CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 19208443.2 which was filed on 2019-Nov-12 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a tunable laser device. The invention further relates to a method to tune a laser beam of a tunable laser and an interferometer system. The invention also relates to a lithographic apparatus.

BACKGROUND

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

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

[0005] In embodiments of a lithographic apparatus, interferometers are used to determine the position of movable objects with high accuracy. A drawback of most known interferometers is that these interferometers are only able to determine relative displacements of the movable object with respect to a reference location. In order to determine an absolute position of the movable object with respect to the reference location a separate zeroing sensor is provided. This zeroing sensor is used to determine an absolute starting position of the movable object. Once this absolute starting position is known, the interferometer may determine a relative displacement of the movable object with respect to this absolute starting position in order to calculate an absolute position of the movable object.

[0006] The zeroing sensor is normally mounted at a specific location at which the absolute starting position of the movable object may be determined. The absolute position of the movable object may therefore only be determined when the movable object is within a relatively small measurement range of the zeroing sensor. The measurement range of the zeroing sensor is typically close to the zeroing sensor, for example within a few centimeters of the zeroing sensor. Each time the measurement of the movable object is started using the interferometer, the movable target has to be brought back into the relatively small measurement range of the zeroing sensor of the position measurement system. This may not only be the case when the lithographic apparatus is started, but for example also when the movable object is shortly out of view of the interferometer, for example when passing behind another movable object.

[0007] As an alternative for a separate zeroing sensor, a tunable laser device can be used to determine an absolute position of a movable object. Such tunable laser device is constructed to provide a radiation beam with a tunable light frequency. Such tunable laser device can be used to provide a measurement beam and a reference beam that are guided along a measurement path and a reference path, respectively, while changing the tunable light frequency of the tunable laser device. Simultaneously, a further measurement beam and a further reference beam from a second laser source can be guided along the measurement path and the reference path. The measurement beam, the reference beam, the further measurement beam and the further reference beam can be measured by a number of detectors in order to determine a first measurement phase value based on the measurement beam, a second measurement phase value based on the further measurement beam, a first reference phase value based on the reference beam and a second reference phase value based on the further reference beam. The absolute position of the movable object may be determined on the basis of the first measurement phase value, the second measurement phase value, the first reference phase value and the second reference phase value.

[0008] A drawback of known tunable laser devices is that the laser beam having a tuned light frequency may have a disturbed tuning trajectory. Such disturbed tuning trajectory may cause difficulties to reliably determine the phase of the signal measured by the interferometer system. In particular high frequency content of the tuning trajectory disturbances may result in an increasingly more difficult challenge for a phase tracking algorithm of the phase measurement device to track the respective phases of the interferometer signal. If the interferometer system is no longer capable of tracking the respective phases of the interferometer signal, the (absolute) position of the movable object cannot be determined with sufficient accuracy. [0009] Another drawback of known tunable laser devices is that the laser beam may have an unstable power during frequency tuning. Such power instabilities during tuning can lead to unpredictable laser currents, and an uncertain laser lifetime.

SUMMARY

[00010] It is an object of the invention to provide a tunable laser device providing a laser beam having a tuned light frequency with a smooth tuning trajectory. It is another object of the invention to provide a method to tune a laser beam having a smooth tuning trajectory. It is another object of the invention to provide a laser beam having a tuned light frequency with a stable power. It is another object of the invention to provide a method to stabilize the power of a laser beam during frequency tuning.

[00011] According to an embodiment of the invention, there is provided a tunable laser device, comprising: a laser source to provide a laser beam having a source light frequency, a tuning device to tune the laser beam having the source light frequency to provide a laser beam having a tuned light frequency, an input device to provide an input signal to the tuning device, and a feedforward device to provide a feedforward signal arranged to compensate disturbance effects caused by the tuning device.

[00012] According to an embodiment of the invention, there is provided a method to tune a laser beam of a tunable laser device, comprising: providing a laser beam having a source light frequency from a laser source, tuning, with a tuning device, the laser beam having the source light frequency to provide a laser beam having a tuned light frequency, and providing a feedforward signal arranged to compensate disturbance effects caused by the tuning device, wherein the feedforward signal is determined on the basis of a comparison of a measured tuning trajectory of the laser beam having the tuned light frequency and a desired the tuning trajectory of the laser beam having the tuned light frequency.

[00013] According to an embodiment of the invention there is provided an interferometer system to determine a position of a movable object, comprising a tunable laser device wherein the tunable laser device comprises: a laser source to provide a laser beam having a source light frequency, a tuning device to tune the laser beam having the source light frequency to provide a laser beam having a tuned light frequency, an input device to provide an input signal to the tuning device, and a feedforward device to provide a feedforward signal arranged to compensate disturbance effects caused by the tuning device.

