Patent Description:
The present invention relates to a method of manufacturing a reflective diffraction grating and an associated inspection apparatus, metrology apparatus and a lithographic apparatus.

A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. Typical wavelengths currently in use are <NUM> (i-line), <NUM>, <NUM> and <NUM>. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range <NUM>-<NUM>, for example <NUM> or <NUM>, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of <NUM>.

In lithographic processes, it is desirable to make frequently measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes or various forms of metrology apparatuses, such as scatterometers. Examples of known scatterometers often rely on provision of dedicated metrology targets, such as underfilled targets (a target, in the form of a simple grating or overlapping gratings in different layers, that is large enough that a measurement beam generates a spot that is smaller than the grating) or overfilled targets (whereby the illumination spot partially or completely contains the target). Further, the use of metrology tools, for example an angular resolved scatterometer illuminating an underfilled target, such as a grating, allows the use of so-called reconstruction methods where the properties of the grating can be calculated by simulating interaction of scattered radiation with a mathematical model of the target structure and comparing the simulation results with those of a measurement. Parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

Scatterometers are versatile instruments which allow measurements of the parameters of a lithographic process by having a sensor in the pupil or a conjugate plane with the pupil of the objective of the scatterometer, measurements usually referred as pupil based measurements, or by having the sensor in the image plane or a plane conjugate with the image plane, in which case the measurements are usually referred as image or field based measurements. Such scatterometers and the associated measurement techniques are further described in patent applications <CIT>, <CIT>, <CIT>, <CIT>, <CIT> or <CIT>.

Aforementioned scatterometers can measure in one image multiple targets from from multiple gratings using light from soft x-ray and visible to near-IR wave range.

As an alternative to optical metrology methods, it has also been considered to use X-rays, including hard X-rays (HXR), and soft X-rays (SXR), or EUV radiation (all of the three may be mentioned together as SXR in following text of this invention for concision reason), for example radiation in a wavelength range between <NUM> and <NUM>, or optionally between <NUM> and <NUM>, or optionally between <NUM> and <NUM>, or optionally between <NUM> and <NUM>. One example of metrology tool functioning in one of the above presented wavelength ranges is transmissive small angle X-ray scattering (T-SAXS as in <CIT>).

Profile (CD) measurements using T-SAXS are discussed by <NPL>. Reflectometry techniques using X-rays (GI-XRS) and extreme ultraviolet (EUV) radiation at grazing incidence are known for measuring properties of films and stacks of layers on a substrate. Within the general field of reflectometry, goniometric and/or spectroscopic techniques can be applied. In goniometry, the variation of a reflected beam with different incidence angles is measured. Spectroscopic reflectometry, on the other hand, measures the spectrum of wavelengths reflected at a given angle (using broadband radiation). For example, EUV reflectometry has been used for inspection of mask blanks, prior to manufacture of reticles (patterning devices) for use in EUV lithography.

It is possible that the range of application makes the use of wavelengths in the soft X-rays or EUV domain not sufficient. Therefore, published patent applications <CIT> and <CIT>) describe hybrid metrology techniques in which measurements made using x-rays and optical measurements with wavelengths in the range <NUM> and <NUM> are combined together to obtain a measurement of a parameter such as CD. A CD measurement is obtained by coupling and x-ray mathematical model and an optical mathematical model through one or more common.

Conventional mirrors for HXR, SXR and EUV with grating patterns on their surface are typically optimized to diffract a relatively large fraction (tens of percents) of the incident radiation; the specularly reflected portion of the light is typically less important.

Reference is made to <CIT> disclosing (see Fig. <NUM>) a grating with a first substructure comprising a ridge with a flat top and a second substructure comprising a trench.

For SXR or EUV metrology, only a small fraction of the specularly reflected should arrive outside the target. This can be translated into a roughness specification for a mirror. Given a fraction of total integrated scatter (TIS) as a requirement, the requirement on root mean square surface error (zRMS) is given by <MAT> where β is the grazing angle of incidence and λ is the wavelength. For example, at λ = <NUM>, β = <NUM> deg, and TIS = <NUM> %, the requirement is zRMS < <NUM>. This is a simple calculation of surface requirements. More sophisticated models may be used.

It is desirable that the mirror-with-diffraction grating produces as little stray light (scattered light) as possible, outside well-defined diffraction orders (including the zeroth order or specular reflection). Roughness specifications for mirrors cannot easily be translated to grating requirements. Conventionally, grating manufacturers have not had design and manufacture methods that can achieve the lowest possible stray light around the zeroth order.

According to a first aspect of the present invention, there is provided a method of manufacturing a reflective diffraction grating for specularly reflecting and diffracting a grazing-incidence beam of radiation incident on the grating, the grating having a periodic structure with a grating period comprising first and second substructures either side of a sidewall facing the incident beam, the method comprising the steps:.

Preferably, the step of determining the configuration of the second substructure and the sidewall comprises determining, using a wavelength of the incident beam and a pitch of the grating periods including the second substructure, a configuration such that any rays of the beam that diffract from the second substructure into a selected non-zero-diffraction-order direction are obscured by the sidewall. This has the effect of eliminating the contribution of stray light to a measured diffraction spectrum. This makes the spectrum measurement more accurate.

Preferably, the step of manufacturing the grating comprises fabricating the grating on a mirror surface. This has the effect of providing efficient specular reflection, which is useful for metrology applications.

