Simultaneous capture of overlapping signals from multiple targets

Abstract

Embodiments of the present application relate to simultaneous capture of overlapping signals from multiple targets. The present invention provides metrology methods and systems in which a detected image is split into at least two pupil plane images at the field plane of the collection path of the optical system of the metrology system. An optical element such as a prism can be used to split the field plane image and multiple targets or target elements can be simultaneously measured by spatially splitting the field plane and / or the illumination source and / or by using two polarization types. Simultaneous capture of multiple targets or target elements increases the throughput of the disclosed metrology system.

Description

[0001] This application is a divisional application of an application filed on July 28, 2017, with application number "201780046742.X" and invention name "Simultaneously capturing overlapping signals from multiple targets". Technical Field

[0002] The present invention relates to the field of scatterometry overlay metrology, and more particularly to pupil plane optical metrology systems. Background Art

[0003] Scatterometric overlay (SCOL) metrology performs overlay measurements on gratings on grating targets, with each grating printed on a different process layer and typically comprising two X targets and two Y targets (X and Y refer to the perpendicular measurement direction), each with equal and opposite programmed overlay offsets. The overlay targets are measured by an optical system having an illumination path, typically employing illumination from a narrow spectral range filled with a subaperture having an illumination pupil, and a collection path, which directs light diffracted by the grating target to a detector(s). The overlay measurement is extracted by comparing the relative intensities of the relative diffraction orders (e.g., +1 and -1 first order diffraction signals) from each target. Figures 1 to 3 is a schematic illustration of a prior art metrology optical system for measuring a scatterometry overlay signal.

[0004] Figure 1 A pupil imaging system 70 is schematically illustrated, comprising an illumination path 61 with an illumination source 62 and a collection path 69 with a detector 66. A beam splitter 65 directs illumination onto a target 63 and directs the reflected signal to the collection path 69 through an objective 64. The optical planes in the pupil imaging system 70 are illustrated as pupil plane 71 and field plane 72, respectively, of the illumination path 61, and field plane 75 and pupil plane 76, respectively, of the collection path 69, defined relative to the target 63 and the objective pupil 74 at field plane 73 (shown schematically). Adjacent images illustrate the illumination pupil 71 (with small NA illumination), the illumination field stop 72 (or point scan coverage), the illuminated target 73, the collection pupil 74 (zeroth and ±1 first order diffraction), the collection field stop 75, and a typical image at the collection pupil 76 on the array sensor (detector) 66. Pupil imaging systems 70 typically employ illumination of a single target with scanned coherence points (71, 73), while collecting the +1st and -1st order diffraction on a single array sensor located at the pupil plane (76), and sequential illumination of additional targets using synchronized capture of the collection pupil - resulting in slow sequential capture of signals from four different targets and requiring movement of the wafer between measurements.

[0005] Figure 2A field imaging system 80 is illustratively shown, including an illumination path 61 with an illumination source 62 and a collection path 69 with a detector 66. A beamsplitter 65 directs illumination onto a target 63 and reflected signals through an objective 64 to the collection path 69. Optical planes in the field imaging system 80 are illustratively shown as pupil planes 81A, 81B (for sequential illumination) of the illumination path 61 and pupil plane 84 and field planes 85A, 85B (for sequential images) of the collection path 69, defined with respect to the target 63 at field plane 82 and objective pupil 83 (shown schematically). The adjacent image illustrates an illumination pupil 81A (with small NA illumination), an illuminated target 82, a collected pupil 83 (zeroth order diffraction, -1st order diffraction and -2nd order diffraction), a typical image at the aperture stop at relay collection pupil 84 and a target image 85A on an array sensor (detector) 66. In a subsequent illumination 81B, a subsequent target image 85B is produced on the array sensor (detector) 66. The field imaging system 80 typically employs illumination of one or more targets from a single tilt angle or the same target(s) with relative tilt angles (81A, 81B), collection of one first order diffraction from the target(s) (or a single relative order diffraction in the case of relative illumination (85A, 85B) while blocking all other orders of diffraction including the zeroth order reflection (84), and isolation of a single diffraction order from each target on the field plane array sensor. Thus, in the field imaging system 80, sequential diffraction capture from four different illumination AOIs (angles of incidence) is slow; the collection NA is small (typically up to 0.40 NA) to block zeroth and higher order diffractions - increasing the size of the collection PSF and causing contamination between adjacent targets and from objects in the periphery; the range of variable wavelengths available for a particular target pitch is limited due to the restrictions imposed on the illumination AOIs and collection AOIs; the ability to mask problematic areas within individual diffraction orders is excluded due to lack of angular resolution within the first order diffraction; and illumination of the targets overfills the field preventing simultaneous illumination of different targets with different polarizations. In this configuration, the spatial extent of the illumination on each target cell must extend beyond the boundaries of each cell. To have illumination with different polarization states on different target cells, there must be sufficient space between the cells so that the illumination for one cell does not contaminate the signal from an adjacent cell. The resulting large separation between cells and the difficulty of introducing spatially controlled polarization makes this practically infeasible.

