Event-based metrology systems and methods

Abstract

A metrology system for determining a characteristic of a structure on a substrate. The metrology system comprises an illumination source to output radiation to an objective. The objective is configured to direct radiation toward the structure on the substrate and direct radiation diffracted from the structure toward a detector. The detector comprises an event based camera to detect changes in intensity of radiation incident on the event based camera, and outputs an event data stream conveying the detected events. The detector comprises an edge detector o receive the event data stream and to output movement detection data representing a movement of the structure on the substrate relative to the objective or camera. The detector comprises a processor to receive the output movement detection data and determine the characteristic of the structure on the substrate based on the movement detection data and 0 properties of the structure on the substrate.

Description

EVENT-BASED METROLOGY SYSTEMS AND METHODSCROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Application No. 63 / 739,816, filed 30 December 2024, and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to metrology systems and method for determining a characteristic of a structure on a substrate. Specifically, it relates to metrology systems comprising an event-based camera.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law’. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] Patterns applied to a semiconductor substrate can be measured and inspected by a variety of different apparatus. The measurements may use different types of sensors to measure and inspect different properties of the pattern. Different types of sensors may have different requirements regarding how a measurement is obtained. The type of sensor used may affect the type of properties that can be measured, the resolution of the measured data, the duration of a measurement, the cost of taking the measurement, requirements on the environment for obtaining a measurement, etc. Described herein are methods and systems for obtaining measurements with properties that meet certain requirements or preferences.SUMMARY

[0006] According to an aspect of the current disclosure there is provided a metrology system for determining a characteristic of a structure on a substrate. The system comprises an illumination source configured to output radiation. The illumination source is optically coupled to an objective, wherein the objective is configured to direct the radiation toward the structure on the substrate, and direct radiation diffracted from the structure toward a detector module. The system further comprises the detector module, which comprises an event based camera, wherein the event based camera is configured to detect events comprising changes in intensity of radiation incident on a plurality of locations on a sensor of the event based camera. The event based camera is configured to output an event data stream conveying the detected events. The detector module further comprises an edge detection processor configured to receive the event data stream as an input and to output movement detection data representing a movement of the structure on the substrate relative to the objective and / or event-based camera. The system also comprises a processor system configured to receive the output movement detection data and determine the characteristic of the structure on the substrate based on the output movement detection data and one or more properties of the structure on the substrate.

[0007] Optionally, the structure may be a moving structure relative to the objective and / or the eventbased camera.

[0008] Optionally, the moving structure may be configured to be moved in a linear motion relative to the objective and / or the event-based camera of the metrology system.

[0009] Optionally, the moving structure may be configured to be moved at a constant velocity relative to the objective and / or the event-based camera of the metrology system.

[0010] Optionally, the moving structure may be configured to move at a speed in a range of 1 mm / s to 1000 mm / s, or at a speed in a range of 3mm / s to 1000 mm / s.

[0011] Optionally, the detected events may comprise detected moving edges in an image formed of radiation received from the moving structure.

[0012] Optionally, the image formed from radiation from the moving structure may comprise diffracted radiation.

[0013] Optionally, the edge detection processor may be configured to identify one or more shapes associated with the moving structure based on the detected moving edges in the image, and identify a moving position of the one or more shape on the sensor.

[0014] Optionally, identifying a moving position of a shape of the one or more shapes may comprise identifying a leading edge of the shape based on an increase in intensity of incident radiation at a location on the sensor, identifying a trailing edge of the shape based on an decrease in intensity of incident radiation at a location on the sensor, wherein the identification of the trailing edge is subsequent to the identification of the leading edge, and determining a position of the shape based on the identified leading edge and the identified trailing edge.

[0015] Optionally, the one or more shapes associated with the moving structure may comprise interference fringes.

[0016] Optionally, the characteristic may comprise alignment.

[0017] Optionally, the structure on a substrate may be an alignment mark.

[0018] Optionally, the radiation may be incoherent radiation.

[0019] Optionally, the illumination source may comprise one or more LED illumination sources.

[0020] Optionally, the one or more LED illumination source may comprise a plurality of LEDs positioned in a ring around the objective.

[0021] Optionally, the illumination source may comprise a plurality of LEDs, and at least some of the LEDs may have different wavelengths.

[0022] Optionally, the illumination source may comprise ambient light.

[0023] Optionally, a sensor may comprise a sensor array of multiple sensor elements.

[0024] Optionally, the system may further comprise an optical system to project at least some of the diffracted radiation onto the detector.

[0025] Optionally, the illumination may be coherent.

[0026] Optionally, the characteristic of the structure may comprise overlay.

[0027] Optionally, the structure may be configured to be held stationary relative to the objective.

[0028] Optionally, the structure may comprise a metrology target.

[0029] Optionally, the metrology target may comprise one or more overlapping and / or nonoverlapping diffraction gratings.

[0030] Optionally, the metrology system may comprise a digital holographic microscopy assembly.

[0031] Optionally, the movement may be vibration-induced movement of the structure on the substrate.

