Measurement system, lithographic apparatus and device manufacturing method

A measurement system to determine a deformation of an object having a front surface, a back surface and a pattern. The measurement system includes a processor system and an interferometer system. The interferometer system has a radiation source and a detector system. The source is configured to emit, to each of a plurality of locations on the object, measurement beams in order to generate, at each of the respective plurality of locations, reflected measurement beams off the front and back surfaces of the object respectively. The detector system is configured to receive the respective reflected measurement beams and output signals representative of the received reflected measurement beams to the processor system. The processor system is configured to receive the signals; determine, based on the signals as received, a characteristic of the object; and determine a deformation of the pattern based on the characteristic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase entry of PCT patent application no. PCT/EP2017/051603, which was filed on Jan. 26, 2017, which claims the benefit of priority of European patent application no. 16159723.2, which was filed on Mar. 10, 2016, and which is incorporated herein in its entirety by reference.

BACKGROUND

Field of the Invention

The present invention relates to a measurement system, a lithographic apparatus and a method for manufacturing a device.

Description of the Related Art

In order to ensure proper operation of the integrated circuit, it is important that the different patterns that are consecutively applied have an accurate match. In order to ensure such a match, care is taken that the substrate is properly positioned, both in the horizontal plane and in vertical direction, relative to the image plane of the pattern. A possible mismatch between an imaged pattern and previously applied pattern may, however, also be caused by a deformation of the pattern. Such a deformation may e.g. be caused by mechanical stresses or by thermal effects such as a non-uniform temperature distribution. When known, these effects may at least partly be compensated, e.g. by adjusting the projection system of the lithographic apparatus. At present, means to assess such a deformation of the pattern are rather limited. In a known arrangement, a temperature of a top surface of a patterning device is determined, e.g. at various locations on the patterning device, by means of IR temperature sensors, the temperature measurements subsequently being used to determine a deformation of the patterning device.

SUMMARY

It is desirable to provide in a more accurate assessment of a deformation of a pattern on a patterning device. Therefore, according to an embodiment of the present invention, there is provided a lithographic apparatus comprising:

an illumination system configured to condition a radiation beam;

a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;a substrate table constructed to hold a substrate;a positioning device configured to position the support relative to the substrate table; anda projection system configured to project the patterned radiation beam onto a target portion of the substrate, wherein the apparatus further comprises a measurement system configured to determine a deformation of the pattern of the patterning device, the measurement system comprising:a processing system andan interferometer system comprising a light source and a detector system; the light source being configured to emit, to each of a plurality of locations on the patterning device, one or more measurement beams in order to generate, at each of the respective plurality of locations, a reflected measurement beam off a front surface of the patterning device and a reflected measurement beam off a back surface of the patterning device; the detector system being configured to receive, for each of the plurality of locations, the respective reflected measurement beams and output one or more signals representative of the received reflected measurement beams to the processing system;wherein the processing system is configured to:receive, for each of the plurality of locations, the one or more signals;determine, based on the plurality of one or more signals as received, a physical characteristic of the patterning device, the physical characteristic being representative of a deformation of the patterning device; and,determine, based on the physical characteristic of the patterning device and a mathematical model of the patterning device, a deformation of the pattern, wherein the physical characteristic is a temperature or a temperature offset at the plurality of locations.

According to another aspect of the present invention, there is provided a measurement system configured to determine a deformation of an object having a front surface and a back surface and being provided with a pattern, the measurement system comprising:a processing system andan interferometer system comprising a light source and a detector system; the light source being configured to emit, to each of a plurality of locations on the object, one or more measurement beams in order to generate, at each of the respective plurality of locations, a reflected measurement beam off the front surface of the object and a reflected measurement beam off the back surface of the object; the detector system being configured to receive, for each of the plurality of locations, the respective reflected measurement beams and output one or more signals representative of the received reflected measurement beams to the processing system;wherein the processing system is configured to:receive, for each of the plurality of locations, the one or more signals;determine, based on the plurality of one or more signals as received, a physical characteristic of the object, the physical characteristic being representative of a deformation of the object; and,determine, based on the physical characteristic of the object and a mathematical model of the object, a deformation of the pattern, wherein the physical characteristic is a temperature or a temperature offset at the plurality of locations.

The object may be a patterning device, the pattern being included in the patterning device and the patterning device being capable of imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam. The deformation may include an in-plane deformation of the pattern.

According to an embodiment of the invention, there is provided a device manufacturing method comprising transferring a pattern from a patterning device onto a substrate using a lithographic apparatus according to the present invention.

The step of transferring the pattern may include controlling a position of the substrate table relative to the support based on the deformation of the pattern. The positioning device may be configured to control the position of the substrate table relative to the support by means of a set point provided to a controller of the positioning device, the set point being based on the deformation of the pattern.

DETAILED DESCRIPTION

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

The lithographic apparatus as schematically shown inFIG. 1further comprises a measurement system MS according to an embodiment of the present invention. The measurement system MS comprises an interferometer system IF2and a processing system PrS. In accordance with the present invention, the measurement system MS is configured to determine a deformation of a pattern that is present on the patterning device or mask MA, as will be explained in more detail below,

