JPS61105408A - Optical measuring instrument - Google Patents

Abstract

PURPOSE:To measure a surface with high precision by holding the distance between the object surface and an object constant and returning the reflected light of measurement light to the same position in the same direction by an optical heterodyne interference length measuring method. CONSTITUTION:Measurement light F1 and reference light F2 from a Zeeman laser 1 which differ in frequency are separated by a prism 4. Then, the measurement light F1 is polarized by a prism 5 and converged on the object surface 9 through a mirror 7 and the objective 8. Its reflected light has a frequency F1+DELTA by a Doppler shift. Its reflected light illuminates a detector 13. Further, part of the reflected light illuminates a detector 11 to detect the inclination of the object surface. Further, the reference light F3 is converted on a mirror 17 and its reflected light is projected on the detector 13 to detect the frequency of beats between the measurement light and reference light, measuring the object surface. Further, tables 29 and 40 are so moved that the objective 8 and object surface 9 at constant distance. Thus, the thickness of the object surface is measured so that the reflected light of the measurement light returns to the same position, so the surface shape is measured with high precision.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is a two-dimensional or non-contact method for optically measuring the surface shape of general free-form surfaces as well as aspherical lenses and aspherical mirrors with high precision. This relates to three-dimensional measurement equipment, and is an optical system that focuses a laser beam onto the surface of the object to be measured using an objective lens, and measures the surface shape by detecting the Doppler shift of the frequency of the reflected light caused by the movement of the surface of the object to be measured. This relates to a measuring device.

Conventional technology Laser beams with two wavelengths are used as measurement beam and reference beam, respectively, and are reflected from different reflecting surfaces.The beams are guided onto the same photodetector again, the beat frequency is detected, and the movement of the reflecting surface is detected. A device that detects the amount of movement of the reflecting surface from the amount of Doppler shift in the frequency of the reflected light caused by this is known as a laser length measuring machine using the optical heterodyne method, and has been commercialized (for example, HP5526). This is known as the most accurate length measuring device with a wide dynamic range. A three-dimensional measuring machine in which this is attached to a three-dimensional moving table is commercially available. In such a conventional device, a corner cube or a mirror is attached to a movable table, and the movement of the movable table is measured using a laser length measuring device. In order to measure the shape of an object to be measured, a measuring probe of some kind is provided, and a moving table is moved along the surface shape of the object to be measured so that the measuring probe comes into contact with almost constant pressure. The three-dimensional coordinates of the position of the object to be measured are measured from the amount.

The above-mentioned 3D measuring machine is configured to measure the 3D coordinates of the object to be measured via a measurement probe, so no matter how accurate the laser length measuring machine itself is, the accuracy of the measurement probe
Measurement accuracy deteriorates due to changes in contact pressure, mechanical backlash, etc. Numerically, the measurement accuracy of a laser length measuring machine is about ±0.01 μm, but the measurement precision of a three-dimensional measuring machine using a measurement probe is at most about 20.2 μm. The present applicant has filed a patent application and patent application for a device that directly irradiates the surface of the object to be measured with laser light and measures the surface shape using the laser length measurement method based on the optical heterodyne method using the reflected light. 57-18976
There is a device described in No. 1. This device focuses measurement light onto the surface of the object to be measured using an objective lens, and uses a focus servo to maintain a constant distance between the objective lens and the surface of the object to be measured.
The measuring glove is equipped with an objective lens or a tilt correction servo that moves the optical path of the incident light in a direction perpendicular to the optical axis so that the incident light and reflected light take almost the same optical path even if the surface to be measured is tilted. The surface shape of the object can be measured using the optical heterodyne method by directly irradiating a laser beam onto the surface of the object to be measured without using the laser beam.

In addition, the applicant has filed a patent application and patent application No. 68-62444.
In this issue, the measurement light is incident on the objective lens at its full aperture, a part of the measurement light reflected from the surface to be measured is separated, and the separated light is separated by a nozzle roller through a convex lens. A method in which a pinhole is placed before and after the focal point and a focus error signal is detected from the difference in intensity of each light passing through the pinhole, and the X! A method for optically measuring coordinates is described.