[0032] Optionally, the movement detection data of the vibration-induced movement may be used to correct vibration-induced error in the measurement of the characteristic of the structure.

[0033] Optionally, the vibration-induced movement may be in a range of 1 nm to 1 pm in a time of the order of 1 ms.

[0034] Optionally, the structure may be a lithographically patterned structure.

[0035] Optionally, the radiation output by the illumination source may comprise one or more wavelengths in a range of 100 nm - 2000 nm, or in a range of 400 nm to 1600 nm.

[0036] Optionally, the radiation output by the illumination source may have a centre wavelength and a bandwidth in a range of 4 nm - 10 nm around the centre wavelength.

[0037] Optionally, the event based camera may have a response time in a range of 5 ps to 15 ps, or below 30 ps.

[0038] According to another aspect of the current disclosure there is provided a metrology apparatus comprising a metrology system as described above.

[0039] According to another aspect of the current disclosure there is provided an inspection apparatus comprising a metrology system as described above.

[0040] According to another aspect of the current disclosure there is provided a lithography apparatus comprising a metrology system as described above.

[0041] According to another aspect of the current disclosure there is provided a litho cell comprising a system according or an apparatus according as described above.BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0043] Figure 1 depicts a schematic overview of a lithographic apparatus;

[0044] Figure 2 depicts a schematic representation of a metrology system for determining a characteristic of a structure on a substrate;

[0045] Figure 3 depicts graphs illustrating the capturing of intensity variation using a frame based camera and an event-based camera;

[0046] Figure 4 depicts a schematic representation of interference fringes incident on a detector;

[0047] Figure 5 depicts examples graphs of cumulative event data and its derivative for a pixel of an event-based camera; and

[0048] Figure 6 depicts an example of vibration-induced movement of interference fringes and its effect on integrated amplitude over time.DETAILED DESCRIPTION

[0049] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0050] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0051] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support inaccordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0052] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0053] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0054] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0055] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0056] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0057] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be movedaccurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0058] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axis, i.e., an x-axis, a y-axis and a z-axis. Each of the three axis is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y-axis is referred to as an Ry-rotation. A rotation around about the z-axis is referred to as an Rz-rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0059] Structures may be lithographically patterned onto a substrate using a lithographic apparatus as described above. These patterned structures may in turn be measured with different types of metrology and / or inspection apparatus, which may form part of or may also be referred to as measurement systems. The different types of measurement systems may measure different types of properties, also referred to as characteristics, of the structure. These properties may for example include focus, dose, alignment, levelling, and / or overlay. Provided herein are solutions for measurement systems that overcome some of the challenges and limitations associated with existing measurement setups.

[0060] In an example implementation, the measurement system may be provided to measure alignment. The system or setup used to measure alignment may for simplicity be referred to as alignment sensor. Some known alignment sensors may measure alignment for example by imaging and interfering dedicated metrology patterns, e.g. alignment marks, that map a substrate prior to exposure of the pattern. Alignment sensor may for example include a camera. A camera may be used in combination with pattern recognition software, which may be used to align an image obtained by a camera with dedicated target masks and or exposed patterns. In another example optical metrology based on digital holography microscopy (DHM) may allow to measure amplitude and phase of the diffracted signal at the sensor location.

[0061] A camera may employ frame-based imaging, in which a sensor captures an entire image, i.e. all radiation intensity at each pixel of the sensor, to form an image frame. Image frames may be recorded at fixed intervals (i.e. frame rate). Frame rates may be limited by camera and or illumination conditions, and frame -based imaging may be challenging in high speed capturing environments. Frame -basedimaging may also be ineffective under challenging illumination conditions. Frame-based imaging may over-sample parts of an image that have not changed between frames, and as a result may require large computing and power resources, as an entire images captured every time a frame is formed. They may export data from every single pixel of a sensor for every single frame. A camera may have a specific exposure time to capture radiation, and movement during the exposure time may cause blur. Therefore, it may be challenging to measure moving targets. As a result, when moving a target into a focal area (also referred to as object field) of a measurement setup, a target may need to decelerate and settle before a measurement is obtained.

[0062] Different types of metrology setups exist for measuring different characteristics. Several example types of setup will now be discussed in more detail. One example type of measurement setup is an alignment sensor. Alignment sensors may be large, expensive, and may be difficult to build. An alignment sensor may for example be a diffraction-based alignment sensor. A diffraction-based alignment sensor may require a metrology target such as a grating mark, also referred to as a grating target. They may struggle to provide alignment data for non-periodic measurement structures. Diffraction-based alignment sensors may use coherent radiation, which may be more expensive than radiation sources providing incoherent radiation. Diffraction-based alignment sensors may also be sensitive to any of deformations of the metrology target grating mark. Deformations to the periodic grating structure may induce a phase shift in the measured signal used to determine a position / alignment of the grating on the substrate. Alignment sensors may also be difficult and / or expensive to multiplex, and are usually designed to only scan or measure a single grating mark at a time. As a result, alignment sensors based on frame-based cameras and using diffraction-based imaging may be slow.

