Optical inspection apparatus and weighing method

Abstract

To provide improved apparatus and methods for semiconductor circuit metrology.SOLUTION: A semiconductor wafer on which at least first and second patterned layers have been deposited in succession is illuminate with at least one illumination beam directed thereto, where the first patterned layer includes a first target feature and the second patterned layer includes a second target feature, the second target feature being overlaid on the first target feature. Images of the first and second target features are captured while varying one or more imaging parameters. The images are processed to identify respective centers of symmetry of the first and second target features in the images and measure variations in the centers of symmetry as a function of the varying image parameters. The variations are applied in measuring an overlay error between the first and second patterned layers.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD

[0001] The present invention relates generally to the manufacture of semiconductor devices, and more particularly to semiconductor circuit metrology to an apparatus and method for use in the same. BACKGROUND ART

[0002] Semiconductor circuits are generally manufactured using photolithographic methods. Photoli thography involves depositing a thin layer of photosensitive polymer (photoresist) over a semiconductor wafer , then patterning the layer using optical or other radiation, whereby portions of the wafer covered by the photoresist are retained. After patterning modification of the wafer by processes such as etching, ion implantation, etc., alters the elemental properties and topography of the wafer while leaving the portions of the wafer covered by the photoresist unaffected. Semiconductor circuit metrology is widely used to measure characteristics of patterned photoresist, such as the topography and location of patterned features

[0003] Accurate positioning of patterned photoresist features relative to underlying previous process layers is critical to ensuring high yield in photolithographic processes Any error in the alignment (registration) of the patterned photoresist relative to the underlying process layers is referred to as an "overlay error" By way of example, for a typical semiconductor circuit with a minimum linewidth of 10 to 14 nm (the so-called 10 nm design rule), the maximum allowable overlay error is 2 to 3 nm. In state-of-the-art semiconductor circuits, the linewidth is 5 ​​​​​​The size is shrinking to nanometers, and consequently, the maximum allowable overlay error is decreasing.

[0004] For visible and near-infrared optical radiation, it occurs within the photoresist layer or beneath the photoresist. Because it can penetrate the body layer, overlay errors are generally measured using optical overlay weighing tools. It is being measured. Optical overlay weighing tools, for example, KLA Corporation. The Archer® series of tools (developed by Milpitas, California, USA) Proxy located within the scribe line (the line separating adjacent semiconductor chips) of a conductive wafer A target (for example, AIM(trademark) overlay target by KLA) is an image It is then processed. By applying an image analysis algorithm to the acquired image, one of the processes within the process layer is performed. The center of symmetry (CoS) of the target features of the pair and the pair within the patterned photoresist layer The CoS of the corresponding target features is located. The overlay error is then applied to them. It is calculated as the distance between the centers of symmetry of the target features in the two layers.

[0005] The terms "optical ray" and "optical radiation" are used to describe the phenomenon of light rays and optical radiation. "Light" and "Beam of Radiation" are the terms used in this book. According to the use in the specification and claims, generally, this refers to visible, infrared, and ultraviolet radiation in general. It refers to that. [Overview of the project] [Problems that the invention aims to solve]

[0006] In the embodiments of the present invention described later, improved apparatus and methods for semiconductor circuit metering are proposed. It will be served. [Means for solving the problem]

[0007] In other words, in the weighing method provided by one embodiment of the present invention, at least The first and second patterned layers are deposited in succession, and within the first patterned layer the first target The second target feature is located within the second patterning layer, and its second... There are few semiconductor wafers in which the target feature overlaps the first target feature. At most, it is illuminated by directing a single light beam. First and second target fee A sequence of images of Cha has one or more imaging parameters across that sequence. The image is captured while changing its shape. By processing the images within that sequence, those images The centers of symmetry of the first and second target features within are identified, and the changes in those centers of symmetry are... The motion is measured as a function of its changing imaging parameters. The measured fluctuation is the 1. Applied when measuring the interlayer overlay error of the second patterned layer.

[0008] In one disclosed embodiment, the first patterning layer includes a process layer, and the second patterning layer includes This includes a resist layer deposited above the process layer.

[0009] In certain embodiments, when capturing a sequence of images, mutual registration The first and second cameras of the system capture the first and second images of the target feature, and the image When processing the images, the change in the center of symmetry is measured by comparing the first and second images. In this embodiment, when capturing the first and second images, the first and second cameras are registered to register A registration image is projected, and the first and second cameras position themselves relative to the registration image. They are aligned. In one exemplary embodiment, when projecting the registration image, A grid pattern is generated by arranging the image of the acquired features, and detection is performed by the first and second cameras respectively. It is projected onto the array of devices.

[0010] In addition to or instead of this, when capturing the first and second images, among the imaging parameters In the first image, one or more of the aforementioned items are set to the first setting, and in the second image, they are set to the second setting. It is set to a certain value. In one disclosed embodiment, one or more of the imaging parameters are set to a certain value. When setting up several cameras, the first and second cameras are set to separate first and second focal positions. Furthermore, the first and second focal positions pass through the first and second sequences of each respective focal position. By stepping the camera, the change in the center of symmetry is measured as a function of the focal position. In an exemplary embodiment, the first and second focal positions are separated by a constant focal distance Δz, In each stage of sequences 1 and 2, both the first and second focal positions are incremented by Δz. It will be done.

[0011] In addition to or instead of the above, at least one illumination beam is provided for each individual first and It includes first and second illumination beams in two polarization states, and when capturing the first and second images... By applying a polarizing beam splitter, the beam reflected from the wafer in the first polarization state The light is directed towards the first camera and is reflected from the wafer in the second polarization state. The light is directed towards the second camera.

[0012] In one embodiment, when capturing a sequence of images, various focus settings are used. - When the image of the get feature is captured and the image is processed, the variation in the center of symmetry is the focus setting. It is measured as a function of tactic.

[0013] In addition to or instead of this, when capturing the image sequence, multiple different wavelengths are used. - When an image of a GET feature is captured and the image is processed, the variation in the center of symmetry is related to the wavelength. It is measured as a number.

[0014] In addition to or instead of this, when capturing a sequence of images, multiple different polarization patterns When the image of the target feature is captured and the image is processed, the change in the center of symmetry occurs. It is measured as a function of the polarization state.

[0015] Furthermore, in addition to or instead of the above, when capturing a sequence of images, at least one At least one aperture of the illumination beam is offset in multiple different ways to target the target. When a feature image is captured and the image is processed, changes in the center of symmetry affect its aperture. It is measured as a function of the offset.

[0016] In one embodiment, a camera is used to capture a sequence of images, and The semiconductor wafer is positioned at various angles and orientations relative to the camera, and images of the target features are captured. Furthermore, when processing the image, the tool-induced shift of the center of symmetry is measured as a function of angle and orientation. It can be done.

[0017] In certain embodiments, when applying the measured variation, 1 The optimal range for one or more imaging parameters is searched, and the values ​​within that optimal range are selected. A recipe for measuring overlay error is created by setting one or more imaging parameters. This is generated. In one disclosed embodiment, when capturing a sequence of images, a semiconductor wafer Images of multiple target features are captured at multiple separate locations above, and the optimal range is determined. When exploring, by applying the variations measured at multiple different locations, semiconductors An optimal range is selected across an area of ​​the wafer.

[0018] In addition to or instead of this, when processing the image, at least one of the target features Individual asymmetries are measured.

[0019] Furthermore, in the weighing method provided by one embodiment of the present invention, at least one is placed on it. A patterned layer is deposited, and multiple bars are oriented parallel to a predetermined axis. A semiconductor wafer having a lattice is directed at at least one illumination beam. It is illuminated by capturing and processing one or more images of the grid, around its axis. Then, the asymmetry of one or more of the bars is revealed. The revealed asymmetry is the putter This is applied when performing quantitative evaluation of the ionized layer.

[0020] In one disclosed embodiment, when capturing one or more images, various focus settings are used. The grid image sequence is captured in the G, and when processing one or more of these images, The variation in the symmetry center of the grid in the image was measured as a function of the focus setting. The asymmetry is elucidated based on the fluctuations. In addition to or instead of this, one or more images When processing, the images of one or more bars and their reflected versions The correlation between the two is calculated, and the degree of asymmetry is derived based on the calculated correlation.

[0021] In addition, in an optical inspection apparatus provided by one embodiment of the present invention, the illumination assembly is On top of that, at least the first and second patterned layers are successively deposited, and the first pattern The first target feature is in the patterned layer, and the second target feature is in the second patterned layer. A semiconductor is equipped with such a feature, where the second target feature overlaps the first target feature. The wafer is configured to be illuminated by directing at least one illumination beam towards it. The rigging assembly captures sequences of images of the first and second target features. The system is configured such that the controller controls one or more images of the device throughout the sequence. By processing the images within that sequence to change the zing parameters, those images Identify the centers of symmetry of the first and second target features within the data and analyze the changes in those centers of symmetry. The measurement should be taken as a function of the changing imaging parameters, and the measured variation It is configured to be applied when measuring the interlayer overlay error between the first and second patterned layers.

[0022] Furthermore, in an optical inspection apparatus provided by one embodiment of the present invention, the illumination assembly is At least one patterned layer is deposited on top of it, and the orientation is determined parallel to a predetermined axis. A semiconductor wafer having a grid with multiple bars is illuminated by at least one light. The imaging assembly is configured to illuminate by directing a bright beam towards its grating. The controller is configured to capture one or more images. By processing multiple images, the asymmetry of one or more bars around their axis can be elucidated. Furthermore, when quantitatively evaluating the patterned layer, the elucidated asymmetry should be applied. It is configured to do so.

