Measuring method and related computer product

Abstract

A method is disclosed, the method comprising: measuring radiation reflected from a measurement target, and decomposing the measured radiation into components, such as Fourier components or spatial components. In addition, a scheme selection method is disclosed, which provides an algorithm for selecting parameters of a measurement device based on a recomputed correlation of the measured radiation based on a single component.

Description

Field of the Invention

[0001] The present invention relates to a method and computer product that can be used for metrology, such as in the manufacture of devices by lithography. Background Art

[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). In such a case, a patterning device (or mask or reticle) can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., a portion including one or more dies) on the substrate (e.g., a silicon wafer). The transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) disposed on the substrate. Usually, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0003] In a lithography process (i.e., a process of developing a device or other structure involved in lithographic exposure, which typically can include one or more associated processing steps, such as resist development, etching, etc.), it is often desirable to measure the created structures, e.g., for process control and verification. Various tools for performing such measurements are known, including scanning electron microscopes that are commonly used to measure critical dimension (CD), and dedicated tools for measuring overlay accuracy, alignment accuracy of two layers of the substrate. More recently, various forms of scatterometers have been developed for use in the lithography field. These devices direct a radiation beam onto a target and measure one or more properties of the scattered radiation, e.g., the intensity at a single reflection angle as a function of wavelength; the intensity at one or more wavelengths as a function of reflection angle; or the polarization as a function of reflection angle, to obtain a "spectrum" from which the property of interest of the target can be determined. The determination of the property of interest can be performed by various techniques: e.g., by reconstructing the target structure by iterative methods (such as rigorous coupled wave analysis or finite element method); library search; and principal component analysis. Summary of the Invention

[0004] In metrology applications, such as in overlay metrology, radiation from a source impinges on a target including overlapping gratings, and the reflected radiation is detected on a sensor. The reflected radiation is the combined result of the individual parts of the incident radiation as it propagates (reflects or transmits) through the metrology target. In the presence of grating asymmetries, such as grating geometry asymmetries, the reflected radiation also contains information about these asymmetries, which may obscure information due to the overlay between the overlapping gratings. In addition, the actual gratings may have asymmetries such as tilts. It may be desirable to be able to measure the overlay accuracy of a lithography process or any other parameter of interest in such a way that the overlay accuracy or any other parameter of interest is invariant to the asymmetries present in the actual metrology gratings.

[0005] In a first aspect of the present invention, there is provided a method comprising: measuring radiation reflected from a metrology target and decomposing the measured radiation into components.

[0006] In a second aspect of the present invention, there is provided a method of measuring a parameter of a lithography process comprising: a) irradiating a metrology target with radiation, b) detecting the scattered radiation from the target, c) changing a parameter of the metrology device, d) repeating steps a) to c) for a plurality of values of the parameter of the metrology device, and e) decomposing the radiation into components.

[0007] In a third aspect of the present invention, there is provided a method of measuring a parameter of a lithography process comprising: a) irradiating a metrology target with radiation; b) detecting the scattered radiation from the target; c) changing a parameter of the metrology device; d) repeating steps a) to c) for a plurality of values of the parameter of the metrology device; e) applying a filter to the measurement results obtained in step d).

[0008] In a fourth aspect of the present invention, there is provided a method of characterizing a lithography process comprising: obtaining a 3D asymmetric map of a bottom grating at a first target location, repeating the obtaining of 3D asymmetric maps for a plurality of targets, and obtaining a target asymmetric map for a wafer based on the above measurement results.

[0009] In a fifth aspect of the present invention, there is provided a method for selecting a parameter of a metrology device comprising: obtaining a plurality of first measurement results at a plurality of first values of the parameter of the metrology device; and calculating a minimum second number of measurement results 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.

[0010] Another aspect of the present invention includes a computer program for performing the method of the first aspect and an associated computer program carrier. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings.

[0012] Figure 1 A lithographic apparatus according to an embodiment of the invention is shown;

[0013] Figure 2 A lithographic cell or cluster according to an embodiment of the invention is shown;

[0014] FIG. 3(a) is a schematic view of a dark-field measurement apparatus for measuring a target according to an embodiment of the invention, the dark-field measurement apparatus using a first reference illumination aperture providing a specific illumination pattern;

[0015] FIG. 3(b) is a schematic detail of the diffraction spectrum of a target for a given illumination direction;

[0016] FIG. 3(c) is a schematic view of a second reference illumination aperture providing an additional illumination pattern when using a measurement apparatus for diffraction-based overlay accuracy measurement;

[0017] FIG. 3(d) is a schematic view of a third reference illumination aperture that combines the first and second pairs of apertures to provide an additional illumination pattern when using a measurement apparatus for diffraction-based overlay accuracy measurement;

[0018] Figure 4 An overview of the form of a target having a plurality of periodic structures (e.g., a plurality of gratings) and a measurement spot on a substrate is depicted;

[0019] Figure 5 Depicts the Figure 4 image of the target obtained in the apparatus of FIG. 3;

[0020] Figure 6 is a flowchart showing the steps of an overlay accuracy measurement method using the apparatus of FIG. 3 and applicable to an embodiment of the invention;

[0021] Figures 7(a) to 7(d) A schematic cross-section of overlapping periodic structures (e.g., gratings) having different overlay accuracy values in a zero region is shown;

[0022] Figure 8 The principle of overlay accuracy measurement in an ideal target structure is shown;

[0023] Figure 9 is a graph of the overlay accuracy sensitivity K of the target versus the wavelength λ (nm), also known as the swing curve; and

[0024] Figure 10 A schematic cross-section of a metrology target is shown.

[0025] Figure 11A graph is shown of values inferred from measurements that are a function of a parameter such as wavelength, which is a metrology tool.

[0026] Figure 12 A graph is shown of values measured as a function of a parameter of a metrology tool inferred from metrology steps according to the present invention. Detailed Description

[0027] Before describing embodiments in detail, it is beneficial to provide an example environment in which the embodiments may be implemented.

[0028] Figure 1 A lithographic apparatus LA is schematically shown. The apparatus includes: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or DUV radiation); a patterning device support or support structure (e.g., a mask table) MT constructed 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 table (e.g., a wafer table) WT constructed 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 system (e.g., a 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 (e.g., including one or more dies) of the substrate W.

[0029] 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 guiding, shaping, or controlling the radiation.

[0030] 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 (e.g., 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 table that may be fixed or movable as required. The patterning device support may ensure that the patterning device (e.g., relative to the projection system) is located in a desired position. Any use herein of the term "reticle" or "mask" may be considered synonymous with the more general term "patterning device".

[0031] The term "patterning device" as used herein should be broadly interpreted as referring to any device that can be used to endow a pattern in a cross-section of a radiation beam so as to create a pattern in a target portion of a substrate. It should be noted that, for example, if the pattern includes phase-shifting features or so-called assist features, the pattern imparted to the radiation beam may not precisely correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device (such as an integrated circuit) created in the target portion.

[0032] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array 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 tilted mirrors impart a pattern in the radiation beam reflected by the mirror matrix.

[0033] As depicted herein, the device is of the transmissive type (e.g., employing a transmissive mask). Alternatively, the device may be of the reflective type (e.g., employing a programmable mirror array of the type described above, or a reflective mask).

[0034] The lithographic apparatus may also be of the type in which at least part of the substrate can be covered by a liquid having a relatively high refractive index (e.g., water) so as to fill the space between the projection system and the substrate. The immersion liquid may also be applied to other spaces in the lithographic apparatus, such as the space between the mask and the projection system. Immersion techniques are well known in the 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 immersed in the liquid, but only means that the liquid is located between the projection system and the substrate during exposure.

[0035] Referring to Figure 1 , the illuminator IL receives a radiation beam from the radiation source SO. For example, when the source is an excimer laser, the source and the lithographic apparatus may be separate entities. In such cases, the source is not considered to form part of the lithographic apparatus, and the radiation beam is transmitted from the source SO to the illuminator IL by means of a beam delivery system BD including, for example, suitable directing mirrors and / or beam expanders. In other cases, such as when the source is a mercury lamp, the source may be an integral part of the lithographic apparatus. If desired, the source SO, the illuminator IL, and the beam delivery system BD together are referred to as the radiation system.

