JP2003065709A - Method for analyzing interference fringe of transparent parallel flat plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that it is necessary to set the distance L between the reference plane and the measurement plane of an optical measuring apparatus which measures a transparent parallel flat plate by a interferometer using wavelength variable laser, so that the ratio of the distance L to the optical thickness nT of the parallel flat plate satisfies the following: nT=3.5L in order to improve the accuracy, but the allowed set error of the distance L is only about 1.4% and unhandy for measuring. SOLUTION: The interferometer system is provided with an apparatus which sets the distance L on the optical axis between the reference plane and the measurement plane so that the ratio of the distance L to the optical thickness nT of the parallel flat plate satisfies the following: 2L<=nT<=9L, and an imaging means which successively photographs nineteen images having information of interference fringes formed by the optical interference of light beams from the reference plane and the plane of parallel flat plate to be measured while varying the central wavelength λ of the emitted light by the value of λ<2> /12L. A calculating operation based on a proposed expression is carried out for the information of nineteen interference fringe images, and the phase information relating to the shape of the plane of parallel flat plate to be measured is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference fringe analysis method for obtaining phase information of the surface shape of a transparent parallel plate or an optical parallel plate which is an object to be measured in an interferometer device using a tunable laser as an observation light source. In particular, regarding the interference fringe noise generated by the reflected output light from both the front surface and the back surface of the parallel plate, and the interference fringe noise generated by the optical interference of the output light multiple-reflected three times between the front surface and the back surface of the parallel plate. It relates to an analysis method.

[0002]

2. Description of the Related Art Conventionally, as an optical measuring device for measuring a transparent parallel plate using a wavelength tunable laser as a light source, for example, K.
Okada, H.Sakuta, T.Ose, and J. Tsujiuchi, "Separa
te measurements of surface shapes and refractive i
ndex inhomogeneity of an optical element using tun
able source phase shifting interferometry, "Appli
ed Optics, vol.29 (1990) 3280-3285. (Reference 1), Twyman Green interferometer, and PJ de Groo
t, "Measurement of transparent plates with wavelen
gth-tuned phase shifting interferometry, "Applied
The Fizeau interferometer device shown in Optics, vol.39 (2000) 2658-2663. (Reference 2) is known.

In such an interferometer, the parallel plate surface to be measured (measurement surface) and the reference surface are illuminated with a monochromatic plane light wave, and the reflected output light from both surfaces is integrated to form an interference fringe.
An image is taken by an image pickup device such as a camera, and the obtained interference fringe image is analyzed to observe and measure the shape (phase change) of the surface to be inspected.

In this measurement, in addition to the reflected output light from the measurement surface (measurement light), the reflected output light from the back surface of the parallel plate and the multiple reflection output light which is reflected and output between the measurement surface and the back surface an odd number of times Exist and are integrated with the output light from the reference surface (reference light) to form interference fringes, which become noise signals. Therefore, it is necessary to remove the influence of these noise signals in the measurement of the plane of parallel plates.

With respect to this point, in the above-mentioned document 1, direct analysis of 60 interference fringe images recorded by the image pickup device every time the wavelength λ of the output light is changed by λ ² / 4L is performed, thereby directly reflecting from the back surface. It succeeded in removing the noise signal generated by light, and the measurement error was λ / 60 (square root squared error) or less.

Further, in the above-mentioned reference 2, the wavelength λ of output light is λ
^2/1 recorded by the imaging device 8L by every changing
By analyzing the three interference fringe images, we succeeded in removing a part of the noise signal created by the direct reflected light from the back surface and the noise signal created by the multiple reflected output light, resulting in a measurement error of λ / 200. .About..lamda. / 50 (maximum minimum error) or less is obtained.

[0007]

The height h at each point of the parallel plate is calculated by the following equation h = λψ (x, y) / 4π which is well known in the reflection type interferometer. In the technology up to now, the maximum measurement accuracy of the parallel plate is λ / 200 by removing the noise signal by the method of Reference 2. However, in order to achieve accuracy with this method, the distance L must be set so that the ratio of the distance L between the reference surface and the measurement surface and the optical thickness nT of the parallel plate satisfies nT = 3.5L. . Distance L at that time
The allowable setting error of is about 1.4%.

In an interference measuring apparatus using a wavelength tunable laser as a light source, the distance L is directly related to the wavelength scanning amount, as can be seen from an example in which measurement is performed every time the wavelength λ of output light is changed by λ ² / 8L. It is desirable that the parameters to be measured can be fixed and measured as much as possible for each device. If this is not done, it is necessary to make adjustments such as changing the measurement time of the imaging device each time the distance is changed. Due to the above-mentioned circumstances, it is extremely inconvenient to construct a measuring instrument that the distance L is adjusted every time for each material of the parallel plate whose thickness and refractive index are slightly different. It also requires additional procedures such as measuring the thickness and refractive index of the flat plate.

