Method for determining parasitic force of actuator of optical component, control method for actuator, optical system, projection system and lithographic apparatus

Abstract

A method of determining a parasitic force of an actuator of an optical component is described, whereby the actuator is mounted to a first frame and configured to control a position of the optical component relative to a second frame. The method comprises: obtaining a first position signal representing a position of the optical component relative to the first frame; performing low-pass filtering on the first position signal; obtaining a second position signal representative of the position of the optical component relative to the second frame; performing high-pass filtering on the second position signal, wherein the high-pass filtering and the low-pass filtering are complementary; a parasitic force of the actuator is determined based on the combined low-pass filtered first position signal and the high-pass filtered second position signal.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 22183960.8 filed on July 8, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a control method for an actuator for positioning an optical component such as a mirror in an EUV lithography apparatus. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. For example, a lithographic apparatus may be used in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate.

[0005] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. A lithographic apparatus may, for example, be configured to generate a patterned beam of radiation and project the beam onto a substrate so that the substrate is patterned. The wavelength of such radiation determines the minimum size of features that can be formed on the substrate. A lithographic apparatus using extreme ultraviolet (EUV) radiation (having a wavelength in the range of 4 nm to 20 nm, such as 6.7 nm or 13.5 nm) may be used to form smaller features on a substrate than a lithographic apparatus using, for example, radiation having a wavelength of 193 nm.

[0006] The manufacture of integrated circuits may often require that multiple layers with specific patterns be produced in sequence and stacked on top of each other. In order to ensure the correct operation of the integrated circuit, the specific pattern needs to be accurately aligned. In order to achieve this, it is important to ensure that the radiation beam including the pattern projected by the lithography equipment is projected at the desired location on the substrate. In order to achieve such accurate positioning of the pattern, it is important to ensure accurate control of all optical components arranged in the optical path of the radiation beam including the pattern.

[0007] In particular, the position of optical components arranged in the optical path between the patterning device and the substrate needs to be accurately controlled. It has been observed that known solutions for controlling the positioning of these optical components may not be accurate enough, in particular when the optical components need to be displaced along a trajectory during projection of the radiation beam. Summary of the invention

[0008] It is an object of the present invention to enable more accurate positioning of optical components in a lithographic apparatus, in particular optical components arranged in the optical path between a patterning device and a substrate.

[0009] According to one aspect of the present invention, there is provided a method for determining a parasitic force of an actuator of an optical component, the actuator being mounted to a first frame and configured to control a position of the optical component relative to a second frame, the method comprising:

[0010] obtaining a first position signal representing a position of the optical component relative to the first frame;

[0011] performing low-pass filtering on the first position signal;

[0012] obtaining a second position signal representing a position of the optical component relative to the second frame;

[0013] Performing high-pass filtering on the second position signal, wherein the high-pass filtering is complementary to the low-pass filtering;

[0014] Based on the combined low pass filtered first position signal and the high pass filtered second position signal, a parasitic force of the actuator is determined.

[0015] According to another aspect of the present invention, there is provided another method of determining parasitic forces of an actuator of an optical component, the actuator being mounted to a frame and configured to control a position of the optical component relative to the frame, the method comprising:

[0016] obtaining a signal representing a position of the optical component relative to the frame;

[0017] performing low-pass filtering on the position signal;

[0018] obtaining a reference signal representing a desired position of the optical component relative to the frame;

[0019] Performing high-pass filtering on the reference signal, wherein the high-pass filtering is complementary to the low-pass filtering;

[0020] Based on the combined low-pass filtered position signal and the high-pass filtered reference signal, parasitic forces of the actuator are determined.

[0021] According to yet another aspect of the present invention, there is provided a method of controlling an actuator, the actuator being mounted to a first frame and configured to control a position of an optical component relative to a second frame, the method comprising:

[0022] obtaining a reference signal representing a desired position of the optical component relative to the second frame, and an optical component position signal representing a measured position of the optical component relative to the second frame;

[0023] determining the parasitic forces of the actuator using the method according to the invention,

[0024] Based on the reference signal, the optical component position signal and the determined parasitic forces, a control signal for controlling the actuator is determined.

[0025] According to another aspect of the present invention, there is provided an optical system, comprising:

[0026] Optical components, such as mirrors;

[0027] an actuator mounted to a first frame of the device, the actuator configured to control a position of the optical component relative to a second frame of the device;

[0028] A control unit, the control unit being configured to:

[0029] o obtaining a reference signal representing a desired position of the optical component relative to the second frame, and an optical component position signal representing a measured position of the optical component relative to the second frame;

[0030] oThe parasitic forces of the actuator are determined by:

[0031] ■ obtaining a first position signal representing the position of the optical component relative to the first frame;

[0032] ■ low-pass filtering the first position signal to obtain a low-pass filtered signal;