[0023] The present invention, along with the detailed description of its embodiments below, can be seen more comprehensively from the following drawings. I hope you will understand. [Brief explanation of the drawing]

[0024] [Figure 1] This is a schematic side view of an optical inspection apparatus for measuring the optical properties of a patterned thin film layer on a semiconductor wafer, according to one embodiment of the present invention. [Figure 2] This is a schematic side view of a grid projector used in an optical inspection apparatus according to one embodiment of the present invention. [Figure 3] This is a schematic representation of an image acquired by a camera in an optical inspection apparatus according to one embodiment of the present invention. [Figure 4] This flowchart schematically illustrates a process for measuring the CoS variation of process layer and resist layer features of an overlay metric proxy target, relating to one embodiment of the present invention. [Figure 5] This is a plot diagram illustrating the focus separation between two cameras in the process shown in Figure 4, relating to one embodiment of the present invention. [Figure 6A] This flowchart schematically illustrates a process for measuring the CoS variation of a feature of an overlay metric proxy target in association with the illumination wavelength, according to one embodiment of the present invention. [Figure 6B] This flowchart schematically illustrates a process for measuring the CoS variation of a feature of an overlay metric proxy target in association with the illumination wavelength, according to one embodiment of the present invention. [Figure 7A] This flowchart schematically illustrates the process of generating a CoS landscape across various focal settings and wavelengths and evaluating sensitivity in one embodiment of the present invention. [Figure 7B] This flowchart schematically illustrates the process of generating a CoS landscape across various focal settings and wavelengths and evaluating sensitivity in one embodiment of the present invention. [Figure 8A]This is a schematic representation of the landscape of tool-induced shift of CoS (CoS_TIS) in the resist layer, relating to one embodiment of the present invention. [Figure 8B] This is a schematic representation of the landscape of corrected CoS (CoS_COR) in the resist layer, relating to one embodiment of the present invention. [Figure 8C] This is a schematic representation of the CoS_TIS landscape in the process layer, relating to one embodiment of the present invention. [Figure 8D] This is a schematic representation of the CoS_COR landscape in the process layer, relating to one embodiment of the present invention. [Figure 9A] This is a schematic representation of the precision landscape in the resist layer according to one embodiment of the present invention. [Figure 9B] This is a schematic representation of the precision landscape in the process layer, relating to one embodiment of the present invention. [Figure 10] This plot diagram relates to one embodiment of the present invention and schematically shows the variation of CoS_TIS as a function of the aperture offset. [Figure 11A] This is a schematic representation of a proxy target image acquired by a camera in an optical inspection apparatus according to one embodiment of the present invention. [Figure 11B] Figure 9A is a schematic cross-sectional view of a grid bar in the proxy target according to one embodiment of the present invention. [Figure 12] A series of plots illustrating the use of image signal correlation for monitoring the asymmetry of features within an overlay proxy target, relating to one embodiment of the present invention. [Modes for carrying out the invention]

[0025] [Overview] The proxy targets for overlay metricing are located on the semiconductor wafer and are sequential (continuous with each other). It is widely used for precise measurement of overlays between patterned layers. This may include, for example, a process layer and a resist layer, and in post-etching applications. It can also be used between two process layers (i.e., in relation to the process layer and the resist layer) Exemplary embodiments will be described later, but the principles of those embodiments will be modified as necessary. (This can be applied to the first and second process layers.)

[0026] However, features within the proxy target (resist layer target features and Both the process layer target feature and the corresponding feature within the device area This is different: the features of a proxy target are usually different from those within the device. Because it has a broad line, it can be used by metrological tools that operate with visible or near-infrared light. It can be broken down and resolved, and because the targets are symmetrical in their design, power The overlay value can be calculated by executing a full symmetry-based image processing algorithm. It is possible. Furthermore, proxy targets are typically valuable "real estate" in their device area. : Placed within the semiconductor wafer scribe line to prevent "real estate" from being seized. The optical distortion of a photolithographic exposure system (scanner) is within the scribe line. These differ from those within the device area, and that is the pattern within the proxy target. The spatial variability differential between the corresponding pattern within the device (as a spatially varying difference) This is connected to the shift.

[0027] Through these design and quantitative considerations (studies), the features of the proxy target are: Regarding lithographic and process effects, what are the device features within the chip area? It reacts differently, and the overlay error measured based on the proxy target is, This may result in an offset for the overlay error in the actual device features. By applying the calibration function, the overlay error measured based on the proxy target is corrected. From this, the precise degree of overlay error in the device area may be derived. However, However, for accurate calibration, a stable and reproducible overlay based on a proxy target is required. Ray measurements become necessary. These, in turn, lead to process-induced effects, such as feature asymmetry. They will end up bearing the burden.

[0028] Furthermore, each proxy target includes a target feature within the photoresist and a preceding pro. Since both of the target features within the seth layer are present, those two sets of target features The turbidity is separated along a direction perpendicular to the semiconductor wafer at a distance of several micrometers. It is possible. Those two sets of target features, in that case, the metric tool The resist layer and the process layer are focused separately, and images are acquired using their respective focus settings. This is how imaging is performed. However, process variation effects and topography and The combination provides the optimal measurement "recipe," i.e., stable and reproducible data from proxy targets. A set of metrological conditions (e.g., focus, illumination numerical aperture, and wave) from which overlay measurement results are obtained. It becomes difficult to find the leader.

[0029] In the embodiments of the present invention described herein, the optical overlay proxy target is This invention provides an optical metrology tool and method that enables the independent identification of two sets of target features. These problems are addressed by doing so. In certain embodiments, the metering tool The two imaging cameras mounted on the vehicle are aligned with each other, but their proxies Focus is achieved at a fixed height difference on the target. Captured by these two cameras By comparing the resulting images, we can identify variations in the optical properties of the proxy target layer, such as focal point variations. The variation in the center of symmetry (CoS) due to imaging parameters such as spectral response and polarization is , it is calibrated and corrected.

[0030] In certain embodiments, a registration image is projected by a projector, for example, a two-dimensional grid. The child's image is projected onto those two cameras. The focus of the measuring tool is then set to each of them. By stepping through a series of stages that are sequentially equal to that fixed height difference, Camera 1 (let's call it CAM1) is positioned at the focal point where the other camera (CAM2) was positioned during the preceding stage. Always bring it to the setting. When CAM1 arrives at its new focal position, CAM2 When the focal point was at that position, CAM1 positioned the image acquired by CAM2. At each focal point, each camera acquires an image of its proxy target. The sequence of acquired images for each of the two sets of target features present in that proxy target is They are aligned with each other after going through various focus settings. Based on these two image sequences This allows us to calculate the variation in CoS due to focus for each of the two sets of target features. Yes, it is possible. Due to the stability of the CoS relative to the focus, these two sets of t-axis This reveals the focus setting at which the image of the acquired feature should be obtained. Through measurement, and consequently, the stable and reproducible overlay error in the semiconductor circuit Proofreading becomes possible.

[0031] In addition to or instead of this, the CoS variability of each set of target features is determined by the illumination wavelength and / or It maps in association with polarization, and by using that mapping, a stable overlay can be created. Measurement can be achieved. That is, in the current embodiments, two-dimensional measurement is possible using wavelength and focal point. The optimal measurement range is identified within the space.

[0032] [Description of optical inspection equipment] Figure 1 shows the optical properties of a patterned thin film layer on a semiconductor wafer 12 according to one embodiment of the present invention. This is a schematic pictorial representation of an optical inspection device 10 for measuring [something].

[0033] The optical inspection device 10 includes an imaging assembly 14, an illumination assembly 16, and an optical relay. The optical inspection device further includes a grid projector 20 and a camera assembly. A module 22, a controller 24, a memory 25, and a semiconductor wafer 12 placed on top thereof. It is equipped with a table 26. The orientation of the device 10 and its constituent members is in Cartesian coordinates 28. It is defined according to the following. The same Cartesian coordinates 28 are shown in the corresponding orientation in the figures shown later. The following uses lowercase x, y, and z to represent the three Cartesian coordinate axes, and uppercase X, The coordinates on the Y and Z axes are expressed using Y and Z.

[0034] The imaging assembly 14 is schematically shown as a single objective lens 30. Instead, assembly 14 can be used for an interferometric objective system (e.g., a Linic interferometer), a dark-field objective system, A phase-contrast objective system, or other suitable types of objective lenses or combinations of lenses and / or mirrors, It can also be considered a feature that is included.

[0035] Objective lens 30 is typically a composite lens with very high optical quality and high numerical values. The aperture (NA) is, for example, NA of 0.7 or more. According to one alternative embodiment, The object lens 30 has a variable NA, which is controlled by the controller 24. It is possible.

[0036] In the illustrated embodiment, the lighting assembly 16 is controlled by the controller 24, It is equipped with individual illuminators 15 and 17, each equipped with its own light source 32 and 33. These can be used to produce one or more discrete tunable wavelengths, or continuous waves (CW) or In pulsed form, across one or more continuous spectra, each beam 34 and 35 Optical radiation consisting of these is emitted independently. At light sources 32 and 33, optical radiation in various polarization states is emitted. For example, it is also possible to emit unpolarized, linearly polarized, or circularly polarized radiation.

[0037] Illuminators 15 and 17 are further connected to two individual light sources 32 and 33 It is equipped with aperture assemblies 36 and 37. Aperture assemblies 36 and 37 These are driven by their respective actuators 38 and 39, thereby providing the assembly 36 Various apertures enter the beam 34, and various apertures provided in the assembly 37 It is brought into the beam 35. Actuators 38 and 39 further bring the individual apartments Within the plane of the assembly, it is also possible to make fine adjustments to the individual apertures of each assembly. The beams 40 and 41 emitted from illuminators 15 and 17, respectively, The beam 43 is collinearly coupled by the splitter 42. Yes. In this type of dual lighting assembly, two illuminators are provided, resulting in device 1 The flexibility of 0 has been enhanced, and the process layer and resist layer on the wafer 12 are phase-independent. Lighting conditions (e.g., wavelength, polarization, and / or NA) can be provided.

