Phase modulator to reduce mark size in alignment

Abstract

An alignment device comprises an illumination system configured to direct one or more illumination beams toward an alignment target and to receive a diffracted beam from the alignment target. The alignment device also comprises a self-referencing interferometer configured to produce two diffracted beamlets, wherein the two diffracted beamlets are orthogonally polarized, rotated 180 degrees relative to each other about an alignment axis, and spatially overlapped. The alignment device also comprises: a beam analyzer configured to produce interference between overlapping components of the diffracted beamlets and to produce two orthogonally polarized optical branches; and a detection system configured to determine the position of the alignment target based on light intensity measurements of the optical branches, wherein the measured light intensity is temporally modulated by a phase modulator.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. application No. 62 / 884,702 filed on August 9, 2019, and the entire contents of said U.S. application are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to alignment apparatus and systems, such as alignment sensor apparatus for lithographic apparatus and systems. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a substrate (usually to a target portion of a substrate). A lithographic apparatus can be used, for example, in the manufacture of an integrated circuit (IC). In that case, a pattern forming device (which is alternatively referred to as a mask or reticle) can be used to produce a circuit pattern to be formed on a single layer of an IC. This pattern can be transferred to a target portion (e.g., a portion including a die, a die, or several dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is typically performed via imaging onto a layer of radiation-sensitive material (resist) disposed on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are patterned continuously. Known lithographic apparatus include: a so-called stepper, in which each target portion is irradiated by exposing the entire pattern to the target portion at one time; and a so-called scanner, in which each target portion is irradiated by scanning the pattern in a given direction (the "scanning" direction) by a radiation beam while simultaneously scanning the target portion parallel or antiparallel to such a scanning direction.

[0005] During a lithography operation, different processing steps may require different layers to be formed successively on a substrate. Therefore, it may be necessary to position the substrate with high accuracy relative to a previous pattern formed on the substrate. Typically, alignment marks are placed on the substrate to be aligned and the alignment marks are positioned with reference to a second object. The lithography equipment may use an alignment device to detect the position of the alignment marks and to align the substrate using the alignment marks to ensure accurate exposure from the mask. The misalignment between alignment marks at two different layers is measured as an overlay error. Therefore, there is a need for a system and method for providing alignment with high accuracy and less variation. Summary of the invention

[0006] Embodiments of an alignment apparatus are described herein.

[0007] According to some embodiments of the present disclosure, an alignment device includes an illumination system, the illumination system being configured to generate an illumination beam, direct the illumination beam toward an alignment target, and receive a diffraction beam reflected from the alignment target, wherein the diffraction beam includes at least one positive diffraction order or one negative diffraction order. The alignment device also includes a self-referencing interferometer, the self-referencing interferometer being configured to receive the diffraction beam, generate diffraction beamlets, wherein the diffraction beamlets are orthogonally polarized, rotated 180 degrees around an alignment axis relative to each other, and spatially overlapped. The alignment device also includes a beam analyzer, the beam analyzer being configured to generate interference intensity between overlapping components of the diffraction beamlets and generate two orthogonally polarized optical branches. The alignment device also includes a detection system, the detection system being configured to determine the position of the alignment target based on a light intensity measurement result of the optical branch, wherein the measured light intensity is modulated by a phase modulator.

[0008] In some embodiments, the illumination system is further configured to generate an off-axis illumination beam at a first position and a second position diametrically opposite to each other in a pupil plane. In some embodiments, the off-axis illumination beam is modulated by the phase modulator.

[0009] In some embodiments, the phase modulator modulates a positive or negative diffraction order of a diffracted beam reflected from the alignment target.

[0010] In some embodiments, the phase modulator modulates the diffracted sub-beams produced by the self-referencing interferometer.

[0011] In some embodiments, the phase modulator modulates the optical branches generated by the beam analyzer.

[0012] In some embodiments, the phase modulator modulates the image of the alignment target at the input of the detection system.

[0013] In some embodiments, the phase modulator is a piezoelectric mirror assembly that generates phase modulation by applying an electrical signal to a piezoelectric material.

[0014] In some embodiments, the phase modulator comprises a liquid crystal modulator, a photoelastic modulator, an electro-optic modulator, a ferroelectric liquid crystal, a micro-mirror array, or a digital spatial light modulator. In some embodiments, the liquid crystal modulator is a potassium dideuterium phosphate (KD*P) crystal.

[0015] In some embodiments, the phase modulator is a wave plate covering half of the pupil plane to change the polarization state of the positive and negative diffraction orders.

[0016] In some embodiments, the phase modulator comprises a transmissive region and a deflecting region, wherein each region comprises two diametrically opposed quadrants, and the deflecting region is wedge-shaped with varying thickness.

[0017] In some embodiments, the phase modulator produces a frequency modulation of the off-axis illumination beam. In some embodiments, the phase modulator is an acousto-optic modulator. In some embodiments, the phase modulator is an internal grating and the frequency modulation of the off-axis illumination beam is produced by scanning the internal grating. In some embodiments, the phase modulator is a half-wave plate and the frequency modulation of the off-axis illumination beam is produced by rotating the half-wave plate.

[0018] Other features and advantages of the present invention and the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. It should be noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein only for illustrative purposes. Based on the teachings contained herein, those skilled in the relevant art will understand additional embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings, which are incorporated herein and form a part of this specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and enable those skilled in the relevant art to make and use the invention.

[0020] Figure 1A is a schematic illustration of a reflective lithography apparatus according to an exemplary embodiment.

[0021] Figure 1B is a schematic illustration of a transmissive lithography apparatus according to an exemplary embodiment.

[0022] Figure 2 is a more detailed schematic illustration of a reflective lithography apparatus according to an exemplary embodiment.

[0023] Figure 3 is a schematic illustration of a lithography cell according to an exemplary embodiment.

[0024] Figure 4 is a schematic illustration of an alignment apparatus according to an exemplary embodiment.

[0025] Figure 5A and Figure 5B An exemplary alignment target designed with a diffraction grating according to some embodiments of the present disclosure is illustrated.

[0026] Figure 6 An exemplary alignment target designed with a two-dimensional diffraction grating according to some embodiments of the present disclosure is illustrated.

[0027] Figure 7An exemplary optical configuration for phase modulation in an alignment device according to some embodiments of the present disclosure is illustrated.

[0028] Figure 8 Another exemplary alignment apparatus according to some embodiments of the present disclosure is illustrated.

[0029] Fig. 9 The polarization states at various locations of light as it propagates through an alignment device according to some embodiments of the present disclosure are illustrated.

[0030] Fig. 10A , Fig. 10B , Fig. 10C , Fig.11A , Fig. 11B , Fig. 12A , Fig. 12B , Fig.13 , Fig.14A , Fig. 14B , Fig.15A , Fig. 15B and Fig.16 An exemplary optical configuration for phase modulation according to some embodiments of the present disclosure is illustrated.

[0031] Features and advantages of the present invention will be apparent from the specific embodiments described below in conjunction with the accompanying drawings, in which like reference numerals identify corresponding elements throughout. In the accompanying drawings, like reference numerals generally indicate identical, functionally similar, and / or structurally similar elements. In addition, typically, the leftmost digit of a reference numeral identifies the drawing in which the reference numeral first appears. Unless otherwise indicated, the drawings provided throughout this disclosure should not be construed as being to scale. DETAILED DESCRIPTION

[0032] This specification discloses one or more embodiments that incorporate the features of the present invention. The disclosed embodiments merely illustrate the present invention. The scope of the present invention is not limited to the disclosed embodiments. The present invention is thus defined by the appended claims.

[0033] The described embodiments and references in this specification to "one embodiment," "an embodiment," "an example embodiment," etc. indicate that the described embodiments may include certain features, structures, or characteristics, but every embodiment may not necessarily include the certain features, structures, or characteristics. Moreover, such phrases do not necessarily refer to the same embodiment. In addition, when a certain feature, structure, or characteristic is described in conjunction with an embodiment, it should be understood that it is within the knowledge of those skilled in the art to implement such feature, structure, or characteristic in conjunction with other embodiments, whether or not explicitly described.

[0034] For ease of description, spatially relative terms such as "below," "beneath," "lower," "above," "on," "upper," etc. may be used herein to describe the relationship of one element or feature to another element or feature as illustrated in the accompanying drawings. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the accompanying drawings. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may be interpreted accordingly.

[0035] As used herein, the term "about" indicates a value of a given quantity that can vary based on a particular technology. Based on a particular technology, the term "about" can indicate, for example, a value of a given quantity that varies within 10% to 30% above or below the value (e.g., ±10%, ±20%, or ±30% of the value).

[0036] The embodiments of the present disclosure may be implemented in hardware, firmware, software, or any combination thereof. The embodiments of the present disclosure may also be implemented as instructions stored on a machine-readable medium that can be read and executed by one or more processors. The machine-readable medium may include any mechanism for storing or transmitting information in a form that can be read by a machine (e.g., a computing device). For example, the machine-readable medium may include a read-only memory (ROM); a random access memory (RAM); a disk storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal (e.g., a carrier wave, an infrared signal, a digital signal, etc.); and the like. In addition, firmware, software, routines, and / or instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only, and such actions are actually caused by computing devices, processors, controllers, or other devices that execute firmware, software, routines, instructions, etc.

[0037] However, before describing such embodiments in further detail, it is instructive to present an example environment in which embodiments of the present disclosure may be implemented.

[0038] Example Lithography System

[0039] Figure 1A and Figure 1BSchematic illustrations of a lithographic apparatus 100 and a lithographic apparatus 100', respectively, in which embodiments of the present invention may be implemented. The lithographic apparatus 100 and the lithographic apparatus 100' each include the following: an illumination system (illuminator) IL, which is configured to condition a radiation beam B (e.g., deep ultraviolet or extreme ultraviolet radiation); a support structure (e.g., a mask stage) MT, which is configured to support a pattern forming device (e.g., a mask, a reticle, or a dynamic pattern forming device) MA and is connected to a first positioner PM configured to accurately position the pattern forming device MA; and a substrate stage (e.g., a wafer stage) WT, which is configured to hold a substrate (e.g., a wafer coated with a resist) W and is connected to a second positioner PW configured to accurately position the substrate W. The lithographic apparatus 100 and 100' also have a projection system PS, which is configured to project a pattern imparted to the radiation beam B by the pattern forming device MA onto a target portion (e.g., including one or more tube cores) C of the substrate W. In lithographic apparatus 100, patterning device MA and projection system PS are reflective. In lithographic apparatus 100', patterning device MA and projection system PS are transmissive.

[0040] The illumination system IL may include various types of optical components for directing, shaping or controlling the radiation beam B, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof.

[0041] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA relative to the reference frame, the design of at least one of the lithographic apparatuses 100 and 100', and other conditions (such as whether 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 may be, for example, a frame or table that may be fixed or movable as required. By using sensors, the support structure MT may ensure that the patterning device MA is in a desired position, for example relative to the projection system PS.

