CROSS-REFERENCE TO RELATED APPLICATION

[0001] The application claims priority of EP application 20216108.9 which was filed on 21 December, 2020 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to an interferometer head for a plane mirror interferometer that enables directionally sensitive measurements, and a method of using such an interferometer to enable directionally sensitive measurements. The invention further relates to a lithographic apparatus, a stage apparatus and a device manufacturing method.

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] Lithographic apparatus usually comprise positioning systems to position an object, wherein use is made of an interferometer system configured to measure a position of the object. In such an interferometer system one or more light beams are directed to a reflective measurement surface such as a mirror or retroreflector on an object to determine an optical path length difference of the one or more light beams compared to a reference optical path length. An interferometer as described may also be used to align light beams, which may be laser beams, from e.g. an interferometer system, with an object. A pointing offset (i.e. a tilt) of a light beam with respect to the measurement surface may be visible in the form of interference fringes. The larger the tilt, the more fringes are to be observed. The tilt of an angled measurement surface with respect to a reference light beam may also be determined. The mirror on the object in general does not have a perfectly flat mirror surface and/or does not extend perfectly in an intended direction, therefore it can be challenging to align a mirror to achieve zero fringes. Moreover, interferometer systems may comprise an inherent tilt, such that even when a zero-position of an interferometer is determined, an offset still remains.

[0006] Known interferometer systems as described above have a read-out accuracy of one fringe, giving rise to a precision of the same order. It may be desirable to calibrate an offset to improve the precision.

[0007] Interferometer systems as described above give rise to the same fringes when the measured tilt is positive or negative. That is to say, while a magnitude of the tilt can be measured, the direction cannot. Thus, tilt (whether inherent or otherwise) cannot be corrected by applying an offset without subsequent trial and error, since the direction of the necessary correction is unknown. Hence, it may be desirable to provide an interferometer head that enables a user to determine both the direction and magnitude of a measured displacement that can be used to align a beam relative to an object, and/or vice versa.

SUMMARY

[0008] Considering the above, it is an object of the invention to provide an interferometer system to assess the alignment of a laser beam, e.g. from an interferometer system, with an object. It is an object of the invention to provide such an interferometer system capable of measuring the direction and magnitude of a measured displacement.

[0009] According to an embodiment of the invention, there is provided an interferometer head configured to: receive an input radiation beam from a light source, receive a first portion of the input beam at a first partially -reflective surface and output a reflected beam from the first partially -reflective surface to a first reference surface, whereupon a first reference beam is reflected and transmitted through the first partially - reflective surface to a first output terminal, receive a second portion of the input beam at a second partially -reflective surface and output a reflected beam from the second partially-reflective surface to a second reference surface, whereupon a second reference beam is reflected and transmitted through the second partially-reflective surface to a second output terminal, transmit a third portion of the input beam towards a measurement surface and receive a reflected beam from the measurement surface, reflect a first portion of the reflected beam from the measurement surface at the second partially-reflective surface to the second output terminal, reflect a second portion of the reflected beam from the measurement surface at the first partially -reflective surface to the first output terminal, wherein the first reference beam and the reflected second portion of the reflected beam from the measurement surface are combined to a first output radiation beam and the second reference beam and the reflected first portion of the reflected beam from the measurement surface are combined to a second output radiation beam, whereby the interferometer head is further configured to implement an offset angle between the first and second output radiation beam. Advantageously, the interferometer head is capable of measuring the direction and magnitude of a measured displacement.

[00010] According to another embodiment of the invention there is provided a method of aligning a radiation beam to an object, the method comprising: directing the radiation beam to a target of the object; inserting an interferometer head according to the invention into the trajectory of the radiation beam, thereby applying the radiation beam as the input radiation beam and having the third portion of the input radiation beam directed to the target as the measurement surface; detecting the first output radiation beam and the second output radiation beam; analysing interference patterns from the detected first and second output radiation beams; determining an alignment angle of the input radiation beam and the object; adjusting an alignment of the input radiation beam and the object, based on the alignment angle. Advantageously, the alignment can be adjusted based on the alignment angle without the need for further measurements or trial and error.

