JP3410051B2 - Shape measuring method and device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shape measuring method and apparatus, and more particularly to irradiating an object to be measured with a light beam emitted from a light source capable of wavelength scanning to determine the shape of the object from the interference fringe intensity distribution. The present invention relates to a shape measuring method and apparatus for measuring.

[0002]

2. Description of the Related Art With the birth of lasers, coherent interferometry based on the monochromaticity of laser light has played an important role in various physical measurements. Conventionally, as a fringe analysis method that has relied on intensity information, a method such as a phase shift method or a Fourier transform method for extracting a fringe phase distribution with high accuracy has been developed, and its sensitivity has reached the nm order. Monochromatic interferometry is suitable for detecting continuous relative changes in optical path difference. But,
Since it cannot cope with discontinuous changes exceeding half a wavelength, the application range is limited. For such a case, there is a method of using a longer wavelength or a large synthetic wavelength obtained by using two or more different wavelengths, but it is not easy to obtain a wide measurement range and sufficient accuracy at the same time. .

On the other hand, in recent years, an external resonator type LD (Laser diode)
Broadband spectral interferometry by wavelength scanning using a broadband wavelength scanning light source typified by e) and dye lasers has attracted attention. Utilizing the wide bandwidth of the light source wavelength, it is expected to be applied to surface shape measurement with discontinuous steps found in many industrial products and tomography in a scattering medium such as a living body. In the wideband spectrum interferometry by wavelength scanning, the intensity fluctuation of the interference signal generated when the optical path difference or the wavelength is changed in the two-beam interference is detected, and the height information is obtained from the fluctuation frequency or period. It is promising as an image measurement method for shapes including continuous steps.

[0004]

However, in the conventional wideband spectrum interferometry method by wavelength scanning, the phase information of the interference fringes is not actively used unlike the monochromatic interferometry method. The accuracy has not reached the level that can be linked to the original sensitivity of the interferometer, and therefore, it is not possible to perform shape measurement with high accuracy, and high accuracy is required.

Therefore, an object of the present invention is to provide a shape measuring method and apparatus which solves the problems of the prior art and enables highly accurate shape measurement by effectively incorporating the phase detection method in the wavelength scanning method. That is.

[0006]

In order to achieve the above object, the shape measuring method of the present invention uses an irradiation light for irradiating an object to be measured with a light beam emitted from a light source capable of wavelength scanning. Separated into light, the object light generated by irradiating the object to be measured with the irradiation light and the reference light are overlapped on the photoelectric surface for photoelectric detection, and depend on the optical path difference between the irradiation light and the reference light. In the shape measuring method for obtaining the interference fringe intensity distribution consisting of the background intensity component and the interference intensity component, and measuring the shape of the DUT, the central wavelength that is approximately the wavelength center is set, and the predetermined wavelength shift from this central wavelength. Setting various scanning wavelengths that are wavelength shifted by an amount, setting at least three predetermined phase shift values at the central wavelength, and for at least three predetermined phase shift values at the scanning wavelength, the scanning wavelength is It is approximated that the predetermined phase shift value at the center wavelength corresponds to the phase shift value at the scanning wavelength and is approximated to be equal to the center wavelength, and for each scanning wavelength, the irradiation light or the reference light is Phase shift by a predetermined phase shift value, obtain the interference fringe intensity distribution for each of the phase shift values, obtain the interference phase value based on the optical path difference in the interference intensity component from the set of the obtained interference fringe intensity distribution, respectively, From the phase difference amount and the wavelength shift amount that are the difference between the interference phase value at the scanning wavelength and the interference phase value at the central wavelength, the wavelength phase that is the ratio between the wavelength shift amount and the phase difference amount. Find the ratio,
It is characterized in that the shape of the object to be measured is measured from the wavelength phase ratio.

The predetermined wavelength scanning width is 1/150 to 1/50 of the center wavelength.