FIG. 2schematically depicts a patterning device100having a front surface100.1and a back surface100.2. On the back surface100.2, a pattern110is provided. Typically, this is a two-dimensional pattern, extending in the XY-plane (the X-direction being perpendicular to the YZ-plane of theFIG. 2), that is to be projected accurately onto a substrate, thereby maintaining a predetermined positional relationship with a pattern that is previously applied to the substrate. Such a patterning device100may e.g. be applied in a lithographic apparatus as shown inFIG. 1. During use, a conditioned beam of radiation120, e.g. DUV radiation, may be projected onto the patterning device100. The patterning device100is typically made of SiO2 or fused silica, which is transparent for the DUV radiation beam120. Typically, the pattern110as applied to the back surface100.2of the patterning device100may be a chromium layer. The patterned chromium layer is not transparent for the DUV radiation120, rather, the DUV radiation120is substantially absorbed by the chromium layer. The absorbed radiation may be converted to heat, increasing the temperature of the chromium layer, e.g. from an environmental temperature of 22° C. to 26° C. this elevated temperature of the chromium layer120may then heat the patterning device100, by means of heat conduction. As a result of this heating, the patterning device100, including the pattern120, may deform. Within the meaning of the present invention, a deformation of the object may refer to a displacement of a point or location on the object relative to a nominal position. A point or location on the object may e.g. have a nominal position with coordinates (x,y,z) and, due the heating, displace to a position (x+Δx, y+Δy, z+Δz). As such, the deformation of the object at position (x,y,z) may be characterized by a displacement vector (Δx, Δy, Δz). Due to the heating of the pattering device100, which may typically be non-uniform, various types of deformations may occur. As a first example of such deformations, in-plane deformations of the pattern110can be mentioned, in-plane deformations being deformations observable in the XY-plane, i.e. parallel to the plane of the pattern110. Such an in-plane deformation may thus be characterized by considering the X- and Y-components of the displacement vector (Δx, Δy, Δz) of different positions of the pattern.

In general, when an object is heated, out-of-plane deformations may also occur. Within the meaning of the present invention, such out-of-plane deformations may e.g. be characterized by considering the Z-component of the displacement vector (Δx, Δy, Δz).

As explained above, the heating of the patterning device can, to a large extent, be attributed to the heating of the pattern110, which is located on the back surface100.2of the patterning device100. Because the pattern110, as a kind of heat source, is thus located on an outer surface100.2of the patterning device100, the temperature distribution may also have a non-uniformity in the Z-direction, i.e. the temperature on the front surface100.1may differ from the temperature at the back surface100.2. Due to this non-uniformity, the patterning device100may e.g. bend. Such a bending of the patterning device, in particular of the pattern, may also be characterized as an out-of-plane deformation of the pattern.

When a deformation of a pattern occurs and no measures are taken, the projection of the pattern onto the substrate, i.e. on the target portions such as target portions C shown inFIG. 1, may be inaccurate. In particular, in-plane deformations of the pattern may cause an alignment error between the projected pattern and a previously applied pattern on the substrate, whereas out-of-plane deformations may cause the image of the pattern to be out of focus, i.e. somewhat blurred, during exposure.

In case a deformation of a pattern occurs and is known, i.e. quantified to some degree, measures may be taken to improve the exposure or projection of the pattern onto the substrate. Such measures may e.g. include adjusting a setting of the illumination system, e.g. illuminator IL as shown inFIG. 1, or the projection system, e.g. projection system PS as shown inFIG. 1, of the exposure apparatus. Alternatively, or in addition, a relative position of the pattering device and the substrate may be controlled based on the deformation of the pattern as determined. In particular, in an embodiment, the lithographic apparatus according to the present invention may comprise a positioning device, such as positioning device PW or PM or a combination thereof, controlling a position of the substrate table relative to the support based on the deformation of the pattern, e.g. based on an in-plane deformation of the pattern. In such embodiment, the positioning device may e.g. be configured to control the position of the substrate table relative to the support by means of a set point. Such a set point may e.g. be provided to a controller or control system of the positioning device, the set point being based on the deformation of the pattern.

With respect to the possible deformations as described, it is worth mentioning that similar considerations may be applied in case reflective patterning devices are used, as e.g. done in lithographic apparatuses using an EUV light source. Such patterning devices may e.g. be provided with a pattern on a front surface of the patterning device, the pattern reflecting the conditioned EUV radiation. In such arrangement, portions of the pattering device that are not provided with a pattern and which are subjected to the EUV radiation may absorb this radiation and heat up, causing a non-uniform temperature distribution.

With respect to the position of the pattern, it may also be noted that the patterning device may also be provided with a cover layer e.g. covering the pattern. In such case, the pattern is thus neither provided on the front surface, nor on the back surface.

It has been proposed to assess the temperature distribution of a patterning device by using temperature sensors. Such an arrangement is schematically shown inFIG. 3.FIG. 3schematically shows a patterning device200having a front surface200.1and a back surface200.2and an array210of infrared (IR) temperature sensors arranged to assess a temperature of the patterning device200by measuring infrared radiation emanating from the patterning device200. As shown, the array210of sensors extends in the X-direction and may e.g. comprise a plurality of temperature sensors arranged adjacent each other in the X-direction. The radiation as measured by the array210of temperature sensors may e.g. be provided to a processing unit220; to determine the temperature of the patterning device200. By displacing the patterning device200relative to the array of sensors in the Y-direction, a two-dimensional temperature profile of the pattering device, i.e. a temperature profile in the XY-plane, may be established. This temperature profile may then be used, e.g. by means of a thermo-mechanical model of the patterning device, to estimate a deformation of the patterning device.

The known arrangement of determining a temperature profile has several drawbacks. The arrangement as shown inFIG. 3essentially captures IR radiation that originates from the front surface200.1of the patterning device200, typically the surface that is not provided with the pattern that is to be projected. As indicated above, there may be a difference between the temperature of the front surface and the temperature of the back surface, due to the heating of the pattern on the bottom surface. The temperature of the front surface200.1may thus be a poor representation of the actual temperature of the back surface200.2while the thermal expansion of the back surface200.2may be considered to cause the deformation of the pattern on the back surface200.2. It may also be worth mentioning that an assessment of the temperature of the top surface200.1does not provide any insight in the average temperature along the Z-direction or a temperature gradient in the Z-direction. As such, the possibilities to determine a deformation of the pattern on a pattering device using an array of infrared temperature sensors is rather limited.