[0006] Figure 3A field imaging system 90 with pupil division is schematically illustrated and includes an illumination path 61 with an illumination source 62 and a collection path 69 with a detector 66. A beam splitter 65 directs illumination onto a target 63 and directs a reflected signal to the collection path 69 through an objective 64. The field imaging system 90 includes a prism 67 at a pupil plane 94 of the collection path 69 that splits the pupil image to produce four field images on the detector 66. The optical planes in the field imaging system 90 are illustrated as pupil plane 91 of the illumination path 61 and pupil plane 94 and field plane 95 of the collection path 69, respectively, defined relative to the target 63 and the objective pupil 93 at field plane 92 (shown schematically). Adjacent images illustrate the illumination pupil 91, the illuminated target 92, a typical image at the collection pupil 93 (one of the zeroth and first order diffraction), the pupil image segmented by prism 67 at relay pupil plane 94, and the target image 95 on the array sensor (detector) 66. Each of the four target images is from a different pupil quadrant (due to the prism shape). Field imaging systems 90 with pupil partitioning require illumination of the target from two opposing pupil quadrants, at 45° relative to the target grating, and require a wavelength and grating pitch selection where the first order X and Y diffraction signals fall within the other two opposing quadrants of the collection pupil. Field imaging systems 90 with pupil partitioning isolate and separate the light in the four pupil quadrants via prisms and form four field images on a common sensor, each consisting of light from one quadrant of the collection pupil. Overlay measurements for each target are extracted from the intensity differences of the target formed by first order diffraction in the quadrants. These operating principles result in a large PSF, which induces contamination between adjacent targets and from peripheral objects, since each target image is formed by a single collection pupil quadrant (at approximately 0.45 NA). Furthermore, the combination of illumination in one pupil quadrant and collection in another quadrant limits wavelength and target pitch, the lack of angular resolution within the first order of diffraction precludes the ability to screen problematic areas within individual diffraction orders, and the illumination overfilling of the target makes simultaneous illumination of different targets with different polarizations practically unfeasible. Summary of the Invention

[0007] The following is a simplified summary to provide an initial understanding of the present invention. The summary does not necessarily identify key elements nor limit the scope of the invention, but merely serves as an introduction to the following description.

[0008] One aspect of the present invention provides a metrology system comprising an optical system having an illumination path and a collection path, wherein at least one detector is at a pupil plane of the collection path, wherein the optical system comprises at least one optical element at at least one field plane of its collection path, the at least one optical element being configured to split a detected image into at least two pupil plane images.

[0009] These additional and / or other aspects and / or advantages of the invention are set forth in the detailed description which follows; may be inferred from the detailed description; and / or may be learned by practice of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] For a better understanding of the embodiments of the invention and to show how the same may be practiced, reference will now be made, by way of example only, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout.

[0011] In the attached figure:

[0012] Figures 1 to 3 is a schematic illustration of a prior art metrology optical system for measuring a scatterometry overlay signal.

[0013] Figure 4 and 5 is a high-level schematic illustration of a pupil imaging optical system according to some embodiments of the present invention.

[0014] Figure 6 is a high-level schematic illustration of a pupil imaging optical system with independent polarization control of multiple target illuminations according to some embodiments of the present invention.

[0015] Figures 7 to 11 is a high-level schematic illustration of a pupil imaging optical system for simultaneously capturing multiple targets on a single common array sensor, according to some embodiments of the present invention.

[0016] Figure 12 , which further includes Figure 12A 、 Figure 12B and Figure 12C , which is a high-level schematic illustration of a system and corresponding schematic images of optical paths according to some embodiments of the present invention.

[0017] Figure 13 is a high-level flow chart illustrating methods according to some embodiments of the present invention. DETAILED DESCRIPTION

[0018] Before setting forth the detailed description, it may be helpful to set forth definitions of certain terms that will be used later in the text.

