NL2023669A

NL2023669A - Apparatus and method for measuring a position of alignment marks

Info

Publication number: NL2023669A
Application number: NL2023669A
Authority: NL
Inventors: Viacheslavovich Voznyi Oleg
Original assignee: Asml Netherlands Bv
Priority date: 2018-08-20
Filing date: 2019-08-20
Publication date: 2020-02-26
Also published as: CN112639623A; CN112639623B; WO2020038629A1

Abstract

An apparatus for measuring a position of each of a plurality of alignment marks on a substrate comprising: an illumination system configured to direct a radiation beam from a radiation source onto the plurality of alignment marks on the substrate, a projection system configured to project images of the plurality of alignment marks from the substrate, the images of the plurality of alignment marks being produced by diffraction of the radiation beam from the plurality of alignment marks; an optical block configured to modulate the images of the plurality of alignment marks projected from the substrate, and Wherein the optical block is configured to project the modulated images of the plurality of alignment marks onto a sensing element to produce signals from Which the position of each of the plurality of alignment marks is determined in parallel.

Description

FIELD [0001] The present invention relates to an apparatus and method for measuring a position of alignment marks.

BACKGROUND [0002] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “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).

[0003] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law’. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0004] In the manufacture of complex devices, typically many lithographic patterning steps are performed, thereby forming functional features in successive layers on the substrate. A critical aspect of performance of the lithographic apparatus is therefore the ability to place the applied pattern correctly and accurately in relation to features laid down in previous layers (by the same apparatus or a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks. Each mark is a structure whose position can be measured at a later time using a position sensor, typically an optical position sensor. The position sensor may be referred to as “alignment sensor” and marks may be referred to as “alignment marks”.

[0005] A lithographic apparatus may include one or more (e.g. a plurality of) alignment sensors by which positions of alignment marks provided on a substrate can be measured accurately. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in current lithographic apparatus is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications of the position sensor have been developed, for example as disclosed in US2015261097A1. Alignment sensors based on image sensors are also known and can be applied for this invention. The contents of all of these publications are incorporated herein by reference. [0006] A mark, or alignment mark, may comprise a series of bars formed on or in a layer provided on the substrate or formed (directly) in the substrate. The bars may be regularly spaced and act as grating lines so that the mark can be regarded as a diffraction grating with a well-known spatial period (pitch). Depending on the orientation of these grating lines, a mark may be designed to allow measurement of a position along the X axis, or along the Y axis (which is oriented substantially perpendicular to the X axis). A mark comprising bars that are arranged at +45 degrees and/or -45 degrees with respect to both the X- and Y-axes allows for a combined X- and Y- measurement using techniques as described in US2009/195768A, which is incorporated by reference.

[0007] The alignment sensor scans each mark optically with a spot of radiation to obtain a periodically varying signal, such as a sine wave. The phase of this signal is analyzed, to determine the position of the mark and. hence, of the substrate relative to the alignment sensor, which, in turn, is fixated relative to a reference frame of a lithographic apparatus. So-called coarse and fine marks may be provided, related to different (coarse and fine) mark dimensions, so that the alignment sensor can distinguish between different cycles of the periodic signal, as well as the exact position (phase) within a cycle. Marks of different pitches may also be used for this purpose.

[0008] Measuring the position of the marks may also provide information on a deformation of the substrate on which the marks are provided, for example in the form of a wafer grid. Deformation of the substrate may occur by, for example, electrostatic clamping of the substrate to the substrate table and/or heating of the substrate when the substrate is exposed to radiation.

[0009] It is desirable to provide an apparatus and method for measuring a position of alignment marks which overcomes or mitigates one or more problems associated with the prior art.

SUMMARY [00010] According to a first aspect of the present invention, there is provided an apparatus for measuring a position of each of a plurality of alignment marks on a substrate comprising: an illumination system configured to direct a radiation beam onto the plurality of alignment marks on the substrate, a projection system configured to project images of the plurality of alignment marks from the substrate, the images of the plurality of alignment marks being produced by diffraction of the radiation beam from the plurality of alignment marks; an optical block configured to modulate the images of the plurality of alignment marks projected from the substrate, and wherein the optical block is configured to project the modulated images of the plurality of alignment marks onto a sensing element configured to produce signals from which the position of each of the plurality of alignment marks is determined in parallel (i.e. substantially at the same time, or simultaneously).

[00011] This has an advantage that the position of the plurality of alignment marks can be measured in parallel (i.e. substantially simultaneously or substantially at the same time) and the plurality of alignment marks can be aligned in parallel. This leads to an improvement in overlay performance without an impact on the productivity of the lithographic apparatus. Furthermore, the alignment sensor can measure any alignment mark on the substrate W without any limitations on the distance between the alignment marks. In addition, no detection grating is required.

[00012] The projection system may be configured such that the images of the plurality of alignment marks are projected into the optical block simultaneously (parallel in time, or substantially at the same time) and the optical block may be configured to project the modulated images of the plurality of the alignment marks onto different parts of the sensing element simultaneously.

[00013] The apparatus may be configured such that the modulated images of the plurality of alignment marks are projected onto the sensing element sequentially.

