Metrology method and apparatus, lithographic system and device manufacturing method.

专利摘要:

公开号:NL2013210A
申请号:NL2013210
申请日:2014-07-18
公开日:2015-02-10
发明作者:Scott Anderson Middlebrooks；Niels Geypen；Hendrik Jan Hidde Smilde；Alexander Straaijer；Maurits Schaar；Markus Gerardus Martinus Kraaij
申请人:Asml Netherlands Bv；
IPC主号:

专利说明:

METROLOGY METHOD AND APPARATUS, LITHOGRAPHIC SYSTEMAND DEVICE MANUFACTURING METHOD
BACKGROUND
Field of the Invention [0001] The present invention relates to methods and apparatus for metrology usable,for example, in the manufacture of devices by lithographic techniques and to methods ofmanufacturing devices using lithographic techniques.
Background Art [0002] A lithographic apparatus is a machine that applies a desired pattern onto asubstrate, 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 patterningdevice, which is alternatively referred to as a mask or a reticle, may be used to generate acircuit pattern to be formed on an individual layer of the IC. This pattern can be transferredonto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitivematerial (resist) provided on the substrate. In general, a single substrate will contain anetwork of adjacent target portions that are successively patterned. Known lithographicapparatus include so-called steppers, in which each target portion is irradiated by exposing anentire pattern onto the target portion at one time, and so-called scanners, in which each targetportion is irradiated by scanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrate parallel or anti parallel tothis direction. It is also possible to transfer the pattern from the patterning device to thesubstrate by imprinting the pattern onto the substrate.
[0003] In lithographic processes, it is desirable frequently to make measurements ofthe structures created, e.g., for process control and verification. Various tools for making suchmeasurements are known, including scanning electron microscopes, which are often used tomeasure critical dimension (CD), and specialized tools to measure overlay, the accuracy ofalignment of two layers in a device. Recently, various forms of scatterometers have beendeveloped for use in the lithographic field. These devices direct a beam of radiation onto atarget and measure one or more properties of the scattered radiation - e.g., intensity at asingle angle of reflection as a function of wavelength; intensity at one or more wavelengthsas a function of reflected angle; or polarization as a function of reflected angle - to obtain a “spectrum” irom which a property of interest of the target can be determined. Determinationof the property of interest may be performed by various techniques: e.g., reconstruction of thetarget structure by iterative approaches such as rigorous coupled wave analysis or finiteelement methods; library searches; and principal component analysis.
[0004] The targets used by somel scatterometers are relatively large gratings, e.g.,40pm by 40pm, 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 itcan be regarded as infinite. However, in order to reduce the size of the targets, e.g., to 10pmby 10pm or less, so they can be positioned in amongst product features, rather than in thescribe lane, metrology has been proposed in which the grating is made smaller than themeasurement spot (i.e., the grating is overfilled). Typically such targets are measured usingdark field scatterometry in which the zeroth order of diffraction (corresponding to a specularreflection) is blocked, and only higher orders processed. Diffraction-based overlay usingdark-field detection of the diffraction orders enables overlay measurements on smallertargets. These targets can be smaller than the illumination spot and may be surrounded byproduct structures on a wafer. Multiple targets can be measured in one image.
[0005] In the known metrology technique, overlay measurement results are obtainedby measuring the target twice under certain conditions, while either rotating the target orchanging the illumination mode or imaging mode to obtain separately the -1st and the -rlstdiffraction order intensities. Comparing these intensities for a given grating provides ameasurement of asymmetry in the grating, and asymmetry in an overlay grating can be usedas an indicator of overlay error.
[0006] Although the known dark-field image-based overlay measurements are fastand computationally very simple (once calibrated), they rely on an assumption that overlay isthe only cause of asymmetry in the target structure. Any other asymmetry in the stack, suchas asymmetry of features within one or both of the overlaid gratings, also causes anasymmetry in the 1st orders. This feature asymmetry which is not related to the overlayclearly perturbs the overlay measurement, giving an inaccurate overlay result. Featureasymmetry in the bottom grating of the overlay grating is a common form of featureasymmetry. It may originate, for example, in wafer processing steps such as chemical-mechanical polishing (CMP), performed after the bottom grating was originally formed.
[0007] Accordingly the skilled person has to choose between, on the one hand, asimple and fast measurement process that provides overlay measurements but is subject toinaccuracies when other causes of asymmetry are present, or, on the other hand, more traditional techniques that are computationally intensive and typically require severalmeasurements of large, underfilled gratings to avoid the pupil image being polluted withsignal contribution from the overlay grating environment, which hampers the reconstructionbased on this pupil image.
SUMMARY
[0008] Therefore, it is desired to make overlay measurements more robust to featureasymmetry contributions to target structure asymmetry and/or distinguish the contributions totarget structure asymmetry that are caused by feature asymmetry from those caused byoverlay (including bias).
[0009] A first aspect provides a method of measuring a parameter of a lithographicprocess, the method comprising the steps of: (a) illuminating target structures on a substrate,the target structures comprising at least a first target structure comprising an overlaid periodicstructure having a first deliberate overlay bias and a second target structure comprising anoverlaid periodic structure having a second deliberate overlay bias; and detecting radiationscattered by each target structure to obtain for each target structure an asymmetrymeasurement representing an overall asymmetry that includes contributions due to (i) thedeliberate overlay bias in the target structure, (ii) an overlay error in a lithographic processduring forming of the target structure and (iii) feature asymmetry within one or more of theperiodic structures; (b) repeating step (a) for a plurality of different illumination conditions;(c) performing a regression analysis on asymmetry measurement data obtained in step (b) byfitting a linear regression model to a planar representation of asymmetry measurements forthe first target structure against asymmetry measurements for the second target structure, thelinear regression model not necessarily being fitted through an origin of the planarrepresentation; and (d) determining the overlay error from a gradient described by the linearregression model.
[0010] Another aspect provides an inspection apparatus for measuring a parameter ofa lithographic process, the apparatus comprising: a support for a substrate having a pluralityof target structures thereon, the target structures comprising at least a first target structurecomprising an overlaid periodic structure having a first deliberate overlay bias and a secondtarget structure comprising an overlaid periodic structure having a second deliberate overlaybias; an optical system being operable to illuminate the targets and detecting radiationscattered by each target to obtain for each target structure and for a plurality of differentillumination conditions, an asymmetry measurement representing an overall asymmetry that includes contributions due to (i) the deliberate overlay bias in the target structure, (ii) anoverlay error in a lithographic process during forming of the target structure and (iii) featureasymmetry within one or more of the periodic structures; a processor arranged to: perform aregression analysis on asymmetry measurement data by fitting a linear regression model to aplanar representation of asymmetry measurements for the first target structure againstasymmetry measurements for the second target structure, the linear regression model notnecessarily being fitted through an origin of the planar representation; and determine theoverlay error from a gradient described by the linear regression model.
[0011] Yet another aspect further provides a computer program product comprisingmachine-readable instructions for causing a processor to perform the processing steps (c) and(d) of a method according to the first aspect as set forth above, on asymmetry data obtainedby illuminating target structures on a substrate, under a plurality of different illuminationconditions, the target structures comprising at least a first target structure comprising anoverlaid periodic structure having a first deliberate overlay bias and a second target structurecomprising an overlaid periodic structure having a second deliberate overlay bias; anddetecting radiation scattered by each target structure to obtain for each target structure anasymmetry measurement representing an overall asymmetry that includes contributions dueto (i) the deliberate overlay bias in the target structure, (ii) an overlay error in a lithographicprocess during forming of the target structure and (iii) feature asymmetry within one or moreof the periodic structures..
