NL2006641A - Methods, systems and apparatus for monitoring integrity of an article, euv optical apparatus incorporating the same. - Google Patents

Description

METHODS, SYSTEMS AND APPARATUS FOR MONITORING INTEGRITY OF AN ARTICLE, EUV OPTICAL APPARATUS INCORPORATING

THE SAME

FIELD

[0001] The present invention relates to methods, systems and apparatus for monitoring integrity of an article operating in a low pressure or vacuum environment. An example of an article that needs monitoring might be a spectral purity filter in an extreme ultraviolet (EUV) optical system, such as an EUV lithographic apparatus.

BACKGROUND

[0002] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0003] A key factor limiting pattern printing is the wavelength λ of the radiation used. In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet (EUV) radiation which is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such EUV radiation is sometimes termed soft x-ray. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.

[0004] EUV sources based on a Sn plasma do not only emit the desired in-band EUV radiation but also out-of-band radiation, most notably in the deep UV (DUV) range (100-400nm). Moreover, in the case of Laser Produced Plasma (LPP) EUV sources, the infrared radiation from the laser, usually at 10.6 pm, presents a significant amount of unwanted radiation. Since the optics of the EUV lithographic system generally have substantial reflectivity at these wavelengths, the unwanted radiation propagates into the lithography tool with significant power if no measures are taken.

[0005] In a lithographic apparatus, out-of-band radiation should be minimized for several reasons. Firstly, resist is sensitive to out-of-band wavelengths, and thus the image quality may be deteriorated. Secondly, unwanted radiation, especially the 10.6 pm radiation in LPP sources, may lead to unwanted heating of the mask, wafer and optics.

[0006] Currently, spectral purity filters (SPFs) may be implemented to bring unwanted radiation within specified limits. SPFs may be of transmissive type or reflective type. Transmassive type SPFs may be formed by a planar article such as a thin membrane or a grid with micro-sized apertures. However, a SPF failure or leakage may occur during operation, which results in partial or total loss of its ability to remove out-of-band radiation and thus may impair the imaging performance and/or damage the machine. Therefore, SPF failure detection methods are required for monitoring SPFs against development of holes. Ideally, the monitoring system should function outside the beam itself.

SUMMARY

[0007] The following presents a simplified summary of the one or more embodiments in order to provide a basic understanding of such embodiments. This summary is not an extensive overview of all contemplated embodiments, and is intended to neither identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0008] According to one or more embodiments, methods, systems and apparatus are provides for monitoring integrity of article.

[0009] According to an embodiment of the present invention, there is provided a method for monitoring integrity of an article operating in a low pressure environment. The method includes directing a beam of electrons toward the article within the environment, the form of the article when intact being expected to stop at least a proportion of the electrons in said beam; and generating a signal to indicate integrity status of the article based at least in part on the proportion of electrons of the beam that are stopped by the article. For example an alarm signal might be generated when at least a part of the article is not stopping an expected proportion of electrons in the beam.

[0010] Where the present text refers to a ‘proportion’ of electrons ‘expected’ to be stopped, this proportion can be determined in different ways, and can be expressed in the design of the apparatus in different ways, as understood by the skilled addressee. The terms are used only to indicate that some test of integrity is based on the proportion of electrons stopped, for discriminating between an article or part of the article being intact and the article or part of the article being non-intact, and do not carry any greater meaning than that. The test does not need to be based on a calculation of a proportion expressed as such, but may be based on a single measurement, from which the proportion stopped can be inferred.

[0011] According to an embodiment of the present invention, there is provided a system for monitoring integrity of an article in a low pressure environment. The system includes an electron beam source module configured to directa beam of electrons toward the article within said environment, the form of article when intact being expected to stop at least a proportion of the electrons in said beam; and a detection module configured to generate a signal to indicate integrity status of the article by identifying when at least a part of the article is not stopping the expected proportion of electrons in the beam.

[0012] Embodiment of the invention are not limited in application to a filter in a lithographic apparatus, but may be applied to monitor the integrity of any article. Such an article may be of a type which is relatively thin, like a membrane or screen, and therefore prone to acquiring holes due to wear or damage. It may be a planar article or curved or dome-shaped article and may function as some kind of filter or barrier in front of another object, or between two chambers of any apparatus involving low pressure or vacuum.

[0013] Embodiments of the invention further provide an optical apparatus including a system for monitoring integrity of an article within the optical apparatus.

[0014] The monitoring techniques disclosed herein can be adapted to other purposes, for example inspection of articles outside their operating environment, or for inspection of manufactured articles for quality control, prior to use. The techniques in some embodiments of the invention may also be adaptable to detect contamination of the article surface, as well as or instead of loss of integrity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0016] Figure 1 depicts schematically a system for monitoring integrity of a planar article according to an embodiment of the present invention;

[0017] Figure 2 depicts schematically a system for monitoring integrity of a planar article according to an embodiment of the present invention;

[0018] Figure 3 depicts schematically a system for monitoring integrity of a planar article according to an embodiment of the present invention;

[0019] Figure 4 depicts schematically a system for monitoring integrity of a planar article according to an embodiment of the present invention;

[0020] Figure 5 depicts schematically a system for monitoring integrity of a planar article according to an embodiment of the present invention;

[0021] Figure 6 depicts schematically an exemplary system for monitoring integrity of a planar article according to an embodiment of the present invention;

[0022] Figure 7 depicts schematically an exemplary system for monitoring integrity of a planar article according to an embodiment of the present invention;

[0023] Figure 8 depicts schematically an exemplary electron beam source module according to an embodiment of the present invention;

[0024] Figure 9 depicts schematically the main features of an EUV lithographic apparatus;

[0025] Figure 10 shows a schematic side view of an exemplary EUV lithographic apparatus;

[0026] Figure 11 depicts Monte-Carlo simulations of electron trajectories after electrons hit a Zirconium membrane from an example of a spectral purity filter part in the EUV lithographic apparatus of Figure 10;

[0027] Figure 12 is a front face view of part of an another example of a grid type spectral purity filter part;

[0028] Figure 13 (a) is a schematic front face view of a small area within the grid type spectral purity filter part of Figure 12;

[0029] Figure 13 (b) is cross sectional view of the same area as shown in Figure 13(a);

[0030] Figure 14(a) depicts an exemplary diagram of an electron beam delivered normal to a portion of a surface of a grid type filter; and

[0031] Figure 14(b) depicts an exemplary diagram of an electron beam delivered at an angle to a portion of a surface of a grid type filter.

[0032] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident, however, that such embodiments may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more embodiments.

