Surface topography detection system and correction method thereof

Abstract

This invention discloses a surface topography detection system and its correction method. The surface topography detection system includes a reference light module, a backlight module, a lens module, and a processing unit. In a first mode, the reference light module receives coaxial light from an objective lens and reflects a portion of the coaxial light to generate a reference light. In a second mode, the backlight module provides uniformly diffused light, which passes through a photomask with a preset pattern to generate patterned light. In the first mode, the lens module captures the reference light and a portion of the coaxial light reflected from the photomask to generate an interference image; in the second mode, it captures the patterned light passing through the photomask to generate a patterned image. The processing unit calculates the error value between the center pixel positions of the interference image and the patterned image using image processing, and generates a correction command based on the error value to correct the reference light module.

Description

[Technical Field]

[0001] This invention relates to an optical system and its calibration method, and more particularly to an optical system and its calibration method for surface topography detection. [Previous Technology]

[0002] As shown in Figure 1, a conventional surface topography detection system 9 includes a light source 90, an objective lens 91, a beam splitter 92, a reflecting mirror 93, and a camera 94. The light source 90 provides coaxial light L1, and the objective lens 91 receives the coaxial light L1 and focuses it at a preset focal length. Generally, the beam splitter 92 is positioned after the objective lens 91 to split the coaxial light L1 into two rays, namely coaxial light L2 and coaxial light L3. When detecting the surface topography of an object, coaxial light L2 is incident on the surface A of the object, and after being reflected by surface A, coaxial light L2 returns to the objective lens 91. Similarly, coaxial light L3 is incident on the reflecting mirror 93, and after being reflected by the reflecting mirror 93, coaxial light L3 also returns to the objective lens 91. At this time, the reflected coaxial light L2 and the reflected coaxial light L3 will interfere with each other on the same optical path (for example, producing interference fringes), and the camera 94 behind the objective lens 91 is used to capture the interference image produced by the reflected coaxial light L2 and the reflected coaxial light L3.

[0003] However, due to thermal expansion and contraction or various undesirable factors, the traditional surface topography detection system 9 may cause the beam splitter 92 and the reflecting mirror 93 to deviate from their preset positions, resulting in measurement errors. Therefore, the industry needs to frequently calibrate the surface topography detection system 9 to ensure that the optical path of the coaxial light L3 reflected by the beam splitter 92 and the reflecting mirror 93 is fixed. Generally, when calibrating the surface topography detection system 9, the position of surface A needs to be fixed in advance. Then, by comparing the contrast and sharpness of the interference fringes in the interference image through image analysis, it can be determined whether the beam splitter 92 and the reflecting mirror 93 of the surface topography detection system 9 have deviated from their preset positions. However, the traditional calibration method can only repeatedly move the beam splitter 92 and the reflecting mirror 93 to find the position with the best contrast and sharpness of the interference fringes. Not only is the calibration efficiency poor, but the accuracy of the measurement also has room for improvement. Therefore, the industry needs a new optical system and its calibration method to improve the calibration efficiency of surface topography detection systems and to quickly determine the optimal position for contrast and sharpness during calibration, thereby reducing measurement errors. [Summary of the Invention]

[0004] The present invention provides a correction method for a surface topography detection system, which determines how to adjust the reference light module by comparing the interferometric image obtained in the first mode and the pattern image obtained in the second mode, thereby quickly completing the correction.

[0005] This invention proposes a correction method for a surface topography detection system. The surface topography detection system includes an objective lens, a reference light module, a backlight module, a lens module, a processing unit, and a photomask. The correction method for the surface topography detection system includes the following steps: in a first mode, coaxial light is emitted from the objective lens; in the first mode, the coaxial light reflected by the reference light module is used to provide reference light; in the first mode, the lens module captures the reference light and the coaxial light reflected by the photomask to generate an interference image; in a second mode, uniformly diffused light is emitted from the backlight module; in the second mode, the uniformly diffused light is passed through a photomask with a preset pattern to generate patterned light; in the second mode, the lens module captures the patterned light passing through the photomask to generate a patterned image; and the processing unit calculates the error value of the center pixel position between the interference image and the patterned image using image processing, and generates a correction command based on the error value to correct the reference light module.

