Reticle clamp, exposure apparatus including reticle clamp, and method

Abstract

Embodiments herein describe methods, devices, and systems for a reticle stage provided with an electrostatic clamp. The disclosure provides an electrostatic clamp system for holding an object by electrostatic clamping force, the clamp system comprising a clamp provided with a plurality of burls to support the object and at least one electrode configured to generate the electrostatic clamping force, and a controller connectable to the at least one electrode and configured to vary the electrostatic clamping force dependent on acceleration of the clamp.

Description

RETICLE CLAMP. EXPOSURE APPARATUS INCLUDING RETICLE CLAMP. AND METHODCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 705,345 which was filed on 9 October 2024, and which is incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates to a reticle clamp, an exposure apparatus including a reticle clamp, and a method for clamping a reticle. The reticle is for instance suitable to impart a pattern to radiation in lithographic processes and systems.BACKGROUND

[0003] An exposure apparatus is a machine constructed to apply a desired pattern onto a substrate. The exposure apparatus can be a lithographic apparatus. An exposure apparatus can be used, for example, in the manufacture of integrated circuits (ICs). An exposure apparatus may, for example, project a pattern of a patterning device (e.g., a mask, a reticle) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate an exposure apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features, which can be formed on the substrate. An exposure apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than an exposure apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] Reticles are critical and sensitive components for imparting patterns on substrates. Reticles contain the chip pattern to be printed on the substrate. A reticle handler may be used to move and position multiple reticles for scanning and / or patterning operations in an exposure apparatus. In EUV lithography, reticles are multilayer reflectors that use interference to reflect light from the pattern. For a description of the reticle stage and the operation thereof, reference is made to WO-20094467-A1, which is incorporated by reference herein in its entirety.

[0006] In the exposure apparatus, the reticle is typically clamped electrostatically on a clamp. The electrostatic clamp may typically be provided with so-called burls, i.e. protrusions, which can support a backside of the reticle. During operation of the exposure apparatus, reticles may need to be replaced. Typically, multiple changes per day are required. In the exposure apparatus, for instance, one reticle stage provided with a first reticle is replaced with another reticle stage carrying a second reticle. Herein, both reticle stages typically have to move from one location to another.

[0007] A problem with the electrostatic clamp of each reticle stage is that during acceleration of the respective stage, the shear stress between the reticle and the burls may cause unwanted material transfer, either from the reticle to the burls or vice versa. The material may cause all sorts of problems. Besides,given the miniscule structures to be created during lithographic processes, any form of contamination is preferably avoided in general.

[0008] The present disclosure aims to provide an improved clamp system, obviating the disadvantages as outlined above.SUMMARY

[0009] Accordingly, the present disclosure provides an electrostatic clamp system for holding an object by electrostatic clamping force, the clamp system comprising: a clamp provided with a plurality of burls to support the object and at least one electrode configured to generate the electrostatic clamping force, and a controller connectable to the electrode and configured to vary the electrostatic clamping force dependent on acceleration of the clamp.

[0010] In an embodiment, the controller is configured to increase the electrostatic clamping force when the clamp starts to accelerate.

[0011] In an embodiment, the controller is configured to vary the clamping force by varying an electrostatic clamping voltage with a slew rate in the range of 5 to 100 kV / sec, for instance 10 to 80 kV / sec, for instance 15 to 50 kV / sec.

[0012] According to another aspect, the disclosure provides an exposure apparatus comprising: an illumination system configured to condition a radiation beam; a support structure 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 projection system configured to project the patterned radiation beam onto a target portion of a substrate, and an electrostatic clamp system for holding an object by electrostatic clamping force, the clamp system comprising: a clamp provided with a plurality of burls to support the object and at least one electrode configured to generate the electrostatic clamping force, and a controller connectable to the electrode and configured to vary the electrostatic clamping force dependent on acceleration of the clamp. The exposure apparatus can be a lithographic apparatus.

[0013] In an embodiment, the electrostatic clamp system includes an electrostatic clamp system as described above.

