CN116472436A

CN116472436A - Mirror spot position calibration method, lithographic apparatus and device manufacturing method

Info

Publication number: CN116472436A
Application number: CN202180078767.4A
Authority: CN
Inventors: J·M·T·A·阿德里安斯
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2020-11-26
Filing date: 2021-10-28
Publication date: 2023-07-21
Also published as: WO2022111940A1; KR20230109640A

Abstract

A method for calibrating spot positions on mirrors of an interferometer system comprising at least two pairs of interferometers for providing redundant measurements of a rotational position of an object about a rotational axis, wherein the method comprises the steps of: a. rotating the object; b. measuring a change in rotational position of the object about the rotational axis using each of the at least two pairs of interferometers; c. determining a deviation of a change in rotational position measured by one of the at least two pairs of interferometers from a change in rotational position measured by the other of the at least two pairs of interferometers or from an average change in rotational position measured by the two or more pairs of interferometers; based on the determined deviation, information about the spot position on the mirror of the one of the at least two pairs of interferometers is obtained.

Description

Mirror spot position calibration method, lithographic apparatus and device manufacturing method

Cross Reference to Related Applications

The present application claims priority from EP application 20209976.8 filed on month 11 and 26 of 2020, which is incorporated herein by reference in its entirety.

Technical Field

The invention relates to a method for calibrating spot positions on mirrors of an interferometer system. The invention also relates to a lithographic apparatus and a device manufacturing method.

Background

A lithographic apparatus is a machine that is configured to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of Integrated Circuits (ICs). The lithographic apparatus may, for example, project a pattern (also commonly referred to as a "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).

With the continued advancement of semiconductor manufacturing processes, the size of circuit elements has been reduced, while the number of functional elements (such as transistors) per device has steadily increased for decades following a trend commonly referred to as "moore's law. To keep pace with moore's law, the semiconductor industry is pursuing technologies that can create smaller and 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 the features that form the pattern on the substrate. Typical wavelengths currently in use are 365nm (i-line), 248nm, 193nm and 13.5nm. Lithographic apparatus using Extreme Ultraviolet (EUV) radiation having a wavelength in the range of 4nm to 20nm, for example 6.7nm or 13.5nm, may be used to form smaller features on a substrate than lithographic apparatus using radiation, for example 193 nm.

Lithographic apparatus typically include a positioning system for positioning an object, wherein an interferometer system configured to measure the position of the object is used. In such interferometer systems, one or more light beams are directed to a mirror on the object to determine an optical path difference of the one or more light beams compared to a reference optical path. The position at which the interferometer beam is incident on a surface, such as a mirror, is indicated by the spot on the surface. To increase the optical path difference caused by the movement of the object, the light beam may be directed to and reflected from the mirror more than once. Examples are so-called 2-way interferometers, in which the light beam is directed to the object twice so that two spots can be distinguished on the corresponding mirror, and 4-way interferometers, in which the light beam is directed to the object four times so that four spots can be distinguished on the mirror.

The mirrors on the object typically do not have a perfectly flat mirror surface and/or do not extend perfectly in the intended direction. For applications requiring accurate measurements of the position of an object, an imperfect mirror may prevent such accurate measurements from being performed, or may be regarded as interference with the respective position signals. Thus, it may be desirable to calibrate the mirrors and use the calibration, for example in the form of a mirror map, to compensate for the position signal of the imperfect mirrors.

When compensating for the position signal of an imperfect mirror, the compensation depends on the position of the beam on the mirror (i.e. the spot position). The prior art compensation scheme uses the expected spot position and ignores the spot position error.

Disclosure of Invention

In view of the above, it is an object of the present invention to provide a method of calibrating the spot position on the mirror of an interferometer system.

According to one embodiment of the present invention, there is provided a method for calibrating the position of a spot on a mirror of an interferometer system comprising at least two pairs of interferometers for providing redundant measurements of the rotational position of an object about a rotational axis, wherein the method comprises the steps of:

a. rotating the object;

b. measuring a change in rotational position of the object about the rotational axis using each of the at least two pairs of interferometers;

c. determining a deviation of a change in rotational position measured by one of the at least two pairs of interferometers from a change in rotational position measured by the other of the at least two pairs of interferometers or from an average change in rotational position measured by the two or more pairs of interferometers; and

d. based on the determined deviation, information about the spot position on the mirror of the one of the at least two pairs of interferometers is obtained.

