Wafer stage system, method, and lithographic apparatus provided with the wafer stage

Abstract

The present disclosure provides a wafer stage system, comprising: a wafer stage (WS) configured to planarly move a wafer arrangeable on the wafer stage relative to a base frame, a balance mass (BM) supporting the wafer stage, and a bearing frame (8) connected to the balance mass via one or more bearings (4, 6), wherein the balance mass is configured to rotate around its center of gravity via a pivot point (20) of the bearing frame.

Description

WAFER STAGE SYSTEM, METHOD, AND LITHOGRAPHIC APPARATUS PROVIDED WITHTHE WAFER STAGECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24188030.1 which was filed on 11 July 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a wafer stage system, to a method of using the wafer stage, and to a lithographic apparatus comprising the wafer stage system. The wafer stage system comprises a substrate stage provided with a balance mass.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] Exposure steps of the semiconductor manufacturing process typically involve lithography. Herein, substrates, such as silicon based wafers, are typically arranged on substrate tables. The substrate table is typically moved or stepped with respect to an incident beam of patterned radiation, to provide the respective pattern to a large number of locations on the respective substrate.

[0006] Given the strong link between throughput and economic viability, there is an ever ongoing drive to speed up the process of stepping or moving the substrate tables. Throughput for lithography based on DUV has already surpassed 300 substrates per hour, and the industry is aiming to speed up to 500 substrates per hour. Herein, the corresponding speed and acceleration of the respective substrate tablesbetween consecutive fields of exposure may easily exceed 100 m / s2, which - like so many parameters in the chip making industry - can be regarded as extreme. With substrate tables having a significant weight, typically exceeding 50 kg, the substrate table is sometimes cheerfully referred to as a "flying (sidewalk) tile". Forces exerted on the substrate tables during acceleration and deceleration may be in the range of multiple g-forces, up to 32 g for state of the art equipment.

[0007] To dampen or counteract opposing forces in the respective machine due to the acceleration and deceleration of the one or more substrate tables within said machine, typically, forces in horizontal direction are absorbed using a balance mass principle. See Figure 2 for an example. The balance mass system however has its limits and the increasing acceleration and resulting forces as described above manage to exceed said limits, resulting in vibrations transmitted to the factory floor and to other equipment.

[0008] US2002093637A1 describes a stage assembly and support system to stabilize a wafer stage base or minimize forces transmitted from the stage assembly to the ground. The wafer stage assembly and support system allow the wafer stage base to move relative the stationary surface in the x and y directions. The base acts as a massive reaction mass and supports the stage through a set of bearings, which are set onto pivoting supports thus allowing the reaction mass (base) to move. The stage assembly may also comprise a set of actuators to control movement of the reaction mass (base).

[0009] US20070052944A1 discloses a lithographic system with a substrate table, a base frame, a balance mass supported by and moveable relative to the base frame and coupled to the substrate table, and at least one supporting member with at least two pivot points attached to the balance mass and to the base frame. A positioning actuator may be provided between the substrate table and a balance mass, and both table and balance mass can freely move in the horizontal plane parallel to the surface of the substrate table support frame. The supporting elements are pivotally attached to the base frame and the balance mass at each end by hinges, which may include ball bearings. The supporting element has pivot points below the center of gravity of the balance mass on either side of the central portion of the supporting element, but no explicit reference to the pivots allowing either the balance mass or the bearing frame to rotate around its center of gravity.

[0010] US20110043781 Al discloses a reticle stage able to move in the XY direction and coupled via an air bearing to a reticle base. In order to cancel a reaction force caused by the movement of the stage, a countermass member which is moved on the base, and a flexure mechanism which supports the countermass member on the base are provided. Flexible coupling is achieved by the pivots so that the moment by the vibration of the reticle base is not transmitted to an intermediate member.

[0011] US2008230677A1 describes a lithographic apparatus with a support and a substrate table to hold a substrate. At least one vibration isolation support device can be provided for supporting an object of the apparatus. The object can be rotatably supported at the vibration isolation support device by way of a rotational support having a center of rotation. The rotational support can have its center of rotation located substantially at the center of gravity of the vibration isolation support device. A rotationalsupport can be used for achieving the rotational disconnection, like a friction free pivotal connection wherein the center of gravity of the vibration isolation support device is located in proximity to the center of rotation of the rotational support. Air bearings are discussed for translational disconnection.

[0012] A consequence of the conventional concept using a balance mass is, however, that the acceleration forces caused by movement of the substrate stage with respect to the balance mass also result in a torque on the corresponding balance mass. The torque results from misalignment of the center of gravity (CoG-WS) of the substrate stage with respect to the center of gravity of the balance mass (CoG-BM) in the vertical direction (z-axis). The torque may be transferred via forces on the balance mass bearings towards the floor. Thus, the vertical forces induced by the torque on the balance mass may induce vibrations in the factory floor, which in turn can negatively and unpredictably impact the performance of nearby systems.

