A substrate comprising a target arrangement, and associated at least one patterning device, lithographic method and metrology method

Abstract

Disclosed is a method of measuring a focus parameter from a focus target, and associated substrate and associated patterning device. The focus target comprises at least a first sub-target and a second sub-target, each having at least a periodic main feature, wherein a respective pitch and / or dimensional parameter of at least some sub-elements of the main feature are configured such that said first sub-target and second sub-target have a respective different best focus value; and wherein each said main feature is formed with a focus dependent center-of-mass and / or pitch. The method comprises obtaining a first measurement signal from said first sub-target and a second measurement signal from said second sub-target; determining a difference signal of said first measurement signal and second measurement signal; and determining said focus parameter from said difference signal.

Description

[Technical Field]

[0001] This invention relates to a target configuration for a measurement system for a photolithography program, and to a method for measuring parameters of a photolithography program. [Previous Technology]

[0002] Lithography equipment is a machine that applies a desired pattern onto a substrate (typically onto a target portion of the substrate). Lithography equipment can be used, for example, in the manufacture of integrated circuits (ICs). In this case, a patterning device (which is alternatively called a mask or magnifying mask) can be used to generate a circuit pattern to be formed on individual layers of the IC. This pattern can be transferred to a target portion (e.g., including a portion of a die, a die, or several dies) on a substrate (e.g., a silicon wafer). Pattern transfer is typically performed by imaging onto a radiation-sensitive material (resist) layer provided on the substrate. Generally, a single substrate will contain a network of sequentially patterned adjacent target portions. During the lithography process, the resulting structure needs to be frequently measured, for example, for process control and verification. Various tools are known for performing such measurements, including scanning electron microscopes, which are often used to measure critical dimensions (CD), and specialized tools used to measure overlap (a measure of the alignment accuracy of two layers in a device). Overlap can be described based on the degree of misalignment between two layers; for example, a reference to a measured overlap of 1 nm can describe a misalignment of 1 nm between two layers.

[0003] Recently, various types of scatterometers have been developed for use in the field of lithography. These devices guide a beam of radiation onto a target and measure one or more properties of the scattered radiation—for example, intensity varying with wavelength at a single reflection angle; intensity varying with reflection angle at one or more wavelengths; or polarization varying with reflection angle—to obtain a "spectrum" that allows for the determination of the target's properties of interest. The determination of the properties of interest can be performed using various techniques: for example, target reconstruction via iterative approaches such as tight coupled-wave analysis or finite element methods; library search; and principal component analysis.

[0004] Conventional scatterometers use relatively large gratings, such as 40 μm by 40 μm, and the measurement beam produces a spot smaller than the grating (i.e., underfilled grating). This simplifies the mathematical reconstruction of the target because the target can be considered infinite. However, to reduce the size of the target, for example to 10 μm by 10 μm or smaller, so that it can be positioned within a product feature rather than a cut, a metric has been proposed that makes the grating smaller than the measurement spot (i.e., overfilled grating). Such targets are typically measured using dark-field scattering measurements, which block zeroth-order diffraction (corresponding to specular reflection) and only process higher orders. Examples of dark-field metrics can be found in International Patent Applications WO 2009 / 078708 and WO 2009 / 106279, the entire contents of which are hereby incorporated by reference. Further developments of the technology have been described in patent publications US20110027704A, US20110043791A, and US20120242970A. Modifications to the equipment for improved yield are described in US2010201963A1 and US2011102753A1. The contents of all these applications are incorporated herein by reference. Overlay measurement of small targets is achieved using diffraction-based overlay detection with dark-field detection of diffraction order. These targets may be smaller than the illumination spot and may be surrounded by product structures on the wafer. The target may contain multiple gratings that can be measured in a single image.

[0005] A crucial parameter in lithography that needs monitoring is the focus. An ever-increasing number of electronic components need to be integrated into ICs. To achieve this integration, it is necessary to reduce the size of the components and thus increase the resolution of the projection system, allowing increasingly smaller details or linewidths to be projected onto the target portion of the substrate. As the critical dimension (CD) in lithography shrinks, the consistency of focus across and between substrates becomes increasingly important. CD is the dimension of one or more features (such as the gate width of a transistor), and variations in this dimension will cause undesirable changes to the physical properties of the features.

[0006] Traditionally, optimal settings are determined by "early wafer delivery," that is, exposing, developing, and measuring the substrate before production operations begin. In early wafer delivery, test structures are exposed in a so-called Focus Energy Matrix (FEM), and the optimal focus and energy (exposure dose) settings are determined from the inspection of these test structures. In recent years, focus metrics have been incorporated into production design to allow for continuous monitoring of focus performance. These metrics should allow for rapid measurement of the focus point to enable rapid performance measurement in high-volume manufacturing. Ideally, the metrics should be small enough to be placed within product features without undue space loss.

[0007] Current test structure design and focusing measurement methods have several drawbacks. It is known that focusing measurement targets require large-pitch sub-resolution features and / or grating structures. Such structures may violate the design rules of lithography equipment users. High-speed detection equipment (such as scatterometers) operating at visible light wavelengths can be used to effectively measure the asymmetry of grating structures. It is known that focusing measurement techniques utilize the fact that focus-sensitive asymmetry can be introduced into structures printed in resist layers through the special design of patterns on patterning devices defining the target structure. For EUV lithography, the generation of sub-resolution features becomes more difficult when printing is performed using radiation with wavelengths less than 20 nm (e.g., 13.5 nm). For EUV lithography, the resist thickness and therefore the thickness of the target structure are smaller. This reduces diffraction efficiency and thus the signal strength usable for focusing measurements.

[0008] For these reasons, there is a general need to develop new technologies for measuring focusing performance in lithography processes (especially in EUV lithography) and for projection-based lithography. [Summary of the Invention]

[0009] A metric will be needed to perform lithography with improved accuracy.

[0010] In a first embodiment, the present invention provides a method for measuring a focusing parameter from a focusing target, the focusing target comprising at least a pair of self-reference sub-targets, the pair of self-reference sub-targets comprising a first sub-target and a second sub-target, wherein each of the first sub-target and the second sub-target comprises at least one periodic principal feature; wherein the spacing and / or size parameters of at least some sub-elements of the principal feature are configured such that the first sub-target and the second sub-target have respectively different optimal focusing values; and wherein each principal feature forms a centroid and / or spacing dependent on a focal point; the method comprising: obtaining a first measurement signal from the first sub-target and obtaining a second measurement signal from the second sub-target; determining a difference signal between the first measurement signal and the second measurement signal; and determining the focusing parameter from the difference signal.

[0011] In a second embodiment, the present invention provides a substrate comprising at least one focusing target for measuring a focusing parameter, the focusing target comprising at least a pair of self-reference sub-targets, the pair of self-reference sub-targets comprising a first sub-target and a second sub-target, wherein each of the first sub-target and the second sub-target comprises at least one periodic main feature; wherein the spacing and / or size parameters of at least some sub-elements of the main feature are configured such that the first sub-target and the second sub-target have respectively different optimal focusing values; and wherein each of the main features forms a focal-dependent centroid and / or spacing.

[0012] In a third embodiment, the present invention provides a substrate comprising at least one focusing target or sub-target thereof, comprising: at least one periodic main feature having a main feature spacing, wherein a centroid and / or the main feature spacing is focally dependent; and a periodic reference feature having a reference feature spacing different from the main feature spacing, the main feature and the reference feature being configured such that scattered radiation from the main feature and the reference feature interferes to form a beat frequency signal or a ripple signal.

[0013] In a fourth state sample, the present invention provides a method for freely measuring a focusing parameter from a focusing target on a substrate of the third state sample, comprising: imaging a superimposed fringe generated by interference between at least one diffraction order from the main feature and at least one corresponding diffraction order from the reference feature to obtain a measurement signal; and determining the focusing parameter from the measurement signal.

[0014] Further features and advantages of the invention are described in detail below with reference to the accompanying drawings, as well as the structure and operation of various embodiments of the invention. It should be noted that the invention is not limited to the specific embodiments described herein. These embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to those skilled in the art based on the teachings contained herein.

