Beam Pointing Monitor and Compensation Systems

An optical system for beam pointing monitoring and compensation is provided. According to an embodiment, a beam pointing monitor and compensation system includes a surface plasmon resonance (SPR) optical element (800). The SPR optical element includes an optical element (801) that includes first (806) and second (802) surfaces. The first and second surfaces of the optical element are substantially parallel to each other. The SPR optical element further includes a first metal layer (803) provided on the second surface of the optical element, a dielectric layer (805) provided on the first metal layer, and a second metal layer (807) provided on the dielectric layer.

FIELD

Embodiments of the present disclosure relate to monitoring and compensation systems and methods for monitoring an angle of incidence of an optical beam and compensating for deviations, suitable for use as part of a lithographic apparatus.

BACKGROUND

During lithographic operation, different processing steps may require different layers to be sequentially formed on the substrate. Accordingly, it may be necessary to position the substrate relative to prior patterns formed thereon with a high degree of accuracy. Generally, alignment marks, which may comprise diffraction gratings are placed on the substrate to be aligned and are located with reference to a second object. Lithographic apparatus may use an alignment system for detecting positions of the alignment marks and for aligning the substrate using the alignment marks to ensure accurate exposure from a mask.

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 may 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 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.

Illumination systems used in lithographic apparatuses, alignment systems, and/or scatterometers can include beam pointing sensors. However, most sensor designs fail to determine beam pointing variation with a precision that is desired.

SUMMARY

In some embodiments of this disclosure, beam pointing monitor methods and systems are provided for more accurate beam pointing measurements. Additionally, in some embodiments of this disclosure, beam pointing compensation systems are provided to compensate for measured beam pointing variations.

According to an embodiment, a beam pointing monitor and compensation system includes a surface plasmon resonance (SPR) optical element. The SPR optical element includes an optical element that includes first and second surfaces. The first and second surfaces of the optical element are substantially parallel to each other. The SPR optical element further includes a first metal layer provided on the second surface of the optical element, a dielectric layer provided on the first metal layer, and a second metal layer provided on the dielectric layer.

In another embodiment, a system includes an illumination system configured to provide a radiation beam and a beam pointing monitor system. The beam pointing monitor system includes a surface plasmon resonance (SPR) optical element. The SPR optical element includes an optical element that includes first and second surfaces. The first and second surfaces of the optical element are substantially parallel to each other. The SPR optical element further includes a first metal layer provided on the second surface of the optical element, a dielectric layer provided on the first metal layer, and a second metal layer provided on the dielectric layer. The beam pointing monitor system is configured to measure an angle of incidence of the radiation beam with respect to a normal to the second surface of the optical element

Yet in another embodiment, a method includes illuminating, with a radiation beam, a surface plasmon resonance (SPR) optical element. The radiation beam is incident on the SPR optical element at an angle of incidence with respect to a normal to the SPR optical element. The SPR optical element comprises an optical element including first and second surfaces, where the first and second surfaces are substantially parallel to each other. The method also includes detecting, using a detector, a reflected radiation beam reflected from the SPR optical element. The SPR optical element provides a SPR of the reflected radiation beam. The method also includes measuring an intensity of the reflected radiation beam and determining the angle of incidence.

In another embodiment, an alignment system includes an illumination system configured to provide a radiation beam and a beam pointing monitor configured to measure an angle of incidence of the radiation beam with respect to a normal to a surface of the beam pointing monitor and configured to determine a beam pointing variation. The alignment system further includes a beam pointing compensator configured to receive a control signal based on the determined beam pointing variation and configured to adjust the angle of incidence of the radiation beam.

Yet in another embodiment, a lithographic apparatus includes a first illumination system configured to illuminate a pattern of a patterning device. The lithographic apparatus further includes a projection system configured to project an image of the pattern on to a target portion of a substrate. The lithographic apparatus also includes a system including a second illumination system configured to provide a radiation beam and a beam pointing monitor and compensation system. The beam pointing monitor and compensation system includes a surface plasmon resonance (SPR) optical element. The SPR optical element includes an optical element comprising first and second surfaces, a first metal layer provided on the second surface of the optical element, a dielectric layer provided on the first metal layer, and a second metal layer provided on the dielectric layer. The beam pointing monitor and compensation system is configured to measure an angle of incidence of the radiation beam with respect to a normal to the second surface of the optical element.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporate the features of this disclosure. The disclosed embodiment(s) merely exemplify the disclosure. The scope of the disclosure is not limited to the disclosed embodiment(s). The disclosure is 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 may be implemented.

Example Reflective and Transmissive Lithographic Systems

FIGS. 1A and 1Bare schematic illustrations of a lithographic apparatus100and lithographic apparatus100′, respectively, in which embodiments of the present disclosure may 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 may 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 may 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 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 and 1B, 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 PPU conjugate 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 a mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.

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, 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 may include a facetted field mirror device222and a facetted 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 elements228,230onto a substrate W held by the wafer stage or substrate table WT.

More elements than shown may generally be present in illumination optics unit IL and projection system PS. The grating spectral filter240may optionally be present, depending upon the type of lithographic apparatus. Further, there may be more mirrors present than those shown in theFIG. 2, for example there may be 1-6 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 reflectors253,254and255, just as an example of a collector (or collector minor). The grazing incidence reflectors253,254and255are 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.

Example Lithographic Cell

FIG. 3shows a lithographic cell300, also sometimes referred to a lithocell or cluster. Lithographic apparatus100or100′ may form part of lithographic cell300. Lithographic cell300may also include apparatus 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 apparatus and delivers then to the loading bay LB of the lithographic apparatus. 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 the supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatus can be operated to maximize throughput and processing efficiency.

In order to ensure that the substrates that are exposed by a lithographic apparatus, such as lithographic apparatus100and/or100′ are exposed correctly and consistently, it is desirable to inspect exposed substrates to measure properties such as overlay errors between subsequent layers, line thicknesses, critical dimensions (CD), etc. If errors are detected, adjustments may be made to exposures of subsequent substrates, especially if the inspection can be done soon and fast enough that other substrates of the same batch are still to be exposed. Also, already exposed substrates may be stripped and reworked—to improve yield—or discarded, thereby avoiding performing exposures on substrates that are known to be faulty. In a case where only some target portions of a substrate are faulty, further exposures can be performed only on those target portions which are good.

An inspection apparatus may be used to determine the properties of the substrates, and in particular, how the properties of different substrates or different layers of the same substrate vary from layer to layer. The inspection apparatus may be integrated into a lithographic apparatus, such as lithographic apparatus100and/or100′ or lithocell300or may be a stand-alone device. To enable most rapid measurements, it is desirable that the inspection apparatus measure properties in the exposed resist layer immediately after the exposure. However, the latent image in the resist has a very low contrast—there is only a very small difference in refractive index between the parts of the resist which have been exposed to radiation and those which have not—and not all inspection apparatus have sufficient sensitivity to make useful measurements of the latent image. Therefore measurements may be taken after the post-exposure bake step (PEB) which is customarily the first step carried out on exposed substrates and increases the contrast between exposed and unexposed parts of the resist. At this stage, the image in the resist may be referred to as semi-latent. It is also possible to make measurements of the developed resist image—at which point either the exposed or unexposed parts of the resist have been removed—or after a pattern transfer step such as etching. The latter possibility limits the possibilities for rework of faulty substrates but may still provide useful information.

