Exposure apparatus and method with reduction of an overlay error

Abstract

An exposure apparatus comprising a substrate table and a projection system comprising: an optical element, an optical element support configured to support the optical element and a sensor configured to generate a signal indicative of an acceleration of the optical element support, a position control system configured to control a position of the target portion of the substrate, the position control system comprising: a dynamic sensitivity model defining the position of the optical element relative to the optical element support as a quasi static and dynamic function of the signal, and an optical sensitivity function defining a position error of the target portion as a function of an estimate of the position of the optical element relative to the optical element support.

Description

EXPOSURE APPARATUS AND METHOD WITH REDUCTION OF AN OVERLAY ERRORCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The application claims priority of EP application 24153208.4 which was filed on 22 January 2024; and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to an exposure apparatus and an exposure method.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law’. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] One or more of the projection lenses in the projection system of the exposure apparatus may vibrate as a result of movements, such as acceleration of the substrate table. The projection lenses are held by a lens barrel. The accelerations may result in movement of internal lens elements with respect to the lens barrel. Such movements of the internal lens elements may cause a shift of the projected image. As the substrate and patterning device are positioned relative to the lens barrel, the shift of the projected image may result in an overlay error.SUMMARY

[0006] Considering the above, it is an object of the invention to reduce an overlay error of the exposure apparatus.

[0007] According to an aspect of the invention, there is provided an exposure apparatus comprising:a substrate table configured to support a substrate, a projection system configured to project a patterned radiation beam onto a target portion of the substrate, the projection system comprising: an optical element, an optical element support configured to support the optical element and a sensor configured to generate a signal indicative of an acceleration of the optical element support, a position control system configured to control a position of the target portion of the substrate, the position control system comprising: a dynamic sensitivity model, the dynamic sensitivity model defining the position of the optical element relative to the optical element support as a quasi static and dynamic function of the signal indicative of the forceon the optical element support, and an optical sensitivity function defining a position error of the target portion as a function of an estimate of the position of the optical element relative to the optical element support, wherein the position control system is configured to estimate the position of the optical element relative to the optical element support using the dynamic sensitivity model and the signal indicative of the acceleration of the optical element support, and correct at least during projection of the patterned radiation beam on the target portion, the position of the target portion using the estimated position of the optical element relative to the optical element support and the optical sensitivity function.

[0008] According to an aspect of the invention, there is provided an exposure method comprising: supporting a substrate by a substrate table, projecting by a projection system a patterned radiation beam onto a target portion of the substrate, the projection system comprising: an optical element, a optical element support configured to support the optical element and a sensor configured to generate a signal indicative of an acceleration of the optical element support, the method comprising controlling by a position control system a position of the target portion of the substrate, the position control system comprising: a dynamic sensitivity model, the dynamic sensitivity model defining the position of the optical element relative to the optical element support as a quasi static and dynamic function of the signal indicative of the acceleration of the optical element support, and an optical sensitivity function defining a position error of the target portion as a function of the estimate of the position of the optical element relative to the optical element support, wherein the method comprises: estimating the position of the optical element relative to the optical element support using the dynamic sensitivity model and the signal indicative of the acceleration of the optical element support andcorrecting at least during projection of the patterned radiation beam on the target portion, the position of the target portion using the estimated position of the optical element relative to the optical element support and the optical sensitivity function.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a schematic overview of a lithographic apparatus according to an embodiment of the invention;Figure 2 depicts a detailed view of a part of the lithographic apparatus of Figure 1;Figure 3 schematically depicts an embodiment of a position control system as may be comprised in the lithographic apparatus;Figure 4 schematically depics a cross sectional view of a part of a projection system;Figure 5 schematically depics a highly schematic cross sectional side view of a part of the exposure apparatus according to an embodiment of the invention; andFigure 6 schematically depicts a highly schematic cross sectional side view of a part of the exposure apparatus according to another embodiment of the invention.DETAILED DESCRIPTION

