JPH0453242B2 - - Google Patents

Abstract

PURPOSE:To measure the distance between surfaces to be inspected irrelevantly to the surface shape without contacting by using a light source which has short coherence distance and splitting light into two, passing the two light beams through respective optical systems and reflecting them by the surface to be inspected, and superposing both light beams on each other and reading interference fringes. CONSTITUTION:The light from the light source 8 is split into two by a polarization beam splitter 19. One light beam is converged on the front lens surface of a lens 1 through an objective lens 24a. The other light beam, on the other hand, is converged on the front lens surface of the lens 1 through an objective lens 24b. A corner cube 21a is moved to equalize the two light beams in optical path length by utilizing variation of interference fringes. Then, the light beams are converged on the rear lens surface and then equalized in optical path length similarly. The optical path length between both lens surfaces of the lens 1 is calculated from the difference between the 1st and the 2nd extents of movement of the corner cube 21a.

Description

Detailed Description of the Invention (Field of Application of the Invention) The present invention relates to optical path length measurement, which uses light interference to measure the optical path length between two test surfaces that reflect light rays, such as both surfaces of a lens. It is related to the device.

(Background of the Invention) Conventionally, devices with a built-in linear encoder have been used to measure the position of the lens surface and the center thickness of the lens.
There is a length measuring machine as shown in Fig. 7. 1 is a lens having a lens surface that is a surface to be inspected; 2 is a vertically movable spindle; 3 is an optical linear scale attached to the spindle 2; 4 is a mount; 5
A sensor is attached to the pedestal 4 and reads the amount of displacement of the linear scale 3, 6 is a bearing that supports the spindle 3 so that it can move accurately and smoothly, and 7 is a stage surface. Note that the linear encoder is composed of the linear scale 3, the sensor 5, and an electronic circuit that processes the output of the sensor 5 and displays the amount of displacement of the linear scale 3.

When measuring the position or center thickness of the upper surface of the lens 1, first lower the spindle 2 to the stage surface 7 with the lens 1 not placed on the stage surface 7, and then set the displayed value of the linear encoder to zero. Set. Next, raise spindle 2 sufficiently high and
is placed on the stage surface 7, and the spindle 2 is lowered to the upper surface of the lens 1. The value displayed by the linear encoder at that time indicates the position of the upper surface of the lens 1, that is, the center thickness of the lens 1 in the case of a biconvex lens as shown in FIG.

The problem with the length measuring machine shown in FIG. 7 is that it is a contact type, so it is easy to scratch the surface to be measured, and there is elasticity due to contact pressure between the tip of the spindle 2 and the surface to be measured or the stage surface 7. Deformation may occur, causing measurement errors, and furthermore, the inner surface of the test object cannot be measured.
For example, it is impossible to measure the distance between the bonded surface and the surface of a bonded lens, or the air gap in a lens system that combines multiple lenses.

On the other hand, non-contact methods include the light cutting method, the triangulation method, and the method of measuring the amount of movement of the lens position that focuses on the test surface, but all of these methods measure the inner surface of the test object as the test surface. In this case, it always depends on the shape of the surface of the object to be tested, and there is a big problem that measurement cannot be performed unless the shape is accurately known. In addition, in the case of a measurement method that uses light, there is a problem in that fluctuations in the air between the measurement device and the surface to be measured affect measurement accuracy.

(Objective of the Invention) An object of the present invention is to solve the above-mentioned problems, to avoid contact with the test surface, to be essentially independent of the surface shape of the test surface, and to improve measurement accuracy between the device and the test surface. An object of the present invention is to provide an optical path length measuring device that is not affected by air fluctuations between the optical path length and the surface.

