Europaisches
`
`Patentamt
`European
`Patent Office
`Office européen
`des brevets
`
`Bescheinigung
`
`Certificate
`
`Attestation
`
`Die angehefteten
`Unterlagen stimmen mit der
`als urspringlich eingereicht
`geltenden Fassung der auf
`dem nachsten Blatt
`bezeichneten europaischen
`Patentanmeldung Uberein.
`
`The attached documents are
`exact copies of the text in
`which the European patent
`application described on the
`following page is deemed to
`have been filed.
`
`Les documentsjoints a la
`présente attestation sont
`conformesautexte,
`considéré comme
`initialement déposé, de la
`demande de brevet
`européen qui est spécifiée a
`la page suivante.
`
`
`
`Patentanmeldung Nr.
`
`Patent application No.
`
`Demande de brevet n°
`
`18199182.9 / EP18199182
`
`The organisation code and number of your priority application, to be used for filing abroad under the Paris
`Convention, is EP18199182.
`
`Der Prasident des Europaischen Patentamts;
`Im Auftrag
`For the President of the European Patent Office
`Le President de l'Office européen des brevets
`p.o.
`
`EPA/EPO/OEB Form 1014
`
`05.17
`
`V. Joseph
`
`

`

`
`
`Anmeldung Nr:
`Application no.:
`Demande no :
`
`18199182.9
`
`Anmelder/ Applicant(s) / Demandeur(s):
`
`ASML Netherlands B.V.
`P.O. Box 324
`5500 AH Veldhoven/NL
`
`Anmeldetag:
`Date offiling:
`Date de dépét :
`
`08.10.18
`
`Bezeichnung der Erfindung / Title of the invention / Titre de I'invention:
`(Falls die Bezeichnung der Erfindung nicht angegeben ist, oder falls die Anmeldung in einer Nicht-Amtssprache des EPA eingereicht
`wurde, siehe Beschreibung beztglich urspriinglicher Bezeichnung.
`If no title is shown, orif the application has been filed in a non-EPO language, please refer to the description for the originaltitle.
`Si aucun titre n'est indiqué, ou si la demande a été déposée dans une langue autre qu'une langue officielle de l'OEB, se référer ala
`description pour le titre original.)
`
`METROLOGY METHOD, PATTERNING DEVICE, APPARATUS AND COMPUTER PROGRAM
`
`In Anspruch genommenePrioritat(en) / Priority(Priorities) claimed / Priorité(s) revendiquée(s)
`Staat/Tag/Aktenzeichen / State/Date/File no. / Pays/Date/Numéro de dépét:
`
`Am Anmeldetag benannte Vertragstaaten / Contracting States designated at date offiling / Etats contractants désignés lors du dépét:
`
`AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LILT LU LV MC MK MT NL NO PL
`PT RO R&SSE SI SK SMTR
`
`EPA/EPO/OEB Form 1014
`
`05.17
`
`2
`
`

`

`2018PO0159EP
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`-1-
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`METROLOGY METHOD, PATTERNING DEVICE, APPARATUS AND
`
`COMPUTER PROGRAM
`
`BACKGROUND
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`Field of the Invention
`
`[0001]
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`The present invention relates to methods and apparatus for metrology usable, for
`
`example, in the manufacture of devices by lithographic techniques and to methods of
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`manufacturing devices using lithographic techniques. The invention further relates to
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`patterning devices and computer program products usable in such methods.
`
`Background Art
`
`[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 intcgrated circuits (ICs). In that instance, a patterning
`
`device, which is alternatively referred to as a mask or a reticle, may be used to generate
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`a circuit pattern to be formed on an individual layer of the IC. This pattern can be
`
`transferred onto a target portion (c.g., including part of, one, or sevcral dics) on a
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`substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a
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`layer of radiauion-sensilive material (resist) provided on the substrate. In general, a
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`single substrate will contain a network of adjacent target portions that are successively
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`patterned. In lithographic processes, itis desirable frequently to make measurements of
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`the structures created, e.g., for process control and verification. Various tools for
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`making such measurements are known,
`
`including scanning electron microscopes,
`
`which are often used to measure crilical dimension (CD), and specialized tools to
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`measure overlay, a measure of the accuracy of alignment of two layers in a device.
