阅读说明：本技术 量测设备 (Measuring equipment ) 是由 M·J·M·范达姆 A·J·登鲍埃夫 N·潘迪 于 2019-07-31 设计创作，主要内容包括：一种用于确定衬底上的结构的关注的特性的量测设备,所述量测设备包括：辐射源,所述辐射源用于产生照射辐射；至少两个照射分支,所述至少两个照射分支用于照射所述衬底上的所述结构,所述照射分支被配置成从不同角度照射所述结构；以及辐射切换器,所述辐射切换器被配置成接收所述照射辐射并且将所述照射辐射的至少一部分转移至所述至少两个照射分支中的可选的一个照射分支。(A metrology apparatus for determining a characteristic of interest of a structure on a substrate, the metrology apparatus comprising: a radiation source for generating illuminating radiation; at least two illumination branches for illuminating the structure on the substrate, the illumination branches configured to illuminate the structure from different angles; and a radiation switch configured to receive the illumination radiation and to divert at least a portion of the illumination radiation to a selectable one of the at least two illumination branches.)

1. A metrology apparatus for determining a characteristic of interest of a structure on a substrate, the metrology apparatus comprising:

-a radiation source for generating illuminating radiation;

-at least two illumination branches for illuminating the structure on the substrate, the illumination branches being configured to illuminate the structure from different angles; and

a radiation switch configured to receive the illumination radiation and to divert at least a portion of the illumination radiation to a selectable one of the at least two illumination branches,

wherein the radiation switch comprises:

a pockels cell configured to control a polarization direction of the illumination radiation and to output a polarization-controlled illumination radiation, and

a polarizing beam splitter optically downstream of the Pockels cell and configured to transmit the polarization-controlled illumination radiation to a first of the at least two illumination branches or reflect the polarization-controlled illumination radiation to a second of the illumination branches depending on the polarization direction of the polarization-controlled illumination radiation.

2. The metrology device of claim 1, further comprising an upstream acousto-optic tunable filter optically arranged upstream of the pockels cell and configured to linearly polarize the illuminating radiation.

3. A metrology apparatus according to claim 1 or 2, further comprising a half-wavelength retarder arranged in one of the first and second illumination branches and configured to rotate the polarization direction of the polarization controlled illumination radiation transmitted or reflected by the polarizing beam splitter in the one of the first and second illumination branches, respectively.

4. The metrology apparatus of any one of the preceding claims, wherein the illumination branches each comprise a respective downstream acousto-optic tunable filter arranged downstream of the radiation switch and configured to switch the respective illumination branch on and off.

5. A metrology apparatus as claimed in any preceding claim further comprising a controller configured to receive data indicative of a wavelength of the illuminating radiation and to control the voltage applied to the pockels cell 70 in dependence on the wavelength of the illuminating radiation.

6. An apparatus according to any preceding claim, wherein the radiation switch comprises a spatial light modulator, and wherein optionally the spatial light modulator comprises a micro mirror device.

7. A metrology apparatus as claimed in any preceding claim wherein the radiation switch comprises an acousto-optic deflector.

8. A metrology apparatus as claimed in any preceding claim wherein the radiation switch comprises at least one beam splitter and a plurality of shutters configured to control transfer of the at least part of the radiation to a selectable one of the at least two illumination branches.

9. A metrology apparatus according to any preceding claim, wherein the illumination branches are configured to illuminate the structure from different angles when viewed along a normal to the structure.

10. An apparatus according to any preceding claim, wherein the illumination branches are configured to illuminate the structure from different angles that are evenly spaced apart from each other.

11. A metrology apparatus according to any preceding claim, wherein the illumination branch comprises at least one optical fibre for illuminating the structure, and wherein optionally the at least one optical fibre is for directly illuminating the structure or the at least one optical fibre is for indirectly illuminating the structure via at least one other optical element.

12. An apparatus according to any preceding claim, the apparatus comprising:

a wavelength selector configured to receive the illuminating radiation and transmit the illuminating radiation having a selected wavelength range, filtering out the illuminating radiation outside the selected wavelength range.

13. An apparatus according to any preceding claim, comprising at least one of:

-a spectrometer, and wherein the radiation switch is configured to selectively transfer at least a portion of the radiation to the spectrometer, an

-a beam dump, and wherein the radiation switch is configured to selectively divert at least a portion of the radiation to the beam dump.

14. A method for determining a parameter of interest of a structure on a substrate, the method comprising:

-generating illuminating radiation;

-receiving the illumination radiation at a radiation switch and diverting at least a portion of the illumination radiation to a selectable one of at least two illumination branches;

-sequentially illuminating the structure from different angles by the at least two illumination branches;

-collecting at least a portion of the radiation diffracted from the structure; and

-receiving and obtaining a record of the collected diffracted radiation at an image sensor,

wherein the radiation switch comprises:

a pockels cell configured to control a polarization direction of the illumination radiation and to output a polarization-controlled illumination radiation, and

a polarizing beam splitter optically downstream of the Pockels cell and configured to transmit the polarization-controlled illumination radiation to a first of the at least two illumination branches or reflect the polarization-controlled illumination radiation to a second of the illumination branches depending on the polarization direction of the polarization-controlled illumination radiation.

15. A lithography unit comprising a metrology apparatus according to one of claims 1 to 13.

Technical Field

The present invention relates to a metrology apparatus for determining a parameter of interest of a structure on a substrate and a method for determining the parameter of interest.

Background

A lithographic apparatus is a machine that is configured to apply a desired pattern onto a substrate. For example, 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 commonly referred to as a "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm and 13.5 nm. Lithographic apparatus using Extreme Ultraviolet (EUV) radiation having a wavelength in the range 4 to 20nm (e.g. 6.7nm or 13.5nm) may be used to form smaller features on a substrate than lithographic apparatus using radiation having a wavelength of, for example, about 193 nm.

Low k₁Lithography can be used for process features having dimensions smaller than the classical resolution limit of the lithographic apparatus. In this process, the resolution formula can be expressed as CD ═ k₁X λ/NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case half a pitch), and k is₁Is an empirical resolution factor. In general, k₁The smaller, the more difficult it becomes to reproduce patterns on the substrate that resemble the shapes and dimensions as planned by the circuit designer in order to achieve a particular electrical function and performance. To overcome these difficulties, complex fine tuning steps may be applied to the lithographic projection apparatus and/or the design layout. These steps include, for example but not limited to: optimization of NA, custom illumination schemes, use of phase-shifting patterning devices, various optimizations in the design layout, such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction"), or other methods commonly defined as "resolution enhancement techniques" (RET)₁Reproduction of the pattern in the case.

Metrology apparatus may be used to measure a parameter of interest of a structure on a substrate. For example, metrology equipment may be used to measure parameters such as critical dimensions, overlap between layers on the substrate, and asymmetry of the pattern on the substrate. The rays of the measuring radiation are used to irradiate the substrate. The radiation is diffracted by structures on the substrate. The diffracted radiation is collected by the objective lens and captured by the sensor.

The intensity of the illumination may be limited by the power of the radiation source and losses in the optical system of the measurement apparatus. The objective lens may have a high numerical aperture, such as about 0.95. Metrology apparatus may require complex optics located downstream of the objective lens, for example to reduce aberrations in the collected diffracted radiation.

Disclosure of Invention

It is an object to provide a metrology apparatus that may have a higher intensity illumination and/or may perform faster measurements.

According to an aspect of the invention, a metrology apparatus for determining a characteristic of interest of a structure on a substrate is provided. According to another aspect, a method for determining a parameter of interest of a structure on a substrate is provided. The metrology apparatus comprises: a radiation source for generating illuminating radiation; at least two illumination branches for illuminating the structure on the substrate, the illumination branches configured to illuminate the structure from different angles; and a radiation switch configured to receive the illumination radiation and to divert at least a portion of the radiation to a selectable one of the at least two illumination branches. Wherein the radiation switch comprises: a pockels cell configured to control a polarization direction of illumination radiation and to output polarization-controlled illumination radiation; and a polarizing beam splitter optically downstream of the pockels cell and configured to transmit the polarization-controlled illumination radiation to a first illumination branch of the at least two illumination branches or reflect the polarization-controlled illumination radiation to a second illumination branch of the illumination branches, depending on a polarization direction of the polarization-controlled illumination radiation.

Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 depicts a schematic overview of an embodiment of a lithographic apparatus;

FIG. 2 depicts a schematic overview of an embodiment of a lithography unit;

fig. 3 depicts a schematic representation of global lithography, which represents the cooperation between three key technologies to optimize semiconductor manufacturing;

FIG. 4 schematically shows how a parameter of interest of a structure on a substrate may be determined according to an embodiment of the invention;

fig. 5 comprises: (a) a schematic diagram of a dark-field scatterometer for measuring an object using a first pair of control perforations according to a comparative example, (b) details of the diffraction spectrum of the object grating for a given illumination direction, (c) second control perforations providing a further illumination pattern when using the scatterometer for diffraction-based overlay measurements, and (d) third control perforations combining the first pair of holes with the second pair of holes;

FIG. 6 shows a metrology apparatus according to an embodiment of the present invention;

figure 7 shows how the illumination spot size is estimated;

FIG. 8 schematically shows an optical connection between components of a metrology apparatus according to an embodiment of the present invention;

FIG. 9 is a top view of an illumination branch illuminating a structure according to an embodiment of the invention;

FIG. 10 schematically depicts part of a metrology apparatus according to an embodiment of the invention; and is

Figure 11 schematically depicts part of a metrology apparatus according to an embodiment of the invention.

