CN113966490A

CN113966490A - Image forming apparatus with a toner supply unit

Info

Publication number: CN113966490A
Application number: CN202080040913.XA
Authority: CN
Inventors: O·V·沃兹纳
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2019-06-03
Filing date: 2020-04-30
Publication date: 2022-01-21
Also published as: WO2020244854A1

Abstract

An image forming apparatus includes: an illumination system operable to direct a beam of radiation onto a first plane; a first support structure operable to support a first marker in a 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

Image forming apparatus with a toner supply unit

Cross Reference to Related Applications

This application claims priority to EP application 19177944.6 filed on 3.6.2019, which is incorporated herein by reference in its entirety.

Technical Field

The present invention relates to an image forming apparatus. In particular, it relates to an image forming apparatus having an alignment system and related methods.

Background

Image forming apparatuses are used in imaging systems, such as lithographic apparatuses. A lithographic apparatus is a machine that is configured to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of Integrated Circuits (ICs). For example, a lithographic apparatus may project a pattern (also commonly referred to as a "design layout" or "design") of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

As semiconductor manufacturing processes continue to evolve, the size of circuit elements has been decreasing, while the number of functional elements (such as transistors) per device has steadily increased over decades, following a trend commonly referred to as 'moore's law. To keep pace with moore's law, the semiconductor industry is seeking technologies that can create smaller and smaller features. To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features patterned on the substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm and 13.5 nm. Lithographic apparatus using Extreme Ultraviolet (EUV) radiation having a wavelength in the range 4nm to 20nm (e.g. 6.7nm or 13.5nm) may be used to form smaller features on a substrate than lithographic apparatus using radiation having a wavelength of, for example, 193 nm.

In the manufacture of complex devices, typically a number of 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 the applied pattern with respect to features formed in previous layers (either by the same apparatus or a different lithographic apparatus). For this purpose, the image forming apparatus of the lithographic apparatus may use one or more identification sets (also interchangeably referred to as markers). Each marker is a structure, for example its position relative to a reference position or another marker 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 marker may be referred to as an "alignment marker".

In addition to lithographic apparatus, there are many other apparatuses to which the invention may be related, such as, for example, mask inspection apparatuses, metrology apparatuses, or any apparatus that measures or processes an object. In these and other devices, alignment may be a critical aspect of device performance.

Measuring alignment can be a time consuming process. For example, for accurate measurement, multiple alignment markers may be used, but some alignment markers may be aligned in sequence, which can make the alignment longer in duration.

It may be desirable to provide an image forming apparatus that overcomes or alleviates one or more problems associated with the prior art. It may also be desirable to provide alternative image forming apparatuses.

Disclosure of Invention

According to a first aspect of the present invention, there is provided an image forming apparatus comprising: an illumination system operable to direct a beam of radiation onto a first plane; a first support structure operable to support a first marker in a 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.

The formation of the combined image may advantageously improve the accuracy of the alignment. The combined image may contain information about the relative positions of the first marker and the second marker. The formation of the combined image may enable the position of the second marker relative to the first marker to be determined, rather than relative to another part of the image forming apparatus. The position of the second marker relative to the first marker enabled by embodiments of the present invention represents the direct positional accuracy of the image forming system.

The formation of the combined image may advantageously reduce the time required for alignment. Determining the alignment distance may 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. Reduced alignment measurement duration can beneficially improve productivity.

The first marker and the second marker may be alignment markers. There may be a first mark in the first plane and a second mark in the second plane. Alternatively, there may be a plurality of first identifiers in the first plane and a plurality of second identifiers in the second plane. The mark may be configured to modify the radiation beam to form a patterned radiation beam. The marking may be in the form of a diffraction grating. In particular, a first mark in a first plane may modify the beam of radiation to form a first patterned beam of radiation.

The markers may be arranged such 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 static in size and shape, for example due to a lack of relative movement between the images of the first and second markers. The combined image may be moved in a plane, e.g. it may be scanned in the plane of the device. The apparatus may also include an image scanning mechanism configured to scan the combined image on a third plane.

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

The illumination system may include an illumination source and illumination optics. The illumination system and the projection system may cooperate to direct the beam of radiation to the first marker and the second marker to form a combined image of the first marker and the second marker. A combined image may be understood as an image containing information about both the first and the second markers.

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

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

The extracted data may depend on the asymmetry of the combined image. The asymmetry may depend on the deviating centroid of one or more features within the combined image. The asymmetric combined image may represent an alignment error. The alignment error may be a non-zero alignment distance between the first marker and the second marker. The alignment error may correspond to an amount of asymmetry in 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 as it represents a fast and relatively easy method for determining alignment errors.

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 an alignment distance between the first marker and the second marker. The modulated combined image may be a modulated version of a combined image of the first mark and the second mark. Modulating the combined image may increase the speed and/or ease with which data may be extracted from the combined image. The modulation system may comprise an optical block. The third plane may be the input plane of the alignment system. The third plane may be the input plane of the modulation system. The radiation beam comprising the modulated combined image may be referred to as a modulated radiation beam. The detector may comprise a sensor. The detector may comprise a multi-pixel detector, such as a CCD or CMOS, or may comprise a single detector, such as a photodiode.

The modulation system may include a self-referencing interferometer and an image scanning mechanism configured to scan the combined image on a third plane. The self-referencing interferometer may be operable to: receiving a combined image of the first marker and the second marker in a third plane; fourier decomposing the combined image of the first marker and the second marker into positive and negative diffraction orders in a pupil plane; introducing a 180 degree rotation between the positive and negative diffraction orders; and positive and negative orders are recombined in a fourth plane (conjugated with the third plane). The combined image of the first marker and the second marker has contributions from positive and negative diffraction orders. When the image scanning mechanism scans the combined image in the third plane, the contributions from the positive and negative diffraction orders are scanned in the opposite direction in the fourth plane (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 and out of phase, producing a modulated combined image. Modulation is achieved by forming a coherent addition of positive and negative diffraction orders.

The use of a self-referencing interferometer can improve alignment speed. The self-referencing interferometer may extract the alignment data spatially rather than computationally, for example. The use of a self-referencing interferometer may reduce the processing power, e.g., computer processing power, required to extract the alignment data.