[0019] As used in this application, the term "target" refers to a periodic structure, typically but not limited to a target unit having a periodic structure (e.g., a grating) above a periodic structure (e.g., a grating). The terms target and target unit are used interchangeably, and the disclosed systems and methods are applicable to either, depending on the implementation details.

[0020] As used in this application, the terms "illumination path" and "collection path" refer to the non-overlapping portions of the optical system before and after the target, respectively.

[0021] The term "field plane" as used in this application refers to any optical plane that is optically conjugate to the object in the optical system, including for example the optical plane in which the image of the object is formed. The term "pupil plane" as used in this application refers to any optical plane that is equivalent to a Fourier plane with respect to the object in the optical image, in particular at the pupil plane in the collection path, each point corresponding to a specific angle of the diffracted light from the object, also referred to as a relay pupil plane.

[0022] The term "polarization" as used in this application refers to electromagnetic radiation in which the electromagnetic field oscillates in a specified direction, which can be with respect to a plane (linear polarization), with respect to a helix (circular polarization), or any other defined direction. The terms "S polarization" and "P polarization" as used in this application refer to any two distinguishable polarization modes, such as different (e.g. orthogonal) linear polarizations, linear and circular polarizations, etc. In particular, the prefixes "S" and "P" are used only to designate different polarization modes and are not limited to linear polarization modes defined with respect to a beamsplitter axis. Also, the polarization modes can be modified during operation.

[0023] In the following description, various aspects of the application are described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present application. However, it will also be apparent to one skilled in the art that the present application can be practiced without the specific details presented herein. Furthermore, well-known features can have been omitted or simplified in order not to obscure the present application. With specific reference to the drawings, it is to be understood that the

[0024] Before at least one embodiment of the present application is explained in detail, it is to be understood that the application is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The application is applicable to other embodiments that can be practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

[0025] Unless otherwise specifically indicated herein, it will be apparent from the following discussion that throughout this specification, discussions utilizing terms such as "process," "compute," "calculate," "determine," "enhance," or the like refer to the actions and / or processes of a computer or computing system, or similar electronic computing device, that manipulate and / or transform data represented as physical (e.g., electronic) quantities within the computing system's registers and / or memories into other data similarly represented as physical quantities within the computing system's memories, registers, or other such information storage, transmission, or display devices.

[0026] A metrology method and system are provided in which a detected image is split into at least two pupil-plane images at a field plane of a collection path of an optical system of the metrology system. Optical elements can be used to split the field plane images, and multiple targets or target units can be measured simultaneously by spatially splitting the field plane and / or illumination source and / or by using two polarization types. Simultaneously capturing multiple targets or target units increases the throughput of the disclosed metrology system.

[0027] Embodiments of the present invention provide efficient and economical systems, methods, and mechanisms for simultaneously measuring multiple overlapping targets with angular resolution within a diffraction order, forming an image of each target with high NA (numerical aperture) to minimize contamination, and more flexible spectral tuning. The disclosed systems and methods provide simultaneous SCOL target overlap measurement; simultaneous illumination of two or more grating-on-grating overlapping targets with multiple illumination beams for efficient illumination; isolation and separation of light from different targets at the collection field plane; forming pupil images from each target on a separate array sensor or spatially separated on a common array sensor (providing angular resolution within each diffraction order); and enabling simultaneous capture of relative diffraction efficiencies from multiple targets.

[0028] In addition, the disclosed systems and methods enhance measurement efficiency by isolating illumination to the interior of the target to minimize contamination between targets and from peripheral objects; providing target-controllable illumination polarization, possibly by collecting polarization to separate signals from different targets before isolation by field position; and implementing programmable illumination and collection field apertures to enable continuous measurement of more than four targets within a single field of view without the need for staged motion between measurements.

[0029] Advantageously, the disclosed systems and methods also allow for adjustment of the illumination AOI (angle of incidence) to achieve a wavelength range for a given target grating pitch; allow for collection of high NA (typically 0.93NA or higher) to minimize the size of the collection PSF (point spread function) and thereby minimize contamination between adjacent targets and from peripheral objects; provide angular resolution of the pupil image to apply angular weighted masking and remove problematic features in the pupil; and use the target's illumination underfill and achieve targeted control of illumination polarization. In particular, by spatially underfilling the illumination of the target cells, the polarization and wavelength of the illumination can be more easily varied for different target cells without risking inter-cell signal contamination.

