Measuring height or distange using chirped pulses of light

Abstract

An apparatus for measuring properties of an object comprises: an illumination system; a detector; and a processor. The illumination system is configured to emit pulsed radiation and direct the pulsed radiation to a beam spot region. The pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with temporal position in the pulse. There may be a fixed relationship between the phase of each wavelength component from different pulses (for example the pulses are formed by a mode-locked laser). The detector is arranged to receive a scattered portion of the pulsed radiation scattered from an object disposed in the beam spot region. The processor is operable to determine at least one beat frequency of radiation received by the detector and to determine one or more properties of the object (e.g. a thickness of a layer of the object) from said at least one beat frequency.

Description

[0001] METHOD AND APPARATUS FOR MEASURING PROPERTIES OF AN OBJECT

[0002] CROSS-REFERENCE TO RELATED APPLICATION

[0003]

[0001] The application claims priority of EP application 24199318.7 which was filed on 09 September, 2024; and which is incorporated herein in its entirety by reference.

[0004] FIELD

[0005]

[0002] The present invention relates to an apparatus for measuring one or more properties of an object. The present invention also relates to a corresponding method for measuring one or more properties of an object. The present invention has particular application in the field of lithography. The object may be a substrate within a lithographic apparatus. Such a substrate may comprise a silicon wafer coated with a photoresist. The present invention also relates to an exposure apparatus (for example a lithographic apparatus) which comprises the apparatus for measuring one or more properties of an object. The present invention also relates to a metrology apparatus which comprises the apparatus for measuring one or more properties of an object.

[0006] BACKGROUND

[0007]

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

[0008]

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

[0009]

[0005] Before exposure of a wafer to patterned radiation in a lithographic apparatus, a topography of the wafer may be determined using apparatus that may be referred to as a level sensor. This measurement of the topography of the wafer may be performed within the lithographic apparatus, for example once the wafer has been clamped to a wafer stage. This information can be used during subsequent exposure of the wafer in order to keep the part of the wafer that is being exposed in a plane of best focus.

[0010]

[0006] It may be desirable to provide new methods and / or apparatus for determining one or more properties of an object (for example a wafer) that may at least partially address one or more problems associated with existing arrangements, whether identified herein or otherwise.

[0011] SUMMARY

[0012]

[0007] According to a first aspect of the present disclosure there is provided an apparatus for measuring one or more properties of an object, the apparatus comprising: an illumination system configured to emit pulsed radiation and direct the pulsed radiation to a beam spot region, wherein the pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with position in the pulse and wherein there is a fixed relationship between the phase of each wavelength component from different pulses; a detector arranged to receive a scattered portion of the pulsed radiation scattered from an object disposed in the beam spot region; and a processor operable to determine at least one beat frequency of radiation received by the detector and to determine one or more properties of the object from said at least one beat frequency.

[0013]

[0008] The apparatus may form part of a lithographic apparatus. The object may be a substrate. The substrate may be a substrate within a lithographic apparatus. Such a substrate may comprise a silicon wafer coated with a photoresist. The lithographic apparatus may be used to expose the substrate to radiation that has been patterned by a reticle or mask. Before the substrate is exposed to the patterned radiation, a shape of the surface of the substrate may be determined. For example a height map of the surface of the substrate may be determined, for example using a level sensor. Subsequently, the measured shape of the surface of the substrate can be used to control a height of the substrate while it is being exposed to the patterned radiation, for example to keep the substrate in a plane of best focus for the image of the patterning device.

[0014]

[0009] One type of known level sensor uses an optical triangulation technique using a broadband radiation source at large angle of incidence. In general, such known level sensors are used to: project a radiation beam onto an object; receive a portion of the radiation beam reflected from the object; and determine the first measurement of the height therefrom. When radiation is incident upon the surface of a silicon wafer, which typically comprises a multilayered stack, a portion of the radiation penetrates into the layers beneath the resist layer. This penetration causes multiple back reflections, resulting in an imbalance in the signal readout for such a level sensor. This effect, the difference between a measured height and a real height due to interference effects of underlying layers, may be referred to as “Apparent Surface Depression” (ASD) or “Height Process Dependency” (HPD).

[0015]

[0010] As used herein, when it is stated that a wavelength of each pulse varies with position in the pulse this may mean temporal position within the pulse. [OH] The apparatus according to the first aspect is advantageous as it allows for a more accurate measurement of height of an object that comprises multiple layers relative to existing level sensors, as now discussed.

[0016]

[0012] The pulsed radiation may be considered to comprise a train of pulses (with specific properties). When the pulsed radiation is incident on an object with structure (for example a multilayered stack) there may be multiple reflections (for example, from each layer). Such multiple reflections will result in a plurality of pulse trains each being time-shifted relative to the others. For example, consider reflection from an object having an upper layer with a thickness of 1 nm. The reflected radiation will comprise two copies of the pulse train that are shifted relative to each other by a time delay of the order of 3 attoseconds (3x1 O'18seconds). Since the pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with position in the pulse, there will be a difference in the wavelengths of the parts of the two copies of the pulse trains that spatially overlap with each other. For example, each portion of the first copy (which was, for example, reflected from the upper surface of the layer) with have a different wavelength to a portion of the second copy (which was, for example, reflected from the lower surface of the layer) that it spatially overlaps with. As a result the two copies of the pulses will interfere and will form a beat frequency (which will be proportional to the time shift between the two copies and a magnitude of the chirp). The beat frequencies in the reflected radiation are related to the thicknesses of the layers in the object. Furthermore, because there is a fixed relationship between the phase of each wavelength component from different pulses, this beat frequency can be observed over a plurality of pulses. Therefore, when the detector receives a scattered portion of the pulsed radiation scattered from an object disposed in the beam spot region, it is effectively stroboscopic sampling the beat frequency using the pulses of the scattered radiation.

[0017]

[0013] Advantageously, the apparatus according to the first aspect allows very small thicknesses (of the order of 1 nanometer) to be measured. Furthermore, the apparatus according to the first aspect can be used to quantify “Apparent Surface Depression” (ASD) or “Height Process Dependency” (HPD) effects and / or can be used to make an accurate height measurement of an object.

[0018]

[0014] When the pulsed radiation is incident on an object it will form a beam spot, which is the intersection of the pulsed radiation beam and the surface of the object. Therefore, in practice the beam spot is partially defined by the object. However, it should be appreciated that the object (which in practice may be a wafer) does not form part of the apparatus. As used herein the term beam spot region is intended to mean a region within the apparatus that the illumination system is configured to direct the pulsed radiation to; and which an object can be disposed in.

[0019]

[0015] According to a second aspect of the present disclosure there is provided an apparatus for measuring one or more properties of an object, the apparatus comprising: an illumination system configured to emit pulsed radiation and direct the pulsed radiation to a beam spot region, wherein the pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with position in the pulse; a detector arranged to receive a scattered portion of the pulsed radiation scattered from an object disposed in the beam spot region; and a processor operable to determine at least one beat frequency of radiation received by the detector and to determine a thickness of a layer of an object from at least one beat frequency of radiation received by the detector.

[0020]

[0016] Advantageously, the apparatus according to the second aspect allows for the thicknesses of layers in an object to be measured. Furthermore, the apparatus according to the second aspect can be used to quantify “Apparent Surface Depression” (ASD) or “Height Process Dependency” (HPD) effects and / or can be used to make an accurate height measurement of an object.

[0021]

[0017] The chirp of the pulsed radiation can be produced in any convenient manner.

[0022]

[0018] Some optional features of the apparatus according to the first aspect of the present disclosure and / or the apparatus according to the second aspect of the present disclosure are now discussed.

[0023]

[0019] The illumination system may comprise: a radiation source operable to produce initial pulsed radiation; and a dispersive medium configured to receive the initial pulsed radiation and to output the pulsed radiation. A duration of the pulses of the pulsed radiation may be greater than that of the initial pulsed radiation.

