JP2877935B2 - Surface accuracy measuring method and measuring device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a technique for easily positioning a reference surface and a measured surface when measuring the state of a curved surface using light interference.

[Conventional technology]

An optical scanning optical system used in a laser beam printer, a laser facsimile, or the like is generally formed of an anamorphic optical system using a cylindrical lens, a toroidal lens, or the like, in order to correct a tilt of a polygon mirror. Note that the cylindrical surface can be considered to be a case where one of the radii of curvature is infinite on the toroidal surface. Therefore, in this specification, the toroidal surface includes the cylindrical surface unless otherwise specified.

These lenses are required to have a surface accuracy of about 0.1 μm in order to increase the density and uniformity of dots formed on the photoconductor. From such a background, it is necessary to measure the toroidal surface with a high accuracy of the wavelength λ or less.

In general, a laser interferometer is widely known as a device for measuring a surface with high accuracy. This interferometer can measure a flat surface or a spherical surface, but can intersect at right angles in a plane such as a toroidal surface. It is not possible to measure a curved surface with a different center of curvature of the main diameter wire.

Therefore, as a conventional method for measuring such a toroidal surface with high accuracy, a "contact needle method" in which a contact needle such as diamond or ruby is brought into contact with a surface to be measured and scanned,
There has been an “optical probe method” or the like in which light is applied to a surface to be measured as a minute spot and the spot is scanned over the entire surface to be measured.

However, in the “contact needle method”, a hard needle is brought into contact with the surface to be measured, so that there is a problem that the surface to be measured is damaged or soiled. Also, the “optical probe method”
Thus, there is no problem that the surface to be measured is damaged or soiled, but there is a problem that it takes a long time to perform the measurement because the surface to be measured is scanned at points.

In order to solve this problem, the applicant has filed Japanese Patent Application No.
No. 126659 proposes a toroidal surface measuring device as shown in FIGS. 8 (a) and 8 (b).

In these figures, reference numeral 1 denotes a light source, and a gas laser or a semiconductor laser having high coherence is used. Reference numerals 2a and 2b denote beam expanders for expanding a narrow light beam from the light source 1 or the like to an appropriate size. Reference numeral 3 denotes a spatial filter that cuts off unnecessary light such as stray light and reflected light. Reference numeral 4 denotes an optical isolator, which comprises a beam splitter 4a and a λ / 4 plate 4b. The light beam expanded by the beam expanders 2a and 2b passes through the objective lens 6 and reaches a toroidal surface 7a as a measured surface of the subject 7. The toroidal surface 7a has main diameter lines AB and CD orthogonal to the vertices. Of these, one main diameter line CD is used as a generating line, and this is formed by rotating along the other main diameter line AB. , After which the main diameter CD is replaced by the G main diameter, AB
Is referred to as an R main diameter wire.

The final surface of the objective lens 6 is a reference surface 6a as a semi-transparent mirror, and is formed as a spherical surface having a predetermined radius of curvature. That is, the center of curvature of the reference surface 6a is the toroidal surface.
It is arranged at a position substantially coincident with the center of the finished curvature of the G main diameter line (CD) 7a. Also, the reference surface 6a or the toroidal surface 7a
Are arranged to be slightly shiftable and / or tiltable in the XZ section. Then, a part of the light incident on the objective lens 6 is reflected by the reference surface 6a, and the rest is transmitted and transmitted to the toroidal surface.
Irradiate 7a.

Reference numeral 8 denotes a turntable for fixing the subject 7, which has a rotation axis coinciding with the center of curvature of the R main diameter line (AB) of the toroidal surface 7a, and is driven by a DC servo motor or a stepping motor (not shown) to measure The toroidal surface 7a, which is a surface, can be scanned along the R main diameter line.

Reference light reflected by reference surface 6a, and toroidal surface 7a
The test light reflected by is returned and superimposed on the optical path that came together. Then, when returning to the optical isolator 4, λ / 4
Due to the action of the plate 4b and the beam splitter 4a, the light is totally reflected by the reflection surface 4c of the beam splitter 4a and reaches the image sensor 10 via the focusing lens 9.

By the way, the reference surface 6a is usually a spherical surface, and the surface to be measured is the toroidal surface 7a. Therefore, interference occurs in an elongated rectangular measurement portion parallel to the G main diameter line whose both surfaces can be regarded as substantially parallel.

Therefore, by moving the objective lens 6 and the focusing lens 9 in the direction of the optical axis, an image 11 of interference fringes can be formed on the image sensor 10 as shown in FIG. ′ Can be measured.

Further, by rotating the turntable 8 along the R main diameter line, it is possible to observe the surface shape, surface roughness, and surface accuracy such as surface undulation of the entire toroidal surface 7a.

Although the above example is a Fizeau interferometer, other interferometers such as a Michelson interferometer can be similarly applied.

[Problems to be solved by the invention]

In the above measurement method, it is necessary to set the measured surface 7a at a predetermined position in order to form an interference fringe on the image sensor 10. That is, the center of curvature of the R main radius line must coincide with the center of rotation of the turntable 8, and the center of curvature of the G main radius line must coincide with the center of curvature of the reference surface 6a.