[0036] The illuminator IL may include an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and / or inner radial extent of the intensity distribution in the pupil plane of the illuminator (commonly referred to as σ - outer and σ - inner respectively) may be adjusted. Additionally, the illuminator IL may include various other components such as an integrator IN and a condenser CO. The illuminator can be used to condition the radiation beam to have a desired uniformity and intensity distribution in the cross-section of the radiation beam.

[0037] The radiation beam B is incident on a patterning device (e.g., a mask) MA held on a patterning device support (e.g., a mask table MT) and is patterned by the patterning device. After passing through the patterning device (e.g., mask) MA, the radiation beam B passes through a projection system PS which focuses the radiation beam onto a target portion C of the substrate W. By means of a second positioner PW and a position sensor IF (e.g., an interferometric device, a linear encoder, a 2 - D encoder or a capacitive sensor), the substrate table WT can be accurately moved, e.g., in order to position different target portions C in the path of the radiation beam B. Similarly, a first positioner PM and another position sensor ( Figure 1 not explicitly shown in the figure) can be used to accurately position the patterning device (e.g., mask) MA relative to the path of the radiation beam B, for example, after mechanical retrieval from a mask library or during scanning.

[0038] 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 shown substrate alignment marks occupy dedicated target portions, they can be located in the spaces between multiple target portions (these are called scribe alignment marks). Similarly, in the case 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. Among the device features, small alignment marks can also be included within the die, in which case it is desirable for the marks to be as small as possible and not to require any imaging or processing conditions different from adjacent features. Embodiments of alignment systems capable of detecting alignment marks are described further below.

[0039] The described apparatus can be used in at least one of the following modes:

[0040] 1. In the step mode, the patterning device support (e.g., mask table) MT and the substrate table WTa remain substantially stationary while the entire pattern imparted to the radiation beam is projected onto the target portion C in one go (i.e., a single static exposure). Then, the substrate table WTa is moved in the X and / or Y direction so that different target portions C can be exposed. In the step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

[0041] 2. In the scanning mode, the patterning device support (e.g., mask table) MT and the substrate table WTa are scanned synchronously while the pattern imparted to the radiation beam is projected onto the target portion C (i.e., single dynamic exposure). The speed and direction of the substrate table WTa relative to the patterning device support (e.g., mask table) MT can be determined by the (reduction) magnification and image inversion characteristics of the projection system PS. In the scanning mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, while the length of the scanning movement determines the height (in the scanning direction) of the target portion.

[0042] 3. In another mode, the patterning device support (e.g., mask table) MT remains substantially stationary to hold the programmable patterning device, and the substrate table WTa is moved or scanned while the pattern imparted to the radiation beam is projected onto the target portion C. In this mode, a pulsed radiation source is typically employed, and the programmable patterning device is updated as needed after each movement of the substrate table WTa or between successive radiation pulses during the scan. This mode of operation can be readily applied to maskless lithography using a programmable patterning device, such as the programmable mirror array of the type described above.

[0043] Combinations and / or variations of the above-described usage modes may also be employed and / or combinations and / or variations of completely different usage modes may also be employed.

[0044] The lithographic apparatus LA is of the so-called dual-table type, having two tables WTa, WTb (e.g., two substrate tables) and two stations (an exposure station and a 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 onto the other substrate table at the measurement station and various preparation steps can be carried out. The preparation steps can include: using a level sensor LS to map the surface of the substrate for control and using an alignment sensor AS to measure the position of alignment marks on the substrate, both sensors being supported by a reference frame RF. If a position sensor IF cannot measure the position of the table when it is at the measurement station as well as at the exposure station, a second position sensor can 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, the other table without a substrate waits at the measurement station (where optionally measurement activities can take place). This other table has one or more measurement devices and can optionally have other tools (e.g., cleaning equipment). When the substrate has completed exposure, the table without a substrate moves to the exposure station to perform, for example, measurement, and the table with the substrate moves to the position where the substrate is unloaded and another substrate is loaded (e.g., the measurement station). These multi-table arrangements enable a significant increase in the throughput of the apparatus.

[0045] AsFigure 2 As shown, the lithography apparatus LA forms part of a lithography cell LC, which is sometimes also referred to as a lithocell or lithocluster, and which further includes equipment for performing one or more pre- and post-exposure processes on a 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 baking plates BK. A substrate handler or robot RO picks up substrates from input / output ports I / O1, I / O2, moves the substrates between different processing devices, and transfers the substrates to the load port LB of the lithography apparatus. These devices, which are generally collectively referred to as the track, are under the control of a track control unit TCU, which is itself controlled by a monitoring system SCS, which also controls the lithography apparatus via a lithography control unit LACU. Thus, different devices can be operated to maximize throughput and processing efficiency.

[0046] In order for a substrate exposed by a lithography apparatus to be correctly and consistently exposed, it is desirable to inspect the exposed substrate to measure one or more properties, such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc. If an error is detected, the exposure of one or more subsequent substrates can be adjusted, especially if the inspection can be performed immediately and quickly enough such that another substrate of the same batch is still being exposed. Also, the exposed substrate can be stripped and reprocessed (to increase yield) or discarded, thus avoiding performing an exposure on a substrate known to be defective. In the case where only some target portions of the substrate are defective, further exposure can be performed only on those good target portions. Another possibility is to adjust the settings of subsequent process steps to compensate for the error, for example, the trimming etch step time can be adjusted to compensate for substrate-to-substrate CD variations caused by the lithography process step.

[0047] An inspection apparatus is used to determine one or more properties of a substrate, in particular how one or more properties of different substrates or different layers of the same substrate vary between layers and / or across substrates. The inspection apparatus may be integrated into a lithographic apparatus LA or a lithographic cell LC, or may be a stand-alone apparatus. To enable the fastest measurements, it is desirable for the inspection apparatus to measure one or more properties of the exposed resist layer immediately after exposure. However, the latent image in the resist has a very low contrast - there is only a very small difference in refractive index between the part of the resist that has been exposed to radiation and the part that has not - and not all inspection apparatuses have sufficient sensitivity to make useful measurements of the latent image. Therefore, the measurements may be made after a post-exposure bake step (PEB), which is typically the first step performed on the exposed substrate and increases the contrast between the exposed and unexposed parts of the resist. At this stage, the image in the resist may be referred to as a semi-latent image. It is also possible to measure the developed resist image - at which point either the exposed or unexposed parts of the resist have 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 purposes.

[0048] Targets used in conventional scatterometers include relatively large periodic structure layouts (e.g., including one or more gratings), e.g., 40 μm × 40 μm. In this case, the measurement beam typically has a spot size smaller than the periodic structure layout (i.e., the layout is not fully filled such that one or more of the periodic structures in the periodic structure are not fully covered by the spot). This simplifies the mathematical reconstruction of the target as it can be considered infinite. However, for example, thus the target can be positioned in product features rather than in scribe lines, and the size of the target has been reduced to, for example, 20 μm × 20 μm or smaller, or 10 μm × 10 μm or smaller. In this case, the periodic structure layout can be made smaller than the measurement spot (i.e., the periodic structure layout is overfilled). Typically, dark field scatterometry is used to measure such targets, in which the zero-order diffraction (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 WO2009 / 078708 and WO2009 / 106279, which are hereby incorporated by reference in their entirety. Further developments of this technique are described in U.S. patent application publications US2011-0027704, US2011-0043791, and US2012-0242970, which are hereby incorporated by reference in their entirety. Diffraction-based overlay accuracy (DBO or μDBO) using dark field detection of diffraction orders enables overlay accuracy measurements of smaller targets. These targets can be smaller than the illumination spot and can be surrounded by product structures on the substrate. In one embodiment, multiple targets can be measured in one image.

[0049] In one embodiment, the target on the substrate can include one or more 1-D periodic gratings, and one or more 1-D periodic gratings are printed such that after development, the bars are formed by solid resist lines. In one embodiment, the target can include one or more 2-D periodic gratings, and one or more 2-D periodic gratings are printed such that after development, one or more of the gratings are formed by solid resist pillars or vias in the resist. Alternatively, the bars, pillars, or vias can be etched into the substrate. The pattern of the grating is sensitive to chromatic aberration in a lithographic projection apparatus (especially the projection system PL), and the illumination symmetry and the presence of such chromatic aberration will manifest themselves as variations in the printed grating. Thus, the measurement data of the printed grating can be used to reconstruct the grating. The parameters of the 1-D grating (such as line width and shape) or the parameters of the 2-D grating (such as pillar or via width or length or shape) can be input into the reconstruction process executed by the processing unit PU from the knowledge of the printing step and / or other measurement processes.