FIG. 4depicts a scatterometer SM1which may be used in the present disclosure. Scatterometer SM1may be integrated into a lithographic apparatus, such as lithographic apparatus100and/or100′ or lithocell300or may be a stand-alone device. It comprises a broadband (white light) radiation projector2which projects radiation onto a substrate W. The reflected radiation is passed to a spectrometer detector4, which measures a spectrum10(intensity as a function of wavelength) of the specular reflected radiation. From this data, the structure or profile giving rise to the detected spectrum may be reconstructed by processing unit PU, e.g., by Rigorous Coupled Wave Analysis and non-linear regression or by comparison with a library of simulated spectra as shown at the bottom ofFIG. 4. In general, for the reconstruction the general form of the structure is known and some parameters are assumed from knowledge of the process by which the structure was made, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer may be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

Another scatterometer SM2that may be used with the present disclosure is shown inFIG. 5. Scatterometer SM2may be integrated into a lithographic apparatus, such as lithographic apparatus100and/or100′ or lithocell300or may be a stand-alone device. Scatterometer SM2may include an optical system1having a radiation source2, a lens system12, a filter13(e.g., interference filter), a reflecting device14(e.g., reference mirror), a lens system15(e.g., a microscopic objective lens system, also referred herein as objective lens system), a partially reflected surface16(e.g., a beam splitter), and a polarizer17. Scatterometer SM2may further include a detector18and a processing unit PU.

In one exemplary operation, the radiation emitted by radiation source2is collimated using lens system12and transmitted through interference filter13and polarizer17, is reflected by partially reflected surface16and is focused onto substrate W via microscope objective lens system15. The reflected radiation then transmits through partially reflecting surface16into a detector18in order to have the scatter spectrum detected. The detector may be located in the back-projected pupil plane11, which is at the focal length of the objective lens system15, however the pupil plane may instead be re-imaged with auxiliary optics (not shown) onto the detector. The pupil plane is the plane in which the radial position of radiation defines the angle of incidence and the angular position defines azimuth angle of the radiation. In one example, the detector is a two-dimensional detector so that a two-dimensional angular scatter spectrum of a substrate target30can be measured. The detector18may be, for example, an array of charge coupled device (CCD) or CMOS sensors, and may use an integration time of, for example, 40 milliseconds per frame.

A reference beam may be used, for example, to measure the intensity of the incident radiation. To do this, when the radiation beam is incident on beam splitter16part of it is transmitted through the beam splitter as a reference beam towards reference minor14. The reference beam is then projected onto a different part of the same detector18or alternatively on to a different detector (not shown).

Interference filter13may include a set of interference filters, which may be available to select a wavelength of interest in the range of, say, 405-790 nm or even lower, such as 200-300 nm. The interference filter may be tunable rather than comprising a set of different filters. A grating could be used instead of interference filters.

Detector18may measure the intensity of scattered light at a single wavelength (or narrow wavelength range), the intensity separately at multiple wavelengths or integrated over a wavelength range. Furthermore, detector18may separately measure the intensity of transverse magnetic- and transverse electric-polarized light and/or the phase difference between the transverse magnetic- and transverse electric-polarized light.

Using a broadband light source (i.e., one with a wide range of light frequencies or wavelengths—and therefore of colors) for a radiation source2may give a large etendue, allowing the mixing of multiple wavelengths. The plurality of wavelengths in the broadband preferably each may have a bandwidth of Δλ and a spacing of at least 2 Δλ (i.e., twice the bandwidth). Several “sources” of radiation can be different portions of an extended radiation source which have been split using fiber bundles. In this way, angle resolved scatter spectra can be measured at multiple wavelengths in parallel. A 3-D spectrum (wavelength and two different angles) can be measured, which contains more information than a 2-D spectrum. This allows more information to be measured which increases metrology process robustness. This is described in more detail in EP1,628,164A, which is incorporated by reference herein in its entirety.

The target30on substrate W may be a 1-D grating, which is printed such that after development, the bars are formed of solid resist lines. The target30may be a 2-D grating, which is printed such that after development, the grating is formed of solid resist pillars or vias in the resist. The bars, pillars or vias may 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. Accordingly, the scatterometry data of the printed gratings is used to reconstruct the gratings. The parameters of the 1-D grating, such as line widths and shapes, or parameters of the 2-D grating, such as pillar or via widths or lengths or shapes, may be input to the reconstruction process, performed by processing unit PU, from knowledge of the printing step and/or other scatterometry processes.

As described above, the target can be on the surface of the substrate. This target will often take the shape of a series of lines in a grating or substantially rectangular structures in a 2-D array. The purpose of rigorous optical diffraction theories in metrology is effectively the calculation of a diffraction spectrum that is reflected from the target. In other words, target shape information is obtained for CD (critical dimension) uniformity and overlay metrology. Overlay metrology is a measuring system in which the overlay of two targets is measured in order to determine whether two layers on a substrate are aligned or not. CD uniformity is simply a measurement of the uniformity of the grating on the spectrum to determine how the exposure system of the lithographic apparatus is functioning. Specifically, CD, or critical dimension, is the width of the object that is “written” on the substrate and is the limit at which a lithographic apparatus is physically able to write on a substrate.

Alignment System with Beam Pointing Monitor and Compensation System According to an Embodiment

FIG. 6illustrates a schematic illustration of an alignment system600that can be implemented as a part of lithographic apparatus100or100′ and/or as part of scatterometers SM1and SM2, according to an embodiment. In an example of this embodiment, alignment system600may be configured to align a substrate (e.g., substrate W) with respect to a patterning device (e.g., patterning device MA). Alignment system600may 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 lithography apparatus100or100′ using the detected positions of the alignment marks. Such alignment of the substrate may ensure accurate exposure of one or more patterns on the substrate.

According to an embodiment, alignment system600may include an illumination system612, an optical system614, an objective system616, an image rotation interferometer626, a detector628, and a signal analyzer630. Illumination system612may be configured to provide an electromagnetic narrow band radiation beam641. In an example, the narrow band radiation beam641may be within a spectrum of wavelengths between about 500 nm to about 900 nm. In another example, the narrow band radiation beam641comprises discrete narrow passbands within a spectrum of wavelengths between about 500 nm to about 900 nm. Yet in another example, radiation beam641may be monochromatic, for example, provided by a monochromatic light source, such as a laser light source in illumination system612. But polychromatic light sources such as LEDs may also be used in illumination system612to provide a polychromatic radiation beam614. It is noted that radiation beam614is not limited to these examples and can include any suitable number and range of wavelengths.