Problems to be Solved by the Invention In the method described in the specification of Japanese Patent Application No. 57-189761 mentioned above, a focus error signal ij) is obtained from a part of the measurement light by the astigmatism method. This method had the following problems. That is, in order to enable measurement even when the slope of the surface to be measured is large, the beam diameter of the incident measurement light must be made sufficiently smaller than the entrance pupil diameter of the objective lens. In addition, the focus servo separates a part of the reflected light of the measurement light and generates an error signal using the astigmatism method, but if the surface to be measured has a large inclination, a part of the reflected light will be lost to the objective lens. Since the lens is blocked by the aperture, there is a problem that a focus error occurs. On the other hand, there is a problem in that when the beam diameter of the incident measurement light is made small, the detection sensitivity of the focus error signal decreases. What's more, the detection sensitivity of the focus error signal is inversely proportional to the square of the numerical aperture (N) of the objective lens, but if you reduce the beam diameter of the incident measurement light, the diameter will actually become smaller. be. As mentioned above, in the method described in the above-mentioned specification, since the focus error signal is obtained from the measurement light, there are contradictory demands to increase the focus sensitivity and to measure a surface with a large inclination. Is there a problem?

Furthermore, in the method described in the above-mentioned Japanese Patent Application No. 58-62444, the measurement light is incident on the objective lens at its full aperture, so the detection sensitivity of the focus error signal is sufficiently high, and the surface to be measured is tilted. Although this method does not cause a focus error even if a part of the reflected measurement light is repeatedly hit by the objective lens aperture, the measurement light that causes optical heterodyne interference may be partly hit repeatedly due to the inclination of the surface to be measured. There was a problem in that it was difficult to measure.

Means for Solving the Problems The present invention, as a means for solving the above problems, separates the light for detecting the focus error signal and the measurement light, and the focus error signal detection light is transmitted to the full aperture of the objective lens. The measurement light is made incident on the beam t to make the focus sensitivity sufficiently high.
'6 is made thinner so that even a surface with a large inclination can be measured using the above-mentioned inclination correction servo. The measurement light uses a He-No Zeeman laser with a wavelength of 633 nm, but the focus (fi detection light has a wavelength of 0.82 nm).
Using a nm semiconductor laser, these lights can be combined and separated by a dichroic mirror.

In order to measure the surface shape of an object to be measured by optical heterodyne interferometry, the present invention uses the above-mentioned means to determine the distance between the objective lens and the position to be measured, regardless of the inclination of the surface to be measured. is constant, and the reflected light of the measurement light always returns to the same direction and position, allowing the thickness components of the object to be measured in two directions to be directly measured with a laser beam. A shape optical measuring device is obtained.

Embodiment In FIG. 1, the frequency F emitted from the Zeeman laser 1
The measurement light of , and the reference light of F2 are separated by the half mirror 3 into light for measuring the X coordinate and two coordinates.

The light for measuring two coordinates is separated by the polarizing prism 4 into measurement light and reference light. The measurement light is reflected by the polarizing prisms 4 and 6, passes through the λ/4 wavelength plate 6, passes through the dichroic mirror 7, and is focused onto the surface to be measured 9 by the objective lens 8.

Since the surface to be measured 9 is moved in the X-Y direction by the moving table 29, the frequency of the reflected light is Doppler-shifted by Δ depending on the displacement velocity of the Z coordinate of the measurement position on the object to be measured, F, +Δ becomes. Here, v Δ=c ′+ (0: speed of light). This reflected measurement light passes through the dichroic baller 7 again and becomes P-polarized light at the λ/4 wavelength plate 6. The S-polarized light is totally reflected, and the P-polarized light is 30%
The light is separated by a transmitting polarizing prism 5, and the reflected light from this polarizing prism 5 completely passes through the polarizing prism 4 and is irradiated onto a photodetector 13.

Polarized light 7' The transmitted light of IJism 6 passes through lens 1o,
The light is irradiated onto the 4-split photodetector 11. The photodetector 11 is
Detects deviations in light distribution caused by the inclination of the surface to be measured,
The error signal of the tilt correction servo is output, and the objective lens holding table 4° is moved by the drive motor 22 in the X direction in FIG.

On the other hand, the reference light having the frequency F2 completely passes through the polarizing prism 4, passes through the λ/4 plate 2 and the metal plate 3, and is focused onto the mirror 27 by the lens 24. The mirror 27 is a moving table 29
The flatness is λ/3o, which is fixed together with the object to be measured 9 on the top.
It is a mirror with good flatness of about (λ=o, e 33 nm). The reflected light from the mirror 27 returns along the same optical path, is totally reflected by the polarizing prism 4, and is irradiated onto the photodetector 13. The beat frequency of the difference between the measurement light and the reference light is detected on the photodetector 13, and the thickness of the surface to be measured is measured from this integration.

