Method, measuring device and target for measuring parameters of patterning process

Abstract

The present disclosure relates to measuring parameters of a patterning process. In one arrangement, a target formed by the patterning process is illuminated. Sub-order diffraction components of radiation scattered from the target are detected and used to determine the parameters of the patterning process.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the priority of European application No. 18213270.4 filed on December 18, 2018 and European application No. 19162436.0 filed on March 13, 2019, the entire contents of these two European applications are incorporated herein by reference. Technical Field

[0003] This description relates to measuring parameters of a patterning process, such as overlay or focus. Background Art

[0004] Lithographic equipment is a machine that applies a desired pattern to a substrate (usually applied to a target portion of a substrate). Lithographic equipment can be used in the manufacture of, for example, integrated circuits (ICs) or other devices designed to be functional. In that case, a pattern forming device (which is alternatively referred to as a mask or reticle) can be used to produce a circuit pattern to be formed on a single layer of a device designed to be functional. This pattern can be transferred to a target portion (for example, a portion including a die, a die or several dies) on a substrate (such as a silicon wafer). The transfer of a pattern is usually performed via imaging onto a radiation-sensitive material (resist) layer disposed on a substrate. Typically, a single substrate will contain a network of adjacent target portions that are continuously patterned.

[0005] The manufacture of devices such as semiconductor devices typically involves processing a substrate (e.g., a semiconductor wafer) using a number of manufacturing processes to form the various features of the device and often multiple layers. These layers and features are typically manufactured and processed using, for example, deposition, photolithography, etching, chemical mechanical polishing, and ion implantation. Multiple devices can be made on multiple dies on a substrate, and then the devices are separated into individual devices. This device manufacturing process can be considered a patterning process. The patterning process involves a pattern transfer step, such as optical and / or nanoimprint lithography using a lithographic apparatus, to provide a pattern on the substrate and typically (but optionally) involves one or more related pattern processing steps, such as resist development by a developing apparatus, baking the substrate using a baking tool, and etching the pattern by an etching apparatus. In addition, one or more measurement processes are involved in the patterning process.

[0006] The metrology process can be used to monitor and / or control the patterning process. Various tools can be used to perform the metrology process, including various forms of scatterometers. These devices direct a radiation beam onto a metrology target and measure one or more characteristics of the scattered radiation. The one or more characteristics of the scattered radiation can yield information about parameters that cause asymmetry in the scattered radiation, such as alignment (overlay) errors and focus errors between different layers.

[0007] In order to minimize the space occupied by structures on the substrate that are used specifically as metrology targets, it is desirable to use relatively small metrology targets, or even to use as metrology targets portions of structures that are intended to form part of the final product being manufactured. Using metrology targets of similar size to product structures can also provide a better representation of product structures and improve metrology. However, reducing the size of the metrology target results in a corresponding reduction in the pitch of the structures in the metrology target. When the pitch becomes significantly shorter than the wavelength of illumination used for measurement, all diffraction orders above the zeroth order may become dissipated, which can interfere with conventional ways of obtaining information about the metrology target from scattered radiation.

[0008] In some metrology arrangements, in particular where the illumination of the target is highly coherent, unwanted interference effects such as fringes may be introduced into the image of the target. The interference effects may disturb the determination of parameters of a patterning process using the image. Summary of the invention

[0009] According to one aspect, a method for measuring parameters of a patterning process is provided, the method comprising: irradiating a target formed by the patterning process; detecting sub-order diffraction components of radiation scattered from the target; and determining the parameters of the patterning process using the detected sub-order diffraction components.

[0010] According to one aspect, a metrology device for measuring parameters of a patterning process is provided, the metrology device comprising: an optical system configured to illuminate a target with radiation and to direct radiation scattered from the target to a detector, wherein: the optical system and the detector are configured to detect sub-order diffraction components of the radiation scattered from the target.

[0011] According to one aspect, a target is provided for use in a method for determining parameters of a patterning process, wherein: the target includes a multilayer structure having a periodic structure formed in two or more layers of the multilayer structure; and the wavelength of a main period of the periodic structure is between 50nm and 400nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0013] Figure 1 schematically depicting an example lithographic apparatus;

[0014] Figure 2 schematically depicting an example lithographic cell or cluster;

[0015] Figure 3A is a schematic diagram of a measurement apparatus for use in measuring a target using a first pair of illumination apertures providing a selected illumination pattern;

[0016] Figure 3B is a schematic detail of the diffraction spectrum of the target for a given illumination direction;

[0017] Figure 3C is a schematic illustration of a second pair of illumination apertures providing an additional illumination mode for diffraction-based overlay measurements;

[0018] Figure 3D is a schematic illustration of a third pair of illumination apertures combining the first pair of apertures with the second pair of apertures;

[0019] Figure 4 Schematically depicting the form of a multi-periodic structure (e.g., multi-grating) target on a substrate and the contour of a measurement spot;

[0020] Figure 5 Schematic diagram of the device in FIG3 Figure 4 An image of the target;

[0021] Figure 6 schematically depicts an optical system of a metrology device using an illumination mode in which incident radiation is blocked by two diametrically opposed quadrants in a pupil plane;

[0022] Figure 7 schematically depicting an infinite grating having grating lines extending perpendicular to the X direction;

[0023] Figure 8 Schematically depicting the Figure 7 The diffraction spectrum of the grating;

[0024] Fig. 9 schematically depicting a finite grating having grating lines extending perpendicular to the X direction;

[0025] Fig.10 Schematically depicting the Fig. 9 The diffraction spectrum of the grating;

[0026] Fig.11 depicts a simulated diffraction spectrum obtained from a target T having an asymmetric structure;

[0027] Fig.12 Describing targets including periodic structures in the X direction;

[0028] Fig.13 and Fig.14 Schematic representation of the formation of sub-order diffraction components Fig.12 An image of a target T;

[0029] Fig.15 and Fig.16 Describe the passages separately Fig.13 and Fig.14The intensity of the edge-shaped local intensity area in the image varies with position;

[0030] Fig.17 depicting a target comprising four spaced-apart subtargets including corresponding periodic structures;

[0031] Fig.18 Describing a target including two spaced apart sub-targets comprising a periodic structure having the same orientation and relative offset;

[0032] Fig.19 Describing includes including Fig.18 A target having two spaced apart sub-targets with periodic structures in different directions and with relatively offset periodic structures;

[0033] Fig. 20 Describing a pupil plane region including a point-symmetric region in which a first sub-order diffraction component and a second sub-order diffraction component are respectively detected;

[0034] Fig.21 depicting a theoretical pupil plane region showing scattering of primary first order radiation where radiation is captured and where radiation is not captured;

[0035] Fig. 22 Depicts a theoretical pupil plane region showing how changes in the orientation of the illuminating radiation can control whether the principal first order component is captured;

[0036] Fig.23 Depicted is a theoretical pupil plane showing how variation in the wavelength of the illuminating radiation can control whether the main first order component is captured. DETAILED DESCRIPTION

[0037] Before describing the embodiments in detail, it is instructive to present an example environment in which the embodiments may be implemented.

[0038] Figure 1 A lithographic apparatus LA is schematically depicted. The apparatus comprises:

[0039] - an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or DUV radiation);

[0040] a support structure (e.g. mask table) MT configured to support a patterning device (e.g. mask) MA and connected to a first positioner PM configured to accurately position the patterning device according to certain parameters;

[0041] a substrate table (e.g. a wafer stage) WT configured to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate according to certain parameters; and

[0042] - a projection system (e.g. a refractive projection lens system) PS, which is configured to project the pattern imparted to the radiation beam B by the pattern forming device MA onto a target portion C (e.g. comprising one or more tube cores) of the substrate W, and the projection system is supported on a reference frame (RF).

[0043] The illumination system may include various types of optical components for directing, shaping, or controlling radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof.

[0044] The support structure supports the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions (such as whether the patterning device is held in a vacuum environment). The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure can be, for example, a frame or a stage, which can be fixed or movable as required. The support structure can ensure that the patterning device is in a desired position, for example relative to a projection system. Any use of the term "reticle" or "mask" herein can be considered synonymous with the more general term "patterning device".

[0045] The term "patterning device" as used herein should be broadly interpreted as referring to any device that can be used to impart a pattern in a target portion of a substrate. In an embodiment, the patterning device is any device that can be used to impart a pattern to a radiation beam in its cross-section so as to produce a pattern in the target portion of the substrate. It should be noted that, for example, if the pattern imparted to the radiation beam includes phase-shifting features or so-called assist features, the pattern may not exactly correspond to the desired pattern in the target portion of the substrate. Typically, the pattern imparted to the radiation beam will correspond to a specific functional layer in a device (such as an integrated circuit) produced in the target portion.

[0046] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography and include mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. An example of a programmable mirror array uses a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incident radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam reflected by the mirror matrix.

[0047] The term "projection system" as used herein should be broadly interpreted to cover any type of projection system suitable for the exposure radiation used or for other factors such as the use of an immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system".

[0048] The projection system PS has an optical transfer function which may be non-uniform and which may affect the pattern imaged onto the substrate W. For unpolarized radiation, these effects can be very well described by two scalar maps or scalar diagrams which describe the transmission (apodization) and relative phase (aberrations) of the radiation exiting the projection system PS as a function of position in the pupil plane. These scalar maps, which may be referred to as transmission maps and relative phase maps, may be expressed as linear combinations of a complete set of basis functions. A particularly suitable set is the Zernike polynomials, which form a set of orthogonal polynomials defined on the unit circle. The determination of each scalar map may involve determining coefficients in such an expansion. Since the Zernike polynomials are orthogonal on the unit circle, the Zernike coefficients may be determined by calculating the inner product of the measured scalar map with each Zernike polynomial in turn and dividing such inner product by the square of the norm of the Zernike polynomial.