DETAILED DESCRIPTION

[0033] Figure 1 depicts schematically a system 50 for monitoring integrity of an article according to an embodiment of the present invention. System 50 includes an electron beam source module 60, an article 120 and a detection module 80. Article 120 is located in a low pressure (near-vacuum) or vacuum environment defined by enclosure 90. An example of such an environment is the interior of an EUV lithography apparatus, but of course vacuum and near-vacuum environments are a feature of many types of scientific and industrial apparatus, and the invention may be applied to the monitoring of such articles generally. The types of articles that can be monitored include in particular articles which are very thin, in comparison with their surface area. Such articles, which may be classified as membrane-like or screen-like, are particularly prone to the formation of holes due to wear or damage. Embodiments of the invention may also be operable to detect contamination of the article surface.

[0034] Electron beam source module 60 is operable to emit an electron beam. Electron beam source module 60 is also operable to deliver the electron beam onto article 120. A controller 82 may be implemented to control system 50 via control signals transmitted through a cable or wirelessly to components of system 50 located at various positions within and outside the vacuum environment. Controller 82 may be part of a controller for a larger apparatus, of which article 120 forms part. In one exemplary embodiment, electron beam source module 60 includes an electron beam source and such an electron beam may for example be a typical electron gun known commonly from the field of cathode ray tube (CRT) technology. In the present examples, the gun may be for example of the type commonly used for oscilloscope CRTs, rather than the type used in CRT for color television and monitor displays. Typically the oscilloscope gun will operate with lower accelerating voltages (for example in the range 1 to 3 kV, say 2 kV) than a television CRT gun (around 30 kV). The kinetic energy and hence the velocity and momentum of electrons leaving the gun are generally determined by this voltage, so that the oscilloscope electrons travel slower and are more easily deflected than those from television gun. The invention is not limited to any particular range of electron energy, but a range from 1 keV to 3 keV, for example 2 keV is considered suitable for the applications to be described. The appropriate energy of the electron beam may be selected over a wide range, according to the material properties and dimensions of article 120. If the energy of the beam is too high, then a significant proportion of the electrons in the beam may have enough energy to penetrate right through the article. Detection will be easier, generally speaking, if the energy of the beam is set so that all or nearly all of the electrons are stopped, when the article is intact.

[0035] In response to directing the electron beam towards article 120, detection module 80 is operable to generate a signal 84 indicating whether there is a hole in article 120 that should not be there, that is whether the article 120 is intact or damaged. The form of article 120, when intact, is capable of stopping at least a proportion of the electrons in the beam, and this proportion is significant enough that a suitably designed detection module 80 can detect when the expected proportion of electrons is not being stopped. Signal 84 indicates the integrity of article 120 by identifying when at least a part of article 120 is not stopping the expected proportion of the electrons in the beam.

[0036] As will be understood by the skilled reader, the proportion of electrons that may be stopped, and the change in this proportion that occurs in the event of a loss of integrity in the article due to wear or damage, will depend greatly on the design of the article, the form of the electron beam produced by source module 60, the form of the detection module 80, as well as the scale and nature of the damage or wear that is occurring. Where the description and clauses refer to the proportion of electrons ‘expected’ to be stopped, this proportion can be determined in different ways, and can be expressed in the design of the apparatus in different ways. The term is used only to indicate that some test based on the proportion of electrons stopped, for discriminating between intact and non-intact articles, or parts of articles, and does not carry any greater meaning than that. The detection module 80 will implement one or more tests on detected signals, to determine whether the expected proportion of electrons are being stopped at some part of the article, in order to generate the integrity signal 84. In some embodiments, for example, a simple threshold test may be implemented to determine whether the predetermined proportion of the electrons in the beam is being stopped. In other embodiments, the test may compare a detected signal with a reference signal that is also time varying, rather than applying a fixed threshold. In other embodiments, where the electron beam is directed at different parts of the article at different times, the test may compare a signal detected at one part with a signal detected at another part, or with a signal detected at the same part at an earlier time. These refinements, which are within the knowledge of a person skilled in the art of measurement and signal processing, can be applied without departing from the principle of detecting when an expected proportion, which may be considered simply as a predetermined proportion, of the electrons in the beam are being stopped by article 120. Such a reference signal may be stored in controller 82, or in detection module.

[0037] Based on this principle, several different types of embodiments are envisaged, and exemplified below. Detection of electrons passing through the article may be performed in a positive way using a collecting element located on the opposite side of the article from source module 60. Electrons passing through the article may be detected in a negative way, by collecting electrons which are absorbed by the article. In either case, detection of electrons may be relatively direct, as when a current is detected in an electrode formed by the collecting element or the article itself, or it may be indirect, as when a light emitting substance such as a phosphor is hit by the electrons, and emits light which can be detected by a camera or other light sensor.

[0038] Based on these different possible principles of operation, in one exemplary embodiment, detection module 80 may include a camera and a detection screen operable together to obtain the signal. In another exemplary embodiment, detection module 80 may include a current meter operable to obtain the signal. In still another exemplary embodiment, detection module 80 may include a combination of a camera, a detection screen and a current meter operable together to obtain the signal. The signal referred to here may be a signal indicating in a binary fashion integrity or non-integrity of the article. The signal may be a variable signal indicating a non-binary degree of confidence or alarm, which can be interpreted by an operator in combination with other observations to determine whether protective action is desired. It may additionally indicate a specific position or positions where defects (holes) are suspected. The signal output by the monitoring system may be a composite of various signals obtained from different detection elements, and/or processed according to different algorithms.

[0039] An operator of a larger apparatus of which the monitored article 120 forms a part will make a judgment as to what action is desired in the event that a failure of integrity is indicated by the monitoring system. The speed of response will depend on the consequences that flow from a failure of the article, and perhaps the degree of failure indicated, in terms of the size and/or number of holes detected. The consequence of a loss of integrity in come cases may be a loss of throughput of the apparatus, or a loss of quality in a product produced by the apparatus. The consequence of a loss of integrity in other cases may be damage to components of the apparatus itself, and possibly danger to operators or the public, or it may be a combination of these. Therefore in some applications, it may be appropriate to provide an automatic shutdown system in response to the signal, so as to shut down the apparatus within a critical time period, as soon as any loss of integrity is detected. In another application, it may be sufficient that a loss of integrity, at least below a certain threshold, is simply logged for reference at the time of planned maintenance. In the following embodiments, a planar article is used as an example of article 120. However, the skilled person can understand that embodiments of the present invention are not limited in application to a planar article, but may be applied to monitor the integrity of any article. Naturally, articles that are very thin in comparison with their extent in other directions are the types of article that are prone to developing holes. Such ‘thin’ articles can be regarded as locally planar, even though they may be curved in one or more dimensions when viewed as a whole.

