Method and system for determining information about a target structure

Abstract

Methods and systems for determining information about a target structure are disclosed. In one arrangement, a value of an asymmetry indicator for the target structure is obtained. The value of the asymmetry indicator represents an amount of overlay-independent asymmetry in the target structure. An error in an initial overlay measurement performed on the target structure at a previous time is estimated. The estimation is performed using a relationship between the obtained value of the asymmetry indicator and an overlay measurement error caused at least in part by the overlay-independent asymmetry. Overlay in the target structure is determined using the initial overlay measurement and the estimated error.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 19188241.4 filed on July 25, 2019, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to determining information about a target structure on a substrate, such as an overlay. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a substrate, usually to a target portion of the substrate. For example, a lithographic apparatus can be used in the manufacture of an integrated circuit (IC). In this case, a patterning device, alternatively referred to as a mask or reticle, can be used to generate a circuit pattern to be formed on each layer of the IC. The pattern can be transferred to a target portion (e.g., including a portion of a die, a die, or several dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is usually via imaging onto a radiation-sensitive material (resist) layer disposed on the substrate. In general, a single substrate will contain a network of adjacent target portions of continuous patterning. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing the entire pattern to the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern by a radiation beam in a given direction ("scanning" direction), while scanning the substrate synchronously parallel to or antiparallel to the direction. The pattern can also be transferred from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0005] In photolithography, it is often desirable to make measurements of the structures being created, for example for process control and verification. Various tools are known for making such measurements, including scanning electron microscopes, which are commonly used to measure critical dimensions (CD), and specialized tools for measuring overlay, i.e., the accuracy of alignment of two layers in a device. More recently, various forms of scatterometers have been developed for use in the field of photolithography. These devices direct a radiation beam onto a target and measure one or more of the following properties of the scattered radiation to obtain a data range from which a property of interest of the target can be determined: for example, intensity at a single reflection angle as a function of wavelength; intensity at one or more wavelengths as a function of reflection angle; or polarization as a function of reflection angle.

[0006] Determination of the properties of interest can be performed by various techniques: for example, reconstruction of the target structure by iterative methods such as rigorous coupled wave analysis or finite element method; library searching; and principal component analysis.

[0007] The targets used by some scatterometers are relatively large gratings, for example, 40μm×40μm, and the measuring beam produces a spot smaller than the grating (i.e., the grating is underfilled). This simplifies the mathematical reconstruction of the target since it can be considered to be infinite. However, in order to reduce the size of the targets, for example, to 10μm×10μm or smaller, so that they can be located between product features rather than in scribe lines, metrology methods have been proposed that make the grating smaller than the measurement spot (i.e., the grating is overfilled). Such targets are typically measured using dark-field scatterometry, in which the zeroth-order diffraction (corresponding to specular reflection) is blocked and only higher-order diffraction is processed. Using dark-field detection of the diffraction orders, diffraction-based overlay enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and can be surrounded by product structures on the substrate. Multiple targets can be measured in one image.

[0008] In known metrology techniques, overlay measurements are obtained by measuring a target twice under specific conditions, while rotating the target or changing the illumination mode or imaging mode to obtain the -1st and +1st diffraction order intensities, respectively. Comparing these intensities for a given grating provides an indicator of overlay.

[0009] While known dark-field image-based overlay measurements are fast and computationally simple (once calibrated), they rely on the assumption that overlay is the only cause of asymmetries in the target structure. Any other asymmetries in the stack, such as asymmetries of features within one or both of the overlaid gratings, may also result in intensity differences between the -1st and +1st orders. Such overlay-independent asymmetries may reduce the accuracy of the overlay measurement. Summary of the invention

[0010] Improved overlay measurements are desired.

[0011] According to aspects of the present invention, a method for determining information about a target structure on a substrate is provided, comprising: obtaining a value of an asymmetry indicator for the target structure, the value of the asymmetry indicator representing an amount of overlay-independent asymmetry in the target structure, the overlay-independent asymmetry being an asymmetry not caused by overlay; estimating an error in an initial overlay measurement performed on the target structure at a previous time, the estimation being performed using a relationship between the obtained value of the asymmetry indicator and an overlay measurement error at least partially caused by overlay-independent asymmetry; and determining the overlay in the target structure using the initial overlay measurement and the estimated error.

[0012] According to aspects of the present invention, a system for determining information about a target structure on a substrate is provided, the system comprising a processor configured to: obtain a value of an asymmetry indicator for the target structure, the value of the asymmetry indicator representing an amount of overlay-independent asymmetry in the target structure, the overlay-independent asymmetry being an asymmetry not caused by overlay; estimate an error in an initial overlay measurement performed on the target structure at a previous time, the estimation being performed using a relationship between the value of the obtained asymmetry indicator and an overlay measurement error at least partially caused by overlay-independent asymmetry; and determine overlay in the target structure using the initial overlay measurement and the estimated error. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Other features and advantages as well as the structure and operation of various embodiments are described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented here for illustrative purposes only. Based on the teachings contained herein, additional embodiments will be apparent to those skilled in the relevant art(s).

[0014] Embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0015] Figure 1 The photolithographic apparatus is depicted;

[0016] Figure 2 Lithographic cells or clusters are depicted;

[0017] Figure 3A Plotted for an overlaid grating with no overlay-independent asymmetry, A + A - Graph of

[0018] Figure 3B A is depicted for an overlay grating with overlay-independent asymmetry. + A - Graph of

[0019] Figure 4 Depicted are two unit cells of an example periodic structure without overlay-independent asymmetry;

[0020] Figure 5 Depicts a symmetrical sidewall angle Figure 4 The unit cell of the periodic structure;

[0021] Figure 6 Depicts a critical size imbalance Figure 4 The unit cell of the periodic structure;

[0022] Figure 7is a flow chart depicting a method for determining information about a target structure;

[0023] Figure 8 is a flow chart depicting a method of obtaining a relationship between a value of an asymmetry indicator and an overlay measurement error;

[0024] Fig. 9 is a flow chart depicting a model-based method for determining sensitivity to different asymmetry modes, asymmetry indicators, and overlay measurement errors;

[0025] Figure 10-13 is asymmetry indicator ( Fig.10 and 11 ) and overlay measurement error ( Fig.12 and 13 ) from simulated scatterometry measurements for different asymmetric modes in the target structure and for different polarizations of the incident radiation;

[0026] Fig.14 Is depicted using reference Figure 9-13 A flow chart describing a method for determining the sensitivity of a method for determining information about a target structure; and

[0027] Fig.15 A system for determining information about a target structure is described.

[0028] Features and advantages of the present invention will become apparent from the detailed description set forth below when taken in conjunction with the accompanying drawings. DETAILED DESCRIPTION

[0029] This specification discloses one or more embodiments incorporating features of the present invention. The disclosed (multiple) embodiments merely illustrate the present invention. The scope of the present invention is not limited to the disclosed (multiple) embodiments. The present invention is defined by the claims appended hereto.

[0030] The described embodiment(s), as well as references to "one embodiment", "an embodiment", "an example embodiment", etc. in the specification, indicate that the described embodiment(s) may include specific features, structures, or characteristics, but each embodiment does not necessarily include the specific features, structures, or characteristics. In addition, these phrases do not necessarily refer to the same embodiment. In addition, when specific features, structures, or characteristics are described in conjunction with an embodiment, it is understood that whether or not explicitly described, it is within the knowledge of those skilled in the art to implement such features, structures, or characteristics in conjunction with other embodiments.

[0031] Embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include a read-only memory (ROM); a random access memory (RAM); a disk storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal, etc. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are merely for convenience, and such actions are actually caused by a computing device, processor, controller, or other device that executes firmware, software, routines, instructions, etc.

