Patent ID: 12197139

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION

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

The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device10and a facetted pupil mirror device11. The faceted field mirror device10and faceted pupil mirror device11together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device10and faceted pupil mirror device11.

After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B′ is generated. The projection system PS is configured to project the patterned EUV radiation beam B′ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors13,14which are configured to project the patterned EUV radiation beam B′ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B′, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors13,14inFIG.1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

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

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

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

FIG.2is a cross-sectional view of an object holder20according to an embodiment of the invention. The object holder20is configured to support an object. In the description below, the invention will be described in a context of the object holder being a substrate holder20that is configured to support a substrate W. However, the object holder is not limited to such a substrate holder20. For example, the object holder may be configured to support a patterning device MA.

In an embodiment, the substrate Table WT comprises the substrate holder20and a substrate stage. The substrate stage comprises a recess into which the substrate holder20is held. The substrate holder20is configured to hold the substrate W relative to the substrate stage of the substrate table WT.

As shown inFIG.2, in an embodiment the substrate holder20comprises a core body21. The core body21is a plate-like disk. As shown inFIG.2, in an embodiment the core body21comprises a plurality of burls22. The burls22are protrusions protruding at the surface of the core body21. As shown inFIG.2, in an embodiment the burls22have distal ends23. The core body21is configured such that the distal ends23are in a support plane24for supporting the substrate W. The underside of the substrate W comes into contact with the distal ends23of the burls22. The position of the underside of the substrate W corresponds to the support plane24. The burls22are arranged so that the substrate W lies generally flat on the substrate holder20.

The burls22are not shown to scale inFIG.2. In a practical embodiment, there can be many hundreds, thousands, or tens of thousands, of burls distributed across a substrate holder20of diameter, e.g., 200 mm, 300 mm or 450 mm. The tips of the burls22have a small area, e.g. less than 1 mm2, so that the total area of all of the burls22on one side of the substrate holder20is less than about 10% of the total area of the total surface area of the substrate holder20. Because of the arrangement of burls22, there is a high probability that any particle that might lie on the surface of the substrate W, substrate holder20or substrate table WT will fall between burls22and will not therefore result in a deformation of the substrate W or substrate holder20. The burl arrangement, which may form a pattern, can be regular or can vary as desired to provide appropriate distribution of force on the substrate W and substrate table WT. The burls22can have any shape in plan but are commonly circular in plan. The burls22can have the same shape and dimensions throughout their height but are commonly tapered. The burls22can project a distance of from about 1 μm to about 5 mm, desirably from about 5 μm to about 250 μm, desirably about 10 μm above the rest of the object-facing surface of the substrate holder20(i.e. the top surface of the electrostatic sheet25). Hence, the distance between the distal ends23of the burls22and the top surface of the electrostatic sheet25in the vertical direction is from about 1 μm to about 5 mm, desirably from about 5 μm to about 250 μm, desirably about 10 μm. The thickness of the core body21of the substrate holder20can be in the range of about 1 mm to about 50 mm, desirably in the range of about 5 mm to 20 mm, typically 10 mm.

The core body21may be made of rigid material. Desirably the material has a high thermal conductivity or a low coefficient of thermal expansion. Desirably the material is electrically conductive. Desirably the material has a high hardness. A suitable material includes SiC (silicon carbide), SiSiC (siliconized silicon carbide), Si3N4 (silicon nitrite), quartz, and/or various other ceramic and glass-ceramics, such as Zerodur™ glass ceramic. The core body21can be manufactured by selectively removing material from a solid disc of the relevant material so as to leave the projecting burls22. A suitable technique to remove material includes electrical discharge machining (EDM), etching, machining and/or laser ablation. The core body21can also be manufactured by growing burls22through a mask. The burls22may be of the same material as the base and can be grown by a physical vapor deposition process or sputtering. In an embodiment, the core body21comprises one or more internal channels (not shown in the Figures). In an embodiment, the core body21comprises a plurality of layers that are bonded together. In an embodiment, the layers are formed of different materials. Merely as one example, in an embodiment the core body21comprises a layer of SiSiC, a layer of glass and another layer of SiSiC in that order. Other combinations of layers are also possible.

As shown inFIG.2, in an embodiment the substrate holder20comprises one or more electrodes26for an electrostatic clamp. A potential difference may be generated in order to provide an electrostatic clamping force between the substrate W and the substrate holder20and/or between the substrate holder20and the substrate stage of the substrate table WT. In an embodiment, the electrodes26are encapsulated between dielectric layers (also known as electrical isolation layers)27,28. The potential difference generated may be of the order of 10 to 5,000 volts. Arrangements using one or more heaters and temperature sensors to locally control the temperature of a substrate are described in U.S. publication no. 2011-0222033, which is incorporated herein by reference in its entirety and the techniques therein may be applied to the techniques herein.

