KR20170023363A - System and method for measuring high height by digital holography microscope - Google Patents

Abstract

According to the present invention, a system of measuring high step difference using a digital holography microscope comprises: a composite wave radiator radiating a composite wave having a longer wavelength than an original wavelength by interference of each different wavelength; a beam splitter dividing and outputting the radiated composite wave into a reference beam route (R) and a measurement beam route (O); a mirror reflecting the composite wave output to the measurement beam route (O), allowing the composite wave to face a sample; an objective lens allowing the composite wave, reflected by the sample, to be incident; and a camera capturing an image of the composite wave passing through the objective lens. As such, the present invention may measure a sample having a high step difference greater than or equal to 100 um using a composite wave.

Description

TECHNICAL FIELD [0001] The present invention relates to a method of measuring a high-stage difference using a digital holography microscope,

The present invention relates to a level measurement system and method, and more particularly, to a system and method for measuring a high level height of a sample using a digital holography microscope.

The holography proposed by Gabor (1948) is a technique for reconstructing three-dimensional information of a specimen by recording and reconstructing the wavefront of the light passing through the specimen. Has been used in various fields due to the recording of the used hologram and the development of the reproduction technique using a computer.

Particularly, in detecting a defect on a display panel, a digital holography microscope (DHM) is used to obtain a step from the reference surface to the top of the defect to detect a defect protruding a predetermined height from a plane (reference plane) Digital Holography Microscope) is used. That is, by using DHM to inspect the display panel, it is possible to reconstruct the three-dimensional information of the defect by recording and reproducing the wavefront of the light passing through the defect.

(x,y)로 나타난다. 도 1은 평면파로 입사하는 빛이 시편을 지난 후, 파면의 왜곡이 생기는 것을 보여준다. 여기서 단파장에 대한 위상차는 식(1)과 같이 표현된다.Generally, when a plane wave is transmitted through a specimen as shown in Fig. 1, the distortion of the wave front due to the thickness h (x, y) of the specimen and the refractive index n _C is retarded

(x, y). Fig. 1 shows that a wavefront is distorted after the light incident on the plane wave passes through the specimen. Here, the phase difference with respect to the short wavelength is expressed by Equation (1).

식(1)

Equation (1)

At this time, if the optical field after passing through the sample is precisely measured, the phase difference information can be extracted, and the thickness of the specimen or the refractive index information of the specimen can be obtained according to the equation (1).

On the other hand, in order to measure a large stepped sample, two synthetic waves are formed by interference of two waveforms having different wavelengths, and a synthetic wave having a wavelength larger than the original wavelength is used. However, it is impossible to measure the high-end difference of more than 100um in the sample measurement method using the synthetic wave.

Korean Patent No. 10-0611078 Korean Patent Publication No. 10-2013-0102311

Accordingly, an object of the present invention is to provide a high-stage difference measurement method using a digital holography microscope for measuring a sample having a high-stage difference of 100um or more by using a synthetic wave .

According to an aspect of the present invention, there is provided a high-stage difference measurement system using a digital holography microscope, including: a composite wave emitting unit that emits a composite wave having a wavelength longer than a first wavelength by interference of different wavelengths; A beam splitter for splitting the emitted composite wave into a reference beam path R and a measurement beam path O and a mirror for reflecting the synthesized wave outputted to the measurement beam path O to the sample, An objective lens to which a composite wave reflected by the sample is incident, and a camera for picking up an image of the composite wave that has passed through the objective lens.

A high-stage difference measuring method using a digital holography microscope according to an embodiment of the present invention includes the steps of obtaining the composite wave having a wavelength longer than a primary wavelength by interference of different wavelengths, Moving the focus so as to align the focal point of the objective lens aligned with the reference plane to the upper portion of the step of the sample; calculating the focus shift distance; And calculating a height of the step based on the moving distance.