[00014] The projection system may comprise an optical element, wherein the optical element is configured to direct the images of the plurality of alignment marks into the optical block sequentially.

[00015] The optical element may be a rotatable mirror, wherein the rotatable mirror may be configured to be rotated through a range of angles such that the images of the plurality of alignment marks are reflected by the rotatable mirror into the optical block sequentially.

[00016] The rotatable mirror may be configured such that the range of angles is such that all the images of the plurality of alignment marks of the illuminated alignment marks are reflected from the rotatable mirror into the optical block, [00017] The sensing element may comprise a plurality of pixels, wherein each of the plurality of pixels of the sensing element may be configured to transform a periodic intensity change caused by each of the plurality of alignment marks being scanned by the radiation beam into the signals, wherein the signals are independent signals for each of the plurality of alignment marks. An advantage of using a sending element with multiple pixels is that the modulated image of the alignment mark can be separated from other images around the alignment mark by selection of the appropriate pixels.

[00018] The optical block may be configured to project each of the modulated images of the plurality of the alignment marks onto more than one of the plurality of pixels of the sensing element and the sensing element may be configured to combine the signals from each of the plurality of pixels which the modulated image of the alignment mark is projected upon.

[00019] The sensing element may comprise a CCD sensor or a CMOS sensor.

[00020] The size of the sensing element may be substantially equal to the size of an exposure slit of a lithographic apparatus.

[00021] The sensing element may be configured to produce the signals such that they may be sampled to construct the independent signals for each of the plurality of alignment marks.

[00022] The sensing element may comprise a single pixel, wherein the single pixel may be configured to transform a periodic intensity change caused by the plurality of alignment marks being scanned into the signals such that the signals may be sampled to construct independent signals for each of the plurality of alignment marks. An advantage of using a single detector (with a single pixel) is that it doesn’t suffer with pixels signal drift which may occur with e.g. CCD detectors.

[00023] The projection system may further comprise a transfer system configured to transfer the radiation beam from a first pupil plane of the projection system to a second pupil plane, wherein the optical element is located in the second pupil plane.

[00024] The apparatus may comprise a radiation source configured to produce the radiation beam.

[00025] The apparatus may comprise a lens to direct the radiation beam onto the plurality of alignment marks.

[00026] The apparatus may comprise a lens to project the images of the plurality of alignment marks from the substrate.

[00027] The lens may be common to both the illumination system and the projection system.

[00028] The optical block may be a self-aligned interferometer.

[00029] According to a second aspect of the present invention, there is provided a metrology apparatus comprising the apparatus as described above.

[00030] According to a third aspect of the present 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 as described above.

[00031] According to a fourth aspect of the present invention, there is provided a method of measuring a position of each of a plurality of alignment marks on a substrate comprising: directing a radiation beam from a radiation source onto the plurality of alignment marks on the substrate using an illumination system; projecting images of the plurality of alignment marks from the substrate using a projection system, the images of the plurality of alignment marks being produced by diffraction of the radiation beam from the plurality of alignment marks; modulating the images of the plurality of alignment marks projected from the substrate in an optical block, and projecting the modulated images of the plurality of alignment marks onto a sensing element to produce signals from which the position of each of the plurality of alignment marks is determined in parallel.

[00032] The method may further comprise projecting the images of the plurality of alignment marks into the optical block simultaneously and projecting the modulated images of the plurality of the alignment marks onto different parts of the sensing element simultaneously.

[00033] The method may further comprise directing the modulated images of the plurality of alignment marks onto the sensing element sequentially.

BRIEF DESCRIPTION OF THE DRAWINGS [00034] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure I depicts a schematic overview of a lithographic apparatus in accordance with an embodiment of the invention;

Figure 2 depicts a schematic block diagram of a known alignment sensor AS;

Figure 3 depicts a schematic diagram of an alignment sensor in accordance with an embodiment of the invention;

Figure 4 depicts a schematic diagram of a sensing element of the alignment sensor in accordance with an embodiment of the invention;

Figure 5 depicts a graph of an electrical signal from a sensing of the alignment sensor in accordance with an embodiment of the invention;

Figure 6 depicts a schematic diagram of an alignment sensor in accordance with an embodiment of the invention;

Figure 7 depicts a schematic diagram of an alignment sensor in accordance with an embodiment of the invention;

Figure 8 depicts a flow diagram of the method for determining a position of alignment marks in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

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

[00035] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[00036] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as 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.

[00037] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to condition the radiation beamB to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[00038] The term “projection system” PS used herein should be broadly interpreted as encompassing various types 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.

[00039] 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 techniques is given in US6952253, which is incorporated herein by reference.

