Projection System Control

Abstract

1. A method for 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 an imaging error caused by movement of one or more lens elements of the projection system due to the measured change in differential pressure during exposure; calculating a lens element adjustment value that compensates for the calculated imaging error; applying the lens element adjustment value; identifying which exposure areas of the substrate were exposed during a delay between the occurrence of the change in differential pressure and the application of the lens element adjustment value; and storing information of the identified exposure areas together with the calculated imaging error.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[01] This application claims priority to European Application No. 22213847.1, filed December 15, 2022, which is incorporated herein by reference in its entirety.

[0002]

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

[0003]

[03] A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that case, a patterning device, also called a mask or reticle, can be used to image a target portion (e.g., including part of a die, or one or more dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist) on it. In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing the entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern with a beam in a certain direction (the "scan" direction) while simultaneously scanning the substrate parallel or anti-parallel to this direction.

[0004]

[04] A problem that can arise is that the lithographic apparatus is affected by the environment it is placed in. For example, if the pressure in the room in which the lithographic apparatus operates changes, this can have a negative effect on the accuracy with which the lithographic apparatus projects a pattern onto a substrate.

[0005]

[05] It would be desirable, for example, to provide a way to obviate or mitigate one or more of the problems of the prior art, whether identified herein or otherwise. Summary of the Invention

[0006]

[06] According to a first aspect of the present invention, there is provided a method of controlling a projection system during exposure of a substrate by a lithographic apparatus, the method comprising obtaining a measurement signal of (or indicative of or representative of) a change in differential pressure across one or more lenses of a projection system of the lithographic apparatus; A method is provided that includes calculating an imaging error caused by movement of one or more lens elements of the projection system due to a differential pressure change, calculating a lens element adjustment value that compensates for the calculated imaging error, applying the lens element adjustment value, determining which exposure areas of the substrate were exposed during a delay between the occurrence of the differential pressure change and the application of the lens element adjustment value, and storing information identifying the exposure areas together with the calculated imaging error.

[0007]

[07] Advantageously, imaging errors of an exposure area are stored so that subsequent exposures of that exposure area can take the imaging errors into account, for example an imaging fingerprint of an exposure area can be replicated for subsequent exposures of the exposure area.

[0008]

[08] The method may further include applying, during a subsequent exposure of the identified area of ​​the substrate, lens element adjustment values ​​that adjust for a (second) imaging error based on the calculated imaging error during exposure of the identified exposure area.

[0009]

[09] The (second) imaging error applied during the subsequent exposure may correspond to the calculated imaging error that is compensated for.

[0010]

[0010] A time delay between the occurrence of the differential pressure change and the application of the lens element adjustment value may be determined, which may take into account the time between the occurrence of the differential pressure change and the taking of a measurement of the differential pressure change.

[0011]

[0011] A time delay between the occurrence of the differential pressure change and the application of the lens element adjustment value may be determined, which may take into account the time to calculate the lens element adjustment value.

[0012]

[0012] The measured differential pressure can be between the pressure below the last lens element of the projection system and the pressure between the penultimate lens element and the previous 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.

[0013]

[0013] The differential pressure may be measured by one or more differential pressure sensors.

[0014]

[0014] More than one measured differential pressure may be used, and thus the acquired measurement signal may include or represent one or more differential pressures measured by one or more pressure sensors.

[0015]

[0015] Determining the time delay between the occurrence of the differential pressure change and the application of the lens element adjustment value may be performed during a calibration that occurs before the start of exposure of the substrate.

[0016] There can be up to 10 specified exposure regions. There can be up to 6 specified exposure regions. There can be up to 3 specified exposure regions.

[0017]

[0017] According to a second aspect of the present invention, there is provided a lithographic apparatus comprising: a substrate support (or substrate table) configured to support or hold a substrate; a projection system configured to project a patterned radiation beam from a patterning device onto the substrate, the projection system including 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 an imaging error caused by movement of one or more lens elements of the projection system due to the occurrence of a change in differential pressure, the controller further configured to calculate a lens element adjustment value that compensates for the calculated imaging error; and a lens element adjuster configured to receive and apply the lens element adjustment value, wherein the controller is further configured to determine and identify which exposure areas of the substrate have been exposed during the delay between the occurrence of the differential pressure change and the application of the lens element adjustment value, and store information identifying the exposure areas together with the calculated imaging error.

[0018] Advantageously, imaging errors of an exposure area are stored so that subsequent exposures of said exposure area can take the imaging errors into account, for example an imaging fingerprint of an exposure area can be replicated for subsequent exposures of the exposure area.

[0019]

[0019] The controller may be further configured to apply a lens element adjustment value that applies a (second) imaging error during subsequent exposure of the substrate based on the calculated imaging error during exposure of the identified exposure area.

