Optical inspection apparatus and metrology 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 generally relates to the manufacture of semiconductor devices, and more particularly to apparatus and methods for semiconductor circuit metrology.

Background Art

[0002] Semiconductor circuits are generally manufactured using photolithographic methods. In photolithography, a thin layer of photosensitive polymer (photoresist) is deposited over a semiconductor wafer, and the layer is patterned using optical radiation or other radiation, and portions of the wafer covered by the photoresist are left intact. After patterning, the wafer is modified by methods such as etching and ion implantation to change the material properties and topography of the wafer, while portions of the wafer covered by the photoresist are not affected.

[0003] Semiconductor circuit metrology is widely used to measure the characteristics of patterned photoresist, such as the topography and location of patterned features. Accurate positioning of the patterned features of the photoresist relative to the underlying process layer is important for ensuring high yield in the photolithographic process. In any case, misregistration of the patterned photoresist relative to the underlying process layer is called "overlay error". As an example, in a typical semiconductor circuit with a minimum line width of 10-14 nm (so-called 10 nm design rule), the maximum allowable overlay error is 2-3 nm. In state-of-the-art semiconductor circuits, the line width is 5 ​​​​​​​​​​​​​​It is being reduced to nm, and accordingly the maximum allowable overlay error has been decreasing.

[0004] For optical radiation in the visible and near-infrared wavelengths, it can pass through the dielectric layer within the photoresist layer or under the photoresist. Therefore, the overlay error is generally measured using an optical overlay metrology tool. In an optical overlay metrology tool, such as the Archer (trademark) series tool by KLA Corporation (Milpitas, California, USA), proxies located within the scribe line (the line separating adjacent semiconductor chips) of a semiconductor wafer are imaged, for example, AIM (trademark) overlay targets by KLA. By applying an image analysis algorithm to the acquired image, the center of symmetry (CoS) of a set of target features within the process layer and the corresponding CoS of the target features within the patterned photoresist layer are located. The overlay error is calculated as the distance between the centers of symmetry of the target features of those two layers.

[0005] The terms "optical ray", "optical radiation", "light", and "beam of radiation" generally refer to all of visible, infrared, and ultraviolet radiation according to their usage in this specification and the claims. SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] In various embodiments of the present invention described below, improved apparatuses and methods for semiconductor circuit metrology are provided. MEANS FOR SOLVING THE PROBLEMS

[0007] That is, in the measurement method provided by an embodiment of the present invention, at least a first and a second patterned layer are successively deposited, a first target feature is provided in the first patterned layer, a second target feature is provided in the second patterned layer, and a semiconductor wafer on which the second target feature overlaps the first target feature is illuminated by directing at least one illumination beam thereon. A sequence of images of the first and second target features is captured while changing one or more imaging parameters over the sequence. By processing the image group within the sequence, the centers of symmetry of the first and second target features within those images are identified, and the variation of those centers of symmetry is measured as a function of the changing imaging parameters. The measured variation is applied when measuring the overlay error between the first and second patterned layers.

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

[0009] In certain embodiments, when capturing a sequence of images, first and second images of the target feature are captured using first and second cameras in a mutual registration state, and when processing the images, the variation of the center of symmetry is measured by comparing the first and second images. In certain embodiments, when capturing the first and second images, a resist registration image is projected onto the first and second cameras, and the first and second cameras are positioned with respect to the resist registration image. ​​​​​​​​​They are superimposed. In one exemplary embodiment, when projecting a registration image, a grid pattern is generated alongside the image of the target feature and projected onto the detector arrays of the first and second cameras respectively. In addition or alternatively, when capturing the first and second images, one or more of the imaging parameters are set to a first setting for the first image and to a second setting for the second image. In one disclosed embodiment, when setting one or more of the imaging parameters, the first and second cameras are set to separate first and second focal positions respectively, and the first and second cameras are stepped through their respective first and second sequences such that the variation of the center of symmetry is measured as a function of the focal position. In one exemplary embodiment, the first and second focal positions are separated by a fixed focal length Δz, and at each step of the first and second sequences, both the first and second focal positions are incremented by Δz. In addition or alternatively, the at least one illumination beam includes first and second illumination beams in separate first and second polarization states, and when capturing the first and second images, a polarization beam splitter is applied so that the light reflected from the wafer in the first polarization state is directed towards the first camera and the light reflected from the wafer in the second polarization state is directed towards the second camera.

[0010] In one embodiment, when capturing a sequence of images, images of the target feature are captured at various focus settings, and when processing the images, the variation of the center of symmetry is determined as a function of the focus settings. One or more of the above are set to a first setting for the first image and to a second setting for the second image. In one disclosed embodiment, when setting one or more of the imaging parameters, the first and second cameras are set to separate first and second focal positions respectively, and the first and second cameras are stepped through their respective first and second sequences such that the variation of the center of symmetry is measured as a function of the focal position. In one exemplary embodiment, the first and second focal positions are separated by a fixed focal length Δz, and at each step of the first and second sequences, both the first and second focal positions are incremented by Δz. In one exemplary embodiment, the first and second focal positions are separated by a fixed focal length Δz, and at each step of the first and second sequences, both the first and second focal positions are incremented by Δz.

[0011] In addition or alternatively, the at least one illumination beam includes first and second illumination beams in separate first and second polarization states, and when capturing the first and second images, a polarization beam splitter is applied so that the light reflected from the wafer in the first polarization state is directed towards the first camera and the light reflected from the wafer in the second polarization state is directed towards the second camera. In one embodiment, when capturing a sequence of images, images of the target feature are captured at various focus settings, and when processing the images, the variation of the center of symmetry is determined as a function of the focus settings.

[0012] In one embodiment, when capturing a sequence of images, images of the target feature are captured at various focus settings, and when processing the images, the variation of the center of symmetry is determined as a function of the focus settings. It is measured as a function of ting.

[0013] In addition to or instead of this, when capturing a sequence of images, images of the target feature are captured at a plurality of different wavelengths, and when processing the images, the variation of the center of symmetry is measured as a function of the wavelength. In addition to or instead of this, when capturing a sequence of images, images of the target feature are captured at a plurality of different polarization states, and when processing the images, the variation of the center of symmetry is measured as a function of the polarization state. It is measured as a function of wavelength.

[0014] Furthermore, in addition to or instead of this, when capturing a sequence of images, images of the target feature are captured with at least one aperture of at least one illumination beam at a plurality of different offsets, and when processing the images, the variation of the center of symmetry is measured as a function of the offset of the aperture. In addition, when capturing a sequence of images, images of the target feature are captured with at least one aperture of at least one illumination beam at a plurality of different offsets, and when processing the images, the variation of the center of symmetry is measured as a function of the offset of the aperture. It is measured as a function of the polarization state.

[0015] Still further, in addition to or instead of this, when capturing a sequence of images, images of the target feature are captured with at least one aperture of at least one illumination beam at a plurality of different offsets, and when processing the images, the variation of the center of symmetry is measured as a function of the offset of the aperture. When capturing a sequence of images, images of the target feature are captured with at least one aperture of at least one illumination beam at a plurality of different offsets, and when processing the images, the variation of the center of symmetry is measured as a function of the offset of the aperture. When capturing a sequence of images, images of the target feature are captured with at least one aperture of at least one illumination beam at a plurality of different offsets, and when processing the images, the variation of the center of symmetry is measured as a function of the offset of the aperture. It is measured as a function of the offset of the aperture.

[0016] Also, in one embodiment, when capturing a sequence of images, a camera is used, and the semiconductor wafer is placed at various angular orientations with respect to the camera, and images of the target feature are captured, and when processing the images, the tool-induced shift of the center of symmetry is measured as a function of the angular orientation. When capturing a sequence of images, a camera is used, and the semiconductor wafer is placed at various angular orientations with respect to the camera, and images of the target feature are captured, and when processing the images, the tool-induced shift of the center of symmetry is measured as a function of the angular orientation. When capturing a sequence of images, a camera is used, and the semiconductor wafer is placed at various angular orientations with respect to the camera, and images of the target feature are captured, and when processing the images, the tool-induced shift of the center of symmetry is measured as a function of the angular orientation. It is measured.

[0017] In certain embodiments, when applying the measured variation, an optimal range of one or more imaging parameters is explored according to the measured variation, and a recipe for overlay error measurement is generated by setting the one or more imaging parameters to values within the optimal range. In one disclosed embodiment, when capturing a sequence of images, a semiconductor wafer In certain embodiments, when applying the measured variation, an optimal range of one or more imaging parameters is explored according to the measured variation, and a recipe for overlay error measurement is generated by setting the one or more imaging parameters to values within the optimal range. In one disclosed embodiment, when capturing a sequence of images, a semiconductor wafer In certain embodiments, when applying the measured variation, an optimal range of one or more imaging parameters is explored according to the measured variation, and a recipe for overlay error measurement is generated by setting the one or more imaging parameters to values within the optimal range. In one disclosed embodiment, when capturing a sequence of images, a semiconductor wafer In certain embodiments, when applying the measured variation, an optimal range of one or more imaging parameters is explored according to the measured variation, and a recipe for overlay error measurement is generated by setting the one or more imaging parameters to values within the optimal range. In one disclosed embodiment, when capturing a sequence of images, a semiconductor wafer Images of a plurality of target features are captured at a plurality of separate locations above, and when searching for the optimal range a range that is optimal over an area of the semiconductor wafer is selected by applying the variations measured at those multiple separate locations.

[0018] In addition to or instead of this, when processing the image, at least one of the target features is measured for its asymmetry.

[0019] Also, in the metrology method provided by an embodiment of the present invention, at least one patterned layer is deposited thereon, and a semiconductor wafer provided with a grid having a plurality of bars oriented parallel to a predetermined axis is illuminated by directing at least one illumination beam thereto. By capturing and processing one or more images of the grid, the asymmetry of one or more of the bars around its axis is elucidated. The elucidated asymmetry is applied when performing a metrological evaluation of the patterned layer.

[0020] In a disclosed embodiment, when capturing one or more images, a sequence of images of the grid is captured at various focus settings, and when processing the one or more images, the variation of the center of symmetry of the grid in the image is measured as a function of the focus setting, and the asymmetry is elucidated based on the measured variation. In addition to or instead of this, when processing the one or more images, the correlation between the image of one or more of the bars and its reflected version of the image is calculated, and the degree of asymmetry is derived based on the calculated correlation.

[0021] In addition, in the optical inspection apparatus provided by an embodiment of the present invention, the illumination assembly , on which at least a first and a second patterned layer are successively deposited, with a first target feature in the first patterned layer and a second target feature in the second patterned layer, and the semiconductor wafer on which the second target feature overlaps the first target feature is configured to be illuminated by directing at least one illumination beam thereon. An imaging assembly is configured to capture a sequence of images of the first and second target features. A controller is configured to process the images within the sequence to identify the centers of symmetry of each of the first and second target features within those images and measure the variation of those centers of symmetry as a function of the changing imaging parameters, and apply the measured variation when measuring the overlay error between the first and second patterned layers. ring layer, and a second target feature in the second patterned layer, and the semiconductor wafer on which the second target feature overlaps the first target feature is configured to be illuminated by directing at least one illumination beam thereon. An imaging assembly is configured to capture a sequence of images of the first and second target features. A controller is configured to process the images within the sequence to identify the centers of symmetry of each of the first and second target features within those images and measure the variation of those centers of symmetry as a function of the changing imaging parameters, and apply the measured variation when measuring the overlay error between the first and second patterned layers. ring layer, and a second target feature in the second patterned layer, and the semiconductor wafer on which the second target feature overlaps the first target feature is configured to be illuminated by directing at least one illumination beam thereon. An imaging assembly is configured to capture a sequence of images of the first and second target features. A controller is configured to process the images within the sequence to identify the centers of symmetry of each of the first and second target features within those images and measure the variation of those centers of symmetry as a function of the changing imaging parameters, and apply the measured variation when measuring the overlay error between the first and second patterned layers. ring layer, and a second target feature in the second patterned layer, and the semiconductor wafer on which the second target feature overlaps the first target feature is configured to be illuminated by directing at least one illumination beam thereon. An imaging assembly is configured to capture a sequence of images of the first and second target features. A controller is configured to process the images within the sequence to identify the centers of symmetry of each of the first and second target features within those images and measure the variation of those centers of symmetry as a function of the changing imaging parameters, and apply the measured variation when measuring the overlay error between the first and second patterned layers. ring layer, and a second target feature in the second patterned layer, and the semiconductor wafer on which the second target feature overlaps the first target feature is configured to be illuminated by directing at least one illumination beam thereon. An imaging assembly is configured to capture a sequence of images of the first and second target features. A controller is configured to process the images within the sequence to identify the centers of symmetry of each of the first and second target features within those images and measure the variation of those centers of symmetry as a function of the changing imaging parameters, and apply the measured variation when measuring the overlay error between the first and second patterned layers. ring layer, and a second target feature in the second patterned layer, and the semiconductor wafer on which the second target feature overlaps the first target feature is configured to be illuminated by directing at least one illumination beam thereon. An imaging assembly is configured to capture a sequence of images of the first and second target features. A controller is configured to process the images within the sequence to identify the centers of symmetry of each of the first and second target features within those images and measure the variation of those centers of symmetry as a function of the changing imaging parameters, and apply the measured variation when measuring the overlay error between the first and second patterned layers. ring layer, and a second target feature in the second patterned layer, and the semiconductor wafer on which the second target feature overlaps the first target feature is configured to be illuminated by directing at least one illumination beam thereon. An imaging assembly is configured to capture a sequence of images of the first and second target features. A controller is configured to process the images within the sequence to identify the centers of symmetry of each of the first and second target features within those images and measure the variation of those centers of symmetry as a function of the changing imaging parameters, and apply the measured variation when measuring the overlay error between the first and second patterned layers. ring layer, and a second target feature in the second patterned layer, and the semiconductor wafer on which the second target feature overlaps the first target feature is configured to be illuminated by directing at least one illumination beam thereon. An imaging assembly is configured to capture a sequence of images of the first and second target features. A controller is configured to process the images within the sequence to identify the centers of symmetry of each of the first and second target features within those images and measure the variation of those centers of symmetry as a function of the changing imaging parameters, and apply the measured variation when measuring the overlay error between the first and second patterned layers. ring layer, and a second target feature in the second patterned layer, and the semiconductor wafer on which the second target feature overlaps the first target feature is configured to be illuminated by directing at least one illumination beam thereon. An imaging assembly is configured to capture a sequence of images of the first and second target features. A controller is configured to process the images within the sequence to identify the centers of symmetry of each of the first and second target features within those images and measure the variation of those centers of symmetry as a function of the changing imaging parameters, and apply the measured variation when measuring the overlay error between the first and second patterned layers. ring layer, and a second target feature in the second patterned layer, and the semiconductor wafer on which the second target feature overlaps the first target feature is configured to be illuminated by directing at least one illumination beam thereon. An imaging assembly is configured to capture a sequence of images of the first and second target features. A controller is configured to process the images within the sequence to identify the centers of symmetry of each of the first and second target features within those images and measure the variation of those centers of symmetry as a function of the changing imaging parameters, and apply the measured variation when measuring the overlay error between the first and second patterned layers.