[0013] The present disclosure aims to provide an improved system and method to obviate or at least mitigate impact of substrate stage acceleration and deceleration.SUMMARY

[0014] The present disclosure provides a wafer stage system, comprising: a wafer stage configured to planarly move a wafer arrangeable on the wafer stage relative to a base frame, a balance mass supporting the wafer stage, and a bearing frame connected to the balance mass via one or more bearings, wherein the balance mass is configured to rotate around its center of gravity via a pivot point of the bearing frame.

[0015] In an embodiment, the bearing frame is pivotably connected to a support structure at the pivot point.

[0016] In an embodiment, the pivot point is elevated with respect to the bearing frame, the bearing frame comprising an elevated middle section.

[0017] In an embodiment, the pivot point of the bearing frame coincides with the balance mass center of gravity.

[0018] In an embodiment, the bearing frame is provided with one or more weights.

[0019] In an embodiment, the wafer stage is adapted to planarly move over a top surface of the balance mass.

[0020] In an embodiment, the system comprises at least one first actuator connected between the bearing frame and the base frame; and at least one suspension mass connected to the bearing frame via at least one second actuator.

[0021] In an embodiment, the at least one second actuator is connected to the base frame via a spring and / or damper.

[0022] In an embodiment, the system comprises a second support frame arranged below the bearing frame and which can rotate with respect to said bearing frame.

[0023] In an embodiment, the second support frame is rotatably connected to the support structure at a second pivot point, wherein the bearing frame is connected to the base frame via first actuators; and wherein the second support frame is provided with at least one suspension mass which is connected to the bearing frame via at least one second actuator.

[0024] In an embodiment, the bearing frame is provided with at least one rotor for providing a reaction torque.

[0025] According to another aspect, the disclosure provides a wafer stage system comprising: a wafer stage configured to planarly move a wafer arrangeable on the wafer stage relative to a base frame, a balance mass supporting the wafer stage, a plurality of balance mass bearings arranged between the balance mass and a bearing frame, wherein the balance mass is provided with at least one rotor.

[0026] According to another aspect, the disclosure provides a wafer stage system comprising: a wafer stage configured to planarly move a wafer arrangeable on the wafer stage relative to a base frame, a balance mass supporting the wafer stage, a plurality of balance mass bearings arranged between the balance mass and a bearing frame, wherein the bearing frame is configured to rotate around its center of gravity via a pivot point of the bearing frame relative to the base frame, and one or move actuators connecting the bearing frame to the balance mass.

[0027] According to another aspect, the disclosure provides a lithographic apparatus comprising at least one wafer stage system as described above.

[0028] According to another aspect, the disclosure provides a method, comprising the steps of: providing a balance mass supporting a wafer stage; connecting a bearing frame to the balance mass via one or more bearings, arranging a wafer on the wafer stage; and planarly moving the wafer stage relative to the balance mass; wherein the balance mass is configured to rotate around its center of gravity via a pivot point of the bearing frame.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] 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;Figure 2 schematically depicts a cross section of a conventional substrate stage;Figure 3 depicts a cross section of an embodiment of a substrate stage according to the present disclosure;Figure 4 depicts a cross section of an embodiment of a substrate stage according to the present disclosure;Figure 5 depicts a diagram exemplifying actuation control of the embodiment of Figure 4;Figure 6 depicts a cross section of an embodiment of a substrate stage according to the present disclosure;Figure 7 depicts a cross section of an embodiment of a substrate stage according to the present disclosure;Figure 8 depicts a cross section of an embodiment of a substrate stage according to the present disclosure; andFigure 9 depicts a cross section of an embodiment of a substrate stage according to the present disclosure.DETAILED DESCRIPTION

[0030] 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).

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning deviceMA 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.

[0039] 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.

[0040] Generally referring to Figure 2, a substrate stage system 1 may comprise a substrate table WS. The substrate table or stage WS is a platform to support and move a substrate, such as a wafer, with respect to a patterning radiation beam, see Figure 1. The substrate stage WS is movably arranged with respect to a base frame BF. The base frame basically represents, or can be considered to be fixedly connected to, the environment.

[0041] For many conventional substrate stage systems, the relatively large horizontal forces due to repeated acceleration and deceleration of the substrate table WS are absorbed using a balance mass principle. Herein, a balance mass unit BM is arranged between the substrate table WS and the base frame BF.

[0042] For instance, a planar motor concept may be implemented to realize the horizonal accelerations of the substrate table WS. Herein, a magnet array may be used, which can be mounted on top of the balance mass BM, forming a so called Magnet Plate. The Magnet Plate is integral to the balance mass BM and contributes to the total mass of the balance mass BM. The magnetic coils of the planar motor concept may be arranged on the bottom of the substrate table WS. In an alternative embodiment, a magnet array formed by (superconductive) magnetic coils is part of the balance mass.

[0043] The balance mass BM is typically connected to bearings 4, 6, which in turn are provided in or on a bearing frame 8. The bearing frame 8 may be connected to the base frame BF via support legs 10, 12. The bearings 4, 6 may be air bearings, and the bearing frame 8 may be an air bearing frame. An actuator 14 or comparable controller may be arranged between a horizontal edge of the balance mass BM and the base frame BF. The actuator or controller element 14 may dampen or obviate horizontal drift of the balance mass with respect to the base frame.