[00014] According to an embodiment of the invention there is provided lithographic apparatus comprising: a mask support constructed to support a patterning device having a pattern, a substrate support constructed to support a substrate; a projection system arranged to project the pattern onto the substrate; and wherein one of the mask support, the substrate support and the projection system comprises a movable object, wherein the lithographic apparatus further comprises an interferometer system comprising a tunable laser device wherein the tunable laser device comprises: a laser source to provide a laser beam having a source light frequency, a tuning device to tune the laser beam having the source light frequency to provide a laser beam having a tuned light frequency, an input device to provide an input signal to the tuning device, and a feedforward device to provide a feedforward signal arranged to compensate disturbance effects caused by the tuning device.

BRIEF DESCRIPTION OF THE DRAWINGS

[00015] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a schematic overview of a lithographic apparatus;

Figure 2 depicts a detailed view of a part of the lithographic apparatus of Figure 1 ;

Figure 3 schematically depicts a position control system;

Figure 4 schematically depicts an embodiment of an interferometer system according to an embodiment of the invention;

Figure 5 shows selection of a set of data points according to a selection criterion;

Figure 6 schematically depicts a tunable laser device according to a first embodiment of the invention;

Figure 7 schematically depicts a tunable laser device according to a second embodiment of the invention; and

Figure 8 schematically depicts a tunable laser device according to a third embodiment of the invention. DETAILED DESCRIPTION

[00016] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[00017] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[00018] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[00019] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[00020] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and/or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and/or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS. [00021] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[00022] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and/or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[00023] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and/or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[00024] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system PMS, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2. Although the substrate alignment marks PI, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks PI, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[00025] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., an x-axis, a y-axis and a z-axis. Each of the three axis is orthogonal to the other two axes. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y-axis is referred to as an Ry-rotation. A rotation around about the z-axis is referred to as an Rz-rotation. The x- axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[00026] Figure 2 shows a more detailed view of a part of the lithographic apparatus LA of

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

[00027] The second positioner PW is arranged to accelerate the substrate support WT by providing a driving force between the substrate support WT and the balance mass BM. The driving force accelerates the substrate support WT in a desired direction. Due to the conservation of momentum, the driving force is also applied to the balance mass BM with equal magnitude, but at a direction opposite to the desired direction. Typically, the mass of the balance mass BM is significantly larger than the masses of the moving part of the second positioner PW and the substrate support WT.

[00028] In an embodiment, the second positioner PW is supported by the balance mass BM.

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

[00029] The position measurement system PMS may comprise any type of sensor that is suitable to determine a position of the substrate support WT. The position measurement system PMS may comprise any type of sensor that is suitable to determine a position of the mask support MT. The sensor may be an optical sensor such as an interferometer or an encoder. The position measurement system PMS may comprise a combined system of an interferometer and an encoder. The sensor may be another type of sensor, such as a magnetic sensor a capacitive sensor or an inductive sensor. The position measurement system PMS may determine the position relative to a reference, for example the metrology frame MF or the projection system PS. The position measurement system PMS may determine the position of the substrate table WT and/or the mask support MT by measuring the position or by measuring a time derivative of the position, such as velocity or acceleration. [00030] The position measurement system PMS may comprise an encoder system. An encoder system is known from for example, United States patent application US2007/0058173A1, filed on September 7, 2006, hereby incorporated by reference. The encoder system comprises an encoder head, a grating and a sensor. The encoder system may receive a primary radiation beam and a secondary radiation beam. Both the primary radiation beam as well as the secondary radiation beam originate from the same radiation beam, i.e., the original radiation beam. At least one of the primary radiation beam and the secondary radiation beam is created by diffracting the original radiation beam with the grating. If both the primary radiation beam and the secondary radiation beam are created by diffracting the original radiation beam with the grating, the primary radiation beam needs to have a different diffraction order than the secondary radiation beam. Different diffraction orders are, for example, +l ^st order, -1 ^st order, +2 ^nd order and -2 ^nd order. The encoder system optically combines the primary radiation beam and the secondary radiation beam into a combined radiation beam. A sensor in the encoder head determines a phase or phase difference of the combined radiation beam. The sensor generates a signal based on the phase or phase difference. The signal is representative of a position of the encoder head relative to the grating. One of the encoder head and the grating may be arranged on the substrate structure WT. The other of the encoder head and the grating may be arranged on the metrology frame MF or the base frame BF. For example, a plurality of encoder heads are arranged on the metrology frame MF, whereas a grating is arranged on a top surface of the substrate support WT. In another example, a grating is arranged on a bottom surface of the substrate support WT, and an encoder head is arranged below the substrate support WT.

