Method for measuring the focus performance of lithographic equipment, pattern forming apparatus and equipment, and device manufacturing method

Abstract

A method for selecting a structure for focus monitoring is disclosed. The method comprises: simulating a focus-dependent parameter's Burson response as a function of focus for one or more different structures; and selecting the structure for focus monitoring during manufacturing based on the results of the simulating step. The simulating step may be performed using computational lithography simulation.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Application No. 62 / 787,973, filed on January 3, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to an inspection apparatus and method which can be used, for example, to perform metrology in device manufacturing by lithography. The invention also relates to such a method for monitoring focus parameters during a lithographic process. 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). A lithographic apparatus can be used, for example, in the manufacture of an integrated circuit (IC). In this case, a pattern forming device (which is alternatively referred to as a mask or reticle) can be used to produce a circuit pattern to be formed on a single layer of the IC. This pattern can be transferred to a target portion (e.g., a portion including a die, a die, or several dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is usually performed via imaging onto a radiation-sensitive material (resist) layer disposed on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are patterned in sequence.

[0005] During lithography, it is desirable to frequently make measurements of the structures produced, for example for process control and verification. Various tools for making these measurements are known, including scanning electron microscopes, which are often used to measure critical dimensions (CD), and specialized tools for measuring overlay (the accuracy of the alignment of two layers in a device). Recently, various forms of scatterometers have been developed for use in the field of lithography. These devices direct a radiation beam onto a target and measure one or more properties of the scattered radiation - such as 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 - 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. The target used by such a scatterometer is a relatively large (e.g. 40 μm by 40 μm) grating, and the measurement beam produces a spot smaller than the grating (i.e., the grating is underfilled or underfilled). Diffraction-based overlay measurement using dark field imaging of diffraction orders enables the measurement of overlay and other parameters of smaller targets. These targets can be smaller than the illumination spot and can be surrounded by product structures on the substrate. Dark field detection in the image plane can be used to effectively separate the intensity from the ambient product structure from the intensity from the overlapping targets.

[0007] Examples of dark field imaging metrology can be found in international patent applications US20100328655A1 and US2011069292A1, the entire contents of which are incorporated herein by reference. Further developments of the technology have been described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets can be smaller than the irradiation spot and can be surrounded by product structures on the wafer. Multiple gratings can be measured in one image using a composite grating target. The contents of all these applications are also incorporated into the present invention by reference.

[0008] An important parameter of the lithography process that needs to be monitored is the focal length. It is expected that an ever-increasing number of electronic components will be integrated into an IC. In order to achieve this integration, it is necessary to reduce the size of the components and therefore increase the resolution of the projection system 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 the focal length across the substrate and between substrates becomes increasingly important. The CD is the size of one or more features (such as the gate width of a transistor) whose variation will cause undesirable variations in the physical properties of the feature.

[0009] Traditionally, optimal settings are determined by "wafer ahead", i.e., substrates that are exposed, developed and measured prior to a production run. In wafer ahead, test structures are exposed in a so-called focus energy matrix (FEM), and the optimal focus and energy (exposure dose) settings are determined from inspection of those test structures. In recent years, focus metrology targets have been included in production designs to allow continuous monitoring of focus performance. These metrology targets should allow rapid measurement of focus to allow rapid performance measurements in high volume manufacturing. Ideally, metrology targets should be small enough that they can be placed in product features without undue loss of space.

[0010] Current test structure designs and focus measurement methods have several disadvantages. Known focus measurement targets require sub-resolution features and / or grating structures with large pitches. These structures may violate the design rules of users of lithographic equipment. The asymmetry of grating structures can be effectively measured using high-speed inspection equipment (such as scatterometers) operating at visible light radiation wavelengths. Known focus measurement techniques exploit the fact that asymmetry in focus sensitivity can be introduced into the structure printed in the resist layer by a special design of the pattern on the pattern forming device that defines the target structure. For EUV lithography, the generation of sub-resolution features becomes more difficult when printing is performed using radiation with a wavelength of less than 20nm (for example 13.5nm). For EUV lithography, the resist thickness and therefore the thickness of the target structure is small. This reduces the diffraction efficiency and therefore the signal strength that can be used for focus measurement.

[0011] For these reasons, in general, there is a need to develop new technologies for the measurement of focus performance during lithography, especially in EUV lithography, and for projection based lithography. Summary of the invention

[0012] The present invention aims to provide an alternative method of measuring focus performance. In some aspects, the present invention aims to provide a method that can be adapted to new environments, such as EUV lithography. In some aspects, the present invention aims to avoid the need to treat sub-resolution features defined in the patterning device.

[0013] In a first aspect, the present invention provides a method for selecting a structure for focus monitoring, the method comprising: simulating, for one or more different structures, a Burson-Johnson response in which a focus-dependent parameter varies with focus; and selecting a structure for focus monitoring during a manufacturing process based on a result of the simulation step.

[0014] In a second aspect, the present invention provides a method for measuring the focusing performance of a lithographic device, the method comprising: (a) obtaining measurement data related to the measurement of the structure selected by the method described in the first aspect after the structure is formed on a substrate; and (b) deriving a measurement result of the focusing performance based at least in part on the measurement data and a calibration model.

[0015] The present invention further provides a measurement apparatus operable to perform the second aspect.

[0016] The present invention further provides a lithography system, the lithography system comprising a lithography device, the lithography device comprising:

[0017] an illumination optical system arranged to illuminate the reflective patterning device;

[0018] a projection optical system arranged to project an image of the patterning device onto a substrate; and

[0019] According to the measuring device of the first aspect of the present invention as described above,

[0020] Wherein the lithographic apparatus is arranged to use measurements of focus performance made by the metrology apparatus when applying patterns to other substrates.

[0021] The present invention further provides a computer program product for implementing the method and apparatus according to the various aspects of the present invention as set out above.

[0022] The present invention further provides a method of manufacturing a device using the method according to the first aspect or the second aspect of the present invention as set out above.