[0033] ■ Performing high-pass filtering on the optical component position signal or the reference signal to obtain a high-pass filtered signal, wherein the high-pass filtering is complementary to the low-pass filtering;

[0034] ■ determining a parasitic force of the actuator based on the combined low-pass filtered signal and the high-pass filtered signal;

[0035] o Determining a control signal for controlling the actuator based on the reference signal, the optical component position signal and the determined parasitic forces. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0037] Figure 1 A lithography system including a lithography apparatus and a radiation source is depicted;

[0038] Figure 2 depicts a control scheme for an actuator configured to control the position or location of an optical component;

[0039] Figure 3 Schematically shows an optical system that can be applied to a projection system of a lithographic apparatus according to the present invention;

[0040] Figure 4 A Bode plot of complementary high-pass and low-pass filters is schematically shown;

[0041] Figure 5 Schematically shows a characteristic force curve of an actuator that can be applied in the present invention;

[0042] Figure 6 A flow chart of a method for determining parasitic forces of an actuator according to the present invention is schematically shown;

[0043] Figure 7 A flow chart schematically shows a control method of an actuator according to the present invention;

[0044] Figures 8 to 10 A control scheme that can be applied according to the present invention is schematically shown;

[0045] Fig.11 Another optical system which may be applied in a projection system of a lithographic apparatus according to the present invention is schematically shown. DETAILED DESCRIPTION

[0046] Figure 1 A lithographic system is shown comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and for supplying the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0047] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Furthermore, the illumination system IL may comprise a faceted field mirror arrangement 10 and a faceted pupil mirror arrangement 11. The faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11 together provide a desired cross-sectional shape and a desired intensity distribution for the EUV radiation beam B. The illumination system IL may comprise other mirrors or arrangements in addition to or instead of the faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11.

[0048] After being so conditioned, the EUV radiation beam B interacts with the pattern forming device MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To this end, the projection system PS may include a plurality of mirrors 13, 14, which are configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B', thereby forming an image MA whose features are smaller than corresponding features on the pattern forming device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is Figure 1 1. Although illustrated in FIG. 1 as having only two mirrors 13, 14, the projection system PS may comprise a different number of mirrors (eg six or eight mirrors).

[0049] The substrate W may include a previously formed pattern. In this case, the lithographic apparatus LA aligns an image formed by the patterned EUV radiation beam B′ with a pattern previously formed on the substrate W.

[0050] A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO in the illumination system IL and / or the projection system PS.

[0051] Figure 1 The radiation source SO shown in is, for example, of a type that may be referred to as a laser produced plasma (LPP) source. A laser system 1 (which may, for example, include a CO2 laser) is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn), provided from, for example, a fuel emitter 3. Although tin is mentioned in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form and may, for example, be a metal or an alloy. The fuel emitter 3 may include a nozzle configured to direct tin, for example, in the form of droplets, along a trajectory toward a plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. The laser energy is deposited into the tin, generating a tin plasma 7 at the plasma formation region 4. During de-excitation and recombination of electrons with ions of the plasma, radiation including EUV radiation is emitted from the plasma 7.

[0052] EUV radiation from the plasma is collected and focused by a collector 5. The collector 5 includes, for example, a near normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 may have a multi-layer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration having two focal points. A first of the focal points may be located at the plasma formation region 4, and a second of the focal points may be located at an intermediate focus 6, as discussed below.

[0053] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may be delivered from the laser system 1 to the radiation source SO by means of a beam delivery system (not shown) comprising, for example, suitable directional mirrors and / or beam expanders, and / or other optical devices. The laser system 1, the radiation system SO and the beam delivery system may be considered together as a radiation system.

[0054] The radiation reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at an intermediate focus 6 to form an image of the plasma present at the plasma formation region 4 at the intermediate focus 6. The image at the intermediate focus 6 serves as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near an opening 8 in an enclosure 9 of the radiation source SO.

[0055] Although Figure 1 The radiation source SO is depicted as a laser produced plasma (LPP) source, but any suitable source, such as a discharge produced plasma (DPP) source, a free electron laser (FEL) may be used to generate EUV radiation.

[0056] The lithographic apparatus LA comprises a position control system PCS such as Figure 2 Such a position control system can, for example, be used to control an actuator as applied in a lithographic apparatus LA, for example to control an optical component as applied in a lithographic apparatus (e.g., Figure 1An actuator for controlling the position of the reflectors 13, 14 of the projection system PS shown in the figure. The position control system PCS shown in the figure includes a set point generator SP, a feedforward controller FF and a feedback controller FB. The position control system PCS provides a drive signal to the actuator ACT. In the control scheme, the actuator ACT drives the optical component OC, which may include a reflector, etc. The driving of the optical component AC by the actuator ACT can cause the displacement of the optical component OC. In the control scheme shown, the displacement or position of the optical component OC is measured using a position measurement system PMS. The position measurement system PMS generates a signal, which is a position signal representing the position quantity of the optical component OC. The set point generator SP generates a signal, which is a reference signal representing the desired position quantity of the optical component OC. For example, the reference signal represents the desired trajectory of the optical component OC. The difference between the reference signal and the position signal forms the input of the feedback controller FB. Based on the input, the feedback controller FB provides at least a portion of the drive signal to the actuator ACT. The reference signal can form the input of the feedforward controller FF. Based on the input, the feedforward controller FF provides at least a part of the drive signal to the actuator ACT.The feedforward FF may utilize information about the dynamic properties of the optical component OC, such as mass, stiffness, resonant modes and eigenfrequencies.

