LITHOGRAPHIC APPARATUS, MULTI-WAVELENGTH PHASE-MODULATED SCANNING METROLOGY SYSTEM AND METHOD

A metrology system includes a radiation source, first, second, and third optical systems, and a processor. The first optical system splits the radiation into first and second beams of radiation and impart one or more phase differences between the first and second beams. The second optical system directs the first and second beams toward a target structure to produce first and second scattered beams of radiation. The third optical system interferes the first and second scattered beams at an imaging detector. The imaging detector generates a detection signal based on the interfered first and second scattered beams. The metrology system modulates one or more phase differences of the first and second scattered beams based on the imparted one or more phase differences. The processor analyzes the detection signal to determine a property of the target structure based on at least the modulated one or more phase differences.

FIELD

The present disclosure relates to a metrology system, for example, an alignment apparatus for measuring a position of a feature on a substrate in lithographic systems.

BACKGROUND

Another lithographic system is an interferometric lithographic system where there is no patterning device, but rather a light beam is split into two beams, and the two beams are caused to interfere at a target portion of the substrate through the use of a reflection system. The interference causes lines to be formed at the target portion of the substrate.

During lithographic operation, different processing steps may require different layers to be sequentially formed on the substrate. Accordingly, it can be necessary to position the substrate relative to prior patterns formed thereon with a high degree of accuracy. Generally, alignment marks are placed on the substrate to be aligned and are located with reference to a second object. A lithographic apparatus may use an alignment apparatus for detecting positions of the alignment marks and for aligning the substrate using the alignment marks to ensure accurate exposure from a mask. Misalignment between the alignment marks at two different layers is measured as overlay error.

In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement can be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of a specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. By contrast, angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.

Errors in alignment of wafers in a lithographic apparatus result in reduced quality, unreliable performance, and reduced yield rates of fabricated devices, which in turn increases time and cost of fabrication of devices.

SUMMARY

Accordingly, it is desirable to improve metrology techniques that allow for more accurate placement of lithographed structures on wafers.

In some embodiments, a metrology system comprises a radiation source, first, second, and third optical systems, and a processor. The first optical system is configured to split the radiation into first and second beams of radiation and impart one or more phase differences between the first and second beams. The second optical system is configured to direct the first and second beams toward a target structure to produce first and second scattered beams of radiation. The third optical system is configured to interfere the first and second scattered beams at an imaging detector. The imaging detector is configured to generate a detection signal based on the interfered first and second scattered beams. The metrology system is configured to modulate one or more phase differences of the first and second scattered beams based on the imparted one or more phase differences. The processor is configured to analyze the detection signal to determine a property of the target structure based on at least the modulated one or more phase differences.

In some embodiments, a lithographic apparatus system comprises an illumination system, a projection system, and a metrology system. The metrology system comprises a radiation source, first, second, and third optical systems, and a processor. The illumination system is configured to illuminate a pattern of a patterning device. The projection system is configured to project an image of the pattern onto a substrate. The first optical system is configured to split the radiation into first and second beams of radiation and impart one or more phase differences between the first and second beams. The second optical system is configured to direct the first and second beams toward a target structure to produce first and second scattered beams of radiation. The third optical system is configured to interfere the first and second scattered beams at an imaging detector. The imaging detector is configured to generate a detection signal based on the interfered first and second scattered beams. The metrology system is configured to modulate one or more phase differences of the first and second scattered beams based on the imparted one or more phase differences. The processor is configured to analyze the detection signal to determine a property of the target structure based on at least the modulated one or more phase differences.

In some embodiments, a method comprises generating radiation. The method further comprises splitting the radiation into first and second beams of radiation using a first optical system. The method further comprises imparting one or more phase differences between the first and second beams using the first optical system. The method further comprises directing the first and second beams toward a target to produce first and second scattered beams of radiation. The method further comprises interfering the first and second scattered beams at an imaging detector. The method further comprises modulating one or more phase differences of the first and second scattered beams. The method further comprises generating a detection signal using the imaging detector. The method further comprises analyzing the detection signal to determine a property of the target structure base on at least the modulated one or more phase differences.

Further features of the present disclosure, as well as the structure and operation of various embodiments, are described in detail below with reference to the accompanying drawings. It is noted that the present disclosure is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

The features of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. Additionally, generally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporate the features of the present disclosure. The disclosed embodiment(s) are provided as examples. The scope of the present disclosure is not limited to the disclosed embodiment(s). Claimed features are defined by the claims appended hereto.

The term “about” as used herein indicates the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the term “about” can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present disclosure can be implemented.

Example Lithographic Systems

FIGS.1A and1Bshow schematic illustrations of a lithographic apparatus100and lithographic apparatus100′, respectively, in which embodiments of the present disclosure can be implemented. Lithographic apparatus100and lithographic apparatus100′ each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatus100and100′ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus100′, the patterning device MA and the projection system PS are transmissive.

The illumination system IL can include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B.

The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatus100and100′, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable, as required. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

The term “patterning device” MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

The terms “inspection apparatus,” “metrology apparatus,” and the like may be used herein to refer to, e.g., a device or system used for measuring a property of a structure (e.g., overlay error, critical dimension parameters) or used in a lithographic apparatus to inspect an alignment of a wafer (e.g., alignment apparatus).

The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

Lithographic apparatus100and/or lithographic apparatus100′ can be of a type having two (dual stage) or more substrate tables WT (and/or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT.

Referring toFIGS.1A and1B, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus100,100′ can be separate physical entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus100or100′, and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (inFIG.1B) including, for example, suitable directing mirrors and/or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus100,100′, for example, when the source SO is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD, if required, can be referred to as a radiation system.

The illuminator IL can include an adjuster AD (inFIG.1B) for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as “σ-outer” and “σ-inner,” respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL can comprise various other components (inFIG.1B), such as an integrator IN and a condenser CO. The illuminator IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section.

Referring toFIG.1A, the radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus100, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2(for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor IF1can be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device (for example, mask) MA and substrate W can be aligned using mask alignment marks M1, M2and substrate alignment marks P1, P2.

Referring toFIG.1B, the radiation beam B is incident on the patterning device (for example, mask MA), which is held on the support structure (for example, mask table MT), and is patterned by the patterning device. 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. The projection system has a pupil conjugate PPU to an illumination system pupil IPU. Portions of radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse a mask pattern without being affected by diffraction at the mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.

The projection system PS projects an image of the mask pattern MP, where the image is formed by diffracted beams produced from the mark pattern MP by radiation from the intensity distribution, onto a photoresist layer coated on the substrate W. For example, the mask pattern MP can include an array of lines and spaces. A diffraction of radiation at the array and different from zeroth order diffraction generates diverted diffracted beams with a change of direction in a direction perpendicular to the lines. Undiffracted beams (i.e., so-called zeroth order diffracted beams) traverse the pattern without any change in propagation direction. The zeroth order diffracted beams traverse an upper lens or upper lens group of the projection system PS, upstream of the pupil conjugate PPU of the projection system PS, to reach the pupil conjugate PPU. The portion of the intensity distribution in the plane of the pupil conjugate PPU and associated with the zeroth order diffracted beams is an image of the intensity distribution in the illumination system pupil IPU of the illumination system IL. The aperture device PD, for example, is disposed at or substantially at a plane that includes the pupil conjugate PPU of the projection system PS.

The projection system PS is arranged to capture, by means of a lens or lens group L, not only the zeroth order diffracted beams, but also first-order or first- and higher-order diffracted beams (not shown). In some embodiments, dipole illumination for imaging line patterns extending in a direction perpendicular to a line can be used to utilize the resolution enhancement effect of dipole illumination. For example, first-order diffracted beams interfere with corresponding zeroth-order diffracted beams at the level of the wafer W to create an image of the line pattern MP at highest possible resolution and process window (i.e., usable depth of focus in combination with tolerable exposure dose deviations). In some embodiments, astigmatism aberration can be reduced by providing radiation poles (not shown) in opposite quadrants of the illumination system pupil IPU. Further, in some embodiments, astigmatism aberration can be reduced by blocking the zeroth order beams in the pupil conjugate PPU of the projection system associated with radiation poles in opposite quadrants. This is described in more detail in US 7,511,799 B2, issued Mar. 31, 2009, which is incorporated by reference herein in its entirety.

With the aid of the second positioner PW and position sensor IF (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor (not shown inFIG.1B) can be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

In general, movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short-stroke actuator only or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks M1, M2, and substrate alignment marks P1, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can be located in spaces between target portions (known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks can be located between the dies.