[0038] Alternatively, the lighting assembly 16 may be equipped with a single illuminator, for example, illuminator 15. Assuming that the lighting conditions for the process layer and the resist layer are those of the light source 32 and aperture acetate The appropriate adjustment of the Nburi 36 may be used to select the appropriate setting. Alternatively, the lighting assemblies may be used. The humb is equipped with more than two illuminators, for example, three or four illuminators. The beams emitted from each illuminator are then connected to an appropriate optical device, such as a beam splitter. They may be combined in a similar manner.

[0039] The optical relay assembly 18 includes beam splitters 44 and 45, and beam splitter assembly The beam splitter assembly 46 includes a beam 46 and lenses 50 and 52. The beam splitters 47 and 48 provided therein use actuators 49, as will be described in detail later. The device 10 can be moved in and out of the optical path. Two sensors are provided in the camera assembly 22. Output arrays 54 and 56 can also be called "cameras," and are CAM1 and CA, respectively. It is labeled as M2. The camera assembly 22 further comprises CAM1 and CAM2 respectively It is equipped with two actuators 58 and 60 that move along the z-axis. In this figure, lens 5 Although 0 and 52 are shown as single lenses, they can be replaced with multiple lenses. It may also be equipped with a mirror.

[0040] The grid projector 20 projects grid images onto CAM1 and C, as will be further described in Figure 2. It is configured to project into AM2. The controller 24 controls the grid projector 20 , memory 25, table 26, light sources 32 and 33, and actuators 38, 39, 49, It is coupled to 58 and 60. The controller 24 typically performs the functions described in this application. Programs programmed within the software and / or firmware to do so A suitable digital and / or It is equipped with an analog interface. Alternatively, the controller 24 may be: Hardwired and / or programmed controllers that perform at least some of the functions of the controller. It is equipped with configurable hardware logic circuits. In Figure 1, for simplicity, the controller 24 is Although shown as a single, solid functional block, in reality, the controller is made up of multiple phases Interconnected control units, and receiving signals shown in the diagram and described in the text. It can be equipped with a suitable interface to support its capabilities.

[0041] Before operating the optical inspection device 10, the semiconductor wafer 12 is placed on the table 26. During operation, under the control of the controller 24, the wafer 12 is moved by the table 26 in x, y and It can be moved along the z-axis or rotated around the z-axis. This is called "focusing".

[0042] To illuminate the wafer 12, the illumination assembly 16 directs the beam 43 of optical radiation to the beam. The beam is radiated towards the plitter 44, where it is reflected into the objective lens 30. Next, the objective lens 30 focuses the beam 43 onto the wafer 12. In a vertical plane (yz plane), the beam 43 is present at the exit from the lighting assembly 16. The cross section shows apertures provided in aperture assemblies 36 and 37, which are suitably arranged and aligned. These apertures modify the shape of the cross-section of beam 43, for example. It is determined to be circular, square, or anamorphic, and along with that, the dimensions of its cross-section are determined. As will be described in detail later, beam 43 is connected to two beams having different wavelengths and / or polarization states. The beams are included, and the cross-sections of each of these two beams are made into aperture assembly 36 And 37 allows for independent control of each phase.

[0043] The apertures in assemblies 36 and 37 are typically conjugate to the entrance pupil of the objective system 30. (In consideration of simplicity, additional optical systems have been omitted from the drawing, and the image is projected onto the entrance pupil.) (and thus the cross-section of the beam 43 emitted from the illuminator assembly 16) This determines the numerical aperture (NA) of the optical radiation illuminating the wafer 12. This is determined. That is, the shape of the light is determined in angular space, for example, circular, square or anamorphic. It can be defined as a hard object, and the full NA of the object system 30 and a small portion of its full NA It can be changed between these two states. According to a certain configuration, the illumination is directed to the objective lens 30 The dark-field image of features on wafer 12 is limited to those with an NA value exceeding the focused NA, and the dark-field image of the features on wafer 12 is obtained. This makes it possible to perform the action.

[0044] The optical radiation illuminating the wafer 12 is directed towards the objective lens 30 by the wafer, that is, by the wafer The feature above is imaged towards the camera assembly 22, and the objective system is traced back. It is reflected. The reflected radiation is received by the objective lens 30 and further received by the beam splitter 44 And it is projected into the beam splitter assembly 46 via 45, where its reflected radiation The beam will strike either beam splitter 47 or beam splitter 48; which one will it be? The actuator 49 directs either of the two beam splitters into its path. It depends on how they are positioned. In this example, beam splitter 47 is wavelength neutral beam. A splitter is a device whose reflection coefficient and transmission coefficient exhibit the same spectral behavior. Beam splitter 48 is a dichroic beam splitter, and it is located in a certain spectral band Δλ. 1. For example, transmit 380-550 nm and another (non-overlapping) spectral band Δλ2 For example, it is configured to reflect light in the 560-800nm ​​range. That is, a beam splitter. When 47 is in the optical path, the reflected radiation is detected by cameras CAM1 and CAM2 respectively. A portion of it is received across its entire spectrum, while the beam splitter 48 receives the light When within the path, the spectrum of the radiation is split, and in CAM1, the spectral band Δλ1 In CAM2, radiation within the spectral band Δλ2 is received. Source 32 emits optical radiation within the spectral band Δλ1, and light source 33 emits optical radiation within the spectral band Δλ2 By emitting optical radiation from within, each of those two layers is illuminated (including the illumination NA). Independent control of these functions becomes possible.

[0045] Alternatively, or in addition to the above, one of the beam splitters 47 and 48 is used to transmit a certain polarization state. It may also be used as a polarizing beam splitter that passes through and reflects the polarization state orthogonal to it. That is, for example, For example, when optical radiation in orthogonal polarization states is emitted from light sources 32 and 33, The radiation from light source 33 is directed towards CAM1, and the radiation from light source 33 is directed towards CAM2. Similar to spectral splitting of illumination, by controlling the polarization of the illumination, the two layers can be related This enables independent control of the lighting. In one embodiment, the beam splitter 48 is dik This is a combination of a Loïc and a polarizing beam splitter.

[0046] The optical radiation transmitted and reflected by the beam splitter within the selected assembly 46 is, Lens 50 focuses on CAM1, and lens 52 focuses on CAM2. The image of wafer 12 is thus captured by CAM1 and CAM2, and controller 2 It is read and processed by method 4.

[0047] Figure 2 is a schematic pictorial representation of a grid projector 20 according to one embodiment of the present invention. The grid projector 20 directs the projector towards cameras CAM1 and CAM2, and these two cameras... A grid image is projected between the elements to be used as a position reference. The grid projector 20 is a light source assembly. The system comprises a bridge 80, a single-mode optical fiber 82, a diffraction assembly 84, and a spatial filter 86. It is.

[0048] In this example, the light source assembly 80 consists of two superluminescent light-emitting diodes. It is equipped with os(sLED) 88 and 90, of which sLED88 emits a wavelength λ1 = Optical radiation at 450 nm is emitted, and sLED90 emits optical radiation at a wavelength of λ2 = 750 nm. The light source 80 is further equipped with lenses 92, 94 and 96 and dichroic beams. It is equipped with a splitter 98. Alternatively, light sources of other types and wavelengths may be used.

[0049] The diffraction assembly 84 is a high-contrast arrangement located between two lenses 102 and 104. The transmission diffraction grating assembly 100 includes, for example, a chromium-on-glass grating assembly. The diffraction grating assembly 100 has orthogonal gratings, which allow light to be y and z It is diffracted in both directions. Through the cooperation of these gratings, the grating projected by the projector 20 The various parts of the child pattern are created.

[0050] The optical radiation emitted by sLED88 and 90 is absorbed by individual lenses 92 and 94. The beam is projected towards the dichroic beam splitter 98. The beam splitter 98 is s The optical radiation emitted by LED88 is allowed to pass through, and the light emitted by sLED90 is also allowed to pass through. Since they are configured to reflect chromatic radiation, the radiation emitted by those two sLEDs These are combined to form a single beam 106. Beam 106 is then filtered by lens 96 to form a single-mode light. The optical radiation is focused into the incident end 108 of fiber 82. The rays exit the fiber through its exit end 110 and enter the diffraction assembly 84, The beam 112 is projected towards the diffraction grating 100 by the 102. Since it is positioned in the focal plane of lens 102, beam 112 is parallel light. The light 112 is diffracted by the lattice assembly 100 to become parallel diffracted light 114, and The focal plane 116 is brought into focus by the element 104.

[0051] The spatial filter 86 is located within the focal plane 116 and rotates by the grid assembly 100. To allow only ±1st order diffracted light (±1st order diffracted light among the multidimensional diffracted light 114) generated by the diffracted light to pass through. It is configured. This function is detailed in insert 118, which shows the yz view. This shows the spatial filter 86 as viewed from the x-axis direction. The spatial filter 86 is transparent The transparent ring 120 is provided on an opaque base 122, for example, a chrome-on-glass base. The ring is formed by removing chromium. ±1st order diffracted light is within ring 120, s The radiation emitted by LED88 is represented as a square 124, and also by sLED90. The more radiated radiation is shown as square 126. The 0th order diffracted light is spatially diffracted. The central part 128 of the filter blocks the diffracted light of order ±1 or higher, and the peripheral part 1 of the spatial filter 86 It is blocked by 30.

[0052] The beam 132 is formed by ±1st order diffracted light after passing through the spatial filter 86. The mutual interference of these beams causes the propagating sinusoidal lattice to (interference between their ±1st order diffracted light These are brought forth as crossing patterns, and they are reflected by the beam splitter 45 (Figure 1) This becomes beam 134. The sinusoidal lattice will be described in detail later in relation to Figure 3. Beam 13 4 is the light reflected from the wafer 12 (Figure 1) toward cameras CAM1 and CAM2. Since it propagates collinearly with respect to chromatic radiation, the mutual resistance of those two cameras will be explained in detail later. Tension is possible.