As shown inFIG.2, in an embodiment the substrate holder20comprises an electrostatic sheet25. The electrostatic sheet25comprises one or more electrodes26. For the electrodes26, in an embodiment two halves of continuous metal film (but isolated from the distal ends23of the burls22) may be separated by a separation distance from each other and deposited to form positive and negative elements of the electrostatic clamp. The separation distance is not particularly limited. In an embodiment the separation distance is at least about 20 μm, optionally 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 an embodiment the separation distance is at most about 2 mm, optionally at most about 1 mm, and optionally at most about 500 μm. In an embodiment the separation distance is about 500 μm. There may therefore be two electrodes26. However, the number of electrodes26in the electrostatic sheet25is not particularly limited and may be one or three or more. Metal lines of the electrodes26may have a layer thickness greater than about 20 nm, desirably greater than about 40 nm. The metal lines desirably have a layer thickness less than or equal to about 1 μm, desirably less than about 500 nm, desirably less than about 200 nm.

An electrode26of an upper electrostatic sheet25may be configured to electrostatically clamp the substrate W to the substrate holder20. An electrode26of a lower electrostatic sheet25may be configured to electrostatically clamp the substrate holder20to the rest of the substrate table WT.

In an embodiment the material of the core body21and the burls22is electrically conductive. For example, in an embodiment the material of the burls22is SiSiC. However, it is not essential for the material of the core body21and the burls22to be electrically conductive. In an embodiment, a grounding layer may be provided that electrically connects the distal ends23of two or more of the burls22(optionally all of the burls22) to ground or a common electrical potential. The grounding layer may be formed by depositing a relatively thick layer of a conductive material. The conductive material is not particularly limited. In an embodiment the conductive material is Cr. In an alternative embodiment the conductive material is CrN. In an embodiment the deposited layer is then patterned to form the grounding layer. The pattern may comprise a series of metal lines that connect together distal ends23of the burls22. Such patterns are sometimes referred to as “Manhattan” patterns. In an alternative embodiment the deposited layer is not patterned. In an embodiment the grounding layer or another layer is arranged to cover a surface of the core body21and/or the burls22. The grounding layer or other layer can help to smoothen the surface to make it easier to clean the surface.

As shown inFIG.2, in an embodiment the electrostatic sheet25comprises an electrode26sandwiched between dielectric layers27,28. As shown inFIG.2, in an embodiment burls22and the electrostatic sheet25are provided on both main surfaces of the substrate holder20. In an alternative embodiment, the burls22and the electrostatic sheet25are provided on only one of the two main surfaces of the substrate holder20. As shown inFIG.2, in an embodiment the electrostatic sheet25is between the burls22. For example, as shown inFIG.2, holes are34are provided in the electrostatic sheet25. The holes34are arranged such that their position corresponds to the burls22of the core body21. The burls22protrude through respective holes34of the electrostatic sheet25such that the electrode26that is sandwiched between the dielectric layers27,28is provided in the region between the burls22.

As shown inFIG.2, in an embodiment the substrate holder20, comprises a bonding material29. In an embodiment, the bonding material has a thickness of at least 100 nm. The bonding material29secures the position of the electrostatic sheet25relative to the core body21. The bonding material29keeps the holes34in the electrostatic sheet25aligned with the burls22. In an embodiment, the burls22are positioned at the centre of respective holes34of the electrostatic sheet25. By providing that the bonding material29has a thickness of at least 100 nm, a minimum level of compliancy of the bond layer is ensured. In general, thicker bonding material is more compliant and less sensitive to stress levels caused by temperature differences at the interface between the core body21and the electrostatic sheet25. In an embodiment, the bonding material29has a thickness of at least 200 nm, optionally at least 500 nm, optionally at least 1 μm, optionally at least 2 μm, optionally at least 5 μm, optionally at least 10 μm, optionally at least 20 μm, optionally at least 25 μm and optionally at least 50 μm. The inventors have found that making the bonding material29at least 25 μm in thickness improves the tolerances of the interface parts to temperature differences (i.e. the bond layer is more compliant). In an embodiment, the bonding material29has a thickness of at most 100 μm. This ensures a minimum level of stability of the bonding material29, which is beneficial for the electrostatic performance of the clamping function. In an embodiment, the bonding material29has a thickness of at most 75 μm, and optionally at most 50 μm. Reducing the thickness of the bonding material29helps to make the bonding material29more stable. Merely as an example, in an embodiment the bonding material29has a thickness in the range of from about 25 μm to about 75 μm.

As shown inFIG.2, in an embodiment the bonding material29is formed in discrete portions that do not connect to each other. There may be some variation in the thickness of the different portions of bonding material29. In an embodiment, the separate portions of bonding material29have substantially the same thickness as each other. As shown inFIG.10, in an alternative embodiment the bonding material29extends continuously in the region between the core body21and the electrostatic sheet25. As shown inFIG.10, in an embodiment the bonding material29extends to the region between the electrostatic sheet25and the burls22.