[0032] Figure 1 A lithographic apparatus LA is schematically depicted. The apparatus comprises 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., mask stage) MT configured to support a patterning device (e.g., mask) MA and connected to a first positioner PM configured to accurately position the patterning device according to specific parameters, a substrate stage (e.g., wafer stage) WT configured to hold a substrate (e.g., a wafer coated with resist) W and connected to a second positioner PW configured to accurately position the substrate according to specific 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., comprising one or more dies) of the substrate W.

[0033] The illumination system may include various types of optical components for directing, shaping, or controlling radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof.

[0034] The patterning device support holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as whether the patterning device is held in a vacuum environment. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support can be a frame or a stage, for example, which can be fixed or movable as required. The patterning device support can ensure that the patterning device is located at a desired position, such as a desired position relative to a projection system. Herein, any use of the term "reticle" or "mask" can be considered synonymous with the more general term "patterning device".

[0035] The term "patterning device" as used herein should be broadly interpreted as referring to any device that can be used to impart a pattern in a cross-section of a radiation beam, such as to create a pattern in a target portion of a substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so-called assist features. Generally, the pattern imparted to the radiation beam will correspond to a specific functional layer in a device created in the target portion, such as an integrated circuit.

[0036] 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 to reflect an incident radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam reflected by the matrix of mirrors.

[0037] The term "projection system" as used herein should be interpreted broadly to include any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic, and electrostatic optical systems, or any combination thereof, as appropriate to the exposure radiation being used, or to other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system."

[0038] As described herein, the device is of the transmissive type (eg, using a transmissive mask). Alternatively, the device may be of the reflective type (eg, using a programmable mirror array of the type described above, or using a reflective mask).

[0039] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and / or two or more mask tables). In such a "multi-stage" machine, the additional tables may be used in parallel, or preparatory steps may be performed on one or more tables while one or more other tables are being used for exposure.

[0040] The lithographic apparatus may also be of a type in which at least a portion of the substrate may be overlaid with a liquid having a relatively high refractive index, such as water, so as to fill the space between the projection system and the substrate. Immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art and are used to increase the numerical aperture of the projection system. The term "immersion" as used herein does not mean that structures 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.

[0041] refer to Figure 1 , the illuminator IL receives a radiation beam from a radiation source SO. For example, when the source is an excimer laser, the source and the lithographic apparatus may be separate entities. In this case, the source is not considered to be part of the lithographic apparatus, and the radiation beam is transferred from the source SO to the illuminator IL by means of a beam delivery system BD, which includes, for example, suitable guide mirrors and / or a beam expander. In other cases, the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. If desired, the source SO and the illuminator IL and the beam delivery system BD may be referred to as a radiation system.

[0042] The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam.

[0043] Generally, at least the outer radial extent and / or the inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the pupil plane of the illuminator can be adjusted. Furthermore, 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 its cross-section.

[0044] The radiation beam B is incident on a patterning device (e.g., a mask) MA, which is held on a patterning device support (e.g., a mask table MT), and is patterned by the patterning device. Having passed the patterning device (e.g., a 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. With the aid of a second positioner PW and a position sensor IF (e.g., an interferometer, a linear encoder, a 2D encoder or a capacitive sensor), the substrate table WT can be precisely moved, for example, in order to position different target portions C on the path of the radiation beam B. Similarly, a first positioner PM and a further position sensor ( Figure 1 The first positioner PM and the support member (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. In general, movement of the patterning device support (e.g. mask table) MT can be achieved with the help of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning) which form part of the first positioner PM. Similarly, movement of the substrate table WT can be achieved using a long-stroke module and a short-stroke module which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the patterning device support (e.g. mask table) MT may be connected to the short-stroke actuator only, or may be fixed.

[0045] Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning device (e.g., mask) MA and substrate W. Although the substrate alignment marks as shown occupy dedicated target portions, they can be located in the spaces between target portions (these are called scribe alignment marks). Similarly, where more than one die is provided on the patterning device (e.g., mask) MA, the mask alignment marks can be located between the dies. Small alignment marks can also be included within the die, between device features, in which case it is desirable that the marks be as small as possible and do not require any imaging or process conditions that are different from those of adjacent features. An alignment system for detecting alignment marks is further described below.

[0046] The described device can be used in at least one of the following modes:

[0047] 1. In step mode, the patterning device support (e.g. mask table) MT and the substrate table WT remain substantially stationary while the entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and / or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

[0048] 2. In scan mode, the patterning device support (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The speed and direction of the substrate table WT relative to the patterning device support (e.g. mask table) MT may be determined by the (de)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width of the target portion in a single dynamic exposure (in the non-scanning direction), while the length of the scanning movement determines the height of the target portion (in the scanning direction).

[0049] 3. In another mode, the programmable patterning device is maintained, the patterning device support (e.g., mask table) MT remains substantially stationary, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, a pulsed radiation source is typically employed, and the programmable patterning device is updated as required after each movement of the substrate table WT, or between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography utilizing a programmable patterning device such as a programmable mirror array of the type described above.

[0050] Combinations and / or variations of the above-described modes of use or entirely different modes of use may also be employed.

[0051] The lithography apparatus LA may be of a so-called dual-stage type having two substrate tables WTa, WTb and two stations - an exposure station and a measuring station - between which the substrate tables may be exchanged. While one substrate on one substrate table is being exposed on the exposure table, another substrate may be loaded onto another substrate table at the measuring station and various preparation steps performed. The preparation steps may include surface control of drawing the substrate using a level sensor LS, and measuring the position of alignment marks on the substrate using an alignment sensor AS. This enables the throughput of the apparatus to be significantly increased. If the position sensor IF cannot measure the position of the substrate while it is located at the measuring station and the exposure station, a second position sensor may be provided to enable the position of the substrate to be tracked at the two stations.

[0052] like Figure 2 As shown, the lithography apparatus LA forms part of a lithography cell LC (sometimes also referred to as a lithography cell or cluster), which also includes devices for performing pre-exposure and post-exposure processes on a substrate. These devices include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH and a baking plate BK. A substrate handler or robot RO picks up substrates from input / output ports I / O1, I / O2, moves them between different process devices, and then transfers them to a feed station LB of the lithography apparatus. These devices, which are generally collectively referred to as tracks, are controlled by a track control unit TCU, which itself is controlled by a supervisory control system SCS, which also controls the lithography apparatus via a lithography apparatus control unit LACU. Thus, the different devices can be operated to maximize throughput and processing efficiency.

[0053] It is known to detect first-order diffracted light from a target structure with overlay error (such as a stacked grating), for example as described in US8339595B2, the entire contents of which are incorporated herein by reference. As described in WO 2015018625, the entire contents of which are incorporated herein by reference, a target can be illuminated to perform measurements related to overlay at the target. As described in more detail in WO2015018625, diffracted radiation can be detected and different orders of diffraction, such as first-order diffraction, can be determined. The overlay at the target can be determined using an asymmetry of the radiation intensity in a particular diffraction order. The intensity asymmetry of the target can be measured using any suitable type of measuring device. For example, as described in WO 2015018625, a micro-diffraction-based overlay (uDBO) metrology device can be used, which can be a stand-alone device or incorporated in a lithography device LA or a lithography cell LC at, for example, a measurement station. The intensity asymmetry can be measured in different ways at different polarizations and wavelengths.