[0053] The spectral content of the sinusoidal lattice propagating as beam 134 is sLED88 and 9 It depends on whether one or both of the 0s are excited and emit optical radiation. Emission wavelength λ1 And matching the spectral characteristics of λ2 with those of the dichroic beam splitter 48. Then, one of those wavelengths is reflected by the beam splitter and the other is transmitted. It can be done.

[0054] Figure 3 shows an image obtained by one of cameras CAM1 and CAM2 according to one embodiment of the present invention. This is a schematic representation of image 150. Image 150 in this example contains AIM(trademark) Proki The image of the target 152 and its proxy target is aligned with the grid projector 20. This includes four grids 154, 156, 158, and 160 projected by A. Target features within IM(trademark) proxy target 152 have x and y axes along There are four resist grids 162 that are oriented in pairs, and also along the x and y axes. It contains four process layer grids 164 that are paired and oriented. In consideration of clarity, only two of the resist grids 162 and two of the process layer grids 164 are framed. Enclose the area, and determine the orientation of one of the areas along the x-axis, and the other area along the y-axis. It is considered to have been ordered.

[0055] Other types of proxy targets use other forms of target features instead. This can happen. For example, a so-called frame-in-frame proxy target within a target The feature consists of a square frame made up of bars.

[0056] For the purpose of calculating the overlay error between the resist layer and the process layer, the controller 24 uses Based on the image of target 152, the X and Y coordinates of the CoS of the resist grid 162 (CoS X,R CoS Y,R ) is calculated, and similarly the X and Y coordinates of the CoS of the process layer grid 164. Mark (CoS) X,P CoS Y,P ) is calculated. (Symmetrical projection onto wafer 12) The difference between the X and Y coordinates of the center results in the overlay error OVL for both X and Y.X =(C oS X,R -CoS X,P ),OVL Y =(CoS Y,R -CoS Y,P is obtained . In view of simplicity, hereinafter, a two-dimensional vector (CoS X ,CoS Y ) is denoted using CoS .

[0057] The gratings 154, 156, 158 and 160 are projected by the grating projector 20 as described above (Figure 2). The spatial division and arrangement of these gratings is achieved, for example, by dividing the gratings in the diffraction grating assembly 10 0 into two pairs of orthogonal gratings. Both cameras CAM1 and CAM2 "see" the gratings 154, 156, 158 and 160, and by using these gratings with the controller 24 (Figure 1) , the positions of the two cameras can be aligned with respect to these gratings in both the x and y directions. As will be described in detail later, this alignment (registra tion) is an essential part of the process for accurate registration of successive images of the target 152 .

[0058] [Disclosure of Resist and Process Target Features in Overlay Metrology Proxy Target The following figures show a wide variety of methods for measuring CoS variation of target features of an overlay proxy target as a function of various imaging parameters, such as focus and illumination wavelength . For convenience and clarity, such methods are described above, in connection with the system structure and components shown in the preceding drawings and in connection with a particular type of proxy target and the target features provided therein. Nevertheless , ​​​This will be obvious to any person skilled in the art who has read this specification. As such, the principles of these methods, with necessary modifications, can be applied to other overlay metrology systems. It can be applied similarly, and it can also be done using other types of proxy targets. Furthermore, by combining the various elements of these different methods, multi-factor CoS measurement and calibration can be performed. It may be possible to enable the correct operation. All such alternative implementations are within the technical scope of the present invention. It is thought that this is the case.

[0059] Figure 4 relates to one embodiment of the process layer of an overlay metric proxy target. and a process for measuring the CoS variation of the resist layer target feature as a function of focus. However, this is flowchart 200, which is schematically depicted. In this process, the light shown in Figure 1 Refer to the inspection device 10, and also refer to Figures 2 and 3. Figure 4 shows The purpose of this process is to associate it with the focus setting of the wafer 12 in the apparatus 10. - Valley proxy target, for example, AIM (trademark) target 152, each layer target The goal is to individually determine the location of the CoS of each feature.

[0060] This process starts in step 202. In step 204, the z-direction is The camera is moved by moving the table 26 and / or by actuators 58 and 60. By doing so, the controller 24 controls the camera CAM1 and to focus on the wafer 12. CAM2 is set. These cameras are moved by the differential dynamic motion of actuators 58 and 60. The focus setting is set to a focal difference of ΔZ (the focus setting in this specification is in wafer space) This refers to the Z coordinate. For example, the focal difference ΔZ is the distance between when cameras CAM1 and CAM2 are in focus. This indicates that each of the x and y planes is separated by a ΔZ on or near the wafer 12. (This is being done.) Further details on the focus separation and focusing of cameras CAM1 and CAM2 can be found in Figure. This will be discussed later in relation to point 5.

[0061] In the first grid registration step 206, both cameras CAM1 and CAM2 , for the grids 154, 156, 158 and 160 projected by the grid projector 20 Alignment is performed. This registration is performed by controller 24. The acquired images of these grids are processed, thereby determining the x and y directions of each camera relative to those grids. The relative position along the direction is searched. To align the cameras with each other, the cameras are placed in a grid. Physically move 154, 156, 158, and 160, or offset the camera / grid space. This can be calculated and used in subsequent processing. The grid has a periodic form and CAM1 and CAM Because the pixels within 2 form a repeating structure, the controller 24 controls (within wafer space) (Regarding x and y coordinates) With accuracy better than 0.1 nm, each camera is positioned relative to those grids. This can be done. Furthermore, the same grid is projected onto each camera, CAM1 and CAM2. Therefore, any spatial shift or vibration of the projected grid is common to both cameras. This occurs in this mode. Each camera is aligned to the same common-mode grid. Therefore, these cameras are aligned with each other with an accuracy of 0.1 nm or better. Yes. In the first acquisition step 208, the image of proxy target 152, specifically the grid, is obtained. Those 162 and 164 are read from cameras CAM1 and CAM2 by controller 24. It is extracted and stored in memory 25.

[0062] In the refocusing step 210, the table 26 moves the wafer 1 along the z-direction by a distance ΔZ. By moving 2, CAM1 is moved to the Z coordinate where CAM2 was before step 210. It is inserted. In the second grid registration step 212, the controller 24 Again, as in the first grid registration step 206, grids 154, 156, 15 Cameras CAM1 and CAM2 are aligned to 8 and 160. The objective is to ensure continuous registration between those two cameras. In CAM1 registration step 214, CAM1 performs CA at the previous focal position. Since it is aligned with the image acquired by M2, the second grid registrations In conjunction with step 212, these two cameras in the xy plane are in relation to the previous focal position. The position will be established. In the second acquisition step 216, the first acquisition step 208 Similarly, the image of proxy target 152 is controlled via cameras CAM1 and CAM2. It is read by the server 24 and stored in memory 25.

[0063] In the discrimination step 218, the controller 24, based on the predetermined sequence of focus stages, It is determined whether another focusing stage is needed. If the answer is yes, this process refocuses. Return to step 210 and continue from there. Once all predetermined focus stages have been taken, In calculation step 220, the controller 24 calculates the various images stored in memory 25. By processing this, the CoS of each lattice 162 and 164 undergoes the focal stage of ΔZ. It is calculated as a function of point setting. This process is terminated in termination step 222. ru.

[0064] In certain embodiments, the direction in which the illumination strikes the wafer 12 (direction of impact: direction of illumination) By adjusting the direction of travel, residual optical errors in the imaging optics of the weighing tool can be corrected. The difference is compensated. For example, in the optical inspection device 10, the controller 24 adjusts the yz plane. By adjusting the position of the aperture assembly 36 using the actuator 38, The residual optical error in the object lens 30 can be compensated for by the grids 162 and 164 respectively. Because the CoS of depends on the position of aperture assembly 36, By measuring CoS as a function of focus for 36 different locations, we can obtain more comprehensive data. It can be collected. In one embodiment, the process described in flowchart 200 is A set of Y and Z coordinates for a percha assembly, e.g., (Y0±n*ΔY, Z0±n*ΔZ) This is performed with respect to; however, Y0 and Z0 are the nominal center positions of the aperture assembly 36, Δ Y and ΔZ represent the increment of the aperture assembly, where n is 0. This is an integer exponent for which values ​​up to the maximum value N are assumed. (See the section titled "Selection of Measurement Conditions") As will be described later, the obtained data can be used to further improve the quality of overlay measurements. can.

[0065] Figure 5 relates to one embodiment of the present invention, showing the focal distance between CAM1 and CAM2 and the projection in Figure 4. This plot diagram schematically shows the various focal stages taken in Seth.

[0066] The positions of the two cameras CAM1 and CAM2 in wafer space using Z coordinates, i.e., wafer 12 The Z coordinate, relative to the given point, is shifted through N focal steps. As noted in the appendix, in the first stage 250, CAM1 is focused on the plane Z=Z0, CAM 2 is focused on the plane Z = Z0 + ΔZ. In the second stage 252, the wafer focus is ΔZ. It is incremented, so that CAM1 becomes Z=Z0+ΔZ and CAM2 becomes Z=Z0+2 Because it is brought into ΔZ, CAM1 is located here, and CAM2 is in the first stage at 250. The focal position becomes the same as before. In the third stage 254, the wafer focal point moves in again by ΔZ. The values ​​are adjusted so that CAM1 is set to Z=Z0+2ΔZ and CAM2 is set to Z=Z0+3ΔZ. This process continues until the Nth stage 256, and the location of CAM1 is Z=Z0+(N- 1) The location of ΔZ and CAM2 is Z = Z0 + NΔZ.

