Precision image processing inspection method for solder mask fit accuracy

Abstract

Method for operating an image processing inspection system for determining a fluorescence image height for capturing a fluorescence image for repeatedly determining the location of a workpiece feature edge located within a layer of fluorescent material on a workpiece, the method comprising: (a) positioning an exposed portion of a surface of the workpiece such that its height can be determined by the image processing inspection system, wherein the exposed portion is not covered by the layer of fluorescent material and has a characteristic surface height along a focal axis with respect to a height within the layer of fluorescent material; (b) setting up the image processing inspection system to determine the height of the exposed portion; (c) determining the height of the exposed portion;(d) Determining a fluorescence image height to be used for fluorescence imaging of the workpiece feature edge located within the layer of fluorescent material, wherein the fluorescence image height is determined in relation to the determined height of the exposed part; and performing at least one of (e) and (f), wherein (e) comprises: (c) storing the determined fluorescence image height in conjunction with a subprogram for later use when acquiring a fluorescence image to be used for inspecting the workpiece feature edge located within the layer of fluorescent material; and (f) comprises: (f) using the fluorescence image height determined in relation to the determined height of the exposed part during the execution of a subprogram when acquiring a fluorescence image to be used for inspecting the workpiece feature edge located within the layer of fluorescent material.

Description

Field of invention The invention relates generally to image processing inspection systems and more specifically to methods for inspecting workpiece features that lie within fluorescent material. background Precision image processing inspection systems (or simply "image processing systems") can be used to obtain precise dimensional measurements of inspected objects and to inspect various other properties of the object. Such systems may include a computer, a camera and optical system, and a precision stage that is movable in multiple directions to allow the camera to scan the features of a workpiece under inspection. An example of a state-of-the-art system available commercially is the QUICK VISION® series of PC-based image processing systems and the QVPAK® software, available from Mitutoyo America Corporation (MAC), located in Aurora, Illinois.The features and operation of the QUICK VISION® series of vision systems and the QVPAK® software are described, for example, in the QVPAK 3D CNC Vision Measuring Machine User's Guide, published in January 2003, and the QVPAK 3D CNC Vision Measuring Machine Operation Guide, published in September 1996, each of which is hereby incorporated in its entirety by reference. This product, exemplified by the QV-302 Pro model, is capable of using a microscope-type optical system to provide images of a workpiece at various magnifications and to move the stage as necessary to traverse the workpiece surface beyond the limits of any single video frame.A single video image typically captures only a portion of the workpiece being observed or inspected, given the desired magnification, measurement resolution, and the limitations of the physical size of such systems. Image processing inspection systems generally employ automated video inspection; U.S. Patent No. US 6,542,180 B1 (the Patent '180) specifies various aspects of such automated video inspection and is incorporated herein in its entirety by reference. As specified in Patent '180, automated video inspection measuring instruments generally possess a programmability that allows the user to define an event sequence for automated inspection for each individual workpiece arrangement. This may be accomplished, for example, by text-based programming, or by a recording mode that sequentially "learns" the inspection event sequence by storing a sequence of machine control instructions corresponding to a sequence of inspection operations performed by a user through a graphical user interface, or by a combination of both methods.Such a recording mode is often referred to as a "learning mode" or "training mode". Once the inspection event sequence is defined in the "learning mode", such a sequence can then be used to automatically capture images of a workpiece during "running mode" (and additionally analyze or inspect them). The machine control instructions, which contain the specific inspection event sequence (i.e., how to capture each image and analyze / inspect each captured image), are generally stored as a "subprogram" or "workpiece program" specific to the particular workpiece setup. For example, a subprogram specifies how to capture each image, such as how to position the camera relative to the workpiece, the lighting level, the magnification, and so on. Furthermore, the subprogram specifies how to analyze / inspect a captured image, for example, by using one or more video tools, such as edge / boundary detection video tools. Video tools (or simply "tools") and other graphical user interface features can be used manually to perform manual inspection and / or machine control operations (in "manual mode"). Their settings and operation can also be recorded during learning mode to create programs for automatic inspection or "subprograms." Video tools can include, for example, edge / boundary detection tools, autofocus tools, shape or pattern matching tools, dimensional measurement tools, and the like. One application for a machine vision inspection system is the inspection of a printed circuit board (PCB), where it may be desirable to measure the fit between a pattern in a solder mask layer and conductive features that are exposed and / or insulated by the solder mask. Prior art methods for measuring solder mask fit are neither fast enough, nor accurate enough, nor robust enough to reliably meet the inspection requirements for the increasingly smaller features encountered in current or future generations of PCB technology. Some solder masks contain fluorescent material. Some known machine vision inspection systems are capable of imaging with light that does not cause fluorescent features on workpieces to fluoresce, and with light that does cause fluorescent features on workpieces to fluoresce. For example, U.S. Patent No.US 5,039,868 A (the patent '868) describes such an inspection system. However, patent '868 relates generally to pattern recognition of features on a printed circuit board and does not address focusing procedures and means for generating high-resolution and highly repeatable measurements of the locations of workpiece feature edges that are obscured by a solder mask layer and / or of associated edge distances or the like that need to be measured with an accuracy on the order of 10 µm or less. Improvements to the inspection methods relating to locating features that are exposed and / or are intended to be isolated by a fluorescent material layer, such as a solder mask layer, would be desirable. Document DE 10 2004 024 785 A1 relates to a method for measuring three-dimensional topographic structures on wafers or components, in which at least one fluorescent topographic structure is scanned with excitation light using a confocal microscope and the fluorescence light emitted from the focal point in the focal plane of the objective, excited by the excitation light, is detected and measurement data are obtained from the position of the focal point and the detected fluorescence signal. Summary This summary is intended to introduce, in a simplified form, a selection of concepts that are described in more detail below. This summary is not intended to provide any main features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. A method for operating an image processing inspection system is provided to determine a reliable and repeatable fluorescence image height, which can be used to acquire a fluorescence image for accurately and repeatably determining the location of a workpiece edge to be inspected within a fluorescent material (e.g., a layer of fluorescent material). In one application, the method can be used as part of a procedure for measuring the fit or overlap of a solder mask layer with respect to a conductive element it covers on a printed circuit board. The method may, in various embodiments, comprise steps including: (a) positioning an exposed portion of a workpiece surface such that its height can be determined by the image processing inspection system, wherein the exposed portion is not covered by the layer of fluorescent material and has a characteristic surface height along a focal axis with respect to a height within the layer of fluorescent material; (b) setting up the image processing inspection system to determine the height of the exposed portion; (c) determining the height of the exposed portion; (d) determining a fluorescence imaging height to be used for fluorescence imaging of a workpiece feature edge located within the layer of fluorescent material, wherein the fluorescence imaging height is determined relative to the determined height of the exposed portion;and performing at least one of (e) and (f), wherein (e) comprises storing the determined fluorescence image height in conjunction with a subprogram for later use when acquiring a fluorescence image used to inspect the workpiece feature edge located within the layer of fluorescent material (e.g., the workpiece feature edge represents a corresponding workpiece feature edge located within a corresponding layer of fluorescent material on a corresponding workpiece being inspected using the subprogram), and (f) comprises using the fluorescence image height determined in relation to the determined height of the exposed part during the execution of a subprogram when acquiring a fluorescence image used to inspect the workpiece feature edge located within the layer of fluorescent material. In some embodiments, the workpiece is a template workpiece, and the method is performed in conjunction with a learning operating mode of the image processing inspection system, which is used to create a subprogram to be used for inspecting workpieces similar to the template workpiece, and the method includes performing steps (a), (b), (c), (d) and (e).In some