WO2013117448A1

WO2013117448A1 - Methods and apparatuses for detecting contaminant particles

Info

Publication number: WO2013117448A1
Application number: PCT/EP2013/051555
Authority: WO
Inventors: Yuli Vladimirsky; James Walsh; Muhammad Arif; Olga Vladimirsky
Original assignee: Asml Holding N.V.
Priority date: 2012-02-07
Filing date: 2013-01-28
Publication date: 2013-08-15
Also published as: TW201337246A; NL2010189A

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

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of US provisional application 61/595,848, which was filed on 7 February 2012, and which is incorporated herein in its entirety by reference.

BACKGROUND

Field

[0002] The invention relates to methods and apparatuses for detecting contaminant particles on an article and particularly to methods, inspection apparatuses, and lithographic apparatuses for detecting contaminant particles on a patterning device.

Background

[0003] Lithography is widely recognized as one of the key steps in the manufacture of integrated circuits (ICs) and other devices and structures. But as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and structures to be manufactured.

[0004] The lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. The lithographic apparatus can be used, for example, in the manufacture of ICs. In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (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 a layer of radiation- sensitive material (resist) provided on the substrate. Generally, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0005] Current lithography systems project pattern features that are extremely small.

Contaminant particles such as dust or other extraneous particulate matter appearing on the surface of the patterning device can adversely affect the resulting product. Any particulate matter that deposits on the patterning before or during a lithographic process is likely to distort features in the pattern being projected onto a substrate. Additionally, hard contaminant particles can damage the reticle. For example, if hard contaminant particles are deposited on a portion of the patterning device that is clamped, the hard contaminant particles can damage the patterning device.

[0006] 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 the surface of the patterning device. Pellicles are used to block particles from reaching the surface of the patterning device. But in some lithographic processes, a pellicle is not used. For example, during EUV lithography processes, pellicles are not used because pellicles attenuate the imaging radiation. When a pellicle does not cover the patterning device, the patterning device is prone to particle contamination that may cause imaging defects and damage the patterning device when clamped.

[0007] Accordingly, inspection and cleaning of the patterning device, for example, an

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

[0008] Patterning devices are typically inspected with scattered light imaging. But scattered light imaging can only determine the size and location of contaminant particles. Scattered light imaging cannot determine other characteristics of contaminant particles such as chemical composition or mechanical properties. Accordingly, there is a need for methods and apparatuses for detecting contaminant particles that can discriminate particle characteristics such as chemical composition and mechanical properties in addition to particle location and size.

SUMMARY

[0009] In an embodiment, a method is provided 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 first image to detect any first, second, third contaminant particles present on the article, the first image indicating a presence and a location of the any first, second, third contaminant particles, the first contaminant particles having a first characteristic, the second contaminant particles having a second characteristic, the third contaminant particles having a third characteristic;

illuminating the article with radiation at one or more second wavelengths suitable for exciting fluorescent radiation from one or more first contaminant particles of the any first contaminant particles at one or more third wavelengths;

filtering out the radiation at the one or more third wavelengths generated from the one or more second contaminant particle using a single or a combination of filters associated with the first, second or third characteristics of the any first, second, third contaminant particles, respectively;

generating a second image of the illuminated article, the second image being formed by the radiation at the one or more third wavelengths, the second image indicating the presence of the one or more second contaminant particles having the second characteristic; and

determining that the any first and third contaminant particles having a characteristic different than the second characteristic of the one or more second contamination particles by comparing the first image to the second image.

In another embodiment, an inspection apparatus is provided for detecting contaminant particles such as one or more first, second and third contaminant particles having a first, a second and a third characteristics respectively on an article under inspection, comprising:

a radiation source configured to generate radiation at one or more first wavelengths and one or more second wavelengths, the one or more second wavelengths being suitable for exciting fluorescent radiation from a first contaminant particle at one or more third wavelengths, the first contaminant particle having the first characteristic; an optical system including 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, 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 a presence 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 from the one or more second contaminant particles, and

to generate a second image of the illuminated article, the second image being formed 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 a presence 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 is different from the first characteristic of the first contamination particle.

