Method of monitoring performance, method of manufacturing devices, computer program and lithographic apparatus

Abstract

A method of monitoring the performance of an immersion lithographic apparatus, the method comprising: moving a test substrate relative to a liquid confinement structure through a series of test route portions whilst projecting exposure radiation through an immersion liquid onto a photosensitive layer on the test substrate such that the photosensitive layer is exposed; wherein the series of test route portions include a first test route portion and a second test route portion; and the second test route portion is symmetric to the first test route portion.

Description

METHOD OF MONITORING PERFORMANCE. METHOD OF MANUFACTURING DEVICES.COMPUTER PROGRAM AND LITHOGRAPHIC APPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23203252.4 which was filed on 12 October 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a method of monitoring the performance of a lithographic apparatus, to a method of manufacturing devices, to a computer program and to a lithographic apparatus.BACKGROUND

[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). Known lithographic apparatus 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 substrate parallel or anti-parallel to this direction.

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law’. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm.

[0005] Further improvements in the resolution of smaller features may be achieved by providing an immersion fluid having a relatively high refractive index, such as water, on the substrate during exposure. The effect of the immersion fluid is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the fluid than in gas. The effect of the immersion fluid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.

[0006] The immersion fluid may be confined to a localized area between the projection system of the lithographic apparatus and the substrate by a fluid handling structure.SUMMARY

[0007] In an immersion lithography process, defects in printed patterns can occur due to the presence of bubbles of gas in the immersion liquid and / or droplets of immersion liquid that are left on the surface of the substrate. Many approaches to minimising the creation of such bubbles and droplets and to ameliorate their effects have been proposed, for example: providing a CO2 environment adjacent the immersion liquid; special coatings on the substrate and substrate support; extracting gas and liquid from a gap between the edge of the substrate and the substrate support; and varying scanning routes and speeds. However, it has not so far been possible to completely eliminate the occurrence of bubbles and droplets and the defects that they cause.

[0008] In many cases, the rate of defects can be reduced (increasing yield) by slowing down scanning speeds or other recipe modifications that reduce throughput. Therefore, a lithographer may choose process parameters that balance throughput and yield to maximise production of functioning devices. To make such a balance, the performance of the lithographic apparatus must be monitored to avoid a change in machine performance (commonly referred to as drift) that would adversely affect the output.

[0009] One approach to monitoring performance is to expose test substrates in between batches of production substrates using stage movements that are a subset of the main production route known to be especially susceptible to the generation of defects. The test substrates are inspected to detect defects immediately, using only minimal process steps to develop the substrate, and the defect rate is determined. If the defect rate is low, then it may be necessary to expose many test substrates to accurately determine the defect rate, which reduces the time available for the exposure of production substrates.

[0010] According to the present invention, there is provided a method of monitoring the performance of an immersion lithographic apparatus, the method comprising: moving a test substrate relative to a liquid confinement structure through a series of test route portions whilst projecting exposure radiation through an immersion liquid onto a photosensitive layer on the test substrate such that the photosensitive layer is exposed; wherein the series of test route portions include a first test route portion and a second test route portion; and the second test route portion is symmetric to the first test route portion

[0011] According to the present invention, there is provided a method of manufacturing devices using an immersion lithographic apparatus, the method comprising, in order:exposing a first batch of production substrates to a device pattern using the immersion lithographic apparatus; performing the method of monitoring the performance of the immersion lithographic apparatus described above; and exposing a second batch of production substrates to the device pattern using the immersion lithographic apparatus.

[0012] According to the present invention, there is provided a computer program comprising computer interpretable code that, when executed by a control system of an immersion lithographic apparatus, causes the immersion lithographic apparatus to perform the method described above.

[0013] According to the present invention, there is provided an immersion lithographic apparatus comprising: a liquid confinement structure; a positioner; a projection system for projecting a radiation beam onto a substrate held by the positioner; and a controller configured to control the positioner and the projection system to: move a test substrate relative to a liquid confinement structure through a series of test route portions whilst projecting the radiation beam through an immersion liquid onto a photosensitive layer on the test substrate such that the photosensitive layer is exposed; wherein the series of test route portions include a first test route portion and a second test route portion; and the second test route portion is symmetric to the first test route portion.

[0014] Further embodiments, features and advantages of the present invention, as well as the structure and operation of the various embodiments, features and advantages of the present invention are described in detail below with reference to the accompanying drawings.DESCRIPTION OF THE DRAWINGS

[0015] 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:Figure 1 depicts a schematic overview of the lithographic apparatus;Figures 2 and 3 depict, in cross-section, two different versions of a fluid handling system for use in a lithographic projection apparatus;Figure 4 is a diagram illustrating an exemplary route for production exposure of a substrate;Figure 5 is a diagram illustrating test route portions employed in an embodiment of the invention;Figure 6 is a diagram illustrating symmetries that may be employed in embodiments of the invention; andFigure 7 is a flow diagram of an embodiment of the invention.

[0016] The features shown in the Figures are not necessarily to scale, and the size and / or arrangement depicted is not limiting. It will be understood that the Figures include optional features which may not be essential to the invention. Furthermore, not all of the features of the apparatus are depicted in each of the figures, and the Figures may only show some of the components relevant for describing a particular feature.DETAILED DESCRIPTION

[0017] 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).