[0063] In order to overcome some of the challenges and limitations with existing alignment sensors as described above, it is proposed herein to make use of an event-based camera. An event-based camera may operate similarly to a human eyes and brain, by using independent receptors which collect information without oversampling. Where a frame-based typical camera may detect an intensity level for pixels across a sensor area, an event space camera may detect intensity changes. An event-based camera therefore does not have to obtain data for every pixel at a specific frame rate. Instead it may operate when an intensity change is detected, which may be referred to as an event. An event may record a level of intensity change for one or more pixels experiencing an intensity change, and may ignore any pixels of a sensor where no intensity changes detected. The result may be data which has captured intensity changes, such as for example movement and specifically moving edges in an image. The resulting image data may comprise information that is less oversampled, and has a smaller size. An event-based camera may operate much faster than a frame-based camera, may require less communication bandwidth, less storage, and / or may create less heat.

[0064] Figure 2 depicts an example metrology system 200 for determining a characteristic of a structure 204 on a substrate 206. The structure may be a lithographically patterned structure. The system comprises an illumination source 208 configured to output radiation 210. The illumination source isoptically coupled to an objective 212. The objective is configured to direct the radiation 210 towards the structure 204 on the substrate 206. The structure may be a metrology target. Radiation may be diffracted from the structure, and diffracted radiation 214 is directed to a detector module 202 . The detector module 202 comprises an event-based camera 216. An event-based camera may comprise one or more sensors configured to receive radiation. The event-based camera is configured to detect events comprising a change of intensity of radiation incident on a plurality of locations on a sensor of the event-based camera. While the objective received radiation from the illumination source and is configured to direction radiation towards the structure, the objective may be considered to be associated with the detector assembly, not the illumination assembly. This may be for example because it contributes to projecting / directing an image of the structure towards the detector module.

[0065] The event-based camera is configured to output an event data stream conveying the detected events. The metrology system further comprises an edge detection processor 218 configured to receive the event data stream from the event-based camera as an input. The edge detection processor can output movement detection data, wherein the movement detection data represents a movement of the structure on the substrate relative to the objective and / or the event-based camera of the metrology system. The metrology system further comprises a processor system 220 configured to receive the output movement detection data. The processor system determines a characteristic of the structure on the substrate based on the output movement detection data and one or more properties of the structure on the substrate.

[0066] An advantage of a metrology system as described in relation to figure 2 is that it captures changes in intensity of the diffracted radiation by using an event-based camera. This may for example enable the metrology system to be more data-efficient, to be better at capturing data at high speeds, etc. An event-based camera may for example have a response time of the order of ps, for example in a range of 5 ps to 15 ps, or below 30 ps.

[0067] As an event-based camera may detect and record intensity changes (differentiation), they do not have an exposure time. This is in contrast to a frame-based camera, which may integrate intensity during an exposure time. As a result of there not being an exposure-time, the captured images may be free from blur. An event-based camera may be operated at high capturing speeds, for example may capture data at a speed equivalent to 10,000 frames per second for a frame-based camera. Because the camera only captures changes in intensity for those pixels that experience the change, less data is generated compared to frame-based imaging. The amount of data generated by an event based camera may be in a range of lOx less to lOOOx less compared to a frame based imaging camera. An event-based camera may have a high dynamic range, for example a dynamic range equal to or greater than 120db.

[0068] The metrology system 200 may be configured to measure a moving structure. The structure may move relative to the rest of the metrology system, specifically relative to the objective 212 and / or the event-based camera 216. In some implementations, the movement of the structure may be a result of the substrate being moved inside the metrology system. A substrate may for example comprise multiple structures to be measured, and may be moved to position the different structures to be measuredin the focal area of the metrology system. The focal area may also be referred to for example as an object field of the event-based camera. In some other examples, the movement may be a result of unwanted vibrations in the system.

[0069] The moving structure may be configured to move in a linear motion relative to the objective and / or the event-based camera of the metrology system. The linear motion may be referred to as a scanning motion. The scanning motion may be performed by a substrate support holding the substrate. The scanning motion may move different structures to be measured through the focal area of the metrology system. The measurement of the structure may be performed during the scanning motion. This may for example mean that the movement of the structure is not stopped in order to obtain the image. This may have an advantage of requiring less time to obtain a measurement of the structure, as no time is needed to decelerate for stopping, measuring, and then accelerating the structure to continue the scanning movement.

[0070] The moving structure may be configured to be moved at a constant velocity relative to the objective and / or the event-based camera of the metrology system. The velocity may be constant during the measurement of the structure. The constant velocity may form part of a scanning motion. The constant nature of the velocity of the structure may be interrupted for example when the movement changes direction, and / or at the beginning or end of a scanning motion. The moving structure may be configured to move at the speed in a range of 1 mm / s to 1000 mm / s, or at a speed in a range of 3mm / s to 1000 mm / s.

