Lithographic apparatus, method of deforming a substrate table and device manufacturing method

A lithographic apparatus includes a projection system, a substrate table, a plurality of sensors, an actuator and a controller. The projection system is configured to project a patterned beam of radiation onto a substrate. The substrate table is configured to support the substrate and to move relative to the projection system. The plurality of sensors is configured to measure a deformation of the substrate table. The actuator is configured to deform the substrate table. The controller is configured to control the actuator to deform the substrate table based on measurements made by the sensors. The plurality of sensors is located on a first side of the substrate table opposite to a second side of the substrate table facing the projection system. The plurality of sensors is substantially stationary relative to the projection system.

FIELD

The present invention relates to a lithographic apparatus, a method of deforming a substrate table and a device manufacturing method.

BACKGROUND

It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. In an embodiment, the liquid is distilled water, although another liquid can be used. An embodiment of the present invention will be described with reference to liquid. However, another fluid may be suitable, particularly a wetting fluid, an incompressible fluid and/or a fluid with higher refractive index than air, desirably a higher refractive index than water. Fluids excluding gases are particularly desirable. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein, or a liquid with a nano-particle suspension (e.g. particles with a maximum dimension of up to 10 nm). The suspended particles may or may not have a similar or the same refractive index as the liquid in which they are suspended. Other liquids which may be suitable include a hydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueous solution.

The substrate is supported by a substrate table. The position of the substrate is controlled to a desired accuracy. Generally, the substrate table moves during use of the lithographic apparatus due to movement of other components in the apparatus. Vibrations within the apparatus can be of the order of the desired accuracy. Therefore, it is a non-trivial problem to control the substrate table to the desired accuracy.

US 2010/0157276 A1 discloses a lithographic apparatus that has, above the substrate table, a metrology frame connected to the projection system. A measurement bar is connected to the metrology frame and extends beneath the substrate table. The end of the measurement bar is positioned directly beneath the projection system and supports an encoder system. The encoder system measures the position of the substrate table at the optical axis by using an encoder grid on the bottom side of the measurement target.

SUMMARY

It is desirable, for example, to further improve the control of the substrate table of a lithographic apparatus.

According to an aspect of the invention, there is provided a lithographic apparatus, comprising a projection system, a substrate table, a plurality of sensors, at least one actuator and a controller. The projection system is for projecting a patterned beam of radiation onto a substrate. The substrate table is configured to support the substrate and to move relative to the projection system. The plurality of sensors is configured to measure a deformation of the substrate table. The at least one actuator is configured to deform the substrate table. The controller is configured to control the actuator to deform the substrate table based on measurements made by the sensors. The plurality of sensors is located on a first side of the substrate table opposite to a second side of the substrate table facing the projection system. The plurality of sensors is substantially stationary relative to the projection system.

According to an aspect of the invention, there is provided a method of deforming a substrate table of a lithographic apparatus. The method comprises measuring a deformation of the substrate table at a plurality of measurement locations located on a first side of the substrate table opposite to a second side facing a projection system, the plurality of measurement locations being substantially stationary relative to the projection system; and controlling at least one actuator based on said measurements to deform the substrate table.

According to an aspect of the invention there is provided a device manufacturing method, comprising: using a lithographic apparatus to project a patterned beam of radiation onto successive substrates supported by at least one substrate table; and using the method mentioned above to deform the substrate table during the use of the lithographic apparatus.

DETAILED DESCRIPTION

The support structure MT holds the patterning device. The support structure MT holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure MT may be a frame or a table, for example, which may be fixed or movable as required. The support structure MT may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”

The illuminator IL may comprise an adjuster AD configured to adjust the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section. Similar to the source SO, the illuminator IL may or may not be considered to form part of the lithographic apparatus. For example, the illuminator IL may be an integral part of the lithographic apparatus or may be a separate entity from the lithographic apparatus. In the latter case, the lithographic apparatus may be configured to allow the illuminator IL to be mounted thereon. Optionally, the illuminator IL is detachable and may be separately provided (for example, by the lithographic apparatus manufacturer or another supplier).

