JPH0256604B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a device for measuring a surface shape using a Michelson interferometer type laser interferometric length measuring device or a laser interferometric length measuring device using an optical heterodyne method. is an optical device used in devices that detect information recorded on a surface or defects, etc., which focuses a laser beam onto the irradiated surface using an objective lens, and from the reflected light, detects some information about the irradiated surface, e.g. It relates to an optical device that detects surface irregularities, reflectance, defects, etc., and measures surface shape, surface movement, etc. from the shift in the wavefront of reflected light or the Doppler shift in frequency.

Configuration of conventional example and its problems In this type of conventional optical device, as shown in FIG. 1, the irradiated surface 3 is located at the focal point of the objective lens 2. When it passes through again, it has the property of returning in the same direction as the incident light. Therefore, when the reflected light is imaged again using a lens, even if the irradiated surface 3 is slightly tilted, the image will be formed at the same point. Therefore, this image forming point becomes a fixed point, and a photodetector is placed at or near this image forming point to detect information. However, in reality, even in such a conventional apparatus, the tolerance for the inclination of the irradiated surface 3 is extremely strict. This is because, as shown in FIG. 1, when the irradiated surface 3 is tilted by θ ₁ , the reflected light is tilted by 2θ _1. Therefore, when the incident light is parallel light, the center position of the reflected light after passing through the objective lens 2 is F ₁ sin2θ changes by ₁ .
Note that F ₁ is the focal length of the objective lens 2. Since this phenomenon has a negative effect on the optical system, the tolerance of the tilt of the irradiated surface in conventional devices is extremely strict. As an example, in a laser interferometer,
Within 0.005°, and within 0.2° for optical discs. For optical disks, the system is designed to fall within this accuracy range, but for interferometric length measuring instruments, a method is usually used in which a corner cube is attached to the object to be measured and the movement of the object is measured by the movement of the corner cube. ing.

Purpose of the Invention The present invention solves the above-mentioned conventional drawbacks. Even if the irradiated surface is tilted, as long as the tilt is within the aperture angle of the objective lens, the reflected light will follow the same path as the incident light. Therefore, it is an object of the present invention to obtain an optical device that has a very large tolerance for the inclination of the irradiated surface.

Configuration of the Invention In order to achieve the above object, an optical device of the present invention comprises:
an objective lens for condensing the emitted light from the light emitting means onto the surface of the object to be irradiated; and the reflection caused by the inclination of the surface of the object to be irradiated by receiving a part of the reflected light from the surface of the object to be irradiated. A first photodetector detects a shift in the position or distribution of light, and the objective lens or light emitting means is aligned perpendicularly to the optical axis in accordance with an error signal obtained from the output of the first photodetector. The light reflected from the surface of the irradiated object takes substantially the same optical path as the incident light.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

First, the basic principle will be explained with reference to FIGS. 2 and 3. When the irradiated surface 4 is tilted by _θ2 , the reflected light is tilted by _2θ2 , and the center position of the reflected light after passing through the objective lens 5 changes by _{F2 sin2θ2} . Note that F ₂ is the focal length of the objective lens 5. However, objective lens 5
Alternatively, if the center of the incident light is translated in parallel by F ₂ sin θ ₂ , the reflected light returns along the same optical path as the incident light.
That is, as shown in FIG. 2, if there is a positional shift in the reflected light, for example, a part of the reflected light is received by the photodetector 6 which is divided into four parts, and an error signal generated in accordance with the positional shift is used to detect a part of the reflected light. The objective lens 5 is moved in a direction perpendicular to the optical axis, and servo is applied so that the position of the reflected light is constant, as shown in FIG. Various shapes of the photodetector and methods of obtaining error signals can be considered. For example, even if the reflected light is not separated by the beam splitter 7, an error signal can be obtained by using a quadrant photodetector 8 having a hole 8a the size of the incident light as shown in FIG. Alternatively, a commercially available optical position detector may be used, in which the voltage generated at both terminals changes depending on the light receiving position.