[0049] The transmission mapping and relative phase mapping are field and system dependent. That is, typically, each projection system PS will have a different Zernike expansion for each field point (i.e., for each spatial location in the image plane of the projection system PS). The relative phase of the projection system PS in its pupil plane can be determined by projecting radiation from, for example, a point-like source in the object plane of the projection system PS (i.e., the plane of the pattern forming device MA) through the projection system PS and using a shearing interferometer to measure the wavefront (i.e., the trajectory of points with the same phase). The shearing interferometer is a common path interferometer and therefore, advantageously, no secondary reference beam is required to measure the wavefront. The shearing interferometer may comprise a diffraction grating (e.g., a two-dimensional grid) in the image plane of the projection system (i.e., the substrate table WT) and a detector arranged to detect an interference pattern in a plane conjugate to the pupil plane of the projection system PS. The interference pattern is related to the derivative of the phase of the radiation with respect to the coordinates in the pupil plane in the shear direction. The detector may comprise an array of sensing elements, such as, for example, a charge coupled device (CCD).

[0050] The projection system PS of the lithographic apparatus may not produce visible fringes, and therefore, a phase stepping technique (such as, for example, moving a diffraction grating) may be used to enhance the accuracy of the wavefront determination. Stepping may be performed in the plane of the diffraction grating and in a direction perpendicular to the scanning direction of the measurement. The stepping range may be one grating period, and at least three (uniformly distributed) phase steps may be used. Thus, for example, three scanning measurements may be performed in the y direction, each scanning measurement being performed for a different position in the x direction. Such stepping of the diffraction grating effectively transforms phase changes into intensity changes, allowing the phase information to be determined. The grating may be stepped in a direction perpendicular to the diffraction grating (z direction) to calibrate the detector.

[0051] The transmission (apodization) of the projection system PS in its pupil plane may be determined by projecting radiation, for example from a point-like source in the object plane of the projection system PS (i.e. the plane of the patterning device MA) through the projection system PS and using a detector to measure the intensity of the radiation in a plane conjugate to the pupil plane of the projection system PS. The same detector may be used as that used to measure the wavefront to determine the aberrations.

[0052] The projection system PS may include a plurality of optical (e.g., lens) elements and may further include an adjustment mechanism AM configured to adjust one or more of the optical elements in order to correct aberrations (phase variations across a pupil plane throughout the field). To achieve this, the adjustment mechanism is operable to manipulate one or more optical (e.g., lens) elements within the projection system PS in one or more different ways. The projection system may have a coordinate system in which the optical axis of the projection system extends in the z direction. The adjustment mechanism is operable to perform any combination of the following: displacing one or more optical elements; tilting one or more optical elements; and / or deforming one or more optical elements. The displacement of the optical element may be performed in any direction (x, y, z, or a combination thereof). The tilting of the optical element is typically performed outside a plane perpendicular to the optical axis by rotating around an axis in the x and / or y direction, but for non-rotationally symmetric aspheric optical elements, rotation around the z axis may be used. The deformation of the optical element may include low-frequency shapes (e.g., astigmatism) and / or high-frequency shapes (e.g., free-form aspheric surfaces). Deformation of the optical element may be performed, for example, by using one or more actuators to apply forces to one or more sides of the optical element and / or by using one or more heating elements to heat one or more selected regions of the optical element. In general, it may be possible to adjust the projection system PS to correct for apodization (variations in transmission across the pupil plane). When designing a patterning device (e.g., mask) MA for the lithographic apparatus LA, a transmission map of the projection system PS may be used. Using computational lithography techniques, the patterning device MA may be designed to at least partially correct for apodization.

[0053] As depicted here, the device is of the transmissive type (eg using a transmissive mask). Alternatively, the device may be of the reflective type (eg using a programmable mirror array of the type mentioned above or using a reflective mask).

[0054] The lithographic apparatus may be of a type having two (dual stage) or more tables (e.g. two or more substrate tables WTa, WTb, two or more patterning device tables, substrate table WTa and table WTb below the projection system without a dedicated substrate for, e.g., facilitating measurement and / or cleaning, etc.). In these "multi-stage" machines, the additional tables may be used in parallel, or preparatory steps may be performed on one or more tables while one or more other tables are being used for exposure. For example, alignment measurements using alignment sensor AS and / or level (height, inclination, etc.) measurements using level sensor LS may be performed.

[0055] The lithographic apparatus may also be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, so as to fill the space between the projection system and the substrate. Immersion liquid may also be applied to other spaces in the lithographic apparatus, such as the space between the patterning device and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of a projection system. The term "immersion" as used herein does not mean that structures such as substrates are necessarily immersed in the liquid, but only means that the liquid is located between the projection system and the substrate during exposure.

[0056] refer to Figure 1 , the illuminator IL receives a radiation beam from a radiation source SO. For example, when the source is an excimer laser, the source and the lithographic apparatus may be separate entities. In such a case, the source is not considered to form a part of the lithographic apparatus, and the radiation beam is transferred from the source SO to the illuminator IL by means of a beam delivery system BD comprising, for example, suitable guide mirrors and / or a beam expander. In other cases, for example, when the source is a mercury lamp, the source may be an integral part of the lithographic apparatus. The source SO and the illuminator IL together with the beam delivery system BD (where necessary) may be referred to as a radiation system.

[0057] The illuminator IL may include an adjuster AD configured to adjust the angular intensity distribution of the radiation beam. Typically, at least the outer radial extent and / or the inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator may be adjusted. In addition, the illuminator IL may include various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam to have a desired uniformity and intensity distribution in its cross-section.

[0058] The radiation beam B is incident on a patterning device (e.g. a mask) MA held on a support structure (e.g. a mask table) MT and is patterned by the patterning device. Having traversed the patterning device MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position sensor IF (e.g. an interferometric device, a linear encoder, a 2D encoder or a capacitive sensor), the substrate table WT can be accurately moved, for example, so that different target portions C are positioned in the path of the radiation beam B. Similarly, the first positioner PM and a further position sensor ( Figure 1 The support structure MT is provided with a plurality of actuators (not explicitly depicted in the figure) to accurately position the pattern forming device MA relative to the path of the radiation beam B. Typically, movement of the support structure MT can be achieved with the help of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning) which form part of the first positioner PM. Similarly, movement of the substrate table WT can be achieved using a long-stroke module and a short-stroke module which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the support structure MT may be connected only to the short-stroke actuator, or may be fixed. The pattern forming device MA and the substrate W may be aligned using pattern forming device alignment marks M1, M2 and substrate alignment marks P1, P2. Although the illustrated substrate alignment marks occupy dedicated target portions, the marks may be located in the space between target portions (these marks are called scribe alignment marks). Similarly, in the case where more than one die is arranged on the pattern forming device MA, the pattern forming device alignment marks may be located between the dies.

[0059] The depicted device can be used in at least one of the following modes:

[0060] 1. In step mode, the support structure MT and substrate table WT are held substantially stationary (i.e. a single static exposure) while the entire pattern imparted to the radiation beam is projected at one time onto a target portion C. The substrate table WT is then shifted in the X-direction and / or Y-direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

[0061] 2. In scan mode, the support structure MT and substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The speed and direction of the substrate table WT relative to the support structure MT may be determined by the (or less) magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width of the target portion in a single dynamic exposure (in the non-scanning direction), while the length of the scanning movement determines the height of the target portion (in the scanning direction).

[0062] 3. In another mode, the support structure MT is held substantially stationary, thereby holding the programmable patterning device, and the substrate table WT is moved or scanned, while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, a pulsed radiation source is typically used, and the programmable patterning device is updated as required after each movement of the substrate table WT, or between successive radiation pulses during a scan. This mode of operation may be readily applied to maskless lithography utilizing a programmable patterning device, such as a programmable mirror array of the type mentioned above.

[0063] Combinations and / or variations on the above-described modes of use or entirely different modes of use may also be employed.

[0064] like Figure 2 As shown, the lithographic equipment LA can form part of a lithographic cell LC (sometimes also referred to as a lithographic cell or cluster), which also includes equipment for performing pre-exposure processes and post-exposure processes on a substrate. Conventionally, these equipment include one or more spin coaters SC for depositing one or more resist layers, one or more developers DE for developing the exposed resist, one or more chill plates CH and / or one or more baking plates BK. The substrate transport device or robot RO picks up one or more substrates from the input / output ports I / O1, I / O2, moves the substrate between different process equipment and transfers the substrate to the feed station LB of the lithographic equipment. These equipment, often collectively referred to as coating and developing systems or tracks, are under the control of a coating and developing system control unit TCU, which itself is controlled by a management control system SCS, which also controls the lithographic equipment via a lithographic control unit LACU. Therefore, different equipment can be operated to maximize production volume and processing efficiency.

[0065] In order to correctly and consistently expose a substrate exposed by a lithographic apparatus, it is desirable to inspect the exposed substrate to measure or determine one or more characteristics, such as overlay (which may be, for example, between structures in superposed layers, or between structures in the same layer that have been separately provided to said layers by, for example, a double patterning process), line thickness, critical dimension (CD), focus offset or material properties. Therefore, the manufacturing facility in which the lithocell LC is located typically also comprises a metrology system MET, which receives some or all of the substrates W after having been processed in the lithocell. The metrology system MET may be part of the lithocell LC, for example, it may be part of the lithographic apparatus LA.

[0066] The measurement results may be provided directly or indirectly to the supervisory control system SCS. If an error is detected, the exposure of subsequent substrates (especially if the inspection can be completed quickly and quickly enough that one or more other substrates of the batch remain to be exposed) and / or the subsequent exposure of the exposed substrate may be adjusted. In addition, the exposed substrate may be stripped and reworked to improve the yield, or discarded, thereby avoiding further processing of substrates known to be defective. In the case where only some target portions of the substrate are defective, additional exposures may be performed only on those target portions that are good.

