Lithography equipment and electrostatic chuck design

Abstract

Embodiments herein describe methods, apparatus, and systems for reducing the electric field at a clamp-reticle interface using an enhanced electrostatic clamp. Specifically, the electrostatic clamp includes a clamp body, an electrode layer disposed on a top surface of the clamp body, and a plurality of burls protruding from a bottom surface of the clamp body, wherein the electrode layer includes a plurality of cutouts at predetermined locations that vertically correspond to the locations of the burls on the top surface of the clamp body.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Patent Application No. 62 / 927,214, filed on October 29, 2019, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates to systems and methods for electrostatic chuck design and electric field reduction in lithographic apparatus. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a substrate (usually to a target portion of the substrate). A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In this example, a pattern forming device (alternatively referred to as a mask or reticle) can be used to generate a circuit pattern to be formed on a single layer of the IC. The pattern can be transferred to a target portion (e.g., including part of a die, one or more dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is usually performed by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are patterned continuously. Known lithographic apparatus include so-called steppers (wherein each target portion is irradiated by exposing the entire pattern to the target portion at once) and so-called scanners (wherein each target portion is irradiated by scanning a pattern with a radiation beam in a given direction ("scanning" direction), while simultaneously scanning target portions parallel or antiparallel to the scanning direction. The pattern can also be transferred from the pattern forming device to the substrate by imprinting the pattern onto the substrate.

[0005] Photolithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and / or structures. However, as the feature sizes manufactured using photolithography become smaller, photolithography is becoming a more critical factor in enabling the manufacture of miniature ICs and other devices and / or structures.

[0006] To project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Compared to lithographic apparatuses that use radiation with a wavelength of, for example, 193 nm, lithographic apparatuses that use extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate.

[0007] Lithographic apparatus using EUV radiation may require that the EUV radiation beam path, or at least a significant portion thereof, must be maintained in a vacuum during lithographic operations. In such a vacuum region of the lithographic apparatus, electrostatic chucks may be used to clamp objects (such as a patterning device and / or a substrate) to structures of the lithographic apparatus (such as a patterning device table and / or a substrate table, respectively).

[0008] In some cases, an electrostatic clamp can be used to hold the reticle in place in the lithographic apparatus. The electrostatic clamp can include electrodes at the top surface of the clamp, with a plurality of burls disposed on the bottom surface of the clamp. When the clamp is energized (e.g., using a clamping voltage) and the reticle is pulled into contact with the burls, the top of the conductive burls may be at a different potential than the back of the reticle. Upon contact, this potential difference can induce a discharge mechanism because the two potentials are equal. This discharge mechanism can lead to material transfer and particle generation, and ultimately damage to the reticle and / or clamp.

[0009] Additionally, backfill gas can be provided at the burl-reticle interface in the electrostatic chuck to promote cooling and heat transfer from the reticle. In some cases, the backfill gas may act as an additional force on the reticle, which may cause the reticle to lose contact with the burl in the electrostatic chuck, resulting in a gap between the reticle and the chuck. To counteract this force, the voltage of the electrostatic chuck can be increased. However, this increase in voltage may cause a charge difference between the burl and the backside of the reticle, generating an electrical discharge that may damage the reticle and / or the chuck. Summary of the Invention

[0010] Thus, the present disclosure provides methods, apparatus, and systems for minimizing electric fields and discharges at the contact points between the burls of an electrostatic chuck and the reticle.

[0011] In some embodiments, an electrostatic chuck includes a chuck body, an electrode layer disposed on a top surface of the chuck body, and a plurality of burls protruding from a bottom surface of the chuck body. The electrode layer includes a plurality of cutouts at predetermined locations that vertically correspond to the locations of the plurality of burls on the bottom surface of the chuck body.

[0012] In some embodiments, a method for enhancing an electrostatic chuck includes manufacturing an electrostatic chuck comprising a chuck body, an electrode layer disposed on a top surface of the chuck body, and a plurality of burls protruding from a bottom surface of the chuck body, the burls configured to contact a back surface of a reticle. The method also includes applying a modification to the electrostatic chuck to reduce an electric field between the burls and the reticle.

[0013] In one embodiment, applying the modification includes reducing the thickness of a conductive coating on the reticle contact surface of the plurality of burls, thereby allowing a voltage difference between the plurality of burls and the reticle to be reduced. In another embodiment, applying the modification includes removing predetermined portions of the electrode layer at locations corresponding to the location of each burl, thereby allowing a plurality of cuts in the electrode layer.

[0014] In another embodiment, applying the modification includes connecting each of the plurality of burls together to provide a virtual ground in the electrostatic chuck. In another embodiment, applying the modification includes grounding the plurality of burls. In another embodiment, applying the modification includes applying a conductive coating to the reticle contact surfaces of the plurality of burls or to the backside of the reticle, the conductive coating comprising an oxide, diamond, or diamond-like carbon (DLC) material.

[0015] In some embodiments, a lithographic apparatus includes an illumination system, a support structure, and a projection system. The illumination system conditions a radiation beam. The support structure is configured to support a patterning device capable of imparting a pattern to the radiation beam in its cross-section to form a patterned radiation beam. The projection system is configured to project the patterned radiation beam onto a target portion of a substrate. The support structure includes an electrostatic clamp. The electrostatic clamp includes a clamp body, an electrode layer disposed on a top surface of the clamp body, and a plurality of protrusions protruding from a bottom surface of the clamp body. The electrode layer includes a plurality of cutouts at predetermined positions that vertically correspond to positions of the plurality of protrusions at the bottom surface of the clamp body.

[0016] Other features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It should be noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Based on the teachings contained herein, additional embodiments will be apparent to those skilled in the relevant art(s). BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the invention and, together with the description, further serve to explain the principles of the invention and enable one skilled in the relevant art(s) to make and use the invention.

[0018] Figure 1 is a schematic diagram of a lithographic apparatus according to an embodiment of the present disclosure.