[0050] Figure 3(a) shows a dark-field metrology apparatus suitable for embodiments of the present invention. The target T (including a periodic structure such as a grating) and diffracted rays are shown in more detail in Figure 3(b). The dark-field metrology apparatus can be a stand-alone device, or incorporated into a lithographic apparatus LA (e.g., at a measurement station), or incorporated into a lithographic cell LC. The optical axis with several branches throughout the apparatus is indicated by the dashed line O. In this apparatus, radiation emitted by output 11 (e.g., a source such as a laser or xenon lamp or an aperture connected to a source) is directed onto the substrate W via an optical system including lenses 12, 14 and an objective 16, via a prism 15. These lenses are arranged in a double sequence in a 4F arrangement. Different lens arrangements can be used as long as it still provides an image of the substrate onto the detector.

[0051] In one embodiment, the lens arrangement allows for obtaining an intermediate pupil plane for spatial frequency filtering. Thus, by defining a spatial intensity distribution in the plane that presents the spatial spectrum of the substrate plane (here called the (conjugate) pupil plane), the angular range of the radiation incident on the substrate can be selected. In particular, this can be achieved, for example, by inserting an aperture plate 13 of a suitable form between lenses 12 and 14 in the plane that is the back-projected image of the objective pupil plane. In the example shown, the aperture plate 13 has different forms, labeled 13N and 13S, allowing for the selection of different illumination modes. The illumination system in this example forms an off-axis illumination mode. In a first illumination mode, the aperture plate 13N provides off-axis illumination from a specified direction, which is designated "north" for illustrative purposes only. In a second illumination mode, the aperture plate 13S is used to provide a similar illumination, but from the opposite direction (labeled "south"). Other illumination modes are possible by using different apertures. The remainder of the pupil plane is ideally dark because any unwanted radiation outside the desired illumination mode can interfere with the desired measurement signal.

[0052] As shown in Fig. 3(b), the target T is placed substantially orthogonal to the optical axis O of the objective 16 with respect to the substrate W. The illumination light I irradiated onto the target T at an angle deviating from the axis O generates a zero-order light ray (solid line 0) and two first-order light rays (dot-chain line +1 and double dot-chain line -1). For a small overfilled target T, these light rays are merely one of many parallel light rays covering the area of the substrate including the measurement target T and other features. Since the aperture in the plate 13 has a finite width (necessary to allow a useful amount of radiation), the incident light I will actually occupy a certain angular range, and the diffracted light rays 0 and +1 / -1 will be spread to some extent. According to the point spread function of the small target, each order +1 and -1 will be further spread over an angular range rather than a single ideal light ray as shown. Note that the pitch and illumination angle of the periodic structure can be designed or adjusted such that the first-order light rays entering the objective are strictly aligned with the central optical axis. Figure 3(a) and 3(b) the light rays shown in are shown slightly off-axis purely to make them more distinguishable in the figure.

[0053] At least the 0th order and +1st order diffracted by the target on the substrate W are collected by the objective 16 and guided back through the prism 15. Returning to Fig. 3(a), both the first and second illumination modes are illustrated by specifying the diametrically opposite apertures labeled North (N) and South (S). When the incident light I comes from the north side of the optical axis, i.e., when the first illumination mode is applied using the aperture plate 13N, the +1 diffracted light ray (labeled +1(N)) enters the objective 16. Conversely, when the second illumination mode is applied using the aperture plate 13S, the -1 diffracted light ray (labeled 1(S)) enters the lens 16. Thus, in one embodiment, for example, after rotating the target or changing the illumination mode or changing the imaging mode to obtain the -1 diffracted order and +1 diffracted order intensities respectively, the measurement results are obtained by measuring the target twice under certain conditions. Comparing these intensities for a given target provides a measurement of the asymmetry of the target, and the asymmetry of the target can be used as an indicator of a parameter (e.g., overlay accuracy error) of the lithography process. In the above case, the illumination mode is changed.

[0054] The beam splitter 17 splits the diffracted beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zero-order and first-order diffracted beams to form a diffracted spectrum (pupil plane image) of the target on the first sensor 19 (e.g., a CCD or CMOS sensor). Each diffracted order hits a different point on the sensor, enabling image processing to compare and contrast the diffracted orders. The pupil plane image captured by the sensor 19 can be used for focus metrology devices and / or to normalize the intensity measurements of the first-order beams. The pupil plane image can also be used for many measurement purposes, such as reconstruction, which will not be described in detail here.

[0055] In the second measurement branch, the optical systems 20, 22 form an image of the target on the substrate W on the sensor 23 (e.g., a CCD or CMOS sensor). In the second measurement branch, the aperture stop 21 is arranged in a plane conjugate to the pupil plane. The aperture stop 21 is used to block the zero-order diffraction beam so that the image DF of the target formed on the sensor 23 is formed by the first-order beam of -1 or +1. The images captured by the sensors 19 and 23 are output to the image processor and controller PU, where the function of the PU will depend on the specific type of measurement being performed. Note that the term "image" used here is in a broad sense. If only one of the -1 and +1 orders exists, an image of such a periodic structural feature (e.g., grating lines) will not be formed.

[0056] The specific forms of the aperture plate 13 and the aperture stop 21 shown in FIG. 3 are merely examples. In another embodiment of the present invention, on-axis illumination of the target is used, and an aperture stop with an off-axis aperture is used to transfer essentially only the first-order diffraction in the diffracted radiation to the sensor. In yet another embodiment, instead of or in addition to the first-order beam, second-order, third-order, and higher-order beams (not shown in FIG. 3) can be used in the measurement.

[0057] To make the illumination suitable for these different types of measurements, the aperture plate 13 can include a plurality of aperture patterns formed around a disk, and the disk rotates to bring the desired pattern to the appropriate position. Note that the aperture plates 13N or 13S are used to measure the periodic structure of the target oriented in one direction (depending on the set X or Y). For the measurement of an orthogonal periodic structure, a 90° and 270° rotation of the target can be achieved. Different aperture plates are shown in FIGS. 3(c) and (d). FIG. 3(c) shows two additional off-axis illumination modes. In the first illumination mode of FIG. 3(c), the aperture plate 13E provides off-axis illumination from a specified direction, which is, for illustrative purposes only, "east" relative to the previously described "north". In the second illumination mode of FIG. 3(c), the aperture plate 13W is used to provide similar illumination but from the opposite direction, which is labeled "west". FIG. 3(d) shows two additional off-axis illumination modes. In the first illumination mode of FIG. 3(d), the aperture plate 13NW provides off-axis illumination from the directions specified as "north" and "west" as previously described. In the second illumination mode, the aperture plate 13SE is used to provide similar illumination but from the opposite directions labeled "south" and "east" as before. The use of these devices and many other variations and applications of the devices are described in, for example, the previously published patent application publications mentioned above.

[0058] Figure 4Depicts an example composite metrology target formed on a substrate. The composite target includes four periodic structures (in this case, gratings) 32, 33, 34, 35 that are positioned closely together. In one embodiment, the periodic structures are positioned closely enough together such that they are all within the measurement spot 31 formed by the illumination beam of the metrology device. In this case, the four periodic structures are thus all illuminated simultaneously and imaged simultaneously on sensors 19 and 23. In an example dedicated to overlay accuracy measurement, the periodic structures 32, 33, 34, 35 themselves are composite periodic structures (e.g., composite gratings) formed by overlapping periodic structures, i.e., the periodic structures are patterned in different layers of the device formed on the substrate W and such that at least one periodic structure in one layer overlaps at least one periodic structure in a different layer. Such a target can have an external dimension within 20 μm × 20 μm or 16 μm × 16 μm. Additionally, all the periodic structures are used to measure the overlay accuracy between a particular pair of layers. To facilitate the target being able to measure more than one pair of layers, the periodic structures 32, 33, 34, 35 can have different overlay offsets, so as to facilitate measuring the overlay accuracy between different layers, where different portions of the composite periodic structure are formed in different layers. Thus, all the periodic structures for one target on the substrate will be used to measure one pair of layers, and all the periodic structures for another identical target on the substrate will be used to measure another pair of layers, where the different biases facilitate differentiation between multiple layer pairs. The meaning of overlay bias will be specifically explained with reference to FIG. 7 below.

