JPH0571958A - Optical measuring device for minute displacement - Google Patents

Abstract

PURPOSE:To detect the minute displacement of an object to be measured without being affected by the inclination thereof by revolving and scanning the reflected beam of a polygon mirror and detecting the diameter of the beam through a slit. CONSTITUTION:A polygon mirror 1 reflects the reflected beam L1 from an object 106 to be measured condensed by an object lens 105 and passed through a 1/4 wavelength plate 104 and a polarizing beam splitter 103 to revolve the same. The beam L1 is incident to a photodetector 2. The beam L2 emitted from a light source 4 is reflected from the other surface of the mirror 1 to be incident to a photodetector 6 to make it possible to realize the state equivalent to such a state that the beam L1 is reflected from the mirror 1 to be incident to the photodetector 2. Then, on the basis of the time when the intensity of the reflected beam becomes a predetermined ratio with respect to the max. value, the width of the predetermined ratio in intensity-of-beam distribution characteristics is calculated and, on the basis of the calculated value, the diameter of the beam reflecting the converging or diffusing degree of the reflected beam can be calculated. since the reflected beam is revolved at this time, the effect of the inclination of the object to be measured is excluded.

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 device for measuring micro-displacement and surface roughness of an object to be measured by optical means.

[0002]

2. Description of the Related Art Conventionally, for example, a critical angle method has been used as an apparatus for detecting a displacement amount of an object to be measured with respect to a focus to measure a minute displacement or surface roughness of the object to be measured (Japanese Patent Laid-Open No. 59-59).
-90007), a method using an astigmatism method (see JP-A-60-186705), or a method using the Foucault method (optical technology contact vol 26, NO1).
1, 1988. P773 to 784) are known.

Of these, the critical angle method utilizes the property that the intensity of a light beam incident near the inherent critical angle of a prism exhibits a sharp change with respect to a minute angle change.

FIG. 4 shows the configuration of a conventional optical micro-displacement measuring device using the critical angle method. In the figure, 101 is a laser light source, 102 is a collimating lens, 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 angles. Prism, 109a,
109b is a two-divided light receiving element.

The laser light from the laser light source 101 is converted into a parallel light flux by the collimator lens 102, 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 a quarter wavelength plate 1.
After being converted into P-polarized light at 04, the polarization beam splitter 103
Is transmitted to the beam splitter 107, where it is dispersed. The split light is reflected by the critical angle prisms 108a and 108b whose reflection surfaces are limited to the critical angle, and is incident on the two-divided light receiving elements 109a and 109b, respectively, and the light amount is detected.

As described above, when the object 106 to be measured is located at the focal point of the objective lens 105, the reflected light becomes parallel light, and the reflectance at the critical angle prisms 108a and 108b is constant for all luminous fluxes. Split light receiving element 109a,
The amount of light received by 109b 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 lower side in the figure of the angular prism 108a and the right side in the figure of the critical angle prism 108b become smaller than the critical angle, and the reflectance decreases, causing a difference in the amount of light received by the two-divided light receiving elements 109a and 109b.

Similarly, the object to be measured 106 is the objective lens 105.
When it is located closer to the focal point of, the reflected light becomes divergent light, which is the opposite of the above case.

As a result, the two-divided light receiving elements 109a, 109
There is a difference in the amount of light received by 9b. The displacement of the object 106 to be measured from the focus position of the objective lens 105 can be detected from such a difference in the amount of light, and the minute displacement and surface roughness of the object 106 can be measured.

On the other hand, the astigmatism method applies astigmatism to a detected image by using a cylindrical lens and converts the displacement amount of the position of the object to be measured into image deformation. In the micro-displacement measuring device according to this method, when the object to be measured is at the in-focus position of the objective lens, the received beam has a circular shape, but when the imaging position is close, it has a vertically elongated elliptical shape. When it is distant, it becomes a horizontally long elliptical shape, so this is photoelectrically converted by a four-division photodiode, the sum of each of the vertical direction and the horizontal direction is calculated, and the output signal of the difference between them is calculated. The output is obtained in proportion to the amount of displacement.

The Foucault micro-displacement measuring apparatus uses a split prism having an optical knife edge effect to split a reflected light beam 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 object to be measured changes, the divergence angle of the reflected light beam from the object to be measured changes, so that the amount of light received by the two-divided photodiodes differs. It is what you want.

