JPH07294231A - Optical surface roughness sensor - Google Patents

Abstract

PURPOSE:To select a measuring range and a resolving power simply and optionally by providing a mechanism for adjusting an effective numerical aperture of an objective lens to condense a measuring light onto a to-be-measured object. CONSTITUTION:A collimator lens 2 collimates a laser light 50 from a semiconductor laser 1 to parallel lights, while an objective lens 6 condenses a laser light 52 to a surface 31 of a steel plate 30. The diameter of beams of the laser light, 51 collimated to parallel lights is adjusted by an aperture 3 between the collimator lens 2 and the objective lens 6. In other words, an effective numerical aperture of the lens 6 condensing the laser light 52 onto the surface 31 of the steel plate 30 is optionally changed, so that, a measuring range and a resolving power are simply and optionally selected to measure a projection and a recess of tone surface 31 of the steel plate 30. When the effective diameter of the aperture 3 is phi3mm or larger, the effective numerical aperture of the lens 6 becomes an actual numerical aperture of the lens 6. and the measuring range and resolving power are nearly fixed. However, when the effective diameter of the aperture 3 is not larger than phi3mm, the effective numerical aperture of the lens 6 is determined by the effective diameter of the aperture 3.

Description

Detailed Description of the Invention

[0001]

The present invention relates to the field of optical measurement,
The present invention relates to an optical surface roughness meter for measuring surface roughness and surface shape in a non-contact manner.

[0002]

2. Description of the Related Art In order to measure the surface shape and surface roughness of an object to be measured in a non-contact manner, as disclosed in Japanese Patent Laid-Open No. 4-65615, an object to be measured is measured in a direction parallel to the surface. While moving the measurement object, the focus error detection optical system is used to measure the change in the reflection or scattering position due to the unevenness of the surface of the measurement object. In the focus error detection optical system, the measurement light is almost focused on the surface of the object to be measured, and the convergence / divergence state of the light reflected or scattered by the surface of the object to be measured is detected to determine the surface position of the object to be measured. taking measurement. Knife edge method, Foucault method, and the like are known as focus error detection optical systems.

The knife edge method will be specifically described with reference to FIG. Point B is the focal position of the objective lens 6. As shown in FIG. 4A, the light 60 from the point B is
After the parallel light 61 is converted into the parallel light 61 by the objective lens 6 and further converted into the convergent light 62 by the condenser lens 7, the knife edge 20 of the converged light 62 is converted into the knife edge 20 by the knife edge 20 arranged so that the apex is located on the optical axis. Half 64 on the same side is blocked and half 6 on the opposite side of knife edge 20 of convergent light 62
3 is focused on the center of the 2-split photodetector 21. The two-part photodetector 21 is arranged at the focal position of the condenser lens 7,
The center of the split photodetector 21 is located on the optical axis. At this time, the differential signals output from the two light receiving portions 22a and 22b with the center of the two-divided photodetector 21 as a boundary become zero. Here, in FIG. 4, reference numeral 15 is a differential amplifier.

However, as shown in FIG. 4B, when light 60 ′ is emitted from point A, which is a position closer to the objective lens 6 than the focal position of the objective lens 6, the light 60 ′ passes through the objective lens 6. Since it becomes the divergent light 61 ′ after that, the converging position (beam waist position) of the convergent light 62 ′ after further passing through the condensing lens 7 is farther from the condensing lens 7 than in the case of the light from the point B. Become. Therefore, the half 63 ′ of the converged light 62 ′ on the side opposite to the knife edge 20 causes the light receiving portion 22 a of the two-split photodetector 21 on the side opposite to the knife edge 20.
The light is mainly emitted. That is, the differential signals output from the two light receiving portions 22a and 22b of the two-divided photodetector 21 are positive. On the contrary, as shown in FIG. 4C, the objective lens 6 is moved from the focus position of the objective lens 6.
When the light 60 ″ is emitted from the point C, which is a position far from the light, the light 60 ″ passes through the objective lens 6 and then converges light 61 ″.
Therefore, the converging position (beam waist position) of the converged light 62 ″ after passing through the condensing lens 7 is closer to the condensing lens 7 than that of the light from the point B. Half 6 of the convergent light 62 "on the opposite side of the knife edge 20
By 3 ″, light is mainly irradiated to the light receiving portion 22b on the same side as the knife edge 20 of the two-divided photodetector 21. That is, the two light receiving portions 22 of the two-divided photodetector 21.
The differential signals output from a and 22b are negative.