[0067] In the measurement system MET, the measurement device is used to determine one or more characteristics of the substrate, and in particular to determine how one or more characteristics of different substrates vary or how different layers of the same substrate vary between different layers. The measurement device can be integrated into the lithography equipment LA or the lithography unit LC, or can be a separate device. In order to achieve fast measurement, it is desirable to make the measurement device measure one or more characteristics of the exposed resist layer immediately after exposure. However, the latent image in the resist has a low contrast - that is, there is only a very small refractive index difference between the part of the resist that has been exposed to the radiation and the part of the resist that has not been exposed to the radiation - and not all measurement devices have sufficient sensitivity to perform useful measurements of the latent image. Therefore, the measurement can be performed after the post-exposure baking step (PEB), which is usually the first step performed on the exposed substrate and increases the contrast between the exposed part and the unexposed part of the resist. At such a stage, the image in the resist can be called a semi-latent image. It is also possible to measure the resist image after development - when either the exposed or unexposed portions of the resist have been removed - or after a pattern transfer step such as etching. The latter possibility limits the possibility of reworking a defective substrate, but can still provide useful information.

[0068] In order to achieve the measurement, one or more targets may be provided on the substrate. In one embodiment, the target is specially designed and may include a periodic structure. In one embodiment, the target is part of a device pattern, such as a periodic structure of the device pattern. In one embodiment, the device pattern is a periodic structure of a memory device (such as a bipolar transistor (BPT) or a bit line contact (BLC) structure).

[0069] In one embodiment, the target on the substrate may include one or more 1-D (one-dimensional) periodic structures (e.g., gratings) that are printed so that after development, the periodic structural features are formed by solid resist lines. In one embodiment, the target may include one or more 2-D (two-dimensional) periodic structures (e.g., gratings) that are printed so that after development, one or more periodic structures are formed by solid resist guide posts or vias in the resist. The bars, guide posts, or vias may alternatively be etched into the substrate (e.g., etched into one or more layers on the substrate).

[0070] In one embodiment, one of the parameters of interest in the patterning process is overlap. Overlap can be measured using dark field scatterometry, where the zero order diffraction (corresponding to specular reflection) is blocked and only higher orders are processed. Examples of dark field measurements can be found in PCT patent application publications WO 2009 / 078708 and WO 2009 / 106279, which are hereby incorporated by reference in their entirety. Further developments of the technology have been described in U.S. patent application publications US2011-0027704, US2011-0043791, and US2012-0242970, which are hereby incorporated by reference in their entirety. Diffraction-based overlap using dark field detection of diffraction orders enables overlap measurement of smaller targets. These targets can be smaller than the irradiation spot and can be surrounded by device product structures on the substrate. In one embodiment, multiple targets can be measured in one radiation capture.

[0071] Figure 3A A measurement device suitable for use in an embodiment for measuring, for example, overlay is schematically shown in FIG. Figure 3BTarget T (comprising a periodic structure such as a grating) and the diffracted rays are illustrated in more detail in . The measurement device may be a separate device or incorporated in a lithographic apparatus LA, for example at a measuring station or in a lithographic cell LC. An optical axis with several branches running through the apparatus is indicated by a dotted line O. In such an apparatus, radiation emitted by an output device 11 (e.g. a source such as a laser or a xenon lamp, or an opening connected to the source) is directed onto a substrate W via a prism 15 by an optical system comprising lenses 12, 14 and an objective lens 16. These lenses are arranged in a double sequence in a 4F arrangement. Different lens arrangements may be used, provided that they still provide an image of the substrate onto a detector.

[0072] In an embodiment, the lens arrangement allows access to the intermediate pupil plane for space-frequency filtering. Therefore, the angular range of radiation incident on the substrate can be selected by the spatial intensity distribution in the plane (here referred to as the (conjugate) pupil plane) that is limited to the spatial spectrum of the substrate plane. In particular, this selection can be made, for example, by inserting an aperture plate 13 of a suitable form between lenses 12 and 14 in the plane of the back-projected image of the objective lens pupil plane. In the illustrated example, two example aperture plates 13 (labeled as 13N and 13S) are shown, which allow different illumination modes to be selected respectively. The illumination system in the current example forms an off-axis illumination mode. In the first illumination mode, the aperture plate 13N provides off-axis illumination from the direction designated as "north" for the sake of description only. In the second illumination mode, the aperture plate 13S is used to provide similar illumination, but provides illumination from the opposite direction marked as "south". By using different holes, other illumination modes are possible. For example, an alternative aperture plate may be provided in which radiation is blocked in two opposing quadrants and passed through two other opposing quadrants, thereby providing the hereinafter mentioned Figure 6 The rest of the pupil plane is desirably dark, since any unwanted radiation outside the desired illumination pattern may interfere with the desired measurement signal.

[0073] like Figure 3BAs shown, the target T is placed so that the substrate W is approximately perpendicular to the optical axis O of the objective lens 16. The illumination ray I that is irradiated onto the target T at an angle offset from the axis O produces a zero-order ray (solid line 0) and two first-order rays (dash-dotted line +1 and dash-dotted line -1). In the case of a small target T with overfill, these rays are only one of many parallel rays that cover the substrate area including the measurement target T and other features. Because the hole in the plate 13 has a finite width (necessary to receive a useful amount of radiation), the incident ray I will actually occupy an angular range, and the diffracted rays 0 and +1 / -1 will be slightly spread out. Each order +1 and -1 will spread further across the angular range, rather than a single ideal ray as shown. It should be noted that the periodic structure pitch and the illumination angle can be designed or adjusted so that the first-order rays entering the objective are closely aligned with the central optical axis. Figure 3A and Figure 3B The rays illustrated in are shown slightly off-axis, just to make them easier to distinguish in the illustration. At least the 0th and +1st orders diffracted by the target on the substrate W are collected by the objective lens 16 and directed back through the prism 15.

[0074] Back to Figure 3A , both the first illumination mode and the second illumination mode are illustrated by specifying diametrically opposed apertures labeled North (N) and South (S). When the incident ray I is from the north side of the optical axis, that is, when the first illumination mode is applied using aperture plate 13N, the +1 diffracted ray labeled +1(N) enters objective lens 16. In contrast, when the second illumination mode is applied using aperture plate 13S, the -1 diffracted ray (labeled -1(S)) is the diffracted ray that enters lens 16. Therefore, in an embodiment, the measurement result is obtained by measuring the target twice under certain conditions (for example, after rotating the target or changing the illumination mode or changing the imaging mode to obtain the -1 diffraction order intensity and the +1 diffraction order intensity separately). Comparing these intensities for a given target provides a measure of the asymmetry in the target, and the asymmetry in the target can be used as an indicator of parameters (such as overlap) of the lithography process. In the situation described above, the illumination mode is changed.

[0075] The beam splitter 17 divides the diffraction beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zero-order diffraction beam and the first order diffraction beams to form a diffraction spectrum (pupil plane image) of the target on a first sensor 19 (e.g., a CCD or CMOS sensor). Each diffraction order hits a different point on the sensor so that image processing can compare and contrast several orders. The pupil plane image captured by the sensor 19 can be used to focus the metrology device and / or normalize the intensity measurement. The pupil plane image can also be used for other measurement purposes such as reconstruction.

[0076] In the second measurement branch, the optical systems 20, 22 form an image of the target on the substrate W on a sensor 23 (e.g., a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is arranged in a plane conjugate to the pupil plane of the objective lens 16. The aperture stop 21 is used to block the zero-order diffraction beam so that the image of the target formed on the sensor 23 is formed by the -1 or +1 order beam. Data about the image measured by the sensors 19 and 23 are output to the processor and controller PU, the functions of which will depend on the specific type of measurement being performed. It should be noted that the term "image" is used in a broad sense. Thus, if only one of the -1 order and the +1 order is present, an image of periodic structural features (e.g., grating lines) will not be formed.

[0077] The specific forms of aperture plate 13 and diaphragm 21 shown in FIG3 are examples only. In another embodiment, coaxial illumination of the target is used, and an aperture diaphragm with an off-axis hole is used so that substantially only one first order diffracted radiation is passed to the sensor. In still other embodiments, 2nd order beams, 3rd order beams and higher order beams (not shown in FIG3 ) may also be used in the measurement instead of or in addition to the first order beam.

[0078] In order to adapt the illumination to these different types of measurements, the aperture plate 13 may include a number of aperture patterns formed around a disk that is rotated to bring the desired pattern into position. It should be noted that aperture plates 13N or 13S are used to measure periodic structures of a target oriented in one direction (X or Y depending on the setup). For measuring orthogonal periodic structures, target rotations of up to 90° and 270° may be implemented. Figure 3C and Figure 3D Different orifice plates are shown in FIG. Figure 3C Two additional types of off-axis illumination modes are shown. Figure 3C In the first illumination mode, aperture plate 13E provides off-axis illumination from a direction designated as "East" relative to the previously described "North" for descriptive purposes only. Figure 3C In a second illumination mode, aperture plate 13W is used to provide similar illumination, but from the opposite direction labeled "West." Figure 3D Two additional types of off-axis illumination modes are shown. Figure 3D In a first illumination mode, aperture plate 13NW provides off-axis illumination from directions designated as "North" and "West" as previously described. In a second illumination mode, aperture plate 13SE is used to provide similar illumination, but from opposite directions designated as "South" and "East" as previously described. Use of these and numerous other variations and applications of the apparatus are described, for example, in the previously disclosed patent application publications referenced above.

