Image forming equipment

Abstract

An image forming device includes: an illumination system operable to direct a radiation beam onto a first plane; a first support structure operable to support a first marker in the first plane, and a second support structure operable to support a second marker in a second plane; a projection system arranged between the first plane and the second plane to form a combined image of the first marker and the second marker in a third plane; and an alignment system operable to extract data from the combined image to determine an alignment distance between the first marker and the second marker.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 19177944.6 filed on June 3, 2019, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to an image forming device, and more particularly to an image forming device with an alignment system and a related method. Background Art

[0004] Image forming devices are used in imaging systems, such as lithographic equipment. A lithographic equipment is a machine configured to apply a desired pattern to a substrate. A lithographic equipment can be used, for example, in the manufacture of integrated circuits (ICs). For example, a lithographic equipment can project a pattern (also commonly referred to as a "design layout" or "design") of a pattern forming device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

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

[0006] In the manufacture of complex devices, typically many lithographic patterning steps are performed to form functional features in successive layers on a substrate. A key aspect of the performance of a lithographic apparatus is the ability to correctly and accurately place an applied pattern with respect to features formed in a previous layer (either by the same apparatus or a different lithographic apparatus). For this purpose, an image forming apparatus of a lithographic apparatus may use one or more sets of identifiers (also interchangeably referred to as marks). Each identifier is a structure whose position relative to a reference position or another identifier can be measured using a position sensor, typically an optical position sensor. The position sensor may be referred to as an "alignment sensor" and the identifier may be referred to as an "alignment identifier."

[0007] In addition to lithographic equipment, there are many other equipment with which the present invention may be relevant, such as, for example, mask inspection equipment, metrology equipment, or any equipment that measures or processes objects. In these and other equipment, alignment may be a key aspect of equipment performance.

[0008] Measuring alignment can be a time-consuming process. For example, for accurate measurement, multiple alignment marks may be used, but some alignment marks may be aligned in sequence, which can make the alignment last longer.

[0009] It may be desirable to provide an image forming apparatus that overcomes or mitigates one or more problems associated with the prior art.It may also be desirable to provide an alternative image forming apparatus. Summary of the invention

[0010] According to a first aspect of the present invention, there is provided an image forming device, comprising: an illumination system operable to direct a radiation beam onto a first plane; a first support structure operable to support a first marker in the first plane, and a second support structure operable to support a second marker in the second plane; a projection system arranged between the first plane and the second plane to form a combined image of the first marker and the second marker in a third plane; and an alignment system operable to extract data from the combined image to determine an alignment distance between the first marker and the second marker.

[0011] The formation of a combined image can advantageously improve the accuracy of the alignment. The combined image can contain information about the relative position of the first identifier and the second identifier. The formation of the combined image can enable the position of the second identifier to be determined relative to the first identifier, rather than relative to the position of another part of the image forming device. The position of the second identifier relative to the first identifier achieved by embodiments of the present invention represents the direct position accuracy of the image forming system.

[0012] The formation of the combined image can advantageously reduce the time required for alignment. Determining the alignment distance can be performed in real time by extracting data from the combined image. The image forming apparatus according to the present invention can determine the alignment distance in a relatively short time, for example between 10 and 20 seconds. The reduced alignment measurement duration can advantageously increase productivity.

[0013] The first mark and the second mark can be alignment marks. There can be one first mark in the first plane and one second mark in the second plane. Alternatively, there can be multiple first marks in the first plane and multiple second marks in the second plane. The mark can be configured to modify the radiation beam to form a patterned radiation beam. The mark can be in the form of a diffraction grating. Specifically, the first mark in the first plane can modify the radiation beam to form a first patterned radiation beam.

[0014] The markers may be arranged so that there is no relative movement between the imaging of the first marker and the second marker in the third plane. Thus, the combined image may be considered to be static in size and shape, for example due to the lack of relative movement between the images of the first marker and the second marker. The combined image may be movable within the plane, for example it may be scanned in the plane of the device. The device may also include an image scanning mechanism configured to scan the combined image in the third plane.

[0015] It will be appreciated that in order to form a combined image of the first identifier and the second identifier, the first plane and the second plane may be conjugate to each other. That is, each point in the first plane may be imaged to a point in the second plane (e.g., by a projection system). The projection system may be configured to direct radiation between the first plane and the second plane. The projection system may be configured to receive a first patterned radiation beam and form an image of one of the first identifiers on one of the second identifiers in the second plane. Additionally, the projection system may be configured to receive a portion of the patterned radiation beam reflected by the second identifier and form a combined image of the first identifier and the second identifier in a third plane.

[0016] The illumination system may include an illumination source and an illumination optical device. The illumination system and the projection system may cooperate to direct the radiation beam to the first identification and the second identification to form a combined image of the first identification and the second identification. The combined image may be understood as an image containing information about both the first identification and the second identification.

[0017] The first plane and / or the second plane may be an object and / or image plane. The first plane and / or the second plane may be an input and / or output plane. The projection system may include a projection lens. The projection system may project the combined image onto a third plane. The third plane may be, for example, an input plane associated with an alignment system.

[0018] The alignment system may be of a through-the-lens type. The alignment system may be operable to measure the relative position of a first identifier in a first plane and a second identifier in a second plane. The alignment distance may be the relative position of an image of a first identifier formed in the second plane (via projection optics) and a second identifier disposed in the second plane. The alignment distance may be a measure of the alignment and / or overlay accuracy of an imaging device, i.e., the accuracy with which a first pattern can be projected onto a second pattern. The alignment distance may be a measure of the alignment between a first identifier in a first plane and a second identifier in a second plane.

[0019] The extracted data may depend on an asymmetry of the combined image. The asymmetry may depend on a deviation from a centroid of one or more features within the combined image. An asymmetric combined image may represent an alignment error. The alignment error may be a non-zero alignment distance between a first identifier and a second identifier. The alignment error may correspond to an amount of asymmetry of the combined image. That is, a more asymmetric combined image may represent a greater alignment error than a more symmetric combined image. Using asymmetry may be advantageous because it represents a quick and relatively easy method for determining an alignment error.

[0020] The alignment system may include: a modulation system operable to form a modulated combined image from the combined image; and a detector operable to extract data from the modulated combined image to determine the alignment distance between the first mark and the second mark. The modulated combined image may be a modulated version of the combined image of the first mark and the second mark. The modulated combined image may increase the speed and / or ease with which data can be extracted from the combined image. The modulation system may include an optical block. The third plane may be an input plane of the alignment system. The third plane may be an input plane of the modulation system. The radiation beam containing the modulated combined image may be referred to as a modulated radiation beam. The detector may include a sensor. The detector may include a multi-pixel detector, such as a CCD or CMOS, or may include a single detector, such as a photodiode.

[0021] The modulation system may include a self-referencing interferometer and an image scanning mechanism configured to scan a combined image on a third plane. The self-referencing interferometer may be operable to: receive a combined image of a first identifier and a second identifier in a third plane; Fourier decompose the combined image of the first identifier and the second identifier into positive and negative diffraction orders in a pupil plane; introduce a 180-degree rotation between the positive and negative diffraction orders; and reorganize the positive and negative orders in a fourth plane (conjugate to the third plane). The combined image of the first identifier and the second identifier has contributions from positive and negative diffraction orders. When the image scanning mechanism scans the combined image on the third plane, the contributions from the positive and negative diffraction orders are scanned on the fourth plane in opposite directions (due to the 180-degree rotation between the positive and negative diffraction orders). The scanning causes the contributions from the positive and negative diffraction orders to move in phase and out of phase, thereby producing a modulated combined image. Modulation is achieved by forming a coherent addition of the positive and negative diffraction orders.

[0022] The use of a self-referencing interferometer may increase the speed of alignment. A self-referencing interferometer may extract alignment data spatially rather than, for example, computationally. The use of a self-referencing interferometer may reduce the processing power, such as computer processing power, required to extract the alignment data.

[0023] The detector may be operable to extract phase shift data from the modulated combined image. The phase shift data may be related to the phase shift between the positive and negative diffraction orders of the combined image of the first identifier and the second identifier. Phase shift data may be extracted from the modulated combined image by determining the intensity of the (oscillating) combined modulated image. The phase shift data may beneficially provide alignment information related to the alignment distance between the first identifier and the second identifier. The detector may integrate the modulated combined image into an electrical signal. The electrical signal may have a maximum intensity. The electrical signal may have a phase shift. The maximum intensity at zero phase shift may indicate perfectly aligned identifiers. A phase shift that is not equal to zero may indicate a poor alignment. A lower intensity at zero phase shift may indicate a poor alignment.

[0024] The modulation system may include: one or more detection gratings in a third plane and an image scanning mechanism configured to scan the combined image on the third plane such that the combined image is scanned relative to the one or more detection gratings.

