METROLOGY METHODS AND RELATED COMPUTER PRODUCTS - Patent application

Abstract

A method is disclosed that includes measuring radiation reflected from a metrology target and decomposing the measured radiation into components, e.g., Fourier or spatial components. Further, a recipe selection method is disclosed that provides an algorithm for selecting parameters of a metrology device based on a recalculated dependence of the measured radiation on a single component.

Description

[Technical field]

[0001] The present invention relates to metrology methods, apparatus, and computer products that can be used, for example, in the manufacture of devices by lithographic techniques. [Background technology]

[0002] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A patterning device, alternatively called a mask or reticle, may then be used to generate the circuit pattern formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on the substrate (e.g. a silicon wafer). Transfer of the pattern is typically by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Generally, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0003]

[0003] In lithography processes (i.e., the development process of devices or other structures that typically involve lithographic exposure, which may include one or more associated processing steps such as developing resist, etching, etc.), it is frequently desirable to make measurements of the structures produced, for example, for process control and verification. Various tools are known for making such measurements, including scanning electron microscopes, often used to measure critical dimensions (CD), and dedicated tools for measuring overlay (the accuracy of the alignment of two layers of a substrate). In recent years, various forms of scatterometers have been developed for use in the lithography field. These devices direct a beam of radiation onto a target and, by measuring one or more properties of the scattered radiation (e.g., intensity at a single reflected angle as a function of wavelength, intensity at one or more wavelengths as a function of reflected angle, or polarization as a function of reflected angle), obtain a "spectrum" from which a property of interest of the target can be determined. Determination of the property of interest can be performed by various techniques, for example, reconstruction of the target structure by iterative techniques such as rigorous coupled wave analysis or finite element method, library searching, and principal component analysis. Summary of the Invention [Means for solving the problem]

[0004]

[0004] In metrology applications, for example in overlay metrology, radiation from a source impinges on a target that includes overlapping gratings, and the reflected radiation is detected on a sensor. The reflected radiation is the result of a combination of various portions of the impinging radiation as they propagate (reflect or transmit) through the metrology target. In the presence of grating asymmetries, for example geometric asymmetries of the gratings, the reflected radiation also contains information about these asymmetries, which may be hidden by the overlay between the overlapping gratings. Furthermore, realistic gratings may have asymmetries such as tilt. It may be desirable to be able to measure the overlay or other parameters of interest of the lithography process in a manner that is invariant to the asymmetries present in the real metrology gratings.

[0005] In a first aspect of the invention, a method is provided that includes measuring radiation reflected from a metrology target and resolving the measured radiation into components.

[0006]

[0006] In a second aspect of the present invention, there is provided a method for measuring a parameter of a lithography process, comprising: a) directing radiation at a metrology target; b) detecting scattered radiation from the target; c) varying a parameter of the metrology apparatus; d) repeating steps a) to c) for multiple values ​​of the parameter of the metrology apparatus; and e) decomposing the radiation into components.

[0007]

[0007] In a third aspect of the present invention, there is provided a method for measuring a parameter of a lithography process, comprising: a) directing radiation at a metrology target; b) detecting scattered radiation from the target; c) varying a parameter of the metrology apparatus; d) repeating steps a) to c) for multiple values ​​of the parameter of the metrology apparatus; and e) filtering the measurements obtained in step d).

[0008]

[0008] In a fourth aspect of the present invention, there is provided a method for characterizing a lithography process, comprising obtaining a 3D asymmetry map of a lower grating at a first target location, repeating the obtaining of 3D asymmetry maps for a number of targets, and obtaining a map of target asymmetry for the wafer based on the above measurements.

[0009]

[0009] In a fifth aspect of the present invention, a method for selecting a parameter of a metrology device is provided, the method comprising obtaining a first plurality of measurements at a first plurality of values ​​of the parameter of the metrology device, and calculating a minimum number of second measurements and associated second values ​​of the parameter of the metrology device such that the second values ​​of the parameter of the metrology device are less than the first value of the parameter of the metrology device.

[0010] Another aspect of the invention comprises a computer program for performing the method of the first aspect, and an associated computer program carrier.

[0011]

[0011] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 depicts a lithographic apparatus according to one embodiment of the invention. [Diagram 2] FIG. 2 illustrates a lithographic cell or cluster in accordance with one embodiment of the present invention. [Figure 3A] FIG. 3(a) is a schematic diagram of a dark field measurement apparatus used to measure a target, according to one embodiment of the present invention, with a first pair of illumination apertures providing an illumination mode. [Figure 3B] FIG. 3(b) is a schematic detail of the diffraction spectrum of the target for a given illumination direction. [Figure 3C]FIG. 3(c) is a schematic diagram of a second pair of illumination apertures that provides an additional illumination mode when using the metrology apparatus for diffraction-based overlay metrology. [Figure 3D]

[0012] Figure 3(d) is a schematic diagram of a third pair of illumination apertures that combines the first and second pairs of apertures to provide an additional illumination mode when using the measurement apparatus for diffraction-based overlay measurement. [Figure 4] FIG. 4 illustrates the configuration of a multi-periodic structure (eg, multi-grating) target and the outline of a measurement spot on a substrate. [Diagram 5] FIG. 5 illustrates an image of the target of FIG. 4 acquired in the apparatus of FIG. [Figure 6] FIG. 6 is a flow chart illustrating the steps of an overlay measurement method that uses the apparatus of FIG. 3 and that is applicable to embodiments of the present invention. [Figure 7A] FIG. 7(a) shows a schematic cross-section of an overlay periodic structure (eg, a grating) having different overlay values ​​in the zero regions. [Figure 7B] FIG. 7(b) shows a schematic cross-section of an overlay periodic structure (eg, a grating) having different overlay values ​​in the zero regions. [Figure 7C] FIG. 7(c) shows a schematic cross-section of an overlay periodic structure (eg, a grating) having different overlay values ​​in the zero regions. [Figure 7D] FIG. 7(d) shows a schematic cross-section of an overlay periodic structure (eg, a grating) having different overlay values ​​in the zero regions. [Figure 8] FIG. 8 illustrates the principle of overlay measurement in an ideal target structure. [Figure 9] FIG. 9 is a graph of overlay sensitivity K, also called the swing curve, versus wavelength λ (nm) for a given target. [Figure 10] FIG. 10 shows a schematic diagram of a cross section of a metrology target. [Figure 11] FIG. 11 shows a graph of values ​​inferred from the measured metrology as a function of a parameter of the metrology tool, such as wavelength. [Figure 12] FIG. 12 shows a graph of values ​​inferred from a metrology step according to the present invention measured as a function of a metrology tool parameter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Before describing the embodiments in detail, it is helpful to provide an example environment in which the embodiments may be implemented.

[0014] 1 depicts a lithographic apparatus LA comprising an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV or DUV radiation), a patterning device support or support structure (e.g. mask table) MT constructed 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, a substrate table (e.g. wafer table) WT constructed to hold a substrate (e.g. resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate according to certain parameters, and a projection system (e.g. refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C of the substrate W (e.g. comprising one or more dies).

[0015]

[0015] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping or controlling radiation.

[0016]

[0016] The patterning device support holds 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 for example whether the patterning device is held in a vacuum environment or not. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support may be a frame or a table, which may be fixed or movable as required. The patterning device support may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms "reticle" or "mask" herein may be considered as synonymous with the more general term "patterning device".

[0017]

[0017] The term "patterning device" as used herein shall be broadly interpreted to refer to any device that can be used to impart a radiation beam having a pattern in its cross-section to a target portion of a substrate to generate a pattern. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so-called assist features. Typically, the pattern imparted to the radiation beam corresponds to a particular functional layer in a device to be generated in the target portion, such as an integrated circuit.

[0018]

[0018] A patterning device may be of a transmissive or reflective type. 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 employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilting mirrors impart a pattern to the radiation beam that is reflected by the mirror matrix.

[0019] As depicted here, the apparatus is of a transmissive type (e.g. employing a transmissive mask) Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as described above, or employing a reflective mask).

