111111
`
`1111111111111111111111111111111111111111111111111111111111111111111111111111
`US 20130329204Al
`
`(19) United States
`c12) Patent Application Publication
`PELLEMANS et al.
`
`(10) Pub. No.: US 2013/0329204 A1
`Dec. 12, 2013
`(43) Pub. Date:
`
`(54) PHOTON SOURCE, METROLOGY
`APPARATUS, LITHOGRAPHIC SYSTEM AND
`DEVICE MANUFACTURING METHOD
`
`(71) Applicant: ASML Netherlands B.V., Veldhoven
`(NL)
`
`(72)
`
`Inventors: Henricus Petrus Maria PELLEMANS,
`Veldhoven (NL); Pavel Stanislavovich
`ANTSIFEROV, Troitsk (RU); Vladimir
`Mihailovitch KRIVTSUN, Troitsk
`(RU); Johannes Matheus Marie DE
`WIT, Belmond (NL); Ralph Jozef
`Johannes Gerardus Anna Maria
`SMEETS, Veldhoven (NL); Gerbrand
`VANDER ZOUW, Eindhoven (NL)
`
`(21) Appl. No.: 13/902,285
`
`(22) Filed:
`
`May24, 2013
`
`Related U.S. Application Data
`
`(60) Provisional application No. 61/658,654, filed on Jun.
`12, 2012.
`
`Publication Classification
`
`(51)
`
`Int. Cl.
`HOSH 1124
`G03F 7120
`(52) U.S. Cl.
`CPC ... HOSH 1124 (2013.01); G03F 7120 (2013.01)
`USPC ........................................ 355/67; 315/111.21
`
`(2006.01)
`(2006.01)
`
`(57)
`
`ABSTRACT
`
`A laser driven light source comprises laser and focusing
`optics. These produce a beam of radiation focused on a
`plasma forming zone within a container containing a gas
`(e.g., Xe). Collection optics collects photons emitted by a
`plasma maintained by the laser radiation to form a beam of
`output radiation. Plasma has an elongate form (L>d) and
`collecting optics is configured to collect photons emerging in
`the longitudinal direction from the plasma. The brightness of
`the plasma is increased compared with sources which collect
`radiation emerging transversely from the plasma. A metrol(cid:173)
`ogy apparatus using the light source can achieve greater accu(cid:173)
`racy and/or throughput as a result of the increased brightness.
`Back reflectors may be provided. Microwave radiation may
`be used instead oflaser radiation to form the plasma.
`
`40
`
`~~~~~
`58~
`
`d
`
`42
`
`.
`
`48
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`11
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`Patent Application Publication Dec. 12, 2013 Sheet 1 of 4
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`US 2013/0329204 A1
`
`Fig. 1
`
`IL
`
`LA
`~-
`
`PW
`
`PW
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`SCx4
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`~(9'@
`~RO
`@)@ @)@
`
`DEx4
`
`Fig. 2
`
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`Patent Application Publication Dec. 12, 2013 Sheet 2 of 4
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`US 2013/0329204 Al
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`23
`
`22
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`Fig. 3
`
`48
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`11
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`Fig. 4
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`Patent Application Publication Dec. 12, 2013 Sheet 3 of 4
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`US 2013/0329204 Al
`
`B (1Q4Wfm2Jnm/sr)
`8
`
`I-"
`
`70
`
`/
`
`7
`
`6
`
`5
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`4
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`800
`
`900
`
`Fig. 5
`
`Fig. 6
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`Patent Application Publication Dec. 12, 2013 Sheet 4 of 4
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`US 2013/0329204 Al
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`92
`
`52 Fig. 7(a)
`
`40
`88
`
`52
`
`_jy
`
`X
`
`Fig. 7 (b)
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`US 2013/0329204 AI
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`Dec. 12, 2013
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`1
`
`PHOTON SOURCE, METROLOGY
`APPARATUS, LITHOGRAPHIC SYSTEM AND
`DEVICE MANUFACTURING METHOD
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`[0001] This application is related to U.S. Provisional Appli(cid:173)
`cation No. 61/658,654, filed Jun. 12, 2012, which is incorpo(cid:173)
`rated by reference herein in its entirety.
