NL2006014A - Stage system calibration method, stage system, and lithographic apparatus comprising such stage system. - Google Patents

Description

STAGE SYSTEM CALIBRATION METHOD, STAGE SYSTEM, AND LITHOGRAPHIC APPARATUS

COMPRISING SUCH STAGE SYSTEM

FIELD

[0001] The present invention relates to a calibration method for calibrating an encoder position measurement system of a stage, to a stage system, and to a lithographic apparatus including such a stage system.

BACKGROUND

[0002] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning’-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0003] In stage systems, such as lithographic apparatus stage systems, a position sensor may be applied to measure a position of the stage. For example, encoder position measurement systems may be applied for such position measurement. Thereto, use may be made of a grid (also referred to as grating), such as a one or two dimensional grid, grid plate, etc., and a sensor head cooperating with the grid. Due its manufacturing process, the grating may not be ideal and may incorporate disturbances. Calibration of grid errors in the position measurement system may be performed by a so-called “fishbone” technique, wherein a pattern is projected repetitively onto the substrate at different mutual locations, and afterwards the distances between the patterns are measured to compare them with the intended distances between the patterns. A

measured difference relates to an error in the measurement system at the location of the exposure. By determining these differences over the complete working range of the position measurement system, an error compensation value can be determined for a large number of locations on the measurement system, that, when applied, corrects the measurement system errors. Commonly, however not exclusively limited thereto, use is made of overlapping or adjacent patterns, such as line patterns, which may in some setups provide fishbone like patterns. It will be understood that this calibration technique can however apply to any type of patterns.

[0004] The fishbone calibration technique is only able to calibrate errors in the grid plate having a low spatial frequency. That is errors which change relatively gradually when moving the stage and the grid plate relative to each other and having a spatial frequency in a frequency range related to the repetition pattern according to the fishbone technique or below. Inaccuracies or other effects that lead to errors at a higher spatial frequency may be difficult or impossible to detect by the above fishbone technique.

[0005] Errors at a higher spatial frequency can be obtained by moving the stage relative to the grid using a low-bandwidth controller. The low-bandwidth of the controller will cause feedback corrections to make the stage follow an irregularity in the grid to be slow. By measuring an input signal of the low-bandwidth controller, information is obtained as to the inaccuracies of the grating in a certain frequency range. This calibration however is prone to disturbances not related to the inaccuracies of the grating, such as measurement noise and/or excitations of resonances in the measurement system and/or stage system. Therefore, an accuracy of such calibration may be limited.

SUMMARY

[0006] It is desirable to improve an accuracy of the calibration of the encoder position measurement system of a stage, in particular a lithographic apparatus stage.

[0007] According to an embodiment of the invention, there is provided a calibration method to calibrate an encoder position measurement system of a stage, the encoder position measurement system including an encoder grid and a sensor head cooperating with the encoder grid, the method including: determining a spatial frequency range of interest of disturbances in the encoder grid; determining a velocity of the stage from the spatial frequency range of interest and a desired temporal frequency range; moving the stage with the determined velocity along a first path using a stage controller controlling a position of the stage, thereby moving the sensor head relative to the encoder grid; during the moving, measuring the position of the stage with respect to the encoder grid by the sensor head; determining a disturbance data map from an output signal of the sensor head, the disturbance data map being representative for the disturbances in the encoder grid in the spatial frequency range of interest; calibrating the encoder position measurement system by applying the determined disturbance data map.

[0008] According to another embodiment of the invention, there is provided a stage system including: a moveable stage; an encoder position measurement system to provide a measurement of a position of the stage, the encoder position measurement system having an encoder grid and a sensor head cooperating with the encoder grid; a control system with a controller to control the position of the stage, the control system being provided with output signals of the sensor head, and the control system being configured to perform the calibration method according to an embodiment of the invention.

[0009] According to a further embodiment of the invention, there is provided a lithographic apparatus including: an illumination system configured to condition a radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, wherein at least one of the support and the substrate table includes a stage system according to an embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments of the invention will now be described, byway of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0011] Figure 1 depicts a lithographic apparatus according to an embodiment of the invention;

[0012] Figure 2 depicts an encoder position measurement system suitable for a lithographic apparatus according to an embodiment of the invention;

[0013] Figure 3 depicts several conversions of errors in the spatial domain to the time domain according to an embodiment of the invention;

[0014] Figure 4 depicts several other conversions of errors in the spatial domain to the time domain according to an embodiment of the invention;

[0015] Figure 5 depicts a path of a stage relative to a grid plate according to an embodiment of the invention; and

[0016] Figure 6 depicts paths of a stage relative to a grid plate according to another embodiment of the invention.