The detector may be operable to extract phase shift data from the modulated combined image. The phase shift data may be related to a phase shift between positive and negative diffraction orders of the combined image of the first marker and the second marker. The phase shift data may be extracted from the modulation combined image by determining the intensity of the (oscillating) combined modulation image. The phase shift data may advantageously provide alignment information related to an alignment distance between the first marker and the second marker. 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 the identity of perfect alignment. A phase shift not equal to zero may indicate a misalignment. A lower intensity at zero phase shift may indicate misregistration.

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.

The use of a detection grating may advantageously allow the collection of alignment data with a relatively easy implementation. By scanning the combined image over the detection gratings in the third plane, a modulated combined image may be produced on the detectors, for example arranged on opposite sides of one or more of the detection gratings. The input plane of the detector may be referred to as the detector plane. The modulation combined image may be an intensity oscillation at the detector plane. The oscillation in intensity may be due to the scanning of the combined image relative to one or more detection gratings. The image may be scanned relative to the detection grating. That is, the image may be scanned over the detection raster. Alternatively or additionally, the detection gratings may be shifted in a third plane so that they scan relative to the combined image.

There may be a single detection grating or there may be multiple detection gratings. The modulation system may separate the radiation beam into a plurality of portions. The modulation system may separate the radiation beam into a first portion and a second portion. The first portion may be directed to a first detection grating. The second portion may be directed to a second detection grating. The first and second detection gratings may be disposed adjacent to the first and second detectors, respectively. The first portion of the radiation beam may be modulated by a first detection grating before entering the first detector. The second portion of the radiation beam may be modulated by a second detection grating before entering the second detector.

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 and second detection gratings may match the diffraction grating periods of the first and second markers. The at least one detection grating may be parallel to a portion of the diffraction grating of one of the markers. The at least one detection grating may be perpendicular to a portion of the diffraction grating of one of the markers.

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 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 the Fourier plane of the modulation system. The fourier plane may be referred to as a pupil plane.

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.

The zero level may be removed. The zeroth order corresponds to no diffraction, i.e. reflection or transmission. The second stage may be removed. Higher levels may be removed. Any combination of diffraction orders may be removed, for example it may be beneficial to remove all but the first order.

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 to an electrical signal.

The first and second markers may comprise diffraction gratings. The diffraction grating may be reflective or transmissive. The diffraction grating may be of various types, such as holographic, ruled or stepped. The marker may comprise two diffraction gratings in the same plane. The indicia may comprise two different diffraction gratings oriented at a non-zero angle to each other in the same plane, e.g., the two diffraction gratings may be perpendicular to each other in the same plane.

The first and second marks 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 marker and the second marker having matching diffraction grating periods are intended to represent that the grating period of the image of the first marker in the second plane is equal to the grating period of the second marker.

The first flag may have a first period. The second identifier may have a second periodicity. The matched first and second grating pairs may be implemented in the form of equal pitches, i.e. the first period may be equal to the second period. Alternatively, matching pairs of first and second gratings may be achieved by having different pitches, wherein the difference in pitch is related to a magnification or reduction factor between the first and second planes, for example due to the projection system. The first period may be scaled by a magnification factor such that it is greater than or less than the second period, wherein the magnification factor is equal to the magnification applied by the projection system.

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

Illuminating multiple markers simultaneously may result in a reduction in the alignment time required for the alignment process. Illuminating multiple markers simultaneously may result in increased productivity. For example, in a lithographic process, reduced alignment time may result in more lithographic processes per hour. Illuminating multiple markers simultaneously may be referred to as parallel alignment. The tag array may include, for example, 3 tags or 50 tags.

Illuminating multiple markers simultaneously may result in increased accuracy. For example, more markers can be used for alignment, which can improve accuracy. The array may include a plurality of identifiers. Ideally, this is a perfect alignment, and the image of the first marker array would perfectly overlap the corresponding array of the second marker.

The image forming apparatus may further include an optical element configured to allow the radiation beam to travel from the illumination optics to the first plane and into the projection system; and directing the combined image to an alignment system. The optical element may be a one-way mirror. A 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, where 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.

The apparatus may be operable in an imaging mode and an alignment mode, and wherein the beam of radiation may be directed through substantially the same portion of the projection system when operating in the imaging mode or the alignment mode. The imaging mode and the alignment mode may be considered as 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 pattern may include determining an alignment distance between the first marker and the second marker.

Operating both 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 prove the accuracy of alignment and/or imaging. This may improve the imaging quality.

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.

The image forming apparatus may have a first optical axis when in the imaging mode and a second optical axis when in the alignment mode. The optical axis may be considered to be the axis along which the radiation beam travels (e.g. the axis along which the chief ray of the radiation beam is along) when the image forming apparatus is in use. The first optical axis and the second optical axis may be considered substantially the same. The marker may be located at or near the first optical axis and the second optical axis. The radiation beam may be directed along or near the optical axis. It will be appreciated that due to physical limitations the radiation beam may not perfectly overlap the optical axis but may be considered to travel efficiently along the same axis.

The image forming apparatus may further include a support scanning mechanism configured to move the first support structure in a first direction in a first plane and to move the second support structure in a second plane in a second direction, wherein the second direction is parallel to but opposite the first direction. Supporting the scanning mechanism may advantageously allow the first indicia to be aligned with the second indicia rather than with a portion of the image forming apparatus.

The support scanning mechanism may provide synchronized scanning of the first marker and the second marker. That is, the support scanning mechanism may scan the first marker and the second marker at a rate such that no relative movement occurs between the image of the first marker and the image of the second marker. I.e. the two identified images are static with respect to each other within the radiation beam. Synchronous scanning may produce a combined image that is shape invariant. The simultaneous scanning may produce a combined image scanned in a third plane.

The support scanning mechanism may form part of the image scanning mechanism. The support scan 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 exposures.