According to some embodiments, alignment system600can further include a beam pointing monitor and compensation system640configured to determine beam pointing variation of a radiation beam with high accuracy and to compensate for the beam pointing variation. Beam pointing monitor and compensation system640can be configured to receive radiation beam641, use a small portion of radiation beam641for beam pointing monitoring and compensation, and pass through radiation beam613.

According to an embodiment, optical system614can include a beam splitter. Optical system614can be configured to receive radiation beam613and split radiation beam613into at least two radiation sub-beams, according an embodiment. In an example, radiation beam613can be split into radiation sub-beams615and617, as shown inFIG. 6. Optical system614can be further configured to direct radiation sub-beam615onto a substrate620placed on a stage622moveable along direction624. Radiation sub-beam615can be configured to illuminate an alignment mark or a target618located on substrate620. Alignment mark or target618can be coated with a radiation sensitive film in an example of this embodiment. In another example, alignment mark or target618can have one hundred and eighty degree symmetry. That is, when alignment mark or target618is rotated one hundred and eighty degrees about an axis of symmetry perpendicular to a plane of alignment mark or target618, rotated alignment mark or target418may be substantially identical to an unrotated alignment mark or target618

As illustrated inFIG. 6, objective system616may be configured to direct diffracted radiation beam619towards image rotation interferometer626, according to an embodiment. Objective system616may comprise any appropriate number of optical elements suitable for directing diffracted radiation beam619. In an example embodiment, diffracted radiation beam619may be at least a portion of radiation beam615that is diffracted from alignment mark618. It should be further noted that even though objective system616is shown to direct radiation beam619towards image rotation interferometer626, the disclosure is not so limiting. It would be apparent to a person skilled in the relevant art that other optical arrangements may be used to obtain the similar result of detecting diffraction signals from alignment mark618.

As illustrated inFIG. 6, image rotation interferometer626can be configured to receive radiation sub-beam617and diffracted radiation beam619through beam splitter614. In an example of this embodiment, image rotation interferometer626can comprise any appropriate set of optical-elements, for example, a combination of prisms that may be configured to form two images of alignment mark618based on the received diffracted radiation beam619. It should be appreciated that a good quality image need not be formed, but that the features of alignment mark618should be resolved. Image rotation interferometer626can be further configured to rotate one of the two images with respect to the other of the two images one hundred and eighty degrees and recombine the rotated and unrotated images interferometrically.

In an embodiment, detector628can be configured to receive the recombined image and detect an interference as a result of the recombined image when alignment axis621of alignment system600passes through a center of symmetry (not shown) of alignment mark618. Such interference may be due to alignment mark618being one hundred and eighty degree symmetrical, and the recombined image interfering constructively or destructively, according to an example embodiment. Based on the detected interference, detector628can be further configured to determine a position of the center of symmetry of alignment mark618and consequently, detect a position of substrate620. According to an example, alignment axis621can be aligned with an optical beam perpendicular to substrate620and passing through a center of image rotation interferometer626.

In a further embodiment, signal analyzer630can be configured to receive signal629including information of the determined center of symmetry. Signal analyzer630can be further configured to determine a position of stage622and correlate the position of stage622with the position of the center of symmetry of alignment mark618. As such, the position of alignment mark618and consequently, the position of substrate620can be accurately known with reference to stage622. Alternatively, signal analyzer630can be configured to determine a position of alignment system600or any other reference element such that the center of symmetry of alignment mark618may be known with reference to alignment system600or any other reference element.

According to some examples, signal analyzer630can include one or more processing units, computing devices, processors, controllers, or other devices executing a firmware, software, routines, instructions, etc. to implement one or more embodiments of this disclosure.

It should be noted that even though a beam splitter614is shown to direct radiation beam615towards alignment mark or target618and to direct reflected radiation beam619towards image rotation interferometer626, the disclosure is not so limiting. It would be apparent to a person skilled in the relevant art that other optical arrangements may be used to obtain the similar result of illuminating alignment mark or target618on substrate620and detecting an image of alignment mark or target618.

According to some embodiments, alignment system600can further include the beam pointing monitor and compensation system640. It is noted that although beam pointing monitor and compensation system640is discussed in accordance with alignment system600, the beam pointing monitor and compensation system640can be part of lithographic apparatus100, lithographic apparatus100′, scatterometer SM1, scatterometer SM2, or any other apparatus to determine beam pointing variation of a radiation beam with high accuracy and to compensate for the beam pointing variation. For example, beam pointing monitor and compensation system640can be used with an illumination system used in apparatus100, lithographic apparatus100′, scatterometer SM1, scatterometer SM2, alignment system600, or any other apparatus.

Also, it is noted that althoughFIG. 6illustrates the beam pointing monitor and compensation system640to be located between illumination system612and optical system614, the beam pointing monitor and compensation system640can be located at other places on the optical path within alignment system600or other systems. For example, the beam pointing monitor and compensation system640can be part of illumination system612. As another example, other optical elements (such as, but not limited to relay lenses, prisms, field stops, etc.) can be provided between illumination system612and optical system614, and the beam pointing monitor and compensation system640can provided within these optical elements. Also, as discussed in more detail below, the beam pointing monitor and compensation system640can be based on and use the optical elements already used within alignment system600or other systems where the beam pointing monitor and compensation system640is being used.

According to some examples, beam pointing monitor and compensation system640is configured to receive radiation beam641from illumination system612. Beam pointing monitor and compensation system640can use a small portion of radiation beam641to measure beam pointing deviation and compensate for any deviation. According to some embodiments, beam pointing monitor and compensation system640can be coupled with signal analyzer630. In this example, beam pointing monitor and compensation system640can send a measured intensity of the radiation beam, a measured incident angle of the radiation beam, and/or a measured beam pointing deviation to signal analyzer630. Signal analyzer630can analyze the measured intensity of the radiation beam, the measured incident angle of the radiation beam, and/or the measured beam pointing deviation and send control signals to beam pointing monitor and compensation system640to compensate for any deviation.

For example, signal analyzer630can receive a measured intensity of a radiation beam from beam pointing monitor and compensation system640, determine the incident angle of the radiation beam, compare the determined incident angle with a reference beam angle, determine the beam pointing deviation, and generate a control signal based on the determined beam pointing deviation. Signal analyzer630can send the control signal to beam pointing monitor and compensation system640to compensate for the measured deviation. According to some examples, beam pointing monitor and compensation system640can be configured to measure an angle of incidence and/or variations in an angle of incidence in the range of pico-radian.

FIG. 7illustrates an exemplary beam pointing monitor and compensation system640according to some embodiments. In this example, beam pointing monitor and compensation system640can include a beam pointing compensator701and a beam pointing monitor system including a surface plasmon resonance (SPR) optical element703and an optical detector705. As noted above, beam pointing compensator701, surface plasmon resonance (SPR) optical element703, and/or optical detector705can be (and/or use) the optical elements already used in alignment system600or any other systems with which beam pointing monitor and compensation system640is being used.