[0023] The following describes in detail other features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention with reference to the accompanying drawings. It should be noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented in the present invention only for illustrative purposes. Based on the teachings contained herein, those skilled in the art will understand additional embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Embodiments of the present 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;

[0025] Figure 1 Describing a lithographic apparatus having a reflective patterning device;

[0026] Figure 2depicts a lithocell or cluster in which lithographic equipment and metrology equipment can be used to perform the method according to the invention;

[0027] 3( a ) and 3 ( b ) schematically illustrate an inspection device adapted to perform angle-resolved scatterometry and dark-field imaging detection methods;

[0028] Figure 4 A schematic diagram depicting overall lithography, showing the synergy between the three key technologies used to optimize semiconductor manufacturing;

[0029] Figure 5 showing the example focus metrology pattern previously described;

[0030] Figure 6 is a graph of a focal length dependent parameter as a function of focal length for a structure including a pair of features having a focus response exhibiting a Persian differential; and

[0031] Figure 7 is a flowchart of a method for monitoring focusing according to an embodiment of the present invention. DETAILED DESCRIPTION

[0032] Before describing embodiments of the present invention in detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

[0033] Figure 1 A lithographic apparatus 100 including a source module SO according to an embodiment of the present invention is schematically depicted. The apparatus comprises:

[0034] - an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation);

[0035] a support structure (e.g. mask table) MT configured to support a patterning device (e.g. mask or reticle) MA and connected to a first positioning device PM configured to accurately position the patterning device;

[0036] a substrate table (e.g. wafer stage) WT configured to hold a substrate (e.g. a resist coated wafer) W and connected to a second positioning device PW configured to accurately position the substrate; and

[0037] - a projection system (eg a reflective projection system) PS configured to project the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C of the substrate W (eg comprising one or more dies).

[0038] 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.

[0039] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as, for example, whether the patterning device is held in a vacuum environment. The support structure may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be, for example, a frame or a stage, which may be fixed or movable as required. The support structure may ensure that the patterning device is in a desired position, for example relative to a projection system.

[0040] The term "patterning device" 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 produce a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a specific functional layer in a device (such as an integrated circuit) produced in the target portion.

[0041] Typically, the pattern forming device used in lithography can be transmissive or reflective. Examples of pattern forming devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography and include mask types such as binary mask types, alternating phase-shift mask types, and attenuated phase-shift mask types, as well as various hybrid mask types. An example of a programmable mirror array uses a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incident radiation beam in different directions. The tilted mirrors impart a pattern to the radiation beam reflected by the mirror matrix.

[0042] Similar to the illumination system, the projection system may include any type of projection system, including refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation used or for other factors such as the use of a vacuum. It may be desirable to use a vacuum for EUV radiation, as other gases may absorb too much radiation. Therefore, a vacuum environment may be provided to the entire beam path by means of a vacuum wall and a vacuum pump.

[0043] As depicted here, the apparatus is of the reflective type (e.g. using a reflective mask). The disclosed focus metrology techniques have been developed specifically for use with a reflective patterning device (reticle), where the illumination is not in a direction perpendicular to the plane of the patterning device surface, but at a slightly oblique angle. In principle, the same techniques can be applied to a transmissive patterning device if, for some reason, the illumination introduces asymmetry. Conventionally, the illumination of a reticle is designed to be symmetrical, but in the case of a reflective reticle this is usually not possible.

[0044] Certain embodiments of the present disclosure use a reflective patterning device to exploit asymmetries in the projection system. Other embodiments are applicable to any kind of projection system.

[0045] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and / or two or more mask tables). In these "multi-stage" machines the extra 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.

[0046] refer to Figure 1 , the illuminator IL receives an extreme ultraviolet radiation beam from a source module SO. Methods for generating EUV light include, but are not necessarily limited to, converting a material having at least one element (e.g., xenon, lithium, or tin) with one or more emission lines in the EUV range into a plasma state. In one such method (often referred to as laser produced plasma "LPP"), the desired plasma can be generated by irradiating a fuel (such as a droplet, stream, or cluster of a material having the desired line emission element) with a laser beam. The source module SO may be a laser ( Figure 1 The source module is a part of an EUV radiation system (not shown) for providing a laser beam for exciting the fuel. The resulting plasma emits output radiation, such as EUV radiation, which is collected using a radiation collector disposed in the source module. For example, when a CO2 laser is used to provide the laser beam for fuel excitation, the laser and the source module may be separate entities.

[0047] In these cases, the laser is not considered to form part of the lithographic apparatus, and the radiation beam is delivered from the laser to the source module by means of a beam delivery system comprising, for example, suitable directing mirrors and / or a beam expander. In other cases, for example when the source is a discharge produced plasma EUV generator (often referred to as a DPP source), the source may be an integral part of the source module.

[0048] The illuminator IL may include an adjuster for adjusting the angular intensity distribution of the radiation beam. Typically, 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 a pupil plane of the illuminator may be adjusted. In addition, the illuminator IL may include various other components, such as a faceted field mirror device and a faceted pupil mirror device. The illuminator may be used to adjust the radiation beam to have a desired uniformity and intensity distribution in its cross section.

[0049] The radiation beam B is incident on a patterning device (e.g. a mask) MA held on the support structure (e.g. a mask table) MT and is patterned by the patterning device. After reflection from the patterning device (e.g. a mask) MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioning device PW and a position sensor PS2 (e.g. an interferometer device, a linear encoder or a capacitive sensor), the substrate table WT can be accurately moved, for example so that different target portions C are positioned in the path of the radiation beam B. Similarly, a first positioning device PM and a further position sensor PS1 can be used to accurately position the patterning device (e.g. a mask) MA relative to the path of the radiation beam B. The patterning device (e.g. a mask) MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

[0050] The depicted device can be used in at least one of the following modes:

[0051] 1. In step mode, the support structure (e.g. mask table) MT and the substrate table WT are kept substantially stationary (i.e. single static exposure) while the entire pattern imparted to the radiation beam is projected at one time onto a target portion C. Next, the substrate table WT is shifted in the X and / or Y direction so that a different target portion C can be exposed.

[0052] 2. In scan mode, the support structure (e.g. mask table) MT and substrate table WT are scanned synchronously (i.e. single dynamic exposure) while a pattern imparted to the radiation beam is projected onto a target portion C. The speed and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

[0053] 3. In another mode, the support structure (e.g. mask table) MT is kept substantially stationary, thereby holding the programmable patterning device, 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 used, 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 may be readily applicable to maskless lithography utilizing a programmable patterning device, such as a programmable mirror array of the type mentioned above.

[0054] Combinations and / or variations of the above-described modes of use, or entirely different modes of use, may also be used.

[0055] It will be appreciated that the lithographic apparatus Figure 1 The diagrams are shown in highly schematic form, but the forms described are all forms necessary for the present disclosure.