[0024]

[0020] Within the dispersive medium, different wavelength components propagate at different speeds. Therefore, the pulses are broadened in the time domain such that there is a correlation between wavelength and time within the individual pulses of the pulsed radiation. In other words, a wavelength at a front of each pulse of the pulsed radiation (i.e. the part of the pulse which exits the dispersive medium first) has a different frequency to the back of the pulsed radiation (i.e. the part of the pulse which exits the dispersive medium last). For example, for a normally dispersive medium a wavelength at a front of each pulse of the pulsed radiation may be greater than a wavelength at the back of each pulse of the pulsed radiation. The wavelength may vary generally linearly with position in each pulse. The pulsed radiation may be said to have a chirp.

[0025]

[0021] The radiation source may be operable to produce initial pulsed radiation having pulse lengths of 1000 femtoseconds or less.

[0026]

[0022] Such a radiation source may be referred to as a femtosecond laser. The radiation source may comprise any type of a short pulsed light source.

[0027]

[0023] The illumination system may comprise a mode-locked laser.

[0028]

[0024] Advantageously, such a mode-locked laser ensures that there is a fixed relationship between the phase of each wavelength component from different pulses. The mode -locked laser may, for example, produce the initial pulsed radiation.

[0029]

[0025] The dispersive medium may comprise an optical fiber.

[0030]

[0026] For example, the dispersive medium may comprise a single mode fiber. Advantageously, such an arrangement allows the amount of temporal broadening of the initial pulsed radiation to be easily selected by selecting a length of the optical fiber. Alternatively, the dispersive medium may comprise a crystal or the like.

[0027] The dispersive medium may be configured to increase a duration of the pulses by a factor of 10 or more.

[0031]

[0028] The apparatus may comprise projection optics operable to direct the pulsed radiation to the beam spot region. The apparatus may comprise detection optics operable to receive a scattered portion of the pulsed radiation from an object disposed in the beam spot region and to direct it to the detector.

[0032]

[0029] The processor may be operable to determine a plurality of beat frequencies of radiation received by the detector and to determine one or more properties of the object from said plurality of beat frequencies.

[0033]

[0030] The processor may be operable to determine a distance from at least one beat frequency of radiation received by the detector.

[0034]

[0031] The processor may be operable to determine a thickness of a layer of an object from at least one beat frequency of radiation received by the detector.

[0035]

[0032] In some embodiments, the apparatus may be referred to as a level sensor.

[0036]

[0033] The processor may be operable to determine a height of a surface of an object from at least one beat frequency of radiation received by the detector.

[0037]

[0034] The apparatus may further comprise: a reference surface; and a first beam splitter configured to direct a first part of the pulsed radiation to the beam spot region and a second part of the pulsed radiation to the reference surface; and the apparatus may be configured to combine a scattered portion of the first part of the pulsed radiation scattered from an object disposed in the beam spot region and a scattered portion of the second part of the pulsed radiation scattered from the reference surface so as to create a combined optical signal that is received by the detector.

[0038]

[0035] In use, a surface to be measured (for example a lithographic substrate or wafer) may be disposed in the beam spot region. Advantageously, this allows for a difference in an optical path length of the first and second parts to be determined. This may allow the height of an object disposed in the beam spot region to be measured or mapped, relative to the reference surface.

[0039]

[0036] In some embodiments, the apparatus may comprise a second beam splitter that is configured to combine a scattered portion of the first part of the pulsed radiation scattered from an object disposed in the beam spot region and a scattered portion of the second part of the pulsed radiation scattered from the reference surface so as to create a combined optical signal that is received by the detector. Alternatively, the scattered portion of the first part of the pulsed radiation scattered from an object disposed in the beam spot region and the scattered portion of the second part of the pulsed radiation scattered from the reference surface may be combined on the detector.

[0040]

[0037] The processor may be configured to determine at least one beat frequency of the combined optical signal and to determine a height from each of the at least one beat frequencies.

[0041]

[0038] The reference surface may be movable.

[0042]

[0039] For example, the reference surface may be movable relative to a support for supporting an object so that an optical path length for the second part of the pulsed radiation can be varied. Advantageously, this may increase a dynamic range of the apparatus. For example, it may allow for a greater range of height variations to be measured. Furthermore, it may allow for a beat frequency that relates to a height of the object to be changed so as to better distinguish it from one or more beat frequencies that are related to thicknesses of layers of the object.

[0043]

[0040] The detector should have sufficient temporal resolution to resolve typical beat frequencies that are generated by (a) one or more layer thicknesses; and / or (b) a height of an object relative to a reference surface. The beat frequencies may, for example, be of the order of MHz.

[0044]

[0041] The detector may have sufficient temporal resolution to resolve the pulses of the pulsed radiation.

[0045]

[0042] A repetition rate of the pulsed radiation produced by the illumination system should be sufficiently high to resolve typical beat frequencies that are generated by (a) one or more layer thicknesses; and / or (b) a height of an object relative to a reference surface.

[0046]

[0043] A repetition rate of the pulsed radiation produced by the illumination system may be greater than 10 MHz.

[0047]

[0044] The apparatus may further comprise a support for supporting an object such that it is positionable in a beam spot region.

[0048]

[0045] The support for supporting a substrate may comprise a substrate holder operable to secure the substrate. For example, the support may comprise a clamp for clamping the substrate to the support.

[0046] The apparatus may further comprise a movement mechanism operable to cause relative movement of the support and the beam spot region.

[0049]

[0047] This may allow the object or substrate to be stepped or scanned through the beam spot region.

[0050]

[0048] According to a third aspect of the present disclosure there is provided an exposure apparatus comprising the apparatus according to the first aspect of the present disclosure and / or the second aspect of the present disclosure.

[0051]

[0049] The exposure apparatus may comprise a lithographic apparatus.

[0052]

[0050] According to a fourth aspect of the present disclosure there is provided a metrology apparatus comprising the apparatus according to the first aspect of the present disclosure and / or the second aspect of the present disclosure.

[0053]

[0051] According to a fifth aspect of the present disclosure there is provided a method of measuring one or more properties of an object, the method comprising: directing pulsed radiation so as to be incident on the object, wherein the pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with position in the pulse and wherein there is a fixed relationship between the phase of each wavelength component from different pulses; receiving a scattered portion of the pulsed radiation scattered from the object; determining at least one beat frequency of the received radiation; and determining one or more properties of the object from said at least one beat frequency.

[0052] The method according to the fifth aspect of the present disclosure may be carried out using the apparatus according to the first aspect of the present disclosure. Directing the pulsed radiation so as to be incident on the object may comprise: positioning the object in a beam spot region and directing the pulsed radiation to the beam spot region.

[0054]

[0053] According to a sixth aspect of the present disclosure there is provided a method for measuring one or more properties of an object, the method comprising: directing pulsed radiation so as to be incident on the object, wherein the pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with position in the pulse; receiving a scattered portion of the pulsed radiation scattered from the object; determining at least one beat frequency of the received radiation; and determining a thickness of a layer of the object from at least one beat frequency of the received radiation.

[0054] The method according to the sixth aspect of the present disclosure may be carried out using the apparatus according to the second aspect of the present disclosure. Directing the pulsed radiation so as to be incident on the object may comprise: positioning the object in a beam spot region and directing the pulsed radiation to the beam spot region.

[0055]

[0055] Some optional features of the method according to the fifth aspect of the present disclosure and / or the method according to the sixth aspect of the present disclosure are now discussed.

[0056]

[0056] The method may further comprise producing the pulsed radiation.

[0057]

[0057] The chirp of the pulsed radiation can be produced in any convenient manner.

[0058]

[0058] Producing the pulsed radiation may comprise: producing initial pulsed radiation; and directing the initial pulsed radiation through a dispersive medium so as to produce the pulsed radiation. A duration of the pulses of the pulsed radiation may be greater than that of the initial pulsed radiation.

[0059] The initial pulsed radiation may have pulse lengths of 1000 femtoseconds or less.

[0059]

[0060] The pulsed radiation may be at least partially produced using a mode-locked laser.