The light emitted from the semiconductor laser 2o is transmitted through a polarizing prism 16, λ
The light passes through the /4 wavelength plate 14, is totally reflected by the dichroic baller 7, enters the objective lens 8 at full aperture, and is focused at approximately the same position as the measurement light irradiation position on the surface to be measured. . If the surface to be measured is tilted, a part of this reflected light will go outside the aperture of the objective lens 8, but a part of it will return to the aperture of the objective lens and will be totally reflected by the dichroic mirror 7 again, and will be reflected by the polarizing prism 1.
5 is totally reflected, passes through the lens 17, and passes through the half baller 41.
The light is divided into equal parts, passes through a pinhole 18 placed before and after the focus, and is irradiated onto the optical detector 19. Since the focusing positions before and after the two pinholes shift according to changes in the distance between the object to be measured and the objective lens, a focus error signal is obtained from the difference in the respective outputs of the photodetectors 19. The objective lens holding table 40 is moved in two directions by the motor 21 so that this focus error signal approaches zero, so that the focusing position of the measurement light and the light of the semiconductor laser is always on the surface to be measured 9. .

Using the focus servo and tilt correction servo described above, the measurement light is reflected from the surface to be measured and returns to the same position and direction as the incident light, regardless of the position or inclination of the surface to be measured.
As shown in FIG. Figure 2 shows the case where the surface to be measured is not tilted, and Figure 3 shows the case where it is significantly tilted. In either case, an accurate focus error signal can be obtained and the optical path of the reflected light of the measurement light can be adjusted. The tilt correction servo operates so that there is no change, and the beat frequency can be detected by interfering with the reference light without any problems.

The objective lens used in this example has an NA of 0.6 and a lens capable of condensing parallel light to a single point, and the focal point -c'o wavefront aberration is RMS (Root Mean 5qua
re) was 0.04λ (λ: 0,633 μm).

Therefore, even if the measurement light passes through the center of the objective lens,
Even if it passes through the peripheral area, the optical path length to the focal point is 0.04
There is only a difference of about X0.633=0.025 μm. or,
Even if there is aberration around the objective lens, if the surface to be measured is tilted and the measurement light passes around the objective lens, the light from the focusing semiconductor laser will also follow the optical path of the measurement light around the objective lens. Since the transmitted light returns, if the focal position shifts due to aberrations, the focus servo is applied to the shifted position, so the effects of the aberrations are corrected to some extent. Since the X coordinate of the measurement position is always the center of the objective lens 8, the X coordinate of the measurement position is the difference between the amount of movement of the moving table 29 and the amount of movement of the objective lens holding table.

Therefore, the baller 3 fixed to each movable table
2.33, the X coordinate can be measured with high precision by irradiating the measurement light F and the reference light y2, respectively, and by using the optical heterodyne method.

Furthermore, although not shown in the figure, the light of the Zeeman laser is further separated, and a rosette is placed on each of the movable tables 40 and 29, and the Y coordinate of the measurement position is also highly accurate using the same method as in the X direction. can be measured.

In the above description, the objective lens holding table 4o is driven in two directions for focus servo.

However, focus servo can also be applied by driving the object holding table 29 in two directions.

In this case, since the mirror 27 is placed at the focal position of the lens 24, if the ε-thief moves in two directions, the focus will shift, but since the lens 24 is a long focal length lens with f=160 mm, The depth of focus is about ±10 mm. Therefore, the object holding table 29 can be moved in two directions by about ±10 mm.

Regarding the tilt correction servo, in the method described above, the objective lens 8 was moved in a direction perpendicular to the two axes, but it is also possible to move the position of the measurement light incident on the objective lens in a direction perpendicular to the Z axis. .

The mirror 27 that reflects the reference light is a mirror with good flatness as described above, so when the moving table 29 is moved in a direction perpendicular to the two axes, even if the moving straightness is not sufficient, measurement errors will occur. Must not be. Since the difference between the shape of the surface to be measured and the surface shape of this mirror is measured, the measurement accuracy is determined by the surface precision of the mirror.

As described above, in the apparatus of this embodiment, not only the two directions but also all the X-Y coordinates are measured by the laser measurement method, so that it is possible to achieve a measurement accuracy QO of about 5 μm for all coordinates.

Another example is as a moving platform for an object to be measured.

It is also possible to use a movable table in polar coordinates, that is, in the R-0 direction, instead of in the X-Y direction.

In the above embodiment, measurement cannot be performed if the surface to be measured is a rough surface. The reason is that the measurement light is focused on a spot with a diameter of about 2 μm on the surface to be measured, but in the case of a rough surface, the reflected light moves in a direction perpendicular to the two axes of the surface to be measured. This is because the direction of the reflected light changes with the amount of movement on the order of microns, which prevents it from interfering with the reference light and makes measurement impossible. Measurement of such rough surfaces is also possible with the apparatus of the present invention, as explained below.

