An elevation pin assembly for loading / unloading a substrate, object table, lithographic apparatus

Abstract

The invention provides an elevation pin assembly for loading / unloading a substrate, the assembly comprising: - a plurality of support pins, the plurality of support pins being higher in number than 3; - an actuator assembly configured to cause a displacement of the plurality of support pins, and - a control unit configured to control the displacement of the plurality of pins to pre-shape the substrate while loading / unloading the substrate.

Description

AN ELEVATION PIN ASSEMBLY FOR LOADING / UNLO APING A SUBSTRATE,OBJECT TABLE, LITHOGRAPHIC APPARATUSCROSS-REFERENCE TO RELATED APPLICATION

[0001] The application claims priority of EP application 23188683.9 which was filed on 31 July, 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to an elevation pin assembly as can e.g. be applied in a stage apparatus of a lithographic apparatus.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’ s law’ . To keep up with Moore’ s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] The semiconductor manufacturing process as applied by a lithographic apparatus may require several hand overs of the substrate from one location to another, whereby the substrate undergoes various processes or process steps. During these processes, the substrate may be clamped to an object table. In order to load / unload a substrate from such an object table, elevation pins are typically used to raise the substrate or lower the substrate to the object table.

[0006] At present, it often occurs that substrates need to be processed which have a certain degree of warpage. It has been observed that the current tools as applied during the loading / unloading of a substrate may be inadequate to ensure a proper loading / unloading of the substrate. In particular, the loading or unloading of a warped substrate may cause wear to the substrate or the object table.SUMMARY

[0007] It is an objective of the present invention to enable an improved loading / unloading of a substrate.

[0008] According to an aspect of the invention, there is provided an elevation pin assembly for loading / unloading a substrate, comprising: a plurality of support pins, the plurality of support pins being higher in number than 3; an actuator assembly configured to cause a displacement of the plurality of support pins, and a control unit configured to control the displacement of the plurality of pins to pre-shape the substrate while loading / unloading the substrate.

[0009] According to an aspect of the invention, there is provided an object table comprising an elevation pin assembly according to the invention.

[0010] According to an aspect of the invention, there is provided a lithographic apparatus comprising an object table according to the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a schematic overview of a lithographic apparatus;Figure 2 depicts a detailed view of a part of the lithographic apparatus of Figure 1;Figure 3 schematically depicts a position control system;Figure 4 depicts a first embodiment of an elevation pin assembly according to the present invention.Figure 5 depicts a second embodiment of an elevation pin assembly according to the present invention.Figure 6 depicts a third embodiment of an elevation pin assembly according to the present invention.Figure 7 depicts a process of loading a substrate using an elevation pin assembly according to the present invention.Figures 8-10 depict support pins as can be applied in an elevation pin assembly according to the present invention.DETAILED DESCRIPTION

[0012] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0013] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0014] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as 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 connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with 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 patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0015] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a 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 radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0016] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0017] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be 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 referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0018] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W maybe carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0019] In order to load / unload a substrate W from the substrate support WT, the substrate support WT may comprise an elevation pin assembly according to the present invention. More details on such an elevation pin assembly are given below.

[0020] In addition to the substrate support WT, the lithographic apparatus LA may comprise 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 a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0021] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0022] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axis, i.e., an x-axis, a y-axis and a z-axis. Each of the three axis is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry -rotation. A rotation around about the z-axis is referred to as an Rz-rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0023] Figure 2 shows a more detailed view of a part of the lithographic apparatus LA of Figure 1. The lithographic apparatus LA may be provided with a base frame BF, a balance mass BM, a metrology frame MF and a vibration isolation system IS. The metrology frame MF supports theprojection system PS. Additionally, the metrology frame MF may support a part of the position measurement system PMS. The metrology frame MF is supported by the base frame BF via the vibration isolation system IS. The vibration isolation system IS is arranged to prevent or reduce vibrations from propagating from the base frame BF to the metrology frame MF.

[0024] The second positioner PW is arranged to accelerate the substrate support WT by providing a driving force between the substrate support WT and the balance mass BM. The driving force accelerates the substrate support WT in a desired direction. Due to the conservation of momentum, the driving force is also applied to the balance mass BM with equal magnitude, but at a direction opposite to the desired direction. Typically, the mass of the balance mass BM is significantly larger than the masses of the moving part of the second positioner PW and the substrate support WT.

[0025] In an embodiment, the second positioner PW is supported by the balance mass BM. For example, wherein the second positioner PW comprises a planar motor to levitate the substrate support WT above the balance mass BM. In another embodiment, the second positioner PW is supported by the base frame BF. For example, wherein the second positioner PW comprises a linear motor and wherein the second positioner PW comprises a bearing, like a gas bearing, to levitate the substrate support WT above the base frame BF.

[0026] The position measurement system PMS may comprise any type of sensor that is suitable to determine a position of the substrate support WT. The position measurement system PMS may comprise any type of sensor that is suitable to determine a position of the mask support MT. The sensor may be an optical sensor such as an interferometer or an encoder. The position measurement system PMS may comprise a combined system of an interferometer and an encoder. The sensor may be another type of sensor, such as a magnetic sensor, a capacitive sensor or an inductive sensor. The position measurement system PMS may determine the position relative to a reference, for example the metrology frame MF or the projection system PS. The position measurement system PMS may determine the position of the substrate table WT and / or the mask support MT by measuring the position or by measuring a time derivative of the position, such as velocity or acceleration.

