Optical metrology method and system

Abstract

A method of determining a positional instability of an image sensor. The method comprises introducing the image sensor to an operational environment. The method comprises performing a first set of measurements of a position of a target image using the image sensor to determine a first set of results indicative of a first position of the image sensor within the operational environment. The method comprises removing the image sensor from the operational environment. The method comprises reintroducing the image sensor to the operational environment. The method comprises performing a second set of measurements of a position of the target image using the image sensor to determine a second set of results indicative of a second position of the image sensor within the operational environment. The method comprises determining the positional instability of the image sensor at least partially based on the first and the second sets of results.

Description

OPTICAL METROLOGY METHOD AND SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24211600.2 which was filed on 7 November 2024 which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to an optical metrology method and system.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).

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

[0005] Known optical metrology methods and systems may not detect a positional instability of an image sensor. It is desirable to provide an improved optical metrology method and system.SUMMARY

[0006] According to a first aspect of the present disclosure, there is provided a method of determining a positional instability of an image sensor. The method comprises a step (a) of introducing the image sensor to an operational environment. The method comprises a step (b) of performing a first set of measurements of a position of a target image using the image sensor to determine a first set of results indicative of a first position of the image sensor within the operational environment. The method comprises a step (c) of removing the image sensor from the operational environment. The method comprises a step (d) of reintroducing the image sensor to the operational environment. The methodcomprises a step (e) of performing a second set of measurements of a position of the target image using the image sensor to determine a second set of results indicative of a second position of the image sensor within the operational environment. The method comprises a step (f) of determining the positional instability of the image sensor at least partially based on the first and the second sets of results.

[0007] The positional instability may comprise a translational movement of the image sensor. The positional instability may comprise a rotational movement of the image sensor. The positional instability may comprise a deformation of the image sensor.

[0008] The image sensor may comprise a plurality of sensing elements. The number of sensing elements may at least partially determine a number of degrees of freedom of positional instability that may be determined. For example, the image sensor may comprise more than seven sensing elements such that translational movement along the X, Y and Z, axes, rotational movement about the X, Y and Z axes, and deformations of the image sensor, may be determined as part of the positional instability.

[0009] Determining the positional instability of the image sensor may comprise comparing a difference between the first and second sets of results to a threshold value.

[0010] The method may comprise repeating steps (c) through (e) to determine a plurality of consecutive sets of results indicative of a plurality of positions of the image sensor within the operational environment.

[0011] Step (f) may comprise determining the positional instability of the image sensor at least partially based on the plurality of consecutive sets of results.

[0012] Each set of results may, for example, comprise one or more of an X translation, Y translation, rotational movement about the Y axis (i.e. Ry) and / or rotational movement about the Z axis (i.e. Rz).

[0013] Step (f) may comprise determining a statistical correlation formed by the plurality of consecutive sets of results.

[0014] Step (f) may comprise comparing the statistical correlation to an expected correlation generated by a mathematical model of mechanics of the image sensor.

[0015] The operational environment may comprise a source of a variable force that acts upon the image sensor and contributes to the positional instability of the image sensor.

[0016] The source may contribute to any change in position over time experienced by the image sensor within the operational environment compared to outside of the operational environment.

[0017] The source may comprise a fluid which forms part of an immersion system configured to increase a numerical aperture of the operational environment.

[0018] According to a second aspect of the present disclosure, there is provided a method of measuring a characteristic of a substrate or an optical system. The method comprises a premeasurement process comprising the method of the first aspect of the present disclosure. The method comprises using the image sensor to measure the characteristic of the substrate or the optical system.

[0019] The characteristic may comprise an optical aberration of the optical system (e.g. a projection system of a lithographic apparatus). The characteristic may comprise a position of an image of a patterning device at the substrate plane. The characteristic may comprise a position of the substrate relative to the patterning device or a radiation beam. The characteristic may comprise a critical dimension of a feature on the substrate. The characteristic may comprise an overlay of a structure or pattern on the substrate.

[0020] According to a third aspect of the present disclosure, there is provided a method of exposing a substrate to a patterned beam of radiation. The method comprises a pre-exposure measurement process comprising the method of the second aspect of the present disclosure.

[0021] According to a fourth aspect of the present disclosure, there is provided an apparatus. The apparatus comprises an image sensor. The apparatus comprises an actuation system configured to introduce the image sensor to an operational environment and remove the image sensor from the operational environment. The apparatus comprises a controller. The controller is configured to (a) use the actuation system to introduce the image sensor to the operational environment. The controller is configured to (b) use the image sensor to perform a first set of measurements of a position of a target image to determine a first set of results indicative of a first position of the image sensor within the operational environment. The controller is configured to (c) use the actuation system to remove the image sensor from the operational environment. The controller is configured to (d) use the actuation system to reintroduce the image sensor to the operational environment. The controller is configured to (e) use the image sensor to perform a second set of measurements of a position of the target image to determine a second set of results indicative of a second position of the image sensor within the operational environment. The apparatus further comprises a processor configured to determine a positional instability of the image sensor at least partially based on the first and second sets of results.

