Method for inferring processing parameters such as focal length and associated apparatus and manufacturing method

Abstract

A method for inferring a value of a first process parameter of a photolithography process is disclosed, the first process parameter being subject to a coupled dependency on a second process parameter. The method includes determining a first metric and a second metric from measurement data, each of the first metric and the second metric depending on both the first process parameter and the second process parameter. The first metric indicates a stronger dependency on the first process parameter than on the second process parameter, and the second metric indicates a stronger dependency on the second process parameter than on the first process parameter. The value of the first process parameter is inferred based on the first and second metrics.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from EP application 19199804.6, filed September 26, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to a metrology apparatus and method, which can be used, for example, to perform metrology when manufacturing devices using photolithography techniques. The present invention also relates to a method for monitoring focus parameters in a photolithography process. Background Art

[0004] A photolithographic apparatus is a machine that applies a desired pattern to a substrate, typically to a target portion of the substrate. A photolithographic apparatus can be used, for example, to manufacture an integrated circuit (IC). In this case, a pattern forming device (alternatively referred to as a mask or reticle) can be used to generate a circuit pattern to be formed on a single layer of the IC. The pattern can be transferred to a target portion (e.g., including a portion of one or more dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is typically via imaging onto a radiation-sensitive material (resist) layer provided on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are continuously patterned.

[0005] In photolithographic processes, 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 (CDs), and specialized tools for measuring overlay (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 properties of the scattered radiation (e.g., intensity as a function of wavelength at a single reflection angle; intensity as a function of reflection angle at one or more wavelengths; or polarization as a function of reflection angle) to obtain a diffraction "spectrum" from which properties of interest of the target can be determined.

[0006] Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. Such scatterometers use a target that is a relatively large grating (e.g., 40 μm by 40 μm), and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is not filled). Examples of dark-field imaging metrology can be found in International Patent Applications US20100328655A1 and US2011069292A1, which are incorporated herein by reference in their entirety. Further developments of this technology are described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A, and WO2013178422A1. These targets can be smaller than the illumination spot and can be surrounded by product structures on the wafer. Using composite grating targets, multiple gratings can be measured in a single image. The contents of all of these applications are also incorporated herein by reference.

[0007] An important parameter of the lithography process that needs to be monitored is the focus. It is expected that more and more electronic components will be integrated into the IC. In order to achieve this, the size of the components needs to be reduced and therefore the resolution of the projection system needs to be increased so that smaller and smaller details or line widths can be projected onto the target portion of the substrate. As the critical dimension (CD) in lithography shrinks, the consistency of focus across and between substrates becomes increasingly important. The CD is the dimension of one or more features (such as the gate width of a transistor) whose variation will cause undesirable changes in the physical properties of the feature. Traditionally, the optimal settings are determined by "pre-sent wafers" (i.e. substrates that are exposed, developed and measured before a production run). In the pre-sent wafers, test structures are exposed in a so-called focus exposure matrix (FEM) and the optimal focus and energy settings are determined by inspecting these test structures.

[0008] Focal length and dose are interrelated terms, so focal length inference is affected by any change in effective dose from the assumed dose. Summary of the Invention

[0009] The present invention aims to solve the problem of the effect of dose on focus inference.

[0010] In a first aspect, the present invention provides a method for inferring the value of a first processing parameter of a lithography process, the method comprising: determining a first metric and a second metric based on measurement data, the measurement data being related to a structure formed on a substrate using a lithography process, each of the first metric and the second metric depending on both the first processing parameter and the second processing parameter, the first metric having different correlations with the first processing parameter and the second processing parameter, and the second metric having different correlations with the second processing parameter and the first processing parameter; and inferring the value of the first processing parameter based on the first metric and the second metric.

[0011] In a second aspect, the present invention provides a method for inferring a focal length value from a target having formed therein a focal length-related asymmetry, which is related to the focal length during formation of the target in a lithography process, the method comprising: determining an asymmetry metric and a sum metric based on measurement data, the asymmetry metric being based on the difference in intensities of complementary diffraction orders, the complementary diffraction orders coming from the diffraction of the measurement radiation after measurement of the target; the sum metric being based on the sum of the intensities of the complementary diffraction orders; and inferring a focal length value based on the asymmetry metric and the sum metric.

[0012] The present invention comprises, in a third aspect, a method for determining a calibration plane for performing calibration in a lithography process; the method comprising: determining a calibration relationship based on calibration measurements associated with at least one structure on a substrate, the at least one structure being formed using a lithography process at different values ​​of a first processing parameter and a second processing parameter, the calibration relationship describing a relationship between a first metric and a second metric for different values ​​of the first processing parameter and the second processing parameter, wherein each of the first metric and the second metric depends on both the first processing parameter and the second processing parameter, the first metric having a stronger correlation with the first processing parameter than with the second processing parameter, and the second metric having a stronger correlation with the second processing parameter than with the first processing parameter; and fitting the calibration plane to a constant value of the second processing parameter.

[0013] The present invention further provides a computer program product comprising machine-readable instructions for causing a processor to execute the method of the first, second and / or third aspects, and provides associated measurement devices, lithography systems and methods for manufacturing devices.

