JPH04208805A - Optical surface roughness measuring apparatus - Google Patents

Abstract

PURPOSE:To enhance measuring accuracy by eliminating the effect due to the fluctuation of a light source by arranging a reference mirror so as to equalize the respective optical lengths from the surface of an object to be measured and the reference mirror to a detection part. CONSTITUTION:The change-over of a highly accurate optical system and a simple optical system is made possible and, in the simple optical system, the length of the light path from the surface 18 on an object to be measured to an optical sensor 62 and that of the light path from a reference mirror 42 to the optical sensor 62 both of which are the lengths of the light paths of measuring beam Lp and reference beam Ls obtained by splitting baser beam L by a polarizing beam splitter 40 are made equal. By this constitution, since the optical lengths of the beams Lp, Ls simultaneously emitted from a light source coincide each other, the fluctuation of frequency due to the disturbance of a light source 30 is cancelled when laser beams allowed to interfere each other in a light detection part and an error can be reduced without receiving the effect of disturbance to enhance measuring accuracy.

Description

[Detailed description of the invention]

[00011

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface roughness measuring device, and more particularly to an optical surface roughness measuring device that measures surface roughness using light interference. [0002] Conventionally, this type of optical surface roughness measuring device has been proposed by the present applicant in the specification and drawings attached to the application No. 2-16935. This device will be explained below with reference to FIG. 3.4. [00031 FIGS. 3 and 4 are configuration diagrams illustrating an example of the above optical surface roughness measuring device. Laser light source device 3
0, a Zeeman laser is used that outputs laser light containing two types of linearly polarized light whose polarization planes are orthogonal to each other and whose frequencies are slightly different, for example, P-polarized light LP and S-polarized light LS. This laser light source 30 and non-polarizing beam splitter 32
The analyzer 33 and reference optical sensor 34 are arranged on a straight line parallel to the X-axis, and the laser light output from the laser light source device 30 is divided into two
It is divided into two parts, of which the non-polarizing beam splitter 32
The laser beam that has passed through is detected by the reference optical sensor 34, and a reference beat signal FB is output. The above P-polarized LP
, If the frequencies of the S-polarized light LS are fP and fS, respectively, the frequency fB of the reference beat signal FB is 1fP-fS. [0004] On the other hand, the laser beam reflected by the non-polarizing beam splitter 32 in the negative direction of the y-axis is expanded in beam diameter by a beam expander 74 arranged on the optical axis, and then reflected by a mirror 75. , enters the revolver 80 arranged below the mirror 75. The revolver 80 includes a bifocal lens 76 and an objective lens 44.
and a polarizing beam splitter 82.
, a simple optical system consisting of a mirror 84 and an objective lens 45 is attached. By manually rotating the revolver 80, it is possible to switch between the high-precision optical system and the simple optical system. A moving table 60 is disposed below the revolver 80 and is moved by a drive device 58 in two-dimensional directions within an xy plane perpendicular to the optical axis of the objective lens 44 . The object to be measured 12 is placed on this moving table 60 . [0005] Also, analyzer 61. A measurement optical sensor 62 is arranged in the positive y-axis direction of the non-polarizing beam splitter 32 and on the same straight line as the optical axis of the beam expander 74. [00061] In the optical surface roughness measuring apparatus configured as above, when the high-precision optical system is switched and arranged as shown in FIG. 3, the laser light from the mirror 75 enters the bifocal lens 76. The bifocal lens 76 in the high-precision optical system is made of optical glass and a birefringent material, and has a property that its refractive index differs depending on the direction of the polarization plane of the incident light beam. Therefore, among the laser light incident on the bifocal lens 76, the P-polarized light LP
is parallel light, and the S-polarized light LS is convergent light and passes through the objective lens 44.
incident on . Since the objective lens 44 is configured such that its front focal point is located at the rear focal point of the bifocal lens 76, the P-polarized light LP incident as parallel light is focused at one point on the surface 18 of the object to be measured 12 as convergent light. On the other hand, the S-polarized light LS, which has entered as convergent light, is converted into parallel light and sent to the object to be measured 12.
A relatively wide range of the surface 18 is irradiated. [0007] The object to be measured 12 is mounted on the moving table 60, and the light P focused on one point on the surface 18 of the object 12 is mounted on the moving table 60.
The polarized light LP is caused by a Doppler shift Δfs that occurs in response to minute irregularities on the surface 18 as the object 12 is moved by the moving table 60, and a doppler shift Δfs caused by vibrations and other disturbances during the movement of the moving table 60. Puller shift Δ
fd, and the frequency of the reflected light is fP+Δfd+Δ
fs. On the other hand, when the S-polarized light LS is irradiated onto a relatively wide range of the surface 18 in the form of a circular parallel beam, the influence of minute irregularities on the surface 18 is averaged out and canceled out as a whole, resulting in Doppler shift due to disturbance. It is only affected by Δfd, and the frequency of the reflected light is fS + Δfd
becomes. The P-polarized light LP is a measurement light, and the S-polarized light LS is a reference light. [00081 P-polarized light LP and S reflected at surface 18
Each of the polarized lights LS follows an optical path opposite to the incident path and is made incident on the non-polarizing beam splitter 32, passes through this and is received by the measurement optical sensor 62, and a measurement beat signal FD is output. This measurement beat signal FD is composed of P polarized light LP and S
This corresponds to the beat due to interference with the polarized light LS, and its frequency fD is fD=l (fP+Δfd+Δfs)−(fS+Δf
d) 1=lfP-fS+Δfsl, and the Doppler shift Δfd due to the disturbance is canceled out. [0009] The measurement bit signal FD and the reference beat signal FB are supplied to the measurement circuit 64, and the frequency fD (=l fP−fs+Δfsl) of the measurement beat signal FD is
from the frequency fB of the reference beat signal FB (=l fP-
By subtracting fS l), only the Doppler shift Δfs due to the irregularities of the surface 18 is extracted, and a signal representing this Doppler shift Δfs is supplied to the control circuit 66. The control circuit 66 is composed of, for example, a microcomputer, and while the driving device 58 sequentially moves the moving table 60 in the x-y directions, the control circuit 66 controls the measuring circuit 64.
The displacement Zs at each position is calculated based on the following equation 1 from the signal (Δfs) supplied by
The overall surface roughness is displayed three-dimensionally on the display 68. [00101