[0022] The processor may be configured to compare a difference between the first and second sets of results to a threshold value as part of determining the positional instability of the image sensor.

[0023] The controller may be configured to repeat steps (c) through (e) to determine a plurality of consecutive sets of results indicative of a plurality of positions of the image sensor.

[0024] The processor may be configured to determine the positional instability of the image sensor at least partially based on the plurality of consecutive sets of results.

[0025] The processor may be configured to determine a statistical correlation formed by the plurality of consecutive sets of results as part of determining the positional instability of the image sensor.

[0026] The processor may be configured to compare the statistical correlation to an expected correlation generated by a mathematical model of mechanics of the image sensor.

[0027] The apparatus may comprise a source of a variable force that acts upon the image sensor and contributes to the positional instability of the image sensor.

[0028] The source may comprise a fluid which forms part of an immersion system configured to increase a numerical aperture of the apparatus.

[0029] According to a fifth aspect of the present disclosure, there is provided an optical metrology system for measuring a characteristic of a substrate or an optical system comprising the apparatus of the fourth aspect of the present disclosure. The image sensor is configured to measure the substrate within the operational environment.

[0030] According to a sixth aspect of the present disclosure, there is provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate in an operational environment, the lithographic apparatus comprising the optical metrology system of the fifth aspect of the present disclosure.

[0031] According to a seventh aspect of the present disclosure, there is provided a computer program comprising computer readable instructions configured to cause a processor to carry out a method according to the first aspect of the present disclosure.

[0032] According to an eighth aspect of the present disclosure, there is provide a computer readable medium carrying a computer program according to the seventh aspect of the present disclosure.

[0033] According to a ninth aspect of the present disclosure, there is provided a computer apparatus. The computer apparatus comprises a memory storing processor readable instructions. The computer apparatus comprises a processor arranged to read and execute instructions stored in said memory. Said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to the first aspect of the present disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 schematically depicts a schematic overview of a lithographic apparatus.Figure 2 schematically depicts an apparatus comprising an image sensor in accordance with the present disclosure.Figure 3 schematically depicts the image sensor of Figure 2 when viewed from above along the Z axis.Figure 4 shows a method of determining a positional instability of an image sensor in accordance with the present disclosure.Figure 5 shows two graphs demonstrating image sensor positional instability that has been determined using the method and apparatus of the present disclosure.DETAILED DESCRIPTION

[0035] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193,157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0036] 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.

[0037] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0038] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0039] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0040] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0041] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may beused 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.

[0042] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The sensor may comprise an image sensor. 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.

[0043] 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 located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0044] 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.

[0045] Figure 2 schematically depicts an apparatus 100 in accordance with the present disclosure. The apparatus 100 comprises an image sensor 110 for measuring a target image (not shown) that may be projected through at least part of the apparatus 100. In the example of Fig. 2, the image sensor 110 forms part of an interferometric sensing system that is typically configured to measure a characteristicof a substrate or an optical system. The characteristic may be one or more optical aberrations associated with one or more optical components of the apparatus 100 (e.g. the projection system PS of the lithographic apparatus LA Fig. 1). The characteristic may also be a position of the image of the patterning device at the substrate plane, or a relative position between the patterning device and the image sensor, or a position of the substrate (e.g. relative to a radiation beam B and / or a patterning device MA as part of a lithographic exposure). For example, the image sensor 110 may be a parallel integrated lens interferometry sensor or a transmissive image sensor. In general, the image sensor 110 is capable of detecting a target image. Such image sensors 110 may be implemented as an integrated measurement system used for system initialization and calibration. Alternatively, such image sensors 110 may be used for monitoring and recalibration “on-demand”.

[0046] With reference to Figs. 1 and 2, the relative phase of a projection system PS in its pupil plane may be determined by projecting radiation, for example from a point-like source in an object plane of the projection system PS (i.e. the plane of the mask MA), through the projection system PS and using a shearing interferometer to measure a wavefront (i.e. a locus of points with the same phase). Shearing interferometers are common path interferometers and therefore, advantageously, no secondary reference beam is required to measure the wavefront. The shearing interferometer may comprise a diffraction grating, for example a two dimensional grid, in an image plane of the projection system PS (i.e. the substrate table WT) and an image sensor 110 arranged to detect an interference pattern in a plane that is conjugate to a pupil plane of the projection system PS. The interference pattern is related to the derivative of the phase of the radiation with respect to a coordinate in the pupil plane in the shearing direction. The image sensor 110 may comprise an array of sensing elements 120 such as, for example, charge coupled devices (CCDs). In a first example measurement process, the diffraction grating is sequentially scanned in two perpendicular directions, which may coincide with axes of a coordinate system of the projection system PS (x and y) or may be at an angle such as, for example, 45 degrees to these axes. Scanning may be performed over an integer number of grating periods, for example one grating period. The scanning averages out phase variations in one direction, allowing phase variations in the other direction to be reconstructed. This allows the wavefront to be determined as a function of both directions.