[0054] Overlay refers to the alignment accuracy between different periodic structures in the target structure (e.g., overlaying gratings in different layers). In some embodiments, the target structure includes multiple pairs of gratings with different corresponding overlay biases (i.e., different intentionally applied overlay offsets). The target structure may also include gratings with different orientations, such as gratings configured to diffract incident radiation in orthogonal X and Y directions, respectively (which may be referred to as X-direction gratings and Y-direction gratings). As an example, the target structure may include an X-direction grating with a bias of +d and an X-direction grating with a bias of -d. A grating with a bias of d means that the grating has an overlay component arranged as follows, so that if both overlay components are accurately printed in their nominal positions, one overlay component will be offset by a distance d relative to the other overlay component. Gratings with biases of +d and -d will have components arranged as follows, so that if perfectly printed, the offsets will be in opposite directions to each other. In an embodiment, the target structure additionally includes a Y-direction grating with a bias of +d and a Y-direction grating with a bias of -d. The target structure may be formed and measured as described in WO2015018625. The size of the offset d of each grating is typically small compared to the width of the grating, such as about 20 nm.

[0055] As described above, in an embodiment, at least one periodic structure (e.g., a grating) in the target structure includes a positive bias offset (e.g., +d) and a negative bias offset (e.g., -d). The intensity asymmetry between different diffraction orders (e.g., between -1 and +1 diffraction orders) may be measured for each positive bias and negative bias portion of the target structure, respectively. The intensity asymmetry value of the positive bias portion of the target structure may be referred to as A + The intensity asymmetry value of the negative bias part of the target structure can be called A - Value. + The value is proportional to the overlay of the positive bias portion of the target structure, with a proportionality constant k, which is called the sensitivity constant. - The value is proportional to the overlay of the negatively biased part of the target structure with a proportionality constant k. The effect of overlay-independent asymmetry can be seen from the different wavelength ranges of the incident radiation. + Value and A - The relationship between the values ​​can be seen in the diagram.

[0056] Figure 3A A depicting the target structure + A -A graph of the overlay without any overlay-independent asymmetry. The target structure is measured at multiple different wavelengths. The wavelengths used are exemplary. In this ideal case, all data points lie on the same straight line for each of the three different overlays considered. In the absence of overlay-independent asymmetry, the straight line would pass through the origin of the graph. Since overlay is proportional to the intensity asymmetry between opposite diffraction orders, the overlay can be determined from the gradient of the line. The overlay ov can be determined by using the following approximate equation:

[0057]

[0058] Where d is the known offset of the target structure being measured, and m = A + / A - .

[0059] Figure 3A Three different measurement sets are shown in FIG. 1 , representing no overlay (ov=0; Figure 3A Line 1), positive overlay (ov>0; usually 0 <ov<20nm; Figure 3A Line 2) and negative engraving (ov<0; usually -20 <ov<0nm; Figure 3A Line 3). Figure 3A The gradient of line 1 (ov = 0) is -1. When the gradient is greater than -1, such as in Figure 3A In line 2, the overlay is greater than 0. When the gradient of the line is less than -1, such as in Figure 3A In line 3, the overlay is less than 0.

[0060] refer to Figure 4-Figure 6 , manufacturing errors lead to defects in the target structure to be used for overlay measurement. These defects can include one or more asymmetries that are not caused by overlay. Each asymmetry that is not caused by overlay can be called an asymmetry that is not related to overlay.

[0061] Figure 4 Two example unit cells of a perfectly formed grating 10 of an example target structure are schematically depicted. Each grating line 12 has perfectly vertical walls. All grating lines 12 have the same width. The grating 10 does not have any overlay-independent asymmetries. If the target structure had further such gratings located in different layers, the misalignment between the two gratings associated with the overlay would result in the target structure having asymmetries that depend only on the overlay. Scattering measurements performed on such a target structure would be able to determine the overlay based on asymmetries in the scattered radiation (e.g., asymmetry between the measured intensities of the +1 and -1 diffraction orders). The absence of any overlay-independent asymmetries in the target structure means that such measurements will be able to determine the overlay with high accuracy.

[0062] Imperfections in the target structure introduce asymmetries that are unrelated to overlay. Figure 5 An example asymmetric pattern due to tilting of the sidewalls 13 of the grating lines 12 is depicted. This asymmetric pattern may be referred to as sidewall asymmetry. The amplitude ΔSWA of the sidewall asymmetry may be defined as the lateral displacement between the top of the sidewall 13 and the bottom of the sidewall 13, as Figure 5 For example, sidewall asymmetry may be caused by defects in the etching process. Figure 6 Another example asymmetric pattern is depicted, caused by critical dimension variations on the target structure. In this case, variations in critical dimension (CD) result in variations in the width of lines 12 of grating 10. This asymmetric pattern may be referred to as CD imbalance. The magnitude of CD imbalance may be defined as ΔCD=ΔCD2-ΔCD1.

[0063] Overlay-independent asymmetries may lead to asymmetries in the scattered radiation, such as an asymmetry between the measured intensities of the +1 and -1 diffraction orders, potentially leading to errors in overlay measurements that rely on asymmetries in the scattered radiation.

[0064] Figure 3B A depicts the target structure with overlay-independent asymmetry + A - Graph of the curve. Figure 3B Line 1 in is for reference only and corresponds to the case where the target structure has no overlay-independent asymmetry and no overlay. Figure 3B Lines 2 and 3 in are the best fit lines through the data points corresponding to two orthogonal polarizations, respectively (line 2 corresponds to the polarization with the electric field vector parallel to the grating direction in the target structure, and line 3 corresponds to the polarization with the electric field vector at 90° to the grating direction in the target structure). Figure 3A Differently, the data points corresponding to lines 2 and 3 cannot be accurately fitted along a straight line. In addition, the best fit line does not pass through the origin.

[0065] The distance of the best fit line to the origin of the graph, which may be referred to as the DTO (distance to origin), provides an asymmetry indicator. The distance to origin (DTO) for each data set is the shortest distance from the origin of the graph to the line, i.e., the shortest distance to the origin of the graph at 90° to the best fit line through the points on the graph. The value of the asymmetry indicator (e.g., DTO) represents the amount of overlay-independent asymmetry in the target structure.

[0066] exist Figure 3B In the example of FIG. 4 , the target structure radiates with different polarizations. Figure 3B Lines 2 and 3 in FIG. 5 show data points obtained from the same target using two different polarizations, respectively. Figure 3B It is shown that the data points of the two polarizations are located on two different lines with different gradients and different DTOs. The DTOs of lines 2 and 3 are given by Figure 3BIndicated by the double-headed arrow in the elliptical area near the origin of the curve graph.

[0067] From the above discussion it can be seen that overlay measurements, which rely on detecting asymmetries in scattered radiation, may be affected by overlay-independent asymmetries in the target structure. Overlay-independent asymmetries are expected to vary significantly between different target structures and may therefore lead to significant errors in overlay measurements. The embodiments described below are intended to improve the extent to which overlay-independent asymmetries are taken into account, thereby improving overlay measurements.

[0068] A preliminary investigation was performed to determine the extent to which overlay-independent asymmetries contribute to the errors in overlay measurements. Multiple target structures were measured at multiple wavelengths and multiple polarization states (including TE and TM polarizations). A + A - A best fit line was derived for each target structure and each polarization. The gradient s of the best fit line was used to calculate the gradient s of the best fit line using the following relationship, based on A + and A - Graph export overlay measurement

[0069]

[0070] The value of the asymmetry indicator (in this case DTO) is obtained for each target structure and each polarization. In addition, a reference tool (which may be an electron beam device) may be used to obtain the overlay OV in each target structure. ref The overlay measurement error can then be determined from the difference: This preliminary investigation shows that a large range of overlay measurement errors is observed and that there is a clear correlation between the overlay measurement errors and the asymmetry indicators, i.e., in this case, the + and A - There is a clear correlation between the DTO derived from the graph and the corresponding values ​​of ΔOV. Therefore, by measuring an asymmetry indicator such as DTO, an expected value of ΔOV can be determined, which can be used to correct initial measurements of overlay that do not account for overlay-independent asymmetries in the target structure.