[0011]

When the object to be measured is tilted in the above-described optical micro-displacement measuring device, the reflected light beam axis is moved in parallel with the incident light beam axis. For example, in the device shown in FIG. 4, when the DUT 106 is tilted by θ,
As shown in FIG. 5, the reflected light beam axis 111 is translated by Δx with respect to the incident light beam axis 110. Then, in this case, when the focal length of the objective lens 105 of the DUT 106 is f, the relationship of Δx = ftan 2θ is established.

By the way, in the apparatus shown in FIG. 4, the reflected light is separated into two light-receiving elements 109a and 109b.
The light is received by, and the above-mentioned deviation of the optical axis Δx
Occurs in opposite directions in the two-divided light receiving elements 109a and 109b, so that the difference in received light amount due to Δx is theoretically offset.

However, the tilt angle θ of the DUT 106 is
Is larger, the reflected light beam is divided into two split light receiving elements 109.
There is a problem in that it becomes out of the light-receiving surface of a and 109b, and it becomes impossible to measure minute displacement and surface roughness.

All the conventional small 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 micro-displacement and surface roughness even when the object to be measured is greatly inclined.

[0016]

The structure of the present invention which achieves the above object is as follows.

By irradiating the object to be measured with a light beam from the light source and detecting the degree of convergence or diffusion of the reflected light beam from the object to be measured, which is based on the amount of deviation from the focus of the object to be measured, An optical micro-displacement measuring device adapted to detect a micro-displacement of an object to be measured based on the displacement amount, the multi-face being driven to rotate the reflected light flux while reflecting the reflected light flux. The mirror and the reflected light flux that is reflected by one surface of the polygon mirror and rotates are detected, the maximum value of the light intensity of this reflected light flux is stored, and the light intensity of the reflected light flux reflected by the next surface of the polygon mirror is stored. It is characterized by further comprising a light beam detecting means for storing a time for which a predetermined ratio to the maximum value is obtained and for obtaining a light beam diameter of the reflected light beam from this time and the rotation speed of the polygon mirror.

[0018]

According to the present invention having the above structure, based on the time when the light intensity of the reflected light flux becomes a predetermined ratio with respect to its maximum value,
The width of the predetermined ratio on the light intensity distribution characteristic can be obtained, and the width of the light flux that reflects the degree of convergence or diffusion of the reflected light flux can be obtained from this width.

At this time, in the present invention, since the reflected light flux is rotated, even if the reflected light flux deviates from the center of the objective lens due to the inclination of the object to be measured, this is not affected.

[0020]

Embodiments of the present invention will now be described in detail with reference to the drawings. Note that the same parts as those in FIG. 4 are denoted by the same reference numerals, and overlapping description will be omitted.

FIG. 1 is an explanatory view showing the outline of the configuration of an embodiment of the present invention. As shown in the figure, the polygon mirror 1 which is a polygon mirror passes through the center of one surface of the polygon mirror 1
The optical axis of the objective lens 105 and the center line along the axial direction of a (the direction perpendicular to the plane of the drawing) and the optical axis of the objective lens 105 are disposed so as to intersect with each other and are rotatable. .. Thus, the polygon mirror 1 is reflected by the DUT 106, condensed by the objective lens 105, and at the same time 1/4.
The reflected light beam L ₁ that has passed through the wave plate 104 and the polarization beam splitter 103 is reflected and rotated.

The photodetector 2 is arranged above the one surface of the polygon mirror 1 in the drawing, passing through the center of the one surface, and on a line orthogonal to the optical axis, and in the axial direction of the rotary shaft 1a. The reflected light beam L _{1 which} is rotated by the polygon mirror ₁ is made incident through the slit 3 extending in parallel.

The light source 4, the collimating lens 5 and the photodetector 6 are for synchronizing the timing of measurement. Of these, the light source 4 and the collimator lens 5 are
The polygon mirror 1 is arranged above the other surface of the polygon mirror 1 at a position which is line-symmetrical with respect to the vertical center line of the polygon mirror 1. Thus, the optical axis of the collimator lens 5 is orthogonal to the optical axis of the objective lens 105 at the center of the other surface of the polygon mirror 1. Further, the photodetector 6 is arranged at a position such that its center is on the optical axis of the objective lens 105. Thus, the light beam L emitted from the light source 4 is in a state equivalent to the state in which the reflected light beam L ₁ is reflected by the one surface of the polygon mirror 1 and enters the photodetector 2.
₂ can be realized by being reflected by the other surface of the polygon mirror 1 and entering the photodetector 6.