[0067] In other words, at each focal stage, CAM1 moves to the focal point where CAM2 was located in the previous stage. Because it is positioned, registration becomes possible between successive stages. The combination of these steps and the lattice registration steps 206 and 212 (Figure 4) Furthermore, controller 24 controls each camera along the x and y directions in a longitudinal (through-focus) manner. To precisely align the lattice and to ensure that each of the lattices 162 and 164 is true to the focal point longitudinally. It becomes possible to calculate S.

[0068] Figures 6A-6B relate to one embodiment of the present invention, and show an overlay metering in relation to the illumination wavelength. The process for measuring the CoS variation of the target feature of a Roxy target is schematicly The flowchart 300 is shown. In this process, the optical detector shown in Figure 1 In addition to using the inspection device 10, Figures 2 and 3 are also referred to. As shown in Figures 6A-6B The purpose of the process is to use overlay processing in relation to the wavelength used by the apparatus 10. For example, the target features of AIM (trademark) target 152 are each Co The objective is to individually determine the position of S. Flowchart 300 shows grid 164, The CoS of grating 162 is calculated as a function of the illumination wavelength in Part 1 301 and Part 2 302. It has.

[0069] This process begins in starting step 303. Beam splitter placement step 30 In step 4, the controller 24 excites the actuator 49, thereby creating a dichroic vibrator. As the optical splitter 48 is brought into the optical path of the device 10, the optical path of the device 10 The radiation is received by CAM1, and the portion of that radiation within the spectral band Δλ1 is received by CAM1. The emission is divided so that the portion within the spectral band Δλ2 is received by CAM2. Yes.

[0070] Subsequently, the process enters the first part 301. In the first illumination step 306, the semiconductor is Eh12 is illuminated with wavelengths λ1 and λ2; however, wavelength λ1 is within the spectral band Δλ1. The wavelength λ2 lies within the spectral band Δλ2, and optical radiation is emitted at wavelength λ1 by the light source 32. Then, optical radiation at wavelength λ2 is emitted by the light source 33. In the first focusing step 308, The controller 24 uses the table 26 and actuators 58 and 60 to control CAM1 is the contrast focus on grid 162, and CAM2 is the contrast focus on grid 164. The focus is set to the contrast. The term "contrast focus" refers to the point where the grid image on each camera is at maximum contrast. This refers to the focal position that exhibits Trust C. Contrast C, on a given camera, refers to the position of the image. Maximum and minimum intensity I of the sub-imagemax and I min Based on this, C=(I max -I min ) / ( I max +I min ) is defined as. First lattice registration step 31 In step 0, the projected grid is as in the first grid registration step 206 (Figure 4). Cameras CAM1 and CAM2 are aligned to points 154, 156, 158, and 160. Then, in the first acquisition step 312, the images of grids 162 and 164 are taken by camera CAM Acquired by 1 and CAM2, and read from those cameras by controller 24, It is stored in memory 25.

[0071] In the first wavelength changing step 314, the controller 24 controls the spectrum using the light source 33. By incrementing the wavelength of optical radiation emitted within the band Δλ² by Δλ, CAM The wavelength of illumination reaching 2 is changed from λ2 to λ2+Δλ. First refocusing step 316 Then, CAM2 is refocused to the contrast focus at the wavelength after that increment. In the second lattice registration step 318, the first lattice registration step As in 310, cameras CAM1 and CA are positioned relative to grids 154, 156, 158 and 160. M2 is aligned. In re-registration step 320, CAM1 is acquired. The same image of grid 162 obtained through CAM1 in step 312 is recombined. The focus is refocused and realigned. That is, the same wavelength and the same focal point are used on the same physical grating 162. The position is continuously adjusted, and this allows CAM1 to act as an "anchor" for section 301. This is established. In the second acquisition step 322, the image of grid 164 is acquired by CAM2. The data is then read from the camera by the controller 24 and stored in the memory 25.

[0072] In the first discrimination step 324, the controller 24 follows a predetermined sequence of wavelength steps. Well, it determines whether another wavelength step is needed within the spectral band Δλ2. If the answer is affirmative... If available, this process returns to the first wavelength changing step 314, and the wavelength of illumination reaching CAM2 The process continues after incrementing again by Δλ. All preset focal lengths Once the point steps are exhausted, the controller 24 will, in the first calculation step 326, Based on the images stored in Mori 25, the CoS of lattice 164 is determined to be in the spectral band Δλ. It is calculated as a function of wavelength passing through the various wavelength steps Δλ within 2.

[0073] This process continues in Part 2, 302, which clarifies the differences between Parts 1 and 2. This is described in detail in Part 2, Steps 328, 330, 332, 334, 340 and 346 correspond to steps 306, 308, 310, 3 in the first part 301. It is very similar to 12, 318 and 324. However, the step in the second part 302 336, 338, 342 and 344 correspond to steps 314, 316 in the first part 301. Unlike 320 and 326, in those steps of the second part, spectral scanning is performed. Unlike in the first part, where the spectral range Δλ² is traversed longitudinally, the spectrum It is performed in a manner that traverses the range Δλ1.

[0074] In the second illumination step 328, the semiconductor wafer is illuminated with 12 wavelengths λ1 and λ2. In the second focusing step 330, the controller 24 adjusts the camera CAM1 on the grid 162. The contrast focus, CAM2, is focused on the contrast focus on grid 164. Third grid In the child registration step 332, cameras CAM1 and CAM2 are on grid 154. Aligned to 156, 158 and 160. In the third acquisition step 334, the grid Images 162 and 164 are acquired by cameras CAM1 and CAM2, and controller 2 The data is read from these cameras by step 4 and stored in memory 25.

[0075] In the second wavelength changing step 336, the controller 24 controls the spectrum of the light source 32. The wavelength of optical radiation emitted within the Torr band Δλ1 is incremented by Δλ. In focusing step 338, CAM1 is incremented to a wavelength Δλ1+Δλ. The camera is refocused to the trust focus. In the fourth grid registration step 340, the camera CAM1 and CAM2 are aligned with grids 154, 156, 158, and 160. In the second re-registration step 342, CAM2 is in the third acquisition step 334 Refocusing and repositioning on the same grid 164 image read from CAM2. This is done, and as a result CAM2 is established as an anchor for the second part 302. In step 344, the image of the grid 162 is acquired by camera CAM1, and the control The data is read from the camera by R24 and stored in memory 25.

[0076] In the second discrimination step 346, the controller 24 follows a predetermined sequence of wavelength steps. Well, it is determined whether or not another wavelength step is needed within the spectral band Δλ1. If the answer is affirmative... If available, this process returns to the second wavelength change step 336, and the wavelength of illumination reaching CAM1 The process continues after incrementing again by Δλ. All preset waves Once the long steps have been taken, the controller 24 will perform the second calculation step 348, lattice The CoS of 162 is calculated as a function of wavelengths passing through various wavelength steps Δλ within the spectral band Δλ1. This process is terminated at termination step 350.

[0077] Similar to the measurement of CoS variation due to focal point, the obtained A for CoS variation due to wavelength Furthermore, as will be explained later in the section titled "Selection of Measurement Conditions," the quality of overlay measurements can be further improved. It can be used to do good.

[0078] The measurement of CoS variation due to illumination wavelength is performed using the polarization state of its optical radiation as an additional parameter. This may be done. In one embodiment, the CoS variation due to wavelength is radiated onto the wafer 12. Measurements are taken regarding the various polarization states of the incoming light. That is, receiving commands from the controller 24. Furthermore, light sources 32 and 33 emit optical radiation in two orthogonal polarization states, illuminating the area. The variation in CoS due to wavelength is measured individually for each polarization state. An alternative embodiment is: EHA12 is illuminated with unpolarized optical radiation, and a specific state of polarization is detected by two cameras CAM1 and This reaches each of CAM2, and its specific state also functions as a polarizer. By the dichroic beam splitter 48, or by its dichroic beam splitter Some aspects are determined by the polarizer appropriately placed between the camera and the two cameras.

[0079] Figures 7A-7B relate to one embodiment of the present invention, and C over various focal settings and wavelengths. The flowchart schematically depicts the process of generating the OS landscape and evaluating its sensitivity. This is Chart 500.

[0080] Scanner-induced overlay errors, such as mispositioning of wafer 12 in the scanner and rotation To capture motion and scanner field distortion, overlay errors are generally measured on the wafer by a number of points. Measurements are taken at individual measurement sites. The process shown in flowchart 500 is wafer This concerns measurements of N sites on 12, and here, n of those sites The layers are numbered (n=1,2,...N). Furthermore, this process consists of a resist layer and a process layer. It will be carried out with respect to both parties.

[0081] CoS was measured in relation to the focal point at 12 different wafer orientations, and of these, the second orientation... Regarding position, the wafer is rotated 180° around the Z-axis. CoS associated with focus Of these, the one in the first direction (arbitrarily set to 0° direction) is CoS0(Z), and the one in the second direction is... CoS 180 (Z) indicates the tool induction of the CoS calculated by controller 24. The starting shift (TIS), i.e., CoS_TIS, is given by CoS_TIS(Z) = (CoS0(Z) + CoS 180 (Z) / 2. The CoS error described by CoS_TIS is the instrument Asymmetry of optical components in the imaging path, for example, due to the asymmetry of the objective lens 30. This is the corrected CoS, i.e., the measured CoS minus CoS_TIS, which is then used as the control. Roller 24 can calculate it as CoS_COR(Z); however, CoS_ COR(Z)=[CoS0(Z)-CoS 180 (Z)] / 2, and "COR" is "correction It represents "after". As mentioned above, CoS is a two-dimensional vector (CoS X CoS Y ) represents , and the notation "CoS" includes both the X and Y coordinates thereof.

[0082] Inaccuracy in CoS measurement caused by mechanical vibration of apparatus 10 can be reduced by acquiring multiple images of proxy target 15 2 a plurality of times and averaging those measurement results.