[0042] 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 so as to create a pattern in a target portion C of the substrate W. The pattern imparted to the radiation beam B may correspond to a specific functional layer in a device created in the target portion C to form an integrated circuit.

[0043] The pattern forming device MA can be a transmissive type (such as in Figure 1B lithographic apparatus 100') or a reflective type (such as in Figure 1A100). Examples of pattern forming devices MA include reticles, masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography and include mask types such as binary, alternating phase shift, or attenuated phase shift, as well as various hybrid mask types. Examples of programmable mirror arrays employ a matrix arrangement of small mirrors, each of which can be individually tilted so that an incident radiation beam is reflected in different directions. The tilted mirrors impart a pattern in a radiation beam B that is reflected by the matrix of small mirrors.

[0044] The term "projection system" PS may encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems or any combination thereof, as appropriate to the exposure radiation used or to other factors such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment may be used for EUV or electron beam radiation, since other gases may absorb too much radiation or electrons. A vacuum environment may therefore be provided to the entire beam path by means of vacuum walls and a vacuum pump.

[0045] The lithographic apparatus 100 and / or the lithographic apparatus 100' may be of a type having two (dual stage) or more substrate tables WT (and / or two or more mask tables). In such a "multi-stage" machine, the additional substrate tables WT may be used in parallel, or preparatory steps may be performed on one or more tables while one or more other substrate tables WT are being used for exposure. In some cases, the additional tables may not be substrate tables WT.

[0046] The lithographic apparatus may also be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, so as to fill the space between the projection system and the substrate. Immersion liquid may also be applied to other spaces in the lithographic apparatus, such as the space between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term "immersion" as used herein does not mean that structures such as substrates are necessarily immersed in the liquid, but only that the liquid is located between the projection system and the substrate during exposure.

[0047] refer to Figure 1A and Figure 1B , the illuminator IL receives a radiation beam from a radiation source SO. For example, when the source SO is an excimer laser, the source SO and the lithographic apparatus 100, 100' may be separate physical entities. In such a case, the source SO is not considered to form part of the lithographic apparatus 100 or 100', and the radiation beam B is delivered to the lithographic apparatus 100 or 100' by means of a beam delivery system BD (in the case of a beam delivery system BD) comprising, for example, suitable directional mirrors and / or a beam expander. Figure 1BIn other cases, for example, when the source SO is a mercury lamp, the source SO may be an integral part of the lithographic apparatus 100, 100'. The source SO and the illuminator IL together with the beam delivery system BD (when necessary) may be referred to as a radiation system.

[0048] The illuminator IL may comprise an adjuster AD (at Figure 1B In general, at least the outer and / or inner radial extent (commonly referred to as the "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 may include various other components (in Figure 1B In the embodiment of the present invention, there are two types of radiation beams (i) and (ii) 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.

[0049] refer to Figure 1A , the radiation beam B is incident on a patterning device (e.g., a mask) MA held on a support structure (e.g., a mask table) MT, and a pattern is formed by the patterning device MA. In the lithographic apparatus 100, the radiation beam B is reflected from the patterning device (e.g., a mask) MA. After reflection from the patterning device (e.g., a mask) MA, the radiation beam B passes through a projection system PS, which focuses the radiation beam B onto a target portion C of a substrate W. With the aid of a second positioner PW and a position sensor IF2 (e.g., an interferometric device, a linear encoder, or a capacitive sensor), the substrate table WT can be accurately moved (e.g., so that different target portions C are positioned in the path of the radiation beam B). Similarly, a first positioner PM and a further position sensor IF1 can be used to accurately position the patterning device (e.g., a mask) MA relative to the path of the radiation beam B. The patterning device (e.g., a mask) MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

[0050] refer to Figure 1B , the radiation beam B is incident on a patterning device (e.g., mask MA) held on a support structure (e.g., mask table MT) and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through a projection system PS which focuses the beam on a target portion C of the substrate W. The projection system has a pupil conjugate PPU to the illumination system pupil IPU. Portions of the radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse the mask pattern without being affected by diffraction at the mask pattern, and produce an image of the intensity distribution at the illumination system pupil IPU.

[0051] The projection system PS projects an image MP' of the mask pattern MP onto a photoresist layer coated on a substrate W, wherein the image MP' is formed by a diffraction beam generated from the marking pattern MP by radiation from the intensity distribution. For example, the mask pattern MP may include a line and space array. Diffraction of radiation at the array and other than the zero-order diffraction produces a steered diffraction beam having a change of direction in a direction perpendicular to the line. The undiffracted beam (i.e., the so-called zero-order diffraction beam) passes through the pattern without any change in propagation direction. The zero-order diffraction beam passes through 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 zero-order diffraction beam is an image of the intensity distribution in the illumination system pupil IPU of the illumination system IL. The hole or aperture device PD is, for example, arranged at or approximately located at a plane including the pupil conjugate PPU of the projection system PS.

[0052] The projection system PS is arranged to capture not only the zero-order diffraction beam, but also the first-order diffraction beam, or the first-order and higher-order diffraction beams (not shown) by means of a lens or lens group L. In some embodiments, dipole illumination for imaging a line pattern extending in a direction perpendicular to the line can be used to utilize the resolution enhancement effect of dipole illumination. For example, the first-order diffraction beam interferes with the corresponding zero-order diffraction beam at the level of the wafer W to produce an image of the line pattern MP at the highest possible resolution and at the process window (i.e., the available depth of focus combined with the allowable exposure dose deviation). In some embodiments, astigmatism aberrations can be reduced by providing radiation poles (not shown) in the opposite quadrants of the illumination system pupil IPU. In addition, in some embodiments, astigmatism aberrations can be reduced by blocking the zero-order beam associated with the radiation pole in the opposite quadrant in the pupil conjugate PPU of the projection system. This situation is described in more detail in US7,511,799 B2, issued on March 31, 2009, which is incorporated herein by reference in its entirety.

[0053] With the help of a second positioner PW and a position sensor IF (e.g. an interferometric device, a linear encoder or a capacitive sensor), the substrate table WT can be accurately moved (e.g. in order to position a different target portion C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor ( Figure 1B ) can be used to accurately position the mask MA relative to the path of the radiation beam B (eg after mechanical retrieval from a mask library or during scanning).

[0054] Typically, movement of the mask table MT can be achieved 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 achieved 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 may be connected only to a short-stroke actuator, or may be fixed. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 may be used to align the mask MA with the substrate W. Although the substrate alignment marks (as shown) occupy dedicated target portions, the substrate alignment marks may be located in the space between the target portions (these marks are referred to as scribe alignment marks). Similarly, in the case where more than one die is provided on the mask MA, the mask alignment marks may be located between these dies.

[0055] The mask table MT and the patterning device MA may be in a vacuum chamber V, wherein an in-vacuum robot IVR may be used to move the patterning device, such as a mask, into and out of the vacuum chamber. Alternatively, when the mask table MT and the patterning device MA are outside the vacuum chamber, an out-of-vacuum robot may be used for various transfer operations similar to the in-vacuum robot IVR. Both the in-vacuum robot and the out-of-vacuum robot need to be calibrated for smooth transfer of any payload (e.g., a mask) to the fixed kinematic mounting table of the transfer station.

[0056] The lithographic apparatus 100 and 100' may be used in at least one of the following modes:

[0057] 1. In step mode, the support structure (e.g. mask table) MT and substrate table WT are kept substantially stationary (i.e. single static exposure) while the entire pattern imparted to the radiation beam B is projected at one time onto a target portion C. Subsequently, the substrate table WT is shifted in the X and / or Y direction so that a different target portion C can be exposed.

[0058] 2. In scan mode, the support structure (e.g. mask table) MT and substrate table WT are scanned synchronously (i.e. single dynamic exposure) as a pattern imparted to the radiation beam B is projected onto a target portion C. The speed and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

[0059] 3. In another mode, the support structure (e.g. mask table) MT is held substantially stationary, thereby holding the 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 may be employed, and the programmable patterning device updated as required after each movement of the substrate table WT or between successive radiation pulses during a scan. This mode of operation may be readily applicable to maskless lithography using a programmable patterning device, such as a programmable mirror array.

[0060] Combinations and / or variations on the described modes of use or entirely different modes of use may also be employed.

[0061] In another embodiment, the lithographic apparatus 100 includes an extreme ultraviolet (EUV) source configured to generate an EUV radiation beam for EUV lithography. Typically, 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.

[0062] Figure 2 The lithographic apparatus 100 is shown in more detail, and the lithographic apparatus includes a source collector apparatus SO, an irradiation system IL, and a projection system PS. The source collector apparatus SO is constructed and arranged so that a vacuum environment can be maintained in the enclosure 20. The source collector apparatus SO includes a source chamber 11 and a collector chamber 12 and is configured to generate and transmit EUV radiation. EUV radiation can be generated by a gas or vapor such as Xe gas, Li vapor, or Sn vapor, wherein a plasma 10 for emitting EUV radiation is generated to emit radiation in the EUV range of the electromagnetic spectrum. The at least partially ionized plasma 10 for emitting EUV radiation can be generated by, for example, a discharge or a laser beam. For efficient generation of radiation, a partial pressure of, for example, 10Pa of Xe, Li, Sn vapor, or any other suitable gas or vapor may be required. In an embodiment, an excited tin (Sn) plasma is provided to generate EUV radiation.

[0063] The radiation emitted by the EUV radiation emitter 10 is transferred from the source chamber 11 to the collector chamber 12 via an optional gas barrier or contaminant trap 30 (also referred to as a contaminant barrier or foil trap in some cases) positioned in or behind an opening in the source chamber 11. The contaminant trap 30 may include a channel structure. The contaminant trap 30 may also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 30 further indicated herein includes at least a channel structure.

[0064] The collector chamber 12 may include a radiation collector CO which may be a so-called grazing incidence collector. The radiation collector CO has an upstream radiation collector side 51 and a downstream radiation collector side 52. Radiation passing through the collector CO may be reflected from the grating spectral filter 40 to be focused in a virtual source point IF. The virtual source point IF is usually called an intermediate focus, and the source collector device is arranged so that the intermediate focus IF is located at or near the opening 19 in the enclosure structure 20. The virtual source point IF is an image of the plasma 10 for emitting radiation. The grating spectral filter 40 is specifically used to suppress infrared (IR) radiation.

[0065] The radiation then passes through an illumination system IL which may include a faceted field mirror arrangement 22 and a faceted pupil mirror arrangement 24 arranged to provide a desired angular distribution of the radiation beam 21 at the patterning device MA and a desired uniformity of radiation intensity at the patterning device MA. When the radiation beam 21 is reflected at the patterning device MA held by the support structure MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 29 onto a substrate W held by a wafer stage or substrate table WT.

[0066] There may typically be more elements than shown in the illumination optics unit IL and the projection system PS. Depending on the type of lithographic apparatus, a grating spectral filter 40 may optionally be present. Additionally, there may be more than Figure 2 More mirrors than the one shown in FIG. 1 may be present in the projection system PS. Figure 2 The reflective elements shown in FIG. 5 may have one to six additional reflective elements.