[Equation 1] is − (2)′) ds cit (χ: laser beam surface) [00111 On the other hand, when switching to the simple optical system shown in FIG.
The reflected laser light is split by a polarizing beam splitter 82 into S-polarized light LS and P-polarized light LP. S polarized light L
The S light is reflected and irradiated onto the mirror 84, and the P-polarized light LP passes through the polarization beam splitter 82 and enters the objective lens 45. The P-polarized light LP incident on the objective lens 45 is condensed onto one point on the surface 18 of the object to be measured 12 as convergent light. [0012] The reflected light from the surface 18 is caused by a Doppler shift Δ that occurs in response to minute irregularities on the surface 18 as the object to be measured 12 is moved by the moving table 60.
fs and Doppler shift Δfd due to vibrations and other disturbances during movement of the moving table 60, the frequency of the reflected light becomes fP+Δfd+Δfs. Further, since the reference mirror 42 is fixed, the frequency of the reflected light from the reference mirror 42 does not change and remains at fS. The P-polarized light LP is a measurement light, and the S-polarized light LS is a reference light. [0013] The reflected lights of the P-polarized light LP and the S-polarized light LS are combined by the polarization beam splitter 82 and sent to the measurement optical sensor 6.
2, and a measurement beat signal FD is output. This measurement beat signal FD corresponds to the beat caused by interference between the P polarized light LP and the S polarized light LS, and its frequency fD is l
★The capsule fP - fS + Δfd + Δfsl, which is affected by the Doppler shift Δfd due to disturbance, but with the simple optical system, the object to be measured with relatively large surface roughness is measured by the objective lens 45 with low magnification, so the surface The influence of disturbance is relatively small compared to roughness. Measurement optical sensor 62
The surface roughness of the surface 18 of the object to be measured 12 is calculated from the measurement beat signal FD received by the measuring device 68 in the same manner as in the high-precision optical system, and is displayed on the display 68. [0014] However, in the above method, when switching to a simple optical system, the optical lengths of the measurement light Lp and the reference light Ls to the measurement optical sensor 62 are not necessarily equal. Therefore, the measurement light Lp and the reference light Ls received by the measurement optical sensor 62 are not irradiated from the laser light source device 30 at the same time. Therefore, when the output frequency of the laser light source device 30 fluctuates over time due to the influence of heat, etc., the measurement light Lp and the reference light Ls received at the same time by the measurement optical sensor 62 each have different frequency fluctuation molecules tl. , ft2. Therefore, the frequency fD of the measurement beat signal FD received by the measurement optical sensor 62 is fD=l (fP+ftl+Δfd+Δfs)−(f
S+ft2)l=lfP-fS+Δfd+Δf s+f
t1-ft2l, and in addition to the disturbance Δfd during measurement, the frequency fluctuation (ftl-ft2) is an error, which prevents accurate measurement. [0015] The present invention has been made to solve the above-mentioned problems, and its purpose is to make the optical lengths of the measurement light and reference light equal when switching to a simple optical system. Another object of the present invention is to provide an optical surface roughness measuring device that can measure surface roughness without being affected by frequency fluctuations of a laser light source device by arranging a reference mirror that reflects a reference light in a simple optical system.
☆☆[0016]