[0040] Figure 2 depicts schematically an embodiment of a system 100 for monitoring integrity of a planar article according to one embodiment of the present invention. System 100 includes an electron beam source 110, a planar article 120, a camera 140 and a detection screen 150. Electron beam source 110, article 120 and detection screen 150 are located in a low pressure environment or in a vacuum environment.

[0041] Electron beam source 110 delivers an beam of electrons represented by circled signs to article 120. The entire surface of the article (or that part of it whose integrity is to be monitored) is covered by the electron beam, either by scanning movement of a narrow beam over the surface or by flooding the whole surface of article 120 with a wide beam. In an embodiment, the electron beam source 110 is a typical electron gun, for example as used in an oscilloscope. In this and in all the embodiments to be described with reference to Figures 2 to 7, some or all components of the electron beam source 110, and in particular the electron gun itself, may be located in a vacuum environment separated wholly or partially from that of the article 120 to be monitored. This modification will be described separately further below, with reference to Figure 8.

[0042] The intact part of the surface of planar article 100 can absorb the electrons of the beam.

As an example, article 120 may be a membrane of metal or other material, which will absorb substantially all of the electrons from the source 110. When there is a hole 130 in article 120, electrons of the beam may go through hole 130.

[0043] For one or more reasons to do with the form of the article 120 or its function, the source 110 in these embodiments is arranged to deliver the electrons obliquely to the surface, rather than from a normal direction. For example, where the article 120 functions as an element in a radiation beam of an optical system, it may be desirable that the monitoring system does not obstruct front and rear views of the article. It may be also that oblique incidence of the beam is desired, so that the electrons are more reliably stopped by the article, when it is intact. An example of such an article will be illustrated further below. In other applications, it may be acceptable or desirable to place the source directly in front of the article. It may even be desired in some embodiments to place the source to one side, but to deflect the electrons so that they impinge on the article from a direction more normal to the surface.

Note that ‘in front’ in this context does not imply any distinction between the two faces of the article: the source could be on either side, according to design preference and practical considerations.

[0044] In this example, detection screen 150 forming part of detection module 80 is located behind article 120. A surface of detection screen 150 is coated with a detection layer 160. In one exemplary embodiment, detection layer 160 may be a phosphor layer, of the type used in CRTs for oscilloscopes. When electrons pass through hole 130 and hit detection layer 160, detection layer 160 emits light. Camera 140 is positioned to be operable to obtain a 2D image of detection layer 160. In one embodiment, camera 140 may be a monochrome camera and may be positioned within or out of the vacuum environment. A spectral filter (not shown) may be provided in or in front of the camera, to attenuate wavelengths of light other than those expected from the phosphor. When electrons of the beam go through hole 130, the electrons hit the detection layer 160 and emit light which is imaged by camera 140. The emitted light shown in the image taken by camera 140 indicates that there is a hole in planar article 120.

[0045] Where source 110 is substantially a point source, the position of the point of light in the image directly represents the position of the hole 130 in the article being monitored. Moreover, the image can reveal the size and number of holes present across the article in a readily-understood form. The size of a small hole may be judged from the intensity of the light spot, provided the intensity of the electron beam is known by calculation or by measurement. The image can be processed by a human viewer and/or by automated signal processing to compare the observed pattern of light spots (if any) with predetermined criteria, and a signal generated to report on the integrity status of the article 120.

[0046] Figure 3 depicts schematically an exemplary system 180 for monitoring integrity of a article according to another embodiment of the present invention. System 180 includes an electron beam source 110, a planar article 120 and a camera 140. Electron beam source 110, article 120 and detection screen 150 are located in a low pressure environment or in a vacuum environment. A surface of article 120 is coated with a detection layer 160. In one exemplary embodiment, detection layer 160 may be a phosphor layer.

[0047] When electrons hit detection layer 160, detection layer 160 emits light. Camera 140 is positioned to be operable to obtain a 2D image of detection layer 160. Since the layer 160 is across the whole surface of the article 120 when intact, light is emitted across the entire field and the camera image is uniformly bright. When there is a hole in article 120, a dark portion in the 2D image can be found. The image seen by the camera 140 in this embodiment will be effectively the negative of the image seen in the embodiment of Figure 2. In another embodiment, the 2D image is compared with a reference 2D image of detection layer 160 obtained when there is no hole in detection layer 160, which can improve the accuracy in spotting a dark portion in the 2D image.

[0048] Figure 4 depicts schematically an exemplary diagram of a system 200 for monitoring integrity of a planar article according to yet another embodiment of the present invention. System 200 includes an electron beam source 110, a planar article 120, a detection screen 210 and a current meter 220. Electron beam source 110, planar article 120 and detection screen 210 are located in a low pressure environment or in a vacuum environment.

[0049] Detection screen 210 is made of a conductive material so that detection screen 210 can act as an electrode collecting electrons passing through hole 130. For example, the detection screen may be made of metal. Current meter 220 is electrically connected to detection screen 210 for detecting a level of current caused by the arrival of electrons from source 110. A change, in particular an increase in electrical current according to current meter 220 indicates that there is or may be a hole in planar article 120. The level of current may be interpreted as indicating a size and/or number of holes. The electron beam in this example simply floods the article surface, so that no positional information is obtained. In other embodiments, described below, the beam is narrower and impinges on different parts of the article at different times, for example to cover the surface of the article in a scanning movement.

[0050] Figure 5 depicts schematically an exemplary diagram of a system 300 for monitoring integrity of a planar article according to yet another embodiment of the present invention. System 300 includes an electron beam source 110, a planar article 120, a deflection plate 310, a current meter 320 and a voltage source 330. Electron beam source 110, planar article 120 and deflection plate 310 are located in a low pressure environment or in a vacuum environment.

[0051] Instead of using a detection screen 210 as shown in system 200, system 300 has a deflection arrangement configured to generate an electrical field between two plates 310, 312. The electrical field which voltage source 330 sets up between the plates 310,312 attracts and collects the electrons passing through hole 130 in planar article 120. Current meter 320 is electrically connected in series with voltage source 330. Current meter 320 measures any electrical current change resulting from an amount of electron collected by plate 310. A change (increase) in electrical current according to current meter 320 indicates that there may be a hole in planar article 120. The electrical field is used to direct electrons from different holes in planar article 120 to the plate 310. Therefore, compared to the detection screen 210 in system 200, the size of plate 310 in system 300 is relatively smaller.

[0052] Figure 6 depicts schematically an exemplary system 400 for monitoring integrity of a planar article according to yet another embodiment of the present invention. System 400 includes an electron beam source 110, a planar article 120, and a current meter 420. Electron beam source 110 and planar article 120 are located in a low pressure environment or in a vacuum environment. Planar article 120 is made of a conductive material like metal.

