CN114910015A - Reconstruction method of white light interference signal - Google Patents
Reconstruction method of white light interference signal Download PDFInfo
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Abstract
The present disclosure describes a method for reconstructing a white light interference signal, comprising: obtaining a target light beam; enabling the target light beam to reach a target point through an interference objective lens, and forming a target reflected light beam at the interference objective lens, wherein the target reflected light beam comprises a first reflected light beam matched with the first light beam and a second reflected light beam matched with the second light beam; obtaining a white light interference signal based on the first reflected light beam and obtaining a compensation interference signal based on the second reflected light beam; obtaining the initial height of the target point based on the white light interference signal, and obtaining the compensation information of the target point based on the compensation interference signal; and compensating the initial height based on the compensation information to obtain a comprehensive measured height of the target point. According to the present disclosure, a reconstruction method capable of compensating for a measurement height of a target point to improve measurement accuracy of the target point can be provided.
Description
Technical Field
The present disclosure relates to an intelligent manufacturing equipment industry, and more particularly, to a method for reconstructing a white light interference signal.
Background
With the development of ultra-precision machining technology, ultra-precision detection technology is also becoming more important. The optical measurement method is widely applied to the field of optical measurement due to the advantages of low cost and high precision. Among them, an optical measurement system based on the interference principle has the advantages of high accuracy and high resolution, and is often used for accurate measurement of physical quantities. The white light interferometer is an ultra-precise measuring device based on a white light interferometry technology, and the measuring precision and accuracy of the white light interferometer are obviously influenced by the environment. Generally speaking, the environmental impact includes two aspects of industrial environmental impact and natural environmental impact, wherein the industrial environmental impact refers to that the ground generates low-frequency vibration due to vibration caused by people and surrounding environment, and then the low-frequency vibration is transferred to the interferometer; the natural environment influence refers to that the interferometer generates certain vibration due to the change of the natural environment, such as air flow, temperature change and the like. The two types of vibration caused by environmental influence belong to environmental vibration. When environmental vibration exists, white light interference fringes generated based on a white light interferometer are prone to generate jitter, for example, interference fringe images are mutually overlapped, deformed, repaired, blurred or burred, and the like, so that a measurement result has large errors; furthermore, the large amplitude of the fringe fluctuation of the plurality of interference patterns will make the interference fringes invisible and impossible to measure.
In the prior art, technologies such as mechanical vibration isolation and synchronous phase shift interference are usually adopted to reduce the influence of environmental vibration on a white light interference measurement result, and a bearing platform with characteristics of good shock resistance, high rigidity and the like is designed from the mechanical structure angle in the mechanical vibration isolation technology to reduce the influence of environmental vibration factors on white light interference microscopic imaging. However, due to the anti-vibration design of the structure, the device is large in size and heavy in weight, the measurement precision is low, the contribution to reconstruction of white light interference signals is limited, and the microscopic 3D appearance of the sample cannot be effectively reconstructed under the environment noises of large vibration, ultralow frequency and high frequency. The synchronous phase shift interference technology is used for simultaneously and rapidly acquiring each frame interferogram with the phase difference of pi/2 by using four CCDs at four spatial positions so as to reduce the influence of environmental vibration on phase shift. The method has high requirement on the consistency of the photoelectric properties of the CCD and each optical device, and the measuring system has complex structure, high control difficulty and high price.
Disclosure of Invention
The present disclosure has been made in view of the above-mentioned state of the art, and an object thereof is to provide a method for reconstructing a white light interference signal capable of compensating for a measurement height of a target point to improve measurement accuracy of the target point.
The present disclosure provides a method for reconstructing a white light interference signal, which is applied to a reconstruction apparatus for measuring a height of a target point of an object to be measured to reconstruct the object to be measured, and the reconstruction method includes: obtaining a target light beam, wherein the target light beam comprises a first light beam and a second light beam, the bandwidth of the first light beam is not less than a first preset value, the bandwidth of the second light beam is not more than a second preset value, and the first preset value is not less than the second preset value; enabling the target light beam to reach the target point through an interference objective lens, and forming a target reflected light beam at the interference objective lens, wherein the target reflected light beam comprises a first reflected light beam matched with the first light beam and a second reflected light beam matched with the second light beam; obtaining a white light interference signal based on the first reflected light beam and obtaining a compensation interference signal based on the second reflected light beam; obtaining the initial height of a target point based on the white light interference signal, and obtaining compensation information of the target point based on the compensation interference signal; and compensating the initial height based on the compensation information to obtain a comprehensive measured height of the target point.
According to the reconstruction method disclosed by the disclosure, the initial height of the target point can be obtained based on the first light beam, and the initial height is compensated based on the second light beam to obtain the comprehensive measurement height of the target point, so that the comprehensive measurement precision of the reconstruction device can be improved, and the reconstruction precision of the reconstruction device can be further improved.
Further, in the reconstruction method according to the present disclosure, optionally, the first beam and the second beam are coupled as the target beam. In this case, the first light beam and the second light beam can be combined into one light beam (i.e., the target light beam) through coupling, which is beneficial to improving the propagation stability of the light beams.
In addition, in the reconstruction method according to the present disclosure, optionally, the target beam is reflected by the first beam splitting unit to the interference objective lens and reaches the target point, the target beam forms a target reflected beam at the interference objective lens, and the target reflected beam reaches the second beam splitting unit through the first beam splitting unit. In this case, the object beam can smoothly reach the interference objective lens and form the object reflected beam by the interference objective lens, whereby the second beam splitting unit can receive the object reflected beam.
In addition, in the reconstruction method according to the present disclosure, optionally, the target reflected light beam is decoupled into a first reflected light beam and a second reflected light beam by the second light splitting unit, the first reflected light beam is transmitted to the first receiving unit, and the second reflected light beam is reflected to the second receiving unit. In this case, the target reflected light beam can be decoupled into a first reflected light beam matched with the first light beam and a second reflected light beam matched with the second light beam based on the second light splitting unit, and then the first receiving unit can convert the first reflected light beam from an optical signal into an electrical signal, and the second receiving unit can convert the second reflected light beam from an optical signal into an electrical signal.
