JPH051970A - Surface shape measuring device - Google Patents

Abstract

(57) [Abstract] [Purpose] To obtain a surface shape measuring device capable of highly accurately measuring an aspherical shape having many aspherical surfaces by using a computer generated hologram. In a surface shape measuring device for measuring a surface shape of an object by an interferometer using a computer generated hologram, the computer generated hologram is arranged so that a spherical wave is obliquely incident from the interferometer, and Place a pinhole in the interferometer that removes unwanted diffracted light that occurs.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface shape measuring device for measuring the surface shape of an aspherical optical element used in an optical system.

[0002]

2. Description of the Related Art In recent years, as optical systems have become more compact and the fields of application have expanded, it has become popular to improve performance by using aspherical surfaces from conventional optical systems having only spherical surfaces. In such cases, the problem is the inspection of the created aspherical surface. In particular, there is a problem in that the amount of aspherical surface becomes large and the interference fringes become too dense in a normal Fizeau type interference system, making measurement impossible in practice.

Therefore, various methods have been conventionally proposed for measuring a large amount of aspherical surface. An example is a method in which a contact or non-contact type probe is scanned on the surface to be inspected and the result is subjected to signal processing. Although this method is general-purpose, it has the drawback that mechanical errors associated with scanning directly affect the measured values. Computer hologram (hereinafter referred to as “CGH”) is a powerful method in such a situation.
A method using is known.

The CGH is created by calculating from a surface shape known as a design value in advance, and when a predetermined incident light is incident, it diffracts with a wavefront corresponding to a desired aspherical amount as a reference wavefront. The diffracted reference wave interferes with the light from the surface to be inspected and measurement is performed. CGH is versatile because it can form an arbitrary wavefront by calculation, and has the advantage of being able to measure the entire surface under test at once in the form of interference fringes.

[0005]

However, in the measurement by the CGH of the above-mentioned conventional example, when the measurement and observation of the surface to be inspected by the interference fringes, unnecessary diffracted light overlaps near the optical axis, and the observation is performed. There was a problem that there are areas that cannot be used. The shape near the optical axis is particularly important when inspecting the shape of an element, but the difficulty of measurement is a serious problem.
The present invention has been made in consideration of the above points, and removes unnecessary diffracted light overlap in the vicinity of the optical axis, and extracts only the desired component light to accurately measure the entire surface under test at high speed. The purpose is to do.

[0006]

According to the present invention, in order to solve the above-mentioned problems, the CGH has an off-axis type structure, and unnecessary diffracted light is removed by making full use of an optical filtering method. This makes it possible to observe the entire test surface. Therefore, as shown in FIG. 2, the CGH is arranged obliquely with respect to the optical axis of the main optical system of the interferometer until it is incident on the CGH, and the optical axis of the test object itself is the same as that of the main optical system or the CGH. It is arranged so that it is not common with the line surface. C
A spherical wave generated by a lens or mirror system that generates a spherical wave in the main optical system enters the GH.
In the main optical system, in order to remove unnecessary diffracted light generated by CGH, a pinhole is arranged at a focal position on the side opposite to the CGH of the spherical wave generating optical system.

[0007]

1 is a schematic view of an aspherical surface measuring device according to a first embodiment of the present invention. In the figure, 1 is a laser light source, 2 and 3
Is a lens, and the lenses 2 and 3 collimate the laser beam 1 into a parallel light beam having a predetermined diameter. The laser light passes through the beam splitter 4 and then enters the spherical wave conversion lens 5. The first surface 6 of the spherical wave conversion lens 5 is a reference plane, and a part of the incident light is reflected by 6 and returns to the original main optical system as a reference wave R.

