Projection system control

Abstract

A method of controlling a projection system during exposure of a substrate by a lithographic apparatus, the method comprising: obtaining a measurement signal of a differential pressure change across one or more lenses of a projection system of the lithographic apparatus; calculating an imaging error caused by movement of one or more lens elements of the projection system due to the differential pressure change measured during the exposure; calculating a lens element adjustment that compensates for the calculated imaging error; adjusting by using the lens element; identifying which exposure regions of the substrate are exposed during a delay between occurrence of the differential pressure change and application of the lens element adjustment; and storing information of the identified exposure area along with the calculated imaging error.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22213847.1, filed on December 15, 2022, and incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to a method of controlling a projection system of a lithographic apparatus and an apparatus configured to control a projection system using the method. The method may form part of a lithographic method and the apparatus may form part of a lithographic apparatus. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a target portion of a substrate. For example, a lithographic apparatus can be used to manufacture integrated circuits (ICs). In this case, a patterning device (alternatively referred to as a mask or reticle) can be used to generate circuit patterns corresponding to the various layers of the IC, and the pattern can be imaged onto a target portion (e.g., including part of a die, a die, or several dies) on a substrate (e.g., a silicon wafer) having a layer of radiation-sensitive material (resist). Typically, a single substrate will contain a network of adjacent target portions that are exposed in succession. Known lithographic apparatus include so-called steppers (each target portion is irradiated by exposing the entire pattern to the target portion at once) and so-called scanners (each target portion is irradiated by scanning the pattern through a beam in a given direction (the "scanning" direction), while synchronously scanning the substrate parallel or antiparallel to that direction).

[0005] A problem that may arise is that the lithographic apparatus is affected by the environment in which the lithographic apparatus is provided. For example, when there are pressure changes in the room in which the lithographic apparatus operates, this may adversely affect the accuracy with which the lithographic apparatus projects a pattern onto a substrate.

[0006] For example, it would be desirable to provide a method that obviates or mitigates one or more problems of the prior art, whether identified herein or elsewhere. Summary of the invention

[0007] According to a first aspect of the present invention, there is provided a method for controlling a projection system during exposure of a substrate by a lithography apparatus, the method comprising: obtaining a measurement signal of a differential pressure change (or an indication or representation of a change) on one or more lenses of the projection system of the lithography apparatus; calculating an imaging error caused by movement of one or more lens elements of the projection system due to the differential pressure change; calculating a lens element adjustment to compensate for the calculated imaging error; applying the lens element adjustment; determining which exposure areas of the substrate are exposed during a delay between the occurrence of the differential pressure change and the application of the lens element adjustment; and storing information identifying these exposure areas together with the calculated imaging error.

[0008] Advantageously, because the exposure region imaging errors are stored, subsequent exposures of these exposure regions can take the imaging errors into account. For example, the imaging fingerprint of the exposure region can be copied for subsequent exposures of the exposure region.

[0009] The method may further comprise, during a subsequent exposure of the identified exposure region of the substrate, applying a lens element adjustment that applies a (second) imaging error based on the imaging error calculated during exposure of the identified exposure region.

[0010] The (second) imaging error applied during the subsequent exposure may correspond to the calculated imaging error and thereby compensate for it.

[0011] The duration of the delay between the occurrence of the differential pressure change and the application of the lens element adjustment may be determined. The determination may take into account the duration between the occurrence of the differential pressure change and the acquisition of a measurement of the differential pressure change.

[0012] The duration of the delay between the occurrence of the differential pressure change and the application of the lens element adjustment can be determined. The determination can take into account the duration of calculating the lens element adjustment.

[0013] The differential pressure measured may be between the pressure beneath a last lens element of the projection system and the pressure between the penultimate lens element and / or the preceding lens element of the projection system.

[0014] The differential pressure can be measured by one or more differential pressure sensors.

[0015] More than one measured differential pressure may be used. Thus, the obtained measurement signal may comprise or represent one or more differential pressures measured by one or more pressure sensors.

[0016] Determining the duration of the delay between occurrence of a differential pressure change and application of a lens element adjustment may occur during a calibration performed before exposure of the substrate begins.

[0017] There can be up to ten identified exposure areas. There can be up to six identified exposure areas. There can be up to three identified exposure areas.

[0018] According to a second aspect of the present invention, there is provided a lithography apparatus comprising: a substrate support (or substrate table) configured to support or hold a substrate; and a projection system configured to project a patterned radiation beam from a pattern forming device onto a substrate, the projection system comprising a plurality of lens elements; one or more pressure sensors configured to obtain differential pressure measurements on one or more lenses of the projection system; a controller configured to calculate an imaging error caused by movement of one or more lens elements of the projection system due to a change in differential pressure, the controller further configured to calculate a lens element adjustment, the lens element adjustment compensating for the calculated imaging error; and a lens element adjuster configured to receive and apply the lens element adjustment, wherein the controller is further configured to determine and identify which exposure areas of the substrate are exposed during a delay between a change in differential pressure and application of the lens element adjustment, and to store information identifying these exposure areas together with the calculated imaging error.

[0019] Advantageously, because the exposure region imaging errors are stored, subsequent exposures of these exposure regions can take the imaging errors into account. For example, the imaging fingerprint of the exposure region can be copied for subsequent exposures of the exposure region.