[00031] The position measurement system PMS may comprise an interferometer system. An interferometer system is known from, for example, United States patent US6,020,964, filed on July 13, 1998, hereby incorporated by reference. The interferometer system may comprise a beam splitter, a mirror, a reference mirror and a sensor. A beam of radiation is split by the beam splitter into a reference beam and a measurement beam. The measurement beam propagates to the mirror and is reflected by the mirror back to the beam splitter. The reference beam propagates to the reference mirror and is reflected by the reference mirror back to the beam splitter. At the beam splitter, the measurement beam and the reference beam are combined into a combined radiation beam. The combined radiation beam is incident on the sensor. The sensor determines a phase or a frequency of the combined radiation beam. The sensor generates a signal based on the phase or the frequency. The signal is representative of a displacement of the mirror. In an embodiment, the mirror is connected to the substrate support WT. The reference mirror may be connected to the metrology frame MF. In an embodiment, the measurement beam and the reference beam are combined into a combined radiation beam by an additional optical component instead of the beam splitter. [00032] The first positioner PM may comprise a long-stroke module and a short-stroke module. The short-stroke module is arranged to move the mask support MT relative to the long-stroke module with a high accuracy over a small range of movement. The long-stroke module is arranged to move the short-stroke module relative to the projection system PS with a relatively low accuracy over a large range of movement. With the combination of the long-stroke module and the short-stroke module, the first positioner PM is able to move the mask support MT relative to the projection system PS with a high accuracy over a large range of movement. Similarly, the second positioner PW may comprise a long- stroke module and a short-stroke module. The short-stroke module is arranged to move the substrate support WT relative to the long-stroke module with a high accuracy over a small range of movement. The long-stroke module is arranged to move the short-stroke module relative to the projection system PS with a relatively low accuracy over a large range of movement. With the combination of the long-stroke module and the short-stroke module, the second positioner PW is able to move the substrate support WT relative to the projection system PS with a high accuracy over a large range of movement.

[00033] The first positioner PM and the second positioner PW each are provided with an actuator to move respectively the mask support MT and the substrate support WT. The actuator may be a linear actuator to provide a driving force along a single axis, for example the y-axis. Multiple linear actuators may be applied to provide driving forces along multiple axis. The actuator may be a planar actuator to provide a driving force along multiple axis. For example, the planar actuator may be arranged to move the substrate support WT in 6 degrees of freedom. The actuator may be an electro-magnetic actuator comprising at least one coil and at least one magnet. The actuator is arranged to move the at least one coil relative to the at least one magnet by applying an electrical current to the at least one coil. The actuator may be a moving-magnet type actuator, which has the at least one magnet coupled to the substrate support WT respectively to the mask support MT. The actuator may be a moving-coil type actuator which has the at least one coil coupled to the substrate support WT respectively to the mask support MT. The actuator may be a voice-coil actuator, a reluctance actuator, a Lorentz-actuator or a piezo-actuator, or any other suitable actuator.

[00034] The lithographic apparatus LA comprises a position control system PCS as schematically depicted in Figure 3. The position control system PCS comprises a setpoint generator SP, a feedforward controller FF and a feedback controller FB. The position control system PCS provides a drive signal to the actuator ACT. The actuator ACT may be the actuator of the first positioner PM or the second positioner PW. The actuator ACT drives the plant P, which may comprise the substrate support WT or the mask support MT. An output of the plant P is a position quantity such as position or velocity or acceleration. The position quantity is measured with the position measurement system PMS. The position measurement system PMS generates a signal, which is a position signal representative of the position quantity of the plant P. The setpoint generator SP generates a signal, which is a reference signal representative of a desired position quantity of the plant P. For example, the reference signal represents a desired trajectory of the substrate support WT. A difference between the reference signal and the position signal forms an input for the feedback controller FB. Based on the input, the feedback controller FB provides at least part of the drive signal for the actuator ACT. The reference signal may form an input for the feedforward controller FF. Based on the input, the feedforward controller FF provides at least part of the drive signal for the actuator ACT. The feedforward FF may make use of information about dynamical characteristics of the plant P, such as mass, stiffness, resonance modes and eigenfrequencies.

[00035] Figure 4 shows an embodiment of an interferometer system 100 according to an embodiment of the invention. The interferometer system 100 is arranged to determine an absolute position of a movable object 200, for example a movable part of a projection system PS of a lithographic apparatus. The movable object 200 may also be a mask support or a substrate support of a lithographic apparatus. The movable object 200 comprises a reflective measurement surface 201.