In addition, in the reconstruction method according to the present disclosure, optionally, the first receiving unit obtains a white light interference signal based on the received first reflected light beam, and the second receiving unit obtains a compensation interference signal based on the received second reflected light beam. In this case, according to the reconstruction method of the present disclosure, it is possible to obtain the initial height of the target point based on the white light interference signal and obtain the compensation information due to the environmental vibration based on the compensation interference signal.
In addition, in the reconstruction method according to the present disclosure, optionally, the first receiving unit receives a first reflected light beam at a preset frame rate to obtain the white light interference signal, and the second receiving unit receives a second reflected light beam at the preset frame rate to obtain the compensation interference signal. In this case, the first receiving unit and the second receiving unit can receive the optical signal synchronously, so that the compensation information obtained based on the second light beam can be matched with the initial height obtained based on the first light beam, and the measurement accuracy of the target point is improved.
In addition, in the reconstruction method according to the present disclosure, optionally, the compensation information of the target point obtained based on the compensation interference signal satisfies a formula:
wherein epsilon j Is the compensation information of the target point in the jth frame, δ j Is the phase change, λ, of the target point at the jth frame due to vibration monitor Is the center wavelength of the second light beam. Thereby, a specific magnitude of the compensation information can be obtained.
In addition, in the reconstruction method according to the present disclosure, optionally, the integrated measured height is obtained by superimposing the initial height and the compensation information. Thus, after obtaining the compensation information, the integrated measured height of the target point can be obtained based on the rule of superposition.
In addition, in the reconstruction method according to the present disclosure, optionally, a phase change of the target point at the j-th frame due to the vibration satisfies a formula:
wherein, I j Is to compensate the light intensity of the interference signal in the jth frame, I j-1 The intensity of the compensating interference signal in the (j-1) th frame, A is a direct current term coefficient of an intensity model of the compensating interference signal, and theta is the wavefront phase of the target point corresponding to the second receiving unit. This makes it possible to obtain a phase change of the target point 21 due to vibration in each frame.
In addition, in the reconstruction method according to the present disclosure, optionally, a wavefront phase of the second receiving unit corresponding to the object point satisfies a formula:
where N is the total number of frames collected, θ k Representing the phase change of the target point in the adjacent frame. This enables to obtain an accurate wavefront phase.
According to the present disclosure, a reconstruction method that compensates for a measured height of a target point to improve measurement accuracy of the target point can be provided.
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The disclosure will now be explained in further detail by way of example only with reference to the accompanying drawings.
Fig. 1 is a schematic diagram illustrating an application scenario of a reconstruction apparatus according to the present disclosure.
Fig. 2 is a block diagram showing the structure of a reconstruction device according to the present disclosure.
Fig. 3 is a schematic diagram showing the overall optical path structure of the reconstruction apparatus according to the present disclosure.
Fig. 4 is a flow chart diagram illustrating a reconstruction method according to the present disclosure.
Fig. 5 is a schematic diagram showing an internal optical path structure of an interference objective lens according to the present disclosure.
FIG. 6 is a schematic diagram illustrating white light interference signals and scan passes in accordance with the present disclosure.
FIG. 7 is a schematic diagram illustrating compensating interference signals and scan runs in accordance with the present disclosure.
Fig. 8 is a schematic diagram illustrating that the first receiving unit and the second receiving unit synchronously receive optical signals according to the present disclosure.
Fig. 9 is a schematic diagram showing a scanning time and a scanning stroke when scanning a target point according to the present disclosure.
Fig. 10 is a schematic flow chart illustrating obtaining compensation information according to the present disclosure.
FIG. 11a is a diagram showing the alignment of an analyte under a first condition based on conventional apparatus and methods
And (4) carrying out three-dimensional reconstruction to obtain a schematic diagram of the three-dimensional surface morphology.
Fig. 11b is a schematic diagram illustrating a three-dimensional surface topography obtained by three-dimensionally reconstructing an object under a first condition by using the reconstruction apparatus and the reconstruction method according to the present disclosure.
Fig. 12a is a schematic view showing a three-dimensional surface topography obtained by three-dimensional reconstruction of an object under test based on conventional apparatus and methods under a second condition.
Fig. 12b is a schematic diagram illustrating a three-dimensional surface topography obtained by three-dimensionally reconstructing the object under the second condition by the reconstruction apparatus and the compensation method according to the present disclosure.
Detailed Description
Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the following description, the same components are denoted by the same reference numerals, and redundant description thereof is omitted. The drawings are schematic and the ratio of the dimensions of the components and the shapes of the components may be different from the actual ones.
It is noted that the terms "comprises" and "comprising," and any variations thereof, in this disclosure, such that a process, method, apparatus, article, or apparatus that comprises or has a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include or have other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
In addition, the headings and the like referred to in the following description of the present disclosure are not intended to limit the content or scope of the present disclosure, but merely serve as a reminder for reading. Such a subtitle should neither be understood as a content for segmenting an article, nor should the content under the subtitle be limited to just the scope of the subtitle.
The present disclosure relates to a reconstruction method of a white light interference signal, which may sometimes be referred to as simply method or reconstruction method hereinafter. In some examples, the method for reconstructing a white light interference signal according to the present disclosure may also be referred to as a reconstruction method having a dual light path, a reconstruction method having a dual light source, a method for reconstructing an object to be measured based on a white light interference signal, a reconstruction method having a compensation function, or a reconstruction method having an anti-vibration function.
According to the reconstruction method, the initial height of the target point can be obtained based on the first light beam, and the initial height is compensated based on the second light beam to obtain the comprehensive measurement height of the target point, so that the comprehensive measurement precision of the reconstruction device can be improved, and the reconstruction precision of the reconstruction device can be improved.
The reconstruction method according to the present embodiment will be described in detail below with reference to the drawings.
Fig. 1 is a schematic diagram illustrating an application scenario of a reconstruction apparatus 10 according to the present disclosure. Fig. 2 is a block diagram showing the structure of the reconstruction device 10 according to the present disclosure. Fig. 3 is a schematic diagram showing the overall optical path structure of the reconstruction apparatus 10 according to the present disclosure.