[0011] In one embodiment, a lithographic apparatus includes a patterning device, a supporting structure for a substrate, a projection optical system for transferring a pattern from the patterning device to the substrate, and an inspection apparatus for detecting contaminant particles on the patterning device. The inspection apparatus can be any one of the above inspection apparatuses.

[0012] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0013] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention. Embodiments of the invention are described, by way of example only, with reference to the accompanying drawings.

[0014] Figure 1 depicts schematically the lithographic apparatus having reflective projection optics.

[0015] Figure 2 is a more detailed view of the apparatus of Figure 1.

[0016] Figure 3 is a more detailed view of an alternative source collector module for the apparatus of Figures 1 and 2. [0017] Figure 4 depicts an alternative example of an EUV lithographic apparatus.

[0018] Figure 5 depicts schematically an apparatus for detecting contaminant particles on an article under inspection according to an embodiment.

[0019] Figure 6 illustrates (a) the spatial distribution of contaminant particles on a patterning device; (b)-(d) images of the patterning device generated using the apparatus of Figure 5; and (e) a discriminated image using the images shown in (b)-

(d).

[0020] Figure 7 depicts schematically another apparatus for detecting contaminant particles on an article under inspection according to another embodiment.

[0021] Figure 8 depicts schematically an apparatus for detecting contaminant particles on an article under inspection according to yet another embodiment.

[0022] Figure 9 is a flowchart for detecting contaminant particles on an article under inspection.

[0023] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION

[0024] This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the claims appended hereto.

[0025] The embodiment(s) described, and references in the specification to "one embodiment," "an embodiment," "an example embodiment," etc., indicate that the embodiments described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. [0026] Embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (for example, a computing 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 (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 convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.

[0027] Before describing such embodiments in more detail, however, it is instructive to present an example environment in which the embodiments may be implemented.

[0028] Figure 1 schematically depicts a lithographic apparatus 100 including a source collector module SO according to one embodiment. Apparatus 100 comprises an illumination system (illuminator) IL configured to condition a radiation beam B (for example, EUV radiation), a support structure (for example, a mask table) MT constructed to support a patterning device (for example, a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device, a substrate table (for example, a wafer table) WT constructed to hold a substrate (for example, a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate, and a projection system (for example, a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (for example, comprising one or more dies) of the substrate W.

[0029] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. [0030] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions such as, for example, whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, that may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example, with respect to the projection system.

[0031] The term "patterning device" should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, for example, an integrated circuit.

[0032] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, programmable LCD panels, and reticles. Masks are well known in lithography and include mask types such as binary, alternating phase- shift, and attenuated phase- shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam that is reflected by the mirror matrix.

[0033] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0034] As shown in Figure 1, apparatus 100 is of a reflective type (for example, employing a reflective mask).

[0035] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such "multiple stage" machines, the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0036] Referring to Figure 1, illuminator IL can receive an extreme ultra violet radiation beam from source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, for example, xenon, lithium, or tin, with one or more emission 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 a droplet, stream, or cluster of material having the required line-emitting element with a laser beam. Source collector module SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam that excites the fuel. The resulting plasma emits output radiation, for example, EUV radiation, which is collected using a radiation collector disposed in source collector module SO. The laser and source collector module SO may be separate entities, for example, when a C02 laser is used to provide the laser beam for fuel excitation.

[0037] In such cases, the laser is not considered to form part of the lithographic apparatus, and the radiation beam is passed from the laser to source collector module SO with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of source collector module SO, for example, when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0038] Illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of illuminator IL can be adjusted. In addition, illuminator IL may comprise various other components such as facetted field and pupil mirror devices. Illuminator IL may be used to condition the radiation beam to have a desired uniformity and intensity distribution in its cross-section.

