CN109163672B - Micro-topography measuring method based on white light interference zero-optical-path-difference position pickup algorithm - Google Patents

Abstract

The invention provides a micro-topography measurement method based on a white light interference zero optical path difference position picking algorithm. A method for measuring microscopic topography based on a white light interference zero optical path difference position picking algorithm, comprising the following steps: S1. Obtaining a white light interference template curve; S2. Scanning a measured object vertically, collecting interference fringes, and obtaining sampling points; S3 .Use the center of gravity method or the extreme value method to quickly and roughly locate the zero optical path difference point; S4. Determine the search range, and take several sampling points within a certain range around the zero optical path difference point as the points to be matched; S5. Use the point to be matched and the template curve in Match within the search range to obtain the zero optical path difference point for precise positioning; S6. Obtain the relative height of the precise surface topography of the object according to the correlation between the zero optical path difference and the topography height, and then reconstruct the 3D topography of the measured object. The algorithm of the invention is simple, the speed is fast, the precision is high, and the anti-noise ability is strong.

Description

A Micro Topography Measurement Based on White Light Interference Zero Optical Path Difference Position Picking Algorithm method

technical field

The invention relates to the technical field of precision optical measurement engineering, and more particularly, to a micro-topography measurement method based on a white light interference zero optical path difference position picking algorithm.

Background technique

The microscopic topography of an object's surface greatly affects its quality and performance. With the continuous development of ultra-precision machining technology, the corresponding ultra-precision detection technology becomes increasingly important. White light interferometry is one of the most important non-contact surface micro-topography optical measurement technologies. It solves the phase ambiguity problem of laser interferometers when the measured surface discontinuity height jumps beyond a quarter wavelength. The basic principle of white light interferometry is as follows: the light emitted by the light source reaches the measured object on the vertical scanning platform through a part of the optical path, and then is reflected as the measurement light, and a part of the light passes through the optical path to the reference mirror and is reflected as the 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 so that the optical path difference between the measurement light and the reference light is less than the coherence length of the light source, the two beams of light interfere to produce white light interference fringes. When the optical path difference between the measurement light and the reference light is equal to zero, that is, the zero optical path difference position, the intensity of the interference signal reaches the maximum. The zero optical path difference position reflects the information of the height of the object, that is, the 3D shape of the object can be reconstructed by locating the zero optical path difference position. Therefore, it is a key step in the white light interferometry technique to precisely locate the zero optical path difference position through the white light interference signal processing algorithm. When 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, which in turn determines the accuracy and speed of reconstructing the 3D topography of the object.

At present, the white light interference signal processing algorithms mainly include: extreme value method, center of gravity method, envelope curve fitting method, phase shift method, spatial frequency domain method, etc. The extreme value method directly uses the maximum light intensity value as the zero optical path difference, which is very simple and fast. The accuracy is mainly determined by the scanning step distance, and it is easily affected by noise, resulting in poor accuracy; the center of gravity method is simple to calculate and fast, but at the same time It is also easily affected by noise, and the accuracy is not high; the accuracy of the white light phase shift method is higher than that of the extreme value method and the center of gravity method, but lower than that of the envelope curve fitting method and the spatial frequency domain method, and the operation speed is moderate; the envelope curve fitting method has Fourier transform The filtering method, the Hilbert transform method, the wavelet transform method, and the direct quadratic polynomial fitting method have high accuracy in general, but the computational load is large and the time consuming is long, which is difficult to meet the requirements of online measurement; the spatial frequency domain method has relatively high accuracy. High precision, but requires Fourier transform and least squares fitting, which requires a large amount of computation.

Therefore, it is of great significance for ultra-precision detection to study the white light interference signal processing algorithm that can quickly and accurately locate the zero optical path difference position to reconstruct the microscopic three-dimensional topography.

SUMMARY OF THE INVENTION

In order to overcome at least one of the above-mentioned defects in the prior art, the present invention provides a micro-topography measurement method based on a white light interference zero optical path difference position pick-up algorithm. The algorithm of the invention is simple, the speed is fast, the precision is high, and the anti-noise ability is strong.

In order to solve the above-mentioned technical problems, the technical solution adopted in the present invention is: a micro-topography measurement method based on a white light interference zero optical path difference position pick-up algorithm, which includes the following steps:

S1. Obtain the white light interference template curve;

S2. Vertically scan the measured object, collect interference fringes, and obtain sampling points;

S3. Use the center of gravity method or the extreme value method to quickly and roughly locate the zero optical path difference;

S4. Determine the search range, and take several sampling points within a certain range around the zero optical path difference point as the points to be matched;

S5. Use the point to be matched and the template curve to match within the search range to obtain the zero optical path difference point for precise positioning;

S6. Obtain an accurate relative height of the surface topography of the object according to the correlation between the zero optical path difference and the topography height, and then reconstruct the 3D topography of the measured object.