[0020] The controller may be further configured to apply, during a subsequent exposure of the substrate, a lens element adjustment that applies a (second) imaging error based on the calculated imaging error during exposure of the identified exposure region.

[0021] The imaging error applied during subsequent exposures may correspond to the calculated imaging error.

[0022] The apparatus may include at least one differential pressure sensor.

[0023] The projection system may include a plurality of differential pressure sensors.

[0024] The differential pressure measurement may be between the pressure beneath the last lens element of the projection system and the pressure between the penultimate lens element and / or the preceding lens element of the projection system.

[0025] One or more pressure sensors may be arranged at the space or volume at the last lens element of the projection lens, at the space or volume at the penultimate lens and / or at the space or volume at the preceding lens of the projection lens.

[0026] The controller may be configured to identify up to ten exposure zones. The controller may be configured to identify up to six exposure zones. The controller may be configured to identify up to three exposure zones.

[0027] Features of different aspects of the invention may be combined together. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference characters indicate corresponding parts, and in which:

[0029] Figure 1 depicts a lithographic apparatus according to an embodiment of the invention;

[0030] Figure 2 Depicting a projection system and controller according to an embodiment of the present invention;

[0031] Figure 3 A method according to an embodiment of the present invention is shown; and

[0032] Figure 4 is a graph illustrating the operation of an embodiment of the present invention. DETAILED DESCRIPTION

[0033] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, magnetic domain memories, liquid crystal displays (LCDs), guidance and detection patterns for thin film magnetic heads, etc. The skilled person will appreciate that in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered synonymous with the more general terms "substrate" or "target portion", respectively. Further, a substrate may be processed more than once, for example to create a multi-layer IC, so that the term substrate as used herein may also refer to a substrate that already contains multiple processed layers.

[0034] As used herein, the terms "radiation" and "beam" encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (eg, having a wavelength of 365, 248, 193, 157 or 126 nm).

[0035] The term "patterning device" as used herein should be broadly interpreted as referring to a device that can be used to impart a radiation beam with a pattern in its cross-section, such as to create a pattern in a target portion of a substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate. Typically, the pattern imparted to the radiation beam will correspond to a specific functional layer in a device (such as an integrated circuit) created in the target portion.

[0036] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array uses a matrix arrangement of small mirrors, each of which can be individually tilted to reflect an incoming radiation beam in different directions; in this way, the reflected beam is patterned.

[0037] The support structure holds the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions such as, for example, whether the patterning device is held in a vacuum environment. The support may use mechanical clamping, vacuum or other clamping techniques, such as electrostatic clamping under vacuum conditions. The support structure may be, for example, a frame or a table, which may be fixed or movable as required, and may ensure that the patterning device is in a desired position, for example relative to a projection system. Any use of the term "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device".

[0038] The term "projection system" as used herein should be broadly interpreted to cover various types of projection systems, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example, the exposure radiation used, or other factors such as the use of immersion liquids. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system."

[0039] The illumination system may also encompass various types of optical components, including refractive, reflective and refractive-reflective optical components for directing, shaping or controlling the radiation beam, and such components may also be referred to below, collectively or individually, as "lenses".

[0040] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and / or two or more support structures). In such a "multi-stage" machine, the additional tables may be used in parallel, or preparatory steps may be performed on one or more tables while one or more other tables are being used for exposure.

[0041] The lithographic apparatus may also be of a type in which the substrate is immersed in a liquid having a relatively high refractive index, such as water, to fill the space between the final element of the projection system and the substrate. Immersion techniques are well known in the art to increase the numerical aperture of projection systems.

[0042] Figure 1A lithographic apparatus according to a particular embodiment of the invention is schematically depicted. The apparatus comprises an illumination system (illuminator) IL for conditioning a radiation beam PB (e.g. UV radiation or DUV radiation), a support structure (e.g. mask table) MT for supporting a patterning device (e.g. mask) MA and connected to a first positioning device PM (for accurately positioning the patterning device relative to an article PL), a substrate table (e.g. wafer stage) WT for holding a substrate (e.g. a resist-coated wafer) W and connected to a second positioning device PW (for accurately positioning the substrate relative to the article PL), and a projection system (e.g. refractive projection system) PL configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0043] As depicted here, the device is transmissive (eg employing a transmissive mask). Alternatively, the device may be at least partially reflective (eg employing a reflective mask or a programmable mirror array of the type cited above).

[0044] The illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In this case, the source is not considered to form part of the lithographic apparatus, and the radiation beam is transferred from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and / or a beam expander. In other cases, the source may be an integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL and, if desired, the beam delivery system BD may be referred to as a radiation system.

[0045] The illuminator IL may include adjustment means AM for adjusting the angular intensity distribution of the beam. Typically, at least the outer and / or inner radial extent (commonly referred to as σ outer and σ inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL typically includes various other components, such as an integrator IN and a condenser CO. The illuminator provides a radiation beam PB that is adjusted so as to have a desired uniformity and intensity distribution in its cross-section.

