Metrology system and lithographic apparatus

Abstract

A sensor head arrangement comprising: a first sensor head being adjustable in only a first two degrees of freedom, the first two degrees of freedom comprising displacement in a first direction and displacement in a second direction, the second direction being different to the first direction; a second sensor head being adjustable in only a second two degrees of freedom, the second two degrees of freedom comprising displacement in the first direction and displacement in a third direction, the third direction being different to the first direction and second direction; and a third sensor head having zero degrees of freedom for adjustment.

Description

METROLOGY SYSTEM AND LITHOGRAPHIC APPARATUSCROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Application No. 63 / 737,109, filed 20 December 2024, and which is incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates to metrology systems, for example, an alignment system for measuring alignment mark positions in lithographic apparatuses and systems.BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which can be a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiationsensitive material (photoresist or simply “resist”) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”- direction) while synchronously scanning the target portions parallel or anti-parallel to this scanning direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0004] During lithographic operation, different processing steps can entail different layers to be sequentially formed on the substrate. Accordingly, it may be necessary to position the substrate relative to prior patterns formed thereon with a high degree of accuracy. Generally, alignment marks are placed on the substrate to be aligned and are located with reference to a second object. A lithographic apparatus can use an alignment apparatus for detecting positions of the alignment marks and for aligning the substrate using the alignment marks to ensure accurate exposure from a mask. Misalignment between the alignment marks at two different layers is measured as overlay error.

[0005] In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters can include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement can be performed on a product substrate and / or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including theuse of scanning electron microscopes and various specialized tools. A fast and non-invasive form of a specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. By contrast, angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.

[0006] Such optical scatterometers can be used to measure parameters, such as critical dimensions of developed photosensitive resist or overlay error (OV) between two layers formed in or on the patterned substrate. Properties of the substrate can be determined by comparing the properties of an illumination beam before and after the beam has been reflected or scattered by the substrate.

[0007] Production speed and throughput is of great importance in lithographic fabrication of ICs and electronic devices. It is desirable for metrology systems used in fabrication to acquire measurements quickly for increasing wafer throughput. To achieve this, parallel metrology systems (e.g., alignment systems) comprising multiple sensor heads for parallel acquisition of metrology data have been proposed.

[0008] It would be desirable to improve on present metrology arrangements for measuring sensor head positions of a parallel metrology system.SUMMARY

[0009] In a first aspect of the invention, there is provided a sensor head arrangement comprising: a first sensor head being adjustable in only a first two degrees of freedom, the first two degrees of freedom comprising displacement in a first direction and displacement in a second direction, the second direction being different to the first direction; a second sensor head being adjustable in only a second two degrees of freedom, the second two degrees of freedom comprising displacement in the first direction and displacement in a third direction, the third direction being different to the first direction and second direction; and a third sensor head having zero degrees of freedom for adjustment.

[0010] In a second aspect of the invention, there is provided method of using the metrology system of the first aspect, the method comprising: adjusting the first sensor head and second sensor head such that the respective focal points of the first sensor head, second sensor head and third sensor head lie substantially on a straight line in at least each of said first dimension and second dimension.

[0011] Further features of various aspects of the present disclosure are described in detail below with reference to the accompanying drawings. It is noted that the present disclosure is not limited to the specific aspects described herein. Such aspects are presented herein for illustrative purposes only.Additional aspects will be apparent to those skilled in the relevant art(s) based on the teachings contained herein.BRIEF DESCRIPTION OF THE DRAWINGS / FIGURES

[0012] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of the present disclosure and to enable those skilled in the relevant art(s) to make and use aspects described herein.

[0013] Figure 1 A shows a reflective lithographic apparatus, according to some aspects;

[0014] Figure IB shows a transmissive lithographic apparatus, according to some aspects;

[0015] Figure 2 shows more details of a reflective lithographic apparatus, according to some aspects;

[0016] Figure 3 shows a lithographic cell, according to some aspects;

[0017] Figures 4A and 4B are schematic illustrations of metrology apparatuses;

[0018] Figure 5(a) is a schematic illustration of a sensor head arrangement in a first configuration prior to aligning their focal points onto a single line,

[0019] Figure 5(b) is a schematic illustration of a sensor head arrangement of Figure 5(a) in a second, intermediate, configuration;

[0020] Figure 5(c) is a schematic illustration of a sensor head arrangement of Figure 5(a) in a third configuration with their focal points aligned onto a single line;

[0021] Figure 6 is a side view schematic illustration of a sensor head with two degrees of freedom in x and y;

[0022] Figure 7 is a top view schematic illustration of the sensor head of Figure 6; and

[0023] Figure 8 is a side view schematic illustration of a sensor head with two degrees of freedom in x and z.

[0024] The features of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and / or structurally similar elements. Additionally, generally, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.DETAILED DESCRIPTION

[0025] The aspects described herein, and references in the specification to “one aspect,” “an aspect,” “an exemplary aspect,” “an example aspect,” etc., indicate that the aspects described can include a particular feature, structure, or characteristic, but every aspect may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the sameaspect. Further, when a particular feature, structure, or characteristic is described in connection with an aspect, it is understood that it is within the knowledge of those skilled in the art to effect such feature, structure, or characteristic in connection with other aspects whether or not explicitly described.

[0026] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, can be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein can likewise be interpreted accordingly.

[0027] The terms “about,” “approximately,” or the like can be used herein to indicate the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the terms “about,” “approximately,” or the like can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

[0028] Aspects of the present disclosure can be implemented in hardware, firmware, software, or any combination thereof. Aspects of the disclosure can also be implemented as instructions stored on a computer-readable medium, which can be read and executed by one or more processors. A machine- readable medium can 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 can include read only memory (ROM); random access memory (RAM); magnetic disk 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. Furthermore, firmware, software, routines, and / or instructions can be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. The term “machine -readable medium” can be interchangeable with similar terms, for example, “computer program product,” “computer-readable medium,” “non-transitory computer- readable medium,” or the like. The term “non-transitory” can be used herein to characterize one or more forms of computer readable media except for a transitory, propagating signal.

[0029] Before describing such aspects in more detail, however, it is instructive to present an example environment in which aspects of the present disclosure can be implemented.

[0030] Example Lithographic Systems

[0031] FIGS. 1A and IB show a lithographic apparatus 100 and a lithographic apparatus 100’, respectively, in which aspects of the present disclosure can be implemented. Lithographic apparatus 100 and lithographic apparatus 100’ each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positionerPM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatus 100 and 100’ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus 100’, the patterning device MA and the projection system PS are transmissive.

[0032] The illumination system IL can include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B.

[0033] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatus 100 and 100’, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

[0034] The term “patterning device” MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

[0035] The patterning device MA can be transmissive (as in lithographic apparatus 100’ of Figure IB) or reflective (as in lithographic apparatus 100 of Figure 1A). Examples of patterning devices MA include reticles, masks, programmable mirror arrays, or programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase shift, or attenuated phase shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam B, which is reflected by a matrix of small mirrors.