[0053] In inspection system 400, electron beam source 110 of system 400 delivers an electron beam to planar article 120 by means of scanning the surface of planar article 120 with a narrow beam, rather than by flooding it with a broad beam. Electron beam scanning a portion of a surface of planar article 120 is shown as 450. When the electron beam hits planar article 120, this will cause a current of electron to flow since the planar article 120 is made of conductive material. Current meter 420 is electrically connected to the planar article 120 and measures the current. When the electron beam scans through planar article 120 and passes through hole 130 that is larger in size than the beam, the beam will pass through planar article 120 and no electrical current is detected by current meter 420. Therefore, in system 400, no electrical current being detected by current meter 420 indicates that there is a hole in planar article 120. Moreover, the timing of the drop in current relative to the timing of the scanning of the beam across the article surface can be detected and used as an indication of the position of the hole in the article. By scanning the beam in a raster pattern like a CRT display monitor, and recording drops in current synchronously with the scan, an image of the location and size of holes can be built up in a signal processing unit. The resolution of this image is limited substantially by the narrowness of the electron beam. If there is a hole that is comparable with the beam size but not so large that the entire beam passes through it, a reduction in current may still be significant enough to be detected and recognised as an indication that a hole is present.

[0054] In addition to acquiring holes due to wear or damage, an article 120 in an operating environment may get a contamination particle 460 on it. Contamination particle 460 is may be made of a non-conductive material. If electrons hit contamination particle 460, they may be absorbed by contamination particle 460 without contributing to the current detected by current meter 420. The skilled person would understand that system 400 in the above embodiment, as shown in Figure 6, can operate to monitor the presence of a contamination particle 460 on article 120. Whether such operation is desirable depends upon the circumstances. The embodiment may be adapted to discriminate between contamination and holes, if desired. One way to do this is to use a combination of detection methods, as will be illustrated below with reference to Figure 7.

[0055] While distinct embodiments have been presented above, the skilled person would understand that to detect and optionally characterize a hole in planar article 120, an integrity monitoring system in a practical embodiment may combine features of more than one of the systems 100,180, 200,300 and 400. Such a combination system 500 is shown in Figure 7 as an example, which combines the features of the systems 180 and 400 together. Such combinations can result more accurate and an improved sensitivity in detecting the presence of a hole in planar article 120, and in avoiding false alarms. The system of Figure 7 can be used for example to discriminate between contaminant particle 460 and a hole in article 120, because, although current meter 420 may not be able to discriminate between these different situations, phosphor layer 160 will only be impacted by electrons which pass through a hole in the article.

[0056] In another embodiment, not illustrated, a scanning electron beam similar to that of the Figure 6 example can be combined with the detection by electrode 210 or 310 in the example of Figures 4 or 5. The monitoring module in that case can obtain a signal that contains information of the position of a hole, by comparing the timing of a rise in the electrode current with the position of the electron beam during its scan.

[0057] Another technique for improving sensitivity while avoiding false alarms is to apply the electron beam intermittently or in a pulsed fashion, so that the detection module 80, in whatever form, can compare measurements of light and/or current with the electron beam on against background levels observed with the electron beam off. Such comparison can be made by taking images with a camera 140 with and without the beam, and subtracting the ‘beam off image from the ‘beam on’ image them before analyzing the image to detect holes. The electron beam may be pulsed with a known frequency or combination of frequencies, and signals from a camera or electrode can be subject to filtering, so that other frequencies, including a DC background level, are attenuated and the ‘hole’ signal becomes relatively stronger. The electron beam may be generated only at known times, and signals detected at other times gated out. Where the operating environment may be expected provide a background level of electron flux or electromagnetic radiation at a known frequency or at known times, the detection module can filter out those frequencies, or gate out signals measured at those known times, so that these background signals do not reduce the signal-to noise ratio in the detection of holes for integrity monitoring of the article.

[0058] Figure 8 depicts schematically an exemplary electron beam source module 600 according to one embodiment of the present invention. The electron beam source module 600 includes an electron beam source 110, an enclosure 610 surrounding electron beam source 110 and an opening 620 in enclosure 610. Enclosure 610 defines a separate vacuum or low-pressure space, within or adjacent to the operating environment of the article being monitored, but isolated from it. In a nearvacuum environment that contains a little gas, hydrogen for example, high-voltages within an electron gun may cause plasma discharge. This may prevent proper operation of the source, and/or create undesirable consequences elsewhere in the operating environment. By isolating some or all parts of source 110 in enclosure 610, the atmosphere can be controlled to exclude the gases that cause electrical discharge, and the discharge is prevented. Apart from that difference, pressures are maintained substantially equal in the two environments, and they are separated by a thin window 620 through which the electron beam exits. In order that the electron beam is not attenuated too much, the window is made as thin as practicable, from a material such as Si, Al, C (diamond) or Be.

[0059] In an embodiment where a narrow electron beam is desired, for example for raster scanning in the embodiment of Figure 6 or 7, the beam may become unsuitably broadened on passing through the window 620. To obtain a narrow beam outside the enclosure 610, a pinhole 630 is provided to select a narrow portion of the electron beam after the electron beam exits from window 620. As an example, the pinhole diameter may be 0.5mm or 1 mm. The distance between the window 620 and the pinhole diameter may be used to determine the angular distribution of the beam. The electron beam from pinhole 630 may be used to scan planar article 120.

[0060] Electron beam source module 600 further includes at least a set of deflection coils 640 driven with appropriate circuitry (not shown) to generate a time-varying electromagnetic field. The electromagnetic field can deflect the electron beam from pinhole 630 in a sequence of movements like a CRT raster scan, so as to scan the surface of planar article 120. This scanning motion 450, which is shown in just one dimension in Figure 8, can be performed in two dimensions to implement the scanning motion 450 used in the systems of Figure 6 or 7. It is understood that electron beam source module 600 may further include another set of deflection coils to generate a second time-varying electromagnetic field so that the surface of planar article 120 may be scanned in two dimensions. In another embodiment, the electromagnetic field can be implemented to control the size, shape and/or direction of the electron beam in a static manner, in addition to or instead of imparting a scanning movement. The size of the beam may be varied, so as to focus on a wider or narrower part of the article surface.

[0061] The electron beam deflected by coils 640 may be used to irradiate the whole surface of planar article 120. Because the deflection is performed with electromagnets rather than electrostatically, the deflection coils 640 need not be at high voltages and can be thus used in the environment of the article without risk of plasma discharge in the atmosphere. Because the deflection coils are located downstream of the pinhole 630, the scanning beam can be very narrow, and allows very sensitive measurement and location of holes 130 in article 120.