Therefore, the position of the light emission point can be detected by this differential signal. The knife edge method is
In this way, the light emission position is detected by detecting the convergent / divergent state of light. On the other hand, the Foucault method uses a split prism instead of the knife edge, and the position of the two-split photodetector is different, but the basic principle is the same as the knife edge method.

FIG. 5 shows an example of a method for measuring surface irregularities using the knife edge method. The laser light 50 emitted from the laser 1 is converted into circular parallel light 51 by the collimator lens 2, and then enters the polarization beam splitter 4. At this time, the laser beams 50 and 51 are linearly polarized light, and the direction of the laser 1 is set so that the polarization direction is parallel to the reflection surface (p-polarized light), that is, parallel to the paper surface. The incident laser beam 51 passes through the polarization beam splitter 4, further passes through the λ / 4 plate 5 and the objective lens 6, and is substantially focused on the surface 31 of the measurement target 30. The substantially condensed laser beam 53 is the measurement target 30.
The light is reflected or scattered by the surface 31 and is transmitted again through the objective lens 6 and the λ / 4 plate 5. When the optical axis of the λ / 4 plate 5 is set to be 45 ° with respect to the polarization direction of the laser light 51, the polarization direction of the laser light 54 transmitted through the λ / 4 plate 5 twice is λ / 4 Laser light 51 that has never transmitted through plate 5
It is rotated by 90 ° with respect to and perpendicular to the reflection surface (s-polarized light), that is, perpendicular to the paper surface. The polarized light becomes s-polarized light, and the laser light 54 that has entered the polarizing beam splitter 4 again is reflected by the polarizing beam splitter 4 this time, becomes convergent light 55 by the condenser lens 7, and then the apex is located on the optical axis. Are reflected in two directions by the right-angled knife-edge mirror 8 arranged in such a manner that the respective reflected lights 56 and 57 are reflected in the respective two directions.
The light receiving portions 13 of the two-divided photodetectors 11 and 12 are condensed at the centers of the divided photodetectors 11 and 12, respectively.
The light is received by a and 13b and the light receiving portions 14a and 14b. Each of the two-divided photodetectors 11 and 12 is arranged at the focal position of the condenser lens 7 folded back by the right-angle knife edge mirror 8, and the center of each of the two-divided photodetectors 11 and 12 is a right-angle knife edge. The mirrors 8 are located on the respective optical axes that are folded back. Each reflected light 56 reflected by the right angle knife edge mirror 8,
Both 57 and 57 correspond to the half 63 of the convergent light 62 in FIG. 4 opposite to the knife edge 20.
That is, for the reflected light 56, the mirror portion 10 on the left side of the apex of the right-angle knife edge mirror 8 acts as a shield, and conversely for the reflected light 57, the mirror portion on the right side of the apex of the right-angle knife edge mirror 8. 9 will act as a shield. Therefore, each two-split photodetector 1
The amount of light received by the light receiving portions 13a and 13b and the light receiving portions 14a and 14b of 1 and 12 changes according to the position where the light is reflected or scattered, that is, the position of the surface 31 of the measured object 30, respectively. Of the two-divided photodetectors 11 and 12 having the same positional relationship,
The emission position can be detected by the differential signal between the output 70a from 14a and the output 70b from the light receiving portions 13b and 14b, similarly to the differential signal in FIG. At this time, 2
The differential signal output from each of the two split photodetectors 11 and 12 is twice the output differential signal of FIG.

On the other hand, the object 30 to be measured is fixed on a moving stage 32 which moves in a direction perpendicular to the laser light 51, and is moved in a direction perpendicular to the laser light 51 by a driving device 33.
At this time, the position at which the laser light 53 focused on the surface 31 of the object to be measured 30 is reflected or scattered is displaced in the optical axis direction in accordance with the unevenness of the surface 31 of the object to be measured 30. In accordance with the displacement, according to the principle of FIG. 4, the outputs 70a from the light receiving portions 13a and 14a and the light receiving portion 13 of the two-divided photodetectors 11 and 12 having the same positional relationship.
The differential signal 71 of the output 70b from b and 14b changes,
The unevenness of the surface 31 of the measured object 30 can be obtained.