[0047] The phase of the sheared wavefront varies at different positions across the pupil plane due to the effect of optical aberrations present within the apparatus 100 (e.g. optical aberrations of optical components of the projection system PS). Such a measurement may be performed at the beginning of a metrology or lithographic process (i.e. before substrates are exposed to patterned radiation) in order to measure and account for optical aberrations present within the apparatus 100. As another example, such a measurement may be performed at the start of each substrate lot as part of a pre-exposure procedure to determine optical aberrations of the lithographic apparatus LA. The optical aberrations may change over time due to, for example, thermal effects, and so it may be desirable to periodically measure the optical aberrations within or between substrate lots.

[0048] Alternatively or additionally, the image sensor 110 of the present disclosure may be used to determine an alignment between components of the apparatus 100 (e.g. between the patterning device MA and the substrate W of the lithographic apparatus LA of Fig. 1), by using the image sensor as a positional anchor point or reference. An example of determining the alignment between a patterning device MA and a substrate W comprises: performing a wafer alignment measurement using a wafer alignment sensor to determine a relative position between the substrate W and the image sensor; performing an image alignment measurement using an image sensor to determine a relative position between the patterning device MA and the image sensor; and combining the results from both measurements to determine an alignment between the patterning device MA and the substrate W. As a positional anchor point or reference for such alignment measurement, the image sensor position is desired to be stable or measurable to be accounted for or compensated.

[0049] The apparatus 100 comprises an immersion system 130 configured to increase a numerical aperture of the apparatus 100 (e.g. to increase a numerical aperture of the projection system PS such that smaller features may be resolved by the lithographic apparatus LA). The immersion system 130 comprises an immersion hood 132. The immersion hood 132 is configured to provide a fluid reservoir 134 between the projection system PS and the substrate support WT. In the example of Fig. 2, the fluid reservoir 134 is filled with a liquid having a relatively high refractive index, e.g. water, provided via inlet / outlet ducts (not shown). The liquid 134 has the effect that the radiation projected though the liquid 110 has a shorter wavelength in the liquid 110 compared to in air or a vacuum, allowing smaller features to be resolved. It is well known that the resolution limit of a projection system PS is determined, inter alia, by the wavelength of the projection beam and the numerical aperture of the system. The presence of the liquid 110 may also be regarded as increasing the effective numerical aperture. Furthermore, at fixed numerical aperture, the liquid 110 is effective to increase the depth of field.

[0050] During a lithographic exposure, the fluid reservoir 134 forms a contactless seal to the substrate W around the image field of the projection system PS so that the liquid is confined to fill a space between the substrate W surface and a final element of the projection system PS. The fluid reservoir 134 may be formed by a seal member (not shown) of the immersion hood 132 positioned below and surrounding the final element of the projection system PS. Liquid is brought into the space below the projection system PS and within the seal member. The seal member may extend a little above the final element of the projection system PS and the liquid level may rise above the final element so that a buffer of liquid is provided. The seal member may conform to the shape of the projection system PS or the final element thereof and may, e.g., be round. At the bottom, the seal member may conform to the shape of the image field, e.g., rectangular, though this need not be the case. The liquid may be confined in the fluid reservoir 134 by a gas seal between the bottom of the seal member of the immersion hood 132 and the surface of the substrate W. The gas seal may be formed by gas, e.g. air, N2 or another inert gas, provided under pressure through an inlet (not shown) to a gap between the seal member of the immersion hood 132 and the substrate W and extracted via an outlet (not shown). An overpressureon the inlet, vacuum level on the outlet and / or a geometry of the gap may be arranged such that there is a high-velocity gas flow inwards that confines the liquid to the fluid reservoir 134.

[0051] In the example of Fig. 2, the substrate support WT and immersion system 130 are positioned such that the fluid reservoir 134 is formed at the image sensor 110. The image sensor 110 is held in a recess formed in the substrate support WT. Seals 140 are provided to prevent unwanted ingress of liquid to the recess and / or internal components of the image sensor 110 and / or substrate support WT. Support structures 151-153 are provided to support the image sensor 110 within the recess of the substrate support WT. In the example of Fig. 2, only two of three support structures 151, 152 are visible due to the cross-sectional view of Fig. 2. All three support structures 151-153 are visible in Fig. 3. The support structures 151-153 may comprise a flexibly resilient material such as, for example, titanium and / or FeNi36.