Mask table MT and patterning device MA can be in a vacuum chamber V, where an in-vacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum chamber. Alternatively, when mask table MT and patterning device MA are outside of the vacuum chamber, an out-of-vacuum robot can be used for various transportation operations, similar to the in-vacuum robot IVR. Both the in-vacuum and out-of-vacuum robots need to be calibrated for a smooth transfer of any payload (e.g., mask) to a fixed kinematic mount of a transfer station.

The lithographic apparatus100and100′ can be used in at least one of the following modes:

2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT can be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO can be employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array.

Combinations and/or variations on the described modes of use or entirely different modes of use can also be employed.

In a further embodiment, lithographic apparatus100includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

Subsequently the radiation traverses the illumination system IL, which can include a faceted field mirror device222and a faceted pupil mirror device224arranged to provide a desired angular distribution of the radiation beam221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation221at the patterning device MA, held by the support structure MT, a patterned beam226is formed and the patterned beam226is imaged by the projection system PS via reflective elements228,229onto a substrate W held by the wafer stage or substrate table WT.

More elements than shown can generally be present in illumination optics unit IL and projection system PS. The grating spectral filter240can optionally be present, depending upon the type of lithographic apparatus. Further, there can be more mirrors present than those shown in theFIG.2, for example there can be one to six additional reflective elements present in the projection system PS than shown inFIG.2.

Collector optic CO, as illustrated inFIG.2, is depicted as a nested collector with grazing incidence reflectors253,254, and255, just as an example of a collector (or collector mirror). The grazing incidence reflectors253,254, and255are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

Exemplary Lithographic Cell

FIG.3shows a lithographic cell300, also sometimes referred to a lithocell or cluster, according to some embodiments. Lithographic apparatus100or100′ can form part of lithographic cell300. Lithographic cell300can also include one or more apparatuses to perform pre- and post-exposure processes on a substrate. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH, and bake plates BK. A substrate handler, or robot, RO picks up substrates from input/output ports I/O1, I/O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus100or100′. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatuses can be operated to maximize throughput and processing efficiency.

Exemplary Inspection Apparatus

In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more alignment apparatuses and/or systems by which positions of marks on a substrate must be measured accurately. These alignment apparatuses are effectively position measuring apparatuses. Different types of marks and different types of alignment apparatuses and/or systems are known from different times and different manufacturers. A type of system widely used in current lithographic apparatus is based on a self-referencing interferometer as described in U.S. Pat. No. 6,961,116 (den Boef et al.). Generally marks are measured separately to obtain X- and Y-positions. A combined X- and Y-measurement can be performed using the techniques described in U.S. Publication No. 2009/195768 A (Bijnen et al.), however. The full contents of both of these disclosures are incorporated herein by reference.

FIG.4Ashows a schematic of a cross-sectional view of an inspection apparatus400that can be implemented as a part of lithographic apparatus100or100′, according to some embodiments. In some embodiments, inspection apparatus400can be configured to align a substrate (e.g., substrate W) with respect to a patterning device (e.g., patterning device MA). Inspection apparatus400can be further configured to detect positions of alignment marks on the substrate and to align the substrate with respect to the patterning device or other components of lithographic apparatus100or100′ using the detected positions of the alignment marks. Such alignment of the substrate can ensure accurate exposure of one or more patterns on the substrate.

In some embodiments, inspection apparatus400can include an illumination system412, a beam splitter414, an interferometer426, a detector428, a beam analyzer430, and an overlay calculation processor432. Illumination system412can be configured to provide an electromagnetic narrow band radiation beam413having one or more passbands. In an example, the one or more passbands can be within a spectrum of wavelengths between about 500 nm to about 900 nm. In another example, the one or more passbands can be discrete narrow passbands within a spectrum of wavelengths between about 500 nm to about 900 nm. Illumination system412can be further configured to provide one or more passbands having substantially constant center wavelength (CWL) values over a long period of time (e.g., over a lifetime of illumination system412). Such configuration of illumination system412can help to prevent the shift of the actual CWL values from the desired CWL values, as discussed above, in current alignment systems. And, as a result, the use of constant CWL values can improve long-term stability and accuracy of alignment systems (e.g., inspection apparatus400) compared to the current alignment apparatuses.

In some embodiments, beam splitter414can be configured to receive radiation beam413and split radiation beam413into at least two radiation sub-beams. For example, radiation beam413can be split into radiation sub-beams415and417, as shown inFIG.4A. Beam splitter414can be further configured to direct radiation sub-beam415onto a substrate420placed on a stage422. In one example, the stage422is movable along direction424. Radiation sub-beam415can be configured to illuminate an alignment mark or a target418located on substrate420. Alignment mark or target418can be coated with a radiation sensitive film. In some embodiments, alignment mark or target418can have one hundred and eighty degrees (i.e., 180°) symmetry. That is, when alignment mark or target418is rotated 180° about an axis of symmetry perpendicular to a plane of alignment mark or target418, rotated alignment mark or target418can be substantially identical to an unrotated alignment mark or target418. The target418on substrate420can be (a) a resist layer grating comprising bars that are formed of solid resist lines, or (b) a product layer grating, or (c) a composite grating stack in an overlay target structure comprising a resist grating overlaid or interleaved on a product layer grating. The bars can alternatively be etched into the substrate. This pattern is sensitive to chromatic aberrations in the lithographic projection apparatus, particularly the projection system PL, and illumination symmetry and the presence of such aberrations will manifest themselves in a variation in the printed grating. One in-line method used in device manufacturing for measurements of line width, pitch, and critical dimension makes use of a technique known as “scatterometry”. Methods of scatterometry are described in Raymond et al., “Multiparameter Grating Metrology Using Optical Scatterometry”, J. Vac. Sci. Tech. B, Vol. 15, no. 2, pp. 361-368 (1997) and Niu et al., “Specular Spectroscopic Scatterometry in DUV Lithography”, SPIE, Vol. 3677 (1999), which are both incorporated by reference herein in their entireties. In scatterometry, light is reflected by periodic structures in the target, and the resulting reflection spectrum at a given angle is detected. The structure giving rise to the reflection spectrum is reconstructed, e.g. using Rigorous Coupled-Wave Analysis (RCWA) or by comparison to a library of patterns derived by simulation. Accordingly, the scatterometry data of the printed gratings is used to reconstruct the gratings. The parameters of the grating, such as line widths and shapes, can be input to the reconstruction process, performed by processing unit PU, from knowledge of the printing step and/or other scatterometry processes.

In some embodiments, beam splitter414can be further configured to receive diffraction radiation beam419and split diffraction radiation beam419into at least two radiation sub-beams, according to an embodiment. Diffraction radiation beam419can be split into diffraction radiation sub-beams429and439, as shown inFIG.4A.

It should be noted that even though beam splitter414is shown to direct radiation sub-beam415towards alignment mark or target418and to direct diffracted radiation sub-beam429towards interferometer426, the disclosure is not so limiting. It would be apparent to a person skilled in the relevant art that other optical arrangements can be used to obtain the similar result of illuminating alignment mark or target418on substrate420and detecting an image of alignment mark or target418.

As illustrated inFIG.4A, interferometer426can be configured to receive radiation sub-beam417and diffracted radiation sub-beam429through beam splitter414. In an example embodiment, diffracted radiation sub-beam429can be at least a portion of radiation sub-beam415that can be reflected from alignment mark or target418. In an example of this embodiment, interferometer426comprises any appropriate set of optical-elements, for example, a combination of prisms that can be configured to form two images of alignment mark or target418based on the received diffracted radiation sub-beam429. It should be appreciated that a good quality image need not be formed, but that the features of alignment mark418should be resolved. Interferometer426can be further configured to rotate one of the two images with respect to the other of the two images 180° and recombine the rotated and unrotated images interferometrically.

In some embodiments, detector428can be configured to receive the recombined image via interferometer signal427and detect interference as a result of the recombined image when alignment axis421of inspection apparatus400passes through a center of symmetry (not shown) of alignment mark or target418. Such interference can be due to alignment mark or target418being 180° symmetrical, and the recombined image interfering constructively or destructively, according to an example embodiment. Based on the detected interference, detector428can be further configured to determine a position of the center of symmetry of alignment mark or target418and consequently, detect a position of substrate420. According to an example, alignment axis421can be aligned with an optical beam perpendicular to substrate420and passing through a center of image rotation interferometer426. Detector428can be further configured to estimate the positions of alignment mark or target418by implementing sensor characteristics and interacting with wafer mark process variations.

In a further embodiment, detector428determines the position of the center of symmetry of alignment mark or target418by performing one or more of the following measurements:1. measuring position variations for various wavelengths (position shift between colors);2. measuring position variations for various orders (position shift between diffraction orders); and3. measuring position variations for various polarizations (position shift between polarizations).