As described above, according to the high level difference measurement method using the digital holography microscope according to the present invention, the high level difference of the sample can be measured using the synthetic wave.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing a configuration in which a plane wave passes through a specimen in general.
2 is a view showing a step difference measurement system using a general reflection type digital holography microscope.
3 is a diagram for helping to understand the principle of making a synthetic wave according to an embodiment of the present invention.
4 is a diagram illustrating a high stage difference measurement system using a digital holography microscope according to an embodiment of the present invention.
5 is a diagram showing measurement and non-measurement areas using a digital holography microscope according to an embodiment of the present invention.
6 is a view showing the step between the defects from the reference plane of the specimen.
7 is a flowchart for explaining a step difference measurement method using a step difference measurement system according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

In the following description of the embodiments of the present invention, descriptions of techniques which are well known in the technical field of the present invention and are not directly related to the present invention will be omitted. In addition, detailed description of components having substantially the same configuration and function will be omitted.

For the same reason, some of the elements in the accompanying drawings are exaggerated, omitted, or schematically shown, and the size of each element does not entirely reflect the actual size. Accordingly, the present invention is not limited by the relative size or spacing depicted in the accompanying drawings.

First, prior to the description of the present invention, a general description of a digital holography microscope (DHM) will be given.

Holography, a technique used to precisely measure an optical field, consists essentially of interference between a scattered sample field (S) through a specimen and a reference field (R) that does not pass through the specimen. The present invention is one of spatial modulation methods in which the reference wavefront R and the scattered wavefront S interfere with small angular difference with respect to the optical axis (DHM) which measures the hologram of a specimen by using an off-axis holography and performs a three-dimensional measurement of the specimen using the hologram.

In order to quantitatively measure the optical field transmitted through the specimen, when the specimen is interfered with a plane wave not transmitted through the specimen with a small angular difference, the optical field on the plane on which the optical camera (CCD) is located is expressed as follows.

식(2)

Equation (2)

(x,y)는 시편에서 발생한 위상차, u_O, v_O 는 작은 각도 차에 의한 x, y축 방향 공간 주파수이다. 상기 식(1)로 표현되는 광학장은 아래 식(3)으로 표현되는 간섭 빛의 세기(intensity)로 카메라를 통해 측정된다.In the above equation (2), | E ₀ | And | E ₁ | are the amplitudes of the reference wavefront and the scattering wavefront, respectively,

(x, y) is the phase difference generated in the specimen, and u _o and v _o are the spatial frequencies in the x and y axes due to the small angular difference. The optical field represented by the above equation (1) is measured through the camera at the intensity of the interference light expressed by the following equation (3).

The above equation (3) can be rewritten as follows.

이다.here,

to be.

From the intensity information of the two-dimensional interference light measured and stored in the CCD camera, the optical field information can be extracted by the following Fourier spectrum using a spatial Fourier transform (Fourier transform).

이다.Of the Fourier spectrum expressed by equation (5), the optical information of the specimen to be measured is

to be.

에 해당하는 부분만 선택한 후 중앙으로 이동하고 이를 역 푸리에 변환을 하게 되면 다음 식 (6)과 같이 시편에서의 광학장을 정량적으로 추출할 수 있게 된다.In order to extract only this information, Fourier spectrum information

And then moves to the center. If the inverse Fourier transform is performed, the optical field in the specimen can be quantitatively extracted as shown in the following equation (6).

Using Radian as a unit of phase difference among the extracted specimen optical field information, this physical quantity can be converted into the absolute thickness information of the specimen using the following equation (1) when the refractive index of the sample is known.

식(1)

Equation (1)

2 is a view showing a step difference measurement system using a general reflection type digital holography microscope.

Referring to FIG. 2, in a short-wavelength reflection type digital holography microscope having a wavelength?, The wavefront of the optical field is reflected and deformed in the specimen, and the deformation against the incident wave is measured as a phase displacement. The phase shift due to specimen reflection is used for 3D phase measurement by the following equation.

식(2)

Equation (2)

Where Δh is the height of the specimen and n is the refractive index of the material containing the specimen. (N = 1 in air and n = 1.33 in water) In a digital holography microscope, the actual phase difference value is measured in modulo 2π as follows:

식(3)

Equation (3)

This is a contour like representation on the map. The contour lines of 360 degrees (2π) are as follows.

Samples with gentle height differences can obtain 3D height information from modulo 2π contour lines using phase unwrapping. In this phase spread function, the height step should be less than the above equidistance.

, It is impossible to measure how many times 2π the phase shift includes (hereinafter referred to as '2π ambiguity').