[0052] The immersion system 130 may exert one or more variable forces on the image sensor 110. For example, a flow of the fluid through inlets and outlets of the immersion hood 132 may exert variable pressures on the image sensor 110. As another example, the flow of the gas seal may exert variable pressures on the image sensor 110. As further examples of sources of variable forces that may act on, and thereby contribute to a positional instability of, the image sensor 110, changes in temperature and / or mechanical vibrations of one or more components of the apparatus 100 and / or electrostatic forces between one or more components of the apparatus 100 may result in variable forces being applied to the image sensor 110. In other apparatus, a vacuum system may act as a source of variable forces that contribute to a positional instability of the image sensor 110. In general, when the image sensor 110 is in an operational environment in which the image sensor 110 is used to perform one or more measurements, a variety of sources may contribute to one or more variable forces being applied to the image sensor 110 that contribute to the positional instability of the image sensor. These forces may cause unwanted changes in position of the image sensor 110. For example, the image sensor 110 position may change relative to one or more components of the apparatus 100 and / or relative to a previously measured position of the image sensor 110. This may in turn contribute to measurement errors (e.g. overlay errors) associated with measurements performed by the image sensor 110. Given that these forces are variable, the changes in position of the image sensor 110 may also vary, making it difficult to predict or otherwise account for the associated measurement errors in advance. In addition, given that these forces are associated with an operating environment of the image sensor 110, measurements performed in an alternative environment in which none or different variable forces are present (e.g. a measurement stage of a lithographic apparatus in which the immersion system 130 is not present) may not be capable of detecting changes in position of the image sensor 110 that are associated with the variable forces present in the operating environment.

[0053] In the example of Fig. 2, the fluid 134 of the immersion system 130 exerts a force 160 proximate one end of the image sensor 110. The force 160 pulls the end of the image sensor 110 towards the immersion hood 132, thereby displacing the image sensor 110 along the Z axis. The change inposition along the Z axis may also be viewed as a tilt “Ry” about the Y axis. Due to the image sensor 110 moving as a whole, and the resisting forces 170 provided by the support structures 151-153 (shown in greater detail in Fig. 3), the image sensor 110 also changes position along the X and Y axes. Figure 3 schematically depicts the image sensor 110 of Fig. 2 when viewed from above along the Z axis. Whilst in practice the support structures 151-153 would not be visible from such a viewpoint, the locations of the support structures 151-153 are shown in Fig. 3 for ease of understanding. As shown in Fig. 3, the pulling force 160 causes a change in height of the proximate support structure 151 and a corresponding change in position of the image sensor along the X and Y axes. Due to the substantially triangular arrangement of three support structures 151-153 in the example of Fig. 3, a change in height of the support structure 151 proximate the pulling force 160 results in the following linear relationship between displacements along the X and Y axes: Y=k*X. A similar linear relationship may exist between displacements along the X axis and rotations about the Y axis (Ry). Different numbers of, and / or different arrangements of, support structures 151-153 may result in different relationships (e.g. different values ofk and / or non-linear relationships) between the positional changes of the image sensor along the different axes. Such systems and relationships may be determined using a mathematical model of mechanics of the image sensor (e.g. using techniques such as trigonometry based on known positional relationships, such as distances and / or angles, between support structures, and / or an appropriate computer program or simulation) . For example, this may comprise determining an expected relationship between a displacement in the Z, X and Y axes as well as other positional variables of an image sensor.

[0054] Referring again to Fig. 2, the change in position of the image sensor 110 causes a corresponding change in position of the sensing elements 120. In the example of Fig. 2, the original positions of the sensing elements 120 are shown as a series of dots that are substantially parallel with the X axis. The changed positions of the sensing elements 120 as a result of the pulling force 160 are shown as a series of dots that are tilted about the Y axis (i.e. non-parallel with the X axis). When unaccounted for, the change in position of the sensing elements 120 introduces measurement errors to measurements performed using the image sensor 110. For example, in a lithographic or metrology tool process, the positional instability of the image sensor caused by forces such as the pulling force 160 may result in an alignment error between the patterning device and the substrate, which in turn may result in an overlay error. Said overlay errors result in manufactured devices having misaligned features across different layers, which can result in the devices being faulty. Said devices may require additional correctional work or may have to be discarded.