[0019] Figure 2 is a perspective schematic diagram of a reticle workbench according to an embodiment of the present disclosure.

[0020] Figure 3 yes Figure 2 A top view of the mask workbench.

[0021] Figure 4A and 4B is a schematic diagram of an electrostatic chuck according to an embodiment of the present disclosure.

[0022] Figure 5 is a three-dimensional perspective view of the top of an electrostatic chuck according to an embodiment of the present disclosure.

[0023] Figure 6A 、 6B 6C are schematic diagrams of a bottom view of a burl in an electrostatic chuck according to an embodiment of the present disclosure.

[0024] Figure 7 is a schematic diagram of a burl-reticle interface in an electrostatic chuck according to an embodiment of the present disclosure.

[0025] Figure 8 FIG. 1 is a schematic diagram of an electrical connection burl in an electrostatic chuck according to an embodiment of the present disclosure.

[0026] Figure 9 is a schematic diagram of a flow chart for enhancing an electrostatic clamp according to an embodiment of the present disclosure.

[0027] 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 accompanying drawings, in which like reference numerals identify corresponding elements throughout. In the accompanying drawings, like reference numerals generally indicate identical, functionally similar, and / or structurally similar elements. Additionally, generally, the leftmost digit(s) of a reference numeral identifies the drawing in which the reference numeral first appears. Unless otherwise indicated, the drawings provided in this disclosure should not be construed as being drawn to scale. DETAILED DESCRIPTION

[0028] This specification discloses one or more embodiments that incorporate the features of the present invention. The disclosed embodiment(s) merely illustrate the present invention. The scope of the present invention is not limited to the disclosed embodiment(s). The present invention is defined by the appended claims.

[0029] The described embodiment(s) and references in the specification to "one embodiment," "an embodiment," "an example embodiment," etc. indicate that the described embodiment(s) may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases do not necessarily refer to the same embodiment. Further, when a particular feature, structure, or characteristic is described in conjunction with an embodiment, it is understood that it is within the knowledge of those skilled in the art to implement such feature, structure, or characteristic in conjunction with other embodiments, whether or not explicitly described.

[0030] Spatially relative terms (such as "below," "beneath," "lower," "above," "upper," etc.) may be used herein for convenience of description to describe the relationship of one element or feature to another element or feature(s) illustrated in the accompanying drawings. Spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the drawings. The device may be otherwise oriented (rotated 90 degrees or at other orientations), and the spatially relative descriptors used herein should be interpreted accordingly.

[0031] As used herein, the term "approximately" indicates a value of a given quantity that may vary based on a particular technology. Based on a particular technology, the term "approximately" may indicate a value of a given quantity that varies within, for example, 10% to 30% of that value (e.g., ±10%, ±20%, or ±30% of the value).

[0032] The embodiments of the present disclosure may be implemented in hardware, firmware, software, or any combination thereof. The embodiments of the present 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 sending information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include a read-only memory (ROM); a random access memory (RAM); a disk storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal (e.g., a carrier wave, an infrared signal, a digital signal, etc.), etc. Further, firmware, software, routines, and / or instructions may be described herein as performing certain actions. However, it should be understood that this description is merely for convenience, and that such actions are in fact caused by a computing device, processor, controller, or other device that executes the firmware, software, routines, instructions, etc.

[0033] However, before describing such embodiments in greater detail, it is beneficial to present an example environment in which embodiments of the present disclosure may be implemented.

[0034] Example lithography system

[0035] Figure 1 A lithographic system is shown comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure (e.g., a mask table) 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.

[0036] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. The illumination system IL may include a faceted field mirror arrangement 10 and a faceted pupil mirror arrangement 11. The faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11.

[0037] After being so 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 this purpose, the projection system PS may comprise a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B' so as to form an image in which features are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as being in Figure 1 While there are only two mirrors 13, 14 in the projection system PS, the projection system PS may comprise a different number of mirrors (eg six or eight mirrors).

[0038] The substrate W may include a previously formed pattern. In this case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B′ with the pattern previously formed on the substrate W.

[0039] A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, illumination system IL and / or projection system PS.

[0040] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source capable of generating EUV radiation.

[0041] Exemplary Reticle Stage

[0042] Figure 2 and 3A schematic diagram of an exemplary reticle table 200 according to some embodiments is shown. The reticle table 200 may include a top table 202, a bottom table 204, a side table 206, and a fixture 300. In some embodiments, the reticle table 200 with the fixture 300 may be implemented in a lithographic apparatus LA. For example, the reticle table 200 may be a support structure MT in the lithographic apparatus LA. In some embodiments, the fixture 300 may be disposed on the top table 202. For example, as Figure 2 As shown, the clamp 300 may be positioned in the center of the top deck 202 with the clamp front face 302 facing vertically away from the top deck 202 .

[0043] In some lithographic apparatuses (e.g., lithographic apparatus LA), a reticle table 200 having a fixture 300 may be used to hold and position a reticle for scanning or patterning operations. In one example, the reticle table 200 may require a high-power drive, a large counterweight, and a heavy frame to support it. In one example, the reticle table 200 may have a large inertia and may weigh more than 500 kg to push and position a reticle weighing approximately 0.5 kg. In order to achieve the reciprocating motion of the reticle typically seen in lithographic scanning or patterning operations, acceleration and deceleration forces may be provided by a linear motor driving the reticle table 200.

[0044] In some embodiments, as Figure 2 and 3 As shown, the reticle stage 200 may include a first encoder 212 and a second encoder 214 for positioning operations. For example, the first encoder 212 and the second encoder 214 may be interferometers. The first encoder 212 may be attached along a first direction, such as the lateral direction of the reticle stage 200 (i.e., the X direction). And the second encoder 214 may be attached along a second direction, such as the longitudinal direction of the reticle stage 200 (i.e., the Y direction). In some embodiments, as Figure 2 and 3 As shown, the first encoder 212 can be orthogonal to the second encoder 214.