[00040] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and/or steps in preparation of a subsequent exposure of the substrate W may be earned out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

Ί [00041] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and/or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[00042] In operation, 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 (design layout) present on 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. With the aid of the second positioner PW and a 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 a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[00043] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axis, i.e., an x-axis, a y-axis and a z-axis. Each of the three axis is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y-axis is referred to as an Ry-rotation, A rotation around about the z-axis is referred to as an Rz-rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[00044] Figure 2 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as is described, for example, in US6961116, and which is incorporated by reference. The invention is not limited to this embodiment of an alignment sensor but may be applied to other types of alignment sensors as well. Radiation source RSO provides a beamRB of radiation of one or more wavelengths, which is diverted by diverting optics onto a mark, such as mark AM located on substrate W, as an illumination spot SP. In this example the diverting optics comprises a spot mirror SM and an objective lens OL. The illumination spot SP, by which the mark AM is illuminated, may be slightly smaller in diameter than the width of the mark itself.

[00045] Radiation diffracted by the mark AM is collimated (in this example via the objective lens OL) into an information-carrying beam IB. The term “diffracted” is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI, e.g. of the type disclosed in US6961116 mentioned above, interferes the beam IB with itself after which the beam is received by a photodetector PD. Additional optics (not shown) may be included to provide separate beams in case more than one wavelength is created by the radiation source RSO. The photodetector may be a single element, or it may comprise a number of pixels, if desired. The photodetector may comprise a sensor array.

[00046] The diverting optics, which in this example comprises the spot mirror SM, may also serve to block zero order radiation reflected from the mark, so that the information-carrying beam IB comprises only higher order diffracted radiation from the mark AM (this is not essential to the measurement, but improves signal to noise ratios).

[00047] Intensity signals SI are supplied to a processing unit PU. By a combination of optical processing in the block SRI and computational processing in the unit PU, values for X- and Y-position on the substrate relative to a reference frame are output.

[00048] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. Coarser measurement techniques are used in conjunction with this to identify which period of a sine w'ave is the one containing the marked position. The same process at coarser and/or finer levels may be repeated at different wavelengths for increased accuracy and/or for robust detection of the mark irrespective of the materials from which the mark is made, and materials on and/or below which the mark is provided. The wavelengths may be multiplexed and de-multiplexed optically so as to be processed simultaneously, and/or they may be multiplexed by time division or frequency division.

[00049] In this example, the alignment sensor and spot SP remain stationary, while it is the substrate W that moves. The alignment sensor can thus be mounted rigidly and accurately to a reference frame, while effectively scanning the mark AM in a direction opposite to the direction of movement of substrate W. The substrate W is controlled in this movement by its mounting on a substrate support and a substrate positioning system controlling the movement of the substrate support. A substrate support position sensor (e.g. an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. A measurement of the position of the marks provided on the substrate support allows the position of the substrate support as determined by the position sensor to be calibrated (e.g. relative to a frame to which the alignment system is connected). A measurement of the position of the alignment marks provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[00050] Figure 3 is a schematic diagram of an apparatus 10 for measuring the position of a plurality of alignment marks 12 on the substrate W. The alignment marks 12 may be periodic gratings. The apparatus 10, which may be referred to as an alignment sensor, is in this embodiment similar to the known alignment sensor AS shown in Figure 2 but has some differences, which will become apparent.

[00051] The apparatus 10 is a measurement apparatus. The apparatus 10 may be, or form part of. a metrology apparatus. A metrology apparatus is used to determine properties of the substrates W, and in particular, how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The apparatus 10 may, for example, be integrated into the lithographic apparatus LA, or may be a stand-alone device. It will be appreciated that the apparatus may be located elsewhere in the lithographic apparatus LA and/or may be used for measuring different alignment marks located on different substrates.

[00052] The alignment sensor 10 includes a radiation source 14 to produce a radiation beam 16 which is used to illuminate the alignment marks 12 on the substrate W. The arrows show the path of the radiation beam 16 through the alignment sensor 10. In other embodiments, the radiation source 14 may not be part of the alignment sensor 14 and may be a separate component.

[00053] The radiation beam 16 passes through an illumination system 18 which is configured to direct the radiation beam 16 onto the alignment marks 12 on the substrate W. The illumination system 18 includes a first illumination lens 20 which collimates the radiation beam 16 and a second illumination lens 22 which focus the radiation beam 16 onto a spot mirror 24. The radiation source 14 image is projected in the pupil plane where the spot mirror 24 is located. The spot mirror 24 reflects the radiation beam 16 onto a lens 26 which is configured to direct (and focus) the radiation beam 16 onto three alignment marks 12. Thus, each of the three alignment marks 12 are illuminated by the radiation beam 16 in parallel (substantially simultaneously/at the same time). In this embodiment, the alignment marks 12 are each illuminated by the radiation beam 16 from the same radiation source 14. However, the alignment marks can be illuminated by different radiation sources at the same time. However, in order to determine the aligned positions of the alignment marks, the same radiation source or a source combination should be used for all alignment marks. The alignment marks 12 being imaged may be located anywhere on the substrate W as long as they can be illuminated by the incoming radiation beam 16, i.e. there is no requirement for the alignment marks 12 to be a specific maximum or minimum distance apart in order to be imaged. It will be appreciated that in other embodiments, the lens 26 may direct the radiation beam 16 onto a different number of alignment marks 12 provided that it is more than one alignment mark 12, i.e. more generally, the lens 26 directs the radiation beam 16 onto a plurality of alignment marks 12.