[0008]

However, the above-mentioned conventional optical surface roughness meter has a problem that the measurement range and the resolution are fixed. That is, a device having a wide measurement range but a low resolution and a device having a narrow measurement range but a high resolution cannot be satisfied by a single device, and a device satisfying the respective specifications is required.

The present invention has been made under the above circumstances, and an object thereof is to provide an optical surface roughness meter capable of easily and arbitrarily selecting a measurement range and resolution. is there.

[0010]

In order to solve the above-mentioned problems, an optical surface roughness meter according to the present invention is an optical system for measuring a surface roughness and a surface shape using a focus error detection optical system in a non-contact manner. The surface roughness meter is characterized by including a mechanism for adjusting the effective numerical aperture (NA) of the objective lens for condensing the measurement light on the object to be measured. , An optical system using the knife edge method or the Foucault method as the focus error detection optical system, and the effective numerical aperture (NA) of the objective lens for condensing the measurement light on the object to be measured. The adjusting mechanism is a mechanism for adjusting the beam diameter of the measurement light.

[0011]

In the focus error detection optical system using the knife edge method or the Foucault method, the measurement range and resolution are determined by the numerical aperture (NA) of the objective lens that collects the measurement light on the object to be measured. Japanese Journal of
Applied Physics, Vol.26 (1987) Supplement 26
-4 pp183-186. This can be qualitatively explained as follows. Since the depth of focus of light condensed by an objective lens with a large numerical aperture is small, if the emission position of the light, that is, the position where the light is reflected or scattered, deviates from the focus position even a little, the divergence and convergence condition of the measurement light becomes Because of the large changes, the measurement range is narrowed, resulting in higher resolution. On the other hand, since the depth of focus of light collected by an objective lens with a small numerical aperture is large, the divergence of measurement light will occur even if the light emission position, that is, the position where the light is reflected or scattered, deviates slightly from the focus position. Since the convergence condition does not change so much, the measurement range becomes wider, resulting in a lower resolution. At this time, if the two-division photodetector is arranged at the focal position of the condenser lens and the influence of aberration can be ignored, the numerical aperture of the condenser lens is
The feature is that it does not affect the measurement range and resolution.

In the present invention, by utilizing this characteristic, the effective numerical aperture of the objective lens for condensing the measurement light on the object to be measured is changed to arbitrarily select the measurement range and the resolution within a certain range. It was made possible.
The numerical aperture of a lens is determined by the effective diameter of the lens and the focal length, but the numerical aperture of the optical system, that is, the effective numerical aperture of the lens is when the beam diameter of the light entering the lens is larger than the effective diameter of the lens. Is equal to the numerical aperture of the lens, but when the beam diameter of the light incident on the lens is smaller than the effective diameter of the lens, it is determined by the beam diameter of the light and the focal length of the lens. That is, the numerical aperture of the optical system, that is, the effective numerical aperture of the lens can be changed by narrowing the beam diameter of light to be smaller than the effective diameter of the lens or changing the focal length of the lens. Therefore, in the focus error detection optical system using the knife edge method or the Foucault method, one method is to provide a mechanism for adjusting the beam diameter of the measurement light incident on the objective lens and adjust the beam diameter of the measurement light. Then, the measurement range and resolution can be easily and arbitrarily selected by arbitrarily changing the numerical aperture of the optical system, that is, the effective numerical aperture of the objective lens. Further, as another method, by providing a mechanism for exchanging objective lenses having different focal lengths and different effective numerical apertures, and exchanging these objective lenses, the numerical aperture as an optical system can be arbitrarily changed. The measurement range and resolution can be easily and arbitrarily selected.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic view showing an embodiment of an optical surface roughness meter using the knife edge method according to the present invention.