embodiments, the method performed in learning mode may further include the following steps: (g) positioning the workpiece feature edge that lies within the layer of fluorescent material in the field of view of the image processing inspection system; (h) positioning the image processing inspection system at the determined fluorescence imaging height; (i) illuminating the field of view using an excitation wavelength profile that causes the fluorescent material to fluoresce and emit fluorescence imaging light; (j) acquiring a fluorescence image of the field of view using the fluorescence imaging height and illuminating the field of view using the excitation wavelength profile; and (k) determining a location of the workpiece feature edge that lies within the fluorescent material based on a location of a corresponding intensity change in the fluorescence image.In some embodiments, steps (g), (h), (i), and (j) can be performed as part of step (d) to evaluate the results and refine a preliminary estimate of the determined fluorescence imaging height. In some embodiments, step (k) can also be performed as part of step (d) to evaluate edge detection results and potentially determine a more effective fluorescence imaging height in step (d). In other applications, steps (g), (h), (i), and (j), and in some cases (k), can simply be performed to evaluate and confirm the effectiveness of the fluorescence imaging height determined in step (d).In some embodiments of the learning mode, step (k) includes setting up the parameters of an edge detection video tool and using this video tool to determine the location of the workpiece feature edge on the template workpiece, and the method further includes a step (I) which includes saving the set-up parameters of the edge detection video tool in conjunction with the subprogram in order to use them later to determine the location of the workpiece feature edge in fluorescence images of workpieces similar to the template workpiece. In some embodiments, the method is performed in conjunction with a run-operating mode of the image processing inspection system by executing a subprogram that includes inspecting the workpiece feature edge located within the fluorescent material on a workpiece similar to a template workpiece used to create the subprogram, and the method comprises performing steps (a), (b), (c), (d), and (f). In such embodiments, step (d) may include determining the fluorescence image height to be used for fluorescence imaging of the workpiece feature edge, retrieving the fluorescence image height information stored in the subprogram in connection with this workpiece feature edge, and determining the fluorescence image height based on this information.For example, in various embodiments, the fluorescence imaging height can be determined as an offset relative to the determined height of the exposed part during the learning mode and stored in the subprogram. This offset can then be retrieved during runtime and added to the height of the exposed part determined during runtime to determine the fluorescence imaging height used during runtime. In some embodiments, the fluorescence imaging height is defined as the same as the determined height of the exposed part (e.g., the offset is missing or is zero). In various embodiments, the method performed in running mode may further include the following steps: (g) positioning the workpiece feature edge that lies within the layer of fluorescent material in the field of view of the image processing inspection system; (h) positioning the image processing inspection system at the determined fluorescence imaging height; (i) illuminating the field of view using an excitation wavelength profile that causes the fluorescent material to fluoresce and emit fluorescence imaging light; (j) capturing a fluorescence image of the field of view using the fluorescence imaging height and illuminating the field of view using the excitation wavelength profile; and (k) determining a location of the workpiece feature edge that lies within the fluorescent material based on a location of a corresponding intensity change in the fluorescence image.In some run mode implementations, step (k) includes configuring an edge detection video tool of the image processing inspection system according to associated parameters stored in the subprogram, and using this edge detection video tool to determine the location of the workpiece feature edge in the fluorescence image. In some embodiments that include performing step (k), the method may further comprise: step (m) determining the location of an edge of the layer of fluorescent material; and (n) determining a measure of a dimensional relationship between the location of the edge of the layer of fluorescent material and the location of the workpiece feature edge that is hidden beneath the layer of fluorescent material. In some such embodiments, the edge of the layer of fluorescent material may advantageously be an edge adjacent to the exposed part, and an image of this edge may be provided using the first set of the image processing inspection system created in step (b), and the edge of the layer of fluorescent material may be determined in this image.In other such embodiments, the edge of the layer of fluorescent material is advantageously included in the fluorescence image of the field of view acquired in step (j), and the edge of the layer of fluorescent material can be determined in this image. In some embodiments (e.g., when the camera of the image processing inspection system is sensitive to a wavelength of the excitation wavelength profile), the image processing inspection system may include a fluorescence imaging filter that blocks at least that wavelength of an excitation wavelength profile used as illumination when acquiring a fluorescence image and transmits at least one wavelength of the fluorescence imaging light emitted by the fluorescent material, and in step (j), acquiring the fluorescence image includes using the fluorescence imaging filter to filter the image light used to form the fluorescence image (e.g., by inserting the fluorescence imaging filter into the imaging path).By blocking the excitation light reflected from different surfaces, the features illuminated by fluorescence within the fluorescent material are more clearly visible in the resulting image. For the best accuracy and reliability in certain applications, it may be advantageous to use an embodiment of the method in which the fluorescence imaging height is determined to fall within the layer of fluorescent material, and / or in which the exposed portion of the workpiece surface has a surface height that lies within a height dimension of the layer of fluorescent material, and / or the exposed portion of the workpiece surface is chosen to be nominally at the same surface height as a surface of a material layer that has the workpiece feature edge hidden beneath the fluorescent material, although implementing these features may not be possible or even desirable in all applications. In some embodiments, the image processing inspection system includes a surface height sensor comprising one of a probe-type sensor, an optical triangulation-type sensor, and a focus signal sensor, and step (a) may include positioning the exposed part in a working area of ​​the surface height sensor, step (b) may include setting up the image processing inspection system to use the surface height sensor to determine the height of the exposed part, and step (c) may include using the surface height sensor to determine the height of the exposed part. In some embodiments (e.g., when no height sensor is used to determine the height of the exposed part), step (a) may include positioning the exposed part in a field of view of the image processing inspection system, step (b) may include setting up the image processing inspection system in a first arrangement to provide an image of at least the exposed part, and step (c) may include determining a focus height of the exposed part based on images of the exposed part acquired at different heights while the image processing inspection system is in the first arrangement, and using this focus height as the determined height of the exposed part. In some embodiments, the image processing inspection system includes controllable illumination that can output not only the excitation wavelength profile used for fluorescence imaging, but also a non-excitation wavelength profile. This non-excitation wavelength profile illuminates the workpiece such that it primarily emits reflected image light and a negligible amount of fluorescence light. In some embodiments, the non-excitation wavelength profile is used in the first arrangement to image the exposed part. However, since the exposed part is not within the fluorescent material and does not fluoresce, in other embodiments the excitation wavelength profile can be used in the first arrangement to provide usable images of the surface of the exposed part. In various embodiments disclosed herein, a fluorescence imaging height, intended for imaging features within a fluorescent material based on its fluorescence light, is determined in relation to the determined height of a non-fluorescent, exposed portion of a surface that is imaged based on reflected illumination. In other words, the "exposed" portion of the surface is not covered with a fluorescent material.Such embodiments enable improved reliability, repeatability and / or accuracy in imaging and inspecting workpiece features located within a layer of fluorescent material, as may be desirable or necessary in solder mask fit measurements, which increasingly exhibit tolerances on the order of µm, or in other applications involving image processing-based inspection within fluorescent materials. Description of the drawing The foregoing aspects and many of the associated advantages of this invention will be easier to evaluate when they become more readily understood by reference to the following detailed description in conjunction with the accompanying drawings, in which Fig. 1 is a sketch showing various typical components of a general-purpose precision image processing inspection system; Fig. 2 is a block diagram of a control system part and an imaging component part of the image processing inspection system of Fig. 1; Fig. 3 is a sketch showing further details of parts of the imaging component part of the image processing inspection system of Fig. 2; Fig. 4 shows a top view and