According to another embodiment of the invention, there is provided a lithographic apparatus including:

an object to be positioned;

an actuator system for positioning an object;

a measurement system comprising an interferometer system comprising at least two pairs of interferometers for providing redundant measurements of a rotational position of an object about a rotational axis; and

a control system for driving the actuator system based on an output of the measurement system, wherein the control system is configured to perform the method according to the invention.

According to another embodiment of the invention, there is provided a device manufacturing method in which a lithographic apparatus according to the invention is used.

Drawings

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

FIG. 1 depicts a schematic diagram of a lithographic apparatus;

FIG. 2 depicts a detailed view of a portion of the lithographic apparatus of FIG. 1;

FIG. 3 schematically depicts a position control system;

FIG. 4 schematically depicts a top view of an interferometer system;

FIG. 5 schematically depicts spot locations on a mirror of the interferometer system of FIG. 4;

FIG. 6 schematically depicts a side view of the object of FIG. 4 after rotating the object about the Y-axis;

FIG. 7A schematically depicts a simplified top view of the interferometer system of FIG. 4 in a first rotational position about an axis parallel to the Z axis; and

FIG. 7B schematically depicts a simplified top view of the interferometer system of FIG. 4 in a second rotational position about an axis parallel to the Z axis.

Detailed Description

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

The terms "reticle," "mask," or "patterning device" used herein may be broadly interpreted as referring to a generic patterning device that can be used to impart an incoming radiation beam with a patterned cross-section that corresponds to a pattern being created in a target portion of the substrate. The term "light valve" may also be used herein. Examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays, in addition to classical masks (transmissive or reflective, binary, phase-shift, hybrid, etc.).

FIG. 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA comprises: an illumination system (also referred to as an 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 configured to hold a substrate (e.g., a resist-coated wafer) 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.

In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example 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 the plane of the patterning device MA.

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".

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. Further information about immersion techniques is given in US 6952253, which is incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel and/or the step of preparing a subsequent exposure of a substrate W may be performed on a substrate W that is positioned on one of the substrate supports WT while another substrate W on the other substrate support WT is used to expose a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement table. The measuring table is arranged to hold the sensor and/or the 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 measuring station may hold a plurality of sensors. The cleaning device may be arranged to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of the system providing the immersion liquid. When the substrate support WT is moved away from the projection system PS, the measurement table can be moved under the projection system PS.

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 a pattern (design layout) present on the patterning device MA. After traversing the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam onto a target portion C of the substrate W. By means of the second positioner PW and 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 focused and aligned positions. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in fig. 1) can 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 M1, M2 and substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between the target portions C, they are referred to as scribe-lane alignment marks.

For the purpose of illustrating the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, namely, an x-axis, a y-axis, and a z-axis. Each of the three axes is orthogonal to the other two axes. The rotation about the x-axis is referred to as Rx rotation. The rotation about the y-axis is referred to as Ry rotation. The rotation about the z-axis is referred to as Rz rotation. The x-axis and the y-axis define a horizontal plane, while the z-axis is in the vertical direction. The Cartesian coordinate system is not limiting of the invention and is for illustration only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to illustrate 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.

FIG. 2 depicts a more detailed view of a portion of the lithographic apparatus LA of FIG. 1. The lithographic apparatus LA may be provided with a chassis BF, a balancing 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 portion of the position measurement system PMS. The metrology frame MF IS supported by the chassis BF via a vibration isolation system IS. The vibration isolation system IS arranged to prevent or reduce the propagation of vibrations from the chassis BF to the metrology frame MF.

The second positioner PW is arranged to accelerate the substrate support WT by providing a driving force between the substrate support WT and a balance mass BM. The driving force accelerates the substrate support WT in a desired direction. Due to conservation of momentum, the driving force is also applied to the balancing mass BM with equal amplitude but in a direction opposite to the desired direction. In general, the mass of the balancing mass BM is substantially greater than the mass of the moving part of the second positioner PW and substrate support WT.

