Off-axis illumination overlay metrology using two diffraction order imaging

Abstract

To provide a method for a measurement and a tool which can increase the accuracy of a measurement, can simplify a measurement process, and can improve the correspondence between a measurement target and a semiconductor device.SOLUTION: In the method, illuminating a target according to a Littrow configuration brings a first measurement signal including a minus-primary diffraction amount and a zero-order diffraction amount and a second measurement signal including a plus-primary diffraction amount and a zero-order diffraction amount in a manner in which the minus-primary diffraction amount of the first measurement signal and a plus-primary diffraction amount of the second measurement signal are diffracted by 180-degrees with reference to an illumination direction. A first measurement for the first measurement signal and a second measurement for the second measurement signal are performed and a measurement index (group) is derived on the basis of the measurements.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to the field of metrology, and more particularly to illumination and measurement configurations that enhance measurement accuracy. [Background technology]

[0002] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of U.S. Provisional Patent Application No. 62,723,944, filed Aug. 28, 2018, which is incorporated herein by reference in its entirety.

[0003] Optical overlay metrology is usually classified into one of two types: imaging or scatterometry. In imaging-based overlay metrology (IBO), the features of the target are printed side-by-side in two separate layers and their lateral shift is measured in the image plane relative to the optical system. In scatterometry or diffraction-based overlay metrology (DBO), the features from different layers are printed on top of each other and the overlay is obtained from the amplitudes of the diffracted orders emanating from the stack. The detailed principles of this metrology are outside the scope of this document.

[0004] Imaging technology can be further categorized into two main configurations: bright-field imaging and dark-field imaging. In bright-field imaging, images are formed with all diffracted orders that fall within the numerical aperture of the system, whereas in dark-field imaging, the zeroth order is blocked. The latter is typically used for superior image contrast / precision. Previous work has indicated that order selection, such as taught by U.S. Pat. No. 6,399,633, incorporated herein by reference in its entirety, can improve measurement accuracy. For example, selecting only the zeroth and ±1st orders leads to improved accuracy at the best contrast focus, as taught by U.S. Pat. No. 6,399,633, incorporated herein by reference in its entirety. Further improvements in accuracy and contrast in this configuration can also be achieved through control of the amplitude and phase of the zeroth order, as taught by U.S. Pat. No. 6,399,633, incorporated herein by reference in its entirety. Prior art has shown that selecting only the ±1st orders can achieve the best results at the expense of signal amplitude. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. WO2007143056 Brochure [Patent Document 2] US Patent Publication No. 20170146915 [Patent Document 3] U.S. Patent No. 7,528,953 [Patent Document 4] U.S. Patent No. 8,456,641 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention provides metrology methods and tools that enable increased metrology accuracy, simplification of the metrology process, and improved metrology target-to-semiconductor device correspondence. [Means for solving the problem]

[0007] The following brief summary provides a basic understanding of the invention. This summary does not necessarily identify the critical elements of the invention or limit the scope, but serves as a prelude to the description that follows.

[0008] One aspect of the present invention provides a method for measuring a metrology target using a metrology tool having an illumination arm and a collecting arm, by illuminating the target with the illumination arm in a Littrow configuration to produce a first measurement signal including -1st order diffraction components and a 0th order diffraction component, and a second measurement signal including +1st order diffraction components and a 0th order diffraction component, in which the -1st order diffraction component of the first measurement signal and the +1st order diffraction component of the second measurement signal are diffracted 180° based on the illumination direction, a first measurement on the first measurement signal and a second measurement on the second measurement signal are performed using the collecting arm, and at least one metrology indicator is derived based on the first and second measurements.

[0009] One aspect of the invention provides a method of measuring a metrology target with a metrology tool having an illumination arm and a collection arm, illuminating the target with the illumination arm to provide a reflected zeroth order diffraction component and -1st and +1st order diffraction components, splitting the reflected zeroth order diffraction component in the collection arm into a first component captured with the -1st order diffraction component and a second component captured with the +1st order diffraction component to provide corresponding first and second measurement results, and deriving at least one metrology indicator from the first and second measurement results.

