Method and apparatus for measuring the topography of an object's surface

Abstract

A method for measuring the topography of a surface of an object includes forming a first image of a pattern on a beam spot area using a radiation beam, moving the object relative to the beam spot area, receiving a portion of the radiation beam reflected from the object and splitting the reflected radiation into a first portion and a second portion such that the first portion of the radiation corresponding to the first portion of the first image is spatially separated from the second portion of the radiation corresponding to the second portion of the first image, determining intensities of the first and second portions of radiation, and determining a height of the object by combining the intensity of the first portion of radiation determined at a first time point with the intensity of the second portion of radiation determined at a second time point.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to EP application 23154103.8, filed January 31, 2023, the entire contents of which are incorporated herein by reference.

[0002]

[0002] The present invention relates to a method for measuring the topography of a surface of an object. The invention also relates to a corresponding apparatus for measuring the topography of a surface of an object. The invention applies in particular to the field of lithography. The object may be a substrate in a lithography apparatus. Such a substrate may comprise a silicon wafer coated with photoresist. The apparatus may be called a level sensor and may form part of the lithography apparatus. The invention also relates to a lithography exposure method using a method or apparatus for measuring the topography of a surface of a substrate. [Background technology]

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

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually decreased, while the number of functional elements, such as transistors, per device has steadily increased for decades, following a trend commonly referred to as "Moore's Law." To accommodate Moore's Law, the semiconductor industry pursues technologies that enable the creation of increasingly smaller features. To project a pattern onto a substrate, a lithography system may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Lithography systems using extreme ultraviolet (EUV) radiation, with wavelengths in the 4 nm to 20 nm range, e.g., 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than lithography systems using radiation with a wavelength of, e.g., 193 nm.

[0005]

[0005] In a lithography apparatus, prior to exposing a wafer to patterned radiation, the topography of the wafer may be determined using a device that may be called a level sensor. This measurement of the wafer topography may be performed within the lithography apparatus, for example, after the wafer has been fixed to a wafer stage. This information may be used during subsequent exposure of the wafer to maintain the exposed portion of the wafer within the best focus plane.

[0006]

[0006] It may be desirable to provide new methods and / or apparatus for determining the topography of a wafer that can at least partially address one or more problems associated with existing configurations, whether or not identified herein. Summary of the Invention

[0007]

[0007] According to a first aspect of the present disclosure, there is provided a method for measuring the topography of a surface of an object, the method comprising: forming a first image of a pattern on a beam spot area using a radiation beam; moving the object relative to the beam spot area; receiving a portion of the radiation beam reflected from the object and splitting the reflected radiation into a first portion and a second portion so that the first portion of the radiation corresponding to the first portion of the first image is spatially separated from the second portion of the radiation corresponding to the second portion of the first image; determining intensities of the first and second portions of radiation; and determining a height of the object by combining the intensity of the first portion of radiation determined at a first time point with the intensity of the second portion of radiation determined at a second time point.

[0008]

[0008] The method according to the first aspect is advantageous as will now be discussed.

[0009] When the height of the object changes, the position of at least a portion of the second image of the pattern will also change, and as a result, this may result in a change in the relative values ​​of the first and second intensities. For example, when the height of the object changes, the position of at least a portion of the second image of the pattern may change relative to a splitting optics configured to split the reflected radiation into a first portion and a second portion.

[0010]

[0010] By determining the height of the substrate by splitting the reflected radiation into a first portion and a second portion and combining the intensities of the first and second portions, the determination of the height may be substantially independent of the intensity of the radiation beam, for example the height may be determined as a differential measurement.

[0011] In a first aspect of the method, the reflected light is split into two portions corresponding to different portions of a first image (formed on the object). Generally, there may be a spatial offset between the first and second portions of the first image (although the first and second portions may spatially overlap). The object being measured may have varying reflectivity across its surface. For example, the object may have localized regions or features with a different reflectivity than the surrounding area of ​​the surface. In this configuration, as the feature moves into (or out of) the beam spot area, the spatial offset between the first and second portions of the first image results in a difference between the first and second intensities due to changes in reflectivity (but not the height of the object). When combining the first and second intensities determined at any given time, the resulting determination of the height of the object will generally contain an error due to changes in the reflectivity of the surface. This is because the first and second portions of the radiation reflect from different areas of the surface.

[0012]

[0012] By combining the intensity of the first portion determined at a first time point with the intensity of the second portion determined at a second time point, the method according to the first aspect enables the method to ensure that the first and second portions correspond to radiation reflected from substantially the same part of the object. Advantageously, this may prevent any step or sudden change in the reflectivity of the object from affecting the height determination.

[0013]

[0013] The object may be a substrate in a lithography apparatus. Such a substrate may include a silicon wafer coated with photoresist. The silicon wafer may include one or more layers that have been previously formed using, for example, a lithography process. Typically, a variety of different materials and / or different feature densities will be present on the surface of such a wafer. This can cause reflectivity variations across the surface of the wafer, as different materials can absorb different portions of incident radiation and different feature densities can result in different amounts of scattering of incident radiation. For example, a 3D-NAND wafer may include features that can reduce the amount of specular reflection of radiation by up to 50%.

[0014] It will be appreciated that determining the height of an object includes determining the height of the object relative to a reference height or position.

[0015]

[0015] The first and second points in time may be selected in response to movement of the object relative to the beam spot area, so that the first and second portions of the combined radiation correspond to radiation reflected from substantially the same portion of the object.

[0016] It will be appreciated that a first image of the pattern is formed in the beam spot area (through which the object is moved), and the reflected radiation is split such that the first and second portions of radiation correspond to different parts of the first image, so that at any given moment the first and second portions of radiation correspond to radiation reflected from two different parts of the object.

[0017] In some embodiments, the first and second portions of the first image (providing the first and second portions of the reflected radiation) may be spatially separated in the direction of object movement. In such embodiments, one or both of the first and second intensities may be shifted in time before being combined to determine height. For example, a time delay may be applied to one of the first and second intensities before combining it with the other of the first and second intensities.

[0018]

[0018] The time difference between the first and second points in time may be given by dividing the spatial offset between the first and second parts of the first image in the direction in which the object is moved relative to the beam spot area by the speed at which the object is moved relative to the beam spot area.

[0019] In some embodiments, the first and second portions may spatially overlap, and in such embodiments, the spatial offset between the first and second portions of the image may be a spatial offset between the centers of mass of the first and second portions of the image, respectively.

[0020]

[0020] The pattern may include at least one feature, and the time difference between the first and second points in time may be given by half the extent of the feature or each feature in the direction in which the object is moved relative to the beam spot area divided by the speed at which the object is moved relative to the beam spot area.

[0021] The or each feature may be a line. The pattern may, for example, include at least one line. In some embodiments, the pattern may include multiple lines. The or each line may have a thickness t in a direction in which the object is moving relative to the beam spot area. The spatial offset between a first portion of the first image and a second portion of the second image may be half this thickness (t / 2). This can be converted to a time delay by dividing by the velocity s of the object relative to the beam spot area. In an embodiment in which the pattern includes multiple lines with a 50% duty cycle having a pitch p (i.e., the spacing between the lines is also t), the spatial offset between a first portion of the first image and a second portion of the second image is one-quarter of the pitch (p / 4). This can also be converted to a time delay by dividing by the velocity s of the object relative to the beam spot area.

[0022]

[0022] Forming a first image of the pattern onto the beam spot area may include providing a radiation beam, patterning the radiation beam using a patterning device, and projecting the patterned radiation onto the beam spot area using a projection optical system.

[0023]

[0023] Moving the object relative to the beam spot area may include scanning the object relative to the beam spot area. Such scanning may be performed at a constant speed or a variable speed. As used herein, scanning the object is intended to mean continuous movement of the object. Alternatively, moving the object relative to the beam spot area may include stepping the object relative to the beam spot area. As used herein, stepping the object is intended to mean movement of the object by multiple successive (time-separated) steps.

[0024]

[0024] The object may be supported by a support, such as a wafer stage in a lithographic apparatus. Moving the object relative to the beam spot area may include moving the support.

[0025]

[0025] Splitting the reflected radiation into a first portion and a second portion may include forming a second image of the pattern on a splitting optical system and using the splitting optical system to direct radiation from the first and second portions of the second image spatially apart.

[0026]

[0026] The position of the second image relative to the splitting optics may determine how much of the reflected radiation is directed towards each of the first and second portions.

[0027] The intensities of the first and second portions of radiation may be determined multiple times at a sampling frequency.

[0028] That is, the intensities of the first and second portions of radiation may be determined multiple times, each determination being separated in time from the previous and subsequent determinations, and the sampling frequency may be the reciprocal of the time interval between the start of one determination and the start of the subsequent determination.

[0029] The first time point may coincide with one of the multiple determinations of the intensities of the first and second portions of radiation, and the second time point may coincide with another of the multiple determinations of the intensities of the first and second portions of radiation.

[0030]

[0030] For example, a desired time delay that is an integer multiple of the time interval between the start of one determination and the start of a later determination may be applied to one of the first and second intensities before it is combined with the other of the first and second intensities.

