WO2016030090A1

WO2016030090A1 - Encoder system calibration method, object positioning system, lithographic apparatus and device device manufacturing method

Info

Publication number: WO2016030090A1
Application number: PCT/EP2015/066872
Authority: WO
Inventors: Frank Auer; Suzanne COSIJNS; Andre Bernardus Jeunink; Willem Koenen; Wouter PRIL; Boudewijn Verhaar; Engelbertus Van der Pasch; Johannes ADRIAENS; Emiel Eussen
Original assignee: Asml Netherlands B.V.
Priority date: 2014-08-29
Filing date: 2015-07-23
Publication date: 2016-03-03
Also published as: NL2015211A

Abstract

The invention relates to an encoder system calibration method, wherein the encoder system is configured to measure a position of an object relative to a reference in a first direction, said encoder system comprising: - a first encoder head cooperating with a first grating to provide a first signal; - a second encoder head cooperating with a second grating to provide a second signal; wherein the method comprises the following steps: a) positioning the object in a predetermined position; b) capturing the first signal when the object is in said predetermined position; c) capturing the second signal when the object is in said predetermined position; and d) calibrating the second grating based on the captured first signal and the respective captured second signal, and wherein steps a) to d) are performed after arranging the encoder system in the object positioning system.

Description

ENCODER SYSTEM CALIBRATION METHOD, OBJECT POSITIONING SYSTEM, LITHOGRAPHIC APPARATUS AND DEVICE DEVICE MANUFACTURING

METHOD BACKGROUND

Cross-reference to related applications

This application claims the benefit of EP applications EP14182842.6, which was filed on 2014-Aug-29, and EP 15169784.4, which was filed on 2015-May-29 and who are incorporated herein in their entirety by reference.

Field of the Invention

The present invention relates to a calibration method for an encoder system. The present invention further relates to an object positioning system, a lithographic apparatus and a method for manufacturing a device.

Description of the Related Art

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. A lithographic apparatus usually comprises one or more objects that need to be accurately positioned such as a support constructed to support the patterning device and/or a substrate table constructed to hold a substrate. The lithographic apparatus therefore may comprise an accurate measurement system including an encoder system to determine the position of the object.

Encoder systems comprise one or more encoder heads cooperating with one or more gratings to provide a signal representative of the position of an object relative to a reference. With the increasing demands with respect to accuracy, the accuracy of the encoder systems needs to increase as well. Current developments to the encoder systems has resulted in an increase of the required calibration effort.

SUMMARY

It is desirable to provide an improved calibration method for an encoder system to obtain an acceptable calibration effort.

According to an embodiment of the invention, there is provided a method for calibration of an encoder system arranged in an object positioning system, wherein the encoder system is configured to measure a position of an object relative to a reference in a first direction, said encoder system comprising:

- a first encoder head cooperating with a first grating to provide a first signal representative of a position of the first encoder head relative to the first grating in the first direction;

- a second encoder head cooperating with a second grating to provide a second signal representative of a position of the second encoder head relative to the second grating in the first direction;

wherein the method comprises the following steps:

a) positioning the object in a predetermined position relative to the reference;

b) creating a captured first signal by capturing the first signal when the object is in said predetermined position;

c) creating a captured second signal by capturing the second signal when the object is in said predetermined position; and

d) calibrating the second grating based on the captured first signal and the respective captured second signal,

and wherein steps a) to d) are performed after arranging the encoder system in the object positioning system. According to a further embodiment of the invention, there is provided an object positioning system comprising:

- an object to be positioned;

- a measurement system for determining the position of the object in one or more degrees of freedom relative to a reference;

- an actuator system for positioning the object;

- a control system configured to drive the actuator system in dependency of an output of the measurement system,

wherein the measurement system comprises an encoder system, said encoder system including a first encoder head, a second encoder head, a first grating and a second grating,

wherein the first encoder head is arranged to cooperate with the first grating to provide a first signal representative of a position of the first encoder head relative to the first grating in a first direction, wherein one of the first encoder head and the first grating is attached to the object and the other one of the first encoder head and the first grating is attached to the reference;

wherein the second encoder is arranged to cooperate with the second grating to provide a second signal representative of a position of the second encoder head relative to the second grating in the first direction, wherein one of the second encoder head and the second grating is attached to the object and the other one of the second encoder head and the second grating is attached to the reference;

wherein the control system comprises calibration data for the first grating,

and wherein the control system is configured to carry out the following steps:

a) positioning the object in a predetermined position relative to the reference in the first direction; b) creating a captured first signal by capturing the first signal when the object is in said predetermined position;

c) applying the calibration data to the captured first signal to create a calibrated first signal;

d) creating a captured second signal by capturing the second signal when the object is in said predetermined position; e) calibrating the second grating based on the calibrated first signal and the captured second signal.

According to another embodiment of the invention, there is provided a lithographic apparatus comprising an object positioning system according to the invention.

According to yet another embodiment of the invention, there is provided a device manufacturing method wherein use is made of an object positioning system according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

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 depicts a lithographic apparatus according to an embodiment of the invention;

Figure 2 depicts schematically an object positioning system according to an embodiment of the invention;

- Figure 3 depicts in more detail a configuration of the measurement system of the object positioning system of Fig. 2; and

Figure 4 depicts in more detail another configuration of the measurement system of the object positioning system of Fig. 2.

Figure 5 depicts a measurement system according to an embodiment of the invention. - Figure 6 depicts two graphs representing signals of the measurement system of Figure 5.