[0071] The structure may be a metrology target. A metrology target may be a structure that has been patterned onto the substrate specifically for measuring one or more characteristics. The metrology target may have a specific design to enable measurement of one or more characteristics. The metrology target may comprise one or more diffraction patterns. The diffraction patterns may for example be a grating structure. A grating structure may for example be a line grating. A line grating may comprise a plurality of periodically spaced straight lines. The lines may have a finite width, height, and length. While they may be referred to as lines, they are three-dimensional structures, and may be approximately rectangular in shape, for example.

[0072] The characteristic to be measured may be alignment. The metrology system may comprise or be an alignment sensor. The structure may be an alignment mark. An alignment mark may comprise one or more periodic gratings. A periodic grating may be a ID periodic grating (e.g. a line grating), or a 2D periodic grating, or may comprise a non-periodic mark.

[0073] The structure may be a diffracting structure, such as an alignment mark described above. Radiation incident on the structure may be diffracted, and at least some of the diffracted radiation may be directed towards the detector module. The detector module may capture at least some of the diffracted radiation, and form an image formed from the captured radiation on the sensor area of the event-based camera. The image may comprise interference fringes formed from interfering radiation ofthe diffracted radiation orders. Any changes in the image of the diffracted radiation on the sensor may be captured by the event-based camera as event data.

[0074] The event-based camera may detect events. An event may indicate a change in intensity on the camera. The detected events may comprise detected moving edges in an image formed on the detector of radiation received from the moving structure. The image formed from radiation received from the moving structure may comprise diffracted radiation, for example diffracted radiation from a diffraction structure. In this example, the image formed on the detector does not represent a diffraction pattern, but rather changes in intensity of the diffraction pattern.

[0075] The edge detection processor may be configured to identify one or more shapes associated with the moving structure, and based on the moving edges represented in the image. In an example of diffraction fringes being formed on the detector, the image may comprise changes in intensity of the diffraction pattern, and the shapes to be identified may be interference fringes. The edge detection processor may for example be configured to identify a moving position (which may also be referred to as a moving location) of the shape by identifying a leading edge of the shape based on an increase in intensity of incident radiation at a first location on the sensor. The edge detection processor may further identify a trailing edge of the shape based on a decrease in intensity of incident radiation at the second location of the sensor. The identification of the trailing edge may be subsequent to the identification of the leading edge. A position of the shape may be determined based on the identified the leading edge and the trailing edge at the first and second positions. For example, a detection of a leading edge and a trailing edge at a certain point in time may indicate that a shape is present between the leading edge and the trailing edge.

[0076] The illumination source may output radiation comprising incoherent radiation. The radiation may be directed, using one or optical elements, from the radiation source to an object field of the eventbased camera in the metrology system. The radiation may therefore illuminate a structure on a substrate present in the object field. The illumination source may for example comprise one or more LEDs (Light Emitting Diode) illumination sources, one or more super luminescent diodes, one or more lasers, ambient light, or combination of any of the above. In an example implementation, the illumination source may comprise a plurality of LEDs. The LEDs may be positioned in a ring around the objective. The LEDs may be configured to output radiation at a plurality of different wavelengths. For example, different LEDs may each be configured to output radiation at one of a plurality of different wavelengths. The LEDs may be tunable across a range of different wavelengths.

[0077] The detector module may comprise one or more detectors, wherein at least one of the detectors comprises an event-based camera. In some examples, the detector module may comprise two or more event-based cameras. In some examples, the detector module may comprise one or more other types of detectors to be used alongside the event-based camera. The other types of detectors may for example include a frame-based camera.

[0078] In an example implementation the moving structure may comprise an alignment mark including a diffracting structure, such as for example ID grating. The structure may be moved linearly at a constant speed through the focal area of the detector setup. Radiation may be directed onto the alignment mark and one or more diffracted radiation orders may be captured and directed towards the detector module. The captured radiation may interfere, forming interference fringes. These interference fringes may for example comprise a plurality of finite dimensional, substantially parallel lines, as illustrated in feature 222 of figure 2. As the diffracting structure may be moved at a constant speed, and linearly through the focal area, the resulting pattern of interference fringes may also move linearly across the detector. Figure 3 comprises a schematic comparison of image capture 300 using a frame-based camera and an event-based camera. The graphs show intensity I at a location of the detector over time t, for example as interference fringes move across the detector at a substantially constant speed. In graph (A), a frame-based camera captures images of radiation incident on the detector at constant time intervals, represented by vertical lines equidistantly spaced along the horizontal axis t, representing time. The line 302 represents intensity at the location over time, and data points 304 at the intersection of the intensity line with the time intervals represent data points captured in subsequent frames measured by the framebased camera.