[0031]

[0031] At least one of the first time point or the second time point may be between two of the multiple determinations of the intensities of the first and second portions of the radiation, and the method may include interpolating between two of the multiple determinations of the intensities of the first and second portions of the radiation.

[0032] For example, a desired time delay that is a non-integer multiple of the time interval between the start of one determination and the start of a later determination may be applied to one of the first and second intensities before it is combined with the other of the first and second intensities. For example, it may be desirable to apply a time delay that is a non-integer multiple of the time interval between the start of one determination and the start of a later determination to the second intensity before it is combined with the first intensity. In such an embodiment, the second intensity used in the height determination may be an intensity obtained by interpolating (e.g., linearly) between two determinations that have a time delay between them.

[0033] The height of the object may be proportional to the difference between the first intensity and the second intensity.

[0034] For example, the height of the object may be determined to be proportional to the difference between the first intensity and the second intensity divided by the sum of the first intensity and the second intensity.

[0035]

[0035] According to a second aspect of the present disclosure, there is provided a lithographic exposure method comprising measuring the topography of a surface of a substrate using a method according to the first aspect of the present disclosure, patterning a radiation beam using a patterning device, and projecting the patterned radiation onto the substrate to form an image of the patterning device on the substrate, wherein the position of the substrate while the patterned radiation is projected onto the substrate is controlled in accordance with the measured topography of the surface of the substrate.

[0036] Advantageously, the measured topography of the surface of the substrate can be used to control the height of the substrate while it is exposed to the patterned radiation, for example to keep the substrate in the plane of best focus of the image of the patterning device. It will be appreciated that the image of the patterning device formed on the substrate may be a diffraction limited image.

[0037]

[0037] Lithographic exposure may be a scanning exposure in which patterning a radiation beam using a patterning device may involve moving the patterning device through the radiation beam, and projecting the patterned radiation onto a substrate to form an image of the patterning device on the substrate may involve moving the substrate so that the image of the patterning device is generally stationary relative to the substrate.

[0038] That is, to image the pattern onto a target area of ​​the substrate, the patterning device is moved or scanned within the illumination field in a scan direction. It will be understood that the substrate is also scanned relative to the illumination field in the plane of the substrate. The movement of the substrate is such that the aerial image of the patterning device is stationary relative to the substrate, and it will be understood that the direction and / or velocity of the substrate may generally differ from the direction and / or velocity of the patterning device (e.g., if the image is inverted and / or if a demagnification factor is applied by the projection system).

[0039]

[0039] According to a third aspect of the present disclosure, there is provided an apparatus for measuring the topography of a surface of an object, the apparatus comprising: a support for supporting the object; projection optics operable to form a first image of the pattern on a beam spot area using a radiation beam; a movement mechanism operable to move the support to move the object supported by the support within the beam spot area; detection optics operable to receive a portion of the radiation beam reflected from the object and split the reflected radiation into a first portion and a second portion so that the first portion of the radiation corresponding to the first portion of the first image is spatially separated from the second portion of the radiation corresponding to the second portion of the first image; a first detector configured to determine an intensity of the first portion of the radiation; a second detector configured to determine an intensity of the second portion of the radiation; and a controller operable to determine a height of the object by combining the intensity of the first portion of the radiation determined at a first point in time with the intensity of the second portion of the radiation determined at a second point in time.

[0040]

[0040] The apparatus may be called a level sensor. The apparatus may form part of a lithographic apparatus.

[0041] The apparatus according to the third aspect is advantageous, as will now be discussed. Because the detection optics are operable to split the reflected radiation into a first portion and a second portion, and the controller is operable to determine the height of the substrate by combining the intensities of the first and second portions, the height determination is substantially independent of the intensity of the radiation beam. For example, the height may be determined as a differential measurement. Furthermore, by combining the intensity of the first portion determined at a first time point with the intensity of the second portion determined at a second time point, the apparatus according to the third aspect enables implementation of a method that ensures that the first and second portions correspond to radiation reflected from substantially the same portion of the object. Advantageously, this may prevent any step or sudden change in the reflectivity of the object from affecting the height determination.

[0042] The controller may be operable to perform a method according to the first aspect of the present disclosure.

[0043] The projection optical system may comprise a projection patterning device and a first imaging optical system configured to form an image of the projection patterning device onto the beam spot area.

[0044]

[0044] The projection patterning device may include a grating. The grating may include a plurality of lines. The lines may be of uniform thickness. The grating may have a 50% duty cycle.

[0045]

[0045] The detection optical system may include a splitting optical system configured to split the reflected radiation into a first part and a second part, and a second imaging optical system configured to receive radiation reflected from an object supported by the support and form a second image of the pattern on the splitting optical system.

[0046]

[0046] The first imaging optics may be substantially equivalent to the second imaging optics.

[0047] The splitting optics, the beam spot area and the projection patterning device all lie in optically conjugate planes. Two planes will be understood to be optically conjugate if all radiation passing through each different point in the first plane is imaged to a different point in the second plane.

[0048]

[0048] An image of the projection patterning device is formed on the splitting optics, and the position of the image indicates the height of the object. In particular, the position of the image relative to the splitting optics indicates the height of the object. As mentioned above, the projection patterning device may include a grating including a plurality of lines. The splitting optics may comprise a plurality of prisms, and the image of each line may be imaged onto one of a plurality of approximately triangular prisms such that a first portion of the line is incident on a first surface of the prism and a second portion of the line is incident on a second surface of the prism. The first portion of the line is directed to a first detector and the second portion of the line is directed to a second detector. As the line moves relative to the prisms (as a result of a change in the height of the object), the amount of radiation directed to each detector changes.

[0049]

[0049] The first and second points in time may be such that the first and second portions of radiation that are combined correspond to radiation reflected from substantially the same portion of the object.

[0050] In some embodiments, the first and second portions of the first image (providing the first and second portions of the reflected radiation) may be spatially offset in the direction of object movement. In such embodiments, one or both of the first and second intensities may be shifted in time before being combined to determine height. For example, a time delay may be applied to one of the first and second intensities before it is combined with the other of the first and second intensities.

[0051]

[0051] The apparatus may further comprise a time shift module configured to apply a time shift to a first signal indicating a first intensity from the first detector and / or a second signal indicating a second intensity from the second detector such that the first signal indicates an intensity of a first portion of the radiation determined at a first time point and the second signal indicates an intensity of a second portion of the radiation determined at a second time point.

[0052]

[0052] The time shift module may be implemented using hardware, software, or a combination of both. Implementing such a time shift module using hardware (e.g., in the data collection module) may allow for greater accuracy. Implementing such a time shift module using software (e.g., as an algorithm executed by the controller) may be a cheaper solution.

[0053]

[0053] The time shift applied to one of the first and second signals may be given by dividing the spatial offset between the first and second parts of the first image in the direction in which the object is moved relative to the beam spot area by the speed at which the object is moved relative to the beam spot area.

[0054] In some embodiments, the first and second portions may spatially overlap, and in such embodiments, the spatial offset between the first and second portions of the image may be a spatial offset between the centers of mass of the first and second portions of the image, respectively.

[0055]

[0055] The pattern may include at least one feature, and the time shift applied to one of the first and second signals may be given by half the extent of the feature or each feature in the direction in which the object is moved relative to the beam spot area divided by the speed at which the object is moved relative to the beam spot area.

[0056] The or each feature may be a line. The pattern may, for example, include at least one line. In some embodiments, the pattern may include multiple lines. The or each line may have a thickness t in the direction of movement of the object relative to the beam spot area. The spatial offset between a first portion of the first image and a second portion of the second image may be half this thickness (t / 2). This can be converted to a time delay by dividing by the velocity s of the object relative to the beam spot area. In an embodiment where the pattern includes multiple lines with a 50% duty cycle having a pitch p (i.e., the spacing between the lines is also t), the spatial offset between a first portion of the first image and a second portion of the second image is one-quarter of the pitch (p / 4). This can also be converted to a time delay by dividing by the velocity s of the object relative to the beam spot area.

[0057] The first and second detectors may be configured to determine the intensities of the first and second portions, respectively, multiple times at a sampling frequency.

[0058]

[0058] That is, the first and second detectors are configured such that the intensities of the first and second portions of radiation may be determined multiple times, each determination being separated in time from the previous and subsequent determinations, and the sampling frequency may be the reciprocal of the time interval between the start of one determination and the start of the subsequent determination.

[0059]

[0059] The first point in time may coincide with one of the multiple determinations of the intensities of the first and second portions of the radiation, and the second point in time may coincide with another of the multiple determinations of the intensities of the first and second portions of the radiation.

[0060]

[0060] For example, a desired time delay that is an integer multiple of the time interval between the start of one determination and the start of a later determination may be applied to one of the first and second intensities before it is combined with the other of the first and second intensities.

[0061]

[0061] In some embodiments, at least one of the first time point or the second time point may be between two of the multiple determinations of the intensities of the first and second portions of the radiation, and the controller may be operable to interpolate between two of the multiple determinations of the intensities of the first and second portions of the radiation.

