WO2013164145A1

WO2013164145A1 - Active torsion mode control for stage

Info

Publication number: WO2013164145A1
Application number: PCT/EP2013/056765
Authority: WO
Inventors: Christopher Ward
Original assignee: Asml Holding N.V.
Priority date: 2012-05-04
Filing date: 2013-03-28
Publication date: 2013-11-07

Abstract

A lithographic apparatus includes an illumination system, a substrate table, a projection system, and a sensor. The illumination system is configured to condition a radiation beam and a support constructed to support a patterning device. The patterning device is capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam. The substrate table is constructed to hold a substrate. The projection system is configured to project the patterned radiation beam onto a target portion of the substrate. The sensor is configured to detect a shear stress along at least a portion of a length of the support and one or more actuators configured to apply one or more torsional mode dampening forces to the support to counteract the detected shear stress.

Description

ACTIVE TORSION MODE CONTROL FOR STAGE BACKGROUND

Field of the Invention

[0001] The present invention relates to a lithographic apparatus and controlling mechanical resonances of components within the apparatus.

Background Art

[0002] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the "scanning" -direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0003] Servo stages, such as the reticle stage or wafer (substrate) stage, will always have mechanical resonances. These mechanical resonances can cause instability of the servo controllers, unless proper care is taken in the controller tuning. Typical design methodology is to design the mechanics to be stiff and lightweight, such that the resonances are at as high a frequency as possible. This method reduces the impact, but is eventually limited by available material properties and dimensional constraints. Even with the resonances at high frequency, it is typically required to use notch or low pass filters in the feedback controller design to ensure controller stability. These filters will always have a non-desired impact on the phase delay behavior and result in degraded servo control performance compared to the ideal case (no resonances and no filters).

[0004] In addition to servo stability and performance issues, the resonances can also cause deformations of the stage (chuck). These resonances can result in motion of the point of interest (the exposure slit in case of reticle scanning), which is not measurable by the feedback measurement system, or vice versa. Ultimately this would lead to degraded servo and imaging performance.

SUMMARY

[0005] Therefore, what is needed is a system and method to actively dampen mechanical resonances, and in particular torsional mode resonance, of a stage within a lithographic apparatus.

[0006] According to an aspect of the present invention, there is provided a lithographic apparatus that includes an illumination system configured to condition a radiation beam, a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, a substrate table constructed to hold a substrate, and a projection system configured to project the patterned radiation beam onto a target portion of the substrate. The lithographic apparatus further includes a sensor configured to detect a shear stress along at least a portion of a length of the support and one or more actuators configured to apply one or more torsional mode dampening forces to the support based on the detected shear stress.

[0007] According to another aspect of the present invention, there is provided an apparatus having a support, a pattern disposed along a length of the support, and one or more actuators coupled to the support. The support is configured to move in response to an actuation signal. The pattern is configured to provide a shear stress measurement along at least a portion of the length of the support when measured by a sensor. The one or more actuators are configured to dampen an excited torsional mode of the support in response to the measured shear stress.

[0008] According to another aspect of the present invention, there is provided a method that includes measuring a shear stress of a support and dampening a torsional mode of the support. The shear stress is measured via a pattern disposed along a length of the support and the torsional mode of the support is dampened via one or more actuators coupled to the support based on the measured shear stress.

[0009] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0010] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention.

[0011] Figure 1 depicts a lithographic apparatus, according to an embodiment of the invention.

[0012] Figure 2 depicts a top view of a support, according to an embodiment of the invention.

[0013] Figure 3 depicts a side view of a support, according to an embodiment of the invention.

[0014] Figures 4-6 depict models of a torsional mode of a support at various views, according to embodiments of the invention.

[0015] Figure 7 depicts a feedback loop for controlling a mechanical resonance of a support, according to an embodiment of the invention.

[0016] Figure 8 depicts a flowchart of a method for dampening a torsional mode of a support, according to an embodiment of the invention.

[0017] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number. DETAILED DESCRIPTION

[0018] This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the claims appended hereto.

