JP2527176B2 - Fringe scanning shearing interferometer - Google Patents

Description

TECHNICAL FIELD The present invention relates to a fringe scanning shearing interferometer.

(Prior Art) In a fringe scanning shearing interferometer, a measurement light beam, that is, a light beam having a measurement wavefront is divided into two optical paths, and one of the divided light beams is offset or sheared with respect to the other. Since the lateral shift amount, that is, the shear amount in the shear directly affects the measurement accuracy, it is necessary to know this with high accuracy.

As a method of performing shear, conventionally, a method of utilizing reflection on the front and back surfaces of a plane-parallel glass, a method of utilizing lateral displacement due to refraction when obliquely transmitting through a plane-parallel glass, a method of using a corner cube prism, etc. are known. ing. However, in all of these, the light flux sheared is a plane wave, and therefore interference fringes do not occur even if they interfere with each other, and therefore high-precision shear amount measurement using interference cannot be performed.

(Object) The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a novel stripe pattern in which the amount of shear can be known with high accuracy and therefore high-accuracy measurement is possible. It is to provide a scanning shearing interferometer.

(Configuration) Hereinafter, the present invention will be described.

The present invention is a fringe scanning shearing interferometer. This device has a selection optical system, a set of a-focal lens system, a beam splitter, two plane mirrors, a piezo element, and a rotation driving means.

The selection optical system is an optical system for arbitrarily selecting the measurement wavefront and the reference plane wavefront.

The set of a-focal lens system is a lens system for relaying the measurement light beam to the light receiving surface when the measurement wavefront is selected.

The beam splitter splits the light beam into two in the optical path sandwiched by the lenses of the a-focal lens system.

The two plane mirrors respectively reflect the two divided light beams and combine them again via the beam splitter.

The piezo element performs fringe scanning by displacing one of the plane mirrors.

The rotation driving means slightly rotates the other plane mirror. The rotation axis of this minute rotation passes through the mirror surface of the other plane mirror and is orthogonal to the incident optical axis to this mirror surface.

The focal point positions of the set of a focal lens systems coincide with the mirror surfaces of the two plane mirrors. Therefore, the rotation axis passes through the focal point position on the other plane mirror.

(Example) Hereinafter, a specific description will be given with reference to the drawings.

FIG. 1 is an explanatory diagram schematically showing only a main part of one embodiment of the present invention. This embodiment is an apparatus for measuring the surface shape of an object. In the figure, reference numeral 10 indicates a laser light source.
The light beam emitted from the laser light source 10 has its beam diameter expanded by the beam expander 12, reflected by the mirror 14, and then enters the beam splitter 16. Laser light source 10, beam expander 12, mirror 14,
It constitutes the light source optical system of the measuring device.

The light beam that has entered the beam splitter 16 is split into two light beams, one of which is incident on the above-described test surface of the object O, that is, an object having a test surface whose shape is to be measured, and the other is on the reference surface of the mirror N. Incident. The reference plane is a plane finished with high precision. Light reflected from the surface to be inspected or the reference surface passes through the beam splitter 16 or is reflected by the beam splitter 16 and is emitted from the beam splitter 16 and is reflected by the mirror 18
Is reflected by and enters the measurement system. Beam splitter
Shutters S1 and S2 are provided between 16 and the object O and the mirror N, and by selectively opening and closing these, reflected light entering the measurement system can be selected. That is, beam splitter 16, mirrors N, 18, shutters S1, S
In the present embodiment, 2 constitutes a selection optical system, and by this selection optical system, any one of the reflected light from the reference surface is selected as the reflected light from the surface to be inspected and made incident on the measurement system. be able to. The reflected light from the surface to be inspected is a measurement light beam and has a measurement wavefront. The wavefront of the reflected light from the reference surface is the reference plane wavefront.

Now, the measurement system includes lenses 20A and 20B, a beam splitter 22, plane mirrors 24 and 28, a piezo element 26, and a rotation driving means 30 that constitute one set of a focal lens system.
And an area sensor 32.