[0018] 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.

[0019] Figure 1 schematically depicts a lithographic apparatus. The lithographic apparatus includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or DUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a substrate table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support WT in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0020] In operation, the illumination system IL receives the radiation beam B 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.

[0021] The term “projection system” 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”.

[0022] The lithographic apparatus is of a type wherein at least a portion of the substrate W may be covered by an immersion liquid having a relatively high refractive index, e.g., water, so as to fdl an immersion space 11 between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US 6,952,253, which is incorporated herein by reference.

[0023] The lithographic apparatus may be of a type having two or more substrate supports WT (also named “dual stage”). In such a “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0024] In addition to the substrate support WT, the lithographic apparatus may comprise a measurement stage (not depicted in figures). The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0025] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be locatedin spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe -lane alignment marks when these are located between the target portions C.

[0026] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axis, i.e., an x-axis, a y-axis and a z-axis. Each of the three axis is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry -rotation. A rotation around about the z-axis is referred to as an Rz- rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0027] Immersion techniques have been introduced into lithographic systems to enable improved resolution of smaller features. In an immersion lithographic apparatus, a liquid layer of immersion liquid having a relatively high refractive index is interposed in the immersion space 11 between a projection system PS of the apparatus (through which the patterned beam is projected towards the substrate W) and the substrate W. The immersion liquid covers at least the part of the substrate W under a final element of the projection system PS. Thus, at least the portion of the substrate W undergoing exposure is immersed in the immersion liquid.

[0028] In commercial immersion lithography, the immersion liquid is water. Typically the water is distilled water of high purity, such as Ultra-Pure Water (UPW) which is commonly used in semiconductor fabrication plants. In an immersion system, the UPW is often purified and it may undergo additional treatment steps before supply to the immersion space 11 as immersion liquid. Other liquids with a high refractive index can be used besides water as the immersion liquid, for example: a hydrocarbon, such as a fluorohydrocarbon; and / or an aqueous solution. Further, other fluids besides liquid have been envisaged for use in immersion lithography.

[0029] In this specification, reference will be made in the description to localized immersion in which the immersion liquid is confined, in use, to the immersion space 11 between the final element and a surface facing the final element. The facing surface is a surface of substrate W or a surface of the supporting stage (or substrate support WT) that is co-planar with the surface of the substrate W. (Please note that reference in the following text to surface of the substrate W also refers in addition or in the alternative to the surface of the substrate support WT, unless expressly stated otherwise; and vice versa). A fluid handling structure IH present between the projection system PS and the substrate support WT is used to confine the immersion liquid to the immersion space 11. The immersion space 11 filled by the immersion liquid is smaller in plan than the top surface of the substrate W and the immersion space 11 remains substantially stationary relative to the projection system PS while the substrate W and substrate support WT move underneath.

[0030] Other immersion systems have been envisaged such as an unconfined immersion system (a so-called ’All Wet’ immersion system) and a bath immersion system. In an unconfined immersion system, the immersion liquid covers more than the surface under the final element. The liquid outside the immersion space 11 is present as a thin liquid film. The liquid may cover the whole surface of the substrate W or even the substrate W and the substrate support WT co-planar with the substrate W. In a bath type system, the substrate W is fully immersed in a bath of immersion liquid.

[0031] The fluid handling structure IH is a structure which supplies the immersion liquid to the immersion space 11, removes the immersion liquid from the immersion space 11 and thereby confines the immersion liquid to the immersion space 11. It includes features which are a part of a fluid supply system. The arrangement disclosed in PCT patent application publication no. WO 99 / 49504 is an early fluid handling structure comprising pipes which either supply or recover the immersion liquid from the immersion space 11 and which operate depending on the relative motion of the stage beneath the projection system PS. In more recent designs, the fluid handling structure extends along at least a part of a boundary of the immersion space 11 between the final element of the projection system PS and the substrate support WT or substrate W, so as to in part define the immersion space 11.

[0032] The fluid handing structure IH may have a selection of different functions. Each function may be derived from a corresponding feature that enables the fluid handling structure IH to achieve that function. The fluid handling structure IH may be referred to by a number of different terms, each referring to a function, such as barrier member, seal member, fluid supply system, fluid removal system, liquid confinement structure, etc..

[0033] Immersion liquid may be used as the immersion fluid. In that case the fluid handling structure IH may be a liquid handling system. In reference to the aforementioned description, reference in this paragraph to a feature defined with respect to fluid may be understood to include a feature defined with respect to liquid.

[0034] A lithographic apparatus has a projection system PS. During exposure of a substrate W, the projection system PS projects a beam of patterned radiation onto the substrate W. To reach the substrate W, the path of the radiation beam B passes from the projection system PS through the immersion liquid confined by the fluid handling structure IH between the projection system PS and the substrate W. The projection system PS has a lens element, the last in the path of the beam, which is in contact with the immersion liquid. This lens element which is in contact with the immersion liquid may be referred to as ‘the last lens element’ or “the final element”. The final element is at least partly surrounded by the fluid handling structure IH. The fluid handling structure IH may confine the immersion liquid under the final element and above the facing surface.