[0071] Figure 7is a flow chart depicting a method for determining information about a target structure on a substrate W. The method includes obtaining a value of an asymmetry indicator for the target structure. The value of the asymmetry indicator represents an amount of overlay-independent asymmetry in the target structure. The overlay-independent asymmetry is not an asymmetry caused by overlay. Thus, overlay-independent asymmetry is not caused by misalignment between periodic structures in different layers of the target structure. In an embodiment, the overlay-independent asymmetry comprises a plurality of asymmetry patterns. In an embodiment, the plurality of asymmetry patterns comprises an asymmetry pattern representing a sidewall angle asymmetry (e.g., as described above with reference to Figure 5 In an embodiment, the plurality of asymmetric patterns include an asymmetric pattern representing asymmetry in a critical dimension (eg, as described above with reference to Figure 6 As described above, it may be referred to as CD imbalance). The plurality of asymmetric modes may include other asymmetric modes.

[0072] In step S1, the target structure is measured using a scatterometry method. A radiation beam is directed onto the target structure and the scattered radiation is detected. As described above, the scatterometry measurement can be performed in a dark field mode. The scatterometry measurement can include separate detection of opposite diffraction orders in order to obtain the intensities of opposite diffraction orders, such as the intensities of the -1 and +1 diffraction orders. In an embodiment, the scatterometry measurement is performed at each wavelength in the wavelength set.

[0073] In step S2, the measurement data from the scatterometry is used to obtain a value of an asymmetry indicator for the target structure. In an embodiment, the value of the asymmetry indicator for the target structure is obtained using information about the asymmetry in the radiation scattered from the target structure at each wavelength in the set of wavelengths. In an embodiment, the information about the asymmetry in the radiation scattered from the target structure comprises differences in detected intensities between different diffraction orders, such as the intensity difference between equal and opposite diffraction orders (e.g., +1 and -1 diffraction orders). In an embodiment, for portions of the target structure having different biases, differences in detected intensities between different diffraction orders are obtained. In an embodiment, the detected intensity difference is obtained for a portion of the target structure having a +d bias to obtain A. + value, and the detection intensity difference is obtained for a portion of the target structure with a -d bias to obtain A - Thus, the target structure may include a composite target structure including portions having different nominal overlay biases. In an embodiment, determining the value of the asymmetry indicator includes fitting a curve to A + A - A graph of a wavelength set, wherein the data points in the graph include at least one data point for each wavelength in a plurality of wavelengths in the wavelength set used in the scatterometry measurement. + A -The graph of the curve includes data points corresponding to the measurement mode, which have differences other than wavelength differences, including, for example, incident angle differences and / or polarization differences. The value of the asymmetry indicator can be obtained by calculating a metric representing the deviation between the characteristic of the fitted curve and the characteristic of the reference curve corresponding to the case where there is no overlay-independent asymmetry, or the deviation between the characteristic of the fitted curve and the characteristic common to a range of reference curves corresponding to the case where there is no overlay-independent asymmetry, but there is a range of different overlays. In an embodiment, the characteristic or common characteristic is that the reference curve or multiple reference curves pass through A + A - In an embodiment, the value of the asymmetry indicator includes the fit curve and A + A - In an embodiment, the fitted curve is a fitted straight line, and the value of the asymmetry indicator includes the distance between the fitted straight line and A + A - The shortest distance between the origins of the graph of (referred to above as the distance to the origin, DTO). In other embodiments, the asymmetry indicator may be defined differently. For example, in one class of embodiments, the asymmetry indicator is defined based on the difference between measured overlay values ​​obtained via measurements performed at different wavelengths (the difference being at least partially caused by overlay-independent asymmetry). For example, the value of the asymmetry indicator in such an embodiment may be obtained by calculating the difference between the measured overlay values ​​obtained at two different wavelengths. In another embodiment, the value of the asymmetry indicator is obtained in a metrology measurement step prior to the lithography step. In such a measurement, the value of the asymmetry indicator may be obtained from the measurement of a grating having a uniform profile of a resist overlaying an underlying grating formed in a semiconductor wafer, such as a measurement of a bottom grating asymmetry (BGA) as known in the art. After the top resist is patterned and developed, such a BGA target may be used for further overlay metrology measurements, or may be an independent BGA target. In another embodiment, the value of the asymmetry indicator is obtained in a metrology measurement step, which may be performed using a metrology device of a lithography device, such as an alignment metrology device. In such measurements, an alignment target may be used. In another embodiment, a method is presented, the method comprising the step of comparing estimated errors of a plurality of values ​​of an asymmetry indicator, the values ​​being obtained from different metrology steps, such as measured on a metrology target, e.g. a DTO, such as measured on a BGA target or such as measured on an alignment target. The method compares the correlation of the estimated errors of a plurality of types of asymmetry indicators and selects the type of asymmetry indicator, which provides the best information about the asymmetry.

[0074] In step S3, the value of the asymmetry indicator (e.g., DTO) obtained in step S2 is used to estimate the error ΔOV in an initial overlay measurement performed on the target structure at a previous time. In an embodiment, the initial overlay measurement is obtained using the measurement data from the scatterometry obtained in step S1. In an embodiment, the error ΔOV in an initial overlay measurement performed on the target structure at a previous time is estimated. + A - The initial overlay measurement is obtained by using the gradient s of the best fit line in the curve graph. Thus, the initial overlay measurement may include the overlay measurement The overlay measure is derived from the gradient s of the best fit line using the following relationship:

[0075]

[0076] The estimation of the error ΔOV is performed based on an expected correlation between the error ΔOV and the value of the asymmetry indicator described above. This correlation is represented as a quantized relationship between the value of the asymmetry indicator (e.g., DTO) and the overlay measurement error ΔOV. The overlay measurement error ΔOV is at least partially caused by an overlay-independent asymmetry, which is represented by the asymmetry indicator. The following description provides an example method for obtaining the relationship, including the use of calibration measurements (which may be referred to as a data-driven method) and / or computer simulations (which may be referred to as a model-based method). The relationship may be determined at an earlier time, and step S3 may include accessing a stored version of the relationship from a computer-accessible memory.

[0077] In step S4, using the initial overlay measurement (e.g., ) and the estimated error (e.g., ΔOV), for example, via the relationship Get the overlay in the target structure.

[0078] Figure 8 is a flow chart illustrating an example data driven method for deriving a relationship between the value of an asymmetry indicator and an overlay measurement error, such as may be found in Figure 7 The method comprises measuring the value of an asymmetry indicator in each of a plurality of calibration structures having different overlay-independent asymmetries, and an overlay measurement error. The different overlay-independent asymmetries may occur via natural variations between calibration structures formed at different times and / or locations. Alternatively, the different overlay-independent asymmetries may be at least partially intentionally induced.

[0079] In step S11, one of the calibration structures is measured. The measurement may be performed using scatterometry. Figure 7 The scattering measurement is performed as described in step S1 of FIG. Figure 8As shown in step S12, information about asymmetry in the radiation scattered from the calibration structure is obtained. The information about asymmetry can be obtained as described above with reference to Figure 7 Thus, each calibration structure may include parts with different overlay biases relative to each other, such as equal and opposite overlay biases +d and -d. Information about asymmetry may include A + Value and A - value.