[0010] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0011] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0012] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system)PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0013] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0014] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0015] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0016] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0017] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0018] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam Bat a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0019] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., an x-axis, a y-axis and a z-axis. Each of the three axes is orthogonal to the other two axes. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry -rotation. A rotation around about the z-axis is referred to as an Rz-rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0020] Figure 2 shows a more detailed view of a part of the lithographic apparatus LA of Figure 1. The lithographic apparatus LA may be provided with a base frame BF, a balance mass BM, a metrology frame MF and a vibration isolation system IS. The metrology frame MF supports the projection system PS. Additionally, the metrology frame MF may support a part of the position measurement system PMS. The metrology frame MF is supported by the base frame BF via the vibration isolation system IS. The vibration isolation system IS is arranged to prevent or reduce vibrations from propagating from the base frame BF to the metrology frame MF.

[0021] The second positioner PW is arranged to accelerate the substrate support WT by providing a driving force between the substrate support WT and the balance mass BM. The driving force accelerates the substrate support WT in a desired direction. Due to the conservation of momentum, the driving force is also applied to the balance mass BM with equal magnitude, but at a direction opposite to the desired direction. Typically, the mass of the balance mass BM is significantly larger than the masses of the moving part of the second positioner PW and the substrate support WT.

[0022] In an embodiment, the second positioner PW is supported by the balance mass BM. For example, wherein the second positioner PW comprises a planar motor to levitate the substrate support WT above the balance mass BM. In another embodiment, the second positioner PW is supported by the base frame BF. For example, wherein the second positioner PW comprises a linear motor and wherein the second positioner PW comprises a bearing, like a gas bearing, to levitate the substrate support WT above the base frame BF.

[0023] The position measurement system PMS may comprise any type of sensor that is suitable to determine a position of the substrate support WT. The position measurement system PMS maycomprise any type of sensor that is suitable to determine a position of the mask support MT. The sensor may be an optical sensor such as an interferometer or an encoder. The position measurement system PMS may comprise a combined system of an interferometer and an encoder. The sensor may be another type of sensor, such as a magnetic sensor, a capacitive sensor or an inductive sensor. The position measurement system PMS may determine the position relative to a reference, for example the metrology frame MF or the projection system PS. The position measurement system PMS may determine the position of the substrate table WT and / or the mask support MT by measuring the position or by measuring a time derivative of the position, such as velocity or acceleration.

[0024] The position measurement system PMS may comprise an encoder system. An encoder system is known from for example, United States patent application US2007 / 0058173A1, filed on September 7, 2006, hereby incorporated by reference. The encoder system comprises an encoder head, a grating and a sensor. The encoder system may receive a primary radiation beam and a secondary radiation beam. Both the primary radiation beam as well as the secondary radiation beam originate from the same radiation beam, i.e., the original radiation beam. At least one of the primary radiation beam and the secondary radiation beam is created by diffracting the original radiation beam with the grating. If both the primary radiation beam and the secondary radiation beam are created by diffracting the original radiation beam with the grating, the primary radiation beam needs to have a different diffraction order than the secondary radiation beam. Different diffraction orders are, for example, +lstorder, -1storder, +2ndorder and -2ndorder. The encoder system optically combines the primary radiation beam and the secondary radiation beam into a combined radiation beam. A sensor in the encoder head determines a phase or phase difference of the combined radiation beam. The sensor generates a signal based on the phase or phase difference. The signal is representative of a position of the encoder head relative to the grating. One of the encoder head and the grating may be arranged on the substrate structure WT. The other of the encoder head and the grating may be arranged on the metrology frame MF or the base frame BF. For example, a plurality of encoder heads is arranged on the metrology frame MF, whereas a grating is arranged on a top surface of the substrate support WT. In another example, a grating is arranged on a bottom surface of the substrate support WT, and an encoder head is arranged below the substrate support WT.