In one embodiment, the second positioner PW is supported by a balanced mass BM. For example, wherein the second positioner PW comprises a planar motor, so that the substrate support WT is suspended above the balanced mass BM. In another embodiment, the second positioner PW is supported by a chassis BF. For example, wherein the second positioner PW comprises a linear motor, and wherein the second positioner PW comprises a bearing, such as a gas bearing, so that the substrate support WT is suspended above the chassis BF.

The position measurement system PMS may comprise any type of sensor suitable for determining the position of the substrate support WT. The position measurement system PMS may comprise any type of sensor suitable for determining the position of the mask support MT. The sensor may be an optical sensor such as an interferometer or encoder. The position measurement system PMS may comprise a combined system of interferometers and encoders. 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 a position relative to a reference (e.g., metrology frame MF or 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 a velocity or acceleration.

The position measurement system PMS may comprise an encoder system. An encoder system is known, for example, from U.S. patent application US2007/0058173A1 filed on 9/7 of 2006, which is incorporated herein by reference. The encoder system includes an encoder head, a grating, and a sensor. The encoder system may receive a primary radiation beam and a secondary radiation beam. The primary radiation beam and 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 generated by diffracting the original radiation beam with a grating. If the primary and secondary radiation beams are generated by diffracting the original radiation beam with a grating, the primary radiation beam needs to have a different diffraction order than the secondary radiation beam. The different diffraction orders are for example +1 order, -1 order, +2 order and-2 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 the phase or phase difference of the combined radiation beam. The sensor generates a signal based on the phase or phase difference. The signal is indicative of the 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 chassis BF. For example, a plurality of encoder heads are arranged on the metrology frame MF, while a grating is arranged on the top surface of the substrate support WT. In another example, the grating is arranged on the bottom surface of the substrate support WT, while the encoder head is arranged below the substrate support WT.

The position measurement system PMS may comprise an interferometer system. An interferometer system is known from, for example, U.S. Pat. No. 6,020,964 to 7/13/1998, which is incorporated herein by reference. The interferometer system can include a beam splitter, a mirror, a reference mirror, and a sensor. The radiation beam is split by a beam splitter into a reference beam and a measurement beam. The measuring 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 the phase or frequency of the combined radiation beam. The sensor generates a signal based on the phase or frequency. The signal is representative of the displacement of the mirror. In one embodiment, the mirror is connected to the substrate support WT. The reference mirror may be connected to a metrology frame MF. In one embodiment, the measurement beam and the reference beam are combined into a combined beam of radiation by additional optical components other than a beam splitter.

The first positioner PM may include a long stroke module and a short stroke module. The short stroke module is arranged to move the mask support MT with a small range of movement relative to the long stroke module, with a high degree of accuracy. The long stroke module is arranged to move the short stroke module with relatively low precision over a large range of movement relative to the projection system PS. By a combination of long-stroke and short-stroke modules, the first positioner PM is able to move the mask support MT within a large range of motion, with high accuracy, relative to the projection system PS. Similarly, the second positioner PW may include a long-stroke module and a short-stroke module. The short stroke module is arranged to move the substrate support WT with high accuracy within a small range of movement relative to the long stroke module. The long stroke module is arranged to move the short stroke module with relatively low precision over a large range of movement relative to the projection system PS. By a combination of the long-stroke module and the short-stroke module, the second positioner PW is able to move the substrate support WT over a large range of motion, with high accuracy, relative to the projection system PS.

Each of the first and second positioners PM and PW is provided with an actuator for moving the mask support MT and substrate support WT, respectively. The actuator may be a linear actuator to provide a driving force along a single axis (e.g., the y-axis). Multiple linear actuators may be employed to provide driving forces along multiple axes. The actuator may be a planar actuator to provide driving force along multiple axes. For example, the planar actuator may be arranged to move the substrate support WT in 6 degrees of freedom. The actuator may be an electromagnetic 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 magnetic actuator having at least one magnet coupled to the substrate support WT and the mask support MT, respectively. The actuator may be a moving coil actuator having at least one coil coupled to the substrate support WT and the mask support MT, respectively. The actuator may be a voice coil actuator, a magneto resistive actuator, a lorentz actuator or a piezoelectric actuator or any other suitable actuator.