[0036] The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation orelectrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0037] Lithographic apparatus 100 and / or lithographic apparatus 100’ can be of a type having two (dual stage) or more substrate tables WT (and / or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT.

[0038] The lithographic apparatus can also be of a type wherein at least a portion of the substrate can be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid can also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid. For example, a liquid can be located between the projection system and the substrate during exposure.

[0039] Referring to FIGS. 1A and IB, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus 100, 100’ can be separate physical entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus 100 or 100’, and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (in Figure IB) including, for example, suitable directing mirrors and / or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus 100, 100’, for example, when the source SO is a mercury lamp. A radiation system can comprise the source SO, the illuminator IL, and / or the beam delivery system BD.

[0040] The illuminator IL can include an adjuster AD (in Figure IB) for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and / or inner radial extent (commonly referred to as “o-outer” and “o-inner,” respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL can comprise various other components (in Figure IB), such as an integrator IN and a condenser CO. The illuminator IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section.

[0041] Referring to Figure 1 A, the radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus 100, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (for example, an interferometric device, linear encoder, or capacitive sensor),the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor IF1 can be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device (for example, mask) MA and substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0042] Referring to Figure IB, the radiation beam B is incident on the patterning device (for example, mask MA), which is held on the support structure (for example, mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. The projection system has a pupil conjugate PPU to an illumination system pupil IPU. Portions of radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse a mask pattern without being affected by diffraction at the mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.

[0043] The projection system PS projects an image of the mask pattern MP, where the image is formed by diffracted beams produced from the mark pattern MP by radiation from the intensity distribution, onto a photoresist layer coated on the substrate W. For example, the mask pattern MP can include an array of lines and spaces. A diffraction of radiation at the array and different from zeroth order diffraction generates diverted diffracted beams with a change of direction in a direction perpendicular to the lines. Undiffracted beams (i.e., so-called zeroth order diffracted beams) traverse the pattern without any change in propagation direction. The zeroth order diffracted beams traverse an upper lens or upper lens group of the projection system PS, upstream of the pupil conjugate PPU of the projection system PS, to reach the pupil conjugate PPU. The portion of the intensity distribution in the plane of the pupil conjugate PPU and associated with the zeroth order diffracted beams is an image of the intensity distribution in the illumination system pupil IPU of the illumination system IL. The aperture device PD, for example, is disposed at or substantially at a plane that includes the pupil conjugate PPU of the projection system PS.

[0044] The projection system PS is arranged to capture (e.g., using a lens or lens group L) the zeroth order diffracted beams, first order diffracted beams, and / or higher order diffracted beams (not shown). In some aspects, dipole illumination for imaging line patterns extending in a direction perpendicular to a line can be used to utilize the resolution enhancement effect of dipole illumination. For example, first- order diffracted beams interfere with corresponding zeroth-order diffracted beams at the level of the wafer W to create an image of the line pattern MP at highest possible resolution and process window (i.e., usable depth of focus in combination with tolerable exposure dose deviations). In some aspects, astigmatism aberration can be reduced by providing radiation poles (not shown) in opposite quadrants of the illumination system pupil IPU. Further, in some aspects, astigmatism aberration can be reduced by blocking the zeroth order beams in the pupil conjugate PPU of the projection system associated withradiation poles in opposite quadrants. This is described in more detail in US 7,511,799 B2, issued Mar. 31, 2009, which is incorporated by reference herein in its entirety.

[0045] With the aid of the second positioner PW and position sensor IFD (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor (not shown in Figure IB) can be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

[0046] In general, movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short-stroke actuator or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks Ml, M2, and substrate alignment marks Pl, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can be located in spaces between target portions (known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks can be located between the dies.

[0047] Mask table MT and patterning device MA can be in a vacuum chamber V, where an in-vacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum chamber. Alternatively, when mask table MT and patterning device MA are outside of the vacuum chamber, an out-of-vacuum robot can be used for various transportation operations, similar to the in-vacuum robot IVR. Both the in-vacuum and out-of-vacuum robots can be calibrated for a smooth transfer of any payload (e.g., mask) to a fixed kinematic mount of a transfer station.

[0048] The lithographic apparatus 100 and 100’ can be used in at least one of the following modes:1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and / or Y direction so that a different target portion C can be exposed.2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT can be determined by the (de-)magnification and image reversal characteristics of the projection system PS.3. In another mode, the support structure (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiationsource SO can be employed and the programmable patterning device is updated as needed after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array.

[0049] Combinations and / or variations on the described modes of use or entirely different modes of use can also be employed.

[0050] In some aspects, lithographic apparatus 100 includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

[0051] In some aspects, lithographic apparatus 100’ includes a deep ultraviolet (DUV) source, which is configured to generate a beam of DUV radiation for DUV lithography. In general, the DUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the DUV radiation beam of the DUV source.

[0052] Figure 2 shows the lithographic apparatus 100 in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector apparatus SO. An EUV radiation emitting plasma 210 can be formed by a discharge produced plasma source. EUV radiation can be produced by a gas or vapor, for example Xe gas, Li vapor, or Sn vapor in which EUV radiation emitting plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. The EUV radiation emitting plasma 210 is created by, for example, an electrical discharge causing at least a partially ionized plasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor, or any other suitable gas or vapor can be used for efficient generation of the radiation. In some aspects, a plasma of excited tin (Sn) (e.g., excited via a laser) is provided to produce EUV radiation.

[0053] The radiation emitted by the EUV radiation emitting plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 can include a channel structure. Contamination trap 230 can also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure.

[0054] The collector chamber 212 can include a radiation collector CO, which can be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point INTF. The virtual source point INTF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged suchthat the intermediate focus INTF is located at or near an opening 219 in the enclosing structure 220. The virtual source point INTF is an image of the EUV radiation emitting plasma 210. Grating spectral filter 240 is used in particular for suppressing infra-red (IR) radiation.

[0055] Subsequently the radiation traverses the illumination system IL, which can include a faceted field mirror device 222 and a faceted pupil mirror device 224 arranged to provide a desired angular distribution of the radiation beam 221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation 221 at the patterning device MA, held by the support structure MT, a patterned beam 226 is formed and the patterned beam 226 is imaged by the projection system PS via reflective elements 228, 229 onto a substrate W held by the wafer stage or substrate table WT.

[0056] More elements than shown can generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 can optionally be present, depending upon the type of lithographic apparatus. Further, there can be more mirrors present than those shown in the Figure 2, for example there can be one to six additional reflective elements present in the projection system PS than shown in Figure 2.

[0057] Collector optic CO, as illustrated in Figure 2, is depicted as a nested collector with grazing incidence reflectors 253, 254, and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254, and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.Example Lithographic Cell

[0058] Figure 3 shows a lithographic cell 300, also sometimes referred to a lithocell or cluster, according to some aspects. Lithographic apparatus 100 or 100’ can form part of lithographic cell 300. Lithographic cell 300 can also include one or more apparatuses to perform pre- and post-exposure processes on a substrate. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH, and bake plates BK. A substrate handler, or robot, RO picks up substrates from input / output ports I / O I , I / O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus 100 or 100’. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatuses can be operated to maximize throughput and processing efficiency.

