System and method

Abstract

The present invention concerns a system, comprising a stage for supporting a substrate and a control system for controlling movement of the stage. The control system is configured to determine an in-situ mass of the substrate, and control a position of the stage depending on the determined in-situ mass of the substrate.

Description

SYSTEM AND METHODCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24211287.8 which was filed on 6 November 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a system and method for controlling movement of a stage, accounting for in-situ mass of the substrate supported by the stage.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.

[0005] A lithographic apparatus may include an illumination system for providing a projection beam of radiation, and a support structure for supporting a patterning device. The patterning device may serve to impart the projection beam with a pattern in its cross-section. The apparatus may also include a projection system for projecting the patterned beam onto a target portion of a substrate.

[0006] In a lithographic apparatus, the substrate to be exposed (which may be referred to as a production substrate) may be held on a substrate support (sometimes referred to as a wafer table). The substrate support may comprise, or be supported on, an actuated stage.

[0007] During activities such as lithography and substrate inspection / assessment, the actuated stage is typically used to move the substrate relative to the rest of the lithographic or inspection / assessment apparatus, particularly with respect to the projection apparatus used to project the pattern or to inspect the substrate. Like other actuated devices, there may be errors in the movement of the stage. These errors may include servo errors, in which it takes longer than is desirable for the stage to arrive at a home or index position, or prolonged settling time errors, in which the stage takes longer than desiredto “settle” into a new direction and / or speed. For stages such as this, typically one of the largest disturbance forces comes from the influence of mass on the force required to achieve a particular acceleration (and therefore also speed and position). There is therefore a desire to reduce errors and settling times in movement of the stage.SUMMARY

[0008] An object of the present invention is to provide a system and method for controlling movement of a stage, accounting for in-situ mass of the substrate supported by the stage. In particular, it is proposed to account for disturbance forces associated with the mass of the substrate in the control of the movement of the stage. For example, errors may result if it is not accounted for that the mass of the substrate on the stage differs from the mass of a nominal wafer, used to calibrate the system. These errors may negatively impact the lithographic or inspection process on the substrate, and / or may reduce throughput due to the increased settling times increasing the time taken to process each substrate.

[0009] In accordance with the present invention is a system, comprising a stage for supporting a substrate and a control system for controlling movement of the stage. The control system is configured to determine an in-situ mass of the substrate, and control a position of the stage depending on the determined in-situ mass of the substrate.

[0010] Also in accordance with the present invention is a method of controlling movement of a stage. The method comprises loading a substrate onto the stage; determining an in-situ mass of the substrate; and controlling a position of the stage depending on the determined in-situ mass of the substrate.

[0011] Further embodiments, features and advantages of the present invention, as well as the structure and operation of the various embodiments features and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the invention will now be described by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:Figure 1 schematically depicts an overview of a lithographic apparatus;Figure 2 depicts a plan view of a substrate support;Figure 3 depicts a cross-sectional view of the substrate support of Figure 2;Figure 4 depicts a cross-sectional view of a system including a stage comprising an actuator to measure a mass of the substrate using pins.Figure 5 depicts a cross-sectional view of a system including a stage comprising an actuator to measure a mass of the substrate using a short-stroke stage of the stage;Figure 6 depicts a cross-sectional view of a system including a stage comprising an actuator to measure a thickness of the substrate using an edge sensor;Figure 7 depicts a cross-sectional view of a system including a stage comprising an actuator to measure a height of an upper surface of the substrate using a height sensor.The features shown in the figures are not necessarily to scale, and the size and / or arrangement depicted is not limiting. It will be understood that the figures include optional features which may not be essential to the invention. Furthermore, not all of the features of the substrate support are depicted in each of the figures, and the figures may only show some of the components relevant for describing a particular feature.DETAILED DESCRIPTION

[0013] 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 436, 405, 365, 248, 193, 157, 126 or 13.5 nm).

[0014] 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.

[0015] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation or DUV 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 substrate table or a substrate support) 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 WT (alternatively referred to as a stage or actuated stage) 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.

[0016] In operation, the illumination system IL receives the radiation beam B 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.

[0017] 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.

[0018] The lithographic apparatus may be of a type wherein at least a portion of the substrate W may be covered by an immersion liquid having a relatively high refractive index, e.g., water, so as to fdl an immersion 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 US 6,952,253, which is incorporated herein by reference.

[0019] The lithographic apparatus may 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 may be 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.

