Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/708—Construction of apparatus, e.g. environment aspects, hygiene aspects or materials
  • G03F7/7095—Materials, e.g. materials for housing, stage or other support having particular properties, e.g. weight, strength, conductivity, thermal expansion coefficient
  • G03F7/70958—Optical materials or coatings, e.g. with particular transmittance, reflectance or anti-reflection properties
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70483—Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
  • G03F7/70591—Testing optical components

Definitions

• the invention relates to a method for qualifying a reflective optical element and a method for determining a thickness profile of a multilayer system and/or a cap layer system of an optical element for reflecting radiation .
• the invention relates to a method for manufacturing multilayer systems with a cap layer system, in particular reflective optical elements for the extreme ultraviolet up to the soft x-ray wavelength range, a corresponding reflective optical element for the extreme ultraviolet up to the soft x-ray wavelength range as well as an EUV-lithography apparatus with at least one such reflective optical element.
• the invention relates to a method for manufacturing a reflective optical element for the extreme ultraviolet up to the soft x-ray wavelength range with a cap layer system of constant thickness as well as an EUV-lithography apparatus with at least one such reflective optical element.
• the invention also relates to methods for determining the phase shift of a standing electromagnetic wave in the extreme ultraviolet up to the soft x-ray wavelength range at the free interface of a multilay& r system.
• the invention further relates to methods for determining the thickn ess of a cap layer system on a multilayer system being preferably used in the manufacturing methods described above.
• the invention relates, moreover, to apparatuses for carrying out these methods.
• the invention also relates to a reflective optical element comprising a multilayer system with a cap layer system with at least one layer consisting of a transition metal, the multilayer system being optimized for an operating wavelength in the extreme ultraviolet up to the short x-ray wavelength range, an EUV-lithography apparatus with at least one such reflective optical element as well as a method for operating of reflective optical elements.
• the invention further relates to an EUV-lithography apparatus and a method for operating such an EUV-lithography apparatus, as well as a reflective optical element.
• Reflective optical elements for the soft x-ray up to the EUV wavelength range such as photo masks or multilayer mirrors, for example, are required particularly for use in the EUV- lithography of semiconductor elements.
• Typical EUV-lithography apparatuses comprise eight or more reflective optical elements.
• the> mirrors In order to still reach a sufficient total intensity of the operating radiation, the> mirrors have to exhibit as high a reflectance as possible, as the total intensity is proportional to the product of the reflectances of the individual mirrors.
• These reflective optical elements should maintain such a high reflectance over their service life. Furthermore, the homogeneity of the reflectance across the surface of the reflective optical elements must be maintained during their service life. The reflectance and the lifetime of such reflective optical elements are particularly impaired by contamination of the surface during irradiation at the operating wavelength through deposition of carbon and oxidation of the surface.
• the reflective optical elements are contaminated by residual gases from the vacuum atmosphere during operation. Thus, molecules of the residual gas are first adsorbed at the surfaces of the reflective optical elements and then broken up by high-energetic photon radiation through emission of secondary electrons and, to some extent, of photoelectrons as well.
• hydrocarbons When hydrocarbons are present in the residual gas atmosphere, a carbon layer is generated which reduces the reflectance of a reflective optical element about 1 % per nm of thickness. For a partial pressure of hydrocarbon of about 10 "9 mbar, a layer with a thickness of 1 nm is obtained already after about 20 hours. As e.g.
• EUV-lithography apparatuses do not allow the necessary throughput with a loss of reflectance of 1 % per reflective optical element, the contamination layer has to be removed in a cleaning process which may last up to 5 hours. Furthermore, such a cleaning process involves the risk that the surface of the reflective optical element is damaged, e.g. roughened up or oxidized, such that the original reflectance cannot be attained again. Residual gas molecules containing oxygen may contribute to the oxidation of the surfaces. Thus, the unprotected surface of a reflective optical element may be destroyed in a few hours.
• the first step in this case is for the substrates to be irradiated in a spatially resolved fashion with sufficiently energetic polychromatic radiation, after which the photoelectron current emanating from the substrate is determined.
• Ultraviolet radiation is used for the characterization. The measurement is restricted to the contamination to be detected, and no characterization of the substrate is made.
• the reflectance of electromagnetic radiation at a multilayer system der ends both on the wavelength ⁇ that is applied to the surface, and on the incidence angle ⁇ thereof with respect to the surface normals.
• the reflectance R ( ⁇ , ⁇ ) of an optical element is yielded in this case from the intensity of the reflected radiation divided by the intensity of the incident radiation.
• the reflectance is measured at a constant incident angle and variable wavelength.
• the reflectance is measured at a constant wavelength ⁇ , and the incident angle ⁇ of the radiation is tuned.
• synchrotron radiation sources or laser-generated and/or discharge- generated plasma sources are used to carry out reflectometric measurements on multilayer systems for the extreme ultraviolet wavelength region (EUV) and the soft x-ray wavelengths region (for example wavelengths from approximately 1 nm to 20 nm).
• EUV extreme ultraviolet wavelength region
• soft x-ray wavelengths region for example wavelengths from approximately 1 nm to 20 nm.
• Synchrotron radiation is very well suited for reflectometric measurements, because synchrotron radiation is a very brilliant, "white” radiation, that is to say can be collimated very well and is very broadband. Moreover, this radiation is available free from contamination in vacuum.
• the polychromatic radiation of a synchrotron or a plasma source is usually collimated and monochromatized for use in reflectometry.
• the radiation prepared in this way is then used for spectral or angularly dependent measurements of reflectance.
• a photodiode for example, can be used as photon detector for the reflected radiation.
• the intensities are measured serially as a rule, that is to say the measurements consist in a time sequence of parameter changes and photon intensity measurements.
• a reference measurement is typically performed at all radiation sources in order to determine the intensity of the incident radiation.
• Reflectometric investigations of reflective optical elements based on multilayer systems certainly cover some properties that are important for their use in EUV lithography, such as, for example, the maximum reflectance, the wavelength at which the maximum reflectance is reached, and the bandwidth of the reflectance curve, it being possible, for example, to deduce compliance with the overall layer thickness specification from the wavelength of the maximum reflectance.
• properties that are important for their use in EUV lithography such as, for example, the maximum reflectance, the wavelength at which the maximum reflectance is reached, and the bandwidth of the reflectance curve
• the light is directed onto a sample having a rotational degree of freedom for changing the angle of incidence.
• photodiodes are used for such reflectance measurements.
• a grid being connected in a conducting fashion to the sample and located in the beam path between the monochromator and the sample is used.
• the multilayer system is connected to an ammeter via a cable.
• Multilayer systems are produced by depositing materials with different refractive indices and/or different absorption coefficients on a substrate in several layers, one upon the other. Those are used as mirrors, particularly in the extreme ultraviolet wavelength range. Other possible applications of multilayer systems are e.g. anti -reflection coatings of optical elements in the visible wavelength range.
• the reflectance of electromagnetic radiation of a multilayer system is based upon interference among the radiation reflected at multiple interfaces of the multilayer system, being described by Bragg's law. This reflectance is therefore of a dispersive nature.
• the reflectance at the interface between two such layers amounts to several per thousands for angles which are larger than the critical angle for electromagnetic radiation in a wavelength range ⁇ 50 nm. For such angles, reflectances of an order of magnitude of up to 70 % can be obtained with multilayer systems. Multilayer systems are therefore used to attain h igh reflectances for large angles relative to layer surfaces and may therefore also be used as dispersive elements.
• a multilayer system for the reflection of short wavelengths consists of successive periods of two or more layers of materials, each with different refractive indices and thicknesses e.g. in the order of magnitude of the wavelength of the reflected radiation.