FIG. 2 is a block diagram showing the luminous flux diameter detecting means according to this embodiment. This luminous flux detection means detects a reflected luminous flux L ₁ which is reflected by the polygon mirror 1 and rotates, and stores the maximum value of the light intensity of this reflected luminous flux L _{1 while} The time when the light intensity of the reflected light beam L ₁ reflected on the surface becomes a predetermined ratio (half in this embodiment) with respect to the maximum value is stored, and the reflected light beam is calculated from this time and the rotation speed of the polygon mirror 1. The light beam diameter of L ₁ is obtained, and the photodetector 2 shown in FIG. 1 and the light source 4, the collimator lens 5, and the photodetector 6 that form the synchronization system are shown.
Is included.

As shown in FIG. 2, the photodetector 2 converts the incident reflected light beam L ₁ into an electric signal, and this reflected light beam L ₁
The light intensity signal S ₁ representing the light intensity of is supplied to the A / D converter 8 via the amplifier 7. The A / D converter 8 converts the light intensity signal S ₁ into a digital signal and supplies the digital signal to the peak hold circuit 9 and the half-value period detection circuit 10. The oscillator 11 supplies a clock pulse S ₂ defining the sampling timing to the A / D converter 8, the peak hold circuit 9 and the half-value period detection circuit 10. Thus, at the rising edge of the clock pulse S ₂ , the light intensity signal S ₁ which is the output signal of the photodetector 2 is taken into the A / D converter 8 and converted into a digital signal, and this digital signal is also held by the peak hold circuit 9 And the half-value period detection circuit 10. The peak hold circuit 9 receives the light intensity signal S during the _first scanning period by the polygon mirror 1, that is, during the period in which the reflected light flux L ₁ is rotated on one surface of the polygon mirror 1.
Memorize the maximum value of ₁ . The half-value period detection circuit 10 sets the half value of the maximum value of the light intensity signal S ₁ that gradually increases in the next scanning period by the polygon mirror 1, that is, in the period in which the reflected light flux L ₁ is rotated on the next one surface of the polygon mirror 1. The time from the time when it reaches the maximum value to the maximum value is gradually reduced, and the time until the time when the value reaches half the maximum value is detected again by referring to the stored contents of the peak hold circuit 9, and the light obtained as a result of this detection is detected. A half-value period signal S ₃ representing a time that is a value that is at least half the maximum value of the intensity signal S ₁ is sent to the displacement conversion unit 14.

The photodetector 6 constituting the synchronous system converts the incident light beam L ₂ into an electric signal, and outputs the trigger signal S ₃ based on the light intensity signal representing the light intensity of the light beam L ₂ to the amplifier 1.
2 to the A / D converter 8 and the counter 13. Thus, the A / D converter 8 starts sampling of the light intensity signal S ₁ by supplying the trigger signal S ₃ generated at the time of detecting the incidence of the light flux L ₂ , for example.
The output state of the counter 13 is High or Low each time the trigger signal S ₃ is supplied, that is, each time the reflected light beam L ₁ is scanned.
It becomes a state.

Depending on the output state of the counter 13, either the peak hold circuit 9 or the half-value period detection circuit 10 operates. That is, it is necessary to identify whether a certain scanning period of the reflected light flux L ₁ is a scanning period for detecting the maximum value of the light intensity signal S ₁ or a scanning period for detecting a half value period of the maximum value. One of the peak hold circuit 9 and the half-value period detection circuit 10 is identified by updating the content of the counter 13 for each scanning and using the output signal as a switching signal to identify which of the scanning periods. Is operated selectively.

The driver 15 drives the motor 16 and sends a rotation speed signal S ₄ representing the rotation speed of the motor 16 to the displacement conversion section 14. The motor 16 is driven by the driver 15 to rotate the polygon mirror 1.

The displacement conversion unit 14 obtains the light beam diameter of the reflected light beam L ₁ based on the half-value period signal S ₃ and the rotation speed signal S ₄ , and converts this light beam diameter into the displacement of the DUT 106 to represent this displacement. Displacement signal S ₅ is transmitted.