a sectional view of features on a sample workpiece aligned to signal strength profiles along a scanning line through a non-fluorescent image of the features and a fluorescent image of the features; and Fig. 5A and Fig.5B Flowcharts show a procedure and routine for operating an image processing inspection system to determine a reliable and repeatable fluorescence imaging height. Description of the invention Fig. 1 is a block diagram of an exemplary image processing inspection system 10, which can be used according to the method described herein. The image processing inspection system 10 includes an image processing measuring machine 12, which is operationally connected to a controlling computer system 14 for the exchange of data and control signals. The controlling computer system 14 is further operationally connected to a monitor or display 16, a printer 18, a joystick 22, a keyboard 24, and a mouse 26 for the exchange of data and control signals. The monitor or display 16 can display a user interface suitable for controlling and / or programming the operations of the image processing inspection system 10. The image processing measuring machine 12 includes a movable workpiece stage 32 and an optical imaging system 34, which may contain a zoom lens or interchangeable lenses. The zoom lens or interchangeable lenses generally provide different magnifications for the images provided by the optical imaging system 34. The image processing inspection system 10 is generally comparable to the QUICK VISION® series of image processing systems and the QVPAK® software described above, as well as to similar, commercially available, state-of-the-art precision image processing inspection systems. The image processing inspection system 10 is also featured in the jointly allocated US patents No. US 7 454 053 B2, US 7 324 682 B2, US patent applications US 2010 / 0 158 343 A1, filed on December 23, 2008, and US 2011 / 0 103 679 A1, filed on December 29, 2011.October 2009, described, which are included here in their entirety by reference. The image processing inspection system 10 can be designed to image and measure workpiece features that fluoresce under suitable excitation light, as well as to image and measure combinations of workpiece surface features that do not fluoresce and workpiece surface features that fluoresce, as detailed below. Fig. 2 is a block diagram of a control system part 120 and an imaging component part 200 of a machine vision inspection system 100. As described in more detail below, the control system part 120 is used to control the imaging component part 200. The imaging component part 200 includes an optical assembly part 205, light sources 220, 230, 230' and 240, and a workpiece stage 210 with a central transparent part 212. The workpiece stage 210 is controllably movable along the X and Y axes, which lie in a plane generally parallel to the surface of the stage on which a workpiece 20 can be positioned. The optical assembly part 205 includes a camera system 260 and an interchangeable lens 250 and may include a lens turret assembly 280 with lenses 286 and 288.As an alternative to the lens turret assembly, a manually interchangeable magnification-changing lens or a zoom lens assembly, or the like, may be included. The optical assembly part 205 is controllably movable along a Z-axis, which is generally orthogonal to the X- and Y-axes, using a controllable motor 294, as described below. In some embodiments, an optional surface height sensor 298 may be included in or attached to the optical assembly part 205. In some embodiments, the surface height sensor 298 may be separate from other sensor components of the optical assembly part 205. In other embodiments, it may share certain components with other systems. For example, in some embodiments, it may emit and / or receive light through the lens 250.In any case, the surface height sensor 298 can be configured to use its schematically depicted height sensing means 298' to determine the height of the surface area of ​​the workpiece 20 along the Z-axis or focus direction. In some cases, the surface height sensor 298 operates in combination with the Z-axis motion control system to determine the height of the surface area. The optional surface height sensor is described in more detail below with reference to Fig. 3. A workpiece 20, or a carrier plate or holder holding a plurality of workpieces 20, which are to be imaged using the image processing inspection system 100, is placed on the workpiece stage 210. The workpiece stage 210 can be controlled to move relative to the optical assembly part 205, so that the interchangeable lens 250 moves between locations on a workpiece 20 and / or between a plurality of workpieces 20. One or more of a stage light 220, a first coaxial light 230, a second coaxial light 230', and a surface light 240 (e.g., a ring light) can each emit source light 222, 232, 232', and / or 242 to illuminate the workpiece or workpieces 20. The light sources 230 and 230' can emit light 232 and 232' along a path that includes a mirror 290, as described in more detail with reference to Fig. 3.The second coaxial light 230' can emit source light 232', which has a wavelength profile that causes certain workpiece materials (e.g., solder mask) to fluoresce, as described in more detail below. The source light is reflected or transmitted as workpiece light 255, or fluorescent workpiece light 255' is emitted, and the workpiece light used for imaging passes through the interchangeable objective lens 250 and the objective turret assembly 280 and is captured by the camera system 260. The image of the workpiece(s) 20 captured by the camera system 260 is output to the control system unit 120 via a signal line 262. The light sources 220, 230, 230', and 240 can be connected to the control system unit 120 via signal lines or buses 221, 231, and 241, respectively.To change the image magnification, the control system part 120 can rotate the lens turret assembly 280 along the axis 284 via a signal line or bus 281 to select a turret lens. In various exemplary embodiments, the optical assembly part 205 is movable in the vertical Z-axis direction with respect to the workpiece stage 210 by means of a controllable motor 294, which drives an actuator, a Bowden cable, or the like to move the optical assembly part 205 along the Z-axis in order to change the focus of the image of the workpiece 20 captured by the camera system 260. The term Z-axis, as used here, refers to the axis to be used to focus the image obtained by the optical assembly part 205. The controllable motor 294 is connected to the input / output interface 130 via a signal line 296. As shown in Fig. 2, the control system part 120, in various exemplary embodiments, includes a controller 125, the input / output interface 130, a memory 140, a workpiece program generator and execution part 170, and a power supply 190. Each of these components, as well as the additional components described below, can be interconnected via one or more data / control buses and / or application programming interfaces, or via direct connections between the various elements. The input / output interface 130 includes an image control interface 131, a motion control interface 132, an illumination control interface 133, a lens control interface 134, and a height sensor interface 139 in embodiments that include the surface height sensor 298. The motion control interface 132 may include a position control element 132a and a velocity / acceleration control element 132b. However, it should be understood that in various exemplary embodiments, such elements may be combined and / or indistinguishable. The illumination control interface 133 includes illumination control elements 133a-133n and 133fl, which, for example, control the selection, power, on / off switch, and, if applicable, the sampling pulse timing control for the various corresponding light sources of the image processing inspection system 100.The lighting control element 133fl can control the selection, power, on / off switch, and, if applicable, the sampling pulse timing for the second coaxial light 230', which can stimulate fluorescent workpiece materials to emit fluorescent imaging light. The height sensor interface 139 can exchange control and / or measurement signals with the surface height sensor 298 and / or other elements via a control and signal bus (not shown separately). Memory 140 contains an image file memory part 141, a workpiece program memory part 142, which may contain one or more subprograms or the like, and a video tool part 143. The video tool part 143 contains the tool part 143a and other similar tool parts (e.g., 143n) and, in some embodiments, may include a fluorescence image edge detection tool 143fl, which determines the graphical user interface, image processing operation, etc., for each of the corresponding tools. The video tool part 143 also contains an inspection area generator 143x, which supports automatic, semi-automatic, and / or manual operations that define various inspection areas which can be processed in different video tools contained in the video tool part 143. In general, the memory unit 140 stores data that can be used to operate the imaging component unit 200, to acquire or capture an image of the workpiece 20 such that the captured image of the workpiece 20 has the desired image properties. The memory unit 140 can also store data from inspection results and can further store data that can be used to operate the image processing inspection system 100 to perform various inspection and measurement operations on the captured images (e.g., partially executed as video tools), either manually or automatically, and to output the results via the input / output interface 130. The memory unit 140 can also contain data that defines a graphical user interface, which can be operated via the input / output interface 130. The signal lines or buses 221, 231, and 241 of the stage light 220, the coaxial lights 230 and 230', and the surface light 240 are all connected to the input / output interface 130. The signal line 262 from the camera system 260 and the signal line 296 from the controllable motor 294 are also connected to the input / output interface 130. Besides carrying image data, the signal line 262 can carry a signal from the controller 125 that triggers image acquisition. One or more display devices (e.g., the display 16 of Fig. 1) and one or more input devices 138 (e.g., the