Method and device for measuring surface topography of object

Abstract

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

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 23154103.8, filed on January 31, 2023, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to a method for measuring the surface topography of an object. The present invention also relates to a corresponding apparatus for measuring the surface topography of an object. The present invention has particular application in the field of photolithography. The object may be a substrate within a photolithography apparatus. Such a substrate may include a silicon wafer coated with photoresist. The apparatus may be referred to as a level sensor and may form part of the photolithography apparatus. The present invention also relates to a photolithography exposure method using the method or apparatus for measuring the surface topography of a substrate. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. For example, a lithographic apparatus can be used in the manufacture of integrated circuits (ICs). For example, a lithographic apparatus can project a pattern (often also referred to as a "design layout" or "design") of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0005] As semiconductor manufacturing processes continue to advance, the size of circuit elements has continued to decrease, while the number of functional elements (such as transistors) per device has been steadily increasing for decades, following a trend generally referred to as 'Moore's Law'. To keep up with Moore's Law, the semiconductor industry has been pursuing technologies that can create smaller and smaller features. In order to project a pattern on a substrate, a lithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features patterned on the substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm and 13.5nm. Compared to lithography apparatuses using radiation with a wavelength of, for example, 193nm, lithography apparatuses using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4nm to 20nm (for example, 6.7nm or 13.5nm) can be used to form smaller features on a substrate.

[0006] Before exposing a wafer to patterned radiation in a lithography apparatus, a device known as a level sensor can be used to determine the topography of the wafer. This measurement of the wafer topography can be performed within the lithography apparatus, for example, once the wafer is clamped to a wafer stage. This information can be used during subsequent exposures of the wafer to maintain the exposed portion of the wafer in the plane of best focus.

[0007] It may be desirable to provide new methods and / or apparatus for determining wafer topography that may at least partially address one or more problems associated with existing arrangements, whether identified herein or otherwise. Summary of the Invention

[0008] According to a first aspect of the present disclosure, a method for measuring the surface morphology of an object is provided, 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 dividing the reflected radiation into a first part and a second part, so that the first part of the radiation corresponding to the first part of the first image is spatially separated from the second part of the radiation corresponding to the second part of the first image; determining the intensities of the first part and the second part of the radiation; and determining the height of the object by combining the intensity of the first part of the radiation determined at the first time and the intensity of the second part of the radiation determined at the second time.

[0009] As will now be discussed, the method according to the first aspect is advantageous.

[0010] As the height of the object changes, the position of at least a portion of the second image of the pattern may also change, which in turn may cause the relative values ​​of the first intensity and the second intensity to change. For example, as 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 beam splitting optical device arranged to split the reflected radiation into the first portion and the second portion.

[0011] By splitting the reflected radiation into a first portion and a second portion and determining the height of the substrate by combining the intensities of the first and second portions, the height may be determined substantially independent of the intensity of the radiation beam. For example, the height may be determined as a differential measurement.

[0012] The method according to the first aspect is of a type in which the reflected light is divided into two parts corresponding to different parts of a first image (formed on the object). Typically, there may be a spatial offset between the first part and the second part of the first image (although the first part and the second part may partially overlap in space). There may be some variation in the reflectivity of the object being measured across the surface. For example, the object may have a local area or feature that has a different reflectivity from the surrounding parts of the surface. With this arrangement, when the feature moves into (or out of) the beam spot area, any spatial offset between the first part and the second part of the first image will result in a difference in the first intensity and the second intensity, which is due to the change in reflectivity (rather than the height of the object). If the first intensity and the second intensity determined at any given time are combined, then typically, the final result of the object height will contain errors due to any variation in the surface reflectivity. This is because the first part and the second part of the radiation are reflected from different areas of the surface.

[0013] By combining the intensity of the first portion determined at a first time and the intensity of the second portion determined at a second time, the method according to the first aspect allows the method to ensure that the first portion and the second portion correspond to radiation reflected from substantially the same part of the object. Advantageously, this prevents any step or sudden change in the reflectivity of the object from affecting the determination of the height.

[0014] The object may be a substrate within a lithographic apparatus. Such a substrate may comprise a silicon wafer coated with photoresist. The silicon wafer may comprise one or more layers previously formed, for example, using a photolithographic process. Typically, such a wafer will have a series of features of varying materials and / or densities on its surface. This can cause the reflectivity of the wafer to vary across its surface, as different materials can absorb different portions of incident radiation, and features of varying densities may cause incident radiation to be scattered by varying amounts. For example, a 3D-NAND wafer may contain features that can reduce the amount of specular reflection of radiation by up to 50%.

[0015] It will be appreciated that determining the height of an object includes determining the height of the object relative to a reference height or location.

[0016] The first time and the second time may be selected in dependence on movement of the object relative to the beam spot area such that the first and second portions of the combined radiation correspond to radiation reflected from substantially the same portion of the object.

[0017] It will be appreciated that a first image of the pattern is formed at the beam spot region (through which the object moves). Because the reflected radiation is split so that the first and second portions of radiation correspond to different portions of the first image, at any given moment the first and second portions of radiation correspond to radiation reflected from two different portions of the object.

[0018] In some embodiments, the first and second portions of the first image (the first and second portions supplying the reflected radiation) may be spatially separated in the direction of object movement. For such embodiments, one or both of the first intensity and the second intensity may be time-shifted before combining the first and second intensities to determine the height. For example, a time delay may be applied to one of the first and second intensities before combining with the other of the first and second intensities.

[0019] The time difference between the first time and the second time can be obtained by dividing the spatial offset between the first part and the second part of the first image in the direction in which the object moves relative to the beam spot area by the speed at which the object moves relative to the beam spot area.

[0020] In some embodiments, the first portion and the second portion may partially overlap spatially.For such embodiments, the spatial offset between the first portion and the second portion of the image may be a spatial offset between the centroids of each of the first portion and the second portion of the image.

[0021] The pattern may comprise at least one feature, and the time difference between the first time and the second time may be given by half the extent of the or each feature in the direction of movement of the object relative to the beam spot area divided by the speed at which the object moves relative to the beam spot area.

[0022] The feature or each feature may be a line. For example, the pattern may include at least one line. In some embodiments, the pattern may include a plurality of lines. The line or each line may have a thickness t in the direction in which the object moves relative to the beam spot area. The spatial offset between the first part of the first image and the second part of the second image may be half the thickness (t / 2). This can be converted into a time delay by dividing by the speed s of the object relative to the beam spot area. For embodiments in which the pattern includes a plurality of lines with a pitch of p and a duty cycle of 50% (i.e., the spacing between the lines is also t), the spatial offset between the first part of the first image and the second part of the second image is a quarter of the pitch (p / 4). Similarly, this can be converted into a time delay by dividing by the speed s of the object relative to the beam spot area.

[0023] Forming a first image patterned on the beam spot area may include providing a radiation beam; patterning the radiation beam with a patterning device; and projecting the patterned radiation onto the beam spot area using projection optics.

[0024] Moving the object relative to the beam spot area may include scanning the object relative to the beam spot area. Such scanning may be at a constant speed or rate or at a variable rate. As used herein, scanning of 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 of the object is intended to mean movement of the object in a plurality of consecutive (temporally separated) steps.

[0025] The object may be supported by a support within the lithographic apparatus, such as a wafer stage. Moving the object relative to the beam spot area may comprise moving the support.

[0026] Splitting the reflected radiation into the first portion and the second portion may include forming a second image of the pattern on the splitting optics, and directing radiation from the first portion and the second portion of the second image using the splitting optics to be spatially separated.

[0027] The position of the second image relative to the beam-splitting optics may determine how much of the reflected radiation is directed to each of the first portion and the second portion.

[0028] The intensities of the first and second parts of the radiation may be determined a plurality of times at a sampling frequency.

[0029] That is, the intensity of the first and second parts of the radiation may be determined a plurality of times, each determination being separated in time from the previous and subsequent determinations.The sampling frequency may be the inverse of the time interval between the start of one determination and the start of a subsequent determination.

[0030] The first time may coincide with one of the plurality of determinations of the intensity of the first and second portions of the radiation.The second time may coincide with another of the plurality of determinations of the intensity of the first and second portions of the radiation.

[0031] 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 subsequent determination may be applied to one of the first and second intensities before being combined with the other of the first and second intensities.

[0032] At least one of the first time or the second time may be between two of the multiple determinations of the intensity of the first and second portions of the radiation, and the method may include interpolating between the two of the multiple determinations of the intensity of the first and second portions of the radiation.