[0012] Yet another aspect further provides a lithographic apparatus comprising theinspection apparatus as set forth above, being operable to apply a device pattern to a series ofsubstrates using a lithographic process, apply target structures to one or more of the series ofsubstrates; measure an overlay parameter of the target structure using a method according tothe first aspect as set forth above; and control the lithographic process for later substrates inaccordance with the result of the method of measuring a parameter.
[0013] A still further aspect provides a method of manufacturing devices wherein adevice pattern is applied to a series of substrates using a lithographic process, the methodincluding inspecting at least one periodic structure formed as part of or beside the devicepattern on at least one of the substrates using a method according to the first aspect as setforth above and controlling the lithographic process for later substrates in accordance withthe result of the inspection method.
[0014] Further features and advantages of the invention, as well as the structure andoperation 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 thespecific embodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent to persons skilled in therelevant art(s) based on the teachings contained herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Embodiments of the invention will now be described, by way of example only,with reference to the accompanying drawings in which: [0016] Figure 1 depicts a lithographic apparatus according to an embodiment of theinvention; [0017] Figure 2 depicts a lithographic cell or cluster according to an embodiment ofthe invention; [0018] Figures 3(a) to 3(d) comprises (a) a schematic diagram of a dark fieldscatterometer for use in measuring targets according to embodiments of the invention using afirst pair of illumination apertures, (b) a detail of diffraction spectrum of a target grating for agiven direction of illumination (c) a second pair of illumination apertures providing furtherillumination modes in using the scatterometer for diffraction based overlay measurementsand (d) a third pair of illumination apertures combining the first and second pair of apertures; [0019] Figure 4 depicts a known form of multiple grating target and an outline of ameasurement spot on a substrate; [0020] Figure 5 depicts an image of the target of Figure 4 obtained in thescatterometer of Figure 3; [0021] Figure 6 is a flowchart showing the steps of an overlay measurement methodusing the scatterometer of Figure 3 and adaptable to form an embodiment of the presentinvention; [0022] Figure 7 is a flowchart expanding on step S6 of the flowchart of Figure 6, inaccordance with an embodiment of the invention; [0023] Figure 8 is a plot of A+ against A- for overlay gratings that have no feature asymmetry; [0024] Figure 9 is a plot of A+ against A- for overlay gratings having featureasymmetry, illustrating a first embodiment of the invention; [0025] Figures 10a and 10b plot of A+ against A- for an overlay grating having nofeature asymmetry and an overlay grating with feature asymmetry, illustrating a secondembodiment of the invention; [0026] Figure 11 is a plot of asymmetry against overlay for an overlay grating withfeature asymmetry; [0027] Figure 12a is a plot of A+ against A- for overlay gratings that have largefeature asymmetry; [0028] Figure 12b is a plot of Α+-Α0 versus A—A0 for an overlay grating comprisinga third bias and large feature asymmetry, illustrating a third embodiment of the invention; [0029] Figure 13 illustrates a composite grating structure having a bias scheme thatcan be used in the third embodiment of the present invention; and [0030] Figure 14 are graphical representations of overlay on a wafer and illustratesthat correcting for process asymmetry minimizes the difference between overlay estimatedwith TE and TM radiation - [0031] Figures 14(a) and 14(b) show representations of uncorrected overlaymeasurements performed over a wafer using, respectively, TE radiation and TM radiation; [0032] Figure 14(c) shows the differences between the measurements of Figures 14(a)and 14(b); [0033] Figures 14(d) and 14(e) show representations of overlay measurementsperformed over a wafer using, respectively, TE radiation and TM radiation and which havebeen corrected in accordance with an embodiment of the invention; [0034] Figure 14(f) shows the differences between the measurements of Figures 14(d)and 14(e).
[0035] The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken in conjunction with thedrawings, in which like reference characters identify corresponding elements throughout. Inthe drawings, like reference numbers generally indicate identical, functionally similar, and/orstructurally similar elements. The drawing in which an element first appears is indicated bythe leftmost digit(s) in the corresponding reference number
DETAILED DESCRIPTION
[0036] This specification discloses one or more embodiments that incorporate thefeatures of this invention. The disclosed embodiment(s) merely exemplify the invention. Thescope of the invention is not limited to the disclosed embodiment(s). The invention is definedby the clauss appended hereto.
[0037] The embodiment(s) described, and references in the specification to "oneembodiment", "an embodiment", "an example embodiment", etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature, stmcture, orcharacteristic. Moreover, such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic is described in connection withan embodiment, it is understood that it is within the knowledge of one skilled in the art toeffect such feature, stmcture, or characteristic in connection with other embodiments whetheror not explicitly described.
[0038] Embodiments of the invention may be implemented in hardware, firmware,software, or any combination thereof. Embodiments of the invention may also beimplemented as instructions stored on a machine-readable medium, which may be read andexecuted by one or more processors. A machine-readable medium may include anymechanism for storing or transmitting information in a form readable by a machine (e.g., acomputing device). For example, a machine-readable medium may include read only memory(ROM); random access memory (RAM); magnetic disk storage media; optical storage media;flash memory devices; electrical, optical, acoustical or other forms of propagated signals, andothers. Further, firmware, software, routines, instructions may be described herein asperforming certain actions. However, it should be appreciated that such descriptions aremerely for convenience and that such actions in fact result from computing devices,processors, controllers, or other devices executing the firmware, software, routines,instructions, etc.
[0039] Before describing embodiments of the invention in detail, it is instructive topresent an example environment in which embodiments of the present invention may beimplemented.
[0040] Figure 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) 1L configured to condition a radiation beam B(e.g., UV radiation or DUV radiation), a patterning device support or support structure (e.g., amask table) MT constructed to support a patterning device (e.g., a mask) MA and connectedto a first positioner PM configured to accurately position the patterning device in accordancewith certain parameters; a substrate table (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist coated wafer) W and connected to a second positioner PW configuredto accurately position the substrate in accordance with certain parameters; and a projectionsystem (e.g., a refractive projection lens system) PS configured to project a pattern impartedto the radiation beam B by patterning device MA onto a target portion C (e.g., including oneor more dies) of the substrate W.
[0041] The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of opticalcomponents, or any combination thereof, for directing, shaping, or controlling radiation.
[0042] The patterning device support holds the patterning device in a manner thatdepends 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 avacuum environment. The patterning device support can use mechanical, vacuum,electrostatic or other clamping techniques to hold the patterning device. The patterningdevice support may be a frame or a table, for example, which may be fixed or movable asrequired. The patterning device support may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use of the terms “reticle’5 or“mask” herein may be considered synonymous with the more general term “patterningdevice.” [0043] The term “patterning device” used herein should be broadly interpreted asreferring 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 thatthe pattern imparted to the radiation beam may not exactly correspond to the desired patternin the target portion of the substrate, for example if the pattern includes phase-shiftingfeatures or so called assist features. Generally, the pattern imparted to the radiation beam willcorrespond to a particular functional layer in a device being created in the target portion, suchas an integrated circuit.