[0020]

[0020] The imaging error applied during subsequent exposures may be consistent with the calculated imaging error.

[0021] The apparatus may include at least one differential pressure sensor.

[0022]

[0022] The projection system may include multiple differential pressure sensors.

[0023]

[0023] The differential pressure measurement can be between the pressure below the last lens element of the projection system and the pressure between the second to last lens element of the projection system and the previous lens element, 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.

[0024]

[0024] One or more pressure sensors may be located in a space or volume in the last lens element of the projection lens, or in a space or volume in the penultimate lens element, or in a space or volume in the lens immediately preceding the projection lens.

[0025] The controller may be configured to identify up to 10 exposure regions. The controller may be configured to identify up to 6 exposure regions. The controller may be configured to identify up to 3 exposure regions.

[0026]

[0026] Features of different aspects of the invention may be combined. [Brief explanation of the drawings]

[0027]

[0027] Some embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which like reference symbols indicate corresponding parts, and in which: [Figure 1] 1 depicts a lithographic apparatus according to an embodiment of the invention; [Figure 2] 1 illustrates a projection system and controller according to an embodiment of the present invention; [Figure 3] 1 illustrates a method according to one embodiment of the present invention. [Figure 4] 4 is a graph illustrating the operation of an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0028]

[0028] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid crystal displays (LCDs), thin film magnetic heads, etc. As will be appreciated by those skilled in the art, in such other applications, any reference to a "wafer" or "die" as used herein may be considered synonymous with the more general terms "substrate" or "target portion," respectively. Furthermore, a substrate may be processed multiple times to create, for example, a multi-layer IC, and therefore the term substrate as used herein may also refer to a substrate that already includes multiple processing layers.

[0029]

[0029] As used herein, the terms "radiation" and "beam" encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having wavelengths of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm).

[0030]

[0030] As used herein, the term "patterning device" should be interpreted broadly to refer to a device that can be used to impart a radiation beam with a pattern in its cross-section so 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 particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0031]

[0031] A 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 employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions, imparting a pattern to the reflected beam.

[0032] The support structure 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 whether or not the patterning device is held in a vacuum environment. The support can use mechanical, vacuum or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required, and which ensures that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms "reticle" or "mask" herein may be considered as synonymous with the more general term "patterning device".

[0033]

[0033] The term "projection system" as used herein should be interpreted broadly to encompass various types of projection systems, including, for example, refractive, reflective, and catadioptric systems, as appropriate for the exposure radiation being used, or other factors such as the use of an immersion fluid. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system."

[0034]

[0034] The illumination system may also include various types of optical components, including refractive, reflective, and reflective-refractive optical components for guiding, shaping, and / or controlling the radiation beam, and such components may hereinafter be referred to collectively or individually as "lenses."

[0035] 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 can be used in parallel, or preliminary steps can be carried out on one or more tables while one or more other tables are used for exposure.

[0036]

[0036] The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index (e.g. water) so as to fill a space between the final element of the projection system and the substrate. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

[0037] 1 schematically depicts a lithographic apparatus according to a particular embodiment of the present invention. The apparatus includes an illumination system (illuminator) IL that conditions a radiation beam PB (e.g. ultraviolet or DUV radiation), a support structure (e.g. mask table) MT that supports a patterning device (e.g. mask) MA and is coupled to a first positioning device PM that precisely positions the patterning device with respect to the component PL, a substrate table (e.g. wafer table) WT that holds a substrate (e.g. resist-coated wafer) W and is coupled to a second positioning device PW that precisely positions the substrate with respect to the component PL, and a projection system (e.g. refractive projection lens) PL that is configured to image a pattern imparted to the radiation beam PB by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0038] As depicted herein, the lithographic apparatus may be of a transmissive type (e.g. employing a transmissive mask), or it may be at least partially reflective (e.g. employing a reflective mask or a programmable mirror array as referred to above).

[0039] 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 such cases, the radiation source is not considered to form part of the lithographic apparatus, and the radiation beam is delivered from the source SO to the illuminator IL using a beam delivery system BD, which may include, for example, suitable directing mirrors and / or beam expanders. In other cases, the radiation source may be an integral part of the lithographic apparatus, for example when the radiation source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0040] The illuminator IL may comprise adjusting means AM for adjusting the angular intensity distribution of the beam. Generally, at least the outer and / or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator may be adjusted. In addition, the illuminator IL will generally comprise various other components, such as an integrator IN and a condenser CO. The illuminator is configured to provide a conditioned radiation beam PB, which is configured to have a desired uniformity and intensity distribution in its cross-section.