[0006] In some embodiments, the center pixel position of the pattern image can be used to indicate the focus position of the pattern light. The processing unit calculates the focus position of the reference light based on the center pixel position of the pattern image and the center pixel position of the interference image. Furthermore, the correction command can adjust the positions of the beam splitter and the reflecting mirror in the reference light module to change the optical path length of the reference light. Additionally, the angle between the upper surface of the photomask and the optical central axis of the objective lens can be less than 90 degrees, and the preset pattern of the photomask can be multiple stripes arranged periodically.

[0007] The present invention provides a surface topography detection system that determines how to adjust the reference light module by comparing an interference image obtained in a first mode with a pattern image obtained in a second mode, thereby quickly completing the correction.

[0008] This invention proposes a surface topography detection system, which operates in a first mode and a second mode during calibration. The surface topography detection system includes a reference light module, a backlight module, a lens module, and a processing unit. The reference light module receives coaxial light from an objective lens in the first mode and reflects a portion of the coaxial light to generate a reference light. The backlight module provides uniformly diffused light in the second mode, which generates patterned light after passing through a photomask with a preset pattern. In the first mode, the lens module captures the reference light and a portion of the coaxial light reflected by the photomask to generate an interference image; in the second mode, it captures the patterned light passing through the photomask to generate a patterned image. The processing unit calculates the error value between the center pixel positions of the interference image and the patterned image using image processing, and generates a calibration command based on the error value to calibrate the reference light module.

[0009] In some embodiments, a photomask is disposed between the backlight module and the objective lens on the optical central axis. The photomask has an upper side surface, the angle between the normal direction of the upper side surface and the optical central axis is less than 90 degrees, and the preset pattern of the photomask can be multiple stripes arranged periodically. Furthermore, the center pixel position of the pattern image can be used to indicate the focusing position of the pattern light. The processing unit calculates the focusing position of the reference light based on the center pixel position of the pattern image and the center pixel position of the interference image. Additionally, the reference light module can include a beam splitter and a reflecting mirror. The beam splitter reflects a portion of the coaxial light to the reflecting mirror, and the reflecting mirror reflects the portion of the coaxial light to generate reference light. The calibration command can adjust the positions of the beam splitter and the reflecting mirror to change the optical path length of the reference light or the object light being measured.

[0010] In summary, unlike traditional correction methods that can only analyze interferometric images for correction, the surface topography detection system and its correction method provided by the present invention can determine how to adjust the reference light module by analyzing interferometric images and pattern images, thereby quickly completing the correction.

Implementation Method

[0019] The features, objectives and functions of the present invention will be further disclosed below. However, the following description is merely an embodiment of the present invention and should not be construed as limiting the scope of the present invention. Any equivalent changes and modifications made in accordance with the claims of the present invention shall still be considered as the essence of the present invention and shall not depart from the spirit and scope of the present invention. Therefore, they should be regarded as further embodiments of the present invention.

[0020] Please refer to Figure 2, which is a schematic diagram illustrating a surface topography detection system according to an embodiment of the present invention. As shown in Figure 2, the surface topography detection system 1 includes a light source 10, an objective lens 11, a reference light module 12, a backlight module 13, a photomask 14, a lens module 15, and a processing unit 16. The light source 10 is used to direct coaxial light toward the objective lens 11, and the objective lens 11 is used to focus the coaxial light. The reference light module 12 is provided on the optical path of the coaxial light, and the reference light module 12 includes a beam splitter 120 and a reflecting mirror 121. The beam splitter 120 is used to reflect a portion of the coaxial light toward the reflecting mirror 121, and the beam splitter 120 can also allow another portion of the coaxial light to pass through. The backlight module 13 is used to emit uniformly diffused light, and the backlight module 13 is disposed behind the photomask 14. The aperture 14 can be defined with two sides: an upper side 14a and a lower side 14b. The upper side 14a faces the objective lens 11, and the lower side 14b faces the backlight module 13. Taking the vertical direction M in Figure 2 as the optical central axis as an example, the light source 10 and the aperture 14 should be on opposite sides of the objective lens 11, and the normal direction N of the upper side 14a of the aperture 14 will be slightly tilted to the optical path of the coaxial light, which can be represented by the vertical direction M shown in Figure 2. The lens module 15 can have a beam splitter 150 and a camera 151 to capture the light beam returning from the objective lens 11. The processing unit 16 can be electrically connected to the camera 151 to analyze the image acquired by the camera 151.

