JPH04286904A - Phase shift grazing incidence interferometer - Google Patents

Abstract

PURPOSE:To introduce a phase shift method which does not require movement of a body to be measured to improve measurement accuracy by providing two polarizing plates to linearly polarize light from a light source and having object light and reference light interfered with each other on an observation surface. CONSTITUTION:Coherent light from a light source 1 is linearly polarized by a first polarizing plate 21 and divided into reference light 8 and object light 9 by a diffraction grid 7 which respectively enter a diffraction grid 12 directly or through a shift plate 11 after being applied to a body 10 to be measured through the phase plate 22. The object light 9 passes through the phase plate 22 twice and is changed in a polarizing direction by 90 deg. and overlays the reference light 8 in the gird 12, while they do not interfere with each other since polarizing directions are perpendicular to each other. The light is incident to a phase element 23, where a phase shift of a known amount can be applied between the reference light 8 and the object light 9 thereby providing a phase shift method. Then the light is transmitted through a second polarizing plate 24 to have the reference light and the object light 9 interfered with each other on an observation surface 15. Thus introduction of the phase shift method allows phase measurement of one or less interference fringe allowing determination of unevenness of stages.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to an oblique incidence interferometer that makes coherent light obliquely incident on the surface of a measurement object, and is used, for example, for non-contact measurement of the surface shape of a measurement object. It is something that will be done.

0002

2. Description of the Related Art FIG. 2 shows an optical system of a conventional oblique incidence interferometer, and FIG. 3 shows an explanatory diagram of its operation. The collimated laser beam enters the diffraction grating 7, and the transmitted light 8 (0th order diffracted light)
and diffracted light 9 (first-order diffracted light). Here, the grating intervals of the diffraction gratings 7 and 12 are the same. The transmitted light 8 directly enters the diffraction grating 12, and after the diffracted light 9 illuminates the measurement surface of the measurement object 10, its reflected light enters the diffraction grating 12, and these two lights cause interference. Here, the transmitted light 8 serves as a reference light, and the diffracted light 9 serves as an object light.

In FIG. 3, the optical path length δ of the reference beam and object beam is
is given by the following equation. δ=(BC+CD)−(BF+FD)+γ/k
...(1) Here, γ is the change in phase given when reflecting off the measurement surface, k = 2π
/λ. Here, if FC=d, δ=2d(1-Sin(i))Sec(i)+γ/
k...(2) And the order N of the interference fringe is N=2d(1-Sin(i))Sec(i/λ)+
γ/2π...(3).

[0004] Thus, the optical path difference of the interference wavefront reaching the observation surface is constant, and for a flat measurement object, this interferometer shows uniformly illuminated interference fringes. When there is a step difference of Δh on the measurement surface, the change in the optical path difference of the part of the light beam incident on the step part is 2Δh (Cos(i)), and the change in the order of the interference fringe is ΔN=2Δh(Cos(i) /λ))
...(4). On the other hand, if the grating spacing of the diffraction gratings 7 and 12 is p,
The diffraction angle θ is λ=p(Sin(θ))
...
It is expressed as (5). From equations (4) and (5), ΔN=2Δh/p

...(6).

[0005] Therefore, unlike a Fizeau interferometer in which the object light is perpendicularly incident, this interferometer has sensitivity reduced by p/λ, and can measure even relatively uneven surfaces.

Next, the phase shift method will be explained using a Twyman-Green interferometer shown in FIG. Coherent parallel light 16 from the light source is split into two by a half mirror 17, and interference occurs between the object light 9 reflected by the measurement object 10 and the reference light 8 reflected by the reference mirror 11. Interference light 18 is observed on observation surface 15 . The phase shift method is
Not only one interferogram but also the measuring object 10 or the reference mirror 1
1 is moved back and forth in the optical path direction to change the phase of the optical path by a known amount, and the phase distribution φ(x,y) of the interference light 18 is obtained from a plurality of interference patterns obtained. In general,
Let the phase change ψ of the object beam or reference beam be 0, π/2, π,
A 4-step method of changing 3π/2 four times is used. The coordinates (x
, y) respectively as I1 , I2 , I3
, I4, the phase distribution φ(x, y) at this time becomes as shown in the following equation. Here, the phase distribution φ(x, y) is
This refers to the phase difference φ between the object beam and the reference beam with respect to the coordinates (x, y) on the interferogram, and is information that three-dimensionally indicates the uneven shape of the surface of the measurement object.

[0007]

[Math 1]

[0008]

[Problems to be Solved by the Invention] Step measurement using a general interferometer such as an oblique incidence interferometer or a Fizeau interferometer has the following drawbacks. ■It is easy to count the number of interference fringes, but it is difficult to measure the phase of one interference fringe (one fringe) or less. ■It is not possible to judge the unevenness of steps.

[0009] To overcome these drawbacks, phase shift methods have recently been frequently used. However, it is difficult to introduce the phase shift method into grazing incidence interferometers, and it has not yet been used. The present inventor has attempted to introduce a phase shift method into a grazing incidence interferometer by moving the measurement object by a known amount using an actuator, and has achieved certain results (see Japanese Patent Application No. 2-405559). However, a technical issue has arisen in which it is desirable to introduce a phase shift method into a grazing-incidence interferometer to further improve measurement accuracy, without physically moving the object to be measured.