[0010] These, additional and / or other aspects and / or advantages of the invention will be set forth in the detailed description which follows, and in some cases may be inferred from the detailed description, and / or may be learned by practice of the invention.

[0011] For a better understanding of embodiments of the present invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like references indicate corresponding elements or parts throughout.

[0012] The accompanying drawings are as follows: [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a high-level schematic block diagram illustrating a metrology tool and method for measuring targets according to some embodiments of the present invention. [Diagram 2] FIG. 1 is a high-level schematic diagram of metrology tools and measurements that may be implemented and performed using variable illumination in accordance with some embodiments of the present invention. [Figure 3-1] FIG. 1 is a high-level schematic diagram of metrology tools and measurements that may be implemented and performed without adaptive optics in accordance with some embodiments of the present invention. [Figure 3-2] FIG. 1 is a high-level schematic diagram of metrology tools and measurements that may be implemented and performed without adaptive optics in accordance with some embodiments of the present invention. [Figure 4] FIG. 1 is a high-level schematic diagram of metrology tools and measurements implemented and performed in accordance with an image splitting configuration in accordance with some embodiments of the present invention. [Figure 5-1] 1 is a high-level flowchart depicting methods according to some embodiments of the present invention. [Figure 5-2] 1 is a high-level flowchart depicting methods according to some embodiments of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] In the following description, various aspects of the present invention are described. For the purpose of explanation, specific configurations and details are described to provide a consistent understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention can be practiced without the specific details presented herein. Furthermore, well-known features have been omitted or simplified in some cases in order not to obscure the present invention. With specific reference to the drawings, it is emphasized that the illustrated matter is exemplary and is intended solely for the purpose of illustrative discussion of the present invention, and is presented to provide what is believed to be the most useful and easily understood description of the principles and conceptual aspects of the present invention. In doing so, no attempt is made to show structural details of the present invention in more detail than is necessary for a fundamental understanding of the present invention, and this specification, taken in conjunction with the drawings, will make clear to those skilled in the art how to practice several aspects of the present invention.

[0015] Prior to describing at least one embodiment of the present invention in detail, it is understood that the application of the present invention is not limited to the detailed configuration and arrangement of the components set forth in the following description or illustrated in the drawings. The present invention is applicable to other embodiments that may be practiced or carried out in various ways, as well as to combinations of the disclosed embodiments. The phraseology and terminology employed herein is for the purpose of description and should not be construed as limiting.

[0016] Unless specifically stated otherwise, as will be apparent from the discussion below, all specific discussions herein utilizing words such as "processing," "calculating," "calculating," "determining," "expanding," "deriving," and the like, are deemed to refer to operations and / or processes of a computer or information processing system or similar electronic information processing device that manipulates and / or converts data represented as physical quantities, e.g., electronic quantities, in the registers and / or memory of the information processing system into other data similarly represented as physical quantities in the memory, registers, or other information storage, transmission, or display device of the information processing system. According to certain embodiments, the illumination technology may consist of electromagnetic radiation in the visible range, ultraviolet light, or even shorter wavelength radiation, e.g., x-rays, and perhaps even particle beams.

[0017] A metrology method and tool are provided that can increase the accuracy of measurement, simplify the measurement process, and improve the correspondence between the metrology target and the semiconductor device. The method provides a first measurement signal including -1st and 0th order diffraction components and a second measurement signal including +1st and 0th order diffraction components by illuminating the target according to the Littrow configuration, in which the -1st order diffraction component of the first measurement signal and the +1st order diffraction component of the second measurement signal are diffracted 180° based on the illumination direction, and performs a first measurement on the first measurement signal and a second measurement on the second measurement signal, and derives metrology indices (group) based on the results. Alternatively or supplementarily, the reflected 0th order diffraction component may be divided to provide a component that interacts with the -1st and +1st order diffraction components.

[0018] Optical overlay tools measure overlay on representative targets that have designs optimized for measurability at optical wavelengths and compatibility with lithography and process steps. As device pitch scales down, these conflicting conditions result in the optimized targets becoming less compatible with the devices (which often leads to overlay bias) or becoming more difficult to measure due to their reduced diffraction efficiency. Advantageously, the manner provided in the disclosed embodiments can increase the sensitivity of the metrology device and facilitate the measurement of low diffraction efficiency targets without compromising measurement accuracy.