Preferably, the mirror surface is curved. This allows the focusing of the specularly reflected beam onto a target, which is useful for metrology applications.

According to the invention, the first substructure comprises a ridge and the second substructure comprises a trench. These are convenient structures to manufacture.

Further according to the invention, the ridge comprises a flat top and the trench comprises a flat floor parallel to the flat top of the ridge. The flat top of the ridge provides efficient specular reflection, which is useful for metrology applications. The flat floor is a convenient structure to manufacture using conventional lithographic processes.

Preferably the step of determining a configuration of the second substructure and the sidewall comprises determining a shape of the trench. The shape is something that can be conveniently controlled by choices in the manufacturing process.

Preferably, the step of determining a configuration of the second substructure and the sidewall comprises determining one or more structural parameters defining an aspect ratio of the trench. An aspect ratio is a useful way to specify the design of a trench.

Preferably, determining one or more structural parameters defining an aspect ratio of the trench comprises satisfying the inequality <MAT> where D is depth of the trench, W is top width of the trench, β is the grazing angle of incidence and β' is the zeroth-order direction or a non-zero-diffraction-order direction. This simple geometric rule is easy to use and it is surprisingly effective at improving the scattering performance on the specular reflectance of the grating so that not too much stray light arrives outside the target.

Preferably, the grating has a varying top width of the trench, and the depth of the trench is selected to satisfy the inequality for a largest top width of the varying top width. This has the effect that specular scattering performance is ensured for all top widths across the grating.

Preferably, the grating has a varying top width of the trench and the depth of the trench is varied to satisfy the inequality in correspondence with the varying width. This has the effect that specular scattering performance is ensured for all top widths across the grating, without requiring the narrowest trenches to be too deep, which may be hard to manufacture.

Preferably, the grating periods are configured with a grating pitch over trench width ratio of over <NUM>, more preferably over <NUM>, most preferably over <NUM>. These increasing ratios progressively provide more specularly reflected light onto the target, which is useful for metrology.

Preferably, the grating periods are configured to specularly reflect a majority of the specularly reflected and diffracted radiation, more preferably over <NUM>%, most preferably over <NUM>%. These increasing percentages progressively provide more specularly reflected light onto the target, which is useful for metrology.

Preferably, the radiation has a wavelength in the range <NUM> to <NUM>, or in the range <NUM> to <NUM>. These are useful ranges of wavelength for metrology applications, particularly in EUV semiconductor manufacturing.

Preferably, the grazing angle of incidence is in the range <NUM> degree to <NUM> degrees, more preferably in the range <NUM> degrees to <NUM> degrees.

According to a second aspect of the present invention there is provided a reflective diffraction grating for specularly reflecting and diffracting a grazing-incidence beam of radiation incident on the grating, the grating having a periodic structure with a grating period comprising first and second substructures either side of a sidewall facing the incident beam, the grating comprising:.

According to a third aspect of the present invention, there is provided an inspection apparatus comprising:.

According to a fourth aspect of the present invention, there is provided a metrology apparatus comprising the inspection apparatus of the third aspect.

According to a fifth aspect of the present invention, there is provided a lithographic apparatus comprising the inspection apparatus of the third aspect.

In the present document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of <NUM>, <NUM>, <NUM>, <NUM> or <NUM>), EUV and X-rays including hard X-rays (HXR) and soft X-rays (SXR) (e.g. in a wavelength range between <NUM> and <NUM> or optionally <NUM> and <NUM> or optionally between <NUM> and <NUM> or optionally between <NUM> and <NUM>).

<FIG> schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (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 support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens 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.

In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

The term "projection system" PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and/or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and/or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered as synonymous with the more general term "projection system" PS.

The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in <CIT>.

The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named "dual stage"). In such "multiple stage" machine, the substrate supports WT may be used in parallel, and/or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and/or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the 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 a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in <FIG>) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks P1, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

In order for the substrates W exposed by the lithographic apparatus LA to be exposed correctly and consistently, it is desirable to inspect substrates to measure properties of patterned structures, such as overlay errors between subsequent layers, line thicknesses, critical dimensions (CD), etc. For this purpose, inspection tools (not shown) may be used. If errors are detected, adjustments, for example, may be made to exposures of subsequent substrates or to other processing steps that are to be performed on the substrates W, especially if the inspection is done before other substrates W of the same batch or lot are still to be exposed or processed.

An inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrates W, and in particular, how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on the substrate W and may, for example, be part of a lithocell (not shown), or may be integrated into the lithographic apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure the properties on a latent image (image in a resist layer after the exposure), or on a semi-latent image (image in a resist layer after a post-exposure bake step PEB), or on a developed resist image (in which the exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

<FIG> illustrates a schematic physical arrangement of a metrology apparatus <NUM> comprising a spectroscopic scatterometer using EUV and/or X-rays radiation in grazing incidence, in which embodiments of the present invention may be implemented, purely by way of example.

Inspection apparatus <NUM> comprises a radiation source <NUM>, illumination system <NUM>, substrate support <NUM>, detection systems <NUM>, <NUM> and metrology processing unit (MPU) <NUM>.