[00054] The lens 26 focuses the radiation beam 16 into illumination spots which are incident on the alignment marks 12. The alignment marks 12 are in a focus (object) plane of the lens 26. The illumination spots may have a diameter that is less than the width of the gratings (alignmen t marks 12) in the X direction. The radiation beam 16 is scanned across the substrate W (i.e. the substrate W is moved in the X direction with respect to the alignment sensor 10) such that the illumination spots move across the gratings 12. The radiation beam 16 is diffracted by the gratings 12 and the diffracted radiation may be considered to be images of the plurality of the gratings 12.

[00055] The images of the gratings 12 (alignment marks) are projected by the same lens 26 that directed the radiation beam 16 onto the alignment marks 12. The lens 26 may be more generally referred to as a projection system (or part of a projection system) which in other embodiments may comprise other optical components. In this embodiment, the lens 26 is common to both the projection system and the illumination system. It will be appreciated that in other embodiments, the projection system and the illumination system may not have a common lens or lenses and may be formed of additional or different optical components to direct and project the radiation beam, e.g. mirrors and beamsplitters.

[00056] The lens 26 is configured to project the images of the alignment marks 12 into an optical block 28 as shown by the arrows in Figure 3. The optical block 28 is a self-referencing interferometer and may be considered to function in the same or similar way as the self-referencing interferometer SRI in Figure 2. For example, the optical block 28 may split the radiation beam into two beams at a pupil plane and apply a 180-degree rotation between the split beams before combining the split beams once more. This may be considered to be a modulation of the radiation beam. In other words, the optical block 28 is configured to modulate the images of the plurality of alignment marks 12 to produce modulated images of the plurality of alignment marles 12, These modulated images allow the position of the alignment marks 12 to be determined as will be described. It will be appreciated that, in other embodiments, the modulation of the images of the alignment marks may be carried out in a different way.

[00057] The use of the optical block 28 means that a detection plate (grating) is not required to be used to determine the position of the alignment marks 12. A detection plate is typically a transparent plate with a detection pattern which is the same as the image of the alignment mark. The detection plate is used to modulate the alignment mark light beam during the image movement along the detection plate due to the alignment mark scan. A detector would collect the image light beam modulated by the detection plate and transform this into an electrical signal which could then be used to determine the position of the alignment mark.

[00058] In this embodiment of the apparatus (alignment sensor) 10, each of the images of the plurality of the alignment marks 12 are projected into the optical block 28 in parallel, i.e. substantially simultaneously (at the same time). The optical block 28 then modulates the images of the alignment marks 12 and produces modulated images of the alignment marks 12. The modulated images of the alignment marks 12 are then projected from the optical block 28 onto a sensing element 30 in parallel (i.e. simultaneously), as shown by the arrows in Figure 3.

[00059] The sensing element 30 may be an intensity sensor. Each of the modulated images of the alignment marks 12 are projected onto different parts of the sensing element 30. The sensing element 30 has a plurality of pixels 32. Thus, the modulated images of the alignment marks 12 are projected onto different pixels 32 of the sensing element 30. The size of the sensing element 30 may be substantially equal to the size of an exposure slit of the lithographic apparatus LA. In this embodiment, the sensing element 30 is a CCD sensor but it will be appreciated that in other embodiments the sensing element 30 may be a different type of sensor, such as a CMOS sensor.

[00060] Figure 4 shows a schematic diagram of the sensing element 30 with the modulated images of the alignment marks 12 shown on the plurality of pixels 32. The optical block 28 is configured to project the modulated images of the alignment marks 12 onto more than one of the plurality of pixels 32 of the sensing element 30. The modulated images of the alignment marks are shown as periodic grating images 34 at the sensing element 30 image sensor plane. The grating images 34 are shown to be on different parts of the sensing element 30 and thus cover different pixels 32. That is, each grating image 34 covers a number of pixels 32 and does not cover the same pixels 32 as any of the other grating images 34. Thus, each grating image 34 can be analysed separately and independently of each of the other grating images 34. Each of the grating images 34 should be projected onto different pixels 32 for two reasons. Firstly, if they are on the same pixels 32 then it will be difficult to separate the overlapped alignment marks 12 signals to determine the aligned positions of the alignment marks 12. Secondly, the optical design assumes that the grating images 34 are located at the same positions with respect to the centre of the sensing element 30 as the actual alignment marks 12 are located with respect to the optical axis of the alignment sensor 10 (i.e. the optical axis of the lens 26 of the alignment sensor 10). In other embodiments, the grating images 34 may be on only one pixel each, as long as they are different pixels.

[00061] As mentioned previously, the radiation beam 16 is diffracted from the alignment marks 12 to produce the images of the alignment marks 12. During the scanning of the alignment marks 12, the illumination spot moves over the alignment marks 12 which produces a periodic intensity change in the modulated images of the alignment marks 12, when viewed by the sensing element 30. The scan of the alignment marks 12 with respect to the alignment sensor 10 leads to the periodic intensity change from zero to maximal intensity (after modulation of the image of the alignment marks 12). This process is described in more detail above with reference to Figure 2.