[0067] like Figure 2 The collector optics CO illustrated in FIG. 1 is depicted as a nested collector with grazing incidence reflectors 53, 54 and 55, merely as an example of a collector (or collector mirror). The grazing incidence reflectors 53, 54 and 55 are arranged axially symmetrically around the optical axis O, and this type of collector optics CO is preferably used in combination with a discharge produced plasma source, generally referred to as a DPP source.

[0068] Exemplary Lithography Cell

[0069] Figure 3A lithography unit 300 is shown, which is sometimes also referred to as a lithography cell or cluster. A lithography apparatus 100 or 100' may form part of a lithography unit 300. A lithography unit 300 may also include one or more devices for performing pre-exposure processes and post-exposure processes on a substrate. Conventionally, these devices include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH, and a baking plate BK. A substrate conveyor or robot RO picks up a substrate from an input / output port I / O1, I / O2, moves the substrate between different process devices, and transfers the substrate to a feed station LB of a lithography apparatus 100 or 100'. These devices (generally referred to as tracks or coating and developing systems) are under the control of a coating and developing system or a track control unit TCU, which itself is controlled by a management control system SCS, which also controls the lithography apparatus via a lithography control unit LACU. Therefore, different devices may be operated to maximize throughput and processing efficiency.

[0070] Exemplary Alignment Apparatus Using Interference

[0071] In order to control the lithography process for accurately placing device features on the substrate, alignment "marks" or "targets" are usually set on the substrate, and the lithography equipment includes one or more alignment devices and / or systems, by which the position of the marks on the substrate must be accurately measured. These alignment devices are actually position measurement devices. Different types of marks and different types of alignment devices and / or systems from different times and different manufacturers are known. The type of system widely used in current lithography equipment is based on the self-reference interferometer described in the U.S. Patent No. 6,961,116 (den Boef et al.) and the U.S. Patent No. 6,628,406 (Kreuzer). Usually, the marks are measured separately to obtain X and Y positions. The combined X and Y measurements can be performed using the technology described in the U.S. Patent No. 8,208,121 (Bijnen et al.). The entire contents of these disclosures are incorporated herein by reference.

[0072] Figure 4A schematic diagram of a cross-sectional view of an alignment device 400 that may be implemented as part of a lithographic apparatus 100 or 100' according to an embodiment is illustrated. In an example of such an embodiment, the alignment device 400 may be configured to align a substrate (e.g., substrate W) relative to a patterning device (e.g., patterning device MA). The alignment device 400 may also be configured to detect the position of alignment marks on the substrate and use the detected position of the alignment marks to align the substrate relative to the patterning device or other components of the lithographic apparatus 100 or 100'. Such alignment of the substrate may ensure accurate exposure of one or more patterns on the substrate.

[0073] According to an embodiment, the alignment device 400 may include an illumination system 402, a beam splitter 414, an interferometer 426, a detector 428, and a signal analyzer 430 according to an example of such an embodiment. The illumination system 402 may be configured to provide an electromagnetic narrowband radiation beam 404 having one or more passbands. In an example, the one or more passbands may be within a spectrum of wavelengths between about 400 nm and about 2.0 μm. In another example, the one or more passbands may be discrete narrow passbands within a spectrum of wavelengths between about 400 nm and about 2.0 μm.

[0074] The beam splitter 414 can be configured to receive the radiation beam 404 and direct the radiation sub-beam 415 onto a substrate 420 placed on a platform 422. In one example, the platform 422 can move along a direction 424. The radiation sub-beam 415 can be configured to illuminate an alignment mark or target 418 located on the substrate 420. In an example of such an embodiment, the alignment mark or target 418 can be coated with a radiation sensitive film. In another example, the alignment mark or target 418 can have a one hundred and eighty degree (i.e., 180°) symmetry. That is, when the alignment mark or target 418 is rotated 180° around an axis of symmetry perpendicular to the plane of the alignment mark or target 418, the rotated alignment mark or target 418 can be substantially the same as the unrotated alignment mark or target 418. The target 418 on the substrate 420 can be: (a) a resist layer grating including gratings formed by solid resist lines, or (b) a product layer grating, or (c) a composite grating stack in an overlay target structure including a resist grating overlaid or interlaced on a product layer grating. The gratings can alternatively be etched into the substrate.

[0075] According to an embodiment, the beam splitter 414 may also be configured to receive the diffracted radiation beam 419 and to direct the diffracted radiation sub-beams 429 towards the interferometer 426 .

[0076] In an example embodiment, the diffracted radiation sub-beam 429 may be at least a portion of the radiation sub-beam 415 that may be reflected from the alignment mark or target 418. In an example of such an embodiment, the interferometer 426 includes any suitable set of optical elements, for example, a combination of prisms that may be configured to form two images of the alignment mark or target 418 based on the received diffracted radiation sub-beam 429. The interferometer 426 may also be configured to rotate one of the two images by 180° relative to the other of the two images and to interferometrically recombine the rotated image and the unrotated image. In some embodiments, the interferometer 426 may be a self-referencing interferometer, which is disclosed in U.S. Pat. No. 6,628,406 (Kreuzer) and is incorporated herein by reference in its entirety.

[0077] In an embodiment, the detector 428 may be configured to receive the recombined image via the interferometer signal 427 and detect interference caused by the recombined image when the alignment axis 421 of the alignment device 400 passes through the center of symmetry (not shown) of the alignment mark or target 418. According to an example embodiment, such interference may be attributed to the alignment mark or target 418 being 180° symmetrical and the recombined images interfering constructively or destructively. Based on the detected interference, the detector 428 may also be configured to determine the position of the center of symmetry of the alignment mark or target 418 and thus detect the position of the substrate 420. According to an example, the alignment axis 421 may be aligned with an optical beam perpendicular to the substrate 420 and pass through the center of the image rotation interferometer 426. The detector 428 may also be configured to estimate the position of the alignment mark or target 418 by implementing sensor characteristics and interacting with wafer marking process variations.

[0078] In another embodiment, the detector 428 determines the location of the center of symmetry of the alignment mark or target 418 by performing one or more of the following measurements:

[0079] 1. Measure the position change for various wavelengths (position shift between colors);

[0080] 2. Measuring the position change for each order (position shift between diffraction orders); and

[0081] 3. Measure the position change for each polarization (position shift between polarizations).

[0082] Such data may be obtained, for example, using any type of alignment sensor, such as a SMASH (Smart Alignment Sensor Hybrid) sensor as described in U.S. Pat. No. 6,961,116, which employs a self-referencing interferometer with a single detector and four different wavelengths, and the alignment signal is extracted in software, or an ATHENA (Advanced Technology for High-Order Enhancement using Alignment) sensor as described in U.S. Pat. No. 6,297,876, which directs each of the seven diffraction orders to a dedicated detector, both of which are incorporated herein by reference in their entirety.

[0083] In an embodiment, signal analyzer 430 may be configured to determine and correlate the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. Thus, the position of alignment mark or target 418 and (therefore) the position of substrate 420 may be accurately known with reference to stage 422.

[0084] In some embodiments, the detector 428 can be a detector array that allows the possibility of accurate stack profile detection. For the detector array, multiple options are possible: multimode fiber bundles; discrete pin detectors per channel; or CCD or CMOS (linear) arrays. The use of multimode fiber bundles enables any dissipative elements to be remotely located for stability reasons. Multiple discrete pin detectors provide a large dynamic range, but each discrete pin detector requires a separate preamplifier. The number of elements is therefore limited. CCD linear arrays provide many elements that can be read out at high speed and are particularly interesting when phase-step detection is used.

[0085] Alignment Marks and Diffraction Gratings

[0086] Figure 5A A cross-sectional view of a diffraction grating 516 according to an embodiment is shown. The diffraction grating 516 may include a pitch or period "Λ" and a grating line width "d" formed on a substrate 520. The efficiency of the high diffraction orders may be determined by parameters of the diffraction grating 516, such as the profile of the grating structure (e.g., a rectangular shape), the film stack thickness "t", the duty cycle "F", etc. The duty cycle "F" may be defined as the ratio of the grating width d to the pitch Λ, i.e., F=d / Λ. The diffraction efficiency may be predicted using rigorous vector wave modeling or various other methods to solve Maxwell's equations, which adequately describe the electromagnetic fields as light propagates.

[0087] In one example, the spectral properties (e.g., diffraction angles) of the diffraction grating 516 can be described by a grating equation. For example, in the case of normal incidence with a radiation beam 515 having a wavelength λ, the m-th order diffraction angle θ can be predicted by the following equation: m：

[0088] Λsin(θ m )=mλ(normal incidence) (1)

[0089] And in the case of an incident angle of θ0, the diffraction equation (1) can be modified to

[0090] Λ(sin(θ m )-sin(θ0))=mλ (2)

[0091] The diffraction grating can be used as an alignment mark (also called an alignment target). The operation of the alignment system can be based on the phase shift between two symmetric high-order diffractions (e.g., between diffracted beams 519-1 and 519-2 with m=+1 and m=-1, respectively). When the diffraction grating 516 is not moving, the frequencies v of the diffracted beams 519-1 and 519-2 can be the same, for example, v·λ=c, where c is the speed of light. In this example, the diffracted beams 519-1 and 519-2 are in phase. If the diffraction grating 516 or the substrate 520 is, for example, moving at a speed v relative to the objective lens 512 or the interferometer (not shown) along the scanning direction 524, stage If the diffracted beam 519-1 moves, the frequency of the diffracted beam 519-2 may increase by Δν due to the Doppler effect, while the frequency of the diffracted beam 519-2 may decrease by Δν. When the diffracted beams 519-1 and 519-2 arrive at the interferometer, the frequency difference may cause a temporal phase shift. The time phase shift can be expressed as:

[0092]

[0093] The other higher-order diffracted beams follow a similar behavior. Thus, the light intensity I measured at the output of the interferometer is SUM It can be a function of the grating offset x0 (the shift from the center of the grating).

[0094]

[0095] By scanning the raster 516 (in Figure 5B The alignment signal can be modulated and the light intensity I SUM Through data processing and analysis, noise and other factors can be filtered out, and the measured light intensity I can be obtained by parameter fitting and SUM The grating offset x0 is determined by extracting Figure 5B In the embodiment, the diffraction grating 516 is oriented in the x or y direction on the substrate 520. In some embodiments, the diffraction grating 516 can be oriented in any other direction on the substrate 520. For example, the diffraction grating 516 can have a 45 degree angle with respect to the x direction (in Figure 5Bnot shown).

[0096] Even as lithography systems provide smaller and smaller patterned features at advanced technology nodes for integrated circuits, lithography systems need to improve accurate alignment between different lithography levels that receive smaller and smaller alignment marks or targets. Alignment marks can often be placed in the scribe lines (areas between chips or dies (product areas)), which are then discarded when the die is cut before packaging. For a given process monitoring area, smaller sizes also allow more alignment marks to be distributed at various locations to improve alignment and / or overlay uniformity across the wafer. Alternatively, for dies with large areas, smaller alignment marks can be placed inside the die to improve alignment across the product chip.