[0083] This process starts at start step 502. In polarization selection step 504, the polarization state (one or more types) of illumination emitted by illumination assembly 16 is selected . In site selection step 506, a site n on wafer 12 is selected. In wavelength selection ste p 507, a wavelength λ is selected. In 0-degree CoS through-focus step 508, as described in connection with FIG. 4 and FIG. 5, CoS is measured throughout the focus Z direction. In 180-degree CoS throug h-focus step 510, the above measurement is repeated, wherein the orientation of wafer 12 is rotated 180° relative to that in step 5 08. In first CoS_TIS step 512, Co S_TIS is calculated by controller 24 for each focus setting Z based on individual CoS values at 0° orientation and 180° orientation as CoS_TIS=(CoS0+CoS 180 ) / 2 . For simplicity, explicit dependence on focus Z, wavelength λ, polarization P and site n is omitted in these formulas. In first CoS_COR step 514, Co S_COR is calculated by controller 24 for each focus setting Z based on individual CoS values at 0° and 180° as CoS_COR=(CoS0-CoS ) / 2 180 calculated as (where explicit reference to the variables is also omitted herein). Wavelength discrimination step 51 In 6, the controller 24 determines whether steps 507 to 51 4 should be re-executed based on a preset list of wavelengths. If the determination is affirmative, in the wavelength increment step 517, the wavelength λ is incremented, and the process continues from step 507.

[0084] After all preset wavelengths are exhausted, the process then, in connection with FIGS. 6A-6B as described, measures CoS in association with wavelengths at the best contrast focus. Although CoS has been measured in association with focal points for all required wavelengths in the preceding steps due to drift of the apparatus 10, part of the CoS measurement results in relation to the focal point may drift along the focal coordinate Z. As will be described in detail below, such drift can be corrected using CoS measurements in relation to wavelengths.

[0085] In 0-degree CoS through wavelength step 518, CoS is measured across the preset spectrum of wavelengths at the best contrast focus. In 180-degree CoS through wavelength step 52 0, the above measurement is repeated, but the orientation of the wafer 12 is rotated by 1 80° relative to that in step 516. In the second CoS_TIS step 522 and the second CoS_COR step 524, CoS_TIS and CoS_COR are calculated by the controller 24 based on the data obtained in steps 518 and 520, respectively, as described in connection with each of the foregoing steps 512 and 514.

[0086] In CoS_TIS stitching step 526, for each wavelength λ, the controller 24 takes the best contrast focus as the focus Z and obtains focus-through CoS_TI from steps 507 to 516 The result obtained for S is the result obtained for CoS_TIS from step 522. The results are compared. If there is a discrepancy between the two results, the longitudinal C at the focal point at wavelength λ is used. By shifting the results related to oS_TIS along the focal coordinate Z, the discrepancy is eliminated. This results in the focal point at adjacent wavelengths, as will be explained in more detail later in relation to Figure 8A. The results of the longitudinal CoS_TIS are "stitched together", and the CoS_TIS in the two-dimensional Zλ space This results in a consistent representation. In CoS_TIS Landscape Step 528, Controller 24 This representation is collected and becomes a set of CoS_TIS values ​​for two variables Z and λ. This collection is called the CoS_TIS landscape.

[0087] In the CoS_TIS derivative step 530, the controller 24 controls the variables Z and λ The second derivative ∂ of CoS_TIS shows the sensitivity of CoS_TIS to fluctuations. 2 Value of CoS_TIS / ∂Z∂λ This is calculated. In the minimum CoS_TIS step 532, the controller 24 calculates this. Within the CoS_TIS landscape, the absolute value of CoS_TIS is in the (Z,λ) plane. One or more two-dimensional areas that fall below a predetermined limit are identified, and as a result, the CoS This indicates the area where the tool-induced shift is minimized, i.e., the area where the CoS error is minimized (C (Different limits may be assigned to the X and Y components of oS_TIS). Minimum CoS_TIS In function step 533, the controller 24 controls its CoS_TIS landscape Within the (Z,λ) plane ∂ 2 The absolute value of CoS_TIS / ∂Z∂λ is not within another predetermined limit. One or more two-dimensional areas that are full are identified, and consequently, the stability of CoS_TIS is determined. The area with the highest property is indicated.

[0088] Similar to CoS_TIS stitching step 526, in CoS_COR stitching step 534, for each wavelength λ, the controller 24 sets the focus Z as the best contrast focus in step 50 the results obtained regarding the through-focus CoS_COR from 7 to 516 are obtained from step 524 and compared with the results obtained for CoS_COR. If there is a discrepancy between these two sets of results , the through-focus CoS_TIS result for wavelength λ is shifted along the focal coordinate Z to eliminate the discrepancy. Thereby, the through-focus CoS_COR results for adjacent wavelengths are "stitched" to obtain a consistent representation of CoS_COR in the Zλ space .

[0089] Similar to CoS_TIS landscape step 528, CoS_COR landscape step 536, this representation is collected by the controller 24, and is the set of CoS_COR values for the two variables Z and λ , that is, what is referred to as the CoS_COR landscape in the present application . In CoS_COR derivative step 538, by the controller 24, the second derivative ∂ that indicates the sensitivity of CoS_COR to changes in the variables Z and λ 2 CoS_COR / the value of ∂Z∂λ is calculated.

[0090] In the minimum CoS_COR derivative step 540, the controller 24 determines that within the CoS _COR landscape, ∂ 2 the absolute value of CoS_COR / ∂Z∂λ is further within another predetermined limit one or more two-dimensional areas that are less than the degree are identified, whereby CoS_COR The area with the highest stability is indicated (CoS_COR is an arbitrary offset relative to 0). Since it may be considered to have, identify the area where CoS_COR is less than a predetermined limit. (There is no theoretical basis for this.)

[0091] In site determination step 542, the controller 24 determines another site on the wafer 12 It is confirmed whether or not it is necessary to measure n. If the result is positive, this process is Returning to step 506, the next site is selected. (Regarding the current polarization) All sites are total Once the measurement is complete, the controller 24 performs a polarization discrimination step 544 to determine the additional polarization. It is confirmed whether or not measurements need to be taken under the current lighting conditions. If the result is positive, Then, the process returns to step 506 and uses the new polarization state of illumination to illuminate all N sites This is measured again. Once all necessary polarization states have been used, the process ends in step 546. This process will now be terminated.

[0092] Figures 8A to 8D relate to one embodiment of the present invention, and the CoS related to the resist and process layer. This is a schematic representation of the _TIS and CoS_COR landscapes. (Figure 8A shows...) This is the CoS_TIS landscape 600 related to the resist layer, as shown in Figure 8B. The CoS_COR landscape 602 related to the resist layer is shown in Figure 8C. The CoS_TIS landscape 604 related to the process layer, shown in Figure 8D, is the process This is the CoS_COR landscape 606 related to the layer. Each landscape 600, 60 2, 604 and 606 relate to a given site n and a given polarization P. A cape is generated for all N sites and for all polarization states of illumination. Due to the docscape, a certain wavelength range (λ min ,λ max ) and a certain focal range (Z min , Z max The individual values ​​of CoS_TIS or CoS_COR in ) are shown. Curve 6 08, 610, 612 and 614, for individual landscapes 600, 602, 604 Regarding 606, the focal position of the best-contrast focus in relation to wavelength is shown.

[0093] The area identified in steps 532, 533, and 540 of Figure 7A is Landscape 6 It is shown within 00-606. That is, within area 616, ∂ 2 CoS_TIS / ∂Z∂ λ is less than the predetermined limit L1, and within area 618, CoS_TIS is less than the predetermined limit L2. Yes, within Area 620 2 CoS_COR / ∂Z∂λ is less than the predetermined limit L3. Inside the rear 622 ∂ 2 CoS_TIS / ∂Z∂λ is less than the predetermined limit L4, and area 62 Within area 4, CoS_TIS is less than the specified limit L5. Within area 626, ∂ 2 CoS_C OR / ∂Z∂λ is less than the predetermined limit L6. Thus, areas 616 and 622 are Each of these resist and process layers shows an area where CoS_TIS stability is high. Areas 618 and 624 are those where the CoS_TIS value is small for those layers, i.e. This indicates areas with minimal measurement tool error. Areas 620 and 626 are respectively... The stability of CoS_COR is high in the t-layer and process layer.

[0094] The concept of "suturing," introduced in steps 526 and 534 of Figure 7A, is schematically shown in Figure 8A. It is drawn in a specific direction. The three lines 628, 630, and 632 represent the function of the focus Z, CoS_T. IS is individual wavelength λ i-1 , λ i and λ i+1 Follow steps 507-516 in Figure 7A This shows the three measured paths. Three points 634, 636 and 6 on curve 608. 38 shows that CoS_TIS, a function of wavelength λ, was measured in steps 518-522 of Figure 7B. It is pointing to the place where it is located. It is on line 628 and intersects with curve 608. The CoS_TIS value is compared to the CoS_TIS value at point 634. If the two values ​​are the same If they are the same, line 628 will not be shifted. Conversely, if those values ​​do not match, line 6 The value on line 628 where 28 (and its CoS_TIS value) intersects curve 608 is It is shifted along the Z direction until it matches the value at point 634. A similar process is performed at point 63 This is repeated with respect to line 630 in relation to 6, and with respect to line 632 in relation to point 638.

[0095] Those three lines 628, 630 and 632 are shifted as needed, and each line and curve 608 When the value at the point where the two lines intersect matches the values ​​at individual points 634, 636, and 638, then These lines are, so to speak, "sewn together" as one. This process applies to all similar lines. Regarding λ min from λ max This is repeated until the CoS_TIS value is calculated longitudinally. Any drift that may occur in the device 10 during measurement is corrected. A similar suture calculation is performed on the landscape 60. This also applies to 2, 604 and 606.