[0025] The use of detection gratings can advantageously allow alignment data to be collected with a relatively easy implementation. By scanning the combined image on the detection gratings in the third plane, a modulated combined image can be produced, for example, on a detector arranged on the opposite side of one or more detection gratings. The input plane of the detector can be referred to as the detector plane. The modulated combined image can be an intensity oscillation at the detector plane. The oscillation of intensity can be due to the scanning of the combined image relative to the one or more detection gratings. The image can be scanned relative to the detection gratings. That is, the image can be scanned on the detection gratings. Alternatively or additionally, the detection gratings can be moved in the third plane so that they are scanned relative to the combined image.

[0026] There may be a single detection grating, or there may be multiple detection gratings. The modulation system may separate the radiation beam into multiple parts. The modulation system may separate the radiation beam into a first part and a second part. The first part may be directed to a first detection grating. The second part may be directed to a second detection grating. The first detection grating and the second detection grating may be arranged adjacent to the first detector and the second detector, respectively. The first part of the radiation beam may be modulated by the first detection grating before entering the first detector. The second part of the radiation beam may be modulated by the second detection grating before entering the second detector.

[0027] The first detection grating and the second detection grating may be oriented perpendicular to each other in the same plane. The first detection grating and the second detection grating may have equal detection grating periods (or pitches). The detection grating periods of the first detection grating and the second detection grating may match the diffraction grating periods of the first and second identifications. At least one detection grating may be parallel to a portion of the diffraction grating of one of the identifications. At least one detection grating may be perpendicular to a portion of the diffraction grating of one of the identifications.

[0028] The modulation system may also be operable to filter out one or more Fourier components from the combined image to form a filtered combined image, and to form a modulated combined image from the filtered combined image. The alignment distance may be determined separately for each Fourier component. Removing one or more Fourier components from the combined image may improve the signal-to-noise ratio. Removing one or more Fourier components may improve the accuracy of the alignment system. Removing one or more Fourier components may be performed by spatial filtering. That is, the Fourier decomposition of the combined image may be formed in a Fourier plane of the modulation system. The Fourier plane may be referred to as a pupil plane.

[0029] The Fourier decomposition may be performed, for example, using a diffraction grating. The Fourier decomposition of the combined image at the pupil plane may be a diffraction pattern. One or more diffraction orders may be removed by reducing or suppressing their transmission through the pupil plane. This may be referred to as spatial filtering.

[0030] The zeroth order may be removed. The zeroth order corresponds to no diffraction, i.e., reflection or transmission. The second order may be removed. Higher orders may be removed. Any combination of diffraction orders may be removed, for example, removing all orders except the first order may be beneficial.

[0031] Alternatively or additionally, the removal of one or more Fourier components may be performed electronically after the modulated combined image collected by the detector has been converted into an electrical signal.

[0032] The first mark and the second mark may include diffraction gratings. The diffraction gratings may be reflective or transmissive. The diffraction gratings may be of various types, such as holographic, ruled or stepped. The mark may include two diffraction gratings in the same plane. The mark may include two different diffraction gratings oriented at a non-zero angle to each other in the same plane, for example, the two diffraction gratings may be perpendicular to each other in the same plane.

[0033] The first and second identifiers may have matching diffraction grating periods. The grating period or diffraction grating may alternatively be referred to as the pitch of the diffraction grating. The first and second identifiers having matching diffraction grating periods are intended to represent that the grating period of the image of the first identifier in the second plane is equal to the grating period of the second identifier.

[0034] The first identification may have a first period. The second identification may have a second period. The matched first grating and second grating pair may be realized in the form of equal pitch, that is, the first period may be equal to the second period. Alternatively, the matched first grating and second grating pair may be realized by having different pitches, wherein the difference in pitch is related to a magnification or reduction factor between the first plane and the second plane, for example due to the projection system. The first period may be scaled by a magnification factor such that it is larger or smaller than the second period, wherein the magnification factor is equal to the magnification applied by the projection system.

[0035] The illumination system may be operable to simultaneously illuminate the first array of markers in the first plane, and the projection system is configured to form an image of the first array of markers onto the corresponding array of second markers and form multiple combined images of each corresponding pair of first and second markers in a third plane.

[0036] Simultaneously illuminating multiple marks may result in a reduction in the alignment time required for an alignment process. Simultaneously illuminating multiple marks may result in an increase in productivity. For example, in a lithography process, reduced alignment time may result in more lithography processes per hour. Simultaneously illuminating multiple marks may be referred to as parallel alignment. The array of marks may include, for example, 3 marks or 50 marks.

[0037] Illuminating multiple markers simultaneously may result in improved accuracy. For example, more markers can be used for alignment, which may improve accuracy. An array may include multiple markers. Ideally, this is a perfect alignment, and the image of the array of first markers will perfectly overlap with the corresponding array of the second marker.

[0038] The image forming device may also include an optical element configured to allow the radiation beam to travel from the illumination optical device to the first plane and enter the projection system; and direct the combined image to the alignment system. The optical element may be a one-way mirror. The one-way mirror may also be referred to as a two-way mirror or a partially reflective / transmissive mirror. The optical element may be a dichroic mirror. The optical element and the projection system may cooperate to direct the combined image to a third plane, wherein the third plane may be conjugate to the first plane and the second plane, and may be an input plane associated with the alignment system.

[0039] The apparatus may be operable in an imaging mode and an alignment mode, and wherein the radiation beam may be directed through substantially the same portion of the projection system when operating in either the imaging mode or the alignment mode. The imaging mode and the alignment mode may be considered to be a first mode and a second mode. The first mode may include projecting an image of an object (e.g., a reticle) in a first plane onto a second plane (e.g., to form the image on a substrate). The alignment mode may include determining an alignment distance between a first identifier and a second identifier.

[0040] Operating the two modes so that the radiation beam is directed through substantially the same portion of the projection system allows alignment and imaging to be performed under similar conditions. For example, the radiation beam may travel through the same portion of the lens in both modes. This may demonstrate accuracy of alignment and / or imaging. This may improve imaging quality.

[0041] The radiation beam may have the same source in both modes, or may have a different source for each mode. The radiation beam may have a first wavelength in the imaging mode and a second wavelength in the alignment mode. The first wavelength and the second wavelength may be the same or may be different.

[0042] When in imaging mode, the image forming device may have a first optical axis and when in alignment mode, a second optical axis. The optical axis may be considered to be the axis along which the radiation beam travels when the image forming device is in use (e.g., the axis along which the principal ray of the radiation beam travels). The first optical axis and the second optical axis may be considered to be substantially the same. The logo may be located at or near the first optical axis and the second optical axis. The radiation beam may be guided along or near the optical axis. It should be understood that due to physical limitations, the radiation beam may not perfectly overlap with the optical axis, but may be considered to effectively travel along the same axis.

[0043] The image forming device may also include a support scanning mechanism configured to move the first support structure in a first direction within a first plane and to move the second support structure in a second plane along a second direction, wherein the second direction is parallel to but opposite to the first direction. The support scanning mechanism may advantageously allow the first indicia to be aligned with the second indicia rather than with a portion of the image forming device.

[0044] The support scanning mechanism may provide synchronized scanning of the first identifier and the second identifier. That is, the support scanning mechanism may scan the first identifier and the second identifier at a rate such that no relative movement occurs between the image of the first identifier and the image of the second identifier. That is, the images of the two identifiers are static relative to each other within the radiation beam. The synchronized scanning may produce a combined image that does not change shape. The synchronized scanning may produce a combined image that is scanned on a third plane.

[0045] The support scanning mechanism may form part of the image scanning mechanism. The support scanning mechanism may form part of the modulation system. When used with an apparatus such as a lithographic apparatus, the support scanning mechanism may include scanning movements associated with normal lithographic exposure.

[0046] According to a second aspect of the invention there is provided a metrology apparatus comprising the apparatus of the first aspect of the invention.The image forming apparatus may also be used for other imaging where providing alignment between the first plane and the second plane may be beneficial.

[0047] According to a third aspect of the invention, there is provided a lithographic apparatus arranged to project a pattern from a pattern forming device onto a substrate, the lithographic apparatus comprising the apparatus of the first aspect of the invention, wherein a first support structure is operable to support the pattern forming device in a first plane, and a second support structure is operable to support the substrate in a second plane.

[0048] The illumination source may be a lithographic light source, such as an illumination source used for normal lithographic exposure. For example, the illumination source may include an EUV or DUV radiation source. Alternatively, the illumination source may be different from the illumination source used for normal lithographic exposure. For example, the illumination source may be of a different wavelength. An illumination source of a different wavelength may be beneficial because it may have a higher transmission and / or reflection through / from the marker. Different wavelengths may be beneficially used to align the illumination source and the lithographic illumination source to allow monitoring of overlap and drift during the lithographic process. That is, overlap and drift can be monitored during and / or between exposure sequences without time-consuming changes to the lithographic settings.