[0020]

[0020] The lithographic apparatus may be of a type in which at least a part of the substrate may be covered by a liquid having a relatively high refractive index, for example water, to fill a space between the projection system and the substrate. Immersion liquid may also be provided in other spaces in the lithographic apparatus, for example between the mask and the projection system. Immersion techniques are well known in the art to increase the numerical aperture of projection systems. The term "immersion" as used herein does not imply that a structure such as the substrate has to be submerged in liquid, but simply that a liquid is located between the projection system and the substrate during exposure.

[0021]

[0021] Referring to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The radiation source and the lithographic apparatus may be separate entities, for example when the radiation source is an excimer laser. In such a case, the radiation source is not considered to form part of the lithographic apparatus, and the radiation beam is passed from the radiation source SO to the illuminator IL using a beam delivery system BD, for example including suitable guiding mirrors and / or a beam expander. In other cases, the radiation source may be an integrated part of the lithographic apparatus, for example when the radiation source is a mercury lamp. The radiation source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0022]

[0022] The illuminator IL may include an adjuster AD configured to adjust the angular intensity distribution of the radiation beam. In general, it is possible to adjust at least the outer and / or inner radius ranges of the intensity distribution within the pupil plane of the illuminator (usually referred to as σ-outer and σ-inner respectively). 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 adjust the radiation beam so that it has a desired uniformity and intensity distribution in its cross-section.

[0023]

[0023] The radiation beam B is incident on and patterned by a patterning device (e.g. mask) MA, which is held on a patterning device support (e.g. mask table MT). After traversing the patterning device (e.g. mask) MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of a substrate W. Using 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 to position a different target portion C in the path of the radiation beam B. Similarly, the patterning device (e.g. mask) MA can be accurately positioned with respect to the path of the radiation beam B using the first positioner PM and another position sensor (not explicitly depicted in Figure 1), for example after a machine search of a mask library or during a scan.

[0024]

[0024] The patterning device (eg mask) MA and substrate W may be aligned with mask alignment marks M. 1 , M 2 and substrate alignment mark P 1 , P 2 . Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between the target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g. mask) MA, the mask alignment marks may be located between the dies. Small alignment markers may also be contained within a die, among other device features, in which case it is desirable for the marker to be as small as possible and not require different imaging or process conditions than adjacent features. Embodiments of alignment systems capable of detecting alignment markers are further described below.

[0025] The depicted apparatus could be used in at least one of the following modes: 1. In step mode, while the entire pattern imparted to the radiation beam is projected onto the target portion C at once (i.e., single static exposure), the patterning device support (e.g., mask) MT and the substrate table WTa basically remain stationary. Next, the substrate table WTa is shifted in the X and / or Y directions so that different target portions C can be exposed. In step mode, the maximum size of the exposure field is limited by the size of the target portion C imaged by a single static exposure. 2. In scan mode, while the pattern imparted to the radiation beam is projected onto the target portion C, the patterning device support (e.g., mask) MT and the substrate table WTa are scanned synchronously (i.e., single dynamic exposure). The speed and direction of the substrate table WTa relative to the patterning device support (e.g., mask) MT may be determined by the magnification (reduction) and image inversion characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion of a single dynamic exposure, and the length of the scanning operation determines the height (in the scanning direction) of the target portion. 3. In another mode, while the pattern imparted to the radiation beam is projected onto the target portion C, the patterning device support (e.g., mask) MT holds the programmable patterning device and basically remains stationary, and the substrate table WTa is moved or scanned. In this mode, generally a pulsed radiation source is used, and the programmable patterning device is updated as needed after each movement of the substrate table WTa or between consecutive radiation pulses during scanning. This mode of operation can be easily applied to maskless lithography using a programmable patterning device such as the programmable mirror array of the type described above.

[0026]

[0026] The above usage modes or combinations and / or variations of completely different usage modes may be used.

[0027]

[0027] The lithographic apparatus LA is of the so-called dual stage type, having two tables WTa, WTb (e.g. two substrate tables) and two stations (exposure station and measurement station) between which the tables can be exchanged. For example, while a substrate on one table is being exposed at the exposure station, another substrate can be loaded on the other substrate table at the measurement station and various preparation steps can be performed. The preparation steps can include mapping of the surface control of the substrate using the level sensor LS and measuring the position of an alignment marker on the substrate using the alignment sensor AS (both sensors are supported by the reference frame RF). If the position sensor IF is not able to measure the position of the table while located at the measurement station and the exposure station, a second position sensor may be provided to enable tracking of the position of the table at both stations. As another example, while a substrate on one table is being exposed at the exposure station, another table without a substrate waits at the measurement station (optionally where measurement activities can take place). This other table has one or more measurement devices and may optionally have other tools (e.g. cleaning apparatus). Once the substrate has completed exposure, the table without the substrate moves to an exposure station, for example to take measurements, and the table with the substrate moves to a location where the substrate is unloaded and another substrate is loaded (e.g. a measurement station). These multi-table arrangements allow for a significant increase in the throughput of the apparatus.

[0028]

[0028] As shown in Fig. 2, the lithographic apparatus LA forms part of a lithographic cell LC, sometimes also called a lithocell or lithocluster, which also includes apparatuses performing one or more pre-exposure and post-exposure processes on the substrate. Conventionally, these include one or more spin coaters SC for depositing a resist layer, one or more developers DE for developing the exposed resist, one or more cooling plates CH, and one or more bake plates BK. A substrate handler or robot RO picks up the substrate from input / output ports I / O1, I / O2, moves it between the different process devices, and delivers it to the loading bay LB of the lithographic apparatus. These devices, often collectively called a track, are under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS (which also controls the lithographic apparatus, by means of a lithography control unit LACU). Thus, by operating the different apparatuses, throughput and processing efficiency can be maximized.

[0029]

[0029] In order for a substrate exposed by a lithographic apparatus to be accurately and consistently exposed, it is desirable to inspect the exposed substrate to measure one or more characteristics, such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc. If an error is detected, adjustments can be made to the exposure of one or more subsequent substrates, especially if the inspection can be done immediately and quickly enough that another substrate of the same batch is still to be exposed. Also, already exposed substrates may be stripped and reworked (to improve yield) or discarded, thereby avoiding exposures on substrates known to be defective. If only some target portions of a substrate are defective, further exposures may be made only on good target portions. Another possibility is to adapt the settings of subsequent process steps to compensate for the error, for example the time of a trim etch step can be adjusted to compensate for substrate-to-substrate CD variations due to lithographic process steps.

[0030]

[0030] Using an inspection device, it is determined how one or more properties of a substrate, and specifically one or more properties of different substrates, or different layers of the same substrate, vary from layer to layer and / or across the substrate. The inspection device may be incorporated into a lithographic apparatus LA or a lithocell LC, or may be a stand-alone device. In order to enable the fastest measurements, it is desirable for the inspection device to measure one or more properties of the exposed resist layer immediately after exposure. However, the latent image of the resist has a very low contrast (there is only a very small difference in refractive index between the part of the resist exposed to radiation and the part of the resist not exposed to radiation), and not all inspection devices have sufficient sensitivity to make useful measurements of the latent image. Therefore, the measurement is the first step customarily performed on the exposed substrate, and may be performed after a post-exposure bake step (PEB) that increases the contrast between the exposed and non-exposed parts of the resist. At this stage, the image of the resist can be called semi-latent. It is also possible to perform measurements of the developed resist image (at this point, the exposed or non-exposed part of the resist has been removed), or after a pattern transfer step such as etching. The latter possibility limits the possibility of reworking defective substrates, but can still provide useful information, for example, for process control.