Figure 7 depicts a side view of the measurement system of Figure 4.

DETAILED DESCRIPTION

Figure 1 schematically depicts a lithographic apparatus LA according to one embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or EUV radiation).

a support structure (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 in accordance with certain parameters; a substrate table (e.g. a wafer table) WTa or WTb 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 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.

The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

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.

The support structure MT supports, i.e. bears the weight of, the patterning device MA. 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 MA. 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 MA is at a desired position, for example with respect to the projection system PS. Any use of the terms "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device."

The term "patterning device MA" 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 such as to create a pattern in a target portion C of the substrate W. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion C of the substrate W, 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 C, such as an integrated circuit. The patterning device MA 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.

The term "projection system PS" 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".

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).

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables) or one substrate table and one calibration stage. In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. The two substrate tables WTa and WTb in the example of Figure 1 are an illustration of this. The invention disclosed herein can be used in a stand-alone fashion, but in particular it can provide additional functions in the pre-exposure measurement stage of either single- or multi-stage apparatuses. In an embodiment, the lithographic apparatus has a measurement table instead of one of the two substrate tables WTa and WTb. Alternatively, the lithographic apparatus has the measurement table in addition to the two substrate tables WTa and WTb. The measurement table is arranged to hold measurement equipment, such as a sensor to measure a property of the radiation beam, but is not configured to hold a substrate W.

The lithographic apparatus LA may also be of a type wherein at least a portion of the substrate W may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system PS and the substrate W. An immersion liquid may also be applied to other spaces in the lithographic apparatus LA, for example, between the patterning device MA and the projection system PS. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term "immersion" as used herein does not mean that a structure, such as a substrate W, must be submerged in liquid, but rather only means that liquid is located between the projection system PS and the substrate W during exposure.

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

The illuminator IL may comprise an adjuster AD for adjusting 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. The radiation beam B is incident on the patterning device MA, which is held on the support structure MT, and is patterned by the patterning device MA. Having traversed the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WTa/WTb 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 positioner PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the patterning device 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 support structure 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 positioner PM. Similarly, movement of the substrate table WTa/WTb 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 support structure MT may be connected to a short-stroke actuator only, or may be fixed. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2. Although the substrate alignment marks PI, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions C. Similarly, in situations in which more than one die is provided on the patterning device MA, the mask alignment marks Ml, M2 may be located between the dies. The depicted apparatus could be used in at least one of the following modes:

In a first mode, the so-called step mode, the support structure MT and the substrate table

WTa/WTb are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WTa/WTb 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.

In a second mode, the so-called scan mode, the support structure MT and the substrate table WTa/WTb are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WTa/WTb relative to the support structure MT 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.

In a third mode, the support structure MT is kept essentially stationary holding a programmable patterning device MA, and the substrate table WTaAVTb is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. In this mode, generally a pulsed radiation source SO is employed and the programmable patterning device MA is updated as required after each movement of the substrate table WTaAVTb 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.

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

Lithographic apparatus LA is of a so-called dual stage type which has two substrate tables WTa and WTb and two stations - an exposure station and a measurement station- between which the substrate tables can be exchanged. While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station so that various preparatory steps may be carried out. The preparatory steps may include mapping the surface of the substrate using a level sensor LS and measuring the position of alignment markers on the substrate using an alignment sensor AS. This enables a substantial increase in the throughput of the apparatus. If the position sensor IF is not capable of measuring the position of the substrate table while it is at the measurement station as well as at the exposure station, a second position sensor may be provided to enable the positions of the substrate table to be tracked at both stations.

The lithographic apparatus LA further includes a control unit LACU which controls all the movements and measurements of the various actuators and sensors described. Control unit LACU also includes signal processing and data processing capacity to implement desired calculations relevant to the operation of the lithographic apparatus LA. In practice, control unit LACU will be realized as a system of many sub-units, each handling the real-time data acquisition, processing and control of a subsystem or component within the lithographic apparatus LA. For example, one processing subsystem may be dedicated to servo control of the substrate positioner PW. Separate units may even handle coarse and fine actuators, or different axes. Another unit might be dedicated to the readout of the position sensor IF. Overall control of the lithographic apparatus LA may be controlled by a central processing unit, communicating with these sub-systems processing units, with operators and with other apparatuses involved in the lithographic manufacturing process.

As described above, the support structure MT and the substrate table WTa/WTb are examples of objects within the lithographic apparatus LA that may need to be positioned (accurately) relative to a reference, e.g. the projection system PS. Other examples of objects that may be positionable are optical elements in the projection system PS.

In order to position objects relative to a reference within the lithographic apparatus LA, the lithographic apparatus LA comprises one or more object positioning systems according to the invention, which will be described in more detail below. Although in the remainder of this description the general term "object" will be used, it will be apparent that this term can be replaced by substrate table WT, support structure MT, optical element, projection system PS, etc., where applicable.

An object positioning system according to the invention is schematically depicted in Fig. 2 and comprises:

- an object OB, e.g. a substrate table WT or support structure MT as shown in Fig. 1, to be positioned relative to a reference RE, e.g. a projection system PS;

- a measurement system MS for measuring the position of the object OB in one or more degrees of freedom, e.g. translational directions X,Y or rotational direction Rz, relative to the reference RE;

- an actuator system AS with one or more actuators for positioning the object OB, preferably by applying a force F to the object OB; and

- a control system CS configured to drive the actuator system AS in dependency of an output

OP of the measurement system MS and a set point SP representing a desired position of the object OB.

