Method and apparatus for inspecting focus measurements

Abstract

A system and method of measuring a focus position in a metrology system includes introducing an optical element into a measurement arm of the metrology system, the optical element configured and arranged to measure a focus position in the metrology system by adding a positive defocus to a first of at least two different positions in an image plane of the metrology system, and adding negative defocus to a second one of the at least two different positions to modify the point spread function at the at least two different positions; irradiating the substrate comprising the feature to be measured by the metrology system with a measurement radiation beam passing from the substrate to the image plane; and controlling a focus position of the substrate based on a first radiation spot size at a first position at the image plane and a second radiation spot size at a second position at the image plane.

Description

[0001] Cross - reference to related applications

[0002] This application claims priority to U.S. Application No. 63 / 423,214, filed on November 7, 2022, which is incorporated herein by reference in its entirety. Technical field

[0003] The present disclosure generally relates to metrology methods and tools for use in a lithographic apparatus, and more particularly, to methods and systems for identifying an in - focus condition in a microscope of an inspection tool. Background art

[0004] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, typically onto a target portion of the substrate. For example, a lithographic apparatus can be used to manufacture integrated circuits (ICs). In such a case, a patterning 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. Such a pattern can be transferred onto a target portion (e.g., a portion including dies, one die, or several dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is typically effected by imaging onto a layer of radiation - sensitive material (resist) provided on the substrate. Generally, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatuses include so - called steppers, in which each target portion is irradiated by exposing the entire pattern onto the target portion at once; and so - called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the "scanning" direction) while synchronously scanning the substrate parallel or anti - parallel to this direction. The pattern can also be transferred from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0005] After imaging, it can be useful to inspect the exposed substrate to measure one or more properties such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc. If an error is detected, the exposure of one or more subsequent substrates can be adjusted. This can be particularly useful, for example, if the inspection can be performed fast enough such that another substrate of the same batch will still be exposed. In addition, an already - exposed substrate can be stripped and reworked (to increase yield) or discarded, thereby avoiding performing an exposure on a substrate known to be faulty. In the case where only some target portions of the substrate are faulty, further exposure can be performed only on the good target portions. Another possibility is to adjust the settings of subsequent process steps to compensate for the error. For example, the time of a trimming etch step can be adjusted to compensate for CD variations from substrate to substrate caused by the lithography process step. Summary of the invention

[0006] In one aspect, there is a method of measuring a focus position in a metrology system, comprising: introducing an optical element into a measurement arm of the metrology system, the optical element being configured and arranged to modify a point spread function at at least two different positions in an image plane of the metrology system by adding positive defocus to a first position of the at least two different positions and adding negative defocus to a second position of the at least two different positions; irradiating the substrate including features to be measured by the metrology system with a measurement radiation beam transmitted from the substrate to the image plane; and controlling a focus position of the substrate based on a first radiation spot characteristic at the first position in the image plane and a second radiation spot characteristic at the second position in the image plane.

[0007] In one aspect, the first radiation spot characteristic is a first radiation spot size, and the second radiation spot characteristic is a second radiation spot size, and controlling the focus position of the substrate includes: changing the focus position of the substrate until the first radiation spot size and the second radiation spot size are equal.

[0008] In one aspect, the first radiation spot characteristic is a first intensity, and the second radiation spot characteristic is a second intensity, and controlling the focus position of the substrate includes: changing the focus position of the substrate until the first intensity and the second intensity are equal.

[0009] In one aspect, a method of measuring a focus position in a metrology system, wherein the method includes: introducing an optical element into a measurement arm of the metrology system, the optical element being configured and arranged to modify a point spread function at at least two different positions in an image plane of the metrology system by adding positive defocus to a first position of the at least two different positions and adding negative defocus to a second position of the at least two different positions; irradiating the substrate including features to be measured by the metrology system with a measurement radiation beam transmitted from the substrate to the image plane, the irradiation pattern of the measurement beam having a plurality of individual sources; comparing a plurality of radiation spot characteristics at the first position in the image plane with a second radiation spot characteristic at the second position in the image plane; and changing the focus position of the substrate until the first radiation spot characteristic is equal to the second radiation spot characteristic.

[0010] In one aspect, there is a system configured to perform the methods described herein.

[0011] In one aspect, there is provided a non - transitory computer program product including machine - readable instructions configured to cause a processor to perform the methods described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments will now be described by way of example only with reference to the drawings, in which:

[0013] Figure 1 Schematically depicts an embodiment of a lithography unit or a lithography cluster;

[0014] Figure 2 Schematically depicts an embodiment of a scatterometer used as a metrology device;

[0015] Figure 3 Schematically depicts another embodiment of a scatterometer used as a metrology device;

[0016] Figure 4 Depicts a composite metrology target formed on a substrate;

[0017] Figure 5A Is a schematic representation of a prior art metrology system, and Figure 5B Is a schematic representation of an embodiment;

[0018] Figure 6 Schematically shows an embodiment of a device for determining focus according to an embodiment;

[0019] Figure 7 Is an example of a focus / defocus curve;

[0020] Figure 8 Is an example of an optical element according to an embodiment;

[0021] Figure 9 Is a schematic diagram of a pinhole source according to an embodiment;

[0022] Figure 10 Shows a point spread function generated by a pinhole source;

[0023] Figure 11 Shows the effect of introducing a phase plate to modify Figure 10 the point spread function; and

[0024] Figure 12A and Figure 12B Shows the correspondence between defocus and intensity (12A) and between defocus and width (12B). Detailed Description

[0025] Before describing the embodiments in detail, it is beneficial to present an example environment in which the embodiments can be implemented.