[0022] Furthermore, in an optical inspection apparatus provided by an embodiment of the present invention, an illumination assembly is configured to illuminate a semiconductor wafer on which at least one patterned layer is deposited and which is provided with a grating having a plurality of bars oriented parallel to a predetermined axis by directing at least one illumination beam thereon. An imaging assembly is configured to capture one or more images of the grating. A controller is configured to elucidate the asymmetry of one or more of the bars around the axis by processing the one or more images and apply the elucidated asymmetry when performing a metrological evaluation of the patterned layer. Furthermore, in an optical inspection apparatus provided by an embodiment of the present invention, an illumination assembly is configured to illuminate a semiconductor wafer on which at least one patterned layer is deposited and which is provided with a grating having a plurality of bars oriented parallel to a predetermined axis by directing at least one illumination beam thereon. An imaging assembly is configured to capture one or more images of the grating. A controller is configured to elucidate the asymmetry of one or more of the bars around the axis by processing the one or more images and apply the elucidated asymmetry when performing a metrological evaluation of the patterned layer. Furthermore, in an optical inspection apparatus provided by an embodiment of the present invention, an illumination assembly is configured to illuminate a semiconductor wafer on which at least one patterned layer is deposited and which is provided with a grating having a plurality of bars oriented parallel to a predetermined axis by directing at least one illumination beam thereon. An imaging assembly is configured to capture one or more images of the grating. A controller is configured to elucidate the asymmetry of one or more of the bars around the axis by processing the one or more images and apply the elucidated asymmetry when performing a metrological evaluation of the patterned layer. Furthermore, in an optical inspection apparatus provided by an embodiment of the present invention, an illumination assembly is configured to illuminate a semiconductor wafer on which at least one patterned layer is deposited and which is provided with a grating having a plurality of bars oriented parallel to a predetermined axis by directing at least one illumination beam thereon. An imaging assembly is configured to capture one or more images of the grating. A controller is configured to elucidate the asymmetry of one or more of the bars around the axis by processing the one or more images and apply the elucidated asymmetry when performing a metrological evaluation of the patterned layer. Furthermore, in an optical inspection apparatus provided by an embodiment of the present invention, an illumination assembly is configured to illuminate a semiconductor wafer on which at least one patterned layer is deposited and which is provided with a grating having a plurality of bars oriented parallel to a predetermined axis by directing at least one illumination beam thereon. An imaging assembly is configured to capture one or more images of the grating. A controller is configured to elucidate the asymmetry of one or more of the bars around the axis by processing the one or more images and apply the elucidated asymmetry when performing a metrological evaluation of the patterned layer. Furthermore, in an optical inspection apparatus provided by an embodiment of the present invention, an illumination assembly is configured to illuminate a semiconductor wafer on which at least one patterned layer is deposited and which is provided with a grating having a plurality of bars oriented parallel to a predetermined axis by directing at least one illumination beam thereon. An imaging assembly is configured to capture one or more images of the grating. A controller is configured to elucidate the asymmetry of one or more of the bars around the axis by processing the one or more images and apply the elucidated asymmetry when performing a metrological evaluation of the patterned layer. Furthermore, in an optical inspection apparatus provided by an embodiment of the present invention, an illumination assembly is configured to illuminate a semiconductor wafer on which at least one patterned layer is deposited and which is provided with a grating having a plurality of bars oriented parallel to a predetermined axis by directing at least one illumination beam thereon. An imaging assembly is configured to capture one or more images of the grating. A controller is configured to elucidate the asymmetry of one or more of the bars around the axis by processing the one or more images and apply the elucidated asymmetry when performing a metrological evaluation of the patterned layer. Furthermore, in an optical inspection apparatus provided by an embodiment of the present invention, an illumination assembly is configured to illuminate a semiconductor wafer on which at least one patterned layer is deposited and which is provided with a grating having a plurality of bars oriented parallel to a predetermined axis by directing at least one illumination beam thereon. An imaging assembly is configured to capture one or more images of the grating. A controller is configured to elucidate the asymmetry of one or more of the bars around the axis by processing the one or more images and apply the elucidated asymmetry when performing a metrological evaluation of the patterned layer.

[0023] The present invention will be more fully understood from the following drawings in conjunction with the detailed description set forth below of its various embodiments. It will be understood more comprehensively.

Brief Description of the Drawings

[0024]

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DETAILED DESCRIPTION OF THE INVENTION

[0025] [Overview] Overlay metrology proxy targets are widely used for precise measurement of overlays between adjacent (continuous with each other) patterned layers on a semiconductor wafer. These layers are For example, it may include a process layer and a resist layer, and in the application after etching It can also be used between two process layers (i.e., in relation to the process layer and the resist layer Although exemplary embodiments will be described later, the principles of those embodiments can be applied to the first process layer and the second process layer with necessary modifications ).

[0026] However, the features within the proxy target (both the resist layer target feature and the process layer target feature) are different from the corresponding features in the device area: The features of the proxy target usually have wider lines than those in the device, so they can be resolved by a metrology tool operating with light in the visible or near-infrared spectrum, and since those targets are symmetric in their design, it is possible to calculate the overlay value by executing a full symmetry-based image processing algorithm : The features of the proxy target usually have wider lines than those in the device, so they can be resolved by a metrology tool operating with light in the visible or near-infrared spectrum, and since those targets are symmetric in their design, it is possible to calculate the overlay value by executing a full symmetry-based image processing algorithm : The features of the proxy target usually have wider lines than those in the device, so they can be resolved by a metrology tool operating with light in the visible or near-infrared spectrum, and since those targets are symmetric in their design, it is possible to calculate the overlay value by executing a full symmetry-based image processing algorithm Moreover, the proxy target is usually placed within the scribe line of the semiconductor wafer so that the valuable "real estate" of its device area is not taken away : The optical distortion of the photolithographic exposure system (scanner) is different in the scribe line from that in the device area, which leads to a spatial variability differential (as a spatially varying difference) shift between the pattern in the proxy target and the corresponding pattern in the device : The optical distortion of the photolithographic exposure system (scanner) is different in the scribe line from that in the device area, which leads to a spatial variability differential (as a spatially varying difference) shift between the pattern in the proxy target and the corresponding pattern in the device : The optical distortion of the photolithographic exposure system (scanner) is different in the scribe line from that in the device area, which leads to a spatial variability differential (as a spatially varying difference) shift between the pattern in the proxy target and the corresponding pattern in the device : The optical distortion of the photolithographic exposure system (scanner) is different in the scribe line from that in the device area, which leads to a spatial variability differential (as a spatially varying difference) shift between the pattern in the proxy target and the corresponding pattern in the device : The optical distortion of the photolithographic exposure system (scanner) is different in the scribe line from that in the device area, which leads to a spatial variability differential (as a spatially varying difference) shift between the pattern in the proxy target and the corresponding pattern in the device : The optical distortion of the photolithographic exposure system (scanner) is different in the scribe line from that in the device area, which leads to a spatial variability differential (as a spatially varying difference) shift between the pattern in the proxy target and the corresponding pattern in the device ).

[0027] Due to these design and metrology considerations, the features of the proxy target are different from the device features in the chip area with respect to lithographic effects and process effects : The optical distortion of the photolithographic exposure system (scanner) is different in the scribe line from that in the device area, which leads to a spatial variability differential (as a spatially varying difference) shift between the pattern in the proxy target and the corresponding pattern in the device will react differently, and the overlay error measured based on the proxy target may have an offset with respect to the overlay error in the actual device feature and may have an offset with respect to the overlay error in the actual device feature By applying a calibration function, the exact degree of the overlay error in the device area may be derived from the overlay error measured based on the proxy target However, accurate calibration requires stable and reproducible overlay measurements based on the proxy target which will in turn be subject to process-induced effects such as feature asymmetry and will be subject to process-induced effects such as feature asymmetry and will be subject to process-induced effects such as feature asymmetry

[0028] Furthermore, each proxy target is provided with both a target feature in the photoresist and a target feature in the underlying process layer, so that these two sets of target features can be separated by a distance of several micrometers along a direction perpendicular to the semiconductor wafer Furthermore, each proxy target is provided with both a target feature in the photoresist and a target feature in the underlying process layer, so that these two sets of target features can be separated by a distance of several micrometers along a direction perpendicular to the semiconductor wafer Furthermore, each proxy target is provided with both a target feature in the photoresist and a target feature in the underlying process layer, so that these two sets of target features can be separated by a distance of several micrometers along a direction perpendicular to the semiconductor wafer In such a case, these two sets of target features are imaged by separately focusing the metrology tool on the resist layer and the process layer and acquiring images at their respective focus settings In such a case, these two sets of target features are imaged by separately focusing the metrology tool on the resist layer and the process layer and acquiring images at their respective focus settings However, due to the combination of process variation effects and topography, it becomes difficult to find an optimal metrology "recipe", i.e., a set of metrology conditions (such as focus, numerical aperture of illumination, and wavelength) that result in stable and reproducible overlay measurement results from the proxy target However, due to the combination of process variation effects and topography, it becomes difficult to find an optimal metrology "recipe", i.e., a set of metrology conditions (such as focus, numerical aperture of illumination, and wavelength) that result in stable and reproducible overlay measurement results from the proxy target However, due to the combination of process variation effects and topography, it becomes difficult to find an optimal metrology "recipe", i.e., a set of metrology conditions (such as focus, numerical aperture of illumination, and wavelength) that result in stable and reproducible overlay measurement results from the proxy target However, due to the combination of process variation effects and topography, it becomes difficult to find an optimal metrology "recipe", i.e., a set of metrology conditions (such as focus, numerical aperture of illumination, and wavelength) that result in stable and reproducible overlay measurement results from the proxy target

[0029] In various embodiments of the invention described herein, optical metrology tools and methods are provided that enable independent resolution of two sets of target features provided in an optical overlay proxy target to address these problems In various embodiments of the invention described herein, optical metrology tools and methods are provided that enable independent resolution of two sets of target features provided in an optical overlay proxy target to address these problems In certain embodiments, the metrology tool The two imaging cameras provided in the loop are focused at a fixed height difference on the proxy target while being aligned with each other. By comparing the images captured by the two cameras, fluctuations in the optical characteristics of the proxy target layer, such as focus fluctuations, fluctuations in the center of symmetry (CoS) due to imaging parameters such as spectral response and polarization, are calibrated and corrected.

[0030] In certain embodiments, a registration image, such as an image of a two-dimensional grid pattern, is projected by the projector towards the two cameras. By stepping the focus of the metrology tool through successive steps each equal to the fixed height difference, one camera (referred to as CAM1) is always brought to the focus setting where the other camera (CAM2) was during the previous step. When CAM1 reaches its new focus position, CAM1 is aligned with the image acquired by CAM2 when CAM2 was at that focus position. At each focus position, an image of the proxy target is acquired by each camera. The acquired image series of each of the two sets of target features provided on the proxy target are aligned with each other through successive focus settings. Based on these two image series, the CoS variation due to focus can be calculated for each of the two sets of target features. From the stability of the CoS with respect to focus, the focus setting at which the images of the two sets of target features should be acquired for stable overlay measurement can be determined. Through stable overlay measurement, calibration of the overlay error in the semiconductor circuit that is stable and reproducible becomes possible. measurement, ultimately, stable and reproducible calibration of the overlay error in the semiconductor circuit becomes possible.