[0039] Radiation beam B is incident on patterning device (for example, a mask or reticle) MA, that is held on the support structure (for example, mask table) MT, and is patterned 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 a target portion C of a substrate W. With the aid of a second positioner PW and a position sensor PS2 (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately, for example, to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device MA with respect to the path of radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0040] 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 table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). Substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

2. In scan mode, support structure MT and substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of substrate 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 a programmable patterning device, and substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0041] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0042] Figure 2 shows apparatus 100 in more detail, including source collector module SO, illumination system IL, and projection system PS. Source collector module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of source collector module SO. An EUV radiation 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, or Sn vapor, in which the very hot plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma 210 is created by, for example, an electrical discharge causing an at least partially ionized plasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor, or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[0043] The radiation emitted by the hot plasma 210 is passed from a source chamber

211 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 in or behind an opening in source chamber 211. Contaminant trap 230 may include a channel structure. Contaminant trap 230 may also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure as known in the art.

[0044] Collector chamber 211 may include a radiation collector CO that may be a so- called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point IF. Virtual source point IF is commonly referred to as the intermediate focus, and source collector module SO can be arranged such that the intermediate 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.

[0045] Subsequently the radiation traverses illumination system IL, which may include a facetted field mirror device 22 and a facetted pupil mirror device 24 arranged to provide a desired angular distribution of a radiation beam 21, at patterning device MA, as well as a desired uniformity of radiation intensity at patterning device MA. Upon reflection of the beam of radiation 21 at patterning device MA, held by support structure MT, a patterned beam 26 is formed, and patterned beam 26 is imaged by projection system PS via reflective elements 28, 30 onto a substrate W held by the wafer stage or substrate table WT. [0046] More elements than shown may generally be present in illumination optics unit IL and projection system PS. Grating spectral filter 240 may optionally be present, depending upon the type of lithographic apparatus. Further, there may be more mirrors present than those shown in the Figures, for example, there may be 1-6 additional reflective elements present in projection system PS than shown in Figure 2.

[0047] Collector optic CO, as illustrated in Figure 2, is depicted as a nested collector with 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 disposed axially 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 called a DPP source.

[0048] Alternatively, source collector module SO may be part of an LPP radiation system as shown in Figure 3. A laser LA is arranged to deposit laser energy into a fuel, such as xenon (Xe), tin (Sn), or lithium (Li) to create the highly ionized plasma 210 with electron temperatures of several 10's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near normal incidence collector optic CO, and focused onto opening 221 in enclosing structure 220.

[0049] Figure 4 shows an alternative arrangement for an EUV lithographic apparatus in which the spectral purity filter SPF is of a transmissive type, rather than a reflective grating. The radiation from source collector module SO in this arrangement follows a straight path from the collector to intermediate focus IF (virtual source point). In alternative embodiments, not shown, the spectral purity filter 11 may be positioned at virtual source point IF or at any point between the collector and virtual source point IF. 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 the previous examples, collector CO may be of the grazing incidence type (Figure 2) or of the direct reflector type (Figure 3).

[0050] 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 by reference herein in their entireties. However, improvements to the particle detection capabilities of these techniques are desired. [0051] A spectroscopic approach for detecting contaminants on a patterning device, such as an EUV lithography reticle, has been proposed in International Patent Application No. PCT/EP2010/059460, which was filed on July 2, 2010, which is incorporated by reference herein in its entirety. Particularly, time-resolved spectroscopy is described. To determine the actual position of contaminant particles on the patterning device, the area inspected is made smaller and smaller. This process requires several measurement steps and adds greatly to the time required for inspection.

[0052] International Patent Application No. PCT/EP2011/067491, filed October 6,

2011, and which claims the benefit of U.S. Provisional Application No. 61/420,075, filed December 6, 2010, discloses an inspection method that uses a detected image formed by secondary radiation emitted by the contaminant material to determine the presence and location of certain contaminant material. The disclosures of the '491 application and the '075 application are also incorporated by reference herein in their entirety. But there is still a need to discriminate contaminant particles that do not emit secondary radiation.