[0059] Example Inspection Apparatus

[0060] In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more alignment apparatuses and / or inspection apparatuses for accurate positioning of marks on a substrate. These alignment apparatuses are effectively position measuring apparatuses. Different typesof marks and different types of alignment apparatuses and / or systems are known from different times and different manufacturers. A type of system widely used in current lithographic apparatus is based on a self -referencing interferometer as described in U.S. Patent No. 6,961,116 (den Boef et al.). Generally marks are measured separately to obtain X- and Y-positions. A combined X- and Y-measurement can be performed using the techniques described in U.S. Publication No. 2009 / 195768 A (Bijnen et al.), however. The full contents of both of these disclosures are incorporated herein by reference.

[0061] Figure 4A shows a cross-sectional view of an inspection apparatus 400 that can be implemented as a part of lithographic apparatus 100 or 100’, according to some aspects. In some aspects, inspection apparatus 400 can be configured to align a substrate (e.g., substrate W) with respect to a patterning device (e.g., patterning device MA). Inspection apparatus 400 can be further configured to detect positions of alignment marks on the substrate and to align the substrate with respect to the patterning device or other components of lithographic apparatus 100 or 100’ using the detected positions of the alignment marks. Such alignment of the substrate can ensure accurate exposure of one or more patterns on the substrate.

[0062] The terms “inspection apparatus,” “metrology system,” or the like can be used herein to refer to, e.g., a device used for measuring a property of a structure (e.g., overlay sensor, critical dimension sensor, or the like), a device or system used in a lithographic apparatus to inspect an alignment of a wafer (e.g., alignment sensor), or the like.

[0063] In some aspects, inspection apparatus 400 can include an illumination system 412, a beam splitter 414, an interferometer 426, a detector 428, a beam analyzer 430, and a processor 432. Illumination system 412 can be configured to provide an electromagnetic narrow band radiation beam 413 having one or more passbands. In an example, the one or more passbands can be within a spectrum of wavelengths between about 500 nm to about 900 nm. In another example, the one or more passbands can be discrete narrow passbands within a spectrum of wavelengths between about 500 nm to about 900 nm. Illumination system 412 can be further configured to provide one or more passbands having substantially constant center wavelength (CWL) values over a long period of time (e.g., over a lifetime of illumination system 412). Such configuration of illumination system 412 can help to prevent the shift of the actual CWL values from the desired CWL values, as discussed above, in current alignment systems. And, as a result, the use of constant CWL values can improve long-term stability and accuracy of alignment systems (e.g., inspection apparatus 400) compared to the current alignment apparatuses.

[0064] In some aspects, beam splitter 414 can be configured to receive radiation beam 413 and split radiation beam 413 into at least two radiation sub-beams. For example, radiation beam 413 can be split into radiation sub-beams 415 and 417, as shown in Figure 4A. Beam splitter 414 can be further configured to direct radiation sub-beam 415 onto a substrate 420 placed on a stage 422. In one example, the stage 422 is movable along direction 424. Radiation sub-beam 415 can be configured to illuminate an alignment mark or a target 418 located on substrate 420. Alignment mark or target 418 can be coated with a radiation sensitive film. In some aspects, alignment mark or target 418 can have one hundredand eighty degrees (i.e., 180°) symmetry. That is, when alignment mark or target 418 is rotated 180° about an axis of symmetry perpendicular to a plane of alignment mark or target 418, rotated alignment mark or target 418 can be substantially identical to an unrotated alignment mark or target 418. The target 418 on substrate 420 can be (a) a resist layer grating comprising bars that are formed of solid resist lines, or (b) a product layer grating, or (c) a composite grating stack in an overlay target structure comprising a resist grating overlaid or interleaved on a product layer grating. The bars can alternatively be etched into the substrate. This pattern is sensitive to chromatic aberrations in the lithographic projection apparatus, particularly the projection system PL, and illumination symmetry and the presence of such aberrations will manifest themselves in a variation in the printed grating. One in-line method used in device manufacturing for measurements of line width, pitch, and critical dimension makes use of a technique known as “scatterometry”. Methods of scatterometry are described in Raymond et al., “Multiparameter Grating Metrology Using Optical Scatterometry”, J. Vac. Sci. Tech. B, Vol. 15, no. 2, pp. 361-368 (1997) and Niu et al., “Specular Spectroscopic Scatterometry in DUV Lithography”, SPIE, Vol. 3677 (1999), which are both incorporated by reference herein in their entireties. In scatterometry, light is reflected by periodic structures in the target, and the resulting reflection spectrum at a given angle is detected. The structure giving rise to the reflection spectrum is reconstructed, e.g. using Rigorous Coupled- Wave Analysis (RCWA) or by comparison to a library of patterns derived by simulation. Accordingly, the scatterometry data of the printed gratings is used to reconstruct the gratings. The parameters of the grating, such as line widths and shapes, can be input to the reconstruction process, performed by processing unit PU, from knowledge of the printing step and / or other scatterometry processes.

[0065] In some aspects, beam splitter 414 can be further configured to receive diffraction radiation beam 419 and split diffraction radiation beam 419 into at least two radiation sub-beams, according to an aspect. Diffraction radiation beam 419 can be split into diffraction radiation sub-beams 429 and 439, as shown in Figure 4A.

[0066] It should be noted that even though beam splitter 414 is shown to direct radiation sub-beam 415 towards alignment mark or target 418 and to direct diffracted radiation sub-beam 429 towards interferometer 426, the disclosure is not so limiting. Other optical arrangements can be used to obtain the similar result of illuminating alignment mark or target 418 on substrate 420 and detecting an image of alignment mark or target 418.

[0067] As illustrated in Figure 4A, interferometer 426 can be configured to receive radiation sub-beam 417 and diffracted radiation sub-beam 429 through beam splitter 414. In an example aspect, diffracted radiation sub-beam 429 can be at least a portion of radiation sub-beam 415 that can be reflected from alignment mark or target 418. In an example of this aspect, interferometer 426 comprises any appropriate set of optical-elements, for example, a combination of prisms that can be configured to form two images of alignment mark or target 418 based on the received diffracted radiation sub-beam 429. It should be appreciated that a good quality image need not be formed. It can be enough to have thefeatures of alignment mark 418 resolved. Interferometer 426 can be further configured to rotate one of the two images with respect to the other of the two images 180° and recombine the rotated and unrotated images interferometrically.