joystick 22, the keyboard 24, and the mouse 26 of Fig. 1) can be connected to the input / output interface 130. The display devices 136 and input devices 138 can be used to display a user interface that may contain various graphical user interface (GUI) components. These components can be used to perform inspection operations and / or create and / or modify subprograms, display images captured by the camera system 260, and / or directly control the imaging component 200. In various exemplary embodiments, when the user uses the image processing inspection system 100 to create a subprogram for the workpiece 20, subprogram instructions are generated either by explicit automatic, semi-automatic, or manual coding using a workpiece programming language and / or by generating the instructions through operation of the image processing inspection system 100 in a learning mode to provide a desired image acquisition training sequence. For example, a training sequence may include positioning a workpiece feature of a sample workpiece in the field of view (FOV), setting light levels, focusing or autofocusing, capturing an image, and providing a training sequence applied to the image (e.g., using video tools).The learning mode works by capturing or recording the sequence(s) and converting them into corresponding subprogram instructions. When the subprogram is executed, these instructions cause the image processing inspection system to replicate the trained image acquisition and inspection operations in order to inspect a workpiece or workpieces that match the sample workpiece used when creating the subprogram. These analysis and inspection methods, used to inspect features in a workpiece image, are typically implemented in various video tools contained in video tool part 143 of memory 140. Many well-known video tools, or simply "tools," are included in commercially available machine vision inspection systems, such as the QUICK VISION® series of machine vision systems and the associated QVPAK® software described above. A particular challenge with general-purpose vision inspection systems is providing procedures and tools that allow relatively untrained users to program such systems with robust inspection workflows that reliably produce accurate measurements. This is especially true for inspection features hidden beneath a fluorescent coating (e.g., a solder mask layer). For example, such coatings may be translucent and / or contain particulate filler materials, making it difficult for automated precision focusing processes to reliably produce a sharp image as desired (particularly for features located beneath or within the fluorescent material) when using conventional illumination and focusing techniques.Furthermore, when using fluorescence imaging techniques, the fluorescent material emits light from its entire volume, meaning there is no precisely defined focus height for the images generated from this emitted light. Therefore, prior art methods do not support accurate and reliable focusing for inspection image acquisition to inspect features hidden beneath a fluorescent coating, especially when it is desired to program the method on a sample workpiece (e.g., during learning mode operations) and then obtain reliable inspection results on similar workpieces subject to significant manufacturing variations in the fluorescent material.This problem is further exacerbated by the fact that tolerances for solder mask misalignment and the like are constantly shrinking, so that the associated inspection repeatability and accuracy for features located within fluorescent materials are desirable to be on the order of 10 µm or less in some applications. Prior art methods for focusing, image acquisition, and image analysis did not provide reliable and robust inspection solutions at these levels of accuracy. Several system features and / or methods disclosed herein address these types of measurement problems. In particular, automatic focusing criteria and methods are provided that generate a fluorescence image which, with good repeatability and accuracy, identifies the location of an underlying edge feature (e.g., of a non-fluorescent material located within the fluorescent material). In some embodiments, the methods disclosed herein can be implemented by operations that utilize known components and video tools (e.g., autofocus tools and edge detection tools). However, in other embodiments, the methods disclosed herein can be implemented by incorporating a specialized fluorescence image edge detection tool, such as the fluorescence image edge detection tool 143fl. For example, the fluorescence image edge detection tool 143fl can be configured to employ fluorescence image focusing user interface functions and / or criteria and methods, as disclosed herein, to enable a relatively simple user to reliably and repeatably operate the image processing inspection system 100 to measure an edge of a workpiece feature that lies within the fluorescent material.In some applications, this can enable the determination of precise dimensional relationships between such an edge and a nearby edge of the fluorescent material (e.g., to measure the fit of a patterned fluorescent material, such as a solder mask layer, with respect to an underlying feature). The Fluorescence Image Edge Detection Tool 143fl can be particularly well suited for printed circuit board inspection (e.g., for measuring the solder mask fit with respect to underlying features on the circuit boards). Automatic Fluorescence Image Focusing Functions, Criteria, and Operations, which can be used separately or in conjunction with the Fluorescence Image Edge Detection Tool 143fl, are described in more detail below. Fig. 3 is a schematic sketch 300 showing an embodiment of controllable illumination elements of the imaging component part 200 (shown in Fig. 2) and an embodiment of the surface height sensor 298. In addition to the elements shown in Fig. 2, sketch 300 shows an optional excitation illumination filter 231' and an optional fluorescence imaging filter 261', which may be included to enhance the controllable illumination usable in various methods disclosed herein. It is also shown that the workpiece 20 contains a fluorescent material 20f and an exposed part 20ex that is not covered by the fluorescent material 20f. As outlined above, the coaxial light 230' can emit source light 232' having an excitation wavelength profile that causes the fluorescent material 20f to fluoresce.The fluorescent workpiece light 255' emitted by the fluorescent material 20f can be received by the camera system 260 to provide fluorescent images. The coaxial light 230 can emit source light 232, and / or the ring light 240 can emit source light 242, which in the most versatile embodiments can each have a "non-excitation wavelength profile" that does not cause the fluorescent material 20f to fluoresce significantly, although this is not necessary in all embodiments. In any case, in various configurations, since the exposed part 20ex does not contain the fluorescent material, any source light (e.g. the source light 232, 242 and / or 232') reflected from the exposed part 20ex can be received by the camera system 260 to provide non-fluorescence images of at least the exposed part, even if the source light contains excitation wavelengths. In many applications, reflected light can be much stronger than the emitted fluorescence light. Fluorescence images can therefore be enhanced in some embodiments by using an optional excitation illumination filter 231' to filter the excitation wavelengths provided by the source 230' and further narrow the band of the excitation wavelength profile in the light 232' to those most effective for stimulating fluorescence. Furthermore, in some embodiments, the semi-silvered mirror 290 can incorporate an optional dichroic filter 290' (e.g., a thin-film filter) designed to reflect as much as possible of the narrowed excitation wavelength profile while transmitting other wavelengths. Thus, all of the excitation wavelengths reflected from the workpiece 20 are essentially blocked from reaching the camera system 260 when a fluorescence image is desired.The optional excitation illumination filter 231' can be movable and positioned so that it does not filter the contents of the source light 232 from the source 230 when the source light 232 is used to provide non-fluorescent images. Alternatively or additionally to the aforementioned fluorescence imaging means, an optional fluorescence imaging filter 261' can be used to prevent all wavelengths other than emitted fluorescence imaging wavelengths from contributing to images in the camera system 260. In principle, the fluorescence imaging filter 261' can provide usable fluorescence images even if a significant amount of source light is reflected from the workpiece 20. However, it blocks non-fluorescence image light. Therefore, the optional excitation illumination filter 231' is movable and positioned so that it does not filter the reflected light when the system is used to provide non-fluorescence images. Based on the above, it can be understood that the clearest fluorescence images are most easily obtained when only an excitation wavelength profile is output to image the workpiece (e.g., from source light 232'). Conversely, the clearest non-fluorescence images are most easily obtained when only a non-excitation wavelength profile is output to image the workpiece (e.g., from source light 232 or 242). It is evident that the special features and elements outlined above for the optical paths, which provide the source light for fluorescence and non-fluorescence imaging, are only exemplary and not limiting. Numerous alternatives for illumination and / or imaging in a manner compatible with the methods disclosed herein are obvious to a person skilled in the art. As described in more detail below, in order to determine a repeatable and reliable focus height for fluorescence images of workpiece features located within a fluorescent material, the height of an exposed portion of the workpiece (that is, a portion not covered with a fluorescent material), such as the exposed portion 420ex, can be determined, and the fluorescence imaging focus height can be determined relative to this determined height of the exposed portion. This can be more reliable than, for example, focusing based on a fluorescence image. In some embodiments, the height of the exposed portion can be determined based on the