Detailed Description

In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. having a wavelength of 365nm, 248nm, 193nm, 157nm or 126 nm) and extreme ultraviolet radiation (EUV, e.g. having a wavelength in the range of about 5-100 nm).

The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general purpose patterning device that can be used to impart an incident radiation beam with a patterned cross-section corresponding to a pattern to be created in a target portion of the substrate; the term "light valve" may also be used in this context. Examples of other such patterning devices, in addition to classical masks (transmissive or reflective; binary, phase-shift, hybrid, etc.), include programmable mirror arrays and programmable LCD arrays.

FIG. 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA comprises: an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g. ultraviolet radiation, DUV radiation or EUV radiation); a mask support (e.g. a mask table) T configured to support a patterning device (e.g. a mask) MA and connected to a support (e.g. a mask table) 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 positioning device 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, for example 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 impart the radiation beam B with a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

The term "projection system" PS as used herein should be broadly interpreted as encompassing any type 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 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 technology is given in US6952253, which is incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such "multi-stage" machines the substrate supports WT may be used in parallel, and/or steps may be performed on a substrate W positioned on one of the substrate supports WT in preparation for subsequent exposure of the substrate W whilst another substrate W on the other substrate support WT is used to expose a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LA may include a metrology stage. The measuring platform is arranged to hold the sensor and/or the 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 platform may hold a plurality of sensors. The cleaning device may be arranged to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of the system providing the immersion liquid. The measurement platform may be moved under the projection system PS while 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 T, and is patterned by the pattern (design layout) present on the patterning device MA. After 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 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 in focus and alignment. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in fig. 1) can 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 substrate alignment marks P1, P2 are shown as occupying dedicated target portions, they may be located in spaces between target portions. When substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe-lane alignment marks.

As shown in fig. 2, the lithographic apparatus LA may form part of a lithographic cell LC (sometimes also referred to as a lithographic cell or (lithographic) cluster), which typically also includes equipment to perform pre-exposure and post-exposure processes on the substrate W. Conventionally, these apparatuses comprise a spin coater SC to deposit a resist layer, a developer DE to expose the resist, a chill plate CH and a bake plate BK, for example to regulate the temperature of the substrate W, for example to regulate the solvent in the resist layer. The substrate handler or robot RO picks up substrates W from the input port I/O1, the output port I/O2, moves them between different process tools, and transfers the substrates W to the load station LB of the lithographic apparatus LA. The devices in the lithography cell, also commonly referred to as coating and development systems or tracks (tracks), are typically under the control of a coating and development system control unit or track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithography apparatus LA, e.g. via a lithography control unit LACU.

In order to expose the substrate W exposed by the lithographic apparatus LA correctly and consistently, the substrate needs to be inspected to measure properties of the patterned structures, such as overlay error, line thickness, Critical Dimension (CD) and the like between subsequent layers. For this purpose, an inspection tool (not shown) may be included in the lithography unit. If an error is detected, for example, adjustments may be made to the exposure of subsequent substrates or other processing steps to be performed on the substrate W, particularly if inspection is performed before other substrates W of the same batch or lot are still to be exposed or processed.

The inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrates W, 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 configured to identify defects on the substrate W and may for example be part of a lithographic cell, or may be integrated into a lithographic apparatus LA, or may even be a separate device. The inspection device may measure the characteristics of a latent image (an image in the resist layer after exposure), a semi-latent image (an image in the resist layer after the post-exposure bake step PEB), or a developed resist image (in which the exposed or unexposed portions of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

Typically, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process, which requires a high accuracy in the determination of the size and placement of the structures on the substrate W. To ensure such high accuracy, the three systems may be combined in a so-called "global" control environment, as schematically depicted in fig. 3. One of the systems is a lithographic apparatus LA which is (actually) connected to a metrology apparatus MT (second system) of the invention and to a computer system CL (third system). The key to this "global" environment is to optimize the cooperation between the three systems to enhance the overall process window and to provide a tight control loop to ensure that the patterning performed by the lithographic apparatus LA remains within the process window. The process window defines a range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device), typically allowing process parameters in a lithographic process or a patterning process to vary within the defined result.

The computer system CL may use (parts of) the design layout to be patterned to predict what resolution enhancement techniques are to be used and perform computational lithography simulations and calculations to determine which mask layouts and lithographic apparatus settings implement the largest overall process window of the patterning process (depicted in fig. 3 by the double arrow in the first scale SC 1). Typically, resolution enhancement techniques are arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where the lithographic apparatus LA is currently operating within the process window (e.g. using input from the metrology apparatus MT) to predict whether a defect (depicted in fig. 3 by the arrow pointing to "0" in the second scale SC 2) is present due to, for example, sub-optimal processing.

The metrology apparatus MT may provide input to the computer system CL to enable accurate simulation and prediction, and may provide feedback to the lithographic apparatus LA to identify possible drift, for example in a calibration or calibration state of the lithographic apparatus LA (depicted in fig. 3 by the plurality of arrows in the third scale SC 3).

In lithographic processes, it is desirable to frequently measure the resulting structures, for example for process control and verification. Different types of metrology equipment MT are known for making such measurements, including scanning electron microscopes or various forms of scatterometry metrology equipment MT. A scatterometer is a multifunctional instrument that allows measurement of parameters of a lithographic process by placing a sensor in the pupil or conjugate plane to the pupil of the scatterometer objective (this measurement is usually referred to as a pupil-based measurement), or by placing a sensor in the image plane or conjugate plane to the image plane (in this case, the measurement is usually referred to as an image or field-based measurement). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753a1, US20120044470A, US20110249244, US20110026032 or ep1,628,164a, which are incorporated herein by reference in their entirety. The aforementioned scatterometers may use light ranging from soft X-rays, as well as visible to near IR wavelengths to measure gratings. The measurement device MT of the present invention may be a diffraction-based scatterometer.

In a first embodiment, the metrology device MT is an angle-resolved scatterometer. In such scatterometers, reconstruction methods may be applied to the measured signals to reconstruct or calculate the properties of the grating. Such a reconstruction may for example be generated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the actual target.

In a second embodiment, the measurement device MT is a spectroscopic scatterometer. In such a spectroscopic scatterometer, radiation emitted by a radiation source is directed onto the target and radiation reflected or scattered from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. a measurement of intensity as a function of wavelength). From this data, the structure or profile of the target producing the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and non-linear regression or by comparison with a library of simulated spectra.

In a third embodiment, the metrology device MT is an ellipsometer. Ellipsometry scatterometers allow for the determination of parameters of a lithographic process by measuring the scattered radiation for each polarization state. Such a metrology apparatus emits polarized light (such as linear, circular or elliptical) by using, for example, a suitable polarizing filter in the illumination section of the metrology apparatus. A source suitable for use in the metrology apparatus may also provide polarized radiation. Various embodiments of prior art ellipsometric scatterometers are described in U.S. patent applications 11/451,599, 11/708,678, 12/256,780, 12/486,449, 12/920,968, 12/922,587, 13/000,229, 13/033,135, 13/533,110, and 13/891,410, which are incorporated by reference herein in their entirety.

In an embodiment of the metrology device MT, the metrology device MT is adapted to measure the overlay of two misaligned gratings or periodic structures by measuring an asymmetry in the reflection spectrum and/or the detection configuration, the asymmetry being related to the degree of overlay. Two (typically, stacked) grating structures may be applied in two different layers (not necessarily consecutive layers) and may be formed at substantially the same location on the wafer. The scatterometer may have a symmetric detection configuration as described, for example, in commonly owned patent application ep1,628,164a, such that any asymmetries are clearly distinguishable. This provides a straightforward way to measure misalignment in the grating. Further examples of measuring the overlay error between two layers containing a targeted periodic structure via the asymmetry of the periodic structure can be found in PCT patent application publication No. WO2011/012624 or U.S. patent application No. US 20160161863, which are incorporated herein by reference in their entirety.

Other parameters of interest may be focus and dose. The focus and dose can be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy) as described in US patent application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure with a unique combination of critical dimension and sidewall angle measurements for each point in a focused energy matrix (FEM-also referred to as a focused exposure matrix) can be used. If these unique combinations of critical dimension and sidewall angle are available, focus and dose values can be uniquely determined from these measurements.

The structure on the substrate, which is the metrology target, may be an ensemble of composite gratings (ensemble) formed by the lithographic process, primarily in resist, but may also be formed after, for example, an etching process. Typically, the pitch and line width of the structures in the grating depend to a large extent on the measuring optical element (in particular the NA of the optical element) in order to be able to capture the diffraction orders from the metrology target. As indicated previously, the diffraction signal may be used to determine a shift (also referred to as "overlap") between two layers or may be used to reconstruct at least a portion of the original grating as produced by the lithographic process. Such reconstruction may be used to provide guidance on the quality of the lithographic process and may be used to control at least a portion of the lithographic process. The target may have a smaller sub-segment configured to mimic the size of the functional portion of the design layout in the target. Due to such sub-segmentation, the target will behave more like a functional portion of the design layout, making the overall process parameter measurement better like a functional portion of the design layout. The target may be measured in an under-filled, i.e. under-filled, mode or an over-filled, i.e. over-filled, mode. In the underfill mode, the measurement beam produces a spot that is smaller than the entire target. In the overfill mode, the measurement beam produces a spot larger than the entire target. In this overfill mode, it is also possible to measure different targets simultaneously, so that different process parameters are determined simultaneously.