[0046] The radiation beam PB is incident on a pattern field device (e.g. a mask) MA, which is held on a support structure (e.g. a reticle or mask table) MT. After traversing the pattern field device MA, the beam PB passes through the projection system PL, which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioning device PW and a position sensor IF (e.g. an interferometer), for example, the substrate table WT can be accurately moved to position a different target portion C in the path of the beam PB. Similarly, the first positioning device PM and a further position sensor (not shown) can be used to position the substrate table WT accurately. Figure 1The support structure MT (as explicitly depicted in Figure 1) can be used to accurately position the patterning device MA relative to the path of the beam PB, for example after mechanical retrieval from a mask library or during scanning. Typically, movement of the stages MT and WT will be achieved with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning) which form part of the positioning devices PM and PW. However, in the case of a stepper (as opposed to a scanner), the support structure MT may be connected to the short-stroke actuator only, or may be fixed. The patterning device MA and substrate W may be aligned using the patterning device alignment marks M1, M2 and the substrate alignment marks P1, P2.

[0047] The lithographic apparatus further comprises a controller 33 configured to determine imaging errors of the projection system and calculate adjustments to be applied to lens elements of the projection system to compensate for the imaging errors. The lithographic apparatus further comprises a first pressure sensor 31 and a second pressure sensor 32 configured to provide pressure measurements to the controller 33.

[0048] The depicted device can be used in the following preferred modes:

[0049] In step mode, the support structure MT and substrate table WT are held substantially stationary while the entire pattern imparted to the beam PB is projected at once onto a target portion C (i.e. a single static exposure). The substrate table WT is then shifted in the X and / or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

[0050] In scan mode, the support structure MT and substrate table WT are scanned synchronously while projecting a pattern imparted to beam PB onto a target portion C (i.e. a single dynamic exposure). The speed and direction of the substrate table WT relative to the support structure MT are determined by the (de-)magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width of the target portion (in the non-scanning direction) in a single dynamic exposure, while the length of the scanning movement determines the height of the target portion (in the scanning direction).

[0051] In another mode, the support structure MT is held substantially stationary, thereby holding the programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the beam PB is projected onto a target portion C. In this mode, a pulsed radiation source is typically employed, and the programmable patterning device is updated as required after each movement of the substrate table WT or between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography, which makes use of a programmable patterning device (such as a programmable mirror array of the type cited above).

[0052] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be employed.

[0053] Figure 2 An example projection system PL is schematically depicted, wherein first to fifth lens elements 21-25 are shown. It is to be understood that the depiction of five lens elements is exemplary only, and that the projection system PL may include any number of lens elements. Figure 2 In FIG. 1 , lens element 25 is the “last” lens element, i.e. the lens element closest to substrate W. A face 26 of the last lens element 25 is directly opposite substrate W. A first pressure P1 prevails in a first volume adjacent to face 26. The first volume may be defined by a substrate compartment 15 in which the substrate table PW is located, or may be defined by other components such as an immersion bath if the lithographic apparatus is an immersion system. The first volume 15 may be referred to as the environment of the bottom side of the last lens element 25.

[0054] In an embodiment, the projection system PL is part of an immersion lithography system, in which an immersion bath containing an immersion medium (e.g., high purity water) may be provided between the last lens element 25 and the substrate W to increase the numerical aperture and thereby the resolution of the lithographic apparatus. In such a system, the last lens element 25 may be connected to the housing 27 of the projection system by a permeable (or "leaky") seal 28 to reduce the pressure gradient across the last lens element 25. In particular, the last lens element 25 may have a higher optical sensitivity and be attached to the housing 27 with a lower stiffness than the penultimate lens element 24, for example.

[0055] A gas source (not depicted) within projection system PL may provide a positive pressure. Thus, there may be a difference between pressure P1 below last lens element 25 and pressure P2 above last lens element 25. This may be referred to as differential pressure. The differential pressure may be referred to as DP12. Pressure P1 in environment 15 below last lens element 25 may change. When this occurs, it will cause a change in differential pressure DP12. Last lens element 25 will move due to differential pressure DP12. For example, if pressure P1 increases, last lens element 25 will move towards penultimate lens element 24 (i.e., at Figure 2 The movement of the last lens element 25 may include a combination of rotation and z-direction (upward or downward) movement. Such movement will adversely affect the accuracy of the lithographic apparatus in projecting the pattern onto the substrate. Due to the influence of the positive pressure provided in the projection system, the change in differential pressure DP12 may take several seconds to decay.

[0056] There may also be differential pressures between other lens elements. When a pressure change occurs in environment 15, it may cause a change in differential pressures elsewhere in projection system PL. For example, a change in differential pressure DP13 may occur between pressure P1 below last lens element 25 and pressure P3 between penultimate lens element 24 and preceding lens element 23. The depicted embodiment measures differential pressure DP12. However, other embodiments may measure differential pressure DP13 (or a different differential pressure).

[0057] In the depicted embodiment, the first pressure sensor 31 is located between the last lens element 25 and the penultimate lens element 24. The second pressure sensor 32 is located on the opposite side of the last lens element 25. In other embodiments, the pressure sensors 31, 32 may be provided at other locations. In general, the pressure sensors 31, 32 may be located at locations that allow the differential pressure DP12 (or different differential pressures) to be measured. The second pressure sensor 32 may be located at other locations in the first region (e.g., other locations in the substrate compartment 15). The pressure sensors 31, 32 may be any pressure sensors suitable for measuring the pressures P1, P2 (e.g., conventional digital pressure sensors).