[0019] It is noted that the disclosed principles, tool 100 and method 200 can be implemented in current and / or developing metrology platforms, thereby improving the accuracy of metrology measurements (e.g., overlay measurements) and providing metrology measurement results that are more closely correlated to actual devices.

[0020] The disclosed embodiments overcome the shortcomings of bright field imaging, such as the possibility that each pair of orders may contribute different errors to overlay, making the total error difficult to control. The disclosed order selection is beneficial in reducing errors, as illustrated in a non-limiting manner in Equation 1, which includes only the three lowest diffraction orders (0th and ±1st orders), where P is the pitch of the periodic target, a n is the nth-order diffraction amplitude, Ψ is the average phase difference between the 0th and ±1st-order diffractions, and δφ and δa1 are the phase difference and amplitude difference between the +1st and -1st orders, respectively. BF represents the applicability of Equation 1 to bright-field imaging, where the term (atan(δa1tanΨ)) represents the amplification of target asymmetry errors in bright-field imaging. x0 represents the position of the target's center of symmetry.

number

[0021] The inventors note that while δφ causes an overlay error that can be factored into the geometric ambiguity due to target asymmetry, the overlay error can be of the order of P / 4 since the second phase term in the first harmonic (atan(δa1tanΨ)) can take any value in the range [-π / 2, +π / 2] for any value of Ψ. For obvious reasons, tanΨ is often referred to as the inaccuracy factor.

[0022] 1 is a high-level schematic block diagram illustrating a metrology tool 100 and a target measurement method 200 (see also FIG. 5) according to some embodiments of the present invention. The metrology tool 100 includes an illumination arm 101 and a collection arm 105 in communication with at least one processing unit 99 (e.g., in a metrology module), which are configured to perform metrology measurements of a metrology target 60 on a wafer, for example, according to the method 200.

[0023] The metrology tool 100 is configured to illuminate the metrology target 60 by the illumination arm 101 according to the Littrow configuration (step 210, see Figure 5) to produce a first measurement signal (see 100A) including a -1st order diffraction component 115B and a 0th order diffraction component 115A from the first incident illumination 110 and a second measurement signal (see 100B) including a +1st order diffraction component 125B and a 0th order diffraction component 125A from the second incident illumination 120, such that the -1st order diffraction component 115B of the first measurement signal and the +1st order diffraction component 125B of the second measurement signal are diffracted 180° with respect to the directions of the incident illuminations 110 and 120, respectively.

[0024] The metrology tool 100 is further configured to perform (step 230) a first measurement on the first measurement signal (including -1st order diffraction component 115B and 0th order diffraction component 115A) and a second measurement on the second measurement signal (including +1st order diffraction component 125B and 0th order diffraction component 125A) by the focusing arm 105, for example using one or more cameras and / or sensors, and to derive at least one metrology indicator based on the first and second measurements (step 240, see further method steps in Figure 5).

[0025] In certain embodiments, the overlay is measured under two different illumination conditions 100A and 100B, with the image (respectively the first and second measurement signals) in each illumination condition being composed of only two orders, 0th and +1th orders and 0th and -1th orders. i By setting the illumination angle to satisfy = ±λ / (2P) (Littrow configuration), the angle of the nth diffraction order is calculated as sinθ n =-sinθ i +nλ / P=λ / P(−(±1) / 2+n). According to non-limiting embodiments, under the assumption that λ≈P, only two orders fit within the numerical aperture, so the angle of the diffraction component for negative angle illumination is sinθ -1 =-λ / (2P),sinθ0=+λ / (2P) (illumination and measurement condition 100A), and those related to positive angle illumination satisfy sinθ1=λ / (2P),sinθ0=-λ / (2P) (illumination and measurement condition 100B). It is noted that while Figure 1 depicts a one-dimensional setting, the same principles can be applied to a two-dimensional setting (see, e.g., Figures 2-4).

