JPH0678894B2 - Surface shape measuring instrument - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface shape measuring instrument for contactlessly measuring the surface shape of a lens used in a camera or the like, particularly an aspherical lens.

[0002]

2. Description of the Related Art Appl.Opt.20 NO.19,1981 pp.3367-3377 is an example of an apparatus for scanning the entire surface of an object to be measured by combining a mirror and a rotational drive of the object to be inspected. There is a description of "Aspheric surface calibrator".

The outline thereof is shown in FIG. 6. The light beam 1 emitted from the light source 1 is expanded by the beam expander 2 and is directed by the beam splitter 3 toward the reference mirror 4, the objective lens 5, and the rotating mirror 6. And a light flux that is focused on the surface of the object 7 to be inspected. The rotating mirror 6 and the test object 7 are rotated as shown by arrows by a rotation driving mechanism (not shown), and the focused light flux scans the entire surface of the test object 7. In this case, the distance between the rotating mirror 6 and the object 7 to be inspected is made to substantially match the radius of curvature of the surface of the object 7 to be inspected. The light beam reflected on the surface of the object 7 travels backward along the same path, is converted into a parallel light beam through the rotating mirror 6 and the objective lens 5, and is superposed on the light beam reflected from the reference mirror 4 in the beam splitter 3. It enters the interference fringe counting unit 8. The interference fringe counting unit 8 counts the movement of the interference fringes according to the amount of deviation of the surface of the test object 7 from the spherical surface, and measures the amount of aspherical surface of the test object 7.

However, in the case of this device, there are drawbacks such that the convex surface cannot be measured in principle and it is difficult to measure a large amount of aspherical surface.

Therefore, as an improved device, the objective lens is constructed so that the light beam is always focused on the object to be inspected, and an object lens having a large amount of aspherical surface or a convex surface can be measured. There is one described in the proceedings of the 25th automatic control federation conference lecture 4042 (P.593 to 594).

The outline thereof is shown in FIG. 7. The light beam 1 emitted from the Zeeman laser 10 is focused on the object 7 through the beam splitter 3, the light beam toward the reference mirror 4, the beam splitter 11, and the objective lens 5. Light flux. The light reflected on the object 7 is converted into a parallel light flux by the objective lens 5, and is split by the beam splitter 11 into a light flux toward the beam splitter 3 and a light flux toward the photodetector 12. The objective lens 5 has a function of being moved in the direction of the arrow a in accordance with a signal from the photodetector 12 by using a known technique so that the incident light beam is always focused on the test object 7, that is, an autofocus function. ing. The object 7 is scanned in a plane (as an XY plane) as indicated by an arrow b.

When the principal ray of the incident light beam does not enter the surface of the object 7 to be inspected perpendicularly, the incident light beam and the reflected light beam from the object 7 follow different optical paths and become laterally offset parallel light beams. From the lens 5 toward the beam splitter 11. This lateral shift amount is measured by the photodetector 12, and the objective lens 5
Is moved in a direction orthogonal to the light flux (direction of arrow c) so that the incident light flux and the reflected light flux overlap. Along with this, the test object 7 is moved by a corresponding amount. The light beam directed to the beam splitter 3 is superposed on the reflected light from the reference mirror 4 in the beam splitter 3 to form an interference fringe, which is counted by the interference fringe counter 8.

The feature of this device is that it has an autofocus function, so that it can measure not only a large amount of aspherical surface or a concave surface, but also a convex surface. However, in order to maintain high accuracy, refer to It is necessary to suppress the relative vibration between the mirror 4 and the object 7 to be inspected. Therefore, it is necessary to fix the reference mirror 4 to the same vibration system as the object 7 to be inspected.

[0009]

However, it is often difficult to satisfy both the vibration condition and the Abbe's error at the same time, and the rigidity of the interference optical system and the accuracy of the vibration system are required to be strict. Environmental conditions are required to reduce the effects of mechanical disturbances such as vibration.

Further, as described above, when the objective lens 5 is laterally moved in the direction of arrow c so that the principal ray is vertically incident on the object to be inspected 7, the position where the principal ray hits the object to be inspected 7 also shifts.
It is necessary to move the test object 7 in parallel by using an appropriate servo mechanism by an amount corresponding to this deviation, and as a result, a high precision servo mechanism is required to drive the test object 7 and the measurement time is increased. It also had the drawback of becoming longer.

