Method for determining the contour of a surface and measuring device for determining the contour of a surface

Abstract

Method for determining the contour of an object's surface along a measuring distance, in which the focus of a microscope objective relative to the surface is varied along a measuring axis (z-axis) over a scan area at measuring points spaced apart from each other along the measuring path and in which height signals representing the height of the surface are derived from images taken during variations in focus, characterized by that along the measuring path the scan area in Rich- The measurement axis (z-axis) of the contour is automatically tracked, with the scan area being shifted towards the object in the measurement direction when the height of the object is locally reduced based on the height signals determined along the measurement path, and shifted away from the object in the measurement direction when the height of the object is locally increased based on the height signals determined along the measurement path. that the contour of the surface is extrapolated from height signals determined along the measuring section to track the scan area.

Description

[0001] The invention relates to a method of the type mentioned in the preamble of claim 1 for determining the contour of a surface along a measuring distance and a measuring device of the type mentioned in the preamble of claim 7 for determining the contour of a surface of an object along a measuring distance.

[0002] In a known method of this type, the focus of a microscope objective relative to the surface is varied at measuring points spaced apart along a measuring axis (z-axis) over a scan range. In this known method, a microscope objective with a high numerical aperture and high magnification is used to achieve high resolution and accuracy in the measuring direction (z-direction), for example, to measure the roughness of a surface. Height signals representing the surface height are derived by evaluating images acquired during the varying focus.

[0003] The scan area (height measurement range) must be selected so that not only individual areas, but the entirety of all surface topographies are not oversaturated. Due to the typically present tilt or curvature of the surface, a large overall scan area must therefore be chosen, which is significantly larger than the scan area required to measure the surface height at a single measurement point.

[0004] Such a procedure is known from DE 10 2007 018 048 A1.

[0005] Due to the use of a microscope objective with a high numerical aperture and high magnification, the measurement field width in a plane perpendicular to the measurement axis (z-axis), i.e., in the x- or y-direction, is only a small 0.2 to 0.3 mm. However, many measurement tasks require a measurement field with a width of approximately 4 mm in the x- or y-direction or even greater. At the same time, a short measurement time is not only necessary or desirable for high throughput, but also for reducing the negative effects of, for example, thermal drift in the measurement setup.

[0006] The lateral xy measurement field can be enlarged by laterally arranging individual topographic fields (measurement fields), each approximately 0.2 mm wide. The measurement fields can be arranged precisely adjacent to one another or overlapping. When the measurement fields are overlapping, image processing methods are used to search for corresponding points, which are then used to align the individual fields. This process is also known as stitching.

[0007] DE 102 54 435 A1 discloses a method for determining the contour of an object's surface along a measuring distance, in which the focus of a lens relative to the surface is varied along a measuring axis (z-axis) over a scan area at measuring points spaced apart from each other along the measuring distance, wherein height signals representing the height of the surface are derived from images taken during the variation of the focus, and wherein the scan area is tracked along the measuring distance in the direction of the measuring axis (z-axis) of the contour.

[0008] German patent DE 199 44 516 A1 discloses a method for capturing the shape of an object, in which a camera image of the object is created and an outline of the object in a first plane, which appears sharp in the camera image, is captured by an evaluation unit connected to the camera. In the known method, the focusing distance of the camera is changed and a sharp outline of the object in a second plane is captured by the evaluation unit. These steps are repeated until a sufficient number of outlines have been captured so that the spatial shape of the object can be represented.

[0009] WO 2003 / 052 347 A2 discloses a method for the three-dimensional measurement of an object's surface using the autofocus principle and a coordinate measuring machine. In the method described in the publication, the area to be measured is divided into sub-areas, and the contrast for each sub-area is captured simultaneously or substantially simultaneously while an optical sensor is moved along its optical axis.

[0010] US Patent 2005 / 0286800A1 discloses a method for imaging a surface in which a relative scanning motion is used between a surface to be imaged and an array of light detectors.

[0011] From the publication “Focus Variation Instruments” (author: Franz Helmi), Optical Measurement of Surface Topography, pages 131-166, Springer Verlag, 05.04.2011, ISBN 978-3-642-12011-4, a method is known in which a surface is measured on the basis of a change in the focus of a microscope objective.

[0012] From the publication Alicona: FOCUS variation, English edition 1, 2010 / 2011 - company publication, it is known that the area-related The measuring range of a corresponding measuring device perpendicular to the measuring axis (z-axis), i.e. in the xy direction, can be increased by joining several individual measurements together in the manner of an xy-stitching.

[0013] The invention is based on the objective of improving the known method with regard to its applicability in manufacturing metrology.

[0014] This problem is solved by the invention specified in claim 1.