[0031] In addition to or instead of this, the variability of the CoS of each set of target features is mapped in association with the illumination wavelength and / or polarization, and by using this mapping, stable overlay measurements can be achieved. That is, in the present embodiments, an optimal measurement range is identified in the two-dimensional space according to the wavelength and focus.

[0032] [Description of the optical inspection device] FIG. 1 is a schematic pictorial view of an optical inspection device 10 for measuring the optical characteristics of a patterned thin film layer on a semiconductor wafer 12 according to an embodiment of the present invention.

[0033] The optical inspection device 10 includes an imaging assembly 14, an illumination assembly 16, and an optical relay assembly 18. The optical inspection device further includes a grating projector 20, a camera assembly 22, a controller 24, a memory 25, and a table 26 on which the semiconductor wafer 12 is placed. The orientation of the device 10 and its component members is defined according to the Cartesian coordinates 28. The same Cartesian coordinates 28 are shown in corresponding orientations in the figures described later. Hereinafter, the three Cartesian coordinate axes are denoted using the lowercase letters x, y, and z, and the coordinates on those axes are denoted using the uppercase letters X, Y, and Z.

[0034] The imaging assembly 14 is schematically shown as a single objective lens 30. Alternatively, the assembly 14 can be provided with an interference objective system (e.g., a Linnik 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.

[0035] The objective lens 30 is typically a compound lens having very high optical quality and a high numerical ​​The aperture (NA), for example, has an NA of 0.7 or greater. According to an alternative embodiment, the objective lens 30 has a variable NA and can be controlled by the controller 24. The objective lens 30 has a variable NA and can be controlled by the controller 24. This is possible.

[0036] In the illustrated embodiment, the illumination assembly 16 is controlled by the controller 24 and includes two illuminators 15 and 17, each having its own light source 32 and 33, which independently emit optical radiation forming respective beams 34 and 35 at one or more discrete adjustable wavelengths, or over a continuous spectrum in CW or pulsed form. The light sources 32 and 33 can also emit optical radiation in various polarization states, such as unpolarized, linearly polarized, or circularly polarized radiation. The illuminators 15 and 17 further include two individual aperture assemblies 36 and 37 connected to their respective light sources 32 and 33. The aperture assemblies 36 and 37 are driven by their respective actuators 38 and 39, which bring various apertures provided in assembly 36 into beam 34 and various apertures provided in assembly 37 into beam 35. The actuators 38 and 39 can further perform fine adjustments for the individual apertures of each assembly within the plane of the respective apertures. The beams 40 and 41 emitted from the illuminators 15 and 17 respectively are collinearly combined by the beam splitter 42 to form beam 43. In this type of dual illumination assembly, the provision of two illuminators enables the device 1... In the illustrated embodiment, the illumination assembly 16 is controlled by the controller 24 and includes two illuminators 15 and 17, each having its own light source 32 and 33, which independently emit optical radiation forming respective beams 34 and 35 at one or more discrete adjustable wavelengths, or over a continuous spectrum in CW or pulsed form. The light sources 32 and 33 can also emit optical radiation in various polarization states, such as unpolarized, linearly polarized, or circularly polarized radiation. In the illustrated embodiment, the illumination assembly 16 is controlled by the controller 24 and includes two illuminators 15 and 17, each having its own light source 32 and 33, which independently emit optical radiation forming respective beams 34 and 35 at one or more discrete adjustable wavelengths, or over a continuous spectrum in CW or pulsed form. The light sources 32 and 33 can also emit optical radiation in various polarization states, such as unpolarized, linearly polarized, or circularly polarized radiation. In the illustrated embodiment, the illumination assembly 16 is controlled by the controller 24 and includes two illuminators 15 and 17, each having its own light source 32 and 33, which independently emit optical radiation forming respective beams 34 and 35 at one or more discrete adjustable wavelengths, or over a continuous spectrum in CW or pulsed form. The light sources 32 and 33 can also emit optical radiation in various polarization states, such as unpolarized, linearly polarized, or circularly polarized radiation. In the illustrated embodiment, the illumination assembly 16 is controlled by the controller 24 and includes two illuminators 15 and 17, each having its own light source 32 and 33, which independently emit optical radiation forming respective beams 34 and 35 at one or more discrete adjustable wavelengths, or over a continuous spectrum in CW or pulsed form. The light sources 32 and 33 can also emit optical radiation in various polarization states, such as unpolarized, linearly polarized, or circularly polarized radiation.

[0037] The illuminators 15 and 17 further include two individual aperture assemblies 36 and 37 connected to their respective light sources 32 and 33. The illuminators 15 and 17 further include two individual aperture assemblies 36 and 37 connected to their respective light sources 32 and 33. The aperture assemblies 36 and 37 are driven by their respective actuators 38 and 39, which bring various apertures provided in assembly 36 into beam 34 and various apertures provided in assembly 37 into beam 35. The aperture assemblies 36 and 37 are driven by their respective actuators 38 and 39, which bring various apertures provided in assembly 36 into beam 34 and various apertures provided in assembly 37 into beam 35. The aperture assemblies 36 and 37 are driven by their respective actuators 38 and 39, which bring various apertures provided in assembly 36 into beam 34 and various apertures provided in assembly 37 into beam 35. The actuators 38 and 39 can further perform fine adjustments for the individual apertures of each assembly within the plane of the respective apertures. The beams 40 and 41 emitted from the illuminators 15 and 17 respectively are collinearly combined by the beam splitter 42 to form beam 43. The beams 40 and 41 emitted from the illuminators 15 and 17 respectively are collinearly combined by the beam splitter 42 to form beam 43. In this type of dual illumination assembly, the provision of two illuminators enables the device 1... The flexibility of 0 is enhanced, and it is independent of the process layer and the resist layer on the wafer 12. Illumination conditions (e.g., wavelength, polarization, and / or NA) can be provided.

[0038] Alternatively, the illumination assembly 16 can be provided with a single illuminator, such as the illuminator 15, and the illumination conditions for the process layer and the resist layer can be selected by appropriate adjustment of the light source 32 and the aperture assembly 36. Further alternatively, the illumination assembly can be provided with more than two illuminators, such as three or four illuminators, and the beams emerging from the individual illuminators can be coupled by appropriate optical devices, such as beam splitters. The optical relay assembly 18 includes beam splitters 44 and 45, a beam splitter assembly 46, and lenses 50 and 52. The beam splitters 47 and 48 provided in the beam splitter assembly 46 can be moved in and out of the optical path of the actuator 10 as will be described in detail later. The two detector arrays 54 and 56 provided in the camera assembly 22 can also be called "cameras" and are denoted as CAM1 and CAM2, respectively. The camera assembly 22 further includes two actuators 58 and 60 for moving CAM1 and CAM2 along the z-axis. In this figure, the lenses 50 and 52 are shown as a single lens, but alternatively, they can be provided with a plurality of lenses and / or mirrors. The grating projector 20 projects a grating image onto CAM1 and C as will be described in more detail according to FIG. 2. The illumination assembly is provided with more than two illuminators, such as three or four illuminators, and the beams emerging from the individual illuminators are coupled by appropriate optical devices, such as beam splitters. The beams emerging from the individual illuminators are coupled by appropriate optical devices, such as beam splitters. The beams emerging from the individual illuminators are coupled by appropriate optical devices, such as beam splitters.

[0039] The optical relay assembly 18 includes beam splitters 44 and 45, a beam splitter assembly 46, and lenses 50 and 52. The beam splitters 47 and 48 provided in the beam splitter assembly 46 can be moved in and out of the optical path of the actuator 10 as will be described in detail later. The two detector arrays 54 and 56 provided in the camera assembly 22 can also be called "cameras" and are denoted as CAM1 and CAM2, respectively. The camera assembly 22 further includes two actuators 58 and 60 for moving CAM1 and CAM2 along the z-axis. The two detector arrays 54 and 56 provided in the camera assembly 22 can also be called "cameras" and are denoted as CAM1 and CAM2, respectively. The camera assembly 22 further includes two actuators 58 and 60 for moving CAM1 and CAM2 along the z-axis. In this figure, the lenses 50 and 52 are shown as a single lens, but alternatively, they can be provided with a plurality of lenses and / or mirrors. In this figure, the lenses 50 and 52 are shown as a single lens, but alternatively, they can be provided with a plurality of lenses and / or mirrors. In this figure, the lenses 50 and 52 are shown as a single lens, but alternatively, they can be provided with a plurality of lenses and / or mirrors.

[0040] The grating projector 20 projects a grating image onto CAM1 and C as will be described in more detail according to FIG. 2. configured to project into the AM2. The controller 24 is the grating projector 20 , the memory 25, the table 26, the light sources 32 and 33, and the actuators 38, 39, 49, 58 and 60. The controller 24 is typically programmed in software and / or firmware to perform the functions described herein, a programmable processor, and suitable digital and / or analog interfaces for connection to other elements of the apparatus 10. Alternatively or in addition, the controller 24 comprises hardwired and / or programmable hardware logic circuitry for performing at least a portion of the functions of the self-controller. In FIG. 1, for simplicity, the controller 24 is shown as a single monolithic functional block, but in reality, the controller can comprise a plurality of interconnected control units, as well as corresponding interfaces for receiving and outputting the signals depicted in the figure and described in the text. Before operating the optical inspection apparatus 10, the semiconductor wafer 12 is placed on the table 26.

[0041] During operation, under the control of the controller 24, the table 26 can move the wafer 12 along the x, y, and z axes, and rotate it about the z axis. The movement along the z axis is called "focusing". To illuminate the wafer 12, a beam 43 of optical radiation is emitted by the illumination assembly 16 towards

[0042] the beam splitter 44, where the beam is reflected into the objective lens 30. Then, the objective lens 30 focuses the beam 43 onto the wafer 12. With respect to its propagation, In a plane perpendicular to the y-axis (yz plane), at the exit from the illumination assembly 16, the beam 43 exhibits a cross-section that is provided with and preferably arranged and aligned with the apertures in the aperture assemblies 36 and 37. By means of those apertures, the shape of the cross-section of the beam 43 is defined, for example, as circular, square or anamorphic, and in conjunction therewith the dimensions of its cross-section are defined. As will be described in detail later, the beam 43 is taken to include two beams having different wavelengths and / or polarization states, and the cross-sections of each of those two beams can be controlled independently of each other by the aperture assemblies 36 and 37.

[0043] The apertures provided in the assemblies 36 and 37 are typically conjugate to the entrance pupil of the objective system 30 (imaged thereon by additional optical systems omitted from the drawings for simplicity) and thus, by the cross-section of the beam 43 exiting from the illuminator assembly 16, the numerical aperture (NA) of the optical radiation illuminating the wafer 12 is defined. That is, the shape of that illumination can be defined, in the angular space, for example, as circular, square or anamorphic, and can be varied between the full NA of the objective system 30 and a fraction of its full NA. According to certain configurations, the illumination can be restricted to have an NA value that exceeds the collecting NA of the objective lens 30, enabling dark-field imaging of features on the wafer 12.

[0044] The optical radiation illuminating the wafer 12 travels back towards the objective system that images the wafer onto the camera assembly 22, i.e., towards the features on that wafer, ​​​​The reflected radiation is received by the objective lens 30 and then passed through the beam splitter 44. and 45 into a beam splitter assembly 46, where the reflected radiation is Will it strike either the beam splitter 47 or the beam splitter 48? The actuator 49 controls which of the two beam splitters is in the path of the beam splitter. In this example, the beam splitter 47 is a wavelength-neutral beam splitter. A splitter, ie one whose reflection and transmission coefficients exhibit the same spectral behavior. The beam splitter 48 is a dichroic beam splitter that splits a certain spectral band Δλ 1, for example, transmits 380-550 nm and transmits a different (non-overlapping) spectral band Δλ2 For example, it is configured to reflect light of 560 to 800 nm. That is, a beam splitter When 47 is in the optical path, the reflected radiation is recorded by the cameras CAM1 and CAM2, respectively. A portion of the light is received across the entire spectrum, while the beam splitter 48 splits the light When in the path, the spectrum of the radiation is divided, and in CAM1, the spectral band Δλ1 At CAM1, radiation within the spectral band Δλ2 is received, and at CAM2, radiation within the spectral band Δλ2 is received. The source 32 emits optical radiation in the spectral band Δλ1 and the source 33 emits optical radiation in the spectral band Δλ2. By emitting optical radiation within the This makes it possible to control the above independently.

[0045] Alternatively or additionally, one of the beam splitters 47 and 48 may be configured to transmit a certain polarization state. Alternatively, a polarizing beam splitter may be used, which transmits one polarized light and reflects the polarized light perpendicular to the first polarized light. For example, when light sources 32 and 33 emit optical radiation with mutually orthogonal polarization states, light source 32 emits The radiation of is directed towards CAM1 and the radiation from the light source 33 is directed towards CAM2. As described above Similar to the spectral splitting of the illumination, by controlling the polarization of the illumination, independent control of the illumination for each of the two layers becomes possible. In one embodiment, the beam splitter 48 is a combination of a dichroic and a polarization beam splitter. The optical radiation transmitted and reflected by the selected beam splitter within the assembly 46 is

[0046] focused onto CAM1 by the lens 50 and onto CAM2 by the lens 52, respectively. The image of the wafer 12 is thus captured by CAM1 and CAM2 and read out and processed by the controller 2 4. Figure 2 is a schematic pictorial view of a grating projector 20 according to an embodiment of the present invention. The grating projector 20 projects a grating image that is used as a position reference between the two cameras towards the cameras CAM1 and CAM2. The grating projector 20 includes a light source assembly 80, a single-mode optical fiber 82, a diffraction assembly 84, and a spatial filter 86.