[0014] According to another aspect, the disclosure provides a method of operating an electrostatic clamp system, the method comprising the steps of: providing an electrostatic clamp system for holding an object by electrostatic clamping force, the clamp system comprising a clamp provided with a plurality of burls to support the object and at least one electrode configured to generate the electrostatic clamping force,arranging an object on the plurality of burls of the clamp, accelerating or decelerating the clamp in accordance with a pattern, and varying the electrostatic clamping force dependent on the acceleration of the clamp.

[0015] In an embodiment, the step of varying the clamping force comprises increasing the electrostatic clamping force when the clamp starts to accelerate.

[0016] In an embodiment, the step of varying the clamping force comprises increasing the electrostatic clamping force between 1-50 milliseconds, preferably between 5-25 milliseconds, more preferably 10- 15 milliseconds, before the clamp starts to accelerate.

[0017] In an embodiment, the step of varying the clamping force comprises increasing an electrostatic voltage provided to the at least one electrode to an upper voltage threshold when the clamp starts to accelerate, and reducing the electrostatic voltage to a lower voltage threshold when the clamp ceases to accelerate.

[0018] In an embodiment, the step of varying the clamping force comprises increasing or decreasing an electrostatic voltage to generate the electrostatic clamping force between a lower force threshold and an upper force threshold.

[0019] In an embodiment, the lower voltage threshold is on the order of 1 to 5 kV, for instance 2 to 3 kV.

[0020] In an embodiment, the upper voltage threshold is in the range of 1.1 to 5 times the lower voltage threshold.

[0021] In an embodiment, the step of varying the clamping force includes varying the electrostatic clamping voltage with a slew rate in the range of 5 to 100 kV / sec, for instance 10 to 80 kV / sec, for instance 15 to 50 kV / sec.

[0022] Further features of the disclosure, as well as the structure and operation of various embodiments of the disclosure, are described in detail below with reference to the accompanying drawings. It is noted that the disclosure 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

[0023] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of the disclosure and to enable a person skilled in the relevant art(s) to make and use the disclosure. Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0024] FIG. 1 shows a schematic illustration of an exposure c apparatus, such as a lithographic apparatus, according to some embodiments.

[0025] FIG. 2 shows a perspective schematic illustration of a reticle stage.

[0026] FIG. 3 shows a top plan view of the reticle stage of Figure 2.

[0027] FIG. 4 shows a perspective schematic illustration of an exemplary reticle stage and reticle exchange apparatus.

[0028] FIG. 5 shows a side view of a schematic illustration of an exemplary reticle stage and reticle exchange apparatus.

[0029] FIG. 6 shows a schematic illustration of a reticle clamp and a reticle on a reticle base plate.

[0030] FIG. 7A shows a diagram of an exemplary cycle of velocity of a reticle stage.

[0031] FIG. 7B shows a diagram of an exemplary cycle of acceleration of the reticle stage of FIG. 7A.

[0032] FIG. 7C shows a diagram of a voltage to operate an electrostatic clamp relating to the cycles of FIGS. 7A and 7B in accordance with an embodiment of a method of the disclosure.

[0033] The features of the present disclosure 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. Additionally, generally, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.DETAILED DESCRIPTION

[0034] This specification discloses one or more embodiments that incorporate the features of this disclosure. The disclosed embodiment(s) merely exemplify the disclosure. The scope of the disclosure is not limited to the disclosed embodiment(s). The disclosure is defined by the claims appended hereto.

[0035] 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.

[0036] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, may be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

[0037] The term “about” as used herein indicates the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the term “about” can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

[0038] Embodiments of the disclosure may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the disclosure 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, and / or 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.

[0039] Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present disclosure may be implemented.

[0040] Exemplary Exposure System

[0041] FIG. 1 shows an exposure system comprising a radiation source SO and an exposure apparatus. The exposure apparatus is a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0042] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL can include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL can include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0043] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS can comprise a plurality of mirrors 13, 14 that are configured toproject the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS can apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 can be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in FIG. 1, the projection system PS can include a different number of mirrors (e.g. six or eight mirrors).

[0044] The substrate W can include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0045] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, can be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0046] The radiation source SO can be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a hydrogen plasma source, a free electron laser (FEL), or any other radiation source that is capable of generating EUV radiation.