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

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

The illumination source may be a lithographic light source, for example an illumination source for normal lithographic exposure. For example, the illumination source may comprise 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 sources may be of different wavelengths. A different wavelength illumination source may be beneficial because it may have higher transmission and/or reflection through/from the marker. Different wavelengths may advantageously be used for aligning the illumination source and the lithographic illumination source to allow monitoring of overlay and drift during the lithographic process. That is, overlay and drift can be monitored during and/or between exposure sequences without time-consuming changes to the lithography settings.

The illumination optics may be lithographic illumination optics, i.e. illumination optics used in normal lithographic exposure. Alternatively, they may be separate, e.g. they may be optimized for different wavelengths.

When used with a lithographic apparatus, the image forming apparatus may improve overlay accuracy, and/or may allow smaller features to be formed. The patterning device may be a patterned reticle. The substrate may be a wafer with resist. The first marker may be supported in a first plane in a position comparable to the position of the pattern during a normal lithographic exposure process. The second marker may be supported in a position in the second plane comparable to a portion of the wafer during a normal lithographic exposure process. That is, the first mark and the second mark may be aligned in positions that are beneficial to ensure accurate alignment of the lithographic process.

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 and a second marker in a second plane of an imaging device, the method comprising: the method includes directing a radiation beam onto a first plane of an imaging device, forming a combined image of a first marker and a second marker, directing the combined image to an alignment system, and extracting data from the combined image to determine an alignment distance.

The formation of the combined image may advantageously improve the accuracy of the alignment. The combined image may contain information about the relative positions of the first marker and the second marker. The formation of the combined image may enable the position of the second marker relative to the first marker to be determined, rather than relative to another part of the image forming apparatus. The position of the second marker relative to the first marker in the combined image achieved by embodiments of the present invention represents the direct positional accuracy of the image forming system.

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

The combined image may travel through the first marker and/or the second marker a second time before traveling to the alignment system. The effect of the second pass through the first marker and/or the second marker on the combined image is negligible.

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.

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

The extracted data may depend on the asymmetry of the combined image. The asymmetry may depend on a deviating centroid of one or more features of the combined image. The asymmetric combined image may represent an alignment error. The alignment error may be a non-zero alignment distance between the first marker and the second marker. The alignment error may correspond to an amount of asymmetry in 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 as it represents a fast and relatively easy method for determining alignment errors.

The method may further 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 the identity of perfect alignment. A phase shift not equal to zero may indicate a misalignment. A lower intensity at zero phase shift may indicate misregistration.

The method may further include scanning the first marker and the second marker relative to each other such that the combined image is scanned relative to the third plane. Scanning may advantageously allow the first indicia to be aligned with the second indicia rather than with a portion of the image forming device.

The scanning may comprise a simultaneous scanning of the first and second identifiers. That is, the first marker and the second marker may be scanned at a rate such that no relative movement occurs between the image of the first marker and the image of the second marker. I.e. the two identified images are static with respect 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. The simultaneous scanning may produce a combined image scanned in a third plane.

When used with an apparatus such as a lithographic apparatus, the scanning may comprise a scanning movement associated with a normal lithographic exposure.

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

The use of a detection grating may advantageously allow the collection of alignment data with a relatively easy implementation. By scanning the combined image over the detection grating in the third plane, a modulated combined image may be produced on a detector arranged, for example, on the opposite side of the detection grating. The input plane of the detector may be referred to as the detector plane. The modulation combined image may be an intensity oscillation at the detector plane. The oscillation in intensity may be due to the scanning of the combined image relative to one or more detection gratings. The image may be raster scanned relative to the detection. That is, the image may be scanned over the detection raster. Alternatively or additionally, the detection gratings may be shifted in a third plane so that they scan relative to the combined image.

According to a fifth aspect of the present invention there is provided an apparatus manufactured using an apparatus according to any one of the first, second or third aspects of the present invention or using a method according to the fourth aspect of the present invention.

Drawings

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

FIG. 1 depicts a schematic overview of a lithographic apparatus according to an embodiment of the invention;

FIG. 2 depicts a schematic overview of an image forming apparatus part of a lithographic apparatus according to an embodiment of the invention;

figures 3 and 4 depict example images formed by markers in an image forming device;

fig. 5 depicts a schematic overview of an alignment system according to an embodiment of the invention;

figure 6 illustrates an example modulation signal for extracting information from a combined image;

fig. 7 depicts a schematic overview of another alignment system according to an embodiment of the invention; and

FIGS. 8A and 8B depict example alignment marks and detection gratings according to embodiments of the present invention.

Detailed Description

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

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

The term "matched" may be used with respect to a diffraction grating. The match can be interpreted as: both may be equal; or may be scaled so that they are equal at the relevant part of the device. For example, the first and second gratings in the first and second planes may be matched separately, with the second grating scaled relative to the first grating, and the scaling being related to a magnification or reduction factor between the first and second planes, e.g. due to a magnification applied by an optical element in the device.

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

For the purpose of elucidating the invention, a cartesian coordinate system is used. The cartesian coordinate system has three axes, namely, an x-axis, a y-axis, and a z-axis. Each of the three axes is orthogonal to the other two axes. Rotation about the x-axis is referred to as Rx rotation. Rotation about the y-axis is referred to as Ry rotation. The rotation around the z-axis is called Rz rotation. The x-axis and y-axis define a horizontal plane (referred to as the xy-plane) while the z-axis is in the vertical direction. The cartesian coordinate system is not limiting to the invention and is used for illustration only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to illustrate the invention. The cartesian coordinate system may be oriented differently, for example such that the z-axis has a component along the horizontal plane.

FIG. 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus may be considered to comprise 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 an imaging apparatus.

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 table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters; a substrate support (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters; and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W. The patterning device MA may comprise a pattern, for example a pattern, to be printed in a lithographic exposure and/or alignment mark.

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

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

Although mirrors and/or lenses may be specifically referenced herein, any suitable optical element may be used. For example, an element may be refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and/or electrostatic or any combination thereof. The elements may be gratings, beam cubes, or any other elements as desired. In some cases, the mirror may be a beam splitter, such as a dichroic mirror, a half-silvered mirror, or any other beam splitting element known in the art.