[0026] Equation group 2 is the signal strength I + (x),I - Represents (x).

number

[0027] Equation 3 represents a non-limiting example overlay of a metric that can be derived from the signals represented by Equation 2, which is the inter-signal phase difference (φ) that corresponds to δφ that can be derived from signal measurements using Equation 2. + -φ - ) can be obtained.

number

[0028] Beneficially, the disclosed metrology tool 100 and method 200 have the following beneficial properties (none of which are limiting of the invention): (i) its amplitude asymmetry does not affect the inaccuracy; (ii) there is no additional source of inaccuracy other than the geometric ambiguity -P / (2π)·δφ since the mean topographic phase is cancelled; (iii) symmetric aberrations are cancelled since both orders follow the same optical path. As a result, the depth of focus depends on the illumination numerical aperture rather than the collection aperture; (iv) there is negligible contamination of higher diffraction orders at the pupil plane since the orders are separated by at least a factor of two compared to the orthogonal illumination condition; and (v) there is no need to provide a shielding element at the collection pupil for accuracy purposes. Some embodiments may lack one or more of the properties listed here, possibly due to specific configurations and practical trade-off issues.

[0029] In any of the disclosed embodiments, a single grab option and corresponding illumination condition (e.g., only one of the left / upper or right / lower illumination poles as shown in FIG. 2) can replace the two different illumination conditions 100A, 100B, so that two measurements are performed at the 0° and 180° wafer orientations. For example, all cells can be measured under a single illumination condition at both the 0° and 180° wafer orientations, but at the expense of illumination condition corrections for wafer motion. Advantageously, in some embodiments, it is not necessary to perform a consistent calibration between the left and right pole positions / sizes under the different illumination conditions 100A, 100B, and yet zero TIS (tool induced shift) measurements can be performed up to the focus reproducibility limit, which is not noticeable due to the large DOF (depth of field), thus compensating for the additional required MAM (move-acquire-measure) time.

[0030] In certain embodiments, furthermore, target size reduction (for the same target design) for a given metrology target design, e.g., by a factor of two by halving the pitch of the periodic structures, is similarly achieved using the disclosed metrology tool 100 and method 200, i.e., with low risk of side lobe clipping due to intra-pupil aperture. In some embodiments, reducing the target pitch by a factor of two while keeping the same wavelength can improve compatibility of the target 60 with devices.

[0031] Certain embodiments of the metrology tool 100 and method 200 further allow for attenuation of the zeroth order at the collection pupil, thereby increasing contrast with only a moderate or negligible effect on the signal amplitude.

[0032] Advantageously, this is in contrast to the prior art, e.g., U.S. Pat. No. 6,399,433, which teaches simultaneous illumination of the target at opposite Littrow angles and obtaining an overlay from a single image composed of the sum of two interference terms 0,+1 or 0,-1, which often results in amplitude differences between diffraction orders associated with topographic phase appearing and leading to magnified inaccuracies. In contrast, the disclosed metrology tool 100 and method 200 according to certain embodiments uses sequential illumination of the target, separation and amplitude difference removal as described in Equations 2 and 3.

[0033] FIG. 2 is a top-down schematic diagram of a metrology tool 100 and measurements 100A, 100B implemented using variable illumination according to some embodiments of the present invention. FIG. 2 presents a non-limiting example implementation of a double glab Littrow illumination two-beam imaging metrology. According to certain embodiments, the illumination arm 101 includes a light source 70 and adaptive optical element(s), such as a digital light processor (DLP) 75, and the collection arm 105 includes sensing elements, such as a camera(s) 90 and / or corresponding sensors and adaptive optical element(s), such as a spatial light modulator (SLM) 95, configured to implement flexible Littrow angle illumination and collection under wavelength variation, e.g., switch between two Littrow configurations in the X and Y directions (see pupil plane illumination FIG. 77, all black circles represent illumination spots in the pupil plane). Both illumination and collection arms 101, 105 are associated with a beam splitter 85 and pass through the objective 80. The illumination beams 110, 120 (also called grabs) are for measurements 100A, 100B, respectively (each illumination beam 110, 120 in the figure consists of two beams corresponding to directions X and Y). -1 exp(iφ -1 )) 115B and 0th diffraction order (T·a0exp(iφ0)) 115A, +1st diffraction order (a1exp(iφ1)) 125B and 0th diffraction order (T·a0exp(iφ0)) 125A are for each measurement direction X, Y, respectively, and T is an additional attenuation factor. It is noted that the adaptive optical element(s) of the embodiments, e.g. DLP, SLM, etc., can be used interchangeably in various embodiments. Contrast improvement can be achieved by appropriately configuring variable illumination and / or collection. If the target 60 has low diffraction efficiency, the zeroth order can be attenuated (per illumination angle) with the same SLM and / or DLP element(s), depending on whether the measurement is performed in transmission or reflection.