[0045] like Figure 2 and 3As shown, the mask workbench 200 may include a clamp 300. The clamp 300 is configured to hold the mask in a fixed plane on the mask workbench 200. The clamp 300 includes a clamp front 302 and can be set on the top table 202. In some embodiments, the clamp 300 can use electrostatic clamping technology to hold and stabilize the object. For example, the clamp 300 can be an electrostatic clamp, which can be configured to electrostatically clamp (i.e., hold) an object, such as a mask, in a vacuum environment. Since EUV radiation needs to be performed in a vacuum environment, a vacuum clamp cannot be used to clamp the mask or mask, and instead an electrostatic clamp can be used. For example, the clamp 300 may include an electrode, a resistive layer on the electrode, a dielectric layer on the resistive layer, and a protrusion protruding from the dielectric layer. In use, a voltage can be applied to the clamp 300, for example, several kV. And a current can flow through the resistive layer so that the voltage at the upper surface of the resistive layer is substantially the same as the voltage of the electrode and an electric field is generated. Furthermore, Coulomb forces (ie, the attraction between oppositely charged particles) will draw the object to the fixture 300 and hold it in place. In some embodiments, the fixture 300 can be a rigid material such as metal, dielectric, ceramic, or a combination thereof.

[0046] Example Electrostatic Chuck Design

[0047] Figure 4A and 4B Schematic diagrams illustrating side views of electrostatic clamps 400 and 410, respectively, according to embodiments of the present disclosure. In some embodiments, electrostatic clamps 400 and 410 represent Figure 2 and 3 An exemplary embodiment of a clamp 300 is shown. Figure 4A An electrostatic chuck 400 is illustrated, comprising a chuck body 404, an electrode layer 406, and a plurality of burls 408. The electrode layer 406 is disposed on a top surface of the electrostatic chuck 400, and the plurality of burls 408 protrude from a bottom surface of the electrostatic chuck 400, wherein the chuck body 404 physically separates the electrode layer 400 from the burls 408.

[0048] In some embodiments, the fixture body 404 can have a height of approximately 100 microns and can include glass, ceramic, and / or polymer materials, such as ultra-low expansion glass (ULE), lithium aluminosilicate glass ceramic, siliconized silicon carbide (SiSiC), benzocyclobutene polymer (BCB), etc. The plurality of burls 408 include a conductive coating and are configured to contact the mask 409. In some embodiments, the plurality of burls can have a height of approximately 10 microns. Although the plurality of burls 408 are shown as having rectangular surfaces that contact the mask 409 for illustrative purposes, it should be understood that the plurality of burls 408 can be cylindrical in shape with rounded edges or rounded surfaces or other planar geometries (e.g., square, elliptical, oval, etc.) that contact the mask 409. Furthermore, although only three burls 408 are shown in FIG. Figure 4A , but any number of burls 408 and any arrangement of burls across the bottom surface of the fixture body 404 (e.g., a grid or random distribution) may be used. In some embodiments, the surface of the burl 408 that contacts the reticle may be referred to herein as the burl top. Figure 4A As illustrated, reticle 409 is shown as having gravity sag caused by gravity pulling downward on reticle 409 as an example.

[0049] In some embodiments, the electrode layer 406 comprises a continuous layer and can be made of any suitable conductive material, such as a metal or metal alloy, for example, aluminum, chromium, platinum, gold, or any combination thereof. In some embodiments, the electrostatic chuck 400 can be powered, and a voltage can be applied to the electrode layer 406 to generate an electric field (e.g., Figure 4A ). The electric field can induce a charge on the surface of reticle 409 that is opposite to the charge of burls 408. The attractive force between burls 408 and reticle 409 can keep reticle 409 in close contact with electrostatic chuck 400. In some embodiments, the back surface of reticle 409 can be coated with a conductive film, which further allows electrostatic chuck 400 to hold reticle 409 in place via the attractive force between burls 408 and the conductive surface of reticle 409.

[0050] In some embodiments, a voltage in the range of 100 to 5000 V may be applied to electrode layer 406 to generate an electrostatic force that overcomes gravity and attracts the back side of reticle 409 to electrostatic chuck 400. However, burl 408, a component of electrostatic chuck 400 that interfaces with the back side of reticle 409, may be at a different electrical potential than reticle 409. The potential difference generated at the burl-reticle interface may result in potential material transfer, particle generation, and / or electrical discharge that may damage the electrostatic chuck and / or reticle.

[0051] In some embodiments, Figure 4B An electrostatic chuck 410 is illustrated that minimizes electric fields and discharges at the contact point between the electrostatic chuck and the reticle.

[0052] In some embodiments, the electrostatic chuck 410 includes a chuck body 414 , an electrode layer 416 , and a plurality of burls 418 .

[0053] In some embodiments, the electrode layer 416 includes a plurality of cutouts 417 or gaps where electrode material has been removed at locations in the top surface of the fixture body 414. In some embodiments, the locations of the plurality of cutouts 417 in the electrode layer 416 can correspond vertically to the locations of the burls 418 in the bottom surface of the fixture body 414. For example, the cutouts 417 can be located directly above the location of the corresponding burl 418 in the electrostatic fixture 414. Each cutout 417 can include a circular cutout (or other planar geometric shape, such as a square, elliptical, oval, etc.) that is concentric with the location of the corresponding burl 418, but is located on the top surface of the fixture body 414 rather than at the bottom surface of the fixture body 414. Although only three burls 418 and three cutouts 417 are present in the electrode layer 416, the electrode layer 416 can be located at a position that corresponds to the locations of the burls 418 in the bottom surface of the fixture body 414. Figure 4B 4, any number of burls 418 and cutouts 417 and any arrangement across the bottom and top surfaces of the chuck body 414, respectively, may be used. In some embodiments, by interrupting the electrode layer 416 with the cutouts 417 near the burls 418, the electric field at the burls 418 may be significantly reduced to improve the contact point between the electrostatic chuck 410 and the reticle 419 and minimize discharge.