[0047] Exemplary Reticle Stage

[0048] FIGS. 2 and 3 show schematic illustrations of an exemplary reticle stage 200, according to some embodiments. Reticle stage 200 can include top stage surface 202, bottom stage surface 204, side stage surfaces 206, and clamp 300. In some embodiments, reticle stage 200 with clamp 300 can be implemented in lithographic apparatus LA. For example, reticle stage 200 can be support structure MT in lithographic apparatus LA. In some embodiments, clamp 300 can be disposed on top stage surface 202. For example, as shown in FIG. 2, clamp 300 can be disposed at a center of top stage surface 202 with clamp frontside 302 facing perpendicularly away from top stage surface 202.

[0049] In some exposure apparatuses, for example, lithographic apparatus LA, a reticle stage 200 with a clamp 300 can be used to hold and position a reticle 408 for scanning or patterning operations. In one example, the reticle stage 200 can require powerful drives, large balance masses, and heavy frames to support it. In one example, the reticle stage 200 can have a large inertia and can weigh over 500 kg to propel and position a reticle 408 weighing about 0.5 kg. To accomplish reciprocating motions of the reticle 408, which are typically found in lithographic scanning or patterning operations, accelerating and decelerating forces can be provided by linear motors that drive the reticle stage 200.

[0050] In some embodiments, as shown in FIGS. 2 and 3, reticle stage 200 can include first encoder 212 and second encoder 214 for positioning operations. For example, first and second encoders 212, 214 can be interferometers. First encoder 212 can be attached along a first direction, for example, a transverse direction (i.e., X-direction) of reticle stage 200. And second encoder 214 can be attached along a second direction, for example, a longitudinal direction (i.e., Y-direction) of reticle stage 200. In some embodiments, as shown in FIGS. 2 and 3, first encoder 212 can be orthogonal to second encoder 214.

[0051] As shown in FIGS. 2 and 3, reticle stage 200 can include clamp 300. Clamp 300 is configured to hold reticle 408 in a fixed plane on reticle stage 200. Clamp 300 includes clamp frontside 302 and can be disposed on top stage surface 202. In some embodiments, clamp 300 can use mechanical, vacuum, electrostatic, or other suitable clamping techniques to hold and secure an object. In some embodiments, clamp 300 can be an electrostatic clamp, which can be configured to electrostatically clamp (i.e., hold) an object, for example, reticle 408 in a vacuum environment. Due to the requirement for EUV radiation to perform in a vacuum environment, vacuum clamps cannot be used to clamp a mask or reticle and instead electrostatic clamps can be used. For example, clamp 300 can include an electrode, a resistive layer on the electrode, a dielectric layer on the resistive layer, and burls projecting from the dielectric layer. In use, a voltage can be applied to clamp 300, for example, several kV. And current can flow through the resistive layer, such that the voltage at an upper surface of the resistive layer will substantially be the same as the voltage of the electrode and generate an electric field. Also, a Coulomb force, attractive force between electrically opposite charged particles, will attract an object to clamp 300 and hold the object in place. In some embodiments, clamp 300 can be a rigid material, for example, a metal, a dielectric, a ceramic, or a combination thereof.

[0052] Exemplary Reticle Exchange Apparatus

[0053] FIG. 4 shows a schematic illustration of an exemplary reticle exchange apparatus 100, according to some embodiments. Reticle exchange apparatus 100 can be configured to minimize reticle exchange time, particle generation, and contact forces or stresses from clamp 300 and / or reticle 408 to reduce damage to clamp 300 and reticle 408 and increase overall throughput in a reticle exchange process, for example, in an exposure apparatus LA.

[0054] As shown in FIG. 4, reticle exchange apparatus 100 can include reticle stage 200, clamp 300, and in-vacuum robot 400. In-vacuum robot 400 can include reticle handler 402.

[0055] Reticle handler 402 can be a rapid exchange device (RED), which is configured to efficiently rotate and minimize reticle exchange time. For example, reticle handler 402 can save time by moving multiple reticles from one position to another substantially simultaneously, instead of serially.