[0026] As Figure 1As shown, the lithographic apparatus LA can form part of a lithographic cell LC (sometimes also referred to as a lithographic cell or lithographic cluster), which also includes equipment for performing one or more pre- and post-exposure processes on a substrate. Conventionally, this equipment includes one or more spin coaters SC for depositing a resist layer, one or more developers DE for developing the exposed resist, one or more chill plates CH, and / or one or more bake plates BK. A substrate transport device or robot RO picks up substrates from the input / output ports I / O1, I / O2, moves the substrates between different process devices, and transfers the substrates to the feed table LB of the lithographic apparatus. These devices, often collectively referred to as the track, are under the control of a track control unit TCU, which is self-controlled via a management control system SCS, and the management control system SCS also controls the lithographic apparatus via a lithography control unit LACU. Thus, different devices can be operated to maximize throughput and processing efficiency.

[0027] In order for the substrates exposed by the lithographic apparatus LA to be exposed correctly and consistently, it is desirable to inspect the exposed substrates to measure one or more properties, such as the overlay error between layers before and after, line thickness, critical dimension (CD), etc. If an error is detected, the exposure of one or more subsequent substrates can be adjusted. This can be particularly useful, for example, if the inspection can be carried out quickly enough such that another substrate of the same batch will still be exposed. In addition, the exposed substrates can be stripped and reworked (to increase throughput) or discarded, thus avoiding performing an exposure on a substrate known to be faulty. In the case where only some target parts of the substrate are faulty, further exposure can be performed only on those good target parts. Another possibility is to adjust the settings of subsequent process steps to compensate for the error. For example, the time of a trimming etch step can be adjusted to compensate for the CD variation from substrate to substrate caused by the lithography process step.

[0028] In an embodiment, the patterning device MA can be provided with a functional pattern (i.e., a pattern that will form part of an operating device). Alternatively or additionally, the patterning device can be provided with a measurement pattern that does not form part of the functional pattern. The measurement pattern can be located, for example, on one side of the functional pattern. The measurement pattern can be used, for example, to measure the alignment of the patterning device relative to the substrate table WT of the lithographic apparatus, or can be used to measure some other parameter (such as overlay). The techniques described herein can be applied to this measurement pattern.

[0029] According to various embodiments of the present invention, measured or simulated wafer features and lithography equipment properties can be used to update the design of the mask to improve performance. In one example, the position of the measurement target (measurement pattern) can be located according to the measured and / or simulated features of the wafer so that the effects of the wafer features and equipment properties are reduced. Alternatively, similar features of the wafer and / or lithography system can be used to update the position and / or orientation of the functional pattern.

[0030] By way of introduction, the operation of an inspection apparatus utilizing a measurement target is described. The inspection apparatus is used to determine one or more properties of a substrate, and in particular, to determine how one or more properties of different substrates or different layers of the same substrate vary between layers and / or across substrates. The inspection apparatus may be integrated into a lithographic apparatus LA or a lithographic cell LC, or may be a stand-alone device. In order to achieve the fastest measurement, it is desirable that the inspection apparatus measures one or more properties in an 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 portion of the resist that has been exposed to radiation and the portion of the resist that has not been exposed to radiation), and not all inspection apparatus have sufficient sensitivity to make useful measurements of the latent image. Therefore, the measurement may be made after a post-exposure bake step (PEB), which is typically the first step performed on the exposed substrate and increases the contrast between the exposed and unexposed portions of the resist. At this stage, the image in the resist may be referred to as a semi-latent image. Measurements can also be made on a developed resist image (when either the exposed or unexposed portions of the resist have been removed), or after a pattern transfer step such as etching. The latter may limit the possibility of reworking defective substrates, but can still provide useful information, for example for process control purposes.

[0031] Figure 2 An embodiment of a scatterometer SM1 is depicted. The scatterometer or scatterometer comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate 6. The reflected radiation is passed to a spectrometer detector 4 which measures the spectrum 10 of the specularly reflected radiation (i.e. a measure of the intensity as a function of wavelength). From this data, the structure or profile resulting from the detected spectrum can be reconstructed by a processing unit PU, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with e.g. Figure 3 A library of simulated spectra is shown at the bottom for comparison. Typically, for the reconstruction, the general form of the structure is known and some parameters are set from knowledge of the process by which the structure was made, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as either a normal incidence scatterometer or an oblique incidence scatterometer.