[0027] The position measurement system PMS may comprise an encoder system. An encoder system is known from for example, United States patent application US2007 / 0058173A1, filed on September 7, 2006, hereby incorporated by reference. The encoder system comprises an encoder head, a grating and a sensor. The encoder system may receive a primary radiation beam and a secondary radiation beam. Both the primary radiation beam as well as the secondary radiation beam originate from the same radiation beam, i.e., the original radiation beam. At least one of the primary radiation beam and the secondary radiation beam is created by diffracting the original radiation beam with the grating. If both the primary radiation beam and the secondary radiation beam are created by diffracting the original radiation beam with the grating, the primary radiation beam needs to have a different diffraction order than the secondary radiation beam. Different diffraction orders are, for example, + 1storder, -1storder, +2ndorder and -2ndorder. The encoder system optically combines theprimary radiation beam and the secondary radiation beam into a combined radiation beam. A sensor in the encoder head determines a phase or phase difference of the combined radiation beam. The sensor generates a signal based on the phase or phase difference. The signal is representative of a position of the encoder head relative to the grating. One of the encoder head and the grating may be arranged on the substrate structure WT. The other of the encoder head and the grating may be arranged on the metrology frame MF or the base frame BF. For example, a plurality of encoder heads are arranged on the metrology frame MF, whereas a grating is arranged on a top surface of the substrate support WT. In another example, a grating is arranged on a bottom surface of the substrate support WT, and an encoder head is arranged below the substrate support WT.

[0028] The position measurement system PMS may comprise an interferometer system. An interferometer system is known from, for example, United States patent US6,020,964, filed on July 13, 1998, hereby incorporated by reference. The interferometer system may comprise a beam splitter, a mirror, a reference mirror and a sensor. A beam of radiation is split by the beam splitter into a reference beam and a measurement beam. The measurement beam propagates to the mirror and is reflected by the mirror back to the beam splitter. The reference beam propagates to the reference mirror and is reflected by the reference mirror back to the beam splitter. At the beam splitter, the measurement beam and the reference beam are combined into a combined radiation beam. The combined radiation beam is incident on the sensor. The sensor determines a phase or a frequency of the combined radiation beam. The sensor generates a signal based on the phase or the frequency. The signal is representative of a displacement of the mirror. In an embodiment, the mirror is connected to the substrate support WT. The reference mirror may be connected to the metrology frame MF. In an embodiment, the measurement beam and the reference beam are combined into a combined radiation beam by an additional optical component instead of the beam splitter.

[0029] The first positioner PM may comprise a long-stroke module and a short-stroke module. The short-stroke module is arranged to move the mask support MT relative to the long-stroke module with a high accuracy over a small range of movement. The long-stroke module is arranged to move the short-stroke module relative to the projection system PS with a relatively low accuracy over a large range of movement. With the combination of the long-stroke module and the short-stroke module, the first positioner PM is able to move the mask support MT relative to the projection system PS with a high accuracy over a large range of movement. Similarly, the second positioner PW may comprise a long-stroke module and a short-stroke module. The short-stroke module is arranged to move the substrate support WT relative to the long- stroke module with a high accuracy over a small range of movement. The long-stroke module is arranged to move the short-stroke module relative to the projection system PS with a relatively low accuracy over a large range of movement. With the combination of the long-stroke module and the short-stroke module, the second positioner PW is able to move the substrate support WT relative to the projection system PS with a high accuracy over a large range of movement.

[0030] The first positioner PM and the second positioner PW each are provided with an actuator to move respectively the mask support MT and the substrate support WT. The actuator may be a linear actuator to provide a driving force along a single axis, for example the y-axis. Multiple linear actuators may be applied to provide driving forces along multiple axis. The actuator may be a planar actuator to provide a driving force along multiple axis. For example, the planar actuator may be arranged to move the substrate support WT in 6 degrees of freedom. The actuator may be an electromagnetic actuator comprising at least one coil and at least one magnet. The actuator is arranged to move the at least one coil relative to the at least one magnet by applying an electrical current to the at least one coil. The actuator may be a moving-magnet type actuator, which has the at least one magnet coupled to the substrate support WT respectively to the mask support MT. The actuator may be a moving-coil type actuator which has the at least one coil coupled to the substrate support WT respectively to the mask support MT. The actuator may be a voice-coil actuator, a reluctance actuator, a Lorentz -actuator or a piezo-actuator, or any other suitable actuator.

[0031] The lithographic apparatus LA comprises a position control system PCS as schematically depicted in Figure 3. The position control system PCS comprises a setpoint generator SP, a feedforward controller FF and a feedback controller FB. The position control system PCS provides a drive signal to the actuator ACT. The actuator ACT may be the actuator of the first positioner PM or the second positioner PW. The actuator ACT drives the plant P, which may comprise the substrate support WT or the mask support MT. An output of the plant P is a position quantity such as position or velocity or acceleration. The position quantity is measured with the position measurement system PMS. The position measurement system PMS generates a signal, which is a position signal representative of the position quantity of the plant P. The setpoint generator SP generates a signal, which is a reference signal representative of a desired position quantity of the plant P. For example, the reference signal represents a desired trajectory of the substrate support WT. A difference between the reference signal and the position signal forms an input for the feedback controller FB. Based on the input, the feedback controller FB provides at least part of the drive signal for the actuator ACT. The reference signal may form an input for the feedforward controller FF. Based on the input, the feedforward controller FF provides at least part of the drive signal for the actuator ACT. The feedforward FF may make use of information about dynamical characteristics of the plant P, such as mass, stiffness, resonance modes and eigenfrequencies.