[0033] For example, a desired time delay may be applied to one of the first and second intensities before being combined with the other of the first and second intensities, the time delay being a non-integer multiple of the time interval between the start of one determination and the start of a subsequent determination. For example, it may be desirable to apply a time delay to the second intensity before being combined with the first intensity, the time delay being a non-integer multiple of the time interval between the start of one determination and the start of a subsequent determination. For such embodiments, the second intensity used to determine the altitude may be an intensity obtained by interpolating (e.g., linearly) between two determinations having a time delay between them.

[0034] The height of the object may be proportional to the difference between the first intensity and the second intensity.

[0035] 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.

[0036] According to a second aspect of the present disclosure, a photolithography exposure method is provided, comprising: measuring the surface morphology of a substrate using the method according to the first aspect of the present disclosure; patterning a radiation beam using a pattern forming device; and projecting the patterned radiation onto the substrate so as to form an image of the pattern forming device on the substrate; wherein the position of the substrate when the patterned radiation is projected onto the substrate is controlled based on the measured surface morphology of the substrate.

[0037] Advantageously, the measured surface topography of the substrate can be used to control the height of the substrate when it is exposed to 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.

[0038] The lithographic exposure may be a scanning exposure such that: patterning the radiation beam using the patterning device may include 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 may include moving the substrate so that the image of the patterning device is generally stationary relative to the substrate.

[0039] That is, the patterning device is moved or scanned in a scan direction across the illumination area in order to image the pattern onto the target area of ​​the substrate. It will be appreciated that the substrate is also scanned relative to the illumination area in the plane of the substrate. The movement of the substrate causes the aerial image of the patterning device to be static relative to the substrate, and it will be appreciated that the orientation and / or velocity of the substrate can generally differ from that of the patterning device (e.g., if the image is inverted and / or if a reduction factor is applied by the projection system).

[0040] According to a third aspect of the present disclosure, there is provided an apparatus for measuring the surface morphology of an object, the apparatus comprising: a support for supporting the object; a projection optical device operable to form a first image of a pattern on a beam spot area using a radiation beam; a moving mechanism operable to move the support so as to move the object supported by the support through the beam spot area; a detection optical device operable to receive a portion of the radiation beam reflected from the object and divide the reflected radiation into a first part and a second part, so that the first part of the radiation corresponding to the first part of the first image is spatially separated from the second part of the radiation corresponding to the second part of the first image; a first detector arranged to determine the intensity of the first part of the radiation; a second detector arranged to determine the intensity of the second part of the radiation; and a controller operable to determine the height of the object by combining the intensity of the first part of the radiation determined at the first time and the intensity of the second part of the radiation determined at the second time.

[0041] The apparatus may be referred to as a level sensor. The apparatus may form part of a lithographic apparatus.

[0042] As will now be discussed, the apparatus according to the third aspect is advantageous. Because the detection optics are operable to separate 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 portion and the second portion, the height can be determined substantially independent of the intensity of the radiation beam. For example, the height can be determined as a differential measurement. Furthermore, by combining the intensity of the first portion determined at a first time and the intensity of the second portion determined at a second time, the apparatus according to the third aspect allows the method to be performed to ensure that the first portion and the second portion correspond to radiation reflected from substantially the same portion of the object. Advantageously, this can prevent any step or sudden change in the reflectivity of the object from affecting the height determination.

[0043] The controller may be operable to implement the method according to the first aspect of the present disclosure.

[0044] The projection optics may comprise: a projection patterning device; and a first imaging optics arranged to form an image of the projection patterning device on the beam spot area.

[0045] The projected pattern forming device may include a grating. The grating may include a plurality of lines. The lines may have a uniform thickness. The grating may have a 50% duty cycle.

[0046] The detection optics may comprise a splitting optic arranged to split the reflected radiation into a first portion and a second portion and a second imaging optic arranged to receive radiation reflected from an object supported by the support and form a second image of the pattern on the splitting optics.

[0047] The first imaging optics may generally be identical to the second imaging optics.

[0048] The beam splitting optics, beam spot region and projection patterning device are all in optically conjugate planes. It will be appreciated that two planes are optically conjugate if all radiation passing through every distinct point in the first plane is imaged onto a distinct point in the second plane.

[0049] An image of the projection pattern forming device is formed on the spectroscopic optical device, the position of the image being indicative of the height of the object. In particular, the position of the image relative to the spectroscopic optical device is indicative of the height of the object. As explained above, the projection pattern forming device may comprise a grating comprising a plurality of lines. The spectroscopic optical device may comprise a plurality of prisms, and the image of each line may be imaged onto one of a plurality of substantially 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 (due to changes in the height of the object), the amount of radiation directed to each of the detectors also changes.

[0050] The first time and the second time may be such that the first and second portions of the combined radiation correspond to radiation reflected from substantially the same portion of the object.

[0051] In some embodiments, the first and second portions of the first image (the first and second portions supplying the reflected radiation) may be spatially offset in the direction of movement of the object. For such embodiments, one or both of the first intensity and the second intensity may be time-shifted before combining the first and second intensities to determine the height. For example, a time delay may be applied to one of the first and second intensities before combining with the other of the first and second intensities.

[0052] The device may also include a time-shift module arranged 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, so that the first signal indicates the intensity of a first portion of the radiation determined at a first time, and the second signal indicates the intensity of a second portion of the radiation determined at a second time.

[0053] The time-shift module can be implemented using hardware, software, or a combination of both. Implementing such a time-shift module using hardware (e.g., in a data acquisition module) can allow for greater accuracy. Implementing such a time-shift module using software (e.g., as an algorithm executed by a controller) can be a cheaper solution.

[0054] The time shift applied to one of the first signal and the second signal can be derived from the spatial offset between the first part and the second part of the first image in the direction in which the object moves relative to the beam spot area divided by the speed at which the object moves relative to the beam spot area.

[0055] In some embodiments, the first portion and the second portion may partially overlap spatially.For such embodiments, the spatial offset between the first portion and the second portion of the image may be a spatial offset between the centroids of each of the first portion and the second portion of the image.

[0056] The pattern may comprise 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 or each feature in the direction of movement of the object relative to the beam spot area divided by the speed at which the object moves relative to the beam spot area.

[0057] The feature or each feature may be a line. For example, the pattern may include at least one line. In some embodiments, the pattern may include a plurality of lines. The line or each line may have a thickness t in the direction in which the object moves relative to the beam spot area. The spatial offset between the first part of the first image and the second part of the second image may be half the thickness (t / 2). This can be converted into a time delay by dividing by the speed s of the object relative to the beam spot area. For embodiments in which the pattern includes a plurality of lines with a pitch of p and a duty cycle of 50% (i.e., the spacing between the lines is also t), the spatial offset between the first part of the first image and the second part of the second image is a quarter of the pitch (p / 4). Similarly, this can be converted into a time delay by dividing by the speed s of the object relative to the beam spot area.

[0058] The first detector and the second detector may be arranged to determine the intensity of the first portion and the second portion, respectively, a plurality of times at a sampling frequency.

[0059] That is, the first detector and the second detector are arranged so that the intensity of the first part and the second part of the radiation can be determined multiple times, each determination being separated in time from the previous determination and the subsequent determination. The sampling frequency can be the inverse of the time interval between the start of one determination and the start of a subsequent determination.

[0060] The first time may coincide with one of the plurality of determinations of the intensity of the first and second portions of the radiation, and the second time may coincide with another of the plurality of determinations of the intensity of the first and second portions of the radiation.

[0061] 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 subsequent determination may be applied to one of the first and second intensities before being combined with the other of the first and second intensities.

[0062] In some embodiments, at least one of the first time or the second time may be between two of the multiple determinations of the intensity of the first and second portions of the radiation, and the controller may be operable to interpolate between the two of the multiple determinations of the intensity of the first and second portions of the radiation.

[0063] For example, a desired time delay may be applied to one of the first and second intensities before being combined with the other of the first and second intensities, the time delay being a non-integer multiple of the time interval between the start of one determination and the start of a subsequent determination. For example, it may be desirable to apply a time delay to the second intensity before being combined with the first intensity, the time delay being a non-integer multiple of the time interval between the start of one determination and the start of a subsequent determination. For such embodiments, the second intensity used to determine the altitude may be an intensity obtained by interpolating (e.g., linearly) between two determinations having a time delay between them.

