(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2006/0103826 A1
`(43) Pub. Date:
`May 18, 2006
`Kok et al.
`
`US 20060103 826A1
`
`(54)
`
`(75)
`
`LITHOGRAPHIC APPARATUS, METHOD OF
`DETERMINING PROPERTIES THEREOF
`AND COMPUTER PROGRAM
`
`Inventors: Haico Victor Kok, Eindhoven (NL);
`Johannes Jacobus Matheus
`Baselmans, Oirschot (NL)
`
`Correspondence Address:
`PILLSBURY WINTHROP SHAW PITTMAN,
`LLP
`P.O. BOX 10500
`MCLEAN, VA 22102 (US)
`
`(73)
`
`Assignee: ASML NETHERLANDS B.V., Veld
`hoven (NL)
`
`(21)
`
`(22)
`
`(51)
`
`(52)
`
`Appl. No.:
`
`10/988,845
`
`Filed:
`
`Nov. 16, 2004
`
`Publication Classi?cation
`
`Int. Cl.
`(2006.01)
`G03B 2 7/54
`US. Cl. ............................................... .. 355/67; 355/53
`
`(57)
`
`ABSTRACT
`
`A lithographic apparatus is arranged to project a patterned
`radiation beam from a patterning device onto a substrate
`using a projection system. The lithographic apparatus com
`prises: an interferometric sensor for measuring the Wave
`front of the radiation beam at the level of the substrate, the
`interferometric sensor having a detector; an actuator for
`displacing the interferometric sensor in a direction along the
`optical axis; a ?rst module for determining the change of
`phase of the Wavefront at each of a plurality of locations on
`the detector of the interferometric sensor, the change of
`phase resulting from displacement of the interferometric
`sensor by the actuator between a ?rst position and a second
`position; and a second module for determining, for each of
`the plurality of locations on the detector, the corresponding
`pupil location at the pupil plane of the projection system
`traversed by the radiation, using the change of phase deter
`mined by the ?rst module and the value of the displacement
`of the interferometric sensor by the actuator, to produce a
`mapping between locations on the detector and correspond
`ing pupil locations. Once the mapping has been obtained, the
`numerical aperture and telecentricity of the projection sys
`tem can be measured.
`
`2%
`
`SL
`
`PP
`
`PH
`
`IAZ
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`Nikon Exhibit 1025 Page 1
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`

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`Patent Application Publication May 18, 2006 Sheet 1 0f 4
`
`US 2006/0103826 A1
`
`Fig. 1
`
`<M2 [MA [M1
`E
`
`S0
`
`B D
`
`lL
`
`AM
`
`IN
`CO
`
`MA
`
`PB
`
`MT
`
`2L
`
`PW
`
`Nikon Exhibit 1025 Page 2
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`

`

`Patent Application Publication May 18, 2006 Sheet 2 0f 4
`
`US 2006/0103826 A1
`
`SL
`
`PP
`
`PH
`
`Nikon Exhibit 1025 Page 3
`
`

`

`Patent Application Publication May 18, 2006 Sheet 3 0f 4
`
`US 2006/0103826 A1
`
`Fig. 4
`x-sheared phase change
`
`y-sheared phase change
`
`a /
`+0.3 contour
`
`-0.5 contour
`
`Fig. 5
`
`Step 1 - Fit
`
`Step 2 - Extrapolate
`
`Nikon Exhibit 1025 Page 4
`
`

`

`Patent Application Publication May 18, 2006 Sheet 4 0f 4
`
`US 2006/0103826 A1
`
`Fig. 6(a)
`
`Fig. 6(b)
`
`12
`
`AZ
`
`Nikon Exhibit 1025 Page 5
`
`