[0049] The illumination optics may be lithographic illumination optics, ie illumination optics used in normal lithographic exposure. Alternatively, they may be separate, for example they may be optimized for different wavelengths.

[0050] When used with a lithographic apparatus, the image forming apparatus can improve overlay accuracy, and / or can allow for the formation of smaller features. The pattern forming device can be a mask having a pattern. The substrate can be a wafer having a resist. The first identifier can be supported in a first plane in a position comparable to the pattern position during a normal lithographic exposure process. During the normal lithographic exposure process, the second identifier can be supported in a second plane in a position comparable to a portion of the wafer. That is, the first identifier and the second identifier can be aligned in a position that is beneficial to ensuring accurate alignment of the lithographic process.

[0051] According to a fourth aspect of the present invention, there is provided a method for measuring an alignment distance between a first marker in a first plane of an imaging device and a second marker in a second plane, the method comprising: directing a radiation beam onto the first plane of the imaging device to form a combined image of the first marker and the second marker, directing the combined image to an alignment system, and extracting data from the combined image to determine the alignment distance.

[0052] The formation of a combined image can advantageously improve the accuracy of the alignment. The combined image can contain information about the relative position of the first identifier and the second identifier. The formation of the combined image can enable the position of the second identifier to be determined relative to the first identifier, rather than relative to another part of the image forming device. The position of the second identifier relative to the first identifier in the combined image achieved by embodiments of the present invention represents the direct position accuracy of the image forming system.

[0053] The formation of the combined image can advantageously reduce the time required for alignment. Determining the alignment distance can be performed in real time by extracting data from the combined image. The method according to the fourth aspect of the invention can determine the alignment distance in a relatively short time, for example between 10 and 20 seconds. The reduced alignment measurement duration can advantageously increase productivity.

[0054] The first mark and the second mark can be alignment marks. There can be one first mark in the first plane and one second mark in the second plane. Alternatively, there can be multiple first marks in the first plane and multiple second marks in the second plane. The mark can be configured to modify the radiation beam to form a patterned radiation beam. The mark can be in the form of a diffraction grating. Specifically, the first mark in the first plane can modify the radiation beam to form a first patterned radiation beam.

[0055] The combined image may pass the first mark and / or the second mark a second time before passing to the alignment system. The second pass through the first mark and / or the second mark has a negligible effect on the combined image.

[0056] It will be appreciated that in order to form a combined image of the first identifier and the second identifier, the first plane and the second plane may be conjugate to each other. That is, each point in the first plane may be imaged to a point in the second plane (e.g., by a projection system). The projection system may be configured to direct radiation between the first plane and the second plane. The projection system may be configured to receive a first patterned radiation beam and form an image of one of the first identifiers on one of the second identifiers in the second plane. Additionally, the projection system may be configured to receive a portion of the patterned radiation beam reflected by the second identifier and form a combined image of the first identifier and the second identifier in a third plane.

[0057] The illumination system may include an illumination source and an illumination optical device. The illumination system and the projection system may cooperate to direct the radiation beam to the first identification and the second identification to form a combined image of the first identification and the second identification. The combined image may be understood as an image containing information about both the first identification and the second identification.

[0058] The first plane and / or the second plane may be an object and / or image plane. The first plane and / or the second plane may be an input and / or output plane. The projection system may include a projection lens. The projection system may project the combined image onto a third plane. The third plane may be, for example, an input plane associated with an alignment system.

[0059] The method according to the fourth aspect of the present invention may use the image forming apparatus according to the first aspect of the present invention.

[0060] The alignment system may be of a through-the-lens type. The alignment system may be operable to measure the relative position of a first identifier in a first plane and a second identifier in a second plane. The alignment distance may be the relative position of an image of a first identifier formed in a second plane (via projection optics) and a second identifier disposed in the second plane. The alignment distance may be a measure of the alignment and / or overlay accuracy of an imaging device. The alignment distance may be a measure of the alignment between a first identifier in a first plane and a second identifier in a second plane.

[0061] The extracted data may depend on an asymmetry of the combined image. The asymmetry may depend on a deviation from a centroid of one or more features of the combined image. An asymmetric combined image may represent an alignment error. The alignment error may be a non-zero alignment distance between the first identifier and the second identifier. The alignment error may correspond to an amount of asymmetry of the combined image. That is, a more asymmetric combined image may represent a larger alignment error than a more symmetric combined image. Using asymmetry may be advantageous because it represents a quick and relatively easy method for determining an alignment error.

[0062] The method may also include transforming the combined image into an electrical signal. The electrical signal may have a maximum intensity. The electrical signal may have a phase shift. The maximum intensity at zero phase shift may indicate a perfectly aligned identity. A phase shift that is not equal to zero may indicate a poor alignment. A lower intensity at zero phase shift may indicate a poor alignment.

[0063] The method may further comprise scanning the first and second markings relative to each other such that the combined image is scanned relative to a third plane.The scanning may advantageously allow the first marking to be aligned with the second marking rather than with a portion of the image forming device.

[0064] The scanning may include synchronous scanning of the first identifier and the second identifier. That is, the first identifier and the second identifier may be scanned at a rate such that no relative movement occurs between the image of the first identifier and the image of the second identifier. That is, the images of the two identifiers are static relative to each other within the radiation beam. This may be referred to as synchronous scanning. Synchronous scanning may produce a combined image that generally does not change shape. Synchronous scanning may produce a combined image that is scanned on a third plane.

[0065] When used with an apparatus such as a lithographic apparatus, scanning may include scanning movements associated with a normal lithographic exposure.

[0066] The method may also include modulating the combined image using a self-referencing interferometer or one or more detection gratings.

[0067] The use of a self-referencing interferometer may increase the speed of alignment. A self-referencing interferometer may extract alignment data spatially rather than, for example, computationally. The use of a self-referencing interferometer may reduce the processing power, such as computer processing power, required to extract the alignment data.

[0068] The use of detection gratings can advantageously allow alignment data to be collected with a relatively easy implementation. By scanning the combined image on the detection gratings in the third plane, a modulated combined image can be produced, for example, on a detector arranged on the opposite side of the detection gratings. The input plane of the detector can be referred to as the detector plane. The modulated combined image can be an intensity oscillation at the detector plane. The oscillation of intensity can be due to the scanning of the combined image relative to one or more detection gratings. The image can be scanned relative to the detection gratings. That is, the image can be scanned on the detection gratings. Alternatively or additionally, the detection gratings can be moved in the third plane so that they are scanned relative to the combined image.

[0069] There may be a single detection grating, or there may be multiple detection gratings. The modulation system may separate the radiation beam into multiple parts. The modulation system may separate the radiation beam into a first part and a second part. The first part may be directed to a first detection grating. The second part may be directed to a second detection grating. The first detection grating and the second detection grating may be arranged adjacent to the first detector and the second detector, respectively. The first part of the radiation beam may be modulated by the first detection grating before entering the first detector. The second part of the radiation beam may be modulated by the second detection grating before entering the second detector.

[0070] According to a fifth aspect of the present invention, there is provided a device manufactured using the apparatus according to any one of the first, second or third aspects of the present invention or using the method according to the fourth aspect of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0072] - Figure 1 depicts a schematic overview of a lithographic apparatus according to an embodiment of the present invention;

[0073] - Figure 2 depicts a schematic overview of an image forming apparatus portion of a lithographic apparatus according to an embodiment of the present invention;

[0074] - Figure 3 and 4 depicts an example image formed by a marker in an image forming device;

[0075] - Figure 5 depicts a schematic overview of an alignment system according to an embodiment of the present invention;

[0076] - Figure 6 illustrates an example modulation signal for extracting information from a combined image;

[0077] - Figure 7 depicts a schematic overview of another alignment system according to an embodiment of the present invention; and

[0078] - Fig. 8A and 8B Example alignment marks and detection gratings according to embodiments of the present invention are depicted. DETAILED DESCRIPTION

[0079] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., wavelengths in the range of about 5 to 100 nm).

[0080] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, corresponding to the pattern to be created in a target portion of the substrate. In this context, the term "light valve" can also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0081] The term "matched" may be used with respect to the diffraction gratings. Matched may be interpreted as: the two may be equal; or may be scaled so that they are equal at the relevant parts of the device. For example, a first grating and a second grating in a first plane and a second plane, respectively, may be matched, wherein the second grating is scaled relative to the first grating, and the scaling is related to an enlargement or reduction factor between the first plane and the second plane, for example due to a magnification applied by optical elements in the device.