[0044] The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, and programmable LCDpanels. 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. Anexample of a programmable mirror array employs a matrix arrangement of small mirrors,each of which can be individually tilted so as to reflect an incoming radiation beam indifferent directions. The tilted mirrors impart a pattern in a radiation beam, which is reflectedby the mirror matrix.
[0045] The term “projection system” used herein should be broadly interpreted asencompassing any type of projection system, including refractive, reflective, caladioplric,magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, asappropriate 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 beconsidered as synonymous with the more general term “projection system”.
[0046] As here depicted, the apparatus is of a transmissive type (e.g., employing atransmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employinga programmable mirror array of a type as referred to above, or employing a reflective mask).
[0047] The lithographic apparatus may be of a type having two (dual stage) or moresubstrate tables (and/or two or more mask tables). In such “multiple stage” machines theadditional tables may be used in parallel, or preparatory steps may be carried out on one ormore tables while one or more other tables are being used for exposure.
[0048] The lithographic apparatus may also be of a type wherein at least a portion ofthe 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 liquidmay also be applied to other spaces in the lithographic apparatus, for example, between themask and the projection system. Immersion techniques are well known in the art forincreasing the numerical aperture of projection systems. The term “immersion” as usedherein does not mean that a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection system and the substrateduring exposure.
[0049] Referring to Figure 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may be separate entities, forexample when the source is an excimer laser. In such cases, the source is not considered toform part of the lithographic apparatus and the radiation beam is passed from the source SOto 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 anintegral 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.
[0050] The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least the outer and/or inner radialextent (commonly referred to as σ-outer and σ-inner, respectively) of the intensitydistribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator ILmay include various other components, such as an integrator IN and a condenser CO. Theilluminator may be used to condition the radiation beam, to have a desired uniformity andintensity distribution in its cross section.
[0051] 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 thepatterning device. Having traversed the patterning device (e.g., mask) MA, the radiationbeam B passes through the projection system PS, which focuses the beam onto a targetportion 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), thesubstrate table WT can be moved accurately, e.g., so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PM and another positionsensor (which is not explicitly depicted in Figure 1) can be used to accurately position thepatterning device (e.g., mask) MA with respect to the path of the radiation beam B, e.g., aftermechanical retrieval from a mask library, or during a scan. In general, movement of thepatterning device support (e.g., mask table) MT may be realized with the aid of a long-strokemodule (coarse positioning) and a short-stroke module (fine positioning), which form part ofthe first positioner PM. Similarly, movement of the substrate table WT may be realized usinga long-stroke module and a short-stroke module, which form part of the second positionerPW. 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.
[0052] Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks Ml, M2 and substrate alignment marks PI, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, they may be located inspaces 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 markersmay also be included within dies, in amongst the device features, in which case it is desirablethat the markers be as small as possible and not require any different imaging or processconditions than adjacent features. The alignment system, which detects the alignmentmarkers is described further below.
[0053] The depicted apparatus could be used in at least one of the following modes: [0054] 1. In step mode, the patterning device support (e.g., mask table) MT andthe substrate table WT are kept essentially stationary, while an entire pattern imparted to theradiation 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 targetportion C can be exposed. In step mode, the maximum size of the exposure field limits thesize of the target portion C imaged in a single static exposure.
[0055] 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 radiationbeam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity anddirection 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 theprojection system PS. In scan mode, the maximum size of the exposure field limits the width(in the non-scanning direction) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanning direction) of thetarget portion.
[0056] 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 tableWT is moved or scanned while a pattern imparted to the radiation beam is projected onto atarget portion C. In this mode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after each movement of the substratetable WT or in between successive radiation pulses during a scan. This mode of operation canbe readily applied to maskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.
[0057] Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.
[0058] Lithographic apparatus LA is of a so-called dual stage type which has twosubstrate tables WTa, WTb and two stations - an exposure station and a measurement station- between which the substrate tables can be exchanged. While one substrate on one substratetable is being exposed at the exposure station, another substrate can be loaded onto the othersubstrate table at the measurement station and various preparatory steps carried out. Thepreparatory steps may include mapping the surface control of the substrate using a levelsensor LS and measuring the position of alignment markers on the substrate using analignment sensor AS. This enables a substantial increase in the throughput of the apparatus. Ifthe position sensor IF is not capable of measuring the position of the substrate table while it isat the measurement station as well as at the exposure station, a second position sensor may beprovided to enable the positions of the substrate table to be tracked at both stations.
[0059] As shown in Figure 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster, which also includesapparatus to perform pre- and post-exposure processes on a substrate. These include spincoaters 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/outputports I/Ol, 1/02, moves them between the different process apparatus and delivers then to theloading bay LB of the lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unit TCU which is itselfcontrolled by the supervisory control system SCS, which also controls the lithographicapparatus via lithography control unit LACU. Thus, the different apparatus can be operated tomaximize throughput and processing efficiency.
[0060] A micro diffraction based overlay (pDBO) metrology apparatus suitable foruse in embodiments of the invention is shown in Figure 3(a). A target grating T anddiffracted rays are illustrated in more detail in Figure 3(b). The pDBO metrology apparatusmay be a stand-alone device or incorporated in either the lithographic apparatus LA, e.g., atthe measurement station, or the lithographic cell LC. An optical axis, which has severalbranches throughout the apparatus, is represented by a dotted line 0. In this apparatus, lightemitted by source 11 (e.g., a xenon lamp) is directed onto substrate W via a beam splitter 15by an optical system comprising lenses 12, 14 and objective lens 16. These lenses may bearranged in a double sequence of a 4F arrangement. A different lens arrangement can beused, provided that it still provides a substrate image onto a detector, and simultaneouslyallows for access of an intermediate pupil-plane for spatial-frequency filtering. Therefore, theangular range at which the radiation is incident on the substrate can be selected by defining aspatial intensity distribution in a plane that presents the spatial spectrum of the substrateplane, here referred to as a (conjugate) pupil plane. Tn particular, this can be done by insertingan 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. In the example illustrated, aperture plate 13has different forms, labeled 13N and 13S, allowing different illumination modes to beselected. The illumination system in the present examples forms an off-axis illuminationmode. In the first illumination mode, aperture plate 13N provides off-axis from a directiondesignated, for the sake of description only, as ‘north’. In a second illumination mode,aperture plate 13S is used to provide similar illumination, but from an opposite direction,labeled ‘south’. Other modes of illumination are possible by using different apertures. Therest of the pupil plane is desirably dark as any unnecessary light outside the desiredillumination mode will interfere with the desired measurement signals.
[0061] As shown in Figure 3(b), target grating T is placed with substrate W normal tothe optical axis O of objective lens 16. A ray of illumination I impinging on grating T froman angle off the axis O gives rise to a zeroth order ray (solid line 0) and two first order rays (dot-chain line +1 and double dot-chain line -1). It should be remembered that with anoverfilled small target grating, these rays are just one of many parallel rays covering the areaof the substrate including metrology target grating T and other features. Since the aperture inplate 13 has a finite width (necessary to admit a useful quantity of light, the incident rays Iwill in fact occupy a range of angles, and the diffracted rays 0 and +1/-1 will be spread out.According to the point spread function of a small target, each order +1 and -1 will be furtherspread over a range of angles, not a single ideal ray as shown. Note that the grating pitchesand illumination angles can be designed or adjusted so that the first order rays entering theobjective lens are closely aligned with the central optical axis. The rays illustrated in Figure3(a) and 3(b) are shown off axis, purely to enable them to be more easily distinguished in thediagram.