[0041] The radiation beam PB is incident on a patterning device (e.g., mask) MA, which is held on a support structure (e.g., reticle or mask table) MT. After passing the patterning device MA, the beam PB passes through the projection system PL, which focuses the beam onto a target portion C of the substrate W. Using the second positioning device PW and a position sensor IF (e.g., an interferometer device), the substrate table WT can be accurately moved, for example to position different target portions C in the path of the beam PB. Similarly, the first positioning device PM and further position sensors (not explicitly shown in FIG. 1 ) can be used to accurately position the patterning device MA with respect to the path of the beam PB, for example after mechanical retrieval from a mask library or during a scan. Typically, movement of the object tables MT and WT will be achieved using 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 a short-stroke actuator only, or may be fixed. Patterning device MA and substrate W may be aligned using patterning device alignment marks M1 and M2 and substrate alignment marks P1 and P2.

[0042] The lithographic apparatus further includes a controller 33 configured to determine an imaging error of the projection system and to calculate adjustments to be applied to lens elements of the projection system to compensate for the imaging error. The lithographic apparatus further includes first and second pressure sensors 31, 32 configured to provide pressure measurements to the controller 33.

[0043] The illustrated apparatus can be used in the following preferred modes:

[0044] In step mode, the support structure MT and substrate table WT are kept essentially stationary while the entire pattern imparted to the beam PB is projected onto the target portion C in one go (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.

[0045] In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the beam PB is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT is 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-scan direction) during a single dynamic exposure, while the length of the scanning motion determines the height of the target portion (in the scan direction).

[0046] In another mode, the support structure MT is kept essentially stationary, 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 the 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 is readily adaptable to maskless lithography employing a programmable patterning device such as a programmable mirror array of the type described above.

[0047] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be employed.

[0048] FIG. 2 shows a schematic representation of an exemplary projection system PL, showing the first to fifth lens elements 21 to 25. The depiction of five lens elements is merely exemplary, and it will be understood that the projection system PL may include any number of lens elements. In FIG. 2, lens element 25 is the “last” lens element, i.e. the lens element closest to the substrate W. A surface 26 of the last lens element 25 directly faces the substrate W. A first pressure P1 is applied to a first volume adjacent to surface 26. The first volume may have its boundary defined by a substrate compartment 15 in which the substrate table PW is located, or may be defined by another component (e.g. an immersion bath if the lithographic apparatus is an immersion system). The first volume 15 may also be referred to as the environment below the last lens element 25.

[0049] In one embodiment, the projection system PL is part of an immersion lithography system in which an immersion bath containing an immersion medium (e.g. highly purified water) is provided between the last lens element 25 and the substrate W, in order to increase the numerical aperture and thereby improve the resolution of the lithographic apparatus. In such a system, the last lens element 25 may be connected to a housing 27 of the projection system by a transparent (or "leaky") seal 28 to reduce pressure gradients across the last lens element 25. In particular, the last lens element 25 may have a higher optical sensitivity than, for example, the penultimate lens element 24, and may be mounted to the housing 27 with a lower stiffness.

[0050] A positive pressure may be provided by a gas source (not shown) in the projection system PL. As a result, a difference may be created between a pressure P1 below the last lens element 25 and a pressure P2 above the last lens element 25. This may be referred to as a differential pressure. The differential pressure may also be referred to as DP12. A change in pressure P1 may occur in the environment 15 below the last lens element 25. When this occurs, a change in the differential pressure DP12 will occur. The last lens element 25 will move as a result of the differential pressure DP12. For example, an increase in pressure P1 will cause the last lens element 25 to move towards the penultimate lens element 24 (i.e., upward in FIG. 2). The movement of the last lens element 25 may include a combination of rotation and movement in the Z direction (upward or downward). This movement will have a negative impact on the accuracy with which a pattern is projected onto a substrate by the lithographic apparatus. Due to the effects of the positive pressure provided in the projection system, it may take several seconds for the change in the differential pressure DP12 to decay.

[0051]

[0051] Pressure differentials may also occur between other lens elements. Pressure changes in the environment 15 may cause changes in pressure differentials elsewhere in the projection system PL. For example, a change in pressure differential DP13 may occur between the pressure P1 below the last lens element 25 and the pressure P3 between the penultimate lens element 24 and the immediately preceding lens element 23. In the embodiment shown, the pressure differential DP12 is measured. However, in other embodiments, the pressure differential DP13 (or another pressure differential) may be measured.

[0052] In the illustrated embodiment, the first pressure sensor 31 is positioned between the last lens element 25 and the penultimate lens element 24. The second pressure sensor 32 is positioned opposite the last lens element 25. In other embodiments, the pressure sensors 31, 32 may be provided in other locations. Typically, the pressure sensors 31, 32 may be positioned in a location that allows measurement of the differential pressure DP12 (or another differential pressure). The second pressure sensor 32 may be positioned elsewhere in the first region (e.g., elsewhere in the substrate compartment 15). The pressure sensors 31, 32 may be any pressure sensors (e.g., conventional digital pressure sensors) that are suitable for measuring pressures P1, P2.