`
`BACKGROUND
`
`[0002]
`1. Field of the Invention
`[0003] The present invention relates to plasma based pho(cid:173)
`ton sources. Such sources may be used for example to provide
`high brightness illumination in methods and for metrology
`usable, for example, in the manufacture of devices by litho(cid:173)
`graphic techniques and to methods of manufacturing devices
`using lithographic techniques.
`[0004] 2. Background Art
`[0005] Photon sources according to the invention may find
`application in a wide range of situations. As an example
`application, we will describe use of the invention as a light
`source in metrology. As a particular field of application of
`metrology, we shall refer for the sake of example to metrology
`in the manufacture of devices by lithography. The terms
`'light' and 'light source' may be used conveniently to refer to
`the generated radiation and the photon source itself, without
`implying any limitation to radiation of visible wavelengths.
`[0006] 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 alternatively
`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.,
`including part of, one, or several dies) on a substrate (e.g., 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 pat(cid:173)
`terned. Known lithographic apparatus include so-called step(cid:173)
`pers, 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 scan(cid:173)
`ning the pattern through a radiation beam in a given direction
`(the "scanning" -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 substrate by imprinting the pattern onto the substrate.
`[0007]
`In lithographic processes, it is desirable frequently
`to make measurements of the structures created, e.g., for
`process control and verification. Various tools for making
`such measurements are known, including scanning electron
`microscopes, which are often used to measure critical dimen(cid:173)
`sion (CD), and specialized tools to measure overlay, the accu(cid:173)
`racy of alignment of two layers in a device. Recently, various
`forms of scatterometers have been developed for use in the
`lithographic field. These devices direct a beam of radiation
`onto a target and measure one or more properties of the
`scattered radiation. From these measured properties a prop(cid:173)
`erty of interest of the target can be determined.
`
`[0008]
`In one commercially available metrology apparatus,
`the light source is a xenon (Xe) arc-discharge lamp. Light
`from this lamp is imaged onto the measurement target
`through an illumination branch of the apparatus sensor, the
`last stage of which consists of a high-NA objective. The
`measurement spot may have a diameter of 25 fllll, for
`example. The spectral distribution of the radiation may be
`broadband or narrowband in nature, and wavelengths may be
`in the near infrared, visible and/or ultraviolet bands. The time
`required for each measurement depends in practice on the
`brightness of the light source at a given wavelength or wave
`range. Future generations of apparatus are desired to provide
`an increased spectral bandwidth and sensor design with lower
`transmittance, while keeping the measurement time the same
`or shorter. Significant source brightness improvements are
`necessary to fulfill these requirements.
`[0009]
`Increasing brightness is not achieved simply by
`increasing the total source power. To increase brightness, a
`higher power must be delivered into the same small spot size.
`Etendue is a measure of how 'spread out' a rays are in an
`optical system. A fundamental property of optical systems is
`that 'etendue' never decreases through the system. The opti(cid:173)
`cal etendue at the target side of the optical system in the
`metrology apparatus is very small (due to the small spot size).
`Therefore the light source must deliver all its energy in a very
`small etendue, in order to provide a real increase in usable
`brightness.
`[0010] Plasma-based photon sources, for example laser
`driven light sources (LDLS) offer higher brightnesses. Plas(cid:173)
`mas are generated in a gaseous medium by the application of
`energy through electric discharge, and laser energy. However,
`the plasma has a finite physical extent and increasing bright(cid:173)
`ness is still a challenge with these sources.
`
`SUMMARY
`
`[0011] The present invention aims to provide a high bright(cid:173)
`ness photon source by alternative means.