The present invention therefore proposes a more direct approach which enables a more accurate assessment of the deformation of a pattern provided on a patterning device. In particular, in the present invention, a measurement system is proposed that is configured to determine a physical characteristic of a patterning device, whereby the physical characteristic represents of a deformation of the patterning device. In the present invention, use is made of an interferometer or interferometer system to measure a thickness or height of a patterning device or an optical path length of a measurement beam through the patterning device. Such a measured height or optical path length may be readily be applied in a mathematical model, as a representation of a deformation of the patterning device, to determine a deformation of a pattern of the patterning device. Alternatively or in addition, the measured height or optical path length may be applied to determine a temperature of the patterning device, in particular an average temperature along the height or along the optical path length as determined. The average temperature along the height of the pattering device may enable a more accurate assessment of the temperature of the back surface and may thus enable a more accurate determination of the deformation of the pattern.

InFIG. 4, a first embodiment of a measurement system400according to the present invention is schematically shown, the measurement system400enabling to determine a deformation of a pattern on a patterning device. In accordance with the present invention, the measurement system400is configured to determine a deformation of an object410having a front surface410.1and a back surface410.2. The object, e.g. a patterning device for use in a lithographic apparatus, is provided with a pattern (not shown) which may be on the front surface410.1, or the back surface410.2or at an interior of the patterning device. The measurement system400as shown comprises an interferometer system420and a processing system430.

In the embodiment as shown, the interferometer system420comprising a light source422and a detector system424, the detector system424comprising a first detector424.1and a second detector424.2. In the embodiment as shown, the interferometer system420is configured, using beam splitters426.1,426.2and426.3, to project a pair of measurement beams440.1,440.2onto a location450on the object. Within the meaning of the present invention, the objects that are subjected to a measurement process typically have a planar shape, e.g. extending in an XY-plane. A location on the object thus refers to a position in the XY-plane which can be characterized by (x, y) coordinates. In the embodiment as shown, the location450is thus accessed by the measurement beams on both the front surface side and the back surface side, whereby the measurement beam440.1is projected on the front surface410.1and the measurement beam440.2is projected on the back surface410.2at substantially the same (x, y) coordinates. In the embodiment as shown, the measurement beams440.1and440.2are reflected off of the respective front and back surfaces410.1and410.2, thereby generating a reflected measurement beam off the front surface410.1of the object450and a reflected measurement beam off the back surface410.2of the object450. The reflected measurement beams are subsequently received by the detectors424.1and424.2. In the embodiment as shown, the detectors424.1and424.2are further configured to receive reflected beams off respective references or reference objects470.1,470.2. In accordance with the present invention, the detector system424is further configured to output one or more signals460.1,460.2representative of the received reflected measurement beams. In the embodiment as shown, the detectors424.1and424.2are thus configured to output signals460.1and460.2and provide them to the processing system430, e.g. to an input terminal430.1of the processing system430. In the embodiment as shown, the interferometer system420can be considered a combination of two Michelson interferometers420.1,420.2sharing the light source422, whereby such interferometers are used to determine a position of the object410, relative to a reference. In particular, in the arrangement as shown, the output signals460.1of the interferometer420.1can be used to determine a Z-position of the front surface410.1of the object410, whereas the output signals460.2of the interferometer420.2can be used to determine a Z-position of the back surface410.2of the object. When combined with knowledge about the distance D in the Z-direction between both interferometers, the thickness or height d of the object at the location450can be determined by the processing system430, based on the signals received.

In accordance with the present invention, the measurement system400is configured to generate the reflected measurement beams at a plurality of different locations, i.e. at different positions of the object relative to the interferometer system in the XY-plane, and provide, for each of the plurality of locations, one or more output signals such as output signals460.1and460.2to the processing system430of the measurement system400. In an embodiment, the processing system430of the measurement system400of the present invention may comprise a memory unit, e.g. for storing the receive signals, and a processing unit comprising a processor or microprocessor or computer or the like for processing the received signals. The processing system may further comprise an input terminal such as input terminal430.1configured to receive input signals such as the signals460.1,460.2that are outputted by the detector system. The processing system as applied in the measurement system according to the present invention may further be configured to output, e.g. via an output terminal of the processing system, any signals such as signals representing results of the processing performed by the processing system, e.g. processing performed by a processing unit of the processing system430.

In accordance with the present invention, the processing system430is configured to determine, based on the plurality of signals as received, i.e. the signals received when measurements at the plurality of locations are performed, a physical characteristic of the patterning device, the physical characteristic being representative of a deformation of the object and determine, based on the physical characteristic and a mathematical model of the object, a deformation of the pattern. In the embodiment as shown inFIG. 4, the signals as received may e.g. be used by the processing system430to determine a deformation of the object in the Z-direction. In particular, the signals as retrieved during the measurement of the Z-position of the front surface410.1and the back surface410.2at the plurality of locations, may e.g. be used by the processing system430to determine the height of the object at the plurality of locations. Variations in the determined height, or a difference between the determined height and an expected, nominal height, may be considered a physical characteristic of the deformation of the object. In case the object has been subjected to a heat load, e.g. in case the object is a patterning device used in a lithographic apparatus, the deformation may e.g. be caused by a heating of the object. In such case, the physical characteristic as determined may also be a temperature or a temperature offset at the plurality of locations. As will be explained in more detail below, the variations in the determined height or optical path length may be used to determine the temperature along the height or optical path length at the plurality of locations.

In accordance with the present invention, the processing system430of the measurement system400is configure to determine, based on the physical characteristic of the object as determined and a mathematical model of the object, a deformation of the pattern. In the embodiment as shown, the height variations as determined for the plurality of locations may be considered deformations of the object in the Z-direction. Using such deformations as input to a mathematical model of the object, the corresponding deformations of the object in the XY-plane, in particular a deformation of a pattern provided on the object and extending in the XY-plane, may be derived.