In many lithographic apparatus a fluid, in particular a liquid, is provided between the final element of the projection system to enable imaging of smaller features and/or increase the effective NA of the apparatus. Embodiments of the invention is described further below with reference to such an immersion apparatus, but may equally be embodied in a non-immersion apparatus. Arrangements for providing liquid between a final element of the projection system and the substrate can be classed into at least two general categories. These are the bath type arrangement and the so called localized immersion system. In the bath type arrangement substantially the whole of the substrate and optionally part of the substrate table is submersed in a bath of liquid. The so called localized immersion system uses a liquid supply system in which liquid is only provided to a localized area of the substrate. In the latter category, the space filled by liquid is smaller in plan than the top surface of the substrate and the area filled with liquid remains substantially stationary relative to the projection system while the substrate moves underneath that area. A further arrangement, to which an embodiment of the invention is directed, is the all wet solution in which the liquid is unconfined. In this arrangement substantially the whole top surface of the substrate and all or part of the substrate table is covered in immersion liquid. The depth of the liquid covering at least the substrate is small. The liquid may be a film, such as a thin film, of liquid on the substrate.

An arrangement which has been proposed is to provide the liquid supply system with a liquid confinement member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. Such an arrangement is illustrated inFIG. 2. The liquid confinement member is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). A seal is formed between the liquid confinement and the surface of the substrate. In an embodiment, a seal is formed between the liquid confinement structure and the surface of the substrate and may be a contactless seal such as a gas seal. Such a system is disclosed in United States patent application publication no. US 2004-0207824.

The fluid handling structure12at least partly contains liquid in the space11between a final element of the projection system PS and the substrate W. A contactless seal16to the substrate W may be formed around the image field of the projection system so that liquid is confined within the space between the substrate W surface and the final element of the projection system PS. The space is at least partly formed by the fluid handling structure12positioned below and surrounding the final element of the projection system PS. Liquid is brought into the space below the projection system and within the fluid handling structure12by liquid inlet13. The liquid may be removed by liquid outlet13.

Many other types of liquid supply system are possible. Embodiments of the present invention is not limited to any particular type of liquid supply system, nor to immersion lithography. Embodiments of the invention can be applied equally in any lithography.

A control system or controller500controls the overall operations of the lithographic apparatus and in particular performs an optimization process described further below. Control system500may be embodied as a suitably-programmed general purpose computer comprising a central processing unit, volatile and non-volatile storage means, input and output devices such as keyboards and screens, connections to networks and interfaces to the various parts of the lithographic apparatus. It will be appreciated that a one-to-one relationship between controlling computer and lithographic apparatus is not necessary. In an embodiment of the invention one computer may control multiple lithographic apparatus. In another embodiment of the invention, multiple networked computers may be used to control one lithographic apparatus. The control system500may be configured to control also associated process devices and substrate handling devices in a lithocell or cluster of which the lithographic apparatus forms a part. The control system500can also be configured to be subordinate to a supervisory control system of a lithocell or cluster and/or an overall control system of a fab.

As shown inFIG. 3, a circular substrate (e.g. wafer) W has a usable area UA slightly smaller than its total surface area, allowing for an edge region that cannot be used to form devices. A standard substrate can have a nominal diameter of 150 mm, 200 mm, 300 mm or 450 mm, for example. At the time of writing, 300 mm is standard for silicon substrates and 450 mm is proposed. Small sizes are used for other semiconductor materials, such as GaAs. Larger sizes have the benefit that each substrate has a greater usable area UA. Embodiments of the present invention are not limited to the use of a particular size or shape of substrate, but may be particularly beneficial when used in combination with larger substrates.

FIG. 4depicts a lithographic apparatus according to an embodiment of the invention. The lithographic apparatus comprises a substrate table WT, a plurality of sensors41, at least four actuators51and a controller500. The substrate table WT is configured to support the substrate W.

The plurality of sensors41are each configured to measure a deformation of the substrate table WT at a measurement location. InFIG. 4, the sensors41are positioned below the substrate table WT. However, this need not be the case. For example as depicted inFIG. 7, the sensors41may be positioned above the substrate table WT. In an embodiment the sensors41are disposed inside the substrate table WT.

At least one of the actuators51is configured to deform the substrate table WT, rather than merely changing the position of the substrate table WT. Deformation of the substrate table WT means that the shape of the substrate table WT changes, alternatively or in addition to the position or orientation of the substrate table WT being changed. In an embodiment the actuators51are configured to bend the substrate table WT.

In an embodiment, the apparatus comprises a plurality (for example three, or six) of actuators configured to change the position and/or orientation of the substrate table WT (but not deform the substrate table WT) and at least one actuator configured to deform the substrate table WT. In an embodiment, all of the actuators that are configured to change the position and/or orientation of the substrate table WT are further configured to deform the substrate table WT.