[0079] In graph (B), the same intensity 306 line is shown, representing a change of intensity over time at a specific location on the detector. The detector in this case is of an event-based camera. Instead of capturing data points on the intensity line at equally spaced time intervals, the event-based camera captures a data point each time there is a change in intensity. This is represented in graph (B) by horizontal lines equidistantly spaced along the vertical axis I, representing intensity. As can be seen from a comparison of graphs (A) and (B), detected events may cluster around increases in intensity 310, and decreases in intensity 312 for the event-based camera. These increases and decreases in intensity may represent leading and trailing edges of an interference fringe, respectively. The event-based camera is better at capturing the change in intensity from low to high, and from high to low, meaning it may be better suited detecting when a leading and / or trailing edge of an interference fringe (or other shape) passes through a location on the detector. Figure 3 provides an illustration on why an event-based camera is better at measuring data related to moving structure is, due to its ability to detect changes in intensity in greater detail than a frame-based camera.

[0080] In an example implementation of edge detection, the edge detection processor may comprise or be connected to a processor with algorithm functionality for associating changes in intensity with a location of the shape, wherein the location of the shape may represent a moving structure in the focal plane of the detection system. Figure 4 depicts a schematic representation of interference fringes 402, and their representation at a specific moment in time on the detector area. The x-axis 406 may represent pixel locations, wherein each number represents an individual pixel, or a subset of pixels, on a detector. The resulting pattern may be approximated by a square wave pattern. The y-axis 404 may represent intensity of radiation incident on the pixel at that moment in time. As the structure is moved, e.g.scanned linearly, across the focal area of the metrology system, this approximated square wave pattern may move across detector area.

[0081] When the detector as an event-based camera, the events registered by a single pixel on the detecting area may comprise a series of positive events, corresponding to an increase in intensity, followed by a series of negative events, corresponding to a decrease in intensity, etc. If the response time is high compared to the scan speed of the moving structure, positive events may cluster on a leading edge of an interference fringe, and negative events may cluster on a trailing edge of an interference fringe.

[0082] Figure 5 depicts example graphs associated with interference fringes being scanned across a detecting area of an event-based camera. The top graph 502 depicts a cumulative sum of events detected at a single pixel of an event-based camera. A reduction in cumulative event intensity may be caused by a series of negative events, and may represent a trailing edge 506. An increase in cumulative event intensity may be caused by a series of positive events, and may represent a leading edge 510. Taking a derivative of the cumulative sum graph may produce impulse events representing trailing edges 508 and leading edges 510, which may be identified for example using peak detection functions or algorithms.

[0083] By determining a cumulative intensity signal and its derivative for each pixel of an event based camera, leading and trailing edges of interference fringes (or other shapes) may be identified with time as they move across different pixels of the detector area. The leading and trailing edges may be used to determine the location of a shape (e,g, of an interference fringe) over time. By using and combining all edge detections detected across all pixels of an event-based camera, the precision of the location and movement determination of the shape may be improved. Precision of the determination of location may be a function of the total number of detected edge detections N. The statistical error may scale as the square root of the total number of detected edge detections V / V. If the dimensions of the target structure (e.g. line grating) are known precisely, edge detections may all contribute constructively to the same known position, the precision of which may be dominated mainly by the statistical error. Being able to determine a location of a fringe on a detector at any one time may lead to improved determination of the characteristic.

[0084] Intensity errors may for example be caused by one or more of the event camera intensity threshold, where low intensity changes are not or incorrectly detected, photon noise, radiation source noise, electrical noise. These may affect determination of an edge detection. Edge detection reproducibility may be quantified as an intensity error divided by a slope of the detected edge as shown in figure 5. The detected position uncertainty may be small for steep image slopes, but may increase as a slope 506 or 510 gets less steep.

[0085] Potential advantages of using an event-based camera for an alignment sensor may further include an opportunity to quantify intra-target characteristics (edge shifts), for example by computing an average mark and determining any deviations from this average. An event-based camera may alsoassist in quantifying characteristics of the detector, for example by computing an average camera and comparing this to characteristics of individual pixels within this camera.

[0086] An intra-target characteristic, or distortions to a target, may be determined for example when each pixel observes the entire target, as it moves through the focal area of the detector system. The observations of the target by each pixel may be integrated to compute a canonical target. Pixel errors, for example between different pixels and / or between observations, may be reduced as a result of such an integration. By removing pixel errors through this integration, any distortions observed within the canonical target may then be interpreted as being a representation of deformations of the target itself.

[0087] Quantifying characteristics of the detector may for example be performed by computing a canonical camera. Each edge of a mark may be observed by each pixel of the event-based camera. These independent observations may be integrated to compute a canonical camera. This integration may average out any differences between detected edges. By removing variation between different fringes through this integration, any differences observed (e.g. delay and / or contrast sensitivity) within the canonical camera may then be interpreted as being a representation of the pixel response of the camera.

[0088] The metrology system of figure 2 can also be used in a Digital Holographic Microscopy (DHM) setup. DHM may be a type of metrology setup for measuring a substrate to determine a characteristic of that substrate. The characteristic may for example be overlay. In a DHM setup, the illumination source may comprise at least two illuminators configured to project radiation onto a target structure to be measured. The radiation beams from the illuminator may be referred to as object beams. The different illuminators may provide radiation onto the target structure at different angles of incidence. The angles of incidence may be designed such that different diffraction orders from radiation originating from different illuminators may be captured by and coupled to the objective. For example a +1 diffracted radiation order from a first illuminator and a - 1 diffracted radiation order from a second illuminator may be coupled into an objective of the metrology setup. The objective may further receive two reference radiation beams, which may be one for each illuminator. Each of the reference radiation beams may be carefully matched to the illumination path or its corresponding illuminator radiation beam. The resulting interference pattern may form a hologram that may encode information from both illumination directions. The characteristic of the target structure on the substrate may be determined for example through computational processing of the hologram measurement data.