FIGS.3-5schematically depict successive stages of a process of manufacturing the substrate holder20. In particular,FIGS.3-5show stages of joining the electrostatic sheet25to the core body21.

As shown inFIG.3, in an embodiment the method of manufacturing the substrate holder20comprises providing a core body21. The core body21comprises a plurality of burls22for supporting the substrate W. As shown inFIG.3, in an embodiment the method comprises applying bonding material29to the core body21. The bonding material29is applied at least to one of the main surfaces of the core body21. The bonding material29is applied between the burls22. As shown inFIG.3, in an embodiment a plurality of discrete portions of bonding material29are applied to the core body21. In an alternative embodiment, the bonding material29is applied substantially continuously across the main surface of the core body21between the burls22. The method for dispensing the bonding material29is not particularly limited. By providing a plurality of discrete portions of bonding material29, the overall volume of the bonding material29may be reduced. This reduces the possibility of excess bonding material29being present near the burls22which may otherwise adversely influence the clamping function of the substrate holder20.

As shown inFIG.4, in an embodiment the method comprises connecting an electrostatic sheet25to a plate30of a tool. The plate30comprises a plurality of recesses31for respective burls22of the core body21. The recesses31line up with respective holes34of the electrostatic sheet25. In an embodiment, the centres of the recesses31align with the centres of the holes34in the electrostatic sheet25. The plate30is configured such that the pattern of recesses31corresponds to the pattern of burls22of the core body21. Accordingly, the disclosure provided above regarding the number and the positioning of the burls22applies equally to the number and positioning of the recesses31of the plate30, as well as to the holes34of the electrostatic sheet25. In an embodiment, in a direction perpendicular to a plane of the plate30, the plate30has a stiffness of at most 105Nm−1. The plate30is configured to be compliant to follow variations in the heights of the burls22.

The way that the plate30is connected to the electrostatic sheet25is not particularly limited. For example, the connection between the plate30and the electrostatic sheet25may be by means of a vacuum and/or electrostatic attraction and/or temporary bonding. In an embodiment, the plate30is positioned with respect to the core body21, for example using a positioner. The positioner is configured to control movement of the plate30when the electrostatic sheet25is connected. The positioner positions the plate30relative to the core body21such that the recesses31of the plate30(and hence also the holes34of the electrostatic sheet25) are aligned with the burls22of the core body21.

The positioner is configured to control movement of the plate30so as to mount the electrostatic sheet25onto the core body21between the burls22. For example, the positioner may lower the plate30downwards onto the core body21. In an alternative embodiment, the positioner is configured to move the core body21upwards towards the plate30such that the burls22extend through the holes34of the electrostatic sheet25.

During the mounting process, the distal ends23of the burls22come into contact with the base of the recesses31. The base of the recesses31stops the electrostatic sheet25and the core body21from moving closer to each other. The depth of the recesses31determines the height at which the electrostatic sheet25is mounted. The depth of the recesses31controls the distance between the bottom of the electrostatic sheet25and the upper surface of the core body21(where the bonding material29is positioned). The depths of the recesses31controls the height that the burls22protrude above the upper surface of the electrostatic sheet25in the manufactured substrate holder20.

In an embodiment, the recesses31have a depth that corresponds to the projected distance of the burls22mentioned above. For example, the recesses31have a depth of about 1 μm to about 5 mm, desirably from about 5 μm to about 250 μm.

In an embodiment, the method of manufacturing the substrate holder20comprises a step of curing the bonding material29. In an embodiment, the bonding material29is cured under vacuum. This helps to avoid inclusions in the bonding material29. However, it is not essential for the bonding material29to be cured. In an alternative embodiment, the bonding material29does not need to be cured.

The type of bonding material used is not particularly limited. In an embodiment the bonding material29comprises an adhesive material. However, it is not essential for the bonding material29to be an adhesive material. Non-adhesive materials can also be used. For example, in an alternative embodiment, the bonding material29is a material used in soldering or welding the electrostatic sheet25to the core body21.

FIG.5schematically shows the moment when the distal ends23of the burls22abut against the base of the recesses31.

In an embodiment, the bonding material29fixes the electrostatic sheet25to the core body21at the height defined by the recesses31of the plate30. In an embodiment, the method comprises disconnecting the plate30from the electrostatic sheet25. As shown inFIG.2, in an embodiment the electrostatic sheet25is provided on each side of the core body21. The method steps described above may be repeated for the opposite surface of the core body21so that an electrostatic sheet25is provided on both surfaces of the core body21.