[0044] Figure 2 indicates the following: horizontal acceleration of the substrate table aws [m / s2]; horizontal acceleration of the balance mass aBM [m / s2] ; mass of the substrate table mws [kg] ; mass ofthe balance mass in HM [kg] ; torque on the balance mass TBM [Nm] ; force dump in vertical direction to the base frame and floor due to the torque fz,dumP[N] ; horizontal controller force fhor [N] ; and horizontal distance or arm of the vertical force(s) directed from the bearing frame to the base frame with respect to the centre of gravity of the balance mass lm[m].

[0045] As exemplified in Figure 2, forces resulting from acceleration and deceleration of the substrate table WS also result in a torque TBM on the balance mass BM. The torque TBM results from a misalignment in vertical direction between the centre of gravity of the substrate table CoGws and the centre of gravity of the balance mass COGBM- Said Z-misalignment of the respective centre of gravities is depicted in Figure 2 as 1COG,Z- The torque may be represented by a principle model description as:TBM=~lcoG,Z ’mWS ’aWS [1]

[0046] The torque TBM is transferred to balance mass bearings 4, 6, through a bearing frame 8, via support legs 10, 12 towards the base frame BF. The legs 10, 12 are typically relatively stiff, so that vertical forces transferred to the bearing frame are also transmitted to the base frame BF. As the base frame is fixed, forces are subsequently transmitted to the floor. The local vertically directed Z-forces fz are dumped to the floor via the base frame, as indicated in Figure 2. Said forces can be described as: fz,dump=(J-CoG,Z 'mWs / ^rn 'aWS P]

[0047] For an exemplary lithographic apparatus LA having exemplary throughput expressed as substrates per hour [sph] , the vertical forces fz,dumP[in Newton] may typically be on the order of [table 1]:

[0048] As exemplified in the table above, with throughputs already at physically very challenging limits, such as 300 substrates per hour, any further increase in throughput will disproportionately increase the resulting vertical forces also. With intended throughput for the next generation of lithographic machines moving in the direction of 400 and even 500 substrates per hour, this would almost double the vertical forces with respect to the currently fastest dry lithographic system. To be able to maintain overlay performance targets, which indicate alignment of respective layers arrangedon the substrate using the lithographic process, options to reduce the exported forces towards the floor and the base frame are required.

[0049] The disadvantage of dumping substrate stage acceleration setpoint forces onto the floor is that nearby surrounding scanner modules that are sensitive to floor vibrations, such as for instance metrology equipment, wafer handlers and reticle handlers, and even other adjacent machines in the factory, can be compromised in their performance. In addition, also the performance of other modules of the lithographic apparatus which are connected via the base frame, such as a mask support system, projection system, etc.), are compromised. Thus, the vibrations may result in overlay degradation and availability issues of equipment. The latter problems scale up in conjunction with the drive for higher throughput while maintaining overlay, focus and availability performance. Herein, not only the setpoint acceleration levels scale up, but also the scan-speed and therefore the scan harmonic frequencies increase, for which many vibration receivers, such as wafer handlers and reticle handlers, are relatively sensitive.

[0050] In order to reduce the impact of the torque disturbance towards a Z-force dump to the floor induced by the substrate stage, several balance mass concepts are disclosed herein and described herein below. Herein, the embodiments depicted in Figures 3 to 9 respectively may be referred to as embodiment A, B, C, D, E and F. In embodiments A, B, C and D, the inertia mass of the balance mass BM and the support frame is used to realize passive absorption of acceleration setpoint induced torque, as outlined by Equation 1 above. This has a particular benefit in relatively low energy consumption. Embodiments E and F seek for correcting the WS induced torque actively on the balance mass.

[0051] Figure 3 depicts a first embodiment of a substrate stage system 1 according to the present disclosure. The basics of the concept is to allow the balance mass BM, which is used to absorb horizontal acceleration forces due to movement of the substrate stage WS, to rotate around its centre of gravity COGBM by creating a pivoting point 20 of the bearing frame 8. The base frame is provided with a support structure 22. The support structure may be, as depicted in Figure 3, a turret, tripod, or pyramid shaped structure. The pivoting point 20 is located at the top end of the support structure 22 and substantially coincides with the centre of gravity of the balance mass. Herein, the balance mass MS is provided with a recess or cavity 24 to allow the support structure 22 to extend into the balance mass at least up to its centre of gravity. The recess 24 may have any shape or form. In a practical embodiment, the recess may have a (potentially truncated) pyramid shape. The bearing frame 8 may be provided with a pivot section 26. The pivot section 26 is intended to conform the shape of the bearing frame 8 to the support structure 22 and the pivot point 20, while allowing the bearing frame to pivot with respect to said pivot point. The pivot section 26 may typically be elevated with respect to other parts of the bearing frame 8, for stability. For instance, the pivot section may be a tripod or pyramid shaped section, having a top end coinciding with the pivot point 20 and the top end of the support structure 22.

[0052] Optionally, radial ends of the bearing frame may be provided with mass elements 28, 30, for additional stability. Optionally, a second controller 32 can be provided between the base frame BF andthe balance mass BM. The second controller 32 can control vertical movement of a radial end of the balance mass with respect to the base frame, for instance to allow rotation around the pivot point 20 while avoiding resonance and limiting deflection in vertical direction.