According to some embodiments, beam pointing compensator701is configured to receive radiation beam641from, for example, illumination system612ofFIG. 6. Beam pointing compensator701can also be configured to receive a control signal708from signal analyzer630ofFIG. 6. Depending on the control signal708, beam pointing compensator701can be configured to adjust radiation beam641to compensate for any measured beam pointing deviations.

The beam pointing monitor and compensation system640can use a surface plasmon resonance (SPR) optical element (such as SPR optical element703) to direct a radiation beam to optical detector705. The beam pointing monitor and compensation system604can utilize SPR effects to measure beam pointing deviation for calibration and correction purposes. SPR can be characterized by a loss of reflected intensity from a surface because of resonant absorption of light due to surface plasmon (SP) excitation. SPR optical element703is configured to receive radiation beam702from beam pointing compensator701, and use a small portion of radiation beam702for beam pointing measurement. Most of received radiation beam702can be transmitted as radiation beam613. Radiation beam613can be used, for example, in alignment system600, as discussed above, for alignment measurements. However, as noted above, radiation beam613can also be used in other systems such as lithographic apparatus100, lithographic apparatus100′, scatterometer SM1, scatterometer SM2, or any other apparatus.

According to some embodiments, SPR optical element703can include another optical element configured to receive radiation beam702and pick off some portion of the radiation beam702for beam pointing monitor and compensation. Additionally or alternatively, the other optical element to pick off some portion of the radiation beam702for beam pointing monitor and compensation is not part of SPR optical element703but can be located between beam pointing compensator701and SPR optical element703. According to some examples, the optical element used to pick off some portion of the radiation beam702for beam pointing monitor and compensation can include a minor, a prism, a beam splitter, or any other suitable optical element.

According to some embodiments, the portion of radiation beam702that is picked off for beam pointing monitor and compensation can include a portion of radiation beam702that is not used in alignment system600(or other systems where the beam pointing monitor and compensation system640is being used.) In one example, the portion of radiation beam702that is picked off for beam pointing monitor and compensation can include a radiation beam with S polarization orientation.

According to some embodiments, SPR optical element703can be configured to reflect radiation704from a small portion of radiation702that SPR optical element703has received. SPR optical element703can direct radiation beam704to optical detector705for measuring the intensity of radiation beam704and therefore, the beam pointing of radiation beam702. According to some examples, optical detector705can send the measured intensity and/or beam pointing706to, for example, signal analyzer630ofFIG. 6for further analysis and also for generating control signal708used for beam pointing compensation.

According to some examples, SPR optical element703can also be controlled by, for example, signal analyzer630. For example, in addition to or alternate to controlling beam pointing compensator701, signal analyzer630can send control signals to SPR optical element703to move SPR optical element703based on the measured beam angle and/or measured beam pointing variations.

Exemplary SPR optical element703are discussed in more detail below with respect toFIGS. 8A-8C, 9, 10, and 11A-11C. Exemplary implementation of beam pointing compensator701are discussed in more detail below with respect toFIGS. 12A-12C.

Exemplary SPR Optical Elements

As discussed above, the beam pointing monitor and compensation systems of the embodiments of this disclosure can use a surface plasmon resonance (SPR) optical element to direct radiation beam to an optical detector. The beam pointing monitor and compensation systems of the embodiments of this disclosure can utilize SPR effects to measure beam pointing deviation for calibration and correction purposes. SPR can be characterized by a loss of reflected intensity from a surface because of resonant absorption of light due to surface plasmon (SP) excitation. The beam pointing monitor and compensation systems of the embodiments of this disclosure can use SPR to determine beam pointing variation of a radiation beam with high accuracy. By measuring the beam pointing variation, the beam pointing monitor and compensation systems of the embodiments of this disclosure can correct and/or compensate for the variation within the optical system.

FIG. 8Aillustrates a cross section of an SPR optical element800and also illustrates an optical detector705, according to some embodiments of this disclosure. In some examples, SPR optical element800is similar to SPR optical element703ofFIG. 7. In some embodiments, SPR optical element800can include an optical element801. Optical element801can include a parallel-plane optical element or a plate optical element. For example, optical element801can include surfaces802and806, which according to some embodiments, are parallel or substantially parallel surfaces. In some examples, surfaces802and806can be disk-shaped or substantially disk-shaped. In some examples, surfaces802and806can be rectangular or substantially rectangular. In some example, surfaces802and806can be square or substantially square. However, optical element801can include other configurations. According to some embodiments, optical element801is an optically transmissive optical element. For example, optical element801can be made of an optically transmissive such as, but not limited to, glass.

SPR optical element800can include an SPR stack804that supports surface plasmon resonance (SPR) on optical element801. SPR can be characterized by a loss of reflected intensity from a surface because of resonant absorption of light due to surface plasmon (SP) excitation. In other words, the intensity of the reflected radiation beam changes compared to the intensity of the incident radiation beam depending on, for example, the incident angle of the incident radiation beam. SPR stack804can be provided on surface802of optical element801and can include a plurality of layers. For example, SPR stack804can include a metal layer803provided on optical element801and a dielectric layer805provided on metal layer803. SPR optical element801can further include a second metal layer807provided on dielectric layer805of SPR stack804. According to some embodiments, metal layer807, SPR stack804, and optical element801, can reflect the radiation beam incident on SPR optical element800and support surface plasmon resonance (SPR).

According to some examples, metal layers803and807can include silver, aluminum, or other suitable metals. Dielectric layer805can include Silicon dioxide (SiO2, also known as silica) or other suitable dielectric materials such as any non-absorbing dielectric material. According to some examples, metal layer803of SPR stack804can have a thickness of about 5 nm to about 40 nm. For example, metal layer803can have a thickness of about 10 nm to about 30 nm. In some embodiments, metal layer807can have a thickness of about 100 nm to about 300 nm. For example, metal layer807can have a thickness of about 150 nm to about 250 nm. In another example, metal layer807can have a thickness in the range of millimeter. According to some embodiments, dielectric layer805of SPR stack804can have a thickness of about 5 μm to about 200 μm. For example, dielectric layer805can have a thickness of about 10 μm to about 100 μm. In another example, dielectric layer805can have a thickness in the range of millimeter. However, the embodiments of this disclosure are not limited to these examples and other materials and other values for thickness can also be used.

According to some examples, the material used for layers803,805, and807, the thicknesses of layers803,805, and807, and the wavelength of the radiation beam incident on SPR optical element800can affect the sensitivity of the beam pointing monitor and compensation systems of the embodiments of this disclosure. Accordingly, the material used for layers803,805, and807, the thicknesses of layers803,805, and807, and the wavelength of the radiation beam incident on SPR optical element800can be optimized to achieve a desired sensitivity.