A shape measuring apparatus of the present invention separates a light source capable of wavelength scanning, an irradiation light for irradiating an object to be measured with a light beam emitted from the light source, and a reference light which serves as a reference, and receives the irradiation light. An interference optical system that superimposes the object light generated by irradiating a measurement object and the reference light on the photocathode, and sets various scanning wavelengths that are wavelength-shifted by a predetermined wavelength shift amount from the central wavelength that is approximately the wavelength center. Wavelength scanning means, and at least three predetermined phase shift values at the central wavelength are set. For at least three predetermined phase shift values at the scanning wavelength, it is approximated that the scanning wavelength is equal to the central wavelength. Then, it is approximated that the predetermined phase shift value at the central wavelength corresponds to the phase shift value at the scanning wavelength, and the irradiation light or the reference light is set to the above for each scanning wavelength. A phase shift means for shifting the phase by a constant phase shift value, and an interference fringe intensity distribution consisting of a background intensity component and an interference intensity component depending on the optical path difference between the irradiation light and the reference light for each of the phase shift values. A signal detection unit to be obtained, and an interference phase value based on an optical path difference in the interference intensity component is obtained from a set of the interference fringe intensity distribution obtained by the signal detection unit, and the interference phase value and the central wavelength at each scanning wavelength. From the phase difference amount and the wavelength shift amount that is the difference with the interference phase value in, the wavelength phase ratio that is the ratio of the wavelength shift amount and the phase difference amount is determined, and the wavelength phase ratio from the measured object. Calculation means for calculating the shape,
It is characterized by including.

The predetermined wavelength scanning width is 1/150 to 1/50 of the center wavelength.

In the above invention, the inventor of the present application has come out of the conventional idea that the phase shift method is not effective unless the phase shift is strictly performed, and came up with a new effective approximation method. is there.

Conventionally, the concept of the phase shift method is not effective unless the phase shift is strictly performed, and the work is extremely complicated because the phase shift is strictly performed for each scanning wavelength, resulting in the wavelength scanning method. However, the phase shift method could not be effectively utilized.

In the present invention, when the phase shift values at various scanning wavelengths are given, it is approximated that the scanning wavelength is equal to the center wavelength and the phase shift value at the center wavelength (for example, 9) is obtained.
0 degree is the phase shift value at the scanning wavelength (for example, 90 degrees).
Since the phase shift work for each scanning wavelength over the wavelength scanning width can be replaced by repeating the phase shift at the central wavelength under a reasonable approximation, it is extremely easy to realize a realistic phase. You can shift.

Further, the wavelength phase ratio, which is the ratio between the wavelength shift amount and the phase difference amount, is calculated from the phase shift amount, which is the difference between the interference phase value at the scanning wavelength and the interference phase value at the center wavelength, and the wavelength shift amount. Since the shape of the object to be measured is determined from the wavelength phase ratio, it differs from the conventional case where only the magnitude of the phase difference amount is focused, and not only the magnitude of the phase difference amount but also the wavelength phase ratio. The information regarding the shape measurement can also be obtained from the sign of.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION A shape measuring method according to the present invention will be described below with reference to the drawings.

FIG. 1 shows a wavelength scanning phase shift interferometer using a broadband wavelength tunable laser. Reference numeral 1 indicates a light source composed of an external resonator type LD, and the light source 1 is a US SD.
This is TC-40 manufactured by L Company (wavelength scanning width 769 to 785 nm, output 500 mW). The light beam emitted from the light source 1 is linearly polarized by a polarization element, collimated by a telescope 2 into a plane wave having a beam diameter of 10 mm, and then incident on a Michelson interferometer 3. In the Michelson interferometer 3, the beam splitter 5 separates the irradiation light 6 and the reference light 7, and the irradiation light 6 is irradiated to the object to be measured 8 and reflected by the object to be measured 8 to generate an object light 10. The light 7 is applied to a PZT mirror 9 which functions as a phase shifter. The reference light 7 and the object light 10 reflected by the PZT mirror 9 are reflected by the beam splitter 5 and then passed through the polarization element 11 to be superimposed on the photoelectric surface of the CCD camera 12 and photoelectrically detected.

The PZT mirror 9 comprises a reference mirror and a PZT element for slightly displacing the reference mirror.

For the sake of convenience of the experiment, the object to be measured 8 is placed before and after the position where the optical path difference is zero between the reference mirror of the PZT mirror 9 and the plane mirror 8a, and 1 mm optically contacted with the plane mirror surface. Thick gauge block (GB) 8
b and.

