CN109163672B - Micro-topography measuring method based on white light interference zero-optical-path-difference position pickup algorithm - Google Patents
Micro-topography measuring method based on white light interference zero-optical-path-difference position pickup algorithm Download PDFInfo
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Abstract
The invention provides a micro-topography measuring method based on a white light interference zero-optical path difference position pickup algorithm. A micro-topography measuring method based on a white light interference zero-optical-path-difference position pickup algorithm comprises the following steps: s1, obtaining a white light interference template curve; s2, vertically scanning a measured object, collecting interference fringes and obtaining a sampling point; s3, quickly and roughly positioning a zero optical path difference point by using a gravity center method or an extreme value method; s4, determining a search range, and taking a plurality of sampling points in a certain range around the zero optical path difference point as points to be matched; s5, matching the points to be matched with the template curve in a search range to obtain precisely positioned zero optical path difference points; s6, obtaining the accurate relative height of the surface appearance of the object according to the correlation between the zero optical path difference and the appearance height, and further reconstructing the 3D appearance of the measured object. The invention has simple algorithm, high speed, high precision and strong noise resistance.
Description
Technical Field
The invention relates to the technical field of precision optical measurement engineering, in particular to a micro-topography measurement method based on a white light interference zero-optical path difference position pickup algorithm.
Background
The microscopic morphology of the surface of an object greatly affects the quality and the service performance of the object. With the continuous development of ultra-precision machining technology, the corresponding ultra-precision detection technology becomes more and more important. The white light interference measurement technology is one of the important non-contact optical measurement technologies for surface micro-topography, and solves the problem of phase ambiguity of a laser interferometer when the discontinuous height jump of the measured surface exceeds a quarter wavelength. The basic principle of white light interferometry is as follows: light emitted by the light source passes through the light path, part of the light reaches a measured object on the vertical scanning platform and then is reflected to be used as measuring light, and part of the light passes through the light path, reaches the reference mirror and then is reflected to be used as reference light. The two beams of reflected light converge and pass through the beam splitter, and are received by the CCD camera. When the vertical scanning platform scans up and down to enable the optical path difference between the measuring light and the reference light to be smaller than the coherence length of the light source, the two beams of light interfere to generate white light interference fringes. When the optical path difference between the measurement light and the reference light is equal to zero, namely, the position of zero optical path difference, the intensity of the interference signal reaches the maximum. The zero optical path difference position reflects the height information of the object, namely the 3D shape of the object can be reconstructed by positioning the zero optical path difference position. Therefore, the accurate positioning of the zero optical path difference position by the white light interference signal processing algorithm is a key step of the white light interference measurement technology. Under the condition that other factors are the same, the white light interference signal processing algorithm directly determines the positioning accuracy and speed of the zero optical path difference position, and further determines the accuracy and speed of the reconstructed object 3D appearance.
At present, the white light interference signal processing algorithm mainly comprises: extreme value method, center of gravity method, envelope curve fitting method, phase shift method, space frequency domain method, etc. The extreme method directly utilizes the position of the maximum light intensity value as a zero optical path difference point, is very simple and quick, has the precision mainly determined by the scanning step distance, and is easily influenced by noise to cause poor precision; the gravity center method is simple in calculation and high in speed, but is easily influenced by noise and is low in precision; the white light phase shift method has higher precision than an extreme method and a gravity center method and lower precision than an envelope curve fitting method and a space frequency domain method, and has moderate operation speed; the envelope curve fitting method comprises a Fourier transform filtering method, a Hilbert transform method, a wavelet transform method, a direct quadratic polynomial fitting method and the like, and has high precision on the whole, but has large calculation amount and long time consumption, and is difficult to meet the requirement of online measurement; the spatial frequency domain method has higher precision, but needs Fourier transform and least square fitting, and has larger operation amount.
Therefore, the study on the white light interference signal processing algorithm capable of quickly and accurately positioning the zero optical path difference position to reconstruct the microscopic three-dimensional morphology has great significance for ultra-precise detection.
Disclosure of Invention
The invention provides a micro-topography measuring method based on a white light interference zero-optical path difference position pickup algorithm, aiming at overcoming at least one defect in the prior art. The invention has simple algorithm, high speed, high precision and strong noise resistance.