[0062] At least the 0 and +1 orders diffracted by the target on substrate W arecollected by objective lens 16 and directed back through beam splitter 15. Returning toFigure 3(a), both the first and second illumination modes are illustrated, by designatingdiametrically opposite apertures labeled as north (N) and south (S). When the incident ray I isfrom the north side of the optical axis, that is when the first illumination mode is appliedusing aperture plate 13N, the +1 diffracted rays, which are labeled +1(N), enter the objectivelens 16. In contrast, when the second illumination mode is applied using aperture plate 13Sthe -1 diffracted rays (labeled -1(S)) are the ones which enter the lens 16.
[0063] A second beam splitter 17 divides the diffracted beams into two measurementbranches. In a first measurement branch, optical system 18 forms a diffraction spectrum(pupil plane image) of the target on first sensor 19 (e.g. a CCD or CMOS sensor) using thezeroth and first order diffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. The pupil plane imagecaptured by sensor 19 can be used for focusing the metrology apparatus and/or normalizingintensity measurements of the first order beam. The pupil plane image can also be used formany measurement purposes such as reconstruction, which are not the subject of the presentdisclosure.
[0064] In the second measurement branch, optical system 20, 22 forms an image ofthe target on the substrate W on sensor 23 (e.g. a CCD or CMOS sensor). In the secondmeasurement branch, an aperture stop 21 is provided in a plane that is conjugate to the pupil-plane. Aperture stop 21 functions to block the zeroth order diffracted beam so that the imageof the target formed on sensor 23 is formed only from the -1 or +1 first order beam. Theimages captured by sensors 19 and 23 are output to image processor and controller PU, the function of which will depend on the particular type of measurements being performed. Notethat the term ‘image’ is used here in a broad sense. An image of the grating lines as such willnot be formed, if only one of the -1 and +1 orders is present.
[0065] The particular forms of aperture plate 13 and field stop 21 shown in Figure 3are purely examples. In another embodiment of the invention, on-axis illumination of thetargets is used and an aperture stop with an off-axis aperture is used to pass substantially onlyone first order of diffracted light to the sensor. In yet other embodiments, 2nd, 3rd and higherorder beams (not shown in Figure 3) can be used in measurements, instead of or in addition tothe first order beams.
[0066] In order to make the illumination adaptable to these different types ofmeasurement, the aperture plate 13 may comprise a number of aperture patterns formedaround a disc, which rotates to bring a desired pattern into place. Alternatively or in addition,a set of plates 13 could be provided and swapped, to achieve the same effect. Aprogrammable illumination device such as a deformable mirror array or transmissive spatialsight modulator can be used also. Moving mirrors or prisms can be used as another way toadjust the illumination mode.
[0067] As just explained in relation to aperture plate 13, the selection of diffractionorders for imaging can alternatively be achieved by altering the pupil-stop 21, or bysubstituting a pupil-stop having a different pattern, or by replacing the fixed field stop with aprogrammable spatial light modulator. In that case the illumination side of the measurementoptical system can remain constant, while it is the imaging side that has first and secondmodes. In the present disclosure, therefore, there are effectively three types of measurementmethods, each with its own advantages and disadvantages. In one method, the illuminationmode is changed to measure the different orders. In another method, the imaging mode ischanged. In a third method, the illumination and imaging modes remain unchanged, but thetarget is rotated through 180 degrees. In each case the desired effect is the same, namely toselect first and second portions of the non-zero order diffracted radiation which aresymmetrically opposite one another in the diffraction spectrum of the target. In principle, thedesired selection of orders could be obtained by a combination of changing the illuminationmodes and the imaging modes simultaneously, but that is likely to bring disadvantages for noadvantage, so it will not be discussed further.
[0068] While the optical system used for imaging in the present examples has a wideentrance pupil which is restricted by the field stop 21, in other embodiments or applicationsthe entrance pupil size of the imaging system itself may be small enough to restrict to the desired order, and thus serve also as the field stop. Different aperture plates are shown inFigures 3(c) and (d) which can be used as described further below.
[0069] Typically, a target grating will be aligned with its grating lines running eithernorth-south or east-west. That is to say, a grating will be aligned in the X direction or the Ydirection of the substrate W. Note that aperture plate 13N or 13S can only be used to measuregratings oriented in one direction (X or Y depending on the set-up). For measurement of anorthogonal grating, rotation of the target through 90° and 270° might be implemented. Moreconveniently, however, illumination from east or west is provided in the illumination optics,using the aperture plate 13E or 13W, shown in Figure 3(c). The aperture plates 13N to 13Wcan be separately formed and interchanged, or they may be a single aperture plate which canbe rotated by 90, 180 or 270 degrees. As mentioned already, the off-axis apertures illustratedin Figure 3(c) could be provided in field stop 21 instead of in illumination aperture plate 13.In that case, the illumination would be on axis.
[0070] Figure 3(d) shows a third pair of aperture plates that can be used to combinethe illumination modes of the first and second pairs. Aperture plate 13NW has apertures atnorth and east, while aperture plate 13SE has apertures at south and west. Provided thatcross-talk between these different diffraction signals is not too great, measurements of both Xand Y gratings can be performed without changing the illumination mode.
[0071] Figure 4 depicts a composite target formed on a substrate according to knownpractice. The composite target comprises four gratings 32 to 35 positioned closely together sothat they will all be within a measurement spot 31 formed by the illumination beam of themetrology apparatus. The four targets thus are all simultaneously illuminated andsimultaneously imaged on sensors 19 and 23. In an example dedicated to overlaymeasurement, gratings 32 to 35 are themselves composite gratings formed by overlyinggratings that are patterned in different layers of the semi-conductor device formed onsubstrate W. Gratings 32 to 35 may have differently biased overlay offsets in order tofacilitate measurement of overlay between the layers in which the different parts of thecomposite gratings are formed. Gratings 32 to 35 may also differ in their orientation, asshown, so as to diffract incoming radiation in X and Y directions. In one example, gratings32 and 34 are X-direction gratings with biases of the -ι-d, -d, respectively. This means thatgrating 32 has its overlying components arranged so that if they were both printed exactly attheir nominal locations one of the components would be offset relative to the other by adistance d. Grating 34 has its components arranged so that if perfectly printed there would bean offset of d but in the opposite direction to the first grating and so on. Gratings 33 and 35 are Y-direction gratings with offsets +d and -d respectively. While four gratings areillustrated, another embodiment might require a larger matrix to obtain the desired accuracy.For example, a 3 x 3 array of nine composite gratings may have biases -4d, -3d, -2d, -d, 0, +d,+2d, +3d, +4d. Separate images of these gratings can be identified in the image captured bysensor 23.