In the embodiment as shown, the interferometer system420may be configured to determine the distance in the Z-direction of the front surface and the back surface of the object at a particular ‘single’ location in the XY-plane. By means of displacing the object410relative to the measurement beams449.1and440.2in both the X-direction and the Y-direction, measurements may be performed that, cover an area, e.g. an area which includes the pattern on the object. By doing so, a two-dimensional grid of measurement data may be obtained, which may e.g. be used by the processing system430to derive a two-dimensional deformation profile of the object in the Z-direction.

Alternatively, the interferometer system420may e.g. be configured to determine the distance in the Z-direction of the front surface and the back surface of the object at an array of different locations in the XY-plane at the same time, e.g. an array of locations having the same Y-coordinate but different X-coordinates. This can e.g. be realized by applying multiple discrete measurement beams arranged in the X-direction or by means of a measurement beam having an elongated cross-section in the X-direction, or a combination thereof. By doing so, multiple measurements corresponding to measurements at different locations having a different X-coordinate may be performed in parallel. In such an arrangement, a relative displacement of the measurement system400and the object need only be enabled in the Y-direction in order to obtain a two-dimension set of measurement data.

In the embodiment as shown, use is made to two Michelson interferometers to determine the height of an object at a particular location. It should be clear that other types of optical measurement systems enabling to determine the height or thickness of an object could be considered as well.

In the embodiment as shown, the height or thickness of the object is determined by projecting a pair of measurement beams440.1and440.2onto respective front and back surfaces410.1and410.2. in such case, use is made of reflected beams that are externally reflected off surfaces of the object. Such an arrangement may e.g. be applied in case the object is not transmissive to the measurement beam or beams, e.g. in case the object is a reflective patterning device.

In case the object is at least partly transmissive to the measurement beams as applied, alternative arrangements could be implemented as well, whereby us is made of an external reflection, e.g. at a front surface of the object and an internal reflection, e.g. at the back surface of the object. Using both reflected measurement beams, an optical path length of the measurement beam through the object can be determined, whereby said optical path length may be subsequently used, in a similar manner as the height assessed in the embodiment shown inFIG. 4, to assess or determine a physical characteristic representing a deformation of the object.

FIG. 5schematically shows an second embodiment of a measurement system500according to the present invention. In accordance with the present invention, the measurement system500is configured to determine a deformation of an object510having a front surface510.1and a back surface510.2. The measurement system comprises an interferometer system520and a processing system530. In the embodiment as shown, the interferometer system520comprises a light source522and a detector system524. In the embodiment as shown, the light source522is configured to generate a measurement beam540comprising two components having different wavelengths or frequencies. In an embodiment, the difference between both wavelength or frequency components is comparatively small. In an embodiment, the light source may be a Zeeman split laser including collimating optics to generate, as a measurement beam540, a collimated beam at two slightly different wavelengths. As shown, the measurement beam540is projected onto a location550on the object510.

In the embodiment as shown, a first component of the measurement beam540is a left-hand circularly polarized beam, whereas the second component of the measurement beam540is a right-hand circularly polarized beam. The interferometer system520further comprises a beam sampler580to sample a portion of the measurement beam520and direct the sampled portion540.1to a reference sensor, e.g. a reference photodiode524.1of the detector system524. In an embodiment, the beam sample580may comprise a plate, e.g. made of glass, having an anti-reflective coating on one side, such that only one side of the plate generates a reflection of the measurement beam540. The sampled portion540.1of the measurement beam is provided to the reference photodiode524.1via an analyzer595. In the embodiment as shown, the analyzer595comprises a linear polarizer. The resulting beam, i.e. the sampled portion540.1of the measurement beam540after being passed through the linear analyzer595will have a so-called beating, a periodic intensity variation. In particular, the intensity of the resulting beam will vary at a frequency corresponding to the frequency difference existing between the two components of the measurement beam540. The portion540.2of the measurement beam540that passes the beam sampler580is projected onto the object510, in particular onto the location550on the object. As shown, part of this portion540.2of the measurement beam540is reflected on the front surface510.1of the object510, while another part540.3propagates into the object510and reflects on the back surface510.2of the object510. The resulting beam542that is reflected off the object, also referred to as the reflected measurement beam542thus comprises a first reflected measurement beam542.1, reflected off the front surface510.1of the object510and a second reflected measurement beam542.2(indicated in dotted line), reflected off the back surface510.2of the object510. This reflected measurement beam542will also, when analyzed, have a beating, i.e. a periodically varying intensity. In the embodiment as shown, the detector system524of the interferometer system520therefore comprises a measurement photodiode524.2for measuring the reflected measurement beam542, after being passed through an analyzer596. In the embodiment as shown, the analyzer596also comprises a linear polarizer. Because the reflected measurement beam542includes a component5412that has travelled or propagated back and forth through the object510, a phase of the beating of the reflected measurement beam will be different from the phase of the beating as observed by the reference photodiode524.1. The phase difference between the beatings or beat components as received by the reference photodiode524.1and the measurement photodiode524.2is a measure for the optical path length of the beam path followed by the part540.3of the measurement beam540that has propagated back and forth through the object510. When, due to a particular heat load on the object510, the object510expands at the location550, the optical path length of the beam part540.3will change, resulting in a change in the phase difference between the beat components. In particular, a change in the phase difference between the beat components can be associated with a temperature difference of the object at the measurement location550, or with a height variation of the object at the measurement location550.