In Fig. 2 the actuator system AS as shown is applying a force F between the object OB and the reference RE. However, it is not necessary per se that the force F is applied to the reference RE. In order to minimize disturbances as a result of the applied forces a so-called separate force frame may be provided which is uncoupled from the reference RE allowing to apply forces F to the object without disturbing the reference RE which is used by the measurement system MS to determine the position of the object relative to the reference RE. In Fig. 2 the measurement system MS is shown as measuring the position of the object OB relative to the reference RE. Although this figure may suggest that a direct measurement is performed, it is also possible that the measurement system MS is configured to measure the position of the object OB relative to another structure. The measurement system MS is considered to measure the position of the object OB in one or more degrees of freedom relative to the reference RE as long as this position can be deducted from the output OP of the measurement system MS. Examples of degrees of freedom which can be measured by the measurement system MS are a X-direction, a Y-direction perpendicular to the X-direction, and a rotational direction Rz about an axis perpendicular to both the X- and Y-direction, commonly referred to as the Z-direction. Other degrees of freedom, such as the Z-direction and/or rotational directions about the X- direction and the Y-direction are also envisaged. Hence, it is not limited to a plane.

The set point SP may be provided to the control system CS by a set point generator SPG. Both the set point generator and the control system CS may be part of a control unit LACU as also depicted in Fig. 1.

Fig. 3 depicts schematically in more detail a top view of a possible measurement system MS including relevant components of the object positioning system of Fig. 2. Components of the object positioning system not relevant to explain the measurement system MS are for clarity reasons not depicted in Fig. 3.

Shown in Fig. 3 are four gratings, namely a first grating Gl, a second grating G2, a third grating G3 and a fourth grating G4. The gratings G1-G4 have a substantially square shape with a cutout. The cutouts of each grating together form an opening O for the projection system PS, so that the radiation beam B produced by the projection system PS is able to pass unhindered between the gratings G1-G4.

Below the gratings G1-G4, the object OB is shown in phantom. In this embodiment, the object OB is a substrate table WT constructed to hold a substrate W. By accurately positioning the object OB (and thus the substrate W) with respect to the projection system PS, the projection system PS is able to project a patterned radiation beam onto a target portion C of the substrate W.

In this embodiment, the gratings G1-G4 are attached to the reference RE of Fig. 2 and thus are attached to a substantially stationary part of the lithographic apparatus LA. The object OB is provided with four encoder heads, namely a first encoder head EHl, a second encoder head EH2, a third encoder head EH3 and a fourth encoder head EH4, each encoder head cooperating with a respective grating G1-G4.

Alternatively, it is possible that the gratings G1-G4 are attached to the object OB and the encoder heads EH1-EH4 are attached to the reference RE. A combination of gratings and encoder heads on the object OB and corresponding gratings and encoder heads on the reference RE is also possible. When the gratings G1-G4 are arranged on the object OB, this may for instance be implemented by arranging ID gratings on a top side or any other side of the object OB and/or a 2D grating on the bottom side of the object OB. A top side of the object OB may face the projection system PS, whereas a bottom side of the object OB may face away from the projection system PS. When the object OB is in a position similar to the drawn position in Fig. 3, the first encoder head EHl is cooperating with the first grating Gl. The first grating Gl comprises periodic features in a first direction FD, so that the first encoder head EHl outputs a first signal representative of a position of the first encoder head EHl relative to the first grating Gl in the first direction FD. The same applies to the third encoder head EH3 and the third grating G3, so that both encoder heads output a signal representative for the position of the object OB in the first direction FD and thus also the angular orientation of the object in the Rz direction can be measured. When the object OB is in a position similar to the drawn position in Fig. 3, the second encoder head EH2 is cooperating with the second grating G2. The second grating G2 comprises periodic features in a second direction SD, so that the second encoder head EH2 outputs a second signal representative of a position of the second encoder head EH2 relative to the second grating G2 in the second direction SD. The same applies to the fourth encoder head EH4 and the fourth grating G4, so that both encoder heads output a signal representative for the position of the object OB in the second direction SD. The combination of the signals can also be used to measure the angular orientation of the object in the Rz direction.

Although the above encoder heads are described as only being able to measure the position in a single direction, i.e. the above encoder heads are described as ID encoder heads, it is possible that one or more of the encoder heads are able to measure a position relative to the respective grating in another direction, e.g a vertical direction perpendicular to the X- and Y-direction, commonly referred to as the Z-direction, but as this is not relevant for the present invention, this possibility will not be explained in more detail. Hence, the encoder heads EH1-EH4 may also be 2D or even 3D encoder heads in case a further direction can be measured.

The first and third gratings Gl, G3 preferably also have periodic features in the second direction SD, and the second and fourth gratings G2, G4 preferably also have periodic features in the first direction FD. This allows the first and third encoder heads EHl, EH3 to also cooperate with either one of the second and fourth gratings G2, G4 and allows the second and fourth encoder heads EH2, EH4 to also cooperate with either one of the first and third gratings Gl, G3. This is advantageous when the object OB is moved away from the opening O, e.g. for a substrate exchange, and an encoder head faces another grating. It is then still possible to determine the position of the encoder head and thus to determine the position of the object OB relative to the reference RE.

