IDENTIFICATION OF PROCESS VARIATIONS DURING PRODUCT MANUFACTURING

Abstract

A method for identifying process variations during a manufacturing process of a product (103), comprising: that at a predetermined stage during the manufacture of a first product (103), several images of an area of ​​the first product (103) are generated using an imaging system (100) with different values ​​of at least one imaging parameter; that the images of the area of ​​the first product (103) are analyzed to generate a first contrast signature for the first product (103) representing the variations in the contrast of the images of the area of ​​the first product (103) with the at least one imaging parameter; that at a predetermined stage during the manufacture of a second product (103), several images of an area of ​​the second product corresponding to the area of ​​the first product (103) are generated using different values ​​of the at least one imaging parameter;that the images of the area of ​​the second product (103) are analyzed to generate a second contrast signature for the second product (103) representing the variations in the contrast of the images of the area of ​​the second product (103) with the at least one imaging parameter; and that the first and second contrast signatures are compared to identify whether a process variation occurred between the manufacture of the first and the second product (103).

Description

CROSS REFERENCE TO RELATED APPLICATIONSThe present patent application claims priority to U.S. Provisional Patent Application Serial No. 62 / 457,781, filed February 10, 2017, and U.S. Provisional Patent Application Serial No. 62 / 591,088, filed November 27, 2017.BACKGROUND OF THE INVENTION1. TECHNICAL FIELDThe present invention relates to the field of quality control in the manufacturing process.2. DISCUSSION OF THE PRIOR ARTManufacturing processes use quality control systems and methods to identify variations in the manufacture of one article and another to ensure the consistency of the process. Sometimes such variations are only detectable in the finished product or are not detectable and only become apparent after much scrap has been produced. Semiconductor devices, such as integrated circuits, may be fabricated in the form of a wafer containing many components. Semiconductor wafers, such as silicon wafers, can be fabricated in a series of layers, each layer consisting of a pattern that must be accurately arranged with respect to the pattern in the adjacent layers. The control of this positioning is referred to as overlay control. In some semiconductor fabrication methods and lithography methods, a metrology target is provided on the wafer to ensure pattern alignment in use. The target may take the form of a set of cells, such as a 2x2 array of rectangular or square cells. Two cells are used for overlay measurement in the X direction and two cells are used for overlay measurement in the Y direction. The target may comprise a diffraction grating. For example, each cell in a target may consist of a diffraction grating. A target may consist of a set of patterns, each pattern being printed and oriented in a different layer, so that measurements in different directions, typically X and Y, are possible.In any manufacturing process, process variations may occur that may be undesirable. By way of example, a lithographic method, an overlay measurement, such as pattern alignment measurement, may not reveal a process variation, which is, for example, a variation in the thickness of an applied layer of material. In some cases, a process variation may result in mismeasuredness in the overlay measurement. Therefore, there is a need for better understanding of process variations and their effects.JP 2006-258 445 A relates to a defect inspection method in which an electron beam is applied to a first region and a second region of a substrate surface on which a circuit is formed. Differences in the contrast signals from the first and second regions are used for detecting a defect.JP 2003-332 396 A relates to an inspection method for a semiconductor component, in which a contrast image of a structure and a contrast image of a reference structure are generated by means of an electron beam. Dimensions are determined in a comparison image formed therefrom.US 2020 / 0 191 657 A1 describes an apparatus for examining a sample with polarized light. Spectral resolution is achieved via spectral filters at the light source.US 2007 / 0 222 464 A1 describes a method in which a region of a sample containing defects is scanned with an electron beam and an inspection image of the region is thereby obtained in each case under two different detection configurations. Each inspection image is compared with a respective reference image to obtain a respective binary voltage contrast image. The binary voltage contrast images are used for further defect analysis.SUMMARY OF THE INVENTIONThe following is a simplified summary that provides an initial understanding of the invention. The summary does not necessarily identify key elements, nor does they limit the scope of the invention. It serves merely as an introduction to the following description.Some embodiments of the present invention provide methods and systems for identifying process variations in the manufacture of products such as semiconductor wafers. According to some embodiments of the invention, a "contrast signature" for a product, such as a wafer or a portion of a wafer, may be generated, for example, at a predetermined stage during fabrication. The contrast signature may be generated when a manufacturing process is operated as desired to be used as a reference, for example. A similar contrast signature can then be generated for another product, such as at the same stage of the fabrication process. The contrast signatures may be compared to determine whether a process variation has occurred.A "contrast signature" is defined herein as a representation of the variation in contrast in an image with respect to at least one imaging parameter. Any known measure of image contrast may be used. The representation may be graphical, and may be displayed to a user, for example, or may be mathematically represented, for example, as a vector or in any other manner in which it may be compared to another contrast signature, for example, by a processor in a computer system. Thus, according to some embodiments of the invention, a process variation may be automatically identified. If desired, an alarm may be generated in response to identifying a variation, or some countermeasures may take place.Embodiments of the invention are not limited to the production of semiconductor wafers. The invention can be used in the manufacture of many other products.These additional and / or other aspects