[0031]

[0031] The target used by conventional scatterometers includes a relatively large periodic structure layout (e.g., including one or more gratings), for example 40 μm×40 μm. In that case, the measurement beam often has a spot size smaller than the periodic structure layout (i.e., the layout is underfilled such that one or more of the periodic structures are not completely covered by the spot). This simplifies the mathematical reconstruction of the target, since it can be considered infinite. However, since the target can be located, for example, in a product feature rather than in a scribe line, the size of the target is reduced, for example, to 20 μm×20 μm or less, or even 10 μm×10 μm or less. In this situation, the periodic structure layout may be made smaller than the measurement spot (i.e., the periodic structure layout is overfilled). Typically, such targets are measured using dark-field scatterometry, in which the zeroth diffraction order (corresponding to specular reflection) is blocked and only higher orders are processed. Examples of dark field metrology can be found in PCT Patent Application Publications WO 2009 / 078708 and WO 2009 / 106279, which are hereby incorporated in their entirety. Further developments of this technology are described in US Patent Application Publications 2011-0027704, 2011-0043791, and 2012-0242970, which are hereby incorporated in their entirety. Diffraction-based overlay with dark field detection of diffraction orders (DBO or μDBO) allows overlay measurements on smaller targets. These targets may be smaller than the illumination spot and may be surrounded by the product structure on the substrate. In some embodiments, multiple targets can be measured in one image.

[0032]

[0032] In an embodiment, the target on the substrate may include one or more 1D periodic gratings printed such that after development, the bars are formed from solid resist lines. In an embodiment, the target may include one or more 2D periodic gratings printed such that after development, the gratings are formed from solid resist pillars or vias in the resist. The bars, pillars, or vias may alternatively be etched into the substrate. The pattern of the grating is sensitive to chromatic aberrations of the lithographic projection apparatus (particularly the projection system PL), and illumination symmetry and the presence of such aberrations will manifest themselves in variations of the printed grating. Thus, the measurement data of the printed grating can be used to reconstruct the grating. Parameters of the 1D grating, such as line width and shape, or parameters of the 2D grating, such as width, length, or shape of the pillars or vias, may be input to the reconstruction process performed by the processing unit PU from knowledge of the printing step and / or other measurement processes.

[0033] A dark-field metrology apparatus suitable for use in an embodiment of the present invention is shown in FIG. 3A. The target T (including a periodic structure such as a grating) and the diffracted rays are illustrated in more detail in FIG. 3B. The dark-field metrology apparatus may be a stand-alone device or may be integrated into the lithography apparatus LA, for example at a measurement station, or into the lithographic cell LC. The optical axis with several branches throughout the apparatus is represented by the dotted line O. In this apparatus, radiation emitted by an output 11 (for example a source such as a laser or a xenon lamp, or an aperture connected to a source) is directed onto the substrate W through a prism 15 by an optical system including lenses 12, 14, and an objective lens 16. These lenses are arranged in a double sequence in a 4F configuration. A different lens arrangement can be used, provided that it still provides an image of the substrate on the detector.

[0034]

[0034] In an embodiment, the lens arrangement allows access of an intermediate pupil plane for spatial frequency filtering. The angular range at which the radiation is incident on the substrate can therefore be selected by defining a spatial intensity distribution in a plane that represents the spatial spectrum of the substrate plane, referred to herein as the (conjugate) pupil plane. In particular, this can be done, for example, by inserting an aperture plate 13 of suitable form between the lenses 12 and 14 in a plane that is a back-projected image of the objective pupil plane. In the illustrated example, the aperture plate 13 has different forms (labeled 13N and 13S), allowing different illumination modes to be selected. The illumination system of the present example forms an off-axis illumination mode. In a first illumination mode, the aperture plate 13N provides off-axis illumination from a direction designated "north" for illustrative purposes only. In a second illumination mode, a similar illumination is provided from the opposite direction, labeled "south", with the aperture plate 13S. Using different apertures, other illumination modes are possible. The remainder of the pupil plane is desirably dark, since unwanted radiation outside the desired illumination mode may interfere with the desired measurement signal.

[0035]

[0035] As shown in FIG. 3(b), the target T is installed in a state where the substrate W is substantially perpendicular to the optical axis O of the objective lens 16. The illumination light ray I that collides with the target T from an angle deviated from the axis O generates a zero-order ray (solid line 0) and two first-order rays (one-dot chain line +1 and two-dot chain line -1). In the case of an overfilled small target T, these rays are just one of many parallel rays that cover the area of the substrate including the metrology target T and other features. Since the aperture of the plate 13 has a finite width (necessary to accept a useful amount of radiation), the incident light ray I actually occupies a certain angular range, and the diffracted rays 0 and +1 / -1 are slightly spread out. According to the point image distribution function of the small target, each of the orders +1 and -1 further spreads over a certain angular range (not a single ideal ray as shown). Note that the pitch of the periodic structure and the illumination angle can be designed or adjusted so that the first-order rays entering the objective lens are exactly aligned with the central optical axis. The rays shown in FIGS. 3(a) and 3(b) are shown slightly off-axis only so that they can be more easily distinguished in the figure. At least the zero-order and +1st-order diffracted by the target on the substrate W are collected by the objective lens 16 and guided back through the prism 15.

[0036] Returning to FIG. 3(a), both the first and second illumination modes are illustrated by designating diametrically opposed apertures, labeled North (N) and South (S). When the incident ray I is from the north side of the optical axis, i.e., when the first illumination mode is applied with the aperture plate 13N, the +1 diffracted ray (labeled +1(N)) enters the objective lens 16. In contrast, when the second illumination mode is applied with the aperture plate 13S, the −1 diffracted ray (labeled −1(S)) is the diffracted ray that enters the lens 16. Thus, in an embodiment, measurements are obtained by measuring the target twice under certain conditions, for example, after rotating the target, or after changing the illumination mode, or after changing the imaging mode, to obtain the −1 and +1 diffraction order intensities separately. Comparing these intensities for a given target provides a measure of the target asymmetry, which can be used as an indicator of a lithography process parameter (e.g., overlay error). In the above circumstances, the lighting mode will be changed.

[0037]

[0037] A beam splitter 17 splits the diffracted beam into two measurement branches. In the first measurement branch, an optical system 18 uses the zeroth and first diffracted 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 processing of the image allows the orders to be compared and contrasted. The pupil plane image captured by the sensor 19 can be used to focus the metrology device and / or to normalize the intensity measurement of the first beam order. The pupil plane image can also be used for a number of measurement purposes, such as reconstruction, which are not described in detail here.

[0038] In the second measurement branch, the optical system 20, 22 forms 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 provided in a plane conjugate with the pupil plane. The aperture stop 21 serves to block the 0th order diffracted beam so that the image DF of the target formed on the sensor 23 is formed from the -1 or +1 order beam. The images captured by the sensors 19 and 23 are output to an image processor and controller PU (whose function depends on the specific type of measurement to be performed). Note that the term "image" is used here in a broad sense. So, if only one of the -1 and +1 orders is present, no image of the periodic structure features (e.g. grating lines) is formed.

[0039]

[0039] The particular configuration of the aperture plate 13 and stop 21 shown in Fig. 3 are merely examples. In another embodiment of the invention, on-axis illumination of the target is used, and an aperture stop with an off-axis aperture is used to send substantially only the first order diffracted radiation to the sensor. In yet other embodiments, second, third and higher order beams (not shown in Fig. 3) can be used in the measurement instead of or in addition to the first order beam.

[0040]

[0040] To make the illumination adaptable to these different types of measurements, the aperture plate 13 may include several aperture patterns formed around a disk, which is rotated to bring the desired pattern into place. Note that aperture plate 13N or 13S is used to measure the periodic structure of the target oriented in one direction (X or Y, depending on the setup). For the measurement of orthogonal periodic structures, a rotation of the target through 90° and 270° may be performed. Different aperture plates are shown in Fig. 3(c) and (d). Fig. 3(c) illustrates two further types of off-axis illumination modes. In the first illumination mode of Fig. 3(c), aperture plate 13E provides off-axis illumination from a direction designated "East" with respect to the aforementioned "North" for illustrative purposes only. In the second illumination mode of Fig. 3(c), a similar illumination is provided from the opposite direction, labeled "West", using aperture plate 13W. Figure 3(d) illustrates two further types of off-axis illumination modes. In the first illumination mode of Figure 3(d), aperture plate 13NW provides off-axis illumination from directions designated "North" and "West" as previously described. In the second illumination mode, aperture plate 13SE is used to provide similar illumination from the opposite directions, labeled "South" and "East" as previously described. These uses, as well as many other variations and applications of the device, are described, for example, in the previously published patent application publications mentioned above.