Regarding the measurement accuracy required in the industrial world, it is not uncommon for parallel plates for etalons to be around λ / 100, and it cannot be said that the current methods have achieved sufficient measurement accuracy.

The present invention uses a new method of analyzing 19 interference fringe images, and the allowable setting error of the distance L, which is one of the above problems, is 10 times or more that of the conventional method.
The purpose is to achieve%. Further, regarding the other problem, the maximum measurement accuracy, the aim is to realize λ / 600.

[0011]

In order to solve the above-mentioned problems, the present invention according to claim 1 relates to the height at each position on the surface of a plane-parallel plate which is transparent at a wavelength λ, or is formed on a substrate. In the method for measuring the height at each position of the surface of a transparent thin film, an illumination light source whose output light is coherent and whose central wavelength λ can be changed with time, and a light flux from the illumination light source is a parallel light flux. After that, an optical system that guides the light onto the reference surface and the parallel plate surface to be measured, and the ratio of the distance L on the optical axis between the reference surface and the measured surface to the optical thickness nT of the parallel plate is 2L. < NT < 9L, and a device for setting the distance L, and interference fringe information obtained by optical interference of light beams from the reference surface and the parallel plate surface to be measured are obtained by using the center wavelength λ of the output light as λ. each varying by ² / 12L, continuously 19 imaging In the interferometer apparatus provided with an image means, interference of 19 sheets obtained by imaging fringe image information I _{- 9}
(X, y), I- ₈ (x, y), ..., I ₀ (x, y
y), I ₁ (x, y), ..., I ₉ (x, y) are subjected to arithmetic processing based on the following equation (1) to obtain phase information ψ (regarding the shape of the parallel plate surface to be measured. x, y) is obtained, which is an interference fringe analysis method. Formula (1)

[Equation 3] Here, the constants ar and br have the following values.

[Equation 4]

According to a second aspect of the present invention, in the interferometer device, the ratio of the distance L on the optical axis between the reference surface and the surface to be measured and the optical thickness nT of the parallel plate is 14L <
The distance L is set so as to satisfy nT < 21L.
This is the interference fringe analysis method described.

According to a third aspect of the present invention, in the interferometer device, the front surface and the back surface of the flat plate are not completely parallel, and when the parallel light is irradiated, the interference fringes produced by the light emitted from both surfaces are 200. The interference fringe analysis method according to claim 1, wherein the interference fringe analysis method is used for an object having an inclination angle that can be observed within this number.

The invention according to claim 4 is characterized in that the interferometer device is a Fizeau interferometer, and the phase difference between the measurement light from the surface to be measured and the reference light from the reference surface is about The interference fringe analysis method according to claim 1, 2 or 3, further comprising means for capturing 19 images by shifting by π / 6.

The invention according to claim 5 is characterized in that the interferometer device is a Mirau type interferometer, and the phase difference between the measurement light from the surface to be measured and the reference light from the reference surface is about π. The interference fringe analysis method according to any one of claims 1 to 3, further comprising means for capturing 19 images by shifting by / 6.

The invention according to claim 6 is the case where the interferometer device is a Fizeau interferometer using a near infrared wavelength as a light source, and the parallel plate is a silicon wafer.
4. The device according to claim 1, further comprising means for capturing 19 images by shifting the phase difference between the measurement light from the surface to be measured and the reference light from the reference surface by about π / 6. This is the interference fringe analysis method described.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Prior to describing a specific example of a method for analyzing interference fringes of a transparent parallel plate in the present invention, the principle by which the above problems can be solved by the present invention will be described. When the wavelength λ of the output light is temporally changed in the interferometer of FIG. 1, the interference fringe intensity which is formed by the light interference of the light flux and is recorded in the imaging device at the time t is expressed by the following equation.

[Equation 5] However, regarding the modulation frequency, assuming that the distance between the reference surface and the measurement surface is L and the wavelength changes by δλ in a unit time,

[Equation 6] Is defined by In Expression 3, the phase value ψ ₁ is the phase information representing the shape of the measurement surface and is the information to be extracted by the analysis method. Also, the term including amplitude s _k in the equation is a noise term generated by the back surface reflection and multiple reflection. FIG. 2 shows the frequency spectrum J (ν) of the signal and noise when the optical thickness of the parallel plate nT = 3L.

The result obtained by capturing the interference fringe image each time the light source wavelength changes by λ ² / 12L and analyzing the phase information by the mathematical formula 1 is represented by the following formula.

[Equation 7]

The first term on the right-hand side of Equation 5 is the phase value to be obtained, and the second term is the error term. Here, the function F is expressed by the following equation.