[0079] Figure 4An example composite metrology target T formed on a substrate is depicted. The composite target comprises four periodic structures (gratings in this case) 32, 33, 34, 35 positioned closely together. In an embodiment, the periodic structure layout can be made smaller than the measurement point (i.e., the periodic structure layout is overfilled). Therefore, in an embodiment, the periodic structures are positioned closely enough together that they are all within the measurement spot 31 formed by the illumination beam of the metrology device. In that case, the four periodic structures are therefore all illuminated simultaneously and imaged simultaneously on the sensor 19 and the sensor 23. In an example dedicated to overlay measurement, the periodic structures 32, 33, 34, 35 are themselves composite periodic structures (e.g., composite gratings) formed by overlapping periodic structures, i.e., the periodic structures are patterned in different layers of a device formed on a substrate W and such that at least one periodic structure in one layer overlaps with at least one periodic structure in a different layer. The outer dimensions of such a target can be within 20 μm×20 μm or within 16 μm×16 μm. In addition, all periodic structures are used to measure the overlap between a specific layer pair. To facilitate the target being able to measure more than a single layer pair, the periodic structures 32, 33, 34, 35 may have overlap offsets that are biased differently so as to facilitate measurement of overlap between different layers formed with different portions of the composite periodic structure. Thus, all periodic structures used for a target on a substrate will be used to measure one layer pair, and all periodic structures used for another identical target on a substrate will be used to measure another layer pair, with the different biases facilitating differentiation between these layer pairs.

[0080] Back to Figure 4 , the periodic structures 32, 33, 34, 35 may also differ in their orientation (as shown) so as to diffract the incident radiation in the X direction and the Y direction. In one example, the periodic structures 32 and 34 are X-direction periodic structures with offsets of +d and -d, respectively. The periodic structures 33 and 35 may be Y-direction periodic structures with offsets of +d and -d, respectively. Although four periodic structures are illustrated, another embodiment may include a larger matrix to obtain the desired accuracy. For example, a 3×3 array of nine composite periodic structures may have offsets of -4d, -3d, -2d, -d, 0, +d, +2d, +3d, +4d. Discrete images of these periodic structures may be identified in the image captured by the sensor 23.

[0081] Figure 5 Shown in the use of Figure 3D The orifice plate 13NW or 13SE is used in the device of Figure 3. Figure 435. Although the sensor 19 cannot distinguish different individual periodic structures 32 to 35, the sensor 23 can distinguish different individual periodic structures 32 to 35. The dark rectangle represents the image field on the sensor, within which the illumination spot 31 on the substrate is imaged into the corresponding circular area 41. Within this field, the rectangular areas 42 to 45 represent the images of the periodic structures 32 to 35. Instead of being positioned in the scribe line or in addition to being positioned in the scribe line, the target can also be positioned in the device product feature. If the periodic structure is located in the device product area, the device feature can also be seen in the periphery of this image field. The processor and controller PU use pattern recognition to process these images to identify the discrete images 42 to 45 of the periodic structures 32 to 35. In this way, the image does not have to be aligned very accurately at a specific location within the sensor frame, which greatly improves the overall throughput of the measurement equipment.

[0082] Once the discrete images of the periodic structure have been identified, the intensity of those individual images can be measured, for example, by averaging or summing the intensity values ​​of selected pixels within the identified area. The intensity and / or other characteristics of the images can be compared to each other. These results can be combined to measure different parameters of the lithography process. Overlay performance is an example of such a parameter.

[0083] Figure 6 Describe the Figure 3A An alternative configuration of the second measurement branch of the embodiment of the present invention is provided, wherein an alternative aperture plate is arranged in the illumination path instead of aperture plate 13N or aperture plate 13S. The alternative aperture plate may be arranged at Figure 3A In order to avoid repetition, Figure 6 The illumination branch is not shown, but can be Figure 3A Any other configuration that is configured or provides optical functionality compatible with the embodiments described below may be used. The first measurement branch may or may not be present. Alternative aperture plate definitions are schematically depicted in Figure 6 , and is labeled 60 in the lower right part of the image. This illumination mode causes radiation not to be incident on the target T from two diametrically opposed quadrants 71 in the pupil plane, and causes radiation to be incident on the target T from the other two diametrically opposed quadrants 72 in the pupil plane. In combination with the target T comprising four periodic structures 32 to 35, this illumination mode results in zero diffraction rays, +1 diffraction rays and -1 diffraction rays in the image as shown by Figure 6The periodic structures 32 to 35 are separated in the pupil plane indicated by the axial view of the pupil plane radiation distribution labeled 74 in the distribution 74. The positions of the +1 and -1 orders of the periodic structures 32 and 34 (which are periodic in the X direction) are labeled X in the distribution 74. The positions of the +1 and -1 orders of the periodic structures 33 and 35 (which are periodic in the Y direction) are labeled Y in the distribution 74. All zero-order radiation from the periodic structures 32 to 35 remains in the quadrant labeled "0" in the distribution 74.

[0084] The group 80 of wedge-shaped optical elements 81 to 84 is arranged in the pupil plane instead of the aperture stop 21. The axial view of the group 80 of wedge-shaped optical elements is labeled 80'. In this example, the group 80 of wedge-shaped optical elements includes four wedge-shaped optical elements 81 to 84. When viewed along the optical axis of the optical system, each wedge-shaped optical element fills a quadrant of a circular area in the pupil plane. Each wedge-shaped optical element 81 to 84 has a lower planar surface and an upper planar surface, and is formed of an optical material having a refractive index different from 1. The lower planar surface is aligned obliquely (in a wedge-shaped manner) relative to the upper planar surface. In the example shown, the maximum gradient of each wedge-shaped optical element 81 to 84 is aligned in the radial direction, as indicated by Figure 6 Each wedge-shaped optical element 81 to 84 has a linear thickness that changes from the optical axis to the radially outermost edge of the wedge-shaped optical element 81 to 84. Each of the wedge-shaped optical elements 81 to 84 receives radiation in a corresponding one of the different quadrants of the pupil plane radiation distribution 74 and redirects the radiation so that an image of the target T is formed on the sensor 23 in a different location on the sensor 23. The axial view of the sensor 23 is in FIG. Figure 623' and shows an example positioning of images 91 to 94 formed by radiation passing through wedge-shaped elements 81 to 84, respectively. As mentioned above, the term "image" is used here in a broad sense. If some of the relevant diffraction orders are not present or not present, the image may not include all details of the target T (e.g. grating lines). In this example, images 91 and 93 correspond to images of the target T formed only by zero-order radiation. Image 92 includes a sub-image 92A formed by +1 diffraction rays from the periodic structure 32, a sub-image 92B corresponding to -1 diffraction rays from the periodic structure 33, a sub-image 92C corresponding to +1 diffraction rays from the periodic structure 34, and a sub-image 92D corresponding to -1 diffraction rays from the periodic structure 35. Image 94 includes a sub-image 94A formed by the -1 diffraction ray from the periodic structure 32, a sub-image 94B corresponding to the +1 diffraction ray from the periodic structure 33, a sub-image 94C corresponding to the -1 diffraction ray from the periodic structure 34, and a sub-image 94D corresponding to the +1 diffraction ray from the periodic structure 35.

[0085] Figure 6 The arrangement allows for simultaneous individual measurement of images of a plurality of periodic structures 32 to 35 in the target T formed by different diffraction components (eg, -1 diffraction rays, +1 diffraction rays, and zero-order rays).

[0086] As mentioned in the introduction to this specification, when the pitch of features in the target T (e.g., grating pitch) becomes significantly shorter than the wavelength of illumination used for measurement, diffracted components such as the +1 and -1 orders may become evanescent or appear at angles that are not practically captureable. Without adaptation, methods that rely on comparing the +1 and -1 orders to detect asymmetry (such as those described above with reference to FIGS. 3 to 4) may not be able to detect asymmetry. Figure 6 The discussed method) is no longer applicable. An alternative approach is to detect asymmetry in the pupil plane of the zeroth order (mirror) scattered radiation, but such asymmetry signals are generally weak. Another alternative approach is to detect asymmetry in the image of the target in a so-called image-based overlay, but when the pitch of features in the target T becomes significantly shorter than the illumination wavelength, the image is expected to disappear due to the loss of the main diffraction orders contributing to the spatial structure in the image.

[0087] The embodiments described below address the above challenges by exploiting the presence of sub-order diffraction components in the radiation scattered from the target T. Some of these embodiments use a method that is nominally configured to measure the +1 and -1 orders in the case where the +1 and -1 orders are dissipated or not captured (such as described above with reference to FIGS. 3 to 4). Figure 6Other embodiments implement image based overlap where the +1 and -1 orders are dissipated or not captured. Still other embodiments provide signal enhancement based on detecting asymmetry in the pupil plane.

[0088] Diffraction of light from any finite aperture causes the angular spread of the light. Figure 7 An infinite grating with grating lines 100 extending perpendicular to the X direction is schematically depicted. The width of each grating line 100 is d1. The pitch of the grating is d2. For an infinite grating such as depicted, which is a good approximation for targets that are large relative to the beam spot, the diffraction spectrum is a comb-like form of infinitely narrow peaks 102, such as Figure 8 Targeted Figure 7 The grating is schematically depicted. Figure 8 is a plot of the normalized intensity Int of the diffracted light versus the normalized k vector of the scattered light parallel to the X direction. The separation of the peaks 102 is defined by the ratio of wavelength to pitch λ0 / d2. The width of the central peak of the intensity envelope 104 is approximately the ratio of wavelength to the width d1 of each grating line 100 λ0 / d1.