[0025] The position measurement system PMS may comprise an interferometer system. An interferometer system is known from, for example, United States patent US6,020,964, filed on July 13, 1998, hereby incorporated by reference. The interferometer system may comprise a beam splitter, a mirror, a reference mirror and a sensor. A beam of radiation is split by the beam splitter into a reference beam and a measurement beam. The measurement beam propagates to the mirror and is reflected by the mirror back to the beam splitter. The reference beam propagates to the reference mirror and is reflected by the reference mirror back to the beam splitter. At the beam splitter, the measurement beam and the reference beam are combined into a combined radiation beam. The combined radiation beam is incident on the sensor. The sensor determines a phase or a frequency ofthe combined radiation beam. The sensor generates a signal based on the phase or the frequency. The signal is representative of a displacement of the mirror. In an embodiment, the mirror is connected to the substrate support WT. The reference mirror may be connected to the metrology frame MF. In an embodiment, the measurement beam and the reference beam are combined into a combined radiation beam by an additional optical component instead of the beam splitter.

[0026] The first positioner PM may comprise a long-stroke module and a short-stroke module. The short-stroke module is arranged to move the mask support MT relative to the long-stroke module with a high accuracy over a small range of movement. The long-stroke module is arranged to move the short-stroke module relative to the projection system PS with a relatively low accuracy over a large range of movement. With the combination of the long-stroke module and the short-stroke module, the first positioner PM is able to move the mask support MT relative to the projection system PS with a high accuracy over a large range of movement. Similarly, the second positioner PW may comprise a long-stroke module and a short-stroke module. The short-stroke module is arranged to move the substrate support WT relative to the long- stroke module with a high accuracy over a small range of movement. The long-stroke module is arranged to move the short-stroke module relative to the projection system PS with a relatively low accuracy over a large range of movement. With the combination of the long-stroke module and the short-stroke module, the second positioner PW is able to move the substrate support WT relative to the projection system PS with a high accuracy over a large range of movement.

[0027] The first positioner PM and the second positioner PW each are provided with an actuator to move respectively the mask support MT and the substrate support WT. The actuator may be a linear actuator to provide a driving force along a single axis, for example the y-axis. Multiple linear actuators may be applied to provide driving forces along multiple axis. The actuator may be a planar actuator to provide a driving force along multiple axis. For example, the planar actuator may be arranged to move the substrate support WT in 6 degrees of freedom. The actuator may be an electromagnetic actuator comprising at least one coil and at least one magnet. The actuator is arranged to move the at least one coil relative to the at least one magnet by applying an electrical current to the at least one coil. The actuator may be a moving-magnet type actuator, which has the at least one magnet coupled to the substrate support WT respectively to the mask support MT. The actuator may be a moving-coil type actuator which has the at least one coil coupled to the substrate support WT respectively to the mask support MT. The actuator may be a voice-coil actuator, a reluctance actuator, a Lorentz -actuator or a piezo-actuator, or any other suitable actuator.

[0028] The lithographic apparatus LA comprises a position control system PCS as schematically depicted in Figure 3. The position control system PCS comprises a setpoint generator SP, a feedforward controller FF and a feedback controller FB. The position control system PCS provides a drive signal to the actuator ACT. The actuator ACT may be the actuator of the first positioner PM or the second positioner PW. The actuator ACT drives the plant P, which may comprise the substratesupport WT or the mask support MT. An output of the plant P is a position quantity such as position or velocity or acceleration. The position quantity is measured with the position measurement system PMS. The position measurement system PMS generates a signal, which is a position signal representative of the position quantity of the plant P. The setpoint generator SP generates a signal, which is a reference signal representative of a desired position quantity of the plant P. For example, the reference signal represents a desired trajectory of the substrate support WT. A difference between the reference signal and the position signal forms an input for the feedback controller FB. Based on the input, the feedback controller FB provides at least part of the drive signal for the actuator ACT. The reference signal may form an input for the feedforward controller FF. Based on the input, the feedforward controller FF provides at least part of the drive signal for the actuator ACT. The feedforward FF may make use of information about dynamical characteristics of the plant P, such as mass, stiffness, resonance modes and eigenfrequencies.