As illustrated inFIG. 8A, radiation beam809is incident on optical element801at an angle θ on surface802of optical element801with respect to the normal on surface802. In some examples, radiation beam809can be radiation beam702ofFIG. 7. In some examples, radiation beam809can be the portion of radiation beam702that is picked off for beam pointing monitor and compensation. Radiation beam704is reflected from surface802of optical element801. Optical detector705receives the radiation beam704and detects the intensity of radiation beam704. According to some embodiments, optical detector705can be configured to measure the intensity of the radiation beam that it receives and can include, for example, a photodiode. For example, optical detector705can include a PN diode or a PIN diode (e.g., a diode with an un-doped intrinsic semiconductor region between a P-type semiconductor and an N-type semiconductor region.) However, it is noted that other optical detectors can be used as optical detector705, such as, but not limited to, CCD sensors, CMOS sensors, etc.

According to some examples, SPR optical element800can use one or more optical elements already used in alignment system600or any other systems with which beam pointing monitor and compensation system640is being used. For example, alignment system600or any other systems with which beam pointing monitor and compensation system640is being used can include one or more parallel-plane optical elements or plate optical elements (e.g., flat surface optical elements.) In this example, SPR stack804and metal layer807can be provided on whole or part of the one or more parallel-plane optical elements or plate optical elements in the alignment system600or any other systems with which beam pointing monitor and compensation system640is being used.

FIG. 9schematically illustrates a plot of percent reflection as a function of incident angle for a given wavelength of incident radiation beam, according to some embodiments. Axis903illustrates the percent reflection and axis905illustrates the incident angle (e.g., angle θ ofFIG. 8A.) Also, as illustrated inFIG. 9, 907depicts the critical angle for optical element801. The critical angle can be an angle with respect to normal to surface802such that radiation beams with incident angles more than the critical angle will reflect entirely from surface802(e.g., the boundary between optical element801and SPR stack804.) In other words, total internal reflection occurs for radiation beams with incident angles more than the critical angle.

Plot900illustrates a resonance of the percent reflection as a function of angle. The resonance can occur in a shape of a comb of notches as the function of angle. For example, plot900illustrates notches901a,901b,901c,901d, etc. As illustrated in plot900, the resonance is a variable resonance. In other words, the width of notches901a,901b,901c,901d, etc. changes when the angle changes from 0 degrees to 90 degrees. Also, as illustrated inFIG. 9, plot900includes notches for angles smaller than the critical angle907and notches for angles greater than critical angle907.

By measuring the intensity of the reflected radiation beam704, optical detector705(alone, or in combination with signal analyzer630) can be configured to determine the angle of incidence of radiation beam809(e.g., angle θ ofFIG. 8A.) According to some embodiments, SPR optical element800can be used for a coarse measurement of the incident angle of radiation beam809. Additionally or alternatively, SPR optical element800can be used for a fine measurement of the incident angle of radiation beam809. For example, by knowing on which notch the measured intensity of reflected radiation beam704is, optical detector705(alone, or in combination with signal analyzer630) can more accurately determine the incident angle of radiation beam809.

According to some embodiments, the beam pointing monitor and compensation systems of this disclosure are configured to measure beam pointing variations and compensate for the variations for any angle of incidence (e.g., angle θ ofFIG. 8A) of the radiation beams. In other words, the beam pointing monitor and compensation systems of this disclosure are not dependent on total internal reflection (TIR) of the beam pointing monitor systems, according to some embodiments. Therefore, the beam pointing monitor and compensation systems of this disclosure can measure beam pointing variations (and compensate for the variations) for angles of incident of the radiation beams that are smaller than a critical angle of the beam pointing monitor systems.

The sensitivity of the beam pointing measurement systems of the embodiments of this disclosure can be determined by the material used for the layers of SPR stack804and metal layer807, the thicknesses of the layers of SPR stack804and metal layer807, and/or the wavelength of the radiation beam809. For example, the number, the width, and/or the depth of notches901a,901b,901c,901d, etc. can be controlled based on the material used for the layers of SPR stack804and metal layer807, the thicknesses of the layers of SPR stack804and metal layer807, and/or the wavelength of the radiation beam809.

According to one example, the material and/or the thickness of metal layer803can control the depth (e.g., the distance along axis903between the maximum and minimum points on each notch) of notches901a,901b,901c,901d, etc. In this example, the material and/or the thickness of metal layer803can be tuned to achieve predetermined depths for notches901a,901b,901c,901d, etc. According to one example, the material and/or the thickness of dielectric layer805can control the width (e.g., the distance along axis905between the maximum and minimum points on each notch) of notches901a,901b,901c,901d, etc. In this example, the material and/or the thickness of dielectric layer805can be tuned to achieve predetermined widths for notches901a,901b,901c,901d, etc. For example, by choosing a thick dielectric layer805, the widths of notches901a,901b,901c,901d, etc. can be made small, and therefore, achieve a more sensitive beam pointing monitor system.

According to some examples, the width, and/or the depth of notches901a,901b,901c,901d, etc. can depend on the wavelength of the radiation beam. In some examples, radiation beam809can include two or more wavelengths. SPR optical element800and optical detector705can use the two or more wavelengths for beam pointing measurements. For example, SPR optical element800and optical detector705can use one wavelength of radiation beam809(a wavelength that has notches901a,901b,901c,901d, etc. with larger width) for coarse measurement and use another wavelength of radiation beam809(a wavelength that has notches901a,901b,901c,901d, etc. with smaller width) for fine measurement. Additionally or alternatively, SPR optical element800and optical detector705can use a combination of the detected radiation of the two wavelengths for beam pointing measurements.

FIG. 10schematically illustrates a plot of percent reflection as a function of incident angle for a given wavelength of incident radiation beam, according to some embodiments. Axis1003illustrates the percent reflection and axis1005illustrates the incident angle. Plot1000illustrates the resonance of the percent reflection as a function of angle for a radiation with S polarization orientation (plot1007), for a radiation with P polarization orientation (plot1009), and for a difference between the S and P polarization orientations (plot1011). As illustrated in plot1000, SPR optical element800can be designed such that the maximum at each notch of plot1009(e.g., point1013) be substantially aligned with the minimum of the corresponding notch of plot1007(e.g., point1015) such that the depth1017of plot1011is increased compared to plots1007and1009. Accordingly, the depth of plot1011can be increased and therefore, the sensitivity of SPR optical element800can be increased.

According to some examples, one or more SPR optical elements and one or more optical detectors can be used to receive radiation beams with S polarization orientation and P polarization orientation. The one or more SPR optical elements and one or more optical detectors can be configured to measure the intensity of the radiation beams with S polarization orientation and P polarization orientation and can be configure to use (alone or with signal analyzer630), for example, plot1000ofFIG. 10to determine the angle of incidence for the radiation beam with S polarization orientation and/or radiation beam with P polarization orientation.