[0057] In order to ensure that the pattern of the patterning device is accurately projected onto the substrate, a projection system arranged in the optical path between the patterning device and the substrate may typically include a plurality of mirrors, for example 4 or more mirrors. Accurate positions of the mirrors are required to ensure that the radiation beam formed with the pattern of the patterning device is projected at a desired location on the substrate. In order to facilitate such accurate positioning, it has been proposed to control the positioning of such mirrors by using a first frame for mounting an actuator, which applies a force to the mirror in order to obtain the desired position or positioning, and using a second frame as a reference frame for the position or positioning of the mirror.

[0058] exist Figure 3 Such an arrangement is schematically shown in FIG. Figure 3 Schematically shown is a reflector 300, typically an optical component such as may be used in applications such as Figure 1 The device LA shown in FIG. 1 is used in a projection system of a lithographic apparatus such as the one shown in FIG. Figure 3 A first frame 310 is schematically shown, the first frame being configured to support or hold an actuator 320 configured to apply a force indicated by arrow 330 to the mirror 300 . Figure 3Also shown is a second frame 340 and a position measurement system 350 configured to measure the position of the mirror 300 relative to the second frame 340. Note that within the meaning of the present invention, the second frame may be referred to as a reference frame or a sensor frame. Within the meaning of the present invention, the first frame 310 on which the actuator 320 is mounted may be referred to as a force frame. Figure 3 It also schematically shows how to achieve position control of the reflector 300. Figure 3 In the embodiment, the difference between a reference r representing the desired position of the reflector 300 relative to the second frame 340 and a measured position p of the reflector 300, which is obtained from the position measurement system 350, is provided to the controller or control unit C. The position measurement system 350 can be, for example, a measurement system based on an interferometer or an encoder. Based on the difference e between the desired position r and the actual position p, the controller C can control the actuator to apply a suitable force to the reflector 300 in order to reduce the difference e. This can be done, for example, by the controller C outputting or generating a control signal Fc, which is provided to the actuator 320 in order to generate the force 330. It can be pointed out that it is also possible to implement Figure 3 The actuator 320 is controlled by other control actions such as feed-forward control not shown.

[0059] In order to position optical components such as mirrors in a projection system of a lithographic apparatus, electromagnetic actuators such as Lorentz actuators are often used. Such an actuator may, for example, comprise a first member provided with a coil, which may, for example, be mounted to a first frame, and a second member provided with a permanent magnet, which may, for example, be mounted to the optical component, such as the mirror. When the coil is supplied with current, a force is generated between the first member and the second member, for example Figure 3 330 indicated in FIG. 330. To control the position of the reflector 300 in this manner, the controller C may rely on certain properties, such as the actuator internal stiffness, or more generally, the actuator parasitic forces. In addition to or in parallel with the applied actuator force, there will typically be a relative force between the frame 310 and the reflector 300 that depends on the relative position between the frame 310 and the reflector 300. This force as a function of the relative position is defined as the internal stiffness. Thus, the internal stiffness may be considered to cause or generate a linear or nonlinear force as a function of position. This linear or nonlinear force or force component may be referred to as a parasitic force of the actuator.

[0060] It has been observed that the presence of non-zero internal stiffness of the actuator applied when positioning the optical component may adversely affect the positioning accuracy of the optical component. In this regard, it can also be pointed out that the applied actuator can generally be combined with or include a support, which is also called a gravity compensator and is configured to at least partially compensate the weight of the optical component. Such a support can, for example, be a passive support or an active support. The passive support can, for example, include a set of permanent magnets configured to generate a repulsive force between the frame 310 and the reflector 300. In addition to the contribution of the actuator, such a support may also suffer from having limited accuracy, resulting in position-dependent forces or parasitic forces.

[0061] The combination of a passive support and an actuator configured to at least partially compensate or counteract the weight of an optical component being controlled may also be referred to as an actuator system.

[0062] Within the meaning of the present invention, an actuator may refer to a stand-alone actuator or to an actuator comprising a passive support or a gravity compensator.

[0063] Due to parasitic forces of the actuator system (i.e., actuator and / or support), the controller C may not always generate the most accurate control signal for the actuator to generate the required force. Typically, controllers or control units are usually reactive. For certain applications or use cases, this may have an adverse effect on performance. In the present invention, a proactive approach is proposed.