[0062] For example, a desired time delay that is a non-integer multiple of the time interval between the start of one determination and the start of a later determination may be applied to one of the first and second intensities before it is combined with the other of the first and second intensities. For example, it may be desirable to apply a time delay that is a non-integer multiple of the time interval between the start of one determination and the start of a later determination to the second intensity before it is combined with the first intensity. In such an embodiment, the second intensity used in the height determination may be an intensity obtained by interpolating (e.g., linearly) between two determinations that have a time delay between them.

[0063]

[0063] The apparatus may further comprise a radiation source operable to generate a radiation beam.

[0064] According to a fourth aspect of the present disclosure, there is provided a lithographic apparatus comprising an apparatus according to the third aspect of the present disclosure.

[0065]

[0065] The lithographic apparatus may further comprise an illumination system operable to illuminate the illumination area, a support structure configured to support the patterning device such that the patterning device can be positioned in the illumination area, a substrate table configured to support a substrate, and a projection system operable to form an image of the patterning device supported by the support structure onto a substrate supported by the substrate table. [Brief explanation of the drawings]

[0066]

[0066] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0067] [Figure 1] 1 shows a schematic overview of a lithographic apparatus; [Figure 2] 2 is a schematic diagram of a level or height sensor that may form part of the lithographic apparatus shown in FIG. 1; [Figure 3] 1 is a schematic diagram of a method for measuring the topography of a surface of an object (e.g., a substrate) according to an embodiment of the present disclosure. [Figure 4] 1 is a schematic diagram of a portion of a substrate showing a beam spot area or measurement location, a first image of a projected grating pattern including two lines, and a first feature on the substrate having a reflectivity different from the rest of the substrate. [Figure 5A] The first and second intensities I1, I2 (determined using the method shown in Figure 3), which comprise radiation from the first and second portions, respectively, of the first image shown in Figure 4, are shown as a function of position in the y-direction on the substrate. [Figure 5B] 5B shows a height determined by combining the first and second intensities I1, I2 shown in FIG. 5A. [Figure 6] 1 is a schematic diagram of a portion of a substrate showing a beam spot area or measurement location, a first image of a projected grating pattern including two lines, and a second feature on the substrate having a different reflectivity than the rest of the substrate. [Figure 7A] The first and second intensities I1, I2 (determined using the method shown in Figure 3), which comprise radiation from the first and second portions, respectively, of the first image shown in Figure 6, are shown as a function of position in the y-direction on the substrate. [Figure 7B] 7B shows a height determined by combining the first and second intensities I1, I2 shown in FIG. 7A. [Figure 8] 1 is a schematic diagram of an interpolation method for applying a time shift to an intensity signal (before combining it with another intensity signal) that is a non-integer multiple of the time interval between two measurements of the intensity signal. [Figure 9] 1 is a schematic diagram of a lithographic exposure method according to an embodiment of the present disclosure; [Figure 10]4 is a schematic diagram of an apparatus for measuring the topography of a surface of an object according to an embodiment of the present disclosure, which may form part of the lithographic apparatus shown in FIG. 1 and which may perform the method shown in FIG. 3. DETAILED DESCRIPTION OF THE INVENTION

[0068]

[0067] In this specification, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having wavelengths in the range of about 5 to 100 nm).

[0069]

[0068] As used herein, the terms "reticle," "mask," or "patterning device" may be broadly interpreted to refer to a general patterning device that can be used to impart an incident radiation beam with a patterned cross-section that corresponds to the pattern to be created on a target portion of a substrate. The term "light valve" may also be used in this context. In addition to standard masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0070] 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA comprises: an illumination system (also called an 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 according to 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 according to 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 the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0071]

[0070] In operation, the illumination system IL receives a radiation beam from the radiation source SO, for example via the 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 the radiation. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0072]

[0071] The term "projection system" PS as used herein should be interpreted broadly to encompass various types of projection systems, including refractive optical systems, catadioptric optical systems, anamorphic optical systems, magnetic optical systems, electromagnetic optical systems and / or electrostatic optical systems, or any combination thereof, as appropriate to the exposure radiation used and / or 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.

[0073]

[0072] The lithographic apparatus LA is of a type in which at least a part of the substrate W may be covered with a liquid having a relatively high refractive index, for example water, to fill a space between the projection system PS and the substrate W, also referred to as immersion lithography. More information about immersion techniques is given in US 6,952,253, which is incorporated herein by reference.

[0074] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also called "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or preparation steps for a subsequent exposure of a substrate W may be performed on a substrate W located on one substrate support WT, while another substrate W on another substrate support WT is being used to expose a pattern onto the other substrate W.

[0075] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is configured to hold a sensor and / or a cleaning device. The sensor may be configured 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 configured to clean part of the lithographic apparatus, for example part of the projection system PS or part of a system for providing immersion liquid. The measurement stage may move below the projection system PS when the substrate support WT is spaced apart from the projection system PS.

[0076] In operation, the radiation beam B is incident on a patterning device, for example a mask MA, which is held on a mask support MT, and is patterned by a pattern (design layout) present on the 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. Using the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example, to position various target portions C at focused and aligned positions in the path of the radiation beam B. Similarly, the first positioner PM, and possibly further position sensors (not explicitly depicted in FIG. 1 ), may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. The patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. The illustrated substrate alignment marks P1, P2 occupy dedicated target portions, but may be located in spaces between the target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are known as scribe-lane alignment marks.

[0077] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes: x, y, and z. Each of the three axes is orthogonal to the other two. Rotation about the x-axis is referred to as Rx rotation. Rotation about the y-axis is referred to as Ry rotation. Rotation about the z-axis is referred to as Rz rotation. The x- and y-axes define a horizontal plane, to which the z-axis is perpendicular. The Cartesian coordinate system is not a limitation of the invention and is used merely for clarity. Alternatively, 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, so that the z-axis has a component along the horizontal plane.

[0078] A topography measurement system, level sensor or height sensor, which may be integrated into a lithographic apparatus, is configured to measure the topography of the upper surface of a substrate (or wafer). A map of the topography of the substrate, also referred to as a height map, may be generated from these measurements, indicating the height of the substrate as a function of position on the substrate. This height map may then be used to correct the position of the substrate during transfer of a pattern to the substrate, to provide an aerial image of the patterning device in focus on the substrate. It will be understood that "height" in this context refers generally to the out-of-plane dimension (also referred to as the Z-axis) relative to the substrate. Typically, a level or height sensor performs measurements at a fixed location (relative to its optics), and relative movement between the substrate and the optics of the level or height sensor results in height measurements at each location across the substrate.

[0079] An example of a level or height sensor LS known in the art is shown schematically in FIG. 2, which illustrates only the principle of operation. In this example, the level sensor LS comprises an optical system including a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO that provides a radiation beam LSB that is patterned by a projection grating PGR of the projection unit LSP. The projection grating PGR may alternatively be referred to as a patterning device PGR. The radiation source LSO may be, for example, a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or unpolarized, such as a polarized or unpolarized laser beam, pulsed or continuous. The radiation source LSO may include multiple radiation sources with different colors or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation but may additionally or alternatively encompass UV and / or IR radiation and any range of wavelengths suitable for reflection from the surface of the substrate.

[0080]

[0079] The projection grating PGR is a periodic grating with a periodic structure that results in a radiation beam BE1 having a periodically varying intensity. The radiation beam BE1 having a periodically varying intensity is directed towards a measurement location MLO on the substrate W at an angle of incidence ANG of 0 to 90 degrees, typically 70 to 80 degrees, relative to an axis normal to the incident substrate surface (Z-axis). The measurement location MLO may alternatively be referred to as the beam spot area MLO. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards a detection unit LSD.

[0081]

[0080] To determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grid DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grid DGR may be identical to the projection grid PGR. The detector DET generates a detector output signal indicative of the received light, which may be output by a photodetector, e.g. indicative of the intensity of the received light, or representing the spatial distribution of the received intensity, which may be output by a camera or a sensor array. The detector DET may comprise any combination of one or more detector types.

[0082]

[0081] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity that depends, inter alia, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0083]

[0082] The projection unit LSP and / or detection unit LSD may include further optical elements such as lenses and / or mirrors along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0084]

[0083] In an embodiment, the detection grating DGR may be omitted and the detector DET may be placed at the position of the detection grating DGR. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0085]

[0084] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots that cover a larger measurement range.

[0086]

[0085] Various height sensors of the general type are disclosed, for example, in US 7,265,364 and US 7,646,471, both of which are incorporated by reference. A height sensor that uses UV radiation instead of visible or infrared radiation is disclosed in US 2010233600 A1, which is incorporated by reference. WO 2016102127 A1, which is incorporated by reference, describes a compact height sensor that uses a multi-element detector to detect and recognize the position of a grid image without the need for a detection grid.

[0087] In general, the detection unit LSD may be configured to split the reflected radiation BE2 into a first part and a second part and to determine the height of the substrate W by combining the intensities of the first and second parts. For example, the height may be determined as a differential measurement. Advantageously, with such an arrangement, the determination of the height of the substrate W may be substantially independent of the intensity of the radiation beam BE1. In practice, the splitting of the radiation into the first and second parts may be realized in various ways.