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`Overlay may be described in terms of the degree of misalignment between the two
`
`layers, for example reference to a measured overlay of [nm may describe a situation
`
`where two layers are misaligned by Inm.
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`[0003]
`
`Recently, various forms of scatterometers have been developed for use in the
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`lithographic field. These devices direct a beam of radiation onto a target and measure
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`one or more properties of the scattered radiation — e.g., intensity at a single angle of
`
`reflection as a function of wavelength; intensity at one or more wavelengths as a
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`2018PO0159EP
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`-2-
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`function of reflected angle; or polarization as a function of reflected angle — to obtain a
`
`“spectrum” from which a property of interest of the target can be determined.
`
`Determination of the property of interest may be performed by various techniques: e.g.,
`
`reconstruction of the target by iterative approaches such as rigorous coupled wave
`
`analysis or finite clement methods; library scarches; and principal componentanalysis.
`
`[0004] The targets used by conventional scatterometers are relatively large, e.g., 40um by
`
`40um, gratings and the measurement beam generates a spot that is smaller than the
`
`grating (i.e., the grating is underfilled). This simplifies mathematical reconstruction of
`
`the target as it can be regardedasinfinite. However, in order to reduce the size of the
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`targets, e.g.,
`
`to 1Oum by 10umor less, e.g., so they can be positioned in amongst
`
`product features, rather than in the scribe lane, metrology has been proposed in which
`
`the grating is made smaller than the measurementspot (i.e., the grating is overfilled).
`
`Typically such targets are measured using dark field scatterometry in which the zeroth
`
`orderof diffraction (corresponding to a specular reflection) is blocked, and only higher
`
`orders processed. Examples of dark field metrology can be foundin international patent
`
`applications WO 2009/078708 and WO 2009/106279 which documents are hereby
`
`incorporated by reference in their entirety. Further developments of the technique have
`
`been described in patent publications US20110027704A, US20110043791A and
`
`US20120242970A. Modifications of the apparatus to improve throughput are described
`
`in US2010201963A 1 and US2011102753A1. The contents of all these applications are
`
`also incorporated herein by reference. Diffraction-based overlay using dark-field
`
`detection of the diffraction orders enables overlay measurements on smaller targets.
`
`These targets can be smaller than the Ulumination spot and may be surrounded by
`
`product structures on a wafer. ‘largets can comprise multiple gratings which can be
`
`measured in one image.
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`[0005] In the known metrology technique, overlay measurement results are obtained by
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`measuring an overlay target twice under certain conditions, while cither rotating the
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`overlay target or changing the illumination mode or imaging mode to obtain separately
`
`the -1%t and the +1* diffraction order intensities. The intensily asymmetry, a comparison
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`30
`
`of these diffraction order intensities, for a given overlay target provides a measurement
`
`of asymmetry in the target. This asymmetry in the overlay target can be used as an
`
`indicator of overlay (undesired misalignment of twolayers).
`
`[0006] In the known method using four distinct sub-targets, a certain portion of the patterned
`
`arca is not usable duc to edge cffects. In semiconductor product designs the efficient
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`2018PO0159EP
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`-3-
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`use of space is very important. The use of only two specific offsets enforces the above
`
`assumption of linearity, which may lead to inaccuracy whenthetrue relationship is non-
`
`linear. To increase the numberof offsets in the known designs used would increase the
`
`space used.
`
`SUMMARYOF THE INVENTION
`
`[0007] It would be desirable to be able to perform metrology of overlay or other performance
`
`parameters with increased accuracy, and/or with less space usedforthe targets.
`
`[0008] The invention in a first aspect provides a method of measuring a performance parameter
`
`of a lithographic process, as defined in appendedclaim1.
`
`[0009] The invention in a second aspect further provides a patterning device for use in a
`
`lithographic apparatus, the patterning device comprising portions that define one or
`
`more device patterns and portions that define one or more metrology patterns, the
`
`metrology patterns including at least one target for use in a methodof the first aspect
`
`of the invention as set forth above,the target having a bias variation between locations
`
`on the target, said bias variation being in an asymmetry-related property.