Source <NUM> in this example comprises a generator of EUV or soft x-ray radiation based on high harmonic generation (HHG) techniques. Such sources are available for example from KMLabs, Boulder Colorado, USA (http://www. Main components of the radiation source are a drive laser <NUM> and an HHG gas cell <NUM>. A gas supply <NUM> supplies suitable gas to the gas cell, where it is optionally ionized by an electric source <NUM>. The drive laser <NUM> may be, for example, a fiber-based laser with an optical amplifier, producing pulses of infrared radiation that may last for example less than <NUM> ns (<NUM> nanosecond) per pulse, with a pulse repetition rate up to several megahertz, as required. The wavelength of the infrared radiation may be for example in the region of <NUM> (<NUM> micron). The laser pulses are delivered as a beam of first radiation <NUM> to the HHG gas cell <NUM>, where in the gas a portion of the radiation is converted to higher frequencies than the first radiation into a beam <NUM> including coherent second radiation of the desired wavelength or wavelengths.

The second radiation may contain multiple wavelengths. If the radiation were monochromatic, then measurement calculations (for example reconstruction) may be simplified, but it is easier with HHG to produce radiation with several wavelengths. The volume of gas within the gas cell <NUM> defines an HHG space, although the space need not be completely enclosed and a flow of gas may be used instead of a static volume. The gas may be for example a noble gas such as neon (Ne) or argon (Ar). N2, <NUM>, He, Ar, Kr, Xe gases can all be considered. These are matters of design choice, and may even be selectable options within the same apparatus. Different wavelengths will, for example, provide different levels of contrast when imaging structure of different materials. For inspection of metal structures or silicon structures, for example, different wavelengths may be selected to those used for imaging features of (carbon-based) resist, or for detecting contamination of such different materials. One or more filtering devices <NUM> may be provided. For example, a filter such as a thin membrane of Aluminum (Al) may serve to cut the fundamental IR radiation from passing further into the inspection apparatus. A grating (not shown) may be provided to select one or more specific harmonic wavelengths from among those generated in the gas cell. Some or all of the beam path may be contained within a vacuum environment, bearing in mind that SXR/EUV radiation is absorbed when traveling in air. The various components of radiation source <NUM> and illumination optics <NUM> can be adjustable to implement different metrology 'recipes' within the same apparatus. For example, different wavelengths and/or polarization can be made selectable.

Depending on the materials of the structure under inspection, different wavelengths may offer a desired level of penetration into lower layers. For resolving the smallest device features and defects among the smallest device features, then a short wavelength is likely to be preferred. For example, one or more wavelengths in the range <NUM>-<NUM> or optionally in the range <NUM>-<NUM> or optionally in the range <NUM>-<NUM> may be chosen. Wavelengths shorter than <NUM> suffer from very low critical angle when reflecting off materials typically of interest in semiconductor manufacture. Therefore, to choose a wavelength greater than <NUM> will provide stronger signals at higher angles of incidence. On the other hand, if the inspection task is for detecting the presence of a certain material, for example to detect contamination, then wavelengths up to <NUM> could be useful.

From the radiation source <NUM>, the filtered beam <NUM> enters an inspection chamber <NUM> where the substrate W including a structure of interest (also known as a target) is held for inspection at a measurement position by substrate support <NUM>. The structure of interest is labeled T. The atmosphere within inspection chamber <NUM> is maintained near vacuum by vacuum pump <NUM>, so that EUV radiation can pass with-out undue attenuation through the atmosphere. The Illumination system <NUM> has the function of focusing the radiation into a focused beam <NUM>, and may comprise for example a two-dimensionally curved mirror, or a series of one-dimensionally curved mirrors, as described in published US patent application <CIT> mentioned above. The focusing is performed to achieve a round or elliptical spot S under <NUM> in diameter, when projected onto the structure of interest. Substrate support <NUM> comprises for example an X-Y translation stage and a rotation stage, by which any part of the substrate W can be brought to the focal point of beam to in a desired orientation. Thus the radiation spot S is formed on the structure of interest. Alternatively, or additionally, substrate support <NUM> comprises for example a tilting stage that may tilt the substrate W at a certain angle to control the angle of incidence of the focused beam on the structure of interest T.

Optionally, the illumination system <NUM> provides a reference beam of radiation to a reference detector <NUM> which may be configured to measure a spectrum and/or intensities of different wavelengths in the filtered beam <NUM>. The reference detector <NUM> may be configured to generate a signal <NUM> that is provided to processor <NUM> and the filter may comprise information about the spectrum of the filtered beam <NUM> and/or the intensities of the different wavelengths in the filtered beam.

Reflected radiation <NUM> is captured by detector <NUM> and a spectrum is provided to processor <NUM> for use in calculating a property of the target structure T. The illumination system <NUM> and detection system <NUM> thus form an inspection apparatus. This inspection apparatus may comprise a soft X-ray and/or EUV spectroscopic reflectometer of the kind described in <CIT>.

If the target T has a certain periodicity, the radiation of the focussed beam <NUM> may be partially diffracted as well. The diffracted radiation <NUM> follows another path at well-defined angles with respect to the angle of incidence then the reflected radiation <NUM>. In <FIG>, the drawn diffracted radiation <NUM> is drawn in a schematic manner and diffracted radiation <NUM> may follow many other paths than the drawn paths. The inspection apparatus <NUM> may also comprise further detection systems <NUM> that detect and/or image at least a portion of the diffracted radiation <NUM>. In <FIG> a single further detection system <NUM> is drawn, but embodiments of the inspection apparatus <NUM> may also comprise more than one further detection system <NUM> that are arranged at different position to detect and/or image diffracted radiation <NUM> at a plurality of diffraction directions. In other words, the (higher) diffraction orders of the focussed radiation beam that impinges on the target T are detected and/or imaged by one or more further detection systems <NUM>. The one or more detection systems <NUM> generates a signal <NUM> that is provided to the metrology processor <NUM>. The signal <NUM> may include information of the diffracted light <NUM> and/or may include images obtained from the diffracted light <NUM>.