The reconstruction method according to the present embodiment can be applied to a reconstruction apparatus 10 shown in fig. 1. In some examples, according to the reconstruction method according to the present embodiment, the height of the target point 21 of the object 20 may be measured. In some examples, the reconstruction method may measure the height of the target point 21 of the object 20 and reconstruct the object 20 (see fig. 3).
In some examples, the height of the target point 21 measured with a single beam may have errors due to the presence of environmental vibrations. Accordingly, reference to an initial height in this disclosure may refer to a measurement taken due to the presence of ambient vibrations, with the resulting initial height having a large error. Compensating for the initial height may refer to reducing errors due to environmental vibrations. In some examples, the height compensated for the initial height may be referred to as a composite measured height. That is, the reconstruction device 10 according to the present embodiment can improve the measurement accuracy of the target point 21.
In some examples, the error may refer to an error in a scanning direction, which may be referred to in fig. 3 (the direction Z points in fig. 3). In other words, the displacement amount (or translation amount) of the target point 21 in the scanning direction due to the environmental vibration is referred to as a translation amount.
In the present embodiment, measuring the height of the target point 21 may include obtaining an initial height of the target point 21 and compensating for the initial height to obtain a comprehensive measured height. Specifically, according to the reconstruction method according to the present embodiment, it is possible to obtain the initial height of the target point 21 based on the first light beam L10 and compensate the initial height based on the second light beam L20 to obtain the integrated measured height. (to be described later in detail)
In some examples, the analyte 20 may be referred to as a sample. The sample can be a semiconductor, a 3C electronic glass screen, a micro-nano material, an automobile part, an MEMS device or other ultra-precise devices. In some examples, the sample may be a device applied in the fields of aerospace, defense, military, and the like.
In some examples, the measurement point at which the reconstruction device 10 measures the object 20 may be referred to as a target point 21. I.e. the reconstruction means 10 may be used to measure the height of the target point 21. The target point 21 may be a point, a line, or an area. In some examples, test object 20 may include at least one target point 21. Measuring one target point 21 can obtain the height of one target point 21, and measuring multiple target points 21 can obtain the height of multiple target points 21. In this case, if only the height of one target point 21 needs to be obtained, the reconstruction device 10 only needs to measure one target point 21; when the reconstruction device 10 measures a plurality of target points 21 of the object 20, the three-dimensional surface topography of the object 20 can be reconstructed based on the heights of the plurality of target points 21. In some examples, the height may be a surface height of the test object 20.
Fig. 4 is a flow chart diagram illustrating a reconstruction method according to the present disclosure.
Referring to fig. 4, the reconstruction method according to the present embodiment may include obtaining a target light beam L1(S200), forming a target reflected light beam L1' based on the target light beam L1 (S400), obtaining a white light interference signal and a compensation interference signal (S600), obtaining an initial height of the target point 21 based on the white light interference signal, obtaining compensation information based on the compensation interference signal (S800), and compensating the initial height based on the compensation information to obtain a comprehensive measured height of the target point 21 (S1000).
In step S200, the target light beam L1 may be obtained. The target light beam L1 may include a first light beam L10 and a second light beam L20.
Referring to fig. 2, the reconstruction apparatus 10 may include a generation module 100. The generation module 100 may be used to generate the target light beam L1.
Referring to fig. 3, in some examples, the generation module 100 may include a first light source 110. The first light source 110 may be used to generate a first light beam L10. The generation module 100 may include a second light source 120. The second light source 120 may be used to generate a second light beam L20. The first light beam L10 and the second light beam L20 can be coupled as the target light beam L1. In this case, the first light beam L10 and the second light beam L20 may be combined into one light beam (i.e., the target light beam L1) through coupling, which is beneficial to improving the propagation stability of the light beams.
In some examples, the bandwidth L10 of the first light beam may be not less than the first preset value. The first preset value may be not less than 100 nm. In some examples, light having a bandwidth not less than a first preset value may be referred to as broadband light. Since broadband light has poor coherence and low coherence length, interference phenomenon is more obvious at a position where the optical path difference is shorter.
In some examples, the first light beam L10 generated by the first light source 110 may be white light. The first light source 110 may serve as a scanning light source for the measurement process. In some examples, the first light beam L10 generated by the first light source 110 may have a wavelength of 400nm-700nm, for example, 400nm, 450nm, 500nm, 550nm, 6000nm, 650nm, or 700 nm. In this case, it can be facilitated for the reconstruction device 10 to obtain image information of the surface of the object 20 to be measured during the measurement.
In some examples, the bandwidth of the second light beam L20 may not be greater than the second preset value. In some examples, the first preset value may be not less than the second preset value. For example, the second preset value may be 50 nm. In some examples, light having a bandwidth not greater than the second preset value may be referred to as narrow-band light. Since the coherence length of the narrow-band light is long, interference can occur throughout the height of the measurement target point 21.
In some examples, the second light beam L20 may be infrared light. In some examples, the wavelength of the second light beam L20 may be not less than 900nm, for example, the wavelength of the second light beam L20 may be 950nm, 980nm, 1000nm, or the like. In this case, the wavelength bands spanned by the first light beam L10 and the second light beam L20 have a sufficiently long interval so that the first light beam L10 and the second light beam L20 can have a higher degree of isolation, and the influence of the second light beam L20 on the first light beam L10 during measurement can be reduced.
Referring to fig. 3, in some examples, the generation module 100 may further include a third light distribution unit 130. The third light splitting unit 130 may be used to couple the first light beam L10 and the second light beam L20 as the target light beam L1.
In some examples, the third light splitting unit 130 may be a dichroic splitter. The third light distribution unit 130 may be disposed at 45 ° to the central axis of the first light source 110, and the central axis of the second light source 120 may be perpendicular to the central axis of the first light source 110. The dichroic beamsplitter can selectively reflect or transmit the light beam based on its wavelength. In this embodiment, the third light splitting unit 130 may be configured to transmit the first light beam L10, and the third light splitting unit 130 may be configured to reflect the second light beam L20. Thereby, the first light beam L10 and the second light beam L20 can be coupled as the target light beam L1 after reaching the third light splitting unit 130.
