JP2915599B2 - Method and apparatus for measuring toroidal surface - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for measuring the state of a curved surface using the interference of light, and more particularly to a curved surface such as a toroidal surface or a cylindrical surface having different centers of curvature of orthogonal main diameter lines in the surface. The measurement of the surface shape and the surface accuracy in the above.

[0002]

2. Description of the Related Art In general, an optical scanning optical system used in a laser beam printer, a laser facsimile, or the like is constituted by an anamorphic optical system using a cylindrical lens, a toroidal lens, or the like in order to correct a tilt of a polygon mirror. You. Note that a 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, a toroidal surface includes a cylindrical surface unless otherwise specified.

[0003] These lenses are required to have a surface precision of about 0.1 µm in order to increase the density and uniformity of dots formed on the photosensitive member. From such a background, it is necessary to measure the toroidal surface with high accuracy of a wavelength or less.

In general, a laser interferometer is widely known as a device for measuring a surface with high precision. This interferometer can measure a flat surface or a spherical surface, but can measure an in-plane surface such as a toroidal surface. Cannot be measured for curved surfaces having different centers of curvature of orthogonal main diameter lines.

[0005] For this reason, a "contact needle system" 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 stained. In the “optical probe method”,
Since the surface to be measured is scanned at a point, there is a problem that the measurement takes time.

Therefore, the applicant of the present invention has proposed a method of measuring a toroidal surface shown in FIGS. 4 (a) and 4 (b) in Japanese Patent Application No. 2-126659.

In FIG. 1, reference numeral 1 denotes a light source, which uses a gas laser or a semiconductor laser having high coherence. 2a,
Reference numeral 2b denotes a beam expander for expanding a narrow light beam from the light source 1 to an appropriate size. Reference numeral 3 denotes a spatial filter for cutting unnecessary light such as ghost light and reflected light. An optical isolator 4 is a beam splitter 4a, λ /
It has four plates 4b and a reflection surface 4c.

The luminous flux 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. This toroidal surface 7
“a” has main diameter lines AB and CD orthogonal to each other at the apex. 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. CD
Is referred to as a G main diameter line, and the main diameter line AB orthogonal thereto is referred to as an R main diameter line.

The final surface of the objective lens 6 is a reference surface 6a as a semi-transparent mirror, and its center of curvature is located at a position substantially coincident with the finished center of curvature of the G main diameter line (CD) of the toroidal surface 7a. Is done. Further, the reference surface 6a or the toroidal surface 7a is arranged so as to be able to slightly shift and / or tilt 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 remaining light is transmitted and irradiates the toroidal surface 7a to be reflected.

Reference numeral 8 denotes a turntable for fixing the object 7 to be measured, 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, a stepping motor or the like (not shown). The toroidal surface 7a, which is the surface to be measured, can be scanned along the R main diameter line.

The coherent light reflected on the reference surface 6a and the toroidal surface 7a returns on the incoming optical path and is superimposed on the reference surface 6a.
A slit-shaped interference fringe 11 is provided for a measurement portion parallel to the R main diameter line, in which the spherical surface and the toroidal surface 7a can be regarded as substantially parallel.
Is formed, and an image is formed on the image sensor 10 shown in FIG. 5 by the focusing lens 9 via the reflection surface 4c of the optical isolator 4.

When the turntable 8 is rotated along the R main diameter line, the surface shape and surface accuracy of the entire toroidal surface 7a can be measured.

[0015]

By the way, on the toroidal surface, a so-called donut or normal type (hereinafter referred to as "the donut type" or "normal type") has a short generating line (G main diameter line) and a long R main diameter line orthogonal thereto. In addition to the “NTS” type, there are a barrel type (hereinafter “BTS”) having a longer generating line and a shorter R main diameter line, and a saddle type (hereinafter “KTS”). FIG. 6 shows a BTS as an example.
Are generated by rotating a generating line (G main diameter line) around a rotation axis 12 passing through the center of curvature O of the R main diameter line.