[00036] The interferometer system 100 is a heterodyne interferometer system comprising a fixed frequency laser source 101. The fixed frequency laser source 101 is constructed to provide a radiation beam with a fixed frequency, and is for example a stabilized HeNe laser source.

[00037] The interferometer system 100 further comprises a tunable laser device 107. The tunable laser device 107 is configured to provide a radiation beam with a tunable light frequency.

[00038] The interferometer system 100 comprises optical systems 108 associated with each of the fixed frequency laser source 101 and the tunable laser device 107. The optical systems 108 are each constructed to split the respective radiation beam into a first beam which is guided along a first optical path and a second beam which is guided along a second optical path.

[00039] In each optical system 108, a first optical frequency shift device 108a is provided in the first optical path and a second optical frequency shift device 108b is provided in the second optical path in order to create a frequency difference between a first frequency of the first beam and a second frequency of the second beam. The first optical frequency shift device 108a and the second optical frequency shift device 108b are for example acousto-optical modulator units that effectively create a frequency difference of for example 4 MHz between the first frequency of the first beam and the second frequency of the second beam. Other devices to create a frequency difference between the first beam and the second beam may also be applied. It is also possible that only in the first optical path or in the second optical path a frequency shift device is arranged to create the desired frequency difference between the first frequency of the first beam and the second frequency of the second beam. The first beam and the second beam originating from the fixed frequency laser source 101 are recombined in the optical system 108 in a recombined radiation with a fixed light frequency. Correspondingly, the first beam and the second beam originating from the tunable laser device 107 are recombined in the optical system 108 in a recombined radiation with a tunable light frequency.

[00040] The recombined radiation beam with fixed light frequency is split, for example by a non-polarizing beam splitter, into a first part and a second part. The first part is directed to interferometer optics 110. The interferometer optics 110 are arranged to split the first part into a first measurement beam and a first reference beam. The first measurement beam is guided along a measurement path 102, having a measurement path length L _x , to the reflective measurement surface 201 on the movable object 200.

After the first measurement beam is reflected by the reflective measurement surface 201, the first measurement beam is recombined with the first reference beam in the interferometer optics 110. The recombined first measurement beam and first reference beam are directed to a detector 103b which is connected to a light sensor device 103. The second part of the laser beam is directed to interferometer optics 111. The interferometer optics 111 is arranged to split the second part into a second measurement beam and a second reference beam. The second measurement beam is guided along a reference path 104, having a reference path length L _ref , to a reflective reference surface 105. After the second measurement beam is reflected by the reflective reference surface 105, the second measurement beam is recombined with the second reference beam in the interferometer optics 111. The recombined second measurement beam and second reference beam are directed to a detector 103c which is connected to the light sensor 103.

[00041] Part of the recombined radiation beam with fixed light frequency is directed to a reference detector 103 a which is connected to the light sensor device 103. This part of the recombined radiation beam with fixed light frequency has not interacted with the reflective measurement surface 201 or the reflective reference surface 105.

[00042] The reflective reference surface 105 is arranged at a fixed location that is used as reference location for measurements. The construction in which the reflective reference surface 105 is provided is therefore intended to be inherently stable, i.e. the location of the reflective reference surface 105 with respect to the interferometer system 100 is constant. The length L _ref of the reference path 104 is therefore also a constant length.

[00043] The reference detector 103a propagates the part of the laser beam onto a light diode of the light sensor device 103. The detector 103b propagates the recombined first measurement beam and first reference beam onto another light diode of the light sensor device 103. The detector 103c propagates the recombined second measurement beam and second reference beam onto yet another light diode of light sensor device 103. The measurements of the light diodes are fed via the light sensor device 103 into a processing unit 106. The processing unit 106 generates a first measurement phase value phl _x based on the input by the detector 103b. Alternatively, the processing unit 106 may generate the first measurement phase value phl _x based on the input by the detector 103b and the input by the reference detector 103a to compensate for disturbances of the laser beam between the fixed frequency laser source 101 and the interferometer optics 110. The processing unit 106 generates a first reference phase value phl _ref based on the input by the detector 103c. Alternatively, the processing unit 106 may generate the first reference phase value phl _ref based on the input by the detector 103c and the input by the reference detector 103a to compensate for disturbances of the laser beam between fixed frequency laser source 101 and the interferometer optics 111. The first measurement phase value phl _x represents a distance or displacement of the measurement object 200, i.e., the measurement path length L _x . The first reference phase value phl _ref represents the reference path length L _ref , which is a constant length.