[0097] Currently, interferometry-based alignment systems rely on raster scanning to produce a modulated alignment signal. Figure 5B Alignment marks along the x and y directions are shown on substrate 520 according to some embodiments. Radiation beam 515 may be scanned across the alignment marks in the x direction and in the y direction. In some embodiments, substrate 520 moves instead of radiation beam 515.

[0098] In one example, to generate reliable alignment signals, long alignment marks are designed to provide sufficient distance for scanning. In image-based alignment systems, small alignment marks can be used. However, due to large data acquisition and processing, resolution and speed may be limited.

[0099] Exemplary Alignment System Using Active Phase Modulation

[0100] Various embodiments according to the present disclosure provide apparatus and methods for aligning with small marks by using active phase modulation. For example, alignment marks as small as 5 μm×5 μm may be sufficient to provide alignment signals with good resolution. Figure 6 5 shows a top view of an exemplary alignment mark or target 618, wherein two-dimensional diffraction gratings are arranged in the x and y directions so that the position of the alignment target 618 can be determined without moving the radiation beam 515. By using active phase modulation, the alignment signals of the alignment target 618 along the x and y directions can be measured simultaneously, and the position of the alignment target 618 in the x and y directions can be determined simultaneously.

[0101] In some embodiments, alignment target 618 may include a diffraction grating oriented in any direction other than the x or y direction. For example, the diffraction grating may form a 45 degree angle with respect to the x or y direction.

[0102] In one example, the alignment device and method require no moving parts and can operate in a static mode. In addition, by using phase modulation, the alignment signal can be modulated at a frequency that is less sensitive to vibrations of the alignment device. Thus, alignment using phase modulation can have less background noise than alignment using raster scanning.

[0103] In some embodiments, when using active phase modulation and alignment target scanning, a counter shift phase may be introduced for high-speed scanning and high-speed alignment.

[0104] Figure 7 and Figure 8 Exemplary alignment devices 700 and 800 according to some embodiments of the present disclosure are illustrated. Alignment devices 700 and 800 can be configured to measure the position of alignment target 618 based on phase modulation of diffracted beam 719 (also referred to as diffracted beam), wherein the phase modulation is generated at various locations in alignment device 700 or 800 by phase modulator 740. Alignment devices 700 and 800 can be created as part of lithography device 100 or 100', or can be created as a separate unit in a lithography cell or lithography cluster 300, and work together with other devices during operation. In some embodiments, alignment device 700 includes an illumination system 702 that generates a coaxial illumination beam 704, and alignment device 800 includes an illumination system 803 that generates an off-axis illumination beam 806. Alignment devices 700 and 800 can also include a projection system 742, a self-referencing interferometer (SRI) 726, a beam analyzer 730, and a detection system 728. In some embodiments, projection system 742 can be part of illumination system 702 and 803.

[0105] In one example, the illumination system 702 and 803 includes a light source 705, wherein the light source 705 emits a spatially coherent coaxial illumination beam 704. The coaxial illumination beam 704 can have an electromagnetic narrowband having one or more passbands. In some embodiments, the coaxial illumination beam 704 can be visible light or infrared light having a wavelength in a range between about 500nm to about 700nm and about 700nm to about 2μm, respectively. In some embodiments, the light source 705 can include one or more radiation sources, each of which produces one or more passbands within a spectrum of wavelengths between about 500nm to about 2μm. In this example, the coaxial illumination beam 704 can be a combination of one or more passbands from one or more radiation sources, and can also have a substantially continuous wavelength. In some embodiments, the wavelength range of the coaxial illumination beam 704 can also include ultraviolet.

[0106] In some embodiments, Figure 8The illumination system 803 of the alignment device 800 in the embodiment includes an off-axis optical component 808 and can generate an off-axis illumination beam 806 from the on-axis illumination beam 704. For off-axis illumination, the radiation source area is limited to a peripheral portion of the pupil that is a certain distance from the optical axis. In some embodiments, the off-axis optical component 808 can include an axicon reflector, one or more beam splitters, or a quadruple lens. A detailed disclosure of off-axis radiation can be found in U.S. Patent No. 9,970,747 (Kreuzer et al.), which is incorporated herein by reference in its entirety.

[0107] Off-axis illumination beam 806 can have an illumination profile 810. In some embodiments, illumination profile 810 can include: (a) a pair of illumination spots positioned diametrically opposite each other with 180° symmetry about the optical axis; and (b) two pairs of illumination spots with 180° symmetry in each pair. In some embodiments, the two pairs of illumination spots are separated by 90° from each other. In some embodiments, the illumination spots of illumination profile 810 are located at an angle α of about 22.5° from the y-axis. Figure 8 The circular area of ​​illumination profile 810 in represents the pupil plane of illumination system 803.

[0108] In some embodiments, the illumination beam 704 can be a polarized beam, for example, having linear polarization, circular polarization, or elliptical polarization. The unpolarized radiation from the light source 705 can be converted into a polarized beam by using a polarizer such as a dichroic mirror, a crystal, or a wire grid polarizer. The two orthogonal polarization directions of the illumination beam 704 can be described as p-polarization and s-polarization. For example, p-polarization has an electric field parallel to the plane of incidence, and s-polarization has an electric field perpendicular to the plane of incidence. In some embodiments, the polarization state of the illumination beam 806 can be different from the incident illumination beam 704 by using one or more polarizers.

[0109] In some embodiments, the alignment device 700 (in Figure 7 ) and alignment device 800 (in Figure 8 704 and 806. The projection system 742 of FIG. 704 includes an objective lens 712 and a beam splitter 714, and is configured to direct the coaxial illumination beam 704 or the off-axis illumination beam 806 toward the alignment target 618 and direct the diffracted beam 719 reflected from the alignment target 618 toward a self-referencing interferometer (SRI) 726. In this example, the coaxial illumination beam 704 is directed toward the alignment target 618 at a normal angle of incidence, while the off-axis illumination beam 806 is directed toward the alignment target 618 at a fixed angle of incidence.

[0110] In one example, during the semiconductor manufacturing process, the wafer can be moved under the main lithography lens. The accuracy of the wafer positioning under this exposure lens can directly determine the quality of the semiconductor product. Therefore, alignment marks can be placed on the wafer starting from the first lithography level to ensure the accuracy of the wafer position. In some embodiments, the alignment target 618 can be a structure formed on a substrate 720 (e.g., a semiconductor wafer) using a previous horizontal lithography mask (not shown) by pattern transfer, and the substrate 720 is located on the platform 722. The materials and film stacks used for the formation of the alignment target 618 can depend on the layout of the alignment target 618 on the previous horizontal lithography mask and the process experienced by the substrate 720. In some embodiments, the alignment target 618 can be made of a radiation-sensitive film (e.g., a resist) or coated with a radiation-sensitive film. The design requirements (e.g., shape and size) for the alignment target 618 depend on the alignment system and alignment method used.

[0111] exist Figure 7 and Figure 8 , beam splitter 714 may be a spot mirror formed by a transmissive cube having a reflective metal layer disposed in the center of the cube. It should be noted that even though beam splitter 714 is shown reflecting illumination beam 704 or 806 toward alignment target 618 and transmitting diffracted beam 719 toward SRI 726, the present disclosure is not so limited. Those skilled in the relevant art will appreciate that other optical arrangements may be used to achieve similar results.

[0112] In one example, objective lens 712 focuses illumination beam 704 or 806 onto alignment target 618 and collects diffracted beam 719 reflected from alignment target 618. Other focusing optical systems may be used to provide similar functionality as known to those skilled in the art.

[0113] In some embodiments, diffracted beam 719 from alignment target 618 includes symmetrically distributed high-order diffracted beams, i.e., m≠0 in equation (1) or (2), for example, +1 and -1, +2 and -2, ..., +n and -n, ..., respectively. Depends on the diffraction angle θ m , diffraction beams of different orders are separated in space. In some embodiments, the diffraction beam 719 includes at least one positive diffraction order or one negative diffraction order.

[0114] In one example, SRI 726 is configured to receive diffracted beam 719 and generate diffracted beamlet 746. SRI 726 can project two superimposed images of alignment target 618 that are rotated 180° relative to each other. Fig. 9As further illustrated in , each pair of positive and negative diffraction orders of diffracted beam 719 produces two pairs of diffracted beamlets 746-1 and 746-2, wherein each pair of diffracted beamlets (e.g., 746-1) includes spatially superimposed components from both the positive and negative diffraction orders of diffracted beam 719. Details of self-referencing interferometers are disclosed in U.S. Pat. Nos. 6,961,116 (Boef et al.) and 6,628,406 (Kreuzer), which are incorporated herein by reference in their entirety.

[0115] In some embodiments, the first polarizer 744 can be placed between the beam splitter 714 and the SRI 726 to adjust the polarization state of the diffracted beam 719 before entering the SRI 726. For example, the first polarizer 744 can be a half-wave plate and can rotate the polarization direction of the diffracted beam 719.

[0116] In some embodiments, a phase compensator 750 and a second polarizer 752 may be implemented between the SRI 726 and the beam analyzer 730. The phase compensator 750 and the second polarizer 752 may adjust the phase and polarization direction of the diffracted sub-beam 746. In some embodiments, the second polarizer 752 may be a half-wave plate. In some embodiments, the phase compensator or phase retarder 750 may be a birefringent crystal made of a material such as tourmaline, calcite (CaCoO3), sodium nitrate (NaNO3), calomel (Hg2Cl2) or rutile (TiO2). In some embodiments, the phase compensator 750 may be made of barium borate (αBBO or BaB2O4). Compared to polarized light along the slow axis, polarized light along the fast axis of the birefringent crystal encounters a lower refractive index and travels through the crystal faster. Therefore, a phase shift may be generated between light with different polarization directions.

[0117] In some embodiments, the beam analyzer 730 can be configured to generate interference intensity between the superimposed components of the diffracted sub-beams 746 and generate two orthogonally polarized optical branches 748. For example, the beam analyzer 730 can transmit light with p polarization and reflect s polarized light. Thus, the p polarization component of the diffracted sub-beams 746 can be transmitted to the optical branch 748-1, and the s polarization component of the diffracted sub-beams 746 can be reflected to the optical branch 748-2. On the optical branch 748-2, the s polarization components of the diffracted sub-beams 746 interfere with each other. Similarly, the p polarization components of the diffracted sub-beams 746 also interfere with each other on the optical branch 748-1.

[0118] In one example, Figure 7 and Figure 8The detection system 728 of the alignment devices 700 and 800 in FIG. 7 generates an incoherent addition of the interfering beams and measures the light intensity of each optical branch 748-1 and 748-2. The detection system 728 can be configured to determine the position of the alignment target 618 based on the light intensity measurement of the optical branch 748, where the light intensity is modulated in some way so as to extract the alignment information among other parameters of the alignment device.

