Measurement systems, lithographic apparatus, device manufacturing method and a method of measuring

A measurement system for measuring a position and/or displacement of an object (40), the measurement system comprising a sensor (20) and a target (45), the sensor comprising an electromagnet (21); a driving circuit (24) configured to drive the electromagnet to generate an alternating magnetic field (AMF); a measuring circuit (25) configured to measure an electrical impedance parameter of the electromagnet; the target being located on a surface (41) of the object that faces the sensor, wherein the target comprises a graphene layer (46), and wherein, in use, when the alternating magnetic field interacts with the target, the alternating magnetic field changes (RMF), altering the electrical impedance parameter of the electromagnet.

FIELD

The present invention relates to measurement systems, a lithographic apparatus, a device manufacturing method, and a method of measuring using the measurement systems.

BACKGROUND

The lithographic apparatus comprises one or more objects that need to be positioned, e.g. a substrate table, a support structure to support a patterning device or an optical element. In order to accurately position the object, displacements and/or the position of the object must be accurately measured. Any error in the measurement of the displacements and/or position of the object may lead to the object being assumed to be in the wrong position or may lead to errors in positioning the object. It is desirable to accurately measure displacements and/or position of an object such that, for example, the object can be moved to a specific location.

Known measurement systems may include the use of an electromagnetic sensor. An electromagnetic sensor has a driving circuit configured to drive an electromagnet to generate an alternating magnetic field. If the electromagnetic sensor is close to the object the alternating magnetic field interacts with the object, and consequently the object affects the alternating magnetic field. Thus the presence of the object affects an electrical impedance parameter of the electromagnet in a manner that depends on the relative position of the object and the electromagnet. Thus location and/or movement of the object relative to the electromagnet can be detected as changes in the electrical impedance parameter. Thus the displacement and/or position of the object may be determined. However, errors in the distance measured by the electromagnetic sensor may be inherent in the measurement and it is desirable to reduce or prevent errors in the measurement of the displacement and/or position of the object.

SUMMARY

It is desirable, for example, to provide an improved measurement system that can accurately measure the displacement and/or position of an object.

According to an aspect of the invention, there is provided a measurement system for measuring a position and/or displacement of an object, the measurement system comprising a sensor and a target, the sensor comprising an electromagnet; a driving circuit configured to drive the electromagnet to generate an alternating magnetic field; and a measuring circuit configured to measure an electrical impedance parameter of the electromagnet; the target being located on a surface of the object that faces the sensor, wherein the target comprises a graphene layer, wherein, in use, when the alternating magnetic field interacts with the target, the alternating magnetic field changes, altering the electrical impedance parameter of the electromagnet.

According to an aspect of the invention, there is provided a measurement system for measuring a displacement and/or a position of an object, the measurement system comprising a sensor comprising an electromagnet; a graphene sensor layer located on the electromagnet; a driving circuit configured to drive the electromagnet to generate an alternating magnetic field; and a measuring circuit configured to measure an electrical impedance parameter of the electromagnet.

According to an aspect of the invention, there is provided a lithographic apparatus comprising a measurement system of any one of the claims.

According to an aspect of the invention, there is provided a device manufacturing method using the lithographic apparatus comprising a measurement system of any one of the claims.

According to an aspect of the invention, there is provided a method of measuring the position of an object using the measurement system of any one claims1to16, the method comprising the steps of: driving the electromagnet to generate an alternating magnetic field; positioning the electromagnetic sensor relative to a target on an object such that the alternating magnetic field interacts with the target and changes the alternating magnetic field, altering the electrical impedance parameter of the electromagnet; measuring an electrical impedance parameter of the electromagnet; and determining the position of the object based on the electrical impedance parameter of the electromagnet.

DETAILED DESCRIPTION

FIG. 1schematically depicts a lithographic apparatus of an embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or DUV radiation);

a support structure (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 support table, e.g. a sensor table to support one or more sensors or a substrate support apparatus60constructed to hold a substrate (e.g. a resist-coated production substrate) W, connected to a second positioner PW configured to accurately position the surface of the table, for example of a substrate W, 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 part of, one, or more dies) of the substrate W.

As here depicted, the lithographic apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the lithographic apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two or more tables (or stage(s) or support(s)), e.g., two or more substrate tables or a combination of one or more substrate tables and one or more sensor or measurement tables. In such “multiple stage” machines the multiple tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. The lithographic apparatus may have two or more patterning device tables (or stage(s) or support(s)) which may be used in parallel in a similar manner to substrate, sensor and measurement tables. The lithographic apparatus may be of a type that has a measurement station, at which there are various sensors for characterizing a production substrate prior to exposure and an exposure station, at which the exposures are commanded out.