[Equation 8]

The optical thickness of the parallel plate to be measured is 2L < nT.
When it is in the range of < 9 L, the frequency spectrum J (ν) of FIG.
_{r 1} and b _r are determined so as to minimize the second term error term on the right side of Expression 5. In this way, the analysis parameters a _r , b
_r is determined as in Equation 2.

FIG. 3 shows the calculation results of the maximum and minimum measurement errors of the phase value ψ ₁ when this analysis method is adopted. In this figure, calculation was performed assuming that the reflectances of the reference surface, the measurement surface and the back surface are all 4%. For comparison, the figure also shows the error when using the conventional analysis method (13-image method) introduced in Document 2 above. According to the method of the present invention, the thickness of the parallel plate is 2
When L < nT < 9L, the error magnitude is λ.
It is / 60 or less, which is superior to the maximum value (λ / 60 in square root error) of Conventional method 1. Further, it is understood from the comparison of FIG. 3 that the present method gives a smaller error in all the ranges than the conventional method 2 and is excellent.

From the results shown in FIG. 3, it is clear that the maximum measurement accuracy of λ / 600 is realized when the optical thickness of the parallel plate to be measured is in the range of 2.8L < nT < 4.1L. Furthermore, assuming that the ratio of the distance L between the reference surface and the measurement surface and the optical thickness nT of the parallel plate is set to satisfy nT = 3.5L, the allowable setting error of the distance L at that time is ΔL It is shown that /L=0.6/3.5=17% is acceptable. Therefore, the allowable error of the conventional method 2 is 1.4%, which is 1
It has more than doubled.

FIG. 4 shows the maximum and minimum measurement error of the phase value ψ ₁ when this analysis method is adopted when the thickness of the parallel plate is further large. In addition to the case where the thickness of the parallel plate is in the range of 2L < nT < 9L, it is possible to measure with a small error in the range of 14L < nT < 21L.

Next, a specific example of an interference fringe analysis method according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a Fizeau interferometer apparatus for carrying out the interference fringe analysis method according to this embodiment. In this embodiment, the case where the above-mentioned predetermined optical surface is the lens surface of the collimator lens will be described as an example.

In this Fizeau interferometer device, the laser light emitted from the monochromatic wavelength tunable laser light source 11 such as a laser diode capable of varying the wavelength λ of the output light is collimated by the collimator lens 12 into a parallel light beam, and the reference plate is used. 1
It is incident on the reference surface 1 of 3 and the measured surface 2 of the measured parallel plate 14. The light beam reflected by the reference surface 1 and the light beam reflected by the measured surface 2 interfere with each other, go backward in the optical path, pass through the semi-transparent mirror 15, and the phase information of the measured surface 2 on the imaging surface of the CCD camera 16. To form an interference fringe.

The interference fringe image information obtained here is subjected to predetermined arithmetic processing in the arithmetic unit 17, and effective and highly accurate interference fringe analysis is performed.

By the way, in the interferometer device using such a variable wavelength laser light source 11, the back surface 3 of the parallel plate is used.
Interference light is generated by the reflected light from the reference surface 1 and the reflected light from the reference surface 1, and the back surface 3 and the measurement surface 2 are exposed three times, five times,
The output light reflected an odd number of times toward the semi-transparent mirror 15 interferes with the output light from the reference surface 1 to generate interference fringe noise in the CCD camera.

These noises cause the accuracy of fringe intensity measurement to deteriorate. Therefore, in the present embodiment, when the distance L is set so that the ratio of the distance L between the reference surface 1 and the measurement surface 2 and the optical thickness nT of the parallel plate satisfies 2L < nT < 9L, these noises are set. Paying attention to the point that the temporal phase change of the fringes is different from the phase change of the signal interference fringes, the reflected light from the back surface 3 and the back surface 3 and the measurement surface 2 are calculated by calculation processing.
It is possible to detect the phase by removing the influence of the output light reflected three times between.

Here, concrete numerical values in the above method will be shown. When the distance L between the reference surface 1 and the measurement surface 2 is 3 mm, and the refractive index of the parallel plate is n = 1.5, the thickness Tmm of the parallel plate is 4 < T < 18 and 28 < T < 42 in the above arrangement. It is possible to measure a parallel plate satisfying the above conditions. At that time, in particular, the thickness Tmm was 5.6 < T < 8.2, 13.8 < T < 16.
1, 29.6 < T < 32.2, 37.8 < T < 40.1
In the range of, the measurement error due to noise is λ / 600
It becomes the following.

Further, the interferometer device is a Fizeau interferometer using a near infrared wavelength as a light source, and a parallel flat plate is a silicon wafer, so that a light source having the same wavelength as observing the inside of the silicon wafer is used. It is possible to selectively detect only the shape of the silicon surface. Conventionally, it was necessary to prepare a light source of another wavelength for surface observation, but by using the same light source, the device can be downsized. Further, by subtracting the measured surface shape from the silicon wafer transmission image, various defects including internal defects of the silicon wafer can be detected.

