CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application no. 62/737,557, which was filed on 27 September 2018 and which is incorporated herein in its entirety by reference.

Field

[0002] The present description relates to a level sensor, in particular a level sensor in a lithographic apparatus.

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as“design layout’’ or “design’’) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] To project a pattern on the substrate, the lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm.

A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on the substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

Summary

[0005] A topographic map of the surface of a substrate to be imaged should be made in order correctly to focus radiation on the substrate during imaging. Optical and capacitance probes used for measuring topographic maps of substrates can suffer from interactions with one or more layers on the substrate beneath the surface which is being measured. In lithography, a substrate may be coated with several layers prior to imaging and these layers are not always the same from one substrate to another. Therefore measuring a topographic map of a substrate in lithography using optical or capacitance probes is problematic because of process dependency.

[0006] A pneumatic sensor does not suffer from process dependency. A pneumatic sensor uses a comparison between gas flows into a reference gap and a measurement gap to generate a signal indicative of a difference in height between the reference gap and the measurement gap. By maintaining the reference gap essentially constant and moving the measurement gap over the surface of the substrate, a map of variations in the height of the surface of the map can be generated. This is because any difference in gas flows is the result of a difference in the size of the gaps and is independent of the process to which the substrate whose topography is being measured has been subjected.

[0007] It is desirable, for example, to provide an improved level sensor system which utilizes a pneumatic sensor in which accuracy is improved.

[0008] According to an aspect, there is provided a level sensor system comprising a pneumatic sensor comprising a reference outlet for forming a reference gap with a reference surface and a measurement outlet for forming a measurement gap with a measurement surface, wherein the pneumatic sensor is configured to make a pneumatic sensor measurement indicative of a difference between a flow of gas out of the reference outlet and a flow of gas out of the measurement outlet; a temperature sensor configured to make a temperature measurement indicative of at least one of the temperature of the reference surface and the temperature of the measurement surface; and a controller configured to adjust the pneumatic sensor measurement based on the temperature measurement to generate a signal indicative of a difference in height between the reference gap and the measurement gap.

[0009] According to an aspect, there is provided a method comprising making a pneumatic sensor measurement indicative of a difference between a flow of gas out of a reference outlet of a pneumatic sensor and a flow of gas out of a measurement outlet of a pneumatic sensor, the pneumatic sensor forming a reference gap between the reference outlet and a reference surface and forming a measurement gap between the measurement outlet and with a measurement surface; making a temperature measurement indicative of one of the temperature of the reference surface and the temperature of the measurement surface; and adjusting the pneumatic sensor measurement based on the temperature measurement to generate a signal indicative of a difference in height between the reference gap and the measurement gap.

Brief Description of the Drawings

[0010] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0011] Figure 1 schematically depicts a lithographic apparatus;

[0012] Figure 2 schematically depicts a level sensor system; and

[0013] Figure 3 is a graph showing temperature dependence of measured height difference.

Detailed Description

[0014] In the present document, the terms“radiation” and“beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm).

[0015] The term“reticle”,“mask” or“patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term“light valve” can also be used in this context. Besides the classic mask

(transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0016] Figure 1 schematically depicts a lithographic apparatus LA of an embodiment. The lithographic apparatus LA comprises:

optionally, an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. t IV radiation or lit IV radiation);

a support structure (e.g. a mask table) T 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 table or wafer table WT constructed to hold a substrate (e.g. a resist-coated production substrate) W, connected to a second positioner PW configured to accurately position a 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.

[0017] In operation, the illuminator IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0018] The term“projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and/or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and/or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the more general term“projection system”. [0019] The lithographic apparatus LA may be of a type having two or more support tables, e.g., two or more support tables or a combination of one or more support tables and one or more cleaning, sensor or measurement tables. For example, the lithographic apparatus LA is a multi-stage apparatus comprising two or more tables located at the exposure side of the projection system PS, each table comprising and/or holding one or more objects. In an example, one or more of the tables may hold a radiation-sensitive substrate. In an example, one or more of the tables may hold a sensor to measure radiation from the projection system. In an example, the multi-stage apparatus comprises a first table configured to hold a radiation-sensitive substrate (i.e., a support table) and a second table not configured to hold a radiation- sensitive substrate (referred to hereinafter generally, and without limitation, as a measurement, sensor and/or cleaning table). The second table may comprise and/or may hold one or more objects, other than a radiation-sensitive substrate. Such one or more objects may include one or more selected from the following: a sensor to measure radiation from the projection system, one or more alignment marks, and/or a cleaning device (to clean, e.g., the liquid confinement structure).