[0056] As shown in FIG. 4, reticle handler 402 can include one or more reticle handler arms 404. In some embodiments, reticle handler arms 404 may be referred to herein as rapid exchange device (RED) grippers. Reticle handler arm 404 can include reticle baseplate 406. Reticle baseplate 406 can be configured to hold an object, for example, reticle 408. In some embodiments, reticle baseplate 406 may be referred to herein as a reticle base frame.

[0057] Reticle baseplate 406 can be an extreme ultraviolet inner pod (EIP) for a reticle. In some embodiments, reticle baseplate 406 includes reticle baseplate frontside 407, and reticle 408 includes reticle backside 409.

[0058] As shown in FIG. 4, reticle baseplate 406 can hold reticle 408 such that reticle baseplate frontside 407 and reticle backside 409 each face top stage surface 202 and clamp frontside 302. Forexample, reticle baseplate frontside 407 and reticle backside 409 can be facing perpendicularly away from top stage surface 202 and clamp frontside 302.

[0059] As shown in FIG. 4, reticle handler arms 404 can be arranged symmetrically about reticle handler 402. For example, reticle handler arms 404 can be spaced from each other by about 90 degrees, 120 degrees, or 180 degrees. In some embodiments, reticle handler arms 404 can be arranged asymmetrically about reticle handler 402. For example, two reticle handler arms 404 can be spaced from each other by about 135 degrees, while another two reticle handler arms 404 can be spaced from each other by about 90 degrees.

[0060] In one example, during a reticle exchange process, reticle handler arm 404 of reticle handler 402 positions reticle 408 on reticle baseplate 406 towards clamp 300 in reticle exchange area 410. As described above, a reticle handoff from reticle handler 402 to clamp 300 includes an unknown reticle position offset, which includes a reticle vertical distance offset (i.e., Z-direction offset) and a reticle tilt offset (i.e., Rx offset and Ry offset). Tilt or excessive non-alignment between clamp 300 and reticle 408 can be a source of particle generation and can damage reticle 408 or clamp 300 over time. Reticle backside 409 and clamp frontside 302 should be in coplanar alignment for a final handoff. Despite calibration, variations still exist due to reticle mechanical and positioning tolerances, which can lead to high comer impacts and unpredictable first contact points between clamp 300 and reticle 408.

[0061] In one example, the reticle exchange process can involve lowering reticle stage 200 with clamp 300, which starts far away from reticle handler 402, as close to reticle 408 as possible until clamp 300 contacts reticle 408 to account for all possible offsets and / or tilts. During a reticle exchange process, reticle stage 200 with clamp 300 can be adjusted in a multi-stage movement.

[0062] As shown in FIG. 6, reticle exchange apparatus 100 can include clamp 300, reticle 408, and reticle baseplate 406. The multi-stage movement can occur in four stages: (1) approach; (2) first contact; (3) full contact; and (4) voltage applied to clamp.

[0063] Clamp 300 can be adjusted in a substantially vertical direction (i.e., Z-direction) toward reticle backside 409. In approach configuration 20, clamp 300 is turned off (i.e., no applied voltage) and reticle handler 402 deactivates the vertical direction (i.e., Z-direction) and tilt (i.e., Rx and Ry, rotation about X-direction and rotation about Y -direction, respectively) servo motors (not shown) of reticle handler arm 404 in reticle exchange area 410.

[0064] Clamp 300 can be adjusted in a substantially vertical direction (i.e., Z-direction) toward reticle backside 409 until clamp 300 makes contact with reticle backside 409. In first contact configuration 30, clamp 300 is turned off and clamp 300 makes contact with reticle backside 409, for example, a comer of reticle 408, and then rotates or tilts about the contact (i.e., Rx and Ry).

[0065] Reticle exchange apparatus 100 can be in a full contact configuration. Herein, clamp 300 can be rotationally adjusted about the contact (i.e., Rx and Ry) toward reticle backside 409 until clamp 300 makes full contact with reticle backside 409. In full contact configuration 40, clamp 300 is turned offand clamp 300 makes full contact with reticle backside 409, for example, all four comers of reticle 408, and is coplanar with reticle backside 409.

