NL2010189A - Methods and apparatuses for detecting contaminant particles. - Google Patents

Abstract

A method and an inspection apparatus is disclosed for detecting contaminant particles such as one or more first, second and third contaminant particles having a first, a second and a third characteristic respectively on an article under inspection. The inspection apparatus includes an optical system having a single or a combination of filters associated with the first, second and third characteristics of the one or more first, second and third contaminant particles to discriminate chemical or mechanical types of the particles. For example, a hard particle and two different types of soft particles can be distinguished by using a color filter or differential filters.

Description

METHODS AND APPARATUSES FOR DETECTING CONTAMINANT

PARTICLES

BACKGROUND

Field

The invention relates to methods and apparatuses for detecting contaminantparticles on an article and particularly to methods, inspection apparatuses, andlithographic apparatuses for detecting contaminant particles on a patterning device.Background

Lithography is widely recognized as one of the key steps in the manufacture ofintegrated circuits (ICs) and other devices and structures. But as the dimensions offeatures made using lithography become smaller, lithography is becoming a morecritical factor for enabling miniature IC or other devices and structures to bemanufactured.

The lithographic apparatus applies a desired pattern onto a substrate, usuallyonto a target portion of the substrate. The lithographic apparatus can be used, forexample, in the manufacture of ICs. In that instance, a patterning device, which isalternatively referred to as a mask or a reticle, may be used to generate a circuitpattern to be formed on an individual layer of the IC. This pattern can be transferredonto a target portion (for example, including part of one or several dies) on a substrate(for example, a silicon wafer). Transfer of the pattern is typically via imaging onto alayer of radiation-sensitive material (resist) provided on the substrate. Generally, asingle substrate will contain a network of adjacent target portions that are successivelypatterned.

Current lithography systems project pattern features that are extremely small.Contaminant particles such as dust or other extraneous particulate matter appearing onthe surface of the patterning device can adversely affect the resulting product. Anyparticulate matter that deposits on the patterning before or during a lithographicprocess is likely to distort features in the pattern being projected onto a substrate.Additionally, hard contaminant particles can damage the reticle. For example, if hardcontaminant particles are deposited on a portion of the patterning device that isclamped, the hard contaminant particles can damage the patterning device.

A pellicle is often used with a patterning device during lithography processes.A pellicle is a thin transparent layer that may be stretched over a frame above thesurface of the patterning device. Pellicles are used to block particles from reachingthe surface of the patterning device. But in some lithographic processes, a pellicle isnot used. For example, during EUV lithography processes, pellicles are not usedbecause pellicles attenuate the imaging radiation. When a pellicle does not cover thepatterning device, the patterning device is prone to particle contamination that maycause imaging defects and damage the patterning device when clamped.

Accordingly, inspection and cleaning of the patterning device, for example, anEUV reticle, before moving the reticle to an exposure position or clamping thepatterning device can be an important aspect of handling a patterning device.Patterning devices are typically cleaned when contamination is suspected from aninspection or from historical statistics. Cleaning usually shortens the patterning devicelifetime, so unnecessary cleaning is to be avoided.

Patterning devices are typically inspected with scattered light imaging. Butscattered light imaging can only determine the size and location of contaminantparticles. Scattered light imaging cannot determine other characteristics ofcontaminant particles such as chemical composition or mechanical properties.Accordingly, there is a need for methods and apparatuses for detecting contaminantparticles that can discriminate particle characteristics such as chemical compositionand mechanical properties in addition to particle location and size.

SUMMARY

In an embodiment, a method is provided for discriminating contaminantparticles on an article under inspection, comprising:illuminating the article with radiation at one or more first wavelengths;detecting the scattered radiation from the illuminated article for generating a firstimage to detect any first, second, third contaminant particles present on the article, thefirst image indicating a presence and a location of the any first, second, thirdcontaminant particles, the first contaminant particles having a first characteristic, thesecond contaminant particles having a second characteristic, the third contaminantparticles having a third characteristic; illuminating the article with radiation at one or more second wavelengths suitable forexciting fluorescent radiation from one or more first contaminant particles of the anyfirst contaminant, particles at one or more third wavelengths; filtering out the radiation at the one or more third wavelengths generated from the oneor more second contaminant particle using a single or a combination of filtersassociated with the first, second or third characteristics of the any first, second, thirdcontaminant particles, respectively; generating a second image of the illuminated article, the second image being formedby the radiation at the one or more third wavelengths, the second image indicating thepresence of the one or more second contaminant particles having the secondcharacteristic; and determining that the any first and third contaminant particles having a characteristicdifferent than the second characteristic of the one or more second contaminationparticles by comparing the first image to the second image.

In another embodiment, an inspection apparatus is provided for detectingcontaminant particles such as one or more first, second and third contaminantparticles having a first, a second and a third characteristics respectively on an articleunder inspection, comprising: a radiation source configured to generate radiation at one or more first wavelengthsand one or more second wavelengths, the one or more second wavelengths beingsuitable for exciting fluorescent radiation from a first contaminant particle at one ormore third wavelengths, the first contaminant particle having the first characteristic;an optical system including a single or a combination of filters associated with thefirst, second and third characteristics of the one or more first, second and thirdcontaminant particles, the optical system configured to illuminate the article with radiation at the one or more first wavelengths,to generate a first image of the illuminated article, the first image indicating apresence and a location of the contaminant particles on the article,to illuminate the article with radiation at the one or more second wavelengths,to filter out the radiation at the one or more third wavelengths generated fromthe one or more second con lam in ant particles, and to generate a second image of the illuminated article, the second image beingformed by the radiation at the one or more third wavelengths, the second image indicating a presence and a location of one or more first contaminant particles on the article; and a processor configured to compare the first image to the second image to determine apresence and a location of the one or more second contaminant particles on the article,the one or more second contaminant particles having the second characteristic that isdifferent from the first characteristic of the first contamination particle.

In one embodiment, a lithographic apparatus includes a patterning device, asupporting structure for a substrate, a projection optical system for transferring apattern from the patterning device to the substrate, and an inspection apparatus fordetecting contaminant particles on the patterning device. The inspection apparatuscan be any one of the above inspection apparatuses.