[0060]

[0061] Advantageously, such a mode-locked laser ensures that there is a fixed relationship between the phase of each wavelength component from different pulses. The mode -locked laser may, for example, produce the initial pulsed radiation.

[0061]

[0062] The method may comprise determining a plurality of beat frequencies of received radiation and determining one or more properties of the object from said plurality of beat frequencies.

[0062]

[0063] The method may comprise determining a distance from at least one beat frequency of the received radiation.

[0063]

[0064] The method according to the fifth aspect of the present disclosure may comprising determining a thickness of a layer of an object from at least one beat frequency of the received radiation.

[0065] The method may comprise determining a height of a surface of an object from at least one beat frequency of the received radiation.

[0064]

[0066] The method may further comprise: splitting the pulsed radiation into a first part that is directed to the object and a second part; directing the second part of the pulsed radiation to a reference surface; and combining a scattered portion of the first part of the pulsed radiation scattered from the object and a scattered portion of the second part of the pulsed radiation scattered from the reference surface so as to create a combined optical signal as the received radiation.

[0065]

[0067] The method may comprise: determining at least one beat frequency of the combined optical signal; and determining a height from each of the at least one beat frequencies.

[0066]

[0068] The method may comprise moving the reference surface.

[0067]

[0069] A repetition rate of the pulsed radiation produced by the illumination system may be greater than 10 MHz.

[0068] BRIEF DESCRIPTION OF THE DRAWINGS

[0069]

[0070] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0070] Figure 1 depicts a schematic overview of a lithographic apparatus;

[0071] Figure 2 is a schematic illustration of a level or height sensor which may form part of the lithographic apparatus shown in Figure 1;

[0072] Figure 3 schematically illustrates the origin of a type of height measurement error (which may be may be referred to as “Apparent Surface Depression” (ASD) error or “Height Process Dependency” (HPD) error) that affects some level sensors and arises due to multiple reflections from a wafer which comprises a multilayered stack;

[0073] Figure 4 schematically illustrates a new type of apparatus for measuring one or more properties of an object according to an embodiment of the present disclosure;

[0074] Figure 5 schematically illustrates how, with the new apparatus shown in Figure 4, a pulsed radiation beam comprising a train of pulses that is incident on an object with structure (for example a multilayered stack) results in multiple reflections, each comprising a pulse train with each of the multiple pulse trains being time-shifted relative to the others;

[0075] Figure 6 schematically illustrates an illumination system that may form part of the new apparatus shown in Figure 4, and which comprises: a radiation source and a dispersive medium 160;

[0076] Figure 7 schematically shows another new apparatus according to an embodiment of the present disclosure that may function as and / or be referred to as a level sensor, the new apparatus shown in Figure 7 may comprise new apparatus shown in Figure 5 and may be implemented as a Mach-Zander interferometer;

[0077] Figure 8 schematically illustrates a new type of method for measuring one or more properties of an object according to an embodiment of the present disclosure; and

[0078] Figure 9 schematically shows an embodiment of the new method shown in Figure 8 with some additional optional steps, the optional steps being shown in dotted lines in Figure 9. DETAILED DESCRIPTION

[0079]

[0071] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 186 nm) and EUV (extreme ultra-violet) radiation (e.g. having a wavelength in the range of about 5-100 nm, e.g. having a wavelength within a range of 4-20 nm, for example 6.7 nm or 13.5 nm).

[0072] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0080]

[0073] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0081]

[0074] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0082]

[0075] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0083]

[0076] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0084]

[0077] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W, on the other substrate support WT, is being used for exposing a pattern on the other substrate W.

[0085]

[0078] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0086]

[0079] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions C. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0087]

[0080] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., an x-axis, a y-axis and a z-axis. Each of the three axes is orthogonal to the other two axes. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry-rotation. A rotation around the z-axis is referred to as an Rz-rotation. The x- axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0081] A topography measurement system, level sensor or height sensor, and which may be integrated in the lithographic apparatus, is arranged to measure a topography of a top surface of a substrate (or wafer). A map of the topography of the substrate, also referred to as height map, may be generated from these measurements indicating a height of the substrate as a function of the position on the substrate. This height map may subsequently be used to correct the position of the substrate during transfer of the pattern on the substrate, in order to provide an aerial image of the patterning device in focus on the substrate. It will be understood that “height” in this context refers to a dimension broadly out of the plane to the substrate (also referred to as Z-axis) . Typically, the level or height sensor performs measurements at a fixed location (relative to its own optical system) and a relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.

[0088]

[0082] An example of a level or height sensor LS as known in the art is schematically shown in Figure 2, which illustrates only the principles of operation. In this example, the level sensor LS comprises an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a beam of radiation LSB which is imparted with a pattern by a projection grating PGR of the projection unit LSP. The projection grating PGR may alternatively be referred to as a patterning device PGR. The radiation source LSO may be, for example, a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO may include a plurality of radiation sources having different colors, or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not restricted to visible radiation, but may additionally or alternatively encompass UV and / or IR radiation and any range of wavelengths suitable to reflect from a surface of a substrate.

[0089]

[0083] The projection grating PGR is a periodic grating comprising a periodic structure resulting in a beam of radiation BE1 having a periodically varying intensity. The beam of radiation BE1 with the periodically varying intensity is directed towards a measurement location MLO on a substrate W having an angle of incidence ANG with respect to an axis perpendicular (Z-axis) to the incident substrate surface between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. The measurement location MLO may alternatively be referred to as the beam spot region MLO. At the measurement location MLO, the patterned beam of radiation BE1 is reflected by the substrate W (indicated by arrows BE2) and directed towards the detection unit LSD.

[0090]

[0084] In order to determine the height level at the measurement location 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 produces a detector output signal indicative of the light received, for example indicative of the intensity of the light received, such as may be output by a photodetector, or representative of a spatial distribution of the intensity received, such as may be output by a camera or sensor array. The detector DET may comprise any combination of one or more detector types.

[0091]

[0085] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal strength as measured by the detector DET, the signal strength having a periodicity that depends, amongst others, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0092]

[0086] The proj ection unit LSP and / or the detection unit LSD may include further optical elements, such as lenses and / or mirrors, along the path of the patterned beam of radiation between the projection grating PGR and the detection grating DGR (not shown).

[0093]

[0087] In an embodiment, the detection grating DGR may be omitted, and the detector DET may be placed at the position where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0094]

[0088] In order to cover the surface of the substrate W effectively, a level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.

[0095]

[0089] Various height sensors of a general type are disclosed for example in US7265364 and US7646471, both incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in US2010233600A1, incorporated by reference. In W02016102127A1, incorporated by reference, a compact height sensor is described which uses a multi -element detector to detect and recognize the position of a grating image, without needing a detection grating.

[0096]

[0090] In general, the detection unit LSD may be arranged such that the reflected radiation BE2 is split into first and second portions and the height of the substrate W is determined by combining the intensities of the first and second portions. For example, the height may be determined as a differential measurement. Advantageously, with such an arrangement, the determination of the height of the substrate W can be substantially independent of the intensity of the radiation beam BEE In practice, the splitting of the radiation into first and second portions may be achieved in a number of different ways.

[0097]

[0091] For example, in some known arrangements, a combination of a polarizer and a shear plate (for example in the form of a Wollaston prism) are used to form two laterally shifted images of the projection grating PGR (each having a different polarization state) on a detection grating DGR. An example of such an arrangement is shown schematically in Figure 5 of US2010233600A1. For example, the projection grating PGR may have a pitch P and a duty cycle of 50% such that the beam of radiation BE1 having a periodically varying intensity comprises a plurality of lines having a thickness of P / 2, adjacent lines being separated by P / 2. The polarizer and a shear plate are arranged to form two images of the projection grating PGR (each having a different polarization state) on the detection grating DGR, one image being laterally shifted relative to the other by P / 2. Downstream of the detection grating DGR the two separate polarization states are each directed to a different detector. The height of the substrate W is determined as being proportional to the difference in the intensities of the two separate polarization states.

