JP2808713B2 - Optical micro displacement measuring device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical micro-displacement measuring device for measuring a micro-displacement and a surface roughness of an object to be measured by optical means.

<Prior Art> Conventionally, as a device for detecting a defocus and measuring a minute displacement and a surface roughness of an object to be measured, for example, a device using a critical angle method (see Japanese Patent Application Laid-Open No. 59-9007) Using the astigmatism method (see JP-A-60-186705) or using the Foucault method (optical technology contact vol26, N
o. 11, 1988, pages 773 to 784).

Here, the critical angle method utilizes the property that the intensity of a light beam incident near the intrinsic critical angle of a prism exhibits a rapid change with respect to a small angle change. FIG. 10 shows the configuration of the optical minute displacement measuring device. In the figure, 101 is a laser light source, 102 is a collimating lens, 103 is a polarizing beam splitter, 104 is 1/4
Wave plate, 105 is an objective lens, 106 is an object to be measured, 107 is a beam splitter, 108a and 108b are critical angle prisms, 109a and 10
Reference numeral 9b denotes a two-division light receiving element. The laser light from the laser light source 101 is converted into a parallel light beam by the collimator lens 102, and is guided to the quarter-wave plate 104 via the polarization beam splitter 103 as S-polarized light. The laser beam guided to the quarter-wave plate 104 is converted into a circularly polarized light beam, and then condensed on the surface of the object 106 via the objective lens 105. After the laser light reflected by the DUT 106 is converted into P-polarized light by the quarter-wave plate 104,
Transmission through the polarization beam splitter 103, beam splitter 1
Guided to 07 and split. Critical angle prisms 108a and 108 whose reflected surfaces are set at critical angles
The light is reflected by b, enters the two-divided light receiving elements 109a and 109b, and the amount of light is detected.

As described above, when the object to be measured 106 is located at the focal point of the objective lens 105, the reflected light is parallel light, and the reflectivity of the critical angle prisms 108a and 108b is constant for all luminous fluxes, and the two-divided light receiving element The light amounts received by 109a and 109b are equal. However, when the DUT 106 is located farther than the focal point of the objective lens 105, the reflected light becomes convergent light, so that the incident angle of the light flux entering the critical angle prisms 108a and 108b is critical with respect to its optical axis. The lower side in the drawing of the angular prism 108a and the lower right side of the drawing in the critical angle prism 108b are smaller than the critical angle, the reflectance is reduced, and a difference occurs in the amount of light received by the two-divided light receiving elements 109a and 109b. Similarly, when the measured object 106 is located at a point closer to the focal point of the objective lens 105, the reflected light becomes divergent light, which is opposite to the above-described case, and is divided into two light receiving elements.
There is a difference between the amounts of light received by 109a and 109b. The displacement of the object 106 from the focal position of the objective lens 105 can be detected from such a difference in the amount of light, and the minute displacement and the surface roughness of the object 106 can be measured.

On the other hand, the astigmatism method is to give astigmatism to a detected image using a cylindrical lens, and to convert the amount of displacement of the position of the measurement object into deformation of the image. When the object to be measured is at the in-focus position of the objective lens, the received light beam has a circular shape, but when the imaging position is close, the light-receiving beam has a vertically long elliptical shape. Then, this is photoelectrically converted by a four-division photodiode, and the sum of each of the vertical and horizontal directions is obtained. Further, the output signal of the difference is obtained, and an output proportional to the displacement of the object to be measured is obtained. Things.

Further, the micro-displacement measuring apparatus according to the Foucault method splits a reflected light beam from an object to be measured into two light beams by using a split prism having an optical knife edge effect, and causes the light beam to enter a two-divided photodiode. When the distance between the object and the surface to be measured changes, the divergence angle of the reflected light beam from the object to be measured changes, so that a difference occurs in the amount of light received by the two-divided photodiode. is there.