[0041]

[0041] Figure 4 illustrates an example of a composite metrology target formed on a substrate. The composite target includes four periodic structures (in this case gratings) 32, 33, 34, 35 positioned close together. In an embodiment, the periodic structures are positioned so close together that they are all located within the measurement spot 31 formed by the illumination beam of the metrology device. Thus, in this case, the four periodic structures are all illuminated simultaneously and imaged simultaneously onto the sensors 19 and 23. In an example dedicated to overlay measurements, 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 the substrate W, and at least one periodic structure of one layer overlays at least one periodic structure of a different layer. Such targets may have outer dimensions within the range of 20 μm x 20 μm, or within the range of 16 μm x 16 μm. Furthermore, all periodic structures are used to measure the overlay between a particular pair of layers. To facilitate targets capable of measuring more than one pair of layers, the periodic structures 32, 33, 34, 35 may have differently biased overlay offsets to facilitate measuring the overlay between different layers in which different parts of a composite periodic structure are formed. Thus, one pair of layers is measured with all periodic structures of a target on a substrate, and another pair of layers is measured with all periodic structures of another same target on a substrate, with the different biases facilitating the distinction between the layer pairs. The meaning of the overlay bias is explained below, with particular reference to FIG. 7.

[0042]

[0042] Figures 7(a)-(c) show schematic cross-sections of overlaying periodic structures (in this case gratings) of each target T with different biases, which can be used for a substrate W, as seen in Figures 3 and 4. A periodic structure with periodicity in the X direction is shown by way of example only. Different combinations of these periodic structures with different biases and with different orientations may be provided.

[0043]

[0043] Starting with Fig. 7(a), a composite overlay target 600 is depicted formed of two layers, denoted L1 and L2. In the bottom layer L1, a first periodic structure (in this case a grating) is formed by features (e.g. lines) 602 and spaces 604 on a substrate 606. In layer L2, a second periodic structure (in this case a grating) is formed by features (e.g. lines) 608 and spaces 610. (The cross section is depicted such that the features 602, 608 extend into the page.) The periodic structure pattern repeats in both layers with pitch P. The lines 602 and 608 are mentioned merely as examples, and other types of features such as dots, blocks, and via holes can be used. In the situation shown in FIG. 7A, there are no overlay errors and biases, and therefore each feature 608 is located exactly on top of a feature 602 of the underlying periodic structure (if the measurement is "line-on-line" - in one embodiment, there may be no overlay error if each feature 608 is located exactly on top of a space 604 (if the measurement is "line-on-trench")).

[0044] In FIG. 7(b), the same target with bias +d is depicted with feature 608 of the upper periodic structure shifted to the right by a distance d relative to feature 602 of the lower periodic structure (distance d is less than pitch P). That is, feature 608 and feature 602 are positioned such that feature 608 would be offset by distance d relative to feature 602 if they were both printed exactly at their nominal locations. The bias distance d may in practice be a few nanometers, e.g., 10 nm, 20 nm, and the pitch P is, for example, in the range of 300-1000 nm, e.g., 500 nm or 600 nm. In FIG. 7C, the same target with bias -d is depicted with feature 608 shifted to the left relative to feature 602. This type of biased target shown in FIG. 7A-C and their use in measurements are described, for example, in the above-mentioned patent application publication.

[0045]

[0045] Furthermore, as alluded to above, although Figures 7(a)-(c) depict features 608 located on features 602 (with or without a small bias of +d or -d applied) referred to as a "line-on-line" target with bias in the zero region, the target may have a program bias of P / 2, half the pitch, such that each feature 608 of the upper periodic structure is located on a space 604 of the lower periodic structure. This is referred to as a "line-on-trench" target. Again, a small bias of +d or -d may be applied. The choice of either a "line-on-line" target or a "line-on-trench" target depends on the application.

[0046] Returning to FIG. 4, the periodic structures 32, 33, 34, 35 may have different orientations as shown to diffract incoming radiation in the X and Y directions. In one example, the periodic structures 32 and 34 are X-direction periodic structures with biases 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 shown, other embodiments may include a larger matrix to obtain the desired accuracy. For example, a 3×3 array of nine composite periodic structures may have biases of -4d, -3d, -2d, -d, 0, +d, +2d, +3d, +4d. Individual images of these periodic structures may be identified in the image captured by the sensor 23.

[0047]

[0047] Figure 5 shows an example of an image that can be formed on and detected by the sensor 23 using the target of Figure 4 in the apparatus of Figure 3, using the aperture plate 13NW or 13SE of Figure 3(d). The sensor 19 cannot resolve the different individual periodic structures 32-35, but the sensor 23 can. The dark rectangle represents the image field on the sensor, in which the illumination spot 31 on the substrate is imaged to a corresponding circular area 41. Within this, the rectangular areas 42-45 represent the images of the periodic structures 32-35. If the periodic structures are located within the product area, the product features may also be visible at the periphery of this image field. The image processor and controller PU processes these images using pattern recognition to identify the individual images 42-45 of the periodic structures 32-35. In this way, the images do not need to be very precisely aligned at a certain location within the sensor frame, which greatly improves the throughput of the measurement apparatus as a whole.

[0048]

[0048] Once the individual images of the periodic structures have been identified, the intensity of these individual images can be measured, for example by averaging or summing selected pixel intensity values ​​within the identified area. The intensities and / or other properties of the images can be compared to each other. These results can be combined to measure different parameters of the lithography process. Overlay accuracy is one example of such a parameter.

[0049]

[0049] Figure 6 illustrates how, for example, using the method described in PCT Patent Application Publication WO2011 / 012624, the overlay error between two layers containing component periodic structures 32-35 is measured by the asymmetry of the periodic structures revealed by comparing the intensity of +1 and -1 order dark field images. In step M1, a structure containing a target including periodic structures 32-35 is produced by processing a substrate (e.g., a semiconductor wafer) one or more times using the lithographic cell of Figure 2. In M2, an image of the periodic structures 32-35 is obtained using one of the first order diffracted beams (e.g., -1) using the metrology apparatus of Figure 3. In one embodiment, a first illumination mode (e.g., an illumination mode generated using aperture plate 13NW) is used. The other first order diffracted beam (+1) can then be used to acquire a second image of the periodic structure (step M3), e.g. by changing the illumination mode or by changing the imaging mode or by rotating the substrate W 180° in the field of view of the metrology apparatus. As a result, the +1 diffracted radiation is captured in the second image. In an embodiment, the illumination mode is changed and a second illumination mode (e.g. an illumination mode generated using aperture plate 13SE) is used. In an embodiment, by performing measurements at 0° and 180° substrate orientations, instrument-induced artifacts such as TIS (instrument-induced errors) can be eliminated.

[0050]

[0050] Note that by including only half of the first order diffracted radiation in each image, the "images" referred to here are not conventional dark-field microscope images. Individual periodic structure features are not resolved. Each periodic structure is represented simply by an area of ​​a certain intensity level. In step M4, a region of interest (ROI) is identified within the image of each component periodic structure from which the intensity level is measured.

[0051]

[0051] Having identified the regions of interest P1, P2, P3, P4 of each individual periodic structure 32-35 and measured its intensity, the asymmetry of the periodic structures and thus, for example, the overlay error can be determined. This is done by the image processor and controller PU in step M5, which compares the intensity values ​​obtained for the +1 and -1 orders of each periodic structure 32-35, thereby identifying the difference between their intensities, i.e. the asymmetry. The term "difference" is not intended to refer only to subtraction. The difference may also be calculated in the form of a ratio. In step M6, the measured asymmetry for several periodic structures, together with knowledge of the overlay bias of these periodic structures, if applicable, is used to calculate one or more performance parameters of the lithographic process in the vicinity of the target T. The performance parameter of interest is the overlay. Other parameters of the performance of the lithographic process can be calculated, such as focus and / or dose. The one or more performance parameters can be fed back for the improvement of the lithographic process (used to improve the measurement and calculation process of FIG. 6 itself, used to improve the design of the target T, etc.).

[0052]

[0052] In one embodiment of determining overlay, Fig. 8 depicts curves 702 illustrating the relationship between the overlay error OV and the measured asymmetry A for an "ideal" target with zero offset and no structural asymmetry within the individual periodic structures forming the overlay target. These graphs are intended to illustrate the principle of determining overlay only, and in each graph the units of the measured asymmetry A and the overlay error OV are arbitrary.