`[0012] The invention in a first aspect provides a plasma(cid:173)
`based photon source apparatus comprising a container for
`containing a gaseous atmosphere, a driving system for gen(cid:173)
`erating radiation, hereinafter referred to as the driving radia(cid:173)
`tion, and forming the driving radiation into at least one beam
`focused on a plasma forming zone within the container, and a
`collecting optical system for collecting photons emitted by a
`plasma maintained by the radiation beam at the plasma loca(cid:173)
`tion and forming the collected photons into at least one beam
`of output radiation. The driving system is configured to main(cid:173)
`tain the plasma in an elongate form having a length along a
`longitudinal axis that is substantially greater than its diameter
`in at least one direction transverse to the longitudinal axis,
`and wherein the collecting optical system is configured to
`collect photons emerging from the plasma from one end of the
`plasma along the longitudinal axis.
`[0013] The driving system may include at least one laser for
`generating the beam of radiation with wavelengths for
`example in the infrared or visible wavebands. The invention is
`thus suitable for application to laser driven light sources. The
`driving system may alternatively be arranged to generate the
`radiation in the microwave range. In either case, the driving
`system may be regarded as a driving optical system, applying
`for example infrared optics or microwave optics as appropri(cid:173)
`ate.
`[0014] As mentioned, the novel photon source may be
`applied in metrology, for example in lithography. The inven-
`
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`2
`
`tion in another aspect provides method of measuring a prop(cid:173)
`erty of structures that have been formed by a lithographic
`process on a substrate, the method comprising the steps of
`illuminating the structures using output radiation of a photon
`source according to the first aspect of the invention, set forth
`above; detecting radiation diffracted by the structures; and
`determining from properties of the diffracted radiation one or
`more properties of the structure.
`[0015] The invention yet further provides an inspection
`apparatus for measuring a property of a structure on a sub(cid:173)
`strate, the apparatus comprising a support for the substrate
`having the structure thereon; an optical system for illuminat(cid:173)
`ing the structure under predetermined illumination condi(cid:173)
`tions and for detecting predetermined portions of radiation
`diffracted by the component target structures under the illu(cid:173)
`mination conditions; a processor arranged to process infor(cid:173)
`mation characterizing the detected radiation to obtain a mea(cid:173)
`surement of the property of the structure. The optical system
`includes a photon source apparatus according to the invention
`as set forth above.
`[0016] The invention yet further provides a lithographic
`system comprising a lithographic apparatus comprising: an
`illumination optical system arranged to illuminate a pattern, a
`projection optical system arranged to project an image of the
`pattern onto a substrate; and an inspection apparatus accord(cid:173)
`ing to an embodiment of the invention as set forth above. The
`lithographic apparatus is arranged to use the measurement
`results from the inspection apparatus in applying the pattern
`to further substrates.
`[0017] The invention yet further provides a method of
`manufacturing devices wherein a device pattern is applied to
`a series of substrates using a lithographic process, the method
`including inspecting at least one composite target structure
`formed as part of or beside the device pattern on at least one
`of the substrates using an inspection method as claimed in
`claim 12 and controlling the lithographic process for later
`substrates in accordance with the result of the inspection
`method.
`[0018] 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 illustrative pur(cid:173)
`poses only. Additional embodiments will be apparent to per(cid:173)
`sons skilled in the relevant art(s) based on the teachings
`contained herein.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0019] The accompanying drawings, which are incorpo(cid:173)
`rated herein and form part of the specification, illustrate the
`present invention and, together with the description, further
`serve to explain the principles of the invention and to enable
`a person skilled in the relevant art(s) to make and use the
`invention.
`[0020] FIG. 1 depicts a lithographic apparatus according to
`an embodiment of the invention.
`[0021] FIG. 2 depicts a lithographic cell or cluster accord(cid:173)
`ing to an embodiment of the invention.
`[0022] FIG. 3 comprises a schematic diagram of an optical
`apparatus incorporating a photon source, the apparatus in this
`example having the form of a scatterometer used in metrol(cid:173)
`ogy.