• the total reflectance of a multilayer system is determined by the order of magnitude of reflection per interface, i.e. the difference of refractive indices on the one hand and absorption coefficients on the other hand.
• the thicknesses of the individual layers are usually constant across the multilayer thickness for each material. Depending on the specifications of the mirror in terms of the reflectance profile, any other imaginable multilayer system is also possible.
• a protecting cap layer system may be provided on the surface of the reflective optical element.
• a cap layer system is a system of one or several layers deposited on top of the periodic multilayer system described before.
• a cap layer system may comprise one or more layers of a transition metal, such as for example ruthenium or iridium or alloys or chemical compounds comprising ruthenium or iridium.
• a topmost cap layer may even be formed due to contamination, e.g. by deposition of a carbon layer.
• the contamination, respectively, degradation of the surface of the reflective optical elements may be favourably affected.
• the thickness of the one or more cap layers it may be accomplished that the reflectance of the reflective optical element does not decrease too strongly despite of the cap layer system.
• Optical elements made of a substrate and a multilayer system being optimized for high reflectances at a given wavelength e.g. photo masks or mirrors for the extreme ultraviolet wavelength range (EUV) are required particularly for use in EUV-lithography of semiconductor elements.
• EUV extreme ultraviolet wavelength range
• layer thickness profiles may be calculated taking into account different relative movements between substrate and source. Thus, it can be determined how the desired thickness distribution may be obtained.
• layer thicknesses actually acquired are measured by abrasion or etching in places of the coating, and measuring the difference in height to the remaining coating with the aid of profile meters. With common profile meters, vertical resolutions of 3 nm minimum can be attained.
• a collector unit for wavelengths smaller than 193 nm up to the EUV wavelength range is known, whose main element is a mirror shell for generating a uniform and telecentric image of the radiation source which comprises a grating structure on its surface.
• This grating structure diffracts different wavelengths into different directions.
• the desired diffraction order, respectively wavelength may be selected.
• the slit stop screens particles which may possibly originate from the radiation source.
• Control loops for the prevention of contaminations of surfaces of reflective optical elements on the basis of multilayer systems in an evacuated system comprising a residual gas atmosphere during the irradiation with operating wavelengths in the EUV-region are already known.
• a photoelectron current generated by photoelectron emission of the irradiated surface of the multilayer system is measured.
• the regulation of the gas composition during irradiation is carried out in dependence of the measured photoelectron current so that by reaching or exceeding a threshold level, a gas is supplied to the closed system and subsequently, before or after reaching another threshold level, the supply of said gas is at least reduced.
• the layer material and/or the layer thickness of at least one layer of the multilayer system should be chosen in such a way that the standing electromagnetic wave being formed during reflection of the incident operating wavelength has a node of the electrical field strength in the region of the free interface of the multilayer system.
• the reflective optical elements are provided with a cap layer which contains one or several transition metals, and, on the other hand, the irradiation at the operating wavelength is carried through in an evacuable closed system having a residual gas atmosphere, wherein a reductive gas or mixture of reductive gases and an oxidizing gas or mixture of oxidizing gases should be present simultaneously in the residual gas atmosphere.
• the partial pressures can be adjusted such that oxidizing and reductive processes on the surface of the reflective optical element are balanced such that no appreciable contamination can take place.
• a residual gas atmosphere consisting of a hydrocarbon, water, and oxygen.
• Lithography apparatuses for the x-ray region which comprise various detectors in order to control the radiation intensity during the irradiation of the mask and the wafer are already known. Through this, deformations of the mirrors which are caused by thermal load are detected and the contamination state of the mirror surfaces is controlled. Particularly for the detection of deformations and the contamination state, it is resorted to the photoelectric effect. The detected deformations are balanced during the irradiation process. If contamination of a mirror is detected, a signal is given that the mirror has to be changed or maintenance has to be carried through.
• cleaning reticles may be provided which are optimized for directing the cleaning beam to the locations to be cleaned.
• a protective layer has been described which considerably reduces the oxidatio n susceptibility, which prolongs the lifetime of reflective optical elements.
• lifetimes of several years have to be achieved.
• a further approach- to avoid losses in reflectance by contamination consists in providing a photocatalytic protective layer, e.g. and oxide of a transition metal, so that during irradiation with EUV-radiation free oxygen radicals are generated which react with the carbon deposits to volatile compounds. Oxygen, water and/or peroxide are supplied where appropriate.
• the object of the invention further consists in providing a method for experimentally determining the phase shift of the standing wave at the free interface of a multilayer system and for determining layer thicknesses, and an arrangement suitable for the purpose, in particular for use in the manufacturing methods mentioned above.
• a further object of the present invention is to provide a reflective optical element for the extreme ultraviolet up to the soft x-ray wavelength range being optimized in terms of contamination, an EUV-lithography apparatus based thereon, as well as a method for operating such an EUV-lithography apparatus.
• the object is further realized in providing an EUV-lithography apparatus with as high a lifetime as possible as well as a method for operating such an EUV- lithography apparatus.
• the invention is realized in a method for manufactu ring a multilayer system with a cap layer system, in particular reflective optical elements for the extreme ultraviolet up to the soft x-ray wavelength range, comprising the steps of:
• the invention is a method for manufacturing a reflectives optical element for the extreme ultraviolet up to the soft x -ray wavelength range with a cap layer system of constant thickness comprising the steps of:
• a further object of the invention is realized by means of methods for determining the phase shift of a standing electromagnetic wave in the extreme ultraviolet up to the soft x-ray wavelength range at the free interface of a multilayer system, having the steps of:
• This object is also achieved by means of methods for determining the thickness of a cap layer system on a multilayer system from the phase shift of a standing electromagnetic wave in the extreme ultraviolet up to the soft x-ray wave range at the free interface of a multilayer system, having a cap layer system, having the steps of:
• the invention is furthermore realized by means of methods for determining the thickness of a cap layer system on a multilayer system from the phase shift of a standing electromagnetic wave in the extreme ultraviolet up to the soft x-ray wave range at the free interface of a multilayer system, having a cap layer system, having the steps of:
• Method steps are advantageously carried out in a spatially resolved fashion, and the experimentally obtained data are advantageously additionally compared with reference data that are obtained from specified multilayer systems, and/or are measured on the multilayer system in the absence of resonance.
• apparatuses for carrying out the methods for determining the phase shift and layer thicknesses as set forth in the claims, the apparatuses having means for spatially and spectrally setting incoming radiation, as well as a vacuum chamber in which a photon detector, a photoelectron detector and a sample holder are arranged, an electrically conducting wide angle element being used as photoelectron detector, and the means for spatially and spectrally setting incoming radiation being optimized to provide a narrowband beam of small beam diameter, or are optimized to provide a narrowband beam of small divergence.
• the devices advantageously have a radiation source for extreme ultraviolet and/or soft x-radiation.
• the sample holder advantageously has three degrees of translational freedom and three degrees of rotational freedom.
• a device is advantageously provided for generating a defined electric field in the vicinity of the sample.
• a reflective optical element for the extreme ultraviolet up to the soft x-ray wavelength range comprising a multilayer system with cap layer system with at least one layer consisting of a transition metal or an alloy, compound or mixture containing a transition metal, being optimized for an operating wavelength in the extreme ultraviolet up to the soft x-ray wavelength range which is characterized in that at least one layer- or cap layer- thickness is chosen in such a way that during irradiation at the operating wavelength a standing electromagnetic wave is formed in such a way that it forms an intensity maximum in the area of the free interface of the reflective optical element.