[0054] Figure 5 is a three-dimensional perspective view of the top of the electrostatic clamp 410 according to an embodiment of the present disclosure. For example, Figure 5 An electrode layer 414 is shown at the top surface of an electrostatic clamp 410. The electrode layer 414 may include cutouts 417 or gaps where electrode material has been removed at locations in the top surface of the clamp that correspond to the locations of burls 418 in the bottom surface of the clamp. In some embodiments, predetermined portions of the electrode layer 414 may be removed by a machining process, etching, laser ablation, etc., thereby allowing the cutouts 417 to be formed. For example, etching may be used to strip away predetermined portions of the electrode layer 414, such as by using chemicals in a wet or dry etching technique. Although only two cutouts 417 having circular areas are present in Figure 5, it should be understood that any number of cutouts 417 having other planar geometries (e.g., square, elliptical, oval, etc.) can be used in electrode layer 414. In some embodiments, each cutout 417 can have a height (e.g., thickness) of approximately 50 to 2000 nanometers (nm) and a diameter of approximately 800 to 1400 microns. In some embodiments, removing 300 μm of the predetermined electrode area around the burl can reduce the electrode area by approximately 7%. Consequently, a higher clamping force and clamping voltage may be required to account for this area reduction.

[0055] Figure 6A 、 6B 6C are schematic diagrams of a bottom view of a burl in an electrostatic clamp according to an embodiment of the present disclosure. Specifically, Figure 6A FIG. 6 shows an electrostatic chuck configuration 600 having an electrode layer 606 and burls 608, wherein the electrode layer 606 is a continuous layer without any cutouts. In some embodiments, the electrode layer 606 and burls 608 may represent exemplary embodiments of the electrode layer 406 and burls 408, respectively, as shown in FIG. Figure 4A The burl 608 and the electrode layer 606 can be separated by the electrostatic clamp body (eg Figure 4A (See chuck body 404 in FIG. ). In some embodiments, burl 608 can have a diameter D of approximately 460 microns and can be configured to contact the backside of the reticle. In some examples, the potential difference between burl 608 and the reticle can cause an electrical discharge, which can damage the reticle and / or the electrostatic chuck.

[0056] Figure 6B FIG. 6 shows an electrostatic chuck configuration 610 having an electrode layer 616, cutouts 617, and burls 618. In some embodiments, electrode layer 616, cutouts 617, and burls 618 may represent exemplary embodiments of electrode layer 416, cutouts 417, and burls 418, respectively. Figure 4B As shown. In some embodiments, the cutout 617 comprises a portion of a predetermined area removed from the electrode layer 616 to reduce the electric field at the burl 618. In some embodiments, by applying the cutout 617 in the electrode layer 616, the area of ​​the electrode layer 616 can be reduced. In some embodiments, the reduction in area may require a higher clamping force to maintain contact between the electrostatic clamp and the mask. Therefore, a higher clamping voltage may be required to compensate for the loss of electrode area and provide sufficient clamping force to maintain contact at the clamp-mask interface. In some embodiments, the clamping force uniformity can be maintained due to spatial filtering achieved by the stiffness of the mask. For example, spatial filtering may occur due to the thickness of the mask. In other words, increasing the thickness of the mask may cause small local variations on the back of the mask to be "blurred" and effectively filtered out from being printed on the front.

[0057] In some embodiments, one or more portions of the electrode layer 616 can be removed, thereby allowing one or more cutouts 617 to include a diameter D of approximately 800 microns. For example, by removing one or more portions of the electrode layer 616 (e.g., cutouts 617 having a diameter D of 800 μm), the electric field at the burl 618 can be reduced by a factor of approximately 8.8, and the clamping voltage can be increased by a factor of approximately 1.07 to maintain the clamping force, resulting in a gain factor of approximately 8.2.

[0058] Figure 6C FIG. 6 shows another configuration 620 of an electrostatic chuck according to an embodiment having an electrode layer 626, a cutout 627, and a burl 628. In some embodiments, the electrode layer 626, the cutout 627, and the burl 628 may represent exemplary embodiments of the electrode layer 416, the cutout 417, and the burl 418, respectively. Figure 4B In configuration 620, one or more portions of electrode layer 626 can be removed, allowing one or more cutouts 627 to include a diameter D of approximately 1400 microns. In some embodiments, a cutout diameter D of approximately 1400 microns can allow the electric field at burl 628 to be reduced by a factor of approximately 125. Additionally, the clamping voltage can be increased by a factor of approximately 1.26 to maintain the clamping force, resulting in a gain factor of approximately 99.

[0059] Exemplary Embodiments for Electric Field Reduction

[0060] In some embodiments, the method further enhances the electrostatic clamp design to minimize electric fields and improve the burl-reticle interface. In some embodiments, the reticle may sag due to gravity and / or backfill gas, thus losing contact even when the clamping voltage is at a significant level (e.g., on the order of hundreds of volts) during reticle stage unloading and re-clamping. This loss of contact may result in micro-gap discharges (e.g., arcing or arcing) in the gap between the clamp and the reticle. Moreover, in some embodiments, contact between the burl top (e.g., the surface of the burl that contacts the reticle) and the reticle backside can only be made when the clamping voltage is hundreds of volts during loading (and the re-contact portion of re-clamping). Additionally, the burl top may typically still be charged with a polarity opposite to that of the previous clamping event (when charged with the opposite polarity). For example, this charge may have a value of approximately 10 to 100 pico-Coulombs (pC).

[0061] In some embodiments, this charge difference can not only cause microgap discharges but also current flow through the rough surface of the burl tops and / or through the rough surface of the top oxide layer of the insulating mask coating. For example, this can result in local temperatures exceeding 10,000 K, localized melting or evaporation of materials (metal and oxide), unavoidable material transfer, and / or microwelding. The charge difference can also cause reticle adhesion and coverage issues for the end user.