[0064] The apparatus may further comprise a radiation source operable to generate a radiation beam.

[0065] According to a fourth aspect of the present disclosure, a lithography apparatus is provided, comprising the apparatus according to the third aspect of the present disclosure.

[0066] The lithographic apparatus may further comprise an illumination system operable to illuminate an illumination area; a support structure configured to support a pattern forming device so that the pattern forming 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 pattern forming device MA supported by the support structure on a substrate supported by the substrate table. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0068] Figure 1 A schematic overview of a photolithographic apparatus is depicted;

[0069] Figure 2 It can be formed Figure 1 A schematic illustration of a level or height sensor of a portion of a lithographic apparatus is shown;

[0070] Figure 3 is a schematic representation of a method for measuring the surface topography of an object (e.g., a substrate) according to an embodiment of the present disclosure;

[0071] Figure 4 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 different reflectivity than the remainder of the substrate;

[0072] Figure 5A The first intensity I1 and the second intensity I2 are shown (using Figure 3 ), which include Figure 4 the radiation of the first and second parts of the first image as a function of the position of the substrate in the y-direction;

[0073] Figure 5B Shown by combining Figure 5A The height determined by the first intensity I1 and the second intensity I2 shown;

[0074] Figure 6 is a schematic diagram of a portion of a substrate illustrating 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 remainder of the substrate;

[0075] Figure 7A The first intensity I1 and the second intensity I2 are shown (using Figure 3 ), which include Figure 6 the radiation of the first and second parts of the first image as a function of the position of the substrate in the y-direction;

[0076] Figure 7B Shown by combining Figure 7A The height determined by the first intensity I1 and the second intensity I2 shown;

[0077] Figure 8 is a schematic representation 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;

[0078] Figure 9 is a schematic representation of a photolithography exposure method according to an embodiment of the present disclosure; and

[0079] Figure 10 is a schematic representation of an apparatus for measuring the surface topography of an object according to an embodiment of the present disclosure, which can form Figure 1 A portion of the lithographic apparatus shown and capable of implementing Figure 3 The method shown. DETAILED DESCRIPTION

[0080] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., wavelengths in the range of about 5 to 100 nm).

[0081] 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 a patterned cross-section to an incoming radiation beam that corresponds to the pattern to be created in a target portion of a substrate. The term "light valve" may also be used in this context. In addition to classical masks (transmissive or reflective, binary, phase-shift, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0082] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., mask table) MT configured to support a patterning device (e.g., 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., wafer stage) WT configured 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., 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.

[0083] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g., via a beam delivery system BD). The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0084] The term "projection system" PS as used herein should be interpreted broadly as covering various types of projection systems, including refractive, reflective, refractive-reflective, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof, as appropriate for 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 should be considered synonymous with the more general term "projection system" PS.

[0085] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US Pat. No. 6,952,253, which is incorporated herein by reference.

[0086] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or a step of preparing a substrate W for subsequent exposure may be performed on a substrate W on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0087] In addition to the substrate support WT, the lithographic apparatus LA can include a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning equipment. The sensors can be arranged to measure properties of the projection system PS or properties of the radiation beam B. The cleaning equipment can be arranged to clean part of the lithographic apparatus, such as part of the projection system PS or part of a system for providing immersion liquid. The measurement platform can be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0088] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA 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 a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (not shown) are arranged in a plurality of positions. Figure 1 The patterning device MA may be accurately positioned relative to the path of the radiation beam B using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may also be located in the space between target portions C. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe lane alignment marks.

[0089] To illustrate the present invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., the x-axis, the y-axis, and the z-axis. Each of the three axes is orthogonal to the other two axes. Rotation around the x-axis is called an Rx rotation. Rotation around the y-axis is called an Ry rotation. Rotation around the z-axis is called an Rz rotation. The x-axis and the y-axis define a horizontal plane, while the z-axis is in the vertical direction. The Cartesian coordinate system does not limit the present invention, but is only used to illustrate. On the contrary, another coordinate system (such as a cylindrical coordinate system) can be used to illustrate the present invention. The orientation of the Cartesian coordinate system can be different, for example, so that the z-axis has a component along the horizontal plane.

[0090] The topography measurement system, level sensor or height sensor that can be integrated in the lithographic apparatus is arranged to measure the topography of the substrate (or wafer). A mapping of the topography of the substrate (also referred to as height mapping) can be generated from these measurements, which indicates the height of the substrate according to the position on the substrate. This height mapping can then be used to correct the position of the substrate during pattern transfer onto the substrate, so as to provide an aerial image of the pattern forming device focused on the substrate. It is to be understood that in this context, "height" refers to the dimension (also referred to as the Z-axis) that is clearly outside the plane for the substrate. Typically, the level or height sensor performs measurement at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level or height sensor causes height measurement to be performed at the position on the entire substrate.

[0091] Examples of level or height sensors LS known in the art are Figure 2 As schematically shown in Figure 2 Only the operating principle is illustrated. In this example, the level sensor LS comprises an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO, which provides a radiation beam LSB that is given a pattern with a projection grating PGR of the projection unit LSP. The projection grating PGR may alternatively be referred to as a pattern forming 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, pulsed or continuous, such as a polarized or unpolarized laser beam. The radiation source LSO may comprise a plurality of radiation sources with different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but may additionally or alternatively cover UV and / or IR radiation and any wavelength range suitable for reflection from the substrate surface.

[0092] The projection grating PGR is a periodic grating comprising a periodic structure that causes the radiation beam BE1 to have a periodically varying intensity. The radiation beam BE1, with its periodically varying intensity, is directed toward a measurement position MLO on a substrate W at an angle of incidence ANG between 0 and 90 degrees, typically between 70 and 80 degrees, relative to an axis perpendicular to the incident substrate surface (the Z-axis). The measurement position MLO may alternatively be referred to as a beam spot area MLO. At the measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed toward a detection unit LSD.

[0093] In order to determine the height level at the measurement position MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET, and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET generates a detector output signal indicative of the received light, for example, an intensity of the received light (such as may be output by a photodetector) or a spatial distribution of the received intensity (such as may be output by a camera or sensor array). The detector DET may comprise any combination of one or more detector types.

[0094] By means of triangulation techniques the height level at the measurement position 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.

[0095] The projection unit LSP and / or the detection unit LSD may comprise further optical elements, such as lenses and / or mirrors, along a path (not shown) of the patterned radiation beam between the projection grating PGR and the detection grating DGR.

[0096] In an embodiment, the detection grating DGR can be omitted and the detector DET can be placed where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0097] 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 a measurement area MLO or an array of light spots covering a larger measurement range.

[0098] For example, various height sensors of the general type are disclosed in US7265364 and US7646471, both of which are incorporated herein by reference. A height sensor that uses UV radiation rather than visible or infrared radiation is disclosed in US2010233600A1, which is incorporated herein by reference. In WO2016102127A1 (incorporated herein by reference), a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without requiring a detection grating.

[0099] Typically, the detection unit LSD can be arranged such that the reflected radiation BE2 is split into a first portion and a second portion, and the height of the substrate W is determined by combining the intensities of the first portion and the second portion. For example, the height can be determined as a differential measurement. Advantageously, with this arrangement, the height of the substrate W can be determined substantially independently of the intensity of the radiation beam BE1. In practice, the splitting of the radiation into the first portion and the second portion can be achieved in a variety of different ways.

[0100] For example, in some known arrangements, a combination of a polarizer and a shear plate (e.g., in the form of a Wollaston prism) is used to form two laterally displaced images of a projection grating PGR (each with a different polarization state) on a detection grating DGR. An example of such an arrangement is schematically shown in FIG5 of US2010233600A1. For example, the projection grating PGR may have a pitch P and a duty cycle of 50%, such that the radiation beam BE1 with periodically varying intensity comprises a plurality of lines of thickness P / 2, with adjacent lines separated by P / 2. The polarizer and shear plate are arranged to form two images of the projection grating PGR (each with a different polarization state) on the detection grating DGR, one image laterally displaced by P / 2 relative to the other. Downstream of the detection grating DGR, the two separate 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 separate polarization states.