[0098]

[0092] In some other known arrangements, rather than splitting the reflected radiation BE2 using two images of the projection grating PGR but having different polarization states, a single image of the projection grating PGR is formed on splitting optics that is arranged to split that single image into first and second portions. Examples of such arrangements are shown schematically in Figure 6 of US2010233600A1 and Figure 2 of W02016102127A1. For example, such arrangements generally comprise splitting optics that is arranged to split the reflected radiation into first and second portions. The splitting optics may 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 splitting optics may be considered to comprise a plurality of prisms and the image of each line of the projection grating PGR may be imaged onto one of the plurality of generally triangular prisms such that a first portion of the line is incident in a first surface of the prism and a second portion of the line is incident in a second surface of the prism. The first portion of the line is directed to the first detector and the second portion of the line is directed to the second detector. As the line moves relative to the prism (as a result of a change in height of the substrate W), the amount of radiation directed to each of the detectors changes.

[0093] One type of level sensor (see, for example, Figure 2 as described above) can be used to measure the height profile of the surface of a substrate W by: projecting a patterned radiation beam BE1 onto a beam spot region MLO; moving the substrate W relative to the beam spot region MLO; receiving a portion of the patterned radiation beam BE2 reflected from the object W and determining the first measurement of the height therefrom. As the height of the substrate W varies, the position of the pattern of the reflected radiation may vary, for example relative to splitting optics arranged to split the reflected radiation into first and second portions. As shown in Figure 3, when radiation is incident upon the surface of a silicon wafer W, which typically comprises a multilayered stack, a portion of the radiation BE1 penetrates into the layers beneath the resist layer. This penetration causes multiple back reflections, resulting in an imbalance in the signal readout for such a level sensor. As a result, the measured height, hm, differs from the actual height, ha, of the wafer W by an amount A. This amount, A, which is the difference between a measured height and a real height due to interference effects of underlying layers, may be referred to as “Apparent Surface Depression” (ASD) error or “Height Process Dependency” (HPD) error.

[0099]

[0094] Some embodiments of the present disclosure relate to a new type of apparatus for measuring one or more properties of an object. Such a new apparatus 100 is now described with reference to Figure 4.

[0100]

[0095] The new apparatus 100 for measuring one or more properties of an object comprises: an illumination system 110; a detector 120; and a processor 130.

[0101]

[0096] The illumination system 110 is configured to emit pulsed radiation 112 and direct the pulsed radiation 112 to a beam spot region 140. The pulses of the pulsed radiation 112 are chirped such that a wavelength of each pulse varies with position in the pulse. As used herein, when it is stated that a wavelength of each pulse varies with position in the pulse this may mean temporal position within the pulse.

[0102]

[0097] When the pulsed radiation 112 is incident on an object W it will form a beam spot, which is the intersection of the pulsed radiation beam 112 and the surface of the object W. Therefore, in practice the beam spot is partially defined by the object W. However, it should be appreciated that the object W (which in practice may be a wafer) does not form part of the apparatus 100. As used herein the term beam spot region 140 is intended to mean a region within the apparatus 100 that the illumination system 110 is configured to direct the pulsed radiation 112 to; and which an object W can be disposed in.

[0103]

[0098] The detector 120 is arranged to receive a scattered portion 122 of the pulsed radiation 112 scattered from an object W disposed in the beam spot region 140.

[0104]

[0099] The processor 130 is operable to determine at least one beat frequency of radiation 122 received by the detector 120 and to determine one or more properties of the object W from said at least one beat frequency.

[0105]

[0100] The detector 120 may comprise any hardware (for example any combination of optical and other elements) that is operable to determine one or more properties of radiation 122 received thereby. For example, the detector 120 may comprise a photodetector, or an array of such photodetectors. In general, the detector 120 may comprise any hardware that is operable to measure a quantity that is indicative of a difference in frequencies of two or more different frequency components or frequency modes. As discussed further below, such different frequency modes may mix to lead to one or more beat frequencies at the sum / difference of the frequencies. For example, the detector 120 may comprise any hardware that can measure, or allow to be determined, an intensity modulation of radiation 122 received thereby and / or a difference in frequencies of two or more frequency components of radiation 122 received thereby.

[0106]

[0101] An output signal of the detector 120 may be indicative of an intensity modulation of radiation 122 received thereby (and this intensity modulation may be dependent on a frequency difference, i.e. it may comprise a beat signal).

[0107]

[0102] The apparatus 100 may form part of a lithographic apparatus (for example of the type shown in Figure 1 and described above). The object may be a substrate W. The substrate W may be a substrate within a lithographic apparatus. Such a substrate W may comprise a silicon wafer coated with a photoresist. The lithographic apparatus LA may be used to expose the substrate W to radiation that has been patterned by a reticle or mask MA. Before the substrate W is exposed to the patterned radiation, a shape of the surface of the substrate W may be determined. For example a height map of the surface of the substrate W may be determined, for example using a level sensor. Subsequently, the measured shape of the surface of the substrate W can be used to control a height of the substrate W while it is being exposed to the patterned radiation, for example to keep the substrate W in a plane of best focus for the image of the patterning device MA.

[0108]

[0103] One type of known level sensor (as described above with reference to Figure 2) uses an optical triangulation technique using a broadband radiation source at large angle of incidence. In general, such known level sensors are used to: project a radiation beam BE1 onto an object W; receive a portion of the radiation beam BE2 reflected from the object W; and determine the first measurement of the height therefrom. As discussed above with reference to Figure 3, when radiation BE1 is incident upon the surface of a silicon wafer W, which typically comprises a multilayered stack, a portion of the radiation penetrates into the layers beneath the resist layer. This penetration causes multiple back reflections, resulting in an imbalance in the signal readout for such a level sensor. This effect, the difference between a measured height and a real height due to interference effects of underlying layers, may be referred to as “Apparent Surface Depression” (ASD) or “Height Process Dependency” (HPD).

[0109]

[0104] In some embodiments of the present disclosure, there is a fixed relationship between the phase of each wavelength component from different pulses. Such embodiments of the new apparatus 100 shown in Figure 4 are advantageous as they allow for a more accurate measurement of height of an object W that comprises multiple layers relative to existing level sensors, as now discussed.

[0110]

[0105] The pulsed radiation 112 may be considered to comprise a train of pulses (with specific properties). When the pulsed radiation 112 is incident on an object W with structure (for example a multilayered stack) there may be multiple reflections (for example, from each layer, as shown in Figure 3 and described above). As shown schematically in Figure 5, such multiple reflections will result in a plurality of pulse trains each being time-shifted relative to the others. For example, referring to Figure 5, consider reflection of a pulse of the pulsed radiation 112 from an object W having an upper layer LI with a thickness of 1 nm. The reflected radiation 122 will comprise two copies 122a, 122b of the pulse train that are shifted relative to each other by a time delay At of the order of 3 attoseconds (3xl018seconds).

[0111]

[0106] Since the pulses of the pulsed radiation 112 are chirped such that a wavelength of each pulse varies with position in the pulse, there will be a difference in the wavelengths of the parts of the two copies 122a, 122b of the pulse trains that spatially overlap with each other. For example, each portion of the first copy 122a (which was, for example, reflected from the upper surface of the layer LI) will have a different wavelength to a portion of the second copy 122b (which was, for example, reflected from the lower surface of the layer L2) that it spatially overlaps with. As a result the two copies of the pulses will interfere and will form a beat frequency (which will be proportional to the time shift At between the two copies and a magnitude of the chirp). The beat frequencies in the reflected radiation 122 are related to the thicknesses of the layers in the object W. Furthermore, because there is a fixed relationship between the phase of each wavelength component from different pulses, this beat frequency can be observed over a plurality of pulses. Therefore, when the detector 130 receives a scattered portion effectively stroboscopic sampling the beat frequency using the pulses of the scattered radiation 122.