[0068] In some aspects, detector 428 can be configured to receive the recombined image via interferometer signal 427 and detect interference as a result of the recombined image when alignment axis 421 of inspection apparatus 400 passes through a center of symmetry (not shown) of alignment mark or target 418. Such interference can be due to alignment mark or target 418 being 180° symmetrical, and the recombined image interfering constructively or destructively, according to an example aspect. Based on the detected interference, detector 428 can be further configured to determine a position of the center of symmetry of alignment mark or target 418 and consequently, detect a position of substrate 420. According to an example, alignment axis 421 can be aligned with an optical beam perpendicular to substrate 420 and passing through a center of image rotation interferometer 426. Detector 428 can be further configured to estimate the positions of alignment mark or target 418 by implementing sensor characteristics and interacting with wafer mark process variations.

[0069] In a further aspect, detector 428 determines the position of the center of symmetry of alignment mark or target 418 by performing one or more of the following measurements:1. measuring position variations for various wavelengths (position shift between colors);2. measuring position variations for various orders (position shift between diffraction orders);3. measuring position variations for various polarizations (position shift between polarizations); and4. measuring intensity difference between opposite orders of a diffraction order pair (e.g., to characterize and correct for asymmetry).

[0070] This data can be obtained using any type of alignment sensor, for example, a SMASH (SMart Alignment Sensor Hybrid) sensor, as described in U.S. Patent No. 6,961,116 that employs a selfreferencing interferometer with a single detector and four different wavelengths, and extracts the alignment signal in software, or Athena (Advanced Technology using High order ENhancement of Alignment), as described in U.S. Patent No. 6,297,876, which directs each of seven diffraction orders to a dedicated detector, which are both incorporated by reference herein in their entireties.

[0071] In some aspects, beam analyzer 430 can be configured to receive and determine an optical state of diffracted radiation sub-beam 439. The optical state can be a measure of beam wavelength, polarization, or beam profile. Beam analyzer 430 can be further configured to determine a position of stage 422 and correlate the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420 can be accurately known with reference to stage 422. Alternatively, beam analyzer 430 can be configured to determine a position of inspection apparatus 400 or any other reference element such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400 or any other reference element. Beam analyzer 430 can be a point or an imaging polarimeter with some form of wavelength-band selectivity. In some aspects, beamanalyzer 430 can be directly integrated into inspection apparatus 400, or connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other aspects.

[0072] In some aspects, beam analyzer 430 can be further configured to determine the overlay data between two patterns on substrate 420. One of these patterns can be a reference pattern on a reference layer. The other pattern can be an exposed pattern on an exposed layer. The reference layer can be an etched layer already present on substrate 420. The reference layer can be generated by a reference pattern exposed on the substrate by lithographic apparatus 100 and / or 100’. The exposed layer can be a resist layer exposed adjacent to the reference layer. The exposed layer can be generated by an exposure pattern exposed on substrate 420 by lithographic apparatus 100 or 100’. The exposed pattern on substrate 420 can correspond to a movement of substrate 420 by stage 422. In some aspects, the measured overlay data can also indicate an offset between the reference pattern and the exposure pattern. The measured overlay data can be used as calibration data to calibrate the exposure pattern exposed by lithographic apparatus 100 or 100’, such that after the calibration, the offset between the exposed layer and the reference layer can be minimized.

[0073] In some aspects, beam analyzer 430 can be further configured to determine a model of the product stack profile of substrate 420, and can be configured to measure overlay, critical dimension, and focus of target 418 in a single measurement. The product stack profile contains information on the stacked product such as alignment mark, target 418, or substrate 420, and can include mark process variation-induced optical signature metrology that is a function of illumination variation. The product stack profile can also include product grating profile, mark stack profile, and mark asymmetry information. An example of beam analyzer 430 is Yieldstar™, manufactured by ASML, Veldhoven, The Netherlands, as described in U.S. Patent No. 8,706,442, which is incorporated by reference herein in its entirety. Beam analyzer 430 can be further configured to process information related to a particular property of an exposed pattern in that layer. For example, beam analyzer 430 can process an overlay parameter (an indication of the positioning accuracy of the layer with respect to a previous layer on the substrate or the positioning accuracy of the first layer with respective to marks on the substrate), a focus parameter, and / or a critical dimension parameter (e.g., line width and its variations) of the depicted image in the layer. Other parameters are image parameters relating to the quality of the depicted image of the exposed pattern.

[0074] In some aspects, an array of detectors (not shown) can be connected to beam analyzer 430, and allows the possibility of accurate stack profile detection as discussed below. For example, detector 428 can be an array of detectors. For the detector array, a number of options are possible: a bundle of multimode fibers, discrete pin detectors per channel, or CCD or CMOS (linear) arrays. The use of a bundle of multimode fibers enables any dissipating elements to be remotely located for stability reasons. Discrete PIN detectors offer a large dynamic range but each need separate pre-amps. The number of elements is therefore limited. CCD linear arrays offer many elements that can be read-out at high speed and are especially of interest if phase-stepping detection is used.

[0075] In some aspects, a second beam analyzer 430’ can be configured to receive and determine an optical state of diffracted radiation sub-beam 429, as shown in Figure 4B. The optical state can be a measure of beam wavelength, polarization, or beam profile. Second beam analyzer 430’ can be identical to beam analyzer 430. Alternatively, second beam analyzer 430’ can be configured to perform one or more of the functions of beam analyzer 430, such as determining a position of stage 422 and correlating the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420, can be accurately known with reference to stage 422. Second beam analyzer 430’ can also be configured to determine a position of inspection apparatus 400, or any other reference element, such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400, or any other reference element.

[0076] In some aspects, second beam analyzer 430’ can be directly integrated into inspection apparatus 400, or it can be connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other aspects. Alternatively, second beam analyzer 430’ and beam analyzer 430 can be combined to form a single analyzer (not shown) configured to receive and determine the optical states of both diffracted radiation sub-beams 429 and 439.

[0077] In some aspects, processor 432 receives information from detector 428 and beam analyzer 430. Processor 432 can create a basic correction algorithm based on the information received from detector 428 and beam analyzer 430, including but not limited to the optical state of the illumination beam, the alignment signals, associated position estimates, and the optical state in the pupil, image, and additional planes. The pupil plane is the plane in which the radial position of radiation defines the angle of incidence and the angular position defines the azimuth angle of the radiation. Processor 432 can utilize the basic correction algorithm to characterize the inspection apparatus 400 with reference to wafer marks and / or alignment marks 418.

[0078] It is increasingly desirable to measure an increasing number of metrology marks per wafer (e.g., in alignment and / or overlay metrology) to enable better corrections, thereby resulting in improved characterization of wafer deformation and therefore better overlay performance. Additionally, many lithographic apparatuses comprise a measurement station for performing alignment metrology which is separate to an exposure station for performing the actual exposure. This enables alignment metrology to be performed in parallel (i.e., on the next wafer to be exposed) to exposing a present wafer. However, as exposures become faster and exposure throughput increases, there is less time for alignment metrology.

[0079] Most current alignment and overlay sensors are of a single-sensor head type. To increase metrology speed and the number of alignment marks measurable per second, multiple sensor head arrangements have been proposed. Such multiple sensor head arrangements enable more than one mark to be measured simultaneously, e.g., one mark per sensor head can be measured at the same time.