[0020] In operation, the radiation beam B is incident on the pattern (design layout) present on patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) T, 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 PMS (e.g. an interferometric device, linear encoder, 2-D encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions C, they may be located in spaces between target portions C (these are known as scribe-lane alignment marks).

[0021] In order that a substrate may be imaged in focus, a height map (topographic map) is generated of its surface topography (so called level sensing). A height map measured using optical or capacitance probes can suffer from process dependency. Process dependency means that the result of the optical or capacitance probe will vary dependent upon one or more processes or layers previously applied to the substrate.

[0022] In order to address the problem of process dependency of optical and capacitance probes, a pneumatic sensor may be used. The pneumatic sensor measures the surface topography of the substrate. The results of the pneumatic sensor are compared with the results of the optical or capacitance probe such that a process dependency can be determined. Knowledge of the process dependency is then used to correct the height map of the substrate as measured by the optical or capacitance probe using a process dependency off-set map.

[0023] An explanation of a pneumatic sensor 10 will now be given with reference to Figure 2, which illustrates an example of a level sensor system 1. The basic principle is to provide a flow of gas out of a reference outlet 100 which forms a reference gap GR with a reference surface R (which is shown in this example as being on a support of the measured surface, but as will be appreciated need not be) and a flow of gas out of a measurement outlet 200 which forms a measurement gap GM with the measurement surface (the surface of the substrate W in the illustrated embodiment). For a given pressure of gas, a gap which is larger will have a lower resistance to flow of gas such that the flow of gas at a greater gap will be larger than the flow of gas when the gap is smaller. The pneumatic sensor 10 generates a pneumatic sensor measurement which is a signal indicative of a difference in height between the reference gap GR and the measurement gap GM. The signal is generated on the basis of the difference in magnitude of flow of gas out of the reference outlet 100 and the flow of gas out of the measurement outlet 200.

[0024] In the pneumatic sensor 10 of Figure 2 a mass flow sensor 300 is used to determine the difference between the flow of gas out of reference outlet 100 and the flow of gas out of measurement outlet 200. Gas is supplied from a gas supply 400 into a shared passageway 410. Optionally a flow resistor 420 restricts the gas flow before the flow of gas is split at an inlet 412 into a first passageway 430 on the reference side and a second passageway 440 on the measurement side. Desirably each of the first passageway 430 and the second passageway 440 receive equal parts of gas from the inlet 412 of the shared passageway 410. A reference restrictor 452 and a measurement restrictor 454 restrict the gas flow in the first passageway 430 and the second passageway 440 respectively. In an embodiment the reference restrictor 452 and the measurement restrictor 454 help ensure equal flows of gas in the first passageway 430 and in the second passageway 440.

[0025] The second passageway 440 terminates in the measurement outlet 200 and the first passageway 430 terminates in the reference outlet 100. The mass flow sensor 300 is for measuring a differential mass flow between the first passageway 430 and the second passageway 440. For this purpose the mass flow sensor 300 is in fluid communication between the first passageway 430 and the second passageway 440 at locations downstream of the reference restrictor 452 and the measurement restrictor 454 and upstream of the reference outlet 100 and the measurement outlet 200, respectively. A difference in the size of the reference gap GR compared to the measurement gap GM results in a different pressure on either side of the mass flow sensor 300. Such a difference in pressure results in differential flow between the first passageway 430 and the second passageway 440 which is measured by the mass flow sensor 300. The magnitude of the measurement made by the mass flow sensor 300 is indicative of the difference in size of the measurement gap GM and the reference gap GR and as such is the pneumatic sensor measurement.

[0026] In an embodiment the size of the reference gap GR is maintained essentially constant. A variation in the size of the measurement gap GM then results in a change in the pressure at the measurement outlet 200 and so a change in the pressure in the second passageway 440. This change in pressure in the second passageway 440 leads to a change in the measurement made by the mass flow sensor 300. The magnitude of the change in mass flow can be correlated to the change in the height of the measurement gap GM.