[0101] In some other known arrangements, instead of using two images of the projection grating PGR having different polarization states to split the reflected radiation BE2, a single image of the projection grating PGR is formed on a splitting optical device arranged to split the single image into a first part and a second part. Figure 6 and WO2016102127A1 Figure 2An example of such an arrangement is schematically shown in . For example, such an arrangement typically includes a spectroscopic optical device that is arranged to split the reflected radiation into a first part and a second part. The spectroscopic optical device can be a ruled grating with a triangular grating profile, which acts as a series of wedges or prisms to redirect the reflected radiation BE2 (according to Snell's law). Such a spectroscopic optical device can be considered to include a plurality of prisms, and the image of each line of the projected grating PGR can be imaged onto one of a plurality of roughly triangular prisms, so that a first part of the line is incident on a first surface of the prism and a second part of the line is incident on a second surface of the prism. The first part of the line is directed to a first detector and the second part of the line is directed to a second detector. As the line moves relative to the prisms (due to changes in the height of the substrate W), the amount of radiation directed to each of the detectors will also change. Embodiments of the present disclosure are particularly suitable for use in level sensors using this type of spectroscopic optical device.

[0102] Some embodiments of the present disclosure relate to a method of measuring the surface topography of an object (e.g., a substrate W), as now referred to Figure 3 discussed.

[0103] Figure 3 is a schematic representation of a method 100 for measuring the surface topography of an object (e.g., a substrate W). For example, the method 100 may use a conventional Figure 2 The level sensor LS of the type shown is implemented.

[0104] Method 100 includes a step 110 of forming a first image of a pattern on a beam spot area with a radiation beam. Forming the first image of the pattern on the beam spot area MLO can include providing a radiation beam LSB; patterning the radiation beam LSB with a patterning device (such as, for example, a projection grating PGR); and projecting the patterned radiation BE1 onto the beam spot area MLO using projection optics. For example, a projection unit LSP can be used to form a first image of the projection grating PGR (pattern) on the measurement location MLO (beam spot area) with the radiation beam LSB.

[0105] The method 100 further includes a step 120 of moving an object (e.g., substrate W) relative to the beam spot region (e.g., measurement position MLO). It will be appreciated that, although described herein as moving the object relative to the beam spot region, in alternative embodiments, the beam spot region may be moved relative to the object (e.g., by moving the projection unit LSP and the detection unit LSD while the object W remains stationary).

[0106] 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 at a constant speed or rate or at a variable rate. As used herein, scanning of 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 of the object W is intended to mean movement of the object in a plurality of consecutive (temporally separated) steps.

[0107] The object W may be supported by a support, such as a wafer stage WT, within the lithographic apparatus LA. Moving the object W relative to the beam spot area MLO may include moving the support WT.

[0108] The method 100 further comprises receiving a portion of the radiation beam reflected from the object (e.g., reflected radiation BE2) and splitting the reflected radiation into a first portion and a second portion 130. In particular, the reflected radiation BE2 is split such that a first portion of the radiation BE2 corresponding to a first portion of the first image (formed at the measurement position MLO) is spatially separated from a second portion of the radiation corresponding to a second portion of the first image.

[0109] The method 100 further comprises a step of determining the intensity of the first and second parts of the radiation 140. For example, this step 140 may be performed by a detector DET which may comprise at least two parts, each part being operable to determine the intensity of one of the first and second parts of the reflected radiation BE2.

[0110] The method 100 further comprises a step 150 of determining the height h of the object W by combining the intensity of the first portion I1 of the radiation BE2 determined at the first time t1 and the intensity of the second portion I2 of the radiation BE2 determined at the second time t2 .

[0111] It will be appreciated that determining the height of the 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 the object W may be proportional to the difference between the first intensity I1(t1) and the second intensity I2(t1) determined at the first time. For example, the height h of the object W may be determined as proportional to the difference between the first intensity I1(t1) and the second intensity I2(t1) divided by the sum of the first intensity and the second intensity. That is, the height h may be derived as follows:

[0112] where α is the gain.

[0113] As the height of the object W changes, for example, the position of at least a portion of the second image of the pattern PGR formed at the detection grating DGR may also change, which in turn may cause the relative values ​​of the first intensity I1 and the second intensity I2 to change. For example, as the height of the object W changes, the position of at least a portion of the second image of the pattern PGR may change relative to a beam splitting optical device arranged to split the reflected radiation into the first part and the second part (or relative to the detector array).

[0114] By dividing the reflected radiation BE2 into a first portion and a second portion and determining the height of the substrate W by combining the intensities of the first and second portions, the height can be determined substantially independently of the intensity of the radiation beam LSB. For example, the height can be determined as a differential measurement (e.g. according to equation (1)).

[0115] Figure 3 The method 100 shown is of a type in which the reflected light BE2 is divided into two parts corresponding to different parts of a first image (formed on the object W). The first image is formed in the beam spot area or measurement position MLO. Typically, there may be a spatial offset between the first and second parts of the first image (although the first and second parts may partially overlap in space). There may be some variation in the reflectivity of the object W being measured across its surface. For example, the object W may have a local area or feature that has a different reflectivity from surrounding parts of the surface. With this arrangement, when the feature moves into (or out of) the beam spot area MLO (for example due to relative movement in step 120), any spatial offset between the first and second parts of the first image will result in a difference in the first and second intensities, which is due to the variation in reflectivity (rather than the height of the object W). As described below with reference to Figures 4 to 7B As discussed further, if the first intensity and the second intensity determined at any given time are combined, then the final determination of the object height will typically contain errors due to any changes in the surface reflectivity of the object W. This is because the first portion and the second portion of the radiation are reflected from different areas of the surface of the object W.

[0116] By combining the intensity of the first portion determined at the first time and the intensity of the second portion determined at the second time, Figure 3 The illustrated method 100 allows the first portion and the second portion to correspond to radiation reflected from substantially the same portion of the object W. Advantageously, this may prevent any steps or sudden changes in the reflectivity of the object W from affecting the determination of the height.

[0117] As described above, the object may be a substrate W within a lithographic apparatus LA. Such a substrate W may comprise a silicon wafer coated with photoresist. The silicon wafer may include one or more layers previously formed, for example, using a photolithographic process. Typically, such a wafer W will have a series of features of varying materials and / or densities on its surface. This can cause the reflectivity of the wafer W to vary across its surface, as different materials may absorb different portions of incident radiation, and features of varying densities may cause different amounts of scattering of the incident radiation. For example, a 3D-NAND wafer may include features that reduce the amount of specular reflection of radiation by up to 50%.

[0118] Although Method 100 Figure 3 100 is shown as five separate steps 110, 120, 130, 140, 150, but this is for ease of understanding only, and it is understood that these steps may be performed in any order. For example, it is contemplated that the step 110 of forming a first image of a pattern on a beam spot area with the radiation beam will be performed before the step 130 of receiving a portion of the radiation beam reflected from the object and dividing the reflected radiation into a first portion and a second portion. However, in practice, the method 100 is particularly well suited for measurements over extended periods of time, and therefore these steps may be performed concurrently. In one embodiment, Figure 3 All steps 110 , 120 , 130 , 140 , 150 shown are performed concurrently during the measurement time.

[0119] Figure 4 is a schematic diagram of a portion of a substrate W showing the beam spot area or measurement position MLO. A first image of a pattern of a projected grating PGR comprising two lines L1, L2 is also shown. In addition, each of the two lines comprises two parts (respectively Figure 4 The first image of the projected grating PGR may be considered to comprise a first portion 210 (comprising the top portions of the two lines L1 , L2) and a second portion 220 (comprising the bottom portions of the two lines L1 , L2).

[0120] For example, by using a spectroscopic optical device to split the light (in Figure 3 ), the first portion 210 and the second portion 220 of the first image correspond to the first portion and the second portion of the radiation reflected by the substrate W. That is, Figure 4 The first and second parts indicated in can be considered as the divided projections on the substrate W achieved by the splitting optical device (only indicated here to illustrate the advantages of the method of the embodiments of the present disclosure). In particular, as Figure 4As shown, dividing the first image into the first portion 210 and the second portion 220 represents a situation where the height of the substrate W is zero (relative to the reference height) because the first portion 210 and the second portion 220 are substantially equal in size.

[0121] As indicated by arrow 230, during method 100 (in step 120), the substrate W is moved in a scanning direction ( Figure 4 The beam spot area (measurement position MLO) is moved in the y direction (in the y direction). Figure 4 The x and y directions shown represent the substrate W (see Figure 1 ) of the target portion C (e.g., comprising one or more dies), and generally, features formed on the substrate tend to be aligned with the x and / or y directions. Note that in Figure 4 In FIG, the lines L1, L2 of the projection grating PGR are arranged at non-zero angles to both the x and y directions. This is to minimize the effects on the height measurement of scattering of the incident radiation beam BE1 from features on the substrate (other than specular reflections).