[00062] Each of the plurality of pixels 32 of the sensing element 30 is configured to transform the periodic intensity change caused by each of the plurality of alignment marks 12 being scanned by the radiation beam 16 into electrical signals (herein after referred to as signals). Due to the modulated images of the alignment marks 12 being on separate parts of the sensing element 30, i.e. covering different pixels 32, each of the modulated images of the alignment marks may be analysed separately and independently. That is, the signals may be considered to be independent signals for each of the plurality of alignment marks

12. Using these signals, the position of each of the alignment marks 12 is determined in parallel, i.e. at the same time or simultaneously.

[00063] As mentioned above, each of the modulated images of the alignment marks 12 are incident on (i.e. projected upon) more than one pixel 32 on the sensing element 30. The sensing element 30 (or another separate component) is configured to combine the signals from each of the plurality of pixels 32 which the modulated image of the alignment mark 12 is projected upon. Thus, the signals from all the pixels 32 covered by the modulated image of the alignment mark 12 can be combined into one electrical signal.

[00064] Figure 5 shows an example of a periodic electrical signal produced from the modulated image of one of the alignment marks 12 on the sensing element 30. The magnitude of the modulated image electrical signal changes from a maximum to a minimum during the scan of the alignment mark 12. In this case, the whole pupil is used and thus the signal shape is triangular. There will be separate similar periodic electrical signals for each of the plurality of alignment marks 12. Due to this, each of the alignment marks 12 may be treated independently. Thus, the aligned position of each of the plurality of alignment marks 12 is independently determined. This determination of the position of the alignment marks 12 occurs in parallel since more than one alignment mark 12 is imaged in parallel (i.e. at the same time). In other embodiments, the shape of the electrical signal may be different (while still changing from a maximum and minimum over the scan of the alignment mark). For example, the signal may be a sine wave in the case of single order usage for the image of the alignment mark (e.g. 1^st or 3^rd order etc.) [00065] The signals (which are intensity signals) are supplied to a processing unit (not shown). The processing unit computationally processes the signals and outputs the position of the alignment marks 12. The processing unit may output values for X- and Y-position on the substrate W relative to a reference frame. The position of the alignment marks 12 may be provided with reference to the substrate W.

[00066] Once the positions of the alignment marks 12 have been determined (i.e. measured by the alignment sensor 10), the aligned positions of the alignment marks 12 may be used to e.g. position patterns (layers) on the substrate W with respect to each other. The processing unit may be used to align the alignment marks 12. To align patterns on a substrate W, more than one alignment mark is required. Since the position of the alignment marks 12 has been determined in parallel, the alignment of the alignment marles 12 may also be carried out in parallel. That is, parallel or simultaneous alignment may be carried out on a plurality of alignment marks 12.

[00067] Other measurement apparatus may include a single channel alignment sensor with a single detector (with or without a detection plate) and may not be able to align multiple alignment marks in parallel. Alternatively, other measurement apparatus may include multiple channel alignment sensors with several alignment sensor channels and several detectors. These multiple channel alignment sensors can align multiple alignment marks in parallel but cannot align the alignment marks with arbitrary positions on the substrate W. This is because the distance between the alignment marks must be, at least, shorter than the distance between the alignment sensor channels. The alignment marks must be in the centre of the channel and the size of the components of the channels required dictate that the alignment marks have to be a specific distance apart. The minimal distance depends on the alignment sensor optical design. It has to be large enough to prevent an optical crosstalk between the neighbour marks. Normally the minimal distance in this sense is much smaller than the distance between the sensor channels which depends on the HW design (lens, mirror size and etc.). The minimal distance also depends on the distance required to have a sufficient accuracy for the wafer alignment using the alignment marks aligned positions. For example, if only 2 alignment marks are used to align the wafer, then a distance between the marks must be normally in tens of mm range to be able to accurately calculate, for example, wafer rotation and expansion.

[00068] The alignment sensor 10 has an advantage that it can measure the position of the plurality of alignment marks 12 in parallel, i.e. at the same time, and align the plurality of alignment marks 12 in parallel. In addition, the alignment sensor 10 does not require a detection grating (plate). Furthermore, the alignment sensor 10 can measure any alignment marks 12 on the substrate W without any limitations on the distance between the alignment marks 12. Furthermore, the alignment sensor 10 is less complex than the multiple channel alignment sensor at least because less components are required (e.g. duplicate components of multiple channels are not needed.) For example, the alignment sensor 10 may only have one optical block and one detector whereas the multiple channel alignment sensor may have several optical blocks and several detectors (i.e. one optical block and one detector per alignment mark being imaged in parallel.) [00069] The alignment sensor 10 may have an advantage that parallel alignment can be performed during a substrate map measurement. Therefore, the number of alignment marks being aligned may be increased without a reduction in productivity. In addition, a productivity improvement may be achieved for lots comprising a plurality of substrates or wafers with the measure side limited sequence. A lithographic apparatus may have a measure side and an expose side. A substrate processing sequence at the measure side of the lithographic apparatus may be: wafer load on the wafer table of the wafer stage chuck, then wafer z (height) map measurement, then wafer alignment on a plurality of wafer alignment marles. At the same time another wafer stage chuck with the loaded wafer performs the expose side wafer processing sequence: lot correction (on 1st wafer of the lot), then reticle alignment, then wafer exposure. If the duration of the measure side sequence is longer than the expose side sequence, then the processing sequence of this lot is named as the measure side limited (from a productivity point of view). Using the alignment sensor 10 means that the wafer z (height) map measurement may be combined with the wafer alignment at the same time. Therefore, since the alignment of the alignment marks is performed in parallel, the number of the wafer alignment marks does not have an impact on the productivity as well. The use of the alignment sensor 10 may lead to an overlay improvement. The alignment sensor 10 has an advantage that the productivity of the lithographic apparatus is independent of the number of the alignment marks. The overlay performance of a lithographic apparatus depends on the number of the alignment marks. That is, the more alignment marks are measured, the more accurate the wafer alignment is and thus there is an improved overlay performance. Therefore, the alignment sensor 10 provides an advantage of improving the overlay performance without an impact on the productivity of the lithographic apparatus.