[0089] In a DHM setup, object beams scattered or diffracted from the target structure may interfere with their associated reference beams. A first object- reference radiation beam pair may exist for a positive diffraction radiation order and a second radiation beam pair may exist for a negative diffraction radiation order. The resulting interference pattern may be referred to as a hologram. The resulting interference pattern may contain interference fringes. The amplitude of the fringes may be measured and used to determine overlay of the target structure. The target structure may be an metrology target, such as an overlay target. In a DHM setup, radiation from the illumination source may be coherent radiation.

[0090] An overlay target may comprise one or more overlapping and / or non-overlapping diffraction gratings. The diffraction gratings may be positioned in different layers of the substrate, and overlay may be determined between the different layers in which the target structures have been patterned. Overlay measurement may be an asymmetry measurement. In a symmetric, static metrology setup, the overlay error in the target structure may be the only source of asymmetry between the signals in the first and second object-reference pairs. As a result, overlay may be determined (i.e. calculated) based on a difference between the signal in the first and second radiation beam pairs.

[0091] The target structure may be held stationary during a DHM measurement, meaning the measurement is not performed while the substrate is being scanned through the focal area of the metrology system. Instead, the substrate may be moved to position the structure in the object field of the DHM metrology system. The structure may be held stationary relative to the objective during the measurement. However, it may be impossible to avoid vibrations, for example as a result of the deceleration to bring the structure to a halt and / or as a result of vibrations in the metrology system itself. The vibration-induced movement of the target may be in a range of 1 nm to 1 pm in a time of the order of 1 ms. The vibrations may negatively affect the quality of the measurement.

[0092] In order to obtain an accurate and precise measurement, obtaining a high fringe contrast is important. It is therefore desirable to avoid or minimise any effects that may reduce the interference fringe contrast on the detector. One effect that may cause interference fringes in a hologram to shift may be path length shifts between the object beam and the reference beam. If the interference fringes shift during integration of radiation on the detector, this may lead to fringe contrast reduction. This may entail a loss of signal, or a reduction of detail in the resulting measurement. Path length shifts may occur at millisecond or sub-millisecond timescales, and may be caused by random vibrations and or settling of the substrate support configured to hold the substrate in place. Movement of the substrate in a DHM metrology system may be unavoidable, as the substrate has to move across the sensor focal area between target structures. Long settling times to wait for any movement-induced vibrations to settle may be costly, as this may reduce throughput of the system.

[0093] Figure 6 depicts a schematic representation of example movement of fringes on a detector as a result of vibrations of the target substrate. The target structure may be stationary, but may experience unintentional vibrations. The fringes 600 may move on the detector over time, as illustrated by the different positions 602 of the signal illustrated in figure 6. If a frame-based camera is used as a detector, the signal incident on the detector may be integrated during an exposure time 604. As a result of the movement 602 of the fringes, the amplitude 606 of the resulting signal may be reduced. Large motions during acquisition of the radiation on the detector may reduce the effective signal amplitude, which may reduce the signal-to-noise ratio. Furthermore, if a loss of fringe contrast is different between the first and second radiation beam pairs, this may introduce a metrology-system-induced asymmetry, which may cause an error in determination of overlay from the measured signal. As overlay measurementsmay be very precise, even small vibrations (e.g. of the order of nanometres, or greater) may affect the resulting measurement.

[0094] The issues regarding vibration-induced errors and lack of precision for overlay measurement in a DHM setup may occur when using an integration-based camera, such as a frame -based camera. It is therefore proposed herein to use the metrology setup as described in relation to figure 2, which uses an event-based camera. An event-based camera may be used to measure vibrations over time. The measured vibrations may be taken into account when determining the characteristic from the DHM measurement. In an example embodiment, an event-based camera may be used instead of a frame-based camera. In another example embodiment an event-based camera may be used in addition to a framebased camera. In such case, an event-based camera may be used as a secondary measurement, to measure vibrations. The vibration measurements may then be used to correct the measured signal of the frame-based camera. An event-based camera may for example measure events with a speed in the order of microseconds. As a result, vibration induced fringe movements may be measured down to a microsecond-order sampling frequency. This may be significantly faster than a traditional frame based camera, which may require an exposure time in the order of milliseconds.

[0095] In an event based stream, event data may comprise one or more of a timestamp indicating when the event was detected, pixel coordinates of the one or more pixels that detected the event (intensity change), an event polarity or sign (where positive may mean an increase in intensity and negative may mean a decrease in intensity), and an amount of change of the intensity.