DETAILED DESCRIPTION

[0017] Figure 1 schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or any other suitable radiation), a patterning device support or mask support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioning device PM configured to accurately position the patterning device in accordance with certain parameters. The apparatus also includes a substrate table (e.g. a wafer table) WT or "substrate support" constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioning device PW configured to accurately position the substrate in accordance with certain parameters. The apparatus further includes 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. including one or more dies) of the substrate W.

[0018] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, to direct, shape, or control radiation.

[0019] The patterning device support 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 patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support may be a frame or a table, for example, which may be fixed or movable as required. The patterning device support 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.”

[0020] The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section so as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0021] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0022] The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, 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”.

[0023] As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

[0024] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables or "substrate supports" (and/or two or more mask tables or "mask supports"). In such “multiple stage” machines the additional tables or supports may be used in parallel, or preparatory steps may be carried out on one or more tables or supports while one or more other tables or supports are being used for exposure.

[0025] The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the patterning device (e.g. mask) and the projection system. Immersion techniques can be used to increase the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that a liquid is located between the projection system and the substrate during exposure.

[0026] Referring to Figure 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and/or a beam expander. In other cases, the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0027] The illuminator IL may include 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 include 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.

[0028] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the patterning device support (e.g., mask table) MT, and is patterned by the patterning device. Having traversed the patterning device (e.g. 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 positioning device PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioning device PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the patterning device support (e.g. mask table) MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioning device PM. Similarly, movement of the substrate table WT or "substrate support" may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the patterning device support (e.g. mask table) MT may be connected to a short-stroke actuator only, or may be fixed. Patterning device (e.g. mask) MA and substrate W may be aligned using patterning device alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g. mask) MA, the patterning device alignment marks may be located between the dies.

[0029] The depicted apparatus could be used in at least one of the following modes: 1. In step mode, the patterning device support (e.g. mask table) MT or "mask support" and the substrate table WT or "substrate support" are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT or "substrate support" is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

2. In scan mode, the patterning device support (e.g. mask table) MT or "mask support" and the substrate table WT or "substrate support" are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT or "substrate support" relative to the patterning device support (e.g. mask table) MT or "mask support" may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.

3. In another mode, the patterning device support (e.g. mask table) MT or "mask support" is kept essentially stationary holding a programmable patterning device, and the substrate table WT or "substrate support" is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or "substrate support" or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0030] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0031] Figure 1 also depicts a control system CS with a controller CON to control the position of a stage composed of the second positioning device PW and the substrate table WT. The control system is provided with output signals of a sensor head of the position sensor IF.

[0032] Figure 2 depicts an encoder position measurement system which is suitable to be used in the lithographic apparatus of Figure 1 to measure a position of a stage system including the substrate table and second positioning device PW or including the support MT and first positioning system PM.

[0033] The measurement system includes a sensor head SH and a grid plate or grating GP, wherein the measurement system is configured to measure a position of the sensor head SH with respect to grid plate GP. The sensor head includes two sensor elements. In this example, an incremental encoder position measurement system is applied, providing a periodic sensor element output signal when moving the sensor head with respect to the grid plate. Position information may be obtained from periodicity and phase of corresponding sensor element output signals of the sensor head. In the embodiment shown, a sensor head assembly is depicted emitting two measurement beams MB1, MB2 towards the grid plate. Due to an interaction with the pattern on the grid plate (which may be one dimensional or two dimensional), beams are returned towards the sensor head at an angle, as schematically depicted in Figure 2, and detected by suitable sensor elements or sensors SE1 ,SE2 of the sensor head. Thus, the sensor head provides for two measurements, namely at A and at B on the grid plate GP. Each of the measurements provides for a sensitivity in horizontal as well as in vertical direction. A sensitivity of the left one of the sensor elements SE1 is schematically indicated by vector ea, while a sensitivity of the right one of the sensor elements SE2 is schematically indicated by eb. It will be appreciated that an angle of ea and eb with respect to horizontal may be small, smaller than indicated in Figure 2. In fact, the angles of ea and eb with respect to the horizontal plane are exaggerated somewhat for illustrative purposes. A measurement of the horizontal position can now be obtained from an addition of ea and eb. A measurement of the vertical position can be obtained from a subtraction of ea and eb, as outlined in the below expressions.

wherein posX and posZ represent a respective horizontal and vertical encoder position information, and ki and k2 are gain factors compensating for the fact that the vectors ea and eb are not exactly directed in the horizontal or vertical direction.