[0014] Other features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, 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. These embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to those skilled in the relevant art based on the teachings contained herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference numerals indicate corresponding parts, and in which:

[0016] Figure 1 The photolithographic apparatus is depicted;

[0017] Figure 2 depicts a lithocell or cluster in which the inspection apparatus according to the invention may be used;

[0018] Figure 3 (a)- Figure 3 (b) shows (a) a schematic diagram of a dark-field scatterometer for measuring a target according to an embodiment of the present invention using a first pair of illumination apertures, and (b) details of the diffraction spectrum of the target grating for a given illumination direction;

[0019] Figure 4 Target forming elements on a reticle suitable for forming a grating on a substrate having focus-related asymmetry are illustrated;

[0020] Figure 5 is a plot of a sum metric versus an asymmetry metric for calibration measurements at three dose levels and multiple focus levels, which may be used in a focus inference method according to an embodiment of the present invention;

[0021] Figure 6 is a graph of the sum metric versus the asymmetry metric illustrating (a) a focal length inference method according to an embodiment of the present invention and (b) a focal length inference method of the prior art;

[0022] Figure 7 Shown are (a) a graph of the asymmetry metric versus focal length and versus sum metric, illustrating a prior art focal length inference method; (b) a graph of the asymmetry metric versus focal length and versus sum metric, illustrating a focal length inference method according to an embodiment of the present invention;

[0023] Figure 8 shows (a) a graph of asymmetry metric versus focal length, (b) a graph of sum metric versus focal length, and (c) a graph of sum metric versus asymmetry metric, illustrating how focal length inference methods according to embodiments of the present invention are more accurate than prior art methods;

[0024] Figure 9 shows (a) a graph of asymmetry metric versus focal length, (b) a graph of sum metric versus focal length, and (c) a graph of sum metric versus asymmetry metric, illustrating how focal length inference methods according to embodiments of the present invention extend the focal length range compared to prior art methods; and

[0025] Figure 10 is a plot of the sum metric versus asymmetry, which illustrates the normalized distance process performance index P known in the art. KPI The difference between the effective dose obtained by the focus inference method according to the embodiment of the present invention. DETAILED DESCRIPTION

[0026] Before describing embodiments of the present invention in detail, it is helpful to describe an example environment in which embodiments of the present invention may be implemented.

[0027] 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 table MT) configured to support a patterning device (e.g., mask) MA and connected to a first positioner PM configured to precisely position the patterning device according to certain parameters; two substrate tables (e.g., wafer stages) WTa and WTb, each configured to hold a substrate (e.g., a resist-coated wafer) W and each connected to a second positioner PW configured to precisely 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 via the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W. A reference frame RF connects the various components and serves as a reference for setting and measuring the position of the patterning device and substrate, as well as features thereon.

[0028] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types, or any combination thereof, for directing, shaping, or controlling radiation.

[0029] 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 can take a variety of forms. The patterning device support can ensure that the patterning device is in a desired position, for example, relative to a projection system.

[0030] The term "patterning device," as used herein, should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section so as to create a pattern in a target portion of the 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. Typically, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device (such as an integrated circuit) being created in the target portion.

[0031] As depicted herein, the device is transmissive (e.g., employing a transmissive patterning device). Alternatively, the device can be reflective (e.g., employing a programmable mirror array of the type described above or employing a reflective mask). Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Any use of the term "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device." The term "patterning device" may also be interpreted as referring to a device that stores pattern information in digital form for use in controlling such a programmable patterning device.

[0032] The term "projection system" as used herein should be broadly interpreted 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 used or other factors (such as use of immersion liquid or use of vacuum). Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system."

[0033] The lithographic apparatus may also be of a type in which at least a portion of the substrate may be covered with a liquid having a relatively high refractive index (e.g., water) to fill the space between the projection system and the substrate. Immersion liquids may also be used in other spaces within the lithographic apparatus (e.g., between the mask and the projection system). Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

[0034] In operation, 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 form part of the lithographic apparatus, and the radiation beam is transferred from source SO to illuminator IL by means of a beam delivery system BD, which includes, for example, appropriate directing mirrors and / or a beam expander. 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, source SO and illuminator IL, together with beam delivery system BD, may be referred to as a radiation system.

[0035] The illuminator IL may, for example, comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam, an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0036] The radiation beam B is incident on and patterned by the patterning device MA, which is held on the patterning device support MT. After passing through the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the help of a second positioner PW and a position sensor IF (e.g., an interferometry device, a linear encoder, a 2-D encoder, or a capacitive sensor), the substrate table WTa or WTb can be accurately moved, for example, in order to position a different target portion C in the path of the radiation beam B. Similarly, a first positioner PM and another position sensor ( Figure 1 The reticle (not explicitly depicted) may be used to precisely position the patterning device (eg, reticle / mask) MA relative to the path of the radiation beam B, for example after mechanical retrieval from a mask library or during scanning.

[0037] Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning device (e.g., reticle / 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, in device features, in which case it is desirable that the marks be as small as possible and not require any imaging or process conditions that are different from adjacent features. An alignment system for detecting alignment marks is described further below.

[0038] The depicted apparatus can be used in various modes. 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 can be determined by the (reduction) magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure area 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). As is known in the art, other types of lithographic apparatus and operating modes are possible. For example, stepping modes are known. In so-called "maskless" lithography, the programmable patterning device remains stationary but with a changing pattern, and the substrate table WT is moved or scanned.

[0039] Combinations and / or variations on the above-described modes of use or entirely different modes of use may also be employed.

[0040] The lithographic apparatus LA is of a so-called dual-stage type (having two substrate tables WTa, WTb and two stations (an exposure station EXP and a measurement station MEA, between which the substrate tables can be exchanged)). While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station and various preparatory steps are performed. This enables the throughput of the apparatus to be increased significantly. The preparatory steps may comprise mapping the surface height profile of the substrate using a level sensor LS and measuring the positions of alignment marks on the substrate using an alignment sensor AS. If the position sensor IF is not able to measure the position of the substrate table at the measurement station as well as at the exposure station, a second position sensor may be provided so that the position of the substrate table relative to the reference frame RF at both stations can be tracked. Other arrangements are known and could be used instead of the dual-stage arrangement shown. For example, other lithographic apparatus are known in which a substrate table and a measurement table are provided. These are docked together when preparatory measurements are performed and then detached when the substrate table undergoes exposure.