`

`US 2006/0103 826 A1
`
`May 18, 2006
`
`LITHOGRAPHIC APPARATUS, METHOD OF
`DETERMINING PROPERTIES THEREOF AND
`COMPUTER PROGRAM
`
`FIELD
`
`[0001] The present invention relates to a method of deter
`mining properties, such as the numerical aperture and tele
`centricity, of the projection lens in a lithographic apparatus
`
`BACKGROUND
`
`[0002] A lithographic apparatus is a machine that applies
`a desired pattern onto a substrate, usually onto a target
`portion of the substrate. A lithographic apparatus can be
`used, for example, in the manufacture of integrated circuits
`(ICs). In that instance, a patterning device, Which is alter
`natively referred to as a mask or a reticle, may be used to
`generate a circuit pattern to be formed on an individual layer
`of the IC. This pattern can be transferred onto a target
`portion (e.g. comprising part of, one, or several dies) on a
`substrate (eg a silicon Wafer). Transfer of the pattern is
`typically via imaging onto a layer of radiation-sensitive
`material (resist) provided on the substrate. In general, a
`single substrate Will contain a netWork of adjacent target
`portions that are successively patterned. Known lithographic
`apparatus include so-called steppers, in Which each target
`portion is irradiated by exposing an entire pattern onto the
`target portion at one time, and so-called scanners, in Which
`each target portion is irradiated by scanning the pattern
`through a radiation beam in a given direction (the “scan
`ning”-direction) While synchronously scanning the substrate
`parallel or anti-parallel to this direction. It is also possible to
`transfer the pattern from the patterning device to the sub
`strate by imprinting the pattern onto the substrate.
`
`[0003] In a lithographic apparatus it is desirable to knoW
`characteristics of the projection lens used for imaging the
`pattern on the patterning device onto the substrate. Such
`characteristics may also be referred to as properties or
`parameters of the projection lens. One such property is the
`numerical aperture (NA) of the lens Which affects the
`imaging of the lithographic apparatus. Knowledge of the
`exact value of the numerical aperture can be used in simu
`lations to determine settings and process WindoWs for the
`lithographic apparatus. In some apparatus, the projection
`lens has an adjustable numerical aperture Which is de?ned
`by elements such as an adjustable diaphragm at a pupil plane
`in the projection lens system. Measurement of the actual
`numerical aperture setting is thus performed.
`[0004] Previously, the numerical aperture has been mea
`sured by imaging defocused pinholes on to a resist-coated
`substrate. The defocusing is performed by placing the pin
`holes on top of a mask (or using a mask upside doWn, such
`that the opaque layer de?ning the pinholes is displaced from
`the usual plane of the patterning device. Dilfractive features,
`such as gratings or arrays, are provided inside the pinholes
`so that the radiation ?lls the complete numerical aperture of
`the projection lens system. HoWever, this technique has the
`problem that extensive measurement analysis of the resist is
`necessary, Which is sloW, and the result is not a simple direct
`measurement performed on the apparatus.
`
`[0005] Another characteristic of a lithographic apparatus
`is the telecentricity of the projection lens and also of the
`illuminator Which provides the radiation beam for the pat
`
`teming device and projection lens. Non-telecentricity of the
`illuminator and projection lens can cause overlay problems.
`The non-telecentricity of the projection lens affects the
`imaging performance. Previously the telecentricity has been
`measured quantitatively by performing overlay measure
`ments at different focus levels of the substrate. HoWever, this
`method also suffers from the problems of being sloW and
`cumbersome, and the method is not very sensitive to tele
`centricity of the projection lens.
`
`SUMMARY OF THE INVENTION
`
`[0006] It is desirable to have knoWledge of the value of the
`numerical aperture of the projection lens system. It is also
`desirable to have information regarding the degree of non
`telecentricity of the projection lens system and also of the
`illuminator.
`
`[0007] According to one aspect of the present invention
`there is provided a lithographic apparatus arranged to project
`a patterned radiation beam from a patterning device onto a
`substrate using a projection system, and comprising:
`
`[0008] an interferometric sensor for measuring the Wave
`front of the radiation beam at the level of the substrate, the
`interferometric sensor having a detector;
`
`[0009] an actuator for displacing the interferometric sen
`sor in a direction along the optical axis;
`[0010] a ?rst module for determining the change of phase
`of the Wavefront at each of a plurality of locations on the
`detector of the interferometric sensor, the change of phase
`resulting from displacement of the interferometric sensor by
`the actuator betWeen a ?rst position and a second position;
`and
`
`[0011] a second module for determining, for each of the
`plurality of locations on the detector, the corresponding
`pupil location at the pupil plane of the projection system
`traversed by the radiation, using the change of phase deter
`mined by the ?rst module and the value of the displacement
`of the interferometric sensor by the actuator, to produce a
`mapping betWeen locations on the detector and correspond
`ing pupil locations.
`[0012] According to another aspect, the present invention
`provides a method for determining properties of a litho
`graphic apparatus, the lithographic apparatus comprising: an
`illumination system con?gured to condition a radiation
`beam;
`[0013] 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;
`[0014]
`a substrate table constructed to hold a substrate;
`[0015] a projection system con?gured to project the pat
`terned radiation beam onto a target portion of the substrate;
`and
`
`[0016] an interferometric sensor for measuring the Wave
`front of the radiation beam at the level of the substrate,
`[0017] Wherein the method comprises:
`[0018] determining the change of phase of the Wavefront
`at each of a plurality of locations on the detector of the
`interferometric sensor, the change of phase resulting from
`
`Nikon Exhibit 1025 Page 6
`
`