[0059] Figures 7(a) to 7(c) Shows a schematic cross-section of overlapping periodic structures (in this case, gratings) of corresponding targets T with different biases. These can be used on the substrate W, as shown in FIGS. 3 and Figure 4 as shown. By way of example only, periodic structures that are periodic in the X direction are shown. Different combinations of these periodic structures with different biases and different orientations can be provided.

[0060] Starting from FIG. 7(a), a composite overlapping target 600 formed in two layers (labeled L1 and L2) is shown. 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 drawn such that features 602, 608 extend into the page.) The periodic structure pattern repeats with a pitch P in both layers. For the purpose of example only, lines 602 and 608 are mentioned, and other types of features such as dots, blocks, and vias can be used. In the case shown in FIG. 7(a), there is no registration accuracy error and no bias, such that each feature 608 is exactly above feature 602 in the bottom periodic structure (where the measurement is "line-to-line"). In an embodiment, no registration accuracy error occurs when each feature 608 is exactly above space 610 (where the measurement is "line-to-groove").

[0061] In FIG. 7(b), the same target with a bias +d is described, such that the feature 608 of the upper periodic structure is shifted to the right by a distance d (the distance d is less than the pitch P) relative to the feature 602 of the lower periodic structure. That is, features 608 and feature 602 are arranged such that if they were both printed exactly at their nominal positions, feature 608 would be offset from feature 602 by a distance d. The bias distance d can actually be a few nanometers, e.g., 10 nm to 20 nm, while the pitch P is, for example, in the range of 300 to 1000 nm, e.g., 500 nm or 600 nm. In FIG. 7(c), the same target with a bias -d is depicted, such that feature 608 is shifted to the left relative to feature 602. This type of biased target shown in FIGS. 7(a) to (c) and its use in measurement are described, for example, in the above-mentioned patent application publication.

[0062] Furthermore, as described above, although FIGS. 7(a) to (c) describe features 608 located above feature 602 (with or without a small bias of +d or -d), which is referred to as a "line-to-line" target with a bias in the zero region, the target can have a programmed bias of P / 2, i.e., half of the pitch, such that each feature 608 in the upper periodic structure is located above space 604 in the lower periodic structure. This is referred to as a "line-to-groove" target. In this case, a small bias of +d or -d can also be applied. The choice between a "line-to-line" target or a "line-to-groove" target depends on the application.

[0063] Back to Figure 4, as shown in the figure, the orientations of the periodic structures 32, 33, 34, 35 can also be different to diffract the incident radiation in the X and Y directions. In one example, the periodic structures 32 and 34 are periodic structures in the X direction with biases of +d and -d respectively. The periodic structures 33 and 35 can be periodic structures in the Y direction with offsets of +d and -d respectively. Although four periodic structures are shown, another embodiment may include a larger matrix to obtain the desired accuracy. For example, a 3×3 array of 9 composite periodic structures can have biases of -4d, -3d, -2d, -d, 0, +d, +2d, +3d, +4d. Separate images of these periodic structures can be identified in the image captured by the sensor 23.

[0064] Figure 5 is shown that can be used in the device of FIG. 3 Figure 4 The target, using the aperture plate 13NW or 13SE from FIG. 3(d), forms an example of an image on the sensor 23 and is detected by the sensor 23. Although the sensor 19 cannot resolve the different individual periodic structures 32 to 35, the sensor 23 can do so. The dark rectangles represent the image fields on the sensor, where the illumination spots 31 on the substrate are imaged into the corresponding circular regions 41. In this circular region, the rectangular regions 42 to 45 represent the images of the periodic structures 32 to 35. If the periodic structures are located in the product region, the product features are also visible at the periphery of this image field. The image processor and controller PU use pattern recognition to process these images to identify the separate images 42 to 45 of the periodic structures 32 to 35. In this way, the images do not have to be very precisely aligned at a specific position within the sensor frame, which greatly improves the throughput of the overall measuring device.

[0065] Once the separate images of the periodic structures have been identified, for example by averaging or summing the selected pixel intensity values within the identified regions, the intensities of these multiple individual images can be measured. The intensities and / or other characteristics of the images can be compared with each other. These results can be combined to measure different parameters of the lithography process. Overlay accuracy performance is an example of such a parameter.

[0066] Figure 6 shows how to measure the overlay accuracy error between two layers containing component periodic structures 32 to 35 through the asymmetry of the periodic structures, as revealed by comparing their intensities in the +1st order and 1st order dark field images, using the method described, for example, in PCT Patent Application Publication No. WO2011 / 012624. In step M1, the substrate (such as a semiconductor wafer) is passed through Figure 2The lithography unit is processed one or more times to create a structure including a target (the target includes periodic structures 32 to 35). At M2, using the metrology device of FIG. 3, an image of the periodic structures 32 to 35 is obtained using one of the first-order diffracted beams (e.g., -1). In an embodiment, a first illumination mode is used (e.g., the illumination mode created using aperture plate 13NW). Then, either by changing the illumination mode, or the imaging mode, or by rotating the substrate W by 180° in the field of view of the metrology device, a second image of the periodic structures can be obtained using another first-order diffracted beam (+1) (step M3). Thus, the +1 diffracted radiation is captured in the second image. In one embodiment, the illumination mode is changed and a second illumination mode is used (e.g., the illumination mode created using aperture plate 13SE). In one embodiment, by performing measurements at substrate orientations of 0° and 180°, tool-induced artifacts such as TIS (tool-induced shift) can be removed.

[0067] Note that the "image" mentioned here is not a conventional dark-field microscopic image by including only half of the first-order diffracted radiation in each image. Individual periodic structure features are not resolved. Each periodic structure will simply be represented by a region of a specific intensity level. In step M4, regions of interest (ROIs) are identified within the image of each component periodic structure, and the intensity level will be measured according to this image.

[0068] Regions of interest P1, P2, P3, P4 have been identified and their intensities measured for each corresponding individual periodic structure 32 to 35. Then, the asymmetry of the periodic structures can be determined and, thus, for example, the overlay accuracy error can be determined. This is done by the image processor and controller PU in step M5 by comparing the intensity values obtained for the +1 and -1 orders for each periodic structure 32 to 35 to identify any differences in their intensities, i.e., the asymmetry. The term "difference" is not intended to refer only to subtraction. The difference can be calculated in ratio form. In step M6, if applicable, the measured asymmetries for multiple periodic structures are used together with the knowledge of the overlay biases of these periodic structures to calculate one or more performance parameters of the lithography process near the target T. The performance parameter of interest is the overlay accuracy. Other performance parameters of the lithography process can be calculated, such as the focal length and / or the dose. One or more performance parameters can be fed back for improving the lithography process, for improving Figure 6 the measurement and calculation process itself, for improving the design of the target T, etc.

[0069] In an embodiment for determining the overlay accuracy, Figure 8Depict curve 702, which illustrates the relationship between the overlay accuracy error OV and the measured asymmetry A for an "ideal" target that has zero offset and no structural asymmetry within the individual periodic structures forming the overlay target. These figures are only for illustrative purposes to determine the principle of overlay accuracy, and in each graph, the units of the measured asymmetry A and the overlay accuracy error OV are arbitrary.

[0070] In the "ideal" cases of FIGS. 7(a) - (c), curve 702 indicates that the measured asymmetry A and the overlay accuracy have a sinusoidal relationship. The period P of the sinusoidal variation corresponds to the period (pitch) of the periodic structure, of course converted to an appropriate scale. In this example, the sinusoidal form is pure, but in actual cases, it may include harmonics. For simplicity, in this example, it is assumed that: (a) only the first-order diffracted radiation from the target reaches the image sensor 23 (or its equivalent in a given embodiment), and (b) the experimental target design is such that: within these first-order diffracted radiations, there is a pure sinusoidal relationship between the intensity and the overlay accuracy results between the upper and lower periodic structures. Whether this actually holds is a function of the optical system design, the wavelength of the illuminating radiation, the pitch P of the periodic structure, and the design and stacking of the target.