According to some examples, SPR optical element800with optical sensor705can be a spatially-distributed beam pointing monitor system. In this example, optical detector705can include a photodiode array, therefore, detecting various spatially-distributed sensor responses across surface802of SPR optical element800. According to some embodiments, an optical polarizing element (not shown) can be included between SPR optical element800and optical detector705. The photodiode array of the optical detector705can detect various spatially-distributed sensor responses across surface802of SPR optical element800. In this example, SPR stack804of SPR optical element800can include a spatial-distributed SPR stack surface pattern. Different subsections of the patterned area can be formed with unique functionality. In this example, the spatially-distributed beam pointing measurement system with combined functionality can be addressed by interrogating individual subsections of the patterned area surface. As one example, adjacent subsections may be devised to detect increasingly larger or smaller nominal beam angles of incidence. For example, SPR stack804can be configured as at least one of laterally defined or patterned such that subsections of SPR stack804can act independently as a function of position. Thus, a selectable range of incident beam angle could be analyzed.

According to some examples, SPR optical element800with optical sensor705can be an electro-optically addressable beam pointing monitor system. In this example, SPR stack804of SPR optical element800can include an electro-optically addressable element, including, for example, an electro-optic surface plasmon resonance (SPR) stack. The electro-optic SPR stack can include at least one of a patterned electro-optic coating for providing a dielectric layer or a segmented and addressable spatially electro-optic coating. In this example, optical detector705can include a photodiode array, thus detecting various spatially-distributed sensor responses across SPR optical element800in response to an electronic input of a voltage V at terminals (not shown) connected to SPR stack804.

According to some examples, the electro-optic SPR stack can be a segmented electro-optically addressable SPR stack. In this example, SPR stack804can have a spatial-distributed SPR stack surface pattern. Different subsections of a patterned area may be formed with unique functionality. In this example, optical detector705can include a photodiode array, thus detecting various spatially-distributed sensor responses across SPR optical element800in response to an electronic input of a voltage V at terminals (not shown) connected to SPR stack804. In this example, each subsection of the patterned area of the electro-optic SPR stack can have its respective terminals and be controlled independently of the other subsections of the patterned area of the electro-optic SPR stack.

FIG. 8Billustrates an SPR optical element820and an optical detector705, according to some embodiments of this disclosure. In some embodiments, SPR optical element820can include an optical element801, which can include a parallel-plane optical element or a plate optical element. According to some embodiments, optical element801is an optically transmissive optical element. SPR optical element820can include a stepped SPR stack that supports surface plasmon resonance (SPR) on optical element801. For example, the stepped SPR stack can include a first SPR stack804aand a second SPR stack804b.

For example, SPR stack804acan include a metal layer803aprovided on optical element801and a dielectric layer805aprovided on metal layer803a. A second metal layer807acan be provided on dielectric layer805a. SPR stack804bcan include a metal layer803bprovided on optical element801and a dielectric layer805bprovided on metal layer803b. A second metal layer807bcan be provided on dielectric layer805b. The incident radiation beam809a(or809b) can be scanned across different thicknesses of the stepped SPR stack to detect various angles and ranges of angle of the incident radiation beam.

Although only two SPR stacks804aand804bare illustrated, the stepped SPR stack can include any number of portions. Also,FIG. 8Billustrates that metal layers803aand803bhave different thicknesses, dielectric layers805aand805bhave different layers, and metal layers807aand807bhave different thickness. However, the embodiments of this disclosure are not limited to this example. For example, metal layers803aand803bcan have same or substantially same thicknesses but layers805aand805bcan have different thicknesses and/or layers807aand807bcan have different thicknesses. For example, metal layers803aand803bcan have same or substantially same thicknesses, metal layers807aand807bcan have same or substantially same thicknesses, but dielectric layers805aand805bcan have different thicknesses. In other words, the thicknesses of layers803a,805a, and807aand the thicknesses of layers803b,805b, and807bcan be designed such that SPR stacks804aand804bresult in a stepped SPR stack.

FIG. 8Cillustrates an SPR optical element840and an optical detector705, according to some embodiments of this disclosure. In some embodiments, SPR optical element840can include an optical element801, which can include a parallel-plane optical element or a plate optical element. According to some embodiments, optical element801is an optically transmissive optical element. SPR optical element840can include a wedged SPR stack that supports surface plasmon resonance (SPR) on optical element801.

For example, the wedged SPR stack can include a metal layer803provided on optical element801and a dielectric layer805provided on metal layer803. A second metal layer807can be provided on dielectric layer805. The incident radiation beam809can be scanned across different thicknesses of the wedged SPR stack to detect various angles and ranges of angle of the incident radiation beam. Scanning of areas of different wedge thicknesses can allow for a single optical detector to act as a variable angle range detector.

FIG. 8Cillustrates the wedged SPR stack where each layer803and805is wedged.FIG. 8Cillustrates that second metal layer807is also wedged. However, the embodiments of this disclosure are not limited to this example. For example, one or more layers of the wedged SPR stack and/or second metal layer807can be substantially parallel-plane plates (e.g., not wedged.) For example, in some embodiments, dielectric layer805and second metal layer807can be wedged while metal layer803is not wedged (e.g., substantially parallel-plane plates.) As another example, dielectric layer805can be wedged, while metal layers803and805are not wedged (e.g., substantially parallel-plane plates).

FIGS. 11A and 11Billustrate exemplary SPR optical elements according to some embodiments.FIG. 11Aillustrates SPR optical element1100with a grating1111. According to some examples, grating1111can be an X-Y diffraction grating. However, the embodiments of this disclosure are not limited to this example and other types of gratings can be provided. According to some embodiments, grating1111is provided on surface1110of optical element1101of SPR optical element1100and is configured to receive the radiation beam1102. SPR optical element1100includes optical element1101, SPR stack1104(which can include a metal layer and a dielectric layer), and metal layer1107, according to some embodiments. SPR optical element1100can be similar to SPR optical elements800,820, and840ofFIGS. 8A-C.

FIG. 11Aalso illustrates four optical detectors1105a-1105dconfigured to receive reflected radiation beams from SPR optical element1100. Exemplary operation of SPR optical element1100with grating1111is discussed with respect toFIGS. 11A and 11B. Incident radiation beam1102is incident on grating1111(not shown inFIG. 11B). According to some examples, grating1111can be configured to diffract radiation beam1102to two more diffracted radiation beams1112a-1112d. According to some examples, diffracted radiation beams1112aand1112care diffracted along the Y-axis direction. Diffracted radiation beams1112band1112dare diffracted along the X-axis direction. In some examples, the angle of diffraction of the diffracted radiation beams1112a-1112dcan depend on the structure of grating1111, such as the pitch of grating11111. Diffracted radiation beams1112a-1112dare reflected from SPR optical element1100. Reflected radiation beams1106a-1106dare received by optical detectors1105a-1105d. Optical detectors1105a-1105d(alone or in combination with signal analyzer630) are configured to determine the angle of incidence of diffracted radiation beams1112a-1112d, and therefore, the angle of incidence of radiation beam1102.

According to some examples, optical detectors1105a-1105d(alone or in combination with signal analyzer630) can use a difference between measured intensities of radiation beams1106band1106dand a difference between measured intensities of radiation beams1106aand1106cto determine the angle of incidence of radiation beam1102.