<Problems to be Solved by the Invention> When the object to be measured is tilted in the above-described optical micro-displacement measuring device, the axis of the reflected light beam moves parallel to the axis of the incident light beam. For example, the device to be measured 10
When 6 is inclined by θ, the reflected light beam axis 111 moves parallel to the incident light beam axis 110 by Δx as shown in FIG. Then, at this time, assuming that the object to be measured 106 has a focal length f of the objective lens 105, a relationship of Δx = ftan2θ holds.

By the way, in the apparatus shown in FIG. 10, the reflected light is split and received by the two-divided light receiving elements 109a and 109b.
Since the directions 109a and 109b occur in opposite directions, the difference in the amount of received light due to Δx is offset. However, when the inclination angle θ of the measured object 106 is large, the reflected light flux deviates from the light receiving surfaces of the two-divided light receiving elements 109a and 109b, so that there is a problem that a minute displacement and a surface roughness cannot be measured. All of the conventional minute displacement measuring devices have the same problem.

In view of such circumstances, it is an object of the present invention to provide an optical micro-displacement measuring device capable of measuring a micro-displacement and a surface roughness even when an object to be measured is greatly inclined.

<Means for Solving the Problems> An optical micro-displacement measuring apparatus that achieves the above object is an illumination optical system that irradiates a light beam from a light source to an object to be measured, and a focus error from a reflected light beam from the object to be measured. An optical micro-displacement measuring device having a focus error detection optical system for detecting, a split optical member for splitting a part of a reflected light beam entering the focus error detection optical system, and a split split by the split optical member. A light receiving element for detecting a displacement of an optical axis to be constrained, and an optical axis moving means for moving an optical axis of a light beam of a reflected light beam entering the split optical member in at least one direction in a plane orthogonal to the optical axis And an electric circuit for obtaining an output for detecting the shift of the optical axis of the reflected light beam from the amount of light received by the light receiving element for detecting the shift of the optical axis, and controlling the optical axis moving means so that the output has a specific value. Then Characterized in that it comprises a control circuit for matching the centers of the reflected light bundle axis of the error detecting optical system of the light receiving element.

<Operation> When the object to be measured is tilted and the optical axis of the reflected light beam shifts, the distribution of the amount of light of the divided light receiving element that receives a part of the reflected light beam via the divided optical member changes. From this, a differential output for detecting the amount of deviation of the optical axis is obtained, and the optical axis moving means is controlled so that it becomes zero, for example, whereby the deviation of the optical axis of the reflected light beam is corrected, and the focus error detection optical system The optical axis of the reflected light beam entering the light receiving element also coincides with the center of the light receiving element.

<Examples> Hereinafter, the present invention will be described based on examples.

FIG. 1 shows an optical micro-displacement measuring apparatus according to an embodiment using a critical angle method for a focus error optical system.

In FIG. 1, 11 is a laser light source, 12 is a collimating lens,
13 is a polarizing beam splitter, 14 is a 1/4 wavelength plate, 15 is an objective lens, 16 is an object to be measured, and a laser beam from a laser light source 11 is converted into a parallel beam by a collimating lens 12,
For example, a 1/4 wavelength plate 1 through the beam splitter 13 with S-polarized light
Guided to 4. The laser light guided to the 波長 wavelength plate 14 is converted into a circularly polarized light beam, and then condensed on the surface of the DUT 16 via the objective lens 15. Then, the laser light reflected by the DUT 16 is turned into P-polarized light by the quarter-wave plate 14, and then passes through the polarization beam splitter 13.

The light beam that has passed through the polarizing beam splitter 13 is reflected by a reflecting mirror 18 held by a reflecting mirror driving means 17 and then split by a beam splitter 19, and one of the split lights is reflected by a critical angle prism 20 and split into two. The light enters the light receiving element 21 and the other of the divided light enters the light receiving element 22 as it is.

Here, the critical angle prism 20 and the two-division light receiving element 21 constitute a focus error detecting optical system based on the critical angle method, and the critical angle prism 20 is moved to the critical angle prism 20 by the displacement of the object to be measured, as described in the conventional section. Since the incident angle of the reflected light beam changes and the amount of light received on both sides of the two-divided light receiving element 21 differs, the minute displacement and the surface roughness of the object 16 can be measured.