Further features and advantages of the invention, as well as the structure andoperation of various embodiments of the invention, are described in detail below withreference to the accompanying drawings. It is noted that the invention is not limitedto the specific embodiments described herein. Such embodiments are presentedherein for illustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form part ofthe specification, illustrate the present invention and, together with the description,further serve to explain the principles of the invention and to enable a person skilledin the relevant art(s) to make and use the invention. Embodiments of the inventionare described, by way of example only, with reference to the accompanying drawings.

Figure 1 depicts schematically the lithographic apparatus having reflectiveprojection optics.

Figure 2 is a more detailed view of the apparatus of Figure 1.

Figure 3 is a more detailed view of an alternative source collector module forthe apparatus of Figures 1 and 2.

Figure 4 depicts an alternative example of an EUV lithographic apparatus.

Figure 5 depicts schematically an apparatus for detecting contaminantparticles on an article under inspection according to an embodiment.

Figure 6 illustrates (a) the spatial distribution of contaminant particles on apatterning device; (b)-(d) images of the patterning device generated using theapparatus of Figure 5; and (e) a discriminated image using the images shown in (b)-(d).

Figure 7 depicts schematically another apparatus for detecting contaminantparticles on an article under inspection according to another embodiment.

Figure 8 depicts schematically an apparatus for detecting contaminantparticles on an article under inspection according to yet another embodiment.

Figure 9 is a flowchart for detecting contaminant particles on an article underinspection.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken in conjunction withthe drawings, in which like reference characters identify corresponding elementsthroughout. In the drawings, like reference numbers generally indicate identical,functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporate thefeatures of this invention. The disclosed embodiment(s) merely exemplify theinvention. The scope of the invention is not limited to the disclosed embodiment(s).The invention is defined by the clauses appended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment,” “an embodiment,” “an example embodiment,” etc., indicate that theembodiments described may include a particular feature, structure, or characteristic,but every embodiment may not necessarily include the particular feature, structure, orcharacteristic. Moreover, such phrases are not necessarily referring to the sameembodiment. Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it is within theknowledge of one skilled in the art to effect such feature, structure, or characteristic inconnection with other embodiments whether or not explicitly described.

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 and executed by one or more processors. A machine-readable medium mayinclude any mechanism for storing or transmitting information in a form readable by amachine (for example, a computing device). For example, a machine-readablemedium 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 (for example, carrier waves,infrared signals, digital signals, etc.); and others. Further, firmware, software,routines, instructions may be described herein as performing certain actions.However, it should be appreciated that such descriptions are merely for convenienceand that such actions in fact result from computing devices, processors, controllers, orother devices executing the firmware, software, routines, instructions, etc.

Before describing such embodiments in more detail, however, it is instructiveto present an example environment in which the embodiments may be implemented.

Figure 1 schematically depicts a lithographic apparatus 100 including a sourcecollector module SO according to one embodiment. Apparatus 100 comprises anillumination system (illuminator) IL configured to condition a radiation beam B (forexample, EUV radiation), a support structure (for example, a mask table) MTconstructed to support a patterning device (for example, a mask or a reticle) MA andconnected to a first positioner PM configured to accurately position the patterningdevice, a substrate table (for example, a wafer table) WT constructed to hold asubstrate (for example, a resist-coated wafer) W and connected to a second positionerPW configured to accurately position the substrate, and a projection system (forexample, a reflective projection system) PS configured to project a pattern imparted tothe radiation beam B by patterning device MA onto a target portion C (for example,comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other typesof optical components, or any combination thereof, for directing, shaping, orcontrolling radiation.

The support structure MT holds the patterning device MA in a manner thatdepends on the orientation of the patterning device, the design of the lithographicapparatus, and other conditions such as, for example, whether or not the patterningdevice is held in a vacuum environment. The support structure can use mechanical,vacuum, electrostatic, or other clamping techniques to hold the patterning device.

The support structure may be a frame or a table, for example, that may be fixed ormovable as required. The support structure may ensure that the patterning device is ata desired position, for example, with respect to the projection system.

The term “patterning device” should be broadly interpreted as referring to anydevice that can be used to impart a radiation beam with a pattern in its cross-sectionsuch as to create a pattern in a target portion of the substrate. The pattern imparted tothe radiation beam may correspond to a particular functional layer in a device beingcreated in the target portion, for example, an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, programmable LCDpanels, and reticles. Masks are well known in lithography and include mask typessuch as binary, alternating phase-shift, and attenuated phase-shift, as well as varioushybrid mask types. An example of a programmable mirror array employs a matrixarrangement of small mirrors, each of which can be individually tilted to reflect anincoming radiation beam in different directions. The tilted mirrors impart a pattern ina radiation beam that is reflected by the mirror matrix.

The projection system, like the illumination system, may include various typesof optical components, such as refractive, reflective, magnetic, electromagnetic,electrostatic or other types of optical components, or any combination thereof, asappropriate for the exposure radiation being used, or for other factors such as the useof a vacuum. It may be desired to use a vacuum for EUV radiation since other gasesmay absorb too much radiation. A vacuum environment may therefore be provided tothe whole beam path with the aid of a vacuum wall and vacuum pumps.

As shown in Figure 1, apparatus 100 is of a reflective type (for example,employing a reflective mask).

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,the additional tables may be used in parallel, or preparatory steps may be carried outon one or more tables while one or more other tables are being used for exposure.

Referring to Figure 1, illuminator IL can receive an extreme ultra violetradiation beam from source collector module SO. Methods to produce EUV lightinclude, but are not necessarily limited to, converting a material into a plasma statethat has at least one element, for example, xenon, lithium, or tin, with one or moreemission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a fuel, such as adroplet, stream, or cluster of material having the required line-emitting element with alaser beam. Source collector module SO may be part of an EUV radiation systemincluding a laser, not shown in Figure 1, for providing the laser beam that excites thefuel. The resulting plasma emits output radiation, for example, EUV radiation, whichis collected using a radiation collector disposed in source collector module SO. Thelaser and source collector module SO may be separate entities, for example, when aC02 laser is used to provide the laser beam for fuel excitation.