[0031]

As described above, according to the interference fringe analysis method of the present invention, in the interferometer device using the variable wavelength illumination light source, the distance L between the reference plane and the parallel plane to be measured and the parallel plane are measured. The interference fringes finally obtained by setting the ratio of the optical thickness nT of the above to a value in a predetermined range and calculating 19 interference fringe image data obtained every time the illumination wavelength is shifted by a predetermined amount. From the phase, the influence of noise interference fringes resulting from multiple reflections on the back surface and inside of the parallel plate can be eliminated. This makes it possible to obtain a highly accurate and highly reliable parallel plate surface shape.

Further, in the invention according to claim 6, when the interferometer device is a Fizeau interferometer using a near infrared wavelength as a light source and the parallel plate is a silicon wafer, the inside of the silicon wafer is observed. It is possible to selectively detect only the shape of the silicon surface using a light source having the same wavelength as that of the above. Conventionally, it was necessary to prepare a light source of another wavelength for surface observation, but by using the same light source, the device can be downsized. Further, by subtracting the measured surface shape from the silicon wafer transmission image, various defects including internal defects of the silicon wafer can be detected.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an interferometer device for carrying out an interference fringe analysis method of the present invention.

FIG. 2 is an explanatory diagram showing a frequency spectrum at a position of an interference fringe signal input to the image pickup apparatus.

FIG. 3 is a graph showing a measurement error (PV value: maximum / minimum error) when a parallel plate is measured by a Fizeau interferometer using the 19-image interference fringe analysis method of the present invention.

FIG. 4 is a graph showing a measurement error (PV value: maximum / minimum error) when a parallel plate is measured by a Fizeau interferometer using the 19-image interference fringe analysis method of the present invention.

[Explanation of symbols]

1 reference plane 2 Surface to be measured 3 Parallel plate back side 11 Tunable laser source 12 Collimator lens 13 Reference plate 14 parallel plates 15 semi-transparent mirror 16 Imaging device 17 arithmetic unit

Claims (6)

[Claims] 1. A method for measuring the height at each position of the surface of a plane-parallel plate transparent at a wavelength λ or the height of each surface of a transparent thin film formed on a substrate, wherein the output light is An illumination light source that is coherent and can change its central wavelength λ with time; an optical system that guides a light flux from the illumination light source onto a reference plane and a parallel plate surface to be measured; A device for setting the distance L such that the ratio of the distance L on the optical axis to the surface to be measured and the optical thickness nT of the parallel plate satisfies 2L < nT < 9L, and the reference surface and the object to be measured. The interference fringe information obtained by the light interference of the light flux from the plane of measurement parallel plate is measured by the central wavelength λ of the output light.
In the interferometer device equipped with an imaging means for continuously capturing 19 images each time λ ² / 12L is changed by λ ² / 12L.
_-9 (x, y), I- ₈ (x, y), ..., I
₀ (x, y), I ₁ (x, y), ..., I ₉ (x, y
An interference fringe analysis method, characterized in that y) is subjected to arithmetic processing based on the following equation (1) to obtain phase information ψ (x, y) regarding the shape of the parallel plate surface to be measured. Expression (1) Here, the constants ar and br have the following values. [Equation 2] 2. In the interferometer device, a ratio of a distance L on the optical axis between the reference surface and the surface to be measured to an optical thickness nT of the parallel plate satisfies 14L < nT < 21L. The interference fringe analysis method according to claim 1, wherein the distance L is set. 3. In the interferometer device, the front surface and the back surface of the flat plate are not perfectly parallel, and when the parallel light is irradiated, the interference fringes formed by the light emitted from the both surfaces can be observed within 200 lines. Use for a thing with a horn 1.
The interference fringe analysis method described. 4. The interferometer device is a Fizeau interferometer, wherein the phase difference between the measurement light from the surface to be measured and the reference light from the reference surface is shifted by about π / 6, and
The interference fringe analysis method according to claim 1, 2 or 3, further comprising means for capturing nine images. 5. The nineteen interferometer devices are Mirau-type interferometers, and the phase difference between the measurement light from the surface to be measured and the reference light from the reference surface is shifted by about π / 6. The interference fringe analysis method according to claim 1, further comprising a unit that captures an image. 6. The interferometer device is a Fizeau interferometer using a near infrared wavelength as a light source, the parallel plate is a silicon wafer, and the measurement light from the surface to be measured and the reference light from the reference surface are used. 4. The interference fringe analysis method according to claim 1, further comprising means for capturing 19 images by shifting the phase difference by about π / 6.