Object holder and manufacturing method

Abstract

The object holder is configured to support an object. The object holder (20) comprises a core body (21) including a plurality of burls (22) having distal ends (23) within a support surface (24) for supporting the object. The object holder further comprises an electrostatic sheet (25) between the burls. The electrostatic sheet comprises an electrode (26) including a metal and metal oxide layers (27, 28) including an oxide of the metal. The electrode is sandwiched between the metal oxide layers.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Application No. 22208395.8, filed November 18, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to an object holder, an object table including such an object holder, a lithographic apparatus including such an object holder, and a method for manufacturing such an object holder. [Background technology]

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

[0004]

[0004] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Lithographic apparatus using extreme ultraviolet (EUV) radiation having 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 than lithographic apparatus using radiation having a wavelength of, e.g., 193 nm.

[0005]

[0005] An object, such as a substrate or a mask, can be held by an object holder. The object holder is configured to clamp the object upon high acceleration of the object, such as a substrate or a mask. For example, a vacuum clamp may be used, whereby the space between the object and the object holder is evacuated or partially evacuated, and the object is pressed against the holder by gas pressure on the opposite surface of the object. However, lithographic apparatuses using EUV radiation may operate in vacuum or near-vacuum conditions, i.e., at low pressure. In such lithographic apparatuses, a vacuum clamp may be inappropriate, and therefore an electrostatic clamp may be used. The lithographic apparatus may include an electrostatic object holder configured to hold the object by electrostatic force. The electrostatic object holder includes a plurality of burls having distal ends within a support surface for supporting the object, and an electrostatic sheet between the burls, the electrostatic sheet being configured to be electrically charged to exert an electrostatic clamping force on the object held by the burls. Summary of the Invention

[0006]

[0006] It is an object of the present invention to provide an improved object holder.

[0007]

[0007] According to one aspect of the present invention, there is provided an object holder configured to support an object, the object holder comprising a core body including a plurality of burls having distal ends within a support surface for supporting the object, and an electrostatic sheet between the burls, the electrostatic sheet comprising an electrode including a metal and a metal oxide layer including an oxide of the metal, the electrode being sandwiched between the metal oxide layers.

[0008] According to another aspect of the invention, there is provided an object table comprising a support member and an object holder according to an aspect of the invention, the object holder being attached to the support member.

[0009]

[0009] According to yet another aspect of the present invention, there is provided a lithographic apparatus configured to project a pattern from a patterning device onto a substrate, the lithographic apparatus including an object holder according to an aspect of the present invention or an object table according to an aspect of the present invention.

[0010]

[0010] According to a further aspect of the present invention, there is provided a method for manufacturing an object holder, the method comprising: providing a metal sheet comprising a metal; creating a plurality of through holes in the metal sheet; providing a metal oxide layer on the metal sheet to form an electrostatic sheet, the metal oxide layer comprising an oxide of the metal; providing a core body comprising a plurality of burls having distal ends within a support surface for supporting an object; and assembling the electrostatic sheet and the core body to form the object holder. [Brief explanation of the drawings]

[0011]

[0011] Some embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0012] - [Figure 1] Figure 1 depicts a lithography system comprising a lithographic apparatus and a radiation source; [Figure 2] Figure 2 shows a cross-sectional view of a part of an object holder for a lithographic apparatus. [Figure 3] Figure 3 shows a cross-sectional view of a part of another embodiment of an object holder. [Figure 4] Figure 4 shows the steps for manufacturing the object holder shown in Figures 2 and 3. [Figure 5] Figure 5 shows the steps for manufacturing the object holder shown in Figures 2 and 3. [Figure 6] Figure 6 shows a cross-sectional view of a portion of an object holder according to an embodiment of the invention. [Figure 7] Figure 7 shows a cross-sectional view of a part of an object holder according to another embodiment of the invention. [Figure 8] FIG. 8 shows a cross-sectional view of another part of an object holder according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] 1 shows a lithography system comprising a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate a beam of EUV radiation B and to provide the beam of EUV radiation B to the lithography apparatus LA. The lithography 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.

[0014] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. In addition, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. Together, the facetted field mirror device 10 and the facetted pupil mirror device 11 provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. In addition to, or instead of, the facetted field mirror device 10 and the facetted pupil mirror device 11, the illumination system IL may include other mirrors or other devices.

[0015] After being conditioned in this way, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is produced. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To that end, the projection system PS may comprise a number of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by a substrate table WT. The projection system PS may apply a demagnification factor to the patterned EUV radiation beam B' to form an image having smaller features than corresponding features on the patterning device MA. For example, a demagnification factor of 4x or 8x may be applied. Although the projection system PS is shown in Figure 1 as having only two mirrors 13, 14, the projection system PS may include a different number of mirrors (for example 6 or 8 mirrors).

[0016]

[0015] The substrate W may include a previously formed pattern, in which case the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B' to the previously formed pattern on the substrate W.

[0017] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided within the source SO, illumination system IL and / or projection system PS. In another embodiment, hydrogen may also be provided within the compartment in which the substrate table WT is located.

[0018]

[0017] The source SO may be a laser-produced plasma (LPP) source, a discharge-produced plasma (DPP) source, a free-electron laser (FEL), or any other source capable of producing EUV radiation.

[0019] 2 is a cross-sectional view of an object holder 20 for a lithographic apparatus. The object holder 20 is configured to support an object. In the following description, the invention is described in the context that the object holder is a substrate holder 20 configured to support a substrate W. However, the object holder is not limited to such a substrate holder 20. For example, the object holder may be configured to support a patterning device MA, such as a mask.

[0020] In one embodiment, the substrate table WT includes a substrate holder 20 and a substrate stage. For example, the substrate stage may include a recess in which the substrate holder 20 is held. The substrate holder 20 is configured to hold the substrate W relative to the substrate stage of the substrate table WT. As another example, a mirror block of the substrate stage may have a flat top surface without cutouts for a clamp. A bottom electrode of the clamp may exert a clamping force on the mirror block, holding the clamp in place on the mirror block.