[0029] One or more of the projection lenses in the projection system of the exposure apparatus may vibrate as a result of movements, such as acceleration of the substrate table. The projection lenses are held by a lens barrel. The accelerations may result in movement of internal lens elements with respect to the lens barrel. The movements of the internal lens elements may cause a shift of the projected image. As the substrate and patterning device are positioned relative to the lens barrel, the shift of the projected image may result in an overlay error.

[0030] It has been proposed to at least partly compensate the line of sight error as may result from an image shift due to a lens vibration. Thereto, an accelerometer is provided on the lens barrel. A quasi-static model of the projection lens is used. Using the model of the projection lens, an acceleration of the lens is estimated on the basis of the measurement of the acceleration by the accelerometer that is provided on the lens barrel. On the basis of the estimated acceleration of the lens, an overlay error is estimated using an optical sensitivity function which expresses the overlay error as a function of the position or acceleration of the lens. The estimated overlay error may be used as a further reference to a position control of the substrate table, so as to at least partly compensate the overlay error by adapting the position of the substrate table.

[0031] Developments in the exposure apparatus tend to result in a lower eigenfrequency of a lens of the projection system. Moreover, a susceptibility of the projection system to overlay error as a result of acceleration of the lens may increase. Still further, higher throughput requirements of the exposure apparatus tend to result in higher accelerations of the substrate table, which may result in higher disturbance forces on the lens barrel, which may in turn result in a higher acceleration of the lens barrel. Moreover, an increase in scan speed of the exposure apparatus may lead to a larger frequency range of the disturbances which may have an impact on overlay.

[0032] According to the present invention, a model is used to predict both the quasi static and the resonant contribution to the image shift of the projection system due to vibrations of an optical element, such as a lens, of the projection system. The parameters of this model may beeigenfrequency and damping of the optical element to estimate the relative displacement with respect to an optical element support, such as a lens barrel. An optical sensitivity of the optical element translates the relative displacement to image shift.

[0033] Figure 4 depicts a detailed, cross sectional view of a part of a projection system. Figure 4 depicts a part of an optical element support OES, such as a lens barrel. The optical element support supports one or more optical elements, such as the optical element OE, e.g. lens element. A vibration in the optical element support may result in a vibration of the optical element. A mass of the optical element support is denoted as mi. A mass of the optical element is denoted as m2. A mounting of the optical element to the optical element support is provided with a stiffness k. A force on the optical element support is denoted as fi, while a position of the optical element support is denoted as xi. As a result of the excitation of the optical element support by the force fi, a force fzi is exerted between the optical element support and the optical element, the force acting via the stiffness k on the mass m2 of the optical element. The relative position of the optical element relative to the optical element support is denoted as X21 while the position of the optical element is denoted as X2.

[0034] Figure 5 depicts a highly schematic cross sectional side view of a part of the exposure apparatus according to an embodiment of the invention. Figure 5 depicts the projection system PS comprising the optical element support OES, such as in the present example a lens barrel, and a plurality of optical elements OE, such as in the present example the lens elements. In the embodiment depicted in figure 5, the sensor is formed by an accelerometer ACC on the optical element support. In the present example, six accelerometers are provided at the optical element support, the accelerometers may for example be configured to provide acceleration signals in 6 degrees of freedom. The exposure apparatus further comprises a position control system K configured to control a position of the target portion of the substrate. The position control system K is provided as input with the signal or signals provided by the accelerometer ACC or accelerometers.

[0035] The position control system comprises a dynamic sensitivity model. The dynamic sensitivity model defines the position of the optical element relative to the optical element support as a quasi static and dynamic function of the signal indicative of the acceleration of the optical element support. The dynamic sensitivity model accordingly estimates by a quasi static and dynamic function the position of the optical element as a fuction of the signal indicative of the acceleration of the optical element support, i.e. in the present example as a function of the acceleration signal as measured by the acceleration sensor. An example of the dynamic sensitivity model will further be described below.

[0036] The position control system further comprises an optical sensitivity function which defines a position error of the target portion as a function of an estimate of the position of the optical element relative to the optical element support. The position error of the target portion may be understood as the position error with respect to an image position of the pattern projected by the projection system onto the substrate. Thus the optical sensitivity function defines how the deviation ofthe position of the optical element relative to the optical element support translates into the position error of the target portion.