This data can be obtained, for example, with any type of alignment sensor, for example a SMASH (SMart Alignment Sensor Hybrid) sensor, as described in U.S. Pat. No. 6,961,116 that employs a self-referencing interferometer with a single detector and four different wavelengths, and extracts the alignment signal in software, or Athena (Advanced Technology using High order ENhancement of Alignment), as described in U.S. Pat. No. 6,297,876, which directs each of seven diffraction orders to a dedicated detector, which are both incorporated by reference herein in their entireties.

In some embodiments, beam analyzer430can be configured to receive and determine an optical state of diffracted radiation sub-beam439. The optical state can be a measure of beam wavelength, polarization, or beam profile. Beam analyzer430can be further configured to determine a position of stage422and correlate the position of stage422with the position of the center of symmetry of alignment mark or target418. As such, the position of alignment mark or target418and, consequently, the position of substrate420can be accurately known with reference to stage422. Alternatively, beam analyzer430can be configured to determine a position of inspection apparatus400or any other reference element such that the center of symmetry of alignment mark or target418can be known with reference to inspection apparatus400or any other reference element. Beam analyzer430can be a point or an imaging polarimeter with some form of wavelength-band selectivity. In some embodiments, beam analyzer430can be directly integrated into inspection apparatus400, or connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other embodiments.

In some embodiments, beam analyzer430can be further configured to determine the overlay data between two patterns on substrate420. One of these patterns can be a reference pattern on a reference layer. The other pattern can be an exposed pattern on an exposed layer. The reference layer can be an etched layer already present on substrate420. The reference layer can be generated by a reference pattern exposed on the substrate by lithographic apparatus100and/or100′. The exposed layer can be a resist layer exposed adjacent to the reference layer. The exposed layer can be generated by an exposure pattern exposed on substrate420by lithographic apparatus100or100′. The exposed pattern on substrate420can correspond to a movement of substrate420by stage422. In some embodiments, the measured overlay data can also indicate an offset between the reference pattern and the exposure pattern. The measured overlay data can be used as calibration data to calibrate the exposure pattern exposed by lithographic apparatus100or100′, such that after the calibration, the offset between the exposed layer and the reference layer can be minimized.

In some embodiments, beam analyzer430can be further configured to determine a model of the product stack profile of substrate420, and can be configured to measure overlay, critical dimension, and focus of target418in a single measurement. The product stack profile contains information on the stacked product such as alignment mark, target418, or substrate420, and can include mark process variation-induced optical signature metrology that is a function of illumination variation. The product stack profile can also include product grating profile, mark stack profile, and mark asymmetry information. An example of beam analyzer430is Yieldstar™, manufactured by ASML, Veldhoven, The Netherlands, as described in U.S. Pat. No. 8,706,442, which is incorporated by reference herein in its entirety. Beam analyzer430can be further configured to process information related to a particular property of an exposed pattern in that layer. For example, beam analyzer430can process an overlay parameter (an indication of the positioning accuracy of the layer with respect to a previous layer on the substrate or the positioning accuracy of the first layer with respective to marks on the substrate), a focus parameter, and/or a critical dimension parameter (e.g., line width and its variations) of the depicted image in the layer. Other parameters are image parameters relating to the quality of the depicted image of the exposed pattern.

In some embodiments, an array of detectors (not shown) can be connected to beam analyzer430, and allows the possibility of accurate stack profile detection as discussed below. For example, detector428can be an array of detectors. For the detector array, a number of options are possible: a bundle of multimode fibers, discrete pin detectors per channel, or CCD or CMOS (linear) arrays. The use of a bundle of multimode fibers enables any dissipating elements to be remotely located for stability reasons. Discrete PIN detectors offer a large dynamic range but each need separate pre-amps. The number of elements is therefore limited. CCD linear arrays offer many elements that can be read-out at high speed and are especially of interest if phase-stepping detection is used.

In some embodiments, a second beam analyzer430′ can be configured to receive and determine an optical state of diffracted radiation sub-beam429, as shown inFIG.4B. The optical state can be a measure of beam wavelength, polarization, or beam profile. Second beam analyzer430′ can be identical to beam analyzer430. Alternatively, second beam analyzer430′ can be configured to perform at least all the functions of beam analyzer430, such as determining a position of stage422and correlating the position of stage422with the position of the center of symmetry of alignment mark or target418. As such, the position of alignment mark or target418and, consequently, the position of substrate420, can be accurately known with reference to stage422. Second beam analyzer430′ can also be configured to determine a position of inspection apparatus400, or any other reference element, such that the center of symmetry of alignment mark or target418can be known with reference to inspection apparatus400, or any other reference element. Second beam analyzer430′ can be further configured to determine the overlay data between two patterns and a model of the product stack profile of substrate420. Second beam analyzer430′ can also be configured to measure overlay, critical dimension, and focus of target418in a single measurement.

In some embodiments, second beam analyzer430′ can be directly integrated into inspection apparatus400, or it can be connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other embodiments. Alternatively, second beam analyzer430′ and beam analyzer430can be combined to form a single analyzer (not shown) configured to receive and determine the optical states of both diffracted radiation sub-beams429and439.

In some embodiments, processor432receives information from detector428and beam analyzer430. For example, processor432can be an overlay calculation processor. The information can comprise a model of the product stack profile constructed by beam analyzer430. Alternatively, processor432can construct a model of the product mark profile using the received information about the product mark. In either case, processor432constructs a model of the stacked product and overlay mark profile using or incorporating a model of the product mark profile. The stack model is then used to determine the overlay offset and minimizes the spectral effect on the overlay offset measurement. Processor432can create a basic correction algorithm based on the information received from detector428and beam analyzer430, including but not limited to the optical state of the illumination beam, the alignment signals, associated position estimates, and the optical state in the pupil, image, and additional planes. The pupil plane is the plane in which the radial position of radiation defines the angle of incidence and the angular position defines the azimuth angle of the radiation. Processor432can utilize the basic correction algorithm to characterize the inspection apparatus400with reference to wafer marks and/or alignment marks418.

In some embodiments, processor432can be further configured to determine printed pattern position offset error with respect to the sensor estimate for each mark based on the information received from detector428and beam analyzer430. The information includes but is not limited to the product stack profile, measurements of overlay, critical dimension, and focus of each alignment marks or target418on substrate420. Processor432can utilize a clustering algorithm to group the marks into sets of similar constant offset error, and create an alignment error offset correction table based on the information. The clustering algorithm can be based on overlay measurement, the position estimates, and additional optical stack process information associated with each set of offset errors. The overlay is calculated for a number of different marks, for example, overlay targets having a positive and a negative bias around a programmed overlay offset. The target that measures the smallest overlay is taken as reference (as it is measured with the best accuracy). From this measured small overlay, and the known programmed overlay of its corresponding target, the overlay error can be deduced. Table 1 illustrates how this can be performed. The smallest measured overlay in the example shown is -1 nm. However this is in relation to a target with a programmed overlay of -30 nm. Consequently the process must have introduced an overlay error of 29 nm.

The smallest value can be taken to be the reference point and, relative to this, the offset can be calculated between measured overlay and that expected due to the programmed overlay. This offset determines the overlay error for each mark or the sets of marks with similar offsets. Therefore, in the Table 1 example, the smallest measured overlay was -1 nm, at the target position with programmed overlay of 30 nm. The difference between the expected and measured overlay at the other targets is compared to this reference. A table such as Table 1 can also be obtained from marks and target418under different illumination settings, the illumination setting, which results in the smallest overlay error, and its corresponding calibration factor, can be determined and selected. Following this, processor432can group marks into sets of similar overlay error. The criteria for grouping marks can be adjusted based on different process controls, for example, different error tolerances for different processes.

In some embodiments, processor432can confirm that all or most members of the group have similar offset errors, and apply an individual offset correction from the clustering algorithm to each mark, based on its additional optical stack metrology. Processor432can determine corrections for each mark and feed the corrections back to lithographic apparatus100or100′ for correcting errors in the overlay, for example, by feeding corrections into the inspection apparatus400.

FIG.5shows a structure of a target518, according to some embodiments. In some embodiments, target518comprises grating structures arranged in four quadrants. Each quadrant’s grating structure is perpendicular to the grating structure of adjacent quadrants. A circle503indicates the size of an illumination spot relative to an area of target518. The area of the illumination spot is smaller than the area of target518(e.g., underfilled). In some embodiments, the area of the illumination spot is larger than the area of target518(e.g., overfilled, not shown). The structure shown for target518can be used in, for example, targets418(FIGS.4A,4B),718(FIG.7), and818(FIG.8).