[0080] Steps S11 and S12 are repeated for each of the sets of wavelengths and / or polarizations (or other scattering settings). In an embodiment, A of the calibration structure is obtained at each wavelength in the set of wavelengths and / or for each polarization. + Value and A - value.

[0081] In step S13, the information about the asymmetry (e.g., A) obtained at each of the wavelength and / or polarization sets is used. + Value and A - value), to obtain the value of the asymmetry indicator (and, as described below, repeated for multiple calibration structures with different overlay-independent asymmetries). Figure 7 The value of the asymmetry indicator is obtained by any means as described in step S2, including, for example, fitting a straight line to A + Value pair A - A graph of the values, and determining the distance to the origin, DTO.

[0082] In step S14, the overlay measurement error ΔOV is obtained. + A - The gradient of the best fit line in the graph s is used to obtain an independent measurement of the overlay in the calibration structure OV using the reference tool. ref , and obtain OV ref and the overlay measurement obtained based on the information of steps S11 and S12 (such as the overlay measurement, ) can be used to obtain the overlay measurement error, as described above.

[0083] Steps S11 , S12 and S13 are repeated ( S14 ) for a plurality of calibration structures having different overlay-independent asymmetries.

[0084] In step S15, for each calibration structure considered, the value of the asymmetry indicator (eg DTO) obtained in step S13 and the corresponding overlay measurement error ΔOV obtained in step S14 are output.

[0085] In step S16, the output of step S15 is then used to obtain a relationship between the value of the asymmetry indicator and the overlay measurement error for use in Figure 7 Step S3. Figure 8 In an example, the relationship is obtained by fitting a curve (e.g., a straight line) to a graph of the value of the asymmetry indicator relative to the corresponding overlay measurement error ΔOV, wherein each data point represents the value of the asymmetry indicator and the corresponding overlay measurement error ΔOV for one of the calibration structures. In an embodiment, the process is performed separately for different polarizations to obtain a first curve and a second curve. The first curve is obtained by fitting a graph of the value of the asymmetry indicator relative to the corresponding overlay measurement error ΔOV, wherein each data point represents, for one of the calibration structures obtained by scattering in step S11 using radiation with a first polarization, the value of the asymmetry indicator and the corresponding overlay measurement error ΔOV. The second curve is obtained by fitting a graph of the value of the asymmetry indicator relative to the corresponding overlay measurement error ΔOV, wherein each data point represents, for one of the calibration structures obtained by scattering in step S11 using radiation with a second polarization, the value of the asymmetry indicator and the corresponding overlay measurement error ΔOV. In an embodiment, the first polarization is orthogonal to the second polarization. In an embodiment, the one of the first and second curves having the highest gradient is used as a basis for deriving the relationship between the value of the asymmetry indicator and the overlay measurement error in order to achieve maximum sensitivity.

[0086] The above data-driven approach is mathematically simple to implement and does not require any mathematical modeling or assumptions about the calibration structure except for some basic target properties such as spacing and induced bias.

[0087] The equation describing the fitted curve provides a representation of the relationship between the value of the asymmetry indicator and the overlay measurement error. The quality of the fit of the curve to the data can provide an indication of the reliability of the representation of the relationship. The quality of the fit (by describing a measure of the accuracy with which the fitted curve fits the data) can be quantified in various ways known in the art of mathematical curve fitting.

[0088] The quality of the fit can depend on the choice of the set of wavelengths represented by the plotted data points. Figure 7In the case where the relationship between the value of the asymmetry indicator and the overlay measurement error in the method is only used for a relatively small set of wavelengths (e.g., less than 10 different wavelengths, optionally less than 5 different wavelengths), the curve fitting of step S16 may be repeated for multiple candidate wavelength sets (including different wavelength combinations). Therefore, the relationship between the value of the asymmetry indicator and the overlay measurement error for multiple candidate wavelength sets may be obtained, and one of the candidate wavelength sets may be selected based on the quality of the correlation between the value of the asymmetry indicator and the overlay measurement error of the candidate wavelength set to preselect the wavelength in the candidate wavelength set. Figure 7 For example, a wavelength set can be selected that allows a curve (e.g., a straight line) to be fitted to the data with high accuracy.

[0089] In some embodiments, the derivation of the relationship between the value of the asymmetry indicator and the overlay measurement error comprises simulating scattering of radiation from the calibration structure for a plurality of simulated overlay-independent asymmetries in the calibration structure. Thus, scatterometry measurements from a series of calibration structures may be simulated, rather than as described above with reference to Figure 8 Scattering measurements are performed on a physical calibration structure as described above. Various methods are known to simulate such scattering. For example, a Monte Carlo simulation can be used to perform the simulation. Such simulation-based methods can be referred to as model-based methods because the simulation of the scattering uses a mathematical model to represent the relevant properties of the calibration structure.

[0090] Fig. 9 is a flowchart depicting an example model-based approach.

[0091] In step S21, a computer is used to simulate scattering from a theoretical calibration structure represented by a mathematical model. The simulation is repeated for a series of theoretical structures, a series of scattered radiation wavelengths, and optionally, a series of different polarizations (e.g., two orthogonal polarizations). The series of theoretical structures may include theoretical structures with multiple different overlays. The series of theoretical structures may also include multiple different overlay-independent asymmetries (e.g., ΔSWA and ΔCD vary within the range of 0nm±1nm). Multiple different overlay-independent asymmetries may include multiple asymmetric amplitudes. Alternatively or additionally, multiple different overlay-independent asymmetries may include multiple asymmetric modes. For example, multiple asymmetric modes may include a first asymmetric mode representing asymmetry of the sidewall angle (which may be referred to as ΔSWA) and a second asymmetric mode representing asymmetry on the critical dimension (which may be referred to as ΔCD).

[0092] In step S22, via the simulation of step S21, values ​​of an asymmetry indicator (eg, DTO) and an overlay measurement error are calculated for a plurality of combinations of overlay, overlay-independent asymmetry and / or polarization.

[0093] In step S23, the calculated value of the asymmetry indicator and the overlay measurement error are used to derive the sensitivity of the asymmetry indicator (e.g., DTO) to each asymmetric mode (e.g., ΔSWA and ΔCD). In this context, the sensitivity of a first quantity to a second quantity represents how the first quantity is expected to change in response to a change in the second quantity. If the sensitivity is known, the change in the first quantity can be derived from the knowledge of the change in the second quantity. In an embodiment, the calculated asymmetry indicator and the overlay measurement error are also used to derive the sensitivity of the overlay measurement error ΔOV to each asymmetric mode (e.g., ΔSWA and ΔCD). In an embodiment, the calculated asymmetry indicator and the overlay measurement error are used to derive the sensitivity of the asymmetry indicator (e.g., DTO) to each asymmetric mode (e.g., ΔSWA and ΔCD) for each of a plurality of incident polarizations (e.g., TE and TM). In an embodiment, the calculated asymmetry indicator and the overlay measurement error are also used to derive the sensitivity of the overlay measurement error ΔOV to each asymmetric mode for each of a plurality of incident polarizations.

[0094] Figure 10-12 Depicted are example graphs of data derived from simulations such as those described above. Fig.10 is a graph of the asymmetry indicator (in this case DTO) versus the value of ΔCD for two different polarizations (TE and TM). Fig.11 is a graph of the asymmetry indicator (DTO) versus ΔSWA value for two different polarizations (TE and TM). Fig.12 is a graph of the overlay measurement error ΔOV versus ΔCD for two different polarizations (TE and TM). Fig.13 is a plot of the overlay measurement error ΔOV versus ΔSWA for two different polarizations (TE and TM). By fitting a straight line to Figure 10-12 For each set of two data points in each of the four graphs of , we can obtain the form The average sensitivity of is equal to the gradient of each line, where a represents the asymmetry mode (e.g., ΔSWA or ΔCD), b represents the value of the asymmetry indicator (e.g., DTO) or the overlay measurement error ΔOV, and P represents polarization.