[0112]

[0107] Advantageously, with a fixed relationship between the phase of each wavelength component from different pulses ofthe pulsed radiation 112, the apparatus 100 shown in Figure 4 allows very small thicknesses (of the order of 1 nanometer) to be measured. Furthermore, such an apparatus 100 can be used to quantify “Apparent Surface Depression” (ASD) or “Height Process Dependency” (HPD) effects and / or can be used to make an accurate height measurement of an object W.

[0113]

[0108] In some embodiments, the processor 130 is operable to determine a thickness of a layer of an object W from at least one beat frequency of radiation 122 received by the detector 120. Advantageously, such embodiments of the apparatus 100 allows for the thicknesses of layers in an object W to be measured. Furthermore, such a new apparatus 100 can be used to quantify “Apparent Surface Depression” (ASD) or “Height Process Dependency” (HPD) effects and / or can be used to make an accurate height measurement of an object.

[0114]

[0109] The chirp of the pulsed radiation 112 can be produced in any convenient manner. One possible embodiment is now described with reference to Figure 6.

[0115] [HO] As shown schematically in Figure 6, in some embodiments, the illumination system 110 may comprise: a radiation source 150 and a dispersive medium 160. The radiation source 150 is operable to produce initial pulsed radiation 152. The dispersive medium 160 is configured to receive the initial pulsed radiation 152 and to output the pulsed radiation 112. A duration Ax' of the pulses of the pulsed radiation 112 is greater than that AT of the initial pulsed radiation 152.

[0116] [Hl] Within the dispersive medium 160, different wavelength components propagate at different speeds. Therefore, the pulses 112 are broadened in the time domain such that there is a correlation between wavelength and time within the individual pulses of the pulsed radiation 112. In other words, a wavelength at a front of each pulse of the pulsed radiation 112 (i.e. the part of the pulse which exits the dispersive medium 160 first) has a different frequency to the back of the pulsed radiation (i.e. the part of the pulse which exits the dispersive medium 160 last). For example, for a normally dispersive medium 160 a wavelength at a front of each pulse of the pulsed radiation 112 may be greater than a wavelength at the back of each pulse of the pulsed radiation 112. The wavelength may vary generally linearly with position in each pulse. The pulsed radiation 112 may be said to have a chirp.

[0117]

[0112] The radiation source 150 may be operable to produce initial pulsed radiation 152 having pulse lengths AT of 1000 femtoseconds or less. Such a radiation source 150 may be referred to as a femtosecond laser. The radiation source 150 may comprise any type of a short pulsed light source.

[0118]

[0113] In some embodiments, the illumination system 110 may comprise a mode-locked laser. For example, the radiation source 150 may comprise a mode-locked laser. Advantageously, such a mode- locked laser ensures that there is a fixed relationship between the phase of each wavelength component from different pulses. The mode-locked laser may, for example, produce the initial pulsed radiation

[0114] In some embodiments, the dispersive medium 160 may comprise an optical fiber. For example, the dispersive medium 160 may comprise a single mode fiber. Advantageously, such an arrangement allows the amount of temporal broadening of the initial pulsed radiation 152 to be easily selected by selecting a length of the optical fiber. Alternatively, the dispersive medium may comprise a crystal or the like.

[0119]

[0115] In some embodiments, the dispersive medium 160 may be configured to increase a duration of the pulses by a factor of 10 or more. In general, the dispersive medium may be configured to increase a duration of the pulses by a factor that is sufficiently large to ensure that the pulses of the pulsed radiation 112 are chirped such that a wavelength of each pulse varies with position in the pulse (for example linearly chirped).

[0120]

[0116] In general, optional elements of the new apparatus 100 shown in Figure 4 are shown in dotted lines.

[0121]

[0117] The apparatus 100 may comprise projection optics 170 operable to direct the pulsed radiation 112 to the beam spot region 140. The apparatus may comprise detection optics 175 operable to receive a scattered portion 122 of the pulsed radiation 112 from an object W disposed in the beam spot region 140 and to direct it to the detector 120.

[0122]

[0118] The apparatus may further comprise a support 180 for supporting an object W such that it is positionable in a beam spot region 140. The support 180 for supporting a substrate W may comprise a substrate holder operable to secure the substrate W. For example, the support may comprise a clamp for clamping the substrate W to the support 180.

[0123]

[0119] The apparatus may further comprise a movement mechanism 182 operable to cause relative movement of the support 180 and the beam spot region 140. The movement mechanism 182 may be operable to cause such movement in a plane of the substrate W, as indicated by arrow 184. This may allow the obj ect W or substrate to be stepped or scanned through the beam spot region 140. Additionally or alternatively, the movement mechanism 182 may be operable to cause such movement in a direction perpendicular to the plane of the substrate W, as indicated by arrow 186.

[0124]

[0120] In some embodiments, the processor 130 may be operable to determine a plurality of beat frequencies of radiation 122 received by the detector 120 and to determine one or more properties of the object W from said plurality of beat frequencies.

[0125]

[0121] In some embodiments, the processor 130 may be operable to determine a distance from at least one beat frequency of radiation 122 received by the detector 120.

[0126]

[0122] An example with some quantitative values is now discussed to promote a better understanding of embodiments of the present disclosure. It will be appreciated that any quantitative values discussed herein are provided merely as examples to illustrate how the embodiments of the present disclosure may work. It will be appreciated that any quantitative values discussed herein are not essential and that the embodiments of the present disclosure are not limited to such values.

[0123] As discussed above with reference to Figure 6, in some embodiments, the pulsed radiation 112 is generated from initial radiation 152 generated by a radiation source 150, which is broadened by a dispersive medium. The radiation source 150 may comprise a femtosecond laser. Typical femtosecond lasers may have bandwidths of the order of 1-100 nm and pulse lengths of -100 fs. Chromatic dispersion can be described in units of fs / nm / m, which means that for every meter of dispersive material radiation propagates through, a pulse with a 1 nm bandwidth will disperse by 1 fs. Typical values for commercial single mode fibres are of the order of 10s of fs / nm / m. Therefore, initial radiation 152 having a pulse duration AT of 100 fs centred at 1030 nm with a bandwidth of 10 nm will be broadened to generate pulsed radiation with a pulse duration Ax' of the order of 5 ps after traveling through an optical fibre with a length of 10 m. As discussed above, alternatively, other pulse durations, wavelengths and bandwidths may be used. For example, the wavelength may be chosen in dependence on the transmission spectra of the layers of the object W.

[0127]

[0124] The beat frequency vBobserved on the detector 120 is proportional to the optical delay At between the two components 122a, 122b that generate the beat and the line width or bandwidth of the pulses, Av, while it is inversely proportional to pulse duration, Ax':

[0128] 1125] For a bandwidth of Av of the order of 3 THz and a dispersed pulse length Ax' of the order of 5 ps an optical delay At of the order of 3 attoseconds (3xl018seconds, for example caused by a layer LI with a thickness of the order of 1 run), the beat frequency vBis of the order of 2 MHz. Such frequencies are easily detectable with commercial photodiodes and readout electronics, which can go up to temporal resolutions of the order of 100 GHz.

[0129]

[0126] It will be appreciated that the amplitude of the beat signal depends on the relative electric field amplitudes from each layer. The ratio of the AC and DC amplitudes can be written as: where E; is the amplitude of the reflected portion from the ith layer. For example, in a simplified case of only two copies of the pulse (for example the simple example illustrated in Figure 5), where the upper surface of the top layer LI reflects 5% of the beam and the lower surface of the thin layer L2 reflects the rest of the pulsed radiation 112, the amplitudes of the AC and DC signals will be 1:21, respectively.

[0127] Another consideration is the repetition rate of the radiation source, which should be high enough to resolve the beat signal (over a plurality of pulses). Commercial femtosecond lasers can go up to repetition rates of the order of 1 GHz, which is sufficiently large to resolve beat frequencies of the order of MHz.

[0130]

[0128] In some embodiments, the apparatus 100 may function as and / or be referred to as a level sensor, as now discussed. In particular, the technique explained above for measuring layer thicknesses with reference to Figures 4 to 6 can be implemented as a Mach-Zander interferometer to measure a surface roughness or height of ab object (e.g. a wafer), as now discussed with reference to Figure 7.