[0088] For example, in some known configurations, a combination of a polarizer and a shear plate (e.g., in the form of a Wollaston prism) is used to form two laterally shifted images of the projection grating PGR (each having a different polarization state) on the detection grating DGR. An example of such a configuration is shown schematically in FIG. 5 of US2010233600A1. For example, the projection grating PGR may have a pitch P and a duty cycle of 50% such that a radiation beam BE1 having a periodically varying intensity includes a plurality of lines having a thickness P / 2, with adjacent lines spaced apart by P / 2. The polarizer and shear plate are configured to form two images of the projection grating PGR (each having a different polarization state) on the detection grating DGR, one image being laterally shifted by P / 2 relative to the other image. Downstream of the detection grating DGR, the two different polarization states are directed to different detectors. The height of the substrate W is determined to be proportional to the intensity difference between the two different polarization states.

[0089]

[0088] In some other known configurations, rather than splitting the reflected radiation BE2 using two images of the projection grating PGR with different polarization states, a single image of the projection grating PGR is formed on a splitting optical system configured to split the single image into a first portion and a second portion. Examples of such configurations are shown schematically in FIG. 6 of US2010233600A1 and FIG. 2 of WO2016102127A1. For example, such configurations typically include a splitting optical system configured to split the reflected radiation into a first portion and a second portion. The splitting optical system may be a ruled grating with a triangular grating profile that acts as a series of wedges or prisms to redirect the reflected radiation BE2 (in accordance with Snell's law). Such a splitting optical system may be considered to include multiple prisms, and the image of each line of the projection grating PGR may be focused on one of multiple generally triangular prisms such that a first portion of the line is incident on a first surface of the prism and a second portion of the line is incident on a second surface of the prism. A first portion of the line is directed to a first detector, and a second portion of the line is directed to a second detector. As the line moves relative to the prism (as a result of changes in the height of the substrate W), the amount of radiation directed to each detector changes. Embodiments of the present disclosure apply particularly to level sensors that use this type of split optics.

[0090] Some embodiments of the present disclosure relate to a method for measuring the topography of a surface of an object (eg, a substrate W), which will now be described with reference to FIG.

[0091] 3 is a schematic illustration of a method 100 for measuring the topography of a surface of an object, such as a substrate W. The method 100 may be carried out, for example, using a level sensor LS generally of the form shown in FIG.

[0092] The method 100 includes a step 110 of forming a first image of a pattern onto the beam spot area using a radiation beam. Forming the first image of the pattern onto the beam spot area MLO may include providing a radiation beam LSB, patterning the radiation beam LSB using a patterning device (such as a projection grating PGR), and projecting the patterned radiation BE1 onto the beam spot MLO area using a projection optics system. For example, a projection unit LSP may be used to form a first image of the projection grating PGR (pattern) onto the measurement location MLO (beam spot area) using the radiation beam LSB.

[0093] The method 100 further comprises a step 120 of moving the object (e.g., substrate W) relative to the beam spot area (e.g., measurement location MLO). Although the object is described here as moving relative to the beam spot area, it will be appreciated that in alternative embodiments the beam spot area may be moved relative to the object (e.g., by moving the projection unit LSP and detection unit LSD while the object W remains stationary).

[0094] Moving the object W relative to the beam spot area MLO may include scanning the object W relative to the beam spot area MLO. Such scanning may be performed at a constant speed or a variable speed. As used herein, scanning the object W is intended to mean continuous movement of the object W. Alternatively, moving the object W relative to the beam spot area MLO may include stepping the object W relative to the beam spot area MLO. As used herein, stepping the object W is intended to mean movement of the object by multiple successive (time-separated) steps.

[0095]

[0094] The object W may be supported by a support, such as a wafer stage WT, in the lithographic apparatus LA. Moving the object W relative to the beam spot area MLO may include moving the support WT.

[0096] The method 100 further includes receiving a portion of the radiation beam reflected from the object (e.g., the reflected radiation BE2) and splitting the reflected radiation into a first portion and a second portion 130. Specifically, the reflected radiation BE2 is split such that the first portion of the radiation BE2 corresponding to a first portion of a first image (formed at the measurement location MLO) is spatially separated from the second portion of the radiation corresponding to a second portion of the first image.

[0097] The method 100 further comprises a step 140 of determining the intensities of the first and second portions of the radiation. For example, this step 140 may be performed by a detector DET which may comprise at least two portions operable to determine the intensity of one of the first and second portions of the reflected radiation BE2.

[0098]

[0097] The method 100 further includes a step 150 of determining the height h of the object W by combining the intensity I1 of the first portion of the radiation BE2 determined at the first time point t1 and the intensity I2 of the second portion of the radiation BE2 determined at the second time point t2.

[0099] It will be understood that determining the height of object W includes determining the height of the object relative to a reference height or position, as is known in the art. The height h of object W may be proportional to the difference between a first intensity I1(t1) and a second intensity I2(t1) determined at a first time point. For example, the height h of object W may be determined as proportional to the difference between the first intensity I1(t1) and the second intensity I1(t1) divided by the sum of the first intensity and the second intensity. That is, the height h is given by:

[0100]

number

[0101] Here, α is the gain.

[0102] A change in height of the object W may, for example, also change the position of at least a part of the second image of the pattern PGR formed on the detection grid DGR, and thus the relative values ​​of the first and second intensities I1, I2. For example, a change in height of the object W may change the position of at least a part of the second image of the pattern PGR relative to the splitting optics (or relative to the detector array) configured to split the reflected radiation into a first and a second part.

[0103] [000100] By determining the height of the substrate W by splitting the reflected radiation BE2 into a first and a second portion and combining the intensities of the first and second portions, the determination of the height may be substantially independent of the intensity of the radiation beam LSB. For example, the height may be determined as a differential measurement (e.g. according to equation (1)).

[0104] 3 is of the type that splits the reflected light BE2 into two portions corresponding to different portions of a first image (formed on the object W). The first image is formed at a beam spot area or measurement location MLO. In general, there may be a spatial offset between the first and second portions of the first image (although the first and second portions may partially spatially overlap). The object W being measured may have varying reflectance across its surface. For example, the object W may have localized regions or features that have a different reflectance than the surrounding area of ​​the surface. In this configuration, when the feature moves into (or out of) the beam spot area MLO (e.g., by relative movement in step 120), the spatial offset between the first and second portions of the first image will result in a difference between the first and second intensities due to differences in reflectance (not the height of the object W). 4-7B, when the first and second intensities determined at any given time are combined, the resulting determination of the height of the object will generally contain errors due to variations in the reflectivity of the surface of the object W, as the first and second portions of the radiation reflect from different areas of the surface of the object W.

[0105] [000102] By combining the intensity of the first portion determined at a first time point with the intensity of the second portion determined at a second time point, the method 100 shown in Figure 3 allows the first and second portions to correspond to radiation reflected from substantially the same portion of the object W. Advantageously, this may prevent any step or sudden change in the reflectivity of the object W from affecting the height determination.

[0106] [000103] As noted above, the object may be a substrate W in a lithography apparatus LA. Such a substrate W may include a silicon wafer coated with photoresist. The silicon wafer may include one or more layers that have been previously formed using, for example, a lithography process. Typically, a variety of different materials and / or different feature densities will be present on the surface of such a wafer W. This can cause variations in reflectivity across the surface of the wafer W, as different materials may absorb different portions of incident radiation and different feature densities can result in different amounts of scattering of the incident radiation. For example, a 3D-NAND wafer may include features that can reduce the amount of specular reflection of radiation by up to 50%.

[0107] 3, method 100 is shown as five separate steps 110, 120, 130, 140, and 150, but this is merely for ease of understanding, and it will be understood that these steps may be performed in any order. For example, step 110 of forming a first image of a pattern on a beam spot area using a radiation beam may be expected to be performed before step 130 of receiving a portion of the radiation beam reflected from an object and splitting the reflected radiation into a first portion and a second portion. However, in practice, method 100 is particularly suited to measurements over long periods of time, and therefore these steps may be performed simultaneously. In one embodiment, all of steps 110, 120, 130, 140, and 150 shown in FIG. 3 are performed simultaneously during the measurement period.

[0108] [000105] Figure 4 is a schematic diagram of a portion of a substrate W, showing the beam spot area or measurement location MLO. Also shown is a first image of the pattern of the projection grating PGR, which includes two lines L1, L2. Each of the two lines also includes two portions (the upper and lower halves of each line L1, L2 in Figure 4). The first image of the projection grating PGR can be considered to include a first portion 210 (which includes the upper portions of the two lines L1, L2) and a second portion 220 (which includes the lower portions of the two lines L1, L2).

[0109] The first and second portions 210, 220 of the first image correspond to the first and second portions of the radiation reflected by the substrate W that are split (in step 130 of the method 100 of FIG. 3 ), for example, by the splitting optics. That is, the first and second portions shown in FIG. 4 may be thought of as a back projection onto the substrate W of the splitting made by the splitting optics (shown here only to illustrate the advantages of the method of an embodiment of the present disclosure). In particular, the splitting of the first image into the first and second portions 210, 220 shown in FIG. 4 represents a situation where the height of the substrate W is zero (with respect to a reference height), since the first and second portions 210, 220 are substantially equal in size.