[0122] Each of the two lines L1, L2 has a thickness t in the direction in which the substrate W is moving 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 substrate W) will typically be greater than the thickness of each of the corresponding lines on the projection grating PGR (a multiple of 1 / cos(ANG)). The spacing between the two lines in the y-direction is also t, so that the pitch p of the first image (in the y-direction) is 2t. In the y-direction, there is a spatial offset 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).

[0123] exist Figure 4 Also shown in FIG. 2 is a feature 240 on wafer W having a different reflectivity than the rest of substrate W. Specifically, the reflectivity of feature 240 on wafer W may be 80% of that of the rest of substrate W. In this example, the extent of feature 240 with reduced reflectivity in the x-direction is equal to or greater than the extent of beam spot area MLO. Thus, feature 240 can be considered to provide a one-dimensional step in reflectivity (in the y-direction). As the substrate is scanned or stepped in the y-direction, feature 240 will move into and subsequently out of beam spot area MLO.

[0124] When the feature 240 moves into (or out of) the beam spot area MLO (e.g., due to the relative movement in step 120), the spatial offset in the y-direction between the first portion 210 and the second portion 220 of the first image results in a difference in the first intensity and the second intensity (as determined in step 140) that is due to the reflectivity variation between the feature 240 and the rest of the substrate W (and not due to the height of the object W). This can be seen from Figure 5Aand 5B see.

[0125] Figure 5A A first intensity I1 and a second intensity I2 (determined in step 140), comprising radiation from a first portion 210 and a second portion 220 of the first image, respectively, are shown as a function of the position of the substrate in the y-direction. Figure 5A (exist Figure 5A As can be seen from the figure (moving from left to right in the figure), as feature 240 moves into beam spot area MLO, first intensity I1 decreases before second intensity I2 decreases (due to the reduced reflectivity of feature 240). Once feature 240 is completely within beam spot area MLO, both first intensity I1 and second intensity I2 decrease due to the reduced reflectivity of feature 240. Furthermore, as feature 240 moves out of beam spot area MLO, first intensity I1 increases back to its nominal value before second intensity I2.

[0126] Figure 5B The height determined by combining the first intensity I1 and the second intensity I2 determined at any given time is shown. In particular, the height is proportional to the difference I2-I1. Recall that Figure 4 The example shown represents the situation when the height of the substrate W is zero (relative to the reference height) because the sizes of the first portion 210 and the second portion 220 are substantially equal. Therefore, when the feature 240 is not in the beam spot area MLO ( Figure 5A and 5B In addition, when the feature 240 is completely located in the beam spot area MLO ( Figure 5A and 5B When the feature 240 is within the center portion of the beam spot area MLO, the height is close to zero. However, due to the reflectivity changes caused by the movement of the feature 240 into or out of the beam spot area MLO, even though the substrate is at zero height, when the feature 240 moves into (or out of) the beam spot area MLO (e.g., due to the relative movement in step 120), the combination of the two simultaneously determined intensities will result in a significant error in the determination of the height of the substrate W. This is because the first portion 210 and the second portion 220 of the radiation are reflected from different areas of the surface of the substrate W.

[0127] Figure 6 Another schematic diagram of a portion of a substrate W showing a beam spot area or measurement location MLO, a first image of a pattern of a projected grating PGR comprising two lines L1 , L2 and a feature 250 , which has a different reflectivity than the rest of the substrate W. Figure 6 The arrangement shown is similar to Figure 4 The only difference between the arrangements shown is that in Figure 6In the example shown, the extent of the reduced reflectivity feature 250 in the x-direction is less than the extent of the beam spot area MLO. Thus, the feature 250 can be considered to provide a two-dimensional step in reflectivity (in the y-direction).

[0128] Figure 7A A first intensity I1 and a second intensity I2 (determined in step 140 ), comprising radiation from a first portion 210 and a second portion 220 , respectively, of the first image, are shown as a function of the position of the substrate in the y-direction. Figure 7B The altitude determined by combining the first intensity I1 and the second intensity I2 determined at any given time is shown.

[0129] In some embodiments, in step 150, the first time and the second time are selected based on the movement of the object W relative to the beam spot area MLO so that the first part and the second part of the combined radiation correspond to radiation reflected from substantially the same part of the object W. It is to be understood that a first image of the pattern is formed at the beam spot area MLO (through which the object W moves). Because the reflected radiation is split so that the first part and the second part of the radiation correspond to different parts of the first image, at any given moment the first part and the second part of the radiation correspond to radiation reflected from two different parts of the object W. In some embodiments, the first part and the second part of the first image (which supply the first part and the second part of the reflected radiation BE2) may be spatially separated in the direction of movement of the object. For example, in Figure 4 and 6 In the example arrangement shown, the first and second portions 210 and 220 of the first image (the first and second portions supplying reflected radiation BE2) are spatially separated by a thickness t of one of two lines L1 and L2 in the direction of movement of the object W (the y-direction). For such embodiments, one or both of the first and second intensities may be time-shifted before being combined to determine the height. For example, a time delay may be applied to one of the first and second intensities before being combined with the other.

[0130] In some embodiments, the time difference Δt between the first time and the second time can be obtained by dividing the spatial offset between the first part and the second part of the first image in the direction in which the object W moves relative to the beam spot area MLO by the speed at which the object W moves relative to the beam spot area MLO. In some embodiments, the first part and the second part can partially overlap in space (e.g., Figure 4 and 6For example, the spatial offset between the first and second parts of the image may be the spatial offset between the centroids of each of the first and second parts of the image. Figure 4 and 6 In the example shown, the height can be determined according to equation (1), where:

[0131] t2=t1-Δt (2)

[0132] And among them:

[0133] and where t is the thickness of the two lines L1, L2 in the first image (formed on the substrate W) in the direction in which the substrate W moves relative to the beam spot area MLO (i.e., the y-direction), and s is the speed at which the object W moves relative to the beam spot area MLO. Recall that the thickness t of each of the lines L1, L2 in the first image (formed on the substrate W) will typically be greater than the thickness of each of the corresponding lines on the projection grating PGR (a multiple of 1 / cos(ANG)).

[0134] In some embodiments, the pattern includes at least one feature (e.g., a line), and the time difference Δt between the first time and the second time 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 speed at which the object W moves relative to the beam spot area MLO. The or each feature may be a line. For example, the pattern may include at least one line. In some embodiments, the pattern may include a plurality of lines. In the plane of the substrate W, in the direction of movement of the object W relative to the beam spot area MLO, the or each line may have a thickness t. The spatial offset between the first portion of the first image and the 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 speed s of the object W relative to the beam spot area MLO. For embodiments in which the pattern includes a plurality of lines with a pitch p and a 50% duty cycle (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). Similarly, this can be converted to a time delay by dividing by the speed s of the object relative to the beam spot area MLO.

[0135] In some embodiments, splitting the reflected radiation BE2 into the first portion and the second portion may include forming a second image of the pattern on a beam splitting optical device (which may typically be arranged at Figure 2The position of the detection grating DGR is shown in FIG, and the radiation from the first and second parts of the second image is directed spatially separated using a beam splitting optical device. The position of the second image relative to the beam splitting optical device can determine how much of the reflected radiation is directed to each of the first and second parts.

[0136] Alternatively, in some embodiments, separating the reflected radiation BE2 into the first portion and the second portion can include forming a second image of the pattern on a detector array comprising a plurality of sensing elements. The detector array can be referred to as a camera, and the individual sensing elements can be referred to as pixels. With this arrangement, a first subset of the sensing elements can be used to determine the intensity of the first portion of the radiation (in step 140), and a second subset of the sensing elements can be used to determine the intensity of the second portion of the radiation.

[0137] In some embodiments, the intensity of the first portion and the second portion of the radiation may be determined multiple times at a sampling frequency f. That is, the intensity of the first portion and the second portion of the radiation may be determined multiple times, with each determination being separated in time from the previous determination and the subsequent determination. The sampling frequency f may be the inverse of a time interval between the start of one determination and the start of a subsequent determination.

[0138] In some embodiments, the first time (determining the intensity of the first portion of radiation BE2 in step 150) coincides with one of the multiple determinations of the intensity of the first and second portions of radiation, and the second time (determining the intensity of the second portion of radiation BE2 in step 150) coincides with another of the multiple determinations of the intensity of the first and second portions of radiation. For example, a desired time delay can be applied to one of the first and second intensities before being combined with the other of the first and second intensities, the time delay being an integer multiple of the time interval between the start of one determination and the start of a subsequent determination.