The lithographic apparatus LA comprises a position control system PCS as shown in fig. 3. The position control system PCS includes a setpoint generator SP, a feedforward controller FF, and a feedback controller FB. The position control system PCS supplies a drive signal to the actuator ACT. The actuator ACT may be an actuator of the first positioner PM or the second positioner PW. The actuators ACT drive the apparatus P, which may comprise a substrate support WT or a mask support MT. The output of the device P is a position quantity such as position or velocity or acceleration. The position quantity is measured with a position measurement system PMS. The position measurement system PMS generates a signal, which is a position signal representing the position quantity of the device P. The setpoint generator SP generates a signal, which is a reference signal representing the desired amount of position of the device P. For example, the reference signal represents a desired trajectory of the substrate support WT. The difference between the reference signal and the position signal forms the input of the feedback controller FB. Based on the input, the feedback controller FB provides at least a portion 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 a portion of the drive signal for the actuator ACT. The feed-forward FF may utilize information about the dynamics of the device P, such as mass, stiffness, resonant mode, and eigenfrequency.

Fig. 4 schematically depicts a practical embodiment of an interferometer system, e.g. as part of a position measurement system PMS configured to measure the position of an object OB, which may be, e.g., a substrate support WT or a mask support MT.

The interferometer system includes multiple pairs of interferometers, three of which are visible in the top view of FIG. 4. Only these three pairs of interferometers will be used throughout the description, but it will be apparent that other interferometers may be provided even though they are omitted from the figures for clarity.

The first pair of interferometers includes a first interferometer IF1 and a second interferometer IF2. The first interferometer IF1 is arranged to provide a first position signal PS1 representing the position of the object OB in the X-direction by directing the first measurement beam MB1 onto the first mirror FM. The second interferometer IF2 is arranged to provide a second position signal PS2 indicative of the position of the object OB in the X-direction by directing the second measurement beam MB2 onto the first mirror FM.

The second pair of interferometers includes a third interferometer IF3 and a fourth interferometer IF4. The third interferometer IF3 is arranged to provide a third position signal PS3 representing the position of the object OB in the Y-direction by directing the third measuring beam MB3 onto the second mirror SM. The fourth interferometer IF4 is arranged to provide a fourth position signal PS4 representing the position of the object OB in the Y-direction by directing the fourth measurement beam MB4 onto the second mirror SM.

The third pair of interferometers includes a fifth interferometer IF5 and a sixth interferometer IF6. The fifth interferometer IF5 is arranged to provide a fifth position signal PS5 representing the position of the object OB in the X-direction by directing the fifth measurement beam MB5 onto the third mirror TM. The sixth interferometer IF6 is arranged to provide a sixth position signal PS6 representing the position of the object OB in the X-direction by directing the sixth measurement beam MB6 onto the third mirror TM.

The first mirror FM and the third mirror TM are arranged on opposite sides of the object OB and extend mainly in the Y direction. The second mirror SM is arranged at one side of the object OB extending mainly in the X direction.

As mentioned above, although not depicted in fig. 4, the interferometer system may comprise an interferometer arranged to provide a position signal representing the position of the object OB in a Z-direction, perpendicular to the X-direction and the Y-direction and thus perpendicular to the drawing plane in fig. 4, by directing the measuring beam onto the mirror.

The interferometer may be a single pass interferometer, wherein the respective measurement beam is directed only once to the corresponding mirror, reflected back to interfere with the reference beam. However, as in this example, one or more, but preferably all, of the interferometers may be multi-pass interferometers in which the respective measuring beam is directed more than once to the corresponding mirror. An example of a mirror is schematically indicated in fig. 5.

Fig. 5 schematically depicts a third mirror TM, but the same can be applied to the first and/or second mirrors FM, SM mutatis mutandis. In this example, the fifth measurement beam MB5 is first directed by the fifth interferometer IF5 to the third mirror TM for reflection from the third mirror TM back to the fifth interferometer IF5 at spot location SP5 a. The fifth interferometer IF5 is configured to direct the fifth measurement beam MB 5a second time towards the third mirror TM to reflect from the third mirror TM back to the fifth interferometer IF5 at the spot location SP5 b.