[00044] The recombined radiation beam with tunable light frequency is split in a further first part and a further second part. Similar to the first part, the further first part is directed to interferometer optics 110. The interferometer optics 110 are arranged to split the further first part into a further first measurement beam and a further first reference beam. The further first measurement beam is guided along the measurement path 102, having the measurement path length L _x , to the reflective measurement surface 201 on the movable object 200. After the further first measurement beam is reflected by the reflective measurement surface 201, the further first measurement beam is recombined with the further first reference beam in the interferometer optics 110. The recombined further first measurement beam and further first reference beam are directed to the detector 103b which is connected to the light sensor device 103. The further second part is directed to interferometer optics 111. The interferometer optics 111 are arranged to split the further second part into a further second measurement beam and a further second reference beam. The further second measurement beam is guided along the reference path 104, having the reference path length L _ref , to the reflective reference surface 105. After the further second measurement beam is reflected by the reflective reference surface 105, the further second measurement beam is recombined with the further second reference beam in the interferometer optics 111. The recombined further second measurement beam and the further second reference beam are directed to the detector 103c which is connected to the light sensor device 103.

[00045] Part of the recombined radiation beam with tunable light frequency is directed to the reference detector 103 a which is connected to the light sensor device 103. This part of the recombined radiation beam with tunable light frequency has not interacted with the reflective measurement surface 201 or the reflective reference surface 105.

[00046] The reference detector 103a propagates the part of the recombined radiation beam with tunable light frequency onto a light diode of the light sensor device 103. The detector 103b propagates the recombined further first measurement beam and further first reference beam onto another light diode of the light sensor device 103. The detector 103c propagates the recombined further second measurement beam and further second reference beam onto yet another light diode of light sensor device 103. The measurements of the light diodes are fed via the light sensor device 103 into the processing unit 106. The processing unit 106 generates a second measurement phase value ph2 _x based on the input by the detector 103b. Alternatively, the processing unit 106 may generate the second measurement phase value ph2 _x based on the input by the detector 103b and the input by the reference detector 103a to compensate for disturbances of the second laser beam between the tunable laser device 107 and the interferometer optics 110. The processing unit 106 generates a second reference phase value ph2 _ref based on the input by the detector 103c. Alternatively, the processing unit 106 may generate the second reference phase value ph2 _ref based on the input by the detector 103c and the input by the reference detector 103a to compensate for disturbances of the second laser beam between the tunable laser device 107 and the interferometer optics 111. The second measurement phase value ph2 _x represents a distance or displacement of the measurement object 200, i.e., the measurement path length L _x . The second reference phase value ph2 _ref represents the reference path length L _ref , which is a constant length.

[00047] The processing unit 106 is arranged to distinguish the first measurement phase value phl _x , the first reference phase value phl _ref , the second measurement phase value ph2 _x , and the second reference phase value ph2 _ref . The first measurement phase value phl _x changes due to movements of the moveable object 200. The second measurement phase value ph2 _x changes due to movements of the movable object 200 and due to a change in the tunable light frequency of the second laser beam. Since the fixed frequency laser source 101 provides a laser beam with a fixed light frequency, the first measurement phase value phl _x is representative for the displacement of the movable object 200, while the second measurement phase value ph2 _x may be representative for displacements of the movable object 200, and also for frequency changes, i.e. wavelength variations, of the light of the tunable laser device 107.

[00048] During measurements with the light sensor device 103, the first measurement phase value phl _x , the second measurement phase value ph2 _x , the first reference phase value phl _ref , and the second reference phase value ph2 _ref may be measured. A combination of the four measured phase values at a single point in time are indicated, in this application, as a data point. Thus, a data point comprises, for a specific point in time, the first measurement phase value phl _x , the second measurement phase value ph2 _x , the first reference phase value phl _ref , and the second reference phase value ph2 _ref .

[00049] Assuming that the movable object 200 will remain at a stationary location during measurement, and the tunable light frequency of the tunable laser device 107 is changed over time, a length ratio L _rat between the measurement path length L _x and the reference path length L _ref can be determined by the processing unit 106 on the basis of the change of the second measurement phase value ph2 _x and the second reference phase value ph2 _ref over time, as caused by the change in the tunable light frequency of radiation beam of the tunable laser device 107, as: [00050] L _rat = L _x /L _ref = Aph2 _x /Aph2 _ref

[00051] Since the length of the reference path L _ref is constant and known, the absolute position of the movable object 200 can consequently be determined as follows: L _x = L _rat * L _ref .

[00052] Thus, the frequency change of the tunable laser device 107 provides sufficient data to calculate the absolute position of the movable object 200 if the movable object 200 remains in a stationary location. However, in practice, the movable object 200 will normally not remain sufficiently stationary to determine the absolute position in this way. The movable object 200 may for instance make a vibrating movement caused by external influences.