[00011] According to another embodiment of the invention there is provided an interferometer head comprising:

- receiving means for receiving an input radiation beam from a light source,

- first path-providing means for providing a first reference path and a first measurement path,

- second path-providing means for providing a second reference path and a second measurement path, wherein a first portion of the input radiation beam is arranged to follow the first reference path to be directed towards a first output terminal via a first reference surface and the first path-providing means, and a second portion of the input radiation beam is arranged to follow the first measurement path to a measurement surface via the second path-providing means, back to the first path-providing means via the second path-providing means and towards the first output terminal, wherein the first path-providing means further provides a first combined optical path, said first combined optical path providing a first output radiation beam, and wherein a third portion of the input radiation beam is arranged to follow the second reference path to be directed towards a second output terminal via a second reference surface and the second path-providing means, and a fourth portion of the input radiation beam is arranged to follow the second measurement path to the measurement surface and towards the second output terminal via the second path-providing means, wherein the second path-providing means further provides a second combined optical path, said second combined optical path providing a second output radiation beam,

- means to induce an offset angle between the first and second output radiation beams,

- means for delivering the first and second output radiation beams to a detector. Advantageously, the interferometer head is capable of measuring the direction and magnitude of a measured displacement.

[00012] According to another embodiment of the invention, there is provided a stage apparatus comprising: an object holder configured to hold an object; a position measurement system configured to provide a radiation beam to a measurement surface of the object holder for measuring a position of the object holder, and an interferometer head according to the invention for aligning the radiation beam to the measurement surface of the object holder.

[00013] According to another embodiment of the invention, there is provided a lithographic apparatus having a beam alignment system comprising an interferometer head according to the invention. The lithographic apparatus further comprises a mask support for holding a patterning device having a pattern, an object holder configured to hold an object, a projection system for projecting the pattern onto the object and a position measurement system configured to provide a radiation beam to a measurement surface of the object holder. Advantageously, the interferometer head can be used to determine the direction of a displacement of a beam relative to the object holder, or vice versa.

[00014] According to another embodiment of the invention, there is provided a device manufacturing method wherein use is made of a lithographic apparatus according to the invention. Advantageously, a more efficient manufacturing method is achieved due to measurement of both the direction and magnitude of a measured displacement.

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 an interferometer head as part of a beam alignment system according to an embodiment of the invention.

Figure 4 schematically depicts an interferometer head as part of a beam alignment system according to another embodiment of the invention. Figure 5 schematically depicts first and second partially-reflective surfaces of an interferometer head according to an embodiment of the invention.

Figure 6 schematically depicts first and second partially-reflective surfaces of an interferometer head according to another embodiment of the invention.

Figure 7 schematically depicts first and second partially-reflective surfaces of an interferometer head according to another embodiment of the invention.

Figure 8 schematically depicts an interferometer head according to an embodiment of the invention wherein reference surfaces are cube corners.

Figure 9 schematically depicts an interferometer head for use with polarised radiation according to an 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 a “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 mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, 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 axes is orthogonal to the other two axes. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry -rotation. A rotation around about the z-axis is referred to as an Rz-rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[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] Figure 3 illustrates an interferometer head 300 as part of a beam alignment system according to an embodiment of the invention e.g. to align beams of the position measurement system PMS. Beam delivery to the interferometer head may be via an input terminal, configured to receive an input radiation beam from a light source. The input radiation may be e.g. a laser beam and may be unpolarized or may have any other state of polarization.

[00031] The interferometer head comprises a first partially-reflective surface 302. The interferometer head 300 is configured to receive a first portion of the input radiation beam 304 at the first partially-reflective surface 302. The first partially-reflective surface 302 may be positioned such that the first portion of the incident radiation beam 304 is incident thereon at an angle of 45°.