To determine the position of the object in three degrees of freedom in the plane of the drawing, namely X-direction, Y-direction and Rz direction, a minimum of three encoder heads is required. The shown embodiment of Fig. 3 uses four encoder heads. This is beneficial in case one of the encoder heads is positioned in the opening O and thus is not able to cooperate with any of the gratings G1-G4. By using four encoder heads EH1-EH4 in this configuration it is ensured that at any time at least three encoder heads can be used to determine the position of the object.

As the output of the encoder heads is only representative for the local position of the encoder head relative to the respective grating, a conversion is required to convert the local position of the encoder head to the position of the object OB, e.g. the position of the center of mass of the object OB. This is usually done using a transformation matrix applied to all sensor signals. However, the transformation matrix assumes that the local position of the encoder head is accurate. Due to imperfections in the grating and deformations due to e.g. residual stresses and mounting forces, the accuracy may be limited. Hence, a calibration may be performed to take into account these imperfections thereby allowing to correct the signal outputted by the encoder head before being inputted to the transformation matrix. The calibration data may be determined by accurately measuring the grating in a special external measuring procedure prior to arranging the grating in the object positioning system.

One of the most critical processes in lithography is the illumination step, which is when the object OB is in a position in which the first encoder head EH1 faces the first grating Gl, the second encoder head EH2 faces the second grating G2, the third encoder head EH3 faces the third grating G3 and the fourth encoder head EH4 faces the fourth grating G4. For accurate movement of the object OB during the illumination step, the calibration function for the first and third gratings is determined in the first direction FD and the calibration function for the second and fourth gratings is determined for the second direction SD.

Using four encoder heads EH1-EH4 and corresponding gratings G1-G4 also has the advantage that the control unit LACU can perform an in situ calibration of the encoder system according to the invention to check the calibration data obtained by the special external measuring procedure. The following example will explain the checking of the calibration data for the third grating G3.

In order to check the calibration data, the control unit LACU of the object positioning system will position the object OB in a predetermined position relative to the reference RE, such that the first encoder head EH1 faces the first grating Gl, the second encoder head EH2 faces the second grating G2, the third encoder head EH3 faces the third grating G3 and the fourth encoder head EH4 faces the fourth grating G4. The third encoder head EH3 cooperates with the third grating G3 to determine a position in the first direction FD. Hence, in order to check the calibration data of the third grating G3 in the first direction FD, the control unit LACU will capture the first signal of the first encoder head EH1 and apply the calibration data of the first grating Gl to the first signal. The control unit LACU will further capture the signal of the third encoder head EH3. Based on the captured first signal and the captured signal of the third encoder head EH3, the third grating G3 can be calibrated resulting in current calibration data that can be compared to the old calibration data. The old calibration data is the calibration data obtained for the third grating G3 by the special external measuring procedure.

By repeating this procedure for other predetermined positions, all calibration data of the third grating G3 can be checked. In this specific embodiment, the control unit LACU will use the position information obtained via the second and fourth encoder heads EH2, EH4 as well.

Preferably, the position information obtained via the second and fourth encoder head EH2, EH4 is used during positioning of the object OB in the predetermined positions for calibration to ensure that the angular orientation in the Rz direction is constant during the calibration process.

Alternatively and/or additionally, the position information may be used to accurately determine the position of the third encoder head EH3 relative to the third grating G3 without using the position information obtained via the third encoder head EH3 itself.

Using similar procedures, all calibration data of the gratings can be checked. This may be useful when for instance a grating is replaced, e.g. during maintenance, and it is desired to check the externally determined calibration data. It may also be advantageous when for instance initially one of the encoder heads is absent and after some operation period of the object positioning system has lapsed the encoder head is added to the encoder system. It is then not necessary per se to obtain calibration data using the special external measuring procedure. The calibration data can then also be obtained using the other encoder heads via a similar in situ calibration method as described above for checking the calibration data.

Fig. 3 may also be representative for a situation prior to upgrading of the encoder system as will be explained with reference to Fig. 4.

Fig. 4 depicts the measurement system MS of Fig. 3 after replacing the second encoder head EH2 of Fig. 3 by a new encoder head EH2' . The new encoder head EH2' is configured to cooperate with any of the gratings, in this drawing the second grating G2, to provide a second signal

representative of a position of the new encoder head EH2' relative to the second grating G2 in the second direction SD and to provide a further signal representative of a position of the new encoder head EH2' relative to the second grating G2 in the first direction FD. Hence, the second encoder head EH2 of Fig. 3 is replaced by an encoder head EH2' capable of measuring the position in an additional degree of freedom compared to the original second encoder head EH2 of Fig. 3.

Replacing encoder heads by new encoder heads with additional degrees of freedom provides possibilities to increase the positioning accuracy of the object positioning system as additional measured degrees of freedom of the object OB allows to determine deformations of the object during accelerating and/or caused by disturbance forces and thus allows to more accurately position the substrate W relative to the projections system PS. As replacing encoder heads is relatively simple, it is an easy and relatively cheap way of upgrading a lithographic apparatus. The gratings G1-G4 do not have to be replaced as they already comprise periodic features in both the first and second directions FD, SD. However, for the new encoder head EH2', no calibration data of the second grating G2 in the first direction FD is available as is for the second direction SD. Hence, if no measures are taken, the accuracy of the new encoder head EH2' in the first direction FD is not sufficient.