In such cases, the laser is not considered to form part of the lithographicapparatus, and the radiation beam is passed from the laser to source collector moduleSO with the aid of a beam delivery system comprising, for example, suitable directingmirrors and/or a beam expander. In other cases the source may be an integral part ofsource collector module SO, for example, when the source is a discharge producedplasma EUV generator, often termed as a DPP source.

Illuminator IL may comprise an adjuster for adjusting the angular intensitydistribution 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 illuminator IL can be adjusted. In addition, illuminatorIL may comprise various other components such as facetted field and pupil mirrordevices. Illuminator IL may be used to condition the radiation beam to have a desireduniformity and intensity distribution in its cross-section.

Radiation beam B is incident on patterning device (for example, a mask orreticle) MA, that is held on the support structure (for example, mask table) MT, and ispatterned by the patterning device. After being reflected from patterning device MA,radiation beam B passes through a projection system PS that focuses the beam onto atarget portion C of a substrate W. With the aid of a second positioner PW and aposition sensor PS2 (for example, an interferometric device, linear encoder, orcapacitive sensor), the substrate table WT can be moved accurately, for example, toposition different target portions C in the path of the radiation beam B. Similarly, thefirst positioner PM and another position sensor PS1 can be used to accurately positionthe patterning device MA with respect to the path of radiation beam B. Patterningdevice MA and substrate W may be aligned using mask alignment marks Ml, M2 andsubstrate alignment marks PI, P2.

The depicted apparatus could be used in at least one of the following modes: 1. In step mode, support structure (for example, mask table) MT and substrate tableWT 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 staticexposure). Substrate table WT is then shifted in the X and/or Y direction so thata different target portion C can be exposed.

2. In scan mode, support structure MT and substrate table WT are scannedsynchronously while a pattern imparted to the radiation beam is projected onto atarget portion C (i.e., a single dynamic exposure). The velocity and direction ofsubstrate table WT relative to support structure MT may be determined by the(de-)magnification and image reversal characteristics of projection system PS.

3. In another mode, support structure MT is kept essentially stationary holding aprogrammable patterning device, and substrate table WT is moved or scannedwhile a pattern imparted to the radiation beam is projected onto a target portionC. In this mode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after each movement ofthe substrate table WT or in between successive radiation pulses during a scan.This mode of operation can be readily applied to maskless lithography thatutilizes programmable patterning device, such as a programmable mirror array ofa type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Figure 2 shows apparatus 100 in more detail, including source collectormodule SO, illumination system IL, and projection system PS. Source collectormodule SO is constructed and arranged such that a vacuum environment can bemaintained in an enclosing structure 220 of source collector module SO. An EUVradiation emitting plasma 210 may be formed by a discharge produced plasma source.EUV radiation may be produced by a gas or vapor, for example, Xe gas, Li vapor, orSn vapor, in which the very hot plasma 210 is created to emit radiation in the EUVrange of the electromagnetic spectrum. The very hot plasma 210 is created by, forexample, an electrical discharge causing an at least partially ionized plasma. Partialpressures of, for example, 10 Pa of Xe, Li, Sn vapor, or any other suitable gas orvapor may be required for efficient generation of the radiation. In an embodiment, aplasma of excited tin (Sn) is provided to produce EUV radiation.

The radiation emitted by the hot plasma 210 is passed from a source chamber211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230(in some cases also referred to as contaminant barrier or foil trap) that is positioned inor behind an opening in source chamber 211. Contaminant trap 230 may include achannel structure. Contaminant trap 230 may also include a gas barrier or acombination of a gas barrier and a channel structure. The contaminant trap orcontaminant barrier 230 further indicated herein at least includes a channel structureas known in the art.

Collector chamber 211 may include a radiation collector CO that may be a so-called grazing incidence collector. Radiation collector CO has an upstream radiationcollector side 251 and a downstream radiation collector side 252. Radiation thattraverses collector CO can be reflected off a grating spectral filter 240 to be focusedin a virtual source point IF. Virtual source point IF is commonly referred to as theintermediate focus, and source collector module SO can be arranged such that theintermediate focus IF is located at or near an opening 221 in enclosing structure 220.Virtual source point IF is an image of radiation emitting plasma 210.

Subsequently the radiation traverses illumination system IL, which mayinclude a facetted field mirror device 22 and a facetted pupil mirror device 24arranged to provide a desired angular distribution of a radiation beam 21, at patterningdevice MA, as well as a desired uniformity of radiation intensity at patterning deviceMA. Upon reflection of the beam of radiation 21 at patterning device MA, held bysupport structure MT, a patterned beam 26 is formed, and patterned beam 26 isimaged by projection system PS via reflective elements 28, 30 onto a substrate Wheld by the wafer stage or substrate table WT.

More elements than shown may generally be present in illumination opticsunit IL and projection system PS. Grating spectral filter 240 may optionally bepresent, depending upon the type of lithographic apparatus. Further, there may bemore mirrors present than those shown in the Figures, for example, there may be 1-6additional reflective elements present in projection system PS than shown in Figure 2.

Collector optic CO, as illustrated in Figure 2, is depicted as a nested collectorwith grazing incidence reflectors 253, 254, and 255, just as an example of a collector(or collector mirror). Grazing incidence reflectors 253, 254, and 255 are disposedaxially symmetric around an optical axis O, and collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often calleda DPP source.

Alternatively, source collector module SO may be part of an LPP radiationsystem as shown in Figure 3. A laser LA is arranged to deposit laser energy into afuel, such as xenon (Xe), tin (Sn), or lithium (Li) to create the highly ionized plasma210 with electron temperatures of several 10's of eV. The energetic radiationgenerated during de-excitation and recombination of these ions is emitted from theplasma, collected by a near normal incidence collector optic CO, and focused ontoopening 221 in enclosing stmcture 220.