As mentioned above, the first positioner PM and the second positioner PW are configured to change the position of the substrate table WT. In an embodiment, the actuators51are further configured to change the orientation and/or position of the substrate table WT. The actuators51are further configured to move the substrate table WT throughout the X-Y plane and in the Z-direction. Here, the Z-direction means the vertical direction.

In an embodiment, the actuators51further perform the function of the second positioner PW mentioned above. In an embodiment the actuators51are in addition to the first positioner PM and the second positioner PW.

The controller500is configured to control the actuators51based on measurements made by the sensors41. The controller500is configured to control the actuators51to deform the substrate table WT such that the substrate table WT has a desired shape.

The sensors41measure the deformation of the substrate table WT so as to determine the present shape of the substrate table WT. In an embodiment the controller500is configured to determine the shape of the substrate table WT based on the measurements made by the sensors41. The controller500can then determine how the substrate table WT may be deformed in order to control the substrate table WT to have the desired shape. The controller500can then control the actuators51to deform the substrate table WT so as to arrive at the desired shape.

Hence, an embodiment of the present invention is a departure from the rigid body philosophy of a substrate table of a standard lithographic apparatus. According to the rigid body philosophy, the substrate table should be stiff in each of the X-, Y- and Z-directions. The purpose of this is to prevent the substrate table from bending. A purpose of preventing bending is to prevent the substrate table from undergoing undesirable deformations during use of the lithographic apparatus. This is because previously it has been very difficult to control the position of the substrate table, and in particular the exact position of the portion of the substrate that is being exposed, particularly when the substrate table undergoes undesirable deformations.

In particular, according to the rigid body philosophy, the substrate table is designed such that the vibration at the first eigenfrequency of the substrate table has a frequency of significantly more than 2 kHz. The purpose of this is to decrease possibility of movement (e.g. vibrations) of other components in the lithographic apparatus causing the substrate table to vibrate at such eigenfrequencies. Here, the first eigenfrequency is the lowest frequency at which vibration occurs. In an embodiment the first eigenfrequency corresponds to the first bending mode.

The first eigenfrequency can be increased by increasing the stiffness of the substrate table. This may be done by, for example, ensuring that the thickness of the substrate table, relative to the area of the substrate table, is more than or equal to a threshold thickness.

As mentioned above, larger substrates W have the benefit of having a greater usable area UA. The larger the substrate W, the larger the substrate table WT should be to support the substrate W. An embodiment of the present invention enables large substrate tables WT to be used, without requiring a corresponding increase in thickness that would be required to ensure its rigidity. This is beneficial because it avoids the mass of the substrate table WT increasing undesirably. By having a large substrate table WT with a low mass, the usable area UA of the substrate W can be large without compromising on the acceleration of the substrate table WT.

In an embodiment, the first eigenfrequency of the substrate table WT is less than or equal to about 2 kHz. Although such a low first eigenfrequency would have previously been problematic, an embodiment of the present invention allows this to be overcome by providing the actuators51that are configured to deform the substrate table WT to have a desired shape. Hence, the actuators51can compensate for any undesirable vibration of the substrate table WT. The actuators can reduce the effect of any such vibration and/or reduce the possibility of vibration occurring undesirably.

In an embodiment, the average vertical thickness of the substrate table WT is less than or equal to about 60 mm, and optionally less than or equal to about 50 mm. In an embodiment, the average vertical thickness of the substrate table WT is less than or equal to about 40 mm, optionally less than or equal to about 30 mm, and optionally about 10 mm. Hence, the substrate table WT can be relatively thin. A thin substrate table WT is more likely to be relatively flexible and is accordingly more likely to be deformed due to movement of other components in the lithographic apparatus during use of the lithographic apparatus. According to an embodiment of the invention, the actuators51can compensate for any such flexing of the substrate table WT by deforming the substrate table WT.

In an embodiment, the substrate table WT has an average diameter parallel to the plane of the substrate table WT of more than or equal to about 300 mm, and optionally more than or equal to about 400 mm. In an embodiment, the lithographic apparatus is used in combination with a substrate W that has a diameter of about 450 mm. In this case, the substrate table WT may have an average diameter in its plane of more than or equal to about 500 mm, and optionally more than or equal to about 600 mm. As the substrate table WT increases in size for a given force, the magnitude of deformation of the substrate table WT increases. Any undesirable deformation can be compensated for by the actuators51of embodiments of the present invention that deform the substrate table WT to have the desired shape.