However, the invention provides a method allowing to increase the accuracy of the new encoder head EH2' in the first direction by making use of the available accurate position information provided for by the first and/or third encoder head EH1, EH3 as will be explained below. The advantage of this method is that the special external measuring procedure in which gratings are accurately measured is not required which saves time and money. To calibrate the second grating G2 in the first direction FD, the object OB is first positioned in a predetermined position relative to the reference RE in the first direction FD and the second direction SD. When the object OB is in the predetermined position, the first signal of the first encoder head EH1 is captured, i.e. the first signal from the encoder head EH1 is captured after calibration data of the first grating Gl is applied to the first signal, and the second signal from the new encoder head EH2' is captured. Based on the captured first signal and the captured second signal, the calibration data at the predetermined position of the second grating G2 can be determined.

By repeating the process for other predetermined positions the entire second grating G2 can be calibrated in the first direction FD.

Although Fig. 4 only relates to the replacement of the second encoder head EH2 by a new encoder head EH2', it will be apparent to the skilled person that the invention may also be applied to the replacement of any other of the encoder heads, so that in a preferred situation all encoder heads EH1-EH4 which were originally only ID or 2D encoder heads are replaced by respectively 2D or 3D encoder heads, thereby increasing the obtained position information that can be used to improve position accuracy of the object OB.

Applicable to all described embodiments above is that preferably during the positioning and capturing steps forces and disturbances applied to the object OB and encoder system are kept below a predetermined threshold corresponding to a desired accuracy of the calibration.

Further, calibrating may include comparing a measured position of a predetermined location based on the captured first signals with a measured position of the predetermined location based on the captured second signals and determining a correction value to be applied to the second signal such that both measured positions correspond to each other.

In an embodiment, additional signals from other encoder heads or sensors are combined with the captured first signal to determine the position of a part of the object that can be compared to the position of the part according to the captured second signal.

Once calibration of a grating has been performed according to the invention, the calibrated grating can also be used to perform the invention again with respect to another grating. With respect to the embodiment of Fig. 4 it has been described how to obtain the calibration data of the second grating G2 in the first direction FD. When subsequently the calibration data of the fourth grating G4 in the first direction FD is to be determined, one can also use the new encoder head EH2' in combination with the second grating G2 and the calibration data thereof in the first direction as an alternative to using the first and/or third encoder head EH1,EH3.

In another embodiment, it is envisaged that for instance next to the second and third grating G2, G3, a fifth and sixth grating are arranged, so that moving range of the object OB is extended compared to the situations in Fig. 3 and 4. When moving the object OB around, it is possible that the first encoder head EH1 faces the second grating G2, the new encoder head EH2' or second encoder head EH2 faces the fifth grating, the third encoder head EH3 faces the sixth grating and the fourth encoder head EH4 faces the third grating G3. By using the calibration data of the second and third gratings G2, G3, whether obtained by a special external measuring procedure or by carrying out the invention, the calibration data of the fifth and sixth gratings may be obtained in one or more directions.

In an embodiment, two or more of the mentioned first, second, third, fourth, fifth and sixth grating may be part of a single grating element and thus the mentioned gratings do not have to be distinct elements.

In an embodiment, the object OB comprises the substrate table WT. Alternatively, the object OB comprises a measurement table. The measurement table may have substantially the same dimensions as the substrate table WT. The measurement table may have more encoder heads than only encoder heads EH1-EH4. After the calibration described above is completed, the

measurement table may be replaced with the substrate table WT. A similar measurement table may be used instead of the support structure MT.

As described above the measurement system MS is arranged to determine a position of the object OB relative to a reference RE. The reference RE may be the projection system PS or a metrology frame. The measurement system MS may be provided with a plurality of sensors. The plurality of sensors may be arranged on the object OB or may be arranged on the reference RE. The plurality of sensors may be arranged substantially stationary to the reference RE, for example on a frame connected to the reference RE.

Each of the plurality of sensors is arranged to cooperate with a respective sensor target portion. In an embodiment, see Figure 5, the plurality of sensors comprises a first sensor S I, a second sensor S2 and a third sensor S3. The first sensor S I is arranged to cooperate with a first sensor target portion so as to provide a first signal representative of the position. The second sensor S2 is arranged to cooperate with a second sensor target portion so as to provide a second signal representative of the position. The third sensor S3 is arranged to cooperate with a third sensor target portion so as to provide a third signal representative of the position.

The first sensor S I, the second sensor S2 and the third sensor S3 may comprise encoder heads EH1-EH4. For example, the first sensor S 1 is encoder head EH1, the second sensor S2 is encoder head EH2 and the third sensor S3 is encoder head EH3. One or more of the sensors S 1-S3 may measure a position of the object OB in both the first direction FD and the second direction SD.

The first sensor target portion, the second sensor target portion and the third sensor target portion may comprise one or more of gratings G1-G4. The first sensor target portion, the second sensor target portion and the third sensor target portion may be portions of only one of gratings G1-G4 or may be portions of more than one of gratings G1-G4.

Alternatively, the first sensor S I, the second sensor S2 and the third sensor S3 are a different types of sensor than an encoder head, for example an interferometer. The first target sensor portion, the second sensor target portion and the third sensor target portion may comprise a reflective element to cooperate with the interferometer.