The overall quality of the measurements of the lithographic parameters using a particular target is determined at least in part by the measurement recipe used to measure the lithographic parameters. The term "substrate measurement recipe" may include measuring one or more parameters of itself, one or more parameters of one or more patterns measured, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, the one or more parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation with respect to the substrate, the direction of the radiation with respect to the pattern on the substrate, and the like. One of the criteria for selecting a measurement recipe may be, for example, the sensitivity or sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. patent application 2016-0161863 and published U.S. patent application US2016/0370717A1, which are incorporated herein by reference in their entirety.

In accordance with an embodiment of the present invention, a metrology device MT, such as a scatterometer, is depicted in FIG. 4. The scatterometer comprises a broadband (white light) radiation projector 2 projecting radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectrometer detector 4 which measures the spectrum 10 of the specularly reflected radiation (i.e. a measurement of the intensity as a function of the wavelength). From this data, the structure or profile that produces the detected spectrum can be reconstructed by the processor PU, for example by rigorous coupled wave analysis and non-linear regression or by comparison with a library of simulated spectra as shown at the bottom of fig. 4. Typically, for reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process used to fabricate the structure, leaving only a few parameters of the structure to be determined from scatterometry data. Such scatterometers may be configured as normal incidence scatterometers or oblique incidence scatterometers.

For a better understanding of the present invention, a metrology apparatus according to a comparative example is described below with reference to fig. 5.

Fig. 5(a) presents a metrology apparatus according to a comparative example, and more particularly a dark field scatterometer. The target TT and the diffracted rays of the measuring radiation with which the target is irradiated are illustrated in more detail in fig. 5 (b). The metrology apparatus shown is of the type known as dark field metrology apparatus. The metrology apparatus may be a stand-alone device, may be comprised in the lithographic apparatus LA (e.g. at a metrology station), or may be comprised in the lithographic cell LC. The optical axis with several branches through the device is indicated by the dashed line O. In this apparatus, light emitted by a source 11 (e.g., a xenon lamp) is directed onto a substrate W via a beam splitter 15 by an optical system comprising lenses 12, 14 and an objective lens 16. The lenses are arranged in a dual order of 4F arrangement. A different lens arrangement may be used as long as it still provides a substrate image onto the detector while allowing access to the intermediate pupil plane for spatial frequency filtering. Thus, the angular range of radiation incident on the substrate, herein referred to as the (conjugate) pupil plane, can be selected by defining a spatial intensity distribution in a plane of the spatial spectrum that represents the substrate plane. This can be done in particular by inserting an aperture plate 13 of suitable form between the lenses 12 and 14 in the plane of the back-projected image of the pupil plane of the objective lens. In the illustrated example, the aperture plate 13 has different forms, designated 13N and 13S, to allow different illumination modes to be selected. The illumination system in this example forms an off-axis illumination mode. In the first illumination mode, the aperture plate 13N provides off-axis (illumination) from a direction designated "north" for ease of description only. In the second illumination mode, the aperture plate 13S is used to provide similar illumination, but illumination from the opposite direction, labeled "south". Other illumination modes are possible by using different apertures. The rest of the pupil plane is desirably dark, since any unnecessary light outside the desired illumination mode may interfere with the desired measurement signal.

As shown in fig. 5(b), the target TT is placed with the substrate W perpendicular to the optical axis O of the objective lens 16. The substrate W may be supported by a support (not shown). A ray I of the measurement radiation impinging on the target TT at an angle off the axis O generates a ray of the zeroth order (solid line 0) and two rays of the first order (the dash-dot line indicates the +1 order, and the two-dot chain line indicates the-1 order). It should be noted that for small targets that are overfilled, these rays are but one of many parallel rays that cover the substrate area that includes the metrology target junction TT and other features. Since the holes in the plate 13 have a limited width (necessary to allow a useful amount of light), the incident radiation I will actually occupy an angular range and the diffracted rays 0 and +1/-1 will spread out slightly. Each order +1 and-1 will further spread out over a range of angles according to the point spread function of a small target, rather than a single ideal ray as illustrated. Note that the grating pitch and illumination angle of the target may be designed or adjusted so that the first order rays entering the objective lens are closely aligned with the central optical axis. The rays shown in fig. 5(a) and 5(b) are shown slightly off axis purely to enable them to be more easily distinguished in the figure.

At least the 0 and +1 orders diffracted by the target TT on the substrate W are collected by the objective lens 16 and directed back through the beam splitter 15. Returning to fig. 5(a), both the first and second illumination modes are illustrated by designating diametrically opposed holes labeled north (N) and south (S). When the incident ray I of the measurement radiation comes from the north side of the optical axis, that is, when the first illumination mode is applied using the aperture plate 13N, the +1 st order diffracted ray labeled +1(N) enters the objective lens 16. In contrast, when the second irradiation mode is applied using the aperture plate 13S, the-1 st order diffracted radiation (labeled-1 (S)) is the radiation that enters the lens 16.

The second beam splitter 17 splits the diffracted beam into two measurement branches. In the first measurement branch, the optical system 18 forms a diffraction spectrum (pupil plane image) of the target structure on a first sensor 19 (e.g. a CCD or CMOS sensor) with the zeroth and first order diffracted beams. Each diffraction order hits a different point on the sensor so that the image processing can compare and contrast multiple orders. The pupil plane image captured by the sensor 19 may be used for focus metrology devices and/or to normalize intensity measurements of the first order beam. The pupil plane image may also be used for many measurement purposes such as reconstruction.

In the second measuring branch, the optical system 20, 22 forms an image of the target TT on a sensor 23 (e.g. a CCD or CMOS sensor). In the second measuring branch, the aperture stop 21 is arranged in a plane conjugate to the pupil plane. The aperture stop 21 acts to block the zeroth order diffracted beam so that the image of the target formed on the sensor 23 is formed by only the-1 or +1 order beams. The images captured by the sensors 19 and 23 are output to a processor PU which processes the images, the function of which will depend on the particular type of measurement being performed. It should be noted that the term "image" is used herein in a broad sense. If only one of the-1 order and the +1 order is present, the image of the raster lines will not be formed, as such.

The particular form of aperture plate 13 and field stop 21 shown in figure 5 is merely exemplary. In another embodiment of the invention, on-axis illumination of the target is used, and an aperture stop having an off-axis aperture is used to pass substantially only one of the diffracted light of the first order to the sensor. In still other embodiments, second, third, and higher order beams (not shown in FIG. 5) may be used in the measurements instead of or in addition to the first order beam.

To accommodate the measurement radiation to these different types of measurements, the aperture plate 13 may include a plurality of aperture patterns formed around a disk that rotates to bring the desired pattern into position. It should be noted that the aperture plate 13N or 13S is only used to measure gratings oriented in one direction (X-direction or Y-direction, depending on the setup). To measure orthogonal gratings, target rotations of 90 ° and 270 ° may be implemented. Different orifice plates are shown in fig. 4(c) and 4 (d). The use of these and many other variations and applications of the apparatus are described in the previously published applications mentioned above.

The rays of the measuring radiation are provided by light emitted by the source 11. This light is directed onto the substrate W via a beam splitter 15 and an objective lens 16, which collects diffracted radiation from the substrate W.

The target TT may include two line gratings in a first direction (e.g., an X direction) and two line gratings in a second direction (e.g., a Y direction) orthogonal to the first direction. The objective lens 16 needs to have a high numerical aperture NA (e.g. about 0.95) in order to collect the radiation diffracted from the grating. As shown in fig. 5, a large number of optical elements are required between the objective lens 16 and the sensors 19 and 23.

It would be good to have a metrology apparatus that may have a higher intensity illumination and/or may perform faster measurements. A metrology apparatus with lower requirements for the numerical aperture of the detection lens (e.g. objective lens 16 in the metrology apparatus of figure 5) would be good. It would be advantageous to have a metrology apparatus with simplified detection optics.

FIG. 6 schematically depicts a metrology apparatus MT according to an embodiment of the invention. The metrology apparatus MT is used to determine a parameter of interest of a structure on the substrate W. The structure is the metrology target TT.

In an embodiment, the measurement device MT comprises a radiation source 32. The radiation source 32 is for generating illuminating radiation. For example, in an embodiment, the radiation source 32 is a laser, such as a white light laser. The radiation source 32 may comprise at least one photonic crystal fiber for transmitting said illuminating radiation. However, the radiation source 32 need not be a white light laser. Other types of laser, or non-laser type sources may be used as radiation source 32.

As depicted in fig. 6, in an embodiment, the metrology apparatus MT comprises at least two illumination branches 51, 52. At least two illumination branches 51, 52 are used for illuminating a target TT on the substrate W. The illumination branches 51, 52 are configured to illuminate the target TT from different angles, as shown for example in fig. 6. The number of illumination branches is not limited to two. For example, there may be three, four, or more than four illumination branches.