and / or advantages of the present invention are set forth in the following detailed description. Where appropriate, the aspects and / or advantages of the present invention may be derived from the detailed description and / or may be learned by practice of the present invention.BRIEF DESCRIPTION OF THE DRAWINGSFor a better understanding of the embodiment of the invention and to show how the same may be carried out, reference will now be made, by way of example only, to the accompanying drawings, in which like reference numerals designate corresponding elements or portions.In the accompanying drawings:FIGS. 1A and 1B are schematic diagrams of a system, in accordance with some embodiments of the present invention;FIG. 2A is an image of a metrology target, FIG. 2B is an enlarged image of a portion of the target, and FIG. 2C is a graphical representation of the change in intensity along a line intersecting the image of FIG. 2B, in accordance with some embodiments of the invention;FIG. 3A is a schematic example of a contrast map, FIG. 3B is the map of FIG. 3A with lines of contrast reversal added, and FIG. 3C shows only the curves of contrast reversal according to some embodiments of the invention;FIGS. 4A, 4B, and 4C schematically show a shift of the lines of contrast reversal according to some embodiments of the invention;FIG. 5 is a schematic graph of contrast variation with wavelength, FIG. 5B is a graph similar to FIG. 5A showing certain points, and FIG. 5C shows only the selected points, in accordance with some embodiments of the invention;FIG. 6 is a flow diagram of a method in accordance with some embodiments of the invention;FIGS. 7A-7C are schematic diagrams illustrating the possibility of using the numerical aperture as a variable imaging parameter in accordance with some embodiments of the invention.DETAILED DESCRIPTION OF THE INVENTIONBefore describing in detail, it may be helpful to set forth definitions of certain terms that are used below.In the following description, various aspects of the present invention will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present invention. However, it will also be apparent to those skilled in the art that the present invention may be practiced without the specific details set forth herein. Furthermore, well-known features may be omitted or simplified in order not to obscure the present invention. With particular reference to the drawings, it is to be understood that the details shown are exemplary and are used for the purpose of illustrative discussion of the present invention only. The drawings are provided in the matter considered to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention as necessary for a basic understanding of the invention. The description, taken together with the drawings, is intended to make it apparent to one skilled in the art how the various forms of the invention are practiced.Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited to the details of construction in its application and the arrangement of the components listed in the following description, or as illustrated in the drawings. The invention may be applied to other embodiments that may be practiced or carried out in various ways or in combinations according to the disclosed embodiments. Moreover, it is to be understood that the terminology and terminology used herein is for the purpose of description and is not to be considered as limiting.Unless otherwise stated, the features of the invention described with respect to one or more embodiments can optionally be included in all other embodiments of the invention.Unless otherwise indicated and as will be apparent from the discussions that follow, it will be appreciated that, during discussions regarding the specification, the terms such as "processing," "computing," "computing," "calculating," "determining," "improving," or the like, refer to the action and / or processes of a computer or computer system or similar electronic computing device that manipulates and / or transforms data transformed as physical quantities, such as electronic quantities, within the registers and / or memories of the computer system into other similarly represented data represented as similarly represented physical quantities within the computer system in their memories, registers, or other such information storage, transmission, or display devices.FIGS. 1A and 1B are schematic diagrams of a system according to some embodiments of the invention used in the manufacture of semiconductor wafers. Systems and methods as described herein may be used in the manufacture of other products, for example, all products that are mapped may generate a contrast signature, as further described herein. The effectiveness of the contrast signature in identifying process variations may depend on the type of product. The system of FIGS. 1A and 1B may be used to obtain images of a region of a wafer surface using the different values of one or more imaging parameters. Thus, the system may be configured such that one or more mapping parameters are variable. The region may be a metrology target, e.g. for overlay measurement, such as a diffraction pattern or diffraction grating.FIG. 1A shows an imaging system 100 configured to direct radiation toward the surface of a wafer and to receive radiation reflected from the wafer to generate images of the wafer. FIG. 1B shows an illumination system 200 that may be used to provide radiation to the imaging unit of FIG. 1A.In the imaging system 100 of FIG. 1A, radiation, such as visible light, is directed toward the surface of a product, in this illustration a wafer 103, located on a support 104. The radiation may be directed to a metrology target. The radiation reflected from the surface of the wafer 103 is received and used to generate images of the wafer, for example in a camera 105. The images can be analyzed in an analysis unit 107. The operation of the imaging system may be controlled by a controller 110.The radiation may be provided to the imaging system 100 via an optical fiber 109. The radiation passes through polarizer 120 and lenses 121-124 to strike beam splitter 130. An annular aperture 127 may be positioned between lenses 122 and 123 and a field aperture 128 may be positioned between lenses 123 and 124, the functions of which are well known to those skilled in the art. A beam splitter 130 can split the radiation in such a way that a part of the radiation is directed via an objective lens system 135 onto the wafer 103 and another part of the radiation is directed via an objective lens system 136 onto a mirror 137. The same pair of objective lens systems 135 and 136 may collect the radiation scattered by the wafer 103 and the radiation reflected by the mirror 137. The same beam splitter 130 may