To aid the alignment and focusing of the spot S with desired product structures, inspection apparatus <NUM> may also provide auxiliary optics using auxiliary radiation under control of metrology processor <NUM>. Metrology processor <NUM> can also communicate with a position controller <NUM> which operates the translation stage, rotation and/or tilting stages. Processor <NUM> receives highly accurate feedback on the position and orientation of the substrate, via sensors. Sensors <NUM> may include interferometers, for example, which can give accuracy in the region of picometers. In the operation of the inspection apparatus <NUM>, spectrum data <NUM> captured by detection system <NUM> is delivered to metrology processing unit <NUM>.

As mentioned, an alternative form of inspection apparatus uses soft X-ray and/or EUV radiation at normal incidence or near-normal incidence, for example to perform diffraction-based measurements of asymmetry. Both types of inspection apparatus could be provided in a hybrid metrology system. Performance parameters to be measured can include overlay (OVL), critical dimension (CD), coherent diffraction imaging (CDI) and at-resolution overlay (ARO) metrology. The soft X-ray and/or EUV radiation may for example have wavelengths less than <NUM>, for example using radiation in the range <NUM>-<NUM>, of optionally in the range from <NUM> to <NUM>. The radiation may be narrowband or broadband in character. The radiation may have discrete peaks in a specific wavelength band or may have a more continuous character.

Like the optical scatterometer used in today's production facilities, the inspection apparatus <NUM> can be used to measure structures within the resist material treated within the litho cell (After Develop Inspection or ADI), and/or to measure structures after they have been formed in harder material (After Etch Inspection or AEI). For example, substrates may be inspected using the inspection apparatus <NUM> after they have been processed by a developing apparatus, etching apparatus, annealing apparatus and/or other apparatus.

<FIG> depicts a schematic representation of radiation path in a metrology apparatus. In the example metrology apparatus depicted in <FIG>, radiation from a nearly diffraction-limited source <NUM> is focused onto a target T on a wafer W. The target is typically a finite-size area on a wafer (e.g. <NUM> × <NUM> or <NUM> × <NUM>). Length scales of grating/mirror nonuniformity of interest (e.g., > <NUM>) depend on target size and length of the light path from grating to target. The radiation has a wavelength in the range <NUM> to <NUM>, preferably in the soft X-ray (SXR) range <NUM> to <NUM>, which may also be described as being part of the EUV range. For accurate metrology, the following points are (among others) important:.

A grazing-incidence (GI) mirror <NUM> focuses an SXR beam onto a target T. Radiation <NUM> scattered from the target is captured by detector <NUM>. A line pattern (grating) on the surface of the mirror diffracts a fraction <NUM> of the SXR radiation towards an array sensor <NUM>, with different wavelength components arriving at different points on the array sensor. The signal from the array sensor can be interpreted as a spectrum.

<FIG> depicts a schematic representation of a grating specularly reflecting and diffracting a grazing-incidence beam of radiation, illustrating geometric parameters. An incoming radiation beam illustrated by ray <NUM> has associated wavefronts <NUM> with a wavelength λ. The beam has a grazing angle of incidence β. The ray <NUM> in this example is incident on a grating <NUM> on mirror <NUM> at an angle φ with respect to the direction of periodicity <NUM> of the grating. When this angle φ is zero, which is the condition for planar diffraction, then the beam has a grazing angle of incidence β, with respect to the grating's direction of periodicity. Planar diffraction occurs if the incident rays and all reflected and diffracted rays are in a (flat) plane. At nonzero φ (for example, φ = <NUM> degrees), rays are on a cone; this case is called "conical diffraction". At the condition for pure conical diffraction (φ = <NUM> degrees), the bottoms of the trenches will never be obscured for any practical trench cross section. However, the embodiments described herein are used with (near) planar diffraction, preferably with φ < <NUM> degrees. Because the incident beam is diverging, φ varies across the beam. Embodiments preferably have numerical apertures below <NUM>, which correspond to half-angles of <NUM> degrees.

The beam is specularly reflected in a zeroth-order direction <NUM> (at an angle that equals β) in the plane <NUM>. The beam is diffracted in one or more diffraction-order direction, in this example a +<NUM>st order direction <NUM> (at an angle that does not equal β).

<FIG> depicts a schematic representation of a cross-section of a reflective diffraction grating on a mirror <NUM>, illustrating grating parameters. A grating period has a trench <NUM> (also known as a groove) with a top width W and a ridge <NUM>, either side of a sidewall <NUM>. The shape of a reflective diffraction grating can be characterized in terms of the main parameters: pitch P spanning the grating period, groove width W, groove depth D. Furthermore, there is the side-wall angle (SWA) and sidewall angle asymmetry (SWAA), as shown in <FIG>. One could introduce more shape parameters, such as the slope of the trench bottom surface (floor) and the curvature of the trench bottom surface. The width of the trench may be small enough and sidewall angles may large enough that there is no flat bottom surface (floor) between the sidewalls. This trench configuration is a v-groove <NUM>.