[0041] like Figure 2As shown in the figure, the lithography apparatus LA forms part of a lithography cell LC (sometimes also called a lithography cell or cluster), which also includes apparatus for performing pre-exposure and post-exposure processing on the substrate. Conventionally, these apparatus include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH and a bake plate BK. A substrate handler or robot RO picks up the substrate from the input / output ports I / O1, I / O2, moves the substrate between the different processing apparatuses, and then passes it to the feed table LB of the lithography apparatus. These apparatuses, which are generally referred to as tracks, are controlled by a track control unit TCU, which is itself controlled by a supervisory control system SCS, which also controls the lithography apparatus via the lithography control unit LACU. Thus, the different apparatuses can be operated to maximize production throughput and processing efficiency.

[0042] In order to ensure that the substrates exposed by the lithography apparatus are correctly and consistently exposed, it is desirable to inspect the exposed substrates to measure properties such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. Accordingly, the manufacturing facility in which the lithography cell LC is located also includes a metrology system MET, which receives some or all of the substrates W that have been processed in the lithography cell. The measurement results are provided directly or indirectly to the supervisory control system SCS. If an error is detected, and in particular if the inspection can be performed quickly and quickly enough so that other substrates of the same batch are still exposed, the exposure of subsequent substrates can be adjusted. Moreover, the exposed substrates can be stripped and reworked to improve the yield or discarded, thereby avoiding further processing on substrates that are known to be defective. In the event that only some target portions of the substrate are defective, further exposure can be performed only on those good target portions.

[0043] In a metrology system MET, an inspection device is used to determine substrate properties, particularly how properties of different substrates or different layers of the same substrate vary from layer to layer. The inspection device can be integrated into the lithography apparatus LA or lithography cell LC, or it can be a standalone device. To achieve the fastest measurement, it is desirable for the inspection device to measure properties in the exposed resist layer immediately after exposure. However, the latent image in the resist has very low contrast (the difference in refractive index between the portions of the resist exposed to radiation and those not exposed to radiation is very small), and not all inspection devices have sufficient sensitivity to make useful measurements of the latent image. Therefore, measurements can be performed after a post-exposure bake (PEB) step, which is typically the first step performed on an exposed substrate and increases the contrast between the exposed and unexposed portions of the resist. At this stage, the image in the resist can be referred to as semi-latent. It is also possible to measure the developed resist image after either the exposed or unexposed portions of the resist have been removed, or after a pattern transfer step (such as etching). The latter possibility limits the rework potential of defective substrates but still provides useful information.

[0044] exist Figure 3 A metrology device suitable for use in embodiments of the present invention is shown in (a). Note that this is only one example of a suitable metrology device. Alternative suitable metrology devices may use EUV radiation, for example, such as those disclosed in WO 2017 / 186483 A1. Figure 3(b) shows the target structure T and the diffracted rays of measurement radiation used to illuminate the target structure in more detail. The metrology apparatus shown is of a type known as a dark-field metrology apparatus. The metrology apparatus can be a stand-alone apparatus or integrated into the lithography apparatus LA, for example at a measurement station, or integrated into the lithography cell LC. An optical axis having multiple branches throughout the apparatus is indicated by a dashed line O. In the apparatus, light emitted by a source 11 (e.g., a xenon lamp) passes through an optical system comprising lenses 12, 14 and an objective lens 16, and is directed via a beam splitter 15 onto a substrate W. These lenses are arranged in a dual sequence of a 4F arrangement. Different lens arrangements can be used, as long as they still provide an image of the substrate to the detector and at the same time allow access to an intermediate pupil plane for spatial frequency filtering. Thus, the angular range of the radiation incident on the substrate can be selected by defining a spatial intensity distribution in a plane that represents the spatial spectrum of the substrate plane (herein referred to as the (conjugate) pupil plane). In particular, this can be accomplished by inserting an aperture plate 13 of suitable form between lens 12 and lens 14 in the plane of the back-projected image of the objective pupil plane. In the example shown, the aperture plate 13 has different forms, labeled 13N and 13S, to allow different illumination modes to be selected. The illumination system of this example forms off-axis illumination modes. In a first illumination mode, aperture plate 13N provides off-axis illumination in a direction designated as 'North' (for illustration only). In a second illumination mode, aperture plate 13S is used to provide 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 desirably dark, as any unnecessary light outside the desired illumination mode would interfere with the desired measurement signal.

[0045] like Figure 3 As shown in (b), a target structure T is placed with the substrate W perpendicular to the optical axis O of the objective lens 16. The substrate W can be supported by a support (not shown). The measurement radiation ray I impinging on the target structure T from an angle offset from the axis O produces a zero-order ray (solid line 0) and two first-order rays (dash-dotted line +1 and dash-dotted line -1). It should be remembered that for small, overpopulated target structures, these rays are just one of many parallel rays covering the area of ​​the substrate including the measurement target structure T and other features. Since the aperture in the plate 13 has a finite width (it must allow a significant amount of light to enter), the incident ray I actually occupies a certain angular range, and the diffracted rays 0 and +1 / -1 will be slightly spread out. Depending on the point spread function of the small target, each order +1 and -1 will be further spread out over a certain angular range, rather than being a single ideal ray as shown. Note that the grating spacing of the target structure and the illumination angle can be designed or adjusted so that the first-order rays entering the objective lens are closely aligned with the central optical axis. Figure 3 (a) and Figure 3The rays illustrated in (b) are shown slightly off-axis purely to make them easier to distinguish in the figure.