[0096] An event-based camera may perform a raster scan, in which the fringes may be scanned through a region of interest, such that events triggered on the event camera measure the full height of the fringe. In DHM, a scan may for example be performed by scanning a delay line forward and / or backward by a few wavelengths. The radiation may be coherent or incoherent, which may depend on the metrology system as described above.

[0097] The substrate may be a lithographically patterned substrate, with the structure being a lithographically patterned structure. The one or more properties of the structure on the substrate may comprise details of the design of the structure (e.g. size, shape, position). The intended design features may be used to model diffraction patterns expected to be measured by the system. The measured data can be compared to the expected patterns, and a difference between them may be used to determine the characteristic, e.g. alignment.

[0098] The radiation output by the illumination source of a metrology system as described herein may comprise one or more wavelengths in a range if 100 nm to 2000 nm, or in a range of 400 nm to 1600 nm. The radiation output by the illumination source may comprise broadband and / or narrowband radiation. The radiation may for example be narrowband radiation comprising a centre wavelength with a bandwidth in a range of 4 nm to 10 nm around the centre wavelength.

[0099] A sensor as described herein may comprise a sensor array with multiple sensor elements. The multiple sensor elements may be interconnected to form a larger sensor area.

[0100] The measurement system comprising an event-based camera described herein may be provided in or connected to one or more of a metrology apparatus, an inspection apparatus, a lithographic apparatus, a litho cell, or an exposure apparatus for a semiconductor manufacturing processes. Methods of measuring a characteristic of a structure on a substrate using a measurement system comprising an event-based camera described herein are also included.In the following, further features, characteristics, and exemplary technical solutions of the present disclosure will be described in terms of clauses that may be optionally claimed in any combination:1. A metrology system for determining a characteristic of a structure on a substrate, the system comprising:an illumination source configured to output radiation, the illumination source optically coupled to an objective, wherein the objective is configured to direct the radiation toward the structure on the substrate, and direct radiation diffracted from the structure toward a detector module;the detector module, comprising:an event based camera, wherein the event based camera is configured to detect events comprising changes in intensity of radiation incident on a plurality of locations on a sensor of the event based camera, wherein the event based camera is configured to output an event data stream conveying the detected events; andan edge detection processor configured to receive the event data stream as an input and to output movement detection data representing a movement of the structure on the substrate relative to the objective and / or event-based camera; anda processor system configured to receive the output movement detection data and determine the characteristic of the structure on the substrate based on the output movement detection data and one or more properties of the structure on the substrate.2. A metrology system according to clause 1, wherein the structure is a moving structure relative to the objective and / or the event-based camera.3. A metrology system according to clause 2, wherein the moving structure is configured to be moved in a linear motion relative to the objective and / or the event-based camera of the metrology system.4. A metrology system according to any of clauses 2 or 3, wherein the moving structure is configured to be moved at a constant velocity relative to the objective and / or the event-based camera of the metrology system.5. A metrology system according to any of clauses 2 -4, wherein the moving structure is configured to move at a speed in a range of 1 mm / s to 1000 mm / s, or at a speed in a range of 3mm / s to 1000 mm / s.6. A metrology system according to any of clauses 2 -5, wherein the detected events comprise detected moving edges in an image formed of radiation received from the moving structure.7. A metrology system according to clause 6, wherein the image formed from radiation from the moving structure comprises diffracted radiation.8. A metrology system according to any of clauses 6 -7, wherein the edge detection processor is configured to identify one or more shapes associated with the moving structure based on the detected moving edges in the image, and identify a moving position of the one or more shape on the sensor. 9. A metrology system according to clause 8, wherein identifying a moving position of a shape of the one or more shapes comprises:identifying a leading edge of the shape based on an increase in intensity of incident radiation at a location on the sensor;identifying a trailing edge of the shape based on an decrease in intensity of incident radiation at a location on the sensor, wherein the identification of the trailing edge is subsequent to the identification of the leading edge; anddetermining a position of the shape based on the identified leading edge and the identified trailing edge.10. A metrology system according to any of clauses 8 - 9, wherein the one or more shapes associated with the moving structure comprise interference fringes.11. A metrology system according to any of clauses 2 - 10, wherein the characteristic comprises alignment.12. A metrology system according to any of clauses 2 - 11, wherein the structure on a substrate is an alignment mark.13. A metrology system according to any of clauses 2 - 12, wherein the radiation is incoherent radiation.14. A metrology system according to any of clauses 2 - 13, wherein the illumination source comprises one or more LED illumination sources.15. A metrology system according to clause 14, wherein the one or more LED illumination sources comprise a plurality of LEDs positioned in a ring positioned around the objective.16. A metrology system according to any of clauses 14 - 15, wherein the illumination source comprises a plurality of LEDs, and wherein at least some of the LEDs have different wavelengths. 17. A metrology system according to any of clauses 2 - 13, wherein the illumination source comprises ambient light.18. A metrology system according to any of clauses 2 - 17, wherein the sensor comprises a sensor array of multiple sensor elements.19. A metrology system according to any of clauses 2 - 18, wherein the system further comprises an optical system to project at least some of the diffracted radiation onto the detector.20. A metrology system according to clause 1, wherein the illumination is coherent.21 A metrology system according to clause 20, wherein the characteristic of the structure comprises overlay.22. A metrology system according to any of clauses 20 -21, wherein the structure is configured to be held stationary relative to the objective.23. A metrology system according to any of clauses 20 - 22, wherein the structure comprises a metrology target.24. A metrology system according to clause 23, wherein the metrology target comprises one or more overlapping and / or non-overlapping diffraction gratings.25. A metrology system according to any of clauses 20- 24, wherein the metrology system comprises a digital holographic microscopy assembly.26. A metrology system according to any of clauses 20 - 25, wherein the movement is vibration-induced movement of the structure on the substrate.27. A metrology system according to clause 26, wherein the movement detection data of the vibration-induced movement is used to correct vibration-induced error in the measurement of the characteristic of the structure.28. A metrology system according to any of clauses 26 - 27, wherein the vibration-induced movement is in a range of 1 nm to 1 pm in a time of the order of 1 ms.29. A metrology system according to any of the preceding clauses, wherein the structure is a lithographically patterned structure.30. A metrology system according to any of the preceding clauses, wherein the radiation output by the illumination source comprises one or more wavelengths in a range of 100 nm - 2000 nm, or in a range of 400 nm to 1600 nm.31. A metrology system according to clause 30, wherein the radiation output by the illumination source has a centre wavelength and a bandwidth in a range of 4 nm - 10 nm around the centre wavelength.32. A metrology system according to any of the preceding clauses, wherein the event based camera has a response time in a range of 5 ps to 15 ps, or below 30 ps.33. A metrology apparatus comprising a metrology system according to any of the preceding clauses.34. An inspection apparatus comprising a metrology system according to any of clauses 1 - 32.35. A lithography apparatus comprising a metrology system according to any of clauses 1 - 32.36. A litho cell comprising a system according to any of clauses 1 - 32, or an apparatus according to any of clauses 33 - 35.