Therefore, the present invention proposes a system and a method for measuring a high-stage difference.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The step difference measuring system according to an embodiment of the present invention is used to measure a step of a sample to detect a defect on a display panel, for example, A method for measuring a sample is disclosed. For reference, it is impossible to measure a high-stage difference of 100 μm or more by using a conventional digital holography microscope, but an embodiment of the present invention for measuring the high-stage difference will be described later.

3 is a diagram for helping to understand the principle of making a synthetic wave according to an embodiment of the present invention.

As shown in FIG. 3, multiple wavelengths are used to increase the size of the measurable height step. To do this, two synthetic waves are generated by interference of two waveforms of different wavelengths λ ₁ and λ ₂ .

은 원 파장 λ₁, λ₂ 보다 매우 크다. Among the two synthetic waves, a wavelength of a synthetic wave having a long wavelength

Is much larger than the original wavelengths? ₁ and? ₂ .

When this composite wave is used, it is possible to measure a sample having a large step because the same effect as that using a very long wavelength is obtained. For example, in the case of λ ₁ = 682.5 nm and λ ₂ = 758.5 nm, the synthesized wave becomes 6.8 μm, and it is possible to measure a sample having a step of 3.4 μm.

The present invention thus acquires a composite wave having a longer wavelength among synthetic waves obtained through interference of two waveforms having different wavelengths.

A system and method for measuring a sample having a high-stage difference using the thus-obtained composite wave will be described. Fig. 5 is a view showing the step between the defects from the reference plane of the specimen, which is referred to in the explanation.

4 is a diagram showing a schematic configuration of a high-stage difference measuring system according to an embodiment of the present invention.

Referring to FIG. 4, the high-stage difference measuring system of the present invention includes a composite wave emitting unit 10, a spatial filter 12, a collimating lens 13, a first beam splitter 14, A first beam splitter 15, a second beam splitter 17, a mirror M, an objective lens 30, and a camera 40.

First, the selection of the objective lens 30 of the digital holography microscope selects a maximum magnification having a field of view (FOV) that can include a defect (specimen in this case) to be measured. The higher the magnification, the shorter the depth of focus (DoF) and the smaller the measurement error.

When the slope of the stepped inclined surface exceeds half of the numerical aperture (NA) of the objective lens 30, a measurement impossible area as shown in Fig. 5 is generated. (For 100um steps, 20x magnification is appropriate.)

After the lens is selected as described above, the height of the stepped portion can be obtained without a phase unwrapping function by using the composite wave emitted through the composite wave emitting portion 10.

Specifically, the focal point of the objective lens 30 is aligned with the reference plane (for example, the plane of the panel). When a dual wavelength (665 um, 678 um) is used, the wavelength of the composite wave is (665 x 678) / (678-665) = 34.682 nm, and the height of the measurable step is 17.3 um. Without phase unwrapping, the step measurement value h (e) is within 17.3 μm.

Here, the composite wave is emitted through the composite wave emitting portion 10 as a composite wave having a longer wavelength out of the two composite waves obtained through the interference of the two waveforms of the different wavelengths λ ₁ and λ ₂ described above. The synthesized wave is incident on the first beam splitter 14 and divided into a reference beam path R and a measurement beam path O and outputted. The composite wave output to the measurement beam path O is reflected by the mirror M and passes through the lens 15 to be reflected from the sample and passes through the objective lens 30 and the second beam splitter 17, (40), thereby obtaining an image for the sample.

Next, the focus is shifted to fit the focus to the top of the step, which is the same as the reference plane.

Here, there are three methods of focusing: manual, region of interest (ROI) image analysis mechanical operation, ROI input intensity analysis mechanical operation, and digital focusing.

In all three cases described above, the reference optical path having a small angular difference from the CCD 40 is closed and only the sample field (object optical path) is measured.

Specifically, the first manual focusing method is a method of finding an image of the upper portion of the step by eye, moving the lens to find a position where the focus is best, and then calculating the moving distance of the lens. This method has the advantage of using the conventional digital holography microscope device as it is, but it can not be used in automatic inspection equipment.

Second, ROI (Region of interest) image analysis The focus movement method using mechanical operation sets the upper part of the step to ROI, changes the focus, and finds the position that best fits through image analysis. The accuracy of the camera autofocus function is important here.