[0062] Figure 7 FIG. 7 is a schematic diagram of a burl-reticle interface 700 in an electrostatic clamp according to an embodiment of the present disclosure. The burl-reticle interface 700 includes an electrostatic clamp 710, a burl 715, a burl top 720, and a reticle 730. Although only one burl 715 is in Figure 7 As shown in FIG, there may be any number of burls 715 in the electrostatic chuck 710 .

[0063] In some embodiments, the burl-reticle interface 700 can be located essentially at the edge of barrier layer breakdown, with nominal values ​​greater than 90 V / um at the top of the burl for a 2 kV clamping voltage, and even higher in certain locations where the field is amplified, such as ridges. For example, barrier layer breakdown can be referred to as sintering, which can occur with internal fields on the order of 100 V / um. In some embodiments, the electric field is substantially higher above the burl top 720 than between adjacent burls 715 because the burls 715 are dielectric (e.g., 110 microns with a dielectric constant ε). r ~5), there is no 10um vacuum gap. Although the electric field between the nodes (in the vacuum gap) is about 70V / um for a 2kV clamping voltage, the electric field on the nodes is greater than 80V / um, such as Figure 7 In some embodiments, when mechanical shear compromises the barrier layer integrity, the barrier breakdown threshold may shift to lower field values, as expected in practical clamping.

[0064] Thus, some embodiments of the present disclosure provide methods and apparatus for improving the burl-mask interface to reduce the electric field by optimizing an electrostatic clamp. In some embodiments, the electrostatic clamp can be optimized to reduce the electric field at the burl-mask interface by applying various modifications to the clamp. One example of a modification that can be applied is to reduce the thickness of a conductive coating on the mask contact surface (e.g., the burl top) of a plurality of burls of the electrostatic clamp. For example, the conductive layer or coating on the burl top comprises titanium nitride (TiN), and for approximately 100 nm of the burl top, the voltage difference between the burl and the mask can be estimated to be on the order of greater than 10 V. In some embodiments, it may be desirable to minimize this voltage difference to less than 1 V, which can be achieved by reducing the thickness of the conductive layer on the burl top to less than 10 nm. Thus, the local field between the burl top and the mask can be reduced by applying a thinner conductive layer on the burl top so that the voltage on the burl top is closer to the voltage on the back of the mask before contact, wherein the voltage difference between the plurality of burls and the mask is reduced.

[0065] In some embodiments, the modification includes minimizing local fields by connecting each of the multiple burls together to provide a virtual ground in the electrostatic chuck. For example, the burls can be electrically connected to effectively combine into a single electrical surface. This modification can maintain the tops of the burls at virtual ground so that the burls and the backside of the reticle are approximately the same voltage (e.g., within the limits of electrode tolerances, which in this context are small, less than about 1% for 2 kV, or about 20 V).

[0066] In some embodiments, a virtual ground may be easier to implement than a full ground because no external interface is required and there is no risk of ground loops in a virtual ground.

[0067] In some embodiments, the tops of the multiple burls in the electrostatic chuck can be physically grounded to minimize the local field at the burl-reticle interface. This modification can keep the burl tops at approximately 0V ground, so that the burls are at the same voltage as the virtually grounded reticle backside.

[0068] Figure 8 Schematic diagram of an electrical connection protrusion in an electrostatic clamp according to an embodiment of the present disclosure. Specifically, Figure 8A plurality of burls 805, burl tops 810, coating 815, and electrode E 820 are shown. In some embodiments, the electrostatic chuck may include four electrodes 820, two of which operate at a positive voltage and the other two electrodes 820 operate at a negative voltage. The plurality of burls 805 may include chromium (Cr) or chromium nitride (CrN), and the burl tops 810 may include titanium nitride (TiN), while the chuck body of the electrostatic chuck may include glass, ceramic, and / or polymer materials such as ultra-low expansion glass (ULE), lithium aluminosilicate glass ceramic, siliconized silicon carbide (SiSiC), benzocyclobutene polymer (BCB), and the like.

[0069] In some embodiments, burl top 810 can be electrically connected to establish a virtual ground. For example, the electrical connection to burl top 810 can be below coating 815, which is a wear-resistant coating that may or may not be conductive. In another example, the electrical connection to burl top 810 can be on the side of coating 815, which may be conductive.

[0070] In some embodiments, a virtual ground can be achieved by interconnecting the burl tops 810 above the positive electrode 820 with the burl tops 810 above the negative electrode 820. In some embodiments, all burl tops 810 can be at 0V (e.g., depending on the precise balance of the electrodes). Thus, the burl tops can be at the same potential as the back of the reticle, which can prevent the risk of welding and discharge when the reticle and burls come into contact. In some embodiments, Figure 8 The configuration shown may include a ground connection to establish a suitable zero voltage for the burl top 810 .

[0071] Some embodiments of modifying the electrostatic clamp to enhance the burl-mask interface may include optimizing the reticle coating, the burls, and the operating conditions. In some embodiments, optimizing the reticle coating may include applying a conductive coating to the reticle contact surfaces of the plurality of burls or the backside of the reticle, wherein the conductive coating comprises an oxide, diamond, or diamond-like carbon (DLC) material. For example, the reticle backside coating or top layer may be optimized by using a material that forms a conductive oxide to prevent the formation of a dielectric barrier in the event of oxidation during manufacturing, storage, and / or cleaning. In some embodiments, semiconductor oxides may be considered for the coating, such as titanium dioxide (TiOx), vanadium oxide (VOx), and the like.

[0072] In some embodiments, a thick, robust top layer on the reticle backside coating can also be used to push the limit of the frit effect to higher voltages and make the reticle less susceptible to mechanical deformation during clamping or use. In some embodiments, the reticle coating may require a compatible cleaning process to maintain the quality and thickness of the layer.