[0101] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0102] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0103] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0104] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0105] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A metrology system for determining a characteristic of a structure on a substrate, the system comprising:an illumination source configured to output radiation, the illumination source optically coupled to an objective, wherein the objective is configured to direct the radiation toward the structure on the substrate, and direct radiation diffracted from the structure toward a detector module;the detector module, comprising:an event based camera, wherein the event based camera is configured to detect events comprising changes in intensity of radiation incident on a plurality of locations on a sensor of the event based camera, wherein the event based camera is configured to output an event data stream conveying the detected events; andan edge detection processor configured to receive the event data stream as an input and to output movement detection data representing a movement of the structure on the substrate relative to the objective and / or event-based camera; anda processor system configured to receive the output movement detection data and determine the characteristic of the structure on the substrate based on the output movement detection data and one or more properties of the structure on the substrate.

2. A metrology system according to claim 1, wherein the structure is a moving structure relative to the objective and / or the event-based camera.

3. A metrology system according to claim 2, wherein the moving structure is configured to be moved in a linear motion relative to the objective and / or the event-based camera of the metrology system.

4. A metrology system according to any of claims 2 or 3, wherein the moving structure is configured to be moved at a constant velocity relative to the objective and / or the event-based camera of the metrology system.

5. A metrology system according to any of claims 2 -4, wherein the moving structure is configured to move at a speed in a range of 1 mm / s to 1000 mm / s, or at a speed in a range of 3mm / s to 1000 mm / s.

6. A metrology system according to any of claims 2 -5, wherein the detected events comprise detected moving edges in an image formed of radiation received from the moving structure.

7. A metrology system according to claim 6, wherein the image formed from radiation from the moving structure comprises diffracted radiation.

8. A metrology system according to any of claims 6 -7, wherein the edge detection processor is configured to identify one or more shapes associated with the moving structure based on the detected moving edges in the image, and identify a moving position of the one or more shape on the sensor.

9. A metrology system according to claim 8, wherein identifying a moving position of a shape of the one or more shapes comprises:identifying a leading edge of the shape based on an increase in intensity of incident radiation at a location on the sensor;identifying a trailing edge of the shape based on an decrease in intensity of incident radiation at a location on the sensor, wherein the identification of the trailing edge is subsequent to the identification of the leading edge; anddetermining a position of the shape based on the identified leading edge and the identified trailing edge.

10. A metrology system according to any of claims 8 -9, wherein the one or more shapes associated with the moving structure comprise interference fringes.

11. A metrology system according to any of claims 2 - 10, wherein the characteristic comprises alignment.

12. A metrology system according to any of claims 2 - 11, wherein the structure on a substrate is an alignment mark.

13. A metrology system according to any of claims 2 - 12, wherein the radiation is incoherent radiation.

14. A metrology system according to any of claims 2 - 13, wherein the illumination source comprises one or more LED illumination sources.

15. A metrology system according to claim 14, wherein the one or more LED illumination sources comprise a plurality of LEDs positioned in a ring positioned around the objective.

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