(Features of the Invention) In order to achieve the above object, the present invention provides a light source with a short coherence length, a collimating optical system that converts the light beam of the light source into a parallel light beam, and a collimating optical system that converts the parallel light beam of the collimating optical system into two mutually parallel light beams. a beam splitting means for splitting the beam into beams with different polarization directions; an optical path length variable means for changing the optical path length of the divided first parallel beam; and a first
a first optical system having a focus-adjustable first objective lens that focuses a parallel light beam on a first test surface and converts a reflected light flux from the first test surface into a parallel light flux; a first focus confirmation means for confirming the focus state of the first objective lens; a length measurement means for measuring the amount of change in the optical path length by the optical path length variable means; a second optical system having a focus-adjustable second objective lens that focuses light onto the second test surface and converts the reflected light beam from the second test surface into a parallel light beam; and the second objective lens. a second focus confirmation means for confirming a focus state with respect to the first optical system; and returning light that is incident on and reflected from the first optical system to the first test surface to the first optical system;
and a separating means for separating a light beam according to a polarization direction in order to return the light incident on and reflected from the second test surface from the second optical system to the second optical system; It is characterized by comprising a beam combining means for re-superimposing the parallel beams passing through the two optical systems, and an interference fringe reading means for finding a specific point of visibility of the interference fringes of the overlapping parallel beams.

(Embodiments of the Invention) As a method for measuring the width of the emission spectrum line of a light source, a method has long been known in which a visibility curve for optical path difference is obtained using a Michelson interferometer and then calculated. The present invention is devised so that this principle can be applied to optical path length measurement.

FIG. 1A shows an optical system according to an embodiment of the present invention,
FIG. 1B shows a portion of the interferometric length measuring device in a plane rotated by 90 degrees from FIG. 1A. In order to explain the polarization direction, movement direction, etc., the orthogonal coordinate system shown in FIG. 1A will be used in principle. That is, the right side is the z-axis, the top is the y-axis, and the bottom from the paper is the x-axis. However, in the interference fringe reading means whose optical axis is not on the z and y axes but is oblique, the optical axis direction is the y axis, and the x axis is downward from the plane of the paper.

8 is a light source, 9 is a heat ray absorption filter, 10 is a condensing lens, 11 is a pinhole, 12 is a collimator,
13 is a bandpass filter that adjusts the spectral width and shape of the light source 8. The light source 8, the heat ray absorption filter 9, and the bandpass filter 12 constitute a light source with a short coherence length, that is, a light beam whose spectral width is wide to some extent. A light source with a short coherence length is expressed in comparison with a laser, and while the coherence length of a laser is usually 100 mm or more, the coherence length of the light source used in the present invention is, for example, about 1 mm or less. In comparison with wavelength, it is about 1000 times or less than one wavelength. When the spectrum is Gaussian and its center wavelength is 550 nm, the half-width of the spectrum is 27 nm in order to set the optical path difference at which the visibility is 1/2 to be 5 μm. In other words, a suitable light source can be easily obtained using a halogen bulb and an interference filter.
However, since only one light source is used, it is preferable to use a light emitting diode with high brightness and high luminous efficiency.

14 is a polarizing plate whose polarization direction is 45 degrees with respect to the x-axis and y-axis; 15 is a beam beam that is partially reflected in order to monitor the variation in the light intensity of the light source 8 and to detect the reflected light from the test surface. 16 is a lens, 17 is a photoelectric element, 18 is a total reflection mirror, 19
is a polarized beam splitter that splits the incident light into two polarized beams whose polarization directions are orthogonal and outputs them in parallel; 20a and 20b are right-angle prism mirrors with total reflection films on two orthogonal surfaces; 21a is a The corner cube 21b, which is arranged to be movable in the y-axis direction and has a driving means, is arranged to be movable in the y-axis direction similarly to the corner cube 21a, and has a driving means and a length measuring circuit for measuring the length of the movement thereof. or fixed corner cubes, 22a, 22
Lenses b, 23a, and 23b constitute objective lenses 24a and 24b that converge a parallel beam of light onto the surface to be measured of the lens 1 in the form of a spot. Lenses 23a, 23
b is movable individually in the z-axis direction, and lenses 22a, 23a and lenses 22b, 23b are movable together in the z-axis direction, and are adjusted so as to be in focus on the surface to be inspected. 25 is a polarizing beam splitter similar to the polarizing beam splitter 19. A first optical system is constituted by the right-angle prism type mirror 20a, a corner cube 21a, and an objective lens 24a, and a second optical system is constituted by a part of the polarizing beam splitter 19, a right-angle prism type mirror 20b, a corner cube 21b, and an objective lens 24b. Ru.