[0064] The object of the present invention is to provide a method for determining parasitic forces of an actuator, whereby said determined parasitic forces can subsequently be applied in an improved control method to control the actuator. A further object of the present invention is that the application of the application of the determined parasitic forces does not cause instability problems of a controller or control unit controlling the actuator.

[0065] In view of the above, those skilled in the art will appreciate that in order to accurately determine the parasitic forces of the actuator, it is necessary to know the relative positions of the frame supporting the actuator and the actuated mirror. Figure 3 From the arrangement shown in FIG. 3 , it can be seen that the fact that the actuator is arranged on the first frame and the position of the reflector is determined or needs to be controlled relative to the second frame makes determining the relative position of the first frame 310 and the reflector 300 complicated.

[0066] It is known to use a position sensor to measure the position of the first frame relative to the reflector. The position sensor is Figure 3denoted by reference numeral 360. However, it has been found that the noise level of the sensor 360 is unacceptably high. Therefore, it has been found that directly using the output signal of the sensor 360 as an indication of the position of the mirror relative to the first frame and using the output signal to derive the parasitic forces of the actuator does not lead to an improvement in the mirror positioning error.

[0067] In order to account for the high noise levels present in the position sensor 360 associated with the first frame 310 , the following method has been devised to determine the parasitic forces of the actuator 320 .

[0068] According to an embodiment of the invention, it is proposed to perform a low-pass filtering on the position signal of the sensor 360 measuring the position of the optical component 300 relative to the first frame 310. However, it has been found that using only such a low-pass filtered position signal is not sufficient to predict or determine the parasitic forces of the actuator 320; due to the low-pass filtering, fast changes in the position of the optical component 300 relative to the second frame 340 are not fully taken into account. Therefore, in addition to the low-pass filtering of the position signal, the method for determining the parasitic forces also utilizes a second position signal representing the position of the optical component relative to the second frame 340.

[0069] In an embodiment of the present invention, the second position signal may be obtained from a position measurement system 350, which is applied to measure the position of the reflector (usually an optical component) relative to the second frame 340. In such an embodiment, the second position signal may correspond to Figure 3 The signal p is shown in FIG.

[0070] In another embodiment of the present invention, a reference r representing a desired position of the optical component 300 relative to the second frame 340 may be used as the second position signal. Note that the reference r may also be referred to as a set point of the optical component relative to the second frame.

[0071] According to the present invention, a high-pass filter is applied to the second position signal representing the position of the optical component relative to the second frame, whereby the high-pass filter is complementary to the low-pass filter. By doing so, and combining the low-pass filtered first position signal and the high-pass filtered second position signal, a combined signal is obtained, which can be used to accurately represent the actual position of the optical component 300 relative to the first frame.

[0072] Within the meaning of the present invention, a high-pass filter HP and a low-pass filter LP are considered complementary when the frequency characteristics of the complementary filters substantially satisfy the condition HP + LP = 1. For example, a second-order low-pass filter HP(s) can be defined as:

[0073]

[0074] Then the complementary high-pass filter HP(s) becomes:

[0075]

[0076] Figure 4 The Bode plots of these filters HP(s) and LP(s) are schematically shown.

[0077] Based on the combined signal, the parasitic force of the actuator can be obtained. In particular, in embodiments of the present invention, the parasitic force characteristics of the actuator or the passive support of the actuator can be used to obtain the parasitic force of the actuator at a given actual position of the optical component 300 relative to the first frame 310.

[0078] Figure 5 Such parasitic force characteristics are schematically shown. Figure 5 The change of the force Fs generated by the passive support is schematically shown, and the force is a function of the position x of the supported optical component relative to the passive support and the frame on which the actuator is mounted. In the curve diagram shown, the nominal support force generated at the nominal position x = 0 is set to zero. As can be seen from the curve diagram, when the relative position between the optical component and the frame of the support and the actuator deviates from the nominal position, the passive support will generate a support force Fs that deviates from the nominal force. When the relative position corresponds to x = x1, the support will generate a support force that deviates from the nominal support force Fd. For a given relative position of the optical component and the frame (the active support and the actuator are mounted on the frame), the relative position is obtained from the combined low-pass filtered first position signal and the high-pass filtered second position signal, and the given parasitic force characteristics of the actuator or the passive support of the actuator, and the parasitic force of the actuator can be determined, for example, as a force component that deviates from the nominal force generated by the passive support or the actuator.

[0079] As mentioned above, an actuator used to control the position of an optical component may also have a varying internal stiffness, i.e. such an actuator may also generate parasitic forces, e.g. expressed as Newtons / ampere in the case of a Lorentz actuator, which is position dependent, i.e. dependent on the relative position of the optical component and the frame supporting the actuator.

