JPH01212304A - Stripe scanning and shearing method - Google Patents

Abstract

PURPOSE:To perform stripe scanning and shearing easily and accurately, by dividing into two a light flux having a surface to be detected and, shifting sideways one of said divided fluxes with respect to the other. CONSTITUTION:A plane parallel transparent plate 1 provided with a reflection preventing film on one surface thereof and a reflecting surface member 2 having at least one plane not provided with a reflection preventing film are opposed in parallel and with a distance at respective planes not provided with a reflection preventing film. A light flux having a surface to be detected is entered with limited incident angles from the plane provided with the reflection preventing film of the transparent plate 1, so that the light flux is divided into two because of the reflection by the pair of the opposed planes not provided with the reflection preventing film. The optical path length is changed for stripe scanning by displacing the transparent plate 1 and the reflecting surface member 2 relatively in a manner to change the distance therebetween. Thus, the stripe scanning and shearing can be conducted easily and accurately.

Description

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

(Prior art) The fringe scanning shearing interferometry method is known as a measurement method that can accurately measure the shape of a test wavefront.

It is applied to aspheric shape measurement, lens performance measurement, surface roughness measurement, etc.

In such a fringe-scanning shearing interference measurement method, high precision is required for shearing, in which a light beam having a test wavefront is divided into two and laterally shifted from each other, and various shearing methods have been proposed.

(Objective) An object of the present invention is to provide a novel fringe scanning/shearing method that can perform fringe scanning and shearing extremely simply and with high precision in a fringe scanning shearing interference measurement method.

(composition) One aspect of the present invention will be explained below.

The fringe scanning/shearing method of the present invention employs a fringe scanning shearing interference measurement method in which a light beam having a test wavefront is divided into two, one light beam is laterally shifted relative to the other light beam by shearing, and the fringe scanning and shearing method This is a method of changing the optical path length of one of the light beams for the purpose of the present invention, and has the following characteristics.

That is, a parallel plane transparent plate provided with an antireflection film on one side, and a reflective surface member having at least one flat surface not provided with an antireflection film, are arranged so that the surfaces not provided with the antireflection film are separated from each other with a gap. The light flux is divided into two by reflection on a pair of opposing planes on which the anti-reflection film is not provided.
To divide.

Then, by relatively displacing the parallel plane transparent plate and the reflective surface member so as to change the size of the gap, the optical path length for stripe scanning is changed.

To explain the method of the present invention in more detail with reference to the drawings, in FIG.
indicates a reflective surface member, and numeral 3 indicates a piezoelectric element.

The parallel plane transparent plate 1 is a transparent plate whose both sides are highly flat-finished and both sides are highly parallelized.

An antireflection film is formed on one side IA, and the other side IB
For example, parallel plane glass can be used as the parallel plane transparent plate on which no antireflection film is formed.

In FIG. 1, the reflective surface member 2 is a parallel plane plate, and is made of the same material as the parallel plane transparent plate 1.

An antireflection film is not formed on one surface 2A of this reflective surface member 2, but an antireflection film is formed on the other surface 2B.

Therefore, the parallel plane transparent plate 1 and the reflective surface member 2 are arranged with the surfaces IB and 2A on the side where the antireflection film is not provided facing each other with a gap in between.

In the example shown in this figure, the parallel plane transparent plate 1 is fixed in the measuring device space, and the reflective surface member 2 is fixed to the piezoelectric element 3 fixed in the device space. 3, it is moved step by step by minute distances in the thickness direction.

When a light beam having a test wavefront W is incident on a parallel plane transparent plate 1 at a finite angle of incidence φ from the side of the surface IA provided with one anti-reflection film as shown in the figure, the light beam enters from the surface IA and a part of it is incident on the surface IB. Hereinafter, this light flux that is reflected by the light beam and proceeds toward the observation surface 8 will be referred to as a measurement light flux, and its wavefront will be referred to as a test wavefront W. On the other hand, the light beam that has passed through the parallel plane transparent plate 1 is reflected by the surface 2A of the reflective surface member 2, passes through the parallel plane transparent plate 1, and similarly heads in the direction of the observation surface 8. This wavefront is called a reference wavefront W°.