Implementation Method

[0016] Before describing the embodiments of the present invention in detail, it is instructive to present an example environment in which the embodiments of the present invention can be implemented.

[0017] Figure 1 schematically depicts a lithography apparatus LA. The apparatus includes: an illumination optics system (illuminator) IL configured to modulate a radiation beam B (e.g., UV radiation or DUV radiation); a patterning device support or support structure (e.g., a mask stage) MT configured to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device according to certain parameters; a substrate stage (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 a projection optics system (e.g., a refractive projection lens system) PS configured to project a pattern imparted by the patterning device MA to the radiation beam B onto a target portion C (e.g., including one or more dies) of the substrate W.

[0018] The illumination optical system may include various types of optical or non-optical components for guiding, shaping or controlling radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of components, or any combination thereof.

[0019] The patterning device support holds the patterning device in a manner dependent on the orientation of the patterning device, the design of the lithography equipment, and other conditions (such as whether the patterning device is held in a vacuum environment). The patterning device support may use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device. The patterning device support may be, for example, a frame or a platform, which may be fixed or movable as needed. The patterning device support ensures that the patterning device is, for example, in the desired position relative to the projection system. Any use of the terms "reduction mask" or "mask" herein may be considered synonymous with the more general term "patterning device".

[0020] The term "patterning device" as used herein should be broadly interpreted as any device that can be used to impart a pattern to a radiation beam in the cross-section of the radiation beam in order to generate a pattern in a target portion of a substrate. It should be noted that, for example, if the pattern imparted to the radiation beam includes phase-shifting features or so-called auxiliary features, the pattern may not precisely 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 the device (such as an integrated circuit) generated in the target portion.

[0021] The patterning device can 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 attenuation phase-shift masks, as well as various hybrid mask types. One example of a programmable mirror array uses a matrix configuration of small mirrors, each of which can be individually tilted so that the incident radiation beam is reflected in different directions. The tilted mirrors impart a pattern to the radiation beam reflected by the mirror matrix.

[0022] As described herein, the device is of the transmissive type (e.g., using a transmissive shield). Alternatively, the device may be of the reflective type (e.g., using a programmable mirror array of the type mentioned above, or using a reflective shield).

[0023] Lithography apparatuses may also fall into the following categories: at least a portion of the substrate may be covered by a liquid (e.g., water) having a relatively high refractive index to fill the space between the projection system and the substrate. A wetting liquid may also be applied to other spaces within the lithography apparatus, such as the space between the mask and the projection system. Immersion techniques are well known in this art for increasing the numerical aperture of the projection system. The term "immersion" as used herein does not mean that a structure such as the substrate must be submerged in the liquid, but only that the liquid is located between the projection system and the substrate during exposure.

[0024] Referring to Figure 1, the illuminator IL receives a radiated beam from the radiation source SO. For example, when the source is an excimer laser, the source and the lithography apparatus can be separate entities. In this case, the source is not considered a component of the lithography apparatus, and the radiated beam is delivered from the source SO to the illuminator IL by means of a beam delivery system BD including, for example, suitable guide mirrors and / or beam expanders. In other cases, for example, when the source is a mercury lamp, the source can be an integral component of the lithography apparatus. The source SO and the illuminator IL, together with the beam delivery system BD (if necessary), can be referred to as the radiation system.

[0025] The illuminator IL may include an adjuster AD for adjusting the angular intensity distribution of the radiated beam. Typically, at least the outer radial range and / or inner radial range (typically referred to as σ_outer and σ_inner, respectively) of the intensity distribution in the pupil plane of the illuminator can be adjusted. Additionally, the illuminator IL may include various other components, such as a beam concentrator IN and a condenser CO. The illuminator can be used to adjust the radiated beam to have a desired uniformity and intensity distribution in its cross-section.

[0026] A radiation beam B is incident on a patterning device (e.g., a mask) MA held on a patterning device support (e.g., a mask stage MT) and is patterned by the patterning device. Having traversed the patterning device (e.g., the mask) MA, the radiation beam B passes through a projection optics system PS, which focuses the beam onto a target portion C of a substrate W, thereby projecting an image of the pattern onto the target portion C. The substrate stage WT can be accurately moved, for example, to position different target portions C within the path of the radiation beam B, using a second locator PW and a position sensor IF (e.g., an interferometer, a linear encoder, a 2D encoder, or a capacitive sensor). Similarly, a first locator PM and another position sensor (not explicitly depicted in FIG1) can be used to accurately position the patterning device (e.g., the mask) MA relative to the path of the radiation beam B, for example, after mechanical capture from a mask library or during scanning.

[0027] Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning device (e.g., mask) MA and the substrate W. Although the substrate alignment marks, as illustrated, occupy a dedicated target portion, these substrate alignment marks can be located in the space between the target portions (these marks are referred to as kerf alignment marks). Similarly, in cases where more than one die is provided on the patterning device (e.g., mask) MA, the mask alignment marks can be located between the dies. Small alignment marks can also be included within the dies of the device features, in which case it is necessary to make the marks as small as possible and without any imaging or procedural conditions different from adjacent features. An alignment system for detecting alignment marks is further described below.

[0028] The lithography apparatus LA in this example is a so-called dual-stage type, having two substrate stages WTa and WTb and two stations—an exposure station and a measurement station—between which the substrate stages are interchangeable. While one substrate on one stage is being exposed at the exposure station, another substrate can be loaded onto the other substrate stage at the measurement station and various preparatory steps can be performed. These preparatory steps may include using a level sensor LS to map the surface control of the substrate and using an alignment sensor AS to measure the position of alignment marks on the substrate. This configuration enables a significant increase in the output of the apparatus.

[0029] The described device can be used in various modes, including, for example, step mode or scanning mode. The construction and operation of lithography devices are well known to those skilled in the art, and further description is unnecessary for understanding the present invention.

[0030] As shown in Figure 2, the lithography equipment LA forms the components of the lithography system, which are referred to as lithography units LC or lithography clusters. The lithography unit LC may also include equipment for performing pre-exposure and post-exposure processes on the substrate. Typically, such equipment includes a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH, and a baking plate BK. A substrate handler or robot RO picks up the substrate from input / output ports I / O1 and I / O2, moves the substrate between different process devices, and delivers the substrate to the loading area LB of the lithography equipment. These devices, often collectively referred to as a coating and developing system (track), are controlled by a coating and developing system control unit TCU, which in turn is controlled by a supervisory control system SCS, which in turn controls the lithography equipment via the lithography control unit LACU. Therefore, different devices can be operated to maximize output and processing efficiency.

[0031] To ensure accurate and consistent exposure of substrates exposed by lithography equipment, it is necessary to inspect the exposed substrates to measure properties such as overlap error between subsequent layers, line thickness, and critical dimension (CD). Therefore, the manufacturing facility with lithography cells (LCs) also includes a metrology system (MET), which houses some or all of the substrates (W) already processed in the lithography cells. The metrology results are provided directly or indirectly to the supervisory control system (SCS). If an error is detected, the exposure of subsequent substrates can be adjusted, especially when inspection can be completed quickly enough so that other substrates in the same batch are still awaiting exposure. Furthermore, exposed substrates can be peeled off and reworked to improve yield or discarded, thereby avoiding further processing of known defective substrates. In cases where only some target portions of a substrate are defective, further exposure can be performed only on those good target portions.

[0032] Within the metrology system (MET), inspection equipment is used to determine the properties of a substrate, specifically, how the properties of different substrates or different layers of the same substrate vary between layers. The inspection equipment can be integrated into a lithography unit (LA) or a lithography cell (LC), or it can be a standalone device. To achieve the fastest possible measurement, the inspection equipment needs to measure the properties of the exposed resist layer immediately after exposure. However, latent images in the resist have extremely low contrast—there is only a very small difference in refractive index between the exposed and unexposed portions of the resist—and not all inspection equipment has sufficient sensitivity to perform useful measurements of latent images. Therefore, measurements can be taken after a post-exposure baking (PEB) step, which is typically the first step performed on the exposed substrate and increases the contrast between the exposed and unexposed portions of the resist. At this stage, the image in the resist can be referred to as a semi-latent. It is also possible to measure the developed resist image—at which point 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 likelihood of reworking defective substrates, but can still provide useful information.