The lithographic apparatus is of a type wherein at least a portion of the substrate W may be covered by a immersion liquid10having a relatively high refractive index, e.g. water such as ultra pure water (UPW), so as to fill an immersion space11between the projection system PS and the substrate W. An immersion liquid10may also be applied to other spaces in the lithography apparatus, for example, between the patterning device MA and the projection system PS. Immersion techniques can be used to increase the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate W, must be submerged in immersion liquid10; rather “immersion” only means that an immersion liquid10is located between the projection system PS and the substrate W during exposure. The path of the patterned radiation beam B from the projection system PS to the substrate W is entirely through immersion liquid10.

The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam B. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator IL can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator IL may be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross-section. Similar to the source SO, the illuminator IL may or may not be considered to form part of the lithographic apparatus. For example, the illuminator IL may be an integral part of the lithographic apparatus or may be a separate entity from the lithographic apparatus. In the latter case, the lithographic apparatus may be configured to allow the illuminator IL to be mounted thereon. Optionally, the illuminator IL is detachable and may be separately provided (for example, by the lithographic apparatus manufacturer or another supplier).

The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device MA. Having traversed the patterning device 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 position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate support apparatus60can be moved accurately, e.g. 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 (which is not explicitly depicted inFIG. 1) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the support structure MT may 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 support apparatus60may 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 support structure MT may be connected to a short-stroke actuator only, or may be fixed. Patterning device MA and substrate W may be aligned using patterning device alignment marks M1, M2and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2as illustrated occupy dedicated target portions, they may be located in spaces between target portions C (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device MA, the patterning device alignment marks M1, M2may be located between the dies.

1. In step mode, the support structure MT and the substrate support apparatus60are 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 support apparatus60is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate support apparatus60are 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 support apparatus60relative to the support structure MT may be determined by the (de)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion C in a single dynamic exposure, whereas the length of the scanning motion (and size of the exposure field) determines the height (in the scanning direction) of the target portion C.

3. In another mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate support apparatus60is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate support apparatus60or 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 of a type as referred to above.

A controller500controls the overall operations of the lithographic apparatus and in particular performs an operation process described further below. Controller500can be embodied as a suitably-programmed general purpose computer comprising a central processing unit, volatile and non-volatile storage means, one or more input and output devices such as a keyboard and screen, one or more network connections and one or more interfaces to the various parts of the lithographic apparatus. It will be appreciated that a one-to-one relationship between controlling computer and lithographic apparatus is not necessary. One computer can control multiple lithographic apparatuses. Multiple networked computers can be used to control one lithographic apparatus. The controller500may also be configured to control one or more associated process devices and substrate handling devices in a lithocell or cluster of which the lithographic apparatus forms a part. The controller500can also be configured to be subordinate to a supervisory control system of a lithocell or cluster and/or an overall control system of a fab.

Arrangements for providing immersion liquid between a final optical element of the projection system PS and the substrate W can be classed into three general categories. These are the bath type arrangement, the so-called localized immersion systems and the all-wet immersion systems. An embodiment of the present invention relates particularly to the localized immersion systems.

Desirably, various components of the lithographic apparatus are accurately positioned. As such, accurate measurement systems are needed to accurately determine the position of components within the lithographic apparatus. Any errors induced in the measurement systems may lead to an error in the placement of one component relative to another which can lead to errors in the location of the patterned radiation beam B incident on the surface of the substrate W and can lead to overlay error.

The position of an object, which may be a component within the lithographic apparatus, may be measured using a measurement system as in the present invention, the measurement system comprising an electromagnetic sensor. For example, an eddy current sensor may be used. Eddy current sensors can be advantageous because they are fairly insensitive to local environmental changes. However, the measurement made by an eddy current sensor has inherent uncertainties in the way in which the measurement is carried out.

The electromagnetic sensor uses an electromagnet to provide an alternating magnetic field, in other words a magnetic field which varies in time. When the sensor is close to the object, the alternating magnetic field interacts with the object (if the object is conductive) and eddy currents are induced beneath the surface of the object. The eddy currents which are generated depend, amongst other things, on the strength of the alternating magnetic field. The closer the electromagnet is to the object, the higher the strength the alternating magnetic field will be in the object. The eddy currents in the object induce a resulting alternating magnetic field in the object. The resulting alternating magnetic field from the object interacts with the alternating magnetic field from the sensor. This induces changes in the alternating magnetic field of the sensor which alters an electrical impedance parameter of the electromagnetic sensor. Thus, the eddy currents cause a change of the electrical impedance parameter, e.g. the inductance, of the electromagnet. The alteration of the electrical impedance parameter can be measured and used to determine the distance of the object from the electromagnetic sensor or changes in the distance. As such, the position of the object may be determined in at least one degree of freedom. Electromagnetic sensors using eddy currents are known, as well as their use as displacement sensors, for example, as described in “Measuring in the Subnanometer Range: Capavitative and Eddy Current Nanodisplacement Sensors” by S. Nihtianov, IEEE Industrial Electronics Magazine (IEM), pp 6-15, Mar. 2014.