[0046] At least the 0th and +1st orders diffracted by the target structure T on the substrate W are collected by the objective lens 16 and directed back through the beam splitter 15. Figure 3 (a) illustrates the first and second illumination modes, designated by diametrically opposed apertures labeled North (N) and South (S). When incident ray I of measurement radiation originates from the north side of the optical axis, i.e., when the first illumination mode is applied using aperture plate 13N, a +1 diffracted ray, labeled +1 (N), enters the objective lens. In contrast, when the second illumination mode is applied using aperture plate 13S, a -1 diffracted ray, labeled 1 (S), enters lens 16.

[0047] A second beam splitter 17 splits the diffracted beam into two measurement branches. In the first measurement branch, an optical system 18 uses the zeroth- and first-order diffracted beams to form a diffraction spectrum (pupil plane image) of the target structure on a first sensor 19 (e.g., a CCD or CMOS sensor). Each diffraction order hits a different point on the sensor, allowing image processing to compare and contrast multiple orders. The pupil plane image captured by sensor 19 can be used to focus the metrology device and / or normalize the intensity measurement of the first-order beam. The pupil plane image can also be used for many measurement purposes, such as reconstruction.

[0048] In the second measurement branch, the optical systems 20, 22 form an image of the target structure T on a sensor 23 (e.g., a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane conjugate to the pupil plane. The aperture stop 21 serves to block the zero-order diffraction beam so that the target image formed on the sensor 23 is formed only by the -1st-order or +1st-order beam. The images captured by the sensors 19 and 23 are output to a processor PU that processes the image, the function of which will depend on the specific type of measurement being performed. Note that the term "image" is used here in a broad sense. If only one of the -1st-order and +1st-order beams is present, no image of such grating lines will be formed.

[0049] When monitoring a lithographic process, it is desirable to monitor the focus of the lithographic beam on the substrate. One known method of determining the focus setting from a printed structure is by measuring the critical dimension (CD) of the printed structure. CD is a measure of the smallest feature (e.g. the line width of an element). The printed structure may be a target specifically formed for focus monitoring, such as a line spacing grating. It is known that CD typically shows a second order response to focus, forming a so-called "Bossung curve" on a graph of CD (y-axis) versus focus (x-axis). The Bossung curve is a substantially symmetrical curve that is substantially symmetrical around a peak representing the best focus. The Bossung curve may be substantially parabolic in shape. This method has several disadvantages. One disadvantage is that the method exhibits low sensitivity near the best focus (due to the parabolic shape of the curve). Another disadvantage is that the method is insensitive to the sign of any defocus (because the curve is primarily symmetrical around the best focus). Furthermore, the method is particularly sensitive to dose and process variations (crosstalk).

[0050] To address these issues, diffraction-based focusing (DBF) was designed. Diffraction-based focusing can use a target-forming feature on a reticle that is designed to print a target with a degree of asymmetry that depends on the focus setting during printing. The degree of this asymmetry can then be measured using scatterometer-based inspection methods, for example by measuring the intensity asymmetry between the intensities of the +1st and -1st order radiation diffracted from the target to obtain a measure of the focus setting. For example, Figure 3 The measurement tool shown in (a) is used to perform this method.

[0051] Figure 4 A purely exemplary DBF target formation design 400 configured for diffraction-based focus measurement is shown to illustrate the basic concept. It includes multiple DBF structures 405, each of which includes a high-resolution substructure 410. The high-resolution substructures 410, atop the base pitch, create an asymmetric resist profile for each DBF structure 405, with the degree of asymmetry depending on the focus. Thus, a metrology tool can measure the degree of asymmetry associated with a target formed using DBF target formation design 400 and convert it to a scanner focus.

[0052] The distance-to-focus (DBF) metrology target should have a unique and preferably monotonic asymmetry signal as a function of target defocus. In this case, the asymmetry signal can describe the difference (e.g., intensity and / or phase difference) in opposite higher diffraction orders (e.g., +1 and -1 diffraction orders).

[0053] It should be understood that the DBF target formation design 400 described above is a relatively simple example used to illustrate the principles of DBF. Many different variations of this method have been described to achieve various improvements. Such improvements may include one or more of the following:

[0054] Get better focus sensitivity,

[0055] Simultaneous dose measurement,

[0056] Minimize dose crosstalk,

[0057] Stay within certain imposed design rules,

[0058] Achieving a monotonic relationship between asymmetry and focal length (e.g., the goal is based on the difference of two Bossung-like signals with a focal length shift between them) and / or

[0059] Suitable for thin resists, such as those used in EUV lithography

[0060] Have characteristics that are more similar to the actual product structure (e.g., have similar resolution).

[0061] WO2017 / 108395 and WO2019 / 110211 (both incorporated herein by reference) are two examples of many such publications that describe a variety of different diffraction-based focusing methods and target designs, all of which are applicable to the concepts disclosed herein. As such, it will be appreciated that the type of target being measured is not important, as long as the target has a focus-related asymmetry such that the intensity asymmetry measurement can be used to infer the focus setting being used.

[0062] Current DBF methods suffer from accuracy issues due to a number of effects. Some DBFs aim to maintain Bossung behavior and therefore have low sensitivity near the optimal focal length, where sensitivity is most important.