[0066] As shown in FIG. 5, reticle exchange apparatus 100 can include clamp controller 360. Clamp controller 360 can be coupled to clamp 300 and be configured to control a position of clamp 300. For example, clamp controller 360 can be configured to control reticle stage 200 to allow compliant movement of clamp 300.

[0067] Clamp controller 360 can be coupled to servo motors or servo actuators (i.e., X-direction, Y- direction, Z-direction, Rx, RY, RZ) of reticle stage 200 and / or clamp 300. For example, clamp controller 360 can control translations of reticle stage 200 with clamp 300 along an x-axis, y-axis, and z-axis (i.e., X-direction, Y-direction, Z-direction) and rotations about the x-axis, y-axis, and z-axis (i.e., Rx, RY, Rz), where the x-axis, y-axis, and z-axis are orthogonal coordinates.

[0068] Embodiment of a Reticle Clamp System and Method

[0069] As described above, a reticle is a critical and sensitive component for imparting patterns on a substrate. The related equipment is equally critical. Conventionally, the electrostatic clamp for clamping the reticle is charged with a certain slew rate, ramping up the voltage up to a peak voltage. The peak voltage is, for instance, in the range of 1 to 5 kV, for instance 2 to 3 kV. The clamping force is typically constant during acceleration.

[0070] According to the present disclosure, an electrostatic clamp varies the clamping force overtime. Particularly while the clamp provided with a reticle is accelerating.

[0071] The diagram of Figure 7A exemplifies the velocity (vertical axis indicating velocity Vel.) of a reticle stage Rs provided with an electrostatic clamp, in relation to time t (horizontal axis). The speed herein is typically along one horizontal direction, for instance along the x-axis of a planar surface. The stage may typically also move in the perpendicular direction, to enable stepped operation.

[0072] For instance, the velocity changes from a certain maximum threshold velocity 700 in one direction, then switches until the chuck moves at another maximum threshold velocity 702 but in the opposite direction. The threshold velocity in either direction is for instance on the order of + / - 6 m / s.

[0073] In the example of Figure 7A, the velocity of the object stage changes between a constant speed section 704, a section 705 during which the speed changes, a second section 706 during which the object stage moves at a constant speed, a second section 707 during which the speed changes, and a third section 708 during which the speed is yet again substantially constant.

[0074] The diagram of Fig. 7B exemplifies the associated acceleration Acc. of the reticle stage Rs. Herein, when the stage moves at a constant speed, the acceleration Acc. is 0. See sections 710, 712, 714 in the diagram. To change the speed or direction of the reticle stage, the stage may first start to accelerate (section 716), and subsequently accelerate at a maximum threshold acceleration (section 718). When the stage approaches a maximum threshold velocity, acceleration of the stage starts to go down, until the stage moves at constant speed, see section 720. When the speed is constant, for instance at the maximum threshold velocity, the acceleration is 0, see section 712. Then, the pattern reverses andthe reticle stage will have to move in the opposite direction. Herein, the stage starts to accelerate in the opposite direction, see section 722, until the stage accelerates at a maximum threshold acceleration (section 724). When the object stage, for instance reticle stage Rs, approaches the maximum threshold velocity 702 in the opposite direction, the stage starts to decelerate, see section 726. When the speed is constant, for instance at the maximum threshold velocity 702, the acceleration is 0, see section 714.

[0075] Exposure apparatuses these days are typically implemented as scanners, meaning that the steps of Figures 7A and 7B are typically repeated multiple times per wafer.

[0076] According to the disclosure, the voltage V applied to the electrostatic clamp 300 clamping the reticle to the reticle stage is varied over time. As exemplified in Figure 7C, according to embodiments of the present disclosure, the clamping voltage V may be varied during movement and acceleration of the reticle stage as described above. The voltage may be varied between a lower voltage threshold 740 and an upper voltage threshold 742. The lower voltage threshold may typically be suitable to provide a suitable minimal electrostatic clamping force to the object, such as the reticle. The upper voltage threshold 742 provides an increased electrostatic clamping force.