[0055] The positional instability introduced to the image sensor 110 by forces such as the pulling force 160 may vary between consecutive operations in the operational environment. Known apparatus and methods are not capable of detecting or accounting for said positional instability of the image sensor. For example, in a first operation of a known lithographic apparatus, an image sensor may be introduced to the operational environment that is the exposure side of the lithographic apparatus and immersionfluid may be introduced to the image sensor. In this example, during the first operation the immersion fluid exerts a first force that causes the image sensor to be in a first position (for example, the image sensor settles in a first position such that the sensing elements are parallel with the x axis). During the first operation, a number of measurements may be performed using the image sensor. After the first operation is completed, the image sensor may be removed from the operational environment by removing the image sensor from the immersion fluid and, for example, passing the image sensor to a measurement side of the lithographic apparatus. A second operation may then be performed, in which the image sensor is reintroduced to the operational environment that is the exposure side of the lithographic apparatus and the immersion fluid may again be introduced to the image sensor. However, unlike the first operation, during the second operation the immersion fluid applies a second force that is different to the first force (e.g. a slightly changed pulling force, such as that shown in Fig. 2) to the image sensor which causes the image sensor to change position from the first position to a second position (for example, the image sensor settles in a second position such that the sensing elements are now non-parallel with the x axis). During the second operation, a number of measurements may be performed using the image sensor, under the now incorrect assumption that the image sensor is still in the first position. Given that the change in position of the image sensor is present for all of the measurements performed during the second operation, none of those measurements can provide any indication that the image sensor has changed position. In known apparatus and methods, no comparison between measurements performed during the first and second operations occurs. As such, the positional instability of the image sensor goes unnoticed, and the resulting errors go unaccounted for.

[0056] It will be appreciated that in known apparatus and methods, all that is eventually detected is an error, such as an overlay error, and that there is no indication of when or where this error was introduced. However, it has been realised that the operational environment comprises sources of variable forces (such as the immersion fluid, the gas seal, etc.) which can cause changes in the position of the image sensor between different operations, and that said changes in position can result in errors, such as overlay errors. The apparatus and method of the present disclosure therefore involves monitoring the position of the image sensor between different operations (i.e. each time the image sensor is removed from, and reintroduced to, the operational environment), such that the positional instability of the image sensor can be detected and accounted for, thereby reducing any associated errors such as overlay errors. A positional instability of the image sensor introduced by the operation environment may be detected as a dramatic change (or “jump”) in measurement results (e.g. relative to a threshold value) between different operations and / or by statistical analysis of the results of measurements of different operations.

[0057] Figure 4 shows a method of determining a positional instability of an image sensor in accordance with the present disclosure . A first step 401 of the method comprises introducing the image sensor 110 to an operational environment 130. For example, with reference to Figs. 1 and 2, the first step 401 may involve a controller CN using an actuation system, such as positioner PW, to introducethe image sensor 110 to the operational environment of the immersion system 130 under the projection system PS.

[0058] A second step 402 of the method comprises performing a first set of measurements of a position of a target image using the image sensor 110 to determine a first set of results indicative of a first position of the image sensor 110 within the operational environment 130. For example, with reference to Figs. 1 and 2, the second step 402 may involve the controller CN using the image sensor 110 to perform a first set of measurements of a position of a target image to determine a first set of results indicative of a first position of the image sensor 110 within the operational environment 130. The target image may, for example, be formed by illuminating a mask alignment mark Ml, M2 and using the projection system PS to project an image of the alignment mark Ml, M2 into the image sensor 110.

[0059] A third step 403 of the method comprises removing the image sensor from the operational environment. For example, with reference to Figs. 1 and 2, the third step 403 may involve the controller CN using the actuation system PW to remove the image sensor 110 from the operational environment 130 (e.g. by removing the image sensor 110 from the immersion system 130 and / or deactivating the immersion system 130).

[0060] A fourth step 404 of the method comprises reintroducing the image sensor 110 to the operational environment. For example, with reference to Figs. 1 and 2, the fourth step 404 may involve the controller CN using the actuation system PW to reintroduce the image sensor 110 to the operational environment 130 (e.g. by reintroducing the image sensor 110 to the immersion system 130 and / or reactivating the immersion system 130).

[0061] A fifth step 405 of the method comprises performing a second set of measurements of a position of the target image using the image sensor 110 to determine a second set of results indicative of a second position of the image sensor 110 within the operational environment 130. For example, with reference to Figs. 1 and 2, the fifth step 405 may involve the controller CN using the image sensor 110 to perform a second set of measurements of a position of the target image (e.g. formed by mask alignment marks Ml, M2) to determine a second set of results indicative of a second position of the image sensor 110 within the operational environment (i.e. within the immersion system 130).

[0062] A sixth step of the method 406 comprises determining the positional instability of the image sensor 110 at least partially based on the first and the second sets of results. For example, with reference to Figs. 1 and 2, the sixth step 406 may involve the use of a processor PR that is configured to determine a positional instability of the image sensor 110 at least partially based on the first and second sets of results.

[0063] Figure 5 shows two graphs demonstrating image sensor positional instability that has been determined using the method and apparatus of the present disclosure. The upper graph shows changes in measured X and Y positions of the image sensor 110 and the lower graph shows changes in measured X and Ry (i.e. rotational movement about the Y axis) positions of the image sensor 110. The dots onthe graphs represent the average measured positions of the image sensor during different operations in an operational environment. For example, a first dot may correspond to a process of introducing the image sensor 110 to the immersion system 130, performing a first set of measurements using the image sensor 110 to determine a first set of results indicative of a first position of the image sensor 110 within the immersion system 130 and determining an average position of the image sensor 110 based on the first set of results. A second dot may correspond to a process of removing the image sensor 110 from the immersion system 130, reintroducing the image sensor 110 to the immersion system 130, performing a second set of measurements using the image sensor 110 to determine a second set of results indicative of a second position of the image sensor 110 within the immersion system 130 and determining an average position of the image sensor 110 based on the second set of results. In the example of Fig. 5, and as shown by the dashed arrow in Fig. 4, third through fourth steps 403-405 have been repeated to determine a plurality of consecutive sets of results indicative of a position of the image sensor within the operational environment, with each dot representing the average measured position of the image sensor for each set of results.