collect the radiation from the wafer and mirror to form a single radiation field, the details of which are sensitive to the distance between the objective 135 and the wafer 103 (focus) in a manner such that the blur may be derived.A portion of the combined radiation may be directed to a focus detector 140, the function of which is further described below. For example, a portion of the combined radiation may be directed to the focus detector 140 through a beam splitter 141 and the lens 143. A portion of the reflected radiation may be directed to a spectrometer 150. For example, a portion of the reflected radiation may be directed to spectrometer 150 through beam splitter 151 and lens 153. The focus detector 140 may perform focus measurements and output signals indicative of measurements of the analysis unit 107. Similarly, spectrometer 150 makes spectral measurements and outputs signals indicative of measurements of analysis unit 107.The camera 105 may be a charge coupled device or a "CCD" array. The camera 105 may be arranged or adjusted to form an image of the reflected radiation at the "image" plane, as is known in the art. For example, the wafer 103 may include a diffraction pattern on its surface. In this case, the reflected radiation is radiation diffracted from the surface of the wafer 103.Various ones of the components forming part of the imaging system 100 of FIG. 1A or the wafer carrier 104 may be moved relative to each other, for example, by one or more motors known to those skilled in the art, but not shown. The operation of the imaging system 100, for example, the operation of motors forming part of the imaging system 100, may be controlled by a controller 110. The operation of the controller 110 may be based in part on signals from the analysis unit 107. The analysis unit 107, or the controller 110, or both, may comprise a computer system having one or more processors. For example, a processor in the analysis unit according to some embodiments of the invention may generate and compare the contrast signatures.According to some embodiments of the invention, images of a wafer may be obtained using different values of one or more imaging parameters. Thus, the controller 110 may control the imaging system 100 to vary one or more imaging parameters between successive image acquisition operations, such as forming a series of images, which are analyzed in the analysis unit 107. For example, controller 110 may control polarizer 120 to change the polarization of radiation from one image and another. The controller may control the position of any lens or carrier 104 for the wafer, such as the lenses in the objective lens system 135 relative to each other or with respect to the carrier 104 for the wafer, to change focus and thereby obtain a series of images each having a different degree of focus. The controller may control the operation of the ring stop 127 to vary the numerical aperture between one image and another. The annular diaphragm may be configured as a double ring, which will be described in detail with reference to FIGS. 7A to 7C. Other imaging parameters that may be varied according to some embodiments of the invention include, but are not limited to, the central wavelength of the radiation used to illuminate the wafer and the bandwidth of the radiation. According to some embodiments of the invention, it is also possible that combinations of parameters are varied to generate a contrast signature representing the variation of contrast in a parameter combination,Referring to FIG. 1B, the illumination system 200 according to some embodiments of the invention includes a radiation source 203. This may be any suitable radiation source known to those skilled in the art. According to some embodiments of the invention, the radiation source 203 may comprise a plurality of radiation sources of different wavelengths and / or bandwidths, one or more of which may be selected, for example, to vary the wavelength and / or bandwidth for illuminating the product or target.As shown in Figure 1B, radiation from source 203 passes through a series of lenses 210-214 and filters 220-226 to optical fiber 109. In operation, the illumination system 200 may vary one or more imaging parameters, such as parameters that, when varied, cause variation in an image captured by an image capture device. For this purpose, the radiation source 203 can be used, for example, as an illumination source. An example of an imaging parameter that can be varied by the control of the illumination system is the wavelength of the radiation. A control system for wafer fabrication may include a variable wavelength illumination system. In this case, some embodiments of the invention may be implemented in a novel manner by operating an existing system such as existing hardware. In accordance with some embodiments of the invention, new hardware or software, including components that vary the wavelength of radiation, may be used to illuminate a wafer to be imaged.In the system illustrated in FIG. 1B, filters 222-226 in an existing wafer manufacturing control system may include, for example, a bandpass and gradient filter, the functions of which are known to those skilled in the art. Filters 220 and 221 may be used to vary the wavelength of radiation supplied to imaging system 100, in accordance with some embodiments of the invention, unless this can be accomplished by filters 222-226.In practice, an illumination system may use radiation polarized in different ways. In this case, the differently polarized radiation can be guided along different channels from the source 203 to the optical fiber 109. For simplicity, only one channel is illustrated in FIG. 1B, but it should be appreciated that a number of channels in a practical system may correspond to the number of polarizations provided.According to some embodiments of the invention, the wavelength of radiation used to illuminate a product, such as a wafer 103, may be varied in any known manner of wavelength variation. A system according to some embodiments of the invention may include a mechanical device to filter a narrow band of wavelengths of light from a broadband source. For example, a broadband radiation source can be split mechanically by means of a prism. Of the split light, a certain wavelength band, or a color, can be mechanically selected, such as by means of a shutter mechanism. In the example shown in FIG. 1B, filters 211 and 212 may be high pass filters or low pass filters, respectively.It will be apparent to one skilled in the art of optics that the order of some of the components illustrated in Figures 1A and 1B, such as lenses and beam splitters, may be varied while still allowing the system to operate as described herein. In particular, as shown, some of the components