[0073] In some embodiments, sintering involves an insulating barrier, such as that formed by most metal oxides. Certain metal oxides (such as molybdenum dioxide (MoO2), ruthenium (IV) oxide (RuO2), etc.) can maintain conductivity while still presenting a wear-resistant top layer, so these may not lead to sintering. Similarly, wear-resistant non-metallic layers (such as DLC or (nano) diamond) can be protected by designing some conductivity, for example by doping with boron (B).

[0074] In some embodiments, the burl of the electrostatic chuck can also be optimized to minimize the electric field at the burl-reticle interface. In some embodiments, the conductive burl top material can be replaced with a fully dielectric burl top material to prevent charge accumulation on the burl top and field amplification at the burl-reticle interface. In some embodiments, the non-conductive or dielectric burl top can be susceptible to triboelectric effects (or contact charging); however, this is not considered a problem due to the conductive reticle backside coating (as opposed to wafer clamping).

[0075] In some embodiments, the conductive burl top material can be replaced with a less conductive burl top material, such as a material with very high sheet resistance. Using a less conductive material to form the burl top can reduce the sintering current and the charge equalization current after polarity switching, thereby maintaining a sufficiently low transient temperature to prevent localized melting and electrochemical migration. In some embodiments, field amplification can also be prevented by avoiding sharp features, edges, and triple points near the burl-reticle interface, including sharp geometric steps such as ridges. In some embodiments, the burl top material can be selected so that the burl material is transferred to the reticle (e.g., an alloy comprising Au / Pt / Ag).

[0076] In some embodiments, the operating conditions of the electrostatic clamp can be optimized to reduce the electric field at the burl-reticle interface. For example, the backfill gas pressure can be removed during re-clamping and unloading of the reticle (e.g., when the reticle is in contact with the top of the burl). This may result in a much lower push-off force and a much lower clamping voltage at which the burl-reticle contact is broken and re-established. The removal of the backfill gas pressure may also indicate a reduction in residual charge on the top of the burl; thus, when contact is re-established after switching polarity (e.g., during re-clamping or an unload-load sequence), less current may be required for equalization. In some embodiments, the backfill gas can be actively pumped out through the supply line to avoid or minimize throughput impacts. For example, it may take a long time to wait for the backfill gas to leak out through a leaky seal. Therefore, the supply line can double as an evacuation line to actively remove the backfill gas.

[0077] In some embodiments, residual charge from the burl tops of opposite polarity can be removed between unloading and loading by exposure to an ionized gas (e.g., ionized by EUV-induced plasma or by a dedicated ionizer). In some embodiments, the dedicated ionizer can be very close to or integrated into the electrostatic chuck (e.g., a hydrogen supply point). In some embodiments, a conductive medium or ionized gas (e.g., plasma) can be applied to the burl-reticle interface while the reticle is in the electrostatic chuck to discharge the burl tops and reticle and maintain a uniform potential.

[0078] In some embodiments, operating conditions can be modified, and the reticle backside can be dehumidified before loading / chucking to prevent residual absorbed water and reduce sintering. In some embodiments, a dry interface between the burl and reticle can only carry current through weak spots in the oxide barrier layer if the burl's roughness matches such weak spots; this may have a limited probability. A moist surface may create surface conductivity on the reticle, meaning that the alignment between the roughness and weak spots may only need to be approximate to create sintering current through the oxide barrier weak spots; this increases the likelihood of sintering. Therefore, dehumidifying the reticle backside will help reduce sintering.

[0079] In some embodiments, the reticle backside coating can be conditioned prior to loading / chucking to strengthen or re-strengthen the top layer (e.g., oxide) to minimize the risk of sintering and / or shear damage. Such reticle conditioning can be performed as a step during reticle cleaning and / or storage requirements.

[0080] In some embodiments, by applying these various modifications to the electrostatic clamps described herein, the electrostatic clamp can be enhanced to reduce the electric field between the burl and the reticle, thereby preventing sticking or adhesion of the reticle and removing high-order drift caused by particles of reticle material formed by re-clamping. Other benefits include improved reticle loading grids and increased design freedom in handling reticle heating (e.g., from more frequent re-clamping).

[0081] Example operation method

[0082] Figure 9 is a flow chart of an exemplary method 900 for enhancing an electrostatic clamp according to an embodiment of the present disclosure. In some embodiments, the method 900 may describe the manufacture and modification of an electrostatic clamp, such as described above with reference to Figures 2 to 8 The electrostatic clamps 400, 410, and / or 710 discussed herein are discussed. It should be understood that the operations shown in method 900 are not exhaustive, and that other operations may also be performed before, after, or between any of the illustrated operations. In various embodiments of the present disclosure, the operations of method 900 may be performed in a different order and / or varied.

[0083] In operation 902, an electrostatic clamp is manufactured. The electrostatic clamp is manufactured to include a clamp body, an electrode layer disposed on the top surface of the clamp body, and a plurality of burls protruding from the bottom surface of the clamp body, the plurality of burls being configured to contact the back side of the mask. In some embodiments, the electrostatic clamp can be manufactured by depositing the electrode layer onto an insulating substrate. The insulating substrate can include glass, ceramic, etc. The insulating layer is then placed and bonded to the electrode layer (or deposited on top of the electrode layer). In some embodiments, the plurality of burls can be formed to protrude from the bottom surface of the insulating substrate (e.g., the clamp body). For example, the plurality of burls can be formed by a photolithography process in which a mask is applied to the clamp surface and unmasked material (e.g., portions of the clamp surface not covered by the mask) is etched away to produce the plurality of burls. In other embodiments, the plurality of burls can be formed by a film deposition process. In some embodiments, the plurality of burls can be coated with a predetermined material, and the coating can be used to ground the mask and / or provide a wear-resistant interface between the burls and the mask.

