JP2625209B2 - 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. 8 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 109b
Denotes a two-divided light receiving element. The laser light from the laser light source 101 is converted into a parallel light beam by the collimating lens 102, and is guided to the quarter-wave plate 104 via the polarizing 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 P-polarized in the quarter wavelength region 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 based on the Foucault method splits a reflected light beam from an object to be measured into two light beams using a split prism having an optical knife edge effect, and causes the light beam to enter a two-segment 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 shown in FIG.
When 106 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 = f tan2θ holds.

By the way, in the device shown in FIG. 8, 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 device that achieves the above object is an illumination optical system that irradiates a light beam from a light source to a measured object, and detects a focus error from a reflected light beam from the measured object. An optical micro-displacement measuring device having a focusing error detection optical system, wherein a split optical member for splitting a part of the reflected light beam entering the focus error detection optical system, and a split light beam split by the split optical member Receiving the divided light receiving element, optical axis reciprocating means for reciprocating the optical axis of the reflected light beam entering the divided optical member in a specific direction in a plane orthogonal to the optical axis, and receiving the divided light receiving element An electric circuit for obtaining at least one differential output for detecting a deviation of the optical axis of the reflected light beam from the amount of light, and focusing an output of the focus error detection optical system when the differential output has a specific value. Mistake It is characterized as difference information.

<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. On the other hand, when the object to be measured is tilted at a constant angle, if the optical axis of the reflected light beam is reciprocated through the optical axis reciprocating means, the distribution of the light amount of the divided light receiving element changes and the focus error information is also changed. There is information when the optical axis shift is corrected during the change.

Therefore, while the optical axis is reciprocated, the distribution of the light quantity of the divided light receiving elements is monitored as a differential output, and the output of the focus error detection optical system when this becomes zero, for example, and the deviation of the optical axis is corrected is calculated as the focus error As information, it is possible to obtain focus error information when there is no tilt.

<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 quarter-wave plate, 15 is an objective lens, 16 is an object to be measured, and a laser hole 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. 9). Is a direction orthogonal to the direction of movement of the reflected light beam axis due to the inclination of. Reference numeral 23 denotes a subtractor, which outputs a difference between the light amounts of the two-divided light receiving elements 22 so that it is possible to know when the optical axis is corrected.

Here, the reflecting mirror driving means 17 is controlled by the driving device controller 24 to always reciprocate the reflecting mirror 18. Therefore, the optical axis of the reflected light beam moves in parallel in response to the reciprocating movement of the reflecting mirror 18, and accordingly, the difference in the amount of light received by the two-divided light receiving element 22 and the focus error detecting optical system The output of the two-segment light receiving element 21 changes. Note that the direction of deviation of the optical axis of the reflected light beam due to the reciprocating movement of the reflecting mirror 18 is a direction orthogonal to the dividing direction of the two-divided light receiving elements 21, 21.

 The principle of the present embodiment will be described in more detail.

First, the reciprocating movement of the reflecting mirror 18, that is, the range of vibration is ±
Assuming that Δm is the deviation of the optical axis due to the inclination of the DUT 16 as shown in FIG. The range ± Δx in which the optical axis shift can be corrected is ± Δx = ± ΔM {1-cos (2)}. Therefore, the inclination θ of the DUT 16 that can be measured is
The range ± θ is Becomes Therefore, by increasing the range of vibration of the reflecting mirror 18, the range of tilt of the object to be measured that can be measured is also increased.

That is, when the optical axis shift is corrected by the movement of the reflecting mirror 18, the output of the two-divided light receiving element 21 of the focus error detecting optical system is an output when the object 16 is not tilted. Therefore, by reading the output, the DUT 16
Focus error information that is not affected by the inclination of

Then, when the optical axis shift is corrected, it is determined by detecting the difference between the light amounts of the two-divided light receiving elements 22. That is, subtraction machine
23, the difference between the light amounts of the respective light receiving elements of the two-division light receiving element 22 is output. When the output becomes a set value, for example, zero, the optical axis deviation is corrected. The focus error is read from the output of the divided light receiving element 21.

As shown in FIG. 3, the two-divided light receiving elements 21 and 21 move in the direction in which the optical axis moves due to the vibration of the reflecting mirror 18 (that is, the direction in which the optical axis shifts when the object to be measured is tilted in the θ direction). : Θ direction), but the light receiving surface is long enough in that direction so that when the DUT is tilted in the direction orthogonal to the θ direction, there is no influence of the decrease in the total amount of received light. And

As described above, in the apparatus according to the present embodiment, the output of the two-divided light receiving elements 21 and 22 is monitored while the reflecting mirror 18 is vibrated within a certain range ± Δm, and When the axis deviation is corrected, the output of the two-divided light receiving element 21 for detecting a focus error is read, thereby detecting a focus error. Becomes possible.

However, since the reflecting mirror 18 must be vibrated for one cycle, if the vibration range ± Δm is increased in order to increase the measurable inclination ± θ of the DUT 16, the measurement time tends to be longer.

Therefore, a configuration as shown in FIG. 4 is effective for shortening the measurement time.