In an embodiment, at least one of the burls22has a stiffness of at most 107Nm−1at its distal end23in a direction within the support plane24. In normal use of the substrate holder20, a direction within the support plane24is horizontal. The stiffness in a direction within the support plane24may be referred to as the horizontal stiffness. The horizontal stiffness refers to the amount of force required to be applied to the distal end23of the burl22in order to move the distal end23horizontally. For example, a horizontal stiffness of 107Nm−1means that a force of 10 N applied horizontally at the distal end23would cause the distal end23to move 1 μm horizontally. By providing that the horizontal stiffness is at most 107Nm−1, the horizontal stiffness is less than that of a known substrate holder in which an electrode is embedded in glass (rather than forming the electrostatic sheet25separately from the core body21that has the burls22). By providing that the burls22extend through the holes34of the electrostatic sheet25, the burls22are longer (in the vertical direction) than the thickness of the electrostatic sheet25. By providing longer burls22, the horizontal stiffness of the burls22can be reduced. In an embodiment, most of the burls22have a stiffness of at most 107Nm−1. In an embodiment, substantially all of the burls22have a stiffness of at most 107Nm−1at their distal ends23in a direction within the support plane24. The horizontal stiffness of the burls22can be controlled by selecting the length of the burls22. Additionally or alternatively, the horizontal stiffness of the burls22can be controlled by selecting the width (i.e. girth) of the burls22.

By reducing the horizontal stiffness of the burls22, the burls22are more flexible in the direction of thermal expansion of the substrate W during exposure. During an exposure process, the substrate W is heated by radiation. The heating of the substrate W causes the substrate W to expand locally. The amount of expansion varies across the substrate W. The expansion of the substrate W can cause parts of the underside of the substrate W to slip relative to the distal ends23of some of the burls22. Such slipping can cause undesirable overlay errors, which can be difficult to predict (and hence difficult to compensate for). By reducing the horizontal stiffness of the burls22, the possibility of the substrate W slipping relative to the burls22is reduced. The expansion of the substrate W imparts a horizontal force on the distal ends23of the burls22. The burls22can flex so as to maintain contact with the substrate W. The burls22can act as a cantilever beam.

In an embodiment, at least one (or most or all) of the burls22has a horizontal stiffness of at most 7×106Nm−1, optionally at most 5×106Nm−1, optionally at most 3×106Nm−1, optionally at most 2×106Nm−1, and optionally at most 106Nm−1at its distal end23. By reducing the horizontal stiffness of the burls22, the variation in contact stiffness of the burls22across the substrate holder22is desirably reduced.

In an embodiment, at least one (or most or all) of the burls22has a horizontal stiffness of at least 105Nm−1, optionally at least 2×105Nm−1, optionally at least 5×105Nm−1, and optionally at least 106Nm−1at its distal end23. This provides a minimum robustness to the burls22.

In an embodiment, the core body21including the burls22is made of a material that has a high Young's modulus. For example, SiSiC has a high Young's modulus. In an embodiment, the burls22have a vertical stiffness of at least 107Nm−1, optionally at least 2×107Nm−1, and optionally at least 3×107Nm−1. For example, the vertical stiffness may be about 3.4×107Nm−1. By increasing the vertical stiffness, focus performance can be improved.

The method of manufacturing the core body21comprising the burls22is not particularly limited. For example, laser ablation, electrical discharge machining and/or powder blasting may be used to produce the burls22.

FIG.6is a close-up view of part of a substrate holder20according to an embodiment of the invention. As shown inFIG.6, in an embodiment at least one of the burls22is surrounded by a trench35. The trench35is formed in the object-facing surface37of the core body21. In an embodiment, the burl22is immediately surrounded by the trench35. By providing the trench35, the length of the burl22is increased without having to increase the thickness of the electrostatic sheet25or the height that the distal end23of the burl22protrudes above the top surface of the electrostatic sheet25. By increasing the length of the burl22, the horizontal stiffness of the burl22can be reduced. The length of the burl22is the vertical distance between the distal end23and the trench floor36. The depth of the trench35is the vertical distance between the trench floor36and the object-facing surface37of the core body21. The object-facing surface37of the core body21is the surface on which the bonding material29is positioned.

The diameter of the distal end23of the burls22is not particularly limited. In an embodiment, the diameter of the distal end23is at least 100 μm, and optionally at least 200 μm. In an embodiment, the diameter at the distal end23is at most 500 μm. For example, in an embodiment the diameter at the distal end23is about 210 μm. The length of the burl22is not particularly limited. In an embodiment, the length of the burls22is at least 200 μm, optionally at least 500 μm and optionally at least 1000 μm. In an embodiment the length of the burls22is at most 2000 μm, and optionally at most 1000 μm. For example, in an embodiment the burls22that are not surrounded by a trench35have a length of about 560 μm. In an embodiment, burls22that are surrounded by a trench35have a length of about 1000 μm. In an embodiment, the core body21comprises at least one burl22that is surrounded by a trench35and at least one burl22that is not surrounded by any such trench. There may be a mixture of burls22that are surrounded by trenches35and burls22that are not surrounded by trenches.