[0034] 3 is a top-down schematic diagram of a metrology tool 100 and measurements 100A, 100B implemented without adaptive optics according to some embodiments of the present invention. An illumination arm 101 can provide illumination 71 and can include a fixed mask 78, e.g., four poles (schematically represented by open circles) for simultaneously illuminating (e.g., corresponding X,Y periodic structures of a target 60) at diametrically opposed Littrow angles. A shutter assembly 130 can include, e.g., a pair of knife-edge shutters mounted on high-speed motors, and can be configured to alternately block poles associated with positive / negative illumination angles, as depicted in the measurement conditions 100A, 100B (each illumination beam 110, 120 in the figure is composed of two beams corresponding to directions X and Y). The collector arm 105 may be equipped with a suitable collection element set, as shown in a non-limiting manner, as two sub-arms with cameras 90A, 90B and zeroth order attenuators 140A, 140B, respectively, capable of providing corresponding measurement results 100A, 100B (also called grabs) to the processing unit 99. The zeroth order attenuation using fixed elements can be achieved by splitting the collected diffracted orders in half and placing two semi-opaque masks 140A, 140B in the collection pupil, each attenuating a different pair of orders. In some embodiments, a variable optical element set, e.g. DLP or SLM, may be used for the zeroth order attenuation, making it possible to avoid splitting the collector arm 105. An additional beam splitter 85A may be used to split the collected radiation into the two sub-arms of the collector arm 105. In some embodiments, optical manipulation in the collection arm allows for a single camera 90 and / or zero order attenuator 140 to be used (not depicted).

[0035] 4 is a top-level schematic diagram of a metrology tool 100 and measurements 100A, 100B implemented in an image splitting configuration according to some embodiments of the present invention. In certain embodiments, metrology tool 100 includes an illumination arm 101 configured to illuminate target 60 with illumination 122 (corresponding to illumination 110+illumination 120), thus providing reflected 0th diffraction order 112 (corresponding to 0th diffraction order 115A+0th diffraction order 125A) and -1st and +1st diffraction orders 115B, 125B, respectively. The disclosed metrology tool 100 further comprises a collection arm 105, which comprises an image splitter 150 (e.g. a prism, e.g. a prism mirror, and potentially via optical element(s) 86, e.g. a mirror) configured to split the reflected 0th diffraction order 122 into a first component 115A captured together with a -1st diffraction order 115B and a second component 125A captured together with a +1st diffraction order 125B, thereby resulting in corresponding first and second measurement results 100A, 100B, respectively. The metrology tool 100 further comprises a processing unit 99 (e.g. associated with the metrology module), which is configured to derive at least one metrology indicator from each of the first and second measurement results 100A, 100B. According to various embodiments, illumination arm 101 and collection arm 105 can be configured to perform illumination and measurement simultaneously in two measurement directions (e.g., X and Y). According to some embodiments, collection arm 105 can include at least one zeroth order attenuator (not shown, see attenuators 140A, 140B) configured to attenuate at least one of the first and second components 115A, 125A of the zeroth order 122.

[0036] In some embodiments of the metrology tool 100, the image splitting configuration depicted in FIG. 4 eliminates imprecision other than ambiguity without degrading the signal level. According to some embodiments, the collection pupil can be split into two halves (four quadrants when measuring periodic structures along two measurement directions, e.g., X and Y) at its mean (or a given ratio, potentially related to the illumination wavelength), for example using a prism mirror 150 or other embodiments disclosed in U.S. Pat. No. 5,399,323, the entire contents of which are incorporated herein by reference. One image 100A can be constructed by interfering the +1 order (125B) with the positive half of the 0 order 122 (the first component 125A), and a second image 100B can be constructed by interfering the -1 order (115B) with the negative part of the 0 order 122 (the second component 115A). In some embodiments, the overlay difference between these two images (100A, 100B) does not depend on the average topographic phase Ψ (see Equation 1), and the signal amplitude decays exclusively linearly with diffraction efficiency, similar to the Littrow method and configuration disclosed above.