[0077] In a practical embodiment, the clamping voltage V may be set at the lower voltage threshold 740 when the stage moves at a stationary speed Vel., i.e. the acceleration is about zero. See for instance diagram section 744. When the stage accelerates, the clamping voltage is ramped up. For instance, when the stage starts to accelerate, the voltage may also start to increase, see diagram section 746. The voltage may increase until the voltage reaches the upper voltage threshold 742.

[0078] At some point during the acceleration, the clamping voltage may be decreased, see line section 748. The lowering of the voltage may start at a suitable point during the cycle. For instance, somewhere in the middle of the section 718, when acceleration is constant, the voltage cycle (Fig. 7C) may switch from increasing to decreasing. The voltage can be lowered, until the voltage reaches the lower voltage threshold. When the stage moves at constant speed, and acceleration is about zero (section 712 in the acceleration cycle), and the voltage can be set at the lower voltage threshold 740, see section 750.

[0079] When the stage starts to accelerate in the opposite direction, the voltage cycle can behave similarly. Herein, the stage accelerates in the opposite direction, see line section 722 (Fig. 7B), the clamping voltage starts to increase, see diagram section 752. The voltage may increase until the voltage reaches the upper voltage threshold 742. For instance, somewhere in the middle of the section 724 (Fig. 7B), when acceleration is constant, the voltage cycle (Fig. 7C) may switch from increasing to decreasing. The voltage can be lowered, see section 754, until the voltage reaches the lower voltage threshold 740. When the stage moves at constant speed, and acceleration is about zero (section 714 in Fig. 7B), the voltage can be set at the lower voltage threshold 740, see section 756.

[0080] Examples of practical embodiment

[0081] In a practical embodiment, the clamping pressure, expressed as a voltage of the electrostatic clamp, may be increased with a slew rate r in the range of 5 to 100 kV / sec, for instance 10 to 80 kV / sec,for instance 15 to 50 kV / sec. The voltage may be increased from the baseline electrode voltage 740 as soon as the pattern part of the reticle has passed the exposure slit.

[0082] In a typical exposure motion profde as shown in Figures 7A and 7B, the total reticle stage turnaround time may be on the order of 20 to 100 milliseconds (ms), for instance about 30 to 60 ms. A velocity set point, comparable to the maximum velocity threshold 700, may be in the range of 2 to 10, for instance about 4 to 8 m / s. The maximum acceleration threshold, see threshold 718, may be in the range of 500 to 1000 m / s2. A theoretical minimum turnaround time can be expressed as 2*(max. velocity ) / (max. acceleration). This turnaround time can be on the order of 15 to 50 ms. In practice, the turnaround time is longer than this theoretical minimum due to tuning of jerk and snap.

[0083] Referring to Fig. 7C, assuming there is about 5 ms available on each side of the cycle of acceleration to ramp up and ramp down the clamping force and the related voltage V, the electrode voltage increase may roughly be on the order of 200 to 500 V.

[0084] At a total turnaround of about 100 ms, the system and method have about 50 ms available to increase the clamping force. The clamping force may be increased at a theoretic maximum of about 100 kV / sec. Herein, 100 kV / sec*50 ms = 5 kV. So, if the lower voltage threshold was 2 kV, the system may push to an upper voltage threshold of 7 kV (2+5 kV). In theory, the upper voltage threshold may be up to 10 kV in total. In a practical embodiment, the upper voltage threshold may be in the range of 1. 1 to 5 times the lower voltage threshold.

[0085] Increased clamping force will lead to an increased normal force and hence increased resistance to reticle slip. The idea here is to “relax” the reticle back to its steady state clamping force during the exposure phase.

[0086] For example, by increasing clamping pressure from the lower voltage threshold 740 to the upper voltage threshold 742 at a certain maximum acceleration setting, an increased resistance to reticle slip can be achieved. In an example, the described increase in clamping force can be represented with a related increase (in %) in clamping pressure. The increase in clamping pressure may vary within a range of, for instance, 10 to 500% with respect to the clamping pressure related to the lower voltage threshold (or, in line with potential maximum electrostatic clamping voltage).