[0034] An accuracy of the encoder position measurement system of Figure 2 is in part determined by an accuracy and repetition accuracy of a pattern of the grating. Moving the sensor head over the grating with a certain velocity converts the measurement information in the grating from the spatial domain to the time domain. How the information is converted depends on a magnitude of the velocity. Low spatial frequencies in the grating may end up as high frequency measurement signals in the encoder position measurement system when moving the sensor head over the grating with relatively large velocity. The mentioned conversion from spatial domain to time domain is elucidated by reference to Figure 3.

[0035] Figure 3 depicts three conversions from errors in the grating in the spatial frequency domain to the temporal frequency domain. On the left hand side of Figure 3 is shown a graph in the spatial frequency domain, i.e. the horizontal axis refers to frequencies in the spatial domain, the frequencies having a unit of 1/m or a similar unit such as 1/mm or 1/pm. In the graph on the left hand side a spatial frequency range SFR is shown depicting the spatial frequency range of interest. The spatial frequency range of interest is delimited by a lowest frequency of interest LF and a highest frequency of interest HF. The lowest frequency of interest LF may be determined by the highest frequency that can be detected by other calibration methods such as the fishbone technique. The highest frequency of interest HF may be determined by the contribution of components above the highest frequency of interest to the overall measurement error. The highest frequency of interest HF may be set such that the contribution of disturbance components above the highest frequency is below a certain predetermined value, e.g. 1% of the overall measurement error. The spatial frequency range of interest may be determined by a preliminary measurement of the grating, wherein the preliminary measurement may be performed with less accuracy as only the frequency range of interest needs to be determined and not the actual disturbances within the spatial frequency range of interest.

[0036] By moving the stage with a certain velocity along a path, thereby moving the two sensor elements relative to the encoder grid, an error with a certain spatial frequency is converted into a measurement signal having a certain temporal frequency.

[0037] On the right hand side of Figure 3, three graphs are shown all in the temporal frequency domain, i.e. the horizontal axes of the graphs indicate frequency having a unit of 1/s (i.e. Hz) or a similar unit, e.g. 1/ms. In the temporal frequency domain, there exists a frequency range DTFR in which the position measurement of the encoder position measurement system is accurate, i.e. the accuracy of the measured position is within certain parameters/specifications. Usually, the accuracy outside this frequency range is poor relative to the accuracy within the frequency range. To measure disturbances in the encoder grid, it is desired to measure the disturbances/errors in this temporal frequency range denoted by DTFR and indicated by the dashed lines in the graphs on the right hand side of Figure 3.

[0038] The three graphs indicate three situations with respect to different velocities of the stage relative to the grating. In situation A, a velocity V1 of the stage is relatively low, and the spatial frequency range of interest SFR is converted to a first temporal frequency range TFR1 which is outside the desired temporal frequency range DTFR. Situation B relates to a velocity V2 of the stage that is relatively high, which converts the spatial frequency range of interest SFR to a second temporal frequency range TFR2 which is also outside the desired temporal frequency range DTFR. Situation C relates to a velocity V3 of the stage converting the spatial frequency range of interest SFR into a third temporal frequency range TFR3 which falls within the desired temporal frequency range DTFR. As the accuracy of the measurement is the highest in the desired temporal frequency range DTFR, it will appreciated that moving the stage with the velocity V3 will result in a more accurate calibration of the encoder position measurement system than the other two velocities.

[0039] Once the spatial frequency range of interest and the desired temporal frequency range have been determined, an optimal velocity for the stage can be determined by choosing the velocity such that the spatial frequency range is converted into a temporal frequency range falling within the desired temporal frequency range.

[0040] Figure 4 depicts an alternative conversion of errors in the spatial frequency domain to the temporal frequency domain in accordance with an embodiment of the invention. On the left hand side a graph in the spatial frequency domain is shown with a spatial frequency range of interest SFR. On the right hand side three graphs are shown in the temporal frequency domain showing a desired temporal frequency range DTFR. In a first attempt to convert the spatial frequency of interest SFR into the desired temporal frequency range DTFR, the stage is moved with a velocity V1. The result is shown in situation A, and as can be seen from this graph, the converted temporal frequency range TFR1 does not fit into the desired temporal frequency range DTFR. In order to accurately measure the errors in the entire spatial frequency range SFR, the stage may in this case be moved twice with a different velocity, so that each time a different portion of the spatial frequency range is within the desired temporal frequency range. This is shown in situation B, in which the stage is moved with a velocity V2 lower than velocity V1, so that portion FP1 of the spatial frequency range of interest is within the desired temporal frequency range and can be measured accurately, and in situation C, in which the stage is moved with a velocity V3 higher than velocity V1, so that portion FP2 of the spatial frequency range of interest is within the desired temporal frequency range and can be measured accurately. The portions FP1 and FP2 may overlap. If required, the spatial frequency range of interest can be divided into more than two portions.