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

The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual stage"). In such "multiple stage" machines the substrate supports WT may be used in parallel, and/or steps in preparation for subsequent exposure of the substrate W may be performed on a substrate W positioned on one of the substrate supports WT while another substrate W on the other substrate support WT is used to expose a pattern on the other substrate W.

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

With the aid of the second positioner PW and position measurement system PMS, the substrate support WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B at focus and alignment positions. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in fig. 1) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B.

The radiation beam B is typically directed 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 hence the radiation beam B. This movement may be referred to as scanning. The mask support and the substrate support may be referred to as support members. Scanning may enable different portions of the mask MA and the substrate W to be scanned through the radiation beam B without moving the radiation beam B itself. The mask support MT and the mask MA may be scanned in a first direction (e.g. along the y-axis). The substrate table WT and substrate W can then be scanned in the opposite direction along the y-axis. The speed of movement of the substrate support WT and the mask support MT may be matched. For example, when a magnification factor is not applied by the projection system PS, the velocities of the mask support MT and the substrate support WT may be equal. Alternatively, due to any magnification imposed by the projection system PS, the speeds of the supports may be scaled such that they move at a comparable speed throughout the radiation beam B given the magnification change. Typically, the velocities of the substrate support WT and the mask support MT are matched such that an aerial image of the mask MA formed in the plane of the substrate W is moved 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, corresponding parts of the mask MA and substrate W pass through the optical axis 10 simultaneously. This type of scanning may be referred to as synchronous scanning.

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 though the mask MA is moved through the radiation beam. In other words, synchronous scanning enables a moving object in the plane of the mask MA to be aligned with a moving object in the plane of the substrate W. For imaging operations, 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.

A perfect alignment may 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) simultaneously. Imperfect alignment may be represented by deviations between objects. Imperfect alignment may be represented by two objects in two conjugate planes, in particular 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 a different time. Imperfect alignment may be represented by an alignment distance. The alignment distance may be a measure of the deviation between two objects, which may be measured relative to a reference position. The alignment distance may be related to a 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 may represent a misalignment between the plurality of projection images.

Imperfect alignment may be addressed or reduced by modifying elements of the imaging device. For example, one or both of the mask or substrate support MT, WT may be moved. Alternatively or additionally, elements in the projection system may be moved such that the optical axis 10 is altered. It may be beneficial to monitor the alignment so that such modifications can be applied. The monitoring and correction of the alignment may be referred to simply as alignment.

Patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks W1, W2. The alignment marks may also be referred to as marks, markings or alignment marks. Although the substrate alignment marks W1, W2 schematically illustrated in FIG. 1 occupy dedicated target portions, they may be located in spaces between target portions. When substrate alignment marks W1, W2 are located between target portions C, these are referred to as scribe-lane alignment marks.

The imaging device of the depicted lithographic apparatus LA is provided with an alignment system AS. An 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 in a lithographic operation.

The lithographic apparatus or other similar imaging apparatus may 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 may also be operated in an alignment mode whereby alignment marks on the first and second planes may be used to ensure that objects on the first plane are correctly aligned with desired locations on the second plane. The imaging mode and the alignment mode may be considered as a first mode and a second mode. Both the first mode and the second mode use a radiation beam. The radiation beam may have a first wavelength in the first mode and a second wavelength in the second mode, wherein the first wavelength and the second wavelength may be the same or may be different. In these first and second modes, the radiation beam B may be generated from the same source, or may have a different source for each mode.

In the embodiment depicted in fig. 1, 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, e.g. 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 apparatus AS, e.g. during alignment operations. The optical element TM may be movable such that it may be positioned in the radiation beam B during the alignment mode and removed from the radiation beam B during the imaging mode.

The optical element TM may be one of various types of optical elements, such as a beam splitting element such as a dichroic mirror, a half silvered mirror, a two-way mirror (also known as a one-way mirror, semi-transparent mirror, or partially transmissive/reflective mirror), a beam cube, or any other optical element or any combination thereof known in the art, depending on the radiation used. The optical element TM can be located at a different position than depicted in FIG. 1, for example, between the mask MA and the projection system PS, or within the illumination system IL.

An imaging device having an alignment system AS will now be described with reference to fig. 2. For 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 that measures or processes an object.

Fig. 2 is a schematic block diagram depicting an imaging apparatus according to an embodiment of the present invention. The 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 illumination optics for directing (and optionally conditioning) the radiation beam B. The imaging device comprises a first plane 20, a second plane 22, a projection system PS arranged between the first plane 20 and the second plane 22, and an alignment system AS. The depicted imaging device also includes two support structures configured to support the article in the first plane 20 and the second plane 22. These can be, for example, the mask support MT which supports the mask MA in the first plane 20 and the substrate support WT which supports the substrate W in the second plane 22.

The radiation beam B is directed to a first plane 20 and the projection system PS directs radiation between the first plane 20 and a second plane 22. The projection system PS may be configured such 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 may be imaged onto the second plane 22. Objects in the second plane 22 may be imaged onto the first plane 20. It should be noted that in fig. 2, the two

planes

20, 22 are parallel to the xy-plane. However, it should be understood that this is illustrative and that the planes may be oriented differently.

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 bi-directional 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.

The imaging device may be provided with alignment markers. One or more first markers M1 can be supported in the first plane 20, for example marker M1 can be associated with the mask MA or mask support MT and can optionally be a fiducial marker. One or more second markers W1 may be supported in the second plane 22, for example marker W1 may be associated with the substrate W or the substrate support WT. In this example, first marker M1 is transmissive and second marker W2 is reflective, but it should be understood that different marker characteristics may be more appropriate with different device geometries. In use, during alignment measurements, markers M1, W1 may be arranged such that, when illuminated by radiation beam B, an image of first marker M1 is formed on second marker W1, and a combined image of first marker M1 and second marker W1 is formed on first marker M1.

The term "combined image" should be understood as the image of first marker M1 overlapping the image of second marker W1 to form a single image. Thus, the combined image may contain information about the first marker M1 and the second marker W1, and the alignment distance between the first marker and the second marker may be determined from the combined image based on the characteristics of the combined image.