Furthermore, the sixth measuring beam MB6 is guided by the sixth interferometer IF6 for the first time to the third mirror TM to be reflected from the third mirror TM back to the sixth interferometer IF6 at the spot position SP6 a. The sixth interferometer IF6 is configured to direct the sixth measurement beam MB 6a second time towards the third mirror TM to reflect from the third mirror TM back to the sixth interferometer IF6 at the spot location SP6 b.

The actual spot position is indicated by a filled circle. Dashed circles are used to indicate the corresponding expected spot locations for which the respective spots are designed. In the prior art, it is assumed that the spot is at the intended position, wherein in practice there is a spot position error compared to the intended spot position due to alignment errors introduced, for example, during assembly.

In this example, the blob position SP5a has a blob position error in the Y-direction only, while the blob position SP5b has a blob position error in both the Y and Z directions. Further, the spot position SP6a has a spot position error only in the Z direction, and the spot position SP6b has a spot position error in both the Y and Z directions.

Each beam portion directed to and reflected from the third mirror TM contributes to the position signal of the respective interferometer. However, since only the total contribution is determined in the interferometer, it is not possible to determine which part may contribute to which beam part. The same applies to spot position errors. Each spot position error contributes to the total error, but it is not possible to attribute a portion of the error to a particular spot position error. As a result, the multi-pass interferometer can be simplified by a single beam with an average spot position, which is the geometric center of the spot position. This simplified description is used in fig. 1, 4, 6, 7A and 7B in addition to fig. 5. In FIG. 5, the expected spot centers IC5 and IC6, respectively, of the average spot positions of the dashed spot positions, and the actual spot centers AC5 and AC6, respectively, of the average spot positions of the solid spot positions are used. Note that although not required by the present invention, the expected speckle centers IC5 and IC6 are at the same level in the Z direction. Furthermore, while the simplified description of a multipass interferometer herein actually involves multiple beam reflections from the mirrors of each interferometer, it also illustrates that the principles of the present invention can be readily applied to a single pass interferometer system as well.

Referring back to fig. 4, the first, second and third pairs of interferometers are each capable of measuring the orientation of object OB about an axis of rotation parallel to the Z direction perpendicular to the X and Y directions, referred to herein as Rz. Thus, two of the three Rz measurements are redundant, which can be used to obtain information about the actual spot centers AC5 and AC6, as will be explained in more detail below.

Fig. 6 depicts a side view of an object OB according to the Z-X plane. As mentioned above, for simplicity reasons, the measurement beam is described as a single beam of a respective mirror at the actual spot center of the spot position of the respective interferometer incident on the object OB. As shown in fig. 5, the actual spot center AC5 is located at a higher level than the actual spot center AC6 in the Z direction, which is reflected in fig. 6 as the fifth measurement beam MB5 being above the sixth measurement beam MB 6. Similarly, the first measurement beam MB1 is above the second measurement beam MB2 and the third measurement beam MB3 is above the fourth measurement beam MB 4.

According to the method of the invention, in this example, the object OB is rotated about an axis parallel to the Y-direction, the axis being perpendicular to the drawing plane and thus to the X-and Y-directions. This direction of rotation will be referred to as Ry. For simplicity, the first, second and third mirrors FM, SM, TM will be considered to be perfectly flat and aligned mirrors unless otherwise indicated.

When the second mirror SM extends in the X and Z directions, rotating the object OB about an axis extending in the Y direction as shown will not affect the Rz measurements of the second pair of interferometers, as the third and fourth measuring beams MB3, MB4 will not experience a change in optical path length. This is because the rotation Ry only rotates the second mirror SM in-plane and is independent of the actual spot centers of the third and fourth interferometers IF3, IF4 on the second mirror SM.