On the other hand, the two-divided light receiving element 22 detects a shift of the optical axis of the reflected light beam due to the inclination of the DUT 16 (see Δx in FIG. 11). 16
Is a direction orthogonal to the direction of movement of the reflected light beam axis due to the inclination of. Incidentally, reference numeral 23 denotes a subtractor, and 24 denotes a controller. The subtractor 23 outputs a difference between the light amounts of the two-divided light receiving elements 22, and the controller 24 controls the reflector driving means 17 based on the output.

This control method will be described in more detail. If the axis of the reflected light beam is shifted, there is a difference in the amount of light received on both sides of the two-divided light receiving element 22, so that when the subtractor 23 outputs this difference in the amount of light, it is controlled so that the output becomes zero, for example. I just need. FIG. 2 is a diagram for explaining a method of controlling the reflecting mirror 18 with respect to the optical axis shift. When the optical axis shifting amount is Δx in the X direction, the amount Δm to be moved of the reflecting mirror 18 is expressed by the following equation. . Note that the angle of reflection at the reflecting mirror 18 is φ.

That is, if the reflecting mirror 18 is moved in parallel by the reflecting mirror driving means 17 by Δm, the optical axis shift is corrected, and the optical axis shift amount becomes zero.

Here, when correcting such an optical axis shift, 2
Let us consider how much the size of the light receiving surface of the divided light receiving element 22 is necessary. In order to receive all of the reflected light flux when the DUT 16 is tilted by the two-divided light receiving element 22, as shown in FIG. 3, the reflected light flux from the DUT 16 becomes divergent light. It is only necessary that all of the light can be received when the measured object 16 has no inclination. Therefore, the focal length of the objective lens 15 is f ₁ , the displacement measurement range is ΔZ, the incident light beam is φ, the light length is 1, and the object 16 moves from the focal point position to the objective lens 15 side by ΔZ / 2. If the luminous flux travels as divergent light from the distance of S when And the diameter φ 'of the light receiving surface is Becomes

If the two-divided light receiving element 22 having such a light receiving surface is used, all inclinations of the DUT 16 can be corrected. When the deviation of the optical axis is corrected by moving the reflecting mirror 18 as described above, the axis of the reflected light beam entering the focus error detection optical system is also corrected at the same time, and the center of the two-part light receiving element 21 and the light of the reflected light beam are reflected. The axis always coincides with the axis, and a stable light amount can be secured, and accurate focus error information can be obtained.

FIG. 4 shows a configuration of an optical minute displacement measuring apparatus according to another embodiment. As shown in the figure, the present embodiment is the same as the embodiment shown in FIG. 1 in that the critical angle method is used for the focus error detection optical system and the movable reflecting mirror is used for the optical axis moving means. The same reference numerals are given to the members shown, and the duplicate description will be omitted.

In the present embodiment, a four-divided light receiving element 22A is used to detect the displacement of the optical axis in two axial directions, and mounted on reflector driving means 17A and 17B, respectively, so that the optical axis can be moved in two axial directions. The light reflected by the two placed reflecting mirrors 18A and 18B is made to enter the beam splitter 19. Here, the reflecting mirror 18A moves the optical axis in the vertical direction in the figure of the light receiving element 22A by its movement, and the reflecting mirror 18B moves the optical axis in the horizontal direction of the light receiving element 22A in the figure by the movement. It is. It should be noted that the focus error detection optical system also has a four-division light receiving element.
21A is used.

In this embodiment, the control for correcting the optical axis shift is performed by setting the light receiving surface of the four-division light receiving element 22A to four of A to D as shown in FIG.
When the optical axis is shifted in the X direction (left direction in the figure), the differential output (A + C)-(B + D) is used, and the optical axis in the Y direction (upward in the figure) If there is a deviation, the deviation of the optical axis is detected by the differential output (A + B)-(C + D), and correction is made by moving the reflecting mirrors 18B and 18A, respectively.