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 marker M1 and the image of the second marker W1. In practice, there may be small variations in size and shape (e.g. due to imperfect synchronous scanning resulting in relative movements between the images of the first marker M1 and the second marker W1), but these small movements may be considered negligible compared to the size of the markers M1, W1. Thus, the combined image may be understood to be generally static in size and shape. If the markers M1, W1 include one or more patterns, the combined image may be understood to have a corresponding pattern that is also generally static. Although generally static in size and/or shape and/or pattern, the combined image may be moved over a plane, for example it may be scanned over a plane, as described in more detail below.

In general, 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 the embodiment of the present invention will be described with reference to a single first marker M1 and a single second marker W1.

In operation, radiation beam B passes through first marker M1. Radiation beam B may be patterned by first mark M1 and form 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 directs the patterned radiation beam PB such that an image of the first mark is formed in the second plane 22. Patterned radiation beam PB reflects from second marking W1. Patterned radiation beam PB is further patterned by second marking W1 and may also be referred to as a patterned radiation beam. At this stage, patterned radiation beam PB may contain information about first mark M1 and second mark W1.

The patterned beam of radiation containing information about first and second marks M1 and W1 is then directed to a third plane where it forms a combined image of the first and second marks. The combined image may be directed to the third plane by the projection system PS and/or other elements of the apparatus. Thus, the third plane may be conjugate to the first plane and the second plane. In the depicted embodiment, the third plane is the same as the first plane 20. The additional optics may refocus the patterned radiation beam PB such that, alternatively or additionally, a combined image is formed at another plane (which is separate from the first plane 20), for example the 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.

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

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

The first and second support members MT, WT are configured such that they can move in their

respective planes

20, 22, for example using the synchronous scanning described above. The scanning may be performed by a support scanning mechanism (not shown). The support scanning mechanism may scan the first marker and the second marker at a rate such that no relative movement occurs between the image of the first marker and the image of the second marker. I.e. the two identified images are static with respect to each other within the radiation beam. Although the images of the first marker M1 and the second marker W1 are static with respect to each other, the combined image may move with respect to the third plane due to the synchronous scanning. Since the first and second markers are static with respect to each other, the combined image does not change size or shape when scanned across the third plane.

An example image according to an embodiment of the present invention is shown in fig. 3. The markings M1, W1 associated with fig. 3 comprise diffraction gratings. That is, markers M1, W1 include periodic structures of high and low transmission/reflection. In this embodiment, marker M1 includes a transmission grating, and second marker W1 includes a reflection grating. The marks have a grating period (also called grating pitch), defined as the distance between successive periodic structures. The mark has a grating duty cycle, defined as the width of the periodic structure. Markers M1, W1 may form images in one or more field planes, where the resulting images are a series of periodic high and low intensity gratings. The image may have a corresponding period that may match the grating period. The image may have a corresponding duty cycle that may match the grating duty cycle. Markers M1, W1 may diffract radiation beam B into a diffraction pattern such that the diffraction pattern may be formed in one or more pupil planes in the device.

The first image 30 represents the transmittance profile of the first marking M1. I.e. it represents the intensity of patterned radiation beam PB after interaction with first mark M1. 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 the intensity variation of the images, for example the intensity in the y-direction.

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

The transmitted first image 32 represents the intensity of the patterned radiation beam PB after interacting with the first marker M1 and then travelling through the projection system PS. 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 the intensity variation of the images, for example the intensity in the y-direction. It should be noted that the transmitted first image 32 includes a portion of the oscillating intensity. This is due to the limited 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).

The second image 33 represents the reflectivity profile of the second indicia. I.e. it represents the intensity of patterned radiation beam PB after interaction with second mark W1 (i.e. if radiation beam B was delivered to second mark W1 before being patterned by first mark 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 the intensity variation of the images, for example the intensity in the y-direction.

The second image 33 has a second pitch P2, a second duty cycle DC2, a second deviation O2 and a second maximum MX 1. In this embodiment, the first and second images have matching pitches (i.e. P1 ≠ P2, taking into account any reduction or magnification factor imposed by the projection system PS), but the duty cycle (DC1 ≠ DC2), the deviation (O1 ≠ O2), and the maximum value (MX1 ≠ MX2) are different.

The third image 34 is a representation of the combined image. I.e. it is a representation of the intensity of patterned radiation beam B after interaction with first mark M1 and second mark W1. 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 the intensity variation of the images, for example 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 the third image 34 is a representation of a combined image in one dimension (along the x-axis) in fig. 3. It should be understood that the x-axis 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 axis 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.

It should be noted that the third image 34 comprises a portion of the oscillation intensity due to the limited numerical aperture of the projection system PS described above. The shape of the third image 34 contains information about the first image 30 and the second image 33 and thus the first alignment marker M1 and the second alignment marker W1, e.g. their relative positions, as now discussed.

In this particular embodiment, the high intensity individual grills within the third image 34 have a stair-step/shoulder configuration. This is due to the difference in shape of the first marker M1 and the second marker W1 (in particular the difference in duty cycle) and thus the difference in shape of the first image 30 and the second image 33 (in particular the difference in duty cycle DC1, DC 2). To better illustrate the stepped configuration, fig. 3 indicates the head 36 and shoulder 38 of the grate. The width of the shoulder 38 may depend on the difference in duty cycle between the first markers M1, W1.

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

The markers M1, W1, and combined image 34 are centered on the intended alignment position 31. In fig. 3, the intended alignment position 31 is represented along the x-axis, but it should be understood that the intended alignment position may be defined in any coordinate system, e.g. it may additionally or alternatively be defined in the y-direction. The expected alignment position 31 represents a reference position in which the first marker M1 and the second marker W1 are expected or expected to be aligned (e.g., by being centered at the position). If either of the first marker M1 or the second marker W1 is not properly aligned, their corresponding images may be centered at a location different from the expected alignment position 31. Due to any such misalignment, the combined image 34 may be asymmetric due to the difference in the positions of the first marker M1 and the second marker W1.

By way of 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 with respect to each other, but are not imaged to the intended location.

Fig. 4 shows a portion of an example combined image 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 asymmetric. To quantify the asymmetry, a 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 measured from the center of the combined image. In this example, the center may be defined as the middle between the distal edge of the first shoulder 38a and the distal edge of the second shoulder 38 b. 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.