[0032] Figure 3Another embodiment of the scatterometer SM2 is shown. In this device, the radiation emitted by the radiation source 2 is focused using the lens system 12 through the interference filter 13 and the polarizer 17, reflected by the partially reflective surface 16, and focused onto the substrate W via the microscope objective 15, which has a high numerical aperture (NA), desirably at least 0.9 or at least 0.95. An immersion scatterometer may even have a lens with a numerical aperture greater than 1. Then, the reflected radiation passes through the partially reflective surface 16 and enters the detector 18 to detect the scattering spectrum. The detector may be located in the back-projected pupil plane 11 at the focal length of the lens 15. However, the pupil plane may alternatively be re-imaged onto the detector 18 using auxiliary optics (not shown). The pupil plane is the plane in which the radial position of the radiation defines the angle of incidence and the angular position defines the azimuthal angle of the radiation. The detector is desirably a two-dimensional detector such that the two-dimensional angular scattering spectrum of the substrate target (i.e., the measurement of the intensity as a function of the scattering angle) can be measured. The detector 18 may be an array of, for example, a CCD or CMOS sensor and may have an integration time of, for example, 40 milliseconds per frame. In addition to or instead of specular radiation, non-specular radiation (i.e., diffraction orders with a magnitude of ±1 or higher) may be used.

[0033] A reference beam is typically used, for example, to measure the intensity of the incident radiation. For this purpose, when the radiation beam is incident on the partially reflective surface 16, a portion of the radiation beam will pass through the surface as a reference beam towards the reference mirror 14. Then this reference beam is projected onto a different portion of the same detector 18.

[0034] One or more interference filters 13 may be used to select wavelengths of interest in a range such as 405 nm - 790 nm or even lower (such as 200 nm - 300 nm). The (multiple) interference filters may be tunable instead of including different sets of filters. A grating may be used instead of or in addition to one or more interference filters.

[0035] The detector 18 may measure the intensity of the scattered radiation at a single wavelength (or narrow wavelength range), the individual intensities at multiple wavelengths, or the intensity integrated over a wavelength range. In addition, the detector may separately measure the intensities of the transverse magnetic (TM) polarized radiation and the transverse electric (TE) polarized radiation, and / or the phase difference between the transverse magnetic polarized radiation and the transverse electric polarized radiation.

[0036] It is possible to use a broadband radiation source 2 (i.e., a radiation source having a wide range of radiation frequencies or wavelengths and thus having color), which gives a large light collection rate, allowing the mixing of multiple wavelengths. The multiple wavelengths in the broadband ideally each have a bandwidth of δλ and a separation of at least 2δλ (i.e., twice the wavelength bandwidth). Several "sources" of the radiation can be different parts of an extended radiation source that have been separated using, for example, an optical fiber bundle. In this way, the angular resolved scattering spectrum can be measured at multiple wavelengths in parallel. A 3-D spectrum (wavelength and two different angles) can be measured, which includes more information than a 2-D spectrum. This allows more information to be measured, thus increasing the robustness of the metrology process. This is described in more detail in U.S. Patent Application Publication No. US2006-0066855, which is incorporated herein by reference in its entirety.

[0037] By comparing one or more properties of the beam before and after being redirected by the target, one or more properties of the substrate can be determined. For example, this can be done by comparing the redirected beam with a theoretical redirected beam calculated using a model of the substrate and searching for the model that gives the best fit between the measured redirected beam and the calculated redirected beam. Typically, a parametric general model is used, and the parameters of the model (e.g., width, height, and sidewall angle of the pattern) are varied until the best match is obtained.

[0038] Two main types of scatterometers are used. A spectroscopic scatterometer directs a broadband radiation beam onto the substrate and measures the spectrum (intensity as a function of wavelength) of the radiation scattered into a specific narrow angular range. An angular resolved scatterometer uses a monochromatic radiation beam and measures the intensity of the scattered radiation (or intensity ratio and phase difference in the case of an ellipsometric configuration) as a function of angle. Alternatively, the measurement signals at different wavelengths can be measured separately and combined at the analysis stage. Polarized radiation can be used to generate more than one spectrum from the same substrate.

[0039] To determine one or more parameters of the substrate, typically a best match is found between a theoretical spectrum generated from a model of the substrate and a measured spectrum generated by the redirected beam as a function of wavelength (spectroscopic scatterometer) or angle (angular resolved scatterometer). To find the best match, there are various methods that can be combined. For example, the first method is an iterative search method, where a first set of model parameters is used to calculate a first spectrum, which is compared with the measured spectrum. Then a second set of model parameters is selected, a second spectrum is calculated, and the second spectrum is compared with the measured spectrum. These steps are repeated with the aim of finding the set of parameters that gives the best matching spectrum. Typically, the information from the comparison is used to guide the selection of subsequent sets of parameters. This process is called an iterative search technique. The model with the set of parameters that gives the best match is considered the best description of the measured substrate.

[0040] A second method is to create a spectral library, with each spectrum corresponding to a particular set of model parameters. Typically, the set of model parameters is chosen to cover all or nearly all possible variations in the substrate properties. The measured spectrum is compared with the spectra in the library. Similar to the iterative search method, the model with the set of parameters corresponding to the spectrum that gives the best match is considered to be the best description of the measured substrate. In this library search technique, interpolation or interpolation techniques can be used to more accurately determine the best set of parameters.