In case a normal incidence of the measurement beam component540.3on the object510is assumed, the optical path length L can be expressed as a function of a temperature offset ΔT by the following equation (1):
L=2·H·(n0+n′·ΔT)(1+α·ΔT)≈L0+2·H·(n′+n0·α)·ΔT(1)
Where:H=the nominal thickness or height of the object510at the measurement location550, i.e. the height in the indicated Z-direction;

α=the thermal expansion coefficient of the object's material;

n0=the refractive index of the object's material at a reference temperature;

n′=the temperature coefficient of the refractive index of the object's material;

ΔT=the temperature offset from the reference temperature (e.g. 22° C.)

L0=the nominal optical path length=2Hn0.It should be pointed out that the temperature offset ΔT refers to an average temperature of the object along the Z-direction at the measurement location550.

The temperature dependency of the phase difference may then be expressed as:

Δy=the occurring phase shift at a temperature offset ΔT;

λ=a nominal wavelength of the measurement beam.

At λ=600 nm, the parameters for fused silica, a typical material used for patterning devices are:n0=1.46;n′=8.86e−6K−1;α=5.5e−7K−1.The optical parameters are taken D. B. Leviton and B. J. Frey, “Temperature-dependent absolute refractive index measurements of synthetic fused silica,” Tech. Rep. arXiv:0805.0091 (2008). This results in a measurement sensitivity of Δφ/ΔT=1.28 rad/K. This sensitivity enables to determine, based on the measured phase shift ΔTthe temperature with a sufficiently high resolution, e.g. 0.2 K or better.
It should also be pointed out that the measured phase shift Δφ may also be used to determine a variation or change ΔH of the height H of the object, by considering that ΔH=H a ΔT.

In the embodiment as shown, the detector system524is configured to output a signal560.2representative of the reflected measurement beams, i.e. the first and second reflected measurement beams542.1,542.2to the processing system530of the measurement system500. In particular, the measurement photodiode524.2of the detector system524is configured to output the signal560.2and provide it to an input terminal530.1of the processing system530. In a similar manner, the reference photodiode524.1of the detector system524is configured to output a signal560.1representative of the sampled measurement beam. Using both signals560.1and560.2, the processing system530may derive a physical characteristic characterizing a deformation of the object510at the location550. In particular, using the above equations, the processing system530may be configured to determine the phase shift Δφ and determine, based on the phase shift Δφ either the temperature offset ΔT of the object at the plurality of locations, or the height variation ΔH of the object at the plurality of locations. Both physical characteristics characterizing a deformation of the object and may be used, in a mathematical model of the object, to determine a deformation of the pattern.

In accordance with the present invention, the measurement system500is configured to capture the reflected measurement beam542from a plurality of locations on the object. In this respect, similar considerations as discussed with reference to the first embodiment may be applied; in order to capture the reflected measurement beam542from a plurality of locations on the object, the object and the measurement system, in particular the measurement beam540, may be displaced relative to each other. Further, rather than applying a measurement beam having a spot-shaped cross-section, the measurement beam540may have an elongated shape, e.g. elongated in the X-direction or may comprise a plurality of spot-shaped beams, e.g. arranged adjacent each other in the X-direction.

In an embodiment, a measurement beam emitted by the light source is converted into a light sheet, e.g. a laser light sheet extending in the X-direction. In such an embodiment, which is schematically shown inFIG. 6, the photodiodes524.1and524.2may be replaced by arrays of photodiodes. In this respect, it should be noted that within the present invention various types of sensors or detectors may be applied in the detector system of the measurement system, ranging from photodiodes to one or two-dimensional arrays of photodiodes or one or two dimensional cameras, e.g. including CCD or CMOS arrays.

InFIG. 6, a third embodiment of a measurement system600according to the present invention is schematically shown. The top portion ofFIG. 6shows a cross-sectional YZ view, whereas the bottom portion schematically shows a top view of the measurement system. The measurement system600comprising an interferometer system comprising a light source622, e.g. a Zeeman laser configure to emit a laser beam640, e.g. a laser beam comprising two components as described with reference toFIG. 5, and a detector system624. In the embodiment as shown, the interferometer system620further comprises a lens system626for converting the laser beam640into a laser light sheet640.1which can be projected, using mirror628of the interferometer system620onto an object610e.g. a patterning device as applied in a lithographic apparatus. In the embodiment as shown, the interferometer system620further comprises a beam sampler680for deflecting a portion of the measurement beam, i.e. the laser light sheet640.1, towards a reference photodiode array624.1of the detector system624. The detector system624further comprises a measurement photodiode array624.2configured to received a reflected measurement beam comprising a first reflected measurement beam reflected off the front surface of the object610and a second reflected measurement beam reflected off the back surface of the object620. The beams or beam portions as received by the reference photodiode array624.1and the measurement photodiode array624.2may be processed, in a similar manner as described above, by means of a processing system630of the measurement system600. In particular, the measurement photodiode array624.2of the detector system624is configured to output the signal660.2and provide it to an input terminal630.1of the processing system630. In a similar manner, the reference photodiode array624.1of the detector system624is configured to output a signal660.1representative of the sampled measurement beam. Using both signals660.1and660.2, the processing system630may derive a physical characteristic representing a deformation of the object110at the measurement locations650, i.e. the area onto which the laser light sheet640.1is projected. By applying a laser light sheet640.1instead of a laser spot, the physical characteristic representing the deformation of the object610, e.g. ΔT or ΔH, may be assessed at a plurality of locations650arranged in the X-direction, without having to displace the object relative to the measurement system in the X-direction. By displacing the object610relative to the measurement system620, in particular the measurement laser light sheet640.1, in the Y-direction, the physical characteristic characterizing a deformation of the object may be determined over an area of the object, e.g. an area covering a pattern that is applied on the object. With respect toFIG. 6, it should be pointed out that, for clarity reasons, additional useful components of the interferometer system, such as polarizers, wave plates, analyzers, etc. are not shown. The use of such components in embodiments of the present invention is explained in more detail below. As will be clear to the skilled person, the use of a laser light sheet as schematically shown inFIG. 6may also be implemented in those embodiments.