FIG.6shows a detected image651, according to some embodiments. In some embodiments, a detected image using a metrology system can comprise a one or more Moiré fringe patterns652, particularly if the metrology system is one that performs image detection on, for example, target518(FIG.5).

In some embodiments, inspection apparatuses and systems described herein can perform measurements at various locations on a target using discrete steps or using a continuous scan (or a mix of both). Embodiments described herein disclose structures and functions for improving accuracy of inspection using scanning operations and modulation of a phase of radiation.

FIG.7shows a metrology system700, according to some embodiments. In some embodiments, metrology system700can also represent a more detailed view of inspection apparatus400(FIGS.4A and4B). For example,FIG.7illustrates a more detailed view of illumination system412and its functions. Unless otherwise noted, elements ofFIG.7that have similar reference numbers (e.g., reference numbers sharing the two right-most numeric digits) as elements ofFIGS.4A and4Bcan have similar structures and functions.

In some embodiments, metrology system700comprises an illumination system712, an optical system710, a detector728, and a processor732. Illumination system712can comprise a radiation source702, an optical fiber704(e.g., a multi-mode fiber), an optical element(s)706(e.g., a lens or lens system), and a diffractive element708(e.g., a grating, adjustable grating, and the like). Optical system710can comprise one or more of optical element706, a blocking element736, a reflective element738(e.g., spot mirror), and an optical element740(e.g., an objective lens).FIG.7shows a non-limiting depiction of metrology system700inspecting a target718(also “target structure”) on a substrate720. The substrate720is disposed on a stage722that is adjustable (e.g., a support structure that can move). It should be appreciated the structures drawn within illumination system712and optical system710are not limited to their depicted positions. For example, diffractive element708can be within optical system710. The positions of structures can vary as necessary, for example, as designed for a modular assembly.

In some embodiments, radiation source702can generate radiation716. Radiation716can be spatially incoherent. Since the output of radiation source702may not be pointed directly toward downstream optical structures, optical fiber704can guide radiation716to downstream optical structures. Optical element(s)706can direct or condition radiation716(e.g., focus, collimate, make parallel, and the like). Diffractive element708can diffract radiation716to generate beams of radiation713and713′ (also first and second beams of radiation). Beam of radiation713can comprise a first non-zero diffraction order from diffractive element708(e.g., +1 order). Beam of radiation713′ can comprise a second non-zero diffraction order from diffractive element708(e.g., -1 order) that is different from the first non-zero diffraction order. Diffractive element708can also generate a zeroth order beam (not labeled). Blocking element736can block the zeroth order beam to allow darkfield measurements. Spot mirror directs beams of radiation713and713′ toward target718. Optical element740focuses beams of radiation713and713′  onto target718such that illumination spots of both beams overlap. The illumination spots can underfill or overfill target718.

In some embodiments, target718can comprise a diffractive structure (e.g., a grating(s) as shown inFIG.5). Target718can reflect, refract, diffract, scatter, or the like, radiation. For ease of discussion, and without limitation, radiation that interacts with a target will be termed scattered radiation throughout. Target718can scatter incident radiation, which is represented by scattered beams of radiation719and719′ (also first and second scattered beams of radiation). Scattered beam of radiation719can represent radiation from beam of radiation713that has been scattered by target718. Similarly, scattered beam of radiation719′ can represent radiation from beam of radiation713′ that has been scattered by target718. Optical element742focuses scattered beams of radiation719and719′ such that scattered beams of radiation719and719′ interfere at detector728. Optical element740directs beams of radiation713and713′ such that they are incident on target718at non-zero incidence angles (e.g., off-axis). The term “off-axis” and “wide-angle” may be used herein to refer to a propagation direction that is oblique with respect to a surface, particularly with respect to a plane of a target. The image at the detector728can be an interference pattern (e.g., a Moiré fringe pattern as shown inFIG.6). Detector728can generate a detection signal based on having received scattered beams of radiation719and719′. Detector728can be an imaging detector (e.g., CCD, CMOS, or the like). In this scenario, the detection signal can comprise a digital or analog representation of an image comprising the interference pattern, which are sent to processor732.

In some embodiments, processor732can analyze the detection signal to determine a property of target718. It should be appreciated that the measurement process can be different depending on the specific property of target718being determined. For example, in the case where the property of target718being determined is an alignment position, a measurement is performed on target718alone. In another example, in the case where the property of target718being determined is an overlay error, the measurement compares target718to a second target. Overlay error determination is a process that compares a first target (on a first layer of fabrication) to a second target (on a second layer of fabrication that is different from the first layer) and determines whether the first and second layers are properly overlayed on top of each other. The first and second targets can be, for example, stacked on top of each other or fabricated side-by-side. Determination of other properties of target718, from either target718alone or in conjunction with another target, can be envisaged (e.g., line width, pitch, critical dimension, and the like). Moreover, while beams of radiation713and713′ are described above as both being incident on target718(i.e., alignment measurement), embodiments can be envisaged where a beam of radiation is directed to another target to allow, for example, overlay error measurements. For example, beams of radiation713and/or713′ can be replicated (e.g., using a beam splitter) for sending to another target.

In some embodiments, analysis performed by processor732can be based on target718having been irradiated by beams of radiation713and713′ (e.g., alignment measurement), which have different diffraction orders (e.g., +1 and -1). The analysis comprises, for example, performing a mathematical fit to the Moiré pattern (e.g., fitting a sine function along the direction of the pitch of the Moiré pattern). Using the information inferred from the mathematical fit, the determined property of target718can be improved and made more accurate. This technique reduces the impact of factors that reduce accuracy of measurements, for example, finite size effects, presence of higher diffraction orders, imperfections in gratings and optics, and the like. It should be appreciated that the mathematical fit is performed on a still image. For example, regions of interest may be selected on target718and/or detected pixels may be assigned weighting to enhance accuracy and robustness of measurement. This is described in more detail in PCT/EP2019/072762, filed on Aug. 27, 2019, which is incorporated by reference herein in its entirety.

In some embodiments, wavefront timings (e.g., phases) of beams of radiation719and719′ can be exploited to enhance accuracy in the determination of the property of target718. Adjusting a phase difference of scattered beams of radiation719and719′ can cause the detected Moiré fringes to move (e.g.,FIG.6). The analysis performed by processor732can be based on the image changing over time (e.g., snapshots of two or more states of the detected image). To adjust the phase difference of scattered beams of radiation719and719′, diffractive element708can be adjusted (e.g., translated perpendicular to beam propagation). Diffractive element708can be actuated. Adjusting diffractive element708can effectively modulate the phase difference of beams of radiation713and713′. Therefore, the phase difference of scattered beams of radiation719and719′ can also be modulated because they are based on beams of radiation713and713′, respectively.

In some embodiments, diffractive element708can comprise an optical filter comprising a periodic optical property. Adjusting diffractive element708can comprise adjusting the periodic optical property. For example, diffractive element708can comprise at least one of a piezo-optic device, an electro-optic device, a liquid crystal device, acousto-optic device, and the like, all of which can have an adjustable diffraction optical structure. The type of modulation of the phase difference can be, for example, sinusoidal, linear, sawtooth, triangle, and the like.

To reiterate, in some embodiments, metrology system700can a phase difference of scattered beams of radiation719and719′ (also “first and second scattered beams”). Adjusting the phase difference of scattered beams of radiation719and719′ can comprise at least one of adjusting diffractive element708, adjusting a position of target718(e.g., movement relative to optical system710), or both adjusting diffractive element708and adjusting a position of target718.

In some embodiments, a signal from one or more detector elements (e.g., pixels) of detector728are analyzed by processor732. For simplicity, the following discussion will focus on one detector element, but it should be appreciated that other detector elements can be used in a similar manner. As the Moiré fringes shift across detector728, a detector element can detect a varying radiation intensity that is characteristic of the modulated phase difference (e.g., sinusoidal, linear, sawtooth, triangle, and the like). Similar to how a mathematical fit was described previously for a still image, here, the mathematical fit is performed with respect to time for the intensity detected at the detector element. The fit can be iterated for all relevant detector elements. One or more detector elements may be disregarded by processor732based on, for example, an algorithm that determines the absence of optical energy or outlier behavior at a detector element. The minimum number of “snapshots” required can be, for example, three for a periodic pattern (e.g., sinusoidal). However, it should be appreciated that higher number and density of data points (e.g., higher sampling rate) can produce more precise mathematical fits. Whether more or fewer data points are used can be determined from, for example, hardware processing constraints. Depending on hardware and computational stresses and costs, fewer or more data points can be used.