of the imaging system 100 may form part of the illumination system 200 and some of the components of the illumination system 200 may form part 100 of the imaging system 100.According to some embodiments of the invention, for a particular wafer or region of the wafer, at a particular stage in the fabrication process, variations in image contrast may be analyzed with an imaging parameter to generate a contrast signature. A contrast signature may be generated in the same manner for a corresponding region of another wafer at the same stage in the fabrication process. The contrast signatures may be compared to identify variation in the manufacturing process.FIGS. 2A to 2C are explanatory diagrams for using contrast according to some embodiments of the invention.FIG. 2A is an image of a metrology target known in the art provided on a wafer for overlay measurement purposes. FIG. 2B is an enlarged view of a component 250 of FIG. 2A, which may be associated with a particular layer, for example. FIG. 2B shows that the component has a visual oscillation between about two values ("black" and "white"). FIG. 2C is a formed, one-dimensional representation of the target component 250, referred to as a "core", formed by the addition of the rows of the image of FIG. 2B (or columns depending on the orientation of the component, or generally region of interest). A core as shown in FIG. 2C has a structure of peaks and valleys corresponding to the maximum intensity I_max and the minimum intensity I_min.The contrast of a repeating pattern is a measure of the visibility of the pattern. A standard method for quantifying or measuring contrast is by means of the "Michelson contrast" definition: ) where I_max and I_min represent the highest and the lowest luminance, respectively. Quantification of contrast according to this equation provides an absolute measure of contrast.For the purposes of some embodiments of this invention, it is useful to assign a sign or direction, positive or negative, to the contrast. This is referred to herein as "labeled contrast" and is useful in identifying "reversal of contrast" as further described herein. For example, if the arrangement of peaks and valleys is such that the first is the I_max, the + sign is associated with the Michelson contrast. If the first peak or valley is I_min, then the sign is assigned to the Michelson contrast. Thus, in each of the embodiments of the invention described herein, the sign of the contrast signature may indicate the direction of contrast. In other words, the contrast signature may indicate variations in the registered contrast, rather than the absolute value or amount of contrast.The assignment of a + or - to the contrast measurement as described above is one of several ways to distinguish contrast measurements. Any other control may be used for differentiation. The contrast signature indicates whether the contrast is positive or negative and can therefore be used to indicate contrast reversal in which the contrast changes from positive to negative or vice versa. This is discussed further below.Embodiments of the invention using the above equations are not limited solely to quantifying contrast. There are several ways to quantify contrast, either of which can be used.In Figure 2B, a region of interest having a positive contrast corresponding to the above definition of positive or negative contrast is shown. If the black and white bars were reversed, this would be determined as negative contrast. The positive and negative values may be assigned to the opposite direction, respectively, provided that the inverse is labeled opposite to each original core.The contrast of an image depends on the imaging parameters, which are determined including, but not limited to, with reference to the illumination details discussed in FIGS. 1A and 1B, such as numerical aperture, polarization, spatial content (e.g., arrangement of bars), color (which may be loosely defined as the spectral distribution - which may be characterized by wavelength and bandwidth). It has been discovered that some variations in a semiconductor manufacturing process result in a difference in the relationship between contrast and an imaging parameter. Therefore, according to some embodiments of the invention, this relationship may be determined for one wafer and the determination may be repeated for another wafer to determine whether a change in process has occurred. This relationship may be determined at least by generating a contrast signature for a wafer or a portion of the wafer that represents the variation in contrast in an image relative to an imaging parameter.The contrast signature may represent variation of contrast in an image with respect to at least one imaging parameter. The representation may be graphical, such as a map or diagram, and may be displayed to a user in accordance with some embodiments of the invention. Additionally or alternatively, the signature may be a mathematical representation, such as a vector or equation, or another mathematical expression. A contrast signature may be in any form in which it may be compared to another contrast signature, for example, but not limited to, visually by a user or visually by a processor in a computer system.A contrast signature may be characteristic of a wafer at a time or time of capturing the images.The contrast signature may comprise one or more features, and according to some embodiments of the invention, identifying a process variation may comprise detecting one or more features in the contrast of the signature, such as a shift in comparing one signature to another. For example, if the contrast signature comprises a graph, the feature may comprise a peak or valley or zero crossing point. In the particular case where the imaging parameter is the focus, a feature in the signature may result from the contrast reversal. Other features of a contrast signature that may be used to compare one contrast signature to another, but are not limited to, the position of the maximum of the gradient of the contrast / contrast signature, position of the maximum contrast, or a particular value of the contrast (i.e., contours of the contrast), and any other characteristic of a contrast map with respect to features of the "mask.".The contrast reversal is a known effect in which, as a result of defocusing the image, a periodic function undergoes inversion: for example, in the example of FIG. 2B, the black stripes become white and, at the same time, the white stripes become black. The change in contrast as defocusing progresses is continuous and therefore there is a point where both black and white become gray - and the contrast becomes zero. This is the reversal point of contrast. For example, referring to FIG. 1A, defocusing may be achieved by varying