[0041] In any method, a sufficient number of data points (wavelengths and / or angles) in the calculated spectrum should be used to achieve an accurate match, typically between 80 and 800 or more data points per spectrum. Using an iterative method, each iteration of each parameter value will involve calculations at 80 or more data points. This is multiplied by the number of iterations required to obtain the correct profile parameters. Thus, many calculations may be required. In practice, this results in a trade-off between accuracy and processing speed. In the library method, a similar trade-off exists between accuracy and the time required to set up the library.

[0042] In any of the above-described scatterometers, the target on the substrate W can be a grating that is printed such that after development, the bars are formed of solid resist lines. Alternatively, the bars can be etched into the substrate. The target pattern is chosen to be sensitive to parameters of interest, such as focus, dose, overlay, chromatic aberration in a lithographic projection apparatus, etc., such that changes in the relevant parameters will manifest as changes in the printed target. For example, the target pattern may be sensitive to chromatic aberration in a lithographic projection apparatus (especially the projection system PL), and the illumination symmetry and the presence of such aberrations will manifest in changes in the printed target pattern. Thus, the scatterometry measurement data of the printed target pattern is used to reconstruct the target pattern. Parameters of the target pattern, such as line width and shape, can be input into the reconstruction process performed by the processing unit PU based on knowledge of the printing step and / or other scatterometry processes. The lines in the target can be composed of subunits that include near-feature or sub-resolution features that together define the lines of the grating, such as those described in US Patent No. 7,466,413.

[0043] Although embodiments of scatterometers have been described herein, other types of metrology devices can be used in the embodiments. For example, a dark-field metrology device such as that described in US Patent No. 8,797,554, which is incorporated herein by reference in its entirety, can be used. Additionally, those other types of metrology devices can use techniques that are completely different from scatterometry.

[0044] For example, the targets described herein can be overlay targets designed for use with Yieldstar stand-alone or integrated metrology tools, and / or alignment targets (such as those commonly used with TwinScan lithography systems), both of which are available from ASML in Veldhoven, the Netherlands.

[0045] Typically, metrology targets used with such systems should be printed on the wafer such that their dimensions meet the design specifications of the particular microelectronic devices to be imaged on the wafer. As the process continues to push the limits of the imaging resolution of lithography equipment into advanced process nodes, design rules and process compatibility requirements place pressure on the selection of appropriate targets. As the targets themselves become more advanced, the use of resolution enhancement techniques (e.g., phase-shift masks and optical proximity correction) is often required, and the printability of targets within the process design rules becomes less certain. Thus, the proposed markers can be tested to confirm their feasibility from the perspective of printability and detectability. In a commercial environment, good overlay marker detectability can be considered a combination of low total measurement uncertainty and short move-acquire-move times, as slow acquisition is detrimental to the overall throughput of the production line. Modern micro-diffraction-based overlay targets (μDBO) can be on the order of about 10 μm on one side, which provides an inherently lower detection signal compared to 2 targets (such as those used in the context of monitoring wafers).

[0046] In addition, once markers meeting the above criteria are selected, there is a possibility that the detectability will vary with process variations (such as film thickness variations, various etch biases, and geometric asymmetries caused by etching and / or polishing processes). Thus, it can be useful to select targets with low detectability variation and low overlay / alignment variation for various process variations. Similarly, the fingerprint (printing characteristics, including, for example, lens aberrations) of the particular machine to be used to produce the microelectronic devices to be imaged will typically affect the imaging and generation of the target markers. Thus, it may be useful to ensure that the markers are fingerprint-resistant, as some patterns will be more or less affected by a particular lithography fingerprint.

[0047] Figure 4Depict a composite metrology target formed on a substrate according to known practice. The composite target includes four gratings 32, 33, 34, 35 that are closely positioned together such that they will all be within the measurement radiation spot 31 formed by the illumination beam of the metrology device. Thus, all four targets are illuminated simultaneously and imaged simultaneously on sensors 4, 18. In an example dedicated to overlay measurement, the gratings 32, 33, 34, 35 themselves are composite gratings formed by overlapping gratings patterned in different layers of a semiconductor device formed on a substrate W. The gratings 32, 33, 34, 35 can have different offset overlaps to facilitate measurement of the overlay between the multiple layers in which the composite grating is formed. The orientations of the gratings 32, 33, 34, 35 can also be different, as shown, to diffract the incident radiation in the X and Y directions. In one example, gratings 32 and 34 are X-direction gratings with offsets of +d and -d, respectively. This means that the overlapping components of grating 32 are arranged such that if all the overlapping components were printed exactly at their nominal positions, one of the components would be offset by a distance d relative to the other. The components of grating 34 are arranged such that if printed perfectly, there should be an offset d, but in the direction opposite to the first grating, and so on. Gratings 33 and 35 can be Y-direction gratings with offsets of +d and -d, respectively. Although four gratings are shown, another embodiment can include a larger matrix to obtain the desired accuracy. For example, a 3×3 array of nine composite gratings can have offsets of -4d, -3d, -2d, -d, 0, +d, +2d, +3d, +4d. The individual images of these gratings can be identified in the images captured by sensors 4, 18.