[0053]

[0053] In the "ideal" situation of Fig. 7(a)-(c), curve 702 shows that the measured asymmetry A has a sinusoidal relationship with the overlay. The period P of the sinusoidal variation corresponds to the period (pitch) of the periodic structure, of course converted to an appropriate scale. The sinusoidal shape is pure in this example, but in a real situation it may contain harmonics. For simplicity, this example assumes that (a) only the first order diffracted radiation from the target reaches the image sensor 23 (or its equivalent in some embodiments), and (b) the experimental target design is such that within these first orders there is a pure sinusoidal relationship between the intensity and the overlay result of the upper and lower periodic structures. Whether this is actually the case depends on the optical system design, the wavelength of the illumination radiation and the pitch P of the periodic structure, as well as the design and stacking of the target.

[0054] As mentioned above, rather than relying on a single measurement, the overlay can be measured using a bias periodic structure. The bias has a known value defined on the patterning device (e.g., reticle) from which it is generated, which serves as an on-substrate calibration of the overlay corresponding to the measurement signal. In the figures, this calculation is shown graphically. In steps M1-M5 of FIG. 6, the asymmetry measurement A +d and A -d are obtained for a component periodic structure (e.g., as shown in Figures 7B and 7C) with biases +d and -d, respectively. Fitting these measurements to a sinusoid gives points 704 and 706 as shown. Knowing the bias, the true overlay error OV can be calculated. The pitch P of the sinusoid is known from the target design. The vertical scale of curve 702 is an unknown coefficient that is not initially known, but can be called the overlay proportionality constant K.

[0055] In the equation terms, the overlay error OV E The relationship between and the intensity asymmetry A is A±d =Ksin(OV E ±d) where the overlay error OV E is expressed on a scale such that the target pitch P corresponds to an angle of 2π radians. The term d is the grating bias of the target (or sub-target) being measured. Using two measurements of a target with different known biases (e.g. +d and -d), the overlay error OV E teeth,

number

[0056]

[0056] Although these measurement techniques are fast and relatively computationally simple (once calibrated), they are based on the assumption that overlay / lateral shift is the only cause of asymmetry. That is, it assumes an "ideal" situation where the target has no structural asymmetry, for example. Structural asymmetry in the stack, such as asymmetry of features in one or both of the superimposed periodic structures, also gives rise to a first order asymmetry in addition to overlay / lateral shift. This structural asymmetry, which is not related to overlay, clearly causes measurement disturbances and leads to inaccurate results.

[0057]

[0057] As an example of structural asymmetry, one or more of the periodic structures of the target may be structurally deformed. For example, one or more sidewalls of the periodic structure features (e.g., grating lines) of the target may not be vertical as intended. As another example, one or more spaces between the periodic structure features of the target (e.g., grating spaces of trenches) may be larger or smaller than intended. Furthermore, one or more features of the periodic structure of the target (e.g., grating lines) may have a smaller or larger width than intended. Additionally, even if the difference from the intended one or more periodic structures of the target is uniform, the difference from the intended one may not be the same for one or more other periodic structures of the target. Structural asymmetry in the lower periodic structure of the composite target is a common form of structural asymmetry. It may result from, for example, a substrate processing step, such as chemical mechanical polishing (CMP), that is performed after the lower periodic structure is initially formed.

[0058]

[0058] Referring to Fig. 7(d), an example of structural asymmetry of the lower periodic structure is depicted in a schematic manner. The features and spaces of the periodic structures of Fig. 7(a)-(c) are shown with perfect square faces, when in reality the features and spaces have some slope and some roughness on the surface. Nevertheless, they are intended to be at least symmetric in profile. The features 602 and / or spaces 604 in the lower periodic structure of Fig. 7(d) no longer have any symmetrical form and are distorted, for example, by one or more processing steps. Thus, for example, the bottom surface of each space 604 is sloped. The sidewall angles of the features and spaces are also asymmetric. If the overlay is measured by the method of Fig. 6 using only two bias periodic structures, the structural asymmetry cannot be distinguished from the overlay, and as a result, the overlay measurement is unreliable.

[0059]

[0059] It has further been found that, in addition to or instead of the structural asymmetry of the target, stacking differences between adjacent periodic structures of a target or between adjacent targets can be a factor that adversely affects the accuracy of measurements such as overlay measurements. Stacking differences can be understood as unintentional differences in the physical configurations between adjacent periodic structures or targets. Stacking differences cause differences in the optical properties (e.g., intensity, polarization, etc.) of the measurement radiation between adjacent periodic structures or targets that are due to causes other than overlay errors, other than intentional biases, and other than structural asymmetries common to the adjacent periodic structures or targets. Stack differences can include thickness differences between adjacent periodic structures or targets (e.g., a difference in the thickness of one or more layers such that one periodic structure or target is higher or lower than another periodic structure or target that is designed to be at substantially equal height), refractive index differences between adjacent periodic structures or targets (e.g., a difference in the refractive index of one or more layers such that the total refractive index of one or more layers of one periodic structure or target differs from the total refractive index of one or more layers of another periodic structure or target despite being designed to have substantially equal total refractive index), material differences between adjacent periodic structures or targets (e.g., a material difference exists for one periodic structure or target from another periodic structure or target that is designed to have substantially the same material). These may include, but are not limited to, differences in the type of material of one or more layers, material uniformity, etc., differences in lattice period of adjacent periodic structures or target structures (e.g., the difference in lattice period for one periodic structure or target with another periodic structure or target designed to have substantially the same lattice period), differences in depth of adjacent periodic structures or targets (e.g., the difference in depth of one periodic structure or target structure due to etching with another periodic structure or target designed to have substantially the same depth), differences in feature width (CD) of adjacent periodic structures or targets (e.g., the difference in width of a feature of one periodic structure or target with another periodic structure or target designed to have substantially the same feature width), and the like.In some examples, the stack difference is introduced in the patterning process by processing steps such as CMP, layer deposition, etching, etc. In an embodiment, the periodic structures or targets are adjacent when they are within 200 μm of each other, within 150 μm of each other, within 100 μm of each other, within 75 μm of each other, within 50 μm of each other, within 40 μm of each other, within 30 μm of each other, within 20 μm of each other, or within 10 μm of each other.

[0060] Intensity asymmetry measurement A +d , A -d The effect of stack difference (which may also be referred to as lattice imbalance between lattices) on (where the subscript indicates the target bias for the target area corresponding to the ROI) is generally A +d =(K+ΔK)sin(OV E +d) A -d =(K-ΔK)sin(OV E -d) where ΔK represents the difference in overlay sensitivity due to stack differences. Thus, the overlay error OV E (assuming it is small) is

number

[0061]

[0061] The stack difference can be considered as a spatial stack parameter variation, i.e., a stack parameter variation across the substrate (target to target). Another problem that can be faced is stack parameter process drift, where one or more of the stack parameters of the target deviate from the optimum over time due to process drift. This can be considered as a temporal stack parameter variation.

[0062]

[0062] It is therefore desirable to derive a combination of target layout, measurement beam wavelength, measurement beam polarization, etc. that produces an accurate measurement of a desired process parameter (e.g., overlay) in the face of structural asymmetries, stack differences, stack parameter process drift, and other process variability, and / or results in a measurement of the desired process parameter that is robust to process variability. Thus, for example, it is desirable to perform measurements using a preferably optimal selection of target-measurement parameter combinations to obtain more accurate process parameter measurements and / or results in a measurement of the desired process parameter that is robust to process variability. This is because the measurement accuracy and / or sensitivity of a target may differ with respect to one or more attributes of the target itself and / or one or more attributes of the measurement radiation provided on the target, such as the wavelength of the radiation, the polarization of the radiation, and / or the intensity distribution (i.e., angular or spatial intensity distribution) of the radiation. In an embodiment, the wavelength range of the radiation is limited to one or more wavelengths selected from a range (e.g., selected from a range of about 400 nm to 900 nm). Furthermore, a choice of different polarizations of the radiation beam may be provided and various illumination shapes may be provided, for example using a number of different apertures, such that it may be desirable to determine an optimized measurement profile for a particular target.

