JP3041205B2 - Reference plate for interferometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reference plate used for an interferometer, and more particularly to a reference plate used for measuring a low-reflectance test surface.

[0002]

2. Description of the Related Art In recent years, in view of improvement in processing accuracy of mechanical, optical or semiconductor parts, a technique for measuring unevenness or surface roughness of a non-processed surface with high accuracy has been required.

[0003] As such a measurement technique, an optical interference generated between a reference plate and the object is measured by an interferometer to obtain the interference fringes, and the interference fringes are analyzed to analyze the surface shape of the object. A device that measures the distance is known.

As the interferometer, for example, a Fizeau type interferometer is widely used. In order to obtain good contrast fringes in this Fizeau-type interferometer,
In particular, the reflectivity of both surfaces is adjusted so that the amount of reflected light from the surface of the subject (hereinafter, referred to as a test surface) and the amount of reflected light from the surface of the reference plate (hereinafter, referred to as a reference surface) are equal. It is necessary. In general, the reference plate is classified into a high reflectance reference plate, a medium reflectance reference plate, a low reflectance reference plate, and the like according to the reflectance of the reference surface, and the low reflectance reference plate is usually a glass plate. Since the polished surface is used as it is, the reflectance is about 4%.

[0005]

By the way, in recent years, an environment for measuring a surface to be inspected having a reflectance of about 1% or less has been provided with the progress of the external light shielding technology and the optical noise reduction technology of the interferometer itself. For this reason, attempts have been made to measure the surface shape of the test surface having such an ultra-low reflectance using an interferometer. However, it has been difficult to obtain interference fringes with good contrast by using the low reflectance reference plate.

On the other hand, as an interference fringe analysis method capable of obtaining a highly accurate analysis result, a relative distance between a test surface and a reference surface is changed, and the interference fringe analysis is performed based on a luminance change at a predetermined position accompanying the change. The fringe scanning method is known, but the fringe scanning method requires an interference fringe image having a good contrast. Therefore, the reflectance of about 1% or less is obtained by using the above-described low reflectance reference plate. It was difficult to measure the surface to be inspected.

The present invention has been made in view of such circumstances, and an interferometer capable of obtaining an interference fringe image with good contrast even on a test surface having an extremely low reflectance of about 1% or less. It is intended to provide a reference plate for use.

[0008]

The reference plate for an interferometer according to the present invention reflects a part of a light beam emitted from a light source,
In a transmission-type interferometer reference plate having a reference surface on one side that transmits so as to irradiate the test object with most of the remaining light flux, the reference surface is subjected to a process of reducing light reflectance. It is characterized by having.

Preferably, the treatment for reducing the light reflectance is a treatment for forming an antireflection film.

[0010] Further, a polished reference surface can be formed on the opposite side of the reference surface.

[0011]

The reference plate for an interferometer according to the present invention has a reference surface which has been subjected to a process for reducing the light reflectance, whereby an interference fringe image having a good contrast can be obtained. Even when measuring a test surface of about 1% or less, which is difficult, the reflectance of the reference surface can be adjusted to the reflectance of the test surface, so that an interference fringe image having a good contrast can be obtained. Can be.

Conventionally, a reference surface treated to increase the reflectance is widely known, but no attempt has been made to create a reference surface having a reflectance lower than that of a normal polished surface. Was.

This is because it has been considered that the reference plane needs to have a reflectance of at least about 4% or more in order to obtain a clear interference fringe image with good contrast. Indeed, the invention of the present application was made from such a breakthrough of the existing concept and a change in the idea in consideration of the progress of the light shielding technology and the noise reduction technology of the interferometer, and the practical value of the technology of the present invention is extremely high. .

Further, if an antireflection film is formed on the reference surface to reduce the light reflectance, the setting of the reflectance is extremely easy and the reliability is high.

If the surface opposite to the reference surface whose reflectance is reduced is a polished reference surface, one reference plate can be used as a surface having a reflectance of about 4% and a reference surface having a reflectance of about 4%. It can be used for measurement of two types of test surfaces including a test surface having a lower reflectance, and the storage space of the reference plate can be reduced.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic view showing a Fizeau-type interferometer apparatus provided with a flat reference plate according to an embodiment of the present invention.

That is, this interferometer device is a laser light source 1
, A lens 4 for converting the laser beam 2 into divergent light 5, a pinhole plate 4 a, a beam splitter 6, a collimator lens 7, and an antireflection film which is arranged so that the parallel light 8 is perpendicular to the parallel light. Reference surface 9 coated with
a, and the remaining parallel light 8
And a reference plate 9 having a circular cross section, which transmits the light through the test surface 10a of the subject 10, and the reference light reflected from the reference surface 9a and the object light reflected from the test surface 10a. A TV camera 11 for observing interference fringes formed by interference is provided.

The reference plate 9 is held by a cylindrical lens barrel (mounting member) 13 via a buffer material 12, and as shown in FIG. 1, a male screw formed on the outer surface of the lens barrel 13 A female threaded portion 15a formed at the lower end of the outer wall 15 of the interferometer device
The reference plate 9 can be fixedly held at a predetermined position by being screwed into the base plate 9.

The reference plate 9 is securely positioned and held in the vertical direction by a holding plate 14 screwed into the inner wall of the lens barrel 13 and a mounting portion 13a projecting inward from the inner wall of the lens barrel 13. Is to be made.

The cushioning member 12 is for preventing the reference plate 9 from being stress-strained by being clamped by the lens barrel 13.