[0020] In addition to the substrate support WT, the lithographic apparatus may comprise a measurement stage (not depicted in Figure 1). 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 mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system PMS, 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 P 1 , P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0022] In this specification, 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 axes is orthogonal to the other two axes. 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] In a lithographic apparatus it is necessary to position with great accuracy the upper surface of a substrate to be exposed in the plane of best focus of the aerial image of the pattern projected by the projection system. To achieve this, the substrate can be held on a substrate support. The surface of the substrate support that supports the substrate can be provided with a plurality of burls whose distal ends can be coplanar in a nominal support plane. The burls, though numerous, may be small in cross- sectional area parallel to the support plane so that the total cross-sectional area of their distal ends is a few percent, e.g. less than 5%, ofthe surface areaofthe substrate. The gas pressure in the space between the substrate support and the substrate may be reduced relative to the pressure above the substrate to create a force clamping the substrate to the substrate support.

[0024] A plan view of a substrate support 1 is shown in Figure 2. A partial cross section of the substrate support 1 is depicted in Figure 3. The substrate support 1 may comprise a main body 10 having an upper surface 11. The main body 10 may form a substantial portion of the substrate support 1. The upper surface 11 may be a top surface of the main body 10 when positioned as shown in Figure 3. That is, the upper surface 11 may be top surface in the Z-direction (the vertical direction).

[0025] The substrate support 1 may comprise a plurality of burls (or protrusions) 20 connected to, and protruding from, the upper surface 11 of the main body 10. Optionally, the substrate support 1 may comprise a plurality of burls (or protrusions) 20 connected to, and protruding from, the lower surface (opposite the upper surface 11) of the main body 10. The plurality of burls 20 may have proximal ends 21, which are situated near the main body 10 when in position, and distal ends 22. The distal ends 22 may be at opposite ends ofthe plurality of burls 20 to the proximal ends 21. That is, the distal ends 22 may be situated at an end of the burl 20 away from the main body 10.

[0026] The plurality of burls 20 may have a central longitudinal axis 23, with the proximal end 21 at one end of the burl 20 and the distal end 22 at the other end of the burl 20 along the central longitudinal axis 23. Thus, each of the plurality of burls 20 may have a central longitudinal axis 23 from the proximal end 21 to the distal end 22.

[0027] The distal ends 22 of the plurality of burls 20 form a support plane for a substrate W. Specifically, the distal ends 22 of the plurality of burls 20 may support a lower surface 31 of the substrateW. An upper surface 32 of the substrate W may be a surface opposite the lower surface 31. The upper surface 32 may be a surface which is configured to receive the radiation beam B.

[0028] The support plane may be formed in a substantially flat plane. Consequently, the substrate W can be positioned on the support plane to also be substantially flat, which can reduce errors in the pattern printed onto the substrate W (i.e., defectivity).

[0029] As shown in Figure 3, the plurality of burls 20 may be substantially frusto-conical, i.e. a truncated cone, or may be conical in shape. They may instead be substantially cylindrical. A frustoconical burl 20 may be stronger than a cylindrical burl 20 and thus have less likelihood of breaking. Preferably the plurality of burls 20 have the same shape as each other.

[0030] The plurality of burls 20 may be connected to the upper surface 11 of the main body 10 in any suitable way. The plurality of burls 20 may be separate components which are attached to the upper surface 11 of the main body 10. Alternatively, the plurality of burls 20 may be integral to the main body 10. In other words, the plurality of burls 20 may be formed as protrusions from the upper surface 11 of the main body 10, i.e. the plurality of burls 20 may be formed as a single part with the main body 10.

[0031] During activities such as lithography and substrate inspection, an actuated stage WT is typically used to support a substrate W disposed on the stage WT and to move the substrate W relative to the rest of the lithographic or inspection / assessment apparatus, particularly with respect to the projection apparatus PS. Like other actuated devices, there may be errors in the movement of the stage. These errors may include servo errors, in which it takes longer than is desirable for the stage to arrive at a home or index position, or prolonged settling time errors, in which the stage takes longer than desired to “settle” into a new direction and / or speed. For stages such as this, typically one of the largest disturbance forces comes from the influence of mass on the force required to achieve a particular acceleration (and therefore also speed and position).

[0032] It is common for the system controlling movement of the stage to be calibrated during a process in which a nominal substrate is supported on the stage. In this way, the disturbance forces associated with a substrate having the same mass as the nominal substrate may be accounted for in the control of the movement of the stage. However, if the mass of a substrate on the stage differs from the mass of the nominal substrate, this may result in errors (such as those described above). These errors may negatively impact the lithographic or inspection process on the substrate, and / or may reduce throughput due to the increased settling times increasing the time taken to process each substrate.