• the invention is realized in an EUV- lithography apparatus with at least one such reflective optical element having an evacuable housing and at least two inlets which open towards the reflective optical element and are used for supplying an oxidizing gas or mixture of gases and a reductive gas or mixture of gases, as well as a method for operating such a reflective optical element in a closed system having a residual gas atmosphere consisting of a hydrocarbon, water, and oxygen, in which the partial pressure of the hydrocarbon is increased in such a way that carbon is deposited on and/or in the topmost layer when the irradiation at the operating wavelength is started, such that the intensity maximum of the forming standing electromagnetic wave is located at the free interface.
• the invention is realized in a method for operating said reflective optical element in a closed system with a residual gas atmosphere comprising a reductive gas fraction and an oxidizing gas fraction, wherein the partial pressures of the gas fractions are adjusted in such a way that oxidizing and reductive reactions on the topmost layer are in equilibrium.
• the invention is realized in a method for operating a reflective optical element for the extreme ultraviolet up to the soft x-ray wavelength range comprising a multilayer system with a topmost cap layer consisting of carbon and/or an oxide in a closed system having a residual gas atmosphere comprising a reductive gas fraction and an oxidizing gas fraction, wherein the partial pressures of the gas fractions are adjusted in such a way that oxidizing and reductive reactions at the topmost cap layer are in equilibrium, and a lithography apparatus with at least one such optical element for the extreme ultraviolet up to the soft x-ray wavelength range consisting of a multilayer system with a topmost cap layer consisting of carbon and/or an oxide, wherein at least one layer- or cap layer thickness is chosen in such a way that during irradiation at the operating wavelength a standing electromagnetic wave is formed in such a way that it forms an intensity minimum in the area of the free interface of the optical element, with an evacuable housing in which the reflective optical element is arranged and at least two inlets which open
• a further aspect of the invention is realized in an EUV-lithography apparatus with at least one photoelectron detector, with appropriate means for adjusting a residual gas atmosphere i nside the EUV-lithography apparatus, and with at least one tunable monoch romator in the optical path such that the incident wavelength can be varied, in particular switched between the operating wavelength and at least one usable wavelength.
• the switching between the operating wavelength and the at least one usable wavelength is preferably realized by a first reflective optical element with a maximum of reflectance at the operating wavelength and at least a second reflective optical element with a maximum of reflectance at the at least one useable wavelength, wherein the first and second reflective optical elements are preferably interchangeable with an operating reticle of the EUV-lithography apparatus.
• the EUV-lithography apparatus comprises a first photon detector for detection of photons at the irradiating wavelength and/or a second photon detector for detection of photons with higher wavelengths corresponding to photon energies below about 90 eV, including photon energies below about 1.65 eV, i.e. in the infrared wavelength range.
• the shifting from the irradiating wavelength to higher wavelengths is done by generating photons with specific wavelengths through controlling the standing electromagnetic wave being formed in the resonant case or by doping of a cap layer system of a reflective optical element for selectively generating photons with defined energies.
• Yet a further aspect of the invention is realized in a method for operating such an EUV-lithography apparatus, wherein at predetermined times an irradiating operation is switched to a detection mode, in which
• the location to be inspected is irradiated selectively and the photoelectron current and, if necessary, the reflectance are measured in dependence of the wavelength by tuning the monochromator;
• the contamination state is identified through determination of the photoelectron current in the region of maximum reflectance, respectively, comparison on the measured photoelectron current data with data modelled for different contamination states.
• Fig. 1 a a manufacturing method for reflective optical elements
• FIG. 1 b a modification of the method shown in figure 1 a;
• Fig.s 2a, b the basic principle of the method for determining the phase shift
• Fig.s 3a, b a first method for determining layer thickness
• Fig.s 4a, b, c, d a second method for determining layer thickness by measuring reflectance and photoelectron current for different carbon thicknesses, and oxidation, respectively
• Fig.s 5a-n spectrally resolved reflectance- and photoelectron current- curves computed for different carbon thicknesses as well as an intensity distribution of a corresponding standing electromagnetic wave being formed in the resonant case
• Fig. 6 a measurement arrangement for measuring the photocurrent and the reflectance
• Fig. 7 a draft of the design principle of a reflective optical element
• Fig. 8 an operating method for a reflective optical element
• Fig.s 9a, b measurement curves of photoelectron current and reflectance for different modes of operation
• Fig. 10 an EUV-lithography apparatus
• Fig. 11 a method for operating the EUV-lithography apparatus shown in fig. 10;
• Fig.s 12a, b, c spectral characteristics of the relative reflectance of reflective optical elements of the EUV-lithography apparatus shown in fig. 10.
• the manufacturing methods for multilayer systems with a cap layer system known so far suffer from that the determination of the thickness of different layers and cap layers, respectively, can be carried through only in a very imprecise way. In particular, only mean layer thicknesses over the depth of the entire multilayer system can be determined by reflectance measurements. However, this information is sufficient for optimization of the coating process of multilayer systems without cap layer system, as far as optimum reflectance is concerned. This problem is more serious in the production of multilayer systems with a cap layer system.
• the cap layer syste m breaks the periodicity such that no reliable information about layer thicknesses can be achieved by mere reflectance measurements. Sticking to the desired multilayer system as precisely as possible is extremely important for cap layer systems in terms of durability properties, in particular contamination resistance, while keeping reflectance as high as possible.
• the measurement principle described herein is based on the fact that changes in the thickness of the cap layer system, which may consist of one or several layers, are resulting in large fluctuations of the photoe lectron current on this coating whereas changes in reflectance are relatively small.
• phase adaptation is absolutely necessary since both the achievable reflectance and the way it changes in the course of the service life, the photoemission and thus the degradation of the surfaces in the course of the service life, and also the wavefront and therefore the imaging properties in the course of the service life are substantially influenced by this phase adaptation.
• One aspect of the invention serves the purpose of measuring optical elements and/or components of optical systems in the spectral range of extreme ultraviolet radiation or soft x-radiation.
• a preferred field of application is the measurement of optics for EUV radiation that achieve high reflectances in a narrow spectral range because of a coating with u p to 100 or more layers with a thickness of a few nanometers that are made from alternating layers of different materials (for example molybdenum and silicon or molybdenum and beryllium).
• Such systems are applied in the field of EUV lithography.
• the spectrally and/or angularly resolved photoelectric characterization proposed here can be used for monitoring the production process and for quality assurance of the components in the EUV lithography system.
• the thickness of the cap layer of a reflective optical element and the spatial homogeneity thereof are determined in the shortest possible measurement period in a cost effective way and with the least possible complexity of apparatus by means of spectrally and/or angularly resolved photoelectric detection.
• the spectrally resolved photoelectric characterization is carried out e.g. with the aid of an arrangement in which a radiation source divergently emits polychromatic EUV radiation and means are provided for collimation of the beam together with means for spectral dispersion. Furthermore, there is need for an electron capture apparatus with a current measuring unit for detecting the emitted photo-induced electrons, as well as a photon detector for measuring the reflectance.
• the object to be characterized is irradiated at a fixed angle to its surface during measurement.
• the object to be characterized is irradiated with a fi-xed wavelength, but at different angles.
• the measurement can also be carried out for specific combinations of wavelength and angle.
• the phase shift of the standing electromagnetic wave is determined by virtue of the fact that the profile of the photoelectric current signal is determined in the region of maximum reflectance.
• the region of maximum reflectance rneans in this case preferably in a region from -4% to +1 %, in particular in a region from - 3 % to +1 %, of the wavelength, or in a corresponding region of the angle ot maximum reflectance.
• a starting premise in this case is that, to a first approximation, the photoelectron current is proportional to the intensity of the standing electromagnetic wave at the location of the free interface to the vacuum. If still further cap layers as for example topmost contamination cap layers are located on the multilayer systems, their outermost surface forms the free interface to the vacuum.
• the phase shift of the electromagnetic wave is arbitrary at the free interface. However, the phase shift can be precisely determined via simultaneous measurement of the reflectance and the photon current.