[0082] The term "conjugate" may be used herein with respect to optical planes. It is to be understood that within an optical system (e.g., a lithographic apparatus), two planes are conjugate if every point in a first plane P is imaged onto a point in a second plane P'. Two or more planes may be conjugate. That is, if a first plane is conjugate to a second plane, and a third plane is conjugate to the second plane, then the third plane may be conjugate to the first plane. Conjugate planes may be referred to as conjugate optical planes or mutually conjugate planes.

[0083] In order to illustrate the present invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., an x-axis, a y-axis, and a z-axis. Each of the three axes is orthogonal to the other two axes. The rotation around the x-axis is called an Rx rotation. The rotation around the y-axis is called an Ry rotation. The rotation around the z-axis is called an Rz rotation. The x-axis and the y-axis define a horizontal plane (called an xy plane), and the z-axis is in the vertical direction. The Cartesian coordinate system does not limit the present invention and is only used to illustrate. On the contrary, another coordinate system such as a cylindrical coordinate system can be used to illustrate the present invention. The orientation of the Cartesian coordinate system can be different, for example, so that the z-axis has a component along the horizontal plane.

[0084] Figure 1 A lithographic apparatus LA is schematically depicted. A lithographic apparatus may be considered to include an image forming apparatus. That is, it uses radiation to form an image of an object on a surface. The image forming apparatus may also be referred to as an imaging system or imaging apparatus.

[0085] The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask stage) MT configured to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters; a substrate support (e.g., a wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W. The patterning device MA may comprise a pattern, e.g., a pattern to be printed in a lithographic exposure and / or alignment mark.

[0086] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g., via a beam delivery system BD). The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0087] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof, as appropriate for the exposure radiation used and / or other factors (such as the use of immersion liquid or the use of a vacuum). Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0088] Although specific reference may be made herein to reflectors and / or lenses, any suitable optical element may be used. For example, the element may be refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic, or any combination thereof. The element may be a grating, a beam cube, or any other element as desired. In some cases, the reflector may be a beam splitter, such as a dichroic mirror, a semi-silvered mirror, or any other beam splitting element known in the art.

[0089] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US6952253, which is incorporated herein by reference.

[0090] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps for preparing a substrate W for subsequent exposure may be performed on a substrate W located on one of the substrate supports WT, while another substrate W on another substrate support WT is used to expose a pattern on another substrate W.

[0091] In a lithographic operation, a radiation beam B is incident on a patterning device (e.g., mask MA) held on a mask support MT and is patterned by a pattern present on the patterning device MA. After passing through the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of a substrate W. This process may also be referred to as a lithographic exposure or imaging operation.

[0092] By means of the second positioner PW and the position measurement system PMS, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B at focus and alignment positions. Similarly, the first positioner PM and possibly another position sensor (not shown) Figure 1 ) can be used to accurately position the pattern forming device MA relative to the path of the radiation beam B.

[0093] The radiation beam B is usually guided along the optical axis 10 of the lithographic apparatus LA. In normal lithographic operation, the mask support MT and the substrate support WT can be moved so that the mask MA and the substrate W pass through the optical axis 10 and thus pass through the radiation beam B. This movement can be called scanning. The mask support and the substrate support can be called support members. Scanning can enable different parts of the mask MA and the substrate W to scan through the radiation beam B without moving the radiation beam B itself. The mask support MT and the mask MA can be scanned in a first direction (for example, along the y-axis). The substrate table WT and the substrate W can then be scanned in opposite directions along the y-axis. The movement speeds of the substrate support WT and the mask support MT can be matched. For example, when the magnification factor is not applied by the projection system PS, the speeds of the mask support MT and the substrate support WT can be equal. Alternatively, due to any magnification applied by the projection system PS, the speed of the supports can be scaled so that they move in the entire radiation beam B at a comparable speed given a magnification change. Typically, the speeds of the substrate support WT and the mask support MT are matched so that the aerial image of the mask MA formed in the plane of the substrate W moves at the same speed and direction as the substrate W (so there is substantially no relative movement between the (aerial) image and the substrate W). This movement ensures that, with correct alignment, the mask MA and corresponding parts of the substrate W pass through the optical axis 10 at the same time. This type of scanning may be referred to as synchronous scanning.

[0094] Synchronous scanning ensures that each portion of the pattern from the mask MA will be imaged onto the same portion of the substrate W, even if the mask MA moves through the radiation beam. In other words, synchronous scanning enables moving objects in the plane of the mask MA to be aligned with moving objects in the plane of the substrate W. For the imaging operation, it may be beneficial to ensure that objects in the plane of the mask MA are well aligned with objects in the plane of the substrate W.

[0095] Perfect alignment can be represented by two objects (i.e., an object in the plane of the mask MA and an object in the plane of the substrate W) passing through the optical axis 10 (or some other reference position or axis) at the same time. Imperfect alignment can be represented by the deviation between the objects. Imperfect alignment can be represented by two objects in two conjugate planes (specifically the planes of the mask MA and the substrate W), each object passing through the optical axis 10 (or some other reference position or axis) at different times. Imperfect alignment can be represented by an alignment distance. The alignment distance can be a measure of the deviation between two objects, which can be measured relative to a reference position. The alignment distance can be related to the time difference between a first object passing through the optical axis (or some other reference position or axis) and a second object passing through the optical axis (or some other reference position or axis). In an imaging operation, the alignment distance can represent a misalignment between multiple projected images.

[0096] Imperfect alignment can be solved or reduced by modifying elements of the imaging device. For example, one or both of the mask or substrate supports MT, WT can be moved. Alternatively or additionally, elements in the projection system can be moved so that the optical axis 10 is modified. It may be beneficial to monitor the alignment so that such modifications can be applied. Monitoring and correction of alignment may be referred to as alignment.

[0097] The pattern forming device MA and the substrate W can be aligned using the mask alignment marks M1, M2 and the substrate alignment marks W1, W2. The alignment marks may also be referred to as marks, marks or alignment marks. Although Figure 1 The substrate alignment marks W1, W2 schematically illustrated in FIG. 3 occupy dedicated target portions, but they may be located in the space between target portions. When the substrate alignment marks W1, W2 are located between target portions C, these are referred to as scribe-lane alignment marks.

[0098] The imaging apparatus of the depicted lithographic apparatus LA is provided with an alignment system AS. The alignment system AS is provided to improve the accuracy with which portions of the mask MA and portions of the substrate W can be aligned during a lithographic operation.

[0099] The lithographic apparatus or other similar imaging apparatus can be operated in an imaging mode, whereby an image is projected from a first plane (i.e., the plane of the mask MA) to a second plane (i.e., the plane of the substrate W). The lithographic apparatus or other similar imaging apparatus can also be operated in an alignment mode, whereby alignment marks on the first plane and the second plane can be used to ensure that an object on the first plane can be properly aligned with a desired location on the second plane. The imaging mode and the alignment mode can be considered to be a first mode and a second mode. Both the first mode and the second mode use a radiation beam. The radiation beam can have a first wavelength in the first mode and a second wavelength in the second mode, where the first wavelength and the second wavelength can be the same or can be different. In these first and second modes, the radiation beam B can be generated from the same source, or there can be different sources for each mode.

[0100] exist Figure 1 In the depicted embodiment, an optical element TM is provided. The optical element TM is arranged to allow a portion of the radiation beam B to pass from the illumination system IL to the mask MA, for example during alignment measurements, and to reflect at least a portion of the radiation travelling from the mask MA towards the illumination system IL and thereby direct the radiation to the alignment device AS, for example during an alignment operation. The optical element TM may be movable so that it can be positioned in the radiation beam B during an alignment mode and removed from the radiation beam B during an imaging mode.

[0101] The optical element TM may be one of various types of optical elements, for example a beam splitting element such as a dichroic mirror, a half-silvered mirror, a two-way mirror (also known as a one-way mirror, a semi-transparent mirror or a partially transmitting / reflecting mirror), a beam cube, or any other optical element known in the art or any combination thereof, depending on the radiation used. The optical element TM may be located in a region adjacent to the optical element. Figure 1 The various locations depicted are, for example, between mask MA and projection system PS, or within illumination system IL.

[0102] Now refer to Figure 2 An imaging apparatus having an alignment system AS is described. For the sake of clarity, the imaging apparatus will be described for use in a lithographic apparatus. However, it should be understood that this is not limiting and that the imaging apparatus may be used for a variety of different image forming applications, such as a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object.

[0103] Figure 2 is a schematic block diagram depicting an imaging device according to an embodiment of the present invention. An illumination system (not shown) provides a radiation beam B, which may also be referred to as beam B or radiation beam B. The illumination system may include an illumination source for forming the radiation beam B and an illumination optical device for guiding (and optionally adjusting) the radiation beam B. The imaging device includes a first plane 20, a second plane 22, a projection system PS disposed between the first plane 20 and the second plane 22, and an alignment system AS. The depicted imaging device also includes two support structures, which are configured to support items in the first plane 20 and the second plane 22. These may be, for example, a mask support MT supporting a mask MA in the first plane 20 and a substrate support WT supporting a substrate W in the second plane 22.