If the irradiated surface 4 deviates from the focal position, the optical path of the reflected light will no longer be constant, which is not good. Therefore, for example, the reflected light is narrowed down by a lens and a cylindrical lens, an error signal is extracted from the generated astigmatism, and the objective lens 5 or the object to be irradiated is moved in the optical axis direction to apply focus servo. The method of extracting a focus error signal from astigmatism is the most common method, but various other methods such as the knife edge method are also known and can be used in the optical device according to the present invention.

The drive mechanism for moving the objective lens 5 or the object to be irradiated using the above-mentioned error signal can be a method of moving a feed screw with a motor, a method of using a linear motor, or the like.

When the optical device according to the present invention is applied to a laser interferometer, it is also possible to directly and precisely measure the thickness of an inclined curved surface. The inclination of the irradiated surface 4 is allowed up to the aperture angle of the objective lens 5. For example, when using the objective lens 5 with NA (numerical aperture) = 0.6, the aperture angle is 36 degrees from the optical axis.
It is possible to sufficiently measure up to the irradiated surface 4 which has an inclination of approximately ±30°. However, in such usage, the beam diameter of the incident light entering the objective lens 5 must be made sufficiently smaller than the entrance pupil diameter of the objective lens 5. For example, in order to be able to measure the irradiated surface 4 with an inclination of ±30° without eclipsing the beam with the objective lens 5 with NA of 0.6, the beam diameter of the incident light should be the entrance pupil diameter of the objective lens 5. It must be less than 1/6 of

Further, when the incident light is diverging light or converging light, if the objective lens 5 is moved in the optical axis direction, the focal position shifts, causing an error. Therefore, in this case, it is better that the incident light be parallel light.

When the objective lens 5 is moved according to the inclination of the irradiated surface 4, the position of the condensing point moves by the amount of movement of the objective lens 5. This strictly holds true if the incident light is parallel light. In curved surface thickness measuring devices, defect inspection devices, and the like that utilize the laser interferometric length measuring device described above, it is necessary to know the position of a measurement point in terms of X-Y coordinates. In this case, the position of the measurement point is determined from the amount of movement of the object to be measured (X ₁ , Y ₁ ) to the amount of movement of the objective lens 5 (X ₂ , Y 1 ).
Y ₂ ) minus (X ₁ − X ₂ , Y ₁ − _{Y 2} ). If the incident light is moved without moving the objective lens 5, the measurement point position does not move.

The laser interferometric length measuring device has an extremely high measurement accuracy of up to 0.01 μm, but when it is combined with the optical device according to the present invention, the effect is very large. For example, it has been considered extremely difficult to precisely measure the shape of an aspherical lens surface in the prior art. The aspheric lens surface is
The value of the thickness Z is determined by the function type for the X-Y coordinates, and the shape is determined. By using the optical device according to the present invention, the value of the thickness Z for any X-Y coordinate position of the lens surface can be measured with the precision of a laser interferometer, so the thickness can be derived from a given function of the actual measurement dimension. This is a system that can automatically measure the deviation from the calculated value in real time.

FIG. 5 shows a surface shape measuring device in which the optical device according to the present invention is applied to a laser interferometric length measuring device using an optical heterodyne method, and this will be explained below. Zeeman laser 9 with oscillation frequencies f ₁ and f ₂
The light emitted from the
It is divided into f ₂ light and f ₁ light polarized in a direction parallel to the plane of the paper. Then, a part of the light is separated by the beam splitter 11, and the beat frequency (f ₁ -f ₂ ) is detected by the photodetector 12. Of the light that has passed through the beam splitter 11, the light at _f2 is sent to the polarizing prism 1.
3 and reflected upward by the fixed mirror 14 to reach the photodetector 15. On the other hand, it is reflected by the surface of the object to be measured 16 at f ₁ , but when the object to be measured 16 moves, the frequency of the reflected light undergoes a Doppler shift depending on the Z component v _Z of the moving speed, and the frequency of the reflected light changes from f ₁ to f ₁ ( 1-2v _Z /c). A part of the reflected light is separated by the polarizing prism 17, and in order to generate the error signals of the position servo and focus servo, the photodetectors 18 and 1
Reach above 9. The polarizing prism 17 has the property of completely transmitting P polarized waves, partially reflecting S polarized waves, and transmitting the rest. The reflected light that has passed through the polarizing prism 17 is totally reflected by the polarizing prism 13 and is sent to the photodetector 15.
reach the top. A beat frequency f ₁ +Δf−f ₂ between f ₂ and f ₁ +Δf is obtained on the photodetector 15, and Δf is determined from the difference between the beat frequency f ₁ −f ₂ obtained on the photodetector 12, Displacement Z is found by integrating this. The measurement accuracy of Z thus obtained is about 0.1 to 0.01 μm. The displacement of the irradiated surface can be measured by measuring the movement of the objective lens 20 in the Z direction, but the measurement accuracy in this case is several μm. In addition, 21 is a beam splitter, 22 is a λ/4 plate, 24 to 26 are lenses, 27 is a cylindrical lens, 28 is a measured value display section,
Reference numeral 29 denotes an object measurement position display section, 30 an objective lens driving device, and 31 an object driving device.