Third, region of interest (ROI) input intensity analysis The focus shift method using mechanical operation sets the upper part of the step to ROI, changes the focus, and finds the position that best fits through input intensity analysis. At this time, the sensitivity to the input of the CCD pixel is important.

Next, the focus shift distance is calculated. Digital holography microscopes use a single-wavelength coherent laser as a light source and recognize the design function of the lens used, so we can calculate the image that is formed when the focus is changed by ray tracing. This is called digital focusing. With this function, the lens shift distance from the reference plane focus to the upper focus point is calculated, and the calculated value is referred to as h (f). This is done automatically, and there is no physical movement of the lens, so it is easy to use in automatic inspection equipment.

Next, the height of the step is finally calculated. That is, the height of the step is calculated on the basis of the previously calculated focusing distance. The calculated focus distance h (f) has an error of ± DoF as shown in FIG.

The calculation of the actual height h (r) of the step can be obtained as follows.

h (f) - DoF? h (r)? h (f)

h (r) = h (e) + N x? / 2 Equation (5)

Here, h (f) is a value measured through a focus shift distance and h (e) is a value measured through a digital holography microscope. Here, h (e) measured through the digital holography microscope has a 2? Ambiguity. Since DoF is known as the characteristic value of the objective lens 30, if the h (r) in the equation (5) is substituted into the equation (4), the N value resulting from the phase discontinuity due to the step difference can be calculated . The calculation conditions at this time are as follows. N is a natural number, and since DoF < / 2, N, the value is only one N. For example, the DoF for a 20x objective lens is 3um, and for the measurements used in the present invention, lambda / 2 = 17.3um.

In other words, one N can be obtained from Eqs. (4) and (5), and the obtained height h (r) is obtained by substituting the obtained N and h (e)

가 된다.
When measuring by applying a short wavelength to DHM at the time of measurement,? Becomes a wavelength to be applied, and when it is measured by applying a combined wavelength (? 1,? 2)

.

As a practical test, a standard sample having various height steps was measured by the step measurement system of the present invention,

10um: measured value (average value): 8.5um

20um: measured value (average value): 18.3um

30um: measured value (average value): 31.1um

50um: measured value (average value): 51.3um

100um: measured value (average value): equal to 101um, and the error is within the DoF regardless of the step height.

Although the upper part of the step is actually flat, it is measured that there is a slight difference in height due to the influence of noise, and the maximum and minimum measured values do not deviate from DoF which is the error range.

FIG. 7 is a flowchart for explaining a high-stage difference measurement method using a step difference measurement system according to an embodiment of the present invention.

Referring to FIG. 7, in step S710, a high-stage difference measurement method according to an embodiment of the present invention acquires a composite wave having a wavelength longer than the original wavelength by interference of different wavelengths.

When using a dual wavelength (665 um, 678 um), the wavelength of the synthetic wave is (665 x 678) / (678-665) = 34.682 nm, and the height of the measurable step is 17.3 um. Without phase unwrapping, the step measurement value h (e) is within 17.3 μm.

In step S720, the focus of the objective lens is adjusted to the reference plane of the sample using the composite wave.

In step S730, the focus of the camera is moved in order to align the focal point of the objective lens with the reference plane above the step of the sample.

Here, there are three methods of focusing: manual, region of interest (ROI) image analysis mechanical operation, ROI input intensity analysis mechanical operation, and digital focusing. When moving the focus, close the reference optical path with a small angle difference from the camera CCD surface in all three cases described above, and pass through only the sample field (object optical path).

Specifically, the first manual focusing method is a method of finding an image of the upper portion of the step by eye, moving the lens to find a position where the focus is best, and then calculating the moving distance of the lens. This method has the advantage of using the conventional digital holography microscope device as it is, but it can not be used in automatic inspection equipment.

Second, ROI image analysis The focusing method using the mechanical operation sets the upper part of the step to ROI, changes the focus, and finds the position that best fits through image analysis. The accuracy of the camera autofocus function is important here.

Third, Analysis of ROI Input Intensity The focus movement method using mechanical operation sets the upper part of the step to ROI, changes the focus, and finds the best focus position through input intensity analysis. At this time, the sensitivity to the input of the CCD pixel is important.