[0053] The following description presents methods and apparatuses for detecting contaminant particles on an article under inspection and for determining characteristics of detected particles. The article to be inspected can be, for example, a lithographic patterning device for generating a circuit pattern to be formed on an individual layer in an integrated circuit. Example patterning devices include a mask, a reticle, or a dynamic patterning device. Example reticles include reticles within any lithography process, for example, EUV lithography and imprint lithography.

[0054] During lithography processes, contaminant particles can be deposited on a patterning device MA. These contaminant particles can have varying characteristics such as different chemical compositions and different mechanical properties. For example, typical contaminant particles during lithographic processes include organic particles, metal oxide or glass particles, and metal or semiconductor particles. Each type of contaminant may have a different mechanical property. For example, organic particles are typically soft. Metal or semiconductor particles are typically hard. And metal oxide or glass particles are typically the hardest contaminant particle deposited on patterning device MA. [0055] When electromagnetic radiation is incident on a surface of a solid, secondary radiation of photons occurs in addition to the regular reflection of the radiation. Many processes generate secondary photon radiation on the surfaces of solids. Such processes include, for example, photoluminescence (PL), inelastic light scattering processes (such as Raman scattering and surface enhanced Raman scattering (SERS)), and elastic light scattering. Other processes such as non-linear generation may be useful in other applications. The efficiency of each of these phenomena depends on the type of material involved. Contaminant particles that accumulate on a surface of patterning device MA, for example, a reticle used in EUV lithographic apparatus, will generally be of a different type of material than the patterning device MA. In the following examples illustrated in Figures 5-9, one or more types of photoluminescence exhibited by at least some of the types of contaminant particle is exploited to detect of contaminant particles and determine characteristics of detected contaminant particles on patterning device MA.

[0056] Figure 5 schematically illustrates an inspection apparatus 500 according to an embodiment. Apparatus 500 includes one or more radiation sources 502. For example, apparatus 500 can include one continuous radiation source 502 that is configured to produce radiation in a wide range of selectable wavelengths, for example, from near infrared (NIR) wavelengths to deep ultraviolet (DUV) wavelengths. In other examples, apparatus 500 includes two or more individual radiation sources 502 that are collectively configured to produce radiation in a wide range of selectable wavelengths, for example, from NIR wavelengths to DUV wavelengths. For simplicity, Figure 5 illustrates only one radiation source 502, the following description references only one radiation source 502. But a person skilled in the art would understand that two or more radiation sources 502 can be configured to collectively illuminate the patterning device MA as described below.

[0057] Radiation source 502 illuminates a patterning device MA, for example, a reticle, using one or more illumination optics. For example, the illumination optics can include one or more mirrors 504 that illuminate patterning device MA with radiation directed from a desired range of angles. Radiation source 502 selectively provides primary radiation at one or more wavelengths. Radiation source 502 can selectively provide broadband radiation with wavelengths ranging from the visible spectrum to NIR, for example. Radiation source 502 can also selectively provide primary radiation at one or more wavelengths that excite photoluminescence in one or more types of contaminant particles such that the particles emit secondary radiation at one or more different wavelengths. Typically, the wavelengths of the secondary radiation are different than the wavelengths of the primary radiation. Different wavelengths of secondary radiation can be emitted by different types of contamination material, or by different photoluminescence processes within the same material.

[0058] Apparatus 500 can include one or more detection optics, for example, an objective lens and an imaging lens that collects the emitted radiation and delivers it to a sensor 506. Sensor 506 can be any suitable 2-dimensional detection device that is capable of generating a full or partial image of patterning device MA under inspection. For example, sensor 506 can be a CCD or CMOS camera. Images generated 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 used to adjust the size of the image field.

[0059] The radiation may be directed at patterning device MA from directly above or obliquely. The illumination optics may comprise reflective and/or transmissive elements. The disclosure of different forms of illumination optics in International Application No. PCT/EP2010/059460, which is hereby incorporated by reference in its entirety.