[0033] This is an increasingly prevalent problem due to the increasing variation in substrate mass. One example of this is that substrates having ever greater numbers of layers are being created. The thickness of the substrates may therefore differ from that of the nominal substrate, resulting in a discrepancy between the mass of the substrate used in practice and the mass of the nominal substrate used for calibration of the system. For example, a nominal substrate may be approximately 775 microns thick. In practice, there are substrates of up to 830 microns thick, with plans for substrates going beyond900 microns thick. A 900 micron thick substrate is likely to be of the order of 20 gram heavier than the nominal substrate (assuming both the substrates and nominal substrate are made of silicon). This difference is large enough to be non-negligible and to have an influence on the level of error in the movement of the stage.

[0034] Furthermore, differences in mass between the nominal substrate and the substrates being created and used in practice may also result from the use of different materials. Whereas traditionally substrates were generally made of silicon, there is an increasing number of substrates being made of different materials. For example, substrates are being made of gallium arsenide (GaAs), which has a density more than double that of silicon. Thus, whereas a nominal 300 mm diameter silicon substrate weighs approximately 128 gram, a gallium arsenide substrate of the same size is likely to weigh approximately 290 gram. There is therefore a problem that systems calibrated using a nominal substrate may have non-negligible errors and / or settling times in the movement of the stage when the stage is supporting a substrate of a different mass.

[0035] With these differences between actual in-situ mass of substrates (e.g., undergoing processes such as lithography or inspection), and the mass of a nominal substrate used in calibration, there may be significant errors. A control system may attempt to reduce these errors by means of feedback control. In particular, during use the control system may determine that the stage WT is not achieving movement according to control targets within acceptable thresholds (for example, target position distance accuracy, or settling time). In other words, the control system may detect errors in the movement of the stage. The control system may be configured to make adjustments to the control in an to attempt to correct for these errors going forward. However, this approach is reactive rather than proactive, meaning some level of errors is experienced before correction is applied in subsequent control commands to compensate for, and aim to reduce or eliminate, the errors. In other words, for each substrate there may be errors at a start of the processing which are gradually compensated for through feedback control. It is therefore desirable to provide a more proactive means of reducing errors in movement of the stage.

[0036] The errors in movement of the stage WT may be reduced by determining a mass of the substrate W disposed on the stage WT to undergo processing (e.g., lithography or inspection), and controlling movement of the stage WT depending on the determined mass. In other words, a substrate W may be loaded onto the stage WT for the purpose of undergoing processing. The mass of the substrate W in-situ on the stage WT, is determined. Then, the position of the stage WT may be controlled depending on the determined in-situ mass of the substrate W. In this way, the in-situ mass of the substrate W may be accounted for in the control applied to the stage WT. As a consequence, errors in the movement of the stage WT, such as overshooting the target position or having a long settling time, may be reduced.

[0037] Preferably, a mass feedforward control of the stage WT may be provided, based on the determined in-situ mass of the substrate W. The in-situ mass of the substrate W may be accounted for,in advance, when controlling the stage WT to perform movement. With this approach, a likelihood of the stage WT reaching its target location and / or speed in an accurate and timely fashion is increased in comparison to a scenario in which the control of the stage WT is based on a nominal substrate, which may have a different mass than that of the substrate W presently disposed on the stage WT.

[0038] The stage WT may be the same as or similar to that described above in reference to Figure 3. For example, the stage may comprise a body 10 having a plurality of burls 20 projecting therefrom. The plurality of burls 20 having distal ends which form a plane to support the substrate W on the stage WT.

[0039] The in-situ mass of the substrate W is optionally based on information input by a user. For example, the user may input the mass of the substrate W. Alternatively, the user may input details related to the size, shape and material properties of the substrate W. For example, the user may input a thickness, diameter and density of a circular disk-shaped substrate W.

[0040] Alternatively, or additionally, to user input, the in-situ mass of the substrate W may be measured for example as the substrate W is being loaded onto the stage WT, or while the substrate W is on the stage WT, or as the substrate W is being unloaded from the stage WT. It is preferable that the in-situ mass of the substrate W is measured as the substrate W is being loaded onto the stage WT or shortly thereafter while the substrate W is on the stage WT. This is because the in-situ mass may be determined while the rest of the apparatus (e.g., for lithography or inspection) is being prepared. In this way, the measuring of the in-situ mass of the substrate W takes place concurrently with other processes and does not increase an amount of time needed to process the substrate W, or increases it by a very small amount of time. Furthermore, the in-situ mass being determined early in the processing of the substrate W enables control of the movement of the stage WT to account for the measured in-situ mass (which would not be possible if the in-situ mass of the substrate were not determined until it was removed from the stage WT.

