37, wherein generating a probe in the vicinity of the optical element and generating an output signal based on the detected interaction are performed while the gas discharge chamber is generating an amplified light beam.

[0055]

[0076] These and other embodiments are within the scope of the following claims.

Claims

1. 1. A metrology apparatus comprising: an optical element in fluid communication with a gain medium of a gas discharge chamber, the probe apparatus being configured to generate a probe proximate the optical element that is exposed to one or more particles; a detection device configured to detect an interaction between the probe and the one or more particles and generate an output signal based on the detected interaction; a processor configured to receive the output signal and estimate a property of the one or more particles; 1. A metrology apparatus comprising:

2. 2. The metrology apparatus of claim 1, wherein the probe apparatus is an optical assembly, the probe is a light sheet, and the detection apparatus configured to detect the interaction includes the detection apparatus configured to capture light generated by the interaction of the light sheet with the one or more particles.

3. The metrology apparatus of claim 2 , wherein the optical assembly includes a laser configured to generate a laser light sheet as the light sheet.

4. The metrology apparatus of claim 3 , wherein the laser is configured to generate light having a wavelength different from a wavelength of light generated from the gain medium in the gas discharge chamber.

5. The metrology apparatus of claim 2 , wherein the detection device comprises a photodiode or a camera.

6. 3. The metrology apparatus of claim 2, wherein an image surface of the detector faces the light sheet such that an extent of the light sheet is observable and imageable.

7. The metrology apparatus of claim 6 , wherein the image surface of the detector faces a surface of the optical element that is in fluid communication with an interior of the gas discharge chamber.

8. 3. The metrology apparatus of claim 2, wherein the light sheet is directed along a path non-parallel to a plane along which an amplified light beam passes through the gas discharge chamber, the amplified light beam being generated by the gain medium during application of energy.

9. The metrology apparatus of claim 2 , wherein the light sheet is directed along a path adjacent to a surface of the optical element.

10. 3. The metrology apparatus of claim 2, wherein the optical element is a window of the gas discharge chamber disposed between an interior of the gas discharge chamber filled with the gain medium and an exterior of the gas discharge chamber, the window hermetically sealing the gas discharge chamber and configured to allow the amplified light beam to pass through the window.

11. The metrology apparatus of claim 10 , wherein the light sheet is directed along a path adjacent a surface of the window that faces the interior of the gas discharge chamber.

12. The metrology apparatus of claim 10 , wherein the light sheet is directed along a path proximate a surface of the window facing the interior of the gas discharge chamber.

13. 3. The metrology apparatus of claim 2, wherein the detection device configured to capture light generated by the interaction of the light sheet with the one or more particles includes capturing light from the light sheet that is scattered or reflected from the one or more particles.

14. 3. The metrology apparatus of claim 2, wherein a probe axis of the light sheet is located in an image plane of the detection device and a long plane of the light sheet is perpendicular to the image plane or the long plane of the light sheet is positioned at an angle between an angle parallel to the image plane and an angle perpendicular to the image plane.

15. 3. The metrology apparatus of claim 2, wherein a probe axis of the light sheet lies in an image plane of the detector and a long plane of the light sheet is parallel to the image plane.

16. 2. The metrology apparatus of claim 1, wherein the processing device configured to estimate properties of the one or more particles comprises a processing device configured to estimate one or more of a number of the one or more particles, a position of the one or more particles, a density of the one or more particles, and a velocity of the one or more particles.

17. 2. The metrology apparatus of claim 1, wherein the probe apparatus generates the probe in the vicinity of the optical element, and the detection apparatus generates the output signal based on the detected interaction while the gas discharge chamber generates an amplified light beam.

18. 20. The metrology apparatus of claim 17, wherein the gas discharge chamber includes a gas that includes a gain medium and electrodes for providing energy to the gain medium such that the gain medium generates a plasma that produces an amplified light beam when a voltage is applied to the electrodes.

19. The metrology apparatus of claim 1 , wherein the probe device and the detection device are disposed within or attached to a housing that holds the optical element.

20. 1. An apparatus for a deep ultraviolet (DUV) gas discharge light source, the apparatus comprising:

1. A metrology apparatus comprising: an optical element in fluid communication with a gain medium of a gas discharge chamber, the probe apparatus being configured to generate a probe proximate the optical element that is exposed to one or more particles; a detection device configured to detect an interaction between the probe and the one or more particles and generate an output signal based on the detected interaction; a processor configured to receive the output signal and estimate a property of the one or more particles; the metrology device, an actuation device configured to receive the estimated characteristic and adjust one or more characteristics of the gas discharge light source based on the estimated characteristic; 1. An apparatus for a deep ultraviolet (DUV) gas discharge light source comprising:

21. The apparatus of claim 20 , further comprising a control device in communication with the processing device and the actuator, the control device configured to analyze the estimated characteristics and analyze performance of the gas discharge chamber based on the analysis of the estimated characteristics.