height corresponding to the best contrast in a set of autofocus images distributed along the Z-axis direction. However, as shown in Fig.Figure 3 shows that in other embodiments the image processing inspection system may include an optional surface height sensor 298, and the system may be configured such that the exposed part 420ex is positioned within a working range of the surface height sensor 298, which can be operated to use its schematically depicted height sensing means 298' to determine the height of the exposed part 420ex of the surface area along the Z-axis or focus direction. In some embodiments, the surface height sensor 298 may include a probe-type sensor, and the height sensing means 298' may include its probe pin. In other embodiments, the surface height sensor 298 may include an optical triangulation-type sensor, e.g.A triangulation sensor in which a height relationship with respect to a surface determines the position of a reflected light beam (which the sensing means 298' may provide) on a light-sensitive detector of the sensor. In other embodiments, the surface height sensor 298 may include a focus signal sensor, e.g., a focus signal sensor in which a height relationship with respect to a surface determines the path of a reflected light beam (which the sensing means 298' may provide) through a lens and to a location on a light-sensitive detector of the sensor. In any case, such sensors can be used to provide a specific height for the exposed part 20ex in various embodiments of the methods disclosed herein, according to known techniques. Fig. 4 shows a top view 400 of a field of view of the image processing inspection system, which shows the features of a sample workpiece part 420 and a sectional view 450 along a section aa (aa shown in view 400) of the features of the sample workpiece part 420. Some dimensions in Fig. 4 are exaggerated for clarity. The upper coordinate axes correspond to the top view 400, and the lower coordinate axes correspond to the section view 450. Aligned below views 400 and 450 are corresponding signal intensity profiles 450 and 460. Each of the signal intensity profiles 450 and 460 represents the change in intensity along a scan line through an image of the features in a non-fluorescence image (e.g., an image illuminated with a first "non-excitation" wavelength profile) and a second fluorescence image of the features (e.g., an image illuminated with a "fluorescence excitation" wavelength profile), respectively.Thus, signal strength profile 450 applies to a conventional image, and signal strength profile 460 applies to a fluorescence image. In this example, the scanning line is positioned in each image such that it corresponds to interface aa. As outlined with reference to Fig. 3, the first wavelength profile can be provided by one or both of the source lights 232 and / or 242 (and / or, in some embodiments, light reflected from the source light 232'), and the excitation wavelength profile can be provided by the source light 232'. The sample workpiece part 420 can, in some cases, be part of a sample workpiece used to create a subprogram in a learning mode, or, in other cases, be a workpiece subjected to inspection operations in a running mode. The features shown in views 400 and 450 include a substrate; a solder mask layer 420f, also referred to as fluorescent material 420f (shown with dotted fill), with edges at the X-axis locations ef1 and ef2; a contact surface 423 (shown with parallel hatching) with edges at the X-axis locations ep1 and ep2; an exposed portion 420ex (e.g., an exposed metallized or soldered portion of the contact surface 423, shown with cross hatching) with edges at the X-axis locations es1 and es2; a conductor track 424 (shown with parallel hatching) with edges at the X-axis locations et1 and et2. The exposed part 420ex can simply be an exposed part of the contact area 423 if the conductor tracks of a printed circuit board are not metallized or soldered at the time of inspection. View 400 also shows an edge detection video tool (or edge tool for short) investigation area ROI-fl ep2 and an associated autofocus video tool (or autofocus tool for short) investigation area ROI-fl AF, as well as an edge tool investigation area ROI-fl et1 and an associated autofocus video tool (or autofocus tool for short) investigation area ROI-fl AF', which are described in more detail below. As is generally known in the art, such investigation areas (ROIs) can be of such a size and positioned in an image that they define the extent of the image to be analyzed using image processing operations that are part of the associated video tool. By convention, the ROI of a video tool (e.g., ROI-fl ep2) can also refer to all operations of the associated video tool, not simply to its area of ​​investigation, and the meaning becomes clear from the context of such a reference. Fig. 4 is an example of a relatively “ideal” fabrication. The edges of the solder mask layer 420f coincide with the edges of the exposed part 420ex, as can be the case if the exposed part is not metallized or if the metallization or solder is applied through a pre-existing solder mask layer 420f. The solder mask layer 420f overlaps and insulates the contact area 423 around its entire perimeter and also completely insulates the conductor track 424. A typical example of a minimum desired insulating “overlap” width dmin for the solder mask layer 420f with respect to the edge of the adjacent conductive element is shown between edges ef2 and ep2. More generally, the overlap dmin along the entire length of each edge of a conductive element is desirable to prevent unwanted electrical short circuits between conductive elements.In some applications, dmin can be on the order of 10 µm or even less. The 420f solder mask layer has a thickness T. In some applications, the thickness T can be on the order of 25–150 µm or more, which can cause the 420f solder mask layer to significantly obscure the depicted edge ep2. It may be desirable to inspect dmin at several representative locations to ensure that the solder mask pattern 420f is correctly aligned with the conductive element pattern. This may require reliable automated imaging and / or edge localization of the conductive elements with an accuracy of less than 10 µm through a relatively thick translucent layer of fluorescent material, which is a challenging problem. Related points are described with reference to profiles 450 and 460. As previously stated, signal intensity profiles 450 and 460 represent the intensity change along a scanning line at location aa in a conventional image and a fluorescence image, respectively. For example, signal intensity profile 450 originates from image light reflected from the surfaces of the fluorescent material 420f and the exposed portion 420ex. Signal intensity profile 450 shows intensity changes at the locations of edge es1 and / or ef1, as well as edge es2 and / or ef2. If the image representing signal intensity profile 450 is automatically focused based on the ROI-AF autofocus tool located on the exposed portion 420ex, then the plane of focus for the image is approximately the determined height plane DHP1, and edges es1 and es2 can primarily determine the location of the intensity changes. If the image, which provides the signal strength profile 450, is based on a surface of the fluorescent material 420f (e.g.If the autofocus tools located on plane P0 (ROI, not shown) automatically focus, then the plane of focus for the image can be closer to plane P0, and edges ef1 and ef2 can primarily determine the location of the intensity changes. However, if the fluorescent material 420f is transparent, it may, in some cases, produce inaccurate and / or unreliable autofocus results. Nevertheless, in both cases, the associated edge locations can be determined based on the intensity changes according to known methods (e.g., at the locations of maximum rate of change of intensity). However, features (e.g., edges) located within the fluorescent material 420f may produce little or no signal in the conventional image of reflected light used for signal strength profile 450. In contrast, a fluorescence image can reveal such hidden features, as shown in signal strength profile 460. Elements and procedures that can be used to acquire a fluorescence image were previously outlined with reference to Fig. 3. (For example, the fluorescent material 420f fluoresces to provide fluorescence image light when excited by the source light 232'.) For descriptive purposes, the signal intensity profile 460 includes a solid signal line indicating the intensity signal derived from a fluorescence image focused at the specified height DHP1, and a dotted signal line showing an intensity signal variation observed in another fluorescence image focused at one of the focus heights FP2 or FP3. This illustrates an important problem associated with determining the location of features that lie within a fluorescent material in a fluorescence image. In particular, the signal strength of the fluorescence image is potentially influenced at various locations by factors including: the amount of diffuse fluorescence light emitted over the entire thickness of the fluorescent material 420f at that particular location, and the reflection of the fluorescence light by the workpiece features located within the fluorescent material near that location, as well as by the image sharpness height and its relationship to the Z-height range of the fluorescent material 420f and the Z-height of the surface(s) adjacent to the features within the fluorescent material (e.g., the Z-height of the contact surface 423).Thus, in the signal strength profile 460 of the intensity signal example, which is provided at a focus height set to DHP1 (the solid line), there is a maximum signal where the fluorescent material 420f is thickest, and a minimum signal where there is no fluorescent material 420f. At the focus height set to DHP1 (the solid line), there is some drop in the signal from the maximum at the edges ep1, ep2, et1, and et2. However, because the image determined at the height level DHP1 is far from the surface of the contact area 423 and the conductor track 424, the effects associated with them are blurred, and the signal drop is not significant. Edge detection based on the associated signal changes may be less reliable and less accurate, or even impossible.For example, the left and right edges of ROI-fl ep1 are marked by the