It can be seen that when the markers M1, W1 are aligned, and thus the combined image is a symmetric combined image 40, the first and second head distances are equal (C ═ D), and the first and second shoulder distances are equal (E ═ F). However, when the markers M1, W1 are misaligned, the combined image is an asymmetric combined image 42, and the first and second shoulder distances are equal (E ═ F), but the first and second head distances 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. Thus, by extracting data about the asymmetry of the combined image 42, the alignment distance between the first marker and the second marker may be determined.

As described above, different types of alignment marks may be used. It should be understood that different types of alignment marks may generate different shapes of combined images. In such instances, the alignment distance may still be determined by evaluating the asymmetry of the combined image.

The formation of the combined image is advantageous because it allows the first marking to be aligned relative to the second marking. Alternative image forming apparatuses (e.g. lithographic apparatuses) use an alignment system that aligns an alignment mark with a part of the apparatus. Alignment with a portion of a device can reduce accuracy, for example if the location of a portion of the device is miscalibrated. For example, a first known image forming apparatus uses an image of a first marker (e.g., on a wafer) and scans it relative to a second (static) marker (e.g., on a reticle), where the second marker forms a detection grating. The known apparatus thus determines the alignment of the first marker relative to the stationary part of the lithographic apparatus. In a second known image forming apparatus, an alignment mark is used on a first plane and an alignment mark is used on a second plane, and separate images of the first and second alignment marks are formed and directed to two separate detectors. Wherein the image recognition may be used to determine the relative position of the two separate images with respect to a part of the image forming apparatus and thereby determine the relative position of the first alignment mark and the second alignment mark.

In the first known image forming apparatus, there is relative movement between the images of the first and second markers (which serve as detection gratings), and therefore a combined image cannot be formed. In the second known image forming apparatus, the individual images of the first marker and the second marker are formed using different parts of the imaging system (or even separate imaging systems), and thus a combined image is not formed. However, the image forming apparatus according to the present invention can form a combined image because there is no relative movement between the image of the first marker M1 and the image of the second marker W1. Therefore, due to the formation of the combined image, the relative positions of the first alignment mark and the second alignment mark can be determined regardless of the position of the image forming apparatus.

It may be beneficial to provide an alignment marker 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 the pattern in an 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. Therefore, it can be said that the imaging operation has the first optical axis. The image forming apparatus when performing an image forming operation may be referred to as operating in an image forming mode.

The alignment procedure may have a second optical axis. The image forming apparatus when performing the aligning operation may be referred to as operating in the aligning mode. Alignment may be less accurate if alignment and imaging are performed by different parts of the device. For example, any effects such as heating that occur during the imaging operation will similarly be present in the alignment operation.

It may be beneficial to have the alignment mode and the imaging mode occur through the same or similar regions of the imaging device. That is, it may be beneficial for the image forming device to direct the beam of radiation 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, e.g. the radiation beam may travel through the same part of the lens in both modes. This may improve the accuracy of alignment and/or imaging.

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

Fig. 5-7 depict example alignment systems AS and methods that may be used to extract data from a combined image to determine an alignment distance. In general, in these examples, the alignment system AS includes a modulation system that can be used to modulate the combined image to form a modulated combined image and a detector that can be used to extract data from the modulated combined image.

Fig. 5 depicts an alignment system AS including a self-referencing interferometer (SRI) 52. The use of self-referencing interferometers to extract positional information of input identifications is described in US 69661116 and US9606442, both of which are incorporated herein by reference. The SRI 52 receives the combined image at the 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 split into diffraction orders (e.g. in the pupil plane of the SRI). SRI 52 applies a relative rotation of 180 degrees between the positive and negative diffraction orders, and the two are recombined to coherently add. The relative rotation can be any rotation that results in a 180 degree difference between the positive and negative orders, for example a 90 degree rotation of the positive order and a-90 degree rotation of the negative order.

The combined image is scanned relative to the input plane 50, for example as a result of a simultaneous scanning of the first marker M1 and the second marker W1, as described above. The combined image may be scanned in a scan direction S. The scanning direction may be, for example, the x-direction. Due to the applied rotation, the positive and negative diffraction orders are scanned in opposite directions (e.g., in opposite directions parallel to the scanning direction S) in the plane of the detector 54. The modulated combined image is generated as positive and negative levels that are scanned relative to each other (moving in and out of phase with each other). The modulated combined image may be intensity modulated, for example, when the positive and negative stages are moved in and out of phase with each other.

Additional optical elements, such as a lens 56, may be provided to shape and/or direct the radiation beam. Typically, the alignment system AS is configured such 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 effecting 180 degrees relative rotation between the positive and negative diffraction orders.

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 additionally, the scanning may be achieved by a synchronous scanning of the support members as 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 the third plane or may be a plane conjugate to the third plane, i.e. may be the fourth plane.

The separation into diffraction orders can be considered a fourier decomposition. There are positive and negative diffraction orders (e.g., +1, +2, +3, and-1, -2, -3), which may 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 portions (e.g., +1 and-1) of the same numbered diffraction orders. 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 a fourier transform plane of the input plane 50 (which may be referred to as a pupil plane).

The modulated combined image may be communicated to a detector 54. The detector 54 may comprise an array of sensing elements (which may each define a pixel of the detector 54), such as a CCD or CMOS detector, as schematically illustrated in fig. 5. Alternatively, the detector may comprise a single detector, such as a photodiode. If a single detector is used, additional scanning mechanisms may be used to sequentially direct different portions of the modulated combined image onto the single detector.

The modulated combined image may be converted to an electrical signal. This signal may be referred to as a modulated signal, such as a modulation of the intensity of the electrical signal as the modulated combined image is scanned along an axis (e.g., the x-axis). The modulation signal can be analyzed to extract data and determine the alignment distance.

Some example signals are shown in fig. 6, which shows the modulation signal along the scan direction S (i.e., when the combined image is scanned across the input plane 50 along the scan direction S). An example of the signal contribution of one fourier harmonic (e.g., +1 and-1 diffraction orders) is shown. An example of a signal contribution from the sum of

negative diffraction orders

60a, 62a,

positive diffraction orders

60b, 62b, and positive and

negative orders

60c, 62c modulated by the SRI (e.g., the signal formed in the plane of detector 54) is shown. The vertical dashed lines are provided as a guide for the eyes to highlight the phase difference between the signals.