[0032] Figure 4 schematically shows a top view (a) and a side view (b) of an elevation pin assembly 400 according to the present invention. In accordance with the present invention, an elevation pin assembly for loading / unloading a substrate is disclosed. In accordance with the present invention, the elevation pin assembly comprises a plurality of support pins 410, the number of support pins 410 being higher than 3. In the embodiment as shown in Figure 4, the elevation pin assembly 400 comprises 4 support pins 410. In Figure 4 (b) the dotted line 402 schematically illustrates a substrate. In accordance with the invention, the plurality of support pins 410 as shown are displaceable. Inparticular, they are displaceable in the indicated Z-direction, which in practice will typically correspond to a vertical direction. As such, in accordance with the invention, the plurality of support pins can be moved upwards or downwards, thereby displacing the top surface 410.1 of the support pins in vertical direction. In accordance with the invention the elevation pin assembly comprises an actuator assembly configured to cause a displacement of the plurality of support pins. In the embodiment as shown, the actuator assembly comprises a plurality of actuators 420 for respectively displacing the plurality of support pins 410. In the embodiment as shown, each support pin of the plurality of support pins 410 is thus equipped with an actuator 420. It can be pointed out that this need not be the case. In an embodiment, as will be illustrated below, two or more support pins may be displaced by a single common actuator.

[0033] In the embodiment as shown, the set of actuators 420 or actuator assembly 420 is mounted to a support frame 440. In such embodiment, the support 404 can e.g. corresponds to a substrate table WT as described above, which support 404 is positioned by a positioning device such as the second positioner PW. In such case, the support frame 440 and the set of actuators 420 can be mounted to the long-stroke module of the second positioner PW. In an alternative embodiment, the support frame 440 can e.g. be mounted to a bottom surface of the support 404 for supporting the substrate 402. Alternatively, the actuator assembly 420 can be integrated or mounted to the support 404 directly.

[0034] In accordance with the present invention, the elevation pin assembly 400 further comprises a control unit 430 for controlling a displacement of the plurality of support pins 410.

[0035] By controlling the displacement of the support pins 410, the control unit 430 can control the height or elevation of the top surface 410.1 of the support pins 410, e.g. relative to a support surface404.1 of a support 404 for the substrate 402. Note that the dotted line 404 schematically illustrates a support onto which the substrate 402 can be supported or held. Such a support can e.g. be the aforementioned substrate table WT. In the embodiment as shown, the support 404 comprises apertures404.2 through which the support pins 410 can protrude. In an embodiment, the support pins 410 can e.g. be displaced, under control of the control unit 430 such that the top surface 410.1 of the plurality of support pins 410 is arranged below the support surface 404.1. By doing so, the substrate 402 can be lowered until it rests on the support surface 404.1. This process whereby a substrate 402 is initially supported by the plurality of support pins 410 and subsequently positioned on a support surface of a support 404 can be referred to as loading a substrate 402. The reverse process, whereby a substrate 402 is initially supported on a support surface 404.1 of a support 404 and subsequently supported by the plurality of support pins 410 when the support pins 410 are elevated, can be referred to as unloading a substrate 402. When unloaded, a gap in vertical direction will typically appear between the support surface 404.1 and a bottom surface of the substrate 402. A gripper may e.g. make use of said gap to grip the substrate 402, e.g. to transport the substrate to a different location.

[0036] In accordance with the present invention, the control unit 430 of the elevation pin assembly is configured to control the displacement of the plurality of pins to pre-shape the substrate whileloading / unloading the substrate. In accordance with the present invention, the displacement of the plurality of support pins is such that a shape of the substrate is changed or modified while the substrate is loaded / unloaded. In particular, in an embodiment of the present invention, a curvature of the substrate can be reduced during the loading or unloading process. In an embodiment, the preshaping of the substrate occurs prior to the substrate contacting the support 404.

[0037] At present, it is observed that substrates that need to be processed, e.g. provided with a pattern in a lithographic apparatus, are no longer flat but are curved, to some extent. This curvature of the substrate is also referred to as warpage. The processing, in particular the handling of warped or curved substrates may pose challenges to known apparatuses. It has been observed that also the loading and unloading of substrates may adversely be affected by the warpage of a substrate. In order to e.g. expose a substrate to a patterned beam of radiation, the substrate is typically clamped onto a support, such as a substrate support WT, e.g. by means of vacuum or electrostatic forces. When a curved or warped substrate needs to be clamped, it may be difficult to obtain the correct clamping of the substrate. The substrate may deform during the process, or the required clamping force may be difficult to achieve. In addition, parts of the substrate may displace relative to the support surface, e.g. in horizontal direction, during the clamping process. This may cause damage to the substrate and / or may cause debris. In addition, it may cause a local positioning inaccuracy of the substrate.

[0038] Using the elevation pin assembly according to the invention, the aforementioned challenges and problems may at least be mitigated. By means of the elevation pin assembly according to the invention, a pre-shaping of the substrate is applied during the loading / unloading of the substrate. As result of this pre-shaping a clamping of the substrate 402 onto a support 404 can be done more easily, with a reduced risk of damaging either the substrate 402 or the support 404. In general, the present invention enables to pre-shape a substrate 402 to a more desirable shape, during a loading / unloading process. In an embodiment, such a more desirable shape can be a substantially flat shape. However, this need not be the case. Alternative pre-shaping processes, whereby the substrate is e.g. pre-shaped towards a slightly curved shape, can be considered as well.