[0035] As depicted in Figure 1, the lithographic apparatus comprises a controller 500. The controller 500 is configured to control the substrate support WT.

[0036] Figure 2 schematically depicts a localized liquid supply system or fluid handling system. The liquid supply system is provided with a fluid handling structure IH (or liquid confinement structure), which extends along at least a part of a boundary of the immersion space 11 between the final element of the projection system PS and the substrate support WT or substrate W. The fluid handling structure IH is substantially stationary relative to the projection system PS in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). In an example, a seal is formed between the fluid handling structure IH and the surface of the substrate W and may be a contactless seal such as a gas seal (such a system with a gas seal is disclosed in EPl, 420, 298) or liquid seal.

[0037] The fluid handling structure IH at least partly confines the immersion liquid in the immersion space 11 between the final element of the projection system PS and the substrate W. The immersion space 11 is at least partly formed by the fluid handling structure IH positioned below and surrounding the final element of the projection system PS. Immersion liquid is brought into the immersion space 11 below the projection system PS and within the fluid handling structure IH by one of liquid openings 13. The immersion liquid may be removed by another of liquid openings 13. The immersion liquid may be brought into the immersion space 11 through at least two liquid openings 13. Which of liquid openings 13 is used to supply the immersion liquid and optionally which is used to remove the immersion liquid may depend on the direction of motion of the substrate support WT.

[0038] The immersion liquid may be confined in the immersion space 11 by a contactless seal such as a gas seal 16 formed by a gas which, during use, is formed between the bottom of the fluid handling structure IH and the surface of the substrate W. The gas in the gas seal 16 is provided under pressure via inlet 15 to the gap between the fluid handling structure IH and substrate W. The gas is extracted via outlet 14. The overpressure on the gas inlet 15, vacuum level on the outlet 14 and geometry of the gap are arranged so that there is a high-velocity gas flow inwardly that confines the immersion liquid. Such a system is disclosed in US 2004 / 0207824, which is hereby incorporated by reference in its entirety. In an example, the fluid handling structure IH does not have the gas seal 16.

[0039] Figure 3 is a side cross-sectional view that depicts a further liquid supply system or fluid handling system. The arrangement illustrated in Figure 3 and described below may be applied to the lithographic apparatus described above and illustrated in Figure 1. The liquid supply system is provided with a fluid handling structure IH (or a liquid confinement structure), which extends along at least a part of a boundary of the immersion space 11 between the final element of the projection system PS and the substrate support WT or substrate W.

[0040] The fluid handling structure IH at least partly confines the immersion liquid in the immersion space 11 between the final element of the projection system PS and the substrate W. The immersion space 11 is at least partly formed by the fluid handling structure IH positioned below and surrounding the final element of the projection system PS. In an example, the fluid handling structureIH comprises a main body member 53 and a porous member 33. The porous member 33 is plate shaped and has a plurality of holes (i.e., openings or pores). The porous member 33 may be a mesh plate wherein numerous small holes 84 are formed in a mesh. Such a system is disclosed in US 2010 / 0045949 Al, which is hereby incorporated by reference in its entirety.

[0041] The main body member 53 comprises supply ports 72, which are capable of supplying the immersion liquid to the immersion space 11, and a recovery port 73, which is capable of recovering the immersion liquid from the immersion space 11. The supply ports 72 are connected to a liquid supply apparatus 75 via passageways 74. The liquid supply apparatus 75 is capable of supplying the immersion liquid to the supply ports 72 through the corresponding passageway 74. The recovery port 73 is capable of recovering the immersion liquid from the immersion space 11. The recovery port 73 is connected to a liquid recovery apparatus 80 via a passageway 79. The liquid recovery apparatus 80 recovers the immersion liquid recovered via the recovery port 73 through the passageway 29. The porous member 33 is disposed in the recovery port 73. Performing the liquid supply operation using the supply ports 72 and the liquid recovery operation using the porous member 33 forms the immersion space 11 between the projection system PS and the fluid handling structure IH on one side and the substrate W on the other side.

[0042] Figure 4 depicts an exemplary route R40 followed by a substrate W for exposure of target portions C001 to Cl 07 on the substrate W. The target portions are exposed in the numbered order. Target portions depicted with dense hatching, e.g. C104, are edge target portions that are exposed at a low speed. Target portions depicted with light hatching, e.g. C085, are edge target portions that are exposed at a low speed, but faster than the densely hatched target portions. Edge target portions are exposed but the exposure motion covers only the part of the target portion that overlaps the substrate W, not the full length of the edge target portion. Route R40 has been calculated by optimizing for throughput and includes long diagonal motions R41, R42. Diagonal motions R41, R42 are transfer moves between the exposure of a target portion at the end of one row and the exposure of the target portion at the beginning of the next. Diagonal motion R41 comes after exposure of edge target portion Cl 04 and repositions the substrate for exposure of edge target portion C105. Diagonal motion R42 comes after exposure of edge target portion C003 and repositions the substrate W for exposure of edge target portion C004.