[0095] from Fig.10 and Fig.11 The derived sensitivity can be expressed as the following sensitivity matrix to define ΔCD / ΔSWA and DTO (DTO TE and DTO TM ) is as follows:

[0096]

[0097] from Fig.12 and13 The derived sensitivity can be expressed as the following sensitivity matrix to define ΔCD / ΔSWA and ΔOV (ΔOV TE and ΔOV TM ) is as follows:

[0098]

[0099] In an embodiment, a verification process is performed to check whether the sensitivity matrix is ​​accurate. In an embodiment, the verification process includes applying large random variations to the model of the calibration target and checking whether the resulting sensitivity remains sufficiently constant. In an alternative embodiment, the verification process includes measuring the value of the asymmetry indicator for a plurality of physical calibration targets on one or more substrates, and using the above reference Fig.10 and Fig.11 The sensitivity obtained as described above is used to obtain estimates for the two asymmetric modes ΔCD and ΔSWA. This can be achieved, for example, via the following equations:

[0100]

[0101] In an embodiment, the process further comprises using the obtained estimates of the two asymmetric modes ΔCD and ΔSWA to calculate the overlay measurement error, ΔOV, for the two polarizations. TE and ΔOV TM , using the reference above Fig.12 and Fig.13 This can be achieved, for example, using the following equation:

[0102]

[0103] In an embodiment, the verification process includes performing, for each calibration target, an overlay measurement error ΔOV calculated using the sensitivity matrix as described above. TE and ΔOV TM , with an overlay measurement error determined by a reference tool such as an electron beam apparatus as described above (e.g., by determining for each polarization: Among them OV ref is determined by a reference tool, and According to A + A - If the correlation between the overlay measurement error determined from the sensitivity matrix and the overlay measurement error determined using the reference tool is high enough (e.g., such that R 2The sensitivity matrix may be used to determine the overlay in the target structure if the correlation parameter is above a predetermined threshold. If the correlation is too low (e.g., such that the correlation parameter is below a predetermined threshold), the sensitivity matrix may be adjusted and the verification process repeated in an iterative process until a sensitivity matrix is ​​obtained with a correlation parameter above a predetermined threshold. The adjustment of the sensitivity matrix may be achieved by adjusting one or more parameters of the simulation used to obtain the sensitivity matrix (such as one or more parameters of a model representing the calibration structure). Alternatively or additionally, the wavelength and / or polarization used in the simulation may be adjusted. Adjusting the wavelength and / or polarization may provide information about an optimal set of wavelengths and / or polarizations for use when measuring the target structure.

[0104] When a sensitivity matrix has been obtained that achieves a satisfactorily high correlation between the overlay measurement error calculated based on the sensitivity matrix and the overlay measurement error determined by the reference tool, the sensitivity matrix provides an asymmetry indicator (e.g., DTO TE and DTO TM ) and the overlay measurement error (e.g., ΔOV TE and ΔOV TM ). Thus, the sensitivity matrix is ​​an example representation of the relationship between the value of the asymmetry indicator and the overlay measurement error for the model-based approach.

[0105] Fig.14 Depicts the use of asymmetric indicator values ​​and the like via references such as above Figure 9-13 Model-based methods are discussed to determine the relationship between overlay measurement errors and methods to determine information about the target structure.

[0106] In step S31, the target structure is measured using a scattering method. The measurement can be performed as described above. Figure 7 Step S1 can be performed in any manner discussed above.

[0107] In step S32, the measurement data from the scatterometry is used to obtain a value for each of the one or more asymmetry indicators of the target structure. Each asymmetry indicator may take any of the forms described above. In an embodiment, the scatterometry is performed for each of a plurality of different polarizations, and the value of the asymmetry indicator comprises a DTO. In the case where the polarization comprises a TE polarization and a TM polarization, the value of the asymmetry indicator obtained may be referred to as a DTO. TE and DTO TM .

[0108] In step S33, each of the one or more overlay-independent asymmetries is estimated using the corresponding values ​​of the asymmetry indicators obtained in step S32 and the stored sensitivities. The stored sensitivities can be obtained from the reference above. Figure 9-13In the simulation described, the sensitivity of the asymmetry indicator to each of the plurality of asymmetry modes (e.g., ΔCD and ΔSWA) and the obtained asymmetry indicator for the target structure are used to calculate each of the one or more overlay-independent asymmetries in the target structure. In this example, step S33 may estimate the overlay-independent asymmetries ΔCD and ΔSWA using the following equations:

[0109]

[0110] As described below, the estimated overlay-independent asymmetry may be used to obtain an improved overlay measurement. The estimated overlay asymmetry may also be of interest in its own right and is therefore output independently (in step S34). In an embodiment, the estimated overlay-independent asymmetry is output for a plurality of target structures at different locations on the substrate W, thereby providing an overlay-independent asymmetry distribution map on the substrate. This information may be used to adjust the lithography process to improve the formation of subsequent target structures (e.g., to reduce overlay-independent asymmetry).

[0111] In step S35, the estimated overlay-independent asymmetry obtained in step S33 is used to estimate the error in each of one or more initial overlay measurements performed on the target structure at a previous time. In an embodiment, the estimate of the error in the initial measurement of overlay is calculated using the sensitivity of the overlay measurement error to each of the plurality of asymmetry patterns (e.g., via a sensitivity matrix) and each of the one or more calculated overlay-independent asymmetries. In an embodiment, as described above, according to A + A - The initial overlay measurement is obtained by the gradient s of the best fit line in the graph of . In this example, where the scatterometry in step S31 is performed using TE and TM polarizations, the initial overlay measurement can be obtained independently for each of the two polarizations. Therefore, step S35 can estimate the error in the initial overlay measurement, ΔOV, for each of the two polarizations. TE and ΔOV TM In this example, this can be achieved using the following equation:

[0112]

[0113] In step S36, using the estimated error in the initial overlay measurement, ΔOV TE and ΔOV TM To correct the initial overlay measurement (derived from the scatterometry in step S31), thereby deriving an improved overlay measurement for output.

[0114] Thus, the model-based approach allows the use of the components of the relationship between the value of the asymmetry indicator and the overlay measurement error (e.g., an appropriate one of the two sensitivity matrices described above) derived from the scattering simulation (e.g., as described above in step S33) to estimate the overlay-independent asymmetry in the target structure. The estimated overlay-independent asymmetry can then be used to perform error estimation in the initial overlay measurement (e.g., as discussed above in step S35).

[0115] As described above, there is a correlation between overlay-independent asymmetry and overlay measurement error. The inventors have recognized that overlay-independent asymmetry will normally have a first component that contributes to the overlay measurement error and a second component that does not contribute to the overlay measurement error. The inventors have also recognized that the second component varies less between different polarizations in scatterometry than the first component. Therefore, by combining measurement data from scatterometry measurements performed at different polarizations, the sensitivity of determining the overlay measurement error can be increased.