`
`[0023] FIG. 4 is a schematic diagram of a novel photon
`source used in the apparatus of FIG. 3 according to a first
`embodiment of the present invention.
`[0024] FIG. 5 is a graph presenting experimental data on
`the relative brightness of an elongate plasma when viewed in
`longitudinal and lateral directions.
`[0025] FIG. 6 is a schematic diagram of a novel photon
`source used in the apparatus of FIG. 3 according to a second
`embodiment of the present invention.
`[0026] FIG. 7 (a) and (b) are schematic views of a novel
`photon source used in the apparatus of FIG. 3 according to a
`third embodiment of the present invention.
`[0027] The features and advantages of the present invention
`will become more apparent from the detailed description set
`forth below when taken in conjunction with the drawings, in
`which like reference characters identify corresponding ele(cid:173)
`ments throughout. In the drawings, like reference numbers
`generally indicate identical, functionally similar, and/or
`structurally similar elements. The drawing in which an ele(cid:173)
`ment first appears is indicated by the leftmost digit(s) in the
`corresponding reference number.
`
`DETAILED DESCRIPTION
`
`[0028] This specification discloses one or more embodi(cid:173)
`ments that incorporate the features of this invention. The
`disclosed embodiment(s) merely exemplify the invention.
`The scope of the invention is not limited to the disclosed
`embodiment(s). The invention is defined by the claims
`appended hereto.
`[0029] The embodiment( s) described, and references in the
`specification to "one embodiment", "an embodiment", "an
`example embodiment", etc., indicate that the embodiment(s)
`described may include a particular feature, structure, or char(cid:173)
`acteristic, but every embodiment may not necessarily include
`the particular feature, structure, or characteristic. Moreover,
`such phrases are not necessarily referring to the same
`embodiment. Further, when a particular feature, structure, or
`characteristic is described in connection with an embodi(cid:173)
`ment, it is understood that it is within the knowledge of one
`skilled in the art to effect such feature, structure, or charac(cid:173)
`teristic in connection with other embodiments whether or not
`explicitly described.
`[0030] Embodiments of the invention may be implemented
`in hardware, firmware, software, or any combination thereof.
`Embodiments of the invention may also be implemented as
`instructions stored on a machine-readable medium, which
`may be read and executed by one or more processors. A
`machine-readable medium may include any mechanism for
`storing or transmitting information in a form readable by a
`machine (e.g., a computing device). For example, a machine(cid:173)
`readable medium may include read only memory (ROM);
`random access memory (RAM); magnetic disk storage
`media; optical storage media; flash memory devices; electri(cid:173)
`cal, optical, acoustical or other forms of propagated signals
`(e.g., carrier waves, infrared signals, digital signals, etc.), and
`others. Further, firmware, software, routines, instructions
`may be described herein as performing certain actions. How(cid:173)
`ever, it should be appreciated that such descriptions are
`merely for convenience and that such actions in fact result
`from computing devices, processors, controllers, or other
`devices executing the firmware, software, routines, instruc(cid:173)
`tions, etc.
`
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`3
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`[0031] Before describing embodiments of the invention in
`detail, it is instructive to present an example environment in
`which embodiments of the present invention may be imple(cid:173)
`mented.
`[0032] FIG. 1 schematically depicts a lithographic appara(cid:173)
`tus LA. The apparatus includes an illumination system (illu(cid:173)
`minator) IL configured to condition a radiation beam B (e.g.,
`UV radiation or DUV radiation), a patterning device support
`or support structure (e.g., a mask table) MT constructed to
`support a patterning device (e.g., a mask) MA and connected
`to a first positioner PM configured to accurately position the
`patterning device in accordance with certain parameters; a
`substrate table (e.g., a wafer table) WT constructed to hold a
`substrate (e.g., a resist coated wafer) Wand connected to a
`second positioner PW configured to accurately position the
`substrate in accordance with certain parameters; and a pro(cid:173)
`jection system (e.g., a refractive projection lens system) PS
`configured to project a pattern imparted to the radiation beam
`B by patterning device MA onto a target portion C (e.g.,
`including one or more dies) of the substrate W.