[0063] A particular issue is the influence of dose and treatment effects on focus measurements. Dose and treatment effects effectively act as crosstalk terms for focus inference, since the intensity asymmetry signal typically used to infer focus from a target also includes dose and crosstalk correlations. The result of this is an effective dose, which may be different from the dose setting used in calibration. Therefore, a method is described herein that enables focus and dose to be inferred simultaneously, thereby obtaining a more accurate focus value that is suitable for the effective dose. Furthermore, the dose information generated by the method can itself be used for process monitoring. In particular, the method comprises inferring focus and dose based on both an asymmetry metric (e.g., intensity asymmetry or the difference of complementary diffraction orders from the target, optionally normalized) and a sum metric (e.g., the sum of complementary diffraction orders from the target).

[0064] The method may include constructing a focus-dose (more specifically, focus-effective dose) calibration relationship based on the relationship between the asymmetry metric and the summation metric. The calibration relationship may take the form of a calibration graph, a calibration grid, or a calibration plane; for example, a graph of the summation metric versus the asymmetry metric. The method then infers the focal length value for the appropriate effective dose based on the focus-dose calibration grid, thereby also inferring the effective dose. Of course, an actual graph need not be constructed, but rather a relationship numerically determined from the input data.

[0065] Currently, the focal length is inferred assuming a certain dose value (e.g., based on a previous calibration or an assumed dose value, e.g., a dose setting used during a lithography process). The inference is then based solely on an asymmetry metric, such as the ΔI metric (difference of complementary orders) or the ΔI / I metric (normalized difference of complementary orders). For example, a calibration curve of the asymmetry metric versus focal length is determined in a calibration step (optionally for different dose levels) and later used to infer the focal length based on the asymmetry measurement (e.g., at the assumed dose level).

[0066] To understand the concepts herein, it should be understood that the asymmetry metric is most inferential for focal length, but also has some dose / treatment dependence, and that the sum metric is most relevant for dose and other treatment factors (which affect effective dose), but also has some focal length dependence. Thus, by explicitly inferring both focal length and (effective) dose simultaneously, the focal length inference result is less relevant for effective dose.

[0067] Figure 5 is a focus-dose calibration grid or {F, D} diagram that can be constructed experimentally in the calibration phase. The figure shows a 4D diagram of the sum metric Sm relative to the asymmetry metric Asym, which is determined for (in this example) multiple focal levels and three dose levels. The dotted lines represent focal length curves fitted to the constant dose line CD of the three dose levels (i.e., each dotted line shows the focal length variation in the {F, D} coordinate space for a constant dose). Each of the solid lines CF is a dose curve that represents a fit to the constant focal length line (i.e., each solid line shows the dose variation in the {F, D} coordinate space for a constant focal length). Only one of each of these lines is marked in the figure. Note that the angle between the constant dose line CD and the constant focal length line CF varies and is typically non-orthogonal.

[0068] After construction, the focus and dose can be inferred by reading the corresponding focal-dose value {F, D} for a given asymmetry metric / sum metric pair from the graph. In particular, this is done by plotting the measurement points corresponding to the measured asymmetry metric / sum metric pairs and inferring the focal length from the corresponding point on an appropriate constant-dose curve (e.g., corresponding to the expected dose; such as the dose setting used) by following the constant-focus line CF from the measurement points to the constant-dose curve. The dose can be inferred from the distance between the measurement point and the curve in that direction.

[0069] Figure 6 (a) is a plot of the sum metric against the asymmetry metric, illustrating the inference method, and Figure 6 (b) is an equivalent diagram illustrating the current approach (eg, focal length inferred using only the asymmetry metric) for comparison. Figure 6 (a) shows a constant dose curve CD and a constant focus line CF corresponding to the measurement point MP (ie corresponding to the measured asymmetry measure and sum measure). Inferred focus value F i The effective dose D is the focal length value in the direction of the constant focal length CF on the constant dose curve CD corresponding to the measurement point MP. e is the distance between the measurement point and the constant dose curve CD in that direction. Figure 6 In (b), the current situation is illustrated on an equivalent diagram. In the current approach, only the asymmetry measure is currently measured, from which the inferred focal length value F is directly inferred. i (ie, the sum metric of the measurement points MP is not used in the inference.) A normalized distance process performance indicator P is sometimes also calculated from the summed intensity values. KPI , for process monitoring; however, this has not been used for focal length inference so far.

[0070] Figure 7 Illustration of the problems with current inference methods. Figure 7 (a) Illustration of the current approach, assuming the dose is a calibrated dose. A plot of the asymmetry measure Asym versus the focal length F that can currently be used, and a plot of the equivalent asymmetry Asym versus the sum Sm are shown. In each case, there are three calibration curves, the best dose curve BD and the best dose + / - 5% curve BD +5％ , BD -5％ The optimal dose BD is the hypothetical dose in this extrapolation. Therefore, the measured asymmetry Asym m This results in the inferred focal length value F0 shown. The asymmetry is also shown relative to the equivalent measurement point MP in the sum curve. Figure 7(b) shows the effect of an effective dose ED that is different from the assumed dose BD on the inferred focal length value F0. In each of the figures, there is an additional effective dose curve ED corresponding to an effective dose that is different from the assumed dose BD. The difference between the effective dose and the assumed dose results in an inferred focal length value F0 based only on the asymmetry metric that is different from the inferred focal length value F0 using the method disclosed herein. i significant differences. Figure 7 (b) also shows the same extrapolation on the asymmetry-sum plot. The sum difference between the effective dose curve and the hypothetical dose curve is described above as the normalized distance process performance indicator P KPI A metric that is sometimes used for monitoring but not for focal length inference.