The illumination branches 51, 52 represent different optical paths for the illumination radiation to reach the target TT. The irradiation radiation transmitted through said irradiation branches 51, 52 is provided by a radiation source 32. The irradiation branches 51, 52 are alternative paths for the irradiation radiation from the radiation source 32 to be directed to the target TT.

As depicted in fig. 6, in an embodiment, the metrology device MT comprises a radiation switch 27. The radiation switch 27 is a switching element. The radiation switch 27 is configured to receive illuminating radiation from the radiation source 32. The radiation switch 27 is configured to divert at least a portion of the radiation to a selectable one of the at least two illumination branches 51, 52.

For example, the radiation switch 27 may be switched between different modes of operation. In the first mode, the radiation switch 27 is configured to receive the illumination radiation and to divert at least a portion of the radiation to the first illumination branch 51. In a second mode, the radiation switch 27 is configured to receive the illumination radiation and to divert at least a portion of the radiation to the second illumination branch 52. The first mode corresponds to the case when the first illumination branch 51 is selected. The second mode corresponds to the case when the second illumination branch 52 is selected. As shown in fig. 6, in an embodiment, the radiation switch is configured to receive a control signal 57 that can be used to select a mode.

In an embodiment, the target TT is illuminated sequentially from different angles by at least two illumination branches 51, 52. In the first time period, the target TT is irradiated by the irradiation radiation from the first irradiation branch 51. In a second time period (after the first time period), the target TT is irradiated by the irradiation radiation from the second irradiation branch 52. In an embodiment, the first time period does not overlap with the second time period. In an embodiment, the second period of time substantially immediately follows the first period of time with substantially no gap between the two periods of time.

In an embodiment, the two illumination branches 51, 52 correspond to complementary angles. For example, if the first illumination branch 51 illuminates the target TT from a first angle, the second illumination branch 52 illuminates the target TT from a complementary angle of 180 ° to the first angle when viewed along the normal to the target TT (i.e., the top view shown in fig. 9). In particular, in an embodiment, the first illumination branch 51 is used to make a dark field measurement originating from the target TT (having a single orientation, e.g. X-orientation), wherein the first diffraction order is collected by the detection lens 16. The second illumination branch 52 is used to make dark field measurements originating from the same target TT with the same orientation, but the complementary diffraction orders are collected by the detection lens 16.

Referring to fig. 9, in an embodiment, different illumination branches 51, 53 are used to determine characteristics of interest of different gratings of the target TT. For example, the first illumination branch 51 may be used to measure radiation diffracted from a grating in the X-direction. The other illumination branch 53 may be used to measure the radiation diffracted from the grating in the Y-direction.

According to the invention, measurements from perpendicular and complementary angles can be made in sequence. Additionally or alternatively, measurements of gratings in different directions may be performed sequentially. This means that the diffracted radiation for these different measurements does not have to be collected simultaneously by the detection lens 16. Alternatively, the detection lens 16 may collect radiation for only one of these measurements at a time.

Embodiments of the present invention contemplate implementing a metrology device MT having a lower NA for the detection lens 16. In an embodiment, the detection lens 16 has a NA of at most 0.9, optionally at most 0.8, optionally at most 0.7, optionally at most 0.6, optionally at most 0.5, and optionally at most 0.4. The detection lens 16 is part of a detection branch of the metrology device MT. The detection lens 16 is configured to collect and transmit a portion of the scattered/reflected radiation in a direction towards the sensor 19. Embodiments of the present invention contemplate a greater degree of design freedom for the detection lens 16 of the metrology apparatus MT. For example, a lens that is simpler or less expensive to manufacture may be used.

As explained above, a radiation switch 27 is provided to subsequently divert radiation to the illumination branches 51, 52. In an embodiment, the radiation switch 27 is configured to switch between illumination branches in less than 1 ms. The fast switching provided by the radiation switch 27 allows measurements to be taken in sequence for a limited period of time. Embodiments of the present invention are expected to enable high-speed continuous measurement of vertical and complementary modes of the grating along the X-direction and Y-direction of the target TT.

In an embodiment, the radiation switch 27 comprises a spatial light modulator. The spatial light modulator is configured to receive illumination radiation from a radiation source 32. The spatial light modulator is configured to apply a spatially varying modulation to the illuminating radiation. In an embodiment, the spatial light modulator comprises a micromirror device. The micromirror device includes a plurality of microscopic small mirrors. The mirror is a micro-electromechanical system. The orientation of the mirrors is controlled by applying a voltage between two electrodes surrounding the mirror array. The direction of the mirror may be controlled so as to divert the illumination radiation to a selectable one of the at least two illumination branches 51, 52. In an embodiment, the spatial light modulator is electrically addressed such that an image on the spatial light modulator is electronically generated and altered. In an alternative embodiment, the spatial light modulator is electrically addressed such that an image on the spatial light modulator is created and altered by illuminating light encoded with the image onto its front or back surface.

The spatial light modulator transfers the illumination radiation with low loss to the illumination branches 51, 52. Embodiments of the present invention contemplate high brightness illumination to achieve the target TT. It may be desirable to perform measurements of the target TT using radiation having different wavelengths. The spatial light modulator may transfer illuminating radiation over a wide range of wavelengths.

As mentioned above, the target TT may be sequentially illuminated from different angles by the illuminating radiation (by using different illumination branches 51, 52). The wavelength of the illuminating radiation may be changed after the measurements have been made using the illuminating radiation from the complete set of different angles. After the wavelength change, measurements at different angles can be repeated. Another complete set of measurements using different angles can be made with varying wavelengths. These operations may be performed in order to obtain sets of measurements corresponding to different wavelengths of the irradiated radiation at different angular instances. The spatial light modulator may be configured to process the full range of wavelengths of the illuminating radiation. In an embodiment, the wavelength of the illuminating radiation may be changed about ten times during the measurement process.

However, the radiation switch 27 does not necessarily comprise a spatial light modulator. Other forms of radiation switches 27 may be used. For example, in an embodiment, the radiation switch 27 comprises an acousto-optic deflector. The acousto-optic deflector may also be referred to as a bragg cell. The acousto-optic deflector uses the acousto-optic effect to diffract and shift the frequency of the illuminating radiation using acoustic waves. For example, in an embodiment, the acousto-optic deflector comprises a nonlinear crystal. The acousto-optic deflector is configured to change an optical property of a material (e.g., a nonlinear crystal) by providing an electromagnetic signal or an acoustic signal. By controlling the manner in which the acousto-optic deflector changes the optical properties of the material, the illumination radiation can be directed to a selectable one of the at least two illumination branches 51, 52.

The acousto-optic deflector has a relatively small number of mechanically moving parts (and possibly no mechanically moving parts). Embodiments of the present invention contemplate high reliability in illumination branch selection with long lifetime.

Neither the spatial light modulator nor the acousto-optic deflector is essential to the invention. Additionally or alternatively, in an embodiment, the radiation switch 27 comprises at least one beam splitter and a plurality of shutters (or shutters) configured to control the transfer of the radiation to a selectable one of the at least two illumination branches 51, 52.

The spatial light modulator and the acousto-optic deflector are capable of diverting the illuminating radiation to the illuminating branches 51, 52 with low loss. In particular, it is not necessary to split the illuminating radiation into different paths and then deliberately cut off some of the paths (thereby losing the radiation). Alternatively, substantially all of the radiation may be directed along the desired optical path.

FIG. 11 schematically depicts part of a metrology apparatus MT in accordance with an embodiment of the present invention. In the embodiment shown in fig. 11, the radiation switch 27 comprises an acousto-optic deflector 75. The acousto-optic deflector 75 is configured to redirect (i.e., deflect) light to various angles. The acousto-optic deflector 75 is configured to deflect light in different directions. The angle of reflection depends on the driving frequency and/or power at which the acoustic wave is excited in the material. In an embodiment, the acousto-optic deflector 75 is configured to deflect radiation in the range from 400nm to 1600 nm.

In an embodiment, the controller is configured to control the drive frequency and/or power used to excite acoustic waves in the material of the acousto-optic deflector 75. The range of angles over which the acousto-optic deflector 75 can redirect light is not particularly limited. In an embodiment, the acousto-optic deflector 75 is configured to deflect light over a range of angles, said range being 0.5 to 1.5 °.

As shown in fig. 11, in an embodiment, the radiation switch 27 comprises a focusing lens 76. The focusing lens 76 is configured to receive the light deflected by the acousto-optic deflector 75 and direct the light to the illumination branches 51-54. The deflection angle provided by the acousto-optic deflector 75 and the focal length of the focusing lens 76 may be selected to provide a sufficiently large spatial deflection of the light beam. For example, in an embodiment, the focusing lens 76 may have a focal length of about 60 mm. If the acousto-optic deflector 75 deflects the light over a range of angles of 1 or more and the focusing lens 76 has a focal length of 60mm, the spatial displacement of the beam can be about 1 mm.

As shown in fig. 11, in an embodiment, the radiation switch 27 comprises a lens array 77. The lens array is configured to couple the radiation beam from the acousto-optic deflector 75 to the optical fibre 30 corresponding to the illumination branches 51 to 54. In an embodiment, the lens array 77 includes a plurality of lenses. In an embodiment, said lens array 77 comprises a lens corresponding to the optical fiber 30 of each illumination branch 51 to 54. In an embodiment, each lens of the lens array 77 has a diameter of at least 1mm, and optionally at most 2 mm.