[0087] Various embodiments of the present systems and methods are disclosed in the subsequent list of numbered clauses. In the following, further features, characteristics, and exemplary technical solutions of the present disclosure will be described in terms of clauses that may be optionally claimed in any combination:1. An electrostatic clamp system for holding an object by electrostatic clamping force, the clamp system comprising: a clamp provided with a plurality of burls to support the object and at least one electrode configured to generate the electrostatic clamping force, and a controller connectable to the at least one electrode and configured to vary the electrostatic clamping force dependent on acceleration of the clamp.2. The clamp system of clause 1, wherein the controller is configured to increase the electrostatic clamping force when the clamp starts to accelerate.3. The clamp system of clause 1 or 2, wherein the controller is configured to vary the clamping force by varying an electrostatic clamping voltage with a slew rate in the range of 5 to 100 kV / sec, for instance 10 to 80 kV / sec, for instance 15 to 50 kV / sec.4. An exposure apparatus comprising: an illumination system configured to condition a radiation beam; a support structure 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 projection system configured to project the patterned radiation beam onto a target portion of a substrate, and an electrostatic clamp system for holding an object by electrostatic clamping force, the electrostatic clamp system comprising: a clamp provided with a plurality of burls to support the object and at least one electrode configured to generate the electrostatic clamping force, and a controller connectable to the at least one electrode and configured to vary the electrostatic clamping force dependent on acceleration of the clamp.5. The exposure apparatus of clause 4, wherein the electrostatic clamp system includes an electrostatic clamp system according to clause 2 or 3.6. A method of operating an electrostatic clamp system, the method comprising the steps of: providing an electrostatic clamp system for holding an object by electrostatic clamping force, the clamp system comprising a clamp provided with a plurality of burls to support the object and at least one electrode configured to generate the electrostatic clamping force, arranging the object on the plurality of burls of the clamp, accelerating or decelerating the clamp in accordance with a pattern, and varying the electrostatic clamping force dependent on the acceleration of the clamp.7. The method of clause 6, wherein the step of varying the clamping force comprises increasing the electrostatic clamping force when the clamp starts to accelerate.8. The method of clause 6 or 7, wherein the step of varying the clamping force comprises increasing the electrostatic clamping force between 1-50 milliseconds, preferably between 5-25 milliseconds, more preferably 10-15 milliseconds, before the clamp starts to accelerate.9. The method of one of clauses 6 to 8, wherein the step of varying the clamping force comprises increasing an electrostatic voltage provided to the at least one electrode to an upper voltage threshold when the clamp starts to accelerate, and reducing the electrostatic voltage to a lower voltage threshold when the clamp ceases to accelerate.10. The method of one of clauses 6 to 9, wherein the step of varying the clamping force comprises increasing or decreasing an electrostatic voltage to generate the electrostatic clamping force between a lower force threshold and an upper force threshold.11. The method of clause 9, wherein the lower voltage threshold is on the order of 1 to 5 kV, for instance 2 to 3 kV.12. The method of clause 9 or 11, wherein the upper voltage threshold is in the range of 1.1 to 5 times the lower voltage threshold.13. The method of one of clauses 6 to 12, wherein the step of varying the clamping force includes varying the electrostatic clamping voltage with a slew rate in the range of 5 to 100 kV / sec, for instance 10 to 80 kV / sec, for instance 15 to 50 kV / sec.

[0088] Although specific reference may be made in this text to a “reticle,” it should be understood that this is just one example of a patterning device and that the embodiments described herein may be applicable to any type of patterning device. Additionally, the embodiments described herein may be used to provide safety support for any object to ensure a clamping failure does not cause the object to fall and damage either itself or other equipment.

[0089] Although specific reference may be made in this text to the use of exposure apparatus in the manufacture of ICs, it should be understood that the exposure 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, 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 unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology unit and / or an inspection unit. 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.

[0090] Although specific reference may have been made above to the use of embodiments of the disclosure in the context of optical lithography, it will be appreciated that the disclosure can 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 can 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.