The wavelength scanning control of the light source 1 and the fine displacement control of the PZT mirror 9 are performed by the computer 13. Light source 1
Wavelength scanning is performed by controlling the drive current of the LD. The wavelength step is Δλst, for example, Δλst
Is 0.2 nm, and the central wavelength (or reference wavelength) λ ₀
The wavelength scanning is performed every 0.2 nm.

The CCD camera 12 obtains the interference fringe intensity distribution I (x, y, Δλ) shown in equation (1), and the computer 13 performs arithmetic processing.

[0020]

[Equation 1] In the interference fringe intensity distribution I (x, y, Δλ), the first term represents the background intensity component and the second term represents the interference intensity component.
x and y represent coordinate values on a plane perpendicular to the traveling direction of the irradiation light 6, and Δλ is the difference between the scanning wavelength λ and the central wavelength (or reference wavelength) λ ₀ as shown in Expression (3). The amount of wavelength shift is shown.

[0021]

[Equation 2]

[Equation 3] The phase component φ (x, y) in the interference intensity component of the interference fringe intensity distribution I (x, y, Δλ) reflects the height distribution h (x, y) of the DUT 8 on the (x, y) plane. And then the object light 10
Due to the optical path difference between the reference light 7 and the reference light 7.

The phase component φ (x, y, Δλ) is given by the equation (2).
As shown in, the height distribution h (x, y), the central wavelength λ _0, and the wavelength shift amount Δλ are represented. here,
φ (x, y) 0 represents the phase component at the central wavelength λ ₀ .

The phase difference component Δφ (x, y, Δλ) represents the difference between the phase component φ (x, y, Δλ) at the scanning wavelength λ and the phase component φ (x, y) _{0 at the} center wavelength λ ₀ . .

The phase component φ (x, y, Δλ) is determined by giving at least three known phase shift values and measuring at least three I (x, y, Δλ). A (x, y) and B (x, y) are erased to obtain each scanning wavelength λ.

Here, the application of at least three known phase shift values is performed by slightly displacing the reference mirror of the PZT mirror 9 by the PZT element.

Specifically, the phase shift is performed by 90 degrees. 5
-Step algorithm is used. Equation (4) is the scanning wavelength λ obtained by the 5-step method, that is, the phase component φ (x, y, in the wavelength shift amount Δλ.
Δλ) is shown. Here, each of I1, ..., I5 is 5
The values of the interference fringe intensity distribution I (x, y, Δλ) measured by phase shifting by 90 degrees by the -step method are shown.

[0027]

[Equation 4] Further, as shown in Expression (5), when the optical path difference between the reference light 7 and the object light 10 at the central wavelength λ ₀ is set to zero, the phase difference component Δφ (x, y, Δλ) and the wavelength shift amount Δ
It can be seen that the ratio to λ is proportional to the height distribution h (x, y) of the DUT 8 on the (x, y) plane.

[0028]

[Equation 5] Therefore, according to the equation (5), the height distribution h (h) of the DUT 8 is obtained by calculating the ratio between the phase difference component Δφ (x, y, Δλ) including the sign and the wavelength shift amount Δλ. x, y)
Can be obtained. Phase difference component Δφ (x, y, Δλ)
The absolute value of the ratio between the wavelength shift amount Δλ and the wavelength shift amount Δλ indicates the magnitude of h (x, y), and the sign of the ratio between the phase difference component Δφ (x, y, Δλ) and the wavelength shift amount Δλ is the object to be measured. 8 is higher or lower than the position where the optical path difference is zero.

Here, what is characteristic of the present invention is that when a phase shift value at the scanning wavelength λ is given,
Is to scan the wavelength lambda is approximate to the phase shift value at the center wavelength lambda ₀ is approximated to be equal to the central wavelength lambda ₀ (e.g., 90 degrees) corresponds to a phase shift value at the scanning wavelength lambda (for example, 90 degrees).

The validity of this approximation holds when the size of the wavelength scanning width λsc is not so large with respect to the size of the central wavelength λ ₀ . Here, the central wavelength λ ₀ is 776 nm
The wavelength scanning width λsc is 8 nm, which is about 1/100 of the center wavelength λ0, and this approximation is sufficiently satisfied.