Reference numeral 26 denotes a beam splitter that guides some light flux to a monitor optical system that monitors whether or not a parallel light flux forms a spot on the surface to be inspected using objective lenses 24a and 24b, and 27, 28, and 29 constitute the monitor optical system. 30 is a quarter-wave plate with the direction in which the phase advances at 45 degrees with respect to the x-axis or z-axis; 31 is a condenser lens;
32 is a beam splitter, 33 is a polarizing beam splitter, 34 and 35 are photoelectric elements, 36 is a polarizing beam splitter, and 37 and 38 are photoelectric elements. Polarizing beam splitter 33 and photoelectric elements 34, 35
are integrally rotatable around the y-axis, and are rotated such that the output signal of the photoelectric element 34 reaches its peak when the optical path length of the first optical system and the optical path length of the second optical system are equal. The photoelectric element 34 detects a polarized light component with a polarization direction of 0° at that position.
The photoelectric element 35 detects a polarized light component with a polarization direction of 90°. Polarizing beam splitter 36 and photoelectric element 3
7 and 38 are integrated and can be rotated around the z-axis, so that the photoelectric element 38 has a polarization direction of 45°.
The photoelectric element 38 detects the polarization component of
The rotation angle is adjusted to detect a 135° polarization component.

Regarding the polarization direction of the polarizing plate 14, if the ratio of the overall transmittance of the first optical system to the entire transmittance of the second optical system is 1, the polarization direction is preferably 45 degrees.
In the case of 1 or more or 1 or less, the first optical system and the second optical system
It is preferable that the polarization direction can be rotated so that (amount of incident light x transmittance) of the optical system becomes equal.

The amount of movement of the corner cube 21a is determined by the interferometer 3.
The length is measured by 9. Regarding the interferometric length measuring device 39, 40 is a laser with a long coherence distance, and a semiconductor laser is preferable in terms of compactness, but it can be replaced with a He--Ne gas laser or the like. 41 is a lens; 42 is a polarizing plate whose polarization direction is set at 45 degrees to the x-axis and y-axis; 43 is a polarizing beam splitter arranged to make the reflected light enter the corner cube 21b; and 44 is the polarizing beam splitter that directs the reflected light to the corner cube 21b. This is a total reflection mirror arranged so as to make the light incident on 21a. Corner cube 21a,
Since the input and output positions of 21b are symmetrical with respect to the apex, the reflected light is emitted with a parallel positional shift and passes through the total reflection mirror 44 and the polarizing beam splitter 43 as shown in FIG. 1B. Total reflection mirror 4
5 to enter the interference fringe measurement optical system. This interference fringe measurement optical system includes a 1/4 wavelength plate 46, a condensing lens 47,
Beam splitter 48, polarizing beam splitter 4
9. 1/4 wavelength plate 30 to photoelectric element 38, which consist of photoelectric elements 50, 51, polarizing beam splitter 52, and photoelectric elements 53, 54, and constitute interference fringe reading means.
It is similar to The interferometric length measuring device 39 includes a photoelectric element 50,
A signal processing circuit (not shown) is provided for processing the output signals of the corners 51, 53, and 54, and this signal processing circuit determines the movement amount y of the corner cube 21a by the laser 40.
Count in integer fractions of the wavelength. Therefore, the movement amount y can be obtained by converting this count value.

The signals output from the photoelectric elements 17, 34, 35, 37, 38 and the interferometric length measuring device 39 are processed by a signal processing system, and an example of the signal processing system is shown in FIG. .

To explain the measurement procedure when measuring the optical path length between both lens surfaces of the lens 1, first, the objective lens 24
A, 24b are focused on the front lens surface of the lens 1 by adjusting while observing with the eye through the eyepiece 29. Next, by moving the corner cube 21a, the optical path lengths of the first optical system and the second optical system are made to match, and the amount of movement y ₀ of the corner cube 21a at that time is taken as the origin of the optical path length measurement. After that, focus only the objective lens 24a on the rear lens surface of the lens 1, move the corner cube 21a, and measure the amount of movement y ₁ until the optical path lengths of the first optical system and the second optical system match. . The optical path length δ between both lens surfaces of the lens 1 is determined by |y ₁ −y ₀ |, and if the refractive index of the lens 1 is n, the thickness d is |y ₁ −y ₀
|/n.

Next, the operation will be explained.