In an embodiment, the controller500is configured to control the actuators51based on the measurements made by the plurality of sensors41so as to maintain the flatness of the substrate table WT. This is a purpose for the deformation of the substrate table WT by the actuators51of the embodiment of the invention. Accordingly, the substrate table WT can be flexible (i.e. not stiff) and the actuators can maintain a flat surface on which the substrate W is supported. This helps to position accurately the portion of the substrate W that is to be exposed in the lithographic apparatus.

In an embodiment, the actuators51are configured to deform the substrate table WT to have a desired shape such that the top surface of the substrate table is not flat, but has a target surface shape. Hence, according to an embodiment of the invention it is possible to deform intentionally the shape of the top surface of the substrate table WT. This may be done to control the substrate table WT such that the desired portion of the substrate W is irradiated by the beam of radiation at a desired level and angle. A target contour shape may be programmed into the controller500.

Hence, the controller500is configured to control the actuators51to deform the substrate table WT so as to achieve the target contour shape at the top surface of the substrate table WT. Meanwhile, the plurality of sensors41measure the deformation of the substrate table WT so as to determine the current shape of the substrate table WT. Hence, the sensors41are part of a feedback loop that enables the controller500to control the shape of the substrate table WT.

For a given shape (e.g. substantially circular, substantially rectangular etc), the substrate table WT may undergo deformation according to known eigenmodes. The eigenmodes depend on the shape of the substrate table WT. An eigenmode is a vibrational mode of the substrate table WT. Each eigenmode is associated with a particular eigenfrequency at which resonance of that eigenmode is likely to occur. The eigenmodes and associated eigenfrequencies of a substrate table WT can be calculated and determined before use of the lithographic apparatus.

Vibrational flexing of the substrate table WT can be described in terms of a superposition of eigenmodes (having different amplitudes). In an embodiment, the sensors41are configured to measure a variation in deformation of the substrate table WT over time. In an embodiment the sensors41output the measurements to the controller500. Based on this information, the controller500can control the actuators51to deform the substrate table WT so as to compensate for the vibrational modes.

Hence, in an embodiment the controller500is configured to control deformation of the substrate table WT in terms of at least one eigenmode. The controller500is configured to control the actuators51based on the characteristics of the at least one eigenmode. The characteristics may be the way in which the substrate table WT flexes according to that particular eigenmode.

In an embodiment, the actuators51are arranged in an array50. The actuators51are distributed substantially evenly across a major surface of the substrate table WT. As depicted inFIG. 4, in an embodiment the actuators51are arranged in an array50. The actuators51are distributed across the major surface of the base plate60. The distribution of the actuators51is selected so as to maximize control and/or performance. In an embodiment the actuators51are distributed substantially evenly. In an embodiment, the density of actuators51per unit area is greater directly under the final lens of the projection system PS than elsewhere. The position of the actuators51overlaps the substrate table WT. In an embodiment, the actuators51are arranged across substantially the entire bottom surface of the substrate table WT.

The purpose of distributing the actuators51across the area of the substrate table WT is to reduce any undesirable deformation of the substrate table WT that is a side effect of driving the substrate table WT with the actuators51so as to move the position or orientation of the substrate table WT. In an embodiment, the lithographic apparatus comprises at least 16 actuators51, at least 24 actuators51in an embodiment, at least 32 actuators51in another embodiment and at least 56 actuators51in an embodiment. Desirably, the larger the substrate table WT, the greater the number of actuators51. With a larger number of actuators51the force exerted by each actuator51may be decreased. The greater the density of actuators51per unit area, the smaller the spatial frequency of control of the substrate table WT. A larger number of actuators51allows compensation for a higher order eigenmode.

The greater the number of positions on the surface of the substrate table WT via which the substrate table WT is driven (so as to change its position), the less the undesirable deformation of the shape of the substrate table WT as a side effect. Accordingly, in an embodiment the lithographic apparatus comprises at least 100 actuators51. The greater the number and/or distribution of the actuators51, the lesser the maximum strain within the substrate table WT underneath the substrate W.

In an embodiment actuators51are positioned at at least two edges of the substrate table WT. The actuators51may drive the substrate table WT in the X-Y direction at the edges of the substrate table WT and/or at positions within a major surface of the substrate table WT (away from the edges).

In an embodiment each actuator51comprises a coil52. In an embodiment, the coil52is disposed within a base plate60. The base plate60is below the substrate table WT.

FIG. 5depicts a plan view of the base plate60. As can be seen inFIG. 5, the coils52corresponding to each actuator51are distributed throughout the base plate60. The distribution may or may not be uniform.