[0053] The pivot point 20 allows the balance mass to absorb vertical forces due to torque, while obviating that said forces are transmitted toward the base frame and the floor. In operation, the vertical forces at the bearings 4, 6, resulting from the mismatch between the centre of gravity of wafer stage WS and balance mass BM respectively, will result in a rotational acceleration of the (air) bearing frame 8. Since the bearing frame has a certain mass inertia, reaction forces at the bearings 4, 6 will be generated passively. The latter is, in terms of low energy consumption, a preferred scenario. Because the bearing frame 8 is only connected to the base frame BF and the floor via the pivot support structure 22, the reaction forces at the bearings will not be transmitted in vertical direction to the floor.

[0054] In a preferred embodiment, the rotation of the bearing frame 8 is provided with constraints in rotations around the Z axis, i.e. Rz. Since the rotation of the balance mass BM is relatively small by design, in practice it is relatively straightforward to provide the constraint. For instance, by providing horizontal struts and / or leaf springs between the bearing frame 8 and the base frame BF.

[0055] Depending on the mass inertia of the bearing frame 8 and the balance mass BM, a resulting rotational displacement < BM of the balance mass BM at the pivot will result in a certain maximum vertical displacement SZBMZWS towards the lower end of the substrate table WS. The amount of allowable local Z-displacement SZBMZWS determines how much extra passive mass 28, 30 may have to be added to the bearing frame 8.

[0056] Note that the extra inertia mass 28, 30 of the bearing frame 8 does not increase the vertical distance 1COG,Z between the respective centres of gravity of the balance mass and the substrate table, as the mass of the balance mass BM can move in horizonal direction with respect to the bearing frame 8, within limits as set by, for instance, the controller 14. Furthermore, rotation of the bearing frame 8 around COGBM is enforced by the pivot. The bearing frame 8 typically has its own centre of gravity CoGframe positioned lower than the COGBM- Therefore, in order to let the frame 8 turn around the pivot 20, a force at the pivot will be generated. This force is the centrifugal force and its magnitude is given

[0057] Herein, rCoG rameis the radius defining over which circular trajectory the CoG of the frame moves with respect to the pivot point 20. By design, <pBMwill be small (for instance on the order of 0.003 rad / s) and pBMwill even be smaller (for instance on the order of 9e-6 rad / s). So, the impact of this force generating vertical and horizontal dump forces on the pivot 20 and therefore on the floor may be regarded as negligible.

[0058] As the substrate table WS can move in horizontal direction, and therefore can be on different XY-positions on the top surface of the balance mass BM, the gravitational forces of the substrate table WS may result in quasi-static tilting of the BM on its pivot 20. To obviate or limit such tilting within apredetermined range, active drift position feedback control, including integrator(s), may be added in the vertical direction. See the vertical controller 32 in Figure 3.

[0059] The drift controller 32 can also be used to optimize a balance between adding extra mass 28, 30 to the bearing frame 8 (to limit SZBMZWS motion) or to except some vertical force (fz,dumP) to the base frame BF as a result of a more aggressive setting of the drift feedback controller 32. Modelling have indicated the improvements of the concept.

[0060] Generally referring to Figure 4, in another embodiment, the added mass 28, 30 to the bearing frame 8 can be significantly reduced. Herein, the vertical controller 32 is arranged between the bearing frame 8 and the base frame BF. The bearing frame 8 is connected to first and second mass elements 40, 42. One or more first actuators 44 may be arranged between the bearing frame 8 and the base frame BF. The bearing frame is, for instance, connected to the mass elements via one or more second actuators 46. The mass elements 40, 42 may be connected to the base frame via a spring 48, 50.

[0061] The second embodiment of the substrate stage system 1 shown in Figure 4 resembles the first embodiment as shown in Figure 3 in terms of operating principle. Hence, the inertia of the balance mass BM and bearing frame 8 is exploited in absorbing the wafer table WS induced torque on the balance mass BM. However, an actuation concept towards the bearing frame 8 is realized that allows for larger actuation forces to be exerted on the bearing frame 8 without increasing the final vertically directed Z- dump forces towards the floor.

[0062] The first actuator(s) 44 and second actuator (s) 46 enable increased actuation forces, which in turn allow to reduce the mass of, or added to, the bearing frame 8 to limit SZBMZWS-

[0063] The total vertical actuation force on the bearing frame 8 may be split into two actuation paths, for instance fAct,i and fAct,2- The first actuation path fAct,i can be used for drift control, including controllers or integrator(s) 14, 32, to keep the balance mass BM levelled in view of quasi-static disturbances. The second actuation path fAct,2 can be used for dumping forces on a reaction or suspension mass 40, 42, indicated in Figure 4 by msusp.

[0064] The low transmissibility (beyond the suspension frequency of msusp) between fAct,2 and the base frame or floor reduces the forces as a result of fAct,2 onto the floor. This enables larger overall controller actuation forces to limit 5ZBM2WS to a set maximum threshold.