[0110] [000107] In (step 120 of) method 100, the substrate W moves in a scan direction (the y direction in Figure 4) relative to the beam spot area (measurement location MLO), as indicated by arrow 230. The x and y directions shown in Figure 4 represent sides of the target portion C (e.g., comprising one or more dies) of the substrate W (see Figure 1), and generally, features formed on the substrate tend to be aligned in the x and / or y directions. It should be noted that the lines L1, L2 of the projection grating PGR are arranged at a non-zero angle relative to both the x and y directions in Figure 4. This is to minimize the effect on the height measurement of scattering (other than specular reflection) of the incident radiation beam BE1 from features on the substrate.

[0111] [000108] Each of the two lines L1, L2 has a thickness t in the direction of movement of the substrate W relative to the beam spot area MLO (i.e., the y direction). Note that the thickness t of each of the lines L1, L2 in the first image (formed on the substrate W) is generally (1 / cos(ANG) times) larger than the thickness of each of the corresponding lines on the projection grating PGR. The spacing in the y direction between the two lines is also t, and the pitch p (in the y direction) of the first image is 2t. There is a spatial offset in the y direction between the first portion 210 and the second portion 220 of the first image equal to t / 2 (where t is the thickness t of the lines L1, L2).

[0112] 4 also shows a feature 240 on the wafer W that has a different reflectivity than the rest of the substrate W. Specifically, the feature 240 on the wafer W may have a reflectivity that is 80% of the rest of the substrate W. In this example, the less reflective feature 240 has an extent in the x-direction that is equal to or greater than the extent of the beam spot area MLO. Thus, the feature 240 may be thought of as providing a one-dimensional step in reflectivity (in the y-direction). As the substrate is scanned or stepped in the y-direction, the feature 240 will move into the beam spot area MLO and then out of the beam spot area MLO.

[0113] [000110] As feature 240 moves into (or out of) beam spot area MLO (e.g., by relative movement in step 120), the spatial offset in the y direction between first portion 210 and second portion 220 of the first image results in a difference between the first and second intensities (determined in step 140) that is due to differences in reflectivity between feature 240 and the rest of substrate W (and not due to the height of object W). This can be seen in Figures 5A and 5B.

[0114] [000111] Figure 5A shows the first and second intensities I1, I2 (determined in step 140) comprising radiation from the first and second portions 210, 220 of the first image, respectively, as a function of position in the y-direction of the substrate. As can be seen from Figure 5A (from left to right in Figure 5A), as the feature 240 moves into the beam spot area MLO, the first intensity I1 decreases before the second intensity I2 decreases (due to a decrease in the reflectivity of the feature 240). Once the feature 240 is fully within the beam spot area MLO, both the first and second intensities I1, I2 decrease due to a decrease in the reflectivity of the feature 240. Furthermore, as the feature 240 moves out of the beam spot area MLO, the first intensity I1 increases and returns to its nominal value before the second intensity I2.

[0115] 5B illustrates the height determined by combining the first and second intensities I1, I2 measured at any given time. Specifically, the height is proportional to the difference I2-I1. Recall that this example shown in FIG. 4 represents a situation in which the height of the substrate W is zero (relative to a reference height) because the first and second portions 210, 220 are substantially equal in size. As a result, when the feature 240 is not within the beam spot area MLO (left and right sides of FIGS. 5A and 5B), the height is zero. Furthermore, when the feature 240 is entirely within the beam spot area MLO (center portions of FIGS. 5A and 5B), the height is close to zero. However, due to changes in reflectivity caused when the feature 240 moves in and out of the beam spot area 240, combining the two simultaneously determined intensities as the feature 240 moves in (or out) of the beam spot area MLO (e.g., due to relative movement in step 120) will result in a large error in the determination of the height of the substrate W, even if the substrate height is zero. This is because the first and second portions 210 and 220 of the radiation are reflected from different areas of the surface of the substrate W.

[0116] [000113] Figure 6 is another schematic illustration of a portion of a substrate W, showing a beam spot area or measurement location MLO, a first image of the pattern of the projection grating PGR including two lines L1, L2, and a feature 250 having a different reflectivity than the rest of the substrate W. The only difference between the configuration shown in Figure 6 and the configuration shown in Figure 4 is that in the example shown in Figure 6, the feature 250 with lower reflectivity has an extent in the x-direction that is smaller than the extent of the beam spot area MLO. Therefore, the feature 250 may be considered to provide a two-dimensional step in reflectivity (in the y-direction).

[0117] [000114] Figure 7A shows the first and second intensities I1, I2 (determined in step 140) comprising radiation from the first and second portions 210, 220 of the first image, respectively, as a function of position in the y-direction of the substrate. Figure 7B shows the height determined by combining the first and second intensities I1, I2 determined at any given time.

[0118] [000115] In some embodiments, in step 150, the first and second points in time may be selected in response to a movement of the object W relative to the beam spot area MLO, such that the combined first and second portions of radiation correspond to radiation reflected from substantially the same portion of the object W. It will be appreciated that a first image of the pattern is formed in the beam spot area MLO (through which the object W moves). The reflected radiation is split such that the first and second portions of radiation correspond to different portions of the first image, so that at any given moment the first and second portions of radiation correspond to radiation reflected from two different portions of the object W. In some embodiments, the first and second portions of the first image (which provide the first and second portions of the reflected radiation BE2) may be spatially separated in the direction of moving the object. 4 and 6, the first and second portions 210, 220 of the first image (which provide the first and second portions of the reflected radiation BE2) are spatially separated in the direction of movement of the object W (y direction) by the thickness t of one of the two lines L1, L2. In such embodiments, one or both of the first and second intensities may be shifted in time before being combined to determine height. For example, a time delay may be applied to one of the first and second intensities before it is combined with the other of the first and second intensities.

[0119] [000116] In some embodiments, the time difference Δt between the first and second time points is given by the spatial offset between the first and second portions of the first image in the direction of movement of the object W relative to the beam spot area MLO, divided by the velocity at which the object W moves relative to the beam spot area MLO. In some embodiments, the first and second portions may spatially overlap (as is the case for the first and second portions 210, 220 of the first image in the examples shown in Figures 4 and 6). In such embodiments, the spatial offset between the first and second portions of the image may be the spatial offset between the centers of mass of the first and second portions of the image, respectively. For example, in the examples shown in Figures 4 and 6, the height may be determined according to equation (1), where

[0120]

number

[0121] and,

[0122]

number

[0123] where t is the thickness of the two lines L1, L2 of the first image (formed on the substrate W) in the direction in which the substrate W is moved relative to the beam spot area MLO (i.e., the y direction), and s is the speed at which the object W is moved relative to the beam spot area MLO. The thickness t of each of the lines L1, L2 in the first image (formed on the substrate W) is generally (1 / cos(ANG) times) larger than the thickness of each of the corresponding lines on the projection grating PGR.

[0124] [000117] In some embodiments, the pattern includes at least one feature (e.g., a line), and the time difference Δt between the first and second time points is given by half the extent of the or each feature in the direction of movement of the object W relative to the beam spot area MLO divided by the velocity of movement of the object W relative to the beam spot area MLO. The or each feature may be a line. The pattern may, for example, include at least one line. In some embodiments, the pattern may include multiple lines. In the plane of the substrate W, the or each line may have a thickness t in the direction of movement of the object W relative to the beam spot area MLO. The spatial offset between a first portion of the first image and a second portion of the second image may be half this thickness (t / 2). This can be converted into a time delay by dividing by the velocity s of the object W relative to the beam spot area MLO. In an embodiment where the pattern includes multiple 50% duty cycle lines with pitch p (i.e., the spacing between the lines is also t), the spatial offset between the first portion of the first image and the second portion of the second image is one-quarter of the pitch (p / 4), which can also be converted to a time delay by dividing by the object's velocity s relative to the beam spot area MLO.

[0125] [000118] In some embodiments, splitting the reflected radiation BE2 into first and second portions may include forming a second image of the pattern on splitting optics (which may be located approximately where the detection grating DGR is shown in Figure 2) and directing radiation from the first and second portions of the second image spatially apart using the splitting optics. The position of the second image relative to the splitting optics may determine how much of the reflected radiation is directed towards each of the first and second portions.

[0126] Alternatively, in some embodiments, splitting the reflected radiation BE2 into the first and second portions may include forming a second image of the pattern on a detector array including a plurality of sensing elements. The detector array may be referred to as a camera, and the individual sensing elements may be referred to as pixels. In such a configuration, a first subset of the sensing elements may be used to determine (in step 140) the intensity of the first portion of the radiation, and a second subset of the sensing elements may be used to determine the intensity of the second portion of the radiation.

[0127] [000120] In some embodiments, the intensities of the first and second portions of radiation may be determined multiple times at a sampling frequency f. That is, the intensities of the first and second portions of radiation may be determined multiple times, each determination separated in time from previous and subsequent determinations. The sampling frequency f may be the reciprocal of the time interval between the start of one determination and the start of a subsequent determination.

[0128] [000121] In some embodiments, the first point in time (at which the intensity of the first portion of radiation BE2 is determined in step 150) coincides with one of the multiple determinations of the intensities of the first and second portions of radiation, and the second point in time (at which the intensity of the second portion of radiation BE2 is determined in step 150) coincides with another of the multiple determinations of the intensities of the first and second portions of radiation. For example, a desired time delay that is an integer multiple of the time interval between the start of one determination and the start of a later determination may be applied to one of the first and second intensities before it is combined with the other of the first and second intensities.