[0019] The embodiment(s) described, and references in the specification to "one embodiment", "an embodiment", "an example embodiment", etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0020] 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 disk 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.

[0021] Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

[0022] Figure 1 schematically shows a lithographic apparatus LAP including a source collector module SO according to an embodiment of the invention. The apparatus includes: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation); a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (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; and a projection system (e.g., a reflective projection 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.

[0023] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0024] The support structure supports, i.e., bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device."

[0025] The term "patterning device" used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0026] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0027] The term "projection system" used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, 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".

[0028] As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

[0029] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0030] The lithographic apparatus may also 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 and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term "immersion" as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

[0031] Referring to figure 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0032] The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross section.

[0033] The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device. 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 position sensor IF (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.

[0034] The depicted apparatus could be used in at least one of the following modes:

[0035] 1. In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

[0036] 2. In scan mode, the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.

[0037] 3. In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0038] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0039] Figure 2 illustrates a top view of a support system 200, according to an embodiment. Support system 200 may represent, for example, either mask table MT or substrate stage WT illustrated in Figure 1. Support system 200 includes a movable body 202 and one or more actuators 206a-d. During certain modes, support system 200 may also include a supported object 204 disposed on movable body 202. For example, supported object 204 may be a patterning device or a wafer (substrate). Support system 200 is not intended to be limited by what object (if any) it supports nor the exact placement of the supported object. Note that the z-axis (or z-direction) is referred to as the vertical axis as an example of one application, however, it should be appreciated that the orientation of moveable body 202 and the coordinate system can have any arbitrary orientation.

[0040] In one example, actuators 206a-d are configured to impart forces upon movable body 202 for at least one of stabilization, translation, rotation, etc., of moveable body 202. In one example, actuators 206a-d can impart forces substantially in the z-direction (into and out of the page) to moveable body 202. The vertical forces can be used to cause various rotations and/or translations of movable body 202. By applying the same vertical force to each of the actuators 206a-d, a translation of the stage in the z-direction (Z) can also be accomplished. In one example, a rotation about the x-axis (R_x) may be performed by applying positive vertical forces from the set of actuators 206a and 206d and negative vertical forces from the set of actuators 206b and 206c, and visa-versa. If the applied forces are equal, then there is zero net force in the z-direction and pure rotation about the x-axis (R_x). In another example, a rotation about the y-axis (R_y) is performed by applying positive vertical forces from the set of actuators 206a and 206b and negative vertical forces from the set of actuators 206c and 206d, and visa- versa. If the applied forces are equal, then there is zero net force in the z-direction and pure rotation about the y-axis (R_y). The ability to use the actuators 206a-d to apply vertical forces with opposite signs between actuators is leveraged for dampening the primary torsional mode associated with moveable body 202 as discussed in further detail below.

[0041] It should be understood that not all four actuators are required to rotate movable body 202 around either the x- or y-axis. For example, only two actuators may be used, such as actuators 206a and 206d, with applied forces of opposite sign to cause a rotation around the y-axis. A similar situation may be realized for rotation around the x-axis. Other examples may involve the use of three actuators to rotate either around the x- or y- axis where the forces applied by each of the three actuators are not equal in magnitude.

[0042] Moveable body 202 may include more actuators than those illustrated in Figure 2.

For example, moveable body 202 may include actuators coupled to the sides or the bottom surface. The various other actuators may be included to impart for, example, x- axis translation, y-axis translation, or R_z rotation. In essence, movable body 202 may include enough actuators to excite moveable body 202 in all six degrees of freedom. In an embodiment, moveable body 202 includes redundant z-axis actuators (e.g., more actuators than are necessary to control R_x, R_y, and Z such that there are more actuators than there are degrees of freedom). The redundancy of the actuators at the corners of moveable body 202 may be used to dampen the primary torsional mode associated with moveable body 202. In an embodiment, applying alternating forces with the redundant z- axis actuators is in the nullspace of the nominal 3 degrees of freedom. The actuators may apply forces in a direction parallel to an axis of translation at each corner of moveable body 202.