But only if the corresponding actual spot centers are not at the same level in the Z direction as shown in fig. 6, the Rz measurements of the first and second pairs of interferometers may be affected by the rotation. The optical path difference caused by the rotation of the first and third mirrors FM, TM is different due to the different level of the measuring beams MB1, MB5 compared to the measuring beams MB2, MB6, respectively. As a result, the position signals PS1, PS2 will change differently and thus result in a change of the Rz measurement value, although the object OB is not rotated about an axis extending in the Z-direction. The same applies to the position signals PS5, the position signals PS5, PS5 will also change differently and thus also result in a change of the Rz measurement value. The change in Rz measurement provides information about the distance between the actual spot centers in the Z direction. Therefore, in the case where the actual spot center is located at the same level in the Z direction, the Rz measurement value should not be changed.

In summary, by rotating the object OB about an axis parallel to the measuring beam of one of the interferometer pairs (in the example of fig. 6, parallel to the second pair of interferometers), information about the distance between the actual spot centers can be obtained in a direction perpendicular to the axis of rotation and perpendicular to the measuring beams of the other pair of interferometers. Thus, in the example of fig. 6, no information about the spot position of the second pair of interferometers is obtained. However, the above procedure may be repeated for rotations about axes extending in the X-direction, such that no change in Rz measurements can be observed using the first and third pairs of interferometers, but a change may occur in the Rz measurements of the second pair of interferometers due to the non-zero distance between the actual spot centers (which is the case in FIG. 6).

Fig. 7A depicts a simplified top view of the interferometer system of fig. 4 and indicates object OB and all measurement beams MB1-MB6. As previously described, all interferometer pairs are capable of providing Rz measurements due to the distance between the actual spot centers of the first and third pairs of interferometers in the Y direction and due to the distance between the actual spot centers of the second pair of interferometers in the X direction.

Fig. 7B depicts the top view of fig. 7A after rotating object OB about an axis extending in the Z-direction. The Rz measurements of all interferometer pairs will change due to rotation, but will change according to the actual distance between the corresponding actual spot centers, which may be different from the distance between the intended spot centers. Thus, the change in Rz measurement due to rotation provides information about the distance between the actual spot centers. The difference between the actual change in the Rz measurement and the expected change in the Rz measurement may be determined, but it is also possible to use only the Rz measurement itself to determine the reference Rz and to determine the difference between the actual change in the Rz measurement and the change in the reference Rz. In the example of fig. 7A and 7B, three pairs of interferometers are used, the following options can be used as reference Rz:

i. Rz measurements provided by the first pair of interferometers;

rz measurements provided by the second pair of interferometers;

rz measurements provided by the third pair of interferometers;

an average of Rz measurements provided by the first pair of interferometers and Rz measurements provided by the second pair of interferometers;

an average of Rz measurement values provided by the first pair of interferometers and Rz measurement values provided by the third pair of interferometers;

an average of Rz measurements provided by the second pair of interferometers and Rz measurements provided by the third pair of interferometers; and

an average of Rz measurements provided by the first pair of interferometers, rz measurements provided by the second pair of interferometers, and Rz measurements provided by the third pair of interferometers.

Thus, the deviation of the change in rotational position measured by one of the at least two pairs of interferometers from the change in rotational position measured by the other of the at least two pairs of interferometers (options i, ii and iii) or from the average change in rotational position measured by the two or more pairs of interferometers (options iv to vii) is determined to obtain information about the distance between the actual spot centers in the X or Y direction.

Since the methods described in the examples of FIGS. 6 and 7A and 7B provide information about the location of the actual spot center in different directions, these methods may be combined to complement each other.

Furthermore, the method described in the examples of FIGS. 6 and 7A and 7B may also be repeated for other rotations to obtain more information about the actual spot center, thereby improving accuracy.

For the example of fig. 6, the orientation of object OB after rotating object OB about Ry and indicated using a solid line may be referred to as a first rotational orientation, while object OB may also have a neutral orientation indicated by a dashed line in fig. 6. The neutral orientation in this example corresponds to alignment with the X-, Y-and Z-directions, which is generally determined by the support structure defining the movement of the object in the X-, Y-and Z-directions. Thus, the method is performed by rotating the object OB from the neutral orientation to the first rotational orientation. The method may be repeated for a second rotational orientation different from the first rotational orientation and different from the neutral orientation. The second rotational orientation may be obtained by rotating the object OB from the neutral orientation in a direction opposite to the rotation required when rotating from the neutral orientation to the first rotational orientation. In other words, the first rotational orientation and the second rotational orientation may be arranged at opposite sides of the neutral orientation.