[0071] As described above, a biased periodic structure can be used to measure overlay accuracy instead of relying on a single measurement. The bias has a known value that is defined in the patterning device (e.g., reticle) that creates the bias, and this known value is used as an on - substrate calibration of the overlay accuracy corresponding to the measurement signal. In the drawings, the calculation is illustrated. In Figure 6 steps M1 to M5, for the component periodic structures with biases +d and -d, asymmetry measurement results A +d and A －d (e.g., as shown in Figure 7(b) and 7(c) ) are obtained respectively. Fitting these measurement results to a sine curve gives the points 704 and 706 shown. Given the known bias, the true overlay accuracy error OV can be calculated. The pitch P of the sine curve is known from the target design. The vertical scale of curve 702 is not known initially, but is an unknown factor that we can call the overlay accuracy scale constant K.

[0072] In equation terms, the relationship between the overlay accuracy error OV E and the intensity asymmetry A is assumed to be:

[0073] A ±d = K sin(OV E ±d)

[0074] where the overlay accuracy error OVE In terms of proportion, 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 measurement results of the target with different known biases (such as +d and -d), the following formula can be used to calculate the overlay accuracy error OV E :

[0075]

[0076] where A +d is the intensity asymmetry measurement result of the +d bias target, and A -d is the intensity asymmetry measurement result of the -d bias target.

[0077] Although these measurement techniques are fast and relatively simple to calculate (once calibrated), they rely on the assumption that the only cause of overlay / lateral offset is asymmetry. That is, assume an "ideal" case where there is no structural asymmetry in the target, for example. Any structural asymmetry in the stack (such as the asymmetry of features within one or two overlapping periodic structures), in addition to the overlay / lateral offset, also results in first-order asymmetry. This structural asymmetry unrelated to the overlay accuracy significantly interferes with the measurement, giving inaccurate results.

[0078] As an example of structural asymmetry, one or more periodic structures of the target can be structurally deformed. For example, one or more sidewalls of the periodic structure features of the target (such as grating lines) may not be vertical as expected. As another example, one or more spacings between the periodic structure features of the target (such as the grating pitch of the trenches) can be greater or less than expected. In addition, one or more features of the periodic structure of the target (such as grating lines) can have a width smaller or larger than expected. Additionally, even if the difference between one or more periodic structures of the target and the expected is uniform, the difference from the expected may be different from the difference for one or more other periodic structures of the target. The structural asymmetry of the lower periodic structure of a composite target is a common form of structural asymmetry. For example, it can originate from a substrate processing step such as chemical mechanical polishing (CMP) performed after the initial formation of the lower periodic structure.

[0079] Referring to FIG. 7(d), an example of the structural asymmetry of the lower periodic structure is schematically shown. When the true features and spaces have a certain slope and a certain roughness on the surface, Figures 7(a) to 7(c)The features and spaces in the periodic structure of Figure 6 are shown as perfect squares. However, their profiles are at least symmetric. The features 602 and / or spaces 604 in the lower periodic structure in Fig. 7(d) no longer have a symmetric form at all, but are distorted by, for example, one or more processing steps. Thus, for example, the bottom surface of each space 604 has been tilted. The sidewall angles of the features and spaces have also become asymmetric. When measuring the overlay accuracy using only two offset periodic structures by the method of Figure 6

[0080] , the structure asymmetry cannot be distinguished from the overlay accuracy, and as a result, the overlay accuracy measurement becomes unreliable.​​​It has further been found that, in addition to or instead of the structural asymmetry of the target, the stack difference between adjacent periodic structures of the target or between adjacent targets may be a factor that adversely affects the accuracy of measurements (such as overlay accuracy measurements). The stack difference can be understood as an undesigned difference in the physical configuration between adjacent periodic structures or adjacent targets. The stack difference results in a difference in the optical properties (such as intensity, polarization, etc.) of the measurement radiation between adjacent periodic structures or adjacent targets, which is due to an inducement different from the overlay accuracy error, different from the intentional bias, and different from the structural asymmetry common to adjacent periodic structures or adjacent targets. The stack difference includes (but is not limited to): a thickness difference between adjacent periodic structures or adjacent targets (for example, a thickness difference of one or more layers causes one periodic structure or target to be higher or lower than another periodic structure or target, which is designed to be at a substantially equal level), a refractive index difference between adjacent periodic structures or adjacent targets (for example, a refractive index difference of one or more layers causes the combined refractive index of one or more layers of one periodic structure or target to be different from the combined refractive index of one or more layers of another periodic structure or target, even though the one or more layers of the other periodic structure or target are designed to have a substantially equal combined refractive index), a material difference between adjacent periodic structures or adjacent targets (for example, a difference in the material type, material uniformity, etc. of one or more layers causes the material of one periodic structure or target to be different from the material of another periodic structure or target that is designed to have substantially the same material), a difference in the grating period of the structures of adjacent periodic structures or adjacent targets (for example, the grating period of one periodic structure or target is different from the grating period of another periodic structure or target that is designed to have substantially the same grating period), a difference in the depth of the structures of adjacent periodic structures or adjacent targets (for example, due to etching, one periodic structure or target is different from another periodic structure or target that is designed to have substantially the same depth in terms of the depth of the structure), a difference in the critical dimension (CD) of the features of adjacent periodic structures or adjacent targets (for example, one periodic structure or target is different from another periodic structure or target that is designed to have substantially the same critical dimension in terms of the critical dimension). In some examples, the stack difference is introduced by processing steps in the patterning process (such as CMP, layer deposition, etching, etc.). In one embodiment, periodic structures or targets are adjacent if 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.

[0081] The stack difference (which may be referred to as the grating imbalance between gratings) on the intensity asymmetry measurement result A+d and A -d (where the subscript indicates the target offset corresponding to the target region of the ROI) can be generally formulated as:

[0082] A +d =(K + ΔK)sin(OV E + d)

[0083] A -d =(K - ΔK)sin(OV E - d)

[0084] where ΔK represents the difference in overlay accuracy sensitivity attributable to stack differences. Thus, the overlay accuracy error OV E (assuming it is small) can be proportional to .

[0085] Stack differences can be considered as spatial stack parameter variations, i.e., stack parameter variations (target-to-target) on the substrate. Another problem that may be encountered is stack parameter process drift, where one or more of the stack parameters of the target drift over time from the optimal value due to process drift. This can be considered as time stack parameter variations.

[0086] Now, in the face of structural asymmetry, stack differences, stack parameter process drift, and any other process variability, it is desirable to derive a combination of target layout, measurement beam wavelength, measurement beam polarization, etc., which will produce an accurate measurement of the desired process parameter (e.g., overlay accuracy), and / or produce a measured value of the desired process parameter that is robust to process variability. Thus, it is desirable to perform measurements, for example, using the desired optimal selection of target-measurement parameter combinations, in order to obtain a more accurate measurement result of the process parameter, and / or produce a measured value of the desired process parameter that is robust to process variability. This is because the measurement accuracy and / or sensitivity of the target can vary with respect to one or more attributes of the target itself and / or one or more attributes of the measurement radiation provided to the target (e.g., the wavelength of the radiation, the polarization of the radiation, and / or the intensity distribution of the radiation (i.e., angular or spatial intensity distribution)). In one 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). Additionally, different polarizations of the radiation beam can be selected, and various illumination shapes can be provided using, for example, multiple different apertures. Thus, it is desirable to determine an optimized measurement profile for a specific target.

[0087] The measurement profile includes one or more parameters of the measurement itself, which can include one or more parameters related to the measurement beam and / or measurement device used to perform the measurement. For example, if the measurement used in a substrate measurement scenario is a diffraction-based optical measurement, one or more parameters of the measurement itself can include the wavelength of the measurement radiation, and / or the polarization of the measurement radiation, and / or the intensity distribution of the measurement radiation, and / or the illumination angle of the substrate relative to the measurement radiation (e.g., the angle of incidence, azimuth angle, etc.), and / or the relative orientation of the pattern on the substrate with respect to the diffracted measurement radiation, and / or the number of measured points or instances of the target, and / or the location of the instances of the target measured on the substrate. One or more parameters of the measurement itself can include one or more parameters of the metrology device used in the measurement, which can include detector sensitivity, numerical aperture, etc.