The light transmitted through the spherical wave conversion lens 5 obliquely enters the CGH 7 as a convergent spherical wave. The CGH 7 is designed with respect to the ideal values of the oblique incident spherical wave and the aspherical surface to be inspected, and the first-order diffracted light of the CGH 7 generates a wavefront equal to the inspected surface. The first-order diffracted light forms a conjugate wavefront on the surface to be inspected 8 having an ideal aspherical surface shape as a design value, is reflected by the surface to be inspected 8 and returns to the CGH 7 again. According to the reciprocity law, the wavefront transmitted through the CGH 7 again becomes a diverging spherical wave corresponding to the spherical wave on which the -1st order diffracted light is incident, reaches the spherical wave generating lens 5, and passes through the spherical wave generating lens 5. Converted to the object wave I. Since this light is the reflected light from the actual surface 8 to be inspected, if the surface 8 to be inspected is ideally formed corresponding to the design value, the transmitted light, that is, the object wave I becomes a perfect parallel light.
Further, when deviating from the ideal value, the position deviating from the parallel light corresponding to the amount is the same as in a normal Fizeau interferometer.

The CGH 7 actually contains diffracted light of various orders. Since only one of them is used for measurement, care must be taken in designing the CGH so that the extra diffracted light does not mix with the measurement light. The following four components have to be considered in the design of the CGH by hitting the path of the light up to this point.

First, it is necessary to consider the reflected light of each diffraction order by 7 of the spherical wave incident on the CGH 7 from the spherical wave conversion lens 5 of the main optical system, and secondly, it is necessary to consider each. It is the transmitted light of the diffraction order. In addition, thirdly, this time, on the contrary, the reflected light of each diffraction order by the CGH 7 of the spherical wave reflected from the surface 8 to be inspected and incident on the CGH 7, and the fourth must be considered. Is transmitted light of each diffraction order by CGH7.

The present invention is characterized by the arrangement of the optical system and the CGH 7 so that the spatial frequency distributions of the light for detecting the diffracted light of these four extra components do not overlap. For example, the optical system of the present invention has an off-axis arrangement. Therefore, the arrangement is such that the 0th-order reflected light of the light incident on the CGH 7 does not return to the spherical wave conversion lens 5. Although it was difficult to separate such diffracted light by the conventional on-axis CGH inspection method, the separation can be easily performed by using the method of the present invention. In this way, the separation of unnecessary diffracted light becomes possible by the arrangement of the optical system itself and the design of the CGH.

In addition to this, an optical filtering method is commonly used to remove the unnecessary diffracted light. Examples of the unnecessary diffracted light of the CGH 7 include the reflected or transmitted diffracted light group of the convergent spherical wave described above and the transmitted diffracted light group incident from the surface 8 to be inspected. Let G be the wavefront created after passing through the wave conversion lens 5.

The light returning from the spherical wave conversion lens 5 is thus in a mixed state of the reference wave R, the object wave I and the unnecessary diffracted light G. These three kinds of waves R, I, and G are reflected by the beam splitter 4 next to the spherical wave conversion lens 5, then transmitted through the lens 9, and reach the pinhole 10.
The pinhole 10 is arranged on the combined focal plane of the spherical wave conversion lens 5 and the lens 9, and serves to perform optical filtering.

In order to clarify the action of optical filtering, the state of light at the position of the pinhole 10 will be described. Considering the wavefront returning from the spherical wave conversion lens 5, the reference wave R is a plane wave propagating in the optical axis direction of the main optical system. In addition, since the object wave I also uses the + 1st order of CGH for the forward and the −1st order of the return, it is a plane wave propagating in the optical axis direction of the main optical system in a form including the information of the surface 8 to be inspected by the reciprocity law. Is becoming Therefore, I and R are superposed as plane waves traveling in the same direction, and the interference between them indicates the shape of the surface to be inspected. On the other hand, the wavefront G can be designed so that its propagation direction does not have a common part with the wavefronts I and R by the arrangement of the optical system and the design of the CGH. At the pinhole 10, the propagation directions of the wavefronts I, R, and G, that is, the spread of light according to the spatial frequency appears, but according to the present invention, I, R, and G are substantially overlapped. It is possible not to have it. Therefore, if the diameter of the pinhole 10 is set appropriately, only the wavefronts I and R can be guided to the interference fringe observation surface 11.

To remove the unnecessary diffracted light G by filtering, basically, as shown in FIG. 2, the CGH 7 is arranged obliquely with respect to the optical axis of the main optical system, and the diffracted light from the CGH 7 is incident on an incident spherical wave. It is realized by the structure of the present invention in which the guide is carried out more obliquely. The diagonal arrangement of CGH7 is CG
It plays a role of not returning the reflected and diffracted light from H7 to the main optical system. On the other hand, the arrangement of obliquely diffracted light from CGH7 is CG.
This means that the 0th-order light of the spherical wave incident on H7 does not reach the surface to be inspected, and is highly effective in removing unnecessary diffracted light.
The oblique diffraction angle of the CGH 7 is determined by the carrier frequency of the CGH 7, and unnecessary diffracted light can be efficiently removed by designing this angle.