[0041] The in-situ mass of the substrate W may optionally be measured a plurality of times, particularly if a mass of the substrate W is expected to change throughout the processing. This is more likely during a lithographic process than an inspection process. For example, the in-situ mass of the substrate W may be determined at a beginning, during and / or at an end of the processing while the substrate W is on the stage WT. In this way, a change of the mass of the substrate W during processing may be accounted for in the control of the movement of the stage WT.

[0042] Although the control is desirably not entirely based on the mass of a nominal substrate, for the reasons provided above, the control system may nonetheless be initially calibrated using a nominal substrate having a predetermined mass. In particular, the control may be set up such that if a substrate W having the predetermined mass were placed on the stage WT, errors resulting from the mass of the substrate W would be minimised. In reality, it is likely that there may be some difference in mass between the substrate W undergoing processing and the predetermined mass of the nominal substrate. Once the in-situ mass of the substrate W undergoing processing has been determined, any differencebetween the determined in-situ mass of the substrate W and the predetermined mass of the nominal substrate may be calculated. The control of the movement of the stage WT may be adjusted based on the calculated difference between the determined in-situ mass of the substrate and the predetermined mass. In other words, the control system is preferably configured to adjust mass feedforward control of the stage based on a difference between the in-situ mass of the substrate and the predetermined mass of the nominal substrate used to calibrate the control system. In this way, the control does not to be established from scratch for each mass of substrate W but is rather established, and adjusted as needed, relative to the control applied to achieve desirable movement of the stage WT when loaded with the nominal substrate.

[0043] The control system is configured, after the in-situ mass has been determined, to control the position of the stage WT to move the stage WT to a target position. In particular, the control system is configured such that the control command to control the position of the stage WT accounts for the measured in-situ mass of the substrate W. In this way, a likelihood of the stage WT, which is supporting the substrate W, reaching the target position accurately is increased compared to if the control system did not account for the in-situ mass of the substrate WT. This is because a likelihood of the stage WT, for example, overshooting the target position due to the influence of the mass of the substrate W is reduced because the in-situ mass of the substrate W is taken into account in the control applied to the stage WT.

[0044] The control system is desirably configured, after moving the stage WT, to assess whether the target position has been reached within a predetermined threshold range. In other words, the control system may be configured to assess whether the target position is reached with an acceptable error margin. For example, the predetermined threshold range may be a distance range and / or a time range. In particular, the distance may be a distance between the target position of the stage WT and an actual position of the stage WT which resulted from the control command intended to set the stage WT at the target position. In this way, the control system may be configured to assess, for example, if the target position has been overshot (with the stage WT travelling past the target position) or undershot (with the stage WT falling short of the target position). Alternatively, or additionally, the predetermined threshold range may include a time range. The time may, for example, be a total time taken between the control being initiated and the stage WT coming to rest at the actual final position of the stage WT. In other words, the time may be a time between the control command being sent, to send the stage WT to the target position, and the stage WT reaching the actual position of the stage WT which resulted from the control command intended to set the stage WT at the target position.

[0045] The control system may be configured to adjust mass feedforward control of the stage if the assessment reveals that the predetermined threshold range has been exceeded. In other words, the control system may be configured to identify if an acceptable error margin, in the movement of the stage WT, has been exceeded. The control system may then be configured to adjust control in an attempt to reduce the error level associated with future control commands issued by the control systemto the stage WT. In this way, the control system may be configured to reduce errors based on feedback to the control system during operation of the system.

[0046] During lithographic and / or inspection processing, the stage WT may move in a scanning pattern (e.g., a zig-zag pattern in the X-Y plane of Figures 4-7). The processing of the substrate W may be performed when the stage WT is moving in the desired direction at a constant speed. It is therefore desirable that the stage WT, which is supporting the substrate W, is able to change direction and reach a stable speed in the new direction quickly and reliably. In other words, it is desirable that the settling time for the stage WT to settle at the constant speed is low. The control system may therefore be configured to determine whether the settling time of the stage WT is within a predetermined threshold settling time. The control system is configured to adjust mass feedforward control of the stage if the predetermined threshold settling time is exceeded. The control system may then be configured to adjust control in an attempt to reduce the settling time associated with future control commands issued by the control system to the stage WT. In this way, the control system may be configured to reduce settling time based on feedback to the control system during operation of the system. For example, the control system may be configured, after the in-situ mass of the substrate W has been determined, to control the stage WT to move in a first direction. The control system may be configured, to then control the stage to move in a second direction (different to the first direction) at a constant speed, accounting for the measured in-situ mass of the substrate W. In one example, the control system is configured to identify a first time at which the stage W starts moving in the second direction, identify a second time at which the stage starts moving at the constant speed, and to assess whether a difference between the first time and the second time is within a predetermined threshold settling time. In another example, the control system is configured to identify a first time at which the command to move in the second direction at the constant speed is sent to the stage WT. In this second example (similarly to the previous example), the control system is configured to identify the second time at which the stage starts moving at the constant speed, and to assess whether a difference between the first time and the second time is within a predetermined threshold settling time. In both examples, the control system is preferably configured to adjust mass feedforward control of the stage if the predetermined threshold settling time is exceeded.