In some examples, the target light beam L1 may reach the interference objective 300 via the first light splitting unit 210 (described later). In some examples, the generation module 100 may further include a first lens unit 140 and a second lens unit 150. And the first and second lens units 140 and 150 may be disposed between the third light splitting unit 130 and the first light splitting unit 210. Here, the first lens unit 140 may be a condensing lens having a function of condensing a light beam, and the second lens unit 150 may be a collimating lens for converting the light beam into collimated light. In this case, the target light beam L1 can be converged into a beam after passing through the first lens unit 140 and then transmitted to the second lens unit 150, and the second lens unit 150 converts the converged target light beam L1 into collimated light and transmits the collimated light to the first light splitting unit 210, whereby the divergence of the target light beam L1 can be reduced and the energy loss of the target light beam L1 can be reduced. In some examples, the generation module 100 may not include the first lens unit 140 and the second lens unit 150.
In step S400, a target reflected light beam L1 'may be formed based on the target light beam L1'
Referring to fig. 2 and 3, the reconstruction apparatus 10 according to the present embodiment may further include a spectroscopic module 200 and an interference objective lens 300. The beam splitting module 200 may be used to change the propagation direction of the light beam, and the interference objective 300 may be used to form the target reflected light beam L1'.
In some examples, the target light beam L1 may be caused to reach the target point 21 via the interference objective lens 300 and form a target reflected light beam L1' at the interference objective lens 300. Specifically, the light splitting module 200 may include a first light splitting unit 210. The first light splitting unit 210 may be configured to receive the target light beam L1. The first light splitting unit 210 may be used to reflect the target light beam L1. In some examples, the first light splitting unit 210 may be used to receive the target light beam L1 and reflect the target light beam L1. Thereby, the target light beam L1 can further change the traveling direction of the target light beam L1 based on the setting of the first light splitting unit 210. In other words, the target light beam L1 may be reflected by the first light splitting unit 210 to the interference objective lens 300 and reach the target point 21.
Fig. 5 is a schematic diagram showing an internal optical path structure of an interference objective 300 according to the present disclosure.
Referring to fig. 5, in some examples, the interference objective 300 may include a fourth light splitting unit 310 and a reference unit 320. Wherein the fourth light splitting unit 310 may be configured to receive the target light beam L1 and split the target light beam L1 into the first and second sub-target light beams L30 and L40. In some examples, the fourth light splitting cell 310 may also be configured to reflect the first sub-target light beam L30 to the reference cell 320 and transmit the second sub-target light beam L40 to the target point 21.
In some examples, the first sub-targeting light beams L30 reflect the reference cell 320 via the fourth light splitting cell 310 and reflect by the reference cell 320 to form first sub-targeting reflected light beams L30'. In some examples, the second sub-targeting light beam L40 may be transmitted to the target point 21 via the fourth light-splitting cell 310 and reflected by the target point 21 to form a second sub-targeting reflected light beam L40'.
In some examples, the first and second sub-target reflected beams L30 ', L40 ' may form the target reflected beam L1 '. Specifically, the fourth light splitting unit 310 may be further configured to synthesize the first and second sub-target reflected light beams L30 ' and L40 ' to form the target reflected light beam L1 '. In other words, the target reflected light beam L1 ' may include a first sub-target reflected light beam L30 ' and a second sub-target reflected light beam L40 '.
In some examples, the target reflected light beam L1 ' may include a first reflected light beam L10 ' matched with the first light beam L10 and a second reflected light beam L20 ' matched with the second light beam L20.
Specifically, the first light beam L10, after reaching the interference objective lens 300, can be split by the fourth light splitting unit 310 into a first sub-beam reflected to the reference unit 320 and a second sub-beam transmitted to the target point 21, and the first sub-beam is reflected by the reference unit 320 to form a first sub-reflected light beam and the second sub-beam is reflected by the target point 21 to form a second sub-reflected light beam. The first sub reflected light beam and the second sub reflected light beam may be combined into the first reflected light beam L10' by the fourth light splitting unit 310.
Similarly, the second light beam L20 arriving at the interference objective lens 300 can be split by the fourth light splitting unit 310 into the third sub-beam reflected to the reference unit 320 and the fourth sub-beam transmitted to the target point 21, and the third sub-beam is reflected by the reference unit 320 to form the third sub-reflected light beam and the fourth sub-beam is reflected by the target point 21 to form the fourth sub-reflected light beam. The third sub reflected light beam and the fourth sub reflected light beam may be combined into the second reflected light beam L20' by the fourth light splitting unit 310.
In summary, the first sub-target light beams L30 may include a first sub-light beam and a third sub-light beam. The second sub-target light beams L40 may include a second sub-beam and a fourth sub-beam. The first sub-target reflected light beams L30' may include a first sub-reflected light beam and a third sub-reflected light beam. The second sub-target reflected light beams L40' may include a second sub-reflected light beam and a fourth sub-reflected light beam. And the first sub-reflected light beam and the second sub-reflected light beam may be combined into the first reflected light beam by the fourth light splitting unit 310. The third sub-reflected light beam and the fourth sub-reflected light beam may be combined into the second reflected light beam by the fourth light splitting unit 310. Thereby, the target reflected light beam L1 ' can include the first reflected light beam L10 ' matching the first light beam L10 and the second reflected light beam L20 ' matching the second light beam L20.
FIG. 6 is a schematic diagram illustrating white light interference signals and scan passes in accordance with the present disclosure. FIG. 7 is a schematic diagram illustrating compensating interference signals and scan runs in accordance with the present disclosure. Where I may be the light intensity and z may be the scanning stroke.
In step S600, a white light interference signal and a compensation interference signal may be obtained.
In some examples, a white light interference signal may be obtained based on the first reflected light beam L10 'and a compensating interference signal may be obtained based on the second reflected light beam L20'.
As described above, the first light beam L10 may be broadband white light. Since broadband light has poor coherence and low coherence length, interference phenomenon is more obvious at a position where the optical path difference is shorter. In some examples, the first sub-reflected light beam and the second sub-reflected light beam may have an optical path difference. When the optical path difference between the first sub reflected light beam and the second sub reflected light beam is not greater than the coherence length of the first light beam L10, the first sub reflected light beam and the second sub reflected light beam may generate an interference phenomenon. In some examples, the interference signal generated by the interference phenomenon of the first sub-reflected light beam and the second sub-reflected light beam may be a white light interference signal. Thus, when the optical path difference between the first sub reflected light beam and the second sub reflected light beam is not greater than the coherence length of the first light beam L10, a white light interference signal can be obtained.