The measurement light flux and the reference light flux interfere with each other on the observation surface 8 to produce interference fringes. Although the test wavefront W and the reference wavefront W° have the same wavefront shape in principle, they are laterally shifted from each other by the shear amount S. This shear amount S is calculated by assuming that the size of the gap between surfaces IB and 2A is T as shown in the figure.

It is given by 2Tsinφ. Therefore, adjust the shear amount S.

This can be done by changing the gap size T (easily possible by manipulating the piezoelectric element), or by changing the incident angle φ
(parallel plane transparent plate, reflective surface member 2 and piezoelectric element 3)
The purpose of providing an anti-reflection film on the surface 2B of the member 2 is to prevent the light flux reflected by this surface 2B from reaching the observation surface 8 as noise. This is to prevent this from happening. Therefore, if the reflected light flux from surfaces other than the surface 2A is substantially prevented from reaching the observation surface 8, the formation of an antireflection film on the reflective surface member can be omitted. Also, surface IB
, 2A, it is desirable to set the materials of the parallel plane transparent plate and the reflective element and the angle of incidence φ so that their reflectances are approximately equal to each other and are on the order of several percent.

This is to increase the contrast of interference fringes generated on the observation surface 8 and to eliminate the influence of high-order reflected light.

For example, if the parallel plane transparent plate 1 and the reflective curved member 2 are made of glass, and the incident angle φ is set so that the reflectance at the surfaces IB and 2A is about 4x, then the reflectance R is equal to the measurement luminous flux. The amplitude intensity of the reference beam is R and (I-R), respectively.
”, -H, but these values are R=0
．． 04 and 0.04, respectively, and are substantially equal to each other, so that the contrast of the interference fringes can be increased. Also page 1 IB, 2A0, 00006
Therefore, the intensity is sufficiently small compared to the intensity of the measurement light flux and the reference light flux, so that it does not act as noise.

The phase difference between the measurement beam and the reference beam is AW (x, y) = W (x, y) - W (x + S, y)
#ldw(x)/dxl·S. Therefore (1/S)S AW(x,y)dx=W(x,y)+
The test wavefront W can be known by an integral operation const. In the above equation, cons t is the wavefront W on the optical axis.
Just take (0,0).

The reflective surface member 2 is moved by the piezoelectric element 3 (specifically PZT) to change the optical path length of the reference beam in stages, and the interference fringe intensity distribution I (X, yI'%) at each optical path length stage is determined. With Fourier expansion.

I (x, y, l, L) = a (x, y) + b
(x, y)cos (k lΔW(x, y)−1J)
1%=λ・n/N, (N=0.1,2.,,,,N-
1) , = 2c/λ. However, λ is the wavelength of the luminous flux.

Fringe intensity at 1% 1(x, y, from IJ = b(x,
y)cos(k AW(x,y))= ,b(x,
y) s in (kΔW(x, y)), and from this the phase difference ΔW(x, y) is AW(x, y) = (18)
jan' (Sl-layer/C3).

Since both the measurement light beam and the reference light beam pass through substantially the same optical path of the parallel plane transparent plate 1, the distortion of the parallel plane transparent plate 1 has almost no effect on the measurement accuracy.In addition, if the reflective surface member 2 is a glass plate, the plane of the glass plate Since one having a degree of λ/20 is easily available, an accuracy of 1710 or more can be obtained even when the distortion of one reflective surface member 2 is taken into consideration.

(Example) Three specific examples are shown below.

The embodiment shown in FIG. 2 applies the present invention. This is a device that tests the performance of lenses. A portion designated by the reference numeral 12 is a portion for carrying out the fringe scanning/shearing method of the present invention, and has been described with reference to FIG. The parallel transparent substrate and the reflective surface member are made of glass plates, and the angle of incidence is set so that the reflectance of the surface on which the antireflection film is not formed is 4%.