In step S740, the focus shift distance is calculated.

Digital holography microscopes use a single-wavelength coherent laser as a light source and recognize the design function of the lens used, so we can calculate the image that is formed when the focus is changed by ray tracing. This is called digital focusing. With this function, the lens shift distance from the reference plane focus to the stepped upper focus point is calculated. This is done automatically, and there is no physical movement of the lens, so it is easy to use in automatic inspection equipment.

In step S750, the height of the step is calculated based on the calculated focus shift distance, and the step is described above.

Although the upper part of the step is actually flat, it is measured that there is a slight difference in height due to the influence of noise, and the maximum and minimum measured values do not deviate from DoF which is the error range.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. , And are not intended to limit the scope of the present invention. It will be apparent to those skilled in the art that other modifications based on the technical idea of the present invention may be practiced without departing from the invention as set forth herein.

10: composite wave emitting portion 12: spatial filter
13: collimating lens 14: first beam splitter
15: Lens 17: Second beam splitter
30: objective lens 40: camera

Claims (9)

In a stepped measurement system using a digital holography microscope,
A composite wave emitting portion that emits a composite wave having a wavelength longer than the original wavelength by interference of different wavelengths;
A beam splitter for dividing the emitted composite wave into a reference beam path (R) and a measurement beam path (O) and outputting the result;
A mirror for reflecting the composite wave output to the measurement beam path (O) and directing it to a sample;
An objective lens for entering a composite wave reflected by the sample; And
And a camera for picking up an image of the composite wave that has passed through the objective lens, wherein the high-order difference measurement system uses the digital holography microscope.
A step measurement method using a measurement system for measuring a step difference by using the digital holography microscope of claim 1, wherein the step difference is a step difference larger than a wavelength of a synthetic wave applied to the holographic microscope,
Obtaining the composite wave having a wavelength longer than the original wavelength by interference of different wavelengths;
Fitting a focus of the objective lens to a reference plane of the sample using the synthetic wave;
Moving the focus to align the focal point of the objective lens with the reference surface above the step of the sample;
Calculating the focus shift distance; And
And calculating a height of the step based on the calculated focus shift distance. 3. The method of claim 2,
And measuring the height of the step difference using the composite wave without applying a phase unwrapping function. 3. The method according to claim 2,
Using one of the following methods: manual operation, region of interest image analysis mechanical operation, region of interest input intensity analysis mechanical operation, digital focusing, Wherein a reference optical path having a small angular difference from the CCD plane is closed and only a sample field (object optical path) is passed through to measure the high stage difference measurement using the digital holography microscope. 4. The method according to claim 3,
Wherein a moving distance of the lens from the lower part to the upper part is obtained by finding the position where the focus is most suitable through the lens through the image processing of the image of the lower part and the upper part of the step, . 4. The method of claim 3, wherein the focus movement method using the ROI image analysis mechanical operation comprises:
Wherein the upper part of the step is set as an ROI, and the position of the lens is changed, and a position where the focus is best is found through image analysis, to determine a high end difference using the digital holography microscope. 4. The method of claim 3, wherein the focus movement method using the ROI input intensity analysis mechanical operation comprises:
Wherein the position of the lens is changed after the upper part of the step is set as an ROI, and a position where the focus is best is found through analysis of the input intensity, thereby measuring a high stage difference using the digital holography microscope. 3. The method according to claim 2,
In the case of the digital holography microscope, a coherent laser of a single wavelength is used as a light source, and a digital focusing function for calculating an image formed when a focus is changed by ray tracing is used to perform a lens shift from the reference plane focus to the step upper focus Wherein the distance is calculated by using a digital holography microscope. 3. The method of claim 2, wherein calculating the height h (r)
h (f) - DoF (Depth of Focus)? h (r)? h
h (r) = h (e) + N?? / 2,
here,
h (f): a value obtained by measuring the height of the stepped portion through the focus shift distance,
h (e) is a value measured with a digital holography microscope, having a value of 2? ambiguity,
DoF: a value known as a characteristic value of the objective lens 30,
N is a natural number, and DoF < / 2 is applied to obtain one N, and h (r) is calculated through the obtained N by using the digital holography microscope.

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