[0021] As shown in FIG. 2, in one embodiment, the substrate holder 20 includes a core body 21. The core body 21 is a plate-shaped disk. As shown in FIG. 2, in one embodiment, the core body 21 includes a plurality of burls 22. The burls 22 are protrusions that protrude from the surface of the core body 21. As shown in FIG. 2, in one embodiment, the burls 22 have distal ends 23. The core body 21 is configured such that the distal ends 23 are within a support surface 24 for supporting the substrate W. The lower surface of the substrate W comes into contact with the distal ends 23 of the burls 22. The position of the lower surface of the substrate W coincides with the support surface 24. The burls 22 are arranged so that the substrate W is approximately flat on the substrate holder 20. The core body 21 may include a cooling function, such as a channel through which a coolant such as water can flow. In this case, the core may be composed of multiple material layers bonded to each other by oxidation bonding, diffusion bonding, or other bonding techniques.

[0022] The burls 22 are not shown to scale in Figure 2. In a practical embodiment, there may be hundreds, thousands, or tens of thousands of burls distributed throughout a substrate holder 20 of, for example, a diameter of 200 mm, 300 mm, or 450 mm. The tips of the burls 22 may be, for example, 1 mm 2 Due to their smaller area, the total area of ​​all burls 22 on one side of the substrate holder 20 is less than about 10% of the total surface area of ​​the substrate holder 20. Due to the arrangement of the burls 22, any particles that may be present on the surface of the substrate W, substrate holder 20, or substrate table WT are likely to fall between the burls 22 and therefore not cause deformation of the substrate W or substrate holder 20. The burl arrangement, which may form a pattern, may be regular or may vary as needed to provide an appropriate force distribution on the substrate W and substrate table WT. On the lower side, the number of burls may be much smaller (e.g., about 150), and the surface area may be larger than 1 mm2. On both the upper and lower sides, trenches may be formed around each burl that protrudes into the core body 21, which has the advantage of reducing the lateral stiffness of the burls. This allows them to adapt without slippage to changes in the shape of the substrate due to thermal changes caused by the thermal load of exposure, and on the lower side, to deformation of the mirror block (i.e., the substrate table body) due to acceleration. The burls 22 can have any shape in a plan view, but are generally circular in a plan view. The burls 22 can have the same shape and dimensions throughout their height, but are generally tapered. The burls 22 can protrude above the remainder of the object-facing surface of the substrate holder 20 (i.e., the upper surface of the electrostatic sheet 25) by a distance of about 1 μm to about 5 mm, preferably about 5 μm to about 250 μm, and preferably about 10 μm. Thus, the vertical distance between the distal ends 23 of the burls 22 and the upper surface of the electrostatic sheet 25 is about 1 μm to about 5 mm, preferably about 5 μm to about 250 μm, and preferably about 10 μm. The thickness of the core body 21 of the substrate holder 20 can be in the range of about 1 mm to about 50 mm, preferably about 5 mm to 20 mm, and typically 10 mm.

[0023]

[0022] The core body 21 may be made of a hard material. The material preferably has a high thermal conductivity and a low coefficient of thermal expansion. Preferably, the material is electrically conductive. Preferably, the material has a high hardness. Suitable materials include SiC (silicon carbide), SiSiC (silicon carbide), Si3N4 (silicon nitride), quartz, and / or various other ceramics and glass ceramics, such as Zerodur® glass ceramic and / or cordierite. The core body 21 can be fabricated by selectively removing material from a solid disk of associated material, leaving protruding burls 22. Suitable techniques for removing material include electrical discharge machining (EDM), etching, machining, and / or laser ablation. The core body 21 can also be fabricated by growing burls 22 through a mask. The burls 22 may be the same material as the substrate and can be grown by a physical vapor deposition process or sputtering. In some embodiments, the core body 21 includes one or more internal channels (not shown). In some embodiments, core body 21 includes multiple layers bonded together. In some embodiments, the multiple layers are formed of different materials. By way of example only, in one embodiment, core body 21 includes a layer of SiSiC, a layer of glass, and another layer of SiSiC, in that order. Other combinations of layers are possible.

[0024] As shown in FIG. 2 , in one embodiment, the substrate holder 20 includes one or more electrodes 26 for electrostatic clamping. A potential difference may be generated to provide an electrostatic clamping force between the substrate W and the substrate holder 20, and / or between the substrate holder 20 and a substrate stage of the substrate table WT. In one embodiment, the electrodes 26 are enclosed between dielectric layers (also known as electrical insulating layers) 27, 28. The generated potential difference may be, for example, on the order of 10 volts to 5,000 volts, and the polarity of the potential difference may be positive or negative. An arrangement using one or more heaters and temperature sensors to locally control the temperature of a substrate is described in U.S. Publication No. 2011-0222033, the entire contents of which are incorporated herein by reference, and the teachings thereof may be applied to the teachings herein.

[0025] As shown in FIG. 2 , in some embodiments, the substrate holder 20 includes an electrostatic sheet 25. The electrostatic sheet 25 includes one or more electrodes 26. In some embodiments, two halves of a continuous metal film (separated from the distal ends 23 of the burls 22) can be positioned a separation distance apart from one another to form the positive and negative elements of the electrostatic clamp. The separation distance is not particularly limited. In some embodiments, the separation distance is at least about 20 μm. In other embodiments, the separation distance is at least about 50 μm, optionally at least about 100 μm, optionally at least about 200 μm, and optionally at least about 500 μm. In some embodiments, the separation distance is at most about 2 mm, optionally at most about 1 mm, and optionally at most about 500 μm. In some embodiments, the separation distance is about 500 μm. Thus, there can be two electrodes 26. However, the number of electrodes 26 on the electrostatic sheet 25 is not particularly limited and may be one, or three or more. For example, at least two electrodes may be required to clamp a substrate with an insulated backside. One electrode may be set to a positive voltage and the other to a negative voltage. This method may not require grounding the substrate, but on average the substrate will be clamped at a potential close to ground (0 V). For applications of electrodes on the underside of the clamp, one electrode may be sufficient to clamp to the mirror block. This is because a voltage difference can be established between the clamp electrode and the mirror block by grounding the conductive coating of the mirror block, generating a force.

[0026] The metal lines of electrode 26 may have a thickness greater than about 20 nm, preferably greater than about 40 nm. The metal lines preferably have a thickness of about 1 pm or less, preferably less than about 500 nm, preferably less than about 200 nm.