In some examples, when the optical path difference between the first sub-reflected light beam and the second sub-reflected light beam is zero, the interference phenomenon of the first sub-reflected light beam and the second sub-reflected light beam is most obvious, and the corresponding white light interference signal can reach a peak value.
In some examples, the direction of the relative displacement of the interference objective 300 and the target point 21 may be referred to as the scanning direction. Referring to fig. 3, the scanning direction may be as shown in the Z direction of fig. 3. In some examples, the relative displacement of the interference objective 300 and the target point 21 as they vary when measuring one target point 21 may be referred to as a scanning stroke.
In some examples, the scan stroke may be no less than the coherence length of the first light beam L10. In this case, the white light interference signal generated by the interference of the first light beam L10 during the scanning process can be displayed completely, and the peak value of the white light interference signal can be accurately determined. (see FIG. 6)
In some examples, the interference phenomenon may occur between the first sub-reflected light beam and the second sub-reflected light beam by adjusting the relative positions of the interference objective lens 300 and the target point 21.
Referring to fig. 2, in some examples, the reconstruction device 10 may further include a drive module 500. The driving module 500 may be configured to adjust the relative positions of the interference objective 300 and the target point 21. In some examples, the drive module 500 may adjust the interference objective 300 away from or close to the target point 21. In other examples, the drive module 500 may adjust the target point 21 away from or close to the interference objective 300. In this case, the relative positions of the interference objective lens 300 and the target point 21 can be changed to change the optical path difference between the first sub reflected light beam and the second sub reflected light beam, and when the optical path difference between the first sub reflected light beam and the second sub reflected light beam is not greater than the coherence length of the first light beam L10, the first sub reflected light beam and the second sub reflected light beam generate an interference phenomenon to generate a white light interference signal.
In some examples, the reconstruction device 10 may also include a carrier module 600. The carrying module 600 can be used for carrying the object 20. That is, the driving module 500 may drive the interference objective 300 and the carrying module 600 to change relative positions therebetween.
In some examples, the driving module 500 may control the relative positions of the interference objective 300 and the carrying module 600 based on a control signal sent by the reconstruction apparatus 10. In other examples, the drive module 500 may be manual.
In some examples, the third sub-reflected light beam and the fourth sub-reflected light beam may have an optical path difference. Since the coherence length of the second light beam L20 is long, interference phenomena may occur in both the third sub-reflected light beam and the fourth sub-reflected light beam throughout the measurement of the height of the target point 21. In other words, the third sub-reflected light beam and the fourth sub-reflected light beam are both interfered to generate a compensation interference signal (see fig. 7) during the entire scanning stroke of measuring one target point 21. In some examples, the interference signal generated by the interference phenomenon that can occur between the third sub-reflected light beam and the fourth sub-reflected light beam may be a compensation interference signal.
In some examples, the light splitting module 200 may further include a second light splitting unit 220. The second light splitting unit 220 may be disposed in the same direction as the first light splitting unit 210. The second light splitting unit 220 may be a dichroic beamsplitter. In some examples, the target reflected light beam L1' may reach the second light splitting unit 220 via the first light splitting unit 210 after exiting through the interference objective lens 300. In other words, the second light splitting unit 220 may be configured to receive the target reflected light beam L1' transmitted through the first light splitting unit 210.
In some examples, the target light beam L1 may be reflected by the first light splitting unit 710 to the interference objective lens 300 and reach the target point 21, the target light beam L1 may form a target reflected light beam L1 'at the interference objective lens 300, and the target reflected light beam L1' may reach the second light splitting unit 720 via the first light splitting unit 710. In this case, the target light beam L1 can smoothly reach the interference objective lens 300, and the target reflected light beam L1 'is formed by the interference objective lens 300, whereby the second light splitting unit 720 can receive the target reflected light beam L1'.
In the reconstruction apparatus 10 according to the present embodiment, the second light splitting unit 220 may be further configured to decouple the target reflected light beam L1 ' into the first reflected light beam L10 ' and the second reflected light beam L20 '. Wherein, the first reflected light beam L10 'may match with the first light beam L10, and the second reflected light beam L20' may match with the second light beam L20. Since the dichroic beamsplitter may selectively reflect or transmit the light beam based on its wavelength, the first reflected light beam L10' may be transmitted through the second light splitting unit 220 and the second reflected light beam L20 may be reflected through the second light splitting unit 220.
The reconstruction device 10 according to the present embodiment may further include a reception module 700. The receiving module 700 may be configured to receive the first reflected light beam L10 'and the second reflected light beam L20'.
In some examples, the receiving module 700 may include a first receiving unit 710 and a second receiving unit 720. Wherein the first receiving unit 710 may be configured to receive the first reflected light beam L10 'transmitted through the second light splitting unit 220, and the second receiving unit 720 may be configured to receive the second reflected light beam L20' reflected through the second light splitting unit 220. In this case, the target reflected light beam L1 ' can be decoupled into the first reflected light beam L10 ' matched with the first light beam L10 and the second reflected light beam L20 ' matched with the second light beam L20 based on the second light splitting unit 220, and the first receiving unit 710 can convert the first reflected light beam L10 ' from an optical signal into an electrical signal, and the second receiving unit 720 can convert the second reflected light beam L20 ' from an optical signal into an electrical signal.
In some examples, the receiving module 700 further includes a third lens unit 730 disposed between the second light splitting unit 220 and the first receiving unit 710, and a fourth lens unit 740 disposed between the second light splitting unit 220 and the second receiving unit 720. In some examples, the third lens unit 730 and the fourth lens unit 740 may be a condensing lens having a function of condensing light beams. In this case, the third lens unit 730 can focus the first reflected light beam L10 'on the first receiving unit 710, and the fourth lens unit 740 can focus the second reflected light beam L20' on the second receiving unit 720.
In some examples, the receiving module 700 may not include the third lens unit 730 and the fourth lens unit 740.