[0047] In this example, the light source assembly 80 includes two high-brightness (superluminescent) light-emitting diodes (sLEDs) 88 and 90. Among them, the sLED 88 emits optical radiation with a wavelength λ1 = 450 nm, and the sLED 90 emits optical radiation with a wavelength λ2 = 750 nm. The light source 80 further includes lenses 92, 94, and 96, and a dichroic beam splitter 98. Alternatively, other types and wavelengths of light sources may be used.

[0048]

[0049] The folding assembly 84 is a high-contrast transmission diffraction grating assembly 100, such as an assembly of a chromium on glass grating, disposed between two lenses 102 and 104. The diffraction grating assembly 100 includes gratings that are orthogonal to each other, by which light is diffracted in both the y and z directions. By the cooperation of these gratings, portions of the grating pattern projected by the projector 20 are created.

[0050] The optical radiation emitted by the sLEDs 88 and 90 is projected by respective lenses 92 and 94 towards the dichroic beam splitter 98. The beam splitter 98 is configured to pass the optical radiation emitted by the sLED 88 and reflect the optical radiation emitted by the sLED 90, so that the radiation emitted by these two sLEDs is combined into a single beam 106. The beam 106 is focused by the lens 96 into the input end 108 of the single-mode optical fiber 82. The optical radiation transmitted through the fiber 82 exits the fiber through its output end 110 and enters the folding assembly 84, and is projected by the lens 102 towards the diffraction grating 100 as a beam 112. Since the output end 110 is disposed in the focal plane of the lens 102, the beam 112 is collimated. The beam 112 is diffracted by the grating assembly 100 into parallel diffracted orders of light 114 and is focused by the lens 104 onto the focal plane 116.

[0051] The spatial filter 86 is disposed within the focal plane 116 and is configured to pass only the ±1st order diffracted light (the ±1st order diffracted light among the diffracted orders of light 114) generated by the diffraction by the grating assembly 100. This function is detailed in the inset view 118, which shows a yz view. ​ , i.e., viewed from the direction of the x-axis, the spatial filter 86 is shown. The spatial filter 86 has a transparent ring 120 on an opaque base 122, and the ring is formed, for example, by removing chromium from a chromium-on-glass base. The ±1st order diffracted light related to the radiation emitted by the sLED 88 is shown as a square 124 within the ring 120, and the light related to the radiation emitted by the sLED 90 is shown as a square 126. The 0th order diffracted light is blocked by the central portion 128 of the spatial filter, and the diffracted light of orders higher than ±1st is blocked by the peripheral portion 130 of the spatial filter 86. The ±1st order diffracted light after passing through the spatial filter 86 forms an expanding beam 132. Due to the mutual interference of these beams, a propagating sine grating is brought about (as the interference pattern between these ±1st order diffracted lights), and they are reflected by the beam splitter 45 (FIG. 1) to become the beam 134. The sine grating will be described in detail later in relation to FIG. 3. The beam 134 propagates collinearly with respect to the optical radiation reflected from the wafer 12 (FIG. 1) towards the cameras CAM1 and CAM2, so the mutual registration of these two cameras is possible as will be described in detail later. The spectral content of the sine grating propagating as the beam 134 depends on whether one or both of the sLEDs 88 and 90 are excited to emit optical radiation. By matching the radiation wavelengths λ1 and λ2 with the spectral characteristics of the dichroic beam splitter 48, one of those wavelengths can be reflected by the beam splitter and the other can be transmitted.

[0052]

[0053]

[0054] ​​​​​​​​​​​​​​​​ FIG. 3 shows an example of an image captured by one of cameras CAM1 and CAM2 according to an embodiment of the present invention. FIG. 1 is a schematic representation of an image 150 that is to be processed using the AIM™ proxy. A dovetail projector 20 is projected along the image of the target 152 and its proxy target. The four gratings 154, 156, 158 and 160 are projected by A. The target features in the IM™ proxy target 152 have Four resist gratings 162 are located at the center and oriented in pairs, and also along the x and y axes. and four process layer gratings 164 arranged in pairs and oriented in parallel. For clarity, only two of the resist gratings 162 and two of the process layer gratings 164 are shown. One of each is oriented along the x-axis and the other along the y-axis. It is stated as such.

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

[0056] For the purpose of calculating the resist layer-to-process layer overlay error, the controller 24 Based on the image of the target 152, the X and Y coordinates of the CoS of the resist grating 162 (CoS X,R ,CoS Y,R ) are calculated, and similarly the X and Y coordinates of the CoS of the process layer lattice 164 are calculated. Mark (CoS) X,P ,CoS Y,P ) is calculated. The difference between the center X and Y coordinates determines the overlay error OVL for each of X and Y.X =(C oS X,R -CoS X,P ),OVL Y =(CoS Y,R -CoS Y,P ) results in . For simplicity, hereinafter, the two-dimensional vector (CoS X , CoS Y ) is denoted .

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

[0058] [Elucidation of Resist and Process Target Features in Overlay Metrology Proxy Target] In the following figures, various methods are shown for measuring the CoS variation of the target features of the overlay proxy target as a function of various imaging parameters such as, for example, focus and illumination wavelength . Such methods have been described above in view of simplicity and clarity and are described in the context of the system structure and components shown in the preceding drawings and in the context of a certain type of proxy target and the target features provided thereon. Nevertheless ​​Those skilled in the art of this technology (so-called persons skilled in the art) who have read this specification will clearly see that the principles of those methods can be similarly applied to other overlay metrology systems with necessary modifications, and can also be done so using other types of proxy targets. Furthermore, the elements of these various methods may be combined to enable multifactor CoS measurement and calibration. All such alternative embodiments are considered to be within the technical scope of the present invention.

[0059] Figure 4 is a flowchart 200 schematically depicting a process for measuring CoS variations of a process layer of an overlay metrology proxy target and a resist layer target feature as a function of focus, according to an embodiment of the present invention. In this process, reference is made to the optical inspection apparatus 10 described in FIG. 1, and FIGS. 2 and 3 are additionally referred to. The purpose of the process depicted in FIG. 4 is to individually elucidate the CoS positions of the target features of each layer of an overlay proxy target, for example, an AIM (trademark) target 152, in association with the focus setting of the wafer 12 in the apparatus 10. This process starts at start step 202. In the focusing step 204, the controller 24 sets the cameras CAM1 and CAM2 to focus on the wafer 12 by moving the table 26 along the z direction and / or moving the cameras by the actuators 58 and 60. The cameras are set to a focus difference of ΔZ by the differential movement of the actuators 58 and 60 (the focus setting in this specification refers to the Z coordinate in the wafer space. For example, the focus difference ΔZ is such that the cameras CAM1 and CAM2 are focused

[0060] This process starts at start step 202. In the focusing step 204, the cameras CAM1 and CAM2 are set by the controller 24 to focus on the wafer 12 by moving the table 26 along the z direction and / or moving the cameras by the actuators 58 and 60. The cameras are set to a focus difference of ΔZ by the differential movement of the actuators 58 and 60 (the focus setting in this specification refers to the Z coordinate in the wafer space. For example, the focus difference ΔZ is such that the cameras CAM1 and CAM2 are focused ​​​​​​​​​​​​​ indicating that each xy plane is separated by ΔZ on or near the wafer 12 (which will be described later in connection with FIG. 5 for further details of the focus separation and focusing of cameras CAM1 and CAM2).

[0061] In the first grating registration step 206, both cameras CAM1 and CAM2 are aligned with the gratings 154, 156, 158, and 160 projected by the grating projector 20. To perform this registration, the acquired images of these gratings are processed by the controller 24, thereby searching for the relative positions of each camera along the x and y directions with respect to these gratings. To align the cameras with each other, the cameras can be physically moved with respect to the gratings 154, 156, 158, and 160, or the camera-grating offset can be calculated and used in subsequent processing. Since the gratings are in a periodic form and the pixels within CAM1 and CAM2 have a repetitive structure, the controller 24 can align each camera with respect to these gratings with an accuracy better than 0.1 nm (with respect to the xy coordinates in the wafer space). Furthermore, since the same gratings are projected onto both cameras CAM1 and CAM2, any spatial shift or vibration of the projected gratings occurs in both of these cameras in a common mode. Since each camera is aligned with respect to these same common-mode gratings, the cameras are aligned with each other with an accuracy better than 0.1 nm. In the first acquisition step 208, the images of the proxy target 152, specifically those of the gratings 162 and 164, are read from the cameras CAM1 and CAM2 by the controller 24 and stored in the memory 25. ​​​​​​​​​

[0062] In the refocusing step 210, the table 26 moves the wafer 1 along the z direction by a distance ΔZ, so that the CAM1 is brought to the Z coordinate where the CAM2 was before step 210. In the second grating registration step 212, the controller 24 again aligns the cameras CAM1 and CAM2 with the gratings 154, 156, 15 8 and 160 as in the first grating registration step 206. The purpose of this step is to ensure continuous registration between those two cameras. In the CAM1 registration step 214, the CAM1 is aligned with the image acquired by the CAM2 at the previous focus position. So, in combination with the second grating registration step 212, the positions of those two cameras in the xy plane are established in relation to the previous focus position. In the second acquisition step 216, similar to the first acquisition step 208, the images of the proxy target 152 are read by the controller 24 through the cameras CAM1 and CAM2 and stored in the memory 25. In the discrimination step 218, the controller 24 determines whether another focus stage is required based on a preset sequence of focus stages. If the answer is yes, the process returns to the refocusing step 210 and continues from there. After all the preset focus stages have been taken, in the calculation step 220, the controller 24 processes the various images stored in the memory 25, so that the CoS of each of the gratings 162 and 164 is calculated as a function of the focus setting through the various focus stages of ΔZ. The process ends at the end step 222.

[0063] In the discrimination step 218, the controller 24 determines whether another focus stage is required based on a preset sequence of focus stages. If the answer is yes, the process returns to the refocusing step 210 and continues from there. After all the preset focus stages have been taken, in the calculation step 220, the controller 24 processes the various images stored in the memory 25, so that the CoS of each of the gratings 162 and 164 is calculated as a function of the focus setting through the various focus stages of ΔZ. The process ends at the end step 222. ​​​​​​​​​​​

[0064] In certain embodiments, by adjusting the direction in which the illumination impinges on the wafer 12 (the impinging direction: the propagation direction of the illumination), the residual optical error in the imaging optical system of the metrology tool is compensated. For example, in the optical inspection apparatus 10, the controller 24 adjusts the position of the aperture assembly 36 in the yz plane by the action of the actuator 38, thereby compensating for the residual optical error in the objective lens 30. Since the CoS of each of the gratings 162 and 164 depends on the position of the aperture assembly 36, by measuring the CoS as a function of focus for a plurality of positions of the aperture assembly 36, more comprehensive data can be collected. In one embodiment, the process described in flowchart 200 is executed for a series of Y and Z coordinates of the aperture assembly, such as (Y0 ± n*ΔY, Z0 ± n*ΔZ) ; where Y0 and Z0 are the nominal center positions of the aperture assembly 36, ΔY and ΔZ represent the incremental increments of the aperture assembly, and n is an integer exponent with values assumed from 0 to N, which is the maximum value. As will be described later in the column entitled "Selection of Measurement Conditions", the quality of the overlay measurement can be further improved using the obtained data. The positions of the two cameras CAM1 and CAM2 in the wafer space with respect to the Z coordinate, i.e., the Z coordinates with respect to the wafer 12, are shifted through N focus levels. On the Z coordinate axis 251 36, more comprehensive data can be collected. In one embodiment, the process described in flowchart 200 is executed for a series of Y and Z coordinates of the aperture assembly, such as (Y0 ± n*ΔY, Z0 ± n*ΔZ) 36, more comprehensive data can be collected. In one embodiment, the process described in flowchart 200 is executed for a series of Y and Z coordinates of the aperture assembly, such as (Y0 ± n*ΔY, Z0 ± n*ΔZ) ; where Y0 and Z0 are the nominal center positions of the aperture assembly 36, ΔY and ΔZ represent the incremental increments of the aperture assembly, and n is an integer exponent with values assumed from 0 to N, which is the maximum value. As will be described later in the column entitled "Selection of Measurement Conditions", the quality of the overlay measurement can be further improved using the obtained data. ; where Y0 and Z0 are the nominal center positions of the aperture assembly 36, ΔY and ΔZ represent the incremental increments of the aperture assembly, and n is an integer exponent with values assumed from 0 to N, which is the maximum value. As will be described later in the column entitled "Selection of Measurement Conditions", the quality of the overlay measurement can be further improved using the obtained data. ; where Y0 and Z0 are the nominal center positions of the aperture assembly 36, ΔY and ΔZ represent the incremental increments of the aperture assembly, and n is an integer exponent with values assumed from 0 to N, which is the maximum value. As will be described later in the column entitled "Selection of Measurement Conditions", the quality of the overlay measurement can be further improved using the obtained data. ; where Y0 and Z0 are the nominal center positions of the aperture assembly 36, ΔY and ΔZ represent the incremental increments of the aperture assembly, and n is an integer exponent with values assumed from 0 to N, which is the maximum value. As will be described later in the column entitled "Selection of Measurement Conditions", the quality of the overlay measurement can be further improved using the obtained data. ; where Y0 and Z0 are the nominal center positions of the aperture assembly 36, ΔY and ΔZ represent the incremental increments of the aperture assembly, and n is an integer exponent with values assumed from 0 to N, which is the maximum value. As will be described later in the column entitled "Selection of Measurement Conditions", the quality of the overlay measurement can be further improved using the obtained data. ; where Y0 and Z0 are the nominal center positions of the aperture assembly 36, ΔY and ΔZ represent the incremental increments of the aperture assembly, and n is an integer exponent with values assumed from 0 to N, which is the maximum value. As will be described later in the column entitled "Selection of Measurement Conditions", the quality of the overlay measurement can be further improved using the obtained data. ; where Y0 and Z0 are the nominal center positions of the aperture assembly 36, ΔY and ΔZ represent the incremental increments of the aperture assembly, and n is an integer exponent with values assumed from 0 to N, which is the maximum value. As will be described later in the column entitled "Selection of Measurement Conditions", the quality of the overlay measurement can be further improved using the obtained data.