[0053]

[0053] Each pressure sensor 31, 32 is connected to the controller 33 and provides a pressure measurement signal to the controller 33. The controller 33 is configured to use the pressure measurements (or pressure measurement signals or representative values ​​thereof) to determine the differential pressure DP12.

[0054]

[0054] The measurement signals provided to the controller 33 may include or represent one or more differential pressures measured by one or more pressure sensors.

[0055] Two or more pressure sensors may be provided on the same surface of the lens elements 21-25. This allows the pressure difference across that surface of the lens elements 21-25 (i.e., the differential pressure across a single lens surface and its change) to be measured. A change in the pressure difference across the surface of the lens elements 21-25 may cause tilt or asymmetric displacement of the lens element. The received or measured differential pressure change may be a differential pressure change across one side of the lens element surface (or lens surface). A calculated image error may be based on this differential pressure change. A lens correction or adjustment may be applied to correct for this change by the controller 33.

[0056] Furthermore, pressure changes across the surfaces of the lens elements 21-25 may change the refractive index of the fluid (e.g., purge gas or immersion liquid) at the lens elements. The change in refractive index may result in (additional) imaging errors. The controller 33 may be arranged to calculate and correct this refractive index change (induced image error). The correction value may be applied by the controller 33 as a correction / adjustment value for the lens elements.

[0057] In another arrangement (not shown), the pressure sensor can be a differential pressure sensor. In such an 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 in the environment 15 below the last lens element 25 (i.e., receives the pressure on the opposite side of the last lens element). Thus, both sides of the differential pressure sensor receive pressure from both sides of the last lens element 25. The differential pressure sensor directly measures the differential pressure DP12. When using this arrangement, the controller 33 does not need to determine the differential pressure DP12 using two pressure measurements, but instead receives the differential pressure DP12 directly. Measuring the differential pressure directly can be more accurate than measuring two pressures and calculating the differential pressure (because the difference between the pressures can be small compared to the value of each pressure). Multiple differential pressure sensors may be provided. Such multiple differential pressure sensors may be configured to measure different differential pressures (e.g., DP12, DP13, etc.).

[0058]

[0058] The projection system PL may include a number of differential pressure sensors, each positioned to measure a pressure difference across a lens element provided in the projection lens.

[0059]

[0059] Both sides of the differential pressure sensor can be mounted in the same space. This allows the differential pressure (and its changes) across a single surface of the lens elements 21-25 to be measured, and this differential pressure can be taken into account when calculating the imaging error. Furthermore, this contribution to the imaging error can be used in the lens adjustment value.

[0060]

[0060] In yet another arrangement, the differential pressure DP12 may be determined by electronics located in front of the controller 33 (for example, using circuitry configured to determine the difference between the two pressure measurements).

[0061] For example, a change in pressure and / or pressure differential may occur when an operator opens the door to a room in which the lithographic apparatus LA is installed. This may occur during exposure of a 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 changes in the pressure differentials DP12, DP13. The imaging errors may include overlay and image focus characteristics.

[0062]

[0062] The controller 33 may be further configured to calculate adjustment values ​​for the lens elements to compensate for the imaging errors. Signals are then sent to the lens elements 21-25 to adjust the lens elements. The adjustment may include, for example, heating or cooling the lens elements, heating or cooling a given region of the lens elements, moving the lens elements, etc.

[0063] The controller 33 may be further configured to determine which exposure areas of the substrate were exposed during a delay between the occurrence of a change in differential pressure (e.g., due to the room door being opened) and the application of the lens element adjustment value. The controller stores information identifying those exposure areas, along with any imaging errors for those exposure areas. The imaging errors for those exposure areas can be taken into account when making subsequent exposures of those exposure areas. For example, the same (or partially the same) imaging errors may be intentionally applied by the lithography apparatus making the subsequent exposures of those areas. The imaging errors are sometimes referred to as overlay fingerprints. Typically, the imaging errors applied to a subsequent exposure may be based on the imaging errors present in a previous exposure of the corresponding exposure area. That is, the applied imaging errors may match the imaging errors that were previously present, or may include some differences. The differences may result, for example, from differences in the characteristics of the pattern being exposed compared to the previously exposed pattern. The applied imaging errors may match the imaging errors that were present in the previous exposure of the corresponding exposure area.

[0064]

[0064] Figure 3 shows a schematic diagram of a method according to one embodiment of the present invention. This method is used when a change in the differential pressure PD12, DP13 occurs. The change in differential pressure is shown as step S0.

[0065] 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 value from the pressure sensors 31, 32). The differential pressure measurement may exceed a threshold value, which indicates a significant change in the differential pressure DP12, DPI3. The change in differential pressure may be caused, for example, by an operator opening and closing the door to the room in which the lithographic apparatus LA is located. The change in pressure may also have other causes. Lithographic exposure of a substrate W is in progress and some exposure areas C of the substrate W may already have been exposed. In this specification, the term "exposure area" is intended to mean an area exposed by the lithographic apparatus LA during a single exposure (e.g. a single exposure field).