In an embodiment, the substrate table WT comprises a magnet array55. The magnet array55forms part of the actuation system for actuating the substrate table WT. The actuators51, each comprising a coil52, can deform the substrate table WT by providing a force to a portion of the magnet array55. The magnet array55is integral to the substrate table WT. In an embodiment the magnet array55comprises a Halbach array.

As depicted inFIG. 4, the plurality of sensors41may be comprised in a sensor array40. In an embodiment, the sensor array40is linked to a reference frame RF above the substrate table WT. The sensors41comprised in the sensor array40are each configured to measure the vertical height of the substrate table WI at a measurement location. This enables non-rigid body control of the substrate table WT via the use of the actuators51.

Any suitable sensor41can be used in embodiments of the invention. In an embodiment, the plurality of sensors41each comprise a capacitance sensor. However, other types of sensors could be used in addition to a capacitance sensor, or as an alternative to a capacitance sensor. For example, the sensors41could be a laser interferometer sensor, or an encoder sensor, for example. In an embodiment the sensors41comprise a focal sensor array. As mentioned above, in an embodiment the flatness of the substrate table WT is maintained by the sensors41and the actuators51. Hence, the tilt of an X-Y plane encoder can be controlled.

In an embodiment, each sensor41is configured to sense the deformation of the substrate table WT at the measurement location via a signal that travels between the measurement location and the sensor41through the coil52of one of the actuators51. In an embodiment, the sensor array40is positioned below the base plate60. In an embodiment, the sensors41are disposed at least partly within the base plate60. Hence, the sensor array41“looks” through the base plate60, where all the coils52are located.

In an embodiment, the base plate60is fixed on a pedestal or an external balance mass. In an embodiment, the pedestal or external balance mass moves only a small amount or not at all, for example less than 1 mm during use of the lithographic apparatus. This allows the sensors41to look through the base plate60during use of the lithographic apparatus.

FIG. 6depicts a cross sectional side view of one of the actuators51. The actuator comprises a coil52. In an embodiment, the actuator51comprises a cooling plate53. The cooling plate may be disposed on an inner surface of the coil52.

With the arrangement depicted inFIGS. 4-6, the plurality of sensors41are each configured to measure the deformation at a bottom surface of the substrate table WT via a signal that is transmitted below the substrate table WT. Hence, the sensors41may be configured to measure the vertical height of the bottom surface of the substrate table WT.

As depicted inFIG. 4, the substrate table WT may comprise a reflective surface45at its bottom surface. Hence, its bottom surface of the substrate table WT may be reflective so as to reflect a signal from the plurality of sensors41. In an embodiment the reflective surface45comprises a two-dimensional grating.

In the case that the sensor array40is positioned below the substrate table WT, the sensor array40may comprise a fiber-based focus sensing array. In an embodiment the fiber-based array comprises a fiber based Fabry Perot interferometer comprising a single-mode fiber mounted into a fiber probe and a sensing element. The space between fiber and reflecting surface is called cavity length. The reflected light is modulated in accordance with the cavity length. In an embodiment the fiber-based array comprises an array of transmit fibers that transmit radiation, a target and an array of receive fibers that capture radiation. The radiation exits the transmit fibers at the tip of a probe, is incident on a target, is captured by the receive fibers and transmitted to the probe. The monitored intensity is proportional to the distance between the probe tip and the target. In an embodiment, the sensor array40may perform fiber based angular measurements. In an embodiment, the sensor array40comprises an inductive sensor array and/or an acoustical sensor array.

A benefit of measuring the deformation of the substrate table WT using a plurality of sensors41positioned below the substrate table WT is that it is possible to measure the deformation of the substrate table at the optical axis of the lithographic apparatus without undesirably interfering with the projection system PS of the lithographic apparatus. It is desirable to measure the deformation of the substrate table WT at the optical axis because it is at this point that the substrate W is exposed. Hence, a measure of the vertical height of the substrate table WT at the optical axis is representative of the vertical position of the die on the substrate W to be exposed. This is helpful to control the focus of the system.

Hence, in an embodiment at least one of the plurality of sensors41measures the deformation of the substrate table WT at the optical axis of a projection system PS of the lithographic apparatus. In an embodiment at least one of the plurality of sensors measures the deformation of the substrate table WT off the optical axis of a projection system PS of the lithographic apparatus. The at least one of the plurality of sensors41measures the deformation of the substrate table WT with respect to a projection system PS and/or a reference frame RF of the lithographic apparatus.