[0043] It will be seen that this production route mostly consists of straight line motions in the scan (+ / -Y) or transverse (+ / -X) directions. This measure, together with the described speed variations for selected edge portions, is effective to reduce defects due to immersion liquid loss (watermarks) or bubbles becoming entrained in the immersion liquid. Other measures and other routes can be devised, but there is always a compromise to be determined between defectivity (rate of defects) and throughput.

[0044] In order for that compromise to be made, it is necessary for the lithographer to know the defectivity rate, the effect of speed variations on defectivity, and any changes in that relationship. Therefore, it is known to interrupt production of devices in a lithographic apparatus to perform tests to monitor the defectivity performance of the lithographic apparatus, usually between batches. These tests may involve exposing a plurality of substrates whilst moving them through a test route, which is a selected portion of the full production route that is sensitive to immersion liquid loss related defects. Substrates exposed for test purposes are referred to herein as test substrates but are generally identical to substrates exposed for the manufacture of devices, which are referred to as production substrates. However, test substrates may not have been subjected to previous exposure and process steps. The number of defects arising in the test substrates is then measured to determine if the lithographic apparatus is performing within specification. Any deviation (drift) from the expected performance may be addressed by maintenance actions or adjusting the recipe for subsequent batches.

[0045] However, if the expected number of defects is low, e.g. a few per substrate or less, it may be necessary to perform test exposures on many test substrates. In this case, the time taken for the test exposures is increased and availability of the lithographic apparatus for production is reduced. Therefore, it is desirable that the requirement for test exposures to monitor performance of the lithographic machine is minimised as far as possible.

[0046] An embodiment of the invention will now be described with reference to Figures 5 to 7. A plurality of test route portions that may be employed in an embodiment of the invention to minimise the number of test substrates that are required to monitor the performance of a lithographic apparatus are depicted in Figure 5. Symmetries that may be used in embodiments of the invention are shown in Figure 6. Figure 7 is a flowchart of a method according to an embodiment of the invention.

[0047] As depicted in Figure 5, test exposure route 100 includes several test route portions 101-106 depicted by dashed or dot-chain outlines. Test route portions 101-106 are designed to be especially sensitive to the generation of defects, for example defects caused by bubbles, floating bubbles and watermarks. Test route portions 101-106 may be selected from a library of routes.

[0048] A first test route portion 101 corresponds to a portion of a production route to be executed in the production of devices in the lithographic apparatus whose performance is to be monitored. Desirably, the first test route portion 101 includes at least one scan section (a period when the substrate moves in a straight line as a constant speed) and a turnaround section (a period in which the substrate moves around a curved path so as to reverse its direction of movement). A second test route portion 102 is symmetric to first test route portion 101. In this case, second test route portion 102 is mirror symmetric to first test route portion 101 about an axis passing through the centre of the substrate W and parallel to the scan direction (Y axis) of production exposures. A third test route portion 103 is symmetric to first test route portion 101. In this case, third test route portion 103 is mirror symmetric to first test route portion 101 about an axis passing through the centre of thesubstrate W and perpendicular to the scan direction (Y axis) of production exposures. A fourth test route portion 104 is symmetric to third test route portion 103. In this case, fourth test route portion 104 is mirror symmetric to third test route portion 103 about an axis passing through the centre of the substrate W and parallel to the scan direction (Y axis) of production exposures.

[0049] It will be understood that the symmetries amongst the first to fourth test route portions 101- 104 may be described in other ways. For example, the fourth test route portion 104 may be described as mirror symmetric with the second test route 102 portion about an axis passing through the centre of the substrate W and perpendicular to the scan direction (Y axis) of production exposures. Also, the fourth test route portion 104 may be described as having 180° rotational symmetry with the first test route portion 101 about an axis passing through the centre of the substrate W and perpendicular thereto (Z axis).

[0050] The symmetries between the first to fourth test route portions 101-104 mean that each of the test route portions can be expected to have substantially the same probabilities of giving rise to defects as each other and to the corresponding portion of the production route. Therefore, the test exposure route including four test route portions 101-104 will have an approximately four times higher probability of giving rise to defects than a test route having only one instance of the test route . This means that the performance of the lithographic apparatus can be determined using fewer test substrates than if only one test route portion were included in the test exposure route. Alternatively, the performance of the lithographic apparatus can be determined more accurately and / or more precisely than if only one test route portion were included in the test exposure route.

[0051] Desirably, the symmetries between test route portions 101 to 104 correspond to symmetries that exist in relevant parts of the liquid confinement structure. For example, if features of the liquid confinement structure that define the immersion space 11, e.g. gas seal 16, have a shape that has mirror symmetries in X- and Y -axes, then desirably there are corresponding mirror symmetries between different test route portions. If the features of the liquid confinement structure that define the immersion space 11 have rotational symmetries, then desirably there are corresponding rotational symmetries between different test route portions. For example, the gas seal 16 may have a shape that is a diamond or rhombus with rounded comers. Having symmetries between test route portions that correspond to the symmetries of the liquid confinement structure, ensures that the extra test route portions have the same probability of generating defects as the corresponding portion of the production route.