[0088] In this context, the pattern being measured (also referred to as the "target" or "target structure") can be a pattern that is optically measured, e.g., the diffraction of the pattern is measured. The pattern being measured can be a pattern that is specifically designed or selected for measurement purposes. Multiple copies of the target can be placed in many locations on the substrate. For example, a substrate measurement scenario can be used to measure overlay accuracy. In one embodiment, a substrate measurement scenario can be used to measure another process parameter (e.g., dose, focus, CD, etc.). In one embodiment, the measurement profile can be used to measure the alignment of the imaged image with respect to the layer of an existing pattern on the substrate. For example, by measuring the relative position of the substrate, the measurement profile can be used to align the patterning device with the substrate.

[0089] Many methods for evaluating and optimizing target-measurement parameter combinations have been described. Such methods are performed prior to production. Thus, once optimized, the selected target-measurement parameter combination will typically be used throughout the production run, i.e., a predetermined measurement profile will be used to measure the targets corresponding to the target design according to the predetermined target-measurement parameter combination. However, as discussed, there may be un-designed stack parameter variations of the targets, resulting in stack differences and / or stack parameter process drifts between the targets. For example, the layer thickness of one or more layers within the stack can vary with the substrate (i.e., target-to-target) and / or over time (i.e., drift). One result of such stack parameter variations can be that the measurement profile is no longer optimal for the target. This can lead to inaccurate measurements of the target. Stack parameter variations can also typically be an indication of process control issues (e.g., process drift), and thus can itself be a useful process monitoring metric.

[0090] Methods for evaluating and optimizing target-measurement parameter combinations can include those that analyze target response sequence data (where the target response sequence data describes the variation of the target response as a function of the measurement profile), in particular one or more parameters of the measurement radiation such as wavelength (e.g., spectral sequence data). In one embodiment, the target response sequence data can represent the oscillatory dependence of measurement data (e.g., an intensity metric obtained as field data (at the image plane) or pupil data (at the pupil plane)) as a function of the wavelength of the measurement radiation. Figure 9 is an example graph of data for measuring the intensity metric of a target. In this particular example, the overlay accuracy sensitivity K at each wavelength λ for single polarization (in this case, linear X polarization). The curve K(λ) has been data-fitted, so this representation can be called a swing curve. It can be understood that it is not necessary to generate a graph as the data can be processed. For measurements at various wavelengths for different single polarizations (e.g., linear Y polarization), similar data graphs can be constructed for the same target. In Figure 9 it, the stack sensitivity and the overlay accuracy sensitivity are plotted for various measurement beam wavelengths. Additionally, although the polarization here is linear X polarization, it can be a different polarization (e.g., linear Y polarization, left-handed elliptically polarized radiation, right-handed elliptically polarized radiation, etc.).

[0091] The intensity metric can be any suitable metric derived from the detected intensity (e.g., intensity asymmetry, overlay accuracy sensitivity K, or stack sensitivity (SS) (which is also the signal contrast)). The stack sensitivity can be understood as a measure of how much the signal intensity changes when the overlay accuracy changes due to diffraction between target (e.g., grating) layers. That is, in an overlay environment, it detects the contrast between the upper periodic structure and the lower periodic structure of an overlay-type target, and thus represents the balance between the diffraction efficiencies of the upper periodic structure and the lower periodic structure. Therefore, it is an exemplary measurement of the measurement sensitivity. In an embodiment, the stack sensitivity is the ratio of the intensity asymmetry to the average intensity. In one embodiment, the stack sensitivity can be expressed by the formula SS = KL / I M , where L is a user-defined constant (e.g., in one embodiment, the value of 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.

[0092] Figure 9 The example of

[0093]

[0094] A(λ)+d and A(λ) -d are intensity asymmetry measurements as a function of wavelength corresponding to offsets +d and -d, respectively, and df(λ) is a dose factor as a function of wavelength. The dose factor can be any function of source intensity and measurement time. In a particular embodiment, it can include the product of source intensity and integration time as a function of wavelength.

[0095] Figure 10 Describes overlapping gratings used in a metrology process. It includes a top grating 101 and a bottom grating 102. In this particular example, a particular form of geometric asymmetry is highlighted, such as the tilt of the bottom grating 102, as defined by angle 103. The top grating is formed on material 104, which in this particular example includes alternating layers of different refractive indices that form part of a semiconductor device. This is a particular example and does not limit the stack in any way.

[0096] As Figure 10 further shown, due to the tilt of the bottom grating, there is a possibility that the registration accuracy, which is defined as the relative distance between the top grating 101 and the bottom grating 102, has different values (e.g., ov1, 131, or ov2, 132, or another ov3, 133). The radiation that forms the radiation beam incident on the detector is formed by, for example, light rays 120, 121, 122, and 123, where light ray 120 is the radiation reflected by the top grating 101, radiation 121 is the radiation reflected by the top of the bottom grating 102, radiation 122 is the radiation reflected by the part of the bottom grating 102 located at a distance D + H from the top grating, and radiation 123 is the radiation reflected by the bottom of the bottom grating 102. As can be seen in this simplified example of wave propagation in the measurement target, all radiation beams 120, 121, 122, and 123 contribute to the radiation that forms the radiation beam incident on the detector. In addition, radiation beam 120 and radiation beam 121 together carry information about ov1, 131, radiation beam 120 and radiation beam 122 together carry information about ov2, 132, and radiation beam 120 and radiation beam 123 together carry information about ov3, 133. Therefore, the radiation incident on the detector carries information about all possible registration accuracies that can be defined for a target that includes geometric asymmetries (such as grating tilt, bottom surface tilt, top tilt in the bottom grating or top grating). Therefore, in the case where the grating contains geometric asymmetries (e.g., tilt), the problem with the current metrology process is being able to discern which registration accuracy value has been measured.

[0097] The propagation of radiation in the metrology target can be further described as a wave having an intensity I, a frequency ω, and a phase as, for example, shown in Equation 1:

[0098]

[0099] where A is the offset, B is the amplitude parameter of the wave, and the frequency ω is proportional to 2πn(D + H) (n is the refractive index, D + H as shown, and t is 1 / wavelength of the light used for radiation). From this description, the interference of wave 120 with propagating waves 121, 122, and 123 can be described according to Equation 1. In other words, the interference of wave 120 with each of waves 121, 122, and 123 (and all possible multiple waves in view of Equation 1 and the stacking geometric parameters) is characterized by a specific frequency (in units of length, such as nanometers), depending on where the wave is reflected back to the detector. Figure 11 Also shown is a parameter 201 inferred from a measurement measurement as a function of a parameter of the measurement device (such as wavelength 202). This dependence makes the shape of the periodic variation of parameter 201 a function of wavelength. The period 203 depends on the total thickness of the stack, i.e., the distance between the top and bottom gratings 101 and 102. A thicker stack has a smaller period, while a thinner stack has a larger period.

[0100] Figure 11 In a first aspect of the present invention, a method is proposed that includes measuring radiation reflected from a measurement target and decomposing the measured radiation into components. In one embodiment, the decomposition of the measured radiation is obtained using the Fourier transform of the measured radiation. In one embodiment, the measured radiation is radiation 201 as shown. The result of decomposing the measurement result into components is further depicted in

[0101] In Figure 11 shown. Figure 12 The Fourier transform of element 201 as a function of wavelength is shown and includes the magnitude of component 302 as a function of frequency (e.g., in nm). The individual components are 310, 311, and 312, which correspond to each of the radiations formed by 120 with 121, 122, and 123. The detection of multiple individual components is considered part of the art, and Fourier transform techniques allow the identification of Figure 12 each of the multiple individual frequency components of the signal shown in Figure 11 shown. The frequency components can also be detected by other techniques, including but not limited to wavelet transforms and Laplace transforms.