[00032] A part of the first portion of the input radiation beam 304 incident on the first partially reflective surface 302 is reflected to emit a reflected beam 306 towards a first reference surface 308. The first reference surface 308 may be a planar mirror. The reflected beam may have orthogonal incidence on the first reference surface. Preferably, the first reference surface 308 is a corner cube. The use of a corner cube is preferred to make the interferometer head less sensitive to its orientation with respect to the input radiation beam 304. Where the first reference surface 308 is a corner cube, the first partially-reflective surface 302 may be positioned such that the first portion of the incident radiation beam 304 is incident thereon at an angle that is smaller or greater than 45°. The reflected beam 306 incident on the first reference surface 308 is reflected therefrom back towards the first partially-reflective surface 302. A portion of this beam is then transmitted through the first partially- reflective surface 302 to create a first reference beam 310, which is directed to a first output terminal (not shown).

[00033] The interferometer head 300 further comprises a second partially-reflective surface 312. A second portion of the input radiation beam 314 is received at the second partially-reflective surface 312. The second partially-reflective surface 312 may be oriented orthogonally with respect to the first partially-reflective surface 302, i.e. such that the angle of incidence of the second portion of the incident radiation 314 is 135° where the angle of incidence of the first portion of the incident radiation on the first partially reflective surface is 45°. However, it may be positioned such that other angles of incidence are achieved. For example, the second partially-reflective surface 312 may be oriented such that the angle of incidence of the second portion of the incident radiation 314 is the same as the angle of incidence of the first portion of the incident radiation 304 on the first partially-reflective surface 302 - as illustrated in the example embodiment of Figure 4. The second partially-reflective surface 312 may be rotated by any angle around the optical axis of the incident beam.

[00034] A part of the second portion of the input radiation beam 314 incident on the second partially-reflective surface 312 is reflected to emit a reflected beam 316 towards a second reference surface 318. The second reference surface 318 may be a planar mirror. Alternatively, the second reference surface 318 is a corner cube. The reflected beam 316 incident on the second reference surface 318 is reflected therefrom back towards the second partially-reflective surface 312. A portion of this beam is then transmitted through the second partially-reflective surface 312 to create a second reference beam 320, which is directed to a second output terminal (not shown).

[00035] The interferometer head 300 is further configured to transmit a third portion of the input radiation beam 322 towards a measurement surface or target 324. The measurement surface or target 324 is reflective and may be a mirror, for example a planar mirror. The third portion of the input radiation beam 322 incident on the measurement surface 324 is accordingly reflected therefrom back to the interferometer head 300. The interferometer head 300 receives the reflected beam 326 from the measurement surface at the second partially-reflective surface 312 and reflects a first portion thereof to create a first measurement beam 328, which is directed to the second output terminal. The first measurement beam 328 is accordingly combined with the second reference beam 320 to create a first output radiation beam.

[00036] A second portion of the received reflected beam 326 from the measurement surface is transmitted through the second partially-reflective surface 312 to the first partially-reflective surface 302, whereupon the incident beam is reflected to create a second measurement beam 330, which is directed to the first output terminal. The second measurement beam 330 is accordingly combined with the first reference beam 310 to create a second output radiation beam.

[00037] Importantly, the interferometer head of the present invention is configured such that an offset is implemented between the signals of the first and second output radiation beams. Preferably, the offset in one of the signals of the first or second output radiation beams is equal and opposite relative to the other of the signals of the first or second output radiation beams. The offset may be implemented via the hardware of the interferometer in a variety of ways. Preferably, the offset is implemented at a location of low beam sensitivity.

[00038] As illustrated in Figure 5, each of the first and second partially-reflective surfaces 500, 502 may be formed from two connected triangular glass prisms, such as is commonly used in known beam-splitters, thereby being part of first and second cubic bodies 504, 506. Any other appropriate form or combination of forms of prism may also be used, e.g. one or more triangular prisms or parallelepipeds, or a combination thereof. The first and second bodies may be arranged adjacent to each other in close proximity. The first and second bodies 504, 506 may not be fixedly connected. A gap may be present between the first and second bodies.