[0084] In operation 904, modifications are applied to the electrostatic chuck to reduce an electric field between the plurality of burls and the reticle. Applying the modifications may include reducing a thickness of a conductive coating on a reticle-contacting surface of the plurality of burls, thereby allowing a voltage difference between the plurality of burls and the reticle to be reduced and / or removing predetermined portions of the electrode layer at locations corresponding to the location of each burl, thereby allowing a plurality of cuts in the electrode layer.

[0085] In some embodiments, the applied modifications may further include connecting each of the plurality of burls together to provide a virtual ground in an electrostatic chuck, grounding the plurality of burls, and / or applying a conductive coating to the mask contact surfaces in the plurality of burls or the back side of the mask, the conductive coating comprising an oxide, diamond, or diamond-like carbon (DLC) material.

[0086] Further aspects of the invention are set out in the numbered clauses below.

[0087] 1. An electrostatic clamp, comprising:

[0088] fixture body;

[0089] an electrode layer disposed on a top surface of the jig body; and

[0090] a plurality of protrusions protruding from the bottom surface of the clamp body,

[0091] The electrode layer includes a plurality of cutouts at predetermined positions vertically corresponding to positions of the plurality of burls at the bottom surface of the clamp body.

[0092] 2. The electrostatic chuck of clause 1, wherein the plurality of burls comprise contact points between the electrostatic chuck and the reticle.

[0093] 3. The electrostatic chuck of clause 1, wherein the plurality of cutouts comprise gaps in the electrode layer that reduce an electric field generated around the plurality of burls.

[0094] 4. The electrostatic chuck of clause 1, wherein the plurality of burls are configured to contact a backside of the reticle.

[0095] 5. The electrostatic chuck according to clause 4, wherein the reticle comprises a conductive coating.

[0096] 6. The electrostatic chuck of clause 1, wherein each burl of the plurality of burls comprises a conductive coating.

[0097] 7. The electrostatic chuck of clause 1, wherein each of the plurality of cutouts has a diameter of approximately 300 to 1400 microns.

[0098] 8. A method for enhancing an electrostatic clamp, the method comprising:

[0099] manufacturing an electrostatic chuck comprising a chuck body, an electrode layer disposed on a top surface of the chuck body, and a plurality of burls protruding from a bottom surface of the chuck body, the plurality of burls configured to contact a back surface of a reticle; and

[0100] Modifications are applied to the electrostatic chuck to reduce the electric field between the plurality of burls and the reticle.

[0101] 9. The method of clause 8, wherein applying comprises reducing a thickness of the conductive coating on a reticle contact surface of the plurality of burls, thereby allowing a voltage difference between the plurality of burls and the reticle to be reduced.

[0102] 10. The method of clause 8, wherein applying comprises removing predetermined portions of the electrode layer at locations corresponding to the location of each burl, thereby allowing for a plurality of cuts in the electrode layer.

[0103] 11. The method of clause 8, wherein applying comprises connecting each of the plurality of burls together to provide a virtual ground in the electrostatic chuck.

[0104] 12. The method of clause 8, wherein applying the modification to the electrostatic clamp comprises grounding the plurality of burls.

[0105] 13. The method of clause 8, wherein applying the modification to the electrostatic chuck comprises applying a conductive coating to the reticle contact surface of the plurality of burls or the backside of the reticle, the conductive coating comprising an oxide, diamond, or diamond-like carbon (DLC) material.

[0106] 14. A lithographic apparatus comprising:

[0107] an illumination system configured to condition the radiation beam;

[0108] a support structure configured to support a patterning device capable of imparting a pattern to the radiation beam in its cross-section to form a patterned radiation beam; and

[0109] a projection system configured to project a patterned radiation beam onto a target portion of the substrate,

[0110] The support structure includes an electrostatic clamp, including:

[0111] fixture body;

[0112] an electrode layer disposed on a top surface of the jig body; and

[0113] a plurality of protrusions protruding from the bottom surface of the clamp body,

[0114] The electrode layer includes a plurality of cutouts at predetermined positions vertically corresponding to positions of the plurality of burls at the bottom surface of the clamp body.

[0115] 15. The lithographic apparatus of clause 14, wherein the plurality of burls comprise contact points between the electrostatic chuck and the patterning device.

[0116] 16. The lithographic apparatus of clause 14, wherein the plurality of cuts comprise gaps in the electrode layer that reduce an electric field generated around the plurality of burls.

[0117] 17. A lithographic apparatus according to clause 14, wherein:

[0118] The patterning device includes a reticle; and

[0119] The plurality of burls are configured to contact a backside of the reticle.

[0120] 18. The lithographic apparatus of clause 17, wherein the reticle comprises a conductive coating.

[0121] 19. The lithographic apparatus of clause 14, wherein each burl of the plurality of burls comprises a conductive coating.

[0122] 20. The lithographic apparatus of clause 14, wherein each of the plurality of kerfs has a diameter of approximately 300 to 1400 microns.

[0123] Final Notes

[0124] Although specific reference may be made herein to a "reticle," it should be understood that this is only one example of a patterning device, and that the embodiments described herein can be applied to any type of patterning device. Additionally, the embodiments described herein can be used to provide secure support for any object to ensure that a clamping failure does not cause the object to fall and damage itself or other devices.

[0125] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it will be understood that the lithographic apparatus described herein may have other applications, such as in the manufacture of guide and detection patterns for integrated optical systems, magnetic domain memories, flat panel displays, LCDs, thin film heads, and the like. The skilled person will appreciate that in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered synonymous with the more general terms "substrate" or "target portion," respectively. The substrates referenced herein may be processed before or after exposure, for example in 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 tool. Where applicable, the present disclosure herein may be applied to such and other substrate processing tools. Further, a substrate may be processed more than once, for example to create a multi-layer IC, so that the term substrate as used herein may also refer to a substrate that already contains multiple processed layers.