[0048] As will be appreciated, such a metrology device itself should be appropriately focused to ensure accurate measurement. To this end, Figure 5A The conventional system schematically shown in includes several optical branches. The substrate W is located in the field of view of the microscope objective 15. The first branch of the system includes the detector 18 and associated optics, which together form the detection branch 40 of the system, which can be, for example, a diffraction-based optical detection branch. The substrate W is illuminated by an illumination branch 42 that includes an illumination source 44. An additional light source 46 provides focused measurement illumination in the focusing branch 48, and the alignment branch 50 includes an alignment camera 52 and its own light source (not shown).

[0049] Figure 5B A metrology system according to an embodiment is schematically shown. In this embodiment, the modification of the detection branch 40' of the system allows the focusing branch 48 to be completely omitted.

[0050] Figure 6 is a schematic diagram of an embodiment of the detection branch 40' of the system. As Figure 5BAs shown, there is a substrate W and a detector 18 at the image plane of the metrology optical system 70. Note that although the schematic diagram only shows a single optical element of the metrology optical system 70, in reality there will be several optical elements that make up the metrology optical system and perform various functions. In addition to the objective lens 15 (which itself can be more complex than the single lens schematically shown), these optical elements can include, for example, relay lenses, condensers, collimators, prisms and / or folding mirrors and semi-reflecting mirrors, projection lenses, etc.

[0051] In some embodiments, an orthogonal wedge optical element (quad-wedge optical element) 72 is included in the metrology optical system 70. This element has the function of spatially separating the diffraction orders received from the substrate at the image plane (detector 18). That is, due to the presence of the quad-wedge optical element, there will be four separate quadrants of the image projected onto the detector 18 (only Q 1 and Q 2 are labeled in the figure).

[0052] Due to this spatial separation at the image plane, the focal length of the metrology optical system 70 may be affected in different ways for each quadrant at the detector 18. In this regard, an optical element 74 is arranged in the metrology optical system 70, which is configured to have a first part 76 and a second part 78. The first part 76 provides positive defocus to the image in the first quadrant Q 1 (i.e., at the first position), and the second part 78 provides negative defocus to the image in the second quadrant Q 2 (i.e., at the second position). This is further shown in Figure 7 , Figure 7 showing Q 1 under positive defocus conditions. That is, the best focus plane (shown as the peak in the focus signal) is to the right of the image plane in the figure. Similarly, Q 2 is under negative defocus conditions, where the best focus plane (maximum amplitude, even though it is a minimum) is to the left of the image plane.

[0053] In an embodiment, the optical element 74 can be moved into and out of the metrology optical system 70, for example, by an actuator. For example, the optical element can be rotated into and out of the optical path, or be slidably actuated into and out of the optical path. In an embodiment, the optical element 74 can be a lens element that can be introduced at or near the pupil plane (or its conjugate plane) of the metrology optical system 70. In an embodiment, the optical element is a lens element as shown in Figure 8 , including a first part 76 and a second part 78. As will be seen in this example, the same defocus is applied to two quadrants, i.e., quadrants Q 1 and Q 3 together, and quadrants Q 2 and Q4 Together. For example, the optical element 74 can be manufactured by combining two lenses, and then the resulting optical element 74 can be moved into and out of the optical path of the metrology optical system 70.

[0054] In operation, the optical element 74 is introduced into the measurement arm of the metrology system (i.e., the metrology optical system 70). As described above, the optical element is configured and arranged at an appropriate position in the metrology optical system 70 to modify its point spread function for at least two different positions by adding positive defocus to a first position among at least two different positions in the image plane of the metrology system and adding negative defocus to a second position among the at least two different positions. Then, the substrate W is irradiated with a measurement radiation beam transferred from the substrate to the image plane to measure surface features thereon to be measured by the metrology system.

[0055] The first radiation spot size at the image plane at the first position (in the first quadrant) is compared with the second radiation spot size at the image plane at the second position (in the second quadrant). If the radiation spot sizes are not equal, the focus position of the substrate is changed until they are equal. As will be understood, more generally, other radiation spot characteristics other than the radiation spot size can be used in the method according to the embodiment. For example, the radiation spot intensity at the measurement point can be used, and the equality of the measured intensities corresponds to the position of best focus.

[0056] In fact, the radiation spot sizes or intensities may never be exactly equal, such that a selected similarity can be specified for a particular substrate pattern. For example, the specified similarity can be expressed as a percentage and can constitute a difference between 0.001% and 0.1%, 1%, or up to 10% or any range between these values. This is because the difference in radiation spot characteristics varies significantly with the aperture of the objective lens 15, the radiation spot size, and the magnification between the substrate and the image plane. Therefore, the radiation spot characteristics should be equal within the system specifications, and the radiation spot characteristics may vary depending on the particular substrate pattern to be measured.

[0057] In Figure 9 In the illustrated embodiment, the aperture 80 having an array of pinholes 82 is used to generate an array of light sources. Each pinhole 82 is used to approximate a point source at a given object field point. As Figure 11 shown, each point source corresponds to a corresponding point spread function 84 at the image plane (detector 18). The point spread function at each point depends on the optical behavior of the optical system between the source and the image plane.