When markers M1, W1 are aligned,

alignment signals

60a, 60b, 60c may be formed. When the markers M1, W1 are misaligned, a

misalignment signal

62a, 62b, 62c may be formed. It can be seen that in the aligned condition, the signals of the positive and

negative stages

60a, 60b are aligned, i.e. there is no phase difference between them. When added, the positive and negative steps may form a modulated signal 60c centered at the desired 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, since it is the maximum amplitude in the case of the alignment markers M1, W1.

However, the signals of the positive and

negative stages

62a, 62b have different phases for the misaligned markers M1, W1. That is, the signals for the positive and

negative stages

62a, 62b are centered around a position that is not aligned with the position 31. This may be referred to as phase shifting. Additionally, as mentioned above, if the image forming device is not properly aligned, there may be some relative movement between the first marker M1 and the second marker W1, which may result in a slight change in the phase shift of the fourier component of the signal.

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

The signal shown in fig. 6 is an example signal of a single fourier harmonic, specifically the first diffraction order (i.e., +1 and-1 diffracted beams). The data on the alignment distance can advantageously be extracted from a single fourier harmonic of the modulated combined image. Thus, it may be desirable to separate the combined image/modulation signal into separate fourier components. This may be performed electronically, for example by analyzing the modulated signal using a computational process.

Alternatively or additionally, the separation of the one or more fourier components may be performed using optical filtering in the image forming device, for example on the combined image or the 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 the transmission of one or more diffraction orders through the pupil plane.

The use of spatial filtering to remove diffraction orders can advantageously improve the signal-to-noise ratio and thus the accuracy of the alignment operation. The use of spatial filtering may also reduce the computational load compared to electronic filtering, since the modulated combined signal can be directly analyzed without applying any further filtering.

The zeroth order diffracted beam (i.e., the beam corresponding to reflection or transmission) may be removed. This may be beneficial to improve accuracy. This may also be beneficial in reducing stray light within the system, which may reduce accuracy and/or may damage components of the apparatus. Higher diffraction orders can be removed. It may be particularly advantageous to remove all diffraction orders except the first diffraction order. Advantageously, the self-referencing interferometer may be used to modulate multiple fourier stages or a single fourier stage.

Different levels of analysis may be beneficial for different alignment applications. For example, a 1 st order (+1 and-1) may benefit the alignment of surface features on the substrate, while a 4 th order (+4 and-4) may provide more accurate alignment of features on the top-coated substrate.

Similarly, different grating pitch sizes may be beneficial for different alignment applications. Advantageously, a diffraction grating of any pitch size may be used in combination with an alignment system comprising a self-referencing interferometer. The use of any pitch dimension may allow more freedom in designing the imaging device. The use of any pitch size may result in improved accuracy.

In some embodiments, the first alignment mark and the second alignment mark may be selected to include diffraction gratings having matching periods. That is, the combined image has a period matching the first alignment mark and the second alignment mark. The diffraction gratings may be of various types, for example holographic, rectilinear or stepped and reflective or transmissive.

An alternative embodiment for extracting data from a combined image is shown in fig. 7. AS seen in this embodiment, the alignment system AS includes two

detectors

54a, 54b, two

detection gratings

70a, 70b, an optical element 74 and a mirror 72. The optical element 74 is arranged to split the patterned radiation beam PB into two portions. A first portion of the patterned radiation beam PB is directed to a first detection grating 70a arranged in front of the first detector 54a, and a second portion of the patterned radiation beam PB is directed to a second detection grating 70b arranged in front of the second detector 54 b. The

detection gratings

70a, 70b may be considered to be located in a detection plane 76 of the alignment system AS, where the detection plane 76 illustrated in fig. 7 is conjugate to a third plane in which the combined image is formed. Additional optical elements, such as a 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. A single detection plane 76 is illustrated in fig. 7, but it should be understood that the

detection gratings

70a, 70b may be arranged in different planes, i.e. there may be multiple detection planes.

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 realized by synchronous scanning of the support members as described above. Scanning the combined image on the detection plane 76 enables the combined image to be scanned over the

detection gratings

70a, 70b and thus modulated by the

detection gratings

70a, 70 b.

The modulation may be an intensity modulation as the combined image is scanned relative to the

detection gratings

70a, 70 b. That is, scanning the combined image relative to the

detection gratings

70a, 70b provides oscillation of the transmitted intensity through the

detection gratings

70a, 70 b. Two modulated combined images associated with the first and second portions of the patterned beam of radiation are formed. The

detectors

54a, 54b may then detect the modulated combined image. The detector may convert the modulated combined image to a modulated signal. Additionally, in this embodiment, one or more fourier components may be removed from the modulation combined image and/or modulation signal(s), for example using one of the methods described above. The alignment distance may be determined based on asymmetry of one or more of the modulated combined images.

The alignment markers M1, W1 may be selected to include diffraction gratings with a period that matches the period of the

detection gratings

70a, 70 b. The

detection gratings

70a, 70b may comprise gratings oriented at a non-zero angle to each other in the same plane. The

detection gratings

70a, 70b may comprise gratings that are oriented at a non-zero angle with respect to the scan direction (e.g., the y-direction). Having the

detection gratings

70a, 70b oriented at a non-zero angle with respect to the scan direction may allow the alignment distance to be determined along multiple axes, e.g., after scanning in only the y-direction, the alignment distance may be calculated in the x-direction and the y-direction. The x and y measurement method using this detection grating is described in US2009/195768A, which is incorporated herein by reference.

In a specific example, as schematically illustrated in fig. 8A, there may be a plurality of first alignment markers M1, M2, M3 forming an identification array. Each alignment mark may include a first grating and a second grating oriented at 90 degrees to each other. The first and second gratings are oriented at +45 degrees and-45 degrees, respectively, with respect to a scan direction S, which may be, for example, in the y-direction. The second alignment marker (not shown) may also comprise a corresponding array of markers, 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).