[0129] Alternatively, in some embodiments, at least one of the first time point (at which the intensity of the first portion of radiation BE2 is determined in step 150) or the second time point (at which the intensity of the second portion of radiation BE2 is determined in step 150) is between two of the multiple determinations of the intensities of the first and second portions of radiation. In such embodiments, method 100 may include interpolating between two of the multiple determinations of the intensities of the first and second portions of radiation.

[0130] [000123] For example, a desired time shift that is a non-integer multiple of the time interval between the start of one determination and the start of a later determination may be applied to one of the first and second intensities before it is combined with the other of the first and second intensities. For example, it may be desirable to apply a time delay Δt that is a non-integer multiple of the time interval between the start of one determination and the start of a later determination to the second intensity (e.g., as defined in equations (2) and (3)) before it is combined with the first intensity. In such an embodiment, the second intensity used in the height determination may be an intensity obtained by interpolating (e.g., linearly) between two determinations that have a time delay between them.

[0131] [000124] Time shifting may be implemented using hardware, software, or a combination of both. Implementing such time shifting using hardware (e.g., in a data collection module) may allow for greater accuracy. Implementing such time shifting using software (e.g., as an algorithm executed by a controller) may be a cheaper solution. An example of a software implementation will now be considered with reference to FIG. 8.

[0132] [000125] Generally, there will be a number of samples (labeled 0 to 9 in Figure 8) determined at a number of y-positions for each of the two intensities I1, I2. In this example, a time shift is applied to one intensity I2, but it will be appreciated that alternatively, the shift may be applied to the other intensity, or both intensities.

[0133] [000126] For each sample, the following steps are predetermined (described with reference to sample number 5):

[0134] [000127] Step 1 is to find for a sample the corresponding y-position Y in the y-position data.

[0135] [000128] In step 2, a shift is applied to this y position Y to determine a shifted y position Y'. The y position samples between which this shifted y position Y' falls are determined. In this example, the shifted y position Y' is between sample number 3 and sample number 4. A ratio (a number between 0 and 1) is determined that quantifies where exactly between sample 3 and sample 4 the shifted y position Y' falls. For example, a ratio of 0 would correspond to the y position of sample 3, a ratio of 1 would correspond to the y position of sample 4, and a ratio between 0 and 1 corresponds to a y position between sample 3 and sample 4. A smaller ratio is closer to sample number 3, and a larger ratio is closer to sample number 4.

[0136] [000129] The ratio β may be given by:

[0137]

number

[0138] where y' is the shifted y position and the sample value y n and y n+1 It has been determined that the

[0139] [000130] In step 3 (schematically shown as 3a and 3b), the ratio β determined in step 2 is used to interpolate between two corresponding samples 3 and 4 of the input raw intensities. That is, a shifted or corrected intensity I' is determined by interpolation, for example according to the following equation:

[0140]

number

[0141] Here I n and I n+1 is the sample value y of the two y positions between which the shifted y position is located. n and y n+1 are the two raw intensity values ​​(inputs) corresponding to

[0142] [000131] In step 4, this interpolated intensity value I' is stored as the shifted output intensity value for sample number 5. This shifted second intensity I2 data set can now be combined with the raw first intensity I1 data set, combining their respective nth data values, and due to the interpolation and shifting process described above, this is equivalent to combining the intensity I1 of the first portion of radiation determined at a first time point t1 with the intensity I2 of the second portion of radiation determined at a second time point t2.

[0143] [000132] If the shifted y position Y' is outside the y position data array, the shifted intensity value will be invalid. This may be the case, for example, in Figure 8, if Y' is determined to be less than sample number 0. In such a case, the shifted y position Y' may optionally be equated to the nearest y position in the data array (e.g., sample number 0 if Y' is determined to be less than sample number 0).

[0144] Some embodiments of the present disclosure relate to lithographic exposure methods, an example of which is shown schematically in Figure 9. The lithographic exposure method 300 comprises measuring a topography of a surface of a substrate W using the method 100 of Figure 3. The lithographic exposure method 300 further comprises patterning 310 a beam of radiation B using a patterning device MA, and projecting 320 the patterned radiation onto the substrate W to form an image of the patterning device MA on the substrate W. The position of the substrate W while the patterned radiation is projected onto the substrate W (in step 320) is controlled in response to the measured topography of the surface of the substrate W (measured in step 100).

[0145] [000134] Advantageously, the measured topography of the surface of the substrate can be used to control the height of the substrate W while it is exposed to the patterned radiation, for example to keep the substrate W in the plane of best focus of the image of the patterning device MA. It will be appreciated that the image of the patterning device MA formed on the substrate W may be a diffraction limited image.

[0146] In some embodiments, the lithographic exposure method 300 comprises a scanning exposure, in which patterning the radiation beam using the patterning device MA comprises moving the patterning device MA through the radiation beam B, and projecting the patterned radiation onto the substrate W to form an image of the patterning device MA on the substrate W comprises moving the substrate W such that the image of the patterning device MA is substantially stationary relative to the substrate W. That is, to image the pattern onto a target area C of the substrate W, the patterning device MA is moved or scanned within the illumination field in a scanning direction. It will be appreciated that the substrate W is also scanned relative to the illumination field in the plane of the substrate W. The movement of the substrate W is such that the aerial image of the patterning device MA is stationary relative to the substrate W, and it will be understood that the direction and / or velocity of the substrate W may generally differ from the direction and / or velocity of the patterning device MA (e.g. if the image is inverted and / or if a demagnification factor is applied by the projection system PS).

[0147] [000136] Some embodiments of the present disclosure relate to an apparatus for measuring the topography of a surface of an object W. One embodiment of such an apparatus 400 is shown in Figure 10. The apparatus 100 may be referred to as a level sensor. The apparatus 100 may form part of a lithographic apparatus LA of the type shown in Figure 1 and described above. The apparatus 400 is generally in the form of a level sensor LS shown in Figure 2.

[0148] The apparatus 400 comprises a support 410 , projection optics 420 , a movement mechanism 430 , detection optics 440 , a first detector 450 , a second detector 460 , and a controller 470 .

[0149] [000138] The support 410 is suitable for supporting an object W. The support 410 may include, for example, a substrate table WT.

[0150] [000139] The projection optics 420 is operable to form a first image of a pattern onto a beam spot area 480 using the radiation beam 422. The projection optics 420 may be generally equivalent to the projection unit LSP shown in Figure 2 and described above, and the beam spot area 480 may be generally equivalent to the measurement location MLO shown in Figure 2 and described above. The projection optics 420 may, for example, comprise a projected patterning device 424 and a first imaging optics 426 configured to form an image of the projected patterning device 424 onto the beam spot area 480. The projected patterning device 424 may include a grating. The grating may include a plurality of lines. The lines may be of uniform thickness. The grating may have a 50% duty cycle. The projected patterning device 424 may be generally equivalent to the projected grating PGR shown in Figure 2 and described above.

[0151] [000140] The movement mechanism 430 is operable to move the support 410 to move an object (e.g., a substrate W) supported by the support 410 within the beam spot region 480. This movement is indicated diagrammatically by arrow 432.

[0152] [000141] The detection optics 440 are operable to receive a portion 442 of the radiation beam reflected from the object W and to split the reflected radiation 442 into a first portion 444 and a second portion 446 such that a first portion 444 of the radiation corresponding to a first portion of the first image is spatially separated from a second portion 446 of the radiation corresponding to a second portion of the first image. The detection optics 440 are generally equivalent to the detection unit LSD shown in Figure 2 and described above.

[0153] [000142] The first detector 450 is configured to determine the intensity of the reflected first portion 444 of the radiation. The second detector 460 is configured to determine the intensity of the reflected second portion 446 of the radiation.

[0154] [000143] The detection optics may comprise splitting optics 448 configured to split the reflected radiation 442 into a first portion 444 and a second portion 446, and second imaging optics 449 configured to receive the radiation 442 reflected from the object W supported by the support 410 and form a second image of the pattern on the splitting optics 448. The first imaging optics 426 may be generally equivalent to the second imaging optics 449.

[0155] [000144] The splitting optics 448, the beam spot area 480, and the projection patterning device 424 all lie in optically conjugate planes. Two planes will be understood to be optically conjugate if all radiation passing through each different point in the first plane is imaged to a different point in the second plane.

[0156] [000145] An image of the projection patterning device 424 is formed on the splitting optics 448, and the position of the image indicates the height of the object W. In particular, the position of the image relative to the splitting optics 448 indicates the height of the object W. As described above, the projection patterning device 424 may include a grating including a plurality of lines. The splitting optics 448 may comprise a plurality of prisms, and the image of each line may be focused onto one of a plurality of generally triangular prisms such that a first portion of the line is incident on a first surface of the prism and a second portion of the line is incident on a second surface of the prism. The first portion of the line is directed to a first detector 450, and the second portion of the line is directed to a second detector 460. As the line moves relative to the prisms (as a result of a change in the height of the object W), the amount of radiation directed to each detector 450, 460 changes.