According to the above embodiment, the light intensity of the reflected light beam L ₁ is based on the time to be half the value for the maximum value, determining the half width of the reflected light beam L ₁ of the light intensity distribution characteristic on be able to. This half width is equal to the reflected light flux L ₁
It reflects the luminous flux diameter of.

More specifically, when the measurement site of the object to be measured 106 is on the objective lens 105 side with respect to the focus of the objective lens 105, the reflected light beam L ₁ becomes diffused light, and on the opposite side of the objective lens 105. However, the degree of diffusion and convergence reflects the displacement from the focus. At this time, the light intensity distribution characteristic of the reflected light flux L ₁ detected by the photodetector 2 becomes the characteristics shown in FIGS. 3A to 3C as the polygon mirror 1 rotates. Therefore, on the light intensity distribution characteristics as shown in FIGS. 3A to 3C, the half value width that is a period in which the light intensity becomes half of the maximum value is detected to calculate the light flux diameter of the reflected light flux L _1. can do.

Incidentally, in FIG. 3A, the measurement site of the object to be measured 106 is closer to the objective lens 10 than the focus of the objective lens 105.
3 is on the focus side, FIG.
(C) shows the case where it is on the opposite side of the objective lens 105 from the focus. These FIG. 3 (a) -FIG. 3 (c)
As shown in, the half widths W ₁ , W ₂ , W in each case
₃ is different, reflecting the luminous flux diameter, that is, the degree of diffusion / convergence of the reflected luminous flux L ₁ .

The detection of the luminous flux diameter as described above is started by the rise of the trigger signal S ₃ based on the photodetector 6 and is the first time of the luminous flux L ₁ reflected by one surface of the polygon mirror 1 which is rotationally driven by the motor 16. At the time of rotation / scanning, the maximum value of the reflected light flux L ₁ is detected in the peak hold period defined by the output state of the counter 13, and
During the second rotation, the scanning of the reflected light beam L ₁ by the next surface of the polygon mirror 1, the output state of the counter 13 is changed, for example, a half-width detection period, the clock pulses by half period detection circuit 10 The half-value period is detected based on S _2, and the half-value period signal S ₃ is transmitted. Then, the displacement conversion unit 14 obtains the luminous flux diameter of the reflected luminous flux L ₁ and detects the displacement of the DUT 106 based on this luminous flux diameter.

At this time, in this embodiment, since the reflected light beam L ₁ is rotated, even if the reflected light beam L ₁ is deviated from the center of the objective lens 105 due to the inclination of the DUT 106, this It will not be affected.

[0035]

As described above in detail with reference to the embodiments, according to the present invention, even if the object to be measured is tilted, the diameter of the reflected light beam can be accurately determined without being affected by the tilt. It is possible to accurately determine the minute displacement of the object to be measured which is uniquely determined by the diameter of the light beam.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an outline of a configuration of an embodiment of the present invention.

FIG. 2 is a block diagram showing a light flux detecting means according to the embodiment.

FIG. 3 is a graph showing a light intensity distribution characteristic of a reflected light flux that changes depending on the position of the object to be measured with respect to the focal position of the objective lens.

FIG. 4 is an explanatory view showing an outline of a configuration of an optical micro-displacement measuring device according to a conventional technique.

FIG. 5 is an explanatory diagram showing a state of a reflected light flux when an object to be measured is tilted.

[Explanation of symbols]

1 Polygon Mirror 2, 6 Photo Detector 3 Slit 4, 101 Light Source 103 Polarizing Beam Splitter 105 Objective Lens 106 Object to be Measured L ₁ Reflected Luminous Flux

Claims (1)

[Claims] 1. A method of irradiating an object to be measured with a light beam from a light source and detecting the degree of convergence or diffusion of the reflected light beam based on the amount of deviation from the focus of the object to be measured from the light beam reflected from the object to be measured. The optical micro-displacement measuring device is configured to detect the micro-displacement of the object to be measured based on the displacement amount by the above-mentioned method. And the reflected light flux that is reflected by one surface of the polygon mirror and rotates,
The maximum value of the light intensity of the reflected light flux is stored, and the time when the light intensity of the reflected light flux reflected by the next surface of the polygon mirror becomes a predetermined ratio with respect to the maximum value is stored. An optical micro-displacement measuring device comprising: a light beam diameter detecting means for obtaining a light beam diameter of a reflected light beam based on a rotation speed of a mirror.