[0102] In another aspect of the present invention, there is provided a method for measuring a parameter of a lithography process, comprising: a) irradiating a metrology target with radiation, b) detecting the scattered radiation from the target, c) changing a parameter of the metrology apparatus, d) repeating steps a) to c) for a plurality of values of the parameter of the metrology apparatus, and e) decomposing the radiation into a plurality of components. In an embodiment of the present invention, the decomposition is a Fourier transform. In an embodiment of the present invention, the method selects components below a threshold of 320. In one embodiment, an inverse decomposition is used and only the selected components are utilized to obtain overlay accuracy. In one embodiment, the inverse decomposition is an inverse Fourier transform. According to U.S. application US2012-0242970, the overlay accuracy is further obtained using prior art methods, the entire content of which is incorporated herein by reference. In one embodiment of the present invention, the threshold 320 is selected such that in Figure 11 only one component 310 remains, and the remaining signal is used in the inverse decomposition step. In this case, the only overlay accuracy measured is ov1, 131 because the only contributing waves of the measured radiation are waves 120 and 121.

[0103] In another aspect of the present invention, obtaining a spectrum as shown in Figure 12 allows the measurement of the phase of each harmonic obtained. In one embodiment, the overlay accuracy can be calculated based on the phase difference of the harmonics in Figure 12 , and the phase difference is obtained for positive and negative first-order diffraction. The Fourier phase of the harmonic corresponding to a specific depth D1 is given by . OVL is the overlay accuracy value, D1 is the depth at which the harmonic is calculated, θ is the general phase, and is the phase proportional to the overlay accuracy. As described above, if the difference between the Fourier phases of specific harmonics is now obtained, this allows the extraction of the overlay accuracy value only from the phases of the Fourier harmonics. It should be understood that this method of calculating overlay accuracy (which includes obtaining the overlay accuracy value from a parameter proportional to the phase of the Fourier harmonics of the measured radiation parameters) is specific to the present invention and provides an alternative method for measuring overlay accuracy in a metrology process, as compared to known methods employed in the prior art. In one aspect of the present invention, there is provided a method for measuring a parameter of a process, comprising: measuring the radiation reflected from a metrology target; decomposing the measured radiation into components; calculating at least two phase values representing each component; and calculating the parameter based on the relationship between the phases.

[0104] In another aspect of the present invention, a method for measuring parameters of a lithography process is provided, comprising: a) irradiating a metrology target with radiation, b) detecting the scattered radiation from the target, c) changing parameters of the metrology apparatus, d) repeating steps a) to c) for a plurality of values of the parameters of the metrology apparatus, e) applying a filter to the measurement results obtained in step d). In one embodiment, the measurement results obtained in step d) are decomposed into individual components. In one embodiment, the filter 320 includes upper and lower limit values defining an individual component. In one embodiment, the filter 320 includes upper and lower limit values defining the interval of a component. In one embodiment, the filter 320 is changed. An advantage of this aspect of the present invention is to measure the overlay accuracy value at a specific depth defined by the filter 320. If the filter 320 is selected as D, only ov1, 131 are measured because all other components in the reflected radiation are removed. If the filter 320 has higher and lower values near the component 311, only ov2, 132 will be measured. In this way, the overlay accuracy is measured and detected at different depths in the stack. In this way, an accurate overlay accuracy can be measured at a specific depth in the stack, which is not contaminated by the contributions of other overlay accuracy values. If the filter 320 is changed, the overlay accuracy values obtained at multiple individual depths in the stack can provide 3D overlay accuracy. Moreover, by appropriate rescaling of the measured values, the method in this aspect of the present invention can provide 3D asymmetry information of the measured stack.

[0105] In another aspect of the present invention, a method for characterizing a lithography process is provided, comprising obtaining a 3D asymmetry map of a bottom grating at a first target position, repeatedly obtaining 3D asymmetry maps of a plurality of targets, and obtaining a target asymmetry map for a wafer based on the above measurements.

[0106] In Figure 11 , measurement signals 201 are obtained at a plurality of wavelengths 202. The decomposition accuracy according to the present invention increases with the number of sampling points for creating Figure 11 the curve graph. However, each measurement point has a related measurement time required for changing the wavelength and performing the measurement. In this regard, too dense sampling results in an increase in the throughput of the metrology process because the required measurements are particularly long. For example, in Figure 11When the curve graph only includes one component, the sampling rate for the number of wavelengths is indicated by the Nyquist criterion. In addition to the theoretical sampling values given by the Nyquist criterion, empirical methods can include, for example, creating the sampling rate through experimental design during the scenario setup phase. In another aspect of the present invention, a method for selecting parameters of a metrology device is provided, including: obtaining a plurality of first measurement results at a plurality of first values of the parameters of the metrology device; and calculating a minimum second number of measurement results and second values associated therewith of the parameters of the metrology device, such that the second values of the parameters of the metrology device are fewer than the first values of the parameters of the metrology device, while still being similar to the key information of the first values of the parameters.

[0107] The above method allows for measuring the overlay accuracy by taking a plurality of measurements at different values of the parameters of the metrology device (such as wavelength, polarization state of the irradiated radiation, or a plurality of incident angles of the irradiated radiation). It is known in the art that in practical situations, when performing measurements on another target, on the same wafer, or on the emitter wafer, such calibration may be inappropriate because the processing conditions, although nominally the same, are actually affected by unwanted variations that are uncontrollable. Therefore, it is desirable to provide a method to obtain the most suitable measurement conditions for the metrology target specific to each measurement. The prior art includes many such methods, which are also known as scenario selection methods. In one aspect of the present invention, the above method for calculating the overlay accuracy can also be adapted to provide the most suitable parameters of the metrology device. Therefore, in one aspect of the present invention, a method is provided that includes: irradiating a target with radiation at a plurality of values of the parameters of the radiation (such as wavelength, polarization, or incident angle); detecting the radiation at the plurality of values of the parameters; and decomposing the measured radiation into a plurality of components. Depending on which overlay accuracy value is of interest, for example Figure 10 the overlay accuracy 131, relevant harmonics are extracted from the measured radiation. Other harmonics are also extracted. In another aspect of this scenario selection method, the correlation of the measured radiation for a single harmonic is recalculated, assuming, for example, a simple sine dependence on the harmonic. This method is not limited to this functional reconstruction, and other methods for reconstructing signals from harmonics can be employed. In another aspect of the scenario selection method, the recalculated correlations of the measured signals based on a plurality of individual harmonics are compared. In one embodiment, the optimal wavelength is such that the recalculated correlation for the harmonic of interest (which corresponds to the overlay accuracy of interest) has a value higher than a first threshold, while the value of the recalculated correlation for the harmonic that is not of interest (because it increases and contaminates the signal) is lower than a second threshold.

[0108] In one embodiment, the selection is obtained digitally based on a search algorithm that has as input values for a first threshold and a second threshold. In one embodiment, the selection can be performed by a skilled operator. In another embodiment of the method of selecting the most suitable wavelength, any wavelength is selected. Additionally, a second wavelength is selected such that it is at a distance within one period of one of the recalculated correlations. In one embodiment, a value of interest is calculated based on the average of two selected wavelengths. In one embodiment, the distance between the two wavelengths is half of one period of at least one of the recalculated correlations. In another embodiment, the distance is one third of the distance between the recalculated correlations.

[0109] In yet another embodiment, by varying the bandwidth of the illumination radiation, suppression of the contribution of unwanted harmonics to the recalculated signal or to the measurement of overlay accuracy can be performed such that the components that cause the unwanted harmonics are suppressed. In one embodiment, the bandwidth is filtered using a rectangular filter characteristic where the bandwidth matches the period or a multiple of the period of the harmonic of interest. In one embodiment, the harmonic of interest is the harmonic that includes the unwanted contribution.

[0110] The above description of the invention applies to 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 intensity of the dark field image of a target is measured at multiple wavelengths. It should be recognized that the metrology device as described in FIG. 3(a) can also provide metrology measurements from a complementary sensor to the sensor used during image acquisition, such as the metrology obtained using sensor 19 of FIG. 3(a). Such measurements are complementary to the measurements obtained by sensor 23 of FIG. 3(a) and contain additional information, such as angular information.