In some embodiments, processor732can determine a property of target718by comparing the information inferred from the mathematical fit to a reference (e.g., information from a fiducial on stage722). For example, the phase information from target718can be compared to the phase information on the fiducial on stage722. Phase information from fiducial on stage722(not shown) can be acquired by performing a measurement on the fiducial that is similar to the measurement on target718.

In some embodiments, the phase difference of scattered beams of radiation719and719′ can be modulated by moving target718(e.g., by translating stage722). However, each detector element on detector728will “see” a different portion of target718as it is translated, which can render the measurement unusable. To address this issue, in some embodiments, detector728can also be translated to match the movement of target718. In some embodiments, processor732can be made cognizant of the translation of target718and associated detector elements (e.g., map a pixel to the spot on target718for each snapshot as target718moves). The image on detector728can be correlated to the movement of target718based on the magnification of the detection optics.

In some embodiments, movable components can be actuated as a continuous scan or in discrete steps. A combination of both movement types can also be used. Discrete steps for acquisition of each data point can stabilize captured images (e.g., clearer images). However, rapid acceleration and deceleration from the discrete steps can stress the actuating hardware. If continuous motion is used, the hardware stress can be mitigated, but the captured images may have some blurring.

FIG.8shows a metrology system800, according to some embodiments. In some embodiments, metrology system800can also represent a more detailed view of metrology system700(FIG.7). Unless otherwise noted, elements ofFIG.8that have similar reference numbers (e.g., reference numbers sharing the two right-most numeric digits) as elements ofFIGS.4A,4B, and7can have similar structures and functions. For simplicity, such structures and their functions will not be reintroduced.

In some embodiments, metrology system800can further comprise a waveform generator844and a frequency doubler846. Waveform generator844can generate a signal having a function (e.g., an oscillating signal), which can be used in the adjusting of diffractive element808. Therefore, the adjusting of diffractive element808can be based on the oscillating signal. The signal from waveform generator844can be sent to frequency doubler846. A frequency doubled signal from frequency doubler846, as a reference signal, can be sent to detector828. In some embodiments, detector828can comprise a lock-in imaging detector (e.g., a lock-in camera).

In some embodiments, lock-in detection can use the principles of lock-in amplifiers to provide sensitive detection and selective filtering of weak or noisy signals and can improve signal-to-noise ratio (SNR). Lock-in amplifier techniques can provide improved accuracy, faster detection times, and reduced noise in an overlay and/or alignment sensor. Further, compact integrated systems, for example, in a single “on chip” detector, can provide a miniaturized detector for measuring a particular characteristic (e.g., alignment) of an alignment mark on a substrate.

In some embodiments, lock-in detection can use what is known as phase-sensitive detection to single out a component of a signal at a specific reference frequency and phase, and can extract the signal from an extremely noisy background. Lock-in detection relies on the orthogonality of sinusoidal functions and can multiply an input signal by a reference signal(s) and integrate the resulting signal over a specified time(s) (e.g., low-pass filter) to extract the desired components (e.g., phase and amplitude).

In some embodiments, lock-in detection can use homodyne detection or heterodyne detection. Homodyne detection uses a single reference frequency (e.g., first frequency ƒ1) to extract the modulated signal. For example, homodyne detection extracts encoded information from an oscillating signal (e.g., phase and/or frequency) by comparing that signal with a standard reference oscillation (e.g., identical to the signal if it carried null information). Heterodyne detection uses two reference frequencies (e.g., first frequency ƒ1and second frequency ƒ2) to extract the modulated signal. For example, heterodyne detection extracts encoded information from an oscillating signal (e.g., phase and/or frequency) by comparing that signal with a standard reference oscillation (e.g., identical to the signal if it carried null information) as well as comparing that signal with a beat frequency (e.g., difference) between the first and second frequencies (e.g., ƒ1- ƒ2). Further, by mixing two frequencies (e.g., ƒ1and ƒ2), a higher frequency than a detector response time can be measured (e.g., ƒ1- ƒ2), and flicker noise (e.g., ⅟ƒ power spectral density) can be reduced.

In some embodiments, adjusting the phase difference of beams of radiation813and813′ can cause twice the amount of change to the phase difference of scattered beams of radiation819and819′. Therefore, detector828can receive a reference signal (e.g., from frequency doubler846) with twice the frequency of the frequency used on diffractive element808. A portion(s) of detector828(e.g., one or more detector elements) contributes to the detection signal a phase and/or frequency of intensity modulation detected at the portion(s) of detector828. The detected phase of intensity modulation is not to be conflated with phases of illumination wavefronts. Processor832can analyze signals from the portion(s) of detector828to determine phase(s) and/or frequency of intensity modulation at the portion(s). Processor832can determine a property of target818by comparing the phase of modulation at each detector element to a reference (e.g., information from a fiducial on stage822or substrate820). A portion of detector828(e.g., one or more detector elements) may be disregarded by processor832based on, for example, an algorithm that determines the absence of optical energy, outlier behavior, and/or noise at a detector element. Processor832can be a processor in the lock-in camera.

In some embodiments, detector828can comprise the electronics for performing lock-in techniques for each detector element (e.g., C2heliCam by Heliotis AG, Lucerne, Switzerland).

In some embodiments, detector828can be disposed away from the location where optical element842focuses scattered beams of radiation819and819′. In this scenario, a fiber array (not shown) can be disposed at the focal point of optical element542. The fiber array can guide the radiation representing the image to detector828.

FIG.9shows a metrology system900, according to some embodiments. In some embodiments, metrology system900can also represent a more detailed view of metrology system700(FIG.7). Unless otherwise noted, elements ofFIG.9that have similar reference numbers (e.g., reference numbers sharing the two right-most numeric digits) as elements ofFIGS.4A,4B,7, and8can have similar structures and functions. For simplicity, such structures and their functions will not be reintroduced.

In some embodiments, metrology system can comprise an optical system908(e.g., beam-splitting element). For disambiguation with respect to other optical systems disclosed herein, optical system908may be referred to as a “first optical system” and identified further by its functions (e.g., beam splitting). Similarly, second, third, and further optical systems may be defined based on their functions. For example, “second” optical system910for directing beams of radiation. In another example, optical element942may be referred to as a “third” optical system for interfering scattered beams of radiation919and919′ interfere at a detector928. Optical system908can comprise a diffractive element (e.g., diffractive element808inFIG.8). In some embodiments, optical system908can comprise a beam splitter, prism, mirror, and the like. In some embodiments, optical system908can be a combination of any of beam splitters, prisms, mirrors, and the like. Metrology system may further comprise one or more phase adjusters948and one or more phase adjusters950. Each of one or more phase adjusters948and/or950can be any of an acousto-optic modulator, an electro-optic modulator, a piezo-optic modulator, a thermo-optic modulator, a variable refractive index device, a variable path length device, and the like. In some embodiments, each of one or more phase adjusters948and/or950can be a combination of any of acousto-optic modulators, electro-optic modulators, a piezo-optic modulators, a thermo-optic modulators, a variable refractive index device, a variable path length device, and the like.

In some embodiments, optical system908can generate beams of radiation913and913′ (e.g., via diffraction or beam splitting of radiation916). Any of beams of radiation913and913′ can be transmitted through one or more phase adjusters948. One or more phase adjusters948can be adjusted to change the phase(s) of beams of radiation913and/or913′. In this manner, the phase difference between beams of radiation913and913′ is adjusted. And, though two phase adjusters948are shown inFIG.9, it should be appreciated that it is possible to adjust the phase difference with just one phase adjuster948. The phase(s) of beams of radiation919and919′ can be similarly adjusted using one or more phase adjusters950. It should be appreciated that one or more phase adjusters950can adjust the phase difference of radiation after scattering from target918, whereas one or more phase adjusters948can adjust the phase difference of radiation before scattering from target918.

In some embodiments, by using one or more phase adjusters948and/or950, optical system908can remain static, in contrast toFIGS.7and8, where a diffractive element was adjustable to allow adjustment of the phase difference. However, in some embodiments, optical system908may be adjustable (e.g., allowed to translate perpendicular to beam direction) in order to adjust the phase difference between beams of radiation913and913′).

FIG.10shows method steps for performing functions described herein, according to some embodiments. The method steps ofFIG.10can be performed in any conceivable order and it is not required that all steps be performed. Moreover, the method steps ofFIG.10described below merely reflect an example of steps and are not limiting. That is, further method steps and functions may be envisaged based upon embodiments described in reference toFIGS.1-9.

At step1002, radiation is generated.