[0047] Figures 4 to 7 show arrangements each including a system to determine the in-situ mass of the substrate W and to control the stage WT based on the determined in-situ mass. In particular, Figures 4 to 7 each show a cross-sectional view of the system in a side view. The system may be comprised as part of a lithographic apparatus configured to form a pattern on the substrate W, such as the lithographic apparatus LA described above in respect to Figure 1, or the system may be comprised in an assessment apparatus configured to inspect the substrate W.

[0048] In preferred arrangements, for example those shown in Figures 4 to 7, the system comprises a mass determination unit configured to measure the in-situ mass of the substrate. The mass determination unit may comprise any suitable means for measuring mass or weight (from which massmay be determined). The mass determination unit may, for example, comprise a spring balance, a strain gauge and / or a load cell.

[0049] The control system may comprise a first controller configured to determine the in-situ mass of the substrate, for example based on user input and / or measurement by the mass determination unit. The control system preferably comprise a second controller configured to control the position of the stage WT. The second controller may be a mass feedforward controller of the stage WT.

[0050] The system may further comprise an actuator configured to apply a force to the substrate W. The control system may be configured to control the actuator. In particular, the first controller may be configured to control the actuator to determine the in-situ mass of the substrate W.

[0051] In the arrangement of Figure 4, the actuator comprises a pin 50 configured to contact the substrate W as the substrate W is loaded onto and / or off of the stage WT (in a Z-direction of Figure 4). As shown in Figure 4, one or more pins 50 may be provided. Each pin 50 is configured to move towards the body 10 of the stage WT when subjected to the load, or weight due to mass, of the substrate W. In particular, the body 10 of the stage WT may define recesses configured to receive the one or more pins 50. The control system is configured to determine the in-situ mass of the substrate based on load on the one or more pins 50. The actuator optionally comprises a plurality of pins 50. Each pin 50 may be configured to move independently. In this way, a mass distribution across the substrate W may be estimated. Alternatively, the pins 50 may be configured to move collectively. The pins 50 may be elevator pins (alternatively referred to as e-pins) configured to receive the substrate W and aid in lowering the substrate W until it contacts the burls 20 for loading of the substrate W onto the stage WT, and to aid in raising the substrate W off the burls 20 for removal of the substrate W off of the stage WT. In this way, the in-situ mass of the substrate W may be determined during a normal, pre-existing and necessary step of the substrate loading / unloading process, without adding time to the overall processing, therefore without reducing throughput. Furthermore, the in-situ mass may be determined by efficiently making use of the existing elevator pin 50 components, instead of by adding the cost and complexity of new components solely for the in-situ mass determination.

[0052] Alternatively, or additionally, the stage WT may comprise the actuator. In this arrangement, the control system is configured to determine the in-situ mass of the substrate based on load on the stage WT. For example, in the arrangement of Figure 5 the stage comprises a short-stroke stage 10a and long-stroke stage 10b. The short-stroke stage 10a is disposed on the long stroke stage 10b. In the arrangement shown in Figure 5, the short-stroke stage 10a comprises the actuator. The control system is configured to determine the in-situ mass of the substrate W based on the load on the short-stroke stage 10a (in a Z-direction of Figure 5) due to the substrate W being supported on the short-stroke stage 10a.

[0053] The long-stroke stage 10b may be configured to make larger movements (in the X-Y plane of Figure 5) compared to the short-stroke stage 10a which may be configured to make smaller movements (in the X-Y plane of Figure 5) than the long-stoke stage 10b. The control system is configured to controlboth the short-stroke stage 10a and the long-stroke stage 10b in order to achieve the desired movement of the substrate W relative to the rest of the apparatus, for example lithographic or assessment apparatus.

[0054] With the arrangement wherein the stage WT comprises the actuator, the in-situ mass of the substrate W may be determined during a normal, pre-existing and necessary step of the substrate loading / unloading process, without adding time to the overall processing, therefore without reducing throughput. Furthermore, the in-situ mass may be determined by efficiently making use of the existing stage WT components, instead of by adding the cost and complexity of new components solely for the in-situ mass determination.