[0058] Each pressure sensor 31, 32 is connected to a controller 33 and provides a pressure measurement signal to the controller 33. The controller 33 is configured to use the pressure measurements (or the pressure measurement signals or representations thereof) to determine a differential pressure DP12.

[0059] The measurement signal provided to controller 33 may include or be representative of one or more differential pressures measured by one or more pressure sensors.

[0060] More than one pressure sensor may be provided at the same surface of the lens elements 21-25. Thus, the pressure difference on the surface of the lens elements 21-25 (i.e., the differential pressure on a single lens surface and its change) may be measured. The change in the pressure difference on the surface of the lens elements 21-25 may cause a tilt or an asymmetric displacement of the lens elements. The received or measured differential pressure change may be a single-sided differential pressure change of the lens element surface (or lens surface). The calculated imaging error may be based on such a differential pressure change. Lens correction or adjustment may be applied by means of the controller 33 to correct such a change.

[0061] In addition, pressure changes at or on the surfaces of the lens elements 21-25 may cause a change in the refractive index of the fluid (e.g. purge gas or immersion liquid) at the lens elements. The change in refractive index may cause (additional) imaging errors. The controller 33 may be arranged to calculate and correct such changes in refractive index (induced imaging errors). The correction may be applied as a lens element correction / adjustment by means of the controller 33.

[0062] In an alternative arrangement (not depicted), the pressure sensor may be a differential pressure sensor. In this arrangement, a tube connects one side of the differential pressure sensor to the space between the last lens element 25 and the penultimate lens element 24. The other side of the differential pressure sensor is located in the environment 15 on the bottom side of the last lens element 25 (i.e., receives the pressure on the opposite side of the last lens element). Therefore, both sides of the differential pressure sensor receive pressure from either side of the last lens element 25. The differential pressure sensor directly measures the differential pressure DP12. In the case of using this arrangement, the controller 33 does not need to use two pressure measurements to determine the differential pressure DP12, but directly receives the differential pressure DP12. Directly measuring the differential pressure may be more accurate than measuring two pressures and calculating the differential pressure (because the difference between the pressures may be small compared to the pressure values). Multiple differential pressure sensors may be provided. The differential pressure sensor may be configured to measure different differential pressures (e.g., DP12, DP13, etc.).

[0063] The projection system PL may comprise a plurality of differential pressure sensors. Each differential sensor may be arranged to measure a pressure difference provided across a lens element in the projection lens.

[0064] Both sides of the differential pressure sensor can be provided in the same space. Thus, the differential pressure (and its changes) across a single surface of the lens elements 21-25 can be measured and taken into account to calculate the imaging error. In turn, this contribution to the imaging error can be used for lens adjustment.

[0065] In yet another alternative arrangement, differential pressure DP12 may be determined by electronics located prior to controller 33 (eg, using a circuit configured to determine the difference between two pressure measurements).

[0066] For example, a change in pressure and / or differential pressure may occur when an operator opens the door of the room where the lithographic apparatus LA is located. This may occur during exposure of the substrate W by the lithographic apparatus LA. The controller 33 may be configured to calculate imaging errors caused by movement of the last lens element 25 and / or other lens elements due to the change in differential pressure DP12, DP13. Imaging errors may include overlay and image focus characteristics.

[0067] The controller 33 may also be configured to calculate lens element adjustments that compensate for imaging errors. Signals are then sent to the lens elements 21-25 to cause adjustments of the lens elements to occur. The adjustments may, for example, include heating or cooling a lens element, heating or cooling a region of a lens element, moving a lens element, etc.

[0068] The controller 33 may also be configured to determine which exposure areas of the substrate are exposed during the delay between a change in differential pressure (e.g., due to the opening of a room door) and the application of lens element adjustment. The controller stores information identifying these exposure areas together with the imaging errors of these exposure areas. The imaging errors of these exposure areas may be taken into account when performing subsequent exposures on these exposure areas. For example, the lithography equipment performing subsequent exposures of these areas may artificially apply the same (or partially the same) imaging errors. The imaging errors may be referred to as overlapping fingerprints. Typically, the imaging errors applied to the subsequent exposures may be based on the imaging errors that existed during the previous exposure of the corresponding exposure area. That is, the imaging errors applied may correspond to the previously existing imaging errors, or may include some differences. For example, differences may be caused by differences in the properties of the exposed pattern compared to the previously exposed pattern. The imaging errors applied may correspond to the imaging errors that existed during the previous exposure of the corresponding exposure area.

[0069] Figure 3 A method according to an embodiment of the invention is schematically depicted. The method is used when a pressure differential PD12, DP13 changes. The differential pressure change is indicated as step S0.

[0070] In a first step S1 of the method, the controller 33 receives a signal of a differential pressure measurement (e.g. receives a signal representing a measurement from the pressure sensors 31, 32). The differential pressure measurement may exceed a threshold value, indicating a significant change in the differential pressure DP12, DP13. For example, a differential pressure change may be caused by an operator opening or closing a door to a room in which the lithographic apparatus LA is provided. The pressure change may also have different causes. Lithographic exposure of a substrate W is ongoing, and some exposure areas C of the substrate W may have been exposed. In this document, the term "exposure area" is intended to mean an area (e.g. a single exposure field) that is exposed during a single exposure of the lithographic apparatus LA.

