Vacuum table and method for clamping warped substrates

Abstract

The present disclosure provides a vacuum table including a table having an upper surface for supporting a substrate, the upper surface being provided with at least two pressure zones, each pressure zone connected to a respective vacuum connector for providing a reduced pressure, and at least one of the pressure zones being provided with a groove extending radially across the upper surface. Each pressure zone may include a corresponding groove connected to a respective vacuum connector and extending along the upper surface, the groove of each pressure zone extending at least circumferentially along the upper surface. The radial grooves may extend like fingers from the corresponding circumferential groove. The radial grooves of one pressure zone extend between the radial grooves of another pressure zone.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Application No. 22213102.1, filed December 13, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a vacuum table and a method for clamping a substrate, which may be a wafer for a lithographic process, and to a lithographic apparatus provided with such a vacuum table. [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 (often referred to as a "design layout" or "design") in a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on the substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to evolve, the dimensions of circuit elements have continually decreased, while the amount of functional elements, such as transistors, per device has steadily increased for decades, following a trend commonly referred to as "Moore's Law." To accommodate Moore's Law, the semiconductor industry pursues technologies that enable the creation of increasingly smaller features. To project patterns onto a substrate, lithography equipment may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. To form smaller features on a substrate than lithography equipment using radiation with a wavelength of, for example, 193 nm, lithography equipment using extreme ultraviolet (EUV) radiation with wavelengths in the range of 4 nm to 20 nm, e.g., 6.7 nm or 13.5 nm, may be used.

[0005]

[0005] To prevent the substrate from moving during patterning, it is typically fixed in a particular position during the lithography process, for example by applying a pressure differential, typically by applying a lower pressure to the side of the substrate facing away from the radiation.

[0006]

[0006] Japanese Patent Application Publication No. H01201936 relates to a plate chuck that uses vacuum to adsorb an object to be exposed in an exposure apparatus for liquid crystal substrates, etc. In a liquid crystal substrate exposure apparatus, the gap between the original (mask) and the object to be exposed (substrate or wafer) is maintained with high precision, and the mask pattern is transferred to the substrate by photolithography. For this purpose, the substrate is vacuum-adsorbed onto a plate chuck that has adsorption grooves.

[0007]

[0007] Japanese Patent Application Publication No. H04159043 relates to a vacuum chuck for holding a workpiece in a machine tool or a vacuum suction method for various conveying devices. The material 8 to be attracted is attracted by a radial groove 4 having a through hole 3 provided in a lower suction plate 1 and a vacuum suction hole 5 provided in an upper suction plate 2. This device has a radial groove. By rotating the lower suction plate relative to the upper suction plate, it is possible to apply vacuum to an area corresponding to the size of the material to be attracted.

[0008]

[0008] Japanese Patent Application Publication No. H07231033 relates to an apparatus for holding plate-shaped substrates, particularly to a projection exposure apparatus for manufacturing semiconductor chips, liquid crystal display elements, etc., and is used to hold semiconductor wafers, ceramic plates, etc. The mounting surface of the wafer holder has multiple ring-shaped recesses (grooves) formed radially from the center of the holder. After the wafer is placed on the mounting surface, the insides of these grooves are depressurized by a vacuum depressurization source, so that almost the entire back surface of the wafer is vacuum-attached to the mounting surface.

[0009] In practice, substrates may warp during many steps of the semiconductor manufacturing process. As used herein, warped or bowed refers to a substrate that is curved in some way. Bow or curvature, as used herein, can be defined as the height difference between the lowest and highest points of a substrate, typically the front or surface of the substrate. For example, a substrate may be designed as a flat disk, but bend slightly into a saddle, hat, trumpet, or other shape. Various mechanisms have been identified, one of which is inherent in the deposition of multiple metal layers, which may act as bimetallic layers at some point in the process, leading to (slight) bowing. Furthermore, the deposition of multiple layers can make it increasingly difficult or impossible to flatten the substrate using clamps or tools.

[0010]

[0010] Conventional vacuum tables for holding substrates have been found to have limited tolerance for curvature or bowing of the substrate. When the curvature exceeds a certain threshold, the vacuum typically leaks, resulting in the substrate no longer adhering to the vacuum table. Because irradiation of the substrate can only be performed if the substrate is held in the correct position during the lithography process, this can and typically results in loss of the substrate, i.e., reduced yield. Summary of the Invention

[0011]

[0011] The present disclosure aims to improve upon the conventional systems described above.