FIG.7is a schematic diagram showing how the horizontal stiffness of the burls

22may vary depending on their position in the substrate holder20. In an embodiment, at least one of the burls22cin peripheral region of the substrate holder20has a lower horizontal stiffness than that of at least one of the burls22ain a central region of the substrate holder20. In an embodiment, the horizontal stiffness of the burls22gradually increases in the radial direction are away from the centre of the substrate holder20. As mentioned above, there are different ways of controlling the horizontal stiffness of the burls22.FIG.7schematically shows the horizontal stiffness being varied by vary the length of the burls22. For example, as shown inFIG.7, in an embodiment the length H of the burls22increases with increasing distance R from the centre of the substrate holder20. Additionally or alternatively, the depth of the trenches35surrounding the burls22may increase with increasing distance R from the centre of the substrate holder20. Additionally or alternatively, the width of the burls22may decrease with increasing distance R from the centre of the substrate holder20. As shown inFIG.7, in an embodiment intermediate burls22bmay have an intermediate height less than the height of the peripheral burls22cgreater than the length of the central burls22a.

When the substrate W expands with respect to the core body21, the relative movement of the substrate W varies depending on the radial position of the substrate W. In general, it may be expected that the relative movement between the substrate W and the core body21may be greatest at the periphery, while at a minimum in the centre of the substrate holder20. This means that the risk of the substrate W slipping relative to the distal ends23of the burls22is greater in the peripheral region of the substrate holder20. By varying the horizontal stiffness of the burls22depending on the radial position, the burls22can be made as flexible as needed for their position. This allows to balance the benefits of the flexible burls with the complexity of manufacturing the core body21. For example, it may be more difficult to manufacture a burl22having lower horizontal thickness. The complexity of manufacturing the core body21may be increased and it may need to be necessary to reduce the possibility of slipping between the substrate W and the substrate holder20.

FIG.8is a close-up view of part of a substrate holder20according to an

embodiment of the invention. As shown inFIG.8, in an embodiment the core body21comprises at least one gas supply passageway38. The gas supply passageway38is configured to supply gas to the radial gap39between the radially outer surface41(shown inFIG.9) of at least one of the burls22and the electrostatic sheet25via a vertical gap40between the core body21and the electrostatic sheet25.FIG.9is a close-up view of the radial gap39between the radially outer surface41of the burl22and the wall42of the electrostatic sheet25that defines the holes34in the electrostatic sheet25. InFIGS.8and9, the double ended arrows represent flow of gas.

In an embodiment, the substrate holder20comprises a thermal conditioner configured to thermally condition the substrate W. The thermal conditioner can be used to control the temperature of the substrate W, for example during an exposure process. In an embodiment the thermal conditioner comprises a circuit through which thermal conditioning fluid flows. In an embodiment the thermal conditioner comprises heaters and sensors controlled to control the thermal conditioning function.

By providing the gas supply passageway38, gas can be supplied between the substrate W and the substrate holder20so as to increase heat transfer between the substrate holder20and the substrate W. This helps with controlling the temperature of the substrate W. As shown inFIG.8, the gas is supplied through the gas supply passageway38directly under the electrostatic sheet25. The gas supply passageway38terminates at an opening in the object-facing surface37of the core body21directly below the electrostatic sheet25. The gas flows in the vertical gap40between the core body21and the electrostatic sheet25and reaches the radial gap39. The gas then fills the gap between the substrate W and the top surface of the electrostatic sheet25, thereby improving thermal conduction between the substrate W and the substrate holder20.

By providing the gas supply passageways38directly below the electrostatic sheet25, flatness of the substrate W can be improved. This is because the gas pathways between the core body21and the lower dielectric layer27of the electrostatic sheet25allow the gas to reach below the substrate W without requiring a hole in the electrostatic sheet25in that location. By not having a hole in the electrostatic sheet25, it is not necessary to have a hole in the high voltage electrode26. By not having a hole in the high voltage electrode26, it is not necessary to have a grounding layer in the hole to shield the electrode26. By not having a hole in the high voltage electrode26, there is no local reduction in the electrostatic clamping force which would otherwise reduce flatness. The electrostatic sheet25is positioned between the gas supply passageway38and the substrate W. In an embodiment, a plurality of such gas supply passageways38are provided across the core body21. There is no particular restriction on the position and number of the gas supply passageways38. An embodiment of the invention is expected to improve uniformity of pressure below the substrate W, without unduly reducing the flatness of the substrate W. The vertical gap40is present in regions between the portions of bonding material29. By increasing the uniformity of the pressure below the substrate W, the mean pressure can be reduced without unduly increasing the possibility of the pressure being too low below parts of the substrate W. By reducing the mean pressure below the substrate W, the effective clamping force holding the substrate W on the substrate holder20is increased. The release time for the gas can be decreased. The type of gas used is not particularly limited. In an embodiment, the gas comprises hydrogen. In an embodiment the electrostatic sheet25comprises holes directly vertically above the gas supply passageway38.