PZT is used as the piezoelectric element.

The test lens 11 is a collimating lens and one point light source 10
The collimation characteristics of the test lens 11 are measured by transmitting laser light from the lens 11 and examining the wavefront of the transmitted light as the test wavefront. Both the measurement light flux and the reference light flux are guided through an imaging lens 13 onto the light receiving surface of an area sensor (CCD camera) 14 serving as an observation surface, and are caused to interfere with each other.

The embodiment shown in FIG. 3 is an apparatus that uses an ultra-precision processed surface such as a polygon mirror as a test surface 16 and measures the surface shape thereof. A parallel laser beam L is irradiated onto a test surface 16 via a half mirror 15, and the reflected light is taken out via the half mirror 15 to examine the test surface. Half mirror 15
The light beam taken out from is subjected to fringe scanning and shearing using the method of the present invention, as in the embodiment shown in FIG. 2, and the measurement light beam and reference light beam are

An area sensor 14 serving as an observation surface is connected through an imaging lens 13.
An image is formed on the light-receiving surface of the two to cause interference between the two.

In the embodiment shown in FIG. 4, the thickness of a thin film 18 of known refractive index formed on a substrate 17 is the object of measurement.

A parallel laser beam is irradiated from the side of the substrate 17, and the transmitted beam is subjected to fringe scanning and shearing using the method of the present invention.
The wavefront of the measurement light beam and the wavefront of the reference light beam are shifted by the shear amount S on the observation surface 8, and interference fringes corresponding to the phase difference ΔW of 1 are generated ('I, 4 t5 (X)).

If the thickness of the thin film 18 is d, the phase difference ΔW is the same as that of the substrate 1.
This is the phase difference between the light that has passed only through the substrate 17 and the light that has passed through the substrate 17 and the thin film 18, and is given by Δen=(no−1)d using the refractive index no of the thin film. On the other hand, the phase difference ΔW known from fringe scanning
As mentioned above, since (1/k)jan'(SN/CS), the film thickness d is d=(λ/2π) ban-'(SN/CS) l/(
No. 1).

Note that if the thickness of the thin film 18 is λ or less, the amount of shear may be arbitrary as long as it is sufficient to generate interference fringes. In this case, the interference fringes can be measured with high precision by fringe scanning regardless of the contrast of the interference fringes. Therefore, the thickness of a thin film with a known refractive index can be measured with extremely high accuracy.

(Effects) As described above, according to the present invention, a novel stripe scanning/shearing method can be provided. Since this method is configured as described above, highly accurate shearing and fringe scanning can be easily achieved using extremely simple equipment.

[Brief explanation of the drawing]

Figure 1 is a diagram for explaining one embodiment of the present invention, Figure 2 is a diagram for explaining one embodiment, Figure 3 is a diagram for explaining another embodiment, and Figure 4 is another embodiment. FIG. 111. Parallel plane transparent plate, 2000 Reflective surface member, 300
．． Piezoelectric element 1 Gome Z area

Claims (1)

[Claims] In the fringe scanning shearing interference measurement method, a light beam having a test wavefront is divided into two, one light beam is laterally shifted relative to the other light beam by shearing, and one of the light beams is divided into two for fringe scanning. A method for changing the optical path length of a light beam, the method comprising: a parallel flat transparent plate having an anti-reflection film on one side; and a reflective surface member having at least one flat surface not provided with the anti-reflection film; The surfaces that are not provided are made to face each other in parallel with a gap between them, and a light beam having the test wavefront is made incident at a finite angle of incidence from the surface of the parallel plane transparent plate on which the antireflection film is provided. The light beam is divided into two by reflection on a pair of opposing planes not provided with an antireflection film, and the parallel plane transparent plate and the reflective surface member are relatively displaced so as to change the size of the gap. A fringe scanning/shearing method characterized by changing the optical path length for fringe scanning by