[0131]

[0129] In some embodiments, the processor 130 may be operable to determine a height of a surface of an object W from at least one beat frequency of radiation received by the detector 130.

[0132]

[0130] In some embodiments, the apparatus 100 further comprises a reference surface 190 and a first beam splitter 200, as shown schematically in Figure 7. The first beam splitter 200 (for example downstream of the dispersive medium 160 discussed above) is configured to direct a first part 112a of the pulsed radiation 112 to the beam spot region 140 and a second part 112b of the pulsed radiation 112 to the reference surface 190.

[0133]

[0131] The apparatus 100 is configured to combine a scattered portion 122a of the first part of the pulsed radiation 112a scattered from an object W disposed in the beam spot region 140 and a scattered portion 122b of the second part of the pulsed radiation 112b scattered from the reference surface 190 so as to create a combined optical signal 122 that is received by the detector 120.

[0134]

[0132] In use, a surface to be measured (for example a lithographic substrate or wafer W) may be disposed in the beam spot region 140. Advantageously, this embodiment allows for a difference in an optical path length of the first and second parts to be determined 112a, 112b. This may allow the height of an object W disposed in the beam spot region 140 to be measured or mapped, relative to the reference surface 190.

[0135]

[0133] In this embodiment, the apparatus 100 comprises a second beam splitter 210 that is configured to combine a scattered portion 122a of the first part 112a of the pulsed radiation 112 scattered from an object W disposed in the beam spot region 140 and a scattered portion 122b of the second part 112b of the pulsed radiation 112 scattered from the reference surface 190 so as to create a combined optical signal 122 that is received by the detector 120. In fact, in this embodiment the apparatus comprises two detectors 120 and the second beam splitter 210 generates two such combined signals 122 each of which is received by a different one of the two detectors 120. However, it will be appreciated that some embodiments the new apparatus 100 may only have one detector 120 and may only detect one of the combined signals 122.

[0136]

[0134] As an alternative to using a second beam splitter 210, the scattered portion 122a ofthe first part 112a of the pulsed radiation 112 scattered from an object W disposed in the beam spot region 140 and the scattered portion 122b of the second part 112b of the pulsed radiation 112 scattered from the reference surface 190 may be combined on the detector 120.

[0135] In embodiments of the new apparatus 100 of the type shown in Figure 7, the processor 130 is configured to determine at least one beat frequency of the combined optical signal(s) 122 and to determine a height from each of the at least one beat frequencies.

[0137]

[0136] It will be appreciated that, in general, each pulse of the scattered portion 122a of the first part 112a of the pulsed radiation 112 scattered from an object W may comprise a plurality of copies of the original pulse 112a that was incident on the object W (and that have reflected from different layers of the object). Therefore, for embodiments of the new apparatus 100 of the type shown in Figure 7, the processor 130 may determine a plurality of beat frequencies, some of which may contain information about a height of the object and some of which may contain information about thicknesses of layers within the object W.

[0138]

[0137] Detecting multiple beat frequencies, where each frequency will be directly proportional to a different layer thickness allows a variation of layer thickness to be estimated and therefore for an HPD error to be reduced. More layers of varying thicknesses will lead to different frequency components which can be measured in the same way as described above. This information will allow the layer thickness variations along a wafer surface to be estimated and, therefore, to reduce the stack dependency of the wafer height measurement.

[0139]

[0138] In some embodiments, the reference surface 190 may be movable. In particular, as indicated schematically by arrow 192 the reference surface 190 may be movable in a direction that is generally perpendicular to a reflective surface 194 of the reference surface 190.

[0140]

[0139] For example, the reference surface 190 may be movable relative to a support for supporting an object so that an optical path length for the second part 112b of the pulsed radiation 112 can be varied. Advantageously, this may increase a dynamic range of the apparatus 100. For example, it may allow for a greater range of height variations to be measured. Furthermore, it may allow for a beat frequency that relates to a height of the object W to be changed so as to better distinguish it from one or more beat frequencies that are related to thicknesses of layers of the object W.

[0141]

[0140] The detector 120 should have sufficient temporal resolution to resolve typical beat frequencies that are generated by (a) one or more layer thicknesses; and / or (b) a height of an object W relative to a reference surface 190. The beat frequencies may, for example, be of the order of MHz.

[0142]

[0141] In some embodiments, the detector 120 may have sufficient temporal resolution to resolve the individual pulses of the pulsed radiation 112.

[0143]

[0142] A repetition rate of the pulsed radiation 112 produced by the illumination system 110 should be sufficiently high to resolve typical beat frequencies that are generated by (a) one or more layer thicknesses; and / or (b) a height of an object relative to a reference surface.

[0144]

[0143] In some embodiments, a repetition rate of the pulsed radiation produced by the illumination system is greater than 10 MHz.

[0145]

[0144] A femtosecond interferometric measurement of phase can be added to the embodiment shown in Figure 7 to also obtain information of the phase delay. In addition to the time of flight measurement, this additional data can be used, for example for calibration. The embodiment shown in Figure 7 constitutes an interferometer, which can be used to measure a relative phase of the pulses 122a, 122b (i.e. a standard interferometry measurement using the two pulses 122a, 122b). In particular, the embodiment shown in Figure 7 can measure the relative phase of: (a) the scattered portion 122a of the first part of the pulsed radiation 112a scattered from an object W disposed in the beam spot region 140; and (b) the scattered portion 122b of the second part of the pulsed radiation 112b scattered from the reference surface 190. Such an arrangement may be less sensitive than, for example, the embodiment described above with reference to Figure 5 but the additional information of the phase of the pulses 122a scattered from the object W relative to that of the pulses 122b scattered from the reference surface 190 can be useful, for example for calibration. For example, a beat frequency measured using the embodiment of Figure 7 (as described above) may be compared to a measured phase delay (measured using a standard interferometry measurement) to identify any discrepancies (and used these to calibrate this embodiment).

[0146]

[0145] Some embodiments of the present disclosure may relate to an exposure apparatus comprising the new apparatus 100 as described above with reference to Figures 4 to 7. The exposure apparatus may comprise a lithographic apparatus. For example, the exposure apparatus may comprise a lithographic apparatus LA of the type shown in Figure 1.

[0147]

[0146] Some embodiments of the present disclosure may relate to a metrology apparatus comprising the new apparatus 100 as described above with reference to Figures 4 to 7.

[0148]

[0147] Some embodiments of the present disclosure may relate to new methods of measuring one or more properties of an object W. Such new methods are now discussed with reference to Figures 8 to 9.

[0149]

[0148] Figure 8 schematically shows a new method 300 according to an embodiment of the present disclosure. Figure 9 schematically shows an embodiment of the new method 300 shown in Figure 8 with some additional optional steps. In general, optional steps of the new method 300 are shown in dotted lines in Figure 9.

[0150]

[0149] The new method 300 comprises a step 310 of directing pulsed radiation 112 so as to be incident on the object W, wherein the pulses of the pulsed radiation 112 are chirped such that a wavelength of each pulse varies with position in the pulse. Directing the pulsed radiation 112 so as to be incident on the object W may comprise: positioning the object W in a beam spot region 140 and directing the pulsed radiation 112 to the beam spot region 140.

[0151]

[0150] The new method 300 further comprises a step 320 of receiving a scattered portion of the pulsed radiation 122 scattered from the object W.

[0152]

[0151] The new method 300 further comprises a step 330 of determining at least one beat frequency of the received radiation 122.

[0153]

[0152] The new method 300 further comprises a step 340 of determining one or more properties of the object W from said at least one beat frequency.

[0153] The new method 300 shown in Figures 8 and 9 may be carried out using the new apparatus 100 as described above with reference to Figures 4 to 7.

[0154]

[0154] In some embodiments, the one or more properties of the object W (determined from said at least one beat frequency at step 340) may comprise a thickness of a layer of the object W from at least one beat frequency of the received radiation 122.