[00039] Alternatively, a surface of the first body may be fixedly connected to a surface of the second body by any appropriate means as would be known to a person skilled in the art, so as to form a single body. The person skilled in the art will readily understand that any number and appropriate form or combination of forms of prism may be used to form the first and second partially-reflective surfaces as part of a single body. For example, in Figure 6 an embodiment is illustrated whereby three triangular glass prisms 600, 602, 604 form a single body 606 with the first and second partiallyre flective surfaces 608, 610. In a further example, Figure 7 illustrates an embodiment whereby two triangular prisms 700, 702 and parallelepiped prism 704 form a single body 706 with the first and second partially -reflective surfaces 708, 710.

[00040] Alternatively, the partially-reflective surfaces may be, e.g. half-silvered mirrors, which may be supported by one or more bodies of the interferometer head.

[00041] In an embodiment the offset may be induced by means of one or more refractive wedges. Such a wedge or wedges may be introduced in the path of the input radiation beam on the first and/or second partially-reflective surfaces. Such a wedge or wedges may alternatively or additionally be positioned adjacent or apart from the first and/or second partially-reflective surfaces in the path of the input radiation beam that is directed towards the measurement surface. Such a wedge or wedges may alternatively or additionally be introduced on the first and/or second reference surfaces, whether one or both of said reference surfaces is a plane mirror or a cube corner. Such a wedge or wedges may alternatively or additionally be introduced on the measurement surface. Figure 8 illustrates an exemplary embodiment of an interferometer head 800 wherein the first reference surface 802 is a cube corner and the second reference surface 804 is a cube corner. In this embodiment the first and second partially-reflective surfaces 806, 808 are oriented orthogonally to each other and each is part of a separate cubic body 810, 812. Refractive wedges 814, 816 are positioned on a surface of each cubic body 810, 812 in the path of the input radiation beam towards the measurement surface. The second wedge compensates for the first wedge. As the exemplary embodiment of Figure 8 illustrates, and as will be readily understood by a person skilled in the art, any one or number of appropriate combinations of the aforementioned means of inducing an offset may be implemented into the interferometer head according to the invention - for example, a wedge or wedges may be positioned adjacent to one or both partially-reflective surfaces in the path of the input radiation beam towards the measurement surface without being positioned on the surface of a body.

[00042] In an embodiment, the incident radiation beam is a polarised radiation beam. An example of an interferometer head 900 of such an embodiment is illustrated in Figure 9. In this case the first and second partially-reflective surfaces 902, 904 are polarising beam splitters. The polarising beam splitters are shown oriented orthogonally to each other, however alternative orientations may be implemented. Additionally, a first waveplate 906, e.g. a 1/8 waveplate, is positioned in the path of the radiation beam 908 between the first polarising beam splitter 902 and the first reference surface 910, a second waveplate 912, e.g. a second 1/8 waveplate, is positioned in the path of the radiation beam 908 between the first polarising beam splitter 902 and the second polarising beam splitter 904, a third waveplate 914, e.g. a third 1/8 waveplate, is positioned in the path of the radiation beam 908 between the second polarising beam splitter 904 and the second reference surface 916 and a fourth waveplate 918, e.g. a fourth 1/8 waveplate, is positioned in the path of the radiation beam 908 between the second polarising beam splitter 904 and the measurement surface 920. In the exemplary embodiment of Figure 9, the first and second polarising beam splitters 902, 904 are contained in first and second bodies 922, 924. The person skilled in the art will readily understand that any appropriate form or combination of forms of body may be used to support the first and second polarising beam splitters, e.g. they may be part of or form a single body. The exemplary body of Figure 9 also illustrates an offset implemented by a tilt of the first reference surface and an equal and opposite tilt of the second reference surface. Similarly as in the case of a non-polarising interferometer head according to the invention, the offset implemented between the first and second output radiation beams may be implemented via the hardware of the interferometer in a variety of ways.

[00043] By inserting the interferometer head according to an embodiment of the invention in the path of a light beam directed towards a reflective measurement surface or target of an object, it may be assessed if said light beam is, for example, directed perpendicular to the target. Each of the combined beams of the first and second output radiation beams contains information regarding the displacement of e.g. the measurement surface, which may be visible in the form of interference fringes. For example, according to an embodiment of the invention, the first output radiation beam may give rise to a first image comprising N fringes and the second output radiation beam may give rise to a second image comprising N+l fringes. Taking the difference of the number of fringes observable between the first and second output radiation beams results in a measurement that not only has a number of fringes to provide a magnitude of displacement, but also provides positive or negative value (e.g. N - (N+l) = -1), thereby providing a directionally sensitive measurement corresponding to an alignment angle of the light beam relative to the object and/or vice versa.