[0030] Figures 4 to 13 These and other aspects of the present invention are illustrated in a non-limiting manner in the figures, and elements from these figures may be combined in any operable combination. The illustration of certain elements in some figures and not in other figures is for illustrative purposes only and is not limiting. Metrology systems and optical systems can be used to generate and measure the overlap between periodic structures at different target layers.

[0031] Figure 4 and 5 1 is a high-level schematic illustration of a pupil imaging optical system 100 according to some embodiments of the present invention. System 100 includes an illumination path 111 having an illumination source 62 and a collection path 119 having detectors 115A, 115B. A beam splitter 65 directs illumination onto two SCOL targets 113 (as an illustrative, non-limiting example) and directs the reflected signal to collection path 119 through objective lens 64. System 100 includes an optical element 110 at a field plane 104 of collection path 119 that splits the field image to produce multiple pupil images on detectors 115A, 115B. Optical element 110 is illustrated as a prism in a non-limiting manner and may include any optical component that enables isolation of pupil distributions generated by different target units (e.g., grating(s), digital microscope(s), etc.). Depending on implementation details, optical element 110 may be configured as reflective or refractive, as exemplified below.

[0032] The optical planes in system 100 are illustrated as pupil plane 101 of illumination path 111 and field plane 104 and pupil plane 105, respectively, of collection path 119, defined relative to target 113 and objective pupil 103 at field plane 102 (shown schematically). Adjacent images illustrate the illumination pupil 101, the illuminated target 113 (at field plane 102), a typical image at collection pupil 103 (zeroth and first order diffraction), the field image segmented by optical element 110 at field plane 104, and pupil images 105A, 105B on array sensors (detectors) 115A, 115B, corresponding to pupil images from the left and right targets in 102, respectively. Figure 5 In FIG. 1 , the system 100 includes a zero-order block 118 at the relay pupil plane 107 that removes the zero-order from the pupil image and pupil planes 105A, 105B.

[0033] In embodiments, any of the disclosed systems 100 may be configured to have any type of illumination coherence, such as incoherent illumination (e.g., illumination source 62 as an arc lamp, LDLS-laser driven light source, speckle laser), a fixed coherent point on the target (e.g., by a laser), and / or a scanned coherent point on the target (e.g., by a laser).

[0034] In embodiments, any of the disclosed systems 100 may be configured with any type of illumination wavelength spectrum, such as a fixed narrow spectrum, a selection from a limited number of discrete narrow spectrums, and / or a selection of a narrow spectrum from a source emitting a broad spectrum.

[0035] In embodiments, any of the disclosed systems 100 can be configured to have any type of light distribution in the illumination pupil (101), such as a fixed light distribution and / or an adjustable or programmable light distribution. In embodiments, any of the disclosed systems 100 can be configured to have any type of light cutoff in the illumination pupil 101, such as a hard aperture cutoff and / or an apodized (gradual) cutoff.

[0036] In embodiments, any of the disclosed systems 100 can be configured to have any type of division of illumination points in the illumination field plane, such as no division (target overfilling), a separate illumination source (fiber optic) specific to each target, a reflective or refractive prism placed before the illumination pupil 101 configured to generate a separate illumination point or patch on each target, and / or a diffraction grating (beam multiplier) in the illumination pupil 101 configured to generate a separate illumination point or patch on each target. Note that in a scanned coherent point configuration, the scanning mechanism may be common to the points of two or more targets.

[0037] In embodiments, any of the disclosed systems 100 can be configured to have any type of illumination polarization, such as a common and fixed illumination polarization over the entire illuminated field (101), a common illumination polarization over the entire illuminated field and switchable between measurements, different illumination polarizations for different targets that are unchangeable (non-switchable), and / or different and switchable illumination polarizations for different targets, the latter option being discussed below. Figure 6 Schematically illustrated in .

[0038] Figure 6 Figure 1 is a high-level schematic illustration of a pupil imaging optical system 100 with independent polarization control of illumination of multiple targets, according to some embodiments of the present invention. The illumination path 111 of system 100 can include an S-polarized illumination source 120-S and a P-polarized illumination source 120-P (e.g., an S-fiber and a P-fiber) with corresponding field stops 121-S and 121-P, respectively, and a polarizing beam splitter (PBS) configured to combine the S-polarized and P-polarized illumination. As illustrated in the adjacent images, in the illustrated configuration, four targets (target units) labeled X1, X2, Y1, and Y2 can be measured simultaneously (polarization directions are schematically illustrated at field plane 102 by lines on corresponding targets 113-S and 113-P). Optionally, illumination sources 120-S and 120-P can be at different wavelength ranges, simultaneously illuminating target 113 at objective field plane 102.