By varying the frequency and/or power of the acoustic wave excited in acousto-optic deflector 75, the deflection angle applied by the acousto-optic deflector 75 can be controlled. Thus, by varying the frequency and/or power input to acousto-optic deflector 75, the deflection angle can be switched quickly to redirect light to selected individual fibers 30 of a particular illumination branch 51-54. This makes it possible to switch the illumination branches 51 to 54 in less than 1 ms.

In an embodiment, the radiation switch 27 is configured to vary the intensity of the radiation applied to the illumination branches 51 to 54. For example, the radiation switch 27 may be configured to vary the illumination intensity between, for example, a medium intensity and a high intensity. This is explained in more detail below.

In an embodiment, the acousto-optic deflector is configured to change the direction of the radiation beam so as to control how much of the radiation beam is coupled to the optical fiber 30 of the corresponding illumination branch 51 to 54. The individual intensities applied to illumination branches 51-54 can be adjusted by detuning the deflection angle provided by acousto-optic deflector 75. When a high radiation intensity is desired, the acousto-optic deflector 75 is configured to provide a deflection angle such that the radiation beam is applied to the center of the optical fiber 30 of a particular illumination branch 51. By directing the radiation beam to the centre of the optical fibre 30, the illumination intensity can be maximised (or almost maximised).

If a lower radiation intensity is required, acoustic light deflector 75 can be controlled to apply a slightly different deflection angle so that the radiation beam is directed to an off-center position (in cross-sectional area) of the optical fiber 30. This results in a reduction in the amount of radiation coupled into the optical fiber 30. This reduces the intensity of the radiation diverted through the optical fiber 30 of the illumination branch 51. In this way, the individual intensities can be adjusted by slightly detuning the angle so that the spot formed on the tip of the fiber 30 is off-center and less light is coupled into the fiber 30.

In this way, small intensity imbalances in the beams passing along the different illumination branches 51 to 54 can be adjusted and corrected. For example, if the energy sensor 55 indicates an intensity imbalance between the illumination branches 51 to 54, the acousto-optic deflector 75 may be controlled to vary the deflection. This may compensate for intensity imbalance and increase the uniformity of intensity across the illumination branches 51 to 54.

It should be noted that the use of acousto-optic deflector 75 to vary the intensity of illumination may be combined with the features of the above-described embodiment shown in FIG. 8. The use of acousto-optic deflector 75 to vary the intensity of illumination may also be used independently of the other features shown in FIG. 8.

FIG. 10 schematically depicts part of a metrology apparatus MT in accordance with an embodiment of the present invention. In the embodiment shown in fig. 10, the radiation switch 27 comprises at least one pockels cell 70. Pockels cell 70 is an electro-optical component configured to control the polarization direction of a radiation beam. The pockels cell is configured such that it generates birefringence in the optical medium induced by an electric field. The birefringence depends on the electric field. When a voltage is applied to the crystal of pockels cell 70, the amount of birefringence thereof changes. Specifically, the amount of birefringence may vary linearly with respect to the applied voltage. Pockels cell 70 acts as a variable retarder.

The voltage applied to the pockels cell 70 is controlled such that the pockels cell 70 applies a variable delay. The variable retarder affects the polarization direction of the radiation input into the pockels cell 70. By controlling the polarization direction of the radiation, the radiation can be directed to the different illumination branches 51 to 54, as explained in more detail below.

As shown in fig. 10, in an embodiment, the metrology apparatus MT comprises a beam splitter 61 configured to receive radiation from the radiation source 32. The beam splitter 61 is configured to direct the radiation along two different branches. The provision of the beam splitter 61 is not essential. In an alternative embodiment, the radiation is directed from the radiation source 32 along one of the two branches shown in fig. 10 (i.e., with only one pockels cell 70). The other branch (with the other pockels cell 70) is optional.

In an embodiment, the wavelength selector 33 comprises an acousto-optic tunable filter 36, the acousto-optic tunable filter 36 being configured to transmit the illuminating radiation of the selected wavelength range based on the frequency and/or power of the acoustic wave excited in the acousto-optic tunable filter 36. In particular, as shown in fig. 10, in an embodiment, the metrology device MT comprises at least one acousto-optic tunable filter 36. The acousto-optic tunable filter 36 may form part of the wavelength selector 33. The acousto-optic tunable filter 36 is configured to select the wavelength of radiation transmitted by the acousto-optic tunable filter 36. In an embodiment, the acousto-optically tunable filter 36 has a bandwidth of about 3 nm.

The acousto-optic tunable filter 36 is configured to linearly polarize the radiation. The radiation output by acousto-optic tunable filter 36 is linearly polarized. The linearly polarized radiation is input into a pockels cell 70, which is located downstream of the acousto-optic tunable filter 36.

As mentioned above, the pockels cell acts as a variable retarder. The pockels cell 70 is configured such that the "fast" and "slow" axes of the retarder are arranged at 45 ° with respect to the plane of polarization of the linearly polarized radiation received by the pockels cell 70. When the voltage applied to the pockels cell 70 is such that the retarder is at 0 °, then the polarization of the radiation is not altered by the pockels cell 70. However, when the voltage applied to the pockels cell 70 is controlled such that the retarder is at 180 °, then the polarization direction of the radiation is rotated by 90 °. The voltage applied to the pockels cell 70 may vary in less than 1ms, so that the switching of the polarization direction may be performed in less than 1 ms.

The voltage that needs to be applied to the pockels cell 70 in order to provide a delay of 180 ° depends on the wavelength of the radiation. The wavelength of the radiation is thus an input to the controller which controls the voltage applied to the pockels cell 70 in order to control the polarization direction of the radiation output by the pockels cell 70.

As shown in fig. 10, in an embodiment the radiation switch 27 comprises a polarizing beam splitter 71 optically downstream of the pockels cell 70. The polarizing beam splitter 71 is configured to transmit or reflect a radiation beam depending on the polarization direction controlled by the pockels cell 70. The radiation output by the pockels cell 70 enters the polarizing beam splitter 71. Depending on the selected polarization direction of the radiation, the radiation is transmitted or reflected by the polarizing beam splitter 71. As shown in fig. 10, in an embodiment radiation switch 27 comprises a half-wavelength retarder 72. The half-wave retarder 72 is configured to rewind or rewind (rotate back) the polarization direction of the radiation.

Both the radiation beam transmitted by the polarizing beam splitter 71 and the radiation beam reflected by the polarizing beam splitter 71 and subsequently transmitted by the half-wave retarder 72 then have the same polarization direction. The two beams correspond to two different illumination branches 51, 52. Thus, the radiation switch 27 is configured to control which illumination branch 51, 52 is used by controlling the voltage applied to the pockels cell 70.

In an embodiment, the metrology device MT comprises at least two acousto-optic tunable filters 36-37, at least one acousto-optic tunable filter being arranged upstream of the radiation switch 27 and at least one acousto-optic tunable filter being arranged downstream of the radiation switch 27. In particular, as shown in fig. 10, in an embodiment each illumination branch 51 to 54 is provided with a further acousto-optic tunable filter 37. The further acousto-optic tunable filter 37 is configured to switch the beam on and off for the corresponding illumination branch 51 to 54.

In an embodiment, the further acousto-optic tunable filter 37 has a larger bandwidth than the acousto-optic tunable filter 36. By requiring the radiation to pass through both acousto-optic tunable filters 36, 37, radiation of unwanted wavelengths can be suppressed more reliably. The switch provided by such an acousto-optic tunable filter 37 may be applied to other embodiments of the present invention.

As shown in fig. 10, there may be two branches, each having a pockels cell 70, so as to provide for irradiating the branches 51 to 54 in total. Alternatively, if only two illumination branches 51, 52 are required, only one branch may be required.

In an embodiment, the beam splitter 61 located immediately downstream of said radiation source 32 is a polarizing beam splitter. This makes it possible to produce two beams with optimal polarization for transmission through the corresponding acousto-optic tunable filter 36. This reduces the amount of energy that would otherwise be lost in the acousto-optic tunable filter 36.

In an embodiment, the light source 32 is a supercontinuum light source.

It should be noted that pockels cell 70 and acousto-optic tunable filters 36 to 37 may be combined with the features of the above-described embodiment shown in fig. 8. Pockels cell 70 and acousto-optic tunable filters 36 to 37 may also be used independently of the other features shown in fig. 8.

In an embodiment, the metrology apparatus MT comprises a detection lens 16. The lens is used to collect at least a portion of the radiation diffracted from the structure (i.e., the target TT). In an embodiment, the lens is similar to the objective lens 16 described above in the context of the metrology apparatus of FIG. 5. In the following description, the lens is described as an objective lens. However, it is not essential that the lens be an objective lens. Alternatively, the lens may be a single lens. The lens may be a single lens, such as a plano-aspheric (plano-asphere) or a biaspheric lens. The lens may be any surface or fresnel lens with a free-form curvature. The lens may comprise a material having a gradient in refractive index.

In an embodiment, the metrology device MT comprises an image sensor 19. The sensor 19 is used to receive and obtain a record of the collected diffracted radiation. The sensor 19 may be similar to the sensor 19 described above in the context of the metrology device shown in FIG. 6.