[0027] The electrode 26 of the upper electrostatic sheet 25 may be configured to electrostatically clamp the substrate W to the substrate holder 20. The electrode 26 of the lower electrostatic sheet 25 may be configured to electrostatically clamp the substrate holder 20 to the remainder of the substrate table WT.

[0028]

[0027] In some embodiments, the material of the core body 21 and the burls 22 is electrically conductive. For example, in some embodiments, the material of the burls 22 is SiSiC. However, it is not necessary that the material of the core body 21 and the burls 22 be electrically conductive. In some embodiments, a ground layer may be provided that electrically connects the distal ends 23 of two or more burls 22 (optionally all of the burls 22) to ground or a common potential. The ground layer may be formed by depositing a relatively thick layer of an electrically conductive material. The electrically conductive material is not particularly limited. In some embodiments, the electrically conductive material is Cr. In another embodiment, the electrically conductive material is CrN. In some embodiments, the deposited layer is then patterned to form the ground layer. The pattern may include a series of metal lines connecting the distal ends 23 of the burls 22 together. Such a pattern may be referred to as a "Manhattan" pattern. In another embodiment, the deposited layer is not patterned. In some embodiments, the ground layer or another layer is disposed to cover the surface of the core body 21 and / or the burls 22. The ground layer or other layer can help lubricate the surface, making it easier to clean.

[0029] As shown in FIG. 2, in one embodiment, the electrostatic sheet 25 includes an electrode 26 sandwiched between dielectric layers 27 and 28. As shown in FIG. 2, in one embodiment, the burls 22 and the electrostatic sheet 25 are provided on both major surfaces of the substrate. In another embodiment, the burls 22 and the electrostatic sheet 25 are provided on only one of the two major surfaces of the substrate holder 20. As shown in FIG. 2, in one embodiment, the electrostatic sheet 25 is located between the burls 22. For example, as shown in FIG. 2, holes 34 are provided in the electrostatic sheet 25. The holes 34 are positioned so that their positions correspond to the burls 22 of the core body 21. The burls 22 protrude through each hole 34 in the electrostatic sheet 25 so that the electrode 26 sandwiched between the dielectric layers 27 and 28 is provided in the region between the burls 22.

[0030] As shown in FIG. 2 , in one embodiment, the substrate holder 20 includes a bonding material 29. In one embodiment, the bonding material has a thickness of at least 100 nm. The bonding material 29 fixes the position of the electrostatic sheet 25 relative to the core body 21. The bonding material 29 maintains the alignment of the holes 34 in the electrostatic sheet 25 with the burrs 22. In one embodiment, the burrs 22 are positioned at the center of each hole 34 in the electrostatic sheet 25. A thickness of at least 100 nm for the bonding material 29 ensures a minimum level of compliance for the bonding layer. Generally, the thicker the bonding material, the more compliant it is and the less sensitive it is to stress levels caused by temperature differences at the interface between the core body 21 and the electrostatic sheet 25. In one embodiment, the thickness of the bonding material 29 is at least 2 micrometers. In another embodiment, the thickness of the bonding material 29 is at least 200 nm, optionally at least 500 nm, optionally at least 1 pm, optionally at least 5 pm, optionally at least 10 pm, optionally at least 20 pm, optionally at least 25 pm, and optionally at least 50 pm. The inventors have discovered that a thickness of the bonding material 29 of at least 25 μm improves the tolerance to temperature differences at the interface (i.e., increases the conformity of the bonding layer). In some embodiments, the thickness of the bonding material 29 is at most 100 pm. This ensures a minimum stability of the bonding material 29, which is beneficial to the electrostatic performance of the clamping function. In some embodiments, the thickness of the bonding material 29 is at most 75 pm, optionally at most 50 pm. Reducing the thickness of the bonding material 29 can increase the stability of the bonding material 29. By way of example only, in some embodiments, the thickness of the bonding material 29 ranges from about 25 pm to about 75 pm.

[0031] As shown in FIG. 2, in some embodiments, the bonding material 29 is formed in separate portions that are not connected to one another. For example, there may be 1,000 glue dots disposed between the burls. The glue dots may be disposed on specific pads machined into the core 21. There may be some variation in the thickness of different portions of the bonding material 29. In some embodiments, the separate portions of the bonding material 29 have substantially the same thickness. As shown in FIG. 10, in another embodiment, the bonding material 29 extends continuously to the region between the core body 21 and the electrostatic sheet 25. As shown in FIG. 10, in some embodiments, the bonding material 29 extends to the region between the electrostatic sheet 25 and the burls 22.

[0032] 3 to 5 show diagrammatically successive stages of the process of manufacturing the substrate holder 20. In particular, FIGS. 3 to 5 show the stage of bonding the electrostatic sheet 25 to the core body 21.

[0033] As shown in FIG. 3 , in an embodiment, a method for manufacturing a substrate holder 20 includes providing a core body 21. The core body 21 includes a plurality of burls 22 for supporting a substrate W. As shown in FIG. 3 , in an embodiment, the method includes applying a binder 29 to the core body 21. The binder 29 is applied to at least one of the major surfaces of the core body 21. The binder 29 is applied between the burls 22. As shown in FIG. 3 , in an embodiment, multiple separate portions of the binder 29 are applied to the core body 21. In another embodiment, the binder 29 is applied substantially continuously across the major surface of the core body 21 between the burls 22. The method for distributing the binder 29 is not particularly limited. By providing multiple separate portions of the binder 29, the overall volume of the binder 29 can be reduced. This reduces the likelihood of excess binder 29 being present near the burls 22, which may adversely affect the clamping function of the substrate holder 20.

[0034] 4, in some embodiments, the method includes coupling the electrostatic sheet 25 to a plate 30 of the tool. The plate 30 includes a plurality of recesses 31 for each of the burls 22 of the core body 21. The recesses 31 fit snugly with respective holes 34 in the electrostatic sheet 25. In some embodiments, the centers of the recesses 31 are aligned with the centers of the holes 34 in the electrostatic sheet 25. The plate 30 is configured such that the pattern of the recesses 31 matches the pattern of the burls 22 of the core body 21. Thus, the disclosure above regarding the number and positioning of the burls 22 applies equally to the number and positioning of the recesses 31 in the plate 30 and the holes 34 in the electrostatic sheet 25. In some embodiments, the plate 30 has a maximum of 10 recesses 31 in a direction perpendicular to the plane of the plate 30. 5 Nm -1 The plate 30 is compliantly configured to accommodate variations in the height of the burls 22.