`
`[0010] The invention in a further provides a metrology apparatus comprising: an illumination
`
`system configured to illuminate with radiation a target; a detection system configured
`
`to detect scattered radiation arising from illumination of the target; wherein said
`
`metrology apparatus is operable to perform the method of the first aspect of the
`
`invention as set forth above.
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`[0011] The invention further provides a computer program comprising processor readable
`
`instructions which, when run on suitable processor controlled apparatus, cause the
`
`processor controlled apparatus to perform the methodofthefirst aspect, and a computer
`
`program carrier comprising such a computer program.
`
`[0012] Further features and advantages of the invention, as well as the structure and operation
`
`of various embodiments of the invention, are described in detail below with reference
`
`to the accompanying drawings.
`
`It is noted that the invention is not limited to the
`
`specific embodiments described herein. Such embodiments are presented herein for
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`30
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`illustrative purposes only. Additional embodiments will be apparent to persons skilled
`
`in the relevantart(s) based on the teachings contained herein.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
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`2018PO0159EP
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`[0013] Embodiments of the invention will now be described, by way of example only, with
`
`reference to the accompanying drawings in which:
`
`Figure 1 depicts a lithographic apparatus according to an embodiment of the
`
`invention;
`
`Figure 2 depicts a lithographic cell or cluster according to an embodimentofthe
`
`invention;
`
`Tigure 3 comprises (a) a schematic diagram of a dark field scatterometer for use
`
`in measuring targets using a first pair of illumination apertures, (b) a detail of diffraction
`
`spectrum of a target grating for a given direction of illumination (c) a second pair of
`
`illumination apertures providing further illumination modesin using the scallerometer
`
`for diffraction based overlay measurements and (d) a third pair of illumination apertures
`
`combining the first and second pair of apertures;
`
`Figure 4 depicts a known form of multiple grating target and an outline of a
`
`measurement spot on a substrate;
`
`Figure 5 depicts an imageof the target of Figure 4 obtainedin the scatterometer
`
`of Figure 3;
`
`Figure 6 depicts a first example of a multiple grating target including continuous
`
`bias features according to an aspect of the present disclosure;
`
`Figure 7 depicts an image of the target of Figure 6 obtained in the scatterometer
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`of Figure 3;
`
`Figure 8 shows in schematic detail the implementation of continuousbias in one
`
`grating of the target of Figure 6 under conditions of (a) zero overlay and (b) non-zero
`
`overlay, according to one embodiment of the present disclosure;
`
`Figure 9 showsin schematic detail the arrangement of continuous bias gratings
`
`in the multiple grating target of Figure 6, according to one embodimentof the present
`
`disclosure;
`
`Figure 10 depicts a second cxample of a modilicd multiple grating target
`
`including continuous bias features according to an aspect of the present disclosure;
`
`Figure 11 depicts an image of the target of Figure 10 obtained in the
`
`30
`
`scatterometer of Figure 3;
`
`Figure 12 (a) shows in schematic detail the implementation of continuous bias
`
`in one grating of the target of Figure 10, while (b) showsthe variation of bias with
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`position in such a grating;
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`Figure 13 shows in schematic detail the implementation of continuous bias in
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`2018PO0159EP
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`-5-
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`two gratings of the multi-grating target of Figure 10 under conditionsof (a) zero overlay
`
`and (b) non-zero overlay, according to one embodimentof the present disclosure;
`
`Figure 14 is a flowchart showing the steps of an overlay measurement method
`
`using the scatterometer of Figure 3;
`
`Figure 15 illustrates (a) signal processing in relation to the first one of the
`
`gratings shown in Figure 13 and (b) signal processing in relation to the other of the
`
`gratings shown in Figure 13,
`
`including graphical illustration of the principles of
`
`calculation of overlay error in one embodimentofthe present disclosure;
`
`Figure 16 shows in schematic detail implementation of continuous bias in two
`
`gratings of a modified mulli-grating target, including the provision of anchor points
`
`according to another embodimentof the present disclosure;
`
`Figure 17 (a) illustrates the inclusion of bias slope changes as an example of
`
`anchor points in the multi-grating target of Figure 16, (b) asymmetry signals obtained
`
`from the gratings shown In Tigure 16 under conditions of non-zero overlay, and (c)
`
`correction of the asymmetry signals using knowledge of the anchor points;
`
`Figure 18 illustrates an example of a grating having multi-step bias as an
`
`alternative to continuous bias, according to another example of the present disclosure;
`
`Figure 19 illustrates a multi-grating target having dual bias gratings according
`
`to another example of the present disclosure;
`
`Figure 20 illustrates an alternative grating target having overlay bias in two
`
`directions, based on L-shaped features;
`
`Figure 21 illustrates a modified version of the grating of Figure 20, modified to
`
`include a multi-step arrangementof bias regions;
`
`Figure 22 illustrates another modified version of the grating of Figure 20,
`
`modified to include continuous bias by rotation of L -shaped features, according to yet
`
`another embodimentof the present disclosure; and
`
`Figures 23 and 24 illustrate targets having multi-step arrangement ofbias
`
`regions in four quadrants, based on the arrangementof Figure 21.