FIG. 6 shows an embodiment in which the astigmatism method is used for the focus error detecting optical system and one movable reflecting mirror is used for the optical axis moving means.
In this embodiment, the configuration for correcting the optical axis shift is the same as that in FIG. 1, and the members having the same functions are denoted by the same reference numerals, and redundant description will be omitted. On the other hand, in the focus error detection optical system, the objective lens 25 is placed behind the beam splitter 19 to converge the light beam, the converged light is split by the beam splitter 27, and after passing through the cylindrical lenses 27a and 27b, Light is received by the quadrant light receiving elements 28a and 28b. In other words, the convergent light is given astigmatism by the cylindrical lenses 17a and 27b to make the light receiving elements 28a and 28b into a vertically or horizontally long ellipse, thereby detecting the deviation of the measured object 16 from the focused position.

FIG. 7 shows an optical minute displacement measuring apparatus according to another embodiment. This embodiment is the same as the apparatus shown in FIG. 6 except that the focus error detection optical system uses the Foucault method, and the same members are denoted by the same reference numerals and redundant description will be omitted.
In the focus error detecting optical system by the Foucault method, the reflected light beam is converted into a convergent light beam by the condenser lens 29 placed behind the beam splitter 19, then split by the light wave splitting prism 30, and each of the split light beams is split into two light receiving elements. Light is received by 31a and 31b, and the displacement of the DUT 16 is measured based on the difference in the amount of light received by the two-divided light receiving elements 31a and 31b.

On the other hand, in the other embodiment shown in FIG. 8, the focus error detection error system uses the critical angle method and is basically the same as the apparatus shown in FIG. And a duplicate description is omitted. In this embodiment, as the optical axis position moving means, the parallel flat glass 33 held by the parallel flat glass driving device 32 is employed. The parallel flat glass 33 is rotated about two axes orthogonal to each other by the driving device 32, whereby the parallel flat glass 33 is inclined with respect to the optical axis of the reflected light beam. The optical axes of the transmitted light beams are translated in two axial directions. Therefore, the light split by the beam splitter 19 is received by the four-division light receiving element 22A, and the optical axis shift in the two axial directions coinciding with the rotation axis of the parallel flat glass 33 is detected. The glass driving device 32 is controlled.

The embodiment shown in FIG. 9 uses a corner cube prism 34 as an optical axis moving means, and can move the optical axis in two axial directions. That is, for example, P-polarized light from the light source 11 is converted into parallel light by the collimating lens 12, passes through the polarizing beam splitter 13, and is guided to the DUT 16 via the / 4 wavelength plate 14 and the condenser lens 15. The reflected light flux becomes S-polarized light, is reflected by the polarization beam splitter 13, is converted into circularly polarized light by the quarter-wave plate 35, and enters the corner cube prism 34. The light reflected by the light beam enters the beam splitter 19 at right angles to the quarter-wave plate 35 and the polarization beam splitter 13. Accordingly, the optical axis of the light beam incident on the beam splitter 19 can be moved in the biaxial directions by parallel moving the corner cube prism 34 in the biaxial directions. The light partially split by the beam splitter 19 is divided into four light receiving elements 22.
The light is received by A and the displacement of the optical axis in the two axes direction is detected.
Is moved to perform optical axis correction.

In this embodiment, the critical angle method is adopted for the focus error detecting optical system, and is basically the same as the apparatus of FIG. 8, for example. Duplicate description will be omitted.

In each of the embodiments described above, even if the DUT 16 is tilted, the deviation of the optical axis is corrected, so that the minute displacement and the surface roughness can be measured irrespective of the tilt of the DUT 16, This is particularly useful when the measured object 16 has a curvature or when the measured object 16 has a depression or the like.

The means for moving the optical axis of the reflected light beam is not limited to the one described above. For example, an optical material having a refractive index larger than 1 can be used instead of the above-mentioned parallel plate glass. The material is not particularly limited, and other prisms such as a conical prism can be used instead of the corner cube prism.