However, one of the main limiting factors determining the accuracy of an electromagnetic sensor using eddy currents in the nanometer and sub-nanometer ranges is the penetration depth, otherwise referred to as the skin depth (δ), of the eddy currents in the object. The skin depth (δ) is defined as:

δ=2ωexc⁢μσ
where μ is the object magnetic permeability, σ is its electrical conductivity, and ωexc=2πfexcwherein fexcis the excitation frequency. The magnetic permeability and electrical conductivity are characterized by the material of the object, not the object itself.

The standard skin depth (δ) is defined as the depth at which the eddy current intensity reaches 1/e (i.e. approximately 37%) of its intensity at the object surface wherein “e” is a mathematical constant. When measuring small displacements or distances (for example, nanometer and/or sub-nanometer ranges), a skin depth (δ) of tens or even hundreds of micrometers can creates a large error, even when the electromagnet of the sensor is almost touching the object. This is because the position of the object is identified by the center of density of the eddy currents (i.e. by an image plane), which is inside the object and not on its surface. The greater the skin depth (δ), the greater the distance from the surface of the object to the center of density of the eddy currents. As the position of the object is measured to the center of density of the eddy currents and not the surface of the object, changes in the skin depth give false indications of changes of the position of the object relative to the electromagnetic sensor. The sensor can be used to measure a distance either directly or by measuring a displacement between an object and the sensor. For example, as depicted inFIG. 2, a sensor may be used to measure the distance X between the surface of a sensing coil and the surface of the object. Ideally, the sensor would measure the actual distance XA. However, the sensor measures the distance XMbetween the image plane in the electromagnet and the image plane in the object. The image plane is the centre of density of the eddy currents and is related to the skin depth in that it is a distance of

δ2
from the surface. Therefore, instead of measuring XA, the distance measured is:

XA+δo2+δe2
wherein the depth of the image plane of the electromagnet of the sensor is at

δe2
and the depth at image plane of the object is at

δo2.
Furthermore, as the skin depth δ depends on conductivity of the object (or sensor), this changes due to temperature and therefore, has a variable error leading to an unstable sensor. Additionally, even if the skin depth is constant and can be accounted for by the measurement sensor, the skin depth may have a disproportionate effect on any measurement errors. Therefore, the skin depth may be one of the main sources of instability and low-resolution in an electromagnetic sensor.

According to the present invention there is proposed a measurement system for measuring a position of an object. The measurement system comprises an electromagnetic sensor to measure the position of an object. The sensor may be an electromagnetic sensor, and is referred to as such from hereon in. The measurement system further comprises a target located on a surface of the object that faces the target. In an embodiment, the measurement system measures the position of the object in a direction substantially perpendicular to a plane formed by the target on a surface of the object. As such, desirably the target is planar on the surface of the object.

A measurement sensor according to an exemplary embodiment of the invention is depicted inFIG. 3. The measurement system comprises an electromagnetic sensor20and a target45, the electromagnetic sensor20comprising an electromagnet21, a driving circuit24configured to drive the electromagnet21to generate an alternating magnetic field and a measuring circuit25configured to measure an electrical impedance parameter of the electromagnet21and the target45being located on an object40, wherein the target45comprises a graphene layer46, and wherein when the alternating magnetic field interacts with the target45, the alternating magnetic field changes, altering the electrical impedance parameter of the electromagnet21. The object may be made of a conductive material, a semiconductor, or an insulator. In an embodiment the target has a higher conductivity than the object.

Providing a target45on a surface41of the object40affects the resulting eddy currents. The target45can be used to decrease the skin depth (δ), thus reducing the distance between the surface of the object40, and the center of density of the eddy currents used to identify the position of the object40. This reduces the error in the position measurement and allows more accurate determination of the position of the object40. The characteristics of the target45may be chosen to minimize the skin depth (δ), in order to improve accuracy of the measurement. Furthermore, if the object40is an insulator, the target45(having high conductivity), may be placed on the surface41of the object40to interact with the electromagnetic sensor20.

The electromagnetic sensor20comprises an electromagnet21, a driving circuit24and a measuring circuit25. The measuring circuit25may be located in a housing23, which is optional. The driving circuit24is used to drive the electromagnet21to generate an alternating electromagnetic field, e.g. an alternating magnetic field. The driving circuit24may be located in a housing22, which is optional.FIG. 4depicts the alternating magnetic field AMF generated by the electromagnet21, although the alternating magnetic field AMF is used and generated in the embodiments of the invention, it is not depicted in the remaining drawings to clearly show the features of the measurement system in the remaining drawings.