For the example of fig. 7A and 7B, after object OB is rotated about Rz, the orientation of object OB is shown in fig. 7B and may be referred to as a first rotational orientation, while object OB may also have a neutral orientation as shown in fig. 7A. The neutral orientation in this example also corresponds to alignment with the X-, Y-and Z-directions, which is generally determined by the support structure defining the movement of the object in the X-, Y-and Z-directions. Thus, the method is performed by rotating the object OB from the neutral orientation to the first rotational orientation. The method may be repeated for a second rotational orientation different from the first rotational orientation and different from the neutral orientation. The second rotational orientation may be obtained by rotating the object OB from the neutral orientation in an opposite direction compared to the rotation required when rotating from the neutral orientation to the first rotational orientation. In other words, the first rotational orientation and the second rotational orientation may be arranged on opposite sides of the neutral orientation.

The above example has been described while assuming that the mirror is perfect, which may not be the case in practice. Since rotation of the object OB may cause the measuring beam and thus the spot to move over the mirror, any deviation from the ideal mirror shape may result in a change in the Rz measurement that is not caused by the deviation of the spot position. In order to distinguish between the different contributions, the method according to the invention may be combined with a method of at least partially calibrating the mirrors of the interferometer.

In one embodiment, referring again to FIG. 6, rotation of the object OB is shown moving the measurement beams MB1 and MB2 and the measurement beams MB5 and MB6 over the respective first and third mirrors FM, TM. The method can be adapted to translate the object in the Z-direction, allowing a pair of measuring beams MB1, MB2 or MB5, MB6 to be located at substantially the same position on the mirror, as in the case of a neutral orientation of the object OB. Additionally, the method may be repeated for the same rotation, but the other pair of measuring beams remains substantially at the same position on the mirror. Such additional measurements may allow to distinguish deviations caused by spot position deviations from deviations caused by imperfect mirrors and spot movements over the mirrors.

In another embodiment, referring again to fig. 7A and 7B, the rotation of object OB shown will move all measuring beams MB1 to MB6 over the respective mirrors FM, SM, TM. The method may be adapted to translate the object in the X and/or Y direction, allowing at least one pair of measuring beams, possibly both pairs of measuring beams, to be at substantially the same position on the mirror, as is the case in the neutral orientation of the object OB. Additionally, the method may be repeated for the same rotation, but using another pair of measuring beams or using another combination of measuring beam pairs that remain substantially at the same position on the mirror. Such additional measurements may allow to distinguish deviations caused by spot position deviations from deviations caused by imperfect mirrors and spot movements on the mirrors.

Although these examples depict the use of three pairs of interferometers, the present invention may also use only two pairs of interferometers. For example, the example of fig. 6 may also be used when one of the first or third pair of interferometers is omitted. Further, when any pair of interferometers is omitted, the example of fig. 7 may also be used.

Although the examples mainly discuss a multipass interferometer and the actual spot center is an average of the spot positions, the invention can also be applied using a single-pass interferometer, in which case the actual spot center coincides with the spot positions and is no longer the average position. However, the same principle still applies.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. 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.

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 devices. Embodiments of the invention may form part of a mask inspection apparatus, metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning apparatus). These devices are commonly referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

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 is not limited to optical lithography and may be used in other applications, for example imprint lithography, where the context allows.

Embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof, as the context allows. 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); a magnetic storage medium; an optical storage medium; a flash memory device; electrical, optical, acoustical or other form 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 that in doing so may cause actuators or other devices to interact with the physical world.

While specific embodiments of the invention have been described above, it should be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative, and not restrictive. It will therefore be apparent to those 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 as described in the numbered clauses below.