[0139] Alternatively, in some embodiments, at least one of the first time (at which the intensity of the first portion of radiation BE2 is determined in step 150) or the second time (at which the intensity of the second portion of radiation BE2 is determined in step 150) is between two of the plurality of determinations of the intensity of the first and second portions of radiation. For such embodiments, method 100 may include interpolating between the two of the plurality of determinations of the intensity of the first and second portions of radiation.

[0140] For example, a desired time shift may be applied to one of the first and second intensities before being combined with the other of the first and second intensities, the time shift being a non-integer multiple of the time interval between the start of one determination and the start of a subsequent determination. For example, it may be desirable to apply a time delay Δt (e.g., defined by equations (2) and (3)) to the second intensity before being combined with the first intensity, the time delay Δt being a non-integer multiple of the time interval between the start of one determination and the start of a subsequent determination. For such embodiments, the second intensity used to determine the altitude may be an intensity obtained by interpolating (e.g., linearly) between two determinations having a time delay between them.

[0141] Time shifting can be implemented using hardware, software, or a combination of both. Implementing this time shifting using hardware (e.g., in a data acquisition module) can allow for greater accuracy. Implementing this time shifting module using software (e.g., as an algorithm executed by a controller) can be a cheaper solution. Figure 8 Discuss examples of software implementations.

[0142] Typically, there will be multiple samples (in Figure 8 0 to 9 in FIG), for each of the two intensities, I1, I2 are determined at a plurality of y positions. In this example, the time shift is applied to one of the intensities, I2, but it will be appreciated that the shift may also be applied to the other or both intensities.

[0143] For each sample, the following steps were performed (refer to sample number 5 for explanation).

[0144] In step 1, for each sample, find the corresponding y position Y in the y position data.

[0145] In step 2, a shift is applied to the y position Y to determine a shifted y position Y'. The samples in the y position between which the shifted y position Y' is located 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 the exact position of the shifted y position Y' between samples 3 and 4. For example, a ratio of 0 corresponds to the y position of sample 3, a ratio of 1 corresponds to the y position of sample 4, and a ratio between 0 and 1 corresponds to a y position between samples 3 and 4. A smaller ratio is closer to sample number 3, and a larger ratio is closer to sample number 4.

[0146] The ratio β can be derived as follows:

[0147]

[0148] where y' is the shifted y position, which has been determined at the sample value y n and yn+1 between.

[0149] In step 3 (schematically indicated as 3a and 3b), interpolation is performed between two corresponding samples 3 and 4 of the input original intensity using the ratio β determined in step 2. That is, a shifted or corrected intensity I' is determined by interpolation, for example according to:

[0150] I′=I n +β·(I n+1 -I n ) (5)

[0151] Among them I n and I n+1 are the two original intensity values ​​(inputs) corresponding to the two y position sample values ​​y at the shifted y position in between n and y n+1 .

[0152] 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 original first intensity I1 data set, but combining the nth data value of each data set, and due to the above-described interpolation and shifting process, this is equivalent to combining the intensity of the first portion I1 of the radiation determined at the first time t1 and the intensity of the second portion I2 of the radiation determined at the second time t2.

[0153] If the shifted y position Y' is outside the y position data array, the shifted intensity value will be invalid. For example, in Figure 8 This may be the case if Y' is determined to be less than sample number 0. Alternatively, for this case, the shifted y position Y' may be equal to the closest y position in the data array (e.g., sample number 0 if Y' is determined to be less than sample number 0).

[0154] Some embodiments of the present disclosure relate to a photolithography exposure method. Figure 9 An example of such a photolithography exposure method 300 is schematically shown in FIG. The photolithography exposure method 300 includes using Figure 3 The method 100 measures the surface topography of the substrate W. The lithographic exposure method 300 further includes: step 310, patterning a radiation beam B using a patterning device MA; and step 320, projecting 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 based on the measured surface topography of the substrate W (as measured in step 100).

[0155] Advantageously, the measured surface topography of the substrate can be used to control the height of the substrate W when it is exposed to 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.

[0156] In some embodiments, the lithographic exposure method 300 includes a scanning exposure such that patterning a radiation beam using the patterning device MA includes moving the patterning device MA through the radiation beam B and projecting the patterned radiation onto the substrate W so as to form an image of the patterning device MA on the substrate W, and includes moving the substrate W such that the image of the patterning device MA is generally stationary relative to the substrate W. That is, the patterning device MA is moved or scanned in a scanning direction through an illumination area in order to image a pattern onto a target area C of the substrate W. It will be appreciated that the substrate W is also scanned relative to the illumination area in the plane of the substrate W. The movement of the substrate W causes the aerial image of the patterning device MA to be stationary relative to the substrate W, and it will be appreciated that, in general, the orientation and velocity of the substrate W may differ from the orientation and velocity of the patterning device MA (e.g., if the image is inverted and / or if a downscaling factor is applied by the projection system PS).

[0157] Some embodiments of the present disclosure relate to a device for measuring the surface topography of an object W. Figure 10 An embodiment of such a device 400 is shown. The device 100 may be referred to as a level sensor. The device 100 may form Figure 1 A portion of a lithographic apparatus LA of the type shown and described above. Apparatus 400 is typically Figure 2 The form of the level sensor LS is shown.

[0158] The apparatus 400 comprises: a support 410 ; a projection optical device 420 ; a movement mechanism 430 ; a detection optical device 440 ; a first detector 450 ; a second detector 460 and a controller 470 .

[0159] The support 410 is suitable for supporting an object W. For example, the support 410 may include a substrate table WT.

[0160] The projection optics 420 are operable to form a first image of a pattern on a beam spot area 480 using a radiation beam 422. The projection optics 420 are generally equivalent to Figure 2 The projection unit LSP shown and described above, and the beam spot area 480 can generally be equivalent to Figure 2The measurement position MLO shown and described above. The projection optical device 420 may, for example, include: a projection pattern forming device 424; and a first imaging optical device 426, which is arranged to form an image of the projection pattern forming device 424 on the beam spot area 480. The projection pattern forming device 424 may include a grating. The grating may include a plurality of lines. The lines may have a uniform thickness. The grating may have a duty cycle of 50%. The projection pattern forming device 424 may generally be equivalent to Figure 2 Projection grating PGR as shown and described above.

[0161] The movement mechanism 430 is operable to move the support 410, thereby moving an object (eg, a substrate W) supported by the support 410 through the beam spot region 480. This movement is schematically indicated by arrow 432.

[0162] 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 the first portion 444 of the radiation corresponding to the first portion of the first image is spatially separated from the second portion 446 of the radiation corresponding to the second portion of the first image. The detection optics 440 are generally equivalent to Figure 2 Detection unit LSD as shown and described above.

[0163] The first detector 450 is arranged to determine the intensity of the first portion 444 of the reflected radiation. The second detector 460 is arranged to determine the intensity of the second portion 446 of the reflected radiation.

[0164] The detection optics may include a beam splitting optic 448 arranged to split the reflected radiation 442 into a first portion 444 and a second portion 446, and a second imaging optic 449 arranged to receive radiation 442 reflected from an object W supported by the support 410 and form a second image of the pattern on the beam splitting optic 448. The first imaging optic 426 may generally be identical to the second imaging optic 449.

[0165] Beam splitting optics 448, beam spot region 480, and projection patterning device 424 are all in optically conjugate planes. It will be appreciated that two planes are optically conjugate if all radiation passing through every distinct point in a first plane is imaged onto a distinct point in a second plane.

[0166] An image of the projection pattern forming device 424 is formed on the spectroscopic optical device 448, and the position of the image is indicative of the height of the object W. In particular, the position of the image relative to the spectroscopic optical device 448 is indicative of the height of the object W. As explained above, the projection pattern forming device 424 may include a grating comprising a plurality of lines. The spectroscopic optical device 448 may include a plurality of prisms, and the image of each line may be imaged 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 (due to changes in the height of the object W), the amount of radiation directed to each of the detectors 450, 460 changes.