Conventionally, in the phase shift method, there is no viewpoint of adopting the above-mentioned approximation, and it is necessary to set a phase shift value (for example, 90 degrees) for each scanning wavelength λ. It was. Therefore, conventionally, the phase shift work has become extremely complicated, and the phase shift method has not been effectively adopted.

In this respect, the inventor of the present application has come out of the conventional idea that the phase shift method is not effective unless the phase shift is strictly performed, and came up with the above approximation method.

By adopting this approximation, the phase shift operation for each scanning wavelength λ over the wavelength scanning width λsc can be replaced by repeating the phase shift at the center wavelength λ _0, so that the phase shift is extremely simple and realistic. Therefore, it becomes possible to effectively incorporate the phase detection method in the wavelength scanning method.

Based on the above approximation, equation (6) shows the principle measurement range hmax. Here, λst is the wavelength step, here 0.2 nm, and λsc is the wavelength scanning width, here 8 nm in the range from 772 nm to 780 nm. Measurement range hmax is ± 0.7
It is 5 mm. The wavelength step λst may be constant for all scanning wavelengths λ, or the wavelength step λst may be different for each scanning wavelength λ.

[0035]

[Equation 6] Next, the result of measuring the shape of the DUT 8 using the above-described approximation using the optical system of FIG. 1 will be described.

It will be shown below that it is possible to detect the step between the flat mirror 8a of the object to be measured 8 and the gauge block 8b.

256 × 2 representing the interference fringe intensity distribution I (x, y, Δλ) shown in the equation (1) by the CCD camera 12.
Image data of 56 × 8 bits can be obtained. For each scanning wavelength λ, the phase component φ (x, y, Δλ) is obtained according to the equation (4) by the 5-step method algorithm. And
For each scanning wavelength λ, the phase difference component Δφ (x,
y, Δλ) and the wavelength shift amount Δλ are obtained. These calculations are performed by the computer 13.

Specifically, the wavelength step λst is 0.2
The scanning wavelength λ is scanned in the range of 772 nm to 780 nm in nm, the least square fitting is performed for each pixel, and the height distribution is obtained from the gradient of the phase change.

In FIG. 2, the horizontal axis represents the scanning wavelength (center wavelength λ ₀ + wavelength shift amount Δλ), and the vertical axis represents the phase difference component Δφ (x,
y, Δλ). In FIG. 2, a corresponds to the coordinate position of the plane mirror 8a, and b corresponds to the coordinate position of the gauge block 8b. Both surfaces of the plane mirror 8a and the gauge block 8b are arranged before and after the position where the optical path difference is zero, but it can be seen that the front and back can be determined by the sign of the phase difference component Δφ (x, y, Δλ).

FIG. 3 shows the phase difference component Δφ (x, y, Δλ).
The result of calculating the height distribution in each pixel from the gradient with respect to the wavelength shift amount Δλ of is shown, and is a three-dimensional display of the obtained step shape. The nominal value of the gauge block (GB) 8b is 1.002 mm, and the scanning wavelength range is 772 nm-7.
The range of 80 nm is 8 nm, and the wavelength shift amount Δλ is measured in 0.2 nm steps.

FIG. 4 shows an example of the cross-sectional data taken along the line aa in FIG. 3, shows the height of the plane mirror 8a along the left vertical axis, and shows the cross-sectional data taken along the line bb in FIG. An example is shown, and the height of the gauge block 8b is shown along the right vertical axis. According to FIG. 4, the height of the gauge block 8b with respect to the plane mirror 8a is 1.001.
mm ± 0.3 μm, the average height of the gauge block 8b is 1.001 mm, and the standard deviation is ± 0.108 μ.
It is m, and it is recognized that the shape can be measured with extremely high accuracy.

As described above, according to the present embodiment, the height distribution of each surface of the plane mirror 8a and the gauge block 8b can be measured with a standard deviation of about 0.1 μm.