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`DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
`
`[0014] Before describing embodiments of the invention in detail, it is instructive to present an
`
`example environment
`
`in which embodiments of the present
`
`invenuion may be
`
`implemented.
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`2018PO0159EP
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`- 6-
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`[0015] Figure 1 schematically depicts a lithographic apparatus LA. The apparatus includes an
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`illumination optical system (illuminator) IL configured to condition a radiation beam B
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`(e.g., UV radiation or DUV radiation), a patterning device support or support structure
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`(e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA
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`and connected to a first positioncr PM configured to accurately position the patterning
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`device in accordance with certain parameters; a substrate table (e.g., a wafer table) WT
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`constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second
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`positioner PW configured to accurately position the substrate in accordancewith certain
`
`parameters; and a projection optical system(e.g., a refractive projection lens system)
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`PS configured to project a pattern imparted to the radiaiion beamB by patterning device
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`MAonto a target portion C (e.g., including one or more dies) of the substrate W.
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`[0016] The illumination optical system may include various types of optical or non-optical
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`components, such asrefractive, reflective, magnetic, electromagnetic, electrostatic or
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`other types of components, or any combination thereof, for directing, shaping, or
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`15
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`controlling radiation.
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`[0017] The patterning device support holds the patterning device in a manner that depends on
`
`the orientation of the patterning device, the design of the lithographic apparatus, and
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`other conditions, such as for example whether or not the patterning device is held in a
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`vacuum environment. The patterning device support can use mechanical, vacuum,
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`20
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`electrostatic or other clamping techniques to hold the patterning device. The patterning
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`device support may be a frame or a table, for example, which may befixed or movable
`
`as required. The patterning device support may ensure that the patterning deviceis at
`
`a desired position, for example with respect to the projection system. Any use of the
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`terms“reticle” or “mask” herein may be considered synonymouswith the more general
`
`term “patterning device.”
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`[0018] The term “patterning device” used herein should be broadly interpreted as referring to
`
`any device that can be used to impart a radiation beamwith a paticrn in ils cross-section
`
`suchas to create a pattern in a target portion of the substrate. It should be notedthat the
`
`pattern imparted to the radiation beam may not exactly correspond to the desired pattern
`
`30
`
`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 radiation
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`beam will correspond to a particular functional layer in a device being created in the
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`target portion, such as an integrated circuit.
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`[0019] The patterning device may be transmissive or reflective. Examples of patterning
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`devices include masks, programmable mirror arrays, and programmable LCD panels.
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`Masks are well known in lithography, and include mask types such as binary,
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`alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types.
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`An example of a programmable mirror array cmploys a matrix arrangement of small
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`mirrors, each of which can be individually tilted so as to reflect an incoming radiation
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`beam in different directions. The tilted mirrors impart a pattern in a radiation beam,
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`whichis reflected by the mirror matrix.
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`[0020] As here depicted, the apparatusis of a transmissive type (e.g., employing a transmissive
`
`mask). Alternatively,
`
`the apparatus may be of a reflective type (e.g., employing a
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`programmable mirror array of a type as referred to above, or employing a reflective
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`mask).