the distance between the objective lens system 135 and the wafer 103. Starting from a suitable distance, an image is generated as shown in Fig. 2B and the distance can be increased or decreased continuously so that in each direction a contrast reversal can occur in the image at a certain distance.Figures 3A-3C illustrate embodiments of the invention in which a contrast signature is generated from images in which two imaging parameters are varied. In the particular embodiment shown in the figures, the two imaging parameters are the varied focus measured, for example, with respect to a distance between the optical components and the wavelength. Acquisition, e.g., acquisition of images using different values of more than one imaging parameter, may be accomplished in various ways. For example, the distance may be set as a component and images may be captured at different wavelengths. Then, the distance may be incremented as a component, and images at different wavelengths may be repeatedly captured. Referring to FIG. 1, this variation of the imaging parameter may be achieved by the controller 110 via control of the operation of the illumination system 200 and the imaging system 100. Throughout the process, the wafer is stationary with respect to the lateral directions, e.g., it is not moved laterally to ensure that each image is an image of the same part of the wafer. This process may be repeated on a same wafer that has undergone the same process at the same stage in the process at a later time. This may be performed, for example, after wafers have been manufactured by the same method to determine whether a process variation has occurred.After images have been taken at different values of two imaging parameters, a contrast signature can be generated in the form of a three-dimensional map, the imaging parameters and contrast representing the three dimensions. For example, the x and y axes of the map are the imaging parameters and contrast may be represented in another dimension, for example, in the z axis or by color representation or in any other manner. FIG. 3A shows a schematic example of such a map, wherein the imaging parameters are the illumination wavelength and the focus, which are determined by a distance, for example, and the contrast is represented along a third axis in the illustrated example in the form of gray levels. The result is a contour map. In other embodiments of the invention, different imaging parameters may be varied and therefore these different imaging parameters may be indicated on the axes.It is noted that, according to some embodiments of the invention, it is possible to compare a contour map of the type shown in FIG. 3A, or any other three-dimensional representation, with other images of the corresponding region of another wafer at the same stage of fabrication to identify a process variation between the fabrication of the two wafers. In other words, a map or three-dimensional representation of the type shown in FIG. 3A may serve as a contrast signature. Any known techniques, including correlation techniques, may be used for comparing the images to identify a process variation.According to some embodiments of the invention, the contrast signature may be simplified for the purpose of comparison. In the map of Fig. 3B, contrast reversal curves were added. These correspond to the points at which the contrast reverses, as described elsewhere herein. In the schematic example of FIG. 3B, the contrast reversal curves are straight lines. FIG. 3C is a graph showing only the lines of contrast reversal. Other contrast data were removed.FIGS. 4A to 4C schematically show how a process variation can be identified from a contrast signature. Minute process variations, as may occur in a semiconductor wafer fabrication process, result in a change in a contrast signature for the surface of the wafer or a portion of the surface of the wafer. The same may also occur in other manufacturing processes. The type of signature change may be related to the type of variation. For example, if the contrast signature is a curve of the type shown in FIG. 3C, some process changes with respect to one or more imaging parameters will result in a shift of a feature of the contrast signature.FIG. 4A is a graph corresponding to the graph of FIG. 3C. A process variation of a certain type may cause a shift in contrast reversal curves with respect to an imaging parameter, such as the wavelength, to a new position, as shown in FIG. 4B. A signed contrast signature generated for a wafer produced by a process subject to variation may look as shown in FIG. 4C. The contrast signatures of FIGS. 4A and 4C can be easily compared, for example automatically, in a computer system to identify the process variation, in order to subsequently take the necessary countermeasures.It should be noted that the embodiments of the invention are not limited to identifying process variations that may occur over time, for example due to wear of machine parts. Embodiments of the invention may be used to identify variations in or differences between processes that are occurring at the same time, such as processes that operate in parallel.A contrast signature can be generated with only one imaging parameter based on the variation in contrast. An example is shown in Figures 5A-5C. FIG. 5A is a graph showing the variation in contrast versus illumination wavelength that a contrast signature can form. In this example, the signed contrast is determined in a manner to indicate whether the contrast is positive or negative with respect to a predetermined gray level. FIG. 5A approximates a graph that could be derived from the map of FIG. 4A. It will be seen from a comparison of Figs. 4A and 5A that the zero-crossing points of Fig. 5A correspond to the contrast reversal lines of Fig. 4A.As shown in the maps of FIGS. 4A-4C, as shown in FIG. 5A, a contrast signature may be reduced to one or more features. FIG. 5B shows the zero crossing points on the graph of FIG. 5A. FIG. 5C shows only the zero crossing points that may be sufficient to form a contrast signature. The zero crossing points may be shifted between one wafer and another wafer in the same manner as the contrast reversal curves and thus may be used to identify a process variation.From FIG. 5B, it can be seen that the exact position at which the contrast reversal occurs is particularly sensitive to a change in the imaging parameter. A small change in wavelength will result in a large contrast change. The same applies to each region on a graph or map in which the change rate of contrast is relatively large. Thus, the selection of points or regions of high rate of change, such as contrast reversal points for comparing one signature with another signature, provides a highly sensitive measurement of