[00053] By selection of data points according to a selection criterion movement of the movable object 200 may be compensated when determining an absolute position of the movable object 200. By compensation of the movement of the movable object 200, the absolute position of the movable object 200 can be determined by the interferometer system 100 itself, i.e. without the need of additional zeroing sensors.

[00054] Since the radiation beam with fixed light frequency allows to determine any relative displacement of the movable object 200, it can be determined whether the movable object 200 is moved during the measurement of the first measurement phase value phl _x and the second measurement phase value ph2 _x . In an embodiment of such method data points are selected from all collected data points for which a relative position of the movable object 200, as measured with the radiation beam of the fixed frequency laser source 101, is the same. According to this method, all data points that are collected by measurements with the interferometer system 100 are compared. Data points for which the first measurement phase value phl _x are the same are selected to form a set of data points. This set of data points resembles a measurement sequence in which the movable object 200 is on the same position at each data point, so it looks as if the movable object 200 has not moved.

[00055] This selection of data points is now explained in more detail with reference to Figure

5. The upper part of Figure 5 shows the first reference phase value phl _ref and the second reference phase value ph2 _ref over time. It can be seen that the constant wavelength of the light of the fixed frequency laser source 101 results in a constant value of the first reference phase value phl _ref . The modulated wavelength change over time of the light as provided by the tunable laser device 107 results in a modulating value of the second reference phase value ph2 _ref . In the lower part of Figure 5, the first measurement phase value phl _x and the second measurement phase value ph2 _x over time are depicted. From the course of the first phase value phl _x , it can be seen that the movable object 200 makes back and forth movements in a position range, for example a vibrating movement. The second measurement phase value ph2 _x shows the additional effect of the modulating frequency of the radiation beam of the tunable laser device 107. [00056] To determine an absolute position of the movable object 200, a plurality of data points having the same first measurement phase value phl _x are selected to form a set of data points. At least 2 data points are required, but having more than 2 data points helps to determine the absolute position more accurately. In Figure 5, as an example, three data points A, B and C are indicated that each have a first measurement phase value phl _x equal to zero. These data points A, B and C can be used as a set of data points. In practice, the set of data points may comprise many more data points. Instead of zero, any other value of the first measurement phase value phl _x can also be selected as long as for each data point within the set of data points the first measurement phase value phl _x is equal. It is advantageous to select a value that is available in many data points.

[00057] For such set of data points, an absolute position of the movable object 200 can be determined as described above, i.e. the length ratio L _mt can be determined on the basis of the second measurement phase value ph2 _x and the second reference phase value ph2 _ref . When the length ratio L _mt is calculated, the absolute position of the movable object 200 can be calculated from the length ratio L _rat and the known length of the reference path 104.

[00058] Other methods to determine the absolute position of the movable object 200 on the basis of the data points may also be applied.

[00059] Figure 6 shows a first embodiment of a tunable laser device 107 that can be used in the interferometer system 100 shown in Figure 4. The tunable laser device 107 is arranged to provide a laser beam having a tuned light frequency TB. The tunable laser device 107 is an external cavity tunable laser device. The tunable laser device 107 comprises a laser source 1, for example a laser diode, to provide a laser beam having a source light frequency SB. An input current device 2 is provided to provide a input current signal to the laser source 1 to emit the laser beam having a source light frequency SB.

[00060] The tunable laser device 1 further comprises a tuning device to tune the laser beam having a source light frequency SB into the laser beam having a tuned light frequency TB. The tuning device comprises a diffraction grating 4 and a mirror element 5 supported by a piezo-actuator 6.

[00061] The laser beam having the source light frequency SB is guided through a lens 3 on the diffraction grating 4. The diffraction grating 4 reflects at least part of the laser beam having a source light frequency SB to a mirror element 5. The mirror element 5 reflects the respective part of the laser beam back to the diffraction grating 4. The mirror element 5 is supported by a piezo-actuator 6. By activation of the piezo-actuator 6, the distance between the diffraction grating 4 and the mirror element 5 can be changed. [00062] The movement of the piezo-actuator 6 is actuated by an input signal 8 provided by an input device 7. Due to this movement of the piezo-actuator 6 and the resulting changing distance between the diffraction grating 4 and the mirror element 5 the frequency of the laser beam can be modulated which results in the laser beam having a tuned light frequency TB that is provided by the tunable laser device 107.

[00063] In order to have a practical useful modulation of the laser beam, the desired modulation shape of the laser beam having a tuned light frequency TB comprises a periodic continuous shape. The input signal 8 which is fed to the piezo-actuator 6 is therefore selected as a periodic continuous signal, for example a sinus signal.