[0058] In this embodiment, the phase plate 86 ( Figure 11is introduced into the optical system to modify the point spread function at the image plane. For each of a plurality of sub-regions of the image plane, the phase plate introduces positive defocus, negative defocus, or no defocus. In regions experiencing positive or negative defocus, the point spread function will decrease in peak intensity and become broader compared to those regions without defocus. The phase plate can be a static optical element that can be moved into and out of the optical path, for example, with an electromechanical stage with one degree of freedom. A transparent spatial light modulator (SLM) can also be used for this purpose.

[0059] The resulting point spread function can be plotted for defocus to determine the focus position. Figure 12A and 12B illustrates the concept.

[0060] In Figure 12A , a curve of the intensity (I) of the point spread function versus the defocus (D) caused by the phase plate is depicted. In Figure 12B , a curve of the full width at half maximum (W) of the point spread function versus the defocus (D) caused by the phase plate is depicted. In each case, defocus at the wafer tends to shift the plotted curves to the left and right.

[0061] Generally, the phase plate 86 should be placed near the image plane of the metrology optical system 70. In this regard, the phase plate can be near an intermediate image plane within the metrology optical system 70, or near the sensor image plane. Since these image planes are conjugate, the phase plate 86 has the same effect at either location. The phase plate 86 can be placed on either side of the wedge optics 72.

[0062] If the phase plate 86 is placed precisely at the image plane, any optical defects (such as surface scratches or debris) will appear in the image at the sensor, which can reduce the measured S / N ratio. If the phase plate is far from the image plane and close to, for example, the pupil plane, the phase shift will not correspond well to a particular PSF and a particular pinhole. Generally, the maximum distance between the phase plate 86 and the image plane depends on the pinhole size and the distance between the pinholes. In cases where there are fewer points and more space between the points, the phase plate can be placed further from the image plane.

[0063] When the phase plate 86 is downstream of the wedge optics 72, the magnification of the image of the radiation spot increases. Additionally, this location can allow different defocus to be applied to the light from the radiation spot when a single radiation spot is imaged into multiple quadrants.

[0064] The phase plate 86 itself can be embodied as a grid of sub-regions in a machined optical material, or presented as an SLM device. In an embodiment, the phase plate can be rotationally symmetric and fabricated from standard optical glass or polymer.

[0065] In an embodiment, illumination mode selection is used at a radiation spot size selector of an optical system to generate a pinhole.

[0066] For a spatial image, the contrast discussed herein includes the image log slope (ILS) and / or the normalized image log slope (NILS), and for a resist, the contrast discussed herein includes the dose sensitivity and / or the exposure latitude.

[0067] As used herein, the terms "optimize", "optimizing", and "optimization" mean adjusting lithography process parameters such that the result and / or process of lithography has more desirable characteristics, such as higher accuracy of the projection of a design layout on a substrate, a larger process window, etc.

[0068] Embodiments of the present invention may take the form of a computer program comprising one or more sequences of machine-readable instructions that describe a method as disclosed herein, or the form of a data storage medium (e.g., a semiconductor memory, a magnetic disk, or an optical disc) in which such a computer program is stored. Additionally, the machine-readable instructions may be included in two or more computer programs. The two or more computer programs may be stored on one or more different memories and / or data storage media.

[0069] For example, the computer program may be included within Figure 1 an inspection device and / or within a control unit LACU, or Figure 1 the inspection device and / or the control unit LACU may include a computer program. In cases where existing devices (e.g., Figure 2 and Figure 3 devices of the type shown) are already in production and / or use, embodiments may be implemented by providing an updated computer program product for causing a processor of the device to execute the method as described herein.

[0070] When one or more computer programs are read by one or more computer processors located within at least one component of an inspection device, any of the controllers described herein may operate individually or in combination. The controllers may each or in combination have any suitable configuration for receiving, processing, and transmitting signals. One or more processors are configured to communicate with at least one controller. For example, each controller may include one or more processors for executing a computer program comprising machine-readable instructions for the above-described method. The controller may include a data storage medium for storing such a computer program and / or hardware for receiving such a medium. Thus, the (s) controller(s) may operate in accordance with the machine-readable instructions of one or more computer programs.

[0071] Embodiments may be further described using the following items:

[0072] 1. A method for measuring a focus position in a metrology system, the method comprising:

[0073] introducing an optical element into a measurement arm of the metrology system, the optical element being configured and arranged to modify a point spread function at at least two different positions in an image plane of the metrology system by adding positive defocus to a first position of the at least two different positions and adding negative defocus to a second position of the at least two different positions;

[0074] irradiating the substrate including features to be measured by the metrology system with a measurement radiation beam transmitted from the substrate to the image plane; and

[0075] controlling a focus position of the substrate based on a first radiation spot characteristic at the first position in the image plane and a second radiation spot characteristic at the second position in the image plane.

[0076] 2. The method according to item 1, wherein the first radiation spot characteristic is a first radiation spot size and the second radiation spot characteristic is a second radiation spot size, and wherein controlling the focus position of the substrate includes: changing the focus position of the substrate until the first radiation spot size and the second radiation spot size are equal.

[0077] 3. The method according to item 1, wherein the first radiation spot characteristic is a first intensity and the second radiation spot characteristic is a second intensity, and wherein controlling the focus position of the substrate includes: changing the focus position of the substrate until the first intensity and the second intensity are equal.