[0043] Figure 3 illustrates a side view of support system 200, according to an embodiment. A pattern 302 is included along at least a portion of a length of moveable body 202. Pattern 302 may be any linear pattern, e.g., a scale, a grating, etc., that can be detected from an external sensor (not shown). Pattern 302 may be included to detect the position of moveable body 202. Similarly, pattern 302 may be used to detect any rotational displacement of moveable body 202.

[0044] In one example, pattern 302 is a diffraction grating having a known pitch and spacing between grating elements. An external optical sensor may be used to reflect light off of the grating and collect the scattered light. In one example, a digital signature of the collected light may be compared to a reference to determine any positional changes of moveable body 202. An interferometric technique may be performed as well by the optical sensor on the grating pattern to determine a positional shift of pattern 302.

[0045] Other types of patterns may be contemplated as well. Pattern 302 may be any one or combination of a magnetized strip, optically reflective/transmissive elements, and conductive elements. The sensor may also be any type of sensor for reading out information from the various pattern types. For example, the sensor may be an optical, magnetic, capacitive, or inductive sensor. The sensor may also be configured to measure eddy currents of areas having different permeability on pattern 302. The sensor may include one or more photodiodes, a CCD array, a CMOS array, or the like for transducing a received optical signal into a voltage. [0046] Pattern 302 is not restricted to only being located on one side of moveable body

202. In an embodiment, pattern 302 exists on at least a portion of two opposite sides of moveable body 202. External sensors may be used to detect a difference in movement between the oppositely located patterns for determining, for example, rotational displacement of moveable body 202.

[0047] Figure 4 illustrates a three dimensional model of the first resonant mode of a reticle stage, according to an embodiment. Various regions 401a-i of the model have been patterned to indicate, for example, the deformation of that region in a given direction. In another example, each of regions 401a-i indicates a level of stress or strain present in that region of the model. Similar patterns are found on the models illustrated in Figures 5 and 6 as well. However, it should be understood that the same pattern may be used in various figures and represent a different characteristic in each figure.

[0048] In one embodiment, region 401a indicates areas of highest total deformation in the model while region 40 li indicate areas of lowest total deformation in the model. The regions 401b-h in-between may indicate levels of deformation that exist between the highest and lowest amount. In this example, the regions of highest total deformation are seen to exist at the corners while the regions of lowest total deformation are seen in the center portion of the stage.

[0049] The torsion mode occurs at a lower frequency than other major resonances of the movable stage, and is easily excitable to become unstable. In one example, the first resonant mode of the reticle stage occurs at around 460 Hertz. A typical method for avoiding instability involves notch filtering the output from multiple axes of servo controllers coupled to the reticle stage.

[0050] The torsional mode illustrated in Figure 4 may be controlled via, for example, dampening using actuators 406a-d positioned at the corners of the reticle stage. In one example, actuators 406a-d are oriented to apply a force substantially aligned with the principle axis of motion resulting from the torsional mode at the actuator locations. However, active dampening control requires accurate measurement of the mode. An example scanning reticle stage has a moving point of interest at the image slit. A typical feedback sensor layout places stationary sensors on a lenstop, with sensor targets on the moving stage. As a result, the feedback sensors' measurement point is not stationary with respect to the chuck, but instead moves relative to the chuck body as the stage is scanned. The torsion mode is primarily a vertical distortion of the chuck. The vertical motion of the torsion mode is position-dependent and therefore the measurement using the standard feedback sensors is time varying. The four corners distort up and down, but the center has zero vertical displacement as observed in Figure 4. At the midpoint the mode is completely unobservable vertically, and in the opposite directions, a sign of the displacement reverses. This makes the vertical sensors scanned over the stage unsuitable for modal control. One possible solution would be to use sensors which are stationary relative to the stage. However, the pre-existing stationary sensors are not of sufficient bandwidth for modal control.