[0119] For a stable alignment apparatus and a static alignment target 618 (e.g., the platform 722 does not move), the light intensity measured by the detection system 728 may be expressed as:

[0120]

[0121]

[0122] Equations 5(a) and 5(b) are similar to Equation 4, except that V stage = 0, and thus the time phase shift However, in order to extract the grating offset x0, light intensity modulation needs to be introduced into equation 5(a) or 5(b). In contrast to moving the platform 722 (or scanning the alignment target) as described in equations (3) and (4), active phase modulation can be used to obtain light intensity modulation and will be described in further detail.

[0123] Fig. 9 The polarization state of the diffracted beam 719 at various locations is illustrated as it propagates through the alignment devices 700 and 800 according to some embodiments of the present disclosure. In this example, for simplicity, only one pair of diffraction orders (e.g., m=+1 and -1) is shown. However, the diffracted beam 719 may also include higher diffraction orders (e.g., m=+2 and -2). Fig. 9 In FIG. 7 , two linearly polarized diffracted beams from an alignment target are illustrated, and the polarization directions are shown with arrows. The diffracted beams 719 may have other polarizations and are not limited to the examples herein. Fig. 9 , the circular area represents the pupil plane of the alignment devices 700 and 800.

[0124] In some embodiments, at a location (1) between the beam splitter 714 and the first polarizer 744, the pair of positive and negative diffraction orders of the diffracted beam 719 are positioned diametrically opposite in the pupil plane. At a location (2) between the first polarizer 744 and the SRI 726, the polarization directions of the positive and negative orders of the diffracted beam 719 are rotated. For example, the first polarizer 744 is a half-wave plate, and the rotation angle depends on the angle of the fast axis of the half-wave plate.

[0125] In one embodiment, each of the positive and negative diffraction orders is split into two diffraction beamlets 746-1 and 746-2 as the diffracted beam 719 propagates through the SRI 726. Each of the diffracted beamlets 746-1 and 746-2 includes components from both the positive and negative diffraction orders of the diffracted beam 719 that are spatially superimposed on each other. Fig. 9 (3) is represented by superposition of +1 and -1. For example, the diffraction beamlet 746-1 may have a p-polarization component of a positive diffraction order of the diffraction beam 719 and an s-polarization component of a negative diffraction order of the diffraction beam 719 that are spatially superimposed on each other. Similarly, the diffraction beamlet 746-2 may have an s-polarization component of a positive diffraction order of the diffraction beam 719 and a p-polarization component of a negative diffraction order of the diffraction beam 719 that are spatially superimposed on each other. In some embodiments, the diffraction beamlets 746-1 and 746-2 are rotated ±90° from the original positions of the positive diffraction order and the negative diffraction order of the diffraction beam 719 in the pupil plane. In other words, the diffraction beamlets 746-1 and 746-2 are rotated 180° relative to each other. Thus, at the output of SRI 726, both diffracted beamlets 746-1 and 746-2 carry alignment information for the positive and negative diffraction orders (eg, +1 and -1) from diffracted beam 719. As shown in FIG.

[0126] The phase shift between the s-polarization and the p-polarization can be adjusted when the diffracted beamlets 746-1 and 746-2 propagate through the phase compensator 750. In addition, the polarization directions of the diffracted beamlets 746-1 and 746-2 can be rotated at the second polarizer 752. The resulting polarization states of the diffracted beamlets 746-1 and 746-2 are shown in Fig. 9 (4) corresponds to part (4) in devices 700 and 800.

[0127] At the position (5) after the beam analyzer 730, each of the diffracted sub-beams 746-1 and 746-2 is divided into two optical branches 748-1 and 748-2. For example, the diffracted sub-beam 746-1 can have both p and s polarization directions rotated 45° at the second polarizer 752. Thus, at the beam analyzer 730, each of the polarizations oriented at 45° can also be decomposed into s polarization and p polarization, wherein the s polarization component is reflected into the optical branch 748-2 and the p polarization component is transmitted into the optical branch 748-1. In some embodiments, the s polarization components of the diffracted sub-beams 746-1 interfere with each other. Similarly, the p polarization components of the diffracted sub-beams 746-1 also interfere with each other. The diffracted sub-beam 746-2 behaves similarly to the diffracted sub-beam 746-1 described above. Therefore, at the position (5), four pairs of interference beams are generated in the devices 700 and 800.

[0128] At position (6), the detection system 728 generates the addition of the interference beams of each optical branch 748-1 and 748-2 and outputs the light intensity as I diff and I sum .

[0129] Active Phase Modulation in Alignment Devices

[0130] In some embodiments, phase modulation may be introduced in alignment apparatuses 700 and 800 without moving platform 722 or the incident beam on alignment target 618 (i.e., without scanning the alignment target). Figure 7 and Figure 8 As shown in FIG. 1 , phase modulators 740 at various locations in alignment apparatus 700 with coaxial illumination and alignment apparatus 800 with off-axis illumination may be used, for example, phase modulators 740 - 1 , 740 - 2 , 740 - 3 , 740 - 4 , and / or 740 - 5 .

[0131] In some embodiments, the phase modulator 740 can be a transmissive phase modulator that covers half of the pupil plane through which one of the optical branches of the light beam propagates. In a transmissive phase modulator, the phase of the light beam can be modulated by changing the birefringence of the material using an applied voltage or pressure. The transmissive phase modulator can include a potassium dideuterium phosphate (KD*P) crystal, a liquid crystal modulator (LCM), a photoelastic modulator (PEM), or an electro-optic modulator (EOM). For example, some KD*P crystals allow phase modulation in one polarization direction and no modulation in an orthogonal polarization direction.

[0132] In some embodiments, the phase modulator 740 may include two components that apply different phase delays that vary over time. By applying two phase delay components and modulating both optical branches of the light beam with different frequencies, the measured light intensity at the detection system 728 may be less sensitive to background noise.

[0133] In some embodiments, the phase modulator 740 may also be a reflection-based arrangement, e.g. Fig. 10A (side view) and Fig. 10B (top view). The piezoelectric mirror assembly 1000 includes two arms 1054 and 1056 having mirrors, wherein the mirror on one of the arms (e.g., arm 1054) can be moved by applying a voltage to a piezoelectric material such as lead zirconate titanate (PZT). Fig. 10A and Fig. 10B1054 can be moved in direction 1058, thereby changing the optical path. In one example, when the light beam enters the piezoelectric mirror assembly 1000, the light beam is split into two optical branches, which are directed to arms 1054 and 1056, respectively. At the output of the piezoelectric mirror assembly 1000, the two optical branches of the light beam acquire an optical path difference and (therefore) a phase shift, wherein the phase shift depends on the voltage applied to the PZT on the arm 1054. As described above, the piezoelectric mirror assembly 1000 can be independent of the polarization state of the light.

[0134] In some embodiments, the phase modulator 740-1 can be a binary amplitude spatial modulator, in which a voltage applied spatially (based on pixels) causes a local binary deflection or reflection of a portion of the incident light beam. Examples of binary amplitude spatial modulators are ferroelectric liquid crystal (FLC), micromirror array (MMA), etc. Ferroelectric liquid crystal is a chiral smectic liquid crystal that changes orientation under an electric field. By rotating the direction of the polarized beam, a quarter wave plate can be produced using FLC. When the FLC is implemented with a beam analyzer, linearly polarized light can be transmitted or reflected depending on the polarization state. The micromirror array consists of pixels that can be manipulated by tilting and flipping and / or by a piston. The piston can be used to locally induce phase delay, while flipping and tilting can be used to reflect a portion of the incident beam. MMA can also be independent of the polarization state of the beam.

[0135] In some embodiments, the phase modulator 740 may be a digital spatial light modulator (SLM), wherein the digital SLM may modulate the phase of the incident light beam for each pixel independently. The digital SLM includes spatially nematic liquid crystal and a micro-mirror array.

[0136] In some embodiments, phase modulator 740 may be any combination of the aforementioned phase modulators.

[0137] Return to reference Figures 7 to 9 , depending on the location of the phase modulator 740 in the alignment apparatus 700 and 800, the light intensity measured at the detection system 728 may be different.

[0138] Active phase modulation using phase modulator 740-1:

[0139] In some embodiments, a phase modulator 740-1 may be placed in the pupil plane between the beam splitter 714 and the first polarizer 744. The phase modulator 740-1 may introduce a phase shift δ between the positive and negative diffraction orders of the diffracted beam 719 and change the measured light intensity at the detection system 728 to:

[0140]

[0141]

[0142] By modulating the phase shift δ over time (i.e., by active phase modulation to introduce a temporal phase shift, e.g. Similar alignment signals (compare to Equation 4) may be generated as the platform 722 is moved or the alignment target 618 is scanned. From the measured light intensity, the position of the alignment target may be determined from the grating shift x0, either by data or by warp fitting.

[0143] Phase modulation using phase modulator 740-2:

[0144] In some embodiments, phase modulator 740-2 may be placed between SRI 726 and phase compensator 750. Assume that phase modulator 740-2 may be placed between diffracted beamlets 746-1 and 746-2 (see Fig. 9 (3)) introduces a phase shift δ between the s-polarization component and the p-polarization component, then the measured light intensity at the detection system 728 can be expressed as:

[0145]

[0146]

[0147] According to equations 7(a) and 7(b), the phase shift introduced by the phase modulator 740-2 changes the amplitude of the light intensity measured at the detection system 728 (i.e., changes the modulation depth). Typically, modulation of the amplitude of the light intensity does not alone provide the desired alignment information. However, the phase modulator 740-2 can be implemented in combination with other forms of modulation (e.g., frequency modulation of the coaxial illumination beam 704 or the off-axis illumination beam 806). Alignment using frequency modulation will be described in detail in a subsequent session herein.

[0148] In some embodiments, wave plate 1059 may be used in conjunction with phase modulator 740-2 to introduce a temporal phase shift similar to that described in Equations 6(a) and 6(b) Fig. 10C An example of a wave plate 1059 according to some embodiments of the present disclosure is illustrated.

[0149] In some embodiments, wave plate 1059 may be placed at Figure 7 and Figure 8 Between the phase modulator 740-2 and SRI726. Fig. 10CAs shown in (ii), wave plate 1059 can change the polarization state of the positive and negative diffraction orders of the diffracted beamlets (e.g., 746-1) aligned with the half pupil of the device. In this example, wave plate 1059 includes two parts. Part 1059-2 is a crystal that is insensitive to polarization, and part 1059-1 is a λ / 2 (half) wave plate, where its fast axis (at Fig. 10C (i) is oriented at 45° relative to the p-polarization or s-polarization direction of the diffracted beamlet (eg, 746-1). In this example, HWP 1059-1 can rotate the polarization direction by 90° relative to the incident beam.

[0150] In some embodiments, the two parts of the wave plate 1059 can be rotated around the angle of the propagation direction of the diffracted beam or diffracted beamlet. Fig. 10C As shown in (i), portion 1059-1 may be rotated 22.5 degrees. In some embodiments, portion 1059-1 may also be rotated 45 degrees, depending on the fast axis orientation.