[0065] To keep the displacements of msuspsufficiently small, to enable the use of relatively inexpensive actuators, and to limit the volume of the design, a certain amount of mass may still required for msusp. However, said mass can be significantly less than the mass as added to the bearing frame 8 in the first embodiment. This comes at the expense of energy consumption of the first and second actuators. A trade of in the design can be made by allowing extra mass to the bearing frame 8.

[0066] When using one or more suspension masses 40, 42, exciting suspended vibrational modes may be required. At this frequency, the transmissibility is relatively high, hence actuation forces on the mass 40, 42 may be amplified and transmitted towards the floor. A typical suspension mode frequency that can be realized with relatively inexpensive, readily available mass suspension springs 48, 50 can bearound 3 Hz. A way to prevent exciting the suspension mode frequency is to use proper filter settings in generating fAct,i and fAct, 2 based upon a total controller actuator input fAct, tot- Figure 5 provides an exemplary illustration how this split actuation path control, associated to the second embodiment of Figure 4, may be realized including reaction mass suspension mode excitation reduction.

[0067] Figure 5 shows a diagram exemplifying actuation path control associated to the embodiment shown in Figure 4. The lower plot in Figure 5 indicates a bode plot 60 of the filter fAct, Split- The filter is shown in the upper diagram of Figure 5. The bode plot also shows the resulting { 1- fAct, Split (s)} bode plot 62 for actuation paths 2 and 1, respectively. The dashed lines 68, 70 without notch indicates forces used to mitigate suspension mode excitation. The solid lines 60, 62 with notch 64, 66 indicate a filter mode used to prevent suspension mode excitation of the suspension mass 40, 42.

[0068] The filter structure that is used for fAct, Split may comprise a second order high-pass filter, to prevent additional DC forces via fAct, 2 on the suspension mass 40, 42 to keep displacement range of the mass minimal, enabling a relatively cheap and straightforward actuator design. Furthermore, a notch filter (see Figure 5) may be added in series to minimize excitation of the suspension mode of the reaction mass. The force fAct,i is now generated, given fAct, tot, via { 1- fAct, split} , which makes that the overall gain from a total actuation command towards the total physical force on the bearing frame 8 is equal to 1. The latter simplifies the design of a FeedBack (FB) and optionally an additional FeedForward (FF) controller to limit 5ZBM2WS-

[0069] Herein, it is preferred that the wafer table WS scan harmonics exceed the designed suspension mode frequency of msusp. It may be relatively difficult to design for suspension frequencies lower than about 1.5 Hz. If the latter is required, the third embodiment as shown in Figure 6 provides a solution where this disadvantage is removed.

[0070] Generally referring to Figure 6, a third embodiment of the substrate stage system 1 resembles the embodiment shown in Figure 4 in terms of operating principle. However, the embodiment creates a suspended reaction mass with a relatively low suspension frequency. Said suspension frequency may, in theory, be about 0. This is done by adding a second rotation frame 80. Herein, the bearing frame may be regarded as a first rotation frame, or rotatable frame. The second rotatable frame 80 has a pivot point 82 at its centre of gravity CoG. At said pivot point 82, the second frame 80 is rotatably connected to the support structure 22. The second frame 80 may be provided with one or more mass elements 84. Said mass elements 84 may be provided at radial ends of the second frame. The second frame 80 may be connected to the bearing frame 8 via, for instance, one or more spring elements 86. The spring elements may comprise a spring, a rubber or elastomer, etc. One or more first actuators 88 may be provided connecting the base frame to the bearing frame 8, to provide first actuation force fAct,i- One or more second actuators 90 may be provided connecting the bearing frame 8 to the second rotatable frame 80, to provide second actuation force fAct, 2.

[0071] In a practical embodiment, resonance frequencies of the second frame 80 exceed the highest scan harmonics of significance (for instance about 50 Hz) to prevent significant frame vibrations. Suchvibrations might be transmitted to the base frame. The second frame 80 may be provided with pairs of opposing reaction masses 84. In operation, said mass elements 84 balance each other above the base frame BF and / or the floor. Hence, creating a very low stiffness spring 86 is relatively simple, or may even be obviated, because the spring elements 86 do not need to support the suspension mass gravity forces anymore.

[0072] To keep the second frame 80 balanced and within actuation range toward the bearing frame 8, a relatively simple drift control-loop may be implemented. Such control loop could include for instance active or passive springs and / or dampers, as illustrated in Figure 6 by means of the spring elements 86.

[0073] For the embodiment shown in Figure 6, it is possible to obviate the pivot point 20 of the bearing frame 8. In other words, it is possible to fixedly connect the bearing frame 8 to the support structure 22. Herein, it is assumed that the exact reaction torque can be generated over time by fAct,2-

[0074] However, having the inertia of the balance mass BM and the bearing frame 8 both contributing to torque absorption, which is enabled by the pivot of both the balance mass and the second frame 80, avoids having to generate an exact reaction torque. The latter provides added benefit, for instance to realize torque isolation with respect to the base frame for higher harmonics, which excite the flexible dynamics of the entire system 1, making those relatively challenging to effectively absorb actively. Furthermore, also energy consumption can be lower, by minimizing the pivoting by using both the mass inertia of the balance mass BM and the bearing frame 8 to enable lowering higher root-mean square (RMS) loads through actuation via fAct,2-

[0075] Generally referring to Figure 7, in another embodiment, the system may comprise one or more first actuators 88 connecting the base frame to the bearing frame 8, to provide first actuation force fAct,i- However, instead of second actuators, the bearing frame 8 is provided with one or more rotor elements 94. The rotors 94 function as disturbance suppression elements. The rotors have a certain mass and inertia, and can be actively rotated.