[0051] Figure 5 illustrates a side view of a three dimensional model of the first resonant mode of the reticle stage from Figure 4, according to an embodiment. The side view provides a picture of the twisting action that occurs as a result of the excited torsion mode. In an embodiment, the various layers of pattern changes indicate changes in the lateral deformation (e.g., stress) in the direction of the y-axis as a result of the torsion mode. Thus, the excited torsion mode causes a relatively large vertical (z-axis) displacement at the corners of the support, but also a smaller horizontal (y-axis or x-axis) displacement along the length of the support.

[0052] In an embodiment, the smaller horizontal motion created by the resonant torsion mode is used to provide modal feedback to the vertical displacement actuators located at the corners of the support. The torsion mode creates a shearing of the chuck, where the top and bottom surfaces move in opposite directions. This shearing motion is of opposite signs on the opposing sides of the support, creating an R_z rotation of the support.

[0053] Figure 6 illustrates a bottom view of a three-dimensional model of the first resonant mode of the reticle stage from Figure 4, according to an embodiment. In an embodiment, the linear stress in the y-direction is observed to be opposite on either side of the reticle stage. For example, the pattern used in region 601a may represent a linear stress at a given magnitude in a first longitudinal direction while the pattern used in region 601b may represent a linear stress at the same magnitude in a second longitudinal direction opposite to the first direction.

[0054] A pattern (e.g., an encoder scale) disposed on both sides of the reticle stage may be used to measure the R_z rotation. In one example, one or more sensors are used to measure the difference between the patterns on opposite sides of the reticle stage to determine the amount of R_z rotation. The R_z motion at the pattern is independent of the stage scan position, and is suitable for use as a feedback signal to dampen the torsion mode. It is also possible to measure the torsion mode via an array of non-scanning sensors situated around the reticle stage, but such a design requires the use of extensive sensing hardware.

[0055] Figure 7 illustrates a feedback loop 700, according to an embodiment. For example, feedback loop 700 can be used for controlling actuators 206 coupled to support system 200. In one example, actuators 206 are controlled to dampen a measured torsion mode, however, any other modes may be measured and dampened as well using a similar feedback design. The feedback system may include a sensor 702, a filter 706, and a controller 710.

[0056] In one example, sensor 702 may be any type of sensor as described earlier with regard to Figure 3. Sensor 702 may continuously collect a signal 701 from a pattern located on support system 200. In another example, sensor 702 may be configured to collect signal 701 at discrete times, periodically, or continuously over a particular time window. Signal 701 may, for example, represent an R_z rotation (e.g., shear stress along the sides) of support system 200 caused by a torsion mode resonance of support system 200. Signal 701 may represent, for example, an optical, capacitive, magnetic, or inductive signal.

[0057] Signal 701 may be transduced into a voltage signal 704 and passed through a filter

706. In one example, filter 706 is a bandpass filter to reduce high frequency noise as well as to reduce the measurement of any R_z rotations not caused by the torsion mode. Filter 706 may also represent a combination of one or more filters, including low-pass filters, high-pass filters, bandpass fitlers, notch filters, etc. Additionally, the one or more filters represented by filter 706 are not limited by any particular order in their arrangement or construction.

[0058] A filtered signal 708 is passed from filter 706 to a controller 710, according to an embodiment, although it should be understood that filter 706 may also be an integrated component of controller 710. Based on a characteristic of filtered signal 708 (such as, for example, an amplitude, phase or frequency), controller 710 applies an actuation signal 712 to the one or more actuators 206 coupled to support system 200. In an embodiment, actuation signal 712 causes actuators 206 at the corners of support system 200 to provide vertical forces to dampen the torsional mode. Referring to the support system illustrated in Figure 2, actuation signal 712 may cause actuators 206a and 206c to provide a force in the +Z direction while actuators 206b and 206d provide a force in the -z direction. Alternating the sign of the applied force between the actuators compensates for the twisting motion caused by the torsional mode as illustrated in Figures 4 and 5. Thus, the torsional mode is dampened and the measured shear stress is counteracted as well. The feedback system may continue to apply torsional mode dampening forces until the measured shear stress falls below a threshold value or is removed altogether. Other applied actuation forces may be contemplated as well to various actuators for the purpose of dampening a variety of excited modes of support system 200 as would be understood to one having skill in the relevant art(s) given the description herein.