[0071] Figure 8 Illustration of how the proposed method and the additional use of the sum metric solve Figure 7 The problem shown. Figure 8 (a) is a plot of the asymmetry metric versus focal length for a simplified (i.e., linear) focal length model. Again, three dose levels are shown. Asymmetry Asym can be described as:

[0072] Asym=c1F+c2D

[0073] where F and D are focal length and dose, respectively, and c1 and c2 are coefficients.

[0074] Figure 8 (b) shows an equivalent diagram of the sum Sm versus focal length F, where the sum can be described as:

[0075] Sm=c3+c4F 2 +c5D

[0076] Where c4 and c5 are coefficients, and c3 is a constant.

[0077] Figure 8 (c) shows that by using two metrics (asymmetry and summation), process variations (e.g., due to dose and other treatment effects) are actually modeled and compensated. Figure 8 (c) is a plot of the equivalent sum versus asymmetry, with the effective dose curve ED added. As can be seen, the inferred dose is the same for the measurement point MP and the equivalent calibration point CP on the best dose BD curve (i.e., both points are on the same constant focal length line CF). If the effective dose changes, the inferred focal length will remain the same.

[0078] The calibration spans the {F, D} space in the (asym, sm) domain, and focus (and dose) inference involves solving the set of equations:

[0079]

[0080]

[0081] Furthermore, the method proposed herein can increase the effective focus range. In existing methods, the asymmetry metric is required to be unique (i.e., monotonic over a sufficient focus range). For example, for 3D-NAND, this currently limits target selection and prohibits additional focus range increase. In the current proposal, the combination of the asymmetry metric and the sum metric needs to be unique. This is generally the case over any full focus range of the generated print target. This is explained by Figure 9 The diagram shows the Figure 8 Similar graph, except for the relationship between asymmetry and focal length ( Figure 9 (a)) has a smaller linear range. Figure 9 The sum in (b) is quadratic in relation to the focal length, Figure 8 Same as in (b). Figure 9 (c) shows that even in this case, a unique focal length inference is still possible. This does assume that the effective dose is small.

[0082] Figure 10 is a plot of the sum versus asymmetry with the values ​​for the three dose settings BD, BD +5％ , BD -5％ and the effective dose curve ED, which illustrates the normalized distance process performance index P KPI The difference between the effective dose obtained in this method is shown. The normalized distance process performance index P KPI , process asymmetry affects PA and effective dose ED V A vector. It can be understood that:

[0083] P KPI 2 =ED 2 -PA 2

[0084] Thus, if the process affects the focal length inference, it will also reduce the apparent normalized distance process performance index P KPI Therefore, effective dose is a better performance indicator with clearer units. However, P KPI Still a valid process flag.

[0085] It should be noted that the above principles apply to any cross-term and any type of inference (including, for example, overlay). Thus, adding additional terms that depend primarily (or at least significantly) on the cross-term (such as the sum term) can be used to infer any lithographic process parameter.

[0086] Thus, a method is disclosed that enables determination of a first treatment parameter (e.g., focal length) and a second treatment parameter (e.g., dose), the method comprising: determining a first metric (e.g., an asymmetry metric) and a second metric (e.g., a sum metric) from measurement data, each of the first metric and the second metric depending on both the first and second treatment parameters, the first metric indicating a different correlation for the first treatment parameter than for the second treatment parameter, the second metric indicating a different correlation for the second treatment parameter than for the first treatment parameter. The first treatment parameter is subject to a coupled correlation of the second treatment parameter. The first metric may indicate a stronger correlation for the first treatment parameter than for the second treatment parameter, and the second metric may indicate a stronger correlation for the second treatment parameter than for the first treatment parameter.

[0087] The asymmetry metric can be based on the difference in the intensities of complementary diffraction orders resulting from diffraction of the measurement radiation after measurement of a target that exhibits a focus-dependent asymmetry. It can optionally be normalized. The complementary diffraction orders can include positive and negative orders of higher diffraction orders of the same index; for example, +1 and -1 diffraction orders. The sum metric can include the sum of the intensities of the same complementary diffraction orders (e.g., from the same measurement). In the case where the target is a dual focal length target, the dual target sum metric can include the difference between the master sum and the slave sum.

[0088] The methods disclosed herein result in one or more of the following:

[0089] Improvements in the robustness of focal length inference due to processing and comparison of effective doses;

[0090] Increased range of available focal lengths;

[0091] · Unequivocal extrapolation of effective dose (which can be used for process monitoring); and

[0092] • Possible improvements in scanner-to-scanner and metrology tool-to-metrology tool robustness.

[0093] Additional embodiments are disclosed in the subsequent numbered clauses:

[0094] 1. A method for inferring a value of a first process parameter of a lithography process, wherein the first process parameter is subject to a coupling dependency of a second process parameter, the method comprising:

[0095] determining a first metric and a second metric based on measurement data relating to at least one structure formed on a substrate using the lithographic process, each of the first metric and the second metric being dependent on both the first and second process parameters, the first metric having a stronger correlation with the first process parameter than with the second process parameter, and the second metric having a stronger correlation with the second process parameter than with the first process parameter; and

[0096] The value of the first processing parameter is inferred based on the first metric and the second metric.

[0097] 2. A method as defined in clause 1, comprising inferring a value of the second processing parameter of the lithography process based on the first metric and the second metric.