Figure 4 shows an alternative arrangement for an EUV lithographic apparatusin which the spectral purity filter SPF is of a transmissive type, rather than a reflectivegrating. The radiation from source collector module SO in this arrangement follows astraight path from the collector to intermediate focus IF (virtual source point). Inalternative embodiments, not shown, the spectral purity filter 11 may be positioned atvirtual source point IF or at any point between the collector and virtual source pointIF. The filter can be placed at other locations in the radiation path, for example,downstream of virtual source point IF. Multiple filters can be deployed. As in theprevious examples, collector CO may be of the grazing incidence type (Figure 2) or ofthe direct reflector type (Figure 3).

It has been proposed to use the presence or absence of a photoluminescence(PL) signal as an indicator of the presence of a defect on a semiconductor substrate,see, for example, JP 2007/258567 or JP 11-304717, which are incorporated byreference herein in their entireties. However, improvements to the particle detectioncapabilities of these techniques are desired.

A spectroscopic approach for detecting contaminants on a patterning device,such as an EUV lithography reticle, has been proposed in International PatentApplication No. PCT/EP2010/059460, which was filed on July 2, 2010, which isincorporated by reference herein in its entirety. Particularly, time-resolvedspectroscopy is described. To determine the actual position of contaminant particleson the patterning device, the area inspected is made smaller and smaller. This processrequires several measurement steps and adds greatly to the time required forinspection.

International Patent Application No. PCT/EP2011/067491, filed October 6,2011, and which clauses the benefit of U.S. Provisional Application No. 61/420,075, filed December 6, 2010, discloses an inspection method that uses a detected imageformed by secondary radiation emitted by the contaminant material to determine thepresence and location of certain contaminant material. The disclosures of the '491application and the '075 application are also incorporated by reference herein in theirentirety. But there is still a need to discriminate contaminant particles that do not emitsecondary radiation.

The following description presents methods and apparatuses for detectingcontaminant particles on an article under inspection and for determiningcharacteristics of detected particles. The article to be inspected can be, for example, alithographic patterning device for generating a circuit pattern to be formed on anindividual layer in an integrated circuit. Example patterning devices include a mask,a reticle, or a dynamic patterning device. Example reticles include reticles within anylithography process, for example, EUV lithography and imprint lithography.

During lithography processes, contaminant particles can be deposited on apatterning device MA. These contaminant particles can have varying characteristicssuch as different chemical compositions and different mechanical properties. Forexample, typical contaminant particles during lithographic processes include organicparticles, metal oxide or glass particles, and metal or semiconductor particles. Eachtype of contaminant may have a different mechanical property. For example, organicparticles are typically soft. Metal or semiconductor particles are typically hard. Andmetal oxide or glass particles are typically the hardest contaminant particle depositedon patterning device MA.

When electromagnetic radiation is incident on a surface of a solid, secondaryradiation of photons occurs in addition to the regular reflection of the radiation. Manyprocesses generate secondary photon radiation on the surfaces of solids. Suchprocesses include, for example, photoluminescence (PL), inelastic light scatteringprocesses (such as Raman scattering and surface enhanced Raman scattering (SERS)),and elastic light scattering. Other processes such as non-linear generation may beuseful in other applications. The efficiency of each of these phenomena depends onthe type of material involved. Contaminant particles that accumulate on a surface ofpatterning device MA, for example, a reticle used in EUV lithographic apparatus, willgenerally be of a different type of material than the patterning device MA. In thefollowing examples illustrated in Figures 5-9, one or more types ofphotoluminescence exhibited by at least some of the types of contaminant particle is exploited to detect of contaminant particles and determine characteristics of detectedcontaminant particles on patterning device MA.

Figure 5 schematically illustrates an inspection apparatus 500 according to anembodiment. Apparatus 500 includes one or more radiation sources 502. Forexample, apparatus 500 can include one continuous radiation source 502 that isconfigured to produce radiation in a wide range of selectable wavelengths, forexample, from near infrared (NIR) wavelengths to deep ultraviolet (DUV)wavelengths. In other examples, apparatus 500 includes two or more individualradiation sources 502 that are collectively configured to produce radiation in a widerange of selectable wavelengths, for example, from NIR wavelengths to DUVwavelengths. For simplicity, Figure 5 illustrates only one radiation source 502, thefollowing description references only one radiation source 502. But a person skilledin the art would understand that two or more radiation sources 502 can be configuredto collectively illuminate the patterning device MA as described below.

Radiation source 502 illuminates a patterning device MA, for example, areticle, using one or more illumination optics. For example, the illumination opticscan include one or more mirrors 504 that illuminate patterning device MA withradiation directed from a desired range of angles. Radiation source 502 selectivelyprovides primary radiation at one or more wavelengths. Radiation source 502 canselectively provide broadband radiation with wavelengths ranging from the visiblespectrum to NIR, for example. Radiation source 502 can also selectively provideprimary radiation at one or more wavelengths that excite photoluminescence in one ormore types of contaminant particles such that the particles emit secondary radiation atone or more different wavelengths. Typically, the wavelengths of the secondaryradiation are different than the wavelengths of the primary radiation. Differentwavelengths of secondary radiation can be emitted by different types ofcontamination material, or by different photoluminescence processes within the samematerial.

Apparatus 500 can include one or more detection optics, for example, anobjective lens and an imaging lens that collects the emitted radiation and delivers it toa sensor 506. Sensor 506 can be any suitable 2-dimensional detection device that iscapable of generating a full or partial image of patterning device MA underinspection. For example, sensor 506 can be a CCD or CMOS camera. Imagesgenerated by sensor 506 can be converted to pixel data and processed in a processing unit PU. Although no magnification is shown in Figure 5, magnification can be usedto adjust the size of the image field.

The radiation may be directed at patterning device MA from directly above orobliquely. The illumination optics may comprise reflective and/or transmissiveelements. The disclosure of different forms of illumination optics in InternationalApplication No. PCT/EP2010/059460, which is hereby incorporated by reference inits entirety.