[0104] The radiation beam B is directed to the first plane 20, and the projection system PS directs the radiation between the first plane 20 and the second plane 22. The projection system PS can be configured so that the first plane 20 and the second plane 22 are conjugate planes. That is, the first plane 20 and the second plane 22 are both field planes of the imaging device. An object in the first plane 20 can be imaged onto the second plane 22. An object in the second plane 22 can be imaged onto the first plane 20. It should be noted that in Figure 2 In FIG. 2 , the two planes 20 , 22 are parallel to the xy plane. However, it should be understood that this is illustrative and the planes may be oriented differently.

[0105] The first plane 20 and the second plane 22 may be considered as an object plane and an image plane, respectively. However, due to the bidirectional nature of the projection system PS, they may be referred to interchangeably. The first plane 20 and the second plane 22 may also be referred to as an input plane and an output plane, respectively, and / or as an object plane and an image plane, respectively.

[0106] The imaging device may be provided with alignment marks. One or more first marks M1 may be supported in a first plane 20, for example the mark M1 may be associated with the mask MA or the mask support MT, and may optionally be a reference mark. One or more second marks W1 may be supported in a second plane 22, for example the mark W1 may be associated with the substrate W or the substrate support WT. In this example, the first mark M1 is transmissive and the second mark W2 is reflective, but it will be appreciated that different marking characteristics may be more appropriate with different device geometries. In use, during alignment measurements, the marks M1, W1 may be arranged so that when irradiated by the radiation beam B, an image of the first mark M1 is formed on the second mark W1, and a combined image of the first mark M1 and the second mark W1 is formed on the first mark M1.

[0107] The term "combined image" should be understood as an image of the first identifier M1 superimposed with an image of the second identifier W1 to form a single image. Therefore, the combined image may contain information about the first identifier M1 and the second identifier W1, and the alignment distance between the first identifier and the second identifier may be determined from the combined image based on the characteristics of the combined image.

[0108] The combined image should be understood to be substantially static in size and shape, i.e., there is substantially no relative movement between the image of the first identifier M1 and the image of the second identifier W1. In practice, there may be small changes in size and shape (e.g., due to relative movement between the images of the first identifier M1 and the second identifier W1 due to imperfect synchronous scanning), but these small movements can be considered negligible compared to the size of the identifiers M1, W1. Therefore, the combined image can be understood to be generally static in size and shape. If the identifiers M1, W1 include one or more patterns, the combined image can be understood to have corresponding patterns that are also generally static. Although generally static in size and / or shape and / or pattern, the combined image can move in a plane, for example, it can be scanned in a plane as described in more detail below.

[0109] Typically, there may be at least one first marker M1 and at least one second marker W1. There may be any number of first markers M1 and any number of second markers W1. For simplicity, the alignment process according to an embodiment of the present invention will be described with reference to a single first marker M1 and a single second marker W1.

[0110] In operation, the radiation beam B passes through the first mark M1. The radiation beam B may be patterned by the first mark M1 and forms a patterned radiation beam PB. It can be said that the patterned radiation beam PB contains information about the first mark M1. The projection system PS guides the patterned radiation beam PB so that an image of the first mark is formed in the second plane 22. The patterned radiation beam PB is reflected from the second mark W1. The patterned radiation beam PB is further patterned by the second mark W1 and may also be referred to as a patterned radiation beam. At this stage, the patterned radiation beam PB may contain information about the first mark M1 and the second mark W1.

[0111] The patterned radiation beam containing information about the first identifier M1 and the second identifier W1 is then directed to a third plane where it forms a combined image of the first identifier and the second identifier. The combined image may be directed to the third plane by the projection system PS and / or other elements of the device. Thus, the third plane may be conjugate to the first plane and the second plane. In the depicted embodiment, the third plane is identical to the first plane 20. Additional optics may refocus the patterned radiation beam PB so that, alternatively or additionally, the combined image is formed at another plane (which is separated from the first plane 20), such as an input plane of the alignment system AS. This separate plane may be referred to as a fourth plane, which may be the same as or different from the third plane.

[0112] The combined image of the first identifier M1 and the second identifier W1 may be referred to as a combined image. It should be understood that in this embodiment, the first identifier is imaged before the second identifier to form the combined image, but in other embodiments, the second identifier may be imaged before the first identifier to form an equivalent combined image. It can be said that the combined image contains information about the first identifier and the second identifier. The combined image can be said to be formed by radiation interacting with the first identifier M1 and the second identifier W1.

[0113] The combined image may be directed to an alignment system AS, for example via a projection system PS and optical elements TM. Using the alignment system AS, data may be extracted from the combined image to determine an alignment distance between the first mark M1 and the second mark W1.

[0114] The first support member MT and the second support member WT are configured so that they can move within their respective planes 20, 22, for example using the above-mentioned synchronous scanning. The scanning can be performed by a support scanning mechanism (not shown). The support scanning mechanism can scan the first and second markers at a rate such that no relative movement occurs between the image of the first marker and the image of the second marker. That is, the images of the two markers are static relative to each other within the radiation beam. Although the images of the first marker M1 and the second marker W1 are static relative to each other, due to the synchronous scanning, the combined image can be moved relative to the third plane. Since the first marker and the second marker are static relative to each other, the combined image does not change size or shape when scanning across the third plane.

[0115] An example image according to an embodiment of the present invention is shown in Figure 3 Indicated in. Figure 3 The associated markers M1, W1 comprise diffraction gratings. That is, the markers M1, W1 comprise periodic structures of high and low transmission / reflection. In this embodiment, the marker M1 comprises a transmission grating and the second marker W1 comprises a reflection grating. The marker has a grating period (also called a grating pitch), which is defined as the distance between consecutive periodic structures. The marker has a grating duty cycle, which is defined as the width of the periodic structure. The markers M1, W1 can form an image in one or more field planes, wherein the resulting image is a series of periodic high-intensity and low-intensity bars. The image can have a corresponding period that can match the grating period. The image can have a corresponding duty cycle that can match the grating duty cycle. The markers M1, W1 can diffract the radiation beam B into a diffraction pattern so that the diffraction pattern can be formed in one or more pupil planes in the device.

[0116] The first image 30 represents the transmittance profile of the first mark M1. That is, it represents the intensity of the patterned radiation beam PB after interacting with the first mark M1. The axis represents the intensity I of the radiation beam in the x-direction, but it should be understood that other directions can be used to measure the intensity changes of these images, such as the intensity in the y-direction.

[0117] The first image 30 has a first period P1, defined as the distance between the center of the first bar and the center of the second bar (or equivalently, the distance between corresponding points on two consecutive bars). The first image 30 has a first duty cycle DC1, defined as the width of the bar. The first image 30 may also have a first deviation O1, defined as the minimum intensity, and a first maximum MX1, defined as the maximum intensity.

[0118] The transmitted first image 32 represents the intensity of the patterned radiation beam PB after interacting with the first marker M1 and then traveling through the projection system PS. The x-axis represents the intensity I of the radiation beam in the x-direction, but it will be appreciated that other directions may be used to measure intensity variations of these images, such as the intensity in the y-direction. It should be noted that the transmitted first image 32 includes portions of oscillating intensity. This is due to the finite numerical aperture of the projection system PS. The projection system PS collects only a limited number of diffraction orders from the patterned radiation beam PB (i.e., the projection system PS collects less than 100% of the patterned radiation beam PB).

[0119] The second image 33 represents the reflectivity profile of the second marker. That is, it represents the intensity of the patterned radiation beam PB after interacting with the second marker W1 (that is, if the radiation beam B was transmitted to the second marker W1 before being patterned by the first marker M1, as may be done in some alternative arrangements). The axis represents the intensity I of the radiation beam in the x-direction, but it will be appreciated that other directions may be used to measure intensity variations in these images, such as the intensity in the y-direction.

[0120] The second image 33 has a second pitch P2, a second duty cycle DC2, a second deviation O2 and a second maximum value MX1. In this embodiment, the first image and the second image have matching pitches (i.e., P1=P2, taking into account any reduction or magnification factors applied by the projection system PS), but different duty cycles (DC1≠DC2), deviations (O1≠O2) and maximum values ​​(MX1≠MX2).

[0121] The third image 34 is a representation of a combined image. That is, it is a representation of the intensity of the patterned radiation beam B after interacting with the first marker M1 and the second marker W1. The x-axis represents the intensity I of the radiation beam in the x-direction, but it will be appreciated that other directions may be used to measure intensity variations in these images, such as the intensity in the y-direction. The third image 34 may also be referred to as a combined image, although in this example, it should be noted that in Figure 3 34 is a representation of the combined image in one dimension (along the x-axis). It should be understood that the x-axes shown for the first image 30, the transmitted first image 32, the second image 33, and the combined image 34 may be different (e.g., have different scales), for example, due to the magnification applied by the projection system. The intensity I-axes for the first image 30, the transmitted first image 32, the second image 33, and the combined image 34 may also be different (e.g., have different scales), for example, due to the magnification applied by the projection system PS.