[0033] Figure 3(a) shows the measuring device. Figure 3(b) shows the target T and the diffracted rays of the measuring radiation used to illuminate the target in more detail. The measuring device shown belongs to the type known as a dark-field measuring device. The measuring device depicted here is merely illustrative to provide an explanation of dark-field metrology. The measuring device can be a standalone unit or incorporated into a lithography device LA, such as at a measuring station, or into a lithography unit LC. The dotted line O represents the optical axis with several branches passing through the device. In this device, the light emitted by the source 11 (e.g., a xenon lamp) is guided onto the substrate W by an optical system including lenses 12, 14 and objective lens 16 via a beam splitter 15. These lenses are configured in a dual sequence of 4F configuration. Different lens configurations can be used, subject to the condition that the lens configuration still provides the substrate image to the detector and simultaneously allows access to the intermediate pupil plane for spatial frequency filtering. Therefore, the range of angles at which radiation is incident on the substrate can be selected by defining the spatial intensity distribution in a plane of the spatial spectrum of the substrate plane (here referred to as the (conjugate) pupil plane). Specifically, this can be achieved by inserting an aperture plate 13 of a suitable form between lenses 12 and 14 in a plane of a back-projected image of the objective pupil plane. In the illustrated example, the aperture plate 13 has different forms, labeled 13N and 13S, thus allowing the selection of different illumination modes. The illumination system in this example forms an off-axis illumination mode. In the first illumination mode, aperture plate 13N provides off-axis illumination from a direction designated "north" for the sake of description only. In the second illumination mode, aperture plate 13S is used to provide similar brightness, but illumination from the opposite direction designated "south". Other illumination modes are possible by using different apertures. The rest of the pupil plane is ideally dark, because any unwanted light outside the desired illumination mode will interfere with the signal to be measured.

[0034] As shown in Figure 3(b), the target T is positioned such that the substrate W is perpendicular to the optical axis O of the objective lens 16. The substrate W can be supported by a support (not shown in the figure). The measuring radiation ray I, which is at an angle to the axis O and irradiates the target T, causes a zeroth-order ray (solid line O) and two first-order rays (dotted chain line +1 and double dotted chain line -1). It should be remembered that, in the case of using a small target with excessive filling, these rays are only one of many parallel rays covering the area of ​​the substrate including the metrological target T and other features. Since the aperture in the plate 13 has a finite width (necessary to receive a useful amount of light), the incident ray I will actually occupy an angular range, and the diffracted rays O and +1 / -1 will be slightly diffused. According to the point spread function of the small target, each +1 and -1 will spread further across an angular range, rather than a single ideal ray as shown. It should be noted that the grating spacing and illumination angle of the target can be designed or adjusted so that the first-order ray entering the objective lens is nearly aligned with the central optical axis. The rays illustrated in Figure 3(a) and Figure 3(b) are shown slightly off-axis, purely to make them easier to distinguish in the diagram.

[0035] At least the 0th and +1st order diffracted by the target T on the substrate W are collected by the objective lens 16 and guided back through the beam splitter 15. Returning to Figure 3(a), both the first and second illumination modes are illustrated by indicating completely opposite apertures labeled North (N) and South (S). When the incident ray I of the measured radiation comes from the north side of the optical axis (i.e., when the first illumination mode is applied using aperture plate 13N), the +1 diffracted ray labeled +1 (N) enters the 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 entering the lens 16.

[0036] The second beam splitter 17 divides the diffracted beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zeroth-order diffracted beam and the first-order diffracted beam to form the diffraction spectrum (pupil plane image) of the target on the first sensor 19 (e.g., a CCD or CMOS sensor). Each diffraction order hits a different point on the sensor, allowing image processing to compare and contrast several orders. The pupil plane image captured by the sensor 19 can be used for focusing measurement devices and / or to normalize the intensity measurement of the first-order beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.

[0037] In the second measurement branch, optical systems 20 and 22 form an image of the target T on sensor 23 (e.g., a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane conjugate with the pupil plane. The aperture stop 21 is used to block the zeroth-order diffracted beam, so that the image of the target formed on sensor 23 is formed only by a -1 or +1 first-order beam. The image captured by sensors 19 and 23 is output to a processor PU that processes the image, the function of which will depend on the specific type of measurement being performed. It should be noted that the term "image" is used here in a broad sense. Therefore, if only one of the -1 and +1 orders exists, an image of the grating lines will not be formed.

[0038] The specific form of the aperture plate 13 and field stop 21 shown in Figure 3 is purely illustrative. In another embodiment of the invention, coaxial illumination of the target is used, and an aperture stop with an off-axis aperture is used to deliver essentially only one first-order diffracted beam to the sensor. In other embodiments, two quadrant apertures may be used. This allows for the simultaneous detection of positive and negative orders, as described in US2010201963A1 mentioned above. As described in US2011102753A1 mentioned above, embodiments with optical wedges (segmented prisms or other suitable elements) in the detection branch can be used to separate the orders for spatial imaging in a single image. In yet another embodiment, second-order, third-order, and higher-order beams (not shown in Figure 3) may be used in the measurement instead of the first-order beam or in addition to the first-order beam. In another embodiment, a segmented prism can be used instead of the aperture stop 21, thereby enabling the simultaneous capture of both +1 and -1 orders at spatially separated locations on the image sensor 23.

[0039] To adapt the measurement of radiation to these different types of measurements, the aperture plate 13 may comprise a plurality of aperture patterns formed around a disk that is rotated to position the desired pattern. It should be noted that the aperture plate 13N or 13S may be used only to measure gratings oriented in one direction (X or Y, depending on the setup). For measuring orthogonal gratings, target rotations of up to 90° and 270° can be implemented. Different aperture plates are shown in Figures 3(c) and (d). The use of such aperture plates and numerous other variations and applications of the apparatus are described in the previously published applications mentioned above.

[0040] Figure 4 depicts a stacked target or composite stacked target formed on a substrate according to known practices. In this example, the stacked target comprises four sub-targets (e.g., gratings) 32 to 35, which are closely positioned together such that they are all within the measurement spot 31 formed by the metric illumination beam of the measuring device. The four sub-stacking targets are thus simultaneously illuminated and simultaneously imaged onto the sensor 23. In an example specifically for stacked measurement, the sub-targets 32 to 35 are themselves composite structures formed by overlay gratings patterned in different layers of a semiconductor device formed on the substrate W. The sub-targets 32 to 35 may have stacked offsets with different biases to facilitate the measurement of stacked targets between layers with different portions of the composite sub-targets. The sub-targets 32 to 35 may also differ in their orientation (as shown) to diffract incident radiation in the X and Y directions. In one example, sub-targets 32 and 34 are X-direction sub-targets with offsets of +d and -d, respectively. Rasteres 33 and 35 are Y-direction sub-targets with offsets of +d and -d, respectively. Individual images of these sub-targets can be identified in the image captured by sensor 23. This is only one example of a stacked target. A stacked target may contain more or fewer than four sub-targets.

[0041] Figure 5 illustrates an example of an image formed on and detected by sensor 23 using the overlapping target of Figure 4 in the device of Figure 3 when using the aperture plate 13NW or 13SE from Figure 3(d). Although the pupil plane image sensor 19 cannot resolve different individual sub-targets 32 to 35, the image sensor 23 can. The shaded area 40 represents the image field on the sensor, in which the illuminated spot 31 on the substrate is imaged into the corresponding circular area 41. Within this circular area, rectangular areas 42 to 45 represent the images of the small overlapping target sub-targets 32 to 35. If the overlapping target is located in the product area, the product features are also visible in the periphery of this image field. The image processor and controller PU use pattern recognition to process these images to identify the individual images 42 to 45 of the sub-targets 32 to 35. In this way, the images do not have to be precisely aligned at a specific location within the sensor frame, which greatly improves the overall output of the measuring device.

[0042] Once individual images of the overlaid target 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 region. The intensity and / or other attributes of these images can be compared with each other. These results can be combined to measure different parameters of the lithography process.