As depicted inFIG. 4, the alternating magnetic field AMF generates eddy currents EC in the target45.FIG. 4depicts exemplary eddy currents EC in the target45only. The eddy currents EC induce a resulting alternating magnetic field RMF in the target45. This resulting alternating magnetic field RMF interacts with the alternating magnetic field AMF as described above.

In an embodiment, the driving circuit24is an electric circuit configured to provide an alternating current to the electromagnet21. The alternating current through the electromagnet21induces the alternating magnetic field AMF. The driving circuit24may be used to control the characteristics, for example, the strength and/or frequency, of the alternating magnetic field AMF. The frequency fexcof the alternating current may be varied, however, it may be of the order of magnitude of 1 kHz to 1 GHz, preferably from 1 kHz to 10 kHz or from 10 kHz to 100 MHz. Alternatively, the frequency fexcmay be even larger than 1 GHz. As described, the frequency affects the skin depth (δ). The larger the frequency, the smaller the skin depth (δ). Therefore, a higher frequency is preferable. For example, the frequency may preferably be higher than 100 kHz, or 1 MHz, or 10 MHz, or 100 MHz or even 1 GHz. However, this may require larger power to drive the electromagnet21, therefore, it may be possible to provide a measurement system with a skin depth (δ) below an acceptable depth using the features of the present invention, which does not require the frequency to be as high as it may otherwise have to be, which can reduce the power required for the measurement system. For example, to reduce the power needed, it may be preferable to have a frequency of less than 1 MHz, or 100 kHz, or 10 kHz.

In an embodiment the measuring circuit25is used to determine a parameter of the electrical impedance of the electromagnet21. The parameter may be determined by detecting parameters relating to the current through and/or voltage across the electromagnet21, e.g. a current through the electromagnet21, a ratio of a voltage over a current through the electromagnet21, a magnitude of a drive current of the driving circuit24driving the electromagnet21, or any other suitable parameter. Preferably, the determined parameter is the inductance.

The measurement system20may further comprise a processing unit26to determine the position of the object40using the measured electrical impedance. The processing unit26may comprise a controller, microprocessor, or any other processing device which is arranged to determine the position of the object40. The processing unit26may be comprised in a controller or microprocessor having other functions in addition to determining the position of the object40. The processing unit26is depicted inFIG. 3as being located in the same housing23as the measuring circuit25, however, the processing unit26and the measuring circuit25may be provided separately.

In an embodiment, the electromagnet21comprises a metal coil (not depicted inFIG. 3). Preferably, the metal coil is formed by closely spaced turns, in other words, the coil is tightly wound. Preferably, the electromagnetic sensor20further comprises a graphene sensor layer48as described in detail below.

The skin depth of a material determines the depth at which the eddy currents occur. The smaller the skin depth, the more precise the determination of the location of the eddy current. Eddy currents may also be generated in the material of the object below the target. The eddy currents within the object adversely affect the accuracy of the sensor. Accordingly, the target should be thick enough to serve as a shield so as to prevent eddy currents from occurring within the object itself, thin enough to be able to have eddy currents concentrated in the target, and the target should have a high enough electrical conductivity to generate large enough currents to be detectable.

Graphene combines both semiconductor and metal properties. In other words, graphene is a semiconductor with zero bandgap. Although graphene has fewer free carriers than metals, it has much higher electron mobility, for example may be approximately 200,000 square centimeters per Volt-second. Therefore, at higher frequencies the generated eddy currents can follow and/or cancel the external magnetic field much more efficiently with minimum losses due to the high mobility of electrons and holes. This reduces or prevents self heating amongst other advantages. Therefore, using the electromagnetic sensor at higher frequencies improves the associated advantages of using graphene. As such, when the excitation frequency is high enough, for example, of the order of a few GHz, the graphene layer may be very thin, for example, a mono-layer of graphene may be used. As such, the thickness of the graphene layer46may be approximately 1 nm to 10 nm. Alternatively, the graphene layer46may comprise several layers of graphene. In this embodiment, the graphene layer46may have a thickness in a range between 1 nm and 50 nm or more preferably in a range between 5 nm and 20 nm. If significantly more layers of graphene are used to form the graphene layer46, the thickness may be higher. For example, in an embodiment, the thickness of the graphene layer46is less than or equal to approximately less than or equal to approximately 5 μm, preferably in a range between approximately 0.5 μm to 2 μm, or more preferably below approximately 0.5 μm.