• the measured photoelectron current curve is compared with an already known photoelectron current curve. This can have been measured in advance on the same or a comparable sample (for example the same multilayer system, but a differen cap layer system or different levels of contamination), or also have been ascertained by model calculations.
• the photoelectron current profile in the region of the maximum reflectance can be used to determine the respective? phase shift with reference to the free interface, which corresponds to the respective spatial positions of the free interface in relation to the multilayer system.
• the difference between the respective positions it is possible by forming the difference between the respective positions to determine the difference in thickness of the two samples or, given known layer thicknesses of the comparison sample, the cap layer thickness of the measured sample, specifically with a resolution of approximately 3 A.
• the measured photoelectron current profile in the region of maximum reflectance is fitted to data modeled for various layer thicknesses.
• the apparatus for carrying out the methods is distinguished in that a wide-angle element such as, for example, a wire grid or a wide-angle plate or even the wall of the vacuum chamber, is used as photoelectron detector. A max ⁇ mum emission angle of the photoelectrons is covered thereby, and so a reliable signal is obtained even with low photoelectron currents.
• a wide-angle element such as, for example, a wire grid or a wide-angle plate or even the wall of the vacuum chamber
• the spatial beam shaping is performed via collimators, for example.
• the main requirements made on the beam in spatial respects a re generally low divergence for a high angular resolution, or a beam spot as small as possible for a high spatial resolution.
• the spectral beam shaping is normally performed via monochromators, for example. It is possible for this purpose to make use, for example, of grid structures, film filters, specular reflectors or the like, particularly in the case of narrowband sources, for example, sources with a line spectrum. Tunable monochromators are preferred in the case of broadband sources, this being, for example, moveable grid structures, if appropriate in combination with a slit.
• Both the spatial and the spectral properties of the beam can be influenced with the aid of focusing monochromators.
• the spectral bandwidth of the monochromator should be as narrow as possible for measurements with a high energy resolution.
• the sample holder has three degrees of translational and rotational freedom, it is easier to carry out spatially resolved measurements at a fixed wavelength and/or fixed incidence angle, or a combination of specific wavelengths and angles. In particular, it is possible thereby to ensure more easily that the entire surface to be tested is scanned uniformly.
• the apparatus has a device for applying an electric field in the vicinity of the sample.
• the photoelectrons can be accelerated onto the wide angle element with the aid of this electric field such that low-energy photoelectrons are also detected. This raises the measuring accuracy. It is, moreover, possible to shield ions that may be present so that they do not falsify the measurement result.
• Laser-induced plasmas, discharge-induced plasmas and relativistic electro ns come into consideration as radiation source, in particular for wavelengths between 1 - 20 nm.
• the synchrotron radiation resulting from relativistic electrons is distinguished by a broad energy spectrum in conjunction with low divergence.
• the two other radiation sources are particularly preferred because of their potential use on a laboratory scale.
• the methods for determining the thickness of a cap layer system on a multilayer system described above can be advantageously used in a method for manufacturing a multilayer system with a cap layer system.
• the location of the maximum of reflectance is primarily determined by the design of the multilayer system as such, whereas the thickness of the cap layer system and its individual layers, respectively, determines the location of the free interface to the vacuum of the total system. Depending on the intensity which the standing wave actually reaches at this free interface, more or less photoelectrons are emitted.
• the intensities of the standing wave can be calculated for arbitrary designs.
• the data determined in such a way is compared with the desired coating design. If there are deviations, the coating parameters available, such as, among others, e.g. pressure, angle, currents, coating masks, movement patterns, and many more, may be adjusted accordingly and then a new coating process may be carried through.
• the coating parameters available, such as, among others, e.g. pressure, angle, currents, coating masks, movement patterns, and many more, may be adjusted accordingly and then a new coating process may be carried through.
• a spatially resolved measurement of the entire surface is highly important.
• the multilayer system with cap layer system may be reasonable to provide a thickness distribution of the cap layer system which is constant across the whole surface or which is variable for compensating e.g. high thermal load, high local risk of contamination, or other.
• the production method is extended in such a way that the coating design is also optimized with its help, namely by testing multilayer systems with cap layen system obtained from simulations for optical systems in which the obtained multilayer system is included in a first pass, and then comparing the simulation results with predetermined specifications, e.g. concerning lifetime or imaging characteristics.
• predetermined specifications e.g. concerning lifetime or imaging characteristics.
• a new optimization loop of the coating process is started, or it is found that the multilayer system with cap layer system meets the specifications.
• the method may further be optimized by testing of the multilayer system during irradiating operation and comparing the test results with predetermined specifications.
• the multilayer system may be measured as a single optical element e.g. in a measurement apparatus as described above, or as a component of an optical system. The latter is particularly recommended if simulations have been carried through for such an optical system beforehand.
• the coating design - if necessary, also the design of the optical system - is modified and a new optimization loop of the coating process is started, or it is found that the multilayer system with cap layer system meets the specifications.
• an EUV source with as high a brilliance as possible, e.g. a synchrotron radiation source, or laser, or discharge-induced plasma source being optimized for small spot sizes of the beam, and one makes sure that the surface of the overall system is completely scanned.
• a coating method as yet known, such as, e.g., electron beam vaporization, sputtering, in particular magnetron sputtering, or other.
• a reflective optical element for the extreme ultraviolet to soft x-ray wavelength range with a cap layer system of constant thickness across the whole surface is produced according to the principles described above.
• one may either be limited to optimization of the coating parameters or may optimize the coating design itself - respectively also the layout of an optical system comprising the reflective optical element - as well. In doing so, an important aim of optimization is the maximization of the lifetime of the reflective optical element while at the same time ensuring a reflectance as high as possible at the operating wavelength.
• a multilayer system consisting of alternating molybdenum and silicon layers.
• a multilayer system is built up periodically.
• intermediate layers, often called barrier layers, are frequently provided which avoid diffusion or intermixing of the individual layers, leading to a high a reflectance over a longer period of time.
• the cap layer system is not built up periodically, but is optimized for protecting the multilayer system underneath from external influences as efficiently as possible.
• the main problem in this context is the contamination which may result in carbon deposits or oxidation of the surface.
• Cap layer systems consisting of e.g. a layer of silicon, a layer of molybdenum, and a topmost silicon layer are highly preferable.
• the topmost silicon layer is transformed at least partially to inert silicon dioxide and/or is covered with carbon by the ambient atmosphere.
• the layer thicknesses of the silicon-molybdenum-silicon cap layer system are chosen in such a way that after growing of a silicon oxide layer and/or a carbon layer, a reflectance as high as possible is still obtained. Further preferred cap layer systems are based upon transition metals.
• the reflective optical according to the invention element may be arranged at arbitrary positions inside EUV-lithography apparatuses, such as in the illumination system or in the projection system. This holds for all reflective optical elements described herein.
• the cap layer system of multilayer systems for reflective optical elements for EUV-lithography is typically optimized with respect to maximum reflectance.
• the cap layer system consists of e.g. a molybdenum layer and an oxidation -resistant layer preferably made of gold, platinum, rhodium, ruthenium, palladium, silver, rhenium, osmium, and/or iridium as a terminating layer.
• the cap layer system as a rule consists of a silicon layer with a thickness of about 2 to 3 nm and a silicon oxide layer with a thickness of about 1 to 2 nm as a terminating layer, where this silicon oxide layer is normally not applied on purpose, but is formed automatically after the coating process by oxidation of the silicon layer, e.g. through removal into the atmosphere or during the set-up.
• the equilibrium state mentioned before is most stable when the intensity maximum of the standing electromagnetic wave being formed is located in the range of the free interface.