[0039] Figure 5 schematically shows another embodiment of an elevation pin assembly 500 according to the present invention. Apart from the location of the plurality of support pins 510 in the assembly 500, the elevation pin assembly 500 substantially corresponds to the elevation pin assembly 400 shown in Figure 4.

[0040] In accordance with the present invention, the elevation pin assembly 500 comprises a plurality of support pins 510, the number of support pints 510 being higher than 3. In the embodiment as shown in Figure 5, the elevation pin assembly 500 comprises 4 support pins 510. In the arrangement as shown, the plurality of support pins 510 comprise a central support pin 510a and 3 outer support pins 510b in a triangular configuration. In Figure 5 (b) the dotted line 502 schematically illustrates a substrate. In the arrangement as shown, the plurality of support pins 510 are displaceable by means of a respective plurality of actuators 520 of an actuator assembly.

[0041] In accordance with the present invention, the elevation pin assembly 500 further comprises a control unit 530 for controlling a displacement of the plurality of support pins 510.

[0042] By controlling the displacement of the support pins 510, the control unit 530 can control the height or elevation of the top surface 510.1 of the support pins 510, e.g. relative to a support surface504.1 of a support 504 for the substrate 502. In particular, the control unit 530 of the elevation pin assembly is configured to control the displacement of the plurality of pins 510 to pre-shape the substrate 502 while loading / unloading the substrate. By doing so, a warpage of a substrate that is being loaded or unloaded can be taken into account and a shape of the substrate can be adjusted, in order to obtain an improved loading or unloading of the substrate.

[0043] Figure 6 schematically shows yet another embodiment of an elevation pin assembly 600 according to the present invention. In accordance with the present invention, the elevation pin assembly 600 comprises a plurality of support pins 610, the number of support pints 610 being higher than 3. In the embodiment as shown in Figure 6, the elevation pin assembly 600 comprises 6 support pins 610. In the arrangement as shown, the plurality of support pins 610 comprise a first set of 3 support pins 610a, arranged in a triangular configuration near a centre of the assembly and 3 outer support pins 610b in a triangular configuration. In Figure 6 (b) the dotted line 602 schematically illustrates a substrate. In the arrangement as shown, the plurality of support pins 610 are displaceable by means of an actuator assembly 620 comprising multiple actuators. In the arrangement as shown, each of the outer support pins 610b is equipped with a dedicated actuator 620b of the actuator assembly 620, for displacing or moving said pin. The first set of pins 610a arranged in the centre of the elevation pin assembly 600 are configured to be displaceable by a common actuator 620a of the actuator assembly 620. In such case, the first set of pins 610a can be arranged on a common frame which can be displaced, e.g. in vertical direction or Z-direction, by the actuator 620a.

[0044] In accordance with the present invention, the elevation pin assembly 600 further comprises a control unit 630 for controlling a displacement of the plurality of support pins 610.

[0045] By controlling the displacement of the support pins 610, the control unit 630 can control the height or elevation of the top surface 610.1 of the support pins 610, e.g. relative to a support surface604.1 of a support 604 for the substrate 602. In particular, the control unit 630 of the elevation pin assembly is configured to control the displacement of the plurality of pins 610 to pre-shape the substrate 602 while loading / unloading the substrate. By doing so, a warpage of a substrate that is being loaded or unloaded can be taken into account and a shape of the substrate can be adjusted, in order to obtain an improved loading or unloading of the substrate.

[0046] As an alternative to the embodiment shown in Figure 6, the first set of support pins 610a may also comprise a single support pin, rather than a set of 3 support pins. Such a single support pin may e.g. be arranged in a central position 612 of the elevation pin assembly. It can further be pointed out that the elevation pin assembly 400 as shown in Figure 4 may also be equipped with one or more additional support pins arranged at or near a centre of the assembly.

[0047] In an embodiment of the present invention, the plurality of support pins as applied in the elevation pin assembly according to the invention, are configured to exert a holding force on the substrate. In such an arrangement, the plurality of support pins, as a whole, are capable of holding the substrate, thus ensuring that the substrate, e.g. substrate 402, 502, 602 described above, remains in a substantially fixed position relative to the support pins or the top surfaces of the support pins. It can be pointed out that, in order to exert a holding force on the substrate, not all support pins need to be able to exert a holding force on the substrate. As such, in an embodiment, a subset of the plurality of support pins is configured to apply a holding force to the substrate.

[0048] In order to create such a holding force, one or more of the plurality of support pins of the elevation pin assembly according to the invention can be configured to exert an attractive force to a bottom surface of the substrate that is being loaded / unloaded. Such an attractive force can e.g. be generated by means of vacuum or electrostatically. This will be explained in more detail below.

[0049] Within the meaning of the present invention, the holding force or attractive force may also be referred to as a clamping force.

[0050] Figure 7 schematically illustrates a process of pre-shaping a substrate while loading / unloading a substrate, using an elevation pin assembly according to the invention.

[0051] Figure 7 (a) schematically illustrates a substrate 702 which is held by a gripper 705 above a substrate support 704. The substrate support 704 comprises an elevation pin assembly 700 according to the invention, which comprises a plurality of support pins 710. In the arrangement as shown, the elevation pin assembly 700 comprises a central pin 710a and multiple, e.g. 3 or 4, support pins 710b arranged radially towards an edge of the substrate 702.

[0052] In the arrangement as shown, the substrate 702 has a curvature, it has an umbrella shape.

[0053] It is further assumed that the central support pin 710a is configured to exert a holding force or clamping force onto the substrate 702.