[0035]

[0034] The manner in which the plate 30 is coupled to the electrostatic sheet 25 is not particularly limited. For example, the coupling between the plate 30 and the electrostatic sheet 25 may be by vacuum and / or electrostatic attraction and / or temporary bonding. One means for generating electrostatic attraction may be applying an electric potential (voltage) between the plate 30 and the sheet 25. In an embodiment, the plate 30 is positioned relative to the core body 21, for example, using a positioner. The positioner is configured to control the movement of the plate 30 when the electrostatic sheet 25 is coupled. The positioner positions the plate 30 relative to the core body 21 so that the recesses 31 in the plate 30 (and therefore the holes 34 in the electrostatic sheet 25) are aligned with the burrs 22 of the core body 21.

[0036] The positioner is configured to control the movement of the plate 30 to attach the electrostatic sheet 25 to the core body 21 between the burls 22. For example, the positioner may lower the plate 30 downward onto the core body 21. In another embodiment, the positioner is configured to move the core body 21 upward toward the plate 30 so that the burls 22 extend through the holes 34 in the electrostatic sheet 25.

[0037] During the attachment process, the distal end 23 of the burl 22 contacts the base of the recess 31. The base of the recess 31 prevents the electrostatic sheet 25 and the core body 21 from moving closer together. The depth of the recess 31 determines the height to which the electrostatic sheet 25 is attached. The depth of the recess 31 controls the distance between the bottom of the electrostatic sheet 25 and the top surface of the core body 21 (where the bonding material 29 is positioned). The depth of the recess 31 controls the height to which the burl 22 protrudes above the top surface of the electrostatic sheet 25 in the manufactured substrate holder 20.

[0038]

[0037] In one embodiment, the recess 31 has a depth corresponding to the above-mentioned protruding distance of the burr 22. For example, the recess 31 has a depth of about 1 μm to about 5 mm, desirably about 5 μm to about 250 μm.

[0039] In some embodiments, the method of manufacturing the substrate holder 20 includes curing the bonding material 29. In some embodiments, the bonding material 29 is cured under vacuum. This helps to avoid inclusions in the bonding material 29. However, it is not necessary for the bonding material 29 to be cured. In other embodiments, the bonding material 29 does not need to be cured.

[0040]

[0039] The type of bonding material used is not particularly limited. In one embodiment, bonding material 29 includes an adhesive material. However, it is not necessary for bonding material 29 to be an adhesive material. A non-adhesive material can also be used. For example, in another embodiment, bonding material 29 is a material used when soldering or welding electrostatic sheet 25 to core body 21, or is solder glass or frit glass.

[0041] 5 shows diagrammatically the moment when the distal end 23 of the burr 22 abuts the base of the recess 31. Recesses can also be formed on a flat plate by depositing a coating where it is necessary to raise it. For example, for 10 μm, multiple coatings can be used.

[0042] 2 to 5, the electrostatic sheet 25 of the object holder described above with reference to Figures 2 to 5 may comprise two glass layers forming a dielectric layer and a conductive coating, such as a chromium coating, on one of the glass layers between them. For example, the conductive coating may be disposed on the underside of the top glass layer.

[0043]

[0042] In the above-mentioned object holder, the electrostatic sheet is made of glass layers, which may be fragile. For example, particles such as dust particles between the object held by the object holder and the electrostatic sheet may exert a strong force on the contact area of ​​the electrostatic sheet, and the particles may come into contact with the electrostatic sheet, increasing the risk of cracking one or both of the glass layers. Furthermore, a high voltage is applied to the conductive coating between the glass layers, resulting in a high voltage difference toward the crowbar, which may increase the risk of dielectric breakdown if such cracks occur.

[0044]

[0043] Figure 6 shows an enlarged view of a portion of a substrate holder 60 according to an embodiment of the present invention. As shown in Figure 6, in an embodiment, the substrate holder 60 comprises a core body 61. The core body 61 may be a plate-like disk with a plurality of burls 62 forming protrusions that protrude from a surface of the core body 61. Distal ends 63 of the burls 62 are disposed within a support surface 64 for supporting a substrate W.

[0045]

[0044] Although only two burls are shown in Figure 6, in an actual embodiment, there may be hundreds, thousands, or tens of thousands of burls distributed across a 60 mm substrate holder. Therefore, the illustration in Figure 6 can be understood to show only a portion of the substrate holder. In one embodiment, the electrode may be shaped like a colander with 1000 holes, with one burl protruding through each hole.

[0046] For further features of core body 61, please refer to core body 21 shown in and described above with reference to FIG.

[0047] Similar to the substrate holder 20 shown in and described with reference to Figure 2, the substrate holder 60 shown in Figure 6 comprises one or more electrodes 66 for electrostatic clamping. A potential difference can be generated between the substrate W and the substrate holder 60, and / or between the substrate holder 60 and the substrate stage of the substrate table WT, to provide an electrostatic clamping force. In an embodiment, the electrodes 66 are encompassed by dielectric layers 67, 68. The generated potential difference may be, for example, in the order of 10 to 5,000 volts.

[0048]

[0047] According to one aspect of the present invention, the electrode includes a metal, and the dielectric layer includes a metal oxide layer, where the metal oxide is an oxide of the metal. Thus, the electrostatic sheet includes a metal oxide layer and an electrode sandwiched between the metal oxide layers, where the electrode includes a metal, and the metal oxide includes an oxide of the metal. The electrostatic sheet including a metal oxide layer and an electrode sandwiched between the metal oxide layers, where the electrode includes a metal, and the metal oxide includes an oxide of the metal, can be strong. Therefore, compared with the glass layer of the electrostatic sheet described with reference to FIG. 2, the electrostatic sheet can be less prone to cracking. Furthermore, because the metal oxide of the metal oxide layer includes an oxide of the same metal as the metal of the electrode, a strong bond is provided between the electrode and the metal oxide layer, thereby improving the strength of the electrostatic sheet.