FIG.10is a close-up view of parts of a substrate holder20according to an embodiment of the invention. As shown inFIG.10, in an embodiment the bonding material29substantially fills the region between the lower surface of the electrostatic sheet25and the facing surface of the core body21. As shown inFIG.10, in an embodiment the bonding material29substantially covers the object-facing surface37of the core body21below the electrostatic sheet25. As shown inFIG.10, in an embodiment the bonding material29surrounds the burl22. As shown inFIG.10, in an embodiment the bonding material29has an upper surface which is lower than the upper surface of the electrostatic sheet25. In an alternative embodiment, the upper surface of the bonding material29is substantially coplanar with the upper surface of the electrostatic sheet25. The embodiment shown inFIG.10is in contrast to other embodiments described above in which the bonding material29is provided in sparse locations (rather than all around the electrostatic sheet25).

In an embodiment the electrode26is connected to a high voltage potential (e.g. about 3200 kV). The electric field arising between the substrate W and the electrostatic sheet25results in an electrostatic attraction force between the substrate holder20and the substrate W. In an embodiment, the core body26and its burls22are electrically grounded (or kept at another controlled potential). There is an electric field present in the region between the electrostatic sheet25and the burls22, as well as in the region between the electrostatic sheet25and the core body21. There is an electric field present in the bonding material29.

As shown inFIG.10, in an embodiment the electrostatic sheet25comprises an electrical insulator44. The electrical insulator44is non-conductive and functions as a high voltage barrier. The electrical insulator44surrounds the electrode26in the plane of the electrode26. The electrical insulator44reduces the possibility of electrical breakdown between the electrode26and the burl22.

Charge can flow across the electrical insulator44. This can undesirably affect the clamping force when the electrode26is at a high voltage. The flow of charge can also undesirably lead to a residual clamping force when the substrate holder20is not used to clamp the substrate W. In an embodiment in which the bonding material29is all around the electrostatic sheet25(e.g. as shown inFIG.10), the surface conductivity of the bonding material29can undesirably impact the electrostatic field and the clamping force.

FIG.11is a close-up view of part of a substrate holder20according to an embodiment of the invention. The arrows indicate the electric field. As shown inFIG.11, in an embodiment the electrostatic sheet25comprises electrostatic shielding45. The electrostatic shielding45is configured to isolate the electrostatic sheet25from other components. In an embodiment the electrostatic shielding45is configured to shield charges in the electrical insulator44from the substrate W and/or from the rest of the core body21and burls22. The electrostatic shielding45may be provided in one or more different positions around the electrostatic sheet25. In an embodiment, the electrostatic shielding45comprises electrically conductive plating at the surface of the electrostatic sheet25. In an embodiment, the electrostatic shielding45is applied by chemical vapour deposition. In an embodiment, the electrostatic shielding45is applied by sputtering. In an embodiment, the electrostatic shielding45is applied by physical vapour deposition. In an embodiment the electrostatic shielding45has a thickness of at least 50 nm, optionally at least 100 nm, optionally at least 200 nm and optionally at least 500 nm. In an embodiment the electrostatic shielding45has a thickness of at most 1000 nm, optionally at most 500 nm, optionally at most 200 nm and optionally at most 100 nm. In an embodiment the thickness of the electrostatic shielding45may be different for different sections of the electrostatic shielding.

FIG.12is a schematic cross-sectional view of part of the electrostatic sheet25according to an embodiment of the invention. As shown inFIG.12, in an embodiment the electrostatic sheet25comprises electrostatic shielding45aat the main surface of the electrostatic sheet25facing the core body21. In the example shown inFIG.11, this is the bottom surface of the electrostatic sheet25. The electrostatic shielding45ais configured to isolate the electrode26from the object-facing surface37of the core body21. In an embodiment, the electrostatic shielding45acovers substantially all of the bottom surface of the electrostatic sheet25. However, this is not necessarily the case. In an alternative embodiment, the electrostatic shielding45acovers only part of the bottom surface of the electrostatic sheet25.

As shown inFIG.12, in an embodiment the electrostatic sheet25comprises electrostatic shielding45bat the hole-defining surface of the electrostatic sheet25. The electrostatic shielding45bis configured to isolate the electrostatic sheet25from the radially outer surface41of at least one of the burls22. As shown inFIG.12, in an embodiment the electrostatic sheet25comprises electrostatic shielding45cat the main surface of the electrostatic sheet25facing away from the core body21. In the example shown inFIG.11, this is the top surface of the electrostatic sheet25. The electrostatic shielding45cis configured to at least partially isolate the electrostatic sheet25from a region of the substrate W vertically above the electrical insulator44. In an embodiment, the electrostatic shielding is electrically grounded (or kept at another controlled potential). The electrostatic shielding45can be present in several locations. In an embodiment, the electrostatic shielding45cis provided as a ring on top of the electrostatic sheet25around the holes34in the electrostatic sheet25. In an embodiment, the different sections of electrostatic shielding45a,45b,45chave different thicknesses.