[0080] In an embodiment of the present invention, the method of obtaining or determining the parasitic force of an actuator as applied to position an optical component may be performed, for example, by a controller or control unit such as Figure 3 The controller C) shown in FIG.

[0081] Figure 6An embodiment of a method for determining parasitic forces of an actuator according to the invention is schematically illustrated. In the embodiment shown, the method is applied in cases where position control or positioning of an optical component relative to a frame is desired, whereby an actuator configured to perform positioning of the optical component is mounted to different frames. Figure 6 In the embodiments of the present invention, the actuator is therefore considered to be mounted to the first frame, while the optical component will be positioned relative to the second frame.

[0082] According to an embodiment of the present invention, a method for determining a parasitic force of an actuator comprises:

[0083] In a first step 400, a first position signal representing a position of an optical component relative to a first frame is obtained.

[0084] In a second step 410, low pass filtering is performed on the first position signal.

[0085] In a third step 420, a second position signal representing the position of the optical component relative to the second frame is obtained.

[0086] In a fourth step 430, high-pass filtering is performed on the second position signal, where the high-pass filtering is complementary to the low-pass filtering.

[0087] And a fifth step 440 of determining a parasitic force of the actuator based on the combined low pass filtered first position signal and the high pass filtered second position signal.

[0088] Regarding the numbering of steps 400 to 440, it may be noted that this numbering does not imply a strict chronological order in which the steps need to be performed. As an example, it may be noted that steps 400 and 410 may be performed in parallel with steps 420 and 430.

[0089] In an embodiment, as shown in the figure, the fifth step 440 may include, for example:

[0090] determining a relative position of the optical component to the first frame based on the combined low pass filtered first position signal and the high pass filtered second position signal, and

[0091] A parasitic force of the actuator is determined based on the relative position and a parasitic force characteristic of the actuator.

[0092] According to an aspect of the present invention, there is also provided a method of controlling an actuator, the method utilizing the parasitic force obtained using the method of obtaining the parasitic force of an actuator according to the present invention.

[0093] Figure 7 Such an actuator control method according to an embodiment of the present invention is schematically illustrated.

[0094] According to an embodiment of the present invention, the control method is intended for an actuator, which is mounted to a first frame and is configured to control the position of an optical component relative to a second frame. The method comprises:

[0095] In a first step 600, a reference signal representing a desired position of the optical component relative to the second frame and an optical component position signal representing a measured position of the optical component relative to the second frame are obtained;

[0096] In a second step 610, the parasitic force of the actuator is determined using the method for determining the parasitic force of the actuator according to the present invention;

[0097] Third step 620: Determine a control signal for controlling the actuator based on the reference signal, the optical component position signal and the determined parasitic forces.

[0098] In an embodiment, the control scheme of the actuator (e.g., as in Figure 2 The control scheme shown in FIG. Figure 6 This can be done in various ways, as will be explained below.

[0099] Figure 8 A first exemplary embodiment of a control concept for an actuator is schematically illustrated, in which the control method according to the invention is implemented.

[0100] exist Figure 8 In , the signal r represents the position reference of the optical component OC to be positioned. The signal r can therefore be considered as the desired position of the optical component OC relative to the reference frame or sensor frame as discussed above. The reference or sensor frame is also referred to as the second frame. The signal r can also be referred to as a reference signal, a position reference signal or a set point signal.

[0101] exist Figure 8 In , the signal p represents the measured position of the optical component OC relative to the reference frame or sensor frame. Therefore, the signals r and p correspond to Figure 3 The signals r and p are shown in Figure 2. Figure 3 , p may thus be the position of optical component 300 measured by position measurement system 350. r and p may, for example, represent a desired position and an actual position in one or more degrees of freedom.

[0102] exist Figure 8 In, p a represents the measured position of the optical component relative to the frame on which the actuator is mounted. In the above, this frame is for example referred to as the force frame or the first frame. Figure 3 , p a This may therefore be the position of the optical component as measured by the position measurement system 360 .

[0103] As in Figure 8 As can be seen in the figure, the position signal p a (i.e., representing the measured position of the optical component relative to the force frame) is subjected to low pass filtering indicated by the low pass filter LP. In a similar manner, the reference signal r is subjected to high pass filtering indicated by the high pass filter HP. The filtered signals 800 and 802 are then combined, for example added. It has been found that the combined signal 804 accurately represents the actual relative position between the optical component and the force frame or the first frame. It can be pointed out that in the absence of the high pass filter contribution obtained from the reference signal r, dynamic effects such as the displacement of the optical component are not fully considered. In other words, due to the variable stiffness or parasitic forces of the actuator or the passive support of the actuator, the displacement of the optical component occurs at a relatively high speed, resulting in a corresponding displacement of the optical component relative to the force frame, which may lead to inaccurate control of the actuator.