Then, the above-mentioned measuring method is applied to the BTS type or K
When applied to the TS type toroidal surface, it is necessary to increase the diameter of the reference surface 6a in accordance with the generating line (G main diameter line),
The interference optical system becomes very expensive. Therefore, a method is considered in which the toroidal surface 7a is rotated 90 ° and placed sideways, interference fringes are generated in parallel with the R main diameter line, and scanning is performed along the G main diameter line. This eliminates the need to increase the diameter of the reference surface 6a. However, as can be seen from FIG. 6, when scanning the measuring portion along the G main meridian, the radius of curvature of the measurement cross-section, so varies in the range r _n from r ₀ in accordance with the distance from the R main meridian , Very difficult.

The present invention has been made to solve this problem, and provides a method and an apparatus which can easily measure the surface shape and surface accuracy of a long toroidal surface of a bus (G main diameter wire) such as BTS or KTS. What you want to do.

[0018]

In order to solve the above-mentioned problems, the present invention irradiates coherent light from the same light source to a surface to be measured and a reference surface serving as a reference, and reflects light reflected from both surfaces. In the method of measuring the surface accuracy by making interference fringes by superimposing,
The coherent light irradiating the surface to be measured is radiated so as to be focused on the rotation axis used to create the toroidal surface, and the G and R main diameter lines orthogonal to each other on the toroidal surface to be measured are Of these, an interference fringe is formed for a measurement portion parallel to the R main diameter line, and the object having a toroidal surface is scanned in parallel with the rotation axis to measure the entire surface.

As a means for fringe analysis, an interference fringe of a measurement portion is imaged on an image sensor by an optical system, and a light intensity signal of the interference fringe from the image sensor is subjected to a Fourier transform method. A configuration may be adopted in which the surface shape and the surface accuracy of the entire toroidal surface are measured by measuring the shape and the surface accuracy and collecting the measurement results of the respective measurement portions obtained by scanning the measurement portion.

The measuring device irradiates coherent light from the same light source to a surface to be measured and a reference surface serving as a reference, and causes reflected light from these two surfaces to interfere with each other to adjust the surface shape and surface accuracy of the surface to be measured. The apparatus for measuring has a reference surface having a predetermined curvature with respect to the curvature of the R main diameter line among the G main diameter line and the R main diameter line orthogonal to each other on the toroidal surface as the surface to be measured. It employs a configuration in which a translation table is provided to support a subject having a surface so as to be movable in parallel with the rotation axis used to create the toroidal surface.

[0021]

The coherent light from the light source is reflected by the reference surface and the surface to be measured and is superimposed and interferes with a measurement portion parallel to the R main diameter line to form interference fringes. This measurement part is scanned in parallel with the rotation axis. However, since the focal point of the coherent light applied to the toroidal surface is always on the rotation axis, it is always moved in focus. .

[0022]

Embodiments of the present invention will be described below with reference to the drawings. 1 (a) and 1 (b) show a measuring device of the present invention.
In the present invention, the toroidal surface 7a is not made of NTS but has a structure similar to that shown in the prior application example of FIG.
Alternatively, they are different in that they are KTS planes and that a translation table 13 is provided instead of the rotation table 8.

In the present invention, the toroidal surface 7a is formed as shown in FIG.
It should be noted that the rotation is 90 ° as compared with that shown in FIG. Translation stand 1
Reference numeral 3 denotes a D (not shown), like the turntable 8 in the conventional example.
It is driven by a C servo motor, a stepping motor, or the like, and can move the DUT 7 in parallel with the rotating shaft 12.

FIGS. 2A to 2D are diagrams showing the interrelationship between the toroidal surface 7a, the reference surface 6a and the rotating shaft 12. FIG.
(a) is a convex BTS, (b) is a concave BTS, (c) is a concave KT
S and (d) show each case of a convex KTS. The reference surface 6a moves relative to the toroidal surface 7a by the movement of the translation table 13, and the state of the movement is exemplarily shown at three places in each of the above-described drawings. Further, the center of curvature O ″ of the reference surface 6a always overlaps the rotation axis 12, and the coherent light emitted from the objective lens 6
It is set so as to converge on the upper point O ″.