<FIG> depicts a schematic geometrical representation of a grating specularly reflecting a grazing-incidence beam of radiation. A grating pitch may be used that is much larger than the SXR wavelength: for example, <NUM> and <NUM> respectively. We can therefore estimate the sensitivities based on quasi-geometrical optics. With reference to <FIG>, part of the incident flux is lost <NUM> and does not specularly reflect due to the depth of the trench and the shadowing effect of the trench edge on the floor of the trench. A portion <NUM> of the incident beam is not specularly reflected in the same zeroth-order direction <NUM> as the beam <NUM> reflected from the trench tops and the beam <NUM> reflected specularly from the center of the trench floor. The incident flux that is reflected <NUM> from the trench floor acquires a phase difference relative to that which is reflected from the top of the ridges <NUM>. This is a consequence of the path length difference arising from the depth of the trench. The waves reflected from the bottom <NUM> and from the top <NUM> add up coherently; in the coherent sum, the phase and amplitude effects are accounted for.

If the manufacturing process of the grating results in local variations of the geometrical parameters in <FIG>, it will result in stray light, just like local height variations in a smooth mirror result in stray light. For a grazing-incidence mirror or grating in a SXR metrology apparatus, we are particularly concerned about local variations on a length scale between <NUM> and <NUM>. This is because the scattering angle is inversely proportional to the length scale of the local variations; sufficiently large scattering angles will be clipped in the light path and sufficiently small scattering angles will not lead to out-of-target illumination (i.e. stray light).

We note that the complex reflection coefficient r (phase and amplitude) of the specular reflection is dependent on the geometrical parameters. The complex reflection coefficient can be calculated using simulation software, which is available commercially or as open source.

It is also possible to make a coarse estimate based on geometrical optics if the grating period is much larger than the wavelength of the light; this results in the formula <MAT> for <MAT> <MAT>.

Here, Rflat is the reflectance (power, not amplitude) for a smooth mirror surface. Eq. 1b can be understood as grooves that are so deep that any light paths via the bottom surface of the grooves (i.e. trench floors) are obscured by the vertical side walls. This does not include light paths back scattered off the sidewall on to the bottom surface.

In this parametrization, we assume that the grooves are etched into a surface that was originally polished to low roughness (e.g. < <NUM>). A manufacturing process may also deposit ridges on top of a polished smooth surface, in which case the formulas would be: <MAT> <MAT>.

Suppose that a particular grating design has parameters P<NUM>, W<NUM>, D<NUM>, resulting in a complex reflection coefficient r<NUM> (assuming a particular grazing angle β and wavelength λ. Local variations ΔW, ΔD (expressed as RMS values) will result in a total integrated scatter <MAT>.

P, W, and D may be determined such that the partial derivatives ∂r/∂W and ∂r/∂D are as small as possible, such that the sensivity to variation in W and D is as small as possible.

In this example, only W and D are considered as parameters with spatial variation. It can be generalized for more parameters by adding the corresponding quadratic terms to the numerator in Eq. (<NUM>).

The optimal combination of parameters depends on how well W and D can be controlled in the patterning process. For example, if W can be controlled with high accuracy, but D cannot, then W and D may be determined such that W < <NUM>D/ tan β; then (according to Eq. 1b), there is no sensitivity to the D parameter.

The equations (<NUM>) and (<NUM>) are approximations. However, rigorous simulations show that narrow, deep grooves allow a much larger (<NUM>×-<NUM>×) tolerance on groove depth than wide, shallow grooves. In metrology apparatus applications with SXR wavelengths of interest <NUM> ≤ λ ≤ <NUM> (rather than any leaked infrared light), grazing angles of incidence in the range <NUM> deg < β < <NUM> deg are useful. Useful pitches are in the range <MAT>
where α<NUM> = <NUM> and α<NUM> = <NUM>. (These constants are the highest and lowest first-order diffraction angles that are useful, in radians). A lithographic patterning process may be considered easiest for a ratio <NUM> < W/P < <NUM> for optical lithography. Low sensitivity to groove depth is achieved roughly for <MAT>.

The factor <NUM> is "a bit more than <NUM>/<NUM>" which is the analytical threshold. Although the pitch is much greater than wavelength, this is not true for the depth (<NUM>-<NUM> in the examples below). Therefore, the wave-like nature of the radiation still affects the reflection/diffraction properties of the grating. It has been found that there is strong drop in sensitivity to groove depth D in the range <NUM> to <NUM>. The threshold may thus be set at for example <NUM>, <NUM>, or <NUM>.

These inequalities define a region in 4D parameter space (P, W, D, β) that would be of interest with the purpose of minimizing sensitivity with respect to the groove depth. The parameter space may include further dimensions, for example if trench-floor tilt or other parameters are included.

If the groove depth is taken according to the above expressions, it tends to result in a large sensitivity with respect to groove width W, all else being equal. Also, deep grooves tend to lead to larger absorption losses. Lowering the sensitivity to both groove depth and groove width can be achieved by choosing the groove width as small as possible and the depth according to Eq. (<NUM>).

A goal for an implementation in a metrology apparatus is a grating with a high reflectance in zeroth order (R0), low diffraction efficiency in first order (R1) and low sensitivities to width and depth variations. Using the above considerations, a number of possible implementations may be considered. A first example is a grating with nominal pitch of <NUM> and duty cycle (W/P) of <NUM>%. The angle of incidence is the smallest angle allowed by optical design considerations. The depth is chosen to fulfil Eq. <NUM> by a margin.