[0104] Based on the combined signal 804, a parasitic force may be determined, for example using a parasitic force characteristic of the actuator or a passive support of the actuator. The parasitic force is calculated by Figure 8 The parasitic force 806 determined (e.g., as in Figure 5 The force correction shown in FIG. 8 ) can then be combined with a control signal 808 generated using the feedforward control FF and the feedback control FB to obtain a corrected control signal 810 which can be applied to control the actuator ACT which controls the position of the optical component OC. Figure 8 Visible in the reference Figure 2 In a manner similar to that indicated, the feedback control FB acts on the difference between the reference signal r and the position signal p, while the feedforward control FF is based on the reference signal r.

[0105] As an alternative to applying a high pass filtering to the reference signal r to obtain a high pass filtered signal such as signal 802, a high pass filtering may also be applied to the signal p representing the measured position of the optical component OC relative to the reference frame. Fig. 9 Such an embodiment is schematically shown in FIG.

[0106] and Figure 8 In contrast, the high-pass filtering HP is now applied to the measured position signal p instead of to the reference r. It has been observed that by doing so, the obtained combined signal 804 now combines the position signal p a The low-pass filtered signal of the optical component OC and the high-pass filtered signal of the position signal p, the combined signal 804 also provides an accurate representation of the actual relative position between the optical component OC and the force frame or the first frame. Figure 8It is used in a similar manner as shown in to determine the required parasitic forces 806.

[0107] In an embodiment of the present invention, the control signal or measurement signal (eg, signal p and p) generated by the controller a ) may need to undergo one or more transformations, for example to take into account different coordinate systems applied, for example, in a controller, a measurement system associated with the optical component OC or the actuator ACT.

[0108] Fig.10 A control scheme according to an embodiment is schematically shown, which illustrates two of such transformations, T1 , T2 .

[0109] As in Fig.10 The control scheme schematically shown in FIG. 1 essentially corresponds to Figure 8 scheme, except the following:

[0110] exist Fig.10 In the figure, the position signal p a is subjected to a transformation T1 in order to take into account the signal p generated by a The different coordinate systems used by the measurement system and the controller that generates the actuator control signals.

[0111] Similarly, the control signal 810 generated by the controller is transformed using transformation T2 to consider the actuator coordinate system.

[0112] With regard to the transformations, e.g. T1 or T2, it can be pointed out that these transformations do not affect the frequency content of the transformed signal. Thus, the applied filtering operation, e.g. low-pass filtering LP, can also be applied before the transformation T1. Alternatively, after an appropriate transformation, the parasitic forces 806 can also be combined with the signal 808 after the transformation T2, instead of before the transformation T2.

[0113] In an embodiment, the present invention also provides a method of determining parasitic forces of an actuator configured to control the position of an optical component, thereby controlling the position of the optical component relative to the same frame as that supporting the actuator. Fig.11 Such an arrangement is schematically shown in FIG.

[0114] Fig.11 A reflector 110 is schematically shown, which is generally an optical component and can be used, for example, in Figure 1 The device LA shown in FIG. 1 is used in a projection system of a lithographic apparatus such as the one shown in FIG. Fig.11 Schematically illustrated is a frame 1110 configured to support or hold an actuator 1120 configured to apply a force indicated by arrow 1130 to the mirror 1100 . Fig.11Also shown is a measurement system 1150 configured to measure the position of the mirror 1100 relative to the frame 1110. Fig.11 It is further schematically shown how the position control of the reflector 300 can be achieved, which is similar to Figure 3 . A controller or control unit C is provided with a difference between a reference r representing a desired position of the reflector 1100 relative to the frame 1110 and a measured position p of the reflector 1100, the measured position p being obtained from a position measurement system 1150. The position measurement system 1150 may be, for example, a measurement system based on an interferometer or an encoder. Based on the difference e between the desired position r and the actual position p, the controller C may control the actuator to apply a suitable force to the reflector 300 in order to reduce the difference e. This may be done, for example, by the controller C outputting or generating a control signal Fc, which is provided to the actuator 320 in order to generate a force 330. It may be noted that it may also be implemented in Figure 3 Other control actions such as feed-forward control not shown in the figure are used to control the actuator 320. Similar to the arrangement with the first frame and the second frame, Fig.11 The arrangement shown in may also suffer from the occurrence of parasitic forces of the actuator 1120, i.e. forces that depend on the relative position of the frame 1110 and the reflector 1100. In order to determine such forces, knowledge of the relative position of the frame 1110 supporting the actuator 1120 and the actuated optical component 1100 is important. The inventors have observed that the application of the position measurement signals obtained from the position measurement system 1150 to determine the parasitic forces may lead to instabilities in the feedback loop of the controller or control unit C. Therefore, in an embodiment of the present invention, the following method for determining the parasitic forces of the actuator is proposed. This method can be applied, for example, in the case where the position of the optical component is controlled by the actuator relative to the same frame as the frame supporting the actuator.