[0012]

[0012] The present disclosure provides: a vacuum table, a table having an upper surface for supporting a substrate; A vacuum table is provided having an upper surface provided with at least two pressure zones, each pressure zone connected to a respective vacuum connector for providing reduced pressure, and at least one of the pressure zones provided with grooves extending radially across the upper surface in the form of fingers from a corresponding circumferential groove.

[0013]

[0013] In one embodiment, each pressure zone includes a corresponding groove connected to a respective vacuum connector and extending along the upper surface, and the groove of each pressure zone extends at least circumferentially along the upper surface.

[0014]

[0014] In one embodiment, the radial grooves extend from the corresponding circumferential grooves like fingers.

[0015] In one embodiment, the radial grooves of one pressure zone extend between the radial grooves of another pressure zone.

[0016] In one embodiment, the upper surface is provided with at least three pressure zones.

[0017]

[0017] In one embodiment, the first pressure zone is located within a first radial distance from the midpoint of the upper surface, and the second pressure zone is located between the first radial distance and the outer periphery of the upper surface.

[0018] In one embodiment, a third pressure zone is located between the second radial distance and the outer periphery of the upper surface.

[0019] In one embodiment, each pressure zone includes at least one opening fluidly connected to a corresponding vacuum connector.

[0020]

[0020] In one embodiment, the grooves of each pressure zone extend inside a boundary of the respective pressure zone, the boundary being selected from a first radial distance, a second radial distance, and an outer periphery of the upper surface.

[0021]

[0021] The groove may have a depth of the order of 1 mm.

[0022]

[0022] The top surface may be provided with a central opening to allow a lifting device to extend therethrough.

[0023]

[0023] In one embodiment, the table includes a control system connectable to the vacuum connector of each pressure zone to control the pressure in each pressure zone independently of the pressure in the other pressure zones.

[0024] The control system may be adapted to control the first reduced pressure independently from the second reduced pressure and / or the third reduced pressure.

[0025]

[0025] The vacuum table may include at least one vacuum source for providing reduced pressure to the vacuum connectors of each pressure zone.

[0026] According to another aspect, the present disclosure provides a lithographic apparatus including at least one vacuum table according to any one of claims 1 to 14.

[0027] According to yet another aspect, the present disclosure provides a method of clamping a substrate to a vacuum table, comprising: positioning the substrate on a top surface of a vacuum table, the top surface being provided with at least two pressure zones, each pressure zone connected to a respective vacuum connector for providing a reduced pressure, at least one of the pressure zones being provided with a groove extending radially across the top surface; applying a first reduced pressure to the first pressure zone until a first threshold is met indicating that the substrate is clamped against the upper surface within the first pressure zone; applying a second reduced pressure to the second pressure zone until a second threshold is met indicating that the substrate is clamped against the upper surface in the second pressure zone; A method is provided, comprising:

[0028]

[0028] In one embodiment, the step of applying the first vacuum pressure and / or the second vacuum pressure includes reducing the pressure within corresponding first or second grooves extending radially across the upper surface of the vacuum table.

[0029]

[0029] The second threshold may be substantially equal to the first threshold.

[0030]

[0030] The method may include applying a third reduced pressure to the third pressure zone until a third threshold is met indicating that the substrate is clamped against the upper surface within the third pressure zone.

[0031]

[0031] The first pressure zone may be located within a first radial distance from a midpoint of the upper surface, the second pressure zone may be located between the first radial distance and the outer periphery of the upper surface, and optionally, the third pressure zone is located between the second pressure zone and the outer periphery of the upper surface.