[00064] However, it has been found that the tuning device itself may cause disturbances to the shape of the tuning trajectory of the laser beam having a tuned light frequency TB. For example, when the desired tuning trajectory for the laser beam having a tuned light frequency TB is a sinus shape, a sinus signal may be used as the input signal 8 for tuning of the light frequency of the laser beam. However due to disturbances caused by the tuning device, for example due to the construction of the mirror element 5 and/or the piezo-actuator 6, the actual tuning trajectory of the laser beam having a tuned light frequency TB may also be disturbed. In particular, high frequency disturbances may occur in the tuning trajectory of the laser beam having a tuned light frequency TB. This may result in an increasingly more difficult challenge for a phase tracking algorithm of the processing device 106 to track the respective phases of the interferometer signals. If the interferometer system is no longer capable of tracking the respective phases of the interferometer signals, the (absolute) position of the movable object 200 cannot be determined with sufficient accuracy.

[00065] To improve the shape of the actual tuning trajectory, i.e. to smoothen the tuning trajectory, a feedforward device may be provided which provides a feedforward signal 9 which is added to the input signal 8. In the shown embodiment, this feedforward device is integrated in the input device 7. In other words, a single input device 7 is arranged to provide an actuation signal to the piezo-actuator 6 comprising a base component, i.e. the sinus shaped input signal, and a feedforward component, i.e. the feedforward signal to compensate for the disturbances caused by the tuning device. In practice, these components may be added to each other in an earlier stage and provided as a single actuation signal optimized for a desired tuning trajectory of the laser beam having a tuned light frequency TB.

[00066] The feedforward signal 9 may be determined by a comparison of a measured tuning trajectory of the laser beam having the tuned light frequency TB on the basis of the input signal 8 and a desired tuning trajectory of the laser beam having the tuned light frequency TB. The feedforward signal 9 is calculated with the aim to provide a tuning trajectory of the laser beam having the tuned light frequency TB which is the same or substantially the same as the desired tuning trajectory. [00067] Iterative steps may be made to further improve the shape of the tuning trajectory. Such iterative step comprises using the determined feedforward signal 9 in combination with the input signal to provide an improved laser beam having a tuned light frequency TB, measuring the resulting tuning trajectory of the laser beam having the tuned light frequency TB, and updating the feedforward signal 9 on the basis of a comparison of the measured resulting tuning trajectory of the laser beam having the tuned light frequency TB and the desired tuning trajectory of the laser beam having the tuned light frequency TB. [00068] These iterative steps may be repeated until the tuning trajectory of the laser beam having a tuned light frequency TB is sufficiently smooth. The tuning trajectory is for example sufficiently smooth when the respective phases of the interferometer signals can be correctly tracked.

[00069] Figure 7 shows an alternative embodiment of a tunable laser device 107 according to an embodiment of the invention. The tunable laser device 107 shown in Figure 7 comprises a laser source 1, for example a laser diode, and a tuning device comprising a diffraction grating 4 and a mirror element 5 supported by a piezo-actuator 6.

[00070] In this embodiment, the feedforward signal 9 to improve the tuning trajectory of the laser beam having the tuned light frequency TB is not provided in the input device 7 that provides an input signal 8 to the piezo-actuator 6, but is provided as an additional signal of the input current device 2. Thus, the current signal provided to the laser source 1 comprises a base component 10, i.e. a base current component to provide the laser beam having the source light frequency SB and a feedforward current component 9 that is used to smoothen the tuned trajectory of the laser beam having the tuned light frequency TB.

[00071] In this embodiment, the input current into the laser source 1 is effectively used to compensate the high frequency content of the disturbances of the tuning trajectory. This input current has a relatively high bandwidth and is therefore suitable to be used to compensate for the disturbances in the tuning trajectory. The input current provides a relatively low tuning range of the frequency of the laser beam, which is substantially smaller than the intended tuning range of the tuning trajectory, but this tuning range of the input current is sufficiently large to compensate for the high frequency disturbances caused by the tuning device.

[00072] In a further embodiment, it is also possible to use both the input current device 2 and the input device 7 to add a feedforward signal/component to the input current signal and the input signal, respectively, to compensate disturbance effects caused by the tuning device.

[00073] Figure 8 shows a third embodiment of a tunable laser device 1. The tunable laser device 1 is an external cavity laser, corresponding to the embodiment of Figure 6, wherein the input device 7 of the piezo-actuator 6 is used to add a feedforward component to the actuation signal of the piezo-actuator 6. In addition to this feedforward, the tunable laser device 1 comprises a feedback device 10 to provide a feedback signal 11 to compensate disturbance effects caused by the tuning device.