[Means for Solving the Problems] In order to achieve this object, the optical surface roughness measuring device of the present invention includes a laser light source device that outputs laser light containing two types of linearly polarized light, and a laser light source device that outputs laser light containing two types of linearly polarized light. Includes a bifocal lens that converts one of the two types of linearly polarized light contained in the laser beam into parallel light and the other into convergent light, and focuses one linearly polarized light onto the surface of the object to be measured as measurement light. and a high-precision optical system that uses the other linearly polarized light as a reference light and irradiates the surface of the object with a sufficiently larger irradiation diameter than the irradiation diameter of the one linearly polarized light, and a laser beam. Only one of the two types of linearly polarized light included is focused on the surface of the object to be measured as measurement light, and the other is irradiated to a reference mirror as reference light, and the detection is performed from the surface of the object to be measured by the measurement light. A simple optical system configured by arranging the reference mirror at a position where the optical length from the reference mirror to the detection section and the optical length of the reference light from the reference mirror to the detection section are equal, and a laser light source device and an object to be measured. and an optical system switching means for switching and arranging only one of the two types of optical systems. [0017]

[Operation] The optical surface roughness measuring device of the present invention having the above configuration has polarization planes that are orthogonal to each other from the laser light source device.
A laser beam containing two types of linearly polarized light with different frequencies is output, and when the surface roughness of the object to be measured is small, the optical system switching means switches to a high-precision optical system including a bifocal lens. If the surface roughness is large, the surface roughness is measured by switching to a simple optical system that does not include a bifocal lens. [0018] When switching to a high-precision optical system including a bifocal lens, the bifocal lens converts two types of linearly polarized light contained in the laser light from the laser light source device into parallel light and parallel light on a common optical axis. One of the two types of linearly polarized light is focused on the surface of the object to be measured as measurement light, and the other linearly polarized light is parallel light and is focused on the surface of the object to be measured. The condensing point of one linearly polarized light is irradiated with an irradiation diameter that is sufficiently larger than the irradiation diameter of the other linearly polarized light. The measurement beat signals of the reflected light from the measured object of one and the other linearly polarized light are as follows:
A phase shift occurs that corresponds to a change in the relative height position between the focal point of one and the other linearly polarized light and the irradiation surface. Therefore, by detecting the frequency shift or phase change in the measurement beat signal, the surface roughness of the object to be measured can be measured. [0019] On the other hand, when switching to a simple optical system that does not include a bifocal lens, only one linearly polarized component of the two types of linearly polarized light contained in the laser beam is focused on the surface of the object to be measured. . A phase shift occurs in the reflected light from the object to be measured, which corresponds to a change in relative height at the focal point, and the surface roughness of the object to be measured can be measured. In addition, at this time, since the optical lengths of the measurement light focused on the surface of the object to be measured and the reference light irradiated to the reference mirror are the same, even if the frequency of the laser light source device fluctuates, the light receiving section When the respective laser beams are interfered with each other, they are canceled and the measurement is not affected. [00201