[0055] The mass determination unit may alternatively, or additionally, comprise a loading arm (not shown in the figures) configured to deposit the substrate W on the stage WT and / or to remove the substrate W from the stage WT. The loading arm may be configured to measure an in-situ mass of the substrate W during loading and / or unloading of the substrate W. The loading arm may be provided instead of, or in addition to, any of the mass determination units described above, for example with respect to Figures 4 to 7.

[0056] Figure 6 depicts an arrangement wherein the mass determination unit comprises an edge sensor 60 configured to measure a thickness of the substrate W. The edge sensor 60 may, for example, be configured to emit and detect optical radiation (e.g., light). The edge sensor 60 may be configured to directly determine the thickness of the substrate W based on a length H of the peripheral surface (i.e.., edge surface) of the substrate W. Alternatively, the edge sensor 60 may be configured to determine a height, or vertical position, of an upper surface of the substrate W in a peripheral region of the substrate W. The position and dimensions of the stage WT may be known, such that a position of the distal ends 22 of the burls 20 are known. The thickness of the substrate W may then be determined by comparison of the vertical position of the upper surface of the substrate W and the known position of the stage WT. In the arrangement of Figure 6, the control system is configured to determine the in-situ mass of the substrate W based on the measured thickness of the substrate W and a predefined density of the substrate W. The predefined density of the substrate W may be entered by a user. Alternatively, the predefined density of a plurality of materials may be stored. The user may input the material and the corresponding predefined density for that material may be retrieved for use by the control system. For example, the control system may be configured to store the predefined densities or to retrieve the predefined densities stored on a storage unit. In the arrangement of Figure 6, the edge sensor 60 is provided instead of the actuator being associated with the stage WT or pins 50 of Figures 4 and 5. In another arrangement, the edge sensor 60 could be provided in addition to an actuator being associated with the stage WT and / or pins 50, as shown in Figures 4 and 5.

[0057] Alternatively, or additionally, the mass determination unit may comprise a height sensor configured to determine a height of an upper surface of the substrate W. In particular, the height sensor may be used to determine a height of the upper surface of the substrate W. The position of the stage WT may be know, such that a position of the distal ends 22 of the burls 20 are known. In this way, thecontrol system is configured to determine the height of the upper surface of the substrate W compared to a position of the distal ends 22 of the burls 20. The lower surface of the substrate W rests on the distal ends 22 of the burls 20 during use. The control system may therefore estimate the thickness of the substrate W based on the determined height of the upper surface of the substrate W and the position of the stage WT. The control system is configured to determine the in-situ mass of the substrate based on the estimated thickness of the substrate W and a predefined density of the substrate W.

[0058] In the arrangement of Figure 7, the height sensor is disposed on an opposite side of the substrate W than the stage WT. In other words, the stage WT supports the lower surface of the substrate W and the height sensor is provided above an upper surface of the substrate W. In particular, the height sensor may form part of a projection system PS. The height sensor may be an optical sensor.

[0059] Although specific reference may be made in this text to the use of a system in the context of the manufacture of ICs, it should be understood that the system described herein may have other applications, such as in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and / or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains one or multiple processed layers.

[0060] 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 may be used in other applications.

[0061] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0062] 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.