Although typically the target sensor portions are manufactured and handled with great care, it may be possible that one of the sensor target portions is damaged. If the sensor target portion is damaged at a location, the sensor cooperating with the sensor target portion at that damaged location may not generate a signal representing the position relative to the sensor target portion properly. The damage may be very small so that only a small part of the sensor target portion is affected. However, even a small damage in a location on the sensor target portion may have a large negative effect on the accuracy of the measurement system MS. This is for example the case when one of the sensors uses the damaged location during a movement of the object OB that requires high accuracy, for example during the illumination step. If a position of a target portion C on the substrate W corresponds with a damaged location on the sensor target portion, the target portion C will not be projected properly, causing a functional loss of the target portion C. The influence of the damaged location may be reduced as follows. The first sensor S I, the second sensor S2 and the third sensor S3 each provide a signal representing the position of the object OB. The position may be in the x-direction, y-direction or z-direction or may be a rotational position in Rx, Ry or Rz. For example each of first sensor S I, the second sensor S2 and the third sensor S3 provides a signal representing an x-position of the movable object OB, see Figure 5. Since three sensors S 1-S3 provide a signal for the x-position, the three sensors S 1-S3 provide redundant measurement information. As shown in Figure 5, all sensors S 1-S3 are located on the object OB at different x-positions. The locations of the sensors S 1-S3 on the object OB are taken into account when providing and processing the signals representing the x-position of the movable object OB. Ideally, the three signals of the three sensors S I -S3 are consistent with each other. The three signals are consistent when each signal represents the x-position in a correct way. However, one of the sensors S 1-S3 may cooperate with a damaged location on a sensor target portion. To explain the invention, it is assumed that the first sensor SI cooperates with the damaged location. However, instead of the first sensor S I, one of the second sensor S2 and the third sensor S3 may cooperate with the damaged location. A sensor may cooperate with a sensor target portion by irradiating the sensor target portion with a beam of radiation. A sensor may cooperate with a sensor target portion by receiving a beam of radiation reflected by the sensor target portion.

Figure 6 shows two graphs. In the left graph, the x-positions as represented by the second signal s2 and the third signal s3 are plotted against x-position represented by the first signal si. In the right graph, the x-position as represented by the first signal si and the third signal s3 are plotted against the x-position as represented by the second signal s2. For clarity reasons, there is an offset between the two lines in each graph, so each line can be seen clearly.

In the left graph, the irregularity IR in the lines indicates that at least one of the sensors S 1-S3 may face a damaged location on a sensor target portion. Since the x-positions as represented by the second signal s2 and the third signal s3 both have the same irregularity IR, the second signal s2 and the third signal s3 are consistent with each other. Therefore, the damaged location affects the first sensor si. That the first sensor si is affected is further illustrated in the right graph. In the right graph of Figure 6, the x-position as represented by the third signal s3 is plotted against the x-position as represented by the second signal s2 as a line without any irregularity IR. So the second signal s2 and the third signal s3 are consistent with each other. The x-position as represented by the first signal si has the irregularity IR and thus the first signal si is inconsistent with the second signal s2 and the third signal s3.

The measurement system MS is arranged to determine an inconsistency of the first signal s 1 compared to the second signal s2 and the third signal s3. The inconsistency may comprise a first inconsistency and a second inconsistency. The first inconsistency is between the first signal s 1 and the second signal s2. The second inconsistency is between the first signal si and the third signal s3.

The measurement system MS may be arranged to correct the inconsistency based on the second signal s2 and the third signal s3. Based on the second signal s2 and the third signal s3, the measurement system MS may determine what the first signal si at the damaged location should have been to be consistent with the second signal s2 and the third signal s3. Based on what the first signal s 1 should have been, the measurement system MS may generate correction data. The measurement system MS may correct the first signal si by using the correction data. The correction data may comprise a correction map. The correction map may comprise correction values for an area surrounding and including the damaged location. The area may be small, for example just large enough to cover the damaged location.

The measurement system MS may use additional position information of the object OB to determine the inconsistency of the first signal si compared to the second signal s2 and the third signal s3. The additional position information may be position information of the object OB in the z-direction, for example when the sensors S 1-S3 are not all at the same z-level. The additional position information may be position information of the object OB in the y-direction for example when the sensors S 1-S3 are not all at the same y-level.

The correction data may depend on an orientation of the object OB. The correction data to correct the x-position may depend on a y-position or a z-position or a tilt in any one of Rx, Ry and Rz. To take such a dependency into account, the correction data may be generated for at least two orientations of the movable object. For example, the two orientations comprise a tilt of the movable object in Ry.

The measurement system MS may have or may cooperate with a controller, such as control system CS or control unit LACU to determine the inconsistency of the first signal si compared to the second signal s2 and the third signal s3.

The sensors S 1-S3 described above do not have to measure exactly in the x-direction, nor do the sensors S 1-S3 all have to measure in the same direction. Each of the sensors S 1-S3 measures in a direction such that the position of the object OB in the x-direction can be determined. For example, as shown in Figure 4, the sensor S 1 may be encoder head EH2. Encoder head EH2 measures in the first direction FD and in the second direction SD. By combining the measurement data from measurements in the first direction FD and the second direction SD, the position of the object OB in the x-direction can be determined. Similar to the x-position, the position of the object OB may be determined in at least one of the other degrees of freedom, y, z, Rx, Ry and Rz. In an embodiment more than three sensors S 1-S3 are used to determine the inconsistency. For example, four sensors such as the four encoder heads EH1-EH4 are used to determine the inconsistency in one or more directions.