[Embodiment] An embodiment embodying the present invention will be described below with reference to the drawings. In this embodiment, the same members as those in the conventional device shown in FIG. 3 are given the same reference numerals, and detailed explanations will be omitted. [0021] FIGS. 1 and 2 are configuration diagrams illustrating an embodiment of the present invention. [00221] The difference between this embodiment and the conventional device is that in the conventional device, the polarizing beam splinter 40 is disposed in a simple optical system, but in this embodiment, the slider 5 is movable in the X direction.
0, and can be inserted and removed in the optical axis between the non-polarizing beam splitter 32 and the beam expander 74, and the reference mirror 42 is placed in the negative direction of the X-axis of the polarizing beam splitter 40. This is the point that was placed. [0023] The slider 50 comprises, for example, a -axis cross roller table and can be moved manually. [00241 As shown in FIG. 1, move the slider 50 in the negative direction of the X axis to remove the polarizing beam splitter 82 from the optical path, and rotate the revolver 80 to set the bifocal lens 76 and objective lens 44. This state is a high-precision optical system. In addition, as shown in FIG. 2, the slider 5
0 in the positive direction of the X-axis, the polarizing beam splitter 82 is inserted into the optical path, and the revolver 80 is rotated to set only the objective lens 45 at a position corresponding to the object to be measured 12. This is a simple optical system. It is. Here slider 5
0 and revolver 80 act as optical system switching means. The objective lens 44 of the high-precision optical system has a relatively high magnification, and the simple optical system has an objective lens 45 with a lower magnification. [0025] The principle of roughness measurement when switching to the high-precision optical system shown in FIG. 1 is exactly the same as that of the conventional device, so its explanation will be omitted. [00261 When switching to the simple optical system shown in FIG. 2, the laser light from the non-polarizing beam splitter 32 is split by the polarizing beam splitter 40 into P-polarized light LP and S-polarized light LS. The P-polarized light LP passes through the polarizing beam splitter 40 and enters the objective lens 45, and the S-polarized light LS is reflected by the polarizing beam splitter 40 and is irradiated onto the reference mirror 42. P-polarized light LP is measurement light, and S-polarized light LS is reference light. Here, the optical length from the polarizing beam splitter 40 to the reference mirror 42 and the optical length from the polarizing beam splitter 40 to the surface 18 of the object to be measured 12 via the beam expander 74, mirror 75, revolver 80, and objective lens 45. A simple optical system is constructed so that the values are equal. The principle of roughness measurement is exactly the same as that of the conventional device, so its explanation will be omitted. [0027] Since the measurement light Lp and the reference light Ls have the same optical length up to the measurement optical sensor 62, the measurement optical sensor 6
The measurement light Lp and the reference light Ls received at 2 are irradiated from the laser light source device 30 at the same time. Therefore, when the output frequency of the laser light source device 30 fluctuates over time due to the influence of heat etc., the measurement light Lp and the reference light Ls received at the same time by the measurement optical sensor 62 have the same frequency fluctuation ft3. is recieving. Therefore, the frequency fD of the measurement beat signal FD received by the measurement optical sensor 62 is I (
fP+ft3+Δfd+Δfs)−(fS+ft3)l
=fP-fS+Δfd+Δfsl, and since the frequency fluctuation numerator t3 is canceled out, it is not affected by the frequency fluctuation. [00281 Although the embodiments of the present invention have been described above in detail based on the drawings, the present invention can also be implemented in other embodiments. [0029] For example, in the conventional apparatus shown in FIGS. 3 and 4, the distance between the polarizing beam splitter 82 and the mirror 84 is set equal to the distance from the polarizing beam splitter 82 to the surface 18 of the object to be measured, and the reference beam and The optical path difference of the measurement light may be eliminated. The point is that the optical system should be configured so that the optical lengths of the reference light and measurement light are equal when switched to the simple optical system. [00301 Furthermore, in the above embodiment, the optical system was switched by manually moving the slider 50 and revolver 80, which are optical system switching means, but the control circuit 66 controls these so that they are automatically linked. Good too. [00311 In addition, in the above embodiment, the laser light source device 3
A Zeeman laser with two orthogonal frequencies was used as the 0, but a laser light source device that uses a frequency shifter equipped with an acousto-optic modulator to create a desired frequency difference between two linearly polarized lights has been used. It is also good. In this case, the reference beat signal FB can also be detected from the drive frequency signal of the acousto-optic modulation element. [0032] Further, in the embodiment, the object to be measured 12 is moved by the moving table 60 driven in the X direction and the y direction.
was configured to be moved in two directions, but the surface roughness may be measured by moving only in one of the X direction and the y direction. [0033] Also, for example, the mirror 75 may be replaced with a semi-transparent mirror and a microscope barrel may be provided above it to monitor the measurement state, or an optical isolator may be inserted between the laser light source device 30 and the non-polarizing beam splitter 32. The present invention can be implemented with various modifications and improvements based on the knowledge of those skilled in the art. [0034] [Effect of the invention] As is clear from the detailed description above,
According to the present invention, it is possible to switch between a high-precision optical system using a bifocal lens and a simple optical system that does not use a bifocal lens, and when switching to the simple optical system, measurement light and reference light can be switched. Since the optical lengths are the same, even if frequency fluctuation occurs in the laser light source device, it can be canceled in the light receiving section and no error occurs.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the configuration of an embodiment embodying the present invention, and is a perspective view showing a case where the surface roughness of a workpiece is measured using a high-precision optical system.