Further, since the object to be inspected 7 is moved in a plane, the reflected light beam is reflected by the objective lens 5 for a deep R convex surface (having a small radius of curvature and a large opening angle) or a concave surface.
However, it also had a drawback that measurement was impossible.

In view of the above problems, the present invention provides a surface shape measuring instrument which can measure a large amount of aspherical surface or a small radius of curvature on both concave and convex surfaces with high accuracy and in a short time. The purpose is to

[0013]

A surface shape measuring instrument according to the present invention comprises a light source which emits a coherent light beam, and a separating / coupling optical system for temporarily separating the light beam into a reference light beam and a test light beam and then combining them. And an inclination correction optical system that moves the test light beam in a direction perpendicular to the optical axis, a focusing optical system that focuses the test light beam on the surface to be inspected, and a light detection that detects the lateral deviation of the light beam caused by the inclination of the surface to be inspected. And a reference mirror that reflects the reference light beam, and an interference fringe counting unit that counts interference fringes generated by combining the reference light beam and the test light beam. A birefringent element that separates the test light beam into two different polarization components is provided on the light source side of the focusing optical system. Further, a Faraday rotator is provided in the path of the test light beam.

[0014]

Since the light beam is moved in the direction perpendicular to the optical axis by the tilt correction optical system, even if the surface to be inspected is a curved surface, it can be moved to the surface to be inspected without moving the incident position with respect to the surface to be inspected. Since the light flux can be made to enter perpendicularly, and therefore it is not necessary to move the objective lens or the object to be measured, it is possible to perform accurate measurement in a short time.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention shown in FIGS. 1 to 4 will be described below by assigning the same reference numerals to the same members as those in the above-mentioned prior art. FIG. 1 shows a basic configuration of the present embodiment. A light source 1 is a light source that emits a visible, infrared or ultraviolet light beam 1 and emits a coherent light beam like a He-Ne laser. Things are used. Polarizing prisms 15, 16 and 17 separate or combine both p and s polarized lights. 1
Reference numeral 8 is a half-wave plate, which is rotatable about its axis. Faraday rotators 19 and 20 are made of glass or crystal and have a function of rotating the polarization planes of both p and s polarized lights which are incident by applying a magnetic field in the traveling direction of the light beam by 45 °. As the glass, FR-5 manufactured by Hoya Glass Co., Ltd. can be used.

Reference numerals 21 and 22 are π / 4 rotators, which rotate the plane of polarization of the incident light beam by 45 ° to the right or left. The beam splitter 11 splits the reflected light beam from the test object 7, sends one to the interference fringe counting unit described later, and directs the other to the photodetector 12. Reference numerals 30 and 31 denote right-angled prisms. The prism 30 is held in a direction perpendicular to the optical axis within the plane of the drawing so as to be driven by a prism moving mechanism (not shown), and the prism 31 is fixed. . Two
Reference numeral 3 is a birefringent prism, for example, a Rochon prism or the like is used, which is fixed at the focal position of the objective lens 5 and supplies the normal light beam and the abnormal light beam having a constant opening angle determined by the wedge angle of the prism to the objective lens 5. To do. These light fluxes are converted into two parallel light fluxes by the objective lens 5 and are focused on two points on the test object 7 which are laterally displaced by, for example, 60 μm.

The birefringent prism 23 is held together with the objective lens 5 by a frictionless linear guide device having a rotation preventing mechanism (not shown), and the object 7 and the objective lens are also held by a linear driving device (not shown). The objective lens 5 is driven so that the distance from the objective lens 5 is always equal to the focal length of the objective lens 5. The photodetector 12 detects the deviation amount (the focus amount) of the distance between the objective lens 5 and the object 7 to be inspected from the focal length of the objective lens 5 and the lateral deviation amount of the light beam by using a known means, and the above-described straight line is detected. The error signal is sent to the drive mechanism and the prism moving mechanism.

Reference numerals 24, 25, 26 and 27 are mirrors. A rectangular prism is used for the reference mirror 4. Therefore, the reference light flux corresponding to both the p and s polarization components has a shape in which the reference light flux just goes backward in the common optical path. Reference numerals 28 and 29 are interference fringe counting units, and for example, a light wave interferometer capable of reversible counting utilizing polarized light disclosed in Japanese Patent Publication No. 51-47344 can be used. The object 7 to be inspected is a glass or plastic lens or metal mold, and is driven to rotate around the optical axis or in the direction of the arrow in FIG.