[0091] 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 disclosure is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0092] The term “substrate” as used herein describes a material onto which material layers are added. In some embodiments, the substrate itself can be patterned and materials added on top of it may also be patterned, or may remain without patterning.

[0093] Although specific reference can be made in this text to the use of the apparatus and / or system according to the disclosure in the manufacture of ICs, it should be explicitly understood that such an apparatus and / or system has many other possible applications. For example, it can be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, LCD panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle,” “wafer,” or “die” in this text should be considered as being replaced by the more general terms “mask,” “substrate,” and “target portion,” respectively.

[0094] While specific embodiments of the disclosure have been described above, it will be appreciated that the disclosure can be practiced otherwise than as described. The description is not intended to limit the disclosure.

[0095] 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 disclosure as contemplated by the inventor(s), and thus, are not intended to limit the present disclosure and the appended claims in any way.

[0096] The present disclosure 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.

[0097] The foregoing description of the specific embodiments will so fully reveal the general nature of the disclosure 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 disclosure. 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.

[0098] The breadth and scope of the present disclosure should not be limited by any of the abovedescribed exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

CLAIMS1. An electrostatic clamp system for holding an object by electrostatic clamping force, the clamp system comprising: a clamp provided with a plurality of burls to support the object and at least one electrode configured to generate the electrostatic clamping force, and a controller connectable to the at least one electrode and configured to vary the electrostatic clamping force dependent on acceleration of the clamp.

2. The clamp system of claim 1, wherein the controller is configured to increase the electrostatic clamping force when the clamp starts to accelerate.

3. The clamp system of claim 1, wherein the controller is configured to vary the clamping force by varying an electrostatic clamping voltage with a slew rate in the range of 5 to 100 kV / sec, for instance 10 to 80 kV / sec, for instance 15 to 50 kV / sec.

4. An exposure apparatus comprising: an illumination system configured to condition a radiation beam; a support structure 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 projection system configured to project the patterned radiation beam onto a target portion of a substrate, and an electrostatic clamp system for holding an object by electrostatic clamping force, the electrostatic clamp system comprising: a clamp provided with a plurality of burls to support the object and at least one electrode configured to generate the electrostatic clamping force, and a controller connectable to the at least one electrode and configured to vary the electrostatic clamping force dependent on acceleration of the clamp.

5. The exposure apparatus of claim 4, wherein the electrostatic clamp system includes an electrostatic clamp system according to claim 2 or 3.

6. A method of operating an electrostatic clamp system, the method comprising the steps of: providing an electrostatic clamp system for holding an object by electrostatic clamping force, the clamp system comprising a clamp provided with a plurality of burls to support the object and at least one electrode configured to generate the electrostatic clamping force,arranging the object on the plurality of burls of the clamp, accelerating or decelerating the clamp in accordance with a pattern, and varying the electrostatic clamping force dependent on the acceleration of the clamp.

7. The method of claim 6, wherein the step of varying the clamping force comprises increasing the electrostatic clamping force when the clamp starts to accelerate.

8. The method of claim 6, wherein the step of varying the clamping force comprises increasing the electrostatic clamping force between 1-50 milliseconds, preferably between 5-25 milliseconds, more preferably 10-15 milliseconds, before the clamp starts to accelerate.

9. The method of claim 6, wherein the step of varying the clamping force comprises increasing an electrostatic voltage provided to the at least one electrode to an upper voltage threshold when the clamp starts to accelerate, and reducing the electrostatic voltage to a lower voltage threshold when the clamp ceases to accelerate.

10. The method of claim 6, wherein the step of varying the clamping force comprises increasing or decreasing an electrostatic voltage to generate the electrostatic clamping force between a lower force threshold and an upper force threshold.

11. The method of claim 9, wherein the lower voltage threshold is on the order of 1 to 5 kV, for instance 2 to 3 kV.

12. The method of claim 9, wherein the upper voltage threshold is in the range of 1.1 to 5 times the lower voltage threshold.

13. The method of claim 6, wherein the step of varying the clamping force includes varying the electrostatic clamping voltage with a slew rate in the range of 5 to 100 kV / sec, for instance 10 to 80 kV / sec, for instance 15 to 50 kV / sec.

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