Effects of the Invention As explained above, according to the present invention, even if the irradiated surface is tilted, as long as the inclination is within the aperture angle of the objective lens, the reflected light will follow the same path as the incident light. It is possible to obtain an optical device with a very large tolerance to the inclination of the surface, and its industrial utility value is extremely large.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of the optical path in the optical system of a conventional device, Figs. 2 and 3 are explanatory diagrams of the principle of an embodiment of the present invention, and Fig. 4 is an explanatory diagram of the optical path in an optical system of an embodiment of the present invention. FIG. 5, which is an explanatory diagram of a photodetector, is a schematic configuration diagram of an optical device in an embodiment of the present invention. 8, 12, 15, 18, 19...photodetector, 9
...Zeeman laser, 10, 22, 23...λ/
4 plates, 11, 21...beam splitter, 13,
17...Polarizing prism, 14...Fixed mirror, 1
6...Object to be measured, 20...Objective lens, 24-2
6... Lens, 27... Cylindrical lens, 28... Measured value display section, 29... Measured object measurement position display section,
30... Objective lens drive device, 31... Measured object drive device.

Claims (1)

[Scope of Claims] 1. An objective lens that focuses the reflected light from the light emitting means onto the surface of the object to be irradiated, and a lens that receives part of the light reflected from the surface of the object to be irradiated to a first photodetector that detects a shift in the position or distribution of the reflected light caused by the tilt; and a driving means for moving in a direction perpendicular to the axis, the optical device having a configuration in which the light reflected from the surface of the irradiated object follows substantially the same optical path as the incident light. 2. Receive part of the reflected light from the surface of the irradiated object,
a second photodetector for detecting a change in the optical path of the reflected light caused by a deviation from a condensing position of the emitted light by the objective lens on the surface of the irradiated object; and the objective lens and the second photodetector. and for converting the optical path of the reflected light into a form capable of obtaining a suitable focus error signal on the second photodetector;
The objective lens or the object to be irradiated is moved in the optical axis direction according to a focus error signal, and the focused position is always maintained on the surface of the object to be irradiated. The optical device according to claim 1, wherein the optical device is configured to be positioned. 3. The optical device according to claim 1 or 2, wherein the beam diameter of the emitted light just before it enters the objective lens is smaller than the entrance pupil of the objective lens. 4. The optical device according to any one of claims 1 to 3, wherein the emitted light is substantially parallel light. 5. The optical device according to any one of claims 1 to 4, comprising means for measuring the amount of movement of the objective lens in a direction perpendicular to the optical axis of the emitted light. 6. Equipped with means for subtracting the measured value of the amount of movement of the objective lens in the direction perpendicular to the optical axis of the synchrotron radiation from the measured value of the amount of movement of the irradiated object in the direction perpendicular to the optical axis. An optical device according to any one of claims 1 to 5. 7. The reflected light from the surface of the object to be irradiated with the synchrotron radiation, and the second synchrotron radiation that is partially separated before the synchrotron radiation reaches the objective lens, or the third synchrotron radiation that is different from the synchrotron radiation, An optical system that causes interference on the same light-receiving surface or photodetector, and interference fringes that occur on the light-receiving surface or the photodetector due to interference between the reflected light and the second or third emitted light. 7. The optical device according to claim 1, further comprising a detection means for detecting information such as displacement of the surface of the irradiated object from a change in beat frequency or a change in beat frequency.