FIG. 2 is a diagram illustrating the configuration of an embodiment embodying the present invention, and is a perspective view showing a case where the surface roughness of an object to be measured is measured using a simple optical system.

FIG. 3 is a diagram illustrating the configuration of a conventional optical surface roughness measuring device, and is a perspective view showing a case where the surface roughness of an object to be measured is measured using a high-precision optical system.

FIG. 4 is a diagram illustrating the configuration of a conventional optical surface roughness measuring device, and is a perspective view showing a case where the surface roughness of an object to be measured is measured using a simple optical system.

[Explanation of symbols]

30 Laser light source device 50 Slider 40 Polarizing beam splitter 42 Reference mirror 80 revolver 76 Bifocal lens 12 Object to be measured 18 Surface 62 Measurement optical sensor LP P polarized light (linear polarized light) LS S polarized light (linear polarized light)

[Figure 1] [Figure 3

Claims (1)

[Claims] 1. A detecting unit for irradiating two types of linearly polarized light whose polarization planes are perpendicular to each other and having different frequencies and detecting the reflected light; An optical surface roughness measuring device that measures the surface roughness of the object to be measured based on the change in the surface roughness includes a laser light source device that outputs a laser beam containing the two types of linearly polarized light, and a laser beam of the laser light source device. It includes a bifocal lens that converts one of the two types of linearly polarized light into parallel light and the other into convergent light, and focuses one linearly polarized light onto the surface of the object to be measured as measurement light, and the other. a high-precision optical system that irradiates the surface of the object to be measured with linearly polarized light as a reference light in parallel light and with an irradiation diameter that is sufficiently larger than the irradiation diameter of the one linearly polarized light; Only one of the two types of linearly polarized light is focused on the surface of the object to be measured as a measurement light, and the other is irradiated to a reference mirror as a reference beam, and the measurement light is transmitted from the surface of the object to the detection section. a simple optical system configured by arranging the reference mirror at a position where the optical length of the reference light and the optical length of the reference light from the reference mirror to the detection section are equal; An optical surface roughness measuring device comprising an optical system switching means for switching and arranging only one of the two types of optical systems in between.