[0056] like Figure 2 As shown in , the lithography equipment LA forms part of the lithography unit LC (sometimes also referred to as a lithography cell or cluster), which also includes equipment for performing pre-exposure processes and post-exposure processes on the substrate. Conventionally, these equipment include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH, and a baking plate BK. The substrate transport device or robot RO picks up the substrate from the input / output ports I / O1, I / O2, moves the substrate between different process equipment, and then transfers the substrate to the feed table LB of the lithography equipment. These devices, which are often collectively referred to as tracks or coating and development systems, are under the control of the track control unit TCU, which itself is controlled by the management control system SCS, which also controls the lithography equipment via the lithography control unit LACU. Therefore, different equipment can be operated to maximize production volume and processing efficiency.

[0057] In order to correctly and consistently expose the substrates exposed by the lithographic apparatus, it is desirable to inspect the exposed substrates to measure properties, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. Therefore, the manufacturing facility in which the lithography unit LC is located also includes a measurement system MET, which receives some or all of the substrates W that have been processed in the lithography unit. The measurement results are provided directly or indirectly to the management control system SCS. If an error is detected, the exposure of subsequent substrates can be adjusted, especially if the inspection can be completed quickly and quickly enough so that other substrates of the same batch are still to be exposed. In addition, the already exposed substrates can be stripped and reworked to improve the yield, or discarded, thereby avoiding further processing of the substrates that are known to be defective. In the case where only some target portions of the substrate are defective, further exposure can be performed only on those target portions that are good.

[0058] In the measurement system MET, the inspection equipment is used to determine the properties of the substrate, and specifically, to determine how the properties of different substrates or different layers of the same substrate vary between different layers. The inspection equipment can be integrated into the lithographic equipment LA or the lithographic unit LC, or can be a separate device. In order to achieve the fastest measurement, it is desirable to make the inspection equipment measure the properties in the exposed resist layer immediately after exposure. However, the latent image in the resist has a very low contrast-there is only a very small refractive index difference between the part of the resist that has been exposed to the radiation and the part of the resist that has not been exposed to the radiation-and not all inspection equipment has enough sensitivity to make useful measurements of the latent image. Therefore, it can be measured after the post-exposure baking step (PEB), which is usually the first step performed on the exposed substrate and increases the contrast between the exposed part and the unexposed part of the resist. At this stage, the image in the resist can be called a semi-latent image. It is also possible to measure the resist image after development - at which point 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 possibility of reworking a defective substrate, but can still provide useful information.

[0059] Fig. 3(a) schematically shows the key elements of an inspection apparatus implementing so-called dark field imaging metrology. The apparatus may be a stand-alone apparatus or incorporated into a lithographic apparatus LA, for example at a measurement station or into a lithographic cell LC. An optical axis with several branches throughout the apparatus is indicated by a dashed line O. The target grating structure T and the diffracted rays are illustrated in more detail in Fig. 3(b).

[0060] As described in the prior applications cited in the prior art, the dark field imaging device of Figure 3(a) can be a component of a multi-purpose angle-resolved scatterometer that can be used instead of or in addition to a spectroscopic scatterometer. In this type of inspection device, the radiation emitted by the radiation source 11 is conditioned by the illumination system 12. For example, the illumination system 12 may include a collimating lens system, a filter, a polarizer and an aperture device 13. The conditioned radiation follows an illumination path IP, in which the conditioned radiation is reflected by a partially reflective surface 15 and is focused into a spot S on the substrate W via a microscope objective 16. The measurement target T can be formed on the substrate W. The lens 16 has a high numerical aperture (NA), preferably at least 0.9 and more preferably at least 0.95. An immersion fluid can be used as needed to obtain a numerical aperture greater than 1.

[0061] In this example, the objective lens 16 is also used to collect radiation that has been scattered by the target. Schematically, a collection path CP for such return radiation is shown. A multipurpose scatterometer may have two or more measurement branches in the collection path. The example shown as a pupil imaging branch includes a pupil imaging optical system 18 and a pupil image sensor 19. An imaging branch is also shown, which will be described in more detail below. In addition, other optical systems and branches will be included in a practical device, for example to collect reference radiation for intensity normalization, for rough imaging of the capture target, for focusing, etc. Details of these optical systems and branches can be found in the prior publications mentioned above.

[0062] Where the metrology target T is provided on a substrate W, this may be a 1-D grating, which is printed so that after development the bars are formed by solid resist lines. The target may be a 2-D grating, which is printed so that after development the grating is formed by solid resist posts or vias in the resist. The bars, posts or vias may alternatively be etched into the substrate. Each of these gratings is an example of a target structure whose properties may be investigated using the inspection apparatus.

[0063] Various components of the illumination system 12 may be adjustable to implement different measurement "options" within the same device. In addition to selecting wavelength (color) and polarization as characteristics of the illuminating radiation, the illumination system 12 may also be adjusted to implement different illumination profiles. The plane of the aperture device 13 is conjugate to the pupil plane of the objective lens 16 and to the plane of the pupil image detector 19. Therefore, the illumination profile defined by the aperture device 13 defines the angular distribution of light incident on the substrate W as a spot S. In order to implement different illumination profiles, the aperture device 13 may be arranged in the illumination path. The aperture device may include different apertures mounted on a movable slide or wheel. It may alternatively include a programmable spatial light modulator. As another alternative, optical fibers may be arranged at different locations in the illumination pupil plane and may be selectively used to transmit light or not transmit light at their respective locations. These variations are discussed and illustrated in the documents cited above.

[0064] In a first example illumination mode, aperture 13N is used and ray 30a is provided so that the angle of incidence is as shown at "I" in Figure 3(b). The path of the zero-order ray reflected by the target T is marked as "0" (not to be confused with the optical axis "O"). In the second illumination mode, aperture 13S is used so that ray 30b can be provided, in which case the angle of incidence and the angle of reflection will be exchanged compared to the first mode. In Figure 3(a), the zero-order rays of the first example illumination mode and the second example illumination mode are marked as 0(13N) and 0(13S), respectively. Both of these illumination modes will be identified as off-axis illumination modes. Many different illumination modes can be implemented for different purposes, including on-axis illumination modes.

[0065] As shown in more detail in FIG. 3( b ), a target grating T as an example of a target structure is placed so that the substrate W is perpendicular to the optical axis O of the objective lens 16. In the case of an off-axis illumination profile, an illumination ray I impinging on the grating T at 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 with a small overfilled target grating, these rays are only one of many parallel rays covering the area of ​​the substrate including the measurement target grating T and other features. Since the beam of illumination rays 30 a has a finite width (necessary to allow a useful amount of light), the incident ray I will in fact occupy an angular range, and the diffracted rays 0 and +1 / −1 will be slightly spread out. According to the point spread function of the small target, each order +1 and −1 will be further spread out over the entire angular range, rather than a single ideal ray as shown.