[0096] Figures 9A and 9B relate to one embodiment of the present invention, and the accuracy of the resist and process layer. This is a schematic representation of the landscape. In this embodiment, the accuracy of the CoS measurement is evaluated. For accuracy measurement, the CoS is measured several times consecutively, for example, 5, 10, or 15 times. Then, an index of its accuracy is calculated. Common indicators include the 3σ value (3 standard deviation) of those measurements. There is a multiplier. In one embodiment, the accuracy of CoS (CoS) is obtained during the aforementioned measurement (Figures 5-7). Measure the value (denoted as _Prec) and associate it with two variables: focal point Z and wavelength λ. A precision landscape is generated.

[0097] In Figure 9A, Landscape 700 represents the accuracy of the CoS of the resist layer, and in Figure 9B... Landscape 702 represents the CoS accuracy of the process layer. Each landscape is Landscapes 600, 602, 604 and in Figures 8A, 8B, 8C, and 8D respectively Similar to 606, in a certain wavelength range (λ min ,λ max ) and a certain focal range (Z min ,Z m ax This represents the individual values ​​of CoS_Prec in Landscape 700 and 70. 2 has its own curves 704 and 706, thereby providing the best contrast in relation to wavelength. The focal position of the resist is shown. Located within landscape 700 and related to the resist layer C Based on the oS_Prec value, the controller 24 sets CoS_Prec to a predetermined limit L Areas 708 and 710, which have values ​​less than 7, have been identified. Similarly, landscape Based on the CoS_Prec value related to the process layer located within the 702, the controller 24 is informed Furthermore, areas 712, 714 and Bi716 has been identified.

[0098] [Selection of measurement conditions] The weighing recipe, i.e., various parameters of the apparatus 10 during overlay weighing (e.g., focus, The settings for wavelength and polarization can have a significant impact on the quality of the measurement results. Details are provided below. Thus, the user of device 10 can achieve a desired balance between various factors, such as stability and accuracy. You can choose to change the measurement conditions. Stratified weighing results are shown in Figures 4 to 9. The result analysis provides users with a sophisticated set of tools, and with that, the user Measurement conditions can be selected while keeping the specific goals in mind. The following are the tools used for this purpose. Two exemplary embodiments illustrating the usage of [the term] are presented.

[0099] Embodiment 1 - In this embodiment, the index M1 is controlled by the controller 24 to measure all measurements Regarding the site and all polarization, Landscape 600, 602, 604 and 606 (Figure 8) Based on Figures A to 8D and Landscapes 700 and 702 (Figures 9A and 9B) It is calculated for each layer.

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[0100] The variables in this index and its components include the wavelength of illumination λ, the focal coordinate Z, the polarization state P, and the layer. There is an L (resist layer or process layer). AVG N This is a flat measurement across N measurement sites. The average, 3σ N This is three times the standard deviation across those N sites, and AVG N Reach 3σ N This includes the effects of process variations across the landscape, encompassing various sites and polarizations. It is done so. For simplicity's sake, the precision is indicated as "Prec".

[0101] Indicator M1 includes CoS_TIS as a contributing factor, and therefore the accuracy of the measurement (T Emphasis is placed on evaluating the error-induced error. LIMIT(M1) is set to a predetermined limit for M1. death, M1(λ,Z,P,L) <LIMIT(M1) By finding the measurement conditions that result in the smallest tool-induced error, the measured overlay error can be minimized. This will result in a difference.

[0102] In the case of the device 10 (Figure 1) having two illuminators 15 and 17, the measurement conditions are stratified By using the index M1, each layer can be optimized independently. If only one data set is available, a compromise must be found between the measurement conditions of the two layers. This would be the case. For example, potential requirements for a single illuminator device are: M1(λ,Z,P,L resist )+M1(λ,Z,P,L process ) <LI MIT' (M1) This is the result; however, L resist ,L process These are the resist and process layers, respectively. Therefore, LIMIT'(M1) is a (different) predetermined limit.

[0103] If CoS_TIS is a certain correction factor related to the tool, then a one-step calibration procedure would It might be possible to correct it. However, the target of the proxy target Because there is a connection between the local geometry of the metric tool and the optical response of the metric tool optical system, on the wafer If multiple locations are measured, CoS_TIS may change for each target. CoS_TIS fluctuations can occur as a result of process variations across the wafer.

[0104] The contribution of CoS_TIS to index M1 is calculated for each aperture assembly 36 and 37. By shifting the aperture horizontally, the reduction can be achieved layer by layer.

[0105] Embodiment 2 - In this embodiment, the indicator M2 is controlled by the controller 24 at indicator M1 The same landscape is used for each layer, however, the formula for index M2 is the same as for index M1. It is different from that.

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[0106] Since the index M2 contains the (second-order) derivatives of both CoS_COR and CoS_TIS, This represents the stability of CoS during overlay error measurement. That is, LIMI Let T(M2) be the predetermined limit for M2. M2(λ,Z,P,L) <LIMIT(M2) By requiring this, the measured overlay error exhibits a high degree of stability. This will result in accurate calibration of measurement overlay errors and robust overlay measurement. We can provide support.

[0107] The same variables are used for the indicator M2 as for M1, and AVG N and 3σ N teeth It is applied to N sites, similar to that in index M1. The lighting assembly 16 illuminates Taking into account whether the configuration has one or two "Ta" elements, the same considerations as in M1 will serve as indicators. This also applies to M2.

[0108] Alternatively or in addition to this, the origins of Landscape 600, 602, 604 and 606 and Land Using data from Scape 700 and 702, we generate other metrics and address various user-specific requests. You may reflect this.

[0109] Figure 10 relates to one embodiment of the present invention, in which the variation of CoS_TIS is the aperture offset. Plot 800 is schematically depicted as a function of .

[0110] Plot 800 shows measurements taken by the device 10 for four measurement sites (n=1, ..., 4). The resulting CoS_TIS fluctuation affects one of the apertures provided in the aperture assembly 36. This is shown in relation to the individual horizontal offset AO. It also shows the individual CoS_T values ​​for various sites. The IS value is shown as line 802. The variation across those four sites, for example, layer control Lastly, due to feature topology, focus variation, site slope, and process variation, line 8 02 exhibits different offsets and inclinations. CoS_TI across those four sites The mean of S, i.e., the function of AO, AVG(CoS_TIS), is shown as line 804. ru.

[0111] The optimal (minimum) CoS_TIS variation across four measurement sites, i.e., 3σ(CoS_TIS S)1 is found at an aperture offset AO1, where CoS_TI S=CoS_TIS1. Non-zero values ​​of AO1 are wafer 12·imaging This indicates that there is a global angular alignment error between the 14 semblages. By choosing an aperture offset larger than AO1, 3σ(CoS_TIS) becomes larger. Although it becomes smaller, AVG(CoS_TIS) becomes smaller. This indicates that there is an opportunity for optimization between and 3σ(CoS_TIS). (Figure 1, apparatus 10) As shown above, by using independent light sources and aperture assemblies, AVG( Independent optimization becomes possible between CoS_TIS and 3σ(CoS_TIS).

[0112] Figures 11A and 11B illustrate the application of the CoS function of the focus in one embodiment of the present invention. Sidewall asymmetry of target features within AIM(trademark) proxy target 152 The explanation of sexuality is schematically depicted.

[0113] Figure 11A is an image of the AIM (trademark) proxy target 152 (shown in Figure 3). This is the case around the resist grid 162 and process layer grid 164 which are oriented along the x-axis. Outline lines are drawn on them. Individual grid bars 902 and 904 of grids 162 and 164 are y It is determined along the axis.

[0114] Figure 11B shows the process layer grid 164 taken along lines 908a and 908b in Figure 11A. These are schematic cross-sectional views of the lattice bars 904a and 904b. The two cross-sectional views are shown together. For illustrative purposes, the grid bars and their x-axis spacings are not shown on the same scale.

[0115] Asymmetric process effects in semiconductor manufacturing processes, such as asymmetric etching, are caused by Figure 11. As shown in B, the grid bar 904 has an asymmetric topographic structure. That is, the left wall 910a of the grid bar 904a is perpendicular to the xy-plane, while the right wall 912a intersects with the xy-plane at a certain oblique angle. Similarly, the left wall of lattice bar 904b While 910b is perpendicular to the xy-plane, the right wall 912b of its grid bar is at a certain angle. It intersects with the xy-plane. Typically, the area occupied by proxy target 152 is small. Since the linear dimension is less than several tens of micrometers, all bars 904 within the grid 164 are as shown in Figure 11. It exhibits the same asymmetry as shown in B. Due to the asymmetry of the grid bars, the focus is As the CoS of grid 164 will shift further, the focus will be as described in more detail below. By utilizing the CoS variation as a function, its asymmetry can be elucidated.

[0116] Asymmetry of grid bars 904a and 904b (and consequently all grid bars 902 and 904 To elucidate the asymmetry, the controller 24 adds a mark on the Z-axis 926. At the focal point, three focus steps 920, 922, and 924 are applied to cameras CAM1 and CAM2. The three images of grid 164 obtained are read out. Controller 24 then The turtle is moved by the z-direction movement of the table 26 and / or by the actuators 58 and 60. Due to the movement of the camera, the contrast focus (Figure 6A-6B) is used to place the camera CAM on the wafer 12. 1 and CAM2 are in focus. At all three focus steps 920, 922 and 924, CAM 1 is focused on a fixed point Z=Z0 in the xy-plane on grid 162, and at that fixed focus... It is aligned with grid 162. That is, its Z coordinate Z0 is used to determine the "anchor" for measurement. A "focal point" is formed.