[0066] In step S2, the controller 33 calculates the imaging error characteristics caused by lens deviations due to changes in the pressure differences DP12 and DP13. As shown in step S3, the exposure area continues to be exposed on the substrate W while the calculation is being performed.

[0067]

[0067] In step S4, corrective lens element adjustment values ​​are calculated. The corrective lens element adjustment values ​​adjust the imaging characteristics of the projection system PL to compensate for imaging errors caused by changes in differential pressure. The exposure area continues to be exposed on the substrate while the calculations are being made.

[0068] In step S5, the imaging error characteristics calculated in step S2 are stored in a memory. The imaging error characteristics are associated with a particular exposure area of ​​the substrate W.

[0069] In step S6, the calculated corrective lens element adjustment values ​​are applied to the projection system PL, so that the imaging errors that were present due to the lens deviations are compensated for (the imaging errors may be substantially eliminated). Subsequent exposures of the substrate W include correction for changes in the differential pressures DP12, DP13.

[0070] There may be a significant delay between the occurrence of a change in differential pressure and the application of the corrective lens element adjustment value to the projection system PL. This delay corresponds to the time it takes to expose multiple exposure areas on the substrate W. For 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 are recorded for these five exposure areas (or other multiple exposure areas).

[0071] In step S7, exposure of the substrate W is completed. The substrate W is removed from the lithographic apparatus LA and processed. The processing may include, for example, one or more of developing photoresist on the substrate, depositing material on the substrate, chemical-mechanical polishing of the substrate, etc.

[0072] In step S8, a subsequent layer is exposed onto the substrate W. The exposure of this subsequent layer may be performed by the same lithographic apparatus LA as the previous exposure, or may be performed by a different lithographic apparatus.

[0073] In step S9, during exposure of a subsequent layer on the substrate, lens adjustment values ​​are applied to the projection system PL of the lithographic apparatus LA to introduce imaging errors. The imaging errors are applied when exposing an exposure area that corresponds to a previously exposed exposure area after the start of the change in differential pressure and before the application of the corrective lens element adjustment values. These exposure areas are sometimes referred to as selected exposure areas (or intermediate exposure areas). For a particular selected exposure area, the applied lens adjustment values ​​produce imaging errors that substantially match the imaging errors that were present when the previous layer was exposed. This can be expressed by the overlay fingerprint during exposure matching the overlay fingerprint of that exposure area when the previous layer was exposed. The pattern features of the area to be exposed are aligned with the pattern features exposed in the previous layer.

[0074]

[0074] The exposure area exposed after the onset of the differential pressure change and before the corrective lens element adjustment value is applied may be referred to as an intermediate exposure area. There may be multiple intermediate exposure areas. For example, there may be up to 10 intermediate exposure areas. For example, there may be up to 6 exposure areas. Also, there may be up to 3 exposure areas.

[0075]

[0075] In one embodiment, the method is performed continuously. In this embodiment, imaging errors resulting from changes in the differential pressure DP12, DP13 can be taken into account during the exposure of subsequent layers. In another embodiment, the execution of the method is triggered when the differential pressure changes beyond a threshold value. The advantage of this embodiment is that computing power is used only when significant errors due to significant changes in differential pressure are expected to occur.

[0076] In one embodiment, the period between the occurrence of the differential pressure change in step S0 and the application of the corrective lens element adjustment value in step S6 is measured. The period may consist of two components. The first component may be a delay resulting from the measurement itself. For example, a period of about a few tenths of a second, for example 0.1 to 0.5 seconds (or other period), may elapse between the occurrence of the differential pressure change and the receipt of the measurement of the differential pressure change by the controller 33. The second component may be a delay resulting from the time required to calculate the corrective lens adjustment value. For example, a period of about 0.1 to 0.5 seconds (or other period) may be required to calculate the corrective lens element adjustment value to be applied to the projection system PL.

[0077] Using knowledge of the delay between the occurrence of a change in differential pressure and receipt of the measured change by the controller 33, and knowledge of the time required to calculate the corrective lens element adjustment value, the controller 33 can determine which exposure areas C on the substrate W have received exposure with imaging errors caused by the change in differential pressure. For example, the controller 33 may determine that an exposure area exposed 0.4 seconds before the pressure change in the chamber is detected will contain imaging errors, and that an exposure area exposed 0.4 seconds after the pressure change in the chamber is detected will contain imaging errors.

[0078]

[0078] When exposing subsequent layers, exposure areas that have imaging errors can be identified. Lens element adjustment values ​​that provide corresponding imaging errors can be applied when exposing these exposure areas. Such lens element adjustment values ​​are not applied when exposing other exposure areas.