[0064] Determining the positional instability of the image sensor 110 may comprise comparing a difference between the first and second sets of results to a threshold value. The processor PR of fig. 1 may be configured to perform this process. The threshold value may be determined using, for example, a mathematical model of mechanics of the image sensor (e.g. using techniques such as trigonometry based on known positional relationships, such as distances and / or angles, between support structures, and / or an appropriate computer program or simulation). Alternatively or additionally, the threshold value may be determined by performing the same process (i.e. performing measurements during different operations in an operational environment) on a reference system in order to determine an expected distribution of measurements in the reference system.

[0065] In the example of the upper graph of Fig. 5, a first group of results 501 are within a threshold value whereas a second group of results 502 are outside of the threshold value. As previously discussed, each dot on the graphs represents an average measured position of the image sensor based on a set of measurement results, whereas each group of dots are multiple average measured positions that are either within the threshold value or outside of the threshold value. The axes of the graphs show normalized unit-less values of distance. In both graphs, the first group of results 501 all have average X positions that are between about -1 and about 0.2, whereas the second group of results 502 all have average X positions that are between about 0.8 and about 1.4. In the upper graph, the first group of results 501 all have average Y positions that are between about -0.1 and about 0.6, whereas the second group of results 502 all have average Y positions that are between about -0.5 and about -0.7. In the lower graph, the first group of results 501 all have average Ry positions that are between about 0 and about -0.2, whereas the second group of results 502 all have average Ry positions that are between about 0.2 and about 0.3.

[0066] Determining the positional instability of the image sensor 110 may comprise determining a statistical correlation formed by the plurality of consecutive sets of results. The processor PR of Fig. 1may be configured to perform this process. With reference to Figs. 2 and 3, as previously discussed, a change in height of the support structure 151 proximate the pulling force 160 results in the following expected relationship between displacements along the X and Y axes: Y=k*X. This represents an expected correlation generated by a mathematical model of mechanics of the image sensor. The line passing through the upper graph of Fig. 5 is a fit to the sets of results, and shows a linear relationship between X and Y as expected. The line passing through the lower graph of Fig. 5 is a fit to the sets of results and shows a positive linear relationship between X and Ry as expected. In general, the statistical correlation may comprise a specific value of an expected relationship (e.g. a specific value of the gradient of a linear relationship) and / or a characteristic of the expected relationship (e.g. the gradient being positive or negative). A p-value of the fitted line to the upper graph of Fig. 5 is approximately 0.000, and a p-value of the fitted line to the lower graph of Fig. 5 is approximately 0.001, both of which demonstrate a strong probability that the sets of results satisfy the expected positional relationship between X and Y / Ry position changes. The p-value may be determined by, for example, statistical means as is done in “null-hypothesis significance testing”. In this example, a threshold of p <0.01 is used to determine whether the correlation is seen as statistically significant or not. Other threshold values may be used. For example, a threshold value of p <0.05 may indicate the presence of a statistically significant correlation (i.e. the presence of an expected relationship between the measured positions of the image sensor). Fitting lines to measured data and / or determining a resulting p-value may be performed using any suitable technique, such as a two-sides T-test, which may be performed using any suitable computer program such as, for example, MATLAB®.

[0067] The method of the present disclosure may form part of a process of measuring a characteristic of a substrate or an optical system. For example, the method of Fig. 4 may form part of a premeasurement process. Once the method of Fig. 4 is complete and a positional instability of the image sensor is understood and accounted for, the image sensor may then be used to measure the characteristic of the substrate or the optical system with improved accuracy. For example, the image sensor may be configured to measure a position of the image of the patterning device at the substrate plane formed by a projection system, or a relative position between the patterning device and the image sensor, thereby measuring a position of the substrate (e.g. relative to a radiation beam B and / or a patterning device MA as part of a lithographic exposure). Alternatively or additionally, the image sensor may be configured to measure a characteristic of a substrate, for example a critical dimension of a feature printed on the substrate, an overlay of features printed on the substrate, etc. The image sensor may, alternatively or additionally be configured to measure a characteristic of an optical system, for example one or more optical aberrations of a projection system as part of a lithographic exposure. The positional instability of the image sensor determined in accordance with the present disclosure may be used by, for example, varying one or more external forces acting on the image sensor until the positional instability is lowered below a certain threshold determined by, for example, overlay specifications. For example, the positional instability of the image sensor determined in accordance with the present disclosure may beused to adjust a suction force applied to the gas seal of the immersion system so as to reduce the positional instability of the image sensor and thereby improve an overlay of a lithographic exposure process. Alternatively or additionally, the positional instability of the image sensor determined in accordance with the present disclosure may be used by, for example, replacing or re-installing hardware such as, for example, the image sensor and / or the immersion system if the positional instability has reached or surpassed a threshold. The method of the present disclosure may form part of a process of exposing a substrate to a patterned beam of radiation, e.g. in a lithographic process. For example, a pre- exposure measurement process involving the abovementioned process of measuring the substrate may be performed to improve an accuracy and / or reproducibility with which a subsequent lithographic exposure is performed.