[0021] In one example, the normal direction N and the vertical direction M of the upper side surface 14a of the photomask 14 should have an angle θ (non-parallel). Here, since the backlight module 13 is flat (its normal direction overlaps the vertical direction M), assuming the photomask 14 can be a flat structure, when the upper side surface 14a and the lower side surface 14b are parallel to each other, the angle θ between the normal direction N and the vertical direction M should also be equal to the angle θ between the backlight module 13 and the lower side surface 14b of the photomask 14. Furthermore, the angle θ between the normal direction N and the vertical direction M is an acute angle, for example, the angle θ can be 5 degrees, 15 degrees, 30 degrees or 45 degrees, and this embodiment is not limited thereto. Since the focal point (or focal plane) of the objective lens 11 is located on the vertical direction M (optical central axis) and at a known fixed distance from the objective lens 11, in practice, the photomask 14 can be approximately positioned at the focal length of the objective lens 11 so that the focal point of the objective lens 11 falls on the upper side surface 14a.

[0022] Those skilled in the art will understand that adjusting the included angle θ can also adjust the position of the focal point of the objective lens 11 on the upper side 14a. For example, adjusting the included angle θ to be larger (the upper side 14a is more tilted) will make the upper side 14a closer to the objective lens 11, allowing the intersection of the focal plane of the objective lens 11 and the upper side 14a (the focusing position) to be closer to the right of Figure 2. Conversely, adjusting the included angle θ to be smaller (the upper side 14a is less tilted) will make the upper side 14a farther away from the objective lens 11, allowing the intersection of the focal plane of the objective lens 11 and the upper side 14a (the focusing position) to be closer to the left of Figure 2. Furthermore, taking the vertical direction M in Figure 2 as the optical center axis as an example, the objective lens 11 and the backlight module 13 should be on opposite sides of the photomask 14. The upper side 14a of the photomask 14 can have a chrome-plated layer, so as to reflect the light from the objective lens 11 onto the upper side 14a, and the photomask 14 also allows the light from the backlight module 13 onto the lower side 14b to pass through.

[0023] When calibrating the surface topography detection system 1, this embodiment sequentially switches the light source 10 and the backlight module 13 on, defining the first mode as turning on the light source 10 and turning off the backlight module 13, and the second mode as turning on the backlight module 13 and turning off the light source 10. For ease of explanation of how to calibrate the surface topography detection system 1, please refer to Figures 2 and 3 together. Figure 3 is a schematic diagram illustrating the operation of the surface topography detection system according to an embodiment of the present invention in the first mode. In this example, when the surface topography detection system 1 operates in the first mode, the light source 10 is on, while the backlight module 13 is off. As shown in the figure, Figure 3 uses dotted chain lines to represent the optical path. The coaxial light L1 emitted from the light source 10 enters the objective lens 11 and is directed towards the reference light module 12 via the objective lens 11. At this time, the coaxial light L1 will be split into coaxial light L2 and coaxial light L3 by the beam splitter 120 of the reference light module 12. The coaxial light L2 passes through the reference light module 12 and is directed towards the upper side 14a of the photomask 14, while the coaxial light L3 is reflected by the beam splitter 120 towards the reflecting mirror 121.