[0071] In step S2, the controller 33 calculates the imaging error characteristics caused by the lens deviation due to the change in the differential pressures DP12, DP13. While the calculation is being performed, the exposure area continues to be exposed on the substrate W, as indicated in step S3.

[0072] In step S4, a corrective lens element adjustment is calculated. The corrective lens element adjustment adjusts the imaging characteristics of the projection system PL to compensate for imaging errors caused by differential pressure changes. While the calculations are being performed, exposure areas continue to be exposed on the substrate.

[0073] In step S5, the imaging error characteristic calculated in step S2 is stored in a memory. The imaging error characteristic is associated with a specific exposure area of ​​the substrate W.

[0074] In step S6, the calculated corrective lens element adjustments are applied to the projection system PL, resulting in compensation for imaging errors present due to lens aberrations (imaging errors may be substantially removed).Subsequent exposure of the substrate W includes correction for changes in the differential pressures DP12, DP13.

[0075] There may be a significant delay between the occurrence of a change in the pressure differential and the application of the corrective lens element adjustment to the projection system PL. The delay may correspond to the time taken to expose a plurality of exposure areas on the substrate W. As an example, there may be a delay corresponding to the exposure of five exposure areas on the substrate W. Referring again to step S5, the calculated error characteristics of these five exposure areas (or other plurality of exposure areas) are recorded.

[0076] In step S7, the exposure of the substrate W is completed. The substrate W is removed from the lithographic apparatus LA and processed. The processing may, for example, include one or more of developing a photoresist on the substrate, depositing a material on the substrate, chemically mechanically polishing the substrate, and the like.

[0077] In step S8, a subsequent layer is exposed on the substrate W. This exposure of the subsequent layer may be performed by the same lithographic apparatus LA as performed the previous exposure, or may be performed by a different lithographic apparatus.

[0078] In step S9, during exposure of a subsequent layer on the substrate, a lens adjustment is applied to the projection system PL of the lithographic apparatus LA to introduce an imaging error. The imaging error is applied when exposing an exposure area corresponding to an exposure area previously exposed after the differential pressure change is started and before the correction lens element adjustment is applied. These exposure areas may be referred to as selected exposure areas (or temporary exposure areas). For a given selected exposure area, the applied lens adjustment generates an imaging error that substantially corresponds to the imaging error that existed when the previous layer was exposed. This can be referenced by saying that the overlap fingerprint during exposure matches the overlap fingerprint of the exposure area when the previous layer was exposed. The pattern features of the exposure area are aligned with the pattern features exposed in the previous layer.

[0079] The exposure region that is exposed after the differential pressure change begins and before the corrective lens element adjustment is applied may be referred to as a temporary exposure region. There may be multiple temporary exposure regions. For example, there may be up to ten temporary exposure regions. For example, there may be up to six exposure regions. There may be up to three exposure regions.

[0080] In an embodiment, the method runs continuously. In this embodiment, imaging errors caused by any changes in differential pressure DP12, DP13 can be taken into account during exposure of subsequent layers. In an alternative embodiment, the method is triggered to run when the differential pressure changes by more than a threshold value. An advantage of this embodiment is that computing power is only used when significant errors due to significant changes in differential pressure are expected to occur.

[0081] In an embodiment, a time period between the differential pressure change occurring in step S0 and the application of the corrective lens element adjustment in step S6 is measured. This time period may consist of two components. The first component may be a delay caused by the measurement itself. For example, a time period of about a few tenths of a second, such as about 0.1 to 0.5 seconds (or some other time period) may pass between the occurrence of the differential pressure change and the controller 33 receiving the measurement of the differential pressure change. The second component may be a delay caused by the time required to calculate the corrective lens adjustment. For example, a time period of about a few tenths of a second, such as about 0.1 to 0.5 seconds (or some other time period) may be required to calculate the corrective lens element adjustment applied to the projection system PL.

[0082] Using knowledge of the time delay between when the differential pressure change occurred and when the controller 33 received the measured change, and knowledge of the time required to calculate the corrective lens element adjustment, the controller 33 can determine which exposure areas C on the substrate W were exposed with imaging errors caused by the differential pressure change. For example, the controller 33 determines that exposure areas exposed 0.4 seconds before the pressure change in the room was detected will include imaging errors, and exposure areas exposed 0.4 seconds after the pressure change in the room was detected will include imaging errors.

[0083] When exposing subsequent layers, exposure regions affected by imaging errors are identified. When exposing these exposure regions, lens element adjustments that provide corresponding imaging errors can be applied. When exposing other exposure regions, no lens element adjustments are applied.

[0084] The corrective lens element adjustment can be applied after an exposure area is exposed and before the next exposure area is exposed. This provides some time for the adjustment to be applied between exposures. In an alternative approach, the adjustment can be applied during exposure.

[0085] Figure 4 is a graph that provides an operational illustration of an embodiment of the present invention. Figure 4 In the graph, the black line (mx-ovl) is a metric that indicates the accuracy of the pattern exposure on the substrate. This metric may be called overlay and is a measure of how well the pattern features are projected at the correct location on the substrate. The correct location may correspond to the location of the previously projected pattern. The left scale of the graph indicates the overlay (zero indicates perfect overlay).