In case a different set of optical design boundary conditions is taken, a smaller angle of incidence on the grating can be allowed for. This changes the table to:.

Both examples above consider a duty cycle of <NUM>%. Ideally, a much smaller duty cycle is used, as this decreases scattering, increases R0 and decreases R1. Such a grating could be described by:.

The grating may be a Variable Line Space (VLS) grating, which has no constant pitch but a pitch that varies gradually across the grating surface. One purpose of the VLS is to minimize aberrations of the diffracted light on the detector. To be on the safe side, the largest pitch should be considered, as this pitch has also the largest width and is this limiting the performance according to Eq. <NUM>. This is described below with reference to <FIG>. Alternatively, if processing allows the groove depth could be varied over the grating (larger local pitch is larger depth) to optimize scattering versus R0 locally. This is described below with reference to <FIG>.

The above description concerns improving the scattering performance on the specular reflection (zeroth-order diffraction). If one wishes to eliminate the contribution of stray light to a measured spectrum, one could use a similar approach: simulations or geometric arguments. The geometric rule in Eq. (<NUM>) can be generalized for the case that the grazing angle of incidence β, with respect to the grating's direction of periodicity, is not equal to the grazing angle of "reflection" β', as is the case with for example the first-order diffraction. The geometric rule becomes <MAT>
where the factor <NUM> means "a bit more than one" to account for the fact that a geometric approximation is not fully correct.

<FIG> depicts a schematic geometrical representation of a grating trench at the depth threshold, D = <NUM> W tan β, for specular reflection from the trench floor being obscured by the sidewall. The left-hand half <NUM> of the trench floor is in shadow for an incoming beam with a grazing angle of incidence β. Specular reflection of the beam from the remaining right-hand half <NUM> of the trench floor in the zeroth-order direction is obscured by the sidewall <NUM> facing the incident beam. There is specular reflection from the top of the ridges <NUM> (as illustrated in <FIG>) in the zeroth-order direction. Thus, when D = <NUM> W tan β the portion of the incident beam that is specularly reflected away from the trench in the same zeroth order direction tends to zero, assuming no diffraction around the top of the sidewall <NUM>.

<FIG> depicts a schematic geometrical representation of a deep grating trench such that specular reflection from the trench floor is obscured by the sidewall.

With reference to <FIG>, a trench and ridge are part of a reflective diffraction grating for specularly reflecting and diffracting a grazing-incidence beam of radiation incident on the grating. The grating has a periodic structure (as shown in <FIG> and <FIG>) with a grating period comprising first (ridge) and second (trench) substructures either side of a sidewall <NUM> facing the incident beam <NUM>. The incident beam has grazing angle of incidence β at the trench with respect to the local direction of periodicity of the grating periods including the trench. The grazing angle of incidence β is in the range <NUM> degree the <NUM> degrees, preferably in the range <NUM> degrees to <NUM> degrees. The ridge is configured to specularly reflect the beam from the flat top <NUM> of the ridge into a specularly reflected beam <NUM> in a zeroth-order direction β'=β. The beam is incident at a grazing angle of incidence β on grating periods including a trench. These grating periods including the trench, thus those which are local to the trench under consideration, are configured with fixed or varying pitch to diffract the beam from the grating periods in one or more non-zero-diffraction-order direction β'≠β (not shown in <FIG>). There are grating periods that do not include the trench under consideration, for example those not local to the trench under consideration. When the grating is curved and/or when the pitch of the grating varies, those grating periods may diffract the beam in a different non-zero-diffraction-order direction with respect to the trench under consideration.

As described above, the shape of the trench may be described by structural parameters top width W and depth D that define the aspect ratio of the trench. The aspect ratio of a substructure such as a trench is the ratio of its sizes in different dimensions, herein defined as W/D. The shape is determined such that any rays of the beam that reflect once from the trench floor in the zeroth-order direction are obscured by the sidewall. In the example of <FIG>, the trench, and therefore the sidewall separating it from the trench top, is determined to have a depth D greater than that shown at the same scale in <FIG>. The trench floor <NUM>, <NUM> and sidewall <NUM> are thus configured such that any rays of the beam that reflect once from the trench floor in the zeroth-order direction are obscured by the sidewall. The flat top of the ridge and flat floor of the trench are parallel. If the mirror surface is curved, then there may be a corresponding but small curvature of the flat top of the ridge and the flat floor of the trench, conforming to curvature of the mirror surface. The greater depth causes over half of the left-hand side <NUM> of the trench floor to be in shadow for the incoming beam <NUM>. Specular reflection of the beam from the remaining right-hand region <NUM> of the trench floor in the zeroth-order direction is all obscured by the sidewall <NUM>. It is obscured from the zeroth-order direction because it is either absorbed or specularly reflected back towards the incident beam. Multiple reflections from the top of the sidewall <NUM>, back onto the trench floor <NUM> and off the other sidewall <NUM> could redirect a negligible portion of the beam in the zeroth-order direction. Similarly, for a particular configuration of grazing-angle of incidence and sidewall angles, a double reflection from the top of the sidewall <NUM>, and off the other sidewall <NUM> could redirect a negligible portion of the beam in the zeroth-order direction. It is negligible because of the losses on reflection of this short-wavelength radiation, and at grazing angles of incidence with respect to the sidewall surfaces of over <NUM> degrees. Thus, the portion of the incident beam that is specularly reflected away from the trench in the same zeroth-order direction as for specular reflection from the ridge tops tends to zero.