[0066] In a branch of the collection path for dark field imaging, the imaging optical system 20 forms an image T' of an object on the substrate W on a sensor 23 (e.g., a CCD or CMOS sensor). An aperture stop 21 is arranged in a plane in the imaging branch of the collection path CP, which plane is conjugate to the pupil plane of the objective lens 16. The aperture stop 20 may also be referred to as a pupil stop. The aperture stop 21 may take different forms, just as the illumination aperture may take different forms. The aperture stop 21 combined with the effective aperture of the lens 16 determines which part of the scattered radiation is used to produce an image on the sensor 23. Typically, the aperture stop 21 is used to block the zero-order diffraction beam so that the image of the object formed on the sensor 23 is formed only from the first-order beam. In an example where two first-order beams are combined to form an image, the image will be a so-called dark field image, which is equivalent to dark field microscopy. As an example of an aperture stop 21, an aperture 21a that only allows on-axis radiation to pass can be used. In case off-axis illumination is used in combination with aperture 21a, only one of the first orders is imaged at a time.

[0067] The image captured by the sensor 23 is output to an image processor and controller PU, the functions of which will depend on the specific type of measurement being performed. For the present purpose, a measurement of the asymmetry of the target structure is performed. The asymmetry measurement results can be combined with the knowledge of the target structure to obtain the measurement results of the performance parameters of the lithography process used to form the target structure. The performance parameters that can be measured in this way include, for example, overlap, focus and dose. A special design of the target is provided to allow these measurements of different performance parameters to be performed via the same basic asymmetry measurement method.

[0068] Referring again to FIG. 3( b) and the first example illumination pattern with rays 30 a, the +1 order diffracted rays from the target grating will enter the objective 16 and contribute to the image recorded at the sensor 23. When the second illumination pattern is used, rays 30 b are incident at an angle opposite to rays 30 b, and thus the -1 order diffracted rays enter the objective and contribute to the image. When off-axis illumination is used, the aperture stop 21 a blocks zero order radiation. As described in the prior publication, the illumination pattern can be defined using off-axis illumination in the X and Y directions.

[0069] By comparing the images of the target grating under these different illumination modes, an asymmetry measurement result can be obtained. Alternatively, an asymmetry measurement result can be obtained by maintaining the same illumination mode but rotating the target. Although off-axis illumination is shown, on-axis illumination of the target can be used instead, and a modified off-axis aperture 21 can be used to pass substantially only one first-order diffracted light to the sensor. In another example, a pair of off-axis prisms 21b are used in combination with the on-axis illumination mode. These prisms have the effect of diverting the +1 order and -1 order to different parts on the sensor 23 so that they can be detected and compared without the need for two consecutive image capture steps. This technology is disclosed in the published patent application US2011102753A1 mentioned above, the contents of which are incorporated herein by reference. Instead of or in addition to the first-order beam, second-order, third-order and higher-order beams (not shown in Figure 3) can also be used in the measurement. As a further variation, the off-axis illumination pattern may be held constant, while the target itself is rotated 180 degrees beneath the objective 16 to capture an image using the opposite diffraction order.

[0070] Typically, the patterning process in the lithographic apparatus 100, LA is one of the most critical steps in the process, which requires a high accuracy in the dimensioning and placement of the structures on the substrate W. In order to ensure this high accuracy, the three systems can be combined in a so-called "holistic" control environment, such as Figure 4 Schematically depicted. One of these systems is the lithographic apparatus LA, which is (in practice) connected to a metrology tool MT (a second system) and to a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithographic apparatus LA remains within the process window. The process window defines a range of process parameters (e.g. dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g. a functional semiconductor device) - typically within which process parameter variations in the lithographic process or patterning process are allowed.

[0071] The computer system CL can use (portions of) the design layout to be patterned to predict which resolution enhancement techniques to use, and perform computational lithography simulations and calculations to determine which mask layout and lithography equipment settings will achieve the maximum overall process window (in Figure 4 Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from the metrology tool MT) to predict whether defects may exist (e.g. due to suboptimal processing) Figure 4), depicted by an arrow pointing to “0” in the second scale SC2.

[0072] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify, for example, possible drift in the calibration state of the lithographic apparatus LA (in Figure 4 SC3 is depicted by multiple arrows in the third scale SC3).

[0073] In the following disclosure, techniques for measuring the focusing performance of a lithography process using oblique illumination on a reflective type pattern forming device will be described. These techniques can be particularly applied in EUV lithography, in which case reflective optics in a near-vacuum environment are required. While printing the product features, a measurement target including certain focus measurement patterns will be printed on the substrate. The asymmetry of these printed patterns will be measured using a diffraction-based technique such as in the device of Figure 3. In order to allow the use of small targets, it will be assumed that the dark field imaging branch of the device will be used to perform these asymmetry measurements. However, diffraction-based measurements of asymmetry can also be performed using the pupil imaging branch. Of course, the device shown in Figure 3 is only an example of an inspection device and method that can be used to measure asymmetry.

[0074] In the context of lithographic equipment operating in the DUV wavelength range, targets for diffraction-based focus (DBF) measurements have been successfully designed and used. A DBF target of a known type is produced by including sub-segmented features in the grating pattern on the mask. These features have a size below the imaging resolution of the lithographic equipment next to the more solid features. Therefore, they are not printed in the resist layer on the substrate as separate features, but they affect the printing of the solid features in a way that is sensitive to focus errors. In particular, the presence of these features produces an asymmetric resist profile for each line in the grating within the DBF measurement target, where the degree of asymmetry depends on the focal length. Therefore, a measurement tool such as the inspection equipment of Figure 3 can measure the degree of asymmetry from a target formed on the substrate and translate this into the scanner focus.

[0075] Unfortunately, known DBF metrology target designs are not suitable for use in all situations. In EUV lithography, the resist film thickness is significantly lower than that used in DUV immersion lithography, resulting in low diffraction efficiency and difficulty in extracting accurate asymmetry information from the diffracted radiation in the scatterometer. In addition, since the resolution of the imaging system is inherently higher in EUV lithography, features with dimensions below the printing resolution of DUV immersion lithography become "solid" features that can be printed by EUV lithography. Providing similar sub-resolution features on EUV reticles is highly impractical and / or may violate the "design rules" of semiconductor manufacturers. Such rules are typically established as a way to constrain feature designs to ensure that printed features meet their process requirements. In any case, working outside of the design rules makes it difficult to simulate the execution of the process on a DBF target, so that the optimal target design and calibration of the focus measurement becomes a matter of trial and error. The expectation of meeting the design rules applies to DBF targets in DUV lithography, not just EUV lithography.