[0115] The method of determining parasitic forces of an actuator of an optical component, the actuator being mounted to a frame and being configured to control a position of the optical component relative to the frame, comprises the following steps:

[0116] Obtain a signal representing the position of the optical component relative to the frame. Fig.11 , which may be, for example, a position signal p obtained from the position measurement system 1150 .

[0117] A low-pass filter is performed on the position signal (eg, position signal p).

[0118] Obtain a reference signal representing the desired position of the optical component relative to the frame. Fig.11 , which may be, for example, a reference signal r representing a desired position of the optical component 1100 .

[0119] Performing high-pass filtering on the reference signal r, wherein the high-pass filtering is complementary to the low-pass filtering;

[0120] The parasitic forces of the actuator are determined based on the combined low pass filtered position signal and the high pass filtered reference signal.

[0121] In this embodiment of the invention, the position signal p of the position measurement system 1150 is not used directly to derive or determine the parasitic forces. Instead, the position signal p is low-pass filtered and the low-pass filtered position signal is combined with a high-pass filtered reference signal. It has been observed that by doing so, the risk of instability in the feedback loop of the controller or control unit C can be mitigated.

[0122] In a similar manner as described above, the method of determining parasitic forces of an actuator may be implemented in a control method of an actuator for controlling the position of an optical component.

[0123] The control method according to the invention can be implemented, for example, in a controller or control unit for an actuator. Such a control unit can, for example, comprise a processing unit or processor and a memory unit for storing data, such as measurement signals. Such a controller or control unit can, for example, have an input to receive input signals, such as measurement signals or reference signals or set points. The controller can also have an output to output, for example, one or more control signals for controlling the actuator, the control signals being obtained by processing the input signals.

[0124] Thus, according to an aspect of the present invention, a control unit is provided, which is configured to execute the control method according to the present invention. Such a control unit can, for example, be applied in an optical system according to the present invention. Such an optical system can, for example, include:

[0125] Optical components, such as mirrors;

[0126] an actuator mounted to a first frame or frames of the system, the actuator being configured to control a position of the optical component relative to a second frame or frames of the system, and

[0127] A control unit configured to execute the method of controlling an actuator according to the invention.

[0128] Such a combination of optical systems according to the invention may for example be applied in a projection system of a lithographic apparatus, for example an EUV lithographic apparatus as described above.

[0129] Although the use of lithographic apparatus in IC manufacturing may be specifically referred to herein, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0130] Although embodiments of the invention are specifically mentioned herein in the context of lithographic equipment, embodiments of the invention may be used in other equipment. Embodiments of the invention may form part of a mask inspection equipment, a metrology equipment, or any equipment that measures or processes objects such as a wafer (or other substrate) or a mask (or other pattern forming device). These equipment may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0131] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0132] Where 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 that can 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 that can be read by a machine (e.g., a computing device). For example, a machine-readable magnetic storage medium may include: read-only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustic or other forms of propagation signals (e.g., carrier waves, infrared signals, digital signals, etc.), etc. In addition, herein, firmware, software, routines, instructions may be described as performing certain actions. However, it should be understood that such descriptions are only for convenience, and that these actions are actually generated 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.

[0133] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in a manner different from that described. The above description is intended to be exemplary and not restrictive. Therefore, it will be appreciated by those skilled in the art that modifications may be made to the described invention without departing from the scope of the aspects set forth below.

[0134] Aspect 13 An optical system, comprising:

[0135] Optical components, such as mirrors;

[0136] an actuator mounted to a first frame of the device, the actuator configured to control a position of the optical component relative to a second frame of the device;

[0137] A control unit, the control unit being configured to:

[0138] obtaining a reference signal representing a desired position of the optical component relative to the second frame and an optical component position signal representing a measured position of the optical component relative to the second frame;

[0139] The parasitic force of the actuator is determined by:

[0140] obtaining a first position signal representing a position of the optical component relative to the first frame;

[0141] performing low-pass filtering on the first position signal to obtain a low-pass filtered signal;

[0142] Performing high-pass filtering on the optical component position signal or the reference signal to obtain a high-pass filtered signal, wherein the high-pass filtering is complementary to the low-pass filtering;

[0143] determining a parasitic force of the actuator based on the combined low pass filtered signal and the high pass filtered signal;

[0144] Based on the reference signal, the optical component position signal and the determined parasitic forces, a control signal for controlling the actuator is determined.

[0145] Aspect 14 The optical system according to Aspect 13, wherein determining the parasitic force comprises:

[0146] determining a relative position of the optical component to the first frame based on the combined low pass filtered signal and the high pass filtered signal;

[0147] The parasitic force is determined based on the relative position and a parasitic force characteristic of the actuator.

[0148] Aspect 15 An optical system according to any one of Aspects 13 to 14, wherein the optical component comprises an EUV mirror.

[0149] Aspect 16 The optical system according to any one of Aspects 13 to 15, further comprising a position measurement system configured to generate the optical component position signal.

