JPS6321503A - Method of measuring fringe scanning interference - Google Patents

Abstract

PURPOSE:To effect easy and positive fringe scanning without using piezo- elements, by arranging in order a plurality of wave-length plates within the preset fringe scanning range. CONSTITUTION:A fringe scanning range is located in a light-path portion which is squeezed between a polarizer 10 and a light-analyser 12 and 2 polarizing elements orthogonally intersecting each other are not separated. And, the fringe scanning is conducted by distributing wave-length plates 40 in the fringe scanning range in order. In other words, in case where the wave-length plates of the same specification are used, the scanning becomes available by increasing one by one the number of wave-length plates to be arranged in the fringe scanning range. And, when using different wave-length plates, the scanning becomes available by sequential exchange of the plates or changing combination of the plates. Thus, easy and positive fringe scanning can be done without using piezo-elements.

Description

[Detailed description of the invention] (Technical field) The present invention relates to a fringe scanning interferometry method.

(Prior Art) The interference measurement method is known as the 1111 method, which includes the Twyman-Green measurement method, the shearing interference measurement method, and the like. Among such interferometric measurement methods, a fringe scanning interferometric measurement method is known. That is, the interference pattern is changed by changing the phase difference between the two light beams that generate the interference pattern, and the multiple interference patterns thus obtained are analyzed to obtain precise interference al! This is a method of determining I. Changing the phase difference between the two light beams in this way is called fringe scanning, and conventionally, this fringe scanning has been performed using a piezo element.

However, piezo elements are expensive, require a dedicated complicated circuit to drive them, and require a high-voltage power source for driving them.

(rT) The present invention has been made in view of the above-mentioned circumstances, and its purpose is to provide a novel fringe scanning interference method that can easily and reliably perform fringe scanning without using piezo elements. The aim is to provide a measurement method.

(Ellipsis) Two aspects of the present invention will be explained below.

The present invention can be applied to interference measurement methods including the Twyman-Green measurement method, shearing interference measurement method, etc.
In order to apply the invention, the interferometric system must have a polarizer and an analyzer.

The fringe scanning interferometry method of the present invention is characterized by the method of performing fringe scanning.

A fringe scanning area, that is, a region where operations for fringe scanning are performed is set at a specific location on the optical path.

That is, the fringe scanning area is sandwiched between a polarizer and an analyzer,
In addition, the two orthogonal polarized light components are set in an optical path portion where they are not separated.

On the other hand, a plurality of wavelength plates are prepared. These wave plates may be the same or different. The same wavelength plates give the same phase difference between the ordinary ray and the extraordinary ray, and this phase difference is set to a predetermined value.

Further, in the case of mutually different wavelength plates, there is a certain relationship between the phase differences provided by each wave plate.

Fringe scanning is performed by sequentially arranging the wave plates in the fringe scanning area. That is, when using the same wavelength plates, this can be done by sequentially increasing the number of wavelength plates arranged in the fringe scanning area by one, then by -, and so on. In addition, if different wavelength plates are used, the wavelength plates placed in the fringe scanning area may be replaced one after another.

Alternatively, fringe scanning can be performed by changing the combination of wave plates arranged in the fringe scanning area.

Hereinafter, description will be given based on specific examples.

FIG. 1 shows an embodiment of the invention.

This embodiment is a fringe scanning shearing interferometry apparatus, and the figure shows the main parts thereof.

In the figure, 1 and 10 are polarizers, 12 is an analyzer, 20 is a Wollaston prism, 30 is an area sensor such as a CCD camera, and 40 is one of the wavelength plates.

The measurement light having the wavefront shape W(X) to be specified enters from the left. Note that the wavefront shape generally depends not only on the X coordinate but also on the y-axis coordinate perpendicular to one drawing, and W(X, Y
), but for simplicity, it is abbreviated as W(x).

Now, when the measurement light passes through the polarizer 10, it becomes linearly polarized light.The linearly polarized state is a state in which the vibration direction of the ordinary ray and the vibration direction of the extraordinary ray are perpendicular to each other, and there is no phase difference between the two rays. .

The linearly polarized measurement light then passes through the wavelength plate 40, but in the transmitted light, there is a gap between the ordinary ray and the extraordinary ray.
A predetermined phase difference δX determined by the wave plate is given. Note that it is usually desirable to adjust the polarization direction of the linearly polarized light and the crystal direction of the wave plate to 45° in order to equalize the light amount between the extraordinary rays.

The measurement light continues to enter the Wollaston prism 20,
Due to the refraction effect of the prism 20, the traveling direction of the ordinary ray,
The traveling directions of the extraordinary rays are separated from each other by an angle of 0. The wavefronts of ordinary rays and extraordinary rays are A(X) and V, respectively.
Let it be B(X).

The measurement light continues to pass through the analyzer 12. As a result, the light beams having wavefronts wA(x) and 1118(X) become coherent, enter the area sensor 30, interfere with each other, and generate an interference pattern.