boundaries ROII and ROIr in signal strength profile 460, and the intensity change indicated by the solid line at edge ep2 is not large. If the edge tool ROI-fl ep1 is trained to find this faint edge in a fluorescence image that is not optimally focused for this edge feature (that is, if the video tool's edge detection parameters are determined based on this pattern signal and stored in a subprogram for inspecting edges on similar parts according to known video tool procedures), the resulting subprogram may not operate reliably. It is conceivable that the results could be even worse if the focus plane were higher in the fluorescent material 420f or at its surface (e.g., plane P0).In contrast, in profile 460 of the example intensity signal, which is provided at a focus height set to FP2 or FP3 (including the signal deviations indicated by the dotted signal lines), there is a more significant drop in the signal at edges ep1, ep2, and et1 because the image focus plane is positioned relative to the surface of contact area 423 and conductor track 424 in such a way that the effects associated with them are either more pronounced on the intensity signal or less blurred in the fluorescence image, or both. Video tool edge detection parameters determined based on this pattern signal (e.g., derived from an optimally focused fluorescence image) and stored in a subprogram can be relatively more reliable and accurate.In most applications, it is desirable to determine the fluorescence image height to best enhance the detection of the desired feature located within the fluorescent material in the resulting fluorescence image. In some embodiments, a window in a graphical user interface for image processing inspection can display an intensity signal profile analogous to profile 460, making it easier for a user to assess the optimal fluorescence image height. Alternatively, such a signal profile can be automatically evaluated as a function of height to determine the fluorescence image sharpness height that provides the maximum intensity signal increase near the desired edge. It should be understood that, although the signal is shown to decrease above contact surface 423 and conductor track 424 in this example, it could increase with a different color or reflectivity, possible fluorescence in the substrate, or with an edge feature made of a different material than these conductive elements at the edge of the feature. However, analogous focus-dependent magnitudes of edge-indicating signal change can still be observed. As outlined above, the plane of focus of a fluorescence image can be an important factor in providing repeatable and accurate detection of the location of hidden features beneath a layer of fluorescent material. With tight feature tolerances (e.g., 10 µm), this factor can become critical. However, automatic focusing using fluorescence images is unreliable because automatic focusing is usually based on image contrast measurements, and the highest contrast image height for a fluorescence image can be unreliable due to variations in flatness, thickness, bubble content, particle content, and specific hidden features within a layer of fluorescent material. This is particularly problematic when image processing inspection workflows and tool parameters (e.g.,(in learning mode) using a sample workpiece, and then an attempt is made to inspect a similar workpiece using identical operations, since tolerances and manufacturing control for fluorescent coatings can be relatively poor compared to many other materials and manufacturing processes used in miniature precision devices. Therefore, it is desirable to provide a sharpness level for fluorescence images according to more repeatable methods, such as those disclosed here. For example, in various embodiments of a method that provides a reliable focus height for fluorescence images, the height of an exposed portion (that is, a portion not covered with a fluorescent material) of a workpiece, such as the exposed portion 420ex, can be determined to provide a reliable reference height. This reference height can then be used as the basis for focusing a fluorescence image. In some embodiments, a height sensor, such as the surface height sensor 298, can be used to determine the height of the exposed portion, as outlined above. However, in other embodiments, the surface height sensor 298 can be omitted and / or the height of the height range of the exposed portion can be determined by performing an autofocus operation on the exposed portion using illumination and imaging methods as described in Fig.3 and / or are outlined below. If the specific height of the exposed part 420ex is determined by an autofocus operation, the autofocus height may be based on the height of best image contrast for the exposed part 420ex, as indicated by a set of non-fluorescent autofocus images (e.g., "non-fluorescent" at least at the location of the exposed part). Then this height of focus determined on the basis of the exposed part (e.g., its height of "best sharpness" or at least a well-focused height) may be the specific height used as the basis for a fluorescence imaging sharpness height. In one embodiment, the determined height based on the exposed portion (e.g., its sharpness height) can be used as the fluorescence imaging sharpness height or plane, particularly if the height of the exposed portion is close to the height of the surface adjacent to the edge feature to be located in the fluorescence image. In other applications, it may be desirable to use the height determined based on the exposed portion as a reference height (that is, a height that has a relatively predictable height relationship to a height of the fluorescent material or to the surface where the edge feature is located in the fluorescent material) and to use a fluorescence imaging sharpness height or plane offset by a defined distance from this reference height. For example, Fig. 4 shows that the focus plane FP2 is offset from the Z-height of the specified height plane DHP1 by a defined Z-offset Off12. The focus plane FP2 may be better suited for capturing a fluorescence image to be used for detecting the edge ep2 located in the edge tool ROI-fl ep2. A similar Z-offset Off13 could be introduced between DHP1 and FP3 if needed. The focus plane FP3 may be better suited for capturing a fluorescence image to be used for detecting the edge et1 located in the edge tool ROI-fl et1. However, more generally, the most suitable offset for any given edge detection can be determined by a user during learning mode on a sample workpiece and stored in a subprogram as a parameter linked to capturing the corresponding fluorescence image for edge detection during run mode. For example, the Z-height at which a desired feature within the fluorescent material (e.g., an edge) is well defined by a change in intensity in a fluorescence image can be determined manually or automatically. The specific Z-height corresponding to the exposed portion (e.g., as determined by a height sensor or an autofocus operation) can be determined, and the Z-offset between these heights can be determined during the learning phase and stored in the subprogram. Then, during the run mode, the fluorescence image height for the corresponding edge feature can be adjusted based on the height of the corresponding exposed portion (e.g., the edge).(as determined by a height sensor or an autofocus operation) by moving by the stored Z-offset to establish a fluorescence image height relative to the determined height of the exposed part, and obtaining a fluorescence image at this fluorescence image height, which is used to determine the location of the edge within the fluorescent material. In various embodiments, the offset is advantageously determined such that the fluorescence image height falls within the layer of fluorescent material. In some applications, it is most advantageous to determine the height of an exposed portion such that it has a surface height within the height of the fluorescent material covering the feature to be imaged at the fluorescence imaging height. In other applications, it is most advantageous to determine the height of an exposed portion such that it has the same surface height as the surface of a material layer containing an edge feature within the fluorescent material to be imaged at the fluorescence imaging height. In some such embodiments, it may be appropriate to simply set the fluorescence imaging height to the same as the determined height of the exposed portion.However, it must be understood that such special choices for the exposed part and the fluorescence imaging height are not limiting and may not be possible or optimal for all workpieces or applications. For some workpieces, the thickness and / or composition of the fluorescent material layer can be highly variable. Therefore, in some embodiments, it may be desirable to determine a fluorescence image height based on more information about such variations, in addition to a reference height determined based on the exposed part, as outlined above. For example, the height of the surface of the fluorescent material 420f and / or its thickness can be determined (e.g., based on surface height sensor measurements or autofocus operations using non-fluorescence imaging, or other known methods). Then, the Z-offset outlined above can be determined, at least partially, based on this additional information (e.g., as the ratio of the thickness to the determined height of the exposed part, or another desired relationship). In some applications, the location of the feature within the fluorescent material is the desired inspection information and can be determined based on the fluorescence image (e.g., by identifying the location of edge ep2 using the edge tool ROI-fl ep2). In other applications, the dimension dmin is the desired inspection information and can be determined based on identifying the location of edge ep2 in the fluorescence image and determining the location of edge ef2 in either a fluorescence or non-fluorescence image (e.g., using another edge tool) and calculating the difference between their locations. In some embodiments, the video tools shown in Fig. 4 can be known types of edge detection tools and autofocus tools, which, together with known motion operations and programming instructions and the like, are used in a sequence that performs operations according to the methods disclosed herein. In other embodiments, the video tools