The former image-side focal point and the latter object-side focal point of the lenses 20A and 20B constituting the a-focal lens system coincide with each other to form a focal point. The focal point of this a-focal lens system coincides with the mirror surfaces of the plane mirrors 24 and 28. With this afocal lens system, the reference surface or the surface to be inspected forms an image on the light receiving surface of the area sensor 32.

The beam splitter 22 splits the light beam into two in the optical path sandwiched by the lenses 20A and 20B forming the a-focal lens system, and one of the two split light beams enters the plane mirror 24 and the other enters the plane mirror 28. To do. The light fluxes returned by the plane mirrors 24 and 28 merge again via the beam splitter 22 and enter the area sensor 32 via the lens 20B.

Further, the piezo element 26 displaces the plane mirror 24 in the direction of the arrow to perform fringe scanning.

The rotation driving means 30 is said to be capable of finely rotating the plane mirror 28 around an axis orthogonal to the incident optical axis. This axis of rotation passes through the converging point of the afocal lens system described above, which coincides with the mirror surface of the plane mirror 28, and is in the direction orthogonal to the drawing in this embodiment.

In addition to these, the measuring device has a computer system as a control calculation means for controlling the process of calculation and measurement on the output of the area sensor 32.

The measurement by the apparatus shown in FIG. 1 will be described below. First, the shear amount S will be described. When the shutter S1 is closed, the shutter S2 is opened, and the laser light source 10 emits light,
The reference plane wavefront is selected as described above. At this time the lens
Area sensor 32 when 20A and 20B are retracted out of the optical path
The light beam incident on is a plane wave. When the inclination angle of the plane mirror 28 with respect to the incident optical axis is θ / 2, the light beam L1 reflected by the plane mirror 28 becomes a light beam L0 reflected by the plane mirror 24, as shown in FIG. 2 (I). On the other hand, the light is incident on the area sensor 32 at an angle of θ. Therefore, the wavefront of the light beam L1 is inclined by the angle θ with respect to the wavefront of the light beam L0. Therefore, an interference fringe corresponding to the tilt angle θ is generated on the light receiving surface, and thus the angle θ can be measured with extremely high accuracy by performing fringe scanning on the interference fringe pattern and performing fringe scanning measurement.

In this state, deploying the lenses 20A and 20B in the optical path,
Light fluxes L01 and L10 reflected by the plane mirrors 24 and 28 are reflected by the lens 20.
After passing through B, the light beams become parallel to each other, and the shear amount S of the quantity light beams L01, L10 is given by f tan θ with f being the focal length of the lens 20B.

If the relative angle between the plane mirrors 24 and 28 is fixed at 90 ° + θ / 2 or 90 ° −θ / 2, the shear amount is f tan θ and is constant as described above. The accuracy required for the relative angle is on the order of seconds or less, and in practice, the above-mentioned angle changes due to thermal deformation of the support system, so that the above-mentioned relative angle cannot be treated as a constant. However, in the present invention, by performing the above-described shear amount measurement prior to the shape measurement, it is possible to know the extremely accurate shear amount for each measurement. The angle θ is measured by the fringe scanning measurement by the fringe scanning as described above, and specifically, for example, the following is performed. That is, by the fringe scanning interferometry method, the analytical equation of the wavefront of the light beam L1 is determined by the method of least squares to determine the tilt angle θ with the wavefront of the light beam L0. The amount of displacement of the plane mirror 24 during the fringe scan is a minute amount of the wavelength of the laser beam, so that there is no substantial displacement between the mirror surface of the plane mirror 24 and the focal point. After the shear amount is obtained in this way, the surface shape of the object O is next measured. That is, when the reflected light from the surface to be inspected is selected by closing the shutter S2 and opening the shutter S1 in FIG. 1, it means that the measurement wavefront is selected by the selection optical system and the area sensor 32 Two light fluxes having a measurement wavefront are laterally displaced from each other by a shear amount S, and
Generate interference fringes. Therefore, fringe scanning is performed to measure the positional difference ΔW between the two light beams. Then, the calculation of W (X) = (1 / S) ∫ΔWdX (dX is the sample pitch) is performed on this ΔW to obtain the shape of the measured wavefront in the X direction, that is, the vertical direction in FIG. .