In a similar manner as described with reference toFIG. 4, the processing systems530and630of the second and third embodiment of the measurement system according to the present invention are further configured to determine, based on the physical characteristic representing the deformation of the object and a mathematical model of the object, a deformation of the pattern.

With respect to the arrangement as shown inFIG. 6, it may be worth nothing that the reference photodiode array624.1may be replaced by a single photodiode which analyses a portion of the laser light sheet640.1that is sampled in a similar manner as described with reference toFIG. 4. As will be understood by the skilled person, the phase of the beating or beat component as detected by the reference photodiode array as shown inFIG. 6will be the same for all photodiodes of the array, since they all receive a sample of the same measurement beam.

In the embodiments as described with reference toFIGS. 5 and 6, the reflected measurement beam as received by the measurement photodiode or the measurement photodiode array is a combination of four beams; the measurement beam as applied in the second and third embodiment comprises two components (having a different frequency), each of which is reflected both at the front surface and at the back surface of the object, resulting in four components in the reflected measurement beam. As a result, the phase difference and the amplitude of the reflected measurement beam as received by the measurement photodiode or photodiode array will depend on the thickness or height H of the object and the reflection coefficients of the front and back surfaces.

In order to simplify the analysis of the received reflected measurement beam, it would be advantageous to ensure that the reflected measurement beam as received by the detector system only comprises two components, the components having a different wavelength and being reflected off of different surfaces.

InFIG. 7, a fourth embodiment of the measurement system700according to the present invention is schematically shown, the embodiment enabling that the reflected measurement beam as received by the detector system only comprises a first reflected measurement beam having a first wavelength and being reflected off the front surface710.1of the object710and a second reflected measurement beam having a second wavelength, different from the first wavelength, and being reflected off the back surface710.2of the object710.

In the embodiment as shown, the selection of the mentioned components is made possible by means of a spatial separation of both components in the measurement beam and a subsequent selective blocking of the reflected measurement beams. The measurement system700as schematically shown inFIG. 7comprises an interferometer system720and a processing system730. The interferometer system720comprises a light source722which is similar to the light source shown inFIG. 4, i.e. a Zeeman split laser configured to emit a measurement beam740comprising two components having different wavelengths, whereby a first component of the measurement beam740is a left-hand circularly polarized beam, whereas the second component of the measurement beam740is a right-hand circularly polarized beam.

The interferometer system720is further configured to provide in a spatial separation of both components of the measurement beam in the Y-direction. In order to realize this, use is made of a quarter-wave plate702and a Wollaston polarizing beam splitter704. The quarter-wave plate702modifies the circularly polarized components of the measurement beam740into linear polarized components, having orthogonal polarizations, which are subsequently split up by the Wollaston polarizing beam splitter in a first component, having the first wavelength and a polarization in the plane of the drawing and a second component, having the second wavelength and a polarization perpendicular to the plane of the drawing.

In the embodiment as shown, the interferometer system720further comprises a lens706for redirecting and focusing the first and second components, spatially separated, onto the object710, in particular onto a location750on the object710. Due to the spatial separation of the first component740.1and second component740.2of the measurement beam, the components being projected onto the object710, a corresponding spatial separation can be realized between the reflected components. By an appropriate dimensioning of the Wollaston polarizing beam splitter704and/or the lens706, one can ensure that the front surface reflection of the second component740.2substantially overlaps or coincides with the back surface reflection of the first component740.1, while the front surface reflection of the first component740.1and the back surface reflection of the second component740.2are spatially separated. By means of an appropriate beam blocker708, e.g. a non-transmissive plate provided with an aperture708.1, the front surface reflection of the first component740.1and the back surface reflection of the second component740.2can be blocked. Subsequently, the overlapping front surface reflection of the second component740.2and the back surface reflection of the first component740.1, indicated by reference number740.3can be provided to a measurement photodiode724.2of the detector system724and analyzed, together with a sample of the measurement beam740that is sampled using a beam sampler780and that is received by a reference photodiode724.1of the detector system724, in a similar manner as discussed above. Similar to the aforementioned embodiments, analyzers795and796are applied in association with the photodiodes724.1and724.2. In the embodiment as shown, the detector system724is configured to output a signal760.2representative of the reflected measurement beams740.3to the processing system730of the measurement system700, In particular, the measurement photodiode724.2of the detector system724is configured to output the signal760.2and provide it to an input terminal730.1of the processing system730. In a similar manner, the reference photodiode724.1of the detector system724is configured to output a signal760.1representative of the sampled measurement beam. Using both signals760.1and760.2, the processing system730may derive a physical characteristic representing a deformation of the object710at the location750.

With respect to the use of the quarter-wave plate702, the Wollaston prism704, the lens706and the beam blocker708which are applied to provide in the aforementioned selection of the reflected measurement beams, it is worth noting that such a selection may be realized in various other manners, without departing from the scope of the invention.

As will be clear to the skilled person, a non-normal angle of incidence has to be applied for the measurement beam components740.1and740.2in order to realize the described selection of reflected measurement beams. However, it is preferred to have the angle of incidence of the measurement beam components740.1and740.2as close to normal as possible because this will reduce the sensitivity of the measurement to small tilt errors, i.e. rotations of the object about the X-axis, relative to the interferometer system.

InFIG. 8, a fifth embodiment of a measurement system800according to the present invention is schematically shown. The embodiment as shown enables the appropriate or desired selection of the reflected measurement beams, i.e. such that the reflected measurement beam as received by the detector system824only comprises a first reflected measurement beam having a first wavelength and being reflected off the front surface of the object and a second reflected measurement beam having a second, different from the first, wavelength and being reflected off the back surface of the object. At the same time, the embodiment does not require the application of a non-normal angle of incidence, i.e. a normal angle of incidence can be applied, thus substantially removing the sensitivity of the measurement to small tilt errors.