`

`US 2006/0103 826 Al
`
`May 18, 2006
`
`displacement of the interferometric sensor along the optical
`axis by an actuator betWeen a ?rst position and a second
`position; and
`[0019] calculating, for each of the plurality of locations on
`the detector, the corresponding pupil location at the pupil
`plane of the projection system traversed by the radiation,
`using the determined change of phase and the value of the
`displacement of the interferometric sensor by the actuator, to
`produce a mapping betWeen locations on the detector and
`corresponding pupil locations.
`[0020] A further aspect of the invention provides a com
`puter program comprising computer-executable code that
`When executed on a computer system instructs the computer
`system to control a lithographic apparatus to perform a
`method as de?ned above.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0021] Embodiments of the invention Will noW be
`described, by Way of example only, With reference to the
`accompanying schematic draWings in Which corresponding
`reference symbols indicate corresponding parts, and in
`Which:
`[0022] FIG. 1 depicts a lithographic apparatus according
`to an embodiment of the invention;
`
`[0023] FIG. 2 shoWs a schematic cross-section of the
`projection lens system during implementation of a method
`embodying the invention;
`[0024] FIG. 3 shoWs tWo schematic examples of interfer
`ence patterns betWeen Wavefronts sheared in orthogonal
`directions;
`[0025] FIG. 4 shoWs plots of phase change as a result of
`defocus across the image of the pupil for x-shearing and
`y-shearing;
`[0026] FIG. 5 schematically illustrates the procedure for
`extending the mapping betWeen detector pixels and points at
`the pupil for regions Which do not receive all three of the
`Zero order and +/— ?rst order di?‘racted beams;
`[0027] FIGS. 6(a) and (b) illustrate schematically the
`effect of defocus on telecentric and non-telecentric systems,
`respectively; and
`[0028] FIG. 7 depicts schematically the coordinate system
`obtained from the calibration procedure together With the
`full pupil of the projection system and the image of the
`intensity distribution from the illumination system, i.e. the
`illumination mode.
`
`DETAILED DESCRIPTION
`
`[0029] FIG. 1 schematically depicts a lithographic appa
`ratus according to one embodiment of the invention. The
`apparatus comprises:
`[0030] an illumination system (illuminator) IL con?gured
`to condition a radiation beam PB (e.g. UV radiation or EUV
`radiation).
`[0031] a support structure (eg a mask table) MT con
`structed to support a patterning device (e. g. a mask) MA and
`connected to a ?rst positioner PM con?gured to accurately
`position the patterning device in accordance With certain
`parameters;
`
`[0032] a substrate table (eg a Wafer table) WT con
`structed to hold a substrate (eg a resist-coated Wafer) W and
`connected to a second positioner PW con?gured to accu
`rately position the substrate in accordance With certain
`parameters; and
`[0033] a projection system (eg a refractive projection
`lens system) PS con?gured 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
`
`[0034] The illumination system may include various types
`of optical components, such as refractive, re?ective, mag
`netic, electromagnetic, electrostatic or other types of optical
`components, or any combination thereof, for directing,
`shaping, or controlling radiation.
`[0035] The support structure supports, i.e. bears the
`Weight of, the patterning device. It holds the patterning
`device in a manner that depends on the orientation of the
`patterning device, the design of the lithographic apparatus,
`and other conditions, such as for example Whether or not the
`patterning device is held in a vacuum environment. The
`support structure can use mechanical, vacuum, electrostatic
`or other clamping techniques to hold the patterning device.
`The support structure may be a frame or a table, for example,
`Which may be ?xed or movable as required. The support
`structure may ensure that the patterning device is at a desired
`position, for example With respect to the projection system.
`Any use of the terms “reticle” or “mask” herein may be
`considered synonymous With the more general term “pat
`terning device.”
`[0036] The term “patteming device” used herein should be
`broadly interpreted as referring to any device that can be
`used to impart a radiation beam With a pattern in its
`cross-section such as to create a pattern in a target portion of
`the substrate. It should be noted that the pattern imparted to
`the radiation beam may not exactly correspond to the desired
`pattern in the target portion of the substrate, for example if
`the pattern includes phase-shifting features or so called
`assist features. Generally, the pattern imparted to the radia
`tion beam Will correspond to a particular functional layer in
`a device being created in the target portion, such as an
`integrated circuit.
`[0037] The patterning device may be transmissive or
`re?ective. Examples of patterning devices include masks,
`programmable mirror arrays, and programmable LCD pan
`els. Masks are Well knoWn in lithography, and include mask
`types such as binary, alternating phase-shift, and attenuated
`phase-shift, as Well as various hybrid mask types. An
`example of a programmable mirror array employs a matrix
`arrangement of small mirrors, each of Which can be indi
`vidually tilted so as to re?ect an incoming radiation beam in
`different directions. The tilted mirrors impart a pattern in a
`radiation beam Which is re?ected by the mirror matrix.
`[0038] The term “projection system” used herein should
`be broadly interpreted as encompassing any type of projec
`tion system, including refractive, re?ective, catadioptric,
`magnetic, electromagnetic and electrostatic optical systems,
`or any combination thereof, as appropriate for the exposure
`radiation being used, or for other factors such as the use of
`an immersion liquid or the use of a vacuum. Any use of the
`term “projection lens” herein may be considered as synony
`mous With the more general term “projection system”.
`
`Nikon Exhibit 1025 Page 7
`
`