Assuming that the distance between the branch point of the ordinary ray and extraordinary ray formed by the Wollaston prism 20 and the light receiving surface of the area sensor 30 is L as shown in the figure, the wavefront VA (X) is equal to VB (X).

A lateral shift occurs by S=2・Ltanθ/2 (1), and these wavefronts WA (X), VB
(X) are inclined to each other by θ. Also, both wavefronts -A
(X).

The phase difference δ between WB(X) is the Wollaston prism 2
The phase difference δ given by 0. (a constant value), using the above-mentioned phase difference δX, δ=δχ+δ. (2) It is possible to have a hailstorm.

Therefore, by replacing the wave plates 10 one after another and changing the phase difference δX in an orderly manner, fringe scanning is performed, and the pattern of the interference fringes is analyzed according to the well-known fringe scanning shearing interferometry method. Shape can be specified. The slope between wavefront WA(X) and 1jB(X) can be corrected computationally.

As is well known, in order to specify the wavefront three-dimensionally, first,
Using the configuration shown in Figure 1, find the shape of the wavefront. Next, measure the wavefront shape (for one line) with the configuration shown in Figure 1 (device configuration) rotated 90 degrees around the optical axis.
A three-dimensional wavefront shape is obtained by combining the wavefront shape obtained by the above determination with the wavefront shape for one line.

Here, when considering the position where the wave plate 10 is arranged, that is, the fringe scanning area in accordance with FIG. 1, it is clear from the figure that this fringe scanning area is located between the polarizer and It is in the area between photons and photons. Also.

When transmitted through the polarizer 10, the extraordinary ray and the ordinary ray are two polarized components orthogonal to each other, but since they are separated by the Wollaston prism 20, the wavelength plate 40 is arranged for stripe scanning. The region is, after all, an optical path region sandwiched between the polarizer and the analyzer, and in which two orthogonal polarized light components are not separated.

In fringe scanning, the phase difference given between the interference lights is 1, usually 0.0, where λ is the wavelength. λ/4. To realize such a phase difference, which is often λ/2 or 3λ/4, using the method of the present invention, for example, the following may be performed.

That is, the phase difference given between the extraordinary rays is usually λ/4.
Prepare three wave plates with wavelengths of λ/2 and 3λ/4,
First, an interference fringe pattern is generated without using a wave plate (that is, with a phase difference of 1 O) and read.

Next, a wave plate giving a phase difference of λ/4 is placed in the fringe scanning area, and the interference fringe pattern is read. Next, the wave plate is replaced, and an interference fringe pattern is generated and read using the phase difference of λ and t2.Finally, the phase difference of 3λ/t2 is generated and read.
Replace it with a wave plate that gives 4.

The generated interference fringe pattern is read. In this way, the phase difference between the extraordinary rays is usually 0°λ/4. λ/2
, 3λ/4.

Alternatively, prepare three wave plates that give a phase of λ/4, first read the interference fringe pattern with a phase difference of 0 (no wave plate installed), and then read the interference fringe pattern with one, two, and three wave plates. It is also possible to realize a phase difference of O9λ/4°λ/2, 3λ/4 by sequentially disposing them in the fringe scanning area and increasing the number of disposed wavelength plates.

In this way, when performing fringe scanning by changing one phase difference by λ/4, it is sufficient to prepare three wavelength plates. Alternatively, a first wave plate providing a phase difference of λ/4 and a λ/2
Using a second wave plate that gives a phase difference of 0, a phase difference of 0 is achieved without any wave plate, and
A phase difference of λ/4 and a phase difference of λ/2 are realized by the second wave plate, and the combination of the first and second wave plates realizes a phase difference of 3λ/4.
It is also possible to realize a phase difference of . In this case, two types of wavelength plates are sufficient to be prepared in advance.

When the unit of phase difference during fringe scanning is λ/3, the phase difference O9
To perform fringe scanning with λ/3 and 2λ/3, the phase difference λ/3
Two wave plates giving a phase difference of λ/3 and a phase difference of 2λ/3 may be used.

In the fringe scanning shearing interferometry method, it is necessary to separate the measurement light into two beams of light, which are then laterally shifted from each other.

In the embodiment shown in FIG. 1, this lateral shift is effected by a Wollaston prism 20 utilizing birefringence.

It is generally possible to separate and shift the ordinary ray and extraordinary ray laterally using a birefringent polarizing element, and a Wollaston prism is an example of a birefringent polarizing element.

This is another example of a birefringent polarizer. Examples using a Savard plate and a Rochon prism are shown in FIGS. 2 and 3, respectively. Items with no risk of confusion are indicated using the same reference numerals in FIGS. 2 and 3. Also in these figures.

+11AI(X) and A2(X) detect the wavefront of the ordinary ray, and WBI(X) and 1ilB2(X) detect the wavefront of the extraordinary ray.