[0078] 4. The method according to any one of items 1-3, wherein the optical element is a lens element introduced adjacent to a pupil plane of the metrology system, and wherein the lens element has a first portion that causes positive defocus at the first position and a second portion that causes negative defocus at the second position.

[0079] 5. The method according to any one of the preceding items, wherein the measurement arm of the metrology system includes a four-wedge optical element adjacent to a pupil plane of the metrology system.

[0080] 6. The method according to any one of items 1-3, wherein the optical element is a phase plate having a first portion that causes positive defocus at the first position and a second portion that causes negative defocus at the second position.

[0081] 7. The method according to item 6, wherein the phase plate is located at a position adjacent to a pupil plane of the metrology system.

[0082] 8. The method according to item 6, wherein the phase plate is located at a position adjacent to the image plane of the measurement system.

[0083] 9. The method according to any one of items 6-8, further comprising: placing a hole array at the object plane of the measurement system.

[0084] 10. The method according to item 9, wherein each hole in the hole array corresponds to a respective position in the image plane of the measurement system.

[0085] 11. The method according to any one of items 6-8, wherein the image plane includes an intermediate image plane.

[0086] 12. The method according to any one of items 6-8, wherein the image plane includes a sensor image plane.

[0087] 13. The method according to any one of the foregoing items, wherein the measurement arm of the measurement system includes a dark-field microscope camera measurement arm.

[0088] 14. The method according to any one of the foregoing items, wherein the optical element includes an optical element selected from the group consisting of: machined optical material, a spatial light modulator device, and a deformable mirror.

[0089] 15. A method for measuring a focus position in a measurement system, wherein the method comprises:

[0090] introducing an optical element into the measurement arm of the measurement system, the optical element being configured and arranged to modify the point spread function at at least two different positions in the image plane of the measurement system by adding positive defocus to a first position of the at least two different positions and adding negative defocus to a second position of the at least two different positions;

[0091] irradiating the substrate including the feature to be measured by the measurement system with a measurement radiation beam transferred from the substrate to the image plane, the irradiation pattern of the measurement beam having a plurality of individual sources;

[0092] comparing a plurality of radiation spot characteristics at the first position at the image plane with second radiation spot characteristics at the second position at the image plane; and

[0093] changing the focus position of the substrate until the first radiation spot characteristics are equal to the second radiation spot characteristics.

[0094] 16. A system for measuring a focus position in a measurement system, the system comprising:

[0095] An optical element that is movable into and out of a measurement arm of the metrology system, the optical element being configured and arranged to modify the point spread function at at least two different locations in the image plane of the metrology system by adding positive defocus to a first location of the at least two different locations and adding negative defocus to a second location of the at least two different locations;

[0096] An illumination source configured to illuminate the substrate including features to be measured by the metrology system with a measurement radiation beam transmitted from the substrate to the image plane; and

[0097] A focus position controller configured to control the focus position of the substrate based on a first radiation spot characteristic at the first location in the image plane and a second radiation spot characteristic at the second location in the image plane.

[0098] 17. The system according to item 16, wherein the first radiation spot characteristic is a first radiation spot size and the second radiation spot characteristic is a second radiation spot size, and wherein the focus position controller is configured to change the focus position of the substrate until the first radiation spot size and the second radiation spot size are equal.

[0099] 18. The system according to item 16, wherein the first radiation spot characteristic is a first intensity and the second radiation spot characteristic is a second intensity, and wherein the focus position controller is configured to change the focus position of the substrate until the first intensity and the second intensity are equal.

[0100] 19. The system according to any one of items 16 - 18, wherein the optical element is a lens element movable into and out of a position adjacent the pupil plane of the metrology system, and wherein the lens element has a first portion that causes the positive defocus at the first location and a second portion that causes the negative defocus at the second location.

[0101] 20. The system according to any one of items 16 - 18, wherein the optical element is a phase plate having a first portion that causes positive defocus at the first location and a second portion that causes negative defocus at the second location.

[0102] 21. The system according to item 20, wherein the phase plate is movable into and out of a position adjacent the pupil plane of the metrology system.

[0103] 22. The system according to item 20, wherein the phase plate is movable into and out of a position adjacent the image plane of the metrology system.

[0104] Although embodiments may have been specifically referred to above in the context of lithography using radiation, it should be understood that embodiments of the invention can be used in other applications (e.g., imprint lithography), and, where the context allows, are not limited to lithography using radiation. In imprint lithography, the topography of the patterning device defines the pattern formed on the substrate. The topography of the patterning device can be pressed into a resist layer provided to the substrate and then the resist is cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. After the resist is cured, the patterning device is removed from the resist, leaving a pattern therein.

[0105] Although in this text the lithographic apparatus may be specifically referred to in the context of use in IC manufacture, it should be understood that the lithographic apparatus described herein may have other applications such as the manufacture of integrated optical systems, the guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms "substrate" or "die" herein may be considered to be synonymous with the more general terms "substrate" or "target portion", respectively. The substrates referred to herein may be processed, before or after exposure, in, for example, a track (a tool generally used to apply a resist layer to a substrate and to develop the exposed resist), a metrology tool, and / or an inspection tool. Where applicable, the disclosure herein can be applied to these and other substrate processing tools. In addition, the substrate may be processed more than once, for example, in order to form a multi-layer IC, so that the term substrate as used herein may also refer to a substrate that already contains multiple processed layers.