[0116] In an embodiment, based on this insight, the Figure 7 A method is provided in which a value of an asymmetry indicator is obtained by combining a first asymmetry value and a second asymmetry value. The first asymmetry value is obtained using information about the asymmetry of a first scattered radiation from a target structure at each of a plurality of different wavelengths of radiation. The second asymmetry value is obtained using information about the asymmetry of a second scattered radiation from a target structure at each of a plurality of different wavelengths of radiation. The first scattered radiation originates from incident radiation having a first polarization. The second scattered radiation originates from incident radiation having a second polarization different from the first polarization. In an embodiment, the first polarization is orthogonal to the second polarization. In one class of embodiments, the first asymmetry value comprises a DTO obtained using TE polarized radiation (as an example of a first polarization), and the second asymmetry value comprises a DTO obtained using TM polarized radiation (as an example of a second polarization).

[0117] In an embodiment, the combination of the first asymmetry value and the second asymmetry value comprises a weighted sum. The weighting of the first asymmetry value and the second asymmetry value may be adjusted to achieve a good match between the estimated overlay measurement error and the actual overlay measurement error. In an embodiment, the weighting of the first asymmetry value in the weighted sum is opposite in sign to the weighting of the second asymmetry value in the weighted sum. Thus, the contribution from the first asymmetry value at least partially cancels the contribution from the second asymmetry value. In an embodiment, the cancellation is such that the second component of the overlay-independent asymmetry (the component that does not contribute to the overlay measurement error) cancels to a greater extent than the first component of the overlay-independent asymmetry (the component that contributes to the overlay measurement error). Therefore, the weighting of the first and second asymmetry values ​​is set to have opposite signs, thereby reducing the strength of the second component of the overlay-independent asymmetry in the signal relative to the first component of the overlay-independent asymmetry, thereby increasing the sensitivity to the first component. In an embodiment, the weighting of the first asymmetry value is equal to and opposite in sign to the weighting of the second asymmetry value, so that the difference between the first asymmetry value and the second asymmetry value is effectively obtained.

[0118] Fig.15 A system 20 for determining information about a target structure on a substrate W is depicted. The system 20 includes a processor 22. The processor 22 may include any known hardware, firmware, or software elements necessary to allow the required data processing operations to be performed (e.g., a CPU, memory, storage, communication bus, etc.). The system 20 also includes a data communication interface 24 to allow data to be input and output from the system 20. The system 20 is configured so that the processor 22 performs the above-referenced Figure 1-14 Any one or more of the methods described for determining information about a target structure on a substrate W. In an embodiment, a scatterometry system 26 is provided for performing scatterometry on a target structure and / or a calibration structure. The scatterometry system may be configured to illuminate a target and detect different diffraction orders, for example as described in US8339595B2 and / or WO2015018625 mentioned above. Measurement data from the scatterometry is provided to a processor 22 via a data communication interface 24. In an embodiment, a computer program may be provided which, when executed by the processor 22, causes the processor 22 to perform the above referenced Figure 1-14 Any one or more of the methods described for determining information about a target structure on a substrate W.

[0119] Any of the above-described methods, systems, or programs may be used as at least a part of a method of manufacturing a device using photolithography technology.

[0120] In addition, the techniques disclosed herein can be applied to large scatterometer targets (also called standard targets) and the overlay in these larger targets can be measured by angle-resolved scatterometry using a pupil image sensor, instead of or in addition to the measurements performed by the dark field imaging branch and sensor.

[0121] While the target structures described above are metrology targets designed and formed specifically for measurement purposes, in other embodiments, properties may be measured on a target that is a functional part of a device formed on a substrate. Many devices have a regular grating-like structure. The terms "target grating" and "target structure" as used herein do not require that the structure be provided specifically for the measurement being performed.

[0122] In association with the physical grating structure of the target implemented on the substrate and the patterning device, an embodiment may include a computer program comprising one or more machine-readable instruction sequences describing a method of measuring the target on the substrate and / or analyzing the measurements to obtain information about the lithography process. The computer program may be, for example, Figure 2 The computer program is executed in the control unit LACU. A data storage medium (e.g., semiconductor memory, magnetic disk or optical disk) having such a computer program stored therein may also be provided. Where existing metrology devices are already in production and / or in use, the present invention may be implemented by providing an updated computer program product for causing a processor to perform the above steps, thereby reducing sensitivity to feature asymmetry to calculate overlay error. The program may optionally be arranged to control an optical system, a substrate support, etc., to perform steps for measuring intensity asymmetry on a suitable plurality of target structures.

[0123] Further existing embodiments are described in the following numbered clauses:

[0124] 1. A method of determining information about a target structure on a substrate, comprising:

[0125] obtaining a value of an asymmetry indicator for the target structure, the value of the asymmetry indicator representing an amount of overlay-independent asymmetry in the target structure, the overlay-independent asymmetry being an asymmetry not caused by overlay;

[0126] estimating an error in an initial overlay measurement performed on the target structure at a previous time, the estimating being performed using a relationship between the obtained value of the asymmetry indicator and an overlay measurement error caused at least in part by overlay-independent asymmetry; and

[0127] Overlay in the target structure is determined using the initial overlay measurement and the estimated error.

[0128] 2. A method according to clause 1, wherein the value of the asymmetry indicator for the target structure is obtained using information about asymmetry in radiation scattered from the target structure at each wavelength in a set of wavelengths.

[0129] 3. A method according to clause 2, wherein the wavelength set is pre-selected by obtaining a relationship between the value of the asymmetry indicator and the overlay measurement error for multiple candidate wavelength sets, and selecting one candidate wavelength set from the multiple candidate wavelength sets based on the quality of the correlation between the value of the asymmetry indicator and the overlay measurement error for the candidate wavelength set.

[0130] 4. A method according to any of the preceding clauses, wherein:

[0131] obtaining a value of the asymmetry indicator by combining the first asymmetry value and the second asymmetry value;

[0132] obtaining the first asymmetry value using information about an asymmetry in first scattered radiation from the target structure at each of a plurality of different wavelengths of the radiation;

[0133] obtaining the second asymmetry value using information about an asymmetry in second scattered radiation from the target structure at each of a plurality of different wavelengths of the radiation;

[0134] The first scattered radiation originates from incident radiation having a first polarization; and

[0135] The second scattered radiation originates from incident radiation having a second polarization, which is different from the first polarization.

[0136] 5. The method of clause 4, wherein the first polarization is orthogonal to the second polarization.

[0137] 6. The method of clause 4 or 5, wherein the combination of the first asymmetry value and the second asymmetry value comprises a weighted sum.

[0138] 7. The method of clause 6, wherein the weighting of the first asymmetry value in the weighted sum is opposite in sign to the weighting of the second asymmetry value in the weighted sum.

[0139] 8. A method according to any of the preceding clauses, further comprising deriving a relationship between the value of the asymmetry indicator and an overlay measurement error.

[0140] 9. The method of clause 8, wherein the derivation of the relationship comprises measuring a value of the asymmetry indicator and an overlay measurement error in each of a plurality of calibration structures, the plurality of calibration structures having different overlay-independent asymmetries.

[0141] 10. A method according to clause 8 or 9, wherein the derivation of the relationship comprises simulating scattering of radiation from a calibration structure for a plurality of simulated overlay-independent asymmetries in the calibration structure.

[0142] 11. The method of clause 10, further comprising: using the relationship derived from the simulation to estimate the overlay-independent asymmetry in the target structure.

[0143] 12. The method of clause 11, wherein the estimated overlay-independent asymmetry is used to perform the estimation of the error in the initial overlay measurement.

[0144] 13. The method of any of clauses 10 to 12, wherein the simulated overlay-independent asymmetry comprises: a plurality of asymmetry magnitudes in each asymmetry mode of a plurality of asymmetry modes.

[0145] 14. The method of clause 13, wherein the plurality of asymmetric patterns comprises:

[0146] a first asymmetric mode representing sidewall angle asymmetry; and

[0147] A second asymmetric mode representing the asymmetry in the critical dimension.