• the described contamination suppression process does not only depend on an appropriate combination of residual gases consisting of reductive and oxidizing gas fractions, such as e.g. hydrocarbon, oxygen, and water on the one hand and a metallic, probably catalytically active surface of the cap layer system on the other hand, but also on the existence of reaction -stimulating free electrons.
• the positioning of the maximum of the standing wave in the vicinity of the free interface causes that a maximum number of reaction -stimulating electrons is always present.
• the cap layer design is chosen in such a way that the free interface is somewhat withdrawn relative to the intensity maximum before the set-up.
• carbon is deposited on the surface.
• the contamination state is controlled through in-situ measurements of the photoelectron current and the reflectance, respectively, of the reflective optical element. Instead of measuring the reflectance simultaneously, one may rely on reflectance curves being calculated with the knowledge of designs of the multilayer system. In any case, by the presence of a carbon layer, the risk of degradation of the surface by unwanted oxidation reactions is also reduced.
• a further advantage of the initial carbon deposition lies in the fact that the carbon is not only accumulated on the topmost layer, but also in the surface range of the matrix of the topmost layer. On the one hand, this avoids further permeation of oxygen and therefore further oxidation of the surface. At the same time, the incorporated carbon itself is largely protected against removal, e.g. through oxidation, because of the strong binding to the matrix. Carbon that is removed nevertheless is continuously re-supplied through the high fraction of hydrocarbons in the residual gas. Excess carbon in the matrix of the surface is continually removed, on the other hand, because of the high fraction of oxidants, such as water and oxygen, in the residual gas.
• the coating design is indeed optimized in such a way that the standing wave being formed in the resonant case develops an intensity minimum at the free interface. In such a way, as few as possible electrons are provided at the surface, resulting in an overall attenuation effect of the reactions which take place at the surface. In such a way, it is made sure that no considerable contamination can take place.
• the lifetime of EUV optics can be attained not only through optimization of e.g. the coating design as described above, but also through an amelioration of EUV-lithography apparatuses as a whole with regard to in-situ control, cleaning and also repair.
• the EUV lithography apparatus comprises at least one photoelectron detector in addition to the usual elements such as e.g. optical elements.
• the latter may also be an ampere meter being connected to the reflective optical element via a cable.
• a supplementary photon detector may also be provided for a simultaneous measurement of reflectance, if it is not relied on reflectance curves calculated with the knowledge of multilayer system designs.
• a tunable monochromator may e.g. consist of a grating of a condenser system of an EUV-lithography apparatus being mounted in a rotatable way.
• the angle of incidence changes and thereby the wavelength of the radiation inside the EUV- lithography apparatus is changed. It is preferred if it is not only possible to switch roughly between two angles of incidence, but if an angular region can be covered in as small as possible steps.
• the residual gas composition may be optimized at least in the region to be inspected. If a residual gas mixture with a reduced amount of contaminants is used, e.g., consisting of hydrocarbon, water, and oxygen, it should be taken care of the fact that the relative ratios of the fractions of the constituents are maintained in such a way that the equilibrium with a reduced amount of contaminants is maintained.
• a residual gas mixture with a reduced amount of contaminants e.g., consisting of hydrocarbon, water, and oxygen
• the region to be inspected is irradiated selectively, and through adjusting of the monochromator the photoelectron current, respectively the reflectance, are measured in dependence of the wavelength in the respective region. From the determination of the spectral characteristics of the photoelectron current and the reflectance and/or the co mparison of the measured photoelectron current with the data modelled for different contamination states, the contamination state can be determined (detection mode). Through this in-situ inspection, the operation state of the EUV- lithography apparatus may be determined with minimal effort.
• the method may be extended in such a way that if first control parameters are exceeded, e.g. particular characteristics of the photoelectron current curve, which indicate an increased contamination, the residual gas atmosphere is modified in such a way that the contamination is reduced (cleaning mode).
• the incident light beam can be used for photocleaning of the respective optical element, e. g. by modifying the beam with respect to its cross section and / or position.
• the light for photocleaning may as well be provided by a second light source of the same wavelength used during operation for the EUV lithography apparatus or of another wavelength suitable for removing contaminations, such as for example DUV Ii ght.
• second control parameters e.g., also particular characteristics of the photoelectron current curve which poi nt to defects on the surface
• the incident beam with respect to its cross-section and its position, so that material can be deposited and/or removed in a spatially defined way (repair mode).
• the detection and cleaning mode operating with a spatial resolution is preferable, too.
• the background of this is that different wavelengths cause different photo electron currents because of a different phase shift of the standing wave formed due to reflection, and, using the overall wavelength spectrum available in the EUV-lithography apparatus for detection, repair and/or cleaning purposes, these may be used selectively.
• a cleaning reticle an appropriate collimator and/or apertures may be used for this purpose.
• the specific cleaning reticle is normally optimized in such a way that it generates the desired illumination at the spots to be inspected inside the projection system in terms of spatial and spectral properties.
• the reticle may also be combined with appropriate apertures.
• the cleaning reticle e.g., the whole surface of the region to be inspected is scanned with a small irradiating spot in an analogous way as in a Braun tube.
• the detection as well as the cleaning and the repair can be performed in a spatially localized manner.
• the size, position and/or wavelength, respectively, bandwidth of the irradiating spot may be changed through an appropriate adjustment.
• a collimator may be considered for control measurements in the illumination system for beam forming.
• An aperture may be used in the illumination system as well as in the projection system.
• Collimators and apertures may also be used in conjunction.
• the at least one adjustable monochromator which preferably is provided with a mechanism for stepless adjustments of position and size, bandwidth, and wavelength of the radiation used for detection, cleaning, and repair, may be adjusted in a stepless way. Thereby, the intensity of the usable radiation may also be varied.
• a commonly available semiconductor detector is used for detection of photons in the reflectance measurement.
• the location to be inspected has to be conductively connected to an electron collecting device.
• the electron collecting device may be e.g. a grating, a metal ring, or a metal cylinder. It is also possible to use the wall of the EUV-lithography apparatus for this purpose, in particular when for the separation of different functional units, such as the projection system or the illumination system, partition walls are present.
• partition walls may also be used for selecting residual gas atmospheres adjusted in dependence of the conditions being present in the respective partitions.
• One may also go so far as to include each optical element in a single compartment.
• the individual mirror compartments may then be separated in the optical path e.g. by optical foil filters.
• the interior of the mirror compartment is connected with the environment through valves.
• the environment may be either the atmosphere of another mirror compartment, or the atmosphere inside the EUV-lithography apparatus, or also a direct gas supply from the exterior.
• valves, gas inlets, gas outlets, or other it may also be achieved that pressure differences inside the surface of a reflective optical element arise. This is particularly advantageous in the cleaning and the repair mode.
• the production method for reflective optical elements for an EUV- lithography apparatus is exemplified.
• a coating design which satisfies theoretical specifications e.g. for the use as a mask or a mirror in the illumination optics or the projection optics and, among other things, may have a cap layer and/or multilayer system with a variable thickness distribution over the surface.
• the multilayer system and the cap layer system may be applied onto a substrate in a way known as such, e.g. by electron beam vaporization or magnetron sputtering.
• the coated substrate is measured in terms of reflectance and photoelectron current, and the thickness distribution is calculated.
• the simultaneous measurement of reflectance and photoelectron current is performed either with dispersion of angle or energy, or with specific combinations of angles and energies. If the desired thickness is reached, the fully coated reflective optical element is inspected by a further synchronous measurement of the reflectance and the photoelectron current. In this way, the design itself can be optimized by testing the reflective optical element in irradiating operation and/or in a simulation of an optical system with respect to fulfilment or non-fulfilment of certain specifications. If necessary, the coating design is adjusted and then a new coating process is carried out which is tested as explained above. If the design as well as the coating process are optimized, one may start with the mass production of the desired reflective optical element based upon the now established optical coating parameters.