[0024] In detail, after coaxial light L2 is incident on the upper side 14a of the photomask 14, it is reflected back by the upper side 14a along the original optical path. Similarly, after coaxial light L3 is incident on the reflecting mirror 121, it is reflected back by the reflecting mirror 121 along the original optical path. As those skilled in the art will understand, since coaxial light L2 and coaxial light L3 are both part of coaxial light L1 before entering the beam splitter 120, the optical paths of coaxial light L2 and coaxial light L3 entering the beam splitter 120 are the same. And, since the optical paths of coaxial light L2 and coaxial light L3 after returning to the beam splitter 120 are also the same when they are incident on the lens module 15, the difference in optical path between coaxial light L2 and coaxial light L3 lies in the difference in distance from the beam splitter 120 to the upper side 14a and the reflecting mirror 121. In other words, assuming the distance from beam splitter 120 to the upper side 14a is D1, the optical path of coaxial light L2 is 2D1 plus the optical path of the same optical path. Assuming the distance from beam splitter 120 to reflecting mirror 121 is D2, the optical path of coaxial light L3 is 2D2 plus the optical path of the same optical path.

[0025] Generally, the distance between the beam splitter 120 and the reflecting mirror 121 should be preset and fixed, that is, the coaxial light L3 passing through the reflecting mirror 121 can also be called the reference light. In addition, due to the optical path difference between the coaxial light L2 and the reference light, interference phenomena will occur. Therefore, when the coaxial light L2 and the reference light are received by the camera 151 of the lens module 15, interference fringes will be seen on the interference image. For ease of explanation, please refer to Figures 3 and 4 together. Figure 4 is a schematic diagram of an interference image according to an embodiment of the present invention. The upper part of Figure 4 illustrates a schematic diagram of the interference image image1, and it can be seen that there are bright and dark interference fringes in the interference image image1. After the image is processed by the processing unit 16, the processing unit 16 can convert the interference fringes in the interference image image1 into a distribution map of contrast and pixel position, and can calculate which pixel position has the highest contrast. For example, assuming that pixel position p1 has the highest contrast, pixel position p1 can be called the center pixel position of the interference image. Of course, this embodiment does not limit how the processing unit 16 converts the interferometric image image1 into a distribution map of grayscale values ​​and pixel positions. Those skilled in the art can freely use image processing methods to accomplish this.

[0026] After obtaining the center pixel position of the interference image, this embodiment can perform another step. Please refer to Figures 2 and 5 together. Figure 5 is a schematic diagram illustrating the operation of the surface topography detection system according to an embodiment of the present invention in the second mode. In this example, when the surface topography detection system 1 operates in the second mode, the light source 10 is off, while the backlight module 13 is on. As shown, the uniformly diffused light provided by the backlight module 13 is emitted through the upper side 14a of the photomask 14, and is received by the camera 151 of the lens module 15 after passing through the objective lens 11. In practice, the photomask 14 may have a preset pattern, such as multiple stripes arranged periodically or other periodic patterns, which is not limited in this embodiment. Thus, the uniformly diffused light passing through the photomask 14 has the optical characteristics of the preset pattern, which is referred to as patterned light in this embodiment. In addition, since the interference signal formed by the patterned light has negligible contrast under the strong light of the backlight module, the poor contrast is negligible.

[0027] To facilitate the explanation of the significance of the patterned light, please refer to Figures 5 and 6 together. Figure 6 is a schematic diagram illustrating a patterned image according to an embodiment of the present invention. The upper part of Figure 6 illustrates a schematic diagram of the patterned image image2. Assuming that the preset pattern is a series of periodically arranged stripes, it can also be seen that there are alternating bright and dark stripes in the patterned image image2. In practice, when the objective lens 11 is focusing, the brightness of the patterned light beam emitted from the upper side 14a corresponding to the focusing position is the greatest, while the brightness of the patterned light beam emitted from other positions on the upper side 14a is relatively small. When the patterned light is received by the lens module 15, a pattern reflecting the optical characteristics of the preset pattern will be seen on the patterned image. Those skilled in the art will understand that if the photomask 14 does not have a preset pattern, the difference in brightness between the beams emitted from the focusing and non-focusing positions may not be obvious, and in particular, it may be difficult to judge the brightness after the lens module 15 receives the light.

[0028] Similarly, after image processing by processing unit 16, processing unit 16 can convert the bright and dark stripes in the pattern image image2 into a distribution map of contrast and pixel positions, and can also calculate which pixel position has the highest contrast. For example, assuming that pixel position p2 has the highest contrast, pixel position p2 can be called the center pixel position of the pattern image. In one example, pixel position p2 can also be regarded as the position where the focus of objective lens 11 is reflected on the photosensitive element.