`[0033] The illumination system may include various types
`of optical components, such as refractive, reflective, mag(cid:173)
`netic, electromagnetic, electrostatic or other types of optical
`components, or any combination thereof, for directing, shap(cid:173)
`ing, or controlling radiation.
`[0034] 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 other conditions, such as for example whether or not the
`patterning device is held in a vacuum environment. The pat(cid:173)
`terning device support can use mechanical, vacuum, electro(cid:173)
`static or other clamping techniques to hold the patterning
`device. The patterning device support may be a frame or a
`table, for example, which may be fixed or movable as
`required. The patterning device support 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 "patterning device."
`[0035] The term "patterning 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 radiation beam will
`correspond to a particular functional layer in a device being
`created in the target portion, such as an integrated circuit.
`[0036] The patterning device may be transmissive or reflec(cid:173)
`tive. Examples of patterning devices include masks, program(cid:173)
`mable mirror arrays, and programmable LCD panels. 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 program(cid:173)
`mable mirror array employs a matrix arrangement of small
`mirrors, each of which can be individually tilted so as to
`reflect an incoming radiation beam in different directions.
`The tilted mirrors impart a pattern in a radiation beam, which
`is reflected by the mirror matrix.
`[0037] The term "projection system" used herein should be
`broadly interpreted as encompassing any type of projection
`system, including refractive, reflective, catadioptric, mag-
`
`netic, 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 synonymous
`with the more general term "projection system".
`[0038] As here depicted, the apparatus is of a transmissive
`type (e.g., employing a transmissive mask). Alternatively, the
`apparatus may be of a reflective type (e.g., employing a pro(cid:173)
`grammable mirror array of a type as referred to above, or
`employing a reflective mask).
`[0039] 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(cid:173)
`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.
`[0040] 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 fill 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 dur(cid:173)
`ing exposure.
`[0041] Referring to FIG. 1, the illuminator IL receives a
`radiation beam from a radiation source SO. 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
`SO to the illuminator IL with the aid of a beam delivery
`system BD including, for example, suitable directing mirrors
`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 illumi(cid:173)
`nator IL, together with the beam delivery system BD if
`required, may be referred to as a radiation system.
`[0042] The illuminator IL may include an adjuster AD for
`adjusting the angular intensity distribution of the radiation
`beam. Generally, at least the outer and/or inner radial extent
`(commonly referred to as a-outer and a-inner, respectively)
`of the intensity distribution in a pupil plane of the illuminator
`can be adjusted. In addition, the illuminator IL may include
`various other components, such as an integrator 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.
`[0043] The radiation beam B is incident on the patterning
`device (e.g., mask) MA, which is held on the patterning
`device support (e.g., mask table MT), and is patterned by the
`patterning device. Having traversed the patterning device
`(e.g., 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 (e.g., an interferometric
`device, linear encoder, 2-D 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
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`4
`
`beam B. Similarly, the first positioner PM and another posi(cid:173)
`tion sensor (which is not explicitly depicted in FIG. 1) can be
`used to accurately position the patterning device (e.g., mask)
`MA with respect to the path of the radiation beam B, e. g., after
`mechanical retrieval from a mask library, or during a scan. In
`general, movement of the patterning device support (e.g.,
`mask table) MT may be realized with the aid of a long-stroke
`module (coarse positioning) and a short-stroke module (fine
`positioning), which form part of the first positioner PM. Simi(cid:173)
`larly, 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 patterning device support (e.g.,
`mask table) MT may be connected to a short-stroke actuator
`only, or may be fixed.
`[0044] 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 marks as illustrated occupy dedicated target por(cid:173)
`tions, 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
`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 sys(cid:173)
`tem, which detects the alignment markers is described further
`below.