[0076] The focus (DBF) metrology target should have a unique and preferably monotonic asymmetry signal as a function of (or varying with) target defocus. In this context, 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). It is also important that the accuracy and sensitivity are high. Other considerations include minimizing dose and other crosstalk effects (e.g., resulting from processing effects), and tool-to-tool matching between inspection tools should be good.

[0077] Figure 5A previous example for solving the proposed problem is illustrated, as described in the published European patent application EP3422102A1. Other focus target examples are described in the (unpublished) European patent application EP18207326.2, which is also published as the Netherlands patent application NL2022031A. These two documents are incorporated herein by reference. The figure shows a small part of a focus measurement pattern in isolation. The repeating unit of this pattern includes a first feature 422 and a second feature 424 spaced apart from each other along a periodic direction. In this example, the periodic direction can be the X direction of the pattern forming device and the substrate. Each first feature 422 includes a grating or other feature, and each 422 grating or other feature has a minimum size w1, which is close to but not less than the resolution limit of the printing step. This value w1 can be, for example, less than 50 nanometers in the periodic direction. The second interval between each first feature 422 and its second closest adjacent second feature 424 has a size w2' and is similar to the size w2 of the second feature 424 itself. Thus, it will be seen that the pattern T comprising a thin first feature and a thicker second feature actually exists in both positive and negative forms. These dimensions w1, w1' and a much larger period P are set, and it should be understood that the dimensions w2 and w2' are much larger than the minimum dimension w1 of the first feature 422, and therefore much larger than the resolution limit of the printing step. The dimensions w2 and w2' can each be, for example, four times more than the dimension w1, five times more than the dimension w1, six times, eight times or ten times more than the dimension w1. Each second feature in the periodic array also includes a sub-feature 426, the minimum dimension of which in a direction transverse to the direction of the periodicity is close to but not less than the resolution limit of the printing step. In this example, the sub-feature is a line protruding asymmetrically from the main body 428 of the first feature. The length of these protruding lines or fingers is marked as w3. The main body 428 of each second feature 424 defines the minimum dimension w4 of the second feature in the direction of the periodicity. Therefore, in this representation, the maximum dimension w2 of the second feature 424 is equal to w3+w4. The minimum dimension of the subfeature in the lateral direction is labeled w5.

[0078] It will be appreciated that only the gap between the sub-feature 426 and the first feature 422 is varied by the focal length. And furthermore, this variation is linear or at least monotonic, resulting in a monotonic asymmetry signal when the target is measured. It is therefore assumed that this gap relies on the different Poisson behavior between the first edge defined by the end of the sub-feature and the second edge defined by the first feature 422. The first edge effectively withdraws in the case of defocus (the length of the sub-feature 426 becomes smaller) while the position of the second edge remains relatively stable. The effective Poisson shift can also originate from a physical height difference: the sub-feature 426 is effectively sampled at a lower resist height than the first feature 422.

[0079] It has been observed that a good principle for targeting is to exploit the difference of two Burson-like signals as a function of the focal offset between the two Burson-like signals. Most existing techniques will produce a single Burson-like signal with no signal information and zero sensitivity around the optimal focal distance. Such a concept is described, for example, in PCT patent application WO2016 / 198283A1, which is incorporated herein by reference.

[0080] As can be appreciated, there is a large design space pertaining to such focusing targets. First, there are a large number of different design principles, of which the examples described in EP3422102A1 and EP18207326.2 / NL2022031A are just examples. For each of these design principles, there are a large number of parameters that can be varied (e.g., pitch, duty cycle, any size of any feature of the target, e.g., Figure 5 Dimensions described and illustrated in the examples of FIG. 1 ). Furthermore, the relationship between focusing performance (e.g., Persian response) and variations within this design space is not fully understood.

[0081] Therefore, it is suggested to simulate the Persian response of the structure, for example by using computational lithography methods, to identify and / or optimize the structural design based on the Persian response. More specifically, in one embodiment, one or more Persian parameters related to the Persian response performance of different structures will be evaluated. Each structure may include, for example, two (or more) features, each feature having a corresponding Persian signal or curve. One such Persian parameter may include a Persian difference parameter. A Persian response including an appropriate Persian difference parameter may include a Persian response having two such Persian-like signals, which are separated by a (e.g., sufficiently large) focal offset. The Persian difference parameter may be expressed in terms of focal offset, or in terms of a target parameter (e.g., critical dimension CD) difference or ΔCD). In the latter case, ΔCD with respect to focal offset may be considered (e.g., a graph of ΔCD varying with focal offset). Another relevant Persian parameter may include a Persian curvature parameter describing the curvature of one or two Persian-like signals.

[0082] Figure 6 The diagram illustrates the principle. Figure 6 A graph including a target parameter P (such as CD) as a function of focal length f illustrates a focus response for a particular target design. The focus response includes a first Persian signal 600 and a second Persian signal 610 separated by a Persian difference df (e.g., a focal length offset between the first Persian signal 600 and the second Persian signal 610). For example, the structure may include a first feature responsible for the first Persian signal 600 and a second feature responsible for the second Persian signal 610. This Persian difference df may be shown to vary monotonically with focal length f. Preferably, the magnitude of this Persian difference should be maximized, or at least large enough to produce a reliable signal (e.g., above a threshold difference). A further consideration is to optimize (e.g., maximize) the Persian curvature of each Persian signal (or at least one Persian signal) in the first Persian signal 600 and the second Persian signal 610. Additional embodiments may include co-optimizing the Poisson difference and the Poisson curvature; for example, to maximize the combination of the two.

[0083] For example, the Persian difference parameter (e.g., when describing a focus offset between the Persian signals 600, 610) and the Persian curvature parameter may be optimized in concert, e.g., to minimize a noise metric. For example, in a plot of the Persian difference versus focus, the noise metric may be, for example, a measure of the amount of noise (on the vertical axis) relative to (e.g., divided by) the local slope of the curve. A similar result may be achieved (e.g., when the Persian difference parameter is ΔCD) by maximizing the slope of a plot of ΔCD versus focus offset over a suitably wide range of focus offsets where ΔCD shows a monotonic response.

[0084] In an embodiment, instead of the Persian difference parameter, the evaluated Persian parameter may comprise the Persian sum (the sum of the two curves). However, this loses the advantage that the difference parameter (at least partially) cancels out the non-focal contributions (e.g. dose effects) that are (partially) common to both Persian curves.