[0151] In some embodiments, Fig. 10C (i), the diffracted beamlets 746-1 and 746-2 are diametrically located in the pupil plane of the wave plate 1059, i.e., the diffracted beamlets 746-1 and 746-2 are 180° apart from each other about the center of the optical axis at the output of the SRI 726. In some embodiments, the diffracted beamlets 746-1 and 746-2 correspond to the x-direction from the alignment target 618 (see Figure 6 ) of the diffraction grating. Fig. 10C (i) Other diffracted beamlets of corresponding diffraction gratings at the output of SRI 726 along other directions, such as the y-direction, and at 45 degree angles are also shown.

[0152] In some embodiments, the wave plate 1059 can be configured so that the diffracted beamlet 746-1 is irradiated onto the HWP portion 1059-1, and the diffracted beamlet 746-2 is irradiated onto the crystal portion 1059-2. Therefore, at the output of the wave plate 1059, the positive and negative diffracted orders of the diffracted beamlet 746-2 maintain the original polarization direction, however, the positive and negative diffracted orders of the diffracted beamlet 746-1 change the polarization direction to the opposite direction. Thus, by using the wave plate 1059, the positive and negative diffracted orders of the diffracted beamlets 746-1 and 746-2 now have the same polarization direction and also carry the same alignment signal.

[0153] A time-dependent phase shift is introduced by using a phase modulator 740-2 after the wave plate 1059, such as Fig. 10C As shown in (ii), the interference intensity can be detected and measured at the detection system 728. Fig. 10C(iii) illustrates an exemplary light intensity measured by the detection system 728, where the phase shift between the two oscillation curves corresponds to the alignment signal (eg, grating offset x0).

[0154] Phase modulation using phase modulator 740-3:

[0155] In some embodiments, a phase modulator 740-3 may also be placed at the output of the beam analyzer 730. In this example, optical branches 748-1 and 748-2 (see Fig. 9 Each optical branch in (5)) can independently receive phase modulation through a separate phase modulator 740-3.

[0156] Phase modulation using phase modulator 740-4:

[0157] In some embodiments, the phase modulator 740-4 can be placed at the input of the detection system 728, in front of the aperture 760. The phase modulator 740-4 can control the region of interest (ROI) by selecting the area of ​​the alignment target to be measured. For example, the phase modulator 740-4 can be placed in front of the aperture 760 where the intermediate image plane exists. The detection system 728 measures the integrated signal via the image, and the intensity of the integrated signal varies as a whole according to the induced phase modulation by moving the platform 722 (i.e., scanning the alignment target) or using phase modulators 740-1, 740-2, 740-3, 740-5, etc. In this example, the phase modulator 740-4 can be a spatial light modulator, thereby allowing the beam to pass for certain areas of the alignment target and blocking other beams. In this way, it can be possible to accommodate the small size of the alignment target 618 without interfering with surrounding structures. The ROI selection function works better when the alignment target 618 is stationary during measurement, ie, the light intensity is modulated using phase modulator 740-1, 740-2, 740-3, or 740-5.

[0158] Thus, the phase modulator 740-4 can selectively remove field regions in the image plane, making the system more robust to interfering signals due to overfilling the alignment target with a beam of larger spot size, more adaptable to changes in the grating profile of the alignment target, and less sensitive to the design of the alignment target. For this purpose, a binary amplitude spatial modulator or a digital SLM can be used for the phase modulator 740-4.

[0159] Phase modulation on off-axis illumination beam:

[0160] In some embodiments, phase modulation of at least one of these off-axis illumination beams may also introduce intensity modulation at the detection system 728. For example, the phase modulator 740-5 may be Figure 8A phase shift δ is introduced into one branch of the off-axis illumination beam 806 in . In this example, the measured light intensity can also be expressed by equations 6(a) and 6(b). However, other optical configurations can also produce phase modulation of the off-axis illumination beam 806.

[0161] Fig.11A An exemplary optical configuration 1100 for phase modulation of off-axis illumination beam 806 is illustrated in accordance with some embodiments of the present disclosure. Phase modulation of off-axis illumination beam 806-1 may be produced by piezoelectric mirror arrangement 1160. The optical path of off-axis illumination beam 806-1 may be modulated by applying an electrical signal to piezoelectric mirror arrangement 1160, thereby producing a phase shift δ between off-axis illumination beams 806-1 and 806-2. By varying the phase shift δ between off-axis illumination beam 806-1 and off-axis illumination beam 806-2 over time, the temporal phase shift may be is introduced into the light intensity measured at the detection system 728, where the light intensity can be expressed by Equations 6(a) and 6(b).

[0162] Fig. 11B Another exemplary optical configuration 1101 for phase modulation of off-axis illumination beam 806 is illustrated in accordance with some embodiments of the present disclosure. In this example, two transmissive phase modulators 1162-1 and 1162-2 are implemented on off-axis illumination beams 806-1 and 806-2. In some embodiments, transmissive phase modulators 1162-1 and 1162-2 may be passive phase compensators, providing a fixed phase shift between off-axis illumination beams 806-1 and 806-2 (as opposed to a change that may vary over time). In some embodiments, transmissive phase modulator 1162-2 may also provide active phase modulation for off-axis illumination beam 806-2 at a different modulation frequency from piezoelectric mirror arrangement 1160 for off-axis illumination beam 806-1. In other words, off-axis illumination beams 806-1 and 806-2 may have phase modulation at two different frequencies. The transmissive phase modulators 1162-1 and 1162-2 may be a liquid crystal modulator (LCM), a photoelastic modulator (PEM), an electro-optic modulator (EOM), or the like.

[0163] Fig. 12A and Fig. 12B 1200 and 1201 are illustrated for an off-axis illumination beam 806 according to some embodiments of the present disclosure. In this example, the off-axis illumination beam 806 has Figure 8 The illumination profiles 810(b) of the two pairs of illumination spots described in FIG. Fig. 12A, the coaxial illumination beam 704 propagates through an optional phase retarder 1264 and may be split into two optical branches at a beam splitter 1266-1, wherein each branch may be further split into two optical branches at beam splitters 1266-2 and 1266-3. Optical branches 806-1 and 806-4 of an off-axis illumination beam 806 (also referred to as off-axis illumination beams 806-1 and 806-4) correspond to a pair of illumination spots separated from each other in diameter, while optical branches 806-2 and 806-3 of the off-axis illumination beam 806 (also referred to as off-axis illumination beams 806-2 and 806-3) correspond to another pair of illumination spots also separated from each other in diameter. For each pair of illumination spots, one of the branches (e.g., 806-1 and 806-2) may be implemented with the aid of phase modulation through phase modulators 1268-1 and 1268-2, respectively. In Fig. 12A In the embodiment of the present invention, optional phase modulators 1268-3 and 1268-4 can be implemented between beam splitter 1266-1 and beam splitter 1266-2 and between beam splitter 1266-1 and beam splitter 1266-3. Optional phase modulators 1268-3 and 1268-4 can provide passive phase compensation or active phase modulation between two pairs of optical branches. Fig. 12B , piezoelectric mirror arrangements 1270-1 and 1270-2 are used as phase modulators for optical branches 806-1 and 806-4, respectively, where optical branches 806-1 and 806-4 correspond to different illumination spots of illumination profile 810(b).

[0164] Return to reference Fig. 12A In some embodiments, phase modulators 1268-1, 1268-2, 1268-3, and 1268-4 may be similar to phase modulator 740 and may also include transmissive phase modulators, such as KD*P crystals, LCMs, PEMs, EOMs, etc. In some embodiments, phase modulators 1268-1, 1268-2, 1268-3, and 1268-4 may be PZT mirror assemblies or binary amplitude spatial modulators, such as FLCs or MMAs. In some embodiments, phase modulators 1268-1, 1268-2, 1268-3, and 1268-4 may be digital SLMs or any combination of the foregoing modulators.

[0165] In some embodiments, phase modulators 1268-3 and 1268-4 may be replaced by frequency modulators to provide separate frequency modulation for optical branch 806. The details of frequency modulation will be described in detail in a subsequent session herein.

[0166] Fig.13An optical configuration 1300 is illustrated for providing phase modulation for an off-axis illumination beam 806 in accordance with some embodiments of the present disclosure. The off-axis illumination beam 806 has an illumination profile 810(b) having two pairs of illumination spots in a pupil plane. In this example, the on-axis illumination beam 704 may be unpolarized. A polarization phase retarder 1372 may convert the illumination beam 704 into polarized illumination and induce a phase shift between an s-polarization component and a p-polarization component. The optical configuration 1300 also includes a four-way beam splitter 1374, wherein the four-way beam splitter 1374 may split the illumination beam 704 into four optical branches 806-1, 806-2, 806-3, and 806-4. In this example, the optical branches 806-1 and 806-3 may have s-polarization, while the optical branches 806-2 and 806-4 may have p-polarization.

[0167] exist Fig.13 , optical configuration 1300 may also include phase modulators 1376-1 and 1376-2. In some embodiments, phase modulator 1376-1 may provide phase modulation for optical branch 806-1, and phase modulator 1376-2 may provide phase modulation for optical branch 806-3. Phase modulators 1376-1 and 1376-2 may be similar to phase modulator 740 and may include similar optics. In some embodiments, phase modulators 1376-1 and 1376-2 may be KD*P crystals, allowing separation of s-polarization from p-polarization by applying different modulation frequencies.

[0168] exist Fig.13 In the embodiment of the present invention, the optical configuration 1300 may further include phase compensators 1378-1 and 1378-2 to adjust the phase shift between the optical branch 806-1 and the optical branch 806-4 and between the optical branch 806-2 and the optical branch 806-3, respectively. Without limiting the scope of the present disclosure, the phase compensators 1378-1 and 1378-2 may also actively modulate the phase of the light beam. In some embodiments, as Fig.13 As shown in FIG. 1 , phase compensators 1378-1 and 1378-2 can be placed on optical branches 806-1 and 806-3, respectively. In some embodiments, phase compensators 1378-1 and 1378-2 can also be placed on optical branches 806-2 and 806-4, respectively. Phase compensators 1378-1 and 1378-2 can be similar to phase modulators 1376-1 and 1376-2 and can include similar optical devices.

[0169] exist Fig.13In the optical configuration 1300 in FIG. 1 , in some embodiments, each illumination spot of the illumination profile 810 ( b ) can have two orthogonally polarized beams. For example, optical branch 806-1 is s-polarized and optical branch 806-4 is p-polarized, or vice versa. Similarly, optical branch 806-2 is s-polarized and optical branch 806-3 is p-polarized, or vice versa. In some embodiments, each illumination spot of the illumination profile 810 ( b ) can have one linearly polarized beam and one unpolarized beam. Phase modulators 1376-1 and 1376-2 and phase compensators 1378-1 and 1378-2 can be configured to modulate polarized beams and unpolarized beams.