[0037] In certain embodiments, the first and second measurements 100A, 100B (e.g., corresponding images) are further calibrated for layer dependent telecentricity. In certain embodiments, focus information is further derived from the telecentricity parameters of the first and second measurements 100A, 100B (e.g., corresponding images), e.g., obtaining each layer focus position from one (or more) grabs, e.g., 100A and / or 100B. In certain embodiments, non-Littrow off-axis illumination is used to measure focus based on shifts in grating and / or cell positions to obtain focus sensitivity without splitting the pupil in the collection path, at the expense of increased sensitivity to symmetric aberrations.

[0038] In certain embodiments, the contrast of low diffraction efficiency targets is further improved by attenuating one or more of the zeroth orders 115A, 125A. Alternatively or additionally, the contrast loss caused by splitting the zeroth order 122 (into two parts for each measurement direction, e.g., into four parts when two measurement directions are used) can be compensated for by various techniques, such as splitting the original signal 122 into four identical copies for the two measurement directions and then applying a single copy to each combination (0th order, +1th order, X ), (0th order, -1st order X ), (0th order, +1st order Y ) and (0th, -1st Y ) can produce different images (not shown) from each copy. Advantageously, by splitting the signal and using only a portion of the 0th order to generate a separate signal with the primary signal (or a component part of it), the amplitude of the 0th order signal can be reduced while still allowing all illumination points to interfere with the 1st order (e.g. the entire signal).

[0039] In any of the disclosed embodiments, metric(s) such as overlay can be derived individually from the phase of the signal resulting from the interference between the individual 0th and ±1st orders.

[0040] Beneficially, the disclosed tool 100 and method 200 may be configured to provide excellent accuracy and precision of IBO in low diffraction efficiency conditions without phase control. For example, in relation to phase and amplitude control techniques taught in U.S. Patent No. 5,399,633 and others, the disclosed embodiments may be simpler to implement, easier to achieve, and less complex to satisfy required system requirements. For example, the disclosed tool 100 and method 200 may be configured to provide methods that seek to minimize or eliminate the linear term a0 (see Equation 1), such as those taught in U.S. Patent No. 5,399,633 and in which the signal amplitude is a1 2 The signal amplitude achieved by the disclosed tool 100 and method 200 is higher, thereby improving the achieved signal-to-noise ratio.

[0041] It is noted that the elements in Figures 1-4 may be combined in any operable combination, and that the fact that an element is depicted in one figure and not in another is merely for illustrative purposes and is not intended as a limitation.

[0042] 5 is a high-level flow chart depicting a method 200 according to some embodiments of the present invention. The steps of the method may be performed in conjunction with the metrology tool 100 described above, which may optionally be configured to perform the method 200. The method 200 may be at least partially implemented by at least one computer processor, such as in a metrology module. In certain embodiments, a computer readable program is configured to perform the relevant steps of the method 200, and a computer readable storage medium embodying the computer readable program constitutes a computer program product. In certain embodiments, a target design file is constructed for each individual target designed according to the method 200 of the embodiments. The method 200 may comprise the following steps, in any order:

[0043] In method 200, a metrology target is measured (step 205) with a metrology tool having an illumination arm and a capture arm by illuminating the target according to a Littrow configuration (step 210), deriving a first metrology signal including −1 and 0 diffraction orders and a second metrology signal including +1 and 0 diffraction orders (step 220), by performing first and second measurements on the first and second metrology signals (step 230), and deriving metrology metric(s), e.g., overlay, from the first and second measurements (step 240). In any of the disclosed embodiments, the first and second measurements can be performed (step 231) under the same illumination conditions at wafer orientations of 0° and 180°, at the expense of modifying the illumination conditions for MAM time.