In some examples, the first receiving unit 710 may obtain a white light interference signal based on the received first reflected light beam L10 ', and the second receiving unit may obtain a compensation interference signal based on the received second reflected light beam L20'. In this case, according to the reconstruction method of the present disclosure, it is possible to obtain the initial height of the target point 21 based on the white light interference signal and obtain the compensation information due to the environmental vibration based on the compensation interference signal.
In some examples, the first receiving unit 710 may be a CDD camera or a CMOS camera. Thus, the first receiving unit 710 can visually display the collected white light interference signal based on the received first reflected light beam L10'.
Referring to fig. 6, in some examples, the white light interference signal may be a sine wave signal modulated by a gaussian envelope. In particular, at the zero optical path difference position, the measurement interference signal may exhibit an interference peak.
In some examples, the second receiving unit 720 may be a photodetector. Preferably, the second receiving unit 720 may be a point photodetector. The second receiving unit 720 can display the collected compensation interference signal in a visualized form based on the received second reflected light beam L20'. In some examples, a point photodetector may have the advantage of high speed, large dynamic range received signals. In this case, using the point photodetector as the second receiving unit 720 enables quick and accurate reception of the light intensity variation information of the target point 21 in the process of measuring the target point 21.
Referring to fig. 7, the compensating interference signal may be a sine wave signal. The compensating interference signal can be displayed completely in the whole scanning process.
Fig. 8 is a schematic diagram illustrating that the first receiving unit 710 and the second receiving unit 720 according to the present disclosure receive optical signals synchronously.
The reconstruction device 10 according to the present embodiment may further include a data processing module 400. In some examples, the data processing module 400 may include a timing synchronization unit 410, a data acquisition unit 420, and a computation unit 430.
In some examples, the timing synchronization unit 410 may be configured to send control signals (e.g., frame synchronization pulses) to the first and second receiving units 710 and 720 so that the first and second receiving units 710 and 720 may receive the first and second reflected light beams L10 'and L20' synchronously. Referring to fig. 8, in some examples, the first receiving unit receives the first reflected light beam at a preset frame rate to obtain a white light interference signal, and the second receiving unit receives the second reflected light beam at a preset frame rate to obtain a compensated interference signal. In this case, the first receiving unit 710 and the second receiving unit 720 can receive the optical signals in synchronization, and thus the compensation information obtained based on the second light beam L20 can be matched with the initial height obtained based on the first light beam L10, improving the measurement accuracy of the target point 21.
In some examples, the turn-on time of the first receiving unit 710 and the second receiving unit 720 may be controlled by a frame synchronization pulse, and the time interval of each frame is the same. In some examples, a total of N frames of sample data may be collected. N may be an even number.
In some examples, a data acquisition unit 420 may be disposed between the first receiving unit 710 and the calculation unit 430. The calculation unit 430 may obtain an initial height of the target point 21 based on the acquired white light interference signal and the compensation interference signal and compensate for the initial height.
As described above, the first receiving unit 710 may be a CDD camera or a CMOS camera. First receiving unit720 may be a point photodetector. The sampling frequency of the first receiving unit 710 may be 50HZ-1KHZ and the sampling frequency of the second receiving unit 720 may be 1MHZ-1 GHZ. Thus, the sampling rate of the second receiving unit 720 may be much greater than the sampling rate of the first receiving unit 710. That is, the time for which the first receiving unit 710 responds to one optical signal may be not less than the response time for which the second receiving unit 720 responds to one optical signal. Referring to fig. 8, let the first receiving unit 710 receive an optical signal for a time τ 1 The second receiving unit 720 receives an optical signal for a time τ 2 In combination with the Frame synchronization pulses (Frame1, Frame2, and Frame3 … …), a time chart of the synchronous collection of the optical signals by the first receiving unit 710 and the second receiving unit 720 can be shown in fig. 8. In this case, the sampling integration time of the second receiving unit 720 may be much lower than that of the first receiving unit 710, so that the vibration information monitored by the second receiving unit 720 can more accurately reflect the environmental vibration.
In step S800, an initial height of the target point 21 may be obtained based on the white light interference signal, and compensation information may be obtained based on the compensation interference signal.
Fig. 9 is a schematic diagram showing the scanning time and the scanning stroke when the target point 21 is scanned according to the present disclosure. Wherein, the scanning travel is z, and the scanning time is t.
As described above, when the optical path difference between the first sub reflected light beam and the second sub reflected light beam in the first reflected light beam L10' is zero, a white light interference signal as shown in fig. 6 can be obtained. In some examples, the initial height of the target point 21 may be obtained based on a white light interference signal at a zero optical path difference position.
In some examples, the target point 21 may be scanned in the Z-direction at a constant velocity and the target point 21 may be acquired at uniform time intervals (or scan steps), each acquired data point being acquired at uniform time intervals (or displacement increments).
Referring to FIG. 9, z may be the relative displacement between the DUT 20 and the interference objective 300, essentially corresponding to the first target reflected light beam L30 'and the second target reflected light beam L40'The optical path difference between them. Ideally, the positions of the target points 21 may be distributed at positions on a straight line in fig. 8. However, in practice, the data points in the graph deviate from a straight line because the presence of environmental vibrations may cause the nominal position at the acquisition time to deviate from the actual position (initial height) of the target point 21. Epsilon 1 、ε 2 、ε 3 And epsilon 4 An error due to environmental vibrations at each acquisition instant (which may be referred to as a position error or a height error) may be represented. The reconstruction method according to the present disclosure can calculate the error (compensation information) in a quantifiable form, and further compensate the initial height of the target point 21.
Fig. 10 is a schematic flow chart illustrating obtaining compensation information in accordance with the present disclosure.
How to obtain the compensation information is explained and derived in detail below:
in step S800, obtaining the compensation information may include obtaining a wavefront phase based on the compensation interference signal (step S820), and obtaining a phase change of the object point 21 due to vibration based on the wavefront phase (step S840).
In S820, the intensity model of the compensated interference signal of the second reflected light beam L20' can be expressed as formula (1):
I(t)＝A+B cos[θ+φ(t)] (1)
where θ can be the wavefront phase, φ (t) can be a time-varying phase shift, and A and B can be the DC term coefficients and AC term coefficients, respectively. I (t) may be light intensity, and t is the scan time.