[0072] Figure 5 shows an example of an image that may be formed on and detectedby the sensor 23, using the target of Figure 4 in the apparatus of Figure 3, using the apertureplates 13NW or 13SE from Figure 3(d). While the pupil plane image sensor 19 cannotresolve the different individual gratings 32 to 35, the image sensor 23 can do so. The darkrectangle represents the field of the image on the sensor, within which the illuminated spot 31on the substrate is imaged into a corresponding circular area 41. Within this, rectangular areas42-45 represent the images of the small target gratings 32 to 35. If the gratings are located inproduct areas, product features may also be visible in the periphery of this image field. Imageprocessor and controller PU processes these images using pattern recognition to identify theseparate images 42 to 45 of gratings 32 to 35. In this way, the images do not have to bealigned very precisely at a specific location within the sensor frame, which greatly improvesthroughput of the measuring apparatus as a whole. However the need for accurate alignmentremains if the imaging process is subject to non-uniformities across the image field. In oneembodiment of the invention, four positions PI to P4 are identified and the gratings arealigned as much as possible with these known positions.
[0073] Once the separate images of the gratings have been identified, the intensitiesof those individual images can be measured, e.g., by averaging or summing selected pixelintensity values within the identified areas. Intensities and/or other properties of the imagescan be compared with one another. These results can be combined to measure differentparameters of the lithographic process. Overlay performance is an important example of sucha parameter, and is a measure of the lateral alignment of two lithographic layers. Overlay canbe defined more specifically, for example, as the lateral position difference between thecenter of the top of a bottom grating and the center of the bottom of a corresponding top¬grating.
[0074] Examples of dark field metrology can be found in international patentapplications WO 2009/078708 and WO 2009/106279 which documents are herebyincorporated by reference in their entirety. Further developments of the technique have beendescribed in patent publications US20110027704A, US20110043791A and US20120123581.The contents of all these applications are also incorporated herein by reference.
[0075] Figure 6 illustrates how, using for example the method described inapplication WO 2011/012624, overlay error between the two layers containing thecomponent gratings 32 to 35 is measured through asymmetry of the gratings, as revealed bycomparing their intensities in the +1 order and -1 order dark field images. At step SI, thesubstrate, for example a semiconductor wafer, is processed through the lithographic cell ofFigure 2 one or more times, to create a stmcture including the overlay targets 32-35. At S2,using the metrology apparatus of Figure 3, an image of the gratings 32 to 35 is obtained usingonly one of the first order diffracted beams (say -1). Then, whether by changing theillumination mode, or changing the imaging mode, or by rotating substrate W by 180° in thefield of view of the metrology apparatus, a second image of the gratings using the other firstorder diffracted beam (+1) can be obtained (step S3). Consequently the +1 diffractedradiation is captured in the second image.
[0076] Note that, by including only half of the first order diffracted radiation in eachimage, the ‘images’ referred to here are not conventional dark field microscopy images. Theindividual grating lines will not be resolved. Each grating will be represented simply by anarea of a certain intensity level. In step S4, a region of interest (ROI) is carefully identifiedwithin the image of each component grating, from which intensity levels will be measured.This is done because, particularly around the edges of the individual grating images, intensityvalues can be highly dependent on process variables such as resist thickness, composition,line shape, as well as edge effects generally.
[0077] FTaving identified the ROI for each individual grating and measured itsintensity, the asymmetry of the grating structure, and hence overlay error, can then bedetermined. This is done by the image processor and controller PU in step S5 comparing theintensity values obtained for +1 and -1 orders for each grating 32-35 to identify anydifference in their intensity, and (S6) from knowledge of the overlay biases of the gratings todetermine overlay error in the vicinity of the target T.
[0078] In the prior applications, mentioned above, various techniques are disclosedfor improving the quality of overlay measurements using the basic method mentioned above.For example, the intensity differences between images may be attributable to differences inthe optical paths used for the different measurements, and not purely asymmetry in the target.The illumination source 11 may be such that the intensity and/or phase of illumination spot31 is not uniform. Corrections can the determined and applied to minimize such errors, byreference for example to the position of the target image in the image field of sensor 23.These techniques are explained in the prior applications, and will not be explained here in further detail. They may be used in combination with the techniques newly disclosed in thepresent application, which will now be described.
[0079] Overlay measurements according to this method assumes that the measuredasymmetry is proportional only to the actual overlay shift between grating layers. However,this is not necessarily the case as the measured asymmetry is also affected by featureasymmetry effects that occur in production of the gratings. These feature asymmetry effectsinclude side-wall angle asymmetry and floor-tilt, and perturb the first order asymmetry-basedoverlay measurement. This will result in a bias on the measurement, and therefore aninaccurate overlay measurement.
[0080] Figure 7 is a flowchart adapting step S6 of the flowchart of Figure 6 to use anA+ versus A- regression to analyze diffraction-based overlay measurements (DBO andpDBO), by determining the asymmetry of the positively biased grating A+ as function of theasymmetry of the negatively biased grating A-. At step S6-1 A+ and A- is determined for anumber or different measured pupil pixels and/or a number or different wavelength-polarization combinations (i.e. for a number of different illumination conditions orillumination “recipes”). Following this, at step S6-2, the determined values of A+ are plottedagainst the determined values of A- to yield the overlay.
[0081] Figure 8 is a plot of A+ against A- for overlay gratings that have no featureasymmetry, such that the only asymmetry present is the asymmetry due to the bias andoverlay. In this case, the relation between A+ and A- lies on a straight line through the origin.Remarkably, all measured wavelength-polariz.ation combinations lie on this line. The slope ofthis line is related to the overlay. The Figure shows four lines: [0082] the dotted line labeled OV=0 is a line indicating zero overlay, having a slope of -1; [0083] the dotted line labeled OV—>co is a line having a slope of +1, indicative ofoverlay approaching infinity [0084] the solid line labeled OV<0 is a line having a slope less than -1 whichindicates overlay less than zero; and [0085] the solid line labeled OV>0 is a line having a slope greater than -1 whichindicates overlay greater than zero; [0086] Additionally, it can be seen that overlay equal to -t-d, where d is the gratingbias, would result in a plotted line along the y-axis; and overlay equal to -d would result in aplotted line along the x-axis.
[0087] It is proposed to use A+ versus A- regression to: [0088] measure the correct overlay as it would be without a contribution attributableto feature asymmetry, by determination of the slope of a line fitted through the data set, theline not necessarily being fitted through the origin; [0089] enable analysis of the feature asymmetry over the wafer via the offset of theline from the origin (i.e. from the intercept term); [0090] perform illumination recipe-optimization by selection of the wavelength-polarization combination(s) that is(are) least sensitive to feature asymmetry.
[0091] Figure 9 is a plot of A+ against A- illustrating the first two of these aspects,such as may be plotted in step S6-2. According to the known method discussed above, datapoints 930 would be fitted with a line 900 through the origin. However, in this embodimentthe data points are fitted according to a best fit method (for example, least squares) by a line910 not necessarily going through the origin (step S6-3). In this way the overlay can still becalculated from the slope of the line 910 (step S6-4); it can be seen that line 910 is parallel toa line 920 indicative of that which would be seen for the same measured structure having nofeature asymmetry. The axis intercept of line 910, that is the offset of line 910 from line 920(a line having the same slope as line 910, but plotted through the origin) indicatesquantitatively the effect of the feature asymmetry (step S6-5).
[0092] With d the overlay-bias of the two symmetrically biased gratings of the targetand slope the slope of line 910, the overlay can be calculated from Figure 9 as (with alinearized relation between the asymmetry and the overlay):
[0093] For a pitch-periodic sine-relation the overlay can similarly be understood as:
where pitch is the grating pitch.