The radiation passing through mirror 504 towards sensor 506 can include arelatively large amount of scattered primary radiation and relatively small amount ofthe secondary radiation emitted by contaminants, if any are present. Thus, apparatus500 can include a filter 508 within the detection optics to selectively filter whichwavelengths reach sensor 506. For example, filter 508 can be configured toselectively block the primary radiation while passing the secondary radiation. Filter508 can include one or more narrow band and/or differential color filter elements.

To filter out the primary radiation, filter 508 can be configured to filter outcertain spectral regions corresponding to the primary radiation wavelengths. Thesespectral regions can be sufficiently narrow that they can be separated using band filterelements along a beam path within the imaging optics without separating the signalinto a spectrum. Accordingly, apparatus 500 images patterning device MA on sensor506 in two-dimensions.

In some embodiments, the imaging optics can include one or morepolarizer/analyzer elements (not shown), the implementation of which would becomeapparent to a person having ordinary skill in the art.

In some embodiments, radiation source 502 can generate primary radiationwavelengths that are not wanted for the inspection of the article usingphotoluminescence. These unwanted wavelengths can optionally be filtered out atradiation source 502, and not in the imaging optics.

Filter 508 can be made from one or more filters elements as described inInternational Patent Application No. PCT/EP2011/067491. Each filter element candefine a particular notch frequency or pass band. The range of wavelengths that passthrough filter 508 and are detected by the sensor 506 can be wide, perhaps rangingfrom ultraviolet (UV) to infrared (IR).

Figure 6A is a plan view of patterning device MA under inspection byapparatus 500 of Figure 5. In some embodiments and as shown in Figures 6A-6E, the field of view of apparatus 500 includes the entire surface of patterning device MA. Inother embodiments, the field of view of apparatus 500 can be smaller than the entirepatterning device MA surface, and multiple images must be obtained to perform acomplete inspection. For example, apparatus 500 can generate a series of imagestaken at stepped intervals to cover the entire area of interest.

As shown in Figure 6A, contaminant particles 602, 604, and 606 are depositedon patterning device MA, for example, an EUV reticle. Contaminant particles 602,604, and 606 can have different characteristics. For example, contaminant particles602, 604, and 606 can be an organic particle (soft), a metal particle (hard), and a metaloxide, respectively. Accordingly, each contaminant particle has a different chemicalcomposition and physical parameters such as hardness or rigidity.

Figure 6B illustrates an image 608 of patterning device MA generated bysensor 506 when radiation source 502 illuminated patterning device MA withbroadband primary radiation. Here, filter 508 selectively allows the primary radiationto pass. Image 608 indicates the location and presence of contaminant particles 602,604, and 606 as bright spots 602', 604', and 606'. Broadband image 608 can indicatesubstantially all the contaminant particles that are deposited on patterning device MA.Broadband image 608 can also show the size of the contaminant particles 602, 604,and 606. But image 608 does not discriminate chemical composition or physicalparameters of the contaminant particles 602, 604, and 606.

To discriminate the chemical composition or physical parameters ofcontaminant particles 602, 604, and 606, one or more images are generated at sensor506 by providing primary radiation at one or more wavelengths known to excitesecondary radiation from the contaminants typically found on the article underinspection. For example, organic particles, metal oxide or glass particles, and metalor semiconductor particles are typically found on patterning device MA. Organicparticles tend to emit secondary radiation when illuminated with radiation at nearultraviolet (NUV) wavelengths, for example, about 400 nm or less. Metal oxide orglass particles tend to emit secondary radiation when illuminated with radiation atDUV wavelengths, for example, about 300 nm or less. And metal or semiconductorparticles do not emit secondary radiation when illuminated with radiation at visibleand NUV wavelengths, for example, about 800 nm or less. Accordingly, todiscriminate organic particles and metal oxide or glass particles, at least two images are generated at sensor 506 by selectively providing primary radiation at NUVwavelengths and at DUV wavelengths, respectively.

Figure 6C illustrates an image 610 of patterning device MA generated bysensor 506 when radiation source 502 illuminates patterning device MA with primaryradiation at one or more NUV wavelengths, which are known to excite secondaryradiation of organic particles. Filter 508 selectively blocks the scattered primaryradiation and any secondary radiation emitted by patterning device MA. Thepresence, size, and location of organic particle 602 are detectable in image 610 asbright spot 602". But image 610 does not indicate the presence of contaminantparticles 604 and 606 because contaminant particles 604 and 606 did not respond tothe primary radiation by emitting secondary radiation at wavelengths that were notblocked by filter 508.

A single filter or a combination of filters as a differential filter can be used asthe filter 508. For example, red, green and blue filters can be used alone or incombination to allow and block a desired w'avelength(s) to discriminate types (soft,hard, softer, and less soft) of contamination particles that may be present on the articlesuch as on a reticle surface.

In one example, the radiation source 502 can illuminate the patterning deviceMA with a radiation at a narrow wavelength '/,L{ to generate signals at wavelengths λι,λ2, and λ3 from different contamination particles such that λ£ΐ < λ] < λ2 < λ3 orilluminate the patterning device MA at a wavelength λb to generate wavelengths λ2, λ3such that /.b < λ2 < λ3 or illuminate the patterning device MA at a wavelength kc togenerate wavelength λ3 such that /.c < /.3. In this way, using a radiation at thewavelength λα the contamination particles emitting fluorescence at the wavelengths λι,λ2, and λ3 can be detected although the signal at the wavelength λ! may be the highestand the signal at the wavelength λ2 may be the lower and the signal at the wavelengthλ3 may be the lowest. And the three signals may overlap each other to some extent.Alternatively, a single radiation with a broader spread at the wavelength λ;, may beused to generate three signals at wavelengths λι, λ2, and λ3 which can be detected ordiscriminated by using one or more differential filters, as discussed above.

Figure 6D illustrates an image 612 of patterning device MA generated bysensor 506 when radiation source 502 illuminates patterning device MA with primaryradiation at one or more DUV wavelengths, which are known to excite secondaryradiation from metal oxide or glass particles. Filter 508 selectively blocks the scattered primary radiation and any secondary radiation emitted by patterning deviceMA. The presence, size, and location of metal oxide or glass particle 604 aredetectable in this image 612 as a bright spot 604".