As shown inFIG.11, for example, in an embodiment the holes34in the electrostatic sheet25are substantially cylindrical. However, this is not necessarily the case.FIGS.13to17are schematic diagrams of alternative embodiments in which the holes34are not completely cylindrical. For example, as shown inFIG.13in an embodiment the holes34are hourglass shaped. This may be easier to manufacture, particularly if powder blasting is used to shape the dielectric layer27,28. In an embodiment, the electrostatic sheet25comprises an upper dielectric layer28between the electrode26and the substrate W. The upper dielectric layer28tapers towards the substrate W. this means that the holes34increases in diameter towards the substrate W across the thickness of the upper dielectric layer28. As shown inFIG.13, in an embodiment the electrostatic sheet25comprises a lower dielectric layer27. The lower dielectric layer27is between the electrode26and the core body21. In an embodiment, the lower dielectric layer tapers towards the core body21. This means that the diameter of the holes34increases towards the core body21across the thickness of the lower dielectric layer27. An embodiment of the invention with hourglass shaped walls34is expected to be easier to manufacture. InFIG.13, both the upper dielectric layer28and the lower dielectric layer27are tapered. In an alternative embodiment, only one of the two dielectric layers27,28is tapered. InFIG.13, the dielectric layers27,28are tapered from both sides of the electrostatic sheet25. As shown inFIG.19, in an alternative embodiment, the electrostatic sheet25is tapered from only one side. For example, both the upper dielectric layer28and the lower dielectric layer27may taper towards the substrate W. Alternatively, both the upper dielectric layer28and the lower dielectric layer27may taper towards the core body21. In an embodiment, the holes34are formed in the electrostatic sheet25by powder blasting from only one side.

As shown inFIGS.13to16, the choice of where the electrostatic shielding45is provided is not particularly limited. As shown inFIG.13, in an embodiment the electrostatic shielding45is provided at the bottom surface of the electrostatic sheet25and inside the holes34but not on top of the electrostatic sheet25.

As shown inFIG.14, in an embodiment the electrostatic shielding45is provided between at least one of the burls22and only one of the two dielectric layers27,28that sandwich the electrode26. For example, as shown inFIG.14, in an embodiment the electrostatic shielding is provided at the bottom surface of the electrostatic sheet25and inside the holes34at the lower dielectric layer27(but not at the upper dielectric layer28).

As shown inFIG.15, in an embodiment22, in an embodiment, the electrostatic shielding45is provided at the bottom surface of the electrostatic sheet25, inside the holes34and at the top of the electrostatic sheet25around the holes34.

By providing the electrostatic shielding45, the possibility of undesirable electrical breakdown is reduced. This is particularly important when the size of the gaps between the electrode26and the burls22may not be well controlled because of manufacturing tolerances. The electrostatic shielding45helps to reduce electrical breakdown that may be caused due to charges flowing through the electrical insulator44or through the dielectric layers27,28. When the bonding material29is provided all around the electrostatic sheet25(for example as shown inFIG.10), the electrode magnetic shielding45reduces the possibility of electrical breakdown as a result of surface conductivity of the bonding materials29. The electrostatic shielding45reduces the possibility of undesirable electrostatic discharge across the gaps around the electrostatic sheet25that may be caused by materials entering those gaps.

As shown inFIG.16, in an embodiment electrostatic shielding45bis provided inside the holes34only for the upper dielectric layer28(but not for the lower dielectric layer27) and on top of the electrostatic sheet25around the holes34.

FIG.17schematically shows a perspective view of part of the substrate holder20according to an embodiment of the invention. As shown inFIG.17, in an embodiment the electrostatic shielding45csurrounding different burls22are electrically connected to each other via conductive lines46(also shown inFIG.16) on the object-facing surface of the upper dielectric layer28of the electrostatic sheet25that is between the electrode26and the substrate W. In an embodiment, the conductive lines46connect the electrostatic shielding45caround the holes34to ground (or another controlled potential). Accordingly, it is not necessary for the electrostatic shielding45aat the bottom surface of the electrostatic sheet25to be provided in order to connect the electrostatic shielding45cto ground.

FIG.18is a schematic plan view of the electrostatic shielding45csurrounding the distal ends23of the burls22connected by the conductive lines46. The electrostatic shielding45cmay appear as doughnuts electrically connected to ground by thin conductive lines46on top of the electrostatic sheet25.

The materials used to make all of the parts of an object holder according to embodiments may be any of the known materials used to manufacture known object holders. In particular, parts of the object holder according to embodiments may be manufactured with materials as disclosed in WO2015/120923A1, WO2014/154428A2 and US2013/0094009A1, the entire contents of which are incorporated herein by reference.