In an embodiment, an inconsistency of the first signal si is determined when the sensor S I faces a damaged location. The object OB may be moved such that one of the other sensors S2, S3 faces and cooperates with the damaged location. When a defect is present in the damaged location, the other sensor S2, S3 also provides an inconsistent signal. In an embodiment, when the object OB is in a first position, the first sensor S 1 cooperates with a first target portion and provides a first signal that is inconsistent with the second signal s2 and the third signal s3. The object OB is moved to a second position such that the second sensor S2 cooperates with the first target portion and provides a further second signal. The further second signal is inconsistent with the concurrent signals from the first sensor S I and third sensor S3. By moving the object OB from the first position to the second position, a damaged location in the first target portion may be determined. The measurement system MS described above may comprise an incremental encoder system. An incremental encoder system has a grating that has a repetitive pattern. The repetitive pattern may comprise a plurality of parallel lines that have the same line thickness and the same pitch relative to each other. The part of the pattern that falls within the length of one pitch is referred to as a period.

Since the pattern is repetitive, the incremental encoder system initially does not have information about in which period the encoder head is measuring. Since the encoder head cannot discriminate between periods, the incremental encoder system needs to be initialized.

To initialize the incremental encoder system, an initialization sensor may be used. The initialization sensor may be any type of sensor that provides absolute position information of the object OB so the incremental encoder system is able to determine in which period the encoder head is measuring. The initialization sensor does not have to be as accurate as the incremental encoder system, as long as the accuracy is sufficient to determine the position of the stage within one period.

In an example, the initialization sensor comprises a radiation source, a reflective member and a detector. The radiation source provides a measurement beam onto the reflective member connected to the object OB. The reflective member, such as a cornercube, reflects the

measurement beam back to the detector. The detector, such as a PSD (position sensitive device), is able to determine a position of the reflective member relative to the detector based on the measurement beam. Alternatively, the initialization sensor comprises an encoder, such as an absolute encoder, or an interferometer or a capacitive sensor.

However, due to for example drift of the grating relative to the initialization sensor, the incremental encoder system may be initialized in the wrong period. This event is referred to as a zeroing jump. The term "zeroing" may be equivalent to the term "initializing". When a zeroing jump has occurred, the incremental encoder system does not indicate the position of the object OB correctly.

With the following method, a zeroing jump can be detected. The inventors have discovered that a zeroing jump can be detected by making use of redundant encoder information. Figure 7 shows a side view of the embodiment of Figure 4. In this example, both encoder heads EH1 and EH2 are used to determine a y-position of the object OB. There is a distance LI between the measurement location of encoder head EH1 on the grating G and the measurement location of encoder head EH2 on the grating G. The signals si and s2 from resp. encoder head EH1 and encoder head EH2 may be consistent or may be inconsistent with each other. The grating G may comprise one or more of the first grating Gl, the second grating G2, the third grating G3 and the fourth grating G4.

When the object OB is rotated with a fixed amount in the Rx-direction, the position of the object OB will be as shown in the dashed lines. The rotation causes a change in the distance between the measurement location of encoder head EH1 on the grating G and the measurement location of encoder head EH2 on the grating G. In this example, the distance increases from distance LI to distance L2. Due to a change to distance L2, the consistency between the signal si and the signal s2 changes from a first inconsistency to a second inconsistency. The difference between the first inconsistency and the second inconsistency can be determined and is referred to as the first inconsistency difference.

After the first inconsistency difference has been determined, the encoder system may be initialized a second time, for example after the encoder system has gone off-line. Similar as above, the signals si and s2 from resp. encoder head EH1 and encoder head EH2 are used to determine the y-position of the object OB. The signals si and s2 from resp. encoder head EH1 and encoder head EH2 may have a third inconsistency. The object OB is rotated with the fixed amount in the Rx-direction and a fourth inconsistency between signals s 1 and s2 is determined. The difference between the third inconsistency and the fourth inconsistency can be determined and is referred to as the second inconsistency difference. When the first inconsistency difference is different from the second inconsistency difference, a zeroing jump may have occurred. When the first inconsistency difference is the same as the second inconsistency difference, the second initialization occurred in the same period as the first initialization. During a first initialization, the signals si and s2 can be used to determine a first difference between the distance LI and the distance L2. During a second initialization, the signals si and s2 can be used to determine a second difference between the distance LI and the distance L2. When the first difference is different from the second difference, a zeroing jump may have occurred. When the first difference is the same as the second difference, the second initialization occurred in the same period as the first initialization.

The fixed amount to rotate the object OB in the Rx-direction may be 500 μrad or any other value sufficiently large to determine the difference in consistency due to a zeroing jump. Instead of determining a zeroing jump in the y-direction, a zeroing jump in the x-direction may be determined in a similar way by rotating the object OB in the Ry-direction.

In an embodiment, there is provided a measurement system for determining a position of an object relative to a reference. The measurement system comprises a plurality of sensors arranged on one of the movable object and the reference. The plurality of sensors comprises a first sensor, a second sensor and a third sensor. The first sensor is arranged to cooperate with a first sensor target portion so as to provide a first signal representative of the position. The second sensor is arranged to cooperate with a second sensor target portion so as to provide a second signal representative of the position. The third sensor is arranged to cooperate with a third sensor target portion so as to provide a third signal representative of the position. The position measurement system is arranged to determine an inconsistency of the first signal compared to the second signal and the third signal. The inconsistency may comprise a first inconsistency and a second inconsistency. The first inconsistency is between the first signal and the second signal. The second inconsistency is between the first signal and the third signal. The measurement system may be arranged to correct the inconsistency based on the second signal and the third signal. The measurement system may be arranged to create a correction map for correcting the inconsistency.