[0052] Figure 5 also depicts a first additional test route portion 105 and a second additional test route portion 106. One of the first additional test route portion 105 and the second additional test route portion 106 corresponds to another portion of the production route that is susceptible to generation of defects. The first additional test route portion 105 and the second additional test route portion 106 have a symmetry as described above. The first and second additional test route portions105, 106 need not be symmetric or even similar in shape to the test route portions 101 to 104. Using the additional test route portions 105, 106 in addition to the test route portions 101-104 described above, the performance of the lithographic apparatus in relation to multiple production route portions can be measured using the same test substrates, subject to space on the test substrate.

[0053] In the case that the performance of the lithographic apparatus is to be measured with respect to multiple portions of the production route, counter-intuitively it is desirable to have more copies of the production route portion that is less likely to give rise to defects so that overall the numbers of defects measured are more similar.

[0054] Desirably, at least one test route portion 101 to 106 includes an edge crossing during which at least a part of the immersion space 11 containing the immersion liquid crosses an edge of the test substrate. Edge crossings are known to have a high probability of giving rise to defects.

[0055] It should be noted that defects that are associated with a route portion, such as a route portion including an edge crossing, that has a high probability of giving rise to defects are not necessarily located in or near the area of the substrate W scanned by that route portion. This is because it may take an unpredictable amount of time for a bubble that is introduced into the immersion liquid at the edge crossing to drift into the path of the radiation beam B where it causes an imaging defect. Similarly, when immersion liquid is left on the substrate W it may be pushed around by subsequent movements of the substrate W before being left at a point some way from its origin. Therefore, as well as the test route portions described above it is desirable that the whole test route covers substantially the entire substrate W in a similar fashion to the production route.

[0056] In some cases, an exposure route may be defined by specifying the dies (or fields) to be scanned as well as the order and direction of scanning. The lithographic apparatus is programmed to join the field scans with standard turnaround moves. A test route may be specified in the same way. Additional parameters, such as scan speed may also be specified. In some cases, parameters of the test route, such as scan speed, may be different than the parameters applied during production. In particular, parameter values chosen for the test route may be such as to increase the probability of defects, e.g. increased scan speed. In that case, it may be desirable to calibrate the probability of defects being generated by the test route relative to the probability of defects being generated in the production route in order that the test process gives an accurate measure of the defect rate of the production route. In other cases, it may be sufficient that the test process is performed periodically so that any change in the results can be detected. Such a change in test results may be usefully taken to indicate a change in performance of the lithographic apparatus, prompting further investigation, even if the exact nature and magnitude of the change in performance of the lithographic apparatus is not known.

[0057] Whilst the invention has been described above as useful for monitoring the performance of a lithographic apparatus over time, it will also be appreciated that the present invention can be used forcomparing the performance of different lithographic apparatus, for the purpose of machine to machine matching.

[0058] As mentioned above, the test substrate is exposed by the radiation beam B whilst the test substrate is moved along the test route. The defects that are to be detected in the present method give rise to local variations in the dose delivered to the photosensitive layer, e.g. because a bubble in the immersion liquid may act as a lens. Defects may therefore be detected by developing and inspecting the exposed photosensitive resist. In some cases, defects may be detectable as a latent image prior to development of the photosensitive resist. It is not necessary that the photosensitive resist be exposed to a pattern in the test method. However, it may be convenient to expose the test substrates to the pattern used for production substrates in order to avoid the need to change the patterning device. Alternatively, a pattern that is especially sensitive to defects may be used in order to make the detection of defects easier.

[0059] Figure 6 is a perspective view useful for understanding different symmetries that may be employed in embodiments of the present invention. As can be seen, a test route portion 110 may be reflected about axes Al and A2 and / or rotated about axis A3. In this example, test route portion 113 is test route portion reflected in axis Al; test route portion 111 is test route portion 110 reflected in axis A2 and test route portion 112 is test route portion 110 rotated about axis A3. These axes A1-A3 are illustrated as mutually orthogonal and centred in the centre of the substrate W. However, it will be appreciated that strict orthogonality is not required. Nor need axes Al to A3 pass through the centre of the substrate W. As is appreciated, test route portions used in the present invention may exhibit combinations of symmetries, e.g. multiple mirror symmetries or mirror symmetries and rotational symmetries so translational symmetries. In Figure 6 the test route portions 110-114 are depicted as a simple L-shape so that the symmetries can be easily recognised but this does not necessarily represent the shape of an actual test route portion.

[0060] A flow chart of an embodiment of the invention is depicted in Figure 7. A first batch of production substrates is exposed in step SI using a production route. One or more test substrates are exposed in step S2 using a test route that includes test route portions as described above. The test substrates are analysed in step S3 to determine a measure of the performance of the lithographic apparatus. As described above, analysis of the test substrates may involve development of the photosensitive resist or detection of a latent image of the resist. Analysis of the test substrates may be as simple as counting defects or involve more detailed analysis of the nature and location of the defects. Based on the analysis of the test substrates it is determined in step S4 whether or not the performance of the lithographic apparatus has changed (drifted).