0010

[Means for Solving the Problems] In order to solve the above problems, in the phase shift oblique incidence interferometer of the present invention, as shown in FIG. In the grazing incidence interferometer, a first polarizing plate 21 linearly polarizes the coherent light from the light source 1, and an observation surface 15.
A phase plate 2 is provided in the optical path of the object light 8.
2 to make the polarization directions of the object beam 9 and the reference beam 8 orthogonal, and a phase shifter 23 that provides a known amount of phase shift between the object beam 8 and the reference beam 9 is used as the light source of the second polarizing plate 24. It is characterized by being placed on the first side.

[0011]

[Operation] In the present invention, the first polarizing plate 21 linearly polarizes the coherent light from the light source 1, and the second polarizing plate 21 linearly polarizes the coherent light from the light source 1, and the second polarizing plate 21 linearly polarizes the coherent light from the light source 1. a plate 24, a phase plate 22 disposed in the optical path of the object light 9 to make the polarization directions of the object light 9 and reference light 8 orthogonal, and a phase shifter 23 disposed on the light source 1 side of the second polarizing plate 24. By using this in combination, it is possible to provide a known amount of phase shift between the object beam 9 and the reference beam 8 in the grazing incidence interferometer, so that the phase shift method can be performed. Therefore, it becomes possible to measure the phase of one fringe or less, and it becomes possible to determine the unevenness of the step.

0012

Embodiment FIG. 1 shows an optical system according to an embodiment of the present invention. The coherent light emitted from the laser light source 1 enters the polarizing plate 21, the objective lens 2, the pinhole 3, the total reflection mirrors 4 and 5, and the collimator lens 6, and becomes parallel light with linear polarization. This parallel light is separated by the diffraction grating 7 into 0th-order diffracted light and 1st-order diffracted light. Diffraction gratings 7 and 12 have the same grating spacing. The 0th-order diffracted light is treated as a reference beam 8, and the 1st-order diffracted beam is treated as an object beam 9. The 0th-order diffracted light directly enters the diffraction grating 12, and the 1st-order diffracted light passes through the phase plate 22 and irradiates the measurement surface, and then enters the diffraction grating 12 through the phase plate 22 again.

The phase plate 22 has the function of shifting the phase of the obliquely incident object light 9 by 1/4 wavelength. Further, the optical crystal axis of this phase plate 22 is set at an angle of 45 degrees with respect to the incident light. In this way, the object light 9 passes through this phase plate 22 twice, so the polarization direction changes by 90 degrees.

These lights are also converted to 0 by the diffraction grating 12.
It is divided into second-order diffracted light and first-order diffracted light. Here, the first-order diffraction light of the reference light 8 and the zero-order diffraction light of the object light 9 overlap. However, since the polarization directions of the object light 9 and the reference light 8 are orthogonal, the object light 9 and the reference light 8 do not interfere here.

Next, this light is made incident on a Babinet-Soleil correction plate which acts as a phase shifter 23. At this time, the object light 9 and the reference light 8 are made to coincide with the fast and slow axes of the Babinet-Soleil correction plate. Here, the phase shift method is performed by operating the Babinet-Soleil correction plate to shift the phase of the object beam 9 or the reference beam 8 by a known amount. Preferably, the reference light 8
The phase of is changed four times: 0, π/2, π, and 3π/2. Thereafter, the object beam 9 and the reference beam 8 are caused to interfere with each other by passing through a polarizing plate 24. The interfering light passes through lens 13 and pinhole 14
interference fringes are observed on the observation surface 15. The observed interference fringes are input into a computer via an image processing device and an A/D converter, and the phase of the object light is calculated using the above-mentioned formula.

Although the Babinet-Soleil correction plate is used as an example of the phase shifter 23 for shifting the phase of the object light or the reference light by a known amount in the embodiment, the present invention is not limited to this. It is also possible to use an electro-optical element having the following.

[0017]

According to the present invention, by introducing a phase shift method into a grazing incidence interferometer, the interval between interference fringes becomes wider than that of a general interferometer, making it possible to measure the phase within one fringe.
Furthermore, there is an effect that the unevenness of the step can be determined. Furthermore, in the configuration of the present invention, a known amount of phase difference is provided between the object light and the reference light by using a plurality of optical elements such as the first and second polarizing plates, the phase plate, and the phase shifter. Therefore, compared to the case where an actuator is used to physically move an object by a known amount, it is possible to provide a more precise phase difference, which also has the effect of improving measurement accuracy.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing an optical system according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram showing a conventional optical system.

FIG. 3 is an explanatory diagram of the operation of a conventional example.

FIG. 4 is an explanatory diagram of the operation of another conventional example.

[Explanation of symbols]

1 Light source 8 Reference light 9 Object light 10 Measurement object 15 Observation surface 21 First polarizing plate 22 Phase plate 23 Phase shifter 24 Second polarizing plate

Claims (1)

[Claims] 1. A grazing-incidence interferometer in which coherent light is obliquely incident on a measurement surface of a measurement object, comprising: a first polarizing plate that linearly polarizes coherent light from a light source; and an object light incident on an observation surface. A second polarizing plate is provided for interfering with the reference light, and a phase plate is arranged in the optical path of the object light so that the polarization directions of the object light and the reference light are orthogonal, and a phase plate is provided between the object light and the reference light. A phase shift grazing incidence interferometer characterized in that a phase shifter giving a known amount of phase shift is disposed on the light source side of the second polarizing plate.