In reality, even with the configuration of the present invention, there is one unnecessary diffracted light. It is the -1st order (or +1) when the light reflected from the surface 8 to be inspected is reflected by the CGH 7.
Next, it is light, but it becomes a divergent spherical wave by reflection and becomes a light that spreads widely at the position of the pinhole 10. Therefore, by appropriately selecting the diameter of the pinhole, the influence on the actual measurement can be almost ignored. ..

As described above, when the structure of the present invention is used, unnecessary diffracted light has a substantially negligible effect.
The problem of the non-measurable area of the surface to be inspected, which has been a problem in the conventional type, is solved, and the measurement with a good S / N ratio can be easily performed.

FIG. 3 is a schematic diagram of a second embodiment of the present invention.
In the figure, the same members as those in the first embodiment are designated by the same reference numerals. The present embodiment is different from the first embodiment in the way of forming the reference wave. In this embodiment, the beam splitter 4
The light reflected by is reflected toward the reference plane 6 and returned to the original beam splitter 4. Other functions, such as a method of removing unnecessary diffracted light, are the same as those in the first embodiment. What is observed on the interference fringe observation surface 11 is the interference of the reflected light from the reference surface 6 and the surface to be inspected, but by making the reference surface completely independent from the surface to be inspected, the pitch of the interference fringes at the time of observation In addition, the fine adjustment can be performed separately from the surface to be inspected by inclining the reference surface.

FIG. 4 is a schematic diagram of a third embodiment of the present invention.
In this case as well, the method of forming the reference wave is different from that in the first embodiment. The light emitted from the laser 1 becomes divergent light by the lens 2 and enters the half mirror 12. After the light transmitted through the half mirror 12 reaches the spherical wave conversion lens 5, it is the same as in the first embodiment, but in this embodiment, the reference wave is reflected by the half mirror 12, and this time by the reference spherical surface 13. Is set. Pinhole 10
The position of is determined in consideration of the lens systems 5 and 9, and the light from the surface to be inspected and the light from the reference spherical surface efficiently interfere with each other, and other unnecessary diffracted light can be removed by filtering.

[0020]

As described above, according to the aspherical surface measuring method by CGH of the off-axis configuration of the present invention, unnecessary diffracted light by CGH, which has been a problem in the past, can be efficiently removed, so that the S / N ratio can be improved. It became possible to measure the inspection surface. As a result, it became possible to measure near the optical axis, which was a problem in the past.

The measuring method by CGH can be widely applied to various aspherical shapes, and in particular, it can be greatly effective by removing the limit that a large amount of aspherical surface cannot be measured by the conventional Fizeau interferometer. Is expected. In the embodiment of the present invention, not only can the conventional problems be overcome with a simple configuration, but also the ease of adjustment of the device can be achieved, and the entire surface can be measured all at once in terms of efficiency. Great effect.

[Brief description of drawings]

FIG. 1 is a diagram showing an aspherical surface measuring system according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a main part of the present invention.

FIG. 3 is a diagram showing an aspherical surface measuring system according to a second embodiment of the present invention.

FIG. 4 is a diagram showing an aspherical surface measuring system according to a third embodiment of the present invention.

[Explanation of symbols] 1 laser 2 lens 3 lens 4 beam splitter 5 spherical wave generating lens 6 reference plane 7 CGH 8 surface to be inspected 9 lens 10 pinhole 11 interference fringe observation surface 12 half mirror 13 reference spherical surface

Claims (1)

Claim: What is claimed is: 1. A surface shape measuring device for measuring a surface shape of an object by an interferometer using a computer generated hologram, wherein the computer generated hologram is arranged so that a spherical wave is obliquely incident from the interferometer. A surface shape measuring apparatus characterized in that a pinhole for removing unnecessary diffracted light generated from the computer generated hologram is arranged in an interferometer.