[0063] Embodiments include the following numbered clauses:1. A system, comprising: a stage for supporting a substrate; and a control system configured to determine an in-situ mass of the substrate, and control a position of the stage depending on the determined in-situ mass of the substrate.2. The system according to clause 1, wherein the control system is configured to determine the in-situ mass of the substrate based on information input by an user.3. The system of either of clauses 1 and 2, wherein the control system comprises a mass determination unit configured to measure the in-situ mass of the substrate.4. The system of clause 3, wherein the mass determination unit comprises a strain gauge.5. The system of any preceding clause, wherein the control system comprises a first controller configured to determine the in-situ mass of the substrate.6. The system of any preceding clause, wherein the control system comprises a second controller configured to control the position of the stage.7. The system of clause 6, wherein the second controller is a mass feedforward controller of the stage.8. The system of any preceding clause, further comprising an actuator configured to apply a force to the substrate, wherein the control system is configured to control the actuator.9. The system of clause 8, when dependent on clause 5, wherein the first controller is configured to control the actuator to determine the in-situ mass of the substrate.10. The system of either of clauses 8 and 9, wherein the actuator comprises a pin configured to contact the substrate as the substrate is loaded onto and / or off of the stage, and wherein the control system is configured to determine the in-situ mass of the substrate based on load on the pin.11. The system of clause 10, wherein the actuator comprises a plurality of pins.12. The system of clause 11, wherein each pin is configured to move independently.13. The system of clause 11, wherein the pins are configured to move collectively.14. The system according to either of clauses 8 and 9, wherein the stage comprises the actuator, and wherein the control system is configured to determine the in-situ mass of the substrate based on load on the stage.15. The system according to clause 14, wherein the stage comprises a long-stroke stage, and a short-stroke stage disposed on the long stroke stage, wherein the short-stroke stage comprises the actuator.16. The system according to any one of clauses 3 to 15, wherein the mass determination unit comprises an edge sensor configured to measure a thickness of the substrate, wherein the control system is configured to determine the in-situ mass of the substrate based on the measured thickness of the substrate and a predefined density of the substrate.17. The system according to any one of clauses 3 to 15, wherein the mass determination unit comprises a height sensor configured to determine a height of an upper surface of the substrate above the stage, wherein the control system is configured to estimate a thickness of the substrate based on the determined height of the upper surface of the substrate, and to determine the in-situ mass of the substrate based on the estimated thickness of the substrate and a predefined density of the substrate.18. The system according to clause 17, wherein the height sensor is disposed on an opposite side of the substrate than the stage.19. The system according to either of clauses 17 and 18, wherein the height sensor is an optical sensor.20. The system according to any preceding clause, wherein the control system is configured to adjust mass feedforward control of the stage based on a difference between the in-situ mass of the substrate and a predetermined mass of a nominal substrate.21. The system according to clause 20, wherein the system is calibrated using the nominal substrate.22. The system according to any preceding clause, wherein the control system is configured, after the in-situ mass has been determined, to control the position of the stage to move the stage to a target position, accounting for the measured in-situ mass of the substrate, assess whether the target position has been reached within a predetermined threshold range, and adjust mass feedforward control of the stage if the predetermined threshold range is exceeded.23. The system according to clause 22, wherein the predetermined threshold range includes a distance range, wherein the distance is a distance between an actual final position of the stage and the target position.24. The system according to either of clauses 22 and 23, wherein the predetermined threshold range includes a total time range, wherein the total time is the time taken between the control being initiated and the stage coming to rest at an actual final position of the stage.25. The system according to any preceding clause, wherein the control system is configured, after the in-situ mass has been determined, to control the stage to move in a first direction, while the stage is moving in the first direction, control the stage to move in a second direction at a constant speed, accounting for the measured in-situ mass of the substrate, identify a first time at which the stage starts moving in the second direction, identify a second time at which the stage starts moving at the constant speed, assess whether a difference between the first time and the second time is within a predetermined threshold settling time, and adjust mass feedforward control of the stage if the predetermined threshold settling time is exceeded.26. The system according to any preceding clause, wherein the system comprises an apparatus.27. The system of clause 26, wherein the apparatus is a lithographic apparatus for forming a pattern on the substrate.28. The system of clause 26, wherein the apparatus is an assessment apparatus for inspecting the substrate.29. A method of controlling movement of a stage, the method comprising loading a substrate onto the stage; determining an in-situ mass of the substrate; controlling a position of the stage depending on the determined in-situ mass of the substrate.30. The method of clause 29, wherein determining the in-situ mass of the substrate comprises measuring the mass of the substrate while the substrate is being loaded on to the stage.31. The method of clause 30, wherein the mass of the substrate is measured using one of: a loading arm used to deposit the substrate on the stage; a pin configured to lower the substrate onto the stage; and the stage used to support the substrate.32. The method of any one of clauses 29 to 31, using the system of any of clauses 1 to 28.33. The method of any one of clauses 29 to 31, comprising calibrating the system using a nominal substrate having a predetermined mass; adjusting control of the position of the stage based on a difference between the determined in-situ mass of the substrate and the predetermined mass.34. The method of any one of clauses 29 to 31, wherein control includes mass feedforward control of the stage.

Claims

CLAIMS1. A system, comprising: a stage for supporting a substrate; and a control system configured to determine an in-situ mass of the substrate, and control a position of the stage depending on the determined in-situ mass of the substrate.

2. The system according to claim 1, wherein the control system is configured to determine the in-situ mass of the substrate based on information input by an user.

3. The system of either of claims 1 and 2, wherein the control system comprises a mass determination unit configured to measure the in-situ mass of the substrate.

4. The system of claim 3, wherein the mass determination unit comprises a strain gauge.

5. The system of any preceding claim, wherein the control system comprises a first controller configured to determine the in-situ mass of the substrate.