Application to EUV lithographic apparatus

[0062] Figure 9 schematically depicts a lithographic apparatus which comprises: a source collector module SO for generating and delivering a beam of radiation (e.g. EUV radiation); an illumination system (illuminator) IL configured to receive radiation from module SO and produce a conditioned radiation beam B; a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g. 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 (e.g. comprising one or more dies) of the substrate W.

[0063] 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.

[0064] 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, which 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.

[0065] 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, such as an integrated circuit.

[0066] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0067] 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.

[0068] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0069] 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.

[0070] Referring to Figure 9, the illumination system IL receives an extreme ultra violet radiation beam from the 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, e.g., 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. The 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 exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0071] 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 the source collector module 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 the source collector module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0072] The illumination system 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 the illumination system can be adjusted. In addition, the illumination system IL may comprise various other components, such as facetted field and pupil mirror devices. The illumination system may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0073] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B. Patterning device (e.g. mask) MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

[0074] The depicted apparatus could be used in at least one of the following modes: 1. In step mode, the support structure (e.g. mask table) MT and the 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). The 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, the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (e.g. mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the 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.

[0075] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0076] Figure 10 shows a schematic side view of an embodiment of an EUV lithographic apparatus 700. It will be noted that, although the physical arrangement is different to that of the apparatus shown in Figure 9, the same modules are present and the principle of operation is similar.

The apparatus includes a source collector module SO in a vacuum housing 703, an illumination system IL in a vacuum housing 704 and a projection system PS in a vacuum housing 705. In source-collector module is a radiation source 707 in which a very hot discharge plasma is created from a gas or vapor, such as for example Xe gas or a vapor of Li, Gd or Sn, so as to emit radiation in the EUV range of the electromagnetic radiation spectrum. In a DPP type source, illustrated, a discharge plasma is created by causing a partially ionized plasma of an electrical discharge to collapse onto the optical axis O. Partial pressures of, for example, 10 Pa 0.1 m bar of Xe, Li, Gd, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, a Sn source as EUV source is applied.

[0077] Instead of DPP source 707, an LPP source can be used in which a CO2 or other laser is directed and focused in a fuel ignition region. The laser beam generator may be a CO2 laser having an infrared wavelength, for example 10.6 micrometer or 9.4 micrometer. Alternatively, other suitable lasers may be used, for example having respective wavelengths in the range of 1 -11 micrometers. Upon interaction with the laser beam, the fuel droplets are transferred into plasma state which may emit, for example, 6.7nm radiation, or any other EUV radiation selected from the range of 5 - 20 nm. EUV is the example of concern here, though a different type of radiation may be generated in other applications. The radiation generated in the plasma is typically gathered by a normal incidence collector, for example an elliptical or other suitable collector.

[0078] The radiation emitted by radiation source 707 passes into a collector optics 710 via a contaminant trap 709 in the form of a gas barrier or "foil trap". The purpose of this contaminant trap is to prevent or at least reduce the incidence of fuel material or by-products impinging on the elements of the optical system and degrading their performance over time. Examples of such contaminant traps are described in United States Patent Nos. 6,614,505 and 6,359,969. Their construction is not material to an understanding of the present invention. However, in some versions they may contribute to the presence of a low pressure hydrogen atmosphere within vacuum housing 703.

[0079] Radiation collector 708 is, for example, a grazing incidence collector comprising a nested array of so-called grazing incidence reflectors. Radiation collectors suitable for this purpose are known from the prior art. Alternatively, as mentioned already, the apparatus can include a normal incidence collector for collecting the radiation. The beam of EUV radiation emanating from the collector 708 will have a certain angular spread, perhaps as much as 10 degrees either side of optical axis O.

[0080] The radiation is focused in a virtual source point 712 (i.e. an intermediate focus or IF) from an aperture in the source-collector housing 703, and enters illumination system IL housing 704. Radiation beam 716 is reflected in illumination system IL via normal incidence reflectors 713,714 onto a reticle or mask positioned on reticle or mask table MT. A patterned beam 717 is formed which is imaged by projection system PS via reflective elements 718,719 onto a substrate mounted on wafer stage or substrate table WT. More elements than shown may generally be present in the illumination system IL and projection system PS.

[0081] Hydrogen or other gas may be provided as a barrier or buffer against contaminant particles at other points in the lithographic apparatus. In particular, a flow of hydrogen into the near-vacuum environment of source collector module SO can be arranged, to impede particles that may try to pass through the intermediate focus (IF) aperture into the projection system. Further, hydrogen gas may be deployed (i) in the vicinity of the patterning device (e.g. reticle) support MT, as a buffer against contaminants from the system contaminating the reticle and (ii) in the vicinity of the wafer support WT, as a buffer against contaminants from the wafer entering the larger vacuum spaces within the system.

[0082] For all these purposes, hydrogen sources HS (some shown schematically, some not shown) are deployed for supplying hydrogen gas to each contaminant trap arrangement. Some sources may supply molecular hydrogen gas (H2) as a simple buffer while others generate H radicals. The invention is not limited to embodiments having a hydrogen atmosphere. Helium is known as another gas that can be used in a contaminant trap.

Spectral purity filters as articles to be monitored

[0083] In an embodiment according to the present invention, EUV lithographic apparatus 700 includes one or more spectral purity filters (SPFs), including for example a filter 720 located in the illumination system IL or a filter 710 located in the source collector module SO. Various types of spectral purity filter are known. Figure 10 shows an embodiment in which the spectral purity filter 710 is of a transmissive type, rather than a reflective grating. The radiation from source collector module SO in this case follows a straight path from the collector to the intermediate focus IF (virtual source point). In an embodiment not illustrated, radiation that traverses collector 708 can be reflected off a grating spectral filter to be focused in the intermediate focus IF. The optical path in that case is not straight but deflected by the reflective type of SPF. Alternatively or in addition, filters such as filter 720 can be placed downstream of the virtual source point 712. Multiple filters can be deployed. For example, a reflection type filter and/or an absorption type filter behind the SPF may be implemented to attenuate the unwanted spectral components in the light path.

[0084] The term “transmissive” used in relation to SPFs means that the wanted radiation (for example, EUV radiation) is substantially transmitted through the filter, while unwanted radiation is substantially blocked, whether by reflection, absorption or a combination of both of them thereof.

[0085] The function of these filters is to minimize the content of unwanted wavelengths of radiation in the beam B, where only the EUV radiation is wanted. Many wavelengths of radiation other than the wanted EUV can be strongly present in the radiation, depending on the type of source 707 which is used. Radiation in DUV and visible wavelengths may be present. In an LPP source based on an infrared laser, for example, there is also a large quantity of infrared radiation in addition to whatever radiation is produced in the plasma. Unwanted radiation wavelengths passing through the apparatus can impair the imaging performance of the apparatus quite directly. Moreover, large quantities of radiation at unwanted wavelengths will contribute to unnecessary heating of optical components such as elements 713, 714,718, and 719 and the environment generally. As well as causing distortion of components, and consequently poor imaging, this heating will often be sufficient to cause permanent damage to the delicate components of the apparatus, or the patterning device MA.