Since the surface shape measuring instrument according to the present embodiment is constructed as described above, the light beam 1 is linearly polarized and has a plane of polarization inclined by 45 ° from the polarized light (p polarized light) in the plane of the drawing. If set, the light beam 1 is separated into p-polarized light and s-polarized light component by the polarization prism 15, and the former passes through the half-wave plate 18 and is converted into elliptically polarized light. This polarization state can be changed by rotating the half-wave plate 18. When the reflectance of the test object 7 is low, for example, when the reflectance is about 4% such as reflection on a glass surface, this rotation angle makes the light incident on the polarization prism 16 elliptically polarized light close to p-polarized light. And the amount of reflected light from the object 7 and the amount of reflected light from the reference mirror 4 are selected to be as similar as possible. On the contrary, when the object to be inspected 7 has a high reflectance like some molds, the plane of polarization is selected in the direction of 45 ° with respect to the polarizing prism 16, and as much light as possible is inspected. And toward the reference mirror 4.

Next, in the polarization prism 16, the p-polarized component of the elliptically polarized light is directed to the Faraday rotator 19, and the s-polarized component is reflected and directed to the reference mirror 4. First, the p-polarized component will be described. The Faraday rotator 19 rotates the plane of polarization by 45 °. Figure 2
Shown in. When the p-polarized light is represented by I in FIG. 2, it changes to the state of, for example, II by passing through the Faraday rotator 19, and if the π / 4 rotator 21 is a left-handed rotator, the p-polarized light by passing through it. Then, the light passes through the polarizing prism 17, passes through the prisms 30 and 31, and reaches the birefringent prism 23.

When a Rochon prism is used as the birefringent prism 23, the p-polarized light proceeds without changing its direction and is focused on the surface of the object 7 to be inspected by the objective lens 5. Then, the p-polarized light undergoes a phase change due to the shape of the object 7 to be examined, then goes back through the same optical path again, and passes through the objective lens 5, the birefringent prism 23, the prisms 31 and 30, and the polarizing prism 17. Further, the plane of polarization changes from the I to II state of FIG. 2 through the π / 4 rotator 21, and further passes through the Faraday rotator 19 to be converted from the II state to the III state, that is, s-polarized light. It is reflected and reaches the beam splitter 11. Here, since the reflected light flux enters the photodetector 12, the defocus amount and the lateral displacement amount of the beam are detected. The light that has passed through the beam splitter 11 is directed to the interference fringe counter 28.

The other s-polarized component of the elliptically polarized light that is incident on the polarizing prism 16 from the left will be described. This is reflected by the polarizing prism 16 and traces the optical path to the reference mirror 4 counterclockwise, and the polarizing prism 16 again. Reflected in the mirror 25
And enters the interference fringe counting unit 29.

In summary, the p-polarized light component of the light beam incident on the polarization prism 16 from the left is incident on the interference fringe counter 28, carrying the information about the shape of the object 7, and the s-polarized light component is the reference light beam, causing interference. It enters the fringe counting unit 29.

The p-polarized light component that has passed through the polarization prism 15 has been described above, but the s-polarized light component reflected by the polarization prism 15 follows exactly the same process. The s-polarized light component is converted into elliptically polarized light by the half-wave plate 18, and enters the polarizing prism 16 from below through the mirrors 24 and 26. The p-polarized light component of the incident light beam is reflected by the polarization prism 16
After passing through, the light path to the reference mirror 4 is traced in the clockwise direction and is incident on the interference fringe counting section 28 via the polarization prism 16. The s-polarized light component is reflected by the polarization prism 16 and then Faraday rotator 20, π / 4 rotator 22, mirror 27, polarization prism 17, prisms 30 and 31, and birefringence prism 23.
The optical path of the extraordinary light having a constant opening angle with respect to the optical path of the p-polarized light is taken through, and is focused by the objective lens 5 to a point laterally displaced from the focusing point of the p-polarized light by the shear amount, and information on the shape of that point is obtained. The same optical path is followed in reverse, and is reflected by the polarizing prism 17 through the objective lens 5, the birefringent prism 23, the prisms 31 and 30, and is converted into p-polarized light through the mirror 27, the π / 4 rotator 22, and the Faraday rotator 20. Then, the light passes through the polarizing prism 16, passes through the mirror 25, and enters the interference fringe counting unit 29.