[0043] The following disclosure illustrates techniques for measuring the focusing performance of lithography processes using oblique illumination on a patterning apparatus of the reflective type. These techniques are particularly applicable to EUV lithography, where reflective optics in a near-vacuum environment are required. While printing product features, metric targets including certain focusing metric patterns are printed on the substrate. The asymmetry of these printed patterns will be measured using a diffraction-based technique, such as that in Figure 3. To allow for the use of small targets, it will be assumed that the dark-field imaging branch of the apparatus will be used to perform these asymmetry measurements. However, a pupil imaging branch can also be used to perform diffraction-based measurements of asymmetry. Of course, the apparatus shown in Figure 3 is merely one example of a detection apparatus and method that can be used to measure asymmetry.

[0044] In the context of lithography equipment operating in the DUV wavelength range, targets measured using diffraction-based focusing (DBF) have been successfully designed and utilized. Known types of DBF targets are generated by including sub-segmented features in a grating pattern on a magnifying mask. These features, adjacent to more solid-state features, have dimensions below the imaging resolution of the lithography equipment. Therefore, they are not printed on the substrate as individual features in the resist layer, but they affect the printing of solid-state features in a manner sensitive to focusing errors. Specifically, the presence of these features creates an asymmetric resist profile for each line in the grating within the DBF measurement target, where the asymmetry depends on the focal point. Therefore, a measurement tool such as that of the detection equipment in Figure 3 can measure the asymmetry from the target formed on the substrate and convert this into the scanner focal point.

[0045] Unfortunately, known DBF metric target designs are not suitable for all situations. In EUV lithography, the resist film thickness is significantly lower than that used in DUV immersion lithography, resulting in low diffraction efficiency and difficulty in extracting accurate asymmetry information from diffracted radiation in the scatterometer. Furthermore, since the resolution of imaging systems is inherently higher in EUV lithography, features smaller than the printing resolution of DUV immersion lithography become "solid" features that can be printed using EUV lithography. Providing similar sub-resolution features on EUV reduction masks is impractical and / or may violate semiconductor manufacturers' "design rules." These rules are typically established as a way to constrain feature design to ensure that printed features meet their procedural requirements. In any case, working outside the design rules makes it difficult to simulate the execution of procedures for DBF targets, turning optimal target design and focus measurement calibration into a trial-and-error process. The desired application of DBF targets conforming to design rules is to DUV lithography, not just EUV lithography.

[0046] Therefore, there is a need for a method and target design for obtaining stronger and / or more robust signals using a scatterometer-based self-focusing target. To achieve this, a focusing target for measuring focusing parameters with respect to a focusing target is disclosed, comprising at least a pair of self-referenced sub-targets, the pair comprising a first sub-target and a second sub-target, wherein each of the first sub-target and the second sub-target comprises at least one periodic principal feature; wherein the spacing and / or size parameters (e.g., critical size CD) of at least some of the sub-elements of the principal feature are configured such that the first sub-target and the second sub-target have different optimal focusing values; and wherein each principal feature forms a focal-dependent centroid and / or spacing measurable using a scatterometer.

[0047] Focusing parameters can be determined by measuring the focused target using a scatterometer. These focusing parameters are related to the focus settings of the photolithography exposure process on which the target is exposed. A substrate is also disclosed, comprising at least one such target and a magnifying mask or mask comprising target forming features for forming at least one such target.

[0048] Therefore, a method for measuring a focusing parameter from a focusing target comprising at least one first sub-target and a second sub-target is also disclosed, wherein each of the sub-targets has at least one periodic main feature, wherein the spacing and / or size parameters of at least some sub-elements of the main feature are configured such that the first sub-target and the second sub-target have different optimal focusing values; and wherein each of the main features forms a centroid and / or spacing dependent on a focal point; the method includes: obtaining a first measurement signal from the first sub-target and obtaining a second measurement signal from the second sub-target; determining a difference signal between the first measurement signal and the second measurement signal; and determining the focusing parameter from the difference signal substrate and the associated patterning device.

[0049] The "centroid" of each sub-target or structure may include the peak position of the first harmonic of the pattern defined by the printed structure.

[0050] To obtain a stronger signal when measuring the centroid of a target, it is proposed that the focused target be configured for self-reference phase measurement of the centroid of sub-targets using a scatterometer. This is particularly suitable for EUV applications with thin resists, because focusing metrics techniques based purely on intensity (or intensity asymmetry) suffer from extremely low signal strength and unreliable inferences (poor regeneration) when measuring known focused targets in such thin resists. In this context, self-reference means comparing the fringe position (phase measurement) indicating the centroid or spacing detected from the first sub-target with the fringe position of the second sub-target.

[0051] Using the first example, in order to obtain a fringe pattern indicating the centroid or spacing, each sub-target can be configured such that the scatterometer can capture the first and second diffraction orders and their phases relative to the measurement centroid / spacing and thus the measurement focus. These orders will interfere to provide a fringe pattern on the scatterometer's detector (camera), the position of which will shift with the focus. By measuring the difference in fringe positions for each sub-target, and thus the difference in centroids, a monotonic relationship between the measurement signal and the focus is obtained.

[0052] Alternatively, each sub-target may include a periodic reference feature having a different distance from the periodic principal feature, such that scattered radiation from the focally dependent principal feature and the reference feature interferes to form a beat frequency signal or a fringe signal. This fringe signal can then be imaged using a scatterometer, wherein the difference in the fringe positions of the two sub-targets is measured to determine the difference in the centroid of each sub-target indicating the focal point.

[0053] In one embodiment, the distance between the main features, Pmain, has a small difference from the distance between the reference features, Pref (e.g., less than 20% or 10% of the difference between the main features). This produces two first-order diffracted beams with slightly different directions. Upon recombining in the image, the superimposed beat pattern becomes visible, where the spacing 1 / Pbeat = 1 / Pref - 1 / Pmain. Thus, the superimposed effect amplifies the phase shift; for example, a 10% spacing difference provides up to a 10-fold amplification of the shift.

[0054] The main feature of each of the sub-objects may comprise a periodic (e.g., 1D or 2D) array of main feature elements, wherein each main feature element comprises a larger first sub-element and a plurality of smaller second sub-elements. The second sub-element may comprise features having a CD and / or spacing at or near the product size, and may therefore be referred to as a resolution-dependent sub-element. For example, the second sub-element may comprise a 1D array of such resolution-dependent second sub-elements parallel to or perpendicular to the longest dimension of the first sub-element. The first sub-element may be a solid element, or it may be a periodic element (e.g., having a CD and / or spacing at or near the product size).

[0055] The principal spacing of the principal features may be large enough to be compatible with a scatterometer device (e.g., as illustrated in Figure 3(a)) (which can be measured by the scatterometer device). Therefore, this principal spacing should provide at least a detectable first order. For example, this principal spacing Pmain may conform to the criterion: 0.7 < λ / Pmain > 1.3, where λ is the wavelength of the radiation being measured. In embodiments where the subtarget does not have a reference feature, this principal spacing should provide detectable first and second orders.

[0056] To impose different optimal focuses on each sub-target, it is proposed that the CD and / or spacing between sub-targets be different according to their resolution characteristics. This could be the CD and / or spacing of the second sub-element, and / or the CD and / or spacing of the first sub-element if the first sub-element is a periodic element. Optimal focus can indicate the focus setting where the CD variation is minimal within a small range (e.g., + / - 5 nm) around the optimal focus and / or the focus setting at the peak or trough of the corresponding Bossung curve of the sub-target; that is, where the CD sensitivity to small focus changes is lowest.

[0057] The distance between these resolution-dependent features and / or CD can be optimized during the lithography calculation step (e.g., using source-mask optimized SMO) to obtain sufficient optimal focus difference between sub-targets and / or maximize the optimal focus difference between sub-targets. Furthermore, additional techniques, such as changing the absorber between two sub-targets on the mask, can be used to further maximize the optimal focus offset; for example, by forming one (or two) targets on the mask using a mask absorber different from the standard Ta-based absorber (e.g., a low-n absorber with a refractive index n < 0.92 and other EUV absorbers).