The shape in the Y direction perpendicular to the drawing in FIG. 1 is obtained by rotating the test surface by 90 degrees in a plane orthogonal to the optical axis or by rotating the plane mirror 28 by 90 degrees around the incident optical axis. This can be obtained by executing the same process as the above, and the shape of the test surface can be known as the wavefront shape by combining the shapes in these respective directions. In the embodiment described above, when measuring the shear amount S, it was necessary to retract the a-focal lens system from the optical path. In the embodiment of FIG. 3, the shear amount can be measured without retracting the a-focal lens system from the optical path. That is, in this embodiment, the selection optical system is composed of the beam splitters 13, 15, 16, 17, the mirror 18, and the shutters S3, S4, S5. Further, in this embodiment, the parallel light flux itself, which is expanded by the beam expander 12 and has a diameter expanded, has a reference plane wavefront.

To measure the amount of shear, the shutters S3 and S5 are closed, the shutter S4 is opened, and the laser light source 10 is made to emit light. At this time, the light flux having the reference plane wavefront enters the plane mirrors 24, 28 through the beam splitters 13, 15, 22 and is reflected by the plane mirrors 24, 28, and then joins again via the beam splitter 22. , Beam splitter 15,13,
It is incident on the area sensor 32 via 17.

Therefore, at this time, since the light flux does not pass through the afocal lens system, it reaches the area sensor 32 while maintaining the flatness of the wavefront. Therefore, the shear amount S can be accurately determined in the same manner as in the above-described embodiment. You can know After measuring the amount of shear, the shutter S4 is closed and the shutters S3 and S5 are opened to select the measurement wavefront, and the configuration of the measurement optical path at this time is the same as that of the embodiment of FIG. Therefore, the measurement can be performed in the same manner as the embodiment of FIG.

(Effect) As described above, according to the present invention, it is possible to provide a novel fringe scanning shearing interferometer. Since this device has the above-mentioned configuration and the shear amount can be accurately known, highly accurate measurement can be realized. In addition, since the amount of shear can be changed by adjusting the minute rotation of the plane mirror, the amount of shear can be changed according to the amount of waviness of the measured wavefront shape, and since the amount of shear can be adjusted only by rotating the plane mirror, it is extremely easy. Is. Various known rotary drive means can be used. However, since the rotation angle is very small, it is preferable to use a piezo element.

[Brief description of drawings]

FIG. 1 is a diagram showing an essential part of one embodiment of the present invention, and FIG.
FIG. 3 is a diagram for explaining the operation of the present invention based on the above embodiment, and FIG. 3 is a diagram for explaining another embodiment of the present invention. O ... Subject, N ... Mirror with reference plane, 18A, 18B ...
… Shutters, 20A, 20B …… A focal lens system lenses, 22 …… Beam splitters, 24,28…
… Plane mirror, 26 …… Piezo element, 30 …… Rotation drive means, 32 …… Area sensor

Claims (1)

(57) [Claims] 1. A selection optical system capable of arbitrarily selecting a measurement wavefront and a reference plane wavefront, and a set of a focal lens system for relaying a measurement light beam onto a light receiving surface when the measurement wavefront is selected, In the optical path sandwiched by the afocal lens system, a beam splitter that splits the light beam into two and two plane mirrors that respectively reflect the two split light beams and join them again via the beam splitter A piezo element for fringe scanning by displacing one of these two plane mirrors, and a rotation driving means for finely rotating the other plane mirror about an axis passing through the mirror surface and orthogonal to the incident optical axis, And a converging point position of the a-focal lens system coincides with a mirror surface position of the two plane mirrors.