[0060] The radiation passing through mirror 504 towards sensor 506 can include a relatively large amount of scattered primary radiation and relatively small amount of the secondary radiation emitted by contaminants, if any are present. Thus, apparatus 500 can include a filter 508 within the detection optics to selectively filter which wavelengths reach sensor 506. For example, filter 508 can be configured to selectively block the primary radiation while passing the secondary radiation. Filter 508 can include one or more narrow band and/or differential color filter elements.

[0061] To filter out the primary radiation, filter 508 can be configured to filter out certain spectral regions corresponding to the primary radiation wavelengths. These spectral regions can be sufficiently narrow that they can be separated using band filter elements along a beam path within the imaging optics without separating the signal into a spectrum. Accordingly, apparatus 500 images patterning device MA on sensor 506 in two-dimensions. [0062] In some embodiments, the imaging optics can include one or more polarizer/analyzer elements (not shown), the implementation of which would become apparent to a person having ordinary skill in the art.

[0063] In some embodiments, radiation source 502 can generate primary radiation wavelengths that are not wanted for the inspection of the article using photoluminescence. These unwanted wavelengths can optionally be filtered out at radiation source 502, and not in the imaging optics.

[0064] Filter 508 can be made from one or more filters elements as described in

International Patent Application No. PCT/EP2011/067491. Each filter element can define a particular notch frequency or pass band. The range of wavelengths that pass through filter 508 and are detected by the sensor 506 can be wide, perhaps ranging from ultraviolet (UV) to infrared (IR).

[0065] Figure 6A is a plan view of patterning device MA under inspection by apparatus 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. In other embodiments, the field of view of apparatus 500 can be smaller than the entire patterning device MA surface, and multiple images must be obtained to perform a complete inspection. For example, apparatus 500 can generate a series of images taken at stepped intervals to cover the entire area of interest.

[0066] As shown in Figure 6A, contaminant particles 602, 604, and 606 are deposited on patterning device MA, for example, an EUV reticle. Contaminant particles 602, 604, and 606 can have different characteristics. For example, contaminant particles 602, 604, and 606 can be an organic particle (soft), a metal particle (hard), and a metal oxide, respectively. Accordingly, each contaminant particle has a different chemical composition and physical parameters such as hardness or rigidity.

[0067] Figure 6B illustrates an image 608 of patterning device MA generated by sensor 506 when radiation source 502 illuminated patterning device MA with broadband primary radiation. Here, filter 508 selectively allows the primary radiation to 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 indicate substantially 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 physical parameters of the contaminant particles 602, 604, and 606.

[0068] To discriminate the chemical composition or physical parameters of contaminant particles 602, 604, and 606, one or more images are generated at sensor 506 by providing primary radiation at one or more wavelengths known to excite secondary radiation from the contaminants typically found on the article under inspection. For example, organic particles, metal oxide or glass particles, and metal or semiconductor particles are typically found on patterning device MA. Organic particles tend to emit secondary radiation when illuminated with radiation at near ultraviolet (NUV) wavelengths, for example, about 400 nm or less. Metal oxide or glass particles tend to emit secondary radiation when illuminated with radiation at DUV wavelengths, for example, about 300 nm or less. And metal or semiconductor particles do not emit secondary radiation when illuminated with radiation at visible and NUV wavelengths, for example, about 800 nm or less. Accordingly, to discriminate organic particles and metal oxide or glass particles, at least two images are generated at sensor 506 by selectively providing primary radiation at NUV wavelengths and at DUV wavelengths, respectively.

[0069] Figure 6C illustrates an image 610 of patterning device MA generated by sensor 506 when radiation source 502 illuminates patterning device MA with primary radiation at one or more NUV wavelengths, which are known to excite secondary radiation of organic particles. Filter 508 selectively blocks the scattered primary radiation and any secondary radiation emitted by patterning device MA. The presence, size, and location of organic particle 602 are detectable in image 610 as bright spot 602". But image 610 does not indicate the presence of contaminant particles 604 and 606 because contaminant particles 604 and 606 did not respond to the primary radiation by emitting secondary radiation at wavelengths that were not blocked by filter 508.