[0089] Fig. 9 and Fig.10 Corresponding to Figure 7 and Figure 8 , the difference is Fig. 9 The grating has a width L x The finite grating is not an infinite grating. In a finite grating, each diffraction order is spread out along the wavenumber axis rather than being infinitely narrow. The width of each diffraction order is the ratio of the wavelength to the width of the grating (λ0 / L x ). In addition, sub-order diffraction components 106 appear between the main diffraction orders. These sub-order diffraction components contain information about the asymmetry in the target T and can be captured even when the main diffraction orders (e.g., +1 and -1 diffraction orders) are dissipated or not captured.

[0090] The main first-order peak 121- to the left of the main zero-order peak 120 is considered to be the main negative first-order peak 121-. Therefore, the main first-order peak 121+ to the right of the main zero-order peak 120 is the main positive first-order peak 121+. The intensity difference of the sub-order diffraction components located between the main zero-order peak 120 and the main negative first-order peak 121- is sensitive to asymmetry in the target (for example, asymmetry caused by overlap, asymmetry in the side wall angle, or focus error effects) compared to the sub-order diffraction components located between the main zero-order peak 120 and the main positive first-order peak 121+. The sub-order diffraction components can be considered to be the sum of the tails of the main diffraction order peaks that arise due to the broadening of those peaks due to the finite size of the target T. Even in the case of target T asymmetry, the tails originating from the main zero-order peak 120 are symmetrical on each side of the main zero-order peak 120. However, the tail from the main positive first order peak will be different from the tail from the main negative first order peak where there is an asymmetry in the target T. This is because the main positive first order peak 121+ and the main negative first order peak 121- are sensitive to the asymmetry in the target T. Therefore, Fig.10 In the direction, when the right tail of the main zero-order peak 120 is superimposed with the left tail of the main positive first-order peak 121+, the resulting sum (defining the sub-diffraction order 106 in such a zone) has a different form from the sum produced by the superposition between the left tail of the main zero-order peak 120 and the right tail of the main negative first-order peak 121-.

[0091] Fig.11 Depicted are simulated diffraction spectra for a single angle of incidence obtained from a target T having an asymmetric structure. The ratio of the wavelength of the illuminating radiation to the pitch of the grating in the target is five (5) so that the only major diffraction order captured by the measurement device (whose outer limits are indicated by the vertical dashed lines) that falls within the angular range (defined relative to the normalized angle NA) is the major zero-order peak 120. Neither the major negative first-order peak 121- nor the major positive first-order peak 121+ is captured. However, due to the finiteness of the grating in the target (the target contains 10 lines), the accompanying sub-order diffraction components (tails) 106 also fall within the NA limit. As described above, the sub-order diffraction components are formed by both the tail from the major zero-order peak 120 and the tails from the major positive first-order peak and the major negative first-order peak (which fall outside the numerical aperture). It is not possible to distinguish the tail of the major zero-order peak from the tails of the first-order peaks because they are combined as an incoherent sum to provide the sub-order diffraction components 106. In Fig.11 It can be appreciated that the amplitude (height) of the sub-order diffraction components 106 on the right side of the main zero-order peak 120 is different from the amplitude (height) of the sub-order diffraction components 106 on the left side of the main zero-order peak 120. This is caused by the difference in the contribution to the corresponding sub-order diffraction components from the (uncaptured) main first-order peak, and can be used to measure asymmetry in the target.

[0092] Based on the above principles, a method for measuring parameters of a patterning process is provided. The method comprises: irradiating a periodic structure in a target T. In an embodiment, the periodic structure comprises a grating. For example, the target T may be as described above with reference to, for example Figures 4 to 6 The target T is a target T formed by a patterning process. The patterning process may include a photolithography process performed using a photolithography device, as described above with reference to Figure 1 The method is described as follows. Sub-order diffraction components of radiation scattered from the periodic structure are detected. As described above, the sub-order diffraction components are sensitive to asymmetries in the periodic structure. The method uses the detected sub-order diffraction components to determine parameters of the patterning process.

[0093] The parameter of the patterning process can be any parameter to which the sub-order diffraction components are sensitive. In general, the parameter that contributes to the change in the symmetry of the target (e.g., the introduction or increase of asymmetry) can be measured. For example, the parameter can include the overlay error between different layers of the target, the side wall angle error in the target, or the focus error during the lithography imaging process.

[0094] In an embodiment, the sub-order diffraction components used in the method are mainly formed by radiation corresponding to a portion of the theoretical diffraction pattern from the periodic structure in the target T, which portion is located between and outside the main first-order peaks 121- and 121+, and outside the main zero-order peak 120 (e.g. Fig.10 depicted in ).

[0095] In an embodiment, the periodic structure includes a main period equal to the dimension of the repeating unit cell in the periodic direction of the periodic structure (for example, the pitch in the X direction of an X-direction periodic structure (such as a grating having lines extending in the Y direction)). The main zero-order peak and the main first-order peak are the main peaks of the theoretical diffraction pattern corresponding to the main period. The main zero-order peak extends from the maximum value of the main zero-order peak to the minimum value of the main zero-order peak (including the first minimum value encountered on each side of the maximum value of the main zero-order peak). Each main first-order peak extends from the maximum value of the main first-order peak to the minimum value of the main first-order peak (for each main first-order peak, including the first minimum value encountered on each side of the maximum value of the main zero-order peak).

[0096] In an embodiment, the periodic structure comprises a diffraction grating having a finite number of grating lines, such that a theoretical diffraction pattern from the grating comprises a plurality of principal peaks located at the same positions as principal peaks in a theoretical diffraction pattern from a diffraction grating having the same periodicity and an infinite number of grating lines. Sub-order diffraction components of the radiation are formed by radiation defined by the structure outside of the principal peaks in the theoretical diffraction pattern.

[0097] Embodiments employing sub-order diffraction components are particularly applicable to cases where the illumination used in the measurement method and periodic structure is such that radiation corresponding to the main first order peak of the theoretical diffraction pattern from the periodic structure either falls outside the numerical aperture of the optical system between the target and the detector or is dissipated. The method is particularly applicable to targets where the pitch of the main period of the periodic structure is between 50 nm and 400 nm, optionally between 50 nm and 300 nm, optionally between 50 nm and 200 nm, optionally between 100 nm and 400 nm, optionally between 200 nm and 400 nm.

[0098] In one class of embodiments, detection of the sub-order diffraction components includes separating radiation corresponding to the sub-order diffraction components from radiation corresponding to the main zero-order peak 120. This is advantageous because zero-order radiation may otherwise dominate the sub-order diffraction components and interfere with obtaining useful information from asymmetries in the sub-order diffraction components.

[0099] In an embodiment, the separation comprises directing the radiation corresponding to the main zero-order peak and the radiation corresponding to the sub-order diffraction components through separate corresponding regions in a pupil plane of the optical system between the target T and the detector 23. For example, the above description with reference to FIGS. 3 to 4 may be used. Figure 5 and Figure 6 This method is implemented with either of the two methods and measurement device configurations described. In each of the two methods, the +1 and −1 diffraction orders are separated from the zeroth order radiation in the pupil plane before reaching the detector 23 .

[0100] In use Figure 3A In an embodiment of the arrangement of , the aperture stop 21 blocks the zeroth order and prevents it from contributing to the image formed on the detector 23. In the case where the +1 and -1 orders are dissipated but the sub-order diffraction components are within the numerical aperture of the measurement device (e.g. Fig.11 In the case of a 3D image of the main zero-order peak 120, the image formed on the detector 23 will be formed only by the sub-order diffraction components. For example, by changing the illumination setting (e.g., exchanging aperture plate 12N for aperture plate 12S), it is possible to compare the image formed by the sub-order diffraction components 106 on one side of the main zero-order peak 120 with the image formed by the sub-order diffraction components 106 on the other side of the main zero-order peak 120.

[0101] In use Figure 6 In an embodiment of the configuration of , the asymmetry between the sub-order diffraction components can be obtained in the same way as the asymmetry between the +1 and -1 diffraction. In fact, Figure 6 Each +1 component indicated in is replaced by a corresponding sub-order diffraction component from the region of the diffraction pattern located between the main positive first-order peak 121+ and the main zero-order peak 120, and Figure 6Each -1 component indicated is replaced by a corresponding sub-order diffraction component 106 from the region of the diffraction pattern located between the main negative first order peak 121- and the main zeroth order peak 120.

[0102] exist Figures 12 to 16 The method is schematically depicted in . Fig.12 A target T is depicted that includes an X-direction periodic structure 32. The periodic structure 32 has an exemplary pitch of 200 nm and is irradiated with 650 nm radiation to produce a Fig.11 Diffraction spectra of the type depicted in and as discussed above. Fig.13 and Fig.14 Schematically depicting an image of a target T formed using sub-order diffraction components. Fig.13 The image shown in is formed by sub-order diffraction components 106 from the region of the diffraction pattern located between the main positive first order peak 121+ and the main zeroth order peak 120. Fig.14 The image shown in is formed by sub-order diffraction components 106 from the region of the diffraction pattern located between the main negative first order peak 121 − and the main zeroth order peak 120 . Fig.13 and Fig.14 Each of the images includes edge-shaped local intensity regions 131 and 132, which correspond to the sub-diffraction orders produced by the periodic structure 32 and extend parallel to the grating lines of the periodic structure 32. Each image also includes edge-shaped local intensity regions 133 and 134 extending orthogonally to the grating lines of the periodic structure 32. The edge-shaped local intensity regions 133 and 134 are produced due to the finite nature of the target T.

[0103] In this case, information about the asymmetry in the target T is encoded in the amplitude asymmetry of the edge-shaped local intensity regions 131 and 132. Fig.15 and Fig.16 Medium picture. Fig.15 Shows the intensity Int as the position X changes along Fig.13 The dotted line B +1 To B' +1 changes. Fig.16 Shows the intensity Int as the position X changes along Fig.14 The dotted line B -1 To B' -1 changes. Fig.15 The peaks 131 and 132 in Fig.16 The height difference compared to encodes information about the asymmetry in the target T. In an embodiment, Fig.15 The sum of peaks 131 and 132 depicted in Fig.16 The sum of peaks 131 and 132 depicted in FIG.