[0157] [000146] Alternatively, in some embodiments, splitting the reflected radiation 442 into the first portion 444 and the second portion 446 may include forming a second image of the pattern (e.g., using second imaging optics 449) on a detector array including a plurality of sensing elements. The detector array may be referred to as a camera, and the individual sensing elements may be referred to as pixels. In such a configuration, a first subset of the sensing elements may be used to determine (in step 140) the intensity of the first portion of the radiation, and a second subset of the sensing elements may be used to determine the intensity of the second portion of the radiation. It should be noted that in such a configuration (in which the splitting optical system 448, first detector 450, and second detector 460 are replaced by a detector array), the detection optical system 440 may be considered to be operable to receive a portion 442 of the radiation beam reflected from the object W and split the reflected radiation 442 into a first portion 444 and a second portion 446 by forming a second image of the pattern (e.g., using a second imaging optical system 449) such that a first portion 444 of the radiation corresponding to a first portion of the first image is spatially separated from a second portion 446 of the radiation corresponding to a second portion of the first image.

[0158] [000147] The controller 470 is operable to determine the height of the object W by combining the intensity of the first portion 444 of the reflected radiation determined at a first time t1 and the intensity of the second portion 446 of the reflected radiation determined at a second time t2.

[0159] 10 is advantageous, as will now be discussed. Because the detection optics 440 are operable to split the reflected radiation into a first portion 444 and a second portion 446, and the controller 470 is operable to determine the height of the substrate W by combining the intensities of the first and second portions 444, 446, the height determination is substantially independent of the intensity of the radiation beam 422. For example, the height may be determined as a difference measurement (e.g., according to equation (1)). Furthermore, the apparatus 400 shown in FIG. 4 enables implementation of a method that ensures that the first and second portions 444, 446 correspond to radiation reflected from substantially the same portion of the object W, by combining the intensity of the first portion 444 determined at a first time point t1 and the intensity of the second portion 446 determined at a second time point t2. Advantageously, this may prevent any step or sudden change in the reflectivity of the object W from affecting the height determination. The controller may be operable to perform the method of FIG. 3, as discussed above.

[0160] [000149] In some embodiments, the first point in time t1 and the second point in time t2 are such that the combined first and second portions 444, 446 of radiation correspond to radiation reflected from substantially the same portion of the object W. In some embodiments, the first and second portions of the first image (providing the reflected first and second portions of radiation) may be spatially offset in the direction of movement of the object W. In such embodiments, one or both of the first and second intensities may be shifted in time before being combined to determine height. For example, a time delay may be applied to one of the first and second intensities before it is combined with the other of the first and second intensities.

[0161] [000150] In some embodiments, the apparatus 400 may further comprise a time shift module 472 configured to apply a time shift to the first signal s1 indicative of the first intensity from the first detector 450 and / or the second signal s2 indicative of the second intensity from the second detector 460, such that the first signal s1 is indicative of the intensity of the first portion 444 of the radiation determined at a first time point t1, and the second signal s2 is indicative of the intensity of the second portion of the radiation determined at a second time point t2.

[0162] [000151] The time shift module 472 may be implemented using hardware, software, or a combination of both. Implementing such a time shift module 472 using hardware (e.g., in the data collection module) may allow for greater accuracy. Implementing such a time shift module 472 using software (e.g., as an algorithm executed by the controller 470) may be a cheaper solution.

[0163] In some embodiments, the time shift applied to one of the first and second signals 444, 446 is given by the spatial offset between the first and second portions of the first image in the direction that the object W is moved relative to the beam spot area 480, divided by the velocity at which the object W is moved relative to the beam spot area 480. In some embodiments, the first and second portions may spatially overlap. In such embodiments, the spatial offset between the first and second portions of the image may be the spatial offset between the centers of mass of the first and second portions of the image, respectively.

[0164] [000153] In some embodiments, the pattern includes at least one feature, and the time shift Δt applied to one of the first and second signals s1, s2 is given by half the extent of the or each feature in the direction of movement of the object W relative to the beam spot area 480, divided by the velocity of movement of the object W relative to the beam spot area 480. The or each feature may be a line. The pattern may, for example, include at least one line. In some embodiments, the pattern may include multiple lines. The or each line may have a thickness t in the direction of movement of the object W relative to the beam spot area 480. The spatial offset between a first portion of the first image and a second portion of the second image may be half this thickness (t / 2). This can be converted to a time delay by dividing by the velocity s of the object W relative to the beam spot area 480. In an embodiment where the pattern includes multiple 50% duty cycle lines with pitch p (i.e., the spacing between the lines is also t), the spatial offset between the first portion of the first image and the second portion of the second image is one-quarter of the pitch (p / 4), which can also be converted to a time delay by dividing by the velocity s of the object W relative to the beam spot area 480.

[0165] In some embodiments, the first and second detectors 450, 460 are configured to determine the intensities of the first and second portions 444, 446, respectively, multiple times at a sampling frequency f. That is, the first and second detectors 450, 460 may be configured such that the intensities of the first and second portions 444, 446 of the radiation may be determined multiple times, with each determination separated in time from the previous and subsequent determinations. The sampling frequency f may be the reciprocal of the time interval between the start of one determination and the start of a subsequent determination.

[0166] In such an embodiment, the first point in time t1 may coincide with one of the multiple determinations of the intensities of the first and second portions of radiation 444, 446, and the second point in time may coincide with another of the multiple determinations of the intensities of the first and second portions of radiation 444, 446. For example, a desired time delay Δt, which is an integer multiple of the time interval between the start of one determination and the start of a later determination, is applied to one of the first and second intensities before it is combined with the other of the first and second intensities.

[0167] [000156] Alternatively, in some other embodiments, at least one of the first time point t1 or the second time point t2 is between two of the multiple determinations of the intensities of the first and second portions 444, 446 of the radiation, and the controller 470 is operable to interpolate between two of the multiple determinations of the intensities of the first and second portions 444, 446 of the radiation.

[0168] [000157] For example, a desired time delay that is a non-integer multiple of the time interval between the start of one determination and the start of a later determination may be applied to one of the first and second intensities before it is combined with the other of the first and second intensities. For example, it may be desirable to apply a time delay that is a non-integer multiple of the time interval between the start of one determination and the start of a later determination to the second intensity before it is combined with the first intensity. In such an embodiment, the second intensity used in the height determination may be an intensity obtained by interpolating (e.g., linearly) between the two determinations that have a time delay between them. Such interpolation may be generally as described above with reference to FIG. 8.

[0169] [000158] In some embodiments, the apparatus 400 may further comprise a radiation source 490 operable to generate the radiation beam 422.

[0170] [000159] Some embodiments of the present disclosure relate to a lithographic apparatus comprising an apparatus 400 of the type shown in Figure 10. The lithographic apparatus may be of the type shown in Figure 1. The lithographic apparatus LA may further comprise an illumination system IL operable to illuminate an illumination area, a support structure MT configured to support a patterning device MA such that the patterning device MA is positionable in the illumination area, a substrate table WT configured to support a substrate W, and a projection system PS operable to form an image of the patterning device MA supported by the support structure MT onto the substrate W supported by the substrate table WT.

[0171] [000160] Although specific reference may be made in this specification to the use of lithographic apparatus in the manufacture of ICs, it should be appreciated that the lithographic apparatus described herein may have other applications, including the manufacture of integrated optical systems, induction and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0172] [000161] Although specific reference may be made herein to embodiments of the invention in connection with lithography apparatus, embodiments of the invention may also 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 objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatus may be generally referred to as lithography tools. Such lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0173] [000162] Although specific reference has been made above to the use of embodiments of the present invention in connection with optical lithography, it will be understood that, where the context permits, the present invention is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0174] [000163] Where the context allows, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present 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, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and the like. Furthermore, firmware, software, routines, and instructions may be described herein as performing certain actions. However, it should be understood that such description is merely for convenience and that such actions actually occur when a computing device, processor, controller, or other device executes the firmware, software, routines, instructions, etc., which in turn may cause actuators or other devices to interact with the physical world.