The configuration from the light source 8 to the bandpass filter 13 creates a parallel light beam with a short coherence distance. This parallel light beam becomes a polarized light beam having an orientation of 45 degrees with respect to the x-axis and the y-axis by the polarizing plate 14.
The luminous flux reflected by the polarizing beam splitter 19 is x
It has a polarization direction in the axial direction, a right-angle prism type mirror 20a, a corner cube 21a, and an objective lens 24.
a, through the polarizing beam splitter 25 to the lens 1
The beam is reflected by the surface to be inspected, and is reflected by the polarizing beam splitter 19 through the same path. On the other hand, the light beam transmitted through the polarizing beam splitter 19 has a polarization direction in the y-axis direction, passes through the right-angle prism type mirror 20b, the corner cube 21b, the objective lens 24b, and the polarizing beam splitter 25, and is reflected on the test surface of the lens 1. and polarizing beam splitter 1 through the same path.
Transmits 9.

When the two orthogonal linearly polarized beams emitted from the polarizing beam splitter 19 enter the quarter-wave plate 30, they become two clockwise and counterclockwise circularly polarized beams, and their combination simply becomes linearly polarized light. The orientation changes depending on the optical path difference between the first optical system and the second optical system. The ratio (intensity) of linearly polarized light to the total amount of light is proportional to visibility, and therefore changes depending on the optical path difference. The intensity and orientation of this linearly polarized light are detected by photoelectric elements 34, 35, 37, and 38.
Note that since the polarization directions of 0° and 180° indicate exactly the same direction, the phase of the output signal from the photoelectric element 34 etc. is twice the polarization direction. For example, a polarization direction of 180° corresponds to a signal phase of 360°. Therefore, photoelectric element 3
4, 35 output signal phase difference and photoelectric element 37,
The phase difference of the output signal of 38 is 180°, and the photoelectric element 34,
37 output signal phase difference and photoelectric elements 35, 38
The phase difference of the output signals is 90°.

In the signal processing system shown in FIG. 2, the photoelectric element 17 receives the light amount fluctuation monitor signal C of the light source 8, the photoelectric element 34 receives the 0° phase polarization intensity signal _A1 , and the photoelectric element 34 receives the 0° phase polarization intensity signal A1.
5 is a 180° phase polarized light intensity signal _A2 , and a photoelectric element 37 is
The photoelectric element 38 outputs a polarized light intensity signal B ₁ of 90° phase and the polarized light intensity signal B ₂ of 270° phase, respectively, and the interferometric length measuring device 39 measures and outputs the moving amount y of the corner cube 21a.

Each signal is offset and gain adjusted by amplifiers 55-59. As a result, if we take the voltage V of the polarized light intensity signal A ₁ amplified by the amplifier 56 on the vertical axis, and take the amount of movement y of the corner cube 21a or the time t when the corner cube 21a is moved at a constant speed on the horizontal axis, then It will look like Figure 3. When the movement amount y ₀ (y ₁ ) with the largest amplitude is zero optical path difference,
Coincident with peak visibility. On the other hand, it is well known that when an oscilloscope is used to plot a sine wave on the horizontal axis and a cosine wave on the vertical axis, the locus will draw a circle as shown in Figure 4. Since the sine wave and the cosine wave are out of phase by 90 degrees, the method to find the amplitude of the polarization intensity signal A ₁ is to use the polarization intensity signal A ₁ and the polarization intensity signal B ₁ which are out of phase by 90 degrees. , respectively, and add them to obtain the square of the amplitude.

However, as can be seen from FIG. 3, the center of vibration of the polarized light intensity signals A ₁ and B ₁ is not at 0 but at V ₀ . This is because the amount of light does not have negative energy. A simple way to reduce the DC component V ₀ to 0 is to subtract a constant voltage _{V 0} from the polarized light intensity signals A ₁ and B ₁ . This method is not wise because it depends on the focus adjustment of the lenses 24a and 24b, the reflectance of the surface to be measured, and the like. Therefore, in Figure 2, the polarized light intensity signal is 180° out of phase from the polarized light intensity signal _A1 .
A ₂ is subtracted by the subtraction circuit 60. Similarly, the polarization intensity signal is 180° out of phase from the polarization intensity signal B ₁ .
B ₂ is subtracted by the subtraction circuit 61. Next, the division circuits 62 and 63 divide (A ₁ -A ₂ ) and (B ₁ -B ₂ ) by the light amount fluctuation monitor signal C, respectively, to correct the light amount fluctuation. Let A and B be the output signals of the division circuits 62 and 63. Multiplying circuit 6 for signals A and B respectively
4,65 and then added by the addition circuit ₆₆ , as shown in _FIG . can get. This corresponds to the visibility curve squared.