[0080] However, present sensor head design means that it is very difficult to accommodate more than one such sensor head in a metrology tool. The sensor heads are typically too large and cannot be efficiently positioned next to each other. They are furthermore too expensive. Much of this bulk and expense of the sensors is due to the precision and / or accuracy requirements of alignment metrology, and because of this, the sensor head is typically able to be accurately and precisely controlled in six degrees of freedom (dof). The apparatus required to enable the necessary actuation to actuate the sensor head in 6 dof is bulky, both in terms of the required actuators and actuation space, and the necessary ancillary tools to support 6 dof control with the required accuracy / precision, e.g., providing for a 6 dof metrology system to measure the sensor head position.

[0081] With a multiple sensor head arrangement, it is not adequate to simply fix the sensor heads together such that the multiple sensor heads can be moved together (e.g., in 6 dof). Because there may be variation in mark pitch (distance between adjacent marks) and / or variation in height between adjacent marks, present sensor head design proposals describe providing for individual sensor head positioning relative to one another (e.g., such that each sensor head can be controlled individually in 6 dof). However, for the reasons provided, this may be impractical due to the bulk and / or cost of such a system.

[0082] As such, a sensor head arrangement is proposed comprising a first sensor head, a second sensor head and a third sensor head, wherein: the first sensor head is adjustable in only a first two degrees of freedom, the first two degrees of freedom comprising linear movement (displacement) in a first direction (e.g., x) and in a second direction (e.g., y), the second direction being different to the first direction; the second sensor head is adjustable in only a second two degrees of freedom, the second two degrees of freedom comprising linear movement (displacement) in the first direction and in a third direction (e.g., z), the third direction being different to the first direction and second direction; and the third sensor head is fixed (i.e., it is not adjustable and has zero degrees of freedom).

[0083] The first direction, second direction and third direction may be all mutually perpendicular, although this is not strictly necessary. By convention, the first or x direction and the second or y direction may together define the substrate plane (and all planes parallel thereto), with the third or z direction being perpendicular to the substrate plane. The y direction may be the scan direction of the lithographic apparatus, assuming a scanner; the concepts are not specifically limited to scanners and can be applied to a stepper apparatus for example.

[0084] By providing a sensor head arrangement where two sensor heads have only two degrees of freedom and the other sensor head fixed to the metrology frame, it is possible to make the sensor heads much smaller than they are presently, e.g., such that the three sensor head arrangement proposed can be accommodated in the volume presently occupied by the present single head sensor. This also means that the sensor heads can be positioned close together.

[0085] Such an arrangement may make it possible to measure three alignment marks simultaneously with (for example) approximately 0.1 nm precision, where the sensor-to-sensor position is on the order of 100 nm in x, y and z. In the non-adjustable directions for each sensor head, the accuracy results directly from the machining accuracy.

[0086] The sensor head arrangement may comprise only these three sensor heads. Where this is the case, it is possible to adjust the sensor heads such that their respective focal points all lie substantially on a straight line in three dimensions. Having all three focal points on a straight line means that wafer stage short-stroke module can align the wafer perfectly (at least in x and y) to this straight line, resulting in zero lateral scan offset, although there may be a small z offset due to wafer non-flatness. Variation in mark pitch in x and y can be accommodated.

[0087] It may be that the adjustable (non-fixed) sensor heads may each comprise sensor heads actuation arrangements where the actuation is only applied in the x-direction, this actuation providing selectable control of the sensor head in its other degree of freedom (i.e., the y-direction and z-direction respectively). In each case, the sensor head may comprise (at least a first actuator and second actuator spaced apart in the z-direction, each of which actuate the sensor head in the x-direction.

[0088] In each case, the sensor head comprises a flexure arrangement (e.g., comprising a plurality of flexures and a plurality of intermediate supports) which provides control of the sensor head positioning in the other degree of freedom based on an actuation delta (i.e., a difference between displacements applied by the first actuator and second actuator), in addition to the control of the sensor head positioning in the x-direction by actuation of the two actuators together. Such sensor heads can be made to be particularly small compared to present sensor arrangements, with actuation in only one direction. Detail of each of these adjustable sensor heads will be described below. However, it can be appreciated that these are only examples and any sensor heads providing only two degrees of freedom (displacement in x / y and displacement in x / z respectively) can be used including arrangements with actuation provided in each of the degrees of freedom.

[0089] Figure 5 illustrates a method of using the proposed sensor head arrangement according to an embodiment. In each of Figures 5(a), 5(b) and 5(c) there is shown a sensor head arrangement 10 comprising a first sensor head 10XY, second sensor head 10XZ and a third sensor head 10F. The first sensor head 10XY comprises 2 dof in x and y, the second sensor head 10XZ comprises 2 dof in x and z and the third sensor head 10F is fixed relative to the metrology frame (not shown), i.e., it has 0 dof. In each Figure, the top portion of the drawing is a side view showing the sensor head arrangement 10 and the relative positions of focal points FP1, FP2, FP3 on the xz plane, while the bottom portion of the drawing is a top-down view showing the relative positions of focal points FP1, FP2, FP3 on the xy (substrate) plane.

[0090] Figure 5(a) shows the sensor head arrangement prior to any adjustment being made. Ideally, the sensor heads would be perfectly aligned such that their respective focal points FP1, FP2, FP3 lie on a common line on a nominal or ideal grid NG. However, this is likely not to be the case due tounintentional offsets, in 6 dof, for each of the sensor heads 10XY, 10XZ, 10F. This is likely to be the case due to one or more of: manufacturing / assembly imperfections, effects of metrology frame movements and / or vibrations, movement / drift in sensor head positions (particular for the adjustable heads). In addition, it may be that different target pitches and / or variation in the target pitch (particularly in x) needs to be accommodated. It can be seen in Figure 5(a) that focal points FP1, FP2, FP3 are not aligned in a straight line in any of x, y or z.

[0091] In Figure 5(b) the second sensor head is actuated to align focal point FP2 with the focal point FP1, FP3 such that they lie on a common line Lnxzon the xz plane. In Figure 5(c), the first sensor head is actuated to align focal point FP1 with focal points FP2, FP3 on the xy plane; i.e., on line Lnxy. As such, in the latter drawing, the 3 focal points FP1, FP2, FP3 of the 3 sensors have been adjusted to the correct pitch in x and y such that they are (e.g., substantially or perfectly) on a straight line. This allows the wafer stage short-stroke module to align the wafer to this straight line resulting in zero lateral scan offset.

[0092] Specific examples of the first sensor head 10XY and second sensor head 10XZ will now be described.

[0093] Figure 6 is a schematic illustration of an arrangement for the first sensor head 10XY , according to such an example. A first actuator 12A contacts one side of the sensor head holder 2A and moves the sensor head holder 2A by a displacement X2 in the x-direction that is generally parallel to the substrate W. Similarly, a second actuator 11 A contacts the intermediate support 1 A and moves the intermediate support 1 A by a movement amount Xi in the x-direction. These actuators 11 A and 12A operate together to control the position and any potential angle of the radiation 5A being emitted from the first sensor head 10XY.