the difference in contrast signature that can be used to identify small changes in the process.The importance of the illumination parameters that are most sensitive to changes is in contrast to that in overlay measurement techniques where high contrast images are desired and imaging parameters can be chosen to have the least effect on contrast. For example, referring again to Figure 5B, the selection of the wavelength for acquisition of the images for overlay measurement is likely to be made at each of the peaks in the curve where the rate of change of contrast with the wavelength is the lowest.Identifying process variations using contrast signatures according to some embodiment of the invention may be used to identify process variations that are not detectable during overlay measurement. These include, but are not limited to, variations in layer thickness and variations in the optical properties of materials such as refractive index.FIG. 6 is a flow diagram of a method in accordance with some embodiments of the invention. The method of FIG. 6 begins at operation 600, at a predetermined stage in the manufacture of a first product, where images are obtained from a region of the product using different values of one or more imaging parameters, such as the focus or wavelength, or both. The terms "first," "second," etc. are used herein simply to distinguish one product from another and do not necessarily imply a temporal or different order. The images may be analyzed at operation 605 to generate a first contrast signature for the first product, representing variation in contrast of one or more imaging parameters.At the same predetermined stage in the production of a second product, which may run later or earlier or in parallel with the production of the first product, a further operation of capturing images is performed at operation 610, using different values of the same one or more mapping parameters. The images obtained at operation 610 are analyzed at operation 615 to generate a second contrast signature for the second product that indicates variations in contrast with respect to one or more imaging parameters.According to some embodiments of the invention, the products are produced in batches. The first and second products are from different batches.The contrast signatures are compared at operation 620 to identify whether a variation has occurred in the process between manufacturing the first and second products. According to some embodiments of the invention, a warning may be generated when a process variation of more than a predetermined amount is identified. For example, a difference in contrast signature, such as a difference in position of one or more features in the contrast signatures, may be compared to a predetermined threshold. A warning is generated when the difference exceeds the threshold value.Embodiments of the invention may also be used to monitor the progress of a process variation and, for example, to represent its progress as a function of time. This may then be correlated with the control parameters of the process using graphs such as, but not limited to, the temperature of the wafer during the etch. This can be used to analyze the cause of variation.Capturing of images with the camera 105 may be performed by the operation of the controller 110. The analysis may be performed in the analysis unit 107, and may possibly be controlled by the controller 107. It may be possible to modify existing systems to implement methods according to some embodiments of the invention. Thus, some embodiments of the invention may include a computer readable medium comprising either transitory or non-transitory instructions that, when implemented in a controller in a product manufacturing system, cause the system to operate according to the methods described herein.Operations 600- 620 may be performed during the manufacture of different pairs of first and second wafers to regularly monitor possible variations in the manufacturing process.FIGS. 7A-7C are schematic diagrams illustrating use of the numerical aperture as variable imaging parameters in accordance with some embodiments of the invention. For example, the imaging parameter mentioned in connection with FIG. 6 may be the numerical aperture. FIG. 7A shows a collimated beam of radiation 700 focused by a lens 710 onto the surface 712 of a layer 714 of a wafer, such as wafer 103. From this figure it is clear that the length of the path of radiation through the layer 714 depends on the angle of incidence. Radiation entering the layer perpendicular to the surface has the shortest path through the layer and the path length increases outwardly from the center of the beam 700.The numerical aperture in an optical system may define the portion of a beam of radiation that passes through each part of the system. Thus, the numerical aperture can be varied to exclude a portion of the radiation beam. The numerical aperture variation has the effect that the range of angles of incidence of the radiation on the surface of the product can be varied. According to some embodiments of this invention, the imaging parameter varied at operation 605 may comprise an angle of incidence or a range of angles of incidence on the surface of the product, which may be restricted, for example, by means of a numerical aperture variation in the range of angles of incidence. This variation can be used to generate different contrast signatures in a similar manner as described herein by varying the wavelength.The limitation of the range of angles of incidence can result in an annular beam for the angle range other than the perpendicular at which incident radiation occurs. FIGS. 7B and 7C show the same arrangement as FIG. 7A, except that beam 700 is reduced to annular beams 720 and 730 having different radii R1and R2, respectively, and widths A1and A2, respectively, which may also be the same. The maximum path length P1of the beam 720 through the layer 714 is longer than the maximum path length P2of the beam 730 through the layer 714. It will be apparent to one skilled in the art that changes in path length through a very thin target, such as in semiconductor fabrication, have a similar effect to varying the wavelength of the incident radiation. Therefore, the variation in angle of incidence can be used to generate a similar contrast signature to that shown in Figures 3A-3C.As noted elsewhere herein, imaging parameters other than wavelength and aperture or angle of incidence may be varied according to embodiments of the invention to generate contrast signatures.Systems and methods according to some embodiments of the invention may be used to identify process variations that may then be examined and corrected, if necessary. For example, a change may be quantified, such as the amount