After the signal (A ² +B ² ) passes through a filter 67 for removing noise, it is converted to the signal A in FIG. 6 by a differentiating circuit 68.
It is differentiated into a waveform like . t ₀ is the amount of movement y ₀ (y ₁ )
This is the time corresponding to , and is the zero crossing point. Pulse generating circuit 69 generates pulse P ₀ shown in FIG. 6B at time t ₀ . On the other hand, the waveform shaping circuit 70
sends a high level signal to the gate 71 to open it when the level of the signal (A ² +B ² ) is above the threshold Lt. This causes the pulse P ₀ to pass through the gate 71.

During the measurement operation, the corner cube 21a is moved in a direction that makes the optical path difference between the first optical system and the second optical system zero, and the length y of the movement is measured by the interference length measuring device 39 and output. However, since the gate 72 is normally closed, the signal is not input to the memory 73.
When the optical path difference between the first optical system and the second optical system becomes zero,
A pulse P ₀ is generated, and this pulse P ₀ is applied to the gate 72.
Since it is opened, the amount of movement y ₀ or y ₁ at that time is stored in memory 7.
3 and is stored as the origin or optical path length of optical path length measurement.

The method explained above measures the amount of movement y ₀ or y ₁ when the peak of visibility is detected, but the amount of movement y ₀ or y ₁ is measured using the inflection point of the visibility curve. You can also. The squared visibility curve shown in FIG. 5 is differentiated twice. The curve becomes the one shown in FIG. 6C, which crosses zero at two inflection points of the visibility squared curve. Generate pulses P ₀₁ and P ₀₂ at these zero cross points t ₀₁ and t ₀₂ , memorize the movement amounts y ₀₁ and y ₀₂ at each time, and calculate y ₀ = (y ₀₁ + y ₀₂ )/2. , the amount of movement y ₀ is obtained. Further, the movement amount y ₁ can be obtained in the same manner.

According to this embodiment, since it is a non-contact method in which the spot light is applied to the surface to be inspected, it is possible to prevent the surface not to be inspected from being scratched or elastically deformed. Furthermore, it is possible to measure the distance between the bonded surface and the surface of a bonded lens and the air spacing between multiple lenses. Furthermore, even when the surface to be measured is the inner surface of a lens, the optical path lengths of the first and second optical systems are not affected by the radius of curvature of the lens surface, making the measurement simple.

Since the space between the present apparatus and the surface to be inspected is included in the optical paths of both the first optical system and the second optical system, the influence of air fluctuations in the space can be canceled out.

In this embodiment, since the interferometric length measuring device 39 is used to measure the moving amount y of the corner cube 21a, there are the following advantages.

(1) Since the axis of movement of the optical path length change by the corner cube 21a and the length measurement axis are the same, no Atsube error occurs in the amount of movement of the corner cube 21a.

(2) The interferometric length measuring device 39 also measures changes in optical path length, and changes in the refractive index of the air due to disturbances, that is, changes in air temperature and pressure, are common to visibility measurements.
The optical path length change itself can be measured accurately.

(3) Become more compact.

(Correspondence between the invention and the embodiments) The light source 8, heat ray absorption filter 9, and bandpass filter 13 correspond to a light source with a short coherence length of the present invention, and the condenser lens 10, pinhole 11, and collimator 13 correspond to a collimating optical system. The polarizing beam splitter 19 corresponds to the beam splitting means and the beam combining means, the corner cube 21a corresponds to the optical path length variable means, the polarizing beam splitter 25 corresponds to the separating means, the beam splitter 26, and the lens 27. , a light beam bending mirror 28 and an eyepiece lens 29
correspond to the first and second focus confirmation means, the interferometric length measuring device 39 corresponds to the length measuring means, and the distance from the 1/4 wavelength plate 30 to the photoelectric element 38 and from the amplifier 55 to the gate 7
1 corresponds to the interference fringe reading means.

(Modification) The interference fringe reading means is not limited to that shown in the illustrated embodiment, and may be replaced by various known means.