[0032] In one embodiment, the method includes controlling the reduced pressure in each of the at least two pressure zones independently of the pressure in the other pressure zones. [Brief explanation of the drawings]

[0033]

[0033] Some embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0034] [Figure 1] 1 shows a schematic view of a lithographic apparatus; [Figure 2A] FIG. 1 is a perspective view of the top of one embodiment of a vacuum table of the present disclosure. [Figure 2B] FIG. 10 is a perspective view of the top of another embodiment of a vacuum table of the present disclosure. [Figure 2C] FIG. 2C is a perspective view of the bottom of the embodiment of FIG. 2A or 2B. [Figure 3A] FIG. 1 is a perspective view of the top of one embodiment of a vacuum table of the present disclosure. [Figure 3B] FIG. 10 is a perspective view of the top of another embodiment of a vacuum table of the present disclosure. [Figure 3C] FIG. 3C is a perspective view of the bottom of the embodiment of FIG. 3A or FIG. 3B. [Figure 4A] FIG. 10 is a diagram of one embodiment of a control scheme for controlling the vacuum table of the present disclosure. [Figure 4B] FIG. 10 is a diagram of another embodiment of a control scheme for controlling the vacuum table of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0035]

[0034] In this specification, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (e.g., extreme ultraviolet radiation having wavelengths in the range of about 5 to 100 nm).

[0036]

[0035] As used herein, the terms "reticle," "mask," or "patterning device" may be broadly interpreted to refer to a general patterning device that can be used to impart an incident radiation beam with a patterned cross-section that corresponds to the pattern to be created on a target portion of a substrate. The term "light valve" may also be used in this context. In addition to typical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0037] 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also called an illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and coupled to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and coupled to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0038]

[0037] In operation, the illumination system IL receives a radiation beam from the radiation source SO, for example via the beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping and / or controlling the radiation. The illuminator IL can be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0039]

[0038] As used herein, the term "projection system" PS should be interpreted broadly to encompass various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optics, or any combination thereof, as appropriate for the exposure radiation being used and / or other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" as used herein should be considered as synonymous with the more general term "projection system" PS.

[0040]

[0039] The lithographic apparatus LA may be of a type in which at least a portion of the substrate is covered by a liquid having a relatively high refractive index (e.g. water) so as to fill a space between the projection system PS and the substrate W, which is also known as immersion lithography. More detailed information about immersion techniques is found in US Pat. No. 6,952,253, which is incorporated herein by reference.

[0041] The lithographic apparatus LA may be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps in preparation for a subsequent exposure of a substrate W may be performed on a substrate W located on one of the substrate supports WT, while a pattern on another substrate W on another substrate support WT is used to expose the other substrate W.

[0042] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example part of the projection system PS or part of a system for providing immersion liquid. When the substrate support WT is away from the projection system PS, the measurement stage may be moved below the projection system PS.

[0043]

[0042] In operation, the radiation beam B is incident on a patterning device (e.g. mask MA), which is held on the mask support MT, and is patterned according to a pattern (design layout) on the patterning device MA. After passing through the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. Using the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the patterning device MA can also be accurately positioned with respect to the path of the radiation beam B using the first positioner PM and possibly further position sensors (not explicitly shown in Figure 1). The patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although substrate alignment marks P1, P2 as illustrated occupy dedicated target portions, they may be located in spaces between the target portions. When substrate alignment marks P1, P2 are located between target portions C, they are known as scribe-lane alignment marks.

[0044]

[0043] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes: x, y, and z. Each of the three axes is orthogonal to the other two. Rotation about the x-axis is called Rx-rotation. Rotation about the y-axis is called Ry-rotation. Rotation about the z-axis is called Rz-rotation. The x- and y-axes define a horizontal plane, while the z-axis is vertical. The Cartesian coordinate system is not a limitation of the invention and is used for clarity only. Instead of Cartesian coordinates, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The Cartesian coordinate system may have a different orientation, for example, the z-axis has a component along the horizontal plane.

[0045] Referring generally to FIG. 2A, the present disclosure relates to a vacuum table 10 including a table 12 having an upper surface 14 for supporting a substrate W. The vacuum table may be included in a substrate table WT (see FIG. 1). The upper surface has an outer periphery or rim 16. The upper surface may be provided with a number of protrusions 18 for supporting the substrate. The protrusions 18 may also be referred to as burls. The burls 18 may have a height of the order of 10-250 μm. For examples of burls and a manufacturing process for the protrusions, see, for example, U.S. Pat. No. 7,940,511.

[0046]

[0045] At least two pressure zones 20, 22 are provided on the upper surface 14. Each pressure zone is connected to a respective vacuum connector 30, 32 for providing reduced pressure.