In an embodiment, the metrology apparatus MT comprises optics 24. The optics 24 are used to transmit the collected diffracted radiation to the sensor 19. Optics 24 may include one or more optical elements, such as lenses, beam splitters, and optical masks.

In an embodiment, the illumination branches 51, 52 comprise at least one optical fiber for illuminating the target TT. As shown in FIG. 6, in an embodiment, at least one optical fiber is used to directly illuminate the target TT. This means that there need not be any optics between the end of the fiber and the target TT. This helps to maintain coherence of the illuminating radiation directed to the target TT.

In a comparative example of the metrology apparatus shown in FIG. 5, the target TT is illuminated by radiation emitted by the source 11 and transmitted through various optical elements including an objective lens 16. The embodiment of the metrology device MT of the present invention is different in that the target TT may be directly illuminated by the optical fiber. This means that when the illuminating radiation is emitted from the optical fiber, the illuminating radiation is not transmitted through other optics, such as any lenses or beam splitters. In particular, the illuminating radiation emitted from the optical fiber is not transmitted through the detection lens 16 before it is incident on the target TT.

However, it is not necessary that the optical fiber directly irradiate the target TT. In an embodiment, at least one optical fiber is used to indirectly illuminate the structure via at least one other optical element. For example, optical elements such as lenses, beam splitters, etc. may be disposed between the end of the optical fiber and the target TT.

Fig. 7 schematically shows the size of a spot 31 of radiation illuminating the target TT. The size of the spot 31 may be estimated using the dimensions shown in fig. 6. With increasing fiber diameterThe size S of the spot 31 increases. The spot 31 may be an ellipse having a width and a length. The spot 31 increases in size S with increasing distance d between the tip 40 of the fiber 30 and the target TT. With increasing angle NA, representing the spread of rays of radiation emitted from the optical fiber 30, the size S of the spot 31 increases. The size S of the spot 31 increases with increasing azimuthal angle θ defined between the normal to the target TT and the central radiation ray from the fiber. In an embodiment, all illumination branches 51, 52 illuminateThe same region of the target TT. The spots 31 of the different illumination branches 51, 52 overlap each other. In an embodiment, the illumination branches 51, 52 illuminate the target TT at the same azimuth angle θ. In an embodiment, the optical fibers 30 of the illumination branches 51, 52 have the same diameter.

In an embodiment, the tip 40 of the at least one optical fiber is positioned between the objective lens 16 and the target TT in a direction parallel to the optical axis O. The optical axis O is defined by the detection lens 16. Specifically, as shown in fig. 6, the tip 40 may be positioned in a volume that: the volume is bounded at one side by a first virtual plane 25 formed by the surface of the substrate W and at the other side by a second virtual plane 26 parallel to the first virtual plane and touching the end of the objective lens 16 facing the substrate W.

An example of this is shown in fig. 6, where the optical fibers of the illumination branches 51, 52 extend down to a position below the end face of the detection lens 16. This may help to position the tip 40 of the fiber close to the target TT. However, it is not necessary that the tip 40 be between the detection lens 16 and the target TT in a direction parallel to the optical axis. For example, the tip 40 may be positioned just above the end face of the detection lens 16, but to one side thereof. This may be done while ensuring that the tip 40 does not interfere with any diffracted radiation from the target TT that may be collected by the detection lens 16.

Figure 8 diagrammatically depicts optical connections between optional components of the metrology apparatus MT. As shown in fig. 8 and described above, the metrology device MT comprises a radiation source 32 and a radiation switch 27. As shown in fig. 8, in an embodiment, the metrology apparatus MT comprises four illumination branches 51 to 54. Two of the illumination branches are used to determine a property of interest of a first set of gratings of the target TT. Two of the illumination branches are used to determine a property of interest of the second set of gratings of the target TT.

As shown in fig. 8, in an embodiment, the metrology device MT includes a wavelength selector 33. The wavelength selector 33 is configured to receive the illuminating radiation. The wavelength selector 33 is configured to transmit illuminating radiation of a selected wavelength range. The wavelength selector 33 is configured to filter out illuminating radiation outside a selected wavelength range. In an embodiment, the selected wavelength range has a bandwidth of about 5 to 15 nm.

As shown in fig. 8, in an embodiment the wavelength selector 33 comprises a plurality of selector elements 34, 35. In an embodiment, the wavelength selector 33 comprises a visible radiation selector unit 34 and an infrared radiation selector unit 35. The visible radiation selector unit 34 is configured to transmit a selected wavelength range within the visible spectrum and filter out other radiation. The infrared radiation selector 35 is configured to transmit a selected wavelength range within the infrared spectrum and filter out other radiation. Further selector units may be provided for other sections of the radiation spectrum.

As depicted in fig. 8, in an embodiment, the wavelength selector 33 comprises a beam splitter 61 for splitting the illumination radiation between the selector units 34, 35. In an embodiment, the wavelength selector 33 comprises a reflector 62 (e.g. a mirror) for recombining the split optical paths into a single optical path for input to the radiation switch 27.

As depicted in fig. 8, in an embodiment, the wavelength selector 33 is optically downstream of the radiation source 32 and optically upstream of the radiation switch 27. The wavelength selector 33 receives illuminating radiation from the radiation source 32. The wavelength selector 33 outputs the selected wavelength range of the illuminating radiation to the radiation switch 27. In an alternative embodiment, the wavelength selector 33 is positioned downstream of the radiation switch 27.

It is not necessary that the measurement device MT comprises a wavelength selector 33. In an embodiment, radiation source 32 comprises a single wavelength source (e.g., a single wavelength laser). For example, a wavelength selector is not necessary when radiation source 32 comprises a single wavelength source. In an embodiment, the radiation source 32 is a tunable single wavelength source of: its output is within a relatively small bandwidth (e.g., a single wavelength) and its small bandwidth center wavelength is selectable.

As shown in fig. 8, in an embodiment, the measurement device MT includes a spectrometer 28. The radiation switch 27 is configured to selectively divert at least a portion of the radiation to a spectrometer 28. The spectrometer 28 is configured to measure spectral components of the illuminating radiation. For example, the spectrometer 28 is configured to measure the wavelength range and distribution of the illuminating radiation. In an embodiment, the spectrometer outputs information received by the radiation source 32 and/or the wavelength selector 33 for providing feedback for controlling the wavelength of the illuminating radiation.

In an embodiment, the radiation switch 27 is configured to continuously transfer the portion of the illuminating radiation to the spectrometer 28. This allows for continuous feedback control of the wavelength of the illuminating radiation used for measurement. In an alternative embodiment, the radiation switch 27 is controlled so as to intermittently transfer at least a portion of said illuminating radiation to the spectrometer 28. This allows for intermittent feedback control of the wavelength of the illuminating radiation. The brightness of the illuminating radiation remains high when it is not transferred to spectrometer 28. Feedback may also be performed continuously rather than intermittently by splitting a small fraction (e.g., 0.1% to 1%) of the output of the wavelength selector 33 in a continuous manner.

As shown in FIG. 8, in an embodiment, the metrology apparatus MT comprises a beam dump 29. The radiation switch 27 is configured to selectively divert at least a portion of the radiation to the beam dump 29. The beam dump 29 is configured to absorb the radiation. The beam dump 29 may be used to prevent the irradiation radiation from being emitted by the irradiation branches 51 to 54 without switching off the radiation source 32. The beam dump 29 may be configured to act as a shutter for the illuminating radiation. However, it is not necessary that the metrology device MT includes a beam dump. For example only, the metrology device MT may not include the beam dump 29 but may have a radiation source 32 that may be turned on and off quickly.

As shown in fig. 8, in an embodiment, the metrology device MT comprises at least two energy sensors 55. Specifically, in an embodiment, each illumination branch includes an energy sensor 55. The energy sensor 55 is configured to measure the intensity of the radiation transmitted through the optical fiber 30 of the corresponding illumination branch 51 to 54.

Each illumination branch 51 to 54 is provided with a corresponding energy sensor 55. The energy sensor 55 helps to control the intensity of the illuminating radiation transmitted through the illuminating branches 51 to 54. In an embodiment, the energy sensor 55 outputs information for feedback control of the intensity of the radiation. For example, the power of the radiation source 32 may be controlled based on information received from the energy sensor 55.

As described above, in an embodiment, two illumination branches 51, 52 are used to measure the first grating (with lines in the X-direction). The other two illumination branches 53, 54 are used to measure a second grating (with lines in the Y-direction). The energy sensor 55 may be used to ensure that the intensity of the radiation transmitted by the two illumination branches 51, 52 for measuring the same grating is the same for both branches. Similar control is performed for both branches for the second raster. In another embodiment, the values measured by the energy sensor 55 are used to normalize the intensity measured on the sensor 19, making it possible to compare the measurements with each other more accurately. Embodiments of the present invention are contemplated to achieve greater accuracy of overlay measurements. This is because the superimposed signal is extremely sensitive to measuring the intensity difference between the two branches of the same grating.