[0070] A single filter or a combination of filters as a differential filter can be used as the filter 508. For example, red, green and blue filters can be used alone or in combination to allow and block a desired wavelength(s) to discriminate types (soft, hard, softer, and less soft) of contamination particles that may be present on the article such as on a reticle surface. [0071] In one example, the radiation source 502 can illuminate the patterning device

MA with a radiation at a narrow wavelength λ_α ίο generate signals at wavelengths λ_1; λ₂, and λ₃ from different contamination particles such that λ_α < λ_\ < λ₂ < λ or illuminate the patterning device MA at a wavelength λ , to generate wavelengths λ₂, λ such that < λ₂ < λ₃ or illuminate the patterning device MA at a wavelength λο to generate wavelength λ₃ such that λο < λ₃. In this way, using a radiation at the wavelength λ_α the contamination particles emitting fluorescence at the wavelengths λ_1; λ₂, and λ can be detected although the signal at the wavelength λ_\ may be the highest and the signal at the wavelength λ₂ may be the lower and the signal at the wavelength λ₃ 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_α may be used to generate three signals at wavelengths λ_1; λ₂, and λ which can be detected or discriminated by using one or more differential filters, as discussed above.

[0072] Figure 6D illustrates an image 612 of patterning device MA generated by sensor 506 when radiation source 502 illuminates patterning device MA with primary radiation at one or more DUV wavelengths, which are known to excite secondary radiation from metal oxide or glass particles. Filter 508 selectively blocks the scattered primary radiation and any secondary radiation emitted by patterning device MA. The presence, size, and location of metal oxide or glass particle 604 are detectable in this image 612 as a bright spot 604".

[0073] But neither image 610 nor image 612 indicates the presence of metal or semiconductor particle 606 because contaminant particle 606 does not emit secondary radiation when illuminated by primary radiation at NUV or DUV wavelengths or at visible wavelengths. Thus, to determine the presence and location of metal or semiconductor particle 606, processing unit PU compares the spectrally- selective images 610 and 612 to the broadband image 608 that indicates the presence of substantially of contaminant particles on patterning device MA. This comparison can include direct comparisons of the images, subtractions of the images, and/or correlations 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, a metal or semiconductor material, that does not emit secondary radiation when illuminated by primary radiation at NUV or DUV wavelengths. [0074] Figure 6E illustrates a discriminated image 614 of patterning device MA showing contaminant particles 602"', 604"', and 606"' with an indication of chemical composition or physical parameters. Processing unit PU can then determine the chemical composition or physical parameter of every containment particle indicated in broadband image 608 by using image 610, image 612, and a comparison of images 610 and 612 to broadband image 608. Knowing what common contaminants that do not emit secondary radiation when illuminated by primary radiation used to generate images 610 and 612, processing unit PU can determine the chemical composition of particles indicated in broadband image 608, but not in images 610 and 612.

[0075] Processing unit PU can deliver an inspection result to an operator, or to an automatic control system of apparatus 500. In embodiments where processing unit PU is physically the same as the control unit of the larger apparatus, the result may of course be delivered internally. The result may be delivered on a dedicated hardware output, or as a message on multi-purpose communication channel. The inspection result indicates at least the presence and location of suspected contamination. It may optionally provide more detailed parameters of what is detected, for example intensity information.

[0076] Figure 7 is a schematic diagram of an inspection apparatus 700 according to another 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 or dichroic mirror, that divides radiation reflected or emitted from the field of view of patterning device MA into first optical branch 720 and second optical branch 722. In first 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 generating a broadband image such as image 608 described above.

[0077] One advantage of having two optical branches 720 and 722 is that the image sensors 606 and 718 can be chosen according to their performance in respective wavebands, rather than having to find a sensor with adequate performance over the entire spectrum of radiation to be detected. Sensitivity and noise performance of the sensors 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 difficult to provide imaging optics with low aberration across such a wide spectrum.

[0078] Figure 8 illustrates another inspection apparatus 800, which again is based on apparatus 500, and again is modified to include second optical branch 722 as in apparatus 700. As shown in Figure 8, second optical branch 722 can include a filter 824. Optical element 716 can be a dichroic mirror that diverts scattered primary radiation into second optical branch 722, where it is focused on image sensor 718. Filter 824 can be a color filter.