At step1004, the radiation is diffracted to generate first and second beams of radiation using an adjustable diffractive element. The first beam comprises a first non-zero diffraction order. The second beam comprises a second non-zero diffraction order different from the first non-zero diffraction order.

At step1006, the first and second beams are directed toward a target structure such that first and second scattered beams of radiation are generated based on the first and second beams, respectively.

At step1008, a phase difference of the first and second scattered beams is adjusted.

At step1010, the first and second scattered beams are interfered at an imaging detector.

At step1012, a detection signal is generated using an imaging detector.

At step1014, the detection signal is analyzed to determine a property of the target structure based on at least the adjusted phase difference using a processor.

The method steps ofFIG.10may be performed in any conceivable order and it is not required that all steps be performed. Moreover, the method steps ofFIG.10described above merely reflect an example of steps and are not limiting. That is, further method steps and functions may be envisaged based upon embodiments described in reference toFIGS.1-9, as well asFIGS.11-15described below.

FIG.11shows an interference pattern1152formed on a detector1128, according to some embodiments. In some embodiments, briefly referencingFIG.9, detector1128can be used as detector928. Interference pattern1152can be formed due to optical element942interfering scattered beams of radiation919and919′ at detector1128. Detector1128can comprise an imaging detector. Detector1128can comprise detector elements1154(e.g., pixels). It was described earlier that the phase difference of scattered beams of radiation919and919′ can be modulated (e.g., modulated sinusoidally over time). Correspondingly, interference pattern1152can appear to move across detector elements1154. When considering only a single detector element1154, the detected illumination intensity at the single detector element1154can increase and/or decrease as the interference pattern1152moves.

In some embodiments, interference pattern1152can be the result of a superposition of a plurality of wavelengths. That is, detector1128can receive the plurality of wavelength, simultaneously, of radiation that has been scattered by a target. Since the phase of each wavelength contribution at detector1128is dependent on the wavelength, the intensity variations at each detector element1154as interference pattern moves may not have a direct correspondence to the modulation of the phase difference. There can be one or more phase difference values corresponding to one or more wavelengths. However, the phase modulation schemes of a metrology apparatus can be configured such that a modulation of phase differences between two beams of radiation (having multiple wavelengths) can be, for example, proportional with respect to wavelength.

FIG.12shows various graphs1256,1258, and1260describing various parameters and effects relating to modulating phase differences of beams of radiation (e.g., beams of radiation919and919′ (FIG.9)), according to some embodiments. In graph1256, the vertical axis represents a value of the modulated phase difference (also “dephasing”) between two beams of radiation. The horizontal axis can represent a progression parameter in arbitrary units a.u.-e.g., time, frame slices, amount by which optical system908(FIG.9) is adjusted, frame slices, or the like. The modulation of the dephasing shown in graph1256is that of a sawtooth pattern, however, it should be understood that the modulation can be any suitable pattern (e.g., sinusoidal). As the dephasing is modulated, a detector (e.g.,1128(FIG.11)) can acquire a sequence of images, denoted by frames1262. Frames1262can comprise a plurality of images. Each image can be divided into portions, with each portion corresponding to pixels1254of the detector.

In some embodiments, graph1258shows a detected intensity signal at a given pixel. The vertical axis of graph1258represents the detected intensity. The horizontal axis represents a progression parameter (e.g., time, frame number, or the like). If the detected signal were to correspond to one wavelength, then the intensity data in graph1258would correspondingly display a signal similar to the dephasing in graph1256(e.g., a sawtooth, sinusoidal, or other pattern having the same periodicity). However, when multiple wavelengths, and resulting phase deviations, are superimposed on the detector, the evolution of the detection signal associated at the given pixel can be as shown in graph1258.

In some embodiments, a processor can analyze the detection signal determine a property of the structure based on the phase difference imparted on the detected beams of radiation. For example, the processor can perform frequency analysis on the signal in graph1258to extract wavelength information, for example, intensity and modulation phase value of each wavelength contribution. The frequency analysis can comprise a demodulation of the information in the detection signal to frequency domain. For example, the demodulation can comprise a Fourier transform.

In some embodiments, graph1260illustrates an example of wavelength decomposition resulting from a frequency analysis of graph1258. The horizontal axis represents wavelength. The vertical axis represents a metric of a property of a target on which metrology was performed (e.g., target518). The metric can be, for example, at least a phase of modulation and/or intensity of each wavelength contribution, or a quantity determined therefrom. The phases of modulation can be mapped to corresponding wavelengths. For example, it was previously mentioned that the modulation of the phase differences can be proportional with respect to wavelengths. The proportionality mapping is provided as a non-limiting example, and therefore other suitable mapping schemes can be used. The property of the target can be, for example, an alignment position of the target. The alignment position can be determined based on the metric. The values of the metric can be based on the phase information determined for the given pixel using frequency analysis.

In some embodiments, multiple graphs1256,1258, and1260can be can be generated, for example, for each of pixels1254. Then, the determining of the property of the target can be based on the frequency analysis of all of pixels1254. The combination of information from multiple pixels can enhance accuracy in determining the property of the target.

In some embodiments, errors in the fabrication of the target causes changes to the scattering behavior of each wavelength. Consequently, the data in graphs1256,1258, and1260can be difficult to reproduce from one fabrication of the target to the next due to fabrication uncertainties. Conversely, an ideal target without uncertainties would result in highly reproducible graphs1256,1258, and1260. Therefore, the method of modulating the phase difference of scattered radiation at multiple wavelengths can be used to discriminate effects of target uncertainties and determine the property of the target to a much higher accuracy than without the use of phase difference modulation.

FIG.13shows a phase modulation portion of an illumination system1312, according to some embodiments. In some embodiments, illumination system1312can also represent an alternative or a more detailed view of illumination systems712(FIG.7) and/or912(FIG.9). Unless otherwise noted, elements ofFIG.13that have similar reference numbers (e.g., reference numbers sharing the two right-most numeric digits) as elements ofFIGS.4A,4B,7-9, and11can have similar structures and functions. For simplicity, such structures and their functions will not be reintroduced.

In some embodiments, illumination system1312comprises an optical system1308. Optical system1308comprises beam splitting elements1364and1364′, optical elements1366and1366′ (e.g., reflectors), and a phase adjuster1348. Phase adjuster1348(e.g., a piezo device) can be in contact with optical element1366′. If desired, a second phase adjuster (not shown) can be mechanically coupled to optical element1366. Beam splitting elements1364and1364′ and optical elements1366and1366′ can be arranged to split an input beam of radiation1316into beams of radiation1313and1313′. Also shown inFIG.13is an optical system1310for directing beams of radiation1313and1313′ onto a target1318. Optical system1310can be external to, or comprised within, illumination system1312.

In some embodiments, beam of radiation1316can interact with beam splitting element1364′ to produce first and second beams. The first beam is directed toward optical element1366′, which is then reflected toward optical system1310as beam of radiation1313′. The second beam continues through to beam splitting element1364. The second beam is directed toward optical system1310as beam of radiation1313. It should be appreciated that beam splitting element1364can be omitted. In this scenario, optical element1366can be used instead of beam splitting element1364to direct beam of radiation1313. The use of beam splitting element1364can be helpful, for example, to preserve structural symmetry and/or to direct scattered radiation from target1318.

In some embodiments, phase adjuster1348can be adjusted (e.g., actuated) along direction1368to adjust a path length of beam of radiation1313′. Direction1368is parallel to a propagation direction of the first beam reflected from optical element1366′. Consequently, phase adjuster1348can be used to dephase beam of radiation1313′ with respect to beam of radiation1313—that is, adjust a phase difference of beams of radiation1313and1313′. The phase difference adjustment can be, for example, periodic (e.g., sawtooth or sinusoidal pattern).

FIG.14shows a phase modulation portion of an illumination system1412, according to some embodiments. In some embodiments, illumination system1412can also represent an alternative or a more detailed view of illumination systems712(FIG.7),912(FIG.9), and/or1312(FIG.13). Unless otherwise noted, elements ofFIG.14that have similar reference numbers (e.g., reference numbers sharing the two right-most numeric digits) as elements ofFIGS.4A,4B,7-9,11, and13can have similar structures and functions. For simplicity, such structures and their functions will not be reintroduced.

In some embodiments, illumination system1412comprises an optical system1408. Optical system1408comprises beam splitting element1464and phase adjusters1448and1448′ (e.g., actuatable retro-reflectors). Beam splitting element1464can split an input beam of radiation1416into beams of radiation1413and1413′. Also shown inFIG.14is an optical system1410for directing beams of radiation1413and1413′ onto a target1418. Optical system1410can be external to, or comprised within, illumination system1412.