[0098] 3. A method according to claim 1 or 2, wherein the first metric comprises an asymmetry metric based on differences in intensities of complementary diffraction orders resulting from diffraction of the measurement radiation after measurement of the at least one structure.

[0099] 4. A method as defined in clause 3, wherein said second metric comprises a sum metric based on the sum of said intensities of said complementary diffraction orders.

[0100] 5. A method as defined in any of the preceding clauses, wherein the at least one structure comprises a target, the target having been formed with a focus-related asymmetry, and the first processing parameter comprises the focus during formation of the target.

[0101] 6. A method as defined in clause 5, wherein said second processing parameter comprises an effective dose during formation of said target.

[0102] 7. A method as defined in any one of the preceding clauses, wherein the inferring step comprises reference to a calibration relationship, the calibration relationship describing the relationship of the first metric to the second metric for different values ​​of the first processing parameter and the second processing parameter.

[0103] 8. A method according to claim 7, wherein the inferring step comprises inferring a value of the first processing parameter at a point on a calibration plane, the calibration plane being fitted to a constant value of the second processing parameter, the point on the calibration plane corresponding to a measurement point described by the first and second metrics in a direction defined by the constant value for the first processing parameter.

[0104] 9. A method as defined in clause 8, wherein the calibration plane fitted to a constant value of the second process parameter is fitted to a set value for the second process parameter.

[0105] 10. The method as defined in clause 8 or 9, further comprising the step of inferring the value of the second process parameter from the distance between the measurement point and the calibration plane in the direction defined by the constant value for the first process parameter.

[0106] 11. A method as defined in any of clauses 7 to 10, further comprising determining the calibration relationship based on calibration measurements for different values ​​of the first process parameter and the second process parameter.

[0107] 12. A method as defined in any one of clauses 7 to 11, wherein the calibration relationship comprises a calibration plot of the first metric relative to the second metric for different values ​​of the first and second processing parameters.

[0108] 13. A method of inferring a focus value from a target, the target having been formed with a focus-related asymmetry, the focus value relating to the focus during formation of the target in a lithographic process, the method comprising:

[0109] determining an asymmetry measure and a summation measure from the measurement data, the asymmetry measure being based on differences in intensities of complementary diffraction orders resulting from diffraction of the measurement radiation after measurement of the target; the summation measure being based on a sum of the intensities of the complementary diffraction orders; and

[0110] The focal length value is inferred based on the asymmetry metric and the sum metric.

[0111] 14. A method as defined in clause 13, wherein the inferring step comprises referring to a calibration relationship, the calibration relationship describing the relationship of the asymmetry measure and the summation measure for different focus values ​​and different dose values.

[0112] 15. A method as defined in clause 14, wherein the inferring step comprises inferring a focus value at a point on a calibration plane fitted to a constant dose value, said point on the calibration plane corresponding to a measurement point described by the asymmetry metric and the sum metric in a direction defined by the constant focus value.

[0113] 16. A method as defined in clause 15, wherein the calibration plane fitted to a constant dose value is fitted to a set dose value.

[0114] 17. The method as defined in clause 16, further comprising the step of determining the effective dose during formation of the target based on the distance between the measurement point and the calibration plane in the direction defined by the constant focus value.

[0115] 18. A method as defined in any one of clauses 14 to 17, further comprising constructing the calibration relationship based on calibration measurements at different focus and dose values.

[0116] 19. A method as defined in any one of clauses 14 to 18, wherein the calibration relationship comprises calibration plots of the asymmetry measure and the summation measure for different values ​​of focus and different values ​​of dose.

[0117] 20. A method of determining a calibration plane for performing calibration in a photolithography process; the method comprising:

[0118] determining a calibration relationship based on calibration measurements associated with at least one structure on the substrate, the at least one structure formed using the lithographic process at different values ​​of a first process parameter and a second process parameter, the calibration relationship describing a relationship of a first metric to a second metric for different values ​​of the first process parameter and the second process parameter, wherein each of the first metric and the second metric depends on both the first process parameter and the second process parameter, the first metric having a stronger correlation with the first process parameter than with the second process parameter, and the second metric having a stronger correlation with the second process parameter than with the first process parameter; and

[0119] A calibration plane is fitted to the constant value of the second processing parameter.

[0120] 21. A method as defined in clause 20, wherein the first metric comprises an asymmetry metric based on differences in intensities of complementary diffraction orders resulting from diffraction of the measurement radiation following measurement of the at least one structure.

[0121] 22. A method as defined in clause 21, wherein said second metric comprises a sum metric based on the sum of said intensities of said complementary diffraction orders.

[0122] 23. A method as defined in any of clauses 20 to 22, wherein the at least one structure comprises a target, the target having been formed with a focus-related asymmetry, and the first processing parameter comprises the focus during formation of the target.

[0123] 24. A method as defined in clause 23, wherein the second processing parameter comprises an effective dose during formation of the target.

[0124] 25. A metrology apparatus for measuring parameters of a lithography process, the metrology apparatus being operable to perform the method of any of clauses 1 to 24.

[0125] 26. A measuring device as defined in clause 25, comprising:

[0126] a support for the substrate having a plurality of targets thereon;

[0127] The optical system used to measure each target; and

[0128] processor.

[0129] 27. A photolithography system comprising:

[0130] A lithographic apparatus comprising:

[0131] an illumination optical system arranged to illuminate the pattern;

[0132] a projection optical system arranged to project an image of the pattern onto a substrate; and

[0133] A measuring device according to clause 25 or 26,

[0134] wherein the lithographic apparatus is arranged to use the inferred value for the first process parameter or to use an inferred focus parameter when determining a control correction when applying the pattern to a further substrate.