[0044] Method 200 may further include configuring the illumination arms to diffract the −1st and +1st diffraction orders of each measurement signal by 180° relative to the illumination direction (step 212). In some embodiments, method 200 may perform the illumination using spatial light modulation and / or digital light processing (step 214) and / or using a pair of shutters alternatingly applied to a quadrupole illumination source, i.e., illuminating simultaneously in two measurement directions (step 216). In some embodiments, at least one of the 0th diffraction orders is attenuated (step 217). In some embodiments, the illumination is performed at multiple wavelengths to provide a flexible Littrow configuration (step 218).

[0045] According to certain embodiments, performing the measurements can be performed by spatial light modulation and / or digital light processing (step 232). In some embodiments, illumination and measurement are performed simultaneously in two measurement directions (step 234).

[0046] According to certain embodiments, the method 200 can further reduce the target size, for example by halving it (step 250) by reducing the pitch of the periodic structure, for example by a factor of two, or can use the disclosed Littrow configuration. According to certain embodiments, the method 200 can illuminate the target with an illumination arm to provide a reflected 0th diffraction order and −1st and +1st diffraction orders (step 222), split the reflected 0th diffraction order with a collection arm into a first component captured with the −1st diffraction order and a second component captured with the +1st diffraction order to provide corresponding first and second measurements (step 224), and derive at least one metric from the first and second measurements (step 240). According to embodiments, the illumination 222 and splitting 224 can be performed simultaneously in two measurement directions for each of the first and second measurements. According to certain embodiments, the method 200 can further attenuate at least one of the first and second components of the 0th diffraction order (step 227). The metric(s) can include the overlay of the target, calculated in Equation 3.

[0047] In certain embodiments, the first and second measurements (e.g., corresponding images) are further calibrated for layer-dependent telecentricity (step 242). Some embodiments further derive focus information from telecentricity parameters of the first and second measurements (e.g., corresponding images) (step 244), e.g., obtaining each layer focus position from one (or more) grabs. Some embodiments obtain focus sensitivity without splitting the pupil in the collection path (step 247), and do so at the expense of increased sensitivity to symmetric aberrations by using non-Littrow off-axis illumination and measuring focus based on shifts in grating and / or cell positions.

[0048] Aspects of the present invention have been described above with reference to flowchart illustrations and / or partial diagrams of methods, apparatus (systems) and computer program products according to embodiments of the present invention. As will be understood, each part of the flowchart illustrations and / or partial diagrams, and combinations of parts of the flowchart illustrations and / or partial diagrams, can be implemented by computer program instructions that can be supplied to a processor of a general purpose computer, special purpose computer or other programmable data processing device to construct a machine, and that can be executed by a processor of the computer or other programmable data processing device to produce means for implementing the functions / acts specified in the flowcharts and / or partial diagrams or portions thereof.

[0049] The computer program instructions may also be stored in a computer readable medium from which a computer or other programmable data processing apparatus or other device can be caused to function in a particular manner, such that storing the instructions in a computer readable medium produces an article of manufacture having embedded therein the instructions which implement or effectuate the functions / acts specified in the flowcharts and / or sub-diagrams, or portions thereof.

[0050] The computer program instructions may be loaded onto a computer or other programmable data processing apparatus or other device and executed in a series of operational steps on the computer or other programmable data processing apparatus or other device to produce a computer-implemented process, thus providing a process for implementing the functions / operations identified in the flowcharts and / or sub-diagrams or portions thereof by the instructions executed on the computer or other programmable apparatus.

[0051] The above flow charts and figures depict the architecture, functionality and operation of potential implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each portion of the flow chart or sub-diagrams may represent a module, segment or portion of code consisting of one or more executable instructions for implementing the specified logical function(s). It should also be noted that, according to some alternative implementations, the functions depicted in the portions may occur in a different order than that depicted in the figures. For example, two portions shown in succession may in fact be executed substantially simultaneously, or the portions may sometimes be executed in the reverse order, depending on the functionality involved. It should also be noted that each portion of the sub-diagrams and / or flow chart depictions, and combinations of portions in the sub-diagrams and / or flow chart depictions, may be implemented by a dedicated hardware-based system that executes the specified functions or operations, or by a combination of dedicated hardware and computer instructions.