In order to realize this embodiment, it is required that the object810that is being examined, is provided with a quarter-wave coating810.3on the front surface810.1. InFIG. 8, reference numbers801,802,803and804respectively refer to symbols indicating a left-hand circularly polarized beam, a right-hand circularly polarized beam, a linear polarized beam in the plane of the drawing and a linear polarized beam perpendicular to the plane of the drawing. It should further be noted that the beams840.1and840.2as emitted by the light source822, e.g. a Zeeman split laser as discussed above, and the reflected beams840.3,840.4,840.5and840.6are shown as spatially separated. Also, the reflected beams are shown at a non-normal angle relative to the front surface810.1. This is merely done for clarity purposes; to more clearly show the different components and their transformations. In practice, the emitted light beams840.1and840.2are assumed to be overlapping and, as a result, the reflected beams will overlap as well and be reflected off the front and back surfaces810.1,810.2at a normal angle, i.e. the angle of incidence of the emitted light beams840.1and840.2. In the embodiment as shown inFIG. 8, the measurement system800comprises an interferometer system820and a processing system830, the interferometer system820comprising a light source822and a detector system824. In the embodiment as shown, the light source822comprises a Zeeman split laser that generates a light beam840.1at wavelength λ1and light beam840.2at wavelength λ2different from λ1. The two light beams840.1and840.2are circularly polarized with opposite rotation directions. A quarter-wave plate805of the interferometer system820converts the light beams840.1and840.2into linearly polarized light beams841.1and841.2that are again at orthogonal polarizations. The beams841.1and841.2are subsequently partially reflected from a beam splitter880.1, e.g. a 50% reflective, not polarization-selective beam splitter, towards a first analyzer890. In the embodiment as shown, the analyzer890is a polarizing filter with the polarization axis at 45 deg. It can be pointed out that any polarizing axis greater than 0 deg. and smaller than 90 deg. could also be applied. The two components of the reflected light beams841.1and841.2will interfere with each other, resulting in a reference signal consisting of components842.1and842.2, said reference signal having a beating or beat component. This signal is received, in the embodiment as shown, by a reference photodiode824.2of the detector system824. The portion of the emitted light beams840.1and840.2that passes through the beam splitter880.1, i.e. light beams843.1and843.2are reflected from the object810, which has a quarter-wave coating810.3on its front surface810.1. The light beams840.3and840.5that are reflected from the top of the quarter-wave coating810.3do not change their polarization. The light beam portions that propagate through the object810, indicated by reference numbers844.1,845.1,844.2and845.2pass through the quarter-wave coating twice and do change their polarization. As a result of the application of the quarter-wave plate805and the quarter-wave coating810.3, the reflected measurement beams840.4and840.5have a different polarization then the reflected measurement beams840.3and840.6, as can be seen inFIG. 8. The reflected beams off the object810are subsequently reflected by the beam splitter880.2towards the photodiode824.2. As such, by applying a second analyzer892, e.g. a polarizing filter that selectively transmits the reflected measurement beams840.4and840.5and blocks the reflected measurement beams840.3and840.6, the desired selection of reflected measurement beams is made, which can be provided to a measurement photodiode824.2of the detector system824of the interferometer system820. The beams or beam portions as received by the reference photodiode824.1and the measurement photodiode824.2may be processed, in a similar manner as described above, by means of a processing system830of the measurement system800. In particular, the measurement photodiode array824.2of the detector system824is configured to output the signal860.2and provide it to an input terminal830.1of the processing system830. In a similar manner, the reference photodiode824.1of the detector system824is configured to output a signal860.1representative of the sampled measurement beam. Using both signals860.1and860.2, the processing system830may derive a physical characteristic representing a deformation of the object810at the measurement location850. The processing system830may further be configured to determine, based on the physical characteristic of the object and a mathematical model of the object, a deformation of a pattern that is provided on the object810.

With respect to the embodiment as shown inFIG. 8, it may be worth mentioning that part of the reflected beams may return towards the light source822and may interfere with the proper stable operation of the light source. In order to mitigate this, a gray filter or attenuator may be provided in the beam path to the tight source822.

The various embodiments of the measurement system according to the present invention that are discussed above thus enable to determine a deformation of a pattern, e.g. a pattern provided on a patterning device, based on a physical characteristic of the object, as determined using an interferometer system. Using the interferometer system, a deformation of the height or the optical path length of the object may be determined at a plurality of locations on the object and applied in a mathematical model of the object to arrive at the deformation of the pattern. Alternatively, a temperature profiled may be derived from the measurements made using the interferometer system and applied in the mathematical model of the object to arrive at the deformation of the pattern.

In an embodiment, the deformation of the pattern as derived is an in-plane deformation. The physical characteristic as determined using the measurement systems as described may be considered an averaged characteristic over the height or thickness of the object. In particular, the temperature offset ΔT as determined at a particular location on the object will represent the average temperature offset along the beam path as followed.