[0106] The patterning device described herein may be referred to as a lithographic patterning device. Thus, the term "lithographic patterning device" can be construed to mean a patterning device suitable for a lithographic apparatus.

[0107] 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); and particle beams (such as ion beams or electron beams).

[0108] The term "lens" can refer to any optical component or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic types of optical components, where the context allows.

[0109] The described embodiments and references in the specification to "embodiments", "examples", etc. indicate that the described embodiments may include specific features, structures, or characteristics, but each embodiment may not necessarily include the specific features, structures, or characteristics. Moreover, such phrases do not necessarily refer to the same embodiment. Further, when a specific feature, structure, or characteristic is described in connection with an embodiment, it should be understood that implementing such feature, structure, or characteristic in connection with other embodiments, whether or not explicitly described, is within the knowledge of those skilled in the art.

[0110] The foregoing description is intended to be illustrative, not limiting. Accordingly, those skilled in the art will understand that the described invention may be modified without departing from the scope of the claims set forth below. For example, in appropriate circumstances, one or more aspects of one or more embodiments may be combined with one or more aspects of one or more other embodiments, or replaced by one or more aspects of one or more other embodiments. Accordingly, such adaptations and modifications are intended to be within the meaning and scope of the equivalents of the disclosed embodiments based on the teachings and guidance presented herein. It should be understood that the language or terminology herein is for the purpose of description by way of example and not of limitation, such that the terminology or language of this specification will be interpreted by those skilled in the art in light of the teachings and guidance. The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the appended claims and their equivalents.

Claims

1. A method for measuring a focus position in a metrology system, wherein the method comprises: introducing an optical element into a measurement arm of the metrology system, the optical element being configured and arranged to modify a point spread function at at least two different positions in an image plane of the metrology system by adding positive defocus to a first position among the at least two different positions and adding negative defocus to a second position among the at least two different positions; irradiating the substrate including features to be measured by the metrology system with a measurement radiation beam transmitted from the substrate to the image plane; and controlling a focus position of the substrate based on a first radiation spot characteristic at the first position in the image plane and a second radiation spot characteristic at the second position in the image plane.

2. The method according to claim 1, wherein the first radiation spot characteristic is a first radiation spot size, and the second radiation spot characteristic is a second radiation spot size, and wherein controlling the focus position of the substrate comprises: changing the focus position of the substrate until the first radiation spot size and the second radiation spot size are equal.

3. The method according to claim 1, wherein the first radiation spot characteristic is a first intensity, and the second radiation spot characteristic is a second intensity, and wherein controlling the focus position of the substrate comprises: changing the focus position of the substrate until the first intensity and the second intensity are equal.

4. The method according to any one of claims 1-3, wherein the optical element is a lens element introduced adjacent to a pupil plane of the metrology system, and wherein the lens element has a first portion that causes positive defocus at the first position and a second portion that causes negative defocus at the second position.

5. The method according to any one of the preceding claims, wherein the measurement arm of the metrology system comprises a four-wedge optical element adjacent to the pupil plane of the metrology system.

6. The method according to any one of claims 1-3, wherein the optical element is a phase plate, the phase plate having a first portion that causes positive defocus at the first position and a second portion that causes negative defocus at the second position.

7. The method according to claim 6, wherein the phase plate is located at a position adjacent to the pupil plane of the metrology system.

8. The method according to claim 6, wherein the phase plate is located at a position adjacent to the image plane of the metrology system.

9. The method according to any one of claims 6-8, further comprising placing an array of holes at an object plane of the metrology system.

10. The method according to claim 9, wherein each hole in the array of holes corresponds to a respective position in the image plane of the metrology system.

11. The method according to any one of claims 6-8, wherein the image plane includes an intermediate image plane.

12. The method according to any one of claims 6-8, wherein the image plane includes a sensor image plane.

13. The method according to any one of the preceding claims, wherein the measurement arm of the metrology system comprises a dark field microscope camera measurement arm.

14. The method according to any one of the preceding claims, wherein the optical element comprises an optical element selected from the group consisting of: a machined optical material, a spatial light modulator device, and a deformable mirror.

15. A method of measuring a focus position in a metrology system, wherein the method comprises: introducing an optical element into a measurement arm of the metrology system, the optical element being configured and arranged to modify a point spread function at at least two different locations in an image plane of the metrology system by adding positive defocus to a first one of the at least two different locations and adding negative defocus to a second one of the at least two different locations; irradiating the substrate comprising features to be measured by the metrology system with a measurement radiation beam transmitted from the substrate to the image plane, the irradiation pattern of the measurement beam having a plurality of individual sources; comparing a plurality of radiation spot characteristics at the first location in the image plane with second radiation spot characteristics at the second location in the image plane; and changing a focus position of the substrate until the first radiation spot characteristics are equal to the second radiation spot characteristics.

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