[0135] 28. A computer program comprising processor-readable instructions which, when executed on suitable processor-controlled means, cause the processor-controlled means to perform the method of any one of clauses 1 to 24.

[0136] 29. A computer program carrier comprising the computer program of clause 28.

[0137] 30. A method of manufacturing a device wherein a device pattern is applied to a series of substrates using a photolithographic process, the method comprising:

[0138] - using a method according to any one of clauses 1 to 24 to monitor the first processing parameter or focus parameter, and

[0139] - controlling the photolithography process of a subsequent substrate based on the inferred value of the first processing parameter or the inferred focus parameter.

[0140] As used herein, the terms "radiation" and "beam" encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365 nm, 355 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5 nm-20 nm), as well as particle beams (such as ion beams or electron beams).

[0141] The term "lens," where the context permits, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components.

[0142] The term target or focus target should not be interpreted as meaning only dedicated targets formed for the specific purpose of metrology or focus metrology, respectively. The term target should be understood to cover any suitable structure on the substrate, including product structures having properties suitable for metrology applications.

[0143] The foregoing description of specific embodiments will fully reveal the overall nature of the invention so that others can easily modify and / or change this specific embodiment for various applications without undue experimentation by applying knowledge within the scope of the art without departing from the overall concept of the invention. Therefore, based on the teachings and guidance given herein, these changes and modifications are intended to fall within the meaning and range of equivalents of the disclosed embodiments. It should be understood that the wording or terminology herein is for the purpose of description by example and not limitation, so that the terms or wording of this specification will be interpreted by those skilled in the art based on the teachings and guidance.

[0144] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be implemented in other ways than those described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A method for inferring a value of a first process parameter and a value of a second process parameter of a photolithography process, the method comprising: determining a first metric and a second metric based on measurement data relating to at least one structure on a substrate formed using the lithographic process, each of the first metric and the second metric depending on both the first process parameter and the second process parameter, the first metric having a dependency on the first process parameter different from a dependency of the first metric on the second process parameter, and the second metric having a dependency on the second process parameter different from a dependency of the second metric on the first process parameter; as well as inferring the value of the first processing parameter and the value of the second processing parameter based on the first metric and the second metric, Wherein the inferring step comprises referencing a calibration relationship describing a relationship of the first metric to the second metric for different values ​​of the first processing parameter and the second processing parameter. 2 . The method of claim 1 , wherein the first metric is more strongly correlated with the first processing parameter than the first metric is with the second processing parameter.

3. The method of claim 1 or 2, wherein the second metric is more strongly correlated with the second processing parameter than the second metric is with the first processing parameter.

4. The method of claim 1, wherein the first metric comprises an asymmetry metric based on differences in intensities of complementary diffraction orders resulting from diffraction of the measurement radiation after measurement of the at least one structure. The method of claim 4 , wherein the second metric comprises a sum metric based on the sum of the intensities of the complementary diffraction orders.

6. The method of claim 1, wherein the at least one structure comprises a target that has been formed with a focus-related asymmetry, and the first processing parameter comprises the focus during formation of the target. The method of claim 6 , wherein the second processing parameter comprises an effective dose during formation of the target.

8. A method of inferring a focus value and a value of an effective dose during formation of a target from a target, the target having been formed with a focus-related asymmetry, the focus value relating to the focus during formation of the target in a lithography process, the method comprising: determining an asymmetry measure and a summation measure from the measurement data, the asymmetry measure being based on differences in intensities of complementary diffraction orders resulting from diffraction of the measurement radiation after measurement of the target; said sum metric being based on a sum of said intensities of said complementary diffraction orders; as well as inferring the focal length value and the effective dose value based on the asymmetry measure and the summation measure, The inferring step comprises referring to a calibration relationship describing a relationship of the asymmetry measure to the sum measure for values ​​of the focal length and the effective dose.

9. A method for determining a calibration plane for performing calibration in a photolithography process; the method comprising: determining a calibration relationship based on calibration measurements associated with at least one structure on the substrate, the at least one structure formed using the lithographic process at different values ​​of a first process parameter and a second process parameter, the calibration relationship describing a relationship of a first metric to a second metric for different values ​​of the first process parameter and the second process parameter, wherein each of the first metric and the second metric depends on both the first process parameter and the second process parameter, the first metric being more strongly dependent on the first process parameter than the first metric is on the second process parameter, and the second metric being more strongly dependent on the second process parameter than the second metric is on the first process parameter; as well as A calibration plane is fitted to the constant value of the second processing parameter.

10. A metrology device for measuring parameters of a lithography process, the metrology device being operable to perform the method according to any one of claims 1 to 9.

11. A photolithography system comprising: A lithographic apparatus comprising: an illumination optical system arranged to illuminate the pattern; a projection optical system arranged to project an image of the pattern onto a substrate; and The measuring device according to claim 10, wherein the lithographic apparatus is arranged to use the inferred value for the first process parameter or to use an inferred focus parameter when determining a control correction when applying the pattern to a further substrate.

12. A computer program product comprising processor-readable instructions which, when executed on a suitable processor-controlled device, cause the processor-controlled device to perform the method according to any one of claims 1 to 9.

13. A method of manufacturing a device wherein a device pattern is applied to a series of substrates using a photolithographic process, the method comprising: - using a method according to any one of claims 1 to 9 to monitor the first processing parameter or focus parameter, and - controlling the photolithography process of a subsequent substrate based on the inferred value of the first processing parameter or the inferred focus parameter.

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