[0047] At least one of the pressure zones 20, 22 is provided with fingers 40, 42, i.e., grooves extending radially across the upper surface. The fingers or radial grooves are connected to the respective vacuum connectors 30, 32 of the corresponding pressure zone. Thus, the radial grooves allow the pressure, typically reduced pressure or vacuum, provided through the connectors 30, 32 to be spread radially across the upper surface. Each pressure zone may include a corresponding groove 50, 52 connected to the respective vacuum connector 30, 32 and extending along the upper surface 14. The grooves 50, 52 of each pressure zone may extend at least circumferentially along the upper surface. In one embodiment, the grooves 50, 52 of each pressure zone may extend at least partially circumferentially along the upper surface. In this specification, "at least partially" means that the circumferential groove may extend along a portion of a circle. For example, the grooves 50, 52 may be composed of multiple sections extending together along a circle. Multiple circular zones may be formed, each with its own dedicated pressure zone. The grooves 50, 52 may extend, for example, over a radius of about 90 degrees. Thus, four or more sections of a radial groove may together constitute a radial groove. The fingers 40, 42 described above may be fluidly connected to the circumferentially extending grooves. The fingers may typically have at least one end that is not connected to another groove.

[0048] As shown in FIG. 2A, the radial grooves 40, 42 extend from the corresponding circumferential grooves 50, 52 in a finger-like manner.

[0049] 2B, at least some of the fingers or radial grooves 40 of one pressure zone 20 may extend between radial grooves 42 of another pressure zone 22. The radial grooves 40 of a particular pressure zone may have different lengths, the latter allowing some fingers to extend between fingers of another pressure zone while other fingers do not.

[0050] The vacuum table may be provided with a central opening 26 to allow a lifting device to extend therethrough. The vacuum table may be adapted to cooperate with a lifting device (not shown). The lifting device may be adapted to move through the central opening 26 between a lifting position, in which the lifting device lifts the substrate from the top surface 14, and a retracted position, in which the substrate W engages the table 10. In this specification, a substrate engaging the table 10 may mean, for example, that the substrate is positioned on the top surface 14 (in the case of a flat top surface without burls 18), or that the substrate is positioned on the protrusions or burls 18.

[0051] 2C, the respective connectors 30, 32 may be located on the bottom 15 of the substrate table 10. The connectors are fluidly connected to respective pressure zones located on the top surface 14. Alternatively, the connectors 30, 32 may be located on the sides of the substrate table 10.

[0052] 3A-3C, in one embodiment, the top surface 14 may include at least three pressure zones 20, 22, 24. Herein, the first pressure zone 20 may be located within a first radial distance from a midpoint of the top surface 14, and the second pressure zone 22 may be located between the first radial distance and a second radial distance that exceeds the first radial distance. The third pressure zone 24 may be located between the second radial distance and the outer periphery 16 of the top surface. Each pressure zone may include at least one opening fluidly connected to a corresponding vacuum connector 30-34.

[0053]

[0052] As illustrated in Figure 3A, the grooves of each pressure zone may extend inside the boundary of the respective pressure zone, the boundary being selected from a first radial distance, a second radial distance, a third radial distance, and the outer periphery of the upper surface.

[0054] 3B, the radial grooves 40, 42, 44 of one or more pressure zones may extend between the fingers of adjacent pressure zones. Thus, the pressure zones may at least partially overlap. This improves control and increases the range of substrate bow that can be processed.

[0055] In one embodiment, the groove may have a depth of about 0.3 to 2 mm, for example, about 1 mm.

[0056] In one embodiment, each pressure zone is provided with one or more openings (not shown) connected to the respective connectors 30, 32, 34. The openings are located within or connected to the grooves of the respective pressure zone. The openings allow gas flow from the openings, and thus from the grooves, to the respective connectors of the pressure zone. In one embodiment, the one or more openings in each pressure zone are located near the inner boundary of the respective pressure zone. In this specification, inner boundary can mean the boundary closest to the midpoint or central opening 26 in the case of a circular top surface 14.

[0057] 4A and 4B , the vacuum table may include or be connectable to a control system 80. The control system may be connected between a vacuum source 90 and the vacuum connectors 30, 32, 34 of each pressure zone. The vacuum source 90 may be connected to each connector using suitable fluid conduits, typically gas lines. The vacuum source 90 may include a pump or other means for reducing pressure and / or generating gas flow. The control system 80 may include at least two valves 82, 84, 86, 88 for opening and closing the connection between the vacuum source 90 and each pressure zone.