In an embodiment each of the first sensor, the second sensor and the third sensor comprises an encoder head. Each of the first sensor target portion, the second sensor target portion and the third sensor target portion may comprise a grating.

In an embodiment, there is provided a method for calibrating a measurement system. The measurement system comprises a first sensor, a second sensor and a third sensor. The method comprises generating, using the first sensor, a first signal representative of a position of an object relative to a reference. The method comprises generating, using the second sensor, a second signal representative of the position. The method comprises generating, using the third sensor, a third signal representative of the position. The method comprises comparing the first signal, the second signal and the third signal with each other. The method comprises determining an inconsistency of the first signal compared with the second signal and the third signal.

Determining the inconsistency may comprise determining a first inconsistency between the first signal and the second signal, and determining a second inconsistency between the first signal and the third signal. The method may comprise generating correction data based on at least one of the second signal and the third signal to correct the inconsistency. The method may comprise generating the correction data for at least two orientations of the object. The at least two orientations may comprise a first tilt and a second tilt different from the first tilt. The measurement system may be arranged in a lithographic apparatus.

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.

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.

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

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

Claims

WHAT IS CLAIMED IS:

1. Method for calibration of an encoder system arranged in an object positioning system, wherein the encoder system is configured to measure a position of an object relative to a reference in a first direction, said encoder system comprising:

- a second encoder head cooperating with a second grating to provide a second signal

representative of a position of the second encoder head relative to the second grating in the first direction;

wherein the method comprises the following steps:

and wherein steps a) to d) are performed after arranging the encoder system in the object positioning system.

2. The method according to claim 1, wherein steps a) to d) are repeated to other predetermined positions of the object relative to the reference.

3. The method according to claim 1 or 2, wherein during steps a) to c) forces and disturbances applied to the object and encoder system are kept below a predetermined threshold corresponding to a desired accuracy of the calibration.

4. The method according to one of claims 1-3, wherein calibrating includes comparing a measured position of a predetermined location based on the captured first signal with a measured position of the predetermined location based on the captured second signal and determining a correction value to be applied to the second signal such that both measured positions correspond to each other.

5. The method according to one of claims 1-4, wherein the encoder system comprises a third grating to cooperate with a further encoder head to provide a further signal representative of a position of the further encoder head relative to the third grating in the first direction, and wherein the method comprises the following steps:

e) positioning the object in a further predetermined position relative to the reference;

f) capturing the second signal when the object is in said further predetermined position;

g) creating a captured further signal by capturing the further signal when the object is in said further predetermined position;

h) repeating steps e) to g) for other further predetermined positions of the object relative to the reference; and

i) calibrating the third grating based on the captured second signals and the respective further signals,

wherein the steps e) to i) are performed after the second grating has been calibrated.

6. The method according to claim 5, wherein the further encoder head is the first encoder head, wherein positioning the object during step a) is carried out such that the first encoder head cooperates with the first grating and the second encoder head cooperates with the second grating, and wherein positioning the object during step e) is carried out such that the first encoder head cooperates with the third grating and the second encoder head cooperates with the second grating.

7. The method according to one of claims 1-4, wherein the encoder system comprises a third grating, and wherein the method comprises the following steps:

e) positioning the object in a further predetermined position relative to the reference such that the first encoder head cooperates with the second grating and such that the second encoder head cooperates with the third grating so that the first signal is representative for the position of the first encoder head relative to the second grating in the first direction and so that the second signal is representative for the position of the second encoder head relative to the third grating in the first direction; f) capturing the first signal when the object is in said further predetermined position; g) capturing the second signal when the object is in said further predetermined position;

i) calibrating the third grating based on the captured first signals and the respective second signals, wherein the steps e) to i) are performed after the second grating has been calibrated.

8. The method according to one of claims 1-7, wherein the first encoder head and the first grating are first arranged in the object positioning system, wherein the second encoder head is arranged in the object positioning system after a period of time of operation of the object positioning system has lapsed, and wherein steps a) to d) are carried out after arranging the second encoder head in the object positioning system.

9. The method according to one of claims 1-8, comprising the following steps to be carried out prior to step a):

1) providing an initial encoder head cooperating with the second grating to provide an initial signal representative of a position of the initial encoder head relative to the second grating in a second direction different from the first direction;

2) calibrating the second grating in the second direction; and

3) replacing the initial encoder head by the second encoder head, so that the second encoder head is able to provide a second signal representative for the position of the second encoder head relative to the second grating in the first direction and an additional signal representative for the position of the second encoder head relative to the second grating in the second direction.

10. Object positioning system comprising:

- an object to be positioned;

- an actuator system for positioning the object;

- a control system configured to drive the actuator system in dependency of an output of the measurement system, wherein the measurement system comprises an encoder system, said encoder system including a first encoder head, a second encoder head, a first grating and a second grating,

wherein the control system comprises calibration data for the first grating,

and wherein the control system is configured to carry out the following steps:

d) creating a captured second signal by capturing the second signal when the object is in said predetermined position;

e) calibrating the second grating based on the calibrated first signal and the captured second signal.

11. A lithographic apparatus comprising an object positioning system according to claim 10.

12. A lithographic apparatus according to claim 11, further comprising:

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, wherein the object is one of the support and the substrate table.

13. Device manufacturing method wherein use is made of an object positioning system according to claim 10.