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`[0021] The lithographic apparatus may also be of a type wherein at least a portion of the
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`substrate may be covered by a liquid having a relatively high refractive index, e.g.,
`
`water, so as to fill a space between the projection system and the substrate. An
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`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
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`knownin the art for increasing the numerical aperture of projection systems. The term
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`“immersion” as used herein does not mean that a structure, such as a substrate, must be
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`submergedin liquid, but rather only means that liquid is located between the projection
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`system and the substrate during exposure.
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`[0022] Referring to Figure 1, the illuminator IL receives a radiation beam from a radiation
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`source SO. The source and the lithographic apparatus may be separate entities, for
`
`example whenthe source is an excimerlaser. In such cases, the source is not considered
`
`to form part of the lithographic apparatus and the radiation beam is passed from the
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`source SO to the illuminator IL with the aid of a beamdelivery system BD including,
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`for cxample, suitable directing mirrors and/or a beam cxpander. In other cases the
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`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
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`delivery system BD if required, may be referred to as a radiation system.
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`[0023] The illuminator IL may include an adjuster AD for adjusting the angular intensity
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`distribution of the radiation beam. Generally, at least the outer and/or inner radial extent
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`(commonly referred to as o-outer and o-inner, respectively) of the intensity distribution
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`in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may
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`2018PO0159EP
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`-8-
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`include various other components, such as an integrator IN and a condenser CO. The
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`illuminator may be used to condition the radiation beam, to have a desired uniformity
`
`and intensity distribution in its cross section.
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`[0024] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is
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`held on the patterning device support (c.g., mask table M'l), and is patterned by the
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`patterning device. Having traversed the patterning device (e.g., mask) MA,
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`the
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`radiation beam B passes through the projection optical system PS, which focuses the
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`beam onto a target portion C of the substrate W, thereby projecting an image of the
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`pattern on the target portion C. With the aid of the second positioner PW and position
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`sensor IF (e.g., an interferometric device, linear encoder, 2-D encoder or capacitive
`
`sensor), the substrate table WT can be movedaccurately, e.g., so as to position different
`
`target portions C in the path of the radiation beamB. Similarly, the first positioner PM
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`and another position sensor (which is not explicitly depicted in Figure 1) can be used
`
`to accurately position the patterning device (e.g., mask) MA with respectto the path of
`
`the radiation beamB, e.g., after mechanical retrieval from a mask library, or during a
`scan.
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`[0025] Patterning device (e.g., mask) MA and substrate W may be aligned using mask
`
`alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate
`
`alignment marksasillustrated 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 onedie is provided on the patterning device
`
`(e.g., mask) MA, the mask alignment marks may be located between the dies. Small
`
`alignment markers may also be included within dies, in amongst the device features, in
`
`which case it is desirable that the markers be as small as possible and not require any
`
`different imaging or process conditions than adjacent features. The alignment system,
`
`which detects the alignment markers is described further below.
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`[0026] Lithographic apparatus LA in this example is of a so-called dual stage type which has
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`two substrate tables WTa, WTb and two stations — an exposure station and a
`
`measurement station — between which the substrate tables can be exchanged. While
`
`30
`
`one substrate on one substrate table is being exposed at the exposure station, another
`
`substrate can be loaded onto the other substrate table at the measurement station and
`
`various preparatory steps carried oul. ‘he preparatory steps may include mapping the
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`surface control of the substrate using a level sensor LS and measuring the position of
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`2018PO0159EP
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`alignment markers on the substrate using an alignment sensor AS. This enables a
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`substantial increase in the throughput of the apparatus.
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`[0027] The depicted apparatus can be used in a variety of modes, including for example a step
`
`mode or a scan mode. The construction and operation of lithographic apparatus is well
`
`knownto those skilled in the art and necd not be described further for an understanding
`
`of the present invention.