FIG. 5 shows the experimental results and the theoretical simulation results of b with respect to changes in the standard deviation σh (mm) of the height measurement when the wavelength scanning width λsc is increased stepwise. Indicates. The standard deviation σh (mm) substantially corresponds to the measurement resolution Δh. According to the experimental results shown in a, it is recognized that the standard deviation of 0.1 μm or less is obtained when the wavelength scanning width λsc is approximately 7 nm or more. This is because the conventional method based on frequency analysis has a measurement resolution Δh of ± 21 μm when an equivalent wavelength scanning width λsc is used. Indicates that accuracy can be obtained.

In FIG. 5, the standard deviation of the experimental result shown in a is larger than that of the theoretical simulation result shown in b. This disagreement is due to the experimental result shown in a.
It has been confirmed that this is due to the influence of the geometrical optical path length error of the optical element and the back surface reflection. Therefore, by removing these error factors, it is possible to bring the experimental result shown in a closer to the theoretical simulation result shown in b. As a result, according to the present embodiment, in the shape measurement. It is possible to reduce the standard deviation to the order of the theoretical simulation result shown in b.

Further, according to FIG. 5, the wavelength scanning width λsc
It is recognized that the standard deviation of the height measurement can be made small if the center wavelength λ ₀ (776 nm) is 1/150 or more.

[0046] When providing a phase shift value in a scanning wavelength lambda, the approximate phase shift value at the center wavelength lambda ₀ and the scanning wavelength lambda is equal to the center wavelength lambda ₀ is to correspond to the phase shift value at the scanning wavelength lambda , Center wavelength λ
_This holds when the size of the wavelength scanning width λ sc is not very large with respect to the size of ₀ , but if the wavelength scanning width λ sc is 1/50 or less of the center wavelength λ ₀ (773 nm), this approximation is sufficiently obtained. To establish. Therefore, if the wavelength scanning width λsc is 1/150 or more and 1/50 or less of the central wavelength λ ₀ ,
Good.

Considering that the conventional frequency analysis method obtained only about 1 μm even when wavelength scanning was performed with a wavelength scanning width of 20 nm or more, according to the present invention, a more accurate result with a narrower wavelength scanning width. You can get
Can be said.

From this result, it is expected that the combination of the wavelength scanning interferometer and the phase analysis can significantly improve the accuracy with an inexpensive light source and can combine the original phase measurement result of the monochromatic interferometer.

[0049]

As described above, according to the configuration of the present invention, when the phase shift values at various scanning wavelengths are given, it is approximated that the scanning wavelength is equal to the central wavelength and the phase shift value at the central wavelength (for example, Since it is approximated that 90 degrees corresponds to the phase shift value at the scanning wavelength (for example, 90 degrees), the phase shift operation for each scanning wavelength over the wavelength scanning width can be replaced by repeating the phase shift at the central wavelength. Therefore, a realistic phase shift can be performed extremely easily.

As a result, the phase shift method can be effectively incorporated in the wavelength scanning method, and the shape measurement can be performed with high accuracy in a narrow wavelength scanning width.

Further, from the phase difference amount which is the difference between the interference phase value at the scanning wavelength and the interference phase value at the center wavelength and the wavelength shift amount, the wavelength phase ratio which is the ratio between the wavelength shift amount and the phase difference amount is obtained. Since the shape of the object to be measured is determined from the wavelength phase ratio, it differs from the conventional case where only the magnitude of the phase difference amount is focused, and not only the magnitude of the phase difference amount but also the wavelength phase ratio. The information regarding the shape measurement can also be obtained from the sign of.

[Brief description of drawings]

FIG. 1 is a block diagram showing a wavelength scanning phase shift interferometer using a broadband wavelength tunable laser for implementing a shape measuring method according to the present invention.

FIG. 2 Scanning wavelength λ and phase difference component Δφ (x, y, Δλ)
The figure which showed the relationship of about the plane mirror (a) and the gauge block (b).

FIG. 3 is a diagram showing a result of calculating a height distribution at each pixel from a gradient of a phase difference component Δφ (x, y, Δλ) with respect to a wavelength shift amount Δλ.

FIG. 4 is a diagram showing a cross-sectional shape of each surface of a plane mirror (a) according to a left-side vertical axis and a gauge block (b) according to a right-side vertical axis.

FIG. 5 is a diagram showing a change in standard deviation of height measurement when the long scanning width λsc is increased stepwise.