[0150] Aspect 17 The optical system according to Aspect 16, wherein the position measurement system comprises an interferometer-based or encoder-based measurement system.

[0151] Aspect 18 A projection system for use in a lithographic apparatus, the projection system comprising the optical system according to any one of aspects 13 to 17.

[0152] Aspect 19 A lithographic apparatus comprises the projection system according to aspect 18.

Claims

1. A method of determining parasitic forces of an actuator of an optical component, the actuator being mounted to a first frame and configured to control a position of the optical component relative to a second frame, the method comprising: obtaining a first position signal representing a position of the optical component relative to the first frame; performing low-pass filtering on the first position signal; obtaining a second position signal representing a position of the optical component relative to the second frame; Performing high-pass filtering on the second position signal, wherein the high-pass filtering is complementary to the low-pass filtering; Based on the combined low pass filtered first position signal and the high pass filtered second position signal, a parasitic force of the actuator is determined.

2. The method according to claim 1, wherein: Determining the parasitic forces of the actuator includes: determining a relative position of the optical component and the first frame based on the combined low pass filtered first position signal and the high pass filtered second position signal; Based on the relative position and a parasitic force characteristic of the actuator, a parasitic force of the actuator is determined.

3. The method according to claim 1 or 2, wherein: The second position signal corresponds to a reference signal representing a desired position of the optical component.

4. A method of controlling an actuator, the actuator being mounted to a first frame and configured to control a position of an optical component relative to a second frame, the method comprising: obtaining a reference signal representing a desired position of the optical component relative to the second frame and an optical component position signal representing a measured position of the optical component relative to the second frame; determining the parasitic forces of the actuator using a method according to any one of claims 1 to 3, Based on the reference signal, the optical component position signal and the determined parasitic forces, a control signal for controlling the actuator is determined.

5. The method according to claim 4, wherein: The second position signal corresponds to the reference signal.

6. The method according to claim 4, wherein: The second position signal corresponds to the optical component position signal.

7. The method according to claim 4, further comprising: The optical component position signal is low-pass filtered.

8. A method of determining parasitic forces of an actuator of an optical component, the actuator being mounted to a frame and configured to control a position of the optical component relative to the frame, the method comprising: obtaining a signal representing a position of the optical component relative to the frame; performing low-pass filtering on the position signal; obtaining a reference signal representing a desired position of the optical component relative to the frame; Performing high-pass filtering on the reference signal, wherein the high-pass filtering is complementary to the low-pass filtering; Based on the combined low pass filtered position signal and the high pass filtered reference signal, parasitic forces of the actuator are determined.

9. The method according to claim 8, wherein: Determining the parasitic forces of the actuator includes: determining a relative position of the optical component and the frame based on the combined low pass filtered position signal and the high pass filtered reference signal; Based on the relative position and a parasitic force characteristic of the actuator, a parasitic force of the actuator is determined.

10. A method of controlling an actuator, the actuator being mounted to a frame and configured to control a position of an optical component relative to the frame, the method comprising: obtaining a reference signal representing a desired position of the optical component relative to the frame and an optical component position signal representing a measured position of the optical component relative to the frame; Determining parasitic forces of the actuator using the method according to claim 8 or 9; Based on the reference signal, the optical component position signal and the determined parasitic forces, a control signal for controlling the actuator is determined.

11. An optical system comprising: Optical components, such as mirrors; an actuator mounted to a first frame of the optical system, the actuator configured to control a position of the optical component relative to a second frame of the optical system; A control unit configured to perform the method according to any one of claims 4 to 7.

12. An optical system comprising: Optical components, such as mirrors; an actuator mounted to a frame of the optical system, the actuator being configured to control a position of the optical component relative to the frame of the optical system; A control unit configured to perform the method according to claim 10.

13. An optical system comprising: Optical components, such as mirrors; an actuator mounted to a first frame of the device, the actuator configured to control a position of the optical component relative to a second frame of the device; A control unit, the control unit being configured to: obtaining a reference signal representing a desired position of the optical component relative to the second frame, and an optical component position signal representing a measured position of the optical component relative to the second frame; The parasitic force of the actuator is determined by: obtaining a first position signal representing a position of the optical component relative to the first frame; performing low-pass filtering on the first position signal to obtain a low-pass filtered signal; Performing high-pass filtering on the optical component position signal or the reference signal to obtain a high-pass filtered signal, wherein the high-pass filtering is complementary to the low-pass filtering; determining a parasitic force of the actuator based on the combined low pass filtered signal and the high pass filtered signal; Based on the reference signal, the optical component position signal and the determined parasitic forces, a control signal for controlling the actuator is determined.

14. A projection system for use in a lithographic apparatus, the projection system comprising an optical system according to any one of claims 11 to 13.

15. A lithographic apparatus comprising a projection system according to claim 14.

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