The reference surface 9a is formed by coating the polished surface with an antireflection film, and is set to have a reflectance of about 1%.

By making the reference surface 9a a plane with an extremely low reflectance, an interference fringe image with good contrast is formed when measuring the test surface 10a having an ultra-low reflectance of about 1%. be able to. That is, this reference plane 9a
Reference light, which is light reflected from the object, and a test surface 10a having an ultra-low reflectance
The amount of light reflected from the object light can be matched with that of the object light, so that an interference fringe image with good contrast can be obtained even if the amount of each of the two reflected lights is small.

The antireflection film is made of, for example, ZrO ₂
And a thin film of MgF ₂ are alternately laminated with a predetermined thickness by using a vapor deposition method or a sputtering method.

The reflectance can be set to a desired value by appropriately changing the thickness of the antireflection film.

The reference plate 9 has a reference surface 9b on the side opposite to the reference surface 9a, which is only polished and has a reflectivity of about 4%. These two reference planes 9
a and 9b are planes having different reflectances from each other,
In order to ensure the contrast of the interference fringes, the operator can appropriately select one of the two reference surfaces 9a and 9b according to the reflectance of the test surface 10a.

When using the reference surface 9b, the lens barrel
By inverting the state of FIG. 1 from that shown in FIG. 1, both reference surfaces 9a and 9b are inverted, and the male screw portion 13c of the lens barrel 13 is screwed into the female screw portion 15a at the lower end of the outer wall 15 to form the lens barrel. 13 is attached to the interferometer device.

Thus, using one reference plate 9, interference fringe measurement can be performed on two types of test surfaces 10a having different reflectivities while maintaining good contrast.

Further, both reference surfaces 9 of the reference plate 9 shown in FIG.
a and 9b are formed so as to be inclined with respect to each other by a predetermined minute angle (wedge angle).

That is, for example, the reference plate device is arranged in a state as shown in FIG.
If it is a, each member is set so that interference fringes due to the reflected light from the reference surface 9a and the reflected light from the test surface 10a are formed on the light receiving surface of the CCD 11. Assuming that the surfaces 9a and 9b are formed to be parallel to each other, interference fringes due to the reflected light from the reference surface 9b and the reflected light from the test surface 10a and the reflected light from both the reference surfaces 9a and 9b. Interference fringes are also formed on the light receiving surface of the CCD 11.

When such unnecessary interference fringes overlap with the interference fringe to be measured, the brightness of the background of the interference fringe to be measured is increased. The contrast of the interference fringes decreases, and the measurement accuracy decreases. Alternatively, normal fringe analysis by the fringe scanning method may not be possible due to the inability to separate both interference fringes during fringe scanning.

Therefore, both reference surfaces 9a and 9 of the reference plate 9 are provided.
b are inclined at a predetermined minute angle with respect to each other, and when the reflected light from the reference surfaces 9a and 9b used at that time is set so as to return to the light receiving surface of the CCD 11, the other reference surfaces The reflected light from 9a and 9b is prevented from returning to the light receiving surface of CCD 11 by a conventional method, that is, by inserting an aperture in the imaging optical system or by increasing the optical path length of the imaging optical system. This solves the problem that the contrast of the measurement interference fringes is reduced due to the generation of unnecessary interference fringes.

If the wedge angle is set too large, an inconvenience associated with refraction of a light beam or the like will occur. Therefore, the wedge angle should be set to such a small angle that such an effect does not become a problem.

It should be noted that the reference plate of the present invention is not limited to the above-described embodiment, but various modifications can be made.

For example, the reflectance of the reference surface whose reflectance has been reduced may be 1% or more. Further, depending on the situation, the reflectance of the reference surface formed on the surface opposite to the reference surface may be reduced. The antireflection film having different antireflection efficiencies may be formed on both surfaces of the reference plate.

Further, instead of coating an antireflection film in order to form a reference surface having a reduced reflectance, the reflectance can be lowered by making this reference surface slightly rough.

In the above embodiment, the case where two reference surfaces formed on the front and back surfaces of the reference plate are used in the same interferometer apparatus is described using an example of a Fizeau type interferometer. Interferometer that uses the light transmitted through the reference plate and reflected from the reference surface as a reference surface, and an interferometer that uses the reflected light from the reference surface as it is as a reference wavefront without transmitting through the reference plate, as in the Twyman type interferometer And may be used. In addition, since the structure can be attached to both sides, it is possible to share the structure.

Further, the shape of the reference surface is not limited to a planar shape, but may be, for example, a spherical shape or an aspherical shape according to the shape of the surface to be inspected.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a state where an interferometer reference plate according to an embodiment of the present invention is applied to a Fizeau interferometer.

[Explanation of symbols]

 Reference Signs List 1 laser light source 2 laser beam 9 reference plate 9a, 9b reference surface 10 subject 10a test surface 11 CCD camera 12 buffer material 13 lens barrel 14 holding plate 15 outer wall

Claims (3)

(57) [Claims] 1. A light source for reflecting a part of a light beam emitted from a light source.
While irradiating most of the remaining light beam to the subject.
Transmission interferometer with a reference plane that transmits light on one side
In use the reference plate, the interferometer reference plate, wherein <br/> that the process of reducing the light reflectance is applied to the reference plane. 2. The interferometer reference plate according to claim 1, wherein the process of reducing the light reflectance is a process of forming an antireflection film. 3. A polished reference surface is provided on a side opposite to the reference surface.
Reference plate for interferometer described.