[0086] The dashed line (mx-decorr.flt) indicates the differential pressure DP13 from the pressure P1 below the last lens element 25 and the pressure P3 between the penultimate lens element 24 and the preceding lens element 23. The scale on the right side of the graph indicates the differential pressure. As can be seen, the differential pressure is initially about 0 and then rises rapidly around 5 seconds. This change in differential pressure may correspond, for example, to an operator opening the door of the room where the lithography apparatus LA is located. Figure 4 As shown, the differential pressure decays over time, although it has not yet reached zero by the end of the depicted 10 second time period.

[0087] Figure 4 The grey (thinner) line in (LOP exposure) indicates the measured differential pressure DP13 received by the controller 33. By comparing the dashed line and the grey line, it can be seen that there is a delay between a change in differential pressure and the controller 33 receiving the corresponding measured differential pressure change.

[0088] Figure 4 The dashed lines in FIG. 1 (LOP queue) show the differential pressure values ​​corresponding to the corrective lens element adjustments. These differential pressure values ​​are calculated based on the corrections calculated for the lens elements. There is a direct relationship between the calculated lens element adjustments and the differential pressure values, and this direct relationship allows the differential pressures corresponding to the lens element adjustments to be plotted in the graph. The differential pressure values ​​occur simultaneously with the corrective lens element adjustments being applied to lens elements 21-25. In other words, there is a direct instantaneous relationship between the differential pressures depicted by the dashed lines and the corrective lens element adjustments applied. As depicted, there is a time delay of approximately 0.8 seconds between the differential pressure change and the lens element adjustment that applies the differential pressure change.

[0089] As can be seen from the black line, the applied corrective lens element adjustment starts to take effect approximately 0.8 seconds after the differential pressure change occurs. After only 6 seconds of exposure time, the overlap drops to around 0.0. The intersection of the black overlap line with the dashed differential pressure line corresponding to the timing of the lens element adjustment shows the correlation between the overlap correction and the differential pressure value corresponding to that correction.

[0090] While the depicted embodiment has a delay of approximately 0.8 seconds, different delays may be suitable for other embodiments.

[0091] exist Figure 2 In the depicted embodiment, the differential pressure DP12 on either side of the last lens element 25 of the projection system PL is used. Figure 4In the graph of FIG. 3 , differential pressure DP13 is used, i.e., the difference between pressure P1 below the last lens element 25 and pressure P3 between the penultimate lens element 24 and the preceding lens element 23. Using differential pressure DP13 may be particularly advantageous and may provide better correction than using differential pressure DP12. Because opening 36 provides some pressure communication to the back of the last lens element 25, changes in differential pressure DP12 may decay relatively quickly. Changes in differential pressure DP13 may decay more slowly and thus continue to cause lens aberrations (and therefore imaging errors) after differential pressure DP12 decays. Therefore, differential pressure DP13 may be more accurately correlated to imaging errors than differential pressure DP12.

[0092] In other embodiments, differential pressures between other parts of the projection system PL may be used.

[0093] Because the stiffness of the mechanical connection of a lens element to its supporting structure may vary, the movement of the lens element in response to a given differential pressure change may vary from lens element to lens element.

[0094] More than one differential pressure may be used. Using more than one differential pressure may advantageously improve the accuracy of the correction provided by embodiments of the present invention. This is because lens element movement and associated imaging errors may be determined more accurately.

[0095] During calibration, the time delay between a change in pressure and the application of a lens element adjustment may be measured. Calibration may be performed before exposure of the substrate begins. For a given lithographic apparatus LA, the time delay may remain constant, in which case the time delay may be measured once. The time delay may change slowly, in which case the time delay may be measured periodically at a time period or time cycle (e.g., weekly, monthly, etc.) that is appropriate to the rate of change. If the software that calculates the lens element adjustment is changed, the time delay may change. In this case, the time delay may be measured when the software is changed. Calibration may include exposing the substrate when a significant pressure change occurs, such as due to opening a door to a room in which the lithographic apparatus is operating. Calibration may include performing measurements on the exposed substrate to determine when a pressure change occurs, and comparing it to a signal received by the controller 33 indicating a pressure change. Calibration may include performing measurements on the exposed substrate W to determine the delay between a pressure change occurring and applying a lens adjustment to correct for the effects of the pressure change.

[0096] According to another embodiment of the present invention, a method for controlling a projection system is provided, wherein the projection system includes a lens adjustment component. The method includes: receiving data of a first pressure in a compartment of the projection system and a second pressure in an environment at the projection system during a first exposure; determining a pressure difference between the first pressure and the second pressure; receiving data of a lens adjustment based on a change in the pressure difference during the first exposure; calculating a lens position error based on a time delay between the change and the lens adjustment provided by the lens adjustment component; and determining a lens position for a second exposure based on the lens position error during the first exposure.

[0097] A person skilled in the art will appreciate that features of different aspects of the invention, as disclosed above, may be combined together.