[0029] It is worth mentioning that this embodiment does not limit the order in which the surface topography detection system 1 operates in the first mode and the second mode. As long as the lens module 15 can acquire the interference image image1 and the pattern image image2 respectively, and the processing unit 16 can calculate the center pixel positions of the interference image and the pattern image, it should fall within the scope of this embodiment. Furthermore, during correction, after the processing unit 16 calculates the center pixel positions of the interference image and the pattern image, it can generate a correction command to adjust the reference light module 12 based on the difference (error value) between the center pixel positions of the interference image and the pattern image.

[0030] In detail, the center pixel position of the pattern image is related to the focal length of the objective lens 11, and therefore should be fixed rather than calibrated. Furthermore, since the focal length of the objective lens 11 should be on the optical central axis, the pattern image should also be brightest at the corresponding optical central axis. In practice, if the optical path lengths of coaxial light L2 and coaxial light L3 are the same, for example, when D1 equals D2, then the center pixel position of the interference image should be the same as the center pixel position of the pattern image. This is because when D1 equals D2, the interference image should have the largest constructive interference fringes (brightest fringes) at the optical central axis, and thus the center pixel position of the interference image will overlap with the center pixel position of the pattern image.

[0031] It should be corrected that the center pixel position of the interferometric image fails to overlap with the center pixel position of the pattern image due to thermal expansion and contraction or other undesirable factors. Please refer to Figures 3 and 7 together. Figure 7 is a schematic diagram illustrating the comparison of an interferometric image and a pattern image according to an embodiment of the present invention. Taking the example of Figure 7, when the center pixel position (pixel position p1) of the interferometric image is not equal to the center pixel position (pixel position p2) of the pattern image, for example, when the difference between pixel position p1 and pixel position p2 is D3, it can be immediately determined that the surface topography detection system 1 has a problem where D1 is not equal to D2. At this time, the surface topography detection system 1 only needs to move the reference light module 12 upward or downward along the optical central axis, that is, increase D1 (decrease D2) or decrease D1 (increase D2), so that pixel position p1 overlaps pixel position p2, and the correction can be completed very quickly.

[0032] To facilitate the explanation of the correction method of the surface topography detection system of the present invention, please refer to Figures 2 to 8. Figure 8 is a flowchart illustrating the steps of the correction method of the surface topography detection system according to an embodiment of the present invention. As shown in the figure, in step S20, in the first mode, the objective lens 11 emits coaxial light L1 generated by the light source 10. In step S21, in the first mode, the coaxial light L3 reflected by the reference light module 12 is used to provide reference light. In step S22, the lens module 15 captures the reference light and the coaxial light L2 reflected by the photomask in the first mode to generate an interference image. In step S23, in the second mode, the backlight module 13 emits uniformly diffused light. In step S24, in the second mode, the uniformly diffused light is passed through the photomask 14 with a preset pattern to generate patterned light. In step S25, the lens module 15 captures the patterned light passing through the photomask 14 in the second mode to generate a patterned image. In step S26, the processing unit 16 calculates the error value of the center pixel position of the interference image and the pattern image by image processing, and generates a correction command based on the error value to correct the reference light module 12.

[0033] In summary, unlike traditional surface topography detection systems, which are not easy to determine whether the reference light module has errors and cannot quickly correct the position of the reference light module, the surface topography detection system demonstrated in this embodiment only needs to sequentially switch on the light source and the backlight module. It can quickly assist the surface topography detection system in making corrections by comparing the center pixel positions of the interference image and the pattern image. [Simplified Explanation of the Diagram]

[0011] Figure 1 is a schematic diagram of a traditional surface morphology detection system.

[0012] Figure 2 is a schematic diagram illustrating a surface topography detection system according to an embodiment of the present invention.

[0013] Figure 3 is a schematic diagram illustrating the operation of a surface topography detection system in a first mode according to an embodiment of the present invention.

[0014] Figure 4 is a schematic diagram illustrating an interference image according to an embodiment of the present invention.