[0086] According to an embodiment of the present invention, planar article 120 may be a transmissive type spectral purity filter (“SPF”), such as a grid type SPF grid or a membrane type SPF.

In one embodiment, purely for example, SPF 710 is a grid type SPF deployed primarily to block (or at least reduce) infrared radiation entering the illumination system IL, while SPF 720 is a membrane type SPF deployed in the illumination system to block or reduce deep ultraviolet (UV) radiation. Both of these SPFs are of the transmissive type, and take the form of generally planar articles, deployed in a plane transverse to the radiation beam. Even a very small hole in the transmissive SPF may allow damaging quantities of unwanted radiation to pass. Inspection of the SPF by direct observation may be difficult, in the environment within the housings 703,704 of the source collector module SO or illumination system IL. Even where a camera could be mounted to observe the filter, distortions that occur in operation, and structural features such as the grid pattern in a grid-type SPF, may make it difficult to discriminate a damaged filter from and intact one. False alarms from the integrity monitoring systems may be very costly in lost throughput, and should be minimized as much as possible.

[0087] In the EUV lithographic apparatus 750, systems 50 and 52 are operable to monitor the integrity of filters 710 and 720 respectively. Each system 50 and 52 may be any of the integrity monitoring systems described above with reference to Figures 1 to 8, or a combination of them. Different monitoring systems may be deployed for each filter, according to their performance under different conditions and constraints of space and cost, considered against the risk of damage associated with failure of each filter.

[0088] In the case where SPF 720 is a membrane-type SPF for EUV lithography, this membrane forms a planar article 120 whose integrity is to be monitored using integrity monitoring system 52. A simple example of such a membrane-type SPF is a metal membrane, for example a Zirconium membrane. Figure 11 depicts an exemplary diagram of Monte-Carlo simulations of electron trajectories after electrons hit a Zirconium membrane, in which the electron beam is normally oriented to the membrane surface and has an energy (corresponding to electron velocity) of 2 keV. In Figure 11, no electron penetrates deeper than 50 nm and so the electrons are reliably absorbed by the membrane. When a hole 130 appears above even a very small size, electrons can pass as illustrate in Figures 2 to 7 and this hole can be detected by the integrity monitoring system.

[0089] In an embodiment of a membrane type SPF for EUV lithography, the simple zirconium membrane may be replaced by another metal, or by a multilayer structure or metals and or other materials such as silicon. Examples of multilayer SPFs are disclosed in European Patent Application Publication No. 2053464 A. As mentioned above, the electron beam energy should be selected with regard to the material properties and dimensions of the article. In the multilayer membrane SPF, these properties depend on a combination of thicknesses and materials. The material properties of article 120 includes for example the charge and abundance of elements used for article 120. For example, to obtain a same penetration depth, for a membrane made of heavier elements, electron beam source module 60 can be adjusted to generate the electron beam with a lower energy, while for article 120 with lighter elements, electron beam source module 60 is operable to generate the electron beam with a higher energy. The best energy value to use can be determined by experiment.

[0090] In the case where SPF 710 is a grid type SPF for EUV lithography, this grid forms a planar article 120 whose integrity is to be monitored using integrity monitoring system 52. An exemplary grid type SPF will be described with reference to Figures 12 and 13. It will be seen that the grid type SPF already includes very small apertures which may pass some electrons. When a hole 130 appears above the normal size, more electrons can pass as illustrate in Figures 2 to 7 and this hole can be detected by the integrity monitoring system 50.

[0091] Figure 12 is a front face view of part of an exemplary spectral purity filter part 802 made according to United States Patent Application No. 61/193,769, filed on 22 December 2008, that may for example be applied as an element of the SPF of a lithographic apparatus. The filter part 802 is configured to transmit extreme ultraviolet (EUV) radiation while substantially blocking a second type of radiation (the ‘unwanted’ radiation) generated by a radiation source. This unwanted radiation may be, for example, infrared (IR) radiation of a wavelength larger than about 1 μηι, particularly larger than about 10 pm. Particularly, the wanted EUV radiation to be transmitted and the unwanted second type of radiation (to be blocked) can emanate from the same radiation source, for example an LPP source SO of a lithographic apparatus.

[0092] Figure 13 (a) is a schematic front face view of a very small area within the filter part of Figure 12, while Figure 13 (b) shows the same part in cross-section on line B-B’. The spectral purity filter in the examples to be described comprises a substantially planar filter part 802 (for example a filter film or filter layer). The filter part 802 has a plurality of (generally parallel) apertures 904 to transmit the extreme ultraviolet radiation and to suppress transmission of the second type of radiation. The face on which radiation impinges from the source SO will be referred to as the front face, while the face from which radiation exits to the illumination system IL can be referred to as the rear face.

[0093] In the example shown, each aperture 904 has parallel sidewalls 906 defining the apertures 904 and extending completely from the front to the rear face. As seen in Figure 12, structural members 808 may extend between portions of the grid part 802, so that the filter part is not too fragile to extend across the entire beam path. The filter part 802 may only be a few micrometers thick, while extending over a diameter of several centimeters. The filter part may be made of a metal such as tungsten (W), in which case it is inherently conductive and suitable for capturing and conducting electrons. In embodiments of the monitoring system where the article itself is used as a conductor for monitoring current, an electrical connection simply needs to be made to the filter part 802 in order to collect the current. It may be necessary to insulate the filter part from its mounting to allow collection of current, unless the current can be collected through the mounting.

[0094] Filter part 802 in other embodiments may be made of a non-metallic material. For example silicon grid SPFs have been proposed. It such embodiments, modification may be made to the filter part to ensure that it captures electrons to the extent necessary for proper functioning of the monitoring system. The same or different modifications may be made to ensure proper conduction of electrons, if the monitoring system is of the type where the article itself is used as a conductor for monitoring current. A silicon part can be doped, for example, to ensure conduction. In some embodiments, the filter part made of silicon will be coated already with metal such as Molybdenum (Mo), to aid reflection of infrared radiation. The same coating may serve to capture and conduct the electrons enough to allow proper functioning of the integrity monitoring system.

[0095] The filter part may be modified by the addition of a phosphor layer 160, where the monitoring system is based on the embodiment of Figure 3.