Alternatively, finding the peak visibility may be performed using the human eye. In that case, the photoelectric element 3
Place human eyes in place of 4, corner cube 2
While moving 1a in the y direction, the length value measured by the interferometric length measuring device 39 is read when the visibility is the best.
Although it is not a good idea for accuracy improvement or automation, it has the advantage of being a simple and inexpensive device.

In the illustrated embodiment, a homodyne interference type is used in which the wavelength of light is only one with a certain spectral width, but a heterodyne interference type is used in which the wavelength is slightly changed between the first optical system and the second optical system. be able to.

Corner cube 21a in the first and second optical systems,
21b, but it can be replaced with a cat's eye made of a lens and a concave or convex surface.

In order to change the optical path length of the first optical system, the refractive index of the medium in the optical path may be changed. In particular, when measuring glass thickness, by inserting glass of standard thickness as is, it is possible to reduce the amount of mechanical change in the optical path length.

Although the polarizing beam splitter 19 serves both as a beam splitting means and a beam combining means, separate means can be used.

In the illustrated embodiment, the optical path lengths of the first and second optical systems on the apparatus side are changed by moving the focus-adjustable objective lenses 24a and 24b only in the optical axis direction and focusing the light on the test surface. However, I replaced it with another objective lens with a known optical path length.
The resulting change in optical path length may be corrected by calculation. It is possible to provide the objective lenses 24a, 24b with an automatic focusing mechanism.

(Effects of the Invention) As explained above, according to the present invention, it is possible to measure the optical path length without contacting the surface to be measured and essentially independent of the shape of the surface of the surface to be measured. can.
Furthermore, measurement accuracy can be made unaffected by fluctuations in the air between the device and the surface to be measured.

[Brief explanation of the drawing]

1A and 1B are a plan view showing an optical system according to an embodiment of the present invention and a front view of a part of the interferometer, and FIG. 2 is a block diagram showing a signal processing system according to an embodiment of the present invention. Figure 3 is a diagram showing the waveform of the polarized light intensity signal,
Figure 4 shows the amplitude of the polarized light intensity signal, Figure 5 shows the visibility squared curve, Figure 6 shows the waveform during signal processing, and Figure 7 shows a conventional length measuring machine. FIG. 1... Lens, 8... Light source, 9... Heat ray absorption filter, 10... Condensing lens, 11... Pinhole, 12... Collimator, 13... Band pass filter, 14... Polarizing plate, 17, 34, 35, 3
7, 38...Photoelectric element, 19, 25, 35, 36
...Polarizing beam splitter, 20a, 20b...
Right angle prism type mirror, 21a, 21b...corner cube, 24a, 24b...objective lens, 2
6... Beam splitter, 27... Lens, 28
...Light beam bending mirror, 29...Eyepiece, 3
0...1/4 wavelength plate, 32...beam splitter,
39...Interferometric length measuring device.

Claims (1)

[Claims] 1. A light source with a short coherence length, a collimating optical system that converts the light beam of the light source into a parallel beam, and a beam splitting system that divides the parallel beam of the collimating optical system into two beams with different polarization directions. means, an optical path length variable means for changing the optical path length of the divided first parallel light beam, and a means for condensing the first parallel light beam onto a first test surface, and a light beam reflected from the first test surface. a first optical system having a focus-adjustable first objective lens that converts the beam into a parallel beam of light again; a first focus confirmation means for confirming the focus state of the first objective lens; a length measuring means for measuring the amount of change in the optical path length; and a length measuring means for condensing the divided second parallel light beam onto a second test surface, and converting the reflected light flux from the second test surface into a parallel light flux again. a second optical system having a second objective lens whose focus is adjustable;
a second focus confirmation means for confirming the focus state of the objective lens; and a second focus confirmation means for confirming the focus state of the objective lens;
back to the optical system, and from the second optical system to the second optical system.
In order to return the light incident on and reflected on the test surface to the second optical system, a separating means separates the light beam according to the polarization direction, and a parallel light beam that passes through the first optical system and the second optical system, respectively. An optical path length measuring device comprising a light beam combining means for re-superimposing the light beams, and an interference fringe reading means for finding a specific point of visibility of the interference fringes of the overlapping parallel light beams. 2. The optical path length measuring device according to claim 1, wherein the first and second focus confirmation means have a common eyepiece.