[0094] Figure 10a is a plot of A+ against A- for (simulated) data of differentpolarization-wavelength combinations, for gratings having no feature asymmetry. It can beseen that all the data fits on the same line, as already discussed. Figure 10b shows a similarplot as that of Figure 10a, but with feature asymmetry present, specifically a 0.5 nm floor-tilt.In both cases data marked by a circle represents TE radiation and data marked by a crossrepresents TM radiation. Although it cannot be seen here, position along the line is largelydetermined by wavelength (for a given polarization) such that shorter (violet) wavelengths tend to be found at the upper end of the line (A+=6 to 8), and the longer (red) wavelengthstend to be found at the lower end of the line.
[0095] From Figure 10b it can be seen that wavelength- and polarization-dependentdeviation from the linear relationship is observed in the region 1000 around the origin.Overlay sensitivity, in this example of a 0.5 nm floor-tilt, is smallest for TE polarization.Furthermore, data with the largest K-value (the proportionality factor between overlay andasymmetry), i.e. the largest sensitivity to overlay, can also be easily identified, this being data1010 which still shows a linear relationship farthest from the origin. The data 1010 in thisexample is for radiation in the short wavelength (violet) region. Consequently a plot such asthis allows selection of an optimal illumination recipe (optional step S6-6) which when usedto measure a grating, yields data 1010 most sensitive to overlay and least dependent onfeature asymmetry.
[0096] In a practical overlay recipe optimization, a number of measurements over thewafer should be performed for different colors and polarizations, such that all possible featureasymmetries on the wafers (e.g. at the edge) are considered. Once the optimum recipe isselected, the measurements can be performed with this single wavelength-polarization-aperture combination.
[0097] If none of the single wavelength-polarization-aperture combinations areproviding sufficient feature asymmetry robustness, it may be possible to identify acombination of 2 or 3 settings using this method combined with the A+ versus A- regressionanalysis explained above. This may be the case where each individual setting yields a cloudof data entries, and the line through 2 to 3 settings shows a non-zero axis cut-off; the slope ofsuch a line would still yield relatively asymmetry robust overlay data. To do this, 2 or 3settings are needed for the actual overlay measurements.
[0098] Figure 11 shows that feature asymmetry results in a vertical offset K0 in thedata plotted on a graph of asymmetry A versus overlay OV. Line 1100 fits data for a gratingstructure having no asymmetry, and line 1110 fits data for a grating structure having someasymmetry. It can be shown that A =K1 sin (OV).
[0099] For very large feature asymmetries, the methods disclosed herein showsignificant deviations from a line. This is illustrated in Figure 12a, which shows simulationdata on an A+ versus A- plot for gratings having large feature asymmetry (note: K0 isconstant over the pupil in this model). As can be seen the data points do not all he on or closeto a line, making fitting to the data very difficult.
[0100] To counter this, in addition to the above methods, a third grating (or gratingpair) can be used, such that the grating structure comprises gratings having three differentbiases. In a specific embodiment, in addition to the +d and -d gratings, there is provided agrating without any bias. This enables the extraction of relative asymmetry, which can beplotted on a graph of Α+-Α0 versus A-AO (Figure lib), where AO is the asymmetry of thezero-biased grating. It can be seen that the resultant data is much less sensitive to featureasymmetry (all data points lie essentially on the same line) which enables the extraction ofthe overlay even in the presence of a large feature asymmetry. It should be noted that K0 isallowed to vary over the pupil in this embodiment. Such measurements can be used todetermine whether there is feature asymmetry is present in the stack. Using this method, theasymmetry A can be calculated as: A= K0 + KI sin (OV).
[0101] This results in an error decrease, compared to the two-bias example, of:
[0102] A suitable grating structure for this method is illustrated in Figure 13. Itcomprises two mutually perpendicular gratings having a negative bias -d, two mutuallyperpendicular gratings having a zero bias and two mutually perpendicular gratings having apositive bias +d. Such a grating structure is directly applicable to small target design as usedfor pDBO targets.
[0103] This method can be combined with others described herein. For example,process-asymmetry sensitivity can be reduced by recipe optimization, experimentallyselecting the least sensitive wavelength and polarization.
[0104] In summary, the overlay analysis- and recipe-selection method using the 1stbias asymmetry vs. 2nd bias asymmetry regression may comprise: [0105] analysis of smallest ‘pupil-sigma’ (recipe setting that fits best on the line, e.g in least-squares sense); [0106] analysis of the processing-asymmetry sensitivity (offset of this line withrespect to the origin, or where there is no linear dependence); a test of model-consistency over the pupil of the ‘linear’ and ‘ATAN’ model; andanalysis of the processing asymmetry over the wafer.
[0107] In addition it should be noted that the proposed recipe selection and overlayanalysis method does not require any stack information to perform.
[0108] For pupil-based analysis, the methods disclosed herein correctly include datapoints over which the K-value (proportionality factor between overlay and asymmetry)changes sign. This potentially extends the usable recipe-settings range for a goodmeasurement, and enables (for example) selection based on other parameters such aslinearity-range.
[0109] It has been disclosed above that overlay can be estimated from the slope of aregressed line 910 such as that show on Figure 9. Feature asymmetry in the stack causes thisregressed line to shift away from the origin. This shift in the regressed line effectivelyobserves the feature asymmetry.
[0110] It is further proposed to correct the estimated overlay across the wafer byutilizing this measured asymmetry shift. Given wafer measurements, the covariance betweenthe estimated overlay and the measured asymmetry shift clearly shows that there is acorrelation between these two measurements over the wafer. Thus, it is proposed to correctthe estimated overlay over the wafer as function of the measured asymmetry shift. To find theoptimal correction, a minimization (for example a linear minimization such as least squares)can be set up which minimizes the difference between overlay estimated with TE polarizedradiation and overlay estimated with TM polarized radiation.
[0111] Such a method may comprise the steps of: [0112] Performing the method steps S6-1 to S6-5 at a number of locations on a waferusing both TE polarized radiation and TM polarized radiation (individually). As a result ofthese measurements, estimates for overlay (line slope) and process asymmetry (line offset) ateach wafer location will be obtained, for both the TE polarized radiation and the TMpolarized radiation.
[0113] Minimizing the difference between overlay estimated with TE polarizedradiation and overlay estimated with TM polarized radiation estimated in the previous step soas to find a correction for the estimated overlay over the wafer as function of the measuredprocess asymmetry.
[0114] Figure 14 illustrates that making corrections for process asymmetry asdescribed minimizes the difference between overlay estimated with TE and TM radiation.Figures 14(a) and 14(b) show representations of uncorrected overlay measurementsperformed over a wafer using, respectively, TE radiation and TM radiation. Figure 14(c) isthe difference between the measurements of Figures 14(a) and 14(b). Figures 14(d) and 14(e)show representations of overlay measurements performed over a wafer using, respectively,TE radiation and TM radiation; and that have been corrected in accordance with this embodiment. Figure 14(f) is the difference between the measurements of Figures 14(d) and14(e). It can be seen clearly that the difference between the TE overlay measurements andTM overlay measurements is smaller for the corrected overlay measurements than for theuncorrected overlay measurements.