[0170] Simultaneous detection of alignment position in X and Y directions

[0171] In some embodiments, this can be accomplished by using a phase modulator (e.g., Figure 7 and Figure 8 The alignment device 700 in FIG. 1 and the phase modulator 740-1 in FIG. 800 simultaneously determine the two-dimensional alignment target 618 in the x and y directions (in FIG. Figure 6 The alignment target 618 may be placed stationarily in the alignment device 700 or 800. In this example, the alignment target 618 may have a diffraction grating with different periods in the x and y directions, for example, a diffraction grating extending parallel to the x-axis may have a period Λ x , and the diffraction grating extending parallel to the y-axis may have a period Λ y , where Λ x ≠Λ y According to Equations 6(a) and 6(b), the light intensity measured at the detection system 728 depends on the period of the diffraction grating Λ. Thus, the measured light intensity can include two different frequencies that can be distinguished or demultiplexed to determine the alignment position in the x and y directions for the alignment target 618.

[0172] In some embodiments, the alignment device 700 (in Figure 7 ) of the coaxial illumination beam 704 and / or the alignment device 800 (in Figure 8 The off-axis illumination beam 806 of FIG. 8 can be used to illuminate the Figure 6 The alignment position in both the x and y directions is detected simultaneously by using the two-dimensional alignment target 618 in the diffraction grating. By scanning the alignment target 618 at an oblique angle to the diffraction grating, the induced frequency difference Δν between the positive and negative diffraction orders of the diffracted beam 719 can be generated in equation (3) in the x and y directions, respectively, i.e., and Where V stage,x and V stage,yare the components of the stage scanning velocity in the x and y directions. Similarly, the measured light intensity may include two different frequencies that may be distinguished or demultiplexed to determine the alignment position in the x and y directions for the alignment target 618.

[0173] In some embodiments, there are two pairs of illumination spots (eg Figure 8 , Fig. 12A , Fig. 12B and Fig.13 The illumination profile 810(b) shown in FIG. 8 can be used for a Figure 6 The alignment target 618 of the two-dimensional diffraction grating in FIG. 618 determines the alignment position in both the x and y directions simultaneously. In this example, one pair of illumination spots can be used to detect the alignment position in the x direction, and another pair can be used to detect the alignment position in the y direction. For example, in Fig. 12A , Fig. 12B or Fig.13 , by modulating off-axis illumination beams 806 - 1 and 806 - 4 at one frequency and modulating off-axis illumination beams 806 - 2 and 806 - 3 at a different frequency, alignment positions in both the x and y directions may be detected for alignment target 618 simultaneously.

[0174] In some embodiments, Figure 7 and Figure 8 The phase modulator 740-1, 740-2, or 740-3 in the alignment devices 700 and 800 can introduce phase modulation to only one diffraction order of the diffraction beam 719 (e.g., m=±1). In some embodiments, the phase modulator 740-1 can introduce different phase modulation to two diffraction orders of the diffraction beam 719 (e.g., m=±1 and ±2). In this example, one diffraction order of the diffraction beam (e.g., m=±1) can provide alignment position information in the x-direction, while the other diffraction order of the diffraction beam (e.g., m=±2) can provide alignment position information in the y-direction. For example, a nematic spatial phase modulator can provide the functionality described herein.

[0175] In some embodiments, Figure 7 and Figure 8The phase modulator 740-3 in the alignment devices 700 and 800 in FIG. 7 can allow specific diffraction orders to pass through and arrive on the optical branches 748-1 and 748-2. In addition to scanning the alignment target 618, the alignment position in the x and y directions can also be determined simultaneously by using the phase modulator 740-3. For example, the phase modulation of the optical branches 748-1 and 748-2 produced by the phase modulator 740-3 can constructively add the phase modulation induced by the scanning of the alignment target 618 in the x direction. Similarly, the phase modulation can destructively add the phase modulation induced by the scanning of the alignment target 618 in the y direction. In this example, two different modulation frequencies can be measured independently for the x and y directions and can be separated by signal analysis.

[0176] Continuously detects alignment position in X and Y directions

[0177] In some embodiments, the alignment position of the two-dimensional alignment target 618 in the x and y directions can be continuously determined. Figure 8 810(b), each pair of illumination spots can provide alignment information from the grating along the x-direction or the y-direction, depending on the scanning direction of the alignment target 618. In this example, the light intensity measured at the detection system 728 is modulated by scanning the two-dimensional alignment target 618.

[0178] In some embodiments, the two pairs of illumination spots of the off-axis illumination beam 806 can be Figure 8 The phase modulator 740-5 in or more specifically by Fig. 12A Phase modulators 1268-1 and 1268-2 in Fig. 12B Phase modulators 1270-1 and 1270-2 or Fig.13 The phase modulators 1376-1 and 1376-2 in the alignment target 618 are modulated. In some embodiments, one pair of illumination spots may be s-polarized and the other pair may be p-polarized. By adjusting the incident angle and orientation of the diffraction grating of the alignment target 618, the grating along the x direction may diffract only one pair of illumination spots, and the grating along the y direction may diffract only the other pair of illumination spots. Thus, it may be feasible to overfill the alignment target 618 with larger illumination spots. By continuously modulating each pair of illumination spots, the alignment position in the x and y directions may be determined independently.

[0179] In some embodiments, Figure 7 and Figure 8The phase modulators 740-1, 740-2 or 740-3 in the embodiment can be used to select a specific diffraction order of the diffracted beam 719, the diffracted beamlet 746 or the optical branch 748. In this example, the diffraction order corresponding to the alignment position in the x or y direction of the alignment target 618 can be selected separately. The alignment position in the x and y directions can therefore be determined sequentially. In this example, a binary amplitude modulator can be used.

[0180] In some embodiments, Figure 7 and Figure 8 The phase modulator 740-4 in can be used to select a region of interest (ROI) from the image plane in front of the aperture 760 to detect a specific direction of the alignment target 618 (e.g., a grating extending along the x-axis). Both the x and y directions of the alignment position can therefore be measured after each other.

[0181] Fig.14A and Fig. 14B Corresponding top and cross-sectional views of beam splitter 1400 are illustrated in accordance with some embodiments of the present disclosure.Beam splitter 1400 may be implemented in place of phase modulator 740-4 that may select an ROI in the image plane in front of aperture 760 in alignment devices 700 and 800.

[0182] according to Fig.14A , the beam splitter 1400 includes two zones: a transmission zone 1488 and a deflection zone 1490. The transmission zone 1488 and the deflection zone 1490 each include two diametrically opposite quadrants. The cross sections along lines 1, 4 and 5 show uniform thickness, while the cross sections along lines 2 and 3 resemble a wedge, i.e., one end is much thicker than the other. The wedge structure of the beam splitter 1400 allows certain components of the diffracted beamlets 748 to be deflected to a field outside the aperture 760. The deflected components of the diffracted beamlets 748 can be x, y, or tilt-rotated diffraction orders when placed at 22.5 degrees to the optical axis. The exemplary design of uniform thickness in the transmission zone 1488 allows for easy adaptation to optical fibers. In some embodiments, the transmission zone 1488 can also be designed to resemble a wedge to reduce the effects of telecentricity.

[0183] In some embodiments, beam splitter 1400 may include two regions: an absorption region (opaque to light) and a transmission region (open), wherein the absorption region may be made of an absorbing material at the wavelength of the illumination beam in alignment devices 700 and 800. The absorption region and the transmission region may also include two diametrically opposed quadrants. This design may utilize a method based on light intensity measurement I sum Determine the alignment position in the x-direction and based on the light intensity measurement I diff Determine the alignment position in the y direction.

[0184] In some embodiments, the wedge angle of the deflection region 1490 of the beam splitter 1400 can be optimized to reduce the effects of telecentricity. For example, the wedge angle of the deflection region 1900 can be zero (uniform thickness) in one quadrant and any non-zero angle (wedge-shaped piece) in another quadrant.

[0185] High-speed alignment detection

[0186] In some embodiments, phase modulation by a spatial phase modulator (e.g., modulator 740-1 or 740-5) can be implemented in conjunction with phase modulation by scanning the alignment target 618. By using phase modulator 740-1 or 740-5, the light intensity measured at the detection system 728 can be lower than the speed V of the moving platform 722. stage The effective frequency of the induced modulation frequency Δν is modulated. In this example, the positive and negative diffraction orders of the diffracted beam 719 experience an increase or decrease in frequency Δν depending on the direction of movement of the platform 722. In some embodiments, the phase modulation from the phase modulator 740-1 or 740-5 can cancel the Δν from the moving platform 722. Thus, although the platform 722 can move at a high speed and can scan the alignment target 618 at a high speed, the position of the alignment target 618 can be determined at a relatively low frequency.

[0187] Align the device's modulation depth (DOM)

[0188] A spatially controllable reflector can deflect a portion of the light beam that is not of interest. Figure 7 and Figure 8 In the alignment devices 700 and 800 in FIG. 1 , phase modulators such as phase modulators 740-1, 740-2, and 740-3 can filter out portions of the diffracted beam 719, diffracted beamlets 746, and optical branches 748 that do not contribute to the alignment signal of the alignment target 618. Only diffracted orders expected from the alignment target 618 are selected and can be propagated through. This functionality will therefore increase the modulation depth (DOM) of the alignment devices 700 and 800, allowing detection of the alignment signal at a lower signal-to-noise ratio level.

[0189] An example of a phase modulator 740-3 that can increase the DOM includes spatial ferroelectric liquid crystal on silicon (FLCoS). After the beam analyzer 730, the sensitivity to temperature-induced changes from the FLCoS is less relevant to the determination of the alignment position of the alignment target 618. In some embodiments, a micromirror array (MMA) can be used for the phase modulator 740-1, 740-2, or 740-3 to increase the DOM.

[0190] Alignment device with frequency modulation

[0191] As described in equation (3), the time phase shift in equation (4) Can be scanned Figure 7 and Figure 8 Target 618 or move Figure 7 and Figure 8 The platform 722 is generated.

[0192] Fig.15A and Fig. 15B Illustrated are exemplary optical configurations 1500 and 1501 according to some embodiments of the present disclosure. In some embodiments, the temporal phase shift It is also possible to modulate the frequency (e.g. by Fig.15A In this example, off-axis illumination beams 806-1 and 806-2 are modulated by AOMs 1580-1 and 1580-2, respectively. This can be expressed as:

[0193]

[0194] where Δν AOM is the frequency difference between AOM 1580-1 and 1580-2.

[0195] In some embodiments, Fig. 15B As shown in the figure, the time phase shift It can be generated by scanning the internal grating 1582. In this example, the off-axis illumination beams 806-1 and 806-2 are diffracted light from the internal grating 1582. By scanning the internal grating 1582 along the scanning direction 1584, a frequency difference Δν can be introduced between the off-axis illumination beams 806-1 and 806-2 grating . Frequency Difference and Time Phase Shift This can be expressed as:

[0196]

[0197]

[0198] Where p and V grating Represents the period and scanning speed of the internal grating 1582.