22. 22. The apparatus of claim 21, wherein the controller is configured to predict a lifetime of the optical element and / or the gas discharge chamber.

23. The apparatus of claim 21 , wherein the actuator is configured to adjust one or more characteristics of a dust particle capture system.

24. 21. The apparatus of claim 20, wherein the particles comprise dust particles generated from the gain medium in the gas discharge chamber.

25. 25. The apparatus of claim 24, wherein the gain medium comprises fluoride and the dust particles comprise metal fluoride particles.

26. 21. The apparatus of claim 20, wherein the gain medium comprises argon fluoride, krypton fluoride, or xenon chloride.

27. 21. The apparatus of claim 20, wherein the metrology device is associated with a powering ring amplifier of the DUV gas discharge light source, and the optical element is a window of the gas discharge chamber of the powering ring amplifier.

28. 30. The apparatus of claim 27, wherein the probe is positioned proximate the window of the gas discharge chamber of the powering ring amplifier that is in fluid communication with a gain medium and that is exposed to one or more particles.

29. 28. The apparatus of claim 27, wherein the window of the gas discharge chamber of the powering ring amplifier is the window at an output side of the gas discharge chamber of the powering ring amplifier.

30. 30. The apparatus of claim 27, wherein the window comprises a crystalline structure configured to transmit light having a wavelength in the DUV range.

31. 31. The apparatus of claim 30, wherein the window comprises calcium fluoride, magnesium fluoride, or fused silica.

32. 21. The apparatus of claim 20, wherein the probe apparatus is an optical assembly including a laser configured to generate a laser light sheet as the probe, and the detection apparatus configured to detect the interaction includes the detection apparatus configured to capture light generated by the interaction of the light sheet with the one or more particles.

33. 33. The apparatus of claim 32, wherein the laser is configured to generate light having a wavelength different from a wavelength of light generated from the gain medium in the gas discharge chamber.

34. 33. The apparatus of claim 32, wherein the detection device comprises a photodiode or a camera.

35. 33. The apparatus of claim 32, wherein the light sheet is directed along a path that is non-parallel to a plane along which an amplified light beam produced by the gain medium during application of energy passes through the gas discharge chamber.

36. 33. The apparatus of claim 32, wherein the laser light sheet is directed along a path adjacent to a surface of the optical element.

37. generating a probe proximate an optical element in fluid communication with a gain medium of a gas discharge chamber, the optical element being exposed to one or more dust particles; detecting an interaction of the generated probe with the one or more dust particles; generating an output signal based on the detected interaction; estimating a characteristic of the one or more dust particles based on the output signal; and A metrology method comprising:

38. 38. The metrology method of claim 37, wherein generating the probe comprises generating a laser light sheet and detecting the interaction comprises capturing light generated by the interaction of the light sheet with the one or more dust particles.

39. 40. The metrology method of claim 38, wherein the laser light sheet has a wavelength different than a wavelength of light generated from the gain medium in the gas discharge chamber.

40. 40. The metrology method of claim 38, wherein capturing the light generated by the interaction of the light sheet with the one or more dust particles comprises capturing light from the laser light sheet that is scattered or reflected from the one or more dust particles.

41. 41. The metrology method of claim 40, wherein capturing the light from the laser light sheet that is scattered or reflected from the one or more dust particles includes generating a potential difference on an exposed surface or generating a two-dimensional image on an exposed surface, the exposed surface receiving the scattered or reflected light from the laser light sheet.

42. 40. The metrology method of claim 38, wherein generating the laser light sheet includes directing the laser light sheet along a path non-parallel to a plane along which an amplified light beam generated by the gain medium passes through the gas discharge chamber during application of energy.

43. 43. The metrology method of claim 42, wherein directing the laser light sheet along the path comprises directing the laser light sheet along a path adjacent to a surface of the optical element.

44. 38. The metrology method of claim 37, wherein estimating the characteristics of the one or more dust particles comprises estimating one or more of a number of the one or more dust particles, a position of the one or more dust particles, a density of the one or more dust particles, and a velocity of the one or more dust particles.

45. 38. The metrology method of claim 37, wherein generating the probe in the vicinity of the optical element and generating the output signal based on the detected interaction occur while the gas discharge chamber is generating an amplified light beam.

Images