[0115] By way of further evidence of the efficacy of this method, overlay wascalculated independently across 3 separate data sets, corresponding to 3 separate wavelengthTE/TM measurement pairs. The differences in the estimated overlay between these 3 setswere considered. It was observed that the RMS of the difference in overlay measured withseparate wavelengths and polarizations was improved by 0.2nm for both the x and ydirection, resulting in a 70% improvement in accuracy.
[0116] Furthermore, it is remarked that the techniques disclosed herein can be appliedto large scatterometer targets, also referred to as standard targets, Using the apparatus ofFigure 3, for example, the overlay in these larger targets can be measured by angle-resolvedscatterometry using the pupil image sensor 19 instead of or in addition to measurements madein the dark-field imaging branch and sensor 23.
[0117] The targets in this proposal also allow for the standard overlay calculationmethods that do not take the bottom grating asymmetry (BGA) into account (‘linear’- and‘ATAN’-method), to be applied from the measurement.
[0118] While the target structures described above are metrology targets specificallydesigned and formed for the purposes of measurement, in other embodiments, properties maybe measured on targets which are functional parts of devices formed on the substrate. Manydevices have regular, grating-like structures. The terms ‘target grating’ and ‘target structure’as used herein do not require that the structure has been provided specifically for themeasurement being performed.
[0119] In association with the physical grating structures of the targets as realized onsubstrates and patterning devices, an embodiment may include a computer programcontaining one or more sequences of machine-readable instructions describing methods ofmeasuring targets on a substrate and/or analyzing measurements to obtain information abouta lithographic process. This computer program may be executed for example within unit PUin the apparatus of Figure 3 and/or the control unit LACU of Figure 2. There may also beprovided a data storage medium (e.g., semiconductor memory, magnetic or optical disk)having such a computer program stored therein. Where an existing metrology apparatus, forexample of the type shown in Figure 3, is already in production and/or in use, the inventioncan be implemented by the provision of updated computer program products for causing a processor to perform the modified step S6 (including steps S6-1 to S6-6) and so calculateoverlay error with reduced sensitivity to feature asymmetry. The program may optionally bearranged to control the optical system, substrate support and the like to perform the steps S2-S5 for measurement of asymmetry on a suitable plurality of target structures.
[0120] Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, it will be appreciated thatthe invention may be used in other applications, for example imprint lithography, and wherethe context allows, is not limited to optical lithography. In imprint lithography a topographyin a patterning device defines the pattern created on a substrate. The topography of thepatterning device may be pressed into a layer of resist supplied to the substrate whereuponthe resist is cured by applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving a pattern in it after the resistis cured.
[0121] The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength ofor about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g.,having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.
[0122] The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractive, reflective,magnetic, electromagnetic and electrostatic optical components.
[0123] The foregoing description of the specific embodiments will so fully reveal thegeneral nature of the invention that others can, by applying knowledge within the skill of theart, readily modify and/or adapt for various applications such specific embodiments, withoutundue experimentation, without departing from the general concept of the present invention.Therefore, such adaptations and modifications are intended to be within the meaning andrange of equivalents of the disclosed embodiments, based on the teaching and guidancepresented herein. It is to be understood that the phraseology or terminology herein is for thepurpose of description by example, and not of limitation, such that the terminology orphraseology of the present specification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.
[0124] The breadth and scope of the present invention should not be limited by any ofthe above-described exemplary embodiments, but should be defined only in accordance withthe following clauss and their equivalents.
[0125] It is to be appreciated that the Detailed Description section, and not theSummary and Abstract sections, is intended to be used to interpret the clauss. The Summaryand Abstract sections may set forth one or more but not all exemplary embodiments of thepresent invention as contemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended clauss in any way.
[0126] The present invention has been described above with the aid of functionalbuilding blocks illustrating the implementation of specified functions and relationshipsthereof. The boundaries of these functional building blocks have been arbitrarily definedherein for the convenience of the description. Alternate boundaries can be defined so long asthe specified functions and relationships thereof are appropriately performed.
[0127] The foregoing description of the specific embodiments will so fully reveal thegeneral nature of the invention that others can, by applying knowledge within the skill of theart, readily modify and/or adapt for various applications such specific embodiments, withoutundue experimentation, without departing from the general concept of the present invention.Therefore, such adaptations and modifications are intended to be within the meaning andrange of equivalents of the disclosed embodiments, based on the teaching and guidancepresented herein. It is to be understood that the phraseology or terminology herein is for thepurpose of description and not of limitation, such that the terminology or phraseology of thepresent specification is to be interpreted by the skilled artisan in light of the teachings andguidance.
[0128] The breadth and scope of the present invention should not be limited by any ofthe above-described exemplary embodiments, but should be defined only in accordance withthe following clauses and their equivalents. Other aspects of the invention are set out as in thefollowing numbered clauses: 1. A method of measuring a parameter of a lithographic process, the method comprisingthe steps of: (a) illuminating target structures on a substrate, the target structures comprising at least afirst target structure comprising an overlaid periodic structure having a first deliberate overlaybias and a second target structure comprising an overlaid periodic structure having a seconddeliberate overlay bias; and detecting radiation scattered by each target structure to obtain foreach target structure an asymmetry measurement representing an overall asymmetry thatincludes contributions due to (i) the deliberate overlay bias in the target structure, (ii) anoverlay error in a lithographic process during forming of the target structure and (iii) featureasymmetry within one or more of the periodic structures; (b) repeating step (a) for a plurality of different illumination conditions to obtainasymmetry measurement data; (c) performing a regression analysis on the asymmetry measurement data by fitting alinear regression model to a planar representation of asymmetry measurements for the firsttarget structure against asymmetry measurements for the second target structure, the linearregression model not necessarily being fitted through an origin of the planar representation;and (d) determining the overlay error from a gradient described by the linear regressionmodel. 2. A method as claused in claus 1 comprising determining the contribution of the overallasymmetry which is due to feature asymmetry from the intercept term of the linear regressionmodel. 3. A method as claused in claus 2 comprising the steps of: determining an overlay correction which is a function of the determined contribution of theoverall asymmetry that is due to feature asymmetry; andcorrecting the overlay error using the overlay correction. 4. A method as claused in claus 3 comprising: performing steps (a) to (d) to determine overlay error measurements for a plurality ofdifferent locations on the substrate; such that overlay error measurements for each locationare obtained using TE polarized radiation and using TM polarized radiation; andcalculating the overlay correction by performing a minimization of the difference betweenmeasured overlay error when measured using TE polarized radiation and measured overlayerror when measured using TM polarized radiation. 5. A method as claused in any preceding claus wherein the linear regression model fittedto the asymmetry measurements is fitted only to asymmetry measurement data lying in theregion of the origin. 6. A method as claused in claus 5 comprising identifying one or more optimalillumination conditions for which feature asymmetry makes minimal contribution to themeasured overall asymmetry, from the plurality of illumination conditions, the optimal illumination conditions being selected from those for which the measured asymmetries lie onor near a offset line, the offset line being that described by the linear regression model butwith zero intercept term such that it lies on the origin. 