`
`[0028] As shown in l'igure 2, the lithographic apparatus LA forms part of a lithographic
`
`system, referred to as a lithographic cell LC or a lithocell or cluster. The lithographic
`
`cell LC may also include apparatus to perform pre- and post-exposure processes on a
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`substrate. Conventionally these include spin coaters SC to deposit resist layers,
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`developers DE to develop exposed resist, chill plates CII and bake plates BK. A
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`substrate handler, or robot, RO picks up substrates from input/output ports I/O1, 1/O2,
`
`moves them between the different process apparatus and delivers then to the loading
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`bay LB of the lithographic apparatus. These devices, which are often collectively
`
`referred to as the track, are under the control of a track control unit TCU whichis itself
`
`controlled by the supervisory control system SCS, which also controls the lithographic
`
`apparatus via lithography control unit LACU. Thus, the different apparatus can be
`
`operated to maximize throughput and processing efficiency.
`
`[0029] In order that the substrates that are exposed by the lithographic apparatus are exposed
`
`correctly and consistently, it is desirable to inspect exposed substrates to measure
`
`properties such as overlay errors between subsequent layers, line thicknesses, critical
`
`dimensions (CD), etc. Accordingly a manufacturing facility in which lithocell LC is
`
`located also includes metrology system MET which receives some or all of the
`
`substrates W that have been processedin the lithocell. Metrology results are provided
`
`directly or indirectly to the supervisory control system SCS. If errors are detected,
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`adjusuments may be made to exposures of subsequent substrates, especially if the
`
`inspection can be done soon and fast cnough that other substrates of the same batch are
`
`still to be exposed. Also, already exposed substrates may be stripped and reworkedto
`
`improve yield, or discarded,
`
`thereby avoiding performing further processing on
`
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`substrates that are known to be faulty.
`
`In a case where only some target portions of a
`
`substrate are faulty, further exposures can be performed only on those target portions
`
`which are good.
`
`[0030] Within metrology system MET, an inspection apparatus is used to determine the
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`propertics of the substrates, and in particular, how the properties of different substrates
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`2018PO0159EP
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`or different layers of the same substrate vary from layer to layer. The inspection
`
`apparatus may be integrated into the lithographic apparatus LA or the lithocell LC or
`
`may be a stand-alone device. To enable most rapid measurements, it is desirable that
`
`the inspection apparatus measure properties in the exposed resist layer immediately
`
`after the exposure. However, the latent image in the resist has a very low contrast —
`
`there is only a very small difference in refractive index between the parts of the resist
`
`which have been exposed to radiation and those which have not — and notall inspection
`
`apparatuses have sufficient sensitivity to make useful measurements of the latent image.
`
`Therefore measurements may be taken after the post-exposure bake step (PEB) which
`
`is customarily the first step carried oul on exposed substrates and increases the contrast
`
`between exposed and unexposed parts of the resist. At this stage, the image in the resist
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`may be referred to as semi-latent.
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`It is also possible to make measurements of the
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`developed resist image — at which point either the exposed or unexposed parts of the
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`resist have been removed — or after a pattern transfer step such as etching. Thelatter
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`possibility limits the possibilities for rework of faulty substrates but may still provide
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`useful information.
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`[0031] A metrology apparatus is shown in Figure 3(a). A target T and diffracted rays of
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`measurement radiation used to illuminate the target are illustrated in more detail in
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`Figure 3(b). The metrology apparatus illustrated is of a type known as a dark field
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`metrology apparatus. The metrology apparatus depicted here is purely exemplary, to
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`provide an explanation of dark field metrology. The metrology apparatus may be a
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`stand-alone device or incorporated in either the lithographic apparatus LA,e.g., at the
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`measurement station, or the lithographic cell LC. An optical axis, which has several
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`branches throughout the apparatus, is represented by a dotted line O. In this apparatus,
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`light emitted by source 11 (e.g., a xenon lamp) is directed onto substrate W via a beam
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`splitter 15 by an optical system comprising lenses 12, 14 and objective lens 16. These
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`lenses arc arranged in a double sequence of a 4F arrangement. A different lens
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`arrangement can be used, provided that it still provides a substrate image onto a
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`detector, and simulianeously allows for access of an intermediate pupil-plane for
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`10
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`15
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`20
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`30
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`spatial-frequency filtering. Therefore, the angular range at which the radiation is
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`incident on the substrate can be selected by defining a spatial intensity distribution in a
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`plane that presents the spatial spectrum of the substrate plane, here referred to as a
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`(conjugate) pupil plane.