`

`US 2006/0103 826 A1
`
`May 18, 2006
`
`[0039] As here depicted, the apparatus is of a transmissive
`type (e. g. employing a transmissive mask). Alternatively, the
`apparatus may be of a re?ective type (eg employing a
`programmable mirror array of a type as referred to above, or
`employing a re?ective mask).
`[0040] The lithographic apparatus may be of a type having
`tWo (dual stage) or more substrate tables (and/or tWo or more
`mask tables). In such “multiple stage” machines the addi
`tional tables may be used in parallel, or preparatory steps
`may be carried out on one or more tables While one or more
`other tables are being used for exposure.
`
`[0041] The lithographic apparatus may also 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 ?ll a space betWeen the projection system and the
`substrate. An immersion liquid may also be applied to other
`spaces in the lithographic apparatus, for example, betWeen
`the mask and the projection system. Immersion techniques
`are Well knoWn in the art for increasing the numerical
`aperture of projection systems. The term “immersion” as
`used herein does not mean that a structure, such as a
`substrate, must be submerged in liquid, but rather only
`means that liquid is located betWeen the projection system
`and the substrate during exposure.
`
`[0042] Referring to FIG. 1, the illuminator IL receives a
`radiation beam from a radiation source S0. The source and
`the lithographic apparatus may be separate entities, for
`example When the source is an excimer laser. In such cases,
`the source is not considered to form part of the lithographic
`apparatus and the radiation beam is passed from the source
`S0 to the illuminator IL With the aid of a beam delivery
`system BD comprising, for example, suitable directing mir
`rors and/or a beam expander. In other cases the source may
`be an integral part of the lithographic apparatus, for example
`When the source is a mercury lamp. The source SO and the
`illuminator IL, together With the beam delivery system BD
`if required, may be referred to as a radiation system.
`
`[0043] The illuminator IL may comprise an adjuster AD
`for adjusting the angular intensity distribution of the radia
`tion beam. Generally, at least the outer and/or inner radial
`extent (commonly referred to as o-outer and o-inner, respec
`tively) of the intensity distribution in a pupil plane of the
`illuminator can be adjusted. In addition, the illuminator IL
`may comprise various other components, such as an inte
`grator IN and a condenser CO. The illuminator may be used
`to condition the radiation beam, to have a desired uniformity
`and intensity distribution in its cross-section.
`
`[0044] The radiation beam B is incident on the patterning
`device (e.g., mask MA), Which is held on the support
`structure (e.g., mask table MT), and is patterned by the
`patterning device. Having traversed the mask MA, the
`radiation beam B passes through the projection system PS,
`Which focuses the beam onto a target portion C of the
`substrate W. With the aid of the second positioner PW and
`position sensor IF (eg an interferometric device, linear
`encoder or capacitive sensor), the substrate table WT can be
`moved accurately, e.g. so as to position different target
`portions C in the path of the radiation beam B. Similarly, the
`?rst positioner PM and another position sensor (Which is not
`explicitly depicted in FIG. 1) can be used to accurately
`position the mask MA With respect to the path of the
`radiation beam B, eg after mechanical retrieval from a
`
`mask library, or during a scan. In general, movement of the
`mask table MT may be realiZed With the aid of a long-stroke
`module (coarse positioning) and a short-stroke module (?ne
`positioning), Which form part of the ?rst positioner PM.
`Similarly, movement of the substrate table WT may be
`realiZed using a long-stroke module and a short-stroke
`module, Which form part of the second positioner PW. In the
`case of a stepper (as opposed to a scanner) the mask table
`MT may be connected to a short-stroke actuator only, or may
`be ?xed. Mask MA and substrate W may be aligned using
`mask alignment marks M1, M2 and substrate alignment
`marks P1, P2. Although the substrate alignment marks as
`illustrated occupy dedicated target portions, they may be
`located in spaces betWeen target portions (these are knoWn
`as scribe-lane alignment marks). Similarly, in situations in
`Which more than one die is provided on the mask MA, the
`mask alignment marks may be located betWeen the dies.
`
`[0045] The depicted apparatus could be used in at least
`one of the folloWing modes:
`
`[0046] 1. In step mode, the mask table MT and the
`substrate table WT are kept essentially stationary, While an
`entire pattern imparted to the radiation beam is projected
`onto a target portion C at one time (i.e. a single static
`exposure). The substrate table WT is then shifted in the X
`and/or Y direction so that a different target portion C can be
`exposed. In step mode, the maximum siZe of the exposure
`?eld limits the siZe of the target portion C imaged in a single
`static exposure.
`
`[0047] 2. In scan mode, the mask table MT and the
`substrate table WT are scanned synchronously While a
`pattern imparted to the radiation beam is projected onto a
`target portion C (i.e. a single dynamic exposure). The
`velocity and direction of the substrate table WT relative to
`the mask table MT may be determined by the (de-)magni
`?cation and image reversal characteristics of the projection
`system PS. In scan mode, the maximum siZe of the exposure
`?eld limits the Width (in the non-scanning direction) of the
`target portion in a single dynamic exposure, Whereas the
`length of the scanning motion determines the height (in the
`scanning direction) of the target portion.
`[0048] 3. In another mode, the mask table MT is kept
`essentially stationary holding a programmable patterning
`device, and the substrate table WT is moved or scanned
`While a pattern imparted to the radiation beam is projected
`onto a target portion C. In this mode, generally a pulsed
`radiation source is employed and the programmable pattem
`ing device is updated as required after each movement of the
`substrate table WT or in betWeen successive radiation pulses
`during a scan. This mode of operation can be readily applied
`to maskless lithography that utiliZes programmable pattem
`ing device, such as a programmable mirror array of a type
`as referred to above.
`
`[0049] Combinations and/or variations on the above
`described modes of use or entirely different modes of use
`may also be employed.
`
`[0050] Previously there has been proposed a measurement
`system for measuring Wave front aberrations of a projection
`lens using the principal knoWn as a “shearing interferom
`eter”. According to this proposal, different portions of the
`projection beam from a particular location at the level of the
`patterning device travel along different paths through the
`
`Nikon Exhibit 1025 Page 8
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`