[0059] Controller 710 may be implemented in software, hardware, or any combination thereof. Controller 710 may utilize standard proportional and derivative control (e.g., a PD controller). Other filters may be included as well for filtering the actuation signal such as low pass or notch filters to reduce noise. Other feedback controller structures may be used as well, such as a lead-lag controller, as would be apparent to one having skill in the relevant art(s). The various parameters for controller 710 may be selected by using standard feedback controller tuning techniques, such as loop shaping methods.

[0060] It should be understood that although support system 200 has been described in terms of being either, for example, the reticle stage or wafer stage of a lithographic apparatus, it should not be limited by such descriptions. For example, support system 200 may represent one or more of the lenses or lens systems within a lithographic apparatus. Alternatively, support system 200 may represent any moving structure having resonant modes that can be dampened to improve the control of the structure.

[0061] Figure 8 illustrates an example torsional mode dampening method 800, according to an embodiment. Method 800 may be carried out using the various components illustrated in Figure 7. It is to be appreciated that method 800 may not require all steps shown or be performed in the order shown.

[0062] Method 800 begins at step 802 where a shear stress of a support is measured. A shear stress is created by a torsional mode of the support being excited. The shear stress may be measured via a pattern located on opposite sides of the support and one or more sensors configured to detect the movement of the patterns on either side. The movement caused by the shear stress may be correlated with an R_z rotation of the support.

[0063] At step 804, the measured shear stress is counteracted by dampening the excited torsional mode of the support. The torsional mode is dampened via one or more actuators coupled to the support. In an embodiment, the actuators are coupled to the corners of the support and apply forces substantially in a vertical (z) direction. Since the torsional mode causes an alternating movement direction at each corner of the support, the actuators may apply the vertical forces in opposing directions to counteract the movement caused by the torsional mode.

[0064] The steps of method 800 may be continually repeated until the torsional mode is completely damped out or dampened below a threshold amount. Method 800 may also include other actions such as steps involving the filtering of the measured shear stress. Additionally, method 800 may include steps directed to performing lithographical procedures such as imparting a pattern to beam of radiation via a patterning device on the support and projecting the patterned beam of radiation onto a target portion of a substrate.

[0065] In one embodiment of the present invention, a stage system is provided for a lithographic apparatus. The stage system comprises a stage and one or more actuators configured to act on the stage for controlling at least three degrees of freedom. For example, the stage may be a reticle stage that includes a chuck, which is configured to hold and scan a reticle. According to one embodiment of the present invention, the three degrees of freedom may be nominal three degrees of freedom Z, Rx, and Ry. The actuators may be configured to provide an input to the stage to create a mechanical resonance mode at a lower frequency than a majority of other resonances. For example, the input to the stage to create the mechanical resonance mode may be in the nullspace of the nominal three degrees of freedom Z, Rx, and Ry.

[0066] The stage system further comprises a sensor configured to measure a horizontal motion of the stage created by the mechanical resonance mode. In the stage system, a controller may be configured to apply a control signal as a feedback signal to the actuators to actively dampen the mechanical resonance mode of the stage. For example, the mechanical resonance mode may be a torsional mode of the stage. And the stage includes a chuck having first and second surfaces and the torsion mode creates a shearing of the chuck, such that the first and second surfaces of the chuck move in opposite directions. The chuck may include two X sides and two Y sides and a shearing motion may be of opposite signs on the two X sides of the chuck.