[0015] The invention is based on the idea of ​​further developing the known method in such a way that the measurement time for determining the contour of a surface, whether along a measuring line or during an area-wide surface measurement, is reduced. In the method according to the invention, the focus of the microscope objective relative to the surface along the measuring axis is varied at each measuring point along the measuring line, and images are captured at the different focal points. A height signal representing the surface height is derived from the images captured at each measuring point. The captured images can be evaluated, for example, based on their contrast, whereby, in particular, an image with maximum contrast can be used to generate a height signal and, using a height reference, to determine the surface height at the respective measuring point.Varying the focus along the measurement axis across the scan area takes more time the greater the height (extent in the measurement direction) of the scan area.

[0016] Based on this, the invention is based on the idea of ​​automatically tracking the scan area along the measuring path in the measuring direction, i.e. in the direction of the measuring axis (z-axis), of the contour.

[0017] According to the invention, the contour of the surface is extrapolated from height signals determined along the measuring path to track the scan area. The scan area can then be adaptively tracked according to the extrapolated contour. For example, if height signals determined along the measuring path indicate that the object's height is locally decreasing, the scan area can be shifted towards the object. Conversely, if the height signals indicate that the object's height is locally increasing, the scan area can be shifted away from the object.

[0018] According to the invention, by tracking the scan area, it is no longer necessary to use a total scan area that captures the entire topography of the surface. Instead, the surface can be scanned with a scan area of ​​relatively small height, i.e., small extent in the measuring direction (z-direction), by tracking the scan area along the contour as described in the invention. This results in a significant time advantage compared to the known method, allowing measurements to be performed much faster. This improves the applicability of the method according to the invention in manufacturing metrology.

[0019] According to the invention, the tracking of the scan area in the direction of the measuring axis (in the measuring direction) is designed such that the scan area is shifted towards the object in the measuring direction when it is determined, based on the height signals determined along the measuring distance, that the height of the object is locally decreasing, and the scan area can be shifted away from the object in the measuring direction when it is determined, based on the height signals determined along the measuring distance, that the height of the object is locally increasing.

[0020] The method according to the invention is suitable for measuring any surface geometry. The surface can be scanned line by line and thus measured two-dimensionally. Alternatively, the surface can be scanned area by area and thus measured three-dimensionally. The method according to the invention is suitable for measuring the shape and / or waviness and / or roughness of workpiece surfaces.

[0021] An advantageous embodiment of the invention provides that the contour of the surface is extrapolated from height signals determined along the measuring path to track the scan area. Based on this extrapolated signal, the scan area can then be tracked along the z-axis and adaptively adjusted to the surface contour. In this embodiment, the extrapolation of the surface contour serves solely to estimate its course and, based on this, to track the scan area. The contour is then precisely measured along the remaining measuring path. Scanning the surface along the measuring path, i.e., in the x- or y-direction, can be continuous or stepwise. According to the invention, still images or moving images can be captured sequentially while varying the focus of the microscope objective along the measuring axis.The recorded images are evaluated using image processing methods.

[0022] According to the invention, a displacement of the scan area away from or towards the object means that a scan area of ​​constant height is displaced in the measuring direction and / or that an upper and / or lower limit of the scan area, as seen in the measuring direction, is displaced relative to the object.

[0023] According to the invention, it is generally sufficient if the scan area has a constant height along the measuring path in the measuring direction. In this embodiment, a scan area of ​​constant height is used along the measuring path. The height of the scan area is chosen to be as small as possible to minimize the measurement time. At the same time, the height of the scan area is chosen to be large enough to capture the entire height structure of the surface of the object being measured without causing an overload during changes in the focus of the microscope objective.

[0024] However, according to the invention, it is also possible to vary the height of the scan area along the measuring path depending on the height signals, as provided for in another embodiment of the invention. For example, the height of the scan area can be reduced if an extrapolation of the surface contour shows that the contour changes with a slight slope. Conversely, to avoid overloading, the scan area can be increased if the extrapolation of the surface contour shows that the contour has a steep slope locally. In this way, the microcontour, for example roughness, of a surface can be determined quickly and reliably using the method according to the invention, even if the shape of the surface changes significantly along the measuring path.

[0025] Another advantageous embodiment of the invention provides that, for three-dimensional measurement of the surface, the surface is scanned in a plane perpendicular to the measurement axis (z-axis). In this embodiment, the lateral xy measurement field can be enlarged by laterally arranging individual topographic fields (measurement fields), for example by means of the known stitching method.

[0026] Another advantageous embodiment of the invention provides that the focusing of the microscope objective is varied automatically.

[0027] According to another embodiment of the inventive method, the shape and / or waviness and / or roughness of the surface is measured.