[0065] FIG. 5 is a plot schematically showing the focus interval between CAM1 and CAM2 and various focus levels taken in the process of FIG. 4 according to an embodiment of the present invention. ; where Y0 and Z0 are the nominal center positions of the aperture assembly 36, ΔY and ΔZ represent the incremental increments of the aperture assembly, and n is an integer exponent with values assumed from 0 to N, which is the maximum value. As will be described later in the column entitled "Selection of Measurement Conditions", the quality of the overlay measurement can be further improved using the obtained data.

[0066] The positions of the two cameras CAM1 and CAM2 in the wafer space with respect to the Z coordinate, i.e., the Z coordinates with respect to the wafer 12, are shifted through N focus levels. On the Z coordinate axis 251 The positions of the two cameras CAM1 and CAM2 in the wafer space with respect to the Z coordinate, i.e., the Z coordinates with respect to the wafer 12, are shifted through N focus levels. On the Z coordinate axis 251 As noted in the first step 250, CAM1 is focused on the plane Z=Z0, and CAM 2 is focused on the plane Z = Z0 + ΔZ. In the second step 252, the wafer focus is shifted by ΔZ. CAM1 is incremented by Z=Z0+ΔZ, and CAM2 is incremented by Z=Z0+ΔZ. This is where CAM1 is located, and CAM2 is located at 250 in the first stage. In a third step 254, the wafer focus is again inclined by ΔZ. CAM1 is brought to Z=Z0+2ΔZ, and CAM2 is brought to Z=Z0+3ΔZ. This process continues until the Nth stage 256, when the location of CAM1 is Z=Z0+(N- 1) ΔZ, the location of CAM2 is Z = Z0 + NΔZ.

[0067] In other words, at each focus step, CAM1 is placed at the focus where CAM2 was located in the previous step. This allows for registration between successive steps. This is done in combination with the steps of performing the grating registration steps 206 and 212 (FIG. 4). Thus, the controller 24 controls each camera along the x and y directions through the focus. The gratings 162 and 164 are precisely aligned across the focal point. It is possible to calculate S.

[0068] 6A-6B illustrate an overlay metrology profile in relation to illumination wavelength according to one embodiment of the present invention. The process of measuring the CoS variation of a target feature of a proxy target is shown in the following schematic. Illustrated is a flow chart 300. In this process, the optical detector shown in FIG. 2 and 3. The purpose of the process is to provide an overlay proxy in conjunction with the wavelengths used by the device 10. Each of the target features of the target, e.g., the AIM™ target 152 The purpose of the flow chart 300 is to individually resolve the positions of the grids 164, S. A first portion 301 and a second portion 302 in which the CoS of the grating 162 is calculated as a function of the illumination wavelength. It has.

[0069] The process begins at start step 303. Beam splitter placement step 30 In the fourth embodiment, the actuator 49 is excited by the controller 24 to generate a dichroic beam. The optical splitter 48 is brought into the optical path of the device 10 so that the optical The radiation is received by CAM1 within the spectral band Δλ1 and the radiation is The portion of the radiation within the spectral band Δλ2 is received by CAM2. become.

[0070] The process then enters the first portion 301. In the first illumination step 306, the semiconductor wafer is Wafer 12 is illuminated with wavelengths λ1 and λ2, where λ1 is within the spectral band Δλ1. , wavelength λ2 is within the spectral band Δλ2, and optical radiation is emitted at wavelength λ1 by light source 32. and optical radiation is emitted by the light source 33 at wavelength λ2. The controller 24 controls the table 26 and the actuators 58 and 60 to Camera CAM1 is the contrast focus on grid 162, and camera CAM2 is the contrast focus on grid 164. The term "contrast focus" refers to the point where the grid images on each camera are focused at maximum contrast. The contrast C is the focal position of the image on a given camera. Maximum and minimum intensities of child image Imax and I min Based on this, C = (I max - I min ) / (I max + I min ) is defined. In the first grid registration step 31 At 0, similar to the first grid registration step 206 (Figure 4), the projected grid 154, 156, 158, and 160 are aligned with cameras CAM1 and CAM2 . Then, in the first acquisition step 312, images of grids 162 and 164 are acquired by cameras CAM 1 and CAM2, read from these cameras by the controller 24, and stored in the memory 25.

[0071] In the first wavelength change step 314, the controller 24 increments the wavelength of the optical radiation emitted by the light source 33 within the spectral band Δλ2 by Δλ, so that the wavelength of the illumination reaching CAM 2 changes from λ2 to λ2 + Δλ. In the first refocusing step 316 , CAM2 is refocused to the contrast focus at the wavelength after the increment. In the second grid registration step 318, similar to the first grid registration step 310, cameras CAM1 and CA M2 are aligned with grids 154, 156, 158, and 160. In the re - registration step 320, CAM1 is refocused and realigned with the image of grid 162 that is the same as the one acquired through CAM1 in the acquisition step 312. That is, it is continuously aligned with the same physical grid 162 at the same wavelength and the same focus, and thereby CAM1 is established as the "anchor" for part 301 . In the second acquisition step 322, an image of grid 164 is acquired by CAM2 . Thus, CAM1 is established as the "anchor" for part 301 . In the second acquisition step 322, an image of grid 164 is acquired by CAM2 It is 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 preset series of wavelength steps to determine whether another wavelength step is required within the spectral band Δλ2. If the answer is affirmative the process returns to the first wavelength change step 314, and the wavelength of the illumination reaching CAM2 is incremented by Δλ again, after which the process continues. When all the preset focus steps are exhausted, in the first calculation step 326 by the controller 24, based on the various images stored in the memory 25, the CoS of the grating 164 is calculated as a function of the wavelength passing through the various wavelength steps Δλ within the spectral band Δλ 2. 2.

[0073] The process continues with a second part 302, which is described in detail to clarify the differences between the first and second parts. Steps 328, 330, 332, 334, 340 and 346 within the second part 302 are identical to the corresponding steps 306, 308, 310, 3 12, 318 and 324 within the first part 301. However, steps 336, 338, 342 and 344 within the second part 302 are different from the corresponding steps 314, 316, 320 and 326 within the first part 301. In those steps of the second part, the spectral scan is not performed in the manner of traversing the spectral range Δλ2 as in the first part, but is performed in the manner of traversing the spectral range Δλ1.

[0074] In the second illumination step 328, the semiconductor wafer 12 is illuminated with wavelengths λ1 and λ2. In the second focusing step 330, the controller 24 causes the camera CAM1 to be on the grating 162 ​​​The contrast focus of, CAM2 is focused on the contrast focus on the grid 164. The third grid In the sub-registration step 332 of the child, cameras CAM1 and CAM2 are aligned with the grids 154, 156, 158, and 160. In the third acquisition step 334, images of the grids 162 and 164 are acquired by cameras CAM1 and CAM2, read from those cameras by the controller 2 4, and stored in the memory 25.

[0075] In the second wavelength change step 336, the controller 24 increments the wavelength of the optical radiation emitted within the spectral band Δλ1 by the light source 32 by Δλ. The second re focus step 338, CAM1 is refocused on the contrast focus at the wavelength Δλ1 + Δλ after the increment. In the fourth grid registration step 340, the cameras CAM1 and CAM2 are aligned with the grids 154, 156, 158, and 160 . In the second re-registration step 342, CAM2 is refocused and realigned with the image of the grid 164 that is the same as that read from CAM2 in the third acquisition step 334 , whereby CAM2 is established as an anchor for the second portion 302. In the fourth acquisition step 344, an image of the grid 162 is acquired by the camera CAM1, read from that camera by the controller 24, and stored in the memory 25. In the second discrimination step 346, the controller 24 determines whether another wavelength step is required within the spectral band Δλ1, following a preset series of wavelength steps. If the answer is affirmative , the process returns to the second wavelength change step 336, and the wavelength of the illumination reaching CAM1 is incremented.

[0076] If so, the process returns to the second wavelength change step 336, and the wavelength of the illumination reaching CAM1 is incremented. If the answer is affirmative , the process returns to the second wavelength change step 336, and the wavelength of the illumination reaching CAM1 Incremented by Δλ again, the process continues. After all the preset wavelength steps are taken, at the second calculation step 348, the CoS of the grating 162 is calculated as a function of the wavelength passing through the harmonic wavelength steps Δλ within the spectral band Δλ1. The process ends at the end step 350.

[0077] Similar to the measurement of the CoS variation due to the focus, the data obtained regarding the CoS variation due to the wavelength can also be used to further improve the quality of the overlay measurement, as described later in the column titled "Selection of Measurement Conditions".

[0078] The measurement of the CoS variation due to the illumination wavelength may be performed with the polarization state of the optical radiation as an additional parameter. In one embodiment, the CoS variation due to the wavelength is measured for various polarization states of the illumination impinging on the wafer 12. That is, upon receiving a command from the controller 24, the optical radiation is emitted in two mutually orthogonal polarization states by the light sources 32 and 33, and the CoS variation due to the illumination wavelength is individually measured for each polarization state. In an alternative embodiment, the wafer 12 is illuminated with unpolarized optical radiation and a particular state of polarization reaches each of the two cameras CAM1 and CAM2, and that particular state is determined by a dichroic beam splitter 48 that also functions as a polarizer, or by a polarizer appropriately disposed between the dichroic beam splitter and the two cameras.

[0079] Figures 7A - 7B schematically depict a flow chart 500 of a process for generating a CoS landscape and evaluating sensitivity over various focus settings and wavelengths according to an embodiment of the present invention. ​​​​​​​​​

[0080] Scanner-induced overlay errors, such as misplacement and rotation of the wafer 12 in the scanner, and scanner field distortion, are generally measured at several measurement sites on the wafer. The process shown in flowchart 500 relates to the measurement of N sites on the wafer 12, where these sites are numbered n (n = 1, 2,... N). Further, this process is performed for both the resist layer and the process layer. For capturing scanner-induced overlay errors, such as misplacement and rotation of the wafer 12 in the scanner, and scanner field distortion, the overlay errors are generally measured at several measurement sites on the wafer. The process shown in flowchart 500 relates to the measurement of N sites on the wafer 12, where these sites are numbered n (n = 1, 2,... N). Further, this process is performed for both the resist layer and the process layer. The process shown in flowchart 500 relates to the measurement of N sites on the wafer 12, where these sites are numbered n (n = 1, 2,... N). Further, this process is performed for both the resist layer and the process layer. (n = 1, 2,... N). Further, this process is performed for both the resist layer and the process layer. The process shown in flowchart 500 relates to the measurement of N sites on the wafer 12, where these sites are numbered n (n = 1, 2,... N). Further, this process is performed for both the resist layer and the process layer.

[0081] CoS is measured in association with the focus at two wafer 12 orientations, and for the second orientation, the wafer is rotated 180° about the Z axis. Of the CoS associated with the focus, the one at the first orientation (arbitrarily set at 0° orientation) is denoted as CoS0(Z), and the one at the second orientation is denoted as CoS (Z). The tool-induced shift (TIS) of CoS, i.e., CoS_TIS, calculated by the controller 24 is given by CoS_TIS(Z) = (CoS0(Z) + CoS (Z)) / 2. The CoS error described by CoS_TIS is due to the asymmetry of the optical members in the imaging path of the apparatus 10, such as the asymmetry of the objective lens 30. The corrected CoS, i.e., the measured CoS minus CoS_TIS, can be calculated by the controller 24 as CoS_COR(Z); where CoS_ 180 COR(Z) = [CoS0(Z) - CoS (Z)] / 2, and "COR" represents "after correction". As described above, CoS represents a two-dimensional vector (CoS , CoS 180 (Z)). The CoS error described by CoS_TIS is due to the asymmetry of the optical members in the imaging path of the apparatus 10, such as the asymmetry of the objective lens 30. The CoS error described by CoS_TIS is due to the asymmetry of the optical members in the imaging path of the apparatus 10, such as the asymmetry of the objective lens 30. The corrected CoS, i.e., the measured CoS minus CoS_TIS, can be calculated by the controller 24 as CoS_COR(Z); where CoS_ COR(Z) = [CoS0(Z) - CoS 180 (Z)] / 2, and "COR" represents "after correction". The corrected CoS, i.e., the measured CoS minus CoS_TIS, can be calculated by the controller 24 as CoS_COR(Z); where CoS_ X , CoS Y ) Therefore, the notation "CoS" includes both its X and Y coordinates.