[0076] Herein, the support structure 22 may be provided with a first stop element 96, such as a shoulder or flange. The bearing frame may be provided with a second stop element 98 fitting within a predetermined tolerance to the first stop element 96. Together, the first and second stop element 96, 98 determine a range of rotation and movement of the bearing frame with respect to the pivot point 20.

[0077] In the embodiment of Fig. 7, an additional reaction torque in response to the balance mass torque TBM can be generated by accelerating and de-accelerating inertias of the rotors 94 mounted on the bearing frame 8.

[0078] Herein below, an example is provided representing a case wherein 100% of the torque TBM has to be absorbed by the rotors 94. Herein, parameters of the lithographic apparatus are assumed as exemplified in Table 1 above. The following rotor parameters are assumed [table 2]:

[0079] Using the parameters indicated above, the reaction torque (for a pure wafer table WS x- acceleration setpoint) would need to be about 122*66*0.13=1047 Nm. The system 1 may, for instance, be provided with two rotational actuators and two rotors 94 per torque actuation direction, as depicted in Figure 7. This means that each actuator would need to generate about 1047 Nm / 4 = 262 Nm.

[0080] The material of the rotor 94 may comprise steel of a suitable composition, strength and durability.

[0081] The time duration of the x-acceleration of the substrate table WS in the test may be assumed to be about 0.017 s. Furthermore, the rotor acceleration or deceleration would be ( '_rot = 262 Nm / J \cl= 1048 rad / s2. Hence, a rotor speed of approximately (1048*0.017*60) / (2*pi) = 170 rpm would be needed.

[0082] Actuators available on the market may be able to provide these types of parameters, and rotational speeds. For instance, an actuator in the QTL 210 series as marketed by Tecnotion, having a height of 65 mm, may be a suitable actuator to drive the rotors 94.

[0083] A point of attention is the air bearing design of the rotors 94 and the connection of the rotors to the bearing frame 8. Herein, typically any connections provoking eigenfrequencies exceeding the highest scan harmonics may have to be absorbed. In a practical embodiment, this indicates eigenfrequencies exceeding 50 Hz.

[0084] Also in the embodiment of Figure 7, optionally, the bearing frame 8 may be fixedly connected to the support structure 22, obviating the pivot point 20. Herein, it is assumed that the exact reaction torque can be generated over time by the torque TAct,2 of the rotors 94.

[0085] However, in a practical embodiment, the latter may not be preferred, in particular not when the connection of the bearing frame to the base frame is relatively stiff, such as when using bearing frame feet 10, 12 towards the base frame, as indicated in Figure 2. The actuated reaction torque would be partially going towards the floor. So, having both the inertia of the balance mass BM and the inertia of the bearing frame 8 contributing to torque absorption (which is enabled by the pivot point 20) is relatively beneficial. Benefits thereof allow to realize torque isolation to the floor for higher scan harmonics, which excite the flexible dynamics of the total structure of the system 1, making it relatively challenging to effectively absorb these. Furthermore, using both the inertia of the balance mass BM and the bearing frame 8 to reduce high RMS loads through the torque TAct,2 caused by rotor actuation is beneficial in terms of lower energy consumption.

[0086] Generally referring to Figure 8, in yet another embodiment, the balance mass may be provided with one or more actively controllable rotors 94. The rotor(s) 94 can be connected to the balance mass via a rotor bearing 100. The balance mass BM may be able to pivot with respect to the bearing frame 8 as in addition to bearings 4, 6, one or more spring elements and / or dampers 102 may be provided connecting the balance mass to the bearing frame 8. The bearing frame itself can be connected to the base frame BF via support legs 10, 12. Although Figure 8 only shows a single rotor 94, the balance mass may be provided with two or more rotors. Rotors and the number of rotors 94 may be scaled up in size compared to example specification from Table 2 above. For instance, the balance mass BM may be provided with four rotors 94, arranged in symmetric fashion with respect to the centre of gravity COGBM- This realizes mass symmetry, which improves stability.

[0087] As with the embodiment of Figure 7, the mass of each rotor 94 may be relatively limited, while still being able to generate a relatively large part of the torque as induced by movement of the wafer table WS. Furthermore, also the size of the rotor 94 and the actuator are relatively small, in terms of required volume. Hence, it is possible to fit one or more rotors 94 in the movable parts of the balance mass BM. Tests have indicated that a typical rotor 94 as suitable for this embodiment may fit in the balance mass plate of the substrate stage system of a current lithographic apparatus.