If the second receiving unit 710 collects the compensated interference signals from the k-th frame to the p-th frame, the interference light intensities of the k-th frame and the p-th frame may be formula (2) and formula (3), respectively:
I k ＝A+B cos(θ) (2)
wherein, I k May be the intensity, I, of the second reflected beam L20' at the k-th frame p May be a second inverseThe intensity of the beam L10' at the p-th frame,
may be a phase increment.
In some examples, equations (2) and (3) may be further derived to obtain the wavefront phase θ of object point 21 of test object 20, equation (4):
in some examples, the phase shift between two frames may be incremented by
("four-step phase shift method"), the wavefront phase θ:
in some examples, the phase truth value at the target point 21 corresponding to the second receiving unit 720 can be set to θ, since the range of the tangent function is [ - π/2, + π/2]The function period is pi, the sequence of light intensities (I) collected from N frames 1 ，I 2 ，I 3 ，L I N ) The interference phase shift of adjacent frames can be calculated:
1 st to 2 nd frames: theta 1 ＝θ+δ 1
Frames 3 to 4: theta 2 ＝Mod π (θ+π+δ 2 )＝θ+δ 2
Frames 5 to 6: theta 3 ＝Mod π (θ+2π+δ 3 )＝θ+δ 3
…
Frames k-1 to k: theta k ＝Mod π (θ+2π+δ k/2 )＝θ+δ k/2
…
Frames N-1 to N: theta N/2 ＝Mod π (θ+2π+δ N/2 )＝θ+δ N/2
Wherein k is 1, 2., N/2,
δ k an additional amount of phase shift due to vibration noise between the 2k-1 to 2k frames.
In some examples, the above N/2 formulas may be added and averaged to obtain equation (6):
when the frame rate of acquisition is sufficient, the averaged vibration component may be close to zero:
therefore, the wavefront phase of the surface of the target point 21 of the object 20 corresponding to the second receiving unit 720 can satisfy the formula (7):
where N is the total number of frames collected, θ k Indicating the phase change of the target point 21 in the adjacent frames (2 k-1 to 2k frames). This enables to obtain an accurate wavefront phase.
In S840, the phase change due to the vibration in each frame j may be calculated based on equation (8), that is, the phase change of the target point 21 due to the vibration in the jth frame may satisfy equation (8):
wherein, I j Is to compensate the light intensity, I, of the interference signal in the j-th frame j-1 Is the light intensity of the compensating interference signal in the j-1 th frame, A is the DC term coefficient of the intensity model of the compensating interference signal, and theta is the corresponding relation of the second receiving unit 720The wavefront phase of the target point 21.
In some examples, the first frame is an initial frame and the phase change due to the vibration may be zero, i.e., δ j 0. This makes it possible to obtain the phase change of the target point 21 due to vibration in each frame based on the formula (8).
In some examples, after arranging a plurality (≧ 3) of second receiving units 720, different amounts of Z-translation can be detected on a plurality of target points 21 at the same time, and can be fitted to a vibration plane, which can characterize the angular wobble noise of target points 21 in addition to the translation noise of target points 21.
In some examples, the compensation information of the target point 21 may be obtained based on a phase change of the target point 21 due to vibration in each frame. In some examples, the compensation information may satisfy equation (9):
wherein, ε j Is the compensation information of the target point 21 at the j-th frame, δ j Is the phase change, λ, of the target point 21 due to vibration at the j-th frame monitor Is the center wavelength of the second light beam. Thereby, a specific magnitude of the compensation information can be obtained.
As described above, the reconstruction method may further include compensating the initial height based on the compensation information to obtain a comprehensive measured height of the target point 21 (S1000).
In some examples, the integrated measured altitude may be obtained by superimposing the initial altitude with the compensation information. The integrated measured height may be obtained, for example, by adding the compensation information to the initial height. Thereby, after obtaining the compensation information, the integrated measured height of the target point 21 can be obtained based on the rule of superposition.
In some examples, the white light interference signal of the first reflected light beam L10' may be made to be at the j-th frame, at the z-directional sequence of positions z j The initial height of the target point 21 is z j Considering the measurement error due to the environmental vibration,the integrated measured height of the target point 21 may be the sum of the initial height and the compensation information: z is a radical of j +ε j 。
In some examples, if the sample data of the jth frame is (z) j ，I j ) Then the sampled data may be modified to:
as can be seen from the above, the uncompensated target point 21 sample data may be a regular sequence of positions (initial heights) or a time sequence. However, by calculating an error (compensation information) due to the environmental vibration and compensating the error to the initial height, it is possible to obtain an irregular but accurate position series (integrated measurement height) or time series of samples.
In some examples, the reconstruction method may obtain a comprehensive measured height of the target points 21 of the object 20 based on the above steps.
In summary, the reconstruction method can obtain the initial height of the target point 21 based on the white light interference signal generated by the first reflected light beam L10'. Compensation information may be obtained based on the compensation interference signal of the second reflected light beam L20' throughout the scan stroke, and the initial height may be compensated based on the compensation information to obtain a resultant measured height. This can improve the measurement accuracy of the target point 21.
In some examples, the reconstruction method may further include performing three-dimensional reconstruction on the object 20 (step S1200). In step S1200, the comprehensive measurement heights of the target points 21 can be obtained, and the object 20 is three-dimensionally reconstructed based on the comprehensive measurement heights of the target points 21 to obtain the three-dimensional surface topography of the object 21.
In some examples, the object 20 may be reconstructed three-dimensionally based on the plurality of corrected sample data. In some examples, the plurality of corrected sampled data may be processed by a non-uniform sampling analysis method, such as signal reconstruction of the object 20 by a non-uniform DFT, to obtain a phase map of the three-dimensional surface topography of the object 20.
Fig. 11a is a schematic diagram showing a three-dimensional surface topography obtained by three-dimensional reconstruction of the object 20 based on the conventional apparatus and method under the first condition. Fig. 11b is a schematic diagram illustrating a three-dimensional surface topography obtained by three-dimensionally reconstructing the object 20 according to the reconstruction apparatus 10 and the reconstruction method of the present disclosure under the first condition.