[0039] It should be explicitly noted that the terms "S-polarization" and "P-polarization" as used in this application refer to any two distinguishable polarization modes, such as different (e.g., orthogonal) linear polarizations, linear polarization and circular polarization, etc. In particular, the prefixes "S" and "P" are used only to designate different polarization modes and are not limited to defining linear polarization modes relative to the beam splitter axis. Although most common implementations may be linear horizontal polarization and linear vertical polarization at the chip, the actual difference in polarization states can be set in a flexible manner. For example, linear horizontal polarization can be used on one channel (e.g., "S") and circular left-hand polarization can be used on another channel (e.g., "P"). "S" and "P" can also designate left-hand circular polarization and right-hand circular polarization or any elliptical polarization. Any of the polarizing beam splitters described below can be replaced by a corresponding non-polarizing beam splitter and possible additional optical elements in the implementation besides the two orthogonal linear polarization states.

[0040] In embodiments, any of the disclosed systems 100 may be configured with any type of illumination field stop, such as no field stop, a single aperture that overfills the target, multiple apertures where each aperture underfills an individual target (see, e.g., Figure 6 and 12 as well as Figure 10The systems 100 described in

[0041] In embodiments, any of the disclosed systems 100 can be configured to have any type of collection pupil (103), such as full-aperture collection (typically greater than 0.90 NA), blocking (118) of only the zeroth order reflection (e.g., see Figure 5 、 7 and 8, and Figure 9 and 10 the systems 100 described in

[0042] In embodiments, any of the disclosed systems 100 can be configured to have any type of collection polarization, such as no polarization filter in the collection pupil 103, a polarization filter common to all targets in collection, and / or different and selectable polarization filters for different targets (e.g., see Figure 11 and 12 the systems 100 described in

[0043] Figures 7 to 11 is a high-level schematic illustration of a pupil imaging optical system 100 that simultaneously captures multiple targets on a single common array sensor 115, in accordance with some embodiments of the present disclosure. In Figure 7 and 8 In the systems 100 described in Figure 7In the figure, adjacent images illustrate the illumination pupil 101, the illuminated target 113 (labeled as X1, X2 at the field plane 102), the collection pupil 103 (zeroth and first order diffraction), a typical image at the relay pupil plane 107 with the zeroth order blocked, the field image segmented by the optical element 110 at the field plane 104, and the pupil image 105, including the pupil images of the two targets X1, X2 in 102 on the array sensor (detector) 115.

[0044] The optical element 110 is configured as Figure 7 The refracting prism and Figure 8 Reflecting prisms in (complete embodiment), where various types of prisms are Figure 8 As alternatives, for example, a reflective prism 110A, and diverging or converging refractive prisms 110B, 110C, respectively. It should be noted that the collection field 104 can be isolated from different targets or target units by any optical means for separating field plane images, such as refractive or reflective prisms 110 (respectively in FIG. Figure 7 、 11 to 12 and Figures 4 to 6 and 8 as non-limiting examples).

[0045] At field plane 104, the field stop may be absent, or the field stop may be a single aperture that is larger than the target. Alternatively or complementary, the collection field stop may include multiple apertures, each of which undersamples a separate target. In some embodiments, the collection field stop may include a single or multiple apertures that specifically block light from undesired targets, such as an S-polarized target on a P-polarized collection channel, as described below in Figure 12 Alternatively or complementary, the collection field stop may include a programmable aperture configured to enable continuous positioning of the aperture to measure more than four targets in a single FOV (field of view) without requiring stage movement.

[0046] Figure 9Schematically illustrates the simultaneous capture of signals of four targets 113 at the following locations on a single array sensor 115: an illumination pupil 101, a target plane 102 (X1, X2 designate target cells having a periodic structure along one direction (designated X), and Y1, Y2 designate target cells having a periodic structure along the other direction (designated Y, perpendicular to X)), a collection pupil plane 103 (designating the points of X zero order and Y zero order and ±1 X and Y first order), a relay pupil 107 with a zero order blocker 118 that removes the X zero order and Y zero order, and a pupil plane image 105 at the detector 115 separated into regions corresponding to the four targets (X1, X2, Y1, Y2) and their corresponding first order diffraction signals. The splitting is performed at the field plane 104 by the optical element 110 at the relay field plane 104. Figure 9 In , the cell space is overfilled by illumination, and the collected light is confined to the interior of the cell by a field stop on the collecting arm that is smaller than the cell.