As in the case of the above-mentioned prior application, the coherent light from the light source 1 is reflected and superimposed on the reference surface 6a and the toroidal surface 7a as the surface to be measured, and is measured in parallel with the R main diameter line. The light interferes with each other and forms an interference fringe 11 on the image sensor 10 by the focusing lens 9. The measurement portion 11 'is scanned in parallel with the rotation axis 12, but as shown in FIGS. 2A to 2D, the focusing point is always on the rotation axis 12, so that the focused state is always maintained. Will be moved.

Even if the radius of curvature of each measurement section changes as the distance from the center line increases, the reference plane 6a and each measurement section are always kept in parallel, so that interference fringes can be formed. The optical path length from the reference plane 6a to the measurement cross section changes according to the distance from the center line 14, and the amount of change can also be calculated from the number of times the contrast of the interference fringes 11 is inverted and the wavelength. The surface shape along the line can be observed.

Returning to FIG. 2, among the coherent light beams reflected by the surface 7a to be measured, the light beams involved in the formation of interference fringes, that is, the light beams that return along the same optical path as the incoming optical path are indicated by arrows in each figure. As shown, the light is directed toward the center of curvature O 'of the G main diameter line. This light beam passes through the optical axis of the objective lens 6 when the reference surface 6a is on the center line 14, but when the reference surface 6a is shifted upward or downward from the center line 14, this light beam also passes through the objective lens 6. Deviated from the optical axis. Therefore, next, how to form the interference fringes 11 in the case of such a shift will be described.

Although the convex BTS shown in FIG. 2A will be described here, the case of another toroidal surface can be considered in the same manner.

FIG. 3 is a view showing in detail the state in which the reference surface 6a has moved above the central axis 14 in FIG. 2 (a). The light emitted from the objective lens 6 is transmitted to the toroidal surface 7.
It converges to an O ″ point on the rotating shaft 12 that creates a, which is nothing but the center of curvature of the reference surface 6a.

Of the converging light beams shown in FIG. 3, a light beam L indicated by a thick line toward the center O 'of the G main diameter line is a reference surface 6a.
That is, since the light ray L is perpendicularly incident on the reference surface 6a and the toroidal surface 7a, the intersection of the light ray L and the reference surface 6a is P, and the intersection of the measured surface 7a is P '. Then, the light rays reflected at the points P and P 'return on the same optical path as that at the time of incidence, but the other light rays pass through an optical path different from the incident light as shown in the figure. Only the light reflected by and is superimposed to form the interference fringes 11, and the other rays have no relation to the interference fringes.

Here, since the reference surface 6a is a spherical surface and its center of curvature O ″ is always on the rotation axis 12, the reference surface 6a
a is a part of the spherical surface S as indicated by a dotted line centered on the point O ″, and the intersection P of the light ray L and the spherical surface S
Is present not only in the section of FIG. 3 but also in all sections including the rotating shaft 12. The locus of the point P is a circle E on the spherical surface S indicated by a two-dot chain line. On the other hand, the locus of the intersection P 'between the light ray L and the toroidal surface 7a is a circle E' on the toroidal surface 7a. Each of the circles E and E ′ is a concentric circle centered on a point on the rotating shaft 12.

Therefore, the portion overlapping the circle E on the reference surface 6a and the portion overlapping the circle E 'on the surface 7a to be measured interfere with each other to form a slit-like interference fringe 11.

If the reference plane 6a shifts vertically from the center line 14, the locus of the point P also shifts from the center line of the reference plane 6a, so that the interference fringe 11 formed on the image sensor 10 in FIG. The image also moves as indicated by the arrow. Therefore, it is desirable to adopt a two-dimensional sensor as the image sensor 10 so that an image can always be formed on the image sensor even when the interference fringes 11 move.

The test wave reflected from the measured surface 7a and reaching the reference surface 6a and the reference wave reflected on the reference circle 6a are both part of the spherical wave centered at the point O ″. On the other hand, since the objective lens 6 may be considered to substantially satisfy the sine condition, the reference wave and the test wave are converted into a plane wave when passing through the objective lens 6.