The grating may be fabricated on a mirror surface which is curved, such as an ellipsoidal mirror as described with reference to <FIG>. Other suitable curved shapes include toroidal, elliptical cylinder, paraboloid, parabolic cylinder, hyperboloid and hyperbolic cylinder. The flat ridge top and trench floor may be slightly curved to conform to curvature of the mirror. A mirror is a substrate that has been polished to obtain a low scattering reflective surface. The surface is polished such that scattering is sufficiently suppressed and a grating is added that (when made according embodiments) will not add more scattering that desired. Holographic methods may be used to pattern the grating, with suitable pattern transfer techniques such as etching.

The structural parameters defining the aspect ratio of the trench are determined to satisfy the inequality <MAT>.

where D is depth of the trench, W is top width of the trench, β is the grazing angle of incidence, with respect to the grating's direction of periodicity, and β' is the zeroth-order direction (β'=β) or a non-zero-diffraction-order direction (β'≠β), with respect to the grating's direction of periodicity. This resolves to Eq. <NUM> when β' = β and <NUM>. 5W of the analytical threshold is changed to <NUM>. 6W to allow for.

Embodiments have a low diffraction efficiency in the first diffraction order and high specular reflectance, with greatly reduced scattering around specular reflection. Thus, the grating periods are configured to specularly reflect preferably a majority of the specularly reflected and diffracted radiation, more preferably over <NUM>% or most preferably over <NUM>%. In an ideal case with a <NUM> or <NUM> degree grazing incidence angle and an optimum mirror material, grating periods are configured to specularly reflect over <NUM>% of the specularly reflected and diffracted radiation (i.e. total reflected and diffracted power). The grating periods are configured with a grating pitch over trench width ratio of preferably over <NUM>, more preferably over <NUM> or most preferably over <NUM>. The radiation has a wavelength in the range <NUM> to <NUM>, preferably in the range <NUM> to <NUM>.

In other examples, no rays of the beam reflect once from the second substructure into the zeroth-order direction, because there is no flat floor in a line of sight with the incident beam. Such examples include v-groove trenches, trenches with a large enough sidewall slope, or trenches with sloping floors (compared to the ridge surface). In such examples, it holds that any rays of the beam that reflect once from the second substructure into the zeroth-order direction are obscured by the sidewall.

<FIG> depicts a schematic geometrical representation of a grating trench at the depth threshold for first-order diffraction from the trench floor being obscured by the sidewall.

The configuration of the trench and the sidewall is determined, using a wavelength of the incident beam and a pitch (P) of the grating periods including the trench, a configuration such that any diffraction of the beam from the trench in a selected non-zero-diffraction-order direction (in this example first-order) is obscured by the sidewall. The structural parameters defining the aspect ratio of the trench are determined to satisfy the inequality of Eq. <NUM>. (with β'≠β).

<FIG> depicts a schematic geometrical representation of a curved grating with divergent incoming rays. Embodiments are implemented in a metrology apparatus with a large numerical aperture NA compared to synchrotron, for example <NUM> < NA < <NUM>. A diverging beam and a curved mirror allow a short path-length and small volume compared to synchrotron. The divergence of the incident beam results in different grazing angles of incidence on the grating α across the grating. If we define the grazing angle of incidence to the middle trench of <FIG> as β<NUM>, then the grazing angle of incidence on the grating α varies from values greater than β<NUM> to values less than β<NUM> across the grating. <FIG> shows inequalities between the various angles, but the sign (< or >) depends on what part of the ellipse is being shown. The inequalities for β and β<NUM> in <FIG> apply to the case where the object focal point is further away from the mirror than the image focal point.

The curvature of the grating and the divergence of the incident beam results in different grazing angles of incidence of the beam incident at the trenches (locally with respect to the reference frame of the respective trench) β across the grating. The effects of beam divergence and curvature add together to make the resulting local value of β. In this example, the grazing angle of incidence β of the beam incident at the trenches varies from values less than β<NUM> to values greater than β<NUM>.

<FIG> depicts a schematic geometrical representation of a curved variable line/space grating. The trench depth is a uniform value D<NUM>. The grating may have a varying top width of the trench top W (from W < W<NUM> to W > W<NUM>) for example because it has a varying pitch and a constant duty cycle which is convenient to manufacture. The right-hand trench has D < <NUM> W tan β therefore some specular reflection of the beam from the trench floor in the zeroth-order direction is not obscured by the sidewall. This may result in unwanted stray light arriving outside the target.

<FIG> depicts a schematic geometrical representation of a curved variable line/space grating with a trench depth at the threshold for specular reflection from the floor of the trench with the widest top being obscured by the sidewall. When the grating has a varying top width of the trench top W, the depth (D > D<NUM>) of the trench may be selected to satisfy the inequality of Eq. <NUM> for a largest top width of the varying top width. Note that also on a flat grating the divergence of the incident beam leads to a variation of the angle of incidence, and a similar optimization may be done in such a case.

