JPH01233310A - Optical method and optical probe for measuring shape of surface - Google Patents

Abstract

PURPOSE:To detect precisely the roughness and undulation of the surface of a substance without suffering the effect of vibration or temperature, by a method wherein two spots, large and small, are projected to the same position of the substance to be measured through the same objective lens and informations contained in reflected lights are taken out separately from each other. CONSTITUTION:A parallel beam of light from a light source 1 is split in two by a polarizing beam splitter 2. The light reflected by a polarizing beam splitter 3 is passed through lenses 15 and 4, bent by a half prism 7, passed through a lens 17 and imaged on the combined focus position F1 of the lenses 15 and 17. This light is reflected on a surface 18 to be measured and reaches a light-sensing element 9. In the light- sensing element 9, a distance between the surface to be measured and the focus F1 is detected by a photodiode Pd. Two split lights are also imaged on a focus F2 after passing through a lens 16. The reflected light from the surface 18 to be measured enters a photo-sensing element 10, which detects a distance between the surface to be measured and the focus F2. The roughness and undulation of the surface of a substance are detected from a difference in size between the spots of two beams.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention optically modulates the surface shape in order to extract only the necessary high spatial frequencies in surface shapes that have roughness, undulations, or longer spatial wavelength components. METHODS AND PROBE FOR MEASUREMENT.

[Conventional technology]

Conventionally, various optical probes have been devised and put into practical use as a method for non-contactly measuring the roughness and shape of a surface. -However, these methods require mechanical guidance with a precision equal to or higher than the resolution of the probe for relative movement between the probe and the surface to be measured. In order to alleviate this difficulty, an optical method was developed that divides a coherent beam into two, detects the difference in height between the two points from the optical path difference between the two beams reaching the reflecting surface, and calculates the surface shape by integrating the difference. A method called the two-point method has also been proposed.

In addition, to measure the shape of a mirror surface, there is a method called laser autocollimation, in which a non-parallel beam is perpendicularly incident on the surface to be measured, differential information about the shape of the mirror surface is detected from the inclination of the reflected light, and the shape is determined from the integral. There is also.

Also, a method called the three-point method is being considered, in which error components of translation and pitching during relative movement are removed from the calculation process of the output by hitting three home bases with the probe.

[Problem to be solved by the invention]

Laser autocollimation and the two-point method can remove translational errors in the surface height direction that occur when the probe and the surface to be measured are moved relative to each other, but pitching errors cannot be removed.

On the other hand, the so-called three-point method requires three probes, so it is not practical in terms of cost and space constraints, and it also has the disadvantage of complicating arithmetic processing.

In these two-point and three-point methods, errors in yawing and rolling that occur during relative movement between the probe and the surface to be measured are
This may cause a larger measurement error than the deviation of the measurement point in measurement using a single probe. In addition, the direction of scanning is - two two-point methods (or three three-point methods).
The direction must be chosen along the line connecting the beam spots. Furthermore, when scanning the probe in a direction perpendicular to the line connecting the spots, a special measurement surface must be prepared so that one beam (or two in the three-point method) is scanned over the correct reference surface. However, with current technology, the weak point of the 3-point method, which in principle has the highest measurement accuracy, is that it requires preparing 3 probes and arranging them in a straight line. That left a big problem.

Therefore, by creating a structure in which the centers of the incident optical axes of the two beams overlap, the present invention eliminates movement errors and the effects of vibration to the same extent or more than the three-point method using only two beams. It is an object of the present invention to provide an optical surface shape measuring method and an optical probe for surface shape measurement, which can have a structure that can be used for surface shape measurement, and which is not subject to spatial constraints like the three-point method or the two-point method.

[Means and effects for solving the problems] The present invention irradiates a surface to be measured with light, receives the reflected light, and determines the height of the surface to be measured based on the position and shape of the image of the reflected light on the light receiving surface. In an optical probe that detects displacement or inclination in the horizontal direction, two spots of different sizes are simultaneously irradiated onto the surface to be measured so that the centers of the spots coincide within the positioning accuracy of the measurement position. By extracting the difference in the height displacement or inclination information of each reflected light beam, it is possible to perform shape measurement by moving the probe and the object relative to each other, and it is possible to obtain mechanical standards with exceptionally high precision. no longer needed. In other words, in terms of movement accuracy, not only pitching and translational errors in the height direction can be removed, but also the effects of errors in all degrees of freedom regarding movement are significantly reduced.

Additionally, since there is no need to worry about errors caused by vibration, rapid scanning becomes possible, allowing shape measurements to be made in a short period of time, and is less susceptible to the effects of optical axis deflection caused by environmental temperature changes.
Since it is possible to perform measurements in a short period of time, it is less susceptible to changes in the shape of the surface to be measured due to temperature changes and drift in the measurement system.