[0049] In some embodiments, the metal includes at least one of aluminum, titanium, and magnesium. The metal may be in the form of pure aluminum, titanium, or magnesium, or any alloy of aluminum, titanium, or magnesium. These metals and their alloys can be oxidized by plasma electrolytic oxidation (PEO), producing a highly crack-resistant sandwich structure. This is because the metal and metal oxide forming the sandwich structure are fabricated from a single metal sheet, the outer surface of which is oxidized to provide a strong bond between the electrode and the dielectric layer. For an aluminum plate oxidized on both sides with 100 micrometers of Al2O3, the aluminum does not need to be extremely thin. Having a relatively thick aluminum plate (200-1000 micrometers) provides a robust substrate holder that can withstand damage from particles compressed between the wafer and the electronic sheet.

[0050]

[0049] In one embodiment, as described below with reference to Figure 8, the electrostatic sheet 65 has a plurality of through holes 74, the burls 62 extend through the through holes, and the edges 73 of the through holes are covered by metal oxide extending between the metal oxide layers.

[0051]

[0050] In one embodiment, the pitch of the through holes in the electrostatic sheet matches the pitch of the burls so that the burls fit snugly with the through holes in the electrostatic sheet.

[0052]

[0051] In one embodiment, the electrostatic sheet extends substantially parallel to the support surface and the distance between the electrostatic sheet and the support surface, i.e., the substrate or wafer held by the substrate holder, is the same, thereby promoting a uniformly distributed electrostatic clamping force.

[0053] FIG. 7 shows an enlarged view of a portion of an object holder according to another embodiment of the present invention. As shown in FIG. 7, the outer surface of the electrostatic sheet facing away from the support surface is coated with a conductive coating 70. The conductive coating can be configured to be connected to ground or another suitable potential. This shields the electrostatic sheet from the core body 61, which can be electrically grounded or held at another suitable potential, preventing or at least reducing electrostatic interaction between the electrostatic sheet and the core body. This reduces the force on the glue 69 between the electrostatic sheet and the core body, which can contribute to the useful life of the glue 69. This can also prevent electrostatic interaction on the underside. The shield can increase the force on the glue. Without the shield, the electrode may be pulled toward the substrate and the core body. With the shield, the electrode may only be pulled toward the substrate, so this force may also act on the glue dots. The force may be, for example, on the order of 10 to 100 kN / m2 or 0.1 to 1 bar.

[0054]

[0053] The object holder according to Fig. 7 further corresponds to the object holder according to Fig. 6. The same reference numbers refer to the same or similar items and the description of Fig. 6 applies analogously to the embodiment shown in Fig. 7.

[0055] In one embodiment, the aluminum oxide layer is coated with polyetheretherketone (PEEK), which can fill the pores of the metal oxide-containing dielectric layer and thus help reduce arcing in such pores.

[0056]

[0055] In one embodiment, the thickness of the metal oxide layer is 50 to 150 micrometers, providing a relatively thin layer with suitable dielectric properties, allowing the distance between one electrode and the other substrate to be relatively short, and therefore a high clamping force to be built.

[0057] In some embodiments, the electrode thickness is 300 to 500 micrometers. A relatively thick electrode comprising a metal can provide a mechanically strong electrode, thereby promoting the mechanical robustness of the entire electrostatic sheet and promoting the resistance of the electrostatic sheet to cracking.

[0058] In one embodiment, the distance between the support surface and the outer surface of the electrostatic sheet facing the support surface is between 1 and 20 micrometers, which, especially when combined with the aforementioned feature that the thickness of the metal oxide layer is between 50 and 150 micrometers, can generate a high clamping force.

[0059]

[0058] In an embodiment, the substrate holder is provided with further electrostatic clamps that allow a surface of the clamp that is parallel to and facing away from the support surface to be clamped to another structure, for example another structure of the lithographic apparatus. Thus, in such an embodiment, the core body comprises a plurality of further burls having distal ends in the plane of the substrate table for supporting the substrate holder on the body of the substrate table, and further electrostatic sheets between the further burls, the further electrostatic sheets comprising metal oxide layers and electrodes sandwiched between the metal oxide layers, and the mirror block plane is parallel to the support surface.

[0060]

[0059] In an embodiment, the metal of the electrode of the further electrostatic sheet comprises aluminum and the metal oxide comprises aluminum oxide. Alternatively, the metal may comprise titanium or magnesium, as described above.

[0061]

[0060] The object holder has been described above in the example of a substrate holder, such as a wafer holder. The object holder described herein may also be a patterning device holder configured to hold a patterning device, such as a mask.

[0062] 8 shows a cross-sectional view of an object holder 60 according to the invention. The object holder 60 is configured to support an object. In the following description, the invention will be described in the context that the object holder is a substrate holder 60 configured to support a substrate W. However, the object holder is not limited to such a substrate holder 60. For example, the object holder may be configured to support a patterning device MA, such as a mask.

[0063] In an embodiment, the substrate table WT comprises a substrate holder 60 and a substrate stage. For example, the substrate stage includes a recess in which the substrate holder 60 is held. The substrate holder 60 is configured to hold the substrate W relative to the substrate stage of the substrate table WT. As another example, the mirror block of the substrate stage may have a flat top surface without cutouts for clamps. A bottom electrode of the clamp may exert a clamping force on the mirror block, holding the clamp in place on the mirror block.

[0064] As shown in FIG. 8 , in an embodiment, the substrate holder 60 includes a core body 61. The core body 61 is a plate-shaped disk. As shown in FIG. 8 , in an embodiment, the core body 61 includes a plurality of burls 62. The burls 62 are protrusions that protrude from the surface of the core body 61. As shown in FIG. 8 , in an embodiment, the burls 62 have distal ends 63. The core body 61 is configured such that the distal ends 63 are within a support surface 64 for supporting the substrate W. The lower surface of the substrate W comes into contact with the distal ends 63 of the burls 62. The position of the lower surface of the substrate W coincides with the support surface 64. The burls 62 are arranged so that the substrate W is approximately flat on the substrate holder 60.