In an embodiment, the thickness of the graphene layer46is substantially uniform. In other words, the thickness of the graphene layer46does not vary substantially in cross-section through the target45, i.e. across the length or width of the target45. In an embodiment wherein the graphene layer46is several layers of graphene, the graphene layer is substantially uniform and may include a variation of approximately 20% or less of the thickness of the graphene layer46, or more preferably, the variation is approximately 10% or less. If the thickness of the graphene layer46is substantially uniform, the outer surface of the target45, in other words, the surface of the graphene layer46that faces the electromagnet21, is substantially parallel to a surface41of the object40on which the target45is located.

In an embodiment, the outer surface of the target45, in other words, the surface of the graphene layer46that faces the electromagnet21, is substantially flat. “Substantially flat” may mean that the roughness average Ra is approximately less than 100 nm, or more preferably 10 nm. Generally, if the roughness average Ra is less than 10%, preferably less than 1%, of the distance between the target45and the electromagnet21, the effect due to the roughness is negligible. The roughness average Rabeing defined as the arithmetic average of absolute values and being calculated using the following equation:

Ra=1n⁢∑i=1n⁢⁢yi
wherein n is the number of data point used in the calculation, and y is the vertical surface position measured from an average surface height, the average surface height being the sum of the heights, divided by the number of points at which the height is measured.

Providing a target45comprising a graphene layer46has the advantage that it can be a very efficient shield against electromagnetic fields, to reduce or prevent eddy currents being generated in the object40below the graphene layer46due to the high electron mobility. This has several advantages. Firstly, the sensitivity of the measurement system may increase. Secondly, changes of the conductivity of the object40may have a reduced effect on the stability of the electromagnetic sensor20. Thirdly, the skin depth (δ) of the eddy currents may be reduced. Therefore, if a given sensitivity of the sensor is required, it may be possible to reduce the excitation frequency without increasing the skin depth (δ) beyond an acceptable amount. Reducing the excitation frequency, for example, to below approximately 1 GHz, or more preferably to approximately 1 MHz to 100 Mz, may be beneficial in that the interface electronics may be simplified and power and/or heat dissipation may be reduced.

Furthermore, providing a target45with high conductivity means that energy losses in the target45may be lower. These losses are mainly due to resistive heating of the target45due to the eddy currents. By heating the target45, its conductivity will change (i.e. it will decrease) and hence the skin depth will change (i.e. it will increase). This will result in variation of the skin depth, which may be called thermal drift, leading to a varying measurement error.

Preferably, the graphene layer46may be formed of a monolayer, i.e. a single atomic layer, of graphene, or more than one layer of graphene, i.e. several single atomic layers of graphene. Several single layers may mean any appropriate number of layers, for example, several may be 2 to 10 layers, or 3 to 5 layers. Several layers of graphene may be referred to as multi-layered graphene. It is preferable to use a monolayer of graphene to maintain high conductivity, however additional layers may be used, i.e. several layers, to prevent or further reduce eddy currents from being generated in the object40below the graphene layer46. A monolayer of graphene may be useful if the alternating magnetic field AMF produced by the electromagnetic sensor20is weak because in this case, only a single monolayer may be needed to effectively reduce or prevent eddy currents in the object. The number of layers may be selected to reach a desired thickness of the overall target45. Graphene has a high mobility of free carriers (electrons) which allow the generation of eddy currents with very low losses very close to the surface of the target45. In other words, a large proportion, or ideally all of the alternating magnetic field AMF which interacts with the target45may cause the resulting alternating magnetic field RMF.

A measurement axis may be defined as perpendicular to the surface41of the object40being measured. The measurement axis AX is depicted inFIG. 3. The cross sectional area of the target At (seeFIG. 5) and the cross-sectional area of the electromagnet Ae (seeFIG. 5) may be determined as the area (in cross section) in a plane orthogonal to the measurement axis AX. In an embodiment, the cross-sectional area of the target At at surface41of the object40is larger than the cross-sectional area of the electromagnet Ae. A plan view is shown inFIG. 5from cross-section AA inFIG. 3. As depicted inFIG. 5, the cross-sectional area of the target At is larger than the cross-sectional area of the electromagnet Ae. In an embodiment, the cross-sectional area of the target At is preferably 1.5 to 5 times larger than the cross-sectional area of the electromagnet Ae, or preferably the cross-sectional area of the target At is preferably 2-3 times larger than the cross-sectional area of the electromagnet Ae. Providing a target with a larger cross-sectional area At than the cross-sectional area of the electromagnet Ae means that higher sensitivity to the position of the target will be achieved, because it increases the interaction between the alternating magnetic field AMF and the target45.