[0028] A measuring device according to the invention for determining the contour of an object's surface along a measuring distance is specified in claim 7. The measuring device specified in claim 7 comprises a microscope objective and a focusing device which, at measuring points spaced apart from one another along the measuring distance, varies the focus of the microscope objective relative to the surface along a measuring axis (z-axis) over a scan area. The measuring device according to the invention further comprises an image acquisition device for capturing images of the surface during the variation of the focus, as well as an evaluation device which, by evaluating the captured images, derives height signals representing the height of the surface.According to the invention, the control device is configured to control the focusing device in such a way that the scan area is automatically tracked along the measuring path in the direction of the measuring axis (z-axis) of the contour. The measuring device according to the invention offers essentially the same advantages as the method according to the invention.

[0029] Advantageous and appropriate embodiments of the measuring device according to the invention are specified in claims 8 to 10. The use of a measuring device according to one of claims 7 to 10 is specified in claim 11.

[0030] Illumination of the object and the acquisition of images of the object can be carried out using any suitable method, for example, by contrast analysis, confocal microscopy, white light microscopy, or any other suitable method. Such methods are generally known to those skilled in the art, for example, from DE 10 2007 018 048 A1, and are therefore not described in detail here.

[0031] The invention is explained in more detail below using an exemplary embodiment with reference to the attached highly schematic drawing.

[0032] It shows: Fig. 1 a highly schematic block diagram of a measuring device according to the invention and Fig. 2. An example of the course of a contour is shown to illustrate the method according to the invention.

[0033] In Fig. Figure 1 shows a highly schematic block diagram illustrating an embodiment of a measuring device 2 according to the invention for determining the contour of a surface 4 of an object 6 along a measuring path. The measuring device 2, which is also referred to as the device below, serves for the three-dimensional measurement of the surface of the object 6. In the illustrated embodiment, the roughness of the surface 4 is measured using the measuring device 2. For this purpose, the measuring device 2 has a microscope objective 8 to which a focusing device 10 is assigned.

[0034] In carrying out a method according to the invention, the microscope objective 8 is moved relative to the surface 4, for example along the Fig. The focusing device 10 varies the focus of the microscope objective 8 relative to the surface 4 at measuring points spaced apart along the measuring path over a scan area. The focus is measured along the y-axis extending into the plane of the drawing and / or along the x-axis perpendicular to the y-axis.

[0035] The microscope objective 8 is further associated with an image acquisition device 12 for capturing images of the surface 4 while varying the focus. Movement of the objective 8 in the x- or y-direction along the measuring path can be continuous or stepwise. The image acquisition device 12 can capture still or moving images of the surface 4, depending on the specific requirements and application. In the illustrated embodiment, the image acquisition device 12 comprises a digital image sensor.

[0036] The image acquisition device 12 is followed by an evaluation device 14, which generates height signals representing the height of the surface at the respective measuring point by evaluating the recorded images and the associated control signals of the focusing device 10.

[0037] According to the invention, a height signal is generated at each measuring point along the measuring path, representing the height of surface 4 (relative to a height reference). By traversing a measuring path, surface 4 can thus be measured two-dimensionally. The height signals along the measuring path define the contour of the surface. To measure surface 4 three-dimensionally, a plurality of line- or strip-shaped measuring fields are placed next to each other. In the method according to the invention, the height profile of surface 4 is thus determined over its entire surface, thereby revealing the three-dimensional topology of surface 4. The method according to the invention is therefore used for measuring three-dimensional surfaces.

[0038] The illumination of surface 4 and the taking of images of surface 4 can be carried out using any suitable method. Since such methods are generally known to those skilled in the art, they will not be explained in detail here.

[0039] According to the invention, a control device 16 is provided which is configured to control the focusing device 10 in such a way that the scan area is automatically followed along the measuring path in the direction of the measuring axis (z-axis), the contour of the surface 4.

[0040] Using the measuring device 2, an embodiment of a method according to the invention for determining the contour of the surface 4 is carried out as follows: Fig. Figure 2 schematically shows the contour of surface 4. Along a measuring path, for example along the x-axis, the focus of the microscope objective 8 is varied relative to surface 4 along the measuring axis (z-axis) over a scan area. The scan area is defined in Fig. 2 is symbolized by a measuring window 18.

[0041] Using the image acquisition device 12, 8 images are taken at each measuring point while varying the focus of the microscope objective, from which the evaluation device 14 derives height signals representing the height of the surface, using control signals from the focusing device 10 and a height reference.

[0042] According to the invention, along the measuring section, which is in Fig. 2 runs in the x-direction, the scan area (measuring window 18) is tracked in the direction of the measuring axis (z-axis) of the contour.

[0043] In the illustrated embodiment, the contour of the surface 4 is extrapolated in the evaluation device 14 from height signals determined along the measuring section.