The identification array may be beneficial for known image forming apparatuses, such as lithographic apparatuses. In a lithographic operation, radiation may be directed onto a portion of the substrate W. The region through which the radiation travels (the region corresponding to the substrate W irradiated by the radiation) may be referred to as a slit. The array of markers may be arranged such that they are disposed in a plurality of locations on the slit. Arranging the array of markers in a plurality of locations on the slit may advantageously enable alignment to be measured and/or monitored at a plurality of locations on the slit, which may therefore improve imaging accuracy over a large portion of the slit.

Three identified arrays are used in this example. However, the tag array may be any number of tags, such as eight tags or fifty tags. The alignment marks on the first and second planes may be arranged such that the image of the first array of marks will perfectly overlap the corresponding array of second marks. Using multiple markers may improve accuracy because more alignment points are used for alignment. Irradiating multiple alignment marks simultaneously may result in reduced alignment time and thus increased productivity of the alignment process. That is, the amount of time required to measure alignment may be reduced. Reduced alignment time may result in increased overall productivity of the imaging process.

As schematically illustrated in fig. 8B, the first detection grating 80a and the second detection grating 80B may comprise detection gratings oriented at 90 degrees to each other. The first detection grating 80a may comprise a grating with lines oriented at +45 degrees with respect to the scanning direction S, and the second detection grating 80b may comprise a grating with lines oriented at-45 degrees with respect to the scanning direction S. The scanning direction S may be, for example, in the y direction. 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 and second marker arrays relative to these detection gratings 80a, 80 b.

When the first markers M1, M2, M3 and the second markers W1, W2, W3 are scanned synchronously, a modulated combined image may 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 may convert the modulated combined image into a modulated signal. The modulation signal can then be modeled to extract the alignment position. Similar to the above example, a misalignment between the first markers M1, M2, M3 and the second markers W1, W2, W3 may result in a phase shift of one or more of the modulation signals, and the alignment distance may then be determined.

The image forming apparatus according to the present invention can provide a faster method of determining the alignment distance. Alternative alignment systems may require 10 to 20 minutes to perform the alignment operation of the substrate. Using the combined image to determine the alignment distance may enable the alignment operation to be performed faster, e.g., within about 10 to 20 seconds per substrate, in accordance with the present invention. Due to the increase in the alignment speed, the alignment operation according to the present invention can be performed during the imaging operation or between subsequent imaging operations.

In another known alignment method, a full resist exposure and development cycle may be performed. That is, a photolithographic wafer covered in photoresist can be exposed to patterned radiation. The resist can then be developed and removed from the development cycle. Information about the alignment may then be extracted. However, this complete process can be a very time consuming activity. The invention detailed above allows real-time alignment information without the need for this development cycle.

According to the embodiments of the present invention, determining the alignment distance using the combined image represents the direct position accuracy of the image forming apparatus, and may enable the alignment operation to be performed more accurately. For example, in a lithographic apparatus including the image forming apparatus described above, the exposure pattern of the mask may be aligned with greater positional accuracy relative to a corresponding pattern on the substrate than other currently known alignment sensors.

The above-described image forming apparatus may also be versatile for many applications, since the design of the alignment marks and thus the characteristics of the combined image may be optimized for a variety of arrangements. Furthermore, although the combined image may be beneficial for calculating the relative alignment between the first marker and the second marker, the device may be generic and may be used to determine the alignment of one marker relative to a part of the device. For example, the alignment operation may be performed using only the identification on the first plane or only the identification on the second plane to determine the alignment of the identification with the device.

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

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative, and not restrictive. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. An image forming apparatus includes

An illumination system operable to direct a beam of radiation 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.

2. The image forming system according to claim 1, wherein the data extracted depends on asymmetry of the combined image.

3. The image forming system according to claim 1 or 2, wherein the alignment system includes:

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 an alignment distance between the first marker and the second marker.

4. The image forming apparatus according to claim 3, wherein the modulation system includes a self-referencing interferometer and an image scanning mechanism configured to scan the combined image on the third plane.

5. The image forming apparatus according to claim 3 or 4, wherein the detector is operable to extract phase shift data from the modulated combined image.

6. The image forming apparatus according to claim 3, wherein the modulation system includes: one or more detection gratings in the 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.

7. The image forming apparatus according to any of claims 3 to 6, 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.

8. The image forming apparatus according to any one of the preceding claims, wherein the first mark and the second mark comprise diffraction gratings.

9. The image forming apparatus according to any one of the preceding claims, wherein the first mark and the second mark have matching diffraction grating periods.

10. An image forming apparatus according to any preceding claim, wherein the illumination system is operable to illuminate a first array of markers in the first plane simultaneously, and the projection system is configured to form images of the first array of markers onto a corresponding second array of markers and to form a plurality of combined images of each pair of corresponding first and second markers in the third plane.

11. The image forming apparatus according to any of the preceding claims, wherein the image forming apparatus is operable in an imaging mode and an alignment mode, and wherein the radiation beam is directed through substantially the same portion of the projection system when operating in the imaging mode or the alignment mode.

12. A metrology apparatus comprising the apparatus of any preceding claim.

13. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, the lithographic apparatus comprising the image forming apparatus of any preceding claim, wherein the first support structure is operable to support the patterning device in the first plane and the second support structure is operable to support the substrate in the second plane.

14. 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 beam of radiation onto the first plane of the imaging device;

forming a combined image of the first marker and the second marker;

directing the combined image to an alignment system;

extracting data from the combined image to determine the alignment distance.

15. The method of claim 14, wherein the data extracted depends on an asymmetry of the combined image.

16. The method of any of claims 14 to 15, further comprising: the first indicia and the second indicia are scanned relative to each other such that the combined image is scanned relative to a third plane.

17. The method of claim 16, further comprising: simultaneously illuminating a first array of markers in the first plane and forming an image of the first array of markers onto a corresponding second array of markers, and forming a plurality of combined images of each corresponding pair of first and second markers in the third plane.

18. The method of any of claims 14 to 17, further comprising: the combined image is modulated using a self-referencing interferometer or one or more detection gratings.