[0058] In the reference feature embodiment, the reference feature may be contained in another layer of the device and / or may be interleaved with the main feature. In a particular instance, the reference features may be interleaved in a periodic direction perpendicular to the main feature. In this embodiment, the vertical spacing PPP of the main features in this vertical direction may be small enough to be unresolved. For example, this vertical spacing PPP may meet the criterion λ / PPP > 1.2, where λ is the wavelength of the radiation being measured.

[0059] Figure 6 is a diagram illustrating how the proposed method and focusing target can be used to monitor the focus. Figure 6(a) shows a first embodiment of a focusing target or its main features including a first sub-target 600a and a second sub-target 600b. Each sub-target 600a, 600b includes respective main feature elements 610a, 610b, and each respective main feature element 610a, 610b includes a first sub-element 620 (in this example, a solid block) and respective second sub-elements 630a, 630b. As will be described, each sub-target may also include reference features (not shown in this diagram) that are staggered in the same layer, quasi-staggered in the same layer, or in different layers (e.g., a reference layer) (staggered in a direction perpendicular to the main spacing of the main features).

[0060] The principal spacing Pmain of the target should be compatible with the scatterometer used to measure the target in order to generate at least a detectable first diffraction order. If a reference feature is provided, generating only a detectable first order is sufficient to enable phase measurement. If the target does not have a reference feature, the principal spacing Pmain should be configured to generate at least a detectable first and second order to enable phase measurement.

[0061] As described, each of the sub-targets 600a and 600b has a different optimal focus setting. This is achieved through a Mask 3D effect, that is, the effect produced by illicit line illumination on a reflective mask that inherently contains 3D patterned features. Specifically, and more specifically, the spacing and / or CD of the second sub-elements 630a may differ from those of the second sub-elements 630b to utilize this Mask 3D effect. In the particular example shown, the spacing Psse1 between the second sub-elements 630a differs from the spacing Psse2 between the second sub-elements 630b. Furthermore, the CD of the second sub-elements 630a differs from that of the second sub-elements 630b. However, only the spacing and CD need to be varied.

[0062] Figure 6(b) illustrates how each printed main feature element 610a, 610b changes with the focus setting of the lithography apparatus used to image the main feature element. The target shown in Figure 6(a) is a target formed on a mask (and printed approximately at optimal focus). Figure 6(b) shows a cross-section of the printed main feature elements 610a, 610b corresponding to the target in Figure 6(a) for five different focus settings. The central image shows the main feature element printed at optimal focus BF, above which are two images of the printed main feature element at two corresponding higher positive defocus values ​​DF+, and below the optimal focus BF image are two images of the printed main feature element at two corresponding higher negative defocus values ​​DF-. In each case, an arrow indicating the centroid of the printed feature is shown.

[0063] Figure 6(c) contains plots 640a and 640b of the measured centroid CoM of the first and second sub-targets relative to the focus f, respectively. Each of these plots contains a Burson curve, as is well known in focus monitoring. An indication of the difference ΔBF between the optimal focus positions of the two sub-targets is also shown. Figure 6(d) shows an equivalent plot of ΔCoM relative to the focus, where ΔCoM is the difference in CoM values ​​between plots 640a and 640b. As can be seen, this difference ΔCoM has a monotonic relationship with the focus. The slope and (therefore) sensitivity of ΔCoM to the focus can be increased by increasing the difference ΔBF between the optimal focus positions.

[0064] As stated, the centroid can be inferred from the position difference (translation) of each sub-target from the interference of the first and second diffraction orders through the imaging fringes, or from the overlapping fringes of the target containing reference features. Other methods include inferring the centroid from the intensity difference between two sub-targets.

[0065] The method may include an initial calibration phase, for example, calibrating the relationship between the centroid difference ΔCoM and the focal point (the relationship illustrated in Figure 6(d)). This may include: printing a calibration focal target array (i.e., the same as the focal target to be used in the manufacturing phase) under different known focal settings (or different focal / dose settings), measuring the centroid difference ΔCoM from the calibration focal target, and determining the relationship between this centroid difference ΔCoM and the known focal settings.

[0066] Figures 7 to 9 illustrate a plurality of example target designs that can be used as alternatives to the embodiment of Figure 6(a). Each of these target designs can be combined with reference features, staggered in the same layer, in the same layer, or in different layers (e.g., reference layers) to generate overlay signals, as will be described. In each case (as in the example of Figure 6(a)), the target may differ in detail from those shown, and may, for example, include different numbers of main feature elements, different sizes, and different distances or numbers between the individual elements shown.

[0067] Figure 7 illustrates a second embodiment of a target (or its main features) comprising a first sub-target 700a and a second sub-target 700b. Each sub-target 700a, 700b comprises respective main feature elements 710a, 710b, and each respective main feature element 710a, 710b comprises respective first sub-elements 720a, 720b and second sub-elements 730. In this example, the main sub-elements 720a, 720b are gratings, and the grating of the first sub-element 720a has a different spacing and / or CD (distance between elements in the illustrated diagram) than the grating of the first sub-element 720b. This provides different optimal focusing for the two sub-targets. In this example, the second sub-element 730, which provides a centroid with a focal point, is perpendicular to the first sub-element and extends from the side of the first sub-target as a finger-like element. In this example, the second sub-element 730 has the same size for both sub-targets 710a, 710b. However, as an alternative to the individual pitch / CD of the grating of the first sub-element, or in addition to the individual pitch / CD of the grating of the first sub-element, it may also differ in the pitch and / or CD between the two sub-targets 710a, 710b.

[0068] It should be noted that this figure is not shown as a complete sub-target that would normally be printed, but rather as individual feature elements printed here under different focus settings (purely for illustration), which have an approximate optimal focus (BF) at the center (note that each sub-target has a different optimal focus) and move to greater defocus in each direction along the arrows. Each feature element may contain the same width / centroid on the mask, and (for a single focus setting) the actual printed target will also contain feature elements with substantially / generally uniform centroids depending on that focus setting.

[0069] Figure 8 illustrates a third embodiment of a target (or its main features) comprising a first sub-target 800a and a second sub-target 800b. Each sub-target 800a, 800b comprises respective main feature elements 810a, 810b, and each respective main feature element 810a, 810b comprises respective first sub-element 820 and second sub-element 830a, 830b. In this example, the main sub-element 820 comprises a solid block. In this example, the second sub-element 830a, 830b, providing a focally dependent centroid, is again perpendicular to the first sub-element and extends from the side of the first sub-element as a finger-like element. In this example, the second sub-element 830a, 830b differs in distance and / or CD between the two sub-targets 810a, 810b. This provides different optimal focus for the two sub-targets. As in Figure 7, this target is represented as being printed along its length at different focal values.

[0070] Figure 9 illustrates a fourth embodiment of a target (or its main features) comprising a first sub-target 900a and a second sub-target 900b. Each sub-target 900a, 900b comprises respective main feature elements 910a, 910b, and each respective main feature element 910a, 910b comprises respective first sub-element 920 and second sub-element 930a, 930b. In this example, the target is symmetrical about its central element in a periodic direction, and in this example, the central element comprises only the first sub-element 920 (i.e., it does not have the significant focal dependence provided by the second sub-element 930a, 930b). In contrast to Figures 7 and 8, the target in Figure 9 is shown as appearing to be printed on a mask or at a single focal level.

[0071] This type of target provides focal-dependent changes in the spacing of the principal feature, which can be measured in the same manner as already described; for example, via fringes imaged from the first and second diffraction orders or also providing superimposed fringes of the reference feature.

[0072] In Figure 9, the target is shown as a symmetrical version of the target in Figure 8. However, the concept can also be applied to the main feature elements of the target in Figure 7 or Figure 6(a) to obtain a target with a focal interdependent distance.