As shown in fig. 8, in an embodiment, each illumination branch 51 to 54 comprises a beam splitter 61 for redirecting radiation towards the energy sensor 55. In an embodiment, the beam splitter 61 is configured to transmit a majority (e.g., about 99%) of the radiation and reflect only a minority (e.g., about 1%) of the radiation toward the energy sensor 55. As shown in fig. 8, the energy sensor 55 is optically located downstream of the optical fiber 30 of the illumination branches 51 to 54. This allows the energy sensor 55 to measure the intensity of the radiation output by the illumination branches 51 to 54. However, the energy sensor 55 may be positioned upstream of the optical fiber 30 (although the measurements made by the energy sensor will then not take into account losses in the optical fiber 30).

It is not necessary that the measuring device MT includes an energy sensor 55. For example, a radiation dose sensor may be used to measure the radiation received at the target TT.

As shown in FIG. 8, in an embodiment, the metrology apparatus MT includes at least two polarizers 56. In particular, in an embodiment, each illumination branch 51 to 54 comprises a polarizer 56. The polarizer 56 is configured to polarize the radiation transmitted through the corresponding illumination branch 51 to 54. As shown in fig. 8, in an embodiment, the polarizer 56 is optically downstream of the optical fiber 30 of the illumination branches 51 to 54. In an embodiment, the polarizer 56 is configured to polarize the illuminating radiation in an appropriate mode just before the radiation hits or hits the target TT.

In an embodiment, polarizer 56 can be controlled to allow transmission of selectable polarizations. For example, the mode of the polarizer may be switched to provide the appropriate polarization of the radiation. In an alternative embodiment, polarizer 56 does not control and provide a fixed polarization of radiation in this manner.

In an embodiment, as shown in FIG. 8, the metrology tool MT comprises a detection branch. The detection branch is configured to collect radiation diffracted and/or scattered from a structure TT on the substrate W. As shown in fig. 6, in an embodiment, the detection branch comprises a detection lens 16 and further optics 24. In an embodiment, the detection branch comprises a sensor 19. In an embodiment, the detection branch comprises a beam splitter.

In an embodiment, the detection branch comprises a processor configured to process data obtained by the sensor 19. In an embodiment, the processor is configured to implement a computational imaging algorithm for correcting aberrations in the diffracted radiation. In the embodiment, the optics 24 of the detection branch are simplified compared to the comparative example shown in fig. 5. Due to the simplified optics, there may be an increase of aberrations in the diffracted radiation. The computational imaging algorithm is used to correct for these aberrations due to the simplified optics.

In an embodiment, the detection branch is configured to collect non-zero diffraction order radiation. For example, in an embodiment, the detection branches 16, 24 are configured to collect the +1 diffraction order and the-1 diffraction order. In an embodiment, the detection branches 16, 24 are not configured to collect specularly reflected radiation.

In an embodiment, the illumination branches 51 to 54 are separate from the detection branches 16, 24. The illumination branches 51 to 54 do not share any components in common with the detection branches 16, 24.

As shown in fig. 8, in an embodiment, the metrology device MT comprises at least one image sensor 19. The image sensor 19 is configured to detect radiation diffracted from the structure TT on the substrate W. As shown in fig. 8, in an embodiment, the metrology device MT comprises a plurality of (e.g. two) image sensors 19. The image sensor 19 may be used to detect radiation in different regions of the complete spectrum. For example, one image sensor may be used to detect visible radiation and another image sensor 19 may be used to detect infrared radiation. As shown in fig. 8, in an embodiment the beam splitter 61 is arranged for splitting the collected radiation between different image sensors 19. However, it is not necessary that there be a plurality of image sensors. In an alternative embodiment, only one image sensor 19 is provided.

As explained above, the various components of the metrology apparatus MT are controllable. For example, the radiation source 32, the wavelength selector 33, the radiation switch 27 and the polarizer 56 may be controlled between different modes. In an embodiment, the metrology apparatus MT includes a controller 58 configured to control one or more of these components.

As explained above, in an embodiment, the metrology apparatus MT comprises one or more components for measuring aspects of radiation. For example, the spectrometer 28 and the energy sensor 55 may measure the wavelength distribution and intensity of the illuminating radiation being used for measurement. In an embodiment, the controller 58 receives information from these measurement components (e.g., the spectrometer 28 and the energy sensor 55). In an embodiment, the controller 58 generates a control signal based on the received information. This allows feedback control of the illuminating radiation for measurement.

FIG. 9 is a plan view of the optical fiber 30 of four separate illumination branches 51 to 54 illuminating a target TT located on a substrate W, according to an embodiment of the present invention. The view in fig. 9 is obtained when viewed along the normal to the target TT. The normal may be parallel to the optical axis O of the detection branches 16, 24 (as shown in fig. 6).

As shown in fig. 9, the optical fibers 30 of the illumination branches 51 to 54 are configured to illuminate the target TT from different angles when viewed along the normal to the target TT. In the example shown in fig. 9, the illumination branches 51 and 52 are used to measure a first set of gratings (with lines in the X-direction) of the target TT. The illumination branches 53 and 54 are used to measure a second grating (with lines in the Y-direction). The illumination branches 51 and 52 illuminate the target TT from complementary angles. This means that the angle β between the illumination branches 51 and 52 is 180 °. Similarly, the illumination branches 53 and 54 illuminate the target TT from complementary angles (i.e., opposite to each other).

In an embodiment, the illumination branches 51 to 54 are configured to illuminate the target TT from different angles evenly spaced apart from each other. For example, as shown in fig. 9, the angle α between adjacent illumination branches (e.g., illumination branches 51 and 54) is 90 °. This means that all four illumination branches 51 to 54 are located at angles evenly spaced from each other. There is a 90 ° angle between each pair of adjacent illumination branches. The illumination branches 51 and 52 are arranged to illuminate the target TT from opposite directions when viewed along a normal to the target TT. The illumination branches 53 and 54 are arranged to illuminate the target TT from opposite directions when viewed along a normal to the target TT.

In an embodiment, the metrology device MT is a diffraction-based scatterometer.

Further embodiments are disclosed in the subsequently numbered aspects:

1. a metrology apparatus for determining a characteristic of interest of a structure on a substrate, the metrology apparatus comprising:

a radiation source for generating illuminating radiation;

at least two illumination branches for illuminating the structure on the substrate, the illumination branches configured to illuminate the structure from different angles; and

a radiation switch configured to receive the illumination radiation and to divert at least a portion of the illumination radiation to a selectable one of the at least two illumination branches.

2. The metrology apparatus of aspect 1, wherein the radiation switch comprises a spatial light modulator.

3. The metrology apparatus of aspect 2, wherein the spatial light modulator comprises a micro-mirror device.

4. The metrology apparatus of aspect 1, wherein the radiation switch comprises an acousto-optic deflector.

5. The metrology apparatus of claim 4, wherein the radiation switch comprises a lens array configured to couple the beam of radiation from the acousto-optic deflector to an optical fiber corresponding to the illumination branch.

6. The metrology apparatus of claim 4, wherein the acousto-optic deflector is configured to change the direction of the radiation beams so as to control how many of the radiation beams are coupled to the optical fibers of the corresponding illumination branch.

7. The metrology apparatus of claim 4, wherein the radiation switch comprises a pockels cell configured to control a polarization direction of a radiation beam.

8. An apparatus according to claim 7, comprising a polarizing beam splitter configured to transmit or reflect a radiation beam depending on the polarization direction controlled by the pockels cell.

9. The metrology apparatus of aspect 1, wherein the radiation switch comprises at least one beam splitter and a plurality of shutters configured to control transfer of the at least a portion of the radiation to a selectable one of the at least two illumination branches.

10. A metrology apparatus in accordance with any preceding aspect, wherein the illumination branches are configured to illuminate the structure from different angles when viewed along a normal to the structure.

11. A metrology apparatus in accordance with any preceding aspect, wherein the illumination branches are configured to illuminate the structure from different angles evenly spaced from each other.

12. A metrology apparatus in accordance with any preceding aspect, wherein the illumination branch comprises at least one optical fibre for illuminating the structure.

13. The metrology apparatus of aspect 12, wherein the at least one optical fiber is used to directly illuminate the structure.

14. The metrology apparatus of aspect 12, wherein the at least one optical fiber is for indirectly illuminating the structure via at least one other optical element.

15. A metrology apparatus in accordance with any preceding aspect, the metrology apparatus comprising:

a wavelength selector configured to receive the illuminating radiation and transmit the illuminating radiation at a selected wavelength range, filtering out the illuminating radiation outside the selected wavelength range.

16. The metrology apparatus of claim 15, wherein the wavelength selector comprises an acousto-optic tunable filter configured to transmit the illuminating radiation of a selected wavelength range based on a frequency and/or power of an acoustic wave excited in the acousto-optic tunable filter.

17. The metrology apparatus of claim 16, comprising at least two acousto-optic tunable filters, at least one acousto-optic tunable filter arranged upstream of the radiation switch and at least one acousto-optic tunable filter arranged downstream of the radiation switch.

18. A metrology apparatus in accordance with any preceding aspect, the metrology apparatus comprising:

a spectrometer;

wherein the radiation switch is configured to selectively transfer at least a portion of the radiation to the spectrometer.

19. A metrology apparatus in accordance with any preceding aspect, the metrology apparatus comprising:

a beam collector;

wherein the radiation switch is configured to selectively divert at least a portion of the radiation to the beam dump.