[0085] It should be noted that Figure 6 This is just an example graph, the actual graph may vary. For example, there may be a target parameter (e.g., CD) offset between the first Persian signal 600 and the second Persian signal 610. Such a CD offset will not affect the proposed method. The first Persian signal 600 and the second Persian signal 610 may also have different curvatures. This will result in curvature residuals in the delta curve, and potential non-monotonicity. However, assuming that the multiple curves have sufficiently unique responses (e.g., the multiple curves do not cross), this will not be a problem for the multi-dimensional calibration curve.

[0086] The structures whose Persian responses are simulated may include specially designed focus targets and / or product structures that form part of the device. When product structures are being evaluated, the simulation may include identifying which product structures provide suitable and / or optimal focus responses (e.g., including a pair of features with sufficient or optimal Persian differential responses). Alternatively or in addition, the simulation may include optimizing any aspect of the design of the product structure (and / or similar product structures), for example, to improve their focus response for focus measurement purposes. In the latter case, the simulation may also include evaluating whether the optimized design will still function correctly. Any type of structure simulated / optimized by computational lithography (e.g., using source mask optimization (SMO) techniques) may be used. These types of structures may be anything from simple line / space patterns through contact holes / vias to more complex logic patterns. Therefore, in terms of simulation, there are no practical restrictions on the structures simulated, but because the measurements of the selected structures should be reliable and robust, there will naturally be measurement considerations.

[0087] Computational lithography describes the use of computer modeling to predict, correct, optimize and verify the imaging performance of the lithography process under a range of patterns, processes and system conditions. In computational lithography simulations, the output of the simulation process will provide information about the predicted shape and size of the printed features on the wafer, such as predicted critical dimensions (CD) and profiles. Such predictions allow for quantitative assessment of the lithography process and whether the process will produce the expected results. Accurate CD predictions rely heavily on the accuracy of the models used in the lithography simulations. Such models can include, for example, image-based 3D mask models or thin mask models.

[0088] Computational lithography includes or is used for, for example, full-chip simulation or verification methods such as LMC (lithography manufacturability check). LMC applies a set of models (e.g., the models used in optical proximity correction (OPC)) to predict the CD (on the substrate) across the entire field. The main purpose of LMC is to highlight the critical patterns in the field before the photomask order is sent to the mask shop. In addition, LCM is also able to extract from the design area that is sensitive to changes in process conditions. LMC basically includes the comparison of the simulated full-chip resist profile with the design target.

[0089] Another computational lithography technique is source mask optimization (SMO), which includes modeling of the illumination profile for optimization of imaging performance. SMO enables optimization of the lithography process at the design, mask, and imaging levels. Use cases include design rule verification and production illumination optimization. More specifically, SMO is typically used to simultaneously co-optimize mask design and source shape to find a combination that achieves the necessary k1 value and a large enough process window for robust or robust manufacturing.

[0090] Therefore, it is proposed to simulate imaging performance on a product, and in particular Persian differentials (or mutual Persian shifts) for candidate structures. Such a simulation will be able to predict which structures (e.g., segments or targets) have the largest Persian differentials, or at least have sufficient Persian differentials for focus monitoring. Alternatively or in addition, the simulation may also determine whether additional features (e.g., features of similar products) can be added, or how the design of a structure (e.g., an actual product structure) can be changed to improve the Persian response. The simulation should also assess what the resulting structure will produce (e.g., still function) when the design of the actual product structure is changed. For example, sufficient Persian differentials may include greater than 5 nm, greater than 10 nm, greater than 15 nm, or greater than 20 nm. It should be noted that the simulation accuracy of the exact Persian shift is not required.

[0091] Because computational lithography techniques are used to simulate the Persian response in at least some embodiments, the simulation can also (e.g., simultaneously and / or as part of a unified overall process) evaluate and / or simulate other aspects of the structure. For example, the simulation can evaluate and identify which structures are critical and / or lithography sensitive. This enables better selection of the most critical lithography, and therefore enables the most sensitive structures to be used for optimization. Other aspects that can be evaluated can include exposure / printing quality, such as printability robustness (e.g., evaluating the accuracy and reproducibility with which a structure can be printed / exposed). In this way, any proposed structural optimization can also be a co-optimization between the Persian response and such exposure / printing quality, particularly in the case where a product structure is used for focus monitoring, the simulation can also evaluate whether the product structure will be produced. Ultimately, the simulation can determine simulation data for any performance parameter.

[0092] In an embodiment, machine learning techniques may be used to pre-select structures that are likely to be sensitive, thereby reducing the evaluation space (e.g., the number of structures that need to be simulated). Possible criteria for training may include, for example, scalability and / or sensitivity. In particular, when computational lithography techniques are used in the manner described above, all structures may be simulated / optimized in any case, and therefore all required data may already exist.

[0093] More specifically, in such a machine learning embodiment, a large set of multiple structures (or structural variables) is input to the trained model. The trained model can output a preselection of structures and / or a suitability score for each structure. The suitability score can be, for example, a scalability value and / or a sensitivity value. The training of the model can be performed based on an existing set of patterns used in computational lithography (e.g., SMO). Since these existing pattern sets will most likely have already been simulated / optimized, at least the sensitivity values ​​will already be known.

[0094] The structures being evaluated may include scribe structures (e.g., dedicated focus targets) and / or intra-die structures. An intra-die structure may include two features that produce a suitable Persian response and are sufficiently close in space. Thus, a structure may include features from two different structures of suitable proximity, and in the context of the present disclosure, the term "structure" refers to any pair (or more pairs) of features that provide a suitable focus response and are close enough for focus monitoring, regardless of whether they are contained within the same functional structure in terms of the functionality of the device. Being close enough or close enough may include, for example, being spaced no more than 3 mm, or no more than 1 mm, or no more than 0.5 mm, or no more than 0.2 mm apart.

[0095] The result of the simulation may be a suitable structure including two features with sufficient Persian differential. This structure may then be exposed multiple times on a substrate with different focal lengths (and optionally, doses). Such a substrate may be a FEM substrate. FEM substrates are known in the art and typically include a substrate coated with photoresist on which a pattern is exposed with a variety of combinations of focal lengths and exposure offsets. The FEM substrate is measured by a metrology tool using, for example, a reconstruction method to determine parameters that depend on focal length (such as one or both of the side wall angle (SWA) and critical dimension (CD). A focus model or a focus-dose model may then be constructed based on these measurements and known focal length values ​​(e.g., as set). The focus-dose model describes and interpolates the relationship between focus and dose and CD and SWA. Using this interpolation model, any single CD and / or SWA measurement result can be converted to focus and dose. For example, this method is described in U.S. patent application US2011-0249244, which is incorporated herein by reference in its entirety.