[0065] The burls 62 are not shown to scale in FIG. 8 . In a practical embodiment, there can be hundreds, thousands, or tens of thousands of burls distributed throughout a substrate holder 60 having a diameter of, for example, 200 mm, 300 mm, or 450 mm. Because the tips of the burls 62 have a small area, for example less than 1 mm, the total area of ​​all of the burls 62 on one side of the substrate holder 60 is less than about 10% of the total surface area of ​​the substrate holder 60. Due to the arrangement of the burls 62, any particles that may be present on the surface of the substrate W, substrate holder 60, or substrate table WT are likely to fall between the burls 62 and therefore not cause deformation of the substrate W or substrate holder 60. The burl arrangement, which may form a pattern, may be regular or may vary as needed to provide an appropriate force distribution on the substrate W and substrate table WT. The burls 62 can have any shape in plan, but are typically circular in plan. The burls 62 can have the same shape and dimensions throughout their height, but are typically tapered. The burls 62 can protrude above the remainder of the object-facing surface of the substrate holder 60 (i.e., the upper surface of the electrostatic sheet 65) by a distance of about 1 micrometer to about 20 micrometers. Thus, the vertical distance between the support surface formed by the distal ends 63 of the burls 62 and the upper surface of the electrostatic sheet 65 is about 1 micrometer to about 20 micrometers. The thickness of the core body 61 of the substrate holder 20 can range from about 1 mm to about 50 mm, desirably from about 5 mm to 20 mm, and typically 10 mm. The burls may be coated with a coating to create favorable mechanical properties for substrate contact, such as a wear-resistant material with a specific coefficient of friction and resistance to damage caused by particles between the substrate and the burls.

[0066]

[0065] The core body 61 may be made of a hard material. Preferably, the material has a high thermal conductivity or a low coefficient of thermal expansion. Preferably, the material is electrically conductive. Preferably, the material has a high hardness. Suitable materials include SiC (silicon carbide), SiSiC (silicon carbide), Si3N4 (silicon nitride), quartz, and / or various other ceramics and glass ceramics, such as Zerodur® glass ceramic and / or cordierite. The core body 61 can be fabricated by selectively removing material from a solid disk of associated material, leaving protruding burls 62. Suitable techniques for removing material include electrical discharge machining (EDM), etching, machining, and / or laser ablation. The core body 61 can also be fabricated by growing burls 62 through a mask. The burls 62 may be the same material as the substrate and can be grown by a physical vapor deposition process or sputtering. In some embodiments, the core body 61 includes one or more internal channels (not shown). In some embodiments, core body 61 includes multiple layers bonded together. In some embodiments, the multiple layers are formed of different materials. By way of example only, in one embodiment, core body 61 includes a layer of SiSiC, a layer of glass, and another layer of SiSiC, in that order. Other combinations of layers are possible.

[0067] 8, in an embodiment, the substrate holder 60 comprises one or more electrodes 66 for electrostatic clamping. A potential difference may be generated to provide an electrostatic clamping force between the substrate W and the substrate holder 60, and / or between the substrate holder 60 and a substrate stage of the substrate table WT. In an embodiment, the electrodes 66 are enclosed between dielectric layers (also known as electrical insulating layers) 27, 28. The generated potential difference may be, for example, from about 10 volts to about 5,000 volts.

[0068] As shown in FIG. 8 , the substrate holder 60 includes an electrostatic sheet 65. The electrostatic sheet 65 includes one or more electrodes 66 comprising a metal. For the electrodes 66, in one embodiment, two halves of the electrode 66 (but separated from the distal end 63 of the burl 62) may be deposited a certain separation distance apart to form the positive and negative elements of the electrostatic clamp. The separation distance is not particularly limited. Thus, there may be two electrodes 66. However, the number of electrodes 66 of the electrostatic sheet 65 is not particularly limited and may be one, or three or more. The electrodes 66 may have a thickness between 300 and 500 micrometers.

[0069] The electrodes 66 of the electrostatic sheet 65 may be configured to electrostatically clamp the substrate W to the substrate holder 60. A further electrostatic clamp may be formed by a further burl 62 and a further electrode 66 arranged on the underside of the object holder. The further electrodes 66 of the further electrostatic sheet 65 may be configured to electrostatically clamp the substrate holder 60 to the remainder of the substrate table WT, for example to its mirror blocks.

[0070]

[0069] In some embodiments, the material of the core body 61 and the burls 62 is electrically conductive. For example, in some embodiments, the material of the burls 62 is SiSiC. However, it is not essential that the material of the core body 61 and the burls 62 be electrically conductive. In some embodiments, a ground layer may be provided that electrically connects the distal ends 63 of two or more burls 62 (optionally all of the burls 62) to ground or a common potential. The ground layer may be formed by depositing a relatively thick layer of an electrically conductive material. The electrically conductive material is not particularly limited.

[0071] As shown in FIG. 8 , in one embodiment, an electrostatic sheet 65 includes an electrode 66 containing a metal sandwiched between dielectric layers 67 and 68 containing a metal oxide. For example, the electrostatic sheet may contain aluminum, and the dielectric layer may contain aluminum oxide. As another example, the electrostatic sheet may contain magnesium, and the dielectric layer may contain magnesium oxide. As yet another example, the electrostatic sheet may contain titanium, and the dielectric layer may contain titanium oxide. The metal-containing electrode structure sandwiched between the metal oxide layers may form a mechanically stable structure that is less susceptible to cracking when subjected to mechanical stress from dust particles between the electrostatic sheet and the substrate, or during substrate holder fabrication. The thickness of the metal oxide layer may be between 50 and 150 micrometers. The oxide formation may be conformal over the entire surface, including the edges of any holes in the plate.

[0072]

[0071] The metal oxide layer can be coated with polyether ether ketone (PEEK) to fill gaps and holes in the metal oxide, making the metal oxide layer less susceptible to arc discharge when an electrostatic voltage is applied to the electrodes of the electrostatic sheet during use.

[0073]

[0072] As explained above with reference to Figure 7, as shown in Figure 8, the side of the electrostatic sheet 65 away from the support surface, i.e., the outer surface facing the core body 61, is provided with a conductive coating which can be grounded for shielding purposes, thereby providing a shield between the electrode and the core body, for example.

[0074] 8, in one embodiment, burls 62 and electrostatic sheets 65 are provided on both major surfaces of the substrate holder 60. In another embodiment, burls 62 and electrostatic sheets 65 are provided on only one of the two major surfaces of the substrate holder 60.