Further, in the step S1, a vertical scanning platform is used to first perform vertical scanning with a certain step size 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 present invention, the white light interference template curve can also be obtained through the theoretical analysis of white light interference and according to various methods such as system parameters. Moreover, there are many different forms of the fitted white light interference template curve expression.

In the present invention, when the hardware of the white light interference system is determined, that is, the numerical value of the light source and the aperture of the objective lens are determined, the specific shape of the white light interference intensity curve can be determined or estimated. Therefore, the fitted white light interference in the present invention Template curve expressions can be:

where I _b is the background light intensity, γ is the fringe contrast, these two parameters can be determined by fitting; _lc and λ ₀ are the coherence length and center wavelength of the light source, respectively, determined by the light source; z is the sampling point position, I (z) is the corresponding light intensity value, which is a known quantity; h ₀ is the position of zero optical path difference, as the only variable.

Further, in the step S2, the object to be measured is placed on a vertical scanning platform for equal-step scanning, and a CCD is used to collect interference fringes, and each pixel of the CCD obtains a series of sampling points.

Further, in the step S3, the rough positioning zero optical path difference point is denoted as N ₀ ; in the step S4, the matching search range is determined as N ₀ ±Δ according to N ₀ , and m samples within a certain range around N ₀ are taken. The point is used as the point to be matched, its position is z, and the corresponding light intensity value is I _z .

Further, in the step S5, the variable h ₀ =N ₀ -Δ of the template curve is initially set, and the position z of the point to be matched is substituted into the white light interference template curve, and the white light interference intensity value I of the template curve at the z position can be obtained. (z). Difference between I(z) and I _z and find the absolute value to get the absolute value of residual, then the sum of the absolute value of residual of m pairs of points is: ε=∑|I(z)-I _z |, change the template curve variable h ₀ , which satisfies N ₀ -Δ≤h ₀ ≤N ₀ +Δ, when the sum of the absolute values of the residuals is the smallest, the corresponding h ₀ is the zero optical path difference position of the precise positioning.

Compared with the prior art, the beneficial effects of the present invention:

The invention makes full use of the simple and fast algorithm of the extreme value method or the center of gravity method, and improves the template matching speed, so that the template matching method has a high operation speed.

The invention makes full use of the determinability or estimability of the shape of the white light interference signal curve, matches the sampling point with the template curve, and achieves the purpose of high-precision three-dimensional topography reconstruction.

The present invention can still maintain high precision when the vertical scanning platform scans with a larger step distance, so the scanning process can be accelerated.

By combining two steps of rough positioning and fine positioning, the invention has simple algorithm, high speed, high precision and strong anti-noise ability.

Description of 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 template obtained in advance in the present invention.

FIG. 3 is a schematic diagram of the rough positioning of the sampling point and the center of gravity method or the extreme value method in the vertical equal-step scanning of the scanning platform in the present invention.

FIG. 4 is a schematic diagram of a search range and a sampling point to be matched in the present invention.

FIG. 5 is a schematic diagram of the matching process between the template curve and the sampling point to be matched in the present invention.

FIG. 6 is a schematic diagram of the completion of the matching between the template curve and the sampling point to be matched in the present invention.

Detailed ways

The accompanying drawings are for illustrative purposes only, and should not be construed as limitations on this patent; in order to better illustrate the present embodiment, some parts of the accompanying drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art It is understandable to the artisan that certain well-known structures and descriptions thereof may be omitted from the drawings. The positional relationships described in the drawings are only for exemplary illustration, and should not be construed as a limitation on the present patent.

As shown in Figure 1, a micro-topography measurement method based on a white light interference zero optical path difference position picking algorithm, which includes the following steps:

S1. Obtain a white light interference template curve. The specific method is as follows: use a vertical scanning platform to perform vertical scanning with a certain step size to obtain dense sampling points, and perform white light interference curve fitting on the sampling points to obtain the white light interference template curve, as shown in Figure 2. When the hardware of the white light interference system is determined, that is, the light source and the aperture of the objective lens are determined, the specific shape of the white light interference intensity curve can be determined or estimated. Therefore, the fitted white light interference template curve in this embodiment expresses The formula can be:

where I _b is the background light intensity, γ is the fringe contrast, these two parameters can be determined by fitting; _lc and λ ₀ are the coherence length and center wavelength of the light source, respectively, determined by the light source; z is the sampling point position, I (z) is the corresponding light intensity value, which is a known quantity; h ₀ is the position of zero optical path difference, as the only variable.