`

`US 2006/0103 826 Al
`
`May 18, 2006
`
`projection lens. This can be achieved by a dilfractive ele
`ment located in the projection beam between the illumina
`tion system and the projection system. The dilfractive ele
`ment, such as a grating, also knoWn as the object grating,
`dilfracts the radiation and spreads it out such that it passes
`through the projection system along a plurality of different
`paths. The dilfractive element is typically located at the level
`at Which the patterning device, eg mask MA is located. The
`dilfractive element can be a grating or can be an array of
`features of suitable siZe, and may be provided Within a
`pinhole. One or more lenses may also be associated With the
`dilfractive element. This assembly as a Whole, located in the
`projection beam betWeen the illuminator and the projection
`system Will be referred to hereafter as the source module.
`
`[0051] Referring to FIG. 2, a source module SM for use
`With an embodiment of the present invention is illustrated.
`It comprises a pinhole plate PP Which is a quartz glass plate
`With an opaque chromium layer on one side, same as a
`reticle, and With a pinhole PH provided in the chromium
`layer. It also comprises a lens SL for focusing the projection
`radiation on to the pinhole. In practice an array of pinholes
`and lenses for different ?eld positions and different slit
`positions are provided, and the lenses can be integrated on
`top of the pinhole plate. The source module should ideally
`generate light Within a Wide range of angles such that the
`pupil of the projection lens is ?lled, or indeed over?lled, for
`numerical aperture measurements, and preferably the pupil
`?lling should be uniform. The use of the lens SL can achieve
`the over-?lling and also increases the light intensity. The
`pinhole PH limits the light to a speci?c location Within the
`?eld. Alternative Ways to obtain uniform pupil ?lling are to
`use a di?‘usor plate (such as an etched ground glass plate) on
`top of the pinhole plate, or an array of microlenses (similar
`to a dilfractive optical element DOE), or a holographic
`dilfusor (similar to a phase-shift mask PSM). The pinhole
`may have some structuring Within, such as sub-resolution
`dilfractive features eg grating patterns, checkerboard pat
`terns, but this is optional, and preferably there is no struc
`turing for embodiments of the invention. Thus speci?c
`source module SM embodiments include; as illustrated in
`FIG. 2; as illustrated but Without the focusing lens and With
`a di?‘usor on top; and a pinhole mask With sub-resolution
`features inside.
`
`[0052] Radiation that has traversed the projection system
`then impinges on a further dilfractive element GR, such as
`a pinhole or a grating, knoWn as the image grating. Referring
`to FIG. 2, the further dilfractive element GR is mounted on
`a carrier plate CP, for example made of quartz. This further
`dilfractive element acts as the “shearing mechanism” that
`generates different dilfractive orders Which can be made to
`interfere With each other. For example, the Zero order may
`be made to interfere With the ?rst order. This interference
`results in a pattern, Which can be detected by a detector to
`reveal information on the Wave front aberration at a par
`ticular location in the image ?eld. The detector DT can be,
`for example, a CCD or CMOS camera Which captures the
`image of the pattern electronically Without using a resist.
`The further diffractive element GR and the detector DT Will