6. The system of any preceding claim, wherein the control system comprises a second controller configured to control the position of the stage.

7. The system of claim 6, wherein the second controller is a mass feedforward controller of the stage.

8. The system of any preceding claim, further comprising an actuator configured to apply a force to the substrate, wherein the control system is configured to control the actuator.

9. The system of claim 8, when dependent on claim 5, wherein the first controller is configured to control the actuator to determine the in-situ mass of the substrate.

10. The system of either of claims 8 and 9, wherein the actuator comprises a pin configured to contact the substrate as the substrate is loaded onto and / or off of the stage, and wherein the control system is configured to determine the in-situ mass of the substrate based on load on the pin.

11. The system of claim 10, wherein the actuator comprises a plurality of pins.

12. The system of claim 11, wherein each pin is configured to move independently.

13. The system of claim 11, wherein the pins are configured to move collectively.

14. The system according to either of claims 8 and 9, wherein the stage comprises the actuator, and wherein the control system is configured to determine the in-situ mass of the substrate based on load on the stage.

15. The system according to claim 14, wherein the stage comprises a long-stroke stage, and a short-stroke stage disposed on the long stroke stage, wherein the short-stroke stage comprises the actuator.

16. The system according to any one of claims 3 to 15, wherein the mass determination unit comprises an edge sensor configured to measure a thickness of the substrate, wherein the control system is configured to determine the in-situ mass of the substrate based on the measured thickness of the substrate and a predefined density of the substrate.

17. The system according to any one of claims 3 to 15, wherein the mass determination unit comprises a height sensor configured to determine a height of an upper surface of the substrate above the stage, wherein the control system is configured to estimate a thickness of the substrate based on the determined height of the upper surface of the substrate, and to determine the in-situ mass of the substrate based on the estimated thickness of the substrate and a predefined density of the substrate.

18. The system according to claim 17, wherein the height sensor is disposed on an opposite side of the substrate than the stage.

19. The system according to either of claims 17 and 18, wherein the height sensor is an optical sensor.

20. The system according to any preceding claim, wherein the control system is configured to adjust mass feedforward control of the stage based on a difference between the in-situ mass of the substrate and a predetermined mass of a nominal substrate.

21. The system according to claim 20, wherein the system is calibrated using the nominal substrate.

22. The system according to any preceding claim, wherein the control system is configured, after the in-situ mass has been determined, to control the position of the stage to move the stage to a target position, accounting for the measured in-situ mass of the substrate, assess whether the target position has been reached within a predetermined threshold range, and adjust mass feedforward control of the stage if the predetermined threshold range is exceeded.

23. The system according to claim 22, wherein the predetermined threshold range includes a distance range, wherein the distance is a distance between an actual final position of the stage and the target position.

24. The system according to either of claims 22 and 23, wherein the predetermined threshold range includes a total time range, wherein the total time is the time taken between the control being initiated and the stage coming to rest at an actual final position of the stage.

25. The system according to any preceding claim, wherein the control system is configured, after the in-situ mass has been determined, to control the stage to move in a first direction, while the stage is moving in the first direction, control the stage to move in a second direction at a constant speed, accounting for the measured in-situ mass of the substrate, identify a first time at which the stage starts moving in the second direction, identify a second time at which the stage starts moving at the constant speed, assess whether a difference between the first time and the second time is within a predetermined threshold settling time, and adjust mass feedforward control of the stage if the predetermined threshold settling time is exceeded.

26. The system according to any preceding claim, wherein the system comprises an apparatus.

27. The system of claim 26, wherein the apparatus is a lithographic apparatus for forming a pattern on the substrate.

28. The system of claim 26, wherein the apparatus is an assessment apparatus for inspecting the substrate.

29. A method of controlling movement of a stage, the method comprisingloading a substrate onto the stage; determining an in-situ mass of the substrate; controlling a position of the stage depending on the determined in-situ mass of the substrate.

30. The method of claim 29, wherein determining the in-situ mass of the substrate comprises measuring the mass of the substrate while the substrate is being loaded on to the stage.

31. The method of claim 30, wherein the mass of the substrate is measured using one of: a loading arm used to deposit the substrate on the stage; a pin configured to lower the substrate onto the stage; and the stage used to support the substrate.

32. The method of any one of claims 29 to 31, using the system of any of claims 1 to 28.

33. The method of any one of claims 29 to 31, comprising calibrating the system using a nominal substrate having a predetermined mass; adjusting control of the position of the stage based on a difference between the determined in-situ mass of the substrate and the predetermined mass.

34. The method of any one of claims 29 to 31, wherein control includes mass feedforward control of the stage.

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