• any kind of thickness distribution may be produced.
• the reflective optical element further comprises a cap layer system with a thickness distribution being constant over the surface (see fig. 1 b).
• the multilayer system is not tested in real irradiating operation, but simulations of an optical system are carried through in which the multilayer system with cap layer system is used. The results are compared to specifications established beforehand in order to decide whether or not the layer design has to be further adjusted.
• further optimization loops may be envisaged in which the multilayer system is tested separately or as a component of said optical system during actual irradiating operation.
• the free interface that is to say the topmost surface in relation to the vacuum, of a multilayer system is measured with radiation in the extreme ultraviolet down to the soft x-ray wavelength range.
• any desired EUV source may be used, e.g. based on relativistic electrons as well as laser- or discharge- induced plasmas.
• Particularly accurate measurements can be obtained with well-collimated beams which lead to a small, intensive irradiati ng spot, which may be obtained e.g. by synchrotron radiation.
• the variable irradiating spot can be used to scan the entire coated surface such that two-dimensional phase shift and thickness distributions can be measured, and geometric coating parameters can also be precisely determined.
• the first step is to define the position of maximum reflectance (marked in fig.s 2a, b by a vertical dash point dash) that visualizes the position of the free interface. Lying to the left thereof is the vacuum, in the case of plotting against the wavelength, and to the right thereof are the cap layer system and th e periodic component of the multilayer system. The profile of the photoelectron current curve is considered at a distance of -3 % to +1 % around the wavelength of maximum reflectance.
• a maximum is located in this area (as in fig. 2a), there is also a maximum of the intensity of the standing wave in the area of the free interface - corresponding to a minimum (as in fig. 2b) or an edge (not illustrated).
• the profile of the photoelectron current curve corresponds to the profile of the intensity of the standing wave in the area of the free interface. There is lateral inversion when plotting this against energy. If angularly resolved measurements are carried out, the curve profile is similar to that in the case of wavelength-resolved measurements.
• the coated or contaminated multilayer system is likewise measured.
• the thickness difference of 35 A results in the present example from the difference between the first phase shift and the second phase shift. An accuracy in the Angstrom range is achieved thereby.
• a second method for determining layer thickness makes use of photoelectron current curves modeled for different thicknesses and to which the measurement curve is fitted.
• the accuracy can be raised by introducing values from measurements in the absence of resonance, or else measurements on precisely specified multilayer systems into the calculation of model curves.
• this type of thickness determination use is made of the fact that differences in thicknesses are expressed only marginally in a change of the reflectance curve (see, for example, fig. 4a for small carbon thicknesses, and fig. 4b for thicker carbon thicknesses), but in significant changes of the photoelectron current curve.
• Fig.s 4a, b, c illustrate by way of example the photocurrent curves measured by energy dispersion, and the partially corresponding modeled curves for small carbon thicknesses of 2 A, 2.5 A, 3 A, 3.5 A (fig. 4a), 3 A, 6 A, 12 A (fig. 4b), 10 A, 20 A, 30 A (fig. 4c) or entirely without a carbon layer, in arbitrary units.
• the reflectance curves for the samples with the respectively thickest carbon layer and for the sample without a carbon layer are also illustrated for the sake of comparison.
• the pure measurement curves are illustrated in fig.s 4a, c.
• the corresponding simulated curves are laid through the measured photoelectron current curve in fig. 4b.
• the position of maximum reflectance is determined very predominantly by the molybdenum silicon multilayer system selected by way of example.
• the position of maximum reflectance is also determined to a slight extent by the topmost cap layer, carbon here, by way of example.
• the presence of a cap layer system composed, for example, of carbon and silicon or else ruthenium has an effect chiefly on the absolute value of maximum reflectance.
• the vacuum interface here specifically the interface of the carbon layer to the vacuum, is denoted as free interface.
• the radiation emanating from a radiation source 1 strikes means for spatial and spectral shaping of the beam characteristic, these being a collimator 2 a nd a monochromator 3 here, by way of example.
• the radiation source is a radiation source for wavelengths in the range from 1 to 20 nm. Suitable for this pu rpose are, for example, laser-induced plasmas or discharge-induced plasmas, or else relativistic electrons.
• the collimator 2 can serve the purpose.of providing a measuring beam of low divergence at the sample location in order to ensure a high angular resolution, or else to provide a small beam diameter at the sample location in order to achieve a high spatial resolution.
• the monochromator 3 serves primarily for selecting the measuring wavelength. It should be as narrowband as possible in order to ensure a high energy resolution. Particularly in conjunction with broadband radiation such as, for example, synchrotron radiation, a tunable monochromator is selected in order to carry out energy-dispersive measurements.
• the monochromator can be constructed, for example, as a moveable grating in combination, if appropriate, with a slit.
• monochromators adapted thereto such as fixed gratings, film filters or specular reflectors, for example.
• Components 2 and 3 can also be of unipartite construction in the form of focusing monochromators.
• the further components of the measuring apparatus are arranged in a vacuum chamber that is indicated by the dashed line 10.
• the sample 6 can be introduced via a load lock 14.
• the sample 6 comprises a multilayer system 8 on a substrate 9, and is fastened on an electrically insulated sample holder 7.
• This sample holder has three degrees of translational freedom and three degrees of rotational freedom, and so the entire surface of the sample 6 can be scanned with the aid of the measuring beam.
• the beam striking the multilayer system 8 is reflected.
• the reflected beam is measured by a photon detector 15 for measuring reflectance.
• photoelectrons induced by the incident radiation emerge from the multilayer system 8. These are indicated by dashed arrows. They are collected by an electron capture apparatus 4.
• the current induced by the incident photoelectrons is measured by an ammeter 13.
• the vacuum wall 10 or else a wide-angle plate or a wire grid, for example, can serve as electro n capture apparatus.
• a defined electric field is applied with the aid of component 5.
• the defined electric potential can be applied by a ring, a cylinder or a grid.
• the component 5 can be of arbitrary complex configuration and is connected to a voltage source 12. It is also ossible, for example, for the sample itself to be set to a specific voltage in order to generate a defined electric potential.
• ammeters in which a specific phase voltage can be set. It is possible correspondingly to make use for component 5 of a voltage unit on which it is also possible to read out currents flowing at component 5.
• the defined electric potential serves not only to direct the electrons onto the electron capture apparatus, but also, in particular, to shield the electrons and the electron capture apparatus from ions that may be present, in particular positively charged ions.
• the measuring accuracy of the photoelectron current curve is thereby raised.
• an ammeter 11 to measure the current flowing in the multilayer system 8 when photoelectrons are emerging.
• a meter in which the voltage can also be set.
• the vacuum chamber 10 there are arranged in the vacuum chamber 10 two or 5 more samples 6 for which the current flowing at the samples is measured by connecting the samples in parallel and/or series with one another to the ammeter 11. The current flowing in one sample during measurement then flows, if appropriate, to the ammeter 11 via the other sample. This is possible because it is not absolute currents that are measured but only the spectralo profile of the photoemission, which is influenced only by the incident radiation.
• a reflective optical element produced e.g. according to fig. 1b, the layer thickness of which can be measured e.g. in an apparatus according to fig. 6, has a design principle shown in fig. 7.
• layer units j, j+1 , n are deposed, in this example, each with four layers 21 , 22, 23, 24.
• These consist in e.g. alternating molybdenum layers 22 and silicon layers 21 with intermediate layers 23, 24 as diffusion barriers. Since the interfaces of the molybdenum and silicon layers 21 , 22 are well-defined because of the diffusion barriers 23 and 24 over quite some period of time, theo maximum reflectance value may longer be maintained.