The graphene layer46may be provided on the object40by any appropriate means. In an embodiment, the graphene layer46is deposited on the surface of the object40. For example, the graphene layer46may be deposited using chemical vapour deposition. The graphene layer46may be processed to achieve the desired thickness of the graphene layer46. For example, the production, deposition and/or processing may comprise exfoliating the graphene layer46, for example using a wedge-shaped tool, using adhesive tape, shearing, sonication, for example using a solvent, optionally with a surfactant, or two immiscible liquids, epitaxy, nanotube slicing, spin coating, supersonic spray, intercalation, using a laser, using microwave assisted oxidation, growing the target45on the object40, reducing graphite oxide, using a carbon dioxide reduction, using sodium ethoxide pyrolysis, using a roll-to-roll manufacturing process and/or using the “Tang-Lau” method. The method of producing, depositing and/or processing the graphene layer46is not limiting.

If for any reason, the graphene layer46is incompatible with the surface of the object40, it may be necessary to additionally provide an adhesive between the graphene layer46and the surface41of the object40. In an embodiment an adhesion promoting layer is deposited on the object40and the graphene layer46is deposited on the adhesion promoting layer. The surface of the object40may be treated by chemical or physical means prior to deposition of the graphene layer46.

In an embodiment, the target45further comprises at least one isolating layer47, for example, as depicted inFIG. 6, the isolating layer47being arranged in-between each graphene layer to mechanically isolate the graphene layers from one another. The following embodiments comprising at least one isolating layer47may be the same as any of the above embodiments except for as herein described. The isolating layer47is configured to substantially mechanically isolate the graphene layers from one another. As such, ideally, each graphene layer (e.g.46a) has no point of contact with another graphene layer (e.g.46b). The isolating layer47is arranged to reduce or prevent tunneling of electrons from one graphene layer to another.

As described above for the graphene layer, providing a very thin graphene layer46(e.g. a mono-layer) and using a high frequency (e.g. in the GHz range) can provide an effective shield to prevent or reduce the generation of eddy currents in the object. However, there are significant benefits working in the sub-GHz frequency range (e.g. in the MHz range). Providing a target with graphene layers46aand46bseparated by isolating layers47allows the skin depth δ to be reduced whilst limiting or reducing self heating even whilst using a lower frequency range, e.g. sub GHz. Preferably, at least one of the graphene layers46aand46b, or preferably all of the graphene layers, are mono-layers of graphene.

In this embodiment, the target45comprises two graphene layers46aand46b. The target45comprising multiple layers (i.e. at least one isolating layer and at least two graphene layers) may be referred to as a multilayer stack. The graphene layers46aand46bmay be the same as the graphene layer46in any of the above embodiments. The isolating layer47of this embodiment is arranged in-between each graphene layer46aand46bto mechanically isolate the graphene layers46aand46bfrom one another. Further isolating layers and graphene layers may be included in the target45. Preferably, the isolating layer(s) and graphene layers alternate with an isolating layer inbetween a graphene layer on either side, to mechanically isolate the graphene layers from each other.

In an embodiment, the isolating layer47may be made of at least one metal. The metal may be highly conductive and preferably is compatible with graphene such that the isolating layer47can be located adjacent to, and in contact with, the graphene layer on either side. In an embodiment, the isolating layer47comprises at least one of copper, silver, gold and/or aluminium and therefore, may be an alloy containing any of these materials. In an embodiment, the isolating layer47may comprise molybdenum disulphate.

The target45made up of the multilayer stack may have a thickness of less than or equal to approximately 5 μm, preferably in a range between approximately 0.5 μm to 2 μm, or more preferably below approximately 0.5 μm. The thickness of the multilayer stack refers to the sum of the thickness of all the layers comprising the multilayer stack. Additionally or alternatively, a thickness of the isolating layer is less than or equal to approximately 1000 nm, preferably in a range between approximately 0.1 nm to 100 nm, or more preferably in a range between approximately 1 nm to 10 nm. Additionally or alternatively, a thickness of the graphene layer is less than or equal to 1000 nm, preferably in a range between 0.1 nm to 100 nm, or more preferably in a range between 1 nm to 10 nm. The thickness of the graphene layers46aand46bmay be substantially the same as each other. The thickness of the isolating layer47may be the same as one or both of the graphene layers46aand46b.

In an embodiment, the target45may comprise 5 to 150 graphene layers46(a or b) with the same number of isolating layers47, preferably, the target45may comprise approximately 20 to 100 graphene layers with the same number of isolating layers. If the graphene layer46is adjacent to the object40(whether or not an adhesive is inbetween) then the target45comprising alternating graphene layers46(a or b) and isolating layers may have one more graphene layer46(a or b) than the number of isolating layers. If the isolating layer47is adjacent to the object40(whether or not an adhesive is inbetween), then the target45comprising alternating graphene layers46and isolating layers47may have one more isolating layer47than the number of graphene layers46(a or b).

In an embodiment, the outer surface of the target45, in other words the surface of the target45facing the electromagnet20, may be substantially parallel to a surface41of the object40on which the target45is located. In other words, the thickness of the target45made up of the multilayer stack is substantially uniform. In this instance, substantially uniform may include a variation of approximately 10% or less of the thickness of the target45, or more preferably, the variation is approximately 5% or less.