But neither image 610 nor image 612 indicates the presence of metal orsemiconductor particle 606 because contaminant particle 606 does not emit secondaryradiation when illuminated by primary radiation at NUV or DUV wavelengths or atvisible wavelengths. Thus, to determine the presence and location of metal orsemiconductor particle 606, processing unit PU compares the spectrally-selectiveimages 610 and 612 to the broadband image 608 that indicates the presence ofsubstantially of contaminant particles on patterning device MA. This comparison caninclude direct comparisons of the images, subtractions of the images, and/orcorrelations of the images. Contaminant particles appearing in broadband image 608,but not in sprectrally-selective images 610 and 612 can be a material, for example, ametal or semiconductor material, that does not emit secondary radiation whenilluminated by primary radiation at NUV or DUV wavelengths.

Figure 6E illustrates a discriminated image 614 of patterning device MAshowing contaminant particles 602"', 604"', and 606'" with an indication of chemicalcomposition or physical parameters. Processing unit PU can then determine thechemical composition or physical parameter of every containment particle indicatedin broadband image 608 by using image 610, image 612, and a comparison of images610 and 612 to broadband image 608. Knowing what common contaminants that donot emit secondary radiation when illuminated by primary radiation used to generateimages 610 and 612, processing unit PU can determine the chemical composition ofparticles indicated in broadband image 608, but not in images 610 and 612.

Processing unit PU can deliver an inspection result to an operator, or to anautomatic control system of apparatus 500. In embodiments where processing unit PUis physically the same as the control unit of the larger apparatus, the result may ofcourse be delivered internally. The result may be delivered on a dedicated hardwareoutput, or as a message on multi-purpose communication channel. The inspectionresult indicates at least the presence and location of suspected contamination. It mayoptionally provide more detailed parameters of what is delected, for example intensityinformation.

Figure 7 is a schematic diagram of an inspection apparatus 700 according toanother embodiment. All the components of apparatus 500 are present and labeled the same, although their characteristics may be modified for reasons described below.Apparatus 700 includes a first optical branch 720 and a second optical branch 722.Second optical branch 722 can have imaging optics and a second sensor 718.Apparatus 700 can include an optical component, for example, a beam splitter ordichroic mirror, that divides radiation reflected or emitted from the field of view ofpatterning device MA into first optical branch 720 and second optical branch 722. Infirst optical branch 720, all radiation reaching sensor 506 passes through filter 508.And in second optical branch 722, all radiation reaching sensor 718 is unfiltered.Accordingly, second optical branch 722 and sensor 718 are well suited for generatinga broadband image such as image 608 described above.

One advantage of having two optical branches 720 and 722 is that the imagesensors 606 and 718 can be chosen according to their performance in respectivewavebands, rather than having to find a sensor with adequate performance over theentire spectrum of radiation to be detected. Sensitivity and noise performance of thesensors 606 and 718 in the bands of interest is likely to be better as a result.Similarly, design of the imaging optics will be easier, as it is very costly and difficultto provide imaging optics with low aberration across such a wide spectrum.

Figure 8 illustrates another inspection apparatus 800, which again is based onapparatus 500, and again is modified to include second optical branch 722 as inapparatus 700. As shown in Figure 8, second optical branch 722 can include a filter824. Optical element 716 can be a dichroic mirror that diverts scattered primaryradiation into second optical branch 722, where it is focused on image sensor 718.Filter 824 can be a color filter.

Information as to the type of contamination present can be useful for examplein the choice and control of decontamination (cleaning) process. Information on thetype of contamination can also be useful for diagnosing the source of contamination,and taking measures to reduce contamination in future. Further, identifying the typeof contamination and location of the contamination is useful to ensure that clampingthe patterning device MA does not damage the patterning device MA.

Figure 9 shows the main process steps for detecting contaminant particles onan article, for example, a reticle for use in a EUV lithography process, usingapparatuses 500, 700, and 800. Inspection apparatuses 500, 700, and 800 can beintegrated within the patterning device housing of a lithographic apparatus so that thepatterning device under inspection is mounted on the same support structure (mask table) MT used during lithographic operations. The support structure MT can bemoved under the inspection apparatus, or equivalently the inspection apparatus ismoved to where the patterning device is already loaded. Alternatively, the patterningdevice MA under inspection may be removed from the immediate vicinity of supportstructure MT to a separate inspection chamber where the inspection apparatus islocated. This latter option avoids crowding the lithographic apparatus with additionalequipment, and also permits the use of processes that would not be permitted orwould be undesirable to perform within the lithographic apparatus itself. Theinspection chamber can be closely coupled to the lithographic apparatus, or quiteseparate from it, according to preference. Alternative inspection apparatuses can beincluded in the same or a different chamber, to allow the detection of different typesof particles by different processes.

Returning to Figure 9, patterning device MA, for example, a reticle, is loadedat step 926 into the inspection apparatus (or the inspection apparatus is brought towhere the patterning device MA is already loaded). Prior to inspection, the patterningdevice may or may not have been used in the lithographic process. Using theinspection apparatus, an image of patterning device MA illuminated with broadbandradiation is acquired at step 928. This image can be a single image of the entirepatterning device, or a set of sub-area images that are processed individually orstitched into a larger image.

At step 930, processing unit PU analyzes the image generated at step 927 todetermine whether contaminant particles are present on patterning device MA and thelocation of such particles. If no particles are detennined to be present on patterningdevice MA, the inspection process can be terminated. But if the particles aredetected, the inspection process continues to step 932 to further discriminate thechemical composition or physical parameters of the contaminant particles.

At step 932, patterning device MA is illuminated with primary radiation at oneor more wavelengths known to excite secondary radiation of certain contaminantparticles. The primary radiation is filtered out, and a second image is generated usingthe secondary radiation. For example, the patterning device MA can be illuminatedwith radiation at NUV wavelengths such that organic particles emit secondaryradiation.