[0058]

[0057] The valves may be controllable between an open position and a closed position. In the open position, each valve allows fluid flow in the corresponding fluid line. In the closed position, the valve blocks fluid flow in the corresponding fluid line. The valves may also be controllable to intermediate positions, i.e., positions between the fully open and fully closed positions. The valves may also be gradually opened or closed, thereby gradually increasing or decreasing the fluid flow through the respective fluid line, thereby allowing the corresponding pressure drop in the corresponding pressure zone to gradually increase or decrease with respect to the fluid flow.

[0059] The control system may include one or more controllers 92, 94, 96, 98. For example, as shown in FIG. 4A, the control system may include one controller 92 controlling each valve. Alternatively, as shown in FIG. 4B, each valve may be provided with its own dedicated controller. According to the present disclosure, fluid flow through the valves connected to each pressure zone, and therefore the corresponding pressure within each pressure zone, may be controlled independently of the pressures within the other pressure zones. The control system may be adapted to control a first reduced pressure within a first pressure zone independently of a second reduced pressure within a second pressure zone and / or a third reduced pressure within a third pressure zone.

[0060]

[0059] The above-mentioned vacuum table 10 may be included in a lithographic apparatus LA, as illustrated with reference to Figure 1. The vacuum table 10 may also be included in a substrate table WT.

[0061] In operation, a vacuum table 10 according to the present disclosure may function as follows. In a typical first step, a substrate may be positioned on the table 10. As used herein, positioning on the table may refer to, for example, positioning the substrate on the upper surface 14 or on the burls 18. Positioning the substrate may include various steps. For example, the substrate may be positioned on the upper surface using a substrate handler such as a robot arm. However, the substrate handler may also position the substrate on a lifting device (not shown) extending through a central opening 26 in the upper surface. The top of the lifting device may include a suction device for clamping the substrate to the lifting device using reduced pressure. Once the substrate is suitably secured to the lifting device, the lifting device may be retracted through the opening 26 until the substrate engages the upper surface 14 of the vacuum table 10.

[0062] It should be noted that a lithography process typically includes at least a measurement step in which the topography of the substrate is measured. The topography provides information about the substrate, including its warpage and curvature. Thus, once a substrate is positioned on the upper surface 14, the system will typically have information available to indicate whether and to what extent a particular part or section of the substrate is raised relative to the upper surface.

[0063]

[0062] In a next step, the method may include applying a first reduced pressure to one or more pressure zones until a first threshold is met indicating that the substrate is clamped against the upper surface within the one or more pressure zones.

[0064]

[0063] The method may then include applying a second reduced pressure to one or more other pressure zones until a second threshold is met indicating that the substrate is clamped against the upper surface within the one or more other pressure zones.

[0065] The selection of pressure zones to select in the initial step and subsequently activate may vary from substrate to substrate, and may typically depend on one or more of the curvature, the degree of bowing (i.e., the maximum difference between the highest and lowest points on the substrate), and the shape of the curvature (e.g., hollow like a bowl, curved like an umbrella, or curled like a taco).

[0066] For example, the method may first activate the vacuum in the pressure zone closest to the substrate's top surface. Thus, for a bowl-shaped substrate, the pressure zone near the center may be activated first, followed by the pressure zone closer to the periphery of the vacuum table. For umbrella-shaped substrates, the reverse may be preferable. Thus, in this case, the outer pressure zones may be activated first, followed by the pressure zones closer to the center of the vacuum table once a good vacuum has been established in the outer pressure zones. For taco-shaped substrates, it may be most advantageous to start with the central pressure zone or the outer pressure zone, depending on which pressure zone initially makes the most contact with each substrate. The control system may operate in a specific manner to minimize or prevent damage to the backside of the wafer. For example, when unclamping a wafer curved like an umbrella, the outer pressure zones may be brought to atmospheric pressure first before the other pressure zones. This prevents normal forces from being applied to the outer contact points as the substrate naturally expands. In one embodiment, the wear resistance of such contact points may be improved by applying, for example, a diamond-like carbon (DLC) coating or similar wear-resistant coating.

[0067]

[0066] The disclosed method benefits from the geometry of each pressure zone including fingers that extend radially across the upper surface of the vacuum table, where the fingers allow the pressure within each pressure zone to extend radially.