`be referred to as the interferometric sensor IS. Convention
`ally, the further dilfractive element GR is located at the level
`of the substrate at the plane of best focus, such that it is at
`a conjugate plane With respect to the ?rst-mentioned dif
`
`fractive element in the source module SM. The detector DT
`is beloW the further diffractive element GR and spaced apart
`from it.
`
`[0053] One proprietary form of an interferometric Wave
`front measurement system implemented on lithography
`tools is knoW as ILIAS (trademark) Which is an acronym for
`Integrated Lens Interferometer At Scanner. This measure
`ment system is routinely provided on lithographic projection
`apparatus.
`[0054] The interferometric sensor essentially measures the
`derivative phase of the Wave front. The detector itself can
`only measure light intensity, but by using interference the
`phase can be converted to intensity. Most interferometers
`require a secondary reference beam to create an interference
`pattern, but this Would be hard to implement in a litho
`graphic projection apparatus. HoWever a class of interfer
`ometer Which does not have this requirement is the shearing
`interferometer. In the case of lateral shearing, interference
`occurs betWeen the Wavefront and a laterally displaced
`(sheared) copy of the original Wavefront. In the present
`embodiment, the further dilfractive element GR splits the
`Wavefront into multiple Wavefronts Which are slightly dis
`placed (sheared) With respect to each other. Interference is
`observed betWeen them. In the present case only the Zero
`and +/— ?rst diffraction orders are considered. The intensity
`of the interference pattern relates to the phase difference
`betWeen the Zero and ?rst diffraction orders. It can be shoWn
`that the intensity I is given by the folloWing approximate
`relation:
`
`Where EO and El are the diffraction e?iciencies for the Zero
`and ?rst diffracted orders, k is the phase stepping distance,
`p is the grating periodicity (in units of Waves), W is the
`Wavefront aberration (in units of Waves) and p is the pupil
`location. In the case of small shearing distances, the Wave
`front phase difference approximates the Wavefront deriva
`tive. By performing successive intensity measurements,
`With a slight displacement of the source module SM With
`respect to the interferometric sensor IS, the detected radia
`tion intensity is modulated (the phase stepping factor k/p in
`the above equation is varied). The ?rst harmonics (With the
`period of the grating as the fundamental frequency) of the
`modulated signal correspond to the diffraction orders of
`interest (0 & +/—l). The phase distribution (as a function of
`pupil location) corresponds to the Wavefront difference of
`interest. By shearing in tWo substantially perpendicular
`directions, the Wavefront difference in tWo directions is
`considered. FIG. 3 gives tWo examples of interference
`patterns betWeen Wavefronts sheared in orthogonal direc
`tions. The detector effectively sees an image of the pupil
`plane of the projection lens; the image of the pupil is
`indicated by the dashed circles in FIG. 3.
`
`[0055] HoWever, there is the problem of hoW to relate a
`particular pixel of the detector to a particular coordinate at
`the pupil plane. Factors which affect this include: the 6
`degrees of freedom in the relative position of the detector
`DT With respect to the grating GR (translations along three
`axes x, y, Z, and rotations about 3 axes Rx, Ry and R2); the
`
`Nikon Exhibit 1025 Page 9
`
`