[0063]

[0063] The measurement profile includes one or more parameters of the measurement itself, which may include one or more parameters related to the measurement beam and / or the measurement device used to perform the measurement. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, the one or more parameters of the measurement itself may include the wavelength of the measurement radiation, and / or the polarization of the measurement radiation, and / or the measurement radiation intensity distribution, and / or the illumination angle of the measurement radiation to the substrate (e.g., the angle of incidence, the azimuth angle, etc.), and / or the relative orientation of the diffraction measurement radiation to the pattern on the substrate, and / or the number of measurement points or instances of the target, and / or the location of the instances of the target measured on the substrate. The one or more parameters of the measurement itself may include one or more parameters of the metrology device used for the measurement, which may include the detector sensitivity, the numerical aperture, etc.

[0064]

[0064] In this context, the pattern to be measured (also called "target" or "target structure") may be a pattern to be optically measured, e.g., the diffraction of which is to be measured. The pattern to be measured may be a pattern specifically designed or selected for measurement purposes. Multiple copies of the target may be placed at many locations on the substrate. For example, a substrate measurement recipe may be used to measure overlay. In an embodiment, a substrate measurement recipe may be used to measure another process parameter (e.g., dose, focus, CD, etc.). In an embodiment, the measurement profile may be used to measure the alignment of a layer of a pattern to be imaged relative to an existing pattern on the substrate, e.g., the measurement profile may be used to align a patterning device to a substrate by measuring the relative position of the substrate.

[0065]

[0065] Several methods for evaluating and optimizing target-measurement parameter combinations have been described. Such methods are performed prior to manufacturing. Thus, once optimized, one or more selected target-measurement parameter combinations are typically used throughout the manufacturing process, i.e., a predetermined measurement profile is used to measure targets of the corresponding target design according to the predetermined target-measurement parameter combination. However, as previously described, there may be unintentional stack parameter variations in the targets that cause stack differences between targets and / or stack parameter process drift. For example, the layer thicknesses of one or more layers in the stack may vary across a substrate (i.e., between targets) and / or over time (i.e., drift). One consequence of this stack parameter variation may be that the measurement profile is no longer optimal for the target. This may result in inaccurate measurements of the target. Also, stack parameter variations may generally be indicative of process control issues (e.g., process drift) and thus may be a useful process monitoring metric in itself.

[0066]

[0066] Methods for evaluating and optimizing target-measurement parameter combinations may include analyzing target response sequence data (e.g., spectral sequence data) describing the measurement profile, in particular the variation of the target response with the variation of one or more parameters of the measurement radiation, such as wavelength. In an embodiment, the target response sequence data may represent the oscillation dependence of the measurement data (e.g., intensity metrics acquired as field data (at the image plane) or as pupil data (at the pupil plane)) as a function of the measurement radiation wavelength. Figure 9 is an example graph of data for a target for an intensity metric, in this particular example, the measurement of the overlay sensitivity K at various wavelengths λ for a single polarization (in this case, linear X-polarization). The curve K(λ) has been fitted using the data above, and thus this representation may be referred to as a swing curve. As will be appreciated, a graph need not be generated, as the data may simply be processed. Similar graphs of data may be constructed for measurements at various wavelengths for a different single polarization (e.g., linear Y-polarization) for the same target. In Figure 9, stack sensitivity and overlay sensitivity are plotted for various measurement beam wavelengths. Furthermore, although the polarization here is linear X-polarized light, it may be a different polarization (such as linear Y-polarized light, left-handed elliptically polarized radiation, right-handed elliptically polarized radiation, etc.).

[0067]

[0067] The intensity metric may be any suitable metric derived from the detected intensity, e.g., intensity asymmetry, overlay sensitivity K, or stack sensitivity (SS) (also signal contrast). Stack sensitivity can be understood as a measure of how much the intensity of the signal changes as the overlay changes due to diffraction between target (e.g., grating) layers. That is, in the overlay situation, it detects the contrast between the upper and lower periodic structures of the overlay target, and thus represents the balance between the diffraction efficiencies of the upper and lower periodic structures. It is therefore an example measure of the sensitivity of the measurement. In one embodiment, stack sensitivity is the ratio of intensity asymmetry to average intensity. In one embodiment, stack sensitivity is expressed as SS=KL / IM where L is a user-defined constant (e.g., in one embodiment, the value L is 20 nm and / or the value of the bias d), and I M is the average intensity of the measurement beam diffracted by the target.

[0068] The example of FIG. 9 shows a swing curve for the overlay sensitivity K(λ) as a function of wavelength λ, where:

number

[0069] 10 shows overlapping gratings used in metrology processes. It includes an upper grating 101 and a lower grating 102. In this particular example, a particular form of geometric asymmetry is highlighted, such as the tilt of the lower grating 102 as defined by angle 103. The upper grating is formed on a material 104, which in this particular example includes alternating layers having different refractive indices that form part of a semiconductor device, this is one particular example and in no way a limitation of the stack.

[0070] 10, due to the tilt of the lower grating, the overlay, defined as the relative distance between the upper grating 101 and the lower grating 102, may have different values, such as ov1, 131, or ov2, 132, or even ov3, 133. The radiation forming the radiation flux impinging on the detector is formed, by way of example, from rays 120, 121, 122, and 123, where ray 120 is the radiation reflected by the upper grating 101, radiation 121 is the radiation reflected by the top of the lower grating 102, radiation 122 is the radiation reflected by a part of the lower grating 102 located at a distance D+H from the upper grating, and radiation 123 is the radiation reflected by the bottom of the lower grating 102. As can be seen from the simplified example of wave propagation in a metrology target, all the radiation fluxes 120, 121, 122, and 123 contribute to the radiation forming the radiation flux impinging on the detector. Furthermore, radiation bundle 120 together with radiation bundle 121 carries information about ov1, 131, radiation bundle 120 together with radiation bundle 122 carries information about ov2, 132, and radiation bundle 120 together with radiation bundle 123 carries information about ov3, 133. Thus, the radiation impinging on the detector carries information about all possible overlays that can be defined for the target, including geometric asymmetries, such as grating tilt, floor tilt, top tilt, etc. It is therefore a challenge of the present metrology process to be able to determine which overlay values ​​are measured when the grating contains geometric asymmetries, such as tilt.

[0071]

[0071] The propagation of radiation in the metrology target can be further described as a wave having intensity I, frequency ω, and phase φ, for example, as seen in Equation 1. I=A+Bcos(ωt+φ) Equation 1

[0072]

[0072] Where A is the offset, B is the amplitude parameter of the wave, the frequency ω is proportional to 2πn(D+H), where n is the refractive index, D+H is as shown in Figure 10, and t is 1 / wavelength of the light used as radiation. From this description, equation 1 can describe the interference of wave 120 with propagating waves 121, 122, and 123. That is, the interference of wave 120 with each of waves 121, 122, and 123 (as well as all of the many waves that are possible given equation 1 and the stack geometric parameters) is characterized by a particular frequency in length units (e.g., nanometers), depending on where the wave is reflected back towards the detector.

[0073] 11 further shows a parameter 201 inferred from a metrology measurement as a function of a parameter of the metrology device, e.g., wavelength 202. This dependence takes the form of a periodic variation of the parameter 201 as a function of wavelength. The period 203 depends on the total thickness of the stack, i.e., the distance between the upper grating 101 and the lower grating 102. The period is shorter for thicker stacks and longer for thinner stacks.

[0074]

[0074] In a first aspect of the invention, a method is proposed that includes measuring radiation reflected from a metrology target and decomposing the measured radiation into components. In an embodiment, the decomposition of the measured radiation is obtained using a Fourier transform of the measured radiation. In an embodiment, the measured radiation is radiation 201 as shown in FIG. 11. The result of the decomposition of the measurement result into components is further shown in FIG. 12. FIG. 12 shows a Fourier transform of element 201 as a function of wavelength, which includes the amplitude of component 302 as a function of frequency (e.g., in nm). The individual components are 310, 311, and 312, which correspond to the radiation formed by 120, 121, 122, and 123, respectively. The detection of the individual components is considered part of the art, and Fourier transform techniques allow the identification of each of the individual frequency components of the signal shown in FIG. 11. The frequency components can also be detected by other techniques, including (but not limited to) wavelet transform and Laplace transform.