[00070] Figure 6 shows another embodiment of the measurement apparatus which is an alignment sensor 40. The alignment sensor 40 is similar to the alignment sensor 10 shown in Figure 3 and the same reference numerals have been used for the same components.

[00071] The alignment sensor 40 includes the same radiation source 14 and illumination system 18 as in Figure 3. The alignment sensor 40 has a projection system 42 which includes the lens 26 but also includes additional components as will be described. The projection system 42 includes a transfer system 44 (an optical system) configured to transfer the radiation beam (i.e. the images of the alignment marks 12) from a first pupil plane PPI of the projection system 42 to a second pupil plane PP2. The second pupil plane PP2 is the pupil of the projection system or the pupil plane of the alignment sensor 10. This is a pupil plane where the diffraction orders of the alignment marks 12 are located. Pupil plane PP2 is, in principle, the same as pupil plane PPI. Located in the second pupil plane PP2 is an optical element, which in this embodiment is a rotatable mirror 46. To image the alignment marks 12 sequentially on the optical block 28 and on the sensing element 50, the rotatable mirror 46 must be located at a pupil plane. Since the first pupil plane PPI is already occupied by the spot mirror 24, the transfer system is used to create the second pupil plane PP2, where the rotatable mirror 46 is located. The exact optical layout is not important, i.e. many different optical layouts may provide the desired results.

[00072] The rotatable mirror 46 performs rotated scans during the scans of the plurality of alignment marks 12. The rotatable mirror 46 is rotatable through a range of angles in directions as indicated by double headed arrow 48. This means that the rotatable mirror 46 may be rotated through the range of angles such that the images of the plurality of alignment marks 12 are reflected by the rotatable mirror 46 into the optical block 28 sequentially (i.e. only one image of one of the alignment marks is permitted to enter the optical block at one time). Thus, the rotatable mirror 46 (optical element) is configured to direct the images of the plurality of alignment marks 12 into the optical block 28 sequentially. The rotatable mirror 46 reflects each time only a single image of an alignment mark 12 (light beam) and blocks the other images of the alignment marks 12 (light beams).

[00073] The range of motion of the rotatable mirror 46 is chosen such that it covers all images of the alignment marks 12 illuminated along the X-axis on the substrate W. In other words, the range of angles of the rotatable mirror 46 is such that all the images of the plurality of alignment marks 12 of the illuminated alignment marks 12 are reflectable (reflected) from the rotatable mirror 46 into the optical block 28. In other embodiments, the optical element may not be a rotatable mirror in the set up described and there may be other optical components which are configured to block all except one of the images of the alignment marks. As an example, to block all but one of the images of the alignment marks, the optical block and the sensing element may be rotated as a single body around the centre of the pupil PP2. More generally, the alignment sensor 10 may be configured such that the modulated images of the alignment marks are projected onto the sensing element sequentially.

[00074] Since only one image of the plurality of alignment marks 12 is directed into the optical block 28, then only a single image of the alignment marks 12 is modulated at any one time in the optical block 28. That is, the images of the plurality of alignment marks 12 are modulated sequentially in the optical block 28. Thus, only a single modulated image of the alignment marks 12 is incident onto a sensing element 50 of the alignment sensor 40 at any one time, i.e. the modulated images of the alignment marks 12 are projected onto the sensing element 50 sequentially. As previously mentioned, the precise configuration of the optical arrangement (e.g. the transfer system 44 and the rotatable mirror 46) is not important. However, in this embodiment, it is important that there is sequential placement of the images of the alignment marks 12 into the optical block 28 and onto the sensing element 50, because a single detector is used as the sensing element 50.