That is, the arc-shaped toroidal section 7a forms an image on the image sensor 10 as a straight line. That is, when a spherical wave is irradiated toward the point O ″ on the rotating shaft 12, a slit-like interference fringe is obtained for an arc parallel to the R main diameter line. However, this interference fringe 11 has a toroidal surface in an ideal state. For example, it is formed linearly on the image sensor 10. This property is a very excellent feature of the measuring method of the present invention.

The data for each cross section of the toroidal surface 7a thus obtained is processed by the Fourier transform method,
The analysis is performed on the entire toroidal surface 7a by scanning in the direction shown in FIG.
No. 9 can be carried out in the same manner.

[0037]

As described above, according to the present invention, the surface shape and the surface accuracy of a surface such as a cylindrical surface or a toroidal surface having different centers of curvature of orthogonal main radial lines on the surface are set to a wavelength λ or less. It can be measured with high accuracy and without contact. Further, the surface shape and surface accuracy of a long toroidal surface of a bus (G main diameter wire) such as BTS or KTS other than NTS can be easily measured. Furthermore, since the reference surface does not need to be enlarged in accordance with the length of the generatrix, the measuring device can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is a view showing a configuration of a measuring apparatus of the present invention, wherein (a) is a front view and (b) is a side view.

FIGS. 2A and 2B are diagrams showing a correlation between a toroidal surface, a reference surface, and a rotation axis, wherein FIG. 2A shows a convex BTS, FIG.
(c) is a diagram showing a concave KTS, and (d) is a diagram showing a convex KTS.

FIG. 3 is a diagram illustrating how interference fringes are formed when a reference plane is shifted above a center line in FIG.

FIGS. 4A and 4B are configuration diagrams of an apparatus for measuring a toroidal surface described in the prior application, wherein FIG. 4A is a front view and FIG. 4B is a side view.

FIG. 5 is a diagram of interference fringes formed on an image sensor.

FIG. 6 is a perspective view of a barrel-shaped toroidal surface (BTS).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 6a Reference surface 7 Subject 7a Toroidal surface 11 Interference fringe 11 'Measurement part 12 Rotation axis 13 Translation stand 14 Center axis OR Center of curvature of R main diameter line O' Center of curvature of G main diameter line O "Curvature of reference plane center

Claims (3)

(57) [Claims] 1. A method of irradiating coherent light from the same light source to a surface to be measured and a reference surface serving as a reference, superimposing reflected light from both surfaces to form interference fringes, and measuring surface accuracy,
The coherent light irradiating the surface to be measured is radiated so as to be focused on the rotation axis used to create the toroidal surface, and the G and R main diameter lines orthogonal to each other on the toroidal surface to be measured are A method for measuring a toroidal surface, comprising: forming an interference fringe on a measurement portion parallel to the R main diameter line; and scanning an object having a toroidal surface in parallel with the rotation axis to measure the entire surface. 2. An image of an interference fringe of a measurement portion is formed on an image sensor by an optical system, and a surface shape and a surface accuracy of the measurement portion are determined by a Fourier transform method from a light intensity signal of the interference fringe from the image sensor. 2. The toroidal surface according to claim 1, wherein the surface shape and the surface accuracy of the entire toroidal surface are measured by a measuring step and a step of accumulating the measurement results of each measurement part obtained according to the scanning of the measurement part. Measurement method. 3. An apparatus for irradiating coherent light from the same light source to a surface to be measured and a reference surface serving as a reference, and interfering reflected light from both surfaces to measure the surface shape and surface accuracy of the surface to be measured. Has a reference surface having a predetermined curvature with respect to the curvature of the R main diameter line among the orthogonal G main diameter line and R main diameter line on the toroidal surface as the surface to be measured, and has a toroidal surface. An apparatus for measuring a toroidal surface, comprising a translation table for supporting a subject so as to be movable in parallel with a rotation axis used for creating the toroidal surface.