From the above, in the interference fringe counting section 28,
The p component of the component transmitted through the polarization prism 15 passes through the Faraday rotator 19 twice to make s
The signal light that has been converted into polarized light and carries the information on the shape of one location of the test object 7 is incident, and the p component of the components reflected by the polarization prism 15 is incident as reference light. Similarly, in the interference fringe counter 29, the s-polarized light of the component reflected by the polarization prism 15 passes through the Faraday rotator 20 twice to be converted into the p-polarized light, which is laterally displaced by the shear amount of the test object 7. The s-polarized light component of the component transmitted through the polarization prism 15 is incident as the reference light. In other words, subject 7
There are two independent interferometers with different polarizations for two points separated by the shear amount of. Since both the reference optical path and the signal optical path are arranged close to the common optical path, the configuration is less likely to be affected by mechanical disturbance such as vibration or air flow, and accuracy of the rotary drive system of the object to be inspected 7. ing.

Further detailed description will be given with reference to FIG.
Represents the amount of aspherical surface L on the vertical axis and the rotation angle θ on the horizontal axis. In FIG. 1, the rotation angle θ is obtained when the object 7 is rotated about its approximate curvature center. It shows the relationship with the aspherical amount L. Since the shape information of the test object 7 is acquired by using two light beams having different polarizations that are laterally offset by the shear amount, the same shape is measured twice. For example, p
If the polarized light flux is measured from O to P in FIG. 3, s
As for polarized light, the same shape is measured from O to P through Q, which is shifted by a shear amount. If the object 7 moves due to mechanical disturbances such as vibrations or imperfections in the rotary drive system during the measurement, for example, the measured value of p-polarized light will have the disturbance indicated by the solid line in the figure. However, almost the same disturbance (shown by the dotted line in the figure) also enters the measurement value of s-polarized light. Therefore, the area between the aspherical curve OP and the horizontal axis θ obtained from the measurement and the aspherical curve O
When the difference in area between PQ and the horizontal axis θ is obtained, the noise component due to disturbance or the like is canceled out, and the true area that does not include noise between the aspherical curve PQ and the horizontal axis θ is obtained. By dividing by the amount of shear, the true value of the amount of aspherical surface at the midpoint of PQ can be obtained.

That is, since the surface shape measuring instrument has the common reference mirror 4 for both p and s polarized lights and has independent interferometers, the aspherical amount L of the object 7 to be inspected is measured. Is characterized in that it can be obtained for each rotation angle without accumulating an error, and this point is different from the prior art.
Therefore, high-accuracy measurement is possible without being affected by mechanical disturbances such as vibration and air flow and the accuracy of the rotation drive mechanism of the object 7. In addition, since it has an autofocus mechanism, it is possible to measure not only large aspherical surfaces and concave surfaces but also convex surfaces, and it is configured to rotate the test object 7 so that it has a deep R Can also be measured.

Next, the operation of causing the light beam to enter the object to be examined perpendicularly will be described. As shown in FIG. 4, when the test object 7 is inclined by β with respect to the incident light beam 1, the reflected light beam is 2
After being deflected by β, the reflected light path shown by the solid line is taken, and after passing through the objective lens 5, it becomes a light beam laterally displaced parallel to the incident light beam and returns to the interferometer. The lateral deviation amount of this light beam is detected by the photodetector 12, and the right angle prism 30
Is parallel-moved by half the lateral displacement of the light flux so that the error signal from the photodetector 12 becomes zero, and the light path shown by the dotted line in FIG. Therefore, the reflected light follows the same optical path as the incident.

In the tilt angle correction optical system shown in this embodiment,
The advantage over the conventional method of moving the objective lens 5 shown in FIG. 7 is that, as shown in FIG. 4, the intersection of the incident light beam 1 and the object 7 moves the incident light beam 1 in parallel as shown by the dotted line. Even though the objective lens 5 is located at exactly the same position except the spherical aberration, there is no need to move the object 7 as shown in FIG. 7, the servo mechanism is simplified and the measurement time is shortened. There is something to do.