[0122] It should be noted that, due to the finite numerical aperture of the projection system PS described above, the third image 34 includes a portion of oscillating intensities. The shape of the third image 34 contains information about the first image 30 and the second image 33, and therefore contains information about the first alignment mark M1 and the second alignment mark W1, such as their relative positions, as now discussed.

[0123] In this particular embodiment, the high intensity individual bars within the third image 34 have a staircase / shoulder-like structure. This is due to the difference in shape (specifically the difference in duty cycle) of the first mark M1 and the second mark W1 and therefore the difference in shape (specifically the difference in duty cycle DC1, DC2) of the first image 30 and the second image 33. To better illustrate the staircase-like structure, Figure 3 The head 36 and the shoulder 38 of the bar are indicated. The width of the shoulder 38 may depend on the difference in duty cycle between the first markers M1 , W1 .

[0124] It should be understood that different types of markers may be used. For example, in some embodiments, the marker may not include a periodic structure and therefore have no pitch. In some embodiments, it may be beneficial to have a pitch that is matched between the first marker M1 and the second marker W1, for example so that the bars of the subsequent first image 30 and the second image 33 overlap. The markers may have equal duty cycles and / or equal deviations and / or equal maximum values. For some embodiments, it may be advantageous to provide markers of different shapes, such as a single line or dot, rather than a diffraction grating.

[0125] The markers M1, W1 and the combined image 34 are centered on the desired alignment position 31. Figure 3 , the expected alignment position 31 is represented along the x-axis, but it will be appreciated that the expected alignment position may be defined in any coordinate system, for example it may additionally or alternatively be defined in the y-direction. The expected alignment position 31 represents a reference position where the first marker M1 and the second marker W1 are expected or intended to be aligned (e.g. by being centered about said position). If either the first marker M1 or the second marker W1 are not properly aligned, their corresponding images may be centered about a position different from the expected alignment position 31. Due to any such misalignment, the combined image 34 may be asymmetric due to the difference in position of the first marker M1 and the second marker W1.

[0126] As a further illustration, if the first image 30 and the second image 33 are both centered at different but equal positions in the x-direction, then the combined image 34 will also be centered at different but equal positions in the x-direction, but will not be asymmetric. This indicates that the alignment marks are aligned relative to each other, but are not imaged at the expected locations.

[0127] Figure 4A portion of an example combined image is shown when the markers M1, W1 are aligned (i.e., aligned combined image 40) and when the markers M1, W1 are misaligned 42 (i.e., misaligned combined image 42). The aligned combined image 40 is symmetrical; the misaligned combined image is asymmetrical. To quantify the asymmetry, the combined image can be defined by a first head distance C and a second head distance D and a first shoulder distance E and a second shoulder distance F, all of which are measured from the center of the combined image. In this example, the center can be defined as the middle between the distal edge of the first shoulder 38a and the distal edge of the second shoulder 38b. The first head distance C and the second head distance D are measured between the center of the head 36 and the first distal end 36a and the second distal end 36b, respectively. The first shoulder distance E and the second shoulder distance F are measured between the center and the distal edge of the first shoulder 38a and the second shoulder 38b, respectively.

[0128] It can be seen that when the markers M1, W1 are aligned, and therefore the combined image is a symmetrical combined image 40, the first head distance and the second head distance are equal (C=D), and the first shoulder distance and the second shoulder distance are equal (E=F). However, when the markers M1, W1 are not aligned, the combined image is an asymmetrical combined image 42, and the first shoulder distance and the second shoulder distance are equal (E=F), but the first head distance and the second head distance are not equal (C≠D). A larger alignment distance may result in a greater inequality between the first head distance C and the second head distance D. Therefore, by extracting data about the asymmetry of the combined image 42, the alignment distance between the first marker and the second marker can be determined.

[0129] As mentioned above, different types of alignment marks can be used. It should be understood that different types of alignment marks can generate combined images of different shapes. In this example, the alignment distance can still be determined by evaluating the asymmetry of the combined image.

[0130] The formation of a combined image is advantageous because it allows the first identifier to be aligned relative to the second identifier. Alternative image forming devices (e.g., lithographic devices) use an alignment system that aligns the alignment identifier with a portion of the device. Alignment with a portion of the device can reduce accuracy, for example if the location of a portion of the device is miscalibrated. For example, a first known image forming device uses an image of a first identifier (e.g., on a wafer) and scans it relative to a second (static) identifier (e.g., on a mask), wherein the second identifier forms a detection grating. The known device thus determines the alignment of the first identifier relative to the stationary portion of the lithographic device. In a second known image forming device, an alignment identifier is used on a first plane and an alignment identifier is used on a second plane, and separate images of the first alignment identifier and the second alignment identifier are formed and directed to two separate detectors. Therein, image recognition can be used to determine the relative position of the two separate images relative to a portion of the image forming device, and thereby determine the relative position of the first alignment identifier and the second alignment identifier.

[0131] In a first known image forming device, there is a relative movement between the images of the first mark and the second mark (which acts as a detection grating), so a combined image cannot be formed. In a second known image forming device, the separate images of the first mark and the second mark are formed using different parts of the imaging system (or even separate imaging systems), so a combined image is not formed. However, the image forming device according to the present invention can form a combined image because there is no relative movement between the image of the first mark M1 and the image of the second mark W1. Therefore, due to the formation of the combined image, the relative position of the first alignment mark and the second alignment mark can be determined regardless of the position of the image forming device.

[0132] It may be beneficial to provide alignment marks at the center of the field, for example, along the optical axis of the device during the alignment process. The imaging operation of the device typically includes placing a pattern in the optical axis in a first plane, and projecting an image of the pattern along the optical axis onto a second plane at the optical axis. Thus, the imaging operation may be said to have a first optical axis. The image forming device when performing an imaging operation may be said to be operating in an imaging mode.

[0133] The alignment procedure may have a second optical axis. An image forming device while performing an alignment operation may be referred to as operating in an alignment mode. If alignment and imaging are performed by different parts of the device, then alignment may not be as accurate. For example, any effects such as heating that occur during the imaging operation will similarly be present in the alignment operation.

[0134] It may be beneficial to have the alignment mode and the imaging mode occur through the same or similar area of ​​the imaging device. That is, it may be beneficial for the image forming device to direct the radiation beam B through substantially the same portion of the projection system PS when in the imaging mode and the alignment mode. This may allow imaging and alignment to be performed under similar conditions, for example the radiation beam may travel through the same portion of the lens in both modes. This may improve the accuracy of alignment and / or imaging.

[0135] The first known device and the second known device described above use alignment marks that are largely off-axis. That is, they are not positioned along the first optical axis of the device (used for imaging mode). On the other hand, the image forming device according to the present invention can provide alignment marks at or near the first optical axis. That is, the first optical axis can be the same as the second optical axis. In other words, the image forming device can guide radiation through the same or similar area of ​​the imaging device in alignment and imaging modes. Therefore, the image forming device according to the present invention can provide more accurate alignment. Additionally, since the alignment mark is close to the optical axis, the alignment process can be performed during or between imaging operations, which may result in increased productivity.

[0136] Figures 5 to 7 Example alignment systems AS and methods that can be used to extract data from a combined image to determine an alignment distance are depicted. Generally, in these examples, the alignment system AS includes a modulation system that can be used to modulate a combined image to form a modulated combined image and a detector that can be used to extract data from the modulated combined image.

[0137] Figure 5 An alignment system AS including a self-referencing interferometer (SRI) 52 is depicted. The use of a self-referencing interferometer to extract position information of an input identification is described in US6961116 and US9606442, both of which are incorporated herein by reference. The SRI 52 receives a combined image at an input plane 50 of the alignment system AS. In this embodiment, the combined image forms the input identification of the SRI. The patterned radiation beam containing the combined image is separated into diffraction orders (e.g., in the pupil plane of the SRI). The SRI 52 applies a relative rotation of 180 degrees between the positive and negative diffraction orders, and the two are recombined to be coherently added. The relative rotation can be a rotation of any angle that results in a difference of 180 degrees between the positive and negative orders, such as rotating the positive order by 90 degrees and rotating the negative order by -90 degrees.