[0104] In cases such as the above example where target T is a composite target comprising multiple sub-targets, it is desirable to arrange the sub-targets in such a way that the images of the sub-targets formed by the sub-order diffraction components are spaced far enough apart that images from different sub-targets do not interfere with each other. In some embodiments, an example of which is depicted in Figures 17 to 19 In an embodiment, each sub-target includes a periodic structure 32 to 35 having a different nominal bias (e.g., a different overlap bias +d or -d) or a different orientation (e.g., an X-direction periodic structure or a Y-direction periodic structure) applied in terms of the parameters of the patterning process. In such an embodiment, the sub-targets may be spaced apart from each other by at least 25%, optionally at least 50%, optionally at least 100% of the maximum lateral dimension of each sub-target. Fig.17 The example depicts a target T comprising four sub-targets corresponding to periodic structures 32 to 35, which are Figure 4 The configuration of target T is similar, except that the individual periodic structures 32 to 35 are spaced farther apart from each other. Therefore, periodic structures 32 and 34 are X-direction periodic structures (schematically indicated by thick arrows) with biases of +d, -d, respectively. Periodic structures 33 and 35 are Y-direction periodic structures with biases of +d and -d, respectively. Each sub-target has a square with a side length of a. Therefore, in this example, the maximum lateral dimension of each sub-target is a. In the alternative case where each sub-target is a rectangle with a side length of a and b, the maximum lateral dimension will be the larger of a and b. Fig.17 The sub-targets in a are spaced by m. Therefore, m should be at least 25% of a. The total target size may correspond to typical target sizes for diffraction-based overlay measurements, such as to have outer dimensions within 20 μm x 20 μm or within 16 μm x 16 μm (i.e., such that 2a+m=20 μm or 16 μm). Fig.18 and Fig.19 Alternative targets T are shown, each comprising sub-targets having a periodic structure having the same orientation as one another but having equal and opposite biases to one another. Fig.18 X-direction periodic structures 32 and 34 are depicted with biases +d and -d. Fig.19 Y-direction periodic structures 33 and 35 are depicted with corresponding biases +d and -d.

[0105] Thus, in some embodiments, the detection of the sub-order diffraction components includes forming a first image and a second image. In an embodiment, a parameter of the patterning process is determined by comparing the first sub-order diffraction component to the second sub-order diffraction component. In some embodiments in which the detection of the sub-order diffraction components includes forming a first image and a second image, the first sub-order diffraction component is detected by measuring a characteristic of the first image (e.g., an average intensity or intensity of one or more edge features), and the second sub-order diffraction component is detected by measuring a characteristic of the second image (e.g., an average intensity or intensity of one or more edge features). In some embodiments, the first sub-order diffraction component may be a sub-order diffraction component formed primarily by radiation corresponding to a portion of the theoretical diffraction pattern that is closer to a main positive first-order peak than to a main negative first-order peak (e.g., corresponding to Fig.10 The second sub-order diffraction component 106 is to the right of the main zero-order peak 120 in FIG. 1 ). The second sub-order diffraction component may be formed primarily from radiation corresponding to a portion of the theoretical diffraction pattern that is closer to the main negative first-order peak than to the main positive first-order peak (e.g., corresponding to Fig.10 The sub-order diffraction component 106 to the left of the main zero-order peak 120 in FIG.

[0106] In the case where the patterning process includes errors in the patterning process (e.g., non-zero overlap), the comparison of the first sub-order diffraction component to the second sub-order diffraction component may include determining the deviation from the nominal relationship between the first sub-order diffraction component and the second sub-order diffraction component that is expected when the error is zero. In an embodiment, the average intensity of an image or a selected area of ​​the image (e.g., an edge area) formed by the first sub-order diffraction component is compared with the average intensity of an image or a selected area of ​​the image (e.g., an edge area) formed by the intensity of the second sub-order diffraction component. In an embodiment, the difference in average intensity is sensitive to the errors in the patterning process and can therefore be used to provide information about the errors. This method can be used in the embodiments described above, which use Figures 3 to 4. Figure 5 and Figure 6 method to detect the sub-order diffraction components.

[0107] In an alternative embodiment, the detection of the sub-order diffraction components comprises detecting the intensity or intensity distribution in the pupil plane. In an embodiment, the illumination mode is such that radiation is not incident on the target T from two diametrically opposed quadrants in the pupil plane, and radiation is incident on the target T from two other diametrically opposed quadrants in the pupil plane, as described above with reference to Figure 6 As discussed above, this may result in the separation of the zeroth order radiation from the sub-order diffraction components in the pupil plane radiation distribution. Fig. 20An example pupil plane region including four quadrants labeled 131, 132, 133, and 134 is depicted. In some embodiments of this type, the first sub-order diffraction component is detected by measuring the intensity or intensity distribution in a first selected region 141 of the pupil plane (the region between the circular boundary in the upper left portion of the depicted pupil plane region and the dashed line), and the second sub-order diffraction component is detected by measuring the intensity or intensity distribution in a second selected region 142 of the pupil plane (the region between the circular boundary in the lower right portion of the depicted pupil plane region and the dashed line). In an embodiment, the average intensity in the first selected region 141 is compared to the average intensity in the second selected region 142. In some embodiments of this type, the first selected region 141 and the second selected region 142 are point symmetric (e.g., in FIG. 1 ). Fig. 20 In this type of embodiment, the asymmetric contribution of the zero-order scattered radiation due to reciprocity-theorem effects complicates the underlying physics: if the structure is asymmetric, the zero-order scattered radiation in point-symmetric pixels of the pupil plane will have different intensities.

[0108] The presence of sub-order diffraction components can also be used to extend the applicability of image-based overlay to smaller pitches. In conventional image-based overlay, an image with positional asymmetry is produced that is sensitive to changes in the overlay in the target. By measuring the asymmetry in the image, information about the asymmetry in the target T can be obtained. When the main structure in the image is lost due to the main first-order peak becoming dissipated, the presence of sub-order diffraction components will mean that information about the asymmetry in the target still exists. This can be used to determine the parameters of the patterning process from the image. In some embodiments of this type, the detection of sub-order diffraction components includes forming an image in an image plane using the sub-order diffraction components, and determining the parameters of the patterning process by comparing a first portion of the image with a second portion of the image. In an embodiment, the comparison of the first portion of the image with the second portion of the image includes comparing the position of a feature in the first portion of the image with the position of a feature in the second portion of the image. For example, the compared features may include spatially well-defined features formed by the sub-order diffraction components, such as spatially local (such as δ) peaks. In an embodiment, the comparison of the first portion of the image with the second portion of the image includes obtaining the position of the center of gravity between the features. In other embodiments, a fitting routine is used to determine feature positioning.By monitoring changes in the position of monitored features (eg, the center of gravity discussed above), information about parameters of the patterning process (eg, overlay or focus) can be obtained.

[0109] In another class of embodiments, the concept of using information from sub-order diffraction components is extended to improve measurements that benefit from accurate knowledge of the location of the image of the target. For example, in situations where the intensity of the image of the target needs to be measured and the intensity is non-uniform due to the presence of interference effects (e.g., interference fringes), accurate knowledge of the location of the image of the target may be beneficial. For example, in the above reference Figures 3A to 5 In embodiments of the type discussed, overlap can be determined by comparing the intensities of separate images formed by the +1 and -1 diffracted radiation. If fringes are present within these images, the intensity in each image is non-uniform. The non-uniformity of each image complicates the derivation of single signal intensities from the images, which in turn complicates or compromises the derivation of overlap. This effect is more likely to be significant when highly coherent illumination patterns are used. If the positioning of the images of the target is accurately known, the impact of the effect can be mathematically reduced or removed.

[0110] The radiation diffracted from the target contains information about various aspects of the target, including information about the target pitch and overall shape (including the size and positioning of the image of the target). Depending on a variety of factors, such as the direction of illumination (illumination pupil position), the numerical aperture of the capture optical system, the wavelength of the radiation, the target pitch, and the target size, the captured image of the target may contain different combinations of information about the target. Fig.21 An example situation with reference to a point in a theoretical pupil plane is schematically depicted. The solid circle 150 represents the position at which the illumination radiation passes through the pupil plane (and hence the direction from which the illumination radiation is incident on the target). In the case where the illumination radiation has an angular spread, the solid circle 150 represents the axis or mean k-vector of the illumination radiation. The solid circle 152 represents the position at which the corresponding zero-order (specular) scattered radiation passes through the pupil plane. Zone 171 represents the numerical aperture of the optical system. Only the scattered radiation within zone 171 will be captured by the optical system and contribute to the detected image of the target. In the pupil plane, the separation between the center of the zero-order radiation at circle 150 and the center of the main first-order radiation at circle 161 is determined by the ratio of wavelength to target pitch (WL / p). The main first-order radiation (e.g., which is used to form an image from which information related to overlap is obtained) will be captured when it passes through zone 171. However, if the main first-order radiation falls outside zone 171 (as indicated by the open circle 162), only the sub-order diffraction components will be detected. These sub-order diffraction components contain information about the location of the image of the target, which information is to be used to obtain information about the target (eg formed by the radiation corresponding to the main non-zero order peak of the radiation scattered from the target).