[0061] If the performance of the lithographic apparatus has changed sufficiently, remedial action in step S5 may be taken. Remedial action may involve some or all of: a maintenance action on the immersion lithographic apparatus; adjustment of an operating parameter of the immersionlithographic apparatus; and adjustment of a parameter of the production route. For example, if the apparatus of the test substrates reveals that there has been an increase in the rate of defects caused by watermarks, the liquid confinement structure may be examined to check if there is a defect in the gas seal 16, such as a blocked outlet. Alternatively, or in addition, a flow rate or pressure of the gas seal 16 might be increased. Further alternatively or in addition, the moving speed of the substrate W during a part of the production route may be reduced.

[0062] Subsequently, an additional batch of production substrates is exposed in step S6, potentially using a modified recipe. It will be appreciated that analysis of the test substrates may take some time, especially if the photoresist is developed, and so the exposure of subsequent production substrates in step S6 may be carried out in parallel with the analysis in step S3 of the test substrates.

[0063] The use of multiple test route portions with different orientations potentially provides additional information about the location and the causes of any changes in the performance of the lithographic apparatus. For example, if there is an increase in watermarks associated with movements in the positive Y direction but not with movements in the negative Y direction in a symmetric test route portion, this may indicate an issue with the trailing part of the gas seal 16 when the substrate W is moving in the positive Y direction.

[0064] The method of the invention may be encoded in a computer program that contains instructions to perform the invention, e.g. autonomously or at the instruction of the lithographer. The computer program may be provided as an upgrade, e.g. a software upgrade, for existing lithographic apparatus. The computer program may be incorporated in the lithographic apparatus at the time of manufacture thereof.

[0065] The present invention may provide a lithographic apparatus. The lithographic apparatus may have any / all of the other features or components of the lithographic apparatus as described above. For example, the lithographic apparatus may optionally comprise at least one or more of a source SO, an illumination system IL, a projection system PS, a substrate support WT, etc..

[0066] Specifically, the lithographic apparatus may comprise the projection system PS configured to project the radiation beam B towards the region of the surface of a substrate W. The lithographic apparatus may further comprise the substrate support WT as described in any of the above embodiments and variations.

[0067] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin film magnetic heads, etc..

[0068] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also beimplemented by instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine -readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0069] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic tools.

[0070] Embodiments of the present invention are further described in the following numbered clauses:

[0071] 1. A method of monitoring the performance of an immersion lithographic apparatus, the method comprising: moving a test substrate relative to a liquid confinement structure through a series of test route portions whilst projecting exposure radiation through an immersion liquid onto a photosensitive layer on the test substrate such that the photosensitive layer is exposed; wherein the series of test route portions include a first test route portion and a second test route portion; and the second test route portion is symmetric to the first test route portion.

[0072] 2. A method according to clause 1 wherein the first test route portion and the second test route portion have a symmetry selected from the group consisting of: mirror symmetry, rotational symmetry and translational symmetry.

[0073] 3. A method according to clause 2 wherein the symmetry corresponds to a symmetry of the liquid confinement structure.

[0074] 4. A method according to clause 1, 2 or 3 wherein the first and second test route portions are mirror symmetric about a line of symmetry passing through the centre of the substrate.

[0075] 5. A method according to clause 4 wherein the test route portions further comprise a third test route portion and a fourth test route portion wherein the third test route portion and the fourth test route portion are symmetric to the first test route portion and the second test route portionrespectively about another line of symmetry passing through the centre of the substrate and desirably perpendicular to the line of symmetry passing through the centre of the substrate.

[0076] 6. A method according to clause 1, 2 or 3 wherein the first test route portion and the second test route portion are rotationally symmetric about an axis passing through the centre of the substrate and perpendicular thereto.

[0077] 7. A method according to any one of the preceding clauses wherein the series of test route portions include a first additional test route portion and a second additional test route portion; and the second additional test route portion is symmetric to the first additional test route portion but asymmetric to the first test route portion or the second test route portion.

[0078] 8. A method according to any one of the preceding clauses wherein at least one test route portion includes at least one scan section during which the test substrate moves in a substantially straight line at a substantially constant speed relative to the liquid confinement structure and at least one turnaround section during which the test substrate moves along a curved path so that its direction of movement relative changes by 90 degrees.

[0079] 9. A method according to any one of the preceding clauses wherein at least one test route portion includes an edge crossing during which at least a part of the immersion space containing the immersion liquid crosses an edge of the test substrate.

[0080] 10. A method according to any one of the preceding clauses further comprising analysing the photosensitive layer after the exposure during the test route portions to determine a performance metric of the immersion lithographic apparatus, e.g. bubble defect rate and / or watermark rate.

[0081] 11. A method according to clause 9 wherein analysing the photosensitive layer comprises developing the photosensitive layer.

[0082] 12. A method according to clause 10 wherein analysing the photosensitive layer comprises detecting a latent image in the photosensitive layer.

[0083] 13. A method of manufacturing devices using an immersion lithographic apparatus, the method comprising, in order: exposing a first batch of production substrates to a device pattern using the immersion lithographic apparatus; performing the method of monitoring the performance of the immersion lithographic apparatus according to any one of the preceding clauses; and exposing a second batch of production substrates to the device pattern using the immersion lithographic apparatus.