At step 934, processing unit PU analyzes the second image generated at step932 to determine whether contaminant particles that emit secondary radiation when exposed to the primary radiation of step 932 are present on patterning device MA.Processing unit PU also can determine the location of any such contaminant particles.

Next at step 936, patterning device MA is illuminated with primary radiationat one or more wavelengths that are different than the wavelengths used in step 933.These wavelengths are known to excite secondary radiation of contaminant particles,for example, metal oxide or glass particles, other than the type of particlediscriminated at step 934. The primary radiation is filtered out, and an image isgenerated using the secondary radiation of the contaminant particles. For example,the patterning device MA can be illuminated with radiation at DUV wavelengths suchthat any metal oxide or glass particles on patterning device MA emit secondaryradiation.

At step 938, processing unit PU analyzes the image generated at step 936 todetermine whether contaminant particles that emit secondary radiation when exposedto the primary radiation of step 936 are present on patterning device MA. Processingunit PU can also can determine the location of any such contaminant particles. Stepssimilar to steps 934-938 can be repeated for any other contaminant particles that maybe deposited on patterning device MA and that would emit secondary radiation whenilluminated with radiation at certain wavelengths.

To discriminate substantially all contaminant particles on patterning deviceMA, including those that do not emit secondary radiation and thus cannot bediscriminated at steps 934-938, processing unit PU can compare the image generatedat step 932 and the image generated at step 936 with the broadband image generatedat step 928. This comparison can include direct comparisons of the images,subtractions of the images, and correlations of the images. Processing unit PUdetermines which particles determined to be present at step 930 are not discriminatedat step 934 or at step 938. Such particles can be contaminant particles that do not emitsecond radiation, for example, metal or semiconductor particles.

Embodiments of the methods and apparatuses of the present disclosure can beused for the inspection of any type of patterning device, for example, a mask or anEUV reticle.

Again, inspection apparatuses 500, 700 and 800 can be an in-lool device, thatis, within a lithographic system, or can be a separate inspection apparatus. As aseparate apparatus, it can be used for purposes of patterning device inspection (forexample, prior to shipping). As an in-tool device, it can perform a quick inspection of a pattering device prior to using the reticle for a lithographic process. It may inparticular be useful to perform inspections in between the lithographic processes, forexample to check after a certain number of exposures whether the patterning is stillclean and to check before clamping the patterning device.

Processing of signals from the sensor may be implemented by processing unitPU in hardware, firmware, software, or any combination thereof. Unit PU may be thesame as a control unit of the lithographic apparatus, or a separate unit, or acombination of the two. Embodiments of the invention of various component parts ofthe invention may also be implemented as instructions stored on a machine-readablemedium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting informationin a form readable by a machine (for example, a computing device). For example, amachine-readable medium may include read only memory (ROM); random accessmemory (RAM); magnetic disk storage media; optical storage media; flash memorydevices; electrical, optical, acoustical or other forms of propagated signals (forexample, carrier waves, infrared signals, digital signals, etc.), and others. Further,firmware, software, routines or instructions may be described herein as performingcertain actions. However, it should be appreciated that such descriptions are merelyfor convenience and that such actions in fact result from computing devices,processors, controllers, or other devices executing the firmware, software, routines,instructions, etc.

It is to be appreciated that the Detailed Description section, and not theSummary and Abstract sections, is intended to be used to interpret the clauses. TheSummary and Abstract sections may set forth one or more but not all exemplaryembodiments of the present invention as contemplated by the inventor(s), and thus,are not intended to limit the present invention and the appended clauses in any way.

The present invention has been described above with the aid of functionalbuilding blocks illustrating the implementation of specified functions andrelationships thereof. The boundaries of these functional building blocks have beenarbitrarily defined herein for the convenience of the description. Alternate boundariescan be defined so long as the specified functions and relationships thereof areappropriately performed.

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 the art, readily modify and/or adapt for various applications such specificembodiments, without undue experimentation, without departing from the generalconcept of the present invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of the disclosedembodiments, based on the teaching and guidance presented herein. It is to beunderstood that the phraseology or terminology herein is for the purpose ofdescription 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 teachingsand guidance.

[0114] The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only inaccordance with the following clauses and their equivalents. Other aspects of theinvention are set out as in the following numbered clauses: 1. A method for discriminating contaminant particles on an article under inspection,comprising: illuminating the article with radiation at one or more first wavelengths;detecting the scattered radiation from the illuminated article for generating a firstimage to detect any first, second, third contaminant particles present on the article, thefirst image indicating a presence and a location of the any first, second, thirdcontaminant particles, the first contaminant particles having a first characteristic, thesecond contaminant particles having a second characteristic, the third contaminantparticles having a third characteristic; illuminating the article with radiation at one or more second wavelengths suitable forexciting fluorescent radiation from one or more first contaminant particles of the anyfirst contaminant particles at one or more third wavelengths; filtering out the radiation at the one or more third wavelengths generated from the oneor more second contaminant particle using a single or a combination of filtersassociated with the first, second or third characteristics of the any first, second, thirdcontaminant particles, respectively; generating a second image of the illuminated article, the second image being formedby the radiation at the one or more third wavelengths, the second image indicating thepresence of the one or more second contaminant particles having the secondcharacteristic; and determining that the any first and third contaminant particles having a characteristicdifferent than the second characteristic of the one or more second contaminationparticles by comparing the first image to the second image.

2. The method of clause f, further comprising: illuminating the article with radiation at one or more fourth wavelengths suitable forexciting fluorescent radiation from one or more third contaminant particles of the anythird contaminant particles at one or more fifth wavelengths; filtering out the radiation at the one or more fifth wavelengths generated from the oneor more third contaminant particles; generating a third image of the illuminated article, the third image being formed bythe radiation at the one or more fifth wavelengths, the third image indicating that theone or more third contaminant particles having the third characteristic; anddetermining that the any first contaminant particles having the first characteristic bycomparing the first, second and third images.

3. The method of clause 1, wherein the one or more first wavelengths comprise one ormore wavelengths in the visible spectrum or in the infrared spectrum.