In this embodiment, the white light interference template curve can also be obtained through the theoretical analysis of white light interference and according to various methods such as system parameters. Moreover, there are many different forms of the fitted white light interference template curve expression.

S2. Vertically scan the measured object, collect interference fringes, and obtain sampling points; the specific method is as follows: place the measured object on a vertical scanning platform for equal-step scanning, use CCD to collect interference fringes, and each pixel of the CCD obtains a series of sampling points point, as shown in Figure 3.

S3. Use the center of gravity method or the extreme value method to quickly and roughly locate the zero optical path difference point, and the rough positioning zero optical path difference point is recorded as N ₀ , as shown in FIG. 3 .

S4. Determine the search range, and take a number of sampling points within a certain range around the zero optical path difference point as the points to be matched; the specific method is as follows: determine the matching search range as N ₀ ±Δ according to N ₀ , and take m within a certain range around N ₀ Each sampling point is used as the point to be matched, its position is z, and the corresponding light intensity value is I _z , as shown in FIG. 4 .

S5. Use the point to be matched and the template curve to match within the search range to obtain the zero optical path difference point of precise positioning; the specific method is as follows: initially set the variable h ₀ =N ₀ -Δ of the template curve, and substitute the position z of the point to be matched To the white light interference template curve, the white light interference intensity value I(z) at the z position of the template curve can be obtained. Difference between I(z) and I _z and find the absolute value to get the absolute value of residual, then the sum of the absolute value of residual of m pairs of points is: ε=∑|I(z)-I _z |, change the template curve variable h ₀ , satisfies N ₀ -Δ≤h ₀ ≤N ₀ +Δ, so that the curve moves within the search range, as shown in FIG. 5 . When the sum of the absolute values of the residuals is the smallest, the corresponding h ₀ is the zero optical path difference position of the precise positioning, as shown in Figure 6.

S6. Obtain an accurate relative height of the surface topography of the object according to the correlation between the zero optical path difference and the topography height, and then reconstruct the 3D topography of the measured object.

Obviously, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those of ordinary skill in the art, changes or modifications in other different forms can also be made on the basis of the above description. There is no need and cannot be exhaustive of all implementations here. Any modification, equivalent replacement and improvement made within the spirit and principle of the present invention shall be included within the protection scope of the claims of the present invention.

Claims (1)

1. a micro-topography measuring method based on white light interference zero optical path difference position pick-up algorithm, is characterized in that, comprises the steps: S1. Use the vertical scanning platform to first perform vertical scanning with a certain step size to obtain dense sampling points, and perform white light interference curve fitting on the sampling points to obtain the white light interference template curve; the fitted white light interference template curve expression is:

where I _b is the background light intensity, γ is the fringe contrast, these two parameters can be determined by fitting; _lc and λ ₀ are the coherence length and center wavelength of the light source, respectively, determined by the light source; z is the sampling point position, I (z) is the corresponding light intensity value, which is a known quantity; h ₀ is the position of zero optical path difference, as the only variable; S2. Place the measured object on a vertical scanning platform for equal-step scanning, use CCD to collect interference fringes, and each pixel of CCD obtains a series of sampling points; S3. Use the center of gravity method or the extreme value method to quickly and roughly locate the zero optical path difference, and the rough positioning zero optical path difference is recorded as N ₀ ; S4. According to N ₀ , determine that the matching search range is N ₀ ±Δ, and take m sampling points within a certain range around N ₀ as the points to be matched, whose positions are z, and the corresponding light intensity value is I _z ; S5. Use the point to be matched and the template curve to match within the search range to obtain the zero optical path difference point for precise positioning; initially set the variable h ₀ =N ₀ -Δ of the template curve, and substitute the position z of the point to be matched to the white light interference template In the curve, the white light interference intensity value I(z) of the template curve at the z position can be obtained; the difference between I( _z ) and Iz and the absolute value can be obtained to obtain the absolute value of the residual error, then the absolute value of the residual error of m pairs of points is divided. The sum is: ε=∑|I(z)-I _z |, change the template curve variable h ₀ to satisfy N ₀ -Δ≤h ₀ ≤N ₀ +Δ, when the sum of the absolute values of the residuals is the smallest, the corresponding The h ₀ is the zero optical path difference position of the precise positioning required; S6. Obtain an accurate relative height of the surface topography of the object according to the correlation between the zero optical path difference and the topography height, and then reconstruct the 3D topography of the measured object.

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