It is known that objects, such as patterning devices as described inFIG. 2, may suffer not only from in-plane deformations, but also from out-of-plane deformations, e.g. curvatures about the X-axis or the Y-axis. Such deformations may e.g. be caused by non-uniform temperature distributions in the object, along the height of the object, i.e. along the Z-direction as indicated inFIG. 2. In order to assess a non-uniformity in the temperature distribution along the height of the object, the following approach may be adopted:

In an embodiment, the measurement system according to the present invention is further equipped with one or more temperature sensors configured to determine a temperature of either the front surface, the back surface or both. In such embodiment, an array of temperature sensors210as shown inFIG. 3may e.g. be applied to determine the temperature profile of the front surface of an object. Using such a measurement system enables to determine both the temperature profile of the front surface of an object, i.e. surface temperature profile, and a temperature profile representing the average temperature offset over the front surface. More specifically, using the temperature sensors and the interferometer system, one may obtain, at each measurement location, a temperature or temperature offset at the front surface and an averaged temperature or averaged temperature offset along the height at the measurement location. In case these temperatures differ, i.e. in case the front surface temperature as measured at the measurement location differs from the average temperature as determined at the measurement location, one can estimate an occurring temperature gradient along the height at the measurement location. Such a temperature gradient may then be determined for each measurement location and applied to a mathematical model e.g. a thermo-mechanical finite element model, to derive a deformation of the object, in particular of a pattern on the object, due to the temperature gradient. Such an approach may result in an estimation of additional deformations of the pattern, in particular an estimation of out-of-plane deformations of the pattern.

The measurement system according to the present invention may e.g. be implemented in a lithographic apparatus according to the present invention, in order to determine a deformation of a pattern provided on a patterning device that is applied in the apparatus to project the pattern onto a substrate.

In an embodiment, a measurement system according to the present invention may be applied to determine a deformation of a particular pattern on a particular patterning device, based on measurements performed in between the exposure of different substrates or different lots of substrates in a lithographic apparatus according to the present invention. Using the measurements, a measurement system according to the present invention may be configured to determine the deformation of the pattern under different operating conditions, e.g. depending on the illumination dosage as applied or depending on the number of exposures performed per unit of time. When this deformation is available, it may e.g. be applied in another lithographic apparatus, e.g. applying the same or a similar patterning device, which need not be equipped with a measurement system according to the present invention. Such an apparatus may then be configured to execute a program to control an operational parameter of the apparatus, in accordance with the deformation of the pattern as determined by the measurement apparatus. As mentioned above, such an operational parameter may e.g. be a setting of an illumination system or projection system of the apparatus or a set point of a positioning device of the apparatus, the positioning device being configured to control a position of the patterning device relative to the substrate during the exposure process.

In an embodiment, there is provided a measurement system configured to determine a deformation of an object having a front surface and a back surface and being provided with a pattern, the measurement system comprising: a processing system and an interferometer system comprising a light source and a detector system; the light source being configured to emit, to each of a plurality of locations on the object, one or more measurement beams in order to generate, at each of the respective plurality of locations, a reflected measurement beam off the front surface of object and a reflected measurement beam off the back surface of the object; the detector system being configured to receive, for each of the plurality of locations, the respective reflected measurement beams and output one or more signals representative of the received reflected measurement beams to the processing system, wherein the processing system is configured to: receive, for each of the plurality of locations, the one or more signals; determine, based on the plurality of one or more signals as received, a physical characteristic of the object, the physical characteristic being representative of a deformation of the patterning device; and determine, based on the physical characteristic of the object and a mathematical model of the object, a deformation of the pattern, wherein the physical characteristic is a temperature or a temperature offset at the plurality of locations.

In an embodiment, the deformation of the patterning device comprises a local deformation of the patterning device at the plurality of locations. In an embodiment, the interferometer system is configured to: project a measurement beam to the front surface of the patterning device, a first portion of the measurement beam being reflected off the front surface, so as to form the reflected measurement beam off the front surface; a second portion of the measurement beam propagating through the patterning device and at least partially reflecting off the back surface, propagating towards the front surface and at least partially propagates outside the patterning device, so as to form the reflected measurement beam off the back surface. In an embodiment, the measurement beam comprises a first component having a first frequency and a second component having a second frequency, the detector system being configured to determine a phase of a beat component of the measurement beam and a phase of a beat component of the reflected measurement beams. In an embodiment, the interferometer system comprises a beam sampler configured to deflect a sample portion of the measurement beam to a first detector of the detector system for determining the phase of the beat component of the measurement beam. In an embodiment, the first component is left-hand circularly polarized and the second component is right-hand circularly polarized; the interferometer system further comprising a first analyzer through which the sample portion propagates prior to the first detector. In an embodiment, the first component is right-hand circularly polarized and the second component is left-hand circularly polarized; the interferometer further comprising a first analyzer through which the sample portion propagates prior to the first detector. In an embodiment, the interferometer system is configured to provide a reflected measurement beam off the front surface having the first frequency and a reflected measurement beam off the back surface having the second frequency to a detector of the detector system and prohibit a reflected measurement beam off the front surface having the second frequency and a reflected measurement beam off the back surface having the first frequency to reach the detector. In an embodiment, the interferometer system is configured to spatially displace the first component relative to the second component prior to impacting the patterning device. In an embodiment, the measurement system further comprises one or more temperature sensors configured to determine a surface temperature profile of the front surface or the back surface or both. In an embodiment, the physical characteristic is a temperature or a temperature offset at the plurality of locations and wherein the processing system is configured to determine a temperature gradient of the patterning device based on the temperature or the temperature offset at the plurality of locations and the surface temperature profile. In an embodiment, the processing system is configured to determine an out-of-plane deformation of the pattern based on the temperature gradient of the object and the mathematical model of the object.

In an embodiment, there is provided a lithographic apparatus comprising: an illumination system configured to condition a radiation beam; a support constructed to support a patterning device comprising a pattern, the patterning device being capable of imparting the radiation beam with the pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; a positioning device configured to position the support relative to the substrate table; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, wherein the apparatus further comprises a measurement system as described herein.

In an embodiment, there is provided a device manufacturing method comprising transferring a pattern from a patterning device onto a substrate using a lithographic apparatus as described herein. In an embodiment, transferring the pattern is preceded by determining a deformation of the pattern by means of the measurement system of the lithographic apparatus, and adjusting a setting of the illumination system or the projection system of the lithographic apparatus based on the deformation of the pattern.