shown in Fig. 4 can be new types of video tools that are specific for fluorescence image edge detection. For example, in one embodiment, a user can select the video tool ROI-fl ep2 from a toolbar in a user interface, which can display the user-adjustable inspection area of ​​ROI-fl ep2 on a real-time video image (e.g., as 420), together with the "linked" autofocus inspection area of ​​ROI-fl AF.The video tool can be configured so that the user can drag and resize the examination area of ​​ROI-fl et1 to a desired exposed area and execute automatic focusing using reflected light (e.g., as outlined above). In one embodiment, the video tool can then implement a fluorescence imaging configuration and display a fluorescence image at the current focus height. The user can then drag and resize the examination area of ​​ROI-fl ep2 to a desired edge and also change the focus height if the current focus height does not produce a desired edge image.The edge tool parameters can then be trained using the best fluorescence image, and the trained edge parameters and the current Z-offset with respect to the specified height of the associated free-standing part can be stored in the subprogram for later use when inspecting similar workpieces. The video tools ROI-fl et1 and ROI-fl AF' can be similarly assigned and trained, or in one embodiment, the video tool ROI-fl et1 can be configured to use previously specified parameters of a free-standing part that are linked to ROI-fl AF when it is in the same field of view, and the video tool area ROI-fl AF' can be omitted. Other video tool implementations and associated graphical user interface features are obvious to a person skilled in the art who has benefited from the general teachings disclosed herein. Figures 5A and 5B show flowcharts 500A and 500B outlining a method for operating an image processing inspection system to determine a reliable and repeatable fluorescence imaging height, such as can be used to acquire a fluorescence image for inspecting the location of a workpiece edge that lies within a layer of fluorescent material. In one embodiment, the method shown in Figures 5A and 5B can be applied, at least in part, by a user selecting and operating the fluorescence image edge detection tool 143fl, shown in Figure 2 and / or as described with reference to an embodiment of the edge tool ROI-fl ep2 shown in Figure 4. In other embodiments, the method can be implemented using various known tools and / or programming steps. The process begins, and at block 505, an exposed portion of a workpiece (that is, a portion not covered by a fluorescent material) is positioned such that its height can be determined by the image processing inspection system, wherein the exposed portion has a characteristic surface height along a focus axis direction (e.g., the Z-axis direction) with respect to a height within a layer of fluorescent material contained on the workpiece. In some embodiments where the image processing inspection system includes a surface height sensor, this may involve positioning the exposed portion within a working area of ​​the surface height sensor. In some embodiments, this may involve positioning the exposed portion within a field of view of an image processing inspection system (e.g., for an autofocus operation).In various applications, the exposed portion can be selected to have a surface height above, within, or below a height range of the fluorescent material. If a bare printed circuit board is the workpiece, the exposed portion of the workpiece can include an exposed part, such as the exposed part 420ex shown in Fig. 4, which may be a metallized or soldered portion of a contact surface, such as contact surface 423, or an exposed portion of a substrate or a populated component, or the like. In a block 510, the image processing inspection system is configured to determine the height of at least the exposed part. In some embodiments where the image processing inspection system includes a surface height sensor, this may include configuring the image processing inspection system to use the surface height sensor to determine the height of the exposed part. In some embodiments, this may include configuring the image processing inspection system in a first arrangement to provide an image of at least the exposed part (e.g., setting up the illumination, etc., for an autofocus operation). In such embodiments, at least the exposed part produces a non-fluorescent image in images acquired using the first arrangement, as outlined above with reference to Fig. 3.In some embodiments, the field of view in the first arrangement can be illuminated using a first “non-excitatory” wavelength profile that does not cause significant fluorescence in the fluorescent material. Various alternative embodiments and considerations regarding the setup of the image processing inspection system for determining the height of the exposed part have been outlined previously (e.g., with reference to Fig. 3). In Block 515, the height of the exposed part is determined. In some embodiments where the image processing inspection system includes a surface height sensor, this may involve using the surface height sensor to determine the height of the exposed part. In some embodiments, this may involve determining a focus height of the exposed part based on images of the exposed part acquired at different heights while the image processing inspection system is in the first arrangement outlined above, and using this focus height as the determined height of the exposed part. Such a focus height, at which the exposed part is sharply imaged, can be determined by known methods (e.g., by analyzing the image contrast as a function of the Z-height). Various considerations for selecting the exposed part to be used in Block 515 have been outlined previously (e.g.,B. with reference to Fig. 4). The flowchart 500A continues via a block A shown in Fig. 5A and Fig. 5B. Figure 5B shows operations that determine a desired fluorescence imaging height to be used for fluorescence imaging of a workpiece feature located within the fluorescent material, where the fluorescence imaging height is determined relative to the determined height of the exposed part (e.g., as determined above). Decision block 520 indicates whether the fluorescence imaging height is determined and stored for the first time (e.g., during learning mode) or whether the fluorescence imaging height is determined in run mode based on previously stored information. Specifically, at decision block 520, if operations are performed to create a subprogram in learning mode, the routine continues at block 525; otherwise (e.g., during run mode), the routine continues at block 530, as further described below.In Block 525, a fluorescence imaging height is determined for use in fluorescence imaging of a workpiece feature located within the fluorescent material—for reasons described above, in relation to the determined height of the exposed part—and the fluorescence imaging height is stored in conjunction with a subprogram for later use (e.g., when a fluorescence image is acquired for inspecting corresponding workpiece features on corresponding workpieces). In some embodiments of the operations in Block 525, the fluorescence imaging height can be stored as an offset dimension relative to the previously determined height of the exposed part, or as otherwise outlined here. In block 530 (e.g., if a subprogram is currently running in run mode), a fluorescence image height for an associated feature located within the fluorescent material of the current workpiece is determined relative to the height of the exposed portion of the current workpiece (previously determined during run mode), and the image processing inspection system is focused on this fluorescence image height. Thus, in block 530, determining the fluorescence image height can involve retrieving the fluorescence image height information stored in a subprogram associated with a feature currently being inspected, as well as determining the fluorescence image height relative to the height of the exposed portion based on this information. Various considerations and alternative embodiments regarding the fluorescence image height have been outlined previously (e.g., with reference to Fig.3 and Fig. 4). The routine continues at block 535, where the workpiece feature (e.g., an edge feature) located within the layer of fluorescent material and belonging to the current fluorescence imaging height is positioned in the field of view of the image processing inspection system. Then, at block 540, the field of view is illuminated using an excitation wavelength profile, causing the fluorescent material to fluoresce. At block 545, a fluorescence image of the field of view is acquired using the fluorescence imaging height and the excitation wavelength profile. Various considerations and alternative embodiments regarding excitation illumination and fluorescence imaging have been outlined previously (e.g., with reference to Figures 3 and 4). Next, at block 550, the location of the workpiece feature within the layer of fluorescent material is determined based on identifying the location of a corresponding intensity change in the fluorescence image acquired at block 545, and the routine ends. For example, with reference to Fig. 4, edge ep2 can be determined based on the intensity change along the scan line at location aa in the scan area of ​​the edge tool ROI-fl ep2, as specified in profile 460. In one embodiment, the intensity change can be identified as the location of the maximum intensity slope or gradient in the neighborhood of edge ep2 according to known methods. (For example, the neighborhood can be specified by a parameter of the edge tool ROI-fl ep2.) It should be evident that the methods disclosed herein provide a more reliable and repeatable fluorescence image height than previously practiced methods and can be used to acquire a fluorescence image for the accurate and repeatable determination of the location of a workpiece edge to be inspected within a fluorescent material. Furthermore, the methods can provide accuracy and repeatability at a higher speed than that available when using conventional fluorescence microscopy focusing techniques. While various preferred and exemplary embodiments of the invention have been presented and described, it is understood that various modifications can be made to them without departing from the spirit and scope of the invention.