[0037] The position control system is configured to estimate the position of the optical element relative to the optical element support using the dynamic sensitivity model and the signal indicative of the acceleration of the optical element support, i.e. in the present example the acceleration signal.

[0038] The position control system is further configured to correct at least during projection of the patterned radiation beam on the target portion, the position of the target portion using the estimated position of the optical element relative to the optical element support and the optical sensitivity function.

[0039] Accordingly, the dynamic sensitivity model is used to estimate the position of the optical element relative to the optical element support on the basis of the acceleration signal, and, using the optical sensitivity function and the estimated position of the optical element relative to the optical element support, the amount of correction of the target portion is determined. The estimation of the position of the optical element relative to the optical element support and the determination of the amount of correction of the target portion may be performed as two separate actions, or a single, combined model may be created which may enable to integrally perform these actions. The position control system may be configured to drive the positioner PW of the substrate table WT to correct the position of the target portion. Furthermore, the position control system may be configured to drive an actuator of the support that supports the patterning device.

[0040] In the present embodiment, the sensor comprises an acceleration sensor and the signal comprises an acceleration signal indicative of an acceleration of the optical element support. Accordingly, use may be made of a sensor that may already be present in the exposure apparatus. Moreover, in the present embodiment, the dynamic sensitivity model expresses a position of the optical element relative to the optical element support as a function of the acceleration of the optical element support. The position of the optical element relative to the optical element support is estimated from the acceleration signal which represents the acceleration of the optical element support. As the dynamic sensitivity model may accurately model a quasi static and dynamic behaviour of the position of the optical element as a function of the acceleration of the optical element support, an accurate estimation of the position of the optical element relative to the optical element support may be provided.

[0041] The dynamic sensitivity model may express the position of the optical element relative to the optical element support by the following equation: x±~ x2_ m2s2x±m2s2+ ds + kwherein xi is the position of the optical element support, X2 is the position of the optical element, m2 is the mass of the optical element, k is a stiffness of the mounting of the optical element to the optical element support, s is a time differentiation operator, and d is a damping between the optical element and the optical element support.

[0042] Figure 6 depicts a highly schematic cross sectional side view of a part of the exposure apparatus according to another embodiment of the invention. Figure 6 depicts the projection system PS comprising the optical element support OES, such as in the present example a lens barrel, and a plurality of optical elements OE, such as in the present example the lens elements. In the embodiment depicted in Figure 6, the sensor is formed by a force sensor that senses a force between the optical element and the optical element support. For example, the force sensor is formed by an optical element actuator OEA, e.g. a piezo electric actuator which actuates the optical element, such as the lens element, relative to the optical element support, such as the lens barrel. By the piezo electric effect, the piezo electric actuator is configured to generate a piezo voltage in response to the force between the optical element and the optical element support. Likewise to the embodiment described with reference to Figure 5, the exposure apparatus further comprises the position control system K configured to control a position of the target portion of the substrate. The position control system K is provided as input with the signal or signals provided by force sensor, i.e, in the present example the piezo electric lens actuator.

[0043] Similarly as described with reference to Figure 5, the position control system comprises a dynamic sensitivity model. In the present embodiment, the dynamic sensitivity model defines the position of the optical element relative to the optical element support as a quasi static and dynamic function of the signal generated by the optical element actuator, i.e. in the present example the lens actuator. The dynamic sensitivity model accordingly estimates by the quasi static and dynamic function the position of the optical element as a fuction of the signal indicative of the acceleration of the optical element support, i.e. in the present example as a function of the optical element actuator signal. An example of the dynamic sensitivity model will further be described below. Likewise as described with reference to Figure 5, the position control system further comprises an optical sensitivity function which defines a position error of the target portion as a function of an estimate of the position of the optical element relative to the optical element support. Thus the optical sensitivity function defines how the deviation of the position of the optical element relative to the optical element support translates into the position error of the target portion.