[0054] Starting from the initial position as illustrated in Figure 7 (a), Figures 7 (b) - (d) illustrate various stages of a loading process of the substrate using the elevation pin assembly 700 according to the invention. Figure 7 (b) illustrates the assembly 700 in a position whereby the support pins 710 have been elevated to a level at which they touch a bottom surface of the substrate 702. Such an elevation of the support pins 710 can be realised by means of the actuator assembly 720 of the elevation pin assembly 700.

[0055] Due to the umbrella shape of the substrate 702, the elevation or the displacement of the central support pin 710a will be larger than the elevation or displacement of the outer support pins 710b of the assembly 700.

[0056] When the substrate 702 is supported by the plurality of support pins 710 of the elevation pin assembly 700, the substrate can in fact be handed over to the elevation pin assembly 700. This can e.g. be done by lowering the gripper 705 and retracting the gripper 705 such that the substate 702 issolely supported by the support pins of the elevation pin assembly 700. Alternatively, or in addition, the support pins 710 can be raised a bit further upward, whereupon the gripper 705 can be retracted.

[0057] In order to ensure that the substrate 702 remains fixed on the support pins 710, the plurality of support pins 710 are, in an embodiment of the present invention, configured to exert a holding force on the substrate 702, thus ensuring that the substrate 702 does not detaches from the support pins 710. In general however, it can be pointed out that the present invention may be applied without the requirement of exerting a holding force on the substrate by the support pins 710. In such case, the gravitational force acting on the substrate 402 is sufficient to hold the substrate. In particular with respect to the pre-shaping of thin or very thin substrates, it can be pointed out that such substrates can be pre-shaped merely by a suitable displacement of plurality of support pins as applied in the elevation pin assembly according to the invention.

[0058] Once the gripper 705 has been retracted, the substrate 702 can be pre-shaped, in accordance with the present invention. This is illustrated in Figure 7 (c). Figure 7 (c) illustrates the stage in the loading process whereby the elevation or displacement of the support pins 710 has been modified so as to adjust the shape of the substrate 702. In particular, the elevation or displacement of the support pins 710 has been adjusted so as to reduce the curvature of the substrate 702. For the example given, this corresponds to exerting a pulling or downward force 712a on the substrate 702, by the central support pin 710a, and / or a pushing force 712b by the radially outward support pins 710b. Within the meaning of the present invention, the force or forces needed to pre- shape the substrate may be referred to as a pre-shaping force. The required pre-shaping force will in general depend on the actual shape of the substrate that is to be loaded / unloaded, the stiffness of the substrate, and the desired shape of the substrate.

[0059] As will be appreciated by the skilled person, the forces exerted on the substrate by the elevation pin assembly according to the invention can be two-fold:- A first force component exerted on the substrate during a loading / unloading process may, as mentioned above, be applied to ensure that the substrate remains attached or held by the support pins. This first component may also be referred to as a clamping force, clamping the substrate to the top surfaces of the plurality of support pins of the elevation pin assembly according to the present invention. Note that not all pins of the plurality of support pins need to be involved in generating this clamping or holding force.- A second force component exerted on the substrate during the loading / unloading process is the pre-shaping force, i.e. the force required to adjust / modify the shape of the substrate.

[0060] In an embodiment, the control of both force components, i.e. the clamping force and the pre-shaping force can be controlled by the control unit of the elevation pin assembly according to the invention. Alternatively, the forces can be controlled independently, e.g. by different control units. As will be understood, the nature of both force components and the manner to apply these components is substantially different; in order to apply the clamping force, i.e. the force component that the substrateremains attached to the support pins, an attractive force is to be generated between a top surface of at least one pin of the plurality of support pins and the bottom surface of the substrate. Such an attractive force can e.g. be generated by means of vacuum or electrostatic forces. Once the support pins are near the bottom surface of the substrate or touch the bottom surface, such a clamping force can be applied, to hold the substrate in its place relative to the plurality of support pins. The pre-shaping force is a force that aims to adjust the shape of the substrate that is being held. As illustrated by the example in Figure 7 (b) / (c), this may require displacing the plurality of support pins while maintaining the clamping force which holds the substrate on the support pins. For the umbrella-shape substrate 702 as shown in Figure 7 (b), this would require exerting a pulling or downward force 712a on the substrate 702, by the central support pin 710a, e.g. by lowering said pin, and / or a pushing force 712b by the radially outward support pins 710b, e.g. by raising said pins. In an embodiment, the displacements of the support pins that hold the substrate 702 are such that the top surfaces of the plurality of support pins are in substantially the same plane. The pre-shaping force needed to achieve this depends on a.o. the amplitude of the warpage of the substrate, the stiffness of the substrate and the desired shape of the substrate. The pre-shaping force is thus to be provided by the actuator assembly 720, in order to shape the substrate 702, e.g. to a more flat shape. Note that during the pre-shaping of the substrate 702, the clamping force holding the substrate onto the support pins 710 should be sufficiently high in order to avoid detaching the substrate from the support pins.

[0061] After pre-shaping the substrate 702, as illustrated in Figure 7 (c), the shape of the substrate 702 complies better with the support surface 704.1 of the support 704 onto which the substrate is to be positioned or held. In particular, by means of the pre-shaping, the substrate 702 can be flattened, up to a certain degree, depending on the number of support pins. As such, in a next stage, illustrated in Figure 7 (d), the substrate 702 can be lowered onto the surface 704.1 of the support 704.