[0027] A difficulty with the above described level sensing is that the pneumatic sensor 10 has been found to be sensitive to temperature variations of either the measurement surface or the reference surface R. This is because the gas flow out of the reference outlet 100 and the measurement outlet 200 can be heated or cooled dependent upon the temperature of the reference surface R or the surface of the measurement surface (in this case the surface of the substrate W). If both surfaces have the same constant temperature this is not a difficulty. However a difference in temperature between the reference surface R and the surface of the substrate W will result in the gas exiting the measurement outlet 200 and reference outlet 100 having different properties. The different properties of the gas (for example viscosity) will result in a change in the mass flow rate measured by the mass flow sensor 300 which can be recorded as a change in measured height of the measurement gap GM. A difference in temperature also has a small impact on pneumatic sensor 10 gain, leading to an undesirable difference in pneumatic sensor measurement between the reference side and the measurement side. If a temperature difference between the measurement surface and reference surface R is essentially constant, this is not a significant problem and can be accounted for during correlation of the measurement of mass flow by the mass flow sensor 300 and the change in height or absolute height of the measurement gap GM. However, a change in temperature of the measurement surface or reference surface R cannot be so compensated.

[0028] Figure 3 illustrates this phenomenon as experimentally measured when the relative positions of the reference surface R and the reference outlet 100 and of the measurement outlet 200 and the substrate W remain constant. The temperature of the substrate W is maintained constant and the temperature of the reference surface R diverges from a reference temperature RT as plotted along the x axis. As can be seen, the measured difference in height between the reference gap GR and measurement gap GM on the y axis varies.

[0029] By using at least one temperature sensor 510, 510a, 510b configured to make a temperature measurement indicative of at least one of a temperature of the reference surface R and the temperature of the measurement surface, the accuracy of the signal indications of height can be improved. This is done by adjusting the output of the pneumatic sensor 10 dependent upon the measured temperature. The output is adjusted based on the temperature measurement to generate a signal indicative of a difference in height between the reference gap GR and the measurement gap compensated for any variation in measured temperature from the reference temperature RT.

[0030] The temperature of a measured surface, such as the substrate W, may be controlled to a high level of accuracy in a lithographic apparatus. For example, the substrate W may be placed on a substrate support 60 before being placed onto the substrate table WT. The substrate support 60 may be thermally controlled, for example by having one or more channels for thermally conditioned fluid or by having one or more electric heat transfer devices (e.g., electric heaters). The variation in temperature may be only of the order of a few mK. However, the top surface of the substrate table WT which defines the reference surface R in this example may not be so well thermally controlled. For example, the temperature of the substrate table WT may vary greatly (compared to that of the substrate W) for example it may vary by up to 100 mK. A variation of only 100 mK in the temperature of the reference surface R could result in a 10 nm error in the measurement of the size of the measurement gap GM.

[0031] In an embodiment, the level sensor system 1 is provided with at least one temperature sensor 510, 510a, 510b. The temperature sensor 510, 510a, 510b is configured to make a temperature measurement indicative of a temperature of the reference surface R and/or of a temperature of the measurement surface (e.g., substrate W).

[0032] In an embodiment the temperature sensor 510 is provided on the reference surface R or measurement surface side of the reference gap GR or measurement gap GM. In an embodiment the temperature sensor 510a, 510b is provided on the pneumatic sensor 10 side of the reference gap GR or measurement gap GM. The pneumatic sensor 10 side of the reference gap GR or measurement gap GM is the side of the reference gap GR or measurement gap GM opposing the reference surface R or measurement surface. One or more advantages and/or disadvantages of both arrangements are described further below.

[0033] In the above described embodiment of Figure 2 the level sensor system 1 is used to measure the surface topography of the substrate W. However, the level sensor system 1 can be used to measure the difference in height between any reference gap GR and any measurement gap GM formed between the measurement outlet 200 and a measured surface. In such embodiments the temperature of the measurement surface may vary as much as that of the reference surface R or more. Therefore in such an embodiment the at least one temperature sensor 510, 510a, 510b may measure the temperature of the measurement surface instead of or in addition to that of the reference surface.

[0034] The way in which the level sensor system 1 uses the output of the at least one temperature sensor 510, 510a, 510b to compensate the output of the mass flow sensor 300 will be described below for the case where only the temperature of the reference surface R is measured and the temperature of the measurement surface is assumed to be constant. However, it will be apparent that the same techniques can be used in the case where the temperature of the measurement surface is measured instead. For the case where the temperature of both the measurement surface and the reference surface R are measured, similar techniques can be used, except that a graph similar to that of Figure 3 is generated experimentally but with a z axis indicating deviation from a reference temperature of the measurement surface. The reference temperature of the measurement surface and of the reference surface R do not need to be the same.