[0044] The position control system is configured to estimate the position of the optical element relative to the optical element support using the dynamic sensitivity model and the signal indicative of the acceleration of the optical element support, i.e. in the present example derived from the force sigal provided by the force sensor, the force signal representing the force between the optical element and the optical element support.

[0045] The position control system is further configured to correct at least during projection of the patterned radiation beam on the target portion, the position of the target portion using the estimated position of the optical element relative to the optical element support and the optical sensitivity function.

[0046] Accordingly, first, the dynamic sensitivity model is used to estimate the position of the optical element relative to the optical element support on the basis of the force sensor signal that senses the force between the optical element and the optical element support, and second, using the optical sensitivity function and the estimated position of the optical element relative to the optical element support, the amount of correction of the target portion is determined. The position control system may be configured to drive the positioner PW of the substrate table WT to correct the position of the target portion. Furthermore, the position control system may be configured to drive an actuator of the support that supports the patterning device.

[0047] In the present embodiment, the sensor comprises a force sensor configured to measure a force between the optical element and the optical element support and wherein the signal indicative of the acceleration of the optical element support comprises a force signal indicative of the force between the optical element and the optical element support.

[0048] Furthermore, in the present embodiment, the dynamic sensitivity model expresses the position of the optical element relative to the optical element support as a function of the force between the optical element and the optical element support, such as measured by the force sensor which measures the force between the optical element and the optical element support. The position of the optical element relative to the optical element support may be accurately estimated from the force signal which represents the force between the optical element and the optical element support. As the dynamic sensitivity model may accurately model a quasi static and dynamic behaviour of the optical element as a function of the force between the optical element and the optical element support, an accurate estimation of the position of the optical element relative to the optical element support may be provided.

[0049] The dynamic sensitivity model may express the position of the optical element relative to the optical element support by the following equation: X2~X1) =k ds~f21wherein xi is the position of the optical element support, x2is the position of the optical element, f2iis the force between the optical element and the optical element support, k is a stiffness of the mounting of the optical element to the optical element support, s is a time differentiation operator, and d is a damping between the optical element and the optical element support.

[0050] In the present example, the projection system comprises an optical element actuator configured to actuate the optical element relative to the optical element support. The actuator is configured to actuate the optical element in response to an actuator drive signal and to generate a response signal in response to an actuation of the optical element. The sensor is formed by the optical element actuator and the signal indicative of the acceleration of the optical element support comprises the response signal. Thereby, the signal generated by the actuator, such as the piezo actuator, may accurately represent the force between the optical element and the optical element support as the sensor is provided in a force path between the optical element and the optical element support.

[0051] The present embodiment aims to use the piezoelectric effect to measure a self-generated voltage from one or more semi-active lens elements during scanning and use this information to correct for image aberrations due to lens internal mode vibrations. The different signals can be multiplied by a correction matrix (based on the lens dependency and vibration mode shape) and provided to the higher bandwidth stage controller as an extra position reference (primarily the substrate table, and optionally the support that supports the patterning device).

[0052] An investigation correlating the frequency domain information contained in the piezos self-generated voltage during normal machine operation to multiple lens internal modes is possible. The identified internal modes may be mainly from the lens elements supported by those piezos, but not limited to. This correlation may be done by processing controlled excitation information with multiple accelerometer measurements (to give the vibration mode shapes) and the piezo measured voltages. Other than that, it is important to note that the signal contains not just the lens element eigenmode, but also other dynamics information from the lens itself (for example, the first lens “banana” mode should be visible there as well).

[0053] The information on the vibration mode shape as comprised in the lens models may translate the amplitude and phase of the voltage measured by each piezo of the lens manipulator and convert it to correction setpoints for the wafer and / or reticle stages (the stages with higher bandwidth and higher correction potential). The measured voltage signals can be processed by the position control system (weighting and combining them to translate the voltages into aberrations and then into correction trajectories for the stages) and injected directly into the wafer / reticle stage control loops. This in indicated in Figure 6 the picture below by the box “K” on the left side.