[00075] In this embodiment, the sensing element 50 is a single detector, i.e. has a single pixel 52. The sensing element 50 may be an intensity sensor. The single pixel 52 is configured to transform a periodic intensity change caused by the plurality of alignment marks 12 being scanned into an electrical signal (similar to the electrical signal shown in Figure 5). Since only a single modulated image of the alignment marks 12 is incident on the sensing element 50 during a particular time period, the signal produced during this time period will only be due to the periodic intensity change from that particular alignment mark 12. As the rotatable mirror 46 is rotated then contributions from the other alignment marks 12 will be included in the signals. The signals may then be sampled to isolate the sample or samples for each alignment mark 12 which can be used to construct the signal for each alignment mark 12. That is, the signals may be sampled to construct independent signals for each of the plurality of alignment marks 12. This sampling may be done in the processing unit or another component.

[00076] Once the signal for each alignment mark 12 is obtained then the position of each alignment mark 12 can be determined in a similar way as with the alignment sensor 10. That is, the signals (which are intensity signals) are supplied to the processing unit to computationally processes the signals and output the position of the alignment marks 12. The processing unit may output values for X- and Y-position on the substrate W relative to a reference frame. The position of the alignment marks 12 may be provided with reference to the substrate W.

[00077] Thus, the position of each of the plurality of alignment marks 12 is independently determined. This determination of the position of the alignment marks 12 occurs in parallel since more than one alignment mark 12 is imaged sequentially for each sampling position of the substrate W scan. Since all the samples of the signals are collected during a single scan of the substrate W with respect to the alignment sensor 40, the positions of the plurality of alignment marks 12 are derived in parallel, i.e. substantially at the same time. Using the positions of the alignment marks 12 determined in parallel, the alignment of the alignment marks 12 may also be carried out in parallel.

[00078] The alignment sensor 40 allows parallel, or simultaneous, alignment to be carried out on a plurality of alignment marks 12 using the sensing element 50 which is a single detector. Using the single detector has the advantage that it doesn’t suffer with pixels signal drift which may occur with CCD detectors. Furthermore, using the single detector has an advantage that an inexpensive detector may be used.

[00079] Figure 7 shows another embodiment of the measurement apparatus which is an alignment sensor 60. The alignment sensor 60 is the same as alignment sensor 40 in Figure 6 except it has a different sensing element. The same reference numerals are used for the same parts as in Figure 6 and for brevity will not be described again.

[00080] The alignment sensor 60 includes a sensing element 62 which in this embodiment has a plurality of pixels 64. The sensing element 62 may be the same as the sensing element 30 in Figure 3 or may be different, e.g. have a different number of pixels etc. The sensing element 62 may be a CCD sensor or a CMOS sensor.

[00081] The alignment sensor 60 functions in the same way as the alignment sensor 40, i.e. the modulated images of the plurality of alignment marks 12 are projected sequentially from the optical block 28 onto the sensing element 62. However, in this case, the modulated image of the alignment mark 12 may cover more than one pixel 64 on the sensing element 62. In this embodiment, it is also important that there is sequential placement of the images of the alignment marks 12 into the optical block 28 and onto the sensing element 60, because the same pixels 64 are used for each alignment mark 12.

[00082] Each of the plurality of pixels 64 of the sensing element 62 is configured to transform a periodic intensity change caused by each of the plurality of alignment marks 12 being scanned by the radiation beam 16 into electrical signals. The sensing element 62 is configured to produce the signals such that they may be sampled to construct the independent signals for each of the plurality of alignment marks 12. Thus, the signals are independent signals for each of the plurality of alignment marks 12 in the same way as described in relation to the sensing element 50 of Figure 6.

[00083] However, the optical block 28 is configured to project each of the modulated images of the plurality of the alignment marks (albeit sequentially) onto more than one of the pixels 64 of the sensing element 62. The sensing element 62 is configured to combine the signals from each of the pixels which the modulated image of the alignment mark 12 is projected on into a combined electrical signal, in a similar way as described in relation to the sensing element 30 in Figure 3.

[00084] The signal or signals (intensity signals) are supplied to the processing unit (not shown) to computationally process the signals and output the position of the alignment marks 12. The position of the alignment marks 12 may then be used to align the alignment marks 12 in parallel.

[00085] An advantage of using the sensing element 62 (multiple pixels) over the sensing element 50 (with the single detector) is that the modulated image of the alignment mark 12 can be separated from other images around the alignment mark 12 by selection of the appropriate pixels. In this way, any impact of neighbouring structures around the alignment mark 12 on the signals may be eliminated or at least reduced. [00086] Figure 8 represents a flow diagram of a method according to an embodiment of the invention. The method is described in relation to the alignment sensor 10 in Figure 3 but it will be appreciated that the method is applicable to the other embodiments with the necessary changes.

[00087] The first step in the method is producing the radiation beam 16 in the radiation source 14 (step 100 of Figure 8). The radiation beam 16 is directed from the radiation source onto the plurality of alignment marks 12 on the substrate W using the illumination system 18 (step 102 in Figure 8). The radiation beam 16 is diffracted by the alignment marks 12 (gratings) which produces the images of the alignment marks

12. The method further comprises projecting the images of the alignment marks 12 (i.e. a radiation beam) from the substrate W (step 104 in Figure 8) into the optical block 28 in parallel (i.e. simultaneously) using the projection system (the lens 26).