A divergent laser beam 50 emitted from the semiconductor laser 1 having an output of 30 mW and a wavelength of 830 nm is placed at a position 20 mm away from the semiconductor laser 1 and has a focal length of 20.
mm, a collimator lens 2 having an effective diameter of 5 mm is converted into a circular parallel laser beam 51 having a beam diameter of 5 mm, and then passed through an aperture 3 having a variable effective diameter, whereby the beam diameter is further changed to an arbitrary size. The parallel laser light 52 becomes parallel laser light 52 and is incident on the polarization beam splitter 4 of 10 mm × 10 mm × 10 mm. The semiconductor laser 1 is He-
A laser such as a Ne laser may be used as long as the amount of light required for measurement is obtained. Further, there is no particular problem as long as the wavelength is about 1 to 2 μm or less. Laser light 50, 51, 52
Is a linearly polarized light, and the direction of the semiconductor laser 1 is set so that its polarization direction is parallel to the reflection surface (p-polarized light), that is, parallel to the paper surface. Therefore, the laser beam 52 incident on the polarization beam splitter 4 is polarized. Transmits through the beam splitter 4, and further has a λ / 4 plate 5 with a focal length of 10 mm and an effective diameter of 3 m.
steel plate 30 which is an object to be measured and which is transmitted through the objective lens 6 of m.
Almost collected on the surface 31 of the. The object to be measured may be a rough surface such as a steel plate or a mirror surface such as an aluminum mirror. The substantially condensed laser light 53 is reflected or scattered by the surface 31 of the steel plate 30 as the measurement target,
It again passes through the objective lens 6 and the λ / 4 plate 5. λ / 4 plate 5
The optical axis of is set to be 45 ° with respect to the polarization direction of the laser light 52, and the polarization direction of the laser light 54 that has been transmitted through the λ / 4 plate 5 twice passes through the λ / 4 plate 5 once. Is rotated by 90 ° with respect to the laser beam 52 which is not transmitted, and becomes perpendicular to the reflection surface (s-polarized light), that is, perpendicular to the paper surface. The polarized light becomes s-polarized light, and the laser light 54 that has entered the polarizing beam splitter 4 again is reflected by the polarizing beam splitter 4 this time and becomes convergent light 55 by the condenser lens 7 having a focal length of 100 mm and an effective diameter of 10 mm. Each reflecting surface arranged so that its apex is located on the optical axis is 10 mm × 14.
The reflected light 56 and 57 are reflected by the 1 mm right-angle knife edge mirror 8 in two directions.
The light receiving portions 13 of the two-divided photodetectors 11 and 12 are condensed at the centers of the divided photodetectors 11 and 12, respectively.
The light is received by a and 13b and the light receiving portions 14a and 14b. The sensitivity of the two-division photodetectors 11 and 12 has a wavelength of 83
0.5 mA / mW near 0 nm, the light receiving parts 13a, 13
The size of each of b and the light receiving portions 14a and 14b is about 0.3 mm × 1.2 mm, and there is a dead zone of 10 μm between the light receiving portions 13a and 13b and the light receiving portions 14a and 14b. Each two-split photodetector 11, and 1
2 is arranged at the focal position of the condenser lens 7 folded back by the right-angle knife edge mirror 8, and the centers of the respective two-divided photodetectors 11 and 12 are on the optical axis folded back by the right-angle knife edge mirror 8. Is located in.
Therefore, each two-split photodetector 11 and 1
2 light receiving portions 13a and 13b, and light receiving portions 14a and 14
The amount of light received by b changes according to the position where the light is reflected or scattered, that is, the position of the surface 31 of the steel plate 30 that is the measurement target, so the same position of each of the two-division photodetectors 11 and 12 is used. The light receiving portion 13a which is related,
The differential signal 7 obtained by differentially amplifying the output 70a from 14a and the outputs 70b from the light receiving portions 13b and 14b by the differential amplifier 15
1 makes it possible to detect the position of the surface 31 of the steel plate 30 to be measured.

On the other hand, the steel plate 30 to be measured is fixed on a moving stage 32 which moves in a direction perpendicular to the laser light 52, and is moved in a direction perpendicular to the laser light 52 by a driving device 33. At this time, the position where the laser beam 53 focused on the surface 31 of the measurement target 30 is reflected or scattered is
Since it is displaced in the optical axis direction corresponding to the unevenness of the surface 31 of the object 30 to be measured, the two-divided photodetectors 11 and 12 having the same positional relationship with each other are accompanied by this displacement.
The differential signals of the outputs 70a from a and 14a and the outputs 70b from the light receiving parts 13b and 14b change, and the unevenness of the surface 31 of the steel plate 30 to be measured can be obtained.