[0073] Figure 10 illustrates a first type of superimposed subtarget 1000 suitable for providing superimposed signals, from which the centroid difference between two such subtargets (or the distance difference between symmetrical targets) can be determined, as described. The upper layer (or the layer of interest for determining the focal value) contains focally dependent subtarget main features 1010. In the lower layer or reference layer and below the subtarget main features 1010 are additional periodic reference features 1020 (e.g., formed without focal dependence). This reference feature 1020 has a reference spacing Pref that is different from (but can be close to) the main feature spacing Pmain. Interference from the (e.g., first) diffraction order of these features 1010, 1020 will produce a superimposed pattern on the detector with focally dependent fringe positions. In this example, the subtarget main feature 1010 is shown as subtarget 600a in Figure 6(a). However, this main feature 1010 may include any of the main feature sub-target configurations illustrated and / or described herein. As previously stated, a sub-target may be paired with another sub-target having a different optimal focus obtained via the masking 3D effect, and provides a self-reference for the fringe position.

[0074] Figure 11 illustrates a second overlay subtarget type 1100 suitable for providing an overlay signal. In this example, the target is formed in a single layer (the layer of interest) and includes subtarget main features 1110 that intersect with the subtarget reference feature 1120 (e.g., formed without focal dependence). In this context, intersect means that the main feature elements and the reference feature elements alternate. Again, the reference spacing Pref is different from (and may be close to) the main feature spacing Pmain. As in the previous example, the subtarget main features 1110 of this second overlay subtarget type 1100 may include any of the main feature subtarget configurations illustrated and / or described herein, and as previously stated, the subtarget may be paired with another subtarget having a different optimal focus obtained via the masking 3D effect.

[0075] Figure 12 illustrates a third type of overlapping sub-target suitable for providing overlapping signals. Again, in this example, the target is formed in a single layer (the layer of interest). However, this configuration is expected to provide improved performance superior to that illustrated in Figure 11. Figure 12(a) shows an exemplary unit cell of this quasi-interlaced configuration, which includes a main feature portion 1210 and a reference feature portion 1220. Two or more of these unit cells are stacked such that the reference feature and the main feature are interlaced (alternated) in a periodic direction perpendicular to the main target spacing P main. The main feature again includes a first sub-element 1220 and a second sub-element 1230. Figure 12(b) shows a target according to this concept, which includes a first sub-target 1240a and a second sub-target 1240b. As previously mentioned, the CD and / or spacing of the sub-elements of the main feature element 1210a are different from the CD and / or spacing of the sub-elements of the main feature element 1210b. Specifically, the second sub-element 1230a shown here has a different CD and / or pitch than the second sub-element 1230b, although the size of the first sub-element varies in this way depending on the actual design of the main feature. As previously mentioned, the main feature may be different from the main feature shown and may include any of the main features described herein (note, for example, that the main feature 1210 in Figure 12(a) has a different design than the main features 1210a and 1210b in Figure 12(b)).

[0076] The vertical spacing P PP of the main feature in this vertical direction can be small enough to be unresolved. For example, this vertical spacing P PP can meet the criterion λ / P PP > 1.2, where λ is the wavelength of the radiation being measured.

[0077] Figure 13 shows an example of an overlay image that can be captured from any of the overlay embodiments described herein. A first overlay image 1300a (e.g., obtained from a first sub-target) and a second overlay image 1300b (e.g., obtained from a second sub-target) are shown. The positions of the individual fringes (e.g., maximum or minimum intensity) can be identified. The difference ΔMF of these overlay fringes will indicate the difference ΔCoM of the centroid of the sub-target (i.e., proportional to it), and can therefore be calibrated relative to defocus as described. Once calibrated, the calibration relationship can be used to translate the overlay fringe difference ΔMF into a focus value. In this way, a self-referenced measurement can be performed on the focusing target.

[0078] The target for focusing on the ripple pattern is currently unknown, and therefore this document also discloses a target or sub-target thereof, comprising: at least one periodic principal feature having a principal feature spacing, wherein the centroid and / or the principal feature spacing is focally dependent; and a periodic reference feature having a reference feature spacing different from the principal feature spacing, the principal feature and the reference feature being configured such that scattered radiation from the principal feature and the reference feature interferes to form a beat frequency signal or a ripple pattern signal.

[0079] In all the embodiments described, each described target may be paired with a mirror or 180-degree rotated version of the other target, enabling autofocus inference to remove / separate the overlapping content. For mirror pairs, any overlapping content will cause stripe shift in the same direction for the pair, while the focus shift will be in the opposite direction, thereby achieving its separation.

[0080] In association with a physical grating structure such as that implemented on a substrate and a patterning apparatus, one embodiment may include a computer program containing one or more sequences of machine-readable instructions that describe methods for measuring the target on the substrate and / or analyzing the measurements to obtain information about the lithography process. This computer program may be executed in, for example, a unit PU in the device of FIG. 3 and / or a control unit LACU of FIG. 2. Data storage media (e.g., semiconductor memory, magnetic disk, or optical disk) may also be provided, in which this computer program is stored. Where existing metrological devices, such as those of the type shown in FIG. 3, are already in production and / or in use, the present invention may be implemented by providing an updated computer program product that causes a processor to perform steps necessary for calculating superposition errors.

[0081] The program may be configured as needed to control the optical system, substrate support and the like to perform the steps necessary to calculate the overlap error for measuring the asymmetry on a suitable plurality of targets.

[0082] Therefore, a target configuration suitable for lithography is disclosed, the target configuration comprising at least two targets positioned within the target configuration such that the target configuration is symmetrical upon rotation. The at least two targets may be positioned within the target configuration such that the measured properties of the at least two targets are symmetrical upon rotation. A method for measuring parameters of lithography is also disclosed, comprising measuring at least two targets of the target configuration by: illuminating the targets with radiation, detecting radiation scattered by the targets, and determining the measured properties of the targets, wherein the properties are symmetrical upon rotation.

[0083] Although reference may be specifically made to the use of embodiments of the invention within the context of optical lithography, it should be understood that the invention can be used in other applications (e.g., imprinting) and is not limited to optical lithography where the context permits. In imprinting, the topography in a patterning apparatus defines a pattern generated on a substrate. The topography of the patterning apparatus can be pressed into a resist layer supplied to the substrate, where the resist is cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. After the resist has cured, the patterning apparatus is removed from the resist, thereby leaving a pattern therein.