[0111] In an embodiment based on the above principles, the detected sub-order diffraction components are used to determine the location of the image of the target on the substrate. The determined location of the image of the target is then used to analyze the intensity of the image of the target to determine parameters of the patterning process (e.g., overlap). In an embodiment, the image of the target is formed so that the image contains significant interference fringes (e.g., because the illumination pattern is highly coherent). In an embodiment, radiation corresponding to a major non-zero order peak of radiation scattered from the target (e.g., +1 or -1 diffracted radiation, as discussed above) is used. Figures 3A to 5 In an example of a patterning process (e.g., an image of the target is formed in a dark field mode, for example), wherein the main zero-order (specularly reflected) radiation is substantially prevented from contributing to the image. The determined positioning of the image of the target makes it possible to correct these fringes during the determination of the parameters of the patterning process (e.g., overlay), thereby improving reliability and / or accuracy.

[0112] In an embodiment, illumination of the target comprises two steps. In a first step, illumination is performed so as to avoid capturing any radiation corresponding to a main non-zero order peak of radiation scattered from the target (e.g., such that radiation corresponding to at least one center of a main first order peak in a theoretical diffraction pattern from the target falls within a region such as Fig.21 The position of the hollow circle 162 in FIG. 1 is outside the numerical aperture 171, rather than Fig.21 In a second step, the illumination is performed so as to allow the capture of radiation corresponding to the main non-zero order peak of the radiation scattered from the target (e.g., such that the radiation corresponding to the center of the main first order peak in the theoretical diffraction pattern from the target falls within a region such as Fig.21 The position of the hollow circle 161 in the figure is the interior of the numerical aperture 171, not Fig.21 The detected sub-order diffraction components are obtained during the first step of illumination, and an image of the target used to determine parameters of the patterning process (e.g., overlap) is obtained during the second step of illumination. The first step and the second step may be performed sequentially, wherein the first step is performed before the second step or the second step is performed before the first step.

[0113] The first and second steps of irradiating can be performed in various ways.

[0114] In the first method, Fig. 22 As schematically depicted in , avoiding capturing any radiation corresponding to a major non-zero order peak of radiation scattered from the target during the first step is achieved at least in part by using different orientations of illumination of the target (e.g., different angles of the illumination axis relative to the target plane) during the first step relative to the second step. Fig. 22Two alternative illumination orientations are depicted, resulting in zero-order (specular) reflections through solid circles 151 and 152 in the pupil plane, respectively. When the illumination radiation is directed through solid circle 151, the main first-order components fall outside the numerical aperture 171 at open circle 161 and are not captured. This illumination configuration is thus suitable for performing a first step of illumination. Conversely, when the illumination radiation is directed through solid circle 152, the main first-order components fall inside the numerical aperture 171 at open circle 162 and are captured. This illumination configuration is thus suitable for performing a second step of illumination.

[0115] In the second method, Fig.23 As schematically depicted in , during the first step, avoiding capturing any radiation corresponding to a major non-zero order peak of radiation scattered from the target is achieved at least in part by illuminating with radiation having a different wavelength during the first step relative to the second step. Fig.23 An example of illumination with three different wavelengths (WL1 to WL3) is depicted. In each case, the zeroth order (specular) reflection passes through the solid circle 152 in the pupil plane, while the difference in wavelength causes the main first order component to pass through a different corresponding one of three different zones in the pupil plane in each case, indicated by the open circles 161, 163 and 164. As indicated by the double arrows, the differences arise due to the different wavelengths causing corresponding differences in the ratio of wavelength to pitch. Therefore, when the wavelength is WL1, the main first order component is captured. This illumination configuration is thus suitable for performing the first step of illumination. Assuming that the differences are large enough to move the radiation outside the numerical aperture 171, the use of longer wavelengths (e.g. WL2) or shorter wavelengths (e.g. WL3) may result in the main first order component not being captured, as indicated by the corresponding open circles 163 and 164 being outside the numerical aperture 171. Any such illumination configuration will be suitable for performing the first step of illumination.

[0116] Additional embodiments according to the present invention are further described in the following numbered aspects:

[0117] 1. A method for measuring parameters of a patterning process, the method comprising:

[0118] irradiating a target formed by the patterning process;

[0119] detecting sub-order diffraction components of radiation scattered from the target; and

[0120] The detected sub-order diffraction components are used to determine the parameters of the patterning process.

[0121] 2. A method according to aspect 1, wherein the sub-order diffraction components are mainly formed by radiation corresponding to the part of the theoretical diffraction pattern from the target, which is located between and outside the main first-order peaks and outside the main zeroth-order peak.

[0122] 3. A method according to clause 2, wherein the detection of the sub-order diffraction components comprises separating radiation corresponding to the sub-order diffraction components from radiation corresponding to the main zero-order peak.

[0123] 4. A method according to aspect 3, wherein the separation includes: directing radiation corresponding to the main zero-order peak and radiation corresponding to the sub-order diffraction components through separate corresponding regions in a pupil plane of the optical system located between the target and the detector.

[0124] 5. A method according to clause 4, wherein the parameters of the patterning process are determined by comparing the first sub-order diffraction component with the second sub-order diffraction component.

[0125] 6. The method according to claim 5, wherein:

[0126] the first sub-order diffraction component is formed primarily of radiation corresponding to a portion of the theoretical diffraction pattern that is closer to the main positive first-order peak than to the main negative first-order peak; and

[0127] The second sub-order diffraction components are mainly formed by radiation corresponding to the part of the theoretical diffraction pattern that is closer to the main negative first order peak than to the main positive first order peak.

[0128] 7. A method according to aspect 5 or 6, wherein:

[0129] The parameters of the patterning process include errors in the patterning process; and

[0130] Comparing the first sub-order diffraction component to the second sub-order diffraction component includes determining a deviation from a nominal relationship between the first sub-order diffraction component and the second sub-order diffraction component that is expected when the error is zero, the deviation thereby providing information about parameters of the patterning process.

[0131] 8. A method according to any one of clauses 5 to 7, wherein the detection of the sub-order diffraction components comprises: detecting the intensity or intensity distribution in a pupil plane.

[0132] 9. The method according to aspect 8, wherein:

[0133] detecting a first sub-order diffraction component by measuring the intensity or intensity distribution in a first selected region of the pupil plane; and

[0134] The second sub-order diffraction component is detected by measuring the intensity or intensity distribution in a second selected area of ​​the pupil plane.

[0135] 10. The method of clause 9, wherein the first selected area and the second selected area are point-symmetric or mirror-symmetric with respect to each other.

[0136] 11. A method according to any one of clauses 5 to 7, wherein the detection of the sub-order diffraction components comprises: forming an image in an image plane using the sub-order diffraction components.

[0137] 12. The method according to claim 11, wherein:

[0138] The forming of the image includes: forming a first image and a second image;

[0139] detecting a first sub-order diffraction component by measuring a property of the first image; and

[0140] The second sub-order diffraction component is detected by measuring a property of the second image.

[0141] 13. The method according to aspect 1, wherein:

[0142] Detecting the sub-order diffraction components includes: forming an image in an image plane using the sub-order diffraction components; and

[0143] Parameters of a patterning process are determined by comparing the first portion of the image to the second portion of the image.

[0144] 14. A method according to clause 13, wherein the comparison of the first part of the image with the second part of the image comprises comparing the position of a feature in the first part of the image with the position of a feature in the second part of the image.

[0145] 15. A method according to clause 1, wherein the detected sub-order diffraction components are used to determine the location of the image of the target.

[0146] 16. The method of clause 15, further comprising: analyzing the intensity of the image of the target using the determined position of the image of the target to determine parameters of a patterning process.

[0147] 17. A method according to clause 16, wherein radiation corresponding to a major non-zero order peak of radiation scattered from the target is used to form an image of the target for determining parameters of a patterning process.

[0148] 18. The method according to aspect 17, wherein:

[0149] In a first step of irradiating the target, the irradiating is performed to avoid capturing any radiation corresponding to a main non-zero order peak of radiation scattered from the target; and

[0150] In a second step of illumination of the target, the illumination is performed so as to allow capturing radiation corresponding to a main non-zero order peak of radiation scattered from the target.

[0151] 19. A method according to clause 18, wherein the detected sub-order diffraction components are obtained during the first step of irradiating and an image of the target used to determine parameters of a patterning process is obtained during the second step of irradiating.

[0152] 20. A method according to aspects 18 or 19, wherein avoiding capturing any radiation corresponding to a major non-zero order peak of radiation scattered from the target during the first step is achieved at least in part by using differently oriented illumination of the target during the first step relative to the second step.

[0153] 21. A method according to any one of aspects 18 to 20, wherein avoiding capturing any radiation corresponding to a major non-zero order peak of radiation scattered from the target during the first step is achieved at least in part by irradiating with radiation having a different wavelength during the first step relative to the second step.

[0154] 22. A method according to any preceding aspect, wherein the target comprises a periodic structure.

[0155] 23. A method according to any preceding aspect, wherein:

[0156] The periodic structure includes a primary period equal to a dimension of a repeating unit cell in a periodic direction of the periodic structure;

[0157] The main zero-order peak and the main first-order peak are the main peaks of the theoretical diffraction pattern corresponding to the main period;

[0158] The main zero-order peak extends from a main zero-order peak maximum to a main zero-order peak minimum; and

[0159] Each main first order peak extends from a main first order peak maximum to a main first order peak minimum.

[0160] 24. A method according to aspect 22 or 23, wherein:

[0161] The periodic structure comprises a diffraction grating having a finite number of grating lines, such that a theoretical diffraction pattern from the diffraction grating comprises a plurality of main peaks located at the same positions as main peaks in a theoretical diffraction pattern from a diffraction grating having the same period and an infinite number of grating lines; and

[0162] Sub-order diffraction components of radiation are formed by radiation confined by structures outside the main peak in the theoretical diffraction pattern.

[0163] 25. The method according to any preceding clause, wherein parameters of the patterning process include focus errors or overlay errors between different layers of the target.

[0164] 26. A method according to any preceding aspect, wherein the illumination and periodic structure are such that radiation from the target corresponding to a main first order peak of a theoretical diffraction pattern falls outside the numerical aperture of an optical system between the target and a detector or is dissipated.