[0094] The actuators 11 A, 12A are shown in Figure 6 to be attached to the intermediate support 1A and the sensor head holder 2A, respectively. However, the actuators 11 A, 12A may be integrated into the body of the intermediate support 1A and the sensor head holder 2 A, respectively. The actuators 11 A, 12A may comprise a solenoid, a piezoelectric actuator, a servo moto, a stepper motor, an electric motor, a pneumatic motor, a hydraulic motor, a linear actuator, a piezostepper, or any combination thereof with or without an additional transmission. In addition, more than one actuator may be provided to control the movement and position of the sensor head holder 2A, and more than one actuator may be provided to control the movement and position of the intermediate support 1A to provide even more control of positioning.

[0095] In one embodiment, the first sensor head 10XY and the sensor head holder 2 A are connected such that there is no relative movement and no degrees of freedom between the two elements. As a result, the first actuator 12A moves together both the sensor head holder 2A and the first sensor head 10XY. In one embodiment, one flexure E2A is positioned on one side (in the -x-direction) of the first sensor head 10XY, while another flexure E3A is positioned on the other side (in the +x-direction) of the first sensor head 10XY. Similarly, one flexure E1A is positioned on one side (in the -x-direction)of the first sensor head 10XY, while another flexure E4A is positioned on the other side (in the +x- direction) of the first sensor head 10XY. However, the embodiment is not limited to this. Both flexures E2A and E3A between the sensor head holder 2A and the intermediate support 1 A may be on the same side of the first sensor head 10XY. Both flexures E2A and E3A may be on the -x-direction side of the first sensor head 10XY, or alternatively, both flexures E2A and E3A may be on the +x-direction side of the first sensor head 10XY. Both flexures E1A and E4A between the intermediate support 1A and the fixed support 3A (e.g., part of a metrology frame) may similarly be on the same side of the first sensor head 10XY as well. Further, though a pair of flexures E2A and E3A are disclosed for coupling between the sensor head holder 2A and the intermediate support 1A and a pair of flexures E1A and E4A are disclosed for coupling between the intermediate support 1A and the fixed support 3 (e.g., part of a metrology frame), the example is not limited to this. There may just be one flexure for coupling between the two components. As a result, it is possible to have just one flexure, such as E2A, positioned between the intermediate support and the sensor head holder 2A. It is also possible to have just one flexure, such as E1A, positioned between the intermediate support 1A and the fixed support 3. On the other hand, it is also possible to have three or more flexures between the sensor head holder 2A and the intermediate support 1 A or between the intermediate support 1 A and the fixed support 3.

[0096] As mentioned previously, the sensor head holder 2A is coupled to the intermediate support 1A by a pair of spaced flexures E2A and E3A. In other words, the flexures E2A and E3A are positioned between the sensor head holder 2A and the intermediate support 1A. In a similar manner, the intermediate support 1A is coupled to the fixed support 3 by flexures E1A and E4A. In other words, the flexures El A and E4A are positioned between the intermediate support 1 A and the fixed support 3.

[0097] As mentioned previously, the fixed support 3 comprises a fixed reference. The fixed support 3 may comprise a single fixed frame or a plurality of fixed frames. Figure 6 shows a plurality of fixed supports 3, which may be connected to the same fixed frame or a plurality of fixed frames.

[0098] The flexures E1A-E4A will be described next. The flexures E1A-E4A are elongated and generally parallel to one another in the z-direction while at rest (not stressed). In one embodiment these flexures are identical elastic elements. The flexures E1A-E4A are shaped as flat elongate sheets or straps that may be made of a metal or other material and may be shaped by electrical discharge machined (EDM) wire as one option. The flexures can be made from various metals, such as titanium, steel, and / or aluminum or another material and are stiff in the plane in which they lie. In one embodiment, the flexures E1A-E4A are formed by electrical discharge machined wire techniques (as a non-limiting example).

[0099] In one embodiment, the flexures E1A-E4A may be of uniform thickness along its length. In another embodiment, each flexure may be thicker in the middle at thinner than at the top and bottom where it meets the intermediate support 1A and the sensor head holder 2A, respectively. Making the flexures thinner in certain locations functions to concentrate the areas of flexure more in the thinner areas in comparison to the thicker areas. Thus, having flexures that are thinner at the ends will tend tokeep the central portions more straight in comparison with the more flexible end portions, In one embodiment, the thickness of the flexures E1A-E4A may range between 0.2-1.5mm.

[0100] As shown in Figure 7, each of the flexures are shown in cross section, illustrating the planes in which they lie, which are angled with respect to the X direction and the Y direction. Otherwise stated, the flexures are slightly rotated about the z-axis. Given its flat configuration, the flexures are relatively stiff in the plane in which they lie, and exhibits maximum flexibility in a direction perpendicular to that plane. As a result of the manner in which their planes are angled relative to the x and y directions, each of the flexures E1A-E4A provides more flexibility in the x-direction than the y-direction, as will be appreciated more fully from descriptions below. As will also be appreciated from below, while the actuators operate to exert movement to the sensor head holder 2 A and the intermediate support 1A in the x-direction to control pitch between the detectors, the angle of the flexure plane enables the actuators to also control a position of the detector in the y-direction (although movement will be a lot less in the y-direction).

[0101] Figure 7 illustrates that in one embodiment, the planes of the flexures E2A and E3A between the sensor head holder 2 A and the intermediate support 1A are parallel to one another and are angled with respect to the planes of the flexures E1A and E4A between the intermediate support 1A and the fixed support 3. The planes of flexures E1A and E4A are parallel to one another as well, as can also be appreciated from Figure 7. In one embodiment, when no external forces are acting on the system, the flexures El A and E4A have a slight rotation Rz = dcp as shown in Figure 7, and the flexures E2A and E3A are preferably but not necessarily made with a slight rotation of the same magnitude in the opposite direction Rz = -dcp, also shown in Figure 7. The angle dcp is the angle that the plane of the flexures El A and E4A forms with respect to the y-direction as shown in Figure 7. Similarly, the angle -dcp is the angle that the plane of the flexures E2A and E3A forms with respect to the y-direction. The flexures El A and E4A are rotated by magnitude dcp in a positive direction, while the flexures E2A and E3A are rotated by the same magnitude dcp but in a negative direction. For example, the angle dcp may be in the range of 1- lOOmrad. In a non-limiting embodiment, the angle dcp may be between 20 to 40 mrad, and in one embodiment, it is approximately 30mrad (e.g., approximately 1.7°).