[0167] Alternatively, in some embodiments, separating the reflected radiation 442 into the first portion 444 and the second portion 446 can include forming a second image of the pattern on a detector array comprising a plurality of sensing elements (e.g., using second imaging optics 449). The detector array can be referred to as a camera, and the individual sensing elements can be referred to as pixels. With this arrangement, a first subset of the sensing elements can be used to determine the intensity of the first portion of the radiation (in step 140), and a second subset of the sensing elements can be used to determine the intensity of the second portion of the radiation. Note that with this arrangement (wherein the beam splitting optics 448, the first detector 450, and the second detector 460 are replaced by the detector array), the detection optics 440 can be considered to be operable to receive a portion 442 of the radiation beam reflected from the object W and separate the reflected radiation 442 into the first portion 444 and the second portion 446 such that, due to forming the second image of the pattern (e.g., using second imaging optics 449), the first portion 444 of the radiation corresponding to the first portion of the first image is spatially separated from the second portion 446 of the radiation corresponding to the second portion of the first image.

[0168] 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 the first time t1 and the intensity of the second portion 446 of the reflected radiation determined at the second time t2.

[0169] As now discussed, Figure 10The apparatus 400 shown is advantageous. Because the detection optics 440 are operable to separate 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 portion 444 and the second portion 446, the height can be determined substantially independent of the intensity of the radiation beam 422. For example, the height can be determined as a differential measurement (e.g., according to equation (1)). Furthermore, by combining the intensity of the first portion 444 determined at a first time t1 and the intensity of the second portion 446 determined at a second time t2, Figure 4 The apparatus 400 shown allows for the implementation of a method to ensure that the first portion 444 and the second portion 446 correspond to radiation reflected from substantially the same portion of the object W. Advantageously, this can prevent any step or sudden change in the reflectivity of the object W from affecting the determination of the height. As discussed above, the controller may be operable to implement Figure 3 method.

[0170] In some embodiments, the first time t1 and the second time t2 are such that the first portion 444 and the second portion 446 of the combined 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 (supposing the first and second portions of the reflected radiation) may be spatially offset in the direction of movement of the object W. For such embodiments, one or both of the first intensity and the second intensity may be time-shifted before being combined to determine the height. For example, a time delay may be applied to one of the first and second intensities before being combined with the other.

[0171] In some embodiments, the device 400 may also include a time-shift module 472, which is arranged to apply a time shift to the first signal s1 indicating the first intensity from the first detector 450 and / or the second signal s2 indicating the second intensity from the second detector 460, so that the first signal s1 indicates the intensity of the first part 444 of the radiation determined at the first time t1, and the second signal s2 indicates the intensity of the second part of the radiation determined at the second time t2.

[0172] The time-shift module 472 can be implemented using hardware, software, or a combination of both. Implementing such a time-shift module 472 using hardware (e.g., in a data acquisition module) can allow for greater accuracy. Implementing such a time-shift module 472 using software (e.g., as an algorithm executed by the controller 470) can be a less expensive solution.

[0173] In some embodiments, the time shift applied to one of the first signal 444 and the second signal 446 is determined by dividing the spatial offset between the first portion and the second portion of the first image in the direction in which the object W moves relative to the beam spot area 480 by the speed at which the object W moves relative to the beam spot area 480. In some embodiments, the first portion and the second portion may partially overlap spatially. For such embodiments, the spatial offset between the first portion and the second portion of the image may be the spatial offset between the center of mass of each of the first portion and the second portion of the image.

[0174] In some embodiments, the pattern may include at least one feature, and the time shift Δt applied to one of the first and second signals s1 and s2 may be determined by dividing the extent of the or each feature in the direction of movement of the object W relative to the beam spot area 480 by the speed of movement of the object W relative to the beam spot area 480 by half. The or each feature may be a line. For example, the pattern may include at least one line. In some embodiments, the pattern may include a plurality of 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 the first portion of the first image and the 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 speed s of the object W relative to the beam spot area 480. For embodiments in which the pattern includes a plurality of lines with a pitch p and a 50% duty cycle (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). Similarly, this can be converted to a time delay by dividing by the speed s of the object W relative to the beam spot area 480.

[0175] In some embodiments, first detector 450 and second detector 460 are arranged to determine the intensity of first portion 444 and second portion 446, respectively, multiple times at a sampling frequency f. That is, first detector 450 and second detector 460 can be arranged so that the intensity of first portion 444 and second portion 446 of radiation can be determined multiple times, with each determination being separated in time from the previous determination and the subsequent determination. The sampling frequency f can be the inverse of the time interval between the start of one determination and the start of a subsequent determination.

[0176] For such an embodiment, the first time t1 can coincide with one of the multiple determinations of the intensity of the first portion 444 and the second portion 446 of the radiation, and the second time can coincide with another of the multiple determinations of the intensity of the first portion 444 and the second portion 446 of the radiation. For example, a desired time delay Δt can be applied to one of the first and second intensities before being combined with the other of the first and second intensities, the time delay Δt being an integer multiple of the time interval between the start of one determination and the start of a subsequent determination.

[0177] Alternatively, in some other embodiments, at least one of the first time t1 or the second time t2 is between two of the multiple determinations of the intensity of the first portion 444 and the second portion 446 of the radiation, and the controller 470 is capable of operating to interpolate between the two of the multiple determinations of the intensity of the first portion 444 and the second portion 446 of the radiation.

[0178] For example, a desired time delay may be applied to one of the first and second intensities before being combined with the other of the first and second intensities, the time delay being a non-integer multiple of the time interval between the start of one determination and the start of a subsequent determination. For example, it may be desirable to apply a time delay to the second intensity before being combined with the first intensity, the time delay being a non-integer multiple of the time interval between the start of one determination and the start of a subsequent determination. For such an embodiment, the second intensity used to determine the altitude may be an intensity obtained by interpolating (e.g., linearly) between two determinations with a time delay between the two. Such interpolation may typically be as described above with reference to Figure 8 Descriptive.

[0179] In some embodiments, apparatus 400 may further include a radiation source 490 operable to generate radiation beam 422 .

[0180] Some embodiments of the present disclosure relate to a lithography apparatus comprising: Figure 10 An apparatus 400 of the type shown. A lithographic apparatus may be 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 on a substrate W supported by the substrate table WT.

[0181] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0182] Although specific reference may be made herein to embodiments of the present invention in the context of a lithographic apparatus, embodiments of the present invention may be used in other apparatuses. Embodiments of the present invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other pattern forming device). These apparatuses may generally be referred to as lithographic tools. Such lithographic tools may utilize vacuum conditions or ambient (non-vacuum) conditions.

[0183] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0184] Where the context permits, 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 others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are merely for convenience, and that such actions are in fact caused by a computing device, processor, controller, or other device executing the firmware, software, routines, instructions, etc., and in doing so, may cause an actuator or other device to interact with the physical world.

[0185] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in other ways than those described. The above description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below. Other aspects of the invention are set forth in the following numbered clauses:

[0186] 1. A method for measuring the surface topography of an object, the method comprising:

[0187] forming a first image of a pattern in a beam spot area using a radiation beam;

[0188] Moving the object relative to the beam spot area;

[0189] receiving a portion of a 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 a first image is spatially separated from the second portion of the radiation corresponding to a second portion of the first image;

[0190] determining an intensity of the first portion and the second portion of the radiation; and

[0191] By combining the intensity of the first portion of the radiation determined at a first time and the intensity of the second portion of the radiation determined at a second time, the height of the object is determined.

[0192] 2. A method according to clause 1, wherein the first time and the second time are selected based on the movement of the object relative to the beam spot area so that the first part and the second part of the combined radiation correspond to radiation reflected from substantially the same part of the object.

[0193] 3. A method according to item 1 or item 2, wherein the time difference between the first time and the second time is obtained by dividing the spatial offset between the first part and the second part of the first image in the direction in which the object moves relative to the beam spot area by the speed at which the object moves relative to the beam spot area.

[0194] 4. A method according to any preceding clause, wherein the pattern comprises at least one feature, and wherein the time difference between the first time and the second time is given by half the extent of the or each feature in the direction of movement of the object relative to the beam spot area, divided by the speed at which the object is moving relative to the beam spot area.

[0195] 5. A method according to any preceding clause, wherein separating the reflected radiation into a first portion and a second portion comprises forming a second image of the pattern on a spectroscopic optical device, and directing radiation from the first portion and the second portion of the second image using the spectroscopic optical device so as to be spatially separated.

[0196] 6. A method according to any preceding clause, wherein the intensities of the first and second parts of the radiation are determined a plurality of times at a sampling frequency.