[0067] Consistent with one embodiment, a scale may be disposed along a length of the stage to provide a shear stress measurement along at least a portion of the length of the stage when measured by the sensor. For instance, the scale may be an encoder scale and the shearing is measured as an Rz rotation. A Rz motion at the scale is independent of a scan position of the stage, and may be used as the feedback signal. A Rz position measurement may be band pass filtered to be used as the feedback signal for controlling the torsion mode. The controller may be tuned based on the feedback signal to control the torsion mode to zero or near zero.

[0068] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0069] 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 may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0070] The terms "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

[0071] The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0072] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g., semiconductor memory, magnetic or optical disk) having such a computer program stored therein.

[0073] It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way.

[0074] The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0075] The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance.

The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A lithographic apparatus comprising:

an illumination system configured to condition a radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beam onto a target portion of the substrate;

a sensor configured to detect a shear stress along at least a portion of a length of the support; and

one or more actuators configured to apply one or more torsional mode dampening forces to the support to counteract the detected shear stress.

2. The lithographic apparatus of claim 1, wherein the one or more actuators are configured to apply the one or more torsional mode dampening forces in a direction substantially aligned with an axis of translation at locations of the one or more actuators caused by a torsional mode of the support.

3. The lithographic apparatus of claim 2, wherein the detected shear stress corresponds to a rotation of the support about an axis passing through a center of the support and substantially parallel to the axis of translation.

4. The lithographic apparatus of claim 1, wherein the one or more actuators are located substantially at corners of the support and apply the forces in opposing directions.

5. The lithographic apparatus of claim 1, wherein the support includes a pattern along a length of the support.

6. The lithographic apparatus of claim 5, wherein the pattern is a diffraction grating.

7. The lithographic apparatus of claim 6, wherein the sensor is configured to detect the shear stress via an optical signal interacting with the diffraction grating.

8. The lithographic apparatus of claim 1, wherein a signal corresponding to the detected shear stress is bandpass filtered.

9. The lithographic apparatus of claim 8, further comprising a controller configured to apply the bandpass filtered signal as a feedback signal to the one or more actuators to dampen the torsional mode of the support.

10. The lithographic apparatus of claim 1, further comprising a controller configured to apply a signal corresponding to the detected shear stress as a feedback signal to the one or more actuators to dampen the torsional mode of the support.

11. The lithographic apparatus of claim 1, further comprising a second sensor configured to detect a second shear stress along at least a portion of a length of the substrate table.

12. The lithographic apparatus of claim 11, further comprising one or more actuators configured to apply one or more torsional mode dampening forces to the substrate table to counteract the detected second shear stress.

13. The lithographic apparatus of claim 12, wherein the number of actuators is greater than a number of degrees of freedom associated with the substrate table.

14. The lithographic apparatus of claim 1, wherein the number of actuators is greater than a number of degrees of freedom associated with the support.

15. An apparatus comprising:

a support configured to move in response to an actuation signal; a scale disposed along a length of the support, wherein the scale is configured to provide a shear stress measurement along at least a portion of the length of the support when measured by a sensor; and

one or more actuators coupled to the support and configured to counteract the measured shear stress by dampening an excited torsional mode of the support.

16. The apparatus of claim 15, wherein the one or more actuators are configured to apply one or more torsional mode dampening forces in a direction substantially aligned with an axis of translation at locations of the one or more actuators caused by a torsional mode of the support.

17. The apparatus of claim 16, wherein the measured shear stress corresponds to a rotation of the support about an axis passing through a center of the support and substantially parallel to the axis of translation.

18. The apparatus of claim 15, wherein the one or more actuators are located substantially at corners of the support and apply forces in opposing directions.

19. The apparatus of claim 15, wherein the scale is a diffraction grating.

20. The apparatus of claim 19, wherein the sensor is configured to measure the shear stress via an optical signal associated with the diffraction grating.

21. The apparatus of claim 15, wherein a signal corresponding to the measured shear stress is passed through a bandpass filter.

22. The apparatus of claim 21, further comprising a controller configured to apply the filtered signal as a feedback signal to the one or more actuators to dampen the torsional mode of the support.