In particular, the metal used for the electrodes26may be Cr or Ti. The metal used on the distal end surfaces of the burls may be CrN or TiN. The insulating parts may be chrome oxide. The core body may be SiSiC. The material used for the electrostatic shielding45may be Cr, CrN or W (although many other materials are possible).

To aid clear explanation, embodiments have been described with reference to upper and lower surfaces of an object holder. The upper and lower surfaces are first and second surfaces of the object holder. The first surface is a surface to which an object may be clamped to. The second surface is a surface that a table may be clamped to. When the object holder is orientated in a horizontal plane, the first surface is an upper surface and the second surface is a lower surface. However, embodiments also include the object holder not being orientated in a horizontal plane.

Embodiments include the object holder being used in any lithographic apparatus. The lithographic apparatus may include any apparatus used in substrate manufacture, testing and inspection, such as an electron-beam inspection apparatus. To aid clear explanation, features of the object holder have been described primarily in the context of the upper side of a substrate holder20clamping to a substrate W. The features of the invention are equally applicable to the lower side of the object holder, for example the lower surface of a substrate holder20clamping to the rest of the substrate table WT. Merely as example, the features relating to the flexible burls22, the trench35and the mounting tool may be applied at the lower side of a substrate holder20.

Embodiments are provided according to the following clauses:

1. An object holder configured to support an object, the object holder comprising:a core body comprising a plurality of burls having distal ends in a support plane for supporting the object; andan electrostatic sheet between the burls, the electrostatic sheet comprising an electrode sandwiched between dielectric layers;wherein the electrostatic sheet is bonded to the core body by a bonding material having a thickness of at least 100 nm.

2. The object holder of clause 1, wherein at least one of the burls has a stiffness of 105-107Nm−1at its distal end in a direction within the support plane.

3. The object holder of clause 1 or 2, wherein at least one of the burls is immediately surrounded by a trench in an object-facing surface of the core body.

4. The object holder of any preceding clause, wherein at least one of the burls in a peripheral region of the object holder has a lower stiffness at its distal end in a direction within the support plane than that of at least one of the burls in a central region of the object holder.

5. The object holder of any preceding clause, wherein the core body comprises at least one gas supply passageway configured to supply gas to a radial gap between a radially outer surface of at least one of the burls and the electrostatic sheet via a vertical gap between the core body and the electrostatic sheet.

6. The object holder of any preceding clause, wherein the electrostatic sheet comprises electrostatic shielding configured to isolate the electrostatic sheet from at least one of a region of the object vertically above an electrical insulator surrounding the electrode in a plane of the electrode, an object-facing surface of the core body and a radially outer surface of at least one of the burls.

7. The object holder of clause 6, wherein the electrostatic shielding is provided between at least one of the burls and only one of two dielectric layers that sandwich the electrode.

8. The object holder of clause 6 or 7, wherein the electrostatic shielding surrounding different burls are electrically connected to each other via conductive lines on an object-facing surface of an upper dielectric layer of the electrostatic sheet between the electrode and the object.

9. The object holder of any preceding clause, wherein the electrostatic sheet comprises a first dielectric layer between the electrode and the object that tapers towards the object or the core body and a second dielectric layer between the electrode and the core body that tapers towards the object or the core body.

10. The object holder of any preceding clause, wherein the bonding material comprises an adhesive material.

11. A tool for mounting an electrostatic sheet comprising an electrode sandwiched between dielectric layers onto a core body comprising a plurality of burls having distal ends in a support plane for supporting an object, the tool comprising a plate comprising a plurality of recesses for respective burls, the plate configured to connect to the electrostatic sheet such that the recesses line up with respective holes in the electrostatic sheet.

12. The tool of clause 11, comprising a positioner configured to control movement of the plate when the electrostatic sheet is connected so as to mount the electrostatic sheet onto the core body between the burls, a depth of the recesses determining a height at which the electrostatic sheet is mounted.

13. The tool of clause 11 or 12, wherein in a direction perpendicular to a plane of the plate the plate has a stiffness of at most 105Nm−1.

14. A method of manufacturing an object holder, the method comprising: providing a core body comprising a plurality of burls having distal ends in a support plane for supporting an object;connecting an electrostatic sheet to a plate comprising a plurality of recesses for respective burls, such that the recesses line up with respective holes in the electrostatic sheet, the electrostatic sheet comprising an electrode sandwiched between dielectric layers; andcontrolling movement of the plate when the electrostatic sheet is connected so as to mount the electrostatic sheet onto the core body between the burls, a depth of the recesses determining a height at which the electrostatic sheet is mounted.

15. A lithographic apparatus comprising an object holder according to any of clauses 1-10.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other 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.

Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may 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 an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

Although specific reference may have been made above to the use of embodiments of the invention in the context of object inspection and optical lithography, it will be appreciated that the invention, where the context allows, is not limited to these contexts and may be used in other applications, for example imprint lithography.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.