7. A method as claused in claus 6 wherein the optimal illumination conditions areselected from those for which the measured asymmetries are furthest away from anymeasured asymmetries not lying on or near the offset line. 8. A method as claused in claus 6 or 7 comprising making subsequent measurements ofstructures on the substrate using one of more of the optimal illumination conditions. 9. A method as claused in any preceding claus wherein the first deliberate overlay bias isa positive overlay bias and the second deliberate overlay bias is a negative overlay bias. 10. A method as claused in claus 9 wherein the first deliberate overlay bias and thesecond deliberate overlay bias are of equal magnitude. 11. A method as claused in any preceding claus wherein step (d) is performed with theassumption that the contribution due to feature asymmetry is constant for all values ofoverlay. 12. A method as claused in any of clauses 1 to 10 wherein the target structures comprise athird target structure having no deliberate overlay bias and the method comprises determiningrelative asymmetry measurements from the difference between asymmetry measurementsobtained in step (b) for the first target structure and the third target structure, and from thedifference between asymmetry measurements obtained in step (b) for the second targetstructure and the third target structure. 13. A method as claused in claus 12 wherein step (c) comprises fitting a linear regressionmodel to a planar representation of the difference of the asymmetry measurements for thefirst target structure and the asymmetry measurements for the third target structure against thedifference of the asymmetry' measurements for the second target structure and the asymmetrymeasurements for the third target structure. 14. A method as claused in claus 12 or 13 wherein step (d) is performed without theassumption that the contribution due to feature asymmetry is constant for all values ofoverlay. 15. A method as claused in any preceding claus comprising an initial step of using alithographic process to form the target structures on a substrate. 16. An inspection apparatus for measuring a parameter of a lithographic process, theapparatus comprising: a support for a substrate having a plurality of target structures thereon, the targetstructures comprising at least a first target structure comprising an overlaid periodic structurehaving a first deliberate overlay bias and a second target structure comprising an overlaidperiodic structure having a second deliberate overlay bias; an optical system being operable to illuminate the targets and detecting radiationscattered by each target to obtain for each target structure and for a plurality of differentillumination conditions, an asymmetry measurement representing an overall asymmetry thatincludes contributions due to (i) the deliberate overlay bias in the target structure, (ii) anoverlay error in a lithographic process during forming of the target structure and (iii) featureasymmetry within one or more of the periodic structures;a processor arranged to: perform a regression analysis on asymmetry measurement data by fitting a linear regressionmodel to a planar representation of asymmetry measurements for the first target structureagainst asymmetry measurements for the second target structure, the linear regression modelnot necessarily being fitted through an origin of the planar representation; anddetermine the overlay error from a gradient described by the linear regression model. 17. An inspection apparatus as claused in claus 16 wherein the processor is operable todetermine the contribution of the overall asymmetry which is due to feature asymmetry fromthe intercept term of the linear regression model. 18. An inspection apparatus as claused in claus 17 being operable to: determine an overlay correction which is a function of the determined contribution of theoverall asymmetry that is due to feature asymmetry; andcorrect the overlay error using the overlay correction. 19. An inspection apparatus as claused in claus 18 being operable to: perform steps (a) to (d) to determine overlay error measurements for a plurality of differentlocations on the substrate; such that overlay error measurements for each location areobtained using TE polarized radiation and using TM polarized radiation; andcalculate the overlay correction by performing a minimization of the difference betweenmeasured overlay error when measured using TE polarized radiation and measured overlayerror when measured using TM polarized radiation. 20. An inspection apparatus as claused in any of clauses 16 to 19 wherein the processor isoperable to fit the linear regression model to only the asymmetry measurement data lying inthe region of the origin. 21. An inspection apparatus as claused in any claus 20 wherein the processor is operableto identify one or more optimal illumination conditions for which feature asymmetry makesminimal contribution to the measured overall asymmetry, from the plurality of illuminationconditions, the optimal illumination conditions being selected from those for which themeasured asymmetries lie on or near a offset line, the offset line being that described by thelinear regression model but with zero intercept term such that it lies on the origin. 22. An inspection apparatus as claused in claus 21 wherein the processor is operable toselect the optimal illumination conditions from those for which the measured asymmetries arefurthest away from any measured asymmetries not lying on or near the offset line. 23. An inspection apparatus as claused in claus 21 or 22 being operable to makesubsequent measurements of structures on the substrate using one of more of the optimalillumination conditions. 24. An inspection apparatus as claused in any of clauss 16 to 23 wherein the firstdeliberate overlay bias is a positive overlay bias and the second deliberate overlay bias is anegative overlay bias. 25. An inspection apparatus as claused in claus 24 wherein the first deliberate overlaybias and the second deliberate overlay bias are of equal magnitude. 26. An inspection apparatus as claused in any of clauses 16 to 25 wherein the processor isoperable to determine the overlay error from a gradient described by the linear regressionmodel with the assumption that the contribution due to feature asymmetry is constant for allvalues of overlay. 27. An inspection apparatus as claused in any of clauses 16 to 25 wherein the targetstructures comprise a third target structure having no deliberate overlay bias and theprocessor is operable to determine relative asymmetry measurements from the differencebetween asymmetry measurements for the first target structure and the third target stmcture,and from the difference between asymmetry measurements for the second target structure andthe third target structure. 28. An inspection apparatus as claused in claus 27 wherein the processor is operable to fita linear regression model to a planar representation of the difference of the asymmetrymeasurements for the first target stmcture and the asymmetry measurements for the thirdtarget stmcture against the difference of the asymmetry measurements for the second targetstmcture and the asymmetry measurements for the third target stmcture. 29. An inspection apparatus as claused in clauses 27 or 28 wherein the processor isoperable to determine the overlay error from a gradient described by the linear regressionmodel with no assumption that the contribution due to feature asymmetry is constant for allvalues of overlay. 30. A lithographic apparatus comprising the inspection apparatus of any of clauses 16 to29, being operable to apply a device pattern to a series of substrates using a lithographic process,apply target structures to one or more of the series of substrates; measure an overlay parameter of the target structure using a method of measuring aparameter as claused in any of clauses 1 to 15; and control the lithographic process for later substrates in accordance with the result of themethod of measuring a parameter. 31. A method of manufacturing devices wherein a device pattern is applied to a series ofsubstrates using a lithographic process, the method including inspecting at least one periodicstructure formed as part of or beside the device pattern on at least one of the substrates usinga method as claused in any of clauses 1 to 15 and controlling the lithographic process for latersubstrates in accordance with the result of the inspection method. 32. A computer program product comprising machine-readable instructions for causing aprocessor to perform the processing steps (c) and (d) of a method according to any of clauses1 to 15, on asymmetry data, the asymmetry data being obtained by: illuminating targetstructures on a substrate, under a plurality of different illumination conditions, the targetstructures comprising at least a first target structure comprising an overlaid periodic structurehaving a first deliberate overlay bias and a second target structure comprising an overlaidperiodic structure having a second deliberate overlay bias; and detecting radiation scattered by each target structure to obtain for each target structure anasymmetry measurement representing an overall asymmetry that includes contributions dueto (i) the deliberate overlay bias in the target structure, (ii) an overlay error in a lithographicprocess during forming of the target structure and (iii) feature asymmetry within one or moreof the periodic structures.

权利要求:
Claims (1)
[1]
A lithography device comprising: an illumination device adapted to provide a radiation beam, a support constructed to support a patterning device, which patterning device is capable of applying a pattern in a cross-section of radiation beam to form a patterned radiation beam; to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projector.

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法律状态:
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