〔Example〕

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

FIGS. 1A and 1B are diagrams showing the basic structure.

In FIG. 1(A), a parallel light beam from a light source 1 is split into two by a polarizing beam splitter 2. In FIG.

The light reflected by the polarizing beam splitter 3 passes through a lens 15,
It passes through the polarizing beam splitter 4, is bent by the half prism 7, passes through the lens 17, and is imaged at the focal position Fl of the combination of lenses 15 and 17. This light is reflected by the surface to be measured 18, passes through the lens 17, the half prism 7, the polarizing beam splitter 6, the reflecting mirror 8, and reaches the light receiving section 9. In the light receiving section 9, the distance between the surface to be measured and the focal point Fl is detected by a photodiode Pd based on a principle called, for example, the astigmatism method.

The other light split by polarizing beam splitter 2 passes through lens 16, is bent by polarizing beam splitter 5.4, passes through half prism 7, lens 17, and enters lens 1.
The image is formed at the focal point F2 of the combined lens combining 6.17. This imaging point is shifted as necessary (forward or backward) in the optical axis direction from the imaging point of the previous beam, and in the case of FIG. 1, it is shifted backward. The second beam reflected from the surface to be measured 18 passes through the half prism 7, is bent by the polarizing beam splitter 6, and enters the light receiving section 10.

In the light receiving section 10, the distance between the surface to be measured and the focal point F2 is detected by a photodiode Pd, for example, based on a principle called non-point convergence.

Note that 11-14 is a 1/4 wavelength plate, which is used to change the polarization angle of the beam.

When the first beam is imaged approximately on the surface to be measured, the spot of the second beam on the surface to be measured is larger than that of the first beam, i.e., the size of the two beam spots is smaller than that of the first beam. To explain the difference with reference to FIG. 1(B), the first beam S split by the polarizing beam splitter 2 forms an image (Fl) on the surface of the surface to be measured 18 via the lens 17.

On the other hand, the second beam P that has gone straight is the lens 17
The same surface to be measured 18 is irradiated so that an image (F2) is formed inside the surface to be measured 18 through the beam.

The state of the two beams on the surface to be measured 18 is divided into a small spot indicated by the symbol T, ie, an optical stylus, and a large spot K, ie, an optical skid surrounding the spot T. The small spot T is narrowed down to the micron order, for example, about 1.2 A11 m, and is used as an optical stylus to detect the roughness of the object surface. In addition, in the case of a large spot 1-K (for example, having a diameter of about 0.4 mm), high frequency components such as roughness are smoothed and do not appear in the output, so the difference in output from the small spot T is taken. By doing this, it is possible to extract roughness information from which waviness and lower frequency components are removed. Therefore, the reflected light of the second beam contains averaged shape information of a wider area range of the surface to be measured. Therefore, the information about the distance to the measured surface obtained from the second beam is
Does not contain information about high spatial frequencies. This functions in the same way as the skid in a mechanical skid-type roughness meter.

Therefore, scan this probe in the direction you want to measure,
By subtracting the distance detection result from the second beam from the distance detection result from the first beam, only the required high spatial frequency components can be extracted. Disturbances due to vibration and errors in the straightness of movement during scanning are equally included in both beams, so if the difference is taken, they are canceled out.

It is also effective to prepare two beams with different thicknesses as a light source and change the size of the Airy disk. At this time, it is also possible to omit lenses 15 and 16 and make Fl and F2 coincide.

Moreover, as an optical system, one of the lenses 15 and 16 may be omitted.

According to the principle of the present invention, any mechanism for distance detection using each beam may be used, so instead of the astigmatism method, for example, the distances between Fl, F2 and the surface to be measured can be detected using a principle called the critical angle method. is also possible.

Furthermore, as a measure against detection errors due to the inclination of the surface to be measured, a well-known method of dividing the reflected light into two beams and observing each beam with a four-split photodiode can be applied as is.

Note that in the principle shown in Figure 1, the two beams are distinguished by their different polarization angles and distributed to separate light receiving systems.
By varying the intensity of the book beam over time at different frequencies,
A configuration in which the reflected light is received by one light receiving system is also possible. In that case, the information of both beams contained in the electrical output of the photoelectric conversion element in the light receiving section is separated using the fluctuating frequency of the light source, converted into position information for each, and then the difference is taken.

It is also possible to attach an actuator to the lens 17 and configure a servo system by phyto-backing the position information detected by the light receiving system of the larger spot, and use the output of the other detection system as a direct measurement value. It is.

FIGS. 2 and 3 show other embodiments of the present invention, which will first be described using FIG. 2.