[0062] Figure 7 (d) schematically shows the substrate 702 supported on the support 704 when the support pins 710 of the elevation pin assembly 700 have been lowered. In particular, the support pins 710 have been lowered to a position whereby the top surfaces of the support pins are substantially flush with the support surface 704.1 of the support 704. As such, in the depicted position, the substrate 702 is in contact with both the support pins 710 and with the support surface 704.1 of the support 704. Due to the pre-shaping of the substrate 702 prior to landing the substrate on the support surface 704.1 of the support 704, the contacting of the substrate 702 and the support 704 can be established substantially without a deformation of the substrate 702. Once lowered to the support 704, the substrate 702 can be clamped to the support, e.g. by a clamping arrangement of the support 704, e.g. a vacuum or electrostatic clamping assembly. In Figure 7 (d), arrows 730 schematically illustrate a clamping force exerted by the support 704 on the substrate 702. Because of the pre-shaping of the substrate, the clamping of the substrate 702 to the support 704 may now be established substantially without requiring a deformation of the substrate 702.

[0063] As will be understood by the skilled person, a deformation of the substrate 702 would occur in case the substrate 702 would be lowered to the support 704 without the pre-shaping action. In particular, when a substrate 702 as shown in Figure 7 (b) would be lowered to the support 704, it would initially, due to the umbrella-shape, only contact the support 704 along the outer edge 702.1 of the substrate 702. When the curved or warped substrate 702 would subsequently be clamped to the support 704, the outer edge 702.1 of the substrate would be forced to displace further outward, thereby possibly damaging either the substrate or the support 704.

[0064] Using the present invention, a substrate 702 that is to be loaded or unloaded is pre-shaped prior to the substrate 702 being supported on the support, thereby avoiding or reducing the risk that the substrate 702 is deformed during a clamping process or unclamping process on the support 704.

[0065] As a result, damage to either the substrate or the support can be avoided or mitigated. In addition, the clamping process, whereby the substrate 702 is handed over from the elevation pin assembly to the support and is held by the support rather than by the elevation pin assembly, is facilitated. In particular, it has been observed that the application of a clamping force, e.g. a vacuum or electrostatic force, can be compromised or rendered difficult when the substrate that is to be clamped on the support has a curved or non-flat shape. In case an umbrella-shaped substrate is positioned on the support 704, it may e.g. be difficult to attract the central portion of the substrate 702 towards the support. Similarly, in case a bowl-shaped substrate is positioned on the support 704, it may e.g. be difficult to attract the outer edge of the substrate 702 towards the support 704. Due to the pre-shaping as provided by the elevation pin assembly according to the invention, such clamping is rendered more easy.

[0066] In accordance with the present invention, the elevation pin assembly comprises a plurality of support pins which can be displaced by an actuator assembly.

[0067] Figure 8 schematically shows a support pin 800 as can be applied in an elevation pin assembly according to the present invention, in a lower position on the left, and an elevated position on the right.

[0068] In the embodiment as shown, a tubular support pin 800 shown, which can e.g. be connected to a vacuum or low-pressure source via supply line 810. In the embodiment as shown, the support pin 800 can be moved upward by means of an actuator 820. The actuator comprising a stationary part 820.1 which is mounted to a frame 830, and a movable part 820.2 which is mounted to the support pin 800. The actuator can e.g. be an electromagnetic actuator such as a Lorentz actuator or a reluctance actuator. A top part of the support pin 800 is configured to provide a support surface 800.1 onto which a substrate can be supported. By means of the vacuum or low-pressure connection, an attractive force can be exerted, when needed, on the substrate that is supported. The actuator 820 as shown may e.g. be controlled by a control unit of the elevation pin assembly in which the support pin 800 is applied.

[0069] In an embodiment, multiple support pins such as support pin 800 are combined and their position is controlled by a common actuator. Referring to Figure 6, the support pins 610a can e.g. bemounted to a common frame which can then be displaced by an actuator such as actuator 820 shown in Figure 8.

[0070] In an embodiment, the support pin as applied in the elevation pin assembly according to the invention comprises a compliant portion. In an embodiment, this compliant portion is part of a tip portion of the support pin. Figure 9 schematically shows a support pin 900 as can be applied in an elevation pin assembly according to the present invention and which has a compliant portion.

[0071] Figure 9 schematically shows a support pin 900 which is similar to the support pin 800, apart from the following. The support pin 900 as schematically shown in Figure 9 comprises, near the tip of the support pin, a compliant portion 910 which allows a deformation, in particular a tilting, of the top portion of the support pin 900. Such a tilting is illustrated on the right, where the support pin 900 is shown in an elevated position. By means of the compliant portion 910 of the support pin, the support surface 900.1 of the support pin 900 can be tilted and may thus comply with an orientation of a surface of a substrate that is to be supported. By doing so, the risk of damaging a substrate that is to be supported can be mitigated. Because of the more conformal contact, the clamping of the substrate on the support surface 900.1 is facilitated, thus enabling a more successful clamping or holding of the substrate on the support pin 900.

[0072] Instead of having a dedicated portion of the support pin to integrate a compliance, the support pin, or a large portion thereof can be made of a flexible material to enable a deformation, e.g. tilting, of the top surface of the support pin.

[0073] In an arrangement whereby multiple support pins are arranged to be moved by a common actuator, it is feasible to integrate a compliance for the multiple support pins as a whole, such that the top surfaces of the multiple support pins can be oriented towards the bottom surface of the substrate that is loaded / unloaded. In order to create the desired compliance, use can e.g. be made of one or more leaf springs or the like. Such one ore more leaf springs can e.g. be applied to provide a compliance to a single support pin or to a set of support pins, e.g. a set of support pins displaceable by a single actuator.