FIG. 11Cillustrates another beam pointing monitor and compensation system, according to some embodiments. In this example, system1140can include two beam pointing monitor systems1145aand1145b. Each of beam pointing monitor systems1145aand1145bcan include an SPR optical element and an optical detector, according to some embodiments. In the exemplary system1140, beam pointing monitor system1145acan be configured to measure beam pointing along, for example, X direction using a radiation beam with S polarization orientation. Also, beam pointing monitor system1145bcan be configured to measure beam pointing along, for example, Y direction using a radiation beam with P polarization orientation.

In this example, radiation beam1147is incident on polarizing beam splitter1141. According to some examples radiation beam1147can be a non-polarized radiation beam. Additionally or alternatively, radiation beam1147can include different polarization information. Radiation beam1147is divided into sub-beams1149and1151. Sub-beam1149, which is reflected from polarizing beam splitter1141can have S polarization orientation, according to some examples. Sub-beam1151, which is passed through polarizing beam splitter1141can have P polarization orientation, according to some examples. Sub-beam1151can further enter beam splitter1143and be reflected as radiation beam1153with P polarization orientation. According to some examples, beam splitter1143is a polarizing beam splitter. Alternatively, beam splitter1143can be a non-polarizing beam splitter.

According to some examples, beam pointing monitor system1145acan be configured to receive radiation beam1149and measure beam pointing of radiation beam1149along, for example, X direction using radiation beam1149with S polarization orientation. According to some examples, beam pointing monitor system1145bcan be configured to receive radiation beam1153and measure beam pointing of radiation beam1153along, for example, Y direction using radiation beam1153with P polarization orientation. Additionally, beam pointing monitor systems1145aand1145b, alone or in combination with, for example, signal analyzer630, can be configured to measure beam pointing of radiation beam1147along, for example, X and Y direction based on the measurements of radiation beams1149and1153.

FIGS. 12A-Cillustrate exemplary beam pointing compensators, according to some embodiments. The beam pointing compensators ofFIGS. 12A-Ccan be used as beam pointing compensator701ofFIG. 7, in some embodiments.

FIG. 12Aillustrates an input fiber assembly1200, which can be used for beam pointing compensation. According to some examples, input fiber assembly1200can include a first portion1201that can be coupled to fiber optics to receive a radiation beam. The input fiber assembly1200can also include a second portion1203configured to output the radiation beam. According to some embodiments, input fiber assembly1200can include actuators1205a-1205cto control the beam pointing of the radiation beam coming out of second portion1203. Although three actuators1205a-1205care illustrated, input fiber assembly1200can include any number (one or more) actuators. According to some examples, actuators1205a-1205ccan include one or more actuator screws, one or more piezoelectric actuators, or any other actuators configured to control input fiber assembly1200. Actuators1205a-1205ccan receive, for example, control signal708ofFIG. 7from signal analyzer630based on the measured beam pointing deviation and control input fiber assembly1200to compensate for the measured deviation.

Beam pointing compensators used in the embodiments of this disclosure can also include opto-mechanical adjustable compensators. For example, beam pointing compensators can include one or more prisms, one or more mirrors, one or more wedges, or other opto-mechanical adjustable compensators configured to correct/compensate for measured beam pointing deviations. For example,FIG. 12Billustrates on exemplary opto-mechanical adjustable compensator1220including wedge prisms1221and1222. According to some examples, wedge prims1221and1222can receive, for example, control signal708ofFIG. 7from signal analyzer630based on the measured beam pointing deviation and control opto-mechanical adjustable compensator1220to compensate for the measured deviation. The compensation can occur by rotating wedge prims1221and1222along the rotation axis1223independently of each other. According to some examples, one wedge prism can be used to change an angle of incidence based on the measure beam pointing deviation.

Beam pointing compensators used in the embodiments of this disclosure can also include electro-optic compensators. For example, beam pointing compensators can include one or more electro-optic cells, one or more acousto-optic beam deflectors, or other electro-optic compensators configured to correct/compensate for measured beam pointing deviations. For example,FIG. 12Cillustrates on exemplary acousto-optic beam deflector1240including transducer1241, a medium1243, and an acoustic absorber1245. According to some examples, transducer1241is configured to generate sound waves within medium1243. The sound waves can travel from transducer1241toward acoustic absorber1245. Radiation beam1247is incident on medium1243. Medium1243and the sound waves within medium1243can act as a grating that diffracts radiation beam1247into diffracted radiation beam1249and transmitted radiation beam1251. The acoustic wavelength of the sound waves within medium1243can control the angle of the diffracted radiation beam1249. The wavelength of the sound waves within medium1243can be controlled with a signal to transducer1241. Additionally or alternatively, the wavelength of the sound waves within medium1243can be controlled by controlling medium1243(e.g., controlling the length, the width, etc. of medium1243.) Transducer1241can receive, for example, control signal708ofFIG. 7from signal analyzer630based on the measured beam pointing deviation and control acousto-optic beam deflector1240to compensate for the measured deviation. The compensation can occur by controlling the wavelength of the sound waves within medium1243.

Although some exemplary systems are shown as exemplary embodiments for beam pointing compensator701, the embodiments of this disclosure are not limited to these examples. Other suitable beam pointing compensators can be used in the embodiments of this disclosure to receive control signals based on beam pointing deviations and to compensate for the deviations.

FIG. 13is a flowchart depicting a method1300, according to an embodiment. For example, method1300can measure beam pointing variations, according to some embodiments. In one example, method1300is performed by beam pointing monitor and compensation system640. It is to be appreciated not all steps may be needed, and the steps may not be performed in the same order as shown inFIG. 13. Reference is made to beam pointing monitor and compensation system640ofFIGS. 6 and 7and SPR optical element800and optical detector750FIG. 8Amerely for convenience of discussion. Other systems may be used to perform the method as will be understood by those skilled in the arts.

At1301, an incident radiation beam, such as radiation beam809is provided to SPR optical element800. At1303, a surface plasmon resonance is provided using, for example, SPR stack804. At1305, radiation beam704is reflected from SPR optical element800. The reflected radiation beam704is a SPR reflected radiation beam, according to some examples. At1307, optical detector705receives the reflected radiation beam704and measures, for example, the intensity of the reflected radiation beam704and/or a percentage of reflectance of the reflected radiation beam704. At1309, based on the measured intensity of the reflected radiation beam704and/or the measured percentage of reflectance of the reflected radiation beam704, optical detector705, alone or in combination with signal analyzer630, determines the angle of incidence (e.g., beam pointing) of radiation beam809. Signal analyzer630, alone or in combination with optical detector705, determines a beam pointing variation by, for example, comparing the measured angle of incidence and a reference angle of incidence. According to some embodiments, the reference angle of incidence is stored in a memory (not shown) accessible by signal analyzer630.