[0047] Figure 10 Schematically illustrates the simultaneous capture of signals from four targets 113 on a single array sensor 115, where the isolated field illumination is at the illumination field stop 101A. Figure 10 In the CMOS image sensor, the cell is not spatially filled with illumination (either by an illumination field stop that is smaller than the cell or by point scanning that remains within the cell boundaries). The collection field stop (if present) is only used to limit stray light. The collection field stop does not play a role in confining light to the cell boundaries.

[0048] It should be noted that the sensor (detector) 115 may include a separate array sensor for each target (e.g. Figures 4 to 6 ) and / or separate array sensors with pupil images located on different areas of the sensor (e.g. Figure 7 、 8 , 10, 12).

[0049] exist Figure 11 In the case of , the target collection is additionally isolated by polarization. Figure 7 The design of the system 100 described in Figure 11 In FIG. 1 , the illumination path 111 includes an S-polarized illumination source 120-S and a P-polarized illumination source 120-P, each having a corresponding field plane aperture 121-S, 121-P, which are combined via a polarization beam splitter 123 to illuminate the target 113. At the collection path 119, after being split in the corresponding optical elements 110-S, 110-P (e.g., Figure 7), the reflected signal and the diffracted signal from the target 113 are split by the polarization beam splitter 125 at the pupil plane 107 to be directed to the detectors 115-S, 115-P at the pupil planes 105-S, 105-P, respectively.

[0050] Figure 12 is a high-level schematic illustration of the system 100 and corresponding schematic images of optical paths according to some embodiments of the present invention. Figure 11 , Figure 12 The capabilities of the system 100 are further enhanced by using: a plurality of S-polarized illumination sources 120-S and a plurality of P-polarized illumination sources 120-P, respectively (e.g., each having at least one single-mode optical fiber, two optical fibers being illustrated in a non-limiting manner), respective scanners 130-S, 130-P (e.g., X / Y piezoelectric scanners), respective pupil apodization apertures 131-S, 131-P at pupil planes 101-S, 101-P, respectively, and respective corresponding (non-polarizing) beam splitters 65-S, 65-P (which deliver corresponding illumination to the polarizing beam splitter 125 and receive corresponding collection beams from the polarizing beam splitter 125). Polarization beam splitter 125 combines illumination from different polarization sources and splits the reflected and diffracted signals from target 113 through beam splitters 65-S, 65-P and optical elements 110-S, 110-P to detectors 115-S, 115-P, respectively, via objective lens 64. Mirrors and additional optical elements can be used to optimize illumination path 111 and collection path 119, respectively.

[0051] Correspondingly, Figure 12 Schematic images further illustrate an X target illuminated and collected on the S-polarized channel (labeled 100-XS), a Y target illuminated and collected on the P-polarized channel (labeled 100-YP), and an embodiment having both an X target and a Y target illuminated and collected on the S-polarized channel (labeled 100-X,YS). The illumination field stops and / or single-mode spot positions on the S and P illumination channels can be used to define which target elements are illuminated by S or P light. Similarly, the field stops on the S and P collection channels can be used to define which target elements are measured using the S or P "analyzer." In this manner, any arrangement of illumination polarization and collection analyzers can be configured.

[0052] For 100-X-S and 100-Y-P, the illumination paths are illustrated by the images of the illumination exit 124-S and 124-P, the apertures 101-S, 101-P and the in-scan points within the field stops at the field planes 101A-S and 101A-P, configured to cause the targets 113 on the wafer to be simultaneously underfilled at the field plane 102 (illustrated schematically as XI, X2 and Yl, Y2, respectively). The corresponding images of the zeroth order diffraction and first order diffraction at the collection pupils 103 can be split by the corresponding collection field prisms 110-S, 110-P and delivered via the collection field stops 104-S, 104-P to simultaneously collect on the array sensors (detectors) 115-S, 115-P, respectively, to produce pupil images.