In an embodiment, the thickness of the isolating layer47is preferably substantially uniform. In other words, the thickness of the isolating layer47does not vary substantially in cross section through the target45, i.e. across the length or width of the target45. In this instance, substantially uniform may include a variation of approximately 10% or less of the thickness of the multilayer stack, or more preferably, the variation is approximately 5% or less. In this embodiment, the thickness and variation of the graphene layer46aand46bmay be as described in relation to the graphene layer46in earlier embodiments.

In an embodiment, the graphene layer46a, graphene layer46band/or isolated layer47may be deposited on top of one another. For example, each layer may be deposited using chemical vapour deposition. Either of the graphene layers46aand/or46b, may be formed using any of the methods for graphene layer46. The isolated layer47may be processed to achieve the desired thickness of each layer. For example, the deposition and/or processing may comprise exfoliating at least one of the layers, for example using a wedge-shaped tool, using adhesive tape, shearing, sonication, for example using a solvent, optionally with a surfactant, or two immiscible liquids, epitaxy, nanotube slicing, spin coating, supersonic spray, intercalation, using a laser, using microwave assisted oxidation and/or growing the layers on top of one another to form the target45on the object40. The method of producing, depositing and/or processing the graphene layers46aor46bor the isolating layer47is not limiting.

If for any reason, the isolating layer47and the graphene layers46are incompatible with each other, it may be necessary to additionally provide an adhesive. In an embodiment an adhesion promoting layer is deposited on the object40and the graphene layer46is deposited on the adhesion promoting layer. The surface of the object40may be treated by chemical or physical means prior to deposition of the graphene layer46. The method of producing, depositing and/or processing the graphene layer46is not limiting.

In an embodiment providing an isolating layer47between graphene layers46means that the target45can be used to effectively reduce or prevent eddy currents in the object40below. Having an additional layer (i.e. by providing at least one the isolating layer) means that only several, or a single, layer of graphene may be used in the graphene layer46, thus maintaining its high conductivity.

In an preferred embodiment, the electromagnetic sensor20may further comprise a graphene sensor layer48on the surface of the electromagnet21. The graphene sensor layer48on the electromagnet21may be the same as the graphene layer46except as herein described. The graphene sensor layer48may have any or all of the advantages described in relation to the graphene layer46. In particular, use of the graphene sensor layer48reduces (or prevents) losses due to heating in the graphene sensor layer48as in the target45due to the use of a highly conductive material. As such, the graphene sensor layer48reduces or prevents thermal drift of the skin depth thus improving the accuracy of the sensor. The graphene sensor layer48may differ from the graphene layer46of the target45in that it is located on the electromagnetic sensor20is not substantially uniform.

As depicted in the exemplary embodiment shown inFIG. 7A, the electromagnet21may comprise a coil. A coil as herein described may be used (with or without the graphene sensor layer48) in any of the above embodiments. As depicted inFIG. 7A, the coil may be a flat coil. An upwards view of the flat coil is depicted inFIG. 7B.FIGS. 7A and 7Bare exemplary and the shape of the electromagnet21is not limiting. In an embodiment, a helical coil may be used. A helical coil may be used which has a longitudinal axis substantially perpendicular to the measurement axis AX. A flat coil, for example as depicted, may have better mechanical stability than a helical coil and it is advantageous in that all of the turns of the coil can be brought close to the target45. In an embodiment, the electromagnetic sensor20may comprise a ferrite material around which the coil is wound. This is done to confine the alternating magnetic field AMF inside the ferrite material and avoid spreading of the alternating magnetic field AMF around the electromagnetic sensor20. However, this type of coil may have low mechanical stability compared to the flat coil. Furthermore, ferrite materials have high permeability at low frequency (which helps to confine the magnetic field inside the ferrite material), but at higher frequencies (in the MHz range) the permeability drops considerably and the positive effect may disappear and the self-heating may increase.

The electromagnetic sensor depicted inFIGS. 7A and 7Bcomprises a graphene sensor layer48on the coil. In this embodiment, the graphene sensor layer48may be patterned on the coil. The pattern may be chosen to optimize the advantageous effects of the graphene sensor layer48, e.g. to reduce or minimize thermal drift.

In an embodiment, the outer surface of the graphene sensor layer48on the surface of the electromagnetic21may have a roughness average RA which is less than approximately 100 nm, or preferably less than 10 nm. Generally, if the roughness average Ra is less than 10%, preferably less than 1%, of the distance between the target45and the electromagnet, the effect due to the roughness is negligible. The roughness of the surface of the graphene sensor layer48may affect transfer characteristics of the electromagnetic sensor20.

