JPH0334002B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an interferometric measuring device particularly suitable for measuring a large area of a surface to be measured when measuring surface accuracy.

Generally, an interference measuring device requires a reflecting mirror having an area larger than the area of the sample as a reference surface. That is, when the area of the sample becomes large, an expensive reference plane that is larger than the area and has good accuracy becomes essential.

An object of the present invention is to provide an interference measuring device that measures the surface accuracy of a sample using a reference surface having an area smaller than that of the sample. In an interference measuring device for observing the test surface by interfering light from the test surface with light from the reference reference surface, a reflective reference surface is diagonally opposed to the test surface. The inspection light consisting of a parallel light beam incident obliquely on the test surface is reflected by the test surface, enters and reflects at the reflective reference surface, and passes through the same point on the test surface again. The present invention is characterized in that it is configured to return to the original optical path, and the inspection light that has passed through the optical path is caused to interfere with the light from the standard reference surface.

The present invention will be described in detail below based on illustrated embodiments.

In FIG. 1, 1 is a laser light emitting device, which emits laser light. Along the optical axis of this laser beam, a light intensity adjustment filter 2 that reduces the intensity of the laser beam, a condensing lens system 3 that includes a pinhole plate that turns the laser beam into a point light source;
A semi-transparent mirror 4 that transmits the laser beam on the outward path and reflects it on the return path, a polarizing plate 5 that polarizes the laser beam in the S direction, a collimator lens system 6 that converts the light from the point light source into a parallel beam, and reflects the light. Semi-transparent reference planes 7 that can generate a reference wavefront and transmit the remaining light are sequentially arranged. Further beyond the semi-transparent reference plane 7, a test surface 8 made of, for example, a glass surface is arranged obliquely, and the parallel light flux emitted from the semi-transparent reference plane 7 is incident at an incident angle of, for example, 60 degrees or more. I'm starting to do that. In addition, in the direction of reflection of this incident light on the test surface 8, a reflection reference plane 9 is arranged perpendicular to the reflected light, so that the light flux incident on the test surface 8 from the semi-transparent reference plane 7 is reflected from the test surface 8. They have a geometrical positional relationship such as the inspection surface 8, the reflective reference plane 9, and returning to the semi-transparent reference plane 7 via the inspection surface 8 again. On the other hand, in the reflection direction of the semi-transparent mirror 4 that reflects the light returning from the collimator lens system 6 directions, an observation surface 10 consisting of a screen or an image pickup tube is disposed, and the cylindrical lens 1
1 to convert the size of the image. Incidentally, this cylindrical lens 11 can be replaced with an alignment lens 12 for adjustment if necessary.

The laser beam emitted from the light emitting device 1 is adjusted to the optimum light intensity by a light intensity adjustment filter 2, and then converted into a point light source from which noise is removed by a condensing lens system 3. This light becomes a spherical light wave, passes through the semi-transparent mirror 4, and is polarized in the S direction by the polarizing plate 5. A part of the light wave that has been flattened by the collimator lens system 6 is reflected by the semi-transparent reference plane 7, becomes a reference wavefront, travels backward in the forward path, and is reflected by the semi-transparent mirror 4, resulting in a point image.
Form PA. On the other hand, the light transmitted through the semi-transparent reference plane 7 enters the test surface 8 obliquely, is reflected there, is reflected toward the reflective reference plane 9, and then obliquely enters the same spot on the test surface 8 again. It is reflected to form a wavefront to be detected. This test wavefront travels backward on the forward path, passes through the semi-transparent reference plane 7 and the collimator lens system 6, is reflected by the semi-transparent mirror 4, and forms a point image PB. Therefore, by inserting an alignment lens 12 in place of the cylindrical lens 11 and rotating and adjusting the reflective reference plane 9, the conjugate images of point images PA and PB can be simultaneously formed on the observation surface 10. There is no inclination between the wavefront and the reference wavefront, and it becomes possible to obtain interference fringes on the observation surface 10 that represent contour lines of the shape of the test wavefront with respect to the reference wavefront.

Since the light enters the surface to be measured 8 from an oblique direction, the shape of the surface to be measured 8 viewed from the direction in which the light travels is reduced in one direction. Therefore, in order to observe the external shape of the surface to be inspected 8 with the correct size ratio,
Next, a cylindrical lens 11 is used as an imaging optical system.
may be inserted and the image may be formed by enlarging the reduced direction compared to the orthogonal direction, or the image may be formed by similarly reducing the orthogonal direction of the reduced direction.
If the cylindrical lens 11 is used in this way, the observation surface 10 by a screen or image pickup tube, etc.
Now, the shape of the surface to be inspected 8 can be observed. However, this enlargement/reduction is not done by optical means such as a lens, but there is of course no problem in obtaining a photoelectric image using an image pickup tube and enlarging or reducing this image by electrical means.

In the present invention, since the light is made to enter the test surface 8 obliquely, the test surface 8 is larger than the transmission reference plane 7 and the reflection reference plane 8, as is clear from FIG. Despite the large area, the entire surface of the test surface 8 can be measured. Also, the amount of unevenness of the surface to be tested 8 is d, and the angle of incidence on the surface to be tested 8 is θ.
Then, the amount of unevenness on the surface to be inspected can be observed as 2×d×cosθ. Therefore, by changing the angle of incidence of light onto the surface to be measured 8, the measurement sensitivity can be changed.