[0015] Figure 5 is a schematic diagram illustrating the operation of a surface topography detection system in a second mode according to an embodiment of the present invention.

[0016] Figure 6 is a schematic diagram illustrating a pattern image according to an embodiment of the present invention.

[0017] Figure 7 is a schematic diagram illustrating the comparison of interferometric images and pattern images according to an embodiment of the present invention.

[0018] Figure 8 is a flowchart illustrating the steps of a correction method for a surface topography detection system according to an embodiment of the present invention. [Biomaterial Storage]

[0035] None

Claims

1. A calibration method for a surface topography detection system, the surface topography detection system comprising an objective lens, a reference light module, a backlight module, a lens module, a processing unit, and a photomask, the calibration method comprising: emitting coaxial light provided by a light source from the objective lens in a first mode; in the first mode, reflecting a portion of the coaxial light from the reference light module to provide a reference light; capturing the reference light and a portion of the coaxial light reflected by the photomask from the lens module in the first mode to generate an interference image; emitting uniformly diffused light from the backlight module in a second mode; in the second mode, passing the uniformly diffused light through the photomask having a preset pattern to generate patterned light. In the second mode, the lens module captures the patterned light passing through the photomask to generate a patterned image; and the processing unit calculates an error value between the center pixel position of the interference image and the patterned image using image processing, and generates a correction command based on the error value to correct the reference light module; wherein turning on the light source and turning off the backlight module is the first mode, and turning on the backlight module and turning off the light source is the second mode.

2. The correction method of the surface topography detection system as described in claim 1, wherein the center pixel position of the pattern image is used to indicate the focus position of the pattern light, and the processing unit calculates the focus position of the reference light based on the center pixel position of the pattern image and the center pixel position of the interference image.

3. The calibration method for the surface topography detection system as described in claim 1, wherein the calibration command is linked to adjust the positions of a beam splitter and a reflecting mirror in the reference light module to change the optical path length of the reference light.

4. The calibration method for the surface topography detection system as described in claim 1, wherein the angle between an upper side of the photomask and an optical central axis of the objective lens is less than 90 degrees, and the preset pattern of the photomask is a plurality of stripes arranged periodically.

5. A surface topography detection system, operating in a first mode and a second mode during calibration, the surface topography detection system comprising: a reference light module for receiving coaxial light from an objective lens in the first mode and reflecting a portion of the coaxial light to generate a reference light; a backlight module for providing uniformly diffused light in the second mode, the uniformly diffused light passing through a photomask having a preset pattern to generate a patterned light; a lens module for capturing the reference light and a portion of the coaxial light reflected by the photomask in the first mode to generate an interference image, and capturing the patterned light passing through the photomask in the second mode to generate a patterned image; and a processing unit for calculating an error value of the center pixel position of the interference image and the patterned image using image processing, and generating a calibration command based on the error value to calibrate the reference light module; wherein the coaxial light is provided by a light source, the first mode is characterized by turning on the light source and turning off the backlight module, and the second mode is characterized by turning on the backlight module and turning off the light source.

6. The surface topography detection system as claimed in claim 5, wherein the photomask is disposed between the backlight module and the objective lens on an optical central axis, the photomask defining an upper side surface, the angle between a normal direction of the upper side surface and the optical central axis being less than 90 degrees.

7. The surface topography detection system as described in claim 6, wherein the preset pattern of the photomask is a plurality of stripes arranged periodically.

8. The surface topography detection system as claimed in claim 5, wherein the center pixel position of the pattern image is used to indicate the focus position of the pattern light, and the processing unit calculates the focus position of the reference light based on the center pixel position of the pattern image and the center pixel position of the interference image.

9. The surface topography detection system as claimed in claim 5, wherein the reference light module includes a beam splitter and a reflector, the beam splitter reflecting a portion of the coaxial light to the reflector, and the reflector reflecting the portion of the coaxial light to generate the reference light.

10. The surface topography detection system as described in claim 9, wherein the correction command is linked to adjust the positions of the beam splitter and the reflecting mirror to change the optical path length of the reference light.

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