[0111] In another aspect of the invention, the use of information available in complementary measurement units is proposed. The method is similar to that regarding Figures 10 to 12The described method, wherein the measurements obtained in a detection complementary to image detection are decomposed. The decomposition elements are determined by the elementary building blocks of light propagation in the complementary plane of the image measurements. Such building blocks can be calculated based on a theoretical model that takes into account all possible radiation paths and the propagation and summation with the target structure. In this regard, the method requires additional optimization steps relative to a method where, in the method, radiation is detected in the image plane of the metrology device. The optimization steps include: determining the relevant elements that form a signal in the complementary plane of the image plane. In one aspect of the invention related to measurements in a plane complementary to the image plane of the metrology device, the elements that form the basis for the decomposition of the measured signal are the spatial patterns or components of the radiation scattered by the target. An important advantage of this method is that the contributions from the various radiation paths within the target structure are obtained simultaneously, which can significantly increase the throughput of the metrology process.

[0112] Although the above has specifically referred to the use of embodiments of the invention in the context of optical lithography, it should be understood that the invention can be used in other applications, such as imprint processes, and, where the context allows, the invention is not limited to optical lithography. In an imprint process, the topography in the patterning device defines the pattern generated on the substrate. The profile of the patterning device can be pressed into a resist layer provided to the substrate, and then the resist is cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. The patterning device is removed from the resist, leaving a pattern therein after the resist is cured.

[0113] Further embodiments of the invention are described in the following numbered clauses:

[0114] 1. A method, comprising:

[0115] measuring radiation reflected from a metrology target, and

[0116] decomposing the measured radiation into components.

[0117] 2. The method according to clause 1, wherein decomposing the measured radiation is obtained by means of a Fourier transform of the measured radiation.

[0118] 3. A method of measuring a parameter of a lithography process, comprising:

[0119] a) irradiating a metrology target with radiation,

[0120] b) detecting the scattered radiation from the target,

[0121] c) changing a parameter of the metrology device,

[0122] d) repeating steps a) to c) for a plurality of values of the parameter of the metrology device, and

[0123] e) Decompose the radiation into components.

[0124] 5. The method according to clause 3, wherein the decomposition is a Fourier transform.

[0125] 6. The method according to clause 3, wherein the method further comprises selecting components less than a threshold of 320.

[0126] 7. The method according to clause 3, wherein the overlay accuracy is generated using inverse decomposition and only the selected components.

[0127] 8. A method for measuring a parameter of a lithography process, comprising:

[0128] a) irradiating a metrology target with radiation,

[0129] b) detecting the scattered radiation from the target,

[0130] c) changing a parameter of the metrology device,

[0131] d) repeating steps a) to c) for a plurality of values of the parameter of the metrology device,

[0132] e) applying a filter to the measurement results obtained in step d).

[0133] 9. The method according to clause 7, wherein the measurement results obtained in step d) are decomposed into individual components.

[0134] 10. The method according to clause 7, wherein the filter 320 comprises upper and lower limit values defining individual components.

[0135] 11. The method according to clause 7, wherein the filter 320 comprises upper and lower limit values defining a component interval.

[0136] 12. The method according to clause 7, wherein the filter 320 is changed.

[0137] 13. A method for characterizing a lithography process, comprising: obtaining a 3D asymmetric map of a bottom grating at a first target position, repeatedly obtaining 3D asymmetric maps of a plurality of targets, and obtaining a target asymmetric map for a wafer based on the above measurement results.

[0138] 14. A method for selecting a parameter of a metrology device, comprising:

[0139] obtaining a plurality of first measurement results at a plurality of first values of the parameter of the metrology device, and

[0140] Calculating a second value associated with a minimum second number of measurement results and a parameter of a measurement device such that the second value of the parameter of the measurement device is less than a first value of the parameter of the measurement device.

[0141] 14. A computer program comprising program instructions operable to perform the method according to any one of clauses 1 to 13 when run on a suitable device.

[0142] 15. A non-transitory computer program carrier comprising the computer program according to clause 14.

[0143] 16. A method of measuring a process parameter, comprising:

[0144] Measuring radiation reflected from a measurement target,

[0145] Decomposing the measured radiation into components,

[0146] Calculating at least two phase values representing each component, and

[0147] Calculating a parameter based on the relationship between the phases.

[0148] 17. A method for selecting a scheme for a measurement process, comprising:

[0149] Irradiating a measurement target with a plurality of parameters of an irradiation source,

[0150] Detecting radiation scattered by the measurement target with the plurality of parameters of the irradiation source,

[0151] Decomposing the measured radiation into components,

[0152] Recalculating the correlation of the measured radiation corresponding to at least one of the components, and

[0153] Selecting a parameter of the irradiation source for a value of the correlation determined relative to a threshold.

[0154] 18. A method, comprising:

[0155] Measuring radiation reflected from a measurement target, and

[0156] Decomposing the measured radiation into spatial components.

[0157] 19. The method according to clause 18, wherein

[0158] The measured radiation is obtained in a plane complementary to the image plane of the measurement device.

[0159] As used herein, the terms "radiation" and "beam" encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of about 365, 355, 248, 193, 157, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5 to 20 nm), as well as particle beams (such as ion beams or electron beams).

[0160] Where context permits, the term "lens" may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electro - static optical components.

[0161] The foregoing description of specific embodiments discloses the general nature of the embodiments of the present invention, and without departing from the general concept of the present invention, enables others skilled in the art to make various applications of these specific embodiments by applying the knowledge within the art, without undue experimentation. Accordingly, based on the teachings and guidance presented herein, such adaptations and modifications are intended to be within the meaning and scope of the equivalents of the disclosed embodiments. It should be understood that the language or terminology herein is for the purpose of description by way of example and not of limitation, such that the terminology or language of this specification will be interpreted by those skilled in the art in light of the teachings and guidance.

[0162] The breadth and scope of the present invention should not be limited by any of the above - described exemplary embodiments, but should be defined only in accordance with the appended claims and their equivalents.

Claims

1. A method for measuring parameters of a lithography process, comprising: measuring a first radiation reflected from a measured measurement target in a metrology device, the measurement target including an upper structure and a lower structure, wherein the lower structure has an inclination and is spaced apart from the upper structure by a predetermined distance along a depth direction of the measurement target, and alternating layers having different refractive indices are provided between the upper structure and the lower structure; changing a parameter, an irradiation mode, or an imaging mode of the metrology device; after the changing, measuring a second radiation reflected from the measurement target; and decomposing the measured radiation into components based on the measured first radiation and the second radiation.

2. The method according to claim 1, wherein decomposing the measured first radiation and the second radiation is obtained by a Fourier transform of the measured radiation.

3. A method for measuring parameters of a lithography process, comprising: a) irradiating a measurement target with radiation, the measurement target including an upper structure and a lower structure, wherein the lower structure has an inclination and is spaced apart from the upper structure by a predetermined distance along a depth direction of the measurement target, and alternating layers having different refractive indices are provided between the upper structure and the lower structure, b) detecting scattered radiation from the target, c) changing a parameter, an irradiation mode, or an imaging mode of the metrology device, d) repeating steps a) to c) for a plurality of values of the parameter of the metrology device, and e) decomposing the radiation into components.

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

5. The method according to claim 3, wherein the method further comprises: selecting components less than a predetermined threshold.

6. The method according to claim 5, wherein the overlay accuracy error is generated using inverse decomposition and only the selected components.

7. A method for measuring parameters of a lithography process, comprising: a) irradiating a measurement target with radiation, the measurement target including an upper structure and a lower structure, wherein the lower structure has an inclination and is spaced apart from the upper structure by a predetermined distance along a depth direction of the measurement target, and alternating layers having different refractive indices are provided between the upper structure and the lower structure, b) detecting scattered radiation from the target, c) changing a parameter, an irradiation mode, or an imaging mode of the metrology device, d) repeating steps a) to c) for a plurality of values of the parameter of the metrology device, e) applying a filter to the measurement results obtained in step d), wherein the measurement results obtained in step d) are decomposed into individual components.

8. The method according to claim 7, wherein the filter includes upper and lower limit values defining individual components.

9. The method according to claim 7, wherein the filter includes upper and lower limit values defining an interval of components.

10. The method according to claim 7, wherein the filter is changed.

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