• the cap layer system 30 On the topmost layer unit n, the cap layer system 30 is attached, which has a constant thickness distribution over the surface in the pr&sent example.
• the cap layer system in this case consists of three layers 31 , 32, 33, each with e.g.5 silicon/molybdenum/silicon, molybdenum/silicon/silicon oxide, or silicon/diffusion barrier/transition metal (both with a topmost cap layer of carbon where appropriate).
• reflective optical elements with transition m&tals such as gold,o platinum, rhodium, ruthenium, palladium, silver, rhenium, osmium and/or iridium, as in the cap layer system of the above example, are typically operated in a residual gas atmosphere with oxidizing and reducing gases, respectively, mixtures of gases for avoiding contamination and increasing lifetime.
• a residual gas atmosphere of a hydrocarbon, water, and oxygen is used.
• hydrocarbons especially hydrocarbons with at least one oxygen atom have been; proved of value, as e.g. ketones and acids.
• MMA methyl methacrylates
• cap layer systems comprising a chemically pure form ot a transition metal, but also for cap layer systems with an alloy, compound, or mixture comprising a transition metal.
• such a residual gas atmosphere also has a positive influence for the operation of reflective optical elements with a topmost cap layer consisting of an oxide or carbon, e.g., a cap layer system of silicon/silicon oxide/carbon, in particular if the reflective optical element is optimized in such a way that an intensity minimum of the standing wave is present at the free interface in the resonant case.
• a topmost cap layer consisting of an oxide or carbon, e.g., a cap layer system of silicon/silicon oxide/carbon
• a particularly effective suppression of contaminations is achieved if one designs the cap layer system with a transition metal in such a way that the maximum intensity of the standing electromagnetic wave formed in the resonant case is located in the region of the free interface, in particular somewhat withdrawn from the free interface (see also fig. 9a).
• the residual gas atmosphere should first be adjusted by increasing the partial pressure of hydrocarbon in such a way that a carbon layer is grown.
• the individual partial pressures should be readjusted until an equilibrium state is reached in which the maximum reflectance and the maximum photoelectron current are located on top of each other (see also fig. 9b). This method is illustrated in fig. 11. Alternatively, it is also possible to choose an equilibrium point in which both maxima are slightly shifted with respect to each other.
• the EUV-lithography apparatus roughly shown in fig. 10 is suited for. It comprises essentially three main components: a part 40 for supplying of a beam, an illumination system 50 for illuminating the reticle 60, and a projection system 70 which serves for imaging the structures on the reticle 60 on the wafer 80.
• the illumination system 50 and the projection system 70 each comprise two mirrors 51 , 52, respectively, 71 , 72.
• all of the mirrors are reflective optical elements of the type described above, i.e. with a cap layer system whose layers have a constant thickness across the mirror surface.
• the illumination system 50 as well as the projection system 70 have valves for adjusting the residual gas atmosphere, in this case e.g. valves 53, 73 for supplying hydrocarbon, valves 54, 74 for supplying water, valves 55, 75 for supplying oxygen, as well as valves 56, 76, used as exhaust valves.
• valves 53, 73 for supplying hydrocarbon
• valves 54, 74 for supplying water
• valves 55, 75 for supplying oxygen
• valves 56, 76 used as exhaust valves.
• spectral narrowing of the radiation originating from a EUV-light source 41 with a continuous spectrum has to be performed, i.e. the bandwidth of the radiation has to be constrained to a narrow range about a center wavelength
• the center wavelength has to be tuned over a sufficiently large wavelength band ("spectral forming").
• a tunable monochromator in the sense of the present application is a device which allows one to fulfil both of these requirements, whether or not these are realized inside the same physical entity or two or more separate physical entities.
• the part 40 of the EUV-lithography apparatus comprises apart from the EUV- light source 41 and a collector 42 a grating 43 as a tunable monochromator.
• the first requirement is fulfilled since the grating 43 serves as a spectral purity filter and the second requirement is met as the grating 43 is rotary mounted such that the angle of incidence may be varied, which allows one to tune the center wavelength in the way described above.
• the spectral narrowing can e.g. be performed if the illumination system 50 of the EUV-lithography apparatus generates a radiation with a sufficiently narrow bandwidth through multiple reflectances at its reflective optical elements.
• spectral narrowing can be achieved by the mirrors 51 , 52 and further reflective optical elements being present in the illumination system 50 (not shown in Fig. 10).
• the radiation incident to the projection system 70 already exhibits a sufficiently narrow bandwidth for the measurement of the photocurrent.
• a very simple possibility of tuning the wavelength is then to use a set of reflective optical elements ("mask blanks"), each having a maximum of reflectance at a different center wavelength.
• mask blanks each having a maximum of reflectance at a different center wavelength.
• the reticle mask 60 has to be exchanged with each mask blank of the set.
• fig. 12a-c The principle of the wavelength tuning performed in this way is illustrated in fig. 12a-c, in which the spectral characteristics of the relative reflectance after the reflection from the reflective optical elements of the EUV-lithography apparatus of fig. 10 are shown, each reflectance curve corresponding to reflectance from one reflective optical element, the reflection at the first reflective optical element (mirror 43) in the illumination system 50 corresponding to the curve with the largest reflectance maximum, the reflection at the last reflective optical element (mirror 72) of the projection system 70 corresponding to the curve with the smallest reflectance maximum. Note, however, that not all of the reflective optical elements of the EUV-lithography apparatus are shown in fig. 10, so that the number of reflectance curves in fig. 12 is larger than the number of reflective optical elements shown in fig. 10.
• each reflection causes a narrowing of the bandwidth and a reduction of reflectance, i.e. a decrease of intensity of the reflected radiation.
• the exact amount of the decrease of intensity is not of interest for the present purposes, so that the reflectance curves of fig. 9 have been conveniently scaled and do not reflect the actual conditions concerning the intensity loss.
• fig. 12a all reflective optical elements inside the EUV-lithography apparatus have the same center wavelength, i.e. a maximum of reflectance at about 135 A, corresponding to the operating wavelength of the EUV-lithography apparatus.
• a reflective optical element with a maximum of reflectance shifted to a higher wavelength e.g. to about 1 36 A
• the spectral characteristics of the reflective optical elements in the beam path after this particular wavelength-shifted reflective optical element also exhibit a maximum of reflectance at the new center wavelength.
• fig. 12c shows - the use of a reflective optical element with a maximum of reflectance being 5 shifted to a wavelength of about 134 A, i.e. below the operating wavelength.
• a series of measurements may be performed with a set of wavelength -shifted optical elements as described in connection with fig. 12, e.g. for an operating wavelength of 135 A with a set of optical elements having 130 A, 131 A, 132 A,o 133 A, 134 A, 135 A, 136 A, 137 A, 138 A, 139 A, and 140 A as a center wavelength. It is also possible to refine the step size to 0.1 A or 0.01 A. The minimal requirement for performing the tuning is the use of two optical elements with different center wavelengths.
• the step size and the number of reflective optical elements used for the measurement of the photocurrent has to bes chosen in such a way that maximal benefit can be achieved with minimal cost.
• the switching of reflective optical elements is in the present embodiment performed by replacing the reticle mask 60 with each of the set of wavelength - shifted reflective optical elements, such that the curves with a non -shifted0 center wavelength of fig.s 12b and 12c correspond to optical elements being present in the illumination system 50, i.e. in the beam path before the reticle 60, and the curves with a shifted center wavelength correspond to reflective optical elements in the projection system 70, i.e. in the beam path behind the reticle 60.5
• masks can be used which do not have an absorption pattern (so-called "mask blanks").