of shift in contrast reversal with respect to an imaging parameter, and it may be determined that an examination is required if there is a variation above a threshold. The type of variation may not be immediately apparent from a difference in the contrast signature, and it may be necessary to perform additional measurements on one or both of the products from which the contrast signature was generated to determine what the variation was.However, once the type of variation, such as increasing or decreasing the layer thickness, is determined, it can be used to build a knowledge base that correlates types of different contrast signatures to types of process variation and, optionally, also relate the measure of the difference of the contrast signature to a certain measure of variation in the process, such as layer thickness. Other of the identified process variations may include, but are not limited to, the composition of a layer and the optical parameters of a layer. According to some embodiments of the invention, a process variation may be identified, for example, from the extent of a change in contrast signature that is greater than a predetermined threshold. Optionally, a warning may be generated in response to identifying a process variation to cause an operator, for example a human, to take a remedial action.Identifying a process variation may be performed by performing one or more measurements on a product to determine the type of variation. This determination may be used to automatically identify the type of variation identified in a future process. Thus, in accordance with some embodiments of the invention, the nature of a process variation may be automatically determined from a difference in contrast signature. Additionally or alternatively, the amount of variation may be determined from a difference in contrast signature. One difference discussed herein is a shift in contrast reversal. Embodiments of the invention are not limited to this difference and other differences in contrast signature and may identify and be attributed to other variations in a manufacturing process. Historical data from previous methods may be used to assign differences in contrast signature to the different types and optionally the measure of variations to process variations of the different types and optionally measures. In accordance with some embodiments of the invention, machine learning may be used to improve the reliability of the assignments of differences to variations.The aspects of the present invention have been described above with reference to flowchart illustrations and / or section diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It is understood that each part of the flowchart illustrations and / or section diagrams, and combinations of parts in the flowchart illustrations and / or section diagrams, may be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, produce means that may be used to implement the functions / acts specified in the flowchart and / or in part diagrams or parts thereof.These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner such that the instructions stored in the computer readable medium produce an article of manufacture, including instructions that use the specified functions / acts in the flowchart and / or a sub-diagram or a portion thereof.The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices that perform a series of operational steps on the computer, other programmable apparatus, or other devices to produce a computer-implemented process such that the instructions present on the computer or other programmable apparatus are executed to implement the functions / acts in the flowchart and / or in a sub-diagram or part thereof.The aforementioned flowcharts and diagrams illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each portion in the flowchart or sub-diagram represents a module, segment, or portion of code that includes one or more executable instructions for implementing the specified logical function(s). It should also be noted that in some alternative implementations, the functions indicated in the section may occur out of the order indicated in the figures. For example, two portions shown in succession may in fact be executed substantially simultaneously, or the portions may sometimes be executed in the reverse order, depending on the functionality involved. It should also be noted that each portion of a sub-diagram and / or flowchart illustration, and combinations of portions of a sub-diagram and / or flowchart illustration, may be implemented using a special purpose hardware-based system to perform the specified functions or acts, or combinations of special purpose hardware and computers.In the above description, an embodiment is an example or implementation of the invention. The various appearances of "one embodiment," "one embodiment," "certain embodiments," or "some embodiments" are not necessarily all referring to the same embodiments. Although various features may be described in the context of the invention with a single embodiment, the features may be provided separately or in any suitable combination. Conversely, although the invention may also be described herein in connection with separate embodiments for clarity, the invention may also be implemented in a single embodiment. Certain embodiments of the invention may include features of various embodiments as disclosed above, and certain embodiments may include elements from other disclosed embodiments as disclosed above. The disclosure of the elements of the invention in the context of a particular embodiment is not to be considered as limiting their use in the specific embodiment. Moreover, it is to be understood that the invention may be practiced or carried out in various ways and that the invention may be implemented in certain embodiments other than those outlined in the specification above.The invention is not limited to these diagrams or to the corresponding descriptions. For example, the flow need not pass through each illustrated box or state, or in exactly the same order as illustrated and described. The meaning of the technical and scientific terms used herein will generally be understood by one of ordinary skill in the art to which the invention pertains, unless otherwise defined. While the invention has been described with reference to a limited number of embodiments, it should not be construed as limiting the scope of the invention, but rather serves to illustrate some of the preferred embodiments. Other possible variations, modifications and applications are also included within the scope of the invention. Accordingly, the scope of the invention should not be limited by the scope of what has been described so far, but only by the claims below and their equivalents.