[0053] In 100-X,Y-S, four fiber exits are illustrated as the illumination exit 124-S, so that the pupil images of all four target cells are simultaneously collected on the detector 115-S. Similarly, four P-polarized illumination exits (not illustrated) can be used and the system 100 can be implemented using four single-mode linearly polarized fibers on two S-illumination channels 120-S and P-illumination channels 120-P, respectively, to generate four S-points and four P-points, each aligned to one of the four targets (target cells) 113. For any given target (cell) 113, either the S-fiber or the P-fiber can be controlled to emit light at a given time, and a common point scan (see scanners 130-S, 130-P, respectively) can be implemented on all S-points and on all P-points. As further illustrated in Figure 12 , the system 100 can include a common illumination aperture for the four S-beams and a common illumination aperture for the four P-beams, with a multi-aperture illumination field stop at the field plane 101A-S, 101A-P on the S-channel and P-channel, respectively, where each aperture is aligned with a separate target. The aperture for the target that is not illuminated can or can not be blocked. The field stop with the aperture at the relay field plane 104-S, 104-P on the S-collection channel and P-collection channel, respectively, can be configured to block the P-illumination targets on the S-collection channel and the S-illumination targets on the P-collection channel (as blocker 118, not shown in Figure 12 , see Figure 5 , 7 , 8). The refractive prisms 110-S, 110-P can be positioned at the field planes on the S-collection channel and P-collection channel, respectively, to separate the light from each of the targets and to locate the pupil images from each of the targets in separate quadrants of the S-array sensor 115-S and P-array sensor 115-P, respectively.

[0054] In some embodiments, the S-beam and the P-beam may be configured to have different spectral wavelengths. Alternatively or additionally, orthogonal target elements (e.g., in the X and Y directions) may be illuminated with different spectral wavelengths (simultaneously in the same objective field of view 102). For example, Figure 12 In FIG. 1 , four S-polarized illumination fibers 120 -S and four P-polarized illumination fibers 120 -P can illuminate each corresponding target with a different spectral wavelength, for example, illuminating an X-directed target and a Y-directed target with different (possibly partially overlapping) spectral wavelength ranges. Similar spectral differentiation can be implemented in (for example) Figure 6 and 11 In the embodiment described in .

[0055] Figure 13 is a high-level flow chart illustrating a method 200 according to some embodiments of the present invention. The method stages may be performed with respect to the system 100 described above, which may optionally be configured to implement the method 200. The method 200 includes splitting a detected image at a field plane of a collection path of an optical system of a metrology system into at least two pupil plane images (stage 210).

[0056] Method 200 may further include simultaneously capturing at least two pupil plane images by at least one detector of the metrology system (stage 220). Method 200 may include detecting the at least two pupil plane images by a single detector (stage 222) and / or by at least two corresponding detectors (stage 224). Method 200 may include simultaneously capturing at least four pupil plane images corresponding to at least four targets (stage 225).

[0057] Method 200 may further include configuring an illumination path of the optical system to have two perpendicularly polarized illumination sources (stage 230) and further include detecting at least two pupil plane images by at least two corresponding detectors (stage 232).

[0058] Method 200 may further include polarizing an illumination path and a collection path of the optical system (stage 240) by configuring the illumination path to include two perpendicularly polarized illumination sources (stage 230), and configuring the collection path to include two perpendicularly polarized detectors (stage 242), and further configuring each of the detectors to detect at least two pupil plane images generated from the corresponding polarized illumination (stage 244).

[0059] Method 200 may further include configuring each of the two vertically polarized illumination sources to include at least two illumination sources that are spatially separated in the field plane of the illumination path (stage 234). Optionally, the at least two illumination sources may be in different wavelength ranges.

[0060] Method 200 may further include scanning at least one of the illumination sources within at least one target cell (stage 250).

[0061] The method 200 may further include blocking the zeroth order diffraction at the pupil plane in the collection path (stage 260).

[0062] In the above description, an embodiment is an example or implementation of the present invention. The appearance of various "one embodiment", "embodiment", "certain embodiments" or "some embodiments" does not necessarily refer to the same embodiment. Although the various features of the present invention can be described in the context of a single embodiment, the features can also be provided separately or in any suitable combination. On the contrary, although the present invention can be described in the context of several separate embodiments for clarity, the present invention can also be implemented in a single embodiment. Certain embodiments of the present invention may include features from different embodiments disclosed above, and certain embodiments may incorporate elements from other embodiments disclosed above. Disclosing the elements of the present invention in the context of a particular embodiment should not be considered as limiting its use in the particular embodiment. In addition, it should be understood that the present invention can be implemented or practiced in various ways, and the present invention can be implemented in certain embodiments other than the embodiments outlined in the above description.