[0155]

[0155] The pulsed radiation 112 (directed to the object W at step 310) may be considered to comprise a train of pulses (with specific properties). When the pulsed radiation 112 is incident on an object W with structure (for example a multilayered stack) there may be multiple reflections (for example, from each layer, as shown in Figure 3 and described above). As shown schematically in Figure 5, and described above, such multiple reflections will result in a plurality of pulse trains each being time-shifted relative to the others. The scattered or reflected radiation 122 (which is received at step 320) will, in general comprise a plurality of copies 122a, 122b of the pulse train 112 that was directed to the object W at step 310 (and which are shifted relative to each other by time delays that are dependent on the thicknesses of the layers of the object W.

[0156]

[0156] Since the pulses of the pulsed radiation 112 are chirped such that a wavelength of each pulse varies with position in the pulse, there will be a difference in the wavelengths of the parts of the multiple copies 122a, 122b of the pulse trains that spatially overlap with each other. As a result the multiple copies of the pulses will interfere and will form at least one beat frequency (which will be proportional to a time shift between two of the copies and a magnitude of the chirp). The beat frequencies in the reflected radiation 122 (which are determined at step 330) are related to the thicknesses of the layers in the object W. Furthermore, because there is a fixed relationship between the phase of each wavelength component from different pulses, this beat frequency can be observed over a plurality of pulses. Therefore, the receipt of the scattered portion 122 of the pulsed radiation from an object W disposed in the beam spot region 140 (at step 320, for example using a detector 130), effectively stroboscopic samples the beat frequency using the pulses of the scattered radiation 122.

[0157]

[0157] In some embodiments, there may be a fixed relationship between the phase of each wavelength component from different pulses of the pulsed radiation 112 (which is directed so as to be incident on the object W at step 310).

[0158]

[0158] Advantageously, with a fixed relationship between the phase of each wavelength component from different pulses of the pulsed radiation 112, the method 300 shown in Figure 8 allows very small thicknesses (of the order of 1 nanometer) to be measured. Furthermore, such a method 300 can be used to quantify “Apparent Surface Depression” (ASD) or “Height Process Dependency” (HPD) effects and / or can be used to make an accurate height measurement of an object W.

[0159]

[0159] In some embodiments, the method 300 may comprise a step 350 of producing the pulsed radiation 112. The chirp of the pulsed radiation 112 can be produced in any convenient manner.

[0160]

[0160] In some embodiments, the step 350 of producing the pulsed radiation may comprise: a step 352 of producing initial pulsed radiation 152; and a step 354 of directing the initial pulsed radiation 152 through a dispersive medium 160 so as to produce the pulsed radiation 112. A duration Ax' of the pulses of the pulsed radiation 112 may be greater than that AT of the initial pulsed radiation 152.

[0161]

[0161] In some embodiments, the initial pulsed radiation 152 may have pulse lengths of 1000 femtoseconds or less.

[0162]

[0162] In some embodiments, the pulsed radiation 112 may be at least partially produced using a mode-locked laser. Advantageously, such a mode-locked laser ensures that there is a fixed relationship between the phase of each wavelength component from different pulses. The mode -locked laser may, for example, produce the initial pulsed radiation 152.

[0163]

[0163] In some embodiments, the method 300 may comprise determining a plurality of beat frequencies of received radiation 122 (for example at step 330) and determining one or more properties of the object W from said plurality of beat frequencies (at step 340).

[0164]

[0164] In some embodiments, the method 300 may comprise determining a distance from at least one beat frequency of the received radiation 122 (for example at step 340). That is, the one or more properties of the object W (determined from said at least one beat frequency at step 340) may comprise a distance (for example a height of an object W or a thickness of a layer in the object W).

[0165]

[0165] In some embodiments, the method 300 may comprise determining a height of a surface of an object W from at least one beat frequency of the received radiation 122 (for example at step 340). That is, the one or more properties of the object W (determined from said at least one beat frequency at step 340) may comprise a height of a surface of an object W. Such embodiments of the method 300 may, for example, be implemented by an apparatus that comprises a reference surface 190. For example, such embodiments of the method 300 may be implemented by an apparatus 100 of the type generally shown in Figure 7.

[0166]

[0166] In some embodiments, the method 300 may further comprise a step 360 of splitting the pulsed radiation 112 into first part 112a that is directed to the object W (at step 310) and a second part 112b.

[0167]

[0167] Such embodiments may comprise a step 370 of directing the second part 112b of the pulsed radiation to a reference surface 190. It will be appreciated that this step 370 may be carried out at substantially the same time as step 310.

[0168]

[0168] Such embodiments may further comprise a step 380 of combining a scattered portion 122a of the first part of the pulsed radiation 112a scattered from the object W (as a consequence of step 310) and a scattered portion 122b of the second part of the pulsed radiation 112b scattered from the reference surface 190 (as a consequence of step 370) so as to create a combined optical signal as the received radiation (which is received at step 320).

[0169]

[0169] The method 300 may comprise determining (at step 330) at least one beat frequency of the combined optical signal (generated at optional step 380) and determining a height from each of the at least one beat frequencies (at step 340).

[0170] In some embodiments, the method 300 may further comprise moving the reference surface 190.

[0170]

[0171] In some embodiments, a repetition rate of the pulsed radiation 112 (which is directed so as to be incident on the object W at step 310 and which optionally is produced at step 350) may be greater than 10 MHz.

[0171]

[0172] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0172]

[0173] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0173]

[0174] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0174]

[0175] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0175]

[0176] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below. Other aspects of the invention are set out as in the following numbered clauses:

[0176] 1. An apparatus for measuring one or more properties of an object, the apparatus comprising: an illumination system configured to emit pulsed radiation and direct the pulsed radiation to a beam spot region, wherein the pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with position in the pulse and wherein there is a fixed relationship between the phase of each wavelength component from different pulses; a detector arranged to receive a scattered portion of the pulsed radiation scattered from an object disposed in the beam spot region; and a processor operable to determine at least one beat frequency of radiation received by the detector and to determine one or more properties of the object from said at least one beat frequency.

[0177] 2. An apparatus for measuring one or more properties of an object, the apparatus comprising: an illumination system configured to emit pulsed radiation and direct the pulsed radiation to a beam spot region, wherein the pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with position in the pulse; a detector arranged to receive a scattered portion of the pulsed radiation scattered from an object disposed in the beam spot region; and a processor operable to determine at least one beat frequency of radiation received by the detector and to determine a thickness of a layer of an object from at least one beat frequency of radiation received by the detector.

[0178] 3. The apparatus of clause 1 or clause 2 wherein the illumination system comprises: a radiation source operable to produce initial pulsed radiation; and a dispersive medium configured to receive the initial pulsed radiation and to output the pulsed radiation, wherein a duration of the pulses of the pulsed radiation is greater than that of the initial pulsed radiation.

[0179] 4. The apparatus of clause 3 wherein the radiation source is operable to produce initial pulsed radiation having pulse lengths of 1000 femtoseconds or less.

[0180] 5. The apparatus of any preceding clause wherein the illumination system comprises a mode- locked laser.

[0181] 6. The apparatus of any preceding clause when dependent either directly or indirectly on clause 3 wherein the dispersive medium comprises an optical fiber.

[0182] 7. The apparatus of any preceding clause when dependent either directly or indirectly on clause 3 wherein the dispersive medium is configured to increase a duration of the pulses by a factor of 10 or more.

[0183] 8. The apparatus of any preceding clause wherein the processor is operable to determine a plurality of beat frequencies of radiation received by the detector and to determine one or more properties of the object from said plurality of beat frequencies. 9. The apparatus of any preceding clause wherein the processor is operable to determine a distance from at least one beat frequency of radiation received by the detector.

[0184] 10. The apparatus of any preceding clause when dependent either directly or indirectly on clause 1 wherein the processor is operable to determine a thickness of a layer of an object from at least one beat frequency of radiation received by the detector.

[0185] 11. The apparatus of any preceding clause wherein the processor is operable to determine a height of a surface of an object from at least one beat frequency of radiation received by the detector.