However, the sensor array40does not have to be below the substrate table WI, or sense deformation of the substrate table WT via signals that are transmitted below the substrate table WT.FIG. 7depicts a lithographic apparatus according to an embodiment of the invention. As depicted inFIG. 7, the lithographic apparatus may comprise a reference frame RF positioned above the substrate table WT. The sensor array40may be attached to the reference frame RF. In an embodiment, each of the sensors41comprises an encoder sensor configured to detect the position of the substrate table WT via an encoder grating. Additionally or alternatively, the sensor array41may comprise at least one interferometer sensor, such as a surface interferometer.

As depicted inFIG. 7, the sensor array40may be configured to measure the deformation at the top surface of the substrate table WT. Additionally or alternatively to an interferometer sensor and an encoder sensor, the sensor array40may comprise a fiber-based focus sensing array, and/or an optical image sensor array.

In an embodiment the plurality of sensors41are each configured to measure the deformation of the substrate table WT via a signal that is transmitted within the substrate table WT. Hence, the sensors41may perform an internal measurement inside the substrate table WT.

Using an internal measurement system, the sensor array40may comprise a capacitance sensor array, a shielded inductive sensor array, interferometer sensors and/or grating sensors.

In an embodiment, the plurality of sensors are each configured to measure the deformation via a signal that corresponds to a deformation of the sensor41within the substrate table WT. The sensors41may each comprise a fiber Bragg grating and/or a piezoelectric element. As the substrate table WT undergoes deformation, the sensor41within the substrate table WT also undergoes a deformation. The deformation of the sensor (e.g. piezoelectric element) is representative of the deformation of the substrate table WT. In this way, it is possible to measure the deformation of the substrate table WT at the optical axis.

In an embodiment the lithographic apparatus has a long stroke and short stroke configuration for moving the substrate table WT. In an embodiment the actuation of the substrate table WT in the X-Y plane is provided by a long stroke stage. The short stroke stage, which is surrounded by the long stroke stage, provides vertical force to the substrate table WI so as to compensate for the force of gravity. The short stroke stage further performs fine positioning in six degrees of freedom. In an embodiment the short stroke stage comprises the substrate table WT.

In an embodiment, a substrate table WT comprises a substrate clamping mechanism. The substrate clamping mechanism may comprise an electrostatic clamp and/or a vacuum clamp. In the case of an extreme ultraviolet lithographic apparatus, an electrostatic clamp is beneficial because it can be used in an environment that is at a very low pressure.

As will be appreciated, any of the above described features can be used with any other feature and it is not only those combinations explicitly described which are covered in this application.

In an embodiment there is provided a lithographic apparatus, comprising a projection system PS, a substrate table WT, a plurality of sensors41, at least one actuator51and a controller500. The projection system PS is for projecting a patterned beam of radiation onto a substrate W. The substrate table WT is configured to support the substrate W and to move relative to the projection system PS. The plurality of sensors41is configured to measure a deformation of the substrate table WT. The at least one actuator51is configured to deform the substrate table WT. The controller500is configured to control the actuator to deform the substrate table WT based on measurements made by the sensors41. The plurality of sensors41are located on a first side of the substrate table opposite to a second side of the substrate table facing the projection system PS. The plurality of sensors41are substantially stationary relative to the projection system PS.

The first side of the substrate table may be below the substrate table. The second side may be above the top side of the substrate table holding the substrate. The sensors may be located on a base plate, a pedestal or an external balance mass. Compared to the movement of the substrate table, the movement a base plate, a pedestal or balance mass is limited, in the range of 0-10 cm, e.g. no more than 5 cm, 1 cm, 5 mm, 1 mm. The sensors are therefore substantially stationary relative to the projection system PS. Having the stationary sensors reduces the dynamical disturbance of wires to the sensors.

At least one of the sensors may be located at the optical axis of the projection system. This way the deformation of the part of the substrate table closest to target portion C can be measured. This allows an accurate amount of deformation by the actuator at the location where the pattern is being projected. This may improve the quality of the projected pattern.

The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm) and extreme ultraviolet (EUV) radiation (e.g. having a wavelength in the range of from 5 to 20 nm).

The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive and reflective optical components.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the embodiments of the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein. Further, the machine readable instruction may be embodied in two or more computer programs. The two or more computer programs may be stored on one or more different memories and/or data storage media.

The controllers described above may have any suitable configuration for receiving, processing, and sending signals. For example, each controller may include one or more processors for executing the computer programs that include machine-readable instructions for the methods described above. The controllers may also include data storage medium for storing such computer programs, and/or hardware to receive such medium.