[0079] Information as to the type of contamination present can be useful for example in the choice and control of decontamination (cleaning) process. Information on the type of contamination can also be useful for diagnosing the source of contamination, and taking measures to reduce contamination in future. Further, identifying the type of contamination and location of the contamination is useful to ensure that clamping the patterning device MA does not damage the patterning device MA.

[0100] Figure 9 shows the main process steps for detecting contaminant particles on an article, for example, a reticle for use in a EUV lithography process, using apparatuses 500, 700, and 800. Inspection apparatuses 500, 700, and 800 can be integrated within the patterning device housing of a lithographic apparatus so that the patterning device under inspection is mounted on the same support structure (mask table) MT used during lithographic operations. The support structure MT can be moved under the inspection apparatus, or equivalently the inspection apparatus is moved to where the patterning device is already loaded. Alternatively, the patterning device MA under inspection may be removed from the immediate vicinity of support structure MT to a separate inspection chamber where the inspection apparatus is located. This latter option avoids crowding the lithographic apparatus with additional equipment, and also permits the use of processes that would not be permitted or would be undesirable to perform within the lithographic apparatus itself. The inspection chamber can be closely coupled to the lithographic apparatus, or quite separate from it, according to preference. Alternative inspection apparatuses can be included in the same or a different chamber, to allow the detection of different types of particles by different processes. [0101] Returning to Figure 9, patterning device MA, for example, a reticle, is loaded at step 926 into the inspection apparatus (or the inspection apparatus is brought to where the patterning device MA is already loaded). Prior to inspection, the patterning device may or may not have been used in the lithographic process. Using the inspection apparatus, an image of patterning device MA illuminated with broadband radiation is acquired at step 928. This image can be a single image of the entire patterning device, or a set of sub-area images that are processed individually or stitched into a larger image.

[0102] At step 930, processing unit PU analyzes the image generated at step 927 to determine whether contaminant particles are present on patterning device MA and the location of such particles. If no particles are determined to be present on patterning device MA, the inspection process can be terminated. But if the particles are detected, the inspection process continues to step 932 to further discriminate the chemical composition or physical parameters of the contaminant particles.

[0103] At step 932, patterning device MA is illuminated with primary radiation at one or more wavelengths known to excite secondary radiation of certain contaminant particles. The primary radiation is filtered out, and a second image is generated using the secondary radiation. For example, the patterning device MA can be illuminated with radiation at NUV wavelengths such that organic particles emit secondary radiation.

[0104] At step 934, processing unit PU analyzes the second image generated at step

932 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.

[0105] Next at step 936, patterning device MA is illuminated with primary radiation at 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 particle discriminated at step 934. The primary radiation is filtered out, and an image is generated using the secondary radiation of the contaminant particles. For example, the patterning device MA can be illuminated with radiation at DUV wavelengths such that any metal oxide or glass particles on patterning device MA emit secondary radiation. [0106] At step 938, processing unit PU analyzes the image generated at step 936 to determine whether contaminant particles that emit secondary radiation when exposed to the primary radiation of step 936 are present on patterning device MA. Processing unit PU can also can determine the location of any such contaminant particles. Steps similar to steps 934-938 can be repeated for any other contaminant particles that may be deposited on patterning device MA and that would emit secondary radiation when illuminated with radiation at certain wavelengths.

[0107] To discriminate substantially all contaminant particles on patterning device

MA, including those that do not emit secondary radiation and thus cannot be discriminated at steps 934-938, processing unit PU can compare the image generated at step 932 and the image generated at step 936 with the broadband image generated at step 928. This comparison can include direct comparisons of the images, subtractions of the images, and correlations of the images. Processing unit PU determines which particles determined to be present at step 930 are not discriminated at step 934 or at step 938. Such particles can be contaminant particles that do not emit second radiation, for example, metal or semiconductor particles.