[0186] 12. The apparatus of any preceding clause further comprising: a reference surface; and a first beam splitter configured to direct a first part of the pulsed radiation to the beam spot region and a second part of the pulsed radiation to the reference surface; and wherein the apparatus is configured to combine a scattered portion of the first part of the pulsed radiation scattered from an object disposed in the beam spot region and a scattered portion of the second part of the pulsed radiation scattered from the reference surface so as to create a combined optical signal that is received by the detector.

[0187] 13. The apparatus of clause 12 wherein the processor is configured to determine at least one beat frequency of the combined optical signal and to determine a height from each of the at least one beat frequencies.

[0188] 14. The apparatus of clause 12 or clause 13 wherein the reference surface is movable.

[0189] 15. The apparatus of any preceding clause wherein the detector has sufficient temporal resolution to resolve the pulses of the pulsed radiation.

[0190] 16. The apparatus of any preceding clause wherein a repetition rate of the pulsed radiation produced by the illumination system is greater than 10 MHz.

[0191] 17. The apparatus of any preceding clause further comprising a support for supporting an object such that it is positionable in a beam spot region.

[0192] 18. The apparatus of clause 17 further comprising a movement mechanism operable to cause relative movement of the support and the beam spot region.

[0193] 19. An exposure apparatus comprising the apparatus of any preceding clause.

[0194] 20. A metrology apparatus comprising the apparatus of any one of clauses 1 to 18.

[0195] 21. A method of measuring one or more properties of an object, the method comprising: directing pulsed radiation so as to be incident on the object, wherein the pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with position in the pulse and wherein there is a fixed relationship between the phase of each wavelength component from different pulses; receiving a scattered portion of the pulsed radiation scattered from the object; determining at least one beat frequency of the received radiation; and determining one or more properties of the object from said at least one beat frequency.

[0196] 22. A method for measuring one or more properties of an object, the method comprising: directing pulsed radiation so as to be incident on the object, wherein the pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with position in the pulse; receiving a scattered portion of the pulsed radiation scattered from the object; determining at least one beat frequency of the received radiation; and determining a thickness of a layer of the object from at least one beat frequency of the received radiation.

[0197] 23. The method of clause 21 or clause 22 further comprising producing the pulsed radiation.

[0198] 24. The method of clause 23 wherein producing the pulsed radiation comprises: producing initial pulsed radiation; and directing the initial pulsed radiation through a dispersive medium so as to produce the pulsed radiation, wherein a duration of the pulses of the pulsed radiation is greater than that of the initial pulsed radiation.

[0199] 25. The method of clause 24 wherein the initial pulsed radiation has pulse lengths of 1000 femtoseconds or less.

[0200] 26. The method or any one of clauses 23 to 25 wherein the pulsed radiation is at least partially produced using a mode-locked laser.

[0201] 27. The method of any one of clause 21 to 26 comprising determining a plurality of beat frequencies of received radiation and determining one or more properties of the object from said plurality of beat frequencies.

[0202] 28. The method of any one of clause 21 to 27 comprising determining a distance from at least one beat frequency of the received radiation.

[0203] 29. The method of any one of clause 21 to 28 when dependent either directly or indirectly on clause 21 comprising determining a thickness of a layer of an object from at least one beat frequency of the received radiation.

[0204] 30. The method of any one of clauses 21 to 29 comprising determining a height of a surface of an object from at least one beat frequency of the received radiation.

[0205] 31. The method of any one of clause 21 to 30 further comprising: splitting the pulsed radiation into a first part that is directed to the object and a second part; directing the second part of the pulsed radiation to a reference surface; and combining a scattered portion of the first part of the pulsed radiation scattered from the object and a scattered portion of the second part of the pulsed radiation scattered from the reference surface so as to create a combined optical signal as the received radiation.

[0206] 32. The method of clause 31 comprising: determining at least one beat frequency of the combined optical signal; and determining a height from each of the at least one beat frequencies.

[0207] 33. The method of clause 31 or clause 32 comprising moving the reference surface.

[0208] 34. The method of any one of clauses 21 to 33 wherein a repetition rate of the pulsed radiation produced by the illumination system is greater than 10 MHz.

Claims

CLAIMS1. An apparatus for measuring one or more properties of an object, the apparatus comprising: an illumination system configured to emit pulsed radiation and direct the pulsed radiation to a beam spot region, wherein the pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with position in the pulse and wherein there is a fixed relationship between the phase of each wavelength component from different pulses; a detector arranged to receive a scattered portion of the pulsed radiation scattered from an object disposed in the beam spot region; and a processor operable to determine at least one beat frequency of radiation received by the detector and to determine one or more properties of the object from said at least one beat frequency.

2. The apparatus of claim 1 wherein the illumination system comprises: a radiation source operable to produce initial pulsed radiation; and a dispersive medium configured to receive the initial pulsed radiation and to output the pulsed radiation, wherein a duration of the pulses of the pulsed radiation is greater than that of the initial pulsed radiation.

3. The apparatus of claim 2 wherein the dispersive medium is configured to increase a duration of the pulses by a factor of 10 or more.

4. The apparatus of any preceding claim wherein the processor is operable to determine a plurality of beat frequencies of radiation received by the detector and to determine one or more properties of the object from said plurality of beat frequencies, and wherein the processor is operable to determine at least one of; a distance from at least one beat frequency of radiation received by the detector, a thickness of a layer of an object from at least one beat frequency of radiation received by the detector, and a height of a surface of an object from at least one beat frequency of radiation received by the detector.

5. The apparatus of any preceding claim further comprising: a reference surface; and a first beam splitter configured to direct a first part of the pulsed radiation to the beam spot region and a second part of the pulsed radiation to the reference surface; and wherein the apparatus is configured to combine a scattered portion of the first part of the pulsed radiation scattered from an object disposed in the beam spot region and a scattered portion of the secondpart of the pulsed radiation scattered from the reference surface so as to create a combined optical signal that is received by the detector.

6. The apparatus of claim 5 wherein the processor is configured to determine at least one beat frequency of the combined optical signal and to determine a height from each of the at least one beat frequencies.

7. The apparatus of any preceding claim further comprising a support for supporting an object such that it is positionable in a beam spot region.

8. The apparatus of claim 7 further comprising a movement mechanism operable to cause relative movement of the support and the beam spot region.

9. An exposure apparatus comprising the apparatus of any preceding claim.

10. A method of measuring one or more properties of an object, the method comprising: directing pulsed radiation so as to be incident on the object, wherein the pulses of the pulsed radiation are chirped such that a wavelength of each pulse varies with position in the pulse and wherein there is a fixed relationship between the phase of each wavelength component from different pulses; receiving a scattered portion of the pulsed radiation scattered from the object; determining at least one beat frequency of the received radiation; and determining one or more properties of the object from said at least one beat frequency.

11. The method of claim 10 further comprising producing the pulsed radiation.

12. The method of claim 11 wherein producing the pulsed radiation comprises: producing initial pulsed radiation; and directing the initial pulsed radiation through a dispersive medium so as to produce the pulsed radiation, wherein a duration of the pulses of the pulsed radiation is greater than that of the initial pulsed radiation.

13. The method of any one of claims lO to 12 comprising determining a plurality of beat frequencies of received radiation and determining one or more properties of the object from said plurality of beat frequencies, wherein the determining one or more properties of the object comprises at least one of; determining a distance from at least one beat frequency of the received radiation, determining a thickness of a layer of an object from at least one beat frequency of the received radiation, anddetermining a height of a surface of an object from at least one beat frequency of the received radiation.

14. The method of any one of claim 10 to 13 further comprising: splitting the pulsed radiation into a first part that is directed to the object and a second part; directing the second part of the pulsed radiation to a reference surface; and combining a scattered portion of the first part of the pulsed radiation scattered from the object and a scattered portion of the second part of the pulsed radiation scattered from the reference surface so as to create a combined optical signal as the received radiation.

15. The method of claim 14 comprising: determining at least one beat frequency of the combined optical signal; and determining a height from each of the at least one beat frequencies.

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