JPH0560557A - Optical method and device for measuring micro displacement - Google Patents

Abstract

PURPOSE:To measure with high accuracy the micro displacement of a subject to be measured having low reflectance by subtracting from the total quantity of reflected flux of light incident on a focal error detecting optical system the quantity of reflected flux of light as a corrected quantity of light, the reflected flux of light being incident on the optical system when a subject to be measured does not exist. CONSTITUTION:Laser light from a semiconductor laser 1 is converted into parallel beams of light by a collimator lens 2. The laser light is slightly deviated from S-polarized light and then part of the light is allowed to pass through a polarizing beam splitter 3 and enter a light quantity detector 10. Light reflected as circularly polarized light at a subject 6 to be measured is guided to a beam splitter 7 via the polarizing beam splitter 3 and is dispersed into a spectrum. The dispersed light is reflected at critical angle prisms 8A, 8B and is detected by two-divided light receiving elements 9A, 9B. The quantity of reflected flux of light which is incident when the subject to be measured does not exist is measured in advance as a corrected quantity of light and the corrected quantity of light is subtracted from the total quantity of reflected flux of light incident on and detected by the light-receiving elements 9A, 9B when the micro deviation of the subject to be measured is measured, so that intensity distribution is found.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical micro-displacement measuring method and apparatus for measuring micro-displacement and surface roughness of an object to be measured.

[0002]

2. Description of the Related Art Conventionally, as an optical micro-displacement measuring apparatus for detecting a micro-displacement and a surface roughness of an object to be measured by detecting a defocus, for example, one using a critical angle method (optical technology contact Vol. 26, No. 11, 1988, p.7.
48-755), using the astigmatism method (Optical Technology Contact Vol. 26, No. 11, 1988, p. 75).
6-762), using the Foucault method (Optical Technology Contact Vol. 26, No 11, 1988, p. 773-
784) and so on.

Here, the critical angle method utilizes the property that the intensity of the light beam incident near the inherent critical angle of the prism exhibits a rapid change with respect to a minute angle change. FIG. 4 shows an example of the configuration of a conventional optical micro-displacement measuring device according to the above. In the figure, 101 is a semiconductor laser which is a laser light source, 102 is a collimating lens,
Reference numeral 103 is a polarization beam splitter, 104 is a quarter wavelength 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 are two-divided light receiving elements, and a semiconductor. The laser light from the laser 101 is collimated lens 10
It is converted into a parallel light flux by 2 and is guided to the quarter-wave plate 104 as S-polarized light through the polarization beam splitter 103.
The laser light guided to the quarter-wave plate 104 is converted into a circularly polarized light beam, and then is condensed on the surface of the DUT 106 via the objective lens 105. The laser light reflected by the DUT 106 is P-polarized by the quarter-wave plate 104, then passes through the polarization beam splitter 103, is guided to the beam splitter 107, and is dispersed. The separated lights are reflected by the critical angle prisms 108A and 108B whose reflection surfaces are set to the critical angles, respectively, and are incident on the two-divided light receiving elements 109A and 109B, respectively, 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 becomes parallel, the reflectance at the critical angle prisms 108A and 108B is constant for all luminous fluxes, and is divided into two. Light receiving element 109A, 1
The amount of light received by 09B becomes equal. However, when the DUT 106 is located farther than the focal point of the objective lens 105, the reflected light becomes a convergent light, so that the incident angle of the light flux entering the critical angle prisms 108A and 108B is critical with respect to the optical axis. The angle prism 108A has a right side in the figure, and the critical angle prism 108b has an upper side in the figure smaller than the critical angle, and the reflectance decreases, which causes a difference in the amount of light received by the two-divided light receiving elements 109A and 109B. Similarly, when the DUT 106 is located closer to the focal point of the objective lens 105, the reflected light becomes divergent light, which is the opposite of the above case,
A difference occurs in the amount of light received by the two-divided light receiving elements 109A and 109B. From such a difference in the amount of light, the DUT 106
Of the displacement of the objective lens 105 from the focus position,
It is possible to measure the minute displacement and the surface roughness of the DUT 106.