[0068]

[0067] Furthermore, as shown in Figures 2B and 3B, the fingers of each pressure zone may partially overlap, which helps to gradually flatten a warped substrate as the substrate is rolled from one pressure zone to the next on a vacuum table.

[0069] The method may include applying a first reduced pressure to a first pressure zone until a first threshold is met indicating that the substrate is clamped against an upper surface in the first pressure zone, and applying a second reduced pressure to a second pressure zone until a second threshold is met indicating that the substrate is clamped against an upper surface in the second pressure zone.

[0070]

[0069] The first vacuum pressure and / or second vacuum pressure can be applied by reducing the pressure in corresponding first or second grooves of each pressure zone extending radially across the upper surface of the vacuum table.

[0071] The first and second thresholds may be selected, for example, from gas flow in the respective pressure lines dropping below a set value and / or a height sensor indicating that the surface of the substrate has dropped below a preset margin relative to the top surface of the vacuum table. The second threshold may be substantially equal to the first threshold.

[0072] The method may include applying a third reduced pressure to the third pressure zone until a third threshold is met, indicating that the substrate is clamped against the upper surface in the third pressure zone. Similar steps may be repeated for any number of subsequent pressure zones greater than three.

[0073] The control system may control the pressure (typically a reduced pressure) in each of the at least two pressure zones independently of the pressure in the other pressure zones. This allows the benefits provided by the vacuum table of the present disclosure to be optimized for a particular substrate shape. As noted above, the shape may vary from round (umbrella-shaped) to hollow (bowl-shaped) to taco-shaped. Control of the pressure zones may be adapted, for example, to start with a reduced pressure near the center of the table and work outward, or vice versa.

[0074] The combination of multiple vacuum zones with burls or protrusions provides yet another advantage. The burls have a limited height. This limited height essentially blocks gas flow between the underside of the substrate and the top surface of table 10. However, the burls support the substrate and allow a limited amount of gas flow, which supports or improves the roll-like movement of the substrate as it flattens on the top surface as the pressure is reduced. In other words, the burls allow a limited amount of gas flow between them, which leads to a reduced pressure and an accelerated pressure drop. The latter generates additional air pressure or torque on the substrate, further increasing the treatable bow.

[0075] Tests and simulations have shown that the vacuum table according to the present disclosure can expand the range of substrate bow that can be corrected. Substrates in this specification particularly relate to semiconductor wafers. Such substrates typically include circular silicon wafers. The diameter of the wafer can be about 300 mm, but can have a variety of sizes. Other substrates include substrates made of other types of materials or combinations of materials that behave as semiconductors under operating conditions.

[0076] The vacuum table of the present disclosure can correct, for example, a warp of more than about 500 μm, for example, more than about 600 μm, for example, more than about 700 μm, for example, more than about 800 μm, for example, more than about 900 μm, or even more than 1 mm, of a semiconductor substrate having a diameter of 300 mm. A warp of up to 2 mm can be corrected using the vacuum table of the present disclosure. The vacuum table of the present disclosure can correct, for example, a warp of more than about 1100 μm, for example, more than about 1200 μm, for example, more than about 1300 μm, for example, more than about 1400 μm, for example, more than about 1500 μm, for example, more than about 1600 μm, for example, more than about 1700 μm, for example, more than about 1800 μm, for example, more than about 1900 μm, or for example, more than about 2000 μm, of a semiconductor substrate. The maximum warp or height difference referred to can be corrected at least for silicon-based substrates. The substrate may include multiple deposited layers of metal and / or semiconductor material to form, for example, microchips, memory devices, etc. This represents a significant improvement in the amount of warpage that can be handled compared to conventional vacuum tables suitable for semiconductor manufacturing processes. For 300 mm semiconductor wafers, conventional vacuum tables are typically inadequate for any type of warpage exceeding 300-400 μm.

[0077] In one embodiment, the (gas) flow rate of each vacuum connection may be in the range of about 10 nl / min to 30 nl / min (normal liters / min), for example about 15-20 nl / min. Nl / min in this specification means 10 liters / min under normal conditions, i.e. 10 l / min at 1 bar (absolute pressure). The pressure drop across the table may be less than 1.0 bar per vacuum channel.