[00044] When the first and second fringe images are electronically detected (e.g., at a complementary metal-oxide-semiconductor (CMOS) camera) the fringe patterns in each image can be analysed. The analysis may be digital, for example by means of software involving e.g. Fourier transform analysis. A given number of fringes can thereby be identified for each of the combined beams of the first and second output radiation beams, and the difference therebetween can provide a directionally sensitive measurement of displacement. Advantageously, any measured offset inherent to a system under investigation, for example in an interferometer, can be corrected without the need for further measurements or trial and error. Based on the directionally sensitive measurement corresponding to an alignment angle, the alignment of the input radiation beam relative to the object and/or the alignment of the object relative to the input radiation beam can be adjusted.

[00045] Implementation of a directionally- sensitive interferometer head according to the invention may advantageously give rise to the possibility of calibrating offsets to improve measurement precision. The calibration can e.g. be done by aligning a laser beam to a mirror at a given distance, such that the beam is reflected back to itself. The resulting number of fringes, i.e. the fringe spatial frequency, is equal to the offset of the tool. Where the accuracy of an intensity profile is improved, it may be possible to achieve a smaller measurement resolution, thus improved measurement accuracy. [00046] The contrast of the fringes is best when the interfering signals have similar intensities. The partially -reflective surfaces of the present invention will induce some loss of beam intensity as radiation traverses the interferometer head. In order to achieve approximately equal intensities in the first and second output radiation beams, the reflectivity of the first partially-reflective surface may be approximately 1/3, and the reflectivity of the second partially-reflective surface may be approximately 1/4, where a value of 1 is total reflection. A preferable optimum reflectivity of the first partially- reflective surface is 0.352 and a preferable optimum reflectivity of the second partially-reflective surface is 0.264. Accordingly, the transmission of the first partially-reflective surface may be approximately 2/3 and the transmission of the second partially-reflective surface may be approximately 3/4. These optimum values are calculated assuming 100% reflectivity on the first and second reference mirrors and the measurement surface. The useable reflectivities of the partially- reflective surfaces are not limited to these values, for example partially-reflective surfaces with a 50:50 reflectivity:transmission ratio may be used. In such an embodiment, additional image processing to improve the contrast may be applied. Alternatively, approximately equal intensities in the first and second output radiation beams may be achieved by filtering the intensity of the reference beams.

[00047] Although specific reference may be made in this text to the use of the invention for beam alignment in a lithographic apparatus, the invention may be applied to any situation where the alignment of a beam relative to a reflective measurement surface or target is to be analysed and/or corrected, and vice versa.

[00048] 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.

[00049] 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.

[00050] 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.

[00051] 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.

[00052] 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. Other aspects of the invention are set-out as in the following numbered clauses.

1. An interferometer head configured to: receive an input radiation beam from a light source, receive a first portion of the input radiation beam at a first partially -reflective surface and output a reflected beam from the first partially -reflective surface to a first reference surface, whereupon a first reference beam is reflected and transmitted through the first partially-reflective surface to a first output terminal, receive a second portion of the input radiation beam at a second partially-reflective surface and output a reflected beam from the second partially-reflective surface to a second reference surface, whereupon a second reference beam is reflected and transmitted through the second partially- reflective surface to a second output terminal, transmit a third portion of the input radiation beam towards a measurement surface and receive a reflected beam from the measurement surface, reflect a first portion of the reflected beam from the measurement surface at the second partially-reflective surface to the second output terminal, reflect a second portion of the reflected beam from the measurement surface at the first partially-reflective surface to the first output terminal, wherein the first reference beam and the reflected second portion of the reflected beam from the measurement surface are combined to a first output radiation beam and the second reference beam and the reflected first portion of the reflected beam from the measurement surface are combined to a second output radiation beam, whereby the interferometer head is further configured to implement an offset angle between the first and second output radiation beam.