On the other hand, in the astigmatism method, astigmatism is applied to a detected image by using a cylindrical lens, and the displacement amount of the position of the object to be measured is converted into image deformation. In the device, when the object to be measured is the focus position of the objective lens, the received light beam has a circular shape, but when the image forming position is close, it has an elongated elliptical shape, and when it is far, it has a horizontal shape. Since it has an elliptical shape, it is photoelectrically converted by a 4-division photodiode, the sum of each of the vertical and horizontal directions is calculated, and the output signal of the difference between them is calculated, and the output proportional to the displacement amount of the measured object is output. Is what you get.

Further, the Foucault micro-displacement measuring apparatus uses a splitting prism having an optical knife edge effect to split a light beam reflected from an object to be measured into two light beams and make them enter a two-divided photodiode. When the distance between the objective lens and the surface to be measured changes, the angle of divergence of the reflected light beam from the object to be measured changes, which causes a difference in the amount of light received by the two-divided photodiodes. It is what you want.

[0007]

In the above-mentioned conventional optical micro-displacement measuring device, it is generally said that the reflectance of the object to be measured 106 needs to be 10% or more, and the reflectance is actually 10%. There is a problem that the measurement accuracy is low when the following small displacement or surface roughness is measured for the object to be measured.

In view of such circumstances, it is an object of the present invention to provide an optical micro-displacement measuring method and device capable of highly accurately measuring micro-displacement and surface roughness of an object having a low reflectance.

[0009]

In the optical micro-displacement measuring method according to the present invention which achieves the above object, a light beam from a light source is irradiated onto an object to be measured by an irradiation optical system and is incident on a focus error detecting optical system. In an optical micro-displacement measuring method for detecting a focus error from the intensity distribution of a reflected light beam, the light amount of the reflected light beam from the irradiation optical system that enters the focus error detection optical system when the object to be measured does not exist is used as the correction light amount. Measured in advance, when measuring a small displacement of the object to be measured, the correction light amount is subtracted from the total light amount of the reflected light beam incident on the focus error detection optical system to obtain an intensity distribution,
Further, the optical micro-displacement measuring device according to the present invention has an irradiation optical system for irradiating the object to be measured with a light beam from a light source, and a distribution detector for detecting an intensity distribution of a reflected light beam incident on an erroneous division detector. An optical micro-displacement measuring device having a focus error detection optical system for detecting a focus error, the light source emitting light splitting means for splitting a light beam from the light source and sending one to the irradiation optical system, and the light source output. A light amount detecting detector that receives the other light beam split by the light splitting means and detects the light amount, and a circuit that subtracts a constant magnification of the output of the light amount detector from the output of the split detector of the focus error detection optical system. And having.

[0010]

Operation: Focus error detection by the focus error detection optical system
Although it is determined by the output difference in the split detector, in order to correct the difference in reflectance of the object to be measured, the displacement difference is usually obtained by dividing the output difference by the total output. However, other than the object to be measured, for example, the reflected light flux from the objective lens in the irradiation optical system will also enter the focus error detection system, and if the reflectance of the object to be measured is small, it will not be in the object to be measured. Has a great influence on the amount of displacement of the reflected light flux. The present invention subtracts the output corresponding to the reflected light flux other than the measured object from the output of the split detector, and accurately measures the displacement amount and the like even when the reflectance of the measured object is small. Further, in the apparatus having the above-described configuration, the subtraction for subtracting the constant magnification of the output of the light quantity detecting detector that receives a part of the light split by the light source outgoing light splitting means from the output of the split detector of the focus error detection optical system. Since it is equipped with a circuit, if the output magnification of the detector for light quantity detection is set so that the output from the subtraction circuit becomes zero in the absence of the object to be measured, the object to be measured is changed according to the fluctuation of the light quantity. The reflected light flux other than the object to be measured is removed.