[0084] Further embodiments of the invention are described in the following numbered clauses: 1. A method for measuring a focus parameter from a focusing target, the focusing target comprising at least a pair of self-reference sub-targets, the pair of self-reference sub-targets comprising a first sub-target and a second sub-target, wherein each of the first sub-target and the second sub-target comprises at least one periodic principal feature; wherein the spacing and / or size parameters of at least some sub-elements of the principal feature are configured such that the first sub-target and the second sub-target have respectively different optimal focus values; and wherein each principal feature forms a focal-dependent centroid and / or spacing; the method comprising: obtaining a first measurement signal from the first sub-target and obtaining a second measurement signal from the second sub-target; determining a difference signal between the first measurement signal and the second measurement signal; and determining the focus parameter from the difference signal. 2. The method of clause 1, wherein the first measurement signal and the second measurement signal are phase signals, and the method comprises using each sub-target to reference the phase of the other sub-target. 3. The method of clause 1 or 2, wherein the primary feature spacing of one of the primary features and the wavelength of the measured radiation are configured to provide at least one detectable first diffraction order from each sub-target. 4. The method of clause 3, wherein the ratio of the wavelength of the measured radiation to one of the primary feature spacings is between 0.7 and 1.3. 5. The method of any of the preceding clauses, comprising capturing at least one first diffraction order and at least one second diffraction order from the measured radiation scattered by each of the first and second sub-targets; imaging interference fringes of the interference between the at least one first diffraction order and the at least one second diffraction order from the first sub-target to obtain the first measurement signal; and imaging interference fringes of the interference between the at least one first diffraction order and the at least one second diffraction order from the second sub-target to obtain the second measurement signal. 6. The method of clause 5, wherein the primary feature spacing of one of the primary features and the wavelength of the measured radiation are configured to provide at least one detectable first diffraction order and a second diffraction order from each sub-target. 7. The method of any one of clauses 1 to 4, wherein each sub-target includes a periodic reference feature having a reference feature spacing different from the main feature spacing, such that scattered radiation from the main feature and the reference feature interferes to form a fringe signal. 8. The method of clause 7, wherein the difference between the main feature spacing and the reference feature spacing is less than 20% of the main feature spacing.9. The method of claim 7 or 8, comprising imaging fringes generated by interference between at least one diffraction order of the main feature of the first sub-target and at least one corresponding diffraction order of the reference feature of the first sub-target to obtain the first measurement signal; and imaging fringes generated by interference between at least one diffraction order of the main feature of the first sub-target and at least one corresponding diffraction order of the reference feature of the second sub-target to obtain the second measurement signal. 10. The method of claim 7, 8 or 9, wherein the reference feature of each sub-target is contained in a layer below the layer of interest containing one of the main features of each sub-target. 11. The method of claim 7, 8 or 9, wherein both the reference feature of each sub-target and the main feature of each sub-target are contained in the same layer. 12. The method of clause 11, wherein for each sub-target, the main feature and the reference feature are staggered in one of the periodic directions of the main feature and the reference feature, such that the main feature element of the main feature alternates with the reference feature element of the reference feature in the periodic direction. 13. The method of clause 11, wherein for each sub-target, the main feature and the reference feature are staggered in a direction perpendicular to the periodicity of the main feature and the reference feature, such that the main feature element of the main feature alternates with the reference feature element of the reference feature in the direction perpendicular to the periodicity of the main feature and the reference feature. 14. The method of clause 13, wherein one of the vertical spacings of the main features in the vertical direction is lower than the resolution limit of the device used to execute the measurement signals. 15. The method of clauses 13 or 14, wherein the ratio of the wavelength of the measured radiation to one of the vertical spacings is greater than 1.2. 16. The method of any one of items 5 to 15, wherein the difference signal includes the difference in position corresponding to the stripes in the first measurement signal and the second measurement signal. 17. The method of any one of the preceding items, wherein each of the main features forms a focally dependent centroid, the centroid of each sub-target describing the peak position of a first harmonic of a pattern defined by the sub-target when printed. 18. The method of any one of items 1 to 16, wherein each of the main features forms a focally dependent distance, the main feature being substantially symmetrical about an axis of symmetry parallel to the periodic direction of the main feature. 19. The method of any one of the preceding items, wherein the main feature of each of the sub-targets comprises a periodic array of main feature elements. 20. The method of item 19, wherein some or all of the main feature elements each comprise a larger first sub-element and a plurality of smaller second sub-elements. 21. The method of clause 20, wherein the second sub-element includes features having a CD and / or a pitch that is at or near one of the product dimensions.22. The method of claim 20 or 21, wherein the second sub-elements are formed in a periodic structure having periodicity in a direction parallel to one of the longest dimensions of the first sub-elements. 23. The method of claim 20 or 21, wherein the second sub-elements are formed in a periodic structure having periodicity in a direction perpendicular to one of the longest dimensions of the first sub-elements. 24. The method of any one of claims 20 to 23, wherein at least some of the sub-elements of the main feature configured such that the first sub-target and the second sub-target have respectively different optimal focus values ​​include the second sub-elements. 25. The method of any one of claims 20 to 24, wherein each of the first sub-elements comprises a solid or single-filled structure. 26. The method of any one of claims 20 to 24, wherein each of the first sub-elements comprises a periodic structure. 27. The method of claim 26, wherein at least some sub-elements of the main feature configured such that the first sub-target and the second sub-target each have a different optimal focus value include the first sub-elements. 28. The method of any of the preceding claims, comprising obtaining a calibrated relationship between the difference signal and the focus parameter to determine the focus parameter from the difference signal. 29. The method of any of the preceding claims, comprising an initial step of exposing the focus target onto a substrate; and measuring the focus target to obtain the first measurement signal and the second measurement signal. 30. The method of claim 29, comprising using a scatterometer to measure the focus target in the measurement step. 31. The method of any of the preceding claims, wherein the focus target is paired with a mirror image or a 180-degree rotated version of the focus target; and the method comprises removing the overlapping content from the determined focus parameter by measuring the measurement signals obtained from the paired focus targets. 32. The method of any of the preceding claims, wherein the size parameter is a critical size. 33. The method of any of the preceding clauses, wherein the focusing parameter describes a focusing setting of a lithography exposure procedure for exposing the focusing target. 34. A substrate comprising at least one focusing target for measuring a focusing parameter, the focusing target comprising at least a pair of self-reference sub-targets, the pair of self-reference sub-targets comprising a first sub-target and a second sub-target, wherein each of the first sub-target and the second sub-target comprises at least one periodic main feature; wherein the spacing and / or size parameters of at least some sub-elements of the main feature are configured such that the first sub-target and the second sub-target have respectively different optimal focusing values; and wherein each main feature forms a focal-dependent centroid and / or spacing. 35. The substrate of clause 34, wherein each sub-target comprises a periodic reference feature having a reference feature spacing different from the main feature spacing, such that scattered radiation from the main feature and the reference feature interferes to form a superimposed signal.36. The substrate of clause 35, wherein the difference between the main feature spacing and the reference feature spacing is less than 20% of the main feature spacing. 37. The substrate of clauses 34, 35, or 36, wherein the reference feature of each sub-target is contained in a layer below the layer of interest containing one of the main features of each sub-target. 38. The substrate of clauses 34, 35, or 36, wherein both the reference feature of each sub-target and the main feature of each sub-target are contained in the same layer. 39. The substrate of clause 38, wherein for each sub-target, the main feature and the reference feature are staggered in a periodic direction of the main feature and the reference feature, such that the main feature element of the main feature alternates with the reference feature element of the reference feature in the periodic direction. 40. A substrate of claim 38, wherein for each sub-target, the main feature and the reference feature are staggered in a direction perpendicular to the periodicity of the main feature and the reference feature, such that the main feature elements of the main feature alternate with the reference feature elements of the reference feature in the direction perpendicular to the periodicity of the main feature and the reference feature. 41. A substrate of any of claims 34 to 40, wherein each main feature has a focally dependent centroid, the centroid of each sub-target describing the peak position of a first harmonic of a pattern defined by the sub-target during printing. 42. A substrate of any of claims 34 to 40, wherein each main feature has a focally dependent spacing, the main feature being substantially symmetrical about an axis of symmetry parallel to the periodicity of the main feature. 43. A substrate of any of claims 34 to 42, wherein the main feature of each of the sub-targets comprises a periodic array of main feature elements. 44. The substrate of claim 43, wherein some or all of the main feature elements each comprise a larger first sub-element and a plurality of smaller second sub-elements. 45. The substrate of claim 44, wherein the second sub-elements comprise features having a CD and / or pitch at or near a product size. 46. The substrate of claim 44 or 45, wherein the second sub-elements are formed in a periodic structure having periodicity in a direction parallel to the longest dimension of the first sub-element. 47. The substrate of claim 44 or 45, wherein the second sub-elements are formed in a periodic structure having periodicity in a direction perpendicular to the longest dimension of the first sub-element. 48. The substrate of any one of claims 44 to 47, wherein at least some of the sub-elements of the main feature configured such that the first sub-target and the second sub-target have respectively different optimal focus values ​​comprise the second sub-elements. 49. The substrate of any one of clauses 44 to 48, wherein each of the first sub-elements comprises a solid or single-filled structure. 50. The substrate of any one of clauses 44 to 48, wherein each of the first sub-elements comprises a periodic structure.51. A substrate of claim 50, wherein at least some of the sub-elements configured such that the first sub-target and the second sub-target each have a different optimal focus value include the first sub-elements. 52. A substrate of any one of claims 34 to 51, wherein the focus target is paired on the substrate with a mirror or 180-degree rotated version of one of the focus targets. 53. A substrate of any one of claims 34 to 52, wherein the dimensional parameter is a critical dimension. 54. A substrate of any one of claims 34 to 53, wherein the focus parameter describes a focus setting for a lithography exposure procedure for exposing one of the focus targets. 55. A substrate comprising at least one focused target or sub-target, comprising: at least one periodic main feature having a main feature spacing, wherein a centroid and / or the main feature spacing is focally dependent; and a periodic reference feature having a reference feature spacing different from the main feature spacing, the main feature and the reference feature being configured such that scattered radiation from the main feature and the reference feature interferes to form a beat frequency signal or a ripple signal. 56. The substrate of claim 55, wherein the difference between the main feature spacing and the reference feature spacing is less than 20% of the main feature spacing. 57. The substrate of claim 55 or 56, wherein the reference feature is contained in a layer below one of the main features of interest containing each sub-target. 58. The substrate of claim 55 or 56, wherein both the reference feature and the main feature are contained in the same layer. 59. A substrate of claim 58, wherein the main feature and the reference feature are staggered in one of the periodic directions of the main feature and the reference feature, such that the main feature element of the main feature alternates with the reference feature element of the reference feature in the periodic direction. 60. A substrate of claim 58, wherein for each sub-target, the main feature and the reference feature are staggered in a direction perpendicular to the periodicity of the main feature and the reference feature, such that the main feature element of the main feature alternates with the reference feature element of the reference feature in the same direction perpendicular to the periodicity of the main feature and the reference feature. 61. A substrate of any of claims 55 to 60, wherein the main feature has a focally dependent centroid, the centroid of each sub-target describing the peak position of a first harmonic of a focal target defined by a pattern during printing. 62. A substrate of any of claims 55 to 60, wherein each main feature has a focally dependent distance, and the main feature is substantially symmetrical about an axis of symmetry parallel to the periodic direction of the main feature. 63. A substrate as described in any one of clauses 55 to 62, wherein the main feature comprises a periodic array of main feature elements. 64. A substrate as described in clause 63, wherein some or all of the main feature elements each comprise a larger first sub-element and a plurality of smaller second sub-elements. 65. A substrate as described in clause 64, wherein the second sub-elements comprise features having a CD and / or a pitch at or near a product dimension.66. The substrate of claim 64 or 65, wherein the second sub-elements are formed in a periodic structure having a periodicity in a direction parallel to one of the longest dimensions of the first sub-elements. 67. The substrate of claim 64 or 65, wherein the second sub-elements are formed in a periodic structure having a periodicity in a direction perpendicular to one of the longest dimensions of the first sub-elements. 68. The substrate of any one of claims 64 to 67, wherein each of the first sub-elements comprises a solid or single-filled structure. 69. The substrate of any one of claims 64 to 67, wherein each of the first sub-elements comprises a periodic structure. 70. The substrate of any one of claims 55 to 69, wherein the focusing target is paired with a mirror image or a 180-degree rotated version of the focusing target on the substrate. 71. The substrate of any one of claims 55 to 70, wherein the focusing parameters describe a focusing setting for exposing one of the focusing targets in a lithography exposure procedure. 72. A patterning apparatus comprising forming the focusing target on the substrate to obtain a target patterned feature of the substrate as described in any of items 34 to 71. 73. A method for measuring a focusing parameter from a focusing target on a substrate as described in any of items 55 to 72, comprising: imaging superimposed fringes generated by interference between at least one diffraction order from the main feature and at least one corresponding diffraction order from the reference feature to obtain a measurement signal; and determining the focusing parameter from the measurement signal. 74. A computer program comprising program instructions operable to cause a measuring device to perform the method as described in any of items 1 to 33 when executed on a suitable device. 75. A non-transitory computer program carrier comprising the computer program as described in item 74. 76. A processing configuration comprising: a non-transitory computer program carrier as described in item 74; and a processor operable to run the computer program. 77. A measuring device comprising the processing configuration as described in clause 76. 78. A lithography apparatus comprising: a patterning device support for supporting a patterning device; a substrate support for supporting a substrate; wherein the lithography apparatus is operable to perform a determined focus parameter value when correcting a further lithography exposure.