• the difference between the mask blanks and a cleaning reticle is simply that the cleaning reticle does not only cause a wavelength shift, but also has a particular absorption pattern for directing the radiation to the areas to be cleaned.
• "defective" masks may be used, i.e. masks which have been designed e.g. as mirrors having their center wavelength at the operating wavelength, but are shifted with respect to the latter due to manufacturing imperfections.
• the thickness of the layers of the multilayer system has to be fixed in an appropriate way. However, it is also possible to shift the center wavelength by application of an adequate cap layer having constant absorption characteristics across the whole surface of the optical element.
• the narrowing of the spectral bandwidth can also be performed by using a EUV source with a sufficiently narrow bandwidth (line source) instead of the EUV source 41 of fig. TO with a continuous spectrum.
• the spectral characteristics of fig. 12 have to be interpreted as an envelope of the line structure of the line source.
• the rotatable grating 43 apart from its use for the measurement of the photocurrent, it allows to switch between an operating wavelength for exposure of the wafer 40 and an usable wavelength for detection of contaminations, cleaning and/or repairing of individual optical elements of the EUV-lithography apparatus. Furthermore, the rotation of the grating 43 also allows to perform angular scans for measuring photocurrent and reflectance in dependence of the angle of incidence, respectively, the wavelength at the reflective optical element 51. For this reason, the reflective optical element 51 and an ampere meter (not shown) are connected via a cable
• the vacuum chamber 90 is used as an electron collector.
• a first photon detector 91 is provided, which may be pivoted into the beam path behind the reflective optical element for measuring purposes. The first photon detector 91 is also connected to the grounded vacuum chamber 90 via a cable
• an ampere- and/or voltage meter (not shown). Also not shown are the wires which direct the measured signals to a computer for receiving and analyzing data.
• the wires which direct the measured signals to a computer for receiving and analyzing data.
• other reflective optical elements respectively, several reflective optical elements may be inspected in-situ with respect to photoelectron current and reflectance, if corresponding detectors are provided.
• ampere meters are used which allow an adjustment of a defined base voltage. It is also possible to use voltage meters which permit readout of current flows.
• the EUV-lithography apparatus shown in fig. 10 may either be operated in the operation mode, in which the mask 60 is illuminated and its structure is imaged onto the wafer 80. However, it is also possible to switch to the detection mode, in which one or more reflective optical elements are tested with regard to contamination.
• the reflective optical element 51 should be tested, as it is exposed to the highest beam load. Therefore, if necessary, the total pressure inside of the illumination system 50 is first minimized.
• the surface of the reflective optical element 51 is irradiated and the photoelectron current and the reflectance are measured in dependence of the wavelength. Through the cooperation of monochromator 43, collimator 42, and aperture 44 it may be achieved that different locations at the surface of the reflective optical elements 51 are measured independently of each other.
• the contamination state of the reflective optical element 51 is determined.
• a cleaning reticle for a measurement of reflective optical elements 71 , 72 of the projection system 74 for illumination purposes, one resorts to a cleaning reticle, if necessary in combination with an appropriate aperture.
• either the operation mode, the cleaning mode, or the repair mode are selected.
• the exceeding of the first control parameter means that the reflective optical element is so strongly contaminated that cleaning has to be carried through.
• Exceeding of the second control parameter means that cleaning is not sufficient, such that the reflective optical element has also to be repaired on its surface.
• the partial pressures of the reductive and oxidizing gas fractions e.g. of hydrocarbon, water, and oxygen, are modified in dependence of the determined contamination, so that during irradiation with the usable radiation, the contamination on the reflective optical element is reduced.
• the incident light beam can be used for photocleaning of the respective optical element.
• the light for photocleaning may as well be provided by a second light source of the operation wavelength or of another DUV or EUV wavelength. If the contamination is removed sufficiently, the operation mode is again selected.
• a cleaning reticle (not shown) in addition to the rotatable grating 43 and the collimator 42, through which the ordinary reticle 16 is replaced in the detecting, cleaning and/or repair mode.
• the structures of the cleaning reticle are optimized in such a way that the optical elements are illuminated selectively, if necessary with spectrally modified radiation, for detecting, cleaning and repair purposes.
• the cleaning reticle may also be mounted in a rotatable and translatable way.
• the switching between the detecting/cleaning and repair mode as well as the operating mode may in particular be accompanied by the following measures: modification of the residual gas composition, modification of possibly applied electrical fields in the region of the trajectories of emitted photoelectrons (e.g. by a ring, grating or cylinder or by setting the reflective optical element to a potential) for directing of the electrons and screening of positive ions and/or modification of the beam characteristics.
• modification of the residual gas composition modification of possibly applied electrical fields in the region of the trajectories of emitted photoelectrons (e.g. by a ring, grating or cylinder or by setting the reflective optical element to a potential) for directing of the electrons and screening of positive ions and/or modification of the beam characteristics.
• the modification of the beam characteristics takes place, among other things, through adjusting of the center wavelength, bandwidth, divergence and the intensity at the monochromator and/or the cleaning reticle, through adjusting of the beam diameter, beam angle and the divergence at the collimator, through adjusting the size of the beam spot and the wavelength through masking, respectively, selecting a reflection order with an aperture.
• a grating, a wire ring or a cylinder as an electron collector device, which are connected electrically to the reflective optical element to be measured or to mass via an ampere meter.
• the reflective optical element to be measured can be connected with an ampere meter through a cable.
• a measuring device with an adjustable voltage may also be used.
• a defined electrical potential is applied by means of an additional component such as e.g. a ring, a cylinder, or a grating. This component may be of an arbitrarily complex shape and is connected to a voltage generator.
• a defined potential may also be applied to the reflective optical element to be measured.
• a second photon detector 94 is arranged in the projection system 70 which is located outside of the beam path.
• the second photon detector 94 allows one to detect radiation having a higher wavelength than the operating wavelength, e.g. a wavelength in the infrared wavelength range above 750 nm (corresponding to an energy of approx. 1.65 eV).
• the second photon detector 94 can be used as a thermal imaging camera generating a spatially resolved two-dimensional image of the optical element 51. Such an image is advantageous already in the operating mode, as the heat transported to each reflective optical element is known beforehand and the image of the temperature distribution contains valuable information about the surface of the inspected optical element.
• a varying two-dimensional energy outflow will result from the variation of narrow-band EUV irradiation in the detection mode because of * a change in the photoelectron characteristics which will show up in the thermal image.
• the thermal image is also of use in the cleaning mode and the repair mode.
• the detection of photons generated by inelastic scattering from the reflective optical elements of an EUV-lithography apparatus is not only advantageous for photon wavelengths in the infrared range, but may also be applied to photons with any wavelength between the operating wavelength (e.g. 13.5 nm, corresponding to approx. 92.5 eV) and wavelengths in the infrared range.
• the principle underlying this detection method is to detect photons at a low energy scale in which its detection is relatively easy, instead of detecting photons with high photon energies, being difficult to detect.
• the method makes use of the loss of photon energy due to inelastic scattering and the fact that valuable information about the surfaces of optical elements may be extracted also f rom inelastically scattered photons.
• Photons unlike electrons, are not influenced by electrical and magnetic fields and may be detected in a robust way by semiconductor detectors.
• the standing electromagnetic wave being formed at the free interface in the resonant case can be used to selectively excite the photons to be detected.
• the method may be extended by using a specific design of the cap layer system, e.g. by doping of the cap layer, such that photons with selected photon energies are excited which lie in the desired detection wavelength range.
• the use of the first photon detector 91 for the measurement of the usual reflectance curves is still possible.
• photons scattered in an inelastic way are emitted in all directions, such that they may be detected by the second photon detector 94 at a geometrically advantageous position in the EUV-lithography apparatus, in particular outside of the beam path.

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