The parallel light beam from the light source 1 is split into two by the polarizing beam splitter 3, and the straight light beam is passed through the l/4 wavelength plate I! , the light is focused on a reflecting mirror 8 via a lens. Then, the light from the point light source 1a with a large divergence angle passes through the collimator lens 17 and becomes a thick parallel ray of light, is bent by the half prism 7, and the zero black light WA1a that heads toward the surface to be measured 18 is at the focal point of the lens 17. The 0 reflected light at the position passes through the half prism 7, the polarizing beam splitter 6, and the converging lens 15, and reaches the light spot position detection unit 9a.The 0 position detection unit 9a passes through the lens 15.
is placed at the focal point.

On the other hand, the narrow beam of parallel light from the point light source 1b with a small divergence angle, which is split by the polarizing beam splitter 3, is reflected by the beam splitter 4, passes through the half prism 7, and passes through the surface to be measured (
Reflective surface) reaches 1B. The reflected light from the surface to be measured 18 passes through the half prism 7, is bent by the polarizing beam splitter 6 and the reflecting mirror 8, passes through the lens 16, and reaches the light spot position detection section 10a.The zero position detection section 10a is connected to the lens 16.
is placed at the focal point.

The information on the tangent to the reflective surface detected by the position detection unit 9a includes:
Information from a wide area where a large spot is incident is averaged and entered. Furthermore, the information on the tangent to the reflective surface detected by the position detection unit 10a includes information on high spatial frequency components corresponding to the spot diameter. From the difference between these two, inclination information that is not affected by disturbance vibration or movement straightness error during scanning can be obtained. By integrating this information and finding it, it is possible to derive the surface shape of the surface to be measured.

Note that even in the principle shown in Figure 2, the two beams are distinguished by their different polarization angles and distributed to separate light receiving systems.
It is also possible to temporally vary the intensity of the two light sources at different frequencies and receive the reflected light with one light receiving system. In that case, the information of both beams contained in the electrical output of the photoelectric conversion element in the light receiving section is separated using the fluctuating frequency of the light source, converted into position information for each, and then the difference is taken.

Figure 3 shows how the principle of the present invention is arbitrarily converted by applicators A and B into a displacement probe that detects displacement information in the height direction by forming an image of a spot on the surface to be measured on a charging conversion element. There are two light sources la and lb. These two light sources 1a
, lb, parallel beams with different luminous fluxes are generated and are incident on the surface to be measured 18 via the half prism 7. At this time, the intensities of the two light sources are given temporal variations of different frequencies, and a spot on the surface to be measured 18 is imaged through the lens 17 on the photoelectric conversion element 9a for position detection. The electrical output of the photoelectric conversion element includes two frequency components corresponding to the frequency of the intensity change of the light source, so after separating these and converting them into position information, the difference between the two is taken to obtain the necessary spatial frequency component. Take out in a form without error.

Note that it is also possible to use two light fluxes with different polarization angles as incident light, and to separate the reflected light into two by a polarizing beam splitter and extract the position information of each.

〔Effect of the invention〕

As detailed above, according to the present invention, two large and small spots are projected at the same position on the object to be measured through the same objective lens, and the information contained in the reflected light is separated and extracted. It is possible to provide an optical surface shape measurement method and an optical probe for surface shape measurement that can detect roughness and waviness of an object surface with high precision without being affected by vibration or temperature.

[Brief explanation of the drawing]

1A and 1B are principle explanatory diagrams showing one embodiment of the present invention, and FIGS. 2 and 3 are principle explanatory diagrams showing other embodiments of the present invention. 1, la, 1b--Light source 2.3, 4, 5.6--Polarizing beam splitter 9.
10-m-light receiving section 9a, 10a--spot position detection section 11, 12.13.14-1/4 wavelength plate 15, 15a
, 15b, 16.17--Lens 18--Measurement surface

Claims (1)

[Claims] 1) Irradiating light onto a surface to be measured, receiving the reflected light, and measuring the displacement or inclination of the surface to be measured in the height direction based on the position and shape of the image of the reflected light on the receiving surface. When using an optical probe for detection, when irradiating two spots of different sizes on the surface to be measured, irradiate them simultaneously so that the centers of the spots coincide within the positioning accuracy of the measured position, An optical surface shape measurement method characterized by extracting the difference in information on displacement and inclination in the height direction of each reflected light beam. 2) The optical surface shape measuring method according to claim 1, wherein one of the two spots having different sizes is focused on the surface to be measured. 3) An optical method that irradiates the surface to be measured with light, receives the reflected light, and detects the displacement or inclination of the surface to be measured in the height direction based on the position and shape of the image of the reflected light on the receiving surface. Using the probe, two spots of different sizes are simultaneously irradiated onto the surface to be measured so that the centers of the spots coincide within the positioning accuracy of the measurement position, and the displacement in the height direction of each reflected light is measured. Alternatively, an optical probe for surface shape measurement is characterized in that it extracts the difference in slope information.