[0197] 7. A method according to clause 6, wherein the first time coincides with one of the multiple determinations of the intensity of the first and second parts of the radiation, and the second time coincides with another of the multiple determinations of the intensity of the first and second parts of the radiation.

[0198] 8. A method according to item 6, wherein at least one of the first time or the second time is between two determinations of multiple determinations of the intensity of the first part and the second part of the radiation, and wherein the method includes interpolating between the two determinations of the multiple determinations of the intensity of the first part and the second part of the radiation.

[0199] 9. A method according to any preceding clause, wherein the height of the object is proportional to the difference between the first intensity and the second intensity.

[0200] 10. A photolithography exposure method, comprising:

[0201] Measuring the surface topography of a substrate using any of the preceding methods;

[0202] patterning the radiation beam using a patterning device; and

[0203] projecting patterned radiation onto the substrate to form an image of the patterning device on the substrate;

[0204] The position of the substrate as patterned radiation is projected onto the substrate is controlled based on the measured surface topography of the substrate.

[0205] 11. A lithographic exposure method according to item 10, wherein the lithographic exposure is a scanning exposure, so that patterning the radiation beam using a pattern forming device includes moving the pattern forming device through the radiation beam, and projecting the patterned radiation onto the substrate so as to form an image of the pattern forming device on the substrate includes moving the substrate so that the image of the pattern forming device is generally stationary relative to the substrate.

[0206] 12. A device for measuring the surface topography of an object, the device comprising:

[0207] Supports, used to support objects;

[0208] projection optics operable to form a first image of the pattern on a beam spot area with the radiation beam;

[0209] a moving mechanism operable to move the support so as to move an object supported by the support through the beam spot area;

[0210] 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;

[0211] a first detector arranged to determine an intensity of a first portion of the radiation;

[0212] a second detector arranged to determine the intensity of the second portion of the radiation; and

[0213] A controller is operable to determine a height of the object by combining an intensity of the first portion of the radiation determined at a first time and an intensity of the second portion of the radiation determined at a second time.

[0214] 13. Apparatus according to clause 12, wherein the controller is operable to carry out the method of any one of clauses 1 to 9.

[0215] 14. Apparatus according to clause 12 or clause 13, wherein the projection optics comprises:

[0216] a projection patterning device; and

[0217] A first imaging optics is arranged to form an image of the projected patterning device on the beam spot area.

[0218] 15. Apparatus according to any one of clauses 12 to 14, wherein the detection optics comprises:

[0219] Splitting optics arranged to split the reflected radiation into a first portion and a second portion; and

[0220] Second imaging optics are arranged to receive radiation reflected from an object supported by the support and to form a second image of the pattern on the beam-splitting optics.

[0221] 16. Apparatus according to any of clauses 12 to 15, wherein the first time and the second time are such that the first and second parts of the combined radiation correspond to radiation reflected from substantially the same part of the object.

[0222] 17. The device according to any one of items 12 to 16 also includes a time-shift module, which is arranged to apply a time shift to the first signal indicating the first intensity from the first detector and / or the second signal indicating the second intensity from the second detector, so that the first signal indicates the intensity of the first part of the radiation determined at the first time, and the second signal indicates the intensity of the second part of the radiation determined at the second time.

[0223] 18. An apparatus according to item 17, wherein the time shift applied to one of the first signal and the second signal is obtained by dividing the spatial offset between the first part and the second part of the first image in the direction of movement of the object relative to the beam spot area by the speed of movement of the object relative to the beam spot area.

[0224] 19. An apparatus according to clause 17 or clause 18, wherein the pattern comprises at least one feature and wherein the time shift applied to one of the first signal and the second signal is given by half the extent of the feature or each feature in the direction of movement of the object relative to the beam spot area divided by the speed at which the object moves relative to the beam spot area.

[0225] 20. Apparatus according to any of clauses 12 to 19, wherein the first detector and the second detector are arranged to determine the intensity of the first portion and the second portion, respectively, a plurality of times at a sampling frequency.

[0226] 21. Apparatus according to clause 20, wherein the first time coincides with one of the plurality of determinations of the intensity of the first and second portions of the radiation, and the second time coincides with another of the plurality of determinations of the intensity of the first and second portions of the radiation.

[0227] 22. An apparatus according to item 20, wherein at least one of the first time or the second time is between two determinations of multiple determinations of the intensity of the first part and the second part of the radiation, and wherein the controller is capable of operating to interpolate between the two determinations of the multiple determinations of the intensity of the first part and the second part of the radiation.

[0228] 23. Apparatus according to any of clauses 11 to 22, further comprising a radiation source operable to generate a radiation beam.

[0229] 24. A lithographic apparatus comprising the apparatus of any one of clauses 11 to 23.

Claims

1. A method for measuring the topography of a surface of an object, the method comprising: forming a first image of a pattern in 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 a 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 intensities of the first portion and the second portion of the radiation; as well as By combining the intensity of the first portion of the radiation determined at a first time and the intensity of the second portion of the radiation determined at a second time, a height of the object is determined.

2. The method of claim 1 , wherein the first time and the second time are selected based on the movement of the object relative to the beam spot area so that the first portion and the second portion of the combined radiation correspond to radiation reflected from substantially the same portion of the object.

3. The method according to claim 1 or claim 2, wherein the time difference between the first time and the second time is obtained by: dividing the spatial offset between the first part and the second part of the first image in the direction in which the object moves relative to the beam spot area by the speed at which the object moves relative to the beam spot area.

4. A method according to any preceding claim, wherein the pattern comprises at least one feature, and wherein the time difference between the first time and the second time is given by: half the extent of the or each feature in the direction of movement of the object relative to the beam spot area divided by the speed at which the object is moving relative to the beam spot area.

5. The method of any preceding claim, wherein dividing the reflected radiation into a first portion and a second portion comprises: A second image of the pattern is formed on a beam splitting optic, and radiation from the first and second portions of the second image is directed so as to be spatially separated using the beam splitting optic.

6. A method according to any preceding claim, wherein the intensities of the first and second parts of the radiation are determined a plurality of times at a sampling frequency.

7. A method according to claim 6, wherein the first time coincides with one of the multiple determinations of the intensity of the first part and the second part of the radiation, and the second time coincides with another of the multiple determinations of the intensity of the first part and the second part of the radiation.

8. A method according to claim 6, wherein at least one of the first time or the second time is between two of the multiple determinations of the intensity of the first and second portions of the radiation, and wherein the method includes interpolating between the two of the multiple determinations of the intensity of the first and second portions of the radiation.

9. A device for measuring the topography of a surface of an object, the device comprising: Supports, used to support objects; projection optics operable to form a first image of the pattern on a beam spot area with the radiation beam; a moving mechanism operable to move the support member so as to move the object supported by the support member through 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 a 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; a first detector arranged to determine an intensity of said first portion of said radiation; a second detector arranged to determine an intensity of said second portion of said radiation; as well as A controller is operable to determine a height of the object by combining the intensity of the first portion of the radiation determined at a first time and the intensity of the second portion of the radiation determined at a second time.

10. The apparatus of claim 9, wherein the projection optics comprises: Projection patterning device; as well as A first imaging optics device is arranged to form an image of the projection patterning device on the beam spot area.

11. The apparatus according to claim 9 or 10, wherein the detection optical device comprises: a beam splitting optic arranged to split the reflected radiation into a first portion and a second portion; as well as Second imaging optics are arranged to receive radiation reflected from an object supported by the support and to form a second image of the pattern on the beam-splitting optics.

12. An apparatus according to any one of claims 9 to 11, wherein the first time and the second time are such that the first and second parts of the combined radiation correspond to radiation reflected from substantially the same part of the object.

13. The device according to any one of claims 9 to 12 further includes a time-shift module, arranged to apply a time shift to the first signal indicating the first intensity from the first detector and / or the second signal indicating the second intensity from the second detector, so that the first signal indicates the intensity of the first part of the radiation determined at a first time, and the second signal indicates the intensity of the second part of the radiation determined at a second time.

14. The apparatus of claim 13 , wherein the time shift applied to one of the first signal and the second signal is derived by dividing the spatial offset between the first portion and the second portion of the first image in the direction in which the object moves relative to the beam spot area by the speed at which the object moves relative to the beam spot area.

15. The apparatus according to any one of claims 9 to 14, wherein the first detector and the second detector are arranged to determine the intensity of the first portion and the second portion, respectively, a plurality of times at a sampling frequency.

Images