However, this adjustment is very delicate and difficult, and if the amount of deviation is large, defocusing or reduction in contrast or interference fringes will not occur at each measurement section.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problem, and has as its object to provide a surface accuracy measuring method and a measuring device which can easily position a reference surface and a measured surface.

[Means for solving the problem]

In order to achieve the above object, the present invention is to irradiate coherent light from the same light source to the surface to be measured and a reference surface to be a reference,
The reference light reflected from the reference surface and the test light reflected from the surface to be measured are superimposed to form an interference fringe, and an image of the interference fringe is formed on the image sensor with a focusing lens to measure surface accuracy. In the method, a screen is placed near a focus point or a beam waist of the reference light, an image of the reference light and an image of the test light are taken on the screen, and the surface to be measured is positioned so that both images overlap. The configuration is adopted.

In addition, a configuration in which an aperture is provided on the screen, or a configuration in which the position of the image of the test light captured on the screen is detected by the image detection unit can be adopted.

As an apparatus of the present invention, the coherent light from the same light source is irradiated to the measured surface and a reference surface serving as a reference, and the reference light reflected from the reference surface and the test light reflected from the measured surface are compared. An apparatus for measuring the surface accuracy by superimposing to form an interference fringe and forming an image of the interference fringe on an image sensor with a focusing lens, wherein a screen is provided in the vicinity of a focus point or a beam waist of the reference light. Is adopted.

Alternatively, it is desirable to adopt a configuration in which an aperture is provided on the screen, or a configuration in which an image detecting means for detecting the position of the image of the test light on the screen is provided.

(Operation)

The reference light reflected on the reference surface forms a small circular image at the focusing unit, and the test light reflected on the other measurement surface forms a slit-shaped image in a direction orthogonal to the interference fringes. . Then, an interference fringe is formed at a portion where the small circular image of the reference light and the slit-like image of the test light overlap.

Therefore, a screen is provided near the focusing part or beam waist of the reference light, the small circular image of the reference light and the image of the test light on the slit are projected, and the surface to be measured is set so that both images overlap on the screen. I do.

After forming an aperture on the screen and positioning the screen so that the above small circular image can pass,
By setting the surface to be measured so that the slit-shaped images overlap each other, only the reference light and the test light that are necessary and sufficient for interference pass through the aperture and remove noise components such as stray light and reflected light. it can.

〔Example〕

 Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows a toroidal surface measuring apparatus according to the present invention. The structure is almost the same as that of FIG.
Is provided in the vicinity of the screen 12, and a circular aperture 12a is formed in the screen 12. In addition,
The screen 12 may be provided near the beam waist instead of near the focusing point.

FIG. 2A shows an image on the screen 12 with the reference light.
13 shows a state in which the image 13 and the image 14 of the test light are formed. Since the reference surface is circular, the reference light forms a small circular image 13. On the other hand, since the surface to be measured 7a is widely irradiated along the R main radial direction (the direction orthogonal to the interference fringes), the test light forms an elliptical or slit image 14. According to the present invention, the two images are monitored on a screen, the object 7 is set so that the two images 13 and 14 overlap on the screen 12 while moving the object 7, and only the luminous flux forming interference fringes is detected. It is intended to pass through the aperture 12a.

Therefore, the screen 12 is supported by moving means (not shown), and is movable in the X, Y, and Z directions. In addition, the diameter of the aperture 12a formed in the screen 12 has a small circular image of the reference light so that stray light and reflected light can be excluded.
It has almost the same diameter as 13. Note that the beam splitter
The diameter of the luminous flux emitted from 4b is equal to that of beam expander 2b.
Since it is determined by NA, it becomes constant regardless of the surface 7a to be measured.

The superposition of the small circular image 13 and the split image 14 is adjusted by rotating the turntable 8 by judging from the moving state of the slit image 14 shown on the screen 12.

In FIG. 2 (b), if the central axes of the reference surface 6a and the surface 7a to be measured coincide with each other as indicated by a solid line, a small circular image of the reference light is obtained.
13 and a slit-shaped image 14 of the test light overlap on the screen 12. However, when the surface 7a to be measured is shifted upward as indicated by the dotted line, the image 14 of the test light is shifted downward as indicated by the dotted line. That is, the subject 7 may be moved in the opposite direction to the image shift.

When the slit-shaped image 14 moves from bottom to top as shown in FIGS. 3A to 3C, it means that the image is rotating around the optical axis of the reference surface 6a of the subject 7 ( Hereinafter, this rotation is referred to as “γ rotation”.)

When the slit-shaped image 14 moves upward from below the small circular image 13 and moves downward again as shown in FIGS. 4 (a) to 4 (c), the subject 7 moves to the position shown in FIG. (Hereinafter, this rotation is referred to as “β rotation”).

When the defocus occurs as shown in FIG. 5, the subject 7 is moved in the optical axis direction and focused because the slit-shaped image 14 has a spread.