[0126] Although specific reference may have been made above to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that the present invention may be used in other applications, such as imprint lithography, and is not limited to optical lithography where the context permits. In imprint lithography, a topography in a patterning device defines a pattern created on a substrate. The topography of the patterning device may be pressed into a resist layer supplied to the substrate, whereupon the resist is cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. After the resist is cured, the patterning device is removed from the resist, leaving a pattern therein.

[0127] It is to be understood that the phraseology and terminology herein are for the purpose of description and not limitation, such that the phraseology or terminology of the present disclosure is to be interpreted by one skilled in the relevant art(s) in light of the teachings herein.

[0128] As used herein, the term "substrate" describes a material to which a layer of material is added. In some embodiments, the substrate itself can be patterned, and the material added thereon can also be patterned, or can remain without patterning.

[0129] While specific reference may be made herein to the use of an apparatus and / or system according to the present invention in the manufacture of ICs, it should be clearly understood that such an apparatus and / or system has many other possible applications. For example, it may be used to manufacture guide and detection patterns for integrated optical systems, magnetic domain memories, LCD panels, thin film magnetic heads, and the like. Those skilled in the art will appreciate that in the context of such alternative applications, any use of the terms "reticle," "wafer," or "die" herein should be considered to be replaced by the more general terms "mask," "substrate," and "target portion," respectively.

[0130] While specific embodiments of the present invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. This description is not intended to limit the invention.

[0131] It should be understood 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.

[0132] The present invention has been described above with the aid of functional building blocks that illustrate the implementation of specified functions and their relationships. For ease of description, the boundaries of these functional building blocks are arbitrarily defined herein. Alternate boundaries can be defined as long as the specified functions and their relationships are properly performed.

[0133] The foregoing description of specific embodiments will fully demonstrate the generality of the present invention. By applying knowledge in the art, others may readily modify and / or adapt such specific embodiments for various applications without undue experimentation and without departing from the general concepts of the present invention. Therefore, based on the teaching and guidance presented herein, such adaptations and modifications are intended to be within the meaning and scope of equivalents to the disclosed embodiments.

[0134] 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

1. An electrostatic clamp, comprising: fixture body; an electrode layer disposed on a top surface of the fixture body; as well as a plurality of burls protruding from a bottom surface of the clamp body, wherein the electrode layer includes a plurality of cutouts at predetermined positions vertically corresponding to positions of the plurality of burls at the bottom surface of the jig body, the plurality of cutouts including at least a first cutout and a second cutout, the first cutout having a first diameter, the second cutout having a second diameter, the second diameter being different from the first diameter, and The first diameter and the second diameter are configured at predetermined locations to reduce an electric field generated around the plurality of burls.

2. The electrostatic chuck according to claim 1 , wherein the plurality of burls comprise contact points between the electrostatic chuck and the reticle, and / or The plurality of cutouts include gaps in the electrode layer that reduce an electric field generated around the plurality of burls. 3 . The electrostatic chuck of claim 1 , wherein the plurality of burls are configured to contact a backside of a reticle, the reticle comprising a conductive coating. The electrostatic chuck of claim 1 , wherein each burl of the plurality of burls comprises a conductive coating. 5 . The electrostatic chuck of claim 1 , wherein each of the plurality of cutouts has a diameter of 300 to 1400 microns.

6. A method for enhancing an electrostatic clamp, the method comprising: manufacturing the electrostatic chuck, the electrostatic chuck including a chuck body, an electrode layer disposed on a top surface of the chuck body, and a plurality of burls protruding from a bottom surface of the chuck body, the plurality of burls configured to contact a back surface of a reticle; and modifying the electrode layer by removing portions of the electrode layer to form a plurality of cuts, the plurality of cuts comprising at least a first cut and a second cut, the first cut having a first diameter, the second cut having a second diameter, the second diameter being different from the first diameter, The first diameter and the second diameter are configured to reduce an electric field between the plurality of burls and the reticle.

7. The method according to claim 6, further comprising: The thickness of the conductive coating on the reticle contacting surfaces of the plurality of burls is reduced, thereby allowing a voltage difference between the plurality of burls and the reticle to be reduced.

8. The method according to claim 6, further comprising: Each of the plurality of burls is connected together to provide a virtual ground in the electrostatic chuck.

9. The method according to claim 6, further comprising: The plurality of burls are grounded.

10. The method according to claim 6, further comprising: A conductive coating is applied to the reticle contact surfaces of the plurality of burls or the backside of the reticle, the conductive coating comprising an oxide, diamond, or diamond-like carbon (DLC) material.

11. A lithographic apparatus comprising: an illumination system configured to condition the radiation beam; a support structure configured to support a patterning device capable of imparting a pattern to the radiation beam in its cross-section so as to form a patterned radiation beam; as well as a projection system configured to project the patterned radiation beam onto a target portion of a substrate, The support structure includes an electrostatic clamp, and the electrostatic clamp includes: fixture body; an electrode layer disposed on a top surface of the fixture body; and a plurality of burls protruding from a bottom surface of the clamp body, wherein the electrode layer includes a plurality of cutouts at predetermined positions vertically corresponding to positions of the plurality of burls at the bottom surface of the jig body, the plurality of cutouts including at least a first cutout and a second cutout, the first cutout having a first diameter, the second cutout having a second diameter, the second diameter being different from the first diameter, and The first diameter and the second diameter are configured at predetermined locations to reduce an electric field generated around the plurality of burls.

12. The lithographic apparatus of claim 11 , wherein the plurality of burls comprise contact points between the electrostatic chuck and the patterning device, and / or The plurality of cutouts include gaps in the electrode layer that reduce an electric field generated around the plurality of burls.

13. The lithographic apparatus of claim 11 , wherein: The pattern forming device includes a reticle; The plurality of burls are configured to contact a back surface of the reticle; The reticle includes a conductive coating; and Each burl of the plurality of burls includes a conductive coating.

14. The lithographic apparatus of claim 11, wherein each of the plurality of cutouts has a diameter of 300 to 1400 microns.

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