[0138] The combined image is scanned relative to the input plane 50, for example as a result of synchronous scanning of the first marker M1 and the second marker W1, as described above. The combined image can be scanned in a scanning direction S. The scanning direction can be, for example, the x-direction. Due to the applied rotation, the positive and negative diffraction orders are scanned in opposite directions (for example, in opposite directions parallel to the scanning direction S) in the plane of the detector 54. A modulated combined image is generated as positive and negative orders are scanned relative to each other (moving in phase and out of phase with each other). The modulated combined image can be an intensity modulation, for example when the positive and negative orders move in phase and out of phase with each other.

[0139] Additional optical elements such as lenses 56 may be provided to shape and / or direct the radiation beam. Typically, the alignment system AS is configured so that the input plane 50 is conjugate to the plane in which the modulated combined image is formed (i.e., the plane of the detector 54) in the absence of optics responsible for achieving a 180 degree relative rotation between the positive and negative diffraction orders.

[0140] Scanning the combined image across the input plane 50 may be performed by an image scanning mechanism, such as a scanning element associated with the alignment system AS. Alternatively or in addition, the scanning may be achieved by synchronized scanning of the support members described above. It should be understood that the input plane 50 may be any relevant plane within the alignment system. The input plane may be a third plane, or may be a plane conjugate to the third plane, i.e., may be a fourth plane.

[0141] The separation into diffraction orders can be considered as a kind of Fourier decomposition. There are positive and negative diffraction orders (e.g. +1, +2, +3... and -1, -2, -3...), which can also be referred to as Fourier components. The term Fourier harmonics may be used herein, which may be understood to mean the positive and negative parts of the same numbered diffraction orders (e.g. +1 and -1). The combined image is decomposed into Fourier components within the SRI. The decomposition may be performed using any suitable optics and may be implemented in a plane that is the Fourier transform plane of the input plane 50 (which may be referred to as the pupil plane).

[0142] The modulated combined image may be transmitted to a detector 54. The detector 54 may include an array of sensing elements (which may each define a pixel of the detector 54), such as a CCD or CMOS detector, e.g. Figure 5 Alternatively, the detector may comprise a single detector, such as a photodiode. If a single detector is used, an additional scanning mechanism may be used to direct different parts of the modulated combined image sequentially onto the single detector.

[0143] The modulated combined image can be converted into an electrical signal. The signal can be referred to as a modulated signal, such as the modulation of the electrical signal intensity when the modulated combined image is scanned along an axis (eg, x-axis). The modulated signal can be analyzed to extract data and determine the alignment distance.

[0144] Some example signals are Figure 6 , which shows the modulated signal along the scan direction S (i.e., when the combined image is scanned along the scan direction S on the input plane 50). An example of the signal contribution of one Fourier harmonic (e.g., the +1 and -1 diffraction orders) is shown. An example of the signal contribution from the negative diffraction orders 60a, 62a, the positive diffraction orders 60b, 62b, and the sum of the positive and negative orders 60c, 62c modulated by the SRI (e.g., the signal formed in the plane of the detector 54) is shown. The vertical dashed lines are provided as a guide to the eye to highlight the phase differences between the signals.

[0145] When the marks M1, W1 are aligned, alignment signals 60a, 60b, 60c may be formed. When the marks M1, W1 are not aligned, misalignment signals 62a, 62b, 62c may be formed. It can be seen that in the aligned case, the signals of the positive and negative levels 60a, 60b are aligned, that is, there is no phase difference between them. When added, the positive and negative levels may form a modulated signal 60c centered on the expected alignment position 31. The modulated signal 60c may have a maximum amplitude 66, which is measured as the maximum amplitude reached by the electrical signal. This may be referred to as the expected maximum amplitude 66 because it is the maximum amplitude in the case of alignment marks M1, W1.

[0146] However, for the misaligned marks M1, W1, the signals of the positive and negative poles 62a, 62b have different phases. That is, the signals for the positive and negative poles 62a, 62b are centered at a position that is not the aligned position 31. This may be referred to as a phase shift. Additionally, as mentioned above, if the image forming device is not properly aligned, there may be some relative movement between the first mark M1 and the second mark W1, which may result in a slight change in the phase shift of the Fourier component of the signal.

[0147] When the signals are added, the modulated signal 62c can have a maximum amplitude 64 that is lower than the expected maximum amplitude 66. The reduced amplitude is due to misalignment of the alignment marks, thereby reducing the amount of transmitted light. Therefore, anything less than the expected maximum amplitude 66 can be considered a deviation from perfect alignment. If the amplitude is less than the expected maximum amplitude 66, the first mark M1 and / or the second mark W1 can be moved (e.g., by moving the first support member MT and / or the second support member MW) to increase the maximum amplitude 64 to the expected maximum amplitude 66. During the alignment procedure, the first mark M1 and / or the second mark W1 can be iteratively moved until the maximum amplitude 64 has been maximized.

[0148] Figure 6 The signal shown is an example signal of a single Fourier harmonic, specifically the first diffraction order (i.e., the +1 and -1 diffraction beams). Data about the alignment distance can be beneficially extracted from a single Fourier harmonic of the modulated combined image. Therefore, it may be desirable to separate the combined image / modulated combined image / modulated signal into separate Fourier components. This may be performed electronically, for example by analyzing the modulated signal using computational processing.

[0149] Alternatively or additionally, separation of one or more Fourier components may be performed using optical filtering in the image forming device, for example on a combined image or a modulated combined image. For example, a component (e.g. comprising one or more slits in a screen) may be provided in a pupil plane of the alignment system AS to remove one or more diffraction orders from the combined image. This may be referred to as spatial filtering, and reduces or suppresses transmission of one or more diffraction orders through the pupil plane.

[0150] The use of spatial filtering to remove diffraction orders can beneficially improve the signal-to-noise ratio and therefore improve the accuracy of the alignment operation. The use of spatial filtering can also reduce the computational load compared to electronic filtering, as the modulated combination signal can be analyzed directly without applying any further filtering.

[0151] The zeroth order diffraction beam (i.e., the beam corresponding to reflection or transmission) can be removed. This can be beneficial for improving accuracy. This can also be beneficial for reducing stray light within the system, which can reduce accuracy and / or can damage components of the device. Higher diffraction orders can be removed. Removing all diffraction orders except the first diffraction order can be particularly beneficial. Advantageously, a self-referencing interferometer can be used to modulate multiple Fourier orders or a single Fourier order.

[0152] Different levels of analysis can be beneficial for different alignment applications. For example, 1 order (+1 and -1) can be beneficial for alignment of surface features on a substrate, while 4 order (+4 and -4) can provide more accurate alignment for features on a substrate coated with a top layer.

[0153] Similarly, different grating pitch sizes may be beneficial for different alignment applications. Advantageously, diffraction gratings of any pitch size may be used in combination with an alignment system including a self-referencing interferometer. The use of any pitch size may allow for greater freedom in designing an imaging device. The use of any pitch size may result in improved accuracy.

[0154] In some embodiments, the first alignment mark and the second alignment mark can be selected to include diffraction gratings with matching periods. That is, the combined image has a period that matches the first alignment mark and the second alignment mark. The diffraction grating can be of various types, such as holographic, ruled or stepped, and reflective or transmissive.

[0155] An alternative embodiment for extracting data from the combined image is Figure 7 As seen in this embodiment, the alignment system AS comprises two detectors 54a, 54b, two detection gratings 70a, 70b, an optical element 74 and a reflector 72. The optical element 74 is arranged to separate the patterned radiation beam PB into two parts. A first part of the patterned radiation beam PB is directed to a first detection grating 70a disposed in front of the first detector 54a, and a second part of the patterned radiation beam PB is directed to a second detection grating 70b disposed in front of the second detector 54b. The detection gratings 70a, 70b can be considered to be located in a detection plane 76 of the alignment system AS, wherein Figure 7 The detection plane 76 is shown conjugate to the third plane in which the combined image is formed. Additional optical elements such as lens 56 may be provided to shape and / or direct one or more portions of the patterned radiation beam PB. In some embodiments, the detection plane 76 may coincide with the third plane. Figure 7 Although not shown in the figure, it should be understood that the detection gratings 70a, 70b can be arranged in different planes, that is, there can be multiple detection planes.

[0156] The modulation is provided by scanning the combined image relative to the detection plane 76 of the alignment system AS. The scanning may be provided by an image scanning mechanism, which may or may not be associated with the alignment system AS. The scanning mechanism may be implemented by synchronized scanning of the support members described above. Scanning the combined image on the detection plane 76 enables the combined image to be scanned on the detection gratings 70a, 70b and thus modulated by the detection gratings 70a, 70b.

[0157] When the combined image is scanned relative to the detection gratings 70a, 70b, the modulation can be an intensity modulation. That is, scanning the combined image relative to the detection gratings 70a, 70b provides an oscillation of the transmission intensity through the detection gratings 70a, 70b. Two modulated combined images associated with the first and second parts of the patterned radiation beam are formed. The detectors 54a, 54b can then detect the modulated combined image. The detector can convert the modulated combined image into a modulated signal. Additionally, in this embodiment, for example, using one of the above methods, one or more Fourier components can be removed from the modulated combined image and / or (multiple) modulated signals. The alignment distance can be determined based on the asymmetry of one or more modulated combined images.