[0098] The method according to an embodiment of the present invention may be performed by a computing device. The device may include a central processing unit ("CPU") connected to a memory. The method described herein may be implemented in a code (software) stored on a memory including one or more storage media, and arranged to be executed on a processor including one or more processing units. The storage medium may be integrated into the CPU and / or separated from the CPU. The code, which may be referred to as an instruction, is configured to be extracted from the memory and executed on the processor to perform operations consistent with the embodiments discussed herein. Alternatively, it is not excluded that some or all of the functionality of the CPU is implemented in a dedicated hardware circuit system or a configurable hardware circuit system such as an FPGA.

[0099] The computing device may include an input configured to enable a user to input data into a software program running on the CPU. The input device may include a mouse, keyboard, touch screen, microphone, etc. The computing device may also include an output device configured to output the measurement results to the user.

[0100] Where the context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented 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 sending information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include a read-only memory (ROM); a random access memory (RAM); a magnetic storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal (e.g., a carrier wave, an infrared signal, a digital signal, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such description is merely for convenience, and such actions are in fact caused by a computing device, processor, controller, or other device that executes firmware, software, routines, instructions, etc., and in doing so may cause an actuator or other device to interact with the physical world.

[0101] Various aspects of the invention are set out in the following clauses.

[0102] 1. A method of controlling a projection system during exposure of a substrate by a lithographic apparatus, the method comprising:

[0103] obtaining a measurement signal indicative of or representative of a change in differential pressure across one or more lenses of a projection system of a lithographic apparatus;

[0104] calculating imaging errors caused by movement of one or more lens elements of the projection system due to changes in differential pressure during exposure;

[0105] calculating lens element adjustments that compensate for the calculated imaging errors;

[0106] Apply lens element adjustments;

[0107] identifying which exposure regions of the substrate are exposed during a delay between occurrence of a change in differential pressure and application of a lens element adjustment; and storing information of the identified exposure regions along with the calculated imaging error.

[0108] 2. A method according to clause 1, wherein the method further comprises, during a subsequent exposure of the identified exposure area of ​​the substrate, applying a lens element adjustment, the lens element adjustment applying a (second) imaging error based on the calculated imaging error during exposure of the identified exposure area.

[0109] 3. A method according to clause 2, wherein the imaging error applied during subsequent exposures corresponds to the calculated imaging error.

[0110] 4. A method according to any preceding clause, wherein the duration of the delay between occurrence of a change in differential pressure and application of the lens element adjustment is determined taking into account the duration between occurrence of the change in differential pressure and obtaining a measurement of the change in differential pressure.

[0111] 5. A method according to any of clauses 1 to 3, wherein the duration of the delay between the occurrence of the differential pressure change and the application of the lens element adjustment is determined, the determination taking into account the duration of calculating the lens element adjustment.

[0112] 6. A method according to any preceding clause, wherein the differential pressure measured is between the pressure below a last lens element of the projection system and the pressure between the penultimate lens element and the preceding lens element of the projection system.

[0113] 7. A method according to any of clauses 1 to 5, wherein the differential pressure measured is between the pressure below a last lens element of the projection system and the pressure above the last lens element of the projection system.

[0114] 8. A method according to any preceding clause, wherein the differential pressure is measured by at least one differential pressure sensor.

[0115] 9. A method according to any preceding clause, wherein more than one measured differential pressure is used.

[0116] 10. A method according to clause 4 or clause 5, wherein the duration of the delay between occurrence of a change in differential pressure and application of the lens element adjustment is determined during a calibration performed before exposure of the substrate commences.

[0117] 11. A method according to any preceding clause, wherein there are up to ten identified exposure regions.

[0118] 12. A lithographic apparatus comprising:

[0119] a substrate table configured to support a substrate;

[0120] a projection system configured to project the patterned radiation beam from the patterning device onto the substrate, the projection system comprising a plurality of lens elements;

[0121] one or more pressure sensors configured to obtain differential pressure measurements across one or more lenses of the projection system;

[0122] a controller configured to calculate imaging errors caused by movement of one or more lens elements of the projection system due to changes in differential pressure;

[0123] The controller is further configured to calculate a lens element adjustment that compensates for the calculated imaging error; and

[0124] A lens element adjuster is configured to receive and apply lens element adjustments; wherein the controller is further configured to determine which exposure areas of the substrate are exposed during a delay between a change in differential pressure and application of the lens element adjustments, and to store information identifying these exposure areas together with the calculated imaging error.

[0125] 13. The lithographic apparatus of clause 12, wherein the controller is further configured to apply, during a subsequent exposure of the substrate, a lens element adjustment that applies a (second) imaging error based on the calculated imaging error during exposure of the identified exposure region.

[0126] 14. A lithographic apparatus according to clause 13, wherein an imaging error applied during a subsequent exposure corresponds to the calculated imaging error.

[0127] 15. A lithographic apparatus according to any of clauses 12 to 14, wherein the apparatus comprises at least one differential pressure sensor arranged to measure a pressure difference across at least one lens element.

[0128] 16. A lithographic apparatus according to clause 15, wherein the projection system comprises a plurality of differential pressure sensors.

[0129] 17. The lithographic apparatus of any of clauses 12 to 16, wherein the differential pressure measurement is between the pressure below a last lens element of the projection system and the pressure between the penultimate lens element and the preceding lens element of the projection system.

[0130] 18. The lithographic apparatus of any of clauses 12 to 16, wherein the differential pressure measurement is between the pressure below a last lens element of the projection system and the pressure above the last lens element of the projection system.