[0082] The inaccuracy of CoS measurement due to mechanical vibration of the apparatus 10 can be reduced by acquiring images of the proxy target 15 multiple times and averaging the measurement results.

[0083] This process starts at the start step 502. In the polarization selection step 504, the (one or more) polarization states of the illumination emitted by the illumination assembly 16 are selected. In the site selection step 506, site n on the wafer 12 is selected. In the wavelength selection step 507, the wavelength λ is selected. In the 0-degree CoS through focus step 508, CoS is measured longitudinally through the focus Z as described in connection with FIGS. 4 and 5. In the 180-degree CoS through focus step 510, the above measurement is repeated, but the orientation of the wafer 12 is rotated 180° with respect to that in step 5 08. In the first CoS_TIS step 512, CoS_TIS is calculated by the controller 24 as CoS_TIS = (CoS0 + CoS ) / 2 based on the individual CoS values at the 0° orientation and 180° orientation for each focus setting Z. For simplicity, the explicit dependence on the focus Z, wavelength λ, polarization P, and site n is omitted in these equations. In the first CoS_COR step 514, CoS_COR is calculated by the controller 24 as CoS_COR = (CoS0 - CoS 180 ) / 2 based on the individual CoS values at 0° and 180° for each focus setting Z (again, the explicit reference to the variables is omitted here). In the wavelength discrimination step 51 For the sake of simplicity, the explicit dependence on the focus Z, wavelength λ, polarization P, and site n is omitted in these equations. In the first CoS_COR step 514, CoS_COR is calculated by the controller 24 as CoS_COR=(CoS0 - CoS ) / 2 based on the individual CoS values at 0° and 180° for each focus setting Z (where the explicit reference to the variables is also omitted here). In the wavelength discrimination step 51 is calculated by the controller 24 based on the individual CoS values at 0° and 180° for each focus setting Z (where the explicit reference to the variables is also omitted here). CoS_COR=(CoS0 - CoS 180 ) / 2 (where the explicit reference to the variables is also omitted here). is calculated (where the explicit reference to the variables is also omitted here). In the wavelength discrimination step 51 In step 6, the controller 24 determines whether steps 507-51 4 should be executed again based on the preset wavelength list. In the case of an affirmative answer, the wavelength increment step 517 increments the wavelength λ, and the process continues from step 507.

[0084] When all the preset wavelengths are exhausted, in this process, subsequently, as described in connection with FIGS. 6A-6B, CoS is measured in association with the wavelength at the best contrast focus. In the previous step, although CoS has been measured in association with the focus at all the required wavelengths, due to the drift of the apparatus 10, some of the measurement results of CoS in relation to the focus may drift along the focus coordinate Z. As will be described in detail below, the drift of that kind can be corrected using the measurement of CoS in relation to the wavelength. In the 0-degree CoS-through-wavelength step 518, CoS is measured in such a manner as to traverse the preset spectrum of the wavelength at the best contrast focus. In the 180-degree CoS-through-wavelength step 52 0, the above measurement is repeated, but the orientation of the wafer 12 is rotated 180° with respect 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 as described in connection with steps 512 and 514 respectively above.

[0085] In the 0-degree CoS-through-wavelength step 518, CoS is measured in such a manner as to traverse the preset spectrum of the wavelength at the best contrast focus. In the 180-degree CoS-through-wavelength step 52 0, the above measurement is repeated, but the orientation of the wafer 12 is rotated 180° with respect to that in step 516. In the second CoS_TIS step 522 and the second CoS_COR step 0, the above measurement is repeated, but the orientation of the wafer 12 is rotated 180° with respect to that in step 516. In the second CoS_TIS step 522 and the second CoS_COR step 80° rotation. 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 as described in connection with steps 512 and 514 respectively above. In the CoS_TIS stitching step 526, for each wavelength λ, the controller 24 sets the focus

[0086] Z as the best contrast focus, and from steps 507-516, CoS_TI longitudinally through the focus Z is used as the best contrast focus, and CoS_TI longitudinally through the focus is obtained from steps 507-516 The results brought about for S are compared with the results brought about for CoS_TIS from step 522. If there is a discrepancy between these two sets of results, the discrepancy is removed by shifting the results for CoS_TIS at wavelength λ along the focal coordinate Z. Thereby, as will be described in more detail in connection with FIG. 8A below, the results of the focal longitudinal CoS_TIS at adjacent wavelengths are "stitched" together, resulting in a consistent representation of CoS_TIS in the two-dimensional Zλ space. In the CoS_TIS landscape step 528, this representation is collected by the controller 24, resulting in a set of CoS_TIS values for the two variables Z and λ. This set is referred to as the CoS_TIS landscape. In the CoS_TIS derivative step 530, the controller 24 calculates the value of the second derivative ∂CoS_TIS / ∂Z∂λ, which indicates the sensitivity of CoS_TIS to variations in the variables Z and λ. In the minimum CoS_TIS step 532, the controller 24 identifies within the CoS_TIS landscape one or more two-dimensional areas in the (Z,λ) plane where the absolute value of CoS_TIS is less than a predetermined limit, thereby indicating the area where the tool-induced shift related to CoS is minimal, i.e., the area where the CoS error is minimal (separate limits may be assigned to the X and Y components of CoS_TIS). In the minimum CoS_TIS derivative step 533, the controller 24 identifies within the CoS_TIS landscape one or more two-dimensional areas in the (Z,λ) plane where the absolute value of ∂CoS_TIS / ∂Z∂λ is less than another predetermined limit, and thus the stability of CoS_TIS is achieved. As will be described in more detail in connection with FIG. 8A below, the results of the focal longitudinal CoS_TIS at adjacent wavelengths are "stitched" together, resulting in a consistent representation of CoS_TIS in the two-dimensional Zλ space. In the CoS_TIS landscape step 528, this representation is collected by the controller 24, resulting in a set of CoS_TIS values for the two variables Z and λ. This set is referred to as the CoS_TIS landscape. As will be described in more detail in connection with FIG. 8A below, the results of the focal longitudinal CoS_TIS at adjacent wavelengths are "stitched" together, resulting in a consistent representation of CoS_TIS in the two-dimensional Zλ space. In the CoS_TIS landscape step 528, this representation is collected by the controller 24, resulting in a set of CoS_TIS values for the two variables Z and λ. This set is referred to as the CoS_TIS landscape. In the CoS_TIS landscape step 528, this representation is collected by the controller 24, resulting in a set of CoS_TIS values for the two variables Z and λ. This set is referred to as the CoS_TIS landscape. In the CoS_TIS landscape step 528, this representation is collected by the controller 24, resulting in a set of CoS_TIS values for the two variables Z and λ. This set is referred to as the CoS_TIS landscape. This set is referred to as the CoS_TIS landscape.

[0087] In the CoS_TIS derivative step 530, the controller 24 calculates the value of the second derivative ∂CoS_TIS / ∂Z∂λ, which indicates the sensitivity of CoS_TIS to variations in the variables Z and λ. In the CoS_TIS derivative step 530, the controller 24 calculates the value of the second derivative ∂CoS_TIS / ∂Z∂λ, which indicates the sensitivity of CoS_TIS to variations in the variables Z and λ. 2 In the CoS_TIS derivative step 530, the controller 24 calculates the value of the second derivative ∂CoS_TIS / ∂Z∂λ, which indicates the sensitivity of CoS_TIS to variations in the variables Z and λ. In the CoS_TIS derivative step 530, the controller 24 calculates the value of the second derivative ∂CoS_TIS / ∂Z∂λ, which indicates the sensitivity of CoS_TIS to variations in the variables Z and λ. In the minimum CoS_TIS step 532, the controller 24 identifies within the CoS_TIS landscape one or more two-dimensional areas in the (Z,λ) plane where the absolute value of CoS_TIS is less than a predetermined limit, thereby indicating the area where the tool-induced shift related to CoS is minimal, i.e., the area where the CoS error is minimal (separate limits may be assigned to the X and Y components of CoS_TIS). In the minimum CoS_TIS step 532, the controller 24 identifies within the CoS_TIS landscape one or more two-dimensional areas in the (Z,λ) plane where the absolute value of CoS_TIS is less than a predetermined limit, thereby indicating the area where the tool-induced shift related to CoS is minimal, i.e., the area where the CoS error is minimal (separate limits may be assigned to the X and Y components of CoS_TIS). In the minimum CoS_TIS step 532, the controller 24 identifies within the CoS_TIS landscape one or more two-dimensional areas in the (Z,λ) plane where the absolute value of CoS_TIS is less than a predetermined limit, thereby indicating the area where the tool-induced shift related to CoS is minimal, i.e., the area where the CoS error is minimal (separate limits may be assigned to the X and Y components of CoS_TIS). In the minimum CoS_TIS step 532, the controller 24 identifies within the CoS_TIS landscape one or more two-dimensional areas in the (Z,λ) plane where the absolute value of CoS_TIS is less than a predetermined limit, thereby indicating the area where the tool-induced shift related to CoS is minimal, i.e., the area where the CoS error is minimal (separate limits may be assigned to the X and Y components of CoS_TIS). In the minimum CoS_TIS derivative step 533, the controller 24 identifies within the CoS_TIS landscape one or more two-dimensional areas in the (Z,λ) plane where the absolute value of ∂CoS_TIS / ∂Z∂λ is less than another predetermined limit, and thus the stability of CoS_TIS is In the minimum CoS_TIS derivative step 533, the controller 24 identifies within the CoS_TIS landscape one or more two-dimensional areas in the (Z,λ) plane where the absolute value of ∂CoS_TIS / ∂Z∂λ is less than another predetermined limit, and thus the stability of CoS_TIS is 2 In the minimum CoS_TIS derivative step 533, the controller 24 identifies within the CoS_TIS landscape one or more two-dimensional areas in the (Z,λ) plane where the absolute value of ∂CoS_TIS / ∂Z∂λ is less than another predetermined limit, and thus the stability of CoS_TIS is In the minimum CoS_TIS derivative step 533, the controller 24 identifies within the CoS_TIS landscape one or more two-dimensional areas in the (Z,λ) plane where the absolute value of ∂CoS_TIS / ∂Z∂λ is less than another predetermined limit, and thus the stability of CoS_TIS is The area with the highest performance is indicated.

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

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

[0090] In the minimum CoS_COR derivative step 540, the controller 24 identifies the (one or more) two-dimensional areas within the CoS _COR landscape where the absolute value of ∂ 2 CoS_COR / ∂Z∂λ is less than another predetermined threshold, thereby obtaining the CoS_COR ​​​​​The area with the highest stability is indicated (CoS_COR can have an arbitrary offset with respect to 0, so there is no theoretical basis for identifying an area where CoS_COR is less than a predetermined limit). In step 542 of site discrimination, the controller 24 checks whether it is necessary to measure another site n on the wafer 12. If the result is affirmative, the process returns to step 506 and the next site is selected. After all sites (with respect to the current polarization) have been measured, in polarization discrimination step 544, the controller 24 checks whether it is necessary to perform measurements with illumination in an additional polarization state. If the result is affirmative, the process returns to step 506 and all N sites are measured again with illumination in a new polarization state. When all necessary polarization states have been exhausted, the process is terminated in end step 546.

[0091] In site discrimination step 542, the controller 24 checks whether it is necessary to measure another site n on the wafer 12. If the result is affirmative, the process returns to step 506 and the next site is selected. (Regarding the current polarization), after all sites have been measured, in polarization discrimination step 544, the controller 24 checks whether it is necessary to perform measurements with illumination in an additional polarization state. If the result is affirmative, the process returns to step 506 and all N sites are measured again with illumination in a new polarization state. When all necessary polarization states have been exhausted, the process is terminated in end step 546. The process is terminated.

[0092] Figures 8A to 8D relate to an embodiment of the present invention and are schematic representation diagrams of CoS_TIS and CoS_COR landscapes related to a resist and a process layer. Shown in Figure 8A is the CoS_TIS landscape 600 related to the resist layer, shown in Figure 8B is the CoS_COR landscape 602 related to the resist layer, shown in Figure 8C is the CoS_TIS landscape 604 related to the process layer, and shown in Figure 8D is the CoS_COR landscape 606 related to the process layer. Each landscape 600, 602, 604, and 606 relates to a given site n and a given polarization P. Similar landscapes are generated for all N sites and for illumination in all polarization states. Each landscape is the CoS_TIS landscape 600 related to the resist layer, shown in Figure 8B is the CoS_COR landscape 602 related to the resist layer, shown in Figure 8C is the CoS_TIS landscape 604 related to the process layer, and shown in Figure 8D is the CoS_COR landscape 606 related to the process layer. Each landscape 600, 602, 604, and 606 relates to a given site n and a given polarization P. Similar landscapes are generated for all N sites and for illumination in all polarization states. ​​​​​​​By dithering, the individual values of CoS_TIS or CoS_COR in a certain wavelength range (λ min , λ max ) and a certain focus range (Z min , Z max ) are represented. Curves 6 08, 610, 612 and 614 show the focus positions of the best contrast foci in relation to the wavelength for the individual landscapes 600, 602, 604 and 606.