[0158] The alignment marks M1, W1 can be selected to include a diffraction grating having a period that matches the period of the detection gratings 70a, 70b. The detection gratings 70a, 70b can include gratings oriented at a non-zero angle to each other in the same plane. The detection gratings 70a, 70b can include gratings oriented at a non-zero angle relative to the scanning direction (e.g., the y-direction). Having detection gratings 70a, 70b oriented at a non-zero angle relative to the scanning direction can allow the alignment distance to be determined along multiple axes, for example, after scanning only in the y-direction, the alignment distance can be calculated in the x-direction and the y-direction. The x and y measurement method using the detection grating is described in US2009 / 195768A, which is incorporated herein by reference.

[0159] In a specific example, Fig. 8A As schematically illustrated in , there may be a plurality of first alignment marks M1, M2, M3 forming a mark array. Each alignment mark may include a first grating and a second grating oriented at 90 degrees to each other. The first grating and the second grating are oriented at +45 degrees and -45 degrees, respectively, relative to a scanning direction S, which may be, for example, in the y-direction. A second alignment mark (not shown) may also include a corresponding array of marks, having substantially the same grating orientation (although as previously discussed, the grating pitch may be scaled depending on the magnification applied by the projection system PS).

[0160] The identification array can be beneficial for known image forming devices (such as lithographic equipment). In a lithographic operation, radiation can be directed onto a portion of a substrate W. The area through which the radiation travels (corresponding to the area of ​​the substrate W illuminated by the radiation) can be referred to as a slit. The identification array can be arranged so that they are set in multiple locations on the slit. Arranging the identification array in multiple locations on the slit can beneficially enable alignment to be measured and / or monitored at multiple locations on the slit, which can therefore improve the imaging accuracy on most of the slits.

[0161] An array of three markers is used in this example. However, the marker array can be any number of markers, such as eight markers or fifty markers. The alignment markers on the first plane and the second plane can be arranged so that the image of the first marker array will perfectly overlap the corresponding array of the second marker. Using multiple markers can improve accuracy because more alignment points are used for alignment. Simultaneously irradiating multiple alignment markers may result in a reduction in alignment time and therefore increase the productivity of the alignment process. That is, the amount of time required to measure the alignment can be reduced. The reduced alignment time may result in an increase in the overall productivity of the imaging process.

[0162] like Figure 8BAs schematically illustrated in FIG. 1 , the first detection grating 80a and the second detection grating 80b may include detection gratings oriented at 90 degrees to each other. The first detection grating 80a may include a grating whose lines are oriented at +45 degrees relative to the scanning direction S, and the second detection grating 80b may include a grating whose lines are oriented at -45 degrees relative to the scanning direction S. The scanning direction S may be in the y direction, for example. This arrangement may allow the alignment distance in the x direction and the alignment distance in the y direction to be determined by scanning the combined image of the first identification array and the second identification array relative to these detection gratings 80a, 80b.

[0163] When the first marker M1, M2, M3 and the second marker W1, W2, W3 are scanned synchronously, a modulated combined image can be formed by the alignment system AS, which contains information about the alignment in the x-direction and the y-direction. The detectors 54a, 54b can convert the modulated combined image into a modulated signal. The modulated signal can then be modeled to extract the alignment position. Similar to the above example, the misalignment between the first marker M1, M2, M3 and the second marker W1, W2, W3 may cause a phase shift in one or more of the modulated signals, and the alignment distance can then be determined.

[0164] The image forming apparatus according to the present invention can provide a faster method of determining the alignment distance. An alternative alignment system may take 10 to 20 minutes to perform an alignment operation of a substrate. According to the present invention, using a combined image to determine the alignment distance can enable the alignment operation to be performed faster, for example within about 10 to 20 seconds per substrate. Due to the increase in alignment speed, the alignment operation according to the present invention can be performed during an imaging operation or between subsequent imaging operations.

[0165] In another known alignment method, a complete resist exposure and development cycle can be performed. That is, a photolithography wafer covered in a photoresist can be exposed to radiation with a pattern. The resist can then be developed and removed from the development cycle. Information about alignment can then be extracted. However, this complete process can be a very time-consuming activity. The invention described in detail above allows real-time alignment information without the need for the development cycle.

[0166] According to an embodiment of the present invention, using a combined image to determine the alignment distance indicates the direct positional accuracy of the image forming device and can enable alignment operations to be performed more accurately. For example, in a lithography apparatus including the above-described image forming apparatus, an exposure pattern of a mask can be aligned with respect to a corresponding pattern on a substrate with greater positional accuracy than with other currently known alignment sensors.

[0167] The image forming apparatus described above may also be generic for many applications, as the design of the alignment marks and therefore the characteristics of the combined image may be optimized for a variety of arrangements. Furthermore, while the combined image may be useful for calculating the relative alignment between the first mark and the second mark, the apparatus may be generic and may be used to determine the alignment of one mark relative to a portion of the apparatus. For example, an alignment operation may be performed using only marks on the first plane or only marks on the second plane to determine the alignment of the marks with the apparatus.

[0168] Although embodiments of the present invention may be specifically referenced in the context of lithographic equipment in parts herein, embodiments of the present invention may be used in other equipment. Embodiments of the present invention may form part of mask inspection equipment, metrology equipment, or any equipment that measures or processes objects such as wafers (or other substrates) or masks (or other pattern forming devices). These equipment may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0169] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in a manner other than as described. The above description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. An image forming device comprising an illumination system operable to direct a radiation beam onto a first plane; a first support structure and a second support structure, the first support structure being operable to support a first marker in the first plane and the second support structure being operable to support a second marker in a second plane; a projection system arranged between the first plane and the second plane to form a combined image of the first mark and the second mark in a third plane; an alignment system operable to extract data from the combined image to determine an alignment distance between the first marker and the second marker; as well as a modulation system operable to form a modulated combined image from said combined image; as well as a detector operable to extract data from the modulated combined image to determine an alignment distance between the first marker and the second marker, The modulation system comprises a self-referencing interferometer and an image scanning mechanism, wherein the image scanning mechanism is configured to scan the combined image on the third plane. 2 . The image forming apparatus according to claim 1 , wherein the extracted data depends on the asymmetry of the combined image.

3. An image forming apparatus according to claim 1, wherein the detector is operable to extract phase shift data from the modulated combined image.

4. The image forming apparatus according to claim 1, wherein the modulation system comprises: One or more detection gratings in the third plane and an image scanning mechanism, the image scanning mechanism being configured to scan the combined image on the third plane such that the combined image is scanned relative to the one or more detection gratings.

5. An image forming apparatus according to any one of claims 1 to 4, wherein the modulation system is further operable to filter out one or more Fourier components from the combined image to form a filtered combined image, and to form the modulated combined image from the filtered combined image. 6 . The image forming apparatus according to claim 1 , wherein the first mark and the second mark include diffraction gratings. 7 . The image forming apparatus according to claim 1 , wherein the first mark and the second mark have matching diffraction grating periods.

8. An image forming device according to any one of claims 1 to 4, wherein the illumination system is operable to simultaneously illuminate a first identification array in the first plane, and the projection system is configured to form an image of the first identification array onto a corresponding second identification array and to form multiple combined images of each pair of corresponding first identifications and second identifications in the third plane.

9. An image forming device according to any one of claims 1 to 4, wherein the image forming device is capable of operating in an imaging mode and an alignment mode, and wherein when operating in the imaging mode or the alignment mode, the radiation beam is directed through substantially the same portion of the projection system.

10. A metrology device comprising the image forming device according to any one of the preceding claims.

11. A lithographic device arranged to project a pattern from a pattern forming device onto a substrate, the lithographic device comprising the image forming device of any one of claims 1 to 9, wherein the first support structure is operable to support the pattern forming device in the first plane, and the second support structure is operable to support the substrate in the second plane.

12. A method for measuring an alignment distance between a first marker in a first plane and a second marker in a second plane of an imaging device, the method comprising: directing a radiation beam onto said first plane of said imaging device; forming a combined image of the first mark and the second mark; directing the combined image to an alignment system; extracting data from the combined image to determine the alignment distance; scanning the first marker and the second marker relative to each other such that the combined image is scanned relative to a third plane; Simultaneously illuminating the first array of markers in the first plane and forming an image of the first array of markers onto a corresponding array of second markers and forming a plurality of combined images of each pair of corresponding first and second markers in the third plane; as well as The combined image is modulated using a self-referencing interferometer or one or more detection gratings.

13. A method according to claim 12, wherein the data extracted depends on the asymmetry of the combined image.

Images