[0054] The same concept may be applied for other scanning elements of the exposure apparatus. Due to their much lower bandwidth the application may not be straight forward, since injecting the measured signal directly in the control loop may not be possible. Therefore, two possibilities are proposed:

[0055] Based on a repetitive behavior of a trajectory during scanning of the exposure apparatus, an excitation mode may be calculated from the setpoints, and a feedforward correction may be applied on the scanning lens elements to correct for multiple aberrations. A knowledge from the real-timevoltage measurements may not used in this case. However, a calibration may be done using these voltages to improve the model.

[0056] As another possibility, a design change may be performed in the scanning elements to make them capable of following trajectories in the applicable frequency range (above an eigen mode of the optical element). The measured voltage signals may be processed by the position control system (weighting and combining them to translate the voltages into aberrations and then into correction trajectories for the substrate table and / or the support that supports the pattening device) and may for example be injected directly into the wafer / reticle stage control loops.

[0057] In an embodiment, the position control system is configured to derive optical element internal modes from the signal indicative of the acceleration of the optical element support. Internal modes of the optical element, such as lens internal modes, may result from the vibration of the optical element support. The internal modes may result in a displacement of the target portion, likewise to the displacement of the optical element relative to the optical element support. The dynamic sensitivity model and the optical sensitivity function may take the optical element internal modes into account.

[0058] The correction of the position of the target portion may be performed in various ways. For example, a correction signal may be sent to the substrate table and / or to the support, e.g. the support of the patterning device. Thereby, the position of the substrate table and / or the position of the support may be adjusted to take account of the deviation of the position of the optical element relative to the optical element support. Accordingly, in an embodiment, the correcting the position of the target portion comprises determining a substrate table correction setpoint using the determined position of the optical element relative to the optical element support and the optical sensitivity function and providing the substrate table correction setpoint to the substrate table.

[0059] Additionally or alternatively, the exposure apparatus comprises the support configured to support the patterning device and the correcting the position of the target portion may comprise determining a support correction setpoint using the determined position of the optical element relative to the optical element support and the optical sensitivity function and providing the support correction setpoint to the support.

[0060] A fast dynamic response may be obtained by providing a feedforward correction to the substrate table and / or to the support. Accordingly, in an embodiment, the correcting the position of the target portion comprises determining a feedforward correction signal using the determined position of the optical element relative to the optical element support and the optical sensitivity function and providing the feedforward correction signal to substantially correct the position of the target portion on the basis of the feedforward correction signal.

[0061] As explained above, the optical element may comprise a lens. Thus, in an embodiment, the optical element comprises a lens and wherein the optical element support comprises a lens barrel.The present invention may likewise be employed with any other optical element, such as a mirror. Correspondingly, the optical element support may comprise a mirror support.

[0062] At low frequency, a displacement of the optical element may be determined by the stiffness between the optical element and the optical element support and the measured optical element support acceleration. According to the present invention, the dynamic sensitivity model estimates the position of the optical element relative to the optical element support as a quasi static and dynamic function of the signal indicative of acceleration of the optical element support (such as the acceleration signal determined by the acceleration sensor on the optical element support or the force signal of the force between the optical element and the optical element support. The dynamic function enables to take account of higher-frequency dynamics of a movement of the optical element relative to the optical element support, hence enabling to accurately estimate the position of the optical element relative to the optical element support even at higher-frequency excitations of the optical element support. The vibrations to which the projection system, hence the optical element support and the optical element, is / are subjected may originate from any source in the exposure apparatus. For example the vibrations may be generated by an acceleration of the substrate table or the support that supports the patterning device. The term vibration may be understood as any force, such as a periodic force, a sinusoidal force or a resonance.

[0063] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0064] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0065] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0066] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine -readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, amachine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0067] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting.