At this time, the collimator lens 2 for collimating the laser light 50 from the semiconductor laser 1 into a circular parallel light and the objective lens 6 for condensing the laser light 52 on the surface 31 of the steel plate 30 to be measured. The beam diameter of the laser light 51 is adjusted by the aperture 3 for adjusting the beam diameter of the laser light 51 that is collimated into circular parallel light, and the laser light is applied to the surface 31 of the steel plate 30 to be measured. By arbitrarily changing the effective numerical aperture of the objective lens 6 that focuses the light 52, the measurement range and resolution can be easily and arbitrarily selected, and the unevenness of the surface 31 of the steel plate 30 to be measured is measured. . The effective diameter of aperture 3 is φ
When it is 3 mm or more, the effective numerical aperture of the objective lens is
It is the actual numerical aperture of the objective lens, and the measurement range and resolution are almost fixed, but when the effective diameter of the aperture 3 is 3 mm or less, the effective numerical aperture of the objective lens is determined by the effective diameter of the aperture 3. Will be done.
For example, when the effective diameter of the aperture 3 is φ3 mm or more, the focal length of the objective lens is 10 mm, the effective numerical aperture of the objective lens is 0.15, and the position where the laser light 53 is reflected or scattered, that is, the object to be measured. The differential signal 71 from the two-divided photodetectors 11 and 12 with respect to the displacement of the position of the surface 31 of the steel plate 30 that is shown in FIG.
It becomes like 0. Also, the effective diameter of the aperture 3 is φ1.
In the case of mm, since the focal length of the objective lens is 10 mm, the effective numerical aperture of the objective lens is 0.05, and the position where the laser beam 53 is reflected or scattered, that is, the position of the surface 31 of the steel plate 30 to be measured. The differential signal 71 from the two-divided photodetectors 11 and 12 with respect to the displacement of 1 shows a monotonic increase in the range of ± 150 μm with respect to the focal position, and becomes like a curve 91 in FIG. Therefore, the effective diameter of the aperture 3 is adjusted and measured so that the measurement range and the resolution match the size of the unevenness of the surface 31 of the steel plate 30 that is the object to be measured, and thereby measurement with high sensitivity is performed. You can

Further, by providing a revolver for replacing the objective lens instead of the aperture 3 and replacing objective lenses having different focal lengths and different effective numerical apertures, the numerical aperture of the optical system can be arbitrarily changed. Since the range and the resolution can be easily and arbitrarily selected, the measurement can be performed with high sensitivity.

FIG. 3 is a schematic view showing an embodiment of an optical surface roughness meter using the Foucault method which is another method of the present invention. Semiconductor laser 1, collimator lens 2, aperture 3, polarization beam splitter 4, λ / 4
Plate 5, Objective Lens 6, Condenser Lens 7, Split Photo Detector 1
1, and 12, the light receiving portion 13a of the two-division photodetector 11,
And 13b, the light receiving portions 14a and 14b of the two-divided photodetector 12, the differential amplifier 15, the measured object 30, the surface 31 of the measured object 30, the moving stage 32, the driving device 3
3, laser light 50, 51, 52, 53, 54, and 5
5, output signal 70a from the light receiving section 13a of the two-split photodetector 11 and the light receiving section 14a of the two-split photodetector 12 and the light receiving section 13b of the two-split photodetector 11 and the light receiving section 14b of the two-split photodetector 12. The output signal 70b from the above and the differential signal 71 between the output signal 70a and the output signal 70b are the same as those of the optical surface roughness meter using the knife edge method shown in FIG. The position of 12 is arranged at the focal point of the condenser lens 7 bent by the prism 16, and the center of each of the two-divided photodetectors 11 and 12 is the prism 1.
6 are located on the respective bent optical axes. In the optical surface roughness meter using the Foucault method shown in FIG. 3, the convergent light 55 transmitted through the condenser lens 7 is the right-angled knife edge prism of the optical surface roughness meter using the knife edge method shown in FIG. Instead of 8, the light is transmitted through a prism 16 arranged so that its apex is located on the optical axis and is separated into two directions, and respective transmitted lights 58 and 59 are divided into respective two-divided photodetectors 11 and 12. Is focused on the center of. Hereinafter, similarly to the optical surface roughness meter using the knife edge method, the unevenness of the surface 31 of the steel plate 30 to be measured can be measured, and a highly sensitive measurement can be performed by the numerical aperture adjusting mechanism. .