In some embodiments, beam of radiation1416can interact with beam splitting element1464to produce first and second beams. The first beam is directed toward phase adjuster1448, which is then reflected toward optical system1410as beam of radiation1413. The second beam is directed toward phase adjuster1448′, which is then reflected toward optical system1410as beam of radiation1413′.

In some embodiments, phase adjuster1448can be adjusted (e.g., actuated) along direction1468to adjust a path length of beam of radiation1413. Direction1468is perpendicular to a propagation direction of the received first beam. By adjusting phase adjuster1448along direction1468, a path length1470of the first beam can be adjusted. Consequently, phase adjuster1448can be used to dephase beam of radiation1413′ with respect to beam of radiation1413—that is, adjust a phase difference of beams of radiation1413and1413′. The phase difference adjustment can be, for example, periodic (e.g., sawtooth or sinusoidal pattern). Correspondingly similar functions can be described with respect to phase adjuster1448′, and direction1468′, and beam of radiation1413′. It should be appreciated that either of phase adjusters1448and1448′ can be replaced with a stationary reflector while still achieving the goal of dephasing adjustment between beams of radiation1413and1413′.

FIG.15shows a phase modulation portion of an illumination system1512, according to some embodiments. In some embodiments, illumination system1512can also represent an alternative or a more detailed view of illumination systems712(FIG.7),912(FIG.9),1312(FIG.13), and/or1512(FIG.14). Unless otherwise noted, elements ofFIG.15that have similar reference numbers (e.g., reference numbers sharing the two right-most numeric digits) as elements ofFIGS.4A,4B,7-9,11,13, and14can have similar structures and functions. For simplicity, such structures and their functions will not be reintroduced.

In some embodiments, illumination system1512comprises an optical system1508. Optical system1508comprises beam splitting element1564and phase adjuster1548. Phase adjuster1548can comprise a plurality of wedge prisms1572. One or more of wedge prisms1572can be adjusted (e.g., actuated). Beam splitting element1564can split an input beam of radiation1516into beams of radiation1513and1513′. Also shown inFIG.15is an optical system1510for directing beams of radiation1513and1513′ onto a target1518. Optical system1510can be external to, or comprised within, illumination system1512.

In some embodiments, beam of radiation1513′ can be passed through phase adjuster1548. Phase adjuster1548can be adjusted to adjust a path length of beam of radiation1513. For example, one or more of actuatable wedge prisms1572can be actuated along direction1568. By adjusting phase adjuster1548along direction1568, a path length of the first beam can be adjusted via adjusting the amount of material traversed by beam of radiation1513′ (e.g., material of higher refractive index). Consequently, phase adjuster1548can be used to dephase beam of radiation1513′ with respect to beam of radiation1513—that is, adjust a phase difference of beams of radiation1513and1513′. The phase difference adjustment can be, for example, periodic (e.g., sawtooth or sinusoidal pattern).

WhileFIGS.13-15describe the use of discrete phase adjusters for modulating phase differences, it should be appreciated that phase adjusting structures described in other figures may be used to modulate phase differences (e.g., diffractive element708(FIG.7)).

Further embodiments are disclosed in the subsequent numbered clauses:

1. A metrology system comprising:a radiation source configured to generate radiation;an adjustable diffractive element configured to diffract the radiation to generate first and second beams of radiation, wherein the first beam comprises a first non-zero diffraction order and the second beam comprises a second non-zero diffraction order different from the first non-zero diffraction order;an optical system configured to direct the first and second beams toward a target structure such that first and second scattered beams of radiation are generated based on the first and second beams, respectively, wherein the metrology system is configured to adjust a phase difference of the first and second scattered beams;an optical element configured to interfere the first and second scattered beams at an imaging detector, wherein the imaging detector is configured to generate a detection signal; anda processor configured to receive and analyze the detection signal to determine a property of the target structure based on at least the adjusted phase difference.

2. The metrology system according to clause 1, wherein the adjusting the phase difference comprises one of adjusting the adjustable diffractive element, adjusting a position of the target structure relative to the optical system, and adjusting the adjustable diffractive element and the position of the target structure.

3. The metrology system according to clause 2, further comprising a support structure configured to support a substrate comprising the target structure and to perform the adjusting the position of the target structure.

4. The metrology system according to clause 2, wherein:the adjustable diffractive element comprises a grating; andthe adjusting the adjustable diffractive element comprises translating the grating.

5. The metrology system according to clause 1, wherein:the adjustable diffractive element comprises an optical filter comprising a periodic optical property; andthe adjusting the adjustable diffractive element comprises adjusting the periodic optical property.

6. The metrology system according to clause 5, wherein the optical filter comprises at least one of acousto-optic device, electro-optic device, piezo-optic device, and liquid crystal device.

7. The metrology system according to clause 1, wherein the property of the target structure comprises an alignment position.

8. The metrology system according to clause 1, further comprising a waveform generator to generate an oscillating signal, wherein the adjusting the diffractive element is based on the oscillating signal.

9. The metrology system according to clause 8, wherein the imaging detector comprises a lock-in camera.

10. The metrology system according to clause 9, wherein the detection signal comprises a phase of intensity modulation at a portion of the lock-in camera.

11. The metrology system according to clause 10, wherein the determining the property of the target structure is based on the phase of intensity modulation at the portion of the lock-in camera.

12. A lithographic apparatus comprising:an illumination system configured to illuminate a pattern of a patterning device;a projection system configured to project an image of the pattern onto a substrate; andmetrology system comprising:a radiation source configured to generate radiation;an adjustable diffractive element configured to diffract the radiation to generate first and second beams of radiation, wherein the first beam comprises a first non-zero diffraction order and the second beam comprises a second non-zero diffraction order different from the first non-zero diffraction order;an optical system configured to direct the first and second beams toward a target structure such that first and second scattered beams of radiation are generated based on the first and second beams, respectively, wherein the metrology system is configured to adjust a phase difference of the first and second scattered beams;an optical element configured to focus and interfere the first and second scattered beams at an imaging detector, wherein the imaging detector is configured to generate a detection signal; anda processor configured to analyze the detection signal to determine a property of the target structure based on at least the adjusted phase difference.

13. The lithographic apparatus according to clause 12, wherein the adjusting the phase difference comprises one of adjusting the adjustable diffractive element, adjusting a position of the target structure relative to the optical system, and adjusting the adjustable diffractive element and the position of the target structure.

14. The lithographic apparatus according to clause 13, wherein the metrology system further comprises a support structure configured to support the substrate comprising the target structure and to perform the adjusting the position of the target structure.

15. The lithographic apparatus according to clause 13, wherein:the adjustable diffractive element comprises a grating; andthe adjusting the adjustable diffractive element comprises translating the grating.

16. The lithographic apparatus according to clause 13, wherein the adjustable diffractive element comprises at least one of acousto-optic device, electro-optic device, piezo-optic device, and liquid crystal device.

17. The lithographic apparatus according to clause 13, wherein the property of the target structure comprises an alignment position.

18. The lithographic apparatus according to clause 13, wherein:the imaging detector comprises a lock-in camera; andthe detection signal comprises a phase of intensity modulation at a portion of the lock-in camera.

19. The lithographic apparatus according to clause 18, wherein the determining the property of the target structure is based on the phase of intensity modulation at the portion of the lock-in camera.

20. A method comprises:generating radiation;diffracting the radiation to generate first and second beams of radiation using an adjustable diffractive element, wherein the first beam comprises a first non-zero diffraction order and the second beam comprises a second non-zero diffraction order different from the first non-zero diffraction;directing the first and second beams toward a target structure such that first and second scattered beams of radiation are generated based on the first and second beams, respectively;adjusting a phase difference of the first and second scattered beams;interfering the first and second scattered beams at an imaging detector;generating a detection signal using the imaging detector; andanalyzing the detection signal to determine a property of the target structure based on at least the adjusted phase difference using a processor.

21. A metrology system comprising:a radiation source configured to generate radiation;a beam-splitting element configured to split the radiation to generate first and second beams of radiation and to adjust a phase difference of the first and second beams;an optical system configured to direct the first and second beams toward a target structure such that first and second scattered beams of radiation are generated based on the first and second beams, respectively, wherein a phase difference of the first and second scattered beams is based on the phase difference of the first and second beams;an optical element configured to interfere the first and second scattered beams at an imaging detector, wherein the imaging detector is configured to generate a detection signal; anda processor configured to receive and analyze the detection signal to determine a property of the target structure based on at least the adjusted phase difference.