[0075]

[0074] As shown in Figure 8, in one embodiment, the electrostatic sheet 65 is located between the burls 62. For example, as shown in Figure 8, through holes 74 are provided in the electrostatic sheet 65. The through holes 74 are arranged so that their positions correspond to the burls 62 of the core body 61. The burls 62 protrude through each through hole 74 in the electrostatic sheet 65 such that the electrodes sandwiched between the dielectric layers 67, 68 are provided in the regions between the burls 62. The edges 73 of the through holes, i.e., the edges facing the burls, are covered with a metal oxide such as aluminum oxide, magnesium oxide, or titanium oxide as described above, to form a dielectric between the electrodes and the burls.

[0076] 8, in one embodiment, the substrate holder 60 includes a bonding material 69. The bonding material 69 fixes the position of the electrostatic sheet 65 relative to the core body 61 so that it does not move. The bonding material 69 maintains the through holes 74 of the electrostatic sheet 65 aligned with the burls 62. In one embodiment, the burls 62 are positioned at the center of each through hole 74 of the electrostatic sheet 65.

[0077]

[0076] An object table according to an aspect of the invention comprises a support member and an object holder according to the invention, the object holder being attached to the support member, which may for example comprise a mirror block of a lithographic apparatus.

[0078] According to yet another aspect of the invention, a lithographic apparatus is configured to project a pattern from a patterning device onto a substrate, the lithographic apparatus comprising an object holder according to the invention or an object table according to the invention. The object may be, for example, a substrate or a patterning device such as a mask. The lithographic apparatus may be the lithographic apparatus described with reference to Figure 1.

[0079]

[0078] Such an object holder can be manufactured as follows: A metal sheet containing metal is provided, and a plurality of through-holes are created in the metal sheet. The through-holes are created in the metal sheet by at least one of sandblasting, electron beam drilling, and CNC machining, which allow relatively small through-holes to be formed in the metal with high precision. Other possibilities for creating through-holes in a metal sheet include laser beam shaping or laser beam etching, in which a laser beam acts on the material, which is then removed with a mild etchant.

[0080]

[0079] A metal oxide layer is applied to a metal sheet to form an electrostatic sheet, the metal oxide layer comprising an oxide of a metal. The metal oxide layer may be applied to the metal sheet by oxidizing the outer surface of an aluminum sheet to form the metal oxide layer. This allows the metal oxide layer and the metal sheet to be manufactured from the same material, providing a highly mechanically stable electrostatic sheet. The electrostatic sheet may be more mechanically robust and less prone to cracking, which may be advantageous in the remaining manufacturing process and handling of the electrostatic sheet. The oxidation may include plasma electrolytic oxidation (PEO). The thickness of the oxide layer can be precisely controlled, allowing the resulting properties to be tailored to the application. For example, a thicker oxide layer improves the dielectric strength. For example, the dielectric strength of aluminum oxide, the main component of the PEO surface layer on aluminum alloys, ranges from 14.6 to 16.7 kV / mm. Another advantage of aluminum oxide is its dielectric constant, approximately 7.5 to 9.4, which may be 50 to 100% higher than that of glass. As a result, the electrodes can be operated at significantly lower voltages while maintaining the desired clamping force. Silica additives can be used. Is the silica additive added to the metal alloy, i.e., before oxidation? The dielectric strength of aluminum with a PEO surface coating containing silica additives is 59-79 kV / mm. With silica additives, it may be possible to obtain 5.9-7.9 kV at a layer thickness of 100 μm. As an alternative to PED, other oxidation processes, such as microarc oxidation (MAO), can be applied to NaAlO2 solutions, providing, for example, 40-100 μm of Al2O3 from Al.

[0081]

[0080] The metal oxide layer can be coated with polyetheretherketone (PEEK), which can fill the pores of the dielectric layer containing the metal oxide and help reduce arcing in these pores.

[0082]

[0081] One side of the electrostatic sheet may be coated with a conductive coating to shield the core body from exposure to electrostatic fields during operation.

[0083]

[0082] Further, in the manufacturing process, a core body is provided that includes a plurality of burls having distal ends within a support surface for supporting an object.

[0084]

[0083] Assembling the electrostatic sheet and the core body to form the object holder. The assembly of the electrostatic sheet and the core body can include assembling the electrostatic sheet and the core body such that one surface of the electrostatic sheet coated with the conductive coating faces away from the support surface in the assembled state.

[0085]

[0084] As mentioned above, in some embodiments, the metal comprises one of aluminum, magnesium, and titanium, and the metal oxide comprises an oxide of one of aluminum, magnesium, and titanium. The aluminum, magnesium, and titanium metals can be oxidized by plasma electrolytic oxidation to form an oxide layer on the electrode.

[0086]

[0085] Although specific reference is made in this specification to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0087] Although specific reference is made herein to embodiments of the invention in relation to lithography apparatus, embodiments of the invention can also be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatus are sometimes referred to generically as lithography tools. Such lithography tools can use vacuum conditions or ambient (non-vacuum) conditions.

[0088]

[0087] Although specific reference has been made above to the use of embodiments of the present invention in the context of optical lithography, it will be understood that the present invention is not limited to optical lithography and may also be used in other applications, such as imprint lithography, where circumstances permit.

[0089]

[0088] While specific embodiments of the present invention have been described above, it will be apparent that the present invention may be practiced in other ways than those described above. The foregoing description is intended to be illustrative and not limiting. Accordingly, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the appended claims.