[0102] As the flexures E1A and E4A are not perpendicular to the x-axis, a movement XI in the x- direction of the intermediate support 1 also yields a small movement Y1 in the y-direction, as seen in the vector diagram in Figure 7. The displacement vectors show that for a displacement XI in the x- direction, a resulting displacement Y 1 in the y-direction is created due to the angle dcp. A trigonometricY1 equation can be used to solve for Yl: tan dxp = — . As a result, Y1 = tan (dtp) * XI. Small-angleY1 approximation can simplify the equation to a linear formula: dxp = — . As a result, Yl is determined to be Yl = dcp*Xl.

[0103] As the flexures E2A and E3A have a small built-in rotation Rz = -dcp, the sensor head holder 2 A will also move in the y-direction during actuation in the x-direction of the actuators 11A and 12 A.Assuming the sensor head holder 2A makes twice the x-stroke compared to the intermediate support 1A (meaning the first actuator 12A moves the sensor head holder 2 A twice the distance in the x- direction compared to the second actuator 11A moving the intermediate support 1A), the following equation can be applied: X2 = 2*X1 (or XI =*X2). For example, a displacement of XI = 3mm would result in a displacement of X2 = 6mm. A displacement Y2 in the y-direction of the sensor head holder 2 during actuation can be determined by the following:Y2 = Y movement of intermediate support 1+ Y relative movement with respect to intermediate support 1= Yl - dtp * (X2 - XI)= dtp * XI — dtp * (2 * XI — XI)= 0

[0104] As a result, whenX2 = 2*X1, then Y2 = 0. In other words, when the first actuator 12A displaces the sensor head holder 2A at exactly twice the distance in the x-direction as compared to the second actuator 11A displacing the intermediate support 1A, the sensor head holder 2 A does not move in the y-direction, and the sensor head holder 2A moves in a straight line along the x-direction. Consequently, the first sensor head 10XY will have a linear movement in the x-direction.

[0105] If, however, the first actuator 12A does not move at exactly double the x-distance compared to the second actuator 11 A, then the sensor head holder 2 A will also move slightly in the y-direction. Using a displacement constant of the second actuator 11 A as 5, the following equation can be applied: XI = X2 + 6. For example, 6 can be in the range of 0.01-0.2mm. In an embodiment, the constant 6 is equal to 0.1mm. Substituting into Y2 yields:

[0106] Simplifying the equation yields Y2 = 2*8*dtp. Tilting the flexures E1A-E4A with Rz = +dcp and Rz = -dcp creates an x-y manipulator. By adjusting the two degrees of freedom X2 and 5 in XI = X2 + 6 results in the equations X2 = X2 and Y2 = 2*6*d<p. It is noted that this is the linearized formula for small dtp. Moving the intermediate support 1A in the x-direction therefore causes fine adjustment of the sensor head holder 2 A in the y-direction.

[0107] Further, the substrate alignment system is not limited to the embodiment described above. Other configurations are also possible. For example, the substrate alignment system could also be used rotated 90° about the x-axis. The substrate alignment system could also be rotated in other various amount about the axes.

[0108] Figure 8 is a schematic illustration of an arrangement for the second sensor head 10XZ, according to an example.

[0109] The height of the second sensor head 10XZ may be manipulated on a sub-micrometer level by making X1=X2+ AX . A small variation AX±creates a AZ movement on the sub-micrometer level that is needed to reach the required z-accuracy for the second sensor head 10XZ.

[0110] In the case where p = 0, the flexures are in the z-direction. As a result, the fine adjustment AZ yields, with ? being a constant depending on the shape of flexures E1Z-E4Z and L0 being the distance between an intermediate support 1Z and a sensor head holder 2Z:For X2= 0, the solution yields AZ = 0 regardless of X2.

[0111] This means, however, that for X2= 0, the initial manufacturing tolerances of AZ being on the order of ±5 um cannot be corrected for to reach 100 nm accuracy.

[0112] As shown in Figure 8, by rotating the flexures E1Z-E4Z a small angle <p, a z-adjustment may be made possible for X2= 0. The AZ adjustment is shown as — Z1for the intermediate support 1Z and — Z2for the sensor head holder 2Z, both of which are in the -z-direction. Figure 8 shows that, in the initial stress-free situation, the flexures E1Z and E4Z are slightly rotated over an angle + p. On the other hand, the flexures E2Z and E3Z are slightly rotated over an angle — p.

[0113] When the second sensor head 10XZ now moves over — q < X2< q, where q is a displacement of the second sensor head 10XZ in the x-direction, z-corrections may be made over the whole stroke by applying a value AXj 0 in the formula Xt= * X2+ AXj as long as tan (p) > ~- As a result, the small angle p allows for z-adjustment for both X20 as well as X2= 0.

[0114] The third sensor head may be simply fixed in some manner to fixed support such as fixed support 3 of the examples of Figures 6 to 8. An arrangement similar to Figure 6 is possible, but where the flexures are all replaced with very stiff members / elements such that the third sensor head is fixed with respect to the fixed support 3. However, any suitable method for fixing the third sensor head to the metrology frame / fixed support 3 may be used.

[0115] In the following, further features, characteristics, and exemplary technical solutions of the present disclosure will be described in terms of clauses that may be optionally claimed in any combination:1. A sensor head arrangement comprising: a first sensor head being adjustable in only a first two degrees of freedom, the first two degrees of freedom comprising displacement in a first direction and displacement in a second direction, the second direction being different to the first direction;a second sensor head being adjustable in only a second two degrees of freedom, the second two degrees of freedom comprising displacement in the first direction and displacement in a third direction, the third direction being different to the first direction and second direction; and a third sensor head having zero degrees of freedom for adjustment.2. A sensor head arrangement as described in clause 1, wherein the sensor head arrangement comprises no more sensor heads other than the first sensor head, second sensor head and third sensor head.3. A sensor head arrangement as described in clause 1 or 2, wherein the first direction and second direction together define a substrate plane of a substrate to be measured by the sensor head arrangement.4. A sensor head arrangement as described in any preceding clause wherein the first sensor head and second sensor head are configured to be adjustable such that the respective focal points of the first sensor head, second sensor head and third sensor head lie substantially on a straight line in at least each of said first dimension and second dimension.5. A sensor head arrangement as described in clause 4, wherein the first sensor head and second sensor head are configured to be adjustable such that the respective focal points of the first sensor head, second sensor head and third sensor head lie substantially on said straight line additionally in said third dimension.6. A sensor head arrangement as described in any preceding clause, wherein said first sensor head and / or said second sensor head each comprise only actuators which actuate in only the first direction.7. A sensor head arrangement as described in clause 6, wherein said first sensor head and / or said second sensor head each comprise a first actuator and a second actuator.8. A sensor head arrangement as described in clause 7, wherein actuation of the first actuator and second actuator of the first sensor head provides controllable displacement of the first sensor head in each of said first two degrees of freedom.9. A sensor head arrangement as described in claim 7 or 8, wherein actuation of the first actuator and second actuator of the second sensor head provides controllable displacement of the first sensor head in each of said second two degrees of freedom.10. A sensor head arrangement as described in any of clauses 6 to 9, wherein said first sensor head and / or said second sensor head each comprise a flexure arrangement to provide said two degrees of freedom.11. A sensor head arrangement as described in clause 10, wherein the flexure arrangement of said first sensor head comprises a plurality of flexures, each of the flexures comprising a rotation around the third direction axis.12. A sensor head arrangement as described in clause 10 or 11, wherein the flexure arrangement of said second sensor head comprises a plurality of flexures, each of the flexures comprising a rotation around the second direction axis.13. A sensor head arrangement as described in any preceding clause, wherein said second direction is perpendicular to the first direction and said third direction is perpendicular to each of the first direction and second direction.14. A metrology system comprising: the sensor head arrangement of any preceding clause; and a metrology frame for supporting the sensor head arrangement.15. A metrology system as described in clause 14, wherein the third sensor head is rigidly fixed to the metrology frame.16. A lithographic apparatus comprising: a patterning device support for supporting a patterning device; a substrate support for supporting a substrate; a projection system being operable to project a patterned beam onto the substrate, subsequent to the beam being patterned by the patterning device; and a metrology system as described in clause 14 or 15, being operable to measure positional information relating to a position of said substrate.17. A lithographic apparatus as described in clause 16, wherein the second direction comprises a scan direction of the lithographic apparatus18. A lithocell, comprising: a lithographic apparatus for applying patterns on a substrate; and the metrology system of clause 14 or 15, being operable to measure a parameter of interest of said patterns.19. A lithocell as described in clausel8, wherein the parameter of interest is overlay or focus.20. A lithocell as described in clause 18 or 19, wherein said lithographic apparatus comprises the lithographic apparatus of clause 16 or 17.21. A method of using the metrology system of clause 14 or 15, comprising: adjusting the first sensor head and second sensor head such that the respective focal points of the first sensor head, second sensor head and third sensor head lie substantially on a straight line in at least each of said first dimension and second dimension. 2. A method as described in clause 21, wherein said adjusting step adjusts the first sensor head and second sensor head such that the respective focal points of the first sensor head, second sensor head and third sensor head lie substantially on said straight line additionally in said third dimension.23. A method as described in clause 21 or 22, comprising adjusting a substrate stage such that a substrate carried on the substrate stage is aligned with said straight line.