[0088] In the embodiment shown in Figure 8, the one or more rotors 94 are substantially similar to the rotors 94 shown in Figure 7. However, the rotors 94 are directly mounted onto the balance mass BM instead of a pivotable bearing frame. The rest of the embodiment of Figure 8 is substantially unchanged, i.e. similar to the system 1 shown in Figure 2. In order to isolate the torque induced by acceleration and other movements of the substrate table WS and obviate said torque from transmitting to the base frame BF, most if not all of said torque will be absorbed by rotation of the one or more rotors 94. This is feasible in practice, yet requires active control of the rotors 94 and consequently consumes energy.

[0089] In addition, the spinning rotors 94 may also be subject to rotations around the Z-axis, i.e. Rz. For instance, if a substrate table WS is accelerating in a horizontal direction, say along the x-axis, on an outer edge of the balance mass BM, this will induce an Rz motion of the balance mass BM. Said motion will generate a torque around the horizontal axis due to the gyroscopic effect of the spinning rotor 94. Said torque may be compensated for. For the example rotors 94 in this disclosure, and the spinning velocity the rotors 94 reach, the gyroscopic effect is expected to be relatively small (~1%) compared to the overall torque induces by acceleration of the substrate table WS.

[0090] The one or more rotors 94 can be provided with electrical power using, for instance, dynamic link power cables (not shown) leading from the base frame BF towards the balance mass BM.

[0091] The embodiment of Figure 8 has an additional benefit, as it can be retrofitted relatively easily to existing systems. The system 1 of Figure 8 fits within the volume available in existing lithographic machines. The latter makes the embodiment of Figure 8 a readily implementable options to mitigate torques and disturbance due to movement of the substrate stage WS, with only limited hardware adaptation.

[0092] Referring to Figure 8, instead of using the rotor based direct torque actuation to directly compensate the torque induced by acceleration of the substrate table WS, one could also limit the torque actuation energy to, for example, only actively dampen to improve the dynamic transfer from substrate table WS acceleration towards dumped Z-force.

[0093] Figure 8 thus provides an alternative balance mass BM concept. Herein, the rotors 94 are used for active damping of, for instance, the rocking mode of the balance mass BM. The latter is illustrated by the balance mass BM which is suspended on relatively weak springs 102 towards the base frame BF and the floor.

[0094] Figure 9 shows yet another embodiment of a substrate stage system 1. The bearing frame can pivot with respect to a pivot point 20 at the top of the support structure 22. One or more actuators 110 are provided, connecting the balance mass BM to the bearing frame 8. The actuators 110 may be linear actuators, for instance comprising a cylinder 112 and piston 114.

[0095] Herein, actuation is considered directly between the balance mass BM and the (air) bearing frame 8. The bearing frame pivot point 20 is moved towards the location of the centre of gravity CoGs of the bearing frame (instead of the centre of gravity of the balance mass BM). The latter is done to minimize translational forces to the base frame BF and pivot when enforcing higher rotational motion of the bearing frame 8.

[0096] With the enabled increased motion range between the balance mass BM and the bearing frame, the inertia of the bearing frame 8 is effectively used to generate relatively high reaction forces in vertical direction to the balance mass BM in order to absorb the torque induces by acceleration of the substrate table WS. The system 1 of Figure 9 is actively controlled, including the air bearing functionality towards the balance mass BM in combination with a vertical actuation of actuators 110. Also, horizontal actuation of the balance mass BM is included, using, for instance, the controllers 14 and 32.

[0097] In a practical embodiment, the typical range of vertical actuation required by actuators 110 may be on the order of several millimetres. The latter would enable higher order of magnitudes of vibrations to be absorbed. The embodiment would be suitable for use with a 500 wafers per hour (wph) lithographic apparatus, as envisaged for future systems. The actuators 110 may be controlled using a control loop between the bearing frame 8 and the balance mass BM. The control loop may track a relative setpoint between the bearing frame and the balance mass BM, allowing to effectively absorb the torque and to provide the correct nominal positioning of the bearing frame 8.

[0098] A second control loop may be used to maintain the magnet plate of the balance mass BM in its correct vertical position, within a certain vertical range with respect to the substrate table WS, i.e. 5ZBM2WS- The second control loop may be a typical low bandwidth control loop with at least an integrator function. The second control loop may optionally include a quasi-static force compensation, to compensated for the gravitation force of the substrate table WS. Said force may change, depending on the location of the substrate table WS in XY-position with respect to the balance mass. If the setpoint feed-forward FF control of the bearing frame controller towards the balance mass BM is optimal, i.e.no disturbances, the second drift control function would not be needed. In this case, also the Z-dump forces from the bearing frame towards the floor will be 0, hence a perfect isolation of torque induced by acceleration of the substrate table WS with respect to the base frame BF.

[0099] Herein below, some generic quantifications of the improvements provided by respective embodiments of the disclosure are provided.

[0100] Via a first order modelling approach, the performance improvement in terms of the dumped Z- force onto the floor is illustrated. As opposed to the conventional BM architecture, where the torque on the balance mass BM is given by Equation 1, in the first embodiment, see Figure 3, the torque is given by:

[0101] To quantify the vertical motion of the balance mass BM at its edge towards the bottom of the substrate table WS, i.e. SZBMZWS, and the dumped Z-force resulting from a feedback controller, the equation of motion for < BM is derived. Based on this SZBMZWS is determined.