FIG. 4 shows an apparatus suitable for measuring the shape of rough surfaces.

This device is used as an alternative to the device of the above-mentioned embodiment, and the lens 42 is simply a cat's eye made of a lens 43, which is attached to the objective lens holding table 4o via the cat's eye fixing part 44, and it is very easy to use. It is removable. Focus servo is applied even if the surface is rough, as long as the reflected light returns to the aperture of objective lens 8.

Therefore, since the distance between the objective lens holding table and the measurement point on the surface to be measured is always constant, the measurement light reflected from the cat's eye fixed on the objective lens holding table is made to interfere with the reference light to hold the objective lens. It becomes possible to measure the amount of movement of the table in two directions.

In this case, the tilt correction servo is not activated. This method is
Since measurement is not performed by directly irradiating the measurement light onto the surface to be measured, the measurement accuracy is lower at 10.22 m compared to the device of the above-mentioned embodiment, but the device of the present invention can easily measure rough surfaces by simply adding a cat's eye. He explained that it can be measured.

As described in detail, the present invention has a structure that allows direct measurement of the shape of the object surface by optical heterodyne interferometric length measurement, and is capable of achieving mechanical precision that is technically possible with conventional devices. It has almost overcome the limitations of measurement accuracy caused by the lack of measurement accuracy, and can measure the shapes of surfaces with various surface shapes with the accuracy of interferometric length measurement, that is, 10.1 to 0.01 μm, and it is possible to measure the shape of surfaces with various surface shapes. Surfaces can also be measured with an accuracy of 10.22 m, and its industrial value is extremely large.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of the optical system of an optical measuring device according to an embodiment of the present invention, and FIGS. 2 and 3 are main parts showing the behavior of the measurement light and focus servo light irradiated onto the object to be measured. Enlarged view,
FIG. 4 is a diagram showing the configuration of an optical system according to another embodiment of the present invention. 2.25, 26, 27, 30, 32, 33.34・”H
+H,; Ra, 3.41... Half Mi 5-14
, 15. 36...Polarized beam distributor, 6...
・Special polarizing beam splitter, 6, 14, 23,
31...λ/4 wavelength plate, 7...Dichroic mirror, 8...Objective lens, 9...
Measured object, 11, 13, 19, 351...Photodetector, 12,37...Polarizing plate, 18...
・Pinhole, 20... Semiconductor laser, 21,
22゜28... Motor, 29... Measured object fixing table, 35... Corner cube,
39...Wave plate, 4°...Objective lens holding table, 42...Lens, 43...
...Baller, 44...Cat's eye fixing part. Name of agent: Patent attorney Toshio Nakao and 1 other person 2nd
Figure 4

Claims (1)

[Claims] (1) A first light emitting means that generates a measurement light with a frequency F_1 and a reference light with a frequency F_2; a first light separation means that separates the optical paths of the measurement light and the reference light; and a first light separation means that separates the optical paths of the measurement light and the reference light; An objective lens for condensing light onto an object surface, the objective lens having an entrance pupil diameter larger than the luminous flux diameter of the measurement light at its entrance pupil; an optical system arranged to cause the measurement light passing through the lens and the reference light to interfere with each other on a first photodetector; and detecting fluctuations in the frequency of a beat signal generated on the first photodetector. , a signal processing means for making it possible to measure the shape of the surface of the object to be measured, and a position of an optical system including the object to be measured, the light source, and the objective lens when the optical axis direction of the objective lens is the Z direction. a first moving means group that is relatively movable in a direction perpendicular to the Z direction; a second moving means that moves the objective lens or the object to be measured in the Z direction; a second light emitting means; a lens for making the light flux diameter of the emitted light from the second light emitting means larger than the light flux diameter of the measurement light at the entrance pupil of the objective lens; a dichroic mirror for mixing and separating the optical paths of the synchrotron radiation and the measurement light; a second photodetector group for detecting a focus error signal from the synchrotron radiation reflected from the surface of the object to be measured; and the focus a focus servo means for moving the second moving means in accordance with an error signal to maintain a constant distance between the object surface to be measured and the objective lens; a third photodetector that receives a portion of the light and detects a positional shift of the reflected light of the measurement light caused by the inclination of the surface of the object to be measured; and an error signal obtained from the output of the third photodetector. depending on the objective lens or the first
a third light emitting means for moving the light emitting means in a direction perpendicular to the Z axis;
and a tilt correction servo means that allows the reflected light of the measurement light to return to the same direction and the same position regardless of the tilt of the object surface to be measured. Optical measuring device equipped with.