[0075]

[0075] In a further aspect of the invention, there is provided a method for measuring a parameter of a lithography process comprising: a) directing radiation to a metrology target; b) detecting scattered radiation from the target; c) varying a parameter of the metrology device; d) repeating steps a)-c) for multiple values ​​of the parameter of the metrology device; and e) decomposing the radiation into components. In an embodiment of the invention, the decomposition is a Fourier transform. In an embodiment of the invention, the method selects components that are smaller than a threshold 320. In an embodiment, the overlay is obtained using an inverse decomposition and utilizing only the selected components. In an embodiment, the inverse decomposition is an inverse Fourier transform. The overlay is further obtained using a state-of-the-art method according to US Patent Application Publication No. 2012-0242970, which is incorporated herein by reference in its entirety. In an embodiment of the invention, the threshold 320 is chosen such that only one component remains, 310 in FIG. 12, and the remaining signal is used in the inverse decomposition step. In this case, the only waves that contribute to the measured radiation are waves 120 and 121, so the only overlays that are measured are ov1, 131.

[0076] In a further aspect of the invention, acquiring a spectrum as shown in Figure 12 allows for the measurement of the phase of each of the acquired harmonics. In an embodiment, the overlay can be calculated from the difference in phase of the harmonics in Figure 12, the difference in phase being acquired for the positive and negative first diffraction orders. The Fourier phase of a harmonic corresponding to a particular depth D1 is given by Φ ±1 =θ(Z+D1)±iφ OVL where OVL is the overlay value, D1 is the depth for which the harmonics are calculated, θ is the general phase, and φ OVL is the phase proportional to the overlay. Now, as mentioned above, if we want to obtain the difference between the Fourier phases of a particular harmonic, Φ +1 -Φ -1 =2φ OVL= 2 x 2π x OVL / pitch, which allows extraction of overlay values ​​only from the phase of said Fourier harmonics. It should be appreciated that such a method of calculating overlay, which involves obtaining overlay values ​​from parameters proportional to the phase of the Fourier harmonics of a measured radiation parameter, is unique to the present invention and provides an alternative method of measuring overlay in a metrology process to known methods used in the state of the art. In one aspect of the present invention, there is provided a method of measuring a parameter of a process comprising measuring radiation reflected from a metrology target, decomposing the measured radiation into components, calculating at least two phase values ​​representative of each component, and calculating a parameter from the relationship between said phases.

[0077]

[0077] In a further aspect of the invention, there is provided a method of measuring a parameter of a lithography process comprising: a) directing radiation to a metrology target; b) detecting scattered radiation from the target; c) varying a parameter of the metrology apparatus; d) repeating steps a)-c) for multiple values ​​of the parameter of the metrology apparatus; and e) filtering the measurement obtained in step d). In an embodiment, the measurement obtained in step d) is resolved into individual components. In an embodiment, the filter 320 includes upper and lower limits that define the individual components. In an embodiment, the filter 320 includes upper and lower limits that define intervals of the components. In an embodiment, the filter 320 varies. An advantage of this aspect of the invention is that it measures the overlay value at a particular depth defined by the filter 320. When the filter 320 is chosen to be D, only ov1, 131 is measured (as all other components in the reflected radiation are filtered out). If the filter 320 has upper and lower limits near the component 311, then only ov2, 132 is measured. In this way, the overlay is measured and probed at different depths of the stack. In this way, it is possible to measure the exact overlay at a particular depth of the stack, i.e., the overlay that is not contaminated by the contribution of other overlay values. If the filter 320 is varied, the overlay values ​​obtained at each depth of the stack may provide a 3D overlay. Also, by appropriate scaling of the measurements, the method of this aspect of the invention may provide 3D asymmetry information of the measured stack.

[0078]

[0078] In a further aspect of the present invention, there is provided a method for characterizing a lithography process comprising obtaining a 3D asymmetry map of a lower grating at a first target location, repeating the obtaining of 3D asymmetry maps for a number of targets, and obtaining a map of target asymmetry for the wafer based on the above measurements.

[0079]

[0079] In Fig. 11, a measurement signal 201 is acquired at multiple wavelengths 202. The accuracy of the resolution according to the invention increases with the number of sample points that make up the graph of Fig. 11. However, each measurement point has an associated measurement time required for changing the wavelength and for making the measurement. In this respect, sampling that is too dense leads to an increase in the throughput of the metrology process, since the measurements required are particularly redundant. For example, if the graph of Fig. 11 includes only one component, the sampling rate in terms of the number of wavelengths is determined by the Nyquist criterion. In addition to the theoretical sampling value given by the Nyquist criterion, an empirical approach may include creating a sampling rate by design of experiment, for example in a recipe setup stage. In a further aspect of the present invention, a method is provided for selecting a parameter of a metrology device, comprising: acquiring a first plurality of measurements at a first plurality of values ​​of the parameter of the metrology device; and calculating a minimum number of second measurements and associated second values ​​of the parameter of the metrology device, such that the second values ​​of the parameter of the metrology device are less than the first values ​​of the parameter of the metrology device, while still resembling the main information of the first values ​​of the parameter.

[0080]

[0080] The above method allows the measurement of overlay by using multiple measurements at different values ​​of a parameter of the metrology device (e.g. wavelength, polarization state of the illumination radiation, or multiple angles of incidence of the illumination radiation). It is known in the art that in practical situations, such calibration may not be suitable when measurements are made on different targets on the same wafer or on different wafers, since the process conditions, although nominally identical, are in fact affected by undesirable variations, which cannot be controlled. It is therefore desirable to provide a method for obtaining the most suitable measurement conditions specific to each measured metrology target. The state of the art includes a number of such methods, also known as recipe selection methods. In one aspect of the invention, the above methodology for calculating overlay can also be adapted to provide the most suitable parameters of the metrology device. Thus, in one aspect of the invention, a method is provided that includes applying radiation to a target at multiple values ​​of a parameter of the radiation, such as wavelength, polarization, or angle of incidence, detecting the radiation at the multiple values ​​of the parameter, and decomposing the measured radiation into components. Depending on which overlay value is of interest (e.g., overlay 131 in FIG. 10), the relevant harmonic is extracted from the measured radiation. Other harmonics are also extracted. In a further aspect of the recipe selection method, the dependence of the measured radiation on a single harmonic is recalculated, for example assuming a simple sine wave dependence on said harmonic. The method is not limited to such a functional reconstruction, and other methods of reconstructing the signal from the harmonics may be used. In a further aspect of the recipe selection method, the recalculated measurement signal dependences on the individual harmonics are compared. In an embodiment, the optimal wavelength is the wavelength where the recalculated dependence on the harmonic of interest (corresponding to the overlay of interest) has a value above a first threshold and the recalculated dependence on the harmonic that is not of interest (because it adds and contaminates the signal) is below a second threshold.

[0081]

[0081] In an embodiment, the selection is obtained numerically based on a search algorithm having the values ​​of the first and second thresholds as input. In an embodiment, the selection may be made by a skilled operator. In a further embodiment of the method for selecting the most suitable wavelength, an arbitrary wavelength is selected. Furthermore, a second wavelength is selected to be a distance within one period of the recalculated dependency. In an embodiment, the value of interest is calculated based on the average value of the two selected wavelengths. In an embodiment, the distance between the two wavelengths is at least half of one period of the recalculated dependency. In another embodiment, the distance is one third of the distance between the recalculated dependencies.

[0082]

[0082] In yet another embodiment, suppression of the contribution of undesired harmonics to the recalculated signal or overlay measurement may be performed by modifying the bandwidth of the illumination radiation such that the components that result in the undesired harmonics are suppressed. In an embodiment, the bandwidth is filtered using a rectangular filter characteristic, where the bandwidth matches the period of the harmonic of interest or a multiple of the period of the harmonic of interest. In an embodiment, the harmonic of interest is the harmonic that induces the undesired contribution.