[00074] The feedback device 10 may be used to further compensate any disturbances caused by the tuning device that are not compensated by the feedforward signal 9. The feedforward signal 9 can only be used to compensate predictable disturbances. It is however possible that the disturbances caused by the tuning device also comprise unpredictable effects. To compensate these unpredictable errors and/or residual errors not compensated by the feedforward signal, the feedback device 10 may be used. The feedback device 10 is arranged to use the actual frequency measured by a frequency measurement device 12 and compare this actual frequency with the desired frequency of the desired tuning trajectory. The difference between the actual frequency and the desired frequency, i.e. the frequency error is used as an additional input signal to the piezo-actuator 6. The feedback device 10 may comprise a controller device 13, such as a PI or PID controller.

[00075] The frequency measurement device 12 may be the light sensor device 103 of the interferometer system 100 or a separate frequency measurement device.

[00076] In the shown embodiment, the feedback signal is fed into the input device 7 that provides an actuation signal to the piezo-actuator 6. In an alternative embodiment, the feedback signal may be fed into the input current device 2. In such an embodiment, the input current is used in a feedback loop to compensate the unpredictable and/or residual errors of the tuning trajectory.

[00077] A feedforward signal may also be used in some applications to stabilize laser power.

In a further embodiment, a tunable laser device is arranged to provide a laser beam having a tuned light frequency TB. The tunable laser device is a distributed feedback (DFB) tunable laser device. The tunable laser device comprises a laser source, for example a laser diode, to provide a laser beam having a source light frequency SB. An input current device is provided to provide a input current signal to the laser source to emit the laser beam having a source light frequency SB.

[00078] The tunable laser device further comprises a tuning device to tune the laser beam having a source light frequency SB into the laser beam having a tuned light frequency TB. The tuning device comprises a thermoelectric cooling (TEC) controller, which is controlled by an input signal provided by an input device.

[00079] The TEC controller applies current to the laser diode temperature controlling element (typically a Peltier cooling element) which changes the temperature and refractive index of the periodically structured element or diffraction grating of the active region of the DFB laser by heating or cooling and in turn changes the source light frequency SB to create a tuned light frequency TB. The power of the source light frequency SB and tuned light frequency TB is temperature dependent as an undesired effect. Thus as the TEC controller heats the laser source during frequency tuning, the laser light power may fluctuate. The fluctuations of the laser power caused by the TEC controller constitute disturbance effects. Due to such disturbances caused by the tuning device, an unstable laser power may be observed during frequency tuning. The power fluctuations may be compensated by optimising the laser current trajectory.

[00080] The power of the laser during a modulation cycle may be measured and the optimal laser current trajectory may be devised to stabilize the laser power during a periodic tuning cycle of the laser. This current trajectory may be devised by a feedback control loop based on the measured laser power, or it may be iteratively determined. To stabilize the laser power, i.e. to smoothen the fluctuations caused by the TEC controller, a feedforward device may be provided which provides a feedforward signal which is added to the input signal. The feedforward signal may be determined by a comparison of a measured current trajectory of the laser beam having the tuned light frequency TB on the basis of the input signal and a desired current trajectory of the laser beam having the tuned light frequency TB. The feedforward signal is calculated with the aim to provide a current trajectory of the laser beam having the tuned light frequency TB which is the same or substantially the same as the desired current trajectory as would be obtained with a feedback control loop with the aim to stabilize laser power).

[00081] Iterative steps may be made to further improve the shape of the current trajectory. Due to the high reproducibility of the DFB laser signal when modulating the frequency there is no need for an active feedback control loop; instead the feedback control signal for a single cycle may be recorded and then input repeatedly as a feedforward signal. Typical feedforward control requires a linear relationship between the setpoints of the TEC and laser current. In the approach disclosed herein, a laser current setpoint is fine-tuned as a freely defined repetitive trajectory that can be chosen as a function of time rather than a function of the TEC setpoint.

[00082] These iterative steps may be repeated until the current trajectory of the laser beam having a tuned light frequency TB is stabilized. The iteratively determined optimum current trajectory is for example sufficiently stabilized when it is identical or almost identical to what would have been found as a current control signal when using a closed loop feedback current signal.

[00083] An alternative method would be to implement an active feedback control using an external laser power sensor to stabilise the laser power. The method of the embodiment described above is a simpler option which advantageously requires no additional hardware once the ideal repetitive current signal has been determined in relation to the repetitive TEC setpoint.

[00084] Use of a feedforward control leads to more predictable currents, therefore there is more certainty of a laser lifetime where the power is stabilized by means of feedforward control, rather than feedback control. Another method to stabilize laser power is to use a variable attenuator. Furthermore, current power control stabilization eliminates the inefficiency of acoustic optical modulator-based power control which results from loss of light. [00085] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[00086] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[00087] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[00088] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions.

However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[00089] 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.