[0068] The apparatus of the present disclosure (e.g. the apparatus 100 of Fig. 2) may form part of a metrology tool or optical metrology system for measuring a characteristic of a substrate or an optical system. For example, the apparatus 100 may be used to determine the positional instability of the image sensor as described above and, once the positional instability has been accounted for, the image sensor may be configured to measure the characteristic of the substrate or the optical system within the operational environment, albeit with an improved accuracy than would otherwise be the case. The optical metrology system may form part of a lithographic apparatus, such as the lithographic apparatus LA of Fig. 1. For example, a pre-exposure measurement process involving measuring a characteristic of the substrate or the optical system may be performed using the optical metrology system to improve an accuracy and / or reproducibility with which a subsequent lithographic exposure is performed.

[0069] 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.

[0070] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0071] 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.

[0072] 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 as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world. For example, a computer program may comprise computer readable instructions configured to cause a processor (such as the processor PR of Fig. 1) to carry out a method according to the present disclosure (e.g. the method of Fig. 4). A computer readable medium may carry the computer program. As another example, a computer apparatus may comprise a memory storing processor readable instructions and a processor (such as the processor PR of Fig. 1) arranged to read and execute instructions stored in said memory. Said processor readable instructions may comprise instructions arranged to control the computer to carry out a method according to the present disclosure (e.g. the method of Fig. 4). Other aspects of the invention are set-out as in the following numbered clauses.1. A method of determining a positional instability of an image sensor, the method comprising the following steps:(a) introducing the image sensor to an operational environment;(b) performing a first set of measurements of a position of a target image using the image sensor to determine a first set of results indicative of a first position of the image sensor within the operational environment;(c) removing the image sensor from the operational environment;(d) reintroducing the image sensor to the operational environment;(e) performing a second set of measurements of a position of the target image using the image sensor to determine a second set of results indicative of a second position of the image sensor within the operational environment;(f) determining the positional instability of the image sensor at least partially based on the first and the second sets of results.2. The method of clause 1, wherein determining the positional instability of the image sensor comprises comparing a difference between the first and second sets of results to a threshold value.3. The method of clause 1 or clause 2, comprising repeating steps (c) through (e) to determine a plurality of consecutive sets of results indicative of a plurality of positions of the image sensor within the operational environment, wherein step (f) comprises determining the positional instability of the image sensor at least partially based on the plurality of consecutive sets of results.4. The method of clause 3, wherein step (f) comprises determining a statistical correlation formed by the plurality of consecutive sets of results.5. The method of clause 4, wherein step (f) comprises comparing the statistical correlation to an expected correlation generated by a mathematical model of mechanics of the image sensor.6. The method of any preceding clause, wherein the operational environment comprises a source of a variable force that acts upon the image sensor and contributes to the positional instability of the image sensor.7. The method of clause 6, wherein the source comprises a fluid which forms part of an immersion system configured to increase a numerical aperture of the operational environment.8. A method of measuring a characteristic of a substrate or an optical system comprising: a pre-measurement process comprising the method of any of clauses 1 to 7; and, using the image sensor to measure the characteristic of the substrate or the optical system.9. A method of exposing a substrate to a patterned beam of radiation, wherein the method comprises a pre-exposure measurement process comprising the method of clause 8.10. An apparatus comprising: an image sensor; an actuation system configured to introduce the image sensor to an operational environment and remove the image sensor from the operational environment; and, a controller configured to perform the following steps:(a) use the actuation system to introduce the image sensor to the operational environment;(b) use the image sensor to perform a first set of measurements of a position of a target image to determine a first set of results indicative of a first position of the image sensor within the operational environment;(c) use the actuation system to remove the image sensor from the operational environment;(d) use the actuation system to reintroduce the image sensor to the operational environment; and,(e) use the image sensor to perform a second set of measurements of a position of the target image to determine a second set of results indicative of a second position of the image sensor within the operational environment, wherein the apparatus further comprises a processor configured to determine a positional instability of the image sensor at least partially based on the first and second sets of results.11. The apparatus of clause 10, wherein the processor is configured to compare a difference between the first and second sets of results to a threshold value as part of determining the positional instability of the image sensor.12. The apparatus of clause 10 or clause 11, wherein the controller is configured to repeat steps (c) through (e) to determine a plurality of consecutive sets of results indicative of a plurality of positions of the image sensor, wherein the processor is configured to determine the positional instability of the image sensor at least partially based on the plurality of consecutive sets of results.13. The apparatus of clause 12, wherein the processor is configured to determine a statistical correlation formed by the plurality of consecutive sets of results as part of determining the positional instability of the image sensor.14. The apparatus of clause 13, wherein the processor is configured to compare the statistical correlation to an expected correlation generated by a mathematical model of mechanics of the image sensor.15. The apparatus of any of clauses 10 to 14, comprising a source of a variable force that acts upon the image sensor and contributes to the positional instability of the image sensor.16. The apparatus of clause 15, wherein the source comprises a fluid which forms part of an immersion system configured to increase a numerical aperture of the apparatus.17. An optical metrology system for measuring a characteristic of a substrate or an optical system comprising the apparatus of any of clauses 10 to 16, wherein the image sensor is configured to measure the characteristic of the substrate or the optical system within the operational environment.18. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate in an operational environment, the lithographic apparatus comprising the optical metrology system of clause 17.19. A computer program comprising computer readable instructions configured to cause a processor to carry out a method according to any one of clauses 1 to 9.20. A computer readable medium carrying a computer program according to clause 19.21. A computer apparatus comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory, wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any one of clauses 1 to 9.