[0019]

According to the present invention, the mechanism for adjusting the effective numerical aperture makes it possible to easily and arbitrarily select the measurement range and resolution.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of an optical surface roughness meter using a knife edge method according to the present invention.

FIG. 2 is a diagram showing an embodiment of the present invention in which the two-division photodetector 11 with respect to the displacement of the position of the surface 31 of the measured object 30
FIG. 5 shows the differential signal 71 from 12 and 12.

FIG. 3 is a schematic view showing an example of an optical surface roughness meter using the Foucault method according to the present invention.

FIG. 4 is a diagram illustrating a focus error detection method by a knife edge method.

FIG. 5 is a schematic diagram showing an example of a conventional optical surface roughness meter.

[Explanation of symbols]

1 Semiconductor Laser 2 Collimating Lens 3 Aperture 4 Polarizing Beam Splitter 5 λ / 4 Plate 6 Objective Lens 7 Condensing Lens 8 Right Angle Knife Edge Mirror 9, 10 Mirror of Right Angle Knife Edge Mirror 11, 12 Split Photo Detector 13a, 13b Light-receiving part 14a of 2-split photodetector 11 and 14b Light-receiving part of 2-split photodetector 15 Differential amplifier 16 Prism 20 Knife edge 21 2-split photodetector 22a Opposite side of 2-split photodetector 21 Light receiving part 22b of the two-sided photodetector 21 on the same side as the knife edge 30 object to be measured 31 surface of object 30 to be measured 32 moving stage 33 drive device 50, 51, 52, 53, 54, 55, 56, 57, 5
8, 59 laser light 60, 60 ', 60 "diverging light 61 parallel light 61' diverging light close to parallel light 61" convergent light near parallel light 62, 62 ', 62 "convergent light 63 knife edge of laser light 62 Half of the other side 63 'Half of the knife edge of the laser light 62' and half of the opposite side 63 "Half of the knife edge of the laser light 62" and half of the opposite side 64 Half of the same side of the knife edge of the laser light 62 'Laser light 62' Half on the same side as the knife edge of 64 ″ Half on the same side as the knife edge of the laser light 62 ″ 70a Output signal from the light receiving section 13a of the two-split photodetector 11 and the light receiving section 14a of the two-split photodetector 12 70b Output signal 71 from the light receiving part 13b of the two-part photodetector 11 and the light receiving part 14b of the two-part photodetector 12 Differential signal between the output signal 70a and the output signal 70b 90 Effective diameter of the aperture 3 is φ With respect to the displacement of the position of the surface 31 of the measured object 30 in the case of 3 mm or more,
When the effective diameter of the differential signal 71 91 aperture 3 from the two-division photodetectors 11 and 12 is φ1 mm,
Differential signal 71 from the two-division photodetectors 11 and 12 with respect to the displacement of the position of the surface 31 of the object 30 to be measured A A position closer to the objective lens 6 than the focus position of the objective lens B B Focus position C of the objective lens 6 Position farther from the objective lens 6 than the focus position of the objective lens 6.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takao Tawaraguchi 5-10-1 Fuchinobe, Sagamihara City, Kanagawa Electronics Research Laboratory, Nippon Steel Corporation

Claims (3)

[Claims] 1. An optical surface roughness meter for measuring a surface roughness and a surface shape in a non-contact manner using a focus error detection optical system, wherein an effective objective lens for condensing measurement light on an object to be measured. An optical surface roughness meter having a mechanism for adjusting a large numerical aperture (NA). 2. The optical surface roughness meter according to claim 1, wherein the focus error detection optical system is an optical system using a knife edge method or a Foucault method. 3. A mechanism for adjusting a beam diameter of the measurement light as a mechanism for adjusting an effective numerical aperture (NA) of an objective lens for condensing the measurement light on an object to be measured. The optical surface roughness meter according to claim 1 or 2, characterized in that.