[0116] The proposed sensor head arrangement may be used for pre-exposure metrology (e.g., alignment as part of an alignment sensor within an alignment station) and / or post-exposure metrology (e.g., overlay / focus or other parameter of interest metrology as part of a scatterometer tool or similar).

[0117] The terms “radiation,” “beam,” “light,” “illumination,” or the like can be used herein to refer to one or more types of electromagnetic radiation, for example, ultraviolet (UV) radiation (for example, having a wavelength / . of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-100 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G- line 436 nm; H-line 405 nm; and / or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in some aspects, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of 5-20 nm.

[0118] Although some aspects of the present disclosure are described in the context of lithographic apparatuses in the manufacture of ICs, it should be understood that lithographic apparatuses described herein can be used in other applications, for example, in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, LCDs, thin-film magnetic heads, etc. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as specific examples of the more general terms “substrate” or “target portion”, respectively. A substrate can be processed before or after exposure in, for example, a track unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) and / or a metrology unit. Where applicable, aspects disclosed herein can be applied to such and other substrate processing tools. Furthermore, a substrate can be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein can also refer to a substrate that already contains multiple processed layers.

[0119] Furthermore, although some aspects of the present disclosure are described in the context of optical lithography, it should be understood that aspects of the present disclosure are not limited to optical lithography. For example, in imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device can be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0120] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0121] The present disclosure has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. The foregoing description of specific aspects will so fully reveal the general nature of the present disclosure that others can, by applying knowledge within the skill of the art, readily modify and / or adapt for various applications such specific aspects, without undue experimentation and without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed aspects, based on the teaching and guidance presented herein.

[0122] It is to be understood that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections can set forth one or more, but not necessarily all, aspects of the present disclosure as contemplated by the inventor(s), and thus, are not intended to limit the present disclosure and the appended claims in any way. The breadth and scope of the protected subject matter should not be limited by any of the above-described aspects, but should be defined in accordance with the following claims and their equivalents.

Claims

CLAIMS1. A sensor head arrangement comprising: a first sensor head being adjustable in only a first two degrees of freedom, the first two degrees of freedom comprising displacement in a first direction and displacement in a second direction, the second direction being different to the first direction; a second sensor head being adjustable in only a second two degrees of freedom, the second two degrees of freedom comprising displacement in the first direction and displacement in a third direction, the third direction being different to the first direction and second direction; and a third sensor head having zero degrees of freedom for adjustment.

2. A sensor head arrangement as claimed in claim 1, wherein the sensor head arrangement comprises no more sensor heads other than the first sensor head, second sensor head and third sensor head.

3. A sensor head arrangement as claimed in claim 1 or 2, wherein the first direction and second direction together define a substrate plane of a substrate to be measured by the sensor head arrangement.

4. A sensor head arrangement as claimed in any preceding claim, wherein the first sensor head and second sensor head are configured to be adjustable such that the respective focal points of the first sensor head, second sensor head and third sensor head lie substantially on a straight line in at least each of said first dimension and second dimension.

5. A sensor head arrangement as claimed in claim 4, wherein the first sensor head and second sensor head are configured to be adjustable such that the respective focal points of the first sensor head, second sensor head and third sensor head lie substantially on said straight line additionally in said third dimension.

6. A sensor head arrangement as claimed in any preceding claim, wherein said first sensor head and / or said second sensor head each comprise only actuators which actuate in only the first direction.

7. A sensor head arrangement as claimed in claim 6, wherein said first sensor head and / or said second sensor head each comprise a first actuator and a second actuator.

8. A sensor head arrangement as claimed in claim 7, wherein actuation of the first actuator and second actuator of the first sensor head provides controllable displacement of the first sensor head in each of said first two degrees of freedom.

9. A sensor head arrangement as claimed in claim 7 or 8, wherein actuation of the first actuator and second actuator of the second sensor head provides controllable displacement of the first sensor head in each of said second two degrees of freedom.

10. A sensor head arrangement as claimed in any of claims 6 to 9, wherein said first sensor head and / or said second sensor head each comprise a flexure arrangement to provide said two degrees of freedom.

11. A sensor head arrangement as claimed in claim 10, wherein the flexure arrangement of said first sensor head comprises a plurality of flexures, each of the flexures comprising a rotation around the third direction axis.

12. A sensor head arrangement as claimed in claim 10 or 11, wherein the flexure arrangement of said second sensor head comprises a plurality of flexures, each of the flexures comprising a rotation around the second direction axis.

13. A sensor head arrangement as claimed in any preceding claim, wherein said second direction is perpendicular to the first direction and said third direction is perpendicular to each of the first direction and second direction.

14. A metrology system comprising: the sensor head arrangement of any preceding claim; and a metrology frame for supporting the sensor head arrangement.

15. A metrology system as claimed in claim 14, wherein the third sensor head is rigidly fixed to the metrology frame.

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