[0199] Fig.16An exemplary optical configuration 1600 according to some embodiments of the present disclosure is illustrated. In some embodiments, frequency modulation can also be introduced by rotating polarizers 1686-1 and 1686-2. For example, a frequency difference between off-axis illumination beam 806-1 and off-axis illumination beam 806-2 can be generated by rotating polarizer 1686-1 clockwise and rotating polarizer 1686-2 counterclockwise. Assuming the rotation speed is Ω, the frequency difference Δν rot and time phase shift This can be expressed as:

[0200] Δν rot =2Ω 9(a)

[0201]

[0202] Further aspects of the invention are set out in the following numbered aspects.

[0203] 1. An alignment device, comprising:

[0204] an illumination system configured to generate one or more illumination beams, direct the one or more illumination beams toward an alignment target, and receive one or more diffracted beams reflected from the alignment target, wherein the one or more diffracted beams include at least one positive diffraction order or one negative diffraction order;

[0205] a self-referencing interferometer configured to receive one or more diffracted beams and produce diffracted beamlets, wherein the diffracted beamlets are orthogonally polarized, rotated 180 degrees relative to each other about an alignment axis, and spatially superimposed;

[0206] a beam analyzer configured to generate interference intensity between superimposed components of the diffracted sub-beams and to generate two orthogonally polarized optical branches; and

[0207] A detection system is configured to determine a position of an alignment target based on light intensity measurements of the optical branches, wherein the measured light intensity is modulated by a phase modulator.

[0208] 2. The alignment apparatus of clause 1, wherein the illumination system is further configured to generate an off-axis illumination beam at first and second positions diametrically opposite to each other in a pupil plane.

[0209] 3. The alignment apparatus of clause 2, wherein the off-axis illumination beam is modulated by the phase modulator.

[0210] 4. The alignment apparatus according to aspect 1, wherein the phase modulator modulates a positive diffraction order or a negative diffraction order of a diffracted beam reflected from an alignment target.

[0211] 5. The alignment apparatus of clause 1, wherein the phase modulator modulates the optical branches generated by the beam analyzer.

[0212] 6. The alignment apparatus of clause 1, wherein the phase modulator modulates an image of the alignment target at an input of the detection system.

[0213] 7. The alignment apparatus according to aspect 1, wherein the phase modulator is a piezoelectric mirror assembly that generates phase modulation by applying an electric signal to a piezoelectric material.

[0214] 8. The alignment apparatus of clause 1, wherein the phase modulator comprises a liquid crystal modulator, a photoelastic modulator, an electro-optic modulator, a ferroelectric liquid crystal, a micro-mirror array, or a digital spatial light modulator.

[0215] 9. The alignment apparatus of clause 8, wherein the liquid crystal modulator is a potassium dideuterium phosphate (KD*P) crystal.

[0216] 10. The alignment apparatus of clause 1, wherein the phase modulator comprises a wave plate covering half of the pupil plane to change the polarization state of the positive and negative diffraction orders.

[0217] 11. An alignment apparatus according to aspect 1, wherein the phase modulator comprises a transmissive region and a deflecting region, wherein each region comprises two diametrically opposed quadrants, and the deflecting region is wedge-shaped with varying thickness.

[0218] 12. The alignment apparatus of clause 1, wherein the phase modulator produces a frequency modulation of the off-axis illumination beam.

[0219] 13. An alignment apparatus according to aspect 12, wherein the phase modulator is an acousto-optic modulator.

[0220] 14. The alignment apparatus of clause 12, wherein the phase modulator is an internal grating and the frequency modulation of the off-axis illumination beam is produced by scanning the internal grating.

[0221] 15. The alignment apparatus of clause 12, wherein the phase modulator is a half-wave plate and the frequency modulation of the off-axis illumination beam is produced by rotating the half-wave plate.

[0222] 16. A lithographic apparatus, comprising:

[0223] an illumination system configured to illuminate the patterning device;

[0224] a projection system configured to project an image of the patterning device onto the wafer; and

[0225] An alignment system, the alignment system comprising:

[0226] an illumination system configured to generate one or more illumination beams, direct the one or more illumination beams toward an alignment target, and receive one or more diffracted beams reflected from the alignment target, wherein the one or more diffracted beams include at least one positive diffraction order or one negative diffraction order;

[0227] a self-referencing interferometer configured to receive one or more diffracted beams and produce diffracted beamlets, wherein the diffracted beamlets are orthogonally polarized, rotated 180 degrees relative to each other about an alignment axis, and spatially superimposed;

[0228] a beam analyzer configured to generate interference intensity between superimposed components of the diffracted sub-beams and to generate two orthogonally polarized optical branches; and

[0229] A detection system is configured to determine a position of an alignment target based on light intensity measurements of the optical branches, wherein the measured light intensity is modulated by a phase modulator.

[0230] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications, such as manufacturing integrated optical systems, guidance and detection patterns for magnetic domain memory, flat panel displays, LCDs, thin film heads, etc. Those skilled in the art will appreciate that in the context of such alternative applications, any use of the term "wafer" or "die" herein may be considered synonymous with the more general term "substrate" or "target portion", respectively. The substrates referred to herein may be processed in, for example, a track or coating and developing system unit (a tool that typically applies a resist layer to a substrate and develops the exposed resist), a metrology unit, and / or an inspection unit before or after exposure. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. In addition, the substrate may be processed more than once, for example to produce a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0231] Although the above may specifically refer to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications such as imprint lithography and is not limited to optical lithography where the context permits. In imprint lithography, the topography in a patterning device defines the pattern produced on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to a substrate, where the resist is cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. After the resist is cured, the patterning device is removed from the resist, leaving a pattern therein.

[0232] It should be understood that the phraseology or terminology herein is for the purpose of description rather than limitation, so that the phraseology or terminology of this specification should be interpreted by those skilled in the relevant art in view of the teachings herein.

[0233] The term "substrate" as used herein describes a material to which a layer of material is added. In some embodiments, the substrate itself can be patterned and the material added on top of the substrate can also be patterned, or the material added on top of the substrate can remain unpatterned.

[0234] Embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form that can be read by a machine (e.g., a computing device). For example, a machine-readable medium may include a read-only memory (ROM); a random access memory (RAM); a disk storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal, and the like. In addition, firmware, software, routines, and / or instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only, and such actions are actually caused by a computing device, a processor, a controller, or other devices that execute firmware, software, routines, and / or instructions.

[0235] The following examples illustrate rather than limit embodiments of the present disclosure. Those skilled in the relevant art will appreciate that other suitable modifications and adaptations of the various conditions and parameters normally encountered in the art are within the spirit and scope of the present invention.

[0236] Although specific reference may be made herein to the use of the apparatus and / or system according to the present invention in IC manufacturing, it should be clearly understood that such apparatus and / or system has a variety of other possible applications. For example, it may be used in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, LCD panels, thin film magnetic heads, etc. Those skilled in the art will appreciate that in the context of such alternative applications, any use of the terms "reticle", "wafer" or "die" herein should be considered to be replaced by the more general terms "mask", "substrate" and "target portion", respectively.

[0237] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced in other ways than as described. The description is not intended to limit the invention.

[0238] It should be understood that the detailed description section, rather than the summary section and the abstract section, is intended to be used to interpret the claims. The summary section and the abstract section may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventors, and therefore, are not intended to limit the present invention and the appended claims in any way.

[0239] The present invention has been described above with the aid of functional blocks that illustrate the implementation of specified functions and their relationships. For ease of description, the boundaries of these functional blocks have been arbitrarily defined herein. Alternative boundaries may be defined as long as the specified functions and their relationships are properly performed.

[0240] The foregoing description of specific embodiments will thus fully reveal the general nature of the invention: others may readily modify and / or adapt such specific embodiments for various applications by applying knowledge understood by those skilled in the art without departing from the general concept of the invention, without undue experimentation. Therefore, based on the teaching and guidance presented herein, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments.

[0241] The breadth and scope of the present invention 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.

Claims

1. An alignment device, comprising: an illumination system configured to generate one or more illumination beams, direct the one or more illumination beams toward an alignment target, and receive one or more diffracted beams reflected from the alignment target, wherein the one or more diffracted beams include at least one positive diffraction order or one negative diffraction order; a self-referencing interferometer configured to receive the one or more diffracted beams and produce diffracted beamlets, wherein the diffracted beamlets are orthogonally polarized, rotated 180 degrees relative to each other about an alignment axis, and spatially superimposed; a beam analyzer configured to generate interference intensity between superimposed components of the diffracted sub-beams and to generate two orthogonally polarized optical branches; as well as a detection system configured to determine the position of the alignment target based on light intensity measurements of the optical branch, wherein the measured light intensity is modulated by a phase modulator, Therein, the phase modulator comprises two components applying different phase delays that vary with time.

2. The alignment apparatus of claim 1, wherein the illumination system is further configured to generate an off-axis illumination beam at first and second locations diametrically opposite to each other in a pupil plane.

3. The alignment apparatus of claim 2, wherein the off-axis illumination beam is modulated by the phase modulator.

4. The alignment device of claim 1, wherein the phase modulator modulates a positive diffraction order or a negative diffraction order of a diffraction beam reflected from the alignment target.

5. The alignment apparatus of claim 1, wherein the phase modulator modulates the optical branches generated by the beam analyzer.

6. The alignment apparatus of claim 1, wherein the phase modulator modulates an image of the alignment target at an input of the detection system.

7. The alignment apparatus according to claim 1, wherein the phase modulator is a piezoelectric mirror assembly that generates phase modulation by applying an electric signal to a piezoelectric material.

8. The alignment device according to claim 1, wherein the phase modulator comprises a liquid crystal modulator, a photoelastic modulator, an electro-optic modulator, a ferroelectric liquid crystal, a micro-mirror array, or a digital spatial light modulator. 9 . The alignment apparatus of claim 8 , wherein the liquid crystal modulator is a potassium dideuterium phosphate (KD*P) crystal.

10. The alignment device of claim 1, wherein the phase modulator comprises a wave plate covering half of a pupil plane to change the polarization states of the positive diffraction orders and the negative diffraction orders.

11. An alignment apparatus according to claim 1, wherein the phase modulator comprises a transmission region and a deflection region, wherein each region comprises two diametrically opposite quadrants, and the deflection region is wedge-shaped with a varying thickness.

12. The alignment apparatus of claim 1, wherein the phase modulator produces a frequency modulation of the off-axis illumination beam.

13. The alignment apparatus of claim 12, wherein the phase modulator is an acousto-optic modulator.

14. The alignment apparatus of claim 12, wherein the phase modulator is an internal grating and the frequency modulation of the off-axis illumination beam is produced by scanning the internal grating.

15. The alignment apparatus of claim 12, wherein the phase modulator is a half-wave plate and the frequency modulation of the off-axis illumination beam is produced by rotating the half-wave plate.

16. A lithographic apparatus comprising: an illumination system configured to illuminate the patterning device; a projection system configured to project an image of the patterning device onto a wafer; as well as An alignment device as claimed in any one of claims 1 to 15.

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