[Explanation of symbols]

1 light source 2 telescope 3 Michelson interferometer 6 Irradiation light 7 Reference light 8 DUT 8a plane mirror 8b gauge block 9 PZT mirror 10 Object light 12 CCD camera 13 Calculator

─────────────────────────────────────────────────── ─── Continuation of the front page Application for application of Article 30, Paragraph 1 of the Patent Act July 23, 1999 “Report of Technical Committee on Image Applied Technology” Vol. 14, No. 1. Announced in "New three-dimensional sensor and its application" (56) Reference JP-A-2-27202 (JP, A) JP-A-3-154801 (JP, A) JP-A-7-306006 (JP, A) ) JP-A-4-297807 (JP, A) JP-A-6-160044 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01B 9/00-11/30

Claims (4)

(57) [Claims] 1. An object generated by separating a light beam emitted from a light source capable of wavelength scanning into an irradiation light for irradiating an object to be measured and a reference light as a reference, and irradiating the irradiation light to the object to be measured. Photoelectric detection is performed by superimposing light and the reference light on the photoelectric surface, and obtains an interference fringe intensity distribution consisting of a background intensity component and an interference intensity component that depend on an optical path difference between the irradiation light and the reference light, and is measured. In the shape measuring method for measuring the shape of an object, a center wavelength that is the wavelength center is set within a predetermined wavelength scanning width, and various scanning wavelengths are set by shifting the center wavelength by a predetermined wavelength shift amount. , Setting at least three predetermined phase shift values at the central wavelength, and regarding the at least three predetermined phase shift values at the scanning wavelength, the scanning wavelength is approximated to be equal to the central wavelength, and the center is approximated. It is approximated that the predetermined phase shift value in the length corresponds to the phase shift value in the scanning wavelength, and for each scanning wavelength, the irradiation light or the reference light is phase-shifted by the predetermined phase shift value, Obtaining the interference fringe intensity distribution for each phase shift value, obtaining the interference phase value based on the optical path difference in the interference intensity component from the set of the obtained interference fringe intensity distribution, the interference phase value at each scanning wavelength and the From the phase difference amount and the wavelength shift amount, which are the difference with the interference phase value at the center wavelength, the wavelength phase ratio, which is the ratio of the wavelength shift amount and the phase difference amount, is obtained, and the measured wavelength phase ratio is measured. A shape measuring method characterized by measuring the shape of an object. 2. The shape measuring method according to claim 1, wherein the predetermined wavelength scanning width is 1/150 to 1/50 of the center wavelength. 3. A light source capable of wavelength scanning, a light beam emitted from the light source, which is divided into an irradiation light for irradiating an object to be measured and a reference light which serves as a reference, and the irradiation light is irradiated to the object to be measured. An interference optical system that superimposes the generated object light and the reference light on the photocathode, and a wavelength scanning unit that sets various scanning wavelengths that are wavelength-shifted by a predetermined wavelength shift amount from the central wavelength that is approximately the wavelength center, At least three predetermined phase shift values are set at the center wavelength, and for at least three predetermined phase shift values at the scanning wavelength, the scan wavelength is approximated to be equal to the center wavelength, and It is approximated that the predetermined phase shift value corresponds to the phase shift value at the scanning wavelength, and the irradiation light or the reference light is positioned by the predetermined phase shift value for each scanning wavelength. Phase shift means for shifting, for each of the phase shift value, a signal detection means for obtaining an interference fringe intensity distribution consisting of a background intensity component and an interference intensity component depending on the optical path difference between the irradiation light and the reference light, Obtain the interference phase value based on the optical path difference in the interference intensity component from the set of the interference fringe intensity distribution obtained by the signal detection means, between the interference phase value at each scanning wavelength and the interference phase value at the central wavelength From the phase difference amount which is a difference and the wavelength shift amount, a wavelength phase ratio which is a ratio of the wavelength shift amount and the phase difference amount is obtained, and an arithmetic means for calculating the shape of the object to be measured from the wavelength phase ratio. A shape measuring device comprising: 4. The shape measuring apparatus according to claim 3, wherein the predetermined wavelength scanning width is 1/150 to 1/50 of the center wavelength.