[0041] If the spatial frequency range of interest is divided into two or more portions, which are each calibrated with a different velocity, then adapted filtering may be used to merge the different calibration results together.

[0042] Figure 5 depicts a grid plate GP with a first path PA along which a stage may move with a certain velocity. In an embodiment of the invention, the stage will move from A to B while measuring the position relative to the grid plate of the stage using a sensor head cooperating with the grid. The movement of the stage is controlled by a stage controller having a bandwidth, in an embodiment, below a desired temporal frequency range, i.e. the bandwidth of the controller will cause feedback corrections to make the stage follow an irregularity in the grid with a frequency in the desired temporal frequency range to be slow. Low-bandwidth in this context means that the controller is configured such that an entire control loop controlling the position of the stage has a relatively low-bandwidth, e.g. the closed loop transfer function of controller, stage and measurement system has a relatively low-bandwidth. When the controller has a low-bandwidth, the input signal to the controller or the measured position of the stage may be used to determine the disturbances in the grid plate. Alternatively a high-bandwidth controller can be used, wherein an output of the controller can be used to determine the disturbances in the encoder grid.

[0043] It will be clear that depending on the controller type (low-bandwidth or high-bandwidth), a disturbance data map can be determined from one or more signals in the control loop, the disturbance data map being representative for the disturbances in the grid in a spatial frequency range of interest. The disturbance data map can then be used to calibrate an output of the sensor head.

[0044] In an embodiment, when a low-bandwidth controller is used, the bandwidth of the control loop is that low that a sensitivity transfer function is one or close to one in the desired temporal frequency range. At least the sensitivity transfer function is constant in the desired temporal frequency range.

[0045] In another embodiment, the stage will move from A to B and from B to A with a certain velocity, after which the measurement results of the reciprocating movements are combined to compensate for phase delays, thereby resulting in a combined measurement result that more accurately represents the errors in the grid plate. In yet another embodiment, the stage is moved multiple times along the path A and B, but as many times from A to B as from B to A, and all measurement results are combined to average the measurement results and compensate for phase delays. Combining the measurements results may be done after some processing such as filtering has been performed on the measurement results. A benefit is that the controller may be configured with less strict demands on phase deviation in the desired temporal frequency range of the sensitivity transfer function of the control loop.

[0046] In an embodiment, the velocity with which the stage is moved relative to the encoder grid is constant for at least a part, preferably a major part of the first path PA. In an embodiment, the stage is accelerated and decelerated at respectively a start of the path and an end of the path, wherein between the acceleration and deceleration phase the velocity of the stage is constant. Further, it may be preferred that during movement of the stage, the motion disturbances to the stage, sensor heads, and encoder grids is as minimal as possible as they may contaminate the position measurements.

[0047] Figure 6 depicts a grid plate 6 showing different paths of the stage relative to the grid plate. First paths PA1 are all in one direction, and second paths PA2 are all in another direction at an angle with the paths PA1, i.e. the paths of the respective set of first and second paths are parallel and adjacent to each other. In this particular embodiment, the first paths PA1 are perpendicular to the second paths PA2. A benefit of measuring the position of the stage along paths in two different mutual directions is that high frequency errors in a direction substantially perpendicular to (or having an obtuse angle with) a path may end up as low frequency signals in the measurement system. By measuring in two directions, information can also be obtained about these high frequency errors in the grid plate and a more accurate calibration can be obtained.

[0048] A benefit of performing the method according to an embodiment of the invention using multiple paths is that calibration of the measurement system can be obtained in an entire working range of the measurement system.

[0049] It is explicitly mentioned here that determining a disturbance data map from an output signal of the sensor head may include one of the following procedures when the output signal of the sensor head includes the nominal signal or setpoint signal which corresponds to the desired movement of the stage: - when the setpoint or nominal signal is available and can be compared to the output signal, taking the difference between the two will yield the disturbances; - when the setpoint or nominal signal is not available or can not readily be compared to the output signal, for instance due to intermediate coordinate transformations, an equation is fitted through the measured data using known parameters of the setpoint or nominal signal (e.g. the setpoint or nominal signal is a flat plane/line for respectively a 2D or 1D situation), and the fitted equation is compared with the output signal to yield the disturbances. Fitting is done, in an embodiment, using least-squares fitting which is the criterion that the sum of squares of differences between the fitted equation and the output signal is minimal.