[0044] If the sampling in the x-direction is, for example, in Fig. If measurements 2 are taken from left to right and the extrapolation of the contour path reveals that the object's height decreases locally, the measurement window is moved towards object 6, i.e., in the Fig. 2 downwards, shifted. The resulting measurement window is in Fig. 2, designated with reference numeral 20.

[0045] If the extrapolation of the contour shows that the object's height decreases locally, the measuring window is moved further away from the measuring device 2, i.e., in Fig. 2 downwards, shifted. The resulting measurement windows are in Fig. Reference numeral 22, 24 is used for reference numeral 26. Reference numeral 26 designates a measuring window at the end of the measuring section.

[0046] If an extrapolation of the contour profile were to show that the surface shape rises locally, the scan area would extend away from object 6, i.e., towards the measuring device 2. Fig. 2 also shifted upwards.

[0047] In the illustrated embodiment, the height of the scan area 18 or 20 to 26 is fixed. However, in a variation of this embodiment, the height of the scan area can be varied, at least section by section, along the measuring path depending on the height signals. In this way, for example, a roughness measurement can still be carried out with a relatively low measurement window even if the surface shape of surface 4 rises or falls sharply in some areas.

[0048] If, despite the tracking of the scan area in the measuring direction along the measuring path according to the invention, the surface should end up lying outside the scan area, the corresponding measuring process can be repeated with appropriate, if necessary local, adjustment of the scan area.

[0049] As from Fig.As can be seen in Figure 2, the height 28 of the scan area in the z-direction is significantly smaller than the total height 30 of the height range scanned in the z-direction. Because, compared to known methods, only a limited height range needs to be scanned by varying the focus of the microscope objective, the measurement speed of the measuring device 2 according to the invention is significantly increased without any limitations regarding measurement accuracy.

[0050] The inventive method or measuring device 2 thus enables the three-dimensional measurement of surfaces in a fast and accurate manner, for example for roughness measurement.

Claims

[1] Method for determining the contour of an object's surface along a measuring distance, in which the focus of a microscope objective relative to the surface is varied along a measuring axis (z-axis) over a scan area at measuring points spaced apart from each other along the measuring path and in which height signals representing the height of the surface are derived from images taken during variations in focus, characterized by , that along the measuring path the scan area in Rich- The measurement axis (z-axis) of the contour is automatically tracked, with the scan area being shifted towards the object in the measurement direction when the height signals determined along the measurement path indicate that the object's height is locally decreasing, and the scan area being shifted away from the object in the measurement direction when the height signals determined along the measurement path indicate that the object's height is locally increasing. that the contour of the surface is extrapolated from height signals determined along the measuring section to track the scan area. [2] Method according to claim 1, characterized by that along the measuring path the scan area in the measuring direction (z-direction) has a constant or approximately constant height. [3] Method according to claim 1 or 2, characterized bythat, at least in sections along the measurement route, the height of the scan area is varied depending on the height signals. [4] Method according to any one of the preceding claims, characterized by , that for three-dimensional measurement of the surface, it is scanned in a plane perpendicular to the measuring axis. [5] Method according to any one of the preceding claims, characterized by , that the focus of the microscope objective is automatically varied. [6] Method according to any one of the preceding claims, characterized by , that the shape and / or the waviness and / or the roughness of the surface is measured. [7] Measuring device for determining the contour of an object's surface along a measuring distance, with a microscope objective, with a focusing device that varies the focusing of the microscope objective relative to the surface along a measuring axis (z-axis) over a scan area at measuring points spaced apart from each other along the measuring path, with an image acquisition device for capturing images of the surface while varying the focus and with an evaluation device that derives height signals representing the height of the surface by evaluating the recorded images, characterized by a control device (16) configured to control the focusing device (10) such that the scan area is automatically adjusted along the measuring path in the direction of the measuring axis (z-axis) of the contour, wherein the control device (16) controls the focusing device (10) such that the scan area is shifted towards the object (6) in the measuring direction when it is determined, based on the height signals determined along the measuring path, that the height of the object (6) is locally decreasing, and that the scan area is shifted away from the object (6) in the measuring direction when it is determined, based on the height signals determined along the measuring path, that the height of the object (6) is locally increasing. wherein, to track the scan area, the evaluation device (14) extrapolates the contour of the surface (4) from height signals determined along the measuring distance. [8] Measuring device according to claim 7, characterized by that the image acquisition device (12) has at least one digital image sensor. [9] Measuring device according to claim 7 or 8, characterized by that the focusing device (10) has at least one piezo drive. [10] Measuring device according to any one of claims 7 to 9, characterized by , that the focusing device (10) automatically varies the focus of the microscope objective (8). [11] Use of a measuring device according to any one of claims 7 to 10 for measuring the shape and / or waviness and / or roughness of a surface.

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