[0096] Figure 14(a) depicts an electron beam delivered in a direction normal to a surface of the grid type filter of Figure 13. Although some electrons may be absorbed by the filter, a significant proportion of electrons pass through the apertures 904 without contact with the sidewalls 906. When there is a hole 130 present in the grid type filter, the proportion of electrons passing through the hole 130 and an aperture 904 in principle increases, which can in principle be detected by any one of or a combination of the above-mentioned systems, such as the detection module 80 of system 50. The difference in electron flux between the damaged and non-damaged conditions in this example may be relatively small, however, so that it becomes a challenge to design the monitoring system to detect small holes 130 without also risking false alarms.

[0097] Figure 14(b) depicts an electron beam delivered at an oblique angle to the grid type filter. The electron beam is delivered onto a grid type filter at an angle with the sidewalls 906. In such a case, the electrons hit the sidewalls 906. Among those, although some electrons are absorbed by the filter due to the contact, some electrons bounce along the sidewalls 906 before passing through the grid type filter. Some or all of these may be detected by the integrity monitoring apparatus, in embodiments where the article itself does not serve as a detector, or is not the only detector. When there is a hole 130 in the grid type filter, the amount of electrons passing through the hole 130 and an aperture 904 are different and the difference is detected by any one of or a combination of the above-mentioned integrity monitoring systems as an embodiment of system 50. Since the beam is delivered at an angle to the filter, more electrons of the electron beam are absorbed by the filter (in its intact portions) and the difference detected can be more significant, as compared to the difference detected when the electron beam is normal to the filter.

[0098] In further alternative embodiments, a SPF may be located practically anywhere in the radiation path. In an embodiment, the SPF is located in a region that receives EUV-containing radiation from the EUV radiation source and delivers the EUV radiation to a suitable downstream EUV radiation optical system, wherein the radiation from the EUV radiation source is arranged to pass through the SPF prior to entering the optical system. In an embodiment, the SPF is in the EUV radiation source. In an embodiment, the SPF is in the EUV lithographic apparatus, such as in the illumination system or in the projection system. In an embodiment, the SPF is located in a radiation path after the plasma but before the collector.

[0099] It will be understood that the apparatus of Figure 10 incorporating one or more systems for monitoring the integrity of a planar article may be used in a lithographic manufacturing process. Such lithographic apparatus may be used in the manufacture of ICs, integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc. It should be appreciated that, in the context of such alternative applications, any use of the term "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer 1C, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[00100] The skilled person in the art can understand that the integrity monitoring systems 50, 52 are not limited to only monitor a SPF in the source collector unit SO or an illumination system IL of an EUV lithographic apparatus. The system 50 can be implemented to monitor the integrity of any SPF or other article in an EUV optical apparatus generally, or in any apparatus where an article is required to function in a vacuum or near-vacuum environment. Alternatively, the system 50 may be implemented outside the operating environment of an EUV lithographic apparatus, for example, to monitor article 120 before or after article 120 is operating in an EUV lithographic apparatus, or to monitor article 120 as a quality control when article 120 is in the process of being manufactured.

[00101] The descriptions above are intended to be illustrative, not limiting. Thus, it should be appreciated that modifications may be made to the present invention as described without departing from the scope of the clauses set out below. For example, the above-mentioned exemplary embodiments may be implemented to detect the integrity of a dome-shaped article. The dome-shaped article may for example be a grid type filter in an optical apparatus such as an EUV lithography.

[00102] The following exemplary embodiments of the present inventions are disclosed.

[00103] In an embodiment, a method is provided for monitoring integrity of an article operating in a low pressure environment, the method comprising: directing a beam of electrons toward the article within said environment, the form of the article when intact being configured to stop at least a significant proportion of the electrons in said beam; and generating a signal to indicate integrity status of the article by identifying when at least a part of the article is not stopping the expected proportion of electrons in the beam.

[00104] In certain embodiments, the signal is generated at least in part by detecting a current flowing in the article itself, a drop in said current indicating that electrons are passing through the article more than expected. The electron beam may be directed at different portions of the article at different times, and wherein the timing of said drop in current is used to generate a signal indicating a location of a defect in the article. For example, the electron beam may be directed at the article in a raster scanning pattern.

[00105] In certain embodiments, a collecting electrode is positioned so as to collect electrons that pass through the article, and wherein said signal is generated at least in part by detecting a current flowing in the collecting electrode, a rise in said current indicating that electrons are passing through the article more than expected. In such embodiments, said electron beam may be directed at different portions of the article at different times, and wherein the timing of said rise in current may be used to generate a signal indicating a location of a defect in the article. For example, said electron beam may be directed at the article in a raster scanning pattern.

[00106] In certain embodiments, a phosphor is applied to one or more elements in the path of said electron beam, and wherein said signal is generated at least in part by observing light emitted by said phosphor when impinged by the electron beam. In such embodiments, said electron beam may be directed at different portions of the article at different times, and wherein the timing of a change in the emitted light may be used to generate a signal indicating a location of a defect in the article. The phosphor may be applied to a collecting screen arranged to receive electrons that pass through the article, a bright portion in a pattern of light formed on said collecting screen indicating the presence of a defect in the article.

[00107] In certain embodiments, said electron beam is generated by an electron source in a vacuum or low pressure environment substantially isolated from that of the article itself. In such embodiment, the environment of the article may include hydrogen gas at low pressure, and the environment of the electron source may substantially exclude said hydrogen gas. Said electron beam may be modified by an electromagnetic arrangement after entering the environment of the article. Said electromagnetic arrangement may be controlled to deflect the electron beam to different portions of the article at different times. In one embodiment, said electron beam passing through an aperture is narrowed before being deflected by said electromagnetic arrangement.

[00108] In certain embodiments, said article is operating within an EUV optical apparatus. In such embodiments, said article may be operating within an EUV lithography apparatus. For example, said article may be a spectral purity filter constructed to attenuate radiation at wavelengths other than a desired range of EUV wavelengths.

[00109] In an embodiment, said article is a membrane or foil. In another embodiment, said article is in the form of a grid.

[00110] In an embodiment, said electron beam is directed at the article at an angle away from a direction normal to a plane of the article.

[00111] In certain embodiments, a system for monitoring integrity of an article operating in a low pressure environment is provided. In such embodiments, said system includes an electron beam source module and a detection module. Said system is operable to perform any of the methods in the above-mentioned embodiments.

[00112] In certain embodiments, an optical apparatus is provided, the optical apparatus comprising: a radiation unit for providing a beam of EUV radiation through an exit aperture; an illumination system for receiving the beam of EUV radiation from the exit aperture of the radiation unit and for conditioning the beam to illuminate a patterning device; a projection system for producing an image of the illuminated patterning device on a substrate, in order to transfer a pattern from the pattern device to the substrate by EUV lithography; at least a system according to any of the systems in the above-mentioned embodiments for monitoring integrity of a filter in the lithographic apparatus, wherein the filter is located in the path of said radiation beam in one of the radiation unit, the illumination system and the projection system.