Claims

A method for identifying process variations during a manufacturing process of a product (103), comprising: at a predetermined stage during the manufacturing of a first product (103) by means of an imaging system (100), generating a plurality of images of a region of the first product (103) using different values of at least one imaging parameter; analyzing the images of the region of the first product (103) to generate a first contrast signature for the first product (103) representing the variations of the contrast of the images of the region of the first product (103) with the at least one imaging parameter; at a predetermined stage during the manufacturing of a second product (103), generating a plurality of images of a region of the second product corresponding to the region of the first product (103) using different values of the at least one imaging parameter; analyzing the images of the area of the second product (103) to generate a second contrast signature for the second product (103) representing the variations in contrast of the images of the area of the second product (103) with the at least one imaging parameter; and comparing the first and second contrast signatures to identify whether a process variation has occurred between the manufacture of the first and second products (103).The method of claim 1, wherein the at least one imaging parameter comprises one or more of: the numerical aperture of the imaging system (100); the wavelength of the illumination radiation; the focus of the imaging system (100); the polarization of the illumination radiation.The method of claim 2, wherein the identifying further comprises detecting a shift in one or more of the features in the first and second contrast signatures with respect to the one or more imaging parameters.The method of claim 1, wherein the at least one mapping parameter comprises two mapping parameters.The method of claim 4, wherein the two imaging parameters comprise focus and wavelength.The method of claim 4, wherein the identifying comprises detecting a shift in one or more of the features in the first and second contrast signatures with respect to the one or more imaging parameters.The method of claim 1, wherein the identifying further comprises detecting a shift in one or more of the features in the first and second contrast signatures with respect to the one or more imaging parameters.The method of claim 1, wherein the first and second signatures each comprise a contrast map.The method of claim 1, wherein the first and second contrast signatures comprise one or more features comprising the values of the at least one mapping parameter at which the rate of change of contrast is highest.The method of claim 1, wherein the analyzing comprises assigning a sign to contrast measurements, whereby the first and second contrast signatures identify a contrast reversal.The method of claim 1, wherein the first and second contrast signatures comprise one or more features having the values of the at least one mapping parameter at which the contrast changes from positive to negative, or vice versa.The method of claim 1, wherein the area comprises a target for overlay measurement.The method of claim 12, wherein the target comprises a diffraction grating.The method of claim 1, wherein the first and second products (103) are produced in batches and the first and second products (103) are from different batches.The method of claim 1, wherein the method comprises repeating the generating, analyzing and comparing in the production of further first and second products (103) to regularly monitor possible variations of the manufacturing process.The method of claim 1, wherein the first and second products are semiconductor wafers (103).A system for identifying process variations during a manufacturing process of a product (103), the system comprising: an illumination system (200) and imaging system (100), wherein the illumination system (200) comprises a radiation source (203), and the imaging system (100) is arranged to direct radiation from the illumination system (200) towards the surface of a product (103) and to receive radiation reflected from the product (103) from the illumination source (203) to generate images of the product (103); a control unit (110) controlling the imaging system (100) or illumination system (200) or both to obtain images of the first and second products (103) at a predetermined stage of manufacturing, wherein different values of the at least one imaging parameter are used; an image analysis unit (107) configured to analyze the images to generate respective contrast signatures for the first and second products (103) representing variations in contrast at the at least one imaging parameter, and to compare the first and second contrast signatures to identify whether a process variation has occurred between manufacturing the first and second products (103).

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