`[0045] The depicted apparatus could be used in at least one
`of the following modes:
`[0046]
`1. In step mode, the patterning device support (e.g.,
`mask table) MT and the substrate table WT are kept essen(cid:173)
`tially stationary, while an entire pattern imparted to the radia(cid:173)
`tion beam is projected onto a target portion Cat 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 field limits the size of the target portion C
`imaged in a single static exposure.
`[0047] 2. In scan mode, the patterning device support (e.g.,
`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 patterning device support (e.g., mask table)
`MT may be determined by the (de-)magnification and image
`reversal characteristics of the projection system PS. In scan
`mode, the maximnm size of the exposure field limits the
`width (in the non-scanning direction) of the target portion in
`a single dynamic exposure, whereas the length of the scan(cid:173)
`ning motion determines the height (in the scanning direction)
`of the target portion.
`[0048] 3. In another mode, the patterning device support
`(e.g., 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 pro(cid:173)
`grammable patterning 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 program-
`
`mabie patterning 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] Lithographic apparatus LA is of a so-called dual
`stage type which has two substrate tables WTa, WTb and two
`stations-an exposure station and a measurement station(cid:173)
`between which the substrate tables can be exchanged. While
`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 out. The preparatory steps may
`include mapping the surface control of the substrate using a
`level sensor LS and measuring the position of alignment
`markers on the substrate using an alignment sensor AS. This
`enables a substantial increase in the throughput of the appa(cid:173)
`ratus. If the position sensor IF is not capable of measuring the
`position of the substrate table while it is at the measurement
`station as well as at the exposure station, a second position
`sensor may be provided to enable the positions of the sub(cid:173)
`strate table to be tracked at both stations.
`[0051] As shown in FIG. 2, the lithographic apparatus LA
`forms part of a lithographic cell LC, also sometimes referred
`to a lithocell or cluster, which also includes apparatus to
`perform pre- and post-exposure processes on a substrate.
`Conventionally these include spin coaters SC to deposit resist
`layers, developers DE to develop exposed resist, chill plates
`CH and bake plates BK. A substrate handler, or robot, RO
`picks up substrates from input/output ports I/0 1, I/02, moves
`them between the different process apparatus and delivers
`then to the loading 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 which
`is itself controlled by the supervisory control system SCS,
`which also controls the lithographic apparatus via lithogra(cid:173)
`phy control unit LACU. Thus, the different apparatus can be
`operated to maximize throughput and processing efficiency.
`[0052] FIG. 3 is a schematic diagram of an optical appara(cid:173)
`tus in the form of a scatterometer suitable for performing
`metrology in conjunction with the lithocell of FIG. 2. The
`apparatus may be used for measuring critical dimensions of
`features formed by lithography, measuring overlay between
`layers and the like. A product feature or dedicated metrology
`target is formed on substrate W. The apparatus may be a
`stand-alone device or incorporated in either the lithographic
`apparatus LA, e.g., at the measurement station, or the litho(cid:173)
`graphic cell LC. An optical axis, which has several branches
`throughout the apparatus, is represented by a dotted line 0. In
`this apparatus, light emitted by source 11 is directed onto
`substrate W via a beam splitter 15 by an optical system
`comprising lenses 12, 14 and objective lens 16. These lenses
`are arranged in a double sequence of a 4F arrangement. A
`different lens arrangement can be used, provided that it still
`provides an image of the source on the substrate, and simul(cid:173)
`taneously allows for access of an intermediate pupil-plane for
`spatial-frequency filtering. Therefore, the angular range at
`which the radiation is incident on the substrate can be selected
`by defining a spatial intensity distribution in a plane that
`presents the spatial spectrum of the substrate plane, here
`referred to as a (conjugate) pupil plane. In particular, this can
`be done by inserting an aperture plate 13 of suitable form
`between lenses 12 and 14, in a plane which is a back-projected
`image of the objective lens pupil plane. For e