[0074] Figure 10 schematically illustrates yet another support pin 1000 as can be applied in an elevation pin assembly according to the present invention. The support pin 1000 also comprises a compliant portion 1010 which allows the top surface 1000.1 to tilt. The support pin 1000 comprises an electrode 1050 via which a clamping force can be exerted on the substrate that is to be loaded / unloaded. In the embodiment as shown, the support pin 1000 can be moved upward by means of an actuator 1020. The actuator comprising a stationary part 1020.1 which is mounted to a frame 1030, and a movable part 1020.2 which is mounted to the support pin 1000. In the embodiment as shown, a position measurement system 1040 is further provided that is configured to measure a vertical position of the support pin 1000. In the embodiment as shown, the position measurement system 1040 comprises a first part 1040.1 mounted to the support pin 1000 and a second part 1040.2 mounted to the frame 1030. The first part 1040.1 can e.g. be a sensor head, whereas the second part1040.2 can e.g. be a scale co-operating with the sensor head. In order to determine the vertical, or Z- position, of the support pin, various position measurement systems could be applied. Examples are capacitive system, encoder based systems or interferometer based system.

[0075] In an embodiment of the present invention, the elevation pin assembly thus comprises a position measurement system configured to measure a vertical position of one or more of the plurality of support pins. The position measurement system may provide position signals to the control unit of the elevation pin assembly. Based on the position signals, the control unit can e.g. determine, in an embodiment of the present invention, the shape or a deformation of the substrate that is supported by the support pins. In particular, during a loading process of a substrate, as e.g. illustrated in Figure 7 (b), the elevation pin assembly according to the invention can be configured to: displace the plurality of support pins towards a bottom surface of the substrate that is to be loaded, resulting in a situation as depicted in Figure 7 (b), receive, from the position measurement system, position signals representing the vertical position of the one or more of the plurality of support pins, and determine, based on the position signals, a required displacement for the plurality of support pins, to pre-shape the substrate.

[0076] Considering the arrangement shown in Figure 7 and assuming that each support pin is equipped with a position measurement system, it will be appreciated that a control unit can determine, based on position signals indicating the position illustrated in Figure 7 (b), how to displace the support pins, either the central pin 710a or the outer pins 710b, in order to arrive at a situation as depicted in Figure 7 (c), i.e. a situation wherein the substrate 702 is substantially flat.

[0077] As an alternative to applying a position measurement system and determining the required displacements, e.g. to flatten the substrate, based on position signals from said measurement system, it may be possible to determine the required displacements based on warpage data which the control unit can receive in advance. Prior to being loaded onto a substrate support table or the like, a substrate may have been conditioned, e.g. in a substrate handler. During such conditioning, warpage information may have been obtained. This warpage information can be applied by the control unit of the elevation pin assembly according to the invention to determine the required displacements to execute the pre-shaping of the substrate during the loading / unloading process.

[0078] With respect to the clamping force as applied to clamp the substrate to the plurality of support pins prior to the pre-shaping of the substrate, it can be pointed out that the pre-shaping forces applied to adjust the shape of the substrate may counteract the clamping force or forces. Considering e.g. the situation as depicted in Figure 7 (b), whereby a substrate 702 with an umbrella-shape is held on supporting pins 710. In order to pre-shape the substrate 702, the central pin 710a needs to be lowered, relative to the outer pins 710b. Due to the stiffness of the substrate however, a lowering of the central portion of the substrate will cause and upward force on the substrate, counteracting the clamping or holding force. If the clamping force is insufficient, the central portion of the substrate 702will detach from the support pin 710a, when said pin is lowered. As such, compared to known elevation pin assemblies which do not apply a pre-shaping of the substrate, it may be required to ensure in the elevation pin assembly according to the invention that the clamping force applied to hold the substrate onto the support pins is sufficient to allow the pre-shaping of the substrate.

[0079] Since the control unit of the elevation pin assembly according to the invention is configured to determine the required displacements to perform the pre-shaping, it could be advantageous to enable the control unit to control the holding or clamping force as well.

[0080] As such, in an embodiment of the present invention, the control unit of the elevation pin assembly can be configured to control the clamping force by means of which the substrate is held on the support pins.

[0081] The elevation pin assembly according to the invention may advantageously be applied in an object table, e.g. an object table for holding a semiconductor substrate as e.g. applied in an exposure apparatus, such as a lithographic apparatus.

[0082] Although specific reference may be made in this text to the use of a 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, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0083] 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.

[0084] 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.