20. A metrology apparatus in accordance with any preceding aspect, comprising at least four illumination branches.

21. A metrology apparatus in accordance with any preceding aspect, wherein the structure comprises a first grating of lines along a first direction and a second grating of lines along a second direction orthogonal to the first direction,

wherein at least two of the illumination branches are used to determine a characteristic of interest of the first grating from different angles and at least two of the illumination branches are used to determine a characteristic of interest of the second grating from different angles.

22. The metrology apparatus of aspect 21, wherein at least two of the illumination branches used to determine a characteristic of interest of the first grating are arranged to illuminate the structure from opposite directions when viewed along a normal to the structure.

23. The metrology apparatus of aspects 21 or 22, wherein at least two of the illumination branches for determining a characteristic of interest of the second grating are arranged to illuminate the structure from opposite directions when viewed along a normal to the structure.

24. A metrology apparatus in accordance with any preceding aspect, comprising at least two energy sensors corresponding to the at least two illumination branches, said energy sensors being configured to measure the intensity of the radiation transmitted through the corresponding illumination branches.

25. The metrology apparatus of any preceding aspect, comprising at least two polarizers corresponding to the at least two illumination branches, the polarizers being configured to polarize the radiation transmitted through the corresponding illumination branches.

26. The metrology apparatus of aspect 25, wherein the polarizer is controllable to allow transmission of selectable polarizations.

27. A metrology apparatus in accordance with any preceding aspect, comprising a detection branch configured to collect radiation diffracted and/or scattered from the structure on the substrate.

28. The metrology apparatus of aspect 27, wherein the detection branch is configured to collect non-zero diffraction order radiation.

29. The metrology apparatus of aspects 27 or 28, wherein the at least two illumination branches are separate from the detection branch.

30. A metrology apparatus in accordance with any preceding aspect, comprising at least one image sensor configured to detect the radiation diffracted from the structures on the substrate.

31. The metrology apparatus of any preceding aspect, comprising a controller configured to control the radiation switch and/or the radiation source.

32. The metrology apparatus of aspect 31, wherein when the metrology apparatus comprises a wavelength selector, the controller is configured to control the wavelength selector.

33. The metrology apparatus of aspects 31 or 32, wherein when the metrology apparatus comprises a polarizer, the controller is configured to control the polarizer.

34. The metrology apparatus of any one of aspects 31-33, wherein when the metrology apparatus comprises an energy sensor, the controller is configured to receive information from the energy sensor, wherein the controller generates a control signal based on the received information.

35. The metrology apparatus of any one of aspects 31-34, wherein when the metrology apparatus comprises a spectrometer, the controller is configured to receive information from the spectrometer, wherein the controller generates a control signal based on the received information.

36. A method for determining a parameter of interest of a structure on a substrate, the method comprising:

generating illuminating radiation;

receiving the illumination radiation at a radiation switch and diverting at least a portion of the illumination radiation to a selectable one of at least two illumination branches;

sequentially illuminating the structure from different angles by the at least two illumination branches;

collecting at least a portion of the diffracted radiation from the structure; and

a record of the collected diffracted radiation is received and obtained at the image sensor.

37. The method of aspect 36, comprising:

changing a wavelength of the illuminating radiation received at the radiation switch; and

sequentially illuminating the structure again with the illuminating radiation of the changed wavelength from different angles by the at least two illumination branches.

38. A lithographic cell comprising the metrology apparatus of one of aspects 1 to 35.

Further embodiments are disclosed in the subsequent list of aspects:

a.) a metrology apparatus for determining a characteristic of interest of a structure on a substrate, the metrology apparatus comprising:

a radiation source for generating illuminating radiation;

at least two illumination branches for illuminating the structure on the substrate, the illumination branches configured to illuminate the structure from different angles; and

a radiation switch configured to receive the illumination radiation and to divert at least a portion of the illumination radiation to a selectable one of the at least two illumination branches,

wherein the radiation switch comprises:

a pockels cell configured to control a polarization direction of the illumination radiation and to output a polarization-controlled illumination radiation, and

a polarizing beam splitter optically downstream of the Pockels cell and configured to transmit the polarization-controlled illumination radiation to a first of the at least two illumination branches or reflect the polarization-controlled illumination radiation to a second of the illumination branches depending on the polarization direction of the polarization-controlled illumination radiation.

b.) the measurement apparatus of aspect a, further comprising an upstream acousto-optic tunable filter optically arranged upstream of the pockels cell and configured to linearly polarize the illuminating radiation.

c.) the metrology apparatus of aspect a or b, further comprising a half-wave retarder arranged in one of the first and second illumination branches and configured to rotate the polarization direction of the polarization-controlled illumination radiation transmitted or reflected by the polarizing beam splitter in the one of the first and second illumination branches, respectively.

d.) the metrology apparatus of any one of the preceding aspects, wherein the illumination branches each comprise a respective downstream acousto-optic tunable filter arranged downstream of the radiation switch and configured to switch the respective illumination branch on and off.

e.) the metrology device of any one of the preceding aspects, further comprising a controller configured to receive data representative of a wavelength of the illuminating radiation and to control a voltage applied to the pockels cell 70 as a function of the wavelength of the illuminating radiation.

f.) the metrology apparatus of any preceding aspect, wherein the radiation switch comprises a spatial light modulator, and wherein optionally the spatial light modulator comprises a micro-mirror device.

g.) a metrology apparatus according to any preceding aspect, wherein the radiation switch comprises an acousto-optic deflector.

h.) the metrology apparatus of any preceding aspect, wherein the radiation switcher comprises at least one beam splitter and a plurality of shutters configured to control transfer of the at least a portion of the radiation to a selectable one of the at least two illumination branches.

i.) a metrology apparatus in accordance with any preceding aspect, wherein the illumination branches are configured to illuminate the structure from different angles when viewed along a normal to the structure.

j.) the metrology apparatus of any preceding aspect, wherein the illumination branches are configured to illuminate the structure from different angles that are evenly spaced apart from each other.

k.) a metrology apparatus according to any preceding aspect, wherein the illumination branch comprises at least one optical fiber for illuminating the structure, and wherein optionally the at least one optical fiber is for directly illuminating the structure or the at least one optical fiber is for indirectly illuminating the structure via at least one other optical element.

l.) a metrology apparatus according to any preceding aspect, the metrology apparatus comprising:

a wavelength selector configured to receive the illuminating radiation and transmit the illuminating radiation at a selected wavelength range, filtering out the illuminating radiation outside the selected wavelength range.

m.) a metrology apparatus according to any preceding aspect, the metrology apparatus comprising at least one of:

-a spectrometer, and wherein the radiation switch is configured to selectively transfer at least a portion of the radiation to the spectrometer, an

-a beam dump, and wherein the radiation switch is configured to selectively divert at least a portion of the radiation to the beam dump.

n.) the metrology device of any preceding aspect, comprising at least two energy sensors corresponding to the at least two illumination branches, the energy sensors being configured to measure the intensity of the radiation transmitted through the corresponding illumination branches.

o.) a metrology apparatus according to any preceding aspect, comprising at least two polarizers corresponding to the at least two illumination branches, the polarizers being configured to polarize the radiation transmitted through the corresponding illumination branches, and wherein optionally the polarizers are controllable to allow transmission of selectable polarizations.

p.) a metrology apparatus according to any preceding aspect, comprising a detection branch configured to collect radiation diffracted and/or scattered from the structure on the substrate.

q.) the metrology apparatus of aspect p), wherein at least one of:

-the detection branch is configured to collect non-zero diffraction order radiation,

-the at least two illumination branches are separate from the detection branch.

r.) a method for determining a parameter of interest of a structure on a substrate, the method comprising:

generating illuminating radiation;

receiving the illumination radiation at a radiation switch and diverting at least a portion of the illumination radiation to a selectable one of at least two illumination branches;

sequentially illuminating the structure from different angles by the at least two illumination branches;

collecting at least a portion of the diffracted radiation from the structure; and

a record of the collected diffracted radiation is received and obtained at the image sensor,

wherein the radiation switch comprises:

a pockels cell configured to control a polarization direction of the illumination radiation and to output a polarization-controlled illumination radiation, and

a polarizing beam splitter optically downstream of the Pockels cell and configured to transmit the polarization-controlled illumination radiation to a first of the at least two illumination branches or reflect the polarization-controlled illumination radiation to a second of the illumination branches depending on the polarization direction of the polarization-controlled illumination radiation.

s.) a lithography unit comprising a metrology apparatus according to one of claims a.) to q ].

Although specific reference is made herein to a "metrology device" or an "inspection device," both terms may also refer to an inspection device or an inspection system. For example, inspection or metrology equipment including embodiments of the present invention may be used to determine characteristics of structures on a substrate or wafer. For example, inspection equipment or metrology equipment including embodiments of the present invention may be used to detect defects in a substrate or structures on a substrate or wafer. In such embodiments, the characteristic of interest of a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate or on the wafer.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic 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 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 a mask (or other patterning device). These devices may be generally referred to as lithographic tools. Such a lithography tool may use vacuum conditions or ambient (non-vacuum) conditions.

Although the foregoing may make specific reference to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may not be limited to optical lithography, and may be used in other applications (e.g. imprint lithography), where the context allows.

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 above description is intended to be illustrative, and not restrictive. 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.