[0093] The areas identified in steps 532, 533 and 540 of FIG. 7A are shown within the landscapes 6 00 to 606. That is, within area 616, ∂ 2 CoS_TIS / ∂Z ∂ λ is less than a predetermined limit L1, and within area 618, CoS_TIS is less than a predetermined limit L2 . Within area 620, ∂ 2 CoS_COR / ∂Z ∂λ is less than a predetermined limit L3. In area 622, ∂ 2 CoS_TIS / ∂Z ∂λ is less than a predetermined limit L4, and within area 62 4, CoS_TIS is less than a predetermined limit L5. Within area 626, ∂ 2 CoS_C OR / ∂Z ∂λ is less than a predetermined limit L6. Thus, areas 616, 622 show areas with high CoS_TIS stability for the resist and process layers respectively, and areas 618 and 624 show where the values of CoS_TIS are small for those layers, that is, where the measurement tool error is small. Areas 620, 626 show areas with high CoS_COR stability for the resist layer and process layer respectively.

[0094] The concept of "stitching" introduced in steps 526 and 534 of FIG. 7A is illustrated schematically in FIG. 8A. is depicted. The three lines 628, 630, and 632 are CoS_T as a function of the focus Z IS for individual wavelengths λ i-1 , λ i and λ i+1 measured at steps 507 - 516 of FIG. 7A for the three paths represented. The three points 634, 636, and 6 38 on curve 608 indicate where CoS_TIS as a function of wavelength λ was measured at steps 518 - 522 of FIG. 7B The CoS_TIS value at the intersection of line 628 with curve 608 is compared with the CoS_TIS value at point 634. If the two values are the same, line 628 is not shifted. Conversely, if they do not match, line 6 28 (and its CoS_TIS value) is shifted along the Z - direction until the value on line 628 at the intersection with curve 608 matches the value at point 634. A similar process is repeated for line 630 in relation to point 63 6 and for line 632 in relation to point 638. The three lines 628, 630, and 632 are shifted as necessary until the values at the intersections of each line with curve 608 match the values at the individual points 634, 636, and 638. At this point, the lines are, so to speak, "stitched" together. This process is repeated for all similar lines from λ

[0095] from λ to λ min to λ max

[0096] thereby correcting for any drift that may occur in apparatus 10 during the measurement of the focus - longitudinal CoS_TIS values. A similar stitching operation is also applied to landscapes 60

[0096] FIGS. 9A and 9B relate to an embodiment of the present invention and pertain to the accuracy of the resist and process layers It is a schematic representation of a landscape. In this embodiment, the accuracy of CoS measurement is evaluated. During accuracy measurement, several consecutive measurements of CoS are performed, for example, 5, 10, or 15 times, and an accuracy index thereof is calculated. A common index is the 3σ value (three times the standard deviation) of those measurements. In one embodiment, during the aforementioned measurements (Figs. 5 to 7), the accuracy of CoS (denoted as CoS_Prec) is measured and associated with two variables, the focus Z and the wavelength λ, to generate an accuracy landscape.

[0097] The landscape 700 in Fig. 9A represents the accuracy of CoS of the resist layer, and the landscape 702 in Fig. 9B represents the accuracy of CoS of the process layer. Each landscape, similar to the landscapes 600, 602, 604, and 606 in Figs. 8A, 8B, 8C, and 8D respectively, represents the individual values of CoS_Prec within a certain wavelength range (λ min ,λ min ,λ max ) and a certain focus range (Z min ,Z m ax 2 has its respective curves 704 and 706, thereby indicating the focus position of the best contrast focus in relation to the wavelength. Based on the CoS_Prec value related to the resist layer within the landscape 700, the controller 24 identifies areas 708 and 710 having values of CoS_Prec less than the preset limit L 7. Similarly, based on the CoS_Prec value related to the process layer within the landscape 702, the controller 24 identifies areas 712, 714, and 716 having values of CoS_Prec less than the preset limit L8.

[0098] ​​​ [Selection of Measurement Conditions] The metrology recipe, i.e., the setting of various parameters (e.g., focus, wavelength, polarization) of the apparatus 10 during overlay metrology, can greatly affect the quality of the measurement results. As detailed below, the user of the apparatus 10 can select the measurement conditions to be changed so that the desired balance is achieved among various factors, such as stability and accuracy. The layer-by-layer measurement result clarifications depicted in FIGS. 4 to 9 provide the user with a sophisticated

[0099] set of tools, thereby enabling the user to select the measurement conditions while keeping in mind the specific goals. Two exemplary embodiments illustrating the usage of these tools are presented below. [Number]

[0100] This metric and the variables within its components include the wavelength λ of illumination, the focus coordinate Z, the polarization state P, and N the layer L (whether it is a resist layer or a process layer). AVG is the average over N measurement sites, 3σ N is three times the standard deviation over those N sites, and AVG N and 3σ N take into account the effects of process variations on the landscape across the sites and polarizations. For the sake of simplicity, the accuracy

[0101] The metric M1 includes CoS_TIS as its contribution factor, and thus places emphasis on the evaluation of the measurement accuracy (tool-induced error). Let LIMIT(M1) be the predetermined limit for M1, and by finding the measurement conditions such that M1(λ,Z,P,L) < LIMIT(M1) the measured overlay error will exhibit the minimum tool-induced error.

[0102] In the case of the apparatus 10 (FIG. 1) having two illuminators 15 and 17, the measurement conditions can be optimized independently for each layer through the use of the layer-specific metric M1. If the apparatus 10 were to have only one illuminator, a compromise between the measurement conditions for the two layers would have to be found. For example, the potential requirements for a single-illuminator apparatus are such that M1(λ,Z,P,L ) + M1(λ,Z,P,L resist ) < LIMIT’(M1) process where L , L are the resist and process layer resist , respectively, and LIMIT’(M1) is a (different) predetermined limit. process

[0103] If CoS_TIS is a fixed correction factor related to the tool, it might be possible to calibrate it through a one-time calibration procedure. However, since there is a connection between the local geometry of the target feature of the proxy target and the optical response of the metrology tool optics, CoS_TIS may vary from target to target when multiple locations on the wafer are measured. This type of CoS_TIS variation can occur as a result of process variations across the wafer.

[0104] The contribution of CoS_TIS to the metric M1 can be reduced for each layer by the lateral shift of the apertures of each aperture assembly 36 and 37.

[0105] Embodiment 2 - In this embodiment, the metric M2 is calculated for each layer by the controller 24 using the same landscape as that of M1. However, the formula for M2 is different from that of M1. [Number]

[0106] Since the metric 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, by setting LIMIT(M2) as a predetermined limit for M2, requiring that M2(λ, Z, P, L) < LIMIT(M2) the measured overlay error exhibits a high degree of stability, and thus accurate calibration and robust overlay metrology of the measured overlay error can be supported.

[0107] The same variables as those for M1 are used for the metric M2, and AVG N and 3σ N are applied to N sites in the same manner as those for the metric M1. Considering whether the illumination assembly 16 has one illuminator or two illuminators, the same considerations as those for M1 are applied to the metric M2.

[0108] Alternatively or in addition, other metrics are generated using data from landscapes 600, 602, 604, and 606 or landscapes 700 and 702 to meet various user requirements.​​ It may be reflected.

[0109] FIG. 10 relates to an embodiment of the present invention, and plot 800 schematically depicts the variation of CoS_TIS as a function of the aperture offset is shown.

[0110] In plot 800, the variation of CoS_TIS measured by the apparatus 10 for four measurement sites (n = 1,..., 4) is shown in association with one of the apertures provided in the aperture assembly 36 horizontal offset AO. The individual CoS_T IS values for various sites are shown as lines 802. Due to the variations across those four sites, such as layer contrast last, feature topology, focus variation, site tilt, and process variation, line 8 02 exhibits separate offsets and slopes. The average of CoS_TI S across those four sites, i.e., AVG(CoS_TIS) as a function of AO, is shown as line 804 is.

[0111] The optimal (minimum) CoS_TIS variation across the four measurement sites, i.e., 3σ(CoS_TI S)1, is found at a certain aperture offset AO1, where CoS_TI S = CoS_TIS1. The non-zero value of AO1 indicates that there is a global angular alignment error between the wafer 12 and the imaging assembly 14. By selecting an aperture offset larger than AO1, although 3σ(CoS_TIS) increases, AVG(CoS_TIS) decreases, indicating that there is an optimization opportunity between AVG(CoS_TIS) and 3σ(CoS_TIS). By using an independent light source and aperture assembly as in the apparatus 10 of FIG. 1, for each layer, AVG( ​​​​Independent optimization between CoS_TIS) and 3σ(CoS_TIS) becomes possible.

[0112] Figures 11A and 11B relate to an embodiment of the present invention and depict the application of CoS as a function of focus to elucidate the sidewall asymmetry of target features within the AIM (trademark) proxy target 152, schematically. is depicted schematically.

[0113] Figure 11A is an image of the AIM (trademark) proxy target 152 (shown in Figure 3) with contour lines drawn around the resist grid 162 and the process layer grid 164 oriented azimuthally along the x-axis The individual grid bars 902 and 904 of grids 162 and 164 are oriented along the y axis.

[0114] Figure 11B is a schematic cross-sectional view of grid bars 904a and 904b taken from the process layer grid 164 along lines 908a and 908b in Figure 11A. To show the two cross-sectional views together the grid bars and their mutual spacing along the x-axis are not shown to the same scale. Due to asymmetric process effects in the semiconductor manufacturing process, such as asymmetric etching, as shown in Figure 11

[0115] B, grid bar 904 has an asymmetric topographical structure: namely, the left sidewall 910a of grid bar 904a is perpendicular to the xy plane, while the right sidewall 912a meets the xy plane at an oblique angle. Similarly, the left sidewall 910b of grid bar 904b is perpendicular to the xy plane, while the right sidewall 912b of that grid bar meets the xy plane at an oblique angle at an oblique angle. Usually, the area occupied by proxy target 152 is small and its linear dimension is tens of μm or less, so that all bars 904 within grid 164 are as shown in Figure 11 and all bars 904 within grid 164 are as shown in Figure 11 Exhibits the same asymmetry as shown in B. Due to the asymmetry of the grid bars, the CoS of grid 164 will shift more towards the focus, so as will be described in more detail below, the CoS variation as a function of the focus can be utilized to elucidate this asymmetry. As a result, the CoS of grid 164 will shift more towards the focus, and as will be described in more detail below, the CoS variation as a function of the focus can be utilized to elucidate this asymmetry. As a result, the CoS of grid 164 will shift more towards the focus, and as will be described in more detail below, the CoS variation as a function of the focus can be utilized to elucidate this asymmetry.

[0116] To elucidate the asymmetry of grid bars 904a and 904b (and thus the asymmetry of all grid bars 902 and 904), the controller 24 reads three images of grid 164 acquired by cameras CAM1 and CAM2 at three focus levels 920, 922, and 924 at the focal point marked on the Z-axis 926. By the controller 24, To elucidate the asymmetry of grid bars 904a and 904b (and thus the asymmetry of all grid bars 902 and 904), the controller 24 reads three images of grid 164 acquired by cameras CAM1 and CAM2 at three focus levels 920, 922, and 924 at the focal point marked on the Z-axis 926. By the controller 24, To elucidate the asymmetry of grid bars 904a and 904b (and thus the asymmetry of all grid bars 902 and 904), the controller 24 reads three images of grid 164 acquired by cameras CAM1 and CAM2 at three focus levels 920, 922, and 924 at the focal point marked on the Z-axis 926. By the controller 24, To elucidate the asymmetry of grid bars 904a and 904b (and thus the asymmetry of all grid bars 902 and 904), the controller 24 reads three images of grid 164 acquired by cameras CAM1 and CAM2 at three focus levels 920, 922, and 924 at the focal point marked on the Z-axis 926. By the controller 24, To elucidate the asymmetry of grid bars 904a and 904b (and thus the asymmetry of all grid bars 902 and 904), the controller 24 reads three images of grid 164 acquired by cameras CAM1 and CAM2 at three focus levels 920, 922, and 924 at the focal point marked on the Z-axis 926. By the controller 24, To elucidate the asymmetry of grid bars 904a and 904b (and thus the asymmetry of all grid bars 902 and 904), the controller 24 reads three images of grid 164 acquired by cameras CAM1 and CAM2 at three focus levels 920, 922, and 924 at the focal point marked on the Z-axis 926. By the controller 24, To elucidate the asymmetry of grid bars 904a and 904b (and thus the asymmetry of all grid bars 902 and 904), the controller 24 reads three images of grid 164 acquired by cameras CAM1 and CAM2 at three focus levels 920, 922, and 924 at the focal point marked on the Z-axis 926. By the controller 24, At all three focus levels 920, 922, and 924, CAM1 is focused on a fixed location within a certain xy plane on grid 162 at Z = Z0, and is aligned with grid 162 at that fixed focus. That is, a "reference focus" for measurement is formed by its Z coordinate Z0. At all three focus levels 920, 922, and 924, CAM1 is focused on a fixed location within a certain xy plane on grid 162 at Z = Z0, and is aligned with grid 162 at that fixed focus. That is, a "reference focus" for measurement is formed by its Z coordinate Z0. At all three focus levels 920, 922, and 924, CAM1 is focused on a fixed location within a certain xy plane on grid 162 at Z = Z0, and is aligned with grid 162 at that fixed focus. That is, a "reference focus" for measurement is formed by its Z coordinate Z0.