In order to solve the technical problems, the invention adopts the technical scheme that: a micro-topography measuring method based on a white light interference zero-optical-path-difference position pickup algorithm comprises the following steps:
s1, obtaining a white light interference template curve;
s2, vertically scanning a measured object, collecting interference fringes and obtaining a sampling point;
s3, quickly and roughly positioning a zero optical path difference point by using a gravity center method or an extreme value method;
s4, determining a search range, and taking a plurality of sampling points in a certain range around the zero optical path difference point as points to be matched;
s5, matching the points to be matched with the template curve in a search range to obtain precisely positioned zero optical path difference points;
s6, obtaining the accurate relative height of the surface appearance of the object according to the correlation between the zero optical path difference and the appearance height, and further reconstructing the 3D appearance of the measured object.
Further, in step S1, a vertical scanning platform is adopted to perform vertical scanning at a certain step length to obtain dense sampling points, and white light interference curve fitting is performed on the sampling points to obtain a white light interference template curve.
In the invention, a white light interference template curve can be obtained by a plurality of methods such as system parameters and the like through the analysis of a white light interference theory. Moreover, the fitted white light interference template curve expression also has a plurality of different forms.
In the present invention, when the white light interference system hardware is determined, that is, when the aperture value of the light source and the objective lens is determined, the specific shape of the white light interference intensity curve can be determined or estimated, so the curve expression of the fitted white light interference template in the present invention can be:
in the formula IbThe background light intensity is used, gamma is the fringe contrast, and the two parameters can be determined through fitting; lcAnd λ0The coherent length and the central wavelength of the light source are respectively determined by the light source; z is the position of the sampling point, and I (z) is the corresponding light intensity value and is a known quantity; h is0The position with zero optical path difference is used as the only variable.
Further, in step S2, the object to be measured is placed on the vertical scanning platform to perform scanning with equal step length, the interference fringes are collected by using the CCD, and each pixel of the CCD obtains a series of sampling points.
Further, in step S3, the coarsely positioning zero optical path difference point is marked as N0(ii) a In the step S4, according to N0Determining a match search range to be N0+ -. DELTA.taking N0M sampling points in a certain range around the sample are used as points to be matched, the positions of the sampling points are z, and the corresponding light intensity values are Iz。
Further, in the step S5, a variable h of the template curve is initially set0＝N0And delta, substituting the position z of the point to be matched into the white light interference template curve to obtain the white light interference intensity value I (z) of the template curve at the z position. Mixing I (z) with IzAnd (4) making a difference and solving an absolute value to obtain a residual absolute value, wherein the sum of the residual absolute values of m pairs of points is as follows: e ═ Σ | I (z) -IzL, changing the template curve variable h0Satisfy N0-Δ≤h0≤N0+ delta, when the sum epsilon of absolute values of residual errors is minimum, corresponding h0Namely the position of the zero optical path difference of the accurate positioning.
Compared with the prior art, the invention has the beneficial effects that:
the method fully utilizes the characteristic of simple and quick algorithm of an extreme value method or a gravity center method, improves the template matching speed, and further ensures that the template matching method has high operation speed.
The invention fully utilizes the determinability or estimability of the curve shape of the white light interference signal, and matches the sampling points with the template curve, thereby achieving the purpose of reconstructing the three-dimensional shape with high precision.
The invention can still keep higher precision when the vertical scanning platform scans at larger step pitch, thereby accelerating the scanning process.
The method combines two steps of coarse positioning and fine positioning, and has the advantages of simple algorithm, high speed, high precision and strong noise resistance.
Drawings
Fig. 1 is a flow chart of the working principle of the present invention.
FIG. 2 is a graph of a white light interference pattern acquired in advance in the present invention.
FIG. 3 is a schematic diagram of the coarse positioning of the sampling point and the gravity center method or the extreme method in the vertical equal-step scanning of the scanning platform according to the present invention.
Fig. 4 is a schematic diagram of the search range and the sample point to be matched in the present invention.
FIG. 5 is a schematic diagram of the matching process between the template curve and the sample points to be matched in the present invention.
FIG. 6 is a schematic diagram of the completion of matching between the template curve and the sample points to be matched in the present invention.
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the patent; for the purpose of better illustrating the embodiments, certain features of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted. The positional relationships depicted in the drawings are for illustrative purposes only and are not to be construed as limiting the present patent.
As shown in fig. 1, a micro-topography measuring method based on a white light interference zero optical path difference position pickup algorithm includes the following steps:
s1, obtaining a white light interference template curve. The specific method comprises the following steps: vertical scanning is firstly carried out by adopting a vertical scanning platform in a certain step length to obtain dense sampling points, and white light interference curve fitting is carried out on the sampling points to obtain a white light interference template curve, as shown in figure 2. When the white light interference system hardware is determined, that is, the aperture value of the light source and the objective lens is determined, the specific shape of the white light interference intensity curve can be determined or estimated, so the curve expression of the fitted white light interference template in the embodiment may be:
in the formula IbThe background light intensity is used, gamma is the fringe contrast, and the two parameters can be determined through fitting; lcAnd λ0The coherent length and the central wavelength of the light source are respectively determined by the light source; z is the position of the sampling point, and I (z) is the corresponding light intensity value and is a known quantity; h is0The position with zero optical path difference is used as the only variable.