[0096] Persand differential parameters for each exposure can then be measured from the FEM substrate (using any suitable metrology method, including scatterometry or SEM / electron beam). The Persand differential parameters may include, for example, the difference between the measured focus-dependent parameters for each of the first and second features of the structure; and more specifically, the CD difference between the two features. The measurements will produce a calibration curve for the Persand differential parameters. However, in the case where the calibration curve is related to the focus offset Persand differential parameters, the Persand curves should be similar (i.e., monotonic calibration curves). For focus metrology during production, the Persand differential parameters are measured on a product substrate, and the calibration curves are used to infer focus performance on the product. The inference algorithm may include any calibration model, including calibration curve fitting, multi-dimensional calibration curves, machine learning models, or any other suitable model.

[0097] The calibration may be based on the Persian differential parameters, or may be directly based on two or more focus-dependent parameter (e.g., CD) inputs. Alternatively, instead of CD, another parameter (which shows a suitable Persian response) may be measured from each feature to determine the Persian differential parameters. For example, the target may be designed so that the CD measurement is converted into an asymmetry or overlay measurement (such as that done for μDBF techniques).

[0098] In an embodiment, the structure may also include a third dose-sensitive (but focus-insensitive) feature to further reduce dose dependence. The method may also include: simulating the dose dependence of such a dose-sensitive feature to optimize its dose response, or at least determine whether its dose response is sufficient. Other features, such as spatial features (which have an inverse Persian signal) may be added to the structure to provide double differential measurements. Therefore, when optimizing / evaluating the target, the simulation may also simulate the Persian response of these spatial features.

[0099] Figure 7is a flow chart describing a method according to an embodiment. At step 700, computational lithography is used to simulate the Persian response of many different structures. At step 710, the structures are evaluated according to their respective Persian parameters. A structure that is considered to be optimal (or at least sufficient) at least in terms of the Persian differential parameters is selected. This step may include the co-optimization of the Persian differential parameters and the Persian curvature parameters. At step 720, a focus calibration curve that varies with the Persian differential parameters (e.g., the CD difference or ΔCD between the features of the structure or the focal length offset) is determined, for example, by exposing and measuring the FEM. At step 730, a product substrate including the one or more structures is exposed. The structure can be formed, for example, in a scribe line or a tube core. At step 740, the Persian differential parameters are measured using a suitable measurement method, and at step 750, the focal length is determined based on the Persian differential parameters and the calibration curve.

[0100] In general, the method of manufacturing devices using the lithography process can be improved by the following operations: performing a focus measurement method as disclosed herein, using the focus measurement method to measure the processed substrate to measure the performance parameters of the lithography process, and adjusting the parameters of the process (especially the focal length) to improve or maintain the performance of the lithography process for subsequent substrate processing.

[0101] While the target structure including the focused metrology pattern described above is a metrology target specifically designed and formed for the purpose of measurement, in other embodiments, properties may be measured on a target that is a functional part of a device formed on the substrate. Many devices have a regular grating-like structure. As used herein, the terms "metrology pattern" and "metrology target" and the like do not require that the structure be provided specifically for the measurement being performed.

[0102] Further embodiments are disclosed in the following aspects:

[0103] 1. A method for selecting a structure for focus monitoring, the method comprising:

[0104] simulating a focal length dependent parameter's Burson response as a function of focal length for one or more different configurations; and

[0105] Based on the results of the simulation step, a structure is selected for focus monitoring during the manufacturing process.

[0106] 2. The method of clause 1, wherein the simulating step is performed using computational lithography simulation.

[0107] 3. A method according to clause 1 or 2, wherein the one or more structures each include at least a first feature and a second feature, the first feature having a first Burson-Williams curve response that varies with focal length for the focal length-dependent parameter and the second feature having a second Burson-Williams curve response that varies with focal length for the focal length-dependent parameter; and wherein the simulating step comprises:

[0108] For each of the one or more structures, values ​​of one or more Perthshire parameters describing a relative relationship between the first Perthshire curve response and a second Perthshire curve response are simulated.

[0109] 4. A method according to aspect 3, wherein the one or more Persian parameters include at least one Persian differential parameter, the Persian differential parameter being based on a simulated relative focal length offset between the first Persian curve response that varies with focal length and the second Persian curve response that varies with focal length.

[0110] 5. The method of clause 4, wherein the relative focus offset is determined as a focus difference between corresponding points of the first Persian curve response as a function of focal length and the second Persian curve response as a function of focal length.

[0111] 6. The method according to aspect 4 or 5, comprising:

[0112] For each of the plurality of structures, simulating a value of a Persian difference parameter; and

[0113] Based on the determined value of the Persian differential parameter, a structure for focus monitoring is selected from the plurality of structures.

[0114] 7. The method according to aspect 6, wherein the selected structure is a structure whose simulated value of the Persian differential parameter exceeds a threshold value.

[0115] 8. The method of clause 7, wherein the Persian differential parameter comprises a focal length offset between the Persian responses of two of the structures.

[0116] 9. The method according to clause 8, wherein the threshold value is 5 nm.

[0117] 10. The method of clause 8, wherein the threshold is 10 nm.

[0118] 11. A method according to any one of aspects 6 to 10, wherein the one or more Persian parameters also include a Persian curvature parameter, the Persian curvature parameter describing the degree of curvature of the first Persian curve response and / or the second Persian curve response, and the selection step includes selecting a structure also based on the determined value of the Persian curvature parameter.

[0119] 12. A method according to any one of aspects 6 to 11, wherein the selected structure is based on a collaborative optimization that tends to maximize the simulated value of the Persian difference parameter and the simulated value of the Persian curvature parameter.

[0120] 13. The method of clause 7, wherein the Persian differential parameter comprises a difference between the focal length dependent parameter relative to a focus parameter or a focal length differential parameter.

[0121] 14. A method according to clause 13, wherein the threshold describes a threshold slope of the focus-dependent parameter relative to a focus parameter or a focus differential parameter.

[0122] 15. The method according to clause 14, wherein the selected structure is a structure whose simulated value of the threshold slope is maximum on a monotonic domain.

[0123] 16. A method according to any one of aspects 6 to 15, wherein:

[0124] The simulating step includes simulating one or more print qualities, and

[0125] The selecting step includes selecting the structure based on a co-optimization of the Persian differential parameter and the one or more print qualities.

[0126] 17. A method according to any one of aspects 6 to 16, comprising applying the trained model to determine a pre-selected subset of said structures from which said selection is to be made in said selecting step.