22. A metrology system comprising:a radiation source configured to generate source radiation;a beam-splitting element configured to split the source radiation to generate first and second beams of radiation;an optical system configured to direct the first and second beams toward a target structure such that first and second scattered beams of radiation are generated based on the first and second beams, respectively;a phase adjuster configured to adjust a phase difference of the first and second scattered beams;an optical element configured to interfere the first and second scattered beams at an imaging detector, wherein the imaging detector is configured to generate a detection signal; anda processor configured to receive and analyze the detection signal to determine a property of the target structure based on at least the adjusted phase difference.

23. The metrology system according to clause 22, wherein the beam-splitting element comprises any one of a beam splitter, a prism, and a mirror.

24. The metrology system according to clause 22, wherein:the beam-splitting element comprises a diffractive element configured to diffract the source radiation to generate the first and second beams; andthe first beam comprises a first non-zero diffraction order and the second beam comprises a second non-zero diffraction order different from the first non-zero diffraction order.

25. The metrology system according to clause 22, wherein the phase adjuster is further configured to adjust a phase of a first one of the first beam, the second beam, the first scattered beam, and the second scattered beam so as to adjust the phase difference.

26. The metrology system according to clause 25, further comprising another phase adjuster configured to adjust a phase of a second one of the first beam, the second beam, the first scattered beam, and the second scattered beam so as to adjust the phase difference.

27. The metrology system according to clause 22, wherein the property of the target structure comprises an alignment position.

28. The metrology system according to clause 22, further comprising a waveform generator to generate an oscillating signal, wherein the adjusting is based on the oscillating signal.

29. The metrology system according to clause 28, wherein the imaging detector comprises a lock-in camera.

30. The metrology system according to clause 29, wherein the detection signal comprises a phase of intensity modulation at a portion of the lock-in camera.

31. The metrology system according to clause 30, wherein the determining the property of the target structure is based on the phase of intensity modulation at the portion of the lock-in camera.

32. A metrology system comprising:a radiation source configured to generate radiation;a first optical system configured to split the radiation into first and second beams of radiation and impart one or more phase differences between the first and second beams;a second optical system configured to direct the first and second beams toward a target structure to produce first and second scattered beams of radiation;a third optical system configured to interfere the first and second scattered beams at an imaging detector, wherein the imaging detector is configured to generate a detection signal based on the interfered first and second scattered beams, wherein the metrology system is configured to modulate one or more phase differences of the first and second scattered beams based on the imparted one or more phase differences; anda processor configured to analyze the detection signal to determine a property of the target structure based on at least the modulated one or more phase differences.

33. The metrology system of clause 32, wherein:the radiation source is further configured to generate multiple wavelengths such that each of the first and second scattered beams comprise a plurality of wavelengths; andthe imaging detector is further configured to receive the plurality of wavelengths, simultaneously, of the interfered first and second scattered beams.

34. The metrology system of clause 33, wherein the processor is further configured to:perform a frequency analysis of the detection signal to determine one or more phases of modulation of the plurality of wavelengths received at one or more portions of the imaging detector; anddetermine the property of the target structure based on the frequency analysis.

35. The metrology system of clause 34, wherein:the frequency analysis comprises a demodulation of the information in the detection signal to frequency domain; andthe determining the one or more phases of modulation comprises mapping each of the one or more phases of modulation to corresponding ones of the plurality of wavelengths.

36. The metrology system of clause 35, wherein the demodulation comprises a Fourier transform.

37. The metrology system of clause 33, wherein the processor is further configured to:perform the frequency analysis of the detection signal to determine one or more phases of modulation of the plurality of wavelengths received at two or more portions of the imaging detector; anddetermine the property of the target structure based on the frequency analysis.

38. The metrology system of clause 32, wherein the metrology system is further configured to modulate the one or more phase differences of the first and second scattered beams proportionally with respect to wavelength.

39. The metrology system of clause 32, wherein the modulating the one or more phase differences of the first and second scattered beams comprises adjusting the imparted one or more phase differences between the first and second beams using the first optical system and/or adjusting a position of the target structure relative to the second optical system.

40. The metrology system of clause 39, wherein the first optical system comprises an adjustable periodic structure; and

the adjusting the imparted one or more phase differences between the first and second beams comprises adjusting the adjustable periodic structure.

41. The metrology system of clause 39, wherein the first optical system comprises:a beam splitting element configured to perform the splitting;first and second optical elements configured to direct the first and second beams, respectively; anda phase adjuster mechanically coupled to the first optical element and configured to adjust the first optical element to perform the adjusting the imparted one or more phase differences between the first and second beams.

42. The metrology system of clause 39, wherein the first optical system comprises:a beam splitting element configured to perform the splitting; andfirst and second retro-reflectors configured to direct the first and second beams, respectively, wherein at least one of the first and second retro-reflectors is further configured to be adjusted to perform the adjusting the imparted one or more phase differences between the first and second beams.

43. The metrology system of clause 39, wherein the first optical system comprises:a beam splitting element configured to perform the splitting; anda phase adjuster comprising wedges, wherein at least one of the wedges is configured to be adjusted to perform the adjusting the imparted one or more phase differences between the first and second beams.

44. The metrology system of clause 32, wherein the property of the target structure comprises an alignment position.

45. The metrology system of clause 32, wherein the imaging detector comprises a lock-in camera.

46. The metrology system of clause 45, wherein the detection signal comprises information about the modulated one or more phase differences of the first and second scattered beams at one or more pixels of the lock-in camera.

47. The metrology system of clause 46, wherein:the processor is configured to analyze the information; andthe determining the property of the target structure is further based on the information.

48. A lithographic apparatus comprising:an illumination system configured to illuminate a pattern of a patterning device;a projection system configured to project an image of the pattern onto a substrate; anda metrology system comprising:a radiation source configured to generate radiation;a first optical system configured to split the radiation into first and second beams of radiation and impart one or more phase differences between the first and second beams;a second optical system configured to direct the first and second beams toward a target structure to produce first and second scattered beams of radiation;a third optical system configured to interfere the first and second scattered beams at an imaging detector, wherein the imaging detector is configured to generate a detection signal based on the interfered first and second scattered beams, wherein the metrology system is configured to modulate one or more phase differences of the first and second scattered beams based on the imparted one or more phase differences; anda processor configured to analyze the detection signal to determine a property of the target structure based on at least the modulated one or more phase differences.

49. The lithographic apparatus of clause 48, wherein:the radiation source is further configured to generate multiple wavelengths such that each of the first and second scattered beams comprise a plurality of wavelengths; andthe imaging detector is further configured to receive the plurality of wavelengths, simultaneously, of the interfered first and second scattered beamsthe processor is further configured to:perform a frequency analysis of the detection signal to determine one or more phases of modulation of the plurality of wavelengths received at one or more portions of the imaging detector; anddetermine the property of the target structure based on the frequency analysis;

50. The lithographic apparatus of clause 48, wherein the property of the target structure comprises an alignment position.

51. A method comprising:generating radiation;splitting the radiation into first and second beams of radiation using a first optical system;imparting one or more phase differences between the first and second beams using the first optical system;directing the first and second beams toward a target to produce first and second scattered beams of radiation;interfering the first and second scattered beams at an imaging detector;modulating one or more phase differences of the first and second scattered beams;generating a detection signal using the imaging detector;analyzing the detection signal to determine a property of the target structure based on at least the modulated one or more phase differences.

Although specific reference can be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, LCDs, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein can be processed, before or after exposure, in for example a track unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology unit and/or an inspection unit. Where applicable, the disclosure herein can be applied to such and other substrate processing tools. Further, the substrate can be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present disclosure is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

The terms “radiation,” “beam,” “light,” “illumination,” and the like as used herein may encompass all types of electromagnetic radiation, for example, ultraviolet (UV) radiation (for example, having a wavelength λ of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-20 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G-line 436 nm; H-line 405 nm; and/or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in some embodiments, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of 5-20 nm.

The term “substrate” as used herein describes a material onto which material layers are added. In some embodiments, the substrate itself can be patterned and materials added on top of it may also be patterned, or may remain without patterning.

Although specific reference can be made in this text to the use of the apparatus and/or system according to the present disclosure in the manufacture of ICs, it should be explicitly understood that such an apparatus and/or system has many other possible applications. For example, it can be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, LCD panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle,” “wafer,” or “die” in this text should be considered as being replaced by the more general terms “mask,” “substrate,” and “target portion,” respectively.

While specific embodiments of the disclosure have been described above, it will be appreciated that embodiments of the present disclosure may be practiced otherwise than as described. The descriptions 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 disclosure as described without departing from the scope of the claims set out below.

The foregoing description of the specific embodiments will so fully reveal the general nature of the present disclosure that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein.

The breadth and scope of the protected subject matter should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.