[0011]

EXAMPLES The present invention will be described below based on examples.

FIG. 1 shows the configuration of an optical micro-displacement measuring device according to an embodiment. In the figure, 1 is a semiconductor laser which is a laser light source, 2 is a collimating lens, 3 is a polarization beam splitter, 4 is a quarter wavelength plate, 5 is an objective lens, 6 is an object to be measured, 7 is a beam splitter, 8A, Reference numeral 8B is a critical angle prism, 9A and 9B are two-divided light receiving elements, and 10 is a light amount detecting detector for receiving the light transmitted through the polarization beam splitter 3. In the apparatus of this embodiment, the semiconductor laser 1
The laser light from is converted into a parallel light beam by the collimator lens 2 and is S-polarized through the polarization beam splitter 3 to be 1
Although it is guided to the / 4 wavelength plate 104, if the laser light from the semiconductor laser 1 is slightly deviated from the S-polarized light, a part of the light will pass through the polarization beam splitter 3 and the light amount detection detector 1
Enter 0. By monitoring the output of the light quantity detecting detector 10, the output fluctuation of the laser light from the semiconductor laser 1 can be detected.

On the other hand, the laser light guided to the quarter-wave plate 4 is converted into a circularly polarized light beam, and then is condensed on the surface of the DUT 6 through the objective lens 5. The laser light reflected by the DUT 6 is P-polarized by the quarter-wave plate 4, then passes through the polarization beam splitter 3, is guided to the beam splitter 7, and is dispersed. The separated lights are reflected by the critical angle prisms 8A and 8B whose reflecting surfaces are set to the critical angles, respectively, and are incident on the two-divided light receiving elements 9A and 9B, and the light amounts are detected. In this embodiment, the two-division light receiving element 9
The displacement and surface roughness of the DUT 6 are obtained from the outputs of A and 9B and the light amount detecting detector 10.

The output of the two-divided light quantity element 9A for the reflected light flux of the object to be measured 6 is A, B, the output of the two-divided light receiving element is C,
Assuming D, the normal displacement meter output is set to the value shown by the following expression 1. Here, (A + B + C +
The reason for dividing by D) is to correct the reflectance of the DUT 6.

[0015]

[Equation 1]

However, in practice, outputs a, b, c, other than the object 6 to be measured, corresponding to the reflected light flux from the objective lens 5, for example.
d is output to the two-divided detectors 9A and 9B. However, a,
b, c and d are constant irrespective of the displacement of the DUT 6 and a =
b = c = d. Therefore, the actual displacement meter output is expressed by the following expression 2.

[0017]

[Equation 2]

That is, the displacement meter output in the normal case is (a
+ B + c + d) is divided by a larger value. This error is not a problem when the reflectance of the DUT 6 is large, but when the reflectance is small, the error of the displacement amount becomes large. That is, as shown in FIG. 2, the inclination when the reflectance is low is smaller than the inclination of the displacement-output characteristic when the DUT 6 has a high reflectance. In this embodiment, such an error is corrected as follows.

A displacement output circuit in this embodiment is shown in FIG. In the figure, 9A, 9B and 10 are the two-divided light receiving elements 9 of FIG.
It corresponds to A, 9B and the detector 10 for detecting the light amount. And
Preamplifiers 11A, 11B, 11C and 11D are respectively connected to the light receiving elements of the two-divided light receiving elements 9A and 9B, and four preamplifiers 12A, 12B, 12C and 12D are connected in parallel to the light amount detecting detector 10. It is connected to the. These preamplifiers 11A to 11D and preamplifier 1
2A to 12D are subtractors 13A, 13B, 13C, 13
It is connected to D, the difference between 11A and 12A in 13A, the difference between 11B and 12B in 13B, 11 in 13C.
The difference between C and 12C, and the difference between 11D and 12D in 13D are output. Also, the subtractor 1
3A and 13C are connected to the adder 14A, and the subtractor 13A
B and 13D are connected to the adder 14B. Further, the difference between the adders 14A and 14B and the sum are obtained by the subtracter 15 and the adder 16, respectively. Then, a value obtained by dividing the output of the subtractor 15 by the output of the adder 16 by the divider 17 is output.