FIG. 14is a flowchart depicting a method1400, according to an embodiment. For example, method1400can measure beam pointing variations and compensate for the variations, according to some embodiments. In one example, method1400is performed by beam pointing monitor and compensation system640. It is to be appreciated not all steps may be needed, and the steps may not be performed in the same order as shown inFIG. 14. Reference is made to beam pointing monitor and compensation system640ofFIGS. 6 and 7and SPR optical element800and optical detector705FIG. 8Amerely for convenience of discussion. Other systems may be used to perform the method as will be understood by those skilled in the arts.

Steps1401-1409of method1400are similar to steps1301-1309of method1300. For example, at1401, an incident radiation beam, such as radiation beam809is provided to SPR optical element800. At1403, a surface plasmon resonance is provided using, for example, SPR stack804. At1405, radiation beam704is reflected from SPR optical element800. The reflected radiation beam704is a SPR reflected radiation beam, according to some examples. At1407, optical detector705receives the reflected radiation beam704and measures, for example, intensity of the reflected radiation beam704and/or a percentage of reflectance of the reflected radiation beam704. At1409, based on the measured intensity of the reflected radiation beam704and/or the measured percentage of reflectance of the reflected radiation beam704, optical detector705, alone or in combination with signal analyzer630, determines the angle of incidence (e.g., beam pointing) of radiation beam809. Signal analyzer630, alone or in combination with optical detector705, determines a beam pointing variation by, for example, comparing the measured angle of incidence and a reference angle of incidence.

At1411, signal analyzer630, as one example, sends a control signal (e.g., control signal708ofFIG. 7) to beam pointing compensator701. At1413, beam pointing compensator701, based on the received control signal, adjusts the angle of incidence (e.g., beam pointing) of, for example, radiation beam641, which will be provided to SPR optical element800. Method1400can continue with steps1403-1413to measure any beam pointing variation based on the adjusted beam pointing and, if needed, to further adjust the beam pointing.

The embodiments may further be described using the following clauses:1. A beam pointing monitor and compensation system, comprising:a surface plasmon resonance (SPR) optical element, comprising:an optical element comprising first and second surfaces, wherein the first and second surfaces are substantially parallel to each other;a first metal layer provided on the second surface of the optical element;a dielectric layer provided on the first metal layer; anda second metal layer provided on the dielectric layer.2. The beam pointing monitor and compensation system of clause1, wherein the optical element comprises an optically transmissive material.3. The beam pointing monitor and compensation system of clause1, wherein the SPR optical element is configured to receive a radiation beam and provide a SPR reflected radiation beam.4. The beam pointing monitor and compensation system of clause3, further comprising:an optical detector configured to receive the SPR reflected radiation beam and to measure an intensity of the received SPR reflected radiation beam.5. The beam pointing monitor and compensation system of clause3, further comprising:a beam pointing compensator configured to control an angle of incidence of the radiation beam on the SPR optical element.6. The beam pointing monitor and compensation system of clause5, wherein:the beam pointing compensator is configured to receive a control signal to control the angle of incidence of the radiation beam on the SPR optical element, andthe control signal is determined based on a measurement of the angle of incidence.7. A system comprising:an illumination system configured to provide a radiation beam; anda beam pointing monitor system, comprising:a surface plasmon resonance (SPR) optical element, comprising:an optical element comprising first and second surfaces, wherein the first and second surfaces are substantially parallel to each other;a first metal layer provided on the second surface of the optical element;a dielectric layer provided on the first metal layer; anda second metal layer provided on the dielectric layer,wherein the beam pointing monitor system is configured to measure an angle of incidence of the radiation beam with respect to a normal to the second surface of the optical element.8. The system of clause7, wherein:the optical element comprises an optically transmissive material, andthe SPR optical element is configured to receive a radiation beam and to provide an SPR reflected radiation beam.9. The system of clause8, wherein the beam pointing monitor system further comprises an optical detector configured to receive the SPR reflected radiation beam and to measure an intensity of the received SPR reflected radiation beam.10. The system of clause8, further comprising:a beam pointing compensator configured to control the angle of incidence of the radiation beam on the SPR optical element,wherein the beam pointing compensator is configured to receive a control signal to control the angle of incidence of the radiation beam on the SPR optical element, the control signal is determined based on a measurement of the angle of incidence, and the measurement of the angle of incidence is determined based on the measured intensity of the received SPR reflected radiation beam.11. A method comprising:illuminating, with a radiation beam, a surface plasmon resonance (SPR) optical element, wherein the radiation beam is incident on the SPR optical element at an angle of incidence with respect to a normal to the SPR optical element, and the SPR optical element comprises an optical element comprising first and second surfaces, the first and second surfaces being substantially parallel to each other;detecting, using a detector, a reflected radiation beam reflected from the SPR optical element, wherein the SPR optical element provides a SPR of the reflected radiation beam;measuring an intensity of the reflected radiation beam; anddetermining the angle of incidence.12. The method of clause11, further comprising:transmitting a control signal to a beam pointing compensator configured to adjust the angle of incidence.13. The method of clause11, further comprising:adjusting, using a beam pointing compensator, the angle of incidence based on the control signal.14. An alignment system comprising:an illumination system configured to provide a radiation beam;a beam pointing monitor configured to measure an angle of incidence of the radiation beam with respect to a normal to a surface of the beam pointing monitor and to determine a beam pointing variation; anda beam pointing compensator configured to receive a control signal based on the determined beam pointing variation and to adjust the angle of incidence of the radiation beam.15. The alignment system of clause14, wherein the beam pointing monitor comprises a surface plasmon resonance (SPR) optical element, the SPR optical element comprising:an optical element comprising first and second surfaces, wherein the first and second surfaces are substantially parallel to each other;a first metal layer provided on the second surface of the optical element;a dielectric layer provided on the first metal layer; anda second metal layer provided on the dielectric layer.16. The alignment system of clause15, wherein the optical element comprises an optically transmissive material.17. The alignment system of clause15, wherein the SPR optical element is configured to receive a radiation beam and provide a surface plasmon resonance (SPR) reflected radiation beam.18. The alignment system of clause17, further comprising:an optical detector configured to receive the SPR reflected radiation beam and to measure an intensity of the received SPR reflected radiation beam.19. A lithographic apparatus comprising:a first illumination system configured to illuminate a pattern of a patterning device;a projection system configured to project an image of the pattern on to a target portion of a substrate; anda system comprising:a second illumination system configured to provide a radiation beam; anda beam pointing monitor and compensation system, comprising:a surface plasmon resonance (SPR) optical element, comprising:an optical element comprising first and second surfaces;a first metal layer provided on the second surface of the optical element;a dielectric layer provided on the first metal layer; anda second metal layer provided on the dielectric layer,wherein the beam pointing monitor and compensation system is configured to measure an angle of incidence of the radiation beam with respect to a normal to the second surface of the optical element.20. The lithographic apparatus of clause19, wherein the first and second surfaces of the optical element are substantially parallel to each other.

The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams. As mentioned above, the term radiation in the context of the driving system may also encompass microwave radiation.