[0050] It is also explicitly mentioned here, that movement of the stage and thus the sensor head relative to the encoder grid is performed, in an embodiment, parallel to a main surface of the encoder grid, but that disturbances of the encoder grid may end up in different output signals of the sensor head, possibly not related to the movement direction, but a direction at an angle or perpendicular to that movement direction, see for instance the sensitivity of the encoder type measurement system of Figure 2 which has a sensitivity in a horizontal and vertical direction relative to the encoder grid. Disturbances of the encoder grid may influence both measurement directions while the disturbances in the measurement directions can be determined/calibrated by moving parallel to a main surface of the encoder grid and not necessarily has to move in a direction substantially perpendicular to the main surface of the encoder grid to resolve the disturbances of the encoder grid in that direction.

[0051] It will be appreciated that the sensor head can be mounted to the stage in one embodiment of the invention. Alternatively, the encoder grid can be mounted to the stage and the sensor head can remain stationary in another embodiment of the invention.

[0052] Motion disturbances caused by electro-magnetic (EM) disturbances during calibration of the disturbance date map may end up in the map. The EM disturbances may be caused by the actuators which may cause position-dependent disturbances, such as cogging. If the disturbance data map calibration is preceded by a calibration to compensate these EM disturbance forces, the accuracy of disturbance data map may be improved.

[0053] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as 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 multiple processed layers.

[0054] 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, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0055] 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 ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

[0056] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0057] 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 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.

[0058] 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 clauses set out below. Other aspects of the invention are set out as in the following numbered clauses: 1. A calibration method for calibrating an encoder position measurement system of a stage, the encoder position measurement system including an encoder grid and a sensor head configured to cooperate with the encoder grid to measure a position of the stage, the method comprising: determining a spatial frequency range of interest of disturbances in the encoder grid; determining a velocity of the stage from the spatial frequency range of interest and a desired temporal frequency range; moving the stage with the determined velocity along a first path using a stage controller configured to control a position of the stage so that the sensor head and the encoder grid move relative to each other; during the moving, measuring the position of the stage; determining a disturbance data map from an output signal of the sensor head, the disturbance data map being representative of the disturbances in the encoder grid in the spatial frequency range of interest; and calibrating the encoder position measurement system based on the determined disturbance data map.

2. The calibration method of clause 1, wherein determining a velocity of the stage includes: dividing the spatial frequency range of interest in portions which each fit in the desired temporal frequency range; determining a velocity of the stage for each portion from a frequency range of the portion and the desired temporal frequency range, and wherein the moving and the measuring are performed for each determined velocity, and the disturbance data map is determined by combining the output signals of the sensor head corresponding to each determined velocity.

3. The calibration method of clause 1, wherein the moving and measuring are repeated for moving the stage in an opposite direction along the first path with the determined velocity, and wherein the output signals of the sensor head of the reciprocating movements along the first path are combined to compensate for phase delays in the output signals, the disturbance data map being determined from the combined and compensated output signals.

4. The calibration method of clause 1, wherein determining a spatial frequency range of interest, determining a velocity of the stage, moving, measuring, determining a disturbance data map and calibrating are performed for multiple first paths being parallel and adjacent to each other.

5. The calibration method of clause 4, wherein determining a spatial frequency range of interest, determining a velocity of the stage, moving, measuring, determining a disturbance data map and calibrating are performed for multiple second paths being parallel and adjacent to each other, the second paths being at an angle to the first paths.

6. The calibration method of clause 4, wherein the second paths are substantially perpendicular to the first paths.

7. The calibration method of clause 1, wherein the controller has a bandwidth below the desired temporal frequency range.

8. The calibration method of clause 1, wherein a transfer function from disturbances in the encoder grid of the stage to measured errors in the output signal of the sensor head is substantially constant in the desired temporal frequency range.

9. A stage system comprising: a moveable stage; an encoder position measurement system to provide a measurement of a position of the stage, the encoder position measurement system having an encoder grid and a sensor head configured to cooperate with the encoder grid; a controller configured to control the position of the stage, the controller being provided with output signals of the sensor head, and the controller being configured to perform the calibration method according to clause 1.

10. A lithographic apparatus comprising: a patterning device support constructed to support a patterning device, the patterning device being capable of imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate support constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, wherein at least one of the supports comprises a stage system according to clause 9.

Claims (1)

A lithography device comprising: an exposure device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.