`

`US 2006/0103 826 Al
`
`May 18, 2006
`
`thickness of the carrier plate CP; the degree of un?atness of
`the detector DT; and the Wedge angle and un?atness of the
`carrier plate CP. It is di?icult to achieve precise mechanical
`speci?cations in the tolerance and alignment of these com
`ponents.
`[0056] One solution to this problem is to apply a calibra
`tion step. The pupil coordinate corresponding to each detec
`tor pixel is calibrated by measuring sheared Wavefronts at
`tWo different focus positions. The different focus positions
`are separated by a displacement of the interferometric sensor
`by an amount )Z, as shoWn in FIG. 2. One of the positions
`could be the plane of best focus, and the other position
`defocused by )Z, but it is not essential that one of the
`positions be the plane of best focus. The shift )Z introduces
`a knoWn amount of aberration, and leads to a unique phase
`change for each pupil position. This is illustrated schemati
`cally in FIG. 4 Which shoWs plots of phase change as a result
`of defocus )Z across the image of the pupil for x-shearing
`and y-shearing. A contour of points With phase change of
`+0.3 for x and a contour of points With phase change of —0.5
`for y are shoWn. The intersection of these contours identi?es
`a pixel With this unique phase change. When the interfero
`metric sensor is moved along the optical axis, i.e. in the Z
`direction in FIGS. 1 and 2, then the derivative phase Will
`change. The exact amount of phase change depends on the
`displacement )Z, and the point in the pupil PU at location p
`traversed by the radiation from source module SM, as
`illustrated in FIG. 2. Therefore for a knoWn displacement or
`defocus )Z and a measured change in phase at a particular
`point on the detector, it is possible to determine the pupil
`point p traversed by the radiation.
`
`[0057] In more detail, sheared Wavefronts upon defocus
`are given by:
`
`Where WX and WY are the Wavefronts sheared in the x and
`y directions respectively, measured upon defocus )Z, at a
`given pixel position k,l as a function of the pupil coordinates
`u.v and With a shear distance s. In these expressions the pupil
`coordinates u,v relate to the sine of the angles Within the tWo
`perpendicular shearing directions, u=sin(theta) and v=sin
`(phi), Where theta and phi are angles With respect to the
`Z-axis and orthogonal With respect to each other, usually in
`the x and y directions. (These coordinates could be norma
`lised to be 1 at the edge of the pupil by dividing by the
`numerical aperture NA of the projection lens, i.e. to give
`pupil position coordinates p). The pair of equations above
`comprise tWo knoWn quantities ( )Z and s), tWo measure
`ments (WX and WY), and tWo unknowns (u, v). For each
`detector pixel one can invert the above pair of relations to
`obtain expressions for the pupil coordinates of that pixel in
`terms of the other quantities. The calculation can be done
`either analytically or numerically for each Adetector pixel.
`Each pixel is given a pupil