[0102] Herein,

[0103] The dumped Z-force results from a feedback controller, which measures the local balance mass Z-position with respect to, for instance, the base frame BF. The resulting closed-loop system, with the WS acceleration setpoint aws as disturbance, is simulated for the parameters as given in Table 1 and other typical parameters for, for instance, a DUV based lithographic system.

[0104] The results of tests and simulations have indicated that the embodiments as disclosed herein can significantly reduce the exported Z-forces towards the floor, compared to conventional balance mass concepts and for future systems having higher throughput in the 500 wph range. As an example, for the model parameters considered in Table 1 and other parameters disclosed herein, the embodiments of the present disclosure can maintain the levels of excitation, currently for the 400 wph system, but at least up to the 500 wph throughput.

[0105] The worst case Z-displacement of the balance mass BM towards the substrate table WS may also remain within reasonable displacement levels when compared to the total fly-height of the substrate table WS with respect to the balance mass BM, i.e. about 1500 pm. To reduce the vertical displacements of the balance mass BM further, without compromising on exported Z-force dump, more mass can be attached to the (air) bearing frame 8. However, adding mass requires structural integrity and volume. If volume is not sufficiently available, the second embodiment as shown in Figure 4 enables to reduce a significant amount of mass on the bearing frame.

[0106] The present disclosure provides several concepts suitable to reduce the impact of the torque disturbance caused by movement and acceleration of the substrate stage towards a z-force (vertically directed force) dump connected to the floor. Multiple concepts are disclosed herein. Concepts A, B, C and D have in common that the inertia mass of the balance mass unit and the support frame is exploited to realize passive absorption of the torque induced by the wafer stage acceleration setpoint. Concepts E and F correct the torque induced by the wafer stage movements directly and / or actively on the horizontally moving balance mass. The most basic version of the embodiments as disclosed herein allows the balance mass to rotate around its center of gravity (CoG) by creating a pivoting point of the conventional bearing frame toward the floor whereby it also includes an air bearing frame.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0111] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A wafer stage system, comprising: a wafer stage (WS) configured to planar ly move a wafer arrangeable on the wafer stage relative to a base frame, a balance mass (BM) supporting the wafer stage, and a bearing frame (8) connected to the balance mass via one or more bearings (4, 6), wherein the balance mass is configured to rotate around its center of gravity via a pivot point (20) of the bearing frame.

2. The system of claim 1 , wherein the bearing frame is pivotably connected to a support structure (22) at the pivot point (20).

3. The system of claim 1 or 2, wherein the pivot point (20) is elevated with respect to the bearing frame, the bearing frame comprising an elevated middle section.

4. The system of claim 3, wherein the pivot point (20) of the bearing frame coincides with the balance mass center of gravity (COGBM).

5. The system of one of the previous claims, wherein the bearing frame is provided with one or more weights (28, 30).

6. The system of one of the previous claims, wherein the wafer stage (WS) is adapted to planarly move over a top surface of the balance mass (BM).

7. The system of one of the previous claims, comprising: at least one first actuator (44) connected between the bearing frame (8) and the base frame (BF); and at least one suspension mass (40, 42) connected to the bearing frame (8) via at least one second actuator (46)8. The system of claim 7, wherein the at least one second actuator (46) is connected to the base frame (BF) via a spring and / or damper (48, 50).

9. The system of one of the previous claims, comprising a second support frame (80) arranged below the bearing frame (8) and which can rotate with respect to said bearing frame.

10. The system of claim 9, wherein the second support frame (80) is rotatably connected to the support structure (22) at a second pivot point (82), wherein the bearing frame (8) is connected to the base frame (BF) via first actuators (88); and wherein the second support frame (80) is provided with at least one suspension mass (84) which is connected to the bearing frame (8) via at least one second actuator (90).

11. The system of one of the previous claims, wherein the bearing frame (8) is provided with at least one rotor (94) for providing a reaction torque.

12. A wafer stage system comprising: a wafer stage (WS) configured to planar ly move a wafer arrangeable on the wafer stage relative to a base frame, a balance mass (BM) supporting the wafer stage , a plurality of balance mass bearings (4, 6) arranged between the balance mass and a bearing frame (8), wherein the balance mass (BM) is provided with at least one rotor (94).

13. A wafer stage system comprising: a wafer stage (WS) configured to planar ly move a wafer arrangeable on the wafer stage relative to a base frame (BF), a balance mass (BM) supporting the wafer stage, a plurality of balance mass bearings (4, 6) arranged between the balance mass and a bearing frame, wherein the bearing frame is configured to rotate around its center of gravity via a pivot point (20) of the bearing frame relative to the base frame, and one or move actuators connecting the bearing frame (8) to the balance mass (BM).

14. Lithographic apparatus comprising at least one wafer stage system according to one of claims 1 to 13.

15. Method, comprising the steps of: providing a balance mass (BM) supporting a wafer stage; connecting a bearing frame (8) to the balance mass via one or more bearings (4, 6), arranging a wafer on the wafer stage (WS); and planar ly moving the wafer stage relative to the balance mass; wherein the balance mass is configured to rotate around its center of gravity via a pivot point

Images