FIG. 5 shows a second embodiment in which the structure of the interferometer is simplified. In the figure, 32 is an optical isolator,
3 is a quarter wavelength plate. The optical isolator 32 is the light source 1
It is a well-known one that cuts off the interference between the interferometer and the interferometer. Instead of the Faraday rotators 19 and 20 and the π / 4 rotators 21 and 22 of the first embodiment, a ¼ wavelength plate 33 is inserted in this embodiment. The p-polarized light that has passed through the polarization prism 16 is converted into circularly polarized light by the quarter-wave plate 33, the p-polarized light component of the circularly polarized light is transmitted by the polarization prism 17, and the s-polarized light component goes out of the system. The p-polarized light that has passed through the polarization prism 17 passes through the beam splitter 11, the birefringence prism 23, and the objective lens 5 and is converged on the object 7 to be inspected. It is incident on the quarter wave plate 33.

Here, the p-polarized light is converted into circularly-polarized light, of which the s-polarized light component is reflected by the polarization prism 16 as signal light and reaches the interference fringe counter 28. Further, the p-polarized component of the circularly polarized light enters the polarizing prism 15 through the polarizing prism 16 and the half-wave plate 18, and a part of the light flux is reflected by the polarizing prism 15 to go out of the system, and the remaining The light flux passes through this and enters the optical isolator 32 and is blocked. Similarly, a part of the s-polarized light beam that enters the quarter-wave plate 33 from the left enters the interference fringe counting unit 29 as signal light, and the rest is lost outside the system or by being blocked by the optical isolator 32. From the above description, it is understood that this embodiment exhibits exactly the same operation as that of the first embodiment shown in FIG. 1 except for the loss of light quantity.

Although the optical system for laterally shifting the beam is omitted in this embodiment, if two right angle prisms are provided between the beam splitter 11 and the birefringent prism 23 as in the first embodiment. Good.

Further, in each of the embodiments described above, the polarization beam splitter is used as the polarization prism 15, but it is also possible to use an ordinary beam splitter. or,
Although a rectangular prism is used as 4 or 30, a cat's eye can also be used. Further, although the case where the rotary drive mechanism is used as the drive mechanism of the object 7 is shown, it goes without saying that a plane drive mechanism can be used.
Further, instead of the objective lens 5, another light converging member such as a hologram lens can be used.

[0034]

As described above, the surface shape measuring instrument according to the present invention is practically capable of measuring a large aspherical amount or a deep R with respect to both concave and convex surfaces with high accuracy and in a short time. It has important advantages.

[Brief description of drawings]

FIG. 1 is a diagram showing an optical system of a surface shape measuring instrument according to a first embodiment of the present invention.

FIG. 2 is a diagram for explaining the rotation of the polarization plane in the above embodiment.

FIG. 3 is a principle diagram of calculating an aspheric amount in the above-described embodiment.

FIG. 4 is a diagram for explaining the operation of the tilt angle correction optical system of the above embodiment.

FIG. 5 is a diagram showing an optical system of a second embodiment.

FIG. 6 is a diagram showing an optical system of a conventional surface shape measuring instrument.

FIG. 7 is a diagram showing an optical system of another conventional surface shape measuring instrument.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 light source 4 reference mirror 5 objective lens 7 test object 11 beam splitter 12 photodetector 15, 16, 17 polarizing prism 18 1/2 wavelength plate 19, 20 Faraday rotator 21, 22 π / 4 rotator 23 birefringent prism 28, 29 Interference fringe counter

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Kawashima 2-43-2 Hatagaya, Shibuya-ku, Tokyo Olympus Optical Co., Ltd. (72) Inventor Kazuo Kawakami 2-43-2 Hatagaya, Shibuya-ku, Tokyo Olympus Optical Co., Ltd. (72) Inventor Toshifumi Uetake 2-43-2 Hatagaya, Shibuya-ku, Tokyo Olympus Optical Co., Ltd. (56) Reference Japanese Patent Publication 5-10602 (JP, B2)

Claims (3)

[Claims] 1. A light source that emits a coherent light beam, a separation / combination optical system that temporarily separates the light beam into a reference light beam and a test light beam, and then combines these, and the test light beam in a direction perpendicular to the optical axis. A tilt correction optical system for moving, a focusing optical system for focusing the test light beam on a test surface, a photodetector for detecting lateral deviation of the light beam caused by the tilt of the test surface, and a reference mirror for reflecting the reference light beam. And an interference fringe counting section for counting interference fringes generated by combining the reference light flux and the test light flux. 2. The surface shape measuring instrument according to claim 1, wherein a birefringence element for separating the test light beam into two different polarization components is provided on the light source side of the focusing optical system. 3. The surface shape measuring instrument according to claim 2, wherein a Faraday rotator is provided in the middle of the path of the test light beam.