Claims

Method for operating an image processing inspection system for determining a fluorescence image height for capturing a fluorescence image for repeatedly determining the location of a workpiece feature edge located within a layer of fluorescent material on a workpiece, the method comprising: (a) positioning an exposed portion of a surface of the workpiece such that its height can be determined by the image processing inspection system, wherein the exposed portion is not covered by the layer of fluorescent material and has a characteristic surface height along a focal axis with respect to a height within the layer of fluorescent material; (b) setting up the image processing inspection system to determine the height of the exposed portion; (c) determining the height of the exposed portion;(d) Determining a fluorescence image height to be used for fluorescence imaging of the workpiece feature edge located within the layer of fluorescent material, wherein the fluorescence image height is determined in relation to the determined height of the exposed part; and performing at least one of (e) and (f), wherein (e) comprises: (c) storing the determined fluorescence image height in conjunction with a subprogram for later use when acquiring a fluorescence image to be used for inspecting the workpiece feature edge located within the layer of fluorescent material; and (f) comprises: (f) using the fluorescence image height determined in relation to the determined height of the exposed part during the execution of a subprogram when acquiring a fluorescence image to be used for inspecting the workpiece feature edge located within the layer of fluorescent material. The method of claim 1, wherein the workpiece is a sample workpiece and the method is carried out in conjunction with a learning operating mode of the image processing inspection system, which is used to create a subprogram to be used for inspecting workpieces similar to the sample workpiece, and the method comprises performing steps (a), (b), (c), (d) and (e). The method of claim 2, further comprising: (g) positioning the workpiece feature edge located within the layer of fluorescent material in a field of view of the image processing inspection system; (h) positioning the image processing inspection system at the determined fluorescence imaging height; (i) illuminating the field of view using an excitation wavelength profile that causes the fluorescent material to fluoresce and emit fluorescence imaging light; (j) capturing a fluorescence image of the field of view using the fluorescence imaging height and illuminating the field of view using the excitation wavelength profile; and (k) determining a location of the workpiece feature edge located within the fluorescent material based on a location of a corresponding intensity change in the fluorescence image. The method of claim 3, wherein step (k) comprises setting up the parameters of an edge detection video tool of the image processing inspection system and using this edge detection video tool to determine the location of the workpiece feature edge on the sample workpiece, and the method further comprises: (1) storing the set-up parameters of the edge detection video tool in conjunction with the subprogram to use them later for determining the location of the workpiece feature edge in fluorescence images of workpieces similar to the sample workpiece. The method of claim 1, wherein the method is carried out in conjunction with a run-operating mode of the image processing inspection system by executing a subprogram which includes inspecting the workpiece feature edge which lies within the fluorescent material on a workpiece which is similar to a sample workpiece which was used to create the subprogram, and the method comprises performing steps (a), (b), (c), (d) and (f), wherein in step (d) determining the fluorescence imaging height to be used for the fluorescence imaging of the workpiece feature edge comprises retrieving the fluorescence imaging height information stored in the subprogram in connection with this workpiece feature edge and determining the fluorescence imaging height based on this information. The method of claim 5, comprising: (g) positioning the workpiece feature edge located within the layer of fluorescent material in the field of view of the image processing inspection system; (h) positioning the image processing inspection system at the determined fluorescence imaging height; (i) illuminating the field of view using an excitation wavelength profile that causes the fluorescent material to fluoresce and emit fluorescence imaging light; (j) acquiring a fluorescence image of the field of view using the fluorescence imaging height and illuminating the field of view using the excitation wavelength profile; and (k) determining a location of the workpiece feature edge located within the fluorescent material based on a location of a corresponding intensity change in the fluorescence image. The method of claim 6, wherein step (k) comprises setting up an edge detection video tool of the image processing inspection system according to associated parameters stored in the subprogram, and using this edge detection video tool to determine the location of the workpiece feature edge in the fluorescence image. The method of claim 6, further comprising: (1) determining the location of an edge of the layer of fluorescent material; and (m) determining a measure of a dimensional relationship between the location of the edge of the layer of fluorescent material and the location of the workpiece feature edge that lies within the layer of fluorescent material. The method of claim 8, wherein the edge of the layer of fluorescent material is an edge contained in the fluorescence image of the field of view acquired in step (j); and in step (1) determining the location of an edge of the layer of fluorescent material comprises locating the edge of the layer of fluorescent material contained in the fluorescence image acquired in step (j). The method of claim 3, wherein the image processing inspection system comprises a fluorescence imaging filter that blocks at least one wavelength of an excitation wavelength profile used as illumination when acquiring a fluorescence image and transmits at least one wavelength of the fluorescence imaging light emitted by the fluorescent material, and in step (j) acquiring the fluorescence image comprises using the fluorescence imaging filter to filter the image light used to form the fluorescence image. Method according to claim 1, wherein the fluorescence imaging height is determined as an offset measure in relation to the determined height of the exposed part. Method according to claim 1, wherein the fluorescence imaging height is determined such that it falls within the layer of fluorescent material. Method according to claim 1, wherein the exposed part is selected such that it has a surface height which falls within a height dimension of the layer of fluorescent material. Method according to claim 13, wherein the exposed part of the surface of the workpiece is selected such that it is nominally at the same surface height as a surface of a material layer which has the workpiece feature edge which lies in the fluorescent material. The method of claim 1, wherein the image processing inspection system comprises a surface height sensor comprising a probe-type sensor, an optical triangulation-type sensor, and a focus signal sensor; step (a) comprises positioning the exposed part in a working area of ​​the surface height sensor; step (b) comprises setting up the image processing inspection system to use the surface height sensor to determine the height of the exposed part; and step (c) comprises using the surface height sensor to determine the height of the exposed part. The method of claim 1, wherein step (a) comprises positioning the exposed part in a field of view of the image processing inspection system; step (b) comprises setting up the image processing inspection system in a first arrangement to provide an image of at least the exposed part; and step (c) comprises determining a focus height of the exposed part based on images of the exposed part acquired at different heights while the image processing inspection system is in the first arrangement, and using this focus height as the determined height of the exposed part. The method of claim 16, wherein: the image processing inspection system comprises a controllable illumination which is controllable to output at least two wavelength profiles, comprising: a non-excitation wavelength profile which illuminates the workpiece such that the workpiece primarily emits reflected image light and an insignificant amount of fluorescence light in response to the non-excitation wavelength profile, so that the non-excitation wavelength profile can be used to acquire non-fluorescence images, and an excitation wavelength profile which causes the layer of fluorescent material to fluoresce and emit a substantial amount of fluorescence imaging light, so that the excitation wavelength profile can be used to acquire the fluorescence image;and in step (b) the first arrangement includes setting up the controllable lighting, outputting the non-excitation wavelength profile and not outputting the excitation wavelength profile.; Method according to claim 17, wherein the controllable lighting comprises a ring light and the first arrangement includes outputting the non-excitation wavelength profile from the ring light. Method according to claim 17, wherein the acquisition of the fluorescence image includes setting up the controllable illumination, outputting the excitation wavelength profile and not outputting the non-excitation wavelength profile.

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