[0084] 14. A method according to clause 13 wherein the production substrates are moved relative to a liquid confinement structure of the immersion lithographic apparatus along a production route during the exposure to the device pattern; and

[0085] at least one test route portion corresponds to a portion of the production route.

[0086] 15. A method according to clause 14 wherein, for at least part of the test route portions, the test substrate is moved faster than production substrates are moved during the corresponding portion of the production route.

[0087] 16. A method according to any one of clauses 13 to 15 further comprising, prior to the exposure of the second production substrates, a step of remedial action based on the results of the step of monitoring, wherein the remedial action is selected from the group consisting of: a maintenance action on the immersion lithographic apparatus; adjustment of an operating parameter of the immersion lithographic apparatus; and adjustment of a parameter of the production route.

[0088] 17. A computer program comprising computer interpretable code that, when executed by a control system of an immersion lithographic apparatus, causes the immersion lithographic apparatus to perform the method according to any one of the preceding clauses.

[0089] 18. An immersion lithographic apparatus comprising: a liquid confinement structure; a positioner; a projection system for projecting a radiation beam onto a substrate held by the positioner; and a controller configured to control the positioner and the projection system to: move a test substrate relative to a liquid confinement structure through a series of test route portions whilst projecting the radiation beam through an immersion liquid onto a photosensitive layer on the test substrate such that the photosensitive layer is exposed; wherein the series of test route portions include a first test route portion and a second test route portion; and the second test route portion is symmetric to the first test route portion.

[0090] 19. An immersion lithographic apparatus according to clause 18 wherein the first test route portion and the second test route portion have a symmetry selected from the group consisting of: mirror symmetry, rotational symmetry and translational symmetry.

[0091] 20. An immersion lithographic apparatus according to clause 19 wherein the symmetry corresponds to a symmetry of the liquid confinement structure.

[0092] 21. An immersion lithographic apparatus according to clause 18, 19 or 20 wherein the first and second test route portions are mirror symmetric about a line of symmetry passing through the centre of the substrate.

[0093] 22. An immersion lithographic apparatus according to clause 21 wherein the test route portions further comprise a third test route portion and a fourth test route portion wherein the third test route portion and the fourth test route portion are symmetric to the first test route portion and the second test route portion respectively about another line of symmetry passing through the centre of the substrate and desirably perpendicular to the line of symmetry passing through the centre of the substrate.

[0094] 23. An immersion lithographic apparatus according to clause 18, 19 or 20 wherein the first test route portion and the second test route portion are rotationally symmetric about an axis passing through the centre of the substrate and perpendicular thereto.

[0095] 24. An immersion lithographic apparatus according to any one of clauses 18 to 23 wherein the series of test route portions include a first additional test route portion and a second additional test route portion; and the second additional test route portion is symmetric to the first additional test route portion but asymmetric to the first test route portion or the second test route portion.

[0096] 25. An immersion lithographic apparatus according to any one of clauses 18 to 24 wherein at least one test route portion includes at least one scan section during which the test substrate moves in a substantially straight line at a substantially constant speed relative to the liquid confinement structure and at least one turnaround section during which the test substrate moves along a curved path so that its direction of movement relative changes by 90 degrees.

[0097] 26. An immersion lithographic apparatus according to any one of clauses 18 to 25 wherein at least one test route portion includes an edge crossing during which at least a part of the immersion space containing the immersion liquid crosses an edge of the test substrate.

[0098] 27. An immersion lithographic apparatus according to any one of clauses 18 to 26 wherein the controller is further configured to control the positioner and the projection system to: expose a first batch of production substrates to a device pattern using the immersion lithographic apparatus before performing the step of moving the test substrate; and then expose a second batch of production substrates to the device pattern using the immersion lithographic apparatus.

[0099] 28. An immersion lithographic apparatus according to clause 27 wherein controller is further configured to control the positioner and the projection system to move the production substrates relative to a liquid confinement structure of the immersion lithographic apparatus along a production route during the exposure to the device pattern; and at least one test route portion corresponds to a portion of the production route.

[0100] 29. An immersion lithographic apparatus according to clause 28 wherein the controller is further configured to control the positioner and the projection system to, for at least part of the testroute portions, move the test substrate faster than production substrates are moved during the corresponding portion of the production route.

[0101] 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.

[0102] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method of monitoring the performance of an immersion lithographic apparatus, the method comprising: moving a test substrate relative to a liquid confinement structure through a series of test route portions whilst projecting exposure radiation through an immersion liquid onto a photosensitive layer on the test substrate such that the photosensitive layer is exposed; wherein the series of test route portions include a first test route portion and a second test route portion; and the second test route portion is symmetric to the first test route portion.

2. The method according to claim 1, wherein the first test route portion and the second test route portion have a symmetry selected from the group consisting of: mirror symmetry, rotational symmetry and translational symmetry, desirably wherein the symmetry corresponds to a symmetry of the liquid confinement structure.

3. The method according to claim 1 or 2, wherein the first and second test route portions are mirror symmetric about a line of symmetry passing through the centre of the substrate, desirably wherein the test route portions further comprise a third test route portion and a fourth test route portion wherein the third test route portion and the fourth test route portion are symmetric to the first test route portion and the second test route portion respectively about another line of symmetry passing through the centre of the substrate and desirably perpendicular to the line of symmetry passing through the centre of the substrate.