4. The method of clause 1, wherein the one or more second wavelengths comprise oneor more wavelengths in the ultraviolet spectrum.

5. The method of clause 1, wherein the one or more second wavelengths comprise oneor more wavelengths in the deep ultraviolet spectrum.

6. The method of clause 1, wherein the article comprises a patterning device for use inoptical lithography.

7. The method of clause 6, wherein the patterning device comprises a reticle for use inEUV lithography.

8. The method of clause 1, wherein the first, second, and third characteristics arechemical composition categories.

9. The method of clause 8, wherein the chemical compositions categories comprise atleast one of the following: an organic composition, a metal composition, and a metaloxide composition.

10. The method of clause 1, wherein the first, second, and third characteristics aremechanical properties.

11. An inspection apparatus for detecting contaminant particles such as one or more first,second and third contaminant particles having a first, a second and a thirdcharacteristics respectively on an article under inspection, comprising: a radiation source configured to generate radiation at one or more first wavelengthsand one or more second wavelengths, the one or more second wavelengths beingsuitable for exciting fluorescent radiation from a first contaminant particle at one ormore third wavelengths, the first contaminant particle having the first characteristic;an optical system including a single or a combination of filters associated with thefirst, second and third characteristics of the one or more first, second and thirdcontaminant particles, the optical system configured to illuminate the article with radiation at the one or more first wavelengths,to generate a first image of the illuminated article, the first image indicating apresence and a location of the contaminant particles on the article,to illuminate the article with radiation at the one or more second wavelengths,to filter out the radiation at the one or more third wavelengths generated fromthe one or more second contaminant particles, and to generate a second image of the illuminated article, the second image beingformed by the radiation at the one or more third wavelengths, the secondimage indicating a presence and a location of one or more first contaminantparticles on the article; and a processor configured to compare the first image to the second image to determine apresence and a location of the one or more second contaminant particles on the article,the one or more second contaminant particles having the second characteristic that isdifferent from the first characteristic of the first contamination particle.

12. The inspection apparatus of clause 11, wherein: the radiation source is further configured to generate radiation at one or more fourthwavelengths suitable for exciting fluorescent radiation from a third contaminantparticle at one or more fifth wavelengths, the third contaminant particle having thethird characteristic; the optical system is further configured to illuminate the article with the radiation at the one or more fourthwavelengths, to filter out radiation at the one or more fifth wavelengths generated from theone or more third contaminant particles, and to generate a third image of the illuminated portion of the article, the thirdimage being formed by the radiation at the one or more fifth wavelengths, thethird image indicating a presence and a location of the one or more thirdcontaminant particles on the article; and the processor is further configured to compare the first, second and third images todetermine the presence and the location of the one or more first contaminant particleson the article.

13. The inspection apparatus of clause 11, wherein the one or more first wavelengthscomprise one or more wavelengths in the visible spectrum or in the infrared spectrum.

14. The inspection apparatus of clause 11, wherein the one or more second wavelengthscomprise one or more wavelengths in the ultraviolet spectrum.

15. The inspection apparatus of clause 11, wherein the one or more second wavelengthscomprise one or more wavelengths in the deep ultraviolet spectrum.

16. The inspection apparatus of clause 11, wherein the article comprises a patterningdevice for use in optical lithography.

17. The inspection apparatus of clause 16, wherein the patterning device comprises areticle for use in EUV lithography.

18. The inspection apparatus of clause 11, wherein the first, second, and thirdcharacteristics are one or more are chemical composition categories.

19. The inspection apparatus of clause 18, wherein the one or more chemicalcompositions categories comprise at least one of the following: an organiccomposition, a metal composition, and a metal oxide composition.

20. The inspection apparatus of clause 11, wherein the first, second, and thirdcharacteristics are mechanical properties.

21. A lithographic apparatus comprising:a supporting structure for a substrate; a projection optical system for transferring a pattern from a patterning device to thesubstrate; and an inspection apparatus for detecting contaminant particles such as one or more first, secondand third contaminant particles having a first, a second and a third characteristicsrespectively on an article under inspection, comprising: a radiation source configured to generate radiation at one or more first wavelengthsand one or more second wavelengths, the one or more second wavelengths beingsuitable for exciting fluorescent radiation from a first contaminant particle at one ormore third wavelengths, the first contaminant particle having the first characteristic;an optical system including a single or a combination of filters associated with thefirst, second and third characteristics of the one or more first, second and thirdcontaminant particles, the optical system configured to illuminate the article with radiation at the one or more first wavelengths,to generate a first image of the illuminated article, the first image indicating apresence and a location of the contaminant particles on the article,to illuminate the article with radiation at the one or more second wavelengths,to filter out the radiation at the one or more third wavelengths generated fromthe one or more second contaminant particles, and to generate a second image of the illuminated article, the second image beingformed by the radiation at the one or more third wavelengths, the secondimage indicating a presence and a location of one or more first contaminantparticles on the article; and a processor configured to compare the first image to the second image to determine apresence and a location of the one or more second contaminant particles on the article, the one or more second contaminant particles having the second characteristic that isdifferent from the first characteristic of the first contamination particle.

22. The inspection apparatus of clause 21, wherein the radiation source is further configured to generate radiation at one or more fourthwavelengths suitable for exciting fluorescent radiation from a third contaminantparticle at one or more fifth wavelengths, the third contaminant particle having thethird characteristic; the optical system is further configured: to illuminate the article with the radiation at the one or more fourthwavelengths, to filter out radiation at the one or more fifth wavelengths generated from theone or more third contaminant particles, and to generate a third image of the illuminated portion of the article, the thirdimage being formed by the radiation at the one or more fifth wavelengths, thethird image indicating a presence and a location of the one or more thirdcontaminant particles on the article; and the processor is further configured to compare the first, second and thirdimages to determine the presence and the location of the one or more firstcontaminant particles on the article.

Claims (1)

A lithography device comprising: an exposure device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being capable of applying a pattern in a cross-section of radiation beam to form a patterned radiation beam; a substrate table constructed 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 projection device.