[0085] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine -readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certainactions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0086] 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. Other aspects of the invention are set-out as in the following numbered clauses.1. An elevation pin assembly for loading / unloading a substrate, comprising: a plurality of support pins, the plurality of support pins being higher in number than 3; an actuator assembly configured to cause a displacement of the plurality of support pins, and a control unit configured to control the displacement of the plurality of pins to pre-shape the substrate while loading / unloading the substrate.2. The assembly according to clause 1, wherein the plurality of support pins is configured to exert a holding force on the substrate.3. The assembly according to clause 2, wherein the holding force is a vacuum force or an electrostatic force.4. The assembly according to clause 2 or 3, wherein the holding force is exerted on the substrate by a subset of the plurality of support pins.5. The assembly according to clause 4, wherein the subset comprises one pin.6. The assembly according to any of the clauses 2 to 5, wherein the control unit is configured to control the holding force.7. The assembly according to any of the preceding clauses, further comprising a support frame onto which the plurality of support pins is displaceably mounted.8. The assembly according to any of the preceding clauses, wherein a tip portion of the plurality of support pins comprises a compliant portion.9. The assembly according to clause 8, wherein the compliant portion is configured to orient a contact surface of the tip portion parallel to a contact surface of the substrate.10. The assembly according to any of the preceding clauses, wherein the plurality of support pins comprises a first set of pins and a second set of pins.11. The assembly according to clause 10, wherein the first set comprises at least 1 pin and wherein the second set comprises at least 3 pins.12. The assembly according to clause 11, wherein each pin of the first set of pins are arranged at a distance DI from a center of the assembly, wherein each pin of the second set of pins are arranged at a distance D2 from a center of the assembly, D2 > DI.13. The assembly according to clause 11 or clause 12, wherein the first set of pins is arranged on a common frame, and wherein an actuator of the actuator assembly is configured to displace the common frame in a vertical direction.14. The assembly according to any of the preceding clauses, further comprising a position measurement system configured to measure a vertical position of one or more of the plurality of support pins.15. The assembly according to clause 14, wherein the position measurement system comprises an encoder based measurement system.16. The assembly according to clause 14 or 15 and referring to clause 2, wherein the control unit is configured to control the holding force, based on the vertical position of the one or more of the plurality of pins.17. The assembly according to any of the clauses 14 to 16, wherein the control unit is configured to, during a loading process: displace the plurality of support pins towards a bottom surface of the substrate that is to be loaded; receive, from the position measurement system, position signals representing the vertical position of the one or more of the plurality of support pins; determine, based on the position signals, a holding force and a displacement for the plurality of support pins, to pre-shape the substrate.18. The assembly according to any of the preceding clauses referring to clause 2, wherein the holding force comprises a clamping force for clamping the substrate onto the support pins and a preshape force, the pre-shape force being caused by the displacement of the plurality of pins to pre-shape the substrate while loading / unloading the substrate.19. The assembly according to any of the preceding clauses, wherein pre-shaping the substrate comprises reducing a curvature or warpage of the substrate.20. An object table comprising a support having a surface for receiving a substrate and an elevation pin assembly according to any of the preceding clauses.21. The object table according to clause 20, wherein control unit of the elevation pin assembly is configured to control the displacement of the plurality of pins to pre-shape the substrate prior to the substrate contacting the support.22. An exposure apparatus comprising an object table according to clause 20 or 21.

Claims

CLAIMS1. An elevation pin assembly for loading / unloading a substrate, comprising: a plurality of support pins, the plurality of support pins being higher in number than 3; an actuator assembly configured to cause a displacement of the plurality of support pins, and a control unit configured to control the displacement of the plurality of pins to pre-shape the substrate while loading / unloading the substrate.

2. The assembly according to claim 1, wherein the plurality of support pins is configured to exert a holding force on the substrate.

3. The assembly according to claim 2, wherein the holding force is a vacuum force or an electrostatic force.

4. The assembly according to claim 2 or 3, wherein the holding force is exerted on the substrate by a subset of the plurality of support pins.

5. The assembly according to claim 4, wherein the subset comprises one pin.

6. The assembly according to any of the claims 2 to 5, wherein the control unit is configured to control the holding force.

7. The assembly according to any of the preceding claims, further comprising a support frame onto which the plurality of support pins is displaceably mounted.

8. The assembly according to any of the preceding claims, wherein a tip portion of the plurality of support pins comprises a compliant portion.

9. The assembly according to claim 8, wherein the compliant portion is configured to orient a contact surface of the tip portion parallel to a contact surface of the substrate.

10. The assembly according to any of the preceding claims, wherein the plurality of support pins comprises a first set of pins and a second set of pins.

11. The assembly according to claim 10, wherein the first set comprises at least 1 pin and wherein the second set comprises at least 3 pins.

12. The assembly according to claim 11, wherein each pin of the first set of pins are arranged at a distance DI from a center of the assembly, wherein each pin of the second set of pins are arranged at a distance D2 from a center of the assembly, D2 > DI.

13. The assembly according to any of the preceding claims, further comprising a position measurement system configured to measure a vertical position of one or more of the plurality of support pins.

14. The assembly according to claim 13 and referring to claim 2, wherein the control unit is configured to control the holding force, based on the vertical position of the one or more of the plurality of pins.

15. The assembly according to claim 13 or 14, wherein the control unit is configured to, during a loading process: displace the plurality of support pins towards a bottom surface of the substrate that is to be loaded; receive, from the position measurement system, position signals representing the vertical position of the one or more of the plurality of support pins; determine, based on the position signals, a holding force and a displacement for the plurality of support pins, to pre-shape the substrate.

16. The assembly according to any of the preceding claims referring to claim 2, wherein the holding force comprises a clamping force for clamping the substrate onto the support pins and a pre-shape force, the pre-shape force being caused by the displacement of the plurality of pins to pre-shape the substrate while loading / unloading the substrate.

17. The assembly according to any of the preceding claims, wherein pre-shaping the substrate comprises reducing a curvature or warpage of the substrate.

18. An object table comprising a support having a surface for receiving a substrate and an elevation pin assembly according to any of the preceding claims.

19. The object table according to claim 18, wherein control unit of the elevation pin assembly is configured to control the displacement of the plurality of pins to pre-shape the substrate prior to the substrate contacting the support.

20. An exposure apparatus comprising an object table according to claim 18 or 19.

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