4. The method according to claim 1 or 2, wherein the first test route portion and the second test route portion are rotationally symmetric about an axis passing through the centre of the substrate and perpendicular thereto.

5. The method according to any of the preceding claims, wherein the series of test route portions include a first additional test route portion and a second additional test route portion; and the second additional test route portion is symmetric to the first additional test route portion but asymmetric to the first test route portion or the second test route portion, and / or wherein at least one test route portion includes at least one scan section during which the test substrate moves in a substantially straight line at a substantially constant speed relative to the liquid confinement structure and at least one turnaround section during which the test substrate moves along a curved path so that its direction of movement relative changes by 90 degrees, and / or wherein at least one test routeportion includes an edge crossing during which at least a part of the immersion space containing the immersion liquid crosses an edge of the test substrate.

6. A method according to any of the preceding claims, further comprising analysing the photosensitive layer after the exposure during the test route portions to determine a performance metric of the immersion lithographic apparatus, e.g. bubble defect rate and / or watermark rate.

7. The method according to claim 6, wherein analysing the photosensitive layer comprises developing the photosensitive layer, or wherein analysing the photosensitive layer comprises detecting a latent image in the photosensitive layer.

8. A method of manufacturing devices using an immersion lithographic apparatus, the method comprising, in order: exposing a first batch of production substrates to a device pattern using the immersion lithographic apparatus; performing the method of monitoring the performance of the immersion lithographic apparatus according to any one of the preceding claims; and exposing a second batch of production substrates to the device pattern using the immersion lithographic apparatus.

9. A method according to claim 8, wherein the production substrates are moved relative to a liquid confinement structure of the immersion lithographic apparatus along a production route during the exposure to the device pattern; and at least one test route portion corresponds to a portion of the production route, desirably wherein, for at least part of the test route portions, the test substrate is moved faster than production substrates are moved during the corresponding portion of the production route, and / or further comprising, prior to the exposure of the second production substrates, a step of remedial action based on the results of the step of monitoring, wherein the remedial action is selected from the group consisting of: a maintenance action on the immersion lithographic apparatus; adjustment of an operating parameter of the immersion lithographic apparatus; and adjustment of a parameter of the production route.

10. A computer program comprising computer interpretable code that, when executed by a control system of an immersion lithographic apparatus, causes the immersion lithographic apparatus to perform the method according to any of the preceding claims.

11. An immersion lithographic apparatus comprising: a liquid confinement structure; a positioner; a projection system for projecting a radiation beam onto a substrate held by the positioner; and a controller configured to control the positioner and the projection system to: move a test substrate relative to a liquid confinement structure through a series of test route portions whilst projecting the radiation beam through an immersion liquid onto a photosensitive layer on the test substrate such that the photosensitive layer is exposed; wherein the series of test route portions include a first test route portion and a second test route portion; and the second test route portion is symmetric to the first test route portion.

12. The immersion lithographic apparatus according to claim 11, wherein the first test route portion and the second test route portion have a symmetry selected from the group consisting of: mirror symmetry, rotational symmetry and translational symmetry, desirably wherein the symmetry corresponds to a symmetry of the liquid confinement structure, and / or wherein the first and second test route portions are mirror symmetric about a line of symmetry passing through the centre of the substrate, desirably wherein the test route portions further comprise a third test route portion and a fourth test route portion wherein the third test route portion and the fourth test route portion are symmetric to the first test route portion and the second test route portion respectively about another line of symmetry passing through the centre of the substrate and desirably perpendicular to the line of symmetry passing through the centre of the substrate.

13. The immersion lithographic apparatus according to claim 12, wherein the first test route portion and the second test route portion are rotationally symmetric about an axis passing through the centre of the substrate and perpendicular thereto.

14. The immersion lithographic apparatus according to any of claims 11-13, wherein the series of test route portions include a first additional test route portion and a second additional test route portion; and the second additional test route portion is symmetric to the first additional test route portion but asymmetric to the first test route portion or the second test route portion, and / or wherein at least one test route portion includes at least one scan section during which the test substrate moves in a substantially straight line at a substantially constant speed relative to the liquid confinement structure and at least one turnaround section during which the test substrate moves along a curved path so thatits direction of movement relative changes by 90 degrees, and / or wherein at least one test route portion includes an edge crossing during which at least a part of the immersion space containing the immersion liquid crosses an edge of the test substrate, and / or wherein the controller is further configured to control the positioner and the projection system to: expose a first batch of production substrates to a device pattern using the immersion lithographic apparatus before performing the step of moving the test substrate; and then expose a second batch of production substrates to the device pattern using the immersion lithographic apparatus.

15. The immersion lithographic apparatus according to claim 14, wherein controller is further configured to control the positioner and the projection system to move the production substrates relative to a liquid confinement structure of the immersion lithographic apparatus along a production route during the exposure to the device pattern; and at least one test route portion corresponds to a portion of the production route, desirably wherein the controller is further configured to control the positioner and the projection system to, for at least part of the test route portions, move the test substrate faster than production substrates are moved during the corresponding portion of the production route.

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