[0078] The pressure differential across the substrate can be on the order of 0.1 to 0.5 bar. As used herein, pressure differential across the substrate may refer to the difference in gas pressure between the top and bottom surfaces of the substrate. The various pressure zones of the vacuum table of the present disclosure can be used to control, and typically reduce, the gas pressure at the bottom of the substrate.

[0079] The vacuum table of the present disclosure allows for dedicated control for each pressure zone. This greatly enhances the ability to tailor and optimize the function of the vacuum table for a particular substrate's warpage. As discussed above, a relatively wide range of maximum height differences for relatively stiff substrates can be corrected and processed. Dedicated control for each pressure zone also allows for optimized clamping for a particular substrate, thereby increasing the variety of warpage that can be processed. For example, a substrate can be essentially rolled from one pressure zone to another on the top surface of the vacuum table, starting from the periphery, starting from the center of the table, or anywhere in between.

[0080]

[0079] The burls or protrusions 18 may typically have a height extending up to 250 μm from the upper surface 14 of the table 10. The burls may have a height of the order of 5 to 200 μm, for example of the order of 10 to 175 μm.

[0081]

[0080] 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. Other possible applications include 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.

[0082] Although specific reference is made herein to embodiments of the invention in the context of lithographic apparatus, embodiments of the invention may 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 commonly referred to as lithography tools. Such lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0083]

[0082] 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 apparent that the present invention is not limited to optical lithography and may, where circumstances permit, be used in other applications, for example imprint lithography.

[0084]

[0083] 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 above description is intended to be illustrative and not limiting. Numerous modifications can be made to the present invention as described above without departing from the scope of the claims set forth below. Other aspects of the present invention are described in the following numbered clauses: 1. A vacuum table comprising: a table having an upper surface for supporting a substrate; the upper surface is provided with at least two pressure zones, each pressure zone connected to a respective vacuum connector for providing reduced pressure, and at least one of the pressure zones is provided with a groove extending radially across the upper surface; Vacuum table. 2. A vacuum table as described in clause 1, wherein each pressure zone includes a corresponding groove connected to a respective vacuum connector and extending along the upper surface, and the groove of each pressure zone extends at least circumferentially along the upper surface. 3. A vacuum table as described in clause 2, wherein the grooves of each pressure zone extend at least partially circumferentially along the upper surface. 4. A vacuum table according to any one of clauses 1 to 3, wherein the radial grooves extend like fingers from the corresponding circumferential grooves. 5. A vacuum table according to one of clauses 1 to 4, wherein the radial grooves of one pressure zone extend between the radial grooves of another pressure zone. 6. A vacuum table according to one of clauses 1 to 5, wherein the upper surface is provided with at least three pressure zones. 7. A vacuum table described in one of clauses 1 to 6, wherein the first pressure zone is positioned within a first radial distance from a midpoint of the upper surface, and the second pressure zone is positioned between the first radial distance and the outer periphery of the upper surface. 8. The vacuum table of clause 7, wherein a third pressure zone is located between the second radial distance and the outer periphery of the upper surface. 9. A vacuum table as described in one of clauses 1 to 8, wherein each pressure zone includes at least one opening fluidly connected to a corresponding vacuum connector. 10. A vacuum table as described in clause 8 or 9, wherein the grooves of each pressure zone extend inside a boundary of the respective pressure zone, the boundary being selected from a first radial distance, a second radial distance, and an outer periphery of the top surface. 11. A vacuum table according to one of clauses 1 to 10, wherein the grooves have a depth of about 1 mm. 12. A vacuum table according to one of clauses 1 to 11, wherein the upper surface is provided with a central opening to allow a lifting device to extend therethrough. 13. A vacuum table according to one of clauses 1 to 12, including a control system connectable to the vacuum connector of each pressure zone for controlling the pressure in each pressure zone independently of the pressure in the other pressure zones. 14. The vacuum table of clause 13, wherein the control system is adapted to control the first vacuum pressure independently from the second vacuum pressure and / or the third vacuum pressure. 15. A vacuum table according to any one of clauses 1 to 14, including at least one vacuum source for providing reduced pressure to the vacuum connectors of each pressure zone. 16. A lithographic apparatus comprising at least one vacuum table according to any one of clauses 1 to 15. 17. A method of clamping a substrate to a vacuum table, comprising: positioning the substrate on a top surface of a vacuum table, the top surface being provided with at least two pressure zones, each pressure zone connected to a respective vacuum connector for providing a reduced pressure, at least one of the pressure zones being provided with a groove extending radially across the top surface; applying a first reduced pressure to the first pressure zone until a first threshold is met indicating that the substrate is clamped against the upper surface within the first pressure zone; applying a second reduced pressure to the second pressure zone until a second threshold is met indicating that the substrate is clamped against the upper surface in the second pressure zone; A method comprising: 18. The method of clause 17, wherein the step of applying the first vacuum pressure and / or the second vacuum pressure includes reducing the pressure in a corresponding first or second groove extending radially across the top surface of the vacuum table. 19. The method of clause 18, wherein the second threshold is substantially equal to the first threshold. 20. Applying a third reduced pressure to the third pressure zone until a third threshold is met indicating that the substrate is clamped against the upper surface within the third pressure zone. 20. The method of clause 17, 18 or 19, comprising: 21. A method according to one of clauses 19 to 20, wherein a first pressure zone is positioned within a first radial distance from a midpoint of the upper surface, a second pressure zone is positioned between the first radial distance and the outer periphery of the upper surface, and optionally a third pressure zone is positioned between the second pressure zone and the outer periphery of the upper surface. 22. A method according to one of clauses 17 to 21, comprising controlling the reduced pressure in each of the at least two pressure zones independently of the pressure in the other pressure zones.