[0165] 27. A method according to any preceding clause, wherein the pitch of the major period of the target is between 50 nm and 400 nm.

[0166] 28. A method according to any preceding aspect, wherein:

[0167] The target includes a plurality of sub-targets, each sub-target including a periodic structure having a different nominal bias applied in the parameters of the patterning process or having a different orientation; and

[0168] The sub-targets are spaced apart from each other by at least 25% of the maximum lateral dimension of each sub-target.

[0169] 29. A measurement device for measuring parameters of a patterning process, the measurement device comprising:

[0170] An optical system configured to illuminate a target with radiation and to direct radiation scattered from the target to a detector, wherein:

[0171] The optical system and detector are configured to detect sub-order diffraction components of radiation scattered from the target.

[0172] 30. A target for use in a method of determining parameters of a patterning process, wherein:

[0173] The object includes a multilayer structure having a periodic structure formed in two or more layers of the multilayer structure; and

[0174] The wavelength of the main period of the periodic structure is between 50 nm and 400 nm.

[0175] 31. The object according to clause 30, wherein:

[0176] The target comprises a plurality of sub-targets, each sub-target comprising a periodic structure having a different nominal bias applied in the parameters of the patterning process or having a different orientation; and

[0177] The sub-targets are spaced apart from each other by at least 25% of the maximum lateral dimension of each sub-target.

[0178] Although specific reference may be made herein to the use of the metrology apparatus in IC manufacturing, it should be understood that the metrology apparatus and processes described herein may have other applications, such as manufacturing integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like. It should be understood by those skilled in the art that in the context of these alternative applications, any use of the term "wafer" or "die" herein may be considered synonymous with the more general term "substrate" or "target portion", respectively. The substrates referred to herein may be processed before or after exposure in, for example, a coating and developing system (a tool that typically applies a resist layer to a substrate and develops the exposed resist), a metrology tool, and / or one or more of various other tools. Where applicable, the disclosure herein may be applied to these and other substrate processing tools. In addition, a substrate may be processed more than once, for example to produce a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0179] Although specific reference may have been made above to the use of embodiments of the present disclosure in the context of optical lithography, it should be understood that the present disclosure may be used in other applications, such as nanoimprint lithography, and is not limited to optical lithography where the context permits. In the case of nanoimprint lithography, the patterning device is an imprint template or mold.

[0180] The terms "radiation" and "beam" as used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of at or about 365 nm, 355 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5 nm-20 nm); as well as particle beams (such as ion beams or electron beams).

[0181] The term "lens" may refer, where the context permits, to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components.

[0182] According to this specification, those skilled in the art will understand the modification and alternative embodiments of various aspects of the present invention. Therefore, this specification and the accompanying drawings should be understood to be illustrative only and for the purpose of teaching those skilled in the art to carry out the general mode of the present invention. It should be understood that the form of the present invention shown and described herein should be regarded as an example of an embodiment. Elements and materials can replace the elements and materials illustrated and described herein, parts and processes can be reversed or omitted, certain features can be used independently, and the features of embodiments or embodiments can be combined, and those skilled in the art will understand all of these after obtaining the benefits of this specification of the present invention. Without departing from the spirit and scope of the present invention as described in the appended claims, changes can be made to the elements described herein. The titles used herein are only for organizational purposes and are not intended to limit the scope of this specification.

Claims

1. A method for measuring parameters of a patterning process, the method comprising: irradiating a target formed by the patterning process; detecting sub-order diffraction components of radiation scattered from the target; as well as Parameters of the patterning process are derived from the detected sub-order diffraction components separated from the main zeroth-order diffraction component and the main first-order diffraction component of the radiation.

2. The method according to claim 1, wherein: The sub-order diffraction components are mainly formed by radiation corresponding to the portion of the theoretical diffraction pattern from the target which is located between and outside the main first order peaks and outside the main zero order peak.

3. The method according to claim 2, wherein: The detection of the sub-order diffraction components includes separating radiation corresponding to the sub-order diffraction components from radiation corresponding to the main zero-order peak.

4. The method according to claim 3, wherein: The separating comprises directing radiation corresponding to the principal zeroth order peak and radiation corresponding to the sub-order diffraction components through separate corresponding regions in a pupil plane of an optical system located between the target and a detector.

5. The method according to claim 4, wherein: Parameters of the patterning process are determined by comparing the first sub-order diffraction component with the second sub-order diffraction component.

6. The method according to claim 5, wherein: The first sub-order diffraction component is formed primarily of radiation corresponding to a portion of the theoretical diffraction pattern that is closer to the main positive first-order peak than to the main negative first-order peak; and The second sub-order diffraction component is mainly formed by radiation corresponding to a portion of the theoretical diffraction pattern that is closer to the main negative first-order peak than to the main positive first-order peak.

7. The method according to claim 5 or 6, wherein: The parameters of the patterning process include errors in the patterning process; and Comparing the first sub-order diffraction component with the second sub-order diffraction component comprises determining a deviation from a nominal relationship between the first sub-order diffraction component and the second sub-order diffraction component expected when the error is zero, the deviation thereby providing information about parameters of the patterning process.

8. The method according to claim 5 or 6, wherein: The detection of the sub-order diffraction components comprises: detecting the intensity or the intensity distribution in the pupil plane.

9. The method according to claim 8, wherein: detecting the first sub-order diffraction component by measuring the intensity or intensity distribution in a first selected region of the pupil plane; and The second sub-order diffraction component is detected by measuring the intensity or intensity distribution in a second selected area of ​​the pupil plane.

10. The method of claim 9, wherein the first selected area and the second selected area are point-symmetric or mirror-symmetric with respect to each other.

11. The method according to claim 5 or 6, wherein the detection of the sub-order diffraction components comprises: An image is formed in an image plane using the sub-order diffraction components.

12. The method according to claim 11, wherein: The forming of the image includes: forming a first image and a second image; detecting the first sub-order diffraction component by measuring a characteristic of the first image; and The second sub-order diffraction component is detected by measuring a property of the second image.

13. The method of claim 1, wherein: The detecting of the sub-order diffraction components comprises: forming an image in an image plane using the sub-order diffraction components; and Parameters of the patterning process are determined by comparing the first portion of the image to the second portion of the image.

14. The method according to claim 13, wherein: The comparison of the first portion of the image to the second portion of the image includes comparing the location of features in the first portion of the image to the location of features in the second portion of the image.

15. The method according to claim 1, wherein: The detected sub-order diffraction components are used to determine the location of the image of the target.

16. The method according to claim 15, further comprising: The intensity of the image of the target is analyzed using the determined location of the image of the target to determine parameters of a patterning process.

17. The method according to claim 16, wherein: Radiation corresponding to a major non-zero order peak of radiation scattered from the target is used to form an image of the target that is used to determine parameters of a patterning process.

18. The method of claim 17, wherein: In a first step of irradiating the target, the irradiating is performed to avoid capturing any radiation corresponding to a main non-zero order peak of radiation scattered from the target; and In a second step of illumination of the target, the illumination is performed so as to allow capturing radiation corresponding to a main non-zero order peak of radiation scattered from the target.

19. The method according to claim 18, wherein the detected sub-order diffraction components are obtained during a first step of irradiating and an image of the target is obtained during a second step of irradiating for determining parameters of a patterning process.

20. The method according to claim 18 or 19, wherein: Avoiding capturing any radiation corresponding to a major non-zero order peak of radiation scattered from the target during the first step is achieved at least in part by using differently directed illumination of the target during the first step relative to the second step.

21. The method according to claim 18 or 19, wherein: Avoiding capturing any radiation corresponding to a major non-zero order peak of radiation scattered from the target during the first step is achieved at least in part by illuminating with radiation having a different wavelength during the first step relative to the second step.

22. A method according to any one of claims 1 to 6, wherein the target comprises a periodic structure.

23. The method of claim 22, wherein: The periodic structure includes a primary period equal to a dimension of a repeating unit cell in a periodic direction of the periodic structure; The main zero-order peak and the main first-order peak are the main peaks of the theoretical diffraction pattern corresponding to the main period; The main zero-order peak extends from a main zero-order peak maximum to a main zero-order peak minimum; and Each main first order peak extends from a main first order peak maximum to a main first order peak minimum.

24. The method of claim 22, wherein: The periodic structure comprises a diffraction grating having a finite number of grating lines, such that a theoretical diffraction pattern from the diffraction grating comprises a plurality of principal peaks located at the same positions as principal peaks in a theoretical diffraction pattern from a diffraction grating having the same period and an infinite number of grating lines; and Sub-order diffraction components of radiation are formed by radiation confined by structures outside the main peak in the theoretical diffraction pattern.

25. The method according to any one of claims 1 to 6, wherein parameters of the patterning process include focus errors or overlay errors between different layers of the target.

26. The method of claim 22, wherein the illumination and periodic structure are such that radiation from the target corresponding to a major first order peak of a theoretical diffraction pattern falls outside a numerical aperture of an optical system between the target and a detector or is dissipated.

27. The method of claim 22, wherein the pitch of the major period of the target is between 50 nm and 400 nm.

28. The method according to any one of claims 1 to 6, wherein: The target includes a plurality of sub-targets, each sub-target including a periodic structure having a different nominal bias applied in the parameters of the patterning process or having a different orientation; and The sub-targets are spaced apart from each other by at least 25% of the maximum lateral dimension of each sub-target.

29. A measurement device for measuring parameters of a patterning process, the measurement device comprising: An optical system configured to illuminate a target with radiation and to direct radiation scattered from the target to a detector, wherein: The optical system and detector are configured to detect asymmetry between sub-order diffraction components of radiation scattered from the target.

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