In FIG. 2, reference numeral 22 indicates a Savard plate.

When the extraordinary ray and the ordinary ray are separated using this Savart plate 22, the traveling directions of the separated rays become parallel to each other after passing through the Savart plate 22, so the wavefront QA
I(X) and WABI(X) overlap on the area sensor 30 in a sideways state without being tilted to each other. The amount of shear, that is, the amount of lateral shift can be adjusted depending on the thickness of the Savart plate 22.

In FIG. 3, reference numeral 24 indicates a Rochon prism.

When the ordinary ray is separated by the Rochon prism 24, the ordinary ray passes straight through the Rochon prism, but
The extraordinary ray travels in a direction tilted by an angle θ1 with respect to the ordinary ray. Therefore, the wavefront wA2(X) and I/B2(X) are
On the area sensor 30, they overlap each other at an angle of 01. The shear amount S1 in this case is determined using the angle θ1 and the distance L1 in FIG.

5i=L1 tanθ1(3) is given by

The phase difference between the wavefronts is δ given by the birefringent polarizer. , if the phase difference given by the wave plate is δX, then it is given by the above equation (1). However, when using a Savart plate, δ. =0.

The method of stripe scanning is the same as in the embodiment shown in FIG.

FIG. 4 shows an example in which the present invention is implemented as a Twyman-Green fringe scanning interferometry method. In FIG. 4 as well, the same reference numerals as in FIG. 1 are used for parts that are likely to cause no confusion.

The light beam incident from the left side of FIG. 4 is first converted into linearly polarized light by the polarizer 10, and then a phase difference is given between the ordinary ray and the extraordinary ray by the wave plate 40. Thereafter, the light flux is divided by the polarizing beam splitter 26, one of which is incident on the reference surface 50 and the other is incident on the measurement surface 60, and is reflected by the 174 wavelength plates 70, , ! When the light beams enter the polarizing beam splitter 26 again via the .to.

Fringe scanning is performed by sequentially arranging a plurality of wavelength plates in a fringe scanning area. For example, the phase difference λ/4. λ/
By preparing three wave plates that give 2,3λ14, and then placing no wave plates (phase difference 0), then placing the three wave plates alternately in the above order, it is possible to always The phase difference between the rays and the extraordinary rays.

Sequentially 0. λ/4. It can be switched to λ/2, 3λ/4. By analyzing the various interference fringe patterns obtained in this way, the shape of the m-plane 60 can be known.

As a method of sequentially arranging wave plates in the stripe scanning area,
As mentioned above, there is a replacement method, that is, a method in which different wavelength plates are replaced, and an addition method, that is, a method in which wavelength plates of the same type are added one by one.

A combination method is possible, ie, a method in which different types of wave plates are exchanged or combined.

In the case of the exchange method, a rotary turret (different kinds of wave plates 40-■ to 40-6
are integrated with a support 42, and the phase difference is changed stepwise by switching the wavelength plate by rotation), or a sliding turret as shown in Figure 5 (■) (different types of wavelength plates are integrated). It is convenient to use a device in which 40-A, 40-B, etc. are integrated with a support 44, and the wave plate disposed in the stripe scanning area is switched by sliding in the longitudinal direction. Moreover, as shown in FIG. 5 (+111), each wavelength plate 4
It is also possible to prepare a plurality of supports 46 for each 00 and perform the exchange method by replacing them.
When a plurality of such wavelength plates are prepared independently, it is convenient to realize a combination method of one additional method. Note that if the amount of light on the area sensor changes due to one combination of wave plates, etc., the amount of light from the light source may be adjusted accordingly.

(Effects) As described above, according to the present invention, a novel fringe scanning interferometry method can be provided.

In this method, fringe scanning is performed by exchanging wave plates, etc., so there is no need to use expensive piezo elements. Therefore,
The cost of measurement equipment can be reduced. Further, the entire optical system can be easily made into a common path, and the influence of disturbance can be reduced.

[Brief explanation of drawings]

Fig. 1 is a diagram showing one embodiment of the present invention, Fig. 2 is a diagram showing another embodiment, Fig. 3 is a diagram showing another embodiment, and Fig. 4 is a diagram showing still another embodiment. , FIG. 5 is a diagram for explaining the usage form of the wave plate. DESCRIPTION OF SYMBOLS 10... Polarizer, 12... Analyzer, 20... Wollaston prism, 30... Area sensor, 40...
... Wave plate.

Claims (1)

[Claims] In a fringe scanning interferometry method using a polarizer and an analyzer, a stripe scanning interferometry method is provided in which the optical path is sandwiched between the polarizer and the analyzer and the two orthogonal polarized components are not separated. A fringe scanning interferometry method characterized in that fringe scanning is performed by setting an area, preparing a plurality of wavelength plates, and sequentially arranging these wavelength plates in the fringe scanning area.