In this embodiment, the measurement system may not comprise the target45. In other words, a graphene sensor layer48of highly conductive material may be formed on the electromagnet21, but not the object40.

A further embodiment includes a method of measuring the position of the object40using the measurement system described in any of the above embodiments. The method comprises driving the electromagnet21to generate an alternating magnetic field AMF; positioning the electromagnetic sensor20relative to a target45on an object40such that the alternating magnetic field AMF interacts with the target45and changes the alternating magnetic field AMF, altering the electrical impedance parameter of the electromagnet21, measuring an electrical impedance parameter of the electromagnet21and determining the position of the object40based on the electrical impedance parameter of the electromagnet21.

Alternatively, the method may measure the position of the object without the target when using any one of the embodiments of the electromagnetic sensor20further comprising the graphene sensor layer48. In this embodiment, the method may comprise the steps of: driving the electromagnet21to generate an alternating magnetic field AMF, positioning the sensor relative to an object40such that the alternating magnetic field AMF interacts with the object and changes the alternating magnetic field AMF, altering the electrical impedance parameter of the electromagnet, measuring an electrical impedance parameter of the electromagnet and determining the position of the object40based on the electrical impedance parameter of the electromagnet.

The measurement system of any of the above embodiments may be used to measure the position of various components of a lithographic apparatus. The lithographic apparatus may be used for manufacturing devices by projecting a beam patterned by a patterning device onto a substrate. Alternatively, the measurement system may be used outside of the lithographic field, where appropriate. In particular, the measurement system may be useful for measuring the position of any object, especially where a high degree of accuracy is required. For example, the measurement system may be used in any scientific and/or precision measurement system for example, electron microscopes, space equipment, etc.

Any of the above embodiments may comprise multiple measurement systems, which may be used to measure the position of the object40in different degrees of freedom. Multiple measurement systems may be used to determine if the object40is tilted. If multiple measurement systems are used, the processing unit26of each measurement system may be separate. However, at least one (but not necessarily all) of the processing units may be combined, for example in a single controller. Additionally or alternatively, the measurement system may be used with any other type of sensor. The measurement system may be integrated with another type of sensor system.

The object40of any of the above embodiments may be any object and is not particularly limiting. The measurement system may be particularly useful in lithographic apparatus due to the need for highly accurate position measurements. The object40may be any component of the lithographic apparatus, for example the patterning device, a component of the projection system and/or the illumination system, the projection system and/or the illumination system, or may be a support arranged to support a component which needs to be accurately positioned, for example the substrate table, the support table and/or the support structure. The object40may be a component used to move a support. The object40is not limited to components within the lithographic apparatus. In particular, one of the advantages of the electromagnetic sensor20as described is that environmental changes around the sensor generally have a small effect on the measurement such that the sensor may be used in many applications, in connection with a lithographic apparatus, or not.

The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 436, 405, 365, 248, 193, 157 or 126 nm). The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive and reflective optical components.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

Any controllers described herein may each or in combination be operable when the one or more computer programs are read by one or more computer processors located within at least one component of the lithographic apparatus. The controllers may each or in combination have any suitable configuration for receiving, processing, and sending signals. One or more processors are configured to communicate with the at least one of the controllers. For example, each controller may include one or more processors for executing the computer programs that include machine-readable instructions for the methods described above. The controllers may include data storage media for storing such computer programs, and/or hardware to receive such media. So the controller(s) may operate according the machine readable instructions of one or more computer programs.

One or more embodiments of the invention may be applied to any immersion lithography apparatus, in particular, but not exclusively, those types mentioned above and whether the immersion liquid is provided in the form of a bath, only on a localized surface area of the substrate, or is unconfined. In an unconfined arrangement, the immersion liquid may flow over the surface of the substrate and/or substrate table so that substantially the entire uncovered surface of the substrate table and/or substrate is wetted. In such an unconfined immersion system, the liquid supply system may not confine the immersion liquid or it may provide a proportion of immersion liquid confinement, but not substantially complete confinement of the immersion liquid.

A liquid supply system as contemplated herein should be broadly construed. In certain embodiments, it may be a mechanism or combination of structures that provides an immersion liquid to a space between the projection system and the substrate and/or substrate table. It may comprise a combination of one or more structures, one or more fluid openings including one or more liquid openings, one or more gas openings or one or more openings for two phase flow. The openings may each be an inlet into the immersion space (or an outlet from a fluid handling structure) or an outlet out of the immersion space (or an inlet into the fluid handling structure). In an embodiment, a surface of the space may be a portion of the substrate and/or substrate table, or a surface of the space may completely cover a surface of the substrate and/or substrate table, or the space may envelop the substrate and/or substrate table. The liquid supply system may optionally further include one or more elements to control the position, quantity, quality, shape, flow rate or any other features of the immersion liquid.