[00088] Once the radiation beam, which holds the information about the images of the alignment marks 12, is in the optical block 28, the optical block 28 modulates the images of the alignment marks (step 106 in Figure 8). The optical block 28 can be, for example, a self-referencing interferometer. In this case, the modulation is carried out by splitting and self-alignment of the radiation beams of the image of the alignment marks 12.

[00089] The next step is projecting the modulated images of the plurality of alignment marks 12 onto a sensing element 30 in parallel (i.e. substantially simultaneously or at the same time) (step 108 in Figure 8). The pixels 32 of the sensing element 30 transform the periodic intensity change caused by each of the alignment marks 12 being scanned by the radiation beam 16 into the electrical signals (step 110 in Figure 8). The signals are independent signals for each of the alignment marks 12.

[00090] The next step is determining the position of each of the plurality of alignment marks 12 in parallel (step 112 in Figure 8). This may be done by computationally processing the signals in the processing unit. Finally, the last step is aligning the plurality of alignment marks 12 on the substrate W in parallel. This may be done by any method that uses the position of alignment marks to align the alignment marks, e.g. to align patterns on a substrate W.

[00091] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[00092] Although specific reference may be made in this text to embodiments of the invention in the context of a 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 mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[00093] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[00094] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world. [00095] 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 descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the clauses set out below. Other aspects of the invention are set out as in the following numbered clauses:

1. An apparatus for measuring a position of each of a plurality of alignment marks on a substrate comprising:

an illumination system configured to direct a radiation beam onto the plurality of alignment marks on the substrate, a projection system configured to project images of the plurality of alignment marks from the substrate, the images of the plurality of alignment marks being produced by diffraction of the radiation beam from the plurality of alignment marks;

an optical block configured to modulate the images of the plurality of alignment marks projected from the substrate, and wherein the optical block is configured to project the modulated images of the plurality of alignment marks onto a sensing element configured to produce signals from which the position of each of the plurality of alignment marks is determined in parallel.

2. The apparatus of clause 1, wherein the projection system is configured such that the images of the plurality of alignment marks are projected info the optical block simultaneously and the optical block is configured to project the modulated images of the plurality of the alignment marks onto different parts of the sensing element simultaneously.

3. The apparatus of clause 1, wherein the apparatus is configured such that the modulated images of the plurality of alignment marks are projected onto the sensing element sequentially.

4. The apparatus of clause 3, wherein the projection system comprises an optical element, wherein the optical element is configured to direct the images of the plurality of alignment marks into the optical block sequentially.

5. The apparatus of clause 4, wherein the optical element is a rotatable mirror, wherein the rotatable mirror is configured to be rotated through a range of angles such that the images of the plurality of alignment marks are reflected by the rotatable mirror into the optical block sequentially.

6. The apparatus of any preceding clause, wherein the sensing element comprises a plurality of pixels, wherein each of the plurality of pixels of the sensing element is configured to transform a periodic intensity change caused by each of the plurality of alignment marks being scanned by the radiation beam into the signals, wherein the signals are independent signals for each of the plurality of alignment marks.

7. The apparatus of clause 6, wherein the optical block is configured to project each of the modulated images of the plurality of the alignment marks onto more than one of the plurality of pixels of the sensing element and the sensing element is configured to combine the signals from each of the plurality of pixels which the modulated image of the alignment mark is projected upon.

8. The apparatus of either of clauses 6-7 when dependent on clauses 3-5, wherein the sensing element is configured to produce the signals such that they may be sampled to construct the independent signals for each of the plurality of alignment marks.

9. The apparatus of clauses 3-5, wherein the sensing element comprises a single pixel, wherein the single pixel is configured to transform a periodic intensity change caused by the plurality of alignment marks being scanned into the signals such that the signals may be sampled to construct independent signals for each of the plurality of alignment marks.

10. The apparatus of any of clauses 3-5 and 9, wherein the projection system further comprises a transfer system configured to transfer the radiation beam from a first pupil plane of the projection system to a second pupil plane, wherein the optical element is located in the second pupil plane.

11. A metrology apparatus comprising the apparatus of any preceding clause,

12. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, the lithographic apparatus comprising the apparatus of clauses 1-10.

13. A method of measuring a position of each of a plurality of alignment marks on a substrate comprising:

directing a radiation beam from a radiation source onto the plurality of alignment marks on the substrate using an illumination system;

projecting images of the plurality of alignment marks from the substrate using a projection system, the images of the plurality of alignment marks being produced by diffraction of the radiation beam from 5 the plurality of alignment marks;

modulating the images of the plurality of alignment marks projec ted from the substrate in an optical block, and projecting the modulated images of the plurality of alignment marks onto a sensing element to produce signals from which the position of each of the plurality of alignment marks is determined in 10 parallel.

14. The method of clause 13, further comprising projecting the images of the plurality of alignment marks into the optical block simultaneously and projecting the modulated images of the plurality of the alignment marks onto different parts of the sensing element simultaneously.

15. The method of clause 13, further comprising directing the modulated images of the plurality of alignment marks onto the sensing element sequentially.

Claims

CONCLUSION

1. A device arranged for exposing a substrate.