In this embodiment, a white light interference template curve may also be obtained by a plurality of methods such as system parameters through white light interference theory analysis. Moreover, the fitted white light interference template curve expression also has a plurality of different forms.
S2, vertically scanning a measured object, collecting interference fringes and obtaining a sampling point; the specific method comprises the following steps: the measured object is placed on a vertical scanning platform to perform equal-step scanning, interference fringes are collected by a CCD, and each pixel of the CCD obtains a series of sampling points, as shown in FIG. 3.
S3, quickly and roughly positioning a zero optical path difference point by using a gravity center method or an extreme value method, and recording the roughly positioned zero optical path difference point as N0As shown in fig. 3.
S4, determining a search range, and taking a plurality of sampling points in a certain range around the zero optical path difference point as points to be matched; the specific method comprises the following steps: according to N0Determining a match search range to be N0+ -. DELTA.taking N0M sampling points in a certain range around the sample are used as points to be matched, the positions of the sampling points are z, and the corresponding light intensity values are IzAs shown in fig. 4.
S5, matching the points to be matched with the template curve in a search range to obtain precisely positioned zero optical path difference points; the specific method comprises the following steps: variable h of initial template curve0＝N0And delta, substituting the position z of the point to be matched into the white light interference template curve to obtain the white light interference intensity value I (z) of the template curve at the z position. Mixing I (z) with IzAnd (4) making a difference and solving an absolute value to obtain a residual absolute value, wherein the sum of the residual absolute values of m pairs of points is as follows: e ═ Σ | I (z) -IzL, changing the template curve variable h0Satisfy N0-Δ≤h0≤N0+ Δ, such that the curve moves within the search range, as shown in fig. 5. When the sum epsilon of the absolute values of the residual errors is minimum, the corresponding h0I.e. the zero optical path difference position of the desired fine positioning, as shown in fig. 6.
S6, obtaining the accurate relative height of the surface appearance of the object according to the correlation between the zero optical path difference and the appearance height, and further reconstructing the 3D appearance of the measured object.
It should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.
Claims (1)
1. A micro-topography measuring method based on a white light interference zero-optical path difference position pickup algorithm is characterized by comprising the following steps:
s1, adopting a vertical scanning platform to perform vertical scanning at a certain step length to obtain dense sampling points, and performing white light interference curve fitting on the sampling points to obtain a white light interference template curve; the expression of the fitted white light interference template curve is as follows:
in the formula IbThe background light intensity is used, gamma is the fringe contrast, and the two parameters can be determined through fitting; lcAnd λ0The coherent length and the central wavelength of the light source are respectively determined by the light source; z is the position of the sampling point, and I (z) is the corresponding light intensity value and is a known quantity; h is0The position with zero optical path difference is taken as a unique variable;
s2, placing the measured object on a vertical scanning platform for scanning with equal step length, collecting interference fringes by using a CCD (charge coupled device), and obtaining a series of sampling points by each pixel of the CCD;
s3, quickly and roughly positioning a zero optical path difference point by using a gravity center method or an extreme value method, and recording the roughly positioned zero optical path difference point as N0；
S4, according to N0Determining a match search range to be N0+ -. DELTA.taking N0M sampling points in a certain range around the sample are used as points to be matched, the positions of the sampling points are z, and the corresponding light intensity values are Iz；
S5, matching the points to be matched with the template curve in a search range to obtain precisely positioned zero optical path difference points; variable h of initial template curve0＝N0Δ, substituting the position z of the point to be matched into the white light interference template curve to obtain a white light interference intensity value I (z) of the template curve at the z position; mixing I (z) with IzAnd (4) making a difference and solving an absolute value to obtain a residual absolute value, wherein the sum of the residual absolute values of m pairs of points is as follows: e ═ Σ | I (z) -IzL, changing the template curve variable h0Satisfy N0-Δ≤h0≤N0+ delta, when the sum epsilon of absolute values of residual errors is minimum, corresponding h0The position of the zero optical path difference of the accurate positioning is obtained;
s6, obtaining the accurate relative height of the surface appearance of the object according to the correlation between the zero optical path difference and the appearance height, and further reconstructing the 3D appearance of the measured object.
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