[0175] While specific embodiments of the present invention have been described above, it will be understood that the invention may be practiced otherwise than as described. The above description is intended to be illustrative and not limiting. Accordingly, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set forth below. Other aspects of the invention are described in the following numbered clauses:

[0176] 1. A method for measuring the topography of a surface of an object, the method comprising: forming a first image of the pattern on a beam spot area using the radiation beam; moving the object relative to the beam spot area; receiving a portion of the radiation beam reflected from the object and splitting the reflected radiation into a first portion and a second portion such that the first portion of the radiation corresponding to the first portion of the first image is spatially separated from the second portion of the radiation corresponding to the second portion of the first image; determining the intensities of the first and second portions of the radiation; and 1. A method comprising: determining a height of an object by combining an intensity of a first portion of radiation determined at a first time point and an intensity of a second portion of radiation determined at a second time point. 2. The method of clause 1, wherein the first and second points in time are selected in response to movement of the object relative to the beam spot area, such that the first and second portions of the radiation that are combined correspond to radiation reflected from substantially the same portion of the object. 3. The method of clause 1 or clause 2, wherein the time difference between the first time point and the second time point is given by dividing the spatial offset between the first and second portions of the first image in a direction in which the object is moved relative to the beam spot area by the velocity at which the object is moved relative to the beam spot area. 4. The method of any one of clauses 1 to 3, wherein the pattern includes at least one feature and the time difference between the first and second time points is given by half the extent of the or each feature in the direction in which the object is moved relative to the beam spot area divided by the speed at which the object is moved relative to the beam spot area. 5. The method of any one of clauses 1 to 4, wherein splitting the reflected radiation into a first portion and a second portion comprises forming a second image of the pattern on a splitting optics, and directing radiation from the first and second portions of the second image spatially apart using the splitting optics. 6. The method of any one of clauses 1 to 5, wherein the intensities of the first and second portions of radiation are determined multiple times at a sampling frequency. 7. The method of clause 6, wherein the first time point coincides with one of the multiple determinations of the intensities of the first and second portions of radiation, and the second time point coincides with another of the multiple determinations of the intensities of the first and second portions of radiation. 8. The method of clause 6, wherein at least one of the first time point or the second time point is between two of the multiple determinations of the intensities of the first and second portions of the radiation, and the method includes interpolating between two of the multiple determinations of the intensities of the first and second portions of the radiation. 9. The method of any one of clauses 1 to 8, wherein the height of the object is proportional to the difference between the first intensity and the second intensity. 10. Measuring the topography of the surface of a substrate using the method of any one of clauses 1 to 9; patterning the radiation beam using a patterning device; and projecting the patterned radiation onto a substrate to form an image of the patterning device on the substrate; A lithographic exposure method in which the position of a substrate while patterned radiation is projected onto the substrate is controlled in response to a measured topography of the surface of the substrate. 11. The lithographic exposure method of clause 10, wherein the lithographic exposure is a scanning exposure, wherein patterning the radiation beam using a patterning device comprises moving the patterning device through the radiation beam, and projecting the patterned radiation onto the substrate to form an image of the patterning device on the substrate comprises moving the substrate such that the image of the patterning device is substantially stationary relative to the substrate. 12. An apparatus for measuring the topography of a surface of an object, the apparatus comprising: a support for supporting the object; a projection optical system operable to form a first image of the pattern on a beam spot area using the radiation beam; a movement mechanism operable to move the support to move an object supported by the support within the beam spot area; detection optics operable to receive a portion of the radiation beam reflected from the object and to split the reflected radiation into a first portion and a second portion such that the first portion of the radiation corresponding to the first portion of the first image is spatially separated from the second portion of the radiation corresponding to the second portion of the first image; a first detector configured to determine an intensity of a first portion of the radiation; a second detector configured to determine an intensity of a second portion of the radiation; and a controller operable to determine the height of the object by combining the intensity of the first portion of the radiation determined at a first time point and the intensity of the second portion of the radiation determined at a second time point. 13. The apparatus of clause 12, wherein the controller is operable to perform the method of any one of clauses 1 to 9. 14. The projection optics comprising: a projection patterning device; and a first imaging optical system configured to form an image of the projection patterning device onto the beam spot area. a splitting optic configured to split the reflected radiation into a first portion and a second portion; and a second imaging optical system configured to receive radiation reflected from an object supported by the support and form a second image of the pattern on the splitting optical system. 16. The apparatus of any one of clauses 12 to 15, wherein the first and second points in time are such that the first and second portions of radiation that are combined correspond to radiation reflected from substantially the same portion of the object. 17. The apparatus of any one of clauses 12 to 16, further comprising a time shift module configured to apply a time shift to a first signal indicative of a first intensity from the first detector and / or a second signal indicative of a second intensity from the second detector, such that the first signal is indicative of an intensity of a first portion of the radiation determined at a first time point and the second signal is indicative of an intensity of a second portion of the radiation determined at a second time point. 18. Apparatus according to clause 17, wherein the time shift applied to one of the first and second signals is given by dividing the spatial offset between the first and second parts of the first image in a direction in which the object is moved relative to the beam spot area by the speed at which the object is moved relative to the beam spot area. 19. The apparatus of clause 17 or clause 18, wherein the pattern includes at least one feature and the time shift applied to one of the first and second signals is given by half the extent of the or each feature in the direction in which the object is moved relative to the beam spot area, divided by the speed at which the object is moved relative to the beam spot area. 20. The apparatus of any one of clauses 12 to 19, wherein the first and second detectors are configured to determine the intensities of the first and second portions, respectively, multiple times at a sampling frequency. 21. The apparatus of clause 20, wherein the first time point coincides with one of the multiple determinations of the intensities of the first and second portions of radiation, and the second time point coincides with another of the multiple determinations of the intensities of the first and second portions of radiation. 22. The apparatus of clause 20, wherein at least one of the first time point or the second time point is between two of the multiple determinations of the intensities of the first and second portions of radiation, and the method includes interpolating between two of the multiple determinations of the intensities of the first and second portions of radiation. 23. The apparatus of any one of clauses 11 to 22, further comprising a radiation source operable to generate a beam of radiation. 24. A lithographic apparatus comprising an apparatus according to any one of clauses 11 to 23.

Claims

1. 1. A method for measuring the topography of a surface of an object, said method comprising: forming a first image of a pattern on a beam spot area using a radiation beam; moving the object relative to the beam spot area; receiving a portion of the radiation beam reflected from the object and splitting the reflected radiation into a first portion and a second portion such that the first portion of the radiation corresponding to a first portion of the first image is spatially separated from a second portion of the radiation corresponding to a second portion of the first image; determining an intensity of the first portion and the second portion of the radiation; determining a height of the object by combining an intensity of the first portion of the radiation determined at a first time point and an intensity of the second portion of the radiation determined at a second time point; A method comprising:

2. the first and second points in time are selected in response to movement of the object relative to the beam spot area, such that the first and second portions of the radiation that are combined correspond to radiation reflected from substantially the same portion of the object. The method of claim 1.

3. a time difference between the first time point and the second time point is given by a spatial offset between the first and second portions of the first image in a direction in which the object is moved relative to the beam spot area divided by a velocity in which the object is moved relative to the beam spot area; The method according to claim 1 or claim 2.

4. 4. The method of claim 1, wherein the pattern includes at least one feature, and the time difference between the first and second time points is given by half the extent of the or each feature in a direction in which the object is moved relative to the beam spot area divided by the speed at which the object is moved relative to the beam spot area.

5. splitting the reflected radiation into a first portion and a second portion includes forming a second image of the pattern on a splitting optics and directing radiation from the first and second portions of the second image spatially apart using the splitting optics.

5. The method according to any one of claims 1 to 4.

6. the intensities of the first and second portions of the radiation are determined a plurality of times at a sampling frequency; 6. The method according to any one of claims 1 to 5.

7. the first time point coinciding with one of a plurality of determinations of the intensities of the first and second portions of the radiation; the second time point coincides with another one of the plurality of determinations of the intensities of the first and second portions of the radiation. The method of claim 6.

8. at least one of the first time point or the second time point is between two of the plurality of determinations of the intensities of the first and second portions of the radiation; the method including interpolating between two of the plurality of determinations of intensities of the first and second portions of the radiation. The method of claim 6.

9. 1. An apparatus for measuring the topography of a surface of an object, said apparatus comprising: a support for supporting the object; a projection optical system operable to form a first image of the pattern on a beam spot area using the radiation beam; a movement mechanism operable to move the support to move an object supported by the support within the beam spot area; detection optics operable to receive a portion of the radiation beam reflected from the object and to split the reflected radiation into a first portion and a second portion such that the first portion of the radiation corresponding to a first portion of the first image is spatially separated from the second portion of the radiation corresponding to a second portion of the first image; a first detector configured to determine an intensity of the first portion of the radiation; a second detector configured to determine an intensity of the second portion of the radiation; a controller operable to determine a height of the object by combining an intensity of the first portion of the radiation determined at a first time point and an intensity of the second portion of the radiation determined at a second time point; A device comprising:

10. The projection optical system a projection patterning device; a first imaging optical system configured to form an image of the projection patterning device onto the beam spot area; 10. The apparatus of claim 9.

11. The detection optical system a splitting optic configured to split the reflected radiation into a first portion and a second portion; a second imaging optical system configured to receive radiation reflected from an object supported by the support and to form a second image of the pattern on the splitting optical system.

11. Apparatus according to claim 9 or claim 10.

12. the first time point and the second time point are such that the first and second portions of the radiation that are combined correspond to radiation reflected from substantially the same portion of the object; 12. Apparatus according to any one of claims 9 to 11.

13. a time shift module configured to apply a time shift to a first signal indicative of the first intensity from the first detector and / or a second signal indicative of the second intensity from the second detector such that the first signal is indicative of an intensity of the first portion of the radiation determined at a first time point and the second signal is indicative of an intensity of the second portion of the radiation determined at a second time point.

13. Apparatus according to any one of claims 9 to 12.

14. the time shift applied to one of the first and second signals is given by a spatial offset between the first and second portions of the first image in a direction in which the object is moved relative to the beam spot area divided by a velocity in which the object is moved relative to the beam spot area; 14. The apparatus of claim 13.

15. the first and second detectors are configured to determine the intensities of the first and second portions, respectively, multiple times at a sampling frequency; 15. Apparatus according to any one of claims 9 to 14.

Images