[0117] At those three focus levels 920, 922, and 924, CAM2 is focused on xy planes having Z coordinates Z1, Z2, and Z3 respectively. During image acquisition, both cameras CAM1 and At those three focus levels 920, 922, and 924, CAM2 is focused on xy planes having Z coordinates Z1, Z2, and Z3 respectively. During image acquisition, both cameras CAM1 and At those three focus levels 920, 922, and 924, CAM2 is focused on xy planes having Z coordinates Z1, Z2, and Z3 respectively. During image acquisition, both cameras CAM1 and At those three focus levels 920, 922, and 924, CAM2 is focused on xy planes having Z coordinates Z1, Z2, and Z3 respectively. During image acquisition, both cameras CAM1 and At those three focus levels 920, 922, and 924, CAM2 is focused on xy planes having Z coordinates Z1, Z2, and Z3 respectively. During image acquisition, both cameras CAM1 and Based on the image read from M1, the controller 24 calculates what is schematically marked as point 928 in Fig. 11B, and stores it in the memory 25. Based on the three images at three focal positions read from CAM2, the controller 24 calculates what are schematically marked as three individual CoS values, points 930, 932, and 934, and stores them in the memory 25. Curve 936 is fitted by the controller 24 to points 928, 930, 932, and 934, which indicates the shift of CoS due to the focus, i.e., serves as a gauge for the cross-sectional asymmetry of the grid bars 902 and 904. Curve 936 can be made into a straight line or a higher-order curve. and stored in the memory 25. Based on the three images at three focal positions read from CAM2, the controller 24 calculates what are schematically marked as three individual CoS values, points 930, 932, and 934, and stores them in the memory 25. Curve 936 is fitted by the controller 24 to points 928, 930, 932, and 934, which indicates the shift of CoS due to the focus, i.e., serves as a gauge for the cross-sectional asymmetry of the grid bars 902 and 904. Curve 936 can be made into a straight line or a higher-order curve. and stored in the memory 25. Curve 936 is fitted by the controller 24 to points 928, 930, 932, and 934, which indicates the shift of CoS due to the focus, i.e., serves as a gauge for the cross-sectional asymmetry of the grid bars 902 and 904. Curve 936 can be made into a straight line or a higher-order curve. which indicates the shift of CoS due to the focus, i.e., serves as a gauge for the cross-sectional asymmetry of the grid bars 902 and 904. Curve 936 can be made into a straight line or a higher-order curve. which indicates the shift of CoS due to the focus, i.e., serves as a gauge for the cross-sectional asymmetry of the grid bars 902 and 904. Curve 936 can be made into a straight line or a higher-order curve.

[0118] The above method can be similarly applied to the process layer grid 164 oriented azimuthally along the y direction and the resist grid 162 oriented azimuthally along both the x and y directions to clarify their cross-sectional asymmetries. Then, by correcting the actual overlay measurement performed on the production wafer, it is possible to address the obvious CoS that may occur as a result of these asymmetries. The above method can be similarly applied to the process layer grid 164 oriented azimuthally along the y direction and the resist grid 162 oriented azimuthally along both the x and y directions to clarify their cross-sectional asymmetries. Then, by correcting the actual overlay measurement performed on the production wafer, it is possible to address the obvious CoS that may occur as a result of these asymmetries. The above method can be similarly applied to the process layer grid 164 oriented azimuthally along the y direction and the resist grid 162 oriented azimuthally along both the x and y directions to clarify their cross-sectional asymmetries. Then, by correcting the actual overlay measurement performed on the production wafer, it is possible to address the obvious CoS that may occur as a result of these asymmetries. The above method can be similarly applied to the process layer grid 164 oriented azimuthally along the y direction and the resist grid 162 oriented azimuthally along both the x and y directions to clarify their cross-sectional asymmetries. Then, by correcting the actual overlay measurement performed on the production wafer, it is possible to address the obvious CoS that may occur as a result of these asymmetries. The above method can be similarly applied to the process layer grid 164 oriented azimuthally along the y direction and the resist grid 162 oriented azimuthally along both the x and y directions to clarify their cross-sectional asymmetries. Then, by correcting the actual overlay measurement performed on the production wafer, it is possible to address the obvious CoS that may occur as a result of these asymmetries.

[0119] In an alternative embodiment, only a single camera, for example, CAM1, is used, and CoS is measured in association with the focus. In this type of measurement, CAM1 is focused while traversing four focal positions Z0, Z1, Z2, and Z3, and the images of the grid 164 are read from CAM1 by the controller 24 at each focal position and stored in the memory 25. Then, the controller 24 calculates individual CoS values based on the images stored in the memory 25. and stored in the memory 25. Then, the controller 24 calculates individual CoS values based on the images stored in the memory 25. and stored in the memory 25. Then, the controller 24 calculates individual CoS values based on the images stored in the memory 25. is calculated, and as described above for two-camera measurement, CoS is calculated in association with the focus. This one-camera measurement method cannot perform anchoring to fixed features and camera mutual registration, so it is more sensitive to the mechanical stability of the apparatus 10 than the two-camera method described above. sensitive.

[0120] The above-described various methods are executed using four focus positions (the "anchor" position Z0 and three focus positions Z1, Z2, and Z3), but instead, fewer or more focus positions may be used. Yes.

[0121] FIG. 12 is a series of plots schematically depicting the use of image signal correlation for monitoring the asymmetry of target features within an overlay proxy target, according to an embodiment of the present invention. As described above, due to the asymmetric process effects of semiconductor manufacturing processes, such as asymmetric etching, the target features of the overlay proxy target may have an asymmetric cross-section. In this embodiment, the monitoring of those asymmetries is utilized (without even evaluating them) for monitoring the semiconductor manufacturing process. asymmetric cross-section. used for monitoring the semiconductor manufacturing process (without even evaluating them).

[0122] FIG. 12 shows the cross-sectional appearance of the grating bar 904a, similar to FIG. 11B. By the controller 24, for example, the acquired image of the bar 904a is read from CAM1, and the image is converted into an image signal and stored in the memory 25. Among the image signals, the portion along the curve 608 (FIG. 8A ) is shown as the curve 1002 in FIG. 12. Based on the image signal of the curve 1002, the controller 24 generates a reflected image signal obtained by reflecting it around the z-axis, that is, the reflected image signal shown as the curve 1004. reflected image signal shown as the curve 1004. is generated.

[0123] The controller 24 calculates two correlation curves 1006 and 1008, namely, the autocorrelation (correlation with itself) curve 1006 of curve 1002 and the cross-correlation curve 1008 between curves 1002 and 1004. Curve 1006 has a maximum value of C and curve 1008 has a maximum value of C. Since curve 1002 has asymmetry, C auto max becomes cross max cross max <C auto max auto max cross m ax The comparison result of C with respect to the maximum value C of the cross-correlation curve 1008 can be used as a measure of the asymmetry of the image signal 1002, and thus as a measure of the effect of generating the asymmetric cross-sectional profile of the target feature within the overlay proxy target. The image of the target feature of the overlay proxy target, for example, the image of bar 904a, can be obtained under various operating conditions of the apparatus 10, such as while changing the wavelength and / or polarization of the illumination of the wafer 12 and / or at various focus settings. The variation of the ratio C

[0124] of one or more of those operating conditions results in a "correlation landscape", which can be further utilized to monitor the semiconductor manufacturing process during the execution of this process. For example, the ratio C cross max / C auto max of the two correlation maximum values can be cross ​​​​​​​​​​​​​​max / C auto max falls below a preset limit, for example, less than 0.8, this can be used to indicate that it is an unacceptable process variation. .

[0125] The above method can be similarly applied to the process layer grid 164 oriented along the y - direction and also to the resist grid 162 oriented along both the x - and y - directions, and thereby the cross - sectional asymmetry thereof can be elucidated.

[0126] By using the above - mentioned embodiments individually or in combination, an optimal overlay metric recipe, i.e., a recipe that provides robust overlay measurement conditions when performing the measurement of overlay errors, can be determined. In such a recipe, optimal focus settings, wavelength, polarization, illumination conditions, and objective lens pupil control are specified. Separate conditions can also be applied individually to the process layer and the resist layer.

[0127] In addition to or instead of this, the information provided by the above - mentioned measurement techniques, for example, regarding the CoS variation due to focus, can be utilized by the controller 24 to enhance the overlay measurement algorithm. The above - mentioned methods can be further generalized to perform simultaneous measurement of overlay errors between multiple pairs of layers and for additional illumination and collection channels.

[0128] As can be observed, the above - mentioned embodiments are cited as examples, and the present invention is not specifically shown and is not limited to what has been described above. Rather, the technical scope of the present invention includes both combinations and sub - combinations of the various features described above, and those Among the modifications and corrections, those that a person skilled in the art would likely conceive upon reading the above description and that are not disclosed in the prior art are also included. ​

Claims

1. An optical inspection apparatus comprising: an illumination device configured to direct at least one illumination beam to illuminate a semiconductor wafer having at least first and second patterned layers successively deposited thereon, the semiconductor wafer comprising a first target feature in the first patterned layer and a second target feature in the second patterned layer that overlies 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, the first camera and the second camera being focused on the semiconductor wafer with a fixed height difference; A controller, identifying a center of symmetry of the first target feature and the second target feature from the first image and the second image; measuring a variation in the center of symmetry by comparing the first image with the second image; A controller configured as follows: An optical inspection device comprising:

2. 2. The apparatus of claim 1, The apparatus, wherein 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.

3. 3. The apparatus of claim 2, An apparatus wherein measuring the variation in the center of symmetry is a function of varying the one or more imaging parameters.

4. 2. The apparatus of claim 1, further comprising: a table configured to support the semiconductor wafer; an objective lens disposed on an optical path from the semiconductor wafer to at least one of the first camera and the second camera; An apparatus comprising:

5. 5. The apparatus of claim 4, 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 being a focus setting; The apparatus, wherein the controller is configured to vary a distance between the semiconductor wafer on the table and the objective lens.

6. 2. The apparatus of claim 1, further comprising: An apparatus comprising: a first actuator configured to move the first camera to adjust a first focal plane of the first camera.

7. 7. The apparatus of claim 6, further comprising: An apparatus comprising: a second actuator configured to move the second camera to adjust a second focal plane of the second camera.

8. The apparatus according to claim 2, the illumination device comprises a first light source for illuminating the semiconductor wafer with a first illumination beam and a second light source for illuminating the semiconductor wafer with a second illumination beam; The apparatus wherein the wavelength of the second illumination beam is different from the wavelength of the first illumination beam.

9. 2. The apparatus of claim 1, The apparatus, wherein the first patterned layer comprises a process layer and the second patterned layer comprises a resist layer deposited above the process layer.

10. 2. The apparatus of claim 1, An apparatus, wherein a first focal plane of the first camera and a second focal plane of the second camera are separated by a fixed focal length Z.

11. 1. A metrology method performed by an optical inspection apparatus, comprising the steps of: directing at least one illumination beam to illuminate a semiconductor wafer having at least first and second patterned layers successively deposited thereon, the semiconductor wafer having a first target feature in the first patterned layer and a second target feature in the second patterned layer that overlies the first target feature; capturing a first image of the first target feature and a second image of the second target feature with 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 in the first image and to a second setting in the second image; processing the first image and the second image, the step including measuring a variation in a center of symmetry as a function of the first setting and the second setting; A method comprising:

12. 12. The method of claim 11, The method, wherein setting one or more of the imaging parameters includes setting the first camera and the second camera to different first and second focus positions, respectively.

13. 12. The method of claim 11, The method, wherein the first setting and the second setting are at different wavelengths.

14. 12. The method of claim 11, The method, wherein the first setting and the second setting are different polarization states.

15. 12. The method of claim 11, The method, wherein the first setting and the second setting are different offsets of at least one aperture of the at least one illumination beam.

16. 12. The method of claim 11, The method, wherein the first setting and the second setting are different angular orientations of the semiconductor wafer relative to the first camera and the second camera.

17. 12. The method of claim 11, The method, wherein directing at least one illumination beam comprises directing a first illumination beam at the semiconductor wafer and directing a second illumination beam at the semiconductor wafer, wherein a wavelength of the second illumination beam is different than a wavelength of the first illumination beam.

18. 12. The method of claim 11, The method, wherein the first patterned layer comprises a process layer and the second patterned layer comprises a resist layer deposited above the process layer.

Images