Furthermore, an advantage of oblique incidence on the surface to be measured 8 is that the surface reflectance of the surface to be measured 8 can be made higher than when the light is incident perpendicularly. This is the first
This is very effective when measuring the surface of parallel flat glass as the test surface 8 as in the embodiment shown in the figure. In other words, when a glass sample is used and light is incident perpendicularly to the glass surface, reflection occurs from both the front and back surfaces of the glass, and interference also occurs at the same time, and the intensity of both reflections is almost equal, resulting in interference fringes with approximately the same intensity. This may cause the surface shape and back surface shape to become indistinguishable. Figure 2 is a graph showing the reflectance of crown glass, where S is S with respect to the sample surface.
It represents polarized light, and P indicates P-polarized light. For example, the surface reflectance of glass is about 4% in the case of normal incidence, and the intensity of reflected light on the front and back surfaces is 4% and 3.7%, respectively, of the intensity of the incident light {= (100-4) x 0.04×(1−0.04)}, which is almost the same level. However, when S-polarized light is incident on a parallel glass surface at an incident angle of 85 degrees, for example, the reflectance of the surface becomes 70%. The intensity of the light reflected by the back surface is 6.3% {= (100-70) x 0.7 x (1-
0.7)}, and the intensity of light due to surface reflection is more than 10 times greater. Therefore, the influence of reflection on the back surface, which is a problem when the light is incident perpendicularly to the glass surface, can be solved by making the S-polarized light obliquely incident.

Note that in order to obtain interference fringes with good contrast on the observation surface 10, it is preferable to match the intensity of the reference wavefront with the intensity of the test wavefront. For this reason, if the reflectance of the surface to be inspected 8 for S-polarized light is A% and the reflectance of the reflective reference plane 9 is 100%, then the reflectance of the transparent reference plane 7 should be selected to approximate the following formula: It becomes desirable.

{2(A/100) ² +1-√4(100) ² +
1)/{2(A/100) ² }×100(%) Figure 3 shows the second embodiment, in which three reflecting mirrors 13, 14, and 15 are used to change the optical path in order to downsize the device. is bent into a substantially circular shape. In this embodiment, two polarizing plates 16 and 17 are installed in front of the condensing lens system 3, and are configured to produce S-polarized light. Note that the same reference numerals as in FIG. 1 indicate the same members, and most of the functions and effects of this embodiment are the same as those of the first embodiment described above.

If the surface 8 to be tested is not a glass surface, for example, an aluminum coated surface, S-polarized light is not required at all. Furthermore, even if the surface 8 to be measured is not a flat surface but a curved surface, measurement is possible if the reflective reference surface is a curved surface that compensates for this. However, there is a problem in that the angle of incidence at each location on the surface to be measured 8 is different, and the sensitivity of the observed interference fringes varies depending on the position.

As explained above, the interference measurement device according to the present invention allows the inspection light to enter the surface to be measured obliquely and retraces the same optical path, so that the area of the reference surface can be smaller than the area of the surface to be measured. There are big advantages.
In addition, since the surface to be tested is reflected twice, information on the amount of unevenness on the surface to be tested is doubled, and more accurate measurements can be made by determining the angle of incidence on the surface to be tested. Become. Furthermore, if the light is S-polarized, even if the surface to be tested is a parallel glass plate, the measurement can be carried out while almost ignoring the influence of the back surface.

[Brief explanation of drawings]

The drawings show an embodiment of the interference measuring device according to the present invention, and FIG. 1 is a block diagram thereof, FIG. 2 is a graph of the surface reflectance of glass, and FIG. 3 is a block diagram of another embodiment. Reference numeral 1 is a laser light emitting device, 2 is a light amount adjustment filter, 3 is a condensing lens, 4 is a semi-transparent mirror, 5, 1
6 and 17 are polarizing plates, 6 is a collimator lens system, 7
8 is a semi-transparent reference plane, 8 is a test surface, 9 is a reflection reference plane, 10 is an observation plane, 11 is a cylindrical lens, and 13, 14, and 15 are reflecting mirrors.

Claims (1)

[Scope of Claims] 1. An interference measuring device for observing the test surface by illuminating the test surface and the standard reference surface and causing light from the test surface and light from the standard reference surface to interfere with each other. In this method, a reflective reference surface is disposed diagonally opposite to the surface to be tested, and an inspection light consisting of a parallel light flux that is obliquely incident on the surface to be tested is reflected by the surface to be tested, and the surface is reflected by the reflective reference surface. The inspection light is configured so that it enters and is reflected and returns to the original optical path via the same point on the test surface again, so that the inspection light that has passed through the optical path interferes with the light from the standard reference surface. An interference measurement device characterized by: 2. The interference measurement device according to claim 1, wherein the standard reference surface made of a semi-transparent mirror is inserted perpendicularly to the light beam between the test surface and a collimator lens system that creates a parallel light beam. 3. The interference pattern obtained by the test wavefront formed by the test surface and the reference wavefront formed by the reference reference surface is expanded or reduced in one direction to a length and width corresponding to the shape of the test surface. An interference measurement device according to claim 1 or 2, wherein the interference measurement device is configured to observe as a ratio. 4. The interference measurement device according to claim 1, wherein the light source is used with S-polarized light relative to the surface to be measured using a polarizing plate. 5. The interference measuring device according to claim 4, wherein the surface to be tested is a glass surface.