Also, the focus error detecting optical system is not particularly limited, and it is needless to say that other known methods can be adopted.

<Effects of the Invention> As described above, according to the optical micro-displacement measuring apparatus of the present invention, the displacement of the optical axis of the reflected light beam due to the inclination of the measured object can be corrected. Even when tilted, the micro displacement and surface roughness can be accurately measured regardless of the tilt, especially when the measured object has a curvature or when the measured object has a dent etc. Useful for

[Brief description of the drawings]

FIG. 1 is a block diagram schematically showing an optical micro-displacement measuring apparatus according to one embodiment of the present invention, FIGS. 2 and 3 are explanatory views for explaining a partial configuration thereof, and FIG. 4 is another embodiment. FIG. 5 is an explanatory view showing the light receiving element, and FIGS.
The drawings are each a schematic configuration diagram showing an optical micro-displacement measuring device according to another embodiment, and FIGS. 10 and 11 are explanatory diagrams for explaining a micro-displacement measuring device according to the prior art. In the drawing, 11 is a light source, 12 is a collimating lens, 13 is a polarizing beam splitter, 14 is a quarter-wave plate, 15 is an objective lens, 16 is an object to be measured, 17, 17A and 17B are reflector driving devices, 18, 18A and 18B are reflecting mirrors, 19 is a beam splitter, 20 is a critical angle prism, 21 is a two-part light receiving element, 21A is a four-part light receiving element, 22 is a two-part light receiving element, 22A is a four-part light receiving element, and 23 is a subtractor , 24 is a drive controller, 25 is a condensing lens, 26 is a beam splitter, 27a and 27b are cylindrical lenses, 28a and 28b are 4-split light receiving elements, 29 is a condensing lens, 30 is a light wave splitting prism, 31a and 31b Reference numeral 32 denotes a parallel plate glass driving device, reference numeral 33 denotes a parallel plate glass, reference numeral 34 denotes a corner cube prism, and reference numeral 35 denotes a quarter-wave plate.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) G01B 11/00-11/30 G01C 3/00-3/32

Claims (2)

(57) [Claims] An optical minute displacement measuring device comprising: an illumination optical system for irradiating a light beam from a light source onto a measured object; and a focus error detection optical system for detecting a focus error from a reflected light beam from the measured object. A splitting optical member that splits a part of the reflected light beam entering the focus error detection optical system; a light-receiving element for detecting a shift of an optical axis that receives the split light beam split by the splitting optical member; An optical axis moving means for moving the optical axis of the light beam of the incoming reflected light beam in at least one direction in a plane orthogonal to the optical axis; and the reflected light beam based on the amount of light received by the light-receiving element for detecting deviation of the optical axis. An electric circuit for obtaining an output for detecting the deviation of the optical axis, and controlling the optical axis moving means so that the output has a specific value, and controlling the center of the light receiving element of the focus error detection optical system and the reflected light beam axis. Match with center Optical minute displacement measuring apparatus characterized by comprising a control circuit for. 2. An optical micro-displacement measuring apparatus comprising: an illumination optical system for irradiating a light beam from a light source onto an object to be measured; and a focus error detection optical system for detecting a focus error from a light beam reflected from the object to be measured. A split optical member for splitting a part of the reflected light beam entering the focus error detection optical system; a light receiving element for receiving at least two split light beams split by the split optical member; and entering the split optical member. An optical axis moving means for moving the optical axis of the light beam of the reflected light beam in at least one direction in a plane perpendicular to the optical axis; and a shift of the optical axis of the reflected light beam from the amount of light received by the divided light receiving elements. An electric circuit for obtaining at least one differential output to be detected; and a center of a light receiving element of the focus error detection optical system and reflected light by controlling the optical axis moving means so that the differential output has a specific value. Optical minute displacement measuring apparatus characterized by comprising a control circuit to match the center of the shaft.