[0063] The present invention is not limited to those figures or corresponding descriptions. For example, the process does not have to proceed along each box or state described, or in exactly the same order as described and illustrated. Unless otherwise defined, the meaning of the technical and scientific terms used herein should be the meaning commonly understood by those skilled in the art to which the present invention belongs. Although the present invention has been described with respect to a limited number of embodiments, these embodiments should not be interpreted as limiting the scope of the present invention, but should be used as illustrations of some preferred embodiments. Other possible variations, modifications and applications are also within the scope of the present invention. Therefore, the scope of the present invention should not be limited by what has been described so far, but by the appended claims and their legal equivalents.

Claims

1. A metering system comprising: An optical system having an illumination path and a collection path, and having at least one detector at a pupil plane of the collection path, wherein the optical system includes at least one optical element at at least one field plane of the collection path, the at least one optical element being configured to split a detected image into at least two pupil plane images, the at least two pupil planes respectively corresponding to different targets, wherein the field plane is optically conjugate with the target on the wafer in the optical system, and wherein the pupil plane is equivalent to a Fourier plane with respect to the target in the at least two pupil plane images.

2. The metrology system of claim 1 , wherein the at least two pupil plane images are detected by a single detector of the optical system.

3. The metrology system of claim 1 , wherein the at least two pupil plane images are detected by at least two corresponding detectors of the optical system. 4 . The metrology system of claim 1 , wherein the at least two pupil plane images include at least four pupil plane images corresponding to at least four targets simultaneously captured by the optical system.

5. The metrology system of claim 1 , wherein the illumination path comprises two perpendicularly polarized illumination sources, and the at least two pupil plane images are detected by at least two corresponding detectors of the optical system.

6. The metrology system of claim 1 , wherein the illumination path and the collection path are polarized, the illumination path comprising two perpendicularly polarized illumination sources, and the collection path comprising two perpendicularly polarized detectors, each perpendicularly polarized detector detecting at least two pupil plane images generated from corresponding polarized illumination.

7. The metrology system of claim 6, wherein each of the two vertically polarized illumination sources comprises at least two illumination sources spatially separated in the field plane of the illumination path.

8. The metrology system of claim 7, wherein the at least two illumination sources are in different wavelength ranges.

9. The metrology system of claim 6, further comprising at least one scanner configured to scan at least one of the illumination sources within at least one target cell.

10. The metrology system of claim 6, further comprising an apodization aperture located at a pupil plane of the illumination path.

11. The metrology system of claim 1 , further comprising at least one zero-order blocker located at a pupil plane in the collection path.

12. A measurement method comprising: A detected image at a field plane of a collection path of an optical system of a metrology system is split into at least two pupil plane images via at least one optical element at the field plane, the at least two pupil planes corresponding to different targets, respectively, wherein the field plane is optically conjugate with the target on the wafer in the optical system, and wherein the pupil plane of the collection path is equivalent to a Fourier plane with respect to the targets in the at least two pupil plane images.

13. The metrology method of claim 12, further comprising simultaneously capturing the at least two pupil plane images by at least one detector of the metrology system.

14. The metrology method of claim 12, further comprising detecting the at least two pupil plane images by a single detector.

15. The metrology method of claim 12, further comprising detecting the at least two pupil plane images by at least two corresponding detectors. 16 . The metrology method of claim 12 , wherein the at least two pupil plane images include at least four pupil plane images corresponding to at least four targets simultaneously captured by the metrology system.

17. The metrology method of claim 12, further comprising configuring an illumination path of the optical system to have two perpendicularly polarized illumination sources, and further comprising detecting the at least two pupil plane images by at least two corresponding detectors.

18. The metrology method of claim 12, further comprising polarizing an illumination path and a collection path of the optical system by configuring the illumination path to include two perpendicularly polarized illumination sources and configuring the collection path to include two perpendicularly polarized detectors, and further configuring each of the detectors to detect at least two pupil plane images generated from the corresponding polarized illumination.

19. The metrology method of claim 18, wherein the two vertically polarized illumination sources are in different wavelength ranges.

20. The metrology method of claim 18, further comprising configuring each of the two vertically polarized illumination sources to comprise at least two illumination sources spatially separated in the field plane of the illumination path.

21. The metrology method of claim 18, further comprising scanning at least one of the illumination sources within at least one target cell.

22. The metrology method of claim 12, further comprising blocking the zeroth order diffraction at a pupil plane in the collection path.

23. The metrology method of claim 12, further comprising underfilling the target on the wafer during illumination of the target.

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