[0052] The embodiments in the above description are examples or implementations of the present invention. The various expressions "an embodiment," "one embodiment," "certain embodiments," or "some embodiments" do not necessarily all refer to the same embodiment. Although various features of the present invention are described in the context of a single embodiment, the features may be provided separately or in any suitable combination. Conversely, although the present invention is described in the context of separate embodiments for clarity, the invention may be embodied in a single embodiment. Features may be incorporated into certain embodiments of the present invention from various embodiments disclosed above, and elements may be incorporated into certain embodiments from other embodiments disclosed above. The fact that elements of the present invention are disclosed in the context of a particular embodiment should not be construed as limiting their use to only that particular embodiment. Furthermore, the present invention may be practiced or embodied in various ways and the present invention may be embodied in some embodiments other than those generally outlined in the above description.

[0053] The present invention is not limited by those figures and corresponding descriptions. For example, the flow need not pass through each illustrated box or state, or in the exact same order as illustrated and described. The meanings of technical and scientific terms used in this application should be understood according to the usual way as understood by those skilled in the art to which the present invention belongs, unless otherwise defined. Although the present invention has been described with reference to a finite number of embodiments, they should not be interpreted as limitations on the scope of the present invention, but rather as examples of some of the preferred embodiments. Other potential variations, modifications, and applications are also within the scope of the present invention. Therefore, the scope of the present invention should be limited not by what has been described above, but by the following claims and their legal equivalents.

Claims

1. 1. A method of measuring a metrology target with a metrology tool having an illumination arm and a collection arm, comprising: Illuminating the metrology target with an illumination arm to provide a reflected zeroth diffraction order and −1st and +1st diffraction orders; a focusing arm for dividing the reflected zeroth order diffraction component into a first component captured together with the −1st order diffraction component and a second component captured together with the +1st order diffraction component, thereby obtaining a first measurement result and a second measurement result; Deriving at least one metric from the first and second measurements; Furthermore, at least one of the first and second components of the zeroth order diffraction is attenuated. method.

2. 2. The method of claim 1, wherein illuminating the metrology target and splitting the zeroth order reflection component are performed simultaneously in two measurement directions for first and second measurements, respectively.

3. 2. The method of claim 1, wherein the at least one metric indicator is an overlay of the metric target and is expressed by the formula: [0010] where OVL represents the overlay of the metrology target, P represents the pitch of the metrology target, and (φ ＋ -φ － ) represents the phase difference between the second measurement signal and the first measurement signal, and x ０ represents the position of the center of interest of the measurement target, and δφ represents the difference between the +1st order analysis and the -1st order analysis.

4. 2. The method of claim 1, further comprising: calibrating the first and second measurements for layer dependent telecentricity.

5. 2. The method of claim 1, further comprising deriving focus information from telecentricity parameters of the first and second measurements.

6. 10. The method of claim 1, further comprising acquiring focus sensitivity data using non-Littrow off-axis illumination and measuring focus based on position shifts in said metrology target.

7. 10. The method of claim 1, wherein the method is at least partially performed by at least one computer processor.

8. A computer program product comprising a computer readable storage medium having a computer readable program embodied thereon, the computer readable program being configured to at least partially perform the method of claim 1.

9. A weighing module comprising a computer program product according to claim 8.

10. an illumination arm configured to illuminate a metrology target to provide a reflected zeroth diffraction order and −1 and +1 diffraction orders; a collection arm including an image splitter configured to split the reflected zeroth diffraction order into a first component captured with the −1st diffraction order and a second component captured with the +1st diffraction order to provide a first measurement and a second measurement; a processing unit configured to derive at least one metric indicator from the first and second measurements; Equipped with The collecting arm includes at least one zeroth order diffraction attenuator configured to attenuate at least one of the first and second components of the zeroth order diffraction. Measuring tool.

11. 11. The metrology tool of claim 10, wherein an illumination arm and a collection arm are configured to illuminate the metrology target and perform the first and second measurements simultaneously in two measurement directions.

12. 11. The metrology tool of claim 10, wherein the processing unit is further configured to at least one of: calibrating the first and second metrology results for layer dependent telecentricity; deriving focus information from telecentricity parameters of the first and second metrology results; and / or acquiring focus sensitivity data using non-Littrow off-axis illumination and measuring focus based on a position shift at the metrology target.

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