In the embodiment shown in FIG. 4, the reflecting mirror 18A is reciprocated.
Is larger than the reflecting mirror 18 of FIG.
A fixed reflecting mirror 25 opposed to 18A is provided in parallel, and reflecting mirror 1
The light beam reflected by 8A enters the reflecting mirror 25, and the light beam reflected by the reflecting mirror 25 is reflected by the reflecting mirror 18A again before entering the beam splitter 19. With this configuration, as shown in FIGS. 5 (a) and 5 (b), the amount of movement of the reflecting mirror required to correct the optical axis deviation Δx due to the tilt of the DUT 16 is reduced. That is, the moving amount in the case of FIG.
m (FIG. 5 (a)) and the moving amount in the case of FIG.
(Fig. 5 (b)) Becomes When the light is reflected twice on the mirror that moves in this way, the amount of movement becomes 1/2 of that when the light is moved once. When the light is reflected n times, the amount of movement Δm ″ becomes m ″ = 1 / n · Δm. As the number increases, the measurement time can be shortened.

The other configuration of the embodiment of FIG. 4 is the same as that of FIG. 1, and the same members are denoted by the same reference numerals and overlapping description will be omitted.

FIG. 6 shows another embodiment. This embodiment is basically the same as the apparatus shown in FIG. 1 using a critical angle method for a focus error detection optical system, and members having the same action are denoted by the same reference numerals and will not be described repeatedly. Omitted. In this embodiment, a parallel plate glass driving device is used as the optical axis reciprocating means.
A parallel flat glass 27 held by 26 is employed. The parallel plate glass 27 is configured to be rotated about an axis orthogonal to the direction of deviation of the optical axis by the driving device 26, whereby the parallel plate glass 27 is moved with respect to the optical axis of the reflected light beam. As a result, the optical axis of the transmitted light beam is translated by the rotation of the parallel flat glass 27.

In the embodiment shown in FIG. 7, the right-angle prism 28 is used as the optical axis reciprocating means, and the optical axis is divided into two by dividing the optical axis into two by moving the right-angle prism 28 in the vertical direction in the figure. It can be moved in a direction perpendicular to the direction. More specifically, for example, P-polarized light from the light source 11 is collimated by the collimating lens 12, passes through the polarizing beam splitter 13, and passes through the quarter-wave plate 14 and the condensing lens 15 to be measured. The reflected light flux is converted into S-polarized light, enters the polarization beam splitter 13, is reflected, is converted into circularly polarized light by the quarter-wave plate 29, and enters the right-angle prism 28. The light reflected by
The light is transmitted through the quarter-wave plate 29 and the polarization beam splitter 13 and enters the beam splitter 19. Therefore, by moving the right-angle prism 28 in the vertical direction in the drawing, the optical axis of the light beam incident on the beam splitter 19 is also moved in parallel.

In this embodiment, the critical angle method is adopted for the focus error detection optical system, and is basically the same as the apparatus of FIG. 1, for example. Duplicate description will be omitted.

In each of the embodiments described above, even when the DUT 16 is tilted, the deviation of the optical axis is corrected and this focus error is read, so that the minute displacement and the surface roughness are measured regardless of the tilt of the DUT 16. This is particularly useful when the DUT 16 has a curvature or when the DUT 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 corner cube prism can be used instead of the right angle prism.

The focus error detecting optical system is not particularly limited, and other known methods such as Foucault method and astigmatism method can be employed. When an optical axis shift occurs in the direction in which the optical axis shifts, it is desirable to adopt a method having a small effect.

<Effects of the Invention> As described above, according to the optical micro-displacement measuring apparatus of the present invention, it is possible to read the focus error when the deviation of the optical axis of the reflection constraint due to the inclination of the measured object is corrected. Even when the DUT is greatly tilted, it can accurately measure its micro-displacement and surface roughness regardless of the tilt, especially when the DUT has a curvature or when the DUT is hollow. This is useful for measurement when there is a problem.

[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 a moving range of the reflecting mirror, and FIG.
FIGS. 7 and 8 are schematic diagrams showing an optical micro-displacement measuring apparatus according to another embodiment, and FIGS. 8 and 9 are explanatory views for explaining a micro-displacement measuring apparatus 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 is a reflector driving device, 18 is a reflector, 19 Is a beam splitter, 20 is a critical angle prism, 21 is a two-part light receiving element, 22 is a two-part light receiving element, 23 is a subtractor, 24 is a drive controller, 25 is a reflector, 26 is a parallel flat glass drive, and 27 is a parallel plate glass drive. Parallel plate glass, 28 is a right angle prism, 29 is a quarter wave plate.

Claims (1)

(57) [Claims] An optical minute 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 detecting optical system for detecting a focus error from a light beam reflected from the object to be measured. A split optical member that splits a part of the reflected light beam entering the focus error detection optical system, a split light receiving element that receives the split light beam split by the split optical member,
An optical axis reciprocating means for reciprocating an optical axis of a reflected light beam entering the split optical member in a specific direction in a plane orthogonal to the optical axis; and a light beam of the reflected light beam based on an amount of light received by the split light receiving element. An electrical circuit that obtains a differential output for detecting axis deviation, wherein the output of the focus error detection optical system when the differential output has a specific value is used as focus error information. Optical micro displacement measuring device.