23. The apparatus of claim 15, further comprising a controller configured to apply a signal corresponding to the detected shear stress as a feedback signal to the one or more actuators to dampen the torsional mode of the support.

24. The apparatus of claim 15, wherein the number of actuators is greater than a number of degrees of freedom associated with the support.

25. A method comprising:

measuring, using a sensor, a shear stress of a support via a pattern disposed along a length of the support; and

counteracting the measured shear stress by dampening a torsional mode of the support via one or more actuators coupled to the support.

26. The method of claim 25, wherein the counteracting comprises applying one or more forces in a direction substantially aligned with an axis of translation at locations of the one or more actuators caused by the torsional mode of the support.

27. The method of claim 26, wherein the measuring comprises measuring a rotation of the support about an axis passing through a center of the support and substantially parallel to the axis of translation.

28. The method of claim 26, further comprising applying the one or more forces to corners of the support.

29. The method of claim 28, wherein the applying further comprises applying the forces in opposing directions.

30. The method of claim 25, further comprising bandpass filtering a signal corresponding to the measured shear stress.

31. The method of claim 30, further comprising applying the bandpass filtered signal as a feedback signal to the one or more actuators to dampen the torsional mode of the support.

32. The method of claim 25, further comprising applying a signal corresponding to the measured shear stress as a feedback signal to the one or more actuators to dampen the torsional mode of the support.

33. The method of claim 25, further comprising:

imparting a pattern to a beam of radiation via a patterning device supported by the support; and

projecting the patterned radiation beam onto a target portion of a substrate.

34. A stage system for a lithographic apparatus, the stage system comprising:

a stage;

one or more actuators configured to act on the stage for controlling at least three degrees of freedom, the one or more actuators are configured to provide an input to the stage to create a mechanical resonance mode at a lower frequency than a majority of other resonances;

a sensor configured to measure a horizontal motion of the stage created by the mechanical resonance mode; and

a controller configured to apply a control signal as a feedback signal to the one or more actuators to actively dampen the mechanical resonance mode of the stage.

35. The stage system according to claim 34, wherein the at least three degrees of freedom are nominal three degrees of freedom Z, Rx, and Ry and the input to the stage to create the mechanical resonance mode is in the nullspace of the nominal three degrees of freedom Z, Rx, and Ry.

36. The stage system according to claim 34, wherein the stage is a reticle stage that includes a chuck, the chuck is configured to hold and scan a reticle.

37. The stage system according to claim 34, wherein the mechanical resonance mode is a torsional mode of the stage.

38. The stage system according to claim 37, wherein: the stage includes a chuck having first and second surfaces and the torsion mode creates a shearing of the chuck, and

the first and second surfaces of the chuck move in opposite directions.

39. The stage system according to claim 38, wherein the chuck includes two X sides and two Y sides and a shearing motion is of opposite signs on the two X sides of the chuck.

40. The stage system according to claim 39, wherein a scale is disposed along a length of the stage and the scale is configured to provide a shear stress measurement along at least a portion of the length of the stage when measured by the sensor.

41. The stage system according to claim 40, wherein the scale is an encoder scale and the shearing is measured as an Rz rotation.

42. The stage system according to claim 40, wherein a Rz motion at the scale is independent of a scan position of the stage, and is used as the feedback signal.

43. The stage system according to claim 40, wherein a Rz position measurement is band pass filtered to use as the feedback signal for controlling the torsion mode.

44. The stage system according to claim 37, wherein the controller is tuned based on the feedback signal to control the torsion mode to zero or near zero.

45. A stage system for a lithographic apparatus, the stage system comprising:

a stage;

a sensor configured to sense a physical parameter value indicative of a shearing stress in the stage such that the shearing stress is created by the mechanical resonance mode; and a controller configured to apply a control signal based on the physical parameter value as a feedback signal to the one or more actuators to actively dampen the mechanical resonance mode of the stage.

46. The stage system according to claim 45, wherein the shearing sense sensed is representative of a deformation of the stage in a direction parallel to a further direction of movement of the stage in the operational use.