[0117] In those three focus stages 920, 922, and 924, CAM2 controls the Z coordinate Z It is focused on the xy plane having 1, Z2 and Z3. During image acquisition, camera CAM1 and Both CAM2s, as shown in the first grid registration step 206 (Figure 4), Since they are aligned with the gratings 154, 156, 158, and 160, between those cameras Then, known lateral registration (registration in the xy plane) is secured. CA Based on the image read from M1, the controller 24 calculates the CoS of the grid 164. The point schematically indicated as 928 in Figure 11B is calculated, and that is stored in memory 2 It is stored in 5. Based on the three images read from CAM2 at three focal positions. The controller 24 then determines three individual CoS values, points 930, 932, and 934. The schematicly noted values ​​are calculated and stored in memory 25. 936 is fitted to points 928, 930, 932 and 934 by controller 24. Therefore, this indicates a shift in CoS due to the focus, i.e., the cross-section of lattice bars 902 and 904. It serves as a measure of surface asymmetry. Treating curve 936 as a straight line also results in a higher-order curve. It is also possible.

[0118] The above method applies to the process layer grid 164 oriented along the y-direction, and the x and y directions. The same method is applied to the resist grid 162 oriented along both sides, and their cross-sectional asymmetry It is also possible to clarify this. Furthermore, the actual overlay measurements performed on production wafers can be supplemented. By correcting these asymmetries, we can address the obvious CoS that can result. Cut.

[0119] An alternative embodiment uses only a single camera, for example CAM1, and associates the focus with There are some measurements in which CoS is measured. In this type of measurement, CAM1 has four focal positions Z0, The image of the grid 164 is brought into focus as it passes through Z1, Z2, and Z3, and the image of the grid 164 is controlled by controller 2 4 reads from CAM1 at each focal position and stores it in memory 25. The controller 24 then calculates the individual CoS values ​​based on the images stored in memory 25. The CoS is calculated, and as mentioned above regarding two-camera measurements, the CoS is calculated in relation to the focus. This single-camera measurement method involves anchoring to fixed features and mutual registration of cameras. Because it is not possible to perform a scan, it is less sensitive to the mechanical stability of the device 10 than the two-camera method mentioned earlier. It's a feeling.

[0120] The above methods involve four focal positions ("anchor" position Z0 and three focal positions Z1, Z2 and Although this is being done using Z3, it is also possible to use fewer or more focal positions instead. stomach.

[0121] Figure 12 relates to one embodiment of the present invention, and the target within the overlay proxy target A series of schematic diagrams illustrating the use of image signal correlation to monitor the asymmetry of the image. This is a plot diagram. As explained earlier, asymmetric process effects in semiconductor manufacturing processes, for example For example, asymmetric etching can cause the target features of the overlay proxy target to The cross-section may be asymmetrical. In this embodiment, the monitoring of such asymmetry is They are using them to monitor semiconductor manufacturing processes (without even evaluating them).

[0122] Figure 12 shows the cross-sectional appearance of the grid bar 904a, similar to Figure 11B. -La 24 reads out the acquired image of bar 904a from CAM1, for example, and that image It is converted into an image signal and stored in memory 25. Of that image signal, curve 608 (Figure 8A) The portion along the curve is shown as curve 1002 in Figure 12. Image signal of curve 1002 Based on this, the controller 24 generates a reflected image signal by reflecting it around the z-axis, i.e., a curved image signal. A reflected image signal, shown as line 1004, is generated.

[0123] The controller 24 controls the two correlation curves 1006 and 1008, i.e., curve 1002. The correlation curve 1006, which is the self-correlation (correlation with itself), and the mutual phase between curves 1002 and 1004. The correlation curve 1008 is calculated. Curve 1006 is C auto max The maximum value It has, and curve 1008 is C cross max It has a maximum value of . Curve 1002 There is asymmetry in C cross max <C auto max This is the result. Autocorrelation curve 1 Maximum value C of 006 auto max The maximum value C of the cross-correlation curve 1008 for cross m ax The comparison results are used as a measure of the asymmetry of image signal 1002, and consequently, overlap A process that generates an asymmetric cross-sectional profile of a target feature within a Roxy target. It can be used as a measure of the effect.

[0124] The image of the target feature of the overlay proxy target, for example, bar 904a The image is captured under various operating conditions of the apparatus 10, for example, the wavelength and / or polarization of the illumination of the wafer 12. This can be achieved by changing and / or using various focus settings. The ratio C of one or more functions among the conditions cross max / C auto max fluctuation This results in a "correlated landscape," which in turn further contributes to the practical application of this process. It can be used to monitor semiconductor manufacturing processes during production. For example, two correlated minimums The ratio of the large value C crossmax / C auto max If it falls below the preset limit, for example, 0.8 If this is the case, it can be used to indicate that the process variation is unacceptable. ru.

[0125] The above method also applies to the process layer grid 164 oriented along the y-direction, as well as the x-direction and direction. This can also be applied to the resist grid 162 which is oriented along both sides, and This will allow us to better understand the asymmetry of those cross-sections.

[0126] By using the above-described embodiments individually or in combination, the optimal overlay weighing method can be achieved. The sip, that is, the overlay error measurement, provides robust overlay measurement conditions when performing the measurement. The recipe can be determined. In such recipes, the optimal focus setting, wavelength, Polarization, illumination conditions, and objective lens pupil control are specified. Different conditions are applied to the process layer and resist. It can also be applied individually to each layer.

[0127] In addition to or instead of this, information obtained by the above-mentioned measurement techniques, for example, Co by focus The controller 24 utilizes the data related to the variation of S, and the overlay metric algorithm is increased. It may be strengthened. Furthermore, the above methods may be used to simultaneously measure the overlay error between multiple pairs of layers. Furthermore, it can be generalized for additional lighting and focusing channels.

[0128] As you can probably guess, the embodiments described above are cited as examples, and the present invention specifically The present invention is not limited to those shown or described above. Rather, the technical scope of the present invention includes the above-mentioned aspects. This encompasses both combinations and subcombinations of various characteristics, and those Among the deformed and modified products, those that a person skilled in the art would likely conceive of after reading the above description and that are otherwise This includes information not disclosed in previous technologies.

Claims

1. An optical inspection device, An illumination device configured to illuminate a semiconductor wafer having at least one illumination beam directed at it, wherein at least first and second patterned layers are continuously deposited, the first patterned layer comprising a first target feature, and the second patterned layer comprising a second target feature overlapping the first target feature; A first camera configured to capture a first image of the first target feature, and a second camera configured to capture a second image of the second target feature, wherein the first camera and the second camera focus on the semiconductor wafer at a certain height difference. It is a controller, The centers of symmetry of the first target feature and the second target feature are identified from the first and second images, The variation at the center of symmetry is measured by comparing the first image and the second image. A controller configured as follows, Equipped with, An optical inspection apparatus in which the controller is configured to change one or more imaging parameters from a first setting for the first image to a second setting for the second image, and the measurement of the variation at the center of symmetry is a function of changing the one or more imaging parameters.

2. The apparatus according to claim 1, further, A table configured to support the aforementioned semiconductor wafer, An objective lens is disposed in the optical path from the semiconductor wafer to at least one of the first camera and the second camera, A device equipped with the following features.

3. The apparatus according to claim 2, The controller is configured to change one or more imaging parameters from a first setting for the first image to a second setting for the second image. The one or more imaging parameters mentioned above are focus settings. An apparatus in which the controller is configured to change the distance between the semiconductor wafer on the table and the objective lens.

4. The apparatus according to claim 1, further, A device comprising a first actuator configured to move the first camera in order to adjust the first focal plane of the first camera.

5. The apparatus according to claim 4, further, A device comprising a second actuator configured to move the second camera in order to adjust the second focal plane of the second camera.

6. The apparatus according to claim 1 The illumination device comprises a first light source for irradiating the semiconductor wafer with a first illumination beam, and a second light source for irradiating the semiconductor wafer with a second illumination beam. An apparatus in which the wavelength of the second illumination beam is different from the wavelength of the first illumination beam.

7. The apparatus according to claim 1, An apparatus in which the first patterned layer includes a process layer, and the second patterned layer includes a resist layer deposited on top of the process layer.

8. The apparatus according to claim 1, An apparatus in which the first focal plane of the first camera and the second focal plane of the second camera are separated by a constant focal distance ΔZ.

9. A weighing method performed by an optical inspection device, The steps of illuminating a semiconductor wafer having at least one illumination beam directed at it, wherein at least first and second patterned layers are continuously deposited, the first patterned layer comprises a first target feature, and the second patterned layer comprises a second target feature overlapping the first target feature; A step of capturing a first image of the first target feature and a second image of the second target feature using a first camera and a second camera, wherein capturing the first image and the second image includes setting one or more imaging parameters to a first setting for the first image and to a second setting for the second image. A step of processing the first image and the second image, the step of measuring the variation at the center of symmetry as a function of the first setting and the second setting, Methods that include...

10. The method according to claim 9, A method comprising setting one or more of the imaging parameters, which includes setting the first camera and the second camera to different first and second focal positions.

11. The method according to claim 9, A method wherein the first setting and the second setting are of different wavelengths.

12. The method according to claim 9, A method wherein the first setting and the second setting are in different polarization states.

13. The method according to claim 9, A method wherein the first setting and the second setting are different offsets of at least one aperture of the at least one illumination beam.

14. The method according to claim 9, A method wherein the first setting and the second setting are different angular orientations of the semiconductor wafer with respect to the first camera and the second camera.

15. The method according to claim 9, A method for directing at least one illumination beam, comprising directing a first illumination beam onto the semiconductor wafer and directing a second illumination beam onto the semiconductor wafer, wherein the wavelength of the second illumination beam is different from the wavelength of the first illumination beam.

16. The method according to claim 9, A method wherein the first patterned layer includes a process layer, and the second patterned layer includes a resist layer deposited on top of the process layer.

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