[0079]

[0079] Corrective lens element adjustments may be applied after an exposure area is exposed and before the next exposure area is exposed, allowing time for the adjustments to be applied between exposures. In another approach, such adjustments may be applied during exposure.

[0080]

[0080] Figure 4 is a graph illustrating the operation of one embodiment of the present invention. The black line in Figure 4 (mx-ovl) is a metric that indicates the accuracy with which a pattern is exposed onto a substrate. This metric is sometimes called overlay and is a measure of the degree to which a pattern feature is projected in the correct location on the substrate. The correct location may match the location of a previously projected pattern. The scale on the left side of the graph indicates overlay (zero indicates perfect overlay).

[0081] The dotted line (mx-decorr.fit) shows the pressure difference DP13 between the pressure P1 below the last lens element 25 and the pressure P3 between the penultimate lens element 24 and the immediately preceding lens element 23. The scale on the right side of the graph shows the pressure difference. As can be seen from the graph, the pressure difference is initially approximately zero and then rises rapidly over a period of approximately 5 seconds. This change in pressure difference may correspond, for example, to an operator opening the door to the room in which the lithography apparatus LA is located. As shown in FIG. 4, the pressure difference decays over time but does not reach zero by the end of the 10-second period shown.

[0082] 4 (LOP exposure) shows the measured differential pressure DP13 received by the controller 33. Comparing the dotted and grey lines, it can be seen that there is a delay between the differential pressure change and the controller 33 receiving the corresponding measured differential pressure change.

[0083] The dashed line (LOP cue) in FIG. 4 indicates a differential pressure value that corresponds to a corrective lens element adjustment value. The differential pressure value is calculated based on the correction values ​​calculated for the lens elements. A direct relationship exists between the calculated lens element adjustment value and the differential pressure value, and this direct relationship allows the differential pressure corresponding to the lens element adjustment value to be plotted on a graph. The differential pressure value occurs simultaneously with the corrective lens element adjustment value applied to lens elements 21-25. In other words, there is a direct instantaneous relationship between the differential pressure shown by the dashed line and the applied corrective lens element adjustment value. As shown, there is a time delay of approximately 0.8 seconds between the differential pressure change and the application of the lens element adjustment value in response to the differential pressure change.

[0084] As can be seen from the black line, the applied corrective lens element adjustment value begins to take effect approximately 0.8 seconds after the differential pressure change occurs. After 6 seconds of exposure time, the overlay drops to approximately zero. The intersection of the black overlay line with the dashed differential pressure line corresponding to the timing of the lens element adjustment shows the correlation between the overlay correction and the differential pressure value that corresponds to that correction.

[0085]

[0085] In the illustrated embodiment, there is a delay of approximately 0.8 seconds, although other delays may be applied in other embodiments.

[0086] In the embodiment shown in Figure 2, a pressure difference DP12 across the last lens element 25 of the projection system PL is used. However, in the graph of Figure 4, a pressure difference DP13 is used, i.e., the difference between the pressure P1 below the last lens element 25 and the pressure P3 between the penultimate lens element 24 and the immediately preceding lens element 23. Using the pressure difference DP13 may be particularly advantageous and may provide better correction than using the pressure difference DP12. Changes in the pressure difference DP12 may decay relatively quickly due to the opening 36 providing some pressure transfer behind the last lens element 25. Changes in the pressure difference DP13 may not decay as quickly, resulting in lens deviations (and therefore imaging errors) continuing to occur even after the pressure difference DP12 has decayed. Therefore, the pressure difference DP13 may correlate more accurately with imaging errors compared to the pressure difference DP12.

[0087] In other embodiments, a pressure differential between other parts of the projection system PL may be used.

[0088]

[0088] The movement of a lens element for a given change in differential pressure may vary from lens element to lens element because the stiffness of the mechanical connection of the lens element to its support structure may vary.

[0089]

[0089] 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, since lens element movement and the associated imaging errors may be more accurately determined.

[0090] The time delay between the pressure change and the application of the lens element adjustment value may be measured during calibration. Calibration may be performed before the start of exposure of a substrate. The time delay may remain constant for a given lithographic apparatus LA, in which case it may be measured only once. The time delay may change slowly, in which case it may be measured periodically at a period appropriate for the rate of change (e.g., weekly, monthly, etc.). The time delay may change when the software calculating the lens element adjustment value is changed. In such cases, the time delay may be measured when the software is changed. Calibration may include exposing a substrate when a significant pressure change is occurring, for example, by opening a door to a room in which the lithographic apparatus is operating. Calibration may include taking measurements of the exposed substrate to determine when the pressure change occurred and comparing this to a signal indicative of the pressure change received by the controller 33. Calibration may include taking measurements of the exposed substrate W to determine the delay between the occurrence of the pressure change and the application of the lens adjustment value to correct for the effects of the pressure change.