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`In particular, this can be done by inserting an aperture plate
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`13 of suitable form between Icnses 12 and 14, in a plane which is a back-projected
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`image of the objective lens pupil plane.
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`In the example illustrated, aperture plate 13
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`has different forms, labeled 13N and 13S, allowing different illumination modes to be
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`selected. The illumination system in the present examples forms an off-axis
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`illumination mode. In the first illumination mode, aperture plate 13N provides off-axis
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`from a direction designated, for the sake of description only, as ‘north’. In a second
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`illumination mode, aperture plate 13S is used to provide similar illumination, but from
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`an opposite direction, labeled ‘south’. Other modes of illumination are possible by
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`using different apertures. The rest of the pupil plane is desirably dark as any
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`unnecessary light outside the desired illumination mode will interfere with the desired
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`10
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`measurementsignals.
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`[0032] As shownin Figure 3(b), target T is placed with substrate W normalto the optical axis
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`15
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`20
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`O of objective lens 16. The substrate W may be supported by a support (not shown). A
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`ray of measurement radiation [ impinging on target T from an angle off the axis O gives
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`rise to a zeroth order ray (solid line 0) and two first order rays (dot-chain line +1 and
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`double dot-chain line -1). It should be remembered that with an overfilled small target,
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`these rays are just one of manyparallel rays covering the area of the substrate including
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`metrology target T and other features. Since the aperture in plate 13 has a finite width
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`(necessary to admit a useful quantity of light, the incident rays I will in fact occupy a
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`range of angles, and the diffracted rays O and +1/-1 will be spread out somewhat.
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`According to the point spread function of a small target, each order +1 and -1 will be
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`further spread over a range of angles, not a single ideal ray as shown. Note that the
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`grating pitches of the targets and the illumination angles can be designed or adjusted so
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`that the first order rays entering the objective lens are closely aligned with the central
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`optical axis. ‘The rays illustrated in Figure 3(a) and 3(b) are shown somewhatoff axis,
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`purely to enable them to be more easily distinguished in the diagram.
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`[0033] At least the O and +1 orders diffracted by the target T on substrate W are collected by
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`objective lens 16 and directed back through beamsplitter 15. Returning to Figure 3(a),
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`both the first and second illumination modes are illustrated, by designating
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`diametrically opposite apertures labeled as north (N) and south (S). When the incident
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`30
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`ray I of measurement radiation is from the north side of the optical axis, that is when
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`the first Ulumination mode is applied using aperture plate 13N, the +1 diffracted rays,
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`which are labeled +1(N), enter the objective lens 16. In contrast, when the second
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`Ulumination mode is applied using aperture plate 13S the -1 diffracted rays
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`(labeled -1(S)) are the ones which enter the Icns 16.
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`2018PO0159EP
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`[0034] A second beamsplitter 17 divides the diffracted beams into two measurement branches.
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`In a first measurement branch, optical system 18 forms a diffraction spectrum (pupil
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`plane image) of the target on first sensor 19 (e.g. a CCD or CMOSsensor) using the
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`zeroth and first order diffractive beams. Each diffraction order hits a different point on
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`the sensor, so that image processing can compare and contrast orders. ‘he pupil plane
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`image captured by sensor 19 can be used for focusing the metrology apparatus and/or
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`normalizing intensity measurements of the first order beam. The pupil plane image can
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`also be used for many measurement purposes such as reconstruction.
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`[0035] In the second measurementbranch, optical system 20, 22 forms an imageof the target
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`T on sensor 23 (e.g. a CCD or CMOSsensor).
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`In the second measurement branch, an
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`aperture stop 21 is provided in a plane that is conjugate to the pupil-plane. Aperture
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`stop 21 functions to block the zeroth order diffracted beam so that the image of the
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`target formed on sensor 23 is formed only from the -1 or +1 first order beam. The
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`images captured by sensors 19 and 23 are output to processor PU which processes the
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`