1. A method for calibrating spot position on a mirror of an interferometer system, the interferometer system comprising at least two pairs of interferometers for providing redundant measurements of a rotational position of an object about a rotational axis, wherein the method comprises the steps of:

a. rotating the object;

b. measuring a change in the rotational position of the object about the rotational axis using each of the at least two pairs of interferometers;

c. determining a deviation of the change in the rotational position measured by one of the at least two pairs of interferometers from the change in the rotational position measured by the other of the at least two pairs of interferometers or from an average change in the rotational position measured by two or more pairs of interferometers; and

2. The method of clause 1, wherein in step a, the object is rotated about the rotation axis.

3. The method of clause 1, wherein the at least two pairs of interferometers comprise a first pair of interferometers configured to measure a position of the object in a first direction perpendicular to the axis and a second pair of interferometers configured to measure a position of the object in a second direction perpendicular to the axis and the first direction, wherein in step a the object rotates about an axis parallel to the first direction, wherein step c comprises determining a deviation of the change in the rotational position measured by the second pair of interferometers compared to the change in the rotational position measured by the first pair of interferometers, and wherein in step d information about the spot position on the mirror of the second pair of interferometers is obtained.

4. The method according to any of clauses 1-3, wherein in step a the object is rotated from a neutral orientation to a first rotational orientation, and wherein steps b-d are repeated after rotating the object to a second rotational orientation, wherein the first rotational orientation and the second rotational orientation are arranged at opposite sides of the neutral orientation.

5. The method of clause 2, wherein in step d, the information obtained provides information about the spot position on the mirror in a direction perpendicular to the axis of rotation.

6. The method of clause 3, wherein in step d, the information obtained provides information about the spot position on the mirror in a direction parallel to the axis of rotation.

7. The method of any of clauses 1-6, wherein the interferometer is a multipass interferometer.

8. The method of any of clauses 1-7, wherein the object is translated to maintain the same spot position on the respective mirrors of at least one pair of interferometers before and after rotating the object.

9. A lithographic apparatus comprising:

an object to be positioned;

an actuator system for positioning the object;

a measurement system comprising an interferometer system comprising at least two pairs of interferometers for providing redundant measurements of a rotational position of the object about a rotational axis; and

a control system for driving the actuator system based on an output of the measurement system, wherein the control system is configured to perform the method according to any of clauses 1-8.

10. The lithographic apparatus of clause 9, further comprising:

an illumination system configured to condition a radiation beam;

a support configured to support a patterning device, the patterning device being capable of imparting the radiation with a pattern in its cross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate; and

a projection system configured to project the patterned beam of radiation onto a target portion of the substrate,

wherein the object is the support or the substrate table.

11. A device manufacturing method, wherein the lithographic apparatus according to clause 9 or 10 is used.

Claims

a. rotating the object;

2. The method of claim 1, wherein in step a, the object is rotated about the rotational axis.

3. The method of claim 1, wherein the at least two pairs of interferometers comprise a first pair of interferometers configured to measure a position of the object in a first direction perpendicular to the axis and a second pair of interferometers configured to measure a position of the object in a second direction perpendicular to the axis and the first direction, wherein in step a the object rotates about an axis parallel to the first direction, wherein step c comprises determining a deviation of the change in the rotational position measured by the second pair of interferometers compared to the change in the rotational position measured by the first pair of interferometers, and wherein in step d information about the spot position on the mirror of the second pair of interferometers is obtained.

4. A method according to any one of claims 1-3, wherein in step a the object is rotated from a neutral orientation to a first rotational orientation, and wherein steps b to d are repeated after rotating the object to a second rotational orientation, wherein the first rotational orientation and the second rotational orientation are arranged at opposite sides of the neutral orientation.

5. The method according to claim 2, wherein in step d the information obtained provides information about the spot position on the mirror in a direction perpendicular to the rotation axis.

6. A method according to claim 3, wherein in step d the information obtained provides information about the spot position on the mirror in a direction parallel to the rotation axis.

7. The method of any one of claims 1-6, wherein the interferometer is a multipass interferometer.

8. The method of any of claims 1-7, wherein the object is translated to maintain the spot position on the respective mirrors of at least one pair of interferometers the same before and after rotating the object.

9. A lithographic apparatus comprising:

an object to be positioned;

an actuator system for positioning the object;

10. The lithographic apparatus of claim 9, further comprising:

an illumination system configured to condition a radiation beam;

a substrate table constructed to hold a substrate; and

wherein the object is the support or the substrate table.

11. A device manufacturing method, wherein the lithographic apparatus according to claim 9 or 10 is used.