As described above, the movement of the image shown in FIGS.
Depending on the installation state, the turntable 8 and the subject 7 are set, and the center of curvature of the R main radius line coincides with the rotation center of the turntable 8 and the center of curvature of the G main radius line coincides with the center of curvature of the reference surface 6a. Let it.

When the surface 7a to be measured is spherical, the slit-shaped image 14 of the test light does not have a slit shape, but is a small circular image like the image of the reference light, but the superposition method is the same. is there.

Next, another embodiment of the method of monitoring the screen 12 will be described.

FIG. 6 shows a monitor camera capable of photographing the screen 12.
15 is used. FIG. 7 shows a screen 16 of the monitor camera 15. An image detecting means using an image sensor or the like in a direction crossing the image 14 of the test light is formed on the image forming surface of the camera.
17 are provided. Since the slit-shaped image 14 is formed on the image detecting means 17, if the output of the image detecting means 17 is detected, the position of the slit-shaped image 14 can be known. Further, it is possible to know whether the rotation is the γ rotation or the β rotation based on the change of the slit image 14 due to the rotation of the turntable 8. On the other hand, the positional relationship between the image detecting means 17 and the aperture 12a is known,
If the positions of the aperture 12a and the small circular image 13 of the reference light are matched in advance, it is possible to know how to move the subject 7.

It is obvious that the image detecting means 17 can be directly attached to the screen 12 in addition to being provided in the monitor camera 15.

As described above, when the image 13 of the reference light and the image 14 of the test light are overlapped,
As shown by the solid line in FIG. 6, only the light beams that overlap and cause interference pass through the aperture 12a, so that stray light and reflected light generated in the interference optical system shown by the dotted line can be effectively cut, and noise Interference fringes with small and clear contrast can be obtained, and accurate measurement of surface shape and surface accuracy can be performed.

In the present invention, the aperture 12 is provided on the screen 12.
Even with a configuration in which a is not provided, the purpose of placing the subject 7 at a regular position can be achieved.

〔The invention's effect〕

As described above, according to the present invention, in a device for measuring a surface shape or surface accuracy, it is easy to accurately set a measured surface with respect to a reference surface in order to form interference fringes. Become like

Further, if an aperture is formed on the screen, stray light and reflected light can be effectively removed, interference fringes with high contrast and no noise can be obtained, and accurate measurement can be performed.

[Brief description of the drawings]

FIG. 1 is a front view showing the configuration of a measuring apparatus according to the present invention. FIG. 2 (a) is a view showing an image of a reference light and an image of a test light on a screen near a convergence point of the reference light. ) Illustrates the relationship between the motion of the image on the screen and the motion of the subject, and FIGS. 3A to 3C illustrate the motion of the image on the screen when the subject rotates γ. 4 (a) to 4 (c) are diagrams for explaining the movement of the image on the screen when the subject makes β rotation, and FIG. 5 is a diagram in which the image of the test light on the screen is defocused. FIG. 6 is a diagram showing an embodiment in which a camera for monitoring a screen is provided, FIG. 7 is a diagram showing a screen of a monitor camera, FIG. 8 is a diagram of a toroidal surface measuring device, FIG. 9A is a front view, FIG. 9B is a side view, and FIG. 9 is a view of interference fringes formed on an image sensor. 1 light source, 6a reference surface, 7 subject, 7a measured surface, 8 turntable, 10 image sensor, 11 interference fringe, 11 'measurement part, 12 …… Screen, 12a ……
Aperture, 13 ... Reference light image, 14 ... Test light image, 17
... Image detecting means, CD: one main diameter line (G main diameter line) orthogonal to the toroidal surface, AB: the other main diameter line (R main diameter line).

Claims (6)

(57) [Claims] A coherent light from the same light source is irradiated on a surface to be measured and a reference surface serving as a reference, and the reference light reflected from the reference surface and the test light reflected from the surface to be measured are superimposed. In the method of forming an interference fringe and forming an image of the interference fringe on an image sensor with a focusing lens to measure surface accuracy, a screen is placed near a focus point or a beam waist of the reference light, and A method of measuring an accuracy of a surface, wherein an image of a reference light and an image of a test light are transferred to a target surface, and the surface to be measured is positioned so that both images overlap each other. 2. The method for measuring surface accuracy according to claim 1, wherein an aperture is provided on the screen, and the screen is positioned so that the image of the reference light passes through the aperture. 3. The method for measuring surface accuracy according to claim 1, wherein the position of the image of the test light imaged on the screen is detected by an image detecting means. 4. A coherent light from the same light source is radiated to a surface to be measured and a reference surface serving as a reference, and the reference light reflected from the reference surface and the test light reflected from the surface to be measured are superimposed. An apparatus for measuring the surface accuracy by forming an interference fringe image on an image sensor with a focusing lens using an image of the interference fringe, characterized in that a screen is provided near a focus point or a beam waist of the reference light. Surface accuracy measuring device. 5. An apparatus according to claim 1, wherein an aperture is provided on the screen. 6. An apparatus for measuring surface accuracy, comprising image detecting means for detecting a position of an image of a test light on a screen.