[0131] 19. A lithographic apparatus according to any of clauses 12 to 18, wherein the controller is configured to identify up to ten exposure regions.

[0132] 20. A computer program comprising computer readable instructions configured to cause a computer to perform a method according to any of clauses 1 to 10.

[0133] 21. A computer readable medium carrying a computer program according to clause 20.

[0134] 22. A lithographic apparatus according to any of clauses 12 to 16, wherein two or more pressure sensors are provided on the same side of a lens surface of one or more lenses to measure pressure differences across the lens surface.

[0135] 23. A method according to any one of clauses 1 to 11, further calculating additional image errors caused by changes in the refractive index of the fluid at the one or more lenses caused by changes in differential pressure.

[0136] 24. A method according to any one of clauses 1 to 5, wherein the differential pressure change is a differential pressure change on a single side of a surface of one or more lenses.

[0137] 25. A method for controlling a projection system, the projection system comprising a lens adjustment component, the method comprising:

[0138] receiving data of a first pressure in a compartment of the projection system and a second pressure in an environment at the projection system during a first exposure;

[0139] determining a pressure difference between a first pressure and a second pressure;

[0140] receiving data of a lens adjustment based on a change in the pressure differential during the first exposure;

[0141] calculating a lens position error based on a time delay between the change and the lens adjustment provided by the lens adjustment component; and

[0142] The lens position for the second exposure is determined based on the lens position error during the first exposure.

[0143] Although specific embodiments of the present invention have been described above, it is to be understood that the present invention may be practiced in a manner other than that described. The above description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A method of controlling a projection system during exposure of a substrate by a lithographic apparatus, the method comprising: obtaining a measurement signal of a change in differential pressure across one or more lenses of a projection system of the lithographic apparatus; calculating imaging errors caused by movement of one or more lens elements of the projection system due to changes in the differential pressure during exposure; calculating a lens element adjustment that compensates for the calculated imaging error; applying said lens element adjustment; identifying which exposure regions of the substrate are exposed during a delay between occurrence of the differential pressure change and application of the lens element adjustment; as well as Information of the identified exposure areas is stored together with the calculated imaging errors.

2. The method according to claim 1, wherein the method further comprises: During a subsequent exposure of the identified exposure region of the substrate, the lens element adjustment is applied, the lens element adjustment applying an imaging error based on the calculated imaging error during exposure of the identified exposure region. 3 . The method of claim 2 , wherein the imaging error applied during the subsequent exposure corresponds to the calculated imaging error.

4. A method according to any preceding claim, wherein the duration of the delay between occurrence of the differential pressure change and application of the lens element adjustment is determined, the determination taking into account the duration between occurrence of the differential pressure change and obtaining the measurement of the differential pressure change.

5. The method of any one of claims 1 to 3, wherein the duration of the delay between occurrence of the differential pressure change and application of the lens element adjustment is determined, the determination taking into account the duration of calculating the lens element adjustment.

6. A method according to any preceding claim, wherein the measured differential pressure is between the pressure below a last lens element of the projection system and the pressure between the penultimate lens element and the preceding lens element of the projection system, or between the pressure below the last lens element of the projection system and the pressure above the last lens element of the projection system.

7. A method according to any preceding claim, wherein the differential pressure is measured by at least one differential pressure sensor.

8. A method according to any preceding claim, wherein more than one measured differential pressure is used.

9. A method according to claim 4 or claim 5, wherein the duration of the delay between occurrence of the differential pressure change and application of the lens element adjustment is determined during a calibration performed before exposure of the substrate commences.

10. A method according to any preceding claim, wherein there are up to ten identified exposure regions.

11. A lithographic apparatus comprising: a substrate table configured to hold a substrate; a projection system configured to project the patterned radiation beam from the patterning device onto the substrate, the projection system comprising a plurality of lens elements; one or more pressure sensors configured to obtain differential pressure measurements across one or more lenses of the projection system; a controller configured to calculate imaging errors caused by movement of one or more lens elements of the projection system due to changes in differential pressure; The controller is further configured to calculate a lens element adjustment that compensates for the calculated imaging error; as well as a lens element adjuster configured to receive and apply said lens element adjustment; in The controller is also configured to determine which exposure regions of the substrate were exposed during a delay between occurrence of the differential pressure change and application of the lens element adjustment, and to store information identifying those exposure regions with the calculated imaging error.

12. The lithographic apparatus of claim 11, wherein the controller is further configured to apply a lens element adjustment during a subsequent exposure of the substrate, the lens element adjustment applying an imaging error based on the calculated imaging error during exposure of the identified exposure region.

13. The lithographic apparatus of claim 12, wherein the imaging error applied during the subsequent exposure corresponds to the calculated imaging error.

14. A lithographic apparatus according to any one of claims 11 to 13, wherein the apparatus comprises at least one differential pressure sensor arranged to measure a pressure difference across at least one lens element.

15. A lithographic device according to any one of claims 11 to 14, wherein the differential pressure measurement value is between the pressure below the last lens element of the projection system and the pressure between the penultimate lens element and the preceding lens element of the projection system, or between the pressure below the last lens element of the projection system and the pressure above the last lens element of the projection system.

Images