[0091] According to a further embodiment of the present invention there is provided a method of controlling a projection system including a lens adjustment means, the method comprising receiving data of a first pressure in a compartment of the projection system and a second pressure in an environment of 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 value 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 value provided by the lens adjustment means, and determining a lens position for a second exposure based on the lens position error during the first exposure.

[0092]

[0092] Those skilled in the art will appreciate that the features of the various aspects of the invention disclosed above can be combined.

[0093]

[0093] A 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") to which a memory is connected. The methods described herein may be implemented in 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 or separate from the CPU. The code, which may be called instructions, is retrieved from the memory and executed on the processor to perform operations according to the embodiments described herein. Alternatively, it is not excluded that some or all of the functions of the CPU may be implemented in dedicated hardware circuits or configurable hardware circuits such as FPGAs.

[0094]

[0094] The computing device may include an input configured to allow a user to input data into the software program running on the CPU. The input device may include a mouse, keyboard, touch screen, microphone, etc. The computing device may further include an output device configured to output measurement results to a user.

[0095]

[0095] Where circumstances permit, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium that 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, or electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.). Also, firmware, software, routines, or instructions may be described herein as performing certain actions. However, it should be understood that such description is for convenience only, and that such actions are in fact due to a computing device, processor, controller, or other device executing the firmware, software, routines, instructions, etc., thereby enabling an actuator or other device to interact with the physical world.

[0096]

[0096] Aspects of the present invention are described in the following clauses. 1. A method of controlling a projection system during exposure of a substrate by a lithographic apparatus, comprising: 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; calculating imaging errors caused by movement of one or more lens elements of the projection system due to differential pressure changes during exposure; calculating lens element adjustments that compensate for the calculated imaging errors; applying a lens element adjustment value; Identifying which exposure areas of the substrate were exposed during a delay between the occurrence of the differential pressure change and the application of the lens element adjustment value; storing information about the identified exposure area along with the calculated imaging error; A method comprising: 2. The method of clause 1, further comprising applying, during a subsequent exposure of the identified exposure area of ​​the substrate, a lens element adjustment value that adjusts the (second) imaging error based on the calculated imaging error during exposure of the identified exposure area. 3. The method of clause 2, wherein the imaging error applied during subsequent exposures corresponds to the calculated imaging error. 4. The method of any of clauses 1 to 3, wherein a time delay between the occurrence of a differential pressure change and the application of the lens element adjustment value is determined, the determination taking into account the time between the occurrence of the differential pressure change and the acquisition of a measurement of the differential pressure change. 5. The method of any of clauses 1 to 3, wherein a time delay between the occurrence of the differential pressure change and the application of the lens element adjustment value is determined, the determination taking into account the time to calculate the lens element adjustment value. 6. The method according to any one of clauses 1 to 5, wherein the measured differential pressure is between the pressure below the last lens element of the projection system and the pressure between the penultimate lens element and the immediately preceding lens element of the projection system. 7. The method according to any one of clauses 1 to 5, wherein the measured differential pressure is between the pressure below the last lens element of the projection system and the pressure above the last lens element of the projection system. 8. The method of any one of clauses 1 to 7, wherein the differential pressure is measured by at least one differential pressure sensor. 9. The method of any of clauses 1 to 8, wherein two or more measured differential pressures are used. 10. The method of clause 4 or clause 5, wherein determining the time delay between the occurrence of the differential pressure change and the application of the lens element adjustment value is performed during a calibration that occurs before the start of exposure of the substrate. 11. The method of any of clauses 1-10, wherein there are up to 10 identified exposure regions. 12. A lithographic apparatus comprising: a substrate table configured to support a substrate; a projection system configured to project a patterned beam of radiation from the patterning device onto a substrate, the projection system including 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 an imaging error caused by movement of one or more lens elements of the projection system due to the occurrence of a change in differential pressure, the controller comprising: a controller further configured to calculate lens element adjustment values ​​that compensate for the calculated imaging errors; a lens element adjuster configured to receive and apply a lens element adjustment value; The lithographic apparatus, wherein the controller is further configured to determine or identify which exposure areas of the substrate were exposed during a delay between the occurrence of the differential pressure change and the application of the lens element adjustment value, and store information identifying the exposure areas together with the calculated imaging error. 13. A lithographic apparatus as described in clause 12, wherein the controller is further configured to apply a lens element adjustment value that applies a (second) imaging error during a subsequent exposure of the substrate based on the calculated imaging error during exposure of the identified exposure area. 14. A lithographic apparatus according to clause 13, wherein the imaging error applied during subsequent exposures corresponds to the calculated imaging error. 15. A lithographic apparatus according to any of clauses 12 to 14, wherein the apparatus includes at least one differential pressure sensor arranged to measure a differential pressure across at least one lens element. 16. A lithographic apparatus according to clause 15, wherein the projection system comprises a plurality of differential pressure sensors. 17. A lithographic apparatus according to any of clauses 12 to 16, wherein the differential pressure measurement is between the pressure below the last lens element of the projection system and the pressure between the penultimate lens element and the immediately preceding lens element of the projection system. 18. A lithographic apparatus according to any of clauses 12 to 16, wherein the differential pressure measurement is between the pressure below the last lens element of the projection system and the pressure above the last lens element of the projection system. 19. A lithographic apparatus according to any of clauses 12 to 18, wherein the controller is configured to identify up to 10 exposure regions. 20. A computer program comprising computer-readable instructions configured to cause a computer to carry out the method according to any one of clauses 1 to 10. 21. A computer readable medium carrying a computer program according to clause 20. 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 a differential pressure across the lens surface. 23. The method of any of clauses 1-11, further comprising calculating additional imaging errors caused by changes in the refractive index of the fluid in one or more lenses due to differential pressure changes. 24. The method of any one of clauses 1 to 5, wherein the differential pressure change is across one side of the surface of one or more lenses. 25. A method of controlling a projection system including lens adjustment means, comprising: receiving data of a first pressure in a compartment of the projection system and a second pressure in an environment of the projection system during a first exposure; Determining a pressure difference between the first pressure and the second pressure; receiving lens adjustment data based on a change in the pressure differential during the first exposure; calculating a lens position error based on a time delay between the change and the lens adjustment value provided by the lens adjustment means; determining a lens position for a second exposure based on the lens position error during the first exposure; A method comprising:

[0097]

[0097] While specific embodiments of the present invention have been described above, it will be apparent that the present invention may be practiced in other ways than those described above. The foregoing description is intended to be illustrative and not limiting. Accordingly, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the appended claims.

Claims

1. 1. A method of controlling a projection system during exposure of a substrate by a lithographic apparatus, comprising the steps of: 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 the pressure differential changes during exposure; calculating lens element adjustments that compensate for the calculated imaging errors; applying the lens element adjustment values; Identifying which exposure areas of the substrate were exposed during a delay between the occurrence of the differential pressure change and the application of the lens element adjustment value; storing information about the identified exposure area along with the calculated imaging error; A method comprising:

2. 2. The method of claim 1, further comprising applying, during a subsequent exposure of the identified exposure area of ​​the substrate, a lens element adjustment value that accounts for the imaging error based on the calculated imaging error during exposure of the identified exposure area.

3. The method of claim 2 , wherein the imaging error applied during the subsequent exposure corresponds to the calculated imaging error.

4. 4. The method of claim 1, wherein the time of the delay between the occurrence of the differential pressure change and the application of the lens element adjustment value is determined, said determining taking into account the time between the occurrence of the differential pressure change and obtaining the measurement of the differential pressure change.

5. 4. The method of claim 1, wherein the time of the delay between the occurrence of the differential pressure change and the application of the lens element adjustment value is determined, said determining taking into account the time to calculate the lens element adjustment value.

6. 6. The method of claim 1, wherein the measured pressure difference is between a pressure below a last lens element of the projection system and a pressure between the penultimate lens element and the previous lens element of the projection system, or between a pressure below a last lens element of the projection system and a pressure above the last lens element of the projection system.

7. The method according to any one of claims 1 to 6, wherein the differential pressure is measured by at least one differential pressure sensor.

8. The method according to any one of claims 1 to 7, wherein two or more measured differential pressures are used.

9. 6. The method of claim 4 or claim 5, wherein determining the time of the delay between the occurrence of the differential pressure change and the application of the lens element adjustment value is performed during a calibration that occurs before exposure of the substrate begins.

10. The method of any of claims 1 to 9, wherein there are up to 10 identified exposure areas.

11. 1. A lithographic apparatus comprising: a substrate table configured to hold a substrate; a projection system configured to project a patterned beam of radiation from the patterning device onto a substrate, the projection system including 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 the occurrence of a change in differential pressure, a controller further configured to calculate lens element adjustment values ​​that compensate for the calculated imaging errors; a lens element adjuster configured to receive and apply the lens element adjustment value; 10. The lithographic apparatus of claim 9, wherein the controller is further configured to determine which exposure areas of the substrate were exposed during a delay between the occurrence of the differential pressure change and the application of the lens element adjustment value, and to store information identifying the exposure areas along with the calculated imaging error.

12. 12. The lithographic apparatus of claim 11, wherein the controller is further configured to apply, during a subsequent exposure of the substrate, lens element adjustment values ​​that account for imaging errors based on the calculated imaging errors during exposure of the identified exposure area.

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 of claims 11 to 13, wherein the apparatus comprises at least one differential pressure sensor arranged to measure a differential pressure across at least one lens element.

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

Images