In the device of this embodiment, the following adjustments are made in advance. First, the DUT 6 is placed at the focal position, and the preamplifiers 11A to 11D are adjusted so that the outputs of the divided light receiving elements 9A and 9B at A, B, C, and D become the same. Next, the amplification factors of the preamplifiers 12A to 12D are adjusted so that the outputs of the subtractors 13A to 13D become zero in the absence of the device under test 6. As a result, the reflections other than those of the DUT 6 are subtracted.
It will be canceled and corrected in A to 13D. In addition, even if the output of the light source 1 fluctuates, the light receiver 10 is correspondingly changed.
The amount of light incident on is also changed, and accordingly, the subtracters 13A to 13A-1
Since the subtraction amount in 3D also varies, appropriate correction is always performed according to the variation in the output of the light source 1.

As described above, in this embodiment, the output (a, b, c,
Since d) is subtracted, an appropriate displacement or the like can be measured even when the reflectance of the DUT 6 is low. It was confirmed as follows that the decrease in measurement accuracy due to an absolute light amount shortage when the reflectance of the DUT 6 was low was not a problem.
For example, the reflectance of the DUT 6 is 1%, the total loss due to each optical element is 50%, and the light amount detectable by the light receiving element is 1 μm.
Assuming that the transmission loss of W and the critical angle prism is 50%, the required light amount P is P = (1 x 10 ^-6 x 4) / (0.01 x 0.5 x 0.5) = 1.6 mW. I won't.

Although the embodiments have been described using the critical angle method, it goes without saying that the present invention can be similarly applied to the methods using the astigmatism method and the Foucault method. Further, the correction method is not limited to the circuits and adjustment methods shown in the embodiments.

[0023]

As described above, according to the present invention, minute displacement and surface roughness of an object having a low reflectance can be measured with high accuracy.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an optical micro-displacement measuring device according to an embodiment.

FIG. 2 is a displacement-output characteristic diagram.

FIG. 3 is a circuit diagram of displacement output of the device of FIG.

FIG. 4 is a configuration diagram showing an optical micro-displacement measuring device according to a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Collimating lens 3 Polarization beam splitter 4 1/4 wavelength plate 5 Objective lens 6 Object to be measured 7 Beam splitter 8A, 8B Critical angle prism 9A, 9B Divided light receiving element 10 Light quantity detection detector

Claims (3)

[Claims] 1. An optical micro-displacement measuring method for irradiating a light beam from a light source to an object to be measured by an irradiation optical system, and detecting a focus error from an intensity distribution of a reflected light beam entering a focus error detection optical system. When there is no measurement object, the light quantity of the reflected light flux from the irradiation optical system that enters the focus error detection optical system is measured in advance as a correction light quantity, and the focus error detection is performed when measuring a small displacement of the DUT. An optical micro-displacement measuring method, characterized in that the intensity distribution is obtained by subtracting the correction light amount from the total light amount of the reflected light beam entering the optical system. 2. An irradiation optical system for irradiating an object to be measured with a light flux from a light source, and a focus error detection optical having a split detector for detecting a focus error from an intensity distribution of a reflected light flux incident on the split detector. An optical micro-displacement measuring device having a system, the light source emitting light splitting means for splitting a light beam from the light source and sending one to the irradiation optical system, and the other of the other split by the light source emitting light splitting means. An optical system having a light amount detecting detector for receiving a light beam and detecting the light amount, and a circuit for subtracting the output of the light amount detector from the overall output of the split detector of the focus error detection optical system. Micro displacement measuring device. 3. The means according to claim 2, wherein the output signal of the light amount detection detector is divided into the same number of outputs as the number of divisions of the division detector of the focus error detection optical system, and the divided outputs are changed. And an optical micro-displacement measuring device.