[0083]

[0083] The above description of the present invention is suitable for metrology where a single value of radiation is detected for each value of a parameter of the metrology device. For example, in the case of dark-field metrology, the average value of the intensity of a dark-field image of a target is measured at multiple wavelengths. As shown in Fig. 3(a), it is recognized that the metrology device can also provide metrology measurements from a complementary sensor to the sensor used in acquiring the image (e.g., metrology obtained using sensor 19 of Fig. 3(a)). Such measurements complement the measurements obtained by sensor 23 of Fig. 3(a) and include further information, e.g., angular information.

[0084]

[0084] In a further aspect of the invention, it is proposed to use information available in a complementary measurement unit. This method is similar to the method described in connection with Figs. 10-12, where a decomposition is performed on measurements acquired in a detection complementary to the image detection. The elements of the decomposition are determined by the basic components of the propagation of light as they exist in a complementary plane to the image measurement. Such components may be calculated on the basis of a theoretical model that considers the propagation and summation of all possible radiation paths with the target structure. In this respect, the method requires an additional optimization step for the way in which radiation is detected in the image plane of the metrology device. The optimization step includes determining the relevant elements that form the signal in the complementary plane to the image plane. In an aspect of the invention that concerns measurements in a complementary plane to the image plane of the metrology device, the elements that form the basis of the decomposition of the measurement signal are the spatial modes or components of the radiation scattered by the target. An important advantage of such a method is the fact that the contributions from the various radiation paths in the target structure are captured simultaneously, a fact that can significantly increase the throughput of the metrology process.

[0085]

[0085] Although specific reference has been made above to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that the present invention may be used in other applications, for example imprint lithography, and is not limited to optical lithography, where circumstances permit. In imprint lithography, a topography in a patterning device defines a pattern to be formed on a substrate. The topography of the patterning device can be pressed into a layer of resist provided on the substrate, where the resist is cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. The patterning device exits the resist after the resist has hardened, leaving a pattern in the resist.

[0086]

[0086] Further embodiments of the present invention are described in the following numbered clauses: 1. Measuring radiation reflected from a metrology target; Resolving the measured radiation into components; A method comprising: 2. The method of claim 1, wherein the decomposition of the measured radiation is obtained using a Fourier transform of the measured radiation. 3. a) directing radiation at a metrology target; b) detecting scattered radiation from the target; and c) changing a parameter of the metrology device; and d) repeating steps a) through c) for multiple values ​​of the parameter of the metrology device; and e) Resolving the radiation into components; 23. A method for measuring a parameter of a lithography process, comprising: 4. The method of claim 3, wherein the decomposition is a Fourier transform. 5. The method of claim 3, wherein the method further comprises selecting components that are less than a threshold 320. 6. The method of claim 3, wherein the overlay is generated using inverse decomposition and utilizing only selected components. 7. a) directing radiation at a metrology target; b) detecting scattered radiation from the target; and c) changing a parameter of the metrology device; and d) repeating steps a) through c) for multiple values ​​of the parameter of the metrology device; and e) filtering the measurements obtained in step d); 23. A method for measuring a parameter of a lithography process, comprising: 8. The method according to clause 7, wherein the measurements obtained in step d) are decomposed into individual components. 9. The method of claim 7, wherein the filter 320 includes upper and lower limits that define the individual components. 10. The method of claim 7, wherein the filter 320 includes upper and lower limits that define an interval for the components. 11. The method of claim 7, wherein the filter 320 is varied. 12. A method for characterizing a lithography process comprising obtaining a 3D asymmetry map of a lower grating at a first target location, repeating obtaining 3D asymmetry maps for multiple targets, and obtaining a map of target asymmetry for the wafer based on the above measurements. 13. Obtaining a first plurality of measurements at a first plurality of values ​​of a parameter of the metrology device; calculating a minimum number of second measurements and an associated second value of the parameter of the metrology device; calculating a second value of the parameter of the metrology device such that the second value is less than the first value of the parameter of the metrology device; A method for selecting a parameter of a metrology device, comprising: 14. A computer program comprising program instructions operable, when executed on any suitable apparatus, to perform the method according to any one of clauses 1 to 13. 15. Non-transitory computer program carriers containing computer programs according to clause 14. 16. Measuring radiation reflected from a metrology target; and Resolving the measured radiation into components; calculating at least two phase values ​​representative of each component; Calculating parameters from the phase relationships; and A method for measuring a parameter of a process comprising: 17. Illuminating a metrology target at multiple parameters of an illumination source; detecting scattered radiation by a metrology target at said multiple parameters of the illumination source; Resolving the measured radiation into components; recalculating the dependence of the measured radiation on at least one of the components; selecting a parameter of the illumination source with respect to the determined dependent value relative to the threshold value; A method for selecting a recipe for a metrology process, comprising: 18. Measuring radiation reflected from a metrology target; and Resolving the measured radiation into spatial components; A method comprising: 19. The measured radiation is acquired at a plane complementary to an image plane of the metrology apparatus; The method described in clause 18.

[0087]

[0087] As used herein, the terms "radiation" and "beam" include all types of electromagnetic radiation, including ultraviolet (UV) (e.g., having wavelengths of about 365, 355, 248, 193, 157, or 126 nm) and extreme ultraviolet (EUV) (e.g., having wavelengths in the range of 5 to 20 nm), as well as particle beams such as ion beams or electron beams.

[0088] The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0089]

[0089] The above description of the specific embodiments makes the general nature of the embodiments of the present invention clear so that others can easily modify and / or adapt the specific embodiments for various applications by applying knowledge of the art without undue experimentation and without departing from the general concept of the present invention. Such adaptations and modifications are therefore intended to be within the meaning and range of equivalents of the disclosed embodiments based on the teaching and guidance presented herein. The expressions or terms in this specification are for the purpose of illustration and description, and not for the purpose of limitation, and therefore the terms or terms in this specification are to be interpreted by those skilled in the art in light of the teachings and guidance.

Claims

1. measuring radiation reflected from a metrology target, the metrology target including an upper structure and a lower structure having a slope and positioned a predetermined distance from the upper structure in a depth direction of the metrology target, the radiation including a plurality of radiation fluxes reflected by the lower structure at different positions in the depth direction; decomposing the measured radiation into components; Including, Each of the resolved components corresponds to one of the plurality of radiant fluxes.

2. The method of claim 1 , wherein the decomposing of the measured radiation is obtained using a Fourier transform of the measured radiation.

3. a) directing radiation at a metrology target, the metrology target including an upper structure and a lower structure having a slope and positioned a predetermined distance from the upper structure along a depth of the metrology target; b) detecting scattered radiation from the metrology target, the scattered radiation including a plurality of beams of radiation reflected by the substructure at different positions in the depth direction; c) varying a parameter of the metrology device; and d) repeating steps a) through c) for multiple values ​​of said parameter of said metrology device; e) decomposing said radiation into components; Including, 11. A method for measuring a parameter of a lithography process, wherein each of the resolved components corresponds to one of the plurality of radiant fluxes.

4. The method of claim 3 , wherein the decomposition is a Fourier transform.

5. The method of claim 3 , further comprising selecting the components that are less than a threshold 320 .

6. The method of claim 5 , wherein the overlay is generated using an inverse decomposition and utilizing only the selected components.

7. a) directing radiation at a metrology target, the metrology target including an upper structure and a lower structure having a slope and positioned a predetermined distance from the upper structure along a depth of the metrology target; b) detecting scattered radiation from the metrology target, the scattered radiation including a plurality of beams of radiation reflected by the substructure at different positions in the depth direction; c) varying a parameter of the metrology device; and d) repeating steps a) through c) for multiple values ​​of said parameter of said metrology device; e) filtering the measurements obtained in step d); Including, 13. A method for measuring a parameter of a lithographic process, wherein the measurements obtained in step d) are decomposed into individual components, each of the decomposed components corresponding to one of the plurality of radiation fluxes.

8. The method of claim 7 , wherein the filter 320 includes upper and lower limits that define the individual components.

9. The method of claim 7 , wherein the filter 320 includes upper and lower limits that define an interval for the components.

10. The method of claim 7 , wherein the filter 320 is varied.

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