[0085] The terms “radiation” and “beam” as used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having wavelengths of about 365 nm, 355 nm, 248 nm, 193 nm, 157 nm or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having wavelengths in the range of 5 nm to 20 nm), as well as particle beams (such as ion beams or electron beams).

[0086] The term "lens" may refer to any or a combination of various types of components (including refractive, reflective, magnetic, electromagnetic and electrostatic components) where the context allows.

[0087] The foregoing description of specific embodiments will fully disclose the general nature of the invention, enabling others to easily modify and / or adapt these specific embodiments for various applications by means of knowledge within the skill level of applying this art without departing from the general concept of the invention, without engaging in improper experimentation. Therefore, based on the teachings and guidance presented herein, such adaptations and modifications are intended to be within the meaning and scope of equivalents of the disclosed embodiments. It should be understood that the wording or terminology used herein is for descriptive and not limiting purposes, such that the terminology or terminology of this specification is to be interpreted by one skilled in the art in accordance with the teachings and guidance.

[0088] Therefore, the breadth and scope of the present invention should not be limited by any of the above exemplary embodiments, but should be defined only by the following claims and their equivalents. [Simplified Explanation of the Diagram]

[0015] Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: FIG1 depicts a lithography apparatus according to an embodiment of the invention; FIG2 depicts a lithography unit or cluster according to an embodiment of the invention; FIG3 includes (a) a schematic diagram of a dark-field scatterometer for measuring a target using a first pair of illumination apertures, (b) details of the diffraction spectrum of a target grating for a given illumination direction, (c) a second pair of illumination apertures providing an additional illumination mode when using the scatterometer for diffraction-based superposition measurements, and (d) a third pair of illumination apertures combining the first pair and the second pair; FIG4 depicts the outline of a multi-grating target of known form on a substrate and the measurement spot; FIG5 depicts an image of the target of FIG4 obtained in the scatterometer of FIG3; Figure 6 illustrates the following: (a) a first example of a focused target according to an embodiment; (b) printed targets obtained for different focus levels; (c) a plot of the measured centroids relative to the focus of two sub-targets of the target in Figure 6(a); and (d) a plot of the difference between the measured centroids relative to the focus from the plot of Figure 6(c); Figure 7 illustrates a second example of a focused target according to an embodiment; Figure 8 illustrates a third example of a focused target according to an embodiment; Figure 9 illustrates a fourth example of a focused target according to an embodiment; Figure 10 illustrates a first example of a superimposed pattern of a focused target according to an embodiment; Figure 11 illustrates a second example of a focused target according to an embodiment; Figure 12 (including Figure 12(a) and Figure 12(b)) illustrates a third example of a focused target according to an embodiment; and Figure 13 illustrates an example of an imaging superimposed pattern obtained when measuring any of the target examples illustrated in Figures 10 to 12.

Claims

1. A substrate comprising at least one focusing target or sub-target thereof, comprising: at least one periodic main feature having a main feature spacing, wherein a centroid and / or the main feature spacing is focally dependent; and a periodic reference feature having a reference feature spacing different from the main feature spacing, the main feature and the reference feature being configured such that scattered radiation from the main feature and the reference feature interferes to form a beat frequency signal or a ripple signal.

2. The substrate of claim 1, wherein the difference between the main feature spacing and the reference feature spacing is less than 20% of the main feature spacing.

3. The substrate of claim 1 or 2, wherein the reference feature is contained in a layer below the layer of interest containing one of the main features of each sub-target.

4. The substrate of claim 1 or 2, wherein both the reference feature and the main feature are contained in the same layer.

5. The substrate of claim 1 or 2, wherein the main feature has been formed with a focally dependent centroid, the centroid of each sub-target describing the peak position of a first harmonic of a focal target defined by a pattern during printing.

6. The substrate of claim 1 or 2, wherein each of the main features is formed with a focal interdependent distance, and the main features are substantially symmetrical about an axis of symmetry parallel to a periodic direction of the main features.

7. The substrate of claim 1 or 2, wherein the main feature comprises a periodic array of one of the main feature elements.

8. The substrate of claim 1 or 2, wherein the focusing target is paired on the substrate with a mirror or 180-degree rotated version of one of the focusing targets.

9. The substrate as requested in item 1 or 2, wherein the focus parameter describes a focus setting of a lithography exposure procedure for exposing one of the focus targets.

10. A patterning apparatus comprising a focus target on the substrate to obtain a target patterned feature of the substrate as claimed in any one of claims 1 to 9.

11. A method for measuring a focusing parameter on a focusing target on a substrate according to any one of claims 1 to 10, comprising: imaging a fringe generated by interference between at least one diffraction order from the main feature and at least one corresponding diffraction order from the reference feature to obtain a measurement signal; and determining the focusing parameter from the measurement signal.

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