[0035] The level sensor system 1 is provided with a controller 700. The controller 700 uses the output of the pneumatic sensor 10 (which is a raw pneumatic sensor measurement indicative of a difference between a flow of gas out of the reference outlet 100 and a flow of gas out of the measurement outlet 200) to generate a signal indicative of a difference in height between the reference gap GR and the

measurement gap GM. The controller 700 adjusts the signal indicative of a difference in height between the reference gap GR and the measurement gap GM based on the temperature measurement made by the temperature sensor 510 in a temperature correction step. That is, the controller 700 can use an experimentally determined relationship between measured temperature and a change in pneumatic sensor measurement, such as illustrated in Figure 3, to correct the raw pneumatic sensor measurement for the measured temperature and thereby generate a signal indicative of a difference in height between the reference gap GR and the measurement gap GM that is improved in accuracy. For example, if the temperature sensor 510, 510a determines the temperature of the reference surface R to be a value RT1 (greater than the reference temperature RT), from Figure 3 it can be seen that the raw pneumatic sensor measurement is expected to be +C greater than is actually the case (because the relationship of Figure 3 happens to be linear, though that may not necessarily be the case). Thus in the correction step the raw pneumatic sensor measurement is adjusted by reducing it by C, to a signal indicative of a difference in height between the reference gap GR and the measurement gap GM. In this way the measurement made by the level sensor system 1 is compensated for any variations in temperature of the reference surface R which can lead to systematic errors, as illustrated in Figure 3.

[0036] In an embodiment the pneumatic sensor measurement is corrected by the controller 700 using a look-up table of temperature measurements. In an additional or alternative embodiment, the controller 700 is configured to adjust the raw pneumatic sensor measurement using a calculation based on the temperature measurement, for example using a formula describing the relationship between the temperature measurement and a correction which needs to be applied to the raw pneumatic sensor measurement to compensate for any deviation from the reference temperature of the reference surface R.

[0037] In an additional or alternative embodiment, the controller 700 may be configured to make the temperature correction step on the basis of first principles. In such a system the controller 700 is configured to adjust the raw pneumatic sensor measurement for a difference in viscosity of gas in the reference gap GR compared to the viscosity of gas in the measurement gap GM. The viscosity of gas may be calculated based at least in part on the temperature measurement made by the temperature sensor 510,

5 lOa, 5 lOb, for example using Sutherland’ s formula where m is the dynamic viscosity

(in Pas) at an input temperature T (in K), mo is the reference viscosity (in Pas) at a reference temperature To (in K) and C is Sutherland’s constant for the gas. The viscosity of gas in the reference gap GR and/or measurement gap GM may additionally be calculated on the basis of a measured temperature or estimated temperature of gas exiting the measurement outlet 200 and/or the reference outlet 100.

[0038] A substrate table WT is normally equipped with at least one temperature sensor 510. In an embodiment, such a temperature sensor 510 may be used in the level sensor system 1. For example, the substrate table WT may include a temperature sensor 510 in or in close proximity with the reference surface R and/or measured surface (where the respective reference or measured surface is on or near the substrate table WT) and a temperature measurement from such a temperature sensor 510 may be used by the controller 700. In other words, the temperature sensor 510 may already exist in or near the reference or measured surface for other purposes, and may in addition be used for the purposes of an embodiment of the present invention. The reference outlet 100 and/or measurement outlet 200 of the pneumatic sensor 10 may be positioned in proximity to the temperature sensor 510. In an alternative or additional embodiment the temperature sensor 510 may be provided specifically for use by the level sensor system 1. The temperature sensor 510 may be in fixed position relative to measured or reference surface (e.g., the substrate table WT). The temperature sensor 510 may be embedded within the substrate table WT and/or the support 60.

[0039] A possible disadvantage of using the temperature sensor 510 of the substrate table WT and/or support 60, and in particular an already existing temperature sensor 510 of the substrate table WT and/or support 60 (that is not provided for the sole purpose of correcting a pneumatic sensor 10 measurement), is that the location of this existing temperature sensor 510 is predetermined and fixed. Accurate temperature compensation by the level sensor system 1 is best if the reference outlet 100 and/or measurement outlet 200 is positioned during measurement close to the temperature sensor 510 to form the respective gap at a position close to the temperature sensor 510. However, such a fixed, predetermined position of the applicable gap may not be optimal for pneumatic sensor 10 measurements. For example, an existing temperature sensor 510 may be located at a position that is subject to larger thermal and magnetic field variations (which may negatively affect the pneumatic sensor 10 measurement performance) than other positions at or on the applicable measured and/or reference surface. The locations in or near the measured and/or reference surface, for example in the surface of the substrate table WT, at which the temperature sensor 510 may be located are limited by space constraints and interaction with other components or sensors embedded in or near the respective surface. The position of the reference and/or measurement gap may not be freely chosen so as to be at an optimal position for pneumatic sensor 10 measurements if the temperature sensor 510 is embedded in the substrate table WT and/or support 60.