[0108] Embodiments of the methods and apparatuses of the present disclosure can be used for the inspection of any type of patterning device, for example, a mask or an EUV reticle.

[0109] Again, inspection apparatuses 500, 700 and 800 can be an in-tool device, that is, within a lithographic system, or can be a separate inspection apparatus. As a separate apparatus, it can be used for purposes of patterning device inspection (for example, 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 in particular be useful to perform inspections in between the lithographic processes, for example to check after a certain number of exposures whether the patterning is still clean and to check before clamping the patterning device.

[0110] Processing of signals from the sensor may be implemented by processing unit

PU in hardware, firmware, software, or any combination thereof. Unit PU may be the same as a control unit of the lithographic apparatus, or a separate unit, or a combination of the two. Embodiments of the invention of various component parts of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine- readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (for example, a computing 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 (for example, carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines or instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.

[0111] It is to be appreciated that the Detailed Description section, and not the

Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way.

[0112] The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0113] The foregoing description of the specific embodiments will so fully reveal the general 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 specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. 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 in accordance with the following claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

A method for discriminating contaminant particles on an article under inspection, comprising:

illuminating the article with radiation at one or more first wavelengths;

The method of claim 1, further comprising:

illuminating the article with radiation at one or more fourth wavelengths suitable for exciting fluorescent radiation from one or more third contaminant particles of the any third contaminant particles at one or more fifth wavelengths;

filtering out the radiation at the one or more fifth wavelengths generated from the one or more third contaminant particles; generating a third image of the illuminated article, the third image being formed by the radiation at the one or more fifth wavelengths, the third image indicating that the one or more third contaminant particles having the third characteristic; and

determining that the any first contaminant particles having the first characteristic by comparing the first, second and third images.

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

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

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

6. The method of claim 1, wherein the article comprises a patterning device for use in optical lithography.

7. The method of claim 6, wherein the patterning device comprises a reticle for use in EUV lithography.

8. The method of claim 1, wherein the first, second, and third characteristics are chemical composition categories.

9. The method of claim 8, wherein the chemical compositions categories comprise at least one of the following: an organic composition, a metal composition, and a metal oxide composition.

10. The method of claim 1, wherein the first, second, and third characteristics are mechanical 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 third characteristics respectively on an article under inspection, comprising:

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 a presence 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 from the one or more second contaminant particles, and

12. The inspection apparatus of claim 11, wherein:

the radiation source is further configured to generate radiation at one or more fourth wavelengths suitable for exciting fluorescent radiation from a third contaminant particle at one or more fifth wavelengths, the third contaminant particle having the third characteristic;

the optical system is further configured

to illuminate the article with the radiation at the one or more fourth wavelengths, to filter out radiation at the one or more fifth wavelengths generated from the one or more third contaminant particles, and

to generate a third image of the illuminated portion of the article, the third image being formed by the radiation at the one or more fifth wavelengths, the third image indicating a presence and a location of the one or more third contaminant particles on the article; and

the processor is further configured to compare the first, second and third images to determine the presence and the location of the one or more first contaminant particles on the article.

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

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

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

16. The inspection apparatus of claim 11, wherein the article comprises a patterning device for use in optical lithography.

17. The inspection apparatus of claim 16, wherein the patterning device comprises a reticle for use in EUV lithography.

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

19. The inspection apparatus of claim 18, wherein the one or more chemical compositions categories comprise at least one of the following: an organic composition, a metal composition, and a metal oxide composition.

20. The inspection apparatus of claim 11, wherein the first, second, and third characteristics 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 the substrate; and

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 third characteristics respectively on an article under inspection, comprising:

22. The inspection apparatus of claim 21, wherein the radiation source is further configured to generate radiation at one or more fourth wavelengths suitable for exciting fluorescent radiation from a third contaminant particle at one or more fifth wavelengths, the third contaminant particle having the third characteristic;

the optical system is further configured:

to illuminate the article with the radiation at the one or more fourth wavelengths,

to filter out radiation at the one or more fifth wavelengths generated from the one or more third contaminant particles, and