Claims

1. a table having an upper surface for supporting a substrate; The upper surface is provided with at least two pressure zones; Each pressure zone is connected to a respective vacuum connector for providing reduced pressure; at least one of the pressure zones is provided with grooves extending radially across the upper surface from a corresponding circumferential groove in a finger-like manner; Vacuum table.

2. each pressure zone including a corresponding groove connected to the respective vacuum connector and extending along the top surface; The vacuum table of claim 1 , wherein the grooves in each pressure zone extend at least circumferentially along the upper surface.

3. The vacuum table of claim 2 , wherein the grooves in each pressure zone extend at least partially circumferentially along the upper surface.

4. A vacuum table according to any preceding claim, wherein the radial grooves of one pressure zone extend between radial grooves of another pressure zone.

5. A vacuum table according to any one of claims 1 to 4, wherein the upper surface is provided with at least three pressure zones.

6. a first pressure zone disposed within a first radial distance from a midpoint of the upper surface; A vacuum table according to any preceding claim, wherein a second pressure zone is located between the first radial distance and the outer periphery of the top surface.

7. A vacuum table according to any one of the preceding claims, wherein each pressure zone comprises at least one opening fluidly connected to the corresponding vacuum connector.

8. A vacuum table according to any preceding claim, wherein the grooves have a depth of the order of 1 mm.

9. A vacuum table according to any preceding claim, wherein the top surface is provided with a central opening to allow a lifting device to extend therethrough.

10. A vacuum table according to any preceding claim, including a control system connectable to the vacuum connector of each pressure zone for controlling the pressure in each pressure zone independently of the pressure in the other pressure zones.

11. An exposure apparatus, such as a lithographic apparatus, comprising at least one vacuum table according to any one of the preceding claims.

12. 1. A method of clamping a substrate to a vacuum table, comprising: positioning the substrate on an upper surface of the vacuum table, the upper surface being provided with at least two pressure zones, each pressure zone connected to a respective vacuum connector for providing a reduced pressure, at least one of the pressure zones being provided with a groove extending radially across the upper surface; applying a first reduced pressure to the first pressure zone until a first threshold is met indicating that the substrate is clamped against the upper surface within the first pressure zone; applying a second reduced pressure to the second pressure zone until a second threshold is met indicating the substrate is clamped against the upper surface within the second pressure zone; A method comprising:

13. 13. The method of claim 12, wherein applying the first vacuum pressure and / or the second vacuum pressure comprises reducing pressure in corresponding first or second grooves extending radially across the top surface of the vacuum table.

14. 14. The method of claim 12 or 13, comprising applying a third reduced pressure to the third pressure zone until a third threshold is met, indicating that the substrate is clamped against the upper surface within the third pressure zone.

15. A method according to any one of claims 12 to 14, comprising controlling the reduced pressure in each of the at least two pressure zones independently of the pressure in the other pressure zones.

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