[0127] 18. The method of any one of clauses 3 to 17, wherein the focus-dependent parameter comprises a critical dimension of each of the first and second features.

[0128] 19. The method of any one of aspects 3 to 18, wherein the simulating step comprises optimizing one or more aspects of the structure to optimize the one or more Persian parameters.

[0129] 20. The method of any one of aspects 3 to 19, wherein the structure comprises a product structure having at least the first feature and the second feature.

[0130] 21. The method according to aspect 20, wherein the simulating step comprises:

[0131] Optimizing one or more aspects of the structure to optimize the one or more Persian parameters, thereby determining an optimized structure; and

[0132] Determine whether the optimized structure will be produced and / or will be printed robustly.

[0133] 22. A method according to any one of clauses 3 to 19, wherein the structure comprises a dedicated focusing target.

[0134] 23. The method of any preceding aspect, wherein the structure comprises a dose-sensitive feature that is not focus-sensitive.

[0135] 24. The method of aspect 23, wherein the simulating step comprises simulating a dose response of the dose-sensitive characteristic.

[0136] 25. The method of any preceding aspect, wherein the structure comprises one or more spatial features having an inverse Persan response.

[0137] 26. A method for measuring focusing performance of a lithographic apparatus, the method comprising:

[0138] (a) obtaining measurement data relating to metrology of a structure selected by a method according to any preceding aspect after the structure has been formed on a substrate; and

[0139] (b) deriving a measure of focusing performance based at least in part on the metrology data and the calibration model.

[0140] 27. The method of clause 26, wherein the measurement data comprises values ​​of Persson differential parameters of the structure formed on the substrate.

[0141] 28. The method of clause 26 or 27, wherein the structure is formed in a scribe line of the substrate.

[0142] 29. The method of clause 26 or 27, wherein the structure is formed in a die region of the substrate.

[0143] 30. A method according to aspect 28 or 29, wherein the structure comprises an actual product structure or a product-like structure.

[0144] 31. A method according to any of clauses 26 to 30, comprising the step of creating the calibration model based on exposure and subsequent measurement of selected structures at different defocus offsets.

[0145] 32. A metrology apparatus for focus performance of a lithographic apparatus, the metrology apparatus being operable to perform a method according to any one of clauses 26 to 31.

[0146] 33. A lithography system comprising:

[0147] A lithographic apparatus, the lithographic apparatus comprising:

[0148] an illumination optical system arranged to illuminate the reflective patterning device;

[0149] a projection optical system arranged to project an image of the patterning device onto a substrate; and

[0150] The measurement device according to aspect 32;

[0151] Wherein the lithographic apparatus is arranged to use measurements of focus performance derived by the metrology apparatus when applying the pattern to a further substrate.

[0152] 34. A lithocell comprising the metrology apparatus according to clause 32 or the lithography system according to clause 33.

[0153] 35. A computer program 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 clauses 1 to 31.

[0154] 36. A method of manufacturing a device, wherein a device pattern is applied to a series of substrates using a photolithographic process, the method comprising:

[0155] measuring the focus performance of the lithography process using a method according to any one of clauses 26 to 31, and

[0156] The lithography process for subsequent substrates is controlled based on the measured focus performance.

[0157] The substrates on which these metrology patterns are formed can be production wafers or experimental wafers in product development. They can also be dedicated metrology wafers, such as monitoring wafers that are processed intermittently as part of an advanced process control (APC) mechanism.

[0158] In association with defining a physical grating structure of the focus metrology pattern implemented on the substrate and pattern forming device, an embodiment may include a computer program comprising one or more sequences of machine-readable instructions describing methods of designing focus metrology patterns, metrology options and / or controlling the inspection apparatus to implement illumination modes and other aspects of those metrology options. This computer program may be executed, for example, in a separate computer system for the design / control process. As mentioned, the unit PU and / or Figure 2 The calculation and control steps are fully or partly performed in the control unit LACU of the computer program. A data storage medium (eg a semiconductor memory, a magnetic disk or an optical disk) in which this computer program is stored may also be provided.

[0159] The terms "radiation" and "beam" as used herein 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 to 20 nm), as well as particle beams, such as ion beams or electron beams.

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

[0161] 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 for selecting a structure for focus monitoring, the method comprising: simulating a focal length-varying Perthson response of a focal length-dependent parameter for a plurality of structures, each of the plurality of structures comprising at least a first feature having a focal length-varying first Perthson curve response for the focal length-dependent parameter and a second feature having a focal length-varying second Perthson curve response for the focal length-dependent parameter; The simulation includes simulating, for each of the plurality of structures, a value of at least a Persian differential parameter based on a relative focus offset between the first Persian curve response that varies with focal length and the second Persian curve response that varies with focal length; and Based on the result of the simulation step, a structure is selected from the plurality of structures for focus monitoring during manufacturing.

2. The method of claim 1, wherein the simulating step is performed using computational lithography simulation.

3. The method of claim 1 , wherein the relative focus offset is determined as a focus difference between corresponding points of the first Persian curve response that varies with focal length and the second Persian curve response that varies with focal length.

4. The method according to claim 1, wherein the selected structure is a structure whose simulated value of the Persian differential parameter exceeds a threshold value.

5. The method of claim 4, wherein the Persian differential parameter comprises a focal length offset between the Persian responses of two of the structures. The method according to claim 4 , wherein the threshold value is 5 nm. The method according to claim 4 , wherein the threshold value is 10 nm.

8. The method according to any one of claims 1 to 7, wherein: The simulating step includes simulating one or more printing qualities, and The selecting step includes selecting the structure based on a co-optimization of the Persian differential parameter and the one or more print qualities.

9. The method according to any one of claims 1 to 7, comprising: The trained model is applied to determine a pre-selected subset of the structures from which the selection is to be made in the selecting step.

10. The method of any one of claims 1 to 7, wherein the structure comprises a product structure having at least the first feature and the second feature.

11. A method for measuring focusing performance of a lithographic apparatus, the method comprising: (a) obtaining measurement data related to metrology of a structure selected by the method according to any one of claims 1 to 10 after the structure is formed on a substrate; as well as (b) deriving a measure of focus performance based at least in part on the metrology data and the calibration model.

12. A metrology apparatus for focus performance of a lithographic apparatus, the metrology apparatus being operable to perform the method of claim 11.

13. 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 11.

14. A method of manufacturing a device, wherein: Applying a device pattern to a series of substrates using a photolithography process, the method comprising: - using the method according to claim 11 to measure the focus performance of the lithography process, and - controlling said lithography process for subsequent substrates depending on the measured focus performance.

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