Referring to fig. 11a and 11b, in some examples, the object 20 may be polished glass having a roughness Sa of 1.5 nm. Under the condition that the environmental vibration is not greater than 5nm (first condition), the three-dimensional surface topography obtained by three-dimensionally reconstructing the object 20 by the conventional apparatus and method may be as shown in fig. 11a, and the three-dimensional surface topography obtained by three-dimensionally reconstructing the object 20 by the reconstruction apparatus 10 and the reconstruction method according to the present disclosure may be as shown in fig. 11 b.
In some examples, the three-dimensional reconstruction of the analyte 20 based on conventional apparatus and methods may result in a surface roughness of 4.501nm and a repeatability of 0.033. The surface roughness of the three-dimensional reconstruction of the object 20 to be measured based on the reconstruction device 10 and the compensation method according to the present disclosure may be 1.546nm, and the repeatability may be 1.438.
As shown in fig. 11a and 11b, in the case of small environmental vibration, the reconstruction of the object 20 is not visually affected by the conventional apparatus and the reconstruction apparatus 10, and the value measured by the conventional apparatus is slightly larger and the repeatability is slightly degraded by the surface roughness index analysis.
Fig. 12a is a schematic diagram showing a three-dimensional surface topography resulting from three-dimensional reconstruction of the test object 20 based on conventional apparatus and methods under a second condition. Fig. 12b is a schematic diagram illustrating a three-dimensional surface topography obtained by three-dimensionally reconstructing the object 20 under the second condition by the reconstruction apparatus 10 and the compensation method according to the present disclosure.
In some examples, the condition where the environmental vibration is about 20nm is referred to as a second condition. In some examples, the three-dimensional reconstruction of the analyte 20 based on conventional apparatus and methods may result in a surface roughness of 7.049nm and a repeatability of 0.003. The surface roughness obtained by three-dimensional reconstruction of the object to be measured 20 based on the reconstruction device 10 and the compensation method according to the present disclosure may be 1.560nm, and the repeatability may be 0.004.
As shown in fig. 12a and 12b, when the environmental vibration amplitude is large, if the conventional reconstruction apparatus 10 is used, the reconstructed image can obviously observe the vibration-induced fringes, and the obtained roughness value is large, especially the repeatability is seriously degraded due to the disturbance of random noise; when the reconstruction apparatus 10 and the reconstruction method of the present disclosure are used, the measured surface roughness and repeatability indexes are substantially consistent with the results in a low noise environment.
According to the reconstruction method disclosed by the disclosure, the initial height of the target point can be obtained based on the first light beam, and the initial height is compensated based on the second light beam to obtain the comprehensive measurement height of the target point, so that the comprehensive measurement precision of the reconstruction device can be improved, and the reconstruction precision of the reconstruction device can be further improved.
While the present disclosure has been described in detail in connection with the drawings and examples, it should be understood that the above description is not intended to limit the disclosure in any way. Those skilled in the art can make modifications and variations to the present disclosure as needed without departing from the true spirit and scope of the disclosure, which fall within the scope of the disclosure.
Claims (10)
1. A method for reconstructing a white light interference signal is applied to a reconstruction device for measuring the height of a target point of an object to be measured so as to reconstruct the object to be measured, and the reconstruction method comprises the following steps: obtaining a target light beam, wherein the target light beam comprises a first light beam and a second light beam, the bandwidth of the first light beam is not less than a first preset value, the bandwidth of the second light beam is not more than a second preset value, and the first preset value is not less than the second preset value; enabling the target light beam to reach the target point through an interference objective lens, and forming a target reflected light beam at the interference objective lens, wherein the target reflected light beam comprises a first reflected light beam matched with the first light beam and a second reflected light beam matched with the second light beam; obtaining a white light interference signal based on the first reflected light beam and obtaining a compensation interference signal based on the second reflected light beam; obtaining the initial height of a target point based on the white light interference signal, and obtaining compensation information of the target point based on the compensation interference signal; and compensating the initial height based on the compensation information to obtain a comprehensive measured height of the target point.
2. The reconstruction method according to claim 1,
the first beam and the second beam are coupled as a target beam.
3. The reconstruction method according to claim 1,
the target light beam is reflected to the interference objective lens through the first light splitting unit and reaches the target point, the target light beam forms a target reflection light beam on the interference objective lens, and the target reflection light beam reaches the second light splitting unit through the first light splitting unit.
4. The reconstruction method according to claim 3,
the second light splitting unit is used for decoupling the target reflected light beam into a first reflected light beam and a second reflected light beam, transmitting the first reflected light beam to the first receiving unit, and reflecting the second reflected light beam to the second receiving unit.
5. The reconstruction method according to claim 4,
the first receiving unit obtains a white light interference signal based on the received first reflected light beam, and the second receiving unit obtains a compensation interference signal based on the received second reflected light beam.
6. The reconstruction method according to claim 5,
the first receiving unit receives a first reflected light beam at a preset frame rate to obtain the white light interference signal, and the second receiving unit receives a second reflected light beam at the preset frame rate to obtain the compensation interference signal.
7. The reconstruction method according to claim 5,
the compensation information of the target point obtained based on the compensation interference signal satisfies the formula:
wherein epsilon j Is the compensation information of the target point in the jth frame, δ j Is the phase change, λ, of the target point at the jth frame due to vibration monitor Is the center wavelength of the second light beam.
8. The reconstruction method according to claim 6 or 7,
obtaining the integrated measured altitude by superimposing the initial altitude and the compensation information.
9. The reconstruction method according to claim 7,
the phase change of the target point at the jth frame caused by vibration meets the formula:
wherein, I j Is to compensate the light intensity, I, of the interference signal in the j-th frame j-1 Is the light intensity of the compensation interference signal in the j-1 frame, a is the direct current term coefficient of the intensity model of the compensation interference signal, and θ is the wavefront phase of the target point corresponding to the second receiving unit.
10. The reconstruction method according to claim 9,
the wavefront phase of the second receiving unit corresponding to the target point satisfies the formula:
where N is the total number of frames collected, θ k Representing the phase change of the target point in the adjacent frame.
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