[00113] While specific embodiments of the invention have been described above, it is to be understood that the embodiments described herein can be implemented in hardware, software, firmware, middleware, microcode, or any combination thereof. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions which, when executed by a computer, control the components of a system described above to monitor integrity of an article.

[00114] Fora hardware implementation, the processing units can be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.

[00115] When the embodiments are implemented in software, firmware, middleware or microcode, program code or code segments, they can be stored in a machine-readable medium, such as a storage component. A code segment can represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment can be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. can be passed, forwarded, or transmitted using any suitable means including memory sharing, message passing, token passing, network transmission, etc.

[00116] Fora software implementation, the techniques described herein can be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes can be stored in memory units and executed by processors. The memory unit can be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art.

[0100] The term "comprising" and “including" as used in the clauses does not exclude other elements or steps. The term "a" or "an" as used in the clauses does not exclude a plurality.

[0101] The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the clauses set out below. Other aspects of the invention are set out as in the following numbered clauses: 1. A method for monitoring integrity of an article operating in a low pressure environment, the method comprising: directing a beam of electrons toward the article within said environment, the form of the article when intact being configured to stop at least a proportion of the electrons in said beam; and generating a signal to indicate integrity status of the article based at least in part on the proportion of electrons of the beam that are stopped by the article.

2. A method as claimed in clause 1, wherein said signal is generated at least in part by detecting a current flowing in the article itself, a reduction in said current indicating that more electrons are passing through the article.

3. A method as claimed in any preceding clause, wherein a collecting electrode is positioned so as to collect electrons that pass through the article, and wherein said signal is generated at least in part by detecting a current flowing in the collecting electrode, an increase in said current indicating that more electrons are passing through the article.

4. A method as claimed in clause 2 or 3, wherein said electron beam is directed at different portions of the article at different times, and wherein the timing of any change in current is used to generate a signal indicating a location of a defect in the article.

5. A method as claimed in any preceding clause, wherein a phosphor is applied to one or more elements in the path of said electron beam, and wherein said signal is generated at least in part by observing light emitted by said phosphor when impinged by the electron beam.

6. A method as claimed in any preceding clause, wherein said electron beam is generated by an electron source in a vacuum or low pressure environment substantially isolated from that of the article itself.

7. A method as claimed in clause 6, wherein said electron beam passing through an aperture is narrowed before being deflected by said electromagnetic arrangement.

8. A method as claimed in any preceding clause, wherein said article is operating within an EUV optical apparatus.

9. A method as claimed in clause 8, wherein said article is a filter within the EUV optical apparatus.

10. A system for monitoring integrity of an article operating in a low pressure environment, comprising: an electron beam source module configured to direct a beam of electrons toward the article within said environment, the form of article when intact being configured to stop at least a proportion of the electrons in said beam; and a detection module configured to generate a signal to indicate integrity status of the article based at least in part on the proportion of electrons of the beam that are stopped by the article.

11. The system as claimed in clause 10, wherein the detection module includes a current meter and said signal is generated at least in part by using the current meter to detect a current flowing in the article itself, a reduction in said current indicating that more electrons are passing through the article.

12. The system as claimed in clause 10 or 11, wherein the detection module includes a collecting electrode to collect electrons that pass through the article, and wherein said signal is generated at least in part by using collecting electrode to detect a current flowing in the collecting electrode, an increase in said current indicating that more electrons are passing through the article.

13. The system as claimed in clause 11 or 12, wherein the detection module is operable to direct said electron beam at different portions of the article at different times, and wherein the timing of any change in current is used to generate a signal indicating a location of a defect in the article.

14. The system as claimed in any of clauses 10 to 13, wherein the detection module includes a phosphor applied to one or more elements in the path of said electron beam, and wherein said signal is generated at least in part by observing light emitted by said phosphor when impinged by the electron beam.

15. The system as claimed in any of clauses 10 to 14, wherein the electron beam source module includes an enclosure and said electron beam is generated by the enclosure in an vacuum or low pressure environment substantially isolated from that of the article itself.

16. The system as claimed in clause 15, wherein the enclosure has an aperture and said electron beam passing through the aperture is narrowed before being deflected by said electromagnetic arrangement.

17. The system as claimed in any of clauses 10 to 16, wherein said article is operating within an EUV optical apparatus.

18. The system as claimed in clause 17, wherein said article is a filter within the EUV optical apparatus.

19. An optical apparatus, comprising: a radiation unit for providing a beam of EUV radiation through an exit aperture; an illumination system for receiving the beam of EUV radiation from the exit aperture of the radiation unit and for conditioning the beam to illuminate a patterning device; a projection system for producing an image of the illuminated patterning device on a substrate, in order to transfer a pattern from the pattern device to the substrate by EUV lithography; and a system according to any of clauses 10 to 18 for monitoring integrity of a filter in the lithographic apparatus, wherein the filter is located in the path of said radiation beam in one of the radiation unit, the illumination system and the projection system.

20. An optical apparatus, comprising: a radiation unit configured to provide a beam of EUV radiation through an exit aperture; an illumination system configured to receive the beam of EUV radiation from the exit aperture of the radiation unit and to condition the beam to illuminate a patterning device; a projection system configured to produce an image of the illuminated patterning device on a substrate, in order to transfer a pattern from the pattern device to the substrate by EUV lithography; and a system configured to monitor integrity of a filter operating in a low pressure environment, the system comprising an electron beam source module configured to direct a beam of electrons toward the filter within said environment, the form of filter when intact being configured to stop at least a proportion of the electrons in said beam, and a detection module configured to generate a signal to indicate integrity status of the filter based at least in part on the proportion of electrons of the beam that are stopped by the filter, wherein the filter is located in the path of said radiation beam in the radiation unit or the illumination system or the projection system.

Claims (1)

1. Een lithografieinrichting omvattende: een belichtinginrichting ingericht voor het leveren van een stralingsbundel; een drager geconstrueerd voor het dragen van een patroneerinrichting, welke patroneerinrichting in staat is een patroon aan te brengen in een doorsnede van de stralingsbundel ter vorming van een gepatroneerde stralingsbundel; een substraattafel geconstrueerd om een substraat te dragen; en een projectieinrichting ingericht voor het projecteren van de gepatroneerde stralingsbundel op een doelgebied van het substraat, met het kenmerk, dat de substraattafel is ingericht voor het positioneren van het doelgebied van het substraat in een brandpuntsvlakvan de projectieinrichting.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 section of the 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.