[0090]

[0089] Example 1. An object holder configured to support an object, the object holder comprising: a core body including a plurality of burls having distal ends within a support surface for supporting an object; Electrostatic sheet between bars and The electrostatic sheet is an electrode including a metal; and a metal oxide layer comprising an oxide of a metal, the electrode being sandwiched between the metal oxide layers. 2. The object holder of example 1, wherein the metal includes at least one of aluminum, titanium, and magnesium. 3. The object holder of example 1 or 2, wherein the electrostatic sheet includes a plurality of through holes, the burrs extend through the through holes, and the edges of the through holes are covered by metal oxide extending between the metal oxide layers. 4. The object holder of example 3, wherein the pitch of the through holes in the electrostatic sheet matches the pitch of the burls so that the burls fit snugly with the through holes in the electrostatic sheet. 5. The object holder of any of Examples 1-4, wherein the electrostatic sheet extends substantially parallel to the support surface. 6. The object holder of any of Examples 1-5, wherein the outer surface of the electrostatic sheet facing away from the support surface is coated with a conductive coating. 7. The object holder according to any of Examples 1 to 6, wherein the aluminum oxide layer is coated with polyetheretherketone (PEEK). 8. The object holder of any of examples 1 to 7, wherein the thickness of the metal oxide layer is between 50 and 150 micrometers. 9. The object holder of any of Examples 1 to 8, wherein the thickness of the electrode is between 300 and 500 micrometers. 10. The object holder of any of Examples 1 to 9, wherein the distance between the support surface and the outer surface of the electrostatic sheet facing the support surface is between 1 and 20 micrometers. 11. The core body is a plurality of further burls having distal ends within the mirror block face for supporting the substrate holder on the mirror block; a further electrostatic sheet between the further bars, the further electrostatic sheet including a metal oxide layer including an oxide of a metal and an electrode sandwiched between the metal oxide layers; 11. The object holder according to any one of Examples 1 to 10, wherein the mirror block surface is parallel to the support surface. 12. The object holder of any of Examples 1 to 11, wherein the metal comprises at least one of aluminum, titanium, and magnesium. 13. The object holder of any of Examples 1-12, wherein the object holder is one of a substrate holder configured to hold a substrate and a patterning device holder configured to hold a patterning device. 14. An object table comprising a support member and an object holder according to any one of Examples 1 to 13, wherein the object holder is attached to the support member. 15. A lithographic apparatus configured to project a pattern from a patterning device onto a substrate, the lithographic apparatus comprising an object holder according to any one of examples 1 to 13 and an object table according to example 14. 16. A method of manufacturing an object holder, comprising: providing a metal sheet comprising a metal; creating a plurality of through holes in a metal sheet; providing a metal oxide layer on a metal sheet to form an electrostatic sheet, the metal oxide layer comprising an oxide of a metal; providing a core body including a plurality of burls having distal ends within a support surface for supporting an object; Assembling the electrostatic sheet and the core body to form an object holder; A method comprising: 17. The method of Example 16, wherein providing a metal oxide layer on the metal sheet includes oxidizing an outer surface of an aluminum sheet to form the metal oxide layer. 18. The method of example 17, wherein the oxidizing comprises plasma electrolytic oxidation (PEO). 19. The method of any of Examples 16-18, further comprising coating the metal oxide layer with polyetheretherketone (PEEK). 20.Through holes are - Sandblasting, - electron beam drilling, -CNC processing, - laser beam shaping, and -Laser beam etching 20. The method of any of Examples 16 to 19, wherein the metal sheet is formed by at least one of the following: 21. The method of any one of Examples 16 to 20, further comprising coating one surface of the electrostatic sheet with a conductive coating, and assembling the electrostatic sheet and the core body includes assembling the electrostatic sheet and the core body such that, in the assembled state, the one surface of the electrostatic sheet coated with the conductive coating faces away from the support surface. 22. The method of any one of Examples 16-21, wherein the metal comprises at least one of aluminum, titanium, and magnesium. 23. The method of any of Examples 16-22, wherein the object holder is one of a substrate holder configured to hold a substrate and a patterning device holder configured to hold a patterning device.

Claims

1. 1. An object holder configured to support an object, the object holder comprising: a core body including a plurality of burls having distal ends within a support surface for supporting the object; an electrostatic sheet between the bars; The electrostatic sheet comprises: an electrode including a metal; and a metal oxide layer comprising an oxide of the metal, the electrode being sandwiched between the metal oxide layers.

2. The object holder of claim 1 , wherein the metal comprises at least one of aluminum, titanium, and magnesium.

3. 3. The object holder of claim 1, wherein the electrostatic sheet includes a plurality of through holes, the burls extending through the through holes, and edges of the through holes being covered by metal oxide extending between the metal oxide layers.

4. 4. The object holder of claim 3, wherein the pitch of the through-holes in the electrostatic sheet matches the pitch of the burls such that the burls fit snugly with the through-holes in the electrostatic sheet.

5. 5. An object holder according to claim 1, wherein the electrostatic sheet extends substantially parallel to the support surface.

6. 6. An object holder according to claim 1, wherein an outer surface of the electrostatic sheet facing away from the support surface is coated with a conductive coating.

7. 7. The object holder according to claim 1, wherein the aluminum oxide layer is coated with polyetheretherketone (PEEK).

8. An object holder according to any one of claims 1 to 7, wherein the thickness of the metal oxide layer is between 50 and 150 micrometers.

9. An object holder according to any of the preceding claims, wherein the thickness of the electrodes is between 300 and 500 micrometers.

10. 10. An object holder according to any preceding claim, wherein the distance between the support surface and the outer surface of the electrostatic sheet facing the support surface is between 1 and 20 micrometers.

11. The core body is a plurality of further burls having distal ends within the mirror block face for supporting the substrate holder on the mirror block; a further electrostatic sheet between the further bars, the further electrostatic sheet including a metal oxide layer including an oxide of the metal and an electrode sandwiched between the metal oxide layers; 11. The object holder of claim 1, wherein the mirror block face is parallel to the support surface.

12. The object holder according to any one of claims 1 to 11, wherein the metal comprises at least one of aluminium, titanium and magnesium.

13. The object holder of any of claims 1 to 12, wherein the object holder is one of a substrate holder configured to hold a substrate and a patterning device holder configured to hold a patterning device.

14. A lithographic apparatus configured to project a pattern from a patterning device onto a substrate, the lithographic apparatus comprising an object holder according to any of claims 1 to 13.

15. 1. A method of manufacturing an object holder, comprising: providing a metal sheet comprising a metal; creating a plurality of through holes in the metal sheet; providing a metal oxide layer on the metal sheet to form an electrostatic sheet, the metal oxide layer comprising an oxide of the metal; providing a core body including a plurality of burls having distal ends within a support surface for supporting the object; assembling the electrostatic sheet and the core body to form the object holder; A method comprising:

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