[0040] In an embodiment, a temperature sensor 5l0a is provided as part of the level sensor system 1, for example on the pneumatic sensor 10 side of the reference gap GR. The temperature sensor 5l0a may be fixedly positioned relative to the reference outlet 100. This allows the position of the reference gap GR on the reference surface R to be chosen freely. The position of the reference gap GR on the reference surface R may be chosen, for example, to be a position that is especially suitable for pneumatic sensor 10 measurements, such as a position on the reference surface R with lower thermal and/or magnetic field variations than other or most positions on the reference surface R.

[0041] In an embodiment, it may be desirable to measure the temperature of the measurement surface. This can be done in addition to or alternatively to measuring the temperature of the reference surface R.

In one embodiment the temperature of the measurement surface is measured by the temperature sensor 510b which is provided on the pneumatic sensor 10 side of the measurement gap GM. In an embodiment the temperature of the measurement surface may be measured by a temperature sensor on the measurement surface side of the measurement gap GM. For example the temperature sensor may be provided adjacent to and/or in contact with the measurement surface.

[0042] The temperature sensor 5l0a, 510b may be a contactless temperature sensor. In the case that the temperature sensor is a contactless temperature sensor, the temperature sensor may comprise an IR temperature sensor. Such a contactless temperature sensor may directly measure the temperature of the applicable surface at the location or a location proximate to the applicable gap, which temperature is the temperature affecting the properties of the gas flow out of the applicable outlet and thus affecting the pneumatic sensor 10 measurement. Thus, a more accurate correction of the pneumatic sensor 10 measurement can be achieved.

[0043] In an embodiment, the temperature sensor 510, 510a, 510b is configured to detect changes in temperature of less than 10 mK. This allows the level sensor system 1 to compensate for temperature variations leading to errors in differences of height between the reference gap GR and the measurement gap GM in the order of nanometers. Such nanometer accuracy is desirable for lithographic processing.

[0044] An advantage of providing a temperature sensor 510a, 510b on the pneumatic sensor 10 side of the measurement gap GM or reference gap GR is that the reference outlet 100 may be moved relative to the reference surface R or may make use of several different reference surfaces R within a lithographic apparatus. However, in an embodiment the reference outlet 100 and the reference surface R are in fixed position relative to each other. In such a case the accuracy of the level sensor system 1 may be increased because the size of the reference gap GR will be very well known and is fixed.

[0045] In an embodiment, the measurement outlet 200 is moveable relative to the measurement surface for example formed by the substrate W. This may mean that the measurement outlet 200 is fixed relative to other components of the level sensor system 1 but that the measurement surface, for example the substrate W, is moveable relative to the remainder of the level sensor system 1 , for example using the substrate table WT.

[0046] In an embodiment, the pneumatic sensor 10 may comprise a plurality of measurement outlets 200 and/or a plurality of reference outlets 100. In such an embodiment, the level sensor system 1 may comprise a plurality of temperature sensors 510, 5l0a, 5l0b, for example one temperature sensor 510, 5l0a to measure the temperature of the reference surface R at each reference outlet 100, and/or one temperature sensor 510b to measure the temperature of the measurement surface at each measurement outlet 200.

[0047] Although the level sensor system 1 has been described above in relation to measuring the surface of a substrate W, the level sensor system 1 can be used for measuring any measurement surface.

[0048] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms“wafer” or“die” herein may be considered as synonymous with the more general terms “substrate” or“target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains one or multiple processed layers.

[0049] Although specific reference may be made in this text to embodiments of the invention in the context of various apparatus, embodiments of the invention may be used in one or more various apparatuses. Embodiments of the invention may form part of or be used with a lithographic apparatus, a patterning device inspection apparatus, an inspection or metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0050] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0051] One or more of the methods as described herein may be performed and/or caused to be performed by a computer system in response to processor executing one or more sequences of one or more instructions contained in a computer-readable medium, such as a memory. Such instructions may be read into a memory from another computer-readable medium, such as a storage device. Execution of the sequences of instructions causes a processor system to perform one or more of the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions. In an embodiment, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0052] The term“computer-readable medium’’ as used herein refers to any medium that participates in providing instructions to processor system for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as a storage device. Volatile media include dynamic memory. Transmission media include coaxial cables, copper wire and fiber optics. Transmission media can also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave, or any other medium from which a computer can read. A non-transitory computer-readable medium is any computer-readable medium of a tangible, physical form such as RAM, ROM, Flash memory, etc.

[0053] The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.