[0148] 15. A method according to clause 13 or 14, wherein the derived relationship is represented by the calculated sensitivity of:

[0149] said asymmetry indicator for each said asymmetry pattern; and

[0150] The overlay error is measured for each of the asymmetric patterns.

[0151] 16. A method according to clause 15, wherein the derived relationship is represented by the calculated sensitivity of:

[0152] said asymmetry indicator for each said asymmetric pattern for each incident polarization of a plurality of incident polarizations; and

[0153] The overlay measurement error for each of the asymmetric modes for each of a plurality of incident polarizations.

[0154] 17. A method according to clause 15 or 16, wherein the overlay independent asymmetry in the target structure is calculated using the sensitivity of the asymmetry indicator for each of the asymmetry patterns and the obtained asymmetry indicator for the target structure.

[0155] 18. A method according to clause 17, wherein the estimate of the error in the initial measurement of overlay is calculated using the sensitivity of the overlay measurement error for each of the asymmetry patterns, and the calculated overlay independent asymmetry.

[0156] 19. A system for determining information about a target structure on a substrate, the system comprising a processor configured to:

[0157] obtaining a value of an asymmetry indicator for the target structure, the value of the asymmetry indicator representing an amount of overlay-independent asymmetry in the target structure, the overlay-independent asymmetry being an asymmetry not caused by overlay;

[0158] estimating an error in an initial overlay measurement performed on the target structure at a previous time, the estimating being performed using a relationship between the obtained value of the asymmetry indicator and an overlay measurement error caused at least in part by overlay-independent asymmetry; and

[0159] Using the initial overlay measurement and the estimated error, overlay in the target structure is determined.

[0160] 20. A computer program comprising instructions which, when executed by a processor, cause the processor to perform the method according to any of clauses 1-18.

[0161] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, such as imprint lithography, and is not limited to optical lithography where the context permits. In imprint lithography, the topography in the patterning device defines the pattern produced on the substrate. The topography of the patterning device may be pressed into a resist layer provided to the substrate, upon which 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 has cured.

[0162] The terms "radiation" and "beam" as used herein include all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

[0163] The term "lens," where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components.

[0164] The above description of the specific embodiments will so comprehensively reveal the general nature of the invention that others can easily modify and / or adapt these specific embodiments to various applications by applying the knowledge of the technical field of the art without departing from the general concept of the invention, without the need for endless experiments. Therefore, based on the teachings and guidance presented herein, such adaptations and modifications are intended to be within the meaning and scope of the equivalent forms of the disclosed embodiments. It should be understood that the phrases or terms here are intended to be illustrative by way of example, rather than restrictive, so that the terms or phrases of this specification will be interpreted by the technicians of this specification according to the teachings and guidance.

[0165] 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 following claims and their equivalents.

Claims

1. A method of determining information about a target structure on a substrate, comprising: obtaining a value of an asymmetry indicator for the target structure, the value of the asymmetry indicator representing an amount of overlay-independent asymmetry in the target structure, the overlay-independent asymmetry being an asymmetry not caused by overlay; estimating an error in an initial overlay measurement performed on the target structure at a previous time, the estimating being performed using a relationship between the obtained value of the asymmetry indicator and an overlay measurement error caused at least in part by overlay-independent asymmetry; and Using the initial overlay measurement and the estimated error, overlay in the target structure is determined. 2 . The method of claim 1 , wherein the value of the asymmetry indicator for the target structure is obtained using information about asymmetry in radiation scattered from the target structure at each wavelength in a set of wavelengths.

3. The method according to claim 2, wherein the wavelength set is pre-selected by: obtaining the relationship between the value of the asymmetry indicator and the overlay measurement error for multiple candidate wavelength sets; and selecting one of the candidate wavelength sets based on the quality of the correlation between the value of the asymmetry indicator and the overlay measurement error for the candidate wavelength set.

4. The method according to claim 2, wherein: obtaining a value of the asymmetry indicator by combining the first asymmetry value and the second asymmetry value; obtaining the first asymmetry value using information about an asymmetry in first scattered radiation from the target structure at each of a plurality of different wavelengths of the radiation; obtaining the second asymmetry value using information about an asymmetry in second scattered radiation from the target structure at each of a plurality of different wavelengths of the radiation; The first scattered radiation originates from incident radiation having a first polarization; as well as The second scattered radiation originates from incident radiation having a second polarization, which is different from the first polarization. The method of claim 4 , wherein the first polarization is orthogonal to the second polarization.

6. The method of claim 4 or 5, wherein the combination of the first asymmetry value and the second asymmetry value comprises a weighted sum. 7 . The method of claim 6 , wherein the weighting of the first asymmetry value in the weighted sum and the weighting of the second asymmetry value in the weighted sum are opposite in sign.

8. A method according to any preceding claim, further comprising deriving a relationship between the value of the asymmetry indicator and an overlay measurement error.

9. The method of claim 8, wherein the deriving of the relationship comprises: A value of the asymmetry indicator and an overlay measurement error are measured in each of a plurality of calibration structures, the plurality of calibration structures having different overlay-independent asymmetries.

10. The method according to claim 8 or 9, wherein the deriving of the relationship comprises: Scattering of radiation from the calibration structure is simulated for a plurality of simulated overlay-independent asymmetries in the calibration structure.

11. The method according to claim 10, further comprising: The overlay-independent asymmetry in the target structure is estimated using the relationship derived from the simulation.

12. The method of claim 11, wherein the estimated overlay-independent asymmetry is used to perform the estimation of the error in the initial overlay measurement.

13. The method of any one of claims 10 to 12, wherein the simulated overlay-independent asymmetry comprises a plurality of asymmetry magnitudes in each asymmetry mode of a plurality of asymmetry modes.

14. The method of claim 13, wherein the plurality of asymmetric patterns comprises: a first asymmetric mode representing asymmetry of the sidewall angles; as well as A second asymmetric mode representing the asymmetry in the critical dimension.

15. A method according to claim 13 or 14, wherein the derived relationship is represented by the calculated sensitivity of: said asymmetry indicator for each said asymmetry pattern; and The overlay error is measured for each of the asymmetric patterns.

16. The method of claim 15, wherein the derived relationship is represented by the calculated sensitivities of: said asymmetry indicator for each said asymmetric pattern for each incident polarization of a plurality of incident polarizations; and The overlay measurement error for each of the asymmetric modes for each of a plurality of incident polarizations.

17. The method according to claim 15 or 16, wherein the overlay-independent asymmetry in the target structure is calculated using the sensitivity of the asymmetry indicator for each of the asymmetry patterns and the obtained asymmetry indicator for the target structure.

18. The method of claim 17, wherein the estimate of the error in the initial measurement of overlay is calculated using the sensitivity of the overlay measurement error for each of the asymmetry patterns and the calculated overlay independent asymmetry.

19. A system for determining information about a target structure on a substrate, the system comprising a processor configured to: obtaining a value of an asymmetry indicator for the target structure, the value of the asymmetry indicator representing an amount of overlay-independent asymmetry in the target structure, the overlay-independent asymmetry being an asymmetry not caused by overlay; estimating an error in an initial overlay measurement performed on the target structure at a previous time, the estimating being performed using a relationship between the obtained value of the asymmetry indicator and an overlay measurement error caused at least in part by overlay-independent asymmetry; and Using the initial overlay measurement and the estimated error, overlay in the target structure is determined.

20. A computer-readable storage medium storing a computer program, wherein the computer program comprises machine-executable instructions, and when the computer program is executed by a processor, the machine is caused to perform the method according to any one of claims 1 to 18.

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