[0073] 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 thatmodifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method of determining a positional instability of an image sensor, the method comprising the following steps:(a) introducing the image sensor to an operational environment;(b) performing a first set of measurements of a position of a target image using the image sensor to determine a first set of results indicative of a first position of the image sensor within the operational environment;(c) removing the image sensor from the operational environment;(d) reintroducing the image sensor to the operational environment;(e) performing a second set of measurements of a position of the target image using the image sensor to determine a second set of results indicative of a second position of the image sensor within the operational environment;(f) determining the positional instability of the image sensor at least partially based on the first and the second sets of results.

2. The method of claim 1, wherein determining the positional instability of the image sensor comprises comparing a difference between the first and second sets of results to a threshold value.

3. The method of claim 1 or claim 2, comprising repeating steps (c) through (e) to determine a plurality of consecutive sets of results indicative of a plurality of positions of the image sensor within the operational environment, wherein step (f) comprises determining the positional instability of the image sensor at least partially based on the plurality of consecutive sets of results.

4. The method of any preceding claim, wherein the operational environment comprises a source of a variable force that acts upon the image sensor and contributes to the positional instability of the image sensor.

5. A method of measuring a characteristic of a substrate or an optical system comprising: a pre-measurement process comprising the method of any of claims 1 to 4; and, using the image sensor to measure the characteristic of the substrate or the optical system.

6. A method of exposing a substrate to a patterned beam of radiation, wherein the method comprises a pre-exposure measurement process comprising the method of claim 5.

7. An apparatus comprising: an image sensor; an actuation system configured to introduce the image sensor to an operational environment and remove the image sensor from the operational environment; and, a controller configured to perform the following steps:(a) use the actuation system to introduce the image sensor to the operational environment;(b) use the image sensor to perform a first set of measurements of a position of a target image to determine a first set of results indicative of a first position of the image sensor within the operational environment;(c) use the actuation system to remove the image sensor from the operational environment;(d) use the actuation system to reintroduce the image sensor to the operational environment; and,(e) use the image sensor to perform a second set of measurements of a position of the target image to determine a second set of results indicative of a second position of the image sensor within the operational environment, wherein the apparatus further comprises a processor configured to determine a positional instability of the image sensor at least partially based on the first and second sets of results.

8. The apparatus of claim 7, wherein the processor is configured to compare a difference between the first and second sets of results to a threshold value as part of determining the positional instability of the image sensor.

9. The apparatus of claim 7 or claim 8, wherein the controller is configured to repeat steps (c) through (e) to determine a plurality of consecutive sets of results indicative of a plurality of positions of the image sensor, wherein the processor is configured to determine the positional instability of the image sensor at least partially based on the plurality of consecutive sets of results.

10. The apparatus of any of claims 7 to 9, comprising a source of a variable force that acts upon the image sensor and contributes to the positional instability of the image sensor.

11. An optical metrology system for measuring a characteristic of a substrate or an optical system comprising the apparatus of any of claims 7 to 10, wherein the image sensor is configured to measure the characteristic of the substrate or the optical system within the operational environment.

12. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate in an operational environment, the lithographic apparatus comprising the optical metrology system of claim 11.

13. A computer program comprising computer readable instructions configured to cause a processor to carry out a method according to any one of claims 1 to 6.

14. A computer readable medium carrying a computer program according to claim 13.

15. A computer apparatus comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory, wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any one of claims 1 to 6.

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