2. The interferometer head of clause 1, wherein the reflectivity of the first partially-reflective surface is approximately 1/3.

3. The interferometer head of clause 1 or clause 2, wherein the reflectivity of the second partially-reflective surface is approximately 1/4.

4. The interferometer head of any of clauses 1-3 wherein at least one of the first and second partially-reflective surfaces is oriented at an angle of 45°.

5. The interferometer head of any of clauses 1-3 wherein the first and second partially-reflective surfaces are oriented orthogonally with respect to each other.

6. The interferometer head of any of clauses 1-5 wherein the offset angle between the first and second output radiation beams is implemented via the orientation angle of one of the first or second reference surfaces.

7. The interferometer head of any of clauses 1-5 wherein the offset angle between the first and second output radiation beams is implemented by means of one or more refractive wedges.

8. The interferometer head of clause 7, wherein one of said one or more refractive wedges is positioned on one of the first or second partially-reflective surfaces.

9. The interferometer head of clause 7 or 8, wherein one of said one or more refractive wedges is positioned on one of the first or second reference surfaces.

10. The interferometer head of any of clauses 7-9, wherein at least one of said one or more refractive wedges is positioned adjacent to one of the first or second partially-reflective surfaces in the path of the input radiation beam towards the measurement surface.

11. The interferometer head of any preceding clause, wherein one of the first or second reflective surfaces is a cube corner.

12. The interferometer head of any preceding clause, wherein the input radiation beam is an unpolarized laser beam. 13. The interferometer head of clause 1, wherein the input radiation beam is polarized.

14. The interferometer head of clause 13, wherein the first and second partially-reflective surfaces are beam-splitting polarizers.

15. A method of aligning a radiation beam to an object, the method comprising: directing the radiation beam to a target of the object; inserting an interferometer head according to any preceding clause into the trajectory of the radiation beam, thereby applying the radiation beam as the input radiation beam and having the third portion of the input radiation beam directed to the target as the measurement surface; detecting the first output radiation beam and the second output radiation beam; analysing interference patterns from the detected first and second output radiation beams; determining an alignment angle of the input radiation beam and the object; adjusting an alignment of the input radiation beam and the object, based on the alignment angle.

16. An interferometer head comprising: receiving means for receiving an input radiation beam from a light source, first path-providing means for providing a first reference path and a first measurement path second path-providing means for providing a second reference path and a second measurement path, wherein a first portion of the input radiation beam is arranged to follow the first reference path to be directed towards a first output terminal via a first reference surface and the first pathproviding means, and a second portion of the input radiation beam is arranged to follow the first measurement path to a measurement surface via the second path-providing means, back to the first path-providing means via the second path-providing means and towards the first output terminal, wherein the first path-providing means further provides a first combined optical path, said first combined optical path providing a first output radiation beam, wherein a third portion of the input radiation beam is arranged to follow the second reference path to be directed towards a second output terminal via a second reference surface and the second pathproviding means, and a fourth portion of the input radiation beam is arranged to follow the second measurement path to the measurement surface and towards the second output terminal via the second path-providing means, wherein the second path-providing means further provides a second combined optical path, said second combined optical path providing a second output radiation beam, means to induce an offset angle between the first and second output radiation beams, means for delivering the first and second output radiation beams to a detector.

17. A stage apparatus comprising: an object holder configured to hold an object; a position measurement system configured to provide a radiation beam to a measurement surface of the object holder for measuring a position of the object holder; and the interferometer head according to one or more of clauses 1-14 for aligning the radiation beam to the measurement surface of the object holder.

18. A lithographic apparatus comprising: a mask support for holding a patterning device having a pattern; an object holder configured to hold an object; a projection system for projecting the pattern onto the object; a position measurement system configured to provide a radiation beam to a measurement surface of the object holder; and the interferometer head according to one or more of clauses 1-14 for aligning the radiation beam to the measurement surface of the object holder.

19. A device manufacturing method wherein use is made of a lithographic apparatus according to clause 18.