<FIG> depicts a schematic geometrical representation of a curved variable line/space grating with a varying trench depth at the threshold for specular reflection from the any trench floor being obscured by the sidewall. As for <FIG>, the grating has a varying top width of the trench W. In this case, rather than selecting a deep trench depth for all trenches, the depth (D) of the trench may be varied (from D < D<NUM> to D > D<NUM>) to satisfy the inequality of Eq. <NUM> in correspondence with the varying width.

The gratings described with reference to <FIG> may be used in an inspection apparatus, such as a metrology apparatus described with reference to <FIG>. The gratings described with reference to <FIG> may also be used in a lithographic apparatus, such as described with reference to <FIG>. These apparatuses then comprise:.

<FIG> is a flowchart of a method of manufacturing a reflective diffraction grating in accordance with an embodiment of the present invention. The method produces a reflective diffraction grating for specularly reflecting and diffracting a grazing-incidence beam of radiation incident on the grating. The radiation may have a wavelength in the range <NUM> to <NUM>, preferably in the range <NUM> to <NUM>. The grating may be as described with reference to <FIG>, having a periodic structure with a grating period comprising first (ridge) and second (trench) substructures either side of a sidewall facing the incident beam.

<NUM>: determining a configuration of the ridge to specularly reflect the beam from the ridge into a specularly reflected beam in a zeroth-order direction β'=β. The beam is incident at a grazing angle of incidence β, with respect to the grating's direction of periodicity, on grating periods including a trench. The grating periods are configured to specularly reflect preferably a majority of the specularly reflected and diffracted radiation, more preferably over <NUM>% or most preferably over <NUM>%. In an ideal case with a <NUM> or <NUM> degree grazing incidence angle and an optimum mirror material, grating periods are configured to specularly reflect over <NUM>% of the specularly reflected and diffracted radiation (i.e. total reflected and diffracted power). The grating periods are configured with a grating pitch over trench width ratio of preferably over <NUM>, more preferably over <NUM> or most preferably over <NUM>.

<NUM>: determining a fixed or varying pitch configuration of grating periods including the trench to diffract the beam from the grating periods in one or more diffraction-order direction β'≠β. This uses information <NUM> including the wavelength of the incident beam and the pitch of the grating periods including the trench under consideration. The ridge comprises a flat top and the trench comprises a flat floor parallel to the flat top of the ridge.

<NUM>: determining, based on a value of the grazing angle of incidence β <NUM> of the beam incident at a trench, a configuration of the trench and a sidewall of its grating period such that any rays of the beam that reflect once from the trench in the zeroth-order direction are obscured by the sidewall. The grazing angle of incidence β may be in the range <NUM> degree to <NUM> degrees, preferably in the range <NUM> degrees to <NUM> degrees. This step of determining the configuration of the trench and the sidewall may comprise determining, using a wavelength of the incident beam and a pitch (P) of the grating periods including the trench, a configuration such that any diffraction of the beam from the trench in a selected non-zero-diffraction-order direction (β'≠β) is obscured by the sidewall. Step <NUM> comprises determining a shape of the trench. This involves determining one or more structural parameters (W, D) defining an aspect ratio of the trench. This comprises satisfying the inequality <MAT> where D is depth of the trench, W is top width of the trench, β is the grazing angle of incidence, with respect to the grating's direction of periodicity, and β' is the zeroth-order direction (β'=β) or a non-zero-diffraction-order direction (β'≠β), with respect to the grating's direction of periodicity. When the grating has a varying top width of the trench (W), the depth (D) of the trench is selected to satisfy the inequality for a largest top width of the varying top width, as described with reference to <FIG>. Alternatively, the depth (D) of the trench is varied to satisfy the inequality in correspondence with the varying width, as described with reference to <FIG>. Another step may be included to optimize floor tilt and/or SWA and/or other parameters.

<NUM>: manufacturing the grating using the determined configurations of the ridge, sidewall and trench. The grating may be manufactured on a mirror surface.

Although specific reference may be made in this text to the use of inspection, metrology and lithographic apparatus in the manufacture of ICs, it should be understood that the apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc..

Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. 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 claims set out below.

Claim 1:
A method of manufacturing a reflective diffraction grating (<NUM>) for specularly reflecting and diffracting a grazing-incidence beam of radiation (<NUM>) incident on the grating (<NUM>), the grating having a periodic structure with a grating period comprising first and second substructures either side of a sidewall (<NUM>) facing the incident beam (<NUM>),
wherein the first substructure comprises a ridge (<NUM>) and the second substructure comprises a trench (<NUM>), and that the ridge (<NUM>) comprises a flat top and the trench (<NUM>) comprises a flat floor parallel to the flat top of the ridge,
wherein the method comprising the steps:
- determining a configuration of the first substructure to specularly reflect the beam (<NUM>), incident at a grazing angle of incidence, with respect to the grating's direction of periodicity, on grating periods including the second substructures, from the first substructure into a specularly reflected beam (<NUM>) in a zeroth-order direction;
- determining a fixed or varying pitch configuration of grating periods including the second substructure to diffract the beam (<NUM>) from the grating periods in one or more non-zero-diffraction-order direction;
- determining, based on the grazing angle of incidence, a configuration of the second substructure and the sidewall (<NUM>) of its grating period such that any rays of the beam (<NUM>) that reflect once from the second substructure into the zeroth-order direction are obscured by the sidewall (<NUM>); and
- manufacturing the grating using the determined configurations of the first and second substructures and sidewall.