Claims

CLAIMS1. An exposure apparatus comprising: a substrate table configured to support a substrate, a projection system configured to project a patterned radiation beam onto a target portion of the substrate, the projection system comprising: an optical element, an optical element support configured to support the optical element and a sensor configured to generate a signal indicative of an acceleration of the optical element support, a position control system configured to control a position of the target portion of the substrate, the position control system comprising: a dynamic sensitivity model, the dynamic sensitivity model defining the position of the optical element relative to the optical element support as a quasi static and dynamic function of the signal indicative of the acceleration of the optical element support, and an optical sensitivity function defining a position error of the target portion as a function of an estimate of the position of the optical element relative to the optical element support, wherein the position control system is configured to estimate the position of the optical element relative to the optical element support using the dynamic sensitivity model and the signal indicative of the acceleration of the optical element support, and correct at least during projection of the patterned radiation beam on the target portion, the position of the target portion using the estimated position of the optical element relative to the optical element support and the optical sensitivity function.

2. The exposure apparatus according to claim 1, wherein the sensor comprises an acceleration sensor and wherein the signal comprises an acceleration signal indicative of an acceleration of the optical element support.

3. The exposure apparatus according to claim 2, wherein the dynamic sensitivity model expresses a position of the optical element relative to the optical element support as a function of the acceleration of the optical element support.

4. The exposure apparatus according to any one of the preceding claims, wherein the sensor comprises a force sensor configured to measure a force between the optical element and the optical element support and wherein the signal indicative of the acceleration of the optical element support comprises a force signal indicative of the force between the optical element and the optical element support.

5. The exposure apparatus according to claim 4, wherein the dynamic sensitivity model expresses the position of the optical element relative to the optical element support as a function of the force between the optical element and the optical element support.

6. The exposure apparatus according to any one of the preceding claims, wherein the projection system comprises an optical element actuator configured to actuate the optical element relative to the optical element support and wherein the actuator is configured to actuate the optical element in response to an actuator drive signal and to generate a response signal in response to an actuation of the optical element, wherein the sensor is formed by the optical element actuator and wherein the signal indicative of the acceleration of the optical element support comprises the response signal.

7. The exposure apparatus according to any one of the preceding claims, wherein the position control system is configured to derive optical element internal modes from the signal.

8. The exposure apparatus according to any one of the preceding claims, wherein the correcting the position of the target portion comprises determining a substrate table correction setpoint using the determined position of the optical element relative to the optical element support and the optical sensitivity function and providing the substrate table correction setpoint to the substrate table.

9. The exposure apparatus according to any one of the preceding claims, wherein the exposure apparatus comprises a support configured to support the patterning device and wherein the correcting the position of the target portion comprises determining a support correction setpoint using the determined position of the optical element relative to the optical element support and the optical sensitivity function and providing the support correction setpoint to the support.

10. The exposure apparatus according to any one of the preceding claims, wherein the correcting the position of the target portion comprises determining a feedforward correction signal using the determined position of the optical element relative to the optical element support and the optical sensitivity function and providing the feedforward correction signal to substantially correct the position of the target portion on the basis of the feedforward correction signal.

11. The exposure apparatus according to any one of the preceding claims, wherein the optical element comprises a lens and wherein the optical element support comprises a lens barrel.

12. The exposure apparatus according to any one of claims 1 - 10, wherein the optical element comprises a mirror and wherein the optical element support comprises a mirror support.

13. An exposure method comprising: supporting a substrate by a substrate table, projecting by a projection system a patterned radiation beam onto a target portion of the substrate, the projection system comprising: an optical element, a optical element support configured to support the optical element and a sensor configured to generate a signal indicative of an acceleration of the optical element support, the method comprising controlling by a position control system a position of the target portion of the substrate, the position control system comprising: a dynamic sensitivity model, the dynamic sensitivity model defining the position of the optical element relative to the optical element support as a quasi static and dynamic function of the signal indicative of the acceleration of the optical element support, and an optical sensitivity function defining a position error of the target portion as a function of the estimate of the position of the optical element relative to the optical element support, wherein the method comprises: estimating the position of the optical element relative to the optical element